Catherine Louis

Laurent Delannoy

Mr Ibrahim Abdellah

Mr Vincent Huc

Mr Cyril

Mrs Mireille

Tayseer Bahry

Luis Hernandez Adame

Cecila Coletta

Maria-Guadalupe Mendez

Mr Oliver Nüsse

my sister Noor Maha Khalil

Keywords: Catalyse Assistée par Plasmon Nanoparticules, Plasmon, Catalyse, Or, Palladium Catalysis Assisted by Plasmon Nanoparticles, Plasmon, Catalysis, Gold, Palladium

Les nanoparticules (NPs) métalliques ont de nombreuses applications en catalyse. Certaines d'entre elle (or, argent, cuivre) présentent une absorption dans le visible résultant de l'excitation du plasmon de surface par un champ électromagnétique extérieur. Nous proposons dans ce projet de coupler les proprietes catalytiques de nanoparticules à base d'or avec leurs propriétés de résonance de plasmon de façon à assister la réaction catalytique par les électrons chauds générés et par l'effet thermique qui en découle. Cette approche de catalyse assiste par plasmon, est générale et peut être utilisée pour une variété de procèdes catalytique exothermiques et endothermiques impliquant des nanoparticules. Cette catalyse assistée par plasmon permet de réaliser des réaction catalytiques à moindre cout énergétique sous lumière solaire.

Les processus catalytique sucitent un interet majeur pour la recherhe et le developpement. Des processus catalytiques photo-actives naturels interviennent dans presque tous les aspets de notre environnement: les cycles de photosynthese nourrissent et colorent les plantes, les reactions enzymatiques essentielles à la suivre des especes, la decontamination d l'eau à la lumiere visible, etc.

Enfin, la conclusion generale met en evidence les principaux resultats obenus au cours de cete these avec des nanostructures plasmonique de Au et de Pd utilisiees pour catalyer differentes reactions. Ces travauz ouvrent des perpectives en catalyse exaltee par plasmon, en particulier pour les applications environnmentales.

Catalytic processes are attracting a major interest for research and development. Naturallyoccurring photo-activated catalytic processes are involved in almost every aspect of our surroundings: photosynthesis cycles provide nurture and color to plants, enzymatic reactions that are crucial for survival of species, decontamination of water under visible light, and the list goes on.

A catalyst, from a chemical point of view, can be simply defined as a chemical entity with specific physio-chemical properties to accelerate and favor the least-energy-demanding reaction route to achieve the desired products. Nowadays, nanostructures of noble and transition metals had been widely developed to serve as catalysts with controlled morphology, variable chemical functionalization, and wide-range of possible crystallographic structures, as well as unique optical properties. In fact, nanoparticles of metallic nature show different properties than metals in their bulk structure. Several factors contribute to the difference in physico-chemical properties such as size, morphology, functionalization, and surrounded environment. These tunable properties are studied and applied in a vast spectrum of applications: industrial and academic research [1]. In particular, plasmon properties of metal nanoparticles have attracted a lot of attention. Indeed, when nanoparticles are under excitation of light, electrons oscillate in phase when excited at a certain wavelength of the electromagnetic wave of incident beam; this phenomenon is called Localized Surface Plasmon Resonance (LSPR). This absorption triggers the intraband transitions and heat dissipation around nanoparticles and allows an efficient and instant energy storing and exchanging throughout a complex mechanisms of interactions between photons and condensed electron gas of the metal. This interesting phenomenon generates localized hot-spots that are rich with electrons and energy in the form of heat. These hot-spots encouraged researchers to find suitable applications in the field of catalysis namely for reactions for environmental protection [2].

Gold, as an inert noble metal in its bulk state, had attracted our interest. Its brilliance, value, and stability drew attention of humans since the beginning of formed communities and societies. Gold is still in the core of today's economics, fashion industry, industrial and medical applications, cosmetics, and even in space shuttles [3]. Michael Faraday was the first who has reported the first preparation of colloidal gold nanoparticles (Figure 1). In his notebook Faraday observes : "The cone was well defined in the fluid by the illuminated particles" [4]. He realized that this cone effect was made because the fluid contained suspended gold particles that were too small to be observed with the scientific apparatus of that time, but which scattered the light to the side. This is known as the Faraday-Tyndall effect, and it is because of this discovery that Faraday is seen as one of the first researchers in nanoscience and nanotechnology [4]. Gold nanoparticles with controlled size, shape, and functionalization have shown promising applications in several domains due to their catalytic properties and their surface plasmon resonance (around 525 nm for quasi-spherical NPs). Different redox reactions activated by Au nanoparticles under illumination have been studied [5,6].

Figure 1. Michael Faraday ( left) and his first prepared solution of gold nanoparticles preserved in the British museum (London)(right). [2] Palladium is a very attractive metal for its applications in different catalytic reactions such as: selective hydrogenation, Suzuki-Miyaura and Heck reactions [7]. Recently, palladium attracts also interest in the field of nanomedicine [8]. Spherical Pd nanoparticles of few nanometers exhibit a plasmon band in the UV while palladium nanosheets show a broad plasmon absorption band in the visible-near infrared domain.

Plasmon properties of metal nanoparticles can be used to enhance the kinetics of catalytic reactions. Few reviews [9][10][11][12][13][14] have already been published on catalysis assisted by plasmonic nanostructures raising several questions such as:

-What are the possible mechanisms by which the catalytic effect takes place? -What are the optimum physico-chemical conditions for a reaction catalyzed by plasmon of nanostructures? And many more. Within this context, in order to bring part of the answers, we chose to study four model reactions:

1-Plasmon induced photo-decomposition of adsorbed para-nitrothiophenol on the surface of spherical gold nanoparticles and nanorods.

2-Photoreduction of Hexacyanoferrate assisted by plasmon of spherical gold nanoparticles under visible light.

3-Suzuki-Miyaura reactions catalyzed by plasmon of palladium nanosheets under visible light.

4-Gas phase oxidation of CO to CO2 assisted by plasmon of gold nanoparticles deposited on a solid support.

It is worth mentioning that catalysis assisted by plasmon of metal nanostructures is an interdisciplinary domain of research that incorporates physics, chemistry as well biology with applications in the medical field for instance. Hence, it is challenging not only to observe this phenomenon, but also to interpret the obtained results. This manuscript aims to provide experimental models of catalytic reactions assisted by plasmon of nanostructures as well as proposing mechanisms for these reactions. This manuscript is organized as follows:

The first chapter is an introduction to nanoscience with a focus on gold nanoparticles, their synthesis and properties, in particular, the localized surface plasmon resonance phenomenon and its applications. This chapter outlines the state of the art in catalysis assisted by plasmon.

The second chapter is devoted to the materials, metal nanoparticle synthesis and the experimental setups used to study the catalytic reactions. The different techniques used for characterization of the nanoparticles and for analysis of the reaction products are also described.

In the third chapter degradation of para-nitrothiophenol assisted by gold plasmon is studied. This degradation is much effective with Au nanoparticles excited under visible light.

The results show that the degradation mechanism is complex as several products are formed. However, evidence is given that surface-catalyzed reactions involving hot electrons occur as (part of) para-nitrothiophenol is reduced to para-aminothiophenol.

In the fourth chapter, we investigate the reduction of ferricyanide (III) ion into ferrocyanide (II) catalyzed by spherical gold nanoparticles (AuNPs) when excited at their localized plasmon resonance. This reduction takes place, at room temperature and under illumination of visible light even without using a reducing agent. This result indicates again that hot electrons are involved in the reaction mechanism.

Suzuki-Heck cross-coupling reactions catalyzed by Pd nanoflowers plasmon are studied in the fifth chapter. The reactions between halogenated benzene compounds and phenylboronic acid in the presence of various bases are carried out in the dark and under visible light irradiation. It is shown that the photocatalytic activity of Pd nanoflowers for Suzuki-Miuyara reactions is remarkably enhanced under visible light irradiation [7].

Finally, the general conclusion highlights the main results obtained during this PhD work with plasmonic Au and Pd nanostructures used to catalyze different reactions. It also gives perspectives of this work, in particular for environmental applications.

In annex, preliminary work on the oxidation of CO to CO2 assisted by plasmon of Au nanoparticles (deposited on different supports) in gas and liquid phase is presented. While nanosciences and nanotechnologies appear as new concepts developed at the end of the 20th century, metallic nanoparticles have already been used since ancient times, in particular to colour glass and ceramics. Nowadays, nanomaterials and nanoparticles are used for many applications in our daily life, such as in the fields of electronics [1], catalysis [2], optics [3], biology [4], and medicine [5]. This first chapter will present a general context and background about metal nanoparticles and their physio-chemical properties.

I-Metal nanoparticles in history and nowadays

The properties of metal nanoparticles are different from the bulk and new properties emerge at the nanoscale. During the last fifty years, there has been a continuous development in understanding and controlling the optical and catalytic properties of noble metals at the nanoscale. A large number of different applications emerged during the previously mentioned development: catalysis, electrocatalysis, photocatalysis, sustainable energy, imaging, sensors, nanomedecine, and even artistic applications [1,6,7]. Our understanding of optical properties of bulk and metallic nanoparticles (NPs)(monometallic NPs and nanoalloys) has enhanced exponentially due to the innovative variety of microscopic characterization techniques and light sources such as pulse modulated fast lasers ranging from femtosecond laser pulses into continuous monochromatic lasers .

The main interest of choosing noble metals as a core of research and applications is a result of an ongoing research in the field of metallic objects and composites. Numerous publications report on the optical and catalytic properties of noble metals and their applications [7,8]. While nanoscience and nanotechnology appear as new concepts developed at the end of the 20th century, many studies showed the existence of metal nanoparticles in ancient art objects giving astonishing optical properties to vases, wine-cups, stained glass in churches, lusters, ceramics, plates, and even jewelry [1,9]. While gold is known since long time for its golden color so appealing in jewelry, gold is also used since antiquity as red dye. For instance, Au NPs have been found in several red glasses or on porcelains decorated with red or pink enamels dating from ancient times. The most famous example of the use of metal nanoparticles concerns a piece of Roman glasswork, the Lycurgus Cup, dating from the fourth century CE, showing a mythological frieze depicting the legend of King Lycurgus and exposed in the British Museum in London. This The opaque green cup turns to a glowing translucent red when light is shined through it. These unusual optical properties are due to the presence of tiny amounts of gold, silver and copper nanoparticles (of about 50 nm) in the glass [1,8]. However, the inability to control the coloration process meant that relatively few glasses of this type were produced, and even fewer survived. The Lycurgus Cup is the outstanding example of this technology in every respectits outstanding cut work and red-green dichroism render it a unique record [1]. However, it is worth mentioning that these metallic amalgams were experimented repetitively for a long time. The craft of metallic alloys is handed from generation to generation. Hence, the practice in itself remained fascinating yet mysterious. Furthermore, one can depict from the historical timeline the overlap between the work of Alchemists and artisans led to formulate a sort of a protocol to synthesize colloidal solutions. Most references refer to the publication of Cassius: De Auro (1685) as a written protocol on synthesis of Cassius Purple [10]. Cassius purple was a colloidal solution used in the manufacturing of vases, cups, and windows. This is a colloidal solution of gold nanoparticles as we now know. Later, in the 19th century, the properties of colloids attracted increasing attention.

Michael Faraday was fascinated by the ruby color of colloidal gold and discovered that the optical properties of gold colloids differ from those of the bulk. In 1857, he reported the first synthesis of Au nanoparticles synthesized by the reduction of gold salts by white phosphorous in solution. This was the first reported observation of quantum size effects [11], and the birth of Nanoscience. Later, the specific properties of metal colloids were explained by Mie [12].

Noble metals had always been at the center of human activities. Copper, silver, and gold played a role in many social aspects of human development. Humans use silver and gold for jewelry as a manifestation of richness and of an upper social class. Gold and silver mines were a source of income and a motivation of dreamy workers during the industrial boost in Latin America and Central Asia.

Throughout the industrial revolution, the awareness of the importance of copper as a conductive metal led to the development of electronics, communication, and many more.

Gold and silver kept their important value as the abundance of such noble metals is declining with time due to reservation and exhaustion of natural resources. Gold plays a key role not only in jewelry, decoration and esthetics, but also in finance and currency rates, and more recently in electronics and nanotechnology. This specific importance of noble metals encouraged researchers to understand their physical and chemical properties. In modern electronic devices, noble metals have a major role, although the full exploitation of their optical properties is at its beginnings.

II-Synthesis of metal nanoparticles

To understand nanoparticles' properties, which change with size and shape, and for applications of nanomaterials, it is important to develop protocols for controlled synthesis of nanoparticles. Various synthesis methods are developed to synthesize nanoparticles and nanomaterials of controlled size, shape, and composition. Thanks to the development of synthesis approaches, and technical tools to study nanomaterials, nanoscience and nanotechnology are showing a very fast development.

Nanoparticles can be prepared by two main approaches: i. "Top-down" approach: by making the materials smaller, that is, by downscaling, using in general physical methods.

ii. Bottom-up" approach by constructing nanomaterials or nanoparticles from small building blocks, that is, by upscaling, using in general chemical or physico-chemical methods In literature, there is a large abundance of innovative methods to synthesize gold nanoparticles (or other plasmonic nanostructures) with controlled size and shape in solutions or on supports. Herein, we will present some of these methods, with a particular focus on synthesis of Au nanoparticles (NPs). Afterwards, we will focus on synthesis methods that were used during this research.

II-1-Physical methods for synthesis of metal nanoparticles

Generally, we refer to physical methods when a physical phenomenon is involved in the production of nanoparticles. It is in general a "top down" approach contrary to the chemical syntheses, which are "bottom up" methods. During this process, one can use a bulk solution of the noble metal and reduce the size by laser, grinding or other methods (not redox processes as used in general in chemical methods).

Recent advances in the fields of photochemistry and radiation chemistry allowed researchers to develop "green" synthesis protocols and alternative methods to synthesize nanostructures. Different light sources can be used to synthesize nanoparticles from "classical" UV-Visible light sources like Xenon lamps or LEDs to lasers. High energy radiations can be also used as a physio-chemical method to prepare nanoparticles and nanomaterials [13]. We describe here two physical methods for synthesis of metal nanoparticles: magnetron sputtering and laser ablation or vaporization.

II-1-1-Magnetron sputtering

Sputter deposition is a physical vapor deposition technique that ejects vapor of the material onto a substrate to synthesize nanoparticles on a solid support. The application of a potential difference between the target and the reactor walls in a rarefied atmosphere allows the creation of a cold plasma. In the presence of an electric field in the rarefied atmosphere, the cathode (the targeted support in this case) will attract the positive charges (the cations).

Actually, an exchange between their momentums will cause sputtering of atoms in the form of neutral particles, which condense on the substrate. The sputtering gas is often an inert gas such as argon or nitrogen. This method has been used to synthesize various metal nanoparticles, including Au NPs (Figure I-5). Nevertheless, the control of the size by this method is limited [14]. In other similar techniques, gold nanoparticles can also be synthesized by Laser Ablation in solution. At low laser flux, the material is heated by the absorbed laser energy and either evaporates or sublimates (Figure I-7). This method leads to the synthesis of well-dispersed and stable colloidal nanoparticles. Laser Ablation of solid targets in liquid environment allows the generation of nanoparticles (NPs) with high purity, easy surface functionability, metastable compositions, or complex structures, including doped nanocrystals, core-shells, hollow microspheres, nanotruffles, or nanocrescents [16].

II-2-Chemical methods for synthesis of metal nanoparticles

A metal salt or a metal complex is used as a precursor in solution or on supports (substrates) and is reduced by chemical agents or via radiation as we will explain later. In general, to obtain stable nanoparticles in solutions, synthesis proceeds in the presence of stabilizing agents (ligands, surfactants or polymers). Stability of colloidal solutions containing metal nanoparticles is either electrostatic or steric (Figure I-8). Electrostatic stability is due to a repulsive force generated by an electric double layer between charged nanoparticles in solution. Steric interactions with the stabilizing agents create a barrier that separates nanoparticles (polymer thiol chains can be an example) and can lead to stability of the colloidal solution. In many chemical methods, gold nanoparticles are produced in solution by reduction of a gold complex (in general chloroauric acid H[Au III Cl4] or potassium chloroauric K[Au III Cl4]). After dissolving H[Au III Cl4], the solution is rapidly stirred while a reducing agent (a strong reducing agent such as NaBH4 or NH2-NH2 or a weak reducing agent such as citrate) is added along with a stabilizing agent (ligand, surfactant or polymers) to limit the size of the nanoparticles and avoid aggregation [3,19,20]. In particular, citrate (a weak reducing agent) can play the double role of reducing and stabilizing agent [21].

II-2-1-Türkevich synthesis of gold nanoparticles

Türkevich method is probably one of the most basic and reproducible method for the synthesis of Au NPs. J. Turkevich et al [21] first published this method in 1951, and then G.

Frens [22] studied this method extensively and developed it by adding the parameter of goldto-reductant ratio to well control the nanoparticle size. Türkevich method is based on the reduction of an Au(III) salt (HAuCl4 or KAuCl4) in aqueous solution by trisodium citrate.

