Three-dimensional foam-like Fe3O4@C core-shell nanocomposites: Controllable synthesis and wideband electromagnetic wave absorption properties

Journal Pre-proofs Research articles Three-dimensional Foam-like Fe3O4@C Core-Shell Nanocomposites:Controllable Synthesis and Wideband Electromagnetic Wave Absorption Properties Xun Meng, Weiwei Yang, Guanghui Han, Yongsheng Yu, Song Ma, Wei Liu, Zhidong Zhang PII: DOI: Reference:

S0304-8853(19)33915-0 https://doi.org/10.1016/j.jmmm.2020.166518 MAGMA 166518

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

16 November 2019 9 January 2020 24 January 2020

Please cite this article as: X. Meng, W. Yang, G. Han, Y. Yu, S. Ma, W. Liu, Z. Zhang, Three-dimensional Foamlike Fe3O4@C Core-Shell Nanocomposites:Controllable Synthesis and Wideband Electromagnetic Wave Absorption Properties, Journal of Magnetism and Magnetic Materials (2020), doi: https://doi.org/10.1016/j.jmmm. 2020.166518

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Three-dimensional Foam-like Fe3O4@C Core-Shell Nanocomposites：Controllable Synthesis and Wideband Electromagnetic Wave Absorption Properties Xun Meng1, Weiwei Yang1*, Guanghui Han1, Yongsheng Yu1*, Song Ma2*, Wei Liu2 and Zhidong Zhang2 1MIIT

Key Laboratory of Critical Materials Technology for New Energy Conversion and

Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China 2Shenyang

National Laboratory for Materials Science, Institute of Metal Research, Chinese

Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China * Corresponding Author. E-mail: [email protected] (Weiwei Yang), [email protected] (Yongsheng Yu) and [email protected] (Song Ma) Abstract: Ferrite and carbon nanocomposites are regarded to be excellent electromagnetic wave absorbers due to their compatible dielectric and magnetic loss capabilities. In this article, the three-dimensional (3D) Fe3O4@C core-shell nanocomposites with foam-like structure have been successfully prepared, which would greatly promote multiple reflection/scattering of the incident microwave. By adjusting the amount of polymeric precursors, the outer carbon can be perfectly controlled within the average thickness range of 9.1 to 34.8 nm. Meanwhile, through the synergy between 3D foam-like Fe3O4 core and outer carbon, the impedance matching conditions of the nanocomposites can be improved. Besides, the electromagnetic wave absorptivity of the nanocomposites presents a significant dependence on the thickness of outer carbon. When the average thickness of outer carbon is 28.3 nm, the absorber shows the most excellent performance, and the minimum reflection loss (RL) value is ﹣55.5 dB with 2.5 mm absorber thickness. Especially, when the thickness is tuned from 1.4 mm to 5.5 mm, the Fe3O4@C exhibits efficient absorption (RL≤﹣20 dB)

in an ultra-wide band (14.9 GHz). Based on the amazing microwave absorption properties, this nanocomposite can be considered as an ideal absorber. Keywords: Fe3O4@C, 3D foam-like structure, core-shell nanocomposites, carbothermal reduction, electromagnetic wave absorption 1. Introduction In recent years, with the in-depth development of electromagnetic (EM) technology, it has made tremendous achievements in military and civilian applications, bringing great convenience to people [1-3]. However, the EM pollution poses a great threat to human health and interferes with the normal operation of other electronics [4,5]. In order to solve these problems, a wide variety of materials have been extensively investigated as electromagnetic wave (EMW) absorbers and they are expected to have the characteristics of light weight, strong and broadband absorption, thin thickness, anti-oxidation and high stability [6]. It is well-acknowledged the absorption of EMW absorbers is decided by the dielectric and magnetic losses, as well as the balance between them to obtain appropriate impedance matching conditions [7,8]. Therefore, the absorbing materials with a single loss mechanism are difficult to meet the ever-increasing demands for EMW absorption. Obviously, the development of magnetic and dielectric nanocomposites with tunable electromagnetic parameters has become an effective method for manufacturing high-performance EMW absorbers. To date, EMW absorbers with magnetic core and dielectric shell have received much attention due to the synergy of the magnetic and dielectric loss modes [9-15]. Fe3O4 attracts people's great interest as a magnetic core material due to its low cost and unique magnetic properties, such as appropriate saturation magnetization value and relatively high Curie temperature, thus offering great application potential as an efficient EMW absorber [16,17]. In the meanwhile, compared with other dielectric additives, carbon shell not only has the inherent advantages of light weight and good chemical stability, but also protects the core material from corrosion, oxidation and pollution [18]. Additionally, the thickness of outer carbon can be adjusted to control the EMW absorbing properties of the core-shell materials. So far, Liu et al. have synthesized Fe3O4@Carbon nanospindles with a core-shell structure,

