Dispersion of metal nanoparticles for aligned carbon nanotube arrays

Article abstract of DOI:10.1063/1.126883

We report that Co metal nanoparticles (an average diameter of 4 nm) chemically synthesized by a reverse micelle method catalyzes the growth of multiwall carbon nanotubes (MWNTs) aligned perpendicular to a substrate. The surface of the nanoparticles is covered with surfactants so that the nanoparticles can be dispersed in organic solvent. The dispersion of the nanoparticles was cast directly onto a plane Si substrate for thermal pyrolysis of acetylene. We have found that the pretreatment of the metal nanoparticles with hydrogen sulfide before the pyrolysis straightens the MWNTs, suggesting sulfurization of the nanoparticle catalyst plays an important role in regular growth of the MWNTs. The dispersion of the nanoparticles offers a conventional and processible approach to synthesize large area aligned MWNT arrays.

Full text of DOI:10.1063/1.126883

Dispersion of metal nanoparticles for aligned carbon nanotube arrays
Hiroki Ago, Toshiki Komatsu, Satoshi Ohshima, Yasunori Kuriki, and Motoo Yumura
Citation: Applied Physics Letters 77, 79 (2000); doi: 10.1063/1.126883
View online: http://dx.doi.org/10.1063/1.126883
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148.251.80.252 On: Mon, 28 Apr 2014 23:47:04

APPLIED PHYSICS LETTERS
VOLUME 77, NUMBER 1
3 JULY 2000
Dispersion of metal nanoparticles for aligned carbon nanotube arrays
Hiroki Ago,a) Toshiki Komatsu,b) Satoshi Ohshima, Yasunori Kuriki, and Motoo Yumura
National Institute of Materials and Chemical Research (NIMC), Tsukuba 305-8565, Japan
͑
Received 7 February 2000; accepted for publication 12 May 2000͒
We report that Co metal nanoparticles ͑an average diameter of 4 nm͒ chemically synthesized by a
reverse micelle method catalyzes the growth of multiwall carbon nanotubes ͑MWNTs͒ aligned
perpendicular to a substrate. The surface of the nanoparticles is covered with surfactants so that the
nanoparticles can be dispersed in organic solvent. The dispersion of the nanoparticles was cast
directly onto a plane Si substrate for thermal pyrolysis of acetylene. We have found that the
pretreatment of the metal nanoparticles with hydrogen sulfide before the pyrolysis straightens the
MWNTs, suggesting sulfurization of the nanoparticle catalyst plays an important role in regular
growth of the MWNTs. The dispersion of the nanoparticles offers a conventional and processible
approach to synthesize large area aligned MWNT arrays. © 2000 American Institute of Physics.
͓
S0003-6951͑00͒05327-4͔
Carbon nanotubes either in their single wall or in multi-
wall form have been extensively studied for macroscopic ap-
plications in many areas, such as electronics, mechanics, and
CVD synthesis of carbon nanotubes. We show that straight
and aligned multiwall carbon nanotubes ͑MWNTs͒ were
formed from the nanoparticles pretreated with hydrogen sul-
1
energy storage. As a method to synthesize carbon nano-
fide (H S) on a plane Si substrate. This array of the MWNTs
2
tubes, chemical vapor deposition ͑CVD͒ in the presence of a
transition metal catalyst has attracted a great deal of interest,
because the CVD proceeds at relatively lower temperature
has potential applications, especially to field emission dis-
plays ͑FEDs͒, because the density of the present MNWT
array is relatively low and the array can be developed to a
large area.
͑
below 1000°C͒ with less carbon impurities compared with
2
–7
arc discharge or laser ablation methods.
Moreover, the
In a reverse micelle method, metal chloride is reduced in
