Colloids and Surfaces A: Physicochem. Eng. Aspects 356 (2010) 156–161
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Colloids and Surfaces A: Physicochemical and
Engineering Aspects
journal homepage: www.elsevier.com/locate/colsurfa
Controlled synthesis of rice ear-like cobalt microcrystals at room temperature
Yunling Li a,b , Jingzhe Zhao b,∗ , Yanchao Zhu a , Dechong Ma b , Yan Zhao a ,
Shengnan Hou b , Feng Yan b , Zichen Wang a
a
b
College of Chemistry, Jilin University, Changchun 130023, PR China
Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
a r t i c l e
i n f o
Article history:
Received 2 November 2009
Received in revised form 6 January 2010
Accepted 8 January 2010
Available online 15 January 2010
Keywords:
Cobalt
Aqueous phase
Chemical reduction
Microcrystals
Magnetic property
a b s t r a c t
In this work, we report a mild chemical procedure to synthesize rice ear-like cobalt microstructures
using hydrazine hydrate as the reductive agent. The as-obtained products were characterized by X-ray
diffraction, scanning electron microscopy, and vibrating sample magnetometry. The results show that
the samples are hexagonal-close-packed (hcp) Co dendritic structures composed of a pronounced trunk
and multiple ears. The length of the main trunk is tens of micrometers, and that of each ear is 3–5 ␮m
with a width of about 1 ␮m. It was found that the chain length of surfactant PEG as directing agent
drastically influenced the morphologies of the produced cobalt crystals. The morphologies of samples
can be manipulated from cauliflower to flower and then to rice ear-like structures only by choosing PEG
of different molecular weight as surfactant. These microstructures exhibited a ferromagnetic behavior at
300 K and might have potential applications in microdevices and other related magnetic devices.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
In the field of crystal growth, considerable attentions have
been focused on dendritic fractals. Dendrites with distinct size,
shape, and chemical behavior provide promises for the design
and fabrication of materials with advanced functionalities [1–6].
In addition, synthesis of dendritic fractals with more complex
micro/nanostructures has attracted great attention, because studies on these dendritic structures are useful to understand their
formation mechanism and to fabricate electronic, photonic or magnetic nanodevices [7–9]. A variety of dendritic crystals such as
metal [10,11], metal oxide [12,13] and chalcogenide [14] has been
studied theoretically and experimentally in prior researches. In this
study, we present an effective approach to fabricate rice ear-like
cobalt microstructures with dendritic morphology.
Metal cobalt has been extensively studied as an important
magnetic material, not only due to its multiple crystal structures (hexagonal-close-packed (hcp) and face-centered-cubic (fcc))
but also because of its structure-dependent magnetic and electronic properties [15]. In recent years, cobalt nanocrystals with
a wide range of morphologies such as monodisperse particles
[16], nanowires [17,18], nanorods [19,20], nanodisc/nanoplatelets
[21,22], nanorings [23], cubic-nanoskeletons [24], two- and
three-dimensional (2D and 3D) superlattices [25] and chain-like
∗ Corresponding author. Tel.: +86 731 8809278; fax: +86 731 8809278.
E-mail addresses: [email protected], zhao [email protected] (J. Zhao).
0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.colsurfa.2010.01.016
structures [26] have been successfully synthesized via different methods. The methods include thermal decomposition of
organometallic precursors [27], electrodeposition technology [28],
template-mediated synthesis [29], magnetic-field-induced process
[30], hydro-/solvothermal methods [31] and solution-phase reduction [32]. Recently, Zhu et al. reported a work on the synthesis
of dendritic cobalt nanocrystals in ethanol medium via a reduction synthetic route, they also studied the influences of the variety
of solvents on the phase and morphology changes of the cobalt
products by substituting ethanol with other solvents (glycol and
glycerin) [33]. Liu et al. prepared highly ordered snowflake-like
cobalt microcrystals via a hydrothermal reduction route, in which
the precipitated Co(OH)2 was reduced by hydrazine hydrate to
achieve Co samples in a slow-release controlled mode [34]. Fu and
co-workers synthesized novel 3D cobalt dendritic superstructures
by a hydrothermal reduction route, their morphology and crystal
structure were controlled by adjusting process parameters such
as hydrothermal time and aqueous hydrazine concentration [35].
