EPR Study of Photo-Induced Surface Modifications of Nanocrystalline TiO₂ Samples

Abstract

The effect of UV-laser irradiation in dynamic vacuum on the surface characteristic of the nanostructured TiO₂ is studied. For these purpose two titania samples, with primary particle size of respectively 5 and 16 nm were outgassed at RT, either in the dark or under UV-laser irradiation. Radicals formed by additional UV-illumination at 77 K in static vacuum or in the presence of O₂ were monitored by means of EPR spectroscopy. The obtained spectra reveal that the studied samples are differently affected by laser illumination. In the case of the TiO₂ materials with the smaller particle size some Ti³⁺ centers are detected upon irradiation at 77 K in static vacuum, and the laser pretreatment greatly increases the concentration of photogenerated radicals formed in the presence of oxygen, especially subsurface O^- and O₂ ^- species. However, in case of the TiO₂ sample with larger crystalline diameter no reduced titanium sites are detected and the effect of irradiation in dynamic vacuum is not so remarkable; oxygen adsorption and subsequent UV-illumination give rise to the formation of O₂H, O^- and O₃ ^- radicals are observed with different proportions depending on the pretreatment conditions. These results are discussed in connection with structural characteristics of these nano-sized materials, along with the possible photoinduced modifications of their surfaces.