La séparation photoélectrochimique de l'eau (PEC) est un moyen direct de produire un combustible solaire tel que l'hydrogène à partir de l'eau. Le goulot d'étranglement de ce processus se situe dans la photoanode, qui est responsable du côté oxydation de la réaction1,2. Dans ce travail, l'utilisation de BiVO4 en tant que photoanode a été largement étudiée afin d'améliorer sa photoactivité. L’optimisation de la synthèse de photoanodes BiVO4 par électrodéposition en couche mince sur du FTO a été réalisée. Les facteurs influant sur l'activité photoélectrochimique, tels que le temps d'électrodéposition, le rapport Bi-KI/benzoquinone-EtOH dans le bain de dépôt et la température de calcination, ont été étudiés à l'aide de la conception composite centrale d'expériences. Les états de surface sur la surface de BiVO4 donnent lieu à des niveaux de défaut pouvant induire une recombinaison électron-trou via le mécanisme de Shockley-Read-Hall5. Afin de minimiser les inefficacités dues à la recombinaison électron-trou et passiver les états de surface, des couches de recouvrement ultra-fines d'Al2O3 et de TiO2 ont été déposées sur les électrodes en film mince BiVO4 d'une manière analogue à l'ALD. Cela a également été réalisé afin de protéger la surface de BiVO4 de la photocorrosion et d’augmenter sa stabilité. Une densité de photocourant de 0,54 mA/cm2 à 1,23 V vs RHE a été obtenue pour les 2 cycles de BiVO4 modifié par Al2O3, comme le montre la Figure 2, soit une amélioration de 54% par rapport à la BiVO4 nue qui démontrait une densité de photocourant de 0,35 mA/cm2. à 1,23 V vs RHE. Une augmentation de 15% de la stabilité de l'électrode de BiVO4 modifiée par Al2O3 a également été observée au cours de 7,5 heures d'irradiation continue. De plus, grâce aux mesures de capacité de surface présentées à la Figure 3, il a été montré que la surcouche de Al2O3 passivait effectivement à passiver les états de surface des électrodes de BiVO4. La nature de la surface de BiVO4 a été étudiée en étudiant la réactivité de la poudre de BiVO4 avec un titrant chimique. L’existence de groupes hydroxyle de surface sur BiVO4 a été confirmée et quantifiée (max. 1,5 OH / nm2) par titrage chimique. La réaction de la poudre de BiVO4 avec une impulsion de AlMe3 et une impulsion de H2O a montré qu'il existait 1,2 molécules de CH4 dégagées par Bi-OH. Dans ce travail, nous avons pu mettre en évidence les facteurs importants dans la synthèse de BiVO4 et leur incidence sur la photoactivité résultante. Nous avons également réussi à passiver les états de surface de BiVO4 en utilisant Al2O3, ce qui n’est pas bien exploré dans la littérature. De plus, nous avons pu sonder et discuter de la nature de la surface de BiVO4. Ceci est une connaissance très fondamentale et le premier rapport à ce sujet, à notre connaissance. Une bonne compréhension de cette surface semi-conductrice importante et de ses interactions facilitera la conception d'un photoanode BiVO4 plus efficace / Photoelectrochemical (PEC) water splitting is a direct way of producing a solar fuel like hydrogen from water. The bottleneck of this process is in the photoanode, which is responsible for the water oxidation side of the reaction1,2. In this work, the use of BiVO4 as a photoanode was extensively studied in order to improve its photoactivity. The optimization of BiVO4 photoanode synthesis via thin film electrodeposition on FTO was performed. The factors affecting the photoelectrochemical activity such as the electrodeposition time, ratio of the Bi-KI to benzoquinone-EtOH in the deposition bath, and the calcination temperature, have been investigated by using the Central Composite Design of Experiments.Surface states on the BiVO4 surface give rise to defect levels, which can mediate electron-hole recombination via the Shockley-Read-Hall mechanism5. In order to protect the BiVO4 surface and minimize the inefficiencies due to electron-hole recombination and passivate the surface states, ultrathin overlayers of Al2O3 and TiO2 were deposited to the BiVO4 thin film electrodes in an ALD-like manner. A photocurrent density of 0.54 mA/cm2 at 1.23 V vs RHE was obtained for the 2 cycles Al2O3-modified BiVO4, which was a 54% improvement from the bare BiVO4 that demonstrated a photocurrent density of 0.35 mA/cm2 at 1.23 V vs RHE. A 15% increase in stability of the Al2O3- modified BiVO4 electrode was also observed over 7.5 hours of continuous irradiation. Moreover, through surface capacitance measurements, it was shown that the Al2O3 overlayer was indeed passivating the surface states of the BiVO4 electrodes. The nature of the BiVO4 surface was studied by investigating the reactivity of powder BiVO4 with a chemical titrant. The existence of