Shape-controlled synthesis and characterization of cobalt oxides hollow spheres and octahedra

Article abstract of DOI:10.1039/c2dt12403c

We demonstrated that single-crystalline cobalt monoxide (CoO) hollow spheres and octahedra could be selectively synthesized via thermal decomposition of cobalt(ii) acetylacetonate in 1-octadecene solvent in the presence of oleic acid and oleylamine. The morphologies and sizes of as-prepared CoO nanocrystals could be controlled by adjusting the reaction parameters. Cobalt oxide (Co ₃O₄) hollow spheres and octahedra could also be selectively obtained via calcination method using corresponding CoO hollow spheres and octahedra as precursors. The morphology, size and structure of the final products were investigated in detail by XRD, SEM, TEM, HRTEM, DSC, TG, and XPS. The results revealed that the electrochemical performance of cobalt oxide hollow spheres is much better than that of cobalt oxide octahedra, which may be related to the degree of crystallinity, size, and morphology of cobalt oxides.

Full text of DOI:10.1039/c2dt12403c

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PAPER
Shape-controlled synthesis and characterization of cobalt oxides hollow
spheres and octahedra†
a,b,c
a
a
a
a
a,b
Rongrong Shi,
Gen Chen, Wei Ma, Dan Zhang, Guanzhou Qiu and Xiaohe Liu*
Received 13th December 2011, Accepted 15th March 2012
DOI: 10.1039/c2dt12403c
We demonstrated that single-crystalline cobalt monoxide (CoO) hollow spheres and octahedra could be
selectively synthesized via thermal decomposition of cobalt(II) acetylacetonate in 1-octadecene solvent in
the presence of oleic acid and oleylamine. The morphologies and sizes of as-prepared CoO nanocrystals
could be controlled by adjusting the reaction parameters. Cobalt oxide (Co O ) hollow spheres and
3
4
octahedra could also be selectively obtained via calcination method using corresponding CoO hollow
spheres and octahedra as precursors. The morphology, size and structure of the final products were
investigated in detail by XRD, SEM, TEM, HRTEM, DSC, TG, and XPS. The results revealed that the
electrochemical performance of cobalt oxide hollow spheres is much better than that of cobalt oxide
octahedra, which may be related to the degree of crystallinity, size, and morphology of cobalt oxides.
1
. Introduction
In recent years, considerable effort has been devoted to prepar-
ing cobalt oxides nanocrystals with controlled morphology and
desired compositions. Several chemical and physicochemical
methods have been employed to prepare cobalt oxides nanocrys-
Inorganic nanocrystals with novel morphologies and desired
compositions have drawn immense attention due to their unique
morphology- and composition-dependent physicochemical prop-
erties and their importance in basic scientific research and poten-
12
13
tals, for example, spray pyrolysis, chemical vapor deposition,
1
4
15
sputtering,
thermal decomposition,
electrospinning tech-
1
–3
16
17
tial technology applications.
Cobalt oxides have aroused
nique, electrochemical and sonochemical synthesis. Recently
18
much more attention in recent years on the basis of their distinc-
tive electronic, magnetic, and catalytic properties and wide
variety of practical and potential applications. It is well known
a variety of novel shapes such as cobalt oxides nanocone,
1
9
20
21
22
23
nanobelt,
nanoring,
nanocube,
nanofiber,
and multishelled hollow sphere
nanorod,
have been
2
4
25,26
nanotube
that CoO and Co O are two especially important forms among
3
4
reported. However, a few routes have been proposed for the syn-
thesis of pure CoO nanocrystals. As far as we know, CoO nano-
crystals have been obtained mainly based on the following two
methods: thermal decomposition of metal–surfactant complexes
the various cobalt oxides based on their distinctive structural fea-
tures and fascinating properties. Especially, CoO, crystallizing in
the rock salt structure, is promising for functional materials
owing to their potential applications based on magnetic, catalytic
and gas-sensing properties.
spinel crystal structure based on a cubic close packing array of
oxide ions, in which Co(II) ions occupy the tetrahedral 8a sites
2
7–29
in noncoordinating solvents
and controlled oxidization of
4
–6
30,31
Co O belongs to the normal
3 4
Co (CO) or metallic cobalt nanocrystals.
