# Selective Growth ofα-Al2O3 Nanowires and Nanobelts

Authors: Yong Zhang; Ruying Li; Xiaorong Zhou; Mei Cai; Xueliang Sun
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

We report the selective growth ofα-Al2O3 nanowires and nanobelts via a catalyst-free chemical vapor deposition process under ambient pressure. By controlling the flow rates of the carrier gas, high-yield production of uniform alumina nanowires with diameter distribution (100 nm–200 nm) was achieved at a high growth rate over 200 μm/hour. Alumina nanobelts with variable width were also synthesized by modulating the carrier gas purge process. Further, the effects of temperatures and carrier gas flow rates on the growth of alumina nanostructures were also investigated. Oxygen partial pressure and supersaturation level of the aluminum suboxide are thought to be important factors in the formation process of the alumina nanowires or nanobelts. The typical growth of the alumina nanowires and nanobelts can be ascribed to vapor-solid (VS) mechanism.


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## Body

## 1. Introduction

Recent
progress in quasione dimensional (Q1D) nanostructured materials has gained increasing
creditability of their advantages in both distinct properties and practical
applications. Aside from nanotubes, the unique structural features of nanowires
or nanobelts make them ideal candidates for both interconnects [1] and functional
units [2, 3] in nanodevice integration. Metal oxide nanowires and nanobelts
have attracted significant scientific interest due to their fascinating
physical properties as well as their potential applications in optical [4],
electronic [5, 6], optoelectronic [7, 8], piezoelectric [9], mechanical [10, 11],
magnetic [12], chemical [13–16] sensors and
nanodevices.As a traditional structure material, aluminum oxide has
featured excellent thermal stability and favorable mechanical properties.
Thanks to peculiar properties recently explored in Q1D
aluminum oxide, such as photoluminescence [17, 18], dielectric response [19],
and selective catalysis [20]; keen interest has been aroused in probing new
frontiers in this field. To date, crystalline and amorphous alumina nanowires,
nanobelts, or nanorods have been synthesized through various techniques,
including catalyst assisted [21–23] or catalyst
free [18, 24, 25] chemical vapor deposition, chemical etching [26, 27],
electrochemical deposition [28], arc-discharge [29], and surfactant-induced
method [20]. As far as the catalyst-assisted growth of nanowires based on
vapor-liquid-solid (VLS) growth mechanism, size of the nanowire diameter is
well defined by the catalyst diameter. However, it still remains a big
challenge of how to control the morphology and size of the Q1D
alumina nanostructures via a catalyst-free method, which is essential to
scale-up fabrication and industrial applications.In this paper, we present a facile and effective way to
selectively grow alumina nanowires or nanobelts under ambient pressure. This
simple and controllable process promises scaled growth of the Q1D
alumina nanostructures for potential industrial applications. The influence of
experimental conditions, such as temperature and carrier gas flow rate, on the
growth of alumina nanowires and nanobelts has been investigated. Further, the
growth mechanism of the nanowires and nanobelts are discussed.


## 2. Experimental

A
hot-wall chemical vapor deposition (HWCVD) system, with an alumina tube (2 inches
outer diameter and 48 inches length) being mounted horizontally at the center,
was employed in the present study. Q1D nanostructured alumina
was fabricated under ambient pressure in such system with reaction temperature
and the flow rate of carrier gas being precisely controlled. Aluminum powder
(A-547, 20 mesh, Fisher Scientific, Pa, USA), used as the starting material, was
contained in an alumina boat and placed at
the center of the alumina tube.For the growth of alumina nanowires, high-purity argon gas
(99.999%) was passed through the tube at a rate of 400 sccm (standard cubic
centimeters per minute) for 30 minutes to purge air out of the furnace. Then,
the furnace was heated to a target temperature ranging from 900°C–1350°C
at a heating rate of 40°C/minute and maintained at the reaction
temperature for 1 hour. Finally, the furnace was cooled down to room temperature
at the end of the reaction. Argon gas was flown through the system at a
controlled rate during the growth process.Unlike the process for the growth of alumina nanowires, the
gas purge step was skipped for the growth of alumina nanobelts. The furnace was directly heated to 1300°C
at a heating rate of 40°C/minute, maintained at 1300°C
for 1 hour, and then cooled down to room temperature. Again, argon gas was
flown through the system at a controlled rate during the process.The synthesized products were examined
initially by using a Hitachi S-4500 field emission scanning electron microscopy
(FESEM) equipped with energy dispersive X-ray (EDX) analysis facility. Further
structural characterization of the Q1D nanostructures was carried out using a
Rigaku-MiniFlex powder X-ray diffraction
spectrometer (XRD), a JEOL 2010 transmission electron microscope (TEM),
and a Tecnai G2 F30 high-resolution transmission electron microscope
(HRTEM).


