Preparation Of ZnS/Bi₂S₃-Based Composite Photocatalyst And Its Performance For Carbon Dioxide Reduction

With the development of industrial technology for the amount of fossil fuels that human used has also increased.Excessive use of fossil energy had caused an increase in atmospheric CO₂content,which leading to increasingly serious issue of energy crisis and global warming.To solve this problem,it is promising that the utilization of solar energy directly converts CO₂ to a series of high-value hydrocarbon fuels such as CH₄,CO and C₂H₅OH.It significantly contributes to relieve the global energy crisis.Using plentiful renewable energy as a driving force can effectively decrease the amount of fossil fuel energy we use.Moreover,it also lower CO₂ emissions to solve the crucial issue for the global warming.As a semiconductor photocatalysts,Metal sulfides are particularly promising for the photocatalytic reduction of CO₂ due to their special photoelectric characteristics,suitable energy band locations,and effective light respone ability.In this work,the electronic structure of the Zn S/Bi₂S₃ material was optimized through interface modulation,defect engineering,and heterostructure to increase the active sites on the surface of Zn S-Bi₂S₃ and boost the photogenerated electron transfer speed.This improved the activity of Zn S-Bi₂S₃and selectivity for photocatalytic CO₂ reduction.By using XPS,UV-vis,EIS,PL,and in situ FTIR characterization to determine the electronic structure,photoelectric properties,and potential reaction mechanism of the Zn S-Bi₂S₃.In this case,we proposed new methods for the design of photocatalysts with high reduction and selectivity.The main research contents of this thesis are as follows:（1）Zn S-Bi₂S₃（ZBS）composite photocatalysts with compact heterogeneous structure and interfacial defects were successfully prepared by a one-step hydrothermal method.The synthesized Zn S-Bi₂S₃ has a heterogeneous structure that can promote photogenerated electron transfer.The unique interfacial defect structure is formed due to the lattice mismatch between the base layer and epitaxial layer,which guides electrons to reach the reaction site quickly to react with CO₂.Under the combined effect of interfacial defects and heterogeneous structure,the charge transfer rate during the photocatalytic reaction is effectively enhanced,which effectively suppresses the secondary recombination of photogenerated electron-hole pairs,provides surface sites for the CO₂reaction,promotes the reaction of photogenerated electrons with CO₂,and thus improves the photocatalytic performance of the material.Density flooding theory（DFT）calculations show that the interaction between the heterojunction interface and interfacial defects enhances the reaction kinetics,reduces the reduction potential of CO₂,and promotes the reaction.Under the combined effect of interfacial defects and heterostructure,photogenerated electrons can be effectively transferred to Bi₂S₃ to participate in the reduction reaction,and the interfacial defects effectively reduce the activation energy barriers of COOH^*and CHO^*,the key intermediates in the reduction of CO₂,enhancing the overall photocatalytic reduction activity.Without any co-catalyst,the conversion yields of CO₂ to CO were 84μmol·g^-1 and 62.2μmol·g^-1 for CH₄ after 5 h of light exposure.（2）The Zn S-Bi₂S₃（S_v/Bi_v-ZBS）composite with double defects(cationic defects（Bi vacancy）and anionic defects（S vacancy）（Bi_v/S_v）was effectively created by using chemical etching.Double defect site increases the surface-active sites and accelerated photogenerated electron transfer.The S_v/Bi_v-ZBS heterojunction possesses more surface-active sites and fast charge transfer rate than monolithic Bi₂S₃ and Zn S due to the synergistic effect of S_v/Bi_v double defects.The surface flaws of S_v/Bi_v-ZBS heterojunction can trap photogenerated electrons,to prevent recombination of photogenerated electron-hole pairs.Then the CO₂ binds to photogenerated electrons can reduce CO₂.The in-situ FT-IR was used to clarify the pathway of CO₂ photoreduction.The S_v/Bi_v-ZBS reduction performance was significantly higher than that of the monolithic one.After 5 hours solar simulation,the yields of CO(152.62μmol·g^-1)and CH₄(57.89μmol·g^-1),in presence of S_v/Bi_v-ZBS,which were 2.08 and 2.9 times higher than those of Zn S and 1.95 and 2.17 times higher than those of Bi₂S₃,respectively.Based on analysis,increasing the reaction active site and accelerating the electron transfer rate are two crucial factors for improving the photocatalytic reduction performance,and defect modification on the material surface is beneficial to enhancing the photocatalytic reduction process.（3）Zn S/Bi₂S₃/Cd S（Zn Bi Cd S）ternary nanocomposite photocatalysts with double heterostructures were prepared by in situ hydrothermal method.The photocatalytic performance of ternary nanocomposite was adjusted by tunning the Cd source was used to investigate their photocatalytic CO₂ reduction performance.We found that the composite created a type-I and type-Z ternary heterostructure,which can ensure a good redox ability of the photogenerated carriers on the Zn S conduction band for the reduction process.The Zn Bi Cd S ternary heterostructure can effectively transfer photogenerated electrons to the surface and react with CO₂ under influence of the double heterostructure,and the Z-type heterostructure between Cd S and Bi₂S₃ suppressed the photogenerated carrier combination.The stronger photocurrent density also indicates the enhanced of photogenerated carriers separation.The formation of Zn-Bi-Cd ternary metallic reactive sites was controlled by tuning the additive amount of Cd elements,and the reaction mechanism was deduced according to in situ FTIR spectra.Thereby,the enhancing the interaction between CO^*reaction intermediate and active site.It significantly improves the selectivity of CO₂ to CH₄photocatalytic reduction.Zn Bi Cd S shows 94.2%selectivity for CH₄ product.It is 14.1,23.4,and5.54 times higher than those of Zn S,Bi₂S₃,and Cd S,respectively.