From an interplay between high-resolution scanning tunnelling microscopy and density functional theory the origin of various point defects on reduced rutile TiO2(110)-(1x1) surfaces was deduced. By means of adsorption and desorption experiments using water and oxygen as probe molecules, the different features observed in scanning tunnelling microscopic images were attributed to bridging oxygen vacancies, oxygen atoms on surface Ti atoms, and single as well as pairs of hydroxyls on bridging oxygen rows. These experimental results were discussed in comparison to previous scanning tunnelling microscopy reports where different assignments of the scanning tunnelling microscopic features were suggested. Based upon density functional theory calculations the interaction of water and oxygen with the reduced TiO2(110) surface was compared with the situation when these molecules encountered a perfect stoichiometric TiO2(110) surface. These density functional theory calculations strongly supported the assignments of the features observed experimentally by scanning tunnelling microscopy. It was explained how to produce clean reduced TiO2(110) surfaces and criteria for ascertaining the cleanliness by scanning tunnelling microscopy were proposed.

Oxygen Vacancies on TiO2(110) and their Interaction with H2O and O2: a Combined High-Resolution STM and DFT Study. Wendt, S., Schaub, R., Matthiesen, J., Vestergaard, E.K., Wahlström, E., Rasmussen, M.D., Thostrup, P., Molina, L.M., Lægsgaard, E., Stensgaard, I., Hammer, B., Besenbacher, F.: Surface Science, 2005, 598[1-3], 226-45