Effect of calcination temperature on structural and optical properties of MAl₂O₄ (M = Ni, Cu, Zn) aluminate spinel nanoparticles

NiAl₂O₄, CuAl₂O₄, and ZnAl₂O₄ aluminate spinel nanoparticles were synthesized by sol-gel auto combustion method using diethanolamine (DEA) as a fuel. The effects of calcination temperature on structure, crystallinity, morphology, and optical properties of MAl₂O₄ (M = Ni, Cu, Zn) have been investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), UV-visible diffuse reflectance spectroscopy (UV-DRS), and photoluminescence (PL) spectroscopy. The XRD and FT-IR results confirm the formation of single-phase spinel structure of NiAl₂O₄, CuAl₂O₄, and ZnAl₂O₄ at 1200, 1000, and 600 ℃, respectively. The direct band gap of these aluminate spinels, calculated from UV-DRS spectra using the Kubelka-Munk function, is found to increase with calcination temperature. The PL spectra demonstrate that NiAl₂O₄ gives the highest blue emission intensity, while CuAl₂O₄ and ZnAl₂O₄ exhibit a very strong violet emission. During fluorescence process, the ZnAl₂O₄ emits visible light in only violet and blue regions, while NiAl₂O₄ and CuAl₂O₄ emissions extend to the green region. It seems therefore that the transition metal type and intrinsic defects in these aluminate powders are responsible for these phenomena.