Controllable synthesis of elliptical Fe3O4@C and Fe3O4/Fe@C nanorings for plasmon resonance-enhanced microwave absorption

Heterostructured nanorings (NRs) with Fe3O4 and/or Fe cores and carbon shells (Fe3O4@C and Fe3O4/Fe@C) were synthesized by a facile and controllable two-step process. The NRs were formed through a synchronous reduction/carbonization/diffusion growth mechanism. Their composition, crystal size, and phase structure could be controlled by selecting the sintering temperature of iron glycolate nanosheets in the presence of acetone. Fe3O4/Fe@C NRs formed at 600 degrees C to 650 degrees C exhibit higher specific saturation magnetization (M-s) than Fe3O4@C NRs obtained at 300 degrees C to 500 degrees C because of increased Fe content and crystal size; the former also shows higher coercivity (H-c) because of large crystal size and surface pinning function. In addition, Fe3O4/Fe@C NRs show lower density and broader absorption bandwidth than Fe3O4@C NRs and Fe3O4 NRs. Fe3O4/Fe@C NRs formed at 600 degrees C with a mass fraction of 40 wt% exhibit an absorption bandwidth (R-L <= -20 dB) of 6.7 GHz and a minimum R-L value of -28.18 dB at 4.94 GHz. The enhanced absorption properties are ascribed to the heterostructured and ring-shape configuration, which generates multiple dielectric relaxations, enhanced electromagnetic parameters, and plasmon resonance absorption.