- Email: [email protected]
Highly efficient and large-scale synthesis of graphene by electrolytic exfoliation
CARBON
4 7 ( 2 0 0 9 ) 3 2 4 2 –3 2 4 6
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Highly efficient and large-scale synthesis of graphene by electrolytic exfoliation Guoxiu Wang*, Bei Wang, Jinsoo Park, Ying Wang, Bing Sun, Jane Yao Institute for Superconducting and Electronic Materials, School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, New South Wales 2522, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
Highly efficient and large-scale synthesis of graphene from graphite was produced by elec-
Received 9 June 2009
trolytic exfoliation using poly(sodium-4-styrenesulfonate) as an effective electrolyte. Scan-
Accepted 16 July 2009
ning and transmission electron microscopy, and atomic force microscopy confirmed the
Available online 19 July 2009
existence of monolayer graphene sheets and stacks containing a few graphene sheets. Raman spectroscopy demonstrated that the as-prepared graphene sheets have low defect content. Based on the measurement of FTIR spectra, the edge-to-face interaction (p–p interaction) between the graphene surface and aromatic rings of poly(sodium-4-styrenesulfonate) could be primarily responsible for producing exfoliation of the graphite electrode to graphene during electrolysis. In contrast to micromechanical exfoliation, electrolytic exfoliation can be scaled up for large-scale and continuous graphene production. 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
Graphene, a monolayer form of carbon with a two-dimensional (2D) honeycomb lattice, has shown many intriguing properties, including high mobility of charge carriers [1,2], unique transport performance [3,4], high mechanical strength [5,6], and extremely high thermal conductivity [7,8]. These fascinating properties render graphene suitable for many technological applications such as graphenebased electronics [9], composite materials [5], molecular gas sensors [10], and energy storage and conversion [11,12]. All of these engineering applications demand massive production of high quality graphene materials. So far, many methods have been developed to produce graphene. These include: (i) Micromechanical cleavage [1]. This approach can only produce a very limited quantity of graphene sheets for fundamental research. (ii) Epitaxial growth via ultra-high vacuum graphitization [13]. This allows the fabrication of patterned graphene structure, which is desir-
able for electronics. However, the combination of high cost and small wafer size limits its application. (iii) Chemical synthesis through oxidation of graphite. The whole process involves oxidation of graphite to graphite oxide, exfoliation of graphite oxide to yield graphene oxide sheets, and chemical or thermal reduction to graphene. Chemical processing inevitably introduces defects in graphene sheets [14,15]. (iv) Chemical vapor deposition (CVD) growth of graphene either on a substrate or substrate free [16,17]. (v) Solvothermal synthesis combined with pyrolysis [18] and liquid phase exfoliation of graphite [19]. Scaling-up production of graphene is still a big challenge. Herein, we report highly efficient synthesis of graphene by electrolytic exfoliation from graphite, which can be easily scaled up for large-scale production. The as-prepared graphene nanosheets are stable in aqueous solution, ready to be isolated as monolayer or multilayer graphene sheets. The capability to produce graphene in large quantity paves the way for versatile practical applications of graphene.
* Corresponding author: Fax: +61 2 42215731. E-mail address: [email protected] (G. Wang). 0008-6223/$ - see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.07.040
CARBON
2.
Experimental
2.1.
Synthesis of graphene by electrolytic exfoliation
4 7 ( 20 0 9 ) 3 2 4 2–32 4 6
High purity graphite rods (/ 6 mm, 99.999%, Aldrich) were used as electrodes. Poly(sodium-4-styrenesulfonate) (PSS, Mw = 70,000, Aldrich) was dissolved in de-ionized (DI) water to form the electrolyte (0.001 M). In a typical synthesis, two graphite rods were placed in an electrolysis cell filled with the electrolyte. A constant potential of 5 V (DC voltage) was applied to the two electrodes (CHI 660 C Electrochemical Workstation). A diagram of the synthesis apparatus is shown in Fig. 1. After 20 min electrolysis, black product gradually appeared at the positive electrode (anode). The exfoliation continued for 4 h. Then the product (a dispersion) was taken from the electrolysis cell. The dispersion was centrifuged at low speed (1000 rpm) to remove large agglomerates. The top of the dispersion was then decanted. This graphene–PSS suspension is very stable in nature. After 6 months storage, there is no any precipitation. To obtain dry graphene powders, the dispersion was washed with DI water and ethanol, and then dried in a vacuum oven at 80 C. The yield of graphene was estimated by weighting the dried graphene powders and the dried sediment. Our electrolytic exfoliation method results in producing graphene at a yield at about 15 wt.%. Graphene paper was also made by vacuum filtration through an anodized aluminum oxide (AAO) membrane (/ 47 mm, 200 nm pore size, Whatman).
