Impact of annealing on ZnFe₂O₄: Structural, optical, magnetic, photocatalytic, and antibacterial properties with DFT insights into optical parameters

Abstract

Zinc ferrite (ZnFe₂O₄) nanoparticles were synthesized using a hydrothermal method and annealed at various temperatures ranging from 250 °C–750 °C. Comprehensive characterization was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), Fourier transform infrared (FTIR) spectroscopy, photoluminescence (PL), and vibrating sample magnetometry (VSM). XRD confirmed the formation of a cubic spinel structure, with crystallite sizes varying between 30 and 40 nm for the annealed nanoparticles. It is observed from the BET analysis that the surface area and pore volume are indirectly proportional to the annealing temperature of the ZnFe₂O₄ nanoparticles. The FTIR spectra confirmed the presence of metal‒oxygen vibrations, particularly ZnO and FeO bonds, and the PL studies revealed emission peaks indicating charge carrier recombination. Magnetic measurements revealed that the 750 °C annealed ZnFe₂O₄ nanoparticles had a saturation magnetization (M_s) of 4.04 emu/g, coercivity (H_c) of 14.28 Oe, and retentivity (M_r) of 0.0029 emu/g. ESR spectroscopy provided a g-value of 2.00 and a peak-to-peak linewidth (ΔHpp) of 67.008 mT. The photocatalytic activity of the nanoparticles was evaluated for the degradation of methylene blue under visible light irradiation, where T_A = 500 °C and T_A = 750 °C ZnFe₂O₄ nanoparticles exhibited the highest degradation efficiencies of 53 %. Antibacterial studies demonstrated that the 750 °C annealed ZnFe₂O₄ nanoparticles were effective against B. subtilis and P. aeruginosa, with zones of inhibition of 22 mm and 24 mm, respectively, comparable to those of the control. Density functional theory (DFT) calculations were performed to obtain insights into the electronic and optical properties of the ZnFe₂O₄ system. The DFT results imply that the electronic structure is fully dominated by the 3d electrons of the transition metal atoms and the 2p of the oxygen atoms. The calculated optical band gap of 1.74 eV closely aligns with the optical properties observed experimentally 1.82 eV for T_A = 500 °C, thereby validating the accuracy of the experimental findings. Additionally, the DFT analysis highlights the isotropic nature of the material with respect to the optical response, offering deeper insight into its suitability for photocatalytic applications. The combination of superior optical, magnetic, and structural properties, along with strong photocatalytic and antibacterial activities, makes zinc ferrite nanoparticles annealed at 750 °C promising candidate for environmental and biomedical applications.