Citrate plays a double role: reducing and stabilizing agent. Indeed, it will reduce Au III into Au 0 and stabilize in the same time the formed Au nanoparticles. Because citrate is a weak reducing agent, the Au III solutions are heated and generally boiled [9]. Türkevich-based methods can be also used for different precursors to form palladium, platinum, and bimetallic nanoparticles (NPs) such as silver-gold NPs.

In this study, we choose Türkevich-Frens method to synthesize Au nanoparticles in solutions because of the easiness and reproducibility of the method and the weak ligand character of citrate, which can be exchanged easily with other stronger ligands.

II-2-2-Deposition-Precipitation Method

Deposition-precipitation (DP) method is widely used by researchers to synthesize metal nanoparticles of controlled size on supports [4,23]. Typically, the supporting material, generally an oxide powder, is immersed into an aqueous solution of HAuCl4 under stirring and fixed pH. Then, the catalyst precursor is recovered via centrifugation followed by washing steps with distilled water and sometimes ammonia to ensure maximum removal of Cl -ions. The latter ions can affect the tuning of gold nanoparticles shape and morphology, as they tend to form undesired products during the thermal reduction of catalyst precursor.

Afterwards, the remaining powder is calcinated (thermally treated under oxygen) in order to achieve certain size and morphology of gold nanoparticles.

Thermal reduction process consists in general in three steps:

(1) A slow rise in temperature until the desired calcination temperature is reached;

(2) A plateau of 2 hours at constant temperature should be maintained;

(3) A cooling-off at the same rate (in absolute value) as the initial temperature rise.

Any variation of temperature during the calcination plateau can affect the size of gold nanoparticles especially, when working on Au nanoparticles on TiO2 [19,23].

Deposition method can also use other "reduction" methods to obtain gold nanoparticles.

The replacement of calcination method can include photoreduction, radiolysis, and even chemical reduction and stabilization.

II-3-Synthesis of metal nanoparticles by radiolysis

Radiolysis is a powerful technique to synthesize metal nanoparticles of controlled size and shape in solutions and on supports [24][25][26]. This technique was largely developed in the Laboratoire de Chimie Physique (LCP, Université Paris-Sud). In this technique, solvated electrons and reducing radicals induced by solvent radiolysis are responsible of reducing the metal precursors [25]. Radiolysis presents the advantage of inducing a homogeneous reduction and nucleation in the whole volume of the sample leading to homogeneous nucleation and growth.

The primary effects of the interaction of high-energy radiation such as particle beams, Xrays or gamma photons with a solution of metal ions are the excitation and the ionization of the solvent molecules, which lead to the formation of primary species. In case of water, these primary species are:

H 2 O → e (𝑎𝑞) -, H 3 O + , H . , OH . , H 2 , H 2 O 2
The solvated electron e (aq) and the hydrogen atom are reducing species able to reduce metal ions to atoms, whereas the hydroxyl radical •OH is an oxidizing species:

E 0 (H 2 O/𝑒 (𝑎𝑞) -) = -2.87 VNHE E 0 (H + /H•) = -2.3 VNHE E 0 (OH  /H2O) =1.
(CH 3 ) 2 CHOH + OH . → (CH 3 ) 2 C . OH + H 2 O (CH 3 ) 2 CHOH + H • → (CH 3 ) 2 C • OH + H 2
The reduction steps of Au III Cl4 into Au 0 in aqueous solution and the formation of gold nanoparticles have been studied by the LCP team [24,25]. Au III is reduced to Au 0, which will coalesce and aggregate to form nanoparticles, via the following pathways: Metal nanoparticles of controlled size can be obtained by controlling the precursor and stabilizer concentrations as well as the dose rates (which fix the reductions kinetics) and the dose (which control the reduction rate).

Au III + e (aq) - → Au II Au III + (CH 3 ) 2 C  OH → Au II + (CH 3 ) 2 CO + H +
Anisotropic nanoparticles, such as Au nanorods [13] or palladium nanosheets and nanoflowers [27][28][29][30] can be obtained using surfactants or ligands, which promote 1D-or 2Dgrowth. In case of bimetallic nanoparticles, the structure (core-shell structures or alloys) can be easily tuned by controlling the dose rate: At low dose rate, core-shell nanoparticles are generally obtained, while nanoalloys are obtained at high dose rate [31].

III-Properties of metal nanoparticles and applications

III-1-Catalytic properties

Chemists have long neglected gold because of its inertness. In 1987, M. Haruta has demonstrated that very small Au nanoparticles (with diameters less than 5 nm) can catalyze the oxidation of CO (which is a very toxic gas) into CO2, even at room temperature [28,30,32]. Compared to platinum catalyst used for the CO oxidation reaction in the catalytic convertors of cars, the reactivity is not only multiplied by a factor close to 10, but it also occurs at much lower temperature (even at room temperature). Since this discovery, gold NPs are attracting a huge attention from the scientific community [28,29].

Usually, these Au nanoparticles are supported on support oxides such as titania or alumina for application in heterogeneous catalysis. Au NPs displayed catalytic properties for many other chemical reactions, such as oxidation of alcohols to acids or aldehydes, the formation of hydrogen peroxide, and selective hydrogenation of C-C bonds and NO [33,34].

III-2-Optical properties

Light and matter interaction has received extensive attention in recent years [29]. Noble metals specifically, exhibit interesting optical properties at the nanoscale. Optical properties of plasmonic nanostructures have been a very interesting subject for physicists and chemists, especially since Mie solution to Maxwell-Garnette equation was published in early 1900 [12].

The ability of acting as nanoscale heat sources under light irradiation is also considered equally puzzling and interesting for many applications.

Plasmonic nanostructures are mainly characterized by their excellent absorption of visible light through localized surface plasmon resonance (LSPR). This resonance can be described as coherent oscillations of the confined surface electrons gas excited by incident electromagnetic radiation, when the excitation frequency matches the characteristic frequency of metal nanostructures. Furthermore, the definition of a Plasmon can be presented as following "a plasmon is a quantum of plasma oscillation" (Figure I-10). In other words, a plasmon is the quantization of the collective longitudinal and/or transversal excitation of a conductive electron gas in a metal. Narayanan et al. published a very interesting review that discusses the physical origin of surface plasmon resonance in gold nanoparticles while focusing on Mie and Maxwell-Garnette theory and reviews the effects of particle size and shape on the resonance conditions [35]. Furthermore, it is known that reducing the size of a nanoparticle has a pronounced effect on the energy level spacing as the system becomes more confined [16,36]. The plasmon resonance phenomenon, which arises from the dipolar nature of the wave-driven collective electron excitation allowed by the small nanoparticle size, is essentially a coherent set of in-phase intraband transitions. Then, NPs appear within the complementary colour, which is red. In case of silver the plasmon is around 420 nm (for the same size) leading to a yellow colour (Figure I-11). Moreover, as the size of gold and silver nanospheres increases, there is a red-shift in the LSPR due to a low frequency for the collective oscillation of electrons, which causes electromagnetic retardation in larger particles. The LSPR absorption is also tuned by the surrounding dielectric environment that is influenced by the solvent effect and substrate effect [37]. exceptional optical properties and their potential applications in nanomedicine and the field of sensors [38][39][40]. Indeed, they present two surface plasmon resonance (SPR) absorption bands corresponding to the electromagnetic wave-driven oscillation of the quasi-free electrons along (longitudinal SPR, LgSPR) and perpendicular (transverse SPR, TrSPR) to the rod long axis (Figure I-12). The selective excitation of one or the other depends on the electric field polarization. Whereas the TrSPR is almost insensitive to the NR morphology, the spectral location of the longitudinal one (LgSPR) can be easily tuned from green to near-infrared by modifying the NR aspect ratio (the aspect ratio being the ratio of the largest axis to the shortest one): The longer the rod is, the smaller the LgSPR frequency. On the basis of these properties, noble metal NRs are thought to be good candidates for different applications, such as nanoparticle mediated hyperthermal therapy against cancer and optical data storage [41].

Wavelength (nm) It is also worth mentioning that noble metals at a nanoscale could act as sources of electrons and heat under illumination. The localized Surface Plasmon Resonance (LSPR) phenomenon could become a very effective way of disposing energy into nanoobjects, especially in the case of noble metals.

When the incident light excites the confined surface electrons, the nanoparticle itself tend to experience a depolarization of the electron gas. The vibrational modes of conductive electrons will vibrate in a coherent manner that will be followed by either an ejection of a hotelectron, or a localized thermal change due to energy input and dissipation in the surrounding medium. Hot electrons originate from the relaxation pattern in which excited electrons relax by electron-electron (e-e) scattering (heat dissipation) [43].

Moreover, LSPR is an efficient way to input energy in a small metallic objects, which will enhance the yield of nano-scale light to heat conversion. Thus, a new door in applied sciences is open for investigation. The applications of LSPR can range from bio-sensors and medical application in cancer therapy, to plasmonic catalysis and solar light harvesting.

Plasmonic solar-cells are for example being developed to enhance light-harvesting mechanisms, thus raising the efficiency of solar cells in terms of photo conversion efficiency (PCE) [44].

III-3-Biological applications

Cancer therapy by hyperthermia can treat tumors by heating, for example using lasers. This is the principle of the "thermal therapy". NPs allow to heat cancer cells very locally and to kill them. Plasmonic NPs absorb light rapidly and the excitation is converted into heat, increasing hyperthermia locally generated by the laser [START_REF] Loo | Technology in Cancer Research & Treatment[END_REF].

The absorption band of the NPs has to be located in the near-infrared region "biological window" from 650 to 900 nm (the wavelength range penetrating living tissues). Because of their optical properties, stability and their biocompatibility, gold nanoparticles are the candidate of choice for this application. Therefore, it is important to optimize the size and shape of gold nanoparticles: Au nanorods, Au bipyramides and Sicore-Aushell NPs are developed for thermal therapy [START_REF] Hirsch | Nanoshell-mediated near-infrared thermal therapy of tumors under magnetic resonance guidance[END_REF][START_REF] Huang | Cancer cell imaging and photothermal therapy in the near-infrared region by using gold nanorods[END_REF][START_REF] Jain | Cell-Specific Radiosensitization by Gold Nanoparticles at Megavoltage Radiation Energies[END_REF][START_REF] Niidome | Development of functional gold nanorods for bioimaging and photothermal therapy[END_REF]. Au nanoparticles can be used in imaging and therapy in the same time (theranostics) using their optical properties. Drug-delivery using photochemical properties of metal nano-objects is also widely studied [START_REF] Farokhzad | Impact of Nanotechnology on Drug Delivery[END_REF][START_REF] Andreu | Nano-objects for Addressing the Control of Nanoparticle Arrangement and Performance in Magnetic Hyperthermia[END_REF]. Photothermal imaging is also being developed as the understanding of the fundamental physics of LSPR is continuing to expand. Individual detection of single-nanometer sized particles in liquid by photothermal microscope was proposed [START_REF] Xiaohua | Cancer Cell Imaging and Photothermal Therapy in the Near-Infrared Region by Using Gold Nanorods[END_REF]. Single metallic nanoparticles imaging for protein detection in cells was proposed using antibodies carrying gold nanoparticles (10 nm) and heated by a laser at 514 nm [START_REF] Huang | Cancer cell imaging and photothermal therapy in the near-infrared region by using gold nanorods[END_REF].

Au NPs can be easily functionalized and used as sensors to detect biological molecules [START_REF] Bedford | Spiky gold shells on magnetic particles for DNA[END_REF][START_REF] Loiseau | Silver-Based Plasmonic Nanoparticles for and Their Use in Biosensing[END_REF][START_REF] Ben Haddada | Gold Nanoparticles Assembly on Silicon and Gold Surfaces: Mechanism, Stability, and Efficiency in Diclofenac Biosensing[END_REF]. They are for example used in the pregnancy tests, where they are functionalized with anti-bodiesanti-β-HCG hormonesfor detection of β-HCG hormones characteristic of pregnancy. Au NPs can also be used for detection of malignant cells or infectious agents such

IV-Catalysis assisted by plasmon of metal nanoparticles

In the following paragraphs, we will try to go over the latest developments in the field of catalysis assisted by plasmonic nanostructures. Since most of the work introduced in this manuscript is focused on gold nanoparticles, we will emphasis on that particular aspect. In addition to photophysical processes, optically excited plasmonic nanoparticles can also activate and induce chemical transformations directly on their surfaces. This phenomenon is called plasmonic catalysis and offers opportunities in reduction or oxidation reactions and in the field of selective chemical synthesis [START_REF] Wang | Photocatalysis for Organic Synthesis and Clean Energy Conversion[END_REF].

In the last decade, application of plasmonic nanoparticles for solar to electrical energy and solar to chemical fuel conversions attracted a lot of interest. In a review, Linic et al.

summarized recent progress in the field of photochemical catalysis on plasmonic metallic nanostructures [44]. The use of LSPR excitation to drive photocatalysis is called plasmonic photocatalysis and falls into two categories: (a) indirect photocatalysis, where excitation of LSPR is used to transfer photon energy to nearby semi-conductors, molecular photocatalysts, and other metals to drive chemical reaction, and (b) direct photocatalysis, where excited metal nanoparticles act as direct catalytic active sites. In the present work we will focus on direct photocatalysis (where the metal nanoparticles are not supported). By precise control of the size, shape, and environment of a plasmonic metal nanostructure, it is possible to design efficient plasmonic photocatalysts for applications.

IV-1-Plasmon assisted catalysis with non-supported metal nanoparticles:

Surface plasmon excitation of colloidal gold nanoparticles with visible light in the presence of H2O2 led to rapid and selective oxidation of sec-phenethyl and benzyl alcohols to acetophenone and benzaldehyde, respectively (Figure I-14). Au NPs were excited by light (through both 532 nm laser and 530 nm LED techniques) or via microwave irradiation [START_REF] Hallett-Tapley | Plasmon-Mediated Catalytic Oxidation of sec-Phenethyl and Benzyl Alcohols[END_REF].

The authors proposed electron transfer with the nanoparticle surface, as well as the participation of peroxyl and ketyl free radicals as fundamental steps in the reaction pathway. This study was one of the first examples of colloidal Au NPs employed in plasmonmediated organic reactions, specifically for alcohol oxidations. These results showed that plasmonic photocatalysis allows the use of more environmentally friendly precursors, such as water and H2O2, rather than the more current and toxic alternatives in oxidation reactions used in organic chemistry. Interestingly, the authors studied and discussed the mechanism. The authors proposed that hydroxyl radicals can be induced by electron transfer to H2O2 or by thermal decomposition of H2O2 and that HO . and . OOH (formed by the reaction of HO . with H2O2) radicals induce oxidation of the alcohols. After exposure to light, the nanoparticles aggregated, which led to a change in color (from red to blue).

The intermediate polarization of the NP surface electrons via plasmon excitation can induce rapid localized heating of the NP surface allowing for harvesting of thermal energy.

The important increase in temperature around the NPs allows chemical reactions requiring high temperatures to be carried out in the microenvironment surrounding the AuNPs at ambient conditions. The induced electromagnetic field generated upon plasmonic excitation can also be used to exploit the photochemical activity of metal nanoparticles. The use of plasmon excitation can amplify the efficiency of many photochemical processes by excitation in the visible region of the electromagnetic spectrum. More importantly, direct excitation of metal nanoparticles can also favor ejection of electrons, and this can be applied in a variety of Plasmon relaxation can lead to thermal effects (T) that can themselves induce supramolecular changes in guest molecules (Ts), chemical change (Tc) sometimes referred as photocatalysis, or changes in the nanoparticle itself (Tn) of either a physical or chemical nature (e.g., oxidation). Under plasmon excitation the nanoparticle can act as an electron donor (Et) or as an electron acceptor (Ht). Red arrows are used to highlight the mechanisms to be discussed in this contribution [43].

Bimetallic nanoparticles have attracted increasing attention for applications in different fields such as catalysis, electrocatalysis, sensing, magnetic storage, etc. In particular, bimetallic nanoparticles based on gold have are attracting a lot of interest because of their optical properties and their application in catalysis and electrocatalysis [START_REF] Hallett-Tapley | Plasmon-Mediated Catalytic Oxidation of sec-Phenethyl and Benzyl Alcohols[END_REF][START_REF] Sandoval | Bimetallic Au-Ag/TiO2 catalyst prepared by deposition-precipitation: High activity and stability in CO oxidation[END_REF][START_REF] Li | Improved activity and stability of dealloyed bimodal nanoporous PtCo catalysts for CO oxidation[END_REF][START_REF] Zhang | Surface plasmon-enhanced zeolite catalysis under light irradiation and its correlation with molecular polarity of reactants[END_REF]. Development of metal nanostructures that exhibit both unique plasmonic and unique catalytic features is critical for application in plasmonic catalysis. Bimetallic AuPd nanowheels (Au core surrounded by metallic Pd) with tunable SPR have been synthesized by Huang et al [START_REF] Huang | Plasmonic and Catalytic AuPd Nanowheels for the Efficient Conversion of Light into Chemical Energy[END_REF].

These AuPd nanowheels can utilize solar energy to drive catalytic reactions, such as the oxidation of benzyl alcohol and Suzuki coupling reactions, at lower temperatures, and with much higher performance compared to conventional heating processes.

