High-yield preparation of uniform cobalt hydroxide and oxide nanoplatelets and their characterization

Article abstract of DOI:10.1021/jp0521149

Cobalt hydroxide nanoplatelets with a uniform hexagonal shape were prepared in high yield (a??95%) by a facile hydrothermal route in the presence of poly(vinylpyrrolidone). This method provides a simple, low-cost, and large-scale route to produce ?2-cobalt hydroxide nanoplatelets with an average diameter of 280 nm and a thickness of ca. 26 nm which show a predominant well-crystalline hexagonal brucite-like phase. Their thermal decomposition produced anisotropic nanoplatelets of cobalt oxides (CoO and Co ₃O₄) under designed temperatures. The products were characterized by transmission electronic microscopy, selected-area electron diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetric, and thermogravimetric analysis. The magnetic properties of the products were investigated by a superconducting quantum interference device magnetometer. Co₃O₄ nanoplatelets exhibit a superparamagnetic behavior, and they might be a promising material to study the magnetic tunneling effect as anisotropic nanostructures. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0521149

19094
J. Phys. Chem. B 2005, 109, 19094-19098
High-Yield Preparation of Uniform Cobalt Hydroxide and Oxide Nanoplatelets and Their
Characterization
Yanglong Hou,† Hiroshi Kondoh,† Masatsugu Shimojo,† Toshihiro Kogure,‡ and
Toshiaki Ohta*,†
Departments of Chemistry and of Earth and Planetary Science, School of Science, The UniVersity of Tokyo,
Tokyo 113-0033, Japan
ReceiVed: April 23, 2005; In Final Form: August 22, 2005
Cobalt hydroxide nanoplatelets with a uniform hexagonal shape were prepared in high yield (∼95%) by a
facile hydrothermal route in the presence of poly(vinylpyrrolidone). This method provides a simple, low-
cost, and large-scale route to produce â-cobalt hydroxide nanoplatelets with an average diameter of 280 nm
and a thickness of ca. 26 nm which show a predominant well-crystalline hexagonal brucite-like phase. Their
thermal decomposition produced anisotropic nanoplatelets of cobalt oxides (CoO and Co3O4) under designed
temperatures. The products were characterized by transmission electronic microscopy, selected-area electron
diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetric, and thermogravimetric
analysis. The magnetic properties of the products were investigated by a superconducting quantum interference
device magnetometer. Co3O4 nanoplatelets exhibit a superparamagnetic behavior, and they might be a promising
material to study the magnetic tunneling effect as anisotropic nanostructures.
Introduction
reports on nanoplatelets or nanosheets of hydroxides due to the
lack of knowledge on their synthesis.24
Low-dimensional nanoscale materials have attracted much
attention in the development of novel optical, electronic,
magnetic, and catalytic materials. Many one-dimensional (1D)
nanoscale materials such as nanotubes, nanobelts, nanorods, and
nanowires have been prepared by different approaches which
can be classified into the following strategies.1,2 First, the growth
of 1D nanoscale materials is achieved in a hard template with
well-confined structures, such as alumina, silica, block polymer,
mica, and membranes.3,4 Second, soft templates are used to
produce 1D nanoscale materials.5,6 Generally, in this process,
surfactants are applied to stabilize the surface of nanonuclei and
kinetically control the growth rates of various facets of nuclei.
