• • • • • • • • • • • • 4.2 Carbon Dots • • • • • • • • • • • • • • • •

Keywords: Nanomaterials, Nanoparticles, Nanodevices, Polyoxometalates, Carbon Dots, POM, CDs, Photocatalysis, Spectroscopy, Ultrafast Transient Absorption, 4.3 Conclusions, 5 {P 2 W 18 }-, 7.3 Proposed Mechanism, 7.4 Conclusions, 8 General Conclusion A.1 Quantum Yield Calculation, A.2 Least-Squares Fitting and Global Analysis, A.3 Quenching Processes and Stern-Volmer Analysis, A.4 Time-Resolved Photoluminescence, A.5 Femtosecond Transient Absorption, A.6 Materials and Synthesis

before nor their properties have been characterised. Through our research we unveiled the dynamics occurring between the two components down to the sub-picosecond time scale, finding that an ultrafast electron transfer takes place from carbon-dots to polyoxometalates.

The nanodevices we have developed overcome many of the complications and limitations previously discussed. By coupling together the two species we have obtained a photoactive system considered inexpensive, stable, non-toxic, and efficient. Such a result could be proven useful for several applications in the domain of green energy production, pollutants degradation, and, more generally, photocatalysis. During the thesis work we have taken the first steps in the development of a completely new family of photocatalytic devices with applications in several different domains.

iii Titre de la thèse : Polyoxométallates photosensibilisés avec des carbon dots : élaboration de photo-anodes pour le craquage photo-électrocatalytique de l'eau Résumé : Récemment, notre société a compris que les réserves énergétiques dont nous disposons diminuent progressivement. La prise de conscience que nous devons changer la façon d'alimenter nos infrastructures a révélé la nécessité de se détourner des combustibles fossiles et de s'orienter vers le développement de solutions renouvelables: la disponibilité des combustibles fossiles va s'épuiser dans un avenir proche et, en raison de son impact élevé sur la pollution et le réchauffement climatique qui en résulte, il est de grande importance qu'une alternative à leur exploitation soit trouvée. Le recours à l'énergie solaire semble être l'une des solutions les plus viables : être capable d'utiliser l'énergie solaire pourrait une fois pour toutes mettre fin à notre dépendance aux énergies fossiles. Le présent travail découle de cette idée, car nous visons à utiliser la partie visible du rayonnement solaire comme source d'énergie nécessaire pour catalyser d'autres réactions utiles. La photocatalyse a plusieurs applications différentes qui vont de la décomposition des polluants à la conversion de l'eau jusqu'à ses composants moléculaires, l'hydrogène et l'oxygène. Ce dernier processus, appelé water splitting, pourrait fournir l'hydrogène nécessaire comme carburant dans notre société.

Dans cette étude, nous explorons le développement d'un type innovant de photocatalyseur à base de polyoxométallates et de carbon dots. Les polyoxométallates constituent une large famille de clusters d'oxydes métalliques polyanioniques. Ces espèces moléculaires se sont avérées être de bons catalyseurs avec des applications dans un large éventail de domaines.

Néanmoins, la quasi-totalité des polyoxométallates ne sont photoactifs que dans l'UV. Une approche pour résoudre ce problème consiste à photosensibiliser les polyoxométallates en les couplant à des composés sensibles à la lumière visible qui agiraient comme des donneurs d'électrons. Cette approche a déjà été explorée ; néanmoins, de nombreuses espèces utilisées comme photosensibilisateurs sont instables, toxiques ou contiennent des métaux rares et coûteux. La solution que nous avons tenté d'explorer pour résoudre ce problème consiste à utiliser des carbon dots, une nouvelle famille de nanoparticules fluorescentes à base de carbone. Ces espèces nanométriques peuvent absorber la lumière visible et se sont avérées capable de transférer facilement l'énergie absorbée aux composés couplés. De plus, ces nanoparticules ne sont pas chères et sont souvent considérées comme non toxiques. Dans notre étude, nous avons réussi à coupler des carbon dots et des polyoxométallates : avec nos résultats, nous visons à donner une preuve de concept pour le développement de dispositifs photocatalytiques où les propriétés proéminentes des polyoxométallates sont alimentées par l'énergie solaire récoltée par les carbon dots. À notre connaissance, ces nanohybrides n'ont jamais été développés auparavant et leurs propriétés n'ont pas été caractérisées. Grâce à nos travaux, nous avons exploré la dynamique se produisant entre les deux composants jusqu'à l'échelle des sub-picosecondes, constatant qu'un transfert d'électrons ultra-rapide a lieu des carbon dots aux polyoxométallates. Les nanodispositifs que nous avons développés surmontent de nombreuses complications et limitations précédemment évoquées concernant la photocatalyse. En couplant ensemble polyoxométallates et carbon dots nous avons obtenu un système photoactif qui est peu coûteux, stable, non toxique et efficace. Un tel résultat pourrait se révéler utile pour plusieurs applications dans le domaine de la production d'énergie verte, de la dégradation des polluants et, plus généralement, de la photocatalyse. Au cours de ce travail de thèse, nous avons fait les premiers pas dans le développement d'une toute nouvelle famille de dispositifs photocatalytiques avec des applications dans plusieurs domaines différents. Abstract: In recent times our society has become aware that the world energy reserves are progressively dwindling. The realisation that we have to change how our infrastructures are powered has unveiled the need to turn away from fossil fuels and move towards the development of renewable solutions: fossil fuel availability is bound to end in the near future and, due to its high toll on pollution and the consequent global warming, it is of uttermost importance that an alternative to their exploitation is found. Turning to solar energy seems to be one of the most viable solutions: being able to utilise the sun power could once and for all stop our dependence on fossil fuels. The present work stems from this very idea, as we aim to use the sun radiation as the power source needed to catalyse useful reactions.

Mots

Photocatalysis has several different applications which go from the decomposition of pollutants to the conversion of water down to its molecular components, hydrogen and oxygen. This latter process, named water splitting, could provide the hydrogen needed as fuel in our society. In this research we explore the development of an innovative type of photocatalyst based on polyoxometalates and carbon-dots. Polyoxometalates are a broad family of polyanionic metal oxide clusters, generally comprised of transition metals in their highest oxidation state linked to oxygen atoms. These molecular species have been found to be good catalysts with applications in a wide array of fields. Nonetheless almost all polyoxometalates are only UV-photoactive. A common approach for solving this issue consists in photosensitising polyoxometalates by coupling them to molecules sensitive to visible light; these sensitisers would act as electron donors thus activating the catalyst. This approach has been previously attempted; nonetheless, many species used as sensitisers are either unstable, toxic, or contain rare and expensive metals. To overcome this issues we tried using carbon-dots, a novel family of fluorescent nanoparticles based on carbon and containing oxygen, hydrogen and nitrogen. This nanosized specie is able to absorb visible light and readily transfer the absorbed energy to coupled compounds.

Moreover, these nanoparticles are extremely cheap and are often considered not-toxic. In our study we managed to couple together carbon-dots and polyoxometalates: with our results we aim to give a proof of concept towards the development of photocatalytic devices where the prominent properties of polyoxometalates are powered via the solar energy harvested by carbon-dots. To our knowledge these nanohybrids have never been developed

Introduction xiii

Introduction en Français xvii

I State of the Art

1 Polyoxometalates 3 1.1 Nomenclature • • • • • • • • • • • • • • • • • • • • • • • • 5 1.2 Synthesis • • • • • • • • • • • • • • • • • • • • • • • • • 6 1.3 Classification • • • • • • • • • • • • • • • • • • • • • • • • 1.4 Structures • • • • • • • • • • • • • • • • • • • • • • • • • 10 1.5 Characteristics • • • • • • • • • • • • • • • • • • • • • • • 19 1.6 Applications • • • • • • • • • • • • • • • • • • • • • • • • 23 1.7 Limitations and Outlook • • • • • • • • • • • • • • • • • • • 36
2 Carbon Dots 39

2.1 Synthetic Approaches • • • • • • • • • • • • • • • • • • • • • 2.2 Properties and Characteristics • • • • • • • • • • • • • • • • • 49 2.3 Applications • • • • • • • • • • • • • • • • • • • • • • • • 60 2.4 Future Perspectives • • • • • • • • • • • • • • • • • • • • • 3 Aim of the Work

II Experimental Results

Introduction

In recent times our society has become aware that the world energy reserves are progressively dwindling. The realisation that we have to change how our infrastructures are powered has unveiled the need to turn away from fossil fuels and to move towards the development of renewable solutions. As our need for more energy keeps on growing the usage of oil and gas to fuel our everyday lives has revealed its limitations: fossil fuel availability is bound to end in the near future and, due to its high toll on pollution and the consequent global warming, it is of uttermost importance that an alternative to their exploitation is found, even before reaching the bottom of our supplies.

As the awareness on this topic has been raising for a while, researchers have already been exploring this issue for several years in order to find a green alternative to hydrocarbons.

Amongst the possible answers to our problems, turning to solar energy seems to be one of the most viable solutions. With its constant and realistically inexhaustible * presence, being able to utilise the sun power could once and for all stop our dependence on fossil fuels.

Despite the optimistic premises, today photovoltaic is still not widely used. The major setback to its widespread usage is due to the fact that we are still not able to efficiently store the harvested energy; in spite of our efforts, the current battery generation is still flawed by the need for rare metals as part of their fabrication which makes production very expensive and thus infeasible on the large scale. This is why research has thus been focusing on ways to convert solar radiation to chemical fuel instead of using it directly.

The present work stems from this idea, as we aim to use the visible part of the sun radiation as the power source needed to catalyse other useful reactions. Photocatalysis has several different applications which go from the decomposition of pollutants to the conversion of water down to its molecular components, hydrogen and oxygen. This latter Introduction process, named water splitting, could provide the hydrogen needed as fuel in our society.

Before being able to reach all these goals, new photocatalysts have yet to be found: in this research we explore the development of an innovative type of photocatalyst based on polyoxometalates and carbon-dots.

Polyoxometalates are a broad family of polyanionic metal oxide clusters, generally comprised of transition metals in their highest oxidation state linked to oxygen atoms.

Common transition metals forming polyoxometalates are molybdenum, tungsten, and vanadium; this variability in their components and the fact that the metal oxide units forming these polyanions can be rearranged into several structures allow for a wide variety of chemical properties. Most importantly, these molecular species have been found to be good catalysts with applications in a wide array of fields. Nonetheless almost all polyoxometalates are only UV-photoactive, a drawback that slows down their possible industry transfer.

A common approach for solving this issue consists in photosensitising polyoxometalates by coupling them to molecules sensitive to visible light; these sensitisers, once excited by solar radiation, would act as electron donors thus activating the catalyst. This approach has been explored by using several compounds such as organic dyes and quantum dots but, once again, this strategy faces several disadvantages: as many species used as sensitisers are either unstable, toxic, or contain rare and expensive metals, this approach is not yet considered feasible.

The solution we tried to explore to solve this issue consists in using carbon-dots, a novel family of fluorescent nanoparticles based on carbon and containing oxygen, hydrogen and nitrogen. This nanosized specie is able to absorb visible light and was found to be able to readily transfer the absorbed energy to coupled compounds, thus making it a possible photosensitiser. Moreover, these nanoparticles are extremely cheap because of the ready availability of all their components and are often considered not-toxic. It is due to all of these properties that since their recent discovery many efforts have been focused on exploring their possibilities. Nowadays, several easy and fast production routes have been developed and in recent times we started pondering their application as inexpensive electron donors.

In our study we managed to couple together carbon-dots and polyoxometalates. With the results herein presented we aim to give a proof of concept towards the development of xv photocatalytic devices where the prominent properties of polyoxometalates are powered via the solar energy harvested by carbon-dots. To our knowledge these nanohybrids have never been developed before nor their properties have been characterised. Through our research we have shown how it is possible to easily obtain these coupled complexes in solution and later explored the characteristics of their interaction through the means of optical spectroscopy, unveiling the dynamics occurring between the two components down to the sub-picosecond time scale. We have thus been able to claim that an ultrafast electron transfer takes place from carbon-dots to polyoxometalates which has then been applied by using the hybrid device to drive the photocatalysis of silver ions.

The nanodevices we have developed overcome many of the complications and limitations previously discussed regarding photocatalysis. By coupling together polyoxometalates and carbon-dots we have obtained a photoactive system which is inexpensive, stable, non-toxic, and efficient. Such a result could be proven useful for several application in the domain of green energy production, pollutants degradation, and, more generally, photocatalysis.

During the thesis work we have taken the first steps in the development of a completely new family of photocatalytic devices with applications in several different domains.

In order to present all of the results, this text is divided in two parts, each with a different focus. In Part I: State of the Art the current research state regarding polyoxometalates and carbon-dots is presented. The following chapters compose this part of the thesis:

Chapter 1: Polyoxometalates presents the field of polyoxometalates, giving some informations on their classification, different structures, characteristic, and capabilities in the field of catalysis and other applications, so to relate them to the topics treated in this thesis and provide a broad view regarding their importance.

Part II: Experimental Results will instead cover the results obtained during the research work, discussing them as they are presented. This section of the work is divided in the following chapters:

Chapter 4: Materials Characterisation shows the experimental characterisation initially performed on each of the materials used during the research work. All of the structural and optical characterisations herein discussed acts as a preliminary knowledge needed to understand the steps followed in order to obtain the results presented in the subsequent chapters.

Introduction en Français

Récemment notre société a compris que les réserves énergétiques dont nous disposons diminuent progressivement. La prise de conscience que nous devons changer la façon dont nos infrastructures sont alimentées a révélé la nécessité de se détourner des combustibles fossiles et de s'orienter vers le développement de solutions renouvelables. Alors que notre besoin de plus d'énergie ne cesse de croître, l'utilisation de pétrole et gaz pour alimenter notre vie quotidienne a révélé ses limites: la disponibilité des combustibles fossiles va terminer dans un avenir proche et, en raison de son coût élevé sur la pollution et le réchauffement climatique qui en résulte, il est de grande importance qu'une alternative à leur exploitation soit trouvée, avant d'atteindre le fond de nos approvisionnements.

La prise de conscience sur ce sujet augmentant depuis un certain temps, les chercheurs explorent déjà cette question depuis plusieurs années afin de trouver une alternative verte aux hydrocarbures. Parmi les réponses possibles à nos problèmes, le recours à l'énergie solaire semble être l'une des solutions les plus viables. Avec sa présence constante et inépuisable de manière réaliste, être capable d'utiliser l'énergie solaire pourrait une fois pour toutes mettre fin à notre dépendance aux énergies fossiles.

Malgré les prémisses optimistes, le photovoltaïque n'est pas encore largement utilisé aujourd'hui. Le principal revers de son utilisation généralisée est dû au fait que nous ne sommes toujours pas en mesure de stocker efficacement l'énergie récoltée; malgré nos efforts, la génération actuelle de batteries est encore déficiente par le besoin de métaux rares dans le cadre de leur fabrication, ce qui rend la production très coûteuse et donc irréalisable à grande échelle. C'est pourquoi la recherche s'est donc concentrée sur les moyens de convertir le rayonnement solaire en carburant chimique au lieu de l'utiliser directement.

Le présent travail découle de cette idée, car nous visons à utiliser la partie visible du Plus important encore, ces espèces moléculaires se sont avérées être de bons catalyseurs avec des applications dans un large éventail de domaines. Néanmoins, presque tous les polyoxométallates ne sont que UV-photoactifs, un inconvénient qui ralentit leur éventuel transfert industrielle.

Une approche courante pour résoudre ce problème consiste à photosensibiliser les polyoxométallates en les couplant à des molécules sensibles à la lumière visible; ces sensibilisateurs une fois excités par le rayonnement solaire agiraient comme des donneurs d'électrons, activant ainsi le catalyseur. Cette approche a été explorée en utilisant plusieurs composés tels que des colorants organiques et des quantum dots mais, encore une fois, cette stratégie faces plusieurs inconvénients: en effet, de nombreuses espèces utilisées comme sensibilisants sont soit instables, toxiques ou contiennent des métaux rares et coûteux.

Pourtant, cette approche n'est pas encore considéré réalisable. Dans notre étude, nous avons réussi à coupler des carbon dots et des polyoxométallates.

Avec les résultats présentés ici, nous visons à donner une preuve de concept pour le développement de dispositifs photocatalytiques où les propriétés proéminentes des polyoxométallates sont alimentées par l'énergie solaire récoltée par les carbon dots. A notre connaissance, ces nanohybrides n'ont jamais été développés auparavant et leurs propriétés n'ont pas été caractérisées. Grâce à nos recherches, nous avons montré comment il est possible d'obtenir facilement ces complexes couplés en solution et exploré les caractéristiques de leur interaction par spectroscopie optique, dévoilant la dynamique se produisant entre les deux composants jusqu'à l'échelle des sub-picosecondes. Nous avons ainsi pu affirmer qu'un transfert d'électrons ultra-rapide a lieu des carbon dots aux polyoxométallates qui a ensuite été appliqué en utilisant le dispositif hybride pour la photocatalyse des ions argent.

Les nanodispositifs que nous avons développés surmontent de nombreuses complications et limitations précédemment évoquées concernant la photocatalyse. En couplant ensemble polyoxométallates et carbon dots nous avons obtenu un système photoactif qui est peu coûteux, stable, non toxique et efficace. Un tel résultat pourrait se révéler utile pour plusieurs applications dans le domaine de la production d'énergie verte, de la dégradation des polluants et, plus généralement, de la photocatalyse. Au cours du travail de thèse, nous avons fait les premiers pas dans le développement d'une toute nouvelle famille de dispositifs photocatalytiques avec des applications dans plusieurs domaines différents.

Introduction en Français différentes. Ici, ils incorporent souvent des hétéroatomes supplémentaires, bien que cela ne se produise pas toujours, qui peut pratiquement consister en n'importe quel élément du tableau périodique; en fait, plus de soixante-dix éléments différents ont rempli ce rôle, 1 avec des représentants de tous les groupes du tableau périodique à l'exception des gaz rares. [START_REF] Pope | Heteropoly and Isopoly Oxometalates[END_REF] Cette extrême variabilité de leur géométrie et de leur composition, fondamentalement incontestée par tous autres composés, permet le réglage fin des propriétés physiques et chimiques des polyoxométallates, ce qui, à son tour, ouvre des possibilités dans un large éventail de domaines d'application.

De nos jours, les polyoxométallates sont considérés comme un matériau clé permettant le développement de dispositifs moléculaires innovants, en particulier dans le domaine de la photoélectrocatalyse. 13 Les caractéristiques les plus significatives de POMs, auxquelles ils doivent leur popularité, sont trouvé parmi leurs propriétés électroniques et redox. Ces clusters d'oxydes métalliques sont en effet capables d'accepter et de libérer plusieurs électrons sans aucune décomposition ni modification de leur structure; 14 ,15 d'ailleurs, en plus de leur capacité à accepter des électrons de manière réversible, les POMs sont également capables de fonctionner comme réservoirs d'électrons; il existe en effet des exemples de polyoxométallates capables d'accepter plusieurs électrons dans leur structure, avec des exemples de 18 voire 32 réductions consécutives décrites dans la littérature. 1 ,16 De plus, les polyoxométallates sont très solubles en milieu aqueux 2 et peuvent être rendus soluble également dans de nombreux autres solvants en fonction du choix du contre-ion. 17 Connaissant ces propriétés significatives, il n'est pas surprenant que les POMs aient été utilisé dans un impressionnant nombre d'applications différentes. En raison de leur variabilité structurelle, 12 bonne réactivité, 13 propriétés redox largement ajustables, 16 leur faible coût et leur faible impact environnemental, 18 les polyoxometalates ont pu s'affirmer comme des matériaux ayant haute relevance dans les domaines de la catalyse, 13 de l'énergie renouvelable, 19 de la biologie, 20 de la médecine, 21 du magnétisme 22 et des science des matériaux. 23 Il y a encore quelques limitations que nous devons surmonter dans la génération actuelle des dispositifs basés sur POM afin d'atteindre leur plein potentiel. Par rapport à il y a moins de cinquante ans, la recherche sur les polyoxométallates a fait d'énormes progrès; depuis l'introduction de techniques cristallographiques abordables, et en particulier la for the fine tuning of polyoxometalates physical and chemical properties, which in turn opens up possibilities in a wide array of applicative fields.

As the history of polyoxometalates is more than two centuries old, there is still some debate on when POMs were observed for the first time. [START_REF]Metal-Oxygen Clusters: The Surface and Catalytic Properties of Heteropoly Oxometalates[END_REF] While some indicate that the first reported synthesis of a POM consists in a molybdenum blue specie dating back to 1793, 4 the first heteropoly molybdenum compound is generally ascribed to the work of Berzelius. [START_REF] Berzelius | [END_REF] In 1826, he observed a yellow precipitate forming due to the addition of ammonium molybdate to phosphoric acid, and the subsequent rapid formation of the blue mixed-valence derivatives of the compound. It was instead in 1862 that the first indication of a tungsten derived POM was given, when Marignac 6 was able to discover and characterise the first heteropoly tungstates by also managing to give their analytical composition. 2

Chapter 1: Polyoxometalates

During the following years many other polyoxometalates were investigated and hundreds of these compounds were described, but still not much was known about their structure.

In 1908 Miolati and Pizzighelli attempted to give their interpretation on the topic 7 by following ionic coordination theory, an approach further developed by Rosenheim and Jaenicke in 1917. 8 Also Pauling contributed to the subject of polyoxometalates structure with an article published in 1929, 9 but it was not until 1933 that the first real evidence was obtained: by analysing powder X-ray diffraction data, the structure of 12-phosphotungstic acid was finally unveiled by Keggin. 10 Further structural studies were few and sparse, so that in a review dated 1971 only 25 X-ray studies were reported, 11 mainly due to the technical limitations existing up to that period. It was just with the advent of new and better characterisation techniquesand especially hardware and software for computerised X-ray crystallography -that the research on polyoxometalates structures could really start thriving; just 20 years later, in a review by Pope and Müller, dozens of polyoxometalates structures and stoichiometries were categorised 1 and their properties and wide range of possible applications highlighted.

As a result of this work, a rapid increase in POMss popularity was sparked and during the 1990s the number of publications regarding this class of supramolecular metal-oxide clusters witnessed a huge growth which has yet to end. 12 Nowadays polyoxometalates are regarded as a key material allowing for the development of innovative molecular based devices, especially in the field of photoelectrocatalysis. 13 Probably the most significant characteristics of POMs, to which they owe their popularity, are found among their electronic and redox properties. These metal-oxide clusters are indeed able to accept and release several electrons without any decomposition or change in their structure; 14 ,15 moreover, in addition to their capability of accepting electrons in a reversible manner, POMs are also able to work as electron-reservoirs; there are indeed examples of polyoxometalates able to accept multiple electrons in their structure, with examples of 18 or even 32 consecutive reductions described in the literature. 1 ,16 Additionally, polyoxometalates are very soluble in aqueous environment 2 and can be made soluble also in many other solvents depending on the counterion choice. 17

Knowing of these significant properties, it should not come as a surprise that POMs have been employed in a staggering number of different applications. Because of their structural versatility, 12 good reactivity, 13 widely tunable redox properties, 16 and their 1.1: Nomenclature low cost and low environmental impact 18 POMs have been able to affirm themselves as relevant materials for the domains of catalysis, 13 renewable energy, 19 biology, 20 medicine, 21 magnetism 22 and materials science. 23 There are still some limitations that we need to overcome in the current generation of POM-based devices in order to unleash their full potential. Compared to less than fifty years ago polyoxometalate research has made huge leaps forward; since the introduction of affordable crystallographic techniques, and especially single crystal X-ray diffraction, we know more than ever about POMs structures and their synthesis, and we have been able

to obtain metal oxide clusters with properties previously unimaginable. Today, with the daily increase in the number of researches regarding polyoxometalates, we are on the right path towards the development of inexpensive and efficient POM-based devices presenting astonishing stability and unmatched catalytic properties.

Nomenclature

Before diving into the ample world of polyoxometalates, some brief considerations on the question of systematic nomenclature for this family of materials should be pointed out. The problem stems from the fact that, if one had to use the IUPAC labelling system when referring to POMs, resulting names would appear cumbersome at the very least, if not totally impractical: for instance, one should use 1.4,1.9,2.5,2.6,3.7,3.8,4.10,5.10,6.11,7.11,8.12,9.12dodeca-µ-oxo-µ 12 -(tetraoxophosphato-O 1,[START_REF] Scheele | Sämmtliche physische und chemische Werke[END_REF]9 ,O [START_REF] Pope | Heteropoly and Isopoly Oxometalates[END_REF][START_REF] Berzelius | [END_REF]6 ,O [START_REF]Metal-Oxygen Clusters: The Surface and Catalytic Properties of Heteropoly Oxometalates[END_REF]7,8 ,O 10,11,12 )tetrakis[tri-µ-oxo-tris(oxomolybdate)] when considering that the cationic part of these compounds can often be changed without affecting neither the resulting structure nor properties, exception made for the solubility in a particular solvent. Moreover, knowing the exact number of water molecules to which a particular POM structure binds to appears trivial for most studies, and thus its indication irrelevant.

Lastly, a further simplification normally pointed out by the presence of curly brackets can be found. This consists in also hiding both the charge of the resulting ion and the totality of oxygen atoms present; in the case of the former polyoxometalate we obtain {P 2 W 18 }. In this case the label fully focuses on the POM properties, as size is indicated by the number of transition metal atoms composing its structure and the eventual presence of heteroatoms readily pointed out.

In the present text all labels indicating each of the compounds used throughout the research will be clearly stated when presenting the relative polyoxometalate, while we redirect to the pertinent reference for each compound cited from the literature during this introductory chapter on POMs.

Synthesis

Polyoxometalates are usually synthesised in aqueous environment. [START_REF] Pope | Heteropoly and Isopoly Oxometalates[END_REF] The most common procedure involved in their preparation normally consists in the acidification of a solution composed of some anion of the chosen transition metal and eventually the needed heteroatom in its anionic form, as shown in the following two examples:

7 MoO 2- 4 + 8 H + --→ [Mo 7 O 24 ] 6-+ 4 H 2 O 12 WO 2- 4 + HPO 4 2-+ 23 H + --→ [PW 12 O 40 ] 3-+ 12 H 2 O
While some POMs structures are directly obtained from such an acidified mixture, additional control is often needed when trying to obtain some well defined or less stable specie; it is not uncommon that excess of some reagent, careful control of the pH and temperature, sequence of addition of the reagents or specific catalysts are required in order to obtain the desired polyoxometalate structure. Afterwards, POMs can be recovered as precipitating salt from the solution by addition of an appropriate basic counterion. It is in this step that polyoxometalates soluble in solvents different that water are obtained, for instance by using tetrabutylammonium as organic cation. Since in this case the obtained salt may be often insoluble in water, when trying to purify the compound it can be sometimes needed to perform the recrystallisation in solvents such as acetone.

Another synthetic approach, followed in particular when trying to obtain more complicated and derived structures, starts instead from preformed POMs. This route is based on the so called lacunary polyoxometalates; as detailed in the following sections regarding the different structures, a lacunary polyoxoanion is obtained by removing some of the transition metal subunits composing the POM itself. This is usually done by simple addition of a base:

α-[P 2 W 18 O 62 ] 6-OH - ---→ α 2 -[P 2 W 17 O 61 ] 10-OH - ---→ α-[P 2 W 16 O 59 ] 12-
The obtained lacunary POM, whose stability changes from case to case, can be successively used in other syntheses, for instance to obtain mixed metals polyoxoanions:

α-[P 2 W 16 O 59 ] 12-+ VO 2+ pH 4-5 ---→ α-[P 2 W 16 V 2 O 62 ] 10-
Several additional synthetic approaches have been developed during the many years of research that has interested polyoxometalates; these include other uncommon routes, which are still being explored today for their interesting purposes and design, such as one-pot hydrothermal synthesis 24 or synthesis in ionic liquids. 25 As this topic is extremely wide and POMs synthesis is not the main focus of this present thesis work, it will not be deepened further.

Classification

With the thousands of different materials found in this class of compounds, a rigorous classification is needed so to understand similarities and differences between the compounds part of this metal-oxide anions family. In the present section an introduction on how POMs structures are formed will be given before transitioning to some of the most common categorisations, that will be presented and briefly described.

As previously mentioned, at the core of polyoxometalates is a transition metal in its highest oxidation state surrounded by oxygen atoms, thus forming a {MO x } unitwith x between 4 and 7. These units usually posses either tetrahedral, square pyramidal, octahedral or pentagonal geometry, as shown in Figure 1.1. To form the full structure, the subunits are linked to each other either by sharing either one, two or three oxygen atoms respectively via corner sharing, edge sharing or face sharing, as shown in Figure 1.3.

The metal inside the unit is commonly referred to as addenda atom; while this is usually either tungsten, molybdenum or vanadium, as proposed by Baker and Glick 27 an addenda atom just needs to be able to change its coordination with oxygen from 4 to 6 upon acidification and have a positive charge high enough so to form double bonds with the terminal oxygen through p π -d π interactions.

This terminal oxygen is the one separated from the addenda metal by the shortest bound length. When referring to it, we have to understand the phenomenon of distortion of the addenda structure; even if one would imagine all of the distances between metal and different oxygens to be equal, this is not what happens. In fact, not all of the oxygen atoms are shared when forming the POM structure: the highly charged addenda atoms exert an intense dipolar attraction towards the adjacent unshared oxygens, especially in the case of those found on the exterior part of the polyoxometalate molecule which are the most polarisable; the others, being either shared between multiple addenda or found on the interior of the structure, feel less the effect. 27 As a result we observe a distorted geometry, in the sense that the metal atom does not appear to be centred in it, while being instead displaced towards the terminal oxygen. This phenomenon is clearly shown in Figure 1.2. Moreover, this asymmetry in the addenda structure has huge implications on the properties of polyoxometalates, which leads to their categorisation in three different types presented in the next subsection.

Types

Based on the coordination environment of the addenda atoms, all polyoxometalates can be classified among three different types. This subdivision, proposed by Pope in 1972, 14 is directly linked to polyoxometalates electronic properties and is based on the number of oxygen atoms which are not shared between different addenda. As previously mentioned, in order to form the different POMs structures the addenda polyhedra are bond together by sharing oxygen atoms via the connection modes shown in Figure 1.3. Nonetheless, external oxygens can be linked to a single central metal atom, being thus unshared; this fact completely changes the addenda properties and gives rise to the following classification:

• Type I POMs are those in which the addenda atoms of all the composing subunits present a single unshared oxygen atom, while all the rest are shared so to form the overall geometry. Polyoxometalates of this type can undergo reversible reduction.

• Type II POMs, instead, are those in which each of the metal addenda presents two unshared oxygen atoms. Type II polyoxometalates are more difficult to reduce and when done it is in an irreversible manner. 1

• Type III POMs, finally, have their structure composed of both kinds of addenda subunits, as each can either present one or two unshared oxygen atoms. Also this type of polyoxometalates can be reversibly reduced.

It probably appears now very clear how the presence of a single unshared oxygen atom in the subunits forming the polyoxometalate has huge consequences on the electrochemical properties of the whole compound itself. According to molecular orbital theory, octahedral metals bound to oxygen with a single unshared oxygen atom -as in the case of addenda in type I and III POMs -possess one vacant t 2g non-bonding orbital which can be populated by one or two electrons via reduction of the complex; in the case of type II POMs, where two non shared oxygen atoms are present, the addenda lacks any unpopulated non-bonding orbital, due to its participation in the π-bonding with oxygen atoms. 28 When reducing this type of POMs, the added electrons must thus populate an anti-bonding orbital, which accounts for lack of stability and irreversibility of the reduction. 2

Categories

Broadly speaking, polyoxometalates can be placed into three different subsets based on their composition and structure:

• Heteropolyanions are all the metal oxide polyanions including one or more heteroanion frameworks in their structure. This framework is usually a tetrahedral {XO 4 } template but different geometries are found, such in the case of the {XO 6 } octahedral template found in Anderson-Evans POMs; here X is usually either P or S but there is no general restriction regarding which element can fulfil this role. In the literature examples of more than 70 different species being incorporated as heteroatoms can be found, with representative from all groups of the periodic table except noble gases. 1 ,2 ,27 Due to their stability, heteropolyanions are the most studied POM family.

• Isopolyanions are instead POMs which are laking any kind of heteroatom and are solely comprised of the metal oxide framework; despite being less stable than their heteroanion counterpart, 29 they display interesting properties such as high charges and, consequently, they can be used as constitutive units for the development of nanodevices. 23 • Molybdenum blue and molybdenum brown reduced POM clusters, lastly, are an interesting class of polyoxometalate whose first report dates back, as previously mentioned, to 1793; nonetheless, it was not until 1995 that their structure was revealed, showing a surprising ring topology. 30 As researchers have shown how it is possible to modify the synthesis conditions so to also obtain spherical POM clusters, 31 this highly reduced class of compounds is considered very appealing for its potential and eventual applications.

Some of the most relevant structures found in these three categories will be presented in the next subsection, in order to give some insight on the wide world of POMs topologies.

Structures

Polyoxometalate structures are numerous and diverse, so that a full description of all the different geometries formed by this class of metal oxide clusters would be impossible.

Moreover, recent advances in POM chemistry has allowed the development of new derived topologies, obtained using as starting point other preformed POMs. As a consequence, in the next part of this first chapter only the classical structures, those renown for historical and stability reasons, will be presented; in the last part of this section, finally, some of the other structures, and in particular those deemed relevant to the purposes of this thesis work, will be described.

Lindqvist

Discovered in 1950, 32 the Lindqvist polyoxometalate is the most representative structure of the isopolyanions category.

Anderson-Evans

In 1937, Anderson suggested a structure for several heteropolyanions composed by six transition metal ions. 33 His proposition was finally confirmed in 1948, when Evans resolved the structure of [TeMo 6 O 24 ] 6 -by studying it via single-crystal X-ray diffraction. 34 The Anderson-Evans heteropolyanions are composed by a central heteroatom forming a {XO 6 } octahedron, which is in turn surrounded by six addenda atoms each forming a {MO 6 } unit; all subunits are linked via oxygen atoms in an edge sharing configuration, as shown in Figure 1.5. In this structure all of the addenda atoms present two mutually unshared oxygens, thus making these kind of POMs part of the type II category.

In addition, two different kinds of POMs are part of this family, based on the oxidation Evans heteropolyanion, in ball-and-stick (left) and polyhedral (right) representation; metal cations are represented in teal colour, heteroatoms in orange, and oxygen anions in red; reproduced from ref. (26 ).

state of the central heteroatom.

Keggin

The first polyoxometalate structure to be ever resolved was that of [PW POMs are of type I.

Finally, this structure can exist as five different isomers, 35 denoted with the greek letters α, β, γ, δ, and ε. These differ from each other by the number of {M 3 O 13 } triads which have been rotated by an angle of π /3 along an axis passing from their centre and intersecting the heteroatom, as shown in Figure 1 cap and belt for the full POM structure; each of the two belt sections, if considered along the heteroatom unit contained within itself, can be viewed as an Anderson-Evans architecture. As for Keggin POMs, inside each triad all {MO 6 } subunits are linked to each other by the edge sharing of two oxygens; the connections between subunits of different belts and caps are instead made by corner sharing. Inside each belt, edge sharing and corner sharing connection modes are alternating so to bind together the {MO 6 } octahedra.

A representation of the structure is shown in Figure 1.8. Also polyoxometalates adhering to this structure, as well as the Keggin ones, are of type I and can thus be reversibly reduced.

As for the Keggin POMs, also the Wells-Dawson structure exists in different isomers.

The α configuration is characterised by the alignment between all terminal oxygen subunits vertices of the two different caps; the same vertices are also aligned to the corner sharing bridged oxygens of the belts. When rotating one of the two {M 3 O 13 } cap triads by a π /3 angle along an axis intersecting both heteroatoms, the β isomer is obtained; in such configuration the terminal oxygen vertices of the rotated triad are now aligned with the edge sharing bridged oxygens of the belt which they are connected to, instead of the corner sharing ones. If the same π /3 rotation is applied to both caps then the isomer indicated by the greek letter γ is obtained; the alignment between terminal oxygens of different caps is then restored as in the case of the α isomer.

In addition, each of these three isomers exist in two different configurations, based on the alignment of the inner heteroatom {XO 4 } tetrahedra. While normally the vertices of both heteroatom subunits are aligned, the α * , β * and γ * isomers are obtained when rotating one of the two tetrahedra -and thus half of the POM structure -by a π angle.

All of the Wells-Dawson isomers are shown in Figure 1.9.

While in the past the procedure commonly followed when synthesising Wells-Dawson

POMs usually yielded a mixture of the α and β isomers, which were afterwards separated in order to obtain solutions of the isolated isomers, 43 synthetic routes directly providing only pure α isomer do nowadays exist. [START_REF] Mbomekalle | [END_REF] Regarding the rest of the six isomers, only the γ and γ * have also been isolated from a synthesis, 45 in line with density functional theory calculations claiming that the stability order goes as follow: 48 has been regarded as very important for green catalytic processes. 49 Other structures have been used

α > β > γ > γ * > β * > α * .
for the development of self-catenated frameworks, 50 as in the case of the Dexter-Silverton POM architecture (Figure 1.11), which has been reported for the first time in 1968. 51 Polyoxometalates are also known for their ability to form very big and symmetrical structures. As mentioned in the introduction to this chapter, the first POM to ever be obtained, whose synthesis was reported by Scheele in 1793, 4 was a kind of molybdenum blue POM cluster; when its structure was finally resolved in 1995, 30 it revealed to be composed by more than 700 atoms, with a molecular mass higher than 24000 g mol -1

(Figure 1.12). Despite the already impressive size, this last metal oxide framework is not even the biggest POM structure ever isolated: in 2002, the synthesis of a protein-sized molybdenum-blue polyoxometalate was reported, composed by 368 Mo atoms and having a molar mass of almost 80000 g mol -1 (Figure 1.13). 52 All of these architectures are just some of the many examples composing the vast polyoxometalate diversity. Again, a complete description of the many different POMs structures which have been reported during their 200 years long history would be impossible. Nonetheless, instead of focusing on any particular geometry, in the following final subsections some derived structures will be presented. These have been chosen for their importance relatively to this work of thesis but are nevertheless relevant for a general comprehension of polyoxometalates architectural versatility; in fact, the following should make clear how POMs properties can be enhanced and expanded by careful incorporation of other components, without having to sacrifice the overall structural stability.

Sandwich polyoxometalates

As briefly anticipated in Section 1.2 when discussing the synthesis of polyoxometalates, a lacunary POM is obtained by the removal of some of the addenda atoms from a preformed structure. This class of polyoxometalate results to be more negative than the starting structure and thus more reactive; as such, many lacunary species cannot be isolated on their own but, those who can, are often used as ligands in the synthesis of coordination compounds.

Transition metal substituted POMs (TMSPs) are clusters formed when the vacant positions left in the lacunary structure of a polyoxometalate is filled with d-block transition metals. The inclusion of these heterometals can have several benefits, from the expansion of the possible POMs structures to the inclusion of new properties based on those of the chosen transition element. As such, many TMSPs have found application in magnetism 54 and water oxidation catalysis 55 ,56 just to name a few.

TMSPs have often been synthesised starting from either Keggin or Wells-Dawson heteropolytungstates, due to the ease of isolation of their lacunary counterparts. Between the many different typologies that can be obtained are found the so-called sandwich -, which can be therefore reacted with the chosen metallic cations in order to form the desired sandwich POM structure; heterophosphates are indicated in the final resulting compound; reproduced from ref. (53 ).

A representation of the general synthetic route for this category of derived polyoxometalates is shown in Figure 1.14. In this scheme, the first step represents the removal of the addenda atoms forming the cap of the POM, which yields the relative lacunary structure; in the second step the sandwich geometry is obtained via the incorporation of transition metal atoms between two lacunary units. This route, herein only representing the particular case involving α-[P 2 W 18 O 62 ] 6 -, represents the general path followed also when starting from different structures, such as the Keggin one; 60 moreover, depending on the overall reaction conditions, the same approach can also result in several other architectures, such as trimeric or tetrameric POMs clusters. 54

Hybrid polyoxometalates

Thanks to their extreme versatility, polyoxometalates can also introduce in their structure organic compounds; this kind of metal oxide clusters are referred to as hybrid POMs, hinting to the mixed organic-inorganic nature of the cluster.

