Bulk morphology and diameter distribution of single-walled carbon nanotubes synthesized by catalytic decomposition of hydrocarbons

Article abstract of DOI:10.1016/S0009-2614(98)00479-5

Long and wide ropes/ribbons of single-walled carbon nanotube (SWNT) bundles with rope diameters of 100 μm and lengths to 3 cm were synthesized by the catalytic decomposition of hydrocarbons. These ropes/ribbons consist of roughly-aligned bundles of aligne

Full text of DOI:10.1016/S0009-2614(98)00479-5

19 June 1998
Ž
.
Chemical Physics Letters 289 1998 602–610
Bulk morphology and diameter distribution of single-walled
carbon nanotubes synthesized by catalytic decomposition of
hydrocarbons
a
b
b
b
b
H.M. Cheng a,b, F. Li , X. Sun , S.D.M. Brown , M.A. Pimenta , A. Marucci ,
c
b,d
G. Dresselhaus , M.S. Dresselhaus
Institute of Metal Research, Academia Sinica, 72 Wenhua Rd., Shenyang, 110015 China
Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
a
b
c
d
Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Received 7 April 1998; in final form 16 April 1998
Abstract
Ž
.
Long and wide ropesrribbons of single-walled carbon nanotube SWNT bundles with rope diameters of 100 mm and
lengths to 3 cm were synthesized by the catalytic decomposition of hydrocarbons. These ropesrribbons consist of
roughly-aligned bundles of aligned SWNTs. The SWNT diameters, as determined from HRTEM images, are 1.69 "0.34
nm, which, in combination with resonant Raman scattering measurements, indicates that our SWNTs have larger diameters
than those synthesized by other techniques. The larger SWNTs are promising for gas-storage applications. The easy
manipulation of the ropes offer special opportunities for their characterization and applications. q 1998 Published by
Elsevier Science B.V. All rights reserved.
1. Introduction
both predicted theoretically and demonstrated experi-
mentally that SWNTs have many especially interest-
ing properties. Since both the laser vaporization and
electric-arc methods may not be the best methods for
obtaining a continuous process for SWNT produc-
Much attention has been paid to both fundamental
and applied research on carbon nanotubes 1–3 since
the discovery of multi-walled carbon nanotubes
w
x
Ž
.
w x
w x
MWNTs by Iijima in 1991 4 . In particular, recent
tion on a commercial scale 14 , more emphasis
should be given from an applications standpoint to
the production of high-purity, high-yield, low-cost,
large-scale and easily handled SWNTs. Recently, we
progress in research on the properties of single-walled
carbon nanotubes SWNTs , such as their atomic
structure and electronic properties 5–8 , gas storage
properties 9 , mechanical properties 10 and prop-
erty enhancement through nanotube modification
Ž
.
w
x
w x
w
x
w x
reported 15 a novel method for synthesizing
SWNTs, the catalytic hydrocarbon decomposition
method, in which benzene is catalytically decom-
posed at 1100–12008C, yielding SWNTs which are
similar, on a nanometer scale, to those obtained by
w
x
11 , has been outstanding, mainly due to the avail-
ability of sufficient quantities of SWNTs that can be
obtained using the pulsed laser vaporization method
w
x
w
x
w x
w x
12 and the electric-arc technique 13 . It has been
laser vaporization 12 and electric-arc 13 tech-
0009-2614r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 98 00479-5

