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NANO EXPRESS
Selective Synthesis of Fe2O3 and Fe3O4 Nanowires Via a SinglePrecursor: A General Method for Metal Oxide Nanowires
Ning Du • Yanfang Xu • Hui Zhang •
Chuanxin Zhai • Deren Yang
Received: 16 April 2010 / Accepted: 7 May 2010 / Published online: 21 May 2010
� The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract Hematite (a-Fe2O3) and magnetite (Fe3O4)
nanowires with the diameter of about 100 nm and the length of
tens of micrometers have been selectively synthesized by a
microemulsion-based method in combination of the calcina-
tions under different atmosphere. The effects of the precursors,
annealing temperature, and atmosphere on the morphology
and the structure of the products have been investigated.
Moreover, Co3O4 nanowires have been fabricated to confirm
the versatility of the method for metal oxide nanowires.
Keywords Nanowires � Sacrifice template �Microemulsion � Calcinations
Introduction
In recent years, one-dimensional (1D) nanostructured
materials have attracted great interest due to their superior
performance in applications such as electronics,optoelec-
tronics, energy storage and conversion, sensing and drug
delivery [1]. Up to now, lots of approaches have been
reported to synthesize 1D nanostructures, such as laser
ablation, chemical vapor deposition (CVD), thermal evap-
oration, hydrothermal/solvethermal process, chemical
reaction, template/precursor-directed methods, and so on
[2–9]. More recently, current existing 1D nanostructures
have been considered as useful precursors to generate other
1D nanostructures that might be difficult to directly syn-
thesize as uniform samples [10–15]. For example, Wang and
Qi reported the synthesis of Ag2S, Ag2Se, and Ag nanofibers
by using Ag2C2O4 as a general precursor, employing both
anioic exchange and redox reactions [11]. Fe3O4 nanotubes
have been prepared by epitaxial coating of Fe3O4 layer on
MgO nanowires and subsequently wet-chemical etching
[12]. Single-crystalline ZnAl2O4 spinel nanotubes could be
fabricated through a spinel-forming interfacial solid-state
reaction of core–shell ZnO–Al2O3 nanowires as precursors
[13]. More recently, we have used carbon nanotubes as
templates to synthesize metal oxide and composite nano-
tubes via layer-by-layer technique [14, 15].
As typical anisotropic magnetic nanomaterials, 1D iron
oxide nanostructures are of great interest because of their
interesting magnetic properties caused by shape anisotropy
[16–18]. However, it is difficult for magnetic iron oxides to
form 1D nanostructures due to their spinel crystal [19].
Recently, great efforts have been made to synthesize 1D iron
oxides nanostructures by means such as oxidation of iron
plates [20, 21], utilization of various templates [22, 23] and
surfactant-assisted hydrothermal and solvothermal [24, 25].
However, it remains challenge to selective synthesize 1D
iron oxides nanostructures with controllable morphology.
Herein, as precursors, FeC2O4�2H2O nanowires were
synthesized by a simple microemulsion-based method.
Ultralong and uniform a-Fe2O3 and Fe3O4 nanowires could
be selectively fabricated by annealing of FeC2O4�2H2O
nanowires under different temperatures and atmospheres.
Co3O4 nanowires were also prepared to confirm the ver-
satility of the method for metal oxide nanowires.
Experimental
All the chemicals are analytical grade without further
purification. The experiment details were as follows: 2.5 g
N. Du � Y. Xu � H. Zhang � C. Zhai � D. Yang (&)
State Key Lab of Silicon Materials and Department of Materials
Science and Engineering, Zhejiang University,
310027 Hangzhou, People’s Republic of China
e-mail: [email protected]
123
Nanoscale Res Lett (2010) 5:1295–1300
DOI 10.1007/s11671-010-9641-y
CTAB (cetyltrimethylammonium bromide) was dissolved
into a mixture of 75 ml of cyclohexane and 2.5 ml of
n-pentanol. After 20 min of stirring, 3.75 ml of 0.1 M
H2C2O4 aqueous solution was introduced into the above
resulting solution, and the mixture was stirred for an
additional 25 min. Finally, 1.25 ml of 0.1 M FeSO4 was
added to the above microemulsion and stirred for 24 h at
room temperature. After the reaction was completed, the
resulting products were centrifugalized, washed with
deionized water and ethanol to remove the ions possibly
remaining in the final product, and finally dried at 80�C in
air. The obtained powders were annealed under O2 (550/
700�C) and Ar/H2 (400�C), respectively.
The obtained samples were characterized by X-ray pow-
der diffraction (XRD) using a Rigaku D/max-ga x-ray dif-
fractometer with graphite monochromatized Cu Ka radiation
(c = 1.54178 A). The morphology and structure of the
samples were examined by field emission scanning electron
microscopy (FESEM, Hitachi S-4800), transmission electron
microscopy (TEM, JEM-200 CX, 160 kV) and high-resolu-
tion transmission electron microscopy (HRTEM, JEOL
JEM-2010) with an energy-dispersive X-ray spectrometer
(EDX). The infrared (IR) spectra were measured with a
Nicolet Nexus FTIR 670 spectrophotometer.
