Microcellular graphene foam for improved broadband electromagnetic interference shielding

Microcellular graphene foam for improved broadband electromagnetic interference shielding

Carbon 102 (2016) 154e160 Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon Microcellular graphene ...

2MB Sizes 0 Downloads 18 Views

Carbon 102 (2016) 154e160

Contents lists available at ScienceDirect

Carbon journal homepage: www.elsevier.com/locate/carbon

Microcellular graphene foam for improved broadband electromagnetic interference shielding Bin Shen a, Yang Li a, Da Yi b, Wentao Zhai a, *, Xingchang Wei b, Wenge Zheng a, ** a

Ningbo Key Lab of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang province, 315201, China b College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou, Zhejiang province, 310027, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 October 2015 Received in revised form 14 January 2016 Accepted 13 February 2016 Available online 16 February 2016

As reported, the foaming of layered graphene films into porous graphene foams could improve their performance for absorbents, catalysis and supercapacitors. Herein, to emphasize the impact of porous structure on electromagnetic interference (EMI) shielding, the direct comparison between graphene film (G-film) and corresponding microcellular graphene foam (G-foam) in terms of EMI shielding efficiency has been investigated in a broadband frequency range of 8.2e59.6 GHz, including X-band, Ku-band, Kband, Ka-band, and U-band. Consequently, despite the lower electrical conductivity of the as-prepared Gfoam, it exhibited an improved average shielding effectiveness (SE) of ~26.3 dB over the entire frequency range in comparison with that of G-film (~20.1 dB). Implication of the results suggested that the foaming of layered graphene films into porous graphene foams could lead to an improvement in EMI shielding, which should be ascribed to the formation of improved internal multiple reflections at the large cell ematrix interfaces owing to the existence of microcellular structure in G-foam. We believe that this research would open up new opportunity for the development of graphene foams in the field of EMI shielding. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction With the rapid development of communication technology along with electronic devices, the electromagnetic interference (EMI) problem has been increasing at a noticeable rate [1e3]. Therefore, designing new high-performance EMI shielding materials with a broadband shielding frequency range is becoming an urgent challenge to be addressed. To date, varieties of nanostructures, especially carbon-based composites, have been widely investigated in order to meet the ideal targets for EMI shielding [2e10]. Graphene, a new class of two-dimensional carbon nanostructure, possesses not only a stable structure but also a high specific surface area and excellent electronic property [11e13], which make it very promising as lightweight EMI shielding materials [14e19]. Early studies focused on dispersing graphene into the

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Zheng).

(W.

http://dx.doi.org/10.1016/j.carbon.2016.02.040 0008-6223/© 2016 Elsevier Ltd. All rights reserved.

Zhai),

[email protected]

insulating polymer matrices to form effective conductive network in composites, thereby improving the EMI shielding performance [16e23]. Despite good EMI shielding effectiveness (SE) of ~20 dB with graphene loading of ~5e15 wt%, their drawbacks, such as brittleness and large effective thickness of several millimeters, have severely hindered their application in some ultrathin flexible electronic devices. In order to take full advantage of graphene's superior electrical property, macroscopic layered graphene films with an electrical conductivity of ~180e1450 S/cm were fabricated through different process, and high EMI SE of ~20e60 dB has been achieved with effective thickness only from a few microns to dozens of micron range [24e28]. Moreover, such graphene films were also characterized by excellent flexibility [24]. Obviously, graphene films represent a more advantageous configuration in comparison with polymer/graphene composites for effective EMI shielding. More recently, the foaming of layered graphene films into porous graphene foams is attracting more and more attention since graphene foams have already exhibited greater potential in many fields including absorbents, catalysis and supercapacitors [29e32]. For instance, the introduction of porous structure in graphene film

