Second-harmonic generation studies were made of polished and etched (110) monocrystalline rutile surfaces. The results suggested that above-bandgap low-energy (4.7eV) ultra-violet irradiation created stable Ti3+ surface defects under ultra-high vacuum. Such defects could be healed by exposing the surface to O2. The results were similar to previously reported data on polished and etched (001) monocrystalline rutile surfaces. Observations of the (001) surfaces had been explained in terms of the photo-desorption and re-adsorption of molecular O that was loosely bound, to Ti3+ defects on the surface, as a Ti4+:O2- complex. In the present study, X-ray photo-electron spectroscopy was used to confirm the type of defect and to measure the density of Ti3+ defects which was created. Defects on the surfaces, which were studied using X-ray photo-electron spectroscopy, were generated by using photons of even lower energy (3.4eV). This indicated that the O species which were removed were very loosely bound. The same defect creation and healing processes were also observed on a nearly defect-free thermally annealed monocrystalline surface. It was not determined whether bridging O ions on the (110) surface, or O2- ions, were the photo-labile species.

Comparative Second Harmonic Generation and X-Ray Photoelectron Spectroscopy Studies of the UV Creation and O2 Healing of Ti3+ Defects on (110) Rutile TiO2 Surfaces. Shultz, A.N., Jang, W., Hetherington, W.M., Baer, D.R., Wang, L.Q., Engelhard, M.H.: Surface Science, 1995, 339, 114-24