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Synthesis of three-dimensional carbon networks decorated with Fe3O4 nanoparticles as lightweight and broadband electromagnetic wave absorber
Journal of Alloys and Compounds 776 (2019) 691e701
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis of three-dimensional carbon networks decorated with Fe3O4 nanoparticles as lightweight and broadband electromagnetic wave absorber Zhennan Liu a, Naiqin Zhao a, b, Chunsheng Shi a, *, Fang He a, Enzuo Liu a, b, Chunnian He a, b a b
School of Materials Science and Engineering, Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin, 300350, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 May 2018 Received in revised form 15 October 2018 Accepted 23 October 2018 Available online 27 October 2018
Three-dimensional (3D) carbon networks decorated with Fe3O4 nanoparticles as lightweight and broadband electromagnetic (EM) wave absorber were in-situ prepared via a simple and large-scale method, combining freeze-drying and high-temperature calcination processes. SEM and TEM results show that 3D carbon network/Fe3O4 (3DC/Fe3O4) composites have interconnected 3D porous carbon networks with submicrometer-sized macropores, and the Fe3O4 nanoparticles are distributed uniformly on the 3D carbon networks. The EM wave absorption performance of 3DC/Fe3O4 can be tuned by changing Fe3O4 contents. The 3DC/Fe3O4 with about 38.2 wt% Fe3O4 exhibits excellent lightweight and broadband EM wave absorption property. The effective absorption bandwidth can reach up 5.95 GHz (11.2e17.15 GHz) at the thickness of 3.0 mm with only 20 wt% filler loading. The minimum RL of 37.8 dB was obtained at 6.95 GHz. The excellent EM wave absorption capability of 3DC/Fe3O4 can be ascribed to good impedance matching, strong dielectric loss ability and unique 3D porous structure. This work demonstrates that the 3DC/Fe3O4 with light weight, broad absorption bandwidth and large-scale production potential can be a promising absorber for practical application. © 2018 Elsevier B.V. All rights reserved.
Keywords: Composite materials 3D carbon networks Fe3O4 nanoparticles In situ synthesis Microwave absorption
1. Introduction In recent years, with the technological advances and the increasing use of electronic equipments, electromagnetic interference (EMI) and electromagnetic pollution induced by electromagnetic (EM) radiation have attracted much attention, which may not only interrupt the normal operation of electronic devices but also be harmful to human health [1e4]. Therefore, it is urgent to develop technologies for solving the growing EM pollution problem, among which the application of EM wave absorbing materials is considered as an effective way [5e7]. The traditional EM wave absorbing materials, such as ferrites and magnetic metals, present strong absorption properties, which can attenuate EM waves effectively. However, these magnetic materials also have many disadvantages, such as large density, serious aggregation problem, poor anti-oxidation and high addition
* Corresponding author. E-mail address: [email protected] (C. Shi). https://doi.org/10.1016/j.jallcom.2018.10.303 0925-8388/© 2018 Elsevier B.V. All rights reserved.
amount in the matrix, which restrict their practical applications in the field of EM wave absorption [8e10]. Hence, it is highly desirable and necessary to explore high-performance EM wave absorbing materials with strong absorption, thin thickness, light weight, wide absorption frequency range and high environmental stability [11,12]. Carbon materials, including carbon fibers [13], carbon nanotubes [14,15], ordered mesoporous carbon [16,17], nanoporous carbon [18,19], expanded graphite [20,21], graphene [22,23], 3D graphene [24] and other carbon materials [25], have been used to combine with the magnetic materials for their features of low density, high electric conductivity, good designability, excellent chemical and thermal stability [10]. Consequently, the carbonbased magnetic composites exhibit enhanced EM wave absorption properties because of significant synergistic or complementary behavior between carbon and magnetic materials. For example, Qiu et al. [12] fabricated magnetite nanoparticle-carbon nanotubehollow carbon fiber composites through chemical vapor deposition technique and chemical reaction. The minimum reflection loss (RL)
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value was 50.9 dB at a frequency of 14.03 GHz with a thickness of 2.5 mm, which showed excellent EM wave absorption performance. Chu et al. [16] prepared ordered mesoporous carbon anchored with FeCo alloy nanoparticles composites via simple impregnation and in situ hydrogen-thermal reduction. The composites provided outstanding EM absorption performance of 73.8 dB. Zhu et al. [26] fabricated magnetic Fe3O4/rGO composites by a one-pot solvothermal method. The minimum RL value of 45 dB was achieved at 8.96 GHz with a sample thickness of 3.5 mm, which exhibited enhanced EM wave absorption performance compared with pure Fe3O4. Li et al. [27] synthesized hierarchical rose-like porous Fe@C via in situ transformation of rose-like iron alkoxide precursor. The composite exhibited the minimum RL value of 71.47 dB with a thin matched thickness of 1.48 mm, which displayed an outstanding EM wave absorption performance. So far, a lot of efforts have been made to synthesize carbonbased composites as EM wave absorbing agents and the strong absorption was obtained for most carbon-based magnetic composites [15,18,21,23]. However, there are still some aspects worth improving. First, the filling contents of the carbon-based absorbing agents in the matrix are relatively high (usually more than 30 wt%) and the effective absorption bandwidth still needs to be expanded. Second, carbon substrates in the carbon-based absorbing materials are usually one-dimensional and two-dimensional, which provide less multiple reflections of EM waves inside the material compared to three-dimensional structures. Third, the weak interface bonding between the magnetic nanoparticles and carbon substrates as well as the strong attractive force among the magnetic nanoparticles lead to the detachment and aggregation of the magnetic nanoparticles, weakening the EM wave absorption property. Moreover, low yields, high costs and complicated preparation methods of the most carbon-based magnetic composites limit their practical applications as absorbers. Hence, developing simple and effective preparation methods to fabricate novel 3D carbon-based magnetic composites with lightweight and broadband EM wave absorption property is still worth investigating. In this work, we fabricated three-dimensional carbon networks tightly anchored with Fe3O4 nanoparticles (3DC/Fe3O4) via a facile method, involving freeze-drying and high-temperature calcination processes. The nanocomposites exhibited unique porous and interconnected network structures, not only reducing the weight of absorbers but also increasing the multiple reflections of EM waves. The obtained 3DC/Fe3O4 nanocomposites exhibited a broad absorption bandwidth of 5.95 GHz (11.2e17.15 GHz) at the thickness of 3.0 mm with only 20 wt% filler loading. The Fe3O4 nanoparticles were distributed evenly on the 3D carbon substrates without aggregation, which ensured the stability of the composites. Moreover, the method may provide a new strategy to fabricate other 3D carbon-based absorbers for practical application. 2. Experimental
and 39.0 g of NaCl (atomic ratio of Fe: C: Na is 1.5: 20: 150) were dissolved in 130 mL deionized water under vigorous magnetic stirring to obtain a homogeneous solution. The solution was then frozen in a refrigerator at 20 C for 24 h. The water in the resulting gel was removed by a freeze-drying technology, and the dry gel was ground to a fine composite powder. During the high-temperature calcination process, 10 g of the composite powder was calcined in a tube furnace at 600 C for 2 h under Ar (80 ccm) and then cooled to room temperature. Finally, the as-synthesized product was washed with deionized water several times to remove NaCl, and the 3DC/Fe3O4 nanocomposite (denoted as 3DC/Fe3O4-2) was obtained after drying at 80 C. Two additional nanocomposites with precursor atomic ratios of Fe: C: Na of 1: 20: 150 and 2: 20: 150 (denoted as 3DC/Fe3O4-1 and 3DC/Fe3O4-3, respectively) were prepared under the same conditions but changing the Fe(NO3)3$9H2O concentration in the precursors. For comparison, pure 3DC was synthesized under the same conditions with 3DC/Fe3O4-2 but without adding the Fe(NO3)3$9H2O. Similarly, two dimensional carbon/Fe3O4 composite (2DC/Fe3O4) was fabricated using a hot-drying method rather than the freeze-drying technology. 2.3. Characterization X-ray diffraction (XRD) measurements were performed on a Rigaku D/max 2500 diffractometer with Cu Ka radiation at a wavelength of 1.5406 Å to identify the phase composition and crystallinity. Raman spectra were recorded on the LabRAM HR Raman spectrometer using laser excitation at 514.5 nm from an argon ion laser source. Thermogravimetry analysis (TGA) was performed with a Perkin-Elmer (TA Instruments) analyzer up to 800 C at a heating rate of 10 C/min in air to determine the content of Fe3O4. The X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Ka radiation. The magnetic property of 3DC/Fe3O4 was measured by a vibrating sample magnetometer (VSM, LDJ9066-1, USA) at room temperature. The electrical conductivity of the samples was tested on a RTS-8 four-point probe tester. Scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, Philips Tecnai G2 F20) were employed to characterize the morphology and structure of the as-synthesized samples. 2.4. Electromagnetic parameters measurements The EM parameters of the as-prepared nanocomposites were measured using an Agilent HP-8722ES vector network analyzer in the frequency range of 1.0e18.0 GHz. The measured samples were prepared by uniformly mixing 20 wt% of the as-prepared nanocomposites with the paraffin matrix and then compressing them into toroidal samples with an outer diameter of 7.00 mm and inner diameter of 3.00 mm.
