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Article
In(OH)3 and In2O3 Nanorod Bundles and Spheres: Microemulsion-Mediated
Hydrothermal Synthesis and Luminescence Properties
Jun Yang, Cuikun Lin, Zhenling Wang, and Jun Lin
Inorg. Chem., 2006, 45 (22), 8973-8979• DOI: 10.1021/ic060934+ • Publication Date (Web): 05 October 2006
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In(OH)3 and In2O3 Nanorod Bundles and Spheres:
Microemulsion-Mediated Hydrothermal Synthesis and Luminescence
Properties
Jun Yang, Cuikun Lin, Zhenling Wang, and Jun Lin*
Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and
Graduate School of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
Received May 29, 2006
Indium hydroxide, In(OH)3, nano-microstructures with two kinds of morphology, nanorod bundles (around 500 nm
in length and 200 nm in diameter) and caddice spherelike agglomerates (around 750−1000 nm in diameter), were
successfully prepared by the cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/n-pentanol microemulsion-
mediated hydrothermal process. Calcination of the In(OH)3 crystals with different morphologies (nanorod bundles
and spheres) at 600 °C in air yielded In2O3 crystals with the same morphology. X-ray diffraction, scanning electron
microscopy, transmission electron microscopy, and photoluminescence (PL) spectra as well as kinetic decays were
used to characterize the samples. The pH values of microemulsion play an important role in the morphological
control of the as-formed In(OH)3 nano-microstructures from the hydrothermal process. The formation mechanisms
for the In(OH)3 nano-microstructures have been proposed on an aggregation mechanism. In2O3 nanorod bundles
and spheres show a similar blue emission peaking around 416 and 439 nm under the 383-nm UV excitation, which
is mainly attributed to the oxygen vacancies in the In2O3 nano-microstructures.
1. Introduction
Semiconductor nanostructures have been attracting in-
creasing attention because of their exceptional properties,
which differ from those of their bulk counterparts, and their
potential applications in optoelectronic devices. Among them,
indium oxide (In2O3) has been investigated extensively for
its semiconducting properties. In2O3 is a very important wide-
band-gap (direct band gap around 3.6 eV), n-type transparent
semiconductor and has been widely used in microelectronic
areas including window heaters, solar cells, liquid-crystal
displays,1 and ultrasensitive gas sensors for detection of O3,2
CO2,3 H2,3,4 NO2,4 and Cl2.5 Inorganic particles often show
unique size- and shape-dependent properties. For instance,
the gas-sensing ability of In2O3 has been shown to increase
significantly by decreasing its particle size.6 It is imaginable
that size- and shape-controllable growth of In2O3 nanopar-
ticles might pave the way to further elevate its performance.
Up to now, various morphologies of In2O3 have been
synthesized via different methods. For example, In2O3
nanobelts through thermal evaporation,7 In2O3 nanowries by
using the vapor-liquid-solid technique,8 In2O3 nanoparticles
prepared from thermal decomposition,9 tin-doped In2O3
whiskers formed by the electron shower physical vapor
deposition process,10 In2O3 nanowire arrays,11 and nanorods12
induced by template-assisted growth have been reported.
However, there are relatively few reports about the hydro-
thermal synthesis of In2O3 nanostructures.13,14
In recent years, soft templates such as reverse micelles or
microemulsions have been widely used as an ideal media to
prepare inorganic nanoparticles.15 As the nanosized water
* To whom correspondence should be addressed. E-mail: jlin@ciac.jl.cn.
(1) Hamberg, I.; Granqvist, C. G. J. Appl. Phys. 1986, 60, 123.
(2) Gagaoudakis, E.; Bender, M.; Douloufakis, E.; Kataarakis, N.;
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Yamazoe, N. Sens, Actuator, B 1998, 46, 139.
(4) Liess, M. Thin Solid Films 2002, 410, 183.
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Weimar, U.; Gopel, W.; Dieguez, A. Sens. Actuators, B 1997, 44,
327.
(7) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947.
(8) Li, C.; Zhang, D.; Han, S.; Liu, X.; Tang, T.; Zhou, C. AdV. Mater.
2003, 15, 143.
