Microwave combustion synthesis, structural, optical and magnetic properties of Zn1−xCoxAl2O4 (0 ⩽ x ⩽ 0.5) spinel nanostructures

Abstract Cobalt doped zinc aluminate spinel type nanostructures were synthesized by microwave combustion method. The structural, vibrational, optical, morphological and magnetic properties were studied by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), diffuse reflectance spectroscopy (DRS), photoluminescence (PL), high resolution scanning electron microscopy (HR-SEM), energy dispersive X-ray diffraction (EDX) and vibrating sample magnetometry (VSM) respectively. The XRD patterns confirmed the formation of single phase ZnAl 2 O 4 cubic spinel without impurities. The lattice parameter increased from 8.081 to 8.116 A with increasing Co 2+ content. The average crystallite size of the nanoparticles estimated using Debey–Scherrer’s method was found to be in the range of 11.80–20.21 nm. The presence of tensile strain in the zinc aluminates was determined from Williamson–Hall (W–H) analysis. FT-IR spectra showed the vibrational stretching frequencies corresponding to the zinc aluminate spinel structure. The direct band gap estimated using Kubelka–Munk method decreased with increasing Co 2+ content (5.01–2.89 eV), due to the formation of sub bands in between the energy gap. Optical absorption spectra confirmed the cobalt substitution by the occurrence of a triplet of bands at 542, 584, and 624 nm, which is the characteristic of Co 2+ ions in the tetrahedral sites. For ZnAl 2 O 4 and Co substituted ZnAl 2 O 4 , PL spectra showed the emission bands in UV as well as in the visible regions, due to the defect centers acting as the trap levels. The elemental compositions of Co, Zn and Al were quantitatively obtained from the EDX analysis. HR-SEM images showed the features of well created pore structures and nano sized grains. Magnetic measurements revealed that the undoped ZnAl 2 O 4 has diamagnetic behavior while the Co doped ZnAl 2 O 4 system has superparamagnetic behavior.