Ab Initio Calculations on the Electronic Structure and Photocatalytic Properties of Two-Dimensional WS2 (0001) Nanolayers of Varying Thickness

2D layered transition metal dichalcogenides (TMDC) structurized in hexagonal 2H c phase possess space group P6(3)/mmc and graphite-type morphology (e.g., WS2 bulk), as well as strong chemical bonds and weak coupling between S-W-S three-plane monolayers (MLs). Ab initio calculations on pristine WS2 (0001) nanolayers are performed using CRYSTAL14 code within the formalism of hybrid density functional theory and Hartree-Fock method (HSE06 Hamiltonian properly adapted to describe tungsten disulphide bulk). An ordered stack of graphene-type WS2 (0001)-oriented MLs (in absence of any point defects including dopants) with thikness varies from 1 to 40 MLs is found to be suitable for photocatalysis since irrespectively of this thickness the bandgap corresponds to the varied range of visible spectrum between its violet and red edges, respectively. The authors calculate the electronic structure of these nanolayers. The highest photocatalytic efficiency is achieved for WS2 2 ML with band gap o epsilon(gap) approximate to 2.06 eV (yellow range of the visible spectrum). Additionally, all the edges of ordered pristine 2H c WS2 (0001) nanosheets (epsilon(VB) and epsilon(CB) levels corresponding to the top of the valence band and the bottom of the conduction band) are properly aligned relative to the oxidation and reduction potentials (epsilon O2/H2O and epsilon H+/H2): epsilon(VB) < epsilon O2/H2O < epsilon H+/H2 < epsilon(CB), which means that probability of photocatalytic electron-hole recombination is rather minimal, being reduced with a decrease of stack thickness.