Third, the intrinsic structures are used to form 1D nanostruc-
tures.7,8 Usually, the materials with hexagonal structure are
favored to form 1D nanostructures under a suitable reaction
condition. Fourth, vapor-liquid-solid (VLS) growth has been
employed to prepare 1D nanostructures.9-11 Recently, it was
found that the low-temperature process based on one or more
strategies described above is more promising to prepare 1D
nanoscale materials. For instance, Ag, Pb, and Pt nanowires
have been synthesized by the polyol process without any
template.12 A number of nanosized materials have also been
produced by the solvothermal approach.13-16 These methods
have some potential advantages, such as low cost, high purity,
and large-scale production. Apart from wire and tube structures,
other anisotropic nanostructures including nanocubes,17,18 nano-
prisms,19 nanotetrapods,20 and nanoplatelets or nanosheets21-23
have been the focus of intensive research for potential applica-
tions in nanoelectronics, data storage, and catalysis. Compared
with those on nanorods or nanowires, there have been few
Cobalt hydroxides can be used as additives to improve the
electrochemical activity of alkaline secondary batteries25 and
the tribological property of lubricating oils.26 Cobalt hydroxide
films show catalytic and reversible electrochromic properties.27,28
In particular, organic groups have been incorporated into the
interlayer region of cobalt hydroxides to form organic magnetic
materials.29 Several approaches have been employed to prepare
nanoplatelets.30 For example, Zeng’s group reported the ligand-
mediated synthesis of brucite-like cobalt hydroxide nanorods/
platelets consisting of linear arrays of assembled “butterflies”.31
The â-Co(OH)2 bulk crystal has a hexagonal structure with
space group p3hm1 and lattice constants of a ) 3.191 Å and c
) 4.664 Å (JCPDS file no. 30-443). This is a layered compound
composed of a Co layer sandwiched by two O layers. Cobalt
hydroxides prefer to grow into nanoplatelets due to their intrinsic
lamellar structures. It is desirable to conduct further structural
analysis for both of these species to understand the correlation
between the geometric and magnetic structures.
Spinel Co3O4 is an important P-type semiconductor due to
its potential applications in ceramic pigments, solid-state sensors,
energy storage as intercalation compounds, rotatable magnets,
heterogeneous catalysts, and electrochromic devices.32-35 Es-
pecially, it is very useful to prepare anisotropic antiferromagnetic
nanostructures for studying the magnetic properties as a function
of geometric structure.34 Some efforts have been focused on
the synthesis of cobalt oxide nanocubes,32 nanorods,36 or
nanotubes.37 Here, we employed a facile solvothermal approach
to synthesize Co(OH)2 nanoplatelets. Cobalt oxide nanoplatelets
were readily obtained by thermal decomposition of Co(OH)2
nanoplatelets. The magnetic properties of cobalt oxide nano-
platelets were also investigated.
* To whom correspondence should be addressed. Fax: 81 3 3812 1896.
E-mail: ohta@chem.s.u-tokyo.ac.jp.
† Department of Chemistry.
‡ Department of Earth and Planetary Science.
10.1021/jp0521149 CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/24/2005

Cobalt Hydroxide and Oxide Nanoplatelet Preparation
J. Phys. Chem. B, Vol. 109, No. 41, 2005 19095
Figure 1. XRD pattern of the as-prepared product (sample A). Indexing
is based on the structure of â-cobalt hydroxides (JCPDS file no. 30-
443).
Figure 2. FTIR spectra from (a) the as-prepared cobalt hydroxide
nanoplatelets (sample A) and (b) PVP.
Experimental Section
Cobalt hydroxide nanoplatelets were prepared by the pre-
cipitation and hydrothermal process. In a typical procedure, 2.5
g of Co(NO3)2‚6H2O and a designed amount (from 3.3 to 0 g)
of poly(vinylpyrrolidone) (PVP) were dissolved in a 20 mL
mixture of ethanol and distilled water with a 1:1 volume ratio
in a three-neck flask. The solution was bubbled with Ar for 30
min, and 50 mL of a 0.4 M NaOH aqueous solution was slowly
added, taking 1.5 h. This caused a color evolution from blue to
red-pink, indicating the phase change from the R to â phase of
Co(OH)2. The reaction suspension was quickly transferred into
an autoclave with a Teflon cup. The autoclave was heated to
120 °C and maintained at that temperature for 6 h. After the
autoclave was cooled to room temperature naturally, red-pink
precipitates were collected by centrifugation. By washing with
acetone and distilled water sequentially, possible excess sur-
factants and byproducts could be removed from the products.
