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Page 2 of 13 Wu et al. Soft Sci 2024;4:42 https://dx.doi.org/10.20517/ss.2024.51

INTRODUCTION
As electronic information technology and devices continue to advance, people’s living standards have been
greatly improved. Nevertheless, the presence of electromagnetic waves (EMWs) is pervasive in everyday
life . The problems of electromagnetic pollution are worsening, affecting not only people’s health but also
[1-4]
[5-7]
the operation of precision electronic instruments . Meanwhile, military equipment such as stealth aircraft,
radar, and precision-guided weapons urgently need to be developed [8-10] . So, outstanding EMW absorbing
materials have important practical value. Among them, crafting ultra-thin absorbers that possess significant
[11]
reflection attenuation and a broad frequency range remains an ongoing challenge . Among various
strategies, the combination of magnetic particles (iron, cobalt, nickel) and carbon materials [graphene,
carbon nanotubes (CNTs), porous carbon] can obtain electromagnetic synergistic composite materials,
improve impedance matching, and enhance absorption of incident EMWs . However, dual magnetic ion-
[12]
[13]
modified carbon-based materials face challenges in achieving efficient EMW absorption .
Metal-organic frameworks (MOFs) are porous crystal materials formed by the coordination of metal ions
and organic groups , so they can be transformed into magnetic-carbon composites, while bringing about
[14]
magnetic loss and dielectric loss. Furthermore, MOF derivatives also have the characteristics of high
dispersion, which can uniformly disperse metal particles into the carbon matrix to form rich heterogeneous
interfaces, significantly enhancing interface polarization, thereby improving EMW absorption
performance [15,16] . For example, Wen et al. developed MOF-derived one-dimensional CNTs-modified Co/C
[17]
composites with strong magnetic losses . Jin et al. designed MOF-derived Ni@nitrogen-doped carbon
(NC) hexagonal nanosheets to obtain electromagnetic synergistic absorbers . However, achieving efficient
[18]
EMW absorption at ultrathin thickness is still extremely challenging. Recent studies have shown that dual
magnetic particles-modified carbon-based materials can bring stronger magnetic attenuation and promote
impedance matching properties, which are anticipated to obtain strong EMW absorption performance at
ultrathin thickness. For instance, Zhu et al. synthesized bimetallic composition Cu/NC@Co/NC complex
materials, with a reflection loss (R ) of -54.13 dB at a thickness of 3 mm . Wang et al. synthesized magnetic
[19]
L
CoFe alloy@C nanocomposites; at a thickness of 3mm, the R registers at -40 dB . Apparently, dual
[20]
L
magnetic particles-modified carbon materials have great potential in terms of ultrathin thickness (≤ 2 mm)
and super EMW absorption (≤ -70 dB).
Herein, dual magnetic particles (hollow CoFe alloy nanospheres and solid Co nanospheres)-modified NC
nanosheets are prepared by a simple chemical precipitation method and pyrolysis process. NC nanosheets
embedded with CoFe alloys and metallic Co nanospheres realize abundant heterogeneous interfaces. The
larger number of NC nanosheets provides opportunities for multiple reflections and scattering. Moreover,
computer simulation technology (CST) simulations confirm the low radar cross section (RCS) of the CoFe/
Co@NC. Therefore, the CoFe/Co@NC heterostructure inherits outstanding EMW absorption capabilities,
achieving an exceptionally high R of -73.8 dB at a minimal thickness of 1.78 mm for optimal impedance
L
matching. At the same time, the effective absorption bandwidth (EAB) can also reach up to 5.4 GHz.

EXPERIMENTAL
Synthesis of CoFe/Co@NC heterostructure
Cobalt nitrate hexahydrate (1.6 mmol) and ferrous sulfate heptahydrate (0.7 mmol) were dissolved in 20 mL
of deionized water. Additionally, 2-Methylimidazole (4 mmol) was mixed with 60 mL of deionized water,
which was subsequently poured into the mixture prepared in the previous step. The obtained mixture was
agitated at ambient temperature for a duration of ten minutes, followed by a period of 4 h for settling. The
resulting solid materials were successively rinsed with ethanol and methanol, and then subjected to drying
in an oven set at a temperature of 70 °C for a period of 12 h. The product was calcined in Ar atmosphere at