Highly conductive interconnected graphene foam based polymer composite

Accepted Manuscript Highly Conductive Interconnected Graphene Foam Based Polymer Composite Yun-Seok Jun, Serubbabel Sy, Wook Ahn, Hadis Zarrin, Lathankan Rasen, Ricky Tjandra, Behnam Meschi Amoli, Boxin Zhao, Gordon Chiu, Aiping Yu PII:

S0008-6223(15)30196-2

DOI:

10.1016/j.carbon.2015.08.079

Reference:

CARBON 10244

To appear in:

Carbon

Received Date: 22 April 2015 Revised Date:

1 July 2015

Accepted Date: 24 August 2015

Please cite this article as: Y.-S. Jun, S. Sy, W. Ahn, H. Zarrin, L. Rasen, R. Tjandra, B.M. Amoli, B. Zhao, G. Chiu, A. Yu, Highly Conductive Interconnected Graphene Foam Based Polymer Composite, Carbon (2015), doi: 10.1016/j.carbon.2015.08.079. 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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Highly Conductive Interconnected Graphene Foam Based Polymer Composite Yun-Seok Jun1, Serubbabel Sy1, Wook Ahn1, Hadis Zarrin1, Lathankan Rasen1, Ricky Tjandra1, Behnam Meschi Amoli1, Boxin Zhao1, Gordon Chiu2, Aiping Yu1∗ Department of Chemical Engineering, University of Waterloo, 200 University Avenue West,

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Waterloo, Ontario, Canada N2L 3G1

Grafoid Inc., 130 Albert Street. Suite 912, Ottawa, Ontario, Canada K1P 5G4

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Abstract

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In this study, the impact of graphene sheet size on the electrical conductivity of interconnected graphene foam polymer composite is thoroughly investigated. Graphene oxide solution is produced from small flake graphite (SFG) (2-15 µm) and large flake graphite (LFG) (>100 µm), respectively. Each solution is used to produce three-dimensional GO foam, which is subsequently heat-treated to produce reduced graphene oxide (RGO) foam. The RGO foams are then infiltrated with poly(dimethylsiloxane) (PDMS) to produce graphene-PDMS (G-PDMS)

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composites. The in-plane electrical conductivity of the G-PDMS composite (0.4 wt %) from LFG reaches ~3.2 S/m, which is more than two orders of magnitude greater than that of GPDMS (1.9 wt%, 1.4 x 10-2 S/m) from SFG. This value is also four orders of magnitude higher than that of the G-PDMS composite prepared from mechanical mixing of 4 wt% RGO powder

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made from SFG with PDMS (4.2 x 10-5 S/m). The though-plane electrical conductivity followed the same trend for SFG and LFG. This reveals that the interconnected graphene foam supplies

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more efficient paths for electron transfer inside the polymer than conventional graphene powder and the use of large sized graphene sheets can significantly improve the electrical properties of G-PDMS.

∗

Corresponding author. Tel: 519 888-4567. E-mail address: [email protected] (Aiping Yu)

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1. Introduction Due to its extraordinary thermal, mechanical, and electrical properties, graphene has been known as an excellent nano-filler to be integrated into polymers [1–5]. Such polymer composites can be

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implemented in many applications, such as electronic devices, energy storage, sensors, electrostatic discharge and electronic magnetic interface shielding [6–8]. Conventional methods for integrating graphene sheets include melt-blending, solution-blending, and in-situ polymerization [9]. However, when graphene is combined into polymers by such approaches, its exceptional properties cannot be fully exploited. This is because the strong π-π interactions

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trigger agglomeration or restacking of the graphene sheets during fabrication, leading to their poor dispersion into the macromolecules and an inferior nano-architecture. To prevent the above-

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mentioned problems, it is necessary to develop a three-dimensional (3D) graphene network (e.g., graphene foam) that does not suffer from restacking or agglomeration [10,11]. The composites fabricated through infiltration with graphene foam can be applied to a variety of applications in electronic devices, energy storage, sensors, and especially electromagnetic interference (EMI) shielding. Metals and metallic composites have been employed as EMI shielding materials, but they suffer from severe oxidation and corrosion, and have poor chemical resistance,

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unfavourably high density, and are difficult to process. However, the composites fabricated based on graphene foam demonstrate numerous advantages including high chemical resistance, great processibility, and significantly improved electrical conductivity [12]. In addition, lightweight and flexibility of such polymer composites make them highly effective and practical EMI

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shielding applications, especially in the areas of aircraft, spacecraft, automobiles, and nextgeneration portable and wearable electronic devices.

