Preparation Of Cadmium Sulfide-based Composite Photocatalysts For Photocatalytic CO₂ Reduction

The large consumption of fossil fuels leads to increasing CO₂ emissions,and the energy crisis and environmental pollution problems are becoming more and more serious.Photocatalytic conversion of CO₂ into valuable solar fuel by solar energy is considered to be one of the most promising ways to solve these two problems.It can not only alleviate the environmental problems caused by CO₂,but also partly replace fossil fuels.The development and utilization of photocatalysts is the most important part to realize efficient photocatalytic reduction of CO₂ by solar energy.At present,more and more research is devoted to the development of efficient photocatalysts,especially visible light responsive photocatalysts.Semiconductor photocatalysts with visible light response,including BiVO₄,BiOI and CdS,have been considered as promising photocatalysts for CO₂ reduction.However,their CO₂ conversion efficiency is not impressive because it is difficult for single photocatalyst to meet the problems such as low separation and high recombination efficiency of photogenerated charges.Meanwhile,single photocatalyst can not provide the ability of CO₂ reduction and water oxidation simultaneously.Therefore,in order to improve the CO₂ conversion efficiency,this project studied the modification of single photocatalyst materials using BiVO₄ and BiOI as the matrix materials,and compounded with CdS to construct Z-scheme heterojunction structure to enhance the photocatalytic reduction of CO₂.?1?BiVO₄ nanosheets were synthesized by using sodium dodecyl benzene sulfonate?SDBS?as surfactant-assisted hydrothermal method.The thickness of BiVO₄ nanosheets was adjusted by adjusting the pH of reaction precursor solution.The pH value was increased from 2.5 to 8.0,and the thickness of BiVO₄ nanosheets was increased from 15³0 to 80¹10 nm.Then,CdS nanoparticles were deposited on the surface of BiVO₄ nanosheets.A series of CdS/BiVO₄ nanocomposites were prepared,and the composites were characterized.The results show that the reduction efficiency of CdS/BiVO₄ composite photocatalyst increases with the decrease of the thickness of BiVO₄ nanosheets.The maximum yields of CH₄ and CO for CdS/BiVO₄ nanocomposite with 55⁷5 nm BiVO₄ are 2.98 and 1.31?mol·g^-1,respectively,after 5 h irradiation.By contrast,the optimal yields of CH₄ and CO for its counterpart with 15³0 nm BiVO₄ attain 8.73 and 1.95?mol·g^-1,respectively.Apart from the increase of the yields,the CdS/BiVO₄ nanocomposites with thinner BiVO₄ nanosheets are more conducive to the formation of CH₄.The enhanced photoactivity of the composites is attributed to the efficient separation of the photoexcited electron-hole pairs via Z-scheme transfer mode.?2?BiOI nanosheets were synthesized by hydrothermal method.A series of CdS/BiOI composites by depositing CdS nanoparticles on the surface of BiOI nanosheets.The morphological,crystalline,band alignment,optical and electronic properties of the prepared samples are intensively investigated and their photoreduction CO₂ activities were performed under visible light irradiation??>400 nm?.The results show that the composites exhibit higher photoreduction CO₂activity than CdS and BiOI.The yields of CO and CH₄ for the optimal composite after 3 h irradiation are 3.32 and 0.54?mol·g^-1,respectively,while those for pure CdS are 0.65 and 0.10?mol·g^-1.Z-scheme charge transfer mode is the main reason for the improvement of photocatalytic activity of composites.At the same time,the construction of Z-scheme heterojunction composite effectively prevented the photocorrosion of CdS and thus,the photocatalyst showed excellent cyclic stability.