Enhancing Built-In Electric Fields through Sulfur-Vacancy Engineering in ZnIn ₂ S ₄ /CdS S-Scheme Heterojunctions for Accelerated Charge Separation and Photocatalysis

S-scheme heterojunctions can resolve the conflict between the limited tunability of the redox capability and light absorption range. However, improving the carrier transport efficiency across the heterojunction interface remains a significant challenge. Herein, sulfur vacancies (Sv) are introduced to optimize the Fermi level of ZnIn 2 S 4 (ZIS), thereby enhancing the intensity of the built-in electric field (BEF) at the Sv-ZnIn 2 S 4 /CdS (Sv-ZIS/CdS) S-scheme heterojunctions interface and providing a strong driving force for interfacial carrier transport. Notably, the BEF intensity of the Sv-ZIS/CdS heterojunction is 1.9 and 3.7 times than that of pristine ZIS and ZIS/CdS composite, respectively. Density functional theory (DFT) calculations, X-ray photoelectron spectroscopy (XPS), Femtosecond transient absorption spectroscopy (fs-TAS) and in situ XPS confirm the existence of a S-scheme electron transfer pathway at the Sv-ZIS/CdS heterojunction interface. Photocatalytic performance tests demonstrate that the degradation rate of tetracycline (TC) by the Sv-ZIS/CdS heterojunctions reaches 99.1% under visible light irradiation, which is significantly higher than that of the CdS, ZIS and ZIS/CdS-10. Photocatalytic mechanism experiments reveal that superoxide radicals ( • O 2 – ) is the primary active species during the TC degradation process. This study provides a valuable reference for the preparation of heterojunction composites for antibiotic wastewater treatment.