Metal-catalyzed growth of In 2O3 nanotowers using thermal

Vol. 36, No. 12
Journal of Semiconductors
December 2015
Metal-catalyzed growth of In2 O3 nanotowers using thermal evaporation
and oxidation method
Liu Jian(刘剑), Huang Shihua(黄仕华)Ž , and He Lü(何绿)
Physics Department, Zhejiang Normal University, Zhejiang 321004, China
Abstract: Large-scale In2 O3 nanotowers with different cross sections were synthesized by a thermal evaporation
and oxidation technique using metal as the catalyst. The morphologies and structural characterizations of In2 O3
nanotowers are dependent on growth processes, such as different metal (Au, Ag or Sn) catalysts, the relative position
of the substrate and evaporation source, growth temperature, gas flow rate, and growth time. In2 O3 nanotowers
cannot be observed using Sn as the catalyst, which indicates that metal liquid droplets play an important role in
the initial stages of the growth of In2 O3 nanotowers. The formation of an In2 O3 nanotower is attributed to the
competitive growth model between a lateral growth controlled by vapor–solid mechanism and an axial vapor–
liquid–solid growth mechanism mediated by metal liquid nanodroplets. The synthesized In2 O3 nanostructures with
novel tower-shaped morphology may have potential applications in optoelectronic devices and gas sensors.
Key words: In2 O3 nanotower; metal-catalyzed growth; thermal evaporation and oxidation; VLS growth mechanism
DOI: 10.1088/1674-4926/36/12/123007
EEACC: 2520
1. Introduction
It is well known that the physical and chemical properties of quasi-one-dimensional metal–oxide nanomaterials are
different from those of bulk materials. Among various metaloxide nanomaterials, indium oxide (In2 O3 / is an important ntype transparent semiconductor with a wide band gap of about
3.6 eV at room temperatureŒ1 . Since Wang et al.Œ2 first fabricated In2 O3 nanobelts using the method of thermal evaporation in 2001, lots of attention was paid to the synthesis and
application of In2 O3 nanostructures. So far, many methods
have been used to synthesize a variety of In2 O3 nanostructures,
such as nanowiresŒ1; 3 7 , nanorodsŒ5 8 , nanotubesŒ9; 10 , and
nanotowersŒ6; 11; 12 . The physical and chemical properties of
In2 O3 nanostructure depend on the morphology, and different
morphologies have different applications. For example, In2 O3
nanowires are applied for efficient field emission devicesŒ13
and gas sensorsŒ14 , and In2 O3 nanoparticles are for toxic gas
sensingŒ15 . Therefore, exploring novel In2 O3 nanostructures
is of great significance for further developing new functional
devices in the future.
In previous reports, most of the efforts were focused on the
synthesis and properties of morphology nanostructures. How
to effectively control the morphology of In2 O3 nanostructure
still remains a challenge due to a limited understanding of its
growth mechanism. In this article, two kinds of novel nanotowers with different cross sections were successfully synthesized
through controlling the relative position of the substrate and
indium source (i.e., adjusting the saturation of indium vapor),
and their growth mechanisms were discussed in detail. In addition, in order to illustrate the role of liquid droplets in the initial
stages of the growth of In2 O3 nanotowers, different metal catalysts (such as Au, Ag and Sn) were investigated. The growth
mechanism analysis is helpful to realize the controlled synthesis of complex nanostructures. The synthesized In2 O3 nanostructures with novel tower-shaped morphology may have potential applications in optoelectronic devices and gas sensors.
2. Experimental details
In2 O3 nanotowers with different cross sections were prepared using thermal evaporation in a conventional horizontal
quartz tube furnace, and the sketch is shown in Figure 1. The
quartz tube has a length of 100 cm and a diameter of 9.6 cm.