However, up to now, there are few reports for the synthesis of dendritic cobalt microcrystals through an aqueous reduction strategy
under mild conditions. In our previous work, we synthesized cobalt
microspheres via a facile solution-phase reduction by introducing
small amount of glycerin or glycol [36]. The hydroxyls (–OH) in
glycerin or glycol are the key factor for the formation of microspheres in that work. In the present work, we select PEG as directing
agent to control the preparation of rice ear-like cobalt microstructures. The variation of chain length of PEG drastically influenced
the morphology of Co microstructures.
Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 356 (2010) 156–161
157
Herein we demonstrate that highly ordered rice ear-like metallic cobalt microcrystals can be successfully synthesized in larger
quantities via a facile aqueous reduction route at room temperature
and atmospheric pressure, in which aqueous hydrazine (N2 H4 ·H2 O)
and sodium hydroxide (NaOH) were used as reductive agent and
alkaline reagent respectively. In this synthetic system, PEG with
varied molecular weight was employed as surfactant to control
the morphologies of the project products. The rice ear-like cobalt
microstructures are expected to bring new opportunities in vast
research and application fields, such as sensors, micro-/nanodevice
and magnetic cells. High yield, simple apparatus and mild conditions would make this synthetic method a good prospect in
large-scale applications. The growth mechanism of the crystals was
also proposed on the basis of experimental results.
2. Experimental
All chemical reagents in this work were of analytical grade purity
and were used as starting materials without further purification.
In a typical procedure, 20 mL of an aqueous solution was first
prepared by dissolving CoCl2 ·6H2 O (2.38 g, 10 mmol), 0.1 g of citric
acid (CA) and a restrained amount of poly(ethylene glycol) (PEG)
in deionized water. NaOH solution (10 mL, 5 M) was subsequently
added into the above solution under vigorous stirring. Then 10 mL
of N2 H4 ·H2 O solution (50 vol%) was added into the system, at once
the color of the solution changed from pink to dark blue. All of
the above manipulations were performed at room temperature.
The molar ratio of CoCl2 ·6H2 O to N2 H4 ·H2 O was 1:8. After one
hour’s time, the reaction finished with the appearance of large
quantity of black precipitates. The precipitates were separated from
the solution by placing a magnet under the container, and washed
several times with distilled water and absolute alcohol to remove
any residual alkali or salt. The resulted Co dendrites were obtained
by drying wet precipitates in a vacuum system at room temperature.
X-ray diffraction (XRD) detection on Co was performed on a Shimadzu model XRD-6000 using Cu K␣ radiation. The morphology
and particle size of the powders were characterized by a scanning
electron microscope (SEM SHIMADZU SSX-550). Samples for SEM
measurement were prepared by taking dried powders using toothpick and pasting them onto conductive adhesive paper at room
temperature. VSM measurements were performed for cobalt powers in a vibrating sample magnetometer (VSM).
Fig. 1. XRD patterns of the as-prepared samples synthesized with PEG(6000) as
surfactants at room temperature.
is tens of micrometers, and that of each ear is about 3–5 ␮m with
width of about 1 ␮m. From the magnified image in Fig. 2b, it can
be clearly seen that the surfaces of the rice ear-like microstructures
are rough, lots of platelets construct the side ears of the microstructures.
3. Results and discussion
3.1. Crystal structure
A typical XRD pattern of the as-prepared black power is shown
in Fig. 1. The characteristic peaks of the sample arise at 2 = 41.54◦ ,
44.54◦ and 47.36◦ , which match the reflection planes of (1 0 0),
(0 0 2) and (1 0 1) of hexagonal-phase cobalt (JCPDS 05-0727).
Broadening of the peaks exhibits nanocrystalline nature of the samples. No characteristic peaks due to the impurities of cobalt oxides
or hydroxide were detected, indicating that Co crystals with high
purity were obtained by our simple aqueous strategy.
3.2. Morphology
Typical SEM images of Co microstructures are presented in Fig. 2
with different magnifications. Fig. 2a of the overall morphology of
the sample reveals that a large amount of rice ear-like metallic
cobalt microstructures can be synthesized by this simple approach.
