Synthesis of Co submicrospheres self-assembled by Co nanosheets via a complexant-assisted hydrothermal approach

Article abstract of DOI:10.1016/j.jmmm.2009.08.028

Co submicrospheres with the typical diameter of ca. 500 nm, and possessing beautiful morphologies composed of dense Co nanosheets ca. 10 nm thick, were synthesized by a facile and low-cost complexant-assisted hydrothermal approach. Magnetic measurement at room temperature indicated the coercivity of the submicrospheres reached 268 Oe, which was much higher than that of bulk Co and of some microstructure cobalt materials reported previously. Hexagonal close-packed (hcp) and face-centered cubic (fcc) cobalt phases in the materials were identified by X-ray diffractometer (XRD). It was revealed that the addition of the complexant sodium tartrate played a crucial role in the formation of the hierarchical architectures of the Co submicrospheres. We believe that the high coercivity of the synthesized submicrospheres may result from their special nano-micro structure, and we suggest that this low-cost and facile synthesis approach can be used for large-scale production of Co magnetic materials with special structures and morphologies, as well as excellent magnetic properties.

Full text of DOI:10.1016/j.jmmm.2009.08.028

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Journal of Magnetism and Magnetic Materials 322 (2010) 30–35
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Synthesis of Co submicrospheres self-assembled by Co nanosheets via a
complexant-assisted hydrothermal approach
Ã
Hao Li a, Zhen Jin a, Huiyu Song b, Shijun Liao b,
a Department of Chemical Engineering, Huizhou University, Huizhou, Guangdong, 516007, People’s Republic of China
b College of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong, 510641, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Co submicrospheres with the typical diameter of ca. 500 nm, and possessing beautiful morphologies
composed of dense Co nanosheets ca. 10 nm thick, were synthesized by a facile and low-cost
complexant-assisted hydrothermal approach. Magnetic measurement at room temperature indicated
the coercivity of the submicrospheres reached 268 Oe, which was much higher than that of bulk Co
and of some microstructure cobalt materials reported previously. Hexagonal close-packed (hcp) and
face-centered cubic (fcc) cobalt phases in the materials were identified by X-ray diffractometer (XRD). It
was revealed that the addition of the complexant sodium tartrate played a crucial role in the formation
of the hierarchical architectures of the Co submicrospheres. We believe that the high coercivity of the
synthesized submicrospheres may result from their special nano-micro structure, and we suggest that
this low-cost and facile synthesis approach can be used for large-scale production of Co magnetic
materials with special structures and morphologies, as well as excellent magnetic properties.
& 2009 Elsevier B.V. All rights reserved.
Received 30 May 2009
Received in revised form
29 July 2009
Available online 27 August 2009
PACS:
75.50.Cc
75.75.+a
Keywords:
Cobalt submicrosphere
Cobalt nanosheet
Complexant
Hydrothermal approach
1. Introduction
through a solvothermal synthesis in polyol using Ru as the
heterogeneous nucleation agent and hexadecylamine as the
structure-directing agent [18]. Yang et al. prepared hexagonal Co
microspheres composed of nanoflakes via a two-step process, in
which cobalt bis (4-pyridine carboxylate) tetrahydrate was used
as precursor of Co [19]. Obviously, noble metals, a considerable
amount of surfactants, or some uncommonly used reagents are
indispensable in most of these approaches, which makes the
synthetic process less attractive because large-scale production of
these microstructures will be costly.
In this study, we report the synthesis of Co submicrospheres
with a typical diameter of ca. 500 nm, composed of Co nanosheets
ca. 10 nm thick. The method was facile and low-cost, employing
the complexant sodium tartrate (ST) to assist in the formation of
hierarchical architectures. The prepared Co submicrospheres were
characterized by X-ray diffractometry (XRD), scanning electron
microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS),
and superconducting quantum interference device (SQUID)
magnetometry. The effects of the complexant on the morphology
of the final products, and the role of ST in the synthesis process,
were also investigated.
Magnetic nanostructures represent a particularly interesting
class of materials for both fundamental scientific research
and potential technological applications [1–3]. In recent years,
controlling the morphologies and structures of magnetic nano-
materials has become the focus of intensive research, because the
electronic, optical, catalytic, and magnetic properties of magnetic
materials depend considerably on their size, shapes, and dimen-
sionality [4–7]. As some of the most important magnetic
materials, Co nanostructures have been of particular interest for
their high saturation magnetization and magnetic coercivity, as
well as their excellent catalytic properties [8–10]. In the past few
years, much attention in this area has been paid to assembling
low-dimensional Co nanostructures into three dimensional
(3D) complex architectures, as a possible way to improve the
magnetic anisotropy of the materials. To date, a number of 3D Co
hierarchical architectures composed of nanosized subunits have
been successfully fabricated by various methods [11–23].
Recently, Zhang et al. reported the synthesis of Co chains
self-assembled by Co submicrospheres with the assistance of
PVP in ethylene glycol solution [15]. Liu et al. demonstrated the
fabrication of hierarchical Co nanoflowers composed of nanorods,
2. Experimental section
Ã
All reagents were of analytic grade and double-distilled water
was used throughout the experiment. In a typical synthetic
Corresponding author. Tel./fax: +86 20 87113586.
E-mail address: chsjliao@scut.edu.cn (S. Liao).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.08.028

