Study on morphologies of Co microcrystals produced by solvothermal method with different solvents

Article abstract of DOI:10.1016/j.materresbull.2010.01.002

Cobalt microcrystals with different sizes and morphologies were fabricated via a facile solvothermal method. The effects of different solvents on the sizes and morphologies are investigated separately. Flowerlike cobalt with around tens of microns in diameter is obtained in ethanol. Meanwhile, of microdisks and octadecahedra with the edge lengths about several microns are fabricated in ethylene glycol and glycerol, respectively. Reduction potential and viscosity rank of the solvents play the important roles in controlling the sizes and morphologies of cobalt. Moreover, room-temperature magnetic measurement shows that the magnetic properties of cobalt microcrystals depend on their morphologies.

Full text of DOI:10.1016/j.materresbull.2010.01.002

Materials Research Bulletin 45 (2010) 373–376
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Study on morphologies of Co microcrystals produced by solvothermal method
with different solvents
*
Lianfeng Duan, Shusheng Jia, Lijun Zhao
Key Laboratory of Automobile Materials (Jilin University), Ministry of Education and Department of Materials Science and Engineering, Jilin University,
Renmin Street No. 5988, Changchun 130022, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Cobalt microcrystals with different sizes and morphologies were fabricated via a facile solvothermal
method. The effects of different solvents on the sizes and morphologies are investigated separately.
Flowerlike cobalt with around tens of microns in diameter is obtained in ethanol. Meanwhile, of
microdisks and octadecahedra with the edge lengths about several microns are fabricated in ethylene
glycol and glycerol, respectively. Reduction potential and viscosity rank of the solvents play the
important roles in controlling the sizes and morphologies of cobalt. Moreover, room-temperature
magnetic measurement shows that the magnetic properties of cobalt microcrystals depend on their
morphologies.
Received 25 June 2009
Received in revised form 8 November 2009
Accepted 4 January 2010
Available online 11 January 2010
Keywords:
A. Magnetic materials
B. Chemical synthesis
B. Crystal growth
D. Microstructure
E. Magnetic properties
ß 2010 Elsevier Ltd. All rights reserved.
1. Introduction
centered cubic (fcc) and the epsilon phases. Generally, the hcp
phase with a high coercivity is of interest in the field of permanent
In recent years, much attention has been focused on the
fabrication of nanoscale and microscale crystals. Ferromagnetic
cobalt (Co) has received considerable attention both theoretically
and experimentally due to its extraordinary electrical, catalytic,
and magnetic properties, which are of interest for basic scientific
research and potential technological applications, for example,
catalysis, electronics, high-density magnetic recording media,
sensors, biotechnology, and biomedical nanotechnology catalysts,
artificial cells, coatings, etc. [1–9]. The size, morphology, and
structure of materials significantly influence their physical and
chemical properties, and therefore their technological applications
[10]. Up to now, various morphologies of ferromagnetic cobalt
crystals such as wires, rods, disks, rings and spheres have been
successfully fabricated [11–15]. Diverse synthetic methodologies,
such as the thermal decomposition of cobalt carbonyl, organome-
tallic precursors, template-mediated synthesis, solvothermal, and
the ultrasonic spray pyrolysis methods, have also been proposed
and demonstrated to prepare cobalt particles [16–20]. In particu-
lar, solvothermal method demonstrates its superiority in control-
ling the shapes of crystals [13,21,22].
magnets, while the fcc phase with a low coercivity can be used as
soft-magnetic materials [23]. The structure, size, and shape play an
important role in defining the properties of the particles as well as
their use in practical applications. Various morphologies of Co
microcrystals with hcp structure were fabricated by solvothermal
method. In addition, the effects of different solvents on the
morphologies of cobalt microcrystals are discussed. Herein, the
metallic Co microcrystals with different morphologies and sizes
were synthesized by solvothermal method using three kinds of
solvents. To investigate the effect of microstructure on the
magnetic properties, we also studied the magnetic properties of
the Co microcrystals with various morphologies.
2. Experimental
All reagents were commercial products with analytical grade
without further purification. In a typical procedure, 0.5 g of
CoCl2Á6H2O was first dissolved in 20 mL of ethanol (S1), ethylene
glycol (S2) or glycerol (S3) under magnetic stirring at room
temperature. The solution was intensively stirred for 20 min, then
1.0 mL of ethylenediamine (EDA) and 1.0 mL hydrazine monohy-
drate N2H4ÁH2O (55 vol.% analytical reagent) were added dropwise
to the solution at room temperature by vigorous stirring for 20 min.
Subsequently, thesolution was transferredintoa30 mLTeflon-lined
stainless steel autoclave. The autoclave was sealed and maintained
at 200 8C for 24 h and then cooled to room temperature naturally.
Co is an important ferromagnetic metal, which can crystallize in
three structures: the hexagonal-close-packed (hcp), the face-
* Corresponding author. Tel.: +86 431 85095878; fax: +86 431 85095876.
E-mail addresses: lijunzhao@jlu.edu.cn, nokiaxf99@yahoo.com.cn (L. Zhao).
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.01.002

