Controlled synthesis of mesoporous β-Ni(OH)2 and NiO nanospheres with enhanced electrochemical performance

Article abstract of DOI:10.1016/j.materresbull.2012.06.014

Uniform mesoporous β-Ni(OH)₂ and NiO nanospheres with hierarchical structures were synthesized by a facile complexation-precipitation method. The effects of ammonia and citrate on the structure and morphology of the products were thoroughly investigated by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption-desorption measurements. The results indicated that ammonia played an important role for the formation of flowerlike spheres assembled from nanosheets. The addition of citrate could remarkably reduce the particle sizes and increase the specific surface areas of flowerlike spheres. A possible formation mechanism based on the experimental results was proposed to understand their growing procedures. β-Ni(OH)₂ and NiO nanospheres prepared with the addition of citrate showed excellent capacitive properties due to their mesoporous structures and large surface areas, suggesting the importance of controlled synthesis of hierarchical nanostructures for their applications.

Full text of DOI:10.1016/j.materresbull.2012.06.014

Materials Research Bulletin 47 (2012) 2120–2125
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Controlled synthesis of mesoporous
b
-Ni(OH)2 and NiO nanospheres with
enhanced electrochemical performance
*
Shengtao Xing, Qian Wang, Zichuan Ma , Yinsu Wu, Yuanzhe Gao
College of Chemistry and Material Sciences, Hebei Normal University, Shijiazhuang 050016, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Uniform mesoporous b-Ni(OH)2 and NiO nanospheres with hierarchical structures were synthesized by a
Received 28 November 2011
Received in revised form 11 April 2012
Accepted 11 June 2012
facile complexation–precipitation method. The effects of ammonia and citrate on the structure and
morphology of the products were thoroughly investigated by X-ray powder diffraction, scanning electron
microscopy, transmission electron microscopy and nitrogen adsorption–desorption measurements. The
results indicated that ammonia played an important role for the formation of flowerlike spheres
assembled from nanosheets. The addition of citrate could remarkably reduce the particle sizes and
increase the specific surface areas of flowerlike spheres. A possible formation mechanism based on the
Available online 19 June 2012
Keywords:
A. Nanostructures
experimental results was proposed to understand their growing procedures.
b-Ni(OH)2 and NiO
B. Chemical synthesis
D. Electrochemical properties
nanospheres prepared with the addition of citrate showed excellent capacitive properties due to their
mesoporous structures and large surface areas, suggesting the importance of controlled synthesis of
hierarchical nanostructures for their applications.
ß 2012 Elsevier Ltd. All rights reserved.
1. Introduction
More recently, spherical superstructures constructed by
nano-substructures have attracted a great deal of interest as
Nickel hydroxide and nickel oxide have received increased
interest because of their excellent electrochemical properties and
potential applications in rechargeable Ni-based alkaline batteries
[1–4]. It has been known that nickel hydroxide has two different
such nanostructured materials exhibit unique and useful
properties in comparison with their bulk counterparts [18–
24]. For example, Jiang et al. synthesized uniform Ni(OH)2
hierarchical nanostructures assembled from ultrathin nano-
flakes by a hydrothermal method and the material exhibited
superior electrochemical properties [25]. Cao et al. reported that
the flowerlike NiO hollow nanosphere composed of twisted
nanosheets showed a high specific capacitance 585 F gÀ1 and
excellent cycling stability [26]. It is well known that the particle
size of spherical superstructure has a significant influence on its
electrochemical properties. There are many factors which might
affect the product size, such as reaction temperature, solvents
used, concentration of the reactants, and the use of surfactant
templates, etc. Cabanas-Polo et al. found that ultrasound
allowed the fabrication of flowerlike particles with sizes ranging
structures,
stable in alkaline solution and quickly transforms into the
-Ni(OH)2 has a well ordered brucite-like structure, which has a
a
-Ni(OH)2 and
b
-Ni(OH)2. The pure
a
-Ni(OH)2 is not
b
-phase.
b
hexagonal lattice and consists of an ordered stacking of well-
oriented Ni(OH)2 layers, without any intercalated species in the
interlayer space [5]. Therefore the
b-phase is more suitable as a
battery cathode than the -phase. NiO is a very prosperous
a
material, being widely used in various fields, such as battery
cathode, catalysis, gas sensors, and magnetic materials [6–9].
Ni(OH)2 can be conveniently synthesized in solutions of Ni(II) by a
chemical precipitation method, while NiO can be conveniently
prepared by thermal decomposition of Ni(OH)2. Up to now,
Ni(OH)2 and NiO with various structures and morphologies have
been synthesized, such as nanoparticle, nanorod, nanowire,
nanosheet and nano/microsphere, and their properties have also
been investigated [10–17]. The results showed that the perfor-
mances of Ni(OH)2 and NiO in most of their applications largely
depended on their crystal structure, morphology and surface area.
in between 0.7 and 1.0
mm in contrast with the 6–8 mm
particles obtained in the absence of ultrasound [27]. Xu et al.
reported that microwave treatment could remarkably reduce
the particle size of 3D flowerlike Ni(OH)2 [5]. Nevertheless, the
self-assembly of nanoscaled particles into complex architectures
with controllable size, shape, and well-defined crystalline
structure has still been one of the main challenges for materials
synthesis and device fabrications. Therefore, the development of
facile methods for the controllable synthesis of hierarchical
nanostructures is highly desirable for the large-scale production
and application of such materials.
* Corresponding author.
E-mail address: mazc@vip.163.com (Z. Ma).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2012.06.014