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31
process, 6 mmol CoCl2 ꢀ 6H2O and 12 mmol ST were dissolved in
3. Results and discussion
102 mL H2O, followed by dropping 30 mL NaOH solution (5 M)
and 16 mL hydrazine hydrate solution (50%) into the above
solution under intense stirring. The total volume of the solution
was ca. 150 mL and the final concentrations of CoCl2, ST, NaOH,
and hydrazine hydrate were 0.04, 0.08, 1, and 1 M, respectively.
The morphology and sizes of the samples were examined by
FE-SEM. Fig. 1a, shows a low-magnification SEM image of the
synthesized materials, in which the spherical particles can be
clearly observed. It is important and interesting that the spherical
particles are neither dense nor simply porous, but have a beautiful
morphology and plentiful hierarchical architectures, as shown in
Fig. 1b; the particles resemble peony flowers, and are composed of
compact Co nanosheets ca. 10 nm thick.
Afterwards, the solution was transferred into
a
Teflon-
lined stainless steel autoclave, which was kept in an air oven
at 393 K for 8 h. After the reaction, the solid precipitate in the
bottom of Teflon cup was collected and washed with double-
distilled water and acetone, then dried in a vacuum oven at 333 K
for 12 h.
The XRD pattern of the Co microspheres in Fig. 1c shows five
peaks at 2y ¼ 41.61, 44.31, 47.41, 51.51, and 75.91, suggesting the
coexistence of hexagonal close-packed (hcp) and face-centered
cubic (fcc) phases of Co in the samples [24]. The hcp and fcc
phases of Co are close in energy because they differ only in the
stacking sequence of the atomic planes in the cubic (111)
direction; mixtures of these two phases have often been observed
in samples prepared by wet-chemical methods [25,26]. The EDS
pattern in Fig. 1d indicates that the sample is essentially pure Co.
A small amount of oxygen (1.2 wt%) was also detected, which may
X-ray diffraction patterns were recorded using a Shimadzu
˚
XD-3A X-ray diffractometer with Cu K
a radiation (l ¼ 1.5406 A).
The morphology and elements of the sample were studied
using a Hitachi 4800 field emission scanning electron microscope
(FE-SEM) equipped with X-ray energy-dispersive spectroscopy.
The M/H hysteresis loop was recorded with
a Quantum
Design MPMS XL-7 superconducting quantum interference device
magnetometer.
100nm
200nm
10µm
Co
Co
Co
O
30
40
50
60
70
80
0
3
6
9
12
15
Energy (keV)
2theta (degree)
Fig. 1. SEM images (a), (b), XRD and EDS patterns (c), (d) of the Co sample.