The well-defined rice ear-like Co microstructures were constructed
by a pronounced trunk and multiple ears. The length of the trunk
Fig. 2. SEM images of the as-synthesized rice ear-like Co microstructures with
PEG(6000) as surfactant: (a) low magnification and (b) high magnification.
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In order to find out the key parameters on the formation of rice
ear Co microstructures, experiments with manipulated parameters were done systematically. Fig. 3 shows the morphologies of
the samples prepared at room temperature via the simple aqueous process with PEG of different molecular weight as controlling
agent. If no surfactant of PEG but parallel amount of glycol (EG) was
added in the solution, cobalt sample with cauliflower-like structures was created, as shown in Fig. 3a. The size of the cauliflower is
about 10 ␮m. When surfactant of PEG(400) was introduced into the
solution, flowerlike Co structures with multiple petals were formed
(Fig. 3b). The size of the flower is about 5–10 ␮m. It is clearly seen
that the flowerlike structures with multiple petals are not in the
same plane but have a 3D structure. Upon increasing the molecular weight of PEG to 2000, rice ear-like structures began to appear,
as shown in Fig. 3c. However, there are still some coexistent flowerlike structures in the sample. As the molecular weight of PEG
increased to 6000, well-defined rice ear-like structures with a pronounced trunk and multiple ears were formed in high yield (Fig. 3d).
However, Co dendrites consist of rice ear-like structures appeared
while the molecular weight of PEG increased to 10k (Fig. 3e). Likewise, we also cannot obtain samples with dominated rice ear-like
morphology when surfactant of PEG 20k was added into the solution (Fig. 3f). These results reveal that the morphology of the Co
microstructures strongly depends on the molecular weight of PEG
with other parameters unchanged. Higher molecular weight benefits the construction of long branched Co microstructures, but it
does not change along with the chain when the chain length of PEG
reached long enough as molecular weight of 10k and 20k.
To further investigate the influences of surfactants, compared
experiments with other surfactants instead of PEG were performed.
The results showed that the morphology of the resulting cobalt
products also depended strongly on the variety of surfactants. Fig. 4
shows the SEM images of the samples with PVP and SDS as surfactants. When other conditions are kept constant, by only substituting
PVP for PEG, Co spherical constructions appeared as shown in
Fig. 4a. The enlarged image of the sample is presented in Fig. 4b,
which indicates that the spheres are assembled by nanoplatelets
with a thickness of tens of nanometers. The spheres conglutinated with each other instead of isolated ones. When SDS was
used instead of PEG, Co particles with anomalous morphology were
obtained, as shown in Fig. 4c. The enlarged image in Fig. 4d further
proves the abnormity of the sample.
Fig. 3. SEM images of the samples prepared at room temperature using PEG with different molecular weight as surfactants: (a) EG, (b) PEG(400), (c) PEG(2000), (d) PEG
(6000), (e) PEG(10k), and (f) PEG(20k).
Y. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 356 (2010) 156–161
159
Fig. 4. SEM images of the samples prepared at room temperature with PVP (a, low magnification; b, high magnification) and SDS (c, low magnification; d, high magnification)
as surfactants.
In addition, we also examined the effects of other synthetic
parameters on the formation of Co rice ear-like microstructures.
We found that reaction temperature is an important factor in determining the rate of the reactions when eliminating the influences
of other parameters. The formal temperature used in our preparation is room temperature. Temperature-dependent experiments
revealed that the rate of the reaction became slower at elevated
reaction temperatures. When the reaction temperature was controlled to be 80 ◦ C, the reduction cannot proceed completely, only
small amount of products were obtained. Furthermore, we found
that a low concentration of CoCl2 (such as 0.25 M) also resulted in
few products with unfinished reaction. The reason of these influences may be the opportunity of particles colliding with each other
reduced, and N2 H4 ·H2 O would volatilize faster at higher temperatures, so higher temperature and low concentration are not of
benefit to the reactions.
3.3. Magnetic properties
In order to directly inspect the magnetism of the prepared particles, we dispersed our sample into water by ultrasonic treatment
and then placed a magnet next to it. When the magnet was placed
near the side wall of the glass vial, the particles formed cm-long
needlelike structures which could be visualized by naked eyes. And
these needles could be perpendicular to the side wall of the glass
vial, as shown in Fig. 5a. The attraction of the particles to the magnet
demonstrates that they are superparamagnetic or ferromagnetic.