and the minimum reflection loss (RL) of the flexible Fe3O4@Carbon@PVDF nanocomposites is ﹣ 38.8 dB [19]. Zou et al. have prepared the Fe3O4@Carbon yolk-shell spherical nanomaterials by a silica-assisted method, which exhibit a significant complementarity between magnetic and dielectric modes with a minimum RL value of ﹣ 18.1 dB [20]. The Fe3O4@C core-shell nanorods have been fabricated by a three-step process by Chen et al. and the minimum RL value was approximately ﹣27.9 dB (14.96 GHz) when the thickness is 2.0 mm. Besides, as the thickness varies from 2.0 to 5.0 mm, the absorption bandwidth of the rod-shaped absorber with RL values less than ﹣18 dB can reach 10.5 GHz [21]. Qiao et al. have developed yolk-shell Fe3O4/C nanochains absorbers with a minimum RL value of ﹣63.09 dB at 11.91 GHz [22]. Liu et al. have prepared Fe2O3 nanosheets via hydrothermal method, then obtained Fe3O4@carbon core-shell nanosheets absorbers by carbothermal reduction method using acetone as carbon source. The absorber with a nanosheet size of 250 nm has the minimum RL of ﹣ 43.95 dB and exhibits efficient absorption (RL ≤ ﹣ 20 dB) over a broadband from 2.08 to 16.40 GHz with thickness ranging from 1.4 to 7.0 mm [23]. However, studies of Fe3O4@carbon EMW absorbers with core-shell structures are still limited

to

zero-dimensional

(0D)

materials

(nanoparticles, nanospheres)

[24-26],

one-dimensional (1D) materials (nanochains, nanorods, nanofibers) [19,22,27,28] and two-dimensional (2D) materials (nanosheets) [23]. It is well known that the microstructure of the absorbing material has a great impact on the attenuation of EMW [22]. Many studies have shown that pore or tunnel structure helps to improve the absorbing properties of composite absorbing materials. For example, the porous Fe3O4@carbon nanocomposites, in which the Fe3O4 nanoparticles embedded in a carbon network, have successfully synthesized by Li et al. Incident microwave could be attenuated by multiple scattering and absorption in the 3D porous structure, thus jointly improving the interaction between the microwave and the nanocomposite, thereby promoting the absorbing properties of the Fe3O4@carbon [29]. Wu's team has produced 3D porous Fe3O4@C microstructured flowers, and the EMW absorption of the composites was improved by multiple reflection/scattering due to the advantages of 3D porous structure [30]. Moreover, the 3D porous structure of the absorber is particularly advantageous for impedance matching [29]. Unfortunately, at present, the morphology of the

composite core-shell structure of Fe3O4 and carbon is mostly granular [26], rod-shaped [19] and sheet-shaped [23], which is easily agglomerated and thus is not conducive to enhancing the multiple reflection of EMW. In this work, we prepared 3D foam-like Fe3O4@C core-shell nanocomposites, studied the relationship between the EMW absorption properties and the thickness of carbon shell and discussed the absorption mechanism. In particular, Fe2O3 prepared by the solution combustion method obtains a 3D porous structure due to gas generation during combustion, and this structure provides multiple reflection/scattering channels of EMW for the absorber prepared later, thereby enhancing EMW absorption performance. The results show that the porous Fe3O4@C with ultra-thin thickness exhibit strong EMW absorption in 2-18 GHz, and the absorption capacity is controllable by adjusting the thickness of outer carbon. The preparation method is not only simple and inexpensive, but also has a rich yield, which provides a meaningful reference for future research. 2. Experimental section 2.1. Chemical reagents Ferric nitrate (Fe(NO3)3·9H2O), glycine (C2H5NO2), absolute ethanol (C2H5OH), resorcinol (C6H6O2), and formaldehyde (HCHO, 38 wt %) are analytically pure purchased from Beijing Yinuokai Technology Co., Ltd. Deionized water was used for all the experiments. 2.2. Synthesis of 3D foam-like Fe2O3 precursor Iron nitrate (Fe(NO3)3·9H2O) and glycine (C2H5NO2) with the required amounts were dissolved in the distilled water. The molar ratio of glycine to ferric nitrate is 0.6. After homogeneous dispersion, the mixture was poured into a beaker, heated until it turned into a gel, further heated, and then the ignition reaction started from a point and spread rapidly. The temperature of the hot plate is maintained at 200 oC during the reaction. The obtained Fe2O3 foam were kept at 700 oC for 1 h in a muffle furnace, then 3D foam-like Fe2O3 precursor can be obtained. 2.3. Synthesis of 3D foam-like Fe3O4@C core-shell nanocomposites The 3D foam-like Fe3O4@C nanocomposites were synthesized by in-situ phenolic polymerization and subsequent carbothermal reduction. Typically, 100, 40, and 0.4 mL each

