Spectroscopic characterization of doped sol-gel silica gels and glasses: evidence of inner-sphere complexation of europium(III)

Eu3+ luminescence, excitation and lifetime spectroscopy measurements have been made during the gelation and densification of Eu3+-doped silica gels prepared by the hydrolysis and condensation of tetraethoxysilane (TEOS). Particular attention was paid to the effect of counter-ion complexation of Eu3+ in gels prepared with different europium precursors. Precursor-dependent spectroscopic variations have been observed. The Eu3+ probe ion was introduced in the initial stage of the sol-gel process by dissolving a hydrated europium salt in the sol. Europium precursors studied include: Eu(NO3)3·6H2O, EuCl3·6H2O, Eu(ClO4)3·6H2O. Sol, gel point and wet gel spectra and lifetimes are interpreted by analogy with spectroscopic studies of the europium salts in mixed ethanol-water solutions. In particular, the energy of the 7F0 → 5D0 excitation spectrum is used to identify complexes containing counter-ions in the inner coordination sphere of Eu3+. Results are presented showing that the NO3− ions have an affinity for the inner coordination sphere of Eu3+ which is greater than the affinity of Cl−. ClO4− ions retain an outer-sphere interaction with Eu3+ throughout the sol-gel process. Thermal densification of the gels destroys the inner-sphere complexation of the counter-ions and, above 300°C, the counter-ion-dependent effects were removed. The results show that the method by which the Eu3+ probe ion is introduced to the system significantly affects the Eu3+ spectra.