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The atomic and electronic structure, formation energy, and the energy barriers for migration have been calculated for the neutral O vacancy point defect ͑F center͒ in cubic SrTiO 3 employing various implementations of density functional theory ͑DFT͒. Both bulk and TiO 2 -terminated ͑001͒ surface F centers have been considered. Supercells of different shapes containing up to 320 atoms have been employed. The limit of an isolated single oxygen vacancy in the bulk corresponds to a 270-atom supercell, in contrast to commonly used supercells containing ϳ40-80 atoms. Calculations carried out with the hybrid B3PW functional show that the F center level approaches the conduction band bottom to within ϳ0.5 eV, as the supercell size increases up to 320 atoms. The analysis of the electronic density maps indicates, however, that this remains a small-radius center with the two electrons left by the missing O ion being redistributed mainly between the vacancy and the 3d͑z 2 ͒ atomic orbitals of the two nearest Ti ions. As for the dynamical properties, the calculated migration energy barrier in the low oxygen depletion regime is predicted to be 0.4 eV. In contrast, the surface F center exhibits a more delocalized character, which leads to significantly reduced ionization and migration energies. Results obtained are compared with available experimental data.

The ground state electronic structure and the formation energies of both TiO 2 and SrTiO 3 nanotubes (NTs) containing C O , N O , S O , and Fe Ti substitutional impurities are studied using first-principles calculations. We observe that N and S dopants in TiO 2 NTs lead to an enhancement of their visible-light-driven photocatalytic response, thereby increasing their ability to split H 2 O molecules. The differences between the highest occupied and lowest unoccupied impurity levels inside the band gap (HOIL and LUIL, respectively) are reduced in these defective nanotubes down to 2.4 and 2.5 eV for N and S doping, respectively. The band gap of an N O +S O co-doped titania nanotube is narrowed down to 2.2 eV (while preserving the proper disposition of the gap edges relatively to the reduction and oxidation potentials, so that HOIL < O 2 /H 2 O < H + /H 2 < LUIL ), thus decreasing the photon energy required for splitting of H 2 O molecule. For C-and Fe-doped TiO 2 NTs, some impurity levels lie in the interval between both redox potentials, which would lead to electron-hole recombination. Our calculations also reveal in sulfur-doped SrTiO 3 NTs a suitable band distribution for the oxygen evolution reaction, although the splitting of water molecules would be hardly possible due to an unsuitable conduction band position for the hydrogen reduction reaction. KeywordsDensity Functional Theory, SrTiO 3 and TiO 2 nanotubes, single-and double-atom dopants, atomic structure, electronic structure, photocatalytic properties. the influence of solar light on semiconducting photoelectrodes in aqueous electrolyte is a potentially clean and renewable source for hydrogen fuel. The process is often considered as artificial photosynthesis, and as such is an attractive and challenging research topic in the field of chemistry and renewable energy. 1-3The efficiency of the water splitting reactions 4 depends on the relative position of the semiconductor band edges (hole and electron energies) with respect to the redox levels, which are defined as measure of the affinity of the semiconducting substance for electrons (its electronegativity) compared with hydrogen. Redox couples in electrochemical reactions are characterized by molecules or ions in a solution which can be reduced and oxidized by a pure electron transfer. 5 This requires the semiconductors to exhibit a proper band alignment relative to the water redox potentials, e.g., the conduction band minimum of the p-type photocathode should be higher than the water reduction potential H + /H 2 , while the valence band maximum of the n-type photoanode, should be lower than the water oxidation potential O 2 /H 2 O. 6 Major limitations for the solar light conversion by photocatalysis relate to the band gap position in the corresponding photocatalytic materials and their stability in an aqueous environment.A number of binary and ternary metal oxide semiconductors have been intensively studied so far. 3,4,6 SrTiO 3 and, especially, TiO 2 (which distinguishes itself due to its superior chemica...

For ab initio simulations on hexagonal single-wall BN and TiO 2 nanotubes (SW NTs), we have applied the formalism of line symmetry groups describing one-periodic (1D) nanostructures with rotohelical symmetry. Both types of NTs can be formed by rolling up the stoichiometric nanosheets of either (i) a (0001) monolayer of BN hexagonal phase or (ii) a three-layer (111) slab of fluorite-type TiO 2 phase. Optimized parameters of the atomic and electronic structure of corresponding slabs and nanotubes have been calculated using hybrid LCAO method as implemented in CRYSTAL code. Their band gaps (∆ε gap ) and strain energies (E strain ) have been analyzed as functions of NT diameter (D NT ). For hexagonal BN and TiO 2 nanotubes, certain qualitative similarities between the ∆ε gap (D NT ) or E strain (D NT ) functions exist despite the different chemical nature.

Using the B3PW hybrid exchange-correlation functional within density-functional theory and employing Gaussian-type basis sets, we calculated the atomic and electronic structures and thermodynamic stability of three double-layered ͑DL͒ SrTiO 3 ͑001͒ surfaces: ͑i͒ SrO-terminated, ͑ii͒ TiO 2 -terminated, and ͑iii͒ ͑2 ϫ 1͒ reconstruction of TiO 2 -terminated SrTiO 3 ͑001͒ recently suggested by Erdman et al. ͓Nature ͑London͒ 419, 55 ͑2002͔͒. A thermodynamic stability diagram obtained from first-principles calculations shows that regular TiO 2 -and SrO-terminated surfaces are the most stable. The stability regions of ͑2 ϫ 1͒ DL TiO 2 -and DL SrO-terminated surfaces lie beyond the precipitation lines of SrO and TiO 2 compounds and thus are less stable with respect to regular SrTiO 3 ͑001͒ surfaces. Analysis of the stability diagram suggests that Sr precipitation on SrTiO 3 surface never occurs. Our simulations show a substantial increase of Ti-O covalency on the DL surfaces as compared to the regular surfaces, which are themselves more covalent than the crystalline bulk. The implications of our calculated results for recent experimental observations are discussed.

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Atomic and electronic structure of regular and O-deficient SrTiO 3 have been studied. Several types of first principles atomistic simulations: Hartree-Fock method, Density Functional Theory, and hybrid HF-DFT functionals, have been applied to periodic models that consider supercells of different sizes (ranging between 40 and 240 atoms). We confirm the ionic character of the Sr-O bonds and the high covalency of the Ti-O 2 substructure. For the stoichiometric cubic crystal; the lattice constant and bulk modulus correctly reproduce the experimental data whereas the band gap is only properly obtained by the B3PW functional. The relaxed geometry around the F center shows a large expansion of the two nearest Ti ions. Moreover, the vacancy formation energy is extremely sensitive to the size and the shape of the supercell as well as the calculation method. The electronic density map indicates the redistribution of two electrons of the missing O atom between the vacancy and 3d atomic orbitals of the two nearest Ti ions, in contrast to the F centers in ionic oxides where the charge centroid does not change.