Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects

TD-DFT
calculations were performed on neutral TinO2n, TinO2n–1, and TinO2n–2 clusters, where n ≤ 7. Calculations show the TinO2n clusters are closed shell systems
containing empty d orbitals and that the partially filled d orbitals
of the suboxide clusters have a profound effect on their structural,
electronic, and topological properties. The low energy photoexcitations
of TinO2n clusters
are all O-2p to Ti-3d transitions, while the open-shell suboxide clusters
are all characterized by d–d transitions that occur at a much
smaller optical gap. Upon low energy photoabsorption, the localization
of the hole is accompanied by a local bond elongation, i.e., polaron
formation, whereas d-electrons are generally delocalized around the
cluster. The properties of the clusters, including the oxygen binding
energies and structures, were calculated to account for the variation
in relative populations found in experimental cluster distributions.
Several TinO2n–2 clusters contain higher symmetry which is reflected in their relative
stability. In particular, the tetrahedral symmetry of Ti4O6 inhibits charge carrier localization and therefore
exhibits higher stability.