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