Graphene enhanced flexible expanded graphite film with high electric, thermal conductivities and EMI shielding at low content

Accepted Manuscript Graphene enhanced flexible expanded graphite film with high electric, thermal conductivities and EMI shielding at low content Yuhang Liu, Jie Zeng, Di Han, Kai Wu, Bowen Yu, Songgang Chai, Feng Chen, Qiang Fu PII:

S0008-6223(18)30296-3

DOI:

10.1016/j.carbon.2018.03.047

Reference:

CARBON 12990

To appear in:

Carbon

Received Date: 25 January 2018 Revised Date:

6 March 2018

Accepted Date: 15 March 2018

Please cite this article as: Y. Liu, J. Zeng, D. Han, K. Wu, B. Yu, S. Chai, F. Chen, Q. Fu, Graphene enhanced flexible expanded graphite film with high electric, thermal conductivities and EMI shielding at low content, Carbon (2018), doi: 10.1016/j.carbon.2018.03.047. 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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Graphical abstract

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Graphene Enhanced Flexible Expanded Graphite Film with High Electric, Thermal Conductivities and

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EMI shielding at Low Content

Yuhang Liu a, Jie Zeng a, Di Han a, Kai Wu a, Bowen Yu a, Songgang Chai b, Feng

College of Polymer Science and Engineering, State Key Laboratory of Polymer

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a

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Chen ∗a, Qiang Fu ∗a

Materials Engineering, Sichuan University, Chengdu 610065, China. b

Shengyi Technology Co.,LTD, Dongguan, 523039, China.

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ABSTRACT

Expanded graphite (EG) films with low oxidation degree exhibit excellent thermal, electric properties and electromagnetic interference (EMI) shielding. However, the

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mechanical brittleness is the major limitation for their applications. To meet this

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challenge, in this work, a small amount of flexible graphene (GE) is introduced to endow EG films with good flexibility and mechanical properties by forming simulate shell structure. Moreover, the influence of oxidation degree and sheet thickness for carbon sheets as well as the lateral size of GE on the film performance is carefully ∗ ∗

Corresponding author. Tel: +86-28-85461795. E-mail: [email protected] (Q. Fu) Corresponding author. Tel: +86-28-85460690. E-mail: [email protected] (F.

Chen) 1

ACCEPTED MANUSCRIPT discussed. As a result, a 431% enhancement of tensile strength from 7.7 Mpa to 40.9 Mpa and little sacrifice of electric, thermal conductivities up to 1467 S cm-1 and 348 W m-1·K-1 are observed with the loading of only 10% large GE (LGE) sheets in EG

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films. Besides, high EMI shielding of 48.3 dB is achieved as the film thickness reaches 43 µm. The excellent flexibility of prepared EG/LGE films can be retained even after more than 1000 times of direct folding. These excellent properties of

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light-weight EG/LGE films, together with their advantages of environmentally

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friendly and facile large-scale fabrication, endow the films with great potential applications in next-generation commercial portable flexible electronics.

1. Introduction

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The fast development of electric devices like smart phone and palm computer has drawn great attention in recent years. However, micro-processors in these devices may generate lots of heat quickly and create high energy hot spots, which can not only

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decrease the performance but also remarkably reduce the life length of these electric

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components. Moreover, these working micro electronics are affected by the function of other nearby devices. Thus, light-weight materials, possessed with supper thermal and electric conductivities as well as high EMI shielding, are needed to meet this challenge.

Carbon materials like expanded graphite (EG), graphene nanosheets (GNS) and reduced graphene oxide (rGO) are ideally satisfied for these materials, which consist of two-dimensional structure and conjugated π-π electronic conjugation for super 2

ACCEPTED MANUSCRIPT electric, thermal transport conductivities and effective EMI shielding [1-5]. Graphene, a newly discovered 2D carbon sheet, possesses both extremely high thermal conductivity of 5000 W m-1·K-1 and electric conductivity of 6000 S cm-1 [6, 7]. Due to

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its excellent properties, thin graphene films and their devices are widely used as thermal, electric conductive and EMI shielding materials [8-11]. In recent years, the preparation of carbon films with high electric and thermal conductivities often starts

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from chemical converted graphene nanosheets (cGNS) or rGO [12-15]. However,

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cGNS or rGO often contains plenty of defects after severe oxidation, which are extremely hard to be removed. Due to incomplete reducing, the prepared films often have a relatively low electric conductivities, such as 298 S cm-1 and 72 S cm-1 reported by Cheng et al. and Li et al, respectively [13, 16]. Thus, reducing at

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extremely high temperature must be applied to recover the rest defects, named graphitizing. For example, Xiang et al, prepared polyethyleneimine (PEI) and cGNS films after thermal annealing at 340 °C for 2 hours as well as cold press, reaching an

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in-plane thermal conductivity of 178 W m-1·K-1 [17]. Tian and his cow workers

