Preparation of cobalt nanoparticles from [bis(salicylidene)cobalt(II)]-oleylamine complex by thermal decomposition

Article abstract of DOI:10.1016/j.jmmm.2007.07.020

[Bis(salicylidene)cobalt(II)] was used as a precursor to prepare cobalt nanoparticles of average diameter 25-35 nm by thermal decomposition. The different combinations of triphenylphosphine and oleylamine were added as surfactants to control the particle size. The resultant samples are characterized by X-ray diffractometer (XRD) and transmission electron microscope (TEM) to depict the phase and morphology. The hysteresis loops of the obtained samples reveal the soft magnet behaviors the enhanced coercivity (H_c) and decreased saturation magnetization (M_s) in contrast to their respective bulk materials.

Full text of DOI:10.1016/j.jmmm.2007.07.020

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) 575–578
www.elsevier.com/locate/jmmm
Preparation of cobalt nanoparticles from
bis(salicylidene)cobalt(II)]–oleylamine complex by thermal
decomposition
[
a,b,
Ã
b
, Fatemeh Davar , Mehdi Mazaheri , Maryam Shaterian
c
b
Masoud Salavati-Niasari
a
Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Iran
b
Department of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Iran
c
Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran
Received 25 April 2007; received in revised form 17 June 2007
Available online 2 August 2007
Abstract
Bis(salicylidene)cobalt(II)] was used as a precursor to prepare cobalt nanoparticles of average diameter 25–35 nm by thermal
[
decomposition. The different combinations of triphenylphosphine and oleylamine were added as surfactants to control the particle size.
The resultant samples are characterized by X-ray diffractometer (XRD) and transmission electron microscope (TEM) to depict the phase
c
and morphology. The hysteresis loops of the obtained samples reveal the soft magnet behaviors the enhanced coercivity (H ) and
decreased saturation magnetization (M ) in contrast to their respective bulk materials.
s
r 2007 Elsevier B.V. All rights reserved.
Keywords: Thermal decomposition; Cobalt; Nanoparticle; Complex
1
. Introduction
cobalt coordination compounds. The preparation of metal
particles by thermal decomposition of complexes becomes
increasingly important mainly due to the easy control of
process conditions, particle size, particle crystal structure,
and purity [7]. Saeki et al. [8] prepared cobalt particles from
the cobalt(II) chloride both by gas–solid reaction and by
gas–gas reaction. They found that the hexagonal crystal-
structured Co particles of several microns in diameter were
produced by the gas–solid reaction while cubic Co
nanoparticles were produced by the gas–gas reaction. Also
recently, these cobalt particles have been synthesized by a
variety of methods including thermal decomposition [9,10],
gas vapor condensation [11,12], and reduction of cobalt
salt [13]. For thermal decomposition, cobalt carbonyl,
Co (CO) , has often been used as a precursor to generate
Metal particles smaller than 100 nm in primary particle
diameter are generally considered as nanoparticles. Such
metal nanoparticles often exhibit very interesting electro-
nic, magnetic, optical, and chemical properties. For
example, their high surface-to-volume ratios have large
fractions of metal atoms at surface available for catalysis
[
1,2]. In the case of cobalt nanoparticles, they are expected
to possess exceptionally high-density magnetic property,
sintering reactivity, hardness levels, excellent impact
resistance properties, etc. Many studies on synthesis and
magnetic properties of nanoscale metal particles and
composites have been reported [3–6]. But, so far, in the
literature there is no work reporting on synthesis of cobalt
nanoparticles by thermal decomposition reaction by using
2
8
nanoparticles. However, this cobalt carbonyl is strongly
toxic and relatively expensive. To avoid these disadvan-
tages, we synthesized cobalt nanoparticles by thermal
Ã
decomposition of [bis(salicylidine)cobalt(II)] ([Co(sal) ]),
2
without any additional reducing agents in this study.
Corresponding author. Tel.: +98 361 5555333; fax: +98 361 5552935.
E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).
0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2007.07.020

