Scanning tunnelling microscopy and photo-emission spectroscopy were used to determine the origin of the band-gap state in rutile TiO2(110). This state had long been attributed to oxygen vacancies (Obvac). However, a recently suggested alternative origin was sub-surface interstitial Ti species. Here, electron bombardment was used to vary the Obvac density while monitoring the band-gap state using photo-emission spectroscopy. The results showed that Ob vac made the dominant contribution to the photo-emission peak and that its magnitude was directly proportional to the Obvac density.

Oxygen Vacancy Origin of the Surface Band-Gap State of TiO2(110). Yim, C.M., Pang, C.L., Thornton, G.: Physical Review Letters, 2010, 104[3], 036806