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Wu et al. Soft Sci 2024;4:42 https://dx.doi.org/10.20517/ss.2024.51 Page 3 of 13

800 °C for 2 h, and after cooling, CoFe/Co@NC can be obtained. It was also compared with powders
pyrolyzed at 600, 700, and 900 °C.

Characterization
The appearance of samples was analyzed using a field-emission scanning electron microscope (FESEM,
acceleration voltage = 5 kV, SU-8010, HITACHI, Japan) and transmission electron microscope (TEM,
acceleration voltage = 200 kV, FEI Talos F200x G2, America). Collect energy dispersive X-ray spectroscopy
(EDS, Oxford, Xplore) using scanning electron microscopy (SEM) apparatus. The crystallographic phases of
the samples were determined using an X-ray diffraction (XRD) analyzer (D8-Advance, Bruker, Germany)
with a Cu-Kα radiation source (λ = 0.15406 nm, 40 kV, 30 mA). XRD patterns from five to 80 degrees were
-1
recorded at a scanning speed of two degrees min . The graphitization degree of carbon in the sample was
analyzed by a Raman spectrometer (LABRAM HR800, HORIBA). The laser wavelength was 532 nm. X-ray
photoelectron spectroscopy (XPS, Thermo SCIENTIFIC Nexsa) was employed to study the chemical
composition in the samples. Al Kα radiation was used as the X-ray source (1,486.68 eV) with a pass energy
of 200 eV for survey spectra and 50 eV for high-resolution spectra. The energy resolution was 0.05 eV. XPS
spectra were fitted using XPSpeak 4.1 software. The specific surface area of the samples was measured by the
N adsorption-desorption isotherms using the Brunauer-Emmett-Teller (BET) method. Barrett-Joyner-
2
Halenda (BJH) strategy of Micromeritics (BRT, ASAP2020M, America) was used to evaluate the specific
surface area and pore size.

Electromagnetic parameter analysis
The electromagnetic parameters of the sample were measured using a vector network analyzer (Agilent
E5071C, USA) in the frequency range of 2-18 GHz. The product was mixed with paraffin to form an EMW
absorption ring with an inner diameter of 3.04 mm and an outer diameter of 7.00 mm. The magnitude of R L
dictates the product’s EMW absorption traits, and is calculated according to .
[21]

1/2
1/2
Z = Z (μ /ε ) tanh[j(2πfd/c)(ε μ ) ] (1)
0
r r
in
r
r
R = 20logǀ(Z - Z )/(Z + Z )ǀ (2)
L
in
0
0
in
Here, Z and Z were the input impedance and free-space impedance, respectively.
in
0
RESULTS AND DISCUSSION
A schematic diagram of CoFe/Co@NC heterostructure synthesis is shown in Figure 1. Firstly, porous CoFe-
MOF cubes are prepared by chemical precipitation method at room temperature, in which Co and Fe ions
are uniformly dispersed in the framework of the MOF. Then, during the calcination in Ar atmosphere, the
MOF is reconfigured to form 2D NC nanosheets. At the same time, Co and Fe are converted into dual
2+
2+
magnetic particles (0D CoFe alloy nanospheres and metallic Co hollow nanospheres), which are tightly
embedded in 2D NC nanosheets to form rich heterogeneous interfaces (CoFe/NC, Co/NC). Furthermore,
the heterostructure assembled from 2D NC nanosheets also provides opportunities for multiple reflections
of EMWs.
From Supplementary Figure 1, it can be seen that the synthesized CoFe-MOF can be assigned to CoFe-ZIF,
which is consistent with previous work . CoFe/Co@NC shows crystalline phases of Co O (PDF#42-1467)
[15]
3
4
[Figure 2A], CoFe alloys (PDF#49-1567) , and metallic Co (PDF#15-0806) . When the temperature is
[23]
[22]
increased, the crystallinity of Co O gradually decreases until the Co O crystalline phase has completely
4
3
4
3
disappeared at 900 °C. In contrast, the crystallinity of CoFe alloys first increases and then decreases with