Is molybdenum necessary for the growth of single-wall carbon nanotubes from CO?

Article abstract of DOI:10.1016/j.cplett.2003.08.023

Catalytic mixtures of molybdenum and cobalt are considered essential for the growth of single-wall carbon nanotubes (SWCNT) from CO by the chemical vapor deposition method. The extensive growth of SWCNT with only cobalt as catalyst was presented. Domains

Full text of DOI:10.1016/j.cplett.2003.08.023

Chemical Physics Letters 379 (2003) 395–400
www.elsevier.com/locate/cplett
Is molybdenum necessary for the growth of single-wall
carbon nanotubes from CO?
a
Aidong Lan , Yan Zhang , Xueyan Zhang , Zafar Iqbal , Haim Grebel
a
b
c
a,
*
a
Electronic Imaging Center and the Electrical and Computer Engineering Department,
New Jersey Institute of Technology, Newark, NJ 07102, USA
b
Characterization Laboratory, NJIT, NJ 07102, USA
Electronic Imaging Center and the Chemistry Department, New Jersey Institute of Technology, Newark, NJ 07102, USA
c
Received 13 June 2003; in final form 13 August 2003
Published online: 16 September 2003
Abstract
Catalytic mixtures of molybdenum and cobalt have been considered essential for the growth of single-wall carbon
nanotubes (SWCNT) from carbon monoxide by the chemical vapor deposition method. Here we demonstrated ex-
tensive growth of SWCNT with only cobalt as catalyst. The typically subsequent stage of annealing the catalyst in the
presence of hydrogen was also eliminated. This process resulted in a near-100% yield of essentially defect-free chiral
tubes. Use of electrochemical deposition of the catalyst seed particles may open the door for the production of massive
electronic interconnect assemblies of these nanotubes.
Ó 2003 Elsevier B.V. All rights reserved.
Single-wall carbon nanotubes (SWCNT) have
stimulated extensive attention recently [1,2]. Their
growth and structure have been a source of nu-
merous studies [3]. One of the growth techniques,
which has now established itself as a mainstream
method involves the catalytic disproportionation
of CO on bimetallic Mo/Co catalysts in a con-
ventional CVD system [4,5]. It was thought that
such a method would require a rough surface to
support the bimetallic catalyst particles. This led
us to synthesize SWCNT within a coherent array
of silica sphere matrix [6]. The tubes formed as
small bundles or, individual strands that were well
separated and had an average individual tube di-
ameter of 0.9 nm. The resulting composite films
were subsequently used to demonstrate the unique
nonlinear optical properties of SWCNT within
the three-dimensionally confining environment of
opalline, photonic crystal structures [7]. Here we
report an extension of this growth method for
SWCNT to include untreated flat substrates while
using Co only as a catalyst, thus opening the door
for a more efficient production of SWCNT.
We used flat substrates of polished silicon
wafers and quartz. The nano-size Co particles
catalytic seeds were deposited by laser ablation,
sputtering and electrochemical techniques. Addi-
tional substrates were made of ordered arrays of
* Corresponding author. Fax: +19735966513.
E-mail address: grebel@njit.edu (H. Grebel).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2003.08.023

