Fast and Facile Preparation of Graphene Oxide and Reduced

Publication Date (Web): July 15, 2009 ... The basic strategy involved the preparation of graphite oxide (GO) from graphite through reaction with benzo...
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3514 Chem. Mater. 2009, 21, 3514–3520 DOI:10.1021/cm901247t

Fast and Facile Preparation of Graphene Oxide and Reduced Graphene Oxide Nanoplatelets Jianfeng Shen, Yizhe Hu, Min Shi, Xin Lu, Chen Qin, Chen Li, and Mingxin Ye* The Special Materials and Technology Center of Fudan University, Department of Materials Science, Fudan University, Shanghai, 200433, China Received May 6, 2009. Revised Manuscript Received June 19, 2009

In this study, we report an inexpensive, massively scalable, fast, and facile method for preparation of graphene oxide and reduced graphene oxide nanoplatelets. The basic strategy involved the preparation of graphite oxide (GO) from graphite through reaction with benzoyl peroxide (BPO), complete exfoliation of GO into graphene oxide sheets, followed by their in situ reduction to reduced graphene oxide nanoplatelets. The mechanism of graphene oxide producing is mainly the generation of oxygencontaining groups on graphene sheets. In addition, inserted BPO and expansion of CO2 evolved during reaction will expand the distance between graphite layers, which are also main factors for exfoliation. Thermogravimetric analysis, Raman spectroscopy, and Fourier transform infrared spectroscopy indicated the successful preparation of GO. X-ray diffraction proved the mechanism of intercalation and exfoliation of graphite. Transmission electron microscopy and atomic force microscopy were used to demonstrate the structure of produced graphene oxide and reduced graphene oxide nanoplatelets. 1. Introduction Graphene, a single layer of carbon atoms in a closely packed honeycomb two-dimensional lattice, has attracted tremendous attention from both the experimental and theoretical scientific communities in recent years.1-8 It is suggested to be a very important material not only for fundamental researches9-15 but also for device applications. In addition to the possibility of low-power, high-density, and high-speed switches, graphene-based devices may also be applied to other areas as atomthick membranes for sensing pressure, as components in *Corresponding author. Fax: 86-021-55664094. E-mail: [email protected].

(1) Novoselov, K. S.; Liang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451. (2) Liu, L.; Ryu, S.; Tomasik, M. R.; Stolyarova, E.; Jung, N.; Hybertsen, M. S.; Steigerwald, M. L.; Brus, L. E.; Flynn, G. W. Nano Lett. 2008, 8, 1965. (3) Calizo, I.; Balandin, A. A.; Bao, W.; Miao, F.; Lau, C. N. Nano Lett. 2007, 7, 2645. (4) Geim, A. K.; Novoselov, K. S. Nature 2007, 6, 183. (5) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. Nature 2007, 446, 60. (6) Ishigami, M.; Chen, J. H.; Cullen, W. G..; Fuhrer, M. S.; Williams, E. D. Nano Lett. 2007, 7, 1643. (7) Graf, D.; Molitor, F.; Ensslin, K.; Stampfer, C.; Jungen, A.; Hierold, C.; Wirtz, L. Solid State Commun. 2007, 143, 44. (8) Yan, J.; Zhang, Y.; Goler, S.; Kim, P.; Pinczuk, P. Solid State Commun. 2007, 143, 39. (9) Ni, Z. H.; Wang, H. M.; Kasim, J.; Fan, H. M.; Yu, T.; Wu, Y. H.; Feng, Y. P.; Shen, Z. X. Nano Lett. 2007, 7, 2758. (10) Niyogi, S.; Bekyarova, E.; Itkis, M. E.; McWilliams, J. L.; Hamon, M. A.; Haddon, R. C. J. Am. Chem. Soc. 2006, 128, 7720. (11) Konatham, D.; Striolo, A. Nano Lett. 2008, 8, 4630. (12) Ouyang, F.; Huang, B.; Li, Z.; Xiao, J.; Wang, H.; Xu, H. J. Phys. Chem. C 2008, 112, 12003. (13) Lomeda, J.; Doyle, C. D.; Kosynkin, D. V.; Hwang, W.; Tour, J. M. J. Am. Chem. Soc. 2008, 130, 16201. (14) Park, S.; An, J.; Piner, R. D.; Jung, I.; Yang, D.; Velamakanni, A.; Nguyen, S. T.; Ruoff, R. S. Chem. Mater. 2008, 20, 6592. (15) Wang, X.; Tabakman, S. M.; Dai, H. J. Am. Chem. Soc. 2008, 130, 8152.

