iron pentacarbonyl porous films enhanced with chitosan for highly efficient broadband electromagnetic interference shielding

iron pentacarbonyl porous films enhanced with chitosan for highly efficient broadband electromagnetic interference shielding

Accepted Manuscript Magnetic, electrically conductive and lightweight graphene/iron pentacarbonyl porous films enhanced with chitosan for highly effic...

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Accepted Manuscript Magnetic, electrically conductive and lightweight graphene/iron pentacarbonyl porous films enhanced with chitosan for highly efficient broadband electromagnetic interference shielding Ji Liu, Hao-Bin Zhang, Yafeng Liu, Qiwei Wang, Zhangshuo Liu, Yiu-Wing Mai, Zhong-Zhen Yu PII:

S0266-3538(17)31687-1

DOI:

10.1016/j.compscitech.2017.08.005

Reference:

CSTE 6860

To appear in:

Composites Science and Technology

Received Date: 12 July 2017 Revised Date:

30 July 2017

Accepted Date: 7 August 2017

Please cite this article as: Liu J, Zhang H-B, Liu Y, Wang Q, Liu Z, Mai Y-W, Yu Z-Z, Magnetic, electrically conductive and lightweight graphene/iron pentacarbonyl porous films enhanced with chitosan for highly efficient broadband electromagnetic interference shielding, Composites Science and Technology (2017), doi: 10.1016/j.compscitech.2017.08.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Magnetic, Electrically Conductive and Lightweight Graphene/Iron Pentacarbonyl Porous Films Enhanced with Chitosan for Highly Efficient Broadband Electromagnetic Interference Shielding

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Ji Liu a,b, Hao-Bin Zhang a,*, Yafeng Liu a, Qiwei Wang a, Zhangshuo Liu a, Yiu-Wing Mai c, Zhong-Zhen Yu a,b,* a

Beijing Key Laboratory of Advanced Functional Polymer Composites, College of Materials

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

Technology, Beijing 100029, China c

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b

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Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Centre for Advanced Materials and Technology, School of Aerospace, Mechanical and

Mechatronic Engineering J07, The University of Sydney, Sydney, NSW 2006, Australia ABSTRACT: Highly efficient and lightweight electromagnetic interference (EMI) shielding

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materials have gained tremendous interests due to the urgent requirement for smart electronic devices and aerospace applications. Herein, we demonstrate a highly efficient hydrazine-induced foaming approach to fabricate magnetic, highly electrically conductive, and lightweight

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graphene/iron pentacarbonyl (IP) porous films for broadband EMI shielding application. The

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chitosan introduced effectively optimizes the microcellular structures by improving the interfacial adhesion between graphene sheets and thus enhances the electrical conduction of the porous films with IP flakes. The resultant porous structure not only reduces the density of the films, but also improves the electromagnetic radiation attenuation by repeated scattering of the incident microwave. The presence of magnetic IP flakes endows the magnetically responsive film with enhanced EMI shielding performance by combining the dielectric and magnetic losses. Thus, the _____________________________________________________ Corresponding author: Fax: +86-10-64428582 E-mail: [email protected] (H.-B. Zhang); [email protected] (Z.-Z. Yu)

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porous film with a small thickness of 0.3 mm and a low density of 0.12 g/cm3 exhibits an excellent broadband EMI shielding performance of >38 dB in the frequency range of 8.2-59.6 GHz with a total bandwidth of 51.4 GHz. These results indicate that the lightweight porous film

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with outstanding magnetic and electrical properties could be used as multifunctional highperformance EMI shielding materials.

