Synthesis of CoFe2O4 nanowire arrays by sol-gel template method

Article abstract of DOI:10.1016/j.cplett.2003.08.090

CoFe₂O₄ nanowire arrays with an average diameter of about 40 nm were prepared in porous anodic aluminum oxide (AAO) template using sol-gel process. Transmission electron microscopy (TEM) diffraction pattern and X-ray diffraction (XRD) showed that the nanowires were polycrystalline phase. Magnetic measurements showed the arrays of nanowires did not show a preferential magnetic orientation, the reason was briefly discussed. The effect of heating rate on the structure and magnetic properties of CoFe ₂O₄ nanowire arrays was investigated. The result showed that the coercivity decreased with the increase of the heating rate.

Full text of DOI:10.1016/j.cplett.2003.08.090

Chemical Physics Letters 379 (2003) 484–489
www.elsevier.com/locate/cplett
Synthesis of CoFe2O4 nanowire arrays
by sol–gel template method
*
Guangbin Ji , Shaolong Tang, Baolong Xu, Benxi Gu, Youwei Du
National Laboratory of Solid State Microstructure and Physics Department, Nanjing University, Nanjing 210093, PR China
Received 14 August 2003; in final form 21 August 2003
Published online: 17 September 2003
Abstract
CoFe2O4 nanowire arrays with an average diameter of about 40 nm were prepared in porous anodic aluminum oxide
(AAO) template using sol–gel process. Transmission electron microscopy (TEM) diffraction pattern and X-ray dif-
fraction (XRD) showed that the nanowires were polycrystalline phase. Magnetic measurements showed the arrays of
nanowires did not show a preferential magnetic orientation, the reason was briefly discussed. The effect of heating rate
on the structure and magnetic properties of CoFe2O4 nanowire arrays was investigated. The result showed that the
coercivity decreased with the increase of the heating rate.
Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction
templates, AAO template is an ideal template to
prepare ordered nanowire arrays because of its
uniform and nearly parallel porous structures [3].
The nanometer-sized ferrite magnetic materials
are the subject of intense research because of their
potential applications in high-density magnetic re-
cording media [4,5]. Among the family of ferrite
materials, cobalt ferrites, used in video tape and
high-density digital recording, are the most com-
mercially significant particulate recording materi-
als. Cobalt ferrite particles [6–10] and films [11–13]
have attracted the interests on the exploration of
recording materials. Though there have been nu-
merous publications about ferrite materials,
CoFe2O4 nanowire arrays have been scarcely re-
ported. Only Pham-Huu et al. [14] have synthesized
CoFe2O4 nanowires inside carbon nanotubes un-
der mild reaction conditions using the confinement
Recently, one of the most exciting areas in
materials science is the study of nanowire arrays
due to their potential applications in fields as
magnetic storage. Among the different preparation
procedures, one of the most widespread is the
template method [1,2]. This method entails syn-
thesizing the desired material within the pores of
a nanoporous host material. Different kinds of
template such as anodic aluminum oxide (AAO),
polymer and nano-channel glass templates have
been widely investigated. Compared with other
* Corresponding author. Fax: +86-25-332-6028.
E-mail addresses: jigbnju@yahoo.com, gbji@ufp.nju.edu.cn
(G. Ji).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2003.08.090

