Influence of Fe and Co/Ni on carbon arc plasma and formation of fullerenes and nanotubes

Article abstract of DOI:10.1021/jp002842q

Carbon nanostructures (fullerenes and nanotubes) were obtained using the carbon arc technique employing homogeneous carbon-iron and carbon-cobalt-nickel anodes. The influence of these catalysts on the arc plasma was studied by optical emission spectroscop

Full text of DOI:10.1021/jp002842q

10708
J. Phys. Chem. A 2000, 104, 10708-10712
Influence of Fe and Co/Ni on Carbon Arc Plasma and Formation of Fullerenes and
Nanotubes
Andrzej Huczko,*,† Hubert Lange,† and Toshiaki Sogabe‡
Department of Chemistry, Warsaw UniVersity, Pasteur 1, 02-093 Warsaw, Poland, and Toyo Tanso Co. Ltd.,
Kagawa 769-1612, Japan
ReceiVed: August 4, 2000
Carbon nanostructures (fullerenes and nanotubes) were obtained using the carbon arc technique employing
homogeneous carbon-iron and carbon-cobalt-nickel anodes. The influence of these catalysts on the arc
plasma was studied by optical emission spectroscopy to determine temperatures and C2 radical content in the
arc zone. The solid products were analyzed by spectrophotometry and mass spectrometry to determine the
C60 yield and relative mass distributions of the higher fullerenes, respectively. SEM and TEM techniques
were used to affirm the presence of carbon nanotubes in the resulting soot product.
Introduction
containing fullerenes were also obtained in a plasma jet
consisting of iron-carbon species combined with a helium
flow.19,20 Some fulleride-iron materials were synthesized by
doping fullerides with iron during the thermal decomposition
of ferrocene.21,22
Knowledge of the basic phenomena involved in the formation
of nanocarbons, which is necessary for optimizing large-scale
applications, is rather limited,23 and the plasma zone in such
processes is usually considered as a “black box”. Surprisingly,
only a few papers of a mostly qualitative character, related to
the diagnostics of the carbon plasma environment, have been
published so far.23-26
The aim of this present study was to investigate the influence
of transition metals, introduced into the carbon arc, on the
plasma characteristics and the formation of fullerenes and
nanotubes. Although the use of catalysts as arc plasma
admixtures is not new, quantitative studies of this kind have
never been reported. The application of homogeneously doped
graphite anodes with a low content of catalyst metals has also
helped in this study to avoid drastic changes of the plasma
parameters, as is the case with the more commonly used anodes,
which are drilled and filled with metals. In fact, the mode of
catalyst introduction into the reaction system can distinctly
influence the process yield. Thus, a much larger quantity of
SWNTs was obtained by Shi et al.27 by using a Y-Ni alloy
composite graphite material as the anode for dc arc discharge,
as compared to the yield via a metallic yttrium and nickel
catalyst mixture.
The arc plasma technique has been known to produce metal
nanopowders and composite nanomaterials for years.1 Specif-
ically, the carbon arc found its application in metallurgy a long
time ago,2 while more recently, carbon nitride, which has
received worldwide attention because its characteristics are close
to those of diamond, was obtained by the carbon plasma
technique.3 Arc-plasma-produced carbon vapors have also been
shown to be the source of fascinating novel nanocarbonss
fullerenes4,5 and nanotubes.6 The carbon arc co-evaporation of
various elements and compounds is known to produce other
interesting carbon nanomaterials, e.g., endohedral fullerenes,7
filled nanotubes,8 nanotubules,9 onions,10 nanoencapsulates,11
and nanospheres.12 Some published results regarding these
syntheses are, however, inconclusive and even contradictory,
mostly due to poor reproducibility and flaws in the experimental
techniques. Marked differences in the yields of carbon nano-
structures, due to the particular element used in the preparation,
have been reported, e.g., in the synthesis of tubules catalyzed
by Cu.13 This is particularly the case in single-wall carbon
nanotube (SWNT) formation, where different catalysts and/or
process parameters are claimed to be the best. When Fe was
used as the catalyst, SWNTs were efficiently grown in a He
atmosphere.14 However, Bethune et al.15 argued that Fe, Ni, and
Ni-Cu mixtures (50:50) do not catalyze the process, while Co
does. Iijima and Ichihashi16 claimed that the synthesis using Fe
is successful only when methane is also present in the reactor.
Vaporization of iron by the carbon arc plasma is also known
to yield other interesting products. Pradeep et al.17 obtained the
FeC60 adduct by contact-arc sublimation of graphite in a partial
atmosphere of Fe(CO)5. On the basis of some properties of the
obtained adduct, the authors concluded that FeC60 was likely
an endohedral species, with the Fe atom inside the C60 cage.
Roth et al.18 have provided evidence for an externally bound
iron-fullerene complex, FeC60+. This was generated also in
the gas phase by a ligand-exchange reaction of Fe(CnH2n)+
(n ) 2-5) ions with preformed C60. Complexes of iron-
During the arcing, optical emission spectroscopy on the arc
zone was carried out to determine temperatures and C2 column
densities across the arc section. The arc interior temperature
plays an important role in the high-yield synthesis of nanocar-
bons, e.g., temperature control is very important for nanotube
growth.28 The C2 radical is only one of the numerous intermedi-
ate carbon species that can be quantitatively measured using
conventional optical emission spectroscopy.29-31 This radical
is generally accepted32 to play an important role in the
mechanism of fullerene and nanotube formation. Hence, it is
of considerable importance to quantitatively study its presence
in the reaction zone. The composition of the solid products was
also studied by various techniques, and gas-phase speciation of
* Corresponding author. E-mail: ahuczko@chem.uw.edu.pl.
† Warsaw University.
‡ Toyo Tanso Co. Ltd.
10.1021/jp002842q CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/26/2000

