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 NTs) 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 < Delta epsilon(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: epsilon(VB) < epsilon(O2/H2O) < epsilon(H+/H2) < epsilon(CB). The values of Delta epsilon(gap) decrease with growth of h(NS) and increase with enlargement of d(NT). 1 ML nanosheet can be considered as a limit of infinite SW NT thickness growth (d(NT)->infinity), which band gap increases up to similar 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 Delta epsilon(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 hNS magnitudes, tuning of d(NT) values provides much higher energy resolution.