B. Shen et al. / Carbon 102 (2016) 154e160

could increase its accessible surface area and enhance solution diffusion in electrochemistry, thereby improving the adsorption capacity of an absorbent and the specific capacitance of a supercapacitor based on graphene foams [29e31]. Because the prior work on flexible graphite (made by compressing exfoliated graphite flakes without a binder) has mentioned that a large surface area of the conductor was preferred for better EMI shielding [33], we suppose that the porous structure in graphene foam should be beneficial to improve its shielding efficiency. However, the direct comparison between graphene film and corresponding graphene foam in terms of EMI shielding efficiency, with the purpose of understanding how the porous structure of material could affect the final shielding performance, has been rarely investigated. Furthermore, although the broadband shielding frequency is important for the practical application of EMI shielding materials, most of the studies focused on the research of graphene-based materials for EMI shielding in X-band (8.2e12.5 GHz) [16e28], and much less effort has been devoted to the exploration of their performance in much broader frequency range. Overall speaking, investigating the impact of porous structure on the EMI shielding efficiency of graphene foam in a broad frequency range and further clarifying the underlying mechanism are necessary. Herein, layered graphene film (G-film) and microcellular graphene foam (G-foam) with quite similar microstructure were fabricated, and the direct comparison of their shielding performance was investigated in a broadband frequency range of 8.2e59.6 GHz, including X-band, Ku-band, K-band, Ka-band, and Uband. Despite the lower electrical conductivity of the as-prepared G-foam, it exhibited an improved average SE total of ~26.3 dB over the entire frequency range in comparison with that of G-film (~20.1 dB), indicating the fact that the foaming of layered graphene films into porous graphene foams could lead to an improvement in EMI shielding. The underlying mechanism for such improvement should be mainly attributed to the formation of improved internal multiple reflections owing to the existence of microcellular structure. Besides, the comparison between G-foam and other reported all-carbon porous materials was also conducted for the sake of highlighting the more advantageous all-carbon configuration of Gfoam. 2. Experimental 2.1. Materials preparation Graphene oxide (GO) film was prepared by direct evaporation of GO suspension under mild heating according to our previous work [24]. The G-foam was fabricated by using a hydrazine-foaming method from GO film. In a typical experiment, 3 ml hydrazine monohydrate (50%) was added into a Teflon vessel (100 ml), and then a piece of GO film was suspended above the hydrazine level in order to avoid direct wetting. After that, the vessel was sealed in a stainless steel autoclave and treated at 90  C for 3 h to obtain the Gfoam. The G-film was prepared by gentle thermal-reducing of the same GO film. In a typical experiment, the GO film was firstly heated to 90  C in order to remove the absorbed water. Then it was further heated to 180  C with a low heating rate (<5  C/min) and maintained at this temperature for some time to obtain the G-film. To ensure that the microstructure of G-film, such as structural integrity and chemical composition, is quite similar with that of Gfoam, the annealing time at 180  C should be controlled at about 2 h. 2.2. Characterizations Scanning

electron

microscopy

(SEM)

observation

was

155

performed with a Hitachi S-4800 field emission SEM at an accelerating voltage of 4 kV. The diffraction behavior of the samples was studied using a Bruker AXS X-ray diffractometer with CuKa radiation at a generator voltage of 40 kV and a generator current of 40 mA. Raman spectra were excited with a laser of 532 nm and record with Labram spectrometer (Super LabRam II system). XPS analysis was carried out a Kratos AXIS ULTRA Multifunctional X-ray Photoelectron Spectroscope using Al (mono) Ka radiation (1488.6 eV) under 1.2  109 Torron. The electrical conductivity of the samples was measured using a standard four-probe method on a Napson Cresbox Measurement System. The average electrical conductivity and corresponding errors of each sample were determined by the three measured values. 2.3. EMI shielding The S parameters (S11 and S21) of the samples were measured with a Rohde & Schwarz ZVA67 vector network analyzer (VNA) using the wave-guide method in X-band, Ku-band, K-band, Kaband, and U-band. The dimension of the sample holder is 22.8  10.0 mm for X-band, 15.7  7.8 mm for Ku-band, 10.6  4.3 mm for K-band, 7.1  3.5 mm for Ka-band, and 4.7  2.3 mm for U-band, respectively. During the measurement, the film-like samples were sandwiched between the waveguide sample holders. The total SE as well as SE absorption and SE reflection were determined based on the measured S parameters as follows:

