Tuning Dielectric Loss of SiO2@CNTs for Electromagnetic Wave Absorption

We developed a simple method to fabricate SiO2-sphere-supported N-doped CNTs (NCNTs) for electromagnetic wave (EMW) absorption. EMW absorption was tuned by adsorption of the organic agent on the precursor of the catalysts. The experimental results show that the conductivity loss and polarization loss of the sample are improved. Meanwhile, the impedance matching characteristics can also be adjusted. When the matching thickness was only 1.5 mm, the optimal 3D structure shows excellent EMW absorption performance, which is better than most magnetic carbon matrix composites. Our current approach opens up an effective way to develop low-cost, high-performance EMW absorbers.


Introduction
With the development of science and technology, the rapid rise of artificial intelligence, the popularity of the smart home, and the extensive application of various electrical and electronic products, people's work efficiency and quality of life has improved. However, at the same time, the widespread use of electronic products also hides huge harms: longterm exposure to electromagnetic radiation will damage human health, but also harms other electronic products' electromagnetic interference, affecting their normal work. These hazards have attracted the attention of many countries in the world, and development of efficient electromagnetic absorption and shielding materials has become the main research direction. Therefore, it is necessary to develop a high-performance electromagnetic wave (EMW) absorber. To improve the efficiency of the unitizations, lightweight absorbers with a thin thickness are required. Carbonaceous materials such as graphene, carbon nanotubes (CNTs) and carbon nanofibers have attracted great attention because of their low mass density, good mechanical and chemical stability and high surface areas [1][2][3][4][5][6][7][8][9]. Carbon nanotubes have received extensive attention and in-depth studies in the field of EMW absorption due to their tubular structure suitable for electron transport, their light weight and good electrical conductivity [10][11][12][13][14][15][16][17]. For example, Lv et al. encapsulated Fe/Fe 3 C nanoparticles (NPs) into N-doped CNTs (NCNTs) and obtained the result that the sample had a reflection loss (R L ) of −46.0 dB and a thickness of 4.97 mm at 3.6 GHz [10]. Chang et al. reported Fe 3 O 4 /PPy/CNT composites with an R L of −25.9 dB with a thickness of 3.0 mm [11]. The reflection loss of ZnFe 2 O 4 @CNT/PVDF composite film prepared by Li et al. was −54.5 dB, with a matching thickness of 2.4 mm [12]. Gong et al. reported SiCN(Fe) fibers with an R L of −47.64 dB and the effective absorption bandwidth of 4.28 GHz [13]. The minimum reflection loss of Fe 3 O 4 /CNTs prepared by Zeng et al. was −51.0 dB, with a matching thickness of 4.4 mm [14]. Recently, a series of magnetic metal alloys (Fe, Co, Ni, etc.) that encapsulated into NCNTs were designed for EMW absorption [15,16]. However, the impedance matching feature of the composites mentioned above needed to be precisely tuned due to highly conductive magnetic metals and CNTs in these composites [17]. Furthermore, there is still room to improve the EMW absorption property of CNT-based absorbers, such as stronger absorption at a lower filler ratio and thinner matching thickness.
Here, we propose a simple method for SiO 2 -sphere-supported NCNTs with embedded Fe 3 C/Fe nanoparticles (NPs) (SiO 2 @Fe 3 C/Fe@NCNT-GT) for EMW absorption. Fe(OH) X was first coated on the surface of SiO 2 spheres [18], and then the organic solvent (terephthalic acid) was adsorbed on the Fe(OH) x surface to form relatively larger metal NPs for the growth of NCNTs with moderate diameters. Compared to the counterpart (SiO 2 @Fe 3 C/Fe@NCNT) without treatment in the organic solvent, the as-prepared SiO 2 @Fe 3 C/Fe@NCNT-GT showed significantly improved EMW absorption performance. At a filler ratio of 25%, the minimum R L (R L, min ) and effective bandwidth of the SiO 2 @Fe 3 C/Fe@NCNT-GT reached −48.43 dB and 4.51 GHz, respectively, while the matching thickness was only 1.5 mm. Paraffin was purchased from Yuyang Wax Industry (Changge, China). It is worth noting that all chemicals were purchased without further treatment before use, and all aqueous solutions were prepared using ultrapure water.

