Abstract Versatile indium oxide tubular nanostructures (well-aligned nanotube arrays, flower-like tubular structures, and square nanotubes) were fabricated by a facile and reliable chemical vapor deposition (CVD) technique, taking advantage of the self-assembly property and substrate-induced epitaxial growth mechanism. The technique has a few advantages, such as low growth temperature, nonexistence of catalyst, template-free synthesis, direct bonding to the semiconductor substrates, etc. This strategy might extend the approach of synthesizing desirable nanostructures of other important low-melting metal oxides for potential applications

Li Ma

Maojun Zheng

Miao Zhong

Yanbo Li

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IOP PUBLISHING NANOTECHNOLOGY
Nanotechnology 18 (2007) 465605 (4pp) doi:10.1088/0957-4484/18/46/465605
Self-assembly of versatile tubular-like
In2O3 nanostructures
Miao Zhong1, Maojun Zheng1,3, Li Ma2 and Yanbo Li1
1 Laboratory of Condensed Matter Spectroscopy and Opto-Electronic Physics, Department of
Physics, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China
2 School of Chemistry and Chemical Technology, Shanghai Jiao Tong University,
Shanghai 200240, People’s Republic of China
E-mail: mjzheng@sjtu.edu.cn
Received 8 August 2007, in final form 16 September 2007
Published 12 October 2007
Online at stacks.iop.org/Nano/18/465605
Abstract
Versatile indium oxide tubular nanostructures (well-aligned nanotube arrays,
flower-like tubular structures, and square nanotubes) were fabricated by a
facile and reliable chemical vapor deposition (CVD) technique, taking
advantage of the self-assembly property and substrate-induced epitaxial
growth mechanism. The technique has a few advantages, such as low growth
temperature, nonexistence of catalyst, template-free synthesis, direct bonding
to the semiconductor substrates, etc. This strategy might extend the approach
of synthesizing desirable nanostructures of other important low-melting
metal oxides for potential applications.
1. Introduction
Since the discovery of carbon nanotubes (NTs) in 1991 [1],
tubular structures have become of great interest because of
their unique properties and numerous potential applications
in optoelectronic devices, biological devices, gas sensors,
advanced catalysis and energy storage [2–8]. Up to now,
various physical and chemical approaches have been developed
to fabricate quasi-tubular formations, such as laser ablation [9],
the solution-phase route [10], the gelation process [11],
chemical vapor deposition [12–14], etc. However, an
intermedia catalyst or a high process temperature is necessary
in some of these methods. In order to fabricate highly aligned
nanotubes, considerable attempts are currently focused on
template-originated methods [15–18], which mostly utilize
a tubular template of an organogelator or surfactant to
encapsulate oxides or use a nanoporous alumina/carbon
template as the substrate to fabricate nanotubes of oxides.
However, there still exists the problem of removing the
templates without destroying the as-prepared structures. Also,
impure components always remain with the oxides.
Indium oxide, a useful wide-band-gap transparent semi-
conductor, has aroused much attention due to its interesting
properties in many important industry fields [19–21]. How-
ever, most of the research is concentrated on constructing
3 Author to whom any correspondence should be addressed.
artificial nanostructures of nanowires or nanoneedles; there
has been less reported on nanotube alignments. In this pa-
per, we demonstrate a simple but effective CVD technique
to construct versatile indium oxide tubular nanostructures:
well-aligned nanotube (NT) arrays, flower-like tubular struc-
tures, and square NTs. This method has a few advantages,
such as substrate-induced epitaxial growth, a structured pattern
depending strongly on tunable conditions and self-assembly
growth processes, low growth temperature, nonexistence of
catalyst, direct bonding to the semiconductor substrates, etc.
We consider that it might provide a new route for low-melting
temperature metals to directly synthesize desirable structures
with controlled composition, crystallinity, and morphology.
2. Experimental details
The synthesis approach for the diverse tubular nanostructures
of In2O3 is briefly composed of two steps: preparation
of a high-quality nanoporous substrate of InP and epitaxial
growth of In2O3 nanostructures via the CVD process. As
reported in our previous work [22], large-scale uniform InP
nanopore arrays were fabricated by means of electrochemical
anodization in HCl solution. Then, the prepared porous
InP (with indium source), loaded in a ceramic boat, was
put into the center of a horizontal quartz tube of a tubular
furnace. First, the quartz tube was evacuated for 10 min
by a mechanical pump. Then the samples were annealed
0957-4484/07/465605+04$30.00 1 © 2007 IOP Publishing Ltd Printed in the UK
Nanotechnology 18 (2007) 465605 M Zhong et al
(e) (f)
(c) (d)
(a) (b)
Figure 1. (a) Plan-view SEM image of square-shaped nanopore arrays of InP. (b)–(e) SEM images of diverse In2O3 tubular structures.