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nanoelectromechanical systems, or in chemical sensing because of their high surface area.16-25 As with any new material that is intended for largescale applications, material availability and processability have been the rate-limiting steps in the evaluation of putative applications of graphene. High-yield production methods for graphene sheets are especially desirable for such applications as composite materials and conductive films.26-30 A number of works about growth and :: (16) Wang, X.; Zhi, L.; Mullen, K. Nano Lett. 2008, 8, 323. (17) Robinson, J. T.; Zalalutdinov, M.; Baldwin, J. W.; Snow, E. S.; Wei, Z.; Sheehan, P.; Houston, B. H. Nano Lett. 2008, 8, 3441. (18) Robinson, J. T.; Perkins, F. K.; Snow, E. S.; Wei, Z.; Sheehan, P. E. Nano Lett. 2008, 8, 3137. (19) Yoo, E.; Kim, J.; Hosono, E.; Zhou, H.; Kudo, T.; Honma, I. Nano Lett. 2008, 8, 2277. (20) Ang, P. K.; Chen, W.; Wee, A. T. S.; Loh, K. P. J. Am. Chem. Soc. 2008, 130, 14392. (21) Dayen, J.; Mahmood, A.; Golubev, D. S.; Roch-Jeune, I.; Salles, P.; Dujardin, E. Small 2008, 4, 716. (22) Si, Y.; Samulski, E. T. Chem. Mater. 2008, 20, 6792. (23) Xu, Y. X.; Bai, H.; Lu, G. W.; Li, C.; Shi, G. Q. J. Am. Chem. Soc. 2008, 130, 5856. (24) 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. (25) Muszynski, R.; Seger, B.; Kama, P. V. J. Phys. Chem. C 2008, 112, 5263. (26) Watcharotone, S.; Dikin, D. A.; Stankovich, S.; Piner, R.; Jung, I.; Dommett, G. H. B.; Evmenenko, G.; Wu, S.; Chen, S.; Liu, C.; Nguyen, S. T.; Ruoff, R. S. Nano Lett. 2007, 7, 1888. (27) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282. (28) Li, X.; Zhang, G.; Bai, X.; Sun, X.; Wang, X.; Wang, E.; Dai, H. Nat. Nanotechnol. 2008, 3, 538. (29) Ramanathan, T.; Abdala, A. A.; Stankovich, S.; Dikin, D. A.; Herrera-alonso, M.; Piner, R. D.; Adamson, D. H.; Schniepp, H. C.; Chen, X.; Ruoff, R. S.; Nguyen, S. T.; Aksay, I. A.; Prud’homme, R. K.; Brinson, L. C. Nat. Nanotechnol. 2008, 3, 327. (30) Hong, W.; Xu, Y.; Lu, G.; Li, C.; Shi, G. Electrochem. Commun. 2008, 10, 1555.

