Mohamed Nawfal

Ghazzal-Dossier Hdr

P Deparis

E M Simonis

M N Gaigneaux

J Ghazzal

H De Coninck

Kebaili

Marija Knezevic

Isabelle Lampre

Marie Erard

David Berardan

Christophe Colbeau-Justin

Mohamed Nawfal Ghazzal

E-Mrs Spring

C Li

G Diress Gesesse

H Remita

M N Ghazzal

C Wang

Oral communication (international) 1. Interfacial charge-carrier transfer and dynamics in CsPbBr3-yXy/TiO2

Titre : Matériau TiO2 nanostructuré pour améliorer la récolte de la lumière et l'activité photocatalytique Mots clés : Photocatalyse, TiO2, collecte de photon, nanostructure Résumé : En s'inspirant du processus complexe de photosynthèse et de l'architecture des feuilles, il apparaît, que le processus de capture de la lumière, la photogénération de charge et leur durée de vie, combinées à la réactivité du photocatalyseur, font partie des principaux défis à relever pour convertir efficacement l'énergie solaire en energie chimique. L'amélioration de l'efficacité photocatalytique d'un matériau photocatalytique nécessite donc : i) une augmentation de la durée de vie des charge photogénérés, ii) d'étendre le spectre d'action du photocatalyseur et l'amélioration de sa capacité à récolter les photons et iii) une augmentation de la réactivité de la surface. Dans ce manuscrit, différentes stratégies ont été étudiées afin d'améliorer l'efficacité photocatalytique du TiO2 pour la génération de H2.

Ces stratégies ont pour objectif commun l'augmentation de leur rendement quantique, de leur réactivité de surface et de la séparation des paires e -/h + pour atteindre une meilleure efficacité photocatalytique. La première partie s'est concentrée sur la conception de différentes architectures qui améliorent la capture de la lumière par le TiO2. Ensuite, la configuration idéale des photocatalyseurs a été explorée en utilisant le concept de "Strong Metal-Support Interaction" comme moyen efficace d'améliorer l'efficacité photocatalytique. Enfin, l'objectif était de proposer de nouvelle architecture de TiO2 qui peuvent augmenter efficacement la densité de porteurs de charge et ainsi la génération d'hydrogène Title : Structured TiO2 material for enhanced light harvesting and photocatalytic activity Keywords : Photocatalysis, TiO2, light harvesting, nanostructure Learning from the complex nature's photosynthesis process and leaf architecture, it appears, in a simplified picture, that light-harvesting, photogeneration of charge carriers, and their lifetime combined with the surface photocatalyst reactivity are among the key challenges driving the efficient conversion of solar energy to chemical fuel. Improving the photocatalytic efficiency of a photocatalytic material requires: i) an increase of the lifetime of the photogenerated charges carriers, ii) broadening the spectrum of absorption response and enhancing the absorbance of photon ability (light harvesting properties) and iii) an increase in the surface reactivity. In this manuscript, Variable strategies were investigated to improve the photocatalytic efficiency of TiO2 toward H2 generation.

These strategies are centered on a common objective, namely increasing their quantum yield, surface reactivity and charge carriers separation to reach improved photocatalytic efficiency. The first part was focus on e design of different architecture that improves the light-harvesting of TiO2 in the nano-and mesoscale. Then, the ideal configuration of photocatalys was explored using the "strong metal-support interaction" concept as an effective way to improve photocatalytic efficiency. Finally, the objective was to propose a new architecture of TiO2 photocatalyst that can efficiently increase the charge carriers density and then increase the hydrogen generation. 2. Co-editor (invited) of a special issue "Green Chemistry for Nanoparticle Synthesis" in journal « Nanomaterials » (Impact Factor 5.719). Appointed member of the jury for the 2018 synchronized session recruitment of assistant engineer in the chemistry department of the University Paris-Saclay. The jury was composed of Gilles Ponchet

Scientific responsabilities

Referee in international

Teaching activities

Master (2003-2004)

The objective of my DEA work was to find a new method to synthesize a photocatalyst (ZnO) supported on glass, cellulose or glass fiber to improve its photocatalytic properties and ovoid the filtration step needed when the photocatalyst is used as a powder. The characterization of the catalyst by XRD shows its crystallization after hydrolysis of the precursor at room temperature. Other techniques, such as BET, SEM, TEM, allowed us to explain the performances of this catalyst (ZnO).

The degradation kinetics of Dimethyldisulfide (DMDS) allowed to evaluate its photocatalytic activity and to compare it to commercial compounds (ZnO and TiO2 P25). The synthesized ZnO was found to have better activity than its commercial equivalent. The degradation of the selected pollutant (DiMethyl DiSulfide -DMDS) was followed by Gas Chromatography. This technique allowed to follow in dynamics the evolution of the concentration of the DMDS as well as the CO2 produced, thus indicating the mineralized fraction of the pollutant. This study also allowed me to highlight the influence of variable parameters on its photocatalytic activity (DMDS flow rate, catalyst mass).

Moreover, IR analyses of the photocatalysts used allowed to identify a certain number of photocatalytic degradation products of DMDS (sulfoxide, sulfone, sulfonate, carbonyls) and to understand its mechanisms.

Ph.D. (November 2004 -Mars 2008)

My Ph.D. thesis followed the same topic of my master work. It aims at the elaboration, by sol-gel process, of thin films of photocatalyst (TiO2) deposited on glass by dip-coating and the development of benchmark reactor to evaluate the efficiency of "self-cleaning" glass. The self-cleaning properties are closely linked to the photocatalytic and photoinduced wetting properties. The first step was to design and build a motorized dip-coating system allowing to obtain films in a reproducible way in terms of thickness, composition, and crystalline structure (after thermal treatment). To do so, a formulation was prepared to obtain a sol that is stable for several weeks and delays the sol-gel transition. Different formulations with various precursor/solvent ratios and pH were tested, and the final choice was a low-viscosity sol with an acidic pH (≈1). Indeed, higher sol viscosity induces a faster polycondensation rate during the xerogel formation during the dip-coating process, leading to the crack of the coating and thereby low adhesion of the TiO2 at the surface of the glass. The sols prepared at higher pH (̴ 11) were much less stable. The kinetics of gelation reaction being favored by the presence of OH -ions (more attracted by the Ti metal), favors the sol-gel transition. Whereas at low pH, the prepared sol was stable for several weeks, which is explained by the electrostatic repulsion between monomers. Working at acidic pH makes the hydrolysis step critical, mainly during the formation of the xerogel on the substrate surface. The next step was to study the effect of the relative humidity present in the air on the rate of polycondensation reactions leading to xerogel formation during dip-coating extraction. Various relative humidity was applied during the dip-coating process to identify optimal conditions and the effect of the surrounding atmosphere on the film's morphology. I have shown through this study that the polycondensation rate during the xerogel formation is very sensitive to the amount of water in the ambient atmosphere. Therefore, the homogeneity and adhesion of sol-gel TiO2 films also were affected. The fast polycondensation induces internal stress in the films and the apparition of cracks.

One of the innovative aspects of my Ph.D. is the development and optimization of a reactor that allows the characterization of the photocatalytic properties of these thin films. It allows to studying the influence of the different parameters, often interdependent, which come into play in the photocatalytic reaction. The measurement principle is to follow in situ the kinetics of decoloration of an aqueous solution of an azo dye: orange II, by UV-vis spectroscopy under irradiation. This device has been dimensioned to measure the activity of samples of the size of a glass plate like a microscope slide. Our photoreactor offers two illumination configurations depending on the location of the sample. When irradiating through the glass support and then through the TiO2 film, it allows to avoid the reduction of the light flux caused by the absorption of part of the photons by the dye solution.

Thus, thanks to this device, we showed that the degradation of Orange II (OII) did not rely on its adsorption on the TiO2 surface but proceeded according to a predominant photocatalytic reaction involving OH • and HO2 • radicals. I performed a comprehensive study on the influence of the different parameters to optimize the operating's conditions of the reactor.

I compared the OII test to the stearic acid test, frequently used to evaluate the photocatalytic activity of self-cleaning glasses. A correlation was found between the pH of the solution and the photocatalytic activity. We have shown that "our" test is sufficiently repeatable, reproducible and economical. Thanks to this system, I was able to highlight the influence of the sodium "poisoning" of TiO2 films through studying the impact of the substrate, and mainly the sodium composition of the glass support, on the photocatalytic properties of thin TiO2 films prepared by sol-gel. Thus the diffusion of sodium ions during the thermal treatment modifies the photocatalytic activity by playing the role of recombination center of the photogenerated charges, which explains the difference of activity between soda-lime glass, Pyrex, and quartz.

