DFT-Based Theoretical Calculations of Nb- and W-Doped Anatase TiO₂: Complex Formation between W Dopants and Oxygen Vacancies

Abstract

The structure and electronic states in Nb-doped TiO 2 (TNO) and W-doped TiO 2 (TWO) having the anatase phase were investigated using a first-principles DFT-based band structure method. In addition to the cases where the dopant substituted for a Ti atom, cells containing a dopant (M Ti , where M ) Nb and W) and an oxygen vacancy (V O ) were calculated to clarify the role of oxygen vacancies in the system. Furthermore, cells containing two dopants and an oxygen vacancy (2M Ti -V O ) and cells with a dopant and two oxygen vacancies (M Ti -2V O ) were calculated. Energetically stable structures were found in the 2W Ti -V O and W Ti -2V O cells sampled, whereas the corresponding structures in TNO did not show any significant energy stabilization. Impurity states were found in the stable 2W Ti -V O and W Ti -2V O structures, and an approach of the two W Ti atoms was found in 2W Ti -V O . These findings suggest the possible formation of complex structures consisting of W Ti dopants and oxygen vacancies. Our results are consistent with recent experiments on TWO by Takeuchi et al., and they rationalize the lower electronic conductivity of TWO versus TNO.

Polaronic interactions between oxygen vacancies in rutile TiO2

“…The first state is the widely-observed closed-shell solution, which has two electrons distributed in between three Ti neighbors [35][36][37]. The Kohn-Sham (KS) energy level of this defect state is relatively deep (0.4~1.0 eV below conduction band minimum [17,33,36,[40][41][42][43][44][45]), in contrast to the experimentally-observed shallow donor states [46][47][48][49]. Thus, they are unlikely to be the major contributor of n-type conductivity in rutile TiO 2 .…”

“…In order to reproduce the correct polaronic behaviors, the self-interaction of electrons must be treated properly to balance the tendency of localization versus delocalization [33][34][35][36][37][38][39][40][41][42][43][44]. Hybrid functionals [33][34][35][36][37][38][39][40] and on-site Coulomb corrections [31,32,[41][42][43][44] are two potential treatments. Using hybrid functionals, several groups of researchers have reported the co-existence of two distinct polaronic states [17,[35][36][37][38].…”

Hydrogen peroxide route to Sn-doped titania photocatalysts

“…Such small but definite lattice expansion was also found in the non-doped anatase obtained from titanium sulfate by the urea hydrolysis ( a  = 3.786 Å and c  = 9.515 Å, [16]) as compared with pure anatase ( a  = 3.782 Å and c  = 9.502 Å, [45]). The non-stoichiometry could be attributed to a cation deficiency presumably due to OH - replacing O 2- excess of O ions (see the XPS results below).…”

Roles of Cu concentration in the photocatalytic activities of Cu-doped TiO 2 from GGA+ U calculations

“…1, corresponding to the Cu atomic percentage (at%) or weight percentage (wt%) of 1.04 at%, 2.08 at%, 3.16 at%, 4.16 at% or 2.48 wt%, 4.94 wt%, 7.36 wt%, 9.76 wt%, respectively. It should be noted that the number of possible lattice arrangements of the Cu atoms greatly increases when Cu concentration increases [29]. Therefore, it is extremely difficult to consider all atomic configurations.…”