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A. Lan et al. / Chemical Physics Letters 379 (2003) 395–400
silica spheres with various sphere diameters. These
were prepared by hydrolysis of tetraethoxysilane
in a mixture of ammonium hydroxide, water and
ethanol [6]. The resulting silica spheres were then
dispersed in ethanol to form a suspension. The
suspension was mixed with methanol solutions of
cobalt nitrate and then dispersed on the substrates.
Cobalt was also successfully laser ablated on
pristine opalline structures.
Cobalt particles were also electrochemically
deposited on tungsten-coated substrates. In these
experiments the tungsten was used as a cathode.
The experiments were conducted in a three-elec-
trode cell with a reference Ag/AgCl electrode and
aqueous electrolyte of 0.25 M Na2SO4 and 1 mM
CoSO4. The applied potential was controlled be-
tween )900 and )1200 mV at potentiostatic tran-
sient. The sizes of the catalytic cobalt particles
determined from scanning electron microscope
images ranged between 20 and 40 nm.
1 kW/cm2. The spatial resolution of the imaging
system was either 1 or 2 lm, depending on the
objective lens. The acquisition time was typically
10 s averaged over 10 scans.
In Figs. 1a and b, we show SEM images of
SWCNT on Si and quartz when using laser-
ablated Co particles. It is evident that bundles with
diameters around 20–30 nm were formed between
the Co particles. Raman spectroscopy clearly in-
dicated that these bundles are SWCNT. As can be
seen from the picture, the growth is directed from
one catalytic point to another to form a bridge.
Although details of the growth are not yet fully
clear, such a process may be essential for a selec-
tive growth, which may lead to the fabrication of
massive electronic interconnects. Fig. 1c presents
AFM images of SWCNT and Co on similar
samples and reiterates the fact that the SWCNT
bundles bridged the gap between two catalytic
points.
The growth of the SWCNT was conducted in a
quartz tube reactor placed in a horizontal tubular
furnace. In the case of cobalt nitrate dispersed in
the opalline structure, we used the following pro-
cedure: following calcination for 30 min in air at
500 °C to decompose the catalyst nitrate to the
corresponding oxide, the samples were heated in
pure hydrogen at 500 °C to convert the oxides to
sub-oxides and metal particles. For the metallic Co
particles, which were deposited by physical or
electroplating techniques, we followed this recipe:
the temperature at the reaction zone was raised to
700 °C, and subsequently pure CO was introduced
at 1 atmosphere and a flow rate of 100 cm3/min.
and kept under these conditions for 30 min. After
completion of the reaction, the reactor was al-
lowed to cool down under flowing argon.
A combination of atomic force microscopy
(AFM), scanning electron microscopy (SEM) and
Raman spectroscopy was used to characterize the
resulting material. The Raman spectra were ac-
quired in a back-scattering geometry using a con-
focal arrangement. Micro-Raman spectrometers
equipped with Peltier stage cooled, charge-coupled
device (CCD) detectors were used. The 514.5 nm
(2.41 eV) line of an argon ion laser or, the 632.8 nm
(1.96 eV) line of a HeNe laser were used to excite
the spectra at a power density of approximately
Previous reports of carbon nanotube (CNT)
produced by CVD methods with Co catalyst re-
sulted in predominantly multi-wall tubes [8]. In
contrast, here we show an unexpected selectivity
growth of single-wall tubes. Variations in the laser
ablation parameters led to different distribution of
the catalytic seeds yet; the average diameter of the
SWCNT bundles shows little dependence on the
seed size (10–20 nm). In addition, systematic ex-
periments were conducted to elucidate the function
of H2 reduction on the Co particles. It turned out
that H2 reduction is not critical to functionalize Co
as a catalyst. This further simplifies the synthesis
process of SWCNT.
In Fig. 2, we show SEM images of directed
growth of SWCNT between islands of the tungsten
electrode. Here, the catalytic Co particles were
grown by electrode position. The distribution and
size of the cobalt particles could be controlled by
the deposition parameters, such as, the potential
and current density. As can be seen from Fig. 2,
domains of aggregated Co particles were formed
on the tungsten electrode. The average domain size
was about 2 lm, with 5 lm spacing between the
domains. As noted previously, the average Co
particle size was determined to be 30 nm. Both the
electrochemically induced particle size and mor-
phology of Co are likely to play important roles in