Result and Discussion
Figure 1 shows the X-ray diffraction (XRD) patterns of the
precursors and the subsequent products after annealing
processes. It can be seen that all the peaks of the precursors
can be assigned to pure ferrous oxalate dehydrate
(FeC2O4�2H2O) (JCPDS: 72-1305). No diffraction peaks
from impurities were found in the sample. When the pre-
cursors were annealed in air at 550/700�C, all the diffrac-
tion peaks of the products can be readily indexed to the
pure rhombohedral phase of a-Fe2O3 (JCPDS: 33-0664).
When the precursors were annealed in Ar/H2 (5% H2) at
400�C, all the diffraction peaks of the products can be
readily indexed to a pure cubic phase of Fe3O4 (JCPDS:
65-3107). The results of XRD demonstrate that
FeC2O4�2H2O can be transformed to a-Fe2O3 and Fe3O4
during the annealing processes under different atmosphere.
The FeC2O4�2H2O nanowires used as the precursors for
the synthesis of a-Fe2O3 and Fe3O4 nanowires were pre-
pared by a simple microemulsion-based method. Figure 2
20 30 40 50 60 70 80
(113
)
(101
0)
(300
)(2
14)
(018
)
(116
)
(024
)
(113
)(110
)
(104
)
(012
)
Fe2O
3-550°C
Fe3O
4
Fe2O
3-720°C
Inte
nsi
ty (
a.u
.)
2θ
FeC2O
4
(012
) (104
)
(110
)
(113
)
(024
)
(116
)
(018
)
(214
)
(101
0)
(300
)
(220
)
(311
)
(400
)
(422
)
(511
)
(440
)
(002
)
(312
)
(114
)
Fig. 1 XRD patterns of as-synthesized products
Fig. 2 SEM images (a), (b) and
TEM images (c), (d) of FeC2O4
nanowires
1296 Nanoscale Res Lett (2010) 5:1295–1300
123
Fig. 3 Morphological and
structural characterization of
Fe2O3 nanowires synthesized at
550�C: a, b SEM images; c–eTEM images; f HRTEM images
Fig. 4 SEM images (a), (b) and
TEM images (c), (d) of Fe2O3
nanowires synthesized at 700�C
Nanoscale Res Lett (2010) 5:1295–1300 1297
123
shows the SEM and the TEM images of the as-synthesized
FeC2O4�2H2O nanostructures. As observed, the obtained
FeC2O4�2H2O nanostructure exhibits the morphology of
the nanowires with the diameter of about 100 nm and the
length of tens of micrometers. Moreover, the surface of the
FeC2O4�2H2O nanowires seems smooth, and no defects can
be observed. When the FeC2O4�2H2O nanowires were
annealed in air at 550�C, porous a-Fe2O3 nanowires were
fabricated. Figure 3 shows the SEM, TEM, and HRTEM
images of the as-synthesized a-Fe2O3 nanowires. It can be
seen that the as-synthesized a-Fe2O3 nanowires retain the
morphology of the FeC2O4�2H2O nanowires (Fig. 3a–d).
However, many nanopores with diameters of about 10–
30 nm appear because of the release of CO2 from
FeC2O4�2H2O during the heat treatments, resulting in the
porous nanowires (Fig. 3e). Such the porous nanowires
may exhibit superior performance in Li-battery and gas
sensors because of the large surface area. Figure 3f shows
the HRTEM image of an a-Fe2O3 nanowire. As can be
seen, two nanocrystals connect together with a grain
boundary between them. There are two kinds of lattice
fringes with lattice spacings of about 0.37 and 0.25 nm,
corresponding to the {1012} plane and {1120} plane of
a-Fe2O3, respectively, which indicates their polycrystalline
nature. Figure 4 shows the SEM and the TEM images of
the products when the annealing temperature was raised up
to 700�C. Compared with the products following a 500�Cannealing process, this sample shows a similar morphology
of nanowires but the nanocrystals that composed of nano-
wires shows better fusion between them due to the higher
annealing temperature. Moreover, quasi-singlecrystalline
a-Fe2O3 nanowires can be observed (Fig. 4c, d). Figure 5
shows the morphology and the structure characterizations
of the products when the FeC2O4�2H2O nanowires were
annealed in Ar/H2 (5% H2) at 400�C. The SEM and the
TEM images indicate that the as-synthesized products
show the morphology of nanowires, which retains the
morphology of the precursors (Fig. 5a–d). However, the
Fig. 5 Morphological and
structural characterization of
Fe3O4 nanowires: a, b SEM
images; c–e TEM images;
f HRTEM images
1298 Nanoscale Res Lett (2010) 5:1295–1300
123
diameters of the nanocrystals and the nanopores that
composed of the nanowires are about 5 and 2 nm (Fig. 5e,
f), respectively, which are smaller than the a-Fe2O3
nanowires due to the lower annealing temperature. The
lattice fringe with a lattice spacing of 0.25 nm corresponds
to the {311} plane of Fe3O4 (Fig. 5f).
