Fabrication and magnetic properties of cobalt nanorod arrays containing a number of ultrafine nanowires electrodeposited within an AAO/SBA-15 template

Article abstract of DOI:10.1016/j.ssc.2011.05.014

Arrays of cobalt nanorods consisting of a number of nanowires have been fabricated by the electrochemical method using an anodic-aluminum oxide (AAO)/mesoporous-silica (SBA-15) composite. Microscopic studies clearly display that each nanorod (with a diameter of ～200 nm) of the array was consisting of a number of cobalt nanowires which exhibit an average diameter of 3 nm. The observed hysteresis loops measured at room temperature indicate that the magnetic shape anisotropy of cobalt mesostructures, i.e. the parallel and perpendicular squarenesses of 0.5 and 0.1, respectively have been estimated. The maximum value of the coercivity measured perpendicular to the sample axis shows a value of 330 Oe and it was found that the coercivity decreases by increasing the temperature which is possibly caused by thermal disturbance inside the arrays.

Full text of DOI:10.1016/j.ssc.2011.05.014

Solid State Communications 151 (2011) 1151–1155
Contents lists available at ScienceDirect
Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Fabrication and magnetic properties of cobalt nanorod arrays containing a
number of ultrafine nanowires electrodeposited within an AAO/SBA-15 template
a,∗
a
a
a
b
c
Guangbin Ji , Zhihong Gong , Yousong Liu , Xiaofeng Chang , Youwei Du , M. Qamar
a
Department of Applied Chemistry, College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, PR China
National Laboratory of Solid State Microstructure and Physics Department of Nanjing University, Nanjing 210093, PR China
Center of Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Arrays of cobalt nanorods consisting of a number of nanowires have been fabricated by the
electrochemical method using an anodic-aluminum oxide (AAO)/mesoporous-silica (SBA-15) composite.
Microscopic studies clearly display that each nanorod (with a diameter of ∼200 nm) of the array was
consisting of a number of cobalt nanowires which exhibit an average diameter of 3 nm. The observed
hysteresis loops measured at room temperature indicate that the magnetic shape anisotropy of cobalt
mesostructures, i.e. the parallel and perpendicular squarenesses of 0.5 and 0.1, respectively have been
estimated. The maximum value of the coercivity measured perpendicular to the sample axis shows a
value of 330 Oe and it was found that the coercivity decreases by increasing the temperature which is
possibly caused by thermal disturbance inside the arrays.
Received 5 October 2010
Received in revised form
4
April 2011
Accepted 11 May 2011
by D.J. Lockwood
Available online 19 May 2011
Keywords:
A. Mesoporous Co nanoarrays
B. AAO/SBA-15 composite templates
C. Electrodeposition
Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved.
1
. Introduction
Recently, mesoporous materials having unique characteristics
such as narrow pore size distribution, high specific surface area,
Iron (Fe), cobalt (Co) and nickel (Ni) magnetic nanomaterials
high pore volume etc, have received particular attention in vari-
ous research areas including hydrogen storage [8], separation [9],
catalysis [10] etc. Efforts have been made to develop mesoporous
materials such as non-metallic mesoporous silica [11], mesoporous
carbon nano-filaments [12], mesoporous titania [13] etc. Among
many mesoporous materials, ordered mesoporous silica has at-
tracted much attention because of the highly ordered hexagonally
arranged mesochannels, thick walls of ∼2 nm [14], adjustable pore
size and high thermal stability [15]. Since the pore size of meso-
porous silica is smaller than that of AAO, it offers many advan-
tages especially the quantum size effect. It has been shown that the
combination of AAO and mesoporous silica results in the forma-
tion of new hierarchical structures where the advantages of both
the templates are utilized [16–19]. It has been found that the SBA-
with 1-D structure have significant importance in fundamental
research as well as in potential technological applications such as
magnetic recording media, sensors and other devices [1–3]. Due to
the high aspect ratio and self-ordered hexagonal pore structure,
anodic aluminum oxide (AAO) templates have been extensively
studied considering the growth of ordered 1-D nanostructures
within their pores [4,5]. The synthesis of cobalt nanowire arrays
with 1-D structure using an AAO template have been reported
in the scientific literature. Huang et al. prepared the arrays of
cobalt nanowires with controlled diameters and identical lengths
of 40 nm by a pulsed electrodeposition method [6]. Ren et al.
fabricated cobalt nanowire arrays with a length of ∼20 nm with
a high coercivity of 1360 Oe through alternating-current (AC)
electrodeposition [7]. It is worth noticing that the AAO templates
have a wide range of porosity starting from some nanometers to
several hundred nanometers. However, the lack of smaller pores
1
5 mesostructures can be directed by the pores of alumina mem-
branes to form hexagonal arrays when they were filled in AAO
pores. Thus the fine and vertical mesochannels (namely, parallel
to the channels of the alumina membrane) which have the charac-
teristics of a narrow pore size distribution, varieties of mesochan-
nels, high specific surface area and a high pore volume can be
effectively provided or obtained. Furthermore, in comparison to
the materials prepared using AAO as a template under identical
conditions, such mesoporous materials synthesized using meso-
porous silica as a template have many evident advantages such
as higher specific surface area, adjustable morphology etc [20,21].
(
a few nanometers) in AAO templates limits their application in
the preparation of 1-D nanomaterials having smaller diameters
which are being applied for the fabrication of nanodevices with
high efficiencies and bioseparation of small molecules.
∗
Corresponding author. Tel.: +86 25 52112902; fax: +86 25 52112626.
E-mail address: gbji@nuaa.edu.cn (G. Ji).
0
038-1098/$ – see front matter Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2011.05.014