R ¼ jS11 j2 ; T ¼ jS21 j2 A¼1RT SEref ðdBÞ ¼ 10 logð1  RÞ

SEabs ðdBÞ ¼ 10 logðT=ð1  RÞÞ

  P SEtotal ðdBÞ ¼ 10 log I ¼ SEref þ SEabs PT where R is reflection coefficient, T is transmission coefficient, and A is absorption coefficient. PI is the incident power, and PT is the transmitted power. To ensure the accuracy of the measurements, three specimens of each sample were selected for testing, and the average SE total together with corresponding errors were determined from the three measured values. 3. Results and discussion Since it is difficult to fabricate graphene foam directly from graphene film, G-foam and G-film here were fabricated by using the same GO film as a precursor. It should be noted that, to strip out other variables that could influence the final EMI shielding performance, the microstructure of G-film, such as structural integrity and chemical composition, was adjusted to the same as that of Gfoam by controlling the fabrication process. In other words, G-foam could be considered as the product fabricated by the direct foaming of G-film, and they would have quite similar microstructure and solid thickness, except for with or without microcellular structure. The overall fabrication process is shown schematically in Fig. 1a. Firstly, GO film was prepared according to the method described in our previous work [24]. SEM observation showed that such GO film exhibited a compacted layer-by-layer nanostructure with a thickness of ~20 mm, as shown in Fig. 1b. XRD analysis of GO film presented a diffraction peak at ~10.3 (Fig. 2a), indicating an interlayer spacing of 0.86 nm. In order to achieve the foaming of GO film simultaneously with the reduction of GO sheets, a hydrazine-

156

B. Shen et al. / Carbon 102 (2016) 154e160

Fig. 1. (a) Schematic representation of the fabrication process of G-film and G-foam; (bec) SEM images showing a layered nanostructure in the cross-section of GO film and G-film; (d) SEM images showing microcellular structure in the cross-section of G-foam. (A colour version of this figure can be viewed online.)

foaming method was employed. During the heat-treatment, the hydrazine vapour would initiate the chemical reduction of GO and yield gaseous species like CO2 and H2O, which could generate enough inner pressure to overcome the van der Waals forces holding GO layers together and realize an effective expansionexfoliation of GO layers [30,34], thereby resulting in the foaming of GO film into G-foam. The cross-sectional SEM views of G-foam, as shown in Fig. 1d, clearly revealed the formation of a cross-linked microcellular structure with cell size beyond several tens microns, as well as the occurrence of ca. 15-fold volume expansion with increased thickness from ~20 mm to ~300 mm. Moreover, the XRD pattern of G-foam only displayed a broad and weak peak around 25 (Fig. 2a), further indicating a less ordered state caused by the formation of microcellular structure [35]. In addition, G-film was also prepared by gentle thermal-reducing of the same GO film. SEM observation (Fig. 1c) together with XRD analysis (Fig. 2a) revealed well stacking of graphene layers with an interlayer spacing of 0.34 nm through the cross-section of G-film. Meanwhile, its thickness decreased from ~20 mm to ~15 mm, which should be attributed to the interlaminar consolidation of graphene layers during the thermal reduction [24,36]. The structural integrity and chemical composition of the samples were evaluated by Raman spectroscopy and XPS. In the Raman spectrum (Fig. 2b), two peaks at 1334 and 1590 cm1 were corresponding to the well-documented D and G bands [34]. The ID/IG intensity ratio determined by integration of the peak areas was an indicator of the degree of disorder and average size of sp2 carbon domains. Compared with GO film, the value of ID/IG increased from ~1.35 to ~1.63 for G-film and ~1.65 for G-foam, indicating the decrease in the average size of sp2 carbon domains, which could be explained by the creation of more numerous but smaller sp2 carbon domains after the reduction [37,38]. The quite similar value of ID/IG for G-film and G-foam also suggested nearly the same structural integrity in them. In the XPS analysis (Fig. 2c), the C1s scan spectrum was fitted into three carbon components with different binding energy: C]C/CeC (284.5 eV), CeO (286.0 eV), and C]O

