Production of fullerenes and single-wall carbon nanotubes by high-temperature pulsed arc discharge

Article abstract of DOI:10.1063/1.481172

Fullerenes and single-wall carbon nanotubes (SWNTs) have been produced for the first time by the high-temperature pulsed arc-discharge technique, which has developed in this laboratory. Fullerenes are identified quantitatively by high-performance liquid chromatography (HPLC), and scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations reveal a significant amount of production of bundles of SWNTs in soot. The pulse arc production of fullerenes and SWNTs favors the high-temperature (≥ 1000°C), long pulses (≥ 1 ms) and a heavy rare gas such as Ar or Kr as a buffer gas. We have found that fullerenes and SWNTs have complementary relationships in their early stage of production. The details of the pulsed arc discharge have been obtained by observing the transition from the pulsed arc discharge to the steady arc discharge while increasing the pulse width.

Full text of DOI:10.1063/1.481172

Production of fullerenes and single-wall carbon nanotubes by high-temperature pulsed
arc discharge
Toshiki Sugai, Hideki Omote, Shunji Bandow, Nobuo Tanaka, and Hisanori Shinohara
Citation: The Journal of Chemical Physics 112, 6000 (2000); doi: 10.1063/1.481172
View online: http://dx.doi.org/10.1063/1.481172
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/112/13?ver=pdfcov
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 112, NUMBER 13
1 APRIL 2000
Production of fullerenes and single-wall carbon nanotubes
by high-temperature pulsed arc discharge
Toshiki Sugai, Hideki Omote, Shunji Bandow,a) Nobuo Tanaka,b)
and Hisanori Shinoharac)
Department of Chemistry, Nagoya University, Nagoya 464-8602, Japan
͑Received 31 August 1999; accepted 7 January 2000͒
Fullerenes and single-wall carbon nanotubes ͑SWNTs͒ have been produced for the first time by the
high-temperature pulsed arc-discharge technique, which has developed in this laboratory. Fullerenes
are identified quantitatively by high-performance liquid chromatography ͑HPLC͒, and scanning
electron microscopy ͑SEM͒ and transmission electron microscopy ͑TEM͒ observations reveal a
significant amount of production of bundles of SWNTs in soot. The pulse arc production of
fullerenes and SWNTs favors the high-temperature (у1000 °C), long pulses (у1 ms) and a heavy
rare gas such as Ar or Kr as a buffer gas. We have found that fullerenes and SWNTs have
complementary relationships in their early stage of production. The details of the pulsed arc
discharge have been obtained by observing the transition from the pulsed arc discharge to the steady
arc discharge while increasing the pulse width. © 2000 American Institute of Physics.
͓S0021-9606͑00͒71113-5͔
were analyzed by mass spectrometry. In previous studies,20,21
we obtained preliminary results on the production of solvent-
extractable fullerenes and SWNTs by this method. One of
the most important advantages of the pulsed arc discharge
over the steady arc discharge is that the pulsed arc period can
be varied over a wide time range (␮sϳs), which allows us to
study the detailed growth processes.
Here, we present the first successful report on the pro-
duction of fullerenes and SWNTs via the high-temperature
pulsed arc-discharge technique. We have found that
fullerenes and SWNTs have been synthesized by the pulsed
arc method, incorporating Ni/Co and Ni/Y–graphite compos-
ite rods as discharge electrodes. The results have shown that
both materials have close relationships in their production
processes.
We have found that the yield of SWNTs starts to in-
crease, with respect to the pulse arc width, when that of
fullerenes begins to decrease. Evidently the formation of
SWNTs is competitive with that of fullerenes, suggesting the
presence of common ͑unknown͒ ‘‘precursors’’ in their early
stage of the growth. We have also revealed the mechanisms
of the pulsed arc and obtained information on the transition
from the pulsed arc discharge to the steady arc discharge by
increasing the pulse width from 50 ␮s to 300 ms.
