Materials Transactions, Vol. 51, No. 8 (2010) pp. 1455 to 1459 #2010 The Mining and Materials Processing Institute of Japan
EXPRESS REGULAR ARTICLE
Production of Graphite Nanosheets by Low-Current Plasma Discharge in Liquid Ethanol Sunghoon Kim1;2 , Ruslan Sergiienko2 , Etsuro Shibata2; * , Yuichiro Hayasaka3 and Takashi Nakamura2 1
Samsung Electro-Mechanics Co. Ltd., 581 Myunghak-Li, Dong-Myon, Yeongi-Gun, Chungcheongham-Do, 339-702, Korea Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan 3 High Voltage Electron Microscope laboratory, Tohoku University, Sendai 980-8577, Japan 2
Graphite nanosheets were produced by low-current plasma discharge in ultrasonically cavitated liquid ethanol. The microstructure, morphology and thickness of the graphite nanosheets were characterized by scanning and transmission electron microscopy, X-ray diffraction, dynamic force microscopy and Raman spectroscopy. The results indicated that the synthesized nanosheets have many folds, curled edges and are up to 11 mm in extent. The graphite nanosheets typically ranged in thickness from 6.7 to 23.5 nm. The proposed method is substrate-free, does not require expensive vacuum equipment and nor does it consume large amounts of electricity. [doi:10.2320/matertrans.M-M2010813] (Received December 4, 2009; Accepted April 20, 2010; Published July 14, 2010) Keywords: plasma discharge, ultrasonic cavitation, graphite nanosheets, transmission electron microscopy, dynamic force microscopy, X-ray diffraction, Raman spectroscopy
1.
Introduction
Graphite nanosheets (GNSs)1–3) (sometimes called carbon nanosheets, nanoflakes,4,5) or nanowalls6,7)) have high surface-to-volume ratio and exceptional electrical and mechanical properties, in addition to high thermal and chemical stability. At the time of writing, carbon nanosheets are being considered as electrode materials and catalyst supports in electrochemical supercapacitors and fuel cells,8,9) lithium-ion batteries10,11) and in field emission5,12) perfomance, and as filler in polymer composites.13,14) In graphite nanosheets, the weak van der Waals bonding between adjacent graphene layers make them promising candidates for exfoliation into single-graphene sheets. The synthesis of graphite nanosheets and single-graphene sheets has been previously demonstrated by using radio-frequency or microwave plasma-enhanced chemical vapor deposition (PECVD),4–7) arc-discharge methods,15,16) chemical reduction of exfoliated graphite oxide17,18) and a solvothermal synthesis.19,20) However, from a technical standpoint, chemical vapor deposition (CVD) and PECVD methods are low yielding and require sophisticated apparatus, controlled atmosphere, gas flow adjustments, and flammable gaseous mixtures. Equally, conventional arcdischarge methods require expensive vacuum equipment and the electric power requirement for arc-discharge usually exceeds 1 kW.15,16) The process of graphite oxidation has been suggested as the most generally effective way to produce GNSs and/or single-graphene sheets in a large quantities and at low cost. However, the GNSs and singlegraphene sheets obtained by this method are usually of comparatively poor quality, mainly due to the introduction of oxygen-containing functional groups during the synthesis, which consequently limits further application, especially as electrically conductive composites and electronic nanodevices. In addition, the time-consuming process of graphite oxidation in the presence of strong acids and oxidants *Corresponding
author, E-mail:
[email protected]
requires specific precautions to minimize the risk of explosion.17) In this study, we propose a new approach for the milligram-scale production of graphite nanosheets using low-current plasma discharge in a liquid ethanol ultrasonic cavitation field. This method is safe, simple, and substratefree, and in contrast to PECVD does not require expensive vacuum equipment. A plasma discharge produced under conditions of ultrasonication is relatively safe, even in an organic liquid because a low electric current is used compared with arc-discharge methods. This method has already been described in the previous paper, but only as a route for the manufacture of carbon nanocapsules.21) 2.