(288.0 eV). Obviously, the intensity of oxygen groups in G-film and G-foam decreased noticeably in relative to that in GO film because of the successful reduction. Moreover, as listed in Fig. 2d, the relative atomic percentages of C]C/CeC, CeO, and C]O species for G-film were ~79%, ~16% and ~5%, which was very similar to that (~77%, ~18% and ~5%) for G-foam, strongly confirming the similar chemical composition in them. The electrical conductivity of the samples was further measured by using the four-probe method. Owing to the presence of microcellular structure that would impair the conductive network, Gfoam showed a lower electrical conductivity of ~3.1 ± 0.8 S/cm in comparison with that (~23.7 ± 3.2 S/cm) of G-film. To investigate the EMI shielding performance, the S parameters (S11 and S21) of the samples were measured with VNA using the wave-guide method in a broadband frequency range of 8.2e59.6 GHz, including X-band, Ku-band, K-band, Ka-band, and U-band (Fig. 3a). The representative SE curves of G-film and G-foam were displayed in Fig. 3b, and it is obvious that the EMI SE of each sample showed a small fluctuation with the respective frequency ranges, which should be attributed to the irregular nature of the conductive networks [10,15,39]. G-film had an average SE total of ~20.1 ± 0.9 dB over the measured bands due to its high electrical conductivity, and its qualified frequency bandwidth of SE total 20 dB (the target value of EMI SE required for practical application) was ~29 GHz as shown in Fig. 3b, covering ~57% of the entire measured bandwidth. However, despite the lower electrical conductivity of G-foam, it possessed an improved average SE total of ~26.3 ± 0.7 dB over the same bands, exhibiting an average enhancement about ~30% compared to that of G-film. Moreover, the qualified frequency bandwidth of G-foam was as wide as ~51 GHz, including the whole measured bandwidth. The above results have suggested that the introduction of microcellular structure could improve the final EMI shielding performance of G-foam. In other words, the foaming of layered graphene films into porous graphene foams could lead to an improvement in EMI shielding. To further clarify the underlying mechanism, the direct

B. Shen et al. / Carbon 102 (2016) 154e160

157

Fig. 2. (a) XRD patterns of GO film, G-film, and G-foam; (b) Raman spectrum of GO film, G-film, and G-foam with a laser of 532 nm; (c) High-resolution XPS spectrum (C 1s) of GO film, G-film, and G-foam; (d) Quantitative analysis of different carbon components calculated from the C1s spectral deconvolution. (A colour version of this figure can be viewed online.)

comparison of SE absorption and SE reflection between G-film and G-foam was also investigated, as shown in Fig. 3c. It is obvious that the value of SE absorption found in G-foam was higher than that found in G-film, while the value of SE reflection in them has not changed much, suggesting that the improvement of SE total in Gfoam was mainly attributed to the enhancement in SE absorption. We conjecture that such enhanced SE absorption in G-foam should be primarily resulted from the formation of improved internal multiple reflections owing to the presence of microcellular structure. To the best of our knowledge, EM reflection generally occurs at the corresponding interface with impedance mismatch, and larger interface area could result in enhanced EM reflection [40,41]. As illustrated in Fig. 4, compared with layered G-film, G-foam with microcellular structure could possess a larger cellematrix interface area. The incident EM waves entering the foam could be repeatedly reflected at these cellematrix interfaces due to the large impedance mismatch between air and graphene layers, which may enhance the transfer of EM energy to be dissipated as heat in the form of

micro-current [40,41], leading to the enhancement in SE absorption. On the other hand, it is anticipated that the EM waves reflecting at these interfaces could not be dissipated in whole, and small part of them, especially the ones close to the input surface of the sample, may pass through the graphene layer and escape out of the sample. As a result, more efficient EM reflection should be detected at the input interface of G-foam. However, what we actually observed was the little changed SE reflection. It may be because that the foaming process could also decease the impedance mismatch at the input interface between G-foam and air, and thus reduce its EM reflection [42]. With the effect of the above two aspects, the value of SE reflection in G-foam has remained essentially unchanged. Furthermore, to further improve the EMI shielding performance, the G-foam samples with different stacked layers were prepared and their corresponding EMI SE was investigated in X-band for an example. As shown in Fig. 5, the average SE total for single-layered sample was ~25.2 ± 0.8 dB, and this value was found to be

158

B. Shen et al. / Carbon 102 (2016) 154e160

Fig. 3. (a) The experiment setup for EMI shielding measurement; (b) SE total of G-film and G-foam in the frequency range of 8.2e59.6 GHz; Corresponding performance enhancement of G-foam were determined based on the values of G-film; (c) SE absorption and SE reflection of G-film and G-foam in the frequency range of 8.2e59.6 GHz. (A colour version of this figure can be viewed online.)