of deionized water, absolute ethanol, and ammonia were placed in a flask and mixed uniformly. Then, 80 mg of Fe2O3 precursor was added to the flask and continuously stirred for 1 h to form a homogeneous solution. Subsequently, a specific amount of resorcinol was added into this homogeneous solution and magnetically stirred at ambient temperature. After 45 min, formaldehyde was added to the flask to initiate phenolic polymerization and kept at normal temperature for another 22 h. After the reaction, the mixed solution was centrifuged to obtain the Fe2O3@PR (phenolic resin) nanocomposites, which were washed with water and ethanol until the solution was colorless, and then dried at 60 oC. In the last step, the Fe2O3@PR nanocomposites undergone a carbothermal reduction process in a tube furnace at 600 oC under Ar atmosphere for 4 h with a heating rate of 5 oC/min. Then the final 3D foam-like Fe3O4@C core-shell nanocomposites can be obtained. In this experiment, the value of the molar ratio of formaldehyde to resorcinol was kept at 2. When the resorcinol was added in molar amounts of 0.6, 1.0, 1.4, and 1.8 mmol, the corresponding products were labeled as S1, S2, S3, and S4. 2.4. Characterization The crystal phases of all products were characterized via Bruker AXS D8-Advanced X-ray diffractometer. The XPS data of the samples was collected by an ESCALAB 250 spectrometer (Thermofisher Co.) for determining the composition and chemical bonding configurations. The morphology and microstructure of the products were observed using SEM (Hitachi S-4800), TEM (JEOL JEM-1400) and HR-TEM (FEI Tecnai G2-F30). Raman spectra were measured using a Thermo Nicolet Almega XR Raman Spectrometer. The magnetic performances of the samples were carried out on a Lakeshore 7404 vibrating sample magnetometer (VSM). 2.5. Measurements of electromagnetic properties The samples containing 50 wt % as-prepared nanocomposites were pressed into rings with 7.00 mm outer diameter, 3.04 mm inner diameter and 2-3 mm thickness for EMW absorption test in which paraffin was used as an adhesive. The EM parameters of the nanocomposites were performed using a vector network analyzer (Agilent N5230A) in 2-18 GHz. 3. Results and discussion

Figure 1 graphically depicts the synthesis process of the 3D foam-like Fe3O4@C core-shell nanocomposites. The 3D Fe2O3 foam (Figure S1) is first prepared by a solution combustion method. Then, the 3D foam-like Fe2O3 is obtained by subsequent heat treatment. Moreover, the Fe2O3@ phenolic resin (PR) is synthesized via in-situ phenolic polymerization on the Fe2O3 surface. After carbothermal reduction in Ar atmosphere, the carbonization of the phenolic resin shell and the reduction of the Fe2O3 core together resulted in the Fe3O4@C core-shell nanocomposites, in the meantime, which still maintains a foam-like morphology well.