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L. Duan et al. / Materials Research Bulletin 45 (2010) 373–376
3. Results and discussion
Fig. 1 gives XRD patterns of the as-prepared cobalt crystals. All
of them are well crystallized. The diffraction peaks for the samples
could be indexed to pure hexagonal-close-packed (hcp) cobalt
phase (JCPDS 05-0727), and no impurities can be detected.
The morphologies of the samples were observed by FESEM.
Fig. 2a is a typical low-magnification SEM image of S1, from which
we can find that flowerlike shapes are the exclusive products,
which means that the flowerlike cobalt can be prepared in large
scale. A high-magnification SEM image of S1 (Fig. 2b) shows that
flowerlike cobalt microcrystals with about 30
mm in diameter.
Furthermore, the rough surfaces of S1 confirmed that flowerlike
shapes consisted of small particles stacked in a random fashion.
SEM image of S2 (Fig. 3a) displays that the product is composed
of 2–3
the SEM image of S3, which shows that the thickness of the
octadecahedra is about 2.5 m, and their width is 1.5–2 m. The
mm-thick and 1.5–2 mm-wide prism-like crystals. Fig. 3b is
Fig. 1. XRD patterns of the as-obtained products (a) S1, (b) S2, and (c) S3.
m
m
special shapes obtained from the solvothermal process, which
means that the polyols may serve as stabilizing reagents to bind
the surfaces of cobalt microcrystals. Furthermore, this serves as an
evidence that the polyols can slow down the growth ratio of the
cobalt crystals, because the dissolution–crystallization process of
the precursor is inhibited.
The products (S1, S2, and S3) were filtered off, washed several times
with distilled water and absolute ethanol, and finally dried in a
vacuum oven at 60 8C for 4 h.
The phases were identified by means of X-ray diffraction (XRD)
with a Rigaku D/max 2500pc X-ray diffractometer with Cu K
scan rate of 0.048 sÀ1
˚
(l) 1.54156 (A) at a . The
a
Further insight into the morphology and microstructure of S3
was gained using TEM and HRTEM. The HRTEM and SAED were
performed in the region of polyhedra as labeled in Fig. 4a which is
the TEM image of S3. The SAED pattern (Fig. 4b) exhibits the good
crystallization of cobalt microcrystals. Furthermore, the SAED
spots can be steadily indexed to (1 0 1), (1 0 3), (0 0 2) facets which
is in good agreement with the XRD data. The lattice fringes of
crystalline S3 can be seen clearly from Fig. 4c, and the interplanar
spacing was measured to be 0.20 nm, corresponding to the (0 0 2)
planar spacing of hcp cobalt. The (0 0 2) plane of hcp-Co
radiation
morphologies were characterized by a JEOL JSM-6700F field-
emission scanning electron microscopy (FESEM) operated at an
acceleration voltage of 5.0 kV. Transmission electron microscopy
(TEM) and high-resolution TEM (HRTEM) observations were
carried out on a JEOL 2100F with an emission voltage of 200 kV.
Magnetic measurements were carried out using a Quantum Design
superconducting quantum interference device (SQUID) magne-
tometer (LakeShore 7307).
Fig. 2. SEM image of S1 (a) low magnification and (b) high magnification.
Fig. 3. SEM images of Co microcrystal (a) S2 and (b) S3.

