EPR study of the surface characteristics of nanostructured TiO2 under UV irradiation

Nanostructured TiO2 samples with primary particle size in the 4-20 nm range were prepared by either hydrothermal (H) or thermal (T) treatment of an am orphous precursor, and their behavior under UV illumination at 77 X was stu died by means of EPR spectroscopy. The samples of the H series present the smallest crystallite size and after irradiation in a vacuum show some Ti3centers. In contrast, under these conditions only weak signals associated w ith oxygenated radicals are observed for the T samples. However, when oxyge n is preadsorbed, several oxygenated complexes (O-, O2-, O2H., and O-3(-)) are photogenerated in proportions that depend on the characteristics of the material. Subsurface O- species are exclusively detected in the case of th e samples of the H series, whereas ozonide radicals and surface O- are stab ilized on materials with larger crystalline domains. These oxygenated compl exes are thermally unstable, and they disappeared after warming to room tem perature in the case of the T samples, but they are transformed to O-2(-) o n the surface of the hydrothermally treated TiO2. Since adsorbed water and different types of free hydroxyls are present on these materials, as reveal ed by FTIR, a number of surface reactions have to be considered in order to account for the formation and stability of such photogenerated species.