Electron-beam-induced sulfur vacancy engineering in hollow NiS2 spheres for enhanced hydrogen evolution

The engineering of surface defects on non-precious metal electrocatalysts is an effective strategy for accelerating the sluggish kinetics of the hydrogen evolution reaction (HER). In this work, hollow NiS 2 spheres were synthesized by a hydrothermal method, followed by a subsequent rapid electron beam annealing (EBA) treatment to generate surface-localized sulfur vacancies while preserving the hollow architecture. X-ray photoelectron spectroscopy, electron energy-loss spectroscopy, and atomic-resolution scanning transmission electron microscopy provide complementary evidence for EBA-induced surface sulfur vacancies and associated changes in the local electronic environment of Ni. The sulfur vacancy engineered NiS 2 ( S v -NiS 2 ) catalyst exhibits a low overpotential of 74.2 mV at 10 mA cm -2 , significantly lower than that of thermally treated NiS 2 ( T -NiS 2 ) (146.4 mV) and pristine NiS 2 (384.1 mV). Electrochemical impedance spectroscopy and C dl -normalized polarization measurements indicate that the improved HER performance is associated with reduced charge-transfer resistance and enhanced intrinsic activity. S v -NiS 2 also maintains stable HER performance for 72 h under the tested conditions. Density functional theory (DFT) calculations demonstrate that sulfur vacancy configurations produce distinct local charge redistribution and electronic structures, with selected vacancy-adjacent Ni sites exhibiting near-thermoneutral hydrogen adsorption free energies. These findings support a relationship between surface sulfur deficiency, local electronic reconstruction, and HER kinetics. Overall, this work demonstrates rapid electron-beam treatment as a promising post-synthesis approach for modifying transition-metal sulfide surfaces for HER electrocatalysis.