a nanoscale water pool surrounded by surfactants to form the
metal nanoparticles stabilized by the surfactants. We em-
ployed didodecyldimethylammonium bromide ͑DDAB͒ and
sodium borohydride (NaBH4) as the cationic surfactant and
catalytic CVD method offers a new route to control over the
alignment and/or direction of carbon nanotubes. Recently, it
has been shown that the CVD method enables directional
growth of carbon nanotubes, which could be developed into
nanoscopic integrated circuits using nanotubes.8
10
reducing agent, respectively. First, DDAB was dissolved in
In the catalytic CVD method, metal nanoparticles are
essential for the growth of carbon nanotubes, because they
work as a catalyst of the nanotube growth; the size and
chemical composition of the metal nanoparticle determines
diameter and structural perfection ͑degree of graphitization͒
toluene with a 10 wt % concentration, followed by dissolving
cobalt chloride (CoCl2 6H2O) to a concentration of 0.005 M.
Under vigorous stirring, 5 M NaBH4 aqueous solution was
added to the toluene solution with a concentration of
Ϫ
2ϩ
͓BH ͔/͓Co ͔ϭ3:1. The solution turned from light blue to
4
2
of the nanotube. It is also noted that metal nanoparticles are
black, suggesting the formation of a colloidal dispersion of
Co nanoparticles. All the above procedures were performed
in a glove box filled with nitrogen gas to avoid oxidation.
The Co nanoparticles were purified by repeating centrifuge
and redispersion in toluene and acetone. The final dispersion
of the Co nanoparticles in acetone was cast onto a plane
crystalline Si substrate and dried at room temperature.
In order to activate and sulfurize the Co nanoparticles,
their cast film was heated in H2S and H2 gas diluted with
nitrogen at 400 °C for 2 h prior to the pyrolysis. For com-
parison, we also examined H2 gas without H2S for the acti-
vation under the same condition. The pyrolytic reaction was
carried out at 800–900 °C in acetylene (C2H2) gas diluted
with nitrogen for 1 h.
Figure 1 shows Co nanoparticles synthesized by the re-
verse micelle method. It is seen that Co nanoparticles are
well separated with a certain distance due to the presence of
the surfactants that cover the surface of the nanoparticles. In
the present condition, an average diameter of the nanopar-
ticles was estimated to be 4 nm from the TEM image. We
note that these nanoparticles can be dispersed in organic sol-
indispensable for the nanotube growth not only in the CVD
method but also in arc discharge or laser ablation methods.1
Metal nanoparticles employed in the CVD synthesis of car-
bon nanotubes have been prepared mainly by the following
two methods. One method is to use nanoporous or mesopo-
,2
5
6
rous substrates, such as zeolites, porous Si, and anodized
9
Al O , in which metal nanoparticles are confined through
2
3
evaporation or impregnation. Another method is to etch a
3
metal substrate or a metal-coated substrate by laser ablation
4
or plasma treatment. The physical etching generates metal-
lic nanoparticles, which play a role as a catalyst for nanotube
growth. For future development, a more simple synthetic
method applicable to plane and large area substrates is very
important.
In this letter, we have synthesized Co metal nanopar-
ticles by a reverse micelle method and applied them for the
a͒
Electronic mail: ago@nimc.go.jp
Also at Japan Fine Ceramics Center ͑JFCC͒, 2-4-3 Nishi-Shinbashi,
b͒
Tokyo 105-0003, Japan.
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003-6951/2000/77(1)/79/3/$17.00 79 © 2000 American Institute of Physics
48.251.80.252 On: Mon, 28 Apr 2014 23:47:04
0
1