## 3. Results and Discussion

### 3.1. Morphological and Structural Characterization

For the nanowires synthesis, when the reaction temperature
was above 1100°C, white cotton-wool-like products could be collected
from inner wall of the alumina boat. In this case, the residue
aluminum/aluminum oxide solidified into a single molten block after heating at
the temperature higher than 1100°C. However, when the reaction temperature was below 1100°C, no
such products could be harvested from the alumina boat. Therefore, the residual
aluminum particles were collected for characterization. In this case, the
particle size becomes much bigger due to melting and solidification after
reaction.Figure1 shows scanning electron
micrographs of the products obtained at various reaction temperatures. Short
nanorods, 5–10 μm in
length and 0.5–1 μm in
diameter, were observed on the surface of residual aluminum particle for the
reaction temperature of 900°C, as shown in Figure 1(a). Thinner
nanorods, of 200–300 nm in diameter, mixed with some nanosheets (indicated by
arrow), were generated on the surface of the residual aluminum particles at the
reaction temperature of 1000°C (see Figure 1(b)). When the
temperature was increased to 1100°C, cotton-wool-like products were
formed on inner wall of the alumina boat. A scanning electron micrograph of such
product is shown in Figure 1(c), revealing nanowires of 100 nm–1μm in diameter. For the reaction temperature of 1300°C,
the scanning electron micrograph of Figure 1(d) shows nanowires with uniform
diameter of 200–400 nm. EDX analysis of the nanowires gave yields of aluminum
and oxygen, indicating the formation of alumina nanostructures. Further, when
the reaction temperature was above 1350°C, the nanowires tended to
become thick and nonuniform in diameter. Therefore, the size dependence of the
nanowires on argon gas flow rate was further investigated by fixing reaction
temperature at 1300°C, as described in the following paragraph.FESEM images show alumina nanowires
synthesized under different temperatures with 200 sccm argon gas. (a) 900°C, (b) 1000°C, (c) 1100°C, (d) 1300°C.
(a)(b)(c)(d)Scanning
electron micrographs of alumina nanostructures grown at 1300°C under
different argon gas flow rates are shown in Figure 2. With argon gas flow rate
in the range of 40–100 sccm, the diameter of the alumina nanowires ranges from
200 to 800 nm (see Figures 2(a) and 2(b)). Interestingly, it was observed that
increased argon gas flow rates favored the growth of thinner alumina nanowires.
Neat alumina nanowires with diameter distribution of 200–400 nm and 100–200 nm
were produced at the argon gas flow rate of 200 sccm and 600 sccm,
respectively, (see Figures 2(c) and 2(d)). Further increase in argon gas flow
rate to above 600 sccm showed little influence on the
morphology and diameter of the nanowires.FESEM images show alumina
nanowires synthesized at 1300°C under different argon gas flow
rates. (a) 40 sccm, (b) 100 sccm, (c) 200 sccm, and (d) 600 sccm.
(a)(b)(c)(d)Further characterization of the nanostructures was carried out using
transmission electron microscopy and X-ray
diffraction spectrometer, as shown in Figure3. The bright field TEM image
reveals that the nanowires are uniform and neat, with the diameter of about 100 nm as shown in Figure 3(a). Figure 3(b) displays XRD pattern of the product, which can be assigned to
rhombohedral structure of α-Al2O3. HRTEM
lattice image of the nanowire in Figure 3(c)
indicates that the lattice spacing is around 0.43 nm, corresponding to
{003} set of planes of α-Al2O3. As
shown in Figure 3(d), the selected area electron diffraction (SAED)
pattern can be fully indexed as rhombohedral structure of α-Al2O3.
By recording the
SAED pattern along the nanowires, it is evident that each individual α-Al2O3 nanowire is a single crystal.(a) Bright field
image, (b) XRD pattern, (c) HRTEM image, and (d) SAED pattern of the Al2O3 nanowire.
(a)(b)(c)(d)Interestingly, it was found
that careful control of argon gas purge process could also modulate the
morphology of alumina nanostructures. As shown in Figures1 and 2, nanowires were produced when a
thirty-minute argon gas purge step was applied. However, under the same
synthesis conditions, but with the argon purge step being skipped,
nanostructures of completely different morphologies were produced, as shown in
Figure 4. At the argon gas flow rate of 40 sccm, only thick nanorods with very
rough surface were generated (see Figure 4(a)). Thin nanobelts, with the widths
ranging from 200 nm to a couple of micrometers,
were obtained with an increased argon flow rate ranging from 150 sccm to
350 sccm (see Figures 4(b) and 4(c)).
When an argon gas flow rate of 600 sccm was applied, a mixture of nanobelts and
nanowires was produced (see Figure 4(d)), which is similar to the observation
in Figure 1(d). Figure 5 shows increased magnification XRD,
SEM, and TEM images of typical alumina nanobelts obtained at 1300°C
at an argon gas flow rate of 150 sccm. The nanobelts are approximately 500 nm
wide and a few tens nanometer thick. The XRD
pattern and selected area diffraction pattern confirm that the nanobelts are α-Al2O3 with
the rhombohedral structure.FESEM images
show alumina nanobelts obtained at 1300°C under different argon gas
flow rates. (a) 40 sccm, (b) 150 sccm, (c) 350 sccm, and (d) 600 sccm.
(a)(b)(c)(d)(a) High
magnification FESEM image, (b) XRD of the Al2O3 nanobelts,
(c) TEM image, (d) HRTEM image, and (e) the corresponding SAED pattern of a
typical Al2O3 nanobelt.
(a)(b)(c)(d)(e)


