Structural and optical properties of Eu3+/Gd3+ ions in silica xerogels and powders obtained by sol-gel method

The xerogels and sol-gel powders with different quantitative composition have been studied based on excitation and emission measurements as well as luminescence decay analysis. The structure of prepared silica sol-gel materials was examined using FT-IR spectroscopy technique. The photoluminescence spectra of Eu3+ ions were registered upon two different excitation wavelengths related to direct excitation of Eu3+ ions (lambda(exc) = 393 nm) and indirect excitation through energy transfer process from Gd3+ to Eu3+ (lambda(exc) = 273 nm). Upon direct excitation of Eu3+ (F-7(0) -> L-5(6) transition) the characteristic emission bands assigned to the D-5(0) -> F-7(J(0-4)) electronic transitions were observed. Also, it was found that enhanced luminescence of Eu3+ is a result of change the excitation parameter through energy transfer phenomenon (S-8(7/2) -> I-6(J) transition of Gd3+) and occurred in both xerogels and powder samples. In this way, a UV photon absorbed by Gd3+ is converted into visible light emitted by Eu3+ ions. Obtained results clearly indicated that high concentration of Gd3+ ions in powders led to particularly strong enhance the emission in red spectral range and allowed for more than 12-times prolongation of luminescence lifetime for the D-5(0) excited state of Eu3+ compared to silica xerogel samples. The values of energy transfer efficiency were also estimated based on luminescence decay times of the P-6(7/2) state of Gd3+ ions. (C) 2016 Elsevier B.V. All rights reserved.