Fabrication and magnetic properties of metallic nanowires via AAO templates

Article abstract of DOI:10.1016/j.jmmm.2009.03.074

Metallic (Ni, Co, Cu and Fe) nanowires were fabricated by electrodeposition into anodic aluminum oxide (AAO) template. In this work, we have studied the effect of the electrode potential on the microstructure and magnetic properties of nanowires. Transmis

Full text of DOI:10.1016/j.jmmm.2009.03.074

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 321 (2009) 2712–2716
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Fabrication and magnetic properties of metallic nanowires via AAO templates
a
a
a,
Ã
b,c
S. Thongmee , H.L. Pang , J. Ding , J.Y. Lin
a
Department of Materials Science and Engineering, National University of Singapore, 117576, Singapore
Department of Physics, National University of Singapore, 117560, Singapore
Institute of Chemical and Engineering Sciences, 627833, Singapore
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Metallic (Ni, Co, Cu and Fe) nanowires were fabricated by electrodeposition into anodic aluminum oxide
(AAO) template. In this work, we have studied the effect of the electrode potential on the microstructure
and magnetic properties of nanowires. Transmission electron microscopy (TEM) results showed that the
metal nanowires were single-crystal. Cu and Ni nanowires had the same orientation along the [2 2 0]
direction, while Co had a preferred orientation along the [10 0] direction. Fe nanowires had a preferred
orientation along the [2 0 0] direction. The growth mechanisms are probably due to the competition
growth of the adjacent grains and the confinement of growth in the nano-sized hole of the AAO
template. Results showed that single crystal or highly textured nanowires had better magnetic
properties compared with that of polycrystal nanowires in terms of coercivity and squareness.
Received 15 November 2008
Received in revised form
8
February 2009
Available online 1 April 2009
Keywords:
Anodic aluminum oxide (AAO) template
Ni, CoFe and Cu nanowires
&
2009 Elsevier B.V. All rights reserved.
1
.
Introduction
following steps: removing the aluminum matrix through etching;
deposition of a conducting metal layer on the back of the AAO
template as the working electrode; electroplating for the forma-
tion of nanowires. To date, many metallic nanowires have been
produced on AAO template, including low melting point metals
such as Au, Ag and Zn nanowires [10–12] and high melting point
metals such as Ni, Fe and Co nanowires [13]. For metallic
nanowires, single-crystal of low melting point nanowires has
been reported and single crystal of high melting point nanowires
has been reported by Pan et al. [14]. In this work, we studied the
effect of the electrode potential on the growth of metallic (Ni, Co,
Fe and Cu) nanowires via AAO template by the electrodeposition
process. The microstructure and magnetic properties of these
nanowires are investigated in detail.
Magnetic nanowires represent an important family of mag-
netic nanostructures due to their potential applications in high-
density recording, high-frequency sensors and other devices as
well as materials for fundamental studies on the magnetic and
transport in dimensionally confined magnetic system [1]. The
anodic aluminum oxide (AAO) template plays an important role in
the synthesis of one-dimensional nanomaterials due to their
particular characters such as controllable pore diameter and
periodicity, extremely narrow distribution of pore size and the
pore has ideal cylindrical shape [2,3]. This offers a promising route
to synthesize a large-area, ordered nanostructure with high aspect
ratio, which is difficult to form by the conventional lithographic
process. Masuda and Fukuda [4] advanced the two-step anodiza-
tion process, leading to perfect hexagonal pore structure for
certain sets of parameters. Thus, the template synthesis has
recently been improved to be an elegant chemical approach for
the fabrication of nanoscale materials and alternative to sophis-
ticated methods such as molecular beam epitaxy and microlitho-
graphy [5]. Chemical methods are widely used to fabricate
nanoscale materials using AAO templates such as alternating
current electrodeposition [6], direct current electrodeposition
2
.