2.1. Materials 3. Results and discussion Ferric nitrate nonahydrate (Fe(NO3)3$9H2O) was purchased from Aladdin Shanghai Biological Technology Co., Ltd., China. Glucose (C6H12O6) and sodium chloride (NaCl) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd., China. All reagents used were of analytical pure grade without further purification. 2.2. Preparation of 3DC/Fe3O4 nanocomposites 3DC/Fe3O4 nanocomposites were prepared via freeze-drying followed by high-temperature calcination processes. In a typical synthesis process, 2.694 g of Fe(NO3)3$9H2O, 2.667 g of C6H12O6
The preparation of 3DC/Fe3O4 nanocomposites is schematically illustrated in Fig. 1. First, NaCl, Fe(NO3)3$9H2O and C6H12O6 were dissolved in deionized water to obtain a homogeneous solution. The solution was then subjected to freeze-drying, during which NaCl particles uniformly coated with thin Fe(NO3)3$9H2OeC6H12O6 composite film were self-assembled into a 3D structure [28,29]. Subsequently, the obtained freeze-drying powders were calcined at 600 C under argon. During the calcination [30], Fe(NO3)3$9H2O was decomposed to Fe2O3 nanoparticles, and glucose was carbonized to amorphous carbon at relatively low temperatures (below
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Fig. 1. Schematic illustration of the fabrication procedure of the 3DC/Fe3O4 nanocomposites.
450 C). At a higher temperature, Fe2O3 nanoparticles anchored on the amorphous carbon were reduced to Fe3O4 nanocrystals by carbon. Meanwhile, Fe3O4 nanoparticles as a catalyst also improved the graphitization of amorphous carbon during consequent thermal treatment. Finally, the 3DC/Fe3O4 nanocomposites were obtained by removing the NaCl template with distilled water. Fig. 2a shows the XRD patterns of the 3DC/Fe3O4 nanocomposites. It can be seen that all the diffraction peaks of the assynthesized samples at 30.1, 35.4 , 43.1, 53.4 , 56.9 and 62.5 are corresponding to the (220), (311), (400), (422), (511) and (440) planes of the face-centered cubic (fcc) structure of magnetite Fe3O4 (JCPDS no. 19e0629), which indicates that well-crystallized Fe3O4 phase is obtained after facile high temperature calcination. Meanwhile, the diffraction peak intensities increase significantly with the increase of the content of Fe(NO3)3$9H2O in the precursor, demonstrating the content of Fe3O4 in the composite can be adjusted easily [31]. In addition, the diffraction peak at 26.5 for graphite carbon can hardly be observed, suggesting low crystallinity of the three-dimensional carbon networks. The contents of Fe3O4 in the 3DC/Fe3O4 nanocomposites are associated with precursor atomic ratio of Fe: C: Na. In order to determine Fe3O4 contents, TGA was performed from room temperature to 800 C at a heating rate of 10 C/min in the air atmosphere, during which carbon was oxidized to CO2 and Fe3O4 was oxidized to Fe2O3. As shown in Fig. 2b, according to the final contents of Fe2O3, the original contents of Fe3O4 in 3DC/Fe3O4-1, 3DC/ Fe3O4-2 and 3DC/Fe3O4-3 samples were calculated to be 15.4 wt %, 38.2 wt % and 57.6 wt %, respectively, which is in agreement with the XRD results. The Raman spectra of the 3DC/Fe3O4 nanocomposites are shown
in Fig. 2c. All samples present two characteristic peaks of D band and G band at about 1335 and 1590 cm1, respectively. The D band corresponds to sp3 hybridized carbon, which is associated with disordered or defective carbon, while the G-band represents a radical CeC stretching mode of sp2 bonded carbon from graphite structures [3,32]. The intensity ratio of D band to G band, ID/IG, indicates the defects and disorders in carbon-based materials [33,34]. The calculated ID/IG ratios for 3DC/Fe3O4-1, 3DC/Fe3O4-2 and 3DC/Fe3O4-3 are 0.814, 0.863 and 0.902, respectively. The ID/IG value of the 3DC/Fe3O4 nanocomposites increases with increasing the Fe3O4 contents, which implies that more defects are introduced in 3DC/Fe3O4. Furthermore, the increased ID/IG value is also associated with the generation of nanocrystalline graphite [5]. Fig. 2d exhibits the field-dependent magnetization of 3DC/Fe3O4 nanocomposites with different Fe3O4 contents, which was measured by a vibrating sample magnetometer at room temperature. Significant hysteresis loops in the M-H cures indicate the ferromagnetic of 3DC/Fe3O4 nanocomposites. The saturation magnetizations (Ms), coercivity (Hc) and remanent magnetization (Mr) are summarized in Table 1. It can be seen that Ms, Hc and Mr values for 3DC/Fe3O4 nanocomposites increase with Fe3O4 contents. The magnetic measurements demonstrate that 3DC/Fe3O4 nanocomposites are ferromagnetic and can be used to attenuate EM waves [35]. Fig. 3 displays the representative SEM images of 3DC/Fe3O4-2. From Fig. 3a, it can be seen that 3DC/Fe3O4 has an interconnected three-dimensional porous network with submicrometer-sized macropores. The porous structure ensures that 3DC/Fe3O4 nanocomposites have a low density, which will be conductive to satisfying the requirement of lightweight. Moreover, the interconnected
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Fig. 2. (a) XRD patterns, (b) TGA curves, (c) Raman spectra and (d) the hysteresis loops of 3DC/Fe3O4 nanocomposites.
Table 1 Magnetic parameters of 3DC/Fe3O4 nanocomposites. Samples
Ms (emu/g)
Hc (Oe)
Mr (emu/g)
3DC/Fe3O4-1 3DC/Fe3O4-2 3DC/Fe3O4-3
23.6 34.8 50.9
92.4 166.1 185.5
4.5 9.1 15.8
3D porous networks provide more propagation paths for EM waves. The magnified SEM image shown in Fig. 3b reveals that many uniform Fe3O4 nanoparticles are homogeneously anchored on the walls of the 3D carbon network without agglomeration. The welldistributed Fe3O4 nanoparticles on the 3D carbon can generate abundant interfacial polarization, which will enhance the loss of
EM waves. Fig. S1 shows the SEM images of 3DC/Fe3O4-1 and 3DC/ Fe3O4-3. Obviously, the microstructures of the samples are similar to 3DC/Fe3O4-2. Nevertheless, it also can be seen that the porous structures are damaged to a certain degree and the Fe3O4 nanoparticles are uneven distributed on the 3DC. The structure and morphology of 3DC/Fe3O4-2 were further investigated by TEM. As shown in Fig. 4a, an interconnected threedimensional porous networks decorated with uniform Fe3O4 nanoparticles was obtained, which was consistent with the results of SEM observation. Besides, it should be noted that the Fe3O4 nanoparticles are still tightly anchored on the 3D carbon networks without detachment and aggregation after a long time sonication for preparing the TEM samples, which indicates the strong bonding between Fe3O4 and carbon networks. Fig. 4b shows the magnified
Fig. 3. (a) SEM and (b) magnified SEM images of 3DC/Fe3O4-2.
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Fig. 4. (a, b) TEM images of 3DC/Fe3O4-2, (c) Size distribution of Fe3O4 NPs on 3DC, (d) HRTEM image of a typical Fe3O4 NP, (eeh) Elemental mapping images of 3DC/Fe3O4-2.
