Magnetic vortex core-shell Fe3O4@C nanorings with enhanced microwave absorption performance

Carbon 157 (2020) 130e139

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Magnetic vortex core-shell Fe3O4@C nanorings with enhanced microwave absorption performance Xiao Wang a, 1, Fei Pan a, 1, Zhen Xiang a, Qingwen Zeng b, Ke Pei b, Renchao Che b, Wei Lu a, * a

Shanghai Key Lab. of D&A for Metal-Functional Materials, School of Materials Science & Engineering, Tongji University, Shanghai, 201804, China Laboratory of Advanced Materials, Department of Materials Science, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai, 200438, China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 August 2019 Received in revised form 26 September 2019 Accepted 13 October 2019 Available online 16 October 2019

Fabricating microwave absorbers (MA) with strong attenuation capability and lightweight is still a challenge problem, which limit their further applications in our daily life. Herein, magnetic vortex coreshell Fe3O4@C nanorings (FNR-C) with excellent microwave absorption property have been successfully prepared by a facile strategy. Electron holography analysis is carried out to detect the magnetic vortex structure of FNR-C. Furthermore, the microwave absorption properties of these samples are investigated in terms of complex permittivity and permeability. The FNR-C exhibits a strong reflection loss value of
61.54 dB at 16.9 GHz with a thickness of 1.50 mm and a low filling ratio of 25%. It’s the first time to take magnetic vortex into discussion. The unexceptionable attenuation ability is mainly attributed to the eddy current loss enhanced by combination of confinement vortex and strain-driven vortex. Besides, thanks to the dielectric feature of carbon, the Fe3O4 core is beneficial for the impedance match. Our findings provide a guidance to the development of nanoferrite@carbon hybrid materials with excellent microwave absorption property from the perspective of magnetic vortex. © 2019 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, with the rapid growth of electronic equipment for communication or detection use, electromagnetic interference pollution and electromagnetic wave (EMW) radiation have become a serious problem, which threats information security and human healthy in our daily life [1,2]. More and more researches concentrated on the microwave shielding materials (ESM) [3,4] and the microwave absorber (MA) [5,6]. In view of energy, incident EMW in MA was converted into thermal and other forms of energy by virtue of dielectric loss and magnetic loss, which makes MA a significant role in dealing with microwave pollution [7]. For practical applications, an excellent MA should have strong absorption, broad frequency band, lightweight and thin matching thickness [8]. The absorption of MA is closely associated with their complex permittivity (εr ¼ ε0
jε00 ), complex permeability (mr ¼ m0
jm00 ) and impedance match [9], all of which depend on their shape, size and

* Corresponding author. E-mail address: [email protected] (W. Lu). 1 Contribute equally to this paper. https://doi.org/10.1016/j.carbon.2019.10.030 0008-6223/© 2019 Elsevier Ltd. All rights reserved.

compositions. In the last decade, various categories of materials are discovered as MA, including carbon nanotube [10], graphene [11], conductive polymer [12], transition metal and their oxides [13], ferrite [14], etc. Compared with other materials, ferrite as a soft magnetic material has good corrosion resistance [15], which makes it a potential candidate to be applied as a practical absorber. Furthermore, the decent permittivity and permeability of ferrites could result in both magnetic loss and dielectric loss in the alternating electromagnetic field [16]. As an vital kind of ferrite materials, Fe3O4 has drawn a great deal of attentions as a absorber because of its low cost and unique properties, such as a suitable saturation magnetization value, good conductivity, adjustable magnetism and high Curie temperature [17,18]. However, Fe3O4 usually suffers from high density and relatively narrow absorption frequency range due to the Snoek’s limit [19]. The Snoek’s limit can be expressed with: (mS1)fr ¼ 23 g  4pMS, where g is the gyromagnetic ratio and MS is the saturation magnetization. There is a tremendous decrease of permeability in high frequency range. Thus, it’s still a challenge for Fe3O4 to have an excellent MA performance with both thin thickness and high frequency capability.

