Synthesis of alumina nanotubes using carbon nanotubes as templates

Article abstract of DOI:10.1016/S0009-2614(02)00874-6

Alumina nanotubes were fabricated using carbon nanotubes (CNT) as templates at 1473 K. The Al₂O₃ nanotubes were polycrystals. Al powders added with CNT were used as solid reactants, where Al₂O₃/Al mixture (1:1 wt %, 74 μm avg dia) was used instead of Al powder. Maximum production of Al₂O₃ nanotubes was obtained with a NH₃ flow rate of ～ 100 sccm. When Ar, instead of NH₃ was used as the protective atmosphere, Al₂O₃ nanowires were fabricated. If some O₂ was added into the NH₃ atmosphere, more Al₂O₃ could be produced, thus the yields of AlN nanowires, Al₂O₃ nanotubes, and CNT coated with Al₂O₃ particles all increased. Decreasing the NH₃ flow rate had a similar effect with adding O₂, because it increased the percentage of O₂ in the chamber. But less NH₃ lowered the production of AlN nanowires and thus higher percentage of Al₂O₃ nanotubes could be obtained.

Full text of DOI:10.1016/S0009-2614(02)00874-6

17 July 2002
Chemical Physics Letters 360 (2002) 579–584
www.elsevier.com/locate/cplett
Synthesis of alumina nanotubes using carbon nanotubes
as templates
*
Yingjiu Zhang, Jun Liu, Rongrui He, Qi Zhang, X. Zhang, Jing Zhu
Electron Microscopy Laboratory, Department of Materials Science and Engineering, School of Materials Science and Engineering,
Tsinghua University, Beijing 100084, People’s Republic of China
Received 20 September 2001; in final form 22 May 2002
Abstract
Alumina nanotubes have been fabricated using carbon nanotubes (CNTs) as templates at 1473 K. The Al2O3 na-
notubes are polycrystals. They are less than 100 nm in outer diameter and tens of nanometer in inner diameter, which is
close to the outer diameters of the templates. Under certain conditions, AlN and Al2O3 nanowires can also be fabri-
cated in this reaction system. Discussions on the growth mechanisms of these nanotubes and nanowires are pre-
sented. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
reaction system. For example, BN nanotubes [12]
and/or ðBNÞxCy/BN nanotubes filled with boron
carbide nanowires [15] could be fabricated in
similar reaction systems.
The discovery of carbon nanotubes (CNTs) [1]
has opened exciting possibilities for fabricating
one-dimensional (1D) nanoscale materials, such as
fabrication of nanocomposites by filling or coating
of CNTs [2–10]. At the same time, CNT-confined
reaction and CNT-substitution reaction have
proved to be useful. Various nanowires, such as
SiC [11], GaN [12], AlN [13], and nanotubes, such
as BN [14], ðBNÞxCy/BN nanotubes filled with
boron carbide nanowires [15] have been fabricated
in this way. Morphologies of the products from
these reactions can be somewhat controlled
[11,16,17] and different types of products could be
obtained with slightly changed parameters in one
Al2O3 is an important structural ceramic for its
low density, high strength and toughness [18]. And
Al2O3 nanotubes could even be of use as catalyst
[8] or templates [19]. Great efforts have been put
on the synthesis of Al2O3 nanotubes, by (1) coat-
ing of CNTs with aluminum isopropoxide [8], (2)
electrochemical anodizing Al (or Al/Si) film [21],
(3) Heating partially hydrolyzed AlCl3 powder at
443–493 K in air or flowing inert gas atmosphere
[20].
In this Letter, Al2O3 nanotubes have been fab-
ricated using carbon nanotubes as templates. And
AlN, Al2O3 nanowires could also be obtained in
this reaction system with slightly changed param-
eters. The growth mechanism is studied, and the
results might be of some help to the fabrication of
* Corresponding author. Fax: +86-10-6277-2507.
E-mail address: jzhu@mail.tsinghua.edu.cn (J. Zhu).
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2614(02)00874-6

