Dependency of tunable microwave absorption performance on morphology-controlled hierarchical shells for core-shell Fe₃O₄@MnO₂ composite microspheres

Core-shell Fe₃O₄@MnO₂ composite microspheres with three different surface architectures, namely mushroom-, honeycomb- and corolla-like morphologies, have been synthesized through a facile two-step method. The components, microstructure, size and morphologies of composite microspheres were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. The N₂ adsorption-desorption isotherms were employed to demonstrate the specific surface areas and porosity. Moreover, vibrating sample magnetometer results manifest that these three composites possess the specific saturation magnetization of 33.7 emu/g, 27.2 emu/g and 23.0 emu/g, respectively. Investigations of microwave absorbing properties indicate that both mushroom-like and corolla-like Fe₃O₄@MnO₂ composite microspheres have very broad absorbing bandwidth in the frequency range of 2–18 GHz, and the corolla-like composites exhibit the strongest absorbing capability with the minimum reflection loss value of −48.5 dB (11.2 GHz), which has rarely been reported yet. In addition, analysis of microwave absorption mechanism reveals that electromagnetic energy absorption mainly derives from matching impedance, conductive loss, multiple scattering and absorption in the cavities, and interfacial polarizations between Fe₃O₄ cores and MnO₂ shells in the composites. Therefore, it is believed that hierarchically structured dielectric shells contribute to the enhancement of microwave absorption performance.