X. Wang et al. / Carbon 157 (2020) 130e139

Two strategies are frequently adopted to overcome these problems. One effective way is to design core-shell structured composites with Fe3O4 as the core and a dielectric shell, especially the carbon materials [20]. On the one hand, dielectric composites were found to effectively reduce the density of the Fe3O4 MA [21] and form a stable protective layer to avoid exposing core to air. On the other hand, carbon materials, for their good conductivity, multiinterface, abundant defect sites, large specific surface area and diversiform surface functional groups [22], have the benefit of reaching good impedance matching and favor dielectric and magnetic loss, all of which are advantageous to enhancing the microwave absorption (MA). For example, Liu [23] et al. demonstrated that Fe3O4/C nanosheets (300 nm) exhibited excellent MA performance, and the reflection loss (RL) value could reach
43.95 dB at 3.92 GHz with a thickness of 4.3 mm. The excellent MA performance was attributed to the impedance matching and multiple dielectric relaxations. Core-shell Fe3O4@MnO2 spheres synthesized by a hydrothermal method showed good MA performance [24], in which the minimum RL value was
48.5 dB at 11.2 GHz with a thickness of 2.5 mm. The second strategy is to affects the propagation and distribution of EMW within the materials by controlling the size, thickness or geometric structure of the materials. Past event, nanotechnology had aroused great interest among researchers accompanied by the appearance of more and more nanosized EMW absorbing materials [25]. At a certain size, the quantum size effect causes the electron energy levels of the nanoparticles to split [26], and some of the splitting energy levels are within the energy range of microwave, thus leading to new absorption channels. At the same time, if the size of nanometer is equal to or less than the wavelength of the relevant wave or the penetration depth of the magnetic field, the periodic boundary conditions of the crystal will be destroyed [27], resulting in some abnormal properties of the materials, which makes it possible to develop a multiband compatible EMW absorbing material. In addition to the size, thickness and geometric structure are the other two key factors. When the thickness of absorbers become smaller than skin depth, it will create an inhibition of the eddy current and decrease the effect of skin effect [28], thereby resulting in ameliorative complex permeability in a high frequency range. As for geometric structure, this strategy is aimed to adjust the magnitude of the anisotropic field or introduce an extra geometrical field [29]. Several shapes of materials exhibit enhanced MA performance due to the existence of magnetic anisotropy, like flake-shaped [30], shuttle-shaped [31] or cabbage-shaped materials [32]. Nanorings have been applied in many fields, such as microwave absorption [33], supercapacitor [34], biomedical applications [35], and photochemical catalysis [36] because of their unique geometry and novel properties. In recent years, by modulating the size and geometrical structure of magnetic nanostructures, a unique magnetic domain of vortex structure has been discovered. Due to closed distribution of magnetic moment, nanostructures with magnetic vortex reduces the stray field, which can effectively weaken the magnetic interaction between nanostructures and avoid the occurrence of agglomeration [35]. In addition, the susceptibility and saturation magnetization of vortex nanostructures exhibit higher value than their counterparts with small size, which can enhance magnetic loss to some extent. At the same time, the MA property of the materials will also be improved owing to the enhancement of eddy current loss result from the common influence of confinement vortex and curling strain [37]. However, few works concentrate on the relationship between magnetic vortex and MA. In this work, we have developed magnetic vortex Fe3O4 nanoring (FNR), Fe3O4@PVP (FNR-PVP) and FNR@C (FNR-C) coreshell hybrid nanorings through a facile hydrothermal method, as

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illustrated in Fig. 1. The resulted core-shell hybrid nanorings (150 nm) come up with a unique magnetic vortex structure and excellent MA property due to well-matched impedance and gentle EMW attenuation ability. The optimized FNR-C show a strong RL values of
61.54 dB at 16.8 GHz with a thickness of only 1.5 mm. Electron holography analysis is carried out to detect the magnetic vortex structure of FNR-C. To best of our acknowledge, it’s the first time to take magnetic vortex into discussion. This work gives inspiration and paradigm to the development of ferrite@carbon hybrid materials in view of vortex, which promotes the design of excellent Ku-band electromagnetic absorption materials. 2. Experimental section All reagents used in our experiments were purchased and used as received without further purification. The relative molecular mass of the polyvinyl pyrrolidone (PVP) was 40000. 2.1. Preparation of Fe2O3 nanoring precursors Fe2O3 NR precursors were synthesized using the following solvothermal approach. Briefly, 1.08 g of ferric trichloride hexahydrate (FeCl36H2O), 15.6 g of sodium sulfate (Na2SO4) and 5.6 mg of sodium dihydrogen phosphate dihydrate (NaH2PO42H2O) were mixed in 200 ml deionized water and continuously stirred for 40 min. The as-designed solution was transferred into a Teflonlined stainless-steel autoclave and then kept at 220  C for 48 h. After naturally cooling to room temperature, the obtained red powders were collected by centrifugation and washed several times with ethanol, and finally dried in a vacuum oven at 60  C for 12 h. 2.2. Preparation of FNR The FNR was prepared by a reduction process with Fe2O3 nanorings. The prepared red powders were put in a silica boat and annealed in a furnace in hydrogen/argon (95% Ar þ 5% H2) atmosphere at 500  C for 2 h with a heating rate of 5  C min
1. At last, the samples were cooled to room temperature under a continuous gas flow. 2.3. Preparation of FNR-PVP and FNR-C In order to fabricate FNR-PVP, 0.1 g FNR and 0.5 g PVP were mixed in 20 ml distilled water and stirred for 12 h and then were washed with distilled water gently for two times. Then, the dried FNR-PVP were annealed in tube furnace at 500  C under Ar atmosphere to obtain the core-shell FNR-C. The heating rate is 5  C/min, and it keeps at the highest temperature for 1 h. 2.4. Characterization The X-ray diffraction (XRD) patterns of all the samples were determined using an X-ray powder diffractometer with Cu-Ka as the irradiation source (l ¼ 1.54 Å). The morphology was characterized by transmission electron microscopy (TEM). The magnetic domain structure as determined using electron holography (JEM 2100F). Raman spectra was measured by a cryogenic matrix isolated Raman spectroscopic system using a 532 nm laser. The room temperature hysteresis loop was performed on vibrating sample magnetometer (VSM manufactured by Lakeshore, Inc.). The electromagnetic parameters of the complex permittivity and complex permeability in the frequency range of 2e18 GHz were measured by an Agilent PNA N5224A vector network analyzer according the coaxial-line theory. All the powders were uniformly dispersed in

