The charge effects on the hydrogen evolution reaction activity of the defected monolayer MoS₂

Doping engineering has proven to be an effective way to tune the hydrogen evolution reaction (HER) activity of MoS₂. Introducing these defects could cause the overall charge imbalance of MoS₂, which makes MoS₂ charged. In order to understand the effect of charge on the HER activity of the defected MoS₂, we systematically investigate the formation energies, hydrogen adsorption Gibbs free energy (), and electronic structures of 3d, 4d, and 5d transition metal (TM) doped monolayer MoS₂ with S vacancies (S_vac) based on the density functional theory (DFT) calculations. According to the formation energy calculation, S_vac in the 0 and -1 charge states (S0vac and S_vac^1-) is found to be stable. of S_vac^1- is -0.16 eV, suggesting its HER catalytic activity is lower than that of Pt (), which is consistent with the experimental results. By substituting the Mo atom with TM atoms, we found that the TM atoms in groups VB-VIIB can promote the generation of S_vac, forming defect complexes (TM_MoS_vac). is greatly affected by the charge state of defects; TM_MoS_vac defects (TM = V, Nb, Ta, Cr, W, Mn, and Re) in -1 charge states exhibited excellent HER activity (). Significantly, W and Re doping can promote the HER activity of MoS₂ independent of the charge state and the Fermi level, which suggests that W and Re doping are most beneficial to improve the HER activity of MoS₂. Therefore, the HER activity of defected MoS₂ is not only influenced by as previously thought, but also by formation energies, charge state and Fermi level position of defects. The underlying physics could be deduced from the charge-induced changes in electronic structures. Our work highlights the defect charge effects on the electrochemical reactions and offers plausible mechanisms of defect charge effects.