Metallic cobalt microcrystals with flowerlike architectures: Synthesis, growth mechanism and magnetic properties

Article abstract of DOI:10.1016/j.materresbull.2009.02.016

Well-defined, three-dimensional (3D) flowerlike metallic Co microcrystals with several radiating hexagonal-tapered petals assembled by particles size of 150-250 nm were fabricated via a facile hydrothermal reduction route under a fixed basic condition. Th

Full text of DOI:10.1016/j.materresbull.2009.02.016

Materials Research Bulletin 44 (2009) 1468–1473
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Metallic cobalt microcrystals with flowerlike architectures: Synthesis, growth
mechanism and magnetic properties
*
Run-Han Wang, Ji-Sen Jiang , Ming Hu
Department of Physics, Center of Functional Nanomaterials and Devices, East China Normal University, Shanghai 200062, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Well-defined, three-dimensional (3D) flowerlike metallic Co microcrystals with several radiating
hexagonal-tapered petals assembled by particles size of 150–250 nm were fabricated via a facile
hydrothermal reduction route under a fixed basic condition. The morphology and structure of the
products were characterized by scanning electronic microscopy (SEM), energy-dispersive X-ray (EDX),
transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray photoelectron
spectroscopy (XPS) and powder X-ray diffraction (XRD). The probable formation mechanism of the
flowerlike Co microcrystals was discussed based on the experimental results. Magnetic properties of Co
microcrystals were investigated by a commercial Physical Properties Measurement System (PPMS). The
Received 14 August 2008
Received in revised form 6 January 2009
Accepted 20 February 2009
Available online 6 March 2009
Keywords:
A. Metals
B. Chemical synthesis
B. Crystal growth
D. Microstructure
D. Magnetic properties
flowerlike products exhibited ferromagnetic characteristics with
a saturation magnetization of
128.1 emu/g and a coercivity of 232.5 Oe at room temperature. Compared to the coercivity value of
bulk Co, the products displayed a remarkable enhanced value due to their special morphology.
ß 2009 Elsevier Ltd. All rights reserved.
1. Introduction
pyrolysis [24], and liquid-phase metal salt reduction [25] have
been developed to fabricate cobalt. Among these methods, the
Morphological control of metals has become a subject of much
intensive research due to its importance in fundamental scientific
research and potential technological applications [1–4]. As one of
the most important transition metals, cobalt displays a wealth of
magnetic [5], electronic [6,7], optical [8], microwave [9] and
catalytic [10] properties. Because the shape of the metal has
significant influence on its properties, there has been an increasing
number of reports on the synthesis of cobalt with various shapes,
such as rods [11,12], chains [13,14], wires [15,16], belts [17], disks
[18], platelets [19], tubes [20], and dendrites [21]. Meanwhile, the
dependence between the shape and the properties of cobalt has
been observed. For example, single-crystal Co nanoplatelets
exhibit a distinct enhanced coercive force attributing to their 2D
structures [22]. Self-assembled devices composed of periodic
arrays of 10-nm-diameter cobalt nanocrystals display spin-
dependent electron transport [6]. Therefore, if we can understand
how to regulate the shape of metals, their chemical and physical
properties may be tailored as desired.
metal carbonyl pyrolysis needs toxic or expensive precursors, and
the reaction often performs at a high temperature, while the
electrochemical deposition generally requires
a complicated
device and intricate processing. In the liquid-phase synthesis,
capping molecules, micelles and complexing agents are usually
applied to control the sizes and shapes of materials [14,17,26,27].
As one of the most common liquid-phase methods, the hydro-
thermal method is often used to prepare high purity materials, and
to control the morphology of materials. Recently, hydrothermal
method has been used to prepare Co. For instance, Xie et al. [17]
prepared 1D single crystal hcp Co nanobelts by a hydrothermal
reduction process at 160 8C. Liu et al. [28] reported a facile
hydrothermal reduction route to fabricate highly ordered 2D
snowflakelike Co microcrystals based on a precipitate slow-release
controlled process. Li and Liao [29] synthesized 3D flowery Co
microcrystals composed of several Co plates via a solvothermal
route in which a water/ethanol mixture was used as a solvent and
ethylenediamine tetraacetic acid sodium (EDTA) was introduced
as a complexing agent, respectively.
In this work, we have synthesized three-dimensional flowerlike
Co microcrystals in large quantities. The radiating hexagonal-
tapered petals of the flowers were formed by assembly of Co
particles sized of 150–250 nm. To the best of our knowledge, no
reports for this unique morphology of metallic Co have been
published up to the present. A formation mechanism of the 3D
flowerlike Co microstructures was proposed based on the
Although materials with different morphologies have been
prepared, selective synthesis of materials with novel morphologies
always turns out to be a great challenge. To date, various synthetic
methods such as electrochemical deposition [23], metal carbonyl
* Corresponding author. Tel.: +86 21 62232001.
E-mail address: jsjiang@phy.ecnu.edu.cn (J.-S. Jiang).
0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.02.016

