Synthesis and characterization of single-walled nanotubes produced with Ce/Ni as catalysts

Article abstract of DOI:10.1016/S0009-2614(00)00235-9

SWNTs were synthesized in high yield in the presence of Ce/Ni as catalyst. Raman and SEM results show that a fraction of the tubes are chiral and have small diameters (the snallest one is 0.79 nm), indicating that Ce plays an important role in the formation. Acid treatment is effective in eliminating impurities but changes the morphology and the electric resistance of mats with increasing treatment time. The resistance of as-grown and short time acid-treated mats follow a 2D VRH behaviour while mats treated for a long time follow a 3D model. The results indicate that hopping conduction occurs on bundles or on tubes and that there are intrinsic links between tubes in as-grown mats.

Full text of DOI:10.1016/S0009-2614(00)00235-9

7
April 2000
Chemical Physics Letters 320 Ž2000. 365–372
www.elsevier.nlrlocatercplett
Synthesis and characterization of single-walled nanotubes
produced with CerNi as catalysts
Bingbing Liu a,b,), T. W a˚ gberg , Eva Olsson , Ruseng Yang , Hongdong Li ,
a
c
b
d
d
b
b
a
Shulin Zhang , Haibin Yang , Guangtian Zou , B. Sundqvist
a
Department of Experimental Physics, Ume a˚ UniÕersity, S-901 87 Ume a˚ , Sweden
National Lab of Superhard Materials, Jilin UniÕersity, Changchun 130 023, China
b
c
Analytical Materials Physics, The A˚ngstr o¨ m Laboratory, Uppsala UniÕersity Box 534, S-751 21 Uppsala, Sweden
d
Department of Physics, Beijing UniÕersity, Beijing 100 871, China
Received 4 January 2000; in final form 10 February 2000
Abstract
SWNTs were synthesized in high yield in the presence of CerNi as catalyst. Raman and SEM results show that a
fraction of the tubes are chiral and have small diameters Žthe snallest one is 0.79 nm., indicating that Ce plays an important
role in the formation. Acid treatment is effective in eliminating impurities but changes the morphology and the electric
resistance of mats with increasing treatment time. The resistance of as-grown and short time acid-treated mats follow a 2D
VRH behaviour while mats treated for a long time follow a 3D model. The results indicate that hopping conduction occurs
on bundles or on tubes and that there are intrinsic links between tubes in as-grown mats. q 2000 Elsevier Science B.V. All
rights reserved.
1
. Introduction
quantities of single-walled nanotubes were available
in the early days, much early research work was
done on multi-walled nanotubes, but after methods
were found to synthesize single-walled nanotubes
with high yield, using either laser evaporation w2x or
an arc discharge method w3x, quantitative studies
have also been possible of the properties of SWNTs.
Recently, other methods, such as solar energy syn-
thesis w4x and pyrolysis of organometallic precursors
w5x, have also been reported. All these methods need
the presence of catalysts. Different catalysts produce
SWNTs with different intrinsic structures and the
study of SWNT production using different catalysts
is therefore important. Such studies will not only
produce SWNTs with different structures, but also
give essential information on their physical and
chemical properties as well as a basic knowledge of
Carbon nanotubes have attracted great interest
from the viewpoints of both fundamental physics and
potential applications, and inspired many experimen-
tal and theoretical investigations since they were first
discovered experimentally w1x. Single-walled nan-
otubes ŽSWNTs. are the best defined nanotube and
thus most suitable for studies of their intrinsic prop-
erties. The electronic properties are predicted to de-
pend on their geometric structures, giving either
metallic or semiconducting properties. As only small
)
Corresponding author. Department of Experimental Physics,
Ume a˚ University, 90187 Ume a˚ , Sweden. Fax: q46-90-786-6673;
e-mail: bingbing.liu@physics.umu.se
0
009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž00.00235-9

