1) The document reports on the tunable growth of indium oxide nanostructures from nanoflute structures containing discrete nanocavities to metal-filled nanotubes. 2) By controlling the partial pressure of indium and oxygen in a chemical vapor deposition process, the morphology can be tuned from nanoflute structures containing hexagonal nanocavities to continuous indium-filled nanotubes. 3) The growth mechanism involves the initial formation of nanoflute structures, followed by out-diffusion of indium metal at lower partial pressures resulting in discrete nanocavities. At higher partial pressures, the cavities connect to form continuous indium-filled nanotubes.

1. Article
pubs.acs.org/JPCC
Tunable Growth of Indium Oxide from Nanoflute to Metal-Filled
Nanotubes
Mukesh Kumar,*,†,‡ V. N. Singh,† B. R. Mehta,† and J. P. Singh*,†
†
Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India
‡
Department of Electrical Engineering, South Dakota State University, EECS Building, Box 2222, Brookings, South Dakota 57007,
United States
ABSTRACT: Here, we report a tunable morphological
transformation of indium oxide (IO) nanoflute, that is,
nanowires embedded with discrete and hollow nanocavities
to indium-filled IO nanotubes by simply controlling the In/O
partial pressure in a thermal chemical vapor deposition
method. The growth of nanoflute is observed as a prestage
for the growth of indium-filled IO nanotubes. The out-
diffusion of indium metal at lower In/O partial pressure results
in the growth of discrete and hexagonal-shaped nanocavities
enclosed by minimum surface energy planes, {111}, which get
connected together for the formation of continuous and indium-filled IO nanotubes at higher In/O partial pressures. The tunable
and controlled growth of indium-filled IO nanotubes may have potential applications in future nanodevices.
I. INTRODUCTION of IO nanowires having hexagonal nanocavities, while
One-dimensional metal oxide nanostructures have attracted comparatively higher indium partial pressure results in the
great attention in recent years because of their potential use in growth of In-filled IO nanotubes.
scientific and technological applications. These nanostructures
II. EXPERIMENTAL DETAILS
offer an opportunity to investigate the effect of dimensionality
and size on the material’s physical properties.1−5 Among them, The vapor phase transport and condensation in the presence of
one of the important metal oxides, indium oxide (IO), has been reducing reagents ethanol have been used for the growth of IO
synthesized in different morphologies due to their wide band nanowires having hexagonal nanocavities and indium-filled IO
gap (∼3.7 eV) and high transparency in the visible region (80− nanotubes on the Si(100) substrate. The In2O3+C (1:1) mixed
90%), which makes them suitable for a number of applications powder as a precursor was placed in an alumina boat and
in nanoelectronics and optoelectronic devices.6−8 Besides the inserted into the heating zone of quartz tube open at both ends.
advantage of IO pyramids, nanowires, and nanorods in a The system was heated to 1000 °C at a rate of 20 °C/min.
number of applications,7,9−12 particularly nanotubes have their Silicon substrates coated with about ∼5 nm gold film were
own advantage and may offer unique applications.13,14 In the placed downstream at a temperature of 960 °C. A small
past few years, much effort has been paid in developing hollow reservoir (5−10 mL) of ethanol was placed in the low
nanostructures including nanotubes and hollow nanoparticles temperature region (∼65 °C) in the upstream direction during
owing due to their unique morphology, which shows their the growth. The partial pressure of reaction species, In and O,
applications in delivery vehicle system, nanofilters, nano- during growth was varied by controlling the flow rate of the
thermometer, and mass sensor with an atomic accu- carrier argon gas at (i) 30 sccm at a deposition pressure of 1
racy.1,13,15−21 The tubular nanostructures with nanoporosity/ Torr (set I) and (ii) 200 sccm at a deposition pressure of 760
nanocavities were reported to have entirely different properties Torr (set II). The deposition time was 1 h, and argon gas flow
than their regular shape and have shown unique enhanced was stopped after the deposition. The furnace was switched off
catalytic and superior optical absorption applications.22 Besides and cooled to room temperature.
the vast applications of hollow nanostructures, there is no clear The morphological and structural details of the grown IO
mechanism for the growth of metal oxide nanotubes except for nanostructures were studied using scanning electron micros-
a few attempts reported in the literature.23−25 For example, copy (SEM) (Zeiss EVO 50) and high-resolution transmission
Morin et al. recently proposed a screw dislocation-driven electron microscopy (HRTEM, Tecnai G20-Stwin at 200 kV).
