Solvothermal synthesis and characterization of cobalt microcrystals with hierarchical radial structure

Article abstract of DOI:10.1016/j.jallcom.2011.10.030

Three-dimensional (3D) hierarchical cobalt microcrystals composed of well-aligned dendrites radiating from the shaft axis were prepared using cobalt chloride hexahydrate as starting materials via a facile hydrothermal route. The structures and morphologie

Full text of DOI:10.1016/j.jallcom.2011.10.030

Journal of Alloys and Compounds 513 (2012) 245–250
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Solvothermal synthesis and characterization of cobalt microcrystals with
hierarchical radial structure
Mingzai Wua,c,∗ , Zhanwen Panga , Xiansong Liua , Guang Lia , Yongqing Maa , Zhaoqi Suna , Lide Zhangb ,
Xiaoshuang Chenc
a Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials, Anhui University, Hefei 230039, PR China
b Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, Anhui, PR China
c The Shanghai Institute of Technical Physics of the Chinese Academy of Sciences, Shang 200083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2011
Received in revised form 8 October 2011
Accepted 11 October 2011
Available online 30 October 2011
Three-dimensional (3D) hierarchical cobalt microcrystals composed of well-aligned dendrites radiating
from the shaft axis were prepared using cobalt chloride hexahydrate as starting materials via a facile
hydrothermal route. The structures and morphologies of products were investigated by X-ray diffrac-
tometer (XRD), field emission scanning electron microscopy (FESEM) and high resolution transmission
electron microscopy (HRTEM) measurements. It was found that the dosages of ethylenediamine, sodium
hydrate played an important role in the morphology control of the final products. A unique growth
mechanism for the radial cobalt microcrystal was tentatively discussed, which offers a fresh and possible
solution to the design and fabrication of materials with hierarchical structure. Additionally, Magnetiza-
tion measurements revealed that the products exhibited ferromagnetic characteristics with a saturation
magnetization of 151.7 emu/g and a coercivity of 355.8 Oe at room temperature. The result shows a
potential application in ferrofluids and other related magnetic application.
Keywords:
Microcrystal
Synthesis
Morphologies
Magnetic properties
Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction
in the controllable synthesis of hierarchical structures, the struc-
tural design and tunable fabrication of a broad range of well-defined
Recently, there has been growing interest in the archi-
tectural control of microstructures with specific dimensions
and well-defined shapes because of their importance in the
morphology-dependent properties and the capability of construct-
ing next generation devices by the “bottom-up” approach [1–3].
The synthesis of precise-shape control and spatial pattern of a
number of novel micro/nanostructures with various superstruc-
tures has been reported [4–7]. For example, dendrites of FeNi3
were reported via a solvothermal synthesis in the absence of
surfactants and the growth mechanism was discussed based on
the diffusion-limited aggregation (DLA) model [8]. Dendrites of
nickel microstructures were synthesized via a solution route. Mag-
netic field-induced aggregation and limited diffusion were used
to explain the formation of dendrite-like nickel [9]. Moreover,
sisal-like cobalt superstructures with novel turreted branches were
fabricated through a solvothermal system and Ostwald ripening
mechanism was believed to be responsible for the formation of
sisal-like cobalt superstructures [10]. Despite the achieved progress
and controllable morphologies is still desired.
Meanwhile, much attention has been paid to magnetic nanopar-
ticles due to their novel physical and chemical properties [11–13].
Among magnetic metal nanoparticles, cobalt particles possess
more outstanding magnetic properties and have potential appli-
cations in many areas including ferrofluids, biomedical systems,
catalysts, optical and mechanic devices, high-density magnetic
recording media [14–23]. The synthesis of cobalt particles with var-
ious morphologies has been reported including nanowires [24,25],
nanorods [26], epitaxial film [27], platelets [28], rings [29], flow-
ers [30] and chain-like structures [31,32]. For example, it has
been reported recently that magnetic cobalt nanoparticles can self-
assemble into rings through dipolar interactions, when dispersed
under appropriate conditions, and show bistable flux closure states
at ambient temperature [29]. Sphere-like and flower-like cobalt
nanostructures with sharp petals were successfully synthesized
via a facile liquid-phase reduction method [30]; while chainlike
cobalt nanostructures composed of submicro spheres could be
obtained with the aid of polyvinyl pyrrolidone (PVP) in ethylene
glycol solution [32]. However, up to now, reports on the synthesis
of hierarchical radial cobalt superstructures through an aqueous
reduction strategy under mild conditions is still lacked.
∗
Corresponding author at: Anhui Key Laboratory of Information Materials and
Devices, School of Physics and Materials, Anhui University, Hefei 230039, PR China.
Tel.: +86 551 5107237; fax: +86 551 5107237.
In this paper, we present the synthesis of 3D radiated cobalt
microcrystals with dendrites as the assembled unit via a facile
E-mail address: mingzaiwu@gmail.com (M. Wu).
0925-8388/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.10.030