Figure 1 ． Simple flow chart for synthesizing 3D foam-like Fe3O4@C core-shell nanocomposites. The crystal phases and compositions of the as-synthesized samples were characterized using X-ray diffractometer. As shown in Figure 2, the precursor (3D foam-like Fe2O3) can be assigned to the rhombohedral structure of α-Fe2O3 phase (JCPDS No. 33-0664). The obvious diffraction peaks show that the crystallinity of the α-Fe2O3 is very high. During high temperature heat treatment, the PR is carbonized, and simultaneously the thermal reduction leads to the conversion of Fe2O3 to Fe3O4. It can be seen that despite the change of shell thickness, the XRD spectra of the S1-S4 after heat treatment at 600 oC present almost the same diffraction peaks matching the magnetic Fe3O4 (JCPDS No. 19-0629). The peaks of carbon are not observed in the XRD patterns, which indicates that the outer carbon after heat treatment is amorphous. Whereas, the outer carbon is evident in the TEM and HR-TEM images below and can be analyzed by Raman spectroscopy and XPS.

Figure 2. XRD patterns of the synthesized products. XPS analysis was carried out to explore the elemental composition and chemical bond types of the 3D foam-like Fe3O4@C nanocomposites. As shown in Figure 3a, the typical signal peaks of iron, oxygen and carbon elements have been found. The obvious peaks at 285 and 531 eV indicate the presence of carbon and oxygen elements, whereas the peaks at 711 and 724 eV correspond to iron element [31]. The high-resolution XPS spectrum for Fe 2p is shown in Figure 3b, and the characteristic peaks corresponding to Fe3+ are located at about 711.5 and 725.2 eV, while the peaks of Fe2+ centered at about 710.4 and 723.4 eV [32]. The molecular formula of the compound can be further determined by calculating atomic ratios. The atomic ratio (γ = Fe3+:Fe2+) can be obtained using the following formula [23]: 𝑛𝑖 𝑛 = (𝐼𝑖 𝐼 ) 𝐸𝑘𝑗 𝐸 𝑗 𝑗 𝑘𝑖

(1)

where ni and nj represent the surface atomic number, Ii and Ij are corresponding the peak area, and Eki and Ekj refer to their respective photoelectron kinetic energy. By calculating the peak areas of Fe2+ and Fe3+ as shown in Figure 3b, the value of γ is about 1.98, which is basically consistent with the Fe3O4 and corroborates the results of XRD analysis. Besides, the O1s peak at 530.1 eV in Figure 3c corresponds to lattice oxygen in Fe3O4, which also further supports the existence of Fe3O4 [33]. Furthermore, the peak located at 531.9 eV in O1s spectrum represents the residual oxygen-containing groups, which can be confirmed in the C1s spectrum (Figure 3d) [34]. The XPS spectrum of C 1s can be fitted into two peaks, corresponding to C−C group (284.6 eV) and C−O group (285.8 eV), respectively [35]. The

characteristic peak at 284.6 eV exhibits an absolutely dominant carbon shell peak, while the peak associated with phenolic resin at 285.8 eV is obviously weak, indicating that most phenolic resins have been converted to carbon after 4 hours of heat treatment at 600 oC. All data analysis results show that the compositions of S1 contain both Fe3O4 and C.

a

b

c

d

Figure 3. XPS survey spectrum of S1 (a) and the corresponding high-resolution XPS spectra of (b): Fe 2p, (c): O 1s and (d): C 1s. The morphology and microstructure of the Fe2O3 and Fe3O4@C samples were observed using SEM, TEM and HRTEM. The SEM image of the prepared Fe2O3 product is shown in Figure 4a. It is apparent that the Fe2O3 is a 3D foam-like morphology composed of chain-like particles winding. The average particle size of the smallest unit of the chain structure is about 100 nm. After phenolic polymerization and carbonization, Fe3O4@C maintains the morphology of Fe2O3 well and the only difference is that the smallest unit is thicker as shown in the SEM images in Figure 4b and c. Moreover, the detailed microstructures of the Fe3O4@C nanocomposites were characterized by TEM. Obviously, it can be found that the Fe3O4@C nanocomposites show core-shell structures, and the surface of carbon shell is uniform and smooth. With the increase of formaldehyde and resorcinol in the polymerization reaction, the average thickness of outer carbon after thermal reduction increased from 9.1 to 34.8 nm (Figure 4d-g). From the HRTEM and SAED images of S3, it can be seen that the lattice structure of the Fe3O4 core is obvious, and its interplanar spacing is 0.48 nm,

corresponding to the (111) plane of Fe3O4 (Figure 4h and i). However, the outer carbon shell cannot be found to have a lattice structure, which further proves the amorphous structure.

a

b

c

d

9.1 nm

300 nm

1 μm

5 μm

e

f 18.7 nm

200 nm

100 nm

g

28.3 nm

h

i

0.48nm

200 nm

(220) (311) (111)

