Improved Photocatalytic CO₂ Reduction via Engineering Zinc Vacancies in S-Scheme ZnO/ZnIn₂S₄ Heterostructures

Defect engineering in semiconductor heterojunctions offers a promising avenue for enhancing the photocatalytic activity. This study demonstrates the rational design of an S-scheme ZnO/ZnIn 2 S 4 (ZIS) heterojunction with enriched zinc vacancies (V Zn ) for efficient photocatalytic CO 2 reduction. Two distinct morphologies of ZnO/ZIS composites were synthesized by modulating the sulfur source during ZIS nucleation, resulting in different V Zn concentrations. The composite with a higher V Zn concentration (B-ZnO/ZIS) exhibited significantly enhanced photocatalytic performance, achieving a CO yield of 233 μmol g –1 under visible light irradiation, which is 27 times higher than that of pristine ZnIn 2 S 4 . This remarkable enhancement is attributed to the synergistic effect of abundant V Zn and the S-scheme heterojunction, promoting efficient charge separation and transfer, as evidenced by a series of photoelectrochemical and physicochemical characterizations. The S-scheme charge transfer mechanism was further corroborated by X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. In situ Fourier-transform infrared (FTIR) spectroscopy revealed the surface intermediates involved in the CO 2 reduction process over B-ZnO/ZIS. This work provides a new strategy for designing high-performance photocatalysts by combining defect engineering and heterojunction construction for efficient CO 2 conversion.