Experiment
In this work, anodic aluminum oxide template was prepared by
two-steps anodizing process [4]. As the starting material, high-
purity (99.999%) aluminum foil was used. Before anodizing, the
aluminum foil was degreased, etched by NaOH to remove
aluminum oxide surface layer, and electropolished. The aluminum
foil was then anodizing in acidic solutions to form a layer of
porous alumina.
[
7,8] and sol–gel [9]. Among the chemical processes approaches to
the fabrication of one-dimensional nanoscale, electrodeposition-
based synthesis method has been widely used. The general
process in the direct current electrodeposition consists of the
The aluminum foil was anodized under constant voltage of
4
0 V in 30 g/l H
step of anodizing, the aluminum oxide film was dissolved in the
mixture of 18 g/l H CrO and 60 g/l H PO at 80 1C for 1 h to
2 2 3
C O for 6 h at room temperature. After the first
2
4
3
4
Ã
remove the aluminum oxide layer. The second step of anodization
was carried out at the same condition as the first step for another
Corresponding author.
E-mail addresses: msedingj@nus.edu.sg, msets@nus.edu.sg (J. Ding).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.03.074

ARTICLE IN PRESS
S. Thongmee et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2712–2716
2713
6
h. The remaining aluminum was removed in 80 g/l CuCl
2
4
the potential was increased to 2.0 V, Cu nanowires showed very
strong (2 2 0) peak and without any other peaks, indicating that
the grown wire is in the single-crystal state, as shown in Fig. 1(c).
Further increasing the applied potential led to polycrystalline
wires again as shown in Fig. 1(d). The XRD results of Ni nanowires
are similar to Cu nanowires. Ni nanowires were single crystal with
an orientation along the [2 2 0] direction when deposited at 2.0 V.
Fig. 2 displays XRD patterns of Co and Fe nanowires. The results in
Fig. 2(a) show the strong hcp-(10 0) peak at higher potential
(2.0 V) without any other peaks, indicating that Co nanowires
have a preferred orientation along the [10 0] direction. For Fe
nanowires, it had a highly textured in the [2 0 0] direction at 2.0 V
since there was still a small (110) peak. It should be noted that we
could not obtain single-crystal Fe nanowires by applying various
potentials.
The single- and polycrystal structures were further confirmed
by HRTEM and selected area electron diffraction (SAED). The
detail of the microstructural evolution of Ni nanowires is shown
in Fig. 3. The samples were prepared by removing the AAO
template in an aqueous solution of 1 M NaOH. It can be seen that
the diameter of nanowires corresponds to the diameter of the
template hole (ꢀ50 nm). The image of Fig. 3(a) shows that at low
potential (0.5 V) the Ni sample result in polycrystalline structure
with non-uniform grains. The polycrystalline structure can be
solution. A subsequent etching was carried out in 5 g/l H PO
3
solution at room temperature to remove the barrier on the bottom
side of the AAO template and widen slightly the pores of the AAO
template. After this process, the diameter of the hole in the back
side of AAO template was adjusted to about 35 nm. The diameter
of pore is about 50 nm with a thickness of about 50 mm.
A conventional three-electrode electrochemical cell was used
to fabricate metal nanowires. A gold layer of thickness of about
100 nm was sputtered on one side of AAO template and used as
working electrode to fabricate metal and alloys nanowires.
Graphite rod and saturated calomel reference electrode (SCE)
were used as the counter and reference electrode, respectively.
The solution of metallic nanowires in this experiment had the
following composition: 200 g/l CoSO
nanowires; 150 g/l NiSO , 13 g/l NiCl
nanowires; 50 g/l CuSO , 10 g/l H BO
FeSO , 45 g/l H BO and 1 g/l ascorbic acid for Fe nanowires. The
4
,
40 g/l
H
3
BO
3
for Co
4
2
and 25 g/l H
3
BO
3
for Ni
4
3
3
for Cu nanowires and 120 g/l
4
3
3
pH value of the electrolyte was maintained at 4 for Co nanowire, 3
for Ni, Cu and Fe nanowires, respectively, adjusted by adding
2 4
H SO . Electrodeposition of nanowires was performed at various
electrode potentials ranging from 0.5 to 4.0 V at room temperature
with SCE. To keep the length of nanowires equal in all the samples,
long deposition time was kept for more than 3 h.