To avoid this problem, we have covered the support with a SiNx layer which will act as a diffusion barrier for the sodium. We have shown that the size of the TiO2 crystallites is correlated to the thickness of the SiNx layer. This led us to suppose that the SiNx/TiO2 interface plays an important role in the crystallization of TiO2 particles and that at low thickness the diffusion barrier is permeable to sodium ions which modify the size of crystallites. Therefore, it is necessary to find a compromise between the thickness, the size of the particles and the photocatalytic efficiency. This last work has been done in collaboration with the group of the LSGS of the Ecole des Mines de Nancy. A joint project was submitted to the region in 2005. It received a favorable response. In this perspective, the efficiency of different sodium diffusion barriers, such as BN, SiO2 or SiNx and the role of the thickness of these barriers on the crystallization of TiO2 affected the photocatalytic activity. A comparative study was performed between TiO2 thin films deposited by the sol-gel dip-coating process and PVD (Physical Vapor Deposition). A device allowing the measurement of the contact angle has been realized. This device allowed me to study the temporal evolution of wettability under UV illumination. This, in addition to the study of the photocatalytic properties, allowed a more precise estimation of the self-cleaning properties of the films. The study was carried out within the framework of a collaboration with the group "flat glasses" of the company Saint-Gobain (Aubervilliers), which made it possible to carry out a comparative study of the photocatalytic properties of commercial selfcleaning glasses (Bioclean®) and those obtained by sol-gel. The measurement of the wetting angle on TiO2 films deposited on different supports showed that sodium also affects the hydrophilic properties of self-cleaning glasses. The study of the degradation of a carbonaceous layer deposited on the surface of these poisoned films allowed us to hypothesize the link between the photocatalytic properties and the hydrophilic properties of the TiO2 layers by showing that the rate of hydrophilic conversion of the surface of the TiO2 films depends on the rate of degradation of a thin hydrophobic hydrocarbon layer adsorbed on the surface.

Teaching assistant (2007-2008)

In the short term, the objective of the research was to define the operating steps allowing to carry out a twin-screw extrusion on the Miscanthus-X-Giganteus to valorize the biomass as fuel within the framework of a common project with the platform of PLASTINOV. The pellets from the plant fiber are used as a natural fuel thus reducing air pollution. Subsequently, a study of the effect of extrusion on the structure of Miscanthus-X-Giganteus was carried out. The results of this project enables to rise funding from ADEM, which allowed the research to continue for a further three years.

Post-Doc position (2008-2012)

The ambition of this project was to introduce to the market a new family of innovative smart materials in the form of ultra-thin surface coatings whose electromagnetic response (optical, thermal) can be adjusted by hygrometry or, more generally, by liquid adsorption.

Within the framework of the HGCM project, my goal was to create porous dielectric mesoporous multilayers and to realize a porous multilayer structure using hybrid sol-gel process (organic/inorganic), which deposited on a glass enables tuning the transparency at will thanks to controlled exchanges of liquid in the structure. It is thus a question, on the basis of a biomimetic approach, of manufacturing a dielectric mirror with switchable reflectivity. By judicious choice of the chemical nature of the materials, the thickness of the dielectric layers and their porosity, it is possible to realize an optical coating with a high reflectivity on a predefined spectral range. The major challenge of this project was to synthesize porous inorganic or hybrid layers whose optical refractive index can be modified by liquid diffusion between adjacent layers. This will result in a structure whose optical properties can be switched at will between a transparent and a reflective state.

The major challenge of this research was to develop a coating free of defects. The coating, whose thin film structure must allow the modulation of the electromagnetic wave in the visible range.

The strategy was to proceed in several steps:

-First, I established a specification by modeling the wave propagation in photonic crystals by the effective medium theory. This was done in order to determine the pore fractions, thicknesses and types of host materials required for the coating (Collaboration with Prof. Olivier Deparis from Université de Namur,).

-Subsequently, a careful study was performed to establish a relationship between the physicochemical properties (sol components, nature of the surfactant...) and textural properties (pore fraction, pore size distribution, pore organization, interconnection of adjacent layers, effective bilayer index, bilayer thickness...) of the thin films. I have developed experimental procedures (chemical formulations, heat treatment, film deposition methodology...) allowing control of the effective index of each of the thin layers of the multilayer structure: by inclusion addition by surfactant self-assembly (micelle formation) in the starting sol and/or by fine-tuning of the refractive index by coupling of high and low refractive index oxides.

-Finally, I proceeded to the characterization of the films by different textural (Ellipsometry Atmospheric Spectroscopy, TEM, SEM, AFM), structural (FTIR, Raman, XPS) and optical (UV-Vis-NIR) techniques. This step allowed me to demonstrate how to link physicochemical properties to textural properties to control the optical response of a Bragg mirror.

-Based on the results of this study, I was able to show different applications for this new type of smart coating, both in optics (Bragg reflection modulation, UV-NIR filter), and in gas detection.

Post-doc position (2014-2016)

The objective of the project is the development of a super-hydrophobic, transparent, resistant and easily applicable deposit on the protective glass of photovoltaic panels and on polymer-based substrates. This will allow to significantly reduce the reflected part of the solar radiation for a wide range of incidence as well as the self-cleaning of these panels (elimination of dust, water, tree leaves, etc...). The main objective is to obtain a maximum yield of photovoltaic cells. In order to develop an industrial process, certain technological constraints are necessary: a simple and economical method; all parameters must be controllable, and the range of values of these must be as wide as possible; the super-hydrophobicity must be as universal as possible (must work with rainwater from China or Belgium) and as durable as possible. The objective was to develop a double roughness surface, the latter being bio-inspired from the Lotus leaf. This project aims at building a superhydrophobic structure robust to disorder according to the model described by Cassie-Baxter.

The dual-scale size of the silica nanoparticles is commonly aimed at producing dual-scale roughness, also called hierarchical roughness (Lotus effect). During the post-doc position, we describe a method to build a stable water-repellant coating with controlled roughness. Hybrid silica nanoparticles are self-assembled over a polymeric surface by alternating consecutive layers. Each one uses homogenously distributed silica nanoparticles of a particular size. The effect of the nanoparticle size of the first layer on the final roughness of the coating is studied. The first layer enables to adjust the distance between the silica nanoparticles of the upper layer, leading to a tunable and controlled final roughness. An optimal size nanoparticle has been found for higher water-repellency. Furthermore, the stability of the coating on polymeric surface (Polycarbonate substrate) is ensured by photopolymerization of hybridized silica nanoparticles using Vinyl functional groups.

Light harvesting ability and photocatalysis 1. Contexte and chalenges

In a dark world, we simply would have nothing. The solar light is the most valuable resource the earth has at its disposal. Light is the main source of energy for all living organisms. Plants, main strainers of life, are crucial in this conversion process and need light for photosynthesis that enables them to make their own food and food for others. Moreover, perhaps more importantly, green leaves store the solar photons by photosynthesis to split water to oxygen, using four protons, and four electrons, which are utilized in the conversion of carbon dioxide to carbohydrates. 2 For years, leaves have fascinated and inspired scientists by the way their elaborated structures and functional components work synergistically to produce a highly complex photosynthesis mechanism, in which light harvesting, photogenerated charge separation, gas/liquid (CO2/H2O) diffusion, and capture, [3][4][START_REF] Nishio | Carbon Fixation Gradients across Spinach Leaves Do Not Follow Interna1 Light Gradients[END_REF][6][7] and catalytic blocks work in harmony to capture solar energy and split efficiently water into oxygen and hydrogen. More fascinating, the leaves' structure and thickness are developed in accordance with the irradiance conditions. The whole hierarchical structure of leaves (Figure 2a) is perfectly designed to manipulate the light energy at all wavelengths of the visible solar spectrum, which essentially increased the light-harvesting (absorption) by controlling the scattering, propagation, and focusing of light. [8][START_REF] Niinemets | Structural determinants of leaf light-harvesting capacity and photosynthetic potentials[END_REF][START_REF] Mathls | [END_REF] The epidermal cells focus light within the leaf by a lens-like mechanism, 11,12 the veins' porous architectures induce a multiple scattering and improve absorbance of light [START_REF] Niinemets | Structural determinants of leaf light-harvesting capacity and photosynthetic potentials[END_REF]13 , the columnar cells, in palisade parenchyma, for example, act as light guides ensuring the light propagation, 14 the spongy mesophyll cells are responsible for the enhanced effective light diffusion path length and light scattering (depending on the incident light angle), 4,6 and the efficient light-harvesting and fast charge separation in the high surface area three-dimensional constructions of interconnected nanolayered thylakoid cylindrical stacks (granum) in the chloroplast. 15 The focusing and propagation of light follow an internal gradient into the structure. The lens-like mechanism ensures optimal light harvesting into the Palissade parenchyma where located chloroplasts, and where "natural photosynthesis" can proceed with successive water splitting and the production of H2 (Photosystem I, PSI) and O2 (Photosystem II, PSII) (eq. 1), followed by CO2 conversion into glucose (eq. 2).