Keywords: A: Functional composites; A: Structural composites; B: Electrical properties; B:

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Magnetic properties 1. Introduction

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The ubiquitous electromagnetic radiations derived from various powerful, portable and smart electronics have become serious concerns for the normal functions of electronic devices and the health of humans. Various electromagnetic interference (EMI) shielding materials and composites have been fabricated to diminish or even eliminate the adverse impact of

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electromagnetic radiations [1-4]. Apparently, conventional heavy, corrosion-susceptible metal materials cannot satisfy the requirements for EMI shielding in emerging areas such as aerospace and portable electronic devices [5]. It is still imperative to prepare lightweight, thin and flexible

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materials with highly efficient EMI shielding efficiency in broad bandwidths. Recently, carbonbased nanomaterials, especially two-dimensional graphene sheets with exceptional electrical,

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thermal and mechanical properties, have been intensively explored as nanofillers of composites or assembled architectures for EMI shielding applications [6,7]. Although polymer/graphene nanocomposites are promising for EMI shielding applications because of their lightweight and enhanced electrical conductivities, the enhancement in EMI shielding efficiency is usually hampered by the electrically insulating polymer matrices [8-11]. Lightweight is another valuable attribute for EMI shielding materials used in aerospace and smart devices. Though the inclusion of numerous microcellular pores indeed decreases the densities of the functional materials, their 2

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mechanical strength and modulus would be reduced with the enlarged volume and thickness [8]. However, this dilemma could be alleviated by preforming three-dimensional (3D) porous architectures for electrically conductive polymer nanocomposites [12], albeit decreasing the

electromagnetic waves through the architectures.

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thickness would also reduce the EMI shielding performances due to the direct transmission of the

Alternatively, freestanding and flexible graphene films compactly assembled by unique sheets

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show obvious superiorities of high electrical conductivity and small thickness over polymer nanocomposites for EMI shielding applications [13-15]. In addition, high-temperature annealing

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endows a graphene oxide (GO) film with a satisfactory EMI shielding effectiveness (SE) of ~20 dB with an extremely small thickness of 8.4 µm [13]. Incorporation of magnetic nanoparticles into graphene films is another efficient way to improve the EMI shielding efficiency with an absorption mechanism [15,16]. For example, an ultrathin (∼2 µm) reduced graphene oxide

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(RGO) film with 56 wt% of Fe3O4 nano-discs exhibited an EMI SE of ~11.2 dB in the frequency range of 2-10 GHz [16]. Song et al. fabricated a freestanding Fe3O4/graphene paper with a thickness of 0.3 mm and a density of 0.78 g/cm3, exhibiting an EMI shielding performance of up

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to 24 dB [15]. Even so, there are still some unresolved issues to be addressed: (a) further increasing the EMI shielding performance, (b) decreasing the density of graphene films,

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especially those containing heavy magnetic particles, and (c) extending the bandwidth rather than mainly focusing on the X-band frequency range. To our best knowledge, few publications have reported on the fabrication of magnetic, conductive, lightweight, thin, flexible and porous graphene films with excellent EMI shielding performances over a broad bandwidth [17]. Herein, we demonstrate a highly efficient hydrazine-induced foaming approach to fabricate magnetic, electrically conductive, flexible and lightweight graphene/iron pentacarbonyl (IP) porous films for broadband EMI shielding applications. The cellular structure of pristine porous 3

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graphene film is optimized by adding chitosan (CS) to enhance the interlayer interactions between the RGO sheets. Moreover, considering the synergetic effect of electrical and magnetic constituents on EMI attenuation, IP flakes are integrated into the conductive porous structure to

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further enhance the EMI shielding performance. The magnetic and conductive porous film with a density of 0.12 g/cm3 and a small thickness of 0.3 mm exhibits a high electrical conductivity of more than 2000 S/m and an excellent broadband EMI shielding performance of >38 dB in the full

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frequency range 8.2-59.6 GHz. The positive effects of CS on the cellular structure and the favorable contribution of magnetic IP particles on shielding performance are also addressed.

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2. Experimental 2.1 Materials

Hydrazine monohydrate (80%) was purchased from Beijing Chemical Factory (China) and graphite flakes (300 mesh) were bought from Huatai Lubricant and Sealing S&T (China). IP

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flakes (Fig. S1) and CS (200-400 mPa·s) were supplied by Nanjing University and Aladdin (China), respectively. Two types of GO sheets were prepared by the modified Staudenmaier method (s-GO) and Hummers method (h-GO), respectively (Figs. S2,S3) [18,19]. s-GO was

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finally used in the present work.