580
Y. Zhang et al. / Chemical Physics Letters 360 (2002) 579–584
one-dimensional nano-scale materials through
CNTs-confined or CNTs-substituted reactions.
under different parameters as shown in Table 1.
Among them, experiments 1, 3, 5 were typical
conditions producing AlN nanowires, Al2O3 na-
notubes and Al2O3 nanowires, respectively, al-
though some AlN nanowires also exist in the
product of experiment 3. Experiments 2 and 4 were
designed to reveal the effect of oxide or oxygen in
the reaction. Experiments 6, 7, 8 were carried out
to testify the necessity of CNTs to the growth of
nanowires or nanotubes.
The products were characterized with scanning
electron microscope (SEM, JSM-6301F, JEOL)
and transmission electron microscope (TEM,
JEM-2010F, JEOL), energy dispersive spectrum
(EDS), and selected area electron diffraction
(SAED).
2. Experimental
The CNTs had been prepared previously
through a modified floating catalyst process [22].
They were all multi-wall CNTs, mainly 10–50 nm
in diameter. The reaction system was similar to
that reported in [13]. Typically, the experiments
were carried out in a conventional furnace with a
horizontal alumina tube, and an alumina crucible
was placed in the hot zone inside the alumina tube,
containing Al and Al2O3 powder mixture covered
with carbon nanotubes. For the maximum pro-
duction of Al2O3 nanotubes (experiment 3), a
stainless steel tube, instead of alumina tube, was
placed in the center of a horizontal furnace as the
reaction chamber, and a BN boat containing solid
reactants was placed in the center of the chamber.
Al powders added with CNTs were used as solid
reactants, except in experiment 1, where Al2O3=Al
mixture (1:1 wt%, 74 lm in average diameter) was
used instead of Al powder. When the system was
heated to 1473 K, NH3 (flow rate: ꢀ100 sccm) was
introduced into the chamber, and the reaction
usually lasted for 60–90 min.
3. Results
In experiment 1, most of the products were
AlN nanowires (Fig. 1a). The average diameter
was 18 nm, slightly larger than that of the pre-
cursor CNTs. The composition of the nanowires
was confirmed by EDS and SAED results. High-
resolution electron microscopic (HREM) image
of an AlN nanowire (Fig. 2a) showed that the
nanowire was a single crystal with some defects,
and it was covered with an amorphous oxide
layer. Corresponding SAED pattern (Fig. 2a, in-
set) indexed to hexagonal AlN, indicated that the
As an effort to study the growth mechanism and
to obtain different products from this reaction
system, a serial of experiments were carried out
Table 1
Synthesis conditions and products
Experiment
Solid
reactantsa
Atmosphere
Boat
Tube
Flow rate
(sccm)
Product
1b
2
Al + Al2O3
Al
NH3
NH3
Al2O3
BN
Al2O3
Stainless steel
300–500
300–500
AlN NWs
Less (AlN
NWs + add.)
3
Al
NH3
BN
Stainless steel
ꢀ100
AlN NWs + more
Al2O3 nanotubes.
AlN NWs + add.
Al2O3 NWs
4
5
6
7
8
Al
Al
Al
Al
Al
NH3 þ ð1–5%ÞO2
BN
BN
Stainless steel
Stainless steel
Al2O3
300–500
300–500
Ar
NH3
Al2O3
Al2O3
Al2O3
AlN particles
Al2O3 particles
Al2O3 whiskers
O2 or the air
Ar
Al2O3
Al2O3
‘NW’ stands for ‘nanowires’, ‘add.’ indicates ‘CNTs coated with some Al2O3 particles + Al2O3 nanotubes’.
a CNTs were also added as solid reactants in all experiments except experiments 6–8.
b The average diameter of the precursor CNTs is 15 nm, while that of the others are 35 nm.

Y. Zhang et al. / Chemical Physics Letters 360 (2002) 579–584
581
Fig. 1. TEM images of the products. (a) AlN nanowires; (b) CNTs coated with Al2O3 particles; (c) Al2O3 nanotubes; (d) Al2O3
nanotubes under larger magnification; (e) an individual Al2O3 nanotube; (f) Al2O3 nanowire with high aspect ratio.
axis of the nanowire was normal to (1 0 0) crystal
planes.
plates. The nanotubes were 10–30 nm in thickness,
and most of them consisted of some small parti-
cles. SAED patterns revealed that these nanotubes
were polycrystalline c-Al2O3. This composition
was further confirmed by EDS result. The atomic
ratio of Al and O is 36:64, in accordance with that
of Al2O3. The element profiles (Fig. 3) across a
nanotube ꢀ80 nm in diameter revealed that more
Al and O existed in the boundaries than in the
center, featuring a tubular geometry due to pro-
jection of cylindrical distribution.
In experiment 2, much less Al2O3 was present in
the reaction system (Table 1) compared with ex-
periment 1. In the product, some CNTs coated
with Al2O3 particles (Fig. 1b) were found, as well
as a small amount of Al2O3 nanotubes, and the
yield of AlN nanowires decreased.
Maximum production of Al2O3 nanotubes was
obtained with a NH3 flow rate of ꢀ100 sccm in
experiment 3 (Figs. 1c–e). The outer diameters of
the nanotubes were larger than 50 nm and the
inner diameters ranged from 10 to 50 nm, similar
to the outer diameter of the CNTs used as tem-
In experiment 4, when 1–5% O2 was added into
the NH3 flow, the yield of all products increased
in comparison with that of experiment 2. The