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H. Li et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 30–35
have resulted from partial oxidization and/or complexant residue
on the sample’s surface.
crystal structures of the Co samples were highly related to the
solvents used in the synthetic process, and the Co products tended
to crystallize as mixed hcp and fcc phases in water solution [27].
This implied that the reaction temperature had little influence on
the crystal structures, in spite of its great impact on the
morphology of the samples.
In a chemical reduction route, reduction temperature has a
direct influence on the formation of crystal nuclei and the crystal
growth rate of nano/micromaterials, thus also affecting the
morphology and crystal structures of the products. In our study,
when the reaction temperature was decreased to 363 K, the
Complexants have been widely utilized to modify the mor-
phology of metallic nanomaterials prepared by chemical reduc-
tion of the corresponding metallic salts [7,12,26,28]. In the present
study, when no complexant was used in the synthesis, the
products were some dendrite-like Co microcrystals (Fig. 3a).
When sodium citrate was substituted for ST as the complexant,
the as-obtained Co products were similar in morphology to those
obtained in the absence of the complexant, implying that sodium
citrate had little effect on the morphology of the products. It
should be mentioned that Li et al. and Xie et al. recently reported
the aqueous phase synthesis of Co microspheres and Co
nanowires with the assistance of citrate acid or citrate sodium
[20,28]. But obviously, the reaction conditions in the present case
were quite different. EDTA is another commonly used complexant
that could be employed to control the morphology of the Co
nanomaterials [7]. But we found that with EDTA as the
complexant, only Co aggregates with irregular morphologies
product was Co microspheres with a typical diameter of ca. 8 mm
(Fig. 2a). Interestingly, numerous flying-saucer-like Co
microcrystals were found embedded in the surface of these
microspheres. We found the reduction reaction could even
progress at room temperature (ca. 300 K), but only some
micron-sized Co spheres with relatively smooth surfaces were
obtained (Fig. 2b). Increasing the temperature to 423 K resulted in
the formation of Co submicrospheres structured by dense Co
nanosheets, similar to those obtained at 393 K but not so beautiful
or well architectured as the latter. It seems that a relatively high
temperature is advantageous in the formation of Co
submicrospheres composed of Co nanosheets, with the optimum
temperature being ca. 393 K.
The crystal structures of the samples (as shown in Fig. 2d)
obtained at different reaction temperatures were found to be
almost the same. In our previous work, it was observed that the
100nm
T= 423 K
T= 393 K
T= 363 K
T= 300 K
30
40
50
60
70
80
2theta (degree)
Fig. 2. SEM images (a)–(c) and the crystal structures (d) of Co sample prepared at different reaction temperatures: (a) T ¼ 300 K (b) T ¼ 363 K (c) T ¼ 423 K.

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300nm
300nm
600nm
Fig. 3. SEM images of the samples prepared without complexing agent (a), and using sodium citrate (b), and EDTA (c) as complexing agents, respectively.
200nm
200nm
Fig. 4. SEM images of Co samples prepared at different molar ratios of ST to CoCl2: (a) 1:1, (b) 3:1.
were obtained (Fig. 3c). Based on these observations, it is clear
that ST is crucial for the formation of the Co submicrospheres
composed of Co nanosheets in our study. Moreover, we found the
amount of ST in the synthesis also played an important role in
determining the final morphology of the products. When the
molar ratio of ST to CoCl2 was decreased from 2:1 to 1:1, Co
aggregates with irregular morphologies as well as Co
nanoplatelets were obtained (Fig. 4a). When the molar ratio was
increased to 3:1, Co submicrospheres composed of Co nanosheets
could again be achieved (Fig. 4b). This suggests that a relatively
high concentration of ST is favorable for the production of Co
submicrospheres.
To further understand the role of ST in the synthesis, we
carried out XRD analysis of a sample obtained at an early stage of
the reaction process (the hydrothermal reaction proceeded for 2 h
in total). The XRD result in Fig. 5a indicates that metallic Co
coexisted with Co(OH)2 in the sample. We also designed another
experiment, in which no N2H4 was introduced into the synthesis
while other experimental conditions were kept unchanged. All the
diffraction peaks in the XRD pattern (Fig. 5b) of the corresponding