(111)

34.8 nm

200 nm

5 nm

51 / nm

Figure 4. (a) SEM image of Fe2O3 precursor, (b and c) SEM images of S3, (d-g) TEM images of S1-S4, (h) HRTEM image and (f) SAED of S3. Furthermore, the graphitization degree of the outer carbon coated on Fe3O4 surface can be confirmed via Raman analysis. In Figure 5a, the signal peaks of D and G bands at 1350 and 1595 cm-1, respectively, can be evidently observed. The peak of D-band indicates the structural disorder and defect of the outer carbon shell, whereas the peak of G-band reveals the planar vibrations of graphite with lattice structure [36,37]. The intensity ratio of the D-peak to the G-peak (ID:IG) was considered as a criterion to determine the degree of disorder of carbon. By comparing the Raman spectra, it can be found that the all values of ID:IG are nearly identical in different samples, which implies the uniform graphitization degree of carbon. The ID:IG values of S1-S4 are relatively large, indicating that the degree of

amorphousness of carbon is high. The results obtained are consistent with those of XRD and HRTEM analyses. Figure 5b exhibits the magnetic hysteresis loops of the S1-S4 measured at ambient temperature. Besides, the detailed relevant magnetic parameters are shown in Table S1. Because of the existence of Fe3O4, all four samples show the soft magnetic properties and can be saturated at 10 kOe. It can be seen that the saturation magnetization (Ms) values of S1, S2, S3 and S4 are 46.4, 44.7, 38.8 and 34.4 emu/g, respectively. Obviously, the Ms values of the Fe3O4@C are significantly lower than those of the pure Fe3O4 as in many previous studies [16,22], which may result from the presence of the non-magnetic carbon shells and the small size of the Fe3O4 nanoparticle units. Due to the increase in the thickness of the outer carbon shells, the Ms and Mr values gradually decrease from the S1 to S4. The Hc values of the as-synthesized Fe3O4@C nanocomposites are higher than those of other different shapes of Fe3O4 (20.69-161.67 Oe) [38-40], and increase with the increase of outer carbon thickness. The larger Hc of the absorbing material suggests a larger magnetic anisotropy, thus would result in a higher resonant frequency in applied electromagnetic field [41].

a

b

Figure 5. Raman spectra of S1-S4 (a), magnetic properties of different samples (b) and the illustration in (b) is a local enlargement of the hysteresis loops. The EMW absorption characteristics are highly correlated with the complex permittivity (εr = ε'﹣jε'') and permeability (μr = μ'﹣jμ'') of the absorbing material, where ε' and ε'' indicate storage and loss abilities of electric energy, μ' and μ'' represent storage and loss capabilities of magnetic energy.

Figure 6 shows the EM parameters of the four different Fe3O4@C samples in 2-18 GHz. As the thickness of outer carbon increases, the values of ε′ of S2, S3 and S4 are significantly increased (Figure 6a), which mainly comes from the fact that the conductivity of outer carbon is higher than that of Fe3O4 core. According to free electron theory, it is beneficial to improve the complex permittivity [19,42]. Meanwhile, the ε' values of S2, S3, and S4 decrease with increasing EMW frequency, implying the typical frequency dispersion behavior due to the presence of carbon [19,43]. The ε″ values of the samples almost follow similar trend as the EMW frequency increases (Figure 6b). In particular, it is necessary to pay attention to the existence of several smooth peaks. The existence of obvious peaks indicates that multiple polarization appear in the samples, which can enhance the EMW absorption performance [29,44]. Moreover, multiple polarization relaxation process can be explained by Debye model theory. According to it, the εr can be expressed as [45], 𝜀𝑠 ― 𝜀∞

(2)

𝜀𝑟 = 𝜀′ +𝑖𝜀′′ = 𝜀∞ + 1 + 𝑖𝜔𝜏0

where ε∞ is the dielectric constant at infinite frequency, εs is the static dielectric constant and τ0 is the relaxation time. From equation 2, it can be deduced that 𝜀𝑠 ― 𝜀∞

(3)

ε′ = 𝜀∞ + 1 + (𝜔𝜏 )2 0

𝜀′′ =

𝜔𝜏0(𝜀𝑠 ― 𝜀∞)

(4)

1 + (𝜔𝜏0)2

Based on above formula, the relation between ε' and ε″ are:

(ε ― ′

𝜀𝑠 + 𝜀∞ 2 2

)

2

+ (𝜀′′) =

(

𝜀𝑠 ― 𝜀∞ 2 2

)

(5)

Equation 5 reveals the relationship between ε′ and ε″, which is named Cole-Cole semicircle. The more than one semicircle in S3 indicates the existence of multiple Debye relaxation processes (Figure S2). Since ε′ and ε″ affect the storage and loss capabilities of the absorbing materials for the incident EMW, it is obvious that if the Fe3O4@C nanocomposites have excellent dielectric loss abilities, the absorption performance for the EMW can be improved. The dielectric loss of Fe3O4@C nanocomposites can be explained in three aspects. Firstly, the structural defects on outer carbon shells can serve as the centers of polarization under the EMW irradiation. Secondly, interfacial polarizations and relative relaxations will occur at the

Fe3O4/carbon interface and carbon/paraffin interface. In addition, as the thickness of the carbon shell increases, the electrical conductivity of the nanocomposite further increases, which facilitates the accumulation of charge at the core-shell interface and promotes the corresponding polarization loss [19]. Thirdly, the special 3D foam-like structure of the nanocomposites increases the scattering or reflection path of incident EMW, which creating more possibilities of dielectric loss and relaxation loss [45]. In a word, the outer carbon shell and special morphology play important roles in modulating the dielectric loss properties of Fe3O4@C nanocomposites.

a

b

c

d

Figure 6. Electromagnetic parameters of S1-S4 in 2-18 GHz: (a) real parts and (b) imaginary parts of permittivity, (c) real parts and (d) imaginary parts of permeability. It is widely accepted that the natural resonance, exchange resonance and eddy current effect are crucial factors affecting the magnetic loss of the EMW absorbers. Figure 6c and d show the μ′ and μ″ of the S1-S4 in 2-18 GHz. The values of μ′ and μ″ of the S1-S4 tend to decrease gradually as the frequency increases. Notably, there are typical resonance phenomena in different frequency ranges in the μ'' spectrum, indicating the presence of natural resonance and exchange resonance. According to previous reports, the resonance peaks of μ″ in the low frequency region (below 10 GHz) should be attributed to natural resonance, and the resonance peaks in the high frequency region (10-18 GHz) should be

attributed to exchange resonance [30]. In addition, the eddy current loss can be evaluated using the following formula: μ′′ = 2𝜋𝜇0(𝜇′)2𝜎𝑑2𝑓 ∕ 3

(6)

where μ0 is vacuum permeability, σ is electrical conductivity and d is diameter of magnetic particles. If the eddy current effect plays a major role in generating magnetic loss, the values of μ″(μ′)-2f

-1

are constant with frequency. The values of μ″(μ′)-2f

-1

for Fe3O4@C

nanocomposites are almost constant in 11-18 GHz (Figure S3), indicating the existence of the eddy current effect. Negative μ″ values can be seen from 11 to 18 GHz in the curve of S1 (Figure 6d). The similar phenomena of negative values of μ″ have been reported in other absorbing materials [29,46], which are considered to be caused by the conversion of magnetic energy into electric energy [47]. The permeability can be precisely calculated via the equation [48]: μ𝑖 =

𝑀𝑠2 𝛼𝑘𝐻𝐶𝑀𝑆 + 𝑏𝜆𝜉

(7)

where Ms, Hc, k, λ and ξ are the saturation magnetization, the coercive field, the proportionality coefficient, the magnetostriction constant and the elastic strain parameter for the crystal, respectively, and a and b are two constants of the material. The high Ms value contributes to increase the initial permeability (μi) of the EMW absorbing material [29]. For the Fe3O4@C nanocomposites, the role of carbon shell is to reduce the value of Ms. In terms of the thickness of the carbon shell, S2-S4 have lower initial permeability than S1. The variation of tan δε and tan δμ with microwave frequency can be used to evaluate the ability of electromagnetic loss (Figure 7). The tan δε values of the four samples show increasing trends with the increase of microwave frequency and the trends of tan δμ are opposite. The S1-S4 have distinct tan δε peaks in certain frequency range, and the S4 has the larger tan δε than the S1, S2 and S3 (Figure 7a), which is consistent with its higher permittivity. Figure 7b shows that the tan δμ values of S1 are larger than those of S2, S3 and S4 in 2-6 GHz. In contrast, S2, S3 and S4 with obvious peaks have higher tan δμ values than S1 in 7-18 GHz, suggesting strong magnetic loss in this region. By comparison, it is known that the values of tan δε and tan δμ of S3 are very close in 2-18 GHz, which indicates a good impedance matching. In addition, the situation of S2 is similar to that of S3, which indicates