The red-pink powders then were dried in a vacuum at 60 °C
for 5 h for further characterization.
Oxide nanoplatelets were obtained by thermal decomposition
of cobalt hydroxide nanoplatelets. Heating Co(OH)2 nanoplate-
lets at 220 °C for 4 h under atmospheric Ar produced CoO,
while heating them at 450 °C for 5 h in air produced Co3O4.
Gray-black powders thus obtained were collected for charac-
terization.
Figure 3. TEM images of the as-prepared â-cobalt hydroxide
nanoplatelets (sample A): (a) survey, (b) a typical SAED pattern, (c)
side view, and (d) HRTEM image of an area selected in (c).
Powder X-ray diffraction (XRD) patterns of the samples were
measured with a Rigaku Mini diffractometer with Cu KR
radiation (λ ) 0.15418 nm). Transmission electron microscopy
(TEM) images were obtained with a Hitachi HF-2000 electron
microscope. Infrared spectra of the samples formed in KBr
platelets were recorded with a Jasco FT/IR 420 spectrometer
to confirm the formation of Co(OH)2. The thermogravimetric
(TG) analysis and differential scanning calorimetric (DSC)
analysis were carried out in air with a Rigaku Thermplus
TG8120 with a heating rate of 5 °C/min. Magnetic studies were
conducted on cobalt hydroxide and oxide nanoplatelets by using
a superconducting quantum interference device (SQUID) mag-
netometer (Quantum Design Co. Ltd.).
is compared with that of PVP in Figure 2. A sharp peak observed
at 3630 cm-1 is assigned to the hydroxyl group in the brucite-
like structure enhanced due to high basicity. No peak appears
at 1384 cm-1, which is a typical absorbance of nitrate anions,30,31
indicating no intercalation of nitrate anions in the sample. The
peak at 1663 cm-1 in sample A might correspond to the peak
at 1676 cm-1 of PVP with a slight shift, indicating chemical
bond formation between PVP and the nanoplatelets. The peaks
in the region of 496-540 cm-1 can be assigned to metal-
oxygen vibrations and metal-OH bending vibrations in the
brucite-like octahedron sheets.30 Thus, the FTIR measurement
also confirms the formation of a brucite-like structure.
Results and Discussion
The morphology of the products was characterized by TEM.
As shown in Figure 3a, one can see that cobalt hydroxides are
well-defined hexagonal platelets with a size of about 280 nm,
showing a relatively narrow size distribution (Supporting
Information Figure SI-1). Figure 3b is a typical SAED pattern,
exhibiting the hexagonal structures of brucite-like â-cobalt
hydroxides. The side-view image (Figure 3c) indicates that the
thickness of the plates is ca. 26 nm. As shown in Figure 3d,
the HRTEM image confirms the layer structure with a lattice
Characterization of Cobalt Hydroxide Nanoplatelets.
Structural information on the products was obtained by XRD
measurement. Figure 1 shows the XRD pattern of a sample
stabilized with 3.3 g of PVP, which is denoted hereafter as
sample A. It exhibits a predominant well-crystalline brucite-
like phase (JCPDS file no. 30-443) in which the 2θ scan has
peaks at 19.0°, 32.4°, and 37.9°, corresponding to the (001),
(100), and (101) diffractions. The FTIR spectrum of sample A

19096 J. Phys. Chem. B, Vol. 109, No. 41, 2005
Hou et al.
Figure 4. TG curve (solid line) and DSC plots (dashed line) of as-
prepared cobalt hydroxide nanoplatelets (sample A).
spacing of 4.6 Å, in accordance with the (001) plane of brucite-
like â-cobalt hydroxides.