3
66
B. Liu et al.rChemical Physics Letters 320 (2000) 365–372
their growth mechanism. However, only a few cata-
lysts, such as the bimetal mixtures NirCoŽFe.,
NirYŽLa. and RhrPd w6–8x, are available for high
yield synthesis of SWNTs. Recently, binary ŽFe, Co,
Ni. alloy nanoparticles were found effective for
SWNT synthesis w9x. Single metals such as Ni and
Co have also been used as catalysts, but it was found
that bimetals are more efficient than single metals
and that Ni plays the main role in the formation of
SWNTs. Also, SWNTs are larger and more abundant
when bimetal catalysts are used w10x. Of other cata-
2. Experimental
The samples were generated by arcing metal ox-
ide impregnated graphite rod in He gas at a pressure
of 66.5 kPa. In brief, 6 mm in diameter graphite rods
were drilled, filled with a mixture of CeO , nanome-
ter Ni Ždiameter -100 nm. and graphite Žin a molar
ratio of CerNirCs1:4:95. and used as anodes.
Prior to arc burning, the composite anodes were
subjected to treatment at 2008C for 2 h in Ar gas.
The arc discharge was created with a current of 100
A and a voltage of 50 V. After arc burning, a
free-standing collar-like deposit Žmat. was produced
around the cathode, similar to the result with NirY
as catalysts reported by Journet et al. w3x. The mats
were studied by Raman spectroscopy ŽRenishaw-
1000. and SEM ŽOxford LEO 1550. and found to
2
lysts, among the rare-earth metals, only the typical
q3 oxide state elements, Y and La, have been
studied and the catalytic effect of other metals for
high yield synthesis are still unknown.
Single-walled nanotubes are always produced to-
gether with some impurities, which are mainly amor-
phous carbon particles or multi-shell carbon parti-
cles, either empty or filled with the catalyst metals
and fullerenes. These impurities need to be elimi-
nated and acids are usually used to remove catalyst
metals and most of the amorphous carbon. The weak
acid, HCl, is used in some experiments, but is not
effective on metals trapped in multi-shell carbon.
Yosida et al. have reported that they cannot remove
Ni even after immersion of samples in HCl for 2
months w11x. Recently, the stronger acid, concen-
contain mainly single-walled nanotubes.
The as-grown mats were purified further in con-
centrated nitric acid for 0.5–2 h and washed with
distilled water until the pH value was over 5, then
dried naturally in Ar gas and annealed at 10008C in
flowing Ar gas for 1 h to remove residual surface
oxides. During the whole procedure we tried to keep
mats in their original shape without sonication, un-
like the experiment reported in Ref. w12x. During the
acid treatment, the samples visibly decreased in size,
indicating a high rate of oxidation. The treated sam-
ples were again characterized by Raman spec-
troscopy and SEM.
trated nitric acid, has been found to eliminate effec-
tively both metals and amorphous carbon while leav-
ing most of the tubes intact in the bundles w12x.
However, the effect of the acid on the morphology
and on the physical properties of bulk samples is
unclear. As the electric transport results for bulk
samples are different from sample to sample and it is
unclear what is the intrinsic behavior of nanotubes
w13x, acid treatment gives us another opportunity to
The as-grown mats and the purified ones were cut
into pieces 5=1=0.2 mm for resistance measure-
3
ments. Silver paint or pressure contacts were applied
in a four-probe configuration. The temperature de-
pendence of resistance was measured in an Oxford
2000 variable temperature cryogenic system using a
current of 20 mA.
study the characteristic properties of mats.
In the present work, we studied the effect of using
the rare-earth element Ce as a catalyst together with
Ni to synthesize SWNTs in high yield by arc evapo-
ration. The oxide state of Ce is typically q4 and
thus different from that of Y and La. Both the
as-grown SWNT mats and samples purified by acid
treatment were characterized by SEM and by Raman
spectroscopy using different excitation lasers. The
temperature dependence of resistance was also stud-
ied and found to vary strongly with acid treatment
time.
3. Results and discussion
Fig. 1a shows a SEM image of an as-grown mat.
It is obvious that there is a large amount of entangled
filaments with diameters from 10 to 30 nm. The
average length between entanglement points is larger
than 100 nm. It is hard to find the filament ends.
Some regions of the samples contain mainly long,
large diameter bundles but only few amorphous car-