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576
2
. Experimental section
that colloidal nanoparticles were generated. The black
solution was aged at 210 1C for 45 min, and was then
cooled to room temperature. The nanoparticles were
precipitated by adding excess ethanol to the solution. The
precipitated nanoparticles were retrieved by centrifugation.
The yield of the overall synthesis was 70% based on the
2
.1. Materials and physical measurements
All the chemicals were of analytical grade and were used
as received without further purification. The precursor
complexes—bis(salicylaldiminato)cobalt(II)—were prepared
according to the procedure described previously [14]. X-ray
diffractometer (XRD) patterns were recorded by a Rigaku
D-max C III XRD using Ni-filtered Cu Ka radiation.
Elemental analyses were carried out with Carlo ERBA
model EA 1108 analyzer. Transmission electron micro-
scopy images were obtained on a Hitachi H-800 transmis-
sion electron microscope (TEM) with an accelerating
voltage of 200 kV. The magnetic measurement was carried
out in a vibrating sample magnetometer (VSM) (BHV-55,
Riken, Japan) at room temperature.
amount of [Co(sal) ]. The nanoparticles could easily be
2
re-dispersed in nonpolar organic solvents, such as hexane
or toluene (Scheme 1). The synthesized particles were
characterized by a TEM, an XRD, and a VSM.
3. Results and discussion
The morphology and particle size of the powder were
determined by transmission electron microscopy (TEM,
CM 200 FEG, Philips, the Netherlands). For preparation
of the TEM sample, the powder was dispersed in high-
purity ethanol via ultrasonic equipment for 2.4 ks. Fig. 1
shows the TEM images of cobalt powders produced at the
conditions described. The morphology of cobalt particles
was spherical in shape, primary particles seemed to be
single crystals of nearly uniform size, and the directional
linkage of particles due to magnetic interaction between
particles was observed. It is observed that particles have a
size ranging from 25 to 35 nm and were also mainly round
in shape.
The X-ray diffraction patterns of the nanoparticle
powders were collected with a Philips X’Pert, Netherlands,
diffractometer equipped with a Cu Ka radiation source.
The XRD pattern of fresh cobalt nanoparticles less than
30 nm in size Fig. 2(a), shows the cubic structure of cobalt.
It is well known that nanometer-sized cobalt is easy to be
oxidized in air. However, from XRD pattern, except for
pure cubic cobalt, no other cobalt oxide phases were
detected, which might result from the protection of oleate
on the surface of nanoparticles. The diffraction peaks are
clearly broadened, which can be the result of the reduced
2
.2. Synthesis of cobalt nanoparticles
The current synthetic procedure is a modified version of
the method developed by Hyeon and co-workers for the
synthesis of nanocrystals for metals that employs the
thermal decomposition of transition metal complexes [15].
Unless otherwise noted, all reactions were carried out in a
dry-argon atmosphere. The overall synthetic procedure is
shown in Scheme 1. In this synthesis cobalt-metal
nanoparticles were prepared by the thermal decomposition
of [Co(sal) ]–oleylamine complex. First, the [Co(sal) ]–
2
2
oleylamine complex was prepared by reaction 0.6 g of
Co(sal) ] and 2 ml of oleylamine. The mixed solution was
[
2
placed in a 50 ml three-neck distillation flask and heated up
to 100 1C for 90 min. During all the experimental processes,
the flask was flushed with high-purity Ar gas to avoid
oxidation. The resulting metal–complex solution was
injected into 5 g of triphenylphosphine at 220 1C. The color
of the solution changed from orange to black, indicating
H
NH2
H
H
H
O
O
O
O
O
O
Oleylamine
Co
Co
O
O
NH2
P
P
P
Thermolysis
Triphenylphosphine
P Co P
P
P
P
Scheme 1.

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577
Fig. 2. X-ray diffraction pattern of (a) nanometer sized fresh Co
nanoparticles and (b) nanometer sized nanoparticles after air exposure.
Co O peaks represented by the pattern, these diffracto-
3
4
grams show that the initially synthesized metallic cobalt
nanoparticles were oxidized to CoO and Co O nanopar-
3
4
ticles. Using the Scherrer formula, based on line broad-
ening, the mean crystallite sizes of the CoO and Co O
3
4
powders were determined to be 19.8 and 21.2 nm,
respectively. Particles of the powder with a larger size
obtained from TEM (25–35 nm) in comparison with their
crystallite nanometer size (CoO: 21.2 and Co O : 16.8) are
3
4
due to the polycrystalline nature of the powders.
The presence of triphenylphospine led the particles to be
well dispersed with no agglomeration, while much larger
particles flocculating together were synthesized in the
absence of triphenylphospine. Triphenylphosphine is a
high-boiling point surfactant with a patulous long-chain
structure providing greater steric hindrance. So, this agent
might slow the addition rate of materials to the nanopar-
ticles during their growth, resulting in much smaller
nanoparticles. Furthermore, the surfactants in the solution
coordinated onto the metal of the nanoparticles, providing
a dynamic organic structure that stabilizes the nanoparticles
Fig. 1. TEM images of cobalt nanoparticles coated with TPP and
oleylamine.
particle size. This indicates that cobalt is all nanoparticle.
While the diffractogram shown in Fig. 2(b) presents the
XRD pattern of nanoparticles after exposing the nano-
particles to air, no peak was related to the cobalt element in
the XRD patterns. As the XRD pattern shown in Fig. 2(b)
exhibits strong peaks for CoO in comparison with weak

