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.