2
8
Herein we demonstrated that single-crystalline CoO hollow
spheres and octahedra could be selectively synthesized in large
quantities by thermal decomposition of cobalt(II) acetylacetonate
and Co(III) ions occupy the octahedral 16d sites. Co O is an
3
4
important magnetic p-type semiconductor that has been demon-
strated to have considerable application in, e.g., heterogeneous
catalysts, gas sensors, electrochromic devices, solar energy
(
Co(acac) ) in 1-octadecene solvent in the presence of oleic acid
2
and oleylamine. The morphologies and sizes of as-prepared CoO
nanocrystals could be controlled by adjusting the reaction para-
meters, such as surfactants and reaction atmosphere. In particu-
lar, we could obtain CoO nanocrystals via a facile thermolysis
method without a protected atmosphere. Co O hollow spheres
7
–11
absorbers, pigments, etc.
3
4
and octahedra could also be obtained via a calcination method
using the corresponding CoO as a precursor. The results revealed
that the electrochemical performance of hollow spheres is much
better compared with that of octahedra, which may be related to
the degree of crystallinity, morphology, and particle size of the
cobalt oxides. It is worthy to note that the current synthetic strat-
egy can be used to synthesize other metal oxides nanocrystals,
and it will have a good prospect in future large-scale applications
due to its high yields and simple reaction apparatus.
a
Department of Inorganic Materials, Central South University,
Changsha, Hunan 410083, People’s Republic of China
b
State Key Laboratory of Powder Metallurgy, Central South University,
Changsha, Hunan 410083, People’s Republic of China. E-mail:
liuxh@csu.edu.cn; Fax: +86-731-88879815; Tel: +86-731-88830543
c
Institute of Materials Science and Engineering, Lanzhou University,
Lanzhou 730000, China
†
Electronic supplementary information (ESI) available: Typical TEM
images of the as-prepared cobalt oxides hollow spheres. See DOI:
0.1039/c2dt12403c
1
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2
. Experimental section
carbonate (DMC) (1 : 1, v/v) was used as the electrolyte. A pure
lithium foil was used as the counter electrode and the samples
under test were used as the working electrode during electroche-
mical measurements. All cells were assembled in an argon-filled
glove box. The electrode capacity was measured by a galvano-
The synthesis uses commercially available reagents: cobalt(II)
acetylacetonate (Co(acac) , analytical grade, 99%, ACROS),
trioctylphosphine oxide (TOPO, analytical grade, 99%,
ACROS), 1-octadecene (ODE, technical grade, 90%, ACROS),
oleic acid (OA, chemical grade, SCRC), oleylamine (OAm, tech-
nical grade, approximate C -content 80–90%, ACROS), hexa-
2
static charge–discharge experiment with a current density of
−1
2
00 mA g at a potential between 0 and 2.5 V.
1
8
decyl trimethyl ammonium bromide (CTAB), polyvinyl
pyrrolidone (PVP) and sodium dodecyl sulfate (SDS) were of
analytical grade and used without any further purification.
All the reactions were conducted in a three-neck flask
equipped with a stirring and heating attachment. In a typical
reaction, 0.2571 g Co(acac)2 (1 mmol) and 0.0364 g CTAB
3
. Results and discussion
Fig. 1A shows the low-magnification TEM image of CoO
obtained by using 0.1 mmol CTAB in the presence of oleic acid
and oleylamine at 260 °C for 2 h under a nitrogen environment.