2.2 Functionalization of GO and Preparation of Modified GO/IP Films

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CS was dissolved in 2 wt% acetic acid solution with a concentration of 10 mg/mL and stirred for 24 h. After GO was dispersed in deionized water with a concentration of 1.0 mg/mL and mildly sonicated for 20 min, the CS solution was added with different GO/CS mass ratios under magnetic stirring. The GO/CS suspension was then mildly sonicated for 60 min with the addition of some NaOH to stabilize the suspension, and the resultant GO/CS composites with different CS contents were designated as m-GO-5, m-GO-10, and m-GO-15. In addition, to fabricate m-GO/IP composite film, the homogeneous GO suspension was first mixed with IP suspension with 4

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different GO/IP mass ratio under mechanical stirring for 60 min, and the CS solution was then added with a GO/CS mass ratio of 95/5. The resultant mixture was sonicated for 15 min to form a homogeneous suspension. Vacuum-assisted filtration was used to assemble the homogeneous

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suspension to m-GO/IP films and these films with different IP contents were designated as mGO/IP5, m-GO/IP10, and m-GO/IP15. For comparison, GO and m-GO films were also fabricated using the vacuum-assisted filtration process.

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2.3 Reduction of GO and Fabrication of Porous Graphene Films

Porous graphene films were fabricated by a hydrazine-induced foaming method. Typically, a

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total of 60 µL hydrazine monohydrate was coated on the surface of the film, and the coated film was then sandwiched by two ceramic wafers and kept at 90 °C in a sealed container for a few hours to obtain graphene (G), modified graphene (m-G), and m-G/IP porous films with GO, mGO, and m-GO/IP films as respective precursors.

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2.4 Characterization

X-ray diffraction (XRD) patterns were recorded with a Rigaku D/Max 2500 X-ray diffractometer using CuKa radiation (1.54 A°) at a generator current of 50 mA and a generator

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voltage of 40 kV with a scanning speed of 4°/min to monitor the structural evolution of the films. Raman spectra were obtained with a Renishaw inVia Raman microscope at an excitation

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wavelength of 514 nm to identify the formation of RGO in the porous films. The chemical compositions of GO and its derivatives were analyzed with a Thermo Fisher Escalab 250 X-ray photoelectron spectroscope (XPS). Microstructures of the nacre-like films and porous films were observed with a Hitachi S4700 field-emission scanning electron microscope (SEM). The volume conductivities were measured using a RTS-8 four-probe resistivity meter (China). Magnetic properties of m-G/IP porous films were characterized with a Lake Shore 7410 vibrating sample magnetometer (VSM, USA) at 300 K. The EMI shielding performances of the samples were 5

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measured using a Wiltron 54169A scalar measurement system at room temperature in the X-band frequency of 8-2 GHz, while their broadband EMI shielding performances were obtained with a Rohde & Schwarz ZVA67 vector network analyzer (VNA) using the wave-guide method in Ku-

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band, K-band, Ka-band, and U-band. The dimensions of the sample holder are 22.8×10 mm2 for X-band, 15.7×7.8 mm2 for Ku-band, 10.8×4.3 mm2 for K-band, 7.1×3.5 mm2 for Ka-band, and 4.7×2.3 mm2 for U band. The total SE (SETotal) and its components of absorption (SEA) and

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reflection (SER) were determined based on the measured S parameters below [1]: (1)

T = 10(S21/10)

(2)

A=1–R–T

(3)

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R = 10(S11/10)

Where R is reflection coefficient, A is absorption coefficient, and T is transmission coefficient. The total EMI SE (SETotal) is the sum of reflection (SER), absorption (SEA), and multiple

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reflections (SEM), and their relationship can be expressed in the following equation [8,20-22]: (4)

SER (dB) = -10log10(1 - R)

(5)

SEA (dB) = -10log10(T/(1 – R))

(6)

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SETotal = SER + SEA + SEM

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3. Results and discussion

3.1 Optimization of Chitosan on Graphene Based Porous Films Although graphene films have excellent electrical and EMI shielding properties [13,23,24], there is still an issue on how to balance the electrical conductivity and density of the films. Herein, a facile hydrazine-induced approach is adopted to fabricate porous graphene films [25]. We choose s-GO film as the precursor of the porous film because the porous film resulted from h-GO usually does not have a satisfactory electrical conductivity for EMI shielding application

