S-O dual vacancy stoichiometry engineering of interfacial BIEF in Co₉S₈-MoO₂ heterojunctions for OER

Constructing heterojunctions with built-in electric fields (BIEF) is a promising strategy for developing high-efficiency oxygen evolution reaction (OER) electrocatalysts. However, precise regulation of BIEF intensity to overcome intrinsic material limitations remains a significant challenge. Herein, we introduce SO dual vacancy stoichiometry as a quantitative descriptor for BIEF modulation. By developing a H₂O₂-selective kinetic etching method, we achieved, for the first time, oxidation-induced desulfurization strategy control over the sulfur-to‑oxygen vacancy ratio (S_V:O_V) at the Co₉S₈-MoO₂ heterojunction interface. Both theoretical calculations(DFT) and experimental data(UPS and KPFM) show that an optimized S_V:O_V stoichiometry of approximately 1:1 maximizes the interfacial work function difference (ΔWF = 1.54 eV), thereby generating the strongest BIEF. This stoichiometry-tuned BIEF induces substantial charge redistribution, downshifts the d-band center of Co active sites, and enhances interfacial electron transfer from 0.953e to 0.992e. Consequently, the energy barrier of the OER rate-determining step (*OH → *O) is reduced by 0.06 eV. The resulting CS_V0.19-MO_V0.22@NF catalyst delivers an ultralow overpotential of 250 mV at 100 mA cm^-2 in 1 M KOH, along with excellent stability. This work establishes dual vacancy stoichiometry as a powerful tool for precisely tailoring interfacial electronic structures and provides a rational design strategy for advanced energy conversion catalysts.