We found that the product mainly consisted of hollow spheres
with an outer diameter of about 250 nm and wall thickness of
about 50 nm. The remarkable contrast between the shells and
centers indicates the hollow nature of the spheres. More details
of structure can be obtained from a further magnified image, as
shown in Fig. 1B. The surfaces of hollow spheres are rough with
some flocculates. High-resolution TEM provided further insight
into the structures of as-prepared products. Fig. 1C shows the
HRTEM image of the area marked in Fig. 1B. The lattice
spacing is calculated to be 0.21 nm, corresponding to d-spacing
of (200) crystal plane of cubic CoO, which further confirmed its
single-crystalline structure. The inset of Fig. 1C is the fast
Fourier transform (FFT) image of the HRTEM result. Fig. 1D
shows the XRD pattern of as-prepared CoO hollow spheres. All
diffraction peaks can be readily indexed to a pure cubic structure
with lattice constant a = 4.263 Å, well consistent with the stan-
dard PDF database (JCPDS file no. 65-2902). Diffraction peaks
of the sample are sharp and narrow, indicative of a relatively
good crystallinity.
(
0.1 mmol) were dissolved into 20 mL ODE with 4 mmol OA
and 10 mmol OAm. The reaction mixture was degassed for
0 min at room temperature using high purity gas of nitrogen.
The solution was heated up to 260 °C with approximately 8 °C
2
−1
min under vigorous stirring. At the beginning of the reaction,
a balloon was used to seal the system and buffer the high
pressure generated during the reaction. After heating under reflux
for 2 h, the solution was then cooled to room temperature, and a
mixture of alcohol and hexane was added to the solution to yield
a waxy precipitate, which was separated by centrifugation.
Finally, the products were dried in an oven at room temperature.
The resulting precipitate was found to be re-dispersible in many
organic solvents, such as n-hexane and chloroform. Shape
control of the products can be achieved by adjusting some of the
parameters, such as the surfactants and reaction atmosphere.
2
.1 Characterization
The obtained products were characterized on a D/max2550 VB
+
(
4
X-ray powder diffractometer (XRD) with Cu-Kα radiation
λ = 1.54178 Å). The operation voltage and current were kept at
0 kV and 40 mA, respectively. The size and morphology of as-
prepared products were determined at 20 kV by a XL30 S-FEG
scanning electron microscope (SEM) and at 160 kV by a
JEM-200CX transmission electron microscope (TEM) and a
JEOL JEM-2010F high-resolution transmission electron micro-
scope (HRTEM). Thermogravimetric analysis and differential
scanning calorimetry (TGA/DSC) were carried out with a
NETZSCH STA-449C simultaneous TG-DTA/DSC apparatus at
−1
a heating rate of 10 K min in flowing air. X-ray photoelectron
spectroscopy (XPS) data were acquired on a VG ECA-LAB
MK2. Sintering process was conducted in box-type resistance
furnace (SX-4-10).
In order to evaluate the electrochemical characteristics, elec-
trodes were fabricated using the powder by mixing 80 wt%
active materials, 10 wt% carbon black and 10 wt% polyvinyli-
dene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone. The
resultant slurries were spread on aluminum foil substrates. After
coating, the electrodes were pressed and dried at 120 °C under
vacuum for 12 h and then pressed between two stainless steel
plates at 1 MPa. Prior to cell assembling, the electrodes with
Fig. 1 (A) Low-magnification TEM image of CoO hollow spheres
obtained by using 0.1 mmol CTAB in the presence of oleic acid and
oleylamine at 260 °C for 2 h under a nitrogen environment; (B) high-
magnification TEM image of CoO hollow spheres; (C) HRTEM image
of the area marked by the square in figure (B); (D) XRD pattern of CoO
hollow spheres. The inset of (C) is the fast Fourier transform image.
2
area of 0.64 cm were dried at 120 °C for 4 h under vacuum.
