Boosting water decomposition by sulfur vacancies for efficient CO2 photoreduction

Abstract

The water oxidation reaction (i.e., OER) is one of the most challenging reaction steps in the overall photocatalytic CO₂ reduction, especially on metal sulfide photocatalysts. Herein, we first demonstrate that well-designed S-vacancies on SnS₂ atomic thin layers (denoted V_S-SnS₂) can directly enhance water oxidation in the overall photocatalytic CO₂ reduction by promoting the formation of molecular O₂. As a result, an average 8.2 times higher CO₂ photoreduction efficiency (CO evolution rate of 25.71 μmol g⁻¹ h⁻¹) was obtained on the champion V_S-SnS₂ (23.07%) catalyst than that on the pristine SnS₂ 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 CO₂ photoreduction and was indeed accelerated on S-vacancy sites (k₁₆₆₀ = −0.033/k₁₆₂₀ = −0.021). Density functional theory (DFT) calculations suggest that S-vacancies facilitate the OER by lowering the energy barriers of intermediate transformations.