Synthesis of High-Density Vertically Aligned Carbon Nanotubes Using Ultrasonic Nebulizer

doi:10.4236/msa.2012.34031

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Materials Sciences and Applications, 2012, 3, 213-217

http://dx.doi.org/10.4236/msa.2012.34031 Published Online April 2012 (http://www.SciRP.org/journal/msa) 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.

Email: [email protected]

Received October 17th, 2011; revised December 27th, 2011; accepted February 22nd, 2012

ABSTRACT

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,

Synthesis of High-Density Vertically Aligned Carbon Nanotubes Using Ultrasonic Nebulizer

214

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

(a)

(b)

(c)

10 µm

(d)

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.

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

4(d).

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)

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

Synthesis of High-Density Vertically Aligned Carbon Nanotubes Using Ultrasonic Nebulizer

216

(a)

(b)

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

(a)

Si substrate

VACNTs

(b)

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

substrate.

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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