### 3.2. Growth Mechanism

For catalyst-assisted growth of alumina and many
semiconductor nanowires, the vapor-liquid-solid (VLS) growth mechanism has been
widely accepted [23, 30]. The presence of
solidified spherical droplets at the tips of nanowires or others is commonly
considered to be evidence of the VLS mechanism. The growth process can be
divided into two stages: the nucleation and growth of eutectic alloy droplets
and the growth of nanowires through the liquid droplets due to supersaturation.
However, no catalyst particles were
observed at the tips of the nanowires or nanobelts through SEM and TEM
investigations in our experiments, indicating that the growth of alumina
nanowires or nanobelts is not governed by the VLS growth mechanism in the
present study. In our experiment, an alumina
boat was used as a container for the
aluminum powder. It was observed that the alumina boat under aluminum powder
was partly dissolved during synthesis process. The degree of such dissolution was dependent on the temperature employed. This indicates
an occurrence of interaction between the alumina boat and the aluminum powder
during the synthesis process. In order to identify
the role of alumina in the reaction process, we used a mixture of metallic
aluminum and alumina powder to take the place of sole metal aluminum powder as
the starting material. While a certain amount of alumina powder was used, the
structure of the products was similar to the case without using the alumina
powder, and the alumina boat was almost not etched. The results indicate that: (1)
the alumina boat which has a similar role to an alumina plate was involved into
the reaction for growing the nanowires or nanobelts; (2) the alumina source for
the product growth can be supplied by extra added alumina powders or plates.For Al–Al2O3 system, various
aluminum-containing vapor species, including Al, AlO, Al2O, and Al2O2 [31], are generated at the temperatures employed in the present study.
Such species may be involved in the synthesis of alumina nanostructured
materials. From a vapor pressure study of aluminum-oxygen system reported by
Brewer and Searcy [31], Al and Al2O are the principal vapor
components under reducing (Al–Al2O3) conditions.Therefore, it is postulated that the formation of the
alumina nanowires or nanobelts is based on the following reactions and phase
transition:(1)4Al(l)+Al2O3(s)⟺3Al2O(g),(2)Al2O(g)+O2(g)⟺Al2O3(s),(3)4Al(l)+3O2(g)⟺2Al2O3(s),(4)4Al(g)+3O2(g)⟺2Al2O3(s).It is known
aluminum suboxide (Al2O) is thermodynamically stable between 1050°C
and 1600°C [32]. At
the temperature above 1100°C, aluminum reacts with the alumina boat and
generates Al2O vapor as reaction (1). There are two available
alumina sources here: one is the reaction product between aluminum and trace
amount of oxygen in the carrier gas or from air
leakage or residue oxygen in the reaction tube, and the other is the
alumina boat. When the supersaturation of the
reactant vapors increases to a level at which nuclei formed, the combination of
Al2O vapor and oxygen inside the chamber can produce nanosized alumina nuclei
attached on the inner wall of the alumina boat which provides preferential
nucleation sites for the alumina nanowire growth. Finally, alumina nanowires or
nanobelts are formed as shown in reaction
(2) through a process of diffusion, collision of atoms, and reaction between
the vapor molecules (including Al2O vapor and O2 in this
experiment), which follows a vapor-solid (VS) growth mechanism. In this case,
the growth of the nanowires/nanobelts can be mainly attributed from the result
of reactions (1) and (2).While
the temperature is in the range of 900°C–1000°C, no
products could be collected from the inner wall of the alumina boat. In this
case, alumina nanorods and nanosheets were observed on the surface of the
residual aluminum particles (see Figures 1(a) and 1(b)). Within this
temperature range, Al2O vapor is metastable. In this case, the nucleation and growth of the alumina
nanostructures may be more dependent on the reactions between the aluminum
liquid/vapor and the oxygen inside or close to the molten aluminum surface as
shown in reactions (3) and (4). The reactions can produce tiny alumina nuclei
in the molten aluminum. Due to
poor wettability of molten aluminum with alumina nuclei [33], high surface
tension of the molten aluminum drives the alumina nuclei to the molten aluminum
surface and
prevents the nuclei from aggregation, which favors the preferetial growth of
the alumina perpendicular to the molten aluminum surface. The subsequent growth of surface nuclei could occur
through attachment kinetics with the reactant species in the dissolved phase as
illustrated in reaction (3) or with gas-phase species as illustrated in
reaction (4).With decreasing temperature (below 900°C), too low
temperature probably is not able to induce the growth of alumina in quasi one
dimension. The alumina obtained could coalesce on the molten surface to form a
crust and result in a thick film over molten aluminum.Another interesting question is why the nanowires and nanobelts can be
selectively grown. Song et al. have reported
that the partial
pressure of oxygen plays a key role in determining the morphology of the Q1D ZnO
nanostructures by VLS growth mode under vacuum conditions, in which Q1D anisotropic growth of ZnO nanowires was
promoted by controllably decreasing oxygen pressure in the reaction chamber [34]. On the other
