The structural and electronic properties of oxygen vacancies (V-Ox) and titanium interstitials (Ti-(i)) in the bulk of the rutile and anatase forms of TiO2 have been investigated with LSD-GGA+U ab initio simulations. In particular, formation energies of the charged and neutral forms of the V-Ox and Ti-(i) defects as well as the corresponding vertical and thermodynamic transition levels have been estimated. The achieved results can reconcile the apparent inconsistency of experimentally observed deep donor levels with the n-type conductivity observed in reduced TiO2. They show indeed that both defects give rise to vertical transition levels about 1 eV below the conduction band (CB), in agreement with experimental measures, and to thermodynamic transition levels close to the CB. That is, these defects behave as deep donors, when looking at vertical transitions, and as shallow donors, when the effects of the structural relaxations are taken into account. A major part of the explanation of this behavior is played by the polaron-like character of the defect states, which was already noted, but not deepened, in literature. Finally, it is shown that the application of the U correction to both Ti and O species gives qualitatively similar results, but with a better agreement to experimental findings, with respect to the application to Ti only. The former approach gives pretty similar results, for both rutile and anatase bulk properties, to those coming from HSE hybrid functional calculations

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The structural, electronic, and vibrational properties of intermediates of the O(2) photoreduction at the (101) TiO(2) (anatase) surface have been investigated by performing ab initio density functional calculations. In detail, a recently proposed approach has been used where molecules on the surface are treated like surface defects. Thus, by applying theoretical methods generally used in the physics of semiconductors, we successfully estimate the location and donor/acceptor character of the electronic levels induced by an adsorbed molecule in the TiO(2) energy gap, both crucial for the surface-molecule charge-transfer processes, and investigate the formation and the properties of charged intermediates. The present approach permits a view of the O(2) photoreduction process through several facets, which elucidates the molecule-surface charge-transfer conditions and reveals the key role played by charged intermediates. A comparison of present results with those of a highly sensitive IR (infrared) spectroscopy study of intermediates of the O(2) photoreduction leads to a deeper understanding of this process and to revised vibrational-line assignments and reaction paths.

The nature of peculiar, short H bonds formed by water molecules in contact with the (101) anatase surface and their effects on the structural and vibrational properties of the first water layers adsorbed on the same surface have been investigated by performing density functional theory (DFT) total energy calculations and ab initio molecular dynamics (AIMD) simulations at different temperatures. Present results show that these short H bonds originate from a water/anatase interface effect related to an electronic charge transfer from surface Ti atoms to surface O atoms, mediated by water molecules. Further, AIMD simulations performed at low temperature indicate that such short H bonds are at the ground of both the atomic arrangements of the water layers and the peculiar features appearing in the corresponding vibrational spectra. The same interface effect significantly influences also the atomic arrangements and the vibrational properties of intermediates of the O 2 photoreduction reaction, which turn out to be involved in similar charge transfer processes as well as in the formation of short H bonds. AIMD simulations show that these short H bonds are still present at room temperature and give estimates of the vibrational frequencies of the same intermediates, which are in a quite good agreement with the experimental findings. Such an agreement supports the unifying theoretical picture proposed here for water molecules and O 2 photoreduction intermediates in contact with the anatase surface.