Reduced and n-Type Doped TiO₂: Nature of Ti³⁺ Species

Abstract

Defect states in reduced and n-type doped titania are of fundamental importance in several technologically important applications. Still, the exact nature of these states, often referred to as “Ti3+ centers”, is largely unclear and a matter of debate. The problem is complicated by the fact that electronic structure calculations based on density functional theory (DFT) in the local density approximation (LDA) or semilocal generalized gradient approximation (GGA) provide results that do not account for many of the experimentally observed fingerprints of the formation of Ti3+ centers in reduced TiO2. Here, we investigate the properties of at least four different types of Ti3+ centers in bulk anatase, (1) 6-fold-coordinated Ti6c 3+ ions introduced by F- or Nb-doping, (2) Ti6c 3+−OH species associated with H-doping, (3) undercoordinated Ti5c 3+ species associated with oxygen vacancies, and (4) interstitial Ti5c 3+ species. The characterization of these different kinds of Ti3+ centers is based on DFT+U and/or hybrid functional calculations, which are known to (partially) correct the self-interaction error of local and semilocal DFT functionals. We found that strongly localized solutions where an excess electron is on a single Ti3+ ion are very close in energy and sometimes degenerate with partly or highly delocalized solutions where the extra charge is distributed over several Ti ions. The defect states corresponding to these different solutions lie at different energies in the band gap of the material. This has important implications for the conductivity mechanism in reduced or n-type doped titania and suggests a significant role of temperature in determining the degree of localization of the trapped charge.

Unraveling the Cooperative Mechanism of Visible-Light Absorption in Bulk N,Nb Codoped TiO2 Powders of Nanomaterials

“…DFT+U has proved to be appropriate to describe Nb anatase doping. [63][64][65] As for the present work, it is crucial to understand how the N and Nb codopants . [63][64][65][66][67][68] The picture becomes quite different when both N and Nb are simultaneously added to the anatase lattice ( Figure 5).…”

“…[63][64][65] As for the present work, it is crucial to understand how the N and Nb codopants . [63][64][65][66][67][68] The picture becomes quite different when both N and Nb are simultaneously added to the anatase lattice ( Figure 5). As already calculated for N doping 9,69 , valence nitrogen states lie in the proximity of the valence band (VB).…”

Surface photochemistry probed by two-photon photoemission spectroscopy

“…These defects are, however, more difficult to characterize. 62 It has been shown that both surface BBOv's [47][48][49]51,52,56,63,64 and subsurface defects [63][64][65][66][67][68][69][70][71][72] are capable of affecting the chemistry on the TiO 2 (110) surface.…”

Surface and interstitial Ti diffusion at the rutile TiO2(110) surface

“…The description of reduced rutile has been an active topic for many years, with much discussion as to whether the dominant species involved are Ti interstitials 15 or oxygen vacancies. 18 Both defects can produce the same signature in ultraviolet photoemission spectroscopy and in density functional theory (DFT) simulations, 19 i.e. reduced Ti 3+ ions.…”