hand, Ye et al. have suggested that the nonequilibrium kinetic
growth caused by high supersaturation level may allow the formation of a
low-surface-energy tip, which favors two-dimensional growth of the nanostructures [35]. Park
et al. have reported that transformation from Q1D growth of ZnO nanowires to Q2D
growth of ZnO nanobelts occurred with increasing supersaturation level of the
reactant vapors governed by VS growth [36]. According to Dalal et al. and Lee
et al.’s reports, higher oxygen partial pressure has been proposed as an
important factor in generating ZnO [37] and Ga2O3 nanobelts [38], respectively, in comparison
with the nanowire growth.In
our case, we propose that purge process can produce low partial pressure of
oxygen inside the chamber though the condition of air leakage, and residue
oxygen still needs to be considered, while skipping the purge process not only
keeps a high oxygen partial pressure but also may trigger a generation of high
supersaturation level of aluminum suboxide vapor during the heating process as
illustrated in the reactions (3), (4), and (1) due  to high oxygen content. So we intend to
attribute the growth of the Al2O3 nanowires to the low
oxygen partial pressure, and the growth of the Al2O3 nanobelts to the combination effect of excessive supersaturation level of the
aluminum suboxide vapor and high oxygen partial pressure in the synthesis
process.In the experiments, there are probably three
sources which provide the oxygen in the synthesis of the Q1D alumina
nanostructures: (1) the argon gas (99.999% purity), which
contains a
small amount of oxygen with the partial pressure of 10−5 atm [39], (2) residue oxygen in the reaction chamber after the
purge, (3) air leakage. Similar to the case that Park
et al. discussed [40], low oxygen partial pressure favors one-dimensional
growth of the products. After the purge
process, most of the oxygen was excluded from the chamber. However, the oxygen
supply coming from air leakage and residue oxygen inside the chamber were not
able to be effectively restrained by the argon gas while a low flow rate of
argon was applied in the growth process. With increasing argon flow rate,
oxygen partial pressure inside the chamber decreased, and gradually closed to
the internal
oxygen partial pressure of the argon gas. This may be one of the reasons why higher argon flow rate
favored the growth of thinner alumina nanowires. While Ar purge process was not conducted, oxygen
partial pressure inside the alumina tube kept
the same as
ambient environment before heating. The furnace was then heated as soon as Ar
gas was introduced. In this circumstance, oxygen partial
pressure decreased inside the alumina tube
while temperature of the furnace increased over the time. On the other hand, high supersaturation level of aluminum
suboxide might be triggered during the heating process due to high oxygen
content inside the chamber.  Depending on the supersaturation level of aluminum suboxide and oxygen partial pressure at
the target temperature, alumina nanostructures with different morphologies were
obtained. At a lower Ar flow rate, the oxygen partial pressure remained very high, and excessive
supersaturation level of the aluminum suboxide was generated when the target temperature
was achieved, allowing quick growth perpendicular to the
longitudinal axis of the product. Therefore, thick column-like structure was
produced (see Figure 4(a)). Proper argon flow rates might activate secondary growth sites and heterogeneous nucleation on the side
of the Q1D structures. This in turn resulted in Q2D growth of the
structures and thus alumina nanobelts were obtained (see Figures 4(b)
and 4(c)).
While Ar flow rate was further increased, the
oxygen partial pressure and supersaturation
level of aluminum suboxide became relative low at the target temperature, which is similar to
the nanowire growth that discussed above. Therefore, both nanowires and
nanobelts can be observed as shown in Figure 4(d). For the growth of the nanobelts, a question may arise,
to what extent the effect of the nonpurge measure can last for growing the
nanobelts because the oxygen partial pressure decreases continuously with
increasing growth time. Figure 6(a) shows a typical SEM image of the nanobelts
(from the same sample as shown in Figure 4(b)). As marked by the white arrows,
a morphological transition from the nanobelt to nanowire is observed. It
reveals that the growth of the nanobelt ends with a wire-like morphology due to
decreasing oxygen partial pressure and supersaturation level of the aluminum
suboxide. This indicates that the growth of the nanobelts is time-dependent and
needs to be carefully designed. Figure 6(b) shows a typical TEM image of the
nanobelt/nanowire structure at the wire-like end. No catalyst particle is
observed at the tip of the nanowire, which conforms to the VS growth model. The
arrow in Figure 6(b) reveals the growth direction of the product.(a) SEM image of the Al2O3 nanobelts showing the transition between the nanobelt and nanowire, (b) TEM
image of a single Al2O3 nanobelt revealing the transition
from belt-like to wire-like structure.
(a)(b)Further
investigation of other processing parameters and precise control of oxygen
partial pressure that affect the morphology of nanostructures generated in
Al–Al2O3 system is currently being undertaken in our
group.