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Graphene foams are typically synthesized by two methods: 1) direct drying of graphene oxide (GO) hydrogels or 2) template guided growth of graphene by chemical vapor deposition (CVD). Polymers can then be subsequently infiltrated into the 3D graphene foams. Since such composites retain the unique interconnected graphene sheets, a high electrical conductivity is achieved as compared to those fabricated by conventional methods [12,13]. For instance, G. Tang et al. prepared graphene foam and infiltrated it with epoxy resin. This composite exhibited electrical conductivity of 0.04 S/m at a loading of only 0.21 vol% filler, which was nine orders of 2

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magnitude higher than those prepared via solution blending [14]. In another work, Z. Chen et al. infiltrated poly(dimethylsiloxane) (PDMS) into graphene foam prepared by CVD. The composite displayed an electrical conductivity of 1000 S/m with a loading of 0.5 wt% [15]. Moreover, M. Chen et al. demonstrated graphene/multiwall carbon nanotube foam combined with PDMS that

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reached an electrical conductivity of 280 S/m at a loading of 1.3 wt% [16]. These studies clearly demonstrate recent advancements in infiltration of polymer materials into 3D graphene foam for improved performance.

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Although significant improvements have been achieved, very few studies have linked the physical characteristics of graphene sheets with the electrical conductivity of the infiltratedcomposites. When the graphene foam is prepared via chemical oxidation of graphite, it is

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essential to fully understand the influence of the starting materials’ characteristics when integrated into polymers. Thus, in this study, the impact of graphene sheet size on the electrical conductivity of the final composites has been explored. The graphene foam was prepared from large flake graphite (LFG) and small flake graphite (SFG), respectively and subsequently infiltrated with PDMS. The graphene-PDMS (G-PDMS) composite produced from LFG exhibited an in-plane electrical conductivity of ~3.2 S/m with a loading of 0.4 wt%, which is

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more than two orders of magnitude greater than those made from SFG (~1.4 x 10-2 S/m at a loading of 1.9 wt%). A variety of characterization including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrical conductivity measurement have been performed to justify the advantages of using

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LFG in fabrication of graphene-based composites.

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2. Materials and Methods 2.1 Materials

LFG (>100 µm) and SFG (2-15 µm) were purchased from Grafoid Inc. and Alfa-Aesar Inc., respectively. Sulfuric acid, phosphoric aicd and hydrogen peroxide (30%) were acquired from Sigma Aldrich. Potassium Permanganate was obtained from EMD Chemicals. PDMS was bought from Dow Corning Inc. 3

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2.2 Preparation of GO, RGO foams, and G-PDMS composite Fig. 1 illustrates the experimental procedures and images of the graphene foams at each stage of preparation. A modified Hummer’s method was used to prepare GO solution from LFG and SFG,

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respectively, and described elsewhere [17]. The prepared GO solution was poured into a housebuilt mold. The lower component of the mold was immersed in a pool of liquid nitrogen, introducing a unidirectional temperature gradient to the GO solution (Fig. 1a). The frozen GO solution was freeze-dried for 72 hr, and the resulting GO foam was thermally reduced in a

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furnace at 900 °C (Fig. 1b). The reduced graphene oxide (RGO) foam was then infiltrated with PDMS as follows (Fig. 1c). Sylgard 184 silicone elastomer base was mixed with its curing agent with 10:1 ratio. The mixture was degased in a vacuum oven at room temperature for 30 min and

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infiltrated into the foam. As seen from these images, the infiltration was carefully controlled and the foam did not suffer from any deformation during this process. The infiltrated G-PDMS composites were sliced into small slabs (0.9 x 0.9 x 0.1 cm) and their electrical conductivities were measured either by a standard 4 point probe station (Fig. 1d) for in-plane conductivity or a potentiostat for through-plane conductivity. Table 1 shows the G-PDMS composites prepared in

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this study.

Fig. 1 - Schematic illustrating G-PDMS fabrication procedure followed by electrical conductivity measurement. Optical images of GO foam (a), RGO foam (b), G-PDMS composite(c), and sliced slab of G-PDMS on a probe fixture (d). (Graphene based foams measured in inches).