that these two samples have good microwave absorption performances. However, the values of tan δε and tan δμ of S1 and S4 are much different over the entire measured frequency range, which causes poor impedance matching conditions that is detrimental to microwave absorption. All the analysis results suggest that both magnetic and dielectric losses in the Fe3O4@C nanocomposites play an important role in the EMW absorption.

b

a

Figure 7. (a) Dielectric loss tangent and (b) magnetic loss tangent of S1-S4. In order to clarify the EMW absorption capacities of the 3D foam-like Fe3O4@C core-shell nanocomposites, the reflection loss (RL) values can be calculated according to the transmit line theory using the formulas below [19]: 𝑍𝑖𝑛 = 𝑍0 𝑍0 =

𝜇𝑟

𝜀𝑟 tanh

[𝑗 ( )

]

2𝜋

𝜇𝑟𝜀𝑟𝑓𝑑

𝑐

() 𝜇0

(8) (9)

𝜀0

|

RL(𝑑𝐵) = 20𝑙𝑜𝑔

|

𝑍𝑖𝑛 ― 𝑍0 𝑍𝑖𝑛 + 𝑍0

(10)

where f, d and c are the measured frequency, thickness and speed of light, respectively. Z0 and Zin represent free space impedance and input impedance. The thickness of the absorbing material has an obvious influence on EMW absorption performances. Figure 8 exhibits the RL values for the S1-S4 with thickness of 1-5.5 mm in 2-18 GHz. It can be seen that all minimum RL values of S2 and S3 are less than ﹣10 dB (90 % absorption) with thicknesses of 1.5-5.5 mm. Moreover, with the increase of thickness, the minimum RL values of S1-S4 gradually move to the low frequency region. The phenomenon is consistent with the quarter-wavelength matching theory [48,49]. However, the minimum RL values of S1 and S4 with high thickness are around ﹣ 10 dB in the low

frequency range, and the RL ability in the high frequency range is poor, which suggest a bad impedance matching performance. Although S4 has higher attenuation constants than S1, S2 and S3 (Figure S4), the impedance matching condition of S4 is poor (Figure S5), which leads to a relatively weak RL capability [45]. Not only the attenuation constant is small but also the impedance matching ability is poor, which indicates a bad absorbing performance of S1. All of the above analysis shows that the EMW absorption performance of the Fe3O4@C can be well controlled by adjusting the outer carbon thickness. By gradually changing the thickness of absorbers from 1.5 to 5.5 mm, the effective absorption bandwidth (EAB, RL ≤ ﹣ 10 dB) of S1-S4 can cover 3.8-10.6, 3.2-18, 2.6-18 and 2.3-13 GHz, respectively. Further analysis, the S1 (Figure 8a and b) reach a minimum RL value of ﹣15.1 dB at 4.6 GHz and has an EAB of 1.7 GHz (3.8-5.5 GHz) with 5.5 mm thickness. As shown in Figure 8c and d, the S2 show excellent absorption performance, obtaining a strong RL value of ﹣ 38.1 dB at 9.1 GHz with 2.4mm thickness and a broad EAB of 5.3 GHz (9.2-14.5 GHz) with 2.0 mm thickness. Figure 8e and f show that when the thickness of absorber is 2.0 mm, the maximum EAB of S3 is 5.3 GHz (8.7-14.0 GHz). Especially ， the S3 exhibits efficient absorption (RL ≤ ﹣ 20 dB) in an ultra-wide band (3.1-18.0 GHz, 14.9 GHz) when the thickness changes from 1.4 to 5.5 mm, which is the best performance of the known materials of its kind (Figure S6 and Table 1). Moreover, the strongest RL value of S3 is ﹣55.5 dB at 8.3 GHz and EAB is 4.0 GHz (6.8-10.8 GHz) with 2.5 mm thickness. Finally, the S4 (Figure 8g and h) has a minimum RL value of ﹣16.1 dB at 2.7 GHz when d = 5.5 mm. In conclusion, the absorption performance of the 3D foam-like Fe3O4@C core-shell nanocomposite (S3) with strong RL in a such wide frequency range (14.9 GHz) is superior to that of known composites of Fe3O4 and carbon [16,19,22,23,50-54].

a

b

c

d

e

f

g

h

Figure 8. Reflection loss values of the 3D foam-like Fe3O4@C core-shell nanocomposites at different thicknesses of 1-5.5 mm. (a, b): S1, (c, d): S2, (e, f): S3 and (g, h): S4.