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fabricated cGNS/GE films and annealed the films at 1600 °C for 2 hours. The prepared cGNS/GE films loading with 20% GE show a thermal diffusivity of 150 mm2 s-1 [18]. Hou and his cow-workers, prepared GE/GE composite films with the observed electric conductivity of 850 S cm-1 and thermal conductivity of 220 W m-1·K-1 after thermal annealing at 1060 °C for 2 hours [19]. Shen et al, fabricated GE films with excellent electrical conductivity of 1000 S cm-1, EMI shielding of 20 dB with the film thickness of only≈8.4 µm and thermal conductivity up to 1100 W 3

ACCEPTED MANUSCRIPT m-1·K-1 via ultrahigh temperature reducing at 2000 °C for 1 hour [14]. Teng and his coworkers prepared GE films by high-speed ball milling together with vacuum filtration. After thermal reducing at 2850 °C for 2 hours and compression at 30 Mpa,

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the resulted GE films show electrical conductivity of 2231 S cm-1 and thermal conductivity of 1529 W m-1·K-1 [20]. However, due to graphitizing at extremely high temperature which leads to the brittleness for the prepared films, almost no

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mechanical properties have been reported in these studies. Besides, in order to prevent

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the rapid release of gas which may lead to the breakdown for the films at this high temperature, a slow speed of temperature rising is also needed. These extraordinary time and energy consuming as well as unpleasant processes may limit the industrial use of these films.

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EG films without any severe oxidation possess very high thermal and electric transport properties. Besides, compared with the preparation process of cGNS and rGO, the simple preparation process endows EG films with better commercial

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applications [21-23]. However, the brittle nature and low mechanical properties often

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limit the use of EG films in many conditions. Often, the tensile strength of net EG films is less than 10 Mpa [24-27]. In our design, graphene oxide (GO), containing plenty of hydrophilic functional groups and having strong π-π interaction with EG sheets, can act as surfactant to disperse EG in water. Besides, GO has abundant hydrophilic functional groups with negative electricity, which can occur electrostatic repulsion on EG surface with negative electricity. The added electrostatic repulsion can effectively prevent the aggregation of EG flakes. Thus, EG flakes packed with 4

ACCEPTED MANUSCRIPT GO shows better stability in water. Moreover, GE can retain good flexible capability and recover certain electric and thermal properties after a relatively low temperature thermal reducing. Thus, a small amount of flexible GE is added to endow EG films

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with good flexibility as well as mechanical properties by forming simulate shell structure and introducing foldable and stretchable wrinkles. Besides, the excellent electric, thermal conductivities and EMI shielding properties of EG films can be

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effectively maintained at the same time. Hence, this new design strategy leads to a

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delicate balance of mechanical and electric, thermal properties. Most importantly, this method towards flexible EG films is simple, cheap, energy saving and environmentally friendly.

In short, a small amount of flexible GE was added into the EG film to enhance

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its mechanical properties by forming simulate shell structure. Firstly, the worm-like EG was exfoliated through the combination of high-speed stirring and sonication. GO

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was added to pack onto the EG surface to disperse EG sheets in water. Then, vacuum filtration and in-situ low temperature thermal reducing (450 °C for 5 minutes) were

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applied to prepare EG/GE films. Besides, mechanical compression was used to get a denser EG/GE film. The results indicate that due to filling the gaps and introducing plenty of wrinkles by GE, the flexibility and tensile strength of EG/GE films can be notably improved by forming simulate shell structure. Meanwhile, the outstanding electric, thermal transport properties and EMI shielding of EG films are effectively kept. Moreover, the use of carbon sheets with thinner sheet thickness as matrix can greatly improve the mechanical properties at the same GE loading. Besides, due to 5

ACCEPTED MANUSCRIPT effective decrease of the intersheet contact resistance and the transportation hindrance of electrons and phonons, caused by large lateral size, the adding of LGE can remarkably improve the comprehensive properties of EG/GE films compared with

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that of adding small GE (SGE) sheets.

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2. Experimental

2.1 Chemicals.

Graphite intercalation compound (GIC) was purchased from Qingdao Jin Ri Lai

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Graphite Co. Ltd. (50 mesh: 270 µm; 200 mesh: 75 µm). The GNS (XTG-P-0762) was obtained from the De yang Carbonene Co. Ltd. China. Potassium permanganate

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(KMnO4, AR >99.5%), Sodium nitrate (NaNO3, AR, >99%), Concentrated sulfuric acid (H2SO4, AR, 95%-98%), Hydrogen peroxide (H2O2, AR, 30%) were obtained

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from Kermal Chemical Reagent Plant, China. All chemicals were directly used without further purification.

