Structural evolution and fluorescence properties of Tb 3+-doped silica xerogels in the gel to glass conversion

Abstract

The Tb 3+-doped silica glasses were prepared by a sol-gel process with appropriate heat treatment. The structural changes during the gel to glass transition of Tb 3+-doped silica xerogels and the effect on fluorescence properties of Tb 3+ were studied in this paper. The results show that the doped silica glasses may be obtained after the xerogels are heat-treated at 800 °C for 24 h. During heat treatment at 400 °C, the residual organic molecules and most of the absorbed water molecules in silica xerogels are removed, and the SiO 4 tetrahedral structural network forms and strengthens gradually with decreasing number of silanol (SiOH) groups. The fluorescence intensity of Tb 3+ ions increases remarkably when the doped xerogel is heat-treated in the range 300–500 °C, and increases slowly above 500 °C(up to 800 °C). This observation leads to a conclusion that the radiative transitions of Tb 3+ ion in the aqueous gels are quenched strongly by the high-frequency vibrations of OH groups from the coordinated water rather than physically absorbed water. The inhomogeneity in the site state of Tb 3+ ions increases with rise in temperature of heat treatment, which broadens the emission bands of Tb 3+ ions doped in silica glasses. Moreover, there will also be an additional interaction owing to the influence of the ligand field of the glasses, which makes the emission bands shift to red and produces Stark splitting of the Tb 3+ ion energy levels.