ARTICLE
pubs.acs.org/JPCC
Three-Dimentional Porous Nano-Ni/Co(OH)2 Nanoflake Composite
Film: A Pseudocapacitive Material with Superior Performance
X. H. Xia, J. P. Tu,* Y. Q. Zhang, Y. J. Mai, X. L. Wang,* C. D. Gu, and X. B. Zhao
State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027,
China
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bS Supporting Information
ABSTRACT: We report a novel three-dimentional (3D) porous nano-Ni/Co(OH)2
nanoflake composite film electrode for potential supercapacitor applications with
both high power and energy capabilities. The 3D porous nano-Ni film with highly
porous nanoramified walls functions as a scaffold to anchor Co(OH)2 nanoflakes to
produce a 3D nanoporous metal/hydroxide nanoflake composite electrode. Co(OH)2 nanoflakes with thicknesses of 20 nm are directly electrodeposited on highly
conductive 3D porous nano-Ni film prepared via a hydrogen bubble template.
Impressively, the Co(OH)2 nanoflake in the composite film exhibits a high specific capacitance of 1920 F g1 at 40 A g1, with
a corresponding energy density as high as 80 W h kg1 at a power density of 11 kW kg1. Moreover, the designed composite film
exhibits excellent cycling stability, making it one of the best electrode materials for high-performance supercapacitors. This work
demonstrates that the 3D porous nanometal/hydroxide nanoflake composite approach is an effective strategy toward supercapacitors with high energy and power densities.
’ INTRODUCTION
With the rapid energy depletion and worsened environmental
pollution, intense research has focused on energy storage and
conversion from alternative energy sources. Of the various power
source devices, supercapacitors, also known as electrochemical
capacitors, have attracted great attention due to their fast
recharge ability, high power performance, long cycle life, and
low maintenance cost.1,2 However, the low energy density of
supercapacitors prevents them from many important applications. It is highly desirable to increase the energy density of
supercapacitors without sacrificing their high power density. An
effective way to boost energy density is to develop electrode
materials with a high specific capacitance. Pseudocapacitive
materials, such as metal oxides/hydroxides and conducting
polymers, are being explored for producing supercapacitors with
increased specific capacitances (several times larger than those of
carbonaceous materials) and high energy densities.3,4 Nevertheless, most of these pseudocapacitive materials are p-type
semiconductors, which are kinetically unfavorable to support
fast electron transport required by high power density.
Despite numerous research efforts, it is still a challenge to
achieve supercapacitors with both high energy and power
densities, which are controlled by the kinetics of the pseudocapacitive electrode. Hence, it is crucial to enhance the kinetics of
ion and electron transport in electrodes and at the electrode/
electrolyte interface.3 Attempts at novel electrode design have
been extensively made. A common adopted strategy is to
construct pseudocapacitive materialconductive matrix composite electrodes.58 Pseudocapacitive materials with high specific
capacitances are integrated into nanoporous-structured conductive matrixes, such as carbonaceous materials (carbon nanotube,
r 2011 American Chemical Society
mesoporous carbon, carbon aerogels, and graphene),916 conducting polymer (poly(3,4-ethylenedioxythiophene)),17 and
dealloyed porous metals (Ni and Au).1820 These porousstructured substrates act as a highly porous conductive network,
enabling a good access of ions and electrons to the active surfaces
leading to enhanced supercapacitor performances.
