Programming a buried interfacial rate hierarchy in a TiO2/CdS S-scheme photocatalyst for selective carrier preservation.

Buried heterointerfaces in particulate photocatalysts are commonly optimized through static band alignment and defect minimization, yet carrier fate is determined by a picosecond competition between productive interfacial sorting and intrinsic localization and trapping. Here, sulfur-supply intensity programs a TiO₂/CdS buried interface while largely preserving bulk CdS loading. X-ray photoelectron spectroscopy defines a chemically accessible interface window through sulfur speciation and surface stoichiometry. Within this window, TC-3-S₆ is electrostatically selected by the largest measured work-function offset (0.19 eV) and the strongest surface photovoltage response, delivering an H₂-evolution rate of 10.1 mmol·g-1·h-1 under simulated AM 1.5G irradiation, 12.6-fold that of pristine CdS. Reaction-medium femtosecond transient absorption resolves a reproducible hierarchy in the early CdS-centered transient response, with effective few-picosecond timescales of approximately 4 ps for pristine CdS, 3 ps for a composition-matched ex situ physical mixture, and 2 ps for the in situ programmed heterojunction. Integrated spectral-area and band-ratio analyses reproduce the same ordering, placing interface-associated redistribution within the native CdS few-picosecond competition window under matched excitation. Enhanced superoxide- and hydroxyl-radical signals, CdS-selective Pt photodeposition, and electrochemical transport measurements show that this kinetic advantage is accompanied by spatial charge separation while retaining both reduction and oxidation capabilities. Together, these orthogonal measurements support redox-preserving carrier sorting within an S-scheme framework. The resulting design principle is transferable: create a chemically accessible buried junction, maximize interfacial electrostatic bias, and ensure that carrier sorting competes effectively within the intrinsic picosecond-loss window, preserving redox-active carriers for surface chemistry.