PHOTOCATALYTIC EFFICIENCY VARIABILITY IN TIO2 PARTICLES

The pre-steady-state photoproduction of hydroxyl radicals ((OH)-O-.) from various TiO2 samples in an aqueous suspension was investigated, using spin trapping with electron paramagnetic resonance (EPR) spectroscopy for detection. This pre-steady-state OH production does not require the presence of a dissolved electron acceptor and thus depends only on processes of oxidation and recombination. The efficiency variations that characterize the steady state, in particular for samples in the anatase and rutile crystal forms of TiO2, are also found under pre-steady-state conditions. Therefore, we propose that variations in the rate constants related to recombination and/or oxidation cause the observed variability in steady-state photocatalytic efficiency. An analysis of the pre-steady-state and steady-state results, based on a simple kinetic model, indicates that the observed variability arises specifically from variations in the rate constant for recombination. In addition, the role of the oxidative power of holes at the valence-band edge was gauged, using size-quantized rutile particles in the form of quantum wires, which were obtained from TICl4 hydrolysis. It is expected that almost the entire bandgap increase of similar to 0.14 eV is imparted to the hole oxidative power. Nonetheless, the effect on the photocatalytic efficiency does not appear to be significant.