Nd³⁺-Induced Modulation of the Structural, Morphological, Magnetic, Optical, and Photocatalytic Activity of Mg–Co–Zn Ferrite Nanoparticles

Developing highly efficient, magnetically recoverable photocatalysts for sustainable wastewater treatment remains a major challenge in environmental remediation. In this work, Nd³⁺-substituted Mg_0.5Co_0.2Zn_0.3Nd_xFe_2−xO₄ (x = 0.00–0.08) spinel nanoferrites were synthesized via the sol–gel citrate combustion method to investigate the influence of rare-earth incorporation on their structural, magnetic, optical, and photocatalytic properties. X-ray diffraction confirmed a single-phase cubic spinel structure with no detectable secondary phases, while Williamson–Hall analysis revealed crystallite sizes ranging from 30.74 to 36.78 nm depending on Nd³⁺ concentration. Vibrating sample magnetometry demonstrated composition-dependent soft magnetic behavior, with the x = 0.06 composition exhibiting the lowest coercivity (92.43 Oe), attractive for low-loss high-frequency applications, while the saturation magnetization decreased from 54.65 to 42.63 emu/g with increasing Nd³⁺ content, remaining sufficiently high for facile magnetic separation and reuse of the catalyst. Optical characterization revealed a non-monotonic variation in the optical band gap with Nd³⁺ content (1.68–1.87 eV), with the lowest value (1.68 eV) for x = 0.08, which is attributed to Nd³⁺-induced lattice distortion and defect states. These electronic structure modifications markedly improved the photocatalytic activity toward methylene blue (MB) decolorization, with the Mg_0.5Co_0.2Zn_0.3Nd_0.08Fe_1.92O₄ nanoferrite achieving 98.40% decolorization within 60 min under visible-light irradiation. Radical-scavenging experiments identified photogenerated holes (h⁺) and superoxide radicals ( ) as the dominant reactive species governing the decolorization process, while recycling tests demonstrated excellent stability, retaining 96.43% of its initial activity after five consecutive cycles. The outstanding photocatalytic efficiency, excellent recyclability, low coercivity, and intrinsic magnetic recoverability demonstrate that Nd³⁺-engineered Mg–Co–Zn nanoferrites are promising multifunctional nanostructured materials for visible-light-driven photocatalysis and magnetically assisted environmental remediation. Their tunable structural, optical, magnetic, and surface properties highlight the potential of rare-earth-engineered ferrite nanomaterials as versatile platforms for sustainable catalytic technologies and emerging energy-related applications.