IR analysis was employed to further confirm the trans-
formation from FeC2O4�2H2O nanowires to a-Fe2O3 and
Fe3O4 nanowires during the thermal treatments. As can be
seen from Fig. 6a, the peaks at 3,358, 1,629, 1,318, and
496 cm-1 are attributed to O–H, C=O, C–O, and Fe–O
functional groups, respectively, indicating the formation of
FeC2O4�2H2O [26]. In order to clarify the difference of IR
spectra between Fe2O3 and Fe3O4, the magnified IR spectra
of Fe2O3 and Fe3O4 were analyzed (Fig. 6b). It can be seen
that there is only one peak at 574 cm-1 for Fe3O4, while
a-Fe2O3 shows two or three peaks which is related with its
structure and size. In addition, c-Fe2O3 also exhibit three
peaks between 500 and 700 cm-1, which is different from
Fe3O4 [27, 28]. The IR analysis combined with TEM
images and XRD pattern can confirm the synthesis of
Fe3O4 nanowires.
As a result, ultralong and uniform a-Fe2O3 and Fe3O4
nanowires were selectively synthesized by the annealing of
FeC2O4�2H2O nanowires under different temperatures and
atmosphere. Figure 7 shows the schematic illustration
diagram for the growth mechanism. As can be seen, when
two microemulsion solutions containing FeSO4 and
H2C2O4 are mixed, FeC2O4 nucleation and irreversible
micellar fusion may be coincident. During the nucleation,
the surfactant molecules of side surfaces of the cylindrical
droplets may adsorb on surface planes of the formed
FeC2O4 nucleus, which may result in the one-dimensional
growth of FeC2O4. Finally, a-Fe2O3 and Fe3O4 nanowires
can be obtained after the calcinations of FeC2O4 nanowires
under different conditions. The morphology, the structure,
and the phase of as-synthesized products could be deter-
mined by the precursors and the annealing conditions.
Based on the above-mentioned analysis, the mechanism
for the formation of a-Fe2O3 and Fe3O4 nanowires can act
as a guideline for the synthesis of the other metal oxide
nanowires. Herein, Co3O4 nanowires were also synthesized
through the similar procedures. Figure 8 shows the as-
synthesized precursor and Co3O4 nanowires. It can be seen
that porous Co3O4 nanowires with diameters of about
200 nm and lengths of several micrometers can be obtained
by the calcinations of CoC2O4 nanowires, which confirms
the versatility of the microemulsion-based method.
Conclusions
In summary, we have developed a microemulsion-based
method in combination of the calcinations to selectively
synthesize a-Fe2O3 and Fe3O4 nanowires with the diameter
of about 100 nm and the length of tens of micrometers. The
FeC2O4�2H2O nanowires used as the precursors were pre-
pared by a simple microemulsion-based method. a-Fe2O3
and Fe3O4 nanowires can be synthesized by the calcina-
tions of FeC2O4�2H2O nanowires under air and H2/Ar at
different temperatures, respectively. Moreover, it is
believed that the approach presented here can be extended
to synthesize other 1D metal oxide nanostructures. Co3O4
C=O
Fe-OC-O
-OH
Fe2O
3-720°C
Tra
nsm
itta
nce
Wavenumbers (cm-1)
FeC2O
4
Fe2O
3-550°C
Fe3O
4
a
4000 3500 3000 2500 2000 1500 1000 500 850 800 750 700 650 600 550 500 450 400
Fe3O
4
Fe2O
3-720°C
Fe2O
3-550°C 574
459534
443526
603Tra
nsm
itta
nce
Wavenumbers (cm-1)
bFig. 6 FTIR spectra of as-
synthesized products
Fig. 7 Schematic illustrations for LBL synthesis of a-Fe2O3 and
Fe3O4 nanowires
Nanoscale Res Lett (2010) 5:1295–1300 1299
123
nanowires have been fabricated to confirm the versatility of
the method for metal oxide nanowires.
Acknowledgments The authors would like to appreciate the
financial supports from 973 Project (No. 2007CB613403), 863 Project
(No. 2007AA02Z476), NSFC (No. 50802086), China Postdoctoral
Science Foundation funded project (No. 20090461350).
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which per-
mits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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a b
c
10 20 30 40 50 60 70
(400
)
(511
)
(422
)
(400
)
(222
)(3
11)
(220
)
Inte
nsi
ty (
a.u
.)
2θ
(111
)
d
Fig. 8 a SEM image of
CoC2O4 nanowires; b SEM
image of Co3O4 nanowires;
c TEM image of Co3O4
nanowires; d XRD of Co3O4
nanowires
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