The morphology and microstructure of the AAO template and
metals: metals nanowires were observed by scanning electron
microscopy (SEM) and transmission electron microscopy (TEM).
The phase structure of resultant nanowires was carried out by
(100)
X-ray diffraction (XRD) with Cu-K
properties were measured by vibration sample magnetometer
VSM) with the applied field either perpendicular or parallel to the
surface of the samples.
a radiation. The magnetic
(
(200)
3.
Results and discussion
(110)
(211)
In order to study the structure of nanowires, X-ray diffraction
40
50
60
70
80
90
100
was firstly used. Fig. 1 shows the XRD patterns of Cu nanowires
prepared by various potentials at room temperature. From the
XRD spectra, Cu nanowires changed from polycrystalline to
single-crystalline state with the increase of applied potentials.
All the Cu nanowires exhibited face-centered cubic (fcc) structure.
As shown in Fig. 1(a), (111) and (2 0 0) peaks of Cu nanowires are
very strong with a relatively weak (2 2 0) peak after deposition at
the potential of 0.5 V. When the potential increased, the intensity
of (2 2 0) peak became stronger and it still accompanied with
2
θ (deg)
Fig. 2. XRD patterns of: (a) Co nanowire and (b) Fe nanowire. The samples
prepared at an applied voltage of 2.0 V.
(111) and (2 0 0) peaks. The intensity of (2 0 0) peak was strongly
decreased (see Fig. 1(b)), indicating a strong (2 2 0) texture. When
(111)
(200)
(220)
(111)
(220)
(200)
(220)
(200)
(220)
(111)
40
50
60
70
θ (deg)
80
90
2
Fig. 3. TEM micrographs and selected area electron diffraction (SAED) patterns of
Ni nanowires: (a) bright field image of polycrystalline Ni, (b) SAED of (a), (c) bright
field image of single-crystal Ni and (d) SAED of (c).
Fig. 1. XRD patterns of Cu nanowires prepared at room temperature by varying
voltages: (a) 0.5 V, (b) 1.0 V, (c) 2.0 V and (d) 4.0 V.

ARTICLE IN PRESS
2714
S. Thongmee et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2712–2716
confirmed by SAED as shown in Fig. 3(b). The results were
consistent with the XRD. When the Ni sample prepared at 2.0 V
was examined, the result showed smooth and uniform structure
preferred to grow in a certain direction until the wire changed to a
single-crystal. The orientation of nuclei of Ni was random and a
texture was induced by the competitive growth between adjacent
grains. The confinement of the nanowire growth in the AAO holes
leads to orientation-preferred growth during the deposition.
Furthermore, the deposition potential will affect the growth of
the ferromagnetic nanowires. The TEM analysis in this work
confirmed the growth mechanisms. The results of single- and
polycrystal Cu and Co were similar to Ni nanowires.
(see Fig. 3(c)). The SAED pattern showed a highly textured
structure with the preferred orientation in the [2 2 0] direction,
as shown in Fig. 3(d). The results of TEM micrographs and SAED
patterns showed that the growth of Ni nanowires at low potential
was random with a polycrystalline structure. Then, the wire
Fig. 4 shows the TEM micrograph of Fe nanowires. Several
nanowires were examined. At low and very high potentials, the
Fe nanowires were found to be only polycrystalline, as shown in
Fig. 4(a). The polycrystalline structure can be further confirmed
by the selected area electron diffraction pattern, as shown in
Fig. 4(b). The pattern indicated
a bcc-structure of the Fe
nanowires, which is consistent with that observed in XRD
spectrum. When the Fe nanowires deposited at 2.0 V were
examined, single-crystal structure could be observed in most of
the areas, as shown in Fig. 4(c). The SAED in Fig. 4(d) shows that Fe
nanowires have a preferred (2 0 0) orientation along the wires and
SAED also revealed the single-crystal structure of the sample.