Table 1. Comparison for Microwave Absorption Performances of Typical Fe3O4@C Composites

To further summarize the electromagnetic wave loss mechanisms, Figure 9 shows the possible loss mechanisms of the EMW in the 3D foam-like Fe3O4@C core-shell nanocomposites. Obviously, the absorption properties of the Fe3O4@C nanocomposites are determined by the impedance matching conditions and some loss modes [57]. Firstly, the outstanding impedance matching condition of the 3D foam-like Fe3O4@C nanocomposites enable more EMW to enter the interior of the absorbing materials, resulting in an excellent EMW absorption capacity. The suitable Z values of the absorbers are achieved by the proper composition, 3D porous structure and synergy of conductive carbon shell and magnetic Fe3O4 core. Secondly, the carbon shell covered on Fe3O4 generates dielectric loss and the Fe3O4 core causes magnetic loss, which all improve the EMW absorption performance. Thirdly, the abundant porous channels of the foam-like structure provide a large number of ways for reflection or scattering and spread of the EMW, which enhances the attenuation of the EMW. Besides, the multiple interfaces including Fe3O4/carbon, carbon/carbon and carbon/paraffin convert EM energy into thermal energy, which observably promotes the EMW loss

performances. Thus, the 3D foam-like Fe3O4@C core-shell nanocomposite can be regarded as an ideal EMW absorber.

Figure 9. Schematic diagram of EMW absorption mechanism of the 3D foam-like Fe3O4@C core-shell nanocomposites. 4. Conclusions In this paper, we successfully synthesized the 3D foam-like Fe3O4@C core-shell nanocomposites by solution combustion method, phenolic polymerization and subsequent carbothermal reduction process. The 3D porous structure is derived from a large number of gas

pore-forming

effects

during

solution

combustion,

which

promotes

multiple

reflection/scattering attenuation of EMW. In addition, the electromagnetic parameters and EMW absorption properties of the 3D foam-like Fe3O4@C core-shell nanocomposites can be controlled by adjusting the outer carbon thickness. Obviously, the absorption performances of the nanocomposites gradually increases and then decreases as the thickness of outer carbon increases. When the outer carbon thickness is 28.3 nm, the minimum RL value of the Fe3O4@C nanocomposites is ﹣55.5 dB with the d = 2.5 mm. In the meanwhile, the efficient

EMW absorption (RL ≤﹣20 dB) is in an ultra-wide band (3.1-18.0 GHz, 14.9 GHz) when the thickness of Fe3O4@C nanocomposite changes from 1.4 to 5.5 mm, which is superior to those of other similar materials reported and can be considered as a promising EMW absorber.

Acknowledgments The National Natural Science Foundation of China supported this work under Grant (No. 51571072, 51871078, 51871219, 51571195 and 51590883) and Heilongjiang Science Foundation (No. E2018028). We are also sincerely grateful for support of the National Key R&D Program of China (No. 2017YFA0206301).

Supporting Information. Figure S1-S6 and Table S1

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1. A novel Fe3O4@C core-shell nanocomposite with a porous foam-like structure was first prepared. 2. The foam-like structure of the precursor was obtained through the subtle formation of pores during the solution combustion process, and the resulting Fe3O4@C core-shell nanocomposites maintained the structure well, which is a novel idea for preparing a porous core-shell structure. 3. Benefiting from unique structural advantages and the synergistic effect of dielectric loss and magnetic loss, the foam-like Fe3O4@C core-shell nanocomposites has better microwave absorption performance than similar materials reported.

TOC

Xun Meng: Conceptualization, Data curation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. Weiwei Yang: Project administration. Guanghui Han: Validation Yongsheng Yu: Project administration, Supervision, Funding acquisition, Writing – review & editing. Song Ma: Resources. Wei Liu: Resources. Zhidong Zhang: Resources.