A synergetic oxide-chalcogenide heterostructure in metallic two-dimensional VSe₂ for the hydrogen-evolution reaction

Transition metal dichalcogenides (TMDCs) have emerged as promising catalysts for the hydrogen evolution reaction (HER) owing to their abundance, low cost, and electronic diversity. However, the role of oxidation in surface reactivity remains underexplored. In this study, we investigate the effects of oxidation on the catalytic properties of metallic VSe₂ films. Using ambient-pressure X-ray photoemission spectroscopy and density functional theory calculations, we revealed that pristine VSe₂ exhibits limited water adsorption and dissociation, leading to poor HER activity. However, controlled oxidation significantly improved water adsorption and dissociation. Notably, the mildly oxidized VSe₂ sample demonstrated enhanced HER performance, surpassing that of both the pristine and highly oxidized samples. Our findings indicate that while increased oxidation promotes better water dissociation, excessive oxidation weakens the binding of the intermediate species (*OH and *H). This study provides critical insights into the role of surface oxidation in optimizing HER activity and offers a pathway for improving TMDC-based catalysts.