To prepare the samples, a high purity (99.99%) indium particle 2 mm in diameter was loaded in a crucible. A cleaned Si
(100) wafer coated with 10 nm Au thin film was placed at
the upside of the indium source, with its surface directly towards the indium particle. The perpendicular distance between
Si wafer and the indium particle was 2 cm. The other cleaned
Si (100) wafer coated with 10 nm Au thin film was horizontally placed at the downstream of the indium source at a distance of 0.5 cm. Then, the crucible was pushed into the center
of the quartz tube that was inserted into a horizontal tube furnace. Before heating, the quartz tube was evacuated to 0.8 Torr
by a vacuum pump, and then filled with 200 sccm high purity N2 for 15 min. Subsequently, the tube furnace was heated
to 1000 ıC with a heating rate of 25 ıC/min, and the heating
time is 60 min under a N2 flow rate of 200 sccm. During the
growth process, the pressure inside the tube furnace remained
at 2 Torr. After the reaction, the tube furnace was cooled to
room temperature. In order to study the effect of growth tem-
* Project supported by the National Natural Science Foundation of China (No. 61076055), the Open Project Program of Surface Physics Laboratory (National Key Laboratory) of Fudan University (No. KF2015_02), the Zhejiang Provincial Science and Technology Key Innovation
Team (No. 2011R50012), and the Zhejiang Provincial Key Laboratory (No. 2013E10022).
† Corresponding author. Email: [email protected]
Received 28 May 2015, revised manuscript received 27 June 2015
© 2015 Chinese Institute of Electronics
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Figure 1. Schematic drawing of the experimental apparatus.
perature, N2 flow rate and growth time on the morphologies of
In2 O3 nanostructures, the synthesis was carried out at a temperature of 1000–1100 ıC with a flow of 50–200 sccm and a
growth time of 5–60 min, respectively. Furthermore, in order
to clarify the growth mechanism of In2 O3 nanotowers, In2 O3
nanostructures were prepared with other metal (Ag and Sn) catalysts.
The morphologies of as-prepared samples were characterized by field-emission scanning electron microscopy (SEM,
Hitachi S-4800). The structural characterizations were investigated by X-ray diffractometer (XRD, X‘Pert PW 3040/60,
Philips) and Raman spectrometer (Renishaw 2000), respectively. In order to evaluate the electrical properties of asprepared samples, current–voltage (I –V / characteristics were
measured by a digital source meter (Keithley 2601) controlled
automatically by a computer. Furthermore, the photoluminescence (PL) properties of as-prepared samples were investigated
at room temperature by using a He–Cd laser line of 325 nm as
an excitation source.
3. Results and discussion
Two kinds of In2 O3 nanotowers with different cross section were successfully synthesized, and their SEM images are
shown in Figure 2. When a Si wafer coated with 10 nm Au
thin film (Si wafer A) was placed at the upside of the indium
source, the tapered In2 O3 nanotowers can be observed on the
surface of the Si wafer, as shown in Figure 2(a). From the magnified SEM image shown in the inset of Figure 2(a), the tapered
nanotower is a periodic layered structure with a decreasing size
from bottom to tip, and each layer of the pillar is the truncated
octahedral configuration. The tapered nanotower with the octagon cross section is defined as nanotower I. The diameter of
nanotower I is about 1–2 m, and the length is about 5–10 m.
However, when Si wafers coated with 10 nm Au thin films
(Si wafer B) were horizontally placed at the downstream of the
indium source, tapered nanotowers with a tetragon cross section (defined as nanotower II here) were observed, as shown in
Figure 2(b). The insets of Figure 2(b) show that nanotower II is
also a periodic layered structure, and has similar morphology
to nanotower I except for the different cross section. It should
be pointed out that no Au nanoparticles were observed at the
tip of nanotowers I and II according to the elemental analysis
results obtained from the SEM-EDS system (not shown here).
