Completely Green Approach for the Preparation of Strong and Highly

Research Article pubs.acs.org/journal/ascecg

Completely Green Approach for the Preparation of Strong and Highly Conductive Graphene Composite Film by Using Nanocellulose as Dispersing Agent and Mechanical Compression Weixing Yang,† Yu Zhang,† Tianyu Liu,† Rui Huang,† Songgang Chai,‡ Feng Chen,*,† and Qiang Fu*,† †

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, China ‡ Guangdong Shengyi Technology Limited Corporation, No. 5 Industrial West Road, Songshan Lake North Industrial Zone, Dongguan 523039, China S Supporting Information *

ABSTRACT: Graphene films receive tremendous attention due to their ultrahigh electrical and thermal conductivities, which show great application prospects in modern electronic devices. However, the brittleness and low strength of graphene films largely limit their use in advanced applications. And the preparation processes of graphene films reported so far are also not completely green. In this work, a novel strong and green graphene composite film with outstanding electromagnetic interference shielding effectiveness (EMI SE), electrical and thermal conductivities was successfully fabricated by using nanofibrillated cellulose (NFC) as dispersing agent and mechanical compression. In this way, graphene nanosheets (GNs) were not only efficiently dispersed in the aqueous solution but also linked together by NFC to enhance mechanical strength of the prepared films. Simultaneously, mechanical compression could powerfully induce strong alignment and increase the contact area of the GNs. As a result, the optimum electrical and thermal conductivities of the obtained films reached up to 988.2 S cm−1 and 240.5 W m−1 K−1, respectively, along with a high tensile strength of 61 MPa and a superior EMI SE of 43 dB with only ≈13 μm in thickness. Even more, the resultant films revealed excellent flame resistance. And the NFC can be removed by burning the films, resulting in complete graphene films with much higher electrical and thermal conductivities. The manufacturing route in our study is facile, cost-effective and completely green for the preparation of strong and highly conductive graphene-based thin films. KEYWORDS: Nanofibrillated cellulose, Graphene, Electrical conductivity, Electromagnetic interference shielding, Thermal conductivity

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INTRODUCTION

To make full use of the conductive properties of GNs, it is essential to achieve the maximum of physical contact between these nanosheets.15,16 In particular, one of the most promising approaches is to reassemble these nanosheets into macrofilms. Until now, several methods to prepare ultrahigh conductive graphene films have been explored extensively.17−22 Among these methods, utilizing thermal or chemical reduction of graphene oxide (GO) is a widely used approach to the preparation of graphene films because GO is suitable for industrial production and can be easily reassembled into films. Besides, the electrical and thermal conductivities can be prominently recovered by thermal or chemical reduction.23−25 Taking the thermal reduction for a example, Shen et al. fabricated the thin graphene film with excellent electrical conductivity of 1000 S cm−1, EMI SE of 20 dB and thermal conductivity of 1100 W m−1 K−1 via ultrahigh temperature

The search for paper-like materials with superior electrical and thermal transport properties is becoming increasingly desirable in tandem with the rapid proliferation of modern electronic devices for efficient electromagnetic interference (EMI) shielding and heat removal.1−5 It is attributed to the fact that the undesirable electromagnetic energy and huge heat emissions generated from the electronic components of these electronic devices would not merely compromise the function and reliability of these devices significantly but also do harm to human health. Materials with preferable electrical and thermal conductivities can provide satisfactory EMI shielding performance with minimal thickness and effectively transfer heat.6−10 Within this context, graphene nanosheets (GNs), as a novel kind of two-dimensional (2D) materials, are commendable candidates for highly conductive materials that could effectively manage EMI and heat owing to their unique structure and superior properties, including ultrahigh electrical and thermal conductivities.11−14 © 2017 American Chemical Society

Received: June 20, 2017 Revised: August 1, 2017 Published: September 1, 2017 9102

DOI: 10.1021/acssuschemeng.7b02012 ACS Sustainable Chem. Eng. 2017, 5, 9102−9113

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ACS Sustainable Chemistry & Engineering

Figure 1. Preparation schematic of GNs/NFC films.

