Structural, optical, and morphological characterization of Cd_xCo_1-xFe₂O₄ spinel ferrite nanoparticles synthesized via...

Cd_xCo_1-xFe₂O₄ (x = 0.25, 0.50, 0.75) spinel ferrite nanoparticles were successfully synthesized using the co-precipitation technique. X-ray diffraction (XRD) analysis confirmed the formation of cubic spinel structure with space group Fd̅3m for all compositions. The structural analysis revealed a non-monotonic dependence of the average crystallite size (D) on Cd content, varying from a minimum of 9.58 nm at x = 0.50 to a maximum of 16.36 nm at x = 0.75. A similar non-linear trend was observed in the defect-related parameters, where the dislocation density (δ), microstrain (ε), and stacking fault probability (SF) reached their highest values at x = 0.50 (δ = 10.90 × 10^-3 lines/nm2, ε = 11.73   ×  10^-3, SF = 7.04  ×  10^-3). Scanning electron microscopy (SEM) images confirmed a transition from spherical nanoparticles to well-defined crystalline facets as cadmium substitution increases. Also, SEM analysis demonstrates that Cd²⁺ substitution significantly promotes grain growth and alters surface topography, providing a mechanism for tuning the microstructural properties of spinel ferrites. Fourier-transform infrared spectroscopy (FTIR) confirmed the formation of the spinel structure through the characteristic metal-oxygen stretching vibrations at tetrahedral (551-587 cm^-1) and octahedral (416-458 cm^-1) sites. The presence of residual organic species from the washing process was also identified. UV-Vis diffuse reflectance spectroscopy revealed that the optical bandgap (E_g) decreases monotonically with increasing Cd²⁺ concentration, from 4.89 eV (x = 0.25) to 4.83 eV (x = 0.50), and further to 4.79 eV (x = 0.75). This bandgap narrowing is attributed to lattice expansion, cation redistribution, and the possible introduction of mid-gap defect states. These findings demonstrate that controlled cadmium substitution provides an effective strategy for tailoring both the structural characteristics and optical bandgap of cobalt ferrite nanoparticles. The composition with x = 0.75, exhibiting the largest crystallite size, lowest defect density, and smallest bandgap, is identified as the most promising candidate for potential applications in photocatalysis and optoelectronics.