Figure 3 shows XRD patterns of the as-synthesized nanotowers. All the marked diffraction peaks in the two samples can be indexed to the body-centered cubic (bcc) structural
Figure 2. SEM images of as-synthesized In2 O3 nanotowers with the
substrates placed at the (a) upside and (b) downstream of the indium
source.
In2 O3 with a lattice constant of 1.011 nm (JCPDS card No.
06-0416). No other impurities (such as In and Au) were detected, revealing a high phase purity of In2 O3 . The diffraction
peak (400) is the most intensive, indicating that most of the obtained In2 O3 nanostructures are preferentially oriented along
the h100i direction.
The Raman scattering measurements were carried out to
further investigate the local structure of the synthesized In2 O3
nanotowers, as shown in Figure 4. Six Raman scattering peaks
located at 109, 133, 307, 367, 496 and 630 cm 1 belong to the
vibration modes of bcc-In2 O3 , which agree well with the reported values in References [16, 17]. The peak at 520 cm 1
which corresponds to the TO phonon mode of the Si crys-
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Liu Jian et al.
Figure 3. XRD pattern of as-synthesized In2 O3 nanotowers with the
substrates placed at the (a) upside and (b) downstream of the indium
source.
Figure 4. Raman spectra of as-synthesized In2 O3 nanotowers with the
substrates placed at the (a) upside and (b) downstream of the indium
source.
tal structure originates from the Si substrates. This agreement
between XRD and Raman results indicates that the obtained
In2 O3 nanotowers are stable in the pure body-centered cubic
(bcc) phase structure without any phase transformation.
Figure 5 shows the I –V curves of the synthesized In2 O3
nanotowers measured in air ambient at room temperature. The
two curves appear to be linear, indicating that an Ohmic contact forms between these nanostructures and Au electrodes. It
can be calculated that the resistances of nanotowers I and II
are 0.776 k and 1.329 k, respectively. As we know, the
conductivity of the In2 O3 nanostructures is strongly related to
surface-adsorbed oxygen moleculesŒ18 . Upon exposure to air
ambient, the electrons in In2 O3 nanostructures ionize the oxygen source from the air to form an oxygen surface layer, which
results in the formation of an electron depletion layer. This process can significantly reduce the conductivity of the In2 O3 nanotowers. As shown in SEM images in Figures 2(a) and 2(b),
nanotower II is longer and denser than nanotower I, so it has
a larger surface-to-volume ratio, which causes the higher resistance. For nanotower II, much more oxygen molecules are
absorbed to the surface of the nanotowers, which generates a
broader electron depletion layer.
Figure 5. I –V curves of as-synthesized In2 O3 nanotowers with the
substrates placed at the (a) upside and (b) downstream of the indium
source.
Figure 6. The room temperature photoluminescence spectra of the assynthesized In2 O3 nanotowers with the substrates placed at the (a)
upside and (b) downstream of the indium source.
Photoluminescence (PL) properties of In2 O3 nanotowers
were also investigated at room temperature, and the results are
shown in Figure 6. PL spectra of the as-synthesized In2 O3 nanotowers exhibits three strong PL peaks centered at 469 nm
(blue emission), 556 nm (green emission) and 607 nm (orange emission) in the range of the visible light region. Furthermore, PL intensity of nanotower II is stronger than that
of nanotower I. As we know, the bulk In2 O3 does not exhibit
the PL property at room temperatureŒ19; 20 . During our experiments, In2 O3 nanotowers are prepared by the thermal evaporation and oxidation technique. Therefore, oxygen or indium vacancies would be generated in these In2 O3 nanotowers because
of partially incomplete oxidation and crystallization. The blue
emission in In2 O3 nanostructures has been observed by another
groupŒ21 , which is mainly attributed to the effect of the oxygen
and indium vacancies. The green–orange emission is attributed
to the deep-level emission which can be assigned to the defect stated induced emission in In2 O3 nanotowers. While the
607 nm orange emission is attributed to the radiative recombination between the electron on oxygen vacancy and the hole
on oxygen vacancy center in the In2 O3 nanotowersŒ19 .