I. INTRODUCTION
Fullerenes and single-wall carbon nanotubes ͑SWNTs͒
have so far provided new nano-scale materials in various
fields. Recent progress in research on the properties of
SWNTs, such as electronic1–3 and atomic structures,4 me-
chanical properties,5 and organic functionalization,6,7 has
been outstanding, mainly due to an efficient production and
purification of SWNTs. Fullerenes and related materials have
been known to exhibit superconductivity,8,9 metal
encapsulation,10,11 polymerization,12 and various organic
functionalization.13 Fullerenes and SWNTs are produced in
macroscopic quantities by the pulsed laser vaporization ͑la-
ser furnace͒ method14 and the electric-arc-discharge
technique.15–17 However, these methods are still far from the
best methods to obtained sufficient quantities of SWNTs
with high yield, high purity, and low cost. In these produc-
tion methods, carbon vapors self assemble into fullerenes
and SWNTs through annealing.14–17 Several studies suggest
that the ‘‘cap structures’’ of the SWNTs, which are the half
spheres of fullerenes, play important roles in the formation
of SWNTs.1,18,19 There should be a close relationship in the
formation process between fullerenes and SWNTs.
In an effort to elucidate the growth mechanism of
fullerenes and SWNTs, we have recently developed a high-
temperature pulsed arc-discharge technique20,21 which is
equipped with a dc pulsed arc-discharge mechanism inside a
high temperature (800 °C–1200 °C) furnace. So far, the
pulsed arc discharge has mainly been used to produce vari-
ous metal22 and carbon clusters23 in the gas phase which
II. EXPERIMENT
The high-temperature pulsed arc-discharge apparatus20,21
consists of a furnace ͑ISUZU KRO-12K͒, a quartz tube
(␾25 mm), electric feedthroughs with insulation quartz
tubes, carbon electrodes, a water cooled trap, and a home-
made pulsed HV power supply ͑cf. Fig. 1͒. The electrodes
were located inside the quarts tube and the furnace to pro-
duce pulsed arc discharges in high-temperature buffer gases.
A buffer gas ͑He, Ar or Kr͒ was passed through the quartz
tube to thermally anneal carbon clusters and particles pro-
a͒
Nanotubulites Project Japan Science and Technology Corporation, Depart-
ment of Physics, Meijo University Shiogamaguchi 1-501, Tenpaku-ku,
Nagoya 468-8502, Japan.
b͒
Department of Applied Physics, School of Engineering, Nagoya Univer-
sity, Nagoya 464-8603, Japan.
Author to whom correspondence should be addressed.
c͒
0021-9606/2000/112(13)/6000/6/$17.00
6000
© 2000 American Institute of Physics
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J. Chem. Phys., Vol. 112, No. 13, 1 April 2000
Production of fullerenes and single-wall carbon nanotubes
6001
FIG. 1. A schematic diagram of a high-temperature pulsed arc discharge
apparatus.
duced by the pulsed arc discharge. The temperature of the
tube was varied between 25 °C and 1100 °C by the furnace.
The flow and pressure of the buffer gas were regulated to
300 cm3/min and to 500 Torr, respectively.
FIG. 2. Pulse width dependence of consumption ratios between an anode
and a cathode.
A power supply ͑PS1͒ can provide pulsed HV voltages
of typically 1.1 kV, 22 A, and 50ϳ3000 ␮s duration. The
repetition rate was normally 3ϳ300 Hz with a constant duty
factor ͑1%͒: The repetition rate was set, for example, to 100
Hz when the duration was 100 ␮s. The second power supply
͑PS2͒ was newly developed for longer pulses ϳ1 s and high
duty factor ϳ10%. To achieve such a high performance, the
power supply utilizes high ͑1.1 kV͒ and low ͑40 V͒ dc volt-
ages, which were used for igniting and maintaining the dis-
charge, respectively. The high voltage is necessary to initiate
the discharge between the electrodes, but the low voltage is
enough to maintain the discharge because of rapid decrease
of the impedance and the voltage drop between the elec-
trodes ͑cf. Sec. IV A͒. In contrast to the steady arc-discharge,
the negative electrode was found to be consumed by the
pulsed arc discharge.20,21
Metal doped composite graphite rods ͑Ni/Co: 0.7/0.7
at. %, and Ni/Y 4.2/1.0 at. %; Toyo Tanso Co. Ltd͒ and pure
graphite rods ͑Toyo Tanso Co. Ltd͒ were cut in sizes of 10
ϫ5ϫ1 mm and were used as the electrodes. The metal com-
posite rods were produced from metal oxide, graphite pow-
der, and a high strength pitch with high-temperature baking
under 1100 °C. The fullerenes, SWNTs and other graphitic
particles drifted within the tube by the gas flow and were
collected on the water cooled copper trap ͑cf. Fig. 1͒.
