Triggering Catalytic Active Sites for Hydrogen Evolution Reaction by Intrinsic Defects in Janus Monolayer MoSSe

Janus transition-metal dichalcogenides have been predicted to be promising candidates for hydrogen evolution reaction (HER) due to their inherent structural asymmetry. However, the effect of intrinsic defects, including vacancies, antisites, and grain boundaries, on their catalytic activity is still unknown. MoSSe provides an ideal platform for studying such defects, since theoretical calculation has indicated that the formation energies of point defects and grain boundaries on MoSSe were lower than that of pristine MoS2 monolayer. In this work, density functional theory is utilized to study all of the possible intrinsic defects on the MoSSe monolayer for HER. The MoSSe monolayer with 4 vertical bar 4, 4 vertical bar 8a, 5 vertical bar 7b, 8 vertical bar 10a GBs, vacancies (V-s, V-s, V-sse, V-mo, V-mos3), and antisite defects (Mo-sse, Se-Mo, S-Mo) shows enhanced HER performance. The adsorption behavior of hydrogen on defects were explained by using a "states-filling" model. The adsorption energy of hydrogen during catalysis changes linearly with the work required to fill unoccupied electronic states within the catalysts. This work could provide a more comprehensive understanding of all of the possible active sites of Janus transition-metal dichalcogenides for HER.