Unique fusiform alumina nanotubes fabricated by combined anodization

Article abstract of DOI:10.1039/c0cc04907g

Alumina nanotubes (ANTs) with unique fusiform morphologies were synthesized via a simple electrochemical route; the fluctuation of the electronic current density during the anodization process is considered to be the main reason for the formation of such new alumina nanostructures.

Full text of DOI:10.1039/c0cc04907g

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Unique fusiform alumina nanotubes fabricated by combined anodizationw
Li Yi, Ling Zhiyuan,* Hu Xing, Liu Yisen and Chang Yi
Received 11th November 2010, Accepted 13th December 2010
DOI: 10.1039/c0cc04907g
Alumina nanotubes (ANTs) with unique fusiform morphologies
were synthesized via a simple electrochemical route; the fluctua-
tion of the electronic current density during the anodization
process is considered to be the main reason for the formation of
such new alumina nanostructures.
finding proper methods for the fabrication of ANTs with
diverse microstructures in oxalic acid-based electrolytes is
significant in both scientific research and commercial applica-
tions. In the present work, a combined anodization process
which is made up of voltage increasing process and constant
current anodization is proposed for the fabrication of unique
fusiform ANTs in an ethanol–oxalic acid electrolyte. During
the fabrication process, it is expected that the fluctuation of
the electronic current density can directly influence the
morphologies of the final products, thus forming ANTs with
unique fusiform morphologies.
Since the first study of carbon nanotubes,1 many investiga-
tions have focused on fabrications and applications of
nanotubes which have many desirable physical and chemical
properties based on their unique nanostructures.2–8 Among
these nanotubes, alumina nanotubes (ANTs) have attracted
much attention because of their good material characteristics,
such as high dielectric constant, excellent insulating property,
high thermal conductivity, chemical inertness, etc., facilitating
their applications in many related fields (e.g., nanodevices and
catalysis).9–11 In past decades, various ANTs have been
successfully fabricated by a number of methods, including
wet-chemical etching of an anodic aluminium oxide (AAO)
film,12,13 direct anodization of Al or Si-based Al films,3,14,15
using nanowires or nanotubes as templates,16,17 or a template-
free hydrothermal synthesis process.18 Among these fabrica-
tion methods, the AAO-based methods are more practical on
account of their relatively simple and low cost processes.3,12–15
However, these methods have inherent drawbacks in controlling
the microstructural parameters (e.g., length and external
diameter) of the as-prepared ANTs. To solve these problems,
Lee et al. have proposed a pulse anodization process.19,20
Although ANTs with a more controllable microstructural
parameter can be obtained by using this process, some
problems still exist: (1) such processes are usually performed
in sulfuric acid electrolytes, corresponding to relatively lower
anodization voltages, thus resulting in a limited regulating
range of the ANTs’ external diameter (equal to the cell size of
the AAO),19,20 (2) the shape of the as-prepared ANTs is
relatively simple, which may limit their applications to a
certain extent. Considering that the anodic aluminium oxides
(AAOs) fabricated in oxalic acid-based electrolytes have a
large regulating range of the microstructural parameters,21–24
During the anodization, it was found that the AAO
fabricated with a setting current density of 500 A mÀ2 (ESIw)
can be easily broken into two parts: a continuous nanoporous
AAO film (Fig. 1) and separated ANTs (Fig. 2). Fig. 1(a) and
(b) show the cross-sectional SEM images of the detached
continuous nanoporous AAO film. It can be seen that the
lower surface in Fig. 1(a) is relatively rough, and is made up of
short separated ANTs (Fig. 1(b)). Obviously, the cell walls in
the AAO film are also made up of two parts with different gray
scales (the rectangular region in Fig. 1(b)). This phenomenon
is in line with the former reports, for the AAO film is
composed of relatively pure anion-free alumina and anion-
contaminated alumina.25,26 In addition, strip voids embedded
Department of Electronic Materials Science and Engineering,
College of Materials Science and Engineering,
Fig. 1 SEM images of the AAO film detached from the entire AAO
(voltage increasing rate: 20 V minÀ1, setting current density: 500 A mÀ2
South China University of Technology, Guangzhou 510640,
People’s Republic of China. E-mail: imzyling@scut.edu.cn;
Fax: +86 20 8711 1224; Tel: +86 20 8711 1224
w Electronic supplementary information (ESI) available: Experimental
and instrumental details, and other diagrams. See DOI: 10.1039/
c0cc04907g
,
0.3 M oxalic acid electrolyte–ethanol = 2 : 1 v/v): (a) the cross-sectional
image; (b) the partial enlarged cross-sectional image near the lower
surface; (c) the image of the lower surface; (d) the partial enlarged image
of (c). Scale bars: 2 mm for (a), 200 nm for (b)–(d).
c
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Chem. Commun., 2011, 47, 2173–2175 2173