The trapped soot was extracted by carbon disulfide and
then analyzed by high-performance liquid chromatography
͑HPLC͒. The HPLC analysis was performed on a buckyprep
column ͑nacalai tesque, ␾4.6ϫ250 mm) with toluene as elu-
ent. The present HPLC system can detect fullerenes at low
concentration as 100 ng/ml. The concentration of fullerenes
in the soot was estimated by using HPLC peak area at 331
nm. The dependence of the fullerene concentration on tem-
perature (25 °C to 1100 °C), HV pulse duration ͑50 ␮s to
300 ms͒ and buffer gas species ͑He, Ar, and Kr͒ was also
measured.
SWNTs. The TEM samples were prepared on a microscope
specimen grid by a drop coating of toluene suspensions of
the soot. The dependence of the production efficiencies of
SWNTs on temperature (25 °C and 1000 °C), HV pulse du-
ration ͑50 ␮s to 300 ms͒ and buffer gases ͑He, Ar, and Kr͒
were also studied. The production of fullerenes from Ni/Co
doped graphite electrodes was also studied to obtain infor-
mation on a competitive formation between fullerenes and
SWNTs.
III. RESULTS
A. Evaporation profiles
Figure 2 shows the consumption ratio between the cath-
ode and the anode of pure graphite. The ratio is around zero
and only the cathode is evaporated in short pulse width range
less than 1 ms. The ratio, however, starts to increase at 3 ms
and keeps increasing up to 300 ms as the pulse width in-
creases, suggesting that the anode is vaporized like the
steady arc discharge. The results indicate that the transition
between the pulsed arc and the steady arc occurs at 3 ms.
Figure 3 shows the vaporization profile of the pure
graphite cathode. To estimate the vaporization profile (V(t))
on the pulse width, the differences between each adjacent
data point in the sampling sequence are used as the following
equation: V(t )ϭ C(t )ϪC(t ) /(t Ϫt ), where t and t
͓
͔
2
2
1
2
1
1
2
are the pulse widths adjacent to each other, and C(tn) is the
averaged ͑each pulse͒ vaporization consumption of the cath-
ode at the specified pulse width of tn . The vaporization rate
steeply decreases as the discharge pulse width increases. The
vaporization rate also depends on the buffer gas. The average
vaporization rates at 1 ms duration are 11, 109, 169 ␮g/s in
He, Ar, and Kr, respectively. The vaporization rate increases
as the mass of the buffer gas increases.
B. Production of fullerenes via pulsed arc discharge
The trapped soot was further characterized by scanning
electron microscopy ͑SEM: Hitachi S-900͒ and transmission
electron microscopy ͑TEM: JEOL JEM 2010͒ to detect
Soot samples of 2–10 mg were obtained in 3–30 hours
from pure carbon electrodes by the present pulsed arc dis-
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6002
J. Chem. Phys., Vol. 112, No. 13, 1 April 2000
Sugai et al.
FIG. 5. Pulse width dependencies of C60 yield in soot produced by pulsed
arc discharge. A distinct peak at 3 ms.
FIG. 3. A vaporization profile of a negative electrode ͑pure graphite͒ against
pulse width. Vaporization rate rapidly decreases with discharge time.
the steady arc discharge ͑5–20 %͒.24,25 The production of
higher fullerenes (C76 , C78 , and C84) is more sensitive to the
pulse duration than those of C60 and C70 ͑cf. Fig. 4͒.
Figure 6 shows the temperature dependence of the C60
production. Fullerenes are produced at higher temperatures
than 800 °C. The yield of C60 increases as the temperature
increases. Similar temperature dependencies have been re-
ported in the laser furnace experiments.26–28
The C60 production in the high-temperature pulsed arc
also depends on the buffer gas. At 1 ms pulsed discharge, the
concentrations of C60 in the soot are 0%, 1.54%, and 1.12%
in He, Ar, and Kr, respectively. Ar and Kr are much more
effective than He for fullerene production.
charge. Figure 4 shows HPLC chromatograms of the extracts
obtained at 25 °C and 1000 °C. The HPLC peak intensity is
normalized by soot weight and thus corresponds to the con-
centration of fullerenes in the soot. C60 and C70 are not pro-
duced at 25 °C, but are effectively produced at 1000 °C, and
the fullerene yield increases as the pulse duration increases.