filtration and mechanical compression. It has been found that NFC could not merely efficiently disperse the GNs in the aqueous solution but also link GNs together to enhance the mechanical strengths of the prepared films. Moreover, the peculiar one-dimensional (1D) structure of NFC could greatly lower the insulating contacts between GNs, thereby obtaining the high electrical and thermal conductivities.9 To optimize the conductive properties of GNs/NFC films, the mechanical compression was employed later to effectively reduce the pore volumes within the film and increase the contact area of GNs, consequently facilitating interaction, electron and phonon transport. To sum up, these resultant GNs/NFC composite films possessed superior electrical, thermal and EMI shielding as well as satisfactory mechanical properties. Although there has been literature associated with graphene/cellulose composite films43−49 and we also have reported the conductive reduced graphene oxide (RGO)/cellulose films for EMI shielding applications,50 the differences between this work and reported literature about graphene/cellulose composite films, including our reported work, have been summarized as follows. In these reported literature, the homogeneous dispersion of graphene in composite films was achieved through the utilization of the good dispersion of GO in water and the following chemical reduction of GO or chemical modification of graphene. But unfortunately, these methods would use the toxic chemicals and irreversibly damage the structure and properties of graphene, thereby resulting in lowered electrical and thermal conductivities. In this work, the NFC was directly employed as dispersing agent for GNs to obtain the high-quality dispersion of GNs in GNs/NFC films. More importantly, the mechanical compression was applied to reduce the pore volumes and significantly facilitate the connections between NFC−GNs and those between GNs−GNs. The obtained electrical and thermal conductivities in this work were much superior than those of these reported graphene/cellulose films. Apart from the mechanical and conductive properties, more comprehensive and in-depth studies about the other crucial properties, such as hydrolytic resistance and flame resistance property, have been carried out in this current study. We postulate that the integration of green preparation route and excellent performances of GNs/NFC composite films as well as comprehensive and in-depth studies about their properties in this study is beneficial to the wide application range of these composite films in modern electronics.

annealing of GO film at 2000 °C.23 However, during the thermal reduction process, the long treatment at high temperature will result in high cost and energy consumption, which makes large-scale industrial production infeasible.3,17 Moreover, the high temperature annealing process may deteriorate the well-stacked film structure and lead to the decrease in mechanical properties.25−27 As for chemical reduction, although it can reduce GO at relatively low temperature, its ability to completely restore the structural damage of GO is inevitably limited, thereby resulting in a lowered electrical conductivity, such as 298 and 72 S cm−1 reported by Cheng et al. and Li et al., respectively.28,29 Additionally, the use of toxic reducing agents and complex cleaning process would restrict its scope of application. To perfectly avoid these problems caused by the reduction of GO, taking physically exfoliated graphene sheets as raw materials to directly prepare graphene films is a fascinating alternative. For instance, graphene nanoplatelets were dispersed by polyethylenimine (PEI) in water and the dispersion was filtrated into graphene film with a electrical conductivity of 880 S cm−1 and a thermal conductivity of 200 W m−1 K−1 as reported by Drzal et al.30 Coleman et al. employed N-methyl-pyrrolidone (NMP) as solvent to exfoliate graphite and fabricate graphite film with a electrical conductivity of 180 S cm−1.31 But unfortunately, the physical exfoliation method of graphene sheets generally requires the special solvents or stabilizer as well as a long enough processing time and merely obtain stable graphene dispersion with a relatively low GNs concentration.32−37 And the solvents or stabilizers here normally possess relative toxicity and are difficult to be completely removed, which are adverse for the green chemical technologies and biomedical applications.38,39 Besides, the subsequent high-temperature annealing treatment, which would limit the large-scale industrial production, is essential to obtain graphene films with superior electrical and thermal properties.22,32 Hence, this approach is inefficient and still not completely green. More importantly, graphene films prepared in these ways are quite brittle and their strength are relatively low, which are unsatisfactory for their applications in practice. For example, Wu et al. and Coleman et al. prepared graphene films with tensile strengths of ≈3 and 15−33 MPa, respectively.15,40 Taking all of the above into account, there is a great demand to develop new, easy processing, sustainable and highly conductive film-like materials. In this work, nanofibrillated cellulose (NFC), considered as an inexhaustible and green biopolymer,41,42 was used as an assisted dispersant to disperse commercially available GNs in water and the stable GNs/NFC dispersion was obtained by sonication for 0.5 h. And then, the dispersion was further processed into highly aligned GNs/NFC films through fast

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EXPERIMENTAL SECTION

Materials. The GNs (XTG-P-0762) and cellulose material (MFC, KY100-S, Celish) were purchased from the Deyang Carbonene Co. Ltd., China and Daicel Chemical Industries, Ltd., Japan, respectively. TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy, 98 wt %) was obtained 9103