In recent years, 3D porous nanometal films prepared by a
hydrogen bubble template have elicited much interest due to
their distinctive structural features and intriguing properties.21,22
These 3D porous nanometal films show highly porous dendritic
walls with a high surface area and an open porous structure,
which make them promising candidates for the construction of
next-generation supercapacitors. To date, several 3D porous
nanometal (Cu, Ni, Au) films have been synthesized via a
hydrogen bubble-templated method,2126 but there are few
reports about supercapacitor applications of the above 3D
porous nanometal films. On the other hand, Co(OH)2 is an
attractive pseudocapacitive material because of its high specific
capacitance, well-defined electrochemical redox activity, and low
cost.27 Herein, we report a novel 3D porous nano-Ni/Co(OH)2
nanoflake composite film electrode for potential supercapacitor
applications with both high power and energy capabilities. The
3D porous nano-Ni/Co(OH)2 nanoflake composite film exhibits a high specific capacitance of 1920 F g1 at 40 A g1, with a
corresponding energy density as high as 80 W h kg1 at a power
density of 11 kW kg1. Moreover, the designed composite film
exhibits excellent cycling stability. Our electrode design protocol
Received: August 23, 2011
Revised:
October 7, 2011
Published: October 10, 2011
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Figure 1. (a, b) SEM and (c, d) TEM images of the 3D porous nano-Ni film. Upper-right insets in (b)(d) correspond to magnified SEM and TEM
images of porous nanoramified walls and the SAED pattern of the porous Ni nanoparticles, respectively. The upper-left inset in (d) is the HRTEM image
of Ni nanoparticles.
could be readily extended to fabricate other 3D porous metal/
hydroxide or metal/oxide composite nanostructures for application in energy storage and conversion. This work demonstrates
that the 3D porous nanometal/hydroxide nanoflake composite
approach is an effective strategy toward supercapacitors with high
energy and power densities.
’ EXPERIMENTAL SECTION
All solvents and chemicals were of reagent quality and were
used without further purification. The cobalt nitrate, sodium
nitrate, ammonia chloride, and nickel chloride were obtained
from Shanghai Chemical Reagent Co. All aqueous solutions were
freshly prepared with high-purity water (18 MΩ 3 cm resistance).
Preparation of 3D Porous Nano-Ni Film. The electrodeposition of 3D porous nano-Ni film was performed in a standard twoelectrode glass cell at 25 °C with an electrolyte consisting of 2 M
NH4Cl and 0.1 M NiCl at a pH value of 3.5, a clean nickel foil
with a size of 2 3 cm2 as the working electrode, and a Pt foil as
the counter electrode. The distance between the two electrodes
was 1 cm, and the electrodeposition was carried out at a constant
current of 2.5 A cm2 for 90 s.
Preparation and Characterization of 3D Porous Nano-Ni/
Co(OH)2 Nanoflake Composite Film. The fabrication process
of the 3D porous nano-Ni/Co(OH)2 nanoflake composite film is
schematically illustrated in Figure S1 (Supporting Information).
The electrodeposition was performed in a standard three-electrode glass cell at 25 °C, with the above 3D porous nano-Ni film
as the working electrode, a saturated calomel electrode (SCE) as
the reference electrode, and a Pt foil as the counter electrode.
The Co(OH)2 film was electrodeposited from an aqueous
solution containing 1.0 M Co(NO3)2 and 0.1 M NaNO3 at a
constant current of 1.0 mA cm2 for 200 s using a Chenhua
CHI660C model electrochemical workstation (Shanghai). The
electrodeposition process of the Co(OH)2 precursor film would
include an electrochemical reaction and a precipitation reaction
expressed as follows:
NO3 þ H2 O þ 2e f NO2 þ 2OH
ð1Þ
Co2þ þ 2OH f CoðOHÞ2
ð2Þ
The weight of Co(OH)2 was measured by calculating the
weight difference between the 3D porous Ni film and the
Co(OH)2 product after electrodeposition. The load weight of
Co(OH)2 in the composite film was approximately 1.0 mg cm2.
For comparison, commercial nickel foam-supported Co(NO3)2
film was also prepared under the same deposition condition.
The samples were characterized by X-ray diffraction (XRD,
Philips PC-APD with Cu Kα radiation). The morphologies of all
samples were observed by field emission scanning electron
microscopy (FESEM, FEI SIRION), transmission electron microscopy (TEM, JEM 200 CX, 160 kV), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010F), and
X-ray photoelectron spectroscopy (XPS, PHI 5700).
Electrochemical Measurements. The electrochemical measurements were carried out in a three-electrode electrochemical
cell containing a 2 M KOH aqueous solution as the electrolyte.
