Origin of Enhancing the Photocatalytic Performance of TiO2 for Artificial Photoreduction of CO2 through a SiO2 Coating Strategy

Artificial photosynthesis that mimics the natural photosynthetic protocol is now regarded as a promising candidate for chemical conversion of CO2 to renewable solar fuels. In this work, core-shell structured TiO2@SiO2 composites have been synthesized via a simple sol-gel method under ambient temperature and pressure and applied to photocatalytic reduction of CO2 with H2O in the gas-phase under simulated solar light irradiation. The results show that compared with bare TiO2, TiO2@SiO2 composites exhibit significantly enhanced adsorption capacity for CO2 and facilitate photogenerated charge carrier separation and thereby enhanced photoactivity toward CO2 reduction. Although the insulating nature of SiO2 inhibits the charge injection from inner TiO2 core through the silica layer to the outer surface, the separation efficiency of charge carriers within the inner pore structure of TiO2@SiO2 is facilitated due to the as-formed Ti-O-Si bonds. In particular, TiO2@30%SiO2, which balances the combined influence of CO2 adsorption and charge carrier separation, acquires the best photoactivity and high selectivity for CO formation. This can be ascribed to the enriched density of adsorbed CO2 and relatively lower electron density on the reactive sites of the samples. In addition, in contrast to bare TiO2, the competitive process of H-2 formation is greatly inhibited over TiO2@SiO2 composites. It is hoped that our work could inspire ongoing interest in utilizing the SiO2 coating method as well as other proper core-shell strategies to tune the activity and selectivity of semiconductor-based materials for artificial photoreduction of CO2 to value-added solar fuels.