S. Xing et al. / Materials Research Bulletin 47 (2012) 2120–2125
2121
In this work, we present a facile complexation–precipitation
method to synthesize spherical hierarchical -Ni(OH)2 3D
architectures. The influences of the precipitator and additive on
the morphology and size of -Ni(OH)2 were thoroughly studied,
which are very important in their applications. The formation
mechanism of hierarchical architectures was proposed. The
morphology-retained NiO with mesoporous structure was readily
electrochemical cell at room temperature. Hg/HgO, Ni foams with
1.0 cm  1.0 cm (coated with acetylene black, polytetrafluoroethy-
lene, Ni(OH)2 or NiO) and 2.5 cm  2.5 cm (coated with acetylene
black, polytetrafluoroethylene and active carbon) were conducted
as the reference electrode, the working electrode and the counter
electrode, respectively. Cyclic voltammetry (CV) measurement
was carried out between 0.1 and 0.6 V in 6 mol/L KOH electrolyte at
a scan rate of 5 mV/s. The constant-current charge–discharge
experiments were performed with a specific current of 500 mA/g.
b
b
obtained by thermal decomposition of the as-synthesized
b-
Ni(OH)2. Finally, the electrochemical properties of -Ni(OH)2 and
b
NiO samples were investigated by cyclic voltammetry and
constant-current charge–discharge cycling techniques.
3. Results and discussion
2. Experimental
3.1. Structures and morphologies of Ni(OH)2 samples
2.1. Synthesis procedures
Fig. 1 shows the XRD patterns of the as-synthesized samples. All
the diffraction peaks can be indexed to single phase
b-Ni(OH)2
All reagents were of analytical grade and used without further
with a crystalline hexagonal structure (JCPDS card 14-0117). No
impurity peaks were found, suggesting the high purity of these
samples. It can be seen that sample Ni(OH)2-25 presents much
sharper peaks than other samples what is related to larger particle
size since the higher the peak width, the smaller the crystallite size
[28].
purification.
b-Ni(OH)2 was prepared by a complexation–precipi-
tation method, in which Ni(NO3)2 and NaOH were used as reagents,
with NH3ÁH2O as a complexing agent. In a typical preparation
process, 1.5 g of Ni(NO3)2Á6H2O was dispersed into 5 mL of ethanol.
Then a certain mount of ammonia solution (25 wt.%) was added
under magnetic stirring, followed by addition of 5 mL of NaOH
(2 M) aqueous solution. After stirring for 10 min, the resultant
suspension was transferred into a 100 mL Teflon-lined stainless
steel autoclave and heated at 100 8C for 24 h. After cooling to room
temperature, the green precipitates were collected by centrifuga-
tion and washed with deionized water and ethanol for several
times, and then dried in an oven overnight at 110 8C. Thereafter,
the products prepared by 0, 5, 15 and 25 mL ammonia solution are
expressed as Ni(OH)2-0, Ni(OH)2-5, Ni(OH)2-15 and Ni(OH)2-25,
respectively, while the product prepared by 25 mL of ammonia
solution and 10 mM sodium citrate was denoted as Ni(OH)2-25C.
Fig. 2 shows the SEM images of the as-synthesized
b-Ni(OH)2.
Irregular thin nanosheets with lengths of about 100–200 nm were
obtained in the absence of ammonia. When 5 mL of ammonia
solution was added, some aggregates composed of nanosheets
were formed (Fig. 2b). The amount of aggregates increased with
increasing the amount of ammonia. When 25 mL of ammonia
solution was added, the products became flowerlike microspheres
with diameters of about 1.5–2.5
mm (Fig. 2d). These microspheres
are composed of large numbers of thin nanosheets with thickness
of about 20–40 nm. As shown in Fig. 2e and f, the sizes of the
flowerlike spheres decreased remarkably with the addition of
sodium citrate. In comparison with Ni(OH)2-25, although Ni(OH)2-
25C has a much smaller particle size of about 300–400 nm, it still
retains the hierarchical 3D complex architectures. The TEM images
in Fig. S1 (Supplementary material) further confirm that both
Ni(OH)2-25 and Ni(OH)2-25C have hierarchical architectures
assembled from dozens of nanosheets.
NiO was obtained by calcination of the as-prepared
air at 400 8C for 2 h.
b-Ni(OH)2 in
2.2. Characterization
Phase identification was carried out by X-ray powder diffrac-
tion (XRD) in a Bruker D8-ADVANCE using Cu K radiation. The
morphology and structure of the synthesized samples were
examined by a field emission scanning electron microscopy
a
The textural information of the
b-Ni(OH)2 samples was also
investigated using N2 adsorption–desorption measurement. As
shown in Fig. 3, the isotherms of Ni(OH)2-0, Ni(OH)2-5, Ni(OH)2-15
and Ni(OH)2-25 are similar to mixed type-II/III isotherms, which
accounts for nonporous or macroporous [29], while the isotherm of
(FESEM Hitachi S-4800, 3 kV) and
a transmission electron
microscopy (TEM Hitachi H-7500, 80 kV). Nitrogen adsorption–
desorption measurements were performed on a Quantachrome
NOVA 4000e surface area and pore size analyzer at 77 K, using the
volumetric method. Specific surface area and pore volume were
calculated by the Brunauer–Emmett–Teller (BET) method. Pore
size distributions were estimated from adsorption branches of
isotherms by the Barrett, Joyner, and Halenda (BJH) method.
2.3. Electrochemical tests
The working electrode was prepared by mixing the electro-
active material (Ni(OH)2 or NiO) with 15 wt.% acetylene black and
5 wt.% polytetrafluoroethylene binder. A slurry of the above
mixture was made using absolute alcohol as solvent which was
subsequently brush-coated onto a nickel foam current collectors
(1.0 cm  1.0 cm). The mesh was dried at 80 8C in air for 5 h for the
removal of the solvent. After drying, the coated mesh was pressed
on a tablet press to 4 MPa. The counter electrode was prepared as
the working electrode except that the mixture was 90 wt.% active
carbon, 5 wt.% acetylene black and 5 wt.% polytetrafluoroethylene
and the nickel foam current collector was 2.5 cm  2.5 cm.
The electrochemical measurements were carried out in a
Fig. 1. XRD patterns of the Ni(OH)2 samples prepared by adding different amounts
of ammonia.
VMP3 electrochemical workstation using
a three-electrode

