Synthesis of cobalt nanoparticles in polymeric membrane and their magnetic anisotropy

Article abstract of DOI:10.1016/j.jmmm.2003.12.449

We systematically synthesized cobalt nanoparticles in the perfluorinated sulfo-cation membrane (MF-4SK) in aqua solution using ion-exchange method. The average radius of cobalt nanoparticles in the polymer film is determined to be 3.5±1.0nm by TEM. FMR me

Full text of DOI:10.1016/j.jmmm.2003.12.449

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1413–1414
Synthesis of cobalt nanoparticles in polymeric membrane and
their magnetic anisotropy
I.-W. Park, M. Yoon*, Y.M. Kim, Y. Kim, J.H. Kim, S. Kim, V. Volkov
Korea Basic Science Institute, 126-16, 5th St. Anam-dong, Sungbuk-ku, Seoul, 136-701, South Korea
Abstract
We systematically synthesized cobalt nanoparticles in the perfluorinated sulfo-cation membrane (MF-4SK) in aqua
solution using ion-exchange method. The average radius of cobalt nanoparticles in the polymer film is determined to be
3
.571.0 nm by TEM. FMR measurements show that the angle dependence of resonance field (H ) follows the
r
characteristics of flat-plate magnetic film and the easy axis lies along the surface of the polymer film. The experimental
results suggest that the magnetic anisotropy comes from magnetized polymer film containing Co nanoparticles.
r 2003 Elsevier B.V. All rights reserved.
PACS: 75.50. Tt; 75.50. Kj
Keywords: Cobalt nanoparticles; MF-4SK; Ion-exchange; Magnetic anisotropy
In material science, nano-sized magnetic materials have
been produced as isolated aggregates, as deposits of
aggregates, such as carbon nanotubes and nanocages filled
with magnetic material, as well as electrodeposits of
magnetic material in nanoporous polycarbonate mem-
branes [1]. Although these methods are pretty powerful to
produce the nano-sized magnetic materials, there are still
problems in production since many of above methods need
high vacuum system. On the other hand, polymeric ion-
exchange system [2] is a good candidate for ferromagnetic
nanoparticles due to the efficiency in synthesis and size
control. The main idea of this new method is as follows.
First, cations of ferromagnetic metals are fixed on charge
groups. After the recovery process, ferromagnetic particles
are obtained according to the electrochemical reaction. It
turns out that the size distribution of particles and the
distance among them depend on the concentration of
transition metal ions in the film and the structure of ion
channels of polymer membrane.
11007100, thickness: 0.190mm) was used. The schematic
picture of the physical structure for MF-4SK is shown in
Fig. 1. In MF-4SK, sulfogroups, counter-ions and water
molecules form ionized channels. It has been shown that
the linear fragment of this channel length is less than
10nm with 4 nm channel width, which is dependent on
2
+
humidity and counter-ions [3]. We used Co as counter-
ion from an aqua solution of hydrous cobalt salt
2
+
(CoCl
2
2
Á 6H O). In this solution, Co ions (Co ) were
+
diffused and substituted for hydrogen ions (H ) at the
sulfonate groups. These Co cations incorporated in MF-
4SK were recovered to metal nanoparticles according to
the electrochemical reaction where NaBH
4
was used as a
2
+
reducing agent. During the recovery process from Co
0
to Co , self-aggregated metal nanoparticles (nCo ) were
0
formed. In Table 1, theoretically calculated concentration
cal
ex
(Con ) and experimentally measured one (Con ) using
ICP-AES technique are compared. At low concentration
2
0
(o1.6 Â 10 ), experimental values determined by mea-
suring concentration of Co ions in the solution before and
after ion exchange process are well matched with
theoretical expectation.
In synthesis, perfluorinated sulfo-cation exchange
polymeric membrane, MF-4SK (molecular weight:
Fig. 2(a) shows FE-TEM image of the cobalt
nanoparticles of the sample, which reveals black dots
with relatively bright stripes. We believe that circular
shaped black dots observed in TEM come from the Co
*
Corresponding author. Tel.: +82-2-920-0719; fax: +82-2-
20-0729.
E-mail addresses: iwpark@kbsi.re.kr (I.-W. Park),
mxy13@kbsi.re.kr (M. Yoon).
9
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.12.449