Fig. 4. Schematic representation of multiple reflections in the microcellular structure of G-foam: the incident waves entering the foam could be repeatedly reflected and scattered (green arrows) between the cellematrix interfaces (black network). (A colour version of this figure can be viewed online.)

substantially improved with increased stacking layers, reaching ~36.5 ± 0.6 dB for double-layered sample and even higher value of ~42.3 ± 0.9 dB for three-layered sample. It is clear that the shielding performance of G-foam increased with increasing the sample thickness, but the increment of SE total is not proportional to thickness increment. The further comparative analysis of SE absorption and SE reflection suggested that the improvements of SE total in G-foam samples with increased thickness mainly came from the contribution of SE absorption, since very similar SE reflection was observed for the three samples with different thickness because of the similar interfacial conditions. The above results were well consistent with those found in some previous work [43], which should be due to the fact that, for the same material, SE absorption is mainly dependent on the sample thickness, while SE reflection primarily relies on the sample interfacial

conditions. Table 1 Listed the recently reported polymer/graphene and allcarbon porous materials and their corresponding shielding performance in the X-band region. Once upon a time, the so-called specific EMI SE (the total SE value divided by density) was always used for the performance comparison between foam-like shielding materials [15,18,20]. However, it is not a scientific way since the value of the specific EMI SE would change along with sample thickness [43]. Therefore, in this work, the performance comparison through directly comparing their total SE values was conducted, and the result demonstrated that our G-foam shows a higher shielding efficiency to other polymer/graphene porous materials with much larger density or sample thickness [15,18,20,22,44], which should be due to the reason that the allgraphene G-foam could effectively eliminate the poor contact

B. Shen et al. / Carbon 102 (2016) 154e160

159

Fig. 5. SE total, SE absorption, and SE reflection of G-foam with different stacked layers in the frequency range of 8.2e12.5 GHz. (A colour version of this figure can be viewed online.)

Table 1 Comparison of the recently reported polymer/graphene and all-carbon porous materials and their corresponding shielding performance. (PMMA: poly(methyl methacrylate), PS: polystyrene, PDMS: poly(dimethyl siloxane), PEI: polyetherimide, PI: polyimide, GN-CNF: graphene nanosheet-carbon nanofiber, GAeCT: graphene aerogel-carbon texture, ALC: aerogel-like carbon). Name

Density (g/cm3)

Thickness (mm)

Conductivity (S/m)

EMI SE (dB)

Refs.

PMMA/graphene PS/graphene PDMS/graphene PEI/graphene PI/graphene GN-CNF GAeCT ALC Flexible graphite G-foam

0.79 0.27 0.06 0.29 0.28 0.08e0.1 0.07 0.112 1.1 0.06

2.4 2.5 1.0 2.3 0.8 0.22e0.27 1.0 10.0 0.79 0.3

3.11 0.22 200 2.2  105 0.8 800 e 96.4 1.3  105 310

~19 ~17.3 ~20 ~11 17e21 26e28 ~15 ~51 ~101.9 ~25.2

20 22 15 18 44 43 45 39 33 This work

between graphene sheets induced by the insulating polymers. Moreover, our G-foam also presents a distinct competitive performance to other all-carbon-based porous materials when sample thickness is in the similar order. For example, the GAeCT hybrid with density of ~0.07 g/cm3 showed an average SE total of ~15 dB at thickness of ~1 mm [43]. However, the G-foam here with lower density of ~0.06 g/cm3 (being roughly estimated by the ratio of mass to volume) exhibited a higher average SE total of ~25.2 dB at smaller thickness of ~300 mm. Such performance is also comparable to that of the GN-CNF hybrid [45], which possessed a density of ~0.08e0.1 g/cm3 and SE total of ~26e28 dB at thickness of ~260 mm, even its electrical conductivity (~800 S/m) was higher than that of G-foam (~310 S/m). Besides, it should be mentioned that the much higher SE value was also reported in ALC and flexible graphite (the frequency range of EMI shielding was 1e2 GHz) [33,39], possibly due to its much thicker sample thickness or much better electrical conductivity. The above-mentioned competitive performance of G-foam should be ascribed to its more advantageous all-carbon configuration. In detail, the GN-CNF and GAeCT hybrids have only the carbon-nanofiber-like networks interfacially modified with some graphene sheets, while G-foam here possesses the continuously cross-linked microcellular structure entirely consisted of large-area graphene layers, which could certainly enhance internal multiple reflections more effectively than the carbon-nanofiber-like networks in GN-CNF and GAeCT hybrids. In addition, more works are underway to further study the influence of cell size and cell density on the shielding performance of G-foam, so as to better illustrate the enhancement mechanism.