2.2 The preparation of GO with different lateral size.

An improved Hummers method was used to prepare ultralarge GO sheets [28]. Firstly, GIC was sieved by using a 50 mesh (270 µm) and 60 mesh sifter (250-300 6

ACCEPTED MANUSCRIPT µm). Then, thermal expansion at 800 ºC for 5 minutes was applied to prepare EG in muffle furnace. For typical GO preparation, 1 g EG, 0.5 g NaNO3 and 100 ml H2SO4 were cooled in an ice water bath with magnetic stirring. 8 g KMnO4 was divided into

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six parts and added into the reaction suspension once every 10 minutes to avoid the temperature rising over 20 ºC. Then, the mixture was further heated to 35 ºC (In 8 minutes) and retained for 2 hours. 200 mL deionized water was slowly added into the

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reaction suspension to avoid the temperature exceeding over 40 ºC. The mixture was

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further oxidized in a water bath set at 98 ºC for another 15 minutes. After the end of reaction, the mixture was put in an ice water bath to cool quickly and plenty of H2O2 was added to remove the unreacted KMnO4 until no bubbles produced. Finally, repeated centrifugation was used to remove the remaining acid. A golden yellow GO

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suspension was obtained.

The obtained GO sheets had a large average lateral size of more than 50 µm, named large GO (LGO) sheets. Then, the medium GO (MGO) sheets were prepared

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by adding extra 8 minutes sonication at 300 W, resulting in an average lateral size

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from 5 to 10 µm. While small GO (SGO) sheets were produced by sonication at 300 W combined with high-speed stirring at 8 kr/min for 30 minutes, leading to an average lateral size of less than 1 µm.

2.3 The preparation of EG/GE films. EG was prepared from GIC (200 mesh: 75 µm) through directly expanding in muffle furnace at 800 ºC for 5 minutes. In order to be contrastive, GNS was also 7

ACCEPTED MANUSCRIPT treated in the same condition. For typical EG/GE films, EG was exfoliated into sheet like through the combination of sonication at 300 W and high-speed stirring at 8 kr/min for 30 minutes. In order to retain the original lateral size of GO, the prepared

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GO with different size distribution was added by magnetic stirring for 15 minutes to form a uniform suspension. Then, the suspension was filtrated by a PTFE membrane (With a diameter of 50 mm and 0.45 µm pore size) by vacuum assisted filtration and

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the films were further dried in a vacuum oven at 60 ºC for 12 hours. The prepared

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EG/GO films were then reduced at 450 ºC for 5 minutes. Finally, a simple 20 Mpa for 5 minutes compression was applied to remove the bubbles formed by thermal reducing and a denser EG/GE film was prepared (Film thickness decreased from 500 µm to 45 µm). The schematic diagram of EG/GE film preparation is shown in Fig. 1.

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The sample name is recorded as EG-GO-X, X represents the weight fraction of GO.

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After thermal reducing, the sample name is identified as EG-GE-X.

Fig. 1. Schematic diagram of the preparation of EG/GE films.

The typical TGA curves show two characteristic reducing peaks of GO (Fig. 2a). The peak around 160 ºC belongs to the removing of functional groups like hydroxyl 8

ACCEPTED MANUSCRIPT groups, carboxyl groups, epoxy groups while the peak at 266 ºC refers to organic sulfur [29]. In order to reduce most of oxygen functional groups in GO, 450 ºC for 5 minutes is applied. After thermal reducing, the C/O ratio increases from 2.4 to 8.76 by

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XPS spectrum, indicating some stable oxygen functional groups can not be removed at this condition (Fig. 2b). Further rising the temperature can remove the rest of oxygen functional groups but the weak point is that graphitizing of GE films can

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cause the loss of flexibility. From TGA curves, a 5% weight loss is observed for

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EG/GO-10, caused by the reducing of GO while EG and EG/GE-10 show almost no weight loss up to 500 ºC, indicating GO in EG films can be effectively reduced (Fig. 2c). Besides, the characteristic XRD peak of GO at 10.4° (Corresponding to the

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interlayer spacing of 8.5 Å) disappears, which also confirms GO is reduced (Fig. 2d).

Fig. 2. (a) TGA curves of GO; (b) XPS data of GO and GE. EG, EG/GO-10 and EG/GE-10 (EG: net EG films): (c) TGA curves; (d) XRD patterns. 9

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2.4 Materials characterization. Scanning electron microscopy (SEM) image was carried out on SEM (JEOL

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JSM-5900LV) at an accelerating voltage of 5 kV. The sample was produced by spin coating on Indium-tin oxide glass (ITO) (0.5 kr/min for 3 seconds and 3 kr/min for 20 seconds). X-Ray diffraction was performed on an X-ray diffractometer (Philips X’Pert

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pro MPD, Holland) with a Cu Kα radiation (λ = 1.5406 Å). All GO samples were

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obtained through freeze-drying. X-ray photoelectron spectroscopy (XPS, AXIS Ultra, KRATOS) was used to characterize the elemental compositions (Including C, O). The mechanical properties were tested on a Dynamic mechanical analyzer (DMA Q800 analyzer TA instruments, U.S.A.). All the samples were processed with 20 mm in

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length and 3 mm in width. The samples were gripped using film tension clamps with a clamp compliance of about 1 N. All tensile tests were conducted in a controlled force mode with a preload force about 0.1 N and a force ramp rate of 0.5 N/min. The

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electric conductivity was measured by a 4-probe method and the equipment was

