thermal oxide synthesis and characterization of fe3o4 nanorods and fe2o3 nanowires

Science in China Series B: Chemistry

© 2009 SCIENCE IN CHINA PRESS
Springer
Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604

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Thermal oxide synthesis and characterization of Fe
3
O
4

nanorods and Fe
2
O
3
nanowires
JIAO Hua
1,2†
& YANG HeQing
1

1
Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Materials Science, Shaanxi Normal
University, Xi’an 710062, China;
2
Department of Chemistry and Chemical Engineering, Weinan Teacher’s University, Weinan 714000, China
Fe
3

O
4
nanorods and Fe
2
O
3
nanowires have been synthesized through a simple thermal oxide reaction
of Fe with C
2
H
2
O
4
solution at 200－600℃ for 1 h in the air. The morphology and structure of Fe
3
O
4

nanorods and Fe
2
O
3
nanowires were detected with powder X-ray diffraction, scanning electron mi-
croscopy and transmission electron microscopy. The influence of temperature on the morphology de-
velopment was experimentally investigated. The results show that the polycrystals Fe
3
O
4
nanorods with
cubic structure and the average diameter of 0.5－0.8 µm grow after reaction at 200－500℃ for 1 h in the

air. When the temperature was 600℃, the samples completely became Fe
2
O
3
nanowires with hexagonal
structure. It was found that C
2
H
2
O
4
molecules had a significant effect on the formation of Fe
3
O
4
nano-
rods. A possible mechanism was also proposed to account for the growth of these Fe
3
O
4
nanorods.
thermal oxide process, nanorods, nanowires, C
2
H
2
O
4
, iron sheet
1 Introduction
Fe

3
O
4
is an important magnetite material having cubic
inverse spinel type structure, which has been widely
used as magnetic fluid and magnetic recording materials,
due to its unique electrical and magnetic properties
[1,2]
.
Nanoscale Fe
3
O
4
has been applied in magnetic ink
[3]
,
electronics and bio-sensitive materials
[4,5]
, high density
magnetic recording media and biomedical fields
[6
－
9]
,
because of its good compatibility with organism and its
electrical and magnetic characteristics of its size and
morphology. Therefore, the preparation of Fe
3
O
4

nanos-
tructures and its properties research are extremely active
in recent years.
At present, different kinds of Fe
3
O
4
nanostructures
have been successfully synthesized via various physical
and chemical methods. For example, the monodisperse
Fe
3
O
4
nanoparticles were prepared by solvothermal and
high temperature organic liquid reflux method
[10
－
13]
, on
the basis of which three-dimensional superlattice has
been assembled
[14]
. Recently, Yu et al. prepared the
structure of octahedron by reflux method
[15]
. The Fe
3
O
4

nanorods, nanowires,
branch-like nanowires, nanochains,
octahedral structure, nanoflakes, peanut-like Fe
3
O
4
,
nanotubes and nanopyramid arrays were prepared by
hydrothermal method
[16
－
23]
, electroprecipitation meth-
od
[24]
, ultrasound irradiation
[25]
, PLD-assisted VLS
[26,27]
,
and microwave plasma chemical vapor deposition tech-
nique (MWCVD)
[28]
, respectively.
Recently, some researchers focused on the investiga-
tion of the synthesis of Fe
3
O
4

nanorods. Wan et al.
[29]

obtained Fe
3
O
4
nanorods with an average diameter of 25
nm, length of 200 nm via hydrothermal reaction of
FeSO
4
·7H
2
O and FeCl
3
at 120℃ for 20 h. Kumar et
al.
[25]
prepared the Fe
3
O
4
nanorods with acetic ferrous
and the stabilizer of cyclodextrin under Ar atmosphere

Received May 5 2008; accepted November 18, 2008
doi: 10.1007/s11426-009-0092-1
†
Corresponding author (email: )
Supported by the Fund of Weinan Teacher’s University (Grant No. 08YKZ008), the

National Natural Science Foundation of China (Grant No. 20573072) and the Doc-
toral Fund of Ministry of Education of China (Grant No. 20060718010)