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)
H.M. Cheng et al.rChemical Physics Letters 289 1998 602–610
603
niques. This growth method allows lower growth
temperatures, permits semi-continuous or continuous
preparation and produces a large quantity of SWNTs
at relatively high purity and low cost.
In this paper, two specific characteristics of
SWNTs synthesized by the catalytic decomposition
of hydrocarbons are reported. One is that the prod-
ucts mostly consist of macroscopic ropes and ribbons
of aligned bundles of SWNTs, compared to the mats
of randomly oriented and entangled carbon filaments
reported by both the laser vaporization and electric-
SWNTs that were obtained were observed in their
as-prepared state using field emission scanning elec-
Ž
.
tron microscopy SEM , transmission electron mi-
Ž
.
croscopy TEM and high-resolution TEM
HRTEM . To further characterize the SWNTs, Ra-
Ž
.
Ž
.
man scattering of the radial breathing mode RBM
of SWNTs in the region around 180cmy1 was mea-
sured, since the frequency of the RBM is inversely
proportional to the tube diameter without any chiral-
ity dependence, according to theoretical predictions
w
x
17–19 .
w
x
arc methods 12,13 . Another characteristic of the
catalytically grown material is that the diameters of
the SWNTs that we obtained, based on the results
from high-resolution transmission electron micro-
3. Results and discussion
3.1. Aligned large ropes and ribbons of SWNT bun-
dles
Ž
.
scope HRTEM images and Raman scattering mea-
surements, are larger than those obtained by the laser
w
x
w x
vaporization 12 and electric-arc 13 techniques.
The bulk morphology of SWNTs prepared by the
w
x
w x
laser vaporization 12 and electric-arc 13 tech-
niques are reported to be a mat of tangled carbon
2. Experimental
Ž
.
filaments SWNT bundles 10 to 20 nm in diameter
and many micrometers long and the axes for the
SWNT bundles are randomly oriented within the
mat. However, in our vapor grown sample under
certain preparation conditions, large quantities of
aligned, long and wide macroscopic ropes and rib-
bons were observed, as shown in Figs. 1 and 2,
which look silver-black and appear to be very light,
and some ribbons are semi-transparent, but free-
standing.
The detailed experimental procedures for the
preparation of SWNTs with our hydrocarbon cat-
alytic decomposition method were described in detail
w
x
elsewhere 15 . Basically, an improved floating cata-
lyst method, similar to that used by Endo et al. for
w
x
the vapor growth of carbon fibers 16 , was em-
ployed in which a vapor-phase catalyst precursor
Ž
.
ferrocene and
a
SWNT growth promoter
Ž
.
thiophene were floated into a horizontal reactor to
Fig. 1 illustrates a photograph showing the actual
achieve semi-continuous growth of SWNTs. During
the preparation of SWNTs, ferrocene was vaporized
and carried into the reaction tube with a mixture of
benzene and thiophene vapors and hydrogen gas.
The vaporized ferrocene was at first reduced by
hydrogen to form atomic iron, allowing atomic iron
to agglomerate into iron clusters or nanoparticles
appropriate for the growth of SWNTs. The reaction
temperature was maintained at 1100–12008C, and
typical synthesis times for our runs have been be-
Ž
.
tween 1 and 30 min but they can be much longer .
The grown nanotubes were transported out of the
reaction zone by the flowing reacted gases and the
nanotubes were collected on a graphite plate or on
the thermocouple-protecting tube and on the tube
wall at the lower-temperature end of the reaction
tube. The morphology and microstructure of the
Fig. 1. Optical image of as-synthesized SWNT product obtained
from the catalytic decomposition of hydrocarbons.

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H.M. Cheng et al.rChemical Physics Letters 289 1998 602–610
Ž .
Fig. 2. SEM images of a rope and ribbon from the as-synthesized product prepared by catalytic decomposition of hydrocarbons. a A low
Ž . Ž . Ž . Ž .
magnification image of a rope.
b
An enlargement of one portion of the rope in a .
c
A low magnification image of a ribbon.
Ž .
d An
Ž . Ž .
enlargement of one portion of the ribbon in c . e A low magnification image of a fine rope. f An enlargement of one portion of the fine
Ž .
rope in e .