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M. Wu et al. / Journal of Alloys and Compounds 513 (2012) 245–250
view of the sample. Interestingly, these dendrites serve as assem-
bling units and grow into a quasi-flower microstructure from the
same centre in a hexagonal symmetrical way. The microcrystal size
is so big that HRTEM image can only be carried on a single den-
drite, as shown in Fig. 2b. Fig. 2c–e shows the HRTEM lattice fringes
taken from the areas labeled with 1–3 in Fig. 2b, respectively. The
perfect fringes reveal the single crystal feature of the dendrite struc-
tures. Furthermore, the planar space of lattice fringes is evaluated
to be 0.20 nm, and 0.22 nm (as shown in Fig. 2c–e), consistent with
the (0 0 2) and (1 0 0) planes of hcp-Co. The selected-area electron
diffraction (SAED) pattern shown in Fig. 2f corresponding to the
area labeled with 2 further confirms the dendrite to be single crys-
talline, and the diffraction spots are indexed as (0 1 0) and (1 0 0)
planes. Based on the planar space of lattice fringes and the indexed
diffraction spots, the growth directions of the main branch and
leaves of the dendrite were supposed to be along [0 0 1] direction,
which agrees well with the XRD result.
200 °C
180 °C
40
50
60
70
80
2θ/(degree)
It was reported that the morphology of the products was depen-
dent on the competition between crystal nucleation and crystal
growth, and the pH value was supposed to exert an impact on both
the rates of crystal nucleation and crystal growth [8,33]. Here, we
show our SEM images of the samples obtained at various dosages of
NaOH in Fig. 3. When the dosage of NaOH is 0.6 g, incomplete leaf-
like structures dominate and no radial structures are observed, as
shown in Fig. 3a. With the increase of NaOH dosage to 1.0 g, the
morphology of the product evolves into a perfect radial pattern,
composed of some dendrite structures radiating from the same
centre as shown in Fig. 3b. The venations of these dendrite crys-
tals could be clearly distinguished. Interestingly, when the dosage
of NaOH is increased to 2.4 g, the venations of dendrite crystals dis-
appear and the sheet-like structures with saw-like edges radiate
from the same centre, as shown in Fig. 3c.
It has been reported that the dosage of ethylenediamine is able
to control the reaction rate and directly related to the formation
of crystals [34]. Here, the controlled experiments are carried out
in order to investigate the relationship of the dosage of ethylene-
diamine and the morphologies of crystals. Fig. 4 shows the SEM
images of the as-obtained cobalt crystals synthesized with differ-
ent dosages of ethylenediamine at 200 ◦C for 2 h. In the absence of
ethylenediamine, the as-obtained samples are composed of saw-
like flakes with the main backbone of dendrite, as shown in Fig. 4a.
When the dosage of the ethylenediamine was 1 ml, flower-like
microstructures were obtained, with leaf–like dendrites radiating
from the same centre, as shown in Fig. 4b. When the ethylene-
diamine dosage was further increased to 10 ml, the flower-like
products grew very heavy with many dendrite-like leafs radiat-
ing from the same centre. In fact, ethylenediamine played two
important roles in the growth process. One is the coordination
agent. It coordinates with Co2+ and form the stable complexing
ions [Co(NH2CH2CH2NH2)2]2+, which reduces the concentration of
free Co2+ ions in solution. As a result, the reduction of Co ions can
be limited to a relatively slow rate, which favors the anisotropic
growth of crystals. The other is surfactant [12]. Ethylenediamine
molecules would be selectively adsorbed on certain crystal faces
with lower surface energy. With the increase of ethylenediamine
dosage, the equilibrium constant for the adsorption of ethylenedi-