The testing cells had a typical two-electrode construction using a
polypropylene microporous sheet as the separator, and 1 M
LiPF6 dissolved in ethylene carbonate (EC) and dimethyl
5982 | Dalton Trans., 2012, 41, 5981–5987
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Generally, the chemical and physical properties of inorganic
materials are directly related to their morphology, size, and phase
composition. Therefore, it is very important to tailor their elec-
tronic, magnetic and optical properties through the morphology
and shape control of the final products. Fig. 2A shows the low-
magnification SEM image of CoO nanocrystals prepared by
using 0.1 mmol CTAB in the presence of oleic acid and oleyl-
amine at 260 °C for 2 h without a protected atmosphere. The
products comprise large quantities of CoO octahedra. More
details of the structure can be obtained from a further magnified
image, as shown in Fig. 2B, from which uniform CoO octahedra
with the size of about 150 nm and edge length of 80–120 nm
can be seen clearly. Fig. 2C is the representative TEM image of
CoO octahedra, which clearly shows those octahedral structures
from different viewing angles. The inset of Fig. 2C is the
HRTEM image taken from individual octahedra. The lattice
spacing was calculated to be 0.21 nm, corresponding to the d-
spacing of (200) crystal plane of cubic CoO. Fig. 2D shows the
XRD pattern of as-prepared octahedral CoO nanocrystals. All
diffraction peaks can be readily indexed to a pure cubic structure
with lattice constant a = 4.274 Å (JCPDS file no. 65-2902),
which indicates that cubic CoO phase can be obtained without a
protected atmosphere.
Fig. 3 Morphologies of CoO nanocrystals prepared by using different
surfactants in the presence of oleic acid and oleylamine at 260 °C for 2 h
under a nitrogen environment: SEM image (A) and TEM image (B) of
CoO nanocrystals prepared with 0.03 g PVP; SEM image (C) of CoO
nanocrystals prepared with 0.1 mmol TOPO; TEM image (D) of CoO
nanocrystals prepared with 0.1 mmol SDS.
The surfactant is also a very important factor influencing the
3
2
morphology of the final products. Fig. 3A and B show the
typical SEM and TEM images of as-prepared product obtained
by using 0.03 g PVP in the presence of oleic acid and oleyl-
amine at 260 °C for 2 h under nitrogen environment. The
product is nearly monodisperse and the heterogeneous contrast
effect from its TEM image (Fig. 3B) suggests that nanocrystals
might have an unsmooth surface. The SEM image confirms this
point. On the whole, these quasi-spherical particles have the
trends of polyhedra. However, it is worth noting that in our pre-
vious research with a similar reaction system, PVP is a more
33
favorable surfactant for the dispersity of nanoparticles. In par-
ticular, when 0.1 mmol TOPO was employed as surfactant to
control morphology of product, CoO nanocrystals with octa-
hedral structures can be obtained, as shown in Fig. 3C. SEM
image shows that the majority morphology of as-prepared CoO
nanocrystals is octahedra. The edge-length of octahedral CoO is
about 80 nm. Fig. 3D is the TEM image of CoO nanocrystals
prepared by using 0.1 mmol SDS in the presence of oleic acid
and oleylamine at 260 °C for 2 h under nitrogen environment,
from which it can be seen that products are composed of irregu-
lar monodisperse nanoparticles in the size range of 20–30 nm.
It should be mentioned that there was a failure to control the
morphology of the products if the reaction was adopted in an air
atmosphere. Ammonia release accompanying the reaction pro-
vides an anaerobic atmosphere. All of these experimental con-
ditions and phenomena ensured the formation of cobalt
monoxide, preventing it from further oxidation. Experimental
results suggest that the reaction atmosphere and surfactant CTAB
may play important roles in the formation of CoO hollow
spheres. As is well-known, most of the methods for preparation
of hollow spheres are template-based ones, in which sacrificial
34,35
templates, either hard or soft ones, are used.
We considered
that nitrogen gas could help the surfactant CTAB to serve as tem-
plate, and CoO crystallites deposited along the template surface,
resulting in formation of the shells around the CTAB vesicles.
Based on the hypothesis above, it is suggested that the amount
of CTAB surfactant used should be effective on the formation of
CoO hollow spheres (see ESI, Fig. S1A and S1B†). When the
amount of CTAB increased from 0.5 mmol to 1.0 mmol, CoO
spheres with hollow interior and floccular structures can be
clearly observed due to the excessive amount of CTAB.