Characterizations and Electromagnetic Parameter Measurement
The morphology and size of the samples were characterized using scanning electron microscopy (SEM; Hitachi SU70, Tokyo, Japan) and transmission electron microscopy (TEM; FEITecnai-F20, Hillsboro, USA). Energy dispersive X-ray spectroscopy (EDX) was performed to confirm the elemental contents of the samples. X-ray diffraction (XRD) data were measured using a Rigaku D/max 2550 V (Tokyo, Japan) with Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) analyses were carried out by using a spectrometer with Mg Kα radiation (ESCALAB 250, Shanghai, China). Raman spectra were recorded on a Raman spectrometer (Xplora Plus, Paris, France) using a 488 nm He−Ne laser. The Brunauer-Emmett-Teller (BET) surface area and pore volume were tested with a Quantachrome Instruments Autosorb-iQ2-MP (Beijing, China) after the composites were vacuum dried at 200°C for 10 h. Fourier-transform infrared (FTIR) spectra of samples were collected using a Nicolet FTIR510 spectrometer (KBr pellet method, 4 cm −1 resolution, Waltham, MA, USA). The electromagnetic wave absorption properties of the absorbing materials were measured using a vector network analyzer (Anritsu MS4644A Vectorstar, Kanagawa, Japan) in the 2-18 GHz range at room temperature.

Synthesis of the SiO 2
Synthesis of the SiO 2 was conducted following the Stöber method. Deionized water (18 mL), absolute ethanol (76 mL) and TEOS (14 mL) were dissolved into NH 4 OH (98 mL), and continuously stirred for 4 h. Then, the colloidal solution was centrifuged, and the resultant was placed in an oven at 100 • C for 12 h to obtain silica microspheres [19,20]. SiO 2 with diameters of about 400 nm (216 mg) were first dispersed in ethanol (72 mL), and ferric acetylacetonate (270.5 mg) was added to the mixture above, sequentially. Distilled water (3.6 mL) and ammonia (2 mL) were added to the mixture with sonication for 15 min. The as-prepared mixture was sealed in a conical flask and stirred at 80°C for 10 h. The precipitate was washed with distilled water and ethanol several times, then centrifuged and dried in a vacuum oven at 40 • C to obtain the SiO 2 @Fe(OH) x [20].

Synthesis of the SiO 2 @Fe 3 C/Fe@NCNT
The SiO 2 @Fe(OH) x was annealed in Ar (the temperature was 800 • C, the time was 30 min and the ramp rate was 5 • C/min) to obtain SiO 2 @Fe 3 C/Fe@NCNT [20].

Synthesis of the SiO 2 @Fe(OH) x -GT
The SiO 2 @Fe(OH) x (200 mg), terephthalic acid (300 mg), pure water (3 mL) and ethanol (3 mL) were added into N,N-Dimethylformamide (DMF) solution (30 mL), and stirred for 30 min. Then, the mixed solution was placed into a 50 mL Teflon container and was treated at a high temperature of 150 • C for 12 h. The precipitate was washed and dried in a 40 • C vacuum oven to obtain the SiO 2 @Fe(OH) x -GT.

Synthesis of the SiO 2 @Fe 3 C/Fe@NCNT-GT
The SiO 2 @Fe(OH) x -GT was annealed in Ar (the temperature was 800 • C, the time was 30 min, and the ramp rate was 5 • C/min) to obtain the SiO 2 @Fe 3 C/Fe@NCNT-GT. The detailed experimental material, structural characterizations and the method are described in the Supplementary Materials.