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Fig. 1. Schematic formation process of FNR-C. (A colour version of this figure can be viewed online.)

paraffin matrix with a filling ratio of 25% and pressed into a toroidal ring with a 4out of 7.00 mm and a 4in of 3.04 mm. The complex permeability and permittivity were obtained from the experimental scattering parameters through the standard Nicolson-Ross and Weir theoretical calculations.

3. Results and discussion The preparation of core-shell FNR-C includes four steps (as illustrated in Fig. 1). A hydrothermal method was used to synthesized a ring-like Fe2O3 precursor by mixing solution of Fe3þ, PO3
4 ,  and SO2
4 at 220 C for two days. It’s worth mentioning that phosphate ions and sulfate ions are two key factors in the formation of nanoring structures. According to Jian’s analysis [38], the contract of phosphate ions and the surface of Fe2O3 affected the dissolution process and sulfate ions further promote the dissolution of Fe2O3 owing to the coordination effect. After that, the Fe3O4 nanorings were obtained by a reduction process under hydrogen and argon atmosphere. In a subsequent step, a PVP layer coating on the FNR was achieved by stirring two substances continuously in distilled water. Herein, PVP not only increased the dispersion of the nanorings, but also act as a carbon source for subsequent reactions. In the step of carbonization, the FNC-PVP were reduced under argon atmosphere at 500  C for 1 h to obtain FNC-C. Finally, the EM testing ring was formed from mixing with FNR-C and paraffin in a ratio of 1:3. The crystallographic structure of FNR, FNR-PVP and FNR-C were investigated by XRD, as shown in Fig. 2a. The diffraction peaks of FNR at 18.3 , 30.1, 35.5 , 37.1, 43.1, 53.5 , 57.0 , 62.6 and 74.1 are assigned to (111), (220), (311), (222), (400), (422), (511), (440), and (533) planes of Fe3O4 (JCPDS No. 89e0691), respectively. Such well-matched diffraction and sharp peaks confirm the high purity and high degree of crystallinity of Fe3O4 FNR. After coating with PVP and carbon, there was not significant change in the diffraction peaks compared with that of FNR, indicating that the Fe3O4 was quite stable and no other components were formed during the adsorption or carbonation process. However, no diffraction peak was indexed to carbon in the FNR-C sample because the carbon layer formed on the surface of the FNR during the carbonization process was comparatively thin. To further confirm the exist of carbon, Raman spectra were applied and two obvious peaks located at 1320 cm
1 (D band) and 1580 cm
1 (G band) can be

distinguished in Fig. 2b. Generally speaking, the D band is associated with disorder or defects in the sp2-hybridized carbon atoms or amorphous carbon deposits, while the G band represents the inplane vibrations of sp2 atoms in a 2D hexagonal graphitic lattice [39]. The ratio of D band to G band (ID/IG) can be used to measure the disorder degree of carbon [40]. As shown in Fig. 2b, the calculated ID/IG value (1.07) of the FNR-C not only confirmed the existence of amorphous carbon, but also suggested abundant defects covering in FNR-C. Moreover, a sharp peak at 670 cm
1 represented the presence of Fe3O4, which is also consisted with the XRD results. In order to prove the existence of the PVP in the FNR-PVP samples, FTIR spectra of the sample FNR-PVP in the wavenumber range of 400e4000 cm
1 are given in Fig. S1. Based on the literatures [41,42], the characteristic peaks of PVP polymer are observed at 2920, 1640, 1420 and 1240 cm
1. In detail, the band at 2920 is connected with the asymmetric and symmetric stretching vibration of CH2. And, the band at 1420 cm
1 is assigned to bending vibrations of CH2. Other bands at 1640 and 1240 cm
1 can be attributed to the stretching and bending vibrations of C]O and CH, respectively [42]. A broad peak located at about 3442 cm
1 represents the stretching vibration of hydroxyl groups (eOH) and hydrogen-bonded (HOH) owing to residual water [43]. In addition to this, the band at 561 cm
1 is assigned to nano-Fe3O4. Fig. 3aec presents the transmission electron microscopy (TEM) images of the synthesized FNR and FNR-C. The TEM images clearly show that a thin carbon shell layer was successfully coated on the Fe3O4 FNR core. The introduction of carbon does not change the ring-shape of FNR. Fig. 3b and c demonstrate the outer diameter (D) of FNR-C is 160 ± 10 nm and the height (H) is 70 ± 10. The aspect ratio (D/H) was calculated to be 2.29 which implied strong shape anisotropy of the FNR-C. Moreover, room temperature hysteresis loops are exhibited in Fig. 3d and e. As we know, the magnetic property affects its magnetic loss behavior, which is better understood with following equations:

m’ ¼ 1 þ ðM = HÞ coss

(1)

m’’ ¼ ðM = HÞ sins

(2)

where M is magnetization, H represents the magnetic field, s means the magnetic lag angle under an external magnetic field. It can be seen that the Ms and Hc values of the FNR and FNR-C are

X. Wang et al. / Carbon 157 (2020) 130e139

133

Fig. 2. (a) XRD pattern of FNR, FNR-PVP and FNR-C. (b) Raman spectra of FNR-C. (A colour version of this figure can be viewed online.)

