It was recalled that bridging oxygen (Obr) vacancies were usually assumed to cause the Ti3d defect state in the band gap of rutile TiO2(110). From high-resolution scanning tunnelling microscopy and photo-electron spectroscopy measurements, it was proposed that Ti interstitials in the near-surface region may be largely responsible for the defect state in the band gap. It was argued that these donor-specific sites played a key role in, and might dictate, the ensuing surface chemistry, such as providing the electronic charge required for O2 adsorption and dissociation. Specifically, a second O2 dissociation channel was identified that occurred within the Ti troughs in addition to the O2 dissociation channel in Obr vacancies. Comprehensive density functional theory calculations supported these experimental observations.

The Role of Interstitial Sites in the Ti3d Defect State in the Band Gap of Titania. Wendt, S., Sprunger, P.T., Lira, E., Madsen, G.K.H., Li, Z., Hansen, J.Ø., Matthiesen, J., Blekinge-Rasmussen, A., Lægsgaard, E., Hammer, B., Besenbacher, F.: Science, 2008, 320[5884], 1755-9