Effect of Calcination Temperature on Photocatalytic and Fenton-like Dye Degradation over Spinel CoCr ₂ O ₄ Nanomaterials

CoCr2O4 is a spinel-type oxide that has attracted attention due to its relatively narrow band gap and multivalent redox capability. Here, we investigate how calcination temperature affects the dye-degradation performance of CoCr2O4 nanoparticles prepared via a hydrothermal route. CoCr2O4 calcined at 500 °C (CCO-500) shows the best activity under visible light, achieving 97% removal of malachite green within 2 h. Increasing the calcination temperature improves crystallinity but decreases cation inversion, yielding an optimum at 500 °C. High removal efficiencies (>85%) are also obtained for methyl orange, crystal violet, and acid orange 7, indicating broad applicability. Notably, CCO-500 exhibits pronounced Fenton-like activity in the dark, attributed to Co2+/Co3+ redox cycling associated with cation inversion, which promotes H2O2 activation and formation of reactive oxidizing species. The catalyst is recyclable, maintaining 85% efficiency after five runs. These results identify CoCr2O4 nanoparticles as versatile catalysts coupling visible-light photocatalysis with Fenton-like oxidation for aqueous environmental remediation.