A. Lan et al. / Chemical Physics Letters 379 (2003) 395–400
397
Fig. 1. SEM images of SWCNTs when using laser-ablated Co particles as catalyst. (a) On a quartz substrate. (b) On a Si substrate. (c)
AFM picture of SWCNT on quartz substrate. Left: amplitude mode (bright spots correspond to height of 50 nm). Right: tapping
mode. Note the bridging effect in all pictures.
the directed growth of the SWCNT. Electro-
chemical deposition of the catalyst followed by
CVD growth would therefore be expected to make
massive electronic interconnects between elec-
trodes for device applications.
Raman spectroscopy was used to further char-
acterize the samples. Fig. 3 shows a typical Raman
spectrum taken from samples with laser-ablated
Co particles. Raman signals at 520 and 939 cmꢀ1
are due to phonons associated with the Si substrate
[9]. The inset shows the low frequency range of the
Raman spectrum of SWCNT with a rich structure
of bands. These bands correspond to the radial
breathing mode (RBM) of single tubes and have
been shown to be inversely proportional to the
diameter of a single SWCNT, dt. Since in this
frequency range, the Raman-active modes of
graphite and graphitic carbons are absent, one can
assess the presence of SWCNT in the samples and
determine its average diameter and type [10–12].

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A. Lan et al. / Chemical Physics Letters 379 (2003) 395–400
Fig. 3. Raman spectrum of SWCNTs produced by decompo-
sition of CO at 700 °C on a flat Si substrate with laser-ablated
catalytic Co particles. The spectrum obtained was excited at
k ¼ 632:8 nm (1.96 eV) using a He–Ne laser. The inset shows
the low frequency Raman lines. The peaks at 520 and at
939 cmꢀ1 correspond to vibration lines of the Si substrate.
can be resonantly excited with 1.96 eV (k ¼
0:6328 lm). Similarly, semiconducting tubes with
diameter around 0.7 and 1.2 nm, and metallic tubes
with diameter around 0.9 nm may be resonantly
excited with 2.54 eV (k ¼ 0:5145 lm). When excited
by 2.54 eV, the Raman spectra in the TM range
(1400–1700 cmꢀ1) display a regular symmetric
shape characteristic of semiconducting tubes rather
than the asymmetrically broadened Fano lineshape
of metallic tubes. A symmetric profile with a dom-
inant peak around x ¼ 1590 cmꢀ1 and two rela-
tively sharp structures around x ¼ 1560 cmꢀ1 and
x ¼ 1550 cmꢀ1 were observed (Fig. 3). In this
spectral region, metallic tubes with a diameter
around 0.9 nm would display an intense, broad and
asymmetric band around x ¼ 1540 cmꢀ1. The ab-
sence of these features indicates that metallic tubes
in the 0.9 nm diameter range are not present in our
sample and the observed spectrum may be attrib-
uted to semiconducting tubes either with 0.7 or
1.2 nm tube diameter. Similar characteristics were
observed with excitation at 1.96 eV and suggests
once again, that 1.1–1.2 nm diameter metallic tubes
Fig. 2. SEM images of SWCNTs at two magnifications. The
catalyst was electrochemically grown Co particles on tungsten
electrodes. Note the connection made by a single tube bundle
between two Co particles on islands of the tungsten electrode.
Raman scattering of SWCNT are governed by
singularities in the electronic density of states due to
its quasi-one-dimensional (1D) nature: the RBM
and tangential mode (TM) lines in the first-order
Raman spectrum are resonantly enhanced when the
laser excitation energy (Elaser) is close to an energy
separation between singular states, such as occurs
for Eii ¼ E11; E22; . . . Resonance in Raman can also
occur for the scattered photon, Elaser ¼ Eii þ Ephonon
for the anti-Stokes processes and Elaser ¼ Eii ꢀ
Ephonon for Stokes processes. The energy of these
allowed Raman-active transitions depends both on
the diameter and on the metallic or, semiconducting
character of the tubes. Following the results of [11],
semiconducting tubes with diameters around 0.8
and metallic tubes with diameters around 1.1 nm

