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 amorphous precursor, and their behavior under UV illumination at 77 K was studied by means of EPR spectroscopy. The samples of the H series present the smallest crystallite size and after irradiation in a vacuum show some Ti³⁺ centers. In contrast, under these conditions only weak signals associated with oxygenated radicals are observed for the T samples. However, when oxygen is preadsorbed, several oxygenated complexes (O^-, O₂^-, O₂H^., and O₃^-) are photogenerated in proportions that depend on the characteristics of the material. Subsurface O^- species are exclusively detected in the case of the samples of the H series, whereas ozonide radicals and surface O^- are stabilized on materials with larger crystalline domains. These oxygenated complexes are thermally unstable, and they disappeared after warming to room temperature in the case of the T samples, but they are transformed to O₂^- on the surface of the hydrothermally treated TiO₂. Since adsorbed water and different types of free hydroxyls are present on these materials, as revealed by FTIR, a number of surface reactions have to be considered in order to account for the formation and stability of such photogenerated species.