B. Liu et al.rChemical Physics Letters 320 (2000) 365–372
367
Fig. 1. SEM images of ŽA. an as-grown mat and ŽB. a mat purified in nitric acid for 2 h.
bon particles, while other regions have a lower den-
sity of smaller bundles. In some places a large
amount of amorphous carbon can also be found.
These structural features might be caused by varia-
tions in the arcing process because of an inhomoge-
neous distribution of catalysts in the anodes. How-
ever, the product still forms as a continuous network
with the nanotubes joining different regions and
without clear boundaries.
for SWNT since MWNTs and graphite exhibit only a
y1
single peak at 1580 cm . The second group, in the
y1
low frequency range near 180 cm , consists of the
radial breathing modes ŽRBM. whose frequencies
depend on the tube diameters. This is also a finger-
print mode, since MWNTs do not show any low
frequency mode and the low frequency mode of
y1
graphite is at 42 cm . The relation between the tube
diameter and the lower Raman frequency can be
Raman spectroscopy was employed to identify
whether the nanometer bundles are single-walled
nanotubes because the Raman spectra of SWNTs
have fingerprint features, quite different from those
of graphite, multi-walled nanotubes or amorphous
carbon. Fig. 2 shows micro-Raman spectra for an
as-grown mat, measured under ambient conditions
with a laser excitation wavelength of 514.5 nm Ža.
and 632.5 nm Žb., respectively. Both Raman spectra
estimated from the formula w15x.
vs223.75rd ,
where d is the diameter of the tube in nanometers
and v the mode frequency of the RBM in cm
The smaller the SWNT diameter, the higher is the
Raman frequency. Insets Ž1. and Ž2. in Fig. 2a show
details of the peak distribution in the low frequency
range. Ž1. is a typical spectrum for most places on
y1
.
are typical for single-walled nanotubes, indicating
that the as-grown mat indeed consists mainly of
SWNTs w14x. The spectra are composed of two main
the sample. The SWNTs produced by arc vaporisa-
y1
tion with NirY usually show peaks near 183 cm
plus another one or two peaks at frequencies lower
y1
groups of lines. The first group consists of lines at
than 200 cm which are resonance excited at 514.5
y1
1
590, 1566 and 1551 cm , so-called tangential
nm, the laser wavelength used in Fig. 2a. Our result
y1
modes, which are related to the Eg graphite mode,
instead shows such a peak at 206 cm in addition
2
y1
split due to the curvature of the rolled-up graphite
sheet. The existence of such a group is a fingerprint
to a peak at 186 cm . From the relation between
diameter and frequency given above, the diameter is

3
68
B. Liu et al.rChemical Physics Letters 320 (2000) 365–372
Fig. 2. Raman spectra for an as-grown mat measured by using different laser excitation wavelengths of: Ža. 514.5 nm and Žb. 632.5 nm.
Insets Ž1.–Ž3. show details of the peak distribution in the low frequency range.
1
.09 and 1.2 nm for 206 and 186 cmy1, respectively.
w18x. We therefore also measured the Raman spec-
trum using a He–Ne laser at 632.5 nm Ž1.96 eV..
The result is shown in Fig. 2b. The high-frequency
tangential modes are insensitive to the laser wave-
length and show only a small shift, while the low-
frequency RBMs are more sensitive. For the RBM
we observed peaks at 165, 186, 192, 217, 250, 268
Recently the value of frequency as a function of
diameter and chirality has been calculated by K u¨ rti
et al. w16,17x. According to their first principle local
density approximation ŽLDA. calculation, the two
main peaks both correspond to chiral tubes, which
are always semiconducting w6x.
y1
We also found a distribution containing numerous
peaks in the low frequency range, as shown in inset
and 281 cm , as shown in inset Ž3. of Fig. 2b. The
observed wavelengths again verify that the diameters
Ž2. of Fig. 1a. The lines fall in the range from 165 to
of those nanotubes are indeed small, as many fre-
quencies are higher than 200 cm . The smallest
y1
y1
2
68 cm , corresponding to diameters of 1.36 to
0
.83 nm. The highest frequency indicates an unusu-
tube is 0.79 nm in diameter. These results suggest
that Ce plays an important role in the formation of
ally small diameter. Such small diameter tubes have
only recently been found in SWNT samples pro-
duced by laser vaporization using RhrPd catalysts
w7,8x, while the peaks for samples produced by arc
discharge using NirY catalysts are usually found at
lower frequencies in the range 148 to 200 cmy1, as
mesaured with an excitation wavelength of 514.5
nm.
The Raman spectra of SWNTs also depend on the
excitation laser wavelength because of resonance
effects. The most detailed data are found using lasers
within a narrow energy range, from 1.7 to 2.2 eV
smaller diameter SWNTs and chiral tubes.
y1
For the peak around 1340 cm
Žthe D-line.,
there are two contributions, mainly from disordered
carbon but with a weak contribution from SWNTs.
This peak is usually used to estimate the purity of
the sample. In the as-grown samples, the intensity of
this peak varies with position, suggesting an inhomo-
geneous distribution of impurities.
The properties of samples purified by nitric acid
vary with treatment time. We found that the samples
treated for 0.5 h were more pure than as-grown

