Core–Shell Mn- and Cu-Doped CoFe ₂ O ₄ @Co ₃ O ₄ Hollow Spheres with Dual Adsorption and Catalytic Function

This work reports the first solvothermal synthesis of Mn- and Cu-doped CoFe₂O₄@Co₃O₄ hollow spheres. The process leads to the formation of core-shell spinel structures via Ostwald ripening. The resulting materials with surface areas up to 115 m²/g were evaluated as bifunctional adsorbent-catalysts for removing Congo Red (CR) dye and oxytetracycline (OTC) from water via combined adsorption and Fenton-like degradation. Mn- and Cu-doping significantly change the morphology of hollow spheres, improving surface charge, redox activity, and pollutant affinity. Mn-doping enhanced the removal efficiency mainly through adsorption, with a minor catalytic contribution. The Mn-3 sample exhibited the highest activity, removing 99% of OTC within 30 min and achieving complete CR elimination within 40 min. XAS and XPS revealed that Co, Mn, and Cu act as redox-active centers, while Fe remains chemically stable. EPR spin-trapping experiments indicated that Cu-3 produces the highest HO^• level, whereas Mn-3 is the most effective generator of singlet oxygen. Postreaction XPS confirmed pollutant adsorption, while recyclability tests revealed that CR degradation remains efficient over three cycles, but OTC removal declines, likely due to stronger chemisorption. These results demonstrate the promise of Mn- and Cu-doped cobalt ferrite hollow spheres as multifunctional materials for advanced water treatment applications.