Rationalizing hydrogen evolution mechanism on the slab of Zn-reduced 2H–MoS2 monolayer by density functional theory calculations

In this paper, the H2 evolution mechanism and activity on Zn-reduced 2H–MoS2 were explored by density functional theory (DFT) calculations based on the possible active sites including nZn-MoS2 and nZn-S vacancy (SV) (n = 1, 2, 3), which were built by replacing one, two, three adjacent Mo atoms in MoS2 and at SV by Zn, respectively. The calculations indicate that the amount of Zn incorporation can significantly affect the H2 evolution activities and mechanisms of Zn-doped MoS2 and Zn-doped SV and Zn-doped SV is more favorable to the production of H2. The formation of 3Zn–MoS2 and 3Zn-SV is less favorable to H2 evolution reactions (HER) due to their too high or low ΔGH. The replacement of one Mo in MoS2 by Zn can reduce the ΔGH of S atom to −0.18 eV, and S bonded by one Zn (SA) in Zn–MoS2 cannot catalyze HER. In 2Zn–MoS2, S bonded by two Zn atoms (SB) catalyzes HER by the Heyrovsky-step-controlled Volmer-Heyrovsky mechanism, and the rate determining step (RDS) has a barrier of 47.8 kcal/mol. Moreover, Zn doping is beneficial for generating Zn-doped SV. Zn-SV catalyzes HER via the same mechanism with 2Zn–MoS2, and the barrier of RDS is 30.7 kcal/mol. After replacing two adjacent Mo atoms of SV with Zn, the resulting 2Zn-SV follows the Volmer-Heyrovsky mechanism to catalyze HER, and the RDS is the Volmer step with a barrier of 27.2 kcal/mol.