Depending on the kind of interactions binding together polyoxometalate and organic compound, hybrid POMs can be divided among two different classes. Complexes of class I are characterised by the presence of weak interactions linking together the two components, such as van-der-Waals forces, hydrogen bounds or ionic interactions; when referring instead to class II hybrid POMs, the organic and inorganic parts are covalently bound one to each other. 63 While class I clusters are appealing for their ease of synthesis, covalently bound complexes often present several advantages, especially when considering the stability of the resulting compound and the enhanced interaction between the two different parts. 61 Several synthetic routes allow to obtain hybrid POMs. Some compounds are directly isolated from a self assembly process when the synthesis proceeds in an organic solvent, 64 while other are obtained via direct grafting of the organic molecule to highly reactive lacunary POMs. 65 In many cases, more complicated strategies have been followed, for instance by performing a methatetical exchange between multiple bonded organic ligands and the polyoxometalate terminal oxygens, which have higher reactivity compared to the other shared oxygen atoms. 66 An interesting approach is based on the use of TMSPs,

where the metal ligands of these complexes can often be substituted for other organic molecules. 67 Finally, an increase in the organic linker complexity can be obtained by derivatisation of a preformed hybrid platform, 62 as shown in Figure 1.15.

Characteristics

In this section, some of the many characteristics and properties shared between different kind of POMs will be presented and their usefulness discussed.

Solubility and behaviour in solution

Polyoxoanions salts are in general very soluble in aqueous environment and, as previously pointed out, varieties soluble in any solvent can be obtained depending on the choice of counterion. 2 ,68 Even if these metal oxide clusters due to their ionicity are highly soluble, some POMs have been observed forming large structures through self-assembly processes. 69 In addition, hybrid POMs can often have amphiphilic properties and be used as surfactants, where the polyanion acts as polar head and the organic part as hydrophobic tail. 70 By careful choice of the hybrid metal oxide cluster, the resulting structure has then been used to obtain different architectures, such as the micelles shown in Figure 1. 16. 71 In this case, two {P 2 V 3 W 15 O 62 } clusters were bound together via an appropriately chosen linker; the resulting inorganic-organic-inorganic architecture displayed an attitude at forming vesicles in solution, whose size could be controlled (60 nm < R h < 120 nm) by changing the hybrid POM concentration and the dielectric constant of the medium. This kind of behaviour is regarded as very interesting for its possible usages, for example as catalytic compartments, nanoreactors, and functional nanoparticles.

Finally, polyoxometalate in solution usually behave as strong acids, with pK a often lower than 1. 72 In general, polyoxometalates are regarded as weak Lewis bases due to the many oxygen atoms found in their structure, able to donate electrons, and as Lewis acids due to the unoccupied orbitals of the metal ions found in their inner skeleton that can thus accept electrons. 13

Electronic and optical properties

Probably the most significant advantage regarding POMs consists in their ability to accept and release several electrons without decomposing or losing their structure. 14 ,15 This property is explained considering the following evidences: firstly, POM anions possess relatively low charge densities, if compared to traditional anions such as ClO 4 -or NO 3 -; also, the partial negative charges found on the outer terminal double-bonded oxide ligands are lower than those found on the embedded inner bridging oxide anions; finally, many POMs have quasi-spherical shapes. All these considerations allow to understand how polyoxometalates are able to exchange electrons via the outer-sphere electron transfer mechanism, which can explain their inherent stability. 73 Being both able to accept and release multiple electrons, when observing the electrochemical properties of polyoxometalates through cyclic voltammetry it is possible to observe several reversible redox peaks, as shown in Figure 1.17 for α-{P (44 ).

can accept up to 32 electrons when protonated: while the first 24 electrons are accepted so to reduce all W VI to W IV , each of the four W IV 3 triads can accept two additional electrons. 1 Examples similar to this are not uncommon and many more can be found, such as in the case of the spherical {V 18 O 42 } isopolyanion, where all metal site can undergo a reduction going from the +V to the +IV oxidation state, thus having accepted a total of 18 electrons. 16 Apart from the obvious application of this properties in the field of catalysis, 13 the ability to accept several electrons has allowed POMs to be used in more exotic fields, such as for spintronics and quantum computing where the control of polyoxometalate charge has been exploited to represent qubits. 74 Regarding the optical properties, in general most of these polyanionic clusters are only able to absorb in the UV region of the light spectrum. The observed transitions are due to p π -d π oxygen to metal charge transfers, with absorption peaks going from 250 to 400 nm; 75 even if sometimes the red tail of the absorption band covers parts of the visible spectrum, this is usually uncommon and absorbance above 400 nm is generally weak. In addition, this absorption process can cause the photoreduction of POM complexes; despite this has been exploited in photocatalytic oxidation processes under simulated solar irradiation, 76 the underlying dynamics are still unexplored. It is believed that the cluster reduction proceeds via an ultrafast electron transfer from a complex formed either with the solvent 76 or a substrate, 77 following the metal-oxide cluster photoexcitation. Nonetheless, few studies have explored the transient spectroscopy of POM excited state, 75 and as such questions about their photoreduction are yet to be fully answered.

Moreover, a clear drawback in the polyoxometalates optical properties resides in their inability to absorb visible radiation. In fact, when using a cut-off filter removing the UV component of the irradiating light, POMs photoconversion efficiencies towards catalytic processes was shown to approach 0%. 76 It appears thus clear how enhancing the absorbance of these kind of complexes at wavelengths above 400 nm could drastically improve their overall usefulness. In this regard, a common strategies involves photosentising polyoxometalates so to expand their capability to absorb light. This has been previously done, for instance by using the cationic dye p-aminoazobenzene which absorbs up to 500 nm; while polyoxometalate {PW 12 } clearly lacks any peak in the visible region of the light spectrum, an hybrid compound obtained from the crystallisation of a solution containing both POM and dye showed the absorbance bands of both components; 78 the isolated hybrid could then used for visible-light driven H 2 generation. While appealing, nowadays researchers are looking for alternatives to the usage of organic dyes, due to their possible instability, cost and often difficult synthesis.

Toxicity

Another appealing POM characteristic regards their non-toxicity. As many studies have researched the possible application of polyoxometalates for their antiviral, 79 antibacterial, 80 and antitumoral 81 properties, it is important to consider their effect on human health.

Studies have been performed both in in-vitro 82 ,83 and in-vivo 84 , and in both cases POMs were found to be non-toxic; nonetheless, other studies suggested how these metal-oxide clusters could have long-term toxicity effects, due to polyoxometalates inherent stability which has also been observed when used in-vivo. 85 It appear thus very important to find easy ways to remove these polyanions from the organism, so to avoid any possible longterm consequence. A plausible approach which has been followed consists in derivatising degradable hybrid polyoxometalates. In ref. (85 ), the effect of an hybrid POM against cancerous glioblastoma cells was studied and the possibility to promote its degradability after use was explored. As a result of the work, it was found that not only it was possible to successfully degrade the complex, but also that its inhibitory effect against glioblastoma cells was retained.

Applications

Because of their enticing properties and unique characteristics, polyoxometalates have found usage in many diverse fields, some of which have been mentioned in the previous section. In the following part of this chapter some of these applications will be explored and presented, in order to give some insight on the numerous applicative possibilities which these polyanionic metal-oxide clusters are able to cover.

Catalysis

Probably most of the research on POMs, when focusing on their possible applications, has investigated their usage as catalysts. Due to their electrochemical properties, they are in fact especially renown for their role in catalytic processes and several reviews regarding this topic can be found in the literature; 13 ,15 ,18 ,86 ,87 in general, POMs have been used both as photocatalysts and electrocatalysts: as each of these fields presents several differences, both in the applicative purpose and underlying mechanisms involved, a certain degree of differentiation is somewhat needed.

Starting from the domain of photocatalysis, polyoxometalates have found application in both oxidative and reductive reactions: in the former case they have been used for the oxidation of alcohols, 88 benzene, 76 ,89 phenols, 90 and alkenes, 91 while in the latter for the reduction of carbon dioxide, 92 ,93 metallic ions, 94 and other pollutants. 95 -98 When talking about their optical properties, it was previously mentioned that polyoxometalates ability to participate in photocatalytic processes stems from their capacity to be photoreduced.

For example, if we take into consideration polyoxotungstates, the photoexcitation process of {WO 6 } octahedral centres can be described by the following equation: 99

[W 6+ -O 2--W 6+ ] hν --→ [W 5+ -O --W 6+ ] *
In this excited state, the formed O -hole centre and the W 5+ trapped electron centre behave as better electron acceptors/donors compared to their ground-state counterpart; this way, the polyoxometalate that was possibly inactive in the dark can suddenly behave as a powerful reagent. 13 This mechanism is independent from the type of metal addenda and as such can be applied to other kinds of POMs. Because of the nature of their excited state, both reductive and oxidative processes can be catalysed by the complex after its photoexcitation, which explains why polyoxometalates have been used for so many different reactions.

Before discussing about the wide field of electrocatalysis, the difference between homogeneous and heterogeneous catalysis should be pointed out. Homogeneous catalysis refers to processes where all catalysts and reagents are in the same phase, or in general when polyoxometalates are dissolved in solution. With the term heterogeneous catalysis we denote instead processes carried out with the catalyst immobilised on the surface of a support. In this regard, common methods for preparing a POM covered electrode as support involve dip coating, polymer incapsulation of the metal-oxide complexes, and electrodeposition. 15 While homogeneous catalysis appears more straightforward and has generally shown superior performances, 56 this approach suffer from a clear drawback: in fact, it can be difficult in such a case to recover the catalyst itself, thus leading to waste and often pollution. In the case of heterogeneous catalysis polyoxometalates can instead be recovered and reutilised; nonetheless, other limitations arise from this approach, the most significant of which being the limited surface area of the support covered by the catalyst which hinders performances. As later explained, a solution to this problem consists in developing nanostructured surfaces by making use of the self-assembly properties of POMs. 100 ,101 In any case, both heterogeneous and homogeneous approaches are regarded as valid when considering POM-catalysed reactions, and either path has been followed for processes such as decomposition of nitrates, nitrites, chlorates and many hydrogen peroxide catalysed reactions; 15 numerous reviews have been written on the topic, also focusing solely on either homogeneous 102 or heterogeneous catalysis. 99 As the field is extremely diverse, it is somewhat more interesting to focus on a precise topic. In particular, POMs have been especially studied for their use as catalysts for water oxidation. This process, otherwise called water splitting, consists in the separation of H 2 O into its components, H 2 and O 2 . 103

The overall reaction allowing this decomposition is described by the following two half reactions:

2 H 2 O --→ 4 H + + O 2 + 4 e - 2 H + + 2 e ---→ H 2
which together yield the following equation:

2 H 2 O --→ 2 H 2 + O 2
The overall thermodynamic potential of this process amounts to 1.23 eV, which corresponds to an infrared photon. 104 As it can be easily inferred from the evidence that water is ubiquitous on our planet despite the relatively low energy needed to achieve this reaction, water splitting is nevertheless kinetically unfavourable; this is mostly due to the fact that such a reaction is a four electrons process. 19 As molecular hydrogen is a very powerful reagent that could be used as green fuel for our daily energy needs, researchers have tried to find viable catalysts able to allow water splitting via solar irradiation.

While probably the most renown semiconductor used for this purpose is TiO 2 , whose photocatalytic water splitting had been already reported in 1972 by Fujishima and Honda, 105 also polyoxometalates have been found usable for this very purpose. Due to their ability to act as electron reservoirs they are in fact able to catalyse the water oxidation reaction and many researches have been exploring this approach. The major breakthrough in this field was achieved in 2008 when two groups, which had been working separately and following different routes, synthesised the same transition metal substituted heteropolyanion while isolating it as two different salts; 55 ,106 Following this finding, more groups kept on working on the same or derived structures in order to achieve more efficient TMSPs capable of water splitting. A common approach

Structure of the [Ru 4 (µ-O) 4 (µ-OH) 2 (H 2 O) 4 (γ-SiW 10 O 36 ) 2 ] 10 - het- eropolyanion; the central {Ru 4 (µ-O) 4 (µ-OH) 2 (H 2 O) 4 } 6+
core is highlighted, with the {Ru 4 } tetrahedron shown in light blue, Ru atoms in blue, µ-O in red and O(H 2 ) in orange (hydrogen atoms omitted for clarity); the two [γ-SiW 10 O 36 ] 8 -lacunary heteropolyanions capping the central core are represented with the {WO 6 } tetrahedron shown in grey, oxygen atoms in red and Si heteroatoms in yellow; reproduced from ref. (55 ).

consists in substituting the expensive ruthenium atoms for less cost-intensive cobalt complexes; this route has nonetheless been sometimes questioned. In fact, while carrying out the catalysis process the instability of Co-containing organic linker could cause its decomposition, thus yielding CoO x species. These molecules are renown for their ability to aid water oxidation themselves. 107 Efforts have thus been focusing on the development of stable cobalt-based polyoxometalates free of any organic linkers, and some examples are present in the literature. 56 ,108 Nonetheless, the usage of POMs allows for some innovative results. For instance, researchers have even been able to decouple the half reaction of H 2 evolution from the water oxidation process. While normally hydrogen and oxygen evolution half reactions proceed synchronously, it has been possible to use polyoxometalates as electron-coupled proton buffer; this means that POMs can take up in their structure the electrons and protons generated during the oxidation of H 2 O, instead of using them to directly produce molecular H 2 and reverts its colour to yellow. The achieved temporal separation could have huge impact for industrial transfer as it can be used for preventing gas mixing in the reaction vessel headspace, both for increased safety and to prevent degradation of the cell components. 109

Nanomaterials

In the domain of nanotechnology polyoxometalates have found use as templates and building units for the construction of nanoscale architectures, bridging the gap between sub-nano and micron scale thanks to their self-assembly abilities. 23 ,112 In fact, having such a tunable and versatile structure, polyoxometalates have been highly appealing in the domain of material science, so to enhance their properties and develop new functional materials. 61 The fact that polyoxometalates act as building blocks allowed them to cover all dimensional architectures, ranging several different size scales. As shown in Figure 1.20, POMs themselves are constituted by a lower tier building block in the form of {MO x } metal oxide addenda. Exactly as these bind to each other so to form the many different polyoxometalate structures previously presented, POMs too are able to self-assemble so to form bigger architectures o increased complexity, with pathways similar to those previously presented that allow lacunary POMs to form sandwich structures and more.

While mostly regarded as molecular units, when considering the sizes that these metal oxide clusters can reach, as in the case of huge molybdenum blue POMs, these complexes are sometimes instead regarded as 0D nanomaterials. This consideration also applies to structures that do not reach these massive scales; for example, ball shaped

{Mo VI 72 Mo V 60 L 30 } n --where L is a ligand that can for example be CH 3 COO -or SO 4 2 -
-acts as a zero-dimensional capsule with pores permeable to water and small cations. 113

Systems such as these have many analogies with fullerenes, 12 and could be used to trap other molecules so to be used as storage capsules or nanoreactors. 23 Controlling the shape of the obtained structure can often be achieved through the use organic cations which are able to give directionality to the architecture when used during the synthesis procedure. This is for instance the case of the tetra-n-butylammonium that is able to encapsulate polyoxometalates forming a (n Bu It is nevertheless more common to find polyoxometalates included as guests in structures such as metal-organic frameworks (MOFs). This idea could have huge consequences in the field of heterogeneous catalysis, especially when considering that bulk POMs have small surface area which somewhat reduces their efficiency; using instead MOFs as support for polyoxometalates could help overcoming this limitation. Such an approach has been previously followed, surprisingly by directly carrying out the synthetic phase as a one-step hydrothermal reaction. By using copper nitrate, benzentricarboxylate (btc) and Keggin POMs as precursors, the process yields crystalline

[(CH 3 ) 4 N] 2 [Cu 2 (btc) 4/3 (H 2 O) 2 ] 6 [H n XM 12 O 40 ] -
where X = Si, Ge, P or As and M = W or Mo -in which the Cu-btc MOF hosts in its matrix the heteropolyanions. 24 The work in which the obtained crystal was presented, also demonstrated how this structure could be used as heterogeneous catalyst and was able to show high activity towards the hydrolysis of esters alongside reproducibility, selectivity, and remarkable stability, ideal characteristics for recycling and reusing the compound.

Until now all of the given examples were characterised by being ordered nanostructured aggregates. Nevertheless, as previously mentioned, polyoxometalates can also form assemblies on an even larger scale. When discussing about POMs behaviour in solution (Section 1.5.1) an example of these kind of structures was given, where an hybrid compound able to self-assemble into micelles was presented; nonetheless, in order to promote the aggregation into vesicles there is no need to utilise hybrid POMs for their amphiphilic properties. In fact, wheel-shaped mixed-valence polyoxomolybdate clusters of type {Mo 154 } was observed to spontaneously form hollow spherical structures with diameters of almost 100 nm, nicknamed blackberries. 117 Each of these vesicles was constituted by ∼ 1165 {Mo 154 } nanowheels and hosted in its interior an isolated water environment; moreover, its surface was interestingly found to be permeable towards small cations but not anions.

In another example, polyoxometalates treated with amino-acids were found forming nanotubes with diameters of 50 -150 nm and lengths of several micrometers. 118 These tyrosine-functionalised Keggin polyoxometalates were obtained in a one-step solid-state chemical reaction at room temperature, a very convenient method that was also able to preserve the Keggin-type structure of the nanotube components. In addition, cyclic voltammetry allowed to show how the three redox waves characterising the reversible reduction of the heteropolyanions were still present; this evidence proves that the electrochemical properties of the starting material were still retained in a new and very convenient nanotubular structure.

Control of polyoxometalates shape and architecture has been achieved even at the micrometer scale: the three-dimensional crystalline solid based purely on Keggin POMs previously presented 116 can be used for controlled microtubes growth. This structure -

described by [(C 4 H 10 NO) 40 (W 72 Mn 12 O 268 X 7 )] n
, where X = Si or Ge -in the presence of polyaromatic phenanthridinium-based cations gives rise to spontaneous formation of small tubes with diameter sizes that could be varied between 20 -130 µm. In addition, it was possible to control their growth direction either by physical modification of the growth path or by applying an opportune voltage. 101 By fusing together different tubes it was found that complicated paths could be drawn and, in addition, the tubular structures were shown to be robust enough to allow flow of solutions within themselves without leakage nor fracture, as shown in Figure 1.21. Such a high control is regarded as ideal for a variety of functions and applications, even though the mechanism of formation is not fully understood yet.

When developing novel devices, POMs self assembly can be exploited to obtain thin films on surfaces, useful for heterogeneous catalysis while also increasing the surface area of the working electrode. This possibility, previously mentioned in Section 1.6.1, is often carried out with direct methods not involving neither careful choice of cations nor organic functionalisation of polyoxometalates, differently from the previously presented examples.

For instance, different types of POMs were deposited as thin films on a quartz substrate by layer-by-layer assembly and coupled to organic visible-absorbing dyes so to enhance their sensitivity to natural light; to do so, the substrates were firstly coated with poly(ethyleneimine) for pretreatment and then immersed in an alternating fashion in POMs and dyes solutions so to obtain multilayer films. The same process was successfully repeated using two different POMs (Keggin [BW 12 O 40 ] 5 -and sandwich complex

[Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 10 -
) and several cationic dyes. In addition, the assembly was found to be both thermally and photochemically stable, showing high potential for dye-sensitised photocatalysis activity under visible light irradiation. 119

Similar methods can also be used in order to functionalise electrodes based on other materials, so to enhance their properties with those of polyoxometalates. For instance, TiO 2 nanotubes grown on a Ti sheet by low-voltage anodisation process were coupled with [PW 12 O 40 ] 3 -simply by dip coating the electrode in a POM solution. In order for the heteropolytungstate molecules to permeate inside the TiO 2 pores, pretreatment of the electrode by poly(diallyldimethylammonium chloride) was sufficient, as this allowed to positively charge the nanotubes walls. 120 The catalytic efficiency of the system was tested by observing the photocatalytic degradation of the organic pollutant bisphenol A; as a result, the electrode was found to be over five times more efficient when incorporating POMs.

Finally, polyoxometalates have been even used to also enhance the properties of dyesensitised solar cells (DSSCs). While only a scarce number of papers can be found in the literature, many of which have uncertain results, Examples such as this allow to understand the potential of POMs in the field of photocurrent generation; the fascinating electronic properties of polyoxometalates could really help overcoming the limitation of the current generation of DSSCs, aiding the need for more efficient charge transport within the device components and hindering the recombination of electrons and holes. 75

Medicine

Polyoxometalates possess several characteristics rendering them attractive for medical applications. The main advantage in this regard is the fact that all of the properties allowing for recognition and reactivity towards biological molecules can be, in these systems, altered and tuned. These include polarity, redox properties, surface charge, size, shape, and acidity. Moreover POMs are, as previously mentioned, mainly regarded as non-toxic

Chapter 1: Polyoxometalates and can be covalently linked to other organic molecules; this opens up the way for more biological compatibility and the possibility to act as carriers for drug-delivery. 21 One of the most fascinating medical applications for polyoxoanions is their usage as antitumoral agents. This approach has been already followed in 1988, when an organic salt of the isopolyanion [Mo 7 O 24 ] 6 -was tested for its ability to inhibit tumor growth in-vivo; 81 in this study, tumor bearing mice were inoculated daily with 100 mg kg -1 of POM. The results proved that use of this compound was effective to inhibit tumor growth when compared to both a control group of non-treated mice and two groups to which clinically approved drugs were administrated (respectively 5-fluorouracil and 1-(4-amino-2-methylpyrimidin-5-yl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride). Moreover, treated mice did not show signs of toxic-effects when administered POMs. Following this research, more works have been focusing on using both the same and other polyoxometalates for cancer treatment; 85 ,122 -124 it is believed that the ability of these metal-oxide compounds to treat cancerous cells derives from their capability to induce cellular apoptosis, as shown in Figure 1.22.

Apart from cancer treatment, polyoxometalates have also been used as antivirals against HIV-1, influenza virus, herpes simplex virus, SARS and more. 79 ,125 Despite showing broad antiviral properties, the underlying mechanism allowing this effect is still not fully understood, as it seems to vary depending on both the virus type and POM structure.

In addition, polyoxometalates have also been used as antibacterials with promising results. In fact, researchers studied several different POMs structures against bacteria which are resistant to β-lactams antibiotics; the tested metal-oxide anions were proven able to penetrate the cell walls and inhibit the gene responsible for the antibiotic resistance.

When afterwards used in conjunction with β-lactams, an enhancement of the antibiotic activity was shown, thus confirming the inhibitory effect against the gene responsible for the resistance. Despite the mechanism responsible for this effect is not yet totally clear, it is supposed that POMs are able to penetrate the cell membrane where they initiate damaging oxidation reaction within the bacterium. In any case, this study points out to the potential of polyoxometalate as powerful disinfectants.

Crystallography

A probably less known field of application for these metal-oxide clusters is that of macromolecule crystallography; here they have found use as crystallisation additives, being able to aid the formation of protein crystals.

The importance of this property stems from the fact that, in order to understand proteins function and their expected interaction with other molecules -especially useful when developing new pharmaceuticals -knowing their three-dimensional structure is firstly needed. In general this is achieved via single-crystal x-ray crystallography, a technique which, as the name suggests, allows to obtain the spacial placement of atoms for a certain molecule, assuming that a pure single crystal of the compound is available. With imperfect lattice structure, though, the resulting three-dimensional positions are obtained with poor accuracy. In the case of proteins, which are soft macromolecules often positively charged, flexibility and mutual repulsion strongly hinder the aggregation needed for lattice formation. Due to the numerous variables influencing the crystallisation process, such as pH, concentration, temperature, ionic strength, and presence of eventual impurities, obtaining protein crystals is nowadays regarded as a trial and error procedure. 20 Polyoxometalates appear as promising candidates in aiding this process. In fact, thanks to their negative charge, they have been shown to participate in several different electrostatic interactions with macromolecules. The formed connections appear to be able to reduce the mutual repulsion of proteins, which then tend to aggregation and crystallisation aided by the increased rigidity of the POM-protein complexes. 128 For example, as shown in As a result, the three-dimensional atom disposition was solved with a resolution of just 3.3 Å. 127 Moreover, the polyoxometalate presence did not influence nor modify significantly the initial protein conformation, as it was observed that the POM-bound structure was similar to that of an active POM-free 30S ribosome subunit. 129

As previously mentioned, there are several different mechanisms allowing for the binding between polyoxometalates and proteins. Due to POMs negative charge, electrostatic interactions are probably the most common in this kind of applications and some studies have demonstrated how changing polyanion structure in order to increase the negative charge resulted in stronger connections. 130 But electrostatic forces are not the only ones keeping together POM-proteins systems; polyoxometalates can indeed also form hydrogen bonds with proton donors and, in particular, with proteins containing amino acids able to act as such. The fact that between these hydrogen donors there also are three amino acids which are positively charged at physiological pH -arginine, histidine, and lysinemakes the formation of an H-bond with POMs even more favourable. In addition, also interactions predominantly based on van der Waals forces 131 and covalent bonds have been described, though the latter was obtained only when the polyoxometalate cluster was synthesised in the presence of the protein and not from a preformed POM. 132

Once again, polyoxometalates result extremely versatile thanks to the properties stemming from their unique structures, allowing for interactions with other species.

Magnetism

Thanks to their ability to accept in their structure many different atoms and a variable number of electrons, spin-localised POMs and mixed-valence POMs have even been used in nanomagnetism, quantum computing, and molecular spintronics for their magnetic properties. 22 The first category, spin-localised POMs, refers to polyoxometalates acting as chelating ligands towards transition and lanthanoid metal ions; when encapsulating these magnetic entities inside the stable POM structure one obtains a well defined magnetic centre. This is usually done starting from lacunary polyoxoanions which results in sandwich polyoxometalates incorporating atoms such as cobalt. For example, the Keggin-derived sandwich

polyoxometalate [Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 10 -is formed by two diamagnetic [PW 9 O 34 ] 9 -
lacunary clusters which encapsulate four Co atoms. As observed via inelastic neutron scattering, the Co II tetramer display ferromagnetic properties arising from an anisotropic exchange due to Co -Co interactions. 133 ,134 In the case of mixed-valence POMs, instead, one makes use of the capability of these metal oxide anions to be easily reduced to change their spin state and thus obtain spindelocalised clusters. For instance, through means which can be both physical or chemical, a system in which same metal atoms are present in different oxydation states can be obtained, as in the case of vanadium-based POMs where V IV -V V sites are present. Due to the natural separation of electrons in the cluster and the ability of POMs to act as electron reservoirs, the strength of the resulting exchange coupling can even be tuned, with evident advantages for consequent applications. As an example, polyvanadate cluster {V 18 O 42 } has a 3d electron population that can be varied between 10 and 18 electrons without changing its overall structure; as the spin organisation is directly linked to the electronic state of the system, magnetic properties of this POM vary for each state. 135 While in the past polyoxometalates have been used as model systems in order to study the interplay between electron delocalisation and magnetic exchange, recently the interest for these classes of POMs has shifted towards their use as single-molecule magnets. 22 These kind of magnets can in fact be used for applications in quantum computing, 136 a field that has seen a growing interest in recent times. For example, [Gd(W 5 O 18 ) 2 ] 9 -and [Gd(P 5 W 30 O 110 )] 12 -polyoxometalates have been studied for their tunable spin properties which makes them single-molecule magnets whose spin could be directly used to represent different qubits states; author of the paper have even got to the point of defining the POM [Gd(P 5 W 30 O 110 )] 12 -as a close to ideal candidate to be used as qubit, since even by applying a very weak magnetic field to the molecule it was possible to initialise the system in the state needed for the application. 137

This optimism revolving around the possible usage of polyoxometalates for applications in the field of quantum computing stems directly from the capability of tuning their magnetic properties by chemical design, an ability which opens up a huge variety of fascinating perspectives for the optimisation of these metal oxide clusters for all kinds of quantum phenomena.

Limitations and Outlook

Nowadays polyoxometalates have found applications in a wide array of different fields. The brief excursus presented in the previous section is just a sample of the many possibilities of these molecular systems; research is now shifting its focus from mastering their synthesis to engineering metal-oxide clusters adapted to well defined uses, trying to foresee their properties and obtain the desired characteristics needed for precise utilisations. Despite the huge adaptability and the appealing properties of POMs, more advances are still required.

In the literature a need for more computational chemistry and simulations is often called out, as it would greatly improve our understanding of charge transfer and different interactions between polyoxometalates and other materials; this would in turn allow us to better develop the needed systems while also shining light on the underlying mechanisms that allow these properties. 75 In fact, even where POMs seem to shine the most we still have no full understanding of the occurring process, such as in the case of water oxidation catalysis. 19 Additional efforts are still ongoing when trying to control polyoxometalates architectures at all scales. For instance, POM based photoanodes, despite the self-assembly attitude of these anionic metal-oxide, do not in general produce high surface areas; this means that generated photocurrents are at least two orders of magnitude lower than the current generation of photoanodes based on other nanocrystalline semiconductors. 75 One approach usually attempted to solve this issue consists in simply increasing the number of deposition cycles for the material, which generally results in having a bigger surface able to participate 1.7: Limitations and Outlook in the photocurrent generation; as observed in some studies, this does not work for POMs as the majority of the obtained films do not result to be electroactive. 138 In fact, charge and mass transport is another major limitation of the current generation of polyoxometalates devices. In order to solve this issue, some attempted to deposit conducting nanoparticles on the photoanode substrate, onto which the POMs where afterwards cast. 139 Moreover, the effect of crystallinity on charge transport is still unclear, as most POM assemblies tend to be non-crystalline. Some attempts have been made to obtain better long-range ordered structures when incorporated in devices, but more has still to be done. 140

In addition, one question that has yet to be answered is whether POMs properties are going to be retained when passing from molecular to bigger scales. 12 If that was the case, polyoxometalates architectures over the micrometer-lengths would open up a complete new field based on the development of functional systems.

Finally, many improvements have yet to be obtained also in the context of POMs sensitised with other species; when trying to increase polyoxometalate photoresponse, the coupling with photo-active materials should allow for better absorption cross-sections and more efficient electron transfer towards the metal-oxide cluster. For instance, there are evidences that depending on the POM structure different quenching abilities arise, sign of a better interaction towards the electron transfer. Nevertheless, we still need to understand what is the cause of this effect in order to learn how to control it. 75 In this present work some efforts have been made to try elucidating this problem but, as it will be later discussed, the high variability in POMs structures and properties presents some challenges when trying to fully understand how each variable contributes to the final result.

Again, polyoxometalates offer properties with elevated potential, are inexpensive, nontoxic, and easily produced on a large scale. The ability to fully control their self-assembly capabilities could easily lead to the development of new functional materials presenting the desired structure and allowing to exploit POMs characteristics. Today we have just started developing POM-based functional devices; while we still have to completely understand the underlying mechanisms responsible for their fascinating properties, it is doubtless that they present high possibilities for many different fields.

Chapter 2

Carbon Dots

Carbon dots (CDs) are nanosized carbonaceous core-shell particles which display very enticing optical properties, such as an intense absorption and a powerful emission. Their serendipitous discovery, occurred while purifying carbon nanotubes, was reported for the first time in 2004 141 but CDs were then rigorously synthesised by Sun et al. only two years later. 142 It is in this report that their characteristics were for the first time unveiled and their peculiar fluorescence described. In the following years, several articles relative to these amazing nanoparticles and their properties were published; today we have yet to fully understand CDs, as their unique features continue to surprise us.

In order to understand how carbon dots managed to suddenly become so relevant in the field of nanotechnology we have to consider the general properties of carbon. This element, mostly recognised for being the fundamental component of life, possess a peculiar electronic configuration, [He]2s 2 2p 2 , which allows for the formation of hybrid orbitals;

because of this hybridisation, carbon presents a very high degree of versatility and is able to form different allotropic structures. As a bulk material, carbon atoms presenting sp 3

hybrid orbitals bind to each other in a tetrahedral geometry giving origin to diamond:

this material possess a band gap of 5.5 eV, which makes it an insulator transparent to visible light. 143 Graphite is instead obtained when carbon atoms are in an sp 2 hybrid configurations which allow the formation of planar binding geometries. Graphitic carbon, whose lattice is characterised by hexagonal patterns, behaves as a semiconductor whose band gap is small enough for the resulting conduction band to be already populated at room temperature. 144 The properties of this allotrope allow to obtain nanosized architectures that are based on them. In fact, the atoms composing graphite are strongly bound to each other only when part of the same plane; instead, the out-of-plane interactions connecting different graphitic layers are much weaker. This characteristic was exploited by Novoselov et al. who, back in 2004 was able to obtain graphene 145 by repeatedly peeling the out-of-plane graphitic layers until managing to isolate a monoatomic-thick plane. This bi-dimensional carbon structure is known for its enticing conductive properties 146 and is regarded as a very promising material for the development of transparent flexible electronics and supercapacitors. 147

Other carbon structures are very similar to that of graphene and can in fact be imagined as obtained by opportunely folding a single graphene sheet, as the unidimensional carbon nanotubes (CNTs) or three-dimensional fullerenes. While all of these structures have been extensively studied for their properties and possible applications, none of them is known for being fluorescent. Even if some research groups have been able to obtain CNTs 148 or graphene 149 capable of some emission, this was possible only by modifying their structure or introducing new molecules onto their surface; moreover, often the resulting fluorescence was only possible in the infrared and with very low efficiencies. This is why being able to directly obtain a carbon-based nanostructure capable of efficient and bright emission made CDs extremely appealing since the first time they appeared on the research scene.

Nowadays nanotechnology is present in every aspect of our daily life and nanoparticles have even found their way in the technology powering modern television displays. It is thus not surprising that concerns about their possible effect on human health and environmental pollution have grown year by year. Many nanoparticles are indeed toxic, 150 mostly due to the kind of elements they are composed of, and often their synthesis can be both expensive and polluting. Fully carbon-based nanoparticles have the power to completely revolutionise this situation: from a cost perspective, carbon sources are present everywhere, thus making CDs very cheap to produce; 151 moreover, being carbon the element of life, it can be naturally harnessed by organisms so that carbon dots do not pose any threat to humans and the environment, as some studies 152 which will later be discussed have pointed out.

In general, CDs are composed of a roughly spherical carbon core whose diameter is often smaller than 10 nm; on their surface a wide variety of organic moieties is found, often including elements such as C, O, H, and N. The most important features of CDs are found in their optical properties. As previously said, these dots present intense absorption bands In the current CDs will be described, trying to elucidate what we currently know on their properties and give some examples of the applications in which they have found a role.

Synthetic Approaches

While CDs were for the first obtained as a by-product of the purification of carbon nanotubes, 141 several synthetic methods have been later developed in order to both obtain control of the nanoparticles characteristics and ease of synthesis. In general, two main pathways can be considered for carbon dots production, the top-down and bottom-up routes. In the following both methodologies will be explored and some examples provided before discussing the post-synthesis purification methods.

Top-down Methods

The top-down synthetic route, as the name suggests, involves using a macroscopic carbonbased precursor which will be broken down to form the carbonic core of CDs. One most common example for this starting material is graphite, which has been used as such in several occasions; 142 ,162 ,163 nonetheless, CDs have also been obtained starting from carbon nanotubes. 141 ,164 This approach generally consists of two steps: while the fist comprises breaking down the macroscopic material and, thus, forming the CDs core, the second step focuses on the passivation of the nanoparticle surface. This latter passage is regarded as very important since it makes these nanoparticles highly soluble and, more importantly, allows CDs to be fluorescent. While this topic will be later explored, several papers have in fact pointed out to a role of the surface on the emission mechanism of carbon dots. 142

Historically, most of the initial synthetic routes to be explored were based on top-down methods; for example, researchers that for the first time obtained CDs produced them using the arc discharge technique. This method, normally approached in order to obtain single-walled carbon nanotubes (SWCNTs), yields a highly impure soot containing the SWCNTs among other by-products. As such, purification steps need to be taken, so to isolate the desired product. Researchers found out that when using HNO 3 to oxidise the arc discharge soot, later neutralised by NaOH, a fluorescent fraction was obtained alongside the nanotubes. This, then isolated via gel electrophoresis separation as shown in Figure 2.2, resulted being composed of spherical carbon nanoparticles, which were finally analysed; 141 arguably, the carbon core of the dots was formed during the arc discharge step, while oxidative treatment in HNO 3 probably allowed for surface passivation.

Successive approaches found in the literature appear to be more focused towards controlling the production procedure. An example consists in producing the initial carbon cores using laser ablation on a graphite substrate. Researchers observed that the obtained fragments were initially not fluorescent, even after acid treatment in HNO 3 . As a conse- quence a further step, whose scheme is shown in Figure 2.3, was taken in order to passivate the surface, consisting in using diamine-terminated oligomeric poly(ethylene glycol) which was able to reacted with the acid-treated particles after mixing both components and heating the obtained suspension at 120 °C. After this step the resulting particles were indeed found to be strongly fluorescent. 142 Other researchers, following the laser ablation step, tried to passivate the surface of the obtained graphitic cores by using different molecules, such as poly(propionylethylenimine-co-ethylenimine), 162 and again emission was observed only after coating the surface with the polymer.

Other approaches found in the literature are based on electrochemical methods, which have been applied using both graphite and carbon nanotubes as starting material: in the first case, 163 a graphite column electrode was electro-oxidised at 3 V (vs SCE) with a Pt wire acting as counter electrode and a 0.1 m NaH 2 PO 4 solution as supporting electrolyte; in the second case 164 multi-walled carbon nanotubes (MWCNTs) grown on a carbon paper via chemical vapour deposition were instead used; the process consisted in cycling the potential of the working electrode between -2 and 2 V (vs Ag/AgClO 4 reference electrode, scan rate 0.5 V s -1 ) against a Pt counter electrode, while a 0.1 m solution of tetrabutylammonium perchlorate (TBAP) in acetonitrile was used as electrolyte. In both cases the solutions changed colour, becoming dark brown, and were found to be fluorescent without additional steps. It is probable that the passivation in this case occurred during the cleavage of the carbon precursor, with the electrolyte itself providing the needed organic moieties; in fact, at least in one case, 164 no CDs could be produced when either KCl or KClO 4 were used rather than TBAP.

Even if initially top-down methods were the main approaches followed for carbon dots synthesis, in recent literature works they are rarely found due to the convenience of bottom-up methods, as explained in the following.

Bottom-up Methods

Nowadays most researchers working on CDs follow bottom-up synthetic route for the production step. The reasoning behind is that these methods, whose name derives from using molecular precursors instead of preformed carbon architectures, are usually both onestep and one-pot approaches; as such they are extremely convenient, being easy to follow and quite rapid. In addition, synthetic approaches solely based on chemistry allow for a better control of the size, shape, and physical properties of the carbon nanoparticles. 165 ,166 When following this route, core formation and surface passivation proceed synchronously.

It has thus been supposed that the presence of organic moieties passivating the surface of the just-formed carbon fragments could actually have a role in limiting the core diameter, thus allowing for better size control. 166

Also in this case several different methods have been developed, among which the most reliable probably is the thermal decomposition route. 165 -168 In this process a molecular carbon-containing precursors is decomposed at high temperature, usually mixed to other reagents, so that the components can then reassemble spontaneously forming the CDs core and surface. Usually the chosen starting material is dissolved in solution, but sometimes also dry precursors can be used; in general, these are placed in a reaction vessel which is then sealed and left reacting for several hours at high temperature.

For example, researchers obtained hydrophilic CDs from 2-(2-aminoethoxy)-ethanol and citric acid; the two precursors were initially dissolved in water, but the solution was then slowly dried until a thick syrup was obtained. This was then transferred to a teflon coated stainless steel autoclave and heated up to 300 °C for 2 h. Finally the obtained fluorescent product was washed with acetone to remove impurities. Depending on the reaction conditions, nanoparticles with different properties are obtained; for instance, the same paper also reports how to obtain organophilic CDs while still using citric acid as precursor: in this case the reagent was dissolved in ethanol rather than water and octadecyl ammonium was added. Again, the solution was then dried and reacted in autoclave using the same conditions and the fluorescent final product was found to be extremely soluble in organic solvents such as toluene, chloroform, tetrahydrofuran, and hexane. 166 Nevertheless, solubility is not the only property that can be controlled during the nanoparticles preparation. For example, a study of 2012 167 was able to point out how the temperature at which the synthesis is carried out can influence both the core formation and surface composition, with huge effects on the optical properties of the resulting product.

In this work, the pyrolytic decomposition of citric acid and ethanolamine was followed,

showing that while at 180 °C only a fluorescent molecular precursor could be formed, this was increasingly consumed if the reaction was carried out at higher temperatures.

In fact, starting from 230 °C a carbon core presenting the molecular precursor on its surface could be formed; this complex gave rise to an emission spectrum composed by and lower quantum yields. A schematic representation linking together temperature of synthesis, emission, and obtained structures is shown in Figure 2.5.