Cyclic voltammetry (CV) measurements and electrochemical
impedance spectroscopy (EIS) tests were performed on a
CHI660c electrochemical workstation (Chenhua, Shanghai). CV
measurements were carried out between 0.1 and 0.6 V at 25 °C,
with the 3D porous nano-Ni/Co(OH)2 nanoflake composite film
as the working electrode, Hg/HgO as the reference electrode, and
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Figure 2. Morphological and structural characterizations of the 3D porous nano-Ni/Co(OH)2 nanoflake composite film: (a, b) SEM images (magnified
SEM image in inset) and (c) TEM image (magnified TEM image in inset); (d) TEM image of an individual Co(OH)2 nanoflake (SAED pattern in
inset); (e) XRD patterns of the (A) 3D porous nano-Ni film and (B) 3D porous nano-Ni/Co(OH)2 nanoflake composite film.
a Pt foil as the counter electrode. EIS tests were made with a
superimposed 5 mV sinusoidal voltage in the frequency range of
100 kHz to 0.01 Hz. The galvanostatic chargedischarge tests
were conducted on a LAND battery program-controlled test
system. The as-prepared electrodes, together with a nickel mesh
counter electrode and an Hg/HgO reference electrode were tested
in a three-compartment system.
’ RESULTS AND DISCUSSION
Self-supported 3D porous nano-Ni film is successfully prepared by a facile cathodic electrodeposition accompanying
hydrogen evolution, as schematically illustrated in Figure S1
(see the Supporting Information). The as-prepared Ni film
exhibits a 3D porous structure with highly porous nanoramified
walls. The typical large pores in the Ni film have sizes of 510 μm
(Figure 1a). The thickness of the porous nano-Ni film is
approximately 50 μm (Figure S2, Supporting Information).
More importantly, the walls of the Ni film consist of numerous
interconnected nanoparticles with diameters of 200300 nm
and show continuous nanopores ranging from 10 to 200 nm
(Figure 1b). As shown in the TEM image (Figure 1c), the branch
of the Ni film exhibits a porous structure with cross-linked
dendritic walls made up of numerous nanoparticles with a size
of about 300 nm. Besides, the selected area electronic diffraction
(SAED) pattern reveals the existence of a Ni phase with a
polycrystalline nature (Figure 1d). The measured lattice spacing
of about 2.06 Å is in good agreement with the (111) planes of the
Ni phase (Figure 1d). The hydrogen bubbles play an important
role in the formation of the unique 3D porous structure. The
hydrogen bubbles, arising from the cathodic reaction on the
nickel foil substrate, create a continuous path from the substrate
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deposition of nickel because there are no nickel ions available.
The growth of nickel toward the hydrogen bubble is inhibited,
leading to nickel deposition only at the interstitial spaces between
hydrogen bubbles. It is noticed that the hydrogen bubbles not
only come from the substrate but also evolve from freshly
deposited nickel. The former leads to the formation of large pores
in the Ni film, whereas the nanopores in the walls result from the
latter. In other words, the hydrogen bubbles act as a dynamic
template for the formation of the 3D porous nano-Ni film.
Typical SEM images of the 3D porous nano-Ni/Co(OH)2
nanoflake composite film are presented in Figure 2a,b. The 3D
porous structure of the Ni film is preserved after loading
Co(OH)2. The surface of the 3D porous nano-Ni film becomes
rough and decorated by many Co(OH)2 nanoflakes with a
thickness of about 20 nm. Additionally, Co(OH)2 nanoflakes
are interconnected with each other, forming a highly porous
sstructure. The Ni nanoparticles in the porous nanoramified walls
are uniformly covered by Co(OH)2 nanoflakes (Figure 2c). The
individual nanoflake exhibits a wrinkle appearance. Diffraction
rings in the SAED pattern of the nanoflake can be well indexed to
the α-Co(OH)2 phase (JCPDS 74-1057) (Figure 2d), supported
by the XRD data (Figure 2e). Taking together the results above, it
is justified that a novel 3D porous nano-Ni/Co(OH)2 nanoflake
composite film has been successfully constructed by successive
electrodeposition methods. Moreover, our electrode design protocol could be readily extended to fabricate other 3D porous
metal/hydroxide or metal/oxide composite nanostructures for
application in energy storage and conversion. For compassion, the
commercial nickel foam-supported Co(OH)2 nanoflake film is
also prepared under the same deposition condition. Apparently,
the nickel foam shows a solid branch covered by interconnected
Co(OH)2 nanoflakes (Figure S3, Supporting Information).