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Figure 7. SEM images of In2 O3 nanotowers synthesized under different growth temperatures, N2 flow rates and growth times. (a) 1000 ıC,
200 sccm, 60 min. (b) 1100 ıC, 200 sccm, 60 min. (c) 1000 ıC, 100 sccm, 60 min. (d) 1000 ıC, 50 sccm, 60 min. (e) 1000 ıC, 200 sccm, 5 min.
(f) 1000 ıC, 200 sccm, 15 min.
Figure 7 shows SEM images of In2 O3 nanotowers synthesized under different growth temperature, N2 flow rate and
growth time while the substrates were placed at the downstream of the indium source. When the growth temperature increases from 1000 to 1100 ıC, the average diameter of the obtained In2 O3 nanotowers becomes larger, as shown in Figures
7(a) and 7(b). This is because at a higher reaction temperature,
the Au layer on the substrate tends to agglomerate and form
larger catalyst nanoparticles. Furthermore, the obtained In2 O3
nanotower always presented a tapered and multilayered nanostructure with a decreasing size from the bottom to the top. As
shown in Figures 7(a), 7(c) and 7(d), we found that the size
and morphologies of the In2 O3 nanotowers are significantly
affected by the N2 flow rate. With the increase of the N2 flow
rates, the size of the In2 O3 nanotower increases. The reason
is that the larger N2 flow rate means a higher concentration of
indium vapor is carried to the substrate. It should be noted that
some of the In2 O3 nanotowers synthesized under the N2 flow
rate of 50 sccm have bent morphologies, as shown in the inset
of Figure 7(d). This may be caused by the indium vapor fluctuating more easily under the N2 flow rate of 50 sccm. While the
N2 flow rate is increased to 100 or 200 sccm, some nanotowers
ended with a truncated octahedral cap can be clearly observed
in Figures 7(c) or 7(a). SEM images of the samples prepared
at 1000 ıC, 200 sccm for 5, 15 and 60 min are shown in Figures 7(e), 7(f) and 7(a), respectively. We can see that the surface
morphologies of the as-prepared samples change gradually as
the growth time is increased. At the growth time of 5 min, irreg-
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Figure 8. SEM images of In2 O3 nanostructures grown with (a) Ag catalyst and (b) Sn catalyst.
Figure 9. Schematic illustration for the growth mechanism of In2 O3 nanotowers I and II.
ular In2 O3 nanopillars are formed on the substrate. When the
growth time increases to 15 min, the irregular nanopillars transform to large-scale nanotowers. The In2 O3 nanotowers have
a periodic layered structure, and each layer of the nanotowers has the truncated octahedral configuration. Further, when
the growth time increases to 60 min, the morphology of In2 O3
nanotowers remains unchanged but the average size becomes
larger.
4. Growth mechanism
In order to explain the growth mechanism of nanotower,
In2 O3 nanostructures induced by other metals (such as Ag and
Sn) are also studied, and their SEM images are shown in Figure 8. In Figure 8(a), a large number of irregular In2 O3 nanotowers can be observed on the substrate using Ag as the catalyst, different from those using Au as the catalyst. However,
using Sn as the catalyst, no nanotowers can be observed, as
shown in Figure 8(b). This is because Sn is easily oxidized
at 1000 ıC in the tube furnace under the pressure of 2 Torr.
Hence, there are no Sn liquid droplets forming on the substrate.
However, Au or Ag are relatively more stable than Sn, and is
easy to form Au or Ag liquid droplets at a growth temperature
of 1000 ıC. This indicates that liquid droplets play an important
role in the initial stages of the growth of In2 O3 nanotowers.
