Organic colloid method to prepare ultrafine cobalt nanoparticles with the size of 2?nm

Article abstract of DOI:10.1016/j.ssc.2007.10.014

Ultrafine Co nanoparticles with the size of ca. 2 nm were prepared by an organic colloid method, in which sodium formate acted as reducing agent, ethylene glycol acted as solvent, sodium citrate acted as both complexing agent and stabilizing agent, respectively. X-ray diffraction (XRD) analysis indicated that the as-prepared Co nanoparticles were in hexagonal close-packed phase, and transmission electron microscope (TEM) images revealed that the size of the well-dispersed Co nanoparticles was as small as 2 nm, and the sizes were distributed in a very narrow region. The hysteresis loop of the as-prepared Co sample measured at room temperature showed a superparamagnetic behavior due to the extremely small size of the products. It was revealed that sodium citrate played a crucial role in decreasing the particle size and narrowing the size distribution.

Full text of DOI:10.1016/j.ssc.2007.10.014

Solid State Communications 145 (2008) 118–121
www.elsevier.com/locate/ssc
Organic colloid method to prepare ultrafine cobalt nanoparticles with the
size of 2 nm
Hao Li, Shijun Liao∗
College of Chemistry, South China University of Technology, Guangzhou, Guangdong, 510641, People’s Republic of China
Received 18 September 2007; received in revised form 14 October 2007; accepted 15 October 2007 by E.V. Sampathkumaran
Available online 18 October 2007
Abstract
Ultrafine Co nanoparticles with the size of ca. 2 nm were prepared by an organic colloid method, in which sodium formate acted as reducing
agent, ethylene glycol acted as solvent, sodium citrate acted as both complexing agent and stabilizing agent, respectively. X-ray diffraction (XRD)
analysis indicated that the as-prepared Co nanoparticles were in hexagonal close-packed phase, and transmission electron microscope (TEM)
images revealed that the size of the well-dispersed Co nanoparticles was as small as 2 nm, and the sizes were distributed in a very narrow region.
The hysteresis loop of the as-prepared Co sample measured at room temperature showed a superparamagnetic behavior due to the extremely small
size of the products. It was revealed that sodium citrate played a crucial role in decreasing the particle size and narrowing the size distribution.
c
ꢀ 2007 Elsevier Ltd. All rights reserved.
PACS: 61.46.Fn; 75.50.-y
Keywords: A. Metals; A. Nanostructures; B.Chemical synthesis
1. Introduction
a few successful examples on the synthesis of Co nanoparticles
in this size regime [13–18]. It seems that the synthesis will be
much more difficult if smaller Co nanoparticles are required.
Recently, Javey et al. obtained 2 nm monodispersed Co
nanoparticles using an electrobeam lithography technique [17].
However it is clear that scaling up of this synthetic procedure
will be rather difficult because of both high operational cost
and tedious synthetic steps. Ledo-Sua´rez et al. also synthesized
2 nm monodispersed Co nanoparticles via an electrochemical
method [18]. But obviously, the usage of some environmentally
unfriendly reagents, such as, acetonitrile, triphenylphosphine
and heptane, in the process made the approach less attractive
from the viewpoint of environment safety.
Recently, our group has reported the formation of ultrafine
(ca. 1 nm) PtRuIr nanocatalyst using an organic colloid
synthesis method [19]. We modified this method and applied
it to the preparation of ultrafine Co nanoparticles. The results
demonstrated that 2 nm Co nanoparticles with narrow size
distribution could be obtained successfully by this route. In this
novel and facile approach, sodium formate acted as reducing
agent, ethylene glycol acted as solvent, sodium citrate acted
as both complexing agent and stabilizing agent, respectively.
The as-prepared Co nanoparticles were characterized by
Magnetic nanoparticles represent a particularly interesting
class of materials for both scientific and technological
explorations [1–4]. In recent years, one of the important
goals in magnetic nanoparticle synthesis was to control the
particle size and narrow the particle size distribution. It
was reported that discrete magnetic nanoparticles with sizes
ranging from 2 nm to 20 nm are of significant importance,
because of the wide application in magnetic storage devices,
ferrofluids, magnetic refrigeration system, magnetic carriers
for targeting and catalysis [5]. As one of the most important
magnetic materials, nanocrystalline Co has spurred intensive
studies not only due to its multiple crystal structures, but also
owing to its size-dependent magnetic, electronic and catalytic
properties [6]. It was demonstrated in the literatures that Co
nanoparticles with diameters of 5–20 nm could be fabricated
by a wide variety of processes [7–12], while preparation of Co
nanoparticles below 5 nm is still challenging and there are only
∗
Corresponding author. Fax: +86 20 87112906.
E-mail address: chsjliao@scut.edu.cn (S. Liao).
c
0038-1098/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2007.10.014

