Structure-luminescence correlations in europium-doped sol-gel ZnO nanopowders

The present study, focused on the sol-gel synthesis of rare-earth-doped zinc oxide, highlights the crucial points involved in the design and development of ZnO/Eu nanosystems. with peculiar and easily tunable photophysical. properties. ZnO nanopowders containing different europium amounts were prepared starting from zinc and europium acetate salts as the sol precursors. The densification process and the evolution of the structural-optical properties were studied as a function of annealing performed in air between 100 and 1000 degrees C. The microstructure and composition of the samples and their dependence on the synthesis procedure were investigated by X-ray diffraction and X-ray photoelectron spectroscopy, whereas the emission properties were studied by photoluminescence spectroscopy in the energy and time domains as a function of the structural evolution. Crystalline ZnO powders in the wurtzite structure were formed after heat treatment at 400 degrees C, with an average nanocrystal size of ca. 20 nm. Stronger annealing conditions allowed a more extended densification of the oxide-based network and the removal of most -OH groups but also promoted the crystallization of EU2O3 at 800 degrees C. The most intense emission bands around 600 nm due to Eu3+ transitions were clearly observed in the annealed samples (T >= 600 degrees C). Interestingly, the observed overlap of the typical Eu3+ red emission with the characteristic green luminescence of the nanostructured ZnO matrix can be synergistically exploited for single or multicolor emission through the proper choice of the excitation wavelength.