2D 1T′-MoS2-WS2van der Waals heterostructure for hydrogen evolution reaction: dispersion-corrected density functional theory calculations

Dispersion-corrected density functional theory calculations were implemented to investigate structural characteristics, as well as the hydrogen evolution reaction (HER) capability of 2D 1T ' phase MoS2-WS(2)van der Waals heterostructures. Two van der Waals corrections were utilized in the study, namely DFT-D3 (semi-empirical-based) and vdW-DF2-B86R (ab-initio-based) corrections. Results show that the DFT-D3 correction stabilized the binding of the monolayers consistent with experimental observations, with binding energy per unit cell of -0.54 eV/cell. The Gibbs free energy of hydrogen adsorption Delta G(ads,H), which is the lone descriptor of HER, were calculated for the two known adsorption sites in the 1T ' phase, termed S1 (sulfur site with elongated bonds, more active for HER) and S2 (sulfur site with compressed bonds, less active for HER). It is revealed that at the van der Waals region, the S1 and S2 sites, acting as a single adsorption site, become active for HER, with significantly lowered value of Delta G(ads,H)at 0.20-0.24 eV. This is linked to the synergistic interaction of the two sites in adsorbing hydrogen. In terms of electronic structure, the enhanced states in the vicinity of the Fermi level for the interacting S1 and S2 sites at the van der Waals region resulted from orbital hybridization among 3pstates of the sulfur sites from the inner top and bottom surfaces. The merging of the two sites at the van der Waals region would result to HER efficiency that is expected to be higher by a factor of 2 compared to that on the top and bottom surfaces. This work has showed that 2D heterostructures could be of importance in catalysis, particularly in HER. Furthermore, it is showed that building a 2D heterostructure could be a good alternative to the application of strain in improving HER capability of 1T ' 2D materials without compromising the adsorption properties of other sites.