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

Abstract

2D layered transition metal dichalcogenides (TMDC) structurized in hexagonal 2H c phase possess space group P63/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 △ϵgap ≈ 2.06 eV (yellow range of the visible spectrum). Additionally, all the edges of ordered pristine 2H c WS2 (0001) nanosheets (ϵVB and ϵCB 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 (ϵnormalO2/normalH2O and ϵnormalH+/H2): ϵVB < ϵnormalO2/normalH2O < ϵnormalH+/H2 < ϵCB, which means that probability of photocatalytic electron‐hole recombination is rather minimal, being reduced with a decrease of stack thickness.

The site-symmetry induced representations of layer groups on the Bilbao Crystallographic Server

“…The phonon symmetry in these crystals has been studied by Molina-Sá nchez & Wirtz (2011), Ribeiro-Soares et al (2014) and Saito et al (2016). However, diperiodic groups were introduced explicitly only by Evarestov (2015), Evarestov et al (2017) and Bocharov et al (2019). In the work of Ribeiro-Soares et al (2014), the symmetry of single layers was described in terms of the related space group P6m2 (No.…”

Impact of Defects on Electronic Properties of Heterostructures Constructed From Monolayers of Transition Metal Dichalcogenides

“…Nevertheless, the experimental data on the photoluminescence of MoS 2 layers agree with the theoretical prediction of conversion from indirect to direct‐gap semiconductor upon moving from bulk to one monolayer . Moreover, the structural parameters obtained taking into account the van der Waals interaction are much closer to the experimental values than the results obtained within common DFT approaches without considering this interaction . Thus, the method we used can be applicable for qualitative analysis and determination of gap modifications under various conditions.…”

“…[14] Moreover, the structural parameters obtained taking into account the van der Waals interaction are much closer to the experimental values than the results obtained within common DFT approaches without considering this interaction. [21] Thus, the method we used can be applicable for qualitative analysis and determination of gap modifications under various conditions.…”

“…Theoretical calculations give gaps of 0.90–1.52 eV (indirect gap) for the bulk of MoS 2 and 1.72–1.86 eV (direct gap) in a MoS 2 monolayer, respectively . For WS 2 the gaps are 1.63 eV in the bulk material and 2.53 eV in one monolayer …”

“…[16][17][18][19][20] For WS 2 the gaps are 1.63 eV in the bulk material and 2.53 eV in one monolayer. [21] Combination of the layers of different TMDs gives a possibility to obtain a directgap hybrid two-dimensional crystal. [12,22] Such structures can be fabricated by means of a layering technique from already formed monomolecular layers or by their layer-by-layer chemical vapor deposition.…”