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J. Phys. Chem. C 2008, 112, 8192–8195
Facile Synthesis and Characterization of Graphene Nanosheets Guoxiu Wang,* Juan Yang, Jinsoo Park, Xinglong Gou, Bei Wang, Hao Liu, and Jane Yao Materials Chemistry Research Group, School of Mechanical, Materials and Mechatronic Engineering, UniVersity of Wollongong, NSW 2522, Australia ReceiVed: NoVember 16, 2007; ReVised Manuscript ReceiVed: March 9, 2008
Graphene nanosheets were produced in large quantity via a soft chemistry synthetic route involving graphite oxidation, ultrasonic exfoliation, and chemical reduction. X-ray diffraction and transmission electron microscopy (TEM) observations show that graphene nanosheets were produced with sizes in the range of tens to hundreds of square nanometers and ripple-like corrugations. High resolution TEM (HRTEM) and selected area electron diffraction (SAED) analysis confirmed the ordered graphite crystal structure of graphene nanosheets. The optical properties of graphene nanosheets were characterized by Raman spectroscopy. 1. Introduction Graphene is a single layer of carbon atoms in a closely packed honeycomb two-dimensional (2D) lattice. Since its discovery by K. S. Novoselov and A. K. Geim in 2004,1 it has attracted numerous investigations into its unique physical, chemical, and mechanical properties, opening up a new research area for materials science and condensed-matter physics, and aiming for a wide-ranging and diversified technological applications.2–6 Owing to the high quality of the sp2 conjugated bond in the carbon lattice, electrons were found to move ballistically in a graphene layer without scattering with mobilities exceeding 15 000 m2 V-1 s-1 at ambient temperature. Furthermore, the charge carriers in graphene crystals mimic relativistic particles with zero rest mass, being described as massless Dirac fermins.2,7 These characteristics drive the dreams of developing graphene based electronics. Although this has not yet been realized, there are real possibilities for a bright future. The preparation of high-quality 2D graphene crystals is the first and most crucial step, not only for fundamental research but also for device applications. Micromechanical cleavage of bulk graphite can only produce graphene flakes in limited quantities. However, the entire process is hard to control. In a later method, ultrathin epitaxial graphene has been grown on single-crystal silicon carbide by vacuum graphitization. This approach allows the fabrication of a patterned graphene structure, which is desirable for electronic applications.8–10 Recently, polystyrene-graphene based composite materials have demonstrated extraordinary room temperature electrical conductivity, leading to the development of a new class of composite materials with enhanced properties and functionalities.11 Graphene oxide papers exhibit high mechanical stiffness and strength, resulting from a unique interlocking-tile arrangement of the nanosize graphene oxide sheets.12 Therefore, graphene has great potential to be massively used as an engineering material with the demand exceeding 1 million tons annually. Micromechanical cleavage and ultrahigh vacuum graphization certainly cannot meet such a high demand in the future. Herein, we report a soft chemical synthesis route toward massive production of graphene nanoplatelets. We prepared high-quality graphene nanosheets through this approach. The * Corresponding author. E-mail:
[email protected].
crystal structure and optical properties of graphene nanosheets were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution TEM (HRTEM), and Raman spectroscopy. 2. Experimental Section Graphene Nanosheet Synthesis. Natural graphite (Aldrich) was used for the preparation of graphene nanosheets. Graphite oxide powders were prepared from it by a modification of Hummers and Offeman’s method.13 Graphite powders were first oxidized by reacting them with concentrated nitric acid and sulfuric acid (1:2 in volume). The reaction vessel was immersed in an ice bath, and potassium chlorate was added slowly. The reaction was allowed to go on for 120 h to fully oxidize graphite into graphite oxide (GO). The GO was thoroughly washed and filtered by deionized water and filtered to remove metal ions and until pH 7. The GO was suspended in a mixture of ethanol and water, and exfoliated through ultrasonication for 1 h. We observed that the suspension gradually evolved into a yellowbrown solution, during which the bulk GO powders were transformed into nanoplatelets. The exfoliated GO was reduced to graphene nanoplatelets by refluxing the GO solution with hydroquinone for 20 h. The final products were then centrifuged, washed, and finally vaccuum-dried. Structural and Raman Spectroscopy Characterization. The evolution of GO during the oxidation process was monitored by X-ray diffraction (XRD; Philips 1730 Diffractometer). Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) analyses were performed on GO and graphene powders by using a JEOL2011 TEM facility. TEM samples were prepared by dispersing GO and graphene dry powders in ethanol to form a homogeneous suspension. Then, the suspension was dropped on 200-mesh copper grid for observation. The Raman spectra of the pristine graphite, GO, and graphene powders were measured by using a Jobin Yvon HR800 confocal Raman system with 632.8 nm diode laser excitation on a 300 lines/mm grating at room temperature. 3. Results and Discussion Natural flake graphite with an average particle size of 250 µm was used as the starting material. As described previously, the inter-graphene layers can be intercalated by various molecular species or ions, during which the interlayer spacing along
10.1021/jp710931h CCC: $40.75 2008 American Chemical Society Published on Web 05/01/2008
Characterization of Graphene Nanosheets
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Figure 1. X-ray diffraction patterns of pristine graphite powders and graphite oxide (GO) at various oxidation stages: 48, 72, and 120 h, respectively.