4. Conclusions In summary, the direct comparison between G-film and corresponding G-foam in terms of EMI shielding efficiency was investigated in a broadband frequency range of 8.2e59.6 GHz, including X-band, Ku-band, K-band, Ka-band, and U-band, and the results suggested that the foaming of layered graphene films into porous graphene foams could lead to an improvement in EMI shielding. The followed analysis indicated that such improvement mainly came from the contribution of SE absorption, which should be a result of the formation of improved internal multiple reflections at the large cellematrix interfaces. Besides, the resultant G-foam exhibited distinctly competitive shielding performance to other allcarbon porous materials, suggesting its more advantageous allcarbon configuration. Acknowledgements The authors are grateful to the financial supports from China Postdoctoral Science Foundation (2015M570531), National Natural Science Foundation of China (51473181, 61274110), and Ningbo Key Lab of Polymer Materials (2010A22001). References [1] N. Li, Y. Huang, F. Du, X. He, X. Lin, H. Gao, et al., Electromagnetic interference (EMI) shielding of single-walled carbon nanotube epoxy composites, Nano Lett. 6 (6) (2006) 1141e1145. [2] Y. Yang, M.C. Gupta, K.L. Dudley, R.W. Lawrence, Novel carbon nanotubepolystyrene foam composites for electromagnetic interference shielding, Nano Lett. 5 (11) (2005) 2131e2134. [3] D.D.L. Chung, Electromagnetic interference shielding effectiveness of carbon materials, Carbon 39 (2) (2001) 279e285.

