Hollow engineering is considered to be an essential subfield in promoting electromagnetic (EM) wave absorption intensity and realizing the lightweight characteristics. However, the enhancement of effective absorption bandwidth (EAB) still faces huge challenges. Herein, HCNs@CoFe₂Se₄-QDs with superior EM wave absorption intensity and ultra-broadband EAB are produced by using tightly arranged SiO₂ spheres as the hard-templates. Specifically, the removal of SiO₂ templates inevitably results in the formation of hollow cavity, which is favorable to optimize the impedance matching and increase the absorption intensity. Besides, the incorporation of selenium powder effectively increases the numbers of heterogeneous interfaces by forming CoFe₂Se₄ quantum dots (QDs) during the pyrolysis process, leading to strengthened interfacial polarization and ultra-broadband EAB. As results, the superior EM wave attenuation with a minimum reflection loss of -67.6 dB and an EAB of 11.4 GHz is achieved with only 20 wt% filler ratio. This design concept of hollow engineering with magnetic QDs provides us an inspiration in optimizing EM wave absorption intensity and simultaneously promoting absorption bandwidth.

Owing to the tunable compositions and versatile functionality, the development of eco-friendly metal–phenolic coordination crystals derivatives is highly anticipated for electromagnetic wave absorption. In this study, three kinds of magnetic hollow carbon spheres (HCSs) with macro-meso-microporous characteristics, including Fe/HCS, Co/HCS, and CoNi/HCS, are successfully fabricated via the co-operative hard template and self-assembling process, in which magnetic particles are encapsulated in carbon shell matrix after the pyrolysis of metal–polyphenol coordination crystals and further subsequent template removal. On the one hand, hierarchical macro-meso-micropores effectively balance the impedance gap between absorbers and air and introduce structural defects or distortion, leading to matched impedance and enhanced dipolar/defect polarization. On the other hand, wrapped magnetic particles provide uncountable hetero-interfaces and induce ferromagnetic resonance, resulting in strengthened interfacial polarization and additional magnetic loss. In particular, enhanced minimum reflection loss (R_L,min) and broadband effective absorption bandwidth (EAB) are achieved with only 10 wt.% filler loading. Specifically, the R_L,min and EAB values are −57.5 dB and 7.2 GHz for Fe/HCS, −50.0 dB and 5.8 GHz for Co/HCS, and −52.1 dB and 6.7 GHz for CoNi/HCS, respectively. Moreover, this work provides us a modular-assembly strategy to regulate the hollow cavity of absorbers and simultaneously manipulates the chemical components of absorbers to regulate electromagnetic wave absorption performance.

Light-weight and exceptional microwave absorption are two vital characteristics for microwave absorbers in practical applications, but still face challenges. Herein, we employ a sacrificial template strategy to fabricate heteroatoms-doped carbon nanocages (CNs) via chemical vapor deposition, in which heteroatoms are simultaneously doped into the carbon frameworks by bubbling flowing source liquid. Compared with CNs, doped heteroatoms, accompanied with the inevitably defective arrangements in the lattice, not only decrease the electrical conductivity and balance the impedance characteristics, but also introduce structural-chemical defects and trigger dominant dipolar/defect polarization. As a result, both the minimum reflection loss (R_L,min) and effective absorption bandwidth (EAB) greatly increase at an ultralow filler loading of 5 wt.% owing to internal hollow void and high specific surface area. The R_L,min values reach −53.6, −43.2, and −50.1 dB for N-CNs, S-CNs, and N,S-CNs with the corresponding EAB of 4.9, 2.5, and 3.1 GHz, respectively. Furthermore, this work provides an effective strategy for the construction of heteroatoms-doped hollow carbon frameworks in large-scale production and the obtained doped carbon nanocages can be used as light-weight and high-performance microwave absorbers.