the c-axis changes from 3.4 to 6.25–7.5 Å.14,15 We employed a mixture of sulfuric acid, nitric acid, and potassium chlorate to oxidize the natural graphite powders in water-based solution. During this process, hydroxyl, carbonyl, epoxy, and peroxy groups were bonded to the edges of basal planes of the graphite structure. Simultaneously, carbon hydrolyzation occurred and the sp2 bonds changed to sp3 bonds. At the same time, H2O, NO3-, or SO42- ions could insert themselves into the graphene layer, inducing an increase in the interlayer spacing.16–18 Although there are many models describing the structure of graphite oxide, the exact structure of GO still remains unclear. We monitored the graphite oxidation process by X-ray diffraction. The intermediate oxidized graphite products were taken from the reaction flask at various oxidation stages. After washing and vacuum-drying, the powders were examined by X-ray diffraction. Figure 1 shows the XRD patterns of the original graphite and the graphite oxide at various oxidation stages. As oxidation proceeds, the intensity of the (002) diffraction line (d-space 3.4 Å at 26.23°) gradually weakened and finally disappeared. Simultaneously, the intensity of the diffraction peak at 11.8° (corresponding to a d-spacing of 0.749 nm) increased with oxidation. After 120 h of chemical treatment, the graphite powders were completely oxidized to graphite oxide. We also noticed that the (002) diffraction line almost completely disappeared after oxidation for 48 h, indicating that the full oxidation of graphite powders can be achieved within a short period. The graphite oxide powders were exfoliated via ultrasonic vibration to produce GO nanoplatelets. The morphology and structure of the GO were observed by TEM analysis. Figure 2a shows a general view of GO nanoplatelets, clearly illustrating the flake-like shapes of graphite oxide particles. Multilayer GO sheets corrugated together with sizes in the range of tens to several hundreds of square nanometers. Figure 2b exhibits a high magnification TEM image of a GO nanosheet, showing a completely amorphous and disordered structure. Selected area electron diffraction (SAED, inset in Figure 2b) further confirmed the disordered nature of the GO nanoplatelets. The SAED pattern of GO shows only diffraction rings and the diffraction dots are unresolved, unambiguously indicating that the GO flakes are amorphous. This result is consistent with XRD analysis. We also found that the GO nanosheets were unstable under electron beam bombardment. After a few minutes exposure to the electron beam during the TEM observation, the GO nanosheets were found to be broken up, which was possibly
Figure 2. (a) TEM image of GO nanosheets. (b) HRTEM image of a single GO nanosheet, illustrating their amorphous character. The inset shows the corresponding SAED pattern.
