Sol–Gel Synthesis of Europium-Doped Al ZnO Nanostructures: Effects on Structural, Morphological, Optical, and Photoluminescence Properties

This study explored the synthesis and characterization of aluminum-doped zinc oxide (AZO) nanostructures and their europium (Eu)-doping. AZO nanostructures were synthesized via a sol–gel method followed by supercritical ethanol drying, with Eu incorporated at 0.5 and 1% concentrations. Various analytical techniques were used to investigate their properties. X-ray diffraction (XRD) confirmed a hexagonal wurtzite structure in both AZO and Eu:AZO nanostructures. While Eu-doping slightly reduced crystal quality, crystallite size remained largely unchanged. Lattice parameters increased due to Zn²⁺ substitution by Eu³⁺. Diffuse reflectance spectroscopy (DRS) revealed an enhancement in reflectance upon Eu-doping, with the average visible reflectance increasing from 79.5 to 85%, while a slight reduction in the bandgap from 3.24 to 3.22 eV was observed. Fourier-transform infrared (FTIR) and attenuated total reflectance (ATR) spectroscopies displayed similar absorption bands across samples, with Eu co-doping reducing the intensity of the Zn–O vibrational band, indicating fewer Zn–O bonds. Scanning electron microscopy (SEM) showed that AZO crystallites, mainly spherical or quasi-spherical, formed toroidal grains with filled centers. Eu-doping promoted greater grain segregation and size, leading to toroidal morphologies with hollow centers. Photoluminescence (PL) spectroscopy revealed emission bands from 360–700 nm, with strong UV emissions at 378 and 389 nm linked to band-to-band and excitonic transitions. Weaker peaks at 366, 538, and 640 nm in the visible region were attributed to structural defects and impurities, while a 420 nm shoulder linked to interstitial zinc decreased with Eu-doping.