ARTICLES
1044 Chinese Science Bulletin Vol. 50 No. 10 May 2005
Chinese Science Bulletin 2005 Vol. 50 No. 10 1044
—
1047
Synthesis ofhighlyordered
SnO
2
/Fe
2
O
3
composite
nanowire arrays by
electrophoretic deposition
method
LI Jianjun
1
, ZHANG Xingtang
1
, CHEN Yanhui
1
,
LI Yuncai
1
, HUANG Yabin
1
, DU Zuliang
1
& LI Tiejin
1,2
1. Key Lab for Special Functional Materials, Henan University, Kaifeng
475001, China;
2. Lab of Photochemistry, Jilin University, Changchun 130021, China
Correspondence should be addressed to Du Zuliang (email: zld@
henu.edu.cn)
Abstract Highlyordered SnO
2
/Fe
2
O
3
composite nano-
wire arrays have been synthesized by electrophoretic deposi-
tion method. The morphology and chemical composition of
SnO
2
/Fe
2
O
3
compositenanowirearrays are characterized by
SEM, TEM, EDX, XPS, and XRD. The results show that the
SnO
2
/Fe
2
O
3
composite nanowires are about 180 nm in width
and tens of microns in length, and they are composed of
small nanoparticles of tetragonal SnO
2
and rhombohedral
ɑ-Fe
2
O
3
with diameters of 10－15 nm. The SnO
2
/Fe
2
O
3
com-
posite nanowires are formed by a series of chemical reac-
tions.
Keywords: SnO
2
/Fe
2
O
3
compositenanowire arrays, electrophoretic
deposition, AAO template, sol particles.
DOI: 10.1360/982004-792
Since the carbon nanotubes (CNTs) were found in 1991,
the studies of one-dimensional nanomaterials, e.g. nano-
tube, nanowire, especially highlyordered nanoarrays,
have attracted much attention due to their particular func-
tions and potential applications. One-dimensional (1D)
nanostructures can be used as building blocks nanoelec-
tronic devices, such as optical storage devices, single-ele-
ctron transport devices, and electronic sensors
[1
—
4]
. Vari-
ous methods have been advanced to synthesize 1D nanos-
tructures, including electrochemical deposition, CVD,
VLS, VS, L-L-S, Sol-Gel, and template method
[5
—
8]
.
Sol-gel processing is a wet chemical route for the synthe-
sis of 1D nanomaterials, motivated by electrostatic attrac-
tion between oppositely charged sol particles and the tem-
plate walls
[9
—
11]
. Recently, a number of groups have suc-
ceeded in synthesizing nanowires using a method of com-
bining sol-gel template processing with electrophoretic
deposition
[12,13]
. In this manner, sol particles were driven
to a certain electrode by the force of external electro-field
and aggregated into the AAO template holes tightly, lead-
ing to the formation of solid nanowire arrays.
For a long time, composite materials of semi-con-
ductors have been studied extensively owing to their
unique optical and electric properties
[14
—
17]
. SnO
2
and
Fe
2
O
3
are both important inorganic semiconductors and
have potential applications in Li-ion batteries, gas sensors,
chemical-catalyst, and magnetic storage devices
[18
—
20]
.
Their composites also attracted great attention owing to
their stable, outstanding gas-sensitive properties and po-
tential application in Li-ion battery electrode
[21
—
23]
. Al-
though some groups have synthesized and studied thin
films and nanopowders of SnO
2
/Fe
2
O
3
composite
[21,22]
, the
synthesis of SnO
2
/Fe
2
O
3
composite nanowires has not
been reported to our knowledge. We have synthesized
homogeneous morphology and highlyordered SnO
2
/Fe
2
O
3
composite nanowirearrays by combining sol-gel template
processing with electrophoretic deposition. The chemical
composition and morphology of the products were char-
acterized by SEM, TEM, EDX, XRD and XPS.
1 Experimental
(ⅰ) Reagent and apparatus. In this work, scanning
electron microscopy (SEM, JEOL JSM-5600LV) and
transmission electron microscopy (TEM, JEOL JEM-
100CX-Ⅱ and JEOL 2010 with EDX) were used to inves-
tigate the morphology and chemical composition of the
nanowires. Samples were sputter-coated with a thin Au
layer prior to observation in the SEM. Through X-ray dif-
fraction (XRD, Phillips X’ Pert Pro MPD) the phase and
crystal structure were determined, and the X-ray photo-
electric spectrum (XPS, KRATOS AXIS ULTR) was used
to characterize the valence of the elements of the
nanowires. The porous anodic aluminum oxide (AAO)
template with a pore diameter of 200nm used in this work
was from Whatman Co., England. All the reagents used in
this experiment, including SnCl
2
.
2H
2
O, FeCl
3
.
