Precursor Concentration-Induced Modulation of Structural and Surface Properties of Cobalt Ferrite for Enhanced Phenol Photodegradation Under UV–Vis Irradiation

Cobalt ferrite (CoFe2O4) photocatalysts were synthesized through a citric acid–ethylene glycol-assisted sol–gel route by systematically varying the Co2+ precursor concentration from 0.1 to 0.5 M. Precursor concentration significantly influenced the structural, optical, surface, and photocatalytic properties of the resulting CoFe2O4. Among the synthesized samples, Sp3, prepared using 0.3 M Co2+ precursor, exhibited the smallest crystallite size (17 nm), the largest (311) d-spacing (0.268 nm), and an indirect optical band gap of approximately 1.3 eV. XPS analysis of the optimized catalyst revealed mixed Co2+/Co3+ and Fe2+/Fe3+ oxidation states together with lattice and defect-associated oxygen species, indicating a redox-active surface environment. Under UV–Vis irradiation using a 350 W xenon (Xe) lamp, Sp3 achieved complete phenol removal within approximately 40 min and exhibited the highest apparent pseudo-first-order rate constant (0.162 min−1) among the synthesized samples. H2O2 addition further accelerated phenol degradation. Reactive-species scavenging experiments identified •OH as the predominant reactive species, with additional contributions from •O2− and h+. TOC removal reached approximately 88% after 15 min under the optimized conditions, demonstrating substantial mineralization. Furthermore, recycling experiments, post-reaction XRD, and ICP-OES analysis demonstrated good structural and chemical stability of the optimized catalyst. These results demonstrate that controlled precursor concentration provides an effective approach for optimizing CoFe2O4 for photocatalytic degradation of phenolic pollutants under UV–Vis irradiation.