We also measured the magnetization of as-synthesized rice ear-like
cobalt powders using a vibrating sample magnetometer (VSM). The
result is shown in Fig. 5b. The saturation magnetization Ms of the
microstructures is found to be 138.96 emu/g, which is very close
to that of bulk cobalt (168 emu/g) [34]. The coercivity (Hc) value is
228 Oe. Compared to the Hc value of bulk Co (a few tens of Oersteds)
Fig. 5. (a) Photograph of the as-prepared rice ear-like Co dispersed in water with the existence of a magnet. (b) Magnetic hysteresis loop of the Co sample at 300 K.
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[17], the obtained Co dendrites exhibit much enhanced coercivity,
which might attribute to the organization of rice ear-like cobalt
microstructures.
bulk metal. The rice ear-like microstructures might have potential
applications in microdevices and magnetic devices.
Acknowledgements
3.4. Possible formation mechanism
From the characterization results, it can be concluded that the
morphologies of the Co microstructures are greatly dependent on
the variety of surfactants, especially the chain length of PEG. The
experimental conditions such as reaction temperature and precursor concentration, also have obvious influences on the structures
of Co crystals. To control over the synthesis, it is necessary to
understand the formation mechanism of the rice ear-like cobalt
microstructures.
Although the exact PEG on crystal growth is not clear, a possible growth mechanism was proposed in this paper. Under the
present conditions, the formation of Co dendrites is based on
self-assembly and directional growth approach concerning the
involvement of PEG chains [37,38]. Citric acid was introduced as
a complexing reagent to control the reaction process. The chemical
reduction can be formulated in the presence of citric acid as follows
[39]:
2[Co(C6 H5 O7 )2 ]4− + N2 H4 + 4OH−
→ 2Co ↓ + N2 ↑ + 4H2 O + 4C6 H5 O7 2−
At the initial stage of the reduction process, small Co particles generated from cobalt(II)–citric complex [Co(C6 H5 O7 )2 ]4−
with blue ␣-Co(OH)2 as intermediates in a basic system. Primary
cobalt particles aggregated to form side ear structure of the project
product by magnetic induction effect [10]. After that multiple ears
assembled along the chain of PEG to form rice ear-like microstructures because of linear molecular structure and abundant oxygen
atoms of PEG molecules. Thus the pronounced rice ear-like Co dendrites evolved through self-assembly of primary Co particles and
template-directing approach by PEG selectively absorbing on crystallographic facets of Co particles [38]. Based on the mentioned
mechanism, chain length of PEG would drastically influence the
growth direction of Co microstructures, which accords with our
experimental results.
As we detected, citric acid is an indispensable reagent for the
reaction and it determines the reaction speed and purity of the
samples, but the key factor to control the morphology of the Co
microcrystals in our research is the chain length of PEG (PEG with
different molecular weights). The effect of PEG is very strong, an
addition of PEG with varied chain length can make the morphologies of Co microcrystals change prominently as shown in Fig. 3a–f.
The morphologies of the produced cobalt crystals were manipulated from cauliflower to flower and then to rice ear-like structure
only by changing the molecular weight of PEG. So the chain length
of PEG plays a key role as directing agent in the growth of rice
ear-like cobalt microstructures.
4. Conclusion
In summary, we have successfully synthesized the rice ear-like
metallic cobalt microstructures via a facile PEG-assisted aqueous
synthetic route at room temperature. The rice ear-like structures
are composed of dozens of well-aligned metal Co ears assembled
radiating from the main trunk. The length of the main trunk is
tens of micrometers, and that of each ear is about 3–5 ␮m with a
width of about 1 ␮m. The shape, structure, and magnetic property
of final products were investigated, and the formation mechanism
of the rice ear-like microstructures was discussed as well. These
microstructures exhibit a ferromagnetic behavior at 300 K and also
have a significantly enhanced magnetic coercivity compared to the
This work was supported by the Scientific Research Foundation
for the Returned Overseas Chinese Scholars, State Education Ministry (SRF for ROCS and SEM), the Natural Science Foundation of
Jilin Province for Excellent Young Scholars (grant no. 20040117)
and also supported by the National Natural Science Foundation of
China (grant no. J0830415)
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