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bought from 4-probe Technology Ltd, China. The measurement of thermogravimetric analyzer (TGA) was performed on TG 209F1 Iris (Netzsch, Germany) under N2 atmosphere with a heating rate of 10 °C/min. To perform the thermal property tests, the value of in-plane thermal conductivity (K) was calculated by the following equation: K = TD × Cp × ρ

(1)

The LFA 447 Nano Flash was used to measure the in-plane thermal diffusivity (TD) 10

ACCEPTED MANUSCRIPT in the previous equation (1). The density (ρ) of these samples were calculated by directly measuring the mass and volume. Besides, specific heat (Cp) was characterized by DSC 1/700 (Mettler-Toledo), in which the specimen was heated at the rate of

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10 °C/min from 20 ºC to 80 ºC, using sapphire as reference sample. The test of frequency for EMI shielding was at the X-band (8.2-12.4 GHz frequency) and all samples were uniformly cut for a diameter of 13 mm. The total EMI shielding (SETotal)

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was calculated by the addition of the reflection (SER) and absorption (SEA) of

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microwave. The power coefficients of absorption (A), transmission (T), and reflection (R) can be obtained from the measured scattering parameters mentioned above and the relationship is described as R+A+T=1. The SER, SEA, and SETotal of the sample can be calculated as follows respectively:

SER= -10log(1-R) SEA= SETotal-SER-SEM

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SETotal= -10logT

(2) (3) (4)

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The SEM is the microwave multiple internal reflections, which can be ignored when

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SETotal ≥ 15 dB.

3. Results and discussion

3.1 The properties of EG/GE and GNS/GE films. Two commercial carbon materials were used to prepare the net carbon films in this study. One is thermally expanded EG while the other is liquid-phase exfoliated GNS. 11

ACCEPTED MANUSCRIPT In order to bring into correspondence with EG, GNS is also thermally treated at 800 °C for 5 minutes. To further characterize EG and GNS, SEM, XRD as well as Raman spectroscopy were employed. The typical SEM images reveal the worm-like

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EG and ball-like GNS are completely exfoliated to sheet like through high-speed stirring exfoliation (Fig. 3a, b and Fig. S1a, b). However, the stiff EG sheets indicate the sheet thickness is still thicker than flexible GNS, which contains many wrinkles

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on its basal planes and edges. The XRD diffraction patterns of both carbon materials

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exhibit a sharp 2 theta degree peak at 26.5°, corresponding to the (002) interlayer spacing of graphite [30]. But compared with EG, the peak of GNS is sharply reduced, indicating the GNS sheets contain less layers (Fig. 3c). C/O ratio is carefully calculated from XPS spectrum (Fig. S2a). GNS includes twice oxygen content than

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EG, which is difficult to be removed at 800 °C thermal annealing. Besides, the ratio of D band (sp3 hybridized carbon defects) and G band (sp2 hybridized carbon double bonds) is collected by Raman spectroscopy (Fig. 3d) [31]. The result indicates that

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GNS contains more defects due to further oxidation degree. Interestingly, EG with AB

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Bernal stacking is reflected by a highly asymmetric G′ band that can be fitted into two Lorentzian peaks (G′3DA and G′3DB) (Fig. S2b), especially like pristine graphite. However, due to the random overlaps of GNS sheets, GNS shows a completely symmetric G′ band that can only be fitted into a single Lorentzian peak (G′2D), revealing the appearance of turbostratic stacking between carbon layers, which also confirms GNS sheets are thinner than EG [32]. SEM images are applied to roughly characterize the sheet thickness. The sheet thickness of EG is more than 400 nm while 12

ACCEPTED MANUSCRIPT GNS is around 15 nm (Fig. S1c, d). To conclude, GNS owns thinner sheet thickness

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but contains higher oxidation degree and defects than EG (Table 1).

Fig. 3. SEM images: (a) EG; (b) GNS. EG and GNS after exfoliation: (c) XRD

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Table 1

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patterns, (d) Raman spectrum.

Comparison of EG with GNS Sheet thickness

C/O ratio

ID/IG

EG

400 nm

64.5

0.107

GNS

15 nm

32.1

0.157

In film preparation, the uniform dispersity of carbon materials in the solution 13

ACCEPTED MANUSCRIPT often has a great influence on the film performance. Thus, organic solvents or surfactants are widely used in the exfoliation process and the dispersion of carbon materials due to their hydrophobicity [33-36]. But the poisonous organic solvents and

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surfactants which are difficult to be removed are harmful to the environment and may influence the properties of the resulted films. Thus, GO, bearing plenty of hydrophilic functional groups like hydroxyl groups, carboxyl groups and having strong interaction

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with carbon materials due to π-π interaction force, is the best matched surfactant for

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carbon materials. Besides, GO has abundant hydrophilic functional groups with negative electricity, which can occur electrostatic repulsion on EG surface with negative electricity. The added electrostatic repulsion can effectively prevent the aggregation of EG flakes. Thus, EG flakes packed with GO shows better stability in