Figure I-18-

Solar spectrum at the top of the atmosphere and at sea level [START_REF] Wu | Design and Fabrication of an Albedo Insensitive Analog[END_REF].

AuNPs have also the potential to use sunlight to efficiently drive chemical reactions and induce photocatalytic processes. These catalytic processes induced by light can be used for synthesis of azo-dyes, which are widely used in the production of dyes, pharmaceutical products, and food additives. Currently azo-dues are synthesized under high pressures and at high temperatures using transition-metal reducing agents and the formed by-products are harmful to the environment [START_REF] Grirrane | Gold-Catalyzed Synthesis of Aromatic Azo Compounds from Anilines and Nitroaromatics[END_REF]. It has been shown for example that Au NPs (of about 6 nm) supported on ZrO2 can drive reduction of nitro-aromatic compounds (in particular nitrobenzene into azobenzene in the presence of isopropanol (which will be converted into acetone) when excited under solar light [START_REF] Zhu | Reduction of Nitroaromatic Compounds on Supported Gold Nanoparticles by Visible and Ultraviolet Light[END_REF]. Isopropanol plays the role of sacrificial donor of protons and electrons. Pineda et al. reported that Au/SiO2 nanoparticles irradiated by a laser at 532 nm catalyze amidation reactions: The production of 4-benzoylmorpholine from benzaldehyde and morpholine via amide formation was obtained with higher conversion rate and higher selectivity when the nanoparticles were irradiated [2].

Plasmonic catalysis can also be used for water treatment. Excitation of 5 nm-Au NPs deposited on SiO2 leads to decomposition of methyl orange dye (used as model pollutant) and trichloroethylene (selected as an example of a real water pollutant) [START_REF] Gomez | Plasmon-enhanced photocatalytic water purification[END_REF]. These oxidative processes are induced by oxidative radicals (O2 .-can be formed by reaction of hot electrons with O2) . as well as HO . formed by the reactions:

O2 •-+ H +  HO2 • (1) 2 HO2 •  H2O2+O2 (2) H2O2+O2 •- • OH+ O2 +OH - (3) H2O2 + h 2 • OH (4)
Plasmon excitation can also be used for functionalization of Au NPs. Reactivity of aryl diazonium salts is commonly used to graft them on surfaces by electro-reduction. Aryl diazonium can also be grafted using UV or visible light (photo-grafting), but with this process the grafting slow or weak, photosensitizers can then be used to enhance its efficiency.

Controlling the surface grafting of species at the nanoscale remains a major challenge for different applications. Nguyen et al. reported plasmon-mediated grafting of aryl films derived from diazonium salts on regular arrays of gold nanostripes. This grafting occurs specifically in the regions of maximum of field enhancement [START_REF] Nguyen | Plasmon-mediated chemical surface functionalization at the nanoscale[END_REF]. The thickness of the aryl film can be monitored by the LSP wavelength and the incident light energy.

IV-3-Plasmonic NPs supported on photocatalytic substrates

TiO2 is a very efficient photocatalyst due to its strong oxidation capacity, high photochemical and biological stability and low cost. Since the discovery of photoinduced decomposition of water on a TiO2 electrode, TiO2-based photocatalysts have attracted wide attention [START_REF] Fujishima | Electrochemical Photolysis of Water at a Semiconductor Electrode[END_REF]. The limitation in TiO2 application, results from low quantum yield due to fast charge carriers (electron/hole e-/h+) recombination and its activation only under UV irradiation because of the value of its band gap (3.2 eV for anatase and 3.0 eV for rutile). UV light constitutes only about 3-4 % of the solar spectrum impinging on the Earth's surface, therefore modification of titania to extent its absorption to visible domain and to enhance its activity is a very active area of research. Surface modification with noble metal (platinum, palladium, silver, gold) nanoparticles (NPs) can result in enhancement of the photo-conversion quantum yield and may allow the extension of the light absorption of wide band-gap semiconductors to the visible light. In particular, plasmonic photocatalysts have appeared as a very promising way to induce a photocatalytic activity of TiO2 in the visible range [7,44].

Different studies report on modification of TiO2 with metal nanoparticles for photocatalytic applications: water depollution and hydrogen production. In the plasmonic photocatalytic Au/TiO2 system, the presence of Au is essential, due to their localized surface plasmon resonance (LSPR). These plasmonic properties can be used to induce photocatalytic activity of the semiconductor material under visible light.

However, the support presents also important effects on the photocatalytic processes, i.e., (i) the electron-hole recombination for anatase and rutile particles has been studied and was found to be higher for rutile than anatase;

V-Conclusion

Nanoscience and Nanotechnology show an increasing interest and a huge development. The fields of potential applications of nanoparticles and nanomaterials are large: chemistry, plasmonics, catalysis, optics, electronics, nanomedicine… Gold has fascinated humans from the ancient time because of its colour and nobelty.

Today Au NPs are attracting increasing attention because of their plasmonic and catalytic properties and their potential applications in different fields. These plasmonic properties can be used to assist catalytic processes using solar light, and with lower energy consumption. The use of plasmonic photocatalysts to catalyze chemical reactions is a new field, and its applications are challenging. In the next chapters, examples of catalysis assisted by plasmon with Au nanoparticles and Pd nanostructures will be presented.

This chapter describes the synthesis and characterization techniques of plasmonic gold nanoparticles and palladium nanoflowers performed in this research as well as the illumination setups and the analytical methods used to follow the chemical reactions.

I-Synthesis of metal nanoparticles and nanostructures I-1-Türkevich synthesis of spherical gold nanoparticles in solution

Spherical gold nanoparticles can be prepared by several methods (chemical, photochemical and radiolytic, and physical methods (cf. Chapter I).

We have chosen the Türkevich method for gold nanoparticles synthesis, as it allows us to work in an aqueous medium with controlled pH at room temperature. This method is based on the reduction of Au III complexes [AuCl4] -by citrate ions, which act as reducing and stabilizing agents.

Türkevich method is probably one of the most basic and reproducible method for synthesis of Au NPs. J. Türkevich et al. [1] first published this method in 1951, and then G.

Frens [2] developed this method, and added the parameter of gold-to-reductant ratio to well

I-2-Synthesis of Palladium Nanoflowers

Ultrathin palladium hexagonal nanosheets were synthesized in emulsions constituted of droplets of toluene containing Pd complexes in water and stabilized by CTAB as surfactant and in quaternary mesophases formed by water, toluene containing Pd complexes, CTAB, and a co-surfactant (pentanol) [4,5]. In this work, we synthesize palladium nanoflowers by radiolysis [5,6]: 20 mL of ethanol solution containing Pd(acac)2 (10 -3 M) were transferred in a small pyrex bottle, which was then sealed with a rubber septum. The solution was flushed by flowing argon, and then CO (1 atm) was carefully passed through the sample at a very low flow rate for 15 min.

Caution: CO is highly toxic gas. Hence, bubbling was done with extreme caution in a very well ventilated hood with external evacuation.

The resulting solution has a light-yellow color. Immediately after CO bubbling, the samples were exposed to gamma irradiation with a 60 Co panoramic source for 1 hour at a dose rate of 3 kGy/h to complete the full reduction of Pd II into Pd 0 . The light yellow solution turned to dark blue after irradiation. The radiolytic reduction of Pd (acac)2 at 10 -3 mol L -1 in ethanol under CO atmosphere leads to Pd nanostructures [5].

It has to be noted that Pd II (acac)2 solutions are stable under CO for many hours: There is no evolution of the absorption spectrum and black large Pd particles (which precipitate) are formed only after few days. Indeed, CO reduces Pd(II) very slowly and the direct reduction by

CO is negligible at the scale of hours.

The Pd nanostructures are stable in ethanol solution for few days in the dark and for few hours under light irradiation.

II-Characterization techniques of the metal nanostructures

Different characterization techniques were used to characterize the Au nanoparticles and the Pd nanoflowers synthesized in this thesis, before and after catalytic reactions.

II-1-UV-Visible absorption spectroscopy

The optical properties of the metal nanoparticles in solution were studied by UV-visible absorption spectroscopy. A beam with a wavelength varying between 180 and 1100 nm passes through a solution in a cuvette. The sample in the cuvette absorbs this UV or visible radiation. The absorbance A() at a certain wavelength λ is governed by the Beer-Lambert law: For metal nanoparticles the characteristic surface plasmon resonance band is observed by absorbance spectroscopy. The spectra of spherical nanoparticles of gold present a SPR located around 520 nm. As the SPR depends on the size and shape of nanoparticles, this observation also gives us an idea about the size of nanoparticles formed. From the absorbance spectroscopy we can also observe scattering, which is due to the size of the particles, their dispersion and also agglomeration in the medium.

𝐴( = 𝑙𝑜𝑔 ( 𝐼 0 𝐼 ) = 𝜀𝑙𝑐
UV-vis absorption spectra of the solutions were recorded on either a Carry 5000 (Agilent) spectrophotometer or a HP8435 diode array spectrophotometer. The various core level binding energies observed in a spectrum can be used to identify all the elements of the periodic table except for hydrogen and helium. Chemical state information can also be extracted because binding energies are sensitive to the chemical environment of the atom.

II-2-Transmission Electron

XPS measurements were performed on a K-Alpha X-Ray Photoelectron Spectrometer

III-Analytical methods

III-1-Electrospray Ionization Mass Spectrometry

Mass spectrometry is an analytical technique that can provide both qualitative (structure)

and quantitative (molecular mass or concentration) information on analyte molecules after their conversion to ions. The molecules of interest are first introduced into the ionisation source of the mass spectrometer, where they are first ionized to acquire positive or negative charges. The ions then travel through the mass analyzer and arrive at different parts of the detector according to their mass/charge (m/z) ratio. Electrospray Ionization -mass spectrometer (ESI-MS) is composed of three basic components: ion source, mass analyzer, and detector. The intact molecular ions are produced in the ionization chamber where the ion source is kept, and then they are transferred in the mass analyzer region via several optics [7].

The mass analyzer sorts and separates the ions according to their mass to charge ratio (m/z value). The separated ions are then passed to the detector systems to measure their concentration,

In the ESI unit (figure II-4), a dilute (less than mM in polar volatile solvent) analyte solution is injected by a mechanical syringe pump through a hypodermic needle or stainless steel capillary at low flow rate (typically 1-20 μL/min). A very high voltage (2-6 kV) is applied to the tip of the metal capillary relative to the surrounding source-sampling cone or heated capillary (typically located at 1-3 cm from the spray needle tip) [7].

III-2-Gas Chromatography coupled with Mass Spectroscopy

Gas Chromatography coupled with mass spectroscopy (GC-MS) is considered as an important analytical tool for natural products analysis, volatile organic compounds, active medical materials and pollutants. GC-MS offers an in-depth knowledge of the sample components as it allows to detect their masses along with their chemical structure.

For example, when analyzing volatile organic compounds (VOC), target analytes are extracted by mixing the sample with water and purge with inert gas (Argon gas in our studies) into an airtight chamber, this is known as purging. Purge-Trap technic is widely used in many petrochemical industries [8]. The trap is a column of adsorbent material at ambient temperature that holds the compounds by returning them to the liquid phase. The trap is then heated and the sample compounds are introduced to the GC-MS column via a volatiles interface, which is a split inlet system. Purge and trap gas chromatography-mass spectrometry system (P&T GC-MS) is particularly suited to volatile organic compounds (VOCs).

Gas chromatography (GC) analyses were performed on a Varian 430-GC gas chromatograph and GC coupled to mass spectrometry (GC-MS) analyses were performed on a Thermo Scientific DSQ apparatus.

III-3-Surface Raman Enhanced Spectroscopy (SERS)

Surface Raman Enhanced Spectroscopy (SERS) was first observed in 1974. It is an extension of normal Raman spectroscopy that relies on electronic and chemical interactions between the excitation laser, analyte of interest, and SERS substrate [9]. Raman signal enhancements as high as 1014 can occur for resonant molecules found in "hot spots" on a SERS substrate because of combined electromagnetic (EM), charge transfer (CT), and resonance signal enhancement mechanisms , [9].

After the centrifugation of sample dispersed in solution at different times of the reaction, the samples were separated into a supernatant and a residue. The resulting residue is a fairly dense aggregation of spherical gold nanoparticles at the bottom of the Eppendorf. The medium of the reaction is aqueous, which raises an issue regarding the resolution of Infra-Red preliminary characterization. Hence, it was necessary to ensure evaporation of water residue in the droplet. This could not be conducted via localized thermal evaporation because of a possible degradation of several organic reagents/pollutants, which is a limitation for the inspection. However, the droplets were left to dry at room temperature, while reserved in dark to avoid possible excitation under visible light.

There were other limitations related to the usage of Infra-Red based techniques. One limitation was the vibrational modes of the ensemble of molecules that cover a wide range of wavenumbers, hence, a limiting overlap between initial and final products. Other obstacle was maintaining a considerable final concentration of reagents remaining in the droplet. The latter can be overcome by preparing series of samples ranging from 10 -3 to 10 -6 M of final concentrations and by enhancing the spectral resolution. Nevertheless, the enhancement of spectral resolution will extend the acquisition time.

Raman and Surface-Enhanced Raman Spectroscopy (SERS) measurements were carried out with a Raman spectrometer (Labram HR evolution (Horiba Sicentific, France SAS Villeneuve d'Ascq). The excitation source was a 633 nm He-Ne laser. The spectral region studied was 400-2000 cm -1 with a spectral resolution of 4 cm -1 . Acquisition time of each spectrum was 1 to 5 seconds. SERS experiments were done in collaboration with Ali Tfayli (Lip(Sys)², EA7357, Faculté de Pharmacie, Châtenay Malabry, Université Paris-Saclay)..

IV-Irradiation setups

In order to carry out a catalytic reaction induced by plasmon, we used visible light (200 to 800 nm) as a source of excitation.

Several parameters were taken into consideration when choosing the light source. Intensity of light source is an important factor. Intensity refers to the number of photons emitted by a light source with a sufficient energy to excite the confined surface electrons. This number can differ depending on optical properties of catalysts, light-exposure geometry, reaction medium, and reflection/absorption characteristics of the catalytic setup [10].

IV-1-Xenon Lamp equipped with a longpass filter

Xenon lamps had been widely used in laboratory experiments, mainly for photocatalysis application. Irradiation was performed using a 300 W Xe arc lamp equipped with a water cell The tunable intensity with a maximum of 18 W will allow us to compare reaction rates when using different light sources.

IV-3-Continuous green laser excitation

The third set-up we used was developed by Bruno Palpant (LPQM, CentraleSupelec) and it is based on a continuous green laser at 532 nm. The laser beam with a defined intensity passes through a sample quartz cuvette. Another light source is used to record the absorption spectra with a specific interval between each reading, hence, the cross-section between laser and light passing through the sample, the absorption spectra is being recorded (Figure II-7).

The intensity of the laser is tunable, and we managed to optimize it at 20 mW per 10% of exposed sample carrier. In addition, a continuous stirring of the sample is arranged to homogenize the irradiated volume to the green light laser irradiation.

V-Chemicals

Most common laboratory chemicals were reagent grade, purchased from commercial sources (Sigma-Aldrich mainly or ACROS) and used without further purification:

Tetrachloroauric acid trihydrate: HAuCl4, 3H2O

Chapter III

Degradation of para-nitrothiophenol by plasmon excitation of gold nanoparticles I-Introduction

For the past decades, nanoscience has become a growing field of interest. In particular, metal nanoparticles (MNPs) have been the subject of numerous research studies because of their unique properties that differ from their bulk counterparts and find applications in various domains, such as optics, electronics, catalysis or biology [1][2][3][4][5]. Among the MNPs, silver (Ag)

and gold (Au) nanoparticles present a strong absorption band in the visible spectral region due to a localized surface plasmon resonance (LSPR) [6]. This resonance corresponds to a coherent oscillation of the free electron gas of the MNPs for which the frequency matches the incident light frequency [1,[7][8][9]. LSPR depends on the metal, the size, the shape and the environment of the NPs and induces several effects i.e. electromagnetic-field enhancement, heat generation, "hot" electrons production, which have been exploited, for instance, in biosensing, photothermal therapy, photovoltaics or (photo-) catalysis [7,[10][11][12][13][14].

Lately, applications of plasmonics in chemistry has arouse with the main objectives to trigger, select chemical reactions and control their yield. Several reviews have already been published describing the optical and thermal properties of the nanoparticles and their main possible effects in chemical reactions [15][16][17][18]. According to the role played by the plasmonic nanoparticles, two classes of reactions are distinguished: plasmon-assisted surface catalysis and plasmon-driven photocatalysis However, the involved mechanisms are not always well understood and sometimes still under debate [13,19,20].

Among the chemical reactions studied, many papers deal with the reactivity of nitroarenes in the presence of nanoparticles [20][21][22]. Indeed, the photo-induced reduction of nitroarenes is often chosen to test the catalytic properties of metal nanoparticles, Paranitrophenol (pNP, 4-nitrophenol) and para-nitrothiophenol (pNTP, 4-nitrobenzenethiol) being used as model pollutants [8,20,[23][24][25][26][27]. The metal nanoparticles serve as catalyst, the reductant being usually NaBH4 (Scheme III-1).

