Effect of Surface-Adsorbed Oxygen on Charge Recombination in a Nanocrystalline TiO2 Film Studied by Ultrasensitive Transient Absorption Spectroscopy

Here, we studied the charge recombinationprocess ina nanocrystallineTiO(2) film by using our newly developed, ultra sensitive,transient absorption spectrometer that can measure absorbance changesof less than 10(-7) at a time resolution of 20 ns.Using the apparatus, high-quality transient absorption signals couldbe obtained even under low excitation light intensities (<10 & mu;Jcm(-2)). We found that the rate of charge recombinationwas strongly enhanced by removing oxygen from the film surface andthat this removal had no effect on the shape of the spectrum, indicatingthat the rate of charge recombination is limited not only by electrontrapping. To explain the experimental results as well as other relatedparadoxical phenomena reported elsewhere in the literature, we proposea model that considers Coulomb repulsion induced by trapped electronsat the particle surface.