surface hydroxyl groups on BiVO4 was confirmed and quantified (max 1.5 OH/nm2) via chemical titration. The reaction of the BiVO4 powder with one pulse of AlMe3 and 1 pulse of H2O showed that there were 1.2 molecules of CH4 evolved per Bi-OH. In this work, we were able to highlight which factors are important in the synthesis of BiVO4, and how they affect the resulting photoactivity. We have also achieved the passivation of the BiVO4 surface states using Al2O3, which is not well-explored in literature. Moreover, we were able to probe and discuss the nature of the BiVO4 surface. This is a very fundamental knowledge and the first report of such, to the best of our knowledge. A good understanding of this important semiconductor surface and its interactions will aid in the design of a more efficient BiVO4 photoanode

La recherche de nouveaux matériaux en tant que photocatalyseurs en lumière visible pour la dépollution de l’environnement (eaux, atmosphères) est un domaine de recherche très actif et suscite l’intérêt d’une large communauté scientifique en Physique, Chimie et Sciences des matériaux. Des recherches exhaustives sont actuellement menées pour améliorer l’efficacité photocatalytique de certaines classes de matériaux photoactifs connus, et pour développer la synthèse de nouveaux matériaux fonctionnels. Dans ce contexte, les semiconducteurs oxydes photoactifs à base de vanadates de bismuth (BiVO4) possédant une bande électronique au milieu du spectre visible, offrent une sérieuse alternative aux photocatalyseurs classiques efficaces (TiO2, ZnO) dont la photo-excitation requiert uniquement la fraction UV du spectre solaire. Le travail effectué dans le cadre de cette thèse est donc dédié aux matériaux à base BiVO4 sous forme d’architectures mésoporeuses ou d’assemblages hybrides associant des groupes organiques à transfert de charges.Deux contributions majeures ont été développées dont la première portant sur la réalisation expérimentale d’architectures mésoporeuses inédites, fonctionnalisées par des groupes organiques sensibilisateurs et l’étude de leurs propriétés électroniques et optiques en vue d’optimiser leurs efficacités photocatalytiques. La deuxième partie porte sur des simulations numériques de nanostructures hybrides par des approches exploitant la méthode DFT, ab-initio ou des modèles de chimie quantique. Des systèmes modèles ont été construits associant des nanoclusters (NC) et des groupes organiques (GO). Les propriétés électroniques et optiques ainsi que les caractéristiques structurelles et vibrationnelles des systèmes (NC-GO) ont été déterminées et confrontées aux données expérimentales. Les phénomènes de transfert de charges impliqués entre les groupes organiques et la structure inorganique ont été caractérisés ainsi que leur rôle dans l’efficacité des réponses photo-catalytiques des systèmes hybrides. / The search for new materials as photocatalysts invisible light for the depollution of the environment (waters, atmospheres) is a very active field of research and attracts the interest of a large scientific community in Physics, Chemistry and Materials Science. Recent research developpements are conducted to improve the photocatalytic efficiency of certain classes of known photoactive materials, and to develop the synthesis of new functional materials. In this context, photoactive oxide semiconductors based on bismuth vanadate (BiVO4) having an electronic band in the middle of the visible spectrum, offer a serious alternative to efficient conventional photocatalysts (TiO2, ZnO) whose photo-excitation requires only the UV fraction of the solar spectrum.The work done in this thesis is therefore dedicated toBiVO4-based materials in the form of mesoporous architectures or hybrid assemblies associating organic groups with charge transfer processes. Two major contributions have been developed, one of which is the experimental realization of novel mesoporous architectures, functionalized by sensitizing organic groups and the study of their electronic and optical properties in order to optimize their photocatalytic efficiencies. The second part deals with numerical simulations of hybrid nanostructures using approaches as the DFT method, ab-initio or quantum chemistry codes. Model systems have been constructed associating BiVO4nanoclusters (NC) and organic groups (GO). The electronic and optical properties as well as the structural and vibrational characteristics of the systems (NC-GO) were determined and compared with the experimental data. The charge transfer phenomena involved between the organic groups and the inorganic structure were characterized as well as their role in the efficiency of photo-catalytic responses of hybrid systems.