Results and Discussion
The diameter of the prepared SiO 2 microspheres were approximately 400 nm. After being coated with an Fe(OH) x layer on the surface of the SiO 2 spheres, the diameter increased from 400 nm to about 500 nm (SiO 2 @Fe(OH) x ). Scanning electron microscopy (SEM) imaging and transmission electron microscopy (TEM) imaging showed that the Fe(OH) x layer was uniformly coated on the surface of the SiO 2 spheres (Figures 1a-c and S1a, Supplementary Materials). In the X-ray diffraction (XRD) pattern, there were two obvious peaks at 2θ 35.02 • and 62.72 • , indicating that the outmost layer of the SiO 2 @Fe(OH) x was mainly composed of weakly crystalline Fe(OH) 3 , which corresponded to JCPDS card no. 22-0346 ( Figure S2a). Energy dispersive X-ray spectroscopy (EDX) element mappings were also confirmed. As shown in Figure 1d, there were Si and O signals in the spherical core region, suggesting that the spherical core region materials were still SiO 2 spheres. Fe and O single elements were obviously present in the area outside the sphere, confirming the composition of the SiO 2 @Fe(OH) 3 . The XRD pattern of the SiO 2 @Fe(OH) x -GT indicated that the weakly crystalline Fe(OH) 3 ( Figure S2a) remained after the treatment, but the rough surface became relatively smooth ( Figure S1b). TEM images show that there was a lamellar structure on the surface of the SiO 2 @Fe(OH) 3 -GT, different from the SiO 2 @Fe(OH) 3 (Figure 1e-g). The peak at 798 and 1431 cm −1 in the Fourier-transform infrared (FTIR) spectra of the SiO 2 @Fe(OH) 3 -GT corresponded to the C-H deformation vibration and the C=C stretching vibration ( Figure S2b). The peak 1659 cm −1 in the spectrum of the SiO 2 @Fe(OH) 3 -GT corresponded to the C=O stretching vibration ( Figure S2b). Thus, the FTIR results indicated the adsorption of terephthalic acid on the Fe(OH) 3 layer. EDX element mappings also confirmed that Fe, O and C single elements were present in the area outside of the sphere, confirming the adsorption of terephthalic acid on the Fe(OH) 3 layer (Figure 1h). the FTIR results indicated the adsorption of terephthalic acid on the Fe(OH)3 layer. EDX element mappings also confirmed that Fe, O and C single elements were present in the area outside of the sphere, confirming the adsorption of terephthalic acid on the Fe(OH)3 layer (Figure 1h). In order to analyze the composition and valence state of the SiO2@Fe3C/Fe@NCNT-GT, XRD and Raman spectra were performed. As shown in Figure 2a, there was a broad diffraction (2θ) from 10° to 30°, corresponding to amorphous SiO2 in the XRD pattern of the SiO2@Fe3C/Fe@NCNT-GT. In the XRD pattern of the SiO2@Fe3C/Fe@NCNT-GT, the peak at 2θ 44.7° and 42.9° can be indexed to (110) planes of the Fe NPs (JCPDS no. 06-0696) and (211) planes of the Fe3C (JCPDS no. 35-0772), in sequence, while the peak at 26.4° is attributed to the NCNTs (JCPDS no. 41-1487). The Raman spectra of the SiO2@Fe3C/Fe@NCNT-GT showed two distinguishable peaks: one at 1325 cm −1 (D band) and the other at 1585 cm −1 (G band) (Figure 2b). Their intensity ratios (ID/IG) for the SiO2@Fe3C/Fe@NCNT-GT was 1.002, which indicated rich defects in the sample. These defects in the SiO2@Fe3C/Fe@NCNT-GT can greatly improve the polarization relaxation and contribute to enhancing the absorption of electromagnetic waves [21]. As shown in Figure 2c, the N2-sorption isotherms of the SiO2@Fe3C/Fe@NCNT-GT displays type-IV loops, revealing that the mesopores existed in the prepared sample. Furthermore, the Brunauer-Emmett-Teller (BET) surface area of the SiO2@Fe3C/Fe@NCNT-GT was 243.54 m 2 g −1 . The illustration in Figure 2c displays pore size distribution diagrams. The pore sizes of the SiO2@Fe3C/Fe@NCNT-GT were centered at 15 nm, and pore volume