2.2.
Characterisation of graphene
Graphene powders were characterized by X-ray diffraction (XRD, Philips 1730 Diffractometer, Cu Ka radiation) to determine the phase purity. Room-temperature Raman spectra of graphene nanosheets were collected with a Jobin Yvon HR800 confocal Raman system with 632.8 nm diode laser excitation on a 1800-line grating. The morphology of the graphene sheets was observed by field emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F). The crystal structures of the individual graphene nanosheets were analyzed by transmission electron microscopy (TEM) and high
Fig. 1 – Diagram of the apparatus for synthesis of graphene via electrolytic exfoliation.
3243
resolution TEM (HRTEM), using a JEOL 2011 TEM facility. Atomic force microscope (AFM) images of graphene sheets were taken in tapping mode using a SPM Dimension 3100 from Veeco. Fourier transform infrared (FTIR) spectra of the graphene dispersion and PSS surfactant were collected on a Nicolet AVATAR 360 FTIR spectrometer equipped with a Smart OMNI sampler with a germanium crystal.
3.
Results and discussion
After electrolysis, a stable graphene dispersion was obtained. This supernatant can be directly filtered to form graphene paper, or washed with DI water and ethanol, then dried in a vacuum oven to obtain bulk powders of graphene (as shown in Fig. S-1). X-ray diffraction was performed on the as-prepared graphene powders and confirmed the graphitic crystal structure of the graphene (Fig. S-2). Raman spectroscopy is a non-destructive technique to characterize graphite materials, in particular to determine the defects, and the ordered and disordered structures of graphene. Fig. 2 shows the Raman spectrum of the bulk dried graphene powders. As a comparison, the Raman spectrum of the graphite rod was also collected and is presented as the inset in Fig. 2. A broad D band (1350 cm 1) and a sharp G band (1580 cm 1) were observed in the Raman spectrum of the graphene powders. The G line represents the in-plane bondstretching motion of the pairs of C sp2 atoms (the E2g phonons); while the D line corresponds to breathing modes of rings or j-point phonons of A1g symmetry [20]. Usually, the graphene powders synthesized by chemical approach show a strong D band in the Raman spectrum with the intensity ratio of ID/IG > 1 due to the defects and partially disordered crystal structure of graphene sheets. We note here that the intensity of the G band is significantly higher than that of the D band, suggesting that the as-prepared graphene has a low defect content [19]. The morphologies of the bulk graphene powders and the crystal structures of individual graphene sheets were analyzed by field emission electron microscopy and transmission electron microscopy. Fig. 3a shows a FESEM image of the graphene nanosheets. The morphology of individual graphene sheets (or stacked multilayer graphene sheets) resembles waves in a crumpled silk veil. As reported previously, crumpling and scrolling are part of intrinsic nature of graphene sheets [21]. Our graphene sheets look transparent under the electron microscope, indicating that the morphological features of the electrolytically synthesized graphene are similar to those graphene prepared by other approaches. Fig. 3b shows a low magnification TEM image of graphene sheets. The sample was prepared by dropping the highly concentrated graphene dispersion on the TEM grid. Most of the graphene sheets are stacked multilayers. These graphene sheets are rippled and entangled. Well diluted graphene dispersion was also prepared for TEM analysis. Fig. 3c shows a TEM view of a few flat graphene sheets in larger sizes (a few square micrometers), in which about 2–3 layers of graphene overlap. Selected area electron diffraction (SAED) was performed on the graphene sheets and the corresponding SAED pattern is shown as the inset in Fig. 3c. The diffraction rings can be fully indexed to the hexagonal
3244
CARBON
4 7 ( 2 0 0 9 ) 3 2 4 2 –3 2 4 6
Intensity (a.u.)