Carbon Arc Plasma, Fullerenes, and Nanotubes
J. Phys. Chem. A, Vol. 104, No. 46, 2000 10709
TABLE 1: Operating Parameters and C60 Fullerene Yield
input sublimation
C60
content
(wt %)
electrode
composition
pressure current voltage power
rate
(kPa)
(A)
(V)
(W)
(mg/s)
C
8
40
80
8
40
80
8
74
71
64
79
70
71
65
76
78
19
23
29
25
28
31
22
28
31
1406
1633
1856
1975
1960
2201
1430
2128
2418
3.9
3.3
3.9
3.3
2.9
3.6
4.2
3.6
3.3
10.5
7.3
4.8
4.4
4.5
6.4
5
C-Fe
C-Co/Ni
40
80
4.5
5
the catalyst in the arc gap was correlated with the metal content
in the cathode deposit and soot.
Experimental Details
The reaction system and experimental procedure have been
described in more detail elsewhere.30 Homogeneous graphite
electrodes 6 mm in diameter and containing 0.6 at. % of either
Fe or a Co-Ni metal mixture were subjected to dc arcing in a
He atmosphere under pressures of 8, 40, and 80 kPa. Experi-
ments with pure graphite anodes were also performed for
comparison. The arc current was adjusted to maintain similar
sublimation rates for the anode (between 3 and 4 mg s-1) for
each electrode type. The electrode gap was kept constant (within
1 mm) using an optoelectronic system. The arc burned in a
stationary mode, and the spatial distribution of temperatures and
C2 radicals remained undisturbed for the duration of the
observations.
The astronomical CCD ST8 camera and coupled monochro-
mator were used for the plasma diagnostics. The C2 content of
the plasma was determined using the self-absorption effect. The
3
3
self-absorption is distinctly pronounced in the d Πg f a Πu
(0-0, 516.5 nm) emission band. The temperature and column
density of C2 (the product of average concentration and column
length along a given chord in the arc cross section) were
estimated by comparing the experimental spectra with those
calculated for various temperatures and column densities of C2.31
The temperatures were also estimated from the well-known
Boltzmann plots, using some of the sufficiently resolved
rotational components of the band. It had been shown earlier33
that in the case of significant self-absorption, the spectra fitting
method and Boltzmann plot lead to temperatures a few hundred
degrees higher and lower than the real values, respectively.
Therefore, the temperatures reported here were the mean values
from the combination of both approaches.
Figure 1. Radial temperature distributions in the arc plasma under
pressures of 1-8, 2-40, and 3-80 kPa.
Soot, produced by the arc sublimation of the test anodes, was
boiled under reflux in dry toluene prior to C60 determination
by conventional spectrophotometric measurements at 329 nm.
For the identification of the higher fullerenes, we analyzed the
soot through the laser desorption TOF MS technique. The
morphology of carbon nanostructures was investigated by SEM
and HRTEM techniques.
Fe (or Co-Ni) at higher pressures when this observation was
more pronounced.
Plasma Temperature and C2 Content. The results of the
spectroscopic measurements of temperatures across a vertical
section between the electrodes are presented in Figure 1. The
temperatures were derived from non-Abel-inverted spectra.
Therefore, the obtained values are to some extent the average
temperatures. Obviously, this is the case mainly for the plasma
columns along and close to the plasma centerline. Generally,
as seen in Figure 1, no great difference exists between the
distributions representing the different anode compositions.
Nevertheless, in the case of the pure graphite electrode, the radial
temperature gradients are slightly higher, probably due to the
lower power input (Table 1). The higher temperatures observed
at lower pressures (curve 1), independent of the anode composi-
tion, are the common feature of the estimated temperature
distributions.
Results and Discussion
The processing variables are presented in Table 1. The
presence of the catalysts in the electrode material resulted in
higher power input compared to that of pure graphite alone.
This is probably due to a higher electrical conductivity of both
the electrode material and the plasma itself. Thus, to maintain
the desired average anode ablation rate of between 3 and
4 mg s-1, we increased the arc power, e.g., from 1860 W
for pure graphite to 2200 W (2400 W) in the presence of