80
Appl. Phys. Lett., Vol. 77, No. 1, 3 July 2000
Ago et al.
FIG. 1. TEM micrograph of Co nanoparticles prepared by a reverse micelle
method. Dispersion of the nanoparticles in toluene was cast onto a polymer-
coated TEM grid for the TEM measurement.
vent and their size is tunable by the water concentration of
1
1
NaBH aqueous solution, which promises processibility
4
and wide variation of the nanoparticles made by the reverse
micelle method.
FIG. 3. High-resolution TEM image of
a
MWNT grown from
H S-pretreated Co metal nanoparticles.
2
The concentrated dispersion of the Co nanoparticles was
cast onto a plane Si substrate and subjected to pyrolysis of
C H gas. The pyrolysis in the presence of the H -pretreated
served of the MWNTs suggests that the as-prepared Co
nanoparticles aggregate to form bigger clusters ͑we use the
term ‘‘clusters’’ for aggregates of the original nanoparticles͒.
Although the MWNTs are not perfectly straight, one can see
that they have a tendency to grow perpendicular to the sub-
strate.
2
2
2
Co nanoparticles resulted in a MWNT array as shown in Fig.
2͑a͒. The MWNTs showed the outer and inner diameters of
20–40 and 8–20 nm, respectively. Because the diameter of
the catalyst tends to correspond to the inner diameter of the
5
MWNTs for the CVD method, the diameter which was ob-
We have found that pretreatment of the Co nanoparticles
with H S gas straightens the MWNTs ͓Fig. 2͑b͔͒. Moreover,
2
the MWNTs grew almost perpendicular to the substrate. We
have also found that the present MWNTs have a wider range
of outer diameters ͑30–80 nm͒ than those of MWNTs gen-
erated from nonsulfurized Co nanoparticles ͑20–40 nm͒. The
corresponding high-resolution transmission electron micros-
copy ͑HR-TEM͒ image of the MWNT shown in Fig. 3 indi-
cates that some MWNTs have a well-graphitized structure.
To study the growth mechanism, bases of the aligned
MWNTs synthesized from H S-treated Co nanoparticles
2
have been investigated ͑Fig. 4͒. We note that the present
nanoparticles are suitable for the study of the growth mecha-
nism because the nanoparticles are not confined in nanopo-
rous substrates. Figure 4͑a͒ indicates a SEM image of the
substrate in the initial stage of the MWNT growth. It is sup-
posed that Co nanoparticles aggregated and formed Co clus-
ters with a droplet-like shape during the heating and the fol-
lowing pyrolysis processes. Interestingly, we have found that
the MWNTs grow from each tip of the droplet-like clusters
as shown in Fig. 4͑b͒. From TEM studies, a metal cluster
was also found at a tip of the MWNT after the growth.
Based on the above observations, we propose a possible
growth mechanism as follows. In the beginning, Co metal
nanoparticles aggregate and react with C H gas, leading to a
2
2
droplet-like cluster ͓Fig. 4͑a͔͒. The cluster should be metal or
metal carbide. Co atoms located at the tip of the cluster is
supposed to be the most reactive in the cluster, because the
atoms at the tip have the highest surface area and, they are
FIG. 2. SEM micrographs of MWNTs grown from Co nanoparticles acti-
vated in H gas prior to the pyrolysis ͑a͒ and MWNTs grown from
2
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H S-pretreated Co ͑b͒.
significantly exposed to C H . Hence, we speculate from
2
2 2
1
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Appl. Phys. Lett., Vol. 77, No. 1, 3 July 2000
Ago et al.
81
of sulfur atoms as well as the growth mechanism of the
present MWNT arrays is interesting and a more detailed dis-
cussion will be reported elsewhere.
The present MWNT array is different from previous
catalytically grown MWNT arrays in terms of the density of
7
Ϫ2
MWNTs. The present sample is less dense (10 cm ) com-
pared with previous aligned MWNT samples.3 Davydov
et al. pointed out that density of emission sites is essential
for FED property and that the less dense array is better for
FED applications.15 This is because an applied electric field
tends to concentrate at each tip of the MWNTs for the low-
density nanotube array. Therefore, we think the present
MWNT array is promising for a FED application. We would
like to note that the arrays can also be used for a high surface
area metal electrode for macroscopic devices.16
,5,7
In summary, we have shown that a cast film of disper-
sion of Co nanoparticles catalyzes growth of the MWNT.
We have found that the addition of sulfur into the nanopar-
ticles stimulates the straight growth of the MWNTs and in-
creases the diameter of the MWNTs. Because the present
method does not require porous substrates, it offers a simple
and processible approach to a large area aligned MWNTs
array. The nanoparticles here are available as dispersion in
solvent so that they could be applied to an ink-jet method for
micropatterning of nanotube arrays.
FIG. 4. SEM micrographs of bases of MWNTs: ͑a͒ metal clusters with a
droplet-like shape which can be regarded as seeds of the MWNTs, and ͑b͒ a
boundary between an area with fully grown MWNTs and an area with
droplet-like clusters.
This work is supported by the Frontier Carbon Technol-
ogy ͑FCT͒ Project of the Ministry of International Trade and
Industry ͑MITI͒, Japan. The authors would like to thank K.
Uchida and F. Ikazaki of NIMC for their valuable discus-
sions and S. Terauchi for help with the experiments.
SEM and TEM images that further pyrolytic reaction occurs
at the tip of the cluster being similar to a standard tip-growth
model. Finally, a MWNT is formed having a metal cluster
1
3
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frequently found together with hollow MWNTs, suggesting
2
1
2
3
the metal cluster at the MWNT tip can have a rod shape.
The effect of H S treatment is also an important issue,
2
because it may be closely related to the growth mechanism.
͑
1997͒.
4
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7
8
9
0
1
2
2
4
00 °C ͑not shown here͒. The metal film treated with H2S
11
gas showed prominent change in the film morphology, re-
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1
1
1
1
2
3
4
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2
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discharge method.14 The arc-discharge method using a
sulfur-doped graphite rod leads to the increase in the diam-
eter of the SWNTs from 1.3–1.5 to 2.7–3.6 nm.1 The role
16
4
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