This journal is © the Owner Societies 2014 Phys. Chem. Chem. Phys., 2014, 16, 9479--9489 | 9479 Cite this: Phys. Chem. Chem. Phys., 2014, 16, 9479 Orientation effects in morphology and electronic properties of anatase TiO 2 one-dimensional nanostructures. I. Nanowires Dmitri B. Migas,* a Andrew B. Filonov, a Victor E. Borisenko a and Natalia V. Skorodumova bc By means of ab initio calculations we have revealed the existence of sizable anisotropy in electronic properties of anatase TiO 2 nanowires with respect to orientation: nanowires with h001i, h100i and h110i axes are found to be direct band-gap, indirect band-gap and degenerate semiconductor materials, respectively. The degenerate semiconducting properties of h110i-oriented TiO 2 nanowires are predicted to be the intrinsic features closely connected with stoichiometry. A band-gap variation with nanowire diameter is also shown to display rather complex behavior characterized by a competition between quantum confinement and surface state effects that is fully compatible with the available contradictory experimental data. Finally, we propose a model to explain the band-gap variation with size in TiO 2 nanowires, nanocrystals and thin films. 1 Introduction Nowadays nanostructures of titanium dioxide (TiO 2 ) such as nanowires (NWs), nanotubes (NTs) and nanocrystals (NCs) are considered to be very promising for various applications. 1–4 TiO 2 NWs in the rutile phase can be readily grown by chemical vapor deposition 5 and thermal evaporation 6,7 involving the vapor–liquid–solid and vapor–solid growth mechanisms. Such NWs possess relatively high crystallinity, which is not the case for TiO 2 NWs in the anatase phase. In fact, the latter NWs as well as anatase TiO 2 NTs and NCs are usually obtained by the so-called ‘wet-chemistry’ methods 1–4 and postgrown heat treat- ment is desirable to improve the crystallinity of these nanostruc- tures. It is also shown that the appearance of TiO 2 NWs, NTs or NCs depends on synthesis conditions. 1–4 In addition, some authors have reported the formation of elongated TiO 2 NCs in the anatase phase, which resemble NWs in shape. 8–12 Relatively short NWs with average sizes of 2.0–2.7 nm (thickness)  28–39 nm (length) have been obtained as well. 13 According to experimental observations there are three different growth orientations of TiO 2 NWs and elongated NCs in the anatase phase: h001i, 8,9,13–15 h100i, 16–18 and h110i. 11 The smallest experimentally observed diameters of anatase TiO 2 NWs were in the range of 4–5 Å. 14 At the same time, rather odd behavior of band-gap changes with diameter of anatase TiO 2 NWs and NCs has often been reported: an expected increase in DE g  1 d 2 dependence of the band-gap (E g ) variation with a decrease in diameters (d) because of the quantum confinement effects is not evident 13,19–25 or band gaps do not change at all. 11,26 In some cases the band-gap of anatase TiO 2 NWs was measured to be even smaller by 0.2 eV compared to the bulk value. 11,25,27 Theoretical investigations of stability and electronic proper- ties of anatase TiO 2 NWs have been performed by ab initio techniques 28–30 mainly focusing on a band-gap variation with diameters, while the details of band structures have not been reported yet. Thin TiO 2 NWs with the h001i growth directions and various cross sections involving different facets have been studied indicating the most stable morphology and changes in the band gap with respect to the perimeter of a NW. 28 The dependencies of electronic properties on the size, surface termi- nation and morphology of h001i- and h100i-oriented TiO 2 NWs were investigated by Iacomino et al. 29 The authors revealed that bare and hydrated TiO 2 NWs showed a band-gap blue shift due to the size confinement, but deviations from the ideal trend have been attributed to NW morphology and crystallinity. 29 Another study of the stability and electronic structure of bare h001i- oriented TiO 2 NWs also indicated a blue shift of band gaps of about 1 eV because of the quantum confinement effects. 30 All previously published theoretical papers have addressed the stability issue of anatase TiO 2 NWs with respect to morphology. 28–30 a Belarusian State University of Informatics and Radioelectronics, P. Browka 6, 220013 Minsk, Belarus. E-mail: migas@bsuir.by b Multiscale Materials Modelling, Department of Materials and Engineering, Royal Institute of Technology (KTH), SE-10044 Stockholm, Sweden c Department of Physics and Astronomy, Uppsala University, Box 516, SE-75121, Uppsala, Sweden Received 26th November 2013, Accepted 26th March 2014 DOI: 10.1039/c3cp54988g www.rsc.org/pccp PCCP PAPER