Magnetic resonance study of the defects influence on the surface characteristics of nanosize anatase

Two anatase samples with crystal size of 11 and 6 nm, prepared by thermal, P11T, and hydrothermal treatments, P6H, of an amorphous TiO2 precursor, have been studied by 1H nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) to obtain information on how the samples defects influence their surface characteristics. The NMR spectra of the hydrated samples were mainly originated by weakly adsorbed water, more abundant on P6H than on P11T, while most of the spectra obtained after evacuation at 473 K were originated by hydroxyls. P6H presented a larger amount of hydroxyls, some of them with significantly different chemical shifts, than P11T. Those particular hydroxyls are generated by water dissociation at oxygen vacancies. The type and intensity of the signals observed in the EPR spectra of the UV irradiated samples depended on the samples preparation method and on the amount of water weakly adsorbed on the samples. For hydrated P6H, photogenerated electrons are stabilized by Ti4+ cations at oxygen vacancies, while these electrons can be delocalized in the P11T conduction band. The stabilization of photogenerated holes as O− radicals by low coordinate bridging O2−, more marked for P6H than for P11T, is favored by weakly adsorbed water. This effect has been attributed to the photodesorption of adsorbed water and protons from acidic bridging hydroxyls, as H3O+ species exchanging protons with adsorbed water, while the hydroxyls bridging O2− anions trap photogenerated holes.