amine molecules on some crystal faces of cobalt may be changed,
resulting in the changes of the growth rate and the morphologies
of the final products.
Fig. 1. XRD patterns of the prepared samples for the reaction temperatures of 180 ◦
C
and 200 ◦C for 4 h with CoCl2·6H2O (2 mmol), NaOH (3 mmol), 1.0 ml of ethylenedi-
amine and 1.0 ml of hydrazine.
hydrothermal process. The influences of temperature, dosage of
sodium hydroxide and ethylenediamine on the morphology of the
products have been investigated. Based on the experiment results,
a fresh and new formation mechanism is proposed. Furthermore,
the magnetic behavior of the cobalt microcrystal is also studied.
2. Experimental
All the reagents are analytical grade and used as received. In a typical procedure,
0.476 g (2 mmol) of CoCl2·6H2O and 1.2 g of NaOH (3 mmol) were first dissolved in
30 ml of distilled water under magnetic stirring. After intensive stirring of 10 min,
1.0 ml of ethylenediamine and 1.0 ml of hydrazine were dropwise added into the
above mixed solution, followed by the continual vigorous stirring for 15 min. The
mixed solution was then transferred to a 50 ml Teflon-lined autoclave, which was
kept at 200 ◦C for 4 h, and cooled to room temperature naturally. The resulted black
product was collected by centrifugation, washed several times with ethanol and
distilled water, and then dried at 60 ◦C for 12 h. Contrast experiments were carried
out by adjusting the dosage of NaOH (0.6–2.4 g) and ethylenediamine (0–10 ml) with
other reaction parameters unchanged.
The microstructures and morphologies of these samples were characterized
by X-ray powder diffraction (XRD) using an 18 kW advanced X-ray diffractome-
˚
ter with Cu K␣ radiation (ꢀ = 1.54056 A), scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). Magnetic hysteretic loop was measured
on a vibrating sample magnetometer (VSM, BHV-55) at room temperature.
3. Results and discussion
The XRD patterns of the as-obtained cobalt crystals synthesized
at different temperature for 4 h with CoCl2·6H2O (2 mmol), NaOH
(3 mmol), 1.0 ml of ethylenediamine and 1.0 ml of hydrazine are
shown in Fig. 1. All the diffraction peaks can be indexed to hcp-
Co, consistent with the standard JCPDS card No. 05-0727. With
the increase of the reaction temperature, the intensities of all the
diffraction peaks are enhanced, indicating that the higher tempera-
ture favors the crystallization of cobalt phase. At 200 ◦C, the relative
intensity ratios of the peak of (0 0 2) to that of (1 0 0), and (1 0 1)
are much higher than those at 180 ◦C, suggesting the preferential
growth of (0 0 1) planes. However, it is found that the reaction time
and the dosages of ethylenediamine, sodium exerted less influence
on the products’ crystallization in terms of XRD pattern character-
ization, which were not shown here.
Fig. 2 shows the typical SEM and TEM images for the prod-
ucts synthesized at 200 ◦C for 4 h with CoCl2·6H2O (2 mmol), NaOH
(3 mmol), 1.0 ml of ethylenediamine and 1.0 ml of hydrazine. Due
to the products’ giant size (about 15 ␮m in size), Fig. 2a shows only
a SEM image of an overall radial microstructure composed of many
dendrites, radiating from the centre with a length of 6 ␮m and a
width of about 3 ␮m. The inset in Fig. 2a shows the enlarged top
The morphological changes induced by the alkalinity have been
reported by many groups. For example, La0.5Ba0.5MnO3 products
with different morphologies (flowerlike, microcube, and nanocube
structures) have been synthesized via the adjustment of reac-
tion solution alkalinity [33]. The formation of single crystalline
FeNi3 dendrites has a great relationship with the concentration
of NaOH of the solution [8]. In fact, the morphological changes