(9) Seo, W. S.; Jo, H. H.; Lee, K.; Park, J. T. AdV. Mater. 2003, 15, 795.
(10) Yumoto, H.; Sako, T.; Gotoh, Y.; Nishiyama, K.; Kaneko, T. J. Cryst.
Growth 1999, 203, 136.
(11) Zheng, M. J.; Zhang, L. D.; Li, G. H. Appl. Phys. Lett. 2001, 79, 839.
(12) Kuo, C. Y.; Lu, S. Y.; Wei, T. Y. J. Cryst. Growth 2005, 285, 400.
Inorg. Chem. 2006, 45, 8973−8979
10.1021/ic060934+ CCC: $33.50 © 2006 American Chemical Society Inorganic Chemistry, Vol. 45, No. 22, 2006 8973
Published on Web 10/05/2006
pools, they have been widely used as spatially constrained
nanoreactors for controlled synthesis of nanoparticles with
a desired narrow size distribution such as Ag nanodisks,16
CdS nanotriangles,17 V2O5 nanowires/nanorods,18 CaCO3
nanowires,19 and BaSO4 filaments cones.20 At the same time,
the combination of microemulsion techniques with a hydro-
thermal/solvothermal process has also been explored for the
preparation of nanocrystals such as ZnS nanocrystallites,21
CdS nanorods,22 SbSO4 nanorods,23 SrCO3 nanostructures,24
Ca10(PO4)6(OH)2 nanofibers,25 ZnO nanowires,26 TiO2 nanoc-
rystallites,27 SnO2 nanorods,28 BaF2 whiskers,29 and molecular
sieve fibers.30
Enlightened by the above reports, we believe that it is
reasonable to prepare In(OH)3 or In2O3 nanostructures via
microemulsion-mediated hydrothermal synthesis. In this
paper, we first report the synthesis of In(OH)3 nano-
microstructures with different morphologies (nanorod bundles
and spheres) in a cetyltrimethylammonium bromide (CTAB)/
water/cyclohexane /n-pentanol microemulsion system under
hydrothermal conditions by carefully controlling fundamental
experimental parameters such as the pH values of the
microemulsion. The obtained In(OH)3 nanorod bundles and
spheres, both consisting of individual nanoparticles with size
ranging from 30 to 55 nm, can be efficiently achieved at a
hydrothermal temperature as low as 140 °C. To the best of
our knowledge, this is the lowest temperature reported by
far for the fabrication of In(OH)3 crystals through the
hydrothermal process. Calcination of the In(OH)3 crystals
yielded In2O3 crystals with the same morphology. Finally,
the optical properties of the resulting In2O3 nano-microstruc-
tures were investigated.
2. Experimental Section
Preparation. All of the chemicals used in our experiment were
of analytical-reagent (A.R.) grade, purchased from Beijing Fine
Chemical Co., Beijing, China. A quaternary microemulsion system
consisting of cetyltrimethylammonium bromide (CTAB)/water/
cyclohexane/n-pentanol was selected for this study. The micro-
emulsion was prepared by dissolving CTAB (1.0 g) in 30 mL of
cyclohexane and 1.5 mL of n-pentanol. The mixing solution was
stirred for 30 min. Then a certain volume of a 0.5 M In(NO3)3
aqueous solution was added to the solution. The molar ratio of water
to CTAB was set at 5, and the pH value of the microemulsion was
changed from 5 to 3 through the addition of drops of concentrated
nitric acid. The solution was stirred for another 60 min. Then the
transparent feedstock was charged into a 45-mL Teflon-lined
stainless steel autoclave and heated at 140 °C for 24 h. After the
autoclave was cooled to room temperature naturally, the precursors
were separated by centrifugation, washing with ethanol and distilled
water several times, and drying in an air atmosphere at 60 °C for
4 h. The final products were retrieved through a heat treatment at
600 °C for 2 h in air. To investigate the growing process for In-
(OH)3 crystals with different morphologies, the hydrothermal
reactions were stopped after designed times of 1.5, 5.5, 16, and 24
h (for nanorod bundles) or 4, 7, 16, and 24 h (for nanorod spheres),
respectively. Then the products were isolated and analyzed.