The peaks at 3 minutes are due to the solvent. The absolute
concentration of C60 in the soot is shown in Fig. 5. The yield
of C60 increases until 3 ms ͑7.9%͒ as the duration increases,
but decreases ͑1.3%͒ abruptly at 10 ms. The yield is rela-
tively constant ͑1–2 %͒ up to 100 ms. At 300 ms duration,
the C60 yield ͑4%͒ again starts to increase. The yield of C60
at 3 ms duration ͑7.9%͒ is comparable to those reported in
C. Production of single-wall carbon nanotubes
SWNTs are also produced via the pulsed arc discharge
from the Ni/Co doped graphite electrodes. The metal par-
ticles obviously play catalytic roles in the production of
SWNTs.15–17 Figure 7 shows TEM images of typical bundles
of nanotubes which consist of individual SWNTs in the soot
FIG. 4. HPLC chromatogram of CS2 extracts of the soot produced by a
high-temperature pulsed arc discharge. Intensities correspond to the concen-
tration in the primary soot. Yields of fullerenes increase as the pulse width
increases.
FIG. 6. Temperature dependencies of C60 yield in soot produced by pulsed
arc discharge. The yield increases as the temperature increases.
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J. Chem. Phys., Vol. 112, No. 13, 1 April 2000
Production of fullerenes and single-wall carbon nanotubes
6003
TABLE I. A summary of production efficiencies of single-wall nanotubes in
various conditions. The double circles, single circles, and triangles represent
high, medium, and low concentrations of the SWNTs in primary soot, re-
spectively. The crosses indicate the absence of SWNT in the soot.
Pulse width / ms
Buffer gas and
temperature
0.05 0.14
1
2
3
10
30 100 300
Ar 25 °C
ϫ
ϫ
᭝
᭺
He 1000 °C
Ar 1000 °C
Kr 1000 °C
ϫ
ϫ
᭺
᭺
᭪
᭺
᭝
᭺
᭪
᭝
᭺
mum conditions for C60 ͑1 ms and 50 ␮s͒ coincide with the
thresholds of the production of SWNTs ͑see positions of the
triangles in Table I͒. This strongly suggests that fullerenes
and SWNTs have close relationships in the production pro-
cesses. The C60 yield of Ar is higher than that of Kr, whereas
Kr is much more effective to produce SWNTs than Ar.
We have also obtained much higher SWNTs yield and
lower fullerenes yield by Ni/Y doped composite rods than
those by Ni/Co doped composite rods. The results will be
reported in a future paper.
IV. DISCUSSION
A. Mechanism of the pulsed arc discharge: an
asymptote to the steady arc
In the steady arc discharge, discharge electrons are ac-
celerated towards the anode by the applied electric field, re-
sulting in vaporization of the anode.24,25,29 However, the ob-
served cathode vaporization in the pulsed arc discharge
indicates a sputtering of the cathode by an ionized buffer gas
such as Arϩ or Heϩ ions.
FIG. 7. TEM images of single-wall nanotubes produced with a buffer gas of
Ar and a pulse width of 3 ms.
The vaporization of the negative electrode presumably
takes place according to the following mechanisms.20,21 Arϩ
or Heϩ ions are produced by the discharge electrons, which
are accelerated toward the negative electrode by a strong
electric field between the electrodes. The accelerated ions
sputter and vaporize the surface of the negative electrode.
obtained at 2 ms pulse width with Ar buffer gas. The relative
yield of the bundles of SWNTs can be evaluated by the SEM
observation and is summarized in Table I. The results show
that the pulse width longer than 1 ms and temperatures
higher than 1000 °C are necessary to produce SWNTs. The
yield increases as the pulse width increases. There is a dip of
the yield of SWNTs at 30 ms, which is probably related to
the transition from the pulsed arc to the steady arc ͑cf. Sec.
III B͒. At 300 ms, thick tubular materials are produced ͑cf.
Fig. 8͒.
