Superb electromagnetic wave-absorbing composites based on large-scale graphene and carbon nanotube films

Graphene has sparked extensive research interest for its excellent physical properties and its unique potential for application in absorption of electromagnetic waves. However, the processing of stable large-scale graphene and magnetic particles on a micrometer-thick conductive support is a formidable challenge for achieving high reflection loss and impedance matching between the absorber and free space. Herein, a novel and simple approach for the processing of a CNT film-Fe3O4-large scale graphene composite is studied. The Fe3O4 particles with size in the range of 20–200 nm are uniformly aligned along the axial direction of the CNTs. The composite exhibits exceptionally high wave absorption capacity even at a very low thickness. Minimum reflection loss of −44.7 dB and absorbing bandwidth of 4.7 GHz at −10 dB are achieved in composites with one-layer graphene in six-layer CNT film-Fe3O4 prepared from 0.04 M FeCl3. Microstructural and theoretical studies of the wave-absorbing mechanism reveal a unique Debye dipolar relaxation with an Eddy current effect in the absorbing bandwidth.

The rapid development of modern electronic equipment and wireless devices has resulted in severe electromagnetic (EM) radiation pollution, which has implications in human health and the normal functioning of electronics. Thus, the design and exploration of effective EM wave absorption materials is of basic scientific importance [1][2][3] . EM waves consist of a time-varying electric and magnetic field, and EM wave absorption is achieved by the attenuation of EM wave energy through dielectric loss and magnetic loss as well as electromagnetic impedance match, which can minimize the reflection of the incident EM wave 4 . Therefore, dielectric loss and magnetic loss are the main factors determining absorption capability. The dielectric tangent loss (tan δ ε = ε″/ε′) is proportional to the dielectric loss, and it describes the dissipation of energy supplied by an external electric field as motion and heat 5 . Magnetic tangent loss (tan δ μ = μ″/μ′) is known as magnetic loss factor or power factor; it is a measure of the energy loss inside a magnetic material due to the phase delay between the applied and induced magnetic fields.
Good impedance matching between the specimen and free space is necessary for enhancing EM wave absorption capability. The impedance matching coefficient |ZinZ0-1| can be expressed as 6 r r r r in 0 1/2 1 /2 And the reflection loss (RL) value is calculated according to the transmission line theory by Equation (2): in 0 i n 0 Here, Z in is the input impedance of the absorber, Z 0 is the characteristic impedance of free space (377 Ω) 7 . ε r = ε′ − jε″ and μ r = μ′ − jμ″ are, respectively, the complex permittivity and permeability of the absorber, c is the velocity of the EM wave in free space, f is the frequency of the EM wave, and d denotes the absorber thickness.
Carbon nanotubes (CNTs) offer the unique potential to be excellent EM wave absorbents. Besides their superior mechanical and thermal properties 8 , the carrier mobility and current carrying capacity of CNTs is up to 10 5 cm 2 V −1 s −1 and 10 9 A cm −1 , respectively 9 . Most of the research effort to date has focused on CNTs decorated with magnetic metal or metal oxide particles for enhancing magnetic attenuation. However, the anticipated improvement in EM wave absorption performance has often been hindered by the poor dispersion of CNTs and their impedance mismatch with free space 10 . CNT films, on the other hand, not only retain the high permittivity of individual CNTs in an isotropic network structure but also greatly facilitate the dispersion of CNTs in the matrix 11 . Furthermore, the dispersion of Fe 3 O 4 nanoparticles on the surface of CNTs can effectively improve permeability and impedance matching 12 . Thus, the CNT film-Fe 3 O 4 absorber provides a highly desirable combination to enhance dielectric and magnetic loss and impedance matching as well as facilitate ease in composites processing.
