Four-faceted nanowires generated from densely-packed TiO2 rutile surfaces: Ab initio calculations

Two-dimensional (2D) slabs and monoperiodic (1D) nanowires orthogonal to the slab surface of rutile-based TiO2 structure terminated by densely-packed surfaces and facets, respectively, have been simulated in the current study. The procedure of structural generation of nanowires (NWs) from titania slabs (2D → 1D) is described. We have simulated: (i) (110), (100), (101) and (001) slabs of different thicknesses as well as (ii) [001]- and [110]-oriented nanowires of different diameters terminated by either four types of related {110} facets or alternating { 1 1 ¯ 0 } and {001} facets, respectively. Nanowires have been described using both the Ti atom-centered rotation axes as well as the hollow site-centered axes passing through the interstitial sites between the Ti and O atoms closest to the axes. For simulations on TiO2 slabs and NWs, we have performed large-scale ab initio Density Functional Theory (DFT) and hybrid DFT–Hartree Fock (DFT–HF) calculations with the total geometry optimization within the Generalized Gradient Approximation (GGA) in the form of the Perdew–Becke–Ernzenhof exchange-correlation functionals (PBE and PBE0, respectively), using the formalism of linear combination of localized atomic functions (LCAO) implemented in CRYSTAL09 code. Both structural and electronic properties of enumerated rutile-based titania slabs and nanowires have been calculated. According to the results of our surface energy calculations, the most stable rutile-based titania slab is terminated by (110) surfaces whereas the energetically favorable [001]-oriented NWs are also terminated by {110} facets only, thus confirming results of previous studies.