## 3.1. Morphological and Structural Characterization

For the nanowires synthesis, when the reaction temperature
was above 1100°C, white cotton-wool-like products could be collected
from inner wall of the alumina boat. In this case, the residue
aluminum/aluminum oxide solidified into a single molten block after heating at
the temperature higher than 1100°C. However, when the reaction temperature was below 1100°C, no
such products could be harvested from the alumina boat. Therefore, the residual
aluminum particles were collected for characterization. In this case, the
particle size becomes much bigger due to melting and solidification after
reaction.Figure1 shows scanning electron
micrographs of the products obtained at various reaction temperatures. Short
nanorods, 5–10 μm in
length and 0.5–1 μm in
diameter, were observed on the surface of residual aluminum particle for the
reaction temperature of 900°C, as shown in Figure 1(a). Thinner
nanorods, of 200–300 nm in diameter, mixed with some nanosheets (indicated by
arrow), were generated on the surface of the residual aluminum particles at the
reaction temperature of 1000°C (see Figure 1(b)). When the
temperature was increased to 1100°C, cotton-wool-like products were
formed on inner wall of the alumina boat. A scanning electron micrograph of such
product is shown in Figure 1(c), revealing nanowires of 100 nm–1μm in diameter. For the reaction temperature of 1300°C,
the scanning electron micrograph of Figure 1(d) shows nanowires with uniform
diameter of 200–400 nm. EDX analysis of the nanowires gave yields of aluminum
and oxygen, indicating the formation of alumina nanostructures. Further, when
the reaction temperature was above 1350°C, the nanowires tended to
become thick and nonuniform in diameter. Therefore, the size dependence of the
nanowires on argon gas flow rate was further investigated by fixing reaction
temperature at 1300°C, as described in the following paragraph.FESEM images show alumina nanowires
synthesized under different temperatures with 200 sccm argon gas. (a) 900°C, (b) 1000°C, (c) 1100°C, (d) 1300°C.
(a)(b)(c)(d)Scanning
electron micrographs of alumina nanostructures grown at 1300°C under
different argon gas flow rates are shown in Figure 2. With argon gas flow rate
in the range of 40–100 sccm, the diameter of the alumina nanowires ranges from
200 to 800 nm (see Figures 2(a) and 2(b)). Interestingly, it was observed that
increased argon gas flow rates favored the growth of thinner alumina nanowires.
Neat alumina nanowires with diameter distribution of 200–400 nm and 100–200 nm
were produced at the argon gas flow rate of 200 sccm and 600 sccm,
respectively, (see Figures 2(c) and 2(d)). Further increase in argon gas flow
rate to above 600 sccm showed little influence on the
morphology and diameter of the nanowires.FESEM images show alumina
nanowires synthesized at 1300°C under different argon gas flow
rates. (a) 40 sccm, (b) 100 sccm, (c) 200 sccm, and (d) 600 sccm.
(a)(b)(c)(d)Further characterization of the nanostructures was carried out using
transmission electron microscopy and X-ray
diffraction spectrometer, as shown in Figure3. The bright field TEM image
reveals that the nanowires are uniform and neat, with the diameter of about 100 nm as shown in Figure 3(a). Figure 3(b) displays XRD pattern of the product, which can be assigned to
rhombohedral structure of α-Al2O3. HRTEM
lattice image of the nanowire in Figure 3(c)
indicates that the lattice spacing is around 0.43 nm, corresponding to
{003} set of planes of α-Al2O3. As
shown in Figure 3(d), the selected area electron diffraction (SAED)
pattern can be fully indexed as rhombohedral structure of α-Al2O3.
By recording the
SAED pattern along the nanowires, it is evident that each individual α-Al2O3 nanowire is a single crystal.(a) Bright field
image, (b) XRD pattern, (c) HRTEM image, and (d) SAED pattern of the Al2O3 nanowire.
(a)(b)(c)(d)Interestingly, it was found
that careful control of argon gas purge process could also modulate the
morphology of alumina nanostructures. As shown in Figures1 and 2, nanowires were produced when a
thirty-minute argon gas purge step was applied. However, under the same
synthesis conditions, but with the argon purge step being skipped,
nanostructures of completely different morphologies were produced, as shown in
Figure 4. At the argon gas flow rate of 40 sccm, only thick nanorods with very
rough surface were generated (see Figure 4(a)). Thin nanobelts, with the widths
ranging from 200 nm to a couple of micrometers,
were obtained with an increased argon flow rate ranging from 150 sccm to
350 sccm (see Figures 4(b) and 4(c)).
When an argon gas flow rate of 600 sccm was applied, a mixture of nanobelts and
nanowires was produced (see Figure 4(d)), which is similar to the observation
in Figure 1(d). Figure 5 shows increased magnification XRD,
SEM, and TEM images of typical alumina nanobelts obtained at 1300°C
at an argon gas flow rate of 150 sccm. The nanobelts are approximately 500 nm
wide and a few tens nanometer thick. The XRD
pattern and selected area diffraction pattern confirm that the nanobelts are α-Al2O3 with
the rhombohedral structure.FESEM images
show alumina nanobelts obtained at 1300°C under different argon gas
flow rates. (a) 40 sccm, (b) 150 sccm, (c) 350 sccm, and (d) 600 sccm.
(a)(b)(c)(d)(a) High
magnification FESEM image, (b) XRD of the Al2O3 nanobelts,
(c) TEM image, (d) HRTEM image, and (e) the corresponding SAED pattern of a
typical Al2O3 nanobelt.
(a)(b)(c)(d)(e)