Scheme III-I. Illustration of reduction of pNTP via a metallic catalyst.

However, in the case of plasmonic nanoparticles under light illumination, it has been shown that nitroarenes react on the surface and may form different products depending on the environment [8,20,25].

Within this context, we have undertaken to study the degradation of para-nitrothiophenol (pNTP) in aqueous solution in the presence of plasmonic gold nanoparticles (AuNP) under visible light irradiation. UV-visible absorption spectroscopy, electrospray-mass spectrometry and surface-enhanced Raman spectroscopy were used to follow the disappearance of pNTP, to analyze the formed products and to get information on the involved mechanisms [28][29][30][31][32][33][34][35].

II-Sample preparations and irradiation

Para-nitrothiophenol (C6H5NO2S) is a highly toxic compound resulting from pesticides treatment of soil and many manufacturing processes. It is a yellow crystalline powder that is It is worth noticing that, in the present study, whatever the set-up used for irradiation, in order to avoid possible two-photon absorption phenomena leading to the direct photodecomposition of pNTP, the light power was limited, i.e. 9 W for the LED reactor, 20 mW for the laser and 150 W for the lamp. Moreover, for the Xe lamp irradiations, a 520 nm longpass filter was used to avoid the direct photoactivation of pNTP.

Otherwise specified, all experiments were performed under ambient atmosphere (i.e. in the presence of dioxygen). Experiments were also carried out at room temperature (23  2 °C)

and the temperature of the solution did not increase by more than 5 degrees during light illumination.

III-Degradation of pNTP followed by UV-visible spectroscopy

The degradation of pNTP is followed by UV-visible spectroscopy as pNTP in aqueous alkaline solution exhibits characteristic absorption bands at 225 and 405 nm. The formed AuNPs also present absorption in the same spectral domain, but the main characteristic LSPR band is shifted towards the visible area, around 530 nm and 520 nm for 30 nm and 15 nm AuNPs, respectively. The described experiments were conducted under ambient atmosphere, i.e., in the presence of dioxygen. However, the formation of pATP corresponds to a reduction reaction.

In consequence, O2 may play a role as an oxidizing agent. So, experiments were also performed under nitrogen atmosphere; for that, the mixture solutions (containing pNTP and Additionally, the degradation of para-nitrophenol, instead of pNTP, was also studied under similar conditions. But, in that case no degradation was observed. That suggests that the thiol group, known to have a good affinity to Au surface, may play a role by favoring the interactions between the molecules and the AuNPs.

IV-Characterizations of the formed products

The monitoring of the degradation of pNTP by UV-visible absorption spectroscopy suggests the formation of pATP and other products. For chemical analysis, centrifugations of the irradiated solutions were performed at room temperature in darkness for 15 minutes with a velocity of 10.000 rpm. Samples were then separated into supernatant solutions and residues.

The residues containing the AuNPs were left to dry at room temperature in darkness to In Table III In the spectrum before illumination (Figure III-11a), the two main peaks observed at 153.9 and 190.9 m/z correspond to pNTP ions and citrate molecules, respectively. However, the spectrum is complex with many peaks. For some specific peaks in order to confirm their attribution, product ion scan (PIS) spectra were recorded to detect fragmentation occurring in the ionic source.

The PIS spectrum obtained from the peak at 185. peaks are observed at 124, 137.9, 153.8, 171.9, 187.9 m/z. Peaks at 137.9 and 153.9 m/z can be attributed to pNP and pNTP anions, respectively. The interpretation of the other peaks could be 124 para-aminobenzenethiolate, 171.9 [pNTP+H2O] or para-aminobenzene sulfonate, 187.9 [para-nitrobenzenesulfenate+H2O].

IV-2-SERS analysis of the AuNPs surface

Information on the AuNPs surfaces was obtained by SERS measurements. These peaks are in accordance with the SERS spectrum of ATP reported in the literature.

Consequently, photoreduction of pNTP to ATP has occurred on the AuNPs surface.

V-Discussion

According to the above results, the disappearance of pNTP requires both the presence of indicates that these species are produced and then disappear during the illumination time.

Taking into account these observations, we may propose the following mechanism.

Plasmon-excitation of AuNPs generates "hot" electrons and "holes"; the formers are responsible of the reduction, while the latters induce oxidation. So, in a first step, reduction of pNTP adsorbed on the AuNPs surface takes place in parallel to oxidation of the thiol group of pNTP molecules in solution (thiol groups are not very stable and easily oxidized). In a second step, the oxidized nitrobenzene are also reduced to form pATP, once they are in close interaction with the AuNP surface, i.e. when they have replaced citrate ions as ligands.

It is to note that, when experiments are carried out under air atmosphere, the electrons may react with the dioxygen and generate O2 -, which is an oxidizing radical. Such reaction favors the oxidation of pNTP of the detriment of its reduction. That may explain why no pNTP is detected in the supernatant.

VI-Conclusion

We have shown that the excitation of the plasmon band of AuNPs is responsible for the disappearance of pNTP indicating that reactions are induced by plasmon excitation, and hot electrons are involved in these reactions. However, adsorption of the molecules on the NPs seems to be an important factor. To validate our proposed mechanism, further experiments are needed. In particular, the above experiments where performed under air atmosphere, i.e. with dioxygen. To unravel the role of dioxygen in the formed products, chemical analysis of the products generated with irradiation experiments carried out under inert atmosphere should be done.

Anyway, these results demonstrate that plasmon-driven chemical reactions are very promising for environmental applications and depollution since pNTP is considered as a model pollutant and can be degraded. Yet the mechanism involving plasmonic nanomaterials, especially in solution, needs further examination.

I-Introduction

Redox reactions are of great importance in environmental catalysis as, for example, reduction of nitrogen oxides (deNOx), CO2 or heavy metals, and generation of hydrogen from water or oxidation of CO or organic pollutants [1]. Platinum, palladium and rhodium are the most used metal catalysts in hydrogenation reactions or in catalytic convertors for deNOx and CO reduction [2]. Catalysis by gold is a domain of increased interest since the discovery by Haruta [3] that small gold nanoparticles (Au-NPs) can catalyze CO reduction at room temperature [4][5][6][7][8].

Au-NPs have also attracted much attention due to their localized surface plasmon resonance (LSPR), i.e., the oscillations of metal free electrons driven by the electric field of the incident light [6,[9][10][11]. There are many applications utilizing the SPR of gold nanoparticles, including subwavelength electromagnetic energy transport, chemical and biological sensors [1,10], surface enhanced Raman scattering (SERS) [12,13], plasmonic photocatalysis [14][15][16],

and photothermal cancer therapy [17][18][19][20], where the plasmonic energy is converted locally into heat that raises the surrounding medium temperature, leading to cancer cell killing. In thermal therapy, hot electrons induced by surface plasmon excitation, lead (after reaction with oxygen or water) to formation of reactive oxygen species (ROS) (HO . , O2 .-and singlet oxygen), which are also responsible in cancer cell killing [21].

Metal nanoparticles such as gold and platinum NPs are efficient catalysts in different reactions with environment applications. These catalytic reactions often involve electron transfers [22,23]. Reduction of ferrocyanate (III) has been taken as a model reaction to study catalysis by metal NPs [22][23][24].

Mulvaney, et al [24] studied redox catalysis by colloidal gold for the reduction of ferricyanide (III) ions by borohydride ions in alkaline aqueous solution. They found that Au-NPs dramatically increase the reaction rate and that Au-NPs also change the mechanism: The reaction is zero order in hexacyanoferrate (III) concentration for the non-catalyzed reaction to first order for the catalyzed reaction. In the latter reaction, Au-NPs act as a reservoir for the electrons and become cathodically polarized. In a first step, borohydride can inject electrons onto Au-NPs, then ferricyanide ions diffuse to the nanoparticle surface and are reduced by excess surface electrons. These nanoparticles serve as a relay for electrons.

El Sayed's group reported on catalysis by metal nanoparticles for the reduction of ferricyanide ion by thiosulfate to ferrocyanide [23,25,26]: This model reaction has been used to compare the catalytic activity of different nanoparticles [24,27]. Size and shape dependent catalytic activity of platinum nanoparticles for this reaction has been demonstrated [23,28]. The study conducted on tetrahedral, cubic and spherical Pt nanoparticles has demonstrated that the kinetic parameters correlate with the fraction of surface atoms located on the corners and edges in each size and shape [28]. It has been also demonstrated that Au-NPs are very efficient catalysts for the reduction of ferricyanide (III) ion to ferrocyanide. El-Sayed et al. reported that catalysis of this reaction by gold nanoparticles was enhanced when the latter were irradiated at their plasmon wavelength [23]. The value of the activation energy of this electron transfer reaction for two different shapes of Au-NPs (nanospheres and nanocages) was measured. This activation energy was found to be similar when using a thermostatic water bath or by using a plasmonic photothermal reactor. This result supported the conclusion that the role of the surface plasmon field on the electron dynamics during the reaction was negligible, and that the main effect was due to temperature increase after plasmon excitation [7,15,16].

2𝐹𝑒(𝐶𝑁)
Recently, the LSPR effect of plasmonic nanostructures has been successfully applied to photocatalysis under visible light irradiation and proved to be very promising [15,29,30].

In the present study, we investigated in details the reduction of ferricyanide (III) ion into ferrocyanide (II) ion catalyzed by spherical Au-NPs and excited at their LSPR band.

Experiments were conducted in the presence (or not) of sodium thiosulfate. This catalysis is enhanced in the presence of Au-NPs with light excitation. We show for the first time that this reduction takes place even without sodium thiosulfate demonstrating the implication of hot electrons in this reduction.

II-Experimental details

Spherical Au-NPs were synthesized by Türkevich method [31][32][33]. Two well-defined sizes of Au-NPs (Au-NPs@citrate) with a mean diameter of 15 and 30 nm, respectively, were obtained by changing the experimental conditions (cf. chapter II). The 30 nm-Au-NPs@citrate solutions were diluted by a factor of four to get the same concentration of Au atoms as in the 15 nm-Au-NPs@citrate solutions. Figure . IV-1 shows the UV-visible spectra and TEM images of the synthesized and used Au-NPs@citrate The maximum of the LSPR band is located at 515

and 525 nm for the solutions containing Au-NPs of a mean size of 15 and 30 nm respectively.

III-Reduction of hexacyanoferrate in the presence of sodium thiosulfate

The studied reaction is the reduction of ferricyanide or hexacyanoferrate, HC-FeIII, by thiosulfate:

2 Fe(CN) 6 (aq) 3- + 2 S 2 O 3 (aq) 2- → 2 Fe(CN) 6 (aq) 4- + S 4 O 6 (aq) 2- (1)
The redox potential of the couples involved in this reaction are E 0 (Fe(CN)6 3-/Fe(CN)6 4-) = +0.36

V and E 0 (S4O6 2-/S2O3 2-) = +0.08 V versus the normal hydrogen electrode at 25°C [34].

Although thermodynamically favorable, this reaction needs to be catalyzed. Solutions The kinetics of the reduction reaction of HC-FeIII by ST is followed by the decrease in the absorbance at 420 nm (Figure IV-4). The reduction is faster for the smaller 15 nm Au-NPs@citrate compared to 30 nm Au-NPs@citrate, and for both sizes the reduction is accelerated by the irradiation. These results confirm that Au-NPs play a role of catalysts in this reaction. As catalysis takes place at the surface of the nanoparticles, the kinetics is enhanced with the specific surface area. Considering the average size of the nanoparticles (15 and 30 nm)

and assuming that they are quasi-spherical, their specific surface areas are respectively 14.5 m² g -1 and 7.3 m² g -1 , respectively.

Moreover, the excitation of the LSPR of the Au-NPs favors the catalytic properties of the Au-NPs. This plasmonic effect could be related to different mechanisms:

1. Electron density exchange between the excited gold nanoparticles and nearby reactants; This suggests that for the initial smallest 15 nm Au-NPs@citrate, assembly and precipitation of the elongated Au-NPs take place rapidly in seconds or minutes (at the scale of the record of the spectra), while for the initial 30 nm Au-NPs@citrate, oblate NPs and smaller aggregates are formed and remain in suspension.

IV-3-Effect of stabilizing agent of the Au-NPs

In the light of the results presented above, we repeated the experiments using Au-NPs stabilized by a non-reducing agent, and so we replaced citrate by para-nitrothiophenol (pNTP) since thiol based compounds are known to be used as stabilizing agents of gold nanoparticles, due to the strong affinity between sulfur and gold atoms. However, it is to keep in mind that during the reaction, the stabilizing agent (pNTP) could also transform [35], and lead to the formation of para-aminothiophenol (pATP), but without significant change in the stabilization of the Au-NPs [32,36].

V-Discussion

The previous results show the disappearance of HC-FeIII catalyzed by Au-NPs under irradiation in the presence, or not of ST. This disappearance is attributed to the reduction of HC-FeIII. To confirm the reaction and to have some insights into the role played by Au-NPs, X-ray photoelectron spectroscopy XPS analysis were carried out to determine the oxidation states of iron and gold. The XPS wide-scan spectra of 30 nm Au-NPs@citrate after reaction under LEDs irradiation at 520 nm for deaerated solutions containing HC-FeIII with ST or without ST (Figure show the presence of gold, iron, carbon, nitrogen, oxygen, potassium and sodium.

Figure IV-14. XPS wide-scan spectra of 30 nm Au-NPs@citrate after reaction under LEDs irradiation at 520 nm for deaerated solutions containing HC-FeIII with ST or without ST.

Oxygen and carbon are due to citrate ions at the surface of the Au-NPs; Sodium can be related to sodium citrate but also to sodium thiosulfate when present, and potassium and nitrogen comes from potassium hexacyanoferrate. bounds [38,39] rather than to Au-citrate [39][40][41]. This assumption is supported by the presence of a contribution at 398. The N1s core-level spectrum was decomposed into three contributions, one corresponding to nitrogene in CN bridges, at 397.7 eV, a second contribution at 398.6 eV, which can be attributed to AuCN [29] and a small contribution at 400.4 eV corresponding to charged nitrogen.

The components attributed to AuCN (in the sample with or without ST) shows that the reaction takes place at the surface of Au-NPs@citrate, leading to an oxidation of Au atoms and ligation of Au-NPs@citrate to CN -. This might account for the small changes observed in the LSPR of Au-NPs@citrate in presence of ST and the decrease in LSPR band in the absence of ST, oxidation being more effective in that case. Indeed, it is well-known that cyanides are used to extract gold from mines by its oxidation and complexation [44], as CN -has a great ability to complex with Au and form a stable complex Au(CN)2 -. In TEM images, observation of small clusters of Au indicate this partial dissolution. Similar dissolution by cyanide ions has already been invoked in the case of size reduction and reshaping of Pt NPs used as catalysts for electron transfer reaction between hexacyanoferrate and thiosulfate [20,25].

To summarize XPS discussion, in the presence of ST Au is much less oxidized than in the absence of ST, because in the former case Au-NPs@citrate work mainly as catalysts in the electron transfer from thiosulfate to Fe III . When ST is not present, the reduction of Fe III occurs as shown by UV-visible spectra and XPS analysis. This reduction takes place either by hot electrons induced by plasmon excitation of Au-NPs, or by citrate. In the former case, the electrons are mainly transferred from Au NPs to Fe III , leading to Au I , which will probably be ligated to CN -present in the solution. The reduction of Fe III may also be partly due to reduction by citrate, which may become possible because of the temperature increase around the Au-NPs@citrate due to the plasmonic photothermal conversion.

VI-Conclusions

Plasmonic catalysis enables to achieve reactions using solar light with less energy and time consumption. The reduction kinetics of ferricyanide (III) ion into ferrocyanide (II) by Au-NPs is enhanced under visible light excitation. This reduction takes place even without sodium thiosulfate and is due to transfer of hot electrons induced by plasmon excitation. The Au-NPs stabilized by citrate reshape during the reaction. However, the shape of the Au-NPs remains stable when they are stabilized by stronger ligands such as para-nitrophenol, and the Au-NPs could then be reused in different catalytic cycles.

This study shed some light on the possible direct applications of catalysis assisted by plasmon of gold nanoparticles excited by visible light. The results demonstrate the effectiveness of using colloidal spherical gold nanoparticles to catalyze redox reactions under visible irradiation.

I-Introduction

Palladium nanostructures have attracted a lot of interest these last decades because of their applications in catalysis and electrocatalysis, in hydrogen storage, in the field of sensors as well as in nanomedicine [1][2][3][4][5][6][7]. Pd is used in a large number of industrially important reactions such as hydrogenation of unsaturated organic compounds and a number of C-C coupling reactions. Moreover, Pd nanoparticles (NPs) are efficient catalysts for environmental pollution abatement and automotive emission control [8][9][10].