𝐻 2 𝑂 → 𝐻 2 + 1 2 𝑂 2 𝐸°= 1.23 𝑒𝑉 (1)
𝐶𝑂 2 + 𝐻 2 𝑂 → 1 6 𝐶 2 𝐻 12 𝑂 6 𝐸°= 1.24 𝑒𝑉 (2)
The reactions are spatially and temporally separated from each other as illustrated in Figure 2b. For a simplified picture, water oxidation first involves photon absorption by a light-harvesting antenna (Lhcb: Chlorophyll a and b), in which the energy is transferred without any loss generating an excited state of the electron donor (P680*). The oxidation (hole transfer) of the tyrosine molecule (TyrZ) by P680 + occurs within the ns-μs scale range. Then, TyrZ is reduced rapidly (μs to ms) by electrons from the Mn complex leaving the hole (×4) localized into the oxygen-evolving complex (OEC), which performs the critical water oxidation and releases O2 and four protons. The subsequent electron transfer from the excited state (P680*) results in the reduction of plastoquinone molecules (QA and QB) -to plastoquinol (PQH2) -and plastocyanin (PC) bound within the cytochrome b6f (Cyt b6f)

complex finally arriving at ferredoxin (2Fe-2S) bound to PSI. These energized electrons reduce protons to produce hydrogen stored in the form of NADPH. [16][17][18] It is worth noting that the water splitting reaction is a four-electron/hole process (four photons), while the NADP + reduction is two electron/hole reactions (two photons). The role of NADPH oxidase appear to be essential in term of hydrogen ion relay or storage that mediate proton transfer for CO2RR. The oxidation and reduction catalysts centers driving the charge carriers separation are then placed at the extremity of PSII and PSI networks, in the way that one-photon electron/hole equivalence of the solar-derived wireless current can be bridged to the four-electron/hole chemistry of water separation (so-called Z-scheme process). 17,19 The water splitting reaction is subsumed by the CO2 reduction reaction (CO2RR) (eq.2). At the equivalent of electron consideration, the production of carbohydrates stores only 0.01 eV more energy than the water splitting reaction and is carried out in the dark. 17 This indicates, that the CO2RR, in the photosynthesis process relies on how the protons released from the water splitting reaction are stored, making the efficiency of the HER determinant in the photosynthesis reaction efficiency. The leaf is perfectly structured to reach optimal management of the incident radiation through reflection, transmission, or absorption. The external cuticle upper epidermis is a thick layer of parenchyma cells composed of high wax, and/or a large number of leaf hairs that significantly reflect the light and regulate the flux penetration into the leaf. The upper epidermis with an ellipsoidal-like structure behaves as a lens focusing the light into the mesophyll layer arranged in columns, forming the palisade mesophyll. The columnar structure acts as a channel transport layer of light, which improves the degree of light scattering (higher absorption in some wavelengths) and reduces energy loss. 20 (b) Cartoon representation of the major compounds of the chloroplast thylakoid membrane of Arabidopsis thaliana (Arabidopsis), together with the enzyme Rubisco, which is extrinsic to the membrane and which catalyzes the first step of the reductive pentose phosphate pathway of carbon dioxide assimilation. 18 Learning from the complex nature's photosynthesis process, it appears, in a simplified picture, that light-harvesting, photogeneration of charge carriers, and their lifetime combined with the surface photocatalyst reactivity are among the key challenges driving the efficient conversion of solar energy to chemical fuel. Improving the photocatalytic efficiency of a photocatalytic material requires: i) an increase of the lifetime of the photogenerated charges carriers, ii) broadening the spectrum of absorption response and enhancing the absorbance of photon ability (light harvesting properties) and

iii) an increase in the surface reactivity.

To provide an overview of the research performed in the field of hydrogen generation by the photocatalysis community and to identify the main topics explored and their evolution, a cooccurrence map for common keywords (extracted from the publications) was produced using "VOSviewer". The results are presented in Figure 3 as nodes connected to each other. Each node represents a keyword describing a subtopic, in which the size is proportional to the number of associated publications and the color indicating the average publication year for those articles. Lines connecting pairs of nodes indicate that at least one paper can be described by both of those keywords.

Other than the main keywords of "TiO2," "photocatalysis," and "hydrogen," which were ubiquitous among most of the works surveyed and as such disregarded, this map revealed that the most frequently investigated subtopics are "co-catalyst (ex. Au, Pt, Pd, Cu, Ag)," "surface plasmon resonance," "heterojunction," and "graphene." This demonstrates that the community is primarily focused upon studying the improvement of the charge carriers separation of TiO2 toward the photocatalytic H2 generation, rather than benchmarking reactors or setups to move forward producing H2 in industrial scale. In addition, several research directions, such as "graphene," "carbon nanotubes," "MXene,"

"surface defects," and "carbon quantum dots," have emerged, mainly to propose alternatives to noble metals. The first question to be handled in my research was how we can provide an efficient architecture that improves the light-harvesting of TiO2 in the nano-and mesoscale. The second question to be addressed was to explore the "strong metal-support interaction" concept as an effective way to improve photocatalytic efficiency. Finally, the objective was to propose a new architecture of TiO2 photocatalyst that can efficiently increase the charge carriers density and then increase the hydrogen generation.

Light harvesting capability of TiO2 at the nanoscale

IUPAC defines a photocatalyst as a "catalyst able to produce, upon absorption of light, chemical transformations of the reaction partners…", 21 which points out that in the dark no photon absorption is possible, and as a consequence no photocatalytic reaction would occurs. Regardless of the photocatalyst's intrinsic properties (optical bandgap, surface defects, etc.), it is evident that the initial step and driving force of photocatalysis is undoubtedly the light harvesting ability (absorption of photons). The speed of light in the medium ranges is around 1-3 × 10 8 m.s -1 , which implies about a hundred femtoseconds to pass through 10 μm thick sample (and even less for TiO2 nanoparticles).

Enhancing the light-harvesting of TiO2 photocatalyst (absorbance factor) is challenging, especially regarding the management of light propagation. Upon illumination of TiO2 photocatalyst, the incident light 𝐼 𝑖 usually splits into several parts as following:

𝐼 𝑖 = 𝐼 𝐴 + 𝐼 𝑇 + 𝐼 𝑅 (eq. 3)
The light is either absorbed (𝐼 𝐴 ), transmitted (𝐼 𝑇 ), or reflected (𝐼 𝑅 ). All the parts are interconnected, and the transmitted or reflected fractions of the light can be modulated in order to increase the absorbance of photons (light-harvesting) by the photocatalyst. The reflected and transmitted parts of the light are both brought by the material structure. Thus, designing photocatalytic material in an optical cavity could lead to an effective enhancement of light absorption through: i) optimal light management by limiting the energy loss under illumination; ii) increasing the residence time of the light within the photocatalyst and increasing its absorbance factor (fraction of light absorbed), and finally iii) expand the optical response of incident light (wavelength range).

Bioinspired photoactive nanolens

Light focusing has been quite a challenge in wide range of applications, such as focusing the light on the reactive surface. We saw above that one of the most intriguing and efficient ways to reach higher efficiency of light-harvesting is to mimic the lens-like epidermal cells nanostructures. The light scattering/reflection abilities are brought about by structural design, usually cylinders or spheres.

Adequate selection of the size and the refractive index of the spherical dielectric nanoparticles will generate a beaming effect (also called photonic nanojets), when illuminated in a plane wave. The light is focused on the output surface, and the nanospheres act as convective photonic nanolens. [START_REF] Chen | Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique References[END_REF][START_REF] Born | [END_REF] The typical approach is to fix a small index contrast between the dielectric nanoparticle and the surrounding medium, generally less than two (n/n0 < 2) 24,25 . However, in order to keep the particle in the nanoscale and optimize the beaming effect, a refractive index contrast is induced by designing a core-shell nanostructure. The final goal is to improve the light-harvesting (absorbance) of TiO2 and Au nanoparticles. 26

