Interface Synergy of Non-Conjugated Polymer and <i>In Situ</i> Phase-Transformation Metal Chalcogenide Enabling Superior Cooperative Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting offers a sustainable route for solar-to-hydrogen conversion but is hindered by sluggish charge transport, rapid photogenerated carrier recombination, and poor stability. Ternary indium-based sulfides have recently emerged as highly attractive candidates, owing to their broad spectral absorption, favorable band alignment, and superior resistance to photocorrosion. However, achieving efficient and durable metal chalcogenide (MC)-based photoanodes still relies heavily on precise interfacial engineering. Here, we rationally design CdIn2S4@bPEI/ZnIn2S4 (CIS@bPEI/ZIS) heterostructured photoanodes via surface modification with a non-conjugated polymer, branched polyethylenimine (bPEI), followed by an in situ cation-exchange transformation. The ultrathin bPEI interlayer, despite being intrinsically insulating, unexpectedly serves as a hole-withdrawing mediator that reinforces the heterojunction interface, thereby accelerating charge transport and prolonging the carrier lifetime. Subsequent coupling with ZnIn2S4 constructs well-defined heterostructures that further optimize charge-transfer pathways. As a result, CIS@bPEI/ZIS photoanodes deliver significantly enhanced charge separation and a superior PEC water oxidation performance. This work highlights a facile, non-conjugated polymer-assisted, phase-modulated strategy for constructing highly efficient MC-based heterostructure photoanodes, providing new insight into the utilization of insulating polymers for the rational design of artificial photosystems in solar-driven PEC water splitting.