Abstract: Fine-tuning the interfacial electronic interaction and surface reactivity of S-scheme heterojunctions is critical for advancing their photocatalytic performance. This study employs density functional theory calculations to systematically investigate the effects of transition metal (TM = Cr, Mn, Fe, Co, and Ni) doping at distinct sites of a CdS/ZnO S-scheme heterojunction: the surface (TM_s), the interface (TM_i), and co-doping at both sites (TM_s+i). The results demonstrate that all doping configurations concurrently enhance both interfacial electron transfer and the hydrogen evolution reaction dynamics. The augmentation of electron transfer across the interface is primarily driven by TM doping at the interface, which reduces the work function of CdS and enlarges the Fermi level discrepancy with ZnO, leading to an enhancement trend of TM_s+i > TM_i > TM_s. Conversely, the optimization of hydrogen adsorption free energy (ΔG_H*) is chiefly governed by surface TM doping, which downshifts the p-band center of S atoms and weakens the S-H bond, resulting in an improvement trend of TM_s+i > TM_s > TM_i. Remarkably, the co-doping configuration exhibits a pronounced synergistic effect, outperforming any single-site doping in optimizing both properties. Furthermore, a clear periodic trend is identified: the promotional effect of TM doping, from Cr to Ni, progressively diminishes for both charge separation and surface reaction, which is linked to the increasing work function and S p-band center. This work highlights the significant potential of a multi-site doping strategy for the synergistic engineering of charge transfer and surface reactions in S-scheme heterojunctions, offering valuable theoretical insights for the precise design of high-efficiency photocatalysts.

Key words: S-scheme heterojunctions, Transition metals, Doping sites, Interfacial electron transfer, Hydrogen adsorption, Density functional theory