The important roles of OH radicals for remote oxidation using TiO(2) photocatalysts were evidenced by the in situ detection of OH radicals in the gas phase using the laser-induced fluorescence (LIF) technique. The appearance of OD-LIF intensities after the exposure of D(2)O vapors over TiO(2) powders and the decrease of the time-resolved signals of OH-LIF intensities with increasing calcined temperatures of TiO(2) powders suggested that the exchangeable water at the TiO(2) surface is the origin of the diffused OH radicals.

Diffusion of OH radicals from UV-irradiated TiO 2 surface to the gas phase was successfully detected using a laser-induced-fluorescence technique for various types of TiO 2 powders. The diffusion time of OH radicals was found to vary with the types of TiO 2 powders and to be affected by the heat treatments of these powders, depending on the treatment temperatures. The diffusion mechanism was discussed based on the characteristic OH-LIF intensities for individual TiO 2 powders and the observations of OD-LIF after the exposure of D 2 O vapors over the TiO 2 powders. The quantum yield of OH radicals diffused from the TiO 2 surface was estimated to be about 5 × 10 -5 by comparing the OH-LIF intensities produced by the 266-nm photolysis of HNO 3.

The adsorption and photodecomposition of seven kinds of amino acids on a TiO2 surface were investigated by zeta potential measurements and 1H NMR spectroscopy in TiO2 aqueous suspension systems. The decomposition rates increased in the order of Phe < Ala < Asp < Trp < Asn < His < Ser. For Phe, Trp, Asn, His, and Ser, the isoelectric point (IEP) of TiO2 shifted to a lower pH with increasing decomposition rates upon adsorption on TiO2, suggesting that the effective adsorption and photocatalytic sites for these amino acids should be the basic terminal OH on the solid surface. Since the amino acids that decomposed faster than the others contain -OH (Ser), -NH (Trp, His), or -NH2 (Asn) in their side chain, they are considered to interact with the basic terminal OH groups more preferably by the side chain and are vulnerable to photocatalytic oxidation. On the other hand, Ala interacts with the acidic bridged OH on TiO2 to cause an IEP shift to a higher pH. The correlation of the surface hydroxyl groups with the photocatalysis of amino acids was verified by the use of calcined TiO2 without surface hydroxyl groups.