The other focus of this research is on graphene, which is well known for its excellent electronic, chemical and physical properties [13][14][15] . Graphene can support current densities greater than 10 8 A cm −2 16 , its carrier mobility can exceed 15,000 cm 2 V −1 s −1 even under ambient conditions 17 , and its real and imaginary parts of the theoretical permittivity exceed 10 4 at frequency less than 400 GHz 18 . Theoretical studies of large-scale graphene layers have shown strong EM wave absorption for normal incident wave. The maximum absorption reaches 50 percent at six layers of graphene and one-third of the incident EM energy is dissipated at a graphene-polymer interface 19,20 . Although large-scale multilayer graphene/methyl methacrylate for EM wave absorption has been suggested theoretically and realized in numerical simulations [18][19][20] , large-scale graphene absorption has never been demonstrated experimentally. This is due to the fact that large-scale graphene suffers from statistical fluctuations in strength and toughness as well as impedance mismatch with free space due to the high permittivity and negligible permeability 21 . Consequently, almost all published work so far on graphene-related EM-absorbing materials has focused on graphene powder; little research has been devoted to the EM wave-absorbing performance of large-scale graphene. Moreover, in order to enhance impedance matching and absorption of both electric and magnetic fields in graphene, the improved decoration and blend of magnetic metal or metal oxide particles have become effective approaches for researchers. Some reports demonstrated that the combination of Fe 3 O 4 22-25 , TiO 2 26 and NiFe 2 O 4 27 magnetic nanoparticles with reduced graphene oxide powders improved the EM wave-absorbing property. However, the magnetic nanoparticles supported on graphene particles are generally aggregated, resulting in uneven distribution 25 . Therefore, the design and processing of high-performance EM-absorbing materials based on CNT and graphene remain a formidable challenge.
Here, a novel and simple approach was adopted to fabricate CNT film-Fe 3 O 4 composites by the solvothermal method. The Fe 3 O 4 nanoparticles on the surface of CNTs not only improved permeability but also enhanced impedance matching of the CNT film-Fe 3 O 4 -graphene composite. Then, the CNT film-Fe 3 O 4 was used as a support substrate to prepare a large-scale graphene (LSG)-CNT film-Fe 3 O 4 composite, which not only retained the dielectric performance of CNT and graphene but also avoided their aggregation.

Results
To enhance the ability of EM wave absorption, highly conductive films (thickness: 2 μm; density: 807 mg cm −3 ) with randomly oriented CNTs coated by Fe 3 O 4 nanoparticles were used as absorbing layers, the single chemical vapor deposition graphene were laid on the surface of CNT-Fe 3 O 4 film as blocking layer ( Supplementary Fig. S1), and the CNT film-Fe 3 O 4 with epoxy resin (EPON TM Resin 862) was impedance matching layer. A schematic illustration of the fabrication process for constructing CNT film-Fe 3 O 4 -graphene-epoxy composites can be found as Supplementary Fig. S2.
The CNT film-Fe 3 O 4 composites were processed using the solvothermal method. The morphology and diameters of the  28 . Figure 1g is a TEM image of a CNT film-Fe 3 O 4 -0.04 M graphene composite. Crystal planes are identified in the selected-area electron diffraction pattern (Fig. 1h), which is consistent with the XRD pattern. Figure 2a  Based on the above magnetic property data and morphology observations, CNT film-Fe 3 O 4 -0.05 M composites with various numbers of layers were fabricated. The values of tan δ ε and tan δ μ , impedance matching coefficient |Z in /Z 0 | and reflection loss of composites consisting of one to seven CNT film-Fe 3 O 4 -0.05 M layers were deduced from the measured data of relative complex permittivity ε r , permeability μ r and thickness d. The effect of the number of layers on the composite EM wave-absorbing properties was studied. The results of Fig. 4a demonstrate that strong peaks of tan δ ε appear when the frequency is higher than 6 GHz. The locations of peaks shift toward lower frequencies with an increase in the number of layers of CNT film-Fe 3 O 4 composite. The peak intensity varies with the number of layers and reaches a maximum value of 1.72 at four layers. Figure 4b depicts the variation of tan δ μ of the CNT film-Fe 3 O 4 composites with frequency and number of layers. Strong peaks of tan δ μ can be seen in the frequency range of 10-17 GHz, and the peak location also shifts toward lower frequencies with increasing number of layers. The peak intensity of tan δ μ decreases from a maximum value of 0.58 at one layer to a minimum value of 0.08 at four layers. It should be noted that the values of tan δ μ become negative in some frequency ranges, indicating that the magnetic energy radiated out from the composites due to the high carrier mobility of CNTs 7 . Overall, the above results demonstrate the dependence of the dielectric and magnetic responses on the number of layers of CNT film-Fe 3 O 4 composite. In addition, the values of tan δ ε are larger than those of tan δ ε for almost the entire frequency range, suggesting that dielectric loss plays a major role in the intrinsic EM wave absorption.
