DFT Study
on Ce-Doped Anatase TiO<sub>2</sub>: Nature of Ce<sup>3+</sup> and
Ti<sup>3+</sup> Centers Triggered by Oxygen Vacancy Formation
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Abstract
A systematic
study of TiO<sub>2</sub> anatase, Ce-doped TiO<sub>2</sub> anatase
with 2.8 and 5.6% dopant concentration and of the systems resulting
from oxygen vacancy formation has been carried out by means of periodic
density functional theory based calculations using PBE, PBE+U, and
hybrid functionals. For each approach, several situations are considered
for the oxygen vacancy formation, differing on the position of the
removed oxygen or on the resulting electronic structure. The hybrid
B3LYP functional and PBE+<i>U</i> approaches provide a physically
meaningful description of localized <i>d</i> and <i>f</i> electrons in Ti<sup>3+</sup> and Ce<sup>3+</sup> species,
respectively. Nevertheless, quasi-degenerate solutions were encountered
featuring either fully localized spin (simple and split) or partially
localized spin. Although standard PBE calculations result always in
fully (unphysical) delocalized solutions, the most stable geometry
thus predicted, in which Ce is six-coordinated and V<sub>O</sub> folded
by 3[TiO<sub>5</sub>], is in agreement with the B3LYP and PBE+<i>U</i> results. The present work provides compelling evidence
that the remarkable catalytic properties of these systems partially
arise from the facilitated oxygen vacancy (V<sub>O</sub>) formation
triggered by the Ce dopant, which is further enhanced when dopant
concentration is increased