The yield is also dependent on the buffer gas used. Kr is
more effective than Ar for the production of SWNTs.
SWNTs were not observed in a He buffer gas. Again, the
temperature and buffer gas dependencies are similar to those
reported in the laser furnace experiment.27
D. Fullerenes vs single-wall carbon nanotubes
Figure 9 shows the C60 yield against the pulse width
produced by Ni/Co doped graphite ͑black bars͒ and pure
graphite ͑white bars͒ in Ar. The yield of Ni/Co doped graph-
ite has a maximum at 1 ms in Ar. However, the yield in-
creases as the pulse width increases in pure graphite. In Kr,
the optimum condition is obtained at 50 ␮s and the yield
(ϳ1%) is much lower than that of Ar (ϳ3%). The opti-
FIG. 8. SEM images of nanotube bundles produced with a buffer gas of Ar
and a pulse widths of 100 ms and 300 ms. Thick tubular materials are
observed at 300 ms.
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6004
J. Chem. Phys., Vol. 112, No. 13, 1 April 2000
Sugai et al.
to those of the laser furnace experiments26–28 and are differ-
ent from those of the steady arc discharge experiments.24,25
The previous laser furnace experiments have shown that
there is a threshold at about 800 °C for the production of
fullerenes.26–28 This suggests that nascent products by the
laser vaporization and the pulsed arc discharge are not
fullerenes; a high temperature thermal annealing is necessary
to transform the nascent cluster products to fullerenes. Car-
bon vapors are, however, subjected to enough annealing to
produce fullerenes due to the hot buffer gas heated by the
´
steady arc discharge per se. The production efficiency of
fullerenes increases as the pulse width increases until 3 ms in
the pulsed arc discharge. The longer pulse has a similar ef-
fect on the production of fullerenes. The annealing after car-
bon vaporization is also provided by the longer pulse, which
is crucial to the growth of fullerenes.20
Bowers and co-workers have shown that cyclic ring car-
bon clusters are annealed into fullerenes by collision heating
with a buffer gas.30,31 The annealing effect of the high tem-
perature buffer gas has been reported by Smalley and co-
workers in the laser furnace experiments.27 The pulsed arc-
discharge experiment also shows the importance of the
annealing effect due to the temperature of the buffer gas and
the pulse width.
The conditions for SWNT formation are also similar to
those of fullerenes produced by the pulsed arc discharge:
higher temperatures and longer pulses are necessary. The
temperature dependence on the SWNTs production in the
pulsed arc discharge is also similar to that of the laser fur-
nace experiments.26–28 The longer pulse (Ͼ1 ms) and the
high-temperature (Ͼ800 °C) conditions in the pulsed arc
discharge can provide an efficient annealing for the carbon
vapor to transform it into SWNTs.
The growth mechanism of SWNTs and the bundles has
not yet fully been understood.1,14,18,19 One of the most in-
triguing aspects of the SWNT growth is that the tube diam-
eter and the corresponding bundle structure vary sensitively
with metal catalysts used in the composite rods.1,32 Obvi-
ously, metal particles/clusters play crucial roles here in the
entire growth processes of SWNTs. The current pulsed arc
experiments show that a certain pulse period (1ϳ10 ms for
Ar buffer gas͒ is required to obtain a sufficient amount of
SWNTs in soot.
FIG. 9. Pulse width dependence of C60 yield produced from Ni/Co doped
graphite and pure graphite in Ar 500 Torr at 1000 °C using PS1. The maxi-
mum yield is obtained at 1 ms in the case of Ni/Co doped graphite, whereas
the yield of pure graphite keeps increasing as the pulse width increase.
The sputtering efficiency of Ar and Kr is much higher than
that of He because of their heavier masses and lower ioniza-
tion potentials.
During the vaporization, various ion species and con-
comitant electrons are produced in plasma, which causes a
decrease of an impedance (R) between the electrodes. The
impedance decrease results in a decrease of voltage drop
(⌬V) between the electrodes, because ⌬V is given by ⌬V
ϭR/(RϩR0)•V0, where R0 and V0 are the output imped-
ance and voltage of the pulsed HV power supply, respec-
tively ͑50 ⍀ and 1.1 kV͒. The voltage drop (⌬V) decreases
from 1.1 kV to about 20 V in about 10 ␮s. This results in the
rapid decrease of the evaporation rate ͑Fig. 3͒ since the ki-
netic energy of the sputtering ions is determined by ⌬V.