R.-H. Wang et al. / Materials Research Bulletin 44 (2009) 1468–1473
1469
Fig. 1. (a) SEM image of the as-synthesized Co flowerlike microcrystals at 120 8C for 8 h. (b) An individual Co flowerlike microcrystal. (c) A single petal of the Co flowerlike
microcrystal.
experimental observations. And the magnetic properties of the
products were also investigated.
electron microscope at an accelerating voltage of 200 kV. Magnetic
measurements for the samples were carried out at room
temperature using a commercial Physical Properties Measurement
System (PPMS, Quantum Design Inc.).
2. Experimental
2.1. Synthesis of flowerlike metallic Co microcrystals
3. Results and discussion
All the reagents were of analytical grade and used without
The morphology of the as-prepared products formed at 120 8C
for 8 h was examined by SEM. Typical SEM images of the products
with different magnifications are given in Fig. 1. Fig. 1a shows the
overall morphology of the sample, which indicates that the
products mainly consist of flowerlike structures. The high-
magnification image of a single Co flowerlike microcrystal shows
that the 3D architecture is constituted by a few petals radiating
from the center (Fig. 1b). Furthermore, each petal shows a
hierarchical structure comprising a tapered trunk and many tight
nanoparticles with size of 150–250 nm (Fig. 1c). The tight particles
form a well-defined hexagon around the trunk (see inset in Fig. 1b).
The phase composition of the as-synthesized flowerlike
products was characterized by XRD, and the XRD pattern of the
sample is shown in Fig. 2a. The characteristic peaks of the as-
further purification. In
a
typical procedure, 0.2108 g of
CoSO4ꢀ7H2O, and 1 g of sodium citrate (Na3C6H5O7ꢀ2H2O) were
dissolved in 30 mL of deionized water. 3 mL hydrazine hydrate
(N2H4ꢀH2O, 85%) and 5 mL NaOH aqueous solution (1 M) were
added dropwise into the above mixture while stirring. The final
mixture was vigorously stirred for 30 min and then transferred
into a 40 mL Teflon-lined stainless steel autoclave for hydro-
thermal treatment at 120 8C for 8 h. After the autoclave was cooled
to room temperature naturally, the gray solid precipitation was
then washed with distilled water and ethanol several times, and
dried under vacuum at 50 8C for 10 h.
2.2. Characterization
prepared products arise at 2u = 41.688, 44.508, 47.468, 62.808 and
The phase identification of the products was accomplished with
75.918, which have been assigned to (1 0 0), (0 0 2), (1 0 1), (1 0 2),
and (1 1 0) planes respectively. All peaks of the products can be
indexed as hexagonal close-packed (hcp) Co (JCPDS no. 05-0727).
No peaks for impurities are detected indicating that the flowerlike
microcrystals consist of pure metallic hcp cobalt. Additionally, the
relative intensity of the peaks corresponding to the (0 0 2)/(1 0 1),
(0 0 2)/(1 1 0) planes are remarkably higher than the standard
values in JCPDS card, suggesting that the preferential crystal
growth orientation of the product was in the [0 0 1] direction. EDX
spectrum of the flowerlike products indicated that the sample is
essentially pure Co (Fig. 2b). Only a very small amount of oxygen
powder X-ray diffraction (XRD) employing a Rigaku D/max 2550 X-
˚
ray diffractometer with Cu Ka radiation (l = 1.5406 A). A scan rate
of 0.028 sꢁ1 was applied to record the pattern in the 2
u range of 30–
808. The scanning electron microscopy (SEM) images and energy-
dispersive X-ray (EDX) analysis were obtained by using a JSM-6460
scanning electron microscope. X-ray photoelectron spectroscopy
(XPS) measurement was carried out on a RBD upgraded PHI-5000C
ESCA system (PerkinElmer). Transmission electron microscopy
(TEM) images and the corresponding selected area electron
diffraction (SAED) were collected with a JEM-2100F transmission
Fig. 2. XRD pattern (a), EDX spectrum (b), XPS spectrum of the as-synthesized flowerlike Co microcrystals at 120 8C for 8 h.