Fig. 2 (A) Low-magnification SEM image of the as-prepared octa-
hedral-like CoO nanocrystals prepared by using 0.1 mmol CTAB in the
presence of oleic acid and oleylamine at 260 °C for 2 h without a pro-
tected atmosphere; (B) high-magnification SEM image of octahedral
CoO nanocrystals selected from (A); (C) TEM image of octahedral CoO
nanocrystals; (D) XRD pattern of octahedral CoO nanocrystals. Inset of
(C) is the corresponding HRTEM image.
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The thermal behavior of CoO nanocrystals oxidized to Co O4
3
was investigated with thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) in the temperature range
−
1
2
5–750 °C with the heating rate of 10 °C min in air, as shown
in Fig. 4. The gradual mass loss (3.39%) in the range 25–290 °C
can be attributed to evaporation of the absorbed organic residues
and water species on the CoO surfaces accompanied by a weak
broad exothermal peak at 240 °C. With the temperature rising,
the obvious gravimetric gain can be seen from Fig. 4. The first
rapid mass increase (2.78%) occurred from 290 °C to 320 °C,
illustrating quick oxidation of the surface of products. The
second mass increase (1.81%) occurred before 445 °C, which
was attributed to the further oxidation of CoO. The DSC curve
of the sample displayed two exothermal peaks in the gravimetric
gain region: one is centered at 290 °C and another at about
3
80 °C. The total mass increase on the curve was smaller than
the theoretical value (7.1%), which may be associated with
partial oxidation of CoO in early stage and organic residues.
Co O4 hollow spheres and octahedra could be selectively
Fig. 5 (A) XRD patterns of inner hollow (curve a) and octahedral
3
obtained via calcination method using corresponding CoO
hollow spheres and octahedra as precursors. Fig. 5A shows XRD
patterns of Co O hollow spheres (curve a) and octahedra (curve
b) obtained by the calcination of corresponding CoO nanocrys-
tals at 600 °C for 2 h in air. All the reflections in the XRD pat-
terns can be indexed to the pure face-centered-cubic phase
(curve b) Co O nanocrystals calcined at 600 °C for 2 h using the corre-
3
4
sponding CoO nanocrystals as precursor, respectively. (B) Low-magnifi-
cation SEM image of Co nanocrystals with inner hollow structures
calcined at 600 °C for 2 h using CoO hollow spheres as precursor. (C)
Low-magnification SEM image of octahedral Co nanocrystals cal-
cined at 600 °C for 2 h using octahedral CoO as precursor. (D) High-
magnification SEM image of octahedral Co nanocrystals. Inset of
B) is the corresponding high-magnification SEM image of Co
hollow spheres.
3 4
O
3
4
3 4
O
3 4
O
(space group: Fd3m (No. 227)) of spinel cobalt oxide with
(
3 4
O
lattice constant a = 8.085 Å (JCPDS file no. 78–1969). No
impurity peaks were observed, which indicates that cubic CoO
was completely converted into the spinel structure Co O . The
3
4
low-magnification SEM image of Co O hollow spheres is
3
4
shown in Fig. 5B and the high-magnification SEM image inset
in Fig. 5B further shows the spheres with a hollow interior.
Fig. 5C and D are low- and high-magnification SEM images of
Co O octahedra, respectively. The morphology analysis of
surface of metastable materials with partially filled valence
bands. It was introduced here to evaluate the surface of as-pre-
pared CoO and Co O nanocrystals. The full survey spectra of
3 4
the two samples are just in coherence with each other and are
shown in Fig. 6, where each of the main peaks are indexed to O
1s, C 1s and Co 2p regions, confirming the cobalt oxides with
nonexistence of impurities. The inserted profile is the binding
energy (BE) spectrum for C 1s electrons of CoO sample. A main
peak emerged at BE of 284.8 eV, which is the standard BE for
amorphous carbon as an inert reference. The surface of the nano-
materials is very vulnerable to being affected by circumstance
because of its special immense surface area and high activity.