ARTICLE IN PRESS
I.-W. Park et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1413–1414
1414
Z
Na+
Na+
Na+ SO -
-
-Na+
3
3
SO
3
SO
3
SO
0
nCo
θ
Na+
SO3 Na
-
+
-
- Na+
SO
3
-
Na+
SO3
SO
3
Fig. 1. Schematic representation of perfluorinated membrane
Y
2
+
0
channel fragments: recovering from Co to Co .
Table 1
Theoretically calculated concentrations Con and experimen-
ex
tally obtained concentration Con
cal
Co-I
Co-II
Co-III
Co-IV
Co-V
X
cal
ex
19
19
19
20
20
20
Con
Con
1.9 Â 10 3.2 Â 10 7.8 Â 10 1.6 Â 10 3.6 Â 10
19 19 19 20
Fig. 3. Geometric coordinates of the FMR.
1.9 Â 10 3.2 Â 10 7.8 Â 10 1.6 Â 10 2.4 Â 10
8
6
6
4
2
0
2
p3/2
4
2
2
p1/2
0
30
60
90
0
30
60
90
20 nm
(a)
θ
(degree)
(b)
). and (b) linewidth (DH) for
θ
(degree)
7
70
780
790
800
810
Fig. 4. (a) The resonance field (H
r
(a)
(b)
Binding energy eV)
Co-IV at room temperature.
Fig. 2. Co-III nanoparticles (a) TEM image. and (b) XPS data.
orientations, shows that DH at y=0ꢀ is the lowest
revealing approximately 1.0 kOe and increases with
nanoparticles in the polymer since they were not seen
before absorption. The average radius of Co nanopar-
ticles in the film was determined to be 3.571.0 nm by
statistical analysis. This image shows the presence of
non-agglomerated small particles homogeneously dis-
persed in the polymer. High resolution XPS scan shows
two binding energy peaks of 2p3/2 (779 eV) and 2p1/2
ꢀ
increasing y, reaching a maximum of 5.2 kOe at y=70 .
In summary, detailed processes of synthesis for cobalt
nanoparticles embedded polymer film using a new
method, ion-exchange, were introduced. The average
radius of Co nanoparticles in the polymer is determined
to be 3.571.0 nm. FMR measurements show that the
magnetic anisotropy comes from magnetized polymer
film revealing the characteristics of the flat-plate-like
magnetic polymer. The easy axis is parallel to the surface
of polymer sample.
(
794 eV) for cobalt as shown in Fig. 2(b).
r
The dependence of the resonance field (H ) for a Co-
embedded MF-4SK at 9.8 GHz (X band) on the applied
field angle (polar angle, y) (See Fig. 3) in the vertical
plane is shown in Fig. 4(a). The lowest resonance field of
1
.8 kOe occurs for the external magnetic field applied
ꢀ
This work has been supported by the NRL of MOST.
ꢀ
parallel (y=0 ) to the sample. The axis of y=0 is
determined to be easy axis since H is the lowest at
r
ꢀ
y=0 . Our experimental evidence indicates that the
resonance field increases with increasing polar angle,
References
ꢀ
reaching 7.0 kOe at y=90 . This phenomenon seems to
be due to the demagnetisation of the flat-plate-like
sample [4]. The angular dependence of the linewidth
[
[
[
1] H. Brune, Surf. Sci. Rep. 31 (1998) 121.
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brane Process, Ellis Horwood Ltd. Chichaster, UK, 1991.
(DH, the peak to peak in the first derivative), which gives
the information of the distribution of the particle
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