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Figure 2. (a) Digital images of the NFC dispersion and GNs dispersions without NFC and with different NFC content. (b, c) SEM images of GNs dispersion with 50% NFC. (d, e) TEM images of GNs dispersion with 50% NFC. All these dispersions were obtained by sonication for 0.5 h. from Sigma-Aldrich. Sodium bromide (NaBr), sodium hypochlorite (NaClO) solution and sodium hydroxide (NaOH) were purchased from Kermel Chemical reagent plant, China. Preparation of GNs/NFC Composite Films. In this work, nanofibrillated cellulose (NFC) was prepared from the MFC on the basis of the reported literature51 and the detailed preparation process is represented in the Supporting Information. And its viscosity-average molecular weight, measured by means of the previously reported method,51,52 was about 5.18 × 104 g mol−1. Figure 1 shows the preparation schematic of GNs/NFC films. First, to obtain the homogeneous GNs/NFC dispersion with the fixed concentration (GNs and NFC) of 5 mg g−1, a certain amount of GNs, the NFC dispersion (6.3 mg g−1) and water were continuously magnetic stirred for 3 h and sonicated (pulsed mode, 100 W, 5 s on and 1 s off) for 0.5 h at ambient temperature. And then, vacuum-assisted filtration was employed to filter the obtained homogeneous GNs/NFC dispersion to prepare GNs/NFC films. The filtration was carried out by using the cellulose ester membrane in which the diameter and pore size were 47 mm and 0.45 μm, respectively. The resultant wet GNs/NFC films were dried under room temperature. Besides, under the same preparation process, the NFC films were also prepared for comparison. For convenience, the as-prepared GNs/NFC films were coded as xGNs/NFC, where x represented the GNs content (wt %) in the GNs/NFC films. For example, the 10GNs/NFC represents the GNs/ NFC films containing 10 wt % GNs and 90 wt % NFC. Lastly, these xGNs/NFC films were further compressed at mechanical compression (150 MPa) for 10 min and the resultant films were marked as xGNs/ NFC-C. Characterization. The structures and morphologies of NFC and GNs were characterized by scanning electron microscopy (SEM, FEI Inspect F, USA) and transmission electron microscopy (TEM, JEM2010). The X-ray photoelectron spectroscopy (XPS) test was carried on Axis Ultra DLD (KRATOS Co., UK). The tensile tests of these obtained films were conducted on the Instron 5567 universal with 1 kN load cell at room temperature with a speed of 20 mm/min and a relative humidity of 60%. All the specimens for the tensile test

possessed uniform size of 30 mm in length and 3 mm wide. And the electrical conductivities of these films were measured by Keithley 6487 picoammeter. The thicknesses of GNs/NFC and GNs/NFC-C films used for tensile tests and electrical conductivity measurements were controlled to be 55 ± 5 μm. The EMI SE tests of the GNs/NFC-C films were conducted on the APC-7 connected with Agilent N5230. Note that the testing frequency for EMI SE was at the X-band and all the test samples possessed uniform diameter of 13 mm. Specifically, the measured scattering parameters (S11 and S12) were employed to calculate the coefficients of reflection (R), absorption (A) and transmission (T). The total EMI SE (SETotal) was the summation of the reflection (SER), absorption (SEA) and multiple internal reflections (SEM) of microware, can be also obtained. The specific measurement process and calculations of SEA, SER, SEM and SETotal are exhibited in the Supporting Information. As for thermal properties tests, the values of cross-plane (K⊥) and in-plane (K) thermal conductivities were calculated by the equations as follows:

K⊥ = α⊥ × C p × ρ

(1)

K = α × Cp × ρ

(2)

The LFA 447 NanoFlash was used to measure the α⊥ (cross-plane thermal diffusivity) and α (in-plane thermal diffusivity) in the previous equations, respectively. The density of these specimen is represented by ρ and can be obtained by the calculation of sample dimension and weight. And Cp (heat capacity) was characterized by DSC 1/700 (Mettler-Toledo), in which the specimen was heated at the rate of 10 °C/min. The measurement of thermogravimetric analyses (TGA) was performed on TG 209F1 Iris (Netzsch, Germany) under N 2 atmosphere with a heating rate of 10 °C/min.

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RESULTS AND DISCUSSION The NFC was prepared by TEMPO-mediated oxidation of MFC. Its average length and diameter were about 0.4 μm and 5 nm as shown in Figure S1. Figure 2a displays that the NFC was 9104


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Figure 3. (a) Digital photos of the 90GNs/NFC-C film exhibiting excellent flexibility. SEM images of fracture surfaces of the 50GNs/NFC (b), 70GNs/NFC (c), 90GNs/NFC (d), 50GNs/NFC-C (e), 70GNs/NFC-C (f) and 90GNs/NFC-C (g). Cross-sectional morphologies of 50GNs/ NFC-C (h) and 90GNs/NFC-C (i) after the tensile strength test.

carbon lattice of GNs and the fluctuations of counterions of NFC.53,54 Then the electrostatic repulsion caused by charged carboxyl groups of the surfaces of NFC and the steric hindrance between NFC and GNs could help GNs stably disperse in aqueous solution.55 In order to prove that NFC tended to attach to the surfaces of GNs, SEM and TEM were further utilized to investigate the morphology of the dispersed GNs with 50% NFC. Figure 2b,c shows the original SEM images, in which the red arrows are referring to NFC. And it can be apparently observed that NFC indeed attached to the surfaces of GNs. Moreover, the TEM results (Figure 2d and Figure S5) further demonstrated that NFC was adsorbed on the surfaces of GNs. Thus, it is reasonable to believe that not only the hydrophobic and attractive interactions between NFC and GNs but also electrostatic repulsion and steric hindrance caused by NFC play crucial roles in acquiring stable and uniform GNs/ NFC dispersions. Owing to these high-quality GNs/NFC dispersions, composite films with various GNs contents could be successfully prepared via fast filtration. To further improve the mechanical and conductive properties, the obtained GNs/ NFC films were treated by mechanical compression. After mechanical compression, the obtained GNs/NFC-C films, especially with high GNs contents, became much thinner and had better flexibility than the corresponding GNs/NFC films (Figure S8). As shown in Figure 3a, the obtained 90GNs/NFCC film showed excellent flexibility and its surface exhibited shining metallic luster. To investigate the stacking and arrangement condition of the GNs and NFC in these films, the representative SEM images of fracture surfaces of GNs/