(f) TEM image of nanotubes from the same sample of image (d); the images on the right are its corresponding SAED pattern and HRTEM
image.
Table 1. Experimental details and obtained morphologies.
Source Process Morphology
Porous InP Condition 1, 500 ◦C Vertical In2O3
nanotube arrays
Porous InP and In Condition 1, 600 ◦C Flower-like
tubular structures
Porous InP and In Condition 2 Square NTs
under different conditions (condition 1: pre-heat the sample
at 450 ◦C for 1 h in low vacuum conditions, then raise the
temperature to 500 ◦C/600 ◦C, and hold for 2 h with Ar and
O2 gas flowing at a rate of 240 and 80 sccm; condition 2:
directly heat the sample at 600 ◦C with Ar and O2 gas kept
flowing in at the same rate). After the annealing treatments,
the furnace was naturally cooled down to room temperature,
and three different kinds of large-scale In2O3 nanostructures
were obtained. (The particular details and corresponding
morphologies are summarized in table 1.)
3. Results and discussion
Figure 1(a) shows a typical scanning electron microscopy
(SEM) image of uniform and square InP nanopore arrays.
The average inner diameter (diagonal length) of pores and
the average wall thickness of the pores were estimated to be
120 nm and 75 nm, respectively. Utilizing such a geometric
surface as substrate, different kinds of tubular structure of
In2O3 are synthesized via the consequent CVD process.
Figure 1(b) shows an SEM image of vertical In2O3 NT arrays.
It shows that the aligned tubes grow straightly on the substrate,
with diameters around 800 nm–1.2 µm and length around
2–6 µm. Figure 1(c) shows an SEM image of the flower-
like tubular structure of In2O3. The individual multilayered
flower-like structure consists of densely packed nanotubes.
Each layer of the petal-like tube is smooth and radially
oriented, with an outer diameter around 3–5 µm. Figures 1(d)
and (e) demonstrate ultralong square In2O3 NTs at different
magnifications. Figure 1(d) is a low-magnification view of the
product, revealing that the NTs are widespread over the whole
substrate with random directions. Figure 1(e) shows a close-up
view of one single NT at a higher magnification, from which
the obviously square and opened-up nozzle can be observed
at one end of the NT, corresponding to the morphology and
dimension of the InP nanopores. This clone structure indicates
that square NTs are directly developed from the InP substrate.
Such structure was further investigated by transmission
electron microscopy (TEM) and high-resolution transmission
electron microscopy (HRTEM) images. Figure 1(f) shows
the TEM (JEOL JEM-2100F) image of a single NT with a
high degree of contrast between the bright central part and
the dark edges, which represent the hollow character of the
tube. The outer diameters of the nanotubes are around 200–
300 nm, retaining the size and similar square shape of the pores
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Nanotechnology 18 (2007) 465605 M Zhong et al
Figure 2. Schematic depiction of the formation of tubular structures.
(This figure is in colour only in the electronic version)
of the InP substrates. The tube body is uniformly straight
and smooth with no macroscopic defects. Its selected area
electron diffraction (SAED) pattern (images on the right-hand
side of figure 1(f)) indicates that the fine structure is single-
crystalline and that it grew along the [100] direction. High-
resolution TEM (HRTEM) was employed to confirm the highly
crystallized character. The interplanar spacing of ∼0.413 nm
agrees well with the spacing of the (211) planes of In2O3
crystal.
We postulate that the nucleation and growth of vertical
NT arrays occurs in three basic steps: (1) dissolution
of nanoporous InP, (2) self-assembly organization of large
nanoscale circles of molten indium, (3) dynamic formation
of In2O3 nuclei circles and homoepitaxial growth of In2O3
tubular structures. A simple mode for the dynamic growth
illustration is proposed, as shown in figure 2. When the
furnace temperature was increased to 450 ◦C and maintained
for 1 h in low vacuum conditions in the pre-heating step,
a rapid dissolution process of nanoporous InP took place.
Phosphor vapor was drawn out by the vacuum pump, while
indium remained on the surface of substrate in liquid state.
Via this step, indium droplets on the surface spontaneously
coalesced together to form closed circles with large nanoscale
diameter. There is one particular question: why were
the large nanoscale circles circular-shaped rather than other
shapes? It is reasonable to assume that this might be related
to the combinational effects of substrate-inducement (porous
morphology, stress, etc) and self-assembly characters. With the
reactant gas O2 flowing into the chamber, the molten indium
circles on the surface first reacted with O2 to synthesize the
In2O3 ring-shaped nuclei and then they gradually developed
into tubular structures. With time, the length of tubes
increased. Meanwhile, the length of tube increased which
prevented other nucleation of In2O3 inside the tubes by
reducing the probability of O2 and indium meeting. Thus, the