growth mechanism for single-crystalline ZnO nanotubes.23 The scanning transmission electron microscopy (STEM) in
In the present study, we propose a unified growth model for
the nanotube formation. We argued that In/O partial pressure Received: December 4, 2011
during the growth determines the final IO tubular morphology. Revised: February 11, 2012
The low partial pressure of indium vapors results in the growth Published: February 13, 2012
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2. The Journal of Physical Chemistry C Article
nanoprobe mode with energy dispersive X-ray (EDX) facility
was used to determine the In/O stoichiometry across the
nanostructures. For precise measurements, these one-dimen-
sional nanostructures were scanned after each EDX data
collection, and the experiment was repeated three times. The
morphology of IO nanostructures changes dramatically for set I
and set II as the carrier gas flow rate and pressure increase. For
gas sensing measurement, the interdigitated electrodes of gold
were thermally deposited on top of the nanostructures and
mounted on the PID controlled heater with the temperature
accuracy of ±1 °C (Excel Instruments). The gas sensing
measurements was performed at 142, 221, and 300 °C with
1000 ppm ethanol. Prior to gas sensing measurement, the
sample was heated at 350 °C for 3 h in cyclic flow of ethanol
and air, respectively, to eliminate the effect of humidity and for
the stability of the sensor. After each cycle, the dry air was
passed over the sensor to avoid the moisture from the ambient
as well as remove the ethanol gas from the chamber. The sensor Figure 2. (a) TEM image of nearly equally spaced nanocavity in IO
response for ethanol gas is calculated as the ratio of resistance nanowire and (b) TEM image of In-filled IO nanotube. Inset of (a)
in air to that in the ethanol gas (Rair/Rgas), where Rair is the and (b) shows higher magnified TEM micrograph of single nanocavity
resistance of sensor in air and Rgas is the resistance in ethanol and In-filled cavity in IO nanotube, respectively. (c) HRTEM of
gas ambient, while the response time is taken as the time nanocavity shows lattice fringes of spacing 0.41 nm, which corresponds
to the (211) plane of IO, and the inset reveals the HRTEM that
required to reach 90% of the saturated value of resistance in the confirms the growth direction of nanowires along the ⟨100⟩ direction.
ethanol gas. (d) HRTEM of In-filled nanotube showing side walls and In-filled
nanocavity. The HRTEM confirms the growth of IO nanotubes along
III. RESULTS AND DISCUSSION the ⟨100⟩ direction.
A. Growth of Nanoflute and Indium-Filled IO Nano-
tubes. Figure 1 shows the TEM micrographs of set I and set II and are nearly equally spaced along the nanowire length.
Interestingly, the nanocavity has a hexagonal shape, as shown in
the inset of Figure 2a. The size of the nanocavity was measured
over more than 100 nanowires and found to vary from 10 to 30
nm with an peak average size of 10−15 nm. Figure 2b shows
the TEM micrograph of IO nanostructure grown with
comparatively higher argon flow rate and at an atmospheric
pressure that resulted in an arrow-like tubular nanostructure.
The structure has a uniform cylindrical base with a typical
diameter of 95 nm and an octahedron tip having the radius of
curvature as low as 5 nm at the end. The cavity and wall
thickness of various IO tubular structures vary from 10 to 60
Figure 1. (a) TEM micrographs of nanocavity decorated IO nanowires nm and 35 to 100 nm, respectively. The HRTEM micrographs
and (b) indium-filled IO nanotubes. The inset shows the of the tip of the IO nanostructures are shown as insets in Figure
corresponding SEM images. The lower In partial pressure during 2b. The images reveal the tubular nature of the structure, which
growth resulted in the growth of nanocavities decorated IO nanowires, is filled with indium metal up to the tip end, which is later
while the higher In partial pressure ended with the In-filled IO
confirmed with STEM-EDX measurements. A high-resolution
nanotubes.
