Infrared Spectra of Photoinduced Species on Hydroxylated Titania Surfaces

We detect transient and persistent diffuse reflectance infrared signals from surface intermediates generated by band gap irradiation of polycrystalline, hydroxylated TiO_2 under controlled atmospheres. Irradiation in vacuo or in the presence of CD_3OD(ads) leads to a new absorbance at 3716 cm^(-1), the decline of a 3647 cm^(-1) band, and a reflectivity loss. Overall reflectivity, which gauges mobile charge, partially recovers in the dark after a few minutes but fully recovers upon exposure to O_2. The 3716 cm^(-1) feature persists for days in a vacuum or dry O_2 but is bleached by the action of (O_2 + H_2O + hν). It is assigned to a Ti(III)O−H^- vibration resulting from electron trapping at acidic Ti(IV)OH centers. Irradiation under O_2 yields a new band at 3683 cm^(-1), ascribed to surface-bound OH radicals that last indefinitely in a vacuum. None of the above events occur on thermally generated TiO_(2-γ) specimens subsequently exposed to H_2O(g). We infer that (1) O_2 is an efficient scavenger of conduction band electrons at the gas−solid interface, (2) the competition for holes between lattice oxygen photodesorption and OH^- oxidation depends on gas pressure and composition, and (3) the trapped carrier buildup eventually results in extended surface reconstruction involving Ti−OH functionalities, giving rise to restructuring hysteresis.