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[17,26]. Fig. 1 shows the microstructure evolution of the unmodified GO films before (Fig. 1a) and after (Fig. 1c,e) the hydrazine-induced foaming process. Clearly, the GO film with highlyoriented GO sheets is converted to porous graphene film with random porous structures due to

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the excessive expansion derived from the weak interlayer interactions (Fig. 1c,e). Hence, CS is used to enhance the interlayer adhesion and to optimize the microcellular structure of the porous film. Indeed, the CS modified graphene (m-G) porous film (Fig. 1d,f) has distinct and continuous

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porous structures inherited from its precursor (Fig. 1b), which contrasts sharply the random structures of its unmodified counterpart (Fig. 1c,e). Additionally, some small pores are observed

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in the porous graphene film (Fig. 1f), which are beneficial for further decreasing the density of the porous film while retaining its reasonable strength and a large strain of >13.5 % (Fig. S4). It is believed that the molecular chains of CS effectively prevent the over-expansion of the film and facilitate the formation of random microcellular structure by providing strong forces to hold the

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sheets together during the hydrazine-induced foaming process (Fig. S5). The optimizing effect of CS is further confirmed by the well-defined structure of graphene/IP porous film (Fig. 2). In the m-GO/IP film (Fig. S6a,b), the IP flakes are uniformly located

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between GO sheets. After foaming, the m-G/IP porous film shows more intact surfaces. By contrast, the unmodified graphene porous film and G/IP porous film present large ruptures on

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their surfaces due to the over-expansion (Fig. S7). The microstructure observation reveals that the IP flakes are embedded within and stuck on the cell walls (Figs. 2 and S6c-f) and there are many small pores in the porous film (Fig. 2c,d). Clearly, the presence of CS effectively controls the foaming process of the graphene films by preventing the formation of large ruptures and thus facilitates the preparation of lightweight and freestanding multifunctional graphene porous films (Fig. 2e).

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To monitor the structural evolution, Fig. 3a and b compares the XRD patterns and Raman spectra of GO, m-GO, m-GO/IP films and m-G/IP porous films. The introduction of CS leads to a shift of the GO characteristic peak towards lower degrees corresponding to an increase in d-

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spacing distance due to the intercalation of CS molecular chains. Also, the broadening of the peak assigned to (002) plane indicates the slightly disordered structure. After treatment with hydrazine, the m-G/IP porous film has a new weak peak at 25.6o instead of 10.6o, suggesting the successful

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reduction of GO. The Raman spectra of GO and its derivatives (Fig. 3b) show two prominent peaks locating at 1598 cm-1 (G band) and 1340 cm-1 (D band) [27], and the presence of CS and IP

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flakes do not obviously alter the position of the main peaks. For m-G/IP porous film, the shift of the G band towards lower wavenumber and the increase in ID/IG ratio are also reflections of the reduction of GO [27]. Besides, the reductions of GO and its derivatives are also reflected in their XPS results, including the increased C/O ratios of the reduced graphene porous films and the

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greatly weakened oxygen-containing peaks (Fig. 3c,d) as compared to the unreduced GO film (Fig. S2c,d) [28]. Note that the presence of CS and IP both decrease the values of C/O ratio of films by introducing more oxygen component, affecting the reduction process (Fig. 3c,d). The

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relatively weak peak assigned to nitrogen for graphene film may come from the reduction reaction with hydrazine. Moreover, there is an additional peak at 286.2 eV corresponding to C-N

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bond and the N1s peak can be fitted into three peaks at 399.6, 400.2 and 401.1 eV, corresponding to amine, amide, and protonated amine groups, respectively [29,30]. All these results strongly suggest the formation of covalent interactions by forming the intermolecular hydrogen bonding and chemical reaction between the amine groups of CS and the carboxyl groups of GO. It is thus reasonable that the molecular chains of CS readily spread and enter the spaces between individual sheets during the filtration process, providing strong forces to hold the graphene sheets together and effectively suppress the over-expansion of the films during the foaming process. 8

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3.2 Magnetic, Electrical and EMI Shielding Properties of Graphene Based Porous Films In view of the strong dependence of the EMI shielding efficiency on electrical conductivity, the influence of CS on the electrical properties of the graphene films is explored (Fig. 4a).