Intensity (a.u.)
G
D
800
1000
1200
1400
G
1600
1800
2000
-1
Raman shift (cm )
800
1000
1200
D
1400
1600
1800
2000
-1
Raman shift (cm ) Fig. 2 – Raman spectrum of the bulk graphene nanosheet powders. The inset is the Raman spectrum of the pristine graphite rod.
Fig. 3 – (a) FESEM image of the bulk graphene powders. (b) TEM image of many graphene sheets. (c) TEM image of a large graphene flake. The inset is the corresponding SAED pattern. (d) HRTEM image of graphene sheets, showing the featureless basal planes and a cross-sectional view of the edges of folded graphene sheets.
graphitic structure, confirming the crystalline nature of the graphene sheets. A high resolution TEM (HRTEM) image of the graphene sheets is shown in Fig. 3d. The majority of the region shown forms a featureless basal plane. We also observed a cross-sectional view due to folding of the graphene sheets. In Fig. 3d, the straight strips marked with an ellipse consist of two layers of graphene sheets; while the curved strips marked with a rectangle are the stack of 4–5 layers of graphene sheets.
The obtained graphene sheets were also analyzed by atomic force microscopy (AFM). Fig. 4a shows an AFM image of a 5 lm · 5 lm mica surface deposited with graphene dispersion. A few large-size graphene flakes and small-size graphene flakes are clearly visible. The zoomed image of graphene flakes and corresponding line scan are shown in Fig. 4b, from which the topographic height of the graphene flake is measured to be about 0.8 nm, indicating the monolayer graphene sheet.
CARBON
3245
4 7 ( 20 0 9 ) 3 2 4 2–32 4 6
771
1038
1126
1176
1007
837
1636
2932
3456
Transmittance (a.u.)
(a)
4000
3500
3000
2500
2000
-1
1500
1000
500
Transmittance (a.u.)
Wavenumber (cm )
(b)
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Transmittance (a.u.)
(c)
Fig. 4 – AFM images of spray deposited graphene flakes: (a) a 5 lm · 5 lm square shows a few large-size graphene flakes and small-size graphene flakes. (b) A zoomed image of graphene flakes. Below the image is a line scan taken horizontally through the image as marked with a red line, from which the height of a small graphene flake and a large graphene flake were determined to be about 0.8 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fourier transform infrared spectra (FTIR) of PSS surfactant, unwashed graphene and washed graphene were collected and are shown in Fig. 5, making it possible to identify the interaction between PSS and graphene. The unwashed graphene exhibits strong PSS absorption features, but the washed sample shows a very weak PSS signal, indicating that most of the bound PSS can be effectively washed away. No new covalent bond can be detected. As proposed by Ruoff and coworkers [22], the edge-to-face interaction between the
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 5 – FTIR spectra of (a) poly(sodium-4-styrenesulfonate) (PSS) surfactant, (b) unwashed graphene powders containing substantial PSS, (c) washed graphene powders. In figure (a), the band at 3456 cm 1 can be assigned to the absorbed H2O due to hydroscopic nature of PSS. The band at 2932 cm 1 corresponds to C–H stretching vibration of the polymer chain. The band at 1636 cm 1 is S@O functional group. The bands at 1176 cm 1 and 1126 cm 1 can be assigned to C–S bond. The bands at 1038 cm 1 and 1007 cm 1 correspond to S–OH. The bands at 837 cm 1 and 771 cm 1 originate from the aromatic ring.
graphene surface and aromatic rings of PSS is primarily responsible for the binding (p–p interaction). PSS is highly hydroscopic. When it is dissolved in water, it will dissociate into Na+ cations and polystyrenesulfonate anions. During the electrolysis process, polystyrenesulfonate anions were forced to move to the positive graphite electrode under the
3246
CARBON
4 7 ( 2 0 0 9 ) 3 2 4 2 –3 2 4 6
electric force and interact with graphite, leading to the electrolytic exfoliation of the graphite rod.
4.
Summary
Graphene sheets can be easily prepared in large quantity by electrolytic exfoliation of graphite under the assistance of PSS surfactant. FESEM, TEM and AFM observations confirmed the existence of monolayer graphene and stacks composed of a few graphene sheets. Raman spectroscopy indicates the low defect content of as-prepared graphene sheets. FTIR spectra show the existence of PSS surfactant on graphene without any covalent binding. The current preparation method can be scaled up for large-scale and continuous production of graphene materials.