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R.-H. Wang et al. / Materials Research Bulletin 44 (2009) 1468–1473
Fig. 3. (a) TEM image of flowerlike Co microcrystals. (b) The selected-area electron diffraction (SAED) pattern of the circled area in (a).
was detected, which was possibly from slight oxidation of the
surface. The existence of CoO on the surface of the sample has been
re-established on the basis of the binding energies of satellite
peaks at 781.0 and 796.7 eV of CoO2p3/2 and CoO2p1/2,
respectively, found in the XPS spectrum (Fig. 2c) [30].
The morphology of the flowerlike Co was further determined
by TEM as shown in Fig. 3a, which is consistent with that
observed by SEM. Additionally, the sample shown in Fig. 3a was
subjected to ultrasonication for 45 min before TEM analysis. It is
noticeable that there is no obvious crack or destruction of the
flowerlike Co after the ultrasonication, indicating a high stability
of the product. Fig. 3b is a typical SAED pattern corresponding to
the circled area in Fig. 3a. These pattern spots demonstrate the
single-crystalline nature of this metallic Co microstructure. The
interplanar spacings, obtained from the SAED pattern, are 2.034,
be formulated as follows:
Co2þ þ 2C6H5O7 ! ½CoðC6H5O7Þ2ꢂ
(1)
(2)
3ꢁ
4ꢁ
2½CoðC6H5O7Þ2ꢂ4ꢁ þ N2H4 þ 4OHꢁ
! 2Co # þ N2 " þ 4C6H5O73ꢁ þ 4H2O
It is well accepted that a slow reaction rate was favorable for
separating the growth step from the nucleation step, which is in
˚
1.895, 1.142 A which can be indexed to the (0 0 2), (1 0 1), and
(1 0 3) planes of the hexagonal Co respectively. The SEAD pattern
also confirms that the growth of the petals of the flowerlike Co
single-crystal was along the [0 0 1] directions, which correspond
to the (0 0 2) planes. It is in complete agreement with the XRD
result.
The flowerlike Co superstructure obtained by the presented
route has a micrometer and nanometer combined structure. It is
widely accepted that this hierarchical structure is formed by the
driving force of thermodynamics, because the supersaturated
solution is not stable [31–33]. Such necessary thermodynamic
nonequilibrium microenvironments can be easily achieved in
wet chemical processes if complexing agents or surfactants are
added into the reaction solutions. Therefore, it is supposed that
the sodium citrate added into our reaction system plays an
important role in the formation of the flowerlike Co micro-
crystals. To investigate the influence of sodium citrate on the
formation of flowerlike Co microcrystals, contrast experiments
were carried out at 120 8C for 8 h with different amount of
sodium citrate. The SEM image of the prepared products shows
that only irregular nanoparticle aggregations were formed in the
absence of sodium citrate (Fig. 4a). When the amount of sodium
citrate increased to 0.5 g, spherical particles and imperfect
flowerlike particles were formed (Fig. 4b). We speculate that
sodium citrate is mainly performing two tasks in this synthesis.
Firstly, sodium citrate is used as a complexing agent. Citrate ions
can coordinate with Co2+ ions to form stable complexing ions
([Co(C6H5O7)2]4ꢁ), which can decrease the concentration of free
Co2+ ions in solution. As a result, the concentration of Co2+ ions
is maintained at a stable level in the solution. After hydrazine
hydrate (N2H4ꢀH2O), a potent reducing agent in alkali solution
[34], is added into the solution, Co2+ ions are released slowly and
reduced in hydrothermal condition, which leads to a relatively
slow rate of generation of Co atoms. The chemical reaction can
Fig. 4. SEM images of the samples prepared at 120 8C for 8 h in the absence (a) and
0.5 g (b) of sodium citrate.

