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Liquid Phase Production of Graphene by Exfoliation of Graphite in Surfactant/Water Solutions Mustafa Lotya,† Yenny Hernandez,† Paul J. King,† Ronan J. Smith,† Valeria Nicolosi,‡ Lisa S. Karlsson,‡ Fiona M. Blighe,† Sukanta De,†,§ Zhiming Wang,† I. T. McGovern,† Georg S. Duesberg,§,| and Jonathan N. Coleman*,†,§ School of Physics, Trinity College Dublin, Dublin 2, Ireland, Department of Materials, UniVersity of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom, CRANN, Trinity College Dublin, Dublin 2, Ireland, and School of Chemistry, Trinity College Dublin, Dublin 2, Ireland Received September 29, 2008; E-mail: colemaj@tcd.ie Abstract: We have demonstrated a method to disperse and exfoliate graphite to give graphene suspended in water-surfactant solutions. Optical characterization of these suspensions allowed the partial optimization of the dispersion process. Transmission electron microscopy showed the dispersed phase to consist of small graphitic flakes. More than 40% of these flakes had <5 layers with ∼3% of flakes consisting of monolayers. Atomic resolution transmission electron microscopy shows the monolayers to be generally free of defects. The dispersed graphitic flakes are stabilized against reaggregation by Coulomb repulsion due to the adsorbed surfactant. We use DLVO and Hamaker theory to describe this stabilization. However, the larger flakes tend to sediment out over ∼6 weeks, leaving only small flakes dispersed. It is possible to form thin films by vacuum filtration of these dispersions. Raman and IR spectroscopic analysis of these films suggests the flakes to be largely free of defects and oxides, although X-ray photoelectron spectroscopy shows evidence of a small oxide population. Individual graphene flakes can be deposited onto mica by spray coating, allowing statistical analysis of flake size and thickness. Vacuum filtered films are reasonably conductive and are semitransparent. Further improvements may result in the development of cheap transparent conductors. 1. Introduction The discovery of monolayer graphene in 20041 has led to the demonstration of a host of novel physical properties in this most exciting of nanomaterials.2 Graphene is generally made by micromechanical cleavage, a process whereby monolayers are peeled from graphite crystals. However, this process has significant disadvantages in terms of yield and throughput. As such, there has been significant interest in the development of a large-scale production method for graphene. In the long term, for many research areas the growth of graphene monolayers3-5 is by far the most desirable route. However, progress has been slow, and, in any case, this technique will be unsuitable for certain applications. Thus, in the medium term, the most promising route is the exfoliation of graphite in the liquid phase to give graphene-like materials. The most common technique has been the oxidation and subsequent exfoliation of graphite to give graphene oxide.6-10 However, this technique suffers from one significant disadvantage; the oxidation process results in the formation of structural defects as evidenced by Raman spectroscopy.6,9 These defects alter the electronic structure of graphene so much as to render it semiconducting.11 These defects are virtually impossible to remove completely; even after annealing at 1100 °C, residual CdO and C-O bonds are observed by X-ray photoelectron spectroscopy.10 Even relatively mild chemical treatments, involving soaking in oleum, result in non-negligible oxidation, which requires annealing at 800 °C to remove.12† School of Physics, Trinity College Dublin. ‡ University of Oxford. § CRANN, Trinity College Dublin. | School of Chemistry, Trinity College Dublin. (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, 666–669. (2) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191. (3) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayo, D.; Li, T. B.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Science 2006, 312, 1191–1196. (4) Ohta, T.; El Gabaly, F.; Bostwick, A.; McChesney, J. L.; Emtsev, K. V.; Schmid, A. K.; Seyller, T.; Horn, K.; Rotenberg, E. New J. Phys. 2008, 10. (5) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Nature 2009, 457, 706- 710. (6) Eda, G.; Fanchini, G.; Chhowalla, M. Nat. Nanotechnol. 