TEM image of 3DC/Fe3O4-2. The particle size distribution histogram shown in Fig. 4c exhibits that the Fe3O4 nanoparticles has a wider size distribution of 10e50 nm, and the average size is about 28 nm. The HRTEM image shown in Fig. 4d depicts the crystalline structures of Fe3O4 nanopaticles. The lattice fringe spacing of 0.253 nm corresponds to the (311) planes of cubic Fe3O4, which is consistent with the XRD results. Fig. 4(eeh) presents the element mapping images of 3DC/Fe3O4-2, demonstrating that C, Fe and O elements are homogeneously distributed in the sample. The TEM images for 3DC/Fe3O4-1 and 3DC/Fe3O4-3 are provided in Fig. S2. It is clear that some areas of 3DC/Fe3O4-1 have no Fe3O4 nanoparticles and 3DC/ Fe3O4-3 shows a slight agglomeration of Fe3O4 nanoparticles, which is consistent with SEM results (Fig. S1). The average size of Fe3O4 nanoparticles for 3DC/Fe3O4-1 and 3DC/Fe3O4-3 are 21 nm and 32 nm, respectively. The elemental composition and chemical binding of 3DC/Fe3O4 was further identified by XPS. As shown in Fig. 5a, the full scan XPS spectra reveals that the 3DC/Fe3O4-2 is completely composed of C, Fe and O elements. Fig. 5b depicts the high-resolution spectra of C 1s. Three peaks with binding energies of 284.6, 285.5 and 288.3 eV are originated from CeC, CeOH and C]O, respectively [36]. The O 1s spectrum, as shown in Fig. 5c, can be divided into three components. The peak positions at the binding energies of 530.1 eV, 531.7 and 533.4 eV are assigned to O2 of Fe3O4, OH and CeO, respectively [8]. The Fe 2p spectra of Fig. 5d shows two peaks at 710.9 and 724.4 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2 of Fe3O4 [37]. The electromagnetic parameters of the paraffin composites containing 20 wt% of the 3DC/Fe3O4 absorber were measured from 1.0 GHz to 18.0 GHz. The electromagnetic parameters include
relative complex permittivity (εr ¼ ε0 -jε00 ) and relative complex permeability (mr ¼ m0 -jm00 ). The real permittivity (ε0 ) and permeability (m0 ) represent the ability to store the electric and magnetic energy, while the imaginary permittivity (ε00 ) and permeability (m00 ) describe the dissipation of electric and magnetic energy [38]. As shown in Fig. 6a and b, both ε0 and ε00 of 3DC/Fe3O4 composites decreased as increasing frequency from 1.0 GHz to 18.0 GHz. Such a phenomenon exists in many carbon materials, such as carbon nanotubes (CNTs), carbon fibers (CFs) and graphene, which is called frequency dispersion behavior [6]. In addition, it can be found that both ε0 and ε00 values of the 3DC/Fe3O4 composites are larger than that of the pure 3DC (Fig. S3) and increase with increasing the Fe3O4 contents, which is opposite to the other graphene-based composites previously reported [5,39]. According to the reported literature, the value of ε00 is mainly affected by the conductivity and polarization [40]. As shown in Fig. S4, the pure 3DC has the lowest electrical conductivity, which mainly caused by the lower degree of graphitizing. As more Fe3O4 nanoparticles were introduced into 3DC, the degree of graphitizing for carbon was improved due to the catalytic effect of Fe3O4 nanoparticles, which leads to increase of electrical conductivity. According to the free electron theory, ε00 z 1/2prfε0, where r is the resistivity, f is the frequency, and ε0 is the dielectric constant of vacuum. It can be deduced that higher electrical conductivity results in higher value of ε00 . Furthermore, the combination of carbon and Fe3O4 nanoparticles improves the degree of polarization, which is also in favor of obtaining high values of ε00 [41]. On the other hand, the increased values of ε0 may be also resulted from the enhanced polarization. Fig. 6c and d illustrate the m0 and m00 values of 3DC/Fe3O4 composites. It can be seen that both m0 and m00 values exhibit slight variation for all samples, which
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Fig. 5. (a) The general XPS, (b) C 1s region, (c) O 1s region and (d) Fe 2p region of 3DC/Fe3O4-2.
indicates minor magnetic loss contributions to EM wave absorption [42]. In generally, the dielectric loss tangent (tan dE ¼ ε00 /ε0 ) and the magnetic loss tangent (tan dM ¼ m00 /m0 ) represent the contributions of dielectric loss and magnetic loss to EM wave absorption. As shown in Fig. 6e and f, it can be found that the dielectric loss tangent increases with increasing the Fe3O4 content over 4.5e18.0 GHz, indicating the incorporation of more Fe3O4 can improve the dielectric loss capacity. Furthermore, the values of tan dE for 3DC/Fe3O4 are higher than tan dM, suggesting dielectric loss makes a major contribution to the EM wave absorption over the frequency range from 1.0 to 18.0 GHz. Conventionally, the dielectric loss mechanism can be analyzed by Cole-Cole semicircles. According to the theory of Debye dipolar relaxation [42], the relationship between ε00 and ε0 can be expressed as (ε0 -(εs þ ε∞)/2)2 þ (ε00 )2 ¼ ((εs-ε∞)/2)2, where εs is the static permittivity, ε∞ is the relative dielectric permittivity at infinite frequency. The plot ε0 versus ε00 should be a single semicircle, which usually denoted as Cole-Cole semicircles. Each semicircle corresponds to one Debye relaxation process [43]. Fig. 7a displays ColeCole semicircles of all samples. It can be found that three semicircles exist in each plot, corresponding to three dielectric relaxation processes, which mainly originate from the interface polarizations between Fe3O4 and carbon, Fe3O4 and paraffin matrix, and carbon and paraffin matrix. In addition, the radius of the semicircles gradually increases with increasing Fe3O4 content, indicating that the contribution of dielectric relaxation to permittivity gradually increases [44,45]. Besides the dielectric loss, magnetic loss is also an important factor for EM attenuation. In general, the magnetic loss originates from hysteresis loss, domain wall resonance, eddy current loss, natural resonance and exchange resonance [46]. The hysteresis loss from irreversible magnetization
only generates in a strong applied field and the domain wall resonance occurs in the lower frequency region (MHz), which can be excluded in this case [43,47]. The eddy current loss can be certified by the equation m00 (m0 )2f1 ¼ 2pm0sd2/3 [15], where m0 is the vacuum permeability, s is the electric conductivity and d is the sample thickness. If magnetic loss originates from eddy current loss, the value of m00 (m0 )2f1 will remain constant over 1e18 GHz [48]. As shown in Fig. 7b, the m00 (m0 )2f1 values of all samples show fluctuation over 1e5 GHz and nearly remain a constant ranging from 5 to 18 GHz, indicating that the magnetic loss mainly result from natural resonance over 1e5 GHz and eddy current loss over 5e18 GHz. On the basis of the measured EM parameters (relative complex permittivity εr and permeability mr), the reflection loss (RL) values are calculated according to the transmission line theory [45,49],
i h Zin ¼ Z0 ðmr =εr Þ1=2 tanh jð2pfd=cÞðmr εr Þ1=2 Z Z0 RL ðdBÞ ¼ 20log in Zin þ Z0
(1)
(2)
where Z0 is the impedance of free space, Zin is the input impedance of the absorber, f is the frequency of electromagnetic waves, d is the thickness of the absorber and c is the velocity of electromagnetic waves in free space. When the values of RL are below 10 dB, 90% of EM wave energy can be absorbed, and the corresponding frequency range is defined as effective absorption bandwidth for practical application. Fig. 8 shows the calculated RL curves of 3DC/Fe3O4-1, 3DC/Fe3O4-2 and 3DC/Fe3O4-3 with different thicknesses in a frequency range from 1.0 to 18.0 GHz. As shown in Fig. 8a, the
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Fig. 6. Frequency dependence of (a) real part (ε0 ) and (b) imaginary part (ε00 ) of the complex permittivity, (c) real part (m0 ) and (d) imaginary part (m00 ) of the complex permeability, (e) dielectric loss tangent (tan dE) and (f) magnetic loss tangent (tan dM) for 3DC/Fe3O4 nanocomposites in the frequency range of 1.0e18.0 GHz.
minimum RL of 3DC/Fe3O4-1 is only 12.5 dB with a thickness of 6.0 mm, showing poor EM wave absorption property. Fig. 8b displays the RL plot of 3DC/Fe3O4-3, it can be seen that 3DC/Fe3O4-3 shows the inferior EM wave absorption performance in the frequency range of 10.0e18.0 GHz, which may be associated with the high permittivity of 3DC/Fe3O4-3. For 3DC/Fe3O4-2 (Fig. 8c), the minimum RL is 37.8 dB at 6.95 GHz with a thickness of 5.5 mm and the effective absorption bandwidth (RL < 10 dB) can reach up 5.95 GHz (11.2e17.15 GHz) with a thickness of only 3 mm, exhibiting good EM wave absorption performance. As listed in Table S1, it can be seen that the 3D/Fe3O4 nanocomposites exhibit advantages in two aspects, i.e., effective absorption bandwidth and filler loading compared with other carbon-based magnetic composites. To analyze the EM wave absorption performance of the 3DC/ Fe3O4 nanocomposites, impedance matching and attenuation constant were investigated. The impedance matching can be
evaluated by the following equation [8,50],
Z ¼ jZin =Z0 j
(3)
where Zin is the input impedance of the absorber, Z0 is the impedance of free space, Z is the modulus of the normalized characteristic impedance (Zin/Z0). When the value of Z is close to 1, most of EM waves can enter into the absorber, indicating that the ideal impedance matching can be achieved. As shown in Fig. 9(aec), the Z values of the 3DC/Fe3O4 nanocomposites were calculated with different thickness in a frequency range from 1.0 to 18.0 GHz. The Z values between 0.9 and 1.1 are marked by red lines. It can be seen that 3DC/Fe3O4-2 possess broader regions that Z values ranged from 0.9 to 1.1 than 3DC/Fe3O4-1 and 3DC/Fe3O4-3, which demonstrates an optimized impedance matching of 3DC/ Fe3O4-2.
