Excitonic states in a (Ti6O12)3 nanotube

The low-lying excited electronic states of a (Ti6O12)3 nanotube are investigated using ab initio self-consistent field configuration interaction theory. The transition energies and moments are calculated and the nature of the orbitals involved is discussed. Transitions correspond to an excitation from an O2p to a nearby Ti3d orbital and singlet-singlet transitions vary in excitation energy from 2.1 eV to 4.3 eV, depending on the oxygen site and environment of the titanium site. Two different structures for the three stacked Ti6O12 rings are found. The occluded Ti structure is lower in energy than a staggered structure by 1.25 eV, only 0.02 eV/bond. Excited electronic states are found to correspond to highly localized holes on oxygen and a highly localized electron in a d orbital on a nearest neighbor titanium. The staggered structure has four absorptions that lie within the intense portion of the solar spectrum, at 585 nm, 472 nm, 471 nm, 423 nm; the occluded structure has one strong absorption, at 532 nm.