G. Ji et al. / Chemical Physics Letters 379 (2003) 484–489
485
effect provided by the carbon tubular template.
Synthesis of cobalt ferrite nanowire arrays by AAO
template has not been reported. In the present
work, we have combined the concept of sol–gel
synthesis and template preparation of nanowires to
fabricate CoFe2O4 nanowire arrays.
the template surface was wiped off. The template
surface was carefully wiped again after drying
under vacuum at 80 °C for 2 h. The sample was
annealed at 500 °C for 10 h in open air. The
temperature was ramped from room temperature
to 500 °C with different heating rate (2.7, 1.3, 0.8,
0.6 °C/min). Then the arrays of CoFe2O4 nanowire
inside the AAO template were obtained.
2. Experimental
2.3. Characterization and magnetic measurement of
CoFe2O4 nanowire arrays
2.1. Preparation of AAO template
Hexagonally ordered porous alumina template
was formed by anodization process as described
previously [15]. Briefly, the starting material,
99.9% pure Al foil, was degreased, annealed at 500
°C for about 2 h in open air. The foil was then dc
anodized in 0.3 M H2SO4 aqueous solution under
a constant voltage of 27 V for 10 h at 0 °C. Sub-
sequently, the pores of the anodic alumina were
widened while keeping the center-to-center dis-
tance fixed by immersing the anodized template in
0.36 M H3PO4 solution for 20 min at 30 °C,
yielding a ordered porous alumina layer contain-
ing straight, parallel pores with an average diam-
eter of 40 nm. The whole sheet was placed into
saturated CuSO4 solution to separate template
membrane with the Al substrate. The AAO tem-
plates was rinsed with distilled water, then dried at
30 °C.
The magnetic properties were measured by a
vibrating sample magnetometer (VSM, Lakeshore,
Model 7300 series) with the applied field either
perpendicular or parallel to the surface of the
samples. Scanning electron microscopy (SEM)
image was recorded with a JSM-6300 microscope.
For SEM sample, small pieces of AAO with
CoFe2O4 nanowire arrays sample were eroded by
an aqueous solution of 5% H3PO4 for 10 min at
30 °C in order to remove the upper part of AAO
and then washed with distilled water several times
and dried in air. The morphology of the CoFe2O4
nanowires was observed by transmission electron
microscopy (TEM, HITACHI-2000) after com-
pletely dissolving AAO template in an aqueous
solution of 1 M NaOH and washing several times
with double-distilled water. A drop of solution was
placed on Cu grids and allowed to dry prior to
electron microscope analysis. The phase structure
characterization of CoFe2O4 nanowire arrays was
carried out by X-ray diffraction (XRD) in the 2h
range 20–80 by means of a diffractometer with
Cu-Ka radiation.
2.2. Preparation of CoFe2O4 nanowire
The metal precursor salts FeCl3 and Co(OAc)2
with a molar ratio of 2:1 were dissolved in a vol-
ume of mixture solution (40 ml) containing citric
acid and ethylene glycol (1:4, molar ratio). The
solution formed a homogeneous mixed solution
under stirring, and then was heated at 140 °C to
induce esterification. A transparent sol was formed
upon evaporation of the excess ethylene glycol.
The viscosity of the sol was dependent on the ex-
tent of ethylene glycol evaporation.
The AAO template was subsequently immersed
in the sol for the desired time under vigorous
stirring at 100 °C. The AAO template in which the
sol was incorporated into the holes was taken out
of when the sol could not flow freely. Excess sol on
3. Results and discussion
3.1. TEM and SEM analysis
Fig. 1a shows the TEM image of AAO template
with an average diameter of 40 nm, the pores are
hexagonally arranged with an interpore distance of
60 nm. The pore diameter, the interpore distance
and the length can be varied over a wide range. A
typical SEM image of CoFe2O4 nanowire arrays is
shown in Fig. 1b. In this image, alumina matrix is

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G. Ji et al. / Chemical Physics Letters 379 (2003) 484–489
Fig. 1. TEM image of AAO template with an interpore distance of 60 nm. The diameter of the pore is about 40 nm (a). SEM image of
the CoFe2O4 nanowire arrays (b).
almost completely dissolved, and only the bottom
adheres together due to the residual alumina. The
nanowire arrays grown by AAO template fill the
nanochannels uniformly and the nanowires are
apparently continuous. The measured diameter of
the nanowires corresponds closely to the pore
diameter.
Fig. 2 shows the TEM images of our sample
with different heating rate prepared by so-gel
template method. The diameter basically equals to
that of pores of the used template. Fig. 2a is the
TEM image of the sample prepared at a heating
rate of 2.7 °C/min. We can see the sample consists
of a lot of obvious grains, which form long grain
Fig. 2. TEM images of CoFe2O4 nanowires annealed at 500 °C for 10 h in open air. The temperature was ramped from room
temperature to 500 °C at different heating rate 2.7 °C/min (a), 1.3 °C/min (b), 0.8 °C/min (c), 0.6 °C/min and corresponding electron
diffraction pattern in the inset on the upper right (d).

