Photoluminescence Properties of Lanthanide-Doped Alumina and YAG Aerogels

In this work, the range of alumina (Al₂O₃) and yttrium aluminum garnet (YAG) aerogels was extended by doping them with lanthanide ions. The aerogels were synthesized by using a universal, epoxide-assisted sol–gel method. They were thermally treated to induce structural changes, which were characterized in more detail by using X-ray diffraction and electron microscopy. The alumina samples showed topotactic phase transformations from boehmite, via γ-alumina to a mixed alumina phase, while the YAG started as an amorphous mixed oxide phase, which crystallized at 1000 °C into pure crystalline YAG. In order to expand the functionalities of the aerogels, they were doped with the rare-earth ions Eu³⁺ and Tb³⁺ (3 mol %). The red or green photoluminescence could be observed only starting from a temperature treatment of 550 °C, which can be related to the defect reduction and crystallinity increase due to phase transformations and sintering processes occurring. For the first time, the photoluminescence quantum yields of luminescent aerogels could be determined. The highest quantum yield of 25.5 ± 1.1% was achieved for the Al₂O₃-Tb-1000 sample.