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water. Furthermore, after in-situ thermal reducing, GE can recover certain electric and thermal conductivities, which hardly effects the properties of carbon matrix. Thus, in this study, we employed GO as surfactant to disperse EG and GNS in water. In order

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to be convenient, GO and EG were mixed all together at the beginning of sonication

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combined with high-speed stirring in this part. Thus, the resultant GO sheets are SGO. The digital photos exhibit that the adding of GO can greatly improve the hydrophile of EG or GNS in water (Fig. 4a and Fig. S3a). The suspension of EG or GNS without GO is layered right away due to hydrophobicity. Once the GO loading reaches more than 5%, EG or GNS can be uniformly dispersed in water for a long time. Another phenomenon which should be noticed is that the supernatant of whether EG/GO-2 or GNS/GO-2 shows no color in sight while water dispersed with only the corresponding 14

ACCEPTED MANUSCRIPT concentration of GO is yellow, indicating GO can successfully pack onto the surface of EG or GNS, caused by strong π-π interaction between EG and GO. Thus, the dispersity of EG and GNS in water can be greatly improved. The digital photos

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exhibit that the net EG films are very brittle (Fig. 4b). In contrast, the adding of only 2% GO can greatly enhance the flexibility of EG films, more remarkable with the increasing of GO loading (Fig. S4a). Besides, after thermal reducing, the flexibility

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originally show excellent mechanical flexibility.

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can be retained. Thanks to the thin sheet thickness and wrinkles, net GNS films

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Fig. 4. Digital photos of (a) EG/GO suspensions; (b) EG/GO and EG/GE films

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(Diameter: 40 mm).

For an industrial application of high-performance films, the mechanical

properties often play a key role. The typical strain-stress curves of EG and GNS films with different GO or GE loading are shown in Fig. 5a-f. It is evident to find both the adding of GO or GE can remarkably increase the mechanical properties. The net EG films are very brittle, with a small elongation at break of 0.35% and tensile strength of 15

ACCEPTED MANUSCRIPT 7.7 Mpa. Due to bad dispersion in water, EG/GO-2 also shows low mechanical strength (Fig. 4a). However, with more than 2% GO loading, the mechanical properties of the prepared EG/GO films are notably improved and continually

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increase with GO content increasing. Compared with the net EG films, a 133% enhancement in tensile strength and 210% in elongation at break are observed for EG/GO-10. Interestingly, after thermal reducing, the tensile strength further increases

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with the sacrifice of elongation at break. Compared with EG/GO-10 before reducing,

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a 49.8% enhancement in tensile strength and 48.1% decrease of elongation at break are discovered for EG/GE-10. With the adding of GO or GE, the same enhancement tendency for GNS/GO and GNS/GE films is discovered. However, the GNS/GE films originally show remarkably stronger mechanical properties than EG/GE films

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(Typical, tensile strength: from 26.9 Mpa to 57.1 Mpa; elongation at break: from 0.56% to 1.46% for EG/GE-10 and GNS/GE-10). The enhancement can be attributed to thin sheet thickness and wrinkles, providing more contact area for each carbon sheet and

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bending space (As confirmed by SEM image: Fig. S5a). The key role of wrinkles will

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be carefully discussed in the following passages.

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Fig. 5. Mechanical properties: typical stress-strain curves of (a) EG/GO films; (b) EG/GE films; (c) GNS/GO films and (d) GNS/GE films. (e) Elongation at break; (f) Tensile strength.

The mechanical properties of free-standing films, consisting 2D flaky materials, often depend on two factors: the lapping degree and interaction force between flaky sheets. And the two influence factors are carefully studied to explain the enhancement 17

ACCEPTED MANUSCRIPT mechanism of GO and GE in EG films. The reason can be explained as follows: due to hydrophobicity of net carbon materials and bad dispersity in water, the cross-sectional morphology of the net EG films show cluttered structure from SEM

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images (Fig. 6a). Adding GO to pack on EG sheet surface can greatly enhance the dispersity of EG in water. The cross-sectional morphology exhibits more ordered structure with the increasing of GE content, caused by stable dispersity and the

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directional flow of water by filtration. Thus, the improved mechanical properties can

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be attributed to the good sheet lapping after adding GO as surfactant. Besides, due to weakening the interaction force between EG and GO sheets, small molecules like water are also an important factor in influencing its mechanical properties. GO, bearing lots of hydrophilic functional groups, often contains bound water and can

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absorb water from air. Thus, the bond water is difficult to be fully removed. TGA is performed to carefully characterize the water content of the prepared EG/GO and EG/GE films (Fig. 6b). Due to the hydrophilic nature of GO, the adding of GO can

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greatly increase the water content of EG/GO films. A 2.5% water content is

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discovered for adding 10% GO while EG/GE-10 contains less than 1% water after reducing. The small amount of water can greatly weaken the interaction force between EG and GO sheets at interface, resulting in increased elongation at break but little improvement of tensile strength. Moreover, π-π interaction between GO or GE and EG is also an important influence factor. In order to quantitatively analyse the functional group changing in GO and GE, XPS spectrum is carried out (Fig. 6c). The binding energy of C-C/C=C bond is assigned at 284.6 eV. The C 1s spectrum of GO 18

