title: Charge trapping and photoadsorption of O2 on dehydroxylated TiO2 nanocrystals--an electron paramagnetic resonance study.
abstract: The interaction of photogenerated charges with molecular oxygen was investigated on TiO2 nanocrystals by means of paramagnetic resonance (EPR) spectroscopy. Compared to photoactivation experiments in vacuum at P < 10(-6) mbar and T = 140 K, the presence of O2 enhances the concentration of persistently trapped electron and hole centres--by a factor of ten--due to the formation of adsorbed O2- species. The photoadsorption of oxygen was also tracked quantitatively by pressure measurements, and the number of trapped charges, hole centres and O2- was found to correspond to ten electron-hole pairs per TiO2 nanocrystal. Conversely, in experiments at P < 10(-6) mbar with one trapped electron-hole pair per particle, charge separation is not persistent and completely reversible with respect to temperature. Heating to 298 K causes the total annihilation of photogenerated and trapped charges.
authors.affiliation: Institut für Materialchemie, Technische Universität Wien, Veterinärplatz 1/GA, 1210 Vienna, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Berger
authors.forename: Martin
authors.initials: M
authors.lastname: Sterrer
authors.forename: Oliver
authors.initials: O
authors.lastname: Diwald
authors.forename: Erich
authors.initials: E
authors.lastname: Knözinger