No abstract

The rutile form of titanium dioxide (r-TiO 2 ) usually shows a photocatalytic activity lower than that of anatase-TiO 2 (a-TiO 2 ). Nevertheless, there are numerous examples of sometimes unexpectedly high activity of r-TiO 2 . This material may appear to be particularly useful when a noncomplete and selective photocatalytic oxidation of organic substrates is required. On the basis of literature examples and our own studies, we compare the photocatalytic activities of r-TiO 2 , a-TiO 2 , and r-TiO 2 /a-TiO 2 composites. Because of a significantly better oxygen adsorption at the surface of r-TiO 2 and a lower redox potential of the excited electron, more efficient O 2•− production takes place at the surface of rutile. As a consequence, generation of 1 O 2 (involving O 2 reduction and the subsequent oxidation of superoxide) and reduction of H 2 O 2 to OH − and HO • are also favored with this material. Therefore, r-TiO 2 can be considered to be a particularly good photocatalyst for activation of molecular oxygen. On the other hand, a-TiO 2 appears to be a stronger oxidant. In its presence, efficient HO • generation (the result of water or surface hydroxyl group oxidation) and oxidation of H 2 O 2 to O 2•− are observed.

The generation of singlet oxygen in aqueous colloids of nanocrystalline TiO2 (anatase) modified by organic chelating ligands forming surface Ti(IV) complexes was studied. Detailed studies revealed a plausible and to date unappreciated influence of near-infrared irradiation on singlet oxygen generation at the surface of TiO2. To detect (1)O2, direct and indirect methods have been applied: a photon counting technique enabling time-resolved measurements of (1)O2 phosphorescence, and fluorescence measurements of a product of singlet oxygen interaction with Singlet Oxygen Sensor Green (SOSG). Both methods proved the generation of (1)O2. Nanocrystalline TiO2 modified with salicylic acid appeared to be the most efficient photosensitizer among the tested materials. The measured quantum yield reached the value of 0.012 upon irradiation at 355 nm, while unmodified TiO2 colloids appeared to be substantially less efficient generators of singlet oxygen with the corresponding quantum yield of ca. 0.003. A photocatalytic degradation of 4-chlorophenol, proceeding through oxidation by OH˙, was also monitored. The influence of irradiation conditions (UV, vis, NIR or any combination of these spectral ranges) on the generation of both singlet oxygen and hydroxyl radicals has been tested and discussed. Simultaneous irradiation with visible and NIR light did not accelerate OH˙ formation; however, for TiO2 modified with catechol it influenced (1)O2 generation. Singlet oxygen is presumably formed according to Nosaka's mechanism comprising O2˙(-) oxidation with a strong oxidant (hole, an oxidized ligand); however, the energy transfer from NIR-excited titanium(iii) centers (trapped electrons) plays also a plausible role.

No abstract

No abstract