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S. Xing et al. / Materials Research Bulletin 47 (2012) 2120–2125
Fig. 2. FESEM images of the Ni(OH)2 samples prepared by adding different amounts of ammonia: (a) Ni(OH)2-0, (b) Ni(OH)2-5, (c) Ni(OH)2-15, (d) Ni(OH)2-25, (e) Ni(OH)2-
25C, and (f) a high-magnification FESEM image of Ni(OH)2-25C.
Ni(OH)2-25C is typically type-IV with hysteresis loops, which
means that the sample has a mesoporous structure. The pore size
distribution curves indicate that the pore size of Ni(OH)2-25C is
much smaller and more uniform than that of Ni(OH)2-25. The
surface areas of Ni(OH)2-0, Ni(OH)2-5, Ni(OH)2-15, Ni(OH)2-25 and
Ni(OH)2-25C were calculated to be about 69, 58, 54, 50 and 184 m2/
g, respectively. The results indicate that Ni(OH)2-25C has a much
larger surface area due to its mesoporous structure and smaller
particle size. Additionally, the surface areas of the products
prepared in the absence of citrate decreased gradually with
increasing the amount of ammonia, which could be due to the
aggregation of these nanosheets.
morphology (Fig. S2a in Supplementary material). Based on the
above results, the formation of flowerlike -Ni(OH)2 sphere is a
b
complexation–precipitation route under hydrothermal condition
(Fig. S3 in Supplementary material). In this synthetic route, NaOH
was used as a precipitating agent, with ammonia as a complexing
agent. When ammonia was added, Ni2+ reacted with ammonia to
form a relatively stable complex, thus decreasing the free Ni2+ ion
concentration in solution and reducing the growth rates of crystals.
In the early stage, the nucleus formation rate was slow and some
aggregated solid nuclei were formed (Fig. S2b in Supplementary
material). During hydrothermal treatment, the nickel ammonia
complex decomposed and the free Ni2+ ion concentration
increased dramatically. Then Ni2+ in solution reacted with OHÀ
3.2. Formation mechanism of flowerlike
b
-Ni(OH)2 structures
rapidly, which is favorable for the formation of b-Ni(OH)2 sheets
[30]. Meanwhile, some NH3 would vaporize from the solution and
generate many bubbles as aggregation and growth center [31].
Finally, flowerlike b-Ni(OH)2 spheres composed of large number of
nanosheets were formed. In addition, the higher pressure caused
by ammonia evaporation may also benefit the formation of
In this experiment, ammonia played an important role for the
formation of flowerlike -Ni(OH)2 structures, and the addition of
sodium citrate remarkably reduced the particle size and increased
the specific surface area of the flowerlike -Ni(OH)2. When
b
b
ammonia was not added, the products were nanosheets, suggest-
ing that Ni2+ could readily react with OHÀ to form nanosheets
under this condition. Whereas the sample prepared by adding
25 mL of ammonia and in the absence of NaOH has an irregular
flowerlike b-Ni(OH)2 sphere [26].
Citrate is an important ligand for metal ions and has been
widely employed as a shape controller through its selective
adsorption on crystal faces. Our previous work found that the