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)
H.M. Cheng et al.rChemical Physics Letters 289 1998 602–610
605
size of the products. It shows that the products
mainly consist of many ropes and ribbons that can be
clearly distinguished to have lengths of several cen-
Ž
.
timeters the longest rope was about 3 to 4 cm and
with diameters of about 0.1 mm or widths of several
millimeters. These ropes and ribbons can be isolated
from one another by hand. If we examine one rope
or ribbon, which can be observed using SEM, we
will see that the rope or ribbon is composed of
thousands of roughly-aligned whisker-like filaments,
as shown in Fig. 2. These filaments were previously
shown to contain SWNT bundles, and TEM observa-
tions of the material in Fig. 2 is shown in Fig. 3. For
example, a rope with a diameter of about 80 mm is
Ž .
shown in Fig. 2 a and many small threads in the
rope are aligned preferentially along the axial direc-
Ž
Ž ..
tion of the rope. A magnified SEM image Fig. 2 b
illustrates that nanotube bundles are roughly aligned
along the axis of the rope, although many bundles
are also twisted with respect to one another. Figs.
Ž .
Ž .
2 c and d show a ribbon with a width of more than
120 mm, in which the SWNT bundles are very well
aligned along the longitudinal direction of the rib-
bon. The thickness of the ribbons in the product is
Ž .
Ž .
typically F10 mm. Figs. 2 e and f show a very
thin rope with a length of more than 200 mm within
the image and a diameter of about 1 mm. We can see
again that the small bundles within the rope are well
aligned along the axis of the rope and that the
apparent diameters of SWNT bundles range from
Ž
Ž ..
several nanometers to about 40 nm Fig. 2 f . A
typical TEM image of the as-synthesized product is
Ž .
given in Fig. 3 a . Here we see that the bundles
always consist of many well-aligned SWNTs and
their diameters, ranging from several nanometers to
40 nm, are well maintained over the entire length,
which is very similar to the SWNT bundles grown
w
x
by other methods 12,13 . In this figure the cross-sec-
tion of a SWNT bundle is seen. The bundle has a
uniform diameter of 20 nm and consists of tens of
SWNTs that are neatly packed.
On the basis of the above observed results, we
conclude that the combination texture of the SWNT
bundles synthesized by our hydrocarbon catalytic
decomposition method, differs in detail from those
produced by other major synthesis methods, and our
SWNT bundles do have certain preferred orienta-
tions and tend to align along their axial direction to
Fig. 3. HRTEM images of the as-synthesized SWNTs by the
Ž .
catalytic decomposition of hydrocarbons.
a
Bundles of aligned
Two bundles of aligned SWNTs and an isolated
Ž .
Ž .
Ž .
c
SWNTs.
SWNT.
b
Two isolated SWNTs.
d
An isolated SWNT with a
An isolated SWNT having the largest
observed diameter of 4.3 nm.
Ž .
e
wavy morphology.