## 3.2. Growth Mechanism

For catalyst-assisted growth of alumina and many
semiconductor nanowires, the vapor-liquid-solid (VLS) growth mechanism has been
widely accepted [23, 30]. The presence of
solidified spherical droplets at the tips of nanowires or others is commonly
considered to be evidence of the VLS mechanism. The growth process can be
divided into two stages: the nucleation and growth of eutectic alloy droplets
and the growth of nanowires through the liquid droplets due to supersaturation.
However, no catalyst particles were
observed at the tips of the nanowires or nanobelts through SEM and TEM
investigations in our experiments, indicating that the growth of alumina
nanowires or nanobelts is not governed by the VLS growth mechanism in the
present study. In our experiment, an alumina
boat was used as a container for the
aluminum powder. It was observed that the alumina boat under aluminum powder
was partly dissolved during synthesis process. The degree of such dissolution was dependent on the temperature employed. This indicates
an occurrence of interaction between the alumina boat and the aluminum powder
during the synthesis process. In order to identify
the role of alumina in the reaction process, we used a mixture of metallic
aluminum and alumina powder to take the place of sole metal aluminum powder as
the starting material. While a certain amount of alumina powder was used, the
structure of the products was similar to the case without using the alumina
powder, and the alumina boat was almost not etched. The results indicate that: (1)
the alumina boat which has a similar role to an alumina plate was involved into
the reaction for growing the nanowires or nanobelts; (2) the alumina source for
the product growth can be supplied by extra added alumina powders or plates.For Al–Al2O3 system, various
aluminum-containing vapor species, including Al, AlO, Al2O, and Al2O2 [31], are generated at the temperatures employed in the present study.
Such species may be involved in the synthesis of alumina nanostructured
materials. From a vapor pressure study of aluminum-oxygen system reported by
Brewer and Searcy [31], Al and Al2O are the principal vapor
components under reducing (Al–Al2O3) conditions.Therefore, it is postulated that the formation of the
alumina nanowires or nanobelts is based on the following reactions and phase
transition:(1)4Al(l)+Al2O3(s)⟺3Al2O(g),(2)Al2O(g)+O2(g)⟺Al2O3(s),(3)4Al(l)+3O2(g)⟺2Al2O3(s),(4)4Al(g)+3O2(g)⟺2Al2O3(s).It is known
aluminum suboxide (Al2O) is thermodynamically stable between 1050°C
and 1600°C [32]. At
the temperature above 1100°C, aluminum reacts with the alumina boat and
generates Al2O vapor as reaction (1). There are two available
alumina sources here: one is the reaction product between aluminum and trace
amount of oxygen in the carrier gas or from air
leakage or residue oxygen in the reaction tube, and the other is the
alumina boat. When the supersaturation of the
reactant vapors increases to a level at which nuclei formed, the combination of
Al2O vapor and oxygen inside the chamber can produce nanosized alumina nuclei
attached on the inner wall of the alumina boat which provides preferential
nucleation sites for the alumina nanowire growth. Finally, alumina nanowires or
nanobelts are formed as shown in reaction
(2) through a process of diffusion, collision of atoms, and reaction between
the vapor molecules (including Al2O vapor and O2 in this
experiment), which follows a vapor-solid (VS) growth mechanism. In this case,
the growth of the nanowires/nanobelts can be mainly attributed from the result
of reactions (1) and (2).While
the temperature is in the range of 900°C–1000°C, no
products could be collected from the inner wall of the alumina boat. In this
case, alumina nanorods and nanosheets were observed on the surface of the
residual aluminum particles (see Figures 1(a) and 1(b)). Within this
