Structural and photoluminescence study of Eu3+/TiO2 xerogels as a function of the temperature using optical techniques

Eu3+/TiO2 xerogels have been obtained from colloidal sols by drying at room conditions. Anatase with traces of brookite phases are obtained from the synthesis and the stability of both with temperature is higher (from 400°C up to 800°C) when Eu3+ is present. Raman phonons have been used to detect the different phases and follow the structural transitions. The observed changes with calcination temperature of anatase modes (Eg,1, B1g,1 Ag/B1g,2, and Eg,3) are found to be mainly related to the grain size. The anatase grain size increases more and at lower temperatures for undoped than for Eu3+-doped samples favoring the transition to rutile. The stability of the brookite phase is also influenced by the doping being higher in the doped xerogels (700°C) than in the undoped ones (400°C). No concentration quenching of the f-f Eu3+ emission bands is observed up to 3% Eu3+ at low temperatures but the maximum emission is found at lower temperatures for higher Eu3+ content may be due to quenching related to Eu3+ migration. The narrower f-f emission bands of the as-prepared samples are consistent with the presence of Eu3+ ions at the surface with a weak interaction with the TiO2 nanoparticles. A diffusion process of Eu3+ ions occurs during calcination from the surface to different positions of the anatase lattice close to the surface producing an inhomogeneous broadening. Finally, the formation of the pyrochlore phase Eu2Ti2O7 is detected which explains the decrease on the emission efficiency and the increase of the Eu3+ environment symmetry.
This study has been supported by the Spanish Ministry of Economy and Competitiveness through projects MAT2012‐31090 and MAT2012‐37276‐C03‐01. Dr. M. Borlaf thanks CSIC for the concession of a grant (JAE‐Pre 083).