A successive paper confirmed the importance of temperature in the mechanism responsible for the nanoparticles formation; 168 here researchers used once again citric acid as carbon-containing precursor, this time mixed to ethylenediamine in an aqueous solution.

Reaction was then carried out for 5 h in a sealed teflon lined autoclave at temperature varying between 150 and 300 °C. Carbonisation degree, observed through elemental analysis, was shown to increase alongside temperature, but once again this lead to aggregation and decrease in quantum yield. In addition, the same study elucidated the importance of surface groups choice on the optical properties of produced carbon dots. By changing quantity and type of reagents, it was found out that if no moieties containing -NH were introduced in the reaction product, as evinced by Fourier transform infrared spectroscopy (FTIR) measures, resulting quantum yields were always lower than 10%; in contrast, by carefully tuning the ratio between citric acid and ethylenediamine, the latter providing the needed nitrogen, yields could reach up to 80%.

Finally, it is worth noticing two very convenient bottom-up methods based on microwave decomposition and ultrasonic treatment. These synthetic routes are especially remarkable for being one-pot processes very easy to follow, 169 often based on inexpensive precursors, 170

and easily carried out on a large scale. 169 ,171 In the case of ultrasonic assisted CDs synthesis, carbonisation leading to core formation is often carried out thanks to the presence of a strong acid. For example, researchers were able to obtain carbon dots from an aqueous solution of glucose to which concentrated HCl was added. 172 The mixture was then ultrasonically treated for 4 h and successively oven-dried at 80 °C for six additional hours.

Such an easy one-step process was able to yield the desired fluorescent carbon nanoparticles.

Microwave decomposition, instead, is another kind of thermal process. As the names suggests, in this technique the heating is carried out in a microwave oven. As the synthesis can even be carried out in a common household microwave, 170 ,173 ,174 this route appears as one of the most affordable among the many allowing for carbon dots production. The first time such a method was reported dates to 2009, when it was already praised for its speed and convenience. 169 The synthesis described in the study is performed starting from an aqueous solution of poly(ethylene glycol) and a saccharide, mixed until fully dissolved before being introduced in a 500 W microwave oven where it was heated for 2 -10 min.

Depending on how long the heating process was carried on for, colour of the obtained solutions changed from transparent to yellow and up to dark brown, sign of carbon dots formation; duration of the process also affected the properties of resulting nanoparticles, both regarding quantum yields and average sizes.

Purification

As previously pointed out, CDs synthesis produce very heterogeneous solutions. Obtained nanoparticles often result having both a broad -compared to their average -size distribution and a wide variety of organic groups present on their surface. As single particle observations have shown how optical properties can change from dot to dot, resulting in sharper emission bands and loss of tunability, 175 researchers have tried developing methods that allow for the isolation of CDs with well defined characteristics.

Very few synthetic methods are able to directly produce well monodisperse carbon nanoparticles. In a communication published in 2013 researchers were able to obtain a sharp size distribution by using reverse micelles as nanoreactors. To do so, carbon tetrachloride was dissolved inside the amphiphilic capsules formed by tetraoctylammonium bromide (TOAB); CCl 4 was then reduced using lithium aluminium hydride so to form the carbon nanoparticles. Afterwards, the now hydrogen terminated surface of the CDs was modified using a platinum-catalysed process which aided the formation of C -C bonds.

The final product size was then obtained analysing atomic force microscopy (AFM) images, yielding a mean diameter of just 1.5 ± 0.3 nm. 176

Nonetheless, examples such as the one just presented are rare in the literature, as many methods are not able to produce nanoparticles displaying such an efficient size control. As such, it is instead more usual to follow several purification steps. For example, a common pathway starts with the removal of bigger carbonaceous aggregates formed as synthesis by-products by centrifugation; afterwards, smaller impurities are eliminated by filtration, only to finally dialyse the resulting solutions in order to finish the purification procedure.

Regarding this last point, as carbon dots display very small diameters, dialysis membrane used need to present very small pore sizes in order avoid nanoparticles loss. It is not thus uncommon to use membranes with a molecular weight cut-off of just 100 g mol -1 . 177

As research on carbon dots advanced, more sophisticated purification methods have been attempted; the need for new techniques has in fact been now claimed for more than Anion-exchange HPLC allows to separate species based on surface charge. By using this method to separate carbon dots it was observed how the initial mixture was actually composed of several entities with different properties. 179 Remarkably, after separation some of the obtained fractions displayed quantum yields significantly higher than the initial heterogeneous solution, while others had completely lost the peculiar emission tunability, usually considered inherent to CDs. In addition, the diameter distribution inside each fraction was unexpectedly sharper. Moreover, researchers observed how there was no clear relationship between size and emission wavelength, possibly excluding quantum confinement effects on the optical properties of the isolated fractions, as later discussed.

Using SEC gives very similar results. Researchers were able to separate a CDs solution into several fractions by using a chromatographic column packed with Sephadex LH-20 and operating at 150 psi. Depending on elution time, three different solutions were obtained, whose average diameters measured via AFM were 2.65 ± 0.48 nm, 2.04 ± 0.57 nm and 1.24 ± 0.43 nm; these, as claimed by the article authors, were among the smallest carbon quantum dots to ever be obtained. Regarding the surface composition of the three fractions, no striking difference was found when analysing their FTIR spectra. Nonetheless, the optical properties of the fractionated entities were different, with one of the three obtained solutions not displaying any tunable fluorescence after separation. Once again, no clear dependence between size and emission properties was found. As hypothesised in the paper, since CDs fluorescence mechanism is strongly affected by surface state and defect presence, these must then be the main characteristics affecting the optical properties, and not the diameter of the nanoparticle core. 182

Nowadays the usage of purification techniques such as these is becoming increasingly more common as it has allowed to isolate particles with better characteristics. Using SEC researchers have been able to isolate CDs fractions with quantum yields as high as 73%. 184 The exceptional optical properties of bright emitting fraction such a this open up new usages for CDs; researchers are still trying to improve and simplify the purification procedures, so that this trend towards nanoparticles with higher quality is probably yet to be over.

Properties and Characteristics

In this section carbon dots characteristics will be detailed and discussed, in order to give a better understanding on why these nanoparticles have attracted so much interest and explain what we currently know on the underlying mechanisms allowing for their peculiar properties.

Core Structure

Carbon dots have been observed several times using both AFM and TEM, techniques that allowed to obtain the diameter of these nanoparticles, generally resulting smaller than 10 nm. 185 Interestingly, reports agree on dots often presenting a broad size distribution. 186

Due to this high variability, several efforts have been focused on controlling CDs size either during and after the production phase, as previously detailed.

In order to study the core structure of these nanoparticles, one of the most reliable approaches consists in the analysis of high-resolution TEM (HRTEM) images and, in particular, of selected area electron diffraction (SAED) patterns. Some initial studies pointed out that carbon atoms composing the core were disposed in a diamond cubic structure. 187 Nonetheless, other researches have pointed out that CDs could be comprised of sp 2 carbon atoms instead of sp 3 , thus leading to a graphitic lattice. 163 ,188 Also in this case results were obtained from the study of HRTEM images, as shown in Figure 2.7, but other techniques have been used too; among these 13 C nuclear magnetic resonance (NMR) spectroscopy 189 allows to identify the intensity of different aliphatic or aromatic carbon while Raman spectroscopy, 164 ,190 used in general when studying the many carbon-based structures, shows the characteristic G, D and 2D bands. When using all these techniques altogether, evidences often pointed towards both sp 2 and disordered carbon presence or yet sp 2 lattice with sp 3 defects. 151 No single core structure is conclusively inherent to CDs;

as research proceeded, its variability was accepted as another characteristic of carbon dots.

Nonetheless, these significant architectural differences have lead some researchers to propose a classification allowing to discriminate some types of carbon dots. 191 In general, when talking about CDs we refer to the whole family of nanosized quasi-spherical carbon based particles. When these nanoparticles are almost bidimensional and formed by just a few-layers of graphene, as is commonly the case for dots obtained by the exfoliation of graphitic material, the produced nanoparticles are instead referred to as graphene quantum dots (GQDs); when the core is instead quasi-spherical and mostly graphitic, these are called carbon quantum dots (CQDs). In both categories, the quantum term refers to the system having a delocalised band structure, with possible effects which will be discussed when exploring the optical properties of these nanoparticles. Finally, when the core is mostly composed of amorphous carbon the nanoparticles are defined as carbon nanodots (CNDs). In general, it can be extremely challenging to discriminate between the different families; this is due to the high heterogeneity present even inside each single group and to the difficulty in recognising the contributions given by structure to resulting electronic properties; because of this, in the present work the generic term carbon dots will be used instead.

In addition, it is not uncommon that other atomic species are introduced as dopants in order to enhance the optical properties of CDs. In fact, if elements other than C are present among the precursors, these can sometimes be incorporated into the core structure and not just onto the surface. Elements normally found in this role are sulphur, 192 boron, 193 , and phosphorous, 194 but without any doubt the one most commonly used is nitrogen. 185 It is supposed that N is particularly efficient in enhancing CDs quantum yields because its introduction leads to the formation of additional surface states. 195 While the actual mechanism is still debated, researchers suppose that, when nitrogen substitutes a carbon atom, it provides an additional delocalised electron to the π * orbital, which tend to increase the radiative lifetime and thus the quantum yield; moreover, presence of N seems to increase the rigidity of the structure, thus also decreasing the probability of non-radiative recombination through phonon generation. 161 Finally, it is worth mentioning that some studies have shown how above a certain threshold in nitrogen content the element passed from being a simple dopant to completely changing the core lattice structure; this was then found to be that of carbon nitride, through analysis of SAED and x-ray diffraction (XRD) patterns. 196 ,197

Surface Characteristics

As previously pointed out, the surface of these carbon particles is covered in organic moieties, as evinced from FTIR measures, x-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). 185 These techniques allow to analyse the composition of the shell and have been used to indicate the presence of several different organic groups such as hydroxyl, carboxyl, carboxylate, and methyl; in addition, when nitrogen is present within some of the synthesis precursors, also amide and amine moieties are often found.

In general, researchers believe that this variability could be either local or global.

In the first case, local variability means that different groups are present on the same particle; with global we instead mean that the observed moities distribution is found across separate particles. These considerations mostly arise from the discussion on the optical heterogeneity of CDs, and especially their tunability, when considering how surface groups can contribute to the energy states allowing for emitting transitions. 175 As such, it will be treated in more detail when dealing with the emission mechanisms of carbon dots.

Nonetheless, the presence of this organic surface also provides very convenient characteristics. Firstly, it is because of it that CDs possess such a high solubility. In fact, in the absence of organic moieties covering the carbon surface, these nanoparticles seem to spontaneously aggregate, as described in the section about bottom-up synthetic routes. 167 ,168 Moreover, due to the presence of -COOH groups, CDs display very often a negative surface charge when in solution. 198 This last property is very useful for coupling carbon dots to other materials, as it easily leads to electrostatic interactions with other charged species.

In addition, a surface covered in functional groups has significant consequences on CDs biocompatibility. Commonly, many particles which have no inherent toxicity can nonetheless be very harmful for human health as, due to their micro-and nano-sizes, they are easily introduced into the organism. Often, due to the lack of interactions towards the organic system, these particles result being non-biocompatible and, as such, stay inside the organism indefinitely. This means that, whether or not composed by carbon, small particles that cannot be naturally disposed of by the body often pose a serious risk of bioaccumulation and possible interference with the organism. 150

Several studies regarding the toxicology of carbon dots have instead managed to show that, both in-vitro 163 ,195 ,199 -201 and in-vivo, 152 ,173 ,202 -204 CDs are usually non toxic. While in general in-vitro studies are limited by the observation of the particles presence not inducing any increase in cellular mortality rates, it is especially interesting to discuss what observed with in-vivo studies. In fact, it has been remarked that carbon dots are easily removed when introduced in the organism. For example, researchers showed that mice which had been injected with CDs were able to eliminate the nanoparticles naturally in less than 24 hours. The author of the study pointed out to the role of the surface in this process;

in fact, due to the interactions between the organism and the dots surface, CDs were observed to diffuse more slowly throughout the body compared to, for instance, CdSe/ZnS nanoparticles. Albeit slow, the organism was able to make use of these interaction to control the route of the nanoparticles which were then readily excreted. 152 More studies such as the one just described have been performed also on other organisms, as shown in Figure 2.8; this kind of researcher has allowed to confirm the non-toxicity of CDs and their viability as fluorophores for bioimaging applications.

Evidences such as these point out to the fact that carbon dots are thus non-toxic and not harmful probably even for human health; if considering how also the precursors normally used in the synthesis of these nanoparticles are not hazardous, CDs can be considered as safe for the environment and green alternatives to other fluorescent semiconductor nanoparticles. Nonetheless, some precaution has still to be taken: in fact, even if CDs themselves are biocompatible, their surface could still be passivated with some dangerous specie. It is thus more correct to say that carbon dots are generally non-toxic, although this can change depending on the kind of organic moieties introduced during the nanoparticle synthesis.

Interaction with Other Species

Carbon dots have shown a remarkable sensitivity to the surrounding environment. This ability has been observed since the first articles on these nanoparticles, as simple changes in pH or addition of other species often lead to a quenching of CDs fluorescence. Nowadays this sensitivity has been tested against pH, 163 ,167 ,205 ions, 158 ,168 ,196 ,206 ,207 molecules, 208 -210 and different solvents; 156 ,211 moreover, it has been used both as a tool to study CDs properties and as proof of concept for possible applications.

The ability of carbon dots to interact with such a wide variety of species is linked to the composition of their surface. As previously said, in this shell are found many distinct organic groups, often with different chemical properties; as a consequence, these enable several possible interaction mechanisms. 185 For example, depending on their protonation state organic groups can take part in electrostatic interactions and, in general, most moieties are know for being able to form either hydrogen bonds or other kinds of weak dipolar interactions. If considering for instance the carboxylate group, which has been commonly found on many CDs obtained via different synthesis, all of the previous cases can be found: in fact, this group easily deprotonates in solution, usually above a pH close to 5, becoming -COO -and thus charging negatively the CDs surface. In addition, this can form hydrogen bonds both as donor and acceptor and, thanks to its polarity, can also take part in dipolar interactions.

It should thus not surprise that the organic groups found on the CDs surface allow for a high sensitivity to the environment. Many articles have evidenced how several different metal ions could cause a quenching of CDs fluorescence; 158 ,168 ,196 ,207 this is considered due to the electrostatic interactions giving rise to a static quenching, allowed by the negative CDs surface and positive ion charge. 157 In addition, carbon dots have been observed to respond to changes in solvent polarity, as different degrees of interactions were formed in solution. 156 The fluorescence itself seems, in fact, to be influenced by the rearrangement of solvent molecules found in proximity of the dot surface; these are caused by changes in the local charge, happening when electrons migrate towards the nanoparticle surface as a consequence of photoexcitation. In fact, the evident sensitivity of CDs emission towards the environment has been used as a proof when discussing the role of the surface on carbon dots optical properties, as detailed in the next section.

In general, the quenching of emission is considered due to an electron transfer from or towards the excited carbon dots. As a consequence, the attitude of CDs to interact with external species can be exploited in order to use them as either electron donors or acceptors. This capacity has been evinced on several occasions; for example, an article of preparative conditions. Authors added different quinones to a CDs synthesis and showed how the position of oxidation and reduction bands, as observed via cyclic voltammetry, was altered by the incorporation of the additives, as shown in Figure 2.9. In addition, to prove how the newly edited CDs electrochemical properties had beneficial effects towards processes involving electron transfer, the photoreduction of methyl viologen was studied in the presence of the different carbon dots species acting as photocatalyst. As a result, it was found out that modified carbon dots were more efficient to catalyse the reaction compared to unmodified ones. 212

Optical Properties

Despite we still lack a full understanding of the underlying mechanisms, the optical capabilities of carbon dots are fundamental to the interest surrounding these nanoparticles.

CDs often present multiple broad absorbance bands that, while usually peaking in the UV region of the light spectrum, in many cases present a tail extending up to the visible interval. 185 Because of this, a CDs solution normally appears to be yellow in colour when observed in regular light.

In general, when these dots are brought to their excited state following irradiation they tend to return to their ground state via radiative transitions, thus emitting light. This characteristic is unique among the carbon based materials, as remarked in the introduction to this chapter. Moreover, as previously mentioned, the emission band is often found in the visible region of the light spectrum. The exact position in wavelength of this band depends on the kind of carbon dots, 213 and as such it is possible to find reports in the literature describing how different emission colours can be obtained following the same synthetic route by just changing the type of precursors used. 214 ,215 Many different CDs have been obtained displaying blue, green or yellow light emission; some have also reported synthetic methods for red emitting dots. 216 ,217 As carbon dots present a huge degree of variability it is hard to point to a unique origin for the electronic transitions responsible for these optical properties. As a common characteristic found for many dots is the presence of sp 2 carbon rings, it is believed that these allow for π → π * transitions giving rise to the absorbance bands found in the UV part of the spectrum. In addition, as also C -O bounds are often present, these are instead accounted for the less energetic UV absorptions, which are linked to n → π * transitions. 218 Nonetheless, when these absorbance bands are excited they seldom lead to emission and result instead in totally dark states or, at most, show low quantum yield blue fluorescence. 219 It is believed that the bright visible emission which CDs are known for originates instead from surface-related defective sites, either in the sp 2 carbon lattice or arising from the presence of doping agents and surface groups. 220 Because of this, the kind of organic moieties present on the particle surface appear to have a fundamental role in generating the mid band-gap states responsible for these bright transitions; as such several efforts have been made trying to control the resulting absorption and emission bands, shifting them as much as possible towards the visible while obtaining higher quantum yields. Some of the efforts and findings made in this regard have been described when talking about CDs core structure and doping, and as such it will not be repeated here.

Among the many papers regarding CDs optical properties, some reported observing upconversion capabilities from these carbon based particles. 221 -224 With the term upconversion we refer to the process in which low energy photons are converted to high energy ones. 225 Despite the clear advantages of such a process, many in the research community working on carbon dots doubt that these nanoparticles actually possess any upconversion capability. In fact, the reported upconverted fluorescence of CDs has been proven to be nothing else than the normal second-order diffraction of the monocromator present inside the spectrofluorometer used during the emission studies. 226 As such, all claims of upconversion from carbon nanoparticles have to be carefully analysed, since such a property until now has never been confirmed.

An enticing property displayed by these families of carbon nanoparticles is their peculiar tunability. This, as mentioned in the introduction to this chapter, is the capability of continuously changing the CDs emission peak position as a function of the excitation wavelength, 185 as shown in Figure 2.10. Such a behaviour results particularly surprising as it goes against the Kasha-Vavilov rule, stating that the same fluorescence emission spectrum is observed irrespective of the excitation wavelength. 227 This is considered true when assuming that the same electronic transitions is being excited and thus that the same absorbance band is responsible for the observed emission.

Several studies have tried to explain the origin of this unique property, but the answer to this question is still debated. In general, it is assumed that CDs tunability stems from an ensemble behaviour which arises from the heterogeneity characterising several of carbon dots properties, such as broad size distribution and huge variety of organic moieties present on their surface. As these properties can have a role in the distribution of the system energy levels, by changing the excitation wavelength a certain subset of nanoparticle is selected and thus different emissions are obtained.

A problem inherent to the tunability question regards whether the heterogeneity is present within each nanoparticle or only among nanoparticles. For example, as previously mentioned when discussing the properties of CDs surface, it is believed that on a single dot can be found several organic groups which lead to emissive transitions; in such a case, tunability should be present even within single dots. In general, their emission mechanism is due to radiative transitions across the semiconductor bandgap, which occur to restore the ground state of the system after excitation.

Of uttermost importance is the ability to control the size of these nanoparticles, as it has strong effects on their emission properties: in fact, the quantum term in their name refers to the quantum confinement effect that affects the system. This property is nothing else that the inverse proportionality between the energy levels of the system and the square of its size; 232 ,233 in practice, as quantum dots shrink their band gap energy increases, which translates to nanoparticles of smaller size emitting light at higher energies and thus more blue-shifted. This effect can be easily understood by recalling the properties displayed by the so-called particle in a box : 228 in this case, the system energy levels get further apart from each other as the box size decreases. In the case of nanoparticles, where the exciton produced is confined inside the particle itself, the resulting dependence between emission wavelength and particle size is basically the same, which explains the dependence between gap energy and inverse of the squared nanoparticle radius. 232

Initial studies on carbon dots assumed that quantum confinement effect had to be present for these particles too. This could explain their tunability if supposing that by changing excitation wavelength one would select nanoparticles of different sizes, which then would result in the emission at different wavelengths. This conclusion has been drawn by observing the emission properties of carbon dots after size separation. Studies advocating for this explanation observed that smaller carbon dots were emitting at shorter wavelengths as shown in Figure 2.12, after selecting them by purification and separation. 155

Nonetheless, this explanation has often been contested. For example, some other researchers observed effects opposite to those expected from quantum confinement and were thus able to rule out this hypothesis, at least for the particular carbon dots analysed. 154

In addition, other studies discussed how the size distribution of CDs did not line up with the observed emission spectra broadening, when supposing it to be caused by quantum

confinement. 234

In addition, the emission is strongly affected by the surface state. It is indeed assumed since the very first articles on CDs synthesis how fluorescence is obtained only after passivation of the surface, which must then have an important role in it. 142 Moreover, several studies have shown how carbon dots optical properties are sensible to the environment, as previously discussed, which is a surface-related and not core-related characteristic as, instead, quantum confinement is. As a consequence, by observing how system state and transitions are extremely affected by the environment surrounding the particle some researchers have concluded that quantum confinement should be excluded:

if the exciton is localised on the surface of the particle it can not be affected by the core quantum confinement. In this case, tunability has instead been explained by observing the heterogeneity of carbon dots surfaces: as the composition of the organic shell affect the optical properties, it is assumed that a CDs solution is an extremely heterogenous system composed of nanoparticles having different surface and that consequently possess different

Applications

Because of their appealing properties, CDs have managed to find a place in many different applications. The biggest appeal comes from the possibility of integrating their optical properties, comparable only to those of other quantum nanoparticles, with all the advantages of carbon based materials, such as the high degree of interaction with other species and especially their biocompatibility. In this section some examples regarding the possibilities covered by carbon dots will be given, so to shine some light on what is to be expected from this family of nanoparticles.

Chemical Sensing

As previously discussed in the section regarding the carbon dots capability to efficiently interact with other species, many articles have observed that the presence of external molecules in solution with these nanoparticles often causes a quenching of their fluorescence.

It does not surprise then to discover that many researchers have exploited this characteristic to make nanosensors based on carbon dots.

The most common example of this application consists in CDs sensing metal ions;

because of the mutual attraction between the negatively charged dot surface and the positive ions, CDs have been found to be very efficient in sensing several different types of metals, such as Hg 2+ , 235 Cu 2+ , 158 Fe 3+ , 236 Pb 2+ , 237 and Ag + , 238 just to name a few.

Surprisingly, CDs are so efficient as chemical sensors that some researchers have been able

to report detection limits as low as 1 fm for Hg 2+ ions. 239 As the main mechanisms causing ions and dots to come together is arguably an electrostatic interaction stemming from the opposite charges of the two species, one could think that the sensing capabilities should not depend on the type of ion. While this is the case for some carbon dots, whose emission was found sensitive to the presence of several different metals, 168 some researchers were able to synthesise CDs with selective sensitivity to a certain ion. For example, carbon dots synthesised starting from glucose via a microwave procedure were found to be extremely selective probes for the detection of Cr(VI). 207

Interestingly, in this case sensing was not achieved by observing the emission intensity;

instead, these particular CDs, normally emitting yellow light, undergo a remarkable blueshift in the presence of Cr(VI) ions, as shown in Figure 2.13. In order to understand what causes this strange behaviour, researchers compared FTIR and XPS spectra of the nanoparticles when the ions were present or not. Remarkably, addition of Cr(VI) lead to a decrease in the amount of oxygen-containing groups; as these moieties have a role in the emission mechanisms, as previously discussed, the presence of chromium seems to have modified the surface and, thus, the emissive properties of CDs, leading to the remarkably selective probing.

In addition, making use of the dual emission of some carbon dots, researchers were able to achieve quantitative sensing of ions when using these nanoparticles as ratiometric probes. 158 ,206 ,240 With dual emission we refer to the fact that some CDs present a fluorescence composed of two distinct bands. As these bands are often linked to different emission mechanisms, they respond differently to alteration in the environment. Consequently, the quenching degree of each of them results dissimilar; after prior calibration, it is thus possible to calculate the concentration of the quenching specie simply from the intensity ratio of the two CDs bands.

Bioimaging and Nanomedicine

Because of their remarkable non-toxicity, carbon nanoparticles have sparked a huge interest towards their possible usage for biological application. For example, reports of carbon dots used as antivirals 241 ,242 and antibacterials 243 can be found in the literature. Nonetheless, one of the most common applications of CDs is found in bioimaging, with several articles which have been reporting how their fluorescence can be exploited for this purpose both in-vitro 162 ,189 ,198 ,244 and in-vivo. 152 ,203 ,204 Remarkably, the multicolour emission of CDs results very versatile for the observation of cells and enhancing their structure. In fact, as shown in Moreover, some CDs have been used for in-vivo identification of tumoral cells. These nanoparticles were synthesised starting from D-glucose and L-aspartic acid; when injected through the intravenous route in a cancer-bearing mice they were found able to spontaneously target cancerous C6 glioma cells. Already after 5 min, as shown in Figure 2.15, these CDs were found to penetrate the blood-brain barrier and start to significantly accumulate in the glioma site of the brain; after 90 min from the injection, while now absent from the rest of the body, it was still possible to observe CDs in the region where the tumor was present. As their fluorescence allow for easy in-vivo identification, this kind of nanoparticles resulted extremely useful for early non-invasive diagnosis. 246

In addition to detection, carbon dots coupled with anticancer drugs could be used to kill cancer cells. This attempt at using CDs for drug-delivery was reported in a study of 2008 by using GQDs passivated with poly(ethylene glycol) (PEG). These nanoparticles were loaded with Rituxan, a CD20+ antibody enabling targeting of specific cancer cells which was able to covalently bond to PEG, and Doxorubicin, an anticancer aromatic drug which was able to bind on GQDs through π-stacking. When tested in-vitro against cancer cells, the modified nanoparticles were able to inhibit the cell viability up to 80%. 159

A different study was instead able to synthesise carbon dots presenting an inherent inhibitive effect towards the growth of cancer cells. Interestingly, these particular nanoparticles were obtained from used green-tea leaves, treated by calcination in an autoclave at 300 °C and purified through centrifugation, filtration and dialysis to remove the bigger agglomerates. These highly soluble CDs were then tested in-vitro on MCF-7 and MDA-MB-231 cancer cells; remarkably, a decrease in the viability of these samples was observed with the increase of nanoparticles concentration. When instead tested on normal MCF-10A cells, CDs exhibited lower toxicities as only 5% of these were influenced by the dots presence. 247 Authors indicate that the inhibitory activity of these nanoparticles stems from the ability to generate reactive oxygen species which, when produced in contact with the cancer cells, are able to induce their apoptosis. Carbon dots such as these result highly appealing for nanomedical applications: in fact, apart from being very effective in causing the apoptosis of cancer cells, they also result inexpensive and convenient, especially if considering their synthetic route.

Optoelectronic

The intense absorption and bright fluorescence of carbon dots make them ideal components for a broad array of devices. In addition, thanks to their capabilities as electron donors and acceptor, CDs can easily integrate with other materials, enhancing their properties. This is especially true when considering their usage for optoelectronic applications.

When seeing them as a green alternative to semiconductor-based nanoparticles, a role which carbon dots could be immediately be thought to cover is that of active layer in light emitting devices. Nonetheless, because of the technical challenges needed to incorporate a new material in such a device, initial studies of CDs in LEDs focused instead on using them as light converters. In fact, when mixed to an epoxy resin they can be used to cover the emitting surface of a pre-existing blue LED, so to instead generate white light for obvious applicative advantages. 248 In any case, some researchers have been able to build LEDs completely based on carbon dots. In such a device, the carbon nanoparticles were synthesised starting from ethylenediamine and phthalic acid by an hydrothermal method in autoclave. To build a LED, the dots were then sandwiched between poly(ethylenedioxythiophene)polystyrene sulfonate and 1,3,5-tris(N-phenylbenzimidazol-2yl)benzene, respectively acting as hole injection layer and electron transport layer. The circuit was finally completed by enclosing the three layers between an indium tin oxide (ITO) glass acting as cathode and an LiF/Al anode. While only the maximum obtained luminance resulted equal to 4.97 cd m -2 , the described device is an interesting proof of concept for what could be achieved by tuning the CDs properties so to achieve higher quantum yields and a smaller energy barrier between hole injection layer and emitting layer. 249 Carbon dots have also been used as active medium for lasers. While some reports of lasing emission from carbon can be found in the literature, 250 ,251 a common point often reported is that, because of either poor physical properties or low optical gain, 252 it is difficult to obtain amplified spontaneous emission from carbon nanoparticles. Nonetheless, recent advances in the purification of CDs have enabled to simplify such a process. In an article from 2018, carbon dots purified via size exclusion chromatography were found to be able to efficiently act as active medium so that lasing emission was observed without the use of external mirror, as shown in Figure 2.16; in fact, the simple 4% reflectance of the quartz cuvette used in the experiments was high enough to behave as an optical cavity when the CDs solution was opportunely pumped via a nanosecond laser source. 184 In this case, the CDs-based laser seemed to closely resemble more traditional dye-based lasers.

Finally, carbon dots have found a place also in the fabrication of solar cells, 253 -255 where they appear to have a dual role. Intuitively, the major contribution from the introduction of CDs in this kind of devices would come from their ability to absorb visible light; because of their powerful absorbance bands, the ability to harvest more of the sun energy would then increase the overall efficiency of the cell. Some researchers, nonetheless, by studying the behaviour of assemblies containing CDs have found out that these nanoparticles have beneficial effects on yet another problem often affecting solar cells, which is charge recombination. A recent report was in fact able to observe through transient absorption measurements how by inserting a thin layer of GQDs in a perovskite solar cell the photogenerated electrons were extracted more quickly; thanks to that, the trapping and recombination of charge carrier, which hinders electron transport through the device, was more efficiently prevented so that the power conversion ratio increased from 8.81% to 10.15%. 160 Other studies reaching the same kind of conclusions can be found in the literature, also when this kind of carbon dots is instead used in TiO 2 -based solar cells. 256 Concluding, it appears that the efficient capability of CDs to act as electron donors could have a huge role in the fabrication of better performing solar cells.

Photocatalysis

Because of their numerous interesting properties, carbon dots are very appealing for photocatalytic applications. Apart from being low-cost and generally non-toxic, the adaptability of their surface and the possibility to interact with other species makes them very suitable for these purposes. In addition, once again their optical properties play a fundamental role in this context, as these allow CDs to convert solar energy and redirect it towards other processes. 257

In many cases these nanoparticles have been simply used as photosensitisers, so to enhance the collection of visible photons of other renown catalysts. In this regard, among the many materials which have been sensitised through carbon dots we can find Fe 2 O 3 , 258

CdS, 259 MoS 2 , 260 and even gold. 261 ,262 Nonetheless, probably the catalyst which has been associated the most to CDs is TiO 2 , 155 ,253 ,263 -266 probably due to the huge appeal of combining together the water splitting capabilities 105 of titania with the green and clean particles that are carbon dots. In many cases, the interaction efficiency of the complex formed by CDs and catalyst was observed through the degradation of organic dyes. Such an application is in fact seen both as interesting for the possibility of removing polluting agents from water and as a benchmark of the photoinduced electron transfer taking part in the redox process, thus verifying the role of carbon nanoparticles. In all cases, the catalytic process was enhanced by the presence of CDs, thanks to their efficient light absorption and the good alignment of the energy bands of sensitisers and catalyst allowing for a photoexcited electron to be transferred from carbon nanoparticles to the active material.

A very interesting example can nonetheless be found in a work where carbon nanoparticles and CoO have been coupled together to achieve efficient water splitting. 267 The CDs-CoO complex was synthesised starting from carbon nanoparticles obtained via an electrochemical route. These were then mixed to Co(CH 3 COO) 2 • 4 H 2 O in a solution composed by n-octanol and ethanol, in which they were left for 2 h while mixing. The formed slurry was then transferred to an autoclave before heating it at 220 °C for 4 h. The so-obtained powder was then tested for water oxidation under visible-light irradiation and compared to pristine CoO, synthesised in a similar manner; as a result, the amount of H 2 and O 2 generated, measured via gas chromatography, was found to be six times higher than the bare cobalt oxide when the complex was used.

Moreover, a number of researches directly using CDs for photocatalytic applications can be found in the literature. For example, carbon dots doped with nitrogen were found able to catalyse the photo-degradation of methyl orange under visible-light irradiation; in just 2 h 90% of the molecule was degraded by the nanoparticles, while in the same amount of time the organic dye resulted to be perfectly stable and as no change in its concentration was found in the absence of CDs. 268 In a different study, carbon nanoparticles once again doped with N were found able to generate H 2 from water. In this paper published in 2017 the nanoparticles were synthesised from citric acid and branched polyethyleneimine via a microwave-assisted thermal route. Remarkably, hydrogen generation was performed at almost neutral pH and did not necessitate addition of any other co-catalyst. The quantum efficiency of these N-CDs for hydrogen generation under UV-visible light was found to be equal to 0.84%. 161 In both the described articles, nitrogen is regarded as very important for carrying out the photocatalytic processes. For example, in the last paper authors claim that the N atoms doping the aromatic carbon domains are responsible for attracting the photogenerated electrons, thus allowing charge separation and enhancing the catalytic H 2 generation.

Whilst we surely still need to further investigate the capabilities of carbon nanoparticles in the domain of photocatalysis, their optical and electronic properties seem to be extremely promising for this applicative context. A higher degree of knowledge on the role of the structure and doping agents in this kind of processes will probably open-up more possibilities for these novel nanoparticles in the future.

Future Perspectives

Since their discovery carbon dots have been able to surprise the research community, thanks to their properties and the huge applicative possibilities where they have found a role. The rapid enhancement of synthetic and purification techniques that this family of carbon based nanomaterials has recently witnessed allowed us to better understand their structure and capabilities. Nowadays having a green, clean and non-toxic family of nanoparticles displaying a high degree of interaction towards other materials appear as an exciting innovation which could revolutionise the field of nanodevice development.

Nonetheless, we are still lacking an answer to several fundamental questions before being able to fully understand CDs and make use of their enticing properties.

While many different synthetic routes have been developed, CDs synthesis still seem to proceed through a trial-and-error pathway. Despite being able to obtain these fluorescent nanoparticles from cheap and easy to access precursors 170 ,247 and via fast one-pot methods, 176 ,221 ,269 the full control of the resulting properties expressed by CDs still seem far from being reached. Only a few researches seem to have explored the many synthetic variables presented in the carbon dots production routes, 161 ,182 and when doing so the role of the used components towards the final optical and electrochemical properties has resulted clearer. It would thus be very beneficial focusing more on controlling the production pathways, so to reach a better understanding of what is possible to obtain. In addition, the high degree of impurities resulting as by-products after the dots formation is now evident. Works on the purification of nanoparticles have shown how the coarse solutions obtained from a synthesis are actually composed by various fractions of different materials, sometimes resulting in carbon dots displaying extremely bright fluorescence. 184 ,191 Finding easier and faster purification routes will surely boost the usage of CDs, opening up new and more efficient applications and shortening the distance separating these nanoparticles from industrial adaptation.

In addition, the lack of understanding of the inner mechanisms allowing the properties of carbon dots has been plaguing this material since its very discovery. As of now, we often hide behind the observation of the huge heterogeneity found in the many different structures and components which characterise these nanoparticles; nonetheless, understanding that there are more common points than differences, as evident from the many shared properties found in any of the CDs categories, can only allow to push further what we currently know.

Right now, several efforts have been focused on finding an answer on these yet unanswered questions. 185 ,219 With the years of CDs research to come, the better understanding that we are obtaining through the use of previously barely explored techniques such as ultrafast spectroscopy 270 ,271 is probably going to help in reaching a conclusion on these fundamental questions. It is to be expected that all these advances will be extremely beneficial not only for a better comprehension of the possibilities and characteristics of carbon dots, but also to their tailoring towards selected applications and the fine-tuning of their properties.

In recent years we have started to find new applications for carbon dots. With each published article, new materials able to efficiently interact with them are found, creating new possibilities previously unexpected. Despite the difficulties which have arisen from the lack of purity displayed by CDs out of synthesis and the still unclear excited states and energy levels distribution of these nanoparticles, 257 several promising results have already been obtained. 161 ,184 ,246 ,249 By combining the recent advances regarding synthesis, purification and understanding of the CDs inner mechanisms, the field of applications covered by carbon nanoparticles is only expected to grow even more quickly in the future. As this innovative nanomaterial has only been discovered less than twenty years ago, the thriving interest for its promising characteristics will only lead to a faster development of the many exciting possibilities carbon dots present.

Chapter 3

Aim of the Work

The objective of this thesis work is obtaining POM-CDs complexes and studying their properties and interaction. Being able to control such a complex is regarded as very appealing from the perspective of the possible nanodevices that would be based on it.

Coupling together polyoxometalate and carbon nanodots would mean being able to harness the advantages that both materials present, especially considering the catalytic POMs capabilities 13 and the optical properties of CDs. 185 In addition, both materials display low toxicity, 84 ,152 an additional properties that applicative transfer would greatly benefit from.

To understand why we consider such an aim as very interesting it is probably better to focus on the limitations displayed by the two species on their own.

Probably the most important point regarding the usage of carbon dots alongside polyoxometalates comes from their significant absorption cross-section. Despite POMs display very promising results when studied as electrocatalysts, the fact that these anions are not photoactive when excited by visible light hinders their possible usage in the broad field of green photocatalysis. A huge focus of the research community is, in fact, nowadays directed towards the development of artificial leaves; these are nothing else than artificial devices which are able to collect energy as natural leaves do, albeit through mechanisms different than photosynthesis so to provide directly usable fuel. 272 As plants are able to convert solar energy and store it in molecules such as carbohydrates, the artificial equivalent should be able to make use of the sun radiation for energy conversion and storage purposes.

From a practical point of view, this kind of devices are usually just required to provide means for splitting water through the use of solar energy, thus producing O 2 and H 2 , even though the final goal would also need absorbing CO 2 so to reduce its amount for clear environmental reasons.

In the case of artificial leaves based on POM-CDs complexes, carbon dots would have the role of photosensitisers, so to absorb the visible radiation part of the solar spectrum before transferring the photoexcited electrons to polyoxometalates; POMs would then perform the actual water splitting, as previously described in Section 1.6.1. This mechanism would allow to overcome the limited visible absorbance of polyoxometalates by simply substituting CDs for the role of photoactive element. As CDs have shown remarkable electron transfer capabilities towards other species 157 this route is expected to be very efficient.

In addition, even if POMs present some of the most attractive catalytic properties, as previously pointed out these supermolecular structures have some charge and mass transport limitation; 75 this is especially true when used as constituent part of nanodevices.

In fact, when used to coat electrode for heterogeneous catalysis purposes, researchers have observed how the efficiency was limited. Very often adding more layers to the coating did not improve performances as expected, pointing towards limitation in charge mobility inside the material. 138 In this regard, CDs could very well be able to provide a solution:

when studying the role of these nanoparticles, especially for the GQDs members of the huge carbon dots varieties, researchers have observed how their presence helped to avoid charge recombination by providing electron traps on their surface which aided separation and thus improved the efficiency of the studied devices. 160 ,256 While these examples were applied in the field of optoelectronic, transferring these finding when using CDs as photosensitisers could highly enhance the performances of the interacting catalysts and, in the case of polyoxometalates, help reducing charge recombination while also improving transport properties.

Apart from these reasons directly linked to the applicative benefits of POM-CDs complexes, there are also fundamental motivations fuelling our interest in coupling together these two materials. It has in fact to be considered that we are still lacking a complete comprehension of the inner mechanisms allowing for the enticing properties of the two species. This is true for polyoxometalates since, has some reviews have claimed, 75 the understanding of the photophysics of POM-sensitisers systems is still incomplete and lagging behind compared to, for example, that of dye-sensitised solar cells. A similar reasoning can also be applied to CDs which present several unanswered question both related to their ability to interact with other species and to their optical properties, as respectively discussed in Sections 2.2.3 and 2.2.4. While we are already witnessing many advances in this regard for both materials, a study of their interaction could further help the ongoing discussion on the topic. Moreover, a comparison between how different POMs structures interact with CDs could help us better understand the role of the existing architectures and eventual heteroatoms presence in both catalytic properties and charge transfer mechanisms of polyoxometalates.

