Construction Of Cs₂AgBiBr₆ Heterojunction And Study Of Its Photocatalytic Performanc

As an emerging semiconductor material,perovskites have been widely used in photovoltaic devices due to their excellent optoelectronic properties,continuously breaking records in solar energy conversion efficiency,and bringing new opportunities for the development of photocatalytic technology.In 2017,Park et al.first applied the organic halide perovskite Methylammonium lead iodide（MAPb I₃）to photocatalytic hydrogen evolution,opening up the exploration of perovskite materials in the field of photocatalysis.However,the instability of organic cations has become a major obstacle in the application of perovskite materials in photocatalysis.Subsequently,the face-centered cubic structure of metal halide perovskite Cs Pb X₃（X=Br,Cl）has been applied in organic synthesis and photocatalytic CO₂ reduction,showing excellent performance.The substitution of organic cations overcame the instability of perovskites,further promoting their application in catalysis.However,the toxicity of lead（Pb）has become another bottleneck in the development of halide perovskite photocatalytic applications.The realization of stable and lead-free metal halide perovskites for photocatalysis is a problem that needs to be urgently solved.Cs₂AgBiBr₆ stands out because of its similar optoelectronic properties to lead-based perovskites.It has been reported that Cs₂AgBiBr₆ has improved stability to thermal decomposition and environmental air,and has been applied in high sensitivity and air-stable photodetectors.In addition,its special bimetallic structure enables flexible tuning of the band gap.Cs₂AgBiBr₆ is usually crystallized in a face-centered cubic structure of Fm-3m space group at room temperature,and there is no competing binary or ternary phase.The interaction between metals can promote the adsorption and activation of hydrogen molecules with moderate strength on the metal atoms.Volonakis et al.confirmed through density functional theory（DFT）calculations based on electron affinity and ionization potential that Cs₂AgBiBr₆ has good hydrogen evolution ability and is a potential material for photocatalytic hydrogen evolution.Bimetallic perovskites represented by Cs₂AgBiBr₆ have great potential in photocatalytic applications.However,to date,the number of reports on bimetallic perovskite catalysts is limited and their catalytic ability is limited.Systematic research on the performance of perovskite catalysts based on optimization strategies is particularly important.In response to the above problem,this paper selects the lead-free halide double perovskite Cs₂AgBiBr₆ as the photocatalyst,and couples defect engineering and piezoelectric effect to achieve efficient photocatalytic hydrogen evolution and degradation of large molecular dyes,respectively,by constructing V_Br-Cs₂AgBiBr₆/WO₃ S-scheme heterostructure and Zn O/Cs₂AgBiBr₆ nanorod array S-scheme heterostructure.The electronic transfer mechanism was also studied.The research achievements obtained are as follows:（1）A catalyst with abundant hydrogen evolution active sites was prepared by solvothermal method,which is a S-scheme heterostructure composed of Cs₂AgBiBr₆ octahedron and WO₃nanorods.It achieves efficient and stable photocatalytic hydrogen evolution performance of lead-free halide double perovskite.The S-scheme heterojunction driven by the Fermi level difference between WO₃ and Cs₂AgBiBr₆ ensures efficient charge transfer at the interface.DFT calculations show that the abundant Br vacancies introduced by atomic thermal vibration on the Cs₂AgBiBr₆（022）crystal surface during solvothermal preparation are effective active sites for photocatalytic hydrogen evolution reaction（HER）.The introduction of active sites by thermal vibration during the construction of heterojunctions by solvothermal method provides a new idea for the design of photocatalytic materials.（2）A piezoelectric photocatalytic lead-free halide double perovskite-based S-scheme heterojunction system based on alcohol was successfully prepared using hydrothermal-assisted magnetron sputtering and spin-coating methods.By using Zn O to provide a piezoelectric field and Cs₂AgBiBr₆ nanoparticles to obtain visible light,efficient dye degradation was achieved.By controlling the excited state of the piezoelectric material Zn O using optical tuning strategy,the coupling regulation of piezoelectric field,photoexcitation and semiconductor properties was achieved.The possible role of piezoelectric,photoelectric and piezoelectric coupling in regulating the photogenerated carrier in the same system was proposed.An internal electric field and piezoelectric field were established in the Zn O/Cs₂AgBiBr₆ heterojunction during the photocatalytic degradation process,achieving efficient piezoelectric photocatalytic efficiency combined with mechanical and solar energy.The optimized Zn O/Cs₂AgBiBr₆ has rate constants of 5.75 and 195.5 times that of photocatalytic and piezoelectric catalytic under UV-visible light irradiation（piezoelectric-photocatalytic）assisted by magnetic stirring,respectively.In addition,the possible piezoelectric photocatalytic mechanism of Zn O in the excited state under UV-visible light irradiation and the non-excited state under visible light irradiation was fully discussed.It opens up a new field for lead-free halide double perovskites in solar and mechanical piezoelectric photocatalytic conversion.The main contents of this paper are as follows:Chapter 1 introduces the research background and significance of this paper.The basic principles of semiconductor photocatalysis technology,the existing problems,the overview,research progress,and main issues of Cs₂AgBiBr₆-based photocatalysts of lead-free halide perovskites are briefly described.In addition,detailed introductions on heterojunction engineering of different photocatalysts are provided.Finally,the topic idea and main research contents of this paper are explained.Chapter 2 and Chapter 3 present the detailed introduction of the research results.Chapter 4 is the conclusion and outlook.While summarizing the work of this paper,the future research is also prospected.