A multifrequency EPR study on organic-capped anatase TiO2 nanocrystals

A multifrequency electron paramagnetic resonance, EPR, study has been carried out on TiO2 nanocrystals (NCs) capped by organic moieties prepared according to both a nonhydrolytic and a hydrolytic procedure, respectively, with spherical or rodlike shape. In particular, EPR measurements have been performed in the 9-95 GHz range, at various temperatures, and with and without UV irradiation. The behavior of the electrons promoted in the conduction band by UV irradiation and of the holes in the valence band has been monitored by means of the generated paramagnetic species that are detectable by EPR, thus enabling the identification of the different sites at which the charges are trapped. In particular, an EPR signal which can be considered a mark of the localization of the photoinduced charge on a carbon of the capping molecule is observed. The presence of paramagnetic species on the surfaces of TiO2 NCs has been used to account for the catalytic performance of such a class of nanostructured material which has been recently proven to present a high catalytic efficiency. The observed carbon radical is suggested to be responsible for the higher catalytic activity of organic capped nanosized catalysts. Indeed, upon irradiation, the intensity of this signal with respect to the bulk Ti3+ signal is larger in the NCs characterized by higher photocatalytic activity; namely, those prepared with a hydrolytic procedure.