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Fig. 7. (a) Cole-Cole semicircles and (b) eddy current loss (denoted by m00 (m0 )2f1) of 3DC/Fe3O4 nanocomposites.
Fig. 8. Reflection loss curves for (a) 3DC/Fe3O4-1, (b) 3DC/Fe3O4-3 and (c) 3DC/Fe3O4-2 with different thickness in the frequency range of 1.0e18.0 GHz.
The attenuation constant (a), representing the internal attenuation ability of the materials, can be calculated by the following formula [51]:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf ðm00 ε00 m0 ε0 Þ þ ðm00 ε00 m0 ε0 Þ2 þ ðm0 ε00 þ m00 ε0 Þ2 a¼ c (4) Fig. 9d displays the calculated attenuation constant of three samples. Evidently, 3DC/Fe3O4-3 exhibits higher a values than 3DC/ Fe3O4-1 and 3DC/Fe3O4-2, indicating that 3DC/Fe3O4-3 has a
stronger attenuation capability for EM waves, which is mainly derived from high dielectric loss [52]. Nevertheless, the impedance matching of 3DC/Fe3O4-3 is bad, as shown in Fig. 9c, resulting in poor EM wave absorption property. The 3DC/Fe3O4-2 with larger a values performs stronger attenuation ability than 3DC/Fe3O4-1. Furthermore, 3DC/Fe3O4-2 also obtained good impedance matching. Therefore, 3DC/Fe3O4-2 shows the excellent EM wave absorption performance. Recently, the quarter-wavelength matching model is usually used to explain the excellent EM wave absorption performance, which is described by the following equation [53]:
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Fig. 9. 3D representation of Z values of (a) 3DC/Fe3O4-1, (b) 3DC/Fe3O4-2 and (c) 3DC/Fe3O4-3; (d) the attenuation constant (a) for 3DC/Fe3O4 composites. The Z values between 0.9 and 1.1 are marked by red lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
tm ¼ nl=4 ¼ nc
. 4fm ðjmr jjεr jÞ1=2 ; ðn ¼ 1; 3; 5 …Þ
(5)
where fm is the peak frequency and tm is the corresponding thickness of the absorber. When the experimental values of tm and fm meet the formula, the reflected EM waves from the air-absorber interface and absorber-metal interface are out of phase by 180 , causing an extinction of them on the air-absorber interface [54]. Fig. S5 shows the calculated fitting cure of above equation and the experimental value of tm and fm. It is clear that the texp m is in good agreement with tfit m , indicating that the excellent EM wave absorption performance of 3DC/Fe3O4-2 can be partly explained by the quarter-wavelength matching model.
As a comparison, the EM wave absorption properties of pure 3DC and 2DC/Fe3O4 were investigated. The measured paraffin samples also contain 20 wt% absorbers. Fig. S6 displays the SEM and TEM images of pure 3DC. It can be seen that the micro-morphology of pure 3DC is similar to 3DC/Fe3O4-2 except for absence of Fe3O4 nanoparticles. The XRD pattern of 2DC/Fe3O4 shown in Fig. S7 confirms the present of magnetite Fe3O4. The SEM and TEM images (Fig. S8) indicate that the 2DC/Fe3O4 has a microstructure of two-dimensional carbon anchored with Fe3O4 nanoparticles. As shown in Fig. 10a, the minimum RL of pure 3DC is only 5.2 dB over 1.0e18.0 GHz, which indicates that the synergistic effect of Fe3O4 and 3DC play an important role on improving EM wave absorption property. 2DC/Fe3O4 (Fig. 10b) obtains the minimum RL of 22.9 dB
Fig. 10. Reflection loss curves for (a) pure 3DC and (b) 2DC/Fe3O4 with different thickness in the frequency range of 1.0e18.0 GHz.
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Fig. 11. Schematic illustration of EM wave propagation and attenuation in 3DC/Fe3O4 nanocomposites.
and the effective absorption bandwidth of 5.59 GHz with a 6.0 mm thickness. Obviously, 3DC/Fe3O4-2 exhibits enhanced EM wave absorption property compared with 2DC/Fe3O4. This result demonstrates that the unique 3D porous structure can attenuate EM waves effectively by providing more channels for the propagation and multiple reflection of the EM waves [6]. Based on the above analysis, it can be summarized that the enhanced EM wave absorption performance with lightweight and broadband features of 3DC/Fe3O4-2 can be mainly attributed to four factors. Fig. 11 depicts the schematic illustration of EM wave propagation and attenuation in 3DC/Fe3O4 nanocomposites. First, the good impedance matching can be obtained by changing the content of Fe3O4 in 3DC/Fe3O4, which ensures most of EM waves can enter into the EM absorbing materials. Second, the strong attenuation ability derived from high dielectric loss can transforms EM energy to heat energy. Third, the unique 3DC porous structure not only makes composites have lightweight but also provides more channels for the propagation and multiple reflection of the EM waves, which improves EM wave absorption performance. Moreover, the mechanism of quarter-wavelength matching model also contributes to the dissipation of EM waves. 4. Conclusions In summary, a novel lightweight and broadband absorber of three-dimensional carbon networks decorated with Fe3O4 nanoparticles (3DC/Fe3O4) was successfully synthesized via freezedrying and high-temperature calcination processes. The 3DC/ Fe3O4 exhibited tunable EM wave absorption property through changing Fe3O4 contents (adjusting initial iron salt concentrations directly). The minimum RL of 3DC/Fe3O4-2 (with about 38.2 wt% Fe3O4) is 37.8 dB at 6.95 GHz. A broad absorption bandwidth of 5.95 GHz (11.2e17.15 GHz) can be achieved at the thickness of 3.0 mm with only 20 wt% 3DC/Fe3O4-2 in paraffin matrix. The excellent EM wave absorption performance of 3DC/Fe3O4-2 nanocomposites can be mainly ascribed to good impedance matching, high dielectric loss and unique 3D porous structure. It is believed
that the 3DC/Fe3O4 with lightweight and broadband features has great potential for realizing practical application. Furthermore, this work also provides a new way to prepare novel carbon-based magnetic composites for EM wave absorption. Acknowledgement The authors acknowledge the financial support by the Science and Technology Support Program of Tianjin (No. 16ZXCLGX00130). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2018.10.303. References [1] Z. Chen, C. Xu, C. Ma, W. Ren, H.M. Cheng, Lightweight and flexible graphene foam composites for high-performance electromagnetic interference shielding, Adv. Mater. 25 (2013) 1296e1300. [2] J. Liu, H.B. Zhang, R. Sun, Y. Liu, Z. Liu, A. Zhou, Z.Z. Yu, Hydrophobic, flexible, and lightweight MXene foams for high-performance electromagnetic-interference shielding, Adv. Mater. 29 (2017). [3] X. Qiu, L. Wang, H. Zhu, Y. Guan, Q. Zhang, Lightweight and efficient microwave absorbing materials based on walnut shell-derived nano-porous carbon, Nanoscale 9 (2017) 7408e7418. [4] Y. Lu, Y. Wang, H. Li, Y. Lin, Z. Jiang, Z. Xie, Q. Kuang, L. Zheng, MOF-derived porous Co/C nanocomposites with excellent electromagnetic wave absorption properties, ACS Appl. Mater. Interfaces 7 (2015) 13604e13611. [5] Y. Lai, S. Wang, D. Qian, S. Zhong, Y. Wang, S. Han, W. Jiang, Tunable electromagnetic wave absorption properties of nickel microspheres decorated reduced graphene oxide, Ceram. Int. 43 (2017) 12904e12914. [6] J. Fang, T. Liu, Z. Chen, Y. Wang, W. Wei, X. Yue, Z. Jiang, A wormhole-like porous carbon/magnetic particles composite as an efficient broadband electromagnetic wave absorber, Nanoscale 8 (2016) 8899e8909. [7] R. Qiang, Y. Du, D. Chen, W. Ma, Y. Wang, P. Xu, J. Ma, H. Zhao, X. Han, Electromagnetic functionalized Co/C composites by in situ pyrolysis of metalorganic frameworks (ZIF-67), J. Alloys Compd. 681 (2016) 384e393. [8] B. Qu, C. Zhu, C. Li, X. Zhang, Y. Chen, Coupling hollow Fe3O4-Fe nanoparticles with graphene sheets for high-performance electromagnetic wave absorbing material, ACS Appl. Mater. Interfaces 8 (2016) 3730e3735. [9] H. Yuan, F. Yan, C. Li, C. Zhu, X. Zhang, Y. Chen, Nickel nanoparticle encapsulated in few-layer nitrogen-doped graphene supported by nitrogen-doped
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis of three-dimensional carbon networks decorated with Fe3O4 nanoparticles as lightweight and broadband electromagnetic wave absorber Zhennan Liu a, Naiqin Zhao a, b, Chunsheng Shi a, *, Fang He a, Enzuo Liu a, b, Chunnian He a, b a b
School of Materials Science and Engineering, Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin, 300350, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 May 2018 Received in revised form 15 October 2018 Accepted 23 October 2018 Available online 27 October 2018
Three-dimensional (3D) carbon networks decorated with Fe3O4 nanoparticles as lightweight and broadband electromagnetic (EM) wave absorber were in-situ prepared via a simple and large-scale method, combining freeze-drying and high-temperature calcination processes. SEM and TEM results show that 3D carbon network/Fe3O4 (3DC/Fe3O4) composites have interconnected 3D porous carbon networks with submicrometer-sized macropores, and the Fe3O4 nanoparticles are distributed uniformly on the 3D carbon networks. The EM wave absorption performance of 3DC/Fe3O4 can be tuned by changing Fe3O4 contents. The 3DC/Fe3O4 with about 38.2 wt% Fe3O4 exhibits excellent lightweight and broadband EM wave absorption property. The effective absorption bandwidth can reach up 5.95 GHz (11.2e17.15 GHz) at the thickness of 3.0 mm with only 20 wt% filler loading. The minimum RL of 37.8 dB was obtained at 6.95 GHz. The excellent EM wave absorption capability of 3DC/Fe3O4 can be ascribed to good impedance matching, strong dielectric loss ability and unique 3D porous structure. This work demonstrates that the 3DC/Fe3O4 with light weight, broad absorption bandwidth and large-scale production potential can be a promising absorber for practical application. © 2018 Elsevier B.V. All rights reserved.
