Preparation of large Co nanosheets with enhanced coercivity by a magnetic-field-assisted solvothermal approach free of surfactants, complexants or templates

Article abstract of DOI:10.1016/j.jmmm.2009.03.048

We presented a magnetic-field-assisted solvothermal route to prepare hexagonal close-packed (hcp) large Co nanosheets free of any surfactants, complexants or templates. Scanning electron microscope (SEM) images showed that the typical size and thickness o

Full text of DOI:10.1016/j.jmmm.2009.03.048

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Journal of Magnetism and Magnetic Materials 321 (2009) 2566–2570
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Preparation of large Co nanosheets with enhanced coercivity by a
magnetic-field-assisted solvothermal approach free of surfactants,
complexants or templates
Ã
Hao Li, Shijun Liao
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:
We presented a magnetic-field-assisted solvothermal route to prepare hexagonal close-packed (hcp)
large Co nanosheets free of any surfactants, complexants or templates. Scanning electron microscope
(SEM) images showed that the typical size and thickness of the Co nanosheets were several microns and
ca. 50 nm, respectively. Magnetic measurement indicated the coercivity of the sample reached 317.3 Oe,
which was much higher than those of bulk Co metal and previously reported Co nanoplatelets. It was
found that the solvents played a crucial role in determining the morphology and the crystal structures
of the Co products, and the introduction of ethanol conduced to the formation of pure hcp phase Co
microcrystal. The external magnetic field was also found to be indispensable for the formation of sheet-
shaped morphology.
Received 15 November 2008
Received in revised form
17 February 2009
Available online 25 March 2009
PACS:
75.50.ꢀy
75.75.+a
Keywords:
& 2009 Elsevier B.V. All rights reserved.
Cobalt
Nanosheet
Solvothermal approach
1. Introduction
have been synthesized by wet chemical methods, such as Co
nanorods [7], Co nanowires [8], Co nanobelts [9], Co nanoplatelets
[10], Co nanobowls [11], Co nanotubes [12] and Co nano-
flowers[13,14]. Obviously, the synthesis of most of these micro-
structures is highly related to the surfactants, complexants or
templates, which play a key role in controlling the morphology of
the products. However, the post-treatment of the templates is
complex and the presence of organic molecules on the surface of
the magnetic nanomaterials has detrimental effect on the
magnetic properties, which may result in difficulties for their
potential application as nanodevices [6,14,15]. Despite significant
progress in the shape-controlled synthesis of metallic micro-
structures, it is still a challenge to synthesize shape-anisotropic Co
nanomaterials without the assistance of surfactants, complexants
or templates. Therefore, it is highly desirable to develop new
methods to prepare the Co nanomaterials with controlled
morphology without using the above-mentioned additives.
On the other hand, several groups have reported that the
existence of a magnetic field could significantly influence the
movement of magnetic nuclei/nanoparticles in solution during
the reaction, which could grow and assemble into one-dimen-
sional nano- or micron-sized wires/rods under an external
magnetic field [16–18]. So far, there has been no report on
the synthesis of multi-dimensional magnetic microcrystals
under external magnetic fields yet. It is still an open question
whether these magnetic nuclei or nanoparticles would inevitably
Cobalt nanomaterials have received much attention owing to
their size-dependent magnetic, electronic and catalytic properties,
which are of interest for the fundamental scientific research and
potential technological applications in, for example, high-density
information storage, magnetic sensors, etc [1–3]. Recently,
controlling the morphology of magnetic nanomaterials has
become a research hotspot in view of the fact that the electronic,
optical, catalytic and magnetic properties of the magnetic metals
depend considerably on their shapes and the textures. It has been
demonstrated that many properties of the shape-anisotropic
magnetic materials with unusual and novel forms significantly
differ from those of the spherical counterparts. For instance, the
plate-like magnetic particles have a higher permeability and a
good microwave absorption property in microwave frequency
range owing to their large shape anisotropy [4]. Compared with
spherical Co nanoparticles, the Co nanomaterials with special
shapes often show enhanced coercivity [5,6], which is highly
desirable for some applications, such as high-density magnetic
recording media and permanent magnets. Up to now, a number of
differently shaped Co nanocrystals and their assembly structures
Ã
Corresponding author. Tel.: +86 20 87112977; fax: +86 20 87112906.
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.2009.03.048

