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2009, 113, 4257–4259 Published on Web 02/20/2009
Simple Method of Preparing Graphene Flakes by an Arc-Discharge Method K. S. Subrahmanyam,† L. S. Panchakarla,† A. Govindaraj,†,‡ and C. N. R. Rao*,†,‡ Chemistry and Physics of Materials Unit and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre for AdVanced Scientific Research, Jakkur P.O., Bangalore 560064 India, and Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012 India ReceiVed: January 27, 2009; ReVised Manuscript ReceiVed: February 15, 2009
Arc discharge between graphite electrodes under a relatively high pressure of hydrogen yields graphene flakes generally containing 2-4 layers in the inner wall region of the arc chamber. The graphene flakes so obtained have been characterized by X-ray diffraction, atomic force microscopy, transmission electron microscopy, and Raman spectroscopy. The method is eminently suited to dope graphene with boron and nitrogen by carrying out arc discharge in the presence of diborane and pyridine respectively. Graphene is one of the most exciting materials being explored today.1-3 It is a one-atom thick sheet of carbon atoms forming six-membered rings and is the basic building block of all other graphitic carbon materials.1 The charge carriers in graphene behave as massless relativistic particles that are described on the basis of Dirac equation rather than the Schro¨dinger equation. Graphene exhibits fascinating properties such as quantum Hall effect at room temperature,4-6 ballistic conduction with high mean free path,1 tunable band gap,7 and high elasticity.8 Singlelayer graphene is generally prepared by micromechanical cleavage of highly ordered pyrolytic graphite (HOPG).9 While this method may suffice for certain physical measurements, it cannot be employed for large-scale preparations and chemical studies. Single-layer graphene has been prepared and deposited on solid substrate by other methods as well, which include heating SiC,10,11 intercalation followed by sonication12 and interaction with polar solvents.13,14 Bilayer graphene has been prepared by plasma enhanced chemical vapor deposition.15,16 Various types of graphene samples have been prepared by thermal exfoliation of graphite oxide.17,18 The number of layers in this preparation is 5-6 or more. Furthermore, graphene prepared by this method contain carboxyl and other functional groups. Another method is the conversion of nanodiamond in an inert atmosphere at high temperatures.18,19 This method generally yields samples containing 5-6 layers with the inclusion of some graphitic particles. Few-layer graphene films have been grown on polycrystalline Ni employing chemical vapor deposition technique.20 We have described a new method of preparing graphene containing 2-4 layers on a relatively large-scale. The procedure involves arc evaporation of graphite electrodes in a hydrogen atmosphere, and makes use of the knowledge that the presence of H2 during the arc discharge process terminates the dangling carbon bonds with hydrogen and prevents the formation of closed structures.21,22 It appears that H2 plays a key role in the formation of graphene by * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. Fax: (+91) 80-2208 2760. † Jawaharlal Nehru Centre for Advanced Scientific Research. ‡ Indian Institute of Science.
10.1021/jp900791y CCC: $40.75
preventing the rolling of sheets into nanotubes and graphitic polyhedral particles. We have noticed a report on the arc discharge of carbon electrodes in the presence of H2 that reports the formation of multiwalled carbon nanotubes (MWNTs) in the central part of the cathode and petal-like graphite sheets consisting of interlaced graphene sheets in the outside region surrounding the cathode.23,24 In the procedure employed by us, we have only collected the deposit formed on the inner walls of the reaction chamber, avoiding the material in the vicinity of the cathode, since the latter tends to contain some quantity of MWNTs and other carbonaceous materials. In this letter, we describe the arc discharge method to prepare graphene samples with 2-4 layers. More importantly, we show that this method is useful to prepare boron- and nitrogen-doped graphene. To prepare pure graphene (HG), direct current arc discharge of graphite evaporation was carried out in a water-cooled stainless steel chamber filled with a mixture of hydrogen and helium in different proportions without using any catalyst. The proportions of H2 and He used in our experiments are H2 (70 torr)/He (500 torr), H2 (100 torr)/He (500 torr), H2 (200 torr)/ He (500 torr), and H2 (400 torr)/He (300 torr). In a typical experiment, a graphite rod (Alfa Aesar with 99.999% purity, 6 mm in diameter and 50 mm long) was used as the anode and another graphite rod (13 mm in diameter and 60 mm in length) was used as the cathode. The discharge current was in the 100-150 A range, with a maximum open circuit voltage of 60 V.25 The arc was maintained by continuously translating the cathode to keep a constant distance of 2 mm from the anode. The deposit collected from the inner walls of the arc chamber contained only graphene flakes unlike the deposit at the cathode which contained MWNTs, carbon onions, and multilayer graphene. The deposit formed on the inner walls was collected and examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), and Raman spectroscopy. AFM samples were prepared by the procedure described elsewhere.17 TEM measurements were carried out by using dispersions in CCl4. The conditions that are favorable for obtaining graphene in the inner walls are the high current (above 100 A), the high voltage (>50 V), and the high pressure of hydrogen (above 200 torr). At lower currents 2009 American Chemical Society
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Figure 1. Noncontact mode AFM images of (a,b) pure (HG) and (c) boron-doped (BG) graphenes with height profiles.