2008, 3, 270– 274. (7) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101–105. (8) Niyogi, S.; Bekyarova, E.; Itkis, M. E.; McWilliams, J. L.; Hamon, M. A.; Haddon, R. C. J. Am. Chem. Soc. 2006, 128, 7720–7721. (9) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558–1565. (10) Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. ACS Nano 2008, 2, 463–470. (11) Jung, I.; Pelton, M.; Piner, R.; Dikin, D. A.; Stankovich, S.; Watcharotone, S.; Hausner, M.; Ruoff, R. S. Nano Lett. 2007, 7, 3569– 3575. Published on Web 02/19/2009 10.1021/ja807449u CCC: $40.75  2009 American Chemical Society J. AM. CHEM. SOC. 2009, 131, 3611–3620 9 3611 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 下划线 Administrator 下划线 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 矩形 Recently, a significant breakthrough was made when two independent groups showed that graphite could be exfoliated in certain solvents to give defect-free monolayer graphene.13,14 This phenomenon relies on using particular solvents, such as N-methyl-pyrrolidone, whose surface energy is so well matched to that of graphene that exfoliation occurs freely.14 However, this process is not without its drawbacks. These solvents are expensive and require special care when handling. In addition, they tend to have high boiling points, making it difficult to deposit individual monolayers on surfaces. Unfortunately, the most useful solvent of all, water, has a surface energy that is much too high to work on its own as an exfoliant for graphene. With these factors in mind, it is easy to see what is needed. We require an alternative, liquid phase process that results in the exfoliation of graphite to give graphene at reasonably high yield. The method should be non-oxidative and should not require high temperature processes or chemical post treatments. In addition, it should be compatible with safe, user-friendly, low boiling-point solvents, preferably water. In this Article, we demonstrate such a method. We disperse graphite in surfactant-water solutions in a manner similar to surfactant aided carbon nanotube dispersion.15-20 By transmis- sion electron microscopy (TEM) analysis, we demonstrate significant levels of exfoliation including the observation of a number of graphene monolayers. Atomic resolution TEM shows the monolayers to be well graphitized and largely defect free. Raman, IR, and X-ray photoelectron spectroscopies also show the graphite/graphene to be relatively defect free and only very slightly oxidized. These dispersions can be vacuum filtered to make thin conductive films and deposited onto surfaces as individual flakes. 2. Results and Discussion 2.1. Optimization of Dispersion Conditions. The absorption coefficient, R, which is related to the absorbance, A, through the Lambert-Beer law (A ) RCl, where C is the concentration and l is the path length), is an important parameter in characterizing any dispersion. To accurately determine R, we prepared a dispersion (∼400 mL) with initial graphite concen- tration, CG,i ) 0.1 mg/mL, and surfactant (sodium dodecylben- zene sulfonate, SDBS) concentration, CSDBS ) 0.5 mg/mL. This was then centrifuged and decanted, and the absorption spectrum was measured (inset of Figure 1). As expected for a quasi two- dimensional material, this spectrum is flat and featureless21 everywhere except below 280 nm where we observe a strong absorption band, which scaled linearly with SDBS concentration but was independent of the graphite concentration; we attribute this band to the SDBS. A precisely measured volume of the dispersion was filtered under high vacuum onto an alumina membrane of known mass. The resulting compact but relatively thick film (∼5 µm) was washed with 1 L of water and dried overnight in a vacuum oven at room temperature. The mass of material in the filtered volume of stock dispersion was then determined using a microbalance. From thermogravimetric (TGA) analysis (not shown) of the dried film, we found that 64 ( 5% of the film was graphitic; the remainder was attributed to residual surfactant. We are not surprised to find so much residual surfactant in these films. Their considerable thickness (∼5 µm) makes it very difficult to wash away the surfactant during film formation. Knowledge of the mass of graphite in the film allowed us to determine the final concentration of the stock dispersion. A sample of the stock dispersion was then serially diluted with 0.5 mg/mL SDBS solution, allowing the measurement of the absorbance per unit length (A/l) versus concentration of graphite (after centrifugation, CG), as shown in Figure 1. A straight line fit through these points gives the absorption coefficient at 660 nm of R ) 1390 mL mg-1 m-1 in reasonable agreement with the value measured for graphite/ graphene in various solvents.14 The non-zero intercept in Figure 1 is attributable to the A/l of residual SDBS in the dispersion (intercept of A/l ) 0.72 m-1 compares with residual absorbance of A/l ≈ 0.5 m-1 for SDBS at CSDBS ) 0.5 mg/mL). Using R for our dispersions, it is possible to determine CG for all subsequent samples. Thus, the fraction of graphite material remaining for any sample after centrifugation (CF) can be calculated from the ratio of dispersed graphite after CF to that before CF: CG/CG,i. Using this fraction-remaining as a gauge, the concentrations CG,i and CSDBS could be optimized. Holding (12) Li, X.; Zhang, G.; Bai, X.; Sun, X.; Wang, X.; Wang, E.; Dai, H. Nat. Nanotechnol. 