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Y. Zhang et al. / Chemical Physics Letters 360 (2002) 579–584
Fig. 2. HREM image of the nanowires (a) AlN; (b) Al2O3.
percentage of Al2O3 nanotubes in the product was
more than that of experiment 2 but less than ex-
periment 3. However, the percentage of CNTs
coated with the Al2O3 particles was more than that
of both experiments 2 and 3.
Fig. 3. Al and O distributions across the Al2O3 nanotubes, 80
nm in diameter. (a) Al; (b) O.
When Ar, instead of NH3 was used as the
protective atmosphere in experiment 5, Al2O3
nanowires were fabricated. Some of them were less
than 20 nm in diameter (Fig. 1f). EDS of these
nanowires were similar to that of the nanotubes,
indicating the Al2O3 composition. HREM image
(Fig. 2b) showed that the nanowire was a single
crystal with some defects, and the growth direction
was perpendicular to {1 1 0} crystal planes.
When Al powders were heated in air or O2 as in
experiment 7, only Al2O3 particles were obtained.
When Al or Al þ Al2O3 mixture was heated in
NH3 atmosphere without CNTs (experiment 6),
the products were mostly particles, with a small
amount of whiskers (Fig. 4a). When Al was heated
in Ar flow, some Al2O3 whiskers could be ob-
tained (Fig. 4b). However, their diameters were
larger than those obtained from CNTs templates
and the diameters were more widely distributed
(Fig. 4c).
4. Discussion
The results of experiments 7, 8, 9 indicate that
CNTs are essential to the fabrication of nanowires
or nanotubes. Although Al nano-powders can re-
act with NH3 at 1473 K [23], this reaction does not
directly produce AlN nanowires at this tempera-
ture [24], especially when much bigger Al particles
was used here. In experiment 1, the CNTs act as
both templates and reactants. CNTs, Al, Al2O3
and NH3 all take part in the reaction producing
AlN nanowires, as have been discussed in previous
work [25].

Y. Zhang et al. / Chemical Physics Letters 360 (2002) 579–584
583
However, relatively much more Al than necessary
exists in the system. When heated, Al vaporizes,
transfers and deposits onto the surfaces of CNTs,
then slowly turns into Al2O3 particles by oxidation
(Fig. 1b). Given enough time and reactants, these
particles would grow into an Al2O3 layer over the
CNTs. Fig. 5 might help to understand this pro-
cess. The end and some inner layers of the CNT
have not disappeared completely, and there still
exist some pores in the Al2O3 layer, which might
be formed during the deposition of Al2O3 parti-
cles. Thus, O2 can get into the CNT from its
broken end or through the pores in the layer.
When oxidation of the CNT is finished and more
Al2O3 is deposited, Al2O3 nanotubes are formed.
Since the Al2O3 layer is formed on the outer sur-
faces of the CNTs, the inner diameters of the
Al2O3 nanotubes are similar to the outer diameters
of the CNTs. In this process, CNTs only act as
templates, which is different from that in experi-
ment 1.
If some O2 is added into the NH3 atmosphere,
as in experiment 4, more Al2O3 can be produced,
thus the yields of AlN nanowires, Al2O3 nanotubes
and CNTs coated with Al2O3 particles all increase.
Decreasing the NH3 flow rate (experiment 3) has a
similar effect with adding O2, because it increases
the percentage of O2 in the chamber. But less NH3
Fig. 4. SEM micrographs of some products. (a) Product of Al
heated in NH3 atmosphere without CNTs; (b) product of Al
heated in air without CNTs; (c) product of Al heated in Ar
atmosphere with CNTs.
In experiment 2, Al2O3 is not added as a reac-
tant, thus it comes only from the partial oxidation
of Al by the remainder oxygen in the chamber.
Since existence of Al2O3 is unavoidable, a small
amount of AlN nanowires can be obtained
through a similar process to that of experiment 1.
Fig. 5. An Al2O3 nanotube that the CNT has not been com-
pletely oxidized.

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Y. Zhang et al. / Chemical Physics Letters 360 (2002) 579–584
lowers the production of AlN nanowires and thus
higher percentage of Al2O3 nanotubes can be ob-
tained.
thors thank Professor S.S. Fan for his help and
discussion.
When NH3 is replaced by Ar atmosphere (ex-
periment 5), obviously no AlN nanowires would
be produced. Nevertheless, only Al2O3 nanowires,
other than Al2O3 nanotubes, are formed. The
reason might be rather complicated. Here is only a
possible explanation: at the reaction temperature,
NH3 turns into N2 and H2, which can prevent the
CNTs from quick oxidation. Therefore, Al2O3
layer over CNTs and then Al2O3 nanotubes could
be formed. While in Ar atmosphere with some O2,
the CNTs are not protected by reductive gas.
When Al2O3 nanowires grow along the CNTs, the
CNTs are oxidized simultaneously. There is little
time between the deposition of Al2O3 and the ox-
idation of the CNTs, thus Al2O3 grows into
nanowires in experiment 5.
Actually, NH3 and CNTs both play an impor-
tant role in the fabrication of Al2O3 nanotubes.
Excessive Al is the key to the synthesis of Al2O3
nanotubes in the reaction system producing AlN
nanowires. It seems that the amount of O2 or oxide
in the reaction system is one of the key factors
determining the production type.
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