However, it should be noted that in our XRD examination, the
peaks other than (2 0 0) could still be observed, indicating that Fe
wires were highly textured with a (2 0 0) orientation.
The magnetic properties of Ni, Co and Fe nanowires embedded
in an AAO template are closely related to their microstructure and
therefore to the growth condition. It is well known that reducing
the nanowires diameter could improve the squareness of the
magnetization hysteresis and raise the coercivity of nanowires.
[15–19]. The magnetization hysteresis loops in Fig. 5 were taken
Fig. 4. TEM micrographs and SAED patterns of Fe nanowires: (a) bright field image
for Ni and Fe nanowires embedded in AAO template at two
different directions, parallel (out-of-plane of AAO template) or
of polycrystalline Fe, (b) SAED of (a), (c) bright field image of single-crystal Fe and
(d) SAED of (c).
Fig. 5. Magnetization curves of Ni and Fe nanowires embedded in the AAO template: (a) Ni nanowires (prepared at 0.5 V), (b) Ni nanowires (prepared at 2.0 V), (c) Fe
nanowires (prepared at 0.5 V) and (d) Fe nanowires (prepared at 2.0 V).

ARTICLE IN PRESS
S. Thongmee et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2712–2716
2715
Table 1
perpendicular (in-plane of AAO template) to the nanowires long
axis. Fig. 5(a) and (b) show hysteresis loops of Ni nanowires by
applying different potentials. From Fig. 5(a), coercivity and
remanence are very low (see Table 1). Increasing the potential
led to an increase in the coercivity and remanence. When the
nanowires are in the single-crystal state, the highest coercivity
and remanence could be obtained, indicating that single-crystal
wires are advantage to the high coercivity and remanence. From
Table 1, single-crystal Ni nanowires prepared at an applied voltage
of 2.0 V have a coercivity of 1000 Oe and squareness of 0.997,
which is much higher than that of polycrystal wires. Following the
same trend, single-crystal Co nanowires also showed a coercivity
of 1320 Oe and a squareness of 0.797. Similarly, high-textured Fe
nanowires showed a coercivity of 1650 Oe and a squareness of
Coercivity and squareness of Ni, Co and Fe nanowires.
Sample (V)
H
c
(Oe) (J)
H
c
(Oe) (?)
M
r
/M
s
(J)
M
r
/M
s
(?)
Ni (0.5)
Ni (2.0)
Co (0.5)
Co (2.0)
Fe (0.5)
Fe (2.0)
580
1030
675
162
143
201
250
194
235
0.486
0.997
0.535
0.797
0.368
0.848
0.066
0.045
0.043
0.035
0.024
0.032
1320
836
1650
0.848. From the results above, it has shown that single crystal or
highly textured nanowires have better magnetic properties than
that of polycrystal nanowires in terms of coercivity and
squareness. It is known that the coercivity and squareness of
metallic nanowires are strongly dependent on the aspect ratio.
Higher coercivities have been achieved in the Ni, Co and Fe
nanowires with a diameter of 5–10 nm [16–19].
Fig. 6 summarized the magnetic property dependence on the
applied potential of Ni, Co and Fe nanowires. From Fig. 6, it is
shown that the applied potential strongly affect the magnetic
properties. For Ni, Co and Fe wires, when the applied voltage is
2.0 V, the coercivity is the largest and the wires preferred single-
crystal growth. The growth of the single-crystal nanowires may be
related to the growth rate. For the small applied potential, the
growth rate is slow. Hence, there is enough time for the wires to
arrange to form polycrystal structure, thus reduce the total system
energy. By increasing the applied potential, the growth rate
increases. The atoms do not have enough time to arrange. The
competition between the AAO walls and the nanowires force the
nanowires growing in one direction. For an example of Ni
nanowires, since (2 2 0) is the highest energy direction, for
minimizing the surface energy of nanowires (facing the AAO
nanowall), the wires prefer to grow in the (2 2 0) direction. If
further increasing the applied potential, the confinement effect
may not work due to the very high growth rate of the nanowires.