10710 J. Phys. Chem. A, Vol. 104, No. 46, 2000
Huczko et al.
small arc gap, the accuracy of the positioning of the target
plasma cross section onto the monochromator slit was within
(0.2 mm only. Thus, even a small horizontal displacement can
yield results from a different arc cross section than was intended.
Yield of C60 and Higher Fullerenes. The final content of
the C60 fullerene in the collected soot is presented in Table 1
(last column). It shows that at low pressures the presence of
the catalysts significantly reduces the yield of C60 while at higher
pressures the yield is independent of the type of graphite
electrode. The use of nonhomogeneous electrodes (drilled and
filled with the metals) with much higher Fe content (5.5 at. %)
yielded approximately the same (4.5 wt %) amount of C60.34
It is known that during the arcing of the graphite, not only
C60 but also higher, mainly C70, fullerenes are formed. Therefore,
the relative mass distributions of higher fullerenes in the solid
products for all tests performed were studied by the LD TOF
MS technique. The resulting mass spectra, normalized to the
C60 peak, are presented in Figure 3. The abundance of the higher
fullerenessparticularly the so-called “missing fullerenes” C72,
C74, and C80sin pure graphite is, as expected, very low.35 The
influence of the catalysts on the formation of those species is
evident, with the role of Fe being quite opposite to that of Co/
Ni. It is clearly seen that for the anode doped with Fe, the higher
fullerene production is distinctly enhanced while for the Co/Ni
admixture the effect is opposite, and especially at greater
pressures, the generation of the higher fullerenes is nearly totally
suppressed. Thus, we can conclude that Fe atoms or clusters
may play a catalytic role in generating these, otherwise missing
fullerenes.
Carbon Nanotubes. When a dc arc forms on a pure graphite
anode, molecular structures known as multiwall carbon nano-
tubes (MWCNTs) are formed. These nanostructures grow
efficiently when the He pressure is great enough (>40 kPa)
and the arc gap is kept small during the arcing (<1 mm). The
nanotubes can only be found in the core of the cathode deposit
and are not detected in other places in the reactor. Coevaporation
of carbon and catalyst (mostly metals) in the arc plasma also
results in the formation of CNTs (mostly as single-walled
structures) at higher pressure, but they are mostly found as a
component of the soot. The results of our experiments indicate
that even small amounts of the catalysts dramatically change
the morphology of the product soot, i.e., the soot is more
compact, much less dusty, and more cloth-like in “texture” in
comparison to the soot resulting from the sublimation of pure
graphite. A weblike product was often found in the discharge
chamber. Such webs have been observed already by research-
ers.28 The soot collected from the reactor walls and the weblike
product were separately analyzed by SEM and TEM. Examples
of the electron microscopy images of soot products obtained at
low- and high-pressure conditions are shown in Figure 4. The
most interesting feature of the soot is the presence of elongated
nanostructures, with single-walled nanotubes clearly visible.
Some of these nanotubes were covered with a layer of
amorphous carbon. In the Fe catalyst, the carbon nanotubules
were found mostly in the weblike material surrounding the
cathode deposit, while for the Co-Ni catalyst, the comparable
nanostructures were found in both the chamber-deposited soot
and the webs. However, significant amounts of the nanotube
bundles up to several nanometers thick were found only in the
webs. The chamber soot contained high concentrations of narrow
nanotubes, together with clustered particles of amorphous carbon
and catalyst accretions. The catalyst particles were also present
in the webs. In contrast to an earlier report,36 we have not found
carbon nanotubes in the cathode “collaret” material.
3
Figure 2. Radial column density distributions of C2 (a Πu, V′ ) 0)
radicals in the arc plasma under pressures of 1-8, 2-40, and 3-80
kPa.
3
The column densities for C2 (a Πu, V ) 0) were calculated
on the basis of the temperatures and the integrated intensities
of the normalized (0,0) vibrational band adjacent to the (1,1)
band (516.5-512.9 nm). The results are shown in Figure 2,
where it can be seen that He pressure strongly influences the
C2 content in the arc plasma. Despite the similar electrode
sublimation rates, more C2 radicals are generated when the
buffer gas pressure in the reactor chamber is higher. Obviously,
this increase is due to the lower expansion rates of the carbon
species, governed by diffusion, at the higher pressure. Compar-
ing the column density distributions for the different anode
compositions, we can see that in the case of the Fe admixture,
the C2 content is lower. This is very intriguing since some
physical properties (e.g., ionization potentials) of the elements
under consideration are close to one another and none of the
catalysts can bond chemically to carbon atoms at plasma
temperatures. It must be admitted, however, that because of the

