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: mseyang@zju.edu.cn

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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