160

B. Shen et al. / Carbon 102 (2016) 154e160

[4] Y. Yang, M.C. Gupta, K.L. Dudley, R.W. Lawrence, Conductive carbon nanofiber-polymer foam structures, Adv. Mater. 17 (16) (2005) 1999e2003. [5] A.P. Singh, M. Mishra, D.P. Hashim, T.N. Narayanan, M.G. Hahm, P. Kumar, et al., Probing the engineered sandwich network of vertically aligned carbon nanotube-reduced graphene oxide composites for high performance electromagnetic interference shielding applications, Carbon 85 (2015) 79e88. [6] A. Ameli, P.U. Jung, C.B. Park, Electrical properties and electromagnetic interference shielding effectiveness of polypropylene/carbon fiber composite foams, Carbon 60 (2013) 379e391. [7] A. Ameli, M. Nofar, S. Wang, C.B. Park, Lightweight polypropylene/stainlesssteel fiber composite foams with low percolation for efficient electromagnetic interference shielding, ACS Appl. Mater. Interfaces 6 (14) (2014) 11091e11100. [8] P. Jin Gyu, L. Jeffrey, Q.F. Cheng, J.W. Bao, S. Jesse, L. Richard, et al., Electromagnetic interference shielding properties of carbon nanotube buckypaper composites, Nanotechnology 20 (41) (2009) 415702. [9] Z. Liu, G. Bai, Y. Huang, Y. Ma, F. Du, F. Li, et al., Reflection and absorption contributions to the electromagnetic interference shielding of single-walled carbon nanotube/polyurethane composites, Carbon 45 (4) (2007) 821e827. [10] M.H. Al-Saleh, W.H. Saadeh, U. Sundararaj, EMI shielding effectiveness of carbon based nanostructured polymeric materials: a comparative study, Carbon 60 (2013) 146e156. [11] X. Du, I. Skachko, A. Barker, E.Y. Andrei, Approaching ballistic transport in suspended graphene, Nat. Nanotechnol. 3 (8) (2008) 491e495. [12] X. Huang, X. Qi, F. Boey, H. Zhang, Graphene-based composites, Chem. Soc. Rev. 41 (2) (2012) 666e686. [13] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, et al., Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (35) (2010) 3906e3924. [14] D.X. Yan, H. Pang, B. Li, R. Vajtai, L. Xu, P.G. Ren, et al., Structured reduced graphene oxide/polymer composites for ultra-efficient electromagnetic interference shielding, Adv. Funct. Mater. 25 (4) (2015) 559e566. [15] 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 (9) (2013) 1296e1300. [16] J. Liang, Y. Wang, Y. Huang, Y. Ma, Z. Liu, J. Cai, et al., Electromagnetic interference shielding of graphene/epoxy composites, Carbon 47 (3) (2009) 922e925. [17] W.L. Song, M.S. Cao, M.M. Lu, S. Bi, C.Y. Wang, J. Liu, et al., Flexible graphene/ polymer composite films in sandwich structures for effective electromagnetic interference shielding, Carbon 66 (2014) 67e76. [18] J.Q. Ling, W.T. Zhai, W.W. Feng, B. Shen, J.F. Zhang, W.G. Zheng, Facile preparation of lightweight microcellular polyetherimide/graphene composite foams for electromagnetic interference shielding, ACS Appl. Mater. Interfaces 5 (7) (2013) 2677e2684. [19] B. Shen, W.T. Zhai, M.M. Tao, J.Q. Ling, W.G. Zheng, Lightweight, multifunctional polyetherimide/[email protected] composite foams for shielding of electromagnetic pollution, ACS Appl. Mater. Interfaces 5 (21) (2013) 11383e11391. [20] H.B. Zhang, Q. Yan, W.G. Zheng, Z. He, Z.Z. Yu, Tough graphene-polymer microcellular foams for electromagnetic interference shielding, ACS Appl. Mater. Interfaces 3 (3) (2011) 918e924. [21] S.T. Hsiao, C.C.M. Ma, H.W. Tien, W.H. Liao, Y.S. Wang, S.M. Li, et al., Using a non-covalent modification to prepare a high electromagnetic interference shielding performance graphene nanosheet/water-borne polyurethane composite, Carbon 60 (2013) 57e66. [22] D.X. Yan, P.G. Ren, H. Pang, Q. Fu, M.B. Yang, Z.M. Li, Efficient electromagnetic interference shielding of lightweight graphene/polystyrene composite, J. Mater. Chem. 22 (36) (2012) 18772e18774. [23] H.B. Zhang, W.G. Zheng, Q. Yan, Z.G. Jiang, Z.Z. Yu, The effect of surface chemistry of graphene on rheological and electrical properties of polymethylmethacrylate composites, Carbon 50 (14) (2012) 5117e5125. [24] B. Shen, W.T. Zhai, W.G. Zheng, Ultrathin flexible graphene film: an excellent thermal conducting material with efficient EMI shielding, Adv. Funct. Mater