stably dispersed in the water and the NFC dispersion was optically transparent. The obtained NFC dispersion remained good stability and uniformity even after being put for 3 months. This can be well explained because a great many of charged carboxyl groups formed on the NFC surface can provide strong electrostatic repulsion between the NFC (Figure S2).52 As for the GNs, its average diameter was about 4.5 μm and the thickness was approximately 6 nm (Figures S3, S4 and S5). To further characterize the GNs, the X-ray photoelectron spectroscopy test was employed. It had been found that the oxygen peak of the GNs was difficult to be observed and the C/O atomic ratio could reach up to 37, and the C 1s spectra revealed that the GNs contained very little oxygenated functional groups (Figure S6a) as well. Moreover, the exceedingly commendable thermal stability of GNs also has been proven by TGA, and the GNs only showed a small amount of weight loss after being exposed to 600 °C (Figure S6b). In this current study, taken the environmental protection into consideration, water and NFC were deliberately selected as solvent and dispersing agent for GNs, respectively. And it could be clearly observed that NFC-assisted dispersed GNs dispersion, even with only 10 wt % NFC, displayed satisfactory stability and homogeneity. Moreover, these GNs/NFC dispersion could maintain homogeneous and stable for 6 h at least (Figure S7). Though the GNs dispersion without NFC demonstrated clear aggregate and sedimentation as shown in Figure 2a. The reasons behind this phenomenon are as follows. First, NFC would attach to the surfaces of GNs owe to the hydrophobic interaction occurring between the hydrophobic faces of NFC and hydrophobic plane of GNs together with attractiveinteraction between the sp2 9105


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Figure 4. (a) Stress−strain curves of GNs/NFC-C films. The comparison of tensile strength (b), elongation at break strain (c) and Young’s modulus (d) of GNs/NFC and GNs/NFC-C films with various GNs loadings.

NFC and GNs/NFC-C films are shown in Figure 3 and Figure S9. These SEM results indicated that the GNs/NFC films presented a compact layered structure when the GNs content was less than or equal to 60 wt %. This was attributed to the effective and stable dispersion of GNs by using NFC as the dispersing agent as well as the directional flow of water caused by filtration. However, when the GNs content was above 70 wt %, the loose and porous layered structures could be easily observed in the GNs/NFC films. The reason may be the weakened dispersion effect of NFC on GNs. Satisfactorily, the GNs/NFC-C films exhibited more ordered and compact layered structure after the mechanical compression, even when the GNs content reached up to 90 wt % (Figure 3e,f,g). Owing to the hydrophobic nature, high content and compact stack of GNs, the 90GNs/NFC-C film showed better water resistance when compared with the GNs/NFC-C films with lower GNs content. And it could remain its original shape even after being soaked in water for 1 week (Figure S10). Moreover, the cross-sectional morphologies of 50GNs/NFC-C (Figure 3h) and 90GNs/NFC-C (Figure 3i) after subsequent tensile strength test illuminated that 1D NFC was tightly packed into a layered structure of 2D GNs, which was helpful to obtain good mechanical strength.9 In conclusion, the mechanical compression could effectively eliminate the pores and align the GNs along the planar direction, consequently resulting in the decrease of the thickness and the more compact layered structure. It is reasonable to conclude that the mechanical compression can effectively improve the mechanical and conductive properties. For practical applications, mechanical property is one of the most important characteristics of the materials. Especially, the materials employed as flexible electronic devices are required to

resist breakage during the manufacturing and application process. The stress−strain curves of GNs/NFC-C films are shown in Figure 4a, which demonstrate that the obtained GNs/ NFC-C films possess favorable mechanical properties even with relatively high GNs contents. Specifically, the 90GNs/NFC-C film displayed a relatively high tensile strength (≈61 MPa), Young’s modulus (≈4.7 GPa) and elongation at break (≈2.1%). Moreover, the average tensile strength, Young’s modulus and elongation at break of both GNs/NFC and GNs/NFC-C films are summarized in Figure 4b−d, and the details are shown in Table S1. For the as-made GNs/NFC films, it could easily observed that the tensile strength and elongation at break decreased rapidly with the augment of the GNs loading. And the Young’s modulus of the GNs/NFC films first increased slowly and then dropped rapidly with the further increase of the GNs content. Briefly, the resultant GNs/NFC films with extremely high GNs contents revealed relatively poor mechanical properties, which was ascribed to the small contact area and weak interaction caused by the pores and unordered stacking among the GNs and NFC. Surprisingly, as for GNs/ NFC-C films, the mechanical properties were far superior to those of the referenced GNs/NFC films. For example, the outstanding tensile strength (≈61 MPa) and Young’s modulus (≈4.7 GPa) of 90GNs/NFC-C film demonstrated 272% and 147% augment than those (16.4 MPa and 1.9 GPa, respectively) of 90GNs/NFC film. Actually, the enhanced mechanical properties of GNs/NFC-C films were the results of eliminated porosity, more ordered and compact layered structure of the GNs through the mechanical compression as well as the enhanced interlayer contact and interactions of GNs. Moreover, the mechanical strength of GNs/NFC films compressed at different temperature has been studied as well. 9106


