Adjusting Nanostructures Of CdS-based Composites For Photocatalytic CO₂ Reduction

Photocatalytic reduction of CO2 to hydrocarbon fuel is a promising avenue to reduce CO2 emission and to produce value-added chemical fuels, which is one of the effective solutions to both global warming and energy shortage concerns. CO2 could be converted into renewable hydrocarbons with a appropriate bandgap semiconductor. Generally, semiconductor with narrow bandgap could absorb more sunlight to generated photoexcited electron and hole, but photocatalytic reaction demand sufficient redox ability for photocatalysts. The low conversion efficiency of solar energy and low selectivity for the reduction products are the critical issues facing the semiconductor photocatalyst. Therefore, it is desirable to develop a photocatalyst with highly selective CO2 photoreduction activity under the visible light. Cadmium sulfide (CdS) is widely used in the photocatalytic fields including water splitting, degradation of organic contaminants,and CO2 reduction,because it is sensitive to visible light, cheap and effective. However, it is toxic and unstable under light. The adjusting of CdS nanostructurescan could affect photogenerated chargecarrier, and thus affects photocatalytic efficiency and products. In this dissertation, a series of CdS-based nanocomposites were fabricated such as CdS/reduced graphene oxide/TiO2 and ZnxCd1-xS solid solution for the CO2 reduction. The details are summarized as follows:(1) An all-solid-state Z-scheme was constructed for photocatalytic reduction of CO2, consisting of CdS nanospheres (CdS) and TiO2 nanoparticles as photocatalysts, and RGO as a solid electron mediator. In this Z-scheme system, the photogenerated electrons from conductor band of TiO2 transfer to RGO, and then recombine with existing holes of CdS, allowing that the photo-generated electrons enrich on the CdS semiconductor and holes on TiO2.The photocatalystic efficiency of the Z-scheme system exhibits remarkable enhancement relative to pure CdS, CdS/TiO2, and CdS/RGO.(2) A series of ZnxCd1-xS solid solutions with the same spherical morphology was successfully synthesized by co-precipitation method. ZnxCd1-xS solid solutions show enhanced efficiency for CO2 photoreduction under visible light irradiation by adjusting the band structure. When the mole ratio of Zn to Cd is 4:1 and 1:4, the as-prepared Zn0.8Cd0.2S sample exhibits the highest efficiency under visible light irradiation. The major products of CO2 photoreduction are CH3OH and CH3CHO and only the Zn0.8Cd0.2S sample with a wide band gap of 2.81 eV can promote the formation of C2H5OH. The high reduction capacity of photogenerated electrons on Zn0.8Cd0.2S sample promotes the formation of reaction intermediates,which can dimerize to form C2H5OH.