Charge Mediated Semiconducting-to-Metallic Phase Transition in Molybdenum Disulfide Monolayer and Hydrogen Evolution Reaction in New 1T′ Phase

The phase transition of single layer molybdenum disulfide (MoS₂) from semiconducting 2H to metallic 1T and then to 1T′ phases, and the effect of the phase transition on hydrogen evolution reaction (HER) are investigated within this work by density functional theory. Experimentally, 2H-MoS₂ has been widely used as an excellent electrode for HER and can get charged easily. Here we find that the negative charge has a significant impact on the structural phase transition in a MoS₂ monolayer. The thermodynamic stability of 1T-MoS₂ increases with the negative charge state, comparing with the 2H-MoS₂ structure before phase transition and the kinetic energy barrier for a phase transition from 2H to 1T decreases from 1.59 to 0.27 eV when 4e^- are injected per MoS₂ unit. Additionally, 1T phase is found to transform into the distorted structure (1T′ phase) spontaneously. On their activity toward hydrogen evolution reaction, 1T′-MoS₂ structure shows comparable hydrogen evolution reaction activity to the 2H-MoS₂ structure. If the charge transfer kinetics is taken into account, the catalytic activity of 1T′-MoS₂ is superior to that of 2H-MoS₂. Our finding provides a possible novel method for phase transition of MoS₂ and enriches understanding of the catalytic properties of MoS₂ for HER.