Results of |Z in /Z 0 | and RL calculated from the relative permittivity and permeability data are given in Fig. 4c and d, respectively. When |Z in /Z 0 | is close to unity, most of the EM waves can effectively enter the composite and then dissipate as heat. Figure 4c shows that when the number of layers is increased, the value of |Z in /Z 0 | of the CNT film-Fe 3 O 4 composites increases and reaches a maximum value of 1.73 at six layers and then decreases to 0.80 at seven layers. Only the values of |Z in /Z 0 | of the CNT film-Fe 3 O 4 composites with five and six layers are close to unity in the frequency range of 10-17 GHz. Figure 4d shows that for the CNT film-Fe 3 O 4 composite with five layers, the minimum value of reflection loss is −19.3 dB, and the frequency bandwidth at −10 dB is 3.6 GHz. For the CNT film-Fe 3 O 4 composite with six layers, the minimum value of reflection loss is −26.6 dB, and the frequency bandwidth at −10 dB is 2.9 GHz. However, although the CNT film-Fe 3 O 4 composites with one, two,  10 M is negative and shows a decreasing trend at frequencies higher than 8.5 GHz (Fig. 4f). Figure 4g shows that the |Z in /Z 0 | values of some composites are close to unity at frequencies higher than 11.7 GHz. The minimum values of reflection loss of the CNT film-Fe 3 O 4 -0.01, 0.04 and 0.05 M composites are -32.0 dB at 17.1 GHz, −43.6 dB at 15.7 GHz and −26.6 dB at 14.5 GHz, respectively, obviously shifting to lower frequency range with increase in FeCl 3 concentrations (Fig. 4h). For the same composites, the bandwidths of the reflection loss at −10 dB are 3.1 GHz, 3.7 GHz and 2.9 GHz, respectively; their bandwidths of reflection loss at −20 dB are 1.2 GHz, 1.5 GHz and 0.3 GHz, respectively. Although the CNT film-Fe 3 O 4 -0.10 M composite has no strong RL peak, the bandwidth at −10 dB is 2.9 GHz.
On the basis of the results summarized above, CNT film-Fe 3 O 4 -LSG composites with six layers of CNT film-Fe 3 O 4 -0.04 M and various numbers of graphene layers have been processed and studied. Although the minimum reflection loss and thickness of the CNT film-Fe 3 O 4 composite established from the above experiments are highly encouraging, the absorption bandwidth of 3.7 GHz is still relatively small. To further improve the EM wave-absorption performance, it is desirable to pursue a CNT film-Fe 3 O 4 -graphene composite in which the graphene layers are separated from one another by a thin layer of epoxy. Figure 5a and b show that tan δ ε increases and tan δ μ decreases to negative values with an increasing number of graphene layers; the ranges of tan δ ε and tan δ μ are, respectively, 0.2-1.6 and −0.4-0.1 for the frequencies of 3-18 GHz, indicating that the dielectric loss plays a major role in EM wave absorption.