1
152
G. Ji et al. / Solid State Communications 151 (2011) 1151–1155
2 3
Al O pore wall
Co nanorods
Fig. 1. Typical SEM images of AAO/SAB-15 template after electrodeposition: (a) top view, (b) side view and cobalt nanorods (c, d).
Therefore, such mesoporous composite materials might have great
potential in various research fields such as spectroscopy, electronic
transmission and magnetic resolution, adsorbtion or carrier of
adsorbent etc. Recently, nanowires of Ag, Ni, and Cu2O with
unprecedented mesostructures, such as coaxially multilayered
stacked-donuts, single- and double-helices within the pores of the
mesoporous silica, have been successfully prepared [22].
for 12 h and calcined at 400 °C for 3 h. The electrodeposition was
2
carried out at 25 °C with a current density of 0.5–1.0 mA/cm
(AC, 50 Hz) using graphite and an AAO/SBA-15 composite template
as the counter-electrode and working electrode, respectively. The
aqueous solution composed of Co(Ac)
2
· 4H2O (0.1 M) and H3BO4
(
0.2 M) was employed as the electrolyte and the deposition time
was controlled to between 6 and 12 h.
In the present investigation, an attempt has been made to
synthesize arrays of cobalt nanorods consisting of a number of
nanowires using AAO/SBA-15 as a template by the AC-induced
electrodeposition process. The phase structure and morphology
was clearly confirmed by means of X-ray diffraction (XRD), field
emission scanning electron microscopy (FE-SEM) and transmission
electron microscopy (TEM). The magnetic properties (such as
perpendicular/parallel coercivity and squarenesses (MR/Ms)) of
as-prepared cobalt nanorod arrays as a function of testing
temperature were systematically investigated by using a vibrating
sample magnetometer (VSM).
2
.2. Characterization
The magnetic properties were measured by a VSM (Lakeshore,
Model 7400 series and VersaLab, Quantum Design) with the
applied field either perpendicular or parallel to the samples axis.
The morphology of as-prepared samples was observed by FE-
SEM (Hitachi S-4800 and Gemini LEO 1530) and TEM (JEOL JEM
2
100). For the FE-SEM observation, small pieces of AAO/SBA-15
with cobalt nanorod arrays were eroded slightly by an aqueous
solution of NaOH (2 M) at room temperature. The samples were
then cleaned with distilled water for several times. For the
TEM (including high resolution TEM, HR-TEM) observation, the
AAO/SBA-15 composite template was immersed in 4 M NaOH
for 4 days at room temperature in order to release the cobalt
nanomaterials from the composite template. A small drop of
suspension was placed on copper grids covered with a carbon thin
film. The phase structure of as-prepared samples was examined by
means of X-ray diffraction (XRD) using Cu-Kα as the X-ray source
2. Experimental section
2
.1. Synthesis
The preparation of the composite template has been described
in detail in our previous published literature [23]. Briefly speaking,
the aluminum foils, after polishing by the electrochemistry
method, were anodized in aqueous solution (0.36 M H3PO4) under
a constant voltage of 120 V at 0 °C. Subsequently, the pores of the
AAO templates were widened in H3PO4 (0.3 M) for 3 h. To prepare
the precursor (sol) of SBA-15, 0.6 g of poly(ethylene oxide)-b-
poly(propylene oxide)-b-poly(ethylene oxide) block copolymer
EO20PO70EO20 (PEO-PPO-PEO, Pluronic P123) was dissolved in
(
λ = 0.15418 nm) with a voltage of 40 kV and a current of 100 mA,
respectively.
3
. Results and discussion
3
.1. Morphology and structural properties
A top view FE-SEM image of the AAO/SBA-15 composite
4
0 mL ethanol followed by the addition of tetraethyl orthosilicate
template is depicted in Fig. 1(a) whose pore diameter was
measured to be around 200 nm. It was observed that most of the
AAO nanochannels were filled with silica nanofibers exhibiting
a mesostructure. Fig. 1(b) shows the side view of the AAO/SBA-
15 composite template, from which it can be seen that the
cobalt nanorods grew in the AAO nanochannels and were oriented
vertically to the AAO membrane. Fig. 1(c) and (d) shows the
arrays of as-electrochemical-deposited cobalt nanorods. From the
(
TEOS, 0.97 g) and aqueous hydrochloric acid (HCl, 0.4 g) to the
mixture, which was stirred for 2 h at room temperature to form the
sol [24]. The AAO templates were immersed in the sol for 24 h to
ensure a complete infiltration of SBA-15. The membrane was taken
out from the sol and the residual sol on the membrane surface
was completely scratched away. The final AAO/SBA-15 composite
template was produced after the membranes were dried at 60 °C