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Overall, the electrical conductivity decreases with the addition of CS and IP. The m-G porous film with 5 wt% of CS exhibits a high conductivity of 2222 S/m, which is very close to that of the graphene porous film (2395 S/m) but much higher than that (213 S/m) of the graphene porous

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film prepared from h-GO [26]. The distinct differences can be explained by the different degrees of reduction of their GO components. The graphene porous film prepared from s-GO has a much

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higher C/O atomic ratio of 14.63 than the graphene porous film prepared from h-GO (8.21) (Fig. S8). The s-GO with more hydroxyl groups and higher initial C/O ratio is easier to obtain a high reduction extent by hydrazine than h-GO with more carboxyl and epoxide groups than s-GO [27,31-33]. The positive effect of CS on well-defined structures may also offset its negative

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insulating effect on the electrical conductivity of the films. The importance of CS is more clearly reflected by the dramatic increase in conductivity from 1499 S/m for the G/IP porous film to 2310 S/m for the m-G/IP porous film. Generally, the graphene porous film tends to show a lower

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conductivity than its un-foamed counterpart owing to the formed pores and generated fractures during the foaming process, and the presence of IP would also aggravate the trends by involving

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gases via decomposition or acting as a new expanding site. Surprisingly, the introduced CS can effectively optimize the microcellular structures by enhancing the interfacial adhesion and avoiding excessive expansion and hence improves the electrical conductivity of the m-G/IP porous film. Clearly, too high loadings of IP flakes would affect the well-defined structures of the porous film, ultimately decreasing the electrical conductivity. In view of the synergetic effect of the electrical and magnetic constituents on EMI attenuation, the IP flakes were integrated into the graphene porous films to further enhance the EMI shielding 9

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performances. Fig. 4b shows the magnetic properties of m-G/IP porous films at 300 K. As expected, the saturation magnetization of m-G/IP foam increases from ~6.7 to ~22.8 emu/g when the IP content is increased from 5 to 15 wt%; but this is still lower than that of neat IP (199.1

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emu/g) due to the presence of non-magnetic components of graphene and CS. The rapid response of the m-G/IP porous film to magnetic field also reflects its magnetic characteristics (Fig. 4c,d). It is seen that movements of the freestanding porous film could be easily controlled with a magnet

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owing to its lightweight feature and strong magnetic susceptibility, implying its potential applications in intelligent devices where EMI shielding is required.

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The integration of lightweight, high electrical conductivity, and strong magnetic properties makes the porous film promising as EMI shielding materials. Compared to the GO film (Fig. S9), graphene porous film reduced with hydrazine (Fig. 5a) exhibits a much better shielding performance due to the distinct electrical conductivities. Interestingly, the structure-optimized m-

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G porous film shows a comparable shielding efficiency to the pristine graphene porous film upon the addition of a small amount of CS. Note that the content of CS should be carefully controlled because the excessive insulating polymer chains would interrupt the interconnected graphene

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network and ultimately decrease the electrical and EMI shielding performances (Fig. S10). Thus, addition of a suitable amount of CS makes the graphene porous film intact and obtains well-

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defined microstructures while retaining the shielding performance to a great extent. Moreover, the benefits of magnetic IP flakes are clearly reflected by the better EMI shielding performance of the m-G/IP porous film than that (32 dB) of m-G porous film (Fig. 5a). The total EMI SE is higher than 38 dB over the whole frequency range of the X-band with a maximum value of 44 dB at 9.8 GHz. Note that although IP indeed enhances the shielding efficiency of m-G films, similar result is not found for the un-reduced GO film. In addition, further increasing the IP content

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decreases the EMI shielding performance (Fig. S11), since too many insulating IP flakes will reduce the electrical conductance by affecting negatively the conducting networks. To emphasize the contribution from dielectric and magnetic losses, Fig. S12 shows the