Figure 3. X-ray diffraction pattern of graphene nanosheets.
caused by the evaporation of the oxygen- and hydrogencontaining functionalized groups. After chemical reduction through refluxing with hydroquinone, GO nanosheets were reduced to graphene nanosheets and restored to an ordered crystal structure. This is evidenced by the reappearance of the (002) diffraction line and disappearance of the diffraction peak at 11.8° in the XRD pattern (Figure 3). During the reduction process of the graphene oxide nanoplatelets by hydroquinone, the yellow-brown solution gradually yielded a black precipitate. Hydroquinone acts as a reducing agent by losing either one H+ from one of its hydroxyls to form a monophenolate ion or two H+ from both hydroxyls to form a diphenolate ion (quinone).19 It is well established that graphite oxides consist of various different types of oxygen functional groups such as hydroxyl, carbonyl, epoxide, lactone, or ether groups. Although the full
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mechanisms of GO reduction are not clear yet, the reduction process must involve the removal of the above oxygen functional groups. Since the GO can be directly converted to the crystalline graphene (d002 ) 3.4 Å), the conjugated graphene network (sp2 carbon) must be reestablished during the reduction process, which is associated with the ring-opening of the epoxides.20,21 Graphene oxide nanosheets are hydrophilic. The GO nanosheet solution in water is stable indefinitely, while graphene nanoplatelets are hydrophobic. We noted that the graphene nanoplatelets can be dispersed into a homogeneous suspension in water via ultrasonic vibration, but the dispersion can only be stable for a few hours due to their hydrophobic nature. The obtained graphene nanosheets were fully analyzed by TEM and HRTEM observations. Figure 4a shows a low magnification TEM image of graphene nanosheets. Large graphene nanosheets (a few hundred square nanometers) were observed to be situated on the top of the copper grid, where they resemble crumpled silk veil waves. Graphene nanosheets were rippled and entangled with each other. They are transparent and exhibit a very stable nature under the electron beam. The most transparent and featureless regions indicated by arrows in Figure 4a are likely to be monolayer graphene nanosheets. We also observed scrolled graphene nanosheets (as shown in Figure 2b). As reported previously,22 corrugation and scrolling are intrinsic to graphene nanosheets. This is because the thermodynamic stability of the 2D membrane results from microscopic crumpling via bending or buckling. Figure 5 shows a high magnification TEM image of a graphene nanosheet. The ordered graphite lattices are clearly visible. However, the disordered regions are also found, indicating that the graphene nanosheets were partially restored to ordered crystal structure. The graphitic laminar structure can be resolved in the ordered region. We performed selected area electron diffraction (SAED) on this region along the [001] zone axis. The SAED pattern is shown as the inset in Figure 5. The well-defined diffraction spots confirm the crystalline structure of the graphene nanoplatelets obtained via chemical reduction of graphite oxides. Therefore, the XRD, HRTEM, and SAED analyses clearly demonstrated that hydroquinone reduction can produce crystalline 2D graphene nanosheets. The thermal behaviors of the GO, graphene nanosheets, and pristine graphite powders were investigated by thermogravmetric analysis (TGA) in dry air. The TGA curves of the pristine graphite, GO, and graphene nanoplatelets are shown in Figure 6. The pristine graphite starts to lose weight at 650 °C due to combustion to carbon dioxide. The GO powders exhibit two steps of mass loss at 200 and 550 °C, which are attributed to the removal of oxygen-containing groups and carbon oxidation, respectively. Graphene nanosheets show a mass loss, starting at an onset temperature of 250 °C, illustrating a much lower thermal stability compared to the bulk graphite powders. Raman spectroscopy is a powerful nondestructive tool to characterize carbonaceous materials, particularly for distinguishing ordered and disordered crystal structures of carbon. The typical features for carbon in Raman spectra are the G line around 1582 cm-1 and the D line around 1350 cm-1. The G line is usually assigned to the E2g phonon of C sp2 atoms, while the D line is a breathing mode of κ-point phonons of A1g symmetry.23,24 The overtone of the D line, the D′ line, is located
Wang et al.
Figure 4. (a) Low magnification TEM image of graphene nanosheets, resembling crumpled silk. The featureless regions indicated by the arrows are monolayer graphene nanosheets. (b) TEM image of a scrolled gaphene nanosheet.