temperature range, Al2O vapor is metastable. In this case, the nucleation and growth of the alumina
nanostructures may be more dependent on the reactions between the aluminum
liquid/vapor and the oxygen inside or close to the molten aluminum surface as
shown in reactions (3) and (4). The reactions can produce tiny alumina nuclei
in the molten aluminum. Due to
poor wettability of molten aluminum with alumina nuclei [33], high surface
tension of the molten aluminum drives the alumina nuclei to the molten aluminum
surface and
prevents the nuclei from aggregation, which favors the preferetial growth of
the alumina perpendicular to the molten aluminum surface. The subsequent growth of surface nuclei could occur
through attachment kinetics with the reactant species in the dissolved phase as
illustrated in reaction (3) or with gas-phase species as illustrated in
reaction (4).With decreasing temperature (below 900°C), too low
temperature probably is not able to induce the growth of alumina in quasi one
dimension. The alumina obtained could coalesce on the molten surface to form a
crust and result in a thick film over molten aluminum.Another interesting question is why the nanowires and nanobelts can be
selectively grown. Song et al. have reported
that the partial
pressure of oxygen plays a key role in determining the morphology of the Q1D ZnO
nanostructures by VLS growth mode under vacuum conditions, in which Q1D anisotropic growth of ZnO nanowires was
promoted by controllably decreasing oxygen pressure in the reaction chamber [34]. On the other
hand, Ye et al. have suggested that the nonequilibrium kinetic
growth caused by high supersaturation level may allow the formation of a
low-surface-energy tip, which favors two-dimensional growth of the nanostructures [35]. Park
et al. have reported that transformation from Q1D growth of ZnO nanowires to Q2D
growth of ZnO nanobelts occurred with increasing supersaturation level of the
reactant vapors governed by VS growth [36]. According to Dalal et al. and Lee
et al.’s reports, higher oxygen partial pressure has been proposed as an
important factor in generating ZnO [37] and Ga2O3 nanobelts [38], respectively, in comparison
with the nanowire growth.In
our case, we propose that purge process can produce low partial pressure of
oxygen inside the chamber though the condition of air leakage, and residue
oxygen still needs to be considered, while skipping the purge process not only
keeps a high oxygen partial pressure but also may trigger a generation of high
supersaturation level of aluminum suboxide vapor during the heating process as
illustrated in the reactions (3), (4), and (1) due  to high oxygen content. So we intend to
attribute the growth of the Al2O3 nanowires to the low
oxygen partial pressure, and the growth of the Al2O3 nanobelts to the combination effect of excessive supersaturation level of the
aluminum suboxide vapor and high oxygen partial pressure in the synthesis
process.In the experiments, there are probably three
sources which provide the oxygen in the synthesis of the Q1D alumina
nanostructures: (1) the argon gas (99.999% purity), which
contains a
small amount of oxygen with the partial pressure of 10−5 atm [39], (2) residue oxygen in the reaction chamber after the
purge, (3) air leakage. Similar to the case that Park
et al. discussed [40], low oxygen partial pressure favors one-dimensional
growth of the products. After the purge
process, most of the oxygen was excluded from the chamber. However, the oxygen
supply coming from air leakage and residue oxygen inside the chamber were not
able to be effectively restrained by the argon gas while a low flow rate of
argon was applied in the growth process. With increasing argon flow rate,
oxygen partial pressure inside the chamber decreased, and gradually closed to
the internal
oxygen partial pressure of the argon gas. This may be one of the reasons why higher argon flow rate
favored the growth of thinner alumina nanowires. While Ar purge process was not conducted, oxygen
