Peroxide and superoxide states of adsorbed O2 on anatase TiO2 (101) with subsurface defects

Density Functional Theory (DFT) calculations within the Generalized Gradient Approximation (GGA) and the GGA + U approach are carried out to investigate the adsorption of O-2 on anatase (101) surfaces having subsurface oxygen vacancies. Our results show that O-2 adsorption is strongly enhanced at sites close to the subsurface defect, whereas dissociation is unfavorable at all sites. The adsorption is accompanied by the transfer of the defect electrons to O-2-derived electronic states in the anatase surface band gap. Peroxide species (O-2(2-), O-O = 1.48 angstrom) are stable when the number of adsorbed O-2 molecules is less or equal the number of defects, whereas superoxide species (O-2(-), O-O = 1.33 angstrom) become more favorable at coverages exceeding approximately 1.5 O-2 molecules per oxygen vacancy.