was 0.568 In order to analyze the composition and valence state of the SiO 2 @Fe 3 C/Fe@NCNT-GT, XRD and Raman spectra were performed. As shown in Figure 2a, there was a broad diffraction (2θ) from 10 • to 30 • , corresponding to amorphous SiO 2 in the XRD pattern of the SiO 2 @Fe 3 C/Fe@NCNT-GT. In the XRD pattern of the SiO 2 @Fe 3 C/Fe@NCNT-GT, the peak at 2θ 44.7 • and 42.9 • can be indexed to (110) planes of the Fe NPs (JCPDS no. 06-0696) and (211) planes of the Fe 3 C (JCPDS no. 35-0772), in sequence, while the peak at 26.4 • is attributed to the NCNTs (JCPDS no. 41-1487). The Raman spectra of the SiO 2 @Fe 3 C/Fe@NCNT-GT showed two distinguishable peaks: one at 1325 cm −1 (D band) and the other at 1585 cm −1 (G band) (Figure 2b). Their intensity ratios (I D /I G ) for the SiO 2 @Fe 3 C/Fe@NCNT-GT was 1.002, which indicated rich defects in the sample. These defects in the SiO 2 @Fe 3 C/Fe@NCNT-GT can greatly improve the polarization relaxation and contribute to enhancing the absorption of electromagnetic waves [21]. As shown in Figure 2c, the N 2 -sorption isotherms of the SiO 2 @Fe 3 C/Fe@NCNT-GT displays type-IV loops, revealing that the mesopores existed in the prepared sample. Furthermore, the Brunauer-Emmett-Teller (BET) surface area of the SiO 2 @Fe 3 C/Fe@NCNT-GT was 243.54 m 2 g −1 . The illustration in Figure 2c displays pore size distribution diagrams. The pore sizes of the SiO 2 @Fe 3 C/Fe@NCNT-GT were centered at 15 nm, and pore volume was 0.568 cm 3 g −1 . X-ray photoelectron spectroscopy (XPS) spectra displayed that there were five elements (Fe, N, O, Si and C) in the SiO 2 @Fe 3 C/Fe@NCNT-GT ( Figure S3). The peaks were at 398.5 (pyridine-N), 399.9 (pyrrolic-N), 401.1 (graphite-Nand) and 404.5 eV (oxide-N) in the XPS spectra of N 1s, respectively (Figure 2d) [22]. The peaks at 709.1 and 721.9 eV in the XPS spectra of Fe 2p can be indexed to metallic Fe. The peaks (717.3 and 733.2 eV) and satellite peaks (712.1 and 724.9 eV) reveal the oxidation state of Fe species in the sample [23][24][25] (Figure 2e). The binding energies of C-C (284.6 eV), C-N (285.7 eV) and C-O (288.7 eV) were observed on the surface of NCNT (Figure 2f) [26,27]. The carbon atom will tend to form unsaturated covalent bonds with the oxygen anion, increasing the charge state and increasing the band gap. This is due to the electron density shifts from the carbocation to the more electronegative oxygen anion, which in turn affects the electron structure. cm³ g −1 . X-ray photoelectron spectroscopy (XPS) spectra displayed that there were five elements (Fe, N, O, Si and C) in the SiO2@Fe3C/Fe@NCNT-GT ( Figure S3). The peaks were at 398.5 (pyridine-N), 399.9 (pyrrolic-N), 401.1 (graphite-Nand) and 404.5 eV (oxide-N) in the XPS spectra of N 1s, respectively (Figure 2d) [22]. The peaks at 709.1 and 721.9 eV in the XPS spectra of Fe 2p can be indexed to metallic Fe. The peaks (717.3 and 733.2 eV) and satellite peaks (712.1 and 724.9 eV) reveal the oxidation state of Fe species in the sample [23][24][25] (Figure 2e). The binding energies of C-C (284.6 eV), C-N (285.7 eV) and C-O (288.7 eV) were observed on the surface of NCNT (Figure 2f) [26,27]. The carbon atom will tend to form unsaturated covalent bonds with the oxygen anion, increasing the charge state and increasing the band gap. This is due to the electron density shifts from the carbocation to the more electronegative oxygen anion, which in turn affects the electron structure. SEM images indicated that the SiO2@Fe3C/Fe@NCNT-GT exhibited 3D morphology, where the NCNTs were grown on the surface of the SiO2 spheres ( Figure S4). Bamboo-like NCNTs in the SiO2@Fe3C/Fe@NCNT-GT are also observed in Figure 3a,b with a length of approximately 1.5 μm. Magnified TEM images show that