Le craquage photo(électro)chimique (PEC) de l’eau par l’énergie solaire est considéré comme une méthode prometteuse de production renouvelable d’hydrogène. Dans ce travail, des hétérostructures à base de BiVO4 et CuFeO2 ont été choisis pour effectuer la réaction d’oxydation et de réduction de l’eau, respectivement. Cependant, les avantages exacts des hétérostructures n’ayant pas encore été complètement élucidés. Ce travail a eu pour objectif d’examiner les propriétés de certaines hétérojonctions à base de BiVO4 et de CuFeO2 par des expériences d’interface. Dans ce but, un certain matériau a été pulvérisé sur un substrat de BiVO4ou de CuFeO2 et des mesures de spectroscopie de photoélectrons ont été effectuées à chaque étape du dépôt. Nous avons ainsi pu interpréter l’alignement des bandes entre le substrat et le matériau pulvérisé, et déterminer l’accordabilité du niveau de Fermi pour les absorbeurs étudiés.Par ailleurs, des hétérostructures à base de particules de CuFeO2 et de BiVO4 anisotropes ont été élaborées par photodéposition. Les performances de ces poudres dans des expériences de craquage photochimique de l’eau ont ensuite été déterminées. / Solar photo(electro)chemical (PEC) water splitting is regarded as a promising ways of renewable hydrogen production. In this work, heterostructures based on BiVO4 and CuFeO2were chosen to perform the water oxidation and water reduction reaction, respectively. However, the exact benefits of the contact materials in these heterostructures have not yet been completelyelucidated. Hence, we opted in this work to investigate the junction properties of certainBiVO4 and CuFeO2 based heterostructures through so called interface experiments, where by a certain contact material was step wise sputtered on to a BiVO4 or CuFeO2 substrate, performing photoelectron spectroscopy measurements in between each deposition step. In this way we could interpret the band alignment between the substrate and the contact material, as well as determine the Fermi level tunability for the studied photoabsorbers. In parallel, new anisotropic CuFeO2and BiVO4 based heterostructured powders were created through photodeposition. These powders were tested as well for their performance in photochemical water splitting.

Reverse combustion is a process converting CO2 into its different reduced/hydrogenated forms while, ideally, oxygen is being released. Understanding how CO2 is interacting/reacting with vanadium (main component in the CB of m-BiVO4) in different oxidation states was our main goal. In this thesis we have attempted to contribute to the ongoing efforts for overcoming the formidable challenges posed by H2 production and CO2 activation. In the process to prove the role of each metal during a reverse combustion process mediated by m-BiVO4, several strategies were followed to prepare pure monoclinic BiVO4 using different starting materials (Chapter 2). Hydrothermal processes in an autoclave were determined as the most efficient way to obtain m-BiVO4. Photoirradiation experiments were performed in-situ and analyzed by EPR, demonstrating that a photoexcited species was generated. EPR spectra were compared with VO2, which suggested that one electron is being transferred from the VB to the CB in the photoexcitation process, in this case forming a vanadium(IV). This experiment suggested that the reduction process of CO2 is possibly occurring through a one-electron transfer process. Several attempts were made unsuccessfully to prepare a bismuth vanadate-like compound containing only vanadium(IV) in its structure. Bi4V2O10 was obtained where the vanadium atom was present in a lower oxidation state but with different Bi/V ratio than in BiVO4. This species does not present any photo-catalytic activity. Instead, it presented mild reactivity in hydrogen formation from formaldehyde in basic media. Photocatalytic experiments on pure m-BiVO4 in the presence of water and CO2 were performed and methanol was obtained as a product. In this process, vanadium leached out from the structure affording a mixture of V(IV) and V(V). On the surface of the remaining m-BiVO4, Bi2O4−x