Keywords: Composite materials 3D carbon networks Fe3O4 nanoparticles In situ synthesis Microwave absorption
1. Introduction In recent years, with the technological advances and the increasing use of electronic equipments, electromagnetic interference (EMI) and electromagnetic pollution induced by electromagnetic (EM) radiation have attracted much attention, which may not only interrupt the normal operation of electronic devices but also be harmful to human health [1e4]. Therefore, it is urgent to develop technologies for solving the growing EM pollution problem, among which the application of EM wave absorbing materials is considered as an effective way [5e7]. The traditional EM wave absorbing materials, such as ferrites and magnetic metals, present strong absorption properties, which can attenuate EM waves effectively. However, these magnetic materials also have many disadvantages, such as large density, serious aggregation problem, poor anti-oxidation and high addition
* Corresponding author. E-mail address: [email protected] (C. Shi). https://doi.org/10.1016/j.jallcom.2018.10.303 0925-8388/© 2018 Elsevier B.V. All rights reserved.
amount in the matrix, which restrict their practical applications in the field of EM wave absorption [8e10]. Hence, it is highly desirable and necessary to explore high-performance EM wave absorbing materials with strong absorption, thin thickness, light weight, wide absorption frequency range and high environmental stability [11,12]. Carbon materials, including carbon fibers [13], carbon nanotubes [14,15], ordered mesoporous carbon [16,17], nanoporous carbon [18,19], expanded graphite [20,21], graphene [22,23], 3D graphene [24] and other carbon materials [25], have been used to combine with the magnetic materials for their features of low density, high electric conductivity, good designability, excellent chemical and thermal stability [10]. Consequently, the carbonbased magnetic composites exhibit enhanced EM wave absorption properties because of significant synergistic or complementary behavior between carbon and magnetic materials. For example, Qiu et al. [12] fabricated magnetite nanoparticle-carbon nanotubehollow carbon fiber composites through chemical vapor deposition technique and chemical reaction. The minimum reflection loss (RL)
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value was 50.9 dB at a frequency of 14.03 GHz with a thickness of 2.5 mm, which showed excellent EM wave absorption performance. Chu et al. [16] prepared ordered mesoporous carbon anchored with FeCo alloy nanoparticles composites via simple impregnation and in situ hydrogen-thermal reduction. The composites provided outstanding EM absorption performance of 73.8 dB. Zhu et al. [26] fabricated magnetic Fe3O4/rGO composites by a one-pot solvothermal method. The minimum RL value of 45 dB was achieved at 8.96 GHz with a sample thickness of 3.5 mm, which exhibited enhanced EM wave absorption performance compared with pure Fe3O4. Li et al. [27] synthesized hierarchical rose-like porous Fe@C via in situ transformation of rose-like iron alkoxide precursor. The composite exhibited the minimum RL value of 71.47 dB with a thin matched thickness of 1.48 mm, which displayed an outstanding EM wave absorption performance. So far, a lot of efforts have been made to synthesize carbonbased composites as EM wave absorbing agents and the strong absorption was obtained for most carbon-based magnetic composites [15,18,21,23]. However, there are still some aspects worth improving. First, the filling contents of the carbon-based absorbing agents in the matrix are relatively high (usually more than 30 wt%) and the effective absorption bandwidth still needs to be expanded. Second, carbon substrates in the carbon-based absorbing materials are usually one-dimensional and two-dimensional, which provide less multiple reflections of EM waves inside the material compared to three-dimensional structures. Third, the weak interface bonding between the magnetic nanoparticles and carbon substrates as well as the strong attractive force among the magnetic nanoparticles lead to the detachment and aggregation of the magnetic nanoparticles, weakening the EM wave absorption property. Moreover, low yields, high costs and complicated preparation methods of the most carbon-based magnetic composites limit their practical applications as absorbers. Hence, developing simple and effective preparation methods to fabricate novel 3D carbon-based magnetic composites with lightweight and broadband EM wave absorption property is still worth investigating. In this work, we fabricated three-dimensional carbon networks tightly anchored with Fe3O4 nanoparticles (3DC/Fe3O4) via a facile method, involving freeze-drying and high-temperature calcination processes. The nanocomposites exhibited unique porous and interconnected network structures, not only reducing the weight of absorbers but also increasing the multiple reflections of EM waves. The obtained 3DC/Fe3O4 nanocomposites exhibited a broad absorption bandwidth of 5.95 GHz (11.2e17.15 GHz) at the thickness of 3.0 mm with only 20 wt% filler loading. The Fe3O4 nanoparticles were distributed evenly on the 3D carbon substrates without aggregation, which ensured the stability of the composites. Moreover, the method may provide a new strategy to fabricate other 3D carbon-based absorbers for practical application. 2. Experimental
and 39.0 g of NaCl (atomic ratio of Fe: C: Na is 1.5: 20: 150) were dissolved in 130 mL deionized water under vigorous magnetic stirring to obtain a homogeneous solution. The solution was then frozen in a refrigerator at 20 C for 24 h. The water in the resulting gel was removed by a freeze-drying technology, and the dry gel was ground to a fine composite powder. During the high-temperature calcination process, 10 g of the composite powder was calcined in a tube furnace at 600 C for 2 h under Ar (80 ccm) and then cooled to room temperature. Finally, the as-synthesized product was washed with deionized water several times to remove NaCl, and the 3DC/Fe3O4 nanocomposite (denoted as 3DC/Fe3O4-2) was obtained after drying at 80 C. Two additional nanocomposites with precursor atomic ratios of Fe: C: Na of 1: 20: 150 and 2: 20: 150 (denoted as 3DC/Fe3O4-1 and 3DC/Fe3O4-3, respectively) were prepared under the same conditions but changing the Fe(NO3)3$9H2O concentration in the precursors. For comparison, pure 3DC was synthesized under the same conditions with 3DC/Fe3O4-2 but without adding the Fe(NO3)3$9H2O. Similarly, two dimensional carbon/Fe3O4 composite (2DC/Fe3O4) was fabricated using a hot-drying method rather than the freeze-drying technology. 2.3. Characterization X-ray diffraction (XRD) measurements were performed on a Rigaku D/max 2500 diffractometer with Cu Ka radiation at a wavelength of 1.5406 Å to identify the phase composition and crystallinity. Raman spectra were recorded on the LabRAM HR Raman spectrometer using laser excitation at 514.5 nm from an argon ion laser source. Thermogravimetry analysis (TGA) was performed with a Perkin-Elmer (TA Instruments) analyzer up to 800 C at a heating rate of 10 C/min in air to determine the content of Fe3O4. The X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Ka radiation. The magnetic property of 3DC/Fe3O4 was measured by a vibrating sample magnetometer (VSM, LDJ9066-1, USA) at room temperature. The electrical conductivity of the samples was tested on a RTS-8 four-point probe tester. Scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, Philips Tecnai G2 F20) were employed to characterize the morphology and structure of the as-synthesized samples. 2.4. Electromagnetic parameters measurements The EM parameters of the as-prepared nanocomposites were measured using an Agilent HP-8722ES vector network analyzer in the frequency range of 1.0e18.0 GHz. The measured samples were prepared by uniformly mixing 20 wt% of the as-prepared nanocomposites with the paraffin matrix and then compressing them into toroidal samples with an outer diameter of 7.00 mm and inner diameter of 3.00 mm.