Characterization. The samples were characterized by powder
X-ray diffraction (XRD) performed on a Rigaku-Dmax 2500
diffractometer with Cu KR radiation (ì ) 0.154 05 nm). The
morphology of the samples was inspected using a field emission
scanning electron microscope (Philips XL 30) equipped with an
energy-dispersive spectrometer and a transmission electron micro-
scope (JEOL-2010, 200 kV). Photoluminescence (PL) excitation
and emission spectra were recorded with a Hitachi F-4500 spec-
trophotometer equipped with a 150-W xenon lamp as the excitation
source at room temperature. The luminescence decay curves were
obtained using a Lecroy Wave Runner 6100 digital oscilloscope
(1 GHz) and a 383-nm UV laser (pulse width ) 4 ns) as the
excitation source.
3. Results and Discussion
3.1. Phase Formation. Figure 1 shows the XRD patterns
of the as-formed products through the hydrothermal process
(a) and those after a heat treatment at 600 °C (b). The XRD
peaks for as-formed samples can be indexed to a cubic lattice
[space group: Im3h(204)] of pure indium hydroxide, In(OH)3,
as shown in Figure 1a. The calculated lattice constant, a )
0.7977 nm, is in good agreement with that (a ) 0.7979 nm)
from the standard card (JCPDS No. 85-1338). After anneal-
(13) Tang, Q.; Zhou, W.; Zhang, W.; Ou, S.; Jiang, K.; Yu, W.; Qian, Y.
T. Cryst. Growth Des. 2005, 5, 147.
(14) Yu, D.; Yu, S. H.; Zhang, S.; Zuo, J.; Wang, D.; Qian, Y. T. AdV.
Funct. Mater. 2003, 13, 497.
(15) (a) Schwuger, M.; Stickdom, K.; Schomacker, R. Chem. ReV. 1995,
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Loy, G. L.; Xu, G. Q. Langmuir 1997, 13, 6427. (c) Feng, P.; Bu, X.;
Stucky, G. D.; Pine, D. J. J. Am. Chem. Soc. 2000, 122, 994.
(16) Maillard, M.; Giorgio, S.; Pileni, M. P. AdV. Mater. 2002, 14, 1084.
(17) (a) Pinna, N.; Weiss, K.; Kongehl, H. S.; Vogel, W.; Urban, J.; Pileni,
M. P. Langmuir 2001, 17, 7982. (b) Pinna, N.; Weiss, K.; Urban, J.;
Pilent, M. P. AdV. Mater. 2001, 13, 261.
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2003, 3, 1131.
(19) Kuang, D. B.; Xu, A. W.; Fang, Y. P.; Ou, H. D.; Liu, H. Q. J. Cryst.
Growth 2002, 244, 379.
(20) (a) Hopwood, J. D.; Stephen, M. Chem. Mater. 1997, 9, 1819. (b) Li,
M.; Stephen, M. Langmuir 2000, 16, 7088.
(21) Xu, S. J.; Chua, S. J. Appl. Phys. Lett. 1998, 73, 478.
(22) Zhang, P.; Gao, L. Langmuir 2003, 19, 208.
(23) Xiang, J. H.; Yu, S. H.; Geng, X.; Liu, B. H.; Xu, Y. Cryst. Growth
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(24) Cao, M. H.; Wu, X. L.; He, X. Y. Langmuir 2005, 21, 6093.
(25) Cao, M. H.; Wang, Y. H.; Guo, C. X. Langmuir 2004, 20, 4784.
(26) Zhang, J.; Sun, L. D.; Pan, H. Y. New J. Chem. 2002, 26, 33.
(27) (a) Wu, M. M.; Long, J. B.; Huang, A. H.; Luo, Y. J. Langmuir 1999,
15, 8822. (b) Andersson, M.; Osterlund, L.; Ljungstrom, S.; Palmqvist,
A. J. Phys. Chem. B 2002, 106, 10674.
(28) Zhang, D. F.; Sun, L. D.; Yin, J. L.; Yan, C. H. AdV. Mater. 2003,
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(29) Cao, M. H.; Hu, C. W.; Wang, E. B. J. Am. Chem. Soc. 2003, 125,
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Figure 1. XRD patterns of the as-formed sample (a) and that after
annealing at 600 °C (b). The standard data for In(OH)3 (JCPDS No. 85-
1338) and In2O3 (JCPDS No. 71-2194) are also presented in the figure for
comparison.
