interaction of TiO2 surface with oxygen (O2) is necessary. The chemistry of oxygen is an important, but often overlooked, component of many chemical and photochemical processes that take place on TiO2-based materials. Oxygen is widely used in photochemical oxidation studies on TiO2 and is usually considered to act primarily as a scavenger of the photoexcited electrons, thus preventing negative charge accumulation on the surface of catalyst. In some solution-phase photochemical studies, the resulting superoxide radical anion (O2−) species are envisioned to participate in homogeneous-phase reactions with water molecules to form hydrogen peroxyl species that may play a role in organic oxidation, either in solution or on the surface. Only a few groups have proposed that surface-bound O2 or O2− species may be directly involved in photooxidation of coabsorbed species. The step of O2− formation upon chemisorption of oxygen requires the presence of surface vacancy sites. No oxygen absorption would occur on a stoichiometric surface. However, the exact nature of absorbed O2 on these TiO2 surfaces is not well understood. Some studies have been carried out for the reduction of TiO2 in oxygen vacancy sites obtained by high-temperature annealing in ultrahigh vacuum (UHV) [12]. TiO2 decomposes at high temperatures and low oxygen pressures or at elevated temperatures in the presence of reducing gases, thereby forming suboxides TiOx with x < 2. Molecular oxygen is typically used by UHV researchers to restore the stoichiometry of ion-sputtered or vacuum-reduced TiO2 surfaces.