Reactivity of Sulfur Vacancy-Rich MoS2 to Water Dissociation

Exposed Mo atoms on the surface of MoS2 can catalyze certain useful chemical reactions, including the dissociation of water to produce hydrogen. However, a comprehensive understanding of water reactivity on defective MoS2 surfaces remains elusive. Here, we use in situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to investigate water dissociation reactions on MoS2 surfaces before and after Ar+ ion beam bombardment. To make the surfaces reactive, we treated them with Ar+ ion beam sputtering first, which created exposed Mo sites. Using ultraviolet photoelectron spectroscopy (UPS) and density of states calculations conducted using density functional theory (DFT), we verified that stripping the surface of sulfur atoms creates metallic surface states that can catalyze water dissociation. At elevated H2O pressures, XPS measurements combined with DFT calculations suggested the presence of four distinct surface species from dissociated water. Specifically, we found that oxides and hydroxides are prominent at the surface, while chemisorbed and physisorbed H2O molecules are also present. This study provides new insights that reveal the prospects of surface-engineered MoS2 as a catalyst for water dissociation.