Yolk–shell Fe–Fe3O4@C nanoparticles with excellent reflection loss and wide bandwidth as electromagnetic wave absorbers in the high-frequency band

[Display omitted] •Fe–Fe3O4@C with a yolk–shell structure is obtained via vacuum carbonization in N2.•The mixture of Fe, Fe3O4, and the carbon layer improves interfacial polarization.•Fe–Fe3O4@C (50 wt% and 1.2 mm thick) exhibits RL of −51 dB and bandwidth of 5.1 GHz.•This Fe–Fe3O4@C presents higher RL with a wider bandwidth than other Fe3O4-based ones. The performance of Fe3O4-based electromagnetic wave-absorbing materials is typically hindered by their low conductivity. Therefore, the introduction of carbon components with a rationally constructed microstructure has evolved as an effective approach for enhancing the electromagnetic properties of metal-oxide-based microwave absorbers. In this study, yolk–shell Fe–Fe3O4@C nanoparticles were synthesized via a hydrothermal–polymerization–vacuum carbonization method under a N2 flow. In addition to a polymer with suitable layer thickness, which is required to obtain a mixed Fe and Fe3O4 phase, an appropriate carbon layer is needed. Furthermore, Fe@C with cavities can be fabricated when the thickness of the polymer layer is higher than the optimum value. The optimized reflection loss, effective bandwidth, and thickness of Fe–Fe3O4@C were −51 dB, 5.1 GHz (12.9–18 GHz), and 1.2 mm, respectively, which were larger than those of most Fe3O4-based absorbing materials reported to date. The excellent microwave absorption performance of Fe–Fe3O4@C was attributed to its excellent electromagnetic properties, including complex permeability and permittivity, and yolk–shell structure, which favored multiple reflections and scatterings and multiple polarizations at the core–cavity and cavity–shell interfaces.