their average diameter and wall thickness were approximately 51 and 12 nm, respectively (Figures 3b,c and S5). There were some NPs, with an average diameter of about 39 nm, embedded in the bamboo-like NCNTs. Figure 3c shows that the NPs were encapsulated in 25-30 layers of the graphene shell in the high-resolution TEM (HRTEM) images. The d-spacing of labeled lattice fringes of 0.20 nm corresponded to the (110) planes of Fe, while the d-spacing of 0.35 nm corresponded to the (002) planes of graphite-carbon (Figure 3d). Notably, a mass of defects were present in the NCNT walls and graphene shell of the SiO2@Fe3C/Fe@NCNT-GT. As SEM images indicated that the SiO 2 @Fe 3 C/Fe@NCNT-GT exhibited 3D morphology, where the NCNTs were grown on the surface of the SiO 2 spheres ( Figure S4). Bamboo-like NCNTs in the SiO 2 @Fe 3 C/Fe@NCNT-GT are also observed in Figure 3a,b with a length of approximately 1.5 µm. Magnified TEM images show that their average diameter and wall thickness were approximately 51 and 12 nm, respectively (Figures 3b,c and S5). There were some NPs, with an average diameter of about 39 nm, embedded in the bamboo-like NCNTs. Figure 3c shows that the NPs were encapsulated in 25-30 layers of the graphene shell in the high-resolution TEM (HRTEM) images. The d-spacing of labeled lattice fringes of 0.20 nm corresponded to the (110) planes of Fe, while the d-spacing of 0.35 nm corresponded to the (002) planes of graphite-carbon (Figure 3d). Notably, a mass of defects were present in the NCNT walls and graphene shell of the SiO 2 @Fe 3 C/Fe@NCNT-GT. As is shown in Figure 3c, these defects are marked with a yellow frame. Defects including lattice distortion, lattice dislocation and fracture edges are considered to have a positive effect on the absorption property of the SiO 2 @Fe 3 C/Fe@NCNT-GT. The distribution of elements in the SiO 2 @Fe 3 C/Fe@NCNT-GT was analyzed by EDX element mapping. There were O and Si signals in the spherical core zone, indicating that the spherical core region mediums were still SiO 2 spheres (Figure 3e). Fe, C and N single Nanomaterials 2021, 11, 2636 6 of 11 elements were present in the zone of the NCNTs, confirming the composition of NCNTs. Compared to the SiO 2 @Fe 3 C/Fe@NCNT [20], the average diameter of the NCNTs in the SiO 2 @Fe 3 C/Fe@NCNT-GT increased from 15 nm to 58 nm, the length of bamboo nodes increased from 15 nm to 50 nm, and the wall thickness increased from 3 to 12 nm. The adsorption of terephthalic acid limited the contact of the Fe(OH) 3 with the reductive gases, leading to the formation of the larger metal NPs. Consequently, the diameter of NCNTs became larger compared to the counterpart without the adsorption of terephthalic acid. is shown in Figure 3c, these defects are marked with a yellow frame. Defects including lattice distortion, lattice dislocation and fracture edges are considered to have a positive effect on the absorption property of the SiO2@Fe3C/Fe@NCNT-GT. The distribution of elements in the SiO2@Fe3C/Fe@NCNT-GT was analyzed by EDX element mapping. There were O and Si signals in the spherical core zone, indicating that the spherical core region mediums were still SiO2 spheres (Figure 3e). Fe, C and N single elements were present in the zone of the NCNTs, confirming the composition of NCNTs. Compared to the SiO2@Fe3C/Fe@NCNT [20], the average diameter of the NCNTs in the SiO2@Fe3C/Fe@NCNT-GT increased from 15 nm to 58 nm, the length of bamboo nodes increased from 15 nm to 50 nm, and the wall thickness increased from 3 to 12 nm. The adsorption of terephthalic acid limited the contact of the Fe(OH)3 with the reductive gases, leading to the formation of the larger metal NPs. Consequently, the diameter of NCNTs became larger compared to the counterpart