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complex permittivity and permeability of m-G/IP in the frequency range of 1-18 GHz. It is obvious that the real part of permittivity (ε′) and imaginary part of permittivity (ε″) of m-G/IP are higher than 17 and the dielectric loss (tan δE=ε″/ε′) is above 1.0 in the X-band, suggesting the

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strong dielectric loss properties derived from the reduced conductive graphene sheets [34]. Fig. S12b,d presents the complex permeability (µ=µ′-jµ″) and magnetic loss (tan δM=µ″/µ′) of m-G/IP,

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which confirm the contribution of the magnetic IP flakes to the overall EMI shielding performance. Often, the imaginary permeability (µ″) equals to zero for non-magnetic materials. Herein, the tan δM and µ″ curves present broad resonance peaks which almost cover the whole Xband, and the resonance peaks are ascribed to the natural resonance of the IP flakes [35]. It

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should be noted that the main microwave attenuation mechanism is the dielectric loss because the tan δM values of the porous composite film are relatively lower [36]. However, both dielectric loss and magnetic loss improve the EMI shielding performance of the m-G/IP porous film.

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The EMI shielding performance of G, m-G, and m-G/IP porous films are compared in term of SETotal, SEA, and SER (Fig. 5a). Apparently, the contribution from SEA is much larger than that

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from SER for all the films, suggesting an absorption-dominant shielding mechanism. Furthermore, the magnetic loss from the magnetic constituent has led to an extra contribution to the shielding efficiency. This can be exemplified by the higher values of SEA and SETotal and the slightly lower SER for m-G/IP porous film as compared to those of G and m-G porous films. Thus, the improved permeability, magnetic loss, and combined effects of dielectric and magnetic losses all improve the lightweight porous film’s EMI shielding performance, consistent with the results reported in different systems including rGO/Fe3O4 nano-disc hybrid film [16], epoxy/graphene composites 11

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filled with Fe3O4 and carbonyl iron [37] among others. Additionally, the porous structure also facilitates repeated scattering of the incident wave and thus helps attenuate the EM radiation (Fig. S13) [38, 39].

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Since a broad effective bandwidth is preferable for EMI shielding applications, we have also characterized the EMI shielding properties of m-G/IP porous film in Ku-band, K-band, Ka-band and U-band (Fig. 5b). The overall shielding performance is improved with increasing frequency.

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The average value of EMI SE is ~38 dB in the X-band, and increases to 47.3 dB in the Ka-band and to 50.3 dB in the U-band with a maximum value of 53.1 dB at 58.4 GHz. Consequently, the

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lightweight porous film presents excellent shielding performances, higher than 38 dB within a very broad frequency range from 8.2 to 59.6 GHz, implying that 99.99% or even 99.999% of the incident radiation would be eliminated. It should be noted that the remarkable EMI shielding capability of the conductive and magnetic porous film mainly originates from the contribution of

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absorption, again confirming the absorption-dominated shielding mechanism. Table 1 compares the EMI shielding performance of our lightweight, magnetic and porous film with those recently reported in the literature. As is well known, the electrical conductivity,

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thickness and density of a material influence its EMI shielding properties. Generally, polymer nanocomposites filled with carbon nanofillers often show moderate electrical conductivity and

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density, thus high thickness is required to achieve satisfactory shielding performance, such as 19 dB at a thickness of 2.4 mm [8], 17.3 dB at 2.5 mm [9], 11 dB at 2.3 mm [10], 21.1-49.2 dB at a thicknesses of 1-4.5 mm [40], and 19.9-57.7 dB at 20-60 mm [41]. When the shielding materials are used in areas of aircraft, spacecraft, and smart devices, they should be thin and lightweight. Recently, Shen et al. [17] fabricated a graphene foam of ~0.06 g/cm3, which shows an electrical conductivity of 310 S/m and an average EMI SE of ~25.2 dB at a thickness of ~0.3 mm. Our lightweight and magnetic m-G/IP porous film exhibits a much higher electrical conductivity of 12