Carbon Arc Plasma, Fullerenes, and Nanotubes
J. Phys. Chem. A, Vol. 104, No. 46, 2000 10711
Figure 4. SEM (a, b, d, and e) and TEM (c and f) images of webs
and soot containing various amounts of nanotubes and irregular clusters
of spherulitic amorphous carbon. C-Fe: (a) web (8 kPa), (b) web (80
kPa). C-Co/Ni: (d) reactor soot (8 kPa), (e) web (80 kPa). C-Fe:
(c) web (80 kPa). C-Co/Ni: (f) web (80 kPa).
a varying distribution is obviously caused by the high-temper-
ature environment (plasma, ∼5000 K; cathode surface, >3200
K) preventing the condensation of the iron vapor. Thus, in the
case of the homogeneous electrodes, an explanation is proposed
to account for the facts that SWNTs do not grow in the cathode
products and that the metal-vapor-catalyzed reactions only occur
in the quenching zone of the reactor.
Acknowledgment. The work was supported by the Com-
mittee for Scientific Research through the Department of
Chemistry, Warsaw University, by Grant 3 T09A 058 16. The
authors also thank Dr. P. Lanigan, De´p, ge´nie chimique,
University de Sherbrooke, Sherbrooke, Canada, for his com-
ments on and corrections to the text.
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