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578
(Fig. 3(b)). The hysteresis loop at cobalt oxide nanopar-
ticles exhibits a ferromagnetic behavior with M about
s
56.2 emu/g. Obviously, the coercivity (H ) of the cobalt
nanoparticles after exposing to air is enhanced while the
c
saturation magnetization (M ) is decreased in contrast to
s
those of the bulk cobalt (166 emu/g and a few tens of
Oersteds). These results indicate that, in spite of decrease in
saturation magnetization, coercivity increases [17].
4. Conclusions
Cobalt nanoparticles have been prepared using thermal
decomposition of [bis(salicylidene)cobalt(II)] in the presence
of TPP and oleylamine. The as-synthesized cobalt particles
show fairly good cubic cobalt crystallinity and are stable in
hydrocarbon solvents against air oxidation. Compared with
the bulk materials, the hysteresis loops of the obtained
samples reveal soft magnet behavior, enhanced coercivity
(H ), and decreased saturation magnetization (M ). The
c s
saturation magnetization, Ms is 64.1 emu/g for cobalt
nanoparticles, which is lower than that of bulk cobalt
particles (Mbulk ¼ 166 emu/g). Meanwhile, the hysteresis
loop of Co nanoparticles after exposing to air also exhibits a
ferromagnetic behavior with saturation magnetization (Ms)
and coercivity (H ) values of about 56.2emu/g.
c
Acknowledgment
Authors are grateful to Council of University of Kashan
for providing financial support to undertake this work.
References
[
[
1] R.W. Siegel, Annu. Rev. Mater. Sci. 21 (1991) 559.
2] R.W. Siegel, Nanophase Materials: Synthesis, Structure, and Proper-
ties, Springer Series in Material Science, 1994, p. 65.
Fig. 3. Magnetization versus applied at 300 K for (a) fresh cobalt
nanoparticles and (b) nanometer sized nanoparticle after air exposure.
[
[
3] C.P. Chen, T.H. Chang, T.F. Wang, Ceram. Int. 27 (2001) 925.
4] H. Hori, T. Teranishi, M. Taki, S. Yamada, M. Miyake,
Y. Yamamoto, J. Magn. Magn. Mater. 226–230 (2001) 1910.
5] L.C. Varanda, M. Jafelicci Jr., G.F. Goya, J. Magn. Magn. Mater.
in solution. The addition of TPP into the mixture of
oleylamine and complex as an additional surfactant
reduced the particle size much further and resulted in very
uniform size distribution (Fig. 2). Oleylamine is known as a
ligand that binds tightly to the metal nanoparticles surface.
The combined effects of TPP and oleylamine were much
more profound than those of individual contributions.
The soft magnet is documented by the hysteresis loop
measured at 300 K as shown in Fig. 3(a). This is almost
immeasurable coercively for cobalt nanoparticles at room
temperature, which is a very typical behavior for a soft
magnet. This indicates that the cobalt particles are soft
magnets and nanosized. The saturation magnetization
[
[
[
[
[
2
26 (2001) 1933.
6] K.W. Edmonds, C. Binns, S.H. Baker, M.J. Maher, S.C. Thornton,
O. Tjernberg, N.B. Brookes, J. Magn. Magn. Mater. 231 (2001) 113.
7] A. Gurav, T. Kodas, T. Pluym, Y. Xiong, Aerosol Sci. Technol. 19
(
1993) 411.
8] Y. Saeki, R. Matsuzaki, H. Nishihara, N. Aoyama, Denki Kagaku 46
1978) 643.
9] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115.
(
[10] C.B. Murray, S. Sun, W. Gaschler, et al., IBM J. Res. Dev. 45 (2001) 47.
[
[
11] X.L. Dong, C.J. Choi, B.K. Kim, Scr. Mater. 47 (2002) 857.
12] Z.H. Wang, C.J. Choi, B.K. Kim, J.C. Kim, Z.D. Zhang, J. Alloys
Compds. 351 (2003) 319.
[
[
13] S. Sun, C.B. Murray, J. Appl. Phys. 85 (1999) 4325.
14] M. Salavati-Niasari, M. Shaterian, M.R. Ganjali, P. Norouzi, J. Mol.
Catal. A 261 (2007) 147.
value, M , obtained by extrapolation seem to reach a
s
[
[
15] H.M. Park, E. Kang, S.U. Son, H.M. Park, M.K. Lee, J. Kim,
K.W. Kim, H. Noh, J. Park, C.J. Bae, J. Park, T. Hyeon, Adv.
Mater. 17 (2005) 429.
maximum of around 64.1 emu/g for cobalt nanoparticles,
which is lower than that of bulk cobalt particles (Mbulk
66 emu/g) [16]. M decreased from 166 emu/g (for bulk
¼
1
cobalt) to about 64.1 emu/g (for cobalt nanoparticle).
s
16] E.P. Wohlfarth, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials,
North-Holland, Amsterdam, 1980, p. 20.
[17] L. Zhang, D. Xue, C. Gao, J. Magn. Magn. Mater. 267 (2003) 111.
Decrease was also observed in M after exposing to air
s