Abstract: Photocatalytic CO₂ reduction reaction (CO₂ RR) is usually limited by the weak adsorption capacity of the catalyst for CO₂ as well as the low product selectivity. In this paper, the electronic properties and catalytic reactive sites of CdIn₂S₄ surface atoms are modulated by Cu doping. Experimental and theoretical calculations show that the coordination environment around the Cd atoms changes due to the charge balance effect after Cu doping, which induces the formation of sulfur vacancies. The sulfur vacancies not only enhanced the adsorption capacity of CO₂, but also acted as charge-enriched centres to provide electrons to the Cu reactive sites and stabilized the reaction intermediates, which led to the highly selective generation of CH₄. Cu-doped CdIn₂S₄ catalysts exhibited excellent performance in photocatalytic reduction of CO₂, and the CH₄ yield of 47.01 μmol∙g^-1∙h^-1 with a selectivity of 97.8%. In this study, the synergistic interaction between sulfur vacancies and metal reactive sites enhances the adsorption and activation of CO₂ and effectively regulates the charge transfer process, providing a new strategy for optimising the performance of semiconductor photocatalysts.

Key words: Cu-doped, Sulfur vacancy, CO₂ reduction, Excellent selectivity of CH₄