TEM image of a single nanowire with nanocavity is shown in
samples along with the SEM images in the inset. Set I samples Figure 2c, which shows the lattice planes. This confirms that
show nanowires grown with hexagonal nanocavities, Figure 1a, the nanocavity resides within the nanowire body but not on the
surface of the nanowire as in the case of titania nanorods as
whereas set II samples show a completely different morphology
reported in literature.22 The interplanar spacing of 0.41 nm in
of indium-filled IO tubular nanostructures, Figure 1b. The
nanowire corresponds to the (211) plane of cubic IO. HRTEM
dense and uniform growth of IO nanostructures in sets I and II of IO nanowire, inset of Figure 2c, reveals that the lattice
is clearly observed in SEM images. The nanowires and fringes correspond to the (200) plane of IO and clearly shows
nanotubes were observed to have nearly the same average the ⟨100⟩ growth direction of nanowires with nanocavity
length of ∼5−12 μm, but their diameters vary from 25 to 90 structures. Figure 2d shows the HRTEM for IO nanotube. The
nm for set I samples and from 40 to 250 nm for set II samples, lattice fringes of IO shell planes and interplanar spacing of two
respectively. The TEM micrograph of set I sample in Figure 1a orthogonal planes as 0.506 and 0.71 nm correspond to (200)
clearly shows the presence of nanocavities within the nanowire and (110) planes of IO nanotube, respectively. Interestingly,
structures, while the TEM micrograph of set II reveals the both nanostructures show the same growth direction of
tubular nanostructures. ⟨100⟩.26,27 The presence of nanocavity in IO nanowire and
A TEM image of a single nanowire having the nanocavities is indium-filled nature of IO nanotubes is confirmed using
clearly shown in Figure 2a along with the high-resolution TEM scanning transmission electron microscopy and energy
image of the single nanocavity as in the inset. The TEM dispersive X-ray analysis (STEM-EDX) study, which is shown
micrograph reveals that these nanocavities are uniform in size in Figure 3a,b. Figure 3 a shows the EDX line profile with an
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3. The Journal of Physical Chemistry C Article
Figure 3. STEM EDX along the radial direction of (a) nanocavities decorated IO nanowires and (b) In-filled IO nanotube along with STEM image
as an inset. The variation of In and O across the nanostructure confirms the presence of nanocavity in nanowire and the In-filled nature of IO
nanotube, respectively.
Figure 4. (a,b) Precursor stage, nucleation stage, growth stage, and final morphology stage for the growth of nanocavities decorated IO nanowires
and In-filled IO nanotube. The different filled spheres in the precursor stage are the symbolic representation of different reaction species. [●]
indium; [gray ●] oxygen; [one gray, two black cluster] oxygen deficient IO; [three gray, two black cluster] IO.
inset showing a line through a single nanocavity. The variation substrate. As the adsorption of reaction species continues, the
of In and O across the nanocavity clearly confirms the presence growth of thermodynamically allowed crystallographic planes
of nanocavity in IO nanowire. The STEM-EDX measurement takes place. Hao et al. have mentioned that the growth rate (r),
on IO tubular structure along its radial direction is shown in perpendicular to three different low-indexed crystallographic
Figure 3b along with the STEM image as the inset. The higher planes in IO, is proportional to the surface energies, γ{111} <
In/O ratio in the central region confirms that the IO nanotube γ{100} < γ{110}.26 So, the growth rate perpendicular to the
is filled with In metal. {111} planes, that is, r{111}, is comparatively slower, and the
B. Growth Mechanism. It was reported that the two {111} planes have a tendency to appear as facets on the fast
different growth mechanisms, vapor−solid (VS) or vapor− growing {110} and {100} planes.26,30 Therefore, as the
liquid−solid (VLS), for the growth of nanostructures are deposition continues, different planes grow according to their
strongly influenced by the reaction kinetics.28,29 In our previous surface energy and lead to the formation 1D IO nanostructures
study, we have established that the partial pressure of incoming with growth direction along ⟨100⟩, which is confirmed by
reaction species changes the growth mechanism from VLS to HRTEM images as shown in Figure 2c and d for both
VS in a conventional vapor transport method.30 Therefore, the nanostructures. The lower partial pressure of In metal in the
sustainability of VLS growth is affected by the partial pressure growth reduced the possibility of In-rich core during the
of the incoming reaction species. In the present case, IO+C precursor state. On the other hand, the indium species that are
carbothermally reduced to oxygen-deficient In2O3−x, and the incorporated and embedded during the growth of nanowires
reducing ambient, ethanol, further makes the growth ambient have a tendency to out-diffusion and contributes in the growth
indium rich. In set II growth conditions, the higher carrier gas of IO at the surface of IO nanowires and leave the vacancies
flow rate at 200 sccm and atmospheric growth pressure, 760 behind, which is shown in the nucleation stage in Figure 4a.