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H.M. Cheng et al.rChemical Physics Letters 289 1998 602–610
form large ropes or ribbons. We consider that the
formation mechanism for large SWNT ropes and
ribbons is most probably related to the following
physical process. The hydrocarbon catalytic decom-
shows a well-aligned SWNT bundle with a diameter
of 30 nm, four parallel SWNTs aligned one-by-one,
and an isolated SWNT with a diameter of 1.7 nm, as
denoted by a, b and c, respectively. Most of the
isolated SWNTs that were observed have diameters
ranging from 1.2 to 2.0 nm, as shown in portion c of
w
x
position method 15 works at normal atmospheric
pressure within a certain gas flow rate, flowing in
one direction. During their synthesis, the SWNTs in
a bundle that are simultaneously grown in the reac-
tion zone are transported out of this zone by the
flowing gases, and the SWNTs adhere to the graphite
plate or to the thermocouple protecting tube or to the
reaction tube wall. The gas phases continuously flow
into the reaction zone and the newly formed nan-
otubes continuously flow out of the reaction zone
with the flowing gas phases. The newly formed
SWNT bundles may adhere to the previously formed
SWNT bundles due to a Van der Waals force, or
they will mechanically twist or knit together to form
Ž .
Ž .
Ž .
Fig. 3 b , portion a of Fig. 3 c and Fig. 3 d . A few
very large SWNTs were also observed, such as those
Ž . Ž
.
shown in portion b of Fig. 3 c 3.2 nm and in Fig.
Ž . Ž
.
3 e 4.3 nm , where the nominal projected diameters
were given in parentheses. Although no SWNTs with
diameters larger than 2 nm were reported in the
as-synthesized form by the laser vaporization and
w
x
electric-arc methods, Nikolaev et al. 20 reported
that as many as 60% of the SWNTs prepared by
laser vaporization coalesced with neighbors after an-
nealing at 1400 or 15008C, resulting in nanotubes
Ž
.
Ž
with twice 2.7 nm and occasionally three times 4.1
Ž
.
Ž
.
long and wide ropes and ribbons. Even during the
nm the diameter of the 10, 10 tube. These results
suggest that big SWNTs, such as 20,20 tubes 2.75
nm or 30,30 tubes 4.12 nm , can also be stabi-
Ž
.
Ž
preparation of samples for characterization, the tubes
tend to adhere to one another or to other surfaces
.
Ž
.
Ž
.
.
like a glass plate . Moreover, since the SWNT bun-
lized and synthesized. In addition, it can be observed
Ž
Ž ..
dles thus formed are very fine and light, it is likely
for the bundles to align along the flowing direction
of the gas phases when the bundles are binding to
each other to form ropes or ribbons. Because other
that some SWNTs Fig. 3 d have a wavy morphol-
ogy, which is considered to be an imperfect struc-
ture. Such structures may be due to the low synthesis
temperature and sulfur addition used in our hydro-
carbon catalytic decomposition method.
w
x
main methods 12,13 for the synthesis of SWNTs
basically involve a low vacuum environment and no
obvious gas phase flow, it is unlikely for them to
obtain long and wide SWNT bundles aligned into
ropes or ribbons. Therefore, this SWNT roperribbon
forming process can be considered to be one of the
unique characteristics of our hydrocarbon catalytic
decomposition method.
The diameters of 74 isolated SWNTs were mea-
sured, with an accuracy of about "0.1 to 0.15 nm,
based on HRTEM images similar to those shown in
Fig. 3. From these measurements a diameter distribu-
tion for our SWNTs was deduced, and the results are
shown in Fig. 4. Since only one SWNT with a
Ž
diameter larger than 3.2 nm 4.3 nm as shown in Fig.
3 e was observed, we put this diameter value into
Ž ..
From Fig. 2, we also observe that some particles
or particle clusters, which are considered to be impu-
rities in SWNTs, such as carbon nanoparticles, cata-
lyst particles and carbon blacks, exist on the surfaces
of or among the SWNT bundles. This suggests that
purification may need to be carried out before a
precise characterization of the physical properties of
the SWNTs is carried out.
the 3.0 to 3.2 nm diameter group of SWNTs to
simplify Fig. 4. We can see that more than 75% of
the SWNTs have diameters ranging from 1.2 to 2
nm, with a Gaussian mean diameter of 1.69"0.34
nm. These results show that the SWNTs synthesized
by our hydrocarbon catalytic decomposition method
have larger diameters than those grown both by the
laser vaporization method, whose diameters have a
w
x
3.2. Diameter distribution of the SWNTs
mean value 12 of 1.38 nm or a Gaussian mean
value of 1.39 "0.1 nm, as fitted by the authors to
w x
We have carried out further HRTEM observations
the STM measurements by Wildoer et al. 6 and by
¨
Ž .
Ž .
w
x
on the as-synthesized samples and Figs. 3 b to e
the electric-arc method 13 , whose diameters are
about 1.4 nm.
Ž .
show typical images of isolated SWNTs. Fig. 3 b