was deposited as a result of the loss of vanadium. The initial idea behind the preparation of a compound different to BiVO4 was to produce a new photocatalyst that preserves the electronic characteristics of vanadium(V) as well as being a semiconductor (Chapter 3). In addition, a higher oxidation state than the vanadium +5 could provide longer electron-hole recombination times and increase lifetime of the photogenerated electrons. By having a +6-oxidation state, such as provided by a Cr atom, it could give a better chance to improve the reduction of CO2 by facilitating oxygen release. Unfortunately, photochemical activity was not observed under any conditions. On the other hand, both monoclinic and orthorhombic BiOHCrO4 were tested for formic acid thermal decomposition. These two unique crystal structures were analyzed by single crystal XRD. The monoclinic isomer displays a much higher thermal resilience and was chosen for the degradation of formic acid studies. During the process, an active species of BiCrO4 was formed and identified. When using vanadium aryloxide compounds in an oxidation state lower than +5 as possible reagents to reduce CO2, interesting results were obtained (Chapter 4). These compounds were prepared aiming at mimicking the reduction of CO2 as performed by hypothetically formed lower valent vanadium. As presented in chapter 2, during the photoirradiation of BiVO4 a new vanadium species is formed. EPR experiments indicated that it could be V(IV). As a result, while vanadium(IV) showed negligible reduction/interaction with CO2, vanadium(III) aryloxide was a powerful reductant. Experiments attempting to control the electron transfer to CO2 resulted in two different outcomes. Firstly, a two-electron transfer from the metal center to CO2 was obtained affording CO and vanadyl(V) tris-aryloxide. Secondly the introduction of a halogen in the metal coordination sphere of a vanadium(III) compound triggered a radical behavior. The use of a compound of vanadium(II) with polydentate oxygen-donor based ligand still yielded CO. However, an intermediate V-O-V moiety was formed in turn performing radical H atom extraction from the solvent through an unprecedented pattern of reactivity. DFT calculations confirmed the nature of the electronic transfer and the formation of V-O-V that acted as an intermediate for the second CO2 interaction. We successfully arrested the reaction to isolate an intermediate and an unprecedented (ONNO)V(OH)-OCO compound was isolated and fully characterized. This CO2 complex provide the second example of a linearly end on bonded CO2 and the first case of such a bonding mode to a transition element. A further study of the reactivity of the vanadium trivalent state was carried out by modifying the ligand to H2ONOO and secondly, by introducing a Cl atom as in LV-Cl (L = ONNO or ONOO) to enable the formation of derivative such as p-methoxy-phenoxide and methoxide ligands via simple ligand substitution. Unfortunately, the (ONOO)2- ligand quenched the reducing power such that no reaction was observed with CO2. Halogen replacement afforded (ONNO)V(p-methoxy-phenoxide)(THF) which displayed no reactivity with CO2, but once the p-methoxy-phenoxide ligand was replaced by a methoxide group, formaldehyde and formate were formed. The DFT proposed mechanism presented an interesting interaction wherein the cis- position in [V(ONNO)]+ is responsible for the H transfer to occur Finally, we have prepared a heterobimetallic system containing Bi-V atoms (Chapter 6). The oxidation states of Bi and V were +3 and +5 respectively. One pot reaction was the most adequate procedure to obtain the heterobimetallic structure. Trasmetallation on Bi compounds by V atoms was observed when attempting to build the heterobimetallic structure using more rational reaction pathways. Attempts to obtain a heterobimetallic structure in oxidation states different than that presented in m-BiVO4 were unsuccessful. When oxidation states lower than +5 for vanadium (vanadium(III-II)) and +3 in bismuth were used, metallic bismuth and untreatable materials with a mixed-valence vanadium were formed.