M. Wu et al. / Journal of Alloys and Compounds 513 (2012) 245–250
247
Fig. 2. (a) SEM images of the as-prepared products; (b) TEM image of a single dendrite structure. (c–e) High-resolution TEM images recorded in different part labeled by the
circle in (b). (f) Electron diffraction image of the as-prepared products. (The dosage of NaOH was 1.2 g and the ethylenediamine was 1 ml.)
Fig. 3. SEM images of the sample prepared at 200 ◦C with CoCl2·6H2O (2 mmol), 1.0 ml of ethylenediamine and 1.0 ml of hydrazine and different dosage of sodium hydroxide:
(a) 0.6 g; (b) 1.0 g; (c) 2.4 g. (The reaction time was 4 h.)

248
M. Wu et al. / Journal of Alloys and Compounds 513 (2012) 245–250
Fig. 4. SEM images of the sample prepared at 200 ◦C with CoCl2·6H2O (2 mmol), NaOH (3 mmol) and 1.0 ml of hydrazine and different dosage of ethylenediamine: (a) 0 ml;
(b) 1 ml; (c) 10 ml. (The reaction time was 2 h.)
induced by the dosage of NaOH and ethylenediamine could be
explained based on the fact that the dosage of NaOH affects the reac-
tion kinetics through tuning the dissolution–deposition equation
of [Co(NH2CH2CH2NH2)2]2+. According to Peng’s theory, the high
chemical potential generated by a high monomer concentration in
the growth solution favors the growth along a certain direction [35].
In our system, the following reaction occurs
diffusion limitation of cobalt nuclei. Finally, the formation of plate-
like cobalt structures is caused [8,33,35].
In order to further understand the growth evolution of the
cobalt microcrystals, the effect of the reaction time (2 h, 4 h and
8 h) is investigated. Interestingly, a few snow-like structures can be
obtained at these three reaction times. Fig. 5a shows a representa-
tive snow-like microstructure with a perfect hexangular symmetry
feature obtained at 200 ◦C for 2 h, as shown in Fig. 5b. The short reac-
tion time implies that the assembling process is very quick. The
formation of snow-like structure is supposed to be an inevitable
intermediate stage in our system. Based on the above SEM results,
a possible mechanism for the growth of radial structure is proposed
as shown schematically in Fig. 6.
Co2+ + 2NH2CH2CH2NH2 → [Co(NH2CH2CH2NH2)2]2+
2[Co(NH2CH2CH2NH2)2]2+ + mN2H4 + 4OH−
→ 2Co + N2↑ + (m − 1)N2H4 + 4H2O
Based on nucleation thermodynamics and reaction balance con-
stant, particle formation depends on the ratio between nucleation
and nuclei growth [37]. At the initial step of the reaction, a few small
cobalt nucleuses are formed due to the hydrate hydrazine reduc-
tion. These nucleuses attract some neighboring newly reduced
cobalt particles and form the trunk of cobalt microstructures
(served as magnetic body). On the trunk surfaces, smaller par-
ticles are further attracted, begin to grow and finally emerge at
60 angles with respect to the trunk. With the proceeding of the
reaction, the dendrite-like microstructures are formed. Under the
synergistic effects of interparticles’ magnetostatic interaction and
diffusion-limited aggregation (DLA) induced by the NaOH, some
early formed cobalt nuclei tend to be adsorbed on heterogeneous
nucleation sites to reduce both the magnetic anisotropic energy and
the surface energy and grow into the basic framework of hexangu-
lar snowflake microstructures with dendrite-like leafs, similar to
the reports on the formation of dendrite Ag structures [38–40]. The
Firstly, complexes [Co(NH2CH2CH2NH2)2]2+ are formed, and
governed by the cationic/anionic mixture reaction system. Then,
these complex ions are reduced to cobalt nuclei under alkaline
conditions. The higher the concentration of OH− is, the larger the
number of cobalt crystal nucleus in the solution is. When the con-
centration of OH− is high enough, the produced cobalt nucleus can
be excessive, and will contribute to the disappearance of the vena-
tions of dendrite crystals to an extent, as shown in Fig. 3c. The
viscosity enhancement of the solution should be the main reason
for the formation of plate-like cobalt microstructure, as described
by the Goddard–Miller equation [36]
ꢁ = ꢁ0 exp (2.5˚)
where ꢁ0 is the viscosity of water, and ˚ is the sodium hydroxide
concentration. The increase of the sodium hydroxide concentra-
tion leads to the exponential increase of solution viscosity and the