The main route for obtaining POM-CDs hybrids followed during the course of this research is based on electrostatic coupling. As previously explained, polyoxometalates are anions, often presenting high negative charges in solution. Even if most of the CDs found in the literature present a negative surface too, which would obviously hinder any electrostatic coupling to POMs presenting the same charge, by carefully choosing the synthetic route and by working at opportune pH, positively charged carbon dots can be nonetheless obtained. In fact, while the negative surface of these nanoparticles is often due to presence of deprotonated carboxylate moieties, a positive charge can arise at mild pH conditions: this happens when -COOH groups are reverted to their neutral state while eventual amines also found within the organic shell become protonated. This means that N rich carbon dots, which often display amines on their surface, would be highly suitable towards the purpose of electrostatic coupling with POMs. Accidentally, high nitrogen content is beneficial to CDs optical properties, as evinced by several literature studies. 195 This means that an electrostatic coupling between POMs and CDs, apart from being perfectly feasible, would also benefit from having a sensitiser displaying good visible-light absorbance cross section and thus high sensibility to solar irradiation.

As far as we know, before this research had started, only one article published in 2014 explored the possibility of bringing together carbon nanoparticles and polyoxometalates. 273

In particular, this paper explores the capabilities of a nanodevice composed by polyoxometalate Ag 3 PW 12 O 40 , Ag nanoparticles, and carbon quantum dots obtained from an electrochemical route. Notably, here CDs do not have any role as photosensitisers.

In fact, as the authors themselves report, absorption of visible light occur at the Ag nanoparticle surface through surface plasmon resonance. It is instead claimed that the CDs role is to be found in two main areas: firstly, thanks to their insolubility they have a stabilisation effect on the rest of the complex, specifically avoiding the dissolution of added. The efficiency of the full device, characterised through the forward oxidation peak current density observed when performing the methanol oxidation reaction, was found to be higher than both that of an electrode solely based on Pt and that of electrodes missing either the CDs or POMs component.

The discussed articles prove how promising POM-CDs hybrids are, especially towards various applications. As noted, polyoxometalates and carbon dots have nonetheless been used alongside other components, so that their role could appear somewhat unclear in some cases. There is still much to be understood about the interactions arising from the coupling of these two species. This work of thesis aims to shine some light on the properties of these innovative materials, trying to elucidate their coupling and the mechanisms underlying their surprising characteristics. Right now, non-toxic photosensitive nanodevices with good electronic properties are very sought out due to the huge amount of possibilities they could cover, especially in the field of green and renewable energy. POMs and CDs both fit into this description and have complementary properties which, when coupled together, can be further enhanced. Focusing on this possibility we explored the fundamental characteristics of the two materials, the underlying mechanisms allowing for the observed electron transfer, and the possible applications that could follow from the highly anticipated further developments of POM-CDs hybrids.

Part II Experimental Results

Chapter 4

Materials Characterisation

In order to give a generic view of the characteristics of the materials mainly used throughout the experiments, the present chapter will present the properties of the chosen polyoxometalates and carbon dots.

Polyoxometalates

Regarding POMs, we have focused on the study of four different species, with {P 2 W 18 } as the compound that has been mostly utilised. In addition, sandwich structures derived from this cluster have been also used, namely {Co 4 P 4 W 30 } and {Zn 4 P 4 W 30 }. In the following sections these compounds will be separately characterised.

Sandwich-Type POMs

As other polyoxometalates have been studied for their interaction with CDs, these compounds will be here briefly presented. In particular, the Wells-

Carbon Dots

In order to find the opportune CDs able to interact with POMs, a literature research was carried out. Among the many different carbon nanoparticles that could be chosen, some of which have been presented in Chapter 2, the focus has been directed towards dots characterised by ease of synthesis, high quantum yields and abundance of nitrogen containing groups on their surface.

While it is clear how a quick and direct synthesis is regarded as extremely advantageous for product availability, the other characteristics have been chosen purely with the coupling of polyoxometalates in mind. To understand why, we have to consider the mechanism responsible for the electron transfer from CDs towards other species, which is the photoinduced electron transfer (PET). 157 For this process to occur, the photoexcited system state needs to be stable until the electron transfer occurs. Having a high quantum yield is a good indicator of this characteristic; in general to achieve good emitting efficiencies, fluorescent materials need to display low non-radiative recombination rates. As a consequence, carbon dots with a high quantum yield were considered having non-radiative recombination pathways occurring with low probabilities, which translates to PET processes not having to compete with this recombination mechanism, thus making the transfer more efficient.

While this assumption explains the first sought after properties, also the second can be explained considering photoinduced electron transfer as the mechanism responsible for the interaction between the two species. In fact, for this mechanism to occur molecular contact * between fluorophore and quencher is required. 227 As such, driving forces able to bring together CDs and POMs are needed. As polyoxometalates are polyanionic species, they could be easily coupled to positively charged materials so to achieve the needed molecular contact through electrostatic interactions. Since carbon dots charge depends on the protonation state of its surface groups, nanoparticles displaying amines on their surface were looked for. In fact, these organic groups are usually protonated at mild pH, a very convenient characteristic for our purposes. Therefore, high nitrogen content was the second property on which the choice of CDs was based.

In the end, cyan-emitting GQDs synthesised via an autoclave hydrothermal treatment of tri-nitropyrene were chosen. The full production route, based on a literature work, 214 is described in Section A.6.2. In the following sections, structural and optical properties of these CDs will be presented.

Structural Properties

The size of the obtained carbon dots was studied via AFM. As shown in Figure 4.6, these nanoparticles display an average height of 3.7 nm, even though the full distribution appears to be quite broad (σ = 2.0 nm) as shown in Figure 4.6C. To obtain the height distribution, 1027 individual grains observed in the topography reported in Figure 4.6A were analysed.

Considering that these nanoparticles are GQDs, it appears that on average four graphene layers are stacked together to form the CDs core. * At least in the approximation described by the Smoluchowski model; nonetheless, other models as well show how the quenching probability for PET exponentially decreases with distance. In particular, the following planar distances were found: three peaks corresponding to 2.1 Å (indicated by the letter a), two additional peaks corresponding to 2.9 Å (b), and several peaks distributed along a ring diffraction pattern corresponding to a distance of 2.5 Å.

In the literature, planar distances of 2.1 Å are usually obtained from HRTEM images of CDs; based on this result, many authors have claimed the underlying carbon structure to be graphitic. Even though we have been able, at least from one image, to observe such a distance, graphitic carbon does not present neither the 2.5 Å nor the 2.9 Å diffraction peaks (based on reference pattern of graphite 2H, 98-003-1170). Due to the previously mentioned difficulty of finding more nanoparticles, the analysis was thus not concluding.

In order to obtain more informations on the underlying core structure of the analysed CDs, Raman spectroscopy was performed. The substrate was then analysed collecting Raman spectra at an excitation wavelength of 325 nm. Resulting spectrum is shown in present. In particular, it is possible to observe both D and G bands, respectively located at 1360 cm -1 and 1500 cm -1 and presenting an intensity ratio of 1.2. In addition, also a composite not fully resolved 2D band located at 2800 cm -1 can be found.

Moving forward, DLS was used in order to obtain the hydrodynamic radius of the nanoparticles. From the number distribution shown in Figure 4.9, an average value R h = 13.6 nm is obtained; once again the distribution appears to be quite broad, this time with σ = 3.8 nm. In addition, the measured size seems to be slightly bigger than what observed via AFM. This is nonetheless not surprising if considering two factors; firstly, the hydrodynamic radius is always going to be bigger than the actual size, due to the presence of the solvation shell surrounding the nanoparticles. This is especially true for species such as carbon dots, on whose surface lies a wide variety of organic groups keen on interacting with the solvent, as explained in Section 2.2.2 and 2.2.3. In addition, AFM allows to measure size only along one dimension; as GQDs cores are formed by graphene layers stacked on top of each others, it is highly possible for vertical and lateral dimensions to not correspond at all. DLS allows to obtain hydrodynamic radii based on the assumption of approximately spherical scatterers, condition here not respected. The obtained R h will nonetheless be later assumed as a valid approximation when needed to calculate the diffusion coefficient of CDs.

Moreover, DLS also allows to measure the electrophoretic mobility of nanoparticles.

This was thus measured for a CDs solution at different pH, so to observe the changes in surface charge due to the protonation state of organic groups present on the dots surface.

In order to adjust pH, diluted HCl was added drop-wise to the initially basic carbon as CDs + , based on the charge of their surface.

In order to understand which organic moieties are responsible for the observed changes in surface charge, FTIR spectroscopy was performed on both CDs -and CDs + so to compare differences between the two species. Resulting spectra are shown in Figure 4.12 and the relative FTIR peaks are reported in Table 4.1. From the identified peaks, it is assumed that the amine moieties are responsible for the positive charge displayed by carbon dots at mildly acidic pH. Their protonation can in fact explain the observed behaviour. Nonetheless, the complex shape of the spectra below 1700 cm -1 makes it difficult to definitely pinpoint the groups responsible for the negative charge observed in basic conditions. While in this region carboxylates vibration peaks are often found, the intense amine signals could cover them. Considering that many carboxylates present a pK a slightly lower than 5 and that the ζ-potential vs. pH curve shown in Figure 4.11 seems to traverse a value of zero around this value, it is assumed that the deprotonation of -COOH to -COO -is actually responsible for the negative surface charge observed in basic conditions. 4.12; their position and the responsible functional groups are indicated.

Optical Properties

The optical properties of CDs in the two previously presented pH conditions -and thus surface charge states -have been studied. All experiments presented in this section have been conducted on solutions having a concentration equal to 0.025 g L -1 .

Both CDs -/+ present absorbance bands which extend into the visible spectral region and, for the positively charged sample, up to 500 nm, as observed in λ exc (nm) CDs -(%) CDs + (%) While this eventuality has not been further characterised, the difference between the two samples is probably evidence of changes in the surface state of the particles, often linked to the CDs optical properties as discussed in Section 2.2.4. Nonetheless, the quantum yield (QY) of each band was estimated using a reference Fluorescein sample in H 2 O at pH 13 (QY = 95%); 227 results are presented in Table 4.2.

400 36 ± 4 11.1 ± 1.2 450 18 ± 2 2.4 ± 0.3
In addition, as commonly found in many carbon nanodots, the emission bands peak position is tunable. As shown in Figure 4.14 for CDs -, excitations between 300 and 500 nm shift the emission of both samples in the region of 430 -530 nm. In addition, it seems that the emission spectrum of the samples is composed by two bands, both of which appear to be tunable; such characteristic could be used to develop ratiometric sensors based on these types of GQDs stable in a wide range of pH values.

The kinetics of the emission bands was thus studied in the nanoseconds time-scale.

Time-resolved (TR) photoluminescence experiments were conducted on both CDs -and CDs + at two different excitation wavelengths, 410 and 450 nm. As for these values of λ exc the two previously observed emission bands seem to be predominant on each other, this choice of excitation should allow to study the full decay behaviour of the sample luminescence.

An example of the obtained data, in particular relative to CDs + excited at 410 nm, is shown in Figure 4.15. From this image-plot, emission at short wavelengths appears to be decaying slower than that found in the longer wavelengths region.

To confirm this observation, spectra taken at different delays from excitation have been normalised and compared. The resulting plot, shown in of the exciting laser were considered as global parameters, as well as the sought time constants τ ; this way it is possible to obtain values able to represent the full studied wavelengths interval, regardless of the spectral asymmetry observed in Figure 4.15.

The obtained τ i are shown in Table 4.3 for both samples and excitation wavelengths.

Based on the fitting procedure we can claim that all data can be represented by a biexponential decay kinetic, evidence which again points towards two different components decaying separately and thus two distinct energy states responsible for the emission.

In order to compare the relative weights of the two components, their fractional amplitude is shown in Figure 4.18; from the results here presented, it appears that both samples present an emission composed by two kinetics whose relevance can be selected by changing the excitation wavelength. The two samples present nonetheless some differences, as CDs + displays lifetimes shorter than those obtained for the negatively charged counterpart. In addition, CDs -seems to present two bands closely overlapping, as their weight does not appear to change depending on the selected emission wavelength as observed in component τ 1 seems to be predominant at short wavelengths while the opposite is true for τ 2 , as evinced from Figure 4.18C and D.

Finally, when observing each component relative to a selected sample the associated time constant is found to decrease with longer excitation wavelengths; it thus seems that the optical tunability found in the steady state photoluminescence peak position is present also in the underlying kinetics.

Based on these evidences, it can be stated that the optical properties of the studied CDs present a high degree of heterogeneity, characterising both spectral shape and emission kinetics. As changing the pH of the studied solution was found to have such significant consequences on the observed characteristics, it is highly probable that the surface state of the nanoparticles have a huge influence on their optical properties. This is nonetheless not surprising, as this topic is largely found and discussed in the literature.

Calculation of the radiative decay rate Γ as ratio between QY and τ can give us informations regarding the underlying emission mechanism. The problem with this approach arises from the fact that, due to the heterogeneity of the CDs samples, contributions from each emitter are difficult to isolate. Nevertheless, the order of magnitude of the radiative decay rate can still be obtained from the average emission lifetime; following this approach, values Γ ≈ 10 extracted from the TA matrix. These are shown in Figure 4.20 for both CDs -and CDs + ; as the signal is often oscillating in time, as evinced by the horizontal features observed in Figure 4.19, reported spectra have been averaged on an interval equal to 10% of the delay associated to each spectrum. This allows to reduce noise and simplify the observation of the kinetic followed by the bands composing the spectra.

It is possible to identify four components present in the spectra of both samples.

The negative signal peaking below 450 nm, close to the excitation pump wavelength and peaking outside of the studied region, is associated to ground-state bleaching (GSB). The other negative signal, recognised as a dip found at 500 nm, is associated to the stimulated emission (SE). Finally, most of the remaining part of the spectra is composed to a positive signal associated to an excited-state absorption (ESA); this appears as composed by two different bands, one found between GSB and SE approximately at 460 nm and another peaking around 570 nm.

As all signals are present in both samples, there are no striking differences in the TA of CDs -and CDs + . Nonetheless, the bands composing the CDs -signal peak at shorter wavelengths compared to CDs + ; for example, the SE band is found at 490 nm for negatively charged particles and at 500 nm for the positive counterpart, while the red-most ESA band peaks at 565 nm in the first case and 580 nm in the other. In addition, bands corresponding to CDs + appear larger than those of CDs -; as shown in As remarked in the previous paragraph, having these best-fitting curves it is possible to reconstruct the original data, as shown in Figure 4.22B for CDs -; as evident from the good overlap between original and reconstructed spectra, it is possible to conclude that the fit procedure has been successful and that the observed TA signal is properly describe as decaying with three different time components. In addition, as it will be later explained in Chapter 5 and 6, also data regarding POM-CDs complexes has been introduced to this same global fitting procedure and can thus be described by DAS displaying the same decay lifetimes. The consequences to this finding will be discussed in the following chapters, when examining the interaction between POMs and CDs.

Regarding the results relative to CDs -and CDs + , the decay associated spectra de- CDs -0.17 ± 0.06 1.2 ± 0.2 17 ± 4 > 1 ns CDs + 0.11 ± 0.02 1.06 ± 0.17 20 ± 3 > 1 ns Table 4.4: Time constants relative to the decay associated spectra of CDs -and CDs + , as obtained by a global least-squares fitting procedure performed on the eigentraces of the samples transient absorption signals.

the two samples; in fact, while both display the GSB and ESA bands previously identified, only in the case of CDs + we can see the negative SE signal. The fact that from the decay associated spectra analysis it is possible to find a decaying SE signal only for CDs + , and not for its negative counterpart, may explain why the positive nanoparticles present a lower QY compared to CDs -. Arguably, the different pH conditions allow for a dissipative mechanism otherwise not occurring.

Finally, regarding the last two DAS of lifetime τ 2 ≈ 1 ps and τ 3 ≈ 20 ps, it is possible to see how these are for the most part characterised by a continuum-like ESA signal. This feature spans the whole investigated region, except for the short-wavelengths part where a GSB band is also found; flat signals such as this have been associated to electrons promoted to the conduction band of the system, consequently free to explore a continuum of energy states; 279 in the case of the studied CDs, the flat part of the DAS should thus be representative of the decay back to the ground state of electrons excited into the graphitic carbon core conduction band. These results appear reasonable if considering that in the literature excitation of the core usually results in either very low quantum yields or completely dark states; 197 this evidence would thus justify associating the flat picoseconds-decaying signals to transitions found within the CDs core. Regarding the weak ESA bands present on top of the continuum-like spectra, mostly noted in the 20 ps DAS of CDs + , are probably due to transitions from the surface back to the core; in fact, these do not contribute to the overall emission -being associated by non-radiative transitions -and present a shape and peak position close to the ESA associated to surface states.

Nitrogen Rich CDs

At the start of Section 4.2 we had discussed how the choice of the particular kind of CDs was conducted. Among the relevant factors considered when valuing the many different carbon dots found in the literature, ease of synthesis, high quantum yields, and abundance of nitrogen were mentioned. Respecting these criteria, whose motivations have been previously explained, other CDs appear as suitable for the purposes of this work of thesis.

In particular, the alternative selected nanoparticles which have been studied are a kind of nitrogen rich subset of the many CDs families, previously studied for their high degree of interaction with the environment 156 and other species in solution. 157 Regarding the aforementioned factors, their synthetic procedure is particularly appealing as it consists in a single very rapid and easy microwave treatment of a citric acid and urea aqueous solution; further details regarding it are found in Section A.6.2, as the procedure is based on a literature work. 173 In addition, reports discussing the optical properties of these carbon dots mention quantum yields as high as 12%, 197 which can be considered very interesting for CDs that do not necessitate any purification step to achieve better emissive characteristics. Concerning the last factor, these dots are known for having nitrogen atoms within the core structure. Due to the high N amount introduced during the synthesis step, this element was found able to bind with carbon atoms to form a non-graphitic lattice: instead of being just either a dopant or a surface passivating agent, in this case the β-C 3 N 4 structure was formed due to nitrogen content. Because of this, these peculiar carbon dots will be labelled N-CDs.

The optical properties of N-CDs are shown in Figure 4.24. The intense absorption spectrum results composed by two bands, peaking at 330 and 400 nm; of these two, the lower energy one extends well into the visible, up to wavelengths higher than 500 nm.

Excitation of this latter band allows to observe an intense fluorescence, in line with the literature reports. Once again, as commonly found for CDs, the N-CDs emission is tunable as it shifts depending on the excitation wavelength: for λ exc = 440 nm the resulting emission band peaks at 530 nm; instead, for λ exc = 500 nm the fluorescence maximum is found at 550 nm.

The surface composition of these nanoparticles has been studied via FTIR spectroscopy, which allows to identify the organic moieties composing it. The resulting measured spectrum is shown in Figure 4.25 and the identified surface functional groups are reported in Table 4.5. From these results, the main components of the organic surface shell of N-CDs are carboxylic acid and amide; this kind of moieties is not uncommon for carbon nanoparticles synthesised in presence of nitrogen.

As a positive surface charge is considered an ideal driving force for the interaction with polyoxometalates, the electrophoretic mobility of N-CDs was measured in a range of different pH, whose value was adjusted by addition of either HCl or NaOH. Despite FTIR results did not allow to identify any functional group which could be protonated in mild acidic conditions, it was decided to nonetheless perform the experiment, as the many intense features of the infrared spectrum could have easily hidden peaks associated to, for instance, amines. The charge distribution measured at pH = 6. in these conditions were observed undergoing rapid flocculation. As in the literature many articles report that CDs are generally unstable in acidic conditions, we believe that N-CDs have precipitated out of solution; in this case, the onset of the large double distribution observed at the reported pH value is just an evidence of this event.

Conclusions

In conclusion, most of the results reported in this chapter concern carbon nanoparticles.

The characterisation herein presented has been done as necessary stepping stone for the study of the interaction further explored in the next chapters. In any case, the results have been largely consistent with the state of the art regarding CDs, as once again their renown heterogeneous optical and structural properties have been remarked.

Nonetheless, a fundamental difference has been found, whose importance will come clear in the development presented in the following chapters. In fact, we have explored how it is possible to find CDs whose surface can display a positive charge. This result is not common, as for instance witnessed when studying N-CDs. Reaching this goal, fundamental for the purposes of this thesis, to our knowledge has never been previously reported in the literature. As most carbon nanoparticles present negative surface charges stemming from the kind of chemical moieties commonly developing during the synthesis step to form the organic shell, interaction with anionic species was to be considered difficult. In fact, the literature is full of evidences of cations able to quench the CDs fluorescence, while changing the counter-ion of the salt used throughout the photoluminescence experiments seldom influenced the resulting quenching.

As later shown, this result will prove very important for the interaction between POMs and CDs, letting us imagine other novel interactions between CDs + and anionic species.

Chapter 5

{P 2 W 18 }-CDs Complexes
In this chapter the interaction between CDs and {P 2 W 18 } will be investigated. From the results of the optical characterisation of CDs -/+ it is clear that the surface chemical groups forming the nanoparticles organic shell are deeply involved in their photoemission cycle; such a result is inferred both considering the differences arising from changes in pH conditions -and thus surface charge -and the discussion regarding the femtosecond transient absorption characterisation. This involvement of the external surface is crucial to the interaction, as demonstrated in the following sections.

Spontaneous formation of {P 2 W 18 }-CDs complexes is obtained simply by addition of {P 2 W 18 } to a solution of CDs. Concentrations used in the course of the experiments will be pointed out for each sample during the description of the followed procedures.

In order to simplify the discussion, each kind of carbon dots will be treated in separate sections. CDs + will be considered firstly, as the opposite charge these nanoparticle present compared to the polyanionic {P 2 W 18 } allows to obtain the most significant interaction.

Later, results obtained for CDs -will be analysed, in order to compare the two surface charge conditions and discuss in more details the nature of the interaction. Lastly, considerations on what observed when using N-CDs will be discussed before the conclusions of the chapter.

Interaction with CDs +

The interaction between CDs + and {P 2 W 18 } has been mainly probed by optical measurements. Initially, modification in absorbance have been observed by comparing the normalised spectra of carbon dots before and after the addition of {P 2 W 18 }. As observed . 227 This is due to the fact that probing the absorbance allows to observe transitions from the sample ground state, which must thus be influenced by external species being already in its proximity before the measurement is carried out. In the case of CDs, as their optical properties are highly reliant on surface states, presence of polyoxometalates seems to significantly influence the ground state responsible for the observed absorbance, thus pointing towards a static interaction or, equivalently, formation of a {P 2 W 18 }-CDs + complex.

As the redshift occurs at wavelengths longer than 400 nm, we can affirm with certainty that the observed differences in the spectra are not merely due to {P 2 W 18 } absorbing in that region. As readily inferred by comparison with the absorbance spectrum of {P 2 W 18 }, it clearly appears that this anion does not contribute in any way to the wavelength region in which the redshift takes place. Moreover, increases in {P 2 W 18 } concentration do not seem to directly translate into further displacement of the band. To understand why that is the case and to analyse eventual quenching effects, the photoluminescence spectra of The absorbance spectrum of CDs + undergoes a redshift upon addition of {P 2 W 18 } as indicated by the arrow (concentrations reported in legend); comparison with the absorbance of {P 2 W 18 } proves that the complex does not contribute on its own to the wavelengths where the CDs + band shifts; all spectra have been arbitrarily normalised. of polyoxometalates, so that already at a concentration of 1 µm only 50% of it can still be observed. To better visualise the undergoing process, data is presented in a Stern-Volmer plot in Figure 5.3. Thanks to this plot we can clearly observe the presence of two linear ranges in concentration, corresponding to two quenching regimes. The strongest of these, as evident by the high slope of the curve, occurs at {P 2 W 18 } concentrations lower than 10 µm;

further increases in concentration bring us into the second region, whose effects develop at concentrations above 10 µm and are clearly less intense. This qualitative observation can be used to explain why the redshift in absorbance does not seem to increase upon further addition of {P 2 W 18 }: as the strongest effect already occurs at low concentrations, increases in the added POM quantity can only slightly affect the system. Resulting additional shifts of the absorbance band are thus too weak to be easily observed.

In order to obtain quantitative informations regarding the magnitude of the {P 2 W 18 } quenching on CDs + , a least-square fitting procedure has been performed on the Stern-Volmer data. Details concerning the curve used as model function are presented in Section A.3.4. Resulting parameters, reported in Table 5.1, indicate the two quenching regimes which had been previously qualitatively identified. The strongest of these displays an accessibility factor f of 80%. As this parameter represent the fraction of fluorophores which is able to interact with the quencher, this result is signifying that the underlying mechanism is not able to completely quench the emission. As such, it is possible to affirm λ exc (nm)

K 1 (m -1 ) f (%) K 2 (m -1 ) 400 (24 ± 7) × 10 5 82.0 ± 1.4 (55.4 ± 1.0) × 10 2 450 (18 ± 8) × 10 5 80 ± 3 (28 ± 12) × 10 2
Table 5.1: Quenching parameters obtained from the least squares fit of the Stern-Volmer plots shown in Figure 5.3; parameters K i are the quenching coefficients while f represents the accessibility factor relative to the process described by

K 1 . R CDs (nm) R POM (nm) D CDs (m 2 s -1 ) D POM (m 2 s -1 ) K (m -1 )
13.6 ± 0.4 0.683 ± 0.012 (181 ± 5) × 10 -13 (360 ± 6) × 10 -12 271 ± 12

Table 5.2: Diffusion limited quenching constant (K) and the parameters used to obtain it relative to the interaction between {P 2 W 18 } and CDs + ; the interacting species radii (R) corresponds to the hydrodynamic radii obtained from the DLS size distribution; diffusion coefficients (D) have been calculated from the Einstein equation; diffusion limited quenching constant calculated from the Smoluchowski equation.

that the first regime is only able to cover part of the explored concentrations -below 10 µm -before yielding to the second process. Nevertheless, both quenching processes are very intense, as evinced by the obtained coefficients. In fact, these are always at least one order of magnitude bigger than the diffusion limited quenching constant, whose value and parameters used for its calculation are shown in Table 5.2.

These results thus indicate that the observed emission quenching can not be caused by dynamic encounters happening because of the Brownian motion of {P 2 W 18 } and CDs + in solution, but is instead due to a static complex which has formed in solution before photoexcitation. To confirm this interpretation, the CDs + photoluminescence has been studied via time-resolved emission spectroscopy in the presence of {P 2 W 18 }. The observed nanosecond decay traces at different POM concentrations, shown in Figure 5.4, appear to proceed in a parallel manner when plotted on a logarithmic y scale, thus implying a constant decay lifetime. Consequently, this data has been analysed by performing a global least-squares fitting procedure; once again, by using a bi-exponential decay as model, it was found that all traces are described by the same lifetime values, equal to those previously found for the emission of CDs + in absence of {P 2 W 18 } (Table 4.3). This lack of changes thus conclusively rules out the possibility of a collisional quenching, which would otherwise cause a decrease in the observed emission decay lifetimes proportional to the quencher concentration. Conclusively, this means that {P 2 W 18 } and CDs + are able in solution to spontaneously form electrostatic {P 2 W 18 }-CDs + complexes.

Additionally, we proceeded to observe these complexes via AFM; the obtained topographies are shown in Figure 5.5A and 5.5B. From the analysis of 200 isolated grains, the size distribution presented in Figure 5.5C was obtained. It appears that two different families contribute to the shown height histogram. As such, analysis was performed separately on grains above and below a 3nm threshold; statistics relative to these two subsets yield average heights of 7.6 nm (σ = 2.3 nm, 98 grains) and 1.0 nm (σ = 0.4 nm, 102 grains).

For a comparison, also the topography of structures formed by {P 2 W 18 } was observed via AFM. As shown in Figure 5.6A and 5.6B, most of the substrate is covered by a uniform plane with approximate thickness of 1 nm. Observation of zones either from this plane or the underlying substrate shows the presence of spherical grains. By collecting from Figure 5.6A the height of 1696 different objects from the lower Si-substrate level the size distribution of these formation is obtained (Figure 5.6C); as the analysis yields an average height (0.9 nm, σ = 0.5 nm) consistent with the size of the Wells-Dawson structure, it is concluded that the observed grains are nothing else than isolated polyoxometalates or, at most, aggregates of a few of them. In addition, also the 1 nm thick plane is assumed to be formed by polyoxometalates forming this bi-dimensional structure thanks to both intra-molecular interactions and binding with the silicon substrate.

When comparing the topographies shown in 5.3, we can now affirm that the smaller ensemble previously indicated is probably due to free POMs which have not formed any complex with CDs + ; these are also present as uniform planes covering Height (nm)

CDs + {P 2 W 18 }-CDs + {P 2 W 18 } Sample Average height ( nm) {P 2 W 18 } 0.9 ± 0.5 {P 2 W 18 }-CDs + 1.0 ± 0.4, 7.6 ± 2.3
CDs + 3.7 ± 2.0 part of the topographies shown in Figure 5.5 and 5.6. Moreover, we believe the larger distribution to be evidence of {P 2 W 18 }-CDs + complexes formed by electrostatic coupling of {P 2 W 18 } over the surface of CDs + ; these complexes thereby display a bigger diameter when compared to carbon dots, as observed from the measured height distributions.

Since we previously evinced that the interaction dynamics occur on a sub-nanosecond time-scale, we therefore proceeded to study the complexes via femtosecond transient absorption spectroscopy. Two different {P 2 W 18 }-CDs + samples were studied, respectively obtained at {P 2 W 18 } concentrations equal to 1.25 µm and 125 µm. These conditions where chosen so to perform the TA experiments on complexes corresponding to the two quenching regimes previously identified. As The shown spectra once again display the usual four signals previously identified in the absence of {P 2 W 18 }. Despite these similarities, a comparison with Figure 4.20B reveals some significant differences found in the GSB and SE regions. As shown in Figure 5.9, in fact, the dip associated to SE appears to be clearly less intense in the case of {P 2 W 18 }-CDs + at all delays from excitation; since the SE is equivalent to fluorescence, this findings is an additional evidence of the quenching of CDs + by {P 2 W 18 }. In addition, while in the absence of {P 2 W 18 } the GSB signal of CDs + significantly decays in the course of the experiment, for {P 2 W 18 }-CDs + complexes the same negative band just seem to undergo a very slight relaxation; as such, it appears that an electron transfer (ET) between CDs + and {P 2 W 18 } has occurred, thus explaining the less intense SE band and the lack of GSB decay. As the electrons responsible for the GSB have been transferred to the POM, the ground state can not be repopulated so that the GSB band kinetic is now lacking any relaxation.

The TA signal of both samples have been analysed by fitting the relative eigentraces;

as already explained, this procedure allows to analyse the decaying components present as visible, the SE dip is less pronounced at all excitation delays; in addition, the GSB signal is able to decay for sample CDs + while seems to persists in the case of {P 2 W 18 }-CDs + complexes. in the TA data and find the time constants associated to their kinetics. The resulting curves fitting the TA traces and the reconstructed spectra are respectively compared to the original data in Figure 5.10A and 5.10B. As previously mentioned, the global fitting procedure yielding these results was performed taking into account data relative to samples both in the absence and presence of {P 2 W 18 }. By including the different datasets altogether, we can conclude that the obtained lifetimes τ i (Table 4.4) are able to represent the kinetics of both CDs + and {P 2 W 18 }-CDs + complexes; in other words, this translates to no new dynamics being present when observing the time-resolved TA of the complexes.

As no dynamics related to the ET from CDs + to {P 2 W 18 } were found in the complexes data, we can conclude that the electron transfer responsible for the quenching must have occurred faster than 110 fs, time relative to the shortest obtained τ .

When observing the DAS relative to {P 2 W 18 }-CDs + , shown in all evidences of quenching have to be found here. In fact, as shown in Figure 5.11B, two notable features are now indicative of the quenching process: firstly we can observe how the GSB band in the presence of {P 2 W 18 } has not decayed, sign of an electron-depleted system as it would be the case for an electron transfer from CDs + to POMs; moreover, the SE signal gets less intense as the {P 2 W 18 } concentration increases, once again pointing towards an emission quenching caused by the ET occurring between the complex components.

When considering the two separated quenching regimes observed in the Stern-Volmer plot (Figure 5.3), it would be expected to observe some differences in the TA experiments performed at low and high {P 2 W 18 } concentrations. The results herein presented, nonetheless, do not point towards an additional quenching mechanism. Moreover, data obtained at 125 µm concentration, due to high scattering, was found to be very noisy. This increase in scattering was caused by nanoparticles precipitating in the experimental setup while the measurement was carried out. These two evidences thus indicate that only a single quenching mechanism occurs, while the second regime observed in the steady state photoluminescence measurements is caused by the nanoparticles precipitating out of solution. At high {P 2 W 18 } concentrations, in fact, the conditions could ideally allow the formation of {P 2 W 18 }-CDs aggregates, similar to the big grains observed via AFM (Figure 5.5A, bottom-left).

In conclusion, the fact that TA data of both CDs + and {P 2 W 18 }-CDs + complexes can be described by the same decay lifetimes evidences how the electron transfer from the excited carbon dots surface towards {P 2 W 18 } takes place through an extremely efficient process occurring in a time span shorter that 110 ps. This result is corroborated by the fact that no new dynamics occur in the system when {P 2 W 18 } is present, as inferred when observing that all transient absorption data can be described by decay associated spectra displaying the same shape and lifetimes; in addition, when observing the longest lived DAS, which represent the situation observed in the steady-state and nanosecond time-resolved experiments, evidences of the happened electron transfer are clearly displayed in the form of a GSB signal not decaying and a SE band less intense in the presence of {P 2 W 18 }.

Interaction with CDs -

The possibility to form complexes composed of anionic {P 2 W 18 } and CDs was also studied when using negatively charged nanoparticles. Despite the two species having the same charge, we tried to investigate whether any interaction was still possible. Once again, the effects of {P 2 W 18 } on CDs -emission were probed by observing the nanoparticles photoluminescence while adding POMs in solution. The obtained emission spectra at different {P 2 W 18 } concentrations are shown in Figure 5.12. While the emission intensity was successfully quenched by {P 2 W 18 } presence, the magnitude of the effect appears smaller compared to what observed when using CDs + , as deduced by comparing these results with those shown in Figure 5.2.

Once again, quantitative analysis was performed by performing a least-squares fitting procedure on the Stern-Volmer plot of the intensity data. As shown in Figure 5.13, the resulting curves are composed by two regimes separated in concentration. Obtained quenching parameters are reported in Table 5.4; these results indicate how the quenching is up to six times less efficient for CDs -compared to what observed for CDs + , thus evidencing the very important role of surface charge in aiding the interaction between {P 2 W 18 } and carbon dots.

Even if the obtained quenching parameters result once again at least one order of magnitude bigger than the diffusion limited coefficient -whose value is indicated in Table 5.2 -the emission kinetics where studied via time-resolved spectroscopy in order λ exc (nm) or not and are therefore equal to those shown in Table 4.4. As previously explained when discussing CDs + , we believe this to be evidence of an extremely efficient interaction between the carbon nanoparticles and the polyoxometalates; in fact, it is indicative of an interaction which must have occurred with a very short sub-picosecond lifetime.

K 1 (m -1 ) f (%) K 2 (m -
The calculated decay associated spectra of {P emission observed in steady-state and nanosecond time-resolved optical experiments, has decreased as a consequence of {P 2 W 18 } presence. Despite the interaction being hindered by the surface charge of CDs -, complexes that are still able to come together result therefore able to display the same properties previously observed when using positive carbon dots. This result seems to indicate that while the charge-driven electrostatic interaction is a significant force aiding the formation of POM-CDs complexes, it is by no means necessary for it to happen. It is instead possible that the static forces keeping the two compounds together stem from other interactions, such as formation of H-bridges between the oxo groups present in the {P 2 W 18 } structure and the hydrogen-rich moieties composing the CDs surface. Even though the efficiency of complexes formation results significantly smaller, as deduced by comparing the steady-state emission quenching results for the two different CDs -/+ samples, the underlying mechanism allowing for the electron transfer from CDs to POM does not differ. Because of the lack of any new dynamic appearing in the TA data, the {P 2 W 18 }-CDs -interaction must occur faster than 170 fs, as this is the lifetime τ 1 associated to the shortest-lived DAS. Nevertheless, in order to clearly identify the source of the POM-CDs interaction further studies would be necessary.

Finally, the DAS relative to {P 2 W 18 }-CDs -complexes obtained at a {P 2 W 18 } concentration equal to 125 µm, shown in Figure 5.18, will be here discussed. Exactly as previously explained, these spectra have also been calculated from the results of the global least-squares fitting procedure performed on the TA matrix eigentraces of all samples.

While this assures that the high-concentration {P 2 W 18 }-CDs -complexes follow the same dynamics of the sample obtained in the absence of {P 2 W 18 }, all of the shown DAS present some striking features previously not observed. In fact, it can be noted how all of the decaying spectra present two ESA bands. Their presence in DAS fully decaying during the TA experiment has been associated to non-radiative transitions from the carbon The evidence that at high {P 2 W 18 } concentration these transient absorption signals rapidly decays seems to consequently indicate how, in the case of CDs -, {P 2 W 18 } presence is able to favour non-radiative decays back to the ground state. This eventuality thus points towards an incompatibility between CDs -and polyoxometalates. Previously we remarked how it is possible to form an interacting complex even though both species present the same charge: it now appears that in order to photosensitise {P 2 W 18 } with CDs -so to improve their properties a careful control of concentration is needed. This is a clear setback for an eventual applicative transfer because of which further studies performed by using CDs -have been abandoned in favour of CDs + .

Interaction with N-CDs

The interaction between polyoxometalates and CDs was finally also studied when using N-CDs. As these nanoparticles display a negative surface charge (Figure 4.27), a situation similar to that observed for CDs -could thus be expected. In order to observe the eventual interaction we therefore proceeded to study the steady-state emission quenching by measuring the photoluminescence spectra of N-CDs in the presence of {P 2 W 18 }. Obtained results, presented in Figure 5.19, indicate that a degree of quenching can, in fact, be achieved.

Nonetheless this effect is very weak, as N-CDs emission was found to be 50% less intense at a {P 2 W 18 } concentration equal to 2.5 mm. In comparison, when using CDs -the same result could be achieved for concentrations approximately equal to 120 µm, which is around Once again, the quenching coefficient was obtained by performing a least-squares fitting procedure. In this case only a single regime could be found, characterised by the parameters reported in Table 5.5. The extremely low value obtained for the quenching parameter K conclusively indicates the interaction between {P 2 W 18 } and N-CDs to be very inefficient.

While it is possible that quenching was in this case due to a dynamic interaction, no K (m -1 ) f (%)

(20 ± 3) × 10 2 54 ± 2
Table 5.5: Quenching parameters obtained from the least squares fit of the Stern-Volmer plots shown in Figure 5.20; parameter K is the quenching coefficient while f represents the accessibility factor relative to the process.

further analysis was performed and N-CDs were considered unable to form complexes with polyoxometalates. Interestingly, even if CDs -were found able to form {P 2 W 18 }-CDs - complexes despite their negative surface charge, the same can not be said for this other type of CDs. While the cause of this could be linked to the different moieties composing the surface of these two kinds of nanoparticles, no further studies aimed at unveiling this question were performed.

Conclusions

In order to group together the results presented in this chapter, these will be recalled in this last section. The main result which has been witnessed concerns the extremely powerful interaction occurring between CDs + and {P 2 W 18 } in solution. What has been observed through several different experimental techniques has allowed us to conclude how these two species are able to spontaneously form {P 2 W 18 }-CDs + complexes which then proceed to interact through a photoinduced electron transfer mechanism. Surprisingly, this was found to proceed with an ultrafast sub-picosecond kinetic; such a result is extremely promising when considering an applicative transfer towards photocatalytic applications.

In addition, the interaction has been also studied for two other different type of CDs, namely CDs -and N-CDs. Both of these are characterised by the negatively charged surface when in solution, albeit distinct results have been obtained for each of these.

In fact, only CDs -have been also shown able to interact with {P 2 W 18 }; once again, the optical characterisation has unveiled an interplay between the two species, albeit the same efficiencies displayed by CDs + could not be achieved. In particular, we were able to witness what we concluded to be the same interaction mechanism also for negative dots. We could thus affirm that this specie is consequently able to form {P 2 W 18 }-CDs -complexes and, once again, their components are capable of interacting through an ultrafast photoinduced electron transfer.