C-C coupling reactions are involved in various organic transformations and polymer syntheses [11][12][13]. C-C bonds forming reactions (such as the Suzuki-Miyaura [14][15][16], Heck [17][18][19], and Sonogoshira [20][21][22] reactions) are catalyzed by transition metals such as palladium. One of the most important chemical reactions for the pharmaceutical and finechemical industries is the Suzuki-Miyaura reaction, a C-C cross-coupling reaction between aryl halides and organoboron compounds that has evolved to become a very useful method to produce highly complex molecules [23]. In the early 1990s, Reetz et al. reported the development of palladium clusters stabilized by alkylammonium cations or polymers and their catalytic activity toward the Suzuki-Miyaura reaction, giving moderate to good yields of the products [24]. A few years later, the group of El-Sayed demonstrated that colloidal Pd nanoparticles were efficient catalysts for the Suzuki-Miyaura reaction in aqueous media, confirming that the reaction is carried out at the surface of the nanoparticles, and that it is easy to separate the nanoparticles from the reaction products due to their solubility in water [25] Palladium-catalyzed C-C coupling reactions involve a thermally activated oxidative addition between the transition metal catalyst and the organic substrates. The more electron-rich is the Pd complex, the faster is this oxidative addition. Very recently, some studies have shown that this charge transfer can be activated by light via a photocatalytic process, allowing a number of different organic transformations [26][27][28]. A study of light-activated Suzuki-Miyaura reactions mediated by Pd-nanodots stabilized on MoS2 semiconductor nanosheets led to the observation of very high turnover frequencies, demonstrating the efficiency of the photocatalytic process. Mechanistic studies of this reaction suggested that an efficient electron-hole pair generation in MoS2 occurs [29]. Pd nanoparticles on semiconducting (2H)WS2 nanosheets, under visible light, have also been shown to catalyze Suzuki-Miyaura reactions at room temperature in protic organic solvents, giving very high yields. Mechanistic studies have shown that the photoexcited electrons produced in the reaction are transferred from the (2H)WS2 nanosheets to palladium nanoparticles, while the photogenerated holes are transferred to the protic organic solvents, thereby increasing the electron density of the palladium and favoring the oxidative addition of the aryl halide [30]. The plasmonic properties of metal nanostructures can also be used to activate or induce catalytic reactions [31]. Small spherical Pd nanoparticles are known to exhibit a plasmon absorption band in the UV range. Moreover, it has been shown that Pd nanodiscs exhibit broad, localized surface plasmon resonance bands with a higher sensitivity to the disc ratio than those of Ag nanodiscs [32]. Trinh et al. demonstrated direct plasmonic harvesting of visible to NIR light energy using Pd nanoplates (with well-defined and tunable longitudinal surface plasmon resonance (LSPR) peak) in a Suzuki coupling reaction between iodobenzene and phenylboronic acid [33] The authors found that the catalytic activity of the Pd nanoplates was 2.5 and 2.7 times higher than that of Pd octahedrons and cubes, respectively, under illumination. By calculating the increase in temperature of the hot electrons in the Pd nanoplates, it has been shown that the enhanced catalytic activity of the Pd nanoplates was mainly a result of the plasmonic photocatalytic effect by plasmon induced hot electrons. A recent review summarizes the advances in heterogeneous photocatalyzed C-C cross-coupling reactions under visible and near-infrared light irradiation [34].

Here, we have synthesized Pd nanoflowers formed by connected nanosheets displaying a broad absorption plasmon band in the visible and near-IR range [35,36]. We study the application of the plasmonic properties of these palladium nanoflowers under visible light irradiation in Suzuki-Miyaura cross-coupling reactions

II-Pd nanoflowers as catalysts

Pd nanoflowers were synthesized by radiolytic reduction of Pd II (acac)2 in ethanol solution after CO bubbling as detailed in chapter II. Pd II can be reduced by solvated electrons and ethanol radicals induced by solvent radiolysis as well as by CO -formed by the reaction of CO with solvated electrons [36]. The initially light-yellow solution turned to a dark blue color after irradiation. Figure V-1 shows the UV-visible absorption spectrum of the solution before and after gamma-irradiation. After irradiation, the absorption peak at 326 nm associated to Pd II (acac)2 complex vanishes and a new band appears at 275 nm (associated to free acetylacetone). While small spherical palladium nanoparticles exhibit a plasmon resonance band in the UV range, in our case, a broad plasmon band in the 400-1200 nm region is observed. Pd nanodisks and nanosheets are known to exhibit broad absorption bands in the visible and near infrared regions [36,37]. previously [35]. for the Suzuki-Miyaura coupling reaction under visible-light illumination gave high yields of the target products, because the activation energy of Suzuki-Miyaura coupling was reduced [41]. In this reported study, plasmon of gold was used to activate the reaction as Au-Pd spherical nanoparticles absorb in the visible range because of Au plasmon. Light absorption due to the LSPR of gold-based nanoparticles was playing an important role in enhancing the catalyst performance. The enhancement of photocatalytic activity was mainly attributed to hot electrons rather than a thermal effect [41]. Pd NPs decorated Au nanorods (NRs) were also used for Suzuki-Miyaura coupling reaction under light illumination, the excitation of plasmon of Au NRs being used to activate the catalytic reaction [42].

V-Conclusion

Plasmonic catalysis enables to achieve reactions using solar light with less energy and time consumption. Palladium nanosheets exhibit a broad plasmon absorption band in the visible-near infrared domain. The catalytic activity of Pd nanosheets for Suzuki--Miyaura reactions was remarkably enhanced under visible light irradiation. The reaction was carried out using iodobenzene, phenylboronic acid and Cs2CO3 as base, in the dark and under visible light irradiation. Indeed, the results showed that the conversion under light irradiation was 96% versus 53% in the dark. Our finding can be explained by the plasmonic effect, which increases the reactivity of palladium nanostructures. The excitation of the plasmon resonance is converted into heat and hot electrons are induced on Pd surface. To optimize the reaction conditions, different bases were used under light irradiation, the kinetics of the reactions showed that Cs2CO3 and K3PO4 were efficient bases for this reaction in the presence of palladium nanosheets. Finally, different haloarenes were subjected to the optimized conditions. The highest conversion was obtained with iodotoluene (80%) and surprisingly the highly activated 1-iodo-or 1-bromo-4-nitrobenzene led to lower conversions. Calculations will be performed to estimate the heat effect induced by plasmon excitation and its contribution on the reaction kinetics. These Pd nanostructures show partial dissolution of Pd and morphological changes during the Suzuki-Miyaura catalytic reaction, and light irradiation induces an enhancement of the kinetics, but could also accelerate Pd dissolution. To enhance the stability of the nanostructures for catalytic applications and increase their turnover frequencies, supporting these nanosheets or modifying them with carbides for example will be tested. These Pd nanoflowers of high surface area could potentially be employed as plasmonic catalysts in other C-C forming reactions, such as Sonogashira, Heck, and Stille reactions.

But, studies are still needed to better understand plasmonic palladium-photocatalyzed organic reactions from fundamental and mechanistic points of view in order to design more effective photocatalysts for a larger number of organic reactions.

Sunlight is abundant, free and of environmentally friendly nature. In chapter III, we show that the degradation mechanism of para-nitrothiophenol (pNTP) is complex as several products were detected by mass spectrometry. The presence of paraaminothiophenol (pATP) indicates that the surface-catalyzed reduction of para-nitrothiophenol in para-aminothiophenol takes place and involves hot electrons. However, to clarify the reduction mechanism, experiments should be conducted under inert atmosphere to get rid of possible oxidation reactions with O2.

The reduction of ferricyanide (III) ion into ferrocyanide (II) ion catalyzed by spherical Au-NPs is described in chapter IV and the results show that this catalysis is enhanced when Au-NPs were excited at their plasmon. This reduction takes place even without sodium thiosulfate, which demonstrates the implication of hot electrons in this reduction. When Au NPs are stabilized by citrate (a weak ligand), they reshape with catalytic cycles. However, their stabilization with a stronger ligand (pNTP) prevents this reshaping; so, stabilization on support should lead to more stable nanoparticles with cycling, and this can open perspectives in catalysis assisted by plasmon.

In chapter V, we have investigated plasmonic catalysis with Pd nanostructures. Indeed, Pd is an efficient catalyst for hydrogenation reactions, environmental pollution abatement and automotive emission control, and C-C coupling reactions. Pd nanoflowers (synthesized by radiolysis) exhibit absorption in the visible and near infrared regions. The photocatalytic activity of Pd nanoflowers for Suzuki-Miuyara reactions is remarkably enhanced under visible light irradiation. The reaction was carried out using iodobenzene, phenylboronic acid and Cs2CO3 as base. Different haloarenes were also used and the highest conversion was obtained with iodotoluene. The excitation of the plasmon resonance is converted into heat and hot electrons are generated. The latter are induced on Pd surface, and are involved in the catalytic mechanism. However, these nanostructures are not very stable with cycling and a dissolution of Pd is observed. To enhance the stability of these nanostructures in catalytic cycles and increase their turnover frequencies, these nanoflowers could be deposited on supports or surface modified with carbides for example. These Pd nanoflowers could also be used for other plasmonic catalytic reactions involving C-C coupling, such as Sonogashira, Heck, and Stille reactions.

Preliminary studies were also conducted in collaboration with Catherine Louis team (Laboratoire de Reactivité des Surfaces, Sorbonne Université) on CO oxidation (into CO2) with Au nanoparticles deposited on oxides (alumina). This reaction is of great importance in environmental catalysis (application in catalytic convertors for example). However, the catalysts were already too active even without illumination. Future experiments will be conducted with less active catalysts: decreasing the Au loading on Al2O3 or using another support such as SiO2, which leads to less active catalysts. The reactions take place under oxygen. If hot electrons are generated, they would react with O2 leading to O2 .-, which can be analyzed by reduction of cytochrome c. The effect of temperature increase to plasmon excitation will also be studied in collaboration with theoretician physicists from Ecole CentraleSupélec.

Utilization of LSPR in photocatalysis under visible-light irradiation can efficiently improve the photocatalytic activity and also modify the product selectivity. Using plasmonic photocatalysts to convert solar light and to catalyze chemical reactions is promising, and the obtained results are encouraging. More studies are required for the physical understanding of plasmonic photocatalysis from fundamental and mechanistic points of view. For example, the determination of the electron-transfer process from plasmonic nanoparticles to the absorbed molecules or to the semiconductor (where they are deposited) is crucial to the photocatalytic properties. Also, the LSPR absorption is sensitive to size, shape and support, and can affect the electron-transfer efficiency and the photocatalytic performance. More fundamental studies and calculations are also needed to understand the effect of heat increase (due to plasmon excitation) on the catalytic reactions.

Plasmonic nanoparticles can be used for greener, safer and more effective processes for chemical reactions. Plasmonic catalysis could be extended to various industrial reactions and reactions of environmental interest. As near infrared constitutes about 45% of the solar spectrum, it is also interesting to use nanostructures with plasmon in the near IR (such as Au nanorods or Pd nanoplates) for plasmonic photocatalysis. Fundamental studies on the mechanisms involved in these plasmonic catalytic reactions will also help to design more effective photocatalysts for a large number of reactions of environmental or industrial interest.

Carbon monoxide is a colorless, odorless, tasteless, but highly toxic gas. It combines with hemoglobin to produce carboxy-hemoglobin, which usurps the space in hemoglobin that normally carries oxygen, which becomes ineffective for delivering oxygen to bodily tissues.

The oxidation of carbon monoxide had been thoroughly studied during the last thirty years, as it is represents a major challenge to reduce its toxic abundance in the atmosphere. This highly exothermic reaction had been part of the refinery processes, vehicle exhaust emissions, water purification processes, industrial waste, and most importantly, medical and environmental aspects.

2CO + O2  2 CO2 ∆H=-394 kJ/mol

Haruta was the first to report that Au nanoparticles less than 5 nm can catalyze the oxidation of CO into CO2 in the gas phase at room temperature [1]. had discovered the reactivity of gold nanoparticles in the enhancement of catalysis properties under certain conditions including size and morphology and the presence of an oxide support. The mechanisms of the oxidation still hold a major debate. Possible oxidation could happen on the cationic gold entities found in the medium with the presence of water coming from gas purging flow. Other mechanisms suggest a co-operative role between the support and the noble metal in the oxidation process. The latter mechanisms tend to be more complicated.

This annex gathers the preliminary results obtained on the oxidation of CO assisted by gold plasmon. The aim of the study was to provide a better understanding on energy damping or hot electrons injection into the system as a direct effect of excitation at wavelength matching the plasmon wavelength. This study has been conducted in collaboration with Catherine Louis and Laurent Delannoy (Laboratoire de Réactivité des Surfaces, Sorbonne Université).

I-Synthesis of gold nanoparticles on support I-1-Deposition-Precipitation (DP) with urea

Deposition-Precipitation (DP) method is widely used by researchers to synthesize metal nanoparticles of controlled size on supports [2]. During thermal treatment, helium is introduced first to push out of any oxygen excess in the U-shaped reactor and valves connected to it. After around 15 minutes of helium purging, H2 gas is purged at a predetermined flow rate.

A typical synthesis of Au/SiO2 with a total content of 0.5 wt% is described as below. 1.3 mL of HAuCl4 solution (19.7 mL) was added to a flask with 24.1 mL of distilled water (1 mM total concentration of Au in 25.4 mL of H2O). Then, 1 g of silica and 38.9 mg of urea were added to the solution and stirred for 3 minutes to form a white suspension The flask was then heated up to 90 ○ C under stirring. The temperature of water bath was kept at 90 ○ C for 2 hours by adding glycerin and continuous magnetic stirring. At this point, the well dispersed suspension turned into a well formed precipitate. Later, 0.194 mL of 2-propanol was added to the suspension in order to scavenge the oxidative . OH radicals generated during gamma irradiation of water solvent. The suspension was first degassed under N2 to remove oxygen.

The flask was irradiated for 67 minutes with a 60 Co panoramic gamma source (dose rate = 4.7 kGy/h). The metal ions (Au III) were reduced by solvated electrons and alcohol radicals induced by solvent radiolysis [3].

I-2-Synthesis after impregnation of gold ions on silica (SiO2) substrate

One of the frequently used methods to synthesize well dispersed mono/poly layered Au/SiO2 is direct impregnation of a gold salt solution directly on silica powder [4]. This method consists of carefully wetting a determined weight of silica powder initially with distilled water then later with a solution containing gold atoms. Afterwards, gentle and continuous mixing using a thick glass rod to achieve a relatively homogenous color for all the sample.

In order to prepare Au/SiO2 4% in mass of Au loading using this method, 3g of SiO2 (Aerosol 300/30) was wetted to ensure incipient humidity. Later, 7.2 mL of a HAuCl4 solution (nau = 6.3×10 -4 mol), till we achieve a homogenous white-yellow powder. The latter powder will be left to dry for 48 to 72 hours.

Then, a urea washing (NH4OH 30 % ) for the dried powder is necessary to ensure controlled diameter of Au/SiO2 nanoparticles, as Cl -anion might interfere leading to larger clusters and perhaps the dissolution of silica. Later, a thermal treatment (calcination in sealed oven heated to 300 Celsius) is required to achieve dark-rose nanoparticles.

Different samples on Al2O3 and SiO2 and with loading about 1% were also prepared ( Gas is pumped into the U-shaped glass reactor from top. As part of the purging process, Helium and oxygen are purged first for around 50 seconds then CO merged in Helium will be next. The initial and final concentrations of both CO and CO2 are detected through a micro-gaschromatography apparatus connected to the output of the reaction setup. Meanwhile, the temperature variations can be monitored by a thermostat that is positioned carefully on the reactor surface.

II-Oxidation of CO II-1-Reactor

II-2-Results

For this experiment, we have to consider different points and raise some questions:

1-Since the reaction is exothermic, how can we separate the heat induced by the SPR from the heat produced during the reaction?

2-In the same manner, do thermal changes inside the LEDs cylinder play a role during the total reaction time?

3-If the catalyst is highly active, is it advisable to deactivate the catalyst?

4-What is the role of reactor position inside the cylinder? How does the intensity of LEDs will modify the final catalytic yield: the concentration of CO2?

First, it is necessary to check the temperature inside the illuminated reactor (LEDs cylinder) when there is no CO gas flow. We have installed sensitive thermostat at the center of the LEDs sphere to measure the temperature changes as a function of time when the intensity of the LEDs is at its maximum. After 60 min of runtime, a 6 C° rise in temperature is measured at the center of the LEDs reactor, and after 3 hours of runtime, the rise in temperature is 7°C.

First, the catalysis experiment was ready to be carried out using Au/Al2O3, 3% Au as catalyst. It seemed favorable to turn on the LEDs cylinder after the conversion of 30% of CO to CO2, because Au/Al2O3 3% is considered highly active, with a conversion percentage of almost 95% according to literature and our data. But, the sample is already too active even without illumination (Figure A-2), so no effect of illumination was detected (Figure A-3).

There are several possible explanations for this phenomenon:

1-The elevated activity of catalyst that is blurring the intended plasmonic effect.

2-The temperature keeps rising then stabilizes around 35 C° inside the LEDs sphere, which makes it more difficult to conduct the experiment under optimized conditions. Obviously as this perturbation is happening; it was not clear to verify this effect.

II-3-Perspectives

The approach would be then to minimize the activity of our catalyst and diffuse the extra temperature by dipping our U-shaped glass reactor into water with ambient temperature to diffuse the extra heat.