S. Xing et al. / Materials Research Bulletin 47 (2012) 2120–2125
2123
Fig. 4. Cyclic voltammograms of the Ni(OH)2 samples prepared by adding different
Fig. 3. Nitrogen adsorption–desorption isotherms (a) of the Ni(OH)2 samples
prepared by adding different amounts of ammonia, and pore-size distribution
curves (b) of the flowerlike Ni(OH)2 samples.
amounts of ammonia.
Ni(OH)2-15, Ni(OH)2-25 and Ni(OH)2-25C, respectively. The result
addition of citric acid could remarkably reduce the particle size and
increase the specific surface area of Mn3O4 nanoparticles [32].
Additionally, Takahashi et al. found that the pore volume of silica
agreed well with the space occupied by citric acid in the CA–silica
composite, and suggested that citric acid can act as a template for
the pore formation [33]. In this experiment, the pore formation
mechanism may also be attributed to the presence of suitable
strength of the citrate–nuclei interaction. The citrate was
suggested to interact with the formed Ni(OH)2 nuclei during the
growth phase, resulting in smaller particle sizes. Ni(OH)2 nano-
sphere was formed under the hydrothermal treatment, and the
citrate might be dispersed uniformly in the Ni(OH)2 framework.
Finally, uniform mesopores can be formed after the citrate is
removed.
demonstrates that Ni(OH)2-25C has a much higher specific
capacitance due to its mesoporous structure and larger surface
area, which benefit electrolyte transport and charge-transfer
reactions.
Since Ni(OH)2-25C exhibited the highest capacitance, its service
life was further investigated by charge–discharge measurement
with a current density of 500 mA/g. As shown in Fig. 5, the
Coulombic efficiency was nearly 100% for each cycle, and no
obvious capacitance decrease was observed after 1000 cycles. All
the above results suggest that the mesoporous
b-Ni(OH)2 with
hierarchical structures is an excellent electrode material for
supercapacitors.
3.3. Electrochemical properties of Ni(OH)2 samples
Electrochemical properties of the as-prepared
b-Ni(OH)2
samples were studied by cyclic voltammetry and constant-current
charge–discharge cycling techniques. The cell assembled in a three
electrode configuration. The CV recorded at a scan rate of 5 mV/s in
6 mol/L KOH aqueous electrolyte is shown in Fig. 4. The anodic
oxidation peak in the range of 0.4–0.5 V can be attributed to the
conversion of nickel hydroxide to nickel oxyhydroxide, while the
reduction peak at 0.1–0.2 V can be associated with the conversion
of oxyhydroxide back to nickel hydroxide [34]. The area under the
CV curve can be used to estimate the capacitance of the system. It
can be seen that the area under the CV curve of Ni(OH)2-25C is
much larger than those of other samples. The specific capacitance
values calculated from the cyclic voltammetry curves were found
to be 946, 743, 682, 547 and 1834 F gÀ1 for Ni(OH)2-0, Ni(OH)2-5,
Fig. 5. Constant-current charge–discharge curves and discharge efficiency (inset) of
Ni(OH)2-25C.