without the adsorption of terephthalic acid. The factors that may enhance the absorption performance of EMW were investigated through the comparison of electromagnetic parameters of the SiO2@Fe3C/Fe@NCNT-GT and the SiO2@Fe3C/Fe@NCNT, including complex permittivity and permeability. They can be expressed separately by the formula εr =ε′ − jε″ and μr = μ′ − jμ″. (ε′ is the real part of permittivity, ε″ is the imaginary part of permittivity, μ′ is the real part of permeability, and μ″ is the imaginary part of permeability) [28][29][30]. As shown in Figure 4a-c, the ε′ values of the SiO2@Fe3C/Fe@NCNT-GT varied in a range of 16.63−9.81, and the ε″ values of the SiO2@Fe3C/Fe@NCNT-GT varied in a range of 6.15−2.44. The permittivity for both samples gradually decreased with the increase in the frequency, which is due to the frequency dispersion effect. The ε′ and ε″ values of the SiO2@Fe3C/Fe@NCNT-GT were larger than those of the SiO2@Fe3C/Fe@NCNT. The dielectric loss tangent (tanδe = ε″/ε′) of the SiO2@Fe3C/Fe@NCNT-GT was also larger. Figure S6 shows that the two samples had very The factors that may enhance the absorption performance of EMW were investigated through the comparison of electromagnetic parameters of the SiO 2 @Fe 3 C/Fe@NCNT-GT and the SiO 2 @Fe 3 C/Fe@NCNT, including complex permittivity and permeability. They can be expressed separately by the formula ε r = ε − jε" and µ r = µ − jµ". (ε is the real part of permittivity, ε" is the imaginary part of permittivity, µ is the real part of permeability, and µ" is the imaginary part of permeability) [28][29][30]. As shown in Figure 4a-c, the ε values of the SiO 2 @Fe 3 C/Fe@NCNT-GT varied in a range of 16.63−9.81, and the ε" values of the SiO 2 @Fe 3 C/Fe@NCNT-GT varied in a range of 6.15−2.44. The permittivity for both samples gradually decreased with the increase in the frequency, which is due to the frequency dispersion effect. The ε and ε" values of the SiO 2 @Fe 3 C/Fe@NCNT-GT were larger than those of the SiO 2 @Fe 3 C/Fe@NCNT. The dielectric loss tangent (tanδ e = ε"/ε ) of the SiO 2 @Fe 3 C/Fe@NCNT-GT was also larger. Figure S6 shows that the two samples had very little difference in the real part of permeability, imaginary part of permeability and magnetic loss tangent (tan δ m ), with a value over 2-18 GHz. The saturation magnetization (M s ), remnant magnetization (M r ) and coercivity (H c ) of the SiO 2 @Fe 3 C/Fe@NCNT-GT were slightly larger than those of the SiO 2 @Fe 3 C/Fe@NCNT ( Figure S7). Thus, the magnetic loss of the SiO 2 @Fe 3 C/Fe@NCNT-GT is not a determining factor for the EMW performance. little difference in the real part of permeability, imaginary part of permeability and magnetic loss tangent (tan δm), with a value over 2-18 GHz. The saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc) of the SiO2@Fe3C/Fe@NCNT-GT were slightly larger than those of the SiO2@Fe3C/Fe@NCNT ( Figure S7). Thus, the magnetic loss of the SiO2@Fe3C/Fe@NCNT-GT is not a determining factor for the EMW performance. In general, the dielectric loss of the absorbing material includes the conduction loss and the polarization relaxation loss within the range of gigahertz. The former can be expressed by the formula (εc″ = σ/ε0ω); the characters σ, ε0 and ω represent the conductivity, the permittivity in a vacuum and the circular frequency, respectively. The latter is expressed by the formula (εp″ =ε″ − εc″). The experimental results showed that the electrical conductivity of the SiO2@Fe3C/Fe@NCNT-GT was higher than that of the SiO2@Fe3C/Fe@NCNT (Table S1). Therefore, the SiO2@Fe3C/Fe@NCNT-GT had increased conductive loss compared to the SiO2@Fe3C/Fe@NCNT (Figure 4d). Meanwhile, the SiO2@Fe3C/Fe@NCNT-GT also had improved polarization losses compared to