Yang et al.
8974 Inorganic Chemistry, Vol. 45, No. 22, 2006
ing at 600 °C in air, the XRD pattern of the resulting sample
exhibits multiple intense peaks, all of which can be perfectly
indexed to the cubic phase [space group: Ia3h(206)] of In2O3,
as shown in Figure 1b. The calculated lattice parameter (a
) 1.0125 nm) for In2O3, is in good agreement with the
known lattice parameter for crystalline In2O3 (a ) 1.0117
nm; JCPDS No. 71-2194). The crystallite size of the samples
can be estimated from the Scherrer equation, D ) 0.941ì/â
cos ı, where D is the average grain size, ì is the X-ray
wavelength (0.154 05 nm), and ı and â are the diffraction
angle and full-width at half-maximum (fwhm) of an observed
peak, respectively.31 The strongest peaks (200) at 2ı )
22.30° and (222) at 2ı ) 30.62° were used to calculate for
the average crystallite size (D) of the In(OH)3 and In2O3
particles, respectively. The estimated average crystallite sizes
are about 60 nm for In(OH)3 particles and 25 nm for In2O3
particles.
The synthesis reactions can be simply expressed as
Under the hydrothermal process, In(OH)3 nano-micro-
structures are prepared by the hydrolysis reaction of In3+ at
140 °C, as described in reaction (1). The dehydration of the
In(OH)3 precursors at 600 °C results in the formation of the
final product In2O3 nano-microstructures, as described in
reaction (2).