Fig. 3. TEM images of (a) FNRs, (b, c) FNR-C; (d) Hysteresis loop of FNR and FNR-C; (e) Saturation magnetization and coercivity force of FNR and FNR-C. (A colour version of this figure can be viewed online.)

70.61 emu$g
1, 230 Oe and 67.35 emu$g
1, 190 Oe, respectively. Generally speaking, the variation of Ms and Hc is related to surface oxidation, shape, particle size, surface disorder state, etc. Herein, the decrease of Ms and Hc value was mainly resulted from the nonmagnetic carbon coating. The MA performance of the nanorings can be evaluated by the reflection loss value (RL). On the basis of transmission line theory [44], RL can be defined with the following formulas:


1=2 Zin ¼ Z0

mr εr

i h tanh jð2pfd = cÞðεr mr Þ1=2

RL ¼ 20logjðZin
Z0 Þ = ðZin þ Z0 Þj

(3)

(4)

Here, Z0 is the impedance of free space, Zin is the input impedance of the absorber, εr is the relative complex permittivity (εr ¼ ε0
jε00 ), mr is the relative complex permeability (mr ¼ m0
jm00 ),

f is the frequency, d is the thickness of the absorber, and c is the velocity of light. Fig. 4 presents the three-dimensional and contour RL maps of the samples with thickness from 1.0 to 5.0 mm in the frequency range of 3.0e18.0 GHz. In this frequency region, FNR-C exhibits much higher MA capability than FNR and FNR-PVP. As showed in Fig. 4a and d, FNR has a relative low MA ability and the minimum RL value is
27.49 dB at 14.9 GHz with an optimal sample thickness of 1.5 mm. From Fig. 4b and e, FNR-PVP displays some improved MA abilities. At 4.0 mm, the minimum RL value is
48.36 dB at 5.9 GHz frequency. As observed in Fig. 4c and f, FNRC exhibits obviously superior absorption performance than the other two, which reaches a strong intensity of
61.54 dB at 16.9 GHz with a thickness of only 1.5 mm. Apart from strong absorption intensity and thin thickness, broad effective frequency bandwidth (EBW; fe ; RL <
10 dB) are demanded for highperformance microwave absorbers. The maximum fe of FNR, FNRPVP and FNR-C is 4.3 (from 13.00 to 17.3 GHz), 3.6 (from 5.5 to

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X. Wang et al. / Carbon 157 (2020) 130e139

Fig. 4. 3D reflection loss and contour map of the samples with different thicknesses (1.0e5.0 mm) from 3.0 to 18.0 GHz: (a, d) FNR, (b, e) FNR-PVP, (c, f) FNR-C. (A colour version of this figure can be viewed online.)

9.1 GHz) and 3.1 (from 10.6 to 14.2 GHz), respectively, as shown in Fig. S3. Moreover, when the thickness ranges from 0.5 to 5.0 mm, all the samples cover exceed 95% of whole testing frequency from 3.0 to 18.0 GHz, which is a rare and inspired phenomenon in ferrite absorbing material. Table 1 listed the microwave absorbing performance of some similar materials reported in recent years. The strong reflection loss, broad effective absorption bandwidth and thin thickness clearly demonstrate the excellent microwave absorbing performance of our magnetic vortex core-shell Fe3O4@C nanorings in comparison with other similar materials reported in literatures. Therefore, considering their attractive advantages, the obtained magnetic vortex core-shell Fe3O4@C nanorings can be considered as an ideal candidate for a high-performance microwave absorber application. From the above analysis, it can be clarified that the coating of carbon greatly improves the MA performance of magnetic vortex FNR with an excellent RL of
61.5 dB at a very low thickness of 1.5 mm. The predicted matching thickness (tm) and the corresponding matching frequency (fm ) are calculated on the basis of the pffiffiffiffiffiffiffiffiffiffiffiffiffiffi equation: tm ¼ nl=4 ¼ nc=ð4fm jεr jjmr jÞ (n ¼ 1,3,5 …), where l is the quarter-wavelength at fm , c is the velocity of light [50]. When

value of tm and fm satisfy this equation, the two reflected EMW from the air-absorber interface and absorber-conductive background interface are out of phase by 180 [51]. Without a doubt, the minimum RL positions of the three samples are in good agreement with the quarter-wavelength matching model in Fig. S4. In general, the variation tendency of the MA properties can be explained by not only suitable impedance matching characteristics but also remarkable attenuation performance of the samples. On one hand, impedance matching ability represents the channel in which EMW can effectively enter into the absorbers [52]. The impedance matching degree of a good MA absorber should be equal or close to that of the free space to achieve zero reflection at the front surface. On the other hand, attenuation is another capacity that EMW is converted into heat or other forms of energy inside absorber due to the existence of various loss mechanisms, such as conductivity loss, polarization loss, natural resonance or eddy current loss [53]. Both aspects, including impedance matching and attenuation, are very crucial to the enhancement of MA properties. The impedance matching properties are determined as the delta function [54]:

Table 1 Microwave absorption performance of some Fe3O4/carbon based materials. sample

filling ratio (wt%)

RLmin (dB)

thickness (mm)

fe (GHz)

ref

Fe3O4/graphite NR Fe3O4/C NR Fe3O4/Fe@C NR Fe3O4@C NS Fe3O4/C NC Fe3O4/graphene capsules FNR-C

29.82 11.95 40.00 21.00 20.00 30.00 25.00


45.80
55.68
28.18
43.95
63.09
32.00
61.45

2.5 6.2 5.0 4.3 3.1 3.5 1.5

2.6 2.4 1.9 3.9 5.3 4.7 ＞3.6

[45] [46] [47] [23] [48] [49] this work

NR: nanorings; NS: nanosheets; NC: nanochains.

X. Wang et al. / Carbon 157 (2020) 130e139

    j△j ¼ sinh2 ðKfdÞ
M 

(5)

where K and M can be calculated by relative complex permittivity and complex permeability. The △ value approaching to 0 indicates an excellent impedance matching degree. Fig. 5aec displays the absolute value maps of delta of the FNR, FNR-PVP and FNP-C samples in the frequency range of 3e18 GHz with a thickness from 0.5 to 5 mm. From the results, it can be clearly seen that the MA of FNR-PVP and FNR-C sample have the larger area close to zero than FNR. This phenomenon not only suggests that the FNR-PVP and FNR-C sample have superiority in the impedance matching, but also proves that the addition of dielectric components can effectively adjust impedance matching. Furthermore, the value of the attenuation constant (a) could also be expressed using the following equation [55]:

pffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ’’ ’’ ’ ’ a¼ pf ðε m
ε m Þ þ ðε’’ m’’
ε’ m’ Þ2
ðε’’ m’ þ ε’ m’’ Þ2 c (6) As shown in Fig. 5d, the attenuation constant a value of FNR is increased from 72 to 253 with the increase of frequency, whereas a value of FNR-PVP and FNR-C is only increased from 38 to 97 and from 46 to 128, respectively. Thus, the FNR possess larger a than that of others, suggesting its outstanding attenuation capability for the incident EM waves. However, the FNR shows a low RL value because of poor impedance matching [56e58]. In order to further understand the influence of material composition and structure on the wave absorption performance, electromagnetic microwave parameters are deemed to be critical factors. The MA performance is determined by the complex

135

permittivity (εr ¼ ε0
jε00 ) and complex permeability (mr ¼ m0
jm00 ). Fig. 6a and b shows the real part (ε0 ) and imaginary part (ε00 ) of complex permittivity over 3.0e18.0 GHz for the samples. According to the EMW energy conversion theory, ε0 represents the storage ability of electric energy and ε00 represents the loss ability of electric energy [59], and all the ε0 curves of the samples depict a decreasing trend as the frequency increases, which is interpreted as the increased polarization hysteresis versus the higher frequency electric field variation [60]. For the FNR, it exhibits the highest real ε0 and sharp descending tendency with decreasing from 16.6 to 11.4 over 3.0e18.0 GHz. For the FNR-PVP sample, the ε0 fluctuates between 8.8 and 8.1 in the investigated frequency range, suggesting a significant decline compared with that of the FNR. After carbonization, the ε0 curve presents a decreasing tendency with moderate fluctuations from 12.4 to 12.2, and the boost of ε0 is the credit of transformation between PVP and C. What’s more, a moderate ε0 value of FNR-PVP and FNR-C are beneficial to impedance matching that lead to high RL value. As for ε00, the values show an inverse ascending trend in the range of 3.0e18.0 GHz. For the FNR, the highest ε00 values can be observed, indicating that it has extravagant dielectric loss capacity. But for FNR-PVP and FNR-C samples, the ε00 values show a rising tendency in the range of 0.4e1.8 and 0.3e2.9, respectively. Furthermore, the dielectric loss tangent (tande) stands for the EM energy loss against the energy stored [61]. Fig. 6c shows that the tande value of the three samples has a similar trend with that of the ε00 . It is distinctly observed that the tande of FNR is higher than that of the other samples in the range 3.0e18.0 GHz. In a general way, there are mainly two kinds of dielectric loss added to the discussion: polarization loss and conductive loss. And it is well known that the polarization loss comes from atomic polarization, electron polarization, dipole polarization and interfacial polarization [62]. The atomic polarization and electron polarization can be

Fig. 5. Calculated delta value maps: (a) FNR, (b) FNR-PVP, (c) FNR-C. (d) Attenuation constant of three samples in the frequency range of 3
18 GHz. (A colour version of this figure can be viewed online.)

