Experimental observations indicate that removing bridging O atoms from the TiO2 rutile (110) surface produces a localised state approximately 0.7eV below the conduction band. The corresponding excess electron density was thought to localise on the pair of Ti atoms neighbouring the vacancy; formally giving two Ti3+ sites. The electronic structure and geometry of the O-deficient TiO2 rutile (110) surfaces was considered by using both gradient-corrected density functional theory (GGA DFT) and density functional theory corrected for on-site Coulomb interactions (GGA+U) to allow a direct comparison of the 2 methods. It was shown that GGA failed to predict the experimentally observed electronic structure, in agreement with previous uncorrected density functional theory calculations for this system. Introducing the +U term encourages localisation of the excess electronic charge, with the qualitative distribution depending on the value of U. For low values of U (≤4.0eV) the charge localises in the sub-surface layers occupied in the GGA solution at arbitrary Ti sites, whereas higher values of U (≥4.2eV) predict strong localisation with the excess electronic charge mainly on the two Ti atoms neighbouring the vacancy. The precise charge distribution for these larger U values was found to differ from that predicted by previous hybrid density functional theory calculations.

A DFT+U Description of Oxygen Vacancies at the TiO2 Rutile (110) Surface. Morgan, B.J., Watson, G.W.: Surface Science, 2007, 601[21], 5034-41