partial pressure inside the alumina tube kept
the same as
ambient environment before heating. The furnace was then heated as soon as Ar
gas was introduced. In this circumstance, oxygen partial
pressure decreased inside the alumina tube
while temperature of the furnace increased over the time. On the other hand, high supersaturation level of aluminum
suboxide might be triggered during the heating process due to high oxygen
content inside the chamber.  Depending on the supersaturation level of aluminum suboxide and oxygen partial pressure at
the target temperature, alumina nanostructures with different morphologies were
obtained. At a lower Ar flow rate, the oxygen partial pressure remained very high, and excessive
supersaturation level of the aluminum suboxide was generated when the target temperature
was achieved, allowing quick growth perpendicular to the
longitudinal axis of the product. Therefore, thick column-like structure was
produced (see Figure 4(a)). Proper argon flow rates might activate secondary growth sites and heterogeneous nucleation on the side
of the Q1D structures. This in turn resulted in Q2D growth of the
structures and thus alumina nanobelts were obtained (see Figures 4(b)
and 4(c)).
While Ar flow rate was further increased, the
oxygen partial pressure and supersaturation
level of aluminum suboxide became relative low at the target temperature, which is similar to
the nanowire growth that discussed above. Therefore, both nanowires and
nanobelts can be observed as shown in Figure 4(d). For the growth of the nanobelts, a question may arise,
to what extent the effect of the nonpurge measure can last for growing the
nanobelts because the oxygen partial pressure decreases continuously with
increasing growth time. Figure 6(a) shows a typical SEM image of the nanobelts
(from the same sample as shown in Figure 4(b)). As marked by the white arrows,
a morphological transition from the nanobelt to nanowire is observed. It
reveals that the growth of the nanobelt ends with a wire-like morphology due to
decreasing oxygen partial pressure and supersaturation level of the aluminum
suboxide. This indicates that the growth of the nanobelts is time-dependent and
needs to be carefully designed. Figure 6(b) shows a typical TEM image of the
nanobelt/nanowire structure at the wire-like end. No catalyst particle is
observed at the tip of the nanowire, which conforms to the VS growth model. The
arrow in Figure 6(b) reveals the growth direction of the product.(a) SEM image of the Al2O3 nanobelts showing the transition between the nanobelt and nanowire, (b) TEM
image of a single Al2O3 nanobelt revealing the transition
from belt-like to wire-like structure.
(a)(b)Further
investigation of other processing parameters and precise control of oxygen
partial pressure that affect the morphology of nanostructures generated in
Al–Al2O3 system is currently being undertaken in our
group.


## 4. Conclusions

We have demonstrated a facile and cost effective way to selective
synthesis of high yield uniformα-Al2O3 nanowires and
nanobelts. This catalyst and vacuum free method is promising in high throughout
production of quasi one-dimensional nanostructured alumina. The typical growth of
the nanowires and nanobelts can be ascribed to vapor-solid (VS) mechanism. It
is suggested that oxygen partial pressure and supersaturation level of the aluminum
suboxide play important roles in determining morphology of the
nanostructures. A lower oxygen partial pressure favors the formation of
alumina nanowires, while the combination effect of a relatively higher oxygen pressure and high
supersaturation level of the aluminum suboxide results in the growth of
alumina nanobelts. Such precise control over morphologies makes it possible for
potential industrial applications of nanostructured Al2O3 materials.



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*Source: 250370-2008-07-27.xml*