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Figure 5. (a) In-plane electrical conductivity (σ) of the GNs/NFC and GNs/NFC-C films. (b) Cross-plane electrical conductivity (σ⊥) of the GNs/ NFC and GNs/NFC-C films.

of GNs/NFC films at the same GNs content. However, it was easily observed that the values of σ⊥ of GNs/NFC and GNs/ NFC-C films were extremely low even when the GNs content was up to 90 wt %, which were vastly inferior to the σ values of these films. For example, the σ⊥ values of 90GNs/NFC and 90GNs/NFC-C films were respectively 0.18 and 0.25 S cm−1, which could be neglected when compared to their superhigh σ (105.2 and 988.2 S cm−1). The theoretical interpretation about the electrical conductive mechanism and the effect of compression treatment on the electrical conductivity for these composite films are represented as follows. First, it is well explained that both incremental σ and σ⊥ could be obtained through increasing the GNs content. The reason is that the increased amount of GNs would result in increasingly more physical contact between GNs and more perfect conductive path. Second, it can be easily concluded from Figure 5 that the increment in the value of σ for the GNs/ NFC films with less than or equal to 30 wt % GNs is more obvious than that for these films with more than 30 wt % GNs. Actually, the reason would be perfectly stated by the SEM results (Figure 3 and Figure S9). For the GNs/NFC films with less than or equal to 30 wt % GNs content, the GNs were orderly unfolded and compactly aligned along the planar direction to form layered structure with large overlapping area, which was all attributed to the effective dispersion effect of NFC on GNs. To further increase the GNs content, the layered structure turned loose and porous due to the poor dispersion of GNs. Moreover, the reason why the σ of the GNs/NFC-C is superior to that of the GNs/NFC is the more ordered and tight stacking of GNs in the layered structure after mechanical compression, consequently achieving the maximum overlapping area and physical contact of GNs.16,22 For these GNs/NFC-C films, the effective dispersion of GNs by NFC and mechanical compression significantly boost highly aligned arrangement and the physical contact of GNs, resulting the superior σ. The theoretical reasons for the low σ⊥ values of GNs/NFC films were the loose layered structure of GNs and the formation of conductive insulating layers of NFC among the GNs. And the improved σ⊥ of GNs/NFC-C films when compared to that of GNs/NFC films was attributed to the mechanical compression that could make the overlapping area larger and the physical contact of GNs more along this direction. Certainly, the σ⊥ values of GNs/NFC-C films were still extremely low due to the high orientation of GNs along the in-plane direction and the constant existence of conductive insulating NFC among the GNs. Taken the outstanding in-plane electrical and mechanical properties into consideration, these GNs/NFC-C films

And the results indicated that the mechanical strength of the resultant films kept almost unchanged when the compression temperature was below 100 °C and worsened following the increasing temperature when the temperature was above 100 °C. This phenomenon was attributed to the degradation of NFC (Figure S11). In conclusion, excellent mechanical performance of the resultant GNs/NFC-C films was successfully achieved by mechanical compression of the GNs/NFC films in this work. Similarly, the effect of the mechanical compression on the electrical conductivity has been investigated (Figure 5). Obviously, the electrical conductivity of GNs/NFC and GNs/NFC-C films increased in the wake of the increasing GNs content and the electrical conductivity of the latter were invariably higher than those of the former with the same composition. As for 10GNs/NFC film, the value of σ was 2.2 S cm−1. With the content of GNs increasing to 30 wt %, the value of σ sharply increased up to 32.7 S cm−1. It should be pointed out that a further increment in the GNs loading would steadily improve the value of σ of the films in spite of only a limited increment and the maximum value was 105.2 S cm−1 for the 90GNs/NFC film. When it comes to the GNs/NFC-C films, it could be briefly concluded that the mechanical compression had a positive effect on the improvement of σ. Specifically, the 10GNs/NFC-C film possessed higher σ (4.7 S cm−1) when compared to 10GNs/NFC film. Similarly, the σ value of 30GNs/NFC-C film was 45.1 S cm−1, much higher than that of 30GNs/NFC film. More intriguingly, the optimal σ could reach up to 988.2 S cm−1 for 90GNs/NFC-C film. Moreover, the σ of GNs/NFC-C films heat-treated at various temperature has been studied (Figure S12). When the treatment temperature was below or equal to 200 °C, the values of σ for all these composite films were almost unaffected by the increasing temperature. Particularly, as for the 90GNs/NFC-C film, its electrical conductivity was almost unaffected by the increasing temperature even up to 400 °C. The reason behind this phenomenon was that 90GNs/NFC-C film contained only a small amount of NFC and the degradation of these NFC had no effect on the physical contact and conductive path of GNs in the 90GNs/NFC-C film. To sum up, when compared to GNs/ NFC composite films, the GNs/NFC-C films possessed far superior electrical property. The σ⊥ values of GNs/NFC and GNs/NFC-C films with various GNs contents have been measured as well, and the results are shown in Figure 5b. As for the GNs/NFC and GNs/NFC-C films, both of their σ⊥ increased with the gradually increasing GNs content and the σ⊥ values of GNs/NFC-C films were always higher than those 9107


