Materials Sciences and Applications, 2012, 3, 213-217 Published Online April 2012 ( 213
Synthesis of High-Density Vertically Aligned Carbon
Nanotubes Using Ultrasonic Nebulizer
Jianhui Zhang1,2, Wei Li1,3, Tetsuo Soga4, Takashi Jimbo4, Takayoshi Tanji1,2
1EcoTopia Science Institute, Nagoya University, Nagoya, Japan; 2Global Research Center for Environment and Energy Based on
Nanomaterials Science, Nagoya, Japan; 3Graduate School of Engineering, Nagoya University, Nagoya, Japan; 4Department of Fron-
tier Materials, Nagoya Institute of Technology, Nagoya, Japan.
Received October 17th, 2011; revised December 27th, 2011; accepted February 22nd, 2012
Vertically aligne d carbon nano tubes (VACNTs) arr ay with high density h ave been synthe sized from a mixture of f erro-
cene and ethanol using ultrasonic nebulizer techniques. Using scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) as well as Raman spectroscopy, investigations reveal that the vertically aligned CNTs are
multi-wall CNTs with well-ordered graphene sheets and are about 20 - 50 nm in diameter. We found that high density
vertically aligned CNTs can be synthesized at 750˚C - 850˚C and that the length and density of vertically aligned CNTs
increase with increasing growth temperatures. In addition, the single-walls CNTs have well-ordered graphene sheet and
are about 2 nm in diameter on silicon substrates at 650˚C.
Keywords: Carbon Nanotubes; Chemical Vapor Deposition; Transmission Electron Microscope
1. Introduction
Since the discovery of carbon nanotubes (CNTs) [1] at-
tempts have been extensively explored to take advantage
of their unique properties for specific applications due to
their unique morphological characteristics [2]. Potential
technical applications in the area of electronic devices [3]
and electron field emissions [4] have been proposed or
explored. To understand their intrinsic properties, it is
necessary to produce highly oriented CNTs [5]. Verti-
cally aligned CNTs can be produced using various che mi-
cal vapor deposition (CVD) methods, such as the hot
filament CVD [6], plasma enhanced CVD [7] and ther-
mal CVD [8]. Among these synthesis methods, the ther-
mal CVD has been widely used, as it provides a way for
large area synthesis of vertically aligned CNTs for elec-
tron emitters. In many cases, the spray pyrolysis CVD
method has been successfully applied in making CNTs
for many years [9]. It is also an important method for the
synthesis of vertically aligned CNTs and considerable
progress has been achieved [10]. By this method of CVD,
the high density oriented CNTs generally need pre-pat-
terned metal catalysts substrate, and a carbon source gas
is introduced into the quartz tube. The high density ori-
ented CNTs are grown on this catalyst-coated substrate
using a pyrolysis of the carbon source gas.
In this study, we demonstrate the synthesis of high
density oriented CNTs using the ultrasonic nebulizer
CVD technique on Si substrate under atmospheric pres-
sure. In this method, the pre-processing of substrate is
not necessary and the purity of the CNTs is high. The
produced vertically aligned CNTs are multi-walled and
exhibit crystallization.
2. Experiment
Figure 1 shows the schematic diagram of the CVD ap-
paratus. The ultrasonic nebulizer is connected to a quartz
tube and a N2 ga s cylinder. Si (100) substrates of size 10
mm × 10 mm were cleaned in acetone and methanol us-
ing an ultrasonicator then in de-ionized water and finally
dried using a nitrogen blower. The substrates were kept
in a quartz boat, which was then placed in the center of
the quartz reaction tube. One side of the quartz tube was
connected to an ultrasonic nebulizer. The ferrocene and
ethanol was used as a catalyst and as carbons source,
respectively. Before flowing the ferrocene/ethanol (0.1 g/
100 ml) mixture to the quartz tube, the electric furnace
should reach to the desired temperature. When the elec-
tric furnace was heated up to the desired temperature the
ultrasonic nebulizer was switched on. The ferrocene/
ethanol mixture was changed to a thin mist. The flow of
N2 was maintained to pass this mist inside the electric
furnace. After the deposition the furnace was switched off
and allowed to cool down to a temperature below 100˚C.
Through this method, the ultrasonic nebulizer (1.65 MHz,
Copyright © 2012 SciRes. MSA
Synthesis of High-Density Vertically Aligned Carbon Nanotubes Using Ultrasonic Nebulizer
Figure 1. Schematic image of the apparatus of the CVD.
Figure 2. SEM image of CNTs on silicon substrate synthe-
sized at 750˚C.
Atom Medical Corp·Model 303) produced a mist of ferro-
cene/ethanol. Finally, produced samples were analyzed
using a scanning electron microscope (SEM, Hitachi S-
3000H), a transmission electron microscope (TEM, JEOL
JEM-3010 EXII) and a Raman spectroscope (JASCO,
NRS-1500W). The excitation wavelength for the Raman
measurement s was 532 nm.
3. Results and Discussions
The ultrasonic nebulizer experiment was carried out in a
temperature range between 650˚C to 950˚C. The dense
and vertically aligned CNTs were observed by SEM on
the substrate as shown in Figure 2. The diameter and
length of the CNTs were about 50 nm and a ten several
microns, respectively.
