Synthesis of Y-junction carbon nanotubes within porous anodic aluminum oxide template

Article abstract of DOI:10.1016/j.ssc.2004.12.009

Y-junction carbon nanotubes with the average diameter about 200 nm were successfully synthesized within porous anodic aluminum oxide template, which was prepared by anodic anodizing aluminum sheet in 1.0 mol/l H₃PO ₄ solution at a constant anodization voltage 90 V.

Full text of DOI:10.1016/j.ssc.2004.12.009

Solid State Communications 133 (2005) 527–529
www.elsevier.com/locate/ssc
Synthesis of Y-junction carbon nanotubes within porous anodic
aluminum oxide template
*
Ling-Bin Kong
State Key Laboratory of Advanced Non-ferrous Materials, Lanzhou University of Technology, Lanzhou 730050, P.R. China
Received 12 September 2004; received in revised form 18 September 2004; accepted 9 December 2004 by H. Akai
Available online 5 January 2005
Abstract
Y-junction carbon nanotubes with the average diameter about 200 nm were successfully synthesized within porous anodic
aluminum oxide template, which was prepared by anodic anodizing aluminum sheet in 1.0 mol/l H3PO4 solution at a constant
anodization voltage 90 V.
q 2004 Elsevier Ltd. All rights reserved.
PACS: 81.40.Cz; 81.15.Pq; 82.45.Cc; 82.45.Qr
Keywords: A. Y-junction carbon nanotubes; A. Anodic aluminum oxide
1. Introduction
Templates of porous AAO are relatively easy to produce and
there is no practical limitation to their size. CNTs confined in
AAO have been obtained by the pyrolysis of some polymeric
materials such as polyacrylonitrile or propylene and by the
catalytic pyrolysis of acetylene within the pores [16,17].
Recent work shows that Y-junction CNTs can be grown
within Y-shaped AAO nanochannels when the voltage
conditions are adjusted in a specific way along a double step
template growth process [18]. Compared with the complex
branching phenomena observed in CNTs produced under
specific arc-discharge condition [19], the Y-shaped CNTs
produced by AAO template are well-aligned and similar in
shape and size. Sui et al. [20] have also prepared CNTs
junctions with the average diameter less than 100 nm by
AAO template. However, Y-junction CNTs with relatively
large diameters have not been investigated. In this paper, we
successfully synthesized Y-junction CNTs with the average
diameter about 200 nm, which may be used as a novel base
material for the development of nanoelectronic devices.
There has been increasing interest in carbon nanotubes
(CNTs) due to the many potential applications such as
electron emitters [1], probes for electrochemistry devices
and scanning probe microscopy [2,3], single electron
transistors [4], quantum wires [5], hydrogen storage media
[6], chemical sensors [7], nanotweezers [8], and so on. For
these purposes, both the development of new manipulation
techniques and alternative synthetic routes must be found.
There has been increasingly interest on the synthesis of Y-
junction CNTs, which might provide nanoelectronics
community with a new material for electronic devices [9–
12]. Recently, it is still a researching focus to find a simple
and cheap method for preparation of Y-junction CNTs.
In relation with the synthetic routes, special attention is
being paid to the production of CNTs confined in a template.
The growth in anodic aluminum oxide (AAO) template is one
of the more promising approaches owing to the possibility of
tailoring the size, density and shape of their pores [13–15].
2. Experiment
* Tel.: C86 931 297 6363; fax: C86 931 280 6962.
E-mail address: konglb@lut.cn.
High-purity aluminum sheets (99.99%, 35!10!
0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2004.12.009