In addition, EM wave attenuation in an absorber is a key factor for achieving EM wave absorption. The EM wave attenuation ability of an absorber can be characterized by the attenuation constant 30 The results of Fig. 5c show that all samples exhibit a high attenuation constant (larger than 150) in the frequency range of 7-18 GHz. The values of α increased with an increasing number of graphene layers, and an obvious peak appears when the number of graphene layers is less than four. The locations of peaks shift toward higher frequencies as the number of graphene layers increases. Figure 5d shows the variations of impedance matching  13.9 GHz. For the case of two graphene layers, the reflection loss is below −10 dB at 10.9-13.5 GHz, and the minimum value is −44.6 dB at 11.9 GHz; the bandwidths of the reflection loss at −10 dB and −20 dB are 2.6 GHz and 1.6 GHz, respectively. However, with the further increase of graphene from three to six layers, the absorbing performance deteriorates sharply; the reflection loss has no bandwidth under −20 dB, and there is only a narrow bandwidth at −10 dB.

Discussion
An important mechanism of dielectric loss, which arises from the polarization and its associated relaxation, can be evaluated by the Cole-Cole semicircle (ε″ versus ε′), and each semicircle is correlated with one Debye relaxation process 7 . The Cole-Cole semicircles of the composite samples are shown in Fig. 6a-e. For the CNT film-Fe 3 O 4 -1L LSG composite, one large Cole-Cole semicircle, which consists of many tiny semicircles, and three small semicircles can be identified in Fig. 6a. With an increasing number of graphene layers, the number of Cole− Cole semicircles increases (Fig. 6b-e), the large semicircle decreases in size, and the tiny semicircles increase in size. The presence of semicircles in the ε″ ~ ε′ relation provides the evidence that there are dual dielectric relaxation processes in the CNT film-Fe 3 O 4 -LSG composites and demonstrates that large-scale graphene improves the intensity of the Debye dipolar relaxation process 31 .
In the microwave frequency band, magnetic loss mainly comes from the eddy current effect, along with natural and exchange resonance 6 . The eddy current loss contribution to the imaginary part of permeability is related to the permeability of vacuum (μ 0 ), thickness (d) and electrical conductivity (σ) of the composite as μ″ = 2πμ 0 (μ′) 2 σd 2 f/3. If the magnetic loss results from the eddy current effect, μ″(μ′) −2 f −1 should be constant when the frequency varies. It is shown in Fig. 6f  A superb EM wave-absorbing material is expected to possess not only a high RL factor and broad absorption bandwidth, but also a fine thickness and, hence, light weight. The reflection loss, absorption bandwidth and thickness achieved in the composites of this work are shown in Fig. 7 and compared with those of previously reported graphene-and CNT-based EM wave-absorbing composites. Compared to other graphene particle or CNT-based absorbers, the absorption bandwidth achieved in this work is 0.6-161.1 percent higher than the others except for the CNT-CoFe 2 O 4 and CNT-La(NO 3 ) 3 absorbers (Fig. 7a); the composite thickness of this work is reduced by 25-70 percent except for the CNT-Ni, -Ag and -La(NO 3 ) 3 absorbers at 0.9, 1 and 1.4 mm, respectively, and the GO-Fe 3 O 4 absorber also at 1.5 mm. However, the maximum reflection loss of this work is 1.2-2.5 times the values of the above five absorbers CNT-Ni, -Ag -CoFe 2 O 4 and -La(NO 3 ) 3 , GO-Fe 3 O 4 (Fig. 7b).
In summary, we have demonstrated that composites composed of large-scale graphene and CNT film-   explanation of nanocarbon-based electromagnetic wave-absorbing composites and opens up the opportunity to apply large-scale CNT and graphene films for EM wave absorption.