the SiO2@Fe3C/Fe@NCNT (Figure 4e) and had a higher attenuation coefficient (Figure 4f). As shown in Figure S8, multiple Cole-Cole semicircles could be found in the curve of the SiO2@Fe3C/Fe@NCNT-GT, confirming the existence of dipole polarization and interfacial polarization relaxation. Therefore, the increased dielectric relaxation loss of the SiO2@Fe3C/Fe@NCNT-GT is relevant to their enhanced dipole and interface polarizations [31][32][33][34][35]. Figure 5 shows the RL-f curves of the two samples with d of 1.5-5.0 mm over 2-18 GHz. It can be found that the SiO2@Fe3C/Fe@NCNT-GT exhibited a better EMW ab- In general, the dielectric loss of the absorbing material includes the conduction loss and the polarization relaxation loss within the range of gigahertz. The former can be expressed by the formula (ε c " = σ/ε 0 ω); the characters σ, ε 0 and ω represent the conductivity, the permittivity in a vacuum and the circular frequency, respectively. The latter is expressed by the formula (ε p " =ε" − ε c "). The experimental results showed that the electrical conductivity of the SiO 2 @Fe 3 C/Fe@NCNT-GT was higher than that of the SiO 2 @Fe 3 C/Fe@NCNT (Table S1). Therefore, the SiO 2 @Fe 3 C/Fe@NCNT-GT had increased conductive loss compared to the SiO 2 @Fe 3 C/Fe@NCNT (Figure 4d). Meanwhile, the SiO 2 @Fe 3 C/Fe@NCNT-GT also had improved polarization losses compared to the SiO 2 @Fe 3 C/Fe@NCNT (Figure 4e) and had a higher attenuation coefficient ( Figure 4f). As shown in Figure S8, multiple Cole-Cole semicircles could be found in the curve of the SiO 2 @Fe 3 C/Fe@NCNT-GT, confirming the existence of dipole polarization and interfacial polarization relaxation. Therefore, the increased dielectric relaxation loss of the SiO 2 @Fe 3 C/Fe@NCNT-GT is relevant to their enhanced dipole and interface polarizations [31][32][33][34][35]. Figure 5 shows the R L -f curves of the two samples with d of 1.5-5.0 mm over 2-18 GHz. It can be found that the SiO 2 @Fe 3 C/Fe@NCNT-GT exhibited a better EMW absorption property than the SiO 2 @Fe 3 C/Fe@NCNT. It should be noted that all of the R L values of SiO 2 @Fe 3 C/Fe@NCNT-GT can exceed −20 dB (Figure 5a), where the minimum value was −48.43 dB with d of only 1.5 mm. However, the R L, min value for the SiO 2 @Fe 3 C/Fe@NCNT was only −16.63 dB with d of 5 mm (Figure 5b). Furthermore, the effective absorption bandwidth (EAB 10 , R L ≤ −10 dB) of the SiO 2 @Fe 3 C/Fe@NCNT-GT was 4.51 GHz, which is superior to that of the SiO 2 @Fe 3 C/Fe@NCNT (2.12 GHz) (Figure 5a,b). Thus, the SiO 2 @Fe 3 C/Fe@NCNT-GT showed a significantly enhanced EMW absorption property in the main parameters, including R L, min , EAB 10 and d values, showing it has potential applications in practical EMW absorption. In addition, our prepared SiO 2 @Fe 3 C/Fe@NCNT-GT had comparable, or better, EMW absorption performance than reported carbon nanotube-based absorbent materials (Figure 5c, Table S2) [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53].

Conclusions
In summary, we fabricated the SiO 2 @Fe 3 C/Fe@NCNT-GT with a moderate diameter for EMW absorption. Compared to the counterpart (SiO 2 @Fe 3 C/Fe@NCNT) without treatment in the organic solvent, the dielectric loss of the as-prepared SiO 2 @Fe 3 C/Fe@NCNT-GT was optimized, the impedance matching characteristics were adjusted, and the absorption performance of EMW was significantly improved. At a filler ratio of 25%, minimum, reflection loss can reach −48.43 dB. In the meantime, effective bandwidth of the SiO 2 @Fe 3 C/Fe@NCNT-GT can reach 4.51 GHz, while the matching thickness is only 1.5 mm, which is better than most magnetic carbon matrix composites. Our present approach opens up an effective way to develop low-cost, high-performance EMW absorbers.