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H.M. Cheng et al.rChemical Physics Letters 289 1998 602–610
607
wider diameter distribution from HRTEM observa-
tions. It is predicted that the Raman cross-section of
the RBM decreases as the diameter increases. Thus
the intensity of the RBM Raman peaks becomes
weak, preventing their experimental observation for
nanotube diameters in the wings of the diameter
distribution, thereby resulting in a relatively narrow
spectrum from the RBM, giving more weight to the
contributions from the smaller diameter SWNTs.
According to theoretical predictions, the fre-
quency of the radial breathing mode is inversely
proportional to the diameter of the SWNTs. Recent
calculations show that the frequency for the RBM
y1
Ž
.
for a 10,10 armchair nanotube is around 162 cm
,
which leads to the expression vs223.75rd, where
v is in units of cmy1 and d in nm 21 . Application
of this expression to the experimental results from
our sample, in which the RBM band is centered
about 158 cmy1, would give an average diameter of
1.42 nm, which is significantly smaller than the
value obtained by HRTEM observations. Similarly,
the average diameter of the laser vaporization sample
Fig. 4. Diameter distribution of the SWNTs obtained by the
catalytic decomposition of hydrocarbons using data obtained from
HRTEM images. The Gaussian fit to these data gives a mean
diameter of 1.69 "0.34 nm for our SWNTs.
w
x
w
x
It is known theoretically 17–19 that the resonant
Raman scattering from SWNTs associated with the
Ž
.
Ž
radial breathing mode RBM depends strongly upon
the diameter of SWNTs. The larger the diameter of
the SWNTs, the lower the frequency of these RBM
peaks. Therefore, resonant Raman scattering mea-
surements of the as-synthesized SWNTs were per-
formed for the RBM spectral region with a laser
excitation energy of 2.54 eV, and the scattering
profile for these results is shown in Fig. 5a. For
comparison, we also measured the Raman band asso-
ciated with the radial breathing mode of the SWNTs
prepared by the laser vaporization method at the
obtained from the maximum of the RBM band about
184 cmy1 and using the above v d versus 1rd
.
Ž .
w
x
expression 21 is around 1.21 nm, which is also
Ž
.
same laser excitation energy Fig. 5b . Comparing
Fig. 5a with b, the RBM frequency of the maximum
intensity of the Raman bands for our SWNTs is
shifted to lower frequencies by about 25 cmy1
,
which indicates that our SWNTs do have larger
diameters than those produced by laser vaporization,
consistent with our HRTEM observations, as dis-
cussed above. However, the RBM spectra for our
SWNT sample is fitted by only two Lorentzian-fitted
curves with peaks at 159 and 154 cmy1 Fig. 5a and
has a smaller bandwidth than that for the laser
Ž
.
Fig. 5. Raman spectrum, using an excitation laser energy of 2.54
eV, of the radial breathing mode for the SWNTs prepared by the
Ž
.
vaporization SWNT sample Fig. 5b which has a
quite broad Raman band composed of more individ-
Ž .
catalytic decomposition of hydrocarbons a and by laser vaporiza-
Ž .
tion b . The dotted curves represent the individual Lorentzian
Ž
.
ual peaks 164, 168, 177, 182, 187, 194, 199 and
curves and the solid curve represents the fit to the experimental
data.
205 cmy1 , although our sample seems to have a