G. Ji et al. / Solid State Communications 151 (2011) 1151–1155
1153
Fig. 2. Typical TEM images of as-prepared cobalt nanorod arrays detached from AAO/SBA-15 template: (a) a bundle of nanowires; (b) and (c) a single nanowire; and
d) representative HR-TEM image.
(
FE-SEM images, the diameter of the nanorods was estimated to be
around 200 nm. The cobalt nanorods, Fig. 1(b) tend to accumulate
and were seen to be broken partially upon the removal of the
template. Additionally, the mass or amount of nanowires existing
in the nanorods can be estimated based on the FE-SEM observation.
In order to get further insight, samples were also analyzed by
means of TEM and a typical TEM image of the as-prepared samples
is depicted in Fig. 2. From Fig. 2(a) and (b), the length and average
diameter of the synthesized nanorods were measured to be
around 3 µm and 200 nm, respectively. The magnified TEM image
Al(311)
5
4
3
2
1
000
000
000
000
000
0
Al(220)
Co(002)
Al(200)
Co(311)
40
(
Fig. 2(c)) clearly showed that each cobalt nanorod consisted of a
number of nanowires which grew irregularly, possibly caused by
the mesoporous structure of SBA-15. As the silica grew with a self-
assembly mechanism, the channels of SBA-15 crossed each other
and formed a network structure which made the cobalt grains
grow along different directions in the meantime. A representative
HR-TEM image depicted in Fig. 2(d) further demonstrated that the
diameter of the disordered nanowire was about 3 nm.
Fig. 3 depicts the XRD pattern of cobalt nanorod arrays which
were prepared using the AAO/SBA-15 composite as a template.
The diffraction peak around 38.65° was indexed to the hexagonal
structure (space group: P63/mmc) of cobalt (JCPDS card no. 05-
35
45
50
55
60
65
70
75
80
2
Theta/ deg.
Fig. 3. The XRD pattern of as-prepared cobalt nanorod arrays within the AAO/SBA-
5 template.
1
1
.0
0
727) which corresponds to the crystal indexes of (311). In
addition, two apparent diffraction peaks with strong intensity,
corresponding to the (220) and (311) crystal indexes of aluminum
around 65.13° and 78.30°, were observed because the Al substrate
was not removed. It was difficult to characterize the diffraction
peak around 44.78° because it resulted either from the crystal
indexes (002) of cobalt or (200) of the Al substrate. However,
the crystal structure investigation revealed the preferential
orientations of (311) and/or (002) for as-deposited Co nanocrystals
as the strong confinement of the porous wall during the crystal
growth within the pores of the SBA-15/AAO composite.
0.5
0
.0
-0.5
-1.0
10000
-
-5000
0
5000
10000
3
.2. Magnetic properties
H (Oe)
Fig. 4. Typical hysteresis loops of the mesoporous Co nanorods measured at room
temperature.
The magnetic properties of as-prepared arrays were studied by
VSM. Fig. 4 presents the hysteresis loops (M–H loops) of the as-
deposited typical cobalt nanorod arrays with the external magnetic
field parallel and perpendicular to the sample axis measured at
room temperature (300 K). The magnetic coercivity/squarenesses
values were around 250 Oe/0.5 and 330 Oe/0.1, for the induced