528
L.-B. Kong / Solid State Communications 133 (2005) 527–529
0.15 mm3) were employed in this experiment. Prior to
anodization, the metal surfaces were degreased, etched in
ethanol by sonicating. After rinsed in distilled water, the
aluminum sheets were electropolished in C2H5OH and
HClO4 solution [V(C2H5OH):V(HClO4)Z4:1] in order to
achieve a smooth surface. It was necessary to immerse the
samples in concentrated acid or alkaline solution for several
minutes to remove the oxide layer formed during the
electropolishing process. All samples were rinsed in
distilled water and then transferred to a nitrogen environ-
ment. The resultant clean aluminum samples were anodic
anodized in 1.0 mol/l H3PO4 solution at 20 8C, with the
aluminum sheets as the counter electrodes.
800 8C. Subsequently, the heating temperature was adjusted
to 700 8C, and a flow of acetylene was initiated at
15 ml/min, then the CNTs were synthesized in the pores
of the AAO template by deposition of carbon atoms from
decomposition of acetylene. After deposition, the acetylene
and hydrogen flows were terminated, then the heating was
turned off and the reactor was cooled to room temperature.
The aluminum oxide template was dissolved by immer-
sion in a 6 mol/l NaOH solution for several hours at room
temperature to release the CNTs. The washed and dried
nanotubes were dispersed in toluene by sonication. A drop
of toluene suspension of the nanotubes was placed on a
transmission electron microscope (TEM, Philips CM120)
grid for structure observation.
In order to obtain several AAO films with different pore
structures, different anodization conditions were used. For
the first set of samples, a single step anodization process was
used. An initial anodization voltage of 50 V of DC was
applied for 1 h and then it was abruptly jumped up to 90 V
and maintained for 1 h. For comparative purpose, the
constant voltage of 50 and 90 V were applied to obtain
template with straight and branched channels, respectively.
The pore bottom of as-obtained AAO film was opened by
chemical etching in 0.3 mol/l H3PO4 aqueous solution at
60 8C for 10 min, and the catalyzer-iron nanoparticles were
deposited on the inner wall of the AAO template pores
before the CVD. This process was accomplished by
immersion of the alumina membrane into a solution of the
0.1 mol/l Fe(NO3)3$9H2O for 10–15 min. After immersion,
the template was dried and placed in a quartz boat and then
inserted into the center of a 40 cm long quartz tube reactor
winded with heating tungsten filament. Atmosphere in the
reactor was purged with a mechanical vacuum pump. And
the Fe ions were reduced to iron by a 1.5 h exposure to
flowing H2 gas (20 ml/min) under vacuum (w150 Pa) at
3. Results and discussion
Fig. 1 shows the TEM image of CNTs grown with the
single step electrochemical oxidization method described
above. When the AAO template anodized at 50 V, the CNTs
with a uniform diameter of 100 mm is observed. However,
when the anodizating voltage increase to 90 V, the diameter
of the CNTs increased to 200 nm, and Y-junction nanotubes
were observed.
It cannot be expected that Y-junction CNTs, such as
those shown in Fig. 1(B), could be formed in a straight hole.
So it is reasonable to conclude that the porous AAO
template has a complex channel structure and the CNTs
perfectly copy this three-dimensional structure of the AAO
template. In the previously described method, a single step
anodization process was used, starting at 50 V and then
changing abruptly to 90 V, with the branched pores being
formed when the higher voltage conditions were used [20].
Fig. 1. TEM images of CNTs released from AAO template which was anodized at (A) 50 V and (B) 90 V.

L.-B. Kong / Solid State Communications 133 (2005) 527–529
In order to know if this abrupt change of the operating Acknowledgements
529
voltage plays some role in the formation of the branched
pore templates, a new anodization test was performed using
a constant working voltage of 90 V. No differences were
observed either in the CNTs structure grown into AAO
templates produced at a constant 90 V operation when
single or double step approaches were used. On the contrary,
when the AAO templates were produced at a constant
operating voltage of 50 V, the CNTs grown in such
templates were straight, in accordance with other reports
[14]. From these results, it is possible to conclude that the
formation of the branches is not caused by the abrupt
increase of the voltage. It is only a characteristic of the high
anodization voltage.
The author is grateful for the financial support by the
Doctorial Foundation, Lanzhou University of Technology,
P.R. China.
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