R.-H. Wang et al. / Materials Research Bulletin 44 (2009) 1468–1473
1471
favor of oriented growth [35]. Secondly, citrate ions can also serve
as a shape modifier. At elevated temperature, the equilibrium
constant for citrate ions binding to certain crystal faces of Co may
change, leading to selective loss of citrate ions on certain crystal
faces and allowing the particles to grow along one axis. Such a role
of citrate has been also reported in the synthesis of large arrays of
(NH3ꢀH2O) was used to tune the pH value of the solution instead of
NaOH, no precipitations was found after hydrothermal treatment
at 120 8C even if the reaction time was prolonged to 16 h. We have
varied NH3ꢀH2O solution with different concentration, such as 5%,
15%, and 25% (v/v). No obviously difference was observed. The
dissimilarity is likely related to the presence of NH4+ ions. It is well
+
oriented ZnO nanostructures [36] and
architectures [37].
b
-NiS 3D flowerlike
known that NH4 ions could coordinate with Co2+ ions to form
[Co(NH3)6]2+ ions, which are more stable than [Co(C6H5O7)2]4ꢁ
ions. Therefore, no Co could be formed at our reaction conditions.
Thus, the choice of alkali is also a key factor for the formation of
flowerlike Co structures.
To further investigate the morphological evolution process of
flowerlike Co microcrystals, a series of experiments were carried
out at different reaction durations (2, 4 and 8 h) while other
parameters remained unchanged. Fig. 6a is the overall SEM image
of products obtained at 120 8C for 2 h, which indicates only
We also find that the concentration of NaOH is crucial to the
formation of flowerlike Co. Contrast experiments have shown that
the flowerlike structures were destroyed and spherical particle
aggregations and short chains were formed, when the initial
concentration of NaOH increased to 2 M keeping other parameters
unchanged (Fig. 5a). When the concentration of NaOH was 3 M,
only spherical particles with diameter of 0.3–1.8
mm were
obtained (Fig. 5b), and no flowerlike structures were found. The
magnified image shows that the surface of the spherical particles is
quite rough (inset in Fig. 5b). Compared with those flowerlike
structures observed in Fig. 1, these differences in the morphologies
should be attributed to the reaction kinetics. It can be concluded
that the higher concentration of NaOH restricted the growth of the
flowerlike Co microcrystals. Chai et al. reported a similar result in
the synthesis of La0.5Ba0.5MnO3 products with different morphol-
ogies (flowerlike, microcube, and nanocube structures) by adjust-
ing the alkalinity of the reaction solution [38]. When ammonia
spherical particles with size of about 1
mm were obtained. The
magnified image shows that each sphere was formed by assembly
of smaller particles. When the hydrothermal time was prolonged
to 4 h, a few flowerlike particles (marked by arrows in Fig. 6c) were
found among the products. The petals of the flowerlike crystal are
shorter (Fig. 6d), indicating the growth of the crystal is still
incomplete. However, when the reaction time was further
prolonged to 8 h, well-defined flowerlike Co crystals with high
yield were obtained (Fig. 1).
In the light of the above experiment results, a possible growth
mechanism is proposed. The schematic illustration of the
formation mechanism of Co flowerlike structure is shown in
Fig. 7. At the early stage of reaction, many nanoparticles with
different sizes appeared in the solution (Fig. 7a). Because of the
interaction between citrate ions binding to nanoparticles by van
der Waals forces and intermolecular hydrogen bonds, Co
nanoparticles would be assembled together (Fig. 7b). With the
reaction proceeding continuously, assembled nanoparticles with a
relatively large size might serve as the crystal nuclei. Due to the
solvation of the Co surface, including hydrogen bonding, ion–
dipole, and dipole–dipole interactions in polar solvents, thermo-
dynamically active sites such as surface defects and active crystal
surfaces were generated on the early formed Co nuclei. These
active sites could then serve as heterogeneous nucleation sites.
Once nuclei were formed at these active sites, the preferential
orientation growth started. The preferential orientation of hcp Co
in our reaction system was along the [0 0 1] directions while citrate
ions were used as shape modifier. As a consequence, many
projecting petals could develop into a complex flowerlike 3D
assembly sharing the same core. When the reactants were
consumed due to particle growth, Ostwald ripening process
occurred, where the larger particles continued to grow, and the
smaller ones got smaller and finally dissolved (from Fig. 7b and c)
[39]. It was reported that the morphology and size of the products
depended on the competition between crystal nucleation and
crystal growth, and the pH value had an impact on both the rate of
crystal nucleation and the rate of crystal growth [38]. In our
reaction system, with a low concentration of NaOH, namely, a low
pH value, the crystal nucleation was slower than the crystal
growth, which led to the formation of flowerlike structures with
relatively larger size. While with a high concentration of NaOH,
namely, a high pH value, the crystal nucleation was faster than the
crystal growth. Therefore, the spherical structures with smaller
size were obtained.
The magnetic hysteresis measurements of the as-synthesized
Co structures were carried out at room temperature in the applied
magnetic field sweeping from ꢁ15 to 15 kOe. Here we have chosen
the flowerlike Co (corresponding to products shown in Fig. 1) and
the spherical Co (corresponding to products shown in Fig. 5b) as
examples to study the magnetic properties of Co microcrystals.
Fig. 5. SEM images of the samples prepared using different concentration of NaOH
solution: (a) 2 M, (b) 3 M. The inset in (b) is a magnified image of single spherical
particle.

