In Situ Sulfidation Engineered Hydroxyl-Enriched SnS/TiO ₂ Heterointerface for Synergistic Charge-Proton Coupled Transfer in Photoelectrochemical Water Splitting

While heterojunction engineering is widely adopted for photoelectrode optimization in the field of photoelectrochemcial water splitting, the critical role of interfacial hydroxyl groups remains underexplored. We introduce an innovative in situ sulfidation strategy to construct hydroxyl-enriched SnS/TiO 2 nanoarrays, where surface hydroxyls act as bifunctional mediators for simultaneous charge and proton transfer. Comprehensive characterization reveals that sulfidation induces hydroxyl enrichment at the heterointerface, evidenced by FTIR peak shift and XPS quantification. Combined experimental and theoretical studies demonstrate the dual role of hydroxyls: (1) As electronic bridges reducing interface resistance and promoting photogenerated carrier separation and transport; (2) As proton relays lowering OER overpotential by 70 mV through facilitating *OH desorption (DFT). The synergistic effects yield exceptional performance: 1.99 mA/cm 2 at 1.23 V vs RHE with 46% IPCE at 350 nm. This work develops a hydroxyl-mediated interfacial engineering strategy, providing insights into charge-proton coupled transfer processes in heterojunction photoelectrodes.