Effect of oxidation on water adsorption and dissociation in epitaxial VSe₂ thin films

Two-dimensional transition metal dichalcogenides (TMDC) have been studied for their remarkable hydrogen evolution reaction (HER) properties and have gained significant attention as earth-abundant and environmentally friendly materials for hydrogen supply. The HER process of water splitting involves the adsorption and dissociation of water molecules, so it is essential to understand these characteristics in order to elucidate the underlying reaction mechanism. However, most of the HER results in TMDCs have been obtained from exfoliated samples or powders, which makes it difficult to control impurities and limits the rigorous analysis of the intrinsic water absorption and dissociation characteristics of the TMDC materials. We have successfully fabricated epitaxial group V TMDCs such as VSe₂ using molecular beam epitaxy (MBE). We then tuned the oxidation of the films and experimentally investigate the water adsorption and dissociation on these epitaxial TMDC thin films by analyzing the chemical bonding characteristics measured by ambient pressure x-ray photoelectron spectroscopy (APXPS) under water atmosphere. With our theoretical analysis, we propose that oxygen sites in TMDC thin films play a critical role in the adsorption and dissociation of water molecules. Our results can provide reference points for water adsorption characteristics in relation to the oxidation states of TMDC thin films, as well as insights into a way to treat surface for development of enhanced water splitting performance.