Heavily impregnated ceria nanoparticles with europium oxide: spectroscopic evidences for homogenous solid solutions and intrinsic structure of Eu3+-oxygen environments

Abstract

We report on the homogeneity, structural and luminescence properties of ceria nanoparticles doped with Eu3+. Eu3+ in concentration of 1, 5 and 10 wt% was incorporated via wetness impregnation into preformed ceria nanoparticles followed by calcination in air at 1000 °C. A remarkable homogeneity of Eu3+-ceria solid solutions is measured for ceria grown by citrate and micro-emulsion methods using Raman, Diffuse Reflectance in UV–Vis, photoluminescence spectroscopies and X-ray diffraction, even for the Eu3+ concentration of 10 wt%. The emission properties of all Eu3+-doped ceria samples are well-characterized by a two main centre model assigned to perturbed and isolated Eu3+ centres. These centres correspond to Eu3+ located in the nearest (local symmetry lower than cubic and Eu3+-oxygen coordination lower than eight) and next-nearest-neighbour positions (cubic local symmetry and eightfold Eu3+-oxygen coordination) to oxygen vacancy, respectively. With increase of Eu3+ concentration, both the oxygen vacancy concentration and the relative contribution of the perturbed Eu3+ centre to the total emission increase. It is established that the characteristic emission and excitation spectra of the two main Eu3+ centres as well as the overall multisite distribution of Eu3+ within ceria lattice are intrinsic properties of Eu3+-doped ceria since these do not depend on synthesis route, nanoparticle size and Eu3+ concentration.