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exfoliation of graphene have been reported.31-36 Among them, micromechanical cleavage is currently the most effective and reliable method to produce high-quality graphene sheets.35 However, the low productivity of this method makes it unsuitable for large-scale applications. It is suggested that working with chemically modified forms of graphene may provide an alternative. Exfoliation of graphite oxide (GO) either by rapid thermal expansion or ultrasonic dispersion has been one of the best approaches to obtain graphene in bulk.37-51 It is found that the century old GO synthesis52 has taken a new turn in recent years as both top-down and bottom-up approaches are being considered to isolate single graphene sheets and probe their electronic properties with and without modification.25 GO is a water-soluble nanomaterial prepared through extensive chemical attack of graphite crystals to introduce oxygen-containing defects in the graphite stack. According to a recently proposed model,12 GO sheets are composed of planar, graphene-like aromatic domains of random sizes interconnected by a network of cyclohexanelike units in chair configuration which are decorated by hydroxyl, epoxy, ether, diol, and ketone groups. These functional groups in GO impart water solubility to the individual sheets, and removal of such groups results in flocculation and precipitation. Compared with other production techniques, this process is attractive because (31) Si, Y.; Samulski, E. T. Nano Lett. 2008, 8, 1679. (32) Worsley, K. A.; Ramesh, P.; Mandal, S. K.; Niyogi, S.; Itkis, M. E.; Haddon, R. C. Chem. Phys. Lett. 2007, 445, 51. (33) Gijie, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B. Nano Lett. 2007, 7, 3394. (34) Wang, G.; Yang, J.; Park, J.; Gou, X.; Wang, B.; Liu, H.; Yao, J. J. Phys. Chem. 2008, 112, 8192. (35) Subrahmanyam, K. S.; Vivekchand, S. R. C.; Govindaraj, A.; Rao, C. N. R. J. Mater. Chem. 2008, 18, 1517. (36) Somani, P. R.; Somani, S. P.; Umeno, M. Chem. Phys. Lett. 2006, 430, 56. (37) McAllister, M. J.; Li, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Chem. Mater. 2007, 19, 4396. (38) Schniepp, H. C.; Li, J.; Mcallister, M. J.; Sai, H.; Herrera-Alonso, M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. J. Phys. Chem. B 2006, 10, 8535. (39) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Nature 2007, 448, 457. (40) 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. (41) Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Carbon 2006, 44, 3342. (42) Paredes, J. I.; Villar-Rodil, S.; Martı´ nez-Alonso, A.; Tasc on, J. M. D. Langmuir 2008, 24, 10560. (43) Wei, Z.; Barlow, D. E.; Sheehan, P. E. Nano Lett. 2008, 8, 3141. (44) Nethravathi, C.; Rajamathi, M. Carbon 2008, 46, 1994. (45) Eda, G.; Lin, Y.; Miller, S.; Chen, C.; Su, W.; Chhowalla, M. Appl. Phys. Lett. 2008, 92, 233305. (46) Park, S.; Lee, K.; Bozoklu, G.; Cai, W.; Nguyen, S. T.; Ruoff, R. S. ACS Nano 2008, 2, 572. (47) Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. ACS Nano 2008, 2, 463. (48) Jung, I.; Dikin, D.; Park, S.; Cai, W.; Mielke, S. L.; Ruoff, R. S. J. Phys. Chem. C 2008, 112, 20264. (49) Yang, X.; Zhang, X.; Liu, Z.Y. Ma; Huang, Y.; Chen, Y. J. Phys. Chem. C 2008, 112, 17554. (50) G omez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Nano Lett. 2007, 7, 3499. (51) Bourlinos, A. B.; Gournis, D.; Petridis, D.; Szab o, T.; Szeri, A.; Dek any, I. Langmuir 2003, 19, 6050. (52) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339.

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of its reliability and exceptionally low material costs. Though this heavily oxidized form of graphene is electrically insulating, but the platelets can be made electrically conductive by exposing them to chemical reductants or heat. To the best of our knowledge, most of the GO presented were obtained based on Hummers and Offeman’s process,52 which is a time-consuming method and offers little control over the degree of functionalization. Herein, we report a fast and facile method for the preparation of GO. Our findings make it possible to process GO materials through low-cost facile processing techniques. Besides, it is shown that the degree of functionalization can be controlled and highly reduced graphene oxide nanoplatelets can be achieved through chemical reduction of prepared graphene oxide. When combined with other methods for large-scale production,53 it will open up enormous opportunities to take advantage of the unique properties of graphene for many technological applications. 2. Experimental Section Chemicals and Materials. Graphite colloidal F-09 (∼10 μm)was purchased from Qingdao BCSM CO.Ltd. Benzoyl peroxide (BPO), Sodium borohydride (NaBH4) were supplied by Shanghai Chemical Reagent Company. Doubly distilled water was used in all the process of aqueous solution preparations and washings. Preparation of Graphene Oxide and Graphene Nanoplatelets. The basic strategy involved the preparation of GO from graphite through reaction with benzoyl peroxide (BPO), complete exfoliation of GO into individual GO sheets, followed by their in situ reduction to produce individual reduced graphene oxide nanoplatelets. The fabrication process is outlined in Figure 1. There are three main steps: (1) Graphite (0.5 g) and BPO (10 g) were ground to a fine powder. This powder was heated in a small beaker at 110 °C for 10 min (Caution! BPO is a strong oxidizer and may explode when heated in a closed container). On completion of the reaction, the mixture was cooled to room temperature and then washed with water for several times until the pH of the filtrate was neutral. The remaining black solid was dried under a vacuum. (2) One-hundred milligrams of GO was dispersed in 100 mL of water and sonicated for 1 h. (3) Two-hundred milligrams of NaBH4 was added to the dispersion. The mixture was stirred for 30 min and heated at 125 °C for 3 h. During the reduction process, the yellow-brown solution gradually yielded a black precipitate. The black solid was isolated by centrifugation, washed with water, and finally dried. Characterization. Water bath sonication was performed with a JYD 1800 L sonicator (100-2000W). Raman spectra were recorded on a Dilor LABRAM-1B multichannel confocal microspectrometer with 514 nm laser excitation. Thermogravimetric analysis (TGA) was conducted with Netzsch TG 209F1 that was fitted to a nitrogen purge gas at 10 °C/min heating rate. Before (53) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; Mcgovern, I. T.; Holland, B.; Byrne, M.; Gun’ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardci, V.; Ferrari, A. C.; Coleman, J. N. Nat. Nanotechnol. 2008, 3, 563.