Figure 2. TEM images of (a) pure (HG) and (b) boron-doped (BG) and (c) nitrogen-doped (NG) graphenes.
and voltages, arc discharge does not occur in the presence of hydrogen. If the content of hydrogen in the arc discharge chamber is decreased, the relative proportion of closed shell polyhedral particles increases, yielding only polyhedral graphitic particles at low H2 pressures. Under optimal conditions, this method yields 10-20 wt % of graphene with respect to the weight of the anode. X-ray diffraction (XRD) patterns of the HG sample collected from the inner walls of the arc chamber show broad (002) reflections around 25° and overlapping (100) and (101) reflections around 45°. Analysis of the (002) reflection showed the HG sample to consist of 3-4 layers. AFM images showed the thickness to be generally around 0.7-1 nm corresponding to 2-3 layers. We show typical AFM images with the height profiles in Figures 1a and b. A TEM image of HG shown in Figure 2a reveals the presence of 2-4 layers, consistent with
the AFM results. The area of the HG samples was generally in the range of 10-40 × 103 nm2. The Raman spectrum of HG (Figure 3) exhibits the characteristic D, G, and the 2D bands of graphene around 1323, 1569, and 2643 cm-1 respectively. It also shows the defect related G′ band as a shoulder around 1600 cm-1. The surface areas of the HG samples, determined by the Brunauer-Emmett-Teller (BET) method, were in the range 270-680 m2/g. The sample with the highest surface are, exhibits a hydrogen uptake of 1 wt % at 1 atm and 77 K and a CO2 uptake of 17 wt % at 1 atm and 195 K.26 An important outcome of the arc-discharge method of preparation described above is its use in doping graphene with boron and nitrogen. Boron-doped graphene (BG) was obtained from the inner walls of the arc chamber by carrying out the discharge in the presence of a mixture of H2 and diborane (B2H6). We have been able to dope graphene (NG) with nitrogen
Letters
Figure 3. Preliminary Raman spectra of pure (HG) and doped (BG and NG) graphene samples.
by carrying out arc discharge in the presence of a H2+pyridine mixture, just as in the case of carbon nanotubes.27 The presence of boron (1-3 wt %) in BG and nitrogen (0.6-1.0 wt %) in NG was confirmed by X-ray photoelectron spectroscopy and electron energy loss spectroscopy. XRD patterns of BG and NG show broad (002) reflections corresponding to the presence of 2-3 layers. AFM images of BG and NG show the presence of bilayer graphene as demonstrated in the case of BG in Figure 1c. TEM images also confirm the 2-3 layer structure of BG and NG (see Figure 2). Raman spectra of BG and NG (Figure 3) show a blue-shifted G band (1576 and 1572 cm-1 in BG and NG, respectively) compared to HG (1569 cm-1). BG and NG show a more intense D band and a less intense 2D band relative to HG. The synthesis, structure, and properties of Band N- doped graphene are now being investigated in detail. In conclusion, we have successfully synthesized graphene with a relatively smaller number of layers by carrying out arc discharge between graphite electrodes in the presence of H2. Graphene is essentially the sole product from the inner wall region of the arc chamber but not from the cathode region. The role of H2 may be to minimize the formation of nanotubes and closed carbon structures. It is noteworthy that graphene can be doped with boron and nitrogen by carrying out the electric discharge in the presence of B2H6 and pyridine, respectively. Synthesis of p- and n-doped graphene samples by employing different sources of boron and nitrogen in the arc discharge process and a study of their properties would be of great importance. References and Notes (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191. (2) Katsnelson, M. I. Mater. Today 2007, 10, 20–27.
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