Effects of Zn/citric acid mole fraction on the structure and luminescence properties of the un-doped and 1.5% Pb²⁺ doped ZnAl₂O₄ powders synthesized by citrate sol-gel method

Un-doped and 1.5% Pb²⁺doped zinc aluminate (ZnAl₂O₄) was successfully prepared at a relatively low temperature (~80 °C) using the sol-gel process. The Zn/citric acid (CA) mole fraction was varied from 0.083 to 1.3. The X-ray diffraction (XRD) data revealed that most of the annealed samples consist of the cubic ZnAl₂O₄ structure with ZnO impurities. The surface morphology of the phosphors was influenced by the Zn/CA mole fractions. The increase in the CA content lead to the morphological evolution and transformation from small particles to rods-like-needles. The nanopowder samples exhibited violet luminescence that varied from 400 to 410 nm, which suggested that the CA content affected the bandgap energy of the host. The results suggest that the emission originated either from the host, Pb²⁺ or ZnO impurities. Emission at 400 nm is attributed to the oxygen vacancies (V₀^∗) in the host and emission peak at 410 nm is attributed to the intrinsic defects emission of ZnO, while the emission at 552 nm is attributed to the second order emission of the excitation wavelength. The emission intensity was influenced by the Zn/CA mole fractions. The CIE colour coordinates of all samples are close to each other in the blue region. Two distinct peaks were observed at around 160 and 385 °C in thermo luminescence (TL) glow curve, which indicates that there are two set of traps.