M. Wu et al. / Journal of Alloys and Compounds 513 (2012) 245–250
249
Fig. 5. (a) SEM images of snow-like structure obtained at 200 ◦C for 2 h with CoCl2·6H2O (2 mmol), NaOH (3 mmol), 1.0 ml of ethylenediamine and 1.0 ml of hydrazine. (b)
Crystal shafts of hexagonal system.
Fig. 6. Schematic illustration of the possible formation process of the products.
growth velocity of c axis [0 0 0 1], is much larger than that in the
ab plane. Thus, the continual growth can be induced along the c
axis. It is the fact that the size of the ripened radial structure is big-
ger than that of the snowflake one, which supports the conclusion
that the radial structures grow from the snowflake ones. With the
consumption of the reactants and ethylenediamine, the equilib-
rium constant of ethylenediamine molecules binding with certain
crystal faces of cobalt, may be changed, leading to the changes of
the growth rate of cobalt crystal faces. Ostwald ripening process
can occur. It is proposed that the formation of trunk part and the
growth of the microstructures are a directed assembled process
followed by the Ostwald ripening, different from the reported tra-
ditional mechanism in Ref. [10]. Fig. 7 shows the detailed evolution
process. The novel growth mechanism offers a unique and possible
solution to the design and preparation of materials with advanced
functionalities. More details about the growth mechanism are still
underway.
The magnetic properties are investigated by the magnetization
Fig. 7. Schematic illustration of the possible formation process of the trunk struc-
ture.
dependence of applied fields at 300 K, as shown in Fig. 8. The
sample synthesized under the conditions of 200 ◦C for 4 h, 0.476 g
of CoCl2·6H2O, 1.2 g of NaOH, 1.0 ml of ethylenediamine, 1.0 ml
of hydrazine is found to possess the hysteretic behavior. The
result reveals that the Co microstructures are ferromagnetic.
The inset is the magnetized hysteretic loops at low applied
field. The values of saturated magnetization (Ms), remnant
magnetization (Mr), and coercivity (Hc) are 151.7 emu/g,
22.42 emu/g, and 355.8 Oe, respectively. The saturation mag-
netization (Ms) for the sample is much close to the corresponding
values for the bulk cobalt (Ms = 168 emu/g). The much higher

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M. Wu et al. / Journal of Alloys and Compounds 513 (2012) 245–250
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Acknowledgements
This work was financed by the 211 project of Anhui Univer-
sity, National Natural Science Foundation of China (50901074,
60976092, 11174002, 50672001), Anhui Provincial Natural Science
Fund (11040606M49), Higher Educational Natural Science Foun-
dation of Anhui Province (KJ2010A012) and Innovation Project of
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