?rea de concentra??o: Produtos e coprodutos. / Submitted by Jos? Henrique Henrique (jose.neves@ufvjm.edu.br) on 2017-10-03T14:42:21Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) wayler_silva_santos.pdf: 9149649 bytes, checksum: 7d300a18f29fb2d1bfa9a3ab5c806ad3 (MD5) / Approved for entry into archive by Rodrigo Martins Cruz (rodrigo.cruz@ufvjm.edu.br) on 2017-10-09T14:02:32Z (GMT) No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) wayler_silva_santos.pdf: 9149649 bytes, checksum: 7d300a18f29fb2d1bfa9a3ab5c806ad3 (MD5) / Made available in DSpace on 2017-10-09T14:02:32Z (GMT). No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) wayler_silva_santos.pdf: 9149649 bytes, checksum: 7d300a18f29fb2d1bfa9a3ab5c806ad3 (MD5) Previous issue date: 2017 / Universidade Federal dos Vales do Jequitinhonha e Mucuri (UFVJM) / Funda??o de Amparo ? Pesquisa do Estado de Minas Gerais (FAPEMIG) / A convers?o da energia da radia??o solar em H2(g), combust?vel verde, usando-se c?lulas fotoeletroqu?micas (PEC) ? uma estrat?gia atraente para armazenar energia e minimizar o uso extensivo de combust?veis f?sseis. Neste trabalho, foram fabricados fotoeletrodos por deposi??o spray pyrolysis de um comp?sito de BiVO4/Bi4V2O11 puro ou dopado com W, depositado diretamente sobre um substrato condutor FTO (sigla para fluorine-doped tin oxide) ou sobre uma camada de SnO2 previamente depositada sobre o substrato FTO. Inicialmente, os materiais foram testados na clivagem molecular da ?gua. Verificou?se a forma??o de uma camada de invers?o de buracos induzida pela perovskita ferroel?trica Bi4V2O11 na interface com BiVO4 de tipo?n criando uma jun??o p?n virtual. A fotovoltagem de sa?da elevada da jun??o, em rela??o a uma heterojun??o p?n convencional, que pode ser ainda aumentada alterando-se a polariza??o e dopando-se o material ferroel?trico com tungst?nio, acarreta diminui??o da recombina??o dos pares el?tron?buracos fotogerados na superf?cie e aumenta a fotocorrente em at? 180%. O comportamento de semicondutores de tipo?p e n quando iluminados sugere o uso potencial da heterojun??o como fotoanodo e fotocatodo em uma c?lula PEC (ou photoelectrochemical cell) com dois fotoeletrodos. Este conceito foi comprovado pela conex?o do fotoanodo BiVO4/Bi4V2O11 dopado com 1% em massa de tungst?nio com o fotocatodo BiVO4/Bi4V2O11 n?o dopado. O sistema formado pelo acoplamento fotoanodo-fotocatodo produziu uma fotovoltagem de 1,54 V, e 0,36% de efici?ncia STH (solar-to-hydrogen efficiency). Um fotoeletrodo BiVO4/Bi4V2O11 dopado com 2% em massa de W foi otimizado. Evidenciou-se a forma??o da camada de invers?o de buracos na interface semicondutora no filme mais denso. Este ?ltimo foi testado em diferentes solu??es, obtendo-se elevadas densidades de corrente em NaAc 0,5 mol L-1, ? medida que se adicionou glicerina e vinha?a. Obteve?se a menor resist?ncia na transfer?ncia de cargas na interface eletrodo/eletr?lito 1,16 k? sob ilumina??o, utilizando a solu??o NaAc 0,5 mol L-1 contendo 20 %v/v de vinha?a. Nessas condi??es, a efici?ncia da convers?o de energia foi aumentada em aproximadamente 100%, e dependendo do potencial aplicado a efici?ncia aumentou cerca de 30% em solu??o contendo 10 %v/v de glicerina, com rela??o ? solu??o NaAc 0,5 mol L-1 pura, demonstrando que glicerina e vinha?a atuam como agentes de sacrif?cio eficazes no sequestro de buracos eletr?nicos, para evitar a recombina??o dos pares de el?tron?buraco, no processo de foto?oxida??o acionado por buracos. / Tese (Doutorado) ? Programa de P?s-gradua??o em Biocombust?veis, Universidade Federal dos Vales do Jequitinhonha e Mucuri, 2017. / The conversion of solar energy into green fuel H2(g) using photoelectrochemical cells (PEC) is an attractive strategy for storing energy and minimizing the extensive use of fossil fuels. In this work, photoelectrodes were prepared by pyrolysis spray deposition from a pure or W?doped BiVO4/Bi4V2O11 composite deposited directly onto an FTO (fluorine-doped tin oxide) electron-conductive substrate or onto a SnO2 layer previously deposited on an FTO substrate. Initially, these materials were tested for molecular water cleavage. It was observed a hole inversion layer induced by the ferroelectric perovskite Bi4V2O11 at the interface of the n?type BiVO4; a virtual p?n junction with high output photovoltage in relation to a conventional p?n heterojunction was also created. Altering the polarization and doping the ferroelectric material with tungsten may increase the p-n heterojunction. This polarization also reduces the