Mechanistic Investigations of Photoelectrochemical Water and Methanol Oxidation on Well-Defined TiO₂ Anatase (101) and Rutile (110) Surfaces

The mechanisms of photoelectrochemical water and methanol oxidation on TiO₂ anatase (101) and rutile (110) surfaces have been studied using the intensity modulated photocurrent spectroscopy (IMPS) technique. The phenomenological rate constants for the photogenerated charge carriers transfer and recombination have been determined at different band bending values in the presence and absence of methanol as well as at different methanol concentrations. The obtained results have been analyzed based on the IMPS theory for a bulk semiconductor surface together with a model to define the nature of the surface-bound intermediates. The results of the analysis indicated that (i) water oxidation proceeds via coupling of two >OH^• _br radicals on both surfaces and the coupling of >OH^• _br radicals is about 1 order of magnitude faster on the anatase (101) surface than that on the rutile (110) surface; (ii) the rate constant of surface recombination at the flatband potential on anatase (101) surface is much lower than that on rutile (110) surface; and (iii) in the presence of methanol, the coupling of the >OH^• _br radicals on the anatase (101) surface is still the dominating reaction whereas on the rutile (110) surface the photogenerated >OH^• _br reacts rapidly with methanol, faster by a factor of 18 times than on anatase (101) surface so that the >OH^• _br coupling is less dominant. On the basis of those findings, it is concluded that the coupling of the >OH^• _br radicals is an essential step to trap the photogenerated holes and reduce the surface recombination on anatase (101) surface in the absence and presence of methanol. The faster reaction of methanol with the >OH^• _br on rutile (110) compared with that on anatase (101), while the former usually exhibits lower photocatalytic activity, indicates that the charge separation (not the charge transfer) is the rate controlling step in the photocatalytic process.