Charge Trapping and Photoadsorption of O2 on Dehydroxylated TiO2 Nanocrystals—An Electron Paramagnetic Resonance Study

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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.