Design of Visible-light driven catalysts for water oxidation and VOC degradation

The PhD thesis involves one or more articles that are either published,submitted or in the process of manuscript preparation. These all chapters are eloborated in context to the understandings and advancements involved during the PhD period. The whole thesis involves insights about synthesis and characterization of BiVO4 in the form of powder as well as thin films. It also describes the ability of BiVO4 powders and thin films in water splitting and volatile organic compound degradation

Synthèse et caractérisation de matériaux nanostructurés BiVO4 dopés par des métaux pour des applications en Photocatalyse

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.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

Chemical, Structural, and Electronic Characterization of the 010 Surface of Single Crystalline Bismuth Vanadate

We have structurally, chemically and electronically characterized the most stable 010 surface of a Mo doped BiVO4 single crystal. Low energy electron diffraction LEED reveals that the surface is not significantly reconstructed from a bulk termination of the crystal. Synchrotron based X ray spectroscopies indicate no surface enhancement of any of the crystal constituents and that the Mo dopant occupies tetrahedral sites by substituting for V at the surface. Using resonant photoemission to study the valence band structure as the V L3 edge is scanned we observe an intra band gap state associated with reduced vanadium formed by the Mo doping. This state is likely associated with small polaron formation at the surface. This feature is enhanced at a photon energy that is not resonant with any of the main features in the absorption spectrum of the pristine BiVO4. This indicates that the additional electron from Mo doping likely induces further distortion of the VO4 tetrahedral units and generates a new conduction band state either by splitting of the V dz2 states or by hybridization of V dzx and V dz2 states. We measure a work function of 5.15 eV for the BiVO4 010 surface. Measurement of the work function allows us to recast the electronic energy levels onto the normal hydrogen electrode scale for comparison to the standard reduction and oxidation potentials of water. This detailed study should provide a basis for future work aimed at a molecular level understanding of BiVO4 electrolyte interfaces used for photoelectrochemical water splittin

Structural, photocatalytic and electroconductive properties of bismuth-substituted CaMoO4

The Ca1−3xBi2xФxMoO4 system (0.025 ​≤ ​x ​≤ ​0.30, where Ф represents cation vacancies) was synthesized and studied. The 0.025 ​= ​x ​≤ ​0.15 compositions show a tetragonal defect scheelite structure. Powder X-ray and neutron diffraction patterns for compositions with 0.15 ​< ​x ​≤ ​0.225 exhibit a tetragonal supercell with asup ​≈ ​√5a, csup ​≈ ​c where a and c are the tetragonal scheelite cell parameters. Transmission electron microscopy shows that Ca0.4Bi0.4MoO4, crystals consist of three crystallographic domains: (1) defect scheelite; (2) tetragonal superlattice and (3) incommensurately modulated. Photocatalytic properties were studied using Rhodamine B water solutions under UV light. Catalytic activity increases with increasing Bi content. The conductivity of 0.15 ​< ​x ​≤ ​0.225 compositions is 10−7 to 10−8 ​S ​cm−1 in the range 500–650 ​°C, while compositions in the range 0.025 ​= ​x ​≤ ​0.15 show conductivity values from 10−3 to 10−8 ​S ​cm−1 from 500 to 800 ​°C. © 2020 Elsevier Inc.The study was done with a support of RSF, projects № 20-73-10048. The travel grant for neutron diffraction work was given by Act 211 Government of the Russian Federation, contract № 02.A03.21.0006. The authors are grateful to the Science and Technology Facilities Council STFC for neutron beam time at the ISIS facility, Rutherford Appleton Laboratory, award No. RB1910306. Dr Ron Smith at ISIS is thanked for his help in neutron data collection

Sheelite-related strontium molybdates: synthesis and characterization

Received: 25.10.2018. Accepted: 17.12.2018. Published: 31.12.2018.The present research is devoted to the cationic-deficient SrMoO4‑based sheelite-related complex oxides. The doping with bismuth to A sublattice and codoping with bismuth and vanadium (to A and B sublattices, respectively) were discussed. The X-Ray powder diffraction and infrared spectroscopy were used to investigate structural characteristics of the complex oxides. In Sr1–1.5xBixMoO4, a superstructural ordering was observed. Conductivity and dielectric loss of ceramic samples are measured using alternating current.This work was financially supported by grant of Russian Foundation for Basic Research, project № 16‑33‑60026

A new series of environment-friendly reddish inorganic pigments based on AFeO3 (A = Ln, Y) with high NIR solar reflectance

Environment-friendly pigments based onAFeO3(A¼La, Pr, Nd, Sm, Gd, Tb, Y or Yb) with high near-infrared (NIR) reflectance were synthesised by a coprecipitation method at 1200 C. The Rietveldrefinement analysis showed single-phase orthorhombic perovskite for all compositions. All pigments,which showed reddish hues, offered good colour stability after mixing these pigments in powder formwith siloxane transparent paint and two different glazes. The powderepaint mixtures produced withGdFeO3, TbFeO3and YFeO3pigments have the highest NIR solar reflectance, reaching values ofR¼50%.The temperature shielding studies conducted using TbFeO3pigmentepaint mixture for a roof coatingyielded a reduction of 3.2 C in comparison to a commercial pigment. Moreover, the glazes that werepigmented using GdFeO3, TbFeO3and YFeO3compositions also presented the most intense reddishcolours. A study of the thermal and chemical stability of the pigment with the highest NIR solarreflectance showed good stability in both cases. The reddish pigments that were prepared can thereforebe good candidates for use in different applications such as cool pigments or pigments for ceramic glazesat high temperatures

Complementary operando insights into the activation of multicomponent selective propylene oxidation catalysts

Two Bi–Mo–Co–Fe–O catalysts were synthesized by flame spray pyrolysis and tested for their catalytic performance in selective oxidation of propylene to acrolein. Pronounced structural changes during temperature–programmed oxidation and reaction were observed by operando X–ray absorption spectroscopy, X–ray diffraction, Raman spectroscopy, and thermogravimetric analysis. During oxidative treatment, mainly binary oxide phases (α–Bi$^{2}$Mo$^{3}$O$^{12}$, β–CoMoO$^{4}$, Fe$^{2}$ (MoO$^{4}$)$^{3}$) were observed, but single (MoO$^{3}$) or ternary (Bi$^{3}$ (FeO$^{4}$)(MoO$^{4}$)$^{2}$) oxides also formed depending on the relative elemental catalyst composition. During propylene oxidation, the reduction of Fe$^{3+}$ to Fe$^{2+}$ led to a strong rise in activity and induced further phase transformations. MoO$^{3}$ was found to be unselective towards acrolein but was essential in binding other single oxides. The formation of β–Co$^{0.7}$Fe$^{0.3}$MoO$^{4}$ and Bi$^{3}$ (FeO$^{4}$)(MoO$^{4}$)$^{2}$ as well as their synergistic interplay with α–Bi$^{2}$Mo$^{3}$O$^{12}$ are key factors for high performance. The combination of complementary operando methods was crucial to reveal new structure–activity/selectivity correlations, therefore bridging the knowledge gap between simplified model systems and complex applied catalysts