Boosting water decomposition by sulfur vacancies for efficient CO photoreduction

The water oxidation reaction ( i.e. , OER) is one of the most challenging reaction steps in the overall photocatalytic CO 2 reduction, especially on metal sulfide photocatalysts. Herein, we first demonstrate that well-designed S-vacancies on SnS 2 atomic thin layers (denoted V S -SnS 2 ) can directly enhance water oxidation in the overall photocatalytic CO 2 reduction by promoting the formation of molecular O 2 . As a result, an average 8.2 times higher CO 2 photoreduction efficiency (CO evolution rate of 25.71 μmol g −1 h −1 ) was obtained on the champion V S -SnS 2 (23.07%) catalyst than that on the pristine SnS 2 catalyst (CO evolution rate of 3.14 μmol g −1 h −1 ) upon white light illumination. Two types of water decompositions (1660 cm −1 /1620 cm −1 ) that vary with the S-vacancy concentration were observed by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), demonstrating the key role of the S-vacancy-induced water decomposition configuration (1660 cm −1 ). Kinetic analysis confirms that the water decomposition reaction is the rate-determining step in the overall CO 2 photoreduction and was indeed accelerated on S-vacancy sites ( k 1660 = −0.033/ k 1620 = −0.021). Density functional theory (DFT) calculations suggest that S-vacancies facilitate the OER by lowering the energy barriers of intermediate transformations. As the rate-determining half reaction (OER) in the overall CO 2 photoreduction, the four-electron-involving OER on S-vacancy sites was favored, therefore facilitating the hole-elimination/proton release and unleashing the CO 2 reduction half reaction.