The Effect of Annealing Time on the Structural and Optical Properties of ZnAl₂O₄:0.01% Cr³⁺ Nanophosphor Prepared via the Sol–Gel Method

Zinc aluminate (ZnAl₂O₄) host and 0.01% Cr³⁺ doped were successfully prepared using the sol–gel method. The annealing time (AT) was varied in the range of 0.5–19 h. The x-ray diffraction results showed that the AT does not affect the crystal structure of the prepared powders. Scanning electron microscopy (SEM) results showed that the morphology of the prepared nanophosphors was influenced by the AT. Energy dispersive x-ray spectroscopy (EDS) confirmed the homogeneous distribution of the constituent elements. Transmission electron microscopy (TEM) suggested that the average crystallites sizes of the ZnAl₂O₄ to be ~20 nm. Ultraviolet–visible (UV–Vis) spectroscopy results revealed that the bandgap (E_g) of the prepared nanophosphor can be tuned by varying the AT. The emission peak at 390 nm is attributed to the intrinsic defects within the host material bandgap. The emission peak at 572 nm is attributed to both contribution from the host and Cr³⁺ (⁴T₁ → ⁴A₂) transition. The maximum PL intensity was observed from the samples annealed for 3 h. The International Commission on Illumination (CIE) chromaticity diagram showed a slight shift on the blue emission with an increase in AT.