Synthesis and photocatalytic activity of plasmonic core-shell nanostructure

As to provide a proof of concept of the size effect size on the light-harvesting ability of photoactive material "TiO2" and thus on the hydrogen generation, core-shell nanostructure (SiO2@Au@TiO2) was constructed using different sizes of the silica core using Stobër method. 27 The size of the silica core was controlled by adjusting the TEOS : Ethanol : H2O : NH4OH proportion, and the solution was aged during 1 h at 40°C. The nanoparticles have their size in the range of 70-450 nm, as shown in Figure 4. Gold nanoparticles (Au NPs), prepared separately following the procedure reported in the ref. 26,28 , were embedded during the synthesis process between the silica core and the TiO2 shell. The photocatalytic performance of core-shell nanocomposites was investigated by following the H2 generation from a solution containing a mixture of methanol/water (1:3 v/v), under UV-visible and visible light illumination (λ > 420 nm) (Figure 5). In the absence of gold nanoparticles (SiO2@TiO2), very low amount of H2 was generated. The introduction of Au NPs into the core-shell nanostructure, even with small amount (0.25 wt%), improved drastically the amount of H2 generated. The kinetics of hydrogen production appeared to be sensitive to the size of the silica core. Indeed, a maximum photoactivity was obtained when the diameter of the silica core was 150 nm (14.53 mmol h -1 g -1 ), even though the photocatalytic reaction was performed using the same amount of photocatalyst and Au/TiO2 ratio. The photocatalytic activity is 5 times higher than the sample with a silica core of 70 nm diameter (2.9 mmol h -1 g -1 ). The kinetics of hydrogen production decreased slightly for larger silica core (k=10.6 mmol h -1 g -1 for dSiO2 = 300 nm), even if it remained relatively higher compared with the smallest size. The largest core-shell nanoparticles showed a drop in photoactivity (k=5.2 mmol h -1 g -1 for dSiO2 = 450 nm), and remain twice as high as the smallest silica diameter. The photocatalytic efficiency of hydrogen production followed systematically the evolution of the LSPR intensity of AuNPs. As the LSPR was enhanced, reaching the maximal absorbance intensity, the hydrogen production rate was the highest (150 nm-SiO2@Au@TiO2). When the LSPR decreased again, the hydrogen production rate decreased, even though it remained higher than that of the composite containing the smallest SiO2 nanoparticles. AuNPs LSPR is known to either assist the photocatalytic reaction through energy transfer 21,[START_REF] Chen | Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique References[END_REF] or mediate the sensitization of TiO2 in the visible range by injecting energetic electrons.

Effect of the core-shell size on the magnetic field -Lens effect or photonic nanojet

To rationalize the relationship between the size of the core-shell nanostructure, the light-harvesting capability and the photocatalytic activity, the field distribution inside the SiO2@Au@TiO2 nanostructure was numerically simulated using the software Comsol. The simulations were computed by Finite Element Method and were realized using the Scattered Field Formulation, i.e., the total electric field was decomposed into an excitation field (called the background field, formulated as a plane wave) and a scattered field, for which the Maxwell's equations are solved. Perfectly Matched Layers (PMLs), used as artificial layers, were considered to be absorbent, non-reflecting and meant preventing spurious reflections at the simulation domain boundaries. All the simulations were conducted inside water as the surrounding medium and at a wavelength of λ = 544 nm, corresponding to maximum absorption of the LSPR. The variation in the electric field distribution in the core-shell nanostructures was tracked for different diameters of the silica core (70, 150, 300, or 450 nm), maintaining the thickness of the TiO2 shell at 10 nm. As predicted by the Mie theory, isolated Au NPs were found to have negligible absorbing and scattering cross sections (mainly because of their small size). Thus, Au NPs barely affect the global field distribution of the SiO2@TiO2 core-shell nanostructure. The complex field distribution patterns are produced and depicted in Figure 6. The scattering effect is strongly dependent on the core diameter of silica and alternating variable refractive index for core-shell nanostructure. From the point of view of geometrical optics, the increase in the scattering field intensity with a more intense forward lobe indicated that the spherical nanoparticles behave as convex nanolens. Resonance manifests itself as two patterns of interferometric fringes in the intensity colormap map for the nanoscale range in Figure 6. These fringes are not observed for SiO2 nanoparticles. Usually, the interferometric fringes are visible at the microscale size range. [START_REF] Born | [END_REF] The use of two materials showing low (nSiO2=1.5) and high (nTiO2=2.5) refractive index intensify the scattering light, leading to interferometric fringes early in the nanoscale.

The scattering effect in the spherical core-shell nanostructures manifests itself as a "beaming effect" with an electromagnetic field enhancement at the exit interface of the spheres, even leading to a focus point at the surface. The core-shell nanostructures act as photonic nanolens (or photonic nanojet).

This leads to increased energy on the outer shell, as the incident beam power must be increased.

To confirm and estimate the influence of the beaming effect on the photocatalytic properties, the squared electric field amplitude was averaged over the inner TiO2 surface, which is proportional to the amount of energy absorbed by AuNPs (Figure 6.b). The results indicate an enhancement of 44.3%

of the energy absorbed in the AuNPs when the core-shell size increased from 70 nm to 150 nm.

Although a less important fact, an additional increase in absorption energy was observed for silica core particles with larger diameters (up to 450 nm). Hence, the aforementioned beaming effect was directly correlated to an enhancement of the electromagnetic energy density to excite the AuNPs and consequently boost the creation of hot electrons assisted by LSPR. The core-shell behaves as a photonic nanolens and presents a "beaming effect", which enhances the light-harvesting ability and the LSPR intensity of AuNPs (starting from 150 nm-SiO2) and then improves the photocatalytic H2 generation. 3. Artificial "Strong-metal support interaction" for higher surface reactivity, selectivity, and charge separation.

Core-shell nanostructures have shown great opportunity to address the light harvesting challenge at the nanoscale range. In this section, the charge carriers mobility and surface reactivity and selectivity toward H2 generation will be discussed. In general, metal nanoparticles, such as Pt and Au, are used to collect the photogenerated electrons either to improve the e -/h + lifetime or act as cocatalyst (reduction site). From a perspective of achieving optimal performance when TiO2 is coupled with Au NPs, two prime factors are of vital importance: i) the efficiency of electron transfer from TiO2 to metal nanoparticles active sites and ii) the available reactive surface. 29 For this purpose, mutual metal-support interactions between metal nanoparticles and TiO2 would offer significant influences on the electronic structure of metal nanoparticles, guarantee an efficient charge transfer, and finally improve the photocatalytic activities.

The concept of strong metal-support interaction (SMSI) was introduced in the 1970s by Tauster et al. 30 The principle of the SMSI method was to induce encapsulation of the metal nanoparticles by metal oxide migration during its thermal reduction (using high-temperature H2). The overlayer obtained by the thermal process was not stable during the catalytic reaction. A re-oxidation or receded SMSI overlayer structure occurs, which induces the inactivation of the catalyst. [31][32][33] A revisited SMSI using HCOx adsorbate has been proposed. 34 The adsorbate mediates the metal oxide overlayer formation at the surface of Rh NPs claiming higher stability during CO2 reduction. In most cases, the SMSI was restricted to metals from groups VI, VII, and VIII. 30,35,36 The surface energy and failure to dissociate H2 molecules necessary for the metal oxide migration, Au NPs were believed to be excluded from forming an SMSI. 37,38 As an alternative, wet-chemistry SMSI was proposed for depositing substoichiometric TiOx at the surface of Au NPs for CO oxidation. 36 The development and studies reporting the SMSI concept were restricted to heterogeneous catalysis. The use of the SMSI concept in photocatalytic H2 generation has not yet been considered for AuNPs. While this method is suitable for catalytic reactions, it seems less applicable in photocatalysis. Indeed, amorphous TiO2

or non-stoichiometric titanium oxide is often obtained under reductive conditions.

To overcome these limitations and extend the SMSI concept to photocatalytic H2 generation, an elegant soft-chemistry method was proposed. Thus, the optimal configuration of the plasmonic photocatalyst and the mechanism occurring during the photocatalytic production of H2 is explored.

The idea was to engineer a plasmonic core-shell based photocatalyst offering two different configurations. The first one is representative of a classic photocatalyst, such as SiO2@TiO2@Au, where the cocatalyst (Au NPs) is deposited at the surface. The second, more original, offers covering Au NPs by a thin TiO2 over layer as an artificial construction of a "strong metal-support interaction"

(SMSI) concept. Thus, the photoactivity, the charge carriers transfer, and the selectivity toward H2 generation were investigated for both systems.

Synthesis of plasmonic core-shell with a reverse configuration

The soft-chemistry process offers the possibility to tune the core-shell system at will. The SMSI concept was achieved by adapting the synthesis process from ref. 26,28 . Sketch of the synthesis process is presented in In comparison with the classical system "SiO2@TiO2@Au", where Au NPs are deposited at the surface of the TiO2 shell (Figure 8a), HR-TEM investigation demonstrated the success of a softchemistry in constructing artificial SMSI through the formation of a thin TiO2 over layer covering the Au NPs (Figure 8b). The HR-TEM images indicate the growth of TiO2 shell in anatase crystalline phase, in agreement with the d-spacing (d=0.351 nm) of lattice fringes. 28 The thickness of the shell is estimated in the range of 4-10 nm, which enables covering Au NPs with an oxide overlayer of 2-3 nm (square of Figure 8b). The results contrast with the previous studies claiming that no SMSI could be constructed with Au NPs, and further demonstrate the success construction of SMSI in other transition metals. The APTMS functionalized Au NPs mediates the growth of TiO2 thin shell and the SMSI is readily formed.