H. Li, S. Liao / Solid State Communications 145 (2008) 118–121
119
transmission electron microscopy (TEM), X-ray diffraction
(XRD) and superconducting quantum interference device
(SQUID) magnetometer.
For comparison, we carried out another experiment, in which
sodium tartrate acted as complexing agent instead, while other
experimental conditions were kept unchanged. The TEM image
of the resultant product is shown in Fig. 2. Nanosized Co
particles could also be obtained, but the size distribution was
large. Additionally, we also made an attempt to use some
other complexing agents, such as ethylenediamine tetraacetic
acid sodium (EDTA), sodium pyrophosphate in our synthesis
route. But we failed to obtain ultrafine Co nanoparticles and the
main products of these reactions were found to be Co(OH)2.
We thought this may result from either the poor solubility of
these complexing agents in ethylene glycol or the low stability
constants of the formed complexes. The role of complexing
agents in this study is interesting but is not well-understood,
and further investigation is still necessary.
2. Experimental section
All reagents were of analytic grade and used without further
purification. In a typical synthetic procedure, 2.5 mmol cobalt
chloride hexahydrate and a certain amount of sodium citrate
were dissolved in 70 mL ethylene glycol under intense stirring,
followed by adding 20 mL ethylene glycol solution of sodium
hydroxide (1 M) to the above solution. For the sake of
simplicity, we defined R as the molar ratio of sodium citrate to
cobalt chloride in the present paper. 10 mmol sodium formate
was then dissolved into the mixed solution. Subsequently,
◦
the temperature of the solution was elevated to 180 C and
It should be pointed out that reduction of divalent cobalt salts
to metallic Co by polyols at elevated temperature (the so-called
polyol process) has been well-documented in the literature [8].
However, the preliminary experiments showed that when R
was 4 and 6, no cobalt nanoparticles were obtained without
adding HCOONa to the reaction medium, so the possibility
of reduction of cobalt (II) citrate complexes to metallic Co by
ethylene glycol could be excluded in these cases. This revealed
that it was HCOONa that initiated the reducing reaction and
ethylene glycol was only used as a solvent at R = 4 and 6.
Fig. 3 shows the XRD patterns of the Co nanoparticles
prepared using different molar ratios of sodium citrate to cobalt
chloride.
held for 8 h. After reaction, the products were collected by
centrifugation and washed with double-distilled water and
acetone, then dried in a vacuum oven at 70 ◦C for 12 h.
X-ray powder diffraction patterns were recorded using a
Shimadzu XD-3A X-ray diffractometer with Cu Kα radiation
˚
(λ = 1.5406 A). The transmission electron microscopy was
carried out in a Philips CM300 transmission electron
microscope operated at an accelerating voltage of 200 kV
and room-temperature M/H hysteresis loop was recorded with
a Quantum Design MPMS XL-7 superconducting quantum
interference device (SQUID) magnetometer.
3. Results and discussion
As could be seen in Fig. 3, at R = 0 and 2, six peaks were
observed at 2θ = 41.4◦, 44.4◦, 47.2◦, 51.5◦, 62.4◦ and 75.8◦
in the XRD patterns, suggesting that these Co samples were
present as both the hexagonal close-packed (hcp) and face-
centered cubic (fcc) structures [21]. It was very interesting to
note that the peak at 2θ = 51.5◦ disappeared as R increased
to 4 and 6, indicating that the corresponding products were in
pure hcp phase Co (space group: P63/mmc1(194); JCPDS:
05-0727, a = 2.503 A, c = 4.060 A). Also, we could see that
the diffraction peaks gradually became broad and the intensities
of the peaks decreased obviously as R increased, revealing a
decrease in the mean size of Co nanoparticles, which was quite
coincident with the TEM results.
Note that no diffraction peaks of impurities, such as, CoO
and Co(OH)2, were observed in all these XRD patterns, which
implied that the Co nanoparticles prepared by our method were
of high purity.
The M/H hysteresis loop (Fig. 4) of the 2 nm Co nanopar-
ticles measured at room temperature (300 K) showed that the
hysteresis and coercivity were nearly undetectable, suggest-
ing that the prepared Co nanoparticles had superparamagnetic
properties at room temperature. The saturation magnetization
was 82.2 emu/g, which was much lower than that of bulk Co
metal (168 emu/g).
The TEM images of cobalt nanoparticles prepared using
different molar ratios of sodium citrate to cobalt chloride are
shown in Fig. 1, which clearly indicated that the dosage of
sodium citrate had a remarkable effect on the Co nanoparticles.
As shown in Fig. 1a, when no sodium citrate was introduced
to the reaction medium, most of the resulting Co nanoparticles
were agglomerated together. In addition, the particle size was
quite large and the size distribution was rather broad. With the
increase in the molar ratio of sodium citrate to cobalt chloride,
the particle size of prepared Co nanoparticles decreased
remarkably. As R increased to 4, the Co nanoparticles were
well-dispersed without agglomeration and the average particle
size significantly decreased despite the fact that the size
distribution from 7 to 20 nm was still somewhat large. And as R
rose to 6, the diameter of the Co nanoparticles dropped further.
High magnification TEM image (Fig. 1e) showed that the Co
particles with an average size of 2 nm were predominantly
spherical and of uniform size.
Based on TEM observation, it was apparent that sodium
citrate, as a complex agent and stabilizer, played a crucial role in
decreasing the particle size and narrowing the size distribution
of the Co nanoparticles. It was generally acknowledged that the
citrate ions could prevent the further growth of nanoparticle
by the complexing of the carbonyl groups with metal ions
and the adsorption on the surface of formed small metallic
nanoparticles. Meanwhile, the adsorbed citrate ions could also
act as a stabilizer and inhibit the agglomeration of nanoparticles
by steric hindrance and/or Coulombic effects [20].
˚
˚
It is known that the energy of a magnetic particle in an
external field is proportional to its size or volume via the
number of magnetic molecules in a single magnetic domain.
When this energy becomes comparable to the thermal energy,
thermal fluctuations will significantly reduce the total magnetic