After the rapid decrease of ⌬V, the pulsed arc asymptoti-
cally approaches to the steady arc discharge with ⌬V
Ϸ20 V.24,25 In this ‘‘quasisteady arc discharge,’’ the vapor-
ization rate decreases gradually ͑cf. Fig. 3͒.
At pulse widths larger than 3 ms, the anode also begins
to vaporize by the discharge electrons as in the steady arc
discharge. This suggests that certain critical pulse widths are
needed to vaporize the anode because of the smaller sputter-
ing and heating effects of electrons than those of the buffer
gas ions. This transition threshold is also supported by an
experiment with a metal cathode and a graphite anode, where
fullerenes are produced only from the vaporization of the
anode.
C. Correlation mechanisms between fullerenes and
nanotubes
Figure 9 shows the strong correlation of production con-
ditions between fullerenes and SWNTs when Ni/Co doped
graphite is used as the electrodes. Evidently the formation of
SWNTs is competitive with that of fullerenes, suggesting the
presence of common ‘‘precursors’’ and common temperature
and time regions in their production processes. One of the
important hypotheses for the growth of SWNTs is that
SWNTs come from metal/carbon fine particles.1,18,19 The
particles segregate the precursors of SWNTs from the par-
ticles during high-temperature annealing as the temperature
of the particles gradually decreases. These metal/carbon fine
particles seem to be the common precursors.
Fullerenes are produced from the graphite cathode as
well as the steady arc discharge, and the threshold pulse
width is also 3 ms ͑cf. Fig. 4͒ because heat up and annealing
of the pulsed arc become enough at 3 ms.
B. Production of fullerenes and single-wall carbon
nanotubes
The observed temperature and buffer gas dependence
͑cf. Fig. 6, and fullerene production of Sec. III B͒ are similar
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J. Chem. Phys., Vol. 112, No. 13, 1 April 2000
Production of fullerenes and single-wall carbon nanotubes
6005
The vaporization profile ͑Fig. 3͒ and the yield of
fullerenes ͑Fig. 5͒ suggest that carbon is not thermally va-
porized like steady arc and high-temperature laser-
vaporization, but is sputtered by the buffer gas ions and rap-
idly cooled by the buffer gas.20,21 During these rapid
vaporization and cooling processes, the vaporized carbon
and metal atoms grow into amorphous carbon/metal par-
ticles, which may have weakly bound incomplete fullerene
molecules.
When the pulsed arc discharge of ϳ1 ms, the electron
and the buffer gas ions collide with the surface of the metal–
carbon particles and sputter out the fullerenes. During the
further discharge ͑1–3 ms͒, the inside temperature of the
particles becomes high enough to be converted into SWNTs.
When the particles are converted into SWNTs, the surface
area of the particles abruptly increases, which may cause
sudden temperature decrease of the particles and the yield of
fullerenes produced from the surface. In the case of pure
graphite, no SWNTs are produced. So the temperature of the
carbon particles does not decrease rapidly compared with
that of Ni/Co doped graphite. The yield of fullerenes, there-
fore, increases in longer pulsed arc ͑1–3 ms͒.
seconds via a suitable pulsed power supply, which enables us
to explore the growth processes of SWNTs and fullerenes.
Furthermore, the pulse arc technique can easily be combined
with high vacuum systems such as time-of-flight mass spec-
trometer and scanning probe microscopes.
ACKNOWLEDGMENTS
The present work has been supported by Grants-in-Aid
for Scientific Research ͑B͒ ͑2͒ ͑No. 09440198 and No.
10554030͒ by the Ministry of Education, Science, Sports and
Culture of Japan and by the Future Program ‘‘Advanced Pro-
cesses for New Carbon Nano-Materials’’ of the Japan Soci-
ety for the Promotion of Science.
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V. SUMMARY
We report the production of fullerenes and SWNTs by
the high-temperature pulsed arc-discharge method, which
has been newly developed in this laboratory, by using Ni/Co
doped graphite and pure graphite electrodes, respectively.
The mechanism of the pulsed arc has been revealed for the
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