600 JIAO Hua et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604
of 0.15 MPa. These synthesized methods of nanorods
usually require organic solvents and complex operation.
In the present investigation, iron sheet as source and
dripping acid solution on the surface of iron sheet were
adopted to prepare Fe
3
O
4
nanorods. The Fe
3
O
4
nanorods
with the rectangular cross-section and approximate
0.5－0.8 μm length were obtained by oxidizing at low
temperature for 1 h. Subsequently, Fe
2
O
3
nanowires in
the range of 100－300 nm were obtained at 600℃.
2 Experimental
2.1 Experimental materials
All the chemical reagents in our experiments are ana-
lytical grade and they are used without further purifica-
tion. Iron (Fe, 99.6%) was obtained from Shaanxi Huaou

Industry Ltd, and oxalic acid (C
2
H
2
O
4
·2H
2
O, analytical
grade) was purchased from Xi’an reagent factory.
2.2 Syntheses of Fe
3
O
4
nanorods
In a typical experiment, a sheet of iron with size of 1×1
cm was polished by the sand paper and dealt with alco-
hol in ultrasonic for 15 min. Then, it was placed in
quartz boat and a drop of oxalic acid (0.75 mol·L
−1
) so-
lution was taken onto the iron surface. After that, the
quartz boat was placed in the oven and maintained at
200, 300, 400, 500 and 600℃ for 1 h with heating rate
of 10℃·min
−1
before being naturally cooled to room
temperature. There was a red and black thin film on the
surface layer of the iron sheet.
2.3 Characterization of products

X-ray powder diffraction (XRD) patterns of the products
were obtained on a Japan Rigaku D/Max-ⅢC diffrac-
tometer at a voltage of 60 kV and a current of 80 mA
with Cu Kα radiation (λ=1.5406 Å), employing a scan-
ning rate of 8° min
−1
in the 2θ ranging from 10° to 70°.
Scanning electron microscopy (SEM) images were ex-
plored on a Holand model FEI Quanta 200 microscope.
Transmission electron microscopy (TEM) images were
taken on a JEOL JEM-3010 transmission electron mi-
croscope at an accelerating voltage of 200 kV.
3 Results and discussion
3.1 SEM analysis
Figure 1(a)－(f) showed the SEM images of samples
synthesized by reactions of C
2
H
2
O
4
with Fe at 300℃
for 1 h and Figure 2(a)－(d) showed the SEM images of
the samples synthesized at 200, 400, 500 and 600℃ for
1 h, respectively. It can be seen clearly that samples
were nanorods with rectangular cross-section and the
size between 0.5－1.0 μm at 200℃, as seen in Figure
2(a). When the reaction temperature was increased to
300℃, the different magnification of the front SEM im-
ages were shown in Figure 1(a), (c)－(f) and the side

SEM image was shown in Figure 1(b). As seen in Figure
1(c) and (e), the shape of nanorods samples were the
appearance of rectangular cross-section and with the
length range of 0.5－0.8 μm. A number of rods were
split along the same axis. The results showed that the
smaller rods and lines were split by the relatively coarse
rods, as seen in Figure 1(d). Figure 1(f) was the
cross-section image of single nanorod with the length of
0.8 μm and width of 0.6 μm under the high multiple.
When the reaction temperature was increased to
400℃, the samples were a small amount of nanolines



Figure 1 SEM images of samples synthesized by reactions of C
2
H
2
O
4

with Fe at 300℃ for 1 h.

JIAO Hua et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604 601
expect for the nanorods of rectangular cross-section, as
shown in Figure 2(b). When the temperature was 500℃,
the morphology of the sample was changed continuously
from nanorods with the size range of 0.3－0.5 µm to
nanowires with the size range of 100－300 nm in Figure
2(c). When the reaction temperature was increased to

600℃, the morphology of the sample was nanowires
with the size range of 100－300 nm, as shown in Figure
2(d). From the analysis of the reaction kinetic, a part of
lower energy molecules became activated as the tem-
perature increased. Later, the increasing chance of the
effective collisions made the reaction rate (ν) increase.
As the temperature was increased, the decomposition
rate of oxalic acid (ν
d
) increased with the gas-liquid in-
terface of the oxalic acid solution contacting with air. At
the same time, the reaction rate (ν
r
) was also increased
with the liquid-solid interface of the oxalic acid solution
contacting with Fe. When the temperature was up to
600℃, ν
d
>ν
r
, the sample morphology was mainly de-
cided by the reaction of water vapor and Fe
[30]
.



Figure 2 SEM images of samples synthesized by reactions of C
2
H

2
O
4

with Fe at different temperatures for 1 h. (a) 200℃; (b) 400℃; (c) 500℃;
(d) 600℃.