Acknowledgments This work was financially supported by the Australian Research Council (ARC) through an ARC Discovery Project (DP0772999) and an ARC Linkage project (LP0775109).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2009.07.040. R E F E R E N C E S
[1] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306:666–9. [2] Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI, Grigorieva IV, et al. Two-dimensional gas of massless Dirac fermions in graphene. Dubonos. Nature 2005;438:197–200. [3] Zhang YB, Tan YW, Stormer HL, Kim P. Experimental observation of the quantum Hall effect and Berry’s phase in graphene. Nature 2005;438:201–4. [4] Heersche HB, Jarillo-Herrero P, Oostinga JB, Vandersypen LMK, Morpurgo AF. Bipolar supercurrent in graphene. Nature 2007;446:56–9. [5] Stankovich S, Dikin DA, Dommett GHB, Rohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite material. Nature 2006;442:282–6. [6] Lee C, Wei X, Kysar JW, Hove J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008;321:385–8.
[7] Yu AP, Ramesh P, Itkis ME, Bekyarova E, Haddon RC. Graphite nanoplatelet–epoxy composite thermal interface materials. J Phys Chem C 2007;111:7565–9. [8] Balandin AA, Ghosh S, Bao WZ, Calizo I, Teweldebrhan D, Miao F, et al. Superior thermal conductivity of single layer graphene. Nano Lett 2008;8:902–7. [9] Li XL, Wang XR, Zhang L, Lee SW, Dai HJ. Chemically derived, ultrasmooth graphene nanoribbon semiconductors. Science 2008;319:1229–32. [10] Schedin F, Geim AK, Morozov SV, Hill EW, Blake P, Katsnelson MI, et al. Detection of individual gas molecules adsorbed on graphene. Nat Mater 2007;6:652–5. [11] Yoo EJ, Kim J, Hosono E, Zhou HS, Kudo T, Honma I. Large reversible Li storage of graphene nanosheet families for use in rechargeable lithium ion batteries. Nano Lett 2008;8:2277–82. [12] Stroller MD, Park S, Zhu Y, An J, Ruoff RS. Graphene-based ultracapacitors. Nano Lett 2008;8:3498–502. [13] Berger C, Song ZM, Li XB, Wu XS, Brown N, Naud C, et al. Electronic confinement and coherence in patterned epitaxial graphene. Science 2006;312:1191–8. [14] Li XL, Zhang GY, Bai XD, Sun XM, Wang XR, Dai HJ. Highly conducting graphene sheets and Langmuir–blodgett films. Nature Nanotechnol 2008;3:538–42. [15] Tung VC, Allen MJ, Yang Y, Kaner RB. High-throughput solution processing of large-scale graphene. Nature Nanotechnol 2009;4:25–9. [16] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al. Largescale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009;457:706–10. [17] Dato A, Radmilovic V, Lee Z, Phillips J, Frenklach M. Substrate-free gas-phase synthesis of graphene sheets. Nano Lett 2008;8:2012–6. [18] Choucair M, Thordarson P, Stride JA. Gram-scale production of graphene based on solvothermal synthesis and sonication. Nature Nanotechnol 2009;4:30–3. [19] Lotya M, Hernandez Y, King PJ, Smith RJ, Nicolosi V, Karlsson LS, et al. Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions. J Am Chem Soc 2009;131:3611–20. [20] Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2000;61:14095–107. [21] Meyer JC, Geim AK, Katsnelson MI, Novoselov KS, Booth TJ, Roth S. The structure of suspended graphene nanosheets. Nature 2007;446:60–3. [22] Stankovich S, Piner RD, Chen XQ, Wu NQ, Nguyen ST, Ruoff RS. Stable aqueous dispersion of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium-4-styrenesulfonate). J Mater Chem 2006;16:155–8.