2008, 3, 538–542. (13) Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.; Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W.; Geim, A. K.; Novoselov, K. S. Nano Lett. 2008, 8, 1704– 1708. (14) Hernandez, Y.; et al. Nat. Nanotechnol. 2008, 3, 563–568. (15) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E.; Schmidt, J.; Talmon, Y. Nano Lett. 2003, 3, 1379–1382. (16) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J. P.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593–596. (17) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y. H.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265–271. (18) Strano, M. S.; Moore, V. C.; Miller, M. K.; Allen, M. J.; Haroz, E. H.; Kittrell, C.; Hauge, R. H.; Smalley, R. E. J. Nanosci. Nanotechnol. 2003, 3, 81–86. (19) Bergin, S. D.; Nicolosi, V.; Cathcart, H.; Lotya, M.; Rickard, D.; Sun, Z. Y.; Blau, W. J.; Coleman, J. N. J. Phys. Chem. C 2008, 112, 972– 977. (20) Sun, Z.; Nicolosi, V.; Rickard, D.; Bergin, S. D.; Aherne, D.; Coleman, J. N. J. Phys. Chem. C 2008, 112, 10692–10699. (21) Abergel, D. S. L.; Fal’ko, V. I. Phys. ReV. B 2007, 75. Figure 1. Absorbance per unit length (λ ) 660 nm) as a function of graphite concentration (after centrifugation) for an SDBS concentration, CSDBS ) 0.5 mg/mL. Graphite concentration before centrifugation was CG,i ) 0.1 mg/mL. NB, the curve does not go through the origin due to the presence of a residual SDBS absorbance. (Intercept of A/l ) 0.72 m-1 compares with residual absorbance of A/l ≈ 0.5 m-1 for SDBS at CSDBS ) 0.5 mg/ mL.) Bottom inset: Absorption spectrum for a sample with CSDBS ) 0.5 mg/mL and CG ) 0.0027 mg/mL. The portion below 400 nm is dominated by the surfactant absorption and has been scaled by a factor of 1/8 for clarity. The portion above 400 nm is dominated by graphene/graphite with some residual SDBS absorption. Top inset: Surfactant-stabilized graphite dispersions (A) before and (B) immediately after centrifugation. Note that the dispersions are almost transparent due to the low concentration of graphite. 3612 J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009 A R T I C L E S Lotya et al. Administrator 矩形 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 下划线 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 下划线 Administrator 高亮 Administrator 高亮 Administrator 下划线 Administrator 矩形 Administrator 高亮 Administrator 高亮 CSDBS constant at a relatively high value of 10 mg/mL, CG was measured as a function of CG,i (Figure 2). Interestingly, we observe an empirical relationship of the form: CG ) 0.01�CG,i. The highest concentration achieved after CF was CG ) 0.05 mg/mL for CG,i ) 14 mg/mL. We have observed concentrations in the range 0.002 mg/mL < CG < 0.05 mg/mL. We note that this is very similar to the range of concentrations generally achieved for surfactant-stabilized nanotube dispersions.22 The largest fraction remaining was ∼3 wt % at CG,i ) 0.1 mg/mL (top inset, Figure 2). This graphite concentration was then fixed and CSDBS varied. Measurement of the fraction remaining showed a broad peak (lower inset, Figure 2), similar to those observed for nanotube-surfactant dispersions.19 The graphitic content was maximized for CSDBS between 0.5 and 1 mg/mL, concentrations very close to the critical micelle concentration (CMC), which is ∼0.7 mg/mL for SDBS.23 The falloff in dispersed graphite below CSDBS ≈ 0.5 mg/mL is reminiscent of the destabilization of nanotube dispersions as the surfactant concentration is reduced below the CMC.19,24 With this in mind, we can hypothesize that the minimum surfactant concentration required for successful dispersion of graphite is the critical micelle concentration. If this is the case, the surfactant concen- tration could possibly be reduced by using alternative surfactants with lower CMC. In this work, to keep the concentration of surfactant to a minimum, all subsequent experiments were performed on standard dispersions with surfactant concentration close to the CMC: CSDBS ) 0.5 mg/mL (also CG,i ) 0.1 mg/ mL). (NB, the fraction remaining in the experiment described in Figure 1, was much smaller than would be expected from the data shown in Figure 2. This is due to the fact that in the former experiment a much larger volume was used resulting in less efficient sonication.) 