The cubic 3d transition metal Co has a partially filled valence
band within which strong electron correlations among the 3d
electrons were localized. It is of great importance to study the
stability and status of atoms special requirements rising from
electrochemical application of cobalt oxides.
3
4
Co O hollow spheres and octahedra reveals the perfect inheri-
3
4
tance from corresponding CoO nanocrystals.
X-ray photoelectron spectroscopy (XPS) is a reliable method
for intensive investigation of samples, especially efficient for
studying the status of atoms less than 10 nm deep from the
CoO and Co O have different coupling into the two possible
3
4
final states giving rise to the main and satellite peaks. It is well
known that cobalt series occupy the identical BE region belong-
ing to Co 2p1/2 and Co 2p3/2 electrons, but the distinction among
many of the cobalt compounds is still vague, as chemical shift of
2+
3+
main peaks in XPS spectra of Co 2p electrons in Co and Co
is not obvious enough. The survey spectra for Co 2p region of
inner hollow CoO nanocrystals (curve a) prepared using
0
2
.1 mmol CTAB with the molar ratio of OA to OAm 4 : 10 at
60 °C for 2 h in nitrogen atmosphere and Co O nanocrystals
3
4
(curve b) sintered at 600 °C for 2 h using inner hollow CoO
Fig. 4 DSC and TGA curves of CoO hollow spheres.
nanocrystals as precursors are shown in Fig. 7. The curves are
5984 | Dalton Trans., 2012, 41, 5981–5987
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38,40–42
which is in agreement with the literature for Co O .
3
4
Unlike in Co(II) compounds, in the low-spin Co(III) compounds,
41
the satellite structure is weak or missing. Co O , a mixed-
3
4
valance oxide, shows a weak satellite structure symptomatic of
42
shake-up from the minor Co(II) component. The (Co 2p1/2
–
Co 2p3/2) energy separation is approximately 15.3 eV, which
is also identical to that of the pure phase Co O in the
3
4
43,44
literature.
The electrochemical performances of as-prepared CoO and
Co O with different morphologies, hollow spheres and octa-
3
4
hedral structures, were investigated. Fig. 8 gives the typical first
discharge curves of Li–CoO and Li–Co O cells made by CoO
3
4
and Co O4 with inner-hollow (filled squares) and octahedral
3
structures (empty circles), between 0 and 2.5 V at a current
density of 200 mA g− . Generally speaking, nanocrystals with
an inner-hollow structure have higher capacity than octahedra,
whether CoO or Co O . Fig. 8A shows that inner-hollow CoO
1
Fig. 6 Full survey XPS spectra of CoO (curve a) and Co
hollow spheres. The inserted profile is the C 1s survey curve.
3 4
O (curve b)
3
4
nanocrystals have an initial discharge capacity as high as
−
1
−1
5
10.8 mAh g , and 116.6 mAh g after 50 cycles. Octahedral
−
1
ones have an initial discharge of at 291.7 mAh g
and
2
9.7 mAh g− after 50 cycles. The discharge curves of Co O4
1
3
hollow spheres electrode shown in Fig. 8B indicated that the dis-
charge voltage decreased sharply from 2.5 V to the discharge
plateau located in the range of 1.0–0.8 V at the first discharge
cycle. The initial capacity of as-prepared Co O hollow spheres
3
4
−
1
reaches 601.2 mAh g . The Co O hollow spheres electrode
3
4
demonstrates a stable reversible lithium storage capacity of
65.9 mAh g− within 50 cycles. Obviously, metal oxide nano-
1
1
crystals with different morphologies have different lithium
storage capacity, and the reason for this difference is compli-
45,46
cated, just as many literature reports have mentioned.