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Figure 1. Procedures used in this study for preparation of graphene oxide and reduced graphene oxide nanoplatelets.

the tests, all the samples were carefully grinded to powders to ensure sufficient diffusion of heat. The measurements were conducted using 6-10 mg samples and weight retention/temperature curves were recorded. Fourier transform infrared (FTIR) spectra were recorded on a NEXUS 670 spectrometer. Transmission ultraviolet-visible (UV-vis) spectra were recorded with an APADA UV-1800PC spectrophotometer. Spectra were acquired from 800 to 200 nm at a scan speed of 200 nm/min and a spectra resolution of 1 nm. X-ray diffraction (XRD) were taken on D/max-rB diffractometer using Cu KR radiation. The investigation of the structure had been performed by Transmission electron microscopy (TEM) using a JEOL JEM-2100F. Atomic force micrographs (AFM) were obtained using a Multimode Nano4 in the tapping mode.

Figure 2. Raman spectra of graphite, GO samples prepared after oxidation for 5 and 10 min, and reduced GO.

3. Results and Discussion Our method for preparation of GO is different from the published ones since it does not involve any added solvents. Its advantage is based on the low melting point of BPO (103-106 °C), enabling us to use it as a solvent directly. It is reported that the half-life of BPO radical at 92 °C is 1 h. At higher temperature (110 °C), it is reasonable to expect that the reaction will be significantly quicker. We believe this is the reason why the reaction reaches completion after 10 min (proved by the absence of further changes in the XRD analysis, see Figure 5). The yield of conversion from graphite to the reduced GO was in excess of 80%. Raman scattering is strongly sensitive to the electronic structure and it has proved to be an essential tool to characterize graphite and graphene materials.54 Figure 2 shows typical Raman spectra of raw graphite, GO samples prepared after oxidation for 5 and 10 min, and reduced GO. In the spectrum of pristine graphite, the peak at 1580 cm-1 (G band) corresponds to an E2g mode of graphite and is related to the vibration of sp2-bonded carbon atoms. A shoulder around 1600 cm-1 on the G band, designated as the D0 band, is defect-related. It can be hardly found in graphite. The peak at 1350 cm-1 (D band, the breathing mode of κ-point phonons of A1 g symmetry) is associated with vibrations of carbon atoms with dangling bonds in plane terminations of disordered graphite. The second-order band (2D) is observed around 2700 cm-1. Though the D and 2D bands cannot be used to determine the number of layers, they are useful to investigate electronic effects.54 After reaction with BPO, the D mode becomes stronger and broader because of the charge transfer between the graphite and BPO, suggesting a higher level of disorder of the graphene layers and defects increased during the functionalization process. Besides, compared to raw graphite, the ratio of the