Experimental Procedures
Low-current plasma discharge was generated between a consumed 50 at% Fe-50 at% Platinum alloy cylindrical anode (3 mm) and the bottom of a titanium ultrasonic horn which serves as a cathode (Fig. 1(a)). The voltage between the anode and cathode was held at 55 V and the upper limit of the current on the electrodes was set at 3.0 A throughout the experiment. The effervescent ultrasonic cavitation field may enhance electrical conductivity due to the high-energy species—radicals, atoms, ions and free electrons that form within it. Generation of the plasma discharge begins with the process of ultrasonic cavitation and pointed end geometry of the consumed anode assists the emission of electrons (e ) from the cathode (Fig. 1(a)). A large amount of graphite nanosheets and Fe-Pt alloy filled carbon nanocapsules were found using a consumed 50 at% Fe-50 at% Pt alloy anode, whilst using a pure Fe anode21) produced a significantly lower number of graphite nanosheets. The nature of the influence of Pt on GNSs synthesis has not yet been resolved. It is possible that Pt plays a key catalytic role in the reduction of liquid ethanol, but in this short paper we are not examining the mechanism of graphite nanosheet formation.
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S. Kim, R. Sergiienko, E. Shibata, Y. Hayasaka and T. Nakamura (b) (a) Ultrasonic generator
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Fig. 1 (a) Schematic view of experimental apparatus. (b) and (c) show typical SEM images of the synthesized graphite nanosheets freely suspended on a carbon TEM microgrid. 1 (b) Curled edges (c) folded central regions indicated by arrows.
After the synthesis of the graphite nanosheets, the material was refluxed in 10–15% hydrogen peroxide aqueous solution at 90 C for 12 hours to remove impurities of amorphous carbon by selective oxidation. The carbon powder sample was then etched in aqua regia for 24 at 40 C to remove exposed Fe-Pt alloy nanoparticles. Finally, the Fe-Pt alloy filled carbon nanocapsules were removed by permanent magnets,21) leaving the dispersed graphite nanosheets in the liquid ethanol. The microstructure, morphology and thickness of graphite nanosheets were characterized by field emission scanning electron microscopy (FE-SEM) (JEOL, JSM-7000F) and transmission electron microscopy (TEM) (80 kV and 300 kV, JEOL), X-ray diffraction (XRD) (Rigaku, RINT2000), dynamic force microscopy (DFM) (NanoNavi/L-trace II) and visible Raman spectroscopy (HoloLab 5000/Raman Rxn1) using a second harmonic of Nd:YAG laser at 532 nm wavelength as an excitation. 3.
Experimental Results and Discussion
Typical SEM images of the synthesized GNSs, freely suspended on a TEM carbon grid, are shown in Figs. 1(b), (c). We measured the average lateral extent of 171 graphite nanosheets using SEM images, and found that the sizes of the GNSs ranged from several hundred nanometers to 11 mm (Fig. 2(f)) with average size of 4–5 mm. The nanosheets are also transparent to an electron beam (Figs. 1(b), (c)) and
irregularly shaped. We frequently observed curled edges (Fig. 1(b), Figs. 2(a), (c) and (d)) and folds (Fig. 1(c)) in the central regions of the synthesized GNSs along with plane featureless regions. As reported previously by Meyer et al.,22) corrugation and scrolling are intrinsic to nanosheets. Selected area electron diffraction (SAED) (Fig. 2(b)) along the [111] zone axis (marked with a circle in Fig. 2(a)) clearly shows the typical hexagonally arranged carbon lattice in the nanosheet, i.e. hexagonal closed packed structure (HCP). The well-defined diffraction spots (as seen in Fig. 2(b)) and layered structures (Figs. 2(c), (d)) confirm the crystalline structure of the graphite nanosheets obtained by low-current plasma discharge in liquid ethanol. Figures 2(c) and (d) shows high-resolution TEM images of the curled edges of two different graphite nanosheets. Aligned graphene layers with an interlayer spacing of about 0.34 nm are clearly visible, corresponding to 002 graphite crystal spacing (from ASTM card # 41-1487). Curled edges can provide a clear TEM signature for the number of graphene layers by direct visualization, since at a curled edge the sheet is locally parallel to the electron beam. Therefore, the thickness of graphite nanosheets can be estimated from high-resolution TEM images (HRTEM).22) Counting the number of graphene layers in the curled edges in the HRTEM images (Figs. 2(c), (d)), revealed 36, and 13 layers and thence the respective thicknesses of graphite nanosheets can be evaluated at about 12.2, and 4.4 nm at an interlayer distance of 0.34 nm.