The
higher discharge capacity of hollow spheres may be attributed to
their unique morphology. The inner hollow structure is favorable
for increasing the interface area between electrode and electro-
lyte, which can result in a higher diffusion rate and faster elec-
trode kinetics, and also can increase the usage factor of the
47
Fig. 7 Co 2p electron XPS spectra for (a) CoO hollow spheres and (b)
Co hollow spheres. Data were acquired with Mg Kα radiation and
were fitted after removal of a linear background.
active component of the electrode. However, the discharge
capacity of octahedral Co O electrode was close to zero, indi-
cating its poor electrode kinetics.
3 4
O
3
4
fitted by pseudo-Voigt function; the binding energy (BE) values
emerged on the fitting lines are listed in Table 1. It was in fact
observed that the BE values of the most intense Co photoelectro-
nic peak (Co 2p) did not allow a clear distinction between CoO
4
. Conclusion
In summary, CoO hollow spheres and octahedra can be selec-
tively synthesized via thermal decomposition of cobalt(II) acety-
lacetonate in 1-octadecene solution using oleic acid and
oleylamine as capping ligands. The morphologies and sizes of
as-prepared CoO nanocrystals can be controlled by adjusting the
reaction parameters. This simple and reliable synthetic strategy
may be carried out to synthesize other metal oxides nanocrystals.
II
III
pure Co (II) and Co O Co Co O . The two major peaks
3
4
2 4
appearing in curve a shown in Fig. 7 are at 780.890 eV and
96.773 eV, separated by 15.9 eV, corresponding to the Co 2p3/2
7
and Co 2p1/2 spin–orbit peaks, respectively, of the CoO phase,
which are consistent with the supposed BE values of earlier
36,37
reported literature.
Furthermore, two satellites located at
Furthermore, Co O4 hollow spheres and octahedra can be
3
approximately 6 eV above the primary binding energy peaks
were detected at 786.278 and 802.796 eV, which were used as a
fingerprint for the recognition of high-spin Co (II) species in
obtained by the calcination of corresponding CoO hollow
spheres and octahedra at 600 °C for 2 h. The electrochemical
performance of cobalt oxide hollow spheres is much better than
that of cobalt oxide octahedra. These cobalt oxide hollow
spheres and octahedra can also be expected to bring new oppor-
tunities for further fundamental research, as well as for techno-
logical applications in catalysts, solid-state sensors, and as anode
materials in Li-ion rechargeable batteries.
38,39
CoO.
In addition, the Co 2p spectrum of Co O shown in
3 4
Fig. 7b yields a Co 2p3/2 peak at 780.591 eV and Co 2p1/2 peak
at 795.935 eV with the corresponding satellites at 784.929 eV
and 803.621 eV, respectively, are just in the same shape and are
the identical BE of Co 2p3/2 and Co 2p1/2 electrons, respectively,
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3 4
Table 1 XPS peak positions (BE in eV) obtained for CoO and Co O hollow spheres compared with the literature values
Samples
Co 2p3/2 (eV)
Satellite (eV)
Co 2p1/2
Satellite (eV)
CoO (this work)
780.890
780.591
780.0–780.919
779.5, 780.743
786.278
784.929
785.339
787.945
796.773
795.935
796.0–796.519
794.5, 795.038
802.796
803.621
802.639
803.045
Co
CoO
Co
3
O
4
(this work)
3
8,41
39–41
3
O
4
Fig. 8 The first discharge curves of the Li–CoO (A) and Li–Co
3
O
4
3 4
(B) cells made by CoO and Co O with different morphologies, hollow spheres
−
1
(
filled squares) and octahedral structures (empty circles), at a current density of 200 mA g . Inset: cycle performance of Li-profiles of the cells at a
−1
current density of 200 mA g at room temperature.
1
1
1
2
7 D. P. Dutta, G. Sharma, P. K. Manna, A. K. Tyagi and S. M. Yusuf, Nano-
technology, 2008, 19, 245609.
8 X. H. Liu, R. Ma, Y. Bando and T. Sasaki, Angew. Chem., Int. Ed., 2010,
Acknowledgements
Financial support of this work by National Natural Science
Foundation of China (No. 50504017) and Fundamental
Research Funds for the Central Universities of China (No.
49, 8253.
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