2.2. Evidence of Exfoliation. To further characterize the exact form of nanocarbons in the dispersions, we conducted a detailed TEM analysis of our standard dispersion. TEM samples were prepared by pipetting a few milliliters of this dispersion onto holey carbon mesh grids (400 mesh). TEM analysis revealed a large quantity of flakes of different types as shown in Figure 3. A small quantity of monolayer graphene flakes was observed (Figure 3A). A larger proportion of flakes were few-layer graphene, including some bilayers and trilayers as shown in Figure 3B and C. In addition, a number of rather disordered flakes with many layers, similar to the one in Figure 3D, were observed. The disorder suggests that these flakes formed by reaggregation of smaller flakes. Finally, a very small number (2) of very large flakes were observed (Figure 3E). It can be shown that these are graphite by the observation of thin multilayers protruding from their edges (Figure 3E, inset). Note that while these large flakes are rare when counted by number, they will contribute disproportionally by mass. It is possible to estimate the number of layers per flake for all but the largest flakes. These data are illustrated in the histogram for the standard dispersion in Figure 4A (the very large flakes are ignored in this histogram). These statistics show a reasonable population of few-layer graphene. For example, ∼43% of flakes had <5 layers. More importantly, ∼3% of the flakes were monolayer graphene. While this value is considerably smaller than that observed for graphene/solvent dispersions,14 working in aqueous systems brings its own advantages. In general, the majority of these few-layer flakes had lateral dimensions of ∼1 µm. Thicker (22) Bergin, S. D.; Nicolosi, V.; Streich, P. V.; Giordani, S.; Sun, Z.; Windle, A. H.; Ryan, P.; Peter, N.; Niraj, P.; Wang, Z.-T. T.; Carpenter, L.; Blau, W. J.; Boland, J. J.; Hamilton J. P.; Coleman, J. N. AdV. Mater. 2008, 20, 1876-1881. (23) Lockwood, N. A.; de Pablo, J. J.; Abbott, N. L. Langmuir 2005, 21, 6805. (24) McDonald, T. J.; Engtrakul, C.; Jones, M.; Rumbles, G.; Heben, M. J. J. Phys. Chem. B 2006, 110, 25339–25346. Figure 2. Graphite concentration after centrifugation (CF) as a function of starting graphite concentration (CSDBS ) 10 mg/mL). Upper inset: The same data represented as the fraction of graphite remaining after CF. Lower inset: Fraction of graphite after centrifugation as a function of SDBS concentration (CG,i ) 0.1 mg/mL). Figure 3. Selected TEM images of flakes prepared by surfactant processing. (A) A monolayer (albeit with a small piece of square debris close to its left-hand edge). (B) A bilayer. (C) A trilayer. (D) A disordered multilayer. (E) A very large flake. Inset: A closeup of an edge of a very large flake showing a small multilayer graphene flake protruding. (F) A monolayer from a sample prepared by sediment recycling. J. AM. CHEM. SOC. 9 VOL. 131, NO. 10, 2009 3613 Liquid Phase Production of Graphene A R T I C L E S Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 矩形 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 矩形 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 下划线 Administrator 下划线 flakes, with more than a few graphene layers per flake, were larger, ranging up to 3 µm in diameter. The sediment remaining after centrifugation can be recycled to improve the overall yield of graphene exfoliation. The sediment was recovered, and fresh (0.5 mg/mL) SDBS solution was added. This sediment dispersion was then processed in the same manner as the original dispersion, and TEM analysis was carried out. In this case, we also observed the presence of isolated monolayer graphene in about 3% of cases (Figure 3F). In addition, the flake thickness distribution shifted toward thinner flakes with large quantities of bilayers and trilayers; 67% of flakes observed had <5 layers (Figure 4B). Notably, there were no large flakes with greater than 10 layers observed, indicating that the reprocessing of recycled sediment gives better exfo- liation than processing of the original sieved graphite. We suggest that the second sonication breaks up the already partially exfoliated chunks of graphite into even smaller pieces from which exfoliation occurs more easily. The ability to easily deposit graphene flakes on a TEM grid allows their detailed characterization using high-resolution TEM (HRTEM). We can use this to confirm the presence of graphene monolayers in these surfactant-stabilized dispersions. Shown in Figure 5A is a HRTEM image of a graphene monolayer similar to that shown in Figure 3A. Significant nonuniformities can be seen, suggesting the presence of residual surfactant. The inset depicts a fast Fourier transform (FFT) of this image. This is equivalent to an electron diffraction pattern. The {1100} spots can clearly be seen. However, the {2110} spots are too faint to see. This intensity difference is the fingerprint of monolayer gra