Defect Engineering of ZnIn2S4 Photocatalysts for Enhanced Hydrogen Evolution Reaction

ZnIn2S4, a visible-light-responsive layered sulfide photocatalyst with a suitable bandgap (~2.4 eV), exhibits considerable potential for the photocatalytic hydrogen evolution reaction (PHER) due to its low toxicity, excellent stability, and appropriate band alignment. Nevertheless, its practical deployment is limited by inherent issues such as rapid charge carrier recombination, scarce surface-active sites, and slow oxidation kinetics. Defect engineering strategies—including sulfur, zinc, and indium vacancies, as well as heteroatom doping—have been developed to mitigate these shortcomings. This review not only summarizes recent advances in these strategies but also elucidates the fundamental physicochemical mechanisms behind the enhanced photocatalytic performance. A systematic quantitative evaluation is presented, highlighting improvements in critical performance metrics such as hydrogen evolution rate, light absorption range, apparent quantum yield (AQY), and charge separation efficiency. Furthermore, the review offers a critical perspective on the current state of defect-engineered ZnIn2S4 systems. Promising future research pathways are outlined, with emphasis on atomic-precision synthesis and operando characterization techniques. Finally, we discuss persistent challenges in the field, including reproducibility in synthesis, long-term operational stability, and scalability toward industrial hydrogen production.