6H
2
O,
formaldehyde solution, hydrochloric acid, and dehydrated
alcohol, were of analysis reagent grade. Water used in all
process was tri-distilled water.
(ⅱ) Synthesisof the colloid. The Fe
3+
colloid was
synthesized using the method of ref. [24]. In a typical
synthesis, 2.7 g FeCl
3
.6H
2
O was dissolved in 40 mL de-
hydrated alcohol, with 5 mL formaldehyde added drop-
wise. The solution was stirred electromagnetically for 2 h
and became brown colloidal (PH value 2-3). Following
ref. [19], 0.40 g SnCl
2
.
2H
2
O was dissolved in 40 mL de-
hydrated alcohol, with 3 mL hydrochloric acid added
dropwise. The solution became a yellowish sol after 2 h of
circumfluence in water bath (80℃), and 24 h of aging at
room temperature. Then, the two types of prepared colloid
were mixed under magnetic stirring. A light brown stable
sol was finally obtained.
(ⅲ) Fabrication ofnanowire arrays. Electrophoretic
deposition occurred in a bi-electrodes system, with a
working electrode (negative electrode) of aluminum and a
ARTICLES
Fig. 1. SEM images of SnO
2
/Fe
2
O
3
compositenanowire arrays. (a) Top
view; (b) side view.
Pt thread counter electrode. The electrodes paralleled to
each other and were set approximately 2.0 cm apart. The
alumina membrane with a surface covered with a thin film
of gold, was fixed on the working electrode. For electro-
phoretic deposition, a potential of 5 V was applied on the
electrodes, and sustained for 2 h. At the end of electro-
phoretic deposition, excess sol was blotted off the mem-
brane with filter paper. Samples prepared in this manner
were annealed at 630℃ for 6 h. By carefully wet chemi-
cal etching with 1 mol/L NaOH part of the alumina mem-
brane was removed before the characterization of SEM,
XRD, and XPS. After all the alumina membrane had been
removed, the samples were studied by TEM and EDX.
2 Results and discussion
(ⅰ) Scanning electron microscope (SEM) analysis.
Figure 1 shows the SEM images of the SnO
2
/Fe
2
O
3
com-
posite nanowirearrays after removing part of the alumina
membrane. Figs. 1 (a) and (b) are the top view and side
view of the as-prepared samples. The nanowires became a
little aggregate owing to lack of support from the mem-
brane which had been dissolved prior to the characteriza-
tion. Obviously, the obtained nanowires have smooth sur-
face, unique morphology, and orderly distribution.
(ⅱ ) Transmission electron microscope (TEM) an-
alysis. Fig. 2 shows the TEM images of the SnO
2
/
Fe
2
O
3
composite nanowires and the corresponding EDX
spectrum. The nanowires are straight and long with
diameters around 180 nm which are a little thinner than
those of the AAO template pores because of the densifica-
tion of the gel during annealing process
[25]
. The lengths of
the nanowires are up to tens of microns, corresponding to
the thickness of the AAO template. The surface of the
nanowires looks smooth in large scale images, but, in fact,
the nanowires are composed of lots of very small
nanoparticles as shown in small scale images. Fig. 2(c) is
the EDX spectrum of a single nanowire, which proves the
existence of Sn, Fe, and O elements in the nanowire.
The presence of the Cu peak is due to the application of
copper grid during TEM observation. A very low peak of
Na can also be observed in the spectrum and this is caused
by the residual of NaOH used for dissolving AAO tem-
plate. The EDX spectrum confirms the exclusive compo-
sition of the as-prepared nanowires with Sn, Fe and O
elements.
(ⅲ) X-ray photoelectron spectrum (XPS) analysis.
Figure 3 is the corresponding XPS spectrum of the sam-
ples as shown in Fig. 1. Fig. 3(a) shows that Sn 3d has
two peaks at 486.8 and 495.2 eV. According to the refer-
ential spectrum, they correspond to 3d
5/2
and 3d
3/2
peaks of
Sn in SnO
2
respectively. The two peaks separate clearly,
which indicates that all Sn in tin oxide is Sn (Ⅳ). In Fig.
3(b), the binding energy of Fe 2p is 710.9 and 724.6 eV
respectively, corresponding to the 2p
3/2
and 2p
1/2
peak of
Fe in ɑ-Fe
2
O
3
. The binding energy of O
1s
in Fig. 3(c) is
530.0 eV, corresponding to the binding energy of oxygen
element in oxide. All the above proves that the as-pre-
pared nanowires are compositeof Fe
2
O
3
and SnO
2
.
(ⅳ) X-ray diffraction analysis. Fig. 4 shows the XRD
pattern of the sample after being annealed in air for 6 h.