intensities (ID/IG) for GO samples is markedly increased, indicating the formation of some sp3 carbon by functionalization. The Raman results described above agree well with those reported by Stankovich, et al., and Xu et al.,23,40 indicating the successful covalent modification of graphene nanoplatelets. After reduction, comparing with raw graphite, the G band of reduced GO is broadened and D/G intensity ratio has been slightly increased. This phenomenon can be attributed to the significant decrease of the size of the in-plane sp2 domains due to oxidation and ultrasonic exfoliation, and partially disordered graphite crystal structure of graphene nanoplatelets.55 Moreover, it is noteworthy that the position of the G-band varied in the order reduced GO>sample after oxidation for 10 min>sample oxidation for 5 min>raw graphite, which may be attributed to the reason that the G-band position increases with a decreasing number of layers in their solid states.54 TGA curves of graphite, GO samples prepared after oxidation for 2, 5, and 10 min, and reduced GO are shown in Figure 3. In agreement with previous reports in the literature, TGA trace of pristine graphite shows little weight loss, which is about 2% below 700 °C. Comparing with the raw graphite, GO shows much lower thermal stability, which is because of the lowered thermal stability due to the reduced van der Walls interaction. Besides, the onset temperature becomes dramatically lower, presumably due to pyrolysis of the labile oxygen-containing functional groups, yielding CO, CO2, and steam. Generally, as oxidation proceeds, the total weight loss increased. After oxidation for 10 min, the weight loss is 18%. The main mass loss takes place around 200-400 °C and is ascribed to the decomposition of labile oxygen functional groups present in the material. There is also a mass loss (∼3%) below 100 °C attributed to the removal of adsorbed

(54) Rao, C. N. R.; Biswas, Kanishka.; Subrahmanyam, K. S.; Govindaraj, A. J. Mater. Chem. 2009, 19, 2457.

(55) Wang, G.; Shen, X.; Wang, B.; Yao, J.; Park, J. Carbon 2009, 47, 2049.

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Figure 3. TGA curves of graphite, GO samples prepared after oxidation for 2, 5, and 10 min, and reduced GO.

water and a slower, steady mass loss (∼7%) over the whole temperature range between 400 and 700 °C, which can be assigned to the removal of more stable oxygen functionalities. These results are easy to understand: the oxidation product of graphite has a layered morphology with oxygen-containing functionality, thereby weakening the van der Waals forces between layers. This will disrupt the hexagonal carbon basal planes on the interior of multilayered stacks of graphene oxide, thus accelerating the process of weight lossing.19 After reduction, the thermal stability of reduced GO is almost as good as the pristine graphite, indicating the fully success of the reduction process. Taken together, Raman, TGA and FTIR data (see Figure S1 in the Supporting Information) suggest that the introduction of oxygen-containing functional groups results in the change of hybridization of the oxidized carbon atoms from planar sp2 to tetrahedral sp3. In addition, after reduction, most of the oxygen functional groups can be removed. Figure 4 shows the dispersion characteristics of prepared samples in water. As optical inspection of dispersions only indicates the presence or absence of particles larger than 10 μm, dispersions of the samples were further characterized by UV/vis spectrometry to obtain quantitative results. The UV-vis spectra were obtained under identical conditions (they were all recorded 3 h later after the dispersions were achieved; the dispersions were diluted by the same factor so that qualitative comparisons could be made). When oxidized, GO still possess a layered structure, but is much lighter in color than graphite because of the loss of electronic conjugation brought about during the oxidation. The vials with parent graphite contain visible precipitates, indicating poor dispersion. With the increasing of oxidation time, the intensity of whole spectral region increased and the dispersion of GO in water becomes better. Moreover, we noted that the reduced graphene oxide nanoplatelets can be dispersed into a homogeneous suspension in water via ultrasonic vibration, though the dispersion can be

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Figure 4. UV/vis spectra and optical micrographs of graphite, GO samples prepared after 2, 4, 5, and 10 min, and reduced GO (left to right) at a concentration of 0.1 mg mL-1 taken after 3 h when the solution had been sonicated with 100 W for 10 min.