24 (28) (2014) 4542e4548. [25] W.L. Song, L.Z. Fan, M.S. Cao, M.M. Lu, C.Y. Wang, J. Wang, et al., Facile fabrication of ultrathin graphene papers for effective electromagnetic shielding, J. Mater. Chem. C 2 (25) (2014) 5057e5064. [26] L. Paliotta, G. De Bellis, A. Tamburrano, F. Marra, A. Rinaldi, S.K. Balijepalli, et al., Highly conductive multilayer-graphene paper as a flexible lightweight electromagnetic shield, Carbon 89 (2015) 260e271. [27] L. Zhang, N.T. Alvarez, M. Zhang, M. Haase, R. Malik, D. Mast, et al., Preparation and characterization of graphene paper for electromagnetic interference shielding, Carbon 82 (2015) 353e359. [28] P. Kumar, F. Shahzad, S. Yu, S.M. Hong, Y.H. Kim, C.M. Koo, Large-area reduced graphene oxide thin film with excellent thermal conductivity and electromagnetic interference shielding effectiveness, Carbon 94 (2015) 494e500. [29] S.J. Yang, J.H. Kang, H. Jung, T. Kim, C.R. Park, Preparation of a freestanding, macroporous reduced graphene oxide film as an efficient and recyclable sorbent for oils and organic solvents, J. Mater. Chem. A 1 (33) (2013) 9427e9432. [30] Z. Niu, J. Chen, H.H. Hng, J. Ma, X. Chen, A leavening strategy to prepare reduced graphene oxide foams, Adv. Mater. 24 (30) (2012) 4144e4150. [31] Y. Ma, Y. Chen, Three-dimensional graphene networks: synthesis, properties and applications, Natl. Sci. Rev. 2 (1) (2015) 40e53. [32] C. Huang, C. Li, G.Q. Shi, Graphene based catalysts, Energy Environ. Sci. 5 (10) (2012) 8848e8868. [33] X. Luo, D.D.L. Chung, Electromagnetic interference shielding reaching 130 dB using flexible graphite, Carbon 34 (10) (1996) 1293e1294. [34] B. Shen, D.D. Lu, W.T. Zhai, W.G. Zheng, Synthesis of graphene by lowtemperature exfoliation and reduction of graphite oxide under ambient atmosphere, J. Mater. Chem. C 1 (1) (2013) 50e53. [35] K. Shu, C. Wang, S. Li, C. Zhao, Y. Yang, H. Liu, et al., Flexible free-standing graphene paper with interconnected porous structure for energy storage, J. Mater. Chem. A 3 (8) (2015) 4428e4434. [36] T. Ghosh, C. Biswas, J. Oh, G. Arabale, T. Hwang, N.D. Luong, et al., Solutionprocessed graphite membrane from reassembled graphene oxide, Chem. Mater. 24 (3) (2012) 594e599. [37] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, et al., Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (7) (2007) 1558e1565. [38] Y. Zhou, Q. Bao, L.A.L. Tang, Y. Zhong, K.P. Loh, Hydrothermal dehydration for the “green” reduction of exfoliated graphene oxide to graphene and demonstration of tunable optical limiting properties, Chem. Mater. 21 (13) (2009) 2950e2956. [39] Y.Q. Li, Y.A. Samad, K. Polychronopoulou, K. Liao, Lightweight and highly conductive aerogel-like carbon from sugarcane with superior mechanical and EMI shielding properties, ACS Sustain Chem. Eng. 3 (7) (2015) 1419e1427. [40] Z. Wang, L. Wu, J. Zhou, Z. Jiang, B. Shen, Chemoselectivity-induced multiple interfaces in MWCNT/[email protected] heterotrimers for whole X-band microwave absorption, Nanoscale 6 (21) (2014) 12298e12302. [41] L. Kong, X. Yin, X. Yuan, Y. Zhang, X. Liu, L. Cheng, et al., Electromagnetic wave absorption properties of graphene modified with carbon nanotube/poly(dimethyl siloxane) composites, Carbon 73 (2014) 185e193. [42] J.M. Thomassin, C. Pagnoulle, L. Bednarz, I. Huynen, R. Jerome, C. Detrembleur, Foams of polycaprolactone/MWNT nanocomposites for efficient EMI reduction, J. Mater. Chem. 18 (7) (2008) 792e796. [43] W.L. Song, X.T. Guan, L.Z. Fan, W.Q. Cao, C.Y. Wang, M.S. Cao, Tuning threedimensional textures with graphene aerogels for ultra-light flexible graphene/texture composites of effective electromagnetic shielding, Carbon 93 (2015) 151e160. [44] Y. Li, X.L. Pei, B. Shen, W.T. Zhai, L.H. Zhang, W.G. Zheng, Polyimide/graphene composite foam sheets with ultrahigh thermostability for electromagnetic interference shielding, RSC Adv. 5 (2015) 24342e24351. [45] W.L. Song, J. Wang, L.Z. Fan, Y. Li, C.Y. Wang, M.S. Cao, Interfacial engineering of carbon nanofiber-graphene-carbon nanofiber heterojunctions in flexible lightweight electromagnetic shielding networks, ACS Appl. Mater. Interfaces 6 (13) (2014) 10516e10523.