Optimizing structural, and optical characteristic of Ni-doped Co-Zn nanoferrites: insights for biological evaluation on maize seed germination

This study aimed to synthesize nickel-doped cobalt–zinc spinel ferrite (Co0.5Zn0.5−xNixFe2O4) nanoparticles and to investigate the influence of Ni²⁺ substitution on their structural, optical, and biological properties, with particular emphasis on their potential application in seed nano-priming. Spinel ferrites with different Ni concentrations (x = 0.0–0.5) were synthesized via the citrate–nitrate auto-combustion method. X-ray diffraction and Rietveld refinement confirmed the formation of a single-phase cubic spinel structure with a prominent (311) reflection, indicating high crystallinity. Increasing Ni²⁺ substitution up to x = 0.3 caused a slight increase in the lattice parameter from 8.397 to 8.398 Å, followed by a decrease to 8.390 Å at x = 0.5, accompanied by variations in crystallite size (42.57–55.63 nm). X-ray density showed a marginal decrease with increasing Ni content, while porosity decreased up to x = 0.3 and then slightly increased. Several structural parameters, including site radii, bond lengths, hopping lengths, and edge lengths, were derived from XRD data. HR-TEM analysis revealed mostly spherical, aggregated nanoparticles, consistent with their magnetic behavior. UV–Vis spectroscopy confirmed semiconducting characteristics, with optical band gaps ranging from 1.44 to 1.59 eV, placing all compositions within the visible-light region and indicating potential photocatalytic applicability. Germination assays on Zea mays seeds demonstrated that low concentrations of Ni-doped nanoferrite (x ≤ 0.3) significantly enhanced germination percentage, seedling length, fresh and dry biomass, and vigor index, whereas higher concentrations exerted negligible effects. Notably, Co0.5Zn0.3Ni0.2Fe2O4 produced the greatest stimulation, increasing seedling length, fresh weight, and dry weight by 45.5, 51.2 and 21.63%, respectively, compared with the control. These results highlight the potential of engineered spinel nanoferrites in agro-nanotechnology and emphasize the importance of optimizing concentrations for effective seed nano-priming and growth enhancement under controlled conditions.