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

Abstract

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 °C to 650 °C exhibit higher specific saturation magnetization (M_s) than Fe₃O₄@C NRs obtained at 300 °C to 500 °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 °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.