G. Ji et al. / Chemical Physics Letters 379 (2003) 484–489
487
chains. The nanowires form gradually with the
slow heating rate (Figs. 2b–d) due to the grain
growing and reaching a quite compact (Fig. 2d).
The TEM diffraction pattern shown in the inset of
Fig. 2d indicates that the CoFe2O4 nanowire is
polycrystalline phase.
From the TEM images, it can be concluded that
the compact nanowire can not be obtained if the
sample is quickly annealed from room temperature
to 500 °C (Figs. 2a,b). For the slow heating rate, the
crystalline quality improves (Figs. 2c–d). When the
heating rate is 0.6 °C/min, the crystalline quality is
quite good. However, if the heating rate below
0.6 °C/min, the crystalline quality is not obviously
improved.
Fig. 3. X-ray diffraction patterns of the CoFe2O4 nanowire
arrays annealed at 500 °C.
are almost the same in shape and the saturation
fields are similar for both measured directions
(Fig. 4). The magnetic properties of this magnetic
nanowire arrays is almost independent on the an-
gle of applied external field. It can be concluded
that the samples do not show a preferential mag-
netic orientation. The results of the magnetic
measurements of our sample do not show a pat-
tern similar to that of the Ni nanowire array
[17,18] reported previously. Here, we give a short
description of the analysis of the magnetic mea-
surement. The total magnetic anisotropy of the
magnetic nanowire arrays is influenced by shape
anisotropy and magneto-crystalline anisotropy.
The growth mechanism of our sample prepared
at different heating rates is below. During the tem-
perature ramping in the tube furnace, the sol within
the AAO template changes into gel and further
spontaneously burns liberating vapors such as H2O,
CO2 [16]. At high heating rate, a large amount of
vapors form in short time, and gush from AAO
channels, resulting in a sample with low particle
density (Fig. 2a). As the heating rate is decreased,
more and more nanometer cobalt ferrite clusters
precipitate and crystallize due to a limited produc-
tion of gas, the particles of cobalt ferrite grow slowly
and the density of cobalt ferrite increases signifi-
cantly which inducing the nanowire formation.
3.2. X-ray diffraction analysis
Fig. 3 shows the XRD pattern of the sample
within AAO templates prepared at 500 °C. Al-
though the background diffraction peaks of Al2O3
are present, the diffraction peaks of CoFe2O4 can
be observed. The average crystallite size of
CoFe2O4 nanowire is estimated to 21 nm using the
Scherrer equation from the prominent (3 1 1) re-
flection of the diffraction pattern. From the XRD
results, the arrays of CoFe2O4 nanowires show
polycrystalline structure in accord with the results
of the TEM diffraction pattern above.
3.3. Magnetic properties
Fig. 4. Hysteresis loops of CoFe2O4 nanowire arrays with a
diameter about 40nm and center to center distance of about 60
nm measured parallel and perpendicular with respect to the
nanowires.
Magnetic measurements show that the in-plane
and perpendicular hysteresis loops of the sample

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G. Ji et al. / Chemical Physics Letters 379 (2003) 484–489
electron microscopy and X-ray diffraction indi-
cated that CoFe2O4 nanowire arrays with poly-
crystalline structure were uniformly assembled
into the ordered nanochannels of AAO. VSM
measurements showed that the arrays of nano-
wires did not exhibit a preferential magnetic ori-
entation. We have investigated the effect of heating
rate on the structure and magnetic properties of
cobalt ferrite nanowire arrays. When the samples
were heated more slowly to the desired annealing
temperatures, the coercivity increased. The mech-
anism of this phenomenon could be attributed to
the increase of the density at slow heating rate and
the improvement of the crystalline quality.
Fig. 5. Coercivity of CoFe2O4 nanowire arrays as a function of
heating rate, measured with the external applied field parallel to
the nanowires.
Acknowledgements
For CoFe2O4 nanowire arrays, the magneto-crys-
talline anisotropy (K1 ¼ 270 ꢀ 103 J/m3) is more
larger than that of Ni nanowire arrays
(K1 ¼ ꢁ5:48 ꢀ 103 J/m3). Therefore, unlike Ni
nanowire arrays, magneto-crystalline anisotropy
of CoFe2O4 nanowire arrays makes the main
contribution to the total anisotropy and the sam-
ples obtained in our experiments are inhomoge-
neous polycrystalline with the magnetic domain
irregularly distributed, resulting in the absence of
perpendicular anisotropy.
This work was supported by the Project No.
50171033 of National Natural Science Foundation
of China and a national key project of funda-
mental research (973, No. G 1999064508)
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