More importantly, further informations on the kind of interaction allowing for the {P 2 W 18 }-CDs formation were unveiled; by noticing how the resulting electron transfer mechanism did not change between CDs + and CDs -, it was possible to conclude that the same mechanism must be involved in the formation of the two different complexes.

As such, we were able to conclude that electrostatic interactions are just driving forces able to bring together {P 2 W 18 } and CDs + while hindering the same process for CDs -.

Nonetheless, CDs -/+ encountering {P 2 W 18 } through diffusion are in both cases able to form hybrid complexes which interact once again through a PET mechanism.

While we had hypothesised the formation of H-bridges between the oxo groups present in the polyoxometalates structure and hydrogen atoms found in the organic moieties of CDs, no conclusive evidence on the kind of interaction keeping together the static complexes was found. In addition, when studying N-CDs no interaction could be proven;

this could either be due to a stronger electrostatic repulsion between these carbon dots and {P 2 W 18 } or the lack of the necessary organic groups on the dots surface involved in the interactions allowing for complexes formation.

As always, further studies are necessary to conclusively unveil the still open questions regarding the studied interaction; nonetheless, the previous results have proven the efficiency of {P 2 W 18 }-CDs + complexes. These are able to interact through a mechanism happening in less than 110 fs, an extremely interesting process because of the speed at which it occurs.

Such a result, albeit surprising, is not completely unheard of in the literature and is known to happen for systems different than POMs and CDs. 280 In addition, literature studies on the interaction of photosensitised POMs have already shown a shortening of the TA signals lifetimes for the photosensitiser, when the latter is coupled to polyoxometalates. 281

Because of these conclusions, in the next chapters CDs + will be the only nanoparticles utilised for the study of interaction with other POMs.

Chapter 6

Interaction with Wells-Dawson

Sandwich-Type POMs

As polyoxometalate {P 2 W 18 } can be considered a model for heteropolyanions, the initial results provided in the previous chapter point towards an efficient interaction between CDs and polyoxometalates. Despite being an archetype structure, there are other POMs presenting more interesting properties, often regarded as very appealing for an applicative transfer. When discussing the state of the art, in particular, the importance of TMSPs was remarked for their possibilities as catalysts. It thus appears clear how photosensitising such kind of structures with carbon dots could result in interesting photocatalytic hybrid devices, if the electron transfer from CDs towards TMSPs was to be proven possible.

In the present chapter results on the interaction between CDs + and two different POMs structures will be presented. Positively charged carbon dots have been selected

following the results discussed in last chapter, which have proven them more reliable for {P 2 W 18 }-CDs interaction than either CDs -or N-CDs. The two TMSPs used throughout these studies have been previously presented in Section 4.1.2 and are {Zn 4 P 4 W 30 } and {Co 4 P 4 W 30 }. Apart from proving how the photointeraction between the two species is possible, we are set to discuss any difference that might arise when compared to the {P 2 W 18 }-CDs + complexes, in order to further elucidate our understanding of that system and of POM-CDs complexes in general.

Moreover, a study of cobalt containing POMs is especially interesting. In fact, many example of such structures have been reported in the literature for their ability to catalyse the water oxidation reaction. Once again being able to obtain stable complexes composed of CDs and Co-based TMSPs would prove extremely crucial, especially considering the importance of performing photocatalytic water splitting for renewable energy purposes.

In the following sections, results relative to {Zn 4 P 4 W 30 } will be presented first. Afterwards, our attention will instead be shifted towards {Co 4 P 4 W 30 } anions. The absorbance spectrum of CDs + undergoes a redshift upon addition of {Zn 4 P 4 W 30 } as indicated by the arrow (concentrations reported in legend); comparison with the absorbance of {Zn 4 P 4 W 30 } proves that the complex does not contribute on its own to the wavelengths where the CDs + band shifts; all spectra have been arbitrarily normalised. 6.1: Stern-Volmer plots of CDs + emission intensity quenched by {Zn 4 P 4 W 30 } (λ exc = 400 nm); the curve best representing the data, also shown in the plot with the corresponding 1-σ confidence interval, has been obtained via a least-squares fitting procedure; points indicated in red have been excluded from the fitting procedure, as relative to an increase in emission intensity instead of a decrease; resulting quenching parameters are reported in the table, with K being the quenching coefficient while f representing the accessibility factor relative to the process.

this value, interestingly, the reverse effect is instead observed: consequent increases in concentration seem to gradually restore the photoluminescence band intensity.

These results can be visualised in a Stern-Volmer plot; as shown in Figure 6. of the Stern-Volmer plot, while unexpected an increase in fluorescence intensity is not inexplicable. To understand why this is the case, we need to consider results discussed in the literature.

As pointed out in Section 2.2.3, the sensitivity of CDs towards the external environment often results in solvatochromic effects. In particular, it has been pointed out 156 how the dielectric and H-bonding interactions of carbon nanoparticles with water molecules usually results in a reduction of their quantum yields. Moreover, a study 157 has been able to show how the interaction of CDs with ions in aqueous solution can screen the nanoparticles from the solvent-related effect, thus causing an increase in the emission intensity of carbon dots.

We believe that what herein observed is a manifestation of this same effect: while at low concentrations interaction between {Zn 4 P 4 W 30 } and CDs + results in a quenching of fluorescence, above a threshold further coupling between the two species occurs in a way which does not cause the electron transfer responsible for the quenching. If comparing these results with what witnessed when using {P 2 W 18 }, also in that case at high concentrations no ET was considered to be happening: the observed decrease in emission intensity was instead associated to aggregation and precipitation of the complexes, as pointed out in Section 5.1. In the case of {Zn 4 P 4 W 30 } we instead believe that no precipitation occurs and further coupling with these anionic specie has instead a screening effect from the solvent interactions, which would otherwise reduce the nanoparticles QY.

Finally, it could seem counter-intuitive that the smaller {P with those of the CDs + DAS but some differences are present nonetheless. Firstly, the shortest-lived spectrum now displays a high noise; we believe this due to scattering from the excitation pump, still relevant at the very short delay of 110 fs. In addition, this signal does not display any ESA band peaking around 460 nm, present instead for CDs + , while the same is found to be more intense in the DAS decaying in 20 ps. Arguably, these differences in the ESA are due to the influence of {Zn 4 P 4 W 30 } on the surface of CDs + following their coupling, as this band has been associated to non-radiative transitions from excited states of the carbon dots surface.

Nevertheless, we can once again affirm that the electron transfer from CDs + to {Zn 4 P 4 W 30 } has occurred through an extremely efficient mechanism taking place in less than 110 fs, lifetime associated to the shortest-lived DAS. We are able to reach this conclusion by once again observing how no new decay kinetics are introduced in the TA signal of CDs + when {Zn 4 P 4 W 30 } are present. As assured by the meaningfulness of the global least-squares fitting procedure, it is consequently possible to represent the evolution of the transient absorption of {Zn 4 P 4 W 30 }-CDs + complexes with the same decay lifetimes also displayed by free CDs + . Because of this, any of the observed effects must have then taken place on a time-scale shorter than those which have been observed via the ultrafast TA experiments.

In fact, effects of the occurred quenching are evidently present in the longest-lived DAS: this signal, once again representing the observables measured during the nanosecond time-resolved and steady state optical characterisation, has undoubtedly been affected by an electron transfer from CDs + to {Zn 4 P 4 W 30 }. As shown in Figure 6.7B, said effects can be readily spotted when comparing the ns-lived DAS of free CDs + and {Zn 4 P 4 W 30 }-CDs + complexes. This plot, including the spectrum of samples obtained at both concentrations, displays how the SE signal continuously decreases when CDs + are involved in the coupling.

Additionally, the GSB band is in the case of {Zn 4 P 4 W 30 }-CDs + complexes still very intense, while previously it was observed to significantly decay.

These two results are consistent with an occurred ET: the ground state of CDs + , normally replenished by ultrafast non-radiative decays, due to the transfer still appears depleted when {Zn 4 P 4 W 30 } are present, as evidenced by the intense GSB found in the long-lived DAS. Moreover, loss of the transferred electrons is also evidenced by the lack of SE band in the signals, as the emission otherwise present has been quenched due to the photoinduced electron transfer process.

In conclusion, {Zn 4 P 4 W 30 }-CDs + complexes seem to behave very similarly to {P 2 W 18 }-CDs + ones. As both hybrid devices have been able to show an extremely strong interaction resulting in fast and highly efficient electron transfer, coupling between POMs and CDs appears confirms itself as very promising. Additionally, evidences of the electron transfer are in the case of {Zn 4 P 4 W 30 }-CDs + even more intense than those observed when using {P 2 W 18 }. This consideration appears to point at the importance of TMSPs in obtaining an effective coupling with CDs + , remarking how structural characteristics of POMs have significant effects on their properties and can be opportunely exploited if chosen carefully. Initially, complexation between the two species was probed by observing how the absorbance bands of CDs + were affected by Co TMSPs. As shown in Figure 6.8, the spectrum of {Co 4 P 4 W 30 } absorbs up to approximately 450 nm, differently than other POMs. As such, differentiating its contributions from changes inherent to the absorbance of CDs + can be, in this case, more difficult. This situation is evident when observing the results obtained at a 12.5 µm concentration, when a very intense band covers almost completely the original CDs + absorbance peaks, hiding their exact position. Nonetheless, the signal measured at the lower {Co 4 P 4 W 30 } concentration of 1.25 µm presents a situation very similar to that observed for both {P 2 W 18 } and {Zn 4 P 4 W 30 }: in fact, the CDs + band seems to undergo a redshift of approximately 10 nm. We believe this to be once again evidence of the formation of complexes between {Co 4 P 4 W 30 } and CDs + , as only such occurrence could explain modifications of the CDs ground state as probed by the measured absorbance.

Following these results, we proceeded to measure how the fluorescence of CDs + responded to {Co 4 P 4 W 30 } additions in solution. By measuring photoluminescence spectra at increasing POM concentrations it was possible to observe the CDs emission being progressively quenched, as shown in Figure 6.9. Moreover, the effect appears to be very To understand the underlying causes of quenching, we proceeded to measure the ultrafast transient absorption of two CDs + samples in which {Co 4 P 4 W 30 } at a concentration of 1.25 µm and 125 µm was added. Spectra at different delays from excitation obtained for the least concentrated {Co 4 P 4 W 30 }-CDs + solution is shown in Figure 6.12. Being used to observe the SE band of CDs + disappearing upon POM addition, this sample presents a surprising features. In fact, when comparing its TA spectra with data obtained for free CDs + (Figure 4.20B) it is possible to notice how the stimulated emission signal is very intense at all delays from excitation, counter-intuitively when expecting an electron transfer. Nonetheless, the GSB band of {Co 4 P 4 W 30 }-CDs + still behaves as expected, since it is still present at long delays from excitation differently than what observed for CDs + : for the latter sample, in fact, it decays during the course of the experiment. The other ESA peaks, finally, do not present significant differences except for the one found at 460 nm which appears to be less intense than before.

In order to better understand the TA data, results from the global least-squares fitting of the eigentraces, which has been performed in a concerted manner on all different POM-CDs + samples, were analysed. A comparison between the reconstructed TA matrix and the originally obtained data is shown in Figure 6 While this feature has been previously observed for other complexes, the same can not be said for the behaviour of the SE band. This seems in fact to slightly increase at a The observed back electron transfer consequently seems to be caused by {Co 4 P 4 W 30 } which have been reduced by the ultrafast ET from CDs + . At the low concentration of 1.25 µm, it appears that the transfer back to the nearby CDs + is a favoured process.

Instead, when enough {Co 4 P 4 W 30 } are present -for instance at 125 µm -these reduced polyoxometalates can transfer the electron to other non-reduced POMs, so that the CDs + SE is not regenerated. It thus seem that when other electron acceptors are present, these Co-containing TMSPs coupled to CDs + can still prove very effective for photocatalytic applications: carbon nanoparticles are indeed proven being able to provide POMs with the harvested photoexcitation, which can then be used to reduce other species.

Conclusions

Through the results presented in the course of this chapter, it was found how the interaction betweep CDs + and POMs was not solely circumscribed to the case of {P 2 W 18 }. On the contrary, the same mechanism could be unveiled for both {Zn 4 P 4 W 30 } and {Co 4 P 4 W 30 }; additionally, the interplay between CDs + and these TMSPs was observed to be even more efficient than before, especially in the case of the Zn-containing POMs. Nonetheless, further mechanisms were observed taking place when considering {Co 4 P 4 W 30 }, evidence which complicated the discussion of the results. Despite these differences, the fact that the ultrafast transient absorption always presents the same relaxation kinetics observed for free CDs + allowed us to affirm that a subpicosecond electron transfer happens as a result of the photoinduced interaction occurring within POM-CDs + complexes. This conclusion was made possible by observing how the intensities of the GSB and SE signals decreased when any of the POMs are present in solution, evidence further corroborated through the analysis of the TA decay associated spectra.

Such a result is extremely positive when considering an eventual transfer towards applications, especially in the domain of photocatalytic reductive reactions. As a consequence, in the next chapter this eventuality will be further considered and explored, trying to confirm how all POM-CDs + are photoactive and how complexes formed with {Zn 4 P 4 W 30 } and {Co 4 P 4 W 30 } are expected to behave more efficiently than when {P 2 W 18 } is instead present.

Chapter 7 Silver Photocatalysis

Because of the promising results presented in the previous chapters, we proceeded to test whether the ultrafast electron transfer between POMs and CDs could be exploited for photocatalytic applications. In order to do so, the light assisted silver nanoparticles formation mediated by POM-CDs + complexes was studied. This approach was chosen as test-bench for our complexes, based on literature reports of photosensitised polyoxometalates able to aid the reduction of Ag + ions and consequent formation of silver clusters under irradiation. 94 Initially, the process was followed by observing the silver plasmon band absorption growing as a function of irradiation time. Absorbance of a solution placed under visible light was regularly measured on a spectrophotometer. In order to only use the visible part of the light spectrum, so to prove how the complexes are effective for solar driven photocatalytic applications, irradiation was carried out using a Xe-lamp as light source; this was equipped with NIR and GG400 filters which allow to cut-off both IR and UV radiation.

As CDs + resulted the most reliable, only these carbon dots were used throughout the experiments. Complexes formed with either {P 2 W 18 }, {Zn 4 P 4 W 30 } or {Co 4 P 4 W 30 } were studied, as based on the previous results all were proven able to interact with the carbon-based nanoparticles. Experiments were performed on aqueous POM-CDs + solutions obtained at a POM concentration equal to 10 µm; the only exception was in the case of {P 2 W 18 }, for which experiments have been repeated after doubling its concentration, indicated by the label 2×{P 2 W 18 }. This was done in order to compare results to the sandwich-type polyoxometalates, as their structure is composed by two trivacant {P 2 W 15 } lacunary subunits embedding four metal atoms. In addition, Ag 2 SO 4 at a concentration of 40 µm was present in solution so to provide the Ag + ions to be reduced. Finally, propan-2-ol (IPA) was used as sacrificial electron donor (concentration equal to 1% by volume). All samples solutions were placed in a long-necked quartz cuvette of 1 cm optical path for the experiments; all samples were de-oxygenated by argon fluxing for half an hour.

In a second moment, presence of Ag nanoparticles was confirmed by TEM and electron diffraction (ED) measurements, in order to study their shape, size, and structure. In the following sections all results will be presented and discussed.

Growth of the Plasmon Band

Results obtained when using {P 2 W 18 }-CDs + compounds to catalysed the formation of Ag nanoparticles are shown in Figure 7.1A. From the measured absorbance it appears clear how the silver plasmon band grows with irradiation time; the band intensity was estimated from its maximum, found at around 460 nm despite slightly shifting with irradiation time.

After 3 h 30 min the growth did not seem to proceed any further, indicating the end of the reaction due to the totality of Ag + ions being finally reduced. The observed band appears being broad and asymmetrically elongated towards the low-energy side of the spectrum. This evidence seems to indicate a large size distribution displayed by the formed nanoparticles. In addition, control experiments were performed either in absence of CDs + or {P 2 W 18 }; in both cases, the plasmon band could hardly be observed, confirming the interplay between the complexes components. The growth kinetics for all experiments are shown in Figure 7.1B. These have been obtained as difference between the absorbance measured at the indicated irradiation time and before irradiation was carried out, in order to exclude contribution from CDs + absorbing in the same region. Indicated points are taken at the wavelength of the plasmon band maximum. As observed, control experiments present a very small growth of the band.

Moreover, when the concentration of {P 2 W 18 } is doubled the photoreduction reaction appears to proceed faster. Nonetheless, this is not surprising as in these conditions more {P 2 W 18 }-CDs + complexes are expected to form in solution and thus aid the light-driven process.

Subsequently, the experiment was repeated using {Zn 4 P 4 W 30 }-CDs + . Resulting measured absorbance is shown in Figure 7.2A. Once again the growth of the Ag plasmon band can be observed proceeding rapidly during irradiation, reaching its maximum at the 3 h 30 min time-mark. Despite these similarities, some differences with the previous case can be found; in particular, the band peak is in this case found at higher energies than before, at 430 nm, and its shape is overall more intense and sharper. Such an evidence seems to indicate how {Zn 4 P 4 W 30 }-CDs + complexes allow to better control the growth of the Ag nanoparticles, which present a smaller and less disperse size as based on the plasmon absorbance shape. Also in this case, control experiments were performed on solutions not containing CDs + , but in this case no band growth could be observed. A comparison of the kinetics for the silver nanoparticles photocatalytic formation is shown for {Zn 4 P 4 W 30 }-CDs + and the control experiments in Figure 7.2B. From the plot it appears clear how these complexes Once again, the process is concluded for an irradiation time of 3 h 30 min, after which the plasmon band maximum does not increases in intensity. Investigating the band shape is in this case more complicated than previously, as {Co 4 P 4 W 30 } contribute to the absorbance in the visible part of the light spectrum where the plasmon resonance occurs. Nevertheless, it is possible to observe how the peak seems to be in this case found at even shorter wavelengths than before as it is placed at 425 nm.

The comparison with the control experiments, performed also for this polyoxometalate complex, are shown as growth kinetics in Figure 7.3B. From these, we can once again conclude how the interaction between POMs and CDs + is crucial for the photocatalytic process we have been observing, as using each of the single components hardly shows the Ag plasmon band growth.

As a last control experiment, we investigated whether it was possible for the catalytic process to occur in absence of light. To do so, solutions containing the POM-CDs + complexes, IPA, and Ag 2 SO 4 were prepared at the same concentrations previously indicated.

Afterwards, the absorbance of each solution was measured, before placing them in the dark.

After 24 h, samples were retrieved and their absorbance measured once again to check for the plasmon band presence. Obtained spectra are shown in Figure 7 at the end of the experiment. In addition, doubling the concentration of {P 2 W 18 } does not allow to obtain complexes matching the photocatalytic abilities of the sandwich-type POMs.

As previously discussed for the ultrafast transient absorption results, CDs + presented an electon transfer considered even stronger when coupled with these polyoxometalates than when coupled to {P 2 W 18 }. Consequently, the differences herein observed for the photocatalytic reduction of Ag + ions appear consistent with the previous results: as observed from the slope of the kinetics at initial irradiation times, TMSPs have the fastest catalytic rates.

Finally, a comparison of the plasmon band obtained with each distinct sample was also performed, so to evidence the differences regarding peak position and shape previously pointed out. The resulting plot is presented in Figure 7 In order to observe single particles and confirm their structure, high resolution TEM images were obtained. Due to instrument availability, only particles formed when using {P 2 W 18 }-CDs complexes could be analysed in this fashion. Results for two nanoparticles of different sizes are shown in Figure 7.8A and 7.8C; while in the former panel it is possible to observe an approximately spherical nanoparticle of diameter equal to 30 nm, the particle shown in the latter presents a bean-like shape and a diameter of approximately 10 nm. In addition, planes visible in this last image seem to present structural stress.

In order to better visualise the planar distances displayed by the two nanoparticles, the FFT of each TEM image was calculated. Resulting peaks are shown in Figure 7.8B and 7.8D respectively for the big and small particle. While the former of the two displays peaks associated to a planar distance of 2.4 Å and 1.4 Å, the latter presents instead peaks corresponding to a distance of 2.0 Å. These last peaks appear to be skewed in the FFT, as expected due to the structural stress evidenced in the corresponding TEM image. All three of the found values correspond to planar distances of the cubic structure of silver (ref. 98-060-4635), thus confirming that the nanoparticles formed thanks to POM-CDs + complexes are indeed silver ones. Indexes relative to each of the planar distances are indicated in the FFT figure.

As the shown analysis is relative to single nanoparticles, in order to achieve results related to the whole sample we proceeded to obtain electron diffraction patterns.

Proposed Mechanism

Considering the previously reported results on the interaction between POMs and CDs + , an ultrafast electron transfer is believed to occur from the photoexcited carbon dots towards the polyoxometalate anions. Such a process would thus lead to the oxidation of the nanoparticles and the reduction of the heteroanionic cluster. We expect a successive process to occur, leading to the regeneration of CDs + through the scavenging of an electron from IPA which would therefore generate the alcohol radical (CH 3 ) 2 C • OH. While the redox potential for the (CH 3 ) 2 C • OH/(CH 3 ) 2 CHOH couple is usually reported close to 0.8 Vvs NHE, 94 ,283 we could not measure the reduction potential of these carbon dots.

Nevertheless, we believe this mechanism to be possible as in the literature electrochemical studies on different CDs structures have observed potentials higher than 1 V vs NHE. 212 Nonetheless, understanding the POM component role results more difficult. In fact, the redox potential of the Ag + /Ag 0 couple is equal to -1.8 V vs NHE as measured by γ-radiolysis: 284 this value is mostly due to contributions from the solvation energy of Ag + ions, as for bulk silver we would otherwise have Ag + /Ag 0 bulk = 0.8 V vs NHE. As such, it results too low to justify direct reduction of Ag + ions by any of the used heteropolyanions. This is due to the fact that the redox potential of {P 2 W 18 }, depending on its reduction state, is at most equal to -0.816 V vs NHE; 285 the same happens for {Zn 4 P 4 W 30 } and {Co 4 P 4 W 30 }, whose reduction potential go as much down as -0.504 V and -0.544 V vs NHE respectively. 53 In practice, none of the used species, whatever their reduction degree, could directly reduce Ag + to Ag 0 as the reaction thermodynamics is not favourable for such a process. While this consideration is in line with the evidence that polyoxometalates not forming complexes with CDs + are not able to catalyse the formation of silver nanoparticles, to understand POMs catalytic role in this process we have to consider effects due to IPA. Nonetheless, we can now understand the role of POMs in the silver reduction process.

Studies on Ag

Up to this point we reached the conclusion that the initial Ag + n cluster is formed thanks to (CH 3 ) 2 C • OH radicals; these have been generated by CDs + , scavenging electrons as a consequence of the photoreduction of POMs. But as the Ag + n cluster increases its nuclearity n the redox potential of the Ag + n /Ag n +1 couple becomes more positive, passing from -1.8 V for n = 1 up to 0.8 V for n → ∞. 288 Consequently, for a certain nuclearity value, the direct reduction of Ag + n by the photoreduced POMs has to become thermodynamically favourable, thus explaining the photocatalytic performances of POM-CDs + complexes.

With this discussion, we believe that the proposed mechanism, reported as a simplified scheme in Figure 7.10, is able to explain the results observed in this last chapter.

Conclusions

With the results presented in this chapter we have managed to show how POM-CDs + complexes are versatile hybrid devices capable of an applicative transfer. Through the photo-reduction of silver ions we have been able to demonstrate how CDs + can act as electron relays to enhance the catalytic performances of polyoxometalates. This kind of evidence shows we have been successful in photosensitising POMs via CDs, result previously considered the aim of this work of thesis. Despite the simple example provided in this last chapter, being able to photocatalyse reductive reactions leaves us very optimistic when considering the wide array of possibilities in reach for the devices we have developed. In fact, all experiments have been conducted only using visible light: in this conditions it was possible to ensure how {P 2 W 18 }-CDs complexes can be used for solar driven photocatalytic processes, ideal outcome when considering the necessity of renewable energies.

The test-bench experiment which has been described clearly indicates the effectiveness of the developed hybrid devices. As such, the electron transfer from photoexcited CDs + towards POMs was proven effective for the reduction of Ag + ions. In addition, the efficiency of sandwich-type polyoxometalates discussed in Section 6.3 was here confirmed, as faster reaction rates and better control on the silver nanoparticles formation was observed. These results were obtained both indirectly, through the analysis of the plasmon bands shape, and directly, as TEM images displayed differently sized silver nanoparticles. Additionally, a mechanism explaining the observed process was proposed based on literary results related to similar systems.

Chapter 8 General Conclusion

Through the experimental results obtained during the doctoral research and presented in the present work of thesis, we have managed to show how polyoxometalates are able to spontaneously form static complexes with carbon dots.

Being able to obtain these results, nonetheless, can not be considered straightforward; in fact, several steps aimed at carefully engineering the final nanodevice and each of its components have been proven necessary for reaching the prefixed goal. As shown in Chapter 4, not many carbon nanoparticles posses surface characteristics displaying a positive charge when dispersed in solution. Only by focusing on literary reports and through experimental attempts it was possible to find the type of CDs best suited for our purposes. Nonetheless, comparing different carbon dots structures has allowed us to shine light on the mechanism underlying the interaction of these compounds with the catalytic anions we have been focusing on: as shown in the course of Chapter 5, surface charge has proven a driving force able to boost the {P 2 W 18 }-CDs complex formation. While this has not been considered absolutely necessary, it was nevertheless the best tool to ensure a proper interaction between the two components.

When observing the effects resulting from the interaction between different POMs structure and CDs + we were able to witness an extremely efficient process occurring within the complexes. Using spectroscopic tools such as femtosecond transient absorption, we unveiled how the electron transfer responsible for the quenching of CDs + luminescence occurred through a sub-picosecond ultrafast kinetic. Such a result, presented in the course of Chapter 5 and 6, was considered extremely promising for an eventual applicative transfer; moreover, we were able to show how all of the different polyoxometalate structures that

Part III

Appendix and References

were acquired with a 3 nm slit and a detector response time of 0.2 s and subsequently treated by subtracting the absorption spectrum of the solvent. Following this procedure, an additional constant baseline, whose value was equal to the minimum of the spectrum, was subtracted in order to minimise scattering contributions.

Steady-state photoluminescence data were obtained using two different instruments.

All of the quenching-related measurements were performed at 25 °C on an Aminco-Bowman Series 2 spectrofluorometer equipped with an external thermostated waterbath for circulation. All other measurements were instead carried out on a JASCO FP6500 spectrofluorometer. In both cases quartz cuvettes of 1 cm optical path were used and, in order to measure CDs emission, the solutions were diluted so to obtain an absorbance approximately equal to 0.1 at the excitation wavelength so to avoid inner filter effects (concentration equal to 2.5×10 -2 g L -1 ). All spectra were obtained using emission and excitation bandwidths both equal to 5 nm and a 1 s response time.

All obtained spectra were treated by subtracting a baseline equal to the minimum value of each spectrum in order to minimise stray light contributions.

All data analysis was performed through self-written Python scripts using packages NumPy and SciPy; least-squares fitting procedure were performed using package lmfit while plots were drawn using Matplotlib.

A.1 Quantum Yield Calculation

Reported quantum yields were obtained using a sample of known QY as reference, which is fluorescein in H 2 O at a pH = 13 having a QY = 95%. 227 The absorbance and emission spectra of both CDs and reference sample were measured in the same conditions and geometry, thus allowing us to calculate quantum yields using Equation A.1:

QY = I I R 1 -10 -A R 1 -10 -A n 2 n 2 R QY R (A.1)
Here subscript R indicates reference-related quantities, I is the integrated intensity of the photoemission band, A is the value of absorbance, and n is the refractive index of the solution. Finally, throughout this work of thesis another powerful tool has been used to analyse experimental data, which is the global analysis. This technique allows to improve results obtained via a least-squares fitting procedure and is especially useful when data is particularly noisy or described by a high number of parameters. Global analysis consists in combining together two or more datasets described by the same model, as long as the dataset share some of the parameters.

For instance, this procedure has been applied when analysing time-resolved photoluminescence data, where all decay traces taken at different emission wavelengths could be described by the same bi-exponential decay model (shown in the relative section, Equation A.26); in addition, parameters σ and t 0 from the used model function are directly linked to the laser excitation pulse, which is the same for all datasets. As such, they are not expected to change for each of the different decay traces. In addition, observation of the experimental data allowed us to conclude that the two lifetimes needed to describe the decay kinetic of the whole emission spectra were independent of excitation wavelength, so that we were able to assume that the value of parameters τ 1 and τ 2 had to be shared by all decay traces. Because of these observation, that data was analysed by a global fitting procedure. Equivalent considerations were made when other data has been analysed with the same technique.

Global analysis thus allows to extend the least-squares fitting procedure to multiple datasets which are simultaneously analysed. In order to do so, once again the goodness-offit has to be minimised; in this case, the global χ 2 G parameter is described by the following equation:

χ 2 G = k j=1 n i=1 1 ε 2 ij y ij -f (x i , p G , p j ) 2 (A.5)
In here, two sums are carried out; while the one running over index i is equivalent to that present in Equation A.3 as it covers the n data points, the other is relative to the k different datasets, each represented by the index j. The major difference between this equation and the previous one comes to the parameters of the model function f (x i , p G , p j ). when the quencher is present, the decay rate is modified due to the introduction of a new non-radiative pathway allowing return to the ground state happening with rate k q [Q], where [Q] is the concentration of the quenching specie and k q is called bimolecular quenching constant. From Equation A.6 and A.7 we obtain:

γ[F * ] 0 = (γ -k q [Q])[F * ] ⇒ ⇒ [F * ] 0 [F * ] = γ + k q [Q] γ = = 1 + k q τ 0 [Q] (A.8)
As previously said, the concentration of fluorophores in the excited state [F * ] is proportional to emission intensity I, so that we can write:

I 0 I = 1 + k q τ 0 [Q] = = 1 + K D [Q]
(A.9) Equation A.9, called Stern-Volmer equation, shows how the emission intensity in the presence of a quencher is expected to decrease linearly as a function of the quencher concentration; the proportionality factor K D = k q τ 0 is called Stern-Volmer constant.

If we now consider the emission lifetimes in the absence of quencher (Equation A.10) and in its presence (Equation A.11), from their ratio we obtain Equation A.12: .12) This is a very interesting results proving that in the case of dynamic quenching the emission lifetime decreases as much as the intensity. When discussing static quenching we collisional quenching affects such a remaining fraction: as previously shown, the amount of fluorophores affected by such a process can be described by Equation A.9. Consequently, the emitting fraction still present after both mechanisms have taken place can be expressed by the product of the two equations, yielding:

τ 0 = γ -1 ⇒ (A.10) τ = (γ + k q [Q]) -1 (A.11) ⇒ τ 0 τ = γ + k q [Q] γ = = 1 + k q τ 0 [Q] (A
I 0 I = (1 + K D [Q]) (1 + K S [Q]) = = 1 + (K D + K S )[Q] + K D K S [Q] 2 (A.20)
Equation A.20 is a second grade polynomial with respect to concentration [Q]; because of this, the Stern-Volmer plots it produces will present the previously discussed curvature.

Another final consideration on quenching has to be taken into account to describe our data. It often happens that when complex molecules are able to fluoresce, the underlying emission process is localised in different parts of the molecule itself. This happens, for instance, in many proteins able to fluoresce thanks to some of the residues composing them. As proteins present three-dimensional structures, it is not uncommon for certain fluorescent residues to be exposed to the external environment while others are instead hidden inside the protein itself. Consequently, it is not hard to imagine how interacting quenchers would differently affect the internal and external fluorophore fractions.

Also in this case the resulting Stern-Volmer plot will not be linear with respect to concentration [Q]; nevertheless, the resulting curve will present a downward concavity, as shown in the example of When the ground state is depopulated due to the interaction with the pump pulse, the now photoexcited molecules are not able to contribute to the sample absorbance any more, thus leading to a decrease in this signal. As such, the TA spectrum will present a negative band in the wavelength region where the absorbance of those molecules was previously found: such a signal is called GSB and, because of its origin, its shape is similar to the absorbance band of the sample.

I 0 I = 1 + K 1 [Q] 1 + K 1 [Q](1 -f ) 1 + K 2 [Q] (A.
Also the SE appears in the TA as a negative signal: in this case, nonetheless, the cause ground-state bleaching (GSB), caused by the absorption of the sample decreasing after excitation due to the depletion of the ground state; stimulated emission (SE), caused by the interaction between the excited sample and the probe signal, which stimulates photon emission and the consequent decay of the system back to the ground state; excited-state absorption (ESA), related to the possibility of new transitions toward higher energy states; right) the hypothetical TA signal composed of the three contributing signals.

is to be found in the probe pulse which, by interacting with the photoexcited molecules, is able to generate stimulated emission. Consequently, the probe intensity is increased by this emission arising from the sample, leading to a negative signal being registered in the TA spectrum. Because of this, the SE signal presents a shape resembling the emission band of the sample, albeit inverted in sign.

Finally, the last contribution to the TA signal comes from the possibilities of the photoexcited molecules to transition to other excited states; as the intensity of the probe will decrease due to the interaction with the molecules, now transitioning to more energetic states, the ESA signal will appear as a positive band in the TA spectrum.

A.5.1 Transient Absorption Setup

As previously mentioned, femtosecond transient absorption measurements were performed on a laboratory-made setup, whose scheme is shown in Figure A.8; this setup is based on a 5 kHz Ti:sapphire femtosecond amplifier (Spectra Physics Solstice-Ace) able to produce 75 fs pulses peaking at 800 nm (FWHM = 30 nm) at 700 µJ per impulse. The produced beam is split in two parts by a beam splitter (80%/20%) to generate the pump and the probe branches respectively.

On the pump arm, the 800 nm beam is frequency-doubled (type I phase-matching) by an ultrathin β-BBO crystal (250 µm) in order to create a 400 nm beam (20% efficiency), which is then isolated from the fundamental by a Schott BG40 filter. The beam is afterwards chopped at 500 Hz and focused on the sample using a parabolic mirror with f = 150 mm.

The polarisation of the pump is controlled by a waveplate.

On the second arm, white light is generated focusing the 800 nm beam in a 1 mm quartz cuvette containing D 2 O, so to generate a broadband pulse extending from 400 to 700 nm.

The probe is focused on the sample by the same parabolic mirror used to focus the pump.

The pump-probe delay is controlled by a motorised delay stage.

The probe and the pump overlap within the sample, which continuously flows in a 200 µm thick flow cell upon the action of a peristaltic pump. The flow is regulated so that every pump pulse hits a fresh portion of the sample. After traversing the sample, the probe beam is dispersed through a Brewster-angle silica prism and focused on the detector by a lens. The spectral resolution of this configuration is 3 nm. The temporal resolution is about 120 fs.

of the singular value decomposition (SVD). The SVD is a mathematical algorithm which allows to decompose a matrix in its factors. This is particularly useful for time-resolved spectral data, as it allows to obtain only the relevant kinetics and spectra while removing the noise components. To do so, as we are going to show, it is necessary to combine SVD and global analysis (Section A.2) so to extract from the original data the decay associated spectra (DAS).

If considering the TA data matrix TA(λ, t) m×n -composed by m lines and n columns respectively relative to the wavelengths λ and delays tvia the SVD algorithm it is possible to decompose it as shown in Equation A.35:

TA(λ, t) m×n = U(λ) m×m × S m×n × V(t) T n×n (A.35) In this equation, matrices U(λ) m×m , S m×n , and V(t) n×n are respectively called the eigenspectra, eigenvalues, and eigentraces matrices; the S m×n matrix has the peculiarity of being diagonal, meaning that all its values are zero except for those found on its diagonal.

This result assumes that the TA signal can be expressed as a linear combination of three components in which U(λ) m×m contains the spectral informations, V(t) n×n the kinetic informations, and the diagonal of S m×n provides the needed weights for each of the other two components; it has to be nevertheless noted that neither the eigenspectra nor the eigentraces possess any inherent physical meaning.

As the values of matrix S m×n provide the weights for the singular value decomposition, only its high s ii values are related to signals relevant to the observed TA of the sample. This is equivalent to saying that low s ii values are instead related to the noise present in

A.6 Materials and Synthesis

All chemicals used throughout the experimental procedures were purchased from Sigma-Aldrich, except for Na Once cooled down, the solution was transferred to a beaker and 150 g of KCl were added while stirring. After 10 min, the formed precipitate was collected via vacuum filtration using a #3 sintered glass funnel and subsequently left to dry. The following day, the powder was collected and redissolved in 650 mL distilled water. Small I 2 pellets were added to the solution while stirring in order to oxidise any reduced W V centres. After 10 min the step was considered complete as the solution colour changed from green to pale yellow and the I 2 pellet was removed via gravity filtration.

The solution was then transferred to a round bottom flask and heated under stirring for 8 days at 80 °C; after the heating was over, the solution was left to cool at room

  -clés : Nanomatériaux ; Nanoparticules ; Nanodispositif ; Polyoxométallate ; Carbon Dots ; POM ; CDs ; Photocatalyse ; Spectroscopie ; Ultrafast Transient Absorption v Thesis title: Polyoxometalate Clusters Photosensitised with Carbon-Dots: a New Generation of Catalytic Nanohybrids for Light Assisted Redox Processes

  rayonnement solaire comme source d'énergie nécessaire pour catalyser d'autres réactions xvii xviii Introduction en Français utiles. La photocatalyse a plusieurs applications différentes qui vont de la décomposition des polluants à la conversion de l'eau jusqu'à ses composants moléculaires, l'hydrogène et l'oxygène. Ce dernier processus, appelé water splitting, pourrait fournir l'hydrogène nécessaire comme carburant dans notre société. Avant de pouvoir atteindre tous ces objectifs, il reste à trouver de nouveaux photocatalyseurs: dans cette recherche, nous explorons le développement d'un type innovant de photocatalyseur à base de polyoxométallates et carbon dots. Les polyoxométallates sont une large famille de clusters d'oxydes métalliques polyanioniques, généralement constitués de métaux de transition dans leur état d'oxydation le plus élevé lié à des atomes d'oxygène. Les métaux de transition souvent rencontrées dans les polyoxométallates sont le molybdène, le tungstène et le vanadium; cette variabilité de leurs composants et le fait que les unités d'oxyde métallique formant ces polyanions peuvent être réarrangées en plusieurs structures permettent une grande variété des propriétés chimiques.

  La solution que nous avons tenté d'explorer pour résoudre ce problème consiste à utiliser des carbon dots, une nouvelle famille de nanoparticules fluorescentes à base de carbone et contenant principalement oxygène, hydrogène et azote. Cette espèce nanométrique peut absorber la lumière visible et s'est avérée capable de transférer facilement l'énergie absorbée aux composés couplés, ce qui en fait un possible photosensibilisateur. De plus, ces nanoparticules ne sont pas chères en raison de la disponibilité immédiate de tous leurs composants et sont souvent considérées comme non toxiques. C'est grâce à toutes ces xix propriétés que depuis leur récente découverte, de nombreux efforts se sont concentrés sur l'exploration de leurs propretés. Plusieurs voies de production simples et rapides ont été développées et ces derniers temps nous avons commencé à réfléchir à leur application en tant que donneurs d'électrons.

Figure 1 . 1 :

 11 Figure 1.1: Different coordination modes for the metal addenda atoms forming the subunits of polyoxometalates; reproduced from ref. (26 ).

Figure 1 . 2 :

 12 Figure 1.2: Distortion of the addenda structure, represented for an octahedral {MO 6 } geometry; the terminal oxygen, towards which the metal addenda is displaced, is clearly indicated; reproduced from ref. (26 ).

Figure 1 . 3 :

 13 Figure 1.3: Different connection modes for the addenda subunits forming the polyoxometalate structure when in {MO 6 } octahedral geometry; reproduced from ref. (26 ).

  With a general formula of [M 6 O 19 ] n -, this metal oxide cluster is composed by six {MO 6 } octahedral units linked via edge sharing and disposed around a central oxygen atom, as shown in Figure 1.4. As in each subunit only one oxygen atom is not shared with other adjacent addenda atoms, polyoxometalates part of this family are of type I and can thus be reversibly reduced.

Figure 1 . 4 :

 14 Figure 1.4: Representation of the Lindqvist isopolyanion, in ball-and-stick (left) and polyhedral (right) representation; metal cations are represented in teal colour while oxygen anions are in red; reproduced from ref. (26 ).

