PHOTOCATALYTIC EFFICIENCY VARIABILITY IN TIO2 PARTICLES

The pre-steady-state photoproduction of hydroxyl radicals ((OH)-O-.) f rom various TiO2 samples in an aqueous suspension was investigated, us ing spin trapping with electron paramagnetic resonance (EPR) spectrosc opy for detection. This pre-steady-state OH production does not requir e 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-ste ady-state conditions. Therefore, we propose that variations in the rat e constants related to recombination and/or oxidation cause the observ ed variability in steady-state photocatalytic efficiency. An analysis of the pre-steady-state and steady-state results, based on a simple ki netic model, indicates that the observed variability arises specifical ly from variations in the rate constant for recombination. In addition , the role of the oxidative power of holes at the valence-band edge wa s 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 photocatal ytic efficiency does not appear to be significant.