L. Duan et al. / Materials Research Bulletin 45 (2010) 373–376
375
Fig. 4. (a) TEM, (b) SAED, and (c) HRTEM images of S3.
corresponds to the [0 0 2] direction which is parallel to the growth
direction as the arrow shown in Fig. 4a.
the critical size. By taking advantage of this key feature, nearly
monodisperse size distribution can be obtained at this stage by
either stopping the reaction (nucleation and growth) quickly or by
supplying a reactant source to keep a saturated condition during
the reaction. The driving force for this spontaneous oriented
attachment is that the elimination of the pairs of high energy
surfaces will lead to a substantial reduction in the surface free
energy from the thermodynamic viewpoint [25]. Because of the
quick nucleation and easy growth in the ethanol solution, EDA has
few effects on the crystal planes. The elimination of the pairs of
crystal planes is random. Hence, the flowerlike morphologies of
cobalt microcrystals are prepared in the ethanol solution by further
crystallization. Moreover, the relatively low reduction ability for
ethanol results in a slow reduction rate, which brings about the
largest crystallite sizes for S1. In addition, the particle sizes can be
controlled by reduction potential of the polyol (Predox) [26]. The
Predox of the polyol is high, and then the size of the particles
becomes smaller. In the present system, the Predox of ethanol is
much weaker than EG and Gly, so the particle sizes of S1 are larger
than these of S2 and S3. To some extent, higher viscosity can slow
down the growth rate and be more advantageous for the isotropic
growth [27]. In the ethylene glycol or the glycerol solvent, the
relatively slow nucleation and growth favor the selective adsorp-
tion of –NH2 on the crystal planes of cobalt crystals, which are good
for the formation of cobalt microdisks and octadecahedra. For the
formation of microdisks, –NH2 functions may bind strongly to
(0 0 1) surfaces of the growing Co crystallites. So the particles grow
preferentially along the [0 0 1] direction and the growth rate along
the [0 0 1] direction is much slower than that along the [1 1 0]
direction, leading to the formation of microdisks with the edge
Murphy [24] had claimed that the preferential adsorption of
molecules and ions in solution onto different crystal facets directs
the growth of particles into various shapes by controlling the
growth rates along different crystal axes. In the ethylene glycol or
glycerol, EDA may play a role to direct the growth of cobalt crystals,
because of the interaction between the –NH2 and the cobalt ions or
the –NH2 selective adsorption on the particle surfaces. Experi-
mental observations suggest that steric interactions among
neighboring adsorbed molecules may have an impact on the
growth rate of the (0 0 2) plane [13]. Moreover, the addition of
N2H4ÁH2O could improve the yields of cobalt powders.
To shed light on the possible growth processes of the Co
microcrystals with various morphologies under different solvents,
schematic illustration was drawn in Fig. 5. It is known that the
physical and chemical properties of the solvents can influence the
solubility, reactivity, and diffusion behavior of the reagents, and
even influence the growth of morphologies of products. On the one
hand, the sequence of boiling point of the three kinds of solvents
from low to high is ethanol, ethylene glycol, and glycerol. On the
other hand, the viscosity ranking of the three kinds of solvents is
ethanol < ethylene glycol < glycerol. Furthermore, hydroxyl num-
bers of the polyols decide the reducing abilities of the three kinds of
solvents. To the ethanol, its lower boiling point leads to the highest
pressure in the closed reaction systems at the same reaction
temperature, which facilitates the growth of cobalt particles; its
lower viscosity helps the quicker nucleation. When the concen-
tration of growth species reduces below the minimum concentra-
tion for nucleation, nucleation stops, whereas the growth
continues until the equilibrium concentration of the precipitated
species is reached. At this stage, kinetic control and focus of size
occurs: the smaller particles grow more rapidly than the larger
ones, because the free energy driving force is larger for smaller
particles than for larger ones, if the particles are slightly larger than
length and height of about 3 and 0.6
mm, respectively. For the
formation of octadecahedra, –NH2 may bind strongly to (0 0 1)
surface of the growing Co crystallites. So the particles grow along
the [0 0 1] is much faster than that along [1 1 0] direction, causing
the formation of octadecahedra; and the (1 0 0) surfaces of S3 have
Fig. 5. Schematic illustration of the possible formation processes of the Co microcrystals with various morphologies under different solvents.