recombination of the photogenerated electron?hole pairs on the surface, to increase the photocurrent to as much as 180%. This characteristic behavior of the p?type and n?type semiconductors when illuminated suggests the use of such a heterojunction as photoanode and photocatode in a PEC cell with two coupled photoelectrodes. This concept was proved to effectively work, by connecting the photovoltaic BiVO4/Bi4V2O11 doped with 1 mass% of W to the non?doped BiVO4/Bi4V2O11 photocathode, which produced a high photovoltage of 1.54 V, and 0.36% STH (solar-to-hydrogen) efficiency. A BiVO4/Bi4V2O11 photoelectrode doped with 2 mass% tungsten was optimized, evidencing the inversion of the hole layer at the semiconductor interface in the resulting denser film. This film tested in different liquid solutions revealed to produce high current densities in 0.5 mol L-1 sodium acetate (NaAc), if glycerol and vinasse were added. The electric resistance in the charge transfer at the electrode/electrolyte interface was relatively low, or 1.16 k? under illumination using 0.5 mol L-1 NaAc solution containing 20 %v/v vinasse. Adding vinasse meant an increase of the energy conversion efficiency corresponding to approximately 100%, and depending on the applied potential efficiency increased by about 30% in solution containing 10% v/v glycerin, relatively to the pure 0.5 mol L-1 NaAc solution. These results evidence that glycerin and vinasse act as effective sacrificing agents, to sequester holes and avoid the recombination of electron?hole pairs in the photo?oxidation driven by holes.

Les matériaux pour la photocatalyse en lumière visible ont attiré un grand intérêt car ils peuvent exploiter tout le spectre d'irradiation solaire notamment afin de détruire des polluants organiques pour l'environnement comme dans la purification de l’eau. Dans ce contexte, le bismuth de vanadates (BiVO4) est digne d'intérêt en raison de sa largeur de bande interdite électronique (~ 2,3 eV) et sa potentielle activité photocatalytique. Des études systématiques ont été menées pour les caractéristiques physico- chimiques de poudres BiVO4 synthétisées par voie hydrothermale et par broyage mécanique à haute énergie. La pertinence de la méthode de mécano-synthèse a été démontrée grâce notamment à son faible coût de fonctionnement, de mise en œuvre facile ainsi que le nombre limité de paramètres et la possibilité d’obtenir des particules à taille réduite (20-100 nm) avec une phase cristalline monoclinique. En couches minces, les matériaux BiVO4 ont été synthétisés par pulvérisation ultrasonique (USP) et par pulvérisation cathodique radiofréquence (rf). Les paramètres pour des dépôts optimaux ont été identifiés permettant d’obtenir des films minces sans fissures, suffisamment denses avec des surfaces texturées à morphologies contrôlées. Les études structurales, vibrationnelles, et les propriétés électroniques et optiques ainsi que leur interprétation grâce à des modèles ont été menées pour une parfaite connaissance des caractéristiques des matériaux BiVO4. Pour les applications visées, BiVO4 sous forme de poudres et de films minces ont été utilisés comme photocatalyseurs pour la dégradation de rhodamine 6G (Rh6) et le bleu de méthylène (MB) sous irradiation en lumière visible. La structure scheelite monoclinique de nanoparticules sphérique de BiVO4 obtenues par mécano-synthèse, ont montré une efficacité améliorée (+50%) de l’activité photocatalytique par rapport à des particules de forme aciculaire obtenues par voie hydrothermale. Dans le cas de films minces, le taux de dégradation du BM est de l’ordre est de 66% pour les films synthétisés par USP alors qu’un taux de 99% a été atteint avec des films obtenus par pulvérisation cathodique rf. Ces travaux valident les propriétés photocatalytiques remarquables de BiVO4 par rapport aux matériaux existants avec des applications prometteuses, notamment dans la résolution de problèmes environnementaux. / Visible light photocatalysts have attracted a great interest since it may exploit the wide solar irradiation spectrum to destroy organic dyes as required for environmental need such as water purification. In this context, bismuth vanadate (BiVO4) is worth of interest due to its narrow band gap (~ 2.3 eV) and the ability to exhibit efficient photocatalytic activity. Systematic studies have been carried out on the physico-chemistry of BiVO4 synthesized as powders by hydrothermal and mechano-chemical techniques. The relevance of ball milling method was demonstrated through its low