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Table 1 - G-PDMS composites prepared in this study. Graphite Size (µm)

GO Conc. (mg/mL)

Loading (wt%)

G-PDMS-1

Large (>100)

5.8

0.4

G-PDMS-2

Large (>100)

11.5

G-PDMS-3

Large (>100)

18.0

G-PDMS-4

Small (2~15)

25

G-PDMS-5

Small (2~15)

35

G-PDMS-6

Small (2~15)

G-PDMS-7*

Small (2~15)

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Composites

0.5

0.6 1.9 1.6 2.2

-

4.0

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* G-PDMS-7 was prepared by mechanical mixing of RGO powder from SFG with PDMS 2.3 Characterizations

The GO and RGO foams as well as G-PDMS were analyzed by SEM (Zesis LEO 1550) for morphology analysis. Individual graphene sheet and their size were observed by TEM (JEOL

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2010F) for size verification. XRD (Brunker AXS D8 Advance) was used to study the crystal structures of GO and RGO foams. Raman spectroscopy (Bruker Senterra, 532nm) was employed to analyze the defects and disorders of GO and RGO foams.

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2.4 Electrical Conductivity Measurement 2.4.1 In-Plane Electrical Conductivity

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The in-plane electrical conductivity (σI) of samples was obtained employing a four-point probe setup equipped with a probe fixture (Cascade microtech Inc.) and a source meter (Keithley 2440 5A Source Meter, Keithley Instruments Inc.). The sheet resistance (Rs) was measured by measuring the voltage drop when a constant current was applied. The electrical conductivity (σI) was calculated by Eq. (1): σI =

 ln2  1 1 1  Ω cm = =  ρ R s t  πt  V

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(1)

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where

is the electrical resistivity, t is the sample thickness, I is the applied current, and V is

the measured voltage drop [18]. 2.4.2 Through-Plane Electrical Conductivity

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The through-plane electrical conductivity (σT) was measured by a two probe setup connected with a multichannel potentiostat (Princeton Applied Research, VersaSTAT MC). The specimen was placed in a fixture designed in our laboratory where the specimen was kept under a consistent pressure. The composite specimen was placed between copper foil electrode, and a

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conductive carbon tape was used to improve the contact surface between the electrodes and the specimen. The electrical resistance (R) was obtained by dividing voltage drop by the current

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applied. The through-plane electrical conductivity (σT) was obtained by Eq. (2): σT =

t R× A

(2)

where R is the electrical resistance, t is the sample thickness, and A is the area of the specimen [19].

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3. Results and Discussion

3.1 Material Characterizations

In Fig.2 TEM images of GO sheets produced from SFG (a) and LFG (d) are shown, respectively.

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It is clear that the individual sheet sizes of GO produced from LFG is considerably greater than that of GO synthesized from SFG. The diagonal graphene sheet size from SFG spans about 10 µm, while the sheet size from LFG reaches 100 µm. It should be pointed out that when imaging

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GO sheets from LFG, one of the smallest GO was chosen for appropriate image since the majority of GO sheets covered the entire TEM grids (200 mesh). The XRD peaks for GO and RGO foams produced from SFG (b) and LFG (e) are presented in Fig. 2 to gain insight into their structural evolution. The interlayer distance (Å) was calculated by Bragg’s law, and the number of layers was estimated from Scherrer-Debye equation (Eq. 3):

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Lc =

0.89 λ β cos (θ )

(3)

where λ is the wavelength of X-ray, β is the Full Width at Half Maximum of peak [20]. The interlayer distance from the (001) peak of GO for SFG and LFG are 8.83 Å (2θ=10.01°) and 8.27

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Å (2θ=10.68°), respectively. This is attributed to the formation of functional groups during oxidation [21]. Upon thermal reduction the interlayer distance for RGO foam SFG and LFG contract to 3.44 Å (2θ=25.92°) and 3.50 Å (2θ=25.43°), respectively owing to removal of functional groups. Furthermore, the estimated number of layers for GO foams from SFG is ~19,

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and this is further reduced to ~7 after the thermal reduction. For LFG, ~15 layers are calculated to be in the GO foams and ~7 layers for RGO foams. This confirms that thermal reduction

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triggers additional exfoliation of the graphene layers.