2.1. Materials 3. Results and discussion Ferric nitrate nonahydrate (Fe(NO3)3$9H2O) was purchased from Aladdin Shanghai Biological Technology Co., Ltd., China. Glucose (C6H12O6) and sodium chloride (NaCl) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd., China. All reagents used were of analytical pure grade without further purification. 2.2. Preparation of 3DC/Fe3O4 nanocomposites 3DC/Fe3O4 nanocomposites were prepared via freeze-drying followed by high-temperature calcination processes. In a typical synthesis process, 2.694 g of Fe(NO3)3$9H2O, 2.667 g of C6H12O6
The preparation of 3DC/Fe3O4 nanocomposites is schematically illustrated in Fig. 1. First, NaCl, Fe(NO3)3$9H2O and C6H12O6 were dissolved in deionized water to obtain a homogeneous solution. The solution was then subjected to freeze-drying, during which NaCl particles uniformly coated with thin Fe(NO3)3$9H2OeC6H12O6 composite film were self-assembled into a 3D structure [28,29]. Subsequently, the obtained freeze-drying powders were calcined at 600 C under argon. During the calcination [30], Fe(NO3)3$9H2O was decomposed to Fe2O3 nanoparticles, and glucose was carbonized to amorphous carbon at relatively low temperatures (below
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693
Fig. 1. Schematic illustration of the fabrication procedure of the 3DC/Fe3O4 nanocomposites.
450 C). At a higher temperature, Fe2O3 nanoparticles anchored on the amorphous carbon were reduced to Fe3O4 nanocrystals by carbon. Meanwhile, Fe3O4 nanoparticles as a catalyst also improved the graphitization of amorphous carbon during consequent thermal treatment. Finally, the 3DC/Fe3O4 nanocomposites were obtained by removing the NaCl template with distilled water. Fig. 2a shows the XRD patterns of the 3DC/Fe3O4 nanocomposites. It can be seen that all the diffraction peaks of the assynthesized samples at 30.1, 35.4 , 43.1, 53.4 , 56.9 and 62.5 are corresponding to the (220), (311), (400), (422), (511) and (440) planes of the face-centered cubic (fcc) structure of magnetite Fe3O4 (JCPDS no. 19e0629), which indicates that well-crystallized Fe3O4 phase is obtained after facile high temperature calcination. Meanwhile, the diffraction peak intensities increase significantly with the increase of the content of Fe(NO3)3$9H2O in the precursor, demonstrating the content of Fe3O4 in the composite can be adjusted easily [31]. In addition, the diffraction peak at 26.5 for graphite carbon can hardly be observed, suggesting low crystallinity of the three-dimensional carbon networks. The contents of Fe3O4 in the 3DC/Fe3O4 nanocomposites are associated with precursor atomic ratio of Fe: C: Na. In order to determine Fe3O4 contents, TGA was performed from room temperature to 800 C at a heating rate of 10 C/min in the air atmosphere, during which carbon was oxidized to CO2 and Fe3O4 was oxidized to Fe2O3. As shown in Fig. 2b, according to the final contents of Fe2O3, the original contents of Fe3O4 in 3DC/Fe3O4-1, 3DC/ Fe3O4-2 and 3DC/Fe3O4-3 samples were calculated to be 15.4 wt %, 38.2 wt % and 57.6 wt %, respectively, which is in agreement with the XRD results. The Raman spectra of the 3DC/Fe3O4 nanocomposites are shown
in Fig. 2c. All samples present two characteristic peaks of D band and G band at about 1335 and 1590 cm1, respectively. The D band corresponds to sp3 hybridized carbon, which is associated with disordered or defective carbon, while the G-band represents a radical CeC stretching mode of sp2 bonded carbon from graphite structures [3,32]. The intensity ratio of D band to G band, ID/IG, indicates the defects and disorders in carbon-based materials [33,34]. The calculated ID/IG ratios for 3DC/Fe3O4-1, 3DC/Fe3O4-2 and 3DC/Fe3O4-3 are 0.814, 0.863 and 0.902, respectively. The ID/IG value of the 3DC/Fe3O4 nanocomposites increases with increasing the Fe3O4 contents, which implies that more defects are introduced in 3DC/Fe3O4. Furthermore, the increased ID/IG value is also associated with the generation of nanocrystalline graphite [5]. Fig. 2d exhibits the field-dependent magnetization of 3DC/Fe3O4 nanocomposites with different Fe3O4 contents, which was measured by a vibrating sample magnetometer at room temperature. Significant hysteresis loops in the M-H cures indicate the ferromagnetic of 3DC/Fe3O4 nanocomposites. The saturation magnetizations (Ms), coercivity (Hc) and remanent magnetization (Mr) are summarized in Table 1. It can be seen that Ms, Hc and Mr values for 3DC/Fe3O4 nanocomposites increase with Fe3O4 contents. The magnetic measurements demonstrate that 3DC/Fe3O4 nanocomposites are ferromagnetic and can be used to attenuate EM waves [35]. Fig. 3 displays the representative SEM images of 3DC/Fe3O4-2. From Fig. 3a, it can be seen that 3DC/Fe3O4 has an interconnected three-dimensional porous network with submicrometer-sized macropores. The porous structure ensures that 3DC/Fe3O4 nanocomposites have a low density, which will be conductive to satisfying the requirement of lightweight. Moreover, the interconnected
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Fig. 2. (a) XRD patterns, (b) TGA curves, (c) Raman spectra and (d) the hysteresis loops of 3DC/Fe3O4 nanocomposites.
Table 1 Magnetic parameters of 3DC/Fe3O4 nanocomposites. Samples
Ms (emu/g)
Hc (Oe)
Mr (emu/g)
3DC/Fe3O4-1 3DC/Fe3O4-2 3DC/Fe3O4-3
23.6 34.8 50.9
92.4 166.1 185.5
4.5 9.1 15.8
3D porous networks provide more propagation paths for EM waves. The magnified SEM image shown in Fig. 3b reveals that many uniform Fe3O4 nanoparticles are homogeneously anchored on the walls of the 3D carbon network without agglomeration. The welldistributed Fe3O4 nanoparticles on the 3D carbon can generate abundant interfacial polarization, which will enhance the loss of
EM waves. Fig. S1 shows the SEM images of 3DC/Fe3O4-1 and 3DC/ Fe3O4-3. Obviously, the microstructures of the samples are similar to 3DC/Fe3O4-2. Nevertheless, it also can be seen that the porous structures are damaged to a certain degree and the Fe3O4 nanoparticles are uneven distributed on the 3DC. The structure and morphology of 3DC/Fe3O4-2 were further investigated by TEM. As shown in Fig. 4a, an interconnected threedimensional porous networks decorated with uniform Fe3O4 nanoparticles was obtained, which was consistent with the results of SEM observation. Besides, it should be noted that the Fe3O4 nanoparticles are still tightly anchored on the 3D carbon networks without detachment and aggregation after a long time sonication for preparing the TEM samples, which indicates the strong bonding between Fe3O4 and carbon networks. Fig. 4b shows the magnified
Fig. 3. (a) SEM and (b) magnified SEM images of 3DC/Fe3O4-2.
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Fig. 4. (a, b) TEM images of 3DC/Fe3O4-2, (c) Size distribution of Fe3O4 NPs on 3DC, (d) HRTEM image of a typical Fe3O4 NP, (eeh) Elemental mapping images of 3DC/Fe3O4-2.