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X. Wang et al. / Carbon 157 (2020) 130e139

Fig. 6. The EM parameters of samples. The real ε0 (a) and imaginary ε00 (b) parts of the complex permittivity, and dielectric loss tande (c), real m0 (d) and imaginary m00 (e) parts of the complex permeability, and magnetic loss tandm (f). (A colour version of this figure can be viewed online.)

excluded because of their minuscule contributions in the microwave range. The highest ε00 and tande value of FNR can be mainly attributed to the dipole polarization, which can be expressed by a Cole-Cole model [63]: 00

εr ¼ ε’
jε ¼ ε∞ þ

εs
ε∞ 1 þ j2pf t

(7)

In the above equation, εs , ε∞ , f and t stands for the static dielectric constant, the dielectric constant at infinite frequency, the frequency and the polarization relaxation time. And then the ε0 and ε00 can be deduced into

ε’ ¼ ε∞ þ

00

ε ¼ ε∞ þ

εs
ε∞

(8)

1 þ ð2pf tÞ2 2pf tðεs
ε∞ Þ

(9)

1 þ ð2pf tÞ2 00

Furthermore, the relationship between ε’ and ε can be written as



ε’


ε
ε 2 00 2 εs þ ε∞  s ∞ þ ðε Þ ¼ 2 2

(10) 00

According to this equation, the curve of ε’ and ε should constitute a single semicircle, called the Cole-Cole semicircle, which represents the Debye dipolar relaxation. Fig. S5a shows the Cole-Cole semicircle of FNR, FNR-PVP and FNR-C in the frequency range of 3e18 GHz, and one Cole-Cole semicircle presents one Debye relaxation process. In detail, enhancement of the Debye dipolar relaxation is accompanied with the increasing number of semicircles, the expanded semicircle radius and higher frequency position [60]. In this case, the FNR show larger numbers of semicircles in high frequency range. Abundant defects play as the polarization centers on the surface, resulting in the generation of Debye dipole polarization. However, the strongest Debye relaxation of FNR not only brings about supreme tande value, but also cause improper impedance matching. The combination of this impedance mismatch and the drastic decrease of the dielectric constant leads

to poor MA performance. On the contrary, only a few semicircles for the curve of the FNR-PVP and FNR-C sample demonstrates that the Debye dipolar relaxation plays non-dominant role in dielectric loss. For this multi-component system, interfacial polarization and conductive loss contribute most. As displayed in Fig. 6c, several resonance peaks located from 10.0 GHz to 16 GHz for FNR-PVP and FNR-C due to interface polarization. On account of the PVP or the amorphous carbon coated on the surface of FNR, a mass of space charges with littery distributions accumulate at the heterogeneous interface (known as the Maxwell-Wagner effect) between Fe3O4 and carbon, contributing to the interfacial polarization. The polarization electrons assembling near the interfacial planes could generate residual holes and gather more electrons driven by the applied high frequency electromagnetic field, which is so available that electromagnetic energy can be quickly converted into thermal energy and improve the MA ability [30]. Furthermore, Conductive loss depends on the electrical conductivity of the composites [64]. Compared with the PVP, carbon shell layer effectively enhances the conductive loss due to their corresponding graphitization, shown in the Raman spectra. Meanwhile, the severe distortion of the ColeCole semicircles further reveals that the polarization relaxation is not the exclusive reason for dielectric loss [65]. As a result, the 00 average ε value from 3.0 to 18.0 GHz increases from 1.13 to 1.60. Herein, FNR-C exhibits excellent MA performance due to the improvement of conductive loss and the introduction of interfacial polarization. Fig. 6d and e shows the changes of m0 and m00 as a function of the three samples. It is widely accepted that m0 represents the storage ability of magnetic energy and m00 represents loss ability of magnetic energy. The magnetic loss ability can be evaluated by magnetic tangent loss (tande ¼ m00 /m0 ), as shown in Fig. 8f. The m0 values of FNR fluctuate around 1.00, and the m0 values of FNR-PVP and FNRC decrease from 1.12 to 0.88 and from 1.38 to 0.73, respectively. This may indicate that the FNR possesses stronger storage abilities of magnetic energy from 6 to 18 GHz, which can be explained by 1=2 Globus equation as follows: m ¼ ðM 2s D=K1 Þ , where D is grain size, and K1 is magnetocrystalline anisotropy constant [66]. In this case, larger Ms values for FNR may contribute to the larger permeability.

X. Wang et al. / Carbon 157 (2020) 130e139

Fig. 7. (a) Off-axis electron holograms of FNR-C. direction of the magnetic induction under field-free conditions following magnetization indicated by color as shown in the color wheel ((blue) right, (sky blue) down, (green) left, (red) up). (b) Schematic illustration for the confinement vortex and strain-driven vortex. (A colour version of this figure can be viewed online.)