3.2. Morphology Control. The pH values of the micro-
emulsion have great effects on the morphology of the
hydrothermal products. Figure 2a shows the SEM images
of the as-formed In(OH)3 samples prepared from the micro-
emulsion system with pH ) 5. It can be seen that the In-
(OH)3 samples are composed of uniform nanorod bundles
(around 500 nm in length and 200 nm in diameter) with an
aspect ratio of about 2.5:1, and the nanorod bundles consist
of individual parallel nanorods with diameters ranging from
30 to 45 nm. Every nanorod is further composed of the
smaller nanosized particles with size ranging from 30 to 45
nm, basically in the same size range as those estimated by
XRD. The inset in Figure 2a is the magnified image of In-
(OH)3 nanorod bundles. After calcination at 600 °C, the
obtained In2O3 samples retain the morphology and size of
In(OH)3, as shown in Figure 2b. The chemical composition
of the In2O3 nanorod bundles was investigated with energy-
dispersive spectrometry (EDS; Figure 2c), which indicates
an atomic ratio of In:O ) 2:2.94. The deviation from In:O
) 2:3 may be due to the experimental error of EDS
experiments. The EDS result gives further support for the
XRD analysis above. To further study the fine structure of
the above nanorod bundles, transmission electron microscopy
(TEM) was performed. Representative TEM micrographs for
the In(OH)3 and In2O3 nanorod bundles are shown in parts
a-d of Figure 3. As can be seen from Figure 3a, In(OH)3
nanorod bundles with a width of about 210 nm and a length
of about 500 nm are obtained, which is composed of parallel
nanorods with a mean diameter of 40 nm. High-resolution
TEM and selected-area electron diffraction (SAED) were
performed in the region of nanorod bundles as labeled in
Figure 3a, and the micrographs are shown in parts b and c
of Figure 3, respectively. The lattice fringes of crystalline
In(OH)3 can be seen clearly (Figure 3b); the SAED images,
which combine partial ring and dot patterns, indicate that
the nanorod is of polycrystalline nature. Figure 3d is the TEM
image of the In2O3 nanorod bundles, which is in agreement
with the SEM results (Figure 2b). The average diameter of
the nanorod (the subunit of nanorod bundles) of In2O3
(31) Zhang, Y. W.; Jin, S.; Tian, S. J.; Li, G. B.; Jia, T.; Liao, C. S.; Yan,
C. H. Chem. Mater. 2001, 13, 372.
In3+ + 3H2O f In(OH)3 + 3H+ (1)
2In(OH)3 f In2O3 + 3H2O (2)
Figure 2. Field emission scanning electron micrsocopy (FE-SEM) images
of the as-formed hydrothermal products of In(OH)3 (a) and the corresponding
In2O3 (b) obtained from the microemulsion with pH ) 5. The inset in part
a is the magnified image of an individual In(OH)3 nanorod bundle. EDS of
the In2O3 nanorod bundles (c) found In and O from the sample with an
atom ratio of In:O ) 2:2.94 (Au from the Au coating and Si from the Si
substrate were used for measurement).
In(OH)3 and In2O3 Nanorod Bundles and Spheres
Inorganic Chemistry, Vol. 45, No. 22, 2006 8975
reduces to 20-30 nm compared with that of In(OH)3 in
Figure 3a (mean diameter of 40 nm), indicating that the
decomposition of In(OH)3 resulted in a reduction of the
particle size in In2O3. There are some dark parts on the TEM
image because those parts may be too thick for the electrons
to penetrate. The SAED of In2O3 nanorod bundles is given
in the inset of Figure 3d, whose strong diffraction rings can
be indexed as (222), (400), and (440) planes of cubic In2O3.
When the pH value of the microemulsion was decreased
to 3, the hydrothermal product of In(OH)3 consisted of
caddice spherelike agglomerates (instead of the nanorod
bundles for pH ) 5), as shown in the SEM image of Figure
4a. The diameters of the spherical agglomerates of In(OH)3
are in the range of 0.75-1 ím. Clearly, the individual In-
(OH)3 sphere seems to be also composed of crooked
nanorods with diameters ranging from 30 to 55 nm, and each
nanorod contains smaller nanosized particles with sizes
ranging from 30 to 55 nm. The initial shape and size of the
products are basically kept in the phase transformation from
In(OH)3 to In2O3 after calcination at 600 °C (Figure 4b).
Figure 4c is the TEM image of the In2O3 spheres. In
agreement with the SEM results (Figure 4b), from Figure
4c we can distinctly see that there are many rods grown on
the surface of the black spheres, which sustains the fact that
the In2O3 spheres are composed of nanorods. These spheres
are well crystallized, which can be corroborated by the SAED
pattern of the sample (inset of Figure 4c). The SAED of the
spheres is consistent with cubic In2O3 with strong ring
patterns due to (222), (400), (440), and (622) planes.
3.3. Formation Mechanism for the In(OH)3 Nano-
microstructures. In principle, crystal growth and crystal
morphology are governed by the degree of supersaturation,
the diffusion of the reaction, the species to the surface of
the crystals, the surface and interfacial energies, and the
structure of the crystals; that is, extrinsic and intrinsic factors,
the crystal structure, and the growth surroundings are
accounted for in the final morphology.13
From the above experimental results, it can be seen that
the pH value of the microemulsion has a significant effect
on the morphologies and sizes of the final products.
Furthermore, it was found that the sizes of the crystals
obtained by this microemulsion-mediated hydrothermal
Figure 3. TEM images of the as-formed hydrothermal products of In-
(OH)3 (a) with its HRTEM (b) and SAED pattern (c) as well as the TEM
image of the In2O3 nanorod bundles (d) with the SAED pattern (inset).
Figure 4. FE-SEM images of the as-formed hydrothermal products of
In(OH)3 (a) and the corresponding In2O3 (b) with its TEM image (the SAED
pattern in the inset) of the In2O3 spheres (c) obtained from the microemulsion
with pH ) 3. The inset in part a is the magnified image showing an
individual In(OH)3 sphere.
Yang et al.
8976 Inorganic Chemistry, Vol. 45, No. 22, 2006
process are much larger than the typical dimensions for
individual microemulsion droplets (around 5-100 nm) under
appropriate reaction conditions.24 Thus, it can be inferred
that both the pH value of the microemulsion and aggregation
and coalescence of individual nanoparticles (formed from
the microemulsion droplets) are responsible for the formation
of products with different morphologies mentioned in the
last section.
The growing process of In(
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