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L. Duan et al. / Materials Research Bulletin 45 (2010) 373–376
Fig. 6. The magnetization curve measured at room temperature for Co microcrystals (a) S1, (b) S2, and (c) S3.
an inclination to disappear. Optional attachment of the cobalt
particles may happen in the ethanol solution; oriented attachment
mechanism may explain the formation of cobalt microdisks and
octadecahedra. With rotation of the adjacent particles, they share
the same crystallographic orientation and subsequent coalescence.
The regular aggregates are formed through the oriented attach-
ment and self-assembly, which are similar to the report on
formation of silver and Co by Fang and Zhu [28,29]. After the self-
assembly process, the loose structures become compact gradually
through further crystallization.
4. Conclusions
In summary, flowerlike, disk-shaped, and polyhedral Co
microcrystals are prepared by the solvothermal method with
different solvents. It has been found that the solvents affect the
crystallite sizes and the morphologies of the Co microcrystals. Both
the reduction potential and the viscosity ranking of the solvents
play pivotal roles in controlling the sizes and the shapes of cobalt
particles. Furthermore, the magnetic properties of cobalt micro-
crystals depend on their morphologies.
In order to investigate the influence of morphologies on the
magnetic properties, the magnetic measurement for the Co
microcrystals is performed. The room-temperature hysteresis
loops of S1, S2, and S3 are shown in Fig. 6. Furthermore, the
magnetic parameters obtained from Fig. 6 are listed in Table 1. To
S1, S2, and S3, the values of saturation magnetization (Ms) are 156,
159, and 165 emu/g, respectively. The values of remnant magneti-
zation (Mr) are 1.4, 2.9, and 4.3 emu/g; the values of coercivity (Hc)
are 24, 44, and 77 Oe. It is generally accepted that when the particle
sizes are larger than the single domain region, the values of Ms and
Mr are increased with the increase of particle sizes. However,
different phenomenon is observed from our experimental results.
The use of various solvents during the solvothermal process may
be responsible for the different results comparing with the above
mention. For S1, it has the lowest values of Ms, Mr, and Hc. For S3, it
owns the highest values of Ms, Mr, and Hc. Especially, the values of
Hc for the samples are different from that of spherical cobalt
particles (375 Oe) and the wire-like cobalt particles (389 Oe) [30].
Magnetization investigation confirms that the samples prepared
by the solvothermal method using different solvents show
relatively good soft magnetism.
Moreover, higher viscosity of solvent can slow down the growth
rate and be more advantageous for the complete crystallization.
Hence, the S3 possesses the highest Ms value. Besides, shape
anisotropy might be responsible for the sequences of coercivity
values for samples. It is well known that the particles with ellipse
morphology had the lowest shape anisotropy. Therefore, we
speculated that disks and octadecahedra had the higher shape
anisotropy energy. Irregular flowerlike shape was close to ellipse,
so they had the lowest coercivity value.
Acknowledgement
This work was supported by the Basic Research Expenses for the
Special Funds of Jilin University.
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Table 1
Magnetic parameters obtained from hysteresis loops.
Samples
S1
S2
S3
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
156
159
165
1.4
24
2.9
49
4.3
77