Figure 5. HRTEM image of a single graphene nanosheet. The inset is the selected area electron diffraction pattern (SAED), which confirms the crystalline nature of the graphene nanosheet.
at 2700 cm-1, while the G′ line (the overtone of the G line) is around 3248 cm-1. Figure 7 shows Raman spectra of pristine graphite and graphene nanosheets. The Raman spectrum of the pristine natural graphite displays a strong G line at 1582 cm-1, a weak D line at 1350 cm-1, a broad D′ line at 2690 cm-1, and a very weak G′ line at 3245 cm-1. In the Raman spectrum of graphene nanosheets, the G band is broadened and shifted upward to 1595 cm-1. At the same time, the intensity of the D band at 1350 cm-1 increases substantially. These phenomena could be attributed to the significant decrease of the size of the in-plane sp2 domains due to oxidation and ultrasonic exfoliation, and partially ordered graphite crystal structure of graphene nanosheets.21 4. Conclusions Graphene nanosheets can be massively prepared by a soft chemistry route. TEM and HRTEM observations confirmed the crystalline nature of graphene nanosheets and also demonstrated that graphene nanosheets naturally corrugated into ripples, like wavy silk. TGA analysis found that graphene nanosheets have
Characterization of Graphene Nanosheets
J. Phys. Chem. C, Vol. 112, No. 22, 2008 8195 acknowledged. We thank Prof. X.Y. Kong for performing the TEM analysis. References and Notes
Figure 6. TGA curves of pristine natural graphite, graphene nanosheets, and exfoliated graphite oxide.
Figure 7. Raman spectra of pristine natural graphite and graphene nanosheets.
much lower thermal stability than pristine natural graphite powders. This is possibly caused by the size effect. Raman spectroscopy measurement indicates the small size of the inplane sp2 domains of graphene nanosheets. Acknowledgment. Financial support by a grant (DP0772999) from the Australian Research Council (ARC) is gratefully
(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197. (3) Zhang, Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201. (4) Aleiner, I. L.; Efetov, K. B. Phys. ReV. Lett. 2006, 97, 236802. (5) Jannik, C. M.; Geim, A. K.; Katsnelson, M. I.; Novoselov, M. I.; Booth, T. J.; Roth, S. Nature 2007, 446, 60. (6) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nat. Mater. 2007, 6, 652. (7) Heersche, H. B.; Jarillo-Herrero, P.; Oostinga, J. B.; Vandersypen, L. M. K.; Morpurgo, A. F. Nature 2007, 446, 56. (8) Charrier, A.; Coati, A.; Argunova, T.; Thibaudau, F.; Garreau, Y.; Pinchaux, R.; Forbeaux, I.; Debever, J.-M.; Sauvage-Simkin, M.; Themlin, J.-M. J. Appl. Phys. 2002, 92, 2479. (9) Berger, C.; Song, Z. M.; Li, T. B.; Li, X. B.; Ogbazghi, A. Y.; Feng, R.; Dai, Z. T.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. J. Phys. Chem. B 2004, 108, 19912. (10) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Nand, C.; Mayon, D.; Li, T. B.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Science 2006, 312, 1191. (11) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, Z. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282. (12) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, T.; Ruoff, R. S. Nature 2007, 448, 457. (13) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (14) Matsuo, Y.; Higashika, S.; Kimura, K.; Miyamoto, Y.; Fukutsuka, T.; Sugie, Y. J. Mater. Chem. 2002, 12, 1592. (15) Viculis, L. M.; Mack, J. J.; Mayer, O. M.; Hahn, H. T.; Kaner, R. B. J. Mater. Chem. 2005, 15, 974. (16) Hontoria-Lucas, C.; López-Peinado, A. J.; López-González, J. de D.; Rojas-Cervantes, M. L.; Martín-Aranda, R. M. Carbon 1995, 33, 1585. (17) He, H. Y.; Riedl, T.; Lerf, A.; Klinowski, J. J. Phys. Chem. 1996, 100, 19954. (18) He, H. Y.; Klinowski, J.; Forster, M.; Lerf, A. Chem. Phys. Lett. 1998, 287, 53. (19) Jia, Y. F.; Demopoulos, G. P. Ind. Eng. Chem. Res. 2003, 42, 72. (20) Bourlinos, A. B.; Gournis, D.; Petridis, D.; Szabó, T.; Szeri, A.; Dékány, I. Langmuir 2003, 19, 6050. (21) Stankovich, S.; Dikin, A. A.; Piner, R. D.; Kohlhass, K. A.; Kleinhammes, A.; Jia, Y. Y.; Wu, Y.; Nguyen, S.; Ruoff, R. S. Carbon 2007, 45, 1558. (22) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. Nature 2007, 446, 60. (23) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53, 1126. (24) Ferrari, A. C.; Robertson, J. Phys. ReV. B 2000, 61, 14095.
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