3.2 XRD analysis
Figure 3(a)－(e) showed the XRD patterns of the sam-
ples prepared from 200－600℃ for 1 h. The samples
were obtained at 200℃ with the two diffraction peaks
corresponding to the cubic structure of Fe (110) (200)
crystal plane (JCPDS No. 06-0696), as shown in Figure
3(a). It indicated that the crystal sample was not com-
plete at 200℃. When the temperature was increased to
300－500℃, the diffraction peak corresponding to the
cubic phase of Fe
3
O
4
(111), (220), (311), (222), (331),
(511) crystal plane (JCPDS No. 65-3107) and hexagonal
phase of Fe
2
O
3
(012), (104), (311), (113), (024), (116),
(214) crystal plane (JCPDS No. 33-0664) became wide
and weak, which indicated that the product was multi-
crystalline structure. Further, the Fe

3
O
4
diffraction peaks
disappeared gradually. When the temperature was up to
600℃, the Fe
3
O
4
diffraction peaks were not obvious,
which indicated that Fe
2
O
3
could be obtained in higher
temperatures.



Figure 3 XRD images of samples synthesized by reactions of C
2
H
2
O
4

with Fe at different temperatures for 1 h. (a) 200℃; (b) 300℃; (c) 400℃;
(d) 500℃; (e) 600℃.

3.3 TEM analysis

In order to determine the detailed crystalline structure,
TEM measurements were employed to investigate the
samples prepared at 300℃ for 1 h. A typical TEM image
of the single Fe
3
O
4
nanorod was shown in Figure 4(a).
The size of the nanorod with the length of 2.4 µm and
the width of 0.5 µm was in good agreement with the
above SEM image shown in Figure 1. Figure 4(b) is the
top TEM image of Figure 4(a). It can be seen from the
Figure 4(b) that the samples of nanorods were fibri-
form-like structure self-assembly. A selected area elec-
tron diffraction (SAED) pattern was presented in Figure
4(c) according to the rectangular frame of Figure 4(b),

602 JIAO Hua et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604


Figure 4 TEM images of samples synthesized by reactions of C
2
H
2
O
4

with Fe at 300℃ for 1 h. (a)－(c)TEM; (d) SAED.

indicating the cubic phase of Fe

3
O
4
(311), (400), (220),
(511) crystal plane diffraction. Meanwhile, the energy
dispersive spectrometer (EDS) was used to analysis the
chemical composition of the sample, and the results can
be seen in Figure 4(d). It can be clearly identified that
the nanorods were composed of Fe and O elements, and
the ratio of the number of atoms Fe and O was about
3︰4. The TEM, SAED and EDS analyses revealed that
Fe
3
O
4
nanorods were of polycrystalline cubic phase
structure.
3.4 The influence of oxalic acid on the morphology
of samples
In order to study the role of oxalic acid, the morphology
of the products from water reacting with Fe sheet at
300℃ and 600℃ for 1 h in the air were investigated,
respectively, as shown in Figure 5. From Figure 5(a), it
can be observed that, when the reaction temperature was
300℃, the surface of the Fe sheet had not shaped regu-
larity morphology, only sporadic small particles. When
the reaction temperature was up to 600℃, the surface of
Fe sheet was nanowires with the size of 100－300 nm in
Figure 5(b). It was obvious that the addition of oxalic
acid was benefit for the formation of nanorods in the

temperature range of 200－500℃.
Figure 6 showed the XRD patterns of the samples
above. It can be seen from the patterns, the two diffrac-
tion peaks were corresponding to the cubic phase Fe
(110) (200) crystal plane (JCPDS No. 06-0696) under


Figure 5 SEM images of samples synthesized by reactions of H
2
O with
Fe at 300℃ (a) and 600℃(b) for 1 h in air.



Figure 6 XRD images of samples synthesized by reactions of H
2
O with
Fe at 300℃ (a) and 600 ℃(b) for 1 h in air.

