Hot‐Injection‐Driven Interfacial Stabilization of Lead‐Free Halide Perovskites via Sulfur‐Vacancy‐Rich MoS ₂ Toward Aqueous Electrocatalysis

Halide perovskites exhibit exceptional optoelectronic and catalytic properties but remain chemically fragile, rapidly degrading under moisture or alkaline conditions. Here, we introduce a synthesis‐driven interfacial engineering strategy that stabilizes the intrinsically unstable lead‐free perovskite by coupling nucleation and heterointerface formation within a single liquid‐phase hot‐injection process. During crystal growth, sulfur‐vacancy‐rich nanosheets conformally encapsulate nanocrystals, forming coherent 2D–3D p–n heterojunctions that suppress oxidation, limit moisture penetration, and enable directional charge transport. The resulting heterostructure exhibits a built‐in potential of 0.67 V, generating an internal electric field that promotes efficient charge separation and enhanced redox stability. Under alkaline conditions, the synthesis‐driven heterointerface sustains bifunctional water splitting in both freshwater and seawater for over 24 h without chlorine evolution or structural degradation. More broadly, this approach provides a generalizable synthesis paradigm for stabilizing moisture‐ and oxidation‐sensitive halide perovskites, enabling their integration into chemically robust and electronically coupled systems for aqueous electrocatalysis and other demanding environments.