Point defects in metal oxides such as ${\text{TiO}}_2$ are key to their applications in numerous technologies. The investigation of thermally induced nonstoichiometry in ${\text{TiO}}_2$ is complicated by the difficulties in preparing and determining a desired degree of nonstoichiometry. We study controlled self-doping of ${\text{TiO}}_2$ by adsorption of 1/8 and 1/16 monolayer Ti at the (110) surface using a combination of experimental and computational approaches to unravel the details of the adsorption process and the oxidation state of Ti. Upon adsorption of Ti, x-ray and ultraviolet photoemission spectroscopy (XPS and UPS) show formation of reduced Ti. Comparison of pure density functional theory (DFT) with experiment shows that pure DFT provides an inconsistent description of the electronic structure. To surmount this difficulty, we apply DFT corrected for on-site Coulomb interaction $\left(\text{DFT}+U\right)$ to describe reduced Ti ions. The optimal value of $U$ is 3 eV, determined from comparison of the computed $\text{Ti}3d$ electronic density of states with the UPS data. $\text{DFT}+U$ and UPS show the appearance of a $\text{Ti}3d$ adsorbate-induced state at 1.3 eV above the valence band and 1.0 eV below the conduction band. The computations show that the adsorbed Ti atom is oxidized to ${\text{Ti}}^{2+}$ and a fivefold coordinated surface Ti atom is reduced to ${\text{Ti}}^{3+}$, while the remaining electron is distributed among other surface Ti atoms. The UPS data are best fitted with reduced ${\text{Ti}}^{2+}$ and ${\text{Ti}}^{3+}$ ions. These results demonstrate that the complexity of doped metal oxides is best understood with a combination of experiment and appropriate computations.

The re-oxidation of slightly reduced TiO₂(110) surfaces by exposure to an oxygen pressure of ~2 × 10^-7mbar in the temperature range 473-1000 K occurs by re-growth of TiO₂overlayers by diffusion of Tiⁿ⁺interstitials from the bulk. Starting with a (1 × 2) reconstructed surface, scanning tunnelling microscope images of the surface reacting under these conditions show that the (1 × 1) islands nucleate within the (1 × 2) layer and grow laterally. As the islands reach a critical size, which is temperature dependent, a new (1 × 2) layer begins to nucleate and grow. At both extremes of the temperature range nucleation of the second layer occurs before coalescence of the (1 × 1) islands, however, at temperatures between 673 and 773 K large areas of (1 × 1) surface form before growth of the second layer. The reaction is cyclic and several layers of TiO₂can be grown in this way.