First-principles Study Of Metal Chalcogenide Photocatalytic Materials And Their Heterostructures

Semiconductor photocatalysis is an effective method to solve environmental pollution and energy shortage.However,the traditional photocatalytic materials limit the further development of photocatalysis due to the wide band gap and low quantum efficiency.Therefore,it is necessary to study some novel photocatalytic materials.Metal sulfide is an important representative of new photocatalytic materials.Due to its various structures,flexible physical and chemical properties,abundant and cheap sources,most of them have potential high-efficiency photocatalytic properties,which provide abundant development for new photocatalytic materials.In this paper,the typical transition metal sulfide Cu_xS_y?1?x/y?2?and heavy metal sulfide Bi₂S₃ are used as the representative,and the basic properties of metal sulfide photocatalytic materials are studied by first-principles calculation method based on density functional theory.The photocatalytic properties are modified by constructing a heterostructure.The main research contents and results are as follows:?1?Structures and properties of Cu_xS_y photocatalytic materials:The electronic structure and optical properties of important metal sulfides Cu_xS_y with different stoichiometries were studied by using the first-principles calculation method based on DFT theory.Cu_xS_y is a self-doping semiconductor that its stoichiometry changes by changing the content of copper vacancies or sulfur vacancies.It is found through research that the band gaps increase after the copper vacancies are generated,and the band gaps decrease after the sulfur vacancies occurs.This change has an effect on the optical properties,and these copper sulfides may therefore produce the localized surface plasmon resonance?LSPR?effect.Different stoichiometries of copper sulfides exhibit different properties,and their photocatalytic properties can be adjusted by adjusting their stoichiometries in photocatalysis.?2?The electronic structure,interface structure and related interfacial properties of Cu₇S₄/MnS heterostructure were investigated.The results show that the interface energy is postive and relatively small,indicating that the heterostructure is stable and easy to prepare.After the interface is formed,a type II heterostructure is formed between the?101?plane of Cu₇S₄ and the?110?plane of MnS,and electrons are transferred from the Cu₇S₄ layer to the MnS layer.Since the direction of the built-in electric field is directed from the Cu₇S₄ layer to the MnS layer,it will promote the transfer of electrons from the Cu₇S₄ layer to the MnS layer,thus achieving a good separation of electron-hole pairs in space and improving photocatalytic performance.?3?The crystal structure,electronic structure and optical properties of five bismuth chalcogenides?including Bi₂O₃,Bi₂S₃,O-Bi₂Se₃,T-Bi₂Se₃,Bi₂Te₃?were investigated.The conclusion are as follows:The electronic structure of Bi₂Q₃ shows obvious similarities and trends.At the same time,there are some differences in their electronic structures due to differences in composition and crystal structure.And the optical properties of bismuth chalcogenides are enhanced with the increase in the atomic number of Group VI elements.Moreover,the microstructure of the crystals also has an important influence on the optical properties.Their differences in electronic structure and optical properties make Bi₂Q₃suitable for different applications.Bi₂O₃ and Bi₂S₃ have good absorption in the ultraviolet and visible regions,and they are often used as photocatalysts.?4?The electronic structure,interface structure and related interface properties of CuS/Bi₂S₃ heterostructure were studied.The following conclusions were drawn:the interface consists of the?001?plane of CuS and the?100?plane of Bi₂S₃,which has small lattice mismatches and a stable interface can be formed.Due to the different bonding modes and chemical environments at the interface,the electronic structure of the interface model has different characteristics compared to the bulk model and the surface model.After the interface is formed,the band edge of Bi₂S₃ moves downward to form a type II heterojunction with CuS,which is favorable for improving the photocatalytic performance.In addition,in the equilibrium state,the direction of the built-in electric field is directed from the Bi₂S₃ layer to the CuS layer,so the photogenerated electron-hole pairs can be spatially separated by the CuS/Bi₂S₃ interface.In addition,the CuS/Bi₂S₃ heterostructure not only improves the light absorption performance of the bulk CuS,but also reduces the photogenerated electron-hole pair recombination of the narrow band gap semiconductor Bi₂S₃,so that the photocatalytic performance is improved.In this paper,the electronic structures and related properties of typical transition metal sulfide Cu_xS_y and heavy metal chalcogenide Bi₂Q₃?Q=O,S,Se,Te?materials were studied,and their basic physicochemical properties were studied comprehensively.The influence of the stoichiometries on the electronic structure and related properties of Cu_xS_y photocatalytic materials was clarified,and the influence of the composition and crystal structure of Bi₂Q₃ materials on its electronic structure and optical properties was deeply understood.On this basis,it was found that some of the metal sulfides needed to be modified.Heterostructures between metal sulfides have received wide attention.Therefore,taking the heterostructures of Cu₇S₄/MnS and Bi₂S₃/CuS as examples,the structure and interfacial properties were studied in depth,and the internal mechanism of their photocatalytic performance enhancement was clarified.The results of these studies provide a reference case and some theoretical support for the development of new metal sulfide photocatalytic materials and their composite heterostructure photocatalytic materials.