Selective visible-light-driven photocatalytic CO2 reduction to CH4 mediated by atomically thin CuIn5S8 layers

Due to the large number of possible products and their similar reduction potentials, a significant challenge in CO2 photoreduction is achieving selectivity to a single product while maintaining high conversion efficiency. Controlling the reaction intermediates that form on the catalyst surface through careful catalyst design is therefore crucial. Here, we prepare atomically thin layers of sulfur-deficient CuIn5S8 that contain charge-enriched Cu–In dual sites, which are highly selective towards photocatalytic production of CH4 from CO2. We propose that the formation of a highly stable Cu–C–O–In intermediate at the Cu–In dual sites is the key feature determining selectivity. We suggest that this configuration not only lowers the overall activation energy barrier, but also converts the endoergic protonation step to an exoergic reaction process, thus changing the reaction pathway to form CH4 instead of CO. As a result, the CuIn5S8 single-unit-cell layers achieve near 100% selectivity for visible-light-driven CO2 reduction to CH4 over CO, with a rate of 8.7 μmol g−1 h−1. Many different molecules can form during photocatalytic reduction of CO2, so identifying catalyst structure–product selectivity relationships is vital. Here, the authors find that sulfur-deficient CuIn5S8 is highly selective to CH4 and suggest that the presence of Cu–In binding sites is key to this behaviour.