Construction And Photocatalytic Performance Of ZnIn₂S₄-based Heterojunction Materials

Although the use of fossil fuels has a huge impact on climate change through the emission of greenhouse gases,they have been used for the past century to meet global energy needs and cannot be replaced.Therefore,in order to build a more environmentally sustainable society,it is crucial to use solar and hydrogen energy rather than fossil fuels to produce renewable energy.Photocatalysis has been found to be a potential method to convert abundant and unlimited solar energy into chemical fuels without additional energy.In addition,it can be used to achieve photocatalytic oxidation of biomass alcohols that have biomass to be converted into high value-added chemicals.In this regard,the ternary metal sulfide ZnIn₂S₄（ZIS）has attracted attention due to its excellent visible-light capability,low cost,excellent chemical stability,and broad interdisciplinary interest.In addition to this,the multilayer nanostructure of ZIS makes it easy for chemical and morphological modulation,as well as combination with other photocatalysts for the construction of heterojunctions.However,the photocatalytic performance of ZIS remains low and its applications are limited due to its slow reaction kinetics and severe photogenerated carrier complexation.Based on the above problems,the work takes ZnIn₂S₄ composites as the research object,optimizes the electronic structure of the materials through structural design,component modulation,elemental doping,the defects construction and heterojunction construction,realizes the high efficiency photocatalysis of ZnIn₂S₄ composites in the visible light,and proposes the photocatalytic reaction mechanism by combining with photoelectrochemical characterization as well as density-functional theory calculations.The research of this thesis includes the following three parts:（1）One-step strategy utilizing a metal-organic framework as a template was used to synthesize three-dimensional ZnIn₂S₄-In₂S₃ hollow hierarchical porous nanotubes as an efficient catalyst for the photocatalytic decomposition of aqueous hydrogen.One-step in situ preparation of hollow ZnIn₂S₄-In₂S₃ heterojunction catalysts resulted from a kinetic process of interior collapsing of In-MIL-68 and growing of ZnIn₂S₄ crystals attached to the outer edges.Critically,the forming of In₂S₃ hollow porous nanotubes and the fabrication of ZnIn₂S₄-In₂S₃ hierarchical heterojunctions with ultra-thin nanosheets were induced by In-MIL-68 at the same time.By modulating the relevant composition of the components of the heterojunctions,the optimal ZnIn₂S₄-In₂S₃-10achieved 5.01 mmol·h^-1·g^-1 hydrogen production efficiency with visible light irradiation in the absence of a co-catalysts,as well as up to 16.23%apparent quantum yield（AQY）at 420 nm,which was significantly better than the photocatalytic yields of In₂S₃-ZnIn₂S₄ heterojunctions synthesized by two-step methods and single-component.It was demonstrated that the fabrication of heterojunction and the formation of hollow porous nanotubes strengthened the absorption of visible light and facilitated the photogenerated carrier separation and transport,which improved the photocatalytic performance of the as-prepared materials,by both theoretical and experimental results.In parallel,there was a novel access to the preparation of MOF-based hollow porous hierarchical photocatalysts as the ZnIn₂S₄-In₂S₃ hierarchical hollow nanotubes demonstrated favorable stability and reusability.（2）A dual system of photocatalytic redox reaction was constructed as an efficient solution to boost photocatalytic H₂ production from water decomposition,and not only solved the problem of water oxidation slowing down in the water decomposition process,but also could be coupled with the biomass converting for the production of high-value-added chemicals.Therefore,by fabricating S-scheme heterojunctions and double defect sites,we have prepared 3D fully spectrum-responsive P-Mo S₂-ZnIn₂S₄-2（P-M-Z-2）floral materials using a hydrothermal method for the effective preparation of H₂ as well as benzyl alcohol oxidation.Herein,the hole-trapped P-doped sites and electron-trapped sulfur vacancies possessed synergistic modulation of charge flow to achieve the separation of photogenerated electrons and holes.The results showed that the optimized P-M-Z-2 had a superior photocatalytic efficacy under visible light irradiation(H₂ yield of 3763.2μmol·h^-1·g^-1 and benzaldehyde yield close to 100%),which was approximately 9 times that of pure ZnIn₂S₄.It was more important to note that the yield of the H₂ of the final P-M-Z-2 after optimization was 31423.6μmol·h^-1·g^-1 with simulated solar irradiation（AM 1.5G）,and the yield of benzaldehyde was 31297.7μmol·h^-1·g^-1.A higher redox potential of the catalyst was retained by the fabrication of S-scheme heterojunctions,enhancing the redox ability as well as the photocatalytic properties,while the heterojunctions facilitated the carrier separation and transport.It not only disclosed the method to improve the photocatalytic activity via modulation of sulfur vacancies by doping and structuring of S-scheme heterojunction,but also offered thoughts for enabling high-efficiency utilization of compounds from biomass platforms.（3）Charge transfer in heterojunctions based on photocatalytic redox systems was a key factor affecting the efficiency and stability of photocatalytic overall water splitting.Herein,near-spherical amorphous In Mo oxides（a-In Mo）synthesized by a hydrothermal method were used as supports for the laterally exfoliated growth of ZnIn₂S₄ nanosheets to form hierarchical a-In Mo-ZnIn₂S₄（In MZ）heterojunctions.The unique hierarchical heterostructure contributed to the exposed active sites,further promoting the involvement of a-In Mo and ZnIn₂S₄ in water oxidation and proton reduction,respectively.The distinctive Z-scheme regulated charge transfer facilitated the spatial separation of photoexcited charges and enhanced the photocorrosion resistance of In MZ,as demonstrated by in situ XPS analysis.The optimized In MZ-90heterojunction exhibited improvements in photocatalytic overall water splitting efficiency(135.4μmol·h^-1·g^-1 for H₂ and 62.5μmol·h^-1·g^-1 for O₂).More than 93.7%overall water splitting activity and intact structure were maintained even after 5（25 h）cycling experiments.In comparison with crystalline In Mo oxide（c-In Mo）,the unique disordered structure of a-In Mo improved the adsorption activity on the catalyst surface,thus In MZ-90 showed superior photocatalytic performance compared with c-In MZ-90.Additionally,the construction of ZnIn₂S₄ and a-In Mo heterojunction further increased the light absorption ability of the catalyst and enhanced the photogenerated electron and hole migration efficiency.This work highlighted the critical role of amorphous-crystalline Z-scheme heterojunctions for improving photocatalytic overall water splitting performance,but also provided guidance for exploring efficient and stable overall water splitting visible-light photocatalysts without sacrificial agents and co-catalysts.