1
154
G. Ji et al. / Solid State Communications 151 (2011) 1151–1155
1
0
0
.0
.5
.0
(a) T=50K
1.0
0.5
(b) T=100K
0.0
-
-
0.5
1.0
-0.5
-1.0
parallel direction
perpendicular direction
parallel direction
perpendicular direction
-
6000
-4000
-2000
0
2000
4000
6000
-6000
-4000
-2000
0
2000
4000
6000
H(Oe)
H(Oe)
1
.0
.5
.0
(c) T=200K
(d) T=300K
1
0
.0
0
0
.5
0.0
-0.5
-1.0
-
-
0.5
parallel direction
perpendicular direction
parallel direction
perpendicular direction
1.0
-
6000
-4000
-2000
0
2000
4000
6000
-
10000
-5000
0
5000
10000
H(Oe)
H(Oe)
Fig. 5. The M–H loops of cobalt nanorod arrays measured at 50 (a), 100 (b), 200 (c) and 300 K (d).
external magnetic field with directions parallel and perpendicular
to the sample axis, respectively. The results mentioned above
clearly demonstrated the strong shape anisotropy of the cobalt
nanorod arrays.
The M–H loops of as-deposited cobalt nanorod arrays measured
at 50, 100, 200 and 300 K are presented in Fig. 5 and the results
indicated little difference between the parallel and perpendicu-
lar coercivity values. However, the observed much lager parallel
squareness showed that the easy axis was along the axis of the
nanorods.
Fig. 6 depicts the changes in coercivity and squareness as a
function of testing temperature. Both the parallel and perpendic-
ular coercivity were found to decrease with the increase in the
measuring temperature and the maximum values were found to
be around 490 and 360 Oe, respectively. It is well known that
the ferromagnetic materials become paramagnetic when the tem-
perature is high enough, which is referred to as the Curie tem-
perature (Tc ), while the cobalt shows ferromagnetism below the
temperature of 1404 K (as shown in Fig. 6). Under the external
magnetic field, the movement rate of molecules increases with the
increase in temperature, below the temperature which is lower
than Tc . The thermal perturbation will partly damage the ordered
arrangement of magnetic moments within the magnetic domain
and hence the spontaneous magnetization was decreased. If the
temperature continues to increase up to Tc , the exchange cou-
pling between two neighboring atoms (or molecules) will disap-
pear suddenly, the magnetic domain disappear as well, and the
ferromagnetism changes to paramagnetism. From the measured
magnetic values, it is reasonable to deduce that the cobalt nanorods
overcame the magnetic anisotropy under the interaction of an ex-
ternal magnetic field and internal heat disturbance. Under identi-
cal conditions, the magnetic anisotropy of the particles is certain
as the thermal disturbance is proportional to the temperature. But
the thermal fluctuation gets smaller when the temperature drops
and this phenomenon requires a greater magnetic field strength to
help to overcome the magnetic anisotropy energy, resulting in the
larger coercivity. The maximum coercivity was, therefore, found
at 50 K. Fig. 6(b) depicts the difference of the changes between
parallel and perpendicular squareness. The parallel squareness in-
creased slowly while the perpendicular to the wire axis decreased
as the testing temperature was increased. The small difference of
coercivity values along the two directions can be attributed to the
disordered cobalt nanowires. Squareness is another parameter in
hysteresis loops which is an indicator of the direction of easy mag-
netization. As the temperature increases, the magnetization easy
axis turned in the direction parallel to the wire axis. This indi-
cated that easy axis magnetization is no longer the direction of
nanowires at low temperature.
4. Conclusions
In this present study, arrays of cobalt nanorods (with diameters
of ∼200 nm) consisting of a number of nanowires (with the
diameter distribution of 3 nm) were successfully prepared by the
electrodeposition method using the AAO/SBA-15 composite as a
template. The shape anisotropy of as-prepared Co nanoarrays was
determined based on the magnetic results. The maximum parallel

G. Ji et al. / Solid State Communications 151 (2011) 1151–1155
1155
520
of as-prepared cobalt nanorod arrays was decreased with the
increase in temperature is possibly due to the thermal disturbance
inside the arrays.
a
4
4
4
3
3
2
2
80
40
00
60
20
80
40
Acknowledgment
Financial support from the National Natural Science Foundation
of China (50701024) and Natural Science Foundation of Jiangsu
Province of China (No: BK2010497) are gratefully acknowledged.
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