Figure 8 : HRTEM images for SiO2@Au@TiO2 and SiO2@TiO2@Au catalysts

Photocatalytic H2 evolution and charge carriers dynamics

The photocatalytic efficiency of the plasmonic core-shell was investigated for hydrogen evolution from 1:3 v/v methanol/water solution, under UV-visible light illumination and shown in Figure 9. A gold-free SiO2@TiO2 core-shell exhibited very low activity for H2 production. In agreement with the reported studies, [39][40][41][42] coupling Au NPs to the core-shell system improves significantly the amount of H2 molecules generated over illumination time.

However, both systems do not show the same photocatalytic efficiency, even though the coreshell are composed of the same compounds, namely, silica, TiO2, and Au NPs. Indeed, covering Au NPs by a thin TiO2 overlayer remarkably boosts the H2 production. The amount of H2 generated was 10 times higher for 1 wt% of SMSI SiO2@Au@TiO2 (Figure 9b), compared to the best performance for the classical system (SiO2@TiO2@Au) (Figure 9a).

The photocatalytic efficiency was compared to 1wt%-Pt/TiO2P25, and the photoactivity was comparable to SMSI SiO2@Au@TiO2 (1 wt% of Au NPs) photocatalyst. It has been reported

that LSPR promotes photocatalytic H2 production. In general, radiative (including near-fields and scattering mechanism) and non-radiative phenomena were suggested to describe the photocatalytic mechanism. 43 The first mechanism involves the interaction of the LSPR of Au NPs with incident light resulting in local enhancement of electromagnetic near -fields at the immediate vicinity of the TiO2 surface. The radiative energy transfer mechanism in the nanocomposite system reaches its maximum when an overlap of the nanoparticle's LSPR band and the semiconductor electronic bandgap is obtained. 44 Therefore, the radiative effect should barely have an impact since the LSPR band (visible) is spectrally misaligned with the TiO2 absorption (UV). Additionally, Au NPs are too small to consider any light scattering of the electric field enhancement. 45 Considering the core-shell configuration of our photocatalysts, mainly the so-called "nonradiative" effect could mostly explain the enhancement of the photocatalytic performance. In the context of hydrogen production, the kinetics of the hydrogen ion reduction at the surface of TiO2 is very low due to its large bandgap, the potential of conduction band and fast recombination charge carriers as well. However, when combined with AuNPs, electrons are collected from TiO 2 to reduce hydrogen ions (under UV light excitation), or are injected to TiO2 (under visible light excitation) raising the rate of H2 generation. Although the nanocomposite core-shell system exhibits improved performance compared to SiO2@TiO2 photocatalyst, it is obvious that the SMSI of Au/TiO2 in the core-shell nanostructure leads to much higher photoefficiency. The charge transfer on both nanocomposite systems is considered, to define how the thin TiO2 overlayer could improve the photocatalytic activity.

As it has been reported, the charge transfer of gold coupled to the TiO2 junction is quite complex. Accordingly, the charge carrier dynamics as well as their lifetime were evaluated using time resolved microwave conductivity (TRMC).

The TRMC signal mainly arises from photogenerated charge carriers produced or transferred at the surface of TiO2 semiconductor. More importantly, the signal is attributed to the electrons density, assuming that the mobility of holes is limited and they are basically trapped in the bulk at the early time range. 46,47 Therefore, the signal is correlated to the electron density at the surface of TiO2 shell after the laser excitation in the early stage (eq. 3), and the decays of free electron number is caused by recombination, trapping and capture by Au NPs 4,27.

∆𝑃(𝑡) 𝑃 = 𝐴 × ∆𝜎(𝑡) = 𝐴𝑒 ∑ ∆𝑛 𝑖 (𝑡) × 𝜇 𝑖 (3)
Figure 9c-d exhibits the TRMC signal obtained from both systems at different AuNPs loading, after 350 nm laser pulses excitation (UV illumination). The signal to noise was about 3 to 1, which is regarding the thickness of the TiO2 overlayer is reasonable. Gold-free SiO 2 @TiO 2 sample exhibit low signal after UV light excitation, which is due to the thin shell layer at the surface (4-10 nm) as shown in Figure 9b. Given the signal decay, it seems obvious that the photogenerated charge carriers density relies on the system. Indeed, after UV light excitation the photogenerated charge carrier density undoubtedly increases or at least remains unchanged for SMSI SiO2@Au@TiO2 compared to the gold-free sample. In the case of the classical system, the signal decreases dramatically. At an early excitation stage, it is suspected that, in SMSI core-shell system, AuNPs collect electrons and possibly inject them back into TiO2, which could translate the increase of the TRMC signal. This complex pathway could rationalize why the quenching of the photogenerated charge carriers is not observed for the SMSI SiO2@Au@TiO2 system. The transfer of hot electrons from AuNPs is a well-known process compared to the opposite trajectory. Electrons obtained under UV illumination by energy higher than the bandgap energy of TiO2 are indeed able to surround the Schottky barrier (SB) and be collected by AuNPs. 29,48 Additionally, hot electrons are injected from the AuNPs to TiO2 under visible light irradiation surrounding the Schottky barrier and flow back into the TiO2. 49 In the classical SiO2@TiO2@Au system, the dramatic decrease of the TRMC signal (low charge carrier density) could be due to: 1) shield effect caused by AuNPs, 2)

surface recombination centers created by the synthesis method, and 3) fast electron scavenge by the metal (<10 ns). 42,50 As discussed above, isolated Au NPs have negligible scattering cross sections (mainly because of their small size), and we can assume that an identical TiO2 surface is obtained for both systems. Therefore, the first hypothesis can be ruled out. The electrons are scavenged by Au NPs and drastically reduce the charge carrier density at the surface of TiO2 acting as a reduction site. To further investigate the adsorption of MeOH over both systems, DFT calculations were performed. This was performed by simulating the adsorption of single methanol molecule on the surface of classical SiO2@TiO2@Au and SMSI SiO2@Au@TiO2. The most stable complex and adsorption energies were found after considering different initial adsorption sites, which the adsorption scenarios were geometrically-optimized at fixed parameters. The methanol adsorbs on the five-coordinated titanium (Ti5c) for both core-shell system. It occurs, as expected, at the periphery of gold cluster interface for SiO2@TiO2@Au with higher adsorption energy (-1.0788 eV) (Figure 11a), compared to SMSI SiO2@Au@TiO2 model (-0.9479 eV) (Figure 11b). The adsorption distance in both systems results is difference by only 0.012 Å, which is not enough to cause the relatively large difference in adsorption energies of 0.13 eV. With stronger adsorption found in SiO2@TiO2@Au, it is speculated that the oxidation follow more likely indirect mechanism via interfacial • OH radicals (eq. 4) that are products of trapping valence holes by surface OH groups or adsorbed water. 51 In this process, • OH radicals lead to the formation of the formation of H2O2, which the dissociation produces O2 (eq. 5 and 6). Then, the oxidation of CH 2 OH • lead to the formaldehyde formation (eq. 7). This later reacts with water and forms formic acid (eq. 8).

CH 3 OH + OH → CH 2 OH + H 2 O • • (4) 2 OH → H 2 O 2 • (5) H 2 O 2 → O 2 + 2H + + 2e - (6)
CH 2 OH • + O 2 → HCOH + HO 2 • ( 7
)
HCOH + H 2 O → HCOOH + H 2 (8)
On the contrary, weaker adsorption of MeOH lead to a direct oxidation via hole scavenging.

The photoinduced charges (e -/h + ) initiate the dissociative adsorption of methanol (eqn. 9 and 10) and hole scavenging. This step is affected by the adsorption/desorption energy of MeOH, which is different for both systems, and thus affects the kinetics "k 1 " of the reaction 1. Another hole scavenging lead to formaldehyde and 𝐻 • (𝑎𝑑𝑠) formation (eq. 11), probably coupled to an injection of an electron at the surface of the photocatalyst. 28 The final step is the formation of H2 via coupling reaction (eq. 12).

CH

2𝐻 • (𝑎𝑑𝑠,𝑇𝑖𝑂 2 ) → H 2(aq) (11)
The reduction of hydrogen ions occurs at the TiO 2 surface in the SMSI SiO 2 @Au@TiO 2 system, while Au could be the reduction site in the case of SiO2@TiO2@Au. It could be concluded from the results that the reduction reaction is more efficient when taking place at the TiO2 surface (Figure 7.d).

These results raise many questions and perspectives regarding the design and the architecture of what could be an ideal photocatalyst. It is undeniable that the core-shell nanostructure behaving as a convex nanolens improves the light-harvesting capability, and covering Au NPs by a thin TiO2 overlayer boosts the kinetics of hydrogen reaction. To provide a deep understanding of the differences made an ANR project is running.