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H. Li, S. Liao / Journal of Magnetism and Magnetic Materials 321 (2009) 2566–2570
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self-assemble into one-dimensional wires or rods under an
external magnetic field and whether some multi-dimensional
magnetic microstructures could be obtained under an external
magnetic field.
After the reaction, the solid was collected and washed with
double-distilled water and acetone, and then dried in a vacuum
oven at 60 1C for 12 h.
X-ray diffraction (XRD) patterns were recorded using
a
In this work, micron-sized cobalt sheets with thickness of ca.
50 nm were synthesized successfully with the assistance of an
external magnetic field by solvothermal method, in which no
surfactants, complexants or templates were used. Additionally,
our experiment results demonstrated that besides one-dimen-
sional Co wires/rods, multi-dimensional Co microcrystals with
special morphology could also be obtained under an external
magnetic field.
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 LEO 1530 VP field emission scanning electron
microscope (FE-SEM) with an X-ray energy-dispersive spectro-
scopy (EDS). The M/H hysteresis loop was recorded with a
Quantum Design MPMS XL-7 superconducting quantum inter-
ference device magnetometer (SQUID).
3. Results and discussion
2. Experimental section
The XRD pattern of Co sample (Fig. 1a) showed five peaks at
2
y ¼ 41.41, 44.21, 47.21, 62.31 and 75.61, which were assigned to
All reagents were of analytic grade and double-distilled water
was used throughout the experiment. In a typical synthetic
procedure, 6 mmol CoCl2 ꢁ 6H2O was dissolved in 48 mL H2O,
followed by dropping 10 mL NaOH solution (15 M) to the CoCl2
solution under intense stirring. Then, 16 mL hydrazine hydrate
solution (50%) and 75 mL ethanol were dropped in sequence to the
above suspension. The volume ratio of water to ethanol in the
resulting suspension with total volume of ca. 150 mL was 1:1, and
the final concentrations of CoCl2, NaOH and hydrazine hydrate
were 0.04, 1 and 1 M, respectively. Afterwards, the suspension was
transferred into a Teflon-lined stainless-steel autoclave that was
placed in a magnetic field with the magnetic field strength of 0.1 T
at the center of the Teflon vessel. The magnetic field was provided
by two pieces of magnets, one was put under the bottom of the
autoclave and the other was put on the top of the autoclave.
Finally, the autoclave, together with the two magnets, was kept in
an air oven at 160 1C for 10 h. The reaction we employed could be
formulated as follows:
the diffractions from the (10 0), (0 0 2), (101), (10 2) and (110)
planes of hexagonal close-packed (hcp) Co (space group: P63/
˚
˚
mmc1(19 4); JCPDS: 05–0727, a ¼ 2.503 A, c ¼ 4.060 A),
respectively. No diffraction peaks of impurities, such as CoO and
Co(OH)2, were observed in the XRD pattern. EDS pattern of the Co
sample (Fig. 1b) indicated that the product was essentially pure
metallic Co. A small amount of oxygen (0.47 wt%) was also
detected, which may result from the slight surface oxidation of
the Co sample. Fig. 1c was the low-magnification SEM image of
cobalt sample, revealing that a large amount of micron-scale
cobalt thin sheets could be obtained by this approach. Fig. 1d
showed that the surface of the Co thin sheets was relatively
smooth, and the inset in Fig. 1d indicated that the quite uniform
thickness of the Co sheets was around ca. 50 nm.
In addition, it should be noted that this reaction was carried
out under the external magnetic field, implying the Co nuclei
would not necessarily grow into one-dimensional nano- or
micron-sized wires/rods under an external magnetic field.
We found that the solvents played an important role in
determining the morphology and crystal structures of the Co
products in the present study, and the introduction of ethanol to
Co2þ þ 2OHꢀ ! CoðOHÞ2
(1)
(2)
2CoðOHÞ2 þ N2H4 ! 2Co þ N2 þ 4H2O
Co
Co
O
Co
30
40
50
60
70
80
0
2
4
6
8
10
12
14
2theta (degree)
Energy (keV)
100nm
500nm
5μm
Fig. 1. XRD (a), EDS (b), patterns and SEM images (c) and (d) of the Co nanosheets.