Production of Graphite Nanosheets by Low-Current Plasma Discharge in Liquid Ethanol
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Fig. 2 (a) shows typical TEM image of the transparent graphite nanosheet. Curled edges, indicated by arrows, exhibit multiple dark lines. (b) shows a SAED pattern of the nanosheet taken from the position marked by a circle in (a). The diameter of the circle is equal to size of a selected area aperture, 950 nm (a). (c), (d) High-resolution TEM images of graphene layers in the curled edges of graphite nanosheets. The number of graphene layers is about 36 and 13, respectively. (e) Histogram of graphite nanosheets thicknesses and the heights of folds and curled edges above substrate level taken from height profiles of DFM images of 70 sheets. (f) Histogram of graphite nanosheets lateral extents taken from SEM images of 171 sheets.
Dynamic force microscopy measurements were employed to quantitatively analyze the thickness of the graphite nanosheets, and some results are shown in Fig. 3. Figures 3(a) and (b) exhibit typical topography images of different graphite nanosheets and height profiles (Figs. 3(c) and (d)) through those graphite nanosheets along the white line as shown in Figs. 3(a) and (b). The minimum step heights measured between the surface of the graphite nanosheets and the silicon substrates were found to be 5.3 and 7.4 nm, which most probably correspond to the actual thicknesses of the GNSs with about 15 and 21 respective graphene layers. However, it cannot be unambiguously shown that the entire graphite nanosheet has uniform thickness because folds in the central regions, and curled edges significantly increase the height profile of sheet above the substrate level. The white areas of folds and curled edges in Figs. 3(a) and (b) depict the highest places above substrate level. The thicknesses of 70 nanosheets were analyzed using height profiles and the histogram is presented in Fig. 2(e). Each sheet was measured in two or three places, and the minimum measured step height of each sheet was considered as the thickness. The most frequently observed minimum step heights ranged from 6.7 to 23.5 nm, and about 50% of measurements showed minimum step heights in the range of 12.5–18 nm. The heights of folds and curled edges above substrate level are also shown in histogram form (Fig. 2(e)). Figure 4 shows the X-ray diffraction (XRD) pattern of the as-prepared carbon powder sample and the purified graphite
nanosheets. The as-prepared carbon powder exhibited the characteristic graphite and face-centered cubic (FCC) (Fe, Pt) diffraction peaks at 26.34 , 40.67 , 47.32 , 69.42 , marked by their indices (002), (111), (200), (220) (top profile in Fig. 4). It was observed that carbon nanocapsules with face centered cubic -(Fe, Pt) core structures were effectively removed by magnetic separation, although some contaminants due to the Pt rich carbon nanocapsules (diffraction peaks at 39.83 and 46.34 , respectively, for Pt(111) and Pt(200)) remained on the surface of the graphite nanosheets (bottom profile in Fig. 4). The XRD pattern of purified graphite nanosheets gives a distinguishable (002) graphite peak at 26.34 . The Raman spectrum may provide more supporting evidence about the nature of the structure and morphology, in particular to determine the defects and the ordered and disordered structures of carbon nanomaterials. Figure 5 compares the Raman spectra of reference graphite powder with synthesized GNSs. It is obvious from this comparison that nanosheets have a graphitic structure. In particular, Fig. 5 shows the D peak (1349 cm1 ), the G peak (1583 cm1 ) and the D0 peak (a shoulder at 1620 cm1 ), which are also seen in microcrystalline graphite,23) indicating that the synthesized nanosheets have a crystalline graphite structure which contains defects. Second-order modes in the range of 2000–3000 cm1 are also present in Fig. 5. The strong peak at 2698 cm1 , the so-called G0 peak, is an overtone of the D peak (2 1349 cm1 ). The small peak at
