Study On Regulatory Strategy And Performance Of BiVO₄-Based Photocatalyst

Energy shortage and environmental pollution problems have seriously restricted the development of human society and become major problems that needed to be solved urgently by countries around the world.It is a general trend of global development to adjust the traditional energy consumption structure and accelerate the transition from fossil energy to renewable energy.Among many renewable and clean energy sources,the development and utilization of solar energy has become the focus of research in recent years.Photocatalytic technology is one of the effective ways of solar energy storage and conversion,which has been widely used in pollutant degradation,fuel preparation,chemical synthesis and other fields,and is regarded as an ideal way to solve the problem of energy shortage and environmental pollution.However,most photocatalysts have the problem of low efficiency,which is difficult to meet the requirements of practical production.Therefore,the development of high-efficiency photocatalysts is the key to realize the efficient storage and conversion of solar energy.Among a variety of photocatalysts,BiVO₄ has shown great advantages in the photocatalytic water splitting,degradation of organic pollutants,photosynthesis of hydrogen peroxide and other fields due to its excellent visible light absorption,chemical stability,low cost and environmental friendliness.However,the relatively low conduction band position of BiVO₄ makes it lack of thermodynamic driving force for some reduction reactions（e.g.,hydrogen evolution and carbon dioxide reduction）.At the same time,there are also problems such as easy recombination of photogenerated carriers and slow surface reaction in the original BiVO₄,which seriously limit its photocatalytic application.In this thesis,aiming at the above problems,typical tetragonal zircon and monoclinic scheeliteite BiVO₄ photocatalysts are selected as research subjects.Based on the relationship between structure and performance,the energy band structure,photogenerated carrier separation and surface reaction site of the BiVO₄ photocatalyst are optimized by introducing epitaxial strain,constructing heterojunction,loading cocatalysts and ion doping strategy,so as to achieve excellent photocatalytic performance.At the same time,the relationship between the structure and performance of photocatalysts are studied by experimental characterization and theoretical calculation methods.The main research contents are as follows:In chapter 1,the research background and significance of photocatalytic technology were introduced.Then,the reaction principle,main influencing factors and application of photocatalysis were outlined.Subsequently,several strategies for improving photocatalytic performance were summarized.In addition,the crystal structure and research progress of BiVO₄,as well as its advantages and existing problems in photocatalytic reaction were introduced.Based on these discussions,the significance of the topic selection and the main research contents of this thesis were introduced.In chapter 2,the effect of strain on the visible-light-drive overall water splitting of tetragonal zircon BiVO₄ photocatalyst was mainly studied.In view of the relatively low conduction band position of BiVO₄,the lack of reducing ability to trigger hydrogen evolution reaction and the serious photogenerated carrier recombination in bulk photocatalyst,based on the theoretical calculation,we proposed to introduce tensile strain into tetragonal zircon BiVO₄ to change its energy band structure and improve the conduction band position,so as to realize the overall water splitting on BiVO₄ photocatalyst.Through the epitaxial growth of BiVO₄ on the FTO substrate,using the lattice mismatch between the substrate and photocatalyst,the tensile strain was successfully introduced into the BiVO₄,which made the conduction band position shift negatively,and the reduction ability was enhanced,satisfying the thermodynamic conditions of photocatalytic overall water splitting reaction.At the same time,according to the charge transfer between different crystal facets,the hydrogen evolution cocatalyst Rh/Cr2O3 and oxygen evolution cocatalyst MnOx were further loaded,which accelerated the spatially separation of photogenerated carriers and surface reaction,and the as-prepared photocatalyst showed excellent overall water splitting activity under visible light irradiation.In chapter 3,the effect of BiVO₄ modified with Ag and MnOx dual cocatalysts on the performance of photocatalytic reduction of CO2 was investigated.Based on the study of chapter 2,we continued to take the tetragonal zircon BiVO₄ as the research object to explore its performance in photocatalytic CO2 reduction.Spatially separated Ag and MnOx were loaded on different crystal facets of BiVO₄ by photodeposition method,which were used as cocatalysts for reduction reaction and oxidation reaction,respectively.Compared with the single cocatalyst loaded Ag-BiVO₄ and MnOx-BiVO₄,the dual cocatalysts loaded Ag-MnOx-BiVO₄ showed better photocatalytic activity of CO2 reduction to CO.Through a series of tests and characterization,it was proved that the loading of dual cocatalysts could be beneficial to the directional migration of photogenerated electrons and holes,thereby inhibiting the recombination of photogenerated carriers and promoting the surface reaction.In chapter 4,based on the previous work,it is difficult for single-component photocatalyst to satisfy the visible light absorption and strong redox potential simultaneously in overall water splitting reaction.We constructed the all-solid-state g-C3N4/ITO/Co-BiVO₄ Z-scheme heterojunction using Co2+doped monoclinic scheelite BiVO₄ as the oxygen evolution photocatalyst,ITO（indium tin oxide）nanoparticles as the electron transport medium,and gC3N4 as the hydrogen evolution photocatalyst,and investigated its photocatalytic overall water splitting performance.ESR,photodeposition experiments,photoelectrochemistry and fluorescence tests proved that the Z-scheme charge transfer mechanism in heterojunction was favorable for the spatially separation of photogenerated carriers,and maintained the strong oxidation and reduction ability of each component.At the same time,ITO as the charge transport medium further accelerated the separation and transfer of photogenerated carriers.Therefore,under full spectrum and visible light irradiation,the g-C3N4/ITO/Co-BiVO₄ composite exhibited better photocatalytic performance than g-C3N4/Co-BiVO₄ heterojunction and single-component photocatalyst.In chapter 5,compared with constructing heterojunction,heterophase junction formed based on phase transition processe could achieve more efficient carrier separation and transfer.BiVO₄ photocatalyst doped with rare earth element yttrium（Y）was prepared by hydrothermal method,and its photocatalytic synthesis of H2O2 was investigated.The test results showed that the photocatalytic synthesis of H2O2 could be significantly improved with appropriate Y doped BiVO₄.Through a series of characterization,it was proved that Y doping induced the crystal structure of BiVO₄ to transition from monoclinic scheelite to tetragonal zircon,thus forming monoclinic/tetragonal BiVO₄ heterophase junction,which was conducive to enhancing carrier transport and effectively inhibiting the recombination of photogenerated electrons and holes.At the same time,theoretical calculation and experimental characterization proved that Y doping enhanced the adsorption of reactant O2 on the surface of BiVO₄,and further promoted the photocatalytic performance of H2O2 generation through the direct two-electron oxygen reduction process.In chapter 6,the research contents and main innovation points of this thesis were comprehensively summarized,as well as the existing problems and deficiencies in the current research work were analyzed.Finally,the future research work was prospected.