Modulating electron density of vacancy site by single Au atom for effective CO$_{2}$ photoreduction

The surface electron density significantly affects the photocatalytic efficiency, especially the photocatalytic CO$_{2}$ reduction reaction, which involves multi-electron participation in the conversion process. Herein, we propose a conceptually different mechanism for surface electron density modulation based on the model of Au anchored CdS. We firstly manipulate the direction of electron transfer by regulating the vacancy types of CdS. When electrons accumulate on vacancies instead of single Au atoms, the adsorption types of CO$_{2}$ change from physical adsorption to chemical adsorption. More importantly, the surface electron density is manipulated by controlling the size of Au nanostructures. When Au nanoclusters downsize to single Au atoms, the strong hybridization of Au 5d and S 2p orbits accelerates the photo-electrons transfer onto the surface, resulting in more electrons available for CO$_{2}$ reduction. As a result, the product generation rate of Au$_{SA}$/Cd$_{1-x}$S manifests a remarkable at least 113-fold enhancement compared with pristine Cd$_{1-x}$S.