Physico-chemical nanomaterials science
Sol-gel derived ZnAl2O4 nanopowders co-doped with Cr3+, Er3+ and Yb3+ ions
V. Hreb1, I. Lutsyuk1, A. Zelinskiy2, A. Luchechko2,
 M. Shpotyuk1, L. Vasylechko1
1 Lviv Polytechnic National University, Bandera 12, 79013, Lviv, 79013, Ukraine
2 Ivan Franko National University of Lviv, Universytetska 1, Lviv, 79000, Ukraine
E-mail: vasyl.m.hreb@lpnu.ua
Low-temperature citrate sol-gel method was used for the synthesis of nanocrystalline ZnAl2O4 spinel-based phosphors co-doped with Cr3+, Er3+ and Yb3+ ions. A series of triple-doped materials of nominal composition ZnAl1.994‑xО4:Cr0.003Er0.003Ybx (x = 0.009, 0.015 and 0.021) were obtained from stoichiometric amounts of Al(NO3)3·9H2O, ZnO, Cr(NO3)3·9H2O, Er2O3 and Yb2O3 as initial precursors. In contrast to the traditional high-temperature solid-state synthesis, the applied method allowed to obtain phase-pure nanopowders of all three series of materials with Cr3+:Er3+:Yb3+ ratios of 1:1:3, 1:1:5 and 1:1:7 at a minimal heat treatment temperature of 700 oC. The average grain size of as-obtained spinel-type materials derived from X-ray diffraction data increases from ~33 nm after initial crystallization at 700 oC up to 73-75 nm in the materials heat-treated at 1200 oC. Spot-check investigation of sample morphology by scanning electron microscopy revealed a platelet-like structure with plates formed by dense packed near spherical grains of 50-150 nm diameter. The unit cell dimensions of triply doped ZnAl2O4:Cr,Er,Yb materials are systematically higher than those for RE-free ZnAl2O4:Cr samples, thus proving effective incorporation of the large Er3+ and Yb3+ ions into ZnAl2O4 lattice. Analysis of obtained structural results shows that the heat treatment temperature effects on the lattice parameter of the spinel structure, which decreases gradually for all three series of the triple doped ZnAl2O4:Cr:Er:Yb powders, becoming practically equal after final annealing at 1200 oC. This observation can be explained (i) by limited solubility of RE ions in ZnAl2O4 crystal matrices and/or (ii) by partial redistribution (inversion) of Al and Zn cations in the tetrahedral and octahedral positions of spinel structure.
The excitation spectra of the ZnAl2O4:Cr:Er:Yb phosphors consist of three wide bands, which correspond to the 4A2→4T1 and 4A2→4T2 transitions in Cr3+ ions in octahedral sites. The evolution of the luminescent characteristics of three series of triple-doped spinel-type materials on the Cr/Er/Yb ratio as well as on the heat treatment temperature of the powders is discussed.
Acknowledgment: The work was partially supported by the National Research Foundation of Ukraine (grant no. 2020.02/0373) and the Ukrainian Ministry of Education and Science (project no. 0121U107736, DB/MODUS).