Evidence for O2- Radical Stabilization at Surface Oxygen Vacancies on Polycrystalline TiO2

The characterization and stability of superoxide radicals (O2-) over polycrystalline TiO2 (Degussa P25) was investigated using electron paramagnetic resonance (EPR) spectroscopy. The adsorbed oxygen molecules act as efficient electron scavengers and were therefore used to indirectly probe the sites of electron transfer at the surface of the anatase component of the mixed phase P25 material. A distribution of various stabilization sites on the surface was identified by analysis of the g values and further confirmed by identification of several well-resolved 17O hyperfine patterns. For the first time, on a polycrystalline TiO2 surface evidence for stabilization of superoxide radicals specifically at anion vacancy sites is presented by EPR. These radicals, labeled [Vac...O2-], are characterized by the spin Hamiltonian parameters of gxx = 2.005, gyy = 2.011, gzz = 2.019, and 17OAxx = 7.64 mT (Ayy = Azz > 1 mT). The [Vac...O2-] radicals exhibit pronounced reactivity under the influence of thermal, photochemical, and chemical treatment compared to the remaining surface O2- anions bound at nonvacancy sites. The extent of site occupancy was found to be sensitive to the oxygen adsorption temperature and the extent of O2- radical migration on the surface. Thus, the stability and lifetime of the surface O2- anions are directly correlated to the structure of the adsorption site itself at the anatase surface of P25.