Novel architecture for light harvesting and H2 generation

The confinement of the light in a periodically structured optical medium (due to multiple Bragg scattering) enables to control of the light propagation and, in particular, the frequency-wavevector dispersion relation, i.e. the photonic band structure. The pioneering work of Yablonovich 52 and John [START_REF] John | PHYSICAL REVIEW LETTERS Strong Localization of Photons in Certain Disordered Dielectric Superlattices[END_REF] proposed the basics of the extraordinary properties of photonic crystals (PCs). Photonic crystals in nature, e.g. butterfly wings, weevil or beetle cuticles, present an amazing and complex structures that manage the propagation of light in such a perfect way that they produce iridescent colors. [START_REF] Vigneron | [END_REF][55][56] PCs can be produced artificially either by alternating high and low refractive index materials [57][58][59][60][61][62] or by self-assembling spherical nanoparticles. [63][64][65][66] PCs designed from 1D, 2D and 3D periodical arrangement of materials interact with electromagnetic radiation and inhibit the propagation of light in well-defined wavelength (energy) ranges and directions (photonic band gap). Many developments in waveguides, 60,62,[START_REF] Mekis | High Transmission through Sharp Bends in Photonic Crystal Waveguides[END_REF][START_REF] Ghazzal | [END_REF][69][70][71][72][73] dielectric mirrors, sensors, 58,60,62,[START_REF] Ghazzal | [END_REF][69][70][71][72][73][74][75] low threshold lasers 76 derive directly from the photonic bandgap property. However, the propagation of light at the bandgap edges has attracted attention. At the lower photon energy edge (red edge) of the photonic band gap, the light standing-wave intensity peaks are primarily localized in the high refractive index dielectric regions of the PC, whereas at the higher photon energy edge (blue edge), they are primarily localized in the low refractive index dielectric regions. The group velocity of light (𝑉 ⃗ 𝑔 = 𝜕𝜔 𝜕𝑘 ⃗ ) is defined as the "local derivative" or "slope" of the photonic band curve in the frequency  versus wave-vector 𝑘 ⃗ diagram.

Near the photonic band edges, this derivative tends to zero, i.e. the photonic band curve flattens. In other word, near the red-edge (or blue-edge) wavelength of the photonic bandgap, the light velocity is slowed down, justifying the term "Slow Photons". 77 The residence time of photons in the PCs increases considerably, which could improve the absorbance factor of the material. The beneficial effect of slowing down photons was first revealed in solar cells. Nishimura et al. achieved an improvement of 26 % of the photogenerated current using TiO2 layer periodically structured in 3D photonic lattice. 78,79 The authors attributed such an enhancement to the improvement of the light harvesting due to the low group velocity of light near the red-edge of the "stop-band" (another name for photonic band gap). Tétreault and co-workers 80,81 reported an improvement of the photon-toelectron conversion of dye sensitized solar cells (based on mesoporous TiO2) when coupled to PCs structure. They attributed this result to multidirectional scattering of photons, back reflection of light (PC acts as a dielectric mirror) and absorption enhancement caused by resonant modes. N. Tétreault and M. Gratzël highlighted in their review the use of PCs as an efficient strategy to increase the photon-to-electron conversion of the next generation of dye-sensitized solar cells. 80 Ozin's group used the concept of "slow Photon" to increase the photoefficiency TiO2 PCs structure. 65,82 They reported that the enhancement of the photocatalytic efficiency was closely related to the position of the Bragg reflection peak wavelength with respect to the electronic band edge of TiO2. Among others, Bao-Lian Su's group investigated further the slow photon effect experimentally. [83][84][85] Theoretical study by Deparis et al. highlighted peculiar properties of the slow photons at the blue edge. 86 Despite the experimentally proven effect of the PC structure on the light harvesting in solar cells and in comparison with the huge number of publications dealing with PCs, only a few studies were dedicated to the exploitation of the photonic properties of TiO2 PC alone 87 or coupled to metallic nanoparticles 88,89 in the context of photocatalytic applications (environmental or energy: hydrogen production). However, these studies have high number of citations by specialized research groups (Figure 12). The synergetic effect of coupling TiO2 PCs structure with the plasmonic properties of NPs has shown promising results. Researchers reported, for instance, a very high photocurrent density production (about 150 μA cm -2 ) under visible light for Au/TiO2(nanotube) 90 , a photo-conversion efficiency of 0.71% for Au/TiO2(inverse opal) which is one of the highest values in photoelectrochemical cells 89 and amplification of the photochemistry of Pt/TiO2 (inverse opal). 65 PCs are now accepted as an effective strategy to increase the photon-to-electron conversion for next-generation dye-sensitized solar cells. 91 But so far, most of these studies have been carried out using synthetic TiO2 opal and inverse opal templates, and no innovative and sustainable strategy has been proposed, making the concept reaching a bottleneck.

In this context, cellulose nanocrystals (CNCs), biosourced material, have the ability to self-assemble into chiral nematic (CN) structure, which can be replicated into inorganic materials. Structuring inorganic materials on the mesoscale using a self-biotemplate assembly process appears as a sustainable method for the synthesis of functional materials. For that purpose, different methods were developed. The first one is based on the impregnation of free-standing cellulose photonic films, the second is a one-pot self-biotemplating sol-gel approach and the third one aim to transfer this strategy to spherical nanospheres.

Elaboration of the nanostructured materials

The transfer of CN structure into inorganic oxide was reported the first time by Dujardin and coworkers. 92 Since then, cellulose nanocrystals (CNCs) have emerged as straightforward biotemplates to produce mesosoporous metal-oxide with chiral nematic structures. The most noticeable discovery is the freestanding silica films obtained by the polycondensation of tetraethyl orthosilane or methyl triethoxisilane precursors dissolved in the CNC aqueous solution. 93 This breakthrough has led to the development of promising functional materials and the design of novel photonic crystals architecture. However, the method was restricted to silica or organosilica oxides, which limits the application to sensing devices and separation of chiral molecules. 94,95 The high sensitivity of precursors (metal chlorides or alkoxides) to humidity, leading to their rapid condensation, destabilizes the chiral nematic phase into isotropic. To overcome this limitation, water stable Ti(acetylacetonate)(acOH) and Titanium diisopropoxyde bis(acetylacetonate) were used to obtain variable morphologies of TiO2 photocatalyst. The solubility and the stability of the titania precursors offer flexibility to use different methods. The impregnation, one-pot sol-gel, and microemulsion were used successfully to structure TiO2 using cellulose nanocrystals as biotemplate (Figure 14).

The impregnation method uses cellulose nanocrystal photonic films which the structure was successfully transferred into inorganic TiO2 films by an approach combining soft chemistry and an evaporation-induced self-assembly (EISA) method. The CNCs photonic films were impregnated by an ethanoic solution containing the precursor of TiO2, followed by drying and calcination steps. 96 The final films exhibited a fingerprint of the chiral nematic structure also observed for the CNCs photonic films. TEM micrographs reveal cylindrical-like pores aligned and separated by TiO2 wall, suggesting a localized helical structure. The helical organization is a periodic succession of parallel pores over distances of a few CNC rod diameters, which orient perpendicularly with respect to the electron beam.

The one-pot biotemplating sol-gel approach enables the synthesis of CNC/TiO2 and CNC/mixed oxide hybrid films. 97 A stable mixture of titanium diisopropoxide bis(acetylacetonate) (TAA) and an aqueous solution of CNCs was used for the elaboration of the films. After removing the CNC biotemplate, the final TiO2 films exhibit a lamellar structure.

Similar formulation was used to obtain spherical nanoparticle with chiral nematic structure. 98 The CNCs were encapsulated in spherical titania-sol microdroplets using an inverse microemulsion method. The CNC self-assembly process was followed from seconds to hours and the key steps that drive the isotropic to Ch nematic phase transition in such aqueous sol were revealed. 98 The CNCs undergo nucleation, coalescence, and migration of tactoids through two coalescence mechanisms, namely, "head-to-tail" and "side-to-side" types.

Evaluation of the photocatalytic performance and the charge carriers dynamics

The photocatalytic activity of these nanostructured materials was evaluated for H2 generation for all samples; impregnated, one-pot and nanospheres. To demonstrate the effect of the chiral nematic structure, time resolved microwave conductivity was used to assess the charge density produced after laser excitation at 350 nm (Figure 14a). The results show an intense TRMC signal (Imax) for the films obtained from the mineralization of CNCs photonic films compared to mesoporous TiO2. This results demonstrate the effect of the chiral nematic structure of TiO2. Au NPs were grown in the TiO2 film (CNTiO2) obtained by impregnation, and the optical and photocatalytic properties were compared to as reference TiO2-P25 (153.1 mmol h -1 g -1 ) (Figure 15a). This result indicates that structuring the TiO2 material in a periodic long-range lamellar structure can considerably improve its photoefficiency for H2 generation without using a cocatalyst. The long-range layered structure improves the light scattering, which provides the TiO2 the capability to improve the photon-to-charge carrier production.