The thermal behavior of â-cobalt hydroxides was investigated
with TG and DSC measurements. Figure 4 shows a typical TG
analysis curve of cobalt hydroxide nanoplatelets coated with
surfactants. The gradual mass loss at lower temperatures can
be attributed to evaporation of the surfactants attached to the
nanoplatelet surfaces. The dominant mass loss profile exhibits
a well-defined decrease over a temperature range of 165-350
°C with an inflection point at about 180 °C. The total mass
loss was estimated to be ∼17%, slightly lower than the
theoretical value (19.4%) calculated from the thermal decom-
position of cobalt hydroxides: Co(OH)2 f CoO + H2O. This
suggests incomplete decomposition even around 350 °C for a
few minutes. The DSC curve shows an endothermic peak at
185 °C, corresponding to the dominant mass loss. Similar
behavior was also observed in the cases of nickel and magne-
sium hydroxides.30
Next, we investigated the influence of PVP concentration on
the morphology of the products. Samples A and B are products
stabilized with 3.3 and 1.5 g of PVP, respectively, while sample
C was prepared without PVP. XRD measurements indicate that
all three of these pink-red colored samples exhibit a typical
brucite-like Co(OH)2 phase (see Supporting Information Figure
SI-2). However, TEM observations show that samples B and C
are composed of semiplatelets with irregular morphologies and
some cracks (Supporting Information Figure SI-3). We can
conclude that the use of PVP is a key to achieve a high yield
(∼95%) of uniform nanoplatelet structures and the amount of
PVP affects the morphology of the products.
Figure 5. (a) TEM image of the CoO nanoplatelets. The inset is an
SAED pattern. (b) A typical HRTEM image of the CoO nanoplatelets
in (a). The lattice spacing of 2.4 Å consists with the (111) plane in
bulk CoO. (c) XRD patterns from (i) the CoO nanoplatelets and (ii)
bulk CoO (Wako Chemicals Ltd.). Indexing is based on the structure
of CoO, JCPDS file no. 48-1719.
Characterization of Cobalt Oxide Nanoplatelets. Generally,
the formation of cobalt oxide nanoplatelets is sensitively
dependent on the reaction conditions and intrinsic structures.
We employed single-crystalline cobalt hydroxide nanoplatelets
as the precursors to prepare cobalt oxide nanoplatelets. On the
basis of the results of the TG analysis, we adopted the condition
of heating at 220 °C under atmospheric Ar to convert Co(OH)2
completely to CoO and that of heating at 450 °C in air to prepare
Co3O4. Figure 5a shows the morphology of the sample annealed
at 220 °C for 4 h under Ar, where one can see that the sample
sustains platelet structures. The HRTEM image (Figure 5b)
indicates a highly crystalline character with a lattice spacing of
2.4 Å, corresponding to the value of the (111) plane of the CoO
phase. To investigate the structure of the sample further, the
XRD pattern was analyzed, as shown in Figure 5c, where all
reflection peaks at (111), (200), and (220) can be indexed to
the peaks of the CoO phase. The lattice constant (a ) 4.26 Å)
is in good agreement with bulk CoO (from Wako Chemicals,
space group Fm3hm, JCPDS file no. 48-1719). No peaks due to
hydroxides were observed, indicating the complete decomposi-
tion of hydroxides under an inert Ar atmosphere. After being
annealed at 450 °C for 5 h in air, the sample was characterized
by TEM and X-ray diffraction. As shown in Figure 6a, the
sample exhibits platelet morphology although there are some
cracks. The HRTEM image shows a well-defined crystalline
structure with a lattice spacing of 4.6 Å, corresponding to the
value of the (111) plane of the Co3O4 phase. The inset in Figure
6b is the fast Fourier transform (FFT) pattern of the sample.
All reflections of XRD patterns in Figure 5c are indexed to the
Co3O4 phase (space group Fd-3m, JCPDS file no. 78-1970),
consistent with the indices of bulk Co3O4 (from Wako Chemi-
cals).
