Oxygen Vacancy Formation on Rutile TiO2(110) and Its Interaction with Molecular Oxygen:  A Theoretical Density Functional Theory Study

Density functional calculations at the B3LYP/6-31G* level were performed to mimic the oxygen vacancy formation on rutile TiO2(110) surface as well as for its interaction with molecular oxygen using extended cluster models. It was shown that the experimental temperature-programmed desorption observation on triply exceeding the concentration of O2 per vacancy site at a low-temperature saturation coverage on TiO2(110) can be well explained without making an assumption on the formation of the tetraoxygen (O4) fragment at the vacancy site, provided that the mechanism of formation of precursor defect sites on rutile TiO2(110) is properly addressed. The results obtained show that there are two kinds of dioxygen molecules adsorbed in a ratio of 1:2. The first kind of dioxygen is directly located at the vacancy site in a nearly perpendicular fashion, while the other involves two dioxygen molecules on next-nearest 5-fold coordinated titanium (Ti5C) sites with additional stabilization through a H-bonding pattern with the two hydroxyl groups nearby from both sides of the vacancy site. The latter adsorption form is expected to undergo a fast desorption, while the former one can be involved in the photooxidation of coadsorbed CO molecules.