Double-beam photoacoustic spectroscopic studies on transient absorption of Titanium(IV) oxide photocatalyst powders

In situ photoabsorption properties of titanium(IV) oxide (TiO2) powders under continuous ultraviolet irradiation were investigated by double-beam photoacoustic (PA) spectroscopy. This PA measurement enabled observation of two kinds of ultraviolet-light-induced intermediate species appearing on various kinds of TiO2 powder samples. Most of the samples (type 1) exhibited photoabsorption due to the production of trivalent titanium (Ti3+) species, while transient absorption assigned to trapped holes or surface peroxy species was also observed for anatase samples with a relatively large specific surface area (type 2). Time-resolved measurements and analyses of the kinetics of photoinduced Ti3+ species suggest that electrons accumulated in type-2 samples have high reactivity toward molecular oxygen compared to type-1 samples. Saturation limits of intensity of the PA signal attributed to Ti3+ species under deaerated conditions in the presence of surface-adsorbed methanol were estimated for both types of samples, and their linear relation with density of Ti3+ species estimated by a conventional photochemical technique was observed. This suggests that the present double-beam PA technique is an alternative feasible method for estimation of density of Ti3+ species, which is a potential measure of density of crystalline defects.