Our catalysts were too active, and it was not possible to study the effect of light irradiation.

Next experiments will be conducted with less active catalysts by decreasing the Au loading on

Résumé de l'état de l'art

Les plasmons sont des oscillations collectives des électrons confinés dans la bande de conduction des nanoparticules métalliques sous l'action de la lumière. Seul un nombre restreint de métaux présente une résonance de plasmon dans le visible, Il s'agit du cuivre, de l'argent et

Introduction

Metallic nanoparticles (NPs) have many applications in catalysis. Some of them (gold, silver, copper) have a visible absorption resulting from the excitation of the surface plasmon by an external electromagnetic field. In this project, we propose to couple the catalytic properties of gold-based nanoparticles with their plasmon resonance properties in order to assist the catalytic reaction with the generated hot electrons and the resulting thermal effect. This plasmon-assisted catalysis approach is general and can be used for a variety of exothermic and endothermic catalytic processes involving nanoparticles. This plasmon-assisted catalysis makes it possible to carry out catalytic reactions at lower energy cost under sunlight.

A summary of the state of the art

Plasmons are collective oscillations of electrons confined in the conduction band of metal nanoparticles under the action of light. Only a small number of metals have plasmon resonance in the visible, this is copper, silver and gold (see Figure 1 above). Of these metals, gold is the easiest to handle because it does not oxidize. In addition, the gold nanoparticles have catalytic properties in the oxidation reactions of molecules such as CO, VOC ... for which the oxide support (TiO2, CeO2) stabilizing the NPs allows the activation of oxygen. Gold is also an interesting catalyst for its high selectivity for hydrogenation reactions, although not very active.

In this case, the support does not intervene in the reaction. Since the last five years, the plasmon resonance properties of gold have begun to be used to support catalytic and photocatalytic reactions. Catalysis exalted by plasmon is still a very recent and very promising area [1,2]. The light energy absorbed by the gold NPs is rapidly converted into heat and lime electrons are generated inducing the exaltation of the catalytic properties.

However, few studies are reported in the literature to study the catalytic exalted by plasmon and understand the origin of this exaltation.

The study of light-heat conversion in metallic NPs and heat transfer to their environment is fairly recent. Thanks to ultrafast optical techniques, it is now possible to understand energy exchange mechanisms, electron transfer mechanisms and their kinetics [3,4]. The thermal relaxation dynamics of metallic NPs in the matrix following the absorption of a light pulse could thus be studied [4].

In addition, if the particles are supported, the heat transfer from the particles does not respect the conventional heat conduction laws. These two effects can allow a significant photothermal heating of the nanoparticles with little optical energy and without significant heating of the support. Plasmon heating with a laser allows accurate control of the location, magnitude and duration of thermal energy. On the other hand, if the nanoparticles are in solution, the thermal energy can be quickly transferred to the solvent. with the surrounding medium Thus, at relatively low laser powers, rapid evaporation of the solvent can be obtained [5]. The possibility of directly heating a nanoparticle of noble metal in a liquid and producing a vapor phase with a laser of appropriate wavelength also makes it possible to study the reactions in the gas phase. Such a reaction catalyzed by the surface of the NP can be controlled rationally (more or less accelerated according to the energy and the luminous power) The surface plasmon resonance plays the most important role in these reactions catalyzed by the surface, and Hot electrons from the surface plasmon cause the catalytic reaction.

The catalyst may also be in solution or in heterogeneous suspension deposited on a support.

The reaction can be carried out with lamp-lit reactors, such as those we have at LCP has been shown, for example, that the plasmon heating of the gold nanoparticles in solution not only provides the heat of reaction, which makes it possible to generate both water but also ethanol vapor locally on the catalysts in solution in a water / ethanol mixture. The reaction products are CO2, CO and H2 and are compatible with the catalytic reforming of ethanol vapor.

Plasmon excitation can also be exploited in the case of photocatalysis. This refers to a catalytic reaction induced by the absorption of photons by a semiconductor that generates an electron and a hole (when the energy of the photon is greater than the gap). TiO2 is the most used photocatalyst but absorbs only in the UV. In addition, the quantum yields of light conversion are low because of high electro-hole recombinations. The addition of metallic nanoparticles (Au, Ag, Cu) makes it possible to increase the quantum efficiency of light conversion on the one hand, by decreasing the recombination-electron hole by trapping the electrons and on the other hand by generating an activity in the visible thanks to the plasmon excitation which allows the electron ejection of the metal towards the conduction band of the semiconductor. In photocatalysis, the predominant effect that allows the increase in photocatalytic activity does not seem to be heating as for catalysis but charge shifts. This manuscript is organized as follows:

The first chapter is an introduction to nanoscience. The synthesis and properties of gold nanoparticles, in particular the phenomenon of localized resonance of surface plasmon are developed. The applications of gold nanoparticles in different fields are presented. This chapter also describes the state of the art of plasmon-assisted catalysis.

The second chapter is devoted to materials, to the synthesis of metallic nanoparticles and to experimental devices used to study catalytic reactions. The various techniques used for the characterization of the nanoparticles and for the analysis of the reaction products are also described.

In the third chapter, we study the degradation of para-nitrothiophenol assisted by gold plasmon.

This degradation is very effective with gold nanoparticles excited under visible light (Figure 5).

The results show that the degradation mechanism is complex because several products are formed. However, there is evidence that surface catalyzed reactions involving hot electrons occur when para-nitrothiophenol (or a portion thereof) is reduced to para-aminothiophenol. In the fourth chapter, we studied the reduction of ferricyanide (III) ion to ferrocyanide (II) catalyzed by spherical gold nanoparticles (AuNPs) when they are excited to their localized plasmon resonance. This reduction takes place at ambient temperature and under illumination (visible light) even without the use of a reducing agent. This result again indicates that hot electrons are involved in the reaction mechanism.

The fifth chapter presents the study of the Suzuki-Miyaura cross-coupling reactions catalyzed by the plasmon of Pd nanoflowers (see Figure 6). Reactions between halogenated benzene compounds and phenyl boronic acid in the presence of various bases are studied in the dark and under irradiation (visible light). It has been shown that the photocatalytic activity of Pd nanofilters for Suzuki-Miuyara reactions is remarkably increased under visible light irradiation [6,7].

Finally, the general conclusion highlights the main results obtained during this thesis with plasmonic nanostructures of Au and Pd used to catalyze different reactions. This work opens perspectives for plasmon-enhanced catalysis, especially for environmental applications. Les processus catalytique sucitent un interet majeur pour la recherhe et le developpement. Des processus catalytiques photo-actives naturels interviennent dans presque tous les aspets de notre environnement: les cycles de photosynthese nourrissent et colorent les plantes, les reactions enzymatiques essentielles à la suivre des especes, la decontamination d l'eau à la lumiere visible, etc.

Enfin, la conclusion generale met en evidence les principaux resultats obenus au cours de cete these avec des nanostructures plasmonique de Au et de Pd utilisiees pour catalyer differentes reactions. Ces travauz ouvrent des perpectives en catalyse exaltee par plasmon, en particulier pour les applications environnmentales.

Title : Catalysis Assisted by Plasmon

Keywords : Nanoparticles, Plasmon, Catalysis, Gold, Palladium Abstract : Metallic nanoparticles (NPs) have many applications in catalysis. Some of them (gold, silver, copper) have a visible absorption resulting from the excitation of the surface plasmon by an external electromagnetic field. In this project, we propose to couple the catalytic properites of gold-based nanoparticles with their plasmon resonance properties in order to assist the catalytic reaction with the generated hot electrons and the resulting thermal effect. This plasmon-assisted catalysis approach is general and can be used for a variety of exothermic and endothermic catalytic processes involving nanoparticles. This plasmon-assisted catalysis makes it possible to carry out catalytic reactions at lower energy cost under sunlight.

Catalytic processes are a of major interst for research and development. Natural photoactivated catalytic processes are involved in almost every aspect of our environment: photosynthetic cycles feed and color plants, enzymatic reactions essential for species survival, decontamination of water with visible light, etc. Finally, the general conclusion highlights the main results obtained during this thesis with plasmonic nanostructures of Au and Pd used to catalyze different reactions. This work opens perspectives for plasmon-enhanced catalysis, especially for environmental applications.

IChapter I :

 I State of the art cup is made of bichromatic glass, which changes color when depending on how the light is shining on it (Figure I-1):

Figure I- 1 .

 1 Figure I-1. Lycurgus Cup (exhibited at the British Museum, London) showing two colors depending on how light is shined upon. It appears green when no light is inside (left), and red when light is shined inside (right) [1,8].

IChapter I :

 I State of the Art .

Figure I- 2 .

 2 Figure I-2. Stained glass window from the Sainte-Chappelle in Paris.

Figure I- 3 .

 3 Figure I-3. An example of "Famille Rose" (or "Pink family") porcelain: the "Mille Fleurs" vase, exposed in the Guimet Museum in Paris.

IChapter I :

 I State of the art

Figure I- 4

 4 Figure I-4 shows the world-wide production of gold according to the United States Geological Survey Center, and one can notice that China is a leader nowadays in the production of gold in metric tons.

Figure I- 4 .

 4 Figure I-4. Worldwide annual gold production according to United States Geological Survey Center.

Figure I- 5 .

 5 Figure I-5. A demonstrative scheme of magnetron sputtering technique and images of Au nanoparticles synthesized by this method [14].

Figure I- 6 .

 6 Figure I-6. A scheme representing laser vaporization technique to prepare nanoparticles, and an example TEM images of synthesized nanoparticles via this method along with their size historgram [15].

IChapter I :

 I State of the art

Figure I- 7 :

 7 Figure I-7: Laser ablation technique used to synthesis gold nanoparticles in solution. Courtesy to Gregory Guisbiers.

Figure I- 8 .

 8 Figure I-8. Electrostatic (citrate) or steric (PVA or PVP polymer) of spherical gold nanoparticles.

Figure I- 9 .

 9 Figure I-9. Reduction and stabilization of gold nanoparticles with long chains of thiols and dendrimers [18].

Figure I- 10 .

 10 Figure I-10. Schematic presentation of polarization of a spherical plasmonic nanoparticle and the continuous oscillation due to dielectric function interactions.

Figure I- 11 .

 11 Figure I-11. Plasmon spectra of spherical Au and Ag nanoparticles in water and the corresponding vials containing these nanoparticles.

Figure I- 12 :

 12 Figure I-12: Transmission electron microscopy image showing Au nanorods (right), and vials containing Au nanorods of different aspect ratios (a-left) and the corresponding absorption spectra (exhibiting longitudinal and transverse plasmon bands) (b-left) [42].

Figure I- 13 :

 13 Figure I-13: Schematic presentation of the two main possible pathways for hot electrons to interact with the surrounding medium [45].

Figure 1 - 14 .

 114 Figure 1-14. Plasmon mediated selective oxidation of sec-phenethyl and benzyl alcohols. to acetophenone and benzaldehyde obtained by irradiation of colloidal Au NPs by LED at 532 nm Aggregation of Au NPs is observed after photocatalytic process (inducing a blue colour because of plasmon coupling) [59].

IChapter I :

 I photo-initiated electron transfer reactions. Figure I-15 outlines the possible dominant pathways following plasmon excitation of metal nanoparticles in the presence of organic molecules in the plasmon-mediated catalytic processes.

Figure 1 - 15 .

 115 Figure 1-15.Possible reaction pathways following plasmon excitation of metal nanoparticles in the presence of organic molecules. In the presence of a suitable receiver, antenna effects (A) can result in excited state processes (As and At). Plasmon relaxation can lead to thermal effects (T) that can themselves induce supramolecular changes in guest molecules (Ts), chemical change (Tc) sometimes referred as photocatalysis, or changes in the nanoparticle itself (Tn) of either a physical or chemical nature (e.g., oxidation). Under plasmon excitation the nanoparticle can act as an electron donor (Et) or as an electron acceptor (Ht). Red arrows are used to highlight the mechanisms to be discussed in this contribution[43].

Figure I- 16 .

 16 Figure I-16. A scheme showing conversion of benzyl alcohol in benzaldehyde and 1-phenylethanol in acetophenone by Au NPs deposited on a support and excited by LED at 530 nm [58].

IChapter I :

 I State of the Art

  photodeposition of gold on fifteen commercial titania, influenced significantly the resultant photocatalytic activities in a different manner under UV and visible light irradiation[START_REF] Kowalska | Visible-light-induced photocatalysis through surface plasmon excitation of gold on titania surfaces[END_REF]. Small gold nanoparticles (Au-NPs) around 2-3 nm on the surface of titanium dioxide work as visible-light absorbers and thermal redox active centers. Au-NPs were synthesized on commercial TiO2 by reduction with tetrakis(hydroxymethyl)phosphonium chloride[START_REF] Méndez-Medrano | Surface Modification of TiO2 with Ag Nanoparticles and CuO Nanoclusters for Application in Photocatalysis[END_REF]. The photocatalytic activity of Au/ TiO2 was evaluated for degradation of phenol, 2-propanol, acetic acid and for H2 production from aqueous methanol solution. This photocatalytic activity was much higher for the plasmonic TiO2 compared to the activity of bare titania. Time resolved microwave conductivity (TRMC) signals showed injection of electrons from Au-NPs to the conduction band of TiO2 under visible-light excitation, due to the activation of localized surface plasmon resonance (LSPR) of the Au-NPs[START_REF] Méndez-Medrano | Surface Modification of TiO2 with Ag Nanoparticles and CuO Nanoclusters for Application in Photocatalysis[END_REF]. Action spectra (AS) correlated with the absorption spectra proving that decomposition of model pollutants is carried out by a photocatalytic mechanism. Au-TiO2 were also active for hydrogen evolution under visible light. It has been also shown that these photocatalysts can be reused several times without appreciable loss of activity.

  control the nanoparticle size. Indeed, citrate ions reduce Au III into Au 0 (Figure II-1) and simultaneously stabilize the formed Au nanoparticles via electrostatic weak-bonds. Because citrate is a weak reducing agent, the gold salt solution is heated and generally boiled [3]. The synthesis of Au-NPs@Citrate starts by boiling a gold salt aqueous solution, then sodium citrate (a solution containing 1% in mass) is added as a reducing and stabilizing agent (Figure II-2).

Figure II- 1 .

 1 Figure II-1. Reduction of Au III complexes by citrate ions.

Figure II- 2 .

 2 Figure II-2. Synthesis of Au nanoparticles (NPs) by Türkevich method: The light yellow solution containing AuCl4 -is boiled and a solution containing citrate ions is added, the mixture turns to grey and then to a red solution, the red color attests the formation of Au nanoparticles.

  I0 and I are the intensities of the incident and transmitted light, respectively. The amount of light that is absorbed by the solution depends on the concentration c of the solute, the path length l of the light through the cuvette and the absorption coefficient  of the solute, i.e. how well the solute absorbs the light at the wavelength . If there is no absorption of the light passing through the solution, the absorbance is 0 and the transmittance is 100% (neglecting the scattering by the cell windows).

  Microscopy (TEM) Transmission Electron Microscopy (TEM) is a powerful imaging tool for the characterization of morphology, lattice coordinates, and distribution of nanostructures. TEM is used to observe the size, shape and morphology of nanoparticles. It probes the internal structure of solids in order to give us an idea about the morphology of the material. When the accelerating electrons are focused on the grid containing the sample, part of the electrons is transmitted by the sample while a part of them are scattered. The electromagnetic lenses in the TEM machine focuses the transmitted electrons into an image or a diffraction pattern and provides us with insight about the sample under observation. The electron diffraction, observed by TEM provides us with the Fourier transform reconstruction. In TEM apparatus, the sample is illuminated using multiple electron beams confined in a high vacuum space, and the transmitted beam makes the image magnified from about fifty to over one million times (Figure II-3). In principle, contrast of the TEM image arises because of the differences in electron density of the elements constituting the sample under investigation. Transmission electronic microscopy (TEM) observations were performed with a JEOL JEM 100 CXII TEM instrument operated at an accelerating voltage of 100 kV and a JEOL JEM 2011 operated at 200 kV. Prior to observations, a few droplets of the metal nanostructure solution were deposited and dried on carbon-coated copper TEM grids under N2 flow. The images were collected using a CCD camera. These characterizations were done in collaboration with Patricia Beaunier, LRS (Sorbonne Université).

Figure II- 3 .

 3 Figure II-3. Interaction between matter and the electron beam for TEM imaging (https://bsp.med.harvard.edu/node/221)

(

  Thermo Fisher Scientific) under ultrahigh vacuum (base pressure in the low 10 -9 mbar), equipped with a monochromatic Al source (AlKα = 1486.7 eV) using a spot size of 400 μm corresponding to an irradiated area of approximately 1 mm 2 . The hemispherical analyzer was operated at 0° take off angle in the Constant Analyzer Energy (CAE) mode, with a pass energy of 200 eV and a step of 1 eV for the acquisition of survey scans and a pass energy of 50 eV and a step of 0.1 eV for the acquisition of narrow scan spectra. Charge compensation was accomplished by means of a "dual beam" flood gun. Drops of a suspension were deposited on silica plates and allowed to dry in the Load Lock prior to transfer in the analysis chamber in order to minimize the contact with air. The recorded XPS spectra were analyzed by means of Avantage software provided by the manufacturer. The C 1s signal 284.8 eV was used as reference for the bonding energy scale. The XPS characterization experiments were done in collaboration with Diana Dragoe (ICMMO, Université Paris-Saclay).