Figure 1 . 5 :

 15 Figure 1.5: Representation of the Anderson-Evans heteropolyanion, in ball-and-stick (left) and polyhedral (right) representation; metal cations are represented in teal colour, heteroatoms in orange, and oxygen anions in red; reproduced from ref. (26 ).

  2 A-type polyoxometalates, of general formula [XM 6 O 24 ] n -, do not contain any protons in their structure since the central atom is in an high oxidation state. In contrast, the B-type, of general formula [X(OH) 6 M 6 O 18 ] n -, presents the heteroatom in a low oxidation state and protons linked to the oxygen atoms of the internal part of the structure.

{M 3 OFigure 1 . 6 .

 316 Figure 1.6. As each of these subunits only contains a single unshared oxygen atom, Keggin

Figure 1 . 6 :

 16 Figure 1.6: Representation of the Keggin heteropolyanion, in ball-and-stick (left) and polyhedral (right) representation; metal cations are represented in teal colour, heteroatoms in orange, and oxygen anions in red; reproduced from ref. (26 ).

Figure 1 . 7 :

 17 Figure 1.7: Representation of the five Keggin heteropolyanion isomers, in polyhedral representation; heteroatoms are represented in orange colour, {M 3 O 13 } metal addenda triads either in purple or teal depending on whether they have been rotated or not; reproduced from ref. (26 ).

Figure 1 . 8 :

 18 Figure 1.8: Representation of the Wells-Dawson heteropolyanion, in ball-and-stick (left) and polyhedral (right) representation; metal cations are represented in teal colour, heteroatoms in orange, and oxygen anions in red; reproduced from ref. (26 ).

Figure 1 . 9 :

 19 Figure 1.9: Representation of the Wells-Dawson heteropolyanion isomers, in polyhedral (side view) and ball-and-stick (top-down view) representation; metal cations are represented in teal colour, heteroatoms in orange, and oxygen anions in red; purple arrows represents rotation of a {M 3 O 13 } cap triad by a π /3 angle while black arrows represent rotation of one half of the cluster by a π angle; reproduced from ref. (26 ).

Figure 1 . 10 :

 110 Figure 1.10: Polyhedral representation of the [NaP 5 W 30 O 110 ] 14 -Preyssler heteropolyanion; reproduced from ref. (49 ).

Figure 1 . 11 :

 111 Figure 1.11: Sketch of the [CeMo 12 O 42 ] 8 - Dexter-Silverton heteropolyanion; reproduced from ref. (51 ).

Figure 1 . 12 :

 112 Figure 1.12: Polyhedral representation of the {Mo 154 } ring molybdenum blue POM; reproduced from ref. (26 ).

Figure 1 . 13 :

 113 Figure 1.13: Polyhedral representation of the {Mo 368 } molybdenum blue POM; reproduced from ref. (26 ).

Figure 1 . 15 :

 115 Figure 1.15: Derivatisation of an Anderson-Evans {MnMo 6 } based hybrid POM; {MnO 6 } octahedra are shown in pink, {MoO 6 } in blue; the two organic linkers capping both sides of the polyoxometalate can be derivatised to form other hybrid structures; reproduced from ref. (61), based on ref.(62 ).

Figure 1 . 16 :

 116 Figure 1.16: By careful choice of the hybrid POM structure, solubility properties of the resulting compound can be used to spontaneously form micelles in solution; left) the inorganic-organicinorganic POM structure which self-assembles into micelles, formed by two {P 2 V 3 W 15 } clusters bound together via an organic linker; right) TEM images of the micelles formed by the compound in a water/acetone solution; reproduced from ref. (71 ).

  Figure 1.18, stems from the fact that it is able to catalyse the oxidation of H 2 O to O 2 in aqueous solution at pH 7 while being stable, a core problem not yet addressed when using POMs for water oxidation.55

Figure 1

 1 Figure 1.18: Structure of the [Ru 4 (µ-O) 4 (µ-OH) 2 (H 2 O) 4 (γ-SiW 10 O 36 ) 2 ] 10 - heteropolyanion; the central {Ru 4 (µ-O) 4 (µ-OH) 2 (H 2 O) 4 } 6+ core is highlighted, with the {Ru 4 } tetrahedron shown in light blue, Ru atoms in blue, µ-O in red and O(H 2 ) in orange (hydrogen atoms omitted for clarity); the two [γ-SiW 10 O 36 ] 8 -lacunary heteropolyanions capping the central core are represented with the {WO 6 } tetrahedron shown in grey, oxygen atoms in red and Si heteroatoms in yellow; reproduced from ref. (55 ).

H 2 .

 2 109 -111 As shown in Figure1.19, through this approach water is initially oxidised, producing oxygen and reducing the heteroanion [H 2 PMo 12 O 40 ] -; this latter polyanion, in order to balance charges, accepts protons and thus becomes [H 4 PMo 12 O 40 ] -, transformation characterised by its turning from yellow to dark blue. In a second moment it is therefore possible to reverse the sign of the potential applied in the electrochemical cell, so to reoxidise the POM which yields back the protons previously accepted in the form of

Figure 1 . 19 :

 119 Figure 1.19: Scheme of the hydrogen-decoupled water splitting achieved by using POMs as electron-coupled protons buffer; a) water is oxidised, producing O 2 , H + and e -; electrons flowing through the circuit reduce the [H 2 PMo 12 O 40 ] -(yellow) present on the right side of the cell which, in turn, accepts the protons passing through the semipermeable membrane separating the compartments, thus becoming [H 4 PMo 12 O 40 ] -(dark blue); b) [H 4 PMo 12 O 40 ] -is reoxidated releasing protons which, migrating to the other cell compartment, react with the electrons flowing through the circuit to yield H 2 ; [H 2 PMo 12 O 40 ] -is at this point regenerated and the cycle can restart; reproduced from ref. (109 ).

Figure 1 . 20 :

 120 Figure1.20: POMs clusters, formed by subnanometer metal-oxide units, display a tendency to self-assemble into bigger structures whose sizes ranges from the tenths to the hundreds of nanometers; as several different architectures ranging from zero-to three-dimensional can be obtained, polyoxometalates are considered building blocks bridging the gap between molecules and nano-systems which have found applications in many different fields; reproduced from ref.(12 ).

Figure 1 . 21 :

 121 Figure 1.21: Controlled growth of POM microtubes; left) growth of a tube over a period of 20 seconds; top right) it is possible to inject fluid into the tube without leakage; center right) preformed tubes can be joined together; lower right) the growth of tubes can be directed into following predefined shapes; based on ref. (101 ), reproduced from ref. (12 ).

  75 a study was able to increase the overall solar-to-electric energy conversion efficiency by 50% in the presence of [PW 12 O 40 ] 3 -, compared to when the Keggin heteropolytungstate was not present.121 The DSSC used in the study was based on ZnO nanoparticles; these were functionalised with POMs directly during their synthetic phase, which consisted in a solvothermal treatment of Zn(CH 3 COO) 2 • 2 H 2 O and CH 3 CH 2 OH in the presence of [PW 12 O 40 ] 3 -at 120 °C for 24 h.

  126 Moreover, POMs were proven directly effective against bacteria themselves, thanks to their photocatalytic properties. A study of 2008 was in fact able to show how under irradiation the heteropolyanion [SiW 12 O 40 ] 4 -was able to inactivate 99.9% of Escherichia Coli bacteria in less than a minute; the same study explored the antibacterial properties of this and other POM stucture against both Gram negative and Gram positive bacteria, finding out remarkable effects both with and without irradiation.80

Figure 1 . 22 :

 122 Figure 1.22: Formation of apoptotic bodies, indicated by arrows, in human gastic MKN45 cancer cell due to in-vitro administration of [Mo 7 O 24 ] 6 -; reproduced from ref. (122 ).

Figure 1 . 23 :

 123 Figure 1.23: Structure of 30S ribosome in surface representation; protein and rRNA portions are respectively shown as green and cyan surfaces; [P 2 W 18 O 62 ] 6 -, used to rigidify the ribosome and increase the resolved crystal structure resolution, is shown in red; based on ref. (127 ), reproduced from ref. (20 ).

Figure 1 .

 1 Figure 1.23, heteropolyanion [P 2 W 18 O 62 ] 6 -was used to treat ribosomal subunits 30S;as a result, the interacting polyoxometalates were demonstrated to induce a beneficial conformational change, meaning that the POM presence was able to freeze the entire vicinity of its environment. Reducing the mobility of zones considered crucial for the flexibility of the macromolecule resulted in trapping the structure in one single conformation.

Figure 2 . 1 :

 21 Figure 2.1: The tunable emission of a CDs solution excited at different wavelengths, as indicated below each picture; reproduced from ref. (153 ).

Figure 2 . 2 :

 22 Figure 2.2: Gel electrophoresis profile under UV light obtained from the purification of SWCNTs treated in HNO 3 and NaOH; A) crude SWCNTs suspension before purification; B ) fluorescent particles, later identified as CDs; C -F ) other less mobile fractions, the last of which are purified SWCNTs; reproduced from ref. (141 ).

Figure 2 . 3 :

 23 Figure 2.3: Carbon nanoparticles obtained from laser ablation of a graphitic substrate can be rendered fluorescent after passivation of the surface by organic polymers; reproduced from ref. (142 ).

  Figure 2.4 shows a transmission electron microscopy (TEM) image of the obtained nanoparticles and their sharp size distribution.

  two separate components, respectively assigned to the fluorescent molecule and carbon dot cores. Further increases in temperature (300 and 400 °C) lead to depletion of the fluorescent molecules until only CDs were only present. Nonetheless, at 400 °C clustering between the nanoparticles became predominant, pointing out to the beneficial role of the surface shell towards particle separation; in fact, as the molecular specie was completely depleted the obtained nanoparticles presented a barren surface, thus tending to aggregation

Figure 2 . 4 :

 24 Figure 2.4: CDs synthesised through a bottomup thermal method display a high size control; A) TEM image of the obtained nanoparticles, with a single CD shown in the inset; B ) size distribution of the observed dots as obtained from the TEM image analysis; reproduced from ref. (166 ).

Figure 2 . 5 :

 25 Figure 2.5: Representation of the emission characteristics of the species obtained from the thermal decomposition of citric acid and ethanolamine; the pyrolysis, whose completion degree is controlled by temperature, allows to consume organic fluorophores in order to build the carbogenic cores of carbon dots; reproduced from ref. (167 ).

Figure 2 . 6 :

 26 Figure 2.6: Different fractions of a CDs solution as obtained by SEC, observed under visible light (top) and UV light (bottom); reproduced from ref. (181 ).

  ten years, in order to achieve the full potential of carbon chemistry in CDs synthesis.178 Nowadays successful approaches often use techniques such as high-performance liquid chromatography (HPLC)179 ,180 or size exclusion chromatography (SEC), 181 -184 which allow to obtain several fractions exhibiting different characteristics, as shown in Figure2.6.

Figure 2 . 7 :

 27 Figure 2.7: A) TEM and B ) HRTEM images of CDs; the indicated 0.21 nm plane spacing is consistent with a graphitic lattice; reproduced from ref. (188 ).

Figure 2 . 8 :

 28 Figure 2.8: Photographs of zebrafish larvae soaked for 10 h in different CDs solutions (concentration indicated above) so to validate the in-vivo biotoxicity and biocompatibility of the nanoparticles; top) brightfield images; bottom) fluorescence images; indicated scale bar is equal to 1 mm; reproduced from ref. (203 ).

  2009 described how CDs fluorescence would be quenched by the presence of 4-nitrotoluene, 2,4-dinitrotoluene, N,N-diethylaniline and diethylamine via a photo-induced electron transfer mechanism. While the first two compounds are known for their capabilities as electron acceptor (reduction potentials respectively -1.19 and -0.9 V vs NHE), the latter are instead electron donors (reduction potentials respectively 0.88 and 1.55 V vs NHE). In all cases the CDs fluorescence was negatively affected by the presence of other compounds and, remarkably, the degree of quenching was found to be dependent on the quencher ability to donate/accept electrons.208 This evidenced was obtained by the different slopes observed in the Stern-Volmer plot of the CDs fluorescence for each of the tested molecules.In conclusion, CDs seem to behave either as acceptors or donors of electrons depending on their redox potential.157 As this property depends on the characteristics of their core structure and surface composition, not all carbon dots would equally behave in the same test case. Researchers have thus tried to customise the electrochemical properties of these nanoparticles in an effort to control the resulting behaviours. In a recent paper it was shown how it is possible to control the redox potential of carbon dots by modulating the

Figure 2 . 9 :

 29 Figure 2.9: A) reaction scheme of the microwave-assisted synthesis of CDs modified with quinones; B ) redox potentials displayed by the different quinone-modified CDs, with the corresponding quinone structure shown on top; being able to control and tune the electrochemical properties of carbon dots is an extremely useful tool when considering their interaction with other species and the possible photocatalytic applications; reproduced from ref. (212 ).

Figure 2 . 10 :

 210 Figure 2.10: Absorbance and tunable emission of CDs as observed by varying the excitation wavelength between 400 and 600 nm with 20 nm increments; intensities normalised to the emission quantum yield measured at the relative excitation wavelength; in the inset, intensity is normalised to the band peak value; reproduced from ref. (142 ).

Figure 2 . 11 :

 211 Figure 2.11: Comparison between the emission spectra of five different CDs as observed via single-molecule emission spectroscopy, excited at two different wavelengths to show the loss of tunability on a single-particle level; reproduced from ref. (175 ).

  Evidences regarding this behaviour seem to vary depending on the type of carbon nanoparticle; until now we still lack means of completely excluding heterogeneity from CDs. 219 Nonetheless, studies of single-molecule emission spectroscopy have shown that, when observing selected carbon dots, no tunability was found on a single-particle level, as shown in Figure2.11. As this technique allows to observe the emission spectrum of a single nanoparticle, in the case of CDs not observing the peculiar tunable emission should demonstrate how each particle behaves as a single emitter and thus that the tunability comes from the ensemble present in solution.156 ,175 Nevertheless, because of the many different CDs families, researchers are careful to claim any generality to these findings.175 In addition, understanding why each dot emits differently is yet another hard challenge.As many explanations have been formulated assuming CDs behave as quantum dots (QDs), it is needed to discuss the characteristics of this other class of nanoparticles before diving in the debate surrounding carbon dots. QDs are nanosized semiconductor particles exhibiting size-dependant optical and electronic properties.228 These are nowadays also used in industrial contexts, due to their long research history and huge potential; in fact, quantum nanoparticles emitting visible light are extremely useful and have been applied in many fields; for example, in medicine they have been used for imaging and diagnostic purposes 229 and in the development of new devices thay have been used as basic components for light-emitting diodes (LEDs).230 ,231

Figure 2 .

 2 Figure 2.12: a) solutions of CDs with different size distribution separated via column chromatography under white light (left) and 365 nm UV light (right); b) emission spectra of the four solution shown in (a), disposed in the same left-to-right order; c) dependency between the observed optical properties and the size distribution of the relative CDs fraction; d ) dependency between band gap energy and graphite fragment size, as obtained via theoretical calculations; reproduced from ref. (155 ).

  absorbance and emission bands. Again, by changing the excitation wavelength different dots are selected and thus different emissions are obtained.In the literature there still is no full agreement regarding the origin of CDs emission and tunability. In general, researchers seem to nowadays converge towards a hypothesis combining both core and surface effects. It appears that quantum confinement influences the energy of the absorbing states, after which the exciton is transferred towards the surface of the dot which alters the emission energy. 185 This explanation is nonetheless a simplification of the real situation: as previously pointed out, with the term carbon dots we refer to different types of carbon-based nanoparticles, which are usually categorised between GQDs, CQDs and CNDs.191 Each of these types of dots seem to behave in a slightly different manner and probably the two hypothesis contributing to the emissive overall behaviour could have different weights for each dot family. As the topic is still open for debate, more research is needed before reaching a full conclusion.

Figure 2 . 13 :

 213 Figure 2.13: The fluorescence response of CDs to different metal ions (at a concentration of 100 µm) as observed under irradiation from a 365 nm UV lamp; these particular carbon nanoparticles are selectively sensitive to the presence of Cr(VI) which causes a clear blueshift of the emission band; reproduced from ref. (207 ).

Figure 2 .

 2 14 by modulating the excitation wavelength and the bands in which light is collected it is possible to choose opportune intervals in which the fluorescence of other molecules is hidden and, making use of the fact that CDs usually permeate in the membrane and cytoplasmic area but not in the cell nucleus, enhance the visibility of certain cellular regions.245

Figure 2 . 14 :

 214 Figure 2.14: Laser scanning confocal microscopy of CDs-labelled L929 cells; excitation wavelength: (a) 405 nm, (b) 488 nm, (c) 543 nm; light collection interval: (a) 432 -490 nm, (b) 516 -551 nm, (c) 595 -657 nm; scale bar: 20 µm; CDs presence remarkably enhances the visibility of different region of the cells thanks to their multicolour emission; reproduced from ref. (245 ).

Figure 2 . 15 :

 215 Figure 2.15: The distribution over time of CDs injected in a glioma-bearing mouse as observed through in-vivo fluorescent bioimaging; the carbon nanoparticles, initially diffusing throughout the body, accumulate in the glioma site while being eliminated from the rest of the organism, allowing for detection of the tumor; modified and reproduced from ref. (246 ).

Figure 2 . 16 :

 216 Figure 2.16: Photo of a CDs solution producing laser emission when amplified by a quartz cuvette acting as optical cavity; the green spot found in the top left of the picture is the laser beam; reproduced from ref. (184 ).

4. 1 . 1 [ 6 -

 116 P 2 W 18 O 62 ]The compound [P 2 W 18 O 62 ] 6 -is a renown heteropolyanion presentingWells-Dawson struc- ture (Figure 1.8). For the sake of clarity, in the rest of this text the abbreviation {P 2 W 18 } will be used when referring to this polyanion. The product has been synthesised following a procedure found in the literature, 44 detailed in Section A.6.1. The 31 P NMR signature of the obtained compound is shown in Figure4.1; the chemical shift of the specie is consistent with the value found in the literature,[START_REF] Mbomekalle | [END_REF] proving purity of the compound.As shown inFigure 4.2 this polyoxometalate does not absorb any visible light; in fact, the absorbance spectrum does not present any feature at wavelengths above 400 nm. Dynamic light scattering (DLS) measurements were performed so to obtain the hydrodynamic radius of the {P 2 W 18 } anions in solution. Resulting number distribution is shown in Figure 4.3, indicating an average value of R h = 0.68 nm. The obtained value is consistent with the size of the Wells-Dawson structure.

Figure 4 . 1 :Figure 4 . 2 :Figure 4 . 3 :

 414243 Figure 4.1: [P 2 W 18 O 62 ] 6 -31 P NMR signature; the indicated peak present a chemical shift value consistent with the one found in the literature for the compound.

Figure 4 . 6 :

 46 Figure 4.6: CDs deposited on a Si substrate as observed through AFM; A and B ) topographies of the compound, respectively on a 10 × 10 µm and 2 × 2 µm area; C ) size distribution of the grains as obtained from the heights of the particles found on (A).

Figure 4 . 7 :Figure 4 . 8 :Figure 4 . 9 :

 474849 Figure 4.7: A and C ) HRTEM images of CDs; B and D) FFT respectively calculated from (A) and (C ); the planar distances responsible for the observed diffraction peaks are indicated.

Figure 4 . 8 ;

 48 Figure 4.8; as indicated, the characteristic Raman bands featured by graphitic carbon are

Figure 4 .

 4 Figure 4.10: ζ-potential distributions displayed by CDs solutions at two different pH values; pH at which the measurements were carried out as well as ζ-potentials values obtained by fitting the distributions are indicated.

Figure 4 . 11 :

 411 Figure 4.11: Dependency between the CDs ζ-potential and pH; by opportunely tuning the pH of the solution in which CDs are dispersed it is possible to control the nanoparticles surface charge.

  Figure 4.10, for the samples obtained at pH 9.84 and 3.97, respectively labelled as CDs - and CDs + . ζ-potential values are obtained by least-squares fitting the distribution with a Gaussian function, as explained in the appendix. The dependency of ζ-potential vs. pH is shown in Figure 4.11; as evident, it is possible to opportunely tune the surface charge of these CDs by changing the solution pH. Therefore, from now on CDs solutions having a pH close to 10 will be referred as CDs -while those having a pH approximately equal to 4

Figure 4 .

 4 13. In the same figure, steady-state photoluminescence spectra obtained by exciting the samples at 400 nm are shown. These are found to be peaking at 480 nm for CDs -and 460 nm for CDs + . The Stokes shift between absorbance and emission of CDs + thus seems to be smaller compared to CDs -; in the literature, a red-shift in the emission bands has often been associated to CDs incorporating a high amount of oxygen species in their surface.

Figure 4 . 13 :Figure 4 . 14 :

 413414 Figure 4.13: Normalised steady state absorption and fluorescence spectra (λ exc = 400 nm) of CDs -/+ , respectively indicated by solid and dashed lines; CDs + appears to have a smaller Stokes shift between excitation and emission bands compared to CDs -.

Figure 4 .Figure 4 . 15 :Figure 4 . 16 :Figure 4 . 17 :

 4415416417 Figure 4.16: Normalised emission spectra of CDs + at different delays from excitation; λ exc = 410 nm.

Figure 4 .Figure 4 . 18 :

 4418 Figure 4.18: The fractional amplitudes of the two exponential components obtained from the bi-exponential fitting procedure of the decay of: A and B ) CDs -; C and D) CDs + ; data obtained at λ exc : A and C ) 410 nm; B and D) 450 nm; the evidence that the relative contribution of the two functions depends on the observed spectral region confirms that the emission of each sample is composed by two separate bands with different kinetics.

  7 s -1 are obtained for both CDs -/+ . Based on the order of magnitude of the calculated radiative decay rate we can conclude that the analysed samples emit due to fluorescence mechanisms, since in the case of phosphorescence we have Γ < 10 3 s -1 . 227 CDs optical properties were further studied on smaller time scales by performing femtosecond transient absorption (TA) experiments. The samples were excited by 400 nm laser pulses of 75 fs duration. Data relative to CDs -is shown in the image-plot presented in Figure 4.19. In order to understand the spectral components giving rise to the observed transient absorption, spectra at different delays from the excitation pulse have been

Figure 4 . 19 :

 419 Figure 4.19: Image-plot of the femtosecond transient absorption of CDs -, obtained at λ exc = 400 nm; identified GSB, SE, and ESA signals are respectively labelled in blue, green, and red; spectra and traces are obtained from this image by taking data respectively on horizontal rows and vertical columns.

Figure 4 .Figure 4 . 21 :

 4421 Figure 4.21: Transient absorption decay traces and relative fitting curves of A) CDs -and B ) CDs + taken at different wavelengths (λ exc = 400 nm); shown data have been averaged in a 10 nm interval around the indicated wavelength.

Figure 4 . 22 :Figure 4 . 23 :

 422423 Figure 4.22: A) The first three eigentraces of the transient absorption data of CDs -and their best-fitting curves used to reconstruct the full signal; B ) comparison between original data and reconstructed transient absorption signal of CDs -; the close resemblance of the two datasets is indicative of a successful fit procedure.

Figure 4 . 24 :

 424 Figure 4.24: Normalised steady state absorption and fluorescence spectra (λ exc = 440 and 500 nm, indicated by coloured arrows) of N-CDs, respectively indicated by solid and dashed lines.

Figure 4 .

 4 Figure 4.25: N-CDs FTIR spectrum, with the identified vibrational peaks indicated; this data allows to identify the presence of both carboxylic acid and amide moieties on the N-CDs surface.

Table 4 . 5 :

 45 Vibrational peaks of N-CDs FTIR spectrum shown in Figure4.25; their position and responsible functional groups are indicated.

  89 and 1.14 is shown in Figure 4.26. Results in approximately neutral conditions are representative of the situation observed at most of the explored pH; as shown in Figure 4.27, N-CDs are negatively charged and stable in solutions up to pH values of 12. When, instead, the experiment is performed in very acidic conditions, the ζ-potential distribution appears to be composed of two different bands (Figure 4.26). By fitting the data at pH = 1.14 with a double Gaussian distribution, average values of -40.3 and 20.7 mV are found; despite this could be regarded as a positive result, N-CDs

Figure 4 .

 4 Figure 4.26: ζ-potential distributions displayed by N-CDs solutions at two different pH values; pH at which the measurements were carried out as well as ζ-potentials values obtained by fitting the distributions are indicated.

Figure 4 . 27 :

 427 Figure 4.27: Dependency between the N-CDs ζ-potential and pH; this kind of CDs appears to always be negatively charged on its surface, except in very acidic conditions.

Chapter 5 :

 5 {P 2 W 18 }-CDs Complexes in Figure 5.1 the bands composing the spectrum appear undergoing a redshift as a result of POMs presence in solution, even at very low micromolar concentrations. Modifications in absorbance due to presence of external species are usually associated to formation of static complexes

CDs

  Figure 5.1: The absorbance spectrum of CDs + undergoes a redshift upon addition of {P 2 W 18 } as indicated by the arrow (concentrations reported in legend); comparison with the absorbance of {P 2 W 18 } proves that the complex does not contribute on its own to the wavelengths where the CDs + band shifts; all spectra have been arbitrarily normalised.

Figure 5 . 2 :

 52 Figure 5.2: Emission spectra of CDs + after addition of {P 2 W 18 } at the indicated concentrations; presence of {P 2 W 18 } in solution induces a quenching in the photoluminescence intensity of CDs + , as indicated by the arrow; data obtained at λ exc = 400 nm.

Figure 5 . 3 :Figure 5 . 4 :

 5354 Figure 5.3: Stern-Volmer plots of CDs + emission intensity quenched by {P 2 W 18 } at two different excitation wavelengths; curves best representing the data, also shown in the plot with the corresponding 1-σ confidence intervals, have been obtained via a least-squares fitting procedure.

Figure 5 .

 5 5 to those obtained for {P 2 W 18 } (Figure 5.6) and CDs (Figure 4.6), as done in Figure 5.7 and Table

Figure 5 . 5 :

 55 Figure 5.5: {P 2 W 18 }-CDs + deposited on a Si substrate as observed through AFM; A and B ) topographies of the compound, respectively on a 10 × 10 µm and 2 × 2 µm area; the tallest grain found on the bottom-left of (A) is 200 nm tall; other tall grains, mostly found on the top-right corner of (A), measure 20 nm; C ) size distribution of the grains as obtained from the heights of the particles found on (B ).

Figure 5 . 6 :

 56 Figure 5.6: {P 2 W 18 } deposited on a Si substrate as observed through AFM; A and B ) topographies of the compound, respectively on a 10 × 10 µm and 2 × 2 µm area; C ) size distribution of the grains as obtained from the heights of the particles found on the lower Si level of (A).

Figure 5 . 7 & Table 5 . 3 :

 5753 Figure 5.7 & Table 5.3: Comparison between the average heights of the indicated samples as obtained from their AFM size distributions (shown in Figure5.6C, 4.6C, and 5.5C); reported uncertainties correspond to the relative σ values.

Figure 5 .

 5 3 shows, in the low concentration sample case we are analysing the first and strongest quenching mechanism; instead, when working at high concentrations we are well into the second and less strong part of the Stern-Volmer plot. Results for the low concentration {P 2 W 18 }-CDs + sample are presented in Figure 5.8.

Figure 5 . 8 :

 58 Figure 5.8: Transient absorption spectra of {P 2 W 18 }-CDs + at different delays from excitation (λ exc = 400 nm); shown spectra have been averaged on a 10% time interval of the indicated delay; the recognised GSB, SE, and ESA signals are respectively marked by blue, green, and red arrows.

Figure 5 . 9 :

 59 Figure 5.9: Transient absorption spectra of CDs + and {P 2 W 18 }-CDs + after A) 420 fs and B ) 60 ps from excitation (λ exc = 400 nm); the positions of GSB and SE bands are indicated by arrows; as visible, the SE dip is less pronounced at all excitation delays; in addition, the GSB signal is able to decay for sample CDs + while seems to persists in the case of {P 2 W 18 }-CDs + complexes.

Figure 5 . 10 :

 510 Figure 5.10: A) transient absorption decay traces and relative fitting curves of {P 2 W 18 }-CDs + complexes taken at different wavelengths (λ exc = 400 nm); shown data have been averaged in a 10 nm interval around the indicated wavelength; B ) comparison between original data and reconstructed transient absorption signal of {P 2 W 18 }-CDs + ; the close resemblance of the datasets shown in both panels is indicative of a successful fit procedure.

Figure 5 .

 5 11, we can reach same conclusions: if comparing these DAS to those obtained in the absence of {P 2 W 18 } (Figure 4.23B) we can readily observe how the DAS shapes barely present any difference in the two cases. Once again, the shortest lived DAS contains a GSB signal, now slightly red-shifted as observed for the steady-state absorbance band of the complexes (Figure 5.1), and the same SE and ESA signals; in addition, decay associated spectra of lifetime τ 2 = 1.06 ps and τ 3 = 20 ps can yet again be described as being composed by a continuum-like ESA signal, which we have previously associated to electrons present in the conduction band of the CDs core, covering the whole studied wavelength region except for the part where the GSB is found.

Figure 5 . 11 :

 511 Figure 5.11: A) decay associated spectra of the transient absorption signal of {P 2 W 18 }-CDs + complexes obtained at a {P 2 W 18 } concentration equal to 1.25 µm; shown DAS have been calculated from the least-squares global fit procedure performed on the relative TA signal eigentraces; to improve legibility, intensity of the shortest lived DAS has been halved as indicated in the legend; B ) comparison between the longest (τ > 1 ns) lived DAS obtained for CDs + and {P 2 W 18 }-CDs + complexes formed at 1.25 µm and 125 µm {P 2 W 18 } concentration.

Figure 5 . 12 :Figure 5 . 13 :

 512513 Figure 5.12: Emission spectra of CDs -after addition of {P 2 W 18 } at the indicated concentrations; presence of {P 2 W 18 } in solution induces a quenching in the photoluminescence intensity of CDs -, as indicated by the arrow; data obtained at λ exc = 400 nm.

Figure 5 . 15 :Figure 5 . 16 :

 515516 Figure 5.15: Transient absorption spectra at different delays from excitation (λ exc = 400 nm) of {P 2 W 18 }-CDs -obtained at a {P 2 W 18 } concentration equal to A) 1.25 µm and B ) 125 µm; shown spectra have been averaged on a 10% time interval of the indicated delay; the recognised GSB, SE, and ESA signals are respectively marked by blue, green, and red arrows.

2 WFigure 5 .

 25 Figure 5.17: A) decay associated spectra of the transient absorption signal of {P 2 W 18 }-CDs - complexes obtained at a {P 2 W 18 } concentration equal to 1.25 µm; shown DAS have been calculated from the least-squares global fit procedure performed on the relative TA signal eigentraces; B ) comparison between the longest (τ > 1 ns) lived DAS obtained for CDs -and {P 2 W 18 }-CDs - complexes formed at 1.25 µm {P 2 W 18 } concentration.

Figure 5 . 18 :

 518 Figure 5.18: Decay associated spectra of the transient absorption signal of {P 2 W 18 }-CDs - complexes obtained at a {P 2 W 18 } concentration equal to 125 µm; shown DAS have been calculated from the least-squares global fit procedure performed on the relative TA signal eigentraces.

20

  times less than what observed in this case. The interaction, already not very strong in the case of CDs -, now appears even less efficient. Quantitative results can be obtained by analysing the Stern-Volmer plot of the N-CDs emission intensity, as shown in Figure 5.20.

Figure 5 . 19 :Figure 5 . 20 :

 519520 Figure 5.19: Emission spectra of N-CDs after addition of {P 2 W 18 } at the indicated concentrations; presence of {P 2 W 18 } in solution induces a slight quenching in the photoluminescence intensity of N-CDs, as indicated by the arrow; data obtained at λ exc = 500 nm.

Figure 6 . 1 :

 61 Figure 6.1: The absorbance spectrum of CDs + undergoes a redshift upon addition of {Zn 4 P 4 W 30 } as indicated by the arrow (concentrations reported in legend); comparison with the absorbance of {Zn 4 P 4 W 30 } proves that the complex does not contribute on its own to the wavelengths where the CDs + band shifts; all spectra have been arbitrarily normalised.

Figure 6 . 2 :

 62 Figure 6.2:Effect of the indicated {Zn 4 P 4 W 30 } concentration on the emission spectra of CDs + ; addition of {Zn 4 P 4 W 30 } up to 20 µm induces a quenching in the photoluminescence intensity of CDs + ; at higher concentrations the emission appears instead to be gradually restored; data obtained at λ exc = 400 nm.

( 20 ± 5 ) 4 Figure 6

 20546 Figure 6.3 & Table6.1: Stern-Volmer plots of CDs + emission intensity quenched by {Zn 4 P 4 W 30 } (λ exc = 400 nm); the curve best representing the data, also shown in the plot with the corresponding 1-σ confidence interval, has been obtained via a least-squares fitting procedure; points indicated in red have been excluded from the fitting procedure, as relative to an increase in emission intensity instead of a decrease; resulting quenching parameters are reported in the table, with K being the quenching coefficient while f representing the accessibility factor relative to the process.

  3, at low concentrations we observe a linear range associated to the quenching. Above 20 µm, instead, a decreasing curve indicates the emission recovery; points associated to this region are shown in red. Additionally, to study the magnitude of quenching a least-squares fitting procedure has been performed, yielding the parameters shown in

Figure 6 .Figure 6 . 6 :

 666 Figure 6.6B. Meaningfulness of the fit is ensured by the good resemblance of the shown signals. Decay associated spectra were then calculated from the parameters obtained via the fitting procedure and are shown in Figure 6.7A. Shapes of the observed signals are consistent

Figure 6 . 7 :

 67 Figure 6.7: A) decay associated spectra of the transient absorption signal of {Zn 4 P 4 W 30 }-CDs + complexes obtained at a {Zn 4 P 4 W 30 } concentration equal to 1.25 µm; shown DAS have been calculated from the least-squares global fit procedure performed on the relative TA signal eigentraces; to improve legibility, intensity of the shortest lived DAS has been halved as indicated in the legend; B ) comparison between the longest (τ > 1 ns) lived DAS obtained for CDs + and {Zn 4 P 4 W 30 }-CDs + complexes formed at 1.25 µm and 125 µm {Zn 4 P 4 W 30 } concentration.

Figure 6 . 12 :Figure 6

 6126 Figure 6.12: Transient absorption spectra at different delays from excitation (λ exc = 400 nm) of {Co 4 P 4 W 30 }-CDs + obtained at a {Co 4 P 4 W 30 } concentration equal to 1.25 µm; shown spectra have been averaged on a 10% time interval of the indicated delay; the recognised GSB, SE, and ESA signals are respectively marked by blue, green, and red arrows.

Figure 6 . 14 :

 614 Figure 6.14: A) decay associated spectra of the transient absorption signal of {Co 4 P 4 W 30 }-CDs + complexes obtained at a {Co 4 P 4 W 30 } concentration equal to 1.25 µm; shown DAS have been calculated from the least-squares global fit procedure performed on the relative TA signal eigentraces; to improve legibility, intensity of the shortest lived DAS has been halved as indicated in the legend; B ) comparison between the longest (τ > 1 ns) lived DAS obtained for CDs + and {Co 4 P 4 W 30 }-CDs + complexes formed at 1.25 µm and 125 µm {Co 4 P 4 W 30 } concentration.

Figure 7 . 1 :

 71 Figure 7.1: A) absorbance of an aqueous solution of {P 2 W 18 }-CDs + , Ag 2 SO 4 , and IPA as measured at different visible-light irradiation times; B ) kinetic of the Ag plasmon absorbance band as observed in aqueous solutions under visible-light irradiation; all samples contain Ag 2 SO 4 , IPA, and the compound indicated in the legend.

Figure 7 . 3 :

 73 Figure 7.3: A) absorbance of an aqueous solution of {Co 4 P 4 W 30 }-CDs + , Ag 2 SO 4 , and IPA as measured at different visible-light irradiation times; B ) kinetic of the Ag plasmon absorbance band as observed in aqueous solutions under visible-light irradiation; all samples contain Ag 2 SO 4 , IPA, and the compound indicated in the legend.

Figure 7 . 7 :

 77 Figure 7.7: TEM images of Ag nanoparticles obtained by visible-light irradiation of aqueous solutions containing Ag 2 SO 4 , IPA, and POM-CDs + complexes; used complexes are: A) {P 2 W 18 }-CDs + ; B ) {Zn 4 P 4 W 30 }-CDs + ; C ) {Co 4 P 4 W 30 }-CDs + .

Figure 7 . 8 :Figure 7 . 9 :

 7879 Figure 7.8: A and C ) HRTEM images of Ag nanoparticles obtained by visible-light irradiation of aqueous solutions containing Ag 2 SO 4 , IPA, and {P 2 W 18 }-CDs + ; B and D) FFT respectively calculated from (A) and (C ); indexes of the planes responsible for the observed diffraction peaks are indicated.

  Figure 7.9A shows the ED image displayed by the nanoparticles shown in Figure 7.7A. As many different particles contribute to the measured diffraction, concentric circles formed by the different peaks can be observed in the resulting image. In order to distinguish to which planar distances these peaks correspond, the ED signal was radially integrated. The resulting radial profile is shown in Figure 7.9B, in which each peak is labelled with its corresponding planar distance. As evident by comparing the peaks position with the planar distances for cubic silver (ref. 98-060-4635) -indicated by vertical dashed lines in the figure -it appears that the observed nanoparticles are indeed all silver.

  + ions in the presence of propan-2-ol have shown how the alcohol radical can initiate the silver reduction involving the following complexation steps: 286 ,287(CH 3 ) 2 C • OH + Ag + --→ [Ag(CH 3 ) 2 C • OH] + [Ag(CH 3 ) 2 C • OH] + + Ag + --→ Ag + 2 + (CH 3 ) 2 CO + H +After these first steps, coalescence between Ag + ions and the initial silver cluster can lead to the formation of Ag nanoparticles: 287Ag 0 + Ag + --→ Ag +This mechanism could explain how it is possible for free CDs + to promote the reduction of Ag + ions, granted that they were able to generate (CH 3 ) 2 C • OH alcohol radicals also in the absence of any kind of POM. It was nevertheless not possible to characterise the eventual electron transfer process between carbon dots and IPA, thus leaving this question open.

Figure 7 . 10 :

 710 Figure 7.10: Simplified schematic representation of the proposed mechanism for the reduction of silver ions mediated by POM-CDs + complexes.

Figure A. 2 :

 2 Figure A.2: left) time-resolved intensity decay of a two-component mixture of anthranilic acid and 2-AP; the dashed line shows the single exponential fit while the solid black line the bi-exponential fit; original data is represented by dots; right) residual plots of the bi-exponential fit (above) and of the single exponential fit (below ); reproduced from ref. (227 ).

  mixture of anthranilic acid (AA) and 2-amino-purine (2-AP) alongside the best-fitting curves obtained from a single exponential and a bi-exponential fitting model. While both models seem to properly represent the original data and can hardly be differentiated, from a comparison of the residual plots relative to the two models it appears clear how the bi-exponential model is the one best-representing the data: in fact, it presents a deviation from the original data homogeneously distributed around zero, while the other model visibly oscillates close to this value. In this case, the single exponential model can be readily excluded. These trial-and-error considerations have been observed for all least-squares fitting analysis performed on data relative to this thesis, if the model was not known a priori.

  These, previously indicated only as p, have now been split in two different sets: the global and local parameters, respectively indicated as p G and p j . While values of parameters p G are shared between all k datasets, local parameters p j can instead change. As previously done, this goodness-of-fit parameter can be minimised through the Levenberg-Marquardt algorithm, thus yielding the best values for both global and local parameters. As more datasets are used for this procedure, it is intuitive how resulting obtained values generally have lower uncertainties. Moreover, all considerations on how to evaluate the proper convergence of the fit procedure are still valid. This makes the global analysis a very powerful tool that can be used when several datasets sharing common parameters are available. These equations represent the situation depicted in the left panel of Figure A.3. Here f (t) is the constant function describing excitation due to continuous irradiation, γ is the fluorescence decay rate and as such is equal to the inverse of emission lifetime τ -1 0 , and [F * ] 0 is the concentration of fluorophores in the excited state in the absence of quencher;

Figure A. 4 :

 4 Figure A.4: Schematic representation of the combination of both static and dynamic quenching processes (left) and the Stern-Volmer plot resulting from such a case (right); reproduced from ref. (227 ).

Figure A. 5 .