This was confirmed by TRMC measurements. The investigation of the TRMC signals' decay, which typically occurs after the e -/h + production during the 8 ns laser pulse, gives useful information about the photocharge lifetime (Figure 15b).

Perspective and conclusion

The light-harvesting capability of a material is one of the key parameters to convert efficiently solar Also, the reaction pathway of H2 generation is different regarding the system.

A biotemplating route has been propose to elaborate variable morphologies of structured TiO2. The sol-gel chemistry combined to cellulose nanocrystals enable to obtain films and spheres containing defined structure. Thus, the films containing chiral nematic structure or organized in lamellar structure were obtained and exhibit improved charge carrier density and photocatalytic activity. The spheres were obtained by combining microemulsion and hydrothermal method. The light scattering induced by the presence of chiral nematic structure is responsible to enhance the light harvesting and thus the photocatalytic H2 generation.

The next step of my research will be focused on investigating the use of Graphdiyne as cocatalyst or to modify it to construct hybrid material for optimal charge carrier separation and hydrogen generation. The preliminary results lead to:

-the development of a bottom-up strategy to obtain nanocomposite (H-graphdiyne/TiO2) based photocatalyst (noble-free metal photocatalyst) for hydrogen generation under visible light excitation (Advanced Functional Materials, 2021, 31 (29), 2100994).

-Simple one-pot synthesis method of graphdiyne/CuO as effective cocatalyst for H2 generation (Angew. Chem.Int. Ed.2022,e202210242).

Grant as coordinator and/or principal investigator 1 . 6 . 7 . 8 . 1 .

 16781 DIM RESPORE, coordinator Title: Hybrid porous nanocomposite for visible light photocatalytic CO2 reduction Period: 2021-2022 Budget: 60k€ Academic partners : ICMMO, LPS 2. SP3, Laboratoire d'excellence NanoSaclay-PALM, co-coordinateur Title: Selective Photocatalysis by Polarized Porous nanoreactors Period: 2021-2023 Budget: 63k€ for the ICP (total 130 k€) Academic partner : LPS 3. Project INGENCAT (ANR, Coordinateur Franc Dumeignil), responsable pour l'ICP Thème: Les nouveaux catalyseurs hétérogènes Period: 2020-2024 Budget: 169k€ for the ICP (570K€ pour le consortium) Academic partners: UCCS, IRCELYON, LC ENS Lyon 4. Z-scheme : Laboratoire d'excellence NanoSaclay, Coordinateur Title: photocatalyst based p-type SiNWs/Semiconductor heterojunction for Hydrogen production. Period: 2018-2019 Budget : 69k€ Academic partner: LPICM 5. ISOGLAC « First Spin-Off » funded by the « Région Wallonne », Belgium, Coordinateur Title: Isolant thermique biosourcée et superhydrophobe. Period : 2015-2017 Budget : 300 k€ Funding obtained by the department of chemistry of the Université Paris-Saclay following the call for proposals " Nouveaux entrants 2018 " for the purchase of a solar simulator to develop a photo-electrochemical cell. Budget: 10 k€ Master 2 de 6 months, DIM RESPORE. Title: Study of the photocatalytic properties of permanently polarized nanotubes Budget : 3700 € Period: 6 months (Septembre 2018-Février 2019) Academic partner: LPS Master 2 de 6 months, IRS-MOMENTOM. Title: Photoactive Composite Nanomaterials for Environment and Energy, Budget: 4500 € Period: 6 months (Février 2018 -Juillet 2019) Edition responsabilities Co-editor of the chapter "Experimental techniques: from steady-state to ultrafast methods" for the Springer Handbook of Inorganic Photochemistry. The publication of the scientific book is planned for January 2022.

Figure 1 :

 1 Figure 1 : Smart coating with bio-inspired hygrochromic optical properties. 1

Figure 2 :

 2 Figure 2 : (a)The schematic of the internal structure of leaves. The leaf is perfectly structured to reach optimal management of the incident radiation through reflection, transmission, or absorption. The external cuticle upper epidermis is a thick layer of parenchyma cells composed of high wax, and/or a large number of leaf hairs that significantly reflect the light and regulate the flux penetration into the leaf. The upper epidermis with an ellipsoidal-like structure behaves as a lens focusing the light into the mesophyll layer arranged in columns, forming the palisade mesophyll. The columnar structure acts as a channel transport layer of light, which improves the degree of light scattering (higher absorption in some wavelengths) and reduces energy loss. 20 (b) Cartoon representation of the major compounds of the chloroplast thylakoid membrane of Arabidopsis thaliana (Arabidopsis), together with the enzyme Rubisco, which is extrinsic to the membrane and which catalyzes the first step of the reductive pentose phosphate pathway of carbon dioxide assimilation.18

Figure 3 :

 3 Figure 3: Overview of the research performed in the field of hydrogen generation by the photocatalysis community.

Figure 4 :

 4 Figure 4 : TEM images of plasmonic core-shell nanostructure with variable diameter of the silica core. (a) 70 nm (insert HR-TEM image showing Au (111) and anatase form of TiO2 shell), (b) 150 nm, (c) 300 nm and d) 450 nm.

12

 12

Figure 5 :

 5 Figure 5 : (a) UV-vis absorbance spectra of plasmonic core-shell nanostructure with different sizes. Hydrogen production under (b) UV-visible and (c) visible light illumination.

Figure 6 :

 6 Figure 6 : (a) Near-field distribution of the square of electric field intensity and the correspondent TEM images of variable size of silica with and without TiO2 shell. (b) Size dependence of the squared electric field amplitude averaged over the inner surface of TiO2 where the AuNPs are located.

Figure 7 .

 7 Two systems were obtained by strict control of the deposition of Au NPs on the silica core before the TiO2 shell's coating (SMSI) or on the TiO2 shell, as a representative of classical system. Different ratios of Au/TiO2 were obtained. The ratio of Au NPs was in the range of 0.25-1.5 wt%, as confirmed by ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) analysis. The ratio of Au NPs match well with the synthesis process and they were homogenously distributed all over the spheres (Figure7b-c). The TEM images revealed a narrower size distribution of Au NPs with an average size ranging from 4.5 to 7 nm.28

Figure 7 :

 7 Figure 7 : (a) scheme of the synthesis process of classical and SMSI core-shell nanostructure. TEM images of samples with variable amount of Au NPs, (b) 0.25 wt%, (c) 0.5 wt%, (d) 1 wt% and € 1.5 wt%.

Figure 9 :Figure 10 :

 910 Figure 9: Photocatalytic hydrogen evolution using classical (a) and (b) SMSI core-shell nanostructure with different amount of AuNPs. TRMC signal obtained under laser excitation for (c) classical and (d) SMSI system.

Figure 11 :

 11 Figure 11 : DFT calculation of the adsorption energies in (a) classical and (b) SMSI systems.

Figure 12 :

 12 Figure 12: Publications records and citations from 2000 to 2022 dedicated to TiO2 photonic crystal for photocatalysis.

  The solubility and self-organization of CNCs in titania-sol enable a successful transfer of the chiral (Ch) nematic structure into hybrid TiOx/C microspheres. Parallel stripe patterns on the surface of microspheres were observed, suggesting a potential transfer of Ch nematic structure to TiOx/C microspheres. The SEM images showed Ch nematic structures with periodic lamellar bands, which were captured into the TiOx/C microsphere wall (Figures 14).This further demonstrated the successful replication of the Ch nematic structure from TAA/CNC microdroplets to hybrid TiOx/C microspheres.

Figure 13 :

 13 Figure 13 : Different strategies used to obtain structure TiO2 photocatalyst using cellulose nanocrystals as a biotemplate.

  The decay of the TRMC signals of photocharges can be divided into three decay stages: charge trapping (𝜏 𝑡𝑟𝑎𝑝 ), charge recombination (𝜏 𝑟𝑒𝑐 ) and surface reactions (𝜏 𝑠𝑢𝑟𝑓 ). The time constant of electron trapping and recombination can be quantitatively estimated from the signal decay using the following equation: 99∆𝑃(𝑡)/𝑃(𝑡) = ∑𝐴 𝑛 • 𝑒𝑥𝑝(-𝑡/𝜏 𝑛 )where 𝐴 𝑛 and 𝜏 𝑛 are proportionality and time constants of each of the three decay processes, respectively. The photocharge lifetime presents in the inset of Figure15bwas found to be correlated with the wall thickness in the lamellar structure. The low charge carrier number produced at the surface of the L-TiO2-1 sample has the longest lifetime, which could be translated by low trapping and recombination yield. This trend evidences that L-TiO2-1 synthesized at a lower TAA/CNC ratio shows weak electron trapping capability, thereby enhancing the proportion of active photocharges for H2 generation. The interfacial charge recombination is likely to occur in the boundaries (interfaces) of the long-range layers with thicker walls where defects could be localized.

