Infrared Spectra of Photoinduced Species on Hydroxylated Titania Surfaces

Abstract

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 3 OD(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 2 O + h ν). 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 2 O(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.