 5 To understand this result, lets consider the emission resulting from two different fluorophore fractions, indicated by letters A and B, of which only the former is accessible to the quencher. In absence of quenching the total emission least-squares fitted using the function presented in Equation A.25:

25 )

 25 scheme is shown inFigure A.6, were recorded on an intensified charge-coupled device (CCD) camera while exciting the sample via a tunable laser system. The latter consists of an optical parametric oscillator pumped by a Q-switched Nd:YAG laser (5 ns pulses at 10 Hz repetition rate). The energy of each laser pulse was on average equal to 0.1 mJ and the spectral resolution of the setup was equal to 4 nm. Luminescence is generated at 1064 nm before being converted to a wavelength chosen between 410 and 2400 nm via three non-linear crystals.

Figure A. 7 :

 7 Figure A.7: Left) diagram of the typical signals observed in a transient absorption experiments:ground-state bleaching (GSB), caused by the absorption of the sample decreasing after excitation due to the depletion of the ground state; stimulated emission (SE), caused by the interaction between the excited sample and the probe signal, which stimulates photon emission and the consequent decay of the system back to the ground state; excited-state absorption (ESA), related to the possibility of new transitions toward higher energy states; right) the hypothetical TA signal composed of the three contributing signals.

Figure

  Figure A.10: A) the weights for the SVD provided by the eigenvalues matrix; points circled in orange are the only one contributing to the signal; B ) image-plot of the original TA signal of CDs -and C ) the same TA signal reconstructed by using only the eigenvalues indicated in (A).

Figure A. 12 :

 12 Figure A.12: α-K 6 P 2 W 18 O 62 • 14 H 2 O crystals obtained from the synthesis, as observed before drying and storage; single crystals were sized from a few millimeters up to around a centimeter.

  et leur synthèse, et nous avons pu obtenir des clusters d'oxydes métalliques ayant propriétés auparavant inimaginables. Aujourd'hui, avec l'augmentation quotidienne du nombre de recherches sur les polyoxométallates, nous sommes sur la bonne voie vers le développement de dispositifs à base de POM peu coûteux, efficaces et présentant une stabilité étonnante et des propriétés catalytiques inégalées. En ce qui concerne les POMs, nous nous sommes concentrés sur l'étude de quatre espèces différentes, dont le {P 2 W 18 } étant le composé le plus utilisé puisque considéré comme une archétype des structures des polyoxométallates. De plus, des structures sandwich dérivées de ce cluster ont également été utilisées, à savoir {Co 4 P 4 W 30 } et {Zn 4 P 4 W 30 }, pour leurs intéressantes caractéristiques. 'elles sont excitées, conduisent à une intense fluorescence visible. De plus, cette émission est également tunable. Quand on parle de CDs tunables, on se réfère à leur capacité à décaler continuellement la position du pic de la bande d'émission en fonction de la longueur d'onde de la source d'excitation. Cela signifie que la même solution de carbon dots peut émettre une lumière bleue, verte ou jaune en fonction de l'énergie d'excitation. 153 De nombreuses études ont tenté d'expliquer cette caractéristique surprenante, considérant souvent l'hétérogénéité de la surface 154 et du core 155 des CDs. Néanmoins, aujourd'hui, nous n'avons pas encore trouvé d'explication satisfaisante pour les mécanismes impliqué dans la tunabilité de la fluorescence des carbon dots.En conclusion, les carbon dots sont peu coûteux, faciles à synthétiser, non toxiques et processus se puisse produire, l'état du système photoexcité a besoin d'être stable jusqu'à ce que le transfert d'électrons se produise. Avoir un rendement quantique élevé est un bon indicateur de cette caractéristique; en général, pour obtenir de bons rendements d'émission, les matériaux fluorescents doivent présenter de faibles taux de recombinaison non radiative. En conséquence, les carbon dots avec un rendement quantique élevé ont été considérés comme ayant des voies de recombinaison non radiative se produisant avec de faibles probabilités, ce qui se traduit par des processus PET n'ayant pas à entrer en compétition avec ce mécanisme de recombinaison, rendant ainsi le transfert plus efficace.

	Carbon Dots

diffraction des rayons X sur monocristal, nous en savons plus que jamais sur les structures xxi des POMs Les carbon dots (CDs) sont des particules nanométriques carbonées noyau-coquille qui présentent des propriétés optiques très intéressantes, telles qu'une absorption intense et une émission puissante. De nos jours, la nanotechnologie est présente dans tous les aspects de notre vie quotidienne et les nanoparticules ont même trouvé une place dans la technologie des écrans des télévisions modernes. Il n'est donc pas surprenant que les inquiétudes quant à leurs effets possibles sur la santé humaine et la pollution de l'environnement se soient accrues d'année en année. De nombreuses nanoparticules sont en effet toxiques, 150 principalement en raison du type d'éléments dont elles sont composées, et souvent leur synthèse peut être à la fois coûteuse et polluante. Les nanoparticules entièrement carbonées ont le pouvoir de révolutionner complètement cette situation: du point de vue des coûts, les sources de carbone sont présentes partout, rendant ainsi les CDs très peu couteux à produire; 151 de plus, étant le carbone l'élément de la vie, il peut être naturellement géré par les organismes afin que les carbon dots ne représentent aucune menace pour l'homme et l'environnement, comme montré par certaines études. 152 En général, les CDs sont composés d'un noyau de carbone à peu près sphérique dont le diamètre est souvent inférieur à 10 nm; à leur surface, on trouve une grande variété de groupements organiques, comprenant souvent des éléments tels que C, O, H et N. Les caractéristiques les plus importantes des CDs se trouvent dans leurs propriétés optiques. Comme dit précédemment, ces nanomatériaux présentent des bandes d'absorption intenses qui, lorsqubiocompatibles; comme en témoignent de nombreuses études, ils sont capables d'interagir fortement à la fois avec leur environnement 156 et d'autres espèces présentes dans leurs alentours. 157 En moins de vingt ans depuis leur découverte, les CDs ont prouvé d'être très prometteur pour de nombreuses applications différentes. Leur efficacité en tant que capteurs chimiques, 158 biomarqueurs, 152 vecteurs thérapeutiques, 159 photosensibilisateurs, 160 et photocatalysts 161 a déjà été étudiée à plusieurs occasions. Malgré leur potentiel élevé, nous manquons encore d'une compréhension complète de leurs mécanismes internes, en particulier par rapport à leurs propriétés optiques, et nous essayons toujours d'optimiser et d'améliorer leur structure et leurs caractéristiques. Les nombreuses questions encore ouvertes sur les carbon dots n'ont fait qu'augmenter l'intérêt autour de cette famille de nanoparticules; trouver les bonnes réponses pourrait ouvrir de nouvelles possibilités dans le domaine des nanotechnologies à base de carbone. Afin de trouver la meilleur structure de CDs au fin d'interagir avec les POMs, une recherche bibliographique a été effectuée. Parmi les nombreuses nanoparticules de carbone qui pourraient être choisies, nous avons cherché des CDs caractérisés par la facilité de synthèse, des rendements quantiques élevés et présentant une abondance de groupes contenant de l'azote sur leur surface. S'il est clair qu'une synthèse rapide et directe est considérée comme extrêmement avantageuse pour la disponibilité du produit, les autres caractéristiques ont été choisies uniquement en tenant compte du couplage avec les polyoxométallates. Pour comprendre pourquoi, nous devons considérer le mécanisme responsable du transfert d'électrons des CDs vers d'autres espèces, qui est le transfert d'électrons photoinduit (PET). 157 Pour xxiii que ce Si cette hypothèse explique les premières propriétés recherchées, la seconde peut également être expliquée en considérant le transfert d'électrons photoinduit comme le mécanisme responsable de l'interaction entre les deux espèces. En fait, pour que ce mécanisme se produise, un contact moléculaire entre le fluorophore et l'extincteur est nécessaire. 227 En tant que tel, il faut trouver comment rassembler les CDs et les POMs ensemble. Comme les polyoxométallates sont des espèces polyanioniques, afin d'obtenir le contact moléculaire nécessaire ils pourraient être facilement couplés à des matériaux chargés positivement par des interactions électrostatiques. Comme la charge des carbon dots dépend de l'état de protonation de ses groupements de surface, des nanoparticules présentant des amines à leur surface ont été recherchées. En fait, ces groupements organiques sont généralement protonés à faible pH, une caractéristique très pratique pour nos besoins. Par conséquent, une teneur élevée en azote était la deuxième propriété sur laquelle le choix de CDs était basé. Au final, des graphene quantum dots (GQDs) émettant du cyan synthétisé par un traitement hydrothermal en autoclave à partir du tri-nitropyrène ont été choisis. Ces carbon dots présentent une surface chargée positivement dans des conditions de pH appropriées.

Ainsi, en fonction de leur état de surface, deux échantillons différents ont été étudiés, à savoir CDs + et CDs -. Avoir une surface positive est extrêmement attrayant pour nos besoins, car l'attraction mutuelle résultant de l'interaction électrostatique avec les POMs anioniques pourrait permettre la formation de complexes POM-CDs.

  But du TravailL'objectif de ce travail de thèse est d'obtenir des complexes POM-CDs et d'étudier leurs propriétés et leur interaction. Pouvoir contrôler un tel complexe est considéré comme très attractif du point de vue des nanodispositifs possibles qui en seraient basés. Coupler polyoxométallate et carbon dots signifierait pouvoir exploiter les avantages des deux matériaux, en particulier compte tenu des capacités catalytiques des POMs 13 et des propriétés optiques des CDs.185 De plus, les deux matériaux présentent une faible toxicité,84 ,152 une propriété supplémentaire dont le transfert applicatif bénéficierait grandement. Pour comprendre pourquoi nous considérons un tel objectif comme très intéressant, il est probablement préférable de se concentrer sur les limitations affichées par les deux espèces seules. même si les POMs présentent certaines des propriétés catalytiques les plus attrayantes, ces structures supermoléculaires ont certaines limitations dans le transport de charge et de masse; 75 cela est particulièrement vrai lorsqu'elles sont utilisées comme partie constitutive de nanodispositifs. En fait, lorsqu'ils sont utilisée pour revêtir un électrode à des fins de catalyse hétérogène, les chercheurs ont observé comment l'efficacité résultante était limitée. Très souvent, l'ajout de couches de POMs supplémentaires au revêtement n'améliorait pas les performances comme prévu, indiquant une limitation de la mobilité des charges à l'intérieur du matériau. 138 À cet égard, les CDs pourrait très bien être en mesure de fournir une solution: quand étudiant le rôle de ces nanoparticules, en particulier pour la variété de carbon dots nommée GQDs, les chercheurs ont observé La principale voie d'obtention des hybrides POM-CDs suivie au cours de cette recherche est basée sur le couplage électrostatique. Comme expliqué précédemment, les polyoxométallates sont des anions, présentant souvent des charges négatives élevées en solution. Même si la plupart des CDs décrits dans la littérature présentent également une surface négative, ce qui entraverait évidemment tout couplage électrostatique aux POMs présentant la même charge, en choisissant soigneusement la voie de synthèse et en travaillant à pH opportun, des carbon dots chargés positivement peuvent néanmoins être obtenus. En fait, alors que la surface négative de ces nanoparticules est souvent due à la présence de fractions carboxylate déprotonées, une charge positive peut apparaître dans des conditions doctoral, à notre connaissance, seulement trois articles supplémentaires sur le thème des complexes POM-CDs ont été publiés par des groupes externes. En 2018, l'auto-assemblage entre POM à base de cobalt et CDs dérivés d'alginate obtenues par une synthèse micro-onde a été rapportée; ce travail a pu montrer des capacités de water splitting très prometteuses basées sur le transfert d'électrons entre les nanoparticules de carbone, agissant comme photosensibilisateurs, et [Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 10 -, travaillant comme catalyseur vers la réaction de dégagement d'oxygène. 274 L'année suivante, une autre recherche s'est plutôt concentrée sur le développement d'un supercondensateur à base de polyaniline (PANI) et incorporant de l'acide phosphomolybdique et des CDs. 275 Dans ce travail, le [PMo 12 O 40 ] 3 -était adsorbé sur la surface été utilisés à côté d'autres composants, de sorte que leur rôle pourrait sembler peu clair dans certains cas. Il reste encore beaucoup à comprendre sur les interactions résultant du couplage de ces deux espèces. Ce travail de thèse vise à mettre en lumière les propriétés de ces matériaux innovants, en essayant d'élucider leur couplage et les mécanismes sous-jacents à leurs caractéristiques surprenantes. Actuellement, nanodispositifs photosensibles non toxiques avec de bonnes propriétés électroniques sont très recherchés en raison de l'énorme quantité de possibilités qu'ils pourraient couvrir, notamment dans le domaine des énergies vertes et renouvelables. les POMs et les CDs s'inscrivent tous deux dans cette description et ont des propriétés complémentaires qui, lorsqu'elles sont couplées ensemble, peuvent être encore améliorées. En nous concentrant sur cette possibilité, nous Ceci a ensuite été confirmé en étudiant le quenching de la fluorescence de CDs + à différentes concentrations de {P 2 W 18 } par analyse de Stern-Volmer. Enfin, la possibilité de rencontres dynamiques a été exclue en effectuant des mesures de photoluminescence résolues en temps. De plus, l'interaction a également été étudiée pour les CDs chargé négativement. Les CDs -se sont ainsi montrés capables d'interagir avec le {P 2 W 18 }; encore une fois, la caractérisation optique a pu dévoiler une interaction entre les deux espèces, même si les mêmes rendements affichés par CDs + n'ont pas pu être atteints. En particulier, nous avons pu assister à ce que nous avons conclu être le même mécanisme d'interaction précédemment observé dans le cas des CDs -aussi. Nous pourrions ainsi affirmer que cette espèce est par conséquent capable de former des complexes {P 2 W 18 }-CDs -et, encore une fois, leurs composants sont capables d'interagir par un transfert d'électrons photoinduit ultra-rapide. Le polyoxométallate {P 2 W 18 } pouvant être considéré comme un modèle pour les hétéropolyanions, ces premiers résultats pointent vers une interaction efficace entre CDs et polyoxométallates. Bien qu'il s'agisse d'une structure d'archétype, il existe d'autres POMs présentant des propriétés plus intéressantes, souvent considérées comme très attractives pour un transfert applicatif. En particulier, les TMSPs présentent des propriétés remarquables en tant que catalyseurs. Il apparaît ainsi clairement comment la photosensibilisation de ce type de structures avec des carbon dots pourrait aboutir à des dispositifs hybrides photocatalytiques très intéressants, si le transfert d'électrons des CDs vers les TMSPs devait être prouvé possible. Ainsi, l'interaction entre CDs + et deux structures POMs différentes a été étudiée. Comme conséquence naturelle des résultats précédemment décrits, nous avons procédé à des tests pour savoir si le transfert d'électrons ultra-rapide entre POMs et CDs pouvait être exploité pour des applications photocatalytiques. Pour ce faire, la formation de nanoparticules d'argent assistée par la lumière et médiée par les complexes POM-CDs + a été étudiée. Cette approche a été choisie comme banc d'essai pour nos complexes, sur la base de rapports de littérature sur polyoxométallates photosensibilisés capables d'aider à la réduction des ions Ag + et à la conséquente formation de clusters d'argent sous irradiation.

	Afin d'étudier l'interaction et l'éventuel transfert d'électrons entre les deux espèces formant
	le complexe, des mesures d'absorption transitoire au femtoseconde (TA) ont été réalisées.

Le point le plus important concernant l'utilisation des carbon dots aux côtés des polyoxométallates provient probablement de leur section efficace d'absorption. Bien que les POMs affichent des résultats très prometteurs lorsqu'ils sont étudiés comme électrocatalyseurs, le fait que ces anions ne soient pas photoactifs lorsqu'ils sont excités par la lumière visible empêche leur utilisation possible dans le vaste domaine de la photocatalyse verte. En fait, aujourd'hui, la communauté de recherche se concentre énormément sur le développement des feuilles artificielles; ce ne sont rien d'autre que des dispositifs artificiels capables de collecter de l'énergie comme le font les feuilles naturelles, bien que par des mécanismes différents que la photosynthèse, afin de fournir un carburant utilisable directement.

272

Comme les plantes sont capables de convertir l'énergie solaire et de la stocker dans des molécules telles que les glucides, l'équivalent artificiel devrait pouvoir utiliser le rayonnement solaire à des fins de conversion et de stockage d'énergie. D'un point de vue pratique, ce type de dispositifs devrait généralement juste pouvoir fournir un moyen d'effectuer le water splitting grâce à l'utilisation de l'énergie solaire, produisant ainsi O 2 et H 2 . Dans le cas des feuilles artificielles à base de complexes POM-CDs, les carbon dots auraient le rôle de photosensibilisants, donc d'absorber la partie du rayonnement visible du spectre solaire avant de transférer les électrons photoexcités aux polyoxométallates; les POMs effectueraient alors le water splitting. Ce mécanisme permettrait de surmonter la limitée absorbance des polyoxométallates en utilisant simplement les CDs dans le rôle d'élément photoactif. Comme les CDs ont montré des capacités de transfert d'électrons remarquables vers d'autres espèces, 157 cette voie est prévue être très efficace. xxv De plus, comment leur présence permettait d'éviter la recombinaison de charge en fournissant des pièges à électrons à leur surface qui facilitaient la séparation des charges et amélioraient ainsi l'efficacité des dispositifs étudiés. 160 ,256 Alors que ces exemples ont été appliqués dans le domaine de l'optoélectronique, le transfert de ces résultats lors de l'utilisation des CDs comme photosensibilisateurs pourrait grandement améliorer les performances des catalyseurs en interagissant et, dans le cas des polyoxométallates, aider à réduire la recombinaison de charge tout en améliorant les propriétés de transport. En dehors de ces raisons directement liées aux bénéfices applicatifs des complexes POM-CDs, il y a aussi des motivations fondamentales qui alimentent notre intérêt à coupler ces deux matériaux. Il faut en effet considérer que nous manquons encore d'une compréhension complète des mécanismes internes permettant les remarquables propriétés des deux espèces. Cela est vrai pour les polyoxométallates puisque, selon certaines reviews, 75 la compréhension de la photophysique des systèmes de POM-sensibilisateur est encore incomplète et en retard par rapport, par exemple, à celle des celles solaires sensibilisées par colorants organiques. Un raisonnement similaire peut également être appliqué aux CDs qui présentent encore plusieurs questions sans réponse, à la fois liées à leur capacité à interagir avec d'autres espèces et à leurs propriétés optiques. Bien que nous assistons déjà à de nombreux progrès à cet égard pour les deux matériaux, une étude de leur interaction pourrait aider davantage la discussion en cours sur le sujet. De plus, une comparaison entre la manière dont différentes structures de POMs interagissent avec les CDs pourrait nous aider à mieux comprendre le rôle des architectures existantes et de l'éventuelle présence d'hétéroatomes sur les propriétés catalytiques et sur les mécanismes de transfert de charge dans les polyoxométallates. de pH faible: cela se produit lorsque les groupes -COOH sont ramenés à leur état neutre tandis que les amines éventuelles également trouvés dans la coquille organique deviennent protonés. Cela signifie que carbon dots riches en N, qui affichent souvent des amines sur leur surface, conviendraient parfaitement au couplage électrostatique avec les POMs. Par accident, une teneur élevée en azote est en plus bénéfique pour les propriétés optiques des CDs, comme le montrent plusieurs études de la littérature. 195 Cela signifie qu'un couplage électrostatique entre POMs et CDs, en plus d'être parfaitement faisable, bénéficierait également d'un sensibilisateur présentant une bonne section efficace d'absorbance de la lumière visible et donc une sensibilité élevée à l'irradiation solaire. À notre connaissance, avant le début de cette recherche, un seul article publié en 2014 explorait la possibilité de réunir des nanoparticules de carbone et des polyoxométallates. 273 En particulier, cet article explore les capacités d'un nanodispositif composé par le polyoxométallate Ag 3 PW 12 O 40 , des nanoparticules d'Ag, et des quantum dots de carbone obtenus par voie électrochimique. Notamment, ici les CDs n'ont aucun rôle de photosensibilisateurs. En fait, comme les auteurs eux-mêmes le rapportent, l'absorption de la lumière visible se produit à la surface de la nanoparticule d'Ag par résonance plasmonique de surface. On suppose plutôt que le rôle des CDs se trouve dans deux domaines principaux: premièrement, grâce à leur insolubilité, ils ont un effet de stabilisation sur le reste du complexe, évitant spécifiquement la dissolution du Ag 3 PW 12 O 40 en solution aqueuse; de plus, il est indiqué que les CDs agissent comme intermédiaire dans le transfert d'électrons entre les nanoparticules d'Ag et les POMs, en aidant à éviter la recombinaison de charge après l'absorption. xxvii Pendant le travail des carbon dots par chimisorption avant de effectuer directement la croissance de PANI sur les hybrides POM-CDs par polymérisation électrochimique de monomères d'aniline. Ce complexe ternaire a été ensuite utilisé comme électrode pour construire un supercondensateur. Le dispositif résultant a montré une capacité spécifique remarquable, une capacité de débit raisonnable et une stabilité de cyclage élevée grâce à la conductivité électrique des CDs permettant un transfert de charge rapide et l'acide phoshomolybdique agissant comme pont entre la chaîne de polyaniline et les nanoparticules de carbone. Puisque ce supercondensateur pourrait alimenter directement certains composants électroniques, comme mentionné par les auteurs de l'article, ces résultats indiquent de nouvelles applications pratiques auparavant imprévues dans lesquelles l'interaction entre POMs et CDs pourrait trouver un rôle. Enfin, en 2020, les chercheurs ont utilisé des carbon dots et le polyoxométallate [Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 10 -comme blocs de construction pour construire un électrocatalyseur à base de Pt pour la réaction d'oxydation du méthanol. 276 Puisque de nos jours les piles à combustible au méthanol sont encore lentes même lorsqu'elles utilisent des catalyseurs à base de platine, les chercheurs se sont concentrés sur l'utilisation des CDs et des POMs pour augmenter leurs performances. L'efficacité du dispositif complet, caractérisée par la densité de courant de pic d'oxydation directe observée lors de la réaction du méthanol, s'est avérée supérieure à la fois à celle d'une électrode basée uniquement sur Pt et à celle des électrodes manquant soit les CDs ou les POMs. Ces articles prouvent à quel point les hybrides POM-CDs sont prometteurs, en particulier pour des applications diverses. Comme indiqué, les polyoxométallates et les carbon dots ont néanmoins avons exploré les caractéristiques fondamentales des deux matériaux, les mécanismes sous-jacents permettant le transfert d'électrons observé et les applications possibles qui pourraient découler des développements ultérieurs très attendus des hybrides POM-CDs. Complexes POM-CDs La formation spontanée de complexes {P 2 W 18 }-CDs est obtenue simplement par addition de {P 2 W 18 } à une solution de CDs. Le résultat principal qui a été observé concerne l'interaction extrêmement puissante qui se produit entre CDs + et {P 2 W 18 } en solution. Les résultats obtenus par plusieurs techniques spectroscopiques nous ont permis de conclure comment ces deux espèces sont capables de former spontanément des complexes {P 2 W 18 }-CDs + qui interagissent ensuite par un mécanisme de transfert d'électrons photoinduit. En particulier, des mesures d'absorption à l'état stationnaire ont montré comment les bandes des CDs étaient influencées par la présence des POMs, signe de formation d'un complexe statique. L'observation et l'analyse des résultats nous ont permis de conclure que l'interaction sous-jacente se déroule par une cinétique ultrarapide aux sous-picosecondes; un tel résultat est extrêmement prometteur si l'on considère un transfert applicatif vers des applications xxix photocatalytiques. Plus important encore, des informations supplémentaires sur le type d'interaction permettant la formation de {P 2 W 18 }-CDs ont été dévoilées; en remarquant comment le mécanisme de transfert d'électrons résultant n'a pas changé entre CDs + et CDs -, il a été possible de conclure que le même mécanisme doit être impliqué dans la formation des deux différents complexes. En tant que tel, nous avons pu conclure que les interactions électrostatiques ne sont que des forces motrices capables de réunir {P 2 W 18 } et CDs + tout en gênant le même processus pour CDs -. Néanmoins, les CDs -/+ rencontrant le {P 2 W 18 } par diffusion sont dans les deux cas capables de former des complexes hybrides qui interagissent à nouveau par un mécanisme PET. Nous émettons l'hypothèse de formation de ponts H entre les groupements oxo présents dans la structure des polyoxométallates et les atomes d'hydrogène trouvés dans les fragments organiques de CDs; néanmoins, aucune preuve concluante sur le type d'interaction gardant ensemble les complexes statiques n'a été trouvée. Comme toujours, d'autres études sont nécessaires pour dévoiler de manière concluante les questions encore ouvertes concernant l'interaction étudiée; néanmoins, les résultats précédents ont prouvé l'efficacité des complexes {P 2 W 18 }-CDs + . Ceux-ci sont capables d'interagir à travers un mécanisme se déroulant en moins de 110 fs, un processus extrêmement intéressant en raison de la vitesse à laquelle il se produit. Un tel résultat, bien que surprenant, n'est pas complètement inconnu dans la littérature et est réputé pour se produire dans des autres systèmes, différents des POMs et des CDs. 280 En outre, études dans la littérature sur l'interaction de POMs photosensibilisés ont déjà montré un raccourcissement de la durée de vie des signaux TA pour le photosensibilisateur, lorsque ce dernier est couplé à des polyoxométallates. 281 Les carbon dots chargés positivement ont été sélectionnés en considérant les précédents résultats, qui ont prouvé qu'ils étaient plus fiables pour l'interaction {P 2 W 18 }-CDs que les CDs -. Les deux TMSPs utilisés pour ces études sont le {Zn 4 P 4 W 30 } et le {Co 4 P 4 W 30 }. En plus de prouver comment la photointeraction entre les deux espèces est possible, nous avons aussi essayé de trouver toutes les différences qui pourrait survenir par rapport aux complexes {P 2 W 18 }-CDs + , afin d'élucider davantage notre compréhension de ce système et des complexes POM-CDs en général. Grâce aux résultats obtenus, basés à nouveau sur les mêmes techniques spectroscopiques, on a découvert comment l'interaction entre les CDs + et les POMs n'était pas uniquement circonscrite au cas du {P 2 W 18 }. Au contraire, le même mécanisme pourrait être dévoilé à la fois pour le {Zn 4 P 4 W 30 } et le {Co 4 P 4 W 30 }; de plus, l'interaction entre CDs + et ces TMSPs s'est avérée encore plus efficace qu'avant, en particulier dans le cas des POMs contenant du Zn. Néanmoins, d'autres mécanismes ont été observés lors de l'examen de {Co 4 P 4 W 30 }, preuve qui a compliqué la compréhension des résultats. Malgré ces différences, le type d'analyse qui a été réalisée sur toutes les données d'absorption transitoire ultra-rapide, conduite par décomposition SVD et étude des DAS, a permis de conclure que les mêmes cinétiques de relaxation observées pour les CDs + libres sont toujours présentes; nous avons ainsi pu affirmer qu'un transfert d'électrons sub-picoseconde se produit à la suite de l'interaction photo-induite se déroulant dans les complexes POM-CDs + . Un tel résultat est extrêmement positif lorsque l'on considère un éventuel transfert vers des applications, notamment dans le domaine des réactions de réduction photocatalytiques. xxxi Photocatalyse d'Ions d'Argent 94 Initialement, le processus a été suivi en observant l'absorption de la bande plasmonique de l'argent, croissant en fonction du temps d'irradiation. L'absorbance d'une solution placée sous lumière visible a été régulièrement mesurée sur un spectrophotomètre. Les complexes formés par les CDs + avec le {P 2 W 18 }, le {Zn 4 P 4 W 30 } ou le {Co 4 P 4 W 30 } ont été étudiés, car tous ont précédemment été trouvés capables d'interagir avec les nanoparticules à base de carbone. Le Ag 2 SO 4 était présent en solution afin de fournir les ions Ag + à réduire. Enfin, le IPA a été utilisé comme donneur d'électrons sacrificiel. La présence de nanoparticules d'Ag a ensuite été confirmée par des mesures TEM et de ED, afin d'étudier leur forme, taille et structure. Les résultats obtenus ont montré comment les complexes POM-CDs + sont des dispositifs hybrides polyvalents capables d'un transfert applicatif. Grâce à la photo-réduction des ions argent, nous avons pu démontrer comment les CDs + peuvent agir comme relais d'électrons pour améliorer les performances catalytiques des polyoxométallates. Ce type de preuve montre comment nous avons réussi à photosensibiliser les POMs par des CDs, résultat précédemment considéré comme but de ce travail de thèse. Être capable de photocatalyser les réactions de réduction nous laisse très optimiste quand on considère le large éventail de possibilités à la portée des appareils que nous avons développés. En fait, toutes les expériences ont été menées uniquement en utilisant la lumière visible: dans ces conditions, il a été possible de s'assurer que les complexes {P 2 W 18 }-CDs peuvent être utilisés pour processus photocatalytiques à énergie solaire, résultat idéal lorsque l'on considère la nécessité des énergies renouvelables.

  ConclusionA travers les résultats expérimentaux obtenus au cours de la recherche doctorale et présentés dans le présent travail de thèse, nous avons réussi à montrer comment les polyoxométallates sont capables de forment spontanément des complexes statiques avec les carbon dots.

	Être capable d'obtenir ces résultats ne peut néanmoins être considéré comme simple; Chapter 1
	en fait, plusieurs étapes visant à concevoir soigneusement le nanodispositif final et chacun
	de ses composants se sont avérées nécessaires pour atteindre l'objectif préfixé. Peu de
	nanoparticules de carbone possèdent des caractéristiques de surface affichant une charge Polyoxometalates
	positive lorsqu'elles sont dispersées en solution, caractéristique révélé idéal pour la formation
	Part I
	State of the Art

In the field of inorganic chemistry, polyoxometalates (POMs) are renown for their versatile topology and unique electronic properties. POMs are inorganic metal-oxide cluster polyanions whose fundamental component is a MO x unit, where M is a transition metal -usually molybdenum, tungsten or vanadium -in its d 0 or d 1 oxidation state. These subunits, which rearrange by sharing oxygen atoms, form structures of different shapes and sizes. Herein they often incorporate additional heteroatoms, albeit this does not always happen, which can virtually consist in any element from the periodic table; in fact, more than seventy different elements have fulfilled this role, 1 with representatives from all groups of the periodic table with the exception of rare gases.

[START_REF] Pope | Heteropoly and Isopoly Oxometalates[END_REF]

This extreme variability in their geometry and composition, basically unchallenged by any other compound, allows

  12 O 40 ] 3 -, studied byKeggin in 1933; 10 due to the importance of his report, finally able to shine light on the yet unanswered question regarding POMs structures, all metal oxide clusters sharing the same geometry are named after him. Fascinatingly, also the first heteropolyanion to ever be reported, 5 [PMo 12 O 40 ] 3 -, shares the same structure.

	Keggin POMs are heteropolyanions with general formula [XM 12 O 40 ] n -and are among
	the most stable of the metal oxide clusters families; as such, the literature is full of reports
	focusing on this kind of polyoxometalates. The heteroatom composing this geometry is
	found in the middle of the Keggin structure, linked to four oxygen atoms in a tetrahedral
	configuration; around it are symmetrically placed the twelve octahedral {MO 6 } units
	containing the addenda atoms. These subunits can be considered as grouped into four

In 1945 Wells proposed a structure 40 for [P 2 W 18 O 62 ] 6 -, a heteropolyanion whose synthesis had been previously reported in 1915. 41 Nonetheless, it was not until 1953 that Dawson was finally able to resolve this very POM structure via X-ray analysis, 42 confirming Wells predictions

  .7. While the α isomer is the most common and corresponds to that firstly characterised by Keggin in 1933, the β one was reported just in 197536 ; the other less stable isomers were isolated only recently, the γ in 200137 , the ε in 200238 and the δ just in 2015.39 , hence the sharing of both name to describe this geometry.The Wells-Dawson polyoxometalate structure is described by the general formula [X 2 M 18 O 62 ] n -. In the centre, the heteroatoms form two tetrahedral {XO 4 } units each coordinating with a single {M 3 O 13 } triad and a {M 6 O 14 } group, respectively acting as

	1.4.4 Wells-Dawson

and several different transition metals is well known and has been extensively studied in the literature;

	polyoxometalates, dimers whose name derives from the resulting final structure. While
	the same kind of geometry has resulted even if starting from different POMs architectures,
	in the following only the Wells-Dawson variety will be presented, focusing in particular on
	those derived from α-[P 2 W 18 O 62 ] 6 -.
	In this case, four metal atoms are sandwiched between two trivacant α-[P 2 W 15 O 56 ] 12 -
	lacunary subunits which have are obtained by removing one of the two {M 3 O 13 } cap triads
	composing the POM used as precursor. The reaction between these lacunary subunits

53 ,57 -59 these types of TMSPs are generally described by the formula [M 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] n -, where M can be one of Mn II , Fe III , Co II , Ni II , Cu II , Zn II or Cd II . 53 Figure

1.14: Reaction scheme of the synthetic route of [M 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] n -; treatment of α-[P 2 W 18 O 62 ] 6 -with Na 2 CO 3 yields α-[P 2 W 15 O 56 ]

  12

  2 W 18 } and β-{P 2 W 18 }. Other than the capability of reversibly accepting electrons, POMs are also able to work as electron-reservoirs. For instance, the Keggin structure polyoxometalate H 4 [SiW 12 O 40 H 6 ]

	Figure 1.17: Voltammograms of α-{P 2 W 18 }
	and β-{P 2 W 18 } 2 × 10 -4 m solutions (0.5 m
	H 2 SO 4 , pH = 0.33); four reversible redox peaks
	can be observed for both polyoxometalate iso-
	mers; reproduced from ref.

  Bu 4 N) 2n [Ag 2 Mo 8 O 26 ] n ; 114 in this POM-based polymer, n Bu 4 N + cations are found to envelop the linear chain. Moreover, by changing solvent other different architectures can be obtained: for example, when using dimethylsulfoxide, the solvent molecules themselves get incorporated inside the structure, yielding an architecture described by (n Bu 4 N) 2n [Ag 2 Mo 8 O 26 (dmso) 2 ] n , which arranges itself in a grid-like pattern by crossing the polymeric chains one on top of each other. This silver-tungsted hybrid POM system is not the only example of bidimensional structures based on polyoxometalates. When using [Ln(H 2 O) 6 ] 3+ cations -with Ln = La 3+ , Sm 3+ , or Ce 3+ -POM clusters were able to coordinate with six of these linkers forming two-dimensional layers of formula [{Ln(H 2 O) 6 } 2 As 8 V 14 O 42 (SO 3 )].

4 N) 2 [Mo 6 O 19 ] entity. This complex, when reacted with silver(I) fluoride in methanol, gave rise to one-dimensional chain structures Chapter 1: Polyoxometalates described by the brute formula (n 115 As evidenced by the previous examples in which cationic linkers were always used to help the structure formation, architectures purely based on polyoxometalates are somewhat rare in the literature. Nonetheless, examples of extended three-dimensional frameworks solely based on POMs can be found and often display interesting properties; for instance, the Kegging POM based framework described by [(C 4 H 10 NO) 40 (W 72 Mn 12 O 268 Si 7 )] n forms a three-dimensional architecture in which POM building blocks are linked together directly via W -O -Mn III coordination bonds. This was obtained by reacting the [γ-SiW 10 O 36 ] 8 - divacant lacunary POM with manganese(II) in the presence of C 4 H 10 NO + and potassium permanganate, which lets the POM subunits crystallise by alternating 3-connected and 4-connected Keggin clusters into an infinite 3D framework. 116 This structure, moreover, was found to incorporate active sites capable of undergoing reversible redox processes without sacrificing stability.

  Ag 3 PW 12 O 40 in aqueous solution; moreover, it is stated that CDs act as electron transfer intermediate between Ag nanoparticles and POMs and help avoiding charge recombination after absorption. Nonetheless, this last point cannot be confirmed as no control experiment of the photocatalytic properties of the complex in the absence of carbon dots is provided, (H 2 O) 2 (PW 9 O 34 ) 2 ] 10 -, working as catalyst towards the oxygen evolution reaction. Lastly, in 2020 researchers used carbon dots and [Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 10 -sandwich CDs composite electrode was obtained. Finally, Pt nanoparticles were deposited by cycling the potential of the as-prepared device between -0.75 and +0.25 V (again vs. Ag/AgCl) in a 0.1 m HCl solution to which 5 mm H 2 PtCl 6 • 6 H 2 O was

	thus rendering impossible a comparison of the nanocomposite efficiency after addition of
	CDs.
	During the doctoral work, to our knowledge, only three additional articles on the topic of
	POM-CDs complexes have been published from external groups. In 2018 the self-assembly
	between a cobalt-based POM and alginate-derived CDs obtained via microwave synthesis
	was reported; this work was able to show very promising water splitting capabilities based
	on the electron transfer between carbon nanoparticles, acting as photosensitisers, and
	[Co 4 polyoxometalate as building blocks to construct a Pt-based electrocatalyst for the methanol
	oxidation reaction. 276 Since nowadays direct methanol fuel cells are still slow even when
	employing platinum based catalysts, researchers focused on using CDs and POMs to increase
	their performances. To do so, a methanol solution of carbon dots was deposited on the

274

The very next year, another research focused instead on developing a supercapacitor based on polyaniline (PANI) and incorporating phosphomolybdic acid and CDs.

275

In this work, [PMo 12 O 40 ] 3 -was adsorbed onto the carbon dots surface via chemisorption before directly growing PANI on the POM-CDs hybrids through electrochemical polymerisation of aniline monomers. This ternary complex was then used as electrode to build a supercapacitor in which sulfuric acid-polyvinyl alcohol was used as gel electrolyte. The resulting device displayed a remarkable specific capacitance, reasonable rate capability, and high cycling stability thanks to the electric conductivity of CDs allowing for fast charge transfer and the phoshomolybdic acid acting as bridge between the polyanilin chain and the carbon nanoparticles. Since this supercapacitor could directly power some electronics, as mentioned by the paper authors, these results indicate new practical applications previously unforeseen in which the interplay between POMs and CDs could find a role. surface of a conductive substrate. After drying, by applying for 1 min a potential of -1.0 V (vs. Ag/AgCl reference electrode) to an electrochemical cell in which the CDs-modified substrate was used as working electrode and a 2 mm [Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 10 -solution as electrolyte, the POM-

  Dawson derived sandwich POMs which have been studied are [Co 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 16 -and [Zn 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 16 - (Figure 1.14); these are respectively abbreviated as {Co 4 P 4 W 30 } and {Zn 4 P 4 W 30 }. While the Co containing compound has been synthesised during the work of thesis following a literature procedure 277 detailed in Section A.6.1, the Zn sandwich POM has been provided by collaborators from the University of Strasbourg (Prof. Laurent Ruhlmann). The 31 P NMR signature of the obtained {Co 4 P 4 W 30 } compound is shown in Figure 4.4. In order to investigate eventual optical properties of these two polyoxometalates, absorbance of 50 µm aqueous solution of the two compounds were obtained. As shown in Figure 4.5, while {Zn 4 P 4 W 30 } is transparent to visible light, {Co 4 P 4 W 30 } presents an absorbance tail covering wavelengths at λ > 400 nm. [Co 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 16 -31 P NMR signature. Absorbance of 50 µm aqueous solutions of [Co 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 16 -and [Zn 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 16 -.

	6.74	Absorbance	0.4 0.6 0.8 1.0	{Co 4 P 4 W 30 } {Zn 4 P 4 W 30 }
			0.2
	0 Figure 4.4: 200 2 4 6 8 10 12 (ppm) 0.0 Figure 4.5:	300 Wavelength (nm) 400 500	600

Table 4 . 1 :

 41 Vibrational peaks of the CDs -and CDs + FTIR spectra shown in Figure

Table 4 . 2

 42

: Quantum yields of the carbon dots emission excited at 400 and 450 nm; values have been calculated using a reference Fluorescein sample (aqueous solution at pH 13, QY = 95%).

Table 4 . 3 :

 43 Time constant associated to the decay traces of CDs -and CDs + at the indicated excitation wavelength, as obtained by global least-squares fitting procedure; the bi-exponential decay seems to best represent the data behaviour.

