EPR Study of the Surface Characteristics of Nanostructured TiO₂ under UV Irradiation

Nanostructured TiO 2 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 Ti 3+ 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 -, O 2 -, O 2 H •, 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 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 O 2 - on the surface of the hydrothermally treated TiO 2 . 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.