Defects at the (1 1 0) surface of rutile TiO2 from ab initio calculations

Defect segregation of oxygen vacancies, protons, titanium vacancies, and the mutual complex between the two latter (i.e. Ruetschi type defects) at the (1 1 0) surface of rutile TiO₂ has been studied through ab initio Density Functional Theory calculations (DFT). Oxygen vacancies and protons form mainly on the 2-fold coordinated oxygen sites, while titanium vacancy related defects are expected to be most prominent at the 5-fold titanium sites. All included defects are shown to be more stable at the surface than in the bulk oxide, and the relative tendency to segregate is most pronounced for oxygen vacancies. Based on the obtained defect segregation energies and previously calculated bulk defect chemistry at lower temperatures, our estimated surface defect formation energies indicate that the surface of TiO₂ can be expected to be dominated by titanium vacancies, protons and their mutual complexes at lower temperatures under wet, oxidizing conditions. Finally, possible routes of dissociative water adsorption on the surface, involving the aforementioned defects are discussed.