processing cost and easy scaling up as well as limited variable parameters to obtain reduced particle sizes down to (20-100 nm). As thin films, BiVO4 were grown by ultrasonic spray pyrolysis (USP) and rf-sputtering techniques. Optimum deposition parameters were identified, leading to the formation of crack free, dense media with textured surfaces composed by controlled morphologies. Analysis of the structural, vibrational, electronic and optical experiments, interpretation and development of models were carried out for deep insight on the properties of BiVO4 materials. For concrete applications, BiVO4 as powders and thin films were used as photocatalysts for the degradation of rhodamine 6G (Rh6) and methylene blue (MB) under visible light irradiation. Monoclinic scheelite structure of spherical-like BiVO4 nanoparticles obtained by mechano-chemical process, have shown 50% more efficient photocatalytic activity compared to acicular-like BiVO4 grains obtained by hydrothermal method. The average degradation rate of MB using USP grown films was found to be 66% during 120 minutes. A significant rate increase in the photocatalytic activity up to 99% was achieved by using rf-sputtered films. Thus, BiVO4 was demonstrated as efficient photocatalysts compared to existing materials with promising applications notably in solving environmental problems.

Le travail de thèse est consacré à la synthèse, l’élaboration et à l’étude des propriétés physiques d’une famille d’oxydes semi-conducteurs BiVO4 sous formes de nanostructures et de films minces incluant un dopage métallique (Metal = Cu, Mo et Ag) dans le but de réaliser des photocatalyseurs efficaces sous irradiation en lumière visible. La synthèse de nanopoudres dopées a ainsi été effectuée par la technique de broyage planétaire à haute énergie ainsi que par la méthode sol-gel. Les matériaux obtenus et les effets de dopage ont été étudiés sur les caractéristiques structurales, électroniques et optiques. En conjuguant des études par XPS, Raman et RPE, nous avons montré que le dopage substitutionnel est effectivement réalisé pour les ions (Cu, Mo) localisés dans les sites cristallins des ions vanadium alors que le dopage par l’élément Ag contribue à former des clusters métalliques localisés à la surface de nanoparticules de BiVO4 formant ainsi des nanocomposites. Les réactions photocatalytiques ont été étudiées par la dégradation de colorants organiques (Acide bleu 113, méthyle orange (MO)) dans des solutions faiblement concentrées. Parmi les ions dopants substitués dans les matrices hôtes, le dopage au cuivre (Cu2+) a montré de meilleures performances en raison d'une augmentation de la densité de charges photo-générées et de la conductivité électrique par rapport au cas du dopage au molybdène. Pour le dopage à l’argent, la formation de clusters métalliques donnent lieu à des effets de résonances plasmoniques qui améliorent l'efficacité photocatlytique à un niveau équivalent à celui du dopage substitutionnel au cuivre. La deuxième contribution de ce travail a porté sur la réalisation par pulvérisation cathodique rf-magnétron de films minces BiVO4 dopés par des éléments Mo et Cu dans des conditions définies par l'atmosphère de dépôt à base de pressions partielles d’un gaz Ar ou d’un mélange Ar / O2 et des températures de substrats variables jusqu'à 450 ° C. Les paramètres optimaux de dépôt ont été identifiés pour réaliser des films cristallins à faible rugosité de surfaces ou à morphologies en nano-îlots. Des études photocatalytiques utilisant des films minces dopés ont été effectuées par la dégradation des colorants organiques (MO) sous rayonnement visible. Ces études montrent que la morphologie des films avec des surfaces spécifiques importante est aussi un facteur d’amplification des performances photocatalytiques des films minces dopés Me-BiVO4. / The thesis work is devoted to the synthesis and investigations of the physical properties of a family of semiconducting oxides based on BiVO4 as nanostructures or thin films including a metal doping (Metal = Cu, Mo and Ag) in order to achieve effective photocatalysts under visible light irradiation. The synthesis of doped nanopowders was carried out by the techniques of high-energy ball milling and sol-gel. The resulting materials and doping effects were characterized on the structural, electronic and optical properties. By combining XPS, Raman and EPR studies, it was shown that the substitutional doping is achieved for the doping ions (Cu, Mo) being located in the lattice sites of the