Figure 3a shows the cyclic voltammograms (CV) curves of the
porous nano-Ni/Co(OH)2 nanoflake composite film and bare
porous nano-Ni film at a scanning rate of 10 mV s1. Interestingly, the 3D porous nano-Ni film shows a strong redox couple
N1/N2, which is due to the reversible reactions of Ni(II)/Ni(III)
formed on the nickel surface in the alkaline electrolyte, supported
by the X-ray photoelectron spectroscopy (XPS) result (Figure S4,
Supporting Information). The 3D porous nano-Ni/Co(OH)2
nanoflake composite film exhibits two redox couples: A1/C1
and A2/C2. The first redox couple A1/C1 corresponds to the
conversion between Co(OH)2 and CoOOH, which can be
simply expressed as follows.27,28
CoðOHÞ2 þ OH T CoOOH þ H2 O þ e
Figure 3. CV curves of the (a) 3D porous nano-Ni film and 3D porous
nano-Ni/Co(OH)2 nanoflake composite film. (b) CV curves of the nickel
foam and Co(OH)2 nanoflake film grown on the nickel foam in the
potential range of 0.1 to 0.6 V at a scanning rate of 10 mV s1 at the
second cycle. (c) CV curves of the 3D porous nano-Ni film, 3D porous
nano-Ni/Co(OH)2 nanoflake composite film, and Co(OH)2 nanoflake
film grown on the nickel foam in the potential region of 0.1 to 0.4 V at a
scanning rate of 10 mV s1 at the second cycle.
to the electrolyteair interface during the electrodeposition
process. Where there is a hydrogen bubble, there will be no
ð3Þ
The second redox couple A2/C2 is much more complex.
Compared to the redox couple N1/N2, the peak potentials of
redox couple A2/C2 shift negatively and the current intensities
are stronger than those of redox couple N1/N2. On the other
hand, for the Co(OH)2 electrode material, another plausible
reaction of CoOOH/CoO2 could occur near 0.4 V (vs Hg/HgO)
in alkaline electrolyte. Therefore, we deduce that the redox
couple A2/C2 is an integrated redox couple, probably due to the
integration of redox couple N1/N2 and redox couple CoOOH/
CoO2. To further demonstrate this hypothesis, we conducted the
CV at a lower scanning rate of 1 mV s1 (Figure S5, Supporting
Information). Note the fact that the original anodic peak A2 splits
into two small peaks X1 and X2, while the peak C2 maintains its
integrity. The redox couple X1/C2 is due to the change between
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Figure 4. Discharge curves of the (a) 3D porous nano-Ni/Co(OH)2
nanoflake composite film and (b) Co(OH)2 nanoflake film grown on the
nickel foam at different discharge current densities. (c) Ragone plot (power
density vs energy density) of the 3D porous nano-Ni/Co(OH)2 nanoflake
composite film and Co(OH)2 nanoflake film grown on the nickel foam.
CoOOH and CoO2, represented by the following reaction:27,28
CoOOH þ OH T CoO2 þ H2 O þ e
ð4Þ
The redox couple X2/C2 belongs to reversible reactions of
Ni(II)/Ni(III). It indicates that the redox couple A2/C2 is an
integrated redox couple consisting of Ni(II)/Ni(III) and
CoOOH/CoO2, and both redox couples have similar reduction
peak potentials. Meanwhile, the nickel foam also exhibits a redox
process Z1/Z2 due to the reversible reactions of Ni(II)/Ni(III)
formed on the nickel surface, but its peak current intensities are
much lower than those of the 3D porous nano-Ni film (Figure 3b).
