Effect of pH on material properties of ZnAl₂O₄:Cr³⁺ nano particles prepared by sol–gel method

ZnAl₂O₄ nanoparticles (NPs) doped with Cr³⁺ (1.0 mol%) were prepared by citric acid assisted sol–gel method at pH values of 3, 5, 7, 9, 11 and 13 with the use of ammonia as pH controller. Analysis of x-ray diffraction (XRD) spectra show that the synthesized ZnAl₂O₄ NPs are cubic structured and the pH did not affect the structure significantly. The average crystallite size increases with an increase in pH. The photoluminescence (PL) intensity was found to be minimum and maximum for the sample synthesized at pH 3 and pH 13, respectively. The PL emission intensity was found to depend on the synthesis pH conditions. The scanning electron microscope (SEM) revealed that the morphology of ZnAl₂O₄:Cr³⁺ NPs changed with the pH values of the solution. Ultraviolet–visible (UV–Vis) measurements show that reflectance is maximum in UV and visible region for sample synthesized under acidic pH of 5. The energy band gap energy was tuned from 3.10 to 3.88 eV with increase in pH value from 3 to 13. It was observed that a strong alkaline nature of the solution enhances the structural and optical properties of ZnAl₂O₄:Cr³⁺ nanostructures.