Tuning interfacial sulfur vacancy level of ZnIn₂S₄ for boosting photocatalytic water purification

Defect engineering provides an effective strategy for coupling photogenerated charge dynamics with interfacial redox reactions. However, the concentration-dependent role of sulfur vacancies (V_S) in ZnIn₂S₄ (ZIS) photocatalysis remains poorly understood. Herein, ZIS photocatalysts with systematically tunable V_S concentrations were synthesized through a one-step solvothermal method by regulating the ethylene glycol/water volume ratio. The optimized V_S-rich ZIS (V_S-ZIS) achieved nearly complete degradation of bisphenol An under visible light irradiation, with an apparent rate constant of 0.039 min^-1, 5.6-fold that of pristine ZIS. Systematic characterizations revealed that V_S accelerated interfacial electron transfer, suppressed bulk charge recombination, and prolonged carrier lifetimes. Mechanistically, V_S served as dual-function sites for electron trapping and molecular oxygen activation, thereby coupling enhanced charge separation with efficient superoxide radical generation. Moreover, V_S-ZIS exhibited broad pollutant applicability, strong resistance to coexisting inorganic ions and humic acid, wide pH adaptability, and consistently high activity in multiple real water matrices. Toxicity prediction and mung bean germination bioassays further confirmed a reduction in the overall ecological risk and phytotoxicity of the treated solution. This work establishes the relationship between V_S concentration, charge dynamics, and reactive species generation, providing mechanistic guidance for the rational design of ZIS-based photocatalysts for water purification.