Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects.

Abstract

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.