Ionic liquid-assisted solvothermal synthesis of hollow CoFe₂O₄ microspheres and their absorbing performances

Hollow CoFe₂O₄ microspheres were successfully synthesized in high yield using a solvothermal method in the presence of long-chain ionic liquid [C₁₈MIM][Br]. Uniform CoFe₂O₄ hollow spheres with a diameter of 100–200 nm and a shell thickness of approximately 10–50 nm are assembled by nanoparticles of 5–10 nm and have cubic spinel structures. A minimum reflection loss of −15.23 dB was observed at 16.98 GHz for hollow CoFe₂O₄ microspheres with a thickness of 5.5 mm, and the effective absorption frequency ranged from 15.28 to 18 GHz. Hollow CoFe₂O₄ microspheres exhibited better microwave absorbing performance than solid microspheres. A possible formation mechanism of hollow CoFe₂O₄ microspheres was discussed.