Figure II- 4 .

 4 Figure II-4. A schematic representation of the ESI-ion source. The interpretation of ESI-MS results can be complex as peaks can become noisy and overlapping. Further fragmentation of interesting peaks to dissect detected mass and verify possible masses corresponding to a certain molecule.

  filter (15 cm) to absorb the near-IR radiation (Figure II-5) and a longpass filter to avoid the direct photoactivation of chemical reagents during irradiation.

Figure II- 5 .

 5 Figure II-5. Xe lamp installation for photocatalytic reactions under UV-Vis selected wavelengths via a cut-off filter.

Figure II- 6 .

 6 Figure II-6. Illustration of LED home-made reactor (left); a photograph of the inside of the reactor and cuvette sample held at the center (right).

Figure II- 7 .

 7 Figure II-7. Laser experimental setup where the cross-section of the two fibers connected to the visible lamp and green laser.

Figure II- 8 .

 8 Figure II-8. Screen shot of the video recorded via CCD camera. One can notice that the temperature rises to its maximum at the point of entry of the continuous green

  partly soluble in water and chloroform; the storage temperature is around 4° C as it has a fusion point of 72-77° C. Aqueous solutions of pNTP (6 × 10 -4 M) were prepared and the pH was adjusted to 11 by addition of 0.1 M NaOH solution in order to form the phenolate anion (pKa = 4.4 [36,37]) which has a characteristic UV-visible electronic absorption band peaking at 405 nm (Figure III.-1).

Figure III- 1 .Scheme III- 2 .

 12 Figure III-1. Absorption spectrum of a 1.64 × 10 -4 M pNTP aqueous solution. Optical path: 1 cm

Figure III- 2 .

 2 Figure III-2. UV-visible absorption spectrum of an aqueous solutions containing both 1.64 × 10 -4 M pNTP, 30 nm AuNP ([Au 0 ] = 1.74 × 10 -4 M), after 40 minutes stirring under darkness, and the sum of the absorption spectra of the aqueous solutions containing either 1.64 × 10 -4 M pNTP or 30 nm AuNP ([Au 0 ] = 1.74 × 10 -4 M). Optical path: 1 cm.

Figure III- 2 Figure III- 3 .Figure III- 4 .Figure III- 5 .

 2345 Figure III-2 presents the UV-visible absorption spectrum of an aqueous solution containing pNTP, and 30 nm AuNPs after 40 minutes of stirring in the dark, as well as the sum of the absorption spectra of the aqueous solution of each compound at the same concentration. It is to note that the absorption spectrum of the mixture of pNTP and Au NPs differs slightly from the expected spectrum obtained from the sum of the spectra of the two compounds. That indicates that, upon mixing, interactions between pNTP and AuNP occur modifying the absorption properties. Moreover, if the mixture is let under stirring in the dark for several hours, a decrease in the absorbance is observed (Figure III-3) This decrease can be explained either by a low degradation of pNTP or by a small change in the absorption properties of pNTP related to adsorption and replacement of citrate on the Au NPs surface. Indeed, due to the presence of thiol group, adsorption of pNTP on Au NPs is favourable and expected as already reported in the literature.

Figure III- 6 .

 6 Figure III-6. Absorbance at 405 nm as a function of time for aqueous solutions containing 1.64 × 10 -4 M pNTP and 30 nm AuNPs under illumination with 525 nm LEDs photoreactor, 532 nm laser beam, or Xe lamp ( > 500 nm).

30nmFigure III- 7 .Figure III- 8 .

 78 Figure III-7.Temporal evolution of the absorption spectra of an aqueous solution containing 1.64 × 10 -4 M pNTP and 30 nm AuNPs ([Au 0 ] = 1.74 × 10 -4 M) under stirring, N2 atmosphere and illumination at 525 nm in the LED photochemical reactor for 480 minutes.

Figure III- 9

 9 Figure III-9 Temporal evolution of the absorption spectra of an aqueous solution containing 1.64 × 10 -4 M pNTP and 15 nm AuNP ([Au 0 ] = 1.74 × 10 -4 M) under stirring and LED illumination 525 nm.

Scheme III- 3 .

 3 Scheme III-3. Representation of products characterization frame-work.

Figure III- 11

 11 Figure III-11 presents the ESI-MS full scan spectra of the supernatant solutions corresponding to pNTP and AuNP mixture solutions before illumination (i.e. after 40 minutes of mixing under darkness) and after 6 hours and 12 hours of illumination by the 525 nm LEDs photoreactor.

After 6 10 .

 610 hours of illumination (Figure III-11b), these two peaks are still present, but the peak at 153. 9 m/z is highly diminished and a new dominant peak has appeared at 185.8 m/z. ESI spectra of solutions containing only the initial compounds: (a) pNTP, (b) trisodium citrate; or (c) para-nitrophenol (pNP) Degradation of para-nitrothiophenol by plasmon excitation of gold nanoparticles Other peaks with significant abundance (> 10%) are located at 202, 213, 218 and 259 m/z. 202 m/z peak is already present in the spectrum of pNTP while 213 and 259 m/z are observed in trisodium citrate spectrum. After 6 hours of illumination (Figure III-11b), these two peaks are still present, but the peak at 153. 9 m/z is highly diminished and a new dominant peak has appeared at 185.8 m/z. Other peaks with significant abundance (> 10%) are located at 202, 213, 218 and 259 m/z. 202 m/z peak is already present in the spectrum of pNTP while 213 and 259 m/z are observed in trisodium citrate spectrum. After 12 hours of illumination, the ESI-MS spectrum of the supernatant (Figure III.11c) is dominated by the peak of citrate at 190.9 m/z; the peak at 153.9 m/z associated to pNTP has disappeared and the peak at 186 m/z has dropped. Three other peaks with an abundance higher than 15% are observed at 110.9, 202 and 212.9 m/z; two of them (110.9 and 212.9 m/z) can be related to citrate (Figure III.10). Hence, it appears that after 12 hours of illumination, there is no more pNTP in solution. By comparison, the ESI-MS spectrum of the supernatant solution obtained from the mixture solution maintained in darkness during 12 hours (Figure III-11d) is much simpler with three dominant peaks at 153.9, 190.9 and 212.9 m/z. The peaks at 190.9 and 212.9 m/z are associated to citrate; the peak at 153.9 m/z is related to pNTP and has decreased compared to the initial spectrum (Figure III-11a) indicating degradation of the compound.

Figure III- 11 .

 11 Figure III-11. ESI spectra of the supernantant solutions obtained after centrifugation of pNTP and AuNPs mixture solutions (a) before illumination (i.e. after 40 minutes of mixing under darkness), (b) after 6 hours and (c) 12 hours of illumination by the LEDs photoreactor, and (d) after 12 hours in dark.

  12 EI-MS spectra obtained by fragmentation of the peak at (a) 185.75 m/z and (b) 218 m/z for the pNTP and AuNPs mixture solution after 6 hours illumination with the LEDs photoreactor 185.75 m/z For the attribution of the peak at 218 m/z, PIS was performed (Figure III-12b). Affiliated

  Figure III.13 shows the SERS spectra of the AuNPs extracted by centrifugation from pNTP and AuNPs mixture solutions before illumination (i.e. after 40 minutes of mixing under darkness) and after 12 hours of illumination by the LEDs photoreactor. After 40 min of mixing in darkness with pNTP, the SERS spectrum of the AuNPs (Figure III-13a) reveals new peaks at 1077, 1332 and 1568 cm -1 . As already reported in the literature, theses peaks are characteristic of pNTP molecule (Figure III-15a), with in particular the Raman intensity at 1332 cm -1 related to (NO2). That gives evidence of the presence of pNTP at the surface of the AuNPs and so, a co-adsorption with citrate. After 12 hours of illumination (Figure III-13b), the Raman intensity of (NO2) disappears and new intensities emerge at 1135, 1388 and 1432 cm -1 .

13 .

 13 Neat SERS spectra of 30 nm AuNPs obtained after centrifugation from pNTP and AuNPs mixture solutions before illumination (i.e. after 40 minutes of mixing under darkness) (a) and after 12 hours of illumination by the LEDs photoreactor (b).

Figure III- 15 .

 15 Figure III-15. Raman spectra of (a) pNTP (b) pATP.

  AuNPs and the excitation of their plasmon band, since almost no degradation is observed either in the absence of AuNPs under light irradiation, or in the presence of AuNPs without light. That indicates that reactions leading to degradation of pNTP are induced by plasmon excitation. Moreover, the absence of degradation of para-nitrophenol in the presence of AuNPs under irradiation, and the detection of pNTP on the surface of the AuNPs show that adsorption of the molecules on the NPs is an important factor, too. UV-visible spectroscopy and SERS measurements show the formation of pATP. This compound is present at the surface of the AuNPs, but not in the solution. That suggests that the reduction reaction of pNTP takes place on the surface and involves "hot" electrons produced by plasmon excitation. Moreover, the formed pATP seems to remain on the AuNPs surface as it is hardly supernatant solution by ESI. Besides, when the reaction is nearly finished after 12 hours of illumination, there is no more pNTP in solution, and only small amounts of nitrobenzene derivatives bearing oxidized thiol group are detected (Figure III.11c). However, these oxidized compounds are in much higher quantity during the illumination (after 6 hours of illumination, Figure III.11b) while they correspond to traces before illumination. That

Figure IV- 1 .

 1 Figure IV-1. UV-Visible spectra and inset TEM images of the spherical gold nanoparticles synthesized by citrate reduction: (a) 15 nm-AuNPs@citrate and (b) 30 nm-AuNPs@citrate. Optical path length: 1 cm.

Figure IV- 2 .

 2 Figure IV-2. Absorbance spectra of (a) Sodium Thiosulfate 10 -3 M, (b) Hexacyanoferrate 10 -3 M (c) Mixture of both reagents under illumination: 20 mL of distilled water under visible irradiation for 120 min under N2 (black spectrum before irradiation and blue spectrum after 120 min irradiation). The spectra were recorded using 2 mm quartz cell.

  containing HC-FeIII and ST (without Au-NPs) are stable at room temperature, even under irradiation, while in the presence of Au-NPs and in the absence of irradiation, the reaction takes place as shown by the disappearance of the absorption bands at 310 and 420 nm associated to HC-FeIII (FigureIV-3). The rate of the reaction depends on the size of the Au-NPs.

Figure IV- 3 .III- 1 -

 31 Figure IV-3. Temporal evolution of the UV-visible absorption spectra of deaerated aqueous solutions containing 5 × 10 -4 M sodium thiosulfate and 5 × 10 -4 M potassium hexacyanoferrate (a) in the presence of 15 nm Au-NPs@citrate under irradiation at λ = 520 nm, (b) in the presence of 15 nm Au-NPs@citrate in the dark, (c) in the presence of 30 nm Au-NPs@citrate under irradiation at λ = 520 nm and (d) in the presence of 30 nm Au-NPs@citrate in the dark. Optical path length: 1 cm.

2 .

 2 Effect of the plasmonic near field enhancement on the electron transfer process between HC-FeIII and ST; 3. Temperature increase due to a rapid conversion of the excited surface plasmon resonance into heat that would eventually accelerate the chemical reaction.It is to note that the temperature increase in the solution during the reaction under irradiation was around 3-4 degrees as measured by a thermos-couple immersed inside the reaction cuvette, but the increase of temperature at the surface of nanoparticles (induced by plasmon resonance) is much higher. Under continuous irradiation, each NP contributes to the global warming of the solution they are surrounded by.

Figure. IV- 4 .III- 2 -

 42 Figure. IV-4. Kinetic trace of the normalized absorbance at 420 nm for the reduction of HC-FeIII in deaerated aqueous solutions containing 5 × 10 -4 M sodium thiosulfate and 5 × 10 -4 M potassium hexacyanoferrate in the presence of 15 nm or 30 nm Au-NPs@citrate in the dark or under LEDs irradiation at 520 nm.

Figure IV- 5 .

 5 Figure IV-5. Temporal evolution of the UV-visible spectra of deaerated aqueous solutions containing 10 -3 M sodium thiosulfate and 10 -3 M potassium hexacyanoferrate under Xe lamp irradiation (equipped with an optical cutoff filter at 450 nm) (a) in the presence of 15 nm Au-NPs@citrate and (b) 30 nm Au-NPs@citrate. Optical path length: 1 cm.

Figure IV- 6 .

 6 Figure IV-6. TEM images of the initial 15 nm Au-NPs@citrate taken at different times during the reduction reaction of hexacyanoferrate (III) by sodium thiosulfate under irradiation with a Xe lamp equipped with a cutoff filter at 450 nm: (a) before irradiation, after (b) 30 minutes, (c) 120 minutes, and (d) 180 minutes of irradiation.

Figure IV- 7 .

 7 Figure IV-7. TEM images of initial 30 nm Au-NPs@citrate taken at different times of irradiation during the reduction reaction of hexacyanoferrate (III) by sodium thiosulfate under irradiation with a Xe lamp equipped with a cutoff filter at 450 nm: (a) before irradiation, and after (b) 30 minutes, (c) 120 minutes, and (d) 180 minutes of irradiation.

Figure IV- 8 .

 8 Figure IV-8. Spectral evolution with time of deaerated aqueous solutions containing 5 × 10 -4 M potassium hexacyanoferrate under LEDs irradiation at 520 nm) in the presence of (a 15 nm Au-NPs@citrate and (b) 30 nm Au-NPs@citrate. Optical path length: 1 cm.

Figure IV- 9 .

 9 Figure IV-9. Kinetic traces of the normalized absorbance at 420 nm of deaerated aqueous solutions containing initially 10 -3 M potassium hexacyanoferrate in the presence of 15 nm or 30 nm Au-NPs@citrate and in the dark or under LEDs irradiation at 520 nm.

Figure IV- 10 .IV- 2 -

 102 Figure IV-10. Temporal evolution of the absorption spectra of a solution containing 15 nm Au-NPs@citrate and 10 -3 M potassium hexacyanoferrate III with no irradiation (in dark) for 180 minutes.

Figure IV- 11 .

 11 Figure IV-11. Temporal evolution of the absorption spectra of a deaerated aqueous solution containing 5 × 10 -4 M potassium hexacyanoferrate in the presence of 15 nm Au-NPs@citrate under Xe lamp irradiation (equipped with an optical cutoff filter at 450 nm). Optical path length: 1 cm.

Figure IV- 12 .

 12 Figure IV-12. Spectral evolution with time of deaerated aqueous solutions containing 10 -3 M potassium hexacyanoferrate in the presence of 15 nm Au-NPs@pNTP under LEDs irradiation at 520 nm (Optical path length: 1 cm).

Figure IV- 12

 12 Figure IV-12 shows the temporal evolution of the UV-visible spectrum of a deaerated solution containing 15 nm Au-NPs@pNTP and HC-FeIII under LEDs irradiation at 520 nm. In that case, while the band at 420 nm due to HC-FeIII drops, no change is observed in the LSPR band of the Au-NPs. That indicates that the 15 nm Au-NPs@pNTP are stable during the reaction under irradiation, what is confirmed by TEM images (Figure IV-13). It is also worth to notice that the disappearance of the absorption band due to Hc-FeIII is quite rapid, nearly as fast as in the presence of ST (Figure IV-3). That proves that, even without a reducing agent such as ST, reduction of HC-FeIII occurs via plasmon excitation of Au-NPs, and consequently hot electrons are involved.

Figure IV- 13 .

 13 Figure IV-13. TEM images of initial 15 nm Au-NPs@pNTP taken at different times during the reduction reaction of hexacyanoferrate (III) under LEDs irradiation at 520 nm: (a) 10 minutes, (b) 30 minutes, and (c) 60 minutes of irradiation.

Fe

  Hexacyanoferrate (III) reduction by electron transfer induced by plasmonic catalysis on gold nanoparticles

  Figure IV-15 presents the XPS spectra recorded for the Au4f, Fe2p and N1s regions.

Figure IV- 15 .

 15 Figure IV-15. XPS spectra of Au4f (top), Fe2p (middle) and N1s (bottom) core levels of 30 nm-Au-NPs@citrate after reaction after reaction under LEDs irradiation at 520 nm (during 60-minutes) for deaerated solutions containing hexacyanoferrate with (left) or without (right) sodium thiosulfate.

  7 eV in the N1s spectrum (Figure IV-15 bottom-right), which has an area equivalent to that of Au + in Au4f [38]. This component at 85.1 eV is also present in the Au4f core-level spectrum of the sample with ST (Figure IV-15 top-left) but to a lesser extent, proving that Au is more protected from oxidation when ST is present. Investigations of Fe2p core-level spectra corroborate further the effect of ST. A complete reduction of Fe III into Fe II is observed in the presence of thiosulfate (Figure IV-15 middle-left), while in the absence of thiosulfate, only 84% of Fe III has been reduced (Figure IV-15 middle right). In Figure IV-15 middle-left, Fe II is easily identified by the presence of a single doublet located at 708.5 (Fe2p3/2) and 721.5 eV (Fe2p1/2), typical values for K3[Fe(CN6)] [42]. This result confirms the hypothesis that, in the presence of ST, at the end of the reaction (after LEDs irradiation for 60 min), there is a complete reduction of Fe III into Fe II in the present experimental conditions. The presence of a second doublet at 709.8 and 723.7 eV in the spectrum of the sample without ST (Figure IV-15 middle right) is indicative of Fe III [27,43].