4 7 ( 2 0 0 9 ) 3 2 4 2 –3 2 4 6
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Highly efficient and large-scale synthesis of graphene by electrolytic exfoliation Guoxiu Wang*, Bei Wang, Jinsoo Park, Ying Wang, Bing Sun, Jane Yao Institute for Superconducting and Electronic Materials, School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, New South Wales 2522, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
Highly efficient and large-scale synthesis of graphene from graphite was produced by elec-
Received 9 June 2009
trolytic exfoliation using poly(sodium-4-styrenesulfonate) as an effective electrolyte. Scan-
Accepted 16 July 2009
ning and transmission electron microscopy, and atomic force microscopy confirmed the
Available online 19 July 2009
existence of monolayer graphene sheets and stacks containing a few graphene sheets. Raman spectroscopy demonstrated that the as-prepared graphene sheets have low defect content. Based on the measurement of FTIR spectra, the edge-to-face interaction (p–p interaction) between the graphene surface and aromatic rings of poly(sodium-4-styrenesulfonate) could be primarily responsible for producing exfoliation of the graphite electrode to graphene during electrolysis. In contrast to micromechanical exfoliation, electrolytic exfoliation can be scaled up for large-scale and continuous graphene production. 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
Graphene, a monolayer form of carbon with a two-dimensional (2D) honeycomb lattice, has shown many intriguing properties, including high mobility of charge carriers [1,2], unique transport performance [3,4], high mechanical strength [5,6], and extremely high thermal conductivity [7,8]. These fascinating properties render graphene suitable for many technological applications such as graphenebased electronics [9], composite materials [5], molecular gas sensors [10], and energy storage and conversion [11,12]. All of these engineering applications demand massive production of high quality graphene materials. So far, many methods have been developed to produce graphene. These include: (i) Micromechanical cleavage [1]. This approach can only produce a very limited quantity of graphene sheets for fundamental research. (ii) Epitaxial growth via ultra-high vacuum graphitization [13]. This allows the fabrication of patterned graphene structure, which is desir-
able for electronics. However, the combination of high cost and small wafer size limits its application. (iii) Chemical synthesis through oxidation of graphite. The whole process involves oxidation of graphite to graphite oxide, exfoliation of graphite oxide to yield graphene oxide sheets, and chemical or thermal reduction to graphene. Chemical processing inevitably introduces defects in graphene sheets [14,15]. (iv) Chemical vapor deposition (CVD) growth of graphene either on a substrate or substrate free [16,17]. (v) Solvothermal synthesis combined with pyrolysis [18] and liquid phase exfoliation of graphite [19]. Scaling-up production of graphene is still a big challenge. Herein, we report highly efficient synthesis of graphene by electrolytic exfoliation from graphite, which can be easily scaled up for large-scale production. The as-prepared graphene nanosheets are stable in aqueous solution, ready to be isolated as monolayer or multilayer graphene sheets. The capability to produce graphene in large quantity paves the way for versatile practical applications of graphene.
* Corresponding author: Fax: +61 2 42215731. E-mail address: [email protected] (G. Wang). 0008-6223/$ - see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.07.040
CARBON
2.
Experimental
2.1.
Synthesis of graphene by electrolytic exfoliation
4 7 ( 20 0 9 ) 3 2 4 2–32 4 6
High purity graphite rods (/ 6 mm, 99.999%, Aldrich) were used as electrodes. Poly(sodium-4-styrenesulfonate) (PSS, Mw = 70,000, Aldrich) was dissolved in de-ionized (DI) water to form the electrolyte (0.001 M). In a typical synthesis, two graphite rods were placed in an electrolysis cell filled with the electrolyte. A constant potential of 5 V (DC voltage) was applied to the two electrodes (CHI 660 C Electrochemical Workstation). A diagram of the synthesis apparatus is shown in Fig. 1. After 20 min electrolysis, black product gradually appeared at the positive electrode (anode). The exfoliation continued for 4 h. Then the product (a dispersion) was taken from the electrolysis cell. The dispersion was centrifuged at low speed (1000 rpm) to remove large agglomerates. The top of the dispersion was then decanted. This graphene–PSS suspension is very stable in nature. After 6 months storage, there is no any precipitation. To obtain dry graphene powders, the dispersion was washed with DI water and ethanol, and then dried in a vacuum oven at 80 C. The yield of graphene was estimated by weighting the dried graphene powders and the dried sediment. Our electrolytic exfoliation method results in producing graphene at a yield at about 15 wt.%. Graphene paper was also made by vacuum filtration through an anodized aluminum oxide (AAO) membrane (/ 47 mm, 200 nm pore size, Whatman).
