Advancing Sustainable Wastewater Treatment with Intermediate ZnFe ₂ O ₄ Nanocatalysts: Tuning Inversion Degree for Solar‐Driven Dye Degradation

Abstract The regeneration performance of ZnFe ₂ O ₄ in wastewater treatment is highly dependent on its degree of inversion. This study systematically investigates the relationship between inversion degree and photocatalytic efficiency of ZFO MNPs for degrading industrial pollutants under solar irradiation. ZFO nanoparticles are synthesized via co‐precipitation and subjected to thermal annealing at temperatures ranging from 550 to 950 °C to control their structural properties. Comprehensive characterization revealed that increasing annealing temperature progressively reduced the inversion degree from 0.54 to zero, as determined by Rietveld refinement of X‐ray diffraction data. This corresponds to a structural transition from intermediate mixed spinel to normal spinel configuration, driven by thermally‐induced iron cation migration within the crystal lattice. The structural evolution directly influenced the electronic properties, causing bandgap redshift and modifying photogenerated electron‐hole pair recombination dynamics, as confirmed through Mott‐Schottky analysis and photocurrent measurements. The inversion degree, along with crystallite size and spin disorder, significantly affected the saturation magnetization of ZFO samples. The molecular simulations revealed that the electrostatic interactions dominate the adsorption of methylene blue (MB) onto ZFO. Photocatalytic experiments demonstrated that the intermediate spinel structure (annealed at 550 °C, x = 0.54) exhibited optimal performance, achieving 97% MB degradation within 70 min under solar irradiation. This enhanced activity is attributed to improved charge separation efficiency and reduced electron‐hole recombination in the inverted spinel structure.