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500nm
5μm
5μm
5μm
Fig. 2. SEM images of Co samples prepared using different solvents: water (a) and (b), water/ethylene glycol (c) and water/glycerol mixture (d).
the reaction system was crucial for the formation of Co
nanosheets. For comparison, the SEM images of the Co products
prepared using water, water/ethylene glycol (1:1 in volume ratio)
and water/glycerol (1:1 in volume ratio) mixture as solvents are
shown in Fig. 2. When water acted as solvent, the product was
three-dimensional flower-like Co microcrystals composed of Co
nanorods radiating from the center. This result confirmed that it
was not necessarily the case that the Co nuclei would self-
assemble to wires/rods under an external magnetic field during
reaction. When water/ethylene glycol was substituted for water/
ethanol as solvent, Co rods with the diameter of 5–10 mm were
obtained. While, when water/glycerol was used as solvent, one-
dimensional Co chains, which consisted of many Co spheres with
the diameter of 2–3 mm, were produced. Compared with the XRD
pattern of Co nanosheets (Fig. 1a), XRD patterns (Fig. 3) of Co
samples prepared using water, water/ethylene glycol and water/
glycerol mixture as solvent showed an additional peak at 51.31,
which could be attributed to the diffraction from the (2 0 0) plane
of face-centered cubic (fcc) Co, suggesting these Co samples were
present as both the hcp and fcc phase [19].
30
40
50
60
70
80
2theta (degree)
Fig. 3. XRD patterns of Co samples prepared using different solvents: water (a),
It has been reported that the morphology and crystal
structures of metallic nanomaterials prepared by solution-chem-
istry reduction route were highly related to the solvents used in
the synthesis [19–21]. On the one hand, the solvents could affect
the reduction rates of metal salts, thus influenced their morphol-
ogy of final products [19,20]. In our previous work, it was found
that the reduction rate of Co salt by N2H4 varied with the solvents
and the introduction of ethanol could remarkably facilitate the
reduction of Co salt [20]. It was possible that a relatively high
reduction rate conduced to the formation of Co nanosheets in the
present study. On the other hand, the viscosity of the solvents,
which would determine the diffusion rate of the generated metal
nuclei, would also be an important factor affecting the morphol-
ogy and crystalline nature of the products. In the water/ethanol
mixture, ethanol could provide a good environment for the
diffusion of nuclei, which was favorable for the anisotropic
growth Co nanocrystals [19]. While in the water/ethylene glycol
and water/glycerol mixture, the relative high viscosity of solvents
would restrain the diffusion of nuclei and thus resulted in the
formation of spherical particles [19]. In these cases, when the
water/ethylene glycol (b) and water/glycerol mixture (c).
magnetic field was presented, the spherical particles tend to align
along the magnetic line of force and favor the formation of Co
wires or rods.
In addition, we found that the external magnetic field was
indispensable for the formation of Co nanosheets. For comparison,
we conducted a contrast experiment in which the external
magnetic field was removed while other experimental conditions
were kept unchanged. Fig. 4a shows the low-magnification SEM
image of the corresponding product, indicating the leaf-like
structure of the Co sample with the thickness of ca. 300 nm and
the size of 3–4 mm. High-magnification SEM (Fig. 4b) showed that
each leaf-like Co microcrystal was composed of a long central
backbone and plenteous of secondary branches, which were
parallel to each other and emerged at the same angle (ca. 601)
with respect to the central backbone. It was interesting that even
without the assistance of external magnetic field the Co nuclei
could grow into leaf-like Co microcrystals, which was quite

