Post annealing immersion: A new technique for studying rare earth ions in porous materials

A novel technique is introduced for studying how rare earth ions are incorporated into the matrix of porous materials. Tb³⁺-doped silica sol-gel is annealed to form porous glass and immersed in an aqueous Gd(NO₃)₃ solution. Re-heating removes the liquid and deposits Gd³⁺ ions on the internal pore surfaces. Spectroscopic investigation of Gd³⁺→Tb³⁺ energy transfer yields information about the location of Tb³⁺ dopant ions relative to pore surfaces. Results from post-annealing immersion experiments indicate that most Tb³⁺ dopant ions are close to pore surfaces when sol-gel glass is annealed to 700 °C. The observed energy transfer decreases with higher annealing temperatures, indicating that Tb³⁺ dopants in densified regions are isolated from Gd³⁺ ions deposited on pore surfaces. Al³⁺ co-doping affects both the overall sample density and the degree of Gd³⁺→Tb³⁺ interaction in post-annealing immersion samples.