continuous decomposition of porous parts beneath or inside the
In2O3 circles, acted as an indium source, and kept the growth
of the In2O3 nanotube.
In order to prove the hypothesis of decomposition of
InP and growth of In2O3, an x-ray diffraction system (XRD)
was employed to investigate the surface parts of the porous
InP substrate at various stages during the growth process. A
comparison of XRD results is presented in figure 3. Figure 3(a)
(c)
(b)
(a)
Figure 3. X-ray diffraction (XRD) patterns of porous InP treated at
different conditions. (a) XRD result from porous InP substrate.
(b) XRD result from porous InP pre-heated in a low-vacuum
chamber at 450 ◦C for 1 h. (c) XRD result from products of In2O3
nanotube arrays.
is the original diffraction spectrum of porous InP; figure 3(b)
shows the result after pre-heating InP at 450 ◦C for 1 h, which
indicates that porous InP substrate is decomposed. Phosphor
was evacuated while indium stayed on the surface, which
supports the supposition of dissolution before. Figure 3(c)
shows a typical diffraction pattern of the final In2O3 product.
No other element such as indium or phosphor was detected.
It is worth noting that porous structures of InP substrates
herein play key roles. First, they lower the decomposition
temperature for easier control of the growth of In2O3
nanotubes. Second, they confine the fluidity of molten indium
so as to facilitate the self-assembly formation of molten indium
circles via the pre-heating step. In contrast, during the same
annealing disposal to bulk InP with a smooth surface, molten
indium joined together into irregular morphologies and none of
the nanostructures was obtained. Third, they make the In2O3
NT grow homoepitaxially along the porous direction, ensuring
growth perpendicular to the substrate plane.
By altering the growth condition, a flower-like structure
of In2O3 was obtained. In contrast to the growth process of
vertical nanotube arrays of In2O3, extra indium was added as
external source and the annealing temperature was raised to
600 ◦C in the fabrication of a flower-like structure, which could
result in indium vapor being supersaturated in the chamber. We
postulate that large indium vapor supersaturation could cause
multi-dimensional nucleation of In2O3 on the molten indium
circles, which gradually evolved into branched nucleation
circles [23, 24], which led to the growth direction of the tubular
nanostructures multiplex. In addition, the increasing annealing
temperature is another influential factor which boosted the
reaction speed of indium and oxygen, causing the nucleation
and growth orientation to be uncertain and difficult to control.
After the whole growth process, multiple nanotubes packed
densely, and a flower-like tubular structure was formed.
In the square NT growth process, porous InP substrate
was directly annealed at 600 ◦C for 2 h with Ar and O2 gas
constantly flowing without the pre-treatment step. The high
annealing temperature of 600 ◦C was used to enhance the
decomposition speed of porous InP and reaction speed between
indium and oxygen. The molten indium decomposed from InP
diffusing into InP, and formed a eutectic on the surface part
of the porous wall, facilitating the formation of In2O3 nuclei.
3
Nanotechnology 18 (2007) 465605 M Zhong et al
And extra indium grains were employed to increase the indium
vapor density to assure the reaction possibility of indium and
O2 for maintaining the growth. Thus, it is deduced that the
decomposition of InP and nucleation of In2O3 herein occurred
simultaneously and continuously. Gradually, the square NTs
were obtained, as described in figure 2. However, increasing
the NT’s length and raising the temperature resulted in the
growth direction of In2O3 NTs being ungovernable, so that all
the NTs are curved with an arbitrary growth orientation. In
a word, the controllable experiment factors, such as reaction
steps, annealing temperature and extra source, have great
effects in the fabrication of various structures.
4. Conclusion
In summary, we have presented a facile and controllable
method for the large-scale fabrication of diverse In2O3
tubular nanostructures. This available strategy of utilizing
self-assembly and substrate-induced properties allowed for
template-free synthesis of a vertically arrayed nanotube
pattern. Furthermore, flower-like tubular structures and square
nanotubes were also successfully fabricated by intentionally
altering the growth conditions. We suggest that this technique
might provide a conceptually different route for synthesizing
desirable nanostructures of other important low-melting point
metal oxides for practical application.
Acknowledgments
This work was supported by the Natural Science Foundation
of China (Grant no. 50572064) and the National Minister of
Education Program for Changjiang Scholars and Innovative
Research Team in University (PCSIRT).
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Self-assembly of versatile tubular-like In 2 O 3 nanostructures

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Self-assembly of versatile tubular-like In 2 O 3 nanostructures

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