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Figure 6. (a) EMI SE of GNs/NFC-C films at the X-band. (b) SETotal, SER and SEA for GNs/NFC-C films at the frequency of 10 GHz. (c) Coefficients of reflection, absorption and transmission for the GNs/NFC-C films at the frequency of 10 GHz. (d) Schematic of EMI shielding of the GNs/NFC-C films. (e) EMI SE for the 90GNs/NFC-C films with various thicknesses. (f) SETotal, SER and SEA for the 90GNs/NFC-C films with various thicknesses at the frequency of 10 GHz.

increase of mobile charge carriers. Furthermore, the power coefficients of absorption, reflection and transmission were calculated to theoretically analyze the shielding mechanism as well. As displayed in Figure 6c, almost all the microwaves transmitted rather than had been absorbed or reflected for the pure NFC films, which indicated no EMI shielding effect. With the incorporation of GNs, the transmission coefficients could be greatly lowered and the reflection coefficients attained much higher than the absorption coefficients except for the 10GNs/ NFC-C films, which demonstrated the reflection of microwaves dominated the EMI shielding mechanism of the GNs/NFC-C films. Although the microwaves absorption and reflection capabilities were improved with the increasing GNs content, the major of microwaves were reflected on the surfaces of the GNs/NFC-C films before being absorbed by reason for impedance mismatches between the air and films caused by the increasing electrical conductivity.50 To make clear the shielding process against microwaves, a schematic is depicted in Figure 6d. First, part of microwaves was reflected at the surface

possessed promising potential as ultrathin EMI shielding materials. The EMI SE of GNs/NFC-C films with varying GNs concentration was measured within the frequency range of 8.2− 12.4 GHz, as exhibited in Figure 6. Clearly, the NFC film represented no EMI shielding performance. As for all the GNs/ NFC-C films, the EMI SE kept almost constant at the X band and increased with the GNs content. When the GNs loading came up to 40 wt %, the value of EMI SE could attain about 22 dB, which was sufficient for practical application (≈ 20 dB). Furthermore, the optimal EMI shielding performance could be obtained for 90GNs/NFC-C film. It was noteworthy that, at the X-band, the 90GNs/NFC-C film with high SE value (around 43 dB) was merely ≈13 μm thick, which was much thinner than the other carbon-based polymer composites with similar SE.6 Besides, to explore the EMI shielding mechanism, the SETotal, SER and SEA were calculated from S parameters.56 Figure 6b revealed that the values of SER, SEA and SETotal gradually climbed with the rising GNs content owing to the 9108


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Figure 7. (a) In-plane (K) and cross-plane (K⊥) thermal conductivity of GNs/NFC-C films. (b) Thermal image of 90GNs/NFC-C, 50GNs/NFC-C and NFC films.

of GNs/NFC-C films due to the impedance mismatches and the rest got into these films. Owing to the high electrical conductivity of the films, most of the incoming microwaves were absorbed, consequently leading to the energy dissipation. Meanwhile, negligible multiple internal reflection happened among the GNs owing to the layer-by-layer structure of the films.57 Noting that the highly aligned GNs layers could effectively attenuate the penetrating microwave by repeating reflection, scattering, and adsorption. The EMI shielding performance of the GNs/NFC-C films was greatly influenced not only by the GNs loading but also by the thickness of the films. Clearly, with the same composition, the value of EMI SE for the GNs/NFC-C films climbed steadily with the increasing thickness when compared the results of Figure 6a with those of Figure S13a. Specially, for the 90GNs/ NFC-C films, the effect of thickness on their EMI shielding performance was characterized, as shown in Figure 6e. It could be clearly seen that the value of EMI SE ascended by degrees in the wake of the increasing thickness and, with the thickness of ≈118 μm, the maximum could reach up to 80 dB. Similarly, Figure 6f represents a gradually increasing tendency for the values of SEA and SETotal following the increasing thickness, but the value of SER slightly decreased. And the power coefficients of 90GNs/NFC-C films revealed a slight increase of absorption coefficients, whereas an opposite trend for reflection coefficients with the increasing thickness (Figure S13b). It could be attributed to that the increasing GNs loading made the interaction between the GNs and electromagnetic microwaves stronger, consequently increasing the absorption loss and attenuating the transmission of microwaves. The slight decreases of SER might be ascribed to the layered structure inside these films and the strong interaction between the GNs and microwaves.6,58 In conclusion, the combination of the highly aligned GNs and the strong interaction between the GNs and electromagnetic microwaves offers the GNs/NFC-C composite films excellent EMI shielding performance. Based on the discussions about SEM results and electrical properties, it can be concluded that the mechanical compression can make the stacking of the GNs more compact and ordered, consequently giving rise to the superior electrical property. And the stacking state of GNs would also be of great importance to the thermal conductivity. Hence, the thermal conductivity measurement of the GNs/NFC-C films has been