ACCEPTED MANUSCRIPT and GE sample demonstrates four types of carbon bonds: C-C/C=C (284.6 eV), C-O (286.6 eV), C=O (287.8 eV), O-C=O (289 eV) [37]. Compared with GO, the characteristic peaks of oxygen functional groups are significantly reduced. Besides,

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the mole fraction of C-C/C=C is increased from 44.2% to 72.8%, indicating some destructive carbon double bonds are repaired. Besides, the ratio of D band (sp3 hybridized carbon defects) and G band (sp2 hybridized carbon double bonds) is

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greatly reduced from 2.13 to 0.613 by Raman spectrum (Fig. 6d). Thus, many

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destructive carbon double bonds are recovered through thermal reducing. The recovered π-π interaction results in strong interaction force between EG and GE sheets. Hence, after thermal reducing, the tensile strength is remarkably improved with the sacrifice of elongation at break. To explain the enhanced flexibility of

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EG/GE films, SEM images are taken to investigate the surface of EG/GE films (Fig. 6e, f). Due to thick sheet thickness, many micro cracks are formed by the direct stack of EG sheets. These cracks easily fracture when folding or stretching. As expected,

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the adding of flexible GE can fill the gaps by forming simulate shell structure.

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Besides, the introduced wrinkles by GE can be stretched when folding or stretching. So, the prepared EG/GE films show excellent flexibility compared with the net EG films. The schematic diagram of foldable and stretchable wrinkles is shown in Fig. 6g.

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Fig. 6. (a) SEM images of cross-sectional morphology of EG/GE films (Scale bar: 25

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µm); (b) TGA curves of EG/GO and EG/GE films; (c) XPS spectrum of GO and GE; (d) Raman spectrum of GO and GE; (e, f) The cross-sectional morphology of EG and

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EG/GE-10; (g) The schematic diagram of foldable and stretchable wrinkles.

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The prepared EG/GE and GNS/GE films were evaluated for their electric

conductivity by 4-point probe method and in-plane TD value at room temperature on laser method (Mechanism: Fig. S6a). Pure EG films reach very high TD up to 295 mm2 s-1 with electric conductivity of 1715 S cm-1 (Fig. 7a, b and Fig. S7a). Due to higher oxidation degree and more defects on their basal planes, net GNS films have a relatively low TD (175 mm2 s-1) and electric (1042 S cm-1) transport properties (Fig. 7c). The gaps, defects and insulators in conductive network will greatly hinder the 20

ACCEPTED MANUSCRIPT transportation of electrons and phonons, resulting in decreased electric and thermal conductivities [38-41]. Despite the fact that the adding of GO is well controlled in a small amount, GO with very high oxidation degree is almost electric and thermal

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insulation. Thus, introducing GO into EG conductive network can greatly increase the transportation hindrance of electrons and phonons. Typically for EG films, with the increasing of GO content, the electric and thermal conductivities show apparently

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decreasing trend. The adding of 10% GO reduces 75% electric conductivity and 41%

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TD property. Notably, after thermal annealing, GO is effectively reduced to GE. The recovered carbon double bonds endow GE with certain electric and thermal conductivities, remarkably reducing the transportation hindrance of electrons and phonons (Schematic diagram: Fig. 7e). Thus, the electric and thermal conductivities

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are greatly improved after thermal reducing. However, the conductivities of GE reduced at this temperature is lower than EG. Thus, the conductivities of EG/GE films are still lower than pure EG films. Typically, with the loading of 10% GE in EG, a 46%

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(≈ 922 S cm-1) reducing for electric conductivity and 34% reducing for TD property

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(194 mm2 s-1) are discovered. The high TD value of the flexible EG/GE and GNS/GE films is competitive to many highly thermal conductive metals, such as aluminum (TD ≈ 84 mm2 s-1) and copper (TD ≈ 112 mm2 s-1). Theoretically, the thermal conductivity is proportional to the TD with a proportionality constant K, where K = TD × Cp × ρ. The experimentally measured density (ρ) was carefully tested for every sample (Fig. S8a). The concentrated density of EG/GE and GNS/GE films is around 1.8 g cm-3 (±0.1 g cm-3). Besides, the specific heat of EG is also tested by DSC 21

ACCEPTED MANUSCRIPT measurement to be 0.749 J g-1·K-1 at 25 °C (Fig. S6b). Thus, the in-plane thermal conductivity of EG/GE-10 is 267 W m-1·K-1 and GNS/GE-10 is 172 W m-1·K-1. Infrared thermal image visually manifests that the heat-transfer speed of EG/GE-10 is

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clearly faster than the rest samples (Fig. 7d).

Fig. 7. EG and GNS films: (a) Electric conductivity, (b, c) In-plane thermal

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conductivity, (d) Infrared thermal image and corresponding temperature distribution (From left to right: EG/GO-10, EG/GE-10, GNS/GO-10, GNS/GE-10), (e) The

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schematic diagram of electric and heat flow after adding GO or GE.