Short and thin CNTs film was observed in the sample
synthesized at 650˚C as show in Figure 3(a). Straight
and vertically aligned CNTs mat were observed in the
sample synthesized at 750˚C - 850˚C as shown in Fig-
ures 3(b) and (c). We can be observed the density of
vertically aligned synthesized at 750˚C - 850˚C, and we
hope that this kind of straight CNTs should be promising
for various field emission electron source applications.
However, we focused on the morphology of the CNTs
synthesized at 950˚C. We only observed kinked CNTs as
products in this deposition as shown in Figure 3(d), the
10 µm
Figure 3. SEM images of CNTs on silicon substrate synthe-
sized at: (a) 650˚C; (b) 750˚C; (c) 850˚C and (d) 950˚C.
Copyright © 2012 SciRes. MSA
Synthesis of High-Density Vertically Aligned Carbon Nanotubes Using Ultrasonic Nebulizer 215
vertically aligned CNTs obviously could not be synthe-
sized at this temperature.
A TEM observation was performed on the synthesized
CNTs at 650˚C - 950˚C. Single-walled CNTs (SWNTs)
can be observed in this sample as shown in Figure 4(a).
The diameter of SWNTs were 2 - 4 nm. As shown in
Figures 4(b) and (c), it can be clearly observed that the
CNTs have straight morphologies, smooth walls, close
end and clean surfaces. The CNTs have a multi-walled
structure with a hollow inside the CNTs. The outer di-
ameters of the CNTs are within a range of 10 - 30 nm.
The inner diameter of the CNTs was typically about 2 - 5
nm. And the kinked CNTs and aggregation of carbon
particles were observed at 950˚C as shown in Figure
Figure 5 shows an EDS spectrum with four strong
peaks, which represent carbon, copper, silicon, and Fe.
This indicates that the growth of VACNTs originated
from Fe or Fe carbide particles on the silicon substrate.
The Cu peak results from a Cu grid.
Figure 6 demonstrates the thermo-gravimetric analysis
(TGA) result of the products, weight starts to reduce
from near 465˚C, the CNTs completely evaporate above
725˚C, while the weight loss of purified CNTs by burn
off starts from 550˚C and completely burns out near
750˚C. The 5 wt% of residual catalyst was measured in
this sample. Thus, it is calculated that the content of the
CNTs in the product is about 95%.
Figure 7(a) shows Raman spectra of the sample syn-
thesized at 650˚C - 950˚C. In the high-frequency region,
the Raman spectra have two characteristic peaks i.e. G
(a) (b)
(c) (d)
Figure 4. TEM images of CNTs on silicon substrate synthe-
sized at: (a) 650˚C; (b) 750˚C; (c) 850˚C and (d) 950˚C.
Figure 5. EDS curve of the VACNTs.
Figure 6. TGA curve of the VACNTs sample synthesized at
750˚C substrate.
and D peaks. The D-peak observed around 1350 cm–1
relates to the presence of defects, while the G-peak
around 1593 cm–1 is associated with the in-plane vibra-
tion of the graphene sheet. Ratios of the G-peak to the
D-peak have been used as an indicator of the amount of
disorder of the CNTs and graphite crystals [11]. The
presence of high intensity of the G-peak provides the
evidence of high graphitic order in CNTs. Both G peak
and D peak are used to determine th e structural quality of
the CNTs. Higher the value of the IG/ID corresponds to
the higher graphitic structure of the CNTs obtained at
650˚C, 750˚C and 850˚C. However, the high intensity of
the D-peak was presented and is higher than G-peak at
Copyright © 2012 SciRes. MSA
Synthesis of High-Density Vertically Aligned Carbon Nanotubes Using Ultrasonic Nebulizer
Figure 7. (a) Raman spectra of the samples synthesized at
650˚C, 750˚C, 850˚C and 950˚C in high frequenc y region; (b)
Raman spectra of the samples synthesized at 650˚C, 750˚C,
850˚C and 950˚C in low frequency r e gion.
950˚C, it explained that the plenty of amorphous carbon
and defects existed at this temperature [12]. Figure 7(b)
shows an expanded view of the low frequency region
[13], in which radial breathing modes (RBMs) (180 - 400
cm–1) of SWNTs are observed for the samples synthe-
sized at 650˚C - 850˚C, which suggested a threshold
temperature somewhere between 650˚C and 850˚C for
SWNTs synthesis under the chosen conditions.
The growth mechanism of VACNTs is still needs fur-
ther study in the CVD method. The possible VACNTs
mechanism in our study cannot be attributed to the
growth of template bu t maybe to the steric of “crowding ”
[13]. Both Fe and ethanol source compounds are broken
Fe particles
Fe/ethanol mist
Si substrate
Si substrate
Figure 8. Schematic diagrams of growth mechanism of
VACNT. (a) Formation of nano-size catalysts from Fe/
ethanol mist; (b) The VACNTs were synthesized on the Si
down to form free Fe and carbon atoms. The Fe atoms
can aggregate into small, nano-sized particles and sputter
down to the Si substrate. The sputter deposition on the Si
substrate will form some growth basis of VACNTs on
the Si substrate. The diffusion of ethanol into these clus-
ters begins to occur soon after the formation of the parti-
cles themselves. The carbon reaches to saturation point
quickly. As the carbon source is supp lied continually, th e
precipitation of graphite starts. The growth of the CNTs
will be directed perpendicular to the Si substrate surface
to form the densely packed, well-aligned mats observed.
This process is illustrated schematically in Figures 8(a)
and (b). We considered that interactions and “crowding”
between CNTs is cause of the good VACNTs.
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