Production of Graphite Nanosheets by Low-Current Plasma Discharge

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Production of Graphite Nanosheets by Low-Current Plasma Discharge

in Liquid Ethanol

Sunghoon Kim1;2, Ruslan Sergiienko2, Etsuro Shibata2;*,

Yuichiro Hayasaka3 and Takashi Nakamura2

1Samsung Electro-Mechanics Co. Ltd., 581 Myunghak-Li, Dong-Myon, Yeongi-Gun, Chungcheongham-Do, 339-702, Korea2Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan3High Voltage Electron Microscope laboratory, Tohoku University, Sendai 980-8577, Japan

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 11mmin extent. The graphite nanosheets typically ranged in thickness from 6.7 to 23.5 nm. The proposedmethodis 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)13) (sometimes called carbon

nanosheets, nanoflakes,4,5) or nanowalls6,7)) have high sur-

face-to-volume ratio and exceptional electrical and mechan-

ical 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 asfiller 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),47) 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, andflammable gaseous mixtures. Equally, conventional arc-

discharge 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 single-

graphene 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 nano-

devices. In addition, the time-consuming process of graphite

oxidation in the presence of strong acids and oxidants

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 substrate-

free, 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 usedcompared 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 throughoutthe experiment. The effervescent ultrasonic cavitation field

may enhance electrical conductivity due to the high-energy

speciesradicals, 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.*Corresponding author, E-mail: [email protected]

Materials Transactions, Vol. 51, No. 8 (2010) pp. 1455 to 1459#2010 The Mining and Materials Processing Institute of Japan EXPRESS REGULARARTICLE
http://dx.doi.org/10.2320/matertrans.M-M2010813http://dx.doi.org/10.2320/matertrans.M-M2010813


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After the synthesis of the graphite nanosheets, the material

was refluxed in 1015% hydrogen peroxide aqueous solution

at 90C 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 40C 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) andtransmission 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 11mm

(Fig. 2(f)) with average size of 45mm. 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 Meyeret 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 ob-tained 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.

gap 100 m

Ultrasonic cavitation

(tiny bubbles)

e-

(Fe50Pt50 alloyanode)

Ethanol

Ti ultrasonichorn cathode

Ethanol decomposition

Metallic vapor

Ultrasonic generator

Plasmadischarge3 A, 55 V

Fe tip

DC powersupply

+

(a)

(b)

(c)

C2Me

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.

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Dynamic force microscopy measurements were employed

to quantitatively analyze the thickness of the graphite nano-

sheets, 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 graphitenanosheet 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 12.518 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 contami-

nants 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) graphitepeak 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 20003000 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

2 4 6 8 10 120

2

4

6

8

10

12

14

16

Numberofnanosheets

Lateral size (m)

0 20 40 60 80 100 120 140 1600

5

10

15

20

25

30

35

40

Height of folds and curled

edges

Thickness of nanosheets

Thickness (nm)

Numberof

measurements

(a)

(b) (d)

(e)

(f)

(c)

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.

P roduction of Graphite Nanosheets by Low-Current P lasma Discharge in Liquid Ethanol 1457


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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 26722679 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

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 conclu-

sion we can say that the synthesized graphite sheets are

multi-layer graphene sheets and most probably contain more

than 10 graphene layers.

(a) (b)

(c) (d)

0 500 1000 1500 2000 2500 3000

5

10

15

20

25

18.6n

m

Heightpro

file(nm)

Lateral size (nm)

5.3nm

0 500 1000 1500 2000 25000

10

20

30

40

50

7.4nm

44.4n

m

Heightprofile(nm)

Lateral size (nm)

Fig. 3 (a), (b) Tapping-mode DFM image of graphite nanosheets deposited on silicon substrates. White areas depict folds and curled

edges with peak heights up to 18.6 and 44.4 nm. (c), (d) Height profiles through the white lines shown in (a) and (b), respectively.

10 20 30 40 50 60 70 80

Intensity(a.u.)

Angle, 2/

As-prepared carbon powder

( Fe, Pt) (220)

( Fe, Pt) (200)

( Fe, Pt) (111)

G (002)

Graphite nanosheets

Pt (200)Pt (111)

G (002)

Fig. 4 X-ray diffraction patterns of the as-prepared carbon powder sample

(top profile) and purified graphite nanosheets (bottom profile).

1000 1250 1500 1750 2000 2250 2500 2750 3000

D+G

(2932)

GI(2D)-2698

2450

DI-1620

G-1583

D-1349

Raman shift (cm-1)

Intensity(a.u.)

Graphite nanosheets

Reference graphite

Fig. 5 Raman spectra of the purified graphite nanosheets and reference

graphite powder (12mm, 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 10mm.

1458 S. Kim, R. Sergiienko, E. Shibata, Y. Hayasaka and T. Nakamura


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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 possessedconsiderable 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 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 0:99)6) and

solvothermal synthesis (ID=IG 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 wassuperior 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

(A) (No. 20686051) from the Ministry of Education, Culture,

Sports, Science and Technology, Japan.

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