4
.
Conclusion
The metallic (Co, Ni, Cu and Fe) nanowires were successfully
prepared by electrodeposition into pores of an AAO template.
Single-crystalline Co, Ni and Cu nanowires could be produced by
using AAO template electrodeposition at
deposition potentials (2.0 V). Potentials lower or higher than
.0 V resulted in polycrystalline nanowires. For the FCC Cu and Ni,
a relatively high
2
the nanowires have the same orientation along the [2 2 0]. Results
indicate that Ni, Co and Fe in the single crystal or highly textured
state has a higher coercivity and remanence than that of
polycrystal nanowires.
References
[
[
[
[
[
1] A. Fert, L. Piraux, J. Magn. Magn. Mater. 200 (1999) 338.
2] X.F. Wang, L.D. Zhang, J. Phys. D 34 (2001) 418.
3] L. Li, J. Mater. Sci. Lett. 20 (2001) 1459.
4] K. Masuda, P. Fukuda, Science 268 (1995) 1466.
5] C.W. Blackledge, J.M. Szarko, A. Dupont, G.H. Chan, E.L. Read, L. Elizabeth,
S.R. Leone, J. Nanosci. Nanotech. 7 (2007) 3336.
[
[
[
6] J.X. Xu, Y. Xu, Appl. Surf. Sci. 253 (2007) 7203.
7] S.W. Lin, S.C. Chang, S.F. Hu, N.T. Jan, J. Magn. Magn. Mater. 282 (2004) 28.
8] J.U. Cho, J.H. Wu, J.H. Min, S.P. Ko, J.Y. Soh, Q.X. Liu, Y.K. Kim, J. Magn. Magn.
Mater. 303 (2006) e281.
Fig. 6. The coercivity and squareness as a function of applied voltage of: (a) Ni
[9] G.S. Wu, T. Xie, X.Y. Yuan, Y. Li, L. Yang, Y.H. Xiao, L.D. Zhang, Solid State
Commun. 134 (2005) 485.
nanowires, (b) Co nanowires and (c) Fe nanowires.

ARTICLE IN PRESS
2716
S. Thongmee et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2712–2716
[
[
10] P. Forrer, F. Schlottig, H. Siegenthaler, M. Textor, J. Appl. Electrochem. 30
2000) 535.
11] M. Tian, J. Wang, J. Kurtz, T.E. Mallouk, M.H.W. Chan, Nano. Lett. 3 (2003)
19.
[15] K. Nielsch, R.B. Wehrspohn, J. Barthel, J. Kirschner, U. Gosele, S.F. Ficher, H.
Kronmuller, Appl. Phys. Lett. 79 (2001) 1360.
[16] H. Zeng, M. Zheng, R. Skomski, D.J. Sellmyer, Y. Liu, L. Menon, S.
Bandyopadhyay, J. Appl. Phys. 87 (2000) 4718.
(
9
[
[
[
12] A.J. Yin, J. Li, W. Jian, A.J. Bennett, J.M. Xu, Appl. Phys. Lett. 79 (2001) 1039.
13] D.J. Sellmyer, M. Zheng, R. Skomski, J. Phys.: Condens. Matter. 13 (2001) R433.
14] H. Pan, B.H. Liu, J.B. Yi, C.K. Poh, S.H. Lim, J. Ding, Y.P. Feng, C.H.A. Huan, J.Y. Lin,
J. Phys. Chem. B 109 (2005) 3094.
[17] Xiaohua Bao, Feiyue Li, Robert M. Metzger, J. Appl. Phys. 79 (1996) 4866.
[18] D. AIMawlawi, N. Coombs, M. Moskovits, J. Appl. Phys. 70 (1991) 4421.
[19] H. Zeng, R. Skomski, L. Menon, Y. Liu, S. Bandyopadhyay, D.J. Sellmyer, Phys.
Rev. B 65 (2002) 134426.