Figure 5. XRD of graphite, GO samples prepared after oxidation for 2, 5, and 10 min, and reduced GO.

stable for only a few hours because of its hydrophobic nature. We also monitored the graphite oxidation process by X-ray diffraction. Figure 5 shows powder XRD results of graphite, GO samples prepared after oxidation for 2, 5, and 10 min, and reduced GO. Raw graphite showed the very strong 002 peak at 26.44°. The intermediate oxidized graphite products were taken from the reaction beaker at various oxidation stages. Spectrum of sample after 2 min oxidation exhibited the same peak as raw graphite (though a little bit weaker). However, after oxidation for 5 min, the peak become even weaker and another peak at 0.4 nm appeared. Complete oxidation is monitored by the total disappearance of the 0.34 nm intergraphene spacing and the appearance of a new one with 0.78 nm d-spacing. Such d-spacing is significantly larger than that of singlelayer pristine graphene (∼0.34 nm). Because the presence of oxygen-containing functional groups attached on both sides of the graphene sheet and the atomic scale roughness arising from structural defects (sp3 bonding) generated on the originally atomically flat graphene sheet, individual

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Figure 6. TEM images of the samples: (a) graphite, (b) GO, and (c) reduced graphene oxide. (d) HRTEM of the reduced graphene oxide (inset is the SAED pattern).

graphene oxide sheets are expected to be thicker than individual pristine graphene sheets. Besides, BPO molecules are expected to insert into the graphite layers, which will also expand the intergraphene spacing. After reduction, we discern a gradual change in the patterns to finally achieve, a randomly ordered carbonaceous layered solid, with basal spacing of 0.34 nm instead of 0.78 nm for the parent GO, indicating that the bulk of the oxygencontaining functional groups is removed from GO. This is consistent with the results of FTIR analysis. In addition, the broad diffraction peak of reduced GO powder hinted that the process could influence the crystallization of the samples in the functionalization process, implying that the extensive conjugated sp2 carbon network is restored. Both the d-spacing value and broadness of this reflection in reduced GO are typical for randomly ordered graphitic platelets. To further characterize the exact structures of nanocarbons in the dispersions, we conducted TEM analysis. TEM samples were prepared by pipetting a few milliliters of dispersion onto holey carbon mesh grids. From TEM image of raw graphite (figure 6a), we can find that the flakes are dark, thick and large, showing the original graphitic structure. With the same ultrasonication condition as GO (100 W, 0.5 h), graphite can not be exfoliated. As to exfoliated GO (Figure 6b), large sheets (a few hundred square nanometers) were observed to be situated on the top of the grid, where they resembled silk veil waves. They were transparent and entangled with each other. The structure of reduced GO sheets is different from GO (Figure 6c). In addition to silklike thin parts, the restacked parts can also be seen. Corrugation and

scrolling are part of the intrinsic nature of graphene nanosheets, which result from the fact that the 2D membrane structure becomes thermodynamically stable via bending.55 Because of scrolling and folding of graphene nanosheets, we would be able to observe the crosssection view of stacked reduced graphene oxide layers. High-magnification TEM (HRTEM) image of reduced GO is shown in Figure 6d. We can find the stacking sheet structure of reduced GO in the TEM cross-sections, and individual layers of the reduced graphene oxide are clearly observed. Though the ordered graphite lattices are clearly visible, the disordered regions are also found, indicating that the reduced GO sheets were partially restored to ordered crystal structure. The inset is the selected area electron diffraction pattern (SAED). It is similar with that in the literature,53 showing both diffraction rings and dots, consistent with HRTEM results. However, comparing with single-layer graphene data in literature,34 the crystal structure of reduced GO is not that complete, confirming that the ordered crystal structure has been partially restored. AFM characterization has been one of the most direct methods of quantifying the degree of exfoliation to graphene level after the dispersion of the powder in a solvent. It is known that the basal planes of the graphene sheets in GO are decorated mostly with epoxide and hydroxyl groups, in addition to carbonyl and carboxyl groups. These oxygen functionalities will alter the van der Walls interactions between the layers and make them hydrophilic, thus facilitating their exfoliation in aqueous media. As a consequence, GO readily forms stable colloidal dispersions of thin graphite oxide sheets in water.

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Figure 7. AFM images of (a) graphene oxide and (b) reduced graphene oxide nanoplatelets.

Figure 8. Possible mechanism of graphene oxide produced in this study.

