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charge transfer; EPR spectroscopy; oxygen; TiO2 nanocrystals; UV-induced charge separation; charge transfer; EPR spectroscopy; oxygen; TiO2; UV-induced charge separation; gas-phase photooxidation; reduced titanium-dioxide; nanostructured TiO2; heterogeneous photocatalysis; surface-properties; rutile surfaces; hydroxyl-groups; spin-resonance; colloidal TiO2; particle-size
Abstract:
The interaction of photogenerated charges with molecular oxygen was investigated on TiO2 nonocrystals by means of paramagnetic resonance (EPR) spectroscopy. Compared to photoactivation experiments in Vacuum at p < 10(-6) mbar and T = 740 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 0,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.
