Synergistic Modulation of ZnIn2S4 by NiS Mediator and Ag2S Heterojunction: In-Situ Interfacial Engineering and Sulfur Vacancies for Efficient Seawater Splitting

Abstract Designing efficient charge transfer channels and modulating surface active sites are pivotal for enhancing photocatalytic water splitting. Herein, we report a ternary NiS-ZnIn2S4/Ag2S (NiS-ZIS/Ag2S) heterostructure fabricated via a facile solvothermal method followed by a photo-assisted impregnation process. A key feature of this synthesis is the in-situ construction of both NiS and Ag2S on ZnIn2S4 nanoflowers through interfacial Ni–S and Ag–S bonds, respectively. In this ternary system, NiS serves as the primary electron sink and hydrogen evolution reaction (HER) active site, efficiently capturing photogenerated electrons from ZnIn2S4 and reducing water to H2. Meanwhile, Ag2S plays an auxiliary yet important role as a hole-extraction center: through the Type-I Ag2S/ZnIn2S4 heterojunction, Ag2S rapidly removes photogenerated holes from ZnIn2S4, suppressing charge recombination and thereby maximizing electron availability for NiS. The in-situ formation of Ni–S and Ag–S bonds simultaneously introduces moderate surface sulfur vacancies (SV), which modulate the local electronic structure of ZnIn2S4, facilitating interfacial charge transfer and enhancing the intrinsic HER activity of NiS sites. Notably, under simulated solar light irradiation in pure seawater, the optimal NiS-ZIS/Ag2S-0.5 catalyst achieves an outstanding H2 evolution rate of 6969 μmol·g–1·h–1, approximately 5.8 times higher than that of pristine ZnIn2S4 (1200 μmol·g–1·h–1). Under pure seawater conditions, the apparent quantum efficiency (AQE) reaches 7.79%, 6.48%, and 4.55% at 380, 400, and 420 nm, respectively. Comprehensive photoelectrochemical measurements, DFT calculations, and in-situ XPS validate the directional charge migration pathway: electrons migrate from ZnIn2S4 to NiS for H2 evolution, while holes transfer to Ag2S for consumption. This work demonstrates that, in this specific ternary system, the Type-I component (Ag2S) is converted into a selective hole extractor through the introduction of a strongly competitive electron acceptor (NiS), thereby overcoming the conventional drawbacks of Type-I heterojunctions and achieving a highly efficient photocatalyst for solar-driven seawater splitting.