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5μm
200nm
Fig. 4. SEM images with different magnifications of Co sample prepared in the absence of the external magnetic field.
(109 Oe [24]), and Co nanoplatelets with diameter of 100 nm and
thickness of 8 nm (218 Oe and 176 Oe in the parallel and
perpendicular directions, respectively [10]). Considering the fact
that the as-prepared large Co nanosheets were randomly oriented
in the sample, Hc could be increased if all nanosheets could be
arrayed in the same direction [10]. It is well acknowledged that
the behavior of the magnetization of magnetic materials is highly
dependent on the types of anisotropy, the most common of which
are crystalline and shape anisotropy. The crystal anisotropic
constants of hcp phase and fcc phase Co were 4.5 ꢂ106 erg/cm3
[25] and 2.7 ꢂ106 erg/cm3 [26], respectively. Thus, for Co,
crystallizing as hcp phase would show higher coercivity owing
to its relatively large crystal anisotropy constant. Furthermore, a
nonspherical polycrystalline specimen can possess shape
anisotropy and a small departure from sphericity in shape
would result in significant increase in coercivity [5]. So, the high
coercivity of the synthesized Co nanosheets could be attributed to
the anisotropic shape of Co nanosheets and the hcp crystal
structure with high crystallinity.
1.0
0.5
0.0
-0.5
-1.0
-10000
-5000
0
5000
10000
H (Oe)
Fig. 5. Magnetic hysteresis loop of the Co nanosheets measured at 300 K.
4. Conclusions
different from previous report that only aggregated Co particles
were obtained when the external magnetic field was removed
[17]. This suggests that there may be some other factors, such as
solvents as mentioned above, controlling the shape-anisotropic
growth of Co nanocrystals.
Large hcp phase Co nanosheets with the thickness of 50 nm
were synthesized by a solvothermal route under an external
magnetic field without the assistance of any surfactants, complex-
ing agents or templates. The solvents played a crucial role in
determining the morphology and the crystal structures of the Co
products. Additionally, the external magnetic field was indis-
pensable for the formation of Co nanosheets. Magnetic measure-
ment of the Co nanosheets at room temperature indicated the
coercivity of the sample came up to 317.3 Oe, which was much
higher than those of bulk Co metal and previously reported Co
nanoplatelet. Our experimental results implied that it was not
necessarily the case that the Co nuclei would self-assemble into
one-dimensional cluster under an external magnetic field during
reaction. Furthermore, multi-dimensional Co microstructures
could also be obtained under an external magnetic field. More
importantly, this facile magnetic-field-assisted solution-phase
route may provide a new method to prepare some anisotropic
magnetic nanocrystals with unusual and novel morphology.
Without the assistance of the magnetic field, the Co nuclei
grew into leaf-like Co microcrystals, which could be described
with the diffusion-limited aggregation (DLA) model [22]. While
when the magnetic field was available, the growth of the Co nuclei
was not only controlled by DLA model, but also affected by the
external magnetic field force. By comparing Fig. 1c and d with
Fig. 4a, it was clear that the Co nanosheets prepared under the
magnetic field were larger and thinner than the leaf-like Co
microcrystals obtained without the magnetic field. This implied
when the magnetic field was presented, the Co nanosheets tend to
grow at the directions along the sheet planes. It was possible that
the newly generated Co nuclei during the reaction were prone to
move to the rim of the existent Co nanosheets under the magnetic
field and then added themselves to the Co nanosheets, thus
resulted in forming large Co nanosheets. The exact role of the
external magnetic field in our synthetic route is interesting but is
not well understood yet and further study is still necessary.
The M/H hysteresis loop (Fig. 5) of powdered Co nanosheets
measured at room temperature showed a ferromagnetic behavior.
Note that the coercivity (Hc) of the large Co nanosheets was
317.3 Oe, showing enhanced coercivity compared to the value of
bulk Co (ca. tens of oersteds at room temperature [23]). The
coercivity was also higher than those of the previously reported
Co nanoplatelets with diameter of 120 nm and thickness of 10 nm
Acknowledgements
The authors would like to thank Professor D.T. Zhang and Dr.
D.X. Wu for their help in SEM experiment. This work was
supported by the National Natural Science Foundation of China
(Project nos. 20673040, 20476034).

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