Fig. 2 - The TEM images of GO from SFG (a) and LFG (d). XRD peaks for GO and RGO foams from SFG (b) and LFG (e), respectively. Raman spectra for GO and RGO foams from SFG (c) and LFG (f)

Raman spectra are widely used to characterize the structural properties of carbonaceous materials, 7

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including disorder and defect structures [22–24]. To further compare the order/disorder degree on the graphene sheets produced from different graphite, Raman spectra for GO and RGO foams synthesized from SFG (c) and LFG (f) are described in Fig. 2. The G band is associated with C-C bond stretching and the D band is attributed to disorders or defects in the graphene sheets. The

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relative intensity ratio of both peaks (ID/IG) is frequently used as a measure of disorder degree. The ID/IG ratio for RGO from SFG is 1.32, while the ratio for RGO from LFG is 1.17. An appropriate quantification of the amount of defects can be obtained by integrating the relative

(

)

I  La = 2.4 × 10 − 10 λ 4  D   IG 

−1

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intensity ratio into Tuinstra & Koenig equation (Eq. 4) to estimate the inter-defect distance (La), (4)

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where λ is the excitation laser wavelength (532 nm) [25] . The inter-defect distance of RGO from LFG was 16.45 nm, and this is higher than that of RGO foam from SFG (14.56 nm). The higher inter-defect distance (La) indicates the lower amount of defects on the graphene sheets. The amount of defects is associated with the amount of edge structures of graphene sheets present in the bulk of nano-composite. The use of LFG reduces the total amount of edge structures in bulk

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RGO foam. Thus, the higher La value obtained for RGO from LFG indicates lesser amount of defects on graphene sheets leading to an improved intrinsic electrical conductivity. This contributes to attaining higher overall electrical conductivity when RGO foam made from LFG is infiltrated with macromolecules compared to that RGO made from SFG. The parameters

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calculated from XRD peaks and Raman spectra are summarized in Table 2.

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Table 2. Parameters estimated from XRD peaks and Raman spectroscopy Raman Spectroscopy

XRD Materials

2θ degree

Interlayer Distance (Å)

FWHM (radian)

Lc(Å)

GO

10.01

8.83

0.00821

169.45

RGO

25.92

3.44

0.05637

25.43

GO

10.68

8.27

0.01110

125.44

RGO

25.43

3.50

0.05364

26.49

Small

La (nm)

19.20

0.97

19.91

7.33

1.32

14.56

15.16

0.96

20.11

7.57

1.17

16.45

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3.2 Morphology of RGO Foam and G-PDMS Composites

ID/IG

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Large

Number of layers

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Graphite Size

The cross and lateral sections of the RGO foams prepared from SFG and LFG are imaged and shown in Fig. 3a-d and Fig. 3e-h, respectively. The SEM images at low magnification confirm that RGO foams are highly porous, and this structure promises a number of future applications by infiltrating with polymers. The SEM images for lateral sections of RGO foams reveal that the

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pores are randomly distributed, but the graphene sheets are well oriented. It is clearly shown that the graphene sheets are aligned along the direction of freezing (arrow) for both sizes of graphite. This confirms that one directional freezing method is an effective way of producing graphene foams exhibiting well-ordered graphene sheets. The SEM images at higher magnifications

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clearly demonstrate that RGO foams from LFG consist of graphene sheets greater in size as compared to those from SFG. This is in line with TEM images showing GO produced from LFG consists of larger size graphene sheets. Furthermore, the graphene sheets from SFG display more

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wrinkles in comparison to the graphene sheets from LFG, probably due to higher defect ratio as indicated by Raman spectra. The graphene sheets from LFG appear to be more flat and less crumpled. This more flat-like and less wrinkled graphene sheets from LFG contribute to reaching higher electrical conductivity for G-PDMS.

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Fig. 3 - The SEM image of RGO foams from SFG for cross (a and c) and lateral (b and d) section. RGO foams from LFG for cross (e and g) and lateral sections (f and h). G-PDMS composites from SFG for cross (i and k) and lateral (j and l) sections. G-PDMS composites from LFG for cross (m and o) and lateral sections (n and p).

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The SEM images of G-PDMS composite demonstrate its combined structure of graphene sheets with PDMS (Fig. 3i-p). The graphene networks that provide the electrical conductivity in the

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composite are clearly observed. The larger size of graphene sheets are observed in the images of G-PDMS from LFG (Fig. 3m-p) as compared to G-PDMS from SFG (Fig. 3i-l). It is also evident that the pores of the RGO foams are completely filled with PDMS and the graphene sheets are well distributed in the G-PDMS. The alignment of graphene sheets along with the direction of freezing is also clearly presented in the SEM images for lateral section. This suggests that RGO foams retain its unique structure after being filled with PDMS, and the infiltration of PDMS does not introduce any discernable deterioration or inhomogeneity to the structure of the RGO foams. Therefore, this method allows the fabrication of flexible and electrically conductive conductors 10

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that overcome the problems of agglomeration and segregation of graphene sheets in the polymercombined composites [11,26]. 3.3 Electrical conductivity of G-PDMS composites