300℃. When the temperature was up to 600℃, the sam-
ples of the diffraction peaks corresponding to the hex-
agonal phase of the Fe
2
O
3
(JCPDS No. 33-0664) indi-
cated that dropping water on the iron surface did not
react at 300℃, and the pure Fe
2
O

3
products were ob-
tained at 600℃. The results of SEM and XRD indicated
that Fe
3
O
4
nanorods on the surface of Fe sheet were
complexation reaction of oxalic acid and iron at a rela-
tively low temperature.
In order to determine the detailed crystalline structure,
TEM measurements were employed to investigate the
samples prepared at 600℃ for 1 h in air, on the surface
of which water dripped, as shown in Figure 7. A typical
TEM image of single Fe
2
O
3
nanowire was shown in
Figure 7(a). The size of the nanowire was in good
agreement with the above SEM image shown in Figure
5(b) with the length of about 100 nm. Figure 7(b) is the
high resolution TEM image of Figure 7(a). It can be
seen from the image that the crystal plane spacing was
0.37 nm, corresponding to the distance of hexagonal

JIAO Hua et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604 603


Figure 7 TEM images of samples synthesized by reactions of H

2
O with
Fe at 600℃ for 1 h. (a),(d)TEM; (b) HRTEM; (c) SAED.

phase of Fe
2
O
3
(012) crystal plane. A SAED pattern was
presented in Figure 7(c) according to Figure 7(a), indi-
cating the hexagonal phase of Fe
2
O
3
[0001] zone axis
diffraction. The growth of nanowires was from the
rough to the fine in Figure 7(d). The TEM and HRTEM
analyses revealed that Fe
2
O
3
nanowires were of single
crystalline hexagonal phase structure.
3.5 Mechanism
Based on the above results, the reaction process was: the
oxalic acid solutions contacted with air and formed an
interface of the gas-liquid phase after oxalic acid drip-
ping on iron surface, which occurred as reaction of ox-
alic acid decomposition (ν
d

). Meanwhile, the reaction of
the oxalic acid solution and Fe happened on the liq-
uid-solid interface (ν
r
). When the Fe sheet with a drop of
acid was placed in the oven, before reaching the de-
composition temperature of 190℃, ν
d
<ν
r
, the oxalic acid
occurred complexation reaction with iron and obtained
ferrous oxalate, as seen eq. (1); latter, the unstable fer-
rous oxalate decomposition became FeO (eq. (2)); fer-
rous oxide was oxidized to Fe
3
O
4
by oxygen in the air
(eq. (3)); the ν
d
>ν
r
was increasing with the temperature
increasing at the same time. When the temperature was
up to 600℃, ν
d
>ν
r
, Fe

2
O
3
nanowires were the result of
Fe reacted with water vapor (eq. (4)).
The chemical reactions can be expressed as:

2 224 24 2
2Fe O 2H C O 2FeC O 2H O
+
+→+ (1)

24 2
FeC O FeO CO + CO→+ (2)

234
6FeO + O 2Fe O→ (3)

22 232
4Fe + O + 2H O Fe O H O→⋅ (4)
Firstly, at a relatively low temperature, Fe
3
O
4
nano-
rods were obtained in situ with oxalic acid solution
dripped on. The ferrous oxalate was obtained via heat
treatment in the air (Figure 8(b)). FeO was obtained
from the unstable ferrous oxalate decomposition.
Whereafter, FeO was oxidized to Fe

3
O
4
grains by oxy-
gen in the air (Figure 8(c)). The nanorods grew from
saturation Fe
3
O
4
grains as the reaction going on (Figure
8(d)). When the reaction temperature was up to 600℃,
the product was only the nanowires due to the high reac-
tion temperature. Actually, the reaction happened be-
tween the water vapor and iron, and the growth process
was depicted in Figure 9. Fe
2
O
3
·nH
2
O grains were
gained in the air under high temperature (Figure 9(a) and
(b)). Fe
2
O
3
·nH
2
O grain began to decompose and became
Fe

2
O
3
nanocrystals as the temperature increased (Figure
9(c)). The nanowires grew from saturation Fe
2
O
3
grains
as the reaction going on (Figure 9(d)).



Figure 8 Schematic diagram of the growth process of Fe
3
O
4
nanorods.



Figure 9 Schematic diagram of the growth process of Fe
2
O
3
nanowires.

4 Conclusions
In summary, we successfully prepared Fe
3

O
4
nanorods
and Fe
2
O
3
nanowires via a simple thermal oxide process.
We investigated the influence of reaction temperature on
the samples morphology. A possible mechanism was

604 JIAO Hua et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 599-604
also proposed to account for the growth of these samples.
These Fe
3
O
4
nanorods and Fe
2
O
3
nanowires have poten-
tial applications in future magnetic materials, sensor
materials and new type of catalysts.

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