Methods
Sample preparation process. CNT film was fabricated by floating catalyst chemical vapor deposition (FCCVD) method 32 . Figure 8 shows the schematics of CNT film fabrication. In this process, the feedstock of ethanol containing 1.2 vol % ferrocene and 0.4 vol % thiophene, carried by Ar/H 2 , was injected at the rate of 20 mL/h into a reactor at high temperature (~1300 °C). Ethanol, ferrocene, and thiophene were used as carbon source, catalyst precursor, and promotor, respectively. CNTs were synthesized and entangled in the hot zone forming a stocking-like aerogel. The CNT aerogel was then blown out continuously from the reactor and collected layer-bylayer by a wheel rotating perpendicularly to the gas flow, which formed macroscopic CNT film (thickness: 2 μm; density: 807 mg/cm 3 ) with randomly oriented CNTs 33 . This CNT film was further densified by spraying ethanol onto it to enhance its mechanical and physical properties.
The single-layer graphene films were synthesized on a Cu substrate using the CVD method. A Cu foil was annealed at 1000 °C for 1.5 h in a quartz tube with a 2 sccm H 2 flow. Subsequently, a 20 sccm CH 4 flow was introduced for graphene growth at 30 mTorr for 0.5 h. Then, the furnace was quickly cooled to room temperature under He flow, and the single-layer graphene on Cu foil was obtained 34 . The size of the graphen was 25 mm × 20 mm.
The CNT film-Fe 3 O 4 composites were prepared in a solvothermal system by the reduction reaction between FeCl 3 and ethylene glycol in the presence of CNT film. FeCl 3 ·6H 2 O was dissolved into 50 mL ethylene glycol followed by the addition of 3.0 NaAc (protective agent) and 10 mL ethylene diamine (EDA) to form a homogeneous solution with magnetic stirring. The mixture was then transferred to a 100 mL Teflon-lined stainless steel autoclave, and 30 cm 2 CNT film was completely immersed in the solution for solvothermal reaction at 200 °C for 10 h (Supplementary Fig. S3). After washed with deionized water five times and dried under vacuum at 70 °C for 20 min, the prepared production was cut uniformly into six pieces with dimensions 25 mm × 20 mm. The loading Fe 3 O 4 prepared at FeCl 3 concentrations of 0.01, 0.2, 0.3, 0.4, 0.5 and 0.10 M were 2, 4, 6, 6, 5 and 3 × 10 −5 g/cm 2 , respectively. The Cu foil of the PMMA-graphene-copper foil was etched away by 0.2gml −1 FeCl 3 . After rinsing with deionized water, the floating PMMA/graphene film was scooped up by CNT film-Fe 3 O 4 film. Then, the precoated PMMA was dissolved by acetone. Finally, the laminated graphene on CNT film-Fe 3 O 4 substrate was obtained.
The individual CNT-Fe 3 O 4 -graphene films were first sprayed with a diluted epoxy solution prepared by mixing an epoxy base agent (EPON TM Resin 862), curing agent (DETDA) and ethyl acetate in the ratio of 10:1:100, followed by degassing in a vacuum oven for 30 min. Next, a CNT film-Fe 3 O 4 -graphene-epoxy composite was prepared by laminating the as fabricated films by spin coating Resin 862 and DETDA in the ratio of 10:1, followed by curing at 100 °C for 1 h. The thickness of one layer of CNT film-Fe 3 O 4 -epoxy composite was about 0.25 mm.
Characterization. X-ray diffraction (XRD) patterns of the composites were measured on a Bruker D8 advance diffractometer using a Cu Kα source (λ = 0.154056 nm). Scanning electron microscopy (SEM) and transmission electron microscope (TEM) images were performed on the Auriga 60 and TEM Talos F200C, respectively. A magnetic study was performed by a vibrating sample magnetometer (VSM, Quantum Design, Versalab). The electromagnetic parameters of the samples were measured using a vector network analyzer (Agilent 8722ES) in the range of 1-18 GHz after a full two-port calibration ( Supplementary Fig. S4-S6). The composite was machined into a toroidal shaped sample with an outer diameter of 7.0 mm and inner diameter of 3.04 mm. Data Availability. All data generated or analysed during this study are included in this published article (and its Supplementary Information files).