Fig. 8. Schematic illustration for the microwave absorption of FNR-C. (A colour version of this figure can be viewed online.)

As to m00 , the m00 values of FNR and FNR-PVP go down from 0.48 to 0.24 and from 0.38 to 0.24, respectively. For FNR-C, the m00 changes from 0.04 to 0.23 in the range of 3.0e18.0 GHz. Several peaks are exhibited in Fig. 6e, indicating the strong magnetic loss ability. In general, the magnetic loss mainly includes magnetic hysteresis, domain wall resonance, eddy current loss, natural resonance and exchange resonance [32]. Magnetic hysteresis can be firstly excluded because it only occurs under a strong applied field. Then, the domain wall resonance exists at a low frequency (＜100 MHz), and thus it can be neglected in the 3.0e18.0 GHz range [67]. The eddy current can be calculated by the following eddy equation [68]:

C0 ¼ m ðm’ Þ
2 ðf Þ
1 ¼ 2pm0 d2 s 00

.

3

(11)

where m0 and s are the permeability of vacuum and the electric conductivity, respectively. If the magnetic loss just originates from the eddy current loss, the C0 value should be constant when the frequency changes. Fig. S5b shows that the C0 significantly changes as the frequency changes, confirming that the eddy current loss is not the only factor. According to the natural resonance equation: 2pf r ¼ gHa [69], where fr is the natural resonance frequency, g is the gyromagnetic ratio, and Ha is the anisotropy energy. The fr will shift toward the low-frequency region when the size of the Fe3O4 nanoring is increased. In addition, the exchange resonance occurs at a higher frequency than the natural resonance [70]. Therefore, it can be speculated that the resonance peaks located at low frequency are relevant to the natural resonance and the others peaks located at high frequency are relevant to the exchange resonance [71e73]. In addition, Fig. 6f shows the downward trend of tandm of all the specimens with increasing frequency (3.0e18.0 GHz). It is

137

noted that the values of tandm follow the order: FNR < FNRPVP < FNR- C. The highest value of FNR-C means more dissipation of magnetic energy of incident microwaves. In addition to the effect of exchange resonance mentioned above in the high frequency range, the dominant reason can be attributed to the enhancement of eddy current loss. As shown in Fig. 7a, it is obviously seen that the vortex domain exists in the nanorings, and each circle represents a potential energy state. In general, when encountering high frequency electromagnetic waves, the eddy current in the vortex structure will respond to this change and force its own vortex to evolve along with the external direction. Due to the influence of its own confinement effect, the eddy current state is hard to be changed so that the electromagnetic wave is consumed in the form of thermal energy under the impact of the vortex structure. Furthermore, the core-shell ring structure has an inner surface and an outer surface, both of which can be regarded as planar two-dimensional structure after curling and bending. This structure generates a lot of strains, which also drives another vortex. At high frequency range, the interaction of confinement vortex and strain-driven vortex effectively enhances the eddy current loss, leading to the enhancement of magnetic loss in FNR-C sample (as shown in Fig. 7b). As we know, higher eddy current losses will induce skin effect. The current aggregating on the surface of the material is harmful to MA properties. However, the impact of skin effect can be ignored in this work due to nanoscale size of FNR-C and weak applied field. In summary, the microwave absorption mechanism of the FNR-C samples is described from the viewpoint of the microstructure (Fig. 8). Firstly, due to the presence of carbon coating layer, the conductivity of the MA is effectively enhanced, The dielectric constant changes stably in the frequency range of 3e18 GHz, which improve the impedance matching to make microwaves successfully enter the MA. Secondly, the interface area and gap of electrical conductivity between Fe3O4 and carbon can cause charge aggregation on the heterogeneous interface and induce interfacial polarization loss. Thirdly, the carbon coating on the surface will produce a dielectric polarization field at nanoscale, which provide a large number of defects and functional groups to act as polarization center, inducing multiple reflection and scattering processes. Such enhanced dielectric polarization processes could effectively dissipate the microwave energy or attenuate microwave irradiations. Fourthly, magnetic loss of composition also contributes to the consumption of incident energy. As showed in the m00 curve, two peaks, including natural resonance in low frequency and exchange resonance in high frequency, have a great effect on the magnetic loss. Fifthly, a magnetic vortex structure not only avoid the accumulation of nanoparticles, but also effectively increase the eddy current loss so as to enhance attenuation constant. Therefore, an appropriate balance between attenuation constant and impedance match cause the outstanding MA performance of the magnetic vortex core-shell Fe3O4@C nanoring material. 4. Conclusion We reported a magnetic vortex core-shell Fe3O4@C nanoring material with excellent microwave absorption properties fabricated by a facile strategy. The permittivity and permeability of samples were conveniently modulated by means of controlling the surface coating state, which results in the variation of impedance matching and reflection loss. The magnetic vortex core-shell Fe3O4@C nanoring absorber shows wonderful microwave absorption performances with minimum RL value of
61.54 dB, extremely low thickness of 1.50 mm and low-rise filling ratio of 25%. The suitable impedance matching and enhanced microwave attenuation which leading to the excellent microwave absorption were mainly on account of the magnetic vortex structure, which improves the