Magnetic Properties of the Products. Both cobalt hydrox-
ides and CoO nanoplatelets have a typical antiferromagnetic
behavior. It was noted that Co3O4 nanoplatelets show super-
paramagnetic behavior. The temperature dependences of mag-

Cobalt Hydroxide and Oxide Nanoplatelet Preparation
J. Phys. Chem. B, Vol. 109, No. 41, 2005 19097
Figure 7. ZFC and FC magnetization curves of the Co3O4 nanoplatelets
in an applied field of 50 Oe.
Figure 8. Hysteresis loops of the Co3O4 nanoplatelets, measured at 2
and 300 K after zero-field cooling. Enlarged loops at 2 K after field
cooling (H ) 5 kOe) are shown in the inset.
As expected, dM/dH has a sharp peak at H ) 0, which is similar
to the behavior observed in manganese-12 acetonate and ferriton.
The shifted hysteresis loop was observed after a field cooling.
The central portions of the ZFC and FC hysteresis loops are
shown in the inset. The FC loop is slightly broadened and shifted
along the direction of the applied field, with an exchange field
of He ) 500 Oe at 2 K, indicating the presence of unidirectional
exchange anisotropy.40
Figure 6. (a) TEM image of the Co3O4 nanoplatelets. (b) XRD patterns
from (i) the Co3O4 nanoplatelets and (ii) bulk Co3O4 (Wako Chemicals
Ltd.). Indexing is based on the structure of Co3O4, JCPDS file no. 78-
1970. (c) A typical HRTEM image of the Co3O4 nanoplatelets in (a).
The inset is the Fourier transform pattern.
Conclusions
We presented a high-yield synthesis of uniform cobalt
hydroxide and oxide nanoplatelets with a size of 280 nm and a
thickness of ca. 26 nm via a facile hydrothermal route. The use
of PVP as a stabilizer is a key to prepare the uniform
nanoplatelet structures, and its amount affects the morphology
of the products. Thermal decomposition was employed to
produce anisotropic nanoplatelets of cobalt oxides. Co3O4
nanoplatelets exhibit superparamagnetic behavior and can be
used for the study of magnetic tunneling as anisotropic
nanostructures.
netization of the Co3O4 nanoplatelets in zero-field-cooled (ZFC)
and field-cooled (FC) conditions are shown in Figure 7. A sharp
maximum is observed at around 35 K (blocking temperature
TB), which is expected to be the freezing temperature of residual
spin moments. Above 35 K, both the ZFC and FC magnetization
curves decrease with temperature. The plots of ø-1 as a function
of temperature show a linear feature, indicating that the
magnetization obeys the Curie-Weiss law with a Θ value of
about 30 K. From the Curie constant, the value of the effective
magnetic moment per ion in the paramagnetic state could be
estimated to be 4.2 µB. The superparamagnetic behavior might
be due to the blocking effect of the resultant spins caused by
incompensation for Co3O4 nanocrystals,34 which is in contrast
with the behavior of bulk antiferromagnetic Co3O4. We found
a small hysteresis loop on the magnetization curve measured at
2 K (<35 K), although the magnetization curve shows no
hysteresis at room temperature, as shown in Figure 8. The field
derivative of the magnetization has been used to detect the
resonant spin tunneling in Mn-12 molecules and ferriton.38,39
Acknowledgment. This work was supported by the Japan
Society for the Promotion of Science (JSPS) and the 21st
Century COE Program from the Ministry of Education, Culture,
Sports, Science and Technology.
Supporting Information Available: Size distribution of
sample A (Figure SI-1), XRD patterns of the as-prepared
products prepared in the presence of different amounts of PVP
(Figure SI-2), and TEM images of samples B and C (Figure
SI-3). This material is available free of charge via the Internet
at http://pubs.acs.org.

19098 J. Phys. Chem. B, Vol. 109, No. 41, 2005
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