		CDs -		CDs +
	λ exc (nm)	τ 1 (ns)	τ 2 (ns)	τ 1 (ns)	τ 2 (ns)
	410	8.9 ± 0.6 6.37 ± 0.10	6.66 ± 0.04 3.78 ± 0.08
	450	7.49 ± 0.12 4.77 ± 0.18	6.27 ± 0.05 2.07 ± 0.14

  Transient absorption spectra of A) CDs -and B ) CDs + at different delays from excitation (λ exc = 400 nm); shown spectra have been averaged on a 10% time interval of the indicated delay; the recognised GSB, SE, and ESA signals are respectively marked by blue, green, and red arrows.

	GSB ESA Delay (ps) 0.27 0.84 2.9 8.8 40 113	TA Signal (mOD)	0 1 2	GSB Delay (ps) ESA 0.27 0.84 2.9 8.8 40 113
	SE	ESA			SE	ESA
				450	500 Wavelength (nm) 550 600	650
	Figure 4.20:

Table 5 . 4 :

 54 Quenching parameters obtained from the least squares fit of the Stern-Volmer plots shown in Figure5.13; parameters K i are the quenching coefficients while f represents the accessibility factor relative to the process described by K 1 .to rule out the possibility of collisional quenching. Resulting decay traces and the relative best-fitting curves are shown in Figure5.14; as previously observed when studying CDs + , trace data presented on a semi-logarithmic plot appear to decay in a parallel manner at all {P 2 W 18 } concentration. Fitting curves have indeed been obtained via a global-least squares minimisation procedure involving TR data obtain before and after {P 2 W 18 } addition; these results thus assure that the CDs -emission kinetic proceeds with the same lifetimes reported in Table4.3, regardless of POM presence. As a consequence, the underlying quenching W 18 } presence; data obtained at λ exc = 450 nm, λ em = 480 nm.

	1 )

  6.1 Interaction with {Zn 4 P 4 W 30 }The interaction between {Zn 4 P 4 W 30 } and CDs + has been studied via spectroscopic techniques. In particular, the optical characterisation was initially performed by observing the effect of {Zn 4 P 4 W 30 } presence in solution on CDs + absorbance. As shown in Figure6.1, upon addition of POMs we can once more witness a redshift of the nanoparticles absorbance above 400 nm. As it is yet again possible to exclude contributions from the {Zn 4 P 4 W 30 } absorbance band, stopping well below 400 nm, this result seem to indicate formation of hybrid {Zn 4 P 4 W 30 }-CDs + static complexes. In this case, the close proximity of anions to CDs + would thus affect the energy states responsible for the observed absorbance band resulting in its redshift.Characterisation was then continued by observing the photoemission intensity of CDs + when {Zn 4 P 4 W 30 } was present in solution at different concentrations. Resulting spectra are shown in Figure6.2; as previously witnessed for {P 2 W 18 }, the emission is quenched upon {Zn 4 P 4 W 30 } addition, at least up to a concentration threshold of 20 µm. Above

	Normalised Absorbance	CDs + 1.25 M {Zn 4 P 4 W 30 } + CDs + 12.5 M {Zn 4 P 4 W 30 } + CDs + {Zn 4 P 4 W 30 }
	300 350 400 450 500 550 600 650 700 Wavelength (nm)

Table 6 .

 6 1. Red points have been excluded from this fit, as not related to a quenching. The obtained parameters seem to indicate a process perfectly in line with what previously observed for {P 2 W 18 }, albeit the obtained accessibility factor results being slightly lower.To prove that the interaction stems from the formation of static complexes and is not, instead, caused by dynamic encounters in solution we proceeded to study the time-resolved fluorescence of CDs + at different {Zn 4 P 4 W 30 } concentrations. Resulting measured traces, shown in Figure6.4, result to once again proceed parallel to each other. As always, a global least-squares fitting procedure was performed to confirm that all kinetics are described by the same lifetimes previously shown in Table4.3 for CDs + . As such, we can exclude the possibility of a collisional quenching and conclude that we are witnessing the formation of {Zn 4 P 4 W 30 }-CDs + complexes.Considering these results, it thus appears highly probable how the mechanism responsible for the observed quenching is the same occurring when using {P 2 W 18 }: an electron transfer from CDs + to POMs. Regarding the differences observed in the second range Nanosecond emission decay traces of CDs + after the addition of {Zn 4 P 4 W 30 } at different concentrations; as evaluated by the best-fitting curves reported on top of the data, CDs + kinetics are represented by biexponential decays having the same lifetimes regardless of {Zn 4 P 4 W 30 } presence; data obtained at λ exc = 450 nm, λ em = 520 nm.

	Intensity (arb. un.)	10 4 10 5 10 6		{Zn 4 P 4 W 30 } ( M) 208 120 59 15 0	Figure 6.4:
		0	10	20 Time (ns)	30

  2 W 18 } anions, differently than {Zn 4 P 4 W 30 }, are able to cause the precipitation of CDs which they couple to. Nevertheless, we have to consider that one of the main factors which prevents aggregation and aids stability in solution is electrostatic repulsion between electrical double layers (EDLs).282 When CDs + and POMs interact, the total charge of the resulting complexes will depend on that of each single specie; while [P 2 W 18 O 62 ] 6 -present an anionic charge of -6 e, that of Transient absorption spectra at different delays from excitation (λ exc = 400 nm) of {Zn 4 P 4 W 30 }-CDs + obtained at a {Zn 4 P 4 W 30 } concentration equal to 1.25 µm; shown spectra have been averaged on a 10% time interval of the indicated delay; the recognised GSB, SE, and ESA signals are respectively marked by blue, green, and red arrows. [Zn 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 16 -is equal to -16 e, and thus significantly more negative. As a consequence, the EDL of {Zn 4 P 4 W 30 }-CDs + will provide a stronger electrostatic repulsion from other complexes compared to {P 2 W 18 }-CDs + , thus explaining the resulting inherent

	TA Signal (mOD)	0 2 4 6	GSB Delay (ps) 0.27 0.84 2.9 113 40 8.8	Figure 6.5:
		-2	ESA	SE	ESA
			450 500 550 600 650 Wavelength (nm)

  6.2 Interaction with {Co 4 P 4 W 30 } Effects caused on CDs + by {Co 4 P 4 W 30 } presence in solution were studied similarly to what previously done for other POM structures.

  The absorbance spectrum of CDs + appears to undergo a redshift upon addition of {Co 4 P 4 W 30 } as indicated by the arrow (concentrations reported in legend); all spectra have been arbitrarily normalised. Emission spectra of CDs + after addition of {Co 4 P 4 W 30 } at the indicated concentrations; presence of {Co 4 P 4 W 30 } in solution induces a quenching in the photoluminescence intensity of CDs + , as indicated by the arrow; data obtained at λ exc = 400 nm. Figure 6.10 & Table 6.2: Stern-Volmer plots of CDs + emission intensity quenched by {Co 4 P 4 W 30 } (λ exc = 400 nm); the curve best representing the data, also shown in the plot with the corresponding 1-σ confidence interval, has been obtained via a least-squares fitting procedure; resulting quenching parameters are reported in the table, with K being the quenching coefficient while f representing the accessibility factor relative to the process. intense as the fluorescence intensity decreased by 50% already at concentrations as low as 2.5 µm. The behaviour of the emission as a function of concentration is presented in a Stern-Volmer plot, shown in Figure 6.10. From this data it is possible to obtain quantitative informations on the undergoing mechanism by performing a least-squares fitting procedure with a single-quenching model. In fact, while all other samples always displayed two different ranges characterised by separate dynamics, the interaction of CDs + and {Co 4 P 4 W 30 } seems to occur via a single mechanism. Parameters describing the experimental data are shown in Table 6.2; while the obtained quenching coefficient results being smaller than those resulting from the interaction of CDs + with both {P 2 W 18 } and {Zn 4 P 4 W 30 }, its magnitude is still indicative of a very strong quenching. As results obtained both from absorbance and photoluminescence measurements seem to point towards a static quenching, and thus formation of {Co 4 P 4 W 30 } complexes, the emission kinetics were observed on a nanoseconds time-scale via time-resolved photoe-Nanosecond emission decay traces of CDs + after the addition of {Co 4 P 4 W 30 } at different concentrations; as evaluated by the best-fitting curves reported on top of the data, CDs + kinetics are represented by bi-exponential decays having the same lifetimes regardless of {Co 4 P 4 W 30 } presence; data obtained at λ exc = 450 nm, λ em = 520 nm. {Co 4 P 4 W 30 }-CDs + complexes.

	2 4 6 8 10 10 3 0 /I) Stern-Volmer Plot (I 10 4 10 5 10 6 Intensity (arb. un.)	{Co 4 P 4 W 30 } ( M) 142 3 0	K (m -1 ) (9.6 ± 1.7) × 10 5 89.9 ± 0.4 f (%) Figure 6.11:
	300 350 400 450 500 550 600 650 700 Wavelength (nm) Normalised Absorbance CDs + 1.25 M {Co 4 P 4 W 30 } + CDs + 12.5 M {Co 4 P 4 W 30 } + CDs + {Co 4 P 4 W 30 } Figure 6.8: 450 500 550 600 650 700 Wavelength (nm) 0.0 0.2 0.4 0.6 0.8 1.0 Intensity (arb. un.) {Co 4 P 4 W 30 } ( M) 0 1 2 9 28 120 208 Figure 6.9: 0 50 100 150 200 Concentration ( M) mission spectroscopy. As previously pointed out, in the case of dynamic quenching the interaction between photoexcited fluorophore and quencher occurs through random en-counters in solution; this causes the emission kinetic decay with a lifetime shorter than that measured in absence of quencher. Nonetheless, as shown in Figure 6.11, all time-resolved CDs + emission traces appear once again to proceed parallel to each other, regardless of the concentration of {Co 0 10 20 30 40 Time (ns)

4 P 4 W 30 } in solution. In fact, we were able to find best-fitting curves representing the shown traces with bi-exponential functions sharing the same decay lifetime and thus prove that the observed quenching is indeed caused by formation of static

  Figure 7.2: A) absorbance of an aqueous solution of {Zn 4 P 4 W 30 }-CDs + , Ag 2 SO 4 , and IPA as measured at different visible-light irradiation times; B ) kinetic of the Ag plasmon absorbance band as observed in aqueous solutions under visible-light irradiation; all samples contain Ag 2 SO 4 , IPA, and the compound indicated in the legend.

	Absorbance	0.2 0.4 0.6 0.8 1.0		{Co 4 P 4 W 30 }-CDs + Before irradiation 4h irradiation	Absorbance	0.1 0.6 0.5 0.2 0.3 0.4		Sample {Co 4 P 4 W 30 }-CDs + {Co 4 P 4 W 30 } CDs +
		300 0.0	400	500 Wavelength (nm) 600	700	800	0.0	0	50 Irradiation time (min) 100 150	200
	Absorbance	0.2 0.4 0.6 0.8		{Zn 4 P 4 W 30 }-CDs + Before irradiation 5h irradiation	Absorbance	0.1 0.6 0.5 0.2 0.3 0.4		Sample {Zn 4 P 4 W 30 }-CDs + {Zn 4 P 4 W 30 } CDs +
		300 0.0	400	500 Wavelength (nm) 600	700	800	0.0	0	50 Irradiation time (min) 100 150	200

  .4, where solid lines and dashed lines respectively indicate results before and after 24 h. As shown in panelFigure 7.4: A and B ) absorbance of an aqueous solution of Ag 2 SO 4 , IPA, and the compound indicated in the legend as measured before (solid lines) and after (dashed lines) leaving the solution in the dark for 24 h; the solution containing {Co 4 P 4 W 30 }-CDs + is the only one displaying growth of the plasmon band without any irradiation, albeit the final intensity resulted significantly lower compared to that of the irradiated sample (Figure7.3A).(A), both {P 2 W 18 }-CDs + samples and the {Zn 4 P 4 W 30 }-CDs + sample do not present any plasmon band growth, sign that the Ag + reduction process is driven by irradiation as expected. Surprisingly, this was not the case for the sample containing {Co 4 P 4 W 30 }-CDs When discussing the ultrafast transient absorption results of {Co 4 P 4 W 30 }-CDs + complexes (Section 6.2), we noticed how {Co 4 P 4 W 30 } were able to transfer electrons back to CDs + after being photoreduced. This polyoxometalate structure was the only one found being able to do so, sign of the unique redox capabilities of this compound. It is possible that even if kept in the dark, part of the Co-based TMSPs were reduced in solution. As a consequence, they were in turn able to catalyse the reduction of Ag + ions thus forming the silver nanoparticles responsible for the observed plasmon band.Nevertheless, the resulting absorbance is not very intense while on the contrary being very broad and asymmetric. As such, we can conclude that the underlying process occurring in the absence of irradiation is not very effective. Consequently, we can still affirm that the previously observed photocatalytic reduction of silver occurs in solution thanks to the interplay between POMs and CDs + .As a comparison between the results obtained for all different POM-CDs + complexes, the plasmon band growth kinetics previously presented are plotted together inFigure 7.5. From this it is possible to conclude how best results are obtained for both TMSPs complexes, with {Co 4 P 4 W 30 }-CDs + producing a band slightly more intense than {Zn 4 P 4 W 30 }-CDs + Normalised absorbance of the Ag plasmon band as measured in an aqueous solution of Ag 2 SO 4 , IPA, and POM-CDs + complexes formed with the compound indicated in the legend; samples have been left under visiblelight irradiation for 3 h 30 min.

	Absorbance	0.0 0.1 0.2 0.3 0.4 0.5 0.6	300 0	400 50 Irradiation time (min) 500 600 Wavelength (nm) No irradiation 700 {P 2 W 18 }-CDs + 2x{P 2 W 18 }-CDs + {Zn 4 P 4 W 30 }-CDs + 100 150 200 POM-CDs + {P2W18} 2x{P2W18} {Zn4P4W30} {Co4P4W30}	800	Absorbance	300 0.0 0.2 0.4 0.6 0.8 Normalised Absorbance	400 400 450 500 550 600 650 700 500 600 700 800 Wavelength (nm) No irradiation {Co 4 P 4 W 30 }-CDs + Wavelength (nm) POM-CDs + {P 2 W 18 } 2x{P 2 W 18 } {Zn 4 P 4 W 30 } {Co 4 P 4 W 30 }
	Figure 7.5: Comparison between the kinetics	Figure 7.6:
	of the Ag plasmon absorbance band as observed
	in different aqueous solutions under visible-light
	irradiation; all samples contain Ag 2 SO 4 , IPA,
	and the compound indicated in the legend.

+ ;

as evident in panel (B ), a plasmon band appeared even if the solution was kept in the dark.

  .6. All spectra have been calculated by subtracting the absorbance measured before irradiation to that obtained after 3 h 30 min; this allows to only isolate the plasmon band, eliminating all contributions to the absorbance due to either CDs + or POMs. In addition, as the band intensities were already discussed by observing the kinetics, spectra have been normalised so to simplify the comparison. As a result, it can be witnessed how complexes obtained at a 10 µm {P 2 W 18 } concentration give rise to the largest plasmon band, sign of a large silver nanoparticles size distribution. Moreover, the observed peak position is in this case found at the longest wavelength; this points towards formation of big silver nanoparticles, which therefore present a plasmon resonance at lower energies. It thus appears that when the electron transfer between CDs + and POMs is more efficient, as is the case for {Zn 4 P 4 W 30 }, {Co 4 P 4 W 30 }, or when the {P 2 W 18 } concentration is increased, POM-CDs + complexes are able to achieve the photocatalytic formation of smaller Ag nanoparticles with improved size-control.7.2 Structure of the NanoparticlesIn order to observe the formed silver nanoparticles and characterise their structure, TEM images of the samples were obtained. Figure7.7 shows results for solutions containing {P 2 W 18 }-CDs + , {Zn 4 P 4 W 30 }-CDs + , and {Co 4 P 4 W 30 }-CDs + -respectively shown in panels (A), (B ), and (C ). As previously discussed when analysing the shape of the plasmon absorbance bands, it once more appears that complexes involving {P 2 W 18 } give rise to the biggest nanoparticles, whose size decreases when considering {Zn 4 P 4 W 30 } and {Co 4 P 4 W 30 }. In general, the particles do not appear to be monodisperse and a broad diameter distribution can be observed via TEM.

  16 [Zn 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] which has been provided by collaborators. Details regarding the materials which have been synthesised during the work of thesis will be presented in the following sections. A.6.1 Polyoxometalates α-K 6 P 2 W 18 O 62 • 14 H 2 O This polyoxometalate salt was synthesised following a literature procedure 44 briefly presented here. The synthesis starts by weighing 300 g of Na 2 WO 4 • 2 H 2 O then dissolved in 350 mL distilled water in a 1 L flask while stirring at room temperature. After complete dissolution, 250 mL of 4 m HCl were added to the Na 2 WO 4 • 2 H 2 O solution at a rate of 2 drops/s. Completed this step, 250 mL of 4 m H 3 PO 4 were added to the same solution at a rate of 4 drops/s. Completed the acid addition, the obtained solution was transferred to a round bottom flask and heated under reflux for 24 h.

du complexe. Ce n'est qu'en se concentrant sur les rapports littéraires et par des tentatives expérimentales qu'il a été possible de trouver le type de CDs le mieux adapté à nos besoins.La charge de surface s'est avérée être une force motrice capable de stimuler la formation du complexe {P 2 W 18 }-CDs. Bien que cela n'ait pas été considéré comme absolument nécessaire, c'était néanmoins la meilleure voie pour assurer une interaction adéquate entre les deux composants. En observant les effets résultant de l'interaction entre les différentes structures de POMs et CDs + , nous avons pu attester un processus extrêmement efficace se produisant au sein des complexes. En utilisant des outils spectroscopiques tels que l'absorption transitoire au femtoseconde, nous avons dévoilé comment le transfert d'électrons responsable du quenching de la luminescence des CDs + s'est produit à travers une cinétique ultrarapide sous-picoseconde. Un tel résultat a été jugé extrêmement prometteur pour un éventuel transfert applicatif: la photocatalyse des nanoparticules d'argent a été une expérience sur banc d'essai indiquant clairement l'efficacité des dispositifs hybrides développés. Le transfert d'électrons des CDs + photoexcité vers les POMs s'est donc avéré efficace pour la réduction des ions Ag + , mécanisme auparavant observé seulement par des moyens indirects. En conclusion, dans le travail présenté nous avons pu développer des complexes hybrides photocatalytiques POM-CDs et montrer leur utilité applicative. Alors que certaines questions sur l'interaction interne et le mécanisme de formation sont encore ouvertes, nous considérons ces résultats très prometteurs. Nous pensons que nos complexes pourraient conduire à un large éventail de nouvelles applications, ouvrant la voie à l'utilisation des POMs sous irradiation par lumière visible dans le domaine de la photocatalyse, tout en utilisant des complexes peu coûteux et faciles à produire formés avec les CDs.

This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 665850.

Chapter 8: General Conclusion have been tested could form complexes with CDs + and interact through the same ultrafast mechanism. Nonetheless differences in efficiency were found, indicating how sandwich-type POM {Zn 4 P 4 W 30 } was the best fit for developing efficient photoactive complexes with carbon nanoparticles.

When studying the photocatalytic reduction of silver ions, these conclusions were finally once again proven: by managing to observe the results of such a test-bench experiment, it was possible to conclusively witness how effective the developed nano-hybrid devices are. While we expect our complexes to find a role in a wider array of applications, these first tests managed to definitely prove the electron transfer mechanism, previously only observed through indirect means. Moreover, the obtained results were able to confirm the higher efficiencies of the studied TMSPs, indicating how it may be possible to better tune our system for increased performances.

In conclusion, in the presented work we have been able to develop photocatalytic POM-CDs hybrid complexes and to show their applicative usefulness. While some questions on the inner interaction and formation mechanism are still open, we consider these results very promising. We believe our complexes could lead to a wide array of novel applications, opening up the way for visible-light driven usage of POMs in the domain of photocatalysis through the usage of inexpensive and easy to produce complexes formed with CDs.

Appendix A

Instruments and Methods

In this chapter the instruments which have been utilised will be briefly presented, alongside the methods applied for data analysis. In particular, we will present how quantum yields for the CDs samples were calculated before explaining what the least-squares fitting procedure and the global analysis techniques are. In addition, we will discuss how the model describing the behaviour observed in the Stern-Volmer plots was chosen. Afterwards, a discussion concerning the time-resolved photoluminescence and ultrafast transient absorption will be treated separately from the other instruments, as both techniques are based on laboratorymade setups. Finally, in the last section the used materials and all followed synthetic procedures will be presented and detailed.

NMR data were obtained on a Bruker UltraShield 400. Solutions were prepared in milli-Q water and measurements were performed at 400 MHz.

DLS measurements, performed to obtain both hydrodynamic radii and surface charge distribution, were carried out on a Malvern Nano ZS model ZEN3600. When measuring R h , measurements were performed using a disposable plastic cuvettes containing 1 mL of sample solution. Results were repeated at different concentrations in order to ensure no sample interaction in solution. DLS size was obtained from the weighed average of the number distribution plot. ζ-potential measurements were performed in an appropriate disposable plastic cuvette. ζ-potential values were obtained by least-squares fitting the charge distribution data with a Gaussian function.

AFM measurements were performed on a NT-MDT model Solver P47. Samples were prepared by depositing a drop of nanoparticle solution on a standard crystalline silicon substrate then left to dry overnight. All measurements were performed in intermittent contact mode using standard silicon AFM tips. Topography images were analysed using the Gwyddion software and corrected as follows: firstly rows were aligned using a 4 th degree polynomial and then the height was corrected by subtracting a 2 nd degree polynomial plane used as background. Particle size was established from the maximal height distribution after selecting individual grains using a height threshold of 0.5 nm.

TEM and ED measurements were performed on a JEOL JEM-2100F Field Emission Electron Microscope. TEM samples were prepared by depositing several drops of sample solution on a FCF200-Cu grid, by drying between each deposition with paper tissues. ED measurements were calibrated using a reference cubic gold sample (ref. 98-004-4362).

Raman samples were prepared by depositing a drop of CDs solution on a silica substrate, then left to dry overnight. Measurements were carried out by the Horiba HR-Evolution multiline MicroRaman using the 325 nm excitation laser with spectral resolution better than 5 cm -1 . The excitation wavelength was chosen so to enhance the measured signal over the sample photoluminescence, based on the fact that Raman scattering cross-section is proportional to λ -4 . The laser power was set to detect the signal and avoid modifying the sample. Optical images are collected with a 40× objective. All data have been treated with a 6 th degree polynomial background subtraction.

FTIR measurements were performed on a Thermo Scientific spectrometer model IR-TF Nicolet iS10. FTIR samples were prepared by mixing a few drops of CDs solution with 100 mg KBr powder. The wet powder was then mixed and left to dry in air before compressing in order to obtain a pellet. A KBr pellet was used to measure the background. All spectra were acquired by averaging together 50 independent measurements. FTIR spectra were treated via the Thermo Fisher Scientific OMNIC 9 application by subtracting a baseline.

Steady-state absorption spectra were obtained on a PerkinElmer LAMBDA 1050 spectrophotometer using quartz cuvettes of 1 cm optical path. All absorption spectra A.1: Quantum Yield Calculation Obtaining this equation is very straightforward if considering that the intensity of emission can be expressed by Equation A.2:

In this, term 1 -10 -A represents the amount of absorbed light, ( n /n 0 ) 2 compensates for refraction at the cuvette interface, and f is the term taking into account losses inherent to the spectrophotometer and spectrofluorometer used to measure the values of A and I.

The intensity modification due to the refraction index term can be obtained for a point emitter as shown in Figure A.1. As knowing the exact value of f is not possible, repeating this measure for a reference of known QY can be done, thus leading to the result presented in Equation A.1. Details regarding this approach can be found in the literature. 227 ,289

A.2 Least-Squares Fitting and Global Analysis

Least-squares fitting analysis is a procedure that allows to test whether a certain mathematical model is consistent with the observed data, while also allowing to obtain values for its parameters which have the highest probability of being correct. Results from this analysis thus allow to represent, or fit, the measured data via the model.

To accomplish the aforementioned goals, this analysis proceeds by minimising the so-called goodness-of-fit parameter χ 2 , given by the following expression:

Here, the original data represented by y i is composed by n measured points; the model function, which depends both from the independent variable x i and from a set of parameters p, is represented by f (x i , p); the uncertainty on y i is represented by ε i .

There are several algorithms which allow to minimise Equation A.3, each with its own advantages. In the course of the data analysis the Levenberg-Marquardt algorithm 290 was chosen, as it is generally considered a fast and robust method for finding local minima, while also allowing to estimate the uncertainties of parameters p. This algorithm is based on an iterative procedure which calculates at each step the value of the model function f (x i , p). This is done by starting from a set of initial parameters p and varying their value at each step; to calculate how much to change the values indicated by p, the Jacobian of χ 2 is calculated. This, being the matrix of the first-order partial derivatives, from a simplified point of view is just indicating the direction in which χ 2 diminishes. As a result, with each iteration the function f (x i , p) will better represent original data y i as their difference decreases.

The main disadvantage of the Levenberg-Marquardt algorithm resides in the fact that it allows to find local minima of the goodness-of-fit; as such, it is highly reliant on a good choice of initial values for the parameters p in order to converge to what is the actual absolute minimum, as otherwise the fit does not provide the best solution. Attempting to reach the best-fitting solution is usually done by trying to opportunely calculate the initial p values from the available data.

Another important consideration on the least-squares fitting analysis has to be made.

In fact, when considering the parameter χ 2 it has to be noted how this represents the collective resemblance between original data y i and the model function f (x i , p), while it does not give any informations regarding them being significantly different at any point x i .

Differently put, this analysis does not inherently give informations regarding whether the chosen model is the one best-representing the measured data. In order to evaluate this last point, it is often useful to observe the residual plot, which is the deviation between original data and model as a function of the independent variable x i :

Such a plot allows to remark eventual differences between y i and f (x i , p); these are expected to be randomly distributed around zero, as otherwise it is highly probable that the chosen model is not adequate to represent the original data. An example of this situation is shown in Figure A.2, displaying the time-resolved intensity decay of a two-component

A.3 Quenching Processes and Stern-Volmer Analysis

With quenching of fluorescence we refer to all processes causing a decrease in the emission intensity of a fluorophore. 227 These can generally be ascribed into two main categories which are dynamic quenching and static quenching (Figure A.3). The former, also named collisional quenching, occurs when the excited fluorophore is brought back to the ground state consequently to a collision occurring with another specie diffusing in solution. As for this process to occur the fluorophore must be in its excited state, such a diffusional interaction must occur during the brief emission lifetime. Such a transient process does not produce any alteration in the nature of the emitting specie; when a static quenching instead occurs, on the contrary, fluorophore and quencher form a bound complex. As such the bound specie is altered in the process: it is possible to say that this kind of quenching acts on the ground state of the system, differently from dynamic quenching which only influences the excited state. This key difference allows to discriminating the two processes, as shown in the following. so that it is possible to write:

will show that this characteristic is exclusive to collisional processes.

To understand how this kind of quenching is linked to the diffusion of fluorophore and quencher in solution we have to analyse the bimolecular quenching constant meaning.

If considering that the value of k q [Q] represents the rate at which the excited system is brought back to the ground state due to collisional processes, we can write:

Such an equation stems from the consideration that the bimolecular quenching constant

indicates the rate at which fluorophores and quenchers collide in solution causing a transition back to the ground state; as nothing assures that all collisions will cause a decay of the system, the proportionality factor f q represents the efficiency of the quenching process. Constant k 0 is instead the diffusion-controlled bimolecular rate, which can be calculated using the Smoluchowski equation:

In Equation A.14 R is the collision radius, D the diffusion coefficient, and N A is

Avogadro's number. The multiplying factor 1000N A is needed when concentration [Q] is expressed in terms of molarity, so to express k 0 in units of m -1 instead of (molecules/m 3 ) -1 .

This equation assumes that both R and D can be expressed as sum of the relative fluorophore and quencher values -respectively indicated by subscripts f and q. Diffusion coefficients are normally calculated via the Einstein-Stokes equation:

Here k B is the Boltzmann's constant, T the temperature, and η the viscosity of the solvent.

By making use of these equations it is possible to calculate the diffusion limited quenching coefficient, obtained for f q = 1. Such a constant represents the maximum value expected for the product k q τ 0 , as it is calculated by assuming maximal quenching efficiency. In this work of thesis it has been calculated as additional proof in order to exclude dynamic quenching processes: as we obtained Stern-Volmer constants K D higher than the diffusion limited quenching coefficient, the observed process could not be due to collisions in solution.

A.3.2 Static Quenching

In the case of static quenching fluorophore and quencher form an interacting bound complex. While the collisional process is time-dependent as previously shown, in the case of static complexes the resulting system immediately returns to the ground state upon In order to obtain the dependency between the decrease in emission intensity and quencher concentration, as previously done for dynamic processes, we have to consider the association constant K S allowing for complex formation:

)

Exactly as previously done for dynamic quenching, the concentration of fluorophores

[F ] can be substituted for the emission intensity I, so to obtain:

Once again, Equation A.18 shows how the emission intensity in the presence of a quencher is expected to decrease linearly as a function of the quencher concentration, regardless on whether the process is dynamic or static. As a consequence, by performing solely steady-state optical measurements it is not possible to obtain informations regarding then underlying quenching process.

We previously claimed that static complexes immediately return to the ground state upon photoexcitation; with this affirmation we actually intended that in the case of static quenching the excited state of the system has an extremely brief lifetime, due to very efficient non-radiative decay pathways stemming from the interacting components of the complex. Such a phenomenon is equivalent to removing part of the fluorophores from the emitting population, as they do not take part in the total fluorescence any more. Because of this, the emission lifetime of the remaining fluorescent population is unaffected so that it is possible to write:

By comparing Equation A.12 and A.19, it is immediately possible to understand how to discriminate between dynamic and static processes: as the observed emission lifetime varies only in the case of collisional quenching, time-resolved measurements allow to readily understand which kind of quenching occurs. It is because of this that the nanosecond time-resolved photoemission of POM-CDs was measured.

Nonetheless, it has to be once again remarked that while dynamic quenching influences the excited state of the emitting system, static quenching affects its ground state. Because of this, in the case of formation of complexes it is sometime possible to observe variation in the position and shape of fluorophore absorbance bands. Despite this, effects on the steady-state absorbance spectrum are often small, so that time-resolved measurements remain the only way to definitively prove the presence or absence of dynamic quenching processes.

A.3.3 Multiple Quenching Processes

It is not uncommon for fluorophores in solution to be affected by both collisional and static quenching processes altogether. In such a case, the resulting I 0 /I Stern-Volmer plot presents a quadratic behaviour, instead of the common linear one observed for either of the two processes acting alone. An example of this is shown in the right panel of To show why this is the case we can separately consider the two processes: as a result of static quenching, part of the emitting population will be removed so that only an amount represented by Equation A.18 will still display fluorescence. We can now consider how can be described as the sum of the intensities of each fraction:

When the quencher is instead present, only the accessible fraction A will be affected by quenching. As we already know that such a situation is depicted by the Stern-Volmer equation, we can write the total intensity as follows:

In this equation, K A is the quenching coefficient relative to the process affecting population A. From Equation A.21 and A.22 it is possible to obtain the following result:

In Equation A.23 parameter f A indicates the fraction of fluorophores accessible to the quencher, represented by: After passing a polarising filter whose purpose is to select the appropriate wavelength -as the generated light is composed by two components different in wavelength and of opposite polarisations -the exciting radiation reaches the sample. Emission from the latter is collected through a slit and directed towards a diffraction reticule (λ blaze = 300 nm, 150 lines/mm) by optical lenses. As the reticule has a spectral resolution of 20 nm mm -1 , slit aperture was set to 250 µm so to achieve a resolution of 5 nm.

The dispersed sample emission spectrum is thus recorded by the intensified CCD camera at a certain delay from the laser pulse -selected thanks to a trigger signal sent by a programmable timing generator -and for a fixed time window of 0.5 ns; as using a CCD allows to record the full sample spectrum via every single acquisition, the spectral evolution can then be obtained by repeating measurements at different delays from excitation.

Cuvettes of 1 cm optical path quartz were used throughout the procedure, filled with a sample solution displaying an absorbance approximately equal to 0.1 at the excitation wavelength so to avoid inner filter effects. Obtained data was treated by removing from each spectrum a linear baseline intercepting points were no emission signal was present; in addition, to obtain the time traces, intensities in an interval of 10 nm around the chosen λ were averaged together so to reduce noise. Time resolved emission traces were then least-squares fitted with the sum of two functions I(t), as the observed CDs emission kinetic can be represented by a bi-exponential decay. Function I(t) represents the convolution (Equation A.26) between an exponential f (t) (Equation A.27) and the instrument response function IRF(t) (Equation A.28), the latter approximated by a Gaussian function:

(A.28)

In the functions, parameter t 0 represents the time at which the sample is excited by the laser pulse, A the amplitude of the exponential at time t = t 0 , and σ the width of the Gaussian function. The bi-exponential decay was chosen for our data through a trial-and-error approach which allowed to exclude other possible decay models.

In order to obtain the two lifetimes τ 1,2 able to represent the full spectral dynamic of free CDs, a global least-squares fitting procedure was performed on five decay traces taken at different wavelengths. In this, parameters t 0 , σ, τ 1 , and τ 2 were set as global parameters, while amplitudes A were left free to vary for each of the dataset. As the emission decay kinetics of POM-CDs complexes proceed parallel to that of free CDs, when repeating the fit procedure on the traces of coupled complexes, lifetimes τ 1 and τ 2 were fixed to the previously obtained values. As the fit properly converged to the original data, the hypotheses that emission lifetimes were not influenced by the presence of POMs was thus confirmed.

A.5 Femtosecond Transient Absorption

Ultrafast transient absorption is a spectroscopic technique in which the changes in absorbance of a sample occurring after its photoexcitation are probed. In a common TA experiment, the photoexcitation is carried out by an impulse lasting some femtoseconds, called pump. Measuring the differences in absorbance is instead done through another signal, called probe, usually presenting a broad spectrum so to allow observation of a wide region of the sample absorbance. By comparing the signal of the probe registered before and after photoexcitation it is possible to observe variations in the sample absorbance spectrum; usually these are obtained as a function of time, so that by controlling the delay between pump and probe it is possible to reconstruct the kinetic of the measured transient The dispersed probe beam is revealed by a 1024 pixels detector system (Glaz Linescan-I) with single-shot capability. A typical signal is obtained by averaging 5000 pumped and 5000 unpumped spectra for each delay, and scanning over the pump-probe delay 10-20

times.

The concentration of the measured CDs + solutions were equal to 2.5 gL -1 in order to have an absorbance equal to 0.3 at 400 nm through the flow cell optical path. The measurements were performed in linear regime with an energy of about 100 nJ per pulse.

All measurements were carried out under the so-called magic angle detection conditions, so that they are not affected by rotational diffusion or depolarisation dynamics.

A.5.2 Corrections to the TA data

The data presented in the thesis was subjected to standard correction procedures which eliminate the effects of group velocity dispersion (GVD) and cross phase modulation (XPM). With GVD we refer to the phenomenon causing signals of different frequency, initially overlapping in time, to acquire different delays as they traverse a medium and is defined as follows:

In here, k is the wave vector, ω is the wave angular frequency, and v g is the group velocity which can be written as:

In Equation A.34, v φ is the phase velocity of the pulse, and n is the refractive index of the medium. As a consequence, because of GVD every spectral component of the pulse is delayed differently, so that the total signal assumes a temporal broadening which depends on both frequency and the refractive index of the medium.

The XPM, instead, is an artefact generated when the probe pulse of the TA interacts with the medium after the passage of the pump signal; the latter does in fact induce alterations in the refractive index of the material, leading to a visible artefact. As previously remarked, because of GVD the various frequencies are influenced differently when the pulse wave passes through the medium, so that the XPM presents a non linear shape.

As such, TA data is corrected by opportunely shifting in time the XPM so to place its full signal at time zero, as shown in Figure A.9. In addition, from the width of the XPM signal it is possible to obtain the time resolution of the TA experiment, which in our case corresponds to 100 fs. After this, the XPM is discarded from successive analysis so to avoid its presence influencing the kinetics relative to the sample.

During discussion of the results it was pointed out how data was analysed by means 450 500 550 600 650

Wavelength (nm)

A.5.3 Analysis Procedure

Maybe the most important aspect of the SVD technique is found in the possibility to fit the whole TA matrix at once. This is done by fitting with an opportune model only the relevant eigentraces -chosen by observing the eigenvalues, as previously discussed. By successfully doing so, it is possible to reconstruct TA(λ, t) m×n with Equation A.35 using the best fitting curves for V(t) n×n . In addition, this method allows to obtain the decay associated spectra (DAS).

DAS are the spectral components of the TA signal decaying with a single well-defined lifetime. Assuming that each mechanism occurring in the system takes place with a certain lifetime, this means that the DAS provides all spectral informations regarding a certain mechanism.

To obtain DAS related to TA data it is necessary to fit the SVD eigentraces. The model f (t) able to represent the kinetics of TA spectra, and thus of its eigentraces, is obtained as the convolution between a multi-exponential decay and the instrument response function (IRF): .36) Here A ij are the pre-exponential amplitudes, τ i the lifetimes associated to each decay, H 0 (t) the Heaviside step function centred at t = 0, and IRF(σ, t) a Gaussian function of width σ representing the instrument response function; in addition, index i represents each of the components of the multi-exponential decay while index j represents each of the eigentraces. These functions f j (t) are thus used as models to fit each of the eigentraces relevant to the signal. As the lifetimes τ i are expected to be shared by the whole signal, the least-squares fitting procedure can be formed through a global analysis. As discussed in Section A.2, this is exactly a situation in which different datasets -the eigentraces -are sharing common variables which behave as global parameters -the lifetimes and everything related to the IRF.

From the result of the fit it is then possible to obtain the DAS using Equation A.37:

In here, U j are the eigenspectra, s jj are the eigenvalues, and A ij are the pre-exponential amplitudes obtained from the fit. This expression means that the DAS are linear combinations of the eigenspectra obtained using the eigenvalues and the decay pre-exponential factors as weights. In matrix notation, this result can be written as:

In addition, it is possible to reconstruct the TA signal as follows:

By combining Equation A. 39, A.37, and A.36 we obtain: Having the DAS relative to a signal is a very powerful tool allowing to simplify the interpretation of the signal itself, as it has been done in the course of this work of thesis.

Since the decay associated spectra represent all the kinetics undergoing in the TA signal and each DAS has a single lifetime associated to it, we can conclude that all mechanism underlying the spectral signatures observed in the transient absorption data must occur with the same lifetimes. This is a very important point for the discussion of our results; in fact, the TA analysis carried out on POM-CDs complexes yielded the same lifetimes observed for CDs not involved in any complex. Consequently, it was concluded that all mechanisms stemming from the interaction of POMs and CDs must occur outside of the probed times, and thus with a lifetime shorter than that associated to the fastest DAS. temperature before being transferred to a fridge at 4 °C for crystallisation.

After 2 days, the crystals (Figure A.12) were recovered via vacuum filtration and stored once dry. In order to increase the yield and recover more sample from the filtrate, still yellow from dissolved product, the precipitation via KCl and the crystallisation steps were repeated two more times. Samples obtained from each of the three purification steps were stored separately.

Na were slowly added while vigorously stirring, with the solution turning red as a result.

Temperature was successively increased to 40 °C and maintained for 15 min to allow the reaction to proceed. Afterwards heating was turned off and the mixture was left to cool down for an additional 5 min, before adding 2.5 mL of a 1 m NaCl aqueous solution. Finally, the product was gravity filtered and left to crystallise in a beaker.

Formed crystals of the final product were recovered after a week.

A.6.2 Carbon Dots

CDs -The synthesis procedure for this type of CDs was adapted from the literature 214 to the scale needed for our experiments. CDs + In order to obtain positively charged carbon dots, diluted HCl was added drop-wise to a 0.25 g L -1 solution of CDs -placed in a beaker (initial pH ≈ 10, as obtained out-of-synthesis). During the whole procedure, pH was controlled using a pH-meter and the sample was vigorously stirred in order to ensure an homogeneous conditions throughout the whole solution.

N-CDs This synthesis is based on another method found in the literature. 173 Said process consists on a single-step synthesis carried out in a microwave oven. In a beaker, 3 g of citric acid and 3 g of urea are dissolved in 10 mL of distilled water. Afterwards, the solution is left to evaporate completely inside a domestic microwave oven operated at 650 W. After 5 min the procedure is complete; inside the beaker a dark-coloured aggregate can be found as product of the precursors combustion. This aggregate is recovered by transferring it to a mortar so to be crushed into a fine powder. This powder is the final N-CDs sample which can then be easily dissolved in an aqueous solution at the appropriate concentration for usage.