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dispersion of particles the Fe3O4@C nanorings and provides confinement vortex in high frequency range. The combination of confinement vortex and stress driven vortices which caused by curved structure of the Fe3O4@C nanorings strengthens the eddy current loss. Therefore, this work gives inspiration and paradigm to the development of nanoferrite@carbon hybrid materials in view of magnetic vortex, which promotes the design of excellent electromagnetic wave absorption materials. Declaration of competing interest The authors declared that they have no conflicts of interest to this work. Acknowledgements This project was supported by the National Natural Science Foundation of China (No 51971162, U1933112, 51671146), the Ministry of Science and Technology of China (2018YFA0209102), the Program of Shanghai Technology Research Leader (18XD1423800), and the Fundamental Research Funds for the Central Universities (22120180096). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.10.030. References [1] J. Liu, R. Che, H. Chen, F. Zhang, F. Xia, Q. Wu, M. Wang, Microwave absorption enhancement of multifunctional composite microspheres with spinel Fe3 O4 Cores and Anatase TiO2 shells, Small 8 (8) (2012) 1214e1221. [2] A. Chaudhary, S. Kumari, R. Kumar, S. Teotia, B.P. Singh, A.P. Singh, S.K. Dhawan, S.R. Dhakate, Lightweight and easily foldable MCMB-MWCNTs composite paper with exceptional electromagnetic interference shielding, ACS Appl. Mater. Interfaces 8 (16) (2016) 10600e10608. [3] Y. Bhattacharjee, D. Chatterjee, S. Bose, Core-multishell heterostructure with excellent heat dissipation for electromagnetic interference shielding, ACS Appl. Mater. Interfaces 10 (36) (2018) 30762e30773. [4] Z. Hu, F. Lu, Y. Liu, L. Zhao, L. Yu, X. Xu, W. Yuan, Q. Zhang, Y. Huang, Construction of anti-ultraviolet “shielding clothes” on poly(p-phenylene benzobisoxazole) fibers: metal organic framework-mediated absorption strategy, ACS Appl. Mater. Interfaces 10 (2018) 43262e43274, https://doi.org/10.1021/ acsami.8b16845. [5] Z. Xiang, Y. Song, J. Xiong, Z. Pan, X. Wang, L. Liu, R. Liu, H. Yang, W. Lu, Enhanced electromagnetic wave absorption of nanoporous Fe3O4 @ carbon composites derived from metal-organic frameworks, Carbon 142 (2019) 20e31. [6] J. Xiong, Z. Xiang, J. Zhao, L. Yu, E. Cui, B. Deng, Z. Liu, R. Liu, W. Lu, Layered NiCo alloy nanoparticles/nanoporous carbon composites derived from bimetallic MOFs with enhanced electromagnetic wave absorption performance, Carbon 154 (2019) 391e401. [7] Y. Ding, L. Zhang, Q. Liao, G. Zhang, S. Liu, Y. Zhang, Electromagnetic wave absorption in reduced graphene oxide functionalized with Fe3O4/Fe nanorings, Nano Res. 9 (7) (2016) 2018e2025. [8] Y. Duan, Z. Xiao, X. Yan, Z. Gao, Y. Tang, L. Hou, Q. Li, G. Ning, Y. Li, Enhanced electromagnetic microwave absorption property of peapod-like MnO@carbon nanowires, ACS Appl. Mater. Interfaces 10 (46) (2018) 40078e40087. [9] L. Huang, J. Li, Z. Wang, Y. Li, X. He, Y. Yuan, Microwave absorption enhancement of porous C@CoFe2O4 nanocomposites derived from eggshell membrane, Carbon 143 (2019) 507e516. [10] Y. Zhang, Y. Liu, X. Wang, Y. Yuan, W. Lai, Z. Wang, X. Zhang, X. Liu, Towards efficient microwave absorption: intrinsic heterostructure of fluorinated SWCNTs, J. Mater. Chem. C 5 (45) (2017) 11847e11855. [11] L. Quan, F.X. Qin, D. Estevez, H. Wang, H.X. Peng, Magnetic graphene for microwave absorbing application: towards the lightest graphene-based absorber, Carbon 125 (2017) 630e639. [12] D.A. Gopakumar, A.R. Pai, Y.B. Pottathara, D. Pasquini, L. Carlos de Morais, M. Luke, N. Kalarikkal, Y. Grohens, S. Thomas, Cellulose nanofiber-based polyaniline flexible papers as sustainable microwave absorbers in the Xband, ACS Appl. Mater. Interfaces 10 (23) (2018) 20032e20043. [13] P. Liu, V.M.H. Ng, Z. Yao, J. Zhou, Y. Lei, Z. Yang, H. Lv, L.B. Kong, Facile synthesis and hierarchical assembly of flowerlike NiO structures with enhanced dielectric and microwave absorption properties, ACS Appl. Mater. Interfaces 9

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