Trapped Energy Level-Assisted Ultrafast Interfacial Charge Transfer in S-Scheme Heterojunctions

S-scheme heterojunctions enable spatial separation of photogenerated carriers, but remain constrained by interfacial electron transfer efficiency. Herein, oxygen vacancy defects are locally formed by removing fluorine atoms from the highly oriented (001) crystal plane of TiO₂ with fluorine doping. These defects can introduce additional doping energy levels, serving as the bench for S-scheme interfacial electron transfer in CdS/TiO₂. As verified by femtosecond transient absorption spectra, in situ irradiated X-ray photoelectron spectra, and theoretical computational simulations, the defect energy levels trap the localized electrons, which participate in the S-scheme interfacial transfer upon photoexcitation. The electron trapping process effectively prolongs the lifetime of photogenerated carriers and retards the charge recombination within each component. Besides, the binding energy shifts and surface potential changes detected by emerging in situ irradiated soft X-ray absorption spectroscopy and in situ irradiated Kelvin probe force microscopy provide conclusive evidence for the CdS/TiO₂ S-scheme heterojunction. Benefitting from trap energy level-assisted S-scheme electron transfer mechanism, the optimal CdS/TiO₂ composite exhibits superb photocatalytic H₂ production performance.