Figure 15 :

 15 Figure 15 : (a) Photocatalytic hydrogen production under UV-vis light excitation for variable TiO2 films obtained by one-pot sol-gel biotemplated method. (b) Charge carriers dynamic.

Figure 16 :

 16 Figure 16 : (a) SEM image of TiOx/C microsphere with wrinkles on the surface and a hole in the center. (b-d) Cross-section SEM images of microspheres showing Ch nematic structures with periodic lamellar bands. (e) XPS spectrum of Ti 2p. (f) WAXS patterns of TiOx/C microspheres. (g) UV-vis spectra of TiO2-P25 and TiOx/C microsphere. (h) Hydrogen generation constant rate of TiOx/C microspheres and commercial TiO2-P25.

  light to chemical energy. Designing and structuring TiO2 in photonic cavities offers an original strategy to improve its light harvesting capability. Core-shell nanostructure constructed of alternated low (SiO2) and high (TiO2) refractive index lead to a beaming effect in the nanoscale. The improvement of photocatalytic hydrogen production was related to the beaming effect which enhance the adsorption energy at the surface of the photocatalyst. In addition, covering Au NPs with a thin TiO2 overlayer demonstrated new pathways of charge carrier transfer and hydrogen generation reaction. It is believed that the charge produced under light illumination at the surface of TiO2 follow different trends depending on the localization of AuNPs. The classical system (where AuNPs is deposited at the surface), the electrons are collected by AuNPs, helping to the charge separation and acting as reduction site. The SMSI system, provide new concept where the charge carrier transfer is different and also the active surface where the hydrogen ion is reduced. The coverage of AuNPs by a thin TiO2 overlayer extends the charge carrier lifetime and boost the photocatalytic production of H2.

Prof. Bor Kae Chang,

  peer-reviewed journals (J. Mater. Chem. C, Appl. Catal. B: Environmental, Nanoscale, RSC advance, Catalysis Today, Materials Research Bulletin, Surface and Coatings Technology, Thin solid films, Scientific Report, International Journal of Hydrogen Energy, ACS applied material Interfaces, ACS applied Nano Materials, Material Today Chemistry, etc.). Department of Chemical and Materials Engineering National Central University, Taiwan. 3

4. Expert for the ANR's APP and the Mission for Transversal and Interdisciplinary Initiatives (MITI) program. Scientific collaborations International 1. Prof. Jérôme Cornil, Laboratory for Chemistry of Novel Materials, Center for Research in Molecular Electronics and Photonics, University of Mons, Belgium 2.

. Prof. Damien DeBecker, Institute

  of Condensed Matter and Nanosciences (IMCN), UCLouvain, Belgium.

4. Prof. Li-Zhu Wu, Key

  Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing, China. 5. Dr. Abdelghani Laachachi, Department of Materials Research and Technology (MRT), Luxembourg Institute of Science and Technology (LIST), Luxembourg. 6. Dr. Youssef Habibi, Department of Materials Research and Technology (MRT), Luxembourg Institute of Science and Technology (LIST), Luxembourg.

Nationales 1. Dr. Erwan Paineau, Université Paris-Saclay, Laboratoire de Physique du Solide, UMR 8502 CNRS, Orsay, France 2. Ass. Prof. Guillaume Bachelier, Univ. Grenoble Alpes, CNRS, Institut Néel, Grenoble, France 3. Dr. Robert Wojcieszak, Univ. Lille, Unité de Catalyse et Chimie du Solide, Lille, France

Doctoral and scientific supervision (since 2016) Tableau 2 :

  Activités d'encadrement d'étudiants en masters, doctorat et stages post-doctorale

	N° Période	First/Last name	type	Research theme
	1	2021-2024	Vien-Quach	Ph.D (3 years)	Core-shell nanostructure for photocatalytic
			Duong		biomass valorization
	2	2019-2022 Marija Knezevic	Ph.D (3 years)	Nanomatériaux Composites Photoactifs pour
					l'Environnement et l'Energie
	3	2019-2023	Qian Xu	Ph.D (4 years)	Photoactive nanostructured material for
					energy and environmental
	4				Photoactive Composite Nanomaterials based
		2018-2019 Marija Knezevic	Master 2	on perovskite quantum dots for Environment
					and Energy
		2018-2022	Cong Wang	Ph.D (4 years)	Design of nanostructured photocatalyst with a
	5				photonic structure for energy
	6	2018-2019	Jian Li	Post-doc	Z-scheme photocatalyst based p-type
				(15 Months)	SiNWs/Semiconductor heterojunction for
					Hydrogen production
			Anna	Master 2	Study of the photocatalytic properties of
	7	2018-2019	Sobolewska	(6 Months)	permanently polarized imogolite nanotubes
				Master 2	Biobased material with photonic structure and
	8	2017-2018	Chunyu Li	(6 Months)	plasmonic effect for hydrogen production
		2017-2018	Zhenpeng Cui	Post-doc	Core-shell nanostructure for visible light
	9			(12 Months)	photocatalytic activation and H2 generation
					Etudes de transfert de charge dans les
	10 2016-2017	Asma	Master 1	systèmes	hybrides
			Merkhoufa	(3 Months)	nanoparticule/chromophore	en	vue
					d'applications en conversion énergétique
			Getaneh-diress	Master 2	Design of photocatalyst with a photonic
	11 2016-2017	Gesesse	(6 Months)	structure for energy and depollution
					applications

Table 3 :

 3 Volume of teaching since my recruitment in September 2016. CM, TP and TD correspond respectively to "lectures", "practical work" and "supervised work".

	Period	Title

PhD

Member « Referee » of the PhD's jury of Dr. Anouchka MAPEMBE KIMENE, « Decarboxylation catalytique de l'acide palmitique en N-pentadecane », defended on the 04 th december 2017. The jury was composed of Prof. Sébastien ROYER (President), Dr. HDR Catherine ESPECEL (Reviewer), Prof. Sophie HERMANS (Reviewer), Ing. Claire DURLIN (Invited member), Ing. Olivier SIMON, (Invited member), Prof. Sébastien PAUL (Director), Dr. Robert WOJCIESZAK (co-director).

for the recruitment of « maître de conférence »

  Appointed member of the selection committee of the CNAM for the recruitment synchronized session 2019 of a lecturer at the Cnam " Physical chemistry, materials and surfaces ". The selection committee was chaired by Mrs. Rachida BENHADDOU-ZERROUKI (PU University of Limoges, President) and Mrs. Isabelle LERAY (DR ENS Cachan, vice-president), and was composed of Dr. Najla FOURATI-ENNOURI, Dr. Fanny HAUQUIER, Prof. Yann MEAR, Prof. Samy REMITA, Dr.

	Caroline CANNIZZO, Prof. Pascal GRIESMAR, Prof. Rachel MEALLET-RENAULT, and Dr.
	Nouredin YAAKOUBI

Jury for recruiting Engineer and Technical Staff for Research and Training (I.T.R.F.)

Implementation and animation of a workshop " depollution by photocatalysis " for the general public. The photocatalysis workshop aims to expose an innovative, economical and ecological process called "photocatalysis". The principle is based on a sensitized photoreaction based on the absorption of a light radiation by the photocatalyst, generally a semiconductor, which allows the treatment of water contaminated by industrial activities, agriculture and household waste.

that of mesoporous TiO2 (meso-TiO2), with and without Au NPs. Au NPs extend the absorbance of TiO2 photocatalyst in the visible range and induce an activation using low energy. 26,28 Au NPs exhibit localized surface plasmon resonance (LSPR), which the intensity is enhanced due to multiple light scattering in the CNTiO2 structure (Figure 14b). The H2 production is significantly improved from 0.81 to 5.52 mmol g -1 after 5 h with a kinetic rate of 0.17 and 1.35 mmol h -1 g -1 , when Au is loaded in meso-TiO2 (Figure 14c andd). The hydrogen production using Au-loaded CNTiO2 is 8 times higher compared to Au-loaded meso-TiO2 composites after 5 h. The films obtained by one-pot sol-gel biotemplating method also demonstrated an improved photocatalytic activity for H2 generation. 97 The Layered TiO2 films show higher photoefficiency compared to that of standard TiO2-P25 whatever the TAA/CNC ratio of the film (Figure 15). The H2 generation rate reached a value of 287.2 mmol h -1 g -1 for L-TiO2-1, which was nearly twice as high