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H. Li, S. Liao / Solid State Communications 145 (2008) 118–121
Fig. 1. Typical TEM images of Co nanoparticles prepared using various molar ratios of sodium citrate to cobalt chloride: (a) R = 0, (b) R = 4, (c, d, e) R = 6, (f)
particle size distribution at R = 6. Inset in Fig. 1e shows the magnified TEM image, in which fringe spacing of cobalt nanocrystals was clearly visible.
moment at a given field [22]. So the magnetic moment
of the as-prepared Co nanoparticles significantly decreased
compared with the corresponding value of bulk Co owing to
its extremely small size. In addition, the electron exchange
between ligand and surface atoms could also quench the
magnetic moment [23]. In the present study, although the Co
nanoparticles were washed before the magnetic measurement,
it was possible that there were still a small amount of citrate
ions adsorbing on the surface of Co nanoparticles, which could
also cause the decrease in saturation magnetization.
4. Conclusion
In summary, 2 nm superparamagnetic Co nanoparticles with
the hexagonal close-packed (hcp) structure were prepared by a
novel and facile approach, in which sodium formate was used

H. Li, S. Liao / Solid State Communications 145 (2008) 118–121
121
It was revealed that sodium citrate played a crucial role in
decreasing the particle size and narrowing the size distribution.
The very efficient route presented here, using only cheap
inorganic reagents and ethylene glycol, should be very suitable
for large-scale production of ultrafine Co nanoparticles with
narrow size distribution due to the simplicity and low cost of the
synthetic process. Furthermore, this approach may be extended
to the preparation of the ultrafine nanoparticles of some other
transition metals and alloys, such as Fe, Ni and their alloys,
which is underway in our laboratory.
Acknowledgements
The authors would like to thank Dr. X.L.Yu at College of
Chemistry, Sun Yat-Sen University for magnetic analysis of
Co sample. This work was supported by the National Natural
Science Foundation of China (Project No. 20476034).
Fig. 2. TEM image of Co nanoparticles prepared using sodium tartrate as
complexing agent.
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