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R.-H. Wang et al. / Materials Research Bulletin 44 (2009) 1468–1473
Fig. 6. SEM images of the samples prepared at 120 8C with different hydrothermal time of 2 h (a and b) and 4 h (c and d) respectively.
Fig. 7. Schematic illustration of the formation process of the flowerlike metallic Co microstructures: from (a) to (b) is the assembly process; from (b) to (d) is the typical
Ostwald ripening process. The insets show SEM images of samples collected after hydrothermal treatment for 2, 4 and 8 h, respectively. The scale bars in all the insets
represent 1
mm.
Both representative hysteresis loops of the as-synthesized flower-
like and spherical Co microcrystals are shown in Fig. 8, which
demonstrate typical ferromagnetic behaviour. The saturation
magnetization value (Ms) of the flowerlike Co is 128.1 emu/g,
and the coercivity value (Hc) is 232.5 Oe. Meanwhile, the Ms of the
spherical Co is 148.7 emu/g, and the Hc is 118.6 Oe. Both Ms of Co
samples are much lower than that of bulk Co (168 emu/g) [40],
which is attributed to the surface oxidation [40,41]. The flowerlike
Co microcrystals exhibit a distinct enhanced coercivity compared
with the value of spherical Co and bulk Co (a few tens of Oersteds at
room temperature [16]). It is well known that magnetic properties
of most materials are strongly affected by some type of anisotropy
including crystal anisotropy, shape anisotropy, stress anisotropy,
externally induced anisotropy, and exchange anisotropy [42]. The
two most common anisotropies in micro-/nanostructured materi-
als are crystalline and shape anisotropy. For flowerlike Co, its hcp
phase, determined in the XRD pattern, would show higher
coercivity owing to its relatively large crystal anisotropy constant.
Furthermore, shape anisotropy is predicted to produce the largest
coercive forces and even a small departure from sphericity in shape
would result in the significant increase of coercivity [43]. Based on
Fig. 8. Magnetic hysteresis loops for the synthesized samples: flowerlike Co
microcrystals and spherical Co microcrystals. The inset shows the low-field part of
the hysteresis loops.

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This work was supported by Shanghai Nanotechnology
Promotion Center (0852nm03200), the PhD Program Scholarship
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