Effects of varying Al_x moles on structure and luminescence properties of ZnAl_xO_1.5x+1:0.1 mol% Tb³⁺ nanophosphors prepared using citrate sol–gel method

Un-doped and ZnAl_xO_1.5x+1:0.1 mol% Tb³⁺ (ZAOT) nano-powders were synthesized via citrate sol-gel method. The Al_x moles were varied in the range of 0.25 ≤ x ≤ 5.0. The X-ray powder diffraction (XRD) data reveal that for x < 1.5, the prepared samples crystal structure consists of mixed phases of the cubic ZnAl₂O₄ and hexagonal ZnO phases, while for x ≥ 1.5 the structure consists of single phase of cubic ZnAl₂O₄. The Raman and Fourier-transform infrared (FTIR) vibrational spectroscopy show the presence of vibrations emanating ZnAl₂O₄ spinel. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show the presence of irregular sphere at x ≥ 2.0 attributed to ZnAl₂O₄. The photoluminescence (PL) spectroscopy reveals emissions from both the host and Tb³⁺ transitions. Emissions from Tb³⁺ are observed at 382, 414, 439, 458 nm and 489, 545, 585, 621 nm, which were attributed to the ⁵D₃ → ⁷F_6,5,4,2 and ⁵D₄ → ⁷F_6,5,4,3, respectively. The results confirm that the Tb³⁺ occupation site depends on the Al_x moles. The International Commission on Illumination (CIE) colour chromaticity shows that the emission colour can be tuned from blue to green by varying the Al_x moles.