Synthesis and photocatalytic activity of Eu3+-doped nanoparticulate TiO2 sols and thermal stability of the resulting xerogels

The synthesis of nanoparticulate TiO2 sols without and with Eu3+ doping (1, 2, or 3 mol%) by the colloidal sol gel method in aqueous media was investigated, with emphasis on the effect of the Eu3+ doping on the peptization time and rheological properties of the sols. It was found that the addition of Eu3+ increasingly retards the peptization process, and also results in sols with greater aggregate sizes which are therefore more viscous, although in all cases the distributions of aggregate sizes are unimodal and the flow behavior is Newtonian. The shifting of the isoelectric point of the sols toward greater pH with increasing Eu3+ doping indicates that the aforementioned trends are due to the chemical adsorption of europium ionic complexes in the form of solvated species. Furthermore, the effect of Eu3+ doping on the ultraviolet visible spectrum and photocatalytic activity of the peptized sols was also explored. It was found that the Eu3+ doping increasingly shifts slightly the absorption edge from the ultraviolet to the visible range, and that its effect on the photocatalytic activity is certainly complex because this is enhanced only if the Eu3+ cations have some electronic transition (charge transfer transition or transitions between the ground state and the excited states) at the wavelength of the incident radiation, in which case the photocatalytic activity first increases with increasing Eu3+ content and then decreases perhaps due to occurrence of Eu-Eu interactions or simply to the greater aggregation state. Finally, the influence of the Eu3+ doping on the thermal stability of the nanoparticulate xerogels resulting from the drying of the peptized sols was also examined by X-ray thermo-diffractometry together with transmission electron microscopy, selected area electron diffractometry, and X-ray energy-dispersive spectrometry. It was found that although the xerogels crystallize all as anatase phase, this is increasingly more thermally stable with increasing Eu3+ doping, displaying a slowed down nanocrystallite growth, delayed onset temperature of the anatase-to-rutile phase transformation, and extended retention temperature of anatase phase. (C) 2013 Elsevier B.V. All rights reserved.