Structural, optical and magnetic characterization of Zn1−xNixAl2O4 (0≤x≤5) spinel nanostructures synthesized by microwave combustion technique

Abstract

Zn1−xNixAl2O4 (0≤x≤0.5) spinel nanostructures were synthesized by microwave combustion technique. Structural, vibrational, optical, morphological and magnetic properties were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), diffuse reflectance (DRS) and photoluminescence (PL) spectroscopy, high resolution scanning electron microscopy (HR-SEM), and vibrating sample magnetometry (VSM), respectively. The XRD pattern confirmed the formation of a single phase cubic spinel structure of ZnAl2O4 (gahnite), except for higher dopant concentrations for which a secondary phase was observed and validated by Rietveld refinement analysis. Different concentration levels of Ni2+ was used as the dopant in the zinc aluminate matrix. Decrease in lattice parameter and increase in porosity were observed on increasing Ni2+ concentration. The average crystallite size of the nanoparticles estimated using Debey–Scherrer׳s method was found to be in the range of 8.89–13.84nm. The William–Hall (W–H) analysis was used to study the effects of lattice strain over the crystallite size. FT-IR spectra showed the vibrational stretching frequencies corresponding to the zinc aluminate spinels. The optical band gap value of undoped zinc aluminate nanostructure is higher than the reported bulk zinc aluminate. The direct band gap estimated using the Kubelka–Munk method decreased with increasing Ni2+ content (5.05–3.98eV), due to the formation of subbands in the energy gap. The HR-SEM images depict the formation of well developed nano-sized clusters with homogeneous well crystallized grains without any agglomerations. The PL characteristics of undoped and Ni2+ doped zinc aluminates are suggestive of defect controlled process. Magnetic measurements revealed that the undoped ZnAl2O4 has diamagnetic behavior while the Ni2+ doped ZnAl2O4 system has superparamagnetic behavior, despite NiO impurities.