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[Display omitted] In this study, hollow Fe 3 O 4 /Fe@C nanocube wave absorbers with different reduction degrees are designed based on the Fe(II)/Fe(III) ion engineering strategy, among which the Fe 3 O 4 @C has the RL min as low as −119.9 dB at 2.3 mm and achieves extremely wide effective absorption bandwidth of 11.4 GHz spanning low-frequency band of 2.8∼7.2 GHz and high-frequency band of 11.0∼18 GHz at 5.0 mm. • A novel ion-engineering and heat treatment process was used to obtain Fe 3 O 4 /Fe@C. • Fe 3 O 4 /Fe@C nanoassemblies have a special structure of hollow core–shell nanocube. • The RL min value of Fe 3 O 4 @C is as high as − 119.9 dB at a thin thickness of 2.3 mm. • Fe 3 O 4 @C achieves an ultra-wideband of 11.4 GHz across high and low frequency bands. Rationally constructing hollow structures with heterogeneous interface is usually an effective strategy to regulate the electromagnetic response behavior for lightweight and strong absorption, but achieving low-frequency and broadband absorption is a major challenge. This work adopts a novel Fe(II)/Fe(III) ions regulation strategy to design and synthesize hollow Fe(OH) X (x = 2, 3) nanocubes, which addresses the disadvantages of time-consuming or using etchant. Subsequently, a unique hollow Fe 3 O 4 /Fe@C nanoboxes with adjustable magnetic composition and uniformly coated shell have been successfully synthesized by in-situ carbonization with the assistance of phenolic resin and hydrogen thermal reduction. Notably, the different magnetic components induce the magnetic coupling assembly and play a crucial role in determining its electromagnetic parameters, whereas Fe 3 O 4 contributes to more dipole polarization and stronger low-frequency response characteristics. For Fe 3 O 4 @C hollow nanoboxes, the minimum reflection loss (RL min) is as low as –119.9 dB at 2.3 mm, and extremely wide effective absorption bandwidth (EAB) of 11.4 GHz is achieved spanning low-frequency band of 2.8 ∼ 7.2 GHz and high-frequency band of 11.0 ∼ 18 GHz at 5.0 mm. Surprisingly, the Fe 3 O 4 @C also has satisfactory EAB 20 (RL＜ –20 dB) reaching 7.1 GHz (14.9 ∼ 18 GHz) at a thin thickness of 1.8 mm. The RL min of other hollow nanoboxes is also below –30 dB, and the EAB is greater than 9 GHz. This study provides a competitive candidate for efficient microwave absorption in complex frequency bands, especially low-frequency bands. [ABSTRACT FROM AUTHOR]

Nowadays, Fe-based microwave absorption materials still face crucial problems of poor oxidation resistance, lack of dielectric loss and narrow effective absorption bandwidth. In this work, we developed a feasible way of calcining and etching SiO 2 microspheres coated Fe3+ and polydopamine (SiO 2 @Fe3+-PDA) to confine Fe nanoparticles (12 nm) in the hollow carbon microspheres with a thickness of 17 nm, which provides more interfaces and polarization sites to enhance microwave attenuation. With the increase of Fe content, the microwave absorption performance of the sample is gradually improved. When Fe content is 19.7 wt%, the sample has a minimum reflection loss (RL) of −30.9 dB and a broadband absorption bandwidth (RL ≤ −10 dB) of 11.9 GHz (6.1–18 GHz) at the thickness of 4.0 mm. The ultra-wide effective absorption bandwidth is ascribed to synergetic effect of Fe nanoparticles and carbon layer, which provide magnetic loss and dielectric loss, respectively. Moreover, the hollow structure extends the transmission path and induces the multiple scattering of the incident electromagnetic waves. The novel hollow Fe/C microspheres are promising as high-efficiency microwave absorbers with strong microwave absorption and broad absorption bandwidth. [Display omitted] • Hollow Fe/C microspheres are fabricated using SiO 2 @Fe3+-PDA microspheres. • Fe nanoparticles are embedded in 17 nm carbon shell. • An ultra-wide effective absorption bandwidth of 11.9 GHz is obtained. • Fe/C-3 sample exhibits a minimum reflection loss value of −30.1 dB. [ABSTRACT FROM AUTHOR]