2.2.
Characterisation of graphene
Graphene powders were characterized by X-ray diffraction (XRD, Philips 1730 Diffractometer, Cu Ka radiation) to determine the phase purity. Room-temperature Raman spectra of graphene nanosheets were collected with a Jobin Yvon HR800 confocal Raman system with 632.8 nm diode laser excitation on a 1800-line grating. The morphology of the graphene sheets was observed by field emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F). The crystal structures of the individual graphene nanosheets were analyzed by transmission electron microscopy (TEM) and high
Fig. 1 – Diagram of the apparatus for synthesis of graphene via electrolytic exfoliation.
3243
resolution TEM (HRTEM), using a JEOL 2011 TEM facility. Atomic force microscope (AFM) images of graphene sheets were taken in tapping mode using a SPM Dimension 3100 from Veeco. Fourier transform infrared (FTIR) spectra of the graphene dispersion and PSS surfactant were collected on a Nicolet AVATAR 360 FTIR spectrometer equipped with a Smart OMNI sampler with a germanium crystal.
3.
Results and discussion
After electrolysis, a stable graphene dispersion was obtained. This supernatant can be directly filtered to form graphene paper, or washed with DI water and ethanol, then dried in a vacuum oven to obtain bulk powders of graphene (as shown in Fig. S-1). X-ray diffraction was performed on the as-prepared graphene powders and confirmed the graphitic crystal structure of the graphene (Fig. S-2). Raman spectroscopy is a non-destructive technique to characterize graphite materials, in particular to determine the defects, and the ordered and disordered structures of graphene. Fig. 2 shows the Raman spectrum of the bulk dried graphene powders. As a comparison, the Raman spectrum of the graphite rod was also collected and is presented as the inset in Fig. 2. A broad D band (1350 cm 1) and a sharp G band (1580 cm 1) were observed in the Raman spectrum of the graphene powders. The G line represents the in-plane bondstretching motion of the pairs of C sp2 atoms (the E2g phonons); while the D line corresponds to breathing modes of rings or j-point phonons of A1g symmetry [20]. Usually, the graphene powders synthesized by chemical approach show a strong D band in the Raman spectrum with the intensity ratio of ID/IG > 1 due to the defects and partially disordered crystal structure of graphene sheets. We note here that the intensity of the G band is significantly higher than that of the D band, suggesting that the as-prepared graphene has a low defect content [19]. The morphologies of the bulk graphene powders and the crystal structures of individual graphene sheets were analyzed by field emission electron microscopy and transmission electron microscopy. Fig. 3a shows a FESEM image of the graphene nanosheets. The morphology of individual graphene sheets (or stacked multilayer graphene sheets) resembles waves in a crumpled silk veil. As reported previously, crumpling and scrolling are part of intrinsic nature of graphene sheets [21]. Our graphene sheets look transparent under the electron microscope, indicating that the morphological features of the electrolytically synthesized graphene are similar to those graphene prepared by other approaches. Fig. 3b shows a low magnification TEM image of graphene sheets. The sample was prepared by dropping the highly concentrated graphene dispersion on the TEM grid. Most of the graphene sheets are stacked multilayers. These graphene sheets are rippled and entangled. Well diluted graphene dispersion was also prepared for TEM analysis. Fig. 3c shows a TEM view of a few flat graphene sheets in larger sizes (a few square micrometers), in which about 2–3 layers of graphene overlap. Selected area electron diffraction (SAED) was performed on the graphene sheets and the corresponding SAED pattern is shown as the inset in Fig. 3c. The diffraction rings can be fully indexed to the hexagonal
3244
CARBON
4 7 ( 2 0 0 9 ) 3 2 4 2 –3 2 4 6
Intensity (a.u.)
Intensity (a.u.)
G
D
800
1000
1200
1400
G
1600
1800
2000
-1
Raman shift (cm )
800
1000
1200
D
1400
1600
1800
2000
-1
Raman shift (cm ) Fig. 2 – Raman spectrum of the bulk graphene nanosheet powders. The inset is the Raman spectrum of the pristine graphite rod.
