Ab initio study of 2H-MoS2 using Hay and Wadt effective core pseudo-potentials for modelling the (100) surface structure

Abstract

Periodic Hartree–Fock and DFT methods were employed to calculate the geometric and electronic properties of bulk 2H-MoS₂ and of its catalytically important (100) edge structure. The core electrons of molybdenum and sulfur were represented by the effective core pseudo-potentials, developed by Hay and Wadt. For the calculations (100) type surface structures were generated by cutting sections from an 2H-MoS₂ crystal, which consists of four, six or eight rows of molybdenum and sulfur atoms. The calculated elastic constants follow the experimental constants and show that 2H-MoS₂ is an anisotropic covalent compound held together by weak dispersion interactions between neighbouring S-Mo-S units. The relaxation of the (100) surface of 2H-MoS₂ leads to an inner relaxation of the Mo atoms and the formation of weakly coupled surface S-S species. As a result of the surface relaxation, the electronic charge of the surface states is different to that of the bulk states: empty Mo d states move into the band gap, and the surface becomes a better electron acceptor. In all calculations the model consisting of six rows of sulfur and molybdenum atoms is the most favourable one, because it provides an accurate description of the 2H-MoS₂ surface structure and enables calculations at reasonable computation time. The calculations based on pseudo-potentials are in good agreement with all-electron calculations and confirm that, in contrast to the semiconducting bulk, the (100) 2H-MoS₂ surface has metallic properties.