Calculation of TiO₂ Surface and Subsurface Oxygen Vacancy by the Screened Exchange Functional

Abstract

The formation energies of oxygen vacancies at different surface and subsurface sites of anatase (101), anatase (001) and rutile (110) surfaces are calculated by screened-exchange (sX) hybrid functional method. Our results show that the oxygen vacancy is more stable on the surface than subsurface for rutile (110), while it is more stable subsurface than on the surface for anatase surfaces. These results are similar to those found by simple density functional theory, but now the sX hybrid functional gives the correct defect localizations. The defects introduce a gap state near the conduction band edge. For the most stable oxygen vacancy site at each TiO 2 surface, the +2 charge state dominates over a wide range of Fermi energies.

Fermi Level Positions and Induced Band Bending at Single Crystalline Anatase (101) and (001) Surfaces: Origin of the Enhanced Photocatalytic Activity of Facet Engineered Crystals

“…For both the (001) and (101) facets, the order of Fermi level positions is ox-TiO 2 < st-TiO 2 < an-TiO 2 < sp-TiO 2 and is related to the different surface electronic structures due to the changed concentration of the different surface states of the different surfaces leading to band bending (space charge layer) effects. In addition, Li et al [19] found that O vacancies are shallow donors with a donor level even above the conduction band minimum at the (101) surface, whereas the donor level at the (001) surface is slightly lower in energy than the calculated gap. As Ti 3+ interstitial and O vacancy give rise to the localized impurity level below the lower end of the conduction band, the amount of these intrinsic defects as well as the surface termination is directly related to the Fermi level position.…”

“…[19] The surface states of the (001) facet near the bulk CBM are lower than those of the (101) facet. It was reported based on a theoretical calculation that the top surfaces of the (001) and (101) facets have surface states above the bulk VBM and below the bulk conduction band minimum.…”

“…The XP valence band spectra show at a first glance the typical valence band emission of TiO 2 bulk material with the typical double layer structure of the O2p π and σ bonding states at binding energy of about 5 and 8 eV, respectively. [19] DGS are conventionally assigned to excess electrons localized on Ti 3+ interstitial and also Ti atoms near O vacancies. The sputtered TiO 2 surfaces show the well-known Ti3d 1 emission lines in the bandgap region.…”

“…The peaks of XP valence band region were fitted with Voigt functions using Gaussian and Lorentzian mixed line shapes. [19] For estimating the concentration of the different states, we have used the XPS data as these are mostly determined by the initial state density, whereas for He UPS data final state density of states cannot be neglected for intensity analysis. The SGS and DGS states are well-established from experimental and theoretical studies, [11,15a,18] while the latter, which is of O 2p character, has been reported in theoretical studies only.…”

“…(CBM). [19] The intensity of surface states for ox-(001) and ox-(101) is decreased, as these surfaces are covered by surface peroxo species. Therefore, the Fermi level at the (001) surface is lower than that at the (101) surface even at defectfree conditions.…”