conducted and the variations of K and K⊥ as a function of GNs concentration are shown in Figure 7a. Note that, for all these GNs/NFC-C films, the heat conductive behaviors along the cross-plane and the in-plane directions of these films were absolutely different. To be specific, the values of K were tremendously higher than those of K⊥ regardless of the composition ratios. For the in-plane direction, only an extremely low K of 1.6 W m−1 K−1 could be obtained for NFC film. The K of 10GNs/NFC-C film increased greatly up to 16.5 W m−1 K−1. Besides, there was a rapidly increasing tendency for K with the gradual increment of GNs loading. The maximum value of K is up to 240.5 W m−1 K−1 for 90GNs/ NFC-C film. In fact, along the planar direction, the GNs were orderly and compactly aligned to form layered structure with large overlapping area. Therefore, the before-mentioned increasing tendency can be well explained theoretically. The increasing GNs content and the mechanical compression treatment made the layered structure more ordered and tighter, resulting in more close packing and less insulating contacts between GNs, which could achieve maximization of the overlapping area and thus improve the K. When it comes to the cross-plane direction, the value of K⊥ increased from 0.3 W m−1 K−1 for NFC film to the maximum, only 1.9 W m−1 K−1, for 90GNs/NFC-C film. It could be ascribed to very low K⊥ of NFC (0.30 W m−1 K−1) and the formation of thermally insulated layers among the GNs along this direction. The heat transport was significantly hindered by thermally insulating NFC between the layered GNs. Furthermore, the significant difference between the K and K⊥ endowed the films with extremely high K/K⊥, namely anisotropy of thermal conductivity (Figure S14). Particularly, the value of K/K⊥ for 90GNs/NFC-C film reached up to 126, at a high level.5,43,45 The thermal photo vividly exhibits that the thermal conductive properties of GNs/NFC-C films could be significantly improved by the increment of the GNs concentration (Figure 7b). As we all known, although graphene films with excellent properties can be prepared by utilizing thermal or chemical reduction of GO,28,60 the utilization of high temperature and toxic reducing agents still impede their practice application greatly. Hence, the preparation of graphene films directly from GNs is a very valuable method. The mechanical, electrical and thermal conductive properties of previously reported graphene 9109


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Table 1. Comparison with Previously Reported Graphene Films Fabricated Directly from Graphene Nanosheetsa ref. no. 36 34 8 18 31 20 39 33 35 40 32 16 30 59 15 22 21 this work

σ (S cm−1)a

preparation method sonication in SC/water (10 h) sonication in nitric acid and sulfuric acid (48−72 h) sonication in nitric acid and sulfuric acid (48−72 h) sonication in SC/water (140 min) sonication in NMP (≥50 h) sonication in nitric acid and sulfuric acid (2−3 days) sonication in SDBS/water (0.5 h), annealing at 250 °C in ArN2 (2 h) ball mill with oxalic acid (12 h) and dispersed in NMP, annealing at 600 °C (2 h) sonication in ODCB (0.5 h), annealing at 400 °C (12 h) sonication in SC/water (400 h), annealing at 500 °C (2 h) sonication in nitric acid and sulfuric acid (72 h), annealing at 1060 °C (2 h) sonication in isopropanol (2 h) and PEI (0.5 h), annealing at 340 °C (2 h), mechanical compression (100 psi, 1 h) sonication in PEI/water (1 h), annealing at 340 °C (2 h), mechanical compression (100 psi, 1 h) sonication in acetone/DMF (20 min), annealing at 250 °C (14 h), mechanical compression (5 MPa) sonication in PEId/water, annealing at 120 °C (1 h) and 340 °C (1 h), high-pressure compression (1000 MPa) ball milling in NMP (6 h), annealing at 2850 °C (2 h), mechanical compression (30 MPa) chemical vapor deposition sonication in NFC/water (0.5 h), mechanical compression (10 min, 150 MPa)

K (W m−1k−1)

TS (MPa)

110 300 220 150 180 386 15 277 15 175 850

880 1443 2200 2231 1136 988.2

12−18 112

15−33 220 178 200 313

3

1529 240.5

22 61

a σ, Electrical conductivity; K, in-plane thermal conductivity; TS, tensile strength; SC, sodium cholate; NMP, N-methyl-pyrrolidone; SDBS, sodium dodecylbenzene sulfonate; ODCB: ortho-dichlorobenzene; PEI, polyethyleneimine; DMF, N,N-dimethylformamide; PEId, polyetherimide.