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3.2 The properties of EG/GE films with different GE size. EG films, possessing excellent electric and thermal conductivities but very brittle,

are chosen as the matrix. GO sheets with different size distribution are applied to strengthen the EG films. First, LGO sheets are prepared through an improved Hummers method by using large graphite flakes and have a weight average size of more than 50 µm by SEM images (Fig. 8a-c). After 8 minutes sonication at 300 W, the sheet size decreases to 5-10 µm, named MGO. The SGO sheets are produced through 22

ACCEPTED MANUSCRIPT the combination of 300 W sonication and 8 kr/min high speed stirring for 30 minutes, resulting in a sheet size less than 1 µm. The SGO sheets are the previously used one. All the GO is added into the EG suspension via magnetic stirring to retain the original

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lateral size.

Fig. 8. SEM images of: (a) SGO; (b) MGO; (c) LGO.

Fig. 9a depicts the typical stress-strain curves of EG films with different sized

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GO or GE sheets. By adding 10% GO, the mechanical properties of EG films show notable improvement. But it is worth mentioning that the mechanical enhancement for

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adding SGO and MGO is almost the same. However, adding LGO exhibits distinct promotion. Compared with net EG films, the corresponding improvements for

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EG/LGO-10 in elongation at break and tensile strength are equally notable, namely 355% and 161%. After reducing, the same enhancement trend as EG/GO-10 and EG/GE-10 is discovered. Compared with net EG films, an improvement of 179% elongation at break and 429% tensile strength is observed for EG/LGE-10. Besides, the mechanical property of EG/LGE-10 is much better than EG/MGE-10 from 32.66 Mpa to 40.92 Mpa. Then, 180º direct folding is used to investigate the flexibility of EG/GE films (Fig. 9b, c). The EG films filled with SGE break into pieces 23

ACCEPTED MANUSCRIPT immediately after the first circle of folding. The same phenomenon is revealed for EG/MGE-10. After less than 5 times folding circles, EG/MGE-10 breaks down into pieces. Interestingly, EG/LGE-10 exhibits excellent folding capability. The micro

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structure can be completely retained even after more than 1000 times folding (Fig. 9d). The reason can be attributed to that LGE performs better mechanical property due to larger aspect ratio and contains more wrinkles, confirmed by SEM images (Fig. 8a-c)

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[42]. These wrinkles can be released when folding or stretching.

24

ACCEPTED MANUSCRIPT Fig. 9. EG films with 10% GO or GE of different size:(a) Typical stress-strain curves; (b-d) Folding property.

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Electric and thermal transportation properties were measured by the methods mentioned above (TD, film thickness and density: Fig. S9a-c). LGO with huge aspect ratio can easily act as obstruction between adjacent EG sheets. The electric and

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thermal conductivities of EG/GO films show decreasing trend with the GO size

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increasing (Fig. 10a, b). However, after thermal reducing, EG/GE films exhibit better electric and thermal conductivities with GE size increasing. The reason can be attributed to the following two factors: due to large aspect ratio, LGE forms less intersheet junctions that act as tunneling barriers, which can greatly decrease the

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electric and thermal resistances. Besides, LGE contains less defects due to fewer edges compared with SGE [42, 43]. Thus, the transportation hindrance of electrons and phonons is remarkably reduced. Typically, EG/LGE-10 exhibits excellent electric

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conductivity (1467 S cm-1) and in-plane thermal conductivity (348 W m-1·K-1),

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compared with EG/MGE-10 of electric conductivity (1177 S cm-1) and in-plane thermal conductivity (306 W m-1·K-1). The high in-plane thermal conductivity of EG/LGE-10 up to 348 W m-1·K-1 is competitive to many highly thermal conductive metals, such as aluminum (K ≈ 237 W m-1·K-1) and copper (K ≈ 401 W m-1·K-1). But the density of EG/LGE-10 of 1.84 g cm-3 is far below copper (Density ≈ 8.96 g cm-3) and aluminum (Density ≈ 2.70 g cm-3). Moreover, the conducting function would endow the EG/GE films with excellent EMI shielding performance. The EMI 25

ACCEPTED MANUSCRIPT shielding of EG films filled with different sized GE (around 43±3 µm thickness) was investigated (Fig. 10c, d). All the EG/GE films show light frequency-dependent EMI shielding. Thus, the average EMI shielding can be used to evaluate the EMI shielding

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effect. The total EMI shielding (SEtotal) is calculated by adding microwave reflection (SER), microwave absorption (SEA), and multiple reflections (SEM). Usually, the SEM is assumed to be very weak and can be ignored when SEtotal is higher than 15 dB. The

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highest average EMI shielding is 52.6 dB, realized by the net EG films due to their

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highest electrical conductivity. With the loading of 10% LGE, little sacrifice of EMI shielding is discovered, achieving 48.3 dB, which exceeds the requirement for commercial shielding applications (20 dB). With the reducing of GE sheet size, the EMI shielding exhibits decreasing trend, which is in accordance with the electric

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property. Thus, the higher electric conductivity endows EG/GE films with better EMI shielding. The results are in accordance with the previously reported works [44, 45]. The EMI coefficients of R, A, T are shown in Fig. 10e. Almost no microwave

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transmits from EG/GE films. Besides, the reflection coefficient is much higher than

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the absorption coefficient, which indicates the reflection of microwaves is the dominant EMI shielding mechanism for EG/GE films.

