Charge Trapping and Photoadsorption of O 2 on Dehydroxylated TiO 2 Nanocrystals—An Electron Paramagnetic Resonance Study

The interaction of photogenerated charges with molecular oxygen was investigated on TiO 2 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 O 2 enhances the concentration of persistently trapped electron and hole centres—by a factor of ten—due to the formation of adsorbed O 2 − species. The photoadsorption of oxygen was also tracked quantitatively by pressure measurements, and the number of trapped charges, hole centres and O 2 − was found to correspond to ten electron–hole pairs per TiO 2 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.