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~2310 S/m and a better shielding performance (~38 dB) over X-band with a low thickness of 0.3 mm and a low density of ~0.12 g/cm3. Hence, integration of thin, lightweight, highly conductive, and highly efficient shielding characteristics makes the porous film promising for EMI shielding

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applications. 4. Conclusion

Magnetic, highly conductive and lightweight graphene/IP porous films are prepared by vacuum -

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assisted filtration followed by hydrazine-induced foaming for highly efficient broadband EMI shielding applications. The magnetic and conductive porous film with a low density of ∼0.12

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g/cm3 and a small thickness of 0.3 mm exhibits a high electrical conductivity of more than 2000 S/m. The introduced CS effectively optimizes the microcellular structures by improving the interfacial adhesion between graphene sheets and thus enhances the conductivity of the m-G/IP porous film. The presence of IP magnetic particles not only makes the porous film magnetically

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responsive, but also enhances the EMI shielding performance by combining with the electrically conductive graphene sheets. The resultant porous film possesses an excellent broadband EMI shielding performance, ~38 dB within the X-band and more than 40 dB in a very broad frequency

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range from 12.4 to 59.6 GHz, including the Ku-band, K-band, Ka-band and U-band, which is attributed to the internal multiple reflections in the highly conductive graphene network and the

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synergetic effect of both electric and magnetic constituents. Further, the magnetic responsiveness of the freestanding film indicates its potential applications in intelligent devices where broadband EMI shielding performances are required. Acknowledgements

Financial support from the National Natural Science Foundation of China (51373011, 51125010, 51533001) and the National

Key Research and Development

(2016YFC0801302) is gratefully acknowledged. 13

Program of China

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Fig. 1. SEM images of (a) GO film, (b) modified GO film, (c,e) graphene porous film, and (d,f)

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modified graphene porous film.

Fig. 2. SEM images of (a,b) microcellular m-G/IP5 porous film, and (c,d) small pores in the mG/IP5 porous film. (e) Schematic showing the effect of CS on interfacial adhesion.

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Fig. 3. (a) XRD patterns and (b) Raman spectra of GO, m-GO, and m-GO/IP films, and m-G/IP porous films; (c) XPS survey scans of Graphene, G/IP, m-G, and m-G/IP porous films; (d) C1s

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and N1s XPS spectra of m-G/IP porous film.

Fig. 4. (a) Electrical conductivities of graphene and its derivative films; (b) Hysteresis loops of m-G/IP porous films; (c,d) Digital images showing the magnetic responsibility of m-G/IP5 porous films.

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Fig. 5. (a) EMI shielding performances of graphene, m-G, and m-G/IP porous films in terms of absorption (SEA) and reflection (SER) in the X-band; (b) EMI shielding performances of the m-

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G/IP porous film in a broad frequency range of 12 to 59.6 GHz.

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Table 1. Comparison of EMI shielding performances of carbon-based porous materials Name

Density (g/cm3)

PMMA/Graphene

0.79

PS/Graphene

EMI SE (dB)

2.4

3.11

8-12

~19

8

0.27

2.5

0.22

8.2-12.4

~17.3

9

PEI/Graphene

0.29

2.3

2.2×10-5

8-12

~11

10

PI/Graphene

0.28

0.8

0.8

8-12

MWNT/WPU Foams

0.039

1-4.5

44.6

PUG Foam

0.027

~20-60

C1000

0.15

2.0

Graphene Foam

0.06

0.3

PDMS/Graphene

0.06

1.0

m-G/IP Porous Film

0.12

0.3

-

17-21

Ref.

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8.2-12.4

~21.1-~49.2

40

8.2-12.4

~19.9-~57.7

41

8.2-12.4

51.2

42

310

8.2-59.6

~25.2

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200

8-12

~20

43

2310

8.2-59.6

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Conductivity Frequency (S/m) range (GHz)

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Thickness (mm)

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This work

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PMMA: poly(methyl methacrylate); PS: polystyrene; PEI: polyetherimide; PI: polyimide; MWNT: multi-walled carbon nanotube; WPU: water-soluble polyurethane; PUG: graphenecoated polyurethane sponge; C1000: phthalonitrile-based carbon foam; and PDMS: poly(dimethyl siloxane).

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