Double-Beam Photoacoustic Spectroscopic Studies on Transient Absorption of Titanium(IV) Oxide Photocatalyst Powders

In situ photoabsorption properties of titanium(IV) oxide (TiO 2 ) 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 TiO 2 powder samples. Most of the samples (type 1) exhibited photoabsorption due to the production of trivalent titanium (Ti 3+ ) 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 Ti 3+ 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 Ti 3+ 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 Ti 3+ 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 Ti 3+ species, which is a potential measure of density of crystalline defects.