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2932 cm1 is attributed to the sum of the D and G peaks. From the position and shape of the G0 (2D) peak as well as the intensity ratio of the 2D peak to the G peak it could be concluded that the synthesized graphite nanosheets are not single-layer graphene sheets.24,25) Single-layer graphene sheets have a single, sharp 2D peak distinctly below 2700 cm1 (at 2672–2679 cm1 )26,27) and the second-order 2D peak is more intense than the G peak.24,25) In our results the intensity of 2D peak in synthesized graphite sheets is about 1.7 times less than the intensity of G peak, and the
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Fig. 5 Raman spectra of the purified graphite nanosheets and reference graphite powder (1–2 mm, Aldrich). Measurements were done from the aggregation of graphite nanosheets deposited on the silicon substrate. Raman spectra were excited with a 532 nm laser using a laser spot size of about 10 mm.
position is at 2698 cm1 . The shape of the 2D peak coincides with those observed in the reference graphite powder (Fig. 5). Graphite sheets with more than 10 graphene layers and bulk graphite exhibit similar 2D peaks.24,25) In conclusion we can say that the synthesized graphite sheets are multi-layer graphene sheets and most probably contain more than 10 graphene layers.
Production of Graphite Nanosheets by Low-Current Plasma Discharge in Liquid Ethanol
The appearance of a D peak at 1349 cm1 is attributed to defects or structural disorder in graphite nanosheets, which are not observed in single crystals of graphite28) and in perfect graphene sheets.24) The defects or structural disorder may be attributed to the curled edges and folded regions of the graphite nanosheets which contribute to the increasing of the D peak signal.27) Our graphite nanosheets possessed considerable curved and corrugated regions (the afore mentioned curled edges and folds in the central regions), as is shown by SEM (Figs. 1(b), (c)), TEM (Figs. 2(a), (c) and (d)) and DFM (Figs. 3(a), (b)) examination. Nevertheless, the observed intensity ratio of the D-to-G peaks (ID =IG ¼ 0:5) in the Raman spectrum of synthesized graphite nanosheets is smaller than that observed for carbon nanosheets produced by using hot filament CVD,5) PECVD (ID =IG ¼ 0:99)6) and solvothermal synthesis (ID =IG ¼ 1:5),19) indicating that our graphite nanosheets have better crystallinity, an observation supported by the sharp SAED pattern shown in Fig. 2(b). 4.
Summary
This paper offers a novel and efficient way of preparing graphite nanosheets using low-current plasma discharge in ultrasonically cavitated liquid ethanol. This method does not need high electric current, nor does it require expensive vacuum equipment and is capable of being scaled-up. The by-products of our method are carbon nanocapsules and amorphous carbon that is easily removed by magnetic separation, hydrogen peroxide and aqua regia acid treatment. The crystalline graphite structure of the nanosheets was confirmed by TEM, XRD and Raman spectroscopy. The crystallinity of the graphite nanosheets produced was superior to that of carbon nanosheets prepared by other methods. The graphite nanosheets range in lateral extent from few hundred nanometers up to 11 mm. Dynamic force microscopy (DFM) measurements showed that the most frequently observed minimum step heights between the surface of graphite nanosheets and silicon substrates ranged from 6.7 to 23.5 nm. The minimum measured step height of each sheet was considered as a measurement of the thickness. The results of the DFM measurements were supported by the HRTEM investigations of curled edges. The synthesized graphite nanosheets can be used as a precursor for production of single- and few-layer graphene sheets. Acknowledgment This work was financially supported by a Grant-in-Aid for Exploratory Research (No. 17656243) and Young Scientists
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