First principles evaluation on photocatalytic suitability of 2H structured and [0001] oriented WS₂ nanosheets and nanotubes

Pristine WS₂ multilayer nanosheets (NSs), which thickness h _NS varies from 1 to 12 monolayers (MLs), as well as single- and multi-walled nanotubes (SW and MW NT_s) of different chirality, which diameter d _NT exceeds 1.9 nm, display photocatalytic suitability to split H₂O molecules. Obviously, such a phenomenon can occur since the band gap of these nanostructures corresponds to the energy range of visible light between the red and violet edges of spectrum (1.55 eV < Δϵ_gap < 2.65 eV). For all the studied WS₂ NSs and NTs, the levels of the top of the valence band and the bottom of the conduction band must be properly aligned relatively to H₂O oxidation and reduction potentials separated by 1.23 eV: ϵ _VB < ϵO₂/H₂O < ϵH+/H₂ < ϵ _CB. The values of Δϵ_gap decrease with growth of h _NS and increase with enlargement of dNT. 1 ML nanosheet can be considered as a limit of infinite SW NT thickness growth (d _NT→∞), which band gap increases up to ∼2.65 eV. First principles calculations have been performed using the hybrid DFT-HF method (HSE06 Hamiltonian) adapted for 2H WS₂ bulk. The highest solar energy conversion efficiency (15-18%) expected at Δϵ_gap = 2.0-2.2 eV (yellow-green range) has been found for 2 ML thick (stoichiometric) WS₂ (0001) NS as well as WS₂ NTs with diameters 2.7-3.2 nm (irrespectively on morphology and chirality indices n of nanotubes). Moreover, unlike discrete variation of h_NS magnitudes, tuning of d _NT values provides much higher energy resolution.