# Core–Shell Mn- and Cu-Doped CoFe2O4@Co3O4 Hollow Spheres with Dual Adsorption and Catalytic Function

## TL;DR

Scientists created hollow spheres with dual functions to remove pollutants from water efficiently.

## Contribution

The first solvothermal synthesis of Mn- and Cu-doped CoFe2O4@Co3O4 hollow spheres with dual adsorption and catalytic functions.

## Key findings

- Mn-3 sample removed 99% of OTC within 30 min and completely eliminated CR within 40 min.
- Co, Mn, and Cu act as redox-active centers, while Fe remains chemically stable.
- CR degradation remains efficient over three cycles, but OTC removal declines due to stronger chemisorption.

## Abstract

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.