vanadium ions. Oppositely, Ag doping contributes to form Ag metal clusters located on the surface of nanoparticles of BiVO4 thereby forming nanocomposites. Photocatalytic reactions were studied by the degradation of organic dyes (Acid Blue 113, methyl orange (MO)) in low concentrated solutions. Among the doping ions substituted in the host matrices, Cu2+ showed better photocatalytic performances because of an increase in the density of photo-generated charges and similar effect on the electrical conductivity compared to the case of Mo doping. In the Ag based nanocomposites, the formation of metal clusters seems to induce surface resonance plasmonic effects that improve the efficiency of photocatalytic reactions with respect to the activity demonstrated for substitutional doping. The second contribution of the thesis work was devoted to BiVO4 thin films deposition by rf sputtering process with Mo and Cu doping under defined synthesis conditions such as the partial pressures of Ar gas or an Ar / O2 mixture and varying the substrate temperatures up to 450 ° C. The optimal deposition parameters have been identified to achieve crystalline films with low roughness surface or alternatively with nano-islands morphologies. Photocatalytic studies using doped thin films were carried out through the degradation of organic dyes (MO) under visible light irradiation. The performed measurements show that the film morphology with high specific surface is also a key factor in the amplification of photocatalytic reactions in metal doped thin films.

Photoelectrodes and photovoltaic devices have been prepared via multiple thin film deposition methods. Aerosol assisted chemical vapour deposition (AACVD), electrodeposition (ED), chemical bath deposition (CBD) and doctor blade technique (DB) have been used to deposit binary and ternary metal oxide films on FTO glass substrates. The prepared thin films were characterised by a combination of SEM (Scanning Electron Microscopy), powder X-ray diffraction, mechanical strength tests and photochemical measurements. Nickel oxide (NiO) thin films prepared by AACVD were determined to have good mechanical strength . with a photocurrent of 7.6 μA cm-2 at 0 V and an onset potential of about 0.10 V. This contrasted with the dark current density of 0.3 μA cm-2 at 0 V. These NiO samples have very high porosity with crystalline columns evidenced by SEM. In comparison with the AACVD methodology, NiO films prepared using a combination of ED and DB show good mechanical strength but a higher photocurrent of 24 μA cm-2 at 0 V and an onset potential of about 0.10 V with a significantly greater dark current density of 7 μA cm-2 at 0 V. The characteristic features shown in the SEM are smaller pores compared to the AACVD method. Copper (II) oxide (CuO) and copper (I) oxide (Cu2O) films were fabricated by AACVD by varying the annealing temperature between 100-325°C in air using a fixed annealing time of 30 min. It was shown by photocurrent density (J-V) measurements that CuO produced at 325 °C was most stable and provided the highest photocurrent of 173 μA cm-2 at 0 V with an onset potential of about 0.23 V. The alignment of zinc oxide (ZnO) nano-rods and nano-tubes fabricated by CBD have been shown to be strongly affected by the seed layer on the FTO substrate. SEM images showed that AACVD provided the best seed layer for aligning the growth of the nano-rods perpendicular to the surface. Nano-rods were successfully altered into nano-tubes using a potassium chloride bath etching method. NiO prepared by both AACVD and the combined ED/DB method were sensitized to absorb more of the solar spectrum using AACVD to deposit CuO over the NiO. A large increase in the photocurrent was observed for the p-type photoelectrode. These p-type photoelectrode showed a photocurrent density of approximately 100 μA cm-2 at 0 V and an onset potential of 0.3 V. This photocathode was then used as a base to produce a solid state p-type solar cell. For the construction of the solid state solar cells several n-type semiconductors were used, these were ZnO, WO3 and BiVO4. WO3 and BiVO4 were successfully produced with BiVO4 proving to be the optimum choice. This cell was then studied more in depth and optimised by controlling the thickness of each layer and annealing temperatures. The best solid state solar cell produced had a Jsc of 0.541 μA cm-2 (541 nA) and a Voc of 0.14 V, TX146 made up of NiO 20 min, CuFe2O4 50 min, CuO 10 min, BiVO4 27 min, using AACVD and then annealed for 30 min at 600°C.