Interstitial oxygen atoms (Oi) in TiO2 were investigated using the density functional theory-based first-principle band structure method. Two polymorphs of TiO2, anatase and rutile structures, were considered. A symmetrical O-O structure was obtained in rutile and was quite similar to the reported structure in anatase. In both polymorphs, Oi traps electrons from the conduction band to the σ* orbital of the O-O structure. The bond length r(O-O) was elongated in this process. The interactions between Oi and Nb or Ta impurities were also surveyed. There was weak attraction for both impurities. A simple statistical analysis was conducted to evaluate the Oi formation in anatase. Two conclusions were drawn from the present results. 1) Oi forms in n-type TiO2 in either the anatase or rutile structure under an O-rich condition. 2) The entropy factor, which originates from the distribution of Oi and from the electrons in the conduction band, makes a small contribution to the free energy at room temperature. The discrepancy between the statistical analysis and the experimental O2 gas annealing process implied the importance of the kinetics of the absorption and migration of Oi. The locality of Oi and Nb/Ta impurities in Nb-doped anatase TiO2 was briefly discussed.

Theoretical Study of the Interstitial Oxygen Atom in Anatase and Rutile TiO2: Electron Trapping and Elongation of the r(O-O) Bond. Kamisaka, H., Yamashita, K.: Journal of Physical Chemistry C, 2011, 115[16], 8265-73