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The (a) in-plane and (b) through-plane electrical conductivity of all the prepared G-PDMS composites is illustrated in Fig. 4. G-PDMS composites based on LFG show significant enhancements in both in-plane and through-plane electrical conductivity. The G-PDMS-1 exhibits an in-plane electrical conductivity of ~3.2 S/m with a loading of 0.4 wt% while the G-

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PDMS-4 shows ~1.4 x 10-2 S/m with a loading of 1.9 wt%. This more than two orders of magnitude gap is a significant difference considering the fact the loading of graphene of small size is four times higher than that of large size. The G-PDMS composites originated from LFG

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also showed improved through-plane electrical conductivity (Fig. 4b). The G-PDMS-2 shows a through-plane electrical conductivity of ~3.2 x 10-2 S/m with a loading of 0.5 wt% whereas the G-PDMS-4 shows ~5.0 x 10-3 S/m with a loading of 1.9 wt%. This significantly improved electrical conductivity is mainly attributed to the fact that G-PDMS-1, G-PDMS-2, and GPDMS-3 consisted of greater size of individual graphene sheets (~100 µm), which is 10 times larger than the small size graphene (~10 µm) is. Furthermore, although the in-plane electrical

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conductivity of G-PDMS is reduced with the use of SFG (G-PDMS-4, 1.4 x 10-2 S/m with a loading of 1.9 wt%), it is still significantly higher than G-PDMS-7 (4.2 x 10-5 S/m with a loading of 4.0 wt%), which was prepared by mechanical mixing RGO powder made from SFG and

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PDMS. This was also observed in through-plane electrical conductivity. This indicates that the interconnected graphene foam provides more efficient paths for electron transfer inside the polymer than conventional graphene powder. In addition, the electrical conductivity does not

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significantly vary with a change of loading of graphene in the composite. For polymer composites fabricated by conventional methods, the percolation theory plays an important role in relationship between the loading of conductive filler and the electrical conductivity [27]. However, the G-PDMS composites retain the three-dimensionally interconnected network of graphene, and once the network is formed, the electron can be moved freely along the graphene network inside the polymer matrix. Thus the popular percolation theory does not play a role for this graphene foam infiltrated composite [14]. 11

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Fig. 4 – (a) In-plane electrical conductivity (σI) and (b) Through-plane electrical conductivity (σT) of G-PDMS composites produced from LFG and SFG, respectively. G-PDMS-7 was produced by mechanical mixing of RGO powder produced from SFG with PDMS.

The use of large sized graphene sheets has several advantages in achieving higher electrical properties of composites. Firstly, individual graphene sheets from LFG shows superior mechanical and electrical properties as it possesses fewer defects during GO production. This is proved by Raman spectra that RGO made from LFG has a lower ID/IG ratio. This is also

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supported by the morphology of graphene sheets, where SEM images shows that large sized graphene sheets have less ripples and wrinkles compared to smaller graphene sheets, indicating that they are mechanically stronger and electrically more conductive. Secondly, since the use of large sized graphene increases overlapped areas between graphene sheets, the contact resistance

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of interconnected graphene network is reduced, and thereby boosting the overall electrical conductivity of the polymer composites. This is in agreement with Hicks’ modeling study that

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graphene sheet area affects nano-composite resistivity more strongly than sheet density, and nano-composites composed of larger sheets are preferred for electrical conductivity applications [28]. Lastly, the large sized graphene sheets diminish the total number of contact junctions on the path for carrying electrons compared to small sized graphene sheets, and therefore enhance the electrical conductivity.

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4. Conclusions In conclusion, we compare the electrical conductivity of G-PDMS from LFG and SFG, respectively. G-PDMS that originated from LFG exhibited significant improvements in both in-

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plane and through-plane electrical conductivity although the loadings of graphene in the GPDMS composites from SFG are considerably higher. G-PDMS fabricated from LFG also showed enhanced through-plane electrical conductivity. This confirms that the size of graphene sheet plays a central role in determining the electrical conductivity of the composite. The greater size of graphene sheet reduces the inter-sheet resistance when fabricated into a bulk composite,

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and the SEM images and Raman spectra from RGO foams suggest that the large sized of graphene sheets possess intrinsically better mechanical and electrical properties. This study

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proves that graphene-polymer composites made of LFG can significantly reveal the excellent properties of graphene and increase the electrical conductivity of the polymer composite. The interconnected graphene foam network provides more efficient paths for electron transfer inside the polymer than conventional graphene powder.

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Acknowledgements

This research was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) as well as Ontario government for Ontario Early Research Award

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Program.

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