On the other hand, the Co(OH)2 nanoflake film grown on the
nickel foam only exhibits one redox couple, P1/P2; another redox
couple at higher potential is not observed in the CV curve. It is
implying that the 3D porous nano-Ni could activate another
redox couple at higher potentials for Co(OH)2. This hypothesis
is supported by the CV test conducted in a narrow potential
range of 0.1 to 0.4 V (Figure 3c). In this potential window, the
3D porous nano-Ni film does not show any redox couples, that is,
a blank potential window. Note that the porous nano-Ni/Co(OH)2 nanoflake composite film exhibits two pairs of redox
peaks and only one for the nickel foam-supported Co(OH)2 film,
indicating that both of the quasi-reversible reactions occur as
shown in eqs 1 and 2 for the porous nano-Ni/Co(OH)2 nanoflake composite film. That is to say, Co(OH)2 nanoflakes grown
on the 3D porous nano-Ni undergo much more sufficient and
complete redox reactions than the counterpart deposited on the
nickel foam. This interesting phenomenon is probably due to the
fact that Co(OH)2 nanoflakes grown on the 3D porous nano-Ni
possess much more reaction active sites due to its high distribution
and large surface area of the active materials in the porous structure.
Obviously, weaker polarization and better reversibility is obtained
for the porous nano-Ni/Co(OH)2 nanoflake composite film. It is
also noticed that the porous nano-Ni/Co(OH)2 nanoflake composite film electrode exhibits higher current densities, indicating that
the porous composite film has higher electrochemical activity than
the Co(OH)2 film grown on the nickel foam.
The load weight of Co(OH)2 in the composite film is approximately 1.0 mg cm2, accounting for 11% by mass. The load weight
of Co(OH)2 grown on the nickel foam is the same as above. The
supercapacitor properties are tested by galvanostatic charge
discharge at different current densities with 2 mA cm2 (corresponding to 2 A g1 based on the mass of Co(OH)2), 4 mA cm2
(4 A g1), 10 mA cm2 (10 A g1), 20 mA cm2 (20 A g1), and
40 mA cm2 (40 A g1), respectively. Discharge curves of the 3D
porous nano-Ni/Co(OH)2 nanoflake composite film and nickel
foam-supported Co(OH)2 nanoflake film at various discharge
current densities are shown in Figure 4a,b. The specific capacitances
of Co(OH)2 nanoflakes are calculated by subtracting the discharge
time of the bare porous nano-Ni film and nickel foam at the same
current density, respectively (Figure S6, Supporting Information).
The specific capacitance of Co(OH)2 nanoflakes is calculated to be
2028 F g1 at 2 A g1 (Figure S7, Supporting Information). It is
noticeable that the specific capacitance is still as high as 1920 F g1
even at 40 A g1. The corresponding area capacitance of Co(OH)2
nanoflakes is measured to be approximately 2.0 F cm2 at 2 A g1
and 1.9 F cm2 at 40 A g1; 95% of capacitance is maintained when
the charge/discharge rate changes from 2 A g1 to 40 A g1, much
higher than those obtained from the counterpart deposited on the
nickel foam (880 F g1 at 2 A g1 and 727 F g1 at 40 A g1, maintaining 82.6% of capacitance). Besides, these values are much higher
than other Co(OH)2 nanoflakes grown on commercial nickel foam
(1473 F g1 at 2 A g1),29 and stainless-steel foil (860 F g1),30 but
lower than those obtained from mesoporous Co(OH)2 nanoflakes
(2646 F g1)31 and Co(OH)2 nanoflakes grown on ultrastable Y
zeolite (3108 F g1).27 Moreover, our values are also much higher
than a nanoporous gold/MnO2 hybrid electrode (1145 F g1).19
Our material also shows higher specific capacitance than many of the
previously reported pseudocapacitive nanomaterials, including Ni(OH)2, NiO, MnO2, Co3O4, RuO2, and their composites with
carbon nanotubes or graphene.5,28,3236
Shown in the Ragone plot (power density vs energy density)
of Co(OH)2 nanoflakes grown on the 3D porous nano-Ni film
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material for supercapacitors with potentially high energy and
power densities. The 3D porous nano-Ni film functions as a
scaffold to anchor Co(OH)2 nanoflakes to produce a 3D
nanoporous metal/hydroxide nanoflake composite electrode.
The 3D porous nano-Ni/Co(OH)2 nanoflake composite film
exhibits superior supercapacitor performance to Co(OH)2 nanoflakes grown on the nickel foam substrate with high energy and
power densities, as well as good cyclic stability, making it one of the
best electrode materials for high-performance supercapacitors.