B. Liu et al.rChemical Physics Letters 320 (2000) 365–372
369
samples and that Raman and SEM showed that the
acid treatment had given no obvious change in the
morphology of the nanotubes in the mat.
SWNTs w22x. Knowledge of the changes in morphol-
ogy will help us to understand the changes in the
electric transport behavior of the mats.
The samples treated for 2 h were even more pure
with hardly any metal-trapping amorphous carbon
impurities. However, their morphology, character-
ized by both SEM and Raman, had changed signifi-
cantly, as shown in Fig. 1b and Fig. 3. Apparently,
the tubes still form bundles with nanometers dimen-
sions, but the bundles are now strongly curved while
the original tubes were much straighter. It is known
that pristine tubes contain disorder, resulting from
defects, bends and twists, etc. w19x. During acid
treatment, the disordered parts are expected to react
with the acid, like amorphous carbon, resulting in
curved tubes. At the same time, both disordered
carbon and metals are removed. In addition to the
increased curvature of the bundles, the continuous
network observed previously seems to have broken
up. The more open part of the pristine network, with
only a few bundles joining the denser regions, might
either be more reactive, or simply react easier be-
cause the acid can more easily come into direct
contact with the tubes. Acids, even mild ones, are
known to open the tips of multi-walled nanotubes
w20x. Dujardin et al. w12x and Sloan et al. w21x tried to
In order to study the changes in the bundles in
detail, Raman spectra were also measured using a
He–Ne laser. In the low frequency RBM range, the
y1
main peak is now at 192 cm , with weak signals at
y1
181 and 217 cm . There is no peak above 217
y1
cm . We measured spectra at several different loca-
tions in several samples. The peak distributions were
the same, indicating that the acid treated samples are
more uniform than the pristine ones. Note that the
y1
relative intensity of the line at 217 cm
is much
lower than in the pristine sample. This result indi-
cates that the small diameter tubes have reacted with
the acid without severe damage to the large diameter
ones. According to the study by Dujardin et al. w12x,
the outer tubes in the bundles protect the inner ones.
This might indicate that in our samples, small diame-
ter tubes are either formed in small diameter bundles
and react easily with the acid, or that the smaller
diameter tubes have more defects.
In the high frequency range, tangential modes are
y1
found at 1591, 1561 and 1539 cm , showing dis-
tinct features. Those features are similar to features
observed in surface enhanced Raman spectra for
individual nanotubes w23x. This suggests that the
distribution of nanotubes is more uniform after acid
treatment, in agreement with our SEM images.
The temperature dependence of resistance for the
as-grown mat in the range from 2 to 300 K is shown
in Fig. 4. The resistance decreased with increasing
temperature. This ‘semiconductor’ behavior in the
observe whether SWNTs are open or not, but this
was difficult because of the low contrast of a SWNT
under TEM. It is likely that the tubes are open as
surface modifications are suggested in HNO -treated
3
resistance also agrees with the Raman results. Plot-
y1r3
ting ln R vs. T
results in a straight line, indicat-
ing that RŽT. obeys the formula w24x
1
rŽdq1.
R
Ž
T
.
sR exp
Ž
T0rT
.
1
Ž .
0
derived for variable range hopping in two dimen-
sions Ž2D VRH, dimension ds2.. The measured
slopes for several samples are almost the same,
giving a T0 of ;525 K. The observation of 2D
VRH is consistent with the results reported else-
where w25x. However, it differs from the 3D VRH
behaviour found in Refs. w11x and w26x, and also
differs from the results of Fisher et al. w27x who
Fig. 3. The Raman spectrum of a mat purified by nitric acid for 2
h, obtained with an excitation wavelength of 632.5 nm. Inset
shows details in the low frequency range.
found that the resistivity decreases with decreasing

3
70
B. Liu et al.rChemical Physics Letters 320 (2000) 365–372
Fig. 4. The temperature dependence of resistance for: Ža. an as-grown mat and Žb. a mat purified by acid for 0.5 h, in the temperature range
y1r3
from 2 K to RT. The plot of ln R vs. T
is fitted by 2D variable range hopping.
Fig. 5. The temperature dependence of resistance for a mat purified by acid for 0.5 h in the temperature range from 2 K to RT. The plot of
y1r4
ln R vs. T
is fitted by 3D VRH.

B. Liu et al.rChemical Physics Letters 320 (2000) 365–372
371
temperature from room temperature to 200 K for
bulk samples.
ences ŽTFR. and by the Chinese Natural Science
Foundation.
The resistance of a mat treated in acid for 0.5 h is
shown in Fig. 4b. It still follows a 2D VRH behav-
ior, but T is slightly decreased to 480 K compared
0
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