Figure 8. (a) 90GNs/NFC-C film burned by the flame and as a fire shield for cotton. (b) Digital photos of the 90GNs/NFC-C-BC film showing excellent flexibility. (c) SEM images of fracture surfaces of the 90GNs/NFC-C-BC film. (d) Cross-sectional morphology of 90GNs/NFC-C-BC film after tensile strength test. (e) Stress−strain curves of the 90GNs/NFC-C-BC and 90GNs/NFC-C films. (f) In-plane electrical conductivity (σ) and thermal conductivity (K) of 90GNs/NFC-C-BC and 90GNs/NFC-C films.

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outstanding electrical conductivity (around 988.2 S cm−1), superior EMI SE (43 dB with ≈13 μm in thickness) and excellent in-plane thermal conductivity (240.5 W m −1 K −1). Furthermore, the 90GNs/NFC-C film revealed excellent flame resistance. And the NFC could be removed by burning the 90GNs/NFC-C film at high temperature, resulting in complete graphene film with much higher electrical and thermal conductivities.

films fabricated directly from GNs through various methods have been researched, and the results are listed in Table 1. Apparently, the electrical and thermal conductivities of 90GNs/ NFC-C film herein are much higher than those of the graphene films fabricated through other methods except for the annealing treatment at high temperature and the treatment under highpressure compression. Moreover, it is well worth pointing out that the 90GNs/NFC-C film possessed the greatest mechanical property compared with these graphene films to our knowledge. More importantly, the preparation process of the 90GNs/ NFC-C film in our work is more facile, cost-effective and completely green. The utilization of NFC as dispersant in water and mechanical compression could avoid the annealing treatment at high temperature, which would increase energy consumption and cost, and solvents or stabilizers with relative toxicity. Combining the excellent performances and superiority of preparation method, in the current study, these strong and highly conductive films show great application prospect in the modern electronics. Excellent flame resistance is very demanded for practical electronic devices, especially for high-power electronics. In this work, the flame resistance test of the 90GNs/NFC-C film was carried out by burning it in the flame generated from alcohol blast burner.61,62 The temperature of the flame could reach to 800 °C and the 90GNs/NFC-C film exhibited good flame retardant performance. When exposed to flame, there was a brief and fast burning for 90GNs/NFC-C film and air pockets formed on this film owing to the presence of NFC (Figure 8a). After burning out NFC, 90GNs/NFC-C film retained its shape and did not support any burning owe to the formation of the GNs framework. Figure 8a displays that a mass of cotton did not burn thanks to the protection of 90GNs/NFC-C film even exposed to flame for 1 min, whereas the cotton caught fire immediately without the protection. Additionally, the thermal stability of 90GNs/NFC-C film was also good, which was confirmed by the TGA shown in Figure S15. More importantly, the 90GNs/NFC-C film after being burned could be mechanical compressed into flexible film (coded as 90GNs/ NFC-C-BC) in which the surface exhibited metallic luster (Figure 8b). With regard to the morphology of the fracture surface, there was no NFC among the GNs after being burned and the layered structure of GNs could be reserved through mechanical compression (Figure 8c,d). However, the mechanical property of 90GNs/NFC-C-BC film dropped from 61 to 29.3 MPa, which might prove that the NFC could efficiently improve the mechanical properties of these GNs/NFC composite films (Figure 8e). Satisfyingly, when compared the electrical and thermal conductive properties with 90GNs/NFCC, the 90GNs/NFC-C-BC possessed higher σ (1559.8 S cm−1) and K (371.9 W m−1 K−1), which indicated that the disappearance of NFC enlarged the overlapping area of GNs, sequentially giving rise to more perfect conductive pathway (Figure 8f). Thus, all the aforementioned features demonstrate that 90GNs/NFC-C film can be used for the protection of sophisticated electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02012. Additional experimental details and results including SEM images, FT-IR spectra, TEM images, AFM images, XPS data, photographs of the samples, TGA date, EMI SE, electrical and thermal conductivities of the samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q. Fu), Tel./Fax: +86-2885461795. *E-mail: [email protected] (F. Chen), Tel./Fax: +86-2885460690. ORCID

Qiang Fu: 0000-0002-5191-3315 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the National Natural Science Foundation of China for the financial support (Grant No. 51573102 and 51421061).

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REFERENCES

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CONCLUSIONS We develop a facile, cost-effective and sustainable route to preparing highly conductive graphene-based films by fast filtration of GNs dispersion with NFC as dispersant followed by mechanical compression. The resultant 90GNs/NFC-C film exhibited strong alignment and large contact area of GNs, which possessed satisfactory tensile strength of 61 MPa, 9111


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