TEM image of 3DC/Fe3O4-2. The particle size distribution histogram shown in Fig. 4c exhibits that the Fe3O4 nanoparticles has a wider size distribution of 10e50 nm, and the average size is about 28 nm. The HRTEM image shown in Fig. 4d depicts the crystalline structures of Fe3O4 nanopaticles. The lattice fringe spacing of 0.253 nm corresponds to the (311) planes of cubic Fe3O4, which is consistent with the XRD results. Fig. 4(eeh) presents the element mapping images of 3DC/Fe3O4-2, demonstrating that C, Fe and O elements are homogeneously distributed in the sample. The TEM images for 3DC/Fe3O4-1 and 3DC/Fe3O4-3 are provided in Fig. S2. It is clear that some areas of 3DC/Fe3O4-1 have no Fe3O4 nanoparticles and 3DC/ Fe3O4-3 shows a slight agglomeration of Fe3O4 nanoparticles, which is consistent with SEM results (Fig. S1). The average size of Fe3O4 nanoparticles for 3DC/Fe3O4-1 and 3DC/Fe3O4-3 are 21 nm and 32 nm, respectively. The elemental composition and chemical binding of 3DC/Fe3O4 was further identified by XPS. As shown in Fig. 5a, the full scan XPS spectra reveals that the 3DC/Fe3O4-2 is completely composed of C, Fe and O elements. Fig. 5b depicts the high-resolution spectra of C 1s. Three peaks with binding energies of 284.6, 285.5 and 288.3 eV are originated from CeC, CeOH and C]O, respectively [36]. The O 1s spectrum, as shown in Fig. 5c, can be divided into three components. The peak positions at the binding energies of 530.1 eV, 531.7 and 533.4 eV are assigned to O2 of Fe3O4, OH and CeO, respectively [8]. The Fe 2p spectra of Fig. 5d shows two peaks at 710.9 and 724.4 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2 of Fe3O4 [37]. The electromagnetic parameters of the paraffin composites containing 20 wt% of the 3DC/Fe3O4 absorber were measured from 1.0 GHz to 18.0 GHz. The electromagnetic parameters include
relative complex permittivity (εr ¼ ε0 -jε00 ) and relative complex permeability (mr ¼ m0 -jm00 ). The real permittivity (ε0 ) and permeability (m0 ) represent the ability to store the electric and magnetic energy, while the imaginary permittivity (ε00 ) and permeability (m00 ) describe the dissipation of electric and magnetic energy [38]. As shown in Fig. 6a and b, both ε0 and ε00 of 3DC/Fe3O4 composites decreased as increasing frequency from 1.0 GHz to 18.0 GHz. Such a phenomenon exists in many carbon materials, such as carbon nanotubes (CNTs), carbon fibers (CFs) and graphene, which is called frequency dispersion behavior [6]. In addition, it can be found that both ε0 and ε00 values of the 3DC/Fe3O4 composites are larger than that of the pure 3DC (Fig. S3) and increase with increasing the Fe3O4 contents, which is opposite to the other graphene-based composites previously reported [5,39]. According to the reported literature, the value of ε00 is mainly affected by the conductivity and polarization [40]. As shown in Fig. S4, the pure 3DC has the lowest electrical conductivity, which mainly caused by the lower degree of graphitizing. As more Fe3O4 nanoparticles were introduced into 3DC, the degree of graphitizing for carbon was improved due to the catalytic effect of Fe3O4 nanoparticles, which leads to increase of electrical conductivity. According to the free electron theory, ε00 z 1/2prfε0, where r is the resistivity, f is the frequency, and ε0 is the dielectric constant of vacuum. It can be deduced that higher electrical conductivity results in higher value of ε00 . Furthermore, the combination of carbon and Fe3O4 nanoparticles improves the degree of polarization, which is also in favor of obtaining high values of ε00 [41]. On the other hand, the increased values of ε0 may be also resulted from the enhanced polarization. Fig. 6c and d illustrate the m0 and m00 values of 3DC/Fe3O4 composites. It can be seen that both m0 and m00 values exhibit slight variation for all samples, which
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Fig. 5. (a) The general XPS, (b) C 1s region, (c) O 1s region and (d) Fe 2p region of 3DC/Fe3O4-2.
indicates minor magnetic loss contributions to EM wave absorption [42]. In generally, the dielectric loss tangent (tan dE ¼ ε00 /ε0 ) and the magnetic loss tangent (tan dM ¼ m00 /m0 ) represent the contributions of dielectric loss and magnetic loss to EM wave absorption. As shown in Fig. 6e and f, it can be found that the dielectric loss tangent increases with increasing the Fe3O4 content over 4.5e18.0 GHz, indicating the incorporation of more Fe3O4 can improve the dielectric loss capacity. Furthermore, the values of tan dE for 3DC/Fe3O4 are higher than tan dM, suggesting dielectric loss makes a major contribution to the EM wave absorption over the frequency range from 1.0 to 18.0 GHz. Conventionally, the dielectric loss mechanism can be analyzed by Cole-Cole semicircles. According to the theory of Debye dipolar relaxation [42], the relationship between ε00 and ε0 can be expressed as (ε0 -(εs þ ε∞)/2)2 þ (ε00 )2 ¼ ((εs-ε∞)/2)2, where εs is the static permittivity, ε∞ is the relative dielectric permittivity at infinite frequency. The plot ε0 versus ε00 should be a single semicircle, which usually denoted as Cole-Cole semicircles. Each semicircle corresponds to one Debye relaxation process [43]. Fig. 7a displays ColeCole semicircles of all samples. It can be found that three semicircles exist in each plot, corresponding to three dielectric relaxation processes, which mainly originate from the interface polarizations between Fe3O4 and carbon, Fe3O4 and paraffin matrix, and carbon and paraffin matrix. In addition, the radius of the semicircles gradually increases with increasing Fe3O4 content, indicating that the contribution of dielectric relaxation to permittivity gradually increases [44,45]. Besides the dielectric loss, magnetic loss is also an important factor for EM attenuation. In general, the magnetic loss originates from hysteresis loss, domain wall resonance, eddy current loss, natural resonance and exchange resonance [46]. The hysteresis loss from irreversible magnetization
only generates in a strong applied field and the domain wall resonance occurs in the lower frequency region (MHz), which can be excluded in this case [43,47]. The eddy current loss can be certified by the equation m00 (m0 )2f1 ¼ 2pm0sd2/3 [15], where m0 is the vacuum permeability, s is the electric conductivity and d is the sample thickness. If magnetic loss originates from eddy current loss, the value of m00 (m0 )2f1 will remain constant over 1e18 GHz [48]. As shown in Fig. 7b, the m00 (m0 )2f1 values of all samples show fluctuation over 1e5 GHz and nearly remain a constant ranging from 5 to 18 GHz, indicating that the magnetic loss mainly result from natural resonance over 1e5 GHz and eddy current loss over 5e18 GHz. On the basis of the measured EM parameters (relative complex permittivity εr and permeability mr), the reflection loss (RL) values are calculated according to the transmission line theory [45,49],
i h Zin ¼ Z0 ðmr =εr Þ1=2 tanh jð2pfd=cÞðmr εr Þ1=2 Z Z0 RL ðdBÞ ¼ 20log in Zin þ Z0
(1)
(2)
where Z0 is the impedance of free space, Zin is the input impedance of the absorber, f is the frequency of electromagnetic waves, d is the thickness of the absorber and c is the velocity of electromagnetic waves in free space. When the values of RL are below 10 dB, 90% of EM wave energy can be absorbed, and the corresponding frequency range is defined as effective absorption bandwidth for practical application. Fig. 8 shows the calculated RL curves of 3DC/Fe3O4-1, 3DC/Fe3O4-2 and 3DC/Fe3O4-3 with different thicknesses in a frequency range from 1.0 to 18.0 GHz. As shown in Fig. 8a, the
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Fig. 6. Frequency dependence of (a) real part (ε0 ) and (b) imaginary part (ε00 ) of the complex permittivity, (c) real part (m0 ) and (d) imaginary part (m00 ) of the complex permeability, (e) dielectric loss tangent (tan dE) and (f) magnetic loss tangent (tan dM) for 3DC/Fe3O4 nanocomposites in the frequency range of 1.0e18.0 GHz.
minimum RL of 3DC/Fe3O4-1 is only 12.5 dB with a thickness of 6.0 mm, showing poor EM wave absorption property. Fig. 8b displays the RL plot of 3DC/Fe3O4-3, it can be seen that 3DC/Fe3O4-3 shows the inferior EM wave absorption performance in the frequency range of 10.0e18.0 GHz, which may be associated with the high permittivity of 3DC/Fe3O4-3. For 3DC/Fe3O4-2 (Fig. 8c), the minimum RL is 37.8 dB at 6.95 GHz with a thickness of 5.5 mm and the effective absorption bandwidth (RL < 10 dB) can reach up 5.95 GHz (11.2e17.15 GHz) with a thickness of only 3 mm, exhibiting good EM wave absorption performance. As listed in Table S1, it can be seen that the 3D/Fe3O4 nanocomposites exhibit advantages in two aspects, i.e., effective absorption bandwidth and filler loading compared with other carbon-based magnetic composites. To analyze the EM wave absorption performance of the 3DC/ Fe3O4 nanocomposites, impedance matching and attenuation constant were investigated. The impedance matching can be
evaluated by the following equation [8,50],
Z ¼ jZin =Z0 j
(3)
where Zin is the input impedance of the absorber, Z0 is the impedance of free space, Z is the modulus of the normalized characteristic impedance (Zin/Z0). When the value of Z is close to 1, most of EM waves can enter into the absorber, indicating that the ideal impedance matching can be achieved. As shown in Fig. 9(aec), the Z values of the 3DC/Fe3O4 nanocomposites were calculated with different thickness in a frequency range from 1.0 to 18.0 GHz. The Z values between 0.9 and 1.1 are marked by red lines. It can be seen that 3DC/Fe3O4-2 possess broader regions that Z values ranged from 0.9 to 1.1 than 3DC/Fe3O4-1 and 3DC/Fe3O4-3, which demonstrates an optimized impedance matching of 3DC/ Fe3O4-2.