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Fig. 3 Schematic diagrams of the as-prepared continuous nano-
Fig. 2 SEM images of the unique fusiform ANTs which are
exposed when the AAO film shown in Fig. 1 is removed. (b) and (d)
are partial enlarged images of the rectangular regions in (a) and (c),
respectively. Scale bars: 2 mm for (a), 400 nm for (b), 500 nm for (c),
and 100 nm for (d).
porous AAO film and separated fusiform ANTs.
in the cell walls (the circular region in Fig. 1(b)) can be
observed, and each of the short ANTs has a continuously
changing external diameter thus forming a fusiform morpho-
logy (Fig. 1(b)). Moreover, the internal pore diameter is
also continuously changing (ESIw, Fig. S1).27,28 Fig. 1(c) and
(d) show the lower surface of the sample shown in Fig. 1(a)
and (b). It can be seen clearly that the short ANTs locate
at the centre of each cell and are surrounded by the hexagonal
skeleton, thus forming a novel ANTs-based nanonozzle
array which has not been reported in previous studies
(Fig. 1(b)–(d)).
Fig. 2 shows the morphologies of the unique fusiform ANTs
which are exposed when the upper AAO film shown in Fig. 1 is
removed. It is apparent that relatively regular variations of the
ANTs’ external diameters occur (Fig. 2(a) and (b)). It has been
known that the mechanical robustness is relatively weaker at
the position where smaller external diameter is formed.19 As a
result, these ANTs can be further broken into several shorter
ANTs by weak external stress (Fig. 2(c)), and these short
ANTs have a unique fusiform morphology with gradually
decreasing external diameters at their ends (Fig. 2(c) and (d)).
Corresponding schematic diagrams of the as-prepared con-
tinuous nanoporous AAO film (Fig. 1) and separated fusiform
ANTs (Fig. 2) are shown in Fig. 3.
Fig. 4 The cross-sectional SEM image of the AAO (setting current
density: 300 A mÀ2). The inset is a partial enlarged image of the
rectangular region with an enlargement factor of 6. Scale bar = 1 mm.
suggested that the variations of the ANTs’ external diameters
can be ascribed to the time-varying Ua.19 However, this
argument may face a challenge, since apparent variations of
the ANTs’ external diameters can also be observed during
constant voltage anodization.24 Moreover, this argument is
difficult to explain the transition from a continuous nano-
porous AAO film to separated ANTs (Fig. 1(b) and 4). Herein
we develop a model to illustrate the formation process of the
as-prepared unique AAO nanostructures. At the beginning of
the anodization, the ia will increase with the Ua. It has been
known that the ia can play a very important role in the
anodization process,29–31 and is made up of ionic current
density (ii) and electronic current density (ie). During the
anodization, the electrolyte species (e.g., HC2O4À) will be
incorporated into the barrier layer of the AAO film. As the
increase of the Ua, these electrolyte species can be further
ionized, and the proportion of ie in the whole ia will be
increased. When the Ua is increased to a critical value, the
avalanche ie will result in a sharp raise of the ia (the circular
region in Fig. 5).31 Meanwhile, the high Ua will produce a large
amount of O2À and OHÀ at the interface of electrolyte/oxide
because of the dissociation of water,32,33 and these anions can
be driven by the high electrical field to cross the AAO barrier
Fig. 4 shows the cross-sectional SEM image of the AAO
fabricated with a setting current density of 300 A mÀ2 (ESIw).
It can be seen that the as-prepared AAO is also made up of
two parts, which is in line with the model shown in Fig. 3(a).
However, it is found that there are relatively firm junctions
between these two parts (the inset in Fig. 4), and the as-prepared
AAO is relatively compact, compared with the sample shown in
Fig. 1 and 2.
To explain the formation of these unique microstructures,
one may examine the Ua and ia transients during the fabrica-
tion process. In Fig. 5, it can be seen that the ia increases
nonlinearly with the linearly increased Ua during the voltage
increasing process, and fluctuation of the Ua can be observed
under the constant current mode. Recently, Lee et al.
c
2174 Chem. Commun., 2011, 47, 2173–2175
This journal is The Royal Society of Chemistry 2011

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In summary, a simple and controllable electrochemical route
was developed for the fabrication of ANTs with unique fusiform
morphologies. The fluctuation of the electronic current density
during the anodization was considered to be the main reason for
the formation of the as-prepared ANTs. Such new alumina
nanostructures may have diverse potential use in many related
fields, such as nanoelectronics, nanocatalysis, drug carrier, etc.
This work was supported by the Fundamental Research
Funds for the Central Universities (2009ZM0095) and the China
Postdoctoral Science Foundation funded project (20100470913).
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anodization voltage (Ua) as a function of the anodization time (t),
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c
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Chem. Commun., 2011, 47, 2173–2175 2175