A. Lan et al. / Chemical Physics Letters 379 (2003) 395–400
399
Table 1
Possible chirality assignments for both the metallic and semiconducting nanotubes in our samples and their xRBM (calculated and
observed) upon the excitation of 632.8 nm (1.96 eV) laser line
Chiral vector
index (n; m)
Diameter of tube
dt (nm)
Radial breathing mode frequency xRBM
(cmꢀ1
Optical transition mode
)
Calculateda
Experiment
Energy (eV)b
Denotationc
(18,0)
(9,9)
1.429
1.238
1.260
1.126
1.100
0.936
0.916
0.829
0.782
168.9
193.0
189.9
211.0
215.7
251.1
256.4
281.9
298.1
168
190
190
216
216
256
256
284
300
1.84
2.02
1.98
2.18
2.28
1.96
2.03
1.93
1.87
E1m1
E1m1
E1m1
E1m1
E1m1
E2s2
E2s2
E2s2
E2s2
(12,6)
(11,5)
(8,8)
(10,3)
(11,1)
(7,5)
(8,3)
a Using the expression xRBM ¼ 223:5=ðdtðnmÞÞ þ 12:5 and assuming a C–C bond distance of 0.144 nm.
b See [11,12].
c Eisi denote ith optical transition of a semiconducting tube while Eimi denotes ith optical transition energy of a metallic tube.
are not present in our samples: the Raman spectra
raises may be attributed to 0.8 nm diameter semi-
conducting tubes. Thus, based on these high-
frequency considerations our results lead to the
conclusion, that our samples contain either 0.7 or
1.2 nm diameter tubes. On the other hand, the
presence of RBM lines in the low-frequency range,
between 250 and 300 cmꢀ1, indicates the abundance
of semiconductor tubes with diameters of 0.9 nm
and smaller (see Table 1) in addition to small
amount of metallic tubes with diameters of 1.1 and
larger. Transmission electron microscopy (TEM) of
SWCNT within the voids of opalline structure
supports the assignment of average tube size of
0.9 nm and smaller. The overall conclusion is that
SWCNT in our samples are predominantly semi-
conductive and are of a size range of 0.7–0.9 nm.
Presence of a weak Raman line near x ¼ 1320
cmꢀ1 (with 1.96 eV excitation) in Fig. 3 is often
identified as the analogous of the D-band in
nanocrystalline graphite, which is assigned to dis-
ordered sp3 defects. The relatively weak intensity
of this line indicates that defects of this type on the
sidewalls of the SWCNTs in our sample are very
few.
at 168 cmꢀ1, which precludes third-resonantly en-
hanced transitions of the semiconductor tubes
when excited by the He–Ne laser line (1.96 eV) –
such transitions are expected in the Raman fre-
quency range between 130 and 160 cmꢀ1. In the
case of metallic tubes we limit our attention to
resonantly enhanced transitions between two ad-
jacent states. We limit our attention to transitions
between adjacent and next-to-adjacent states for
semiconductor tubes. The weak peaks presented at
x ¼ 384 cmꢀ1 and x ¼ 428 cmꢀ1 are considered as
second-harmonic phonons associated with those
of the lines at 190 and 216 cmꢀ1, respectively. The
assignment of low frequency peaks is given in
Table 1. We based this assignment on two criteria:
(i) large Raman intensity values are expected only
when the transition energy Eii, is close to the en-
ergy of the laser line, Elaser and (ii) considering the
trigonal warping effect [13], large Raman intensity
values are expected for armchair or, chiral na-
notubes with a large chiral angle. Judging from
Table 1 and Fig. 3, most SWCNT in our samples
can be assigned as chiral. Whether our synthesis
method can be further developed to control the
chirality of SWCNT produced is still an open
question and further study along this direction is
underway.
The low frequency Raman spectral region from
x ¼ 168 cmꢀ1 to x ¼ 300 cmꢀ1, corresponds to a
large distribution of tube diameter (0.78–1.43 nm)
and therefore, many different tube geometries need
to be considered. Our notch filter starts to cut-off
In summary, we have demonstrated that very
high quality SWCNT can be grown on flat sur-
faces by use of only Co as a catalyst and pure CO

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A. Lan et al. / Chemical Physics Letters 379 (2003) 395–400
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This research was funded in part by ARO
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