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Fig. 7. (a) Cole-Cole semicircles and (b) eddy current loss (denoted by m00 (m0 )2f1) of 3DC/Fe3O4 nanocomposites.
Fig. 8. Reflection loss curves for (a) 3DC/Fe3O4-1, (b) 3DC/Fe3O4-3 and (c) 3DC/Fe3O4-2 with different thickness in the frequency range of 1.0e18.0 GHz.
The attenuation constant (a), representing the internal attenuation ability of the materials, can be calculated by the following formula [51]:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf ðm00 ε00 m0 ε0 Þ þ ðm00 ε00 m0 ε0 Þ2 þ ðm0 ε00 þ m00 ε0 Þ2 a¼ c (4) Fig. 9d displays the calculated attenuation constant of three samples. Evidently, 3DC/Fe3O4-3 exhibits higher a values than 3DC/ Fe3O4-1 and 3DC/Fe3O4-2, indicating that 3DC/Fe3O4-3 has a
stronger attenuation capability for EM waves, which is mainly derived from high dielectric loss [52]. Nevertheless, the impedance matching of 3DC/Fe3O4-3 is bad, as shown in Fig. 9c, resulting in poor EM wave absorption property. The 3DC/Fe3O4-2 with larger a values performs stronger attenuation ability than 3DC/Fe3O4-1. Furthermore, 3DC/Fe3O4-2 also obtained good impedance matching. Therefore, 3DC/Fe3O4-2 shows the excellent EM wave absorption performance. Recently, the quarter-wavelength matching model is usually used to explain the excellent EM wave absorption performance, which is described by the following equation [53]:
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Fig. 9. 3D representation of Z values of (a) 3DC/Fe3O4-1, (b) 3DC/Fe3O4-2 and (c) 3DC/Fe3O4-3; (d) the attenuation constant (a) for 3DC/Fe3O4 composites. The Z values between 0.9 and 1.1 are marked by red lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
tm ¼ nl=4 ¼ nc
. 4fm ðjmr jjεr jÞ1=2 ; ðn ¼ 1; 3; 5 …Þ
(5)
where fm is the peak frequency and tm is the corresponding thickness of the absorber. When the experimental values of tm and fm meet the formula, the reflected EM waves from the air-absorber interface and absorber-metal interface are out of phase by 180 , causing an extinction of them on the air-absorber interface [54]. Fig. S5 shows the calculated fitting cure of above equation and the experimental value of tm and fm. It is clear that the texp m is in good agreement with tfit m , indicating that the excellent EM wave absorption performance of 3DC/Fe3O4-2 can be partly explained by the quarter-wavelength matching model.
As a comparison, the EM wave absorption properties of pure 3DC and 2DC/Fe3O4 were investigated. The measured paraffin samples also contain 20 wt% absorbers. Fig. S6 displays the SEM and TEM images of pure 3DC. It can be seen that the micro-morphology of pure 3DC is similar to 3DC/Fe3O4-2 except for absence of Fe3O4 nanoparticles. The XRD pattern of 2DC/Fe3O4 shown in Fig. S7 confirms the present of magnetite Fe3O4. The SEM and TEM images (Fig. S8) indicate that the 2DC/Fe3O4 has a microstructure of two-dimensional carbon anchored with Fe3O4 nanoparticles. As shown in Fig. 10a, the minimum RL of pure 3DC is only 5.2 dB over 1.0e18.0 GHz, which indicates that the synergistic effect of Fe3O4 and 3DC play an important role on improving EM wave absorption property. 2DC/Fe3O4 (Fig. 10b) obtains the minimum RL of 22.9 dB
Fig. 10. Reflection loss curves for (a) pure 3DC and (b) 2DC/Fe3O4 with different thickness in the frequency range of 1.0e18.0 GHz.
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Fig. 11. Schematic illustration of EM wave propagation and attenuation in 3DC/Fe3O4 nanocomposites.
and the effective absorption bandwidth of 5.59 GHz with a 6.0 mm thickness. Obviously, 3DC/Fe3O4-2 exhibits enhanced EM wave absorption property compared with 2DC/Fe3O4. This result demonstrates that the unique 3D porous structure can attenuate EM waves effectively by providing more channels for the propagation and multiple reflection of the EM waves [6]. Based on the above analysis, it can be summarized that the enhanced EM wave absorption performance with lightweight and broadband features of 3DC/Fe3O4-2 can be mainly attributed to four factors. Fig. 11 depicts the schematic illustration of EM wave propagation and attenuation in 3DC/Fe3O4 nanocomposites. First, the good impedance matching can be obtained by changing the content of Fe3O4 in 3DC/Fe3O4, which ensures most of EM waves can enter into the EM absorbing materials. Second, the strong attenuation ability derived from high dielectric loss can transforms EM energy to heat energy. Third, the unique 3DC porous structure not only makes composites have lightweight but also provides more channels for the propagation and multiple reflection of the EM waves, which improves EM wave absorption performance. Moreover, the mechanism of quarter-wavelength matching model also contributes to the dissipation of EM waves. 4. Conclusions In summary, a novel lightweight and broadband absorber of three-dimensional carbon networks decorated with Fe3O4 nanoparticles (3DC/Fe3O4) was successfully synthesized via freezedrying and high-temperature calcination processes. The 3DC/ Fe3O4 exhibited tunable EM wave absorption property through changing Fe3O4 contents (adjusting initial iron salt concentrations directly). The minimum RL of 3DC/Fe3O4-2 (with about 38.2 wt% Fe3O4) is 37.8 dB at 6.95 GHz. A broad absorption bandwidth of 5.95 GHz (11.2e17.15 GHz) can be achieved at the thickness of 3.0 mm with only 20 wt% 3DC/Fe3O4-2 in paraffin matrix. The excellent EM wave absorption performance of 3DC/Fe3O4-2 nanocomposites can be mainly ascribed to good impedance matching, high dielectric loss and unique 3D porous structure. It is believed
that the 3DC/Fe3O4 with lightweight and broadband features has great potential for realizing practical application. Furthermore, this work also provides a new way to prepare novel carbon-based magnetic composites for EM wave absorption. Acknowledgement The authors acknowledge the financial support by the Science and Technology Support Program of Tianjin (No. 16ZXCLGX00130). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2018.10.303. References [1] Z. Chen, C. Xu, C. Ma, W. Ren, H.M. Cheng, Lightweight and flexible graphene foam composites for high-performance electromagnetic interference shielding, Adv. Mater. 25 (2013) 1296e1300. [2] J. Liu, H.B. Zhang, R. Sun, Y. Liu, Z. Liu, A. Zhou, Z.Z. Yu, Hydrophobic, flexible, and lightweight MXene foams for high-performance electromagnetic-interference shielding, Adv. Mater. 29 (2017). [3] X. Qiu, L. Wang, H. Zhu, Y. Guan, Q. Zhang, Lightweight and efficient microwave absorbing materials based on walnut shell-derived nano-porous carbon, Nanoscale 9 (2017) 7408e7418. [4] Y. Lu, Y. Wang, H. Li, Y. Lin, Z. Jiang, Z. Xie, Q. Kuang, L. Zheng, MOF-derived porous Co/C nanocomposites with excellent electromagnetic wave absorption properties, ACS Appl. Mater. Interfaces 7 (2015) 13604e13611. [5] Y. Lai, S. Wang, D. Qian, S. Zhong, Y. Wang, S. Han, W. Jiang, Tunable electromagnetic wave absorption properties of nickel microspheres decorated reduced graphene oxide, Ceram. Int. 43 (2017) 12904e12914. [6] J. Fang, T. Liu, Z. Chen, Y. Wang, W. Wei, X. Yue, Z. Jiang, A wormhole-like porous carbon/magnetic particles composite as an efficient broadband electromagnetic wave absorber, Nanoscale 8 (2016) 8899e8909. [7] R. Qiang, Y. Du, D. Chen, W. Ma, Y. Wang, P. Xu, J. Ma, H. Zhao, X. Han, Electromagnetic functionalized Co/C composites by in situ pyrolysis of metalorganic frameworks (ZIF-67), J. Alloys Compd. 681 (2016) 384e393. [8] B. Qu, C. Zhu, C. Li, X. Zhang, Y. Chen, Coupling hollow Fe3O4-Fe nanoparticles with graphene sheets for high-performance electromagnetic wave absorbing material, ACS Appl. Mater. Interfaces 8 (2016) 3730e3735. [9] H. Yuan, F. Yan, C. Li, C. Zhu, X. Zhang, Y. Chen, Nickel nanoparticle encapsulated in few-layer nitrogen-doped graphene supported by nitrogen-doped
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