Understanding the air stability of defective MoSand the oxidation effect on the surface HER activity.

The defective single layer MoS(SL-MoS) with high defect concentrations has shown promising electrocatalytic potential, but it is also highly reactive with gas molecules. The study of electro-chemical activity on gas doped defective SL-MoSis of importance yet still scarcely discussed. Herein, we performed density functional theory calculations to study the adsorption and chemical activity of four major air molecules on the defective SL-MoSunder different defect concentrations, and evaluated the influence on the hydrogen evolution reaction activity. The Nand COmolecules are in physisorption states, HO molecule is in molecular chemisorption state, while Ocan be strongly captured and dissociated into atomic O, which repair the S-vacancy and form O-doped structure. Further study showed that compared to the inert S surface of pure MoS, the O incorporation greatly enhance the surface reactivity. Using H adsorption as the test probe, the adsorption of H becomes stronger with the increasing oxygen concentration. We further unravel the electronic origins underlying the catalytic activity. The lowest unoccupied electronic states are shown to correlate linearly with the activity, and thus can be used as an electronic descriptor to characterize the electrocatalytic activity.