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H.M. Cheng et al.rChemical Physics Letters 289 1998 602–610
smaller than the average nanotube diameters for the
als. They estimated that, if hydrogen at the density of
solid hydrogen with a hexagonal close-packed lattice
Ž
laser vaporization sample 1.38 nm reported from
w
x
Ž
.
X-ray diffraction results 12 and 1.39 nm by our
Gaussian fit to the STM measurements by Wildoer et
al. 6 . Although the reason for this difference is not
were to fill the entire hollow of typical 10,10
SWNTs and the interstitial volume between these
SWNTs, then the total hydrogen uptake would be
¨
w x.
w
x
Ž
.
yet fully understood, it may be explained by the
decreases in the Raman cross-section of the RBM
with increasing tube diameter, as discussed above.
This discrepancy may also be ascribed to the under-
HrC s 114 atom% for 10,10 SWNTs, but for
larger SWNTs with a 2 nm diameter, the amount of
hydrogen that could be stored within the nanotube
core at the solid hydrogen density would be in-
Ž
.
w
x
estimation of the RBM peak frequency for the 10,10
creased to a HrC ratio of about 250 atom%, using
nanotube in the theoretical prediction. If we fit the
the same set of assumptions. On the basis of their
Ž
.
w
x
experimental results of the mean diameter 1.38 nm
prediction 23 , we estimate that the amount of hy-
drogen that could be stored within our SWNTs with
a median diameter of 1.69 nm could be as high as
about 150 atom%, a rather optimistic value for prac-
tical energy-related applications.
y1
Ž
.
and maximum RBM Raman peak 184 cm
tained from the laser vaporization sample, the above
expression will be adjusted to be vs254rd. With
this expression, we can obtain a mean diameter of
1.61 nm from the RBM Raman peak 158 cm
our sample, which is much closer to the HRTEM
results. In addition, Kahn and Lu 22 pointed out
that the RBM frequency of SWNTs in a bundle is
expected to be higher than that of an isolated SWNT
because of interactions between nanotubes. Since
most of our SWNTs are observed to be in bundle
form, the measured RBM frequency could be higher,
which, as a consequence, results in a lower estima-
tion of the diameters of SWNTs.
The growth mechanism of larger-diameter SWNTs
from the catalytic decomposition of hydrocarbons
with the addition of sulfur is not yet clear. One
possibility attributes the larger diameters to the low
growth temperature in the catalytic hydrocarbon de-
composition method. The growth temperature for our
ob-
y1
Ž
.
w x
for
Dillon et al. 9 investigated experimentally the
hydrogen absorption capacity of SWNTs with a mean
diameter of about 1.2 nm in comparison with that of
activated carbon. They showed that hydrogen can
condense to a high density inside the narrow core of
SWNTs under conditions that do not induce adsorp-
tion within a standard mesoporous activated carbon.
The hydrogen uptake capacity of their SWNTs was
found to be 5–10 wt%, or an HrC ratio of about
60–120 atom%. They also pointed out that the
amount of hydrogen stored for a practical vehicle
powered by a fuel cell required system densities
w
x
w
x
approaching 6.5 wt% and 62 kg H2
m
y3, and no
storage technology is currently capable of meeting
these goals. However, they predicted that SWNTs
with diameters of 1.63 nm and 2.0 nm would come
close to the target densities and could operate near
room temperatures if modest H2 over-pressures
compensated for the lower heats of adsorption ex-
pected in the larger cavities. High energy storage
efficiencies can also be achieved, if the system could
be operated at or near ambient temperatures and
pressures. Therefore, larger diameter SWNTs synthe-
sized by our hydrocarbon catalytic decomposition
method appear to be very promising for this applica-
tion.
Ž
.
method is relatively low 1100–12008C , compared
to the laser-generated and electric-arc-generated tem-
Ž
.
peratures of graphite targets about 30008C for other
growth methods. This lower growth temperature may
result in larger-diameter SWNTs, since the strain
energy per atom for curving a graphene sheet into a
cylinder is much higher at smaller diameter. In addi-
tion, the growth rate of SWNTs at lower temperature
is slower, which gives more time to make larger tube
diameters.
Since the long and wide ropes and ribbons of
aligned SWNT bundles can be easily handled and
manipulated by hand or with the aid of a low-mag-
nification optical microscope, the availability of such
nanotubes will be of importance for many property
characterizations and application explorations of
3.3. Expected applications for the aboÕe two charac-
teristics of SWNTs
w
x
Brown et al. 23 have recently discussed the
reversible hydrogen uptake in carbon-based materi-

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H.M. Cheng et al.rChemical Physics Letters 289 1998 602–610
609
SWNTs. For example, by incorporating such an
Acknowledgements
aligned SWNT rope into a resin matrix to form a
mini-composite structure, the mechanical properties
of the SWNTs can be characterized through a combi-
nation of experimental measurements and modeling
predictions for composite materials. This research is
in progress and prelimininary results show that it is
promising.
The IMR group acknowledges NSFC grant No.
59672024 for support of this research and the work
at MIT was supported in part by NSF grant DMR
95-10093. The authors owe sincere thanks to Mr.
Y.L. Wei and Ms. G. Su for their help in sample
preparation and to Dr. M.J. Matthews for valuable
discussions.
4. Conclusions
1. The products synthesized by the catalytic decom-
position of hydrocarbons consist mainly of long
and wide, macroscopic ropes or ribbons of aligned
SWNT bundles with rope diameters of about 100
mm or ribbon widths of several millimeters and
lengths of about 3 cm. The microstructure obser-
vations show that the ropes consist of many
roughly-aligned bundles and the bundles consist
of well-aligned SWNTs.
2. The formation of aligned ropes and ribbons of
SWNT bundles is considered to be one of the
unique characteristics of the hydrocarbon catalytic
decomposition method, mainly due to the action
of the van der Waals force, the flow of the gas
phases in one direction and the mechanical inter-
locking and twisting of the bundles.
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w
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Ž
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