Mechanistic Insights Into In‐Phase SrTiO ₃ (110)/ZnIn ₂ S ₄ (102) S‐Scheme Heterojunctions for Multifunctional Hydrogen Evolution

ABSTRACT Rational design of facet‐engineered heterojunctions is critical for advancing multifunctional hydrogen evolution through various technologies that rely on sustainable resources. Herein, we report the synthesis of in‐phase SrTiO ₃ (110)/ZnIn ₂ S ₄ (102) facet‐rich heterojunctions, featuring a highly aligned interface that simultaneously enhances charge separation and boosts hydrogen evolution activity across photochemical, electrochemical, and photoelectrochemical processes. Detailed structural and spectroscopic analyses, including optical studies, XPS, EIS, Mott‐Schottky measurements, electron localization function, and difference charge density mapping, reveal an S‐scheme charge transfer pathway facilitated by strong interfacial coupling and favorable band alignment. The exposure of S‐rich ZnIn ₂ S ₄ facets provides optimal hydrogen adsorption sites, while the (110)/(102) facet synergy promotes directional charge transport and visible‐light‐driven activity. Consequently, the optimized 7.5 wt.% ZnIn ₂ S ₄ ‐SrTiO ₃ (7.5ZIS) heterostructure achieves a ∼10‐fold increase in photochemical H ₂ evolution compared to pristine SrTiO ₃ and ZnIn ₂ S ₄ , with an AQY of 35.7% at 450 nm. Additionally, it delivers impressive HER current densities at lower overpotential values in electrochemical (651 mV) and photoelectrochemical (584 mV) processes, without utilizing expensive conductive substrates like Ni foam. This study offers deep mechanistic insight into facet‐selective S‐scheme design and positions oxide‐sulfide heterojunctions as promising candidates for scalable H ₂ production.