Figure V- 1 .

 1 Figure V-1. Absorption spectrum of an ethanol solution containing 10 -3 mol L -1 of Pd(acac)2 under 1 atm of CO for different gamma-irradiation doses: Black line: 0 kGy, red: 2 kGy, green 3 kGy. Dose rate 3 kGy h -1 . Optical path = 1 mm.

Figure V- 2 .

 2 Figure V-2. TEM images of the synthesized Pd nanoflowers; In inset: their electron diffraction patterns.

Figure V- 6 .

 6 Figure V-6. Catalytic cycle of Suzuki-Miyaura reaction.

Figure V- 7 .

 7 Figure V-7. TEM images of palladium nanostructures after 30 minutes (a and b) and 3 hours (c and d) of catalytic reaction under visible light showing the evolution of the Pd nanoflowers formed by nanosheets into smaller Pd nanoparticles of few nanometers.

Figure A- 1 .

 1 Figure A-1. Experimental setup used to study the oxidation of CO to CO2 catalyzed by gold nanoparticles either supported on Al2O3, and SiO2 or in solution like above.

Figure A- 2 .

 2 Figure A-2. Evolution of CO and CO2 as a function of time of reaction without irradiation of LED light source. Note that the high activity of the catalyst made a total conversion with no irradiation; Then the CO2 concentration started to go down as the activity decreases. The reaction is too fast.

  de l'or (voir Figure 1 en dessus). Parmi ces métaux, l'or est le plus facile à manipuler car il ne s'oxyde pas. Par ailleurs, les nanoparticules d'or présentent des propriétés catalytiques dans les réactions d'oxydation de molécules telles que CO, COV… pour lesquelles le support oxyde (TiO2, CeO2) stabilisant les NPs permet l'activation de l'oxygène. L'or est aussi un catalyseur intéressant pour sa forte sélectivité pour des réactions d'hydrogénation bien que peu actif. Dans ce cas, le support n'intervient pas dans la réaction.

Figure 1 -

 1 Figure 1-Présentation schématique de la polarisation d'une nanoparticule plasmonique sphérique et de l'oscillation continue due aux interactions de la fonction diélectrique.

Figure 2 -

 2 Figure 2-Présentation schématique des deux principales voies possibles d'interaction des électrons chauds avec le milieu entourent.

Figure 6 -

 6 Figure 6-Illustration de la réaction de Suzuki-Miyaura catalysée par des nanofleurs de palladium.

Figure 1 -

 1 Figure 1-Schematic presentation of the polarization of a spherical plasmonic nanoparticle and the continuous oscillation due to the interactions of the dielectric function.

Figure- 2

 2 demonstrate two major processes related to the excitation of the plasmonic band a nanoparticle.

Figure 2 -

 2 Figure 2-Schematic presentation of the two main possible ways of interaction of hot electrons

  Catalytic processes are of major interest for research and development. Natural photoactivated catalytic processes are involved in almost every aspect of our environment: photosynthetic cycles feed and color plants, enzymatic reactions essential for species survival, decontamination of water with visible light, etc. Nanoparticles of a metallic nature have properties different from those of metals in their overall structure. Several factors contribute to the difference in physicochemical properties such as size, morphology, functionalization and the surrounding For reactions in the gas phase, reactors from the Surface Reactivity Laboratory (LRS, Sorbonne University) (collaboration with the Catherine Louis team) have been adapted to produce thin films and allow irradiation under visible light or under excitation with a lamp LEDs of wavelength resignation corresponding to that of the plasmon. In a second step, we studied in collaboration with the ICMMO, Suzuki-Miyaura reactions catalyzed by visible palladium nanofillers of palladium.

Figure 5 64 × 10 - 4 M

 564104 Figure 5 Temporal evolution of the absorption spectra of an aqueous solution containing 1.64 × 10 -4 M pNTP and 30 nm AuNPs ([Au0] = 1.74 × 10 -4 M) with stirring and illumination at 525 nm in the photochemical reactor with LEDs for 480 minutes.

Figure 6

 6 Figure 6 Illustration of the Suzuki-Miyaura reaction catalyzed by palladium nanofillers.

  Au I + (CH 3 ) 2 C  OH → Au 0 or + (CH 3 ) 2 CO + H + Au I + H  → Au 0 + H +

	I	Chapter I : State of the art
		Au I + e (aq) -	→ Au 0
		2 Au II → Au I + Au III
			20

IV-2-Plasmon assisted catalysis via supported metal nanoparticles

	I	Chapter I : State of the art
		However, in catalysis, for practical applications, to prevent NPs from aggregation or
		sintering and to recover the catalysts after reactions, metal nanoparticles are in general
		supported on solid substrates. Different studies report on plasmonic catalysis conducted by
		metal nanoparticles on different supports [63].
		Different inert supports are commonly used to deposit metal nanoparticles for application
		in catalysis such as: alumina (Al2O3), Silica (SiO2), ceria (CeO2), zirconia (ZrO2), zeolites,
		hydrotalcite (Mg6Al2(CO3)(OH)16) [4]. The first example of plasmonic catalysis by metal NPs
		on inert supports was reported by Hallet-Tapley et al. who showed the conversion of benzyl
		alcohol in benzaldehyde and 1-phenylethanol in acetophenone by Au NPs deposited on
		hydrotalcyte, alumina or zeolites (1% in mass) and excited by LED at 530 nm (Figure I-16)
		[58].

Table II -

 II

	Gold salt solution	Sodium citrate	Ratio of citrate	Au NPs' mean diameter
	[KAuCl4]	solution	to gold	determined by TEM (nm)
	20 mL at 0.25 mM	1 mL at 34 mM	6.8	15
	20 mL at 1 mM	0.8 mL at 34 mM	1.0	30

1 provides the concentrations and the citrate/gold ratios used to synthesize welldispersed quasi-spherical gold nanoparticles with a mean diameter of 15 or 30 nm.

Table II -

 II

1. Experimental conditions of concentrations and ratios [citrate]/[Au III ] for AuNPs@Citrate of 15 and 30 nm.

  -1 are also listed possible products with their formula and mass.

	Chemical entity	Formula	Monoisotopic	Isotopes
			Mass
	4-NTP	C6H5NO2S	155.0041	M+1 (6.6%)
				M+2 (4.4%)
	Nitrophenol	C6H5NO3	139.0269	M+1 (6.7%)
	DMAB Dimer (4,4'-[-1,2-Diazenediyl] dibenzenethiol)	C12H10N2S2	246.0285	M+1 (13%) M+2 (9%)
	3-[4-(1,3-benzothiazol-2-yl)-3-oxo-8-[2-(2-thienyl)vinyl]-1-thia-4,6,7-triazaspiro[4.4]non-7-en-6-yl]	C24H18N4O4S4	554.0211	M+1 (17%) M+2 (24%)
	benzenesulfonic acid

Table III -

 III

1. Possible products (name, formula and mass) obtained by the degradation of pNTP IV-1-1-ESI-MS spectra of the irradiated solutions

Table A -

 A 1). The size of the gold nanoparticles is around 2-3 nm.

	Sample	Au (%)
	Au/SiO2 1%	0.64
	Au/SiO2_R01 1%	0.67
	Au/SiO2_R02 1%	0.67
	average loading	0.66
	Au/Al2O3 1%	0.74
	Au/Al2O3 _R01 1%	0.73
	Au/Al2O3_R02 1%	0.75
	average loading	0.74

Table A - 1 .

 A1 Loading of gold determined by XRF analysis data for the different samples prepared

concentration time of experiment Evolution of CO 2 concentration as a function of time (NO IRRADIATION) Figure A-3.

  Evolution of CO and CO2 concentration as a function of time of reaction under illumination with LEDs light source. Note that because of the high activity of the catalyst, it is very difficult to determine the actual effect of AuNPs plasmon;

		Evolution of CO concentration as a function of time (NO
		IRRADIATION)
		1,2
	concentration	1
	CO
		10:48:00 11:16:48 11:45:36 12:14:24 12:43:12 13:12:00 13:40:48 14:09:36 14:38:24 15:07:12
		Time of experiment
		1,2
		1
		0,8
		0,6
	CO 2	0,2 0,4
		0
		10:48:00 11:16:48 11:45:36 12:14:24 12:43:12 13:12:00 13:40:48 14:09:36 14:38:24 15:07:12

CO concentration Time of experiment Evolution of CO concentration as a function of time (IRRADIATION)

	0,29
	0,28
	0,27
	0,26
	0,25
	0,24
	0,23
	14:38:24	14:52:48	15:07:12	15:21:36	15:36:00	15:50:24	16:04:48
	0,74
	0,73
	0,72
	0,71
	0,7
	0,69
	0,68
	0,67
	14:38:24	14:52:48	15:07:12	15:21:36	15:36:00	15:50:24	16:04:48

CO 2 Concentration time of experiment Evolution of CO 2 concentration as a function of time (IRRADIATION) Introduction Les

  nanoparticules (NPs) métalliques ont de nombreuses applications en catalyse. Certaines d'entre elles (or, argent, cuivre) présentent une absorption dans le visible résultant de l'excitation du plasmon de surface par un champ électromagnétique extérieur.

	Nous proposons dans ce projet de coupler les propriétés catalytiques de nanoparticules à base
	d'or avec leurs propriétés de résonance de plasmon de façon à assister la réaction catalytique
	par les électrons chauds générés et par l'effet thermique qui en découle. Cette approche de
	catalyse assistée par plasmon, est générale et peut être utilisée pour une variété de procédés
	catalytiques exothermiques et endothermiques impliquant des nanoparticules. Cette catalyse
	assistée par plasmon permet de réaliser des réactions catalytiques à moindre coût énergétique
	sous lumière solaire.

VChapter V: Plasmonic catalysis for Suzuki-Miyaura cross-coupling reaction using Palladium nanoflowers

Chapter V: Plasmonic catalysis for Suzuki-Miyaura cross-coupling reaction using Palladium nanoflowers

Acknowledgements

helped me a lot during the last stretches of manuscript writing. I sincerely hope we collaborate again in the future. I acknowledge C'Nano IdF and the DIM NanoK pour the financial support of the project and for the PhD scholarship. Many thanks to Mr. Bruno Palpant from LPQM (Ecole-Centrale Supelec) and Mrs. discussions. I thank Mrs Patricia Beaunier (LRS,

II

Chapter II: Experimental setups, materials and methods

Chapter II

Experimental setups, materials and methods

Chapter IV

Hexacyanoferrate (III) reduction by electron transfer induced by plasmonic catalysis on gold nanoparticles

Chapter V Plasmonic catalysis for Suzuki-Miyaura cross-coupling reaction using Palladium nanoflowers V Chapter V: Plasmonic catalysis for Suzuki-Miyaura cross-coupling reaction using Palladium nanoflowers Previous studies show that CO plays an important role in defining the morphology of the nanosheets and the nanoflowers and intermediate unstable palladium carbonyl clusters are probably involved in the nucleation/growth process [35,36].

The XPS spectrum of Pd nanoflowers exhibit two main asymmetric peaks at 335 and 340.2 eV (Figure V-3). This double feature, separated by 5.2 eV, is specific to the 3d5/2 -3d3/2 spin-orbit splitting of Pd3d core shell. The component Pd3d3/2 has a more pronounced asymmetry due to the interference coming from the Pd3d5/2 plasmon loss peak, at around 342 eV [38,39] The peak positions, aligned following the aliphatic component of C1s set at 284.8

eV, together with their asymmetry and the presence of the plasmon loss peak confirms the metallic nature of Pd. It is to note that the generated Pd nanostructures are stable in ethanol solution for few days in the dark and for few hours under light irradiation.

III-Catalytic study of the Suzuki-Miyaura cross coupling reactions III-1-Experimental details

For the catalytic reaction, a Schlenk tube equipped with a magnetic stirring bar was charged with haloarene (0.54 mmol), boronic acid (0.6 mmol), base (1 mmol) and 4 mL of ethanol solution containing 0.43 mg of palladium nano-sheets. The flask was evacuated and backfilled with argon three times. The reaction mixture was irradiated under stirring with a 300 W Xe lamp equipped with a water cell filter to absorb the near-IR radiation and a 475 nm cut-off filter to avoid direct photoactivation of the aromatic compounds. The mixture was then filtered and analyzed by gas chromatography (GC) or gas chromatography coupled with mass spectrometry (GC-MS).

III-2-Reaction between iodobenzene and phenylboric acid

These Pd nanoflowers exhibiting absorption plasmon in the visible range in ethanol solutions were used as catalysts in Suzuki-Miyaura cross coupling reaction between iodobenzene and phenylboronic acid using Cs2CO3 as base (Scheme V-1).

Scheme V-1. Reaction between iodobenzene (1a) and phenyl boric acid (1b) catalyzed by Pd nanoflowers.

The reaction was performed under argon atmosphere, at room temperature, either in the dark or under visible light irradiation (>475 nm). The formation of biphenyl was monitored by GC over a period of 240 min (Figure V-4). After 5 min of latency, light irradiation induces an increase both in the reaction rate and in the conversion. The kinetics of the reaction was much faster and the conversion rate observed for this reaction was dramatically higher under irradiation compared to that carried out in the dark. After 20 min of reaction 57% conversion was observed under irradiation, and only 9% in the dark. The reaction continued to proceed quickly under light illumination to reach 90% conversion at 60 min and 96 % after 240 min, while the reaction in the dark went on slowly up to 53% conversion at 240 min. These results confirm that photocatalysis is indeed taking place. Pd nanoflowers are absorbing in the visible region as shown in Figure 1.

The excitation of the plasmon resonance can be converted into heat and hot electrons can be induced on the surface of the nanostructures. These two effects may also be reinforced by the formation of "hot spots", where the electric fields are concentrated. This is likely to accelerate the initial oxidative addition reactions and the subsequent steps of the Suzuki-Miyaura reaction as well, (i.e. the transmetallation and the reductive elimination reactions).

Under similar conditions as those described above, the reaction was carried out using In contrast to Cs2CO3, no induction time at the beginning of the reaction is observed by using the other bases. With KOH and K3PO4, the reaction initiated very quickly. Over 54% conversion was noted after 5 min. The reaction with K2CO3 is the slowest (only 47% conversion after 15 min), and the least efficient with a maximum conversion of 85%. The reactions reach maximum conversions of 89% for KOH and 94% K3PO4 after 240 min. To conclude, the best bases for this reaction were found to be Cs2CO3 and K3PO4.

III-3-Reactions between other haloarenes and phenylboric acid

In order to confirm that the reaction is a true cross-coupling reaction and not a simple homocoupling, other halogenated compounds were employed, such as 4-iodotoluene, 1-iodo-4-nitrobenzene and 1-bromo-4-nitrobenzene. The reactions were carried out under the same conditions, with light irradiation for 240 min and using K3PO4 as base (Scheme V-2).

Scheme V-2. Scope of the reaction catalyzed by Pd nanoflowers.

In these reactions only cross-coupling products were observed. The conversion was very good (80%) in the case of 4-iodotoluene, while, surprisingly, the conversion was lower with the very activated 1-iodo-4-nitrobenzene (60% of conversion), and very low with 1-bromo-4nitrobenzene (11% of conversion). Irradiation of plasmonic Pd nanostructures induces hot electrons and holes, therefore the Pd nanostructures are charged. So, the adsorption of more polar organic halides is favored, which facilitates the first step of the reaction (the oxidative addition of the aryl halide). This can explain the difference in reactivity with 4-iodotoluene, 1iodo-4-nitrobenzene and 1-bromo-4-nitrobenzene.

III-4-Proposed mechanism

Wang et al. reported plasmonic Suzuki-Miyaura coupling reactions under light activation (using a laser and under solar irradiation) on Au-Pd nanostructures formed by PdNPs on the surface of Au nanorods [40]. They demonstrated that light was absorbed by the Au cores and PdNPs promoted the Suzuki coupling reactions. Au-Pd nanoalloys supported on ZrO2 tested

Annex

Gas phase oxidation of CO to CO 2 assisted by plasmon of gold nanoparticles

Al2O3 or using another support such as SiO2, which leads to less active catalysts. Another possibility is to prepare nanoparticles with larger sizes (3-5 nm).

The experiments could be conducted in a tube filled with distilled water (where the Ushaped reactor will be immersed) at room temperature to minimize the increase of temperature induced by the LED and to diffuse the heat produced by the reaction.

Synthèse de thèse en Français.

Catalyse Assistée par Plasmon.

Table des matières

Résumé des travaux et principaux résultats

Summary of work and main results

In a first step, we studied reactions catalyzed by gold nanoparticles synthesized by the Türkevich method (reduction of gold complexes by citrate).