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H. Li et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 30–35
product can be assigned to Co(OH)2, and the typical SEM images
(Fig. 6) indicate that these Co(OH)2 products are numerous
nanosheets ca. 20 nm thick and 50–200 nm in diameter.
prepared Co(OH)2 nanosheets (Fig. 6) were used instead of CoCl2
as the reagent in the absence of ST. We found that no Co
submicrospheres were produced without the assistance of ST;
instead, the products were dendritic Co microcrystals (Fig. 7),
quite similar in morphology to the sample obtained with CoCl2 as
the reagent in the absence of ST (Fig. 3a). Based on our
experimental results, it is clear that ST played a key role in the
second step, i.e., sustaining the nanosheet architecture and
assisting in the formation of Co nanosheets, which would self-
assemble into submicrospheres to lower the energy of the system,
due to the high surface energy of the nanosheets and the strong
magnetic interaction.
To determine the magnetic properties of the as-prepared Co
submicrospheres, magnetic measurements were carried out at
room temperature (300 K). The results are shown in Fig. 8, which
demonstrates that the sample exhibits typical ferromagnetic
properties at room temperature. The saturation magnetization
(Ms) and remnant magnetization (Mr) of the sample are 157.2 and
11.0 emu/g, respectively. Ms is lower than that of bulk Co
(168 emu/g), which may be attributed to the existence of oxides
and/or organic residue on the surface of the sample [30]. As
shown in Fig. 8b, the coercivity of the as-prepared Co
microspheres composed of Co nanosheets at room temperature
can reach 268 Oe, which is much higher than that of bulk Co (ca.
tens of oersteds at room temperature [31]). The coercivity is also
higher than those of recently reported Co chains self-assembled
by Co submicrospheres (90 Oe, 295 K [15]), and Co flowerlike
architectures (197 Oe, 295 K [16]). It is widely acknowledged that
the magnetization behavior of magnetic materials is closely
associated with their shape and crystalline anisotropy [32]. The
enhanced coercivity of the as-synthesized Co submicrospheres
can be attributed to the high shape anisotropy of the Co
nanosheets in the submicrospheres. However, this was still
much lower than that of the flowerlike Co microcrystals
composed of Co nanoplatelets reported in our early work
(407.3 Oe, 300 K [7]), which may be due to the relatively small
aspect ratio and low crystallinity of Co nanosheets in the as-
prepared Co submicrospheres.
The solubility product constant (Ksp) of Co(OH)2 is 10ꢁ14.23
,
while the complex stability constant of Co2+–tartaric acid is only
102.1 [29]. So, in a strong basic solution the Co2+–ST complex is not
stable from a thermodynamics viewpoint, and the ST ligands will
be replaced by OHꢁ. Considering the XRD results in Fig. 5, it is
reasonable to conclude that the actual precursor of Co was
Co(OH)2 rather than Co2+–ST complex. Recently, Zhang et al.
proposed a mechanism to explain the formation of the flowerlike
Co architecture [16], which included three steps: (1) Co(OH)2
platelets were formed as an intermediate product; (2) Co products
sustained the platelet structure of Co(OH)2 in the presence of the
surfactant during chemical reduction; and (3) Co nanosheets
diffused and assembled together as a result of strong magnetic
interaction and of the drive of entropy. According to this
mechanism, the formation of Co nanosheets is, in the present
study, the key step in the production of Co submicrospheres. To
clarify this, an additional experiment was done in which the as-
Co(OH)2
Co
(b)
(a)
30
40
50
60
70
80
2 theta (degree)
4. Conclusion
Fig. 5. XRD patterns of the samples obtained at the reaction time of 2 h (a) and
obtained in the absence of reducing agent (b).
In summary, Co submicrospheres self-assembled by dense Co
nanosheets with a thickness of ca. 10 nm were synthesized via a
50nm
200nm
Fig. 6. Typical SEM images of the Co sample obtained in the absence of reducing
Fig. 7. SEM images of the Co sample prepared by using Co(OH)2 nanosheets as
agent.
reagent in the absence of ST.

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200
150
100
50
30
20
10
0
0
-50
-10
-20
-30
-100
-150
-200
-10000
-5000
0
5000
10000
-400
-200
0
200
400
H (Oe)
H (Oe)
Fig. 8. Room-temperature magnetic hysteresis loops of the Co submicrospheres (a) and magnified magnetic hysteresis loops at the low field (b).
complexant-assisted hydrothermal approach, in which sodium
tartrate was used to assist the formation of a hierarchical
architecture. Magnetic measurement at room temperature in-
dicated the coercivity of the as-prepared submicrospheres
reached 268 Oe, which was much higher than the coercivity of
either bulk Co or similar microstructures reported previously. It
was revealed that complexant, sodium tartrate, was crucial for the
formation of the Co submicrospheres. The facile complexant-
assisted solution route presented here used only low-cost
inorganic reagents, making the method very suitable for large-
scale production of Co submicrospheres with hierarchical nanos-
tructures.
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Acknowledgements
The authors would like to thank Ms. J.Y. Liao for her help in the
SEM experiments. This work was supported by the National
Natural Science Foundation of China (Project nos. 20673040,
20476034).
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