Site-specific catalytic activity in exfoliated MoS₂ single-layer polytypes for hydrogen evolution: Basal plane and edges

We performed ab initio calculations on the basic set of MoS₂ single-layer materials, namely the 1H, 1T and 1T′ polytypes, to lay a theoretical framework on the emerging breakthrough-discoveries of high activity towards the hydrogen evolution reaction (HER) in exfoliated MoS₂ and related materials. Our calculations show that for exfoliated MoS₂, 1T′ is the most HER active polytype, with active sites both on the basal plane and at the edges of the layered grains. In comparison, the basal planes of the 1H and 1T polytypes are HER inactive and their edge-sites are not as active as those of the 1T′ polytype. We also found that 1T-MoS₂ is unstable and easily transforms into 1T′-MoS₂, and the 1T′ phase is metastable with a considerable barrier >0.7 eV to bar its transformation into the most stable 1H phase. Further, unlike the case of exfoliated WS₂, the HER activity of exfoliated MoS₂ is not so critically affected by the lattice strain. In addition, in contrast to the 1T′-WS₂, the 1T′-MoS₂ is not metallic but has a very small bandgap of 0.1-0.2 eV. Hence, 1T′-MoS₂ should have a high enough conductivity and other suitable properties for it to function as an electrochemical HER catalyst.