The improvement in energy density without sacrificing power
density suggests that our electrode design protocol may be
attractive for fabrication of new types of higher energy and power
electrochemical devices using a 3D porous nanometal architecture.
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’ ASSOCIATED CONTENT
Figure 5. Cycling performances of the 3D porous nano-Ni/Co(OH)2
nanoflake composite film and Co(OH)2 nanoflake film grown on the
nickel foam at 2 mA cm2 (corresponding to 2 A g1 based on the mass
of Co(OH)2).
(Figure 4c), the Co(OH)2 nanoflakes deliver an impressive high
energy density of 80 W h kg1 at a high power density of 11 kW
kg1, higher than the nickel foam-supported Co(OH)2 film
(30.5 W h kg1 at 11 kW kg1), superior to other hydroxidebased pseudocapacitive composites. Besides, the composite film
exhibits excellent pseudocapacitance retention with 1918 F g1
after 2000 cycles, maintaining 94.7% of the maximum value
(Figure 5; see Figure S6, Supporting Information, for details),
higher than the nickel foam-supported Co(OH)2 film (648 mA h g1
with 73.6%), demonstrating good capacity retention of the
porous composite film. Several features make the Co(OH)2
nanoflakes grown on the 3D porous nano-Ni film unique
building blocks for high power and energy storage and release.
First, Co(OH)2 nanoflakes are directly grown on the Ni nanoparticles of the porous walls. Such intimate binding affords facile
electron transport between individual Co(OH)2 nanoflakes and
Ni nanoparticles. Second, the porous structure shortens the
transportation/diffusion path for both electrons and ions, leading
to faster kinetics and higher utilization of active material,37,38
confirmed by the electrochemical impedance spectroscopy (EIS)
result (Figure S9, Supporting Information). It is well accepted
that the semicircle reflects the electrochemical reaction impedance of the film electrode and the straight line indicates the
diffusion of the electroactive species. A bigger semicircle means a
larger charge-transfer resistance, and a higher slope signifies a
lower diffusion rate. Obviously, the porous nano-Ni/Co(OH)2
nanoflake composite film exhibits a much smaller semicircle and
slower slope. It is concluded that the porous composite film has a
much lower charge-transfer resistance and ion diffusion resistance than the Co(OH)2 film grown on the nickel foam,
indicating that the porous composite film is favorable for charge
transfer and ion diffusion. Third, the high surface area of the 3D
nanoporous architecture favors the efficient contact between
active materials and electrolytes, providing more active sites for
electrochemical reactions. All these features contribute to the
high specific capacitance and high energy and power densities of
the 3D porous nano-Ni/Co(OH)2 composite film.
’ CONCLUSIONS
In summary, we have demonstrated a 3D porous nano-Ni/Co(OH)2 nanoflake composite film as an interesting pseudocapacitive
bS
Supporting Information. Schematic illustration for the
formation of the 3D porous nano-Ni/Co(OH)2 nanoflake composite film (Figure S1); SEM image of the side view of the 3D
porous nano-Ni/Co(OH)2 nanoflake composite film (Figure S2);
morphological and structural characterizations of the Co(OH)2
nanoflake film grown on the nickel foam substrate (Figure S3);
XPS spectra of the 3D porous nano-Ni film after CV test (Figure
S4); CV curve of the 3D porous nano-Ni/Co(OH)2 nanoflake
composite film at a scanning rate of 1 mV s1 (Figure S5);
discharge curves of the 3D porous nano-Ni film and nickel foam at
different discharge current densities (Figure S6); specific capacitances at different discharge current densities (Figure S7); cycling
performances of discharge electric quantity of the 3D porous
nano-Ni film and 3D porous nano-Ni/Co(OH)2 nanoflake composite film at 2 mA cm2 (Figure S8); and EIS plots of two film
electrodes at 100% depth of discharge (Figure S9). This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Tel: (86)-571-87952573. Fax: (86)-571-87952856. E-mail:
[email protected] (J.P.T.), [email protected] (X.L.W.).
’ ACKNOWLEDGMENT
The authors would like to acknowledge financial support from
the China Postdoctoral Science Foundation (Grant No.
20100481401).
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