Selectivity control of photocatalytic CO2 reduction over ZnS-based nanocrystals: A comparison study on the role of ionic cocatalysts

Taking copper doped ZnS (ZnS:Cu) nanocrystals as the main body of photocatalyst, the influence of different base transition metal ions (M2+ = Ni2+, Co2+, Fe2+ and Cd2+) on photocatalytic CO2 reduction in inorganic reaction system is investigated. Confined single-atom Ni2+, Co2+, and Cd2+ sites were created via cation-exchange process and enhanced CO2 reduction, while Fe2+ suppressed the photocatalytic activity for both water and CO2 reduction. The modified ZnS:Cu photocatalysts (M/ZnS:Cu) demonstrated tunable product selectivity, with Ni2+ and Co2+ showing high selectivity for syngas production and Cd2+ displaying remarkable formate selectivity. DFT calculations indicated favorable H adsorption free energy on Ni2+ and Co2+ sites, promoting the hydrogen evolution reaction. The selectivity of CO2 reduction products was found to be sensitive to the initial intermediate adsorption states. *COOH formed on Ni2+ and Co2+ while *OCHO formed on Cd2+, favoring the production of CO and HCOOH as the main products, respectively. This work provides valuable insights for developing efficient solar-to-fuel platforms with controlled CO2 reduction selectivity.