Torr, may increase the partial pressure of In/O vapors in The low growth pressure, nearly 3 orders of magnitude less
contrast to set I growth conditions of gas flow rate at 30 sccm than the atmospheric pressure in set II growth conditions, may
and nearly 3 orders lower growth pressure of 1 Torr. The also promote the out-diffusion of In. The out-diffusion of metal
change in the ratio of In/O partial pressures along with the in the formation of the nanocavity has already been discussed
difference in the pressure of growth chamber may be for the growth of cobalt oxide and iron oxide hollow
responsible for this structural transformation. nanoparticles and nanocrystals.19,31 The out-diffusion of the
The different stages such as precursor, nucleation, growth, In metal during growth may be sustained and supported by the
and final morphology stages are shown in Figure 4. In the lower growth pressure of 1 Torr. As the out-diffusion continues,
nucleation stages, the reaction species condense on the silicon the numbers of vacancies increase and no longer separate, but
substrate and form the nuclei of oxygen-deficient IO on silicon they coalesce inside IO nanowire bulk and start to form a
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4. The Journal of Physical Chemistry C Article
nanovoid structure, as shown in the growth stage in Figure 4a. of the above-mentioned factors result in the formation of a
These nanovoids combine and form a nanocavity within IO bottom-truncated octahedron. As the reaction proceeds further,
nanowires bulk; the final morphology of nanocavities decorated the reagent species get adsorbed directly at the bottom and on
IO nanowires is shown in the morphology stage in Figure 4a. the facet planes of the growing structure. Because the facet
The self-catalytic grown nanowires consist of nanocavities with planes of the octahedrons are the minimum surface energy
the cavity size varying from 10 to 30 nm and have hexagonal planes and have low binding energy and high desorption
appearance, as shown in Figure 2a. More specifically, the probability of incoming reaction species, therefore, the growth
hexagonal appeared nanocavity may be the projection of three- along these plane is minimum.35 It is quite probable that the In-
dimensional octahedron cavities inside the IO nanowire along rich reaction species initially adsorbed on the facets of the
the [111] direction.32 The octahedron cavity is truncated in octahedrons are transferred to the bottom of the structure. As
nature as is clearly revealed from the inset of Figure 2a. The the growth continues, the excess In may phase separate from
growth of truncated octahedron nanocavity has also been In2O3 and get encapsulated inside the octahedron structures.
reported in Si at SiC thin film/Si substrate by annealing at 1050 Indium inside the core is sucked from the base of the growth,
°C.32 Some of the nanocavity in IO nanowires gives a circular due to the good wetting properties between In and IO surfaces
appearance, but that could be due to the more complex nature and the capillary effect in the nanosize cavity.25 Figure 4b shows
of polygon nanocavity in IO nanowires. The growth of IO the growth stage of In-filled IO nanotube. Because of the low
nanowires with nanocavity is governed by self-catalytic vapor− melting point of indium metal filled inside the IO nanosize
solid growth followed by the out-diffusion of indium-rich cavity, the large vapor pressure at growth temperature may
species from IO nanowires and the formation of nanovoids, the exert a continuous thrust in upward growth direction. The
Kirkendall effect.29,31−34 The hexagonal shape of the nanocavity vapor pressure of macroscopic indium heated at 923 °C is 1
may be considered as due to the cubic symmetry of IO crystal. Pa.36 It has been reported that the vapor pressure has an inverse
It is understandable that the minimum energy planes like (111) dependence on the diameter of the cavity of nanotube
may form the facets for the growth of IO octahedron structures.37 Therefore, the large upward thrust of melted
structures.26−30 The growth of nanocavity may be considered indium in the nanosize cavity may result in the growth of
similar to the growth of IO octahedron structures having the indium-filled IO nanotube.
{111} facets but with no IO atoms.32 In 3D, nanocavity may In the case of nanocavities decorated IO nanowires where the
visualize as the truncated octahedron structures with no IO growth pressure is nearly 3 orders of magnitude smaller than
atoms in IO nanowires. The angle between {111} and the other the atmospheric pressure, the initial In embedded may be at
occurred planes is ∼35°, which indicates the possibility of nanoscale. Figure 5a reveals the observed initial diameter of In
{110}, which is another higher surface energy plane in IO. So,
the truncated octahedron nanocavity is enclosed with {111}
planes and {110} planes. The (110) plane is not the minimum
energy plane, but the occurrence of these planes may be
energetically possible because it may reduce the entire surface
energy of the nanocavity.32 The free energy per unit volume
favors to enlarge the nanocavity, but the surface energy of
surrounded cogrowth IO planes disfavors that. So, the total
minimization energy determines the size of the nanocavity in
IO nanowires. It is important to note that the size of different
nanocavities in IO nanowires has the maximum size in the
range of 10−15 nm in IO nanowires. The formation of Figure 5. (a) Initial stage of nanovoids formation for the final growth
hexagonal shape voids has also been reported in Si(111).32 In of nanocavities decorated IO nanowires. (b) Partially In-filled IO
Si(111), the presence of higher surface energy planes was nanotube.
