Abstract:The utilization of renewable solar energy to convert CO2 into high-value-added chemicals and fuels is of great significance for mitigating the greenhouse effect and achieving carbon neutrality. Although ZnIn2S4 (ZIS) is a promising photocatalyst for CO2 reduction, its practical application is severely limited by rapid photogenerated carrier recombination and photocorrosion. Constructing heterojunctions with g-C3N4 (CN) is an effective strategy; however, conventional methods often result in poor interfacial contact and complex reaction systems. In this study, an electrostatic self-assembly strategy was employed to combine negatively charged ultrathin ZIS nanosheets with protonated, positively charged g-C3N4 (pCN) at pH 3, forming a ZnIn2S4/pCN (ZIS/pCN) heterojunction with a 2D/2D face-to-face configuration and enabling controllable interface engineering. Without the use of sacrificial agents or noble-metal cocatalysts, the optimal ZIS/pCN（5∶3） composite exhibited significantly enhanced photocatalytic activity for CO2 photoreduction in a gas-solid reaction system, achieving CO and CH4 evolution rates of 14.72 and 1.42 μmol·g−1·h−1, respectively, which are 10.2 and 7.5 times higher than those of pure ZIS, along with good photostability. Among the composites, ZIS/pCN (5∶3) also showed the highest CH4 selectivity, indicating that the optimized 2D/2D heterojunction kinetically favors a multi-electron transfer pathway. This composite effectively addresses the intrinsic limitations of single-component materials, including severe charge recombination and active-site masking in ZIS, as well as the poor interfacial contact in conventional heterojunctions. Optical characterizations, X-ray photoelectron spectroscopy (XPS), and photoelectrochemical analyses demonstrate that the heterojunction broadens visible-light absorption and establishes a built-in electric field through interfacial electron interactions. Upon light irradiation, both pCN and ZIS are excited to generate electron-hole pairs. Driven by the built-in electric field, charge carriers follow a possible S-scheme transfer pathway at the interface, in which high-energy electrons retained in the conduction band of pCN reduce CO2 to CO and CH4, while holes in the valence band of ZIS participate in water oxidation to supply protons for the reaction. The 2D/2D face-to-face configuration provides a large and intimate contact interface, serving as a continuous pathway for charge transfer and significantly reducing transport resistance. This unique heterostructure markedly enhances charge separation efficiency and reaction kinetics. This work provides a reliable interface engineering strategy for the design of efficient and stable photocatalysts for CO2 reduction, highlighting the synergistic effects of precisely constructed heterojunctions in boosting photocatalytic performance. Close-