S-scheme unidirectional transmission of CdS-CuO heterojunction benefits for superior photocatalytic hydrogen evolution efficiency

The efficiency and stability of catalysts for photocatalytic hydrogen evolution (PHE) are largely governed by the charge transfer behaviors across the heterojunction interfaces. In this study, CuO, a typical semiconductor featuring a broad spectral absorption range, is successfully employed as the electron acceptor to combine with CdS for constructing a S-scheme heterojunction. The optimized photocatalyst (CdS-CuO2:1) delivers an exceptional hydrogen evolution rate of 18.89 mmol/(g·h), 4.15-fold higher compared with bare CdS. X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible diffuse reflection absorption spectroscopy (UV-vis DRS) confirmed the S-scheme band structure of the composites. Moreover, the surface photovoltage (SPV) and electron paramagnetic resonance (EPR) indicated that the photogenerated electrons and photogenerated holes of CdS-CuO2:1 were respectively transferred to the conduction band (CB) of CdS with a higher reduction potential and the valence band (VB) of CuO with a higher oxidation potential under illumination, as expected for the S-scheme mechanism. Density-functional-theory calculations of the electron density difference (EDD) disclose an interfacial electric field oriented from CdS to CuO. This built-in field suppresses charge recombination and accelerates carrier migration, rationalizing the markedly enhanced PHE activity. This study offers a novel strategy for designing S-scheme heterojunctions with high light harvesting and charge utilization toward sustainable solar-to-hydrogen conversion.