attributed to the higher substrate temperature during growth
(1050 °C) in comparison to the 850 °C, which results in the embedded during the growth of nanocavities embedded IO
formation of triangular voids enclosed within (111) planes. In nanowires and confirmed the proposed growth mechanism,
our case, the growth temperature is 960 °C, which is higher while the presence of indium in IO nanotubes was confirmed
than the 850 °C and, that may promote the appearance of using STEM-EDX elsewhere.38 Nearly 2−4 nm voids at growth
higher surface energy planes to enclose the nanocavity in IO temperature may exert an order of magnitude higher pressure
nanowires and reduce the total surface energy of nanosize than nanocavity of 10−60 nm in In-filled IO nanotubes and
cavity. As In/O partial pressure increases by increasing the gas results in the In out-diffusion and hence the formation of
flow rate, 200 sccm, and working pressure, 760 Torr, the nanoscale voids (Figure 5a). These voids further coalesce and
growth ambient becomes In-rich and supports the growth of form the hexagonal nanocavity inside the IO nanowire. Some of
indium-filled IO nanotubes. The different stages during the the nanotubes grown with set II were observed partially filled
growth of indium-filled IO nanotube are shown in Figure 4b. with In, Figure 5b, indicating the possibility of In out-diffusion
The precursor stage in Figure 4b describes the formation of In- from the thin IO shell during the growth.38
rich nuclei on the silicon substrate. With time, the excess Finally, the representative gas sensing response of nanocavity
indium has a tendency to re-evaporate and/or oxidize except at decorated IO nanowires is shown in Figure 6a. The inset shows
the vicinity of substrate. The bottom of the growing structure is the electrodes used to measure the gas sensing. The effect of
in contact with the substrate and does not get sufficient oxygen substrate temperature on gas sensor response and response
to form the IO. So, initially the growth along this plane is time for both IO nanoflute and IO nanotube structures is
relatively slower. Other IO planes are expected to grow shown in Figure 6b. The sensor response of IO nanoflute and
according to the surface energy value: γ{111}< γ{100}< γ{110}.26 All indium-filled IO nanotubes increases from 2.5 to 18 and from
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5. The Journal of Physical Chemistry C Article
Figure 6. (a) The representative sensing response of nanocavity decorated nanowires measured with gold interdigitated electrodes at 300 °C for
1000 ppm ethanol and (b) sensor response and response time for nanocavity decorated nanowires and indium-filled IO nanotubes as a function of
temperature.
1.1 to 3.7, while the response time decreases from 16 to 3 s and pressure than atmosphere in the growth chamber induced the
50 to 20 s, respectively, as temperature increases from 142 to out-diffusion of indium metal and results in the formation of
300 °C. The interaction of ethanol gas and chemisorbed oxygen indium voids in the growing IO nanowire. As the growth
on the surface of IO nanostructures is responsible for the continues, these nanovoids coalesce, and this results in the
change in the resistance of sensor. The possible reaction growth of discrete and hexagonal-shaped nanocavities enclosed
between ethanol and chemisorbed oxygen is as follows:39 by minimum surface energy planes of IO, {111}. The
atmospheric growth pressure and nearly 1 order of magnitude
6O− + C2H5OH(g) → 3H2O(g) + 2CO2 (g) + 6e− higher gas flow rate result in a higher In/O partial pressure and
cause the formation of continuous and indium-filled IO
The reactions produce more electrons and thus increase the
nanotubes.
■
conductivity of n-type IO nanostructures upon exposure to
ethanol. The temperature-dependent sensor response can be
understand on the basis of temperature-dependent interaction AUTHOR INFORMATION
of ethanol and chemisorbed oxygen on the surface of Corresponding Author
nanowires. The surface of oxide nanostructures can be covered *E-mail: mukesh.iitdelhi@gmail.com (M.K.); jpsingh@physics.
by chemisorbed oxygen in the form of O2−, O−, or O2− iitd.ac.in (J.P.S.).
depending on the sample temperature. The O2− is the most
unstable due to its high charge.40 Therefore, the possibility of Notes
its occurrence at the surface is very small. The O2− is the The authors declare no competing financial interest.
■
dominant species near room temperature, but the surface
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