According to the experimental results of SEM-EDS and
XRD, no Au catalyst droplet can be observed at the tip of In2 O3
nanotowers. Therefore, the growth process of In2 O3 nanotowers should not be interpreted as the VLS mechanism. To sum
up what has been discussed above, a competitive growth mechanism, i.e., periodical 1-D growth controlled by a VLS mechanism and persistent 0-D growth controlled by a VS mechanism
has been proposed to explain the formation of In2 O3 nanotowers, which is similar to Yan et al.Œ10 . The schematic diagram
of the growth mechanism is illustrated in Figure 9.
At the first stage of heating, the spherical Au liquid
droplets (Figure 9(a)) are first formed on top of the Si (100)
substrate surface and then used as a catalyst for subsequent
growth of In2 O3 nanotowers. Then, the indium vapor gradually evaporated out of the indium particle dissolved into the
Au liquid droplets to form Au–In alloy droplets. The concentration of indium vapor was insufficient at this stage, and was
not enough to maintain the continuous 1-D growth, which led
to the 0-D growth beginning. For In2 O3 with a bcc structure, it
is well-known that the growth rates perpendicular to different
planes are proportional to their surface energies. The relationships among three low-index crystallographic planes of surface energies correspond to f111g < f100g < f110g Œ22; 23 .
So, the (110) plane with the high energy plane would disappear
earlier, meanwhile, the (111) plane with the low energy plane
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would be preserved. Finally, an octahedron with its bottom and
the other five corners truncated (Figure 9(b)) for nanotower I or
an octahedron with its bottom truncated only (Figure 9(b0 )) for
nanotower II is formed on the substrate through the VS mechanism.
To the development process of those truncated octahedrons, we speculated that it was mainly caused by different
saturation of indium vapor. Because Si wafer A is further away
from the indium source than Si wafer B, the saturation of indium vapor around Si wafer A is lower than that around Si
wafer B. When the reaction proceeds, abundant indium was
evaporated, leading to a relatively higher saturation vapor adsorbed onto the truncated octahedron and finally reaching the
critical value of 1-D growth along the h100i crystalline direction. With the beginning of the 1-D growth mediated by Au
liquid nanodroplets, the saturation of indium vapor starts to decrease, and then a new 0-D growth starts again. It should be
noticed that, during the whole growth process, the 0-D growth
is continuous without interruption. When a new 0-D growth
restarted again, a new part of the nanotower came into being
(Figures 9(c) and 9(c0 )). Finally, the periodical 1-D and persistent 0-D growth resulted in the formation of nanotowers I and II
(Figures 9(d) and 9(d0 )). The decreasing size of the nanotower
from bottom to tip could be ascribed to the different growth
timeŒ12 . The growth time of nanotowers with the truncated octahedron formed at the bottom is longer than ones at the top,
thus it certainly possesses a larger size. An octahedral shaped
cap formed on the top of some nanotowers can be ascribed to
a low saturation of indium vapor when the tube furnace began
to cool downŒ24 .
5. Conclusions
In summary, two kinds of In2 O3 nanotowers with octagon
and tetragon cross section were successfully synthesized using
Au catalyst by the thermal evaporation and oxidation method.
Raman and XRD spectra reveal that the obtained In2 O3 nanotowers are stable in the pure body-centered cubic (bcc) phase
structure. The room temperature photoluminescence spectra
of the obtained In2 O3 nanotowers exhibits three strong luminescence bands centered at 469 nm, 556 nm, and 607 nm in
the range of the visible light region. The size and morphologies of In2 O3 nanotowers are mainly affected by the N2 flow
rate and growth time, whereas the growth temperature only
changes the diameter of In2 O3 nanotowers. Finally, a competitive growth mechanism, i.e., periodical 1-D growth controlled
by a VLS mechanism and persistent 0-D growth controlled
by a VS mechanism has been proposed to explain the formation process of the In2 O3 nanotower with a different cross section. The synthesized In2 O3 nanostructures with novel towershaped morphology may have potential applications in optoelectronic devices and gas sensors.
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