Fig. 3 – (a) FESEM image of the bulk graphene powders. (b) TEM image of many graphene sheets. (c) TEM image of a large graphene flake. The inset is the corresponding SAED pattern. (d) HRTEM image of graphene sheets, showing the featureless basal planes and a cross-sectional view of the edges of folded graphene sheets.
graphitic structure, confirming the crystalline nature of the graphene sheets. A high resolution TEM (HRTEM) image of the graphene sheets is shown in Fig. 3d. The majority of the region shown forms a featureless basal plane. We also observed a cross-sectional view due to folding of the graphene sheets. In Fig. 3d, the straight strips marked with an ellipse consist of two layers of graphene sheets; while the curved strips marked with a rectangle are the stack of 4–5 layers of graphene sheets.
The obtained graphene sheets were also analyzed by atomic force microscopy (AFM). Fig. 4a shows an AFM image of a 5 lm · 5 lm mica surface deposited with graphene dispersion. A few large-size graphene flakes and small-size graphene flakes are clearly visible. The zoomed image of graphene flakes and corresponding line scan are shown in Fig. 4b, from which the topographic height of the graphene flake is measured to be about 0.8 nm, indicating the monolayer graphene sheet.
CARBON
3245
4 7 ( 20 0 9 ) 3 2 4 2–32 4 6
771
1038
1126
1176
1007
837
1636
2932
3456
Transmittance (a.u.)
(a)
4000
3500
3000
2500
2000
-1
1500
1000
500
Transmittance (a.u.)
Wavenumber (cm )
(b)
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Transmittance (a.u.)
(c)
Fig. 4 – AFM images of spray deposited graphene flakes: (a) a 5 lm · 5 lm square shows a few large-size graphene flakes and small-size graphene flakes. (b) A zoomed image of graphene flakes. Below the image is a line scan taken horizontally through the image as marked with a red line, from which the height of a small graphene flake and a large graphene flake were determined to be about 0.8 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fourier transform infrared spectra (FTIR) of PSS surfactant, unwashed graphene and washed graphene were collected and are shown in Fig. 5, making it possible to identify the interaction between PSS and graphene. The unwashed graphene exhibits strong PSS absorption features, but the washed sample shows a very weak PSS signal, indicating that most of the bound PSS can be effectively washed away. No new covalent bond can be detected. As proposed by Ruoff and coworkers [22], the edge-to-face interaction between the
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 5 – FTIR spectra of (a) poly(sodium-4-styrenesulfonate) (PSS) surfactant, (b) unwashed graphene powders containing substantial PSS, (c) washed graphene powders. In figure (a), the band at 3456 cm 1 can be assigned to the absorbed H2O due to hydroscopic nature of PSS. The band at 2932 cm 1 corresponds to C–H stretching vibration of the polymer chain. The band at 1636 cm 1 is S@O functional group. The bands at 1176 cm 1 and 1126 cm 1 can be assigned to C–S bond. The bands at 1038 cm 1 and 1007 cm 1 correspond to S–OH. The bands at 837 cm 1 and 771 cm 1 originate from the aromatic ring.
graphene surface and aromatic rings of PSS is primarily responsible for the binding (p–p interaction). PSS is highly hydroscopic. When it is dissolved in water, it will dissociate into Na+ cations and polystyrenesulfonate anions. During the electrolysis process, polystyrenesulfonate anions were forced to move to the positive graphite electrode under the
3246
CARBON
4 7 ( 2 0 0 9 ) 3 2 4 2 –3 2 4 6
electric force and interact with graphite, leading to the electrolytic exfoliation of the graphite rod.
4.
Summary
Graphene sheets can be easily prepared in large quantity by electrolytic exfoliation of graphite under the assistance of PSS surfactant. FESEM, TEM and AFM observations confirmed the existence of monolayer graphene and stacks composed of a few graphene sheets. Raman spectroscopy indicates the low defect content of as-prepared graphene sheets. FTIR spectra show the existence of PSS surfactant on graphene without any covalent binding. The current preparation method can be scaled up for large-scale and continuous production of graphene materials.
Acknowledgments This work was financially supported by the Australian Research Council (ARC) through an ARC Discovery Project (DP0772999) and an ARC Linkage project (LP0775109).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2009.07.040. R E F E R E N C E S
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