The peaks indexed with and without a box represent the
diffraction peaks of Fe
2
O
3
and SnO
2
respectively. The
broad peak between 15° and 35° is from the uncovered
glass substrate for the little amount of samples. All the
diffraction peaks of Fe
2
O
3
are well corresponding to the
literature of rhombohedral ɑ-Fe
2
O
3
(JCPDF number
24-0072), and the three peaks of SnO
2
correspond to the
diffraction peaks of (110), (200), and (301) crystal faces of
tetragonal SnO
2
(JCPDF number 46-1088) respectively,
suggesting that the obtained nanowires are composed of
highly crystalline rhombohedral ɑ-Fe
2
O
3
and tetragonal
SnO
2
. The widened diffraction peaks indicate that the
SnO
2
/Fe
2
O
3
composite nanowires are composed of very
small nanoparticles
[26]
. According to Sherrer formula, the
even diameters of the nanoparticls are 10—15 nm, which
is in agreement with the investigation of TEM.
The mechanism of the formation of the nanowires can
be explained as follows. FeCl
3
.
6H
2
O was dissolved in
Chinese Science Bulletin Vol. 50 No. 10 May 2005 1045
ARTICLES
Fig. 2. TEM images of SnO
2
/Fe
2
O
3
compositenanowire array (a) ×
1500; (b) ×10000; (c) EDX spectrum.
dehydrated alcohol followed by the addition of formalde-
hyde solution, and stable Fe(OH)
3
sol was obtained after
enough stirring. At the same time, Sn(OH)
2
sol was
formed by 2 h of circumfluence of the alcohol solution of
SnCl
2
.
2H
2
O. The colloidal particles in the sol are all typi-
cal double-layer structure. Because the inner part of the
colloidal particle (inside of the slip plane) is positively
charged
[12,27]
, the colloidal particles moved to the cathode
Fig. 3. XPS of SnO
2
/Fe
2
O
3
compositenanowire array in AAO template.
(a) Sn 3d; (b) Fe 2p; (c) O 1s.
when an external electric field was applied to the elec-
trodes. They aggregated into the pores of AAO template
and coagulated. While the as-prepared samples were an-
nealed at 630℃ in air for 6 h, Sn(OH)
2
and Fe(OH)
3
in
the gel were dehydrated and oxidized, leading to the for-
mation of SnO
2
and Fe
2
O
3
respectively. The reaction
equations are as follows:
2Sn(OH)
2
→ 2SnO+2H
2
O↑
1046 Chinese Science Bulletin Vol. 50 No. 10 May 2005
ARTICLES
2Fe(OH)
3
→ Fe
2
O
3
+3H
2
O↑
2SnO+O
2
→ 2SnO
2
As a result, the gel was translated into SnO
2
/Fe
2
O
3
com-
posite nanowires because of the confinement of the AAO
template pores. The high annealing temperature caused
high crystalline of tetragonal SnO
2
and rhombohedral
ɑ-Fe
2
O
3
. The diameters of the prepared nanowires are a
little smaller than those of the AAO template pores due to
the densification reaction during sintering process
[28]
.
Fig. 4. XRD spectrum ofcompositenanowire array in AAO template.
The peak numbers in and out of panes represent diffraction peaks of
Fe
2
O
3
and SnO
2
, respectively.
3 Conclusion
Highly ordered SnO
2
/Fe
2
O
3
compositenanowirearrays
have been synthesized by electrophoretic method using
AAO as template. The results of characterization show
that the obtained nanowires are composed of
well-crystalline tetragonal SnO
2
and rhombohedral
ɑ-Fe
2
O
3
. The diameters of the nanowires are around 180
nm, a little smaller than those of the pores of the AAO
template, due to the densification reaction in the annealing
process. SnO
2
/Fe
2
O
3
composite nanowires have better
electron directional transport property than the thin films
and powders ofcomposite SnO
2
/Fe
2
O
3
, and are well ar-
rayed, which makes it convenient to fabricate nanodevice
with higher sensitivity. It is believed that the SnO
2
/Fe
2
O
3
composite nanowires will find extensive applications in
gas sensors, etc.
Acknowledgement This work was supported by the Prophase Project
of “973” Plan (Grant No. 2002CCC02700) and the National Natural
Science Foundation of China (Grants No. 20371015 and 90306010).
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Chinese Science Bulletin Vol. 50 No. 10 May 2005 1047
. photo-
luminescence of highly ordered TiO
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nanowire arrays, Appl. Phys.
Lett., 2001, 78: 1125－1127.
11. Zhou, Y. K., Huang, J., Li, H. L., Synthesis of highly ordered. Bulletin 2005 Vol. 50 No. 10 1044
—
1047
Synthesis of highly ordered
SnO
2
/Fe
2
O
3
composite
nanowire arrays by
electrophoretic deposition
method