26

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Fig. 10. EG films reinforced with different sized GE: (a) Electric conductivity; (b) In-plane thermal conductivity. (c) EMI shielding in the frequency range of 8.2–12.4 GHz (43±3 µm); (d) SEA, SER, and SEtotal at the frequency range of 10.3 GHz. (e) EMI coefficients of reflection (R), absorption (A) and transmission (T) at the

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frequency range of 10.3 GHz.

As is known to all, GE films with excellent electric and thermal properties can be

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prepared by chemical reducing or extremely high temperature thermal reducing of GO

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or GNS. The preparation process of GO and GNS is very complex. Besides, the limitation of high temperature reducing and toxic reducing agents still limit their industrial applications. Hence, the preparation of net carbon films directly obtained from the combination of EG and GE is a very simple method. The previously reported net carbon films for their mechanical, electric and thermal properties via various methods has been studied in Table 2. Typically, the electric and thermal conductivities of EG/LGE-10 are much higher than most reported films except for the film after 27

ACCEPTED MANUSCRIPT 2850 °C thermal reducing. But it is worth noticing that our EG/LGE-10 exhibits the best mechanical properties compared with these net carbon films. Furthermore, the preparation process of EG/LGE-10 is completely green, simple and cost-efficient. The

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using of GO as surfactant completely avoids the using of organic solvents and surfactants, which is hardly to be removed and may influence the properties of the prepared films. The mechanical flexibility with high electric, thermal properties and

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efficient EMI shielding of EG/GE films may generate great applications in next

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generation electronics.

Table 2. Comparison with previously reported net carbon films. Ref. no.

Method

[46]

Sonication in SC/water (10 h)

[47]

Sonication in HNO3 and H2SO4 (48−72 h)

σ (S cm−1)

300

180

[49]

Sonication in HNO3 and H2SO4 (2−3 days)

386

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Sonication in NMP (≥50 h)

112

137mm2 s-1

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Sonication in alcohol−water mixture for 20h annealing at 1060 °C for 2 h

850

220

Sonication in SDBS/water (0.5 h)

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[36]

18

Sonication in alcohol and oxidative acid treatments annealing at 1100 °C(1 h)

[19]

Tensile strength (MPa)

110

[48]

[18]

K (W m−1 k−1)

annealing at 250 °C in ArN2 (2 h)

15

[50] Ball mill with oxalic acid (12 h) and dispersed in NMP annealing at 600 °C (2 h)

277

[51] Sonication in ODCB (0.5 h), annealing at 400 °C (12 h)

15

[52] Sonication in SC/water (400 h), annealing at 500 °C (2 h)

175

[20]

Ball milling in NMP (6 h),

annealing at 2850 °C (2 h), mechanical compression (30 MPa) [53]

33

Chemical vapor deposition

2231

1529

1136

22

This work Stirring and sonication for 0.5h, annealing at 450 °C (5 min) mechanical compression at 20 MPa (5min)

1467

348

SC, sodium cholate; NMP, N-methyl-pyrrolidone; SDBS, sodium dodecylbenzene sulfonate; ODCB: ortho-

28

40.9

ACCEPTED MANUSCRIPT dichlorobenzene; DMF, N,N-dimethylformamide.

4. Conclusion

In summary, the net carbon films combined with EG and GE are prepared by a simple,

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green and cost-efficient method, which combines vacuum filtration with thermal annealing, following by compression. The relatively mild reducing condition as well as short time process makes it possible for this film to meet the business applications

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and retain good flexibility for GE. The introduction of flexible GE with large lateral

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size to form simulate shell structure can greatly improve the mechanical properties of the net EG films with little sacrifice of electric, thermal conductivities and EMI shielding. The prepared EG films filled with 10% LGE show excellent electric (1467 S cm-1), thermal (348 W m-1·K-1) conductivities and good mechanical strength (40.9

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Mpa). Besides, high EMI shielding of 48.3 dB is achieved as the film thickness reaches 43 µm. Notably, EG/LGE-10 exhibits good flexibility even after 1000 times direct folding. To further improve the over-all properties of the prepared net carbon

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films, EG with lower oxidation degree and thinner sheet thickness is needed. The

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combination of EG and GE could be considered as a new alternative way to produce excellent light weight, thermal and flexible conducting films with efficient EMI shielding.

Notes

The authors declare no competing financial interest.

29

ACCEPTED MANUSCRIPT ACKNOWLEDGMENTS

We would like to express our sincere thanks to the National Natural Science

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Foundation of China for financial support (Grant No. 51573102 and 51721091).

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