We found that sufficiently dilute colloidal suspensions of GO prepared with the aid of ultrasound are clear, homogeneous and stable. AFM image confirm that with higher ultrasonication power (450 W for 300s), graphene oxide and reduced graphene oxide can be formed. Evaporated dispersion of GO is comprised of isolated graphitic sheets (Figure 7a) and graphene oxide has lateral dimensions of several decades of nanometers. The cross-sectional view of the typical AFM image of the exfoliated GO indicated that the average thickness of GO sheets is ∼1.3 nm, being somewhat larger than the interlayer spacing of GO (0.78 nm) measured by XRD. As described previously, the graphite layers can be intercalated by BPO molecules, during which the interlayer spacing along the

c-axis will change. During this process, hydroxyl, carbonyl, epoxy groups were bonded to the edges of basal planes of the graphite structure. Simultaneously, carbon hydroxylation occurred and the sp2 bonds changed to sp3 bonds. Because a pristine graphene sheet is atomically flat with a well-known van der Waals thickness of 0.34 nm, graphene oxide sheets are expected to be “thicker” because of the presence of covalently bonded oxygen and the displacement of the sp3-hybridized carbon atoms slightly above and below the original graphene plane; thus we can expect that these sheets are uniformly graphene monolayers and bilayers. As to reduced graphene oxide nanoplatelets, the AFM image (Figure 7b) reveals that the sheets display height

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variations at two length scales: some flat areas covered with 0.2-0.4 nm “bumps” and some high points showing average height of about 1.5 nm. We relate the bumpy texture of the flat regions to the presence of dead space because of the extensive edge functionalization employed during our approach. Thus, it can be concluded that complete exfoliation of reduced graphene oxide nanoplatelets down to one to three layers is indeed achieved under these conditions, because the intersheet distance for reduced graphene oxide nanoplatelets is 0.34 nm. The mechanism of the reaction between graphite and BPO is still unclear, we tentatively speculate that the whole process (Figure 8) contains three steps: (1) When the melted BPO is mixed with graphite, edge-to-face noncovalent aromatic interactions between graphene surface and the aromatic rings of BPO might be responsible for the BPO intercalating into the nanosheets.56 (2) When the temperature of the system gets higher, exfoliation of graphite occurs because of the decomposition of the intercalated BPO. Upon heating, they will exfoliate violently because of volatile gaseous species released from the intercalate (at the same time, free radical is generated). This is the reason why the d-spacing increased from 0.34 to 0.78 nm (see the XRD analysis). (3) The produced free radical may react with O2 and CO2 in the air and GO is achieved. After ultrasonication of GO in water, graphene oxide is produced. However, our analysis still leaves open the question of how the free radical react with O2 and CO2. The answer to this intriguing query constitutes a fertile research area that awaits further investigations by both experimenters and theorists. (56) Wang, G.; Shen, X.; Wang, B.; Yao, J.; Park, J. Carbon 2009, 47, 1359.

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4. Conclusion In conclusion, we presented a route to graphene oxide and isolated reduced graphene oxide nanoplatelets that is amenable to bulk production. The mechanism of graphene oxide producing is mainly the generation of oxygen-containing groups on graphene sheets (hydrophilic nature and increased d-spacing). These abundant functional groups weaken the van der Waals interactions between the layers of GO and make them hydrophilic, which is the reason for the occurrence of single-layer graphene oxide in aqueous media. In addition, expansion of CO2 evolved into the interstices between the graphene sheets during the reaction is a main factor for exfoliation. This work suggests that ordinary graphite, when treated directly by appropriate chemical means, can readily generate stable GO without the need for any solvents. These results should facilitate the manipulation and processing of graphene-based materials for different applications. With further surface modifications, reduced graphene oxide nanoplatelets that is soluble in different solvents should be accessible, thereby further expediting the application of graphene. Supporting Information Available: FTIR spectra of graphite, GO samples prepared after oxidation for 2, 5, and 10 min, and reduced GO (Figure S1); parallel process under the same condition without O2 (Figure S2 and S3); electrical conductivities of the samples (Table S1); SEM images of raw graphite and GO samples (Figure S4); AFM image of GO under low ultrasonication power (Figure S5); and UV/vis analysis of spectra of GO samples prepared after 10 min oxidation (Figure S6) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.