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S. Xing et al. / Materials Research Bulletin 47 (2012) 2120–2125
Fig. 6. XRD patterns of the NiO samples obtained by calcination of Ni(OH)2-25 and
Ni(OH)2-25C.
Fig. 8. Cyclic voltammograms of the flowerlike NiO samples.
3.4. Structures and morphologies of flowerlike NiO samples
Since nickel oxide with the unchanged nanostructure can be
obtained via a thermal decomposition of
b-Ni(OH)2, we synthe-
sized flowerlike NiO by calcination of Ni(OH)2-25 and Ni(OH)2-25C
in air at 400 8C for 2 h. XRD patterns of the products are shown in
Fig. 6. The products are of good crystallinity and all the diffraction
peaks can be indexed to the face-centered cubic NiO phase (JCPDS
card 04-0835). No peaks from
that -Ni(OH)2 has been completely transformed to NiO after
heated at 400 8C for 2 h.
b-Ni(OH)2 were observed, indicating
b
The morphology and size of the as-synthesized nickel oxide
were examined by SEM and TEM. As shown in Fig. 7, both NiO-25
and NiO-25C have flowerlike structures, which are similar to those
of the Ni(OH)2 precursors. The TEM images in Fig. S4 (Supplemen-
tary material) further confirm that the hierarchical architectures
built from nanosheets can be maintained after thermal decompo-
Fig. 9. Constant-current charge–discharge curves and discharge efficiency (inset) of
NiO-25C.
sition of
b-Ni(OH)2.
3.5. Electrochemical properties of flowerlike NiO samples
curve is constant, indicating their excellent pseudocapacitance
behaviors. The discharge efficiency of NiO-25C was also examined. It
was found that the sample had a high discharge efficiency (Fig. 8,
inset). Moreover, the sample supercapacitor still retained almost
100% of initial capacitance over 1000 cycles.
The above results suggest that both Ni(OH)2-25C and NiO-25C
have a relatively high supercapacitance and excellent capacitance
retention due to their mesoporous structures and large surface
areas. Reducing the particle sizes of hierarchical nanostructures
could significantly improve their electrochemical properties. The
Electrochemical properties of the NiO samples were also
investigated by cyclic voltammetry and constant-current charge–
discharge cycling techniques. As shown in Fig. 8, a pair of anodic and
cathodic peaks can be observed in the curves, which can be due to
the reversible reactions between NiO and NiOOH. The specific
capacitance values for NiO-25 and NiO-25C were calculated to be
170 and 290 F gÀ1, respectively. The last twenty charge–discharge
procedures during the charging and discharging steps are shown in
Fig. 9. The charging curves are very symmetric to their discharging
counterparts in the whole potential region and the slope of every
mesoporous Ni(OH)2 nanosphere showed
a higher specific
capacitance (1834 F gÀ1) than many of the previously reported
Fig. 7. SEM images of the NiO samples obtained by calcination of Ni(OH)2-25 and Ni(OH)2-25C: (a) NiO-25, (b) NiO-25C.

S. Xing et al. / Materials Research Bulletin 47 (2012) 2120–2125
2125
nanomaterials [1,14,21,35]. Furthermore, it showed a good cycling
References
stability. Therefore, these mesoporous nanospheres may have
potential applications in electrochemical storage.
[1] H. Wang, H.S. Casalongue, Y. Liang, H. Dai, J. Am. Chem. Soc. 132 (2010) 7472.
[2] M.R. Palacin, Chem. Soc. Rev. 38 (2009) 2565.
[3] M.A. Kiani, M.F. Mousavi, S. Ghasemi, J. Power Sources 195 (2010) 5794.
[4] S.R. Ovshinsky, M.A. Fetcenko, J. Ross, Science 260 (1993) 176.
[5] L. Xu, Y.S. Ding, C.H. Chen, L. Zhao, C. Rimkus, R. Joesten, S.L. Suib, Chem. Mater. 20
(2008) 308.
[6] X. Wang, L. Li, Y. Zhang, S. Wang, Z. Zhang, L. Fei, Y. Qian, Cryst. Growth Des. 6
(2006) 2163.
4. Conclusions
In summary, this work present a facile complexation–precipita-
tionmethodtosynthesizemesoporousb-Ni(OH)2 nanosphereswith
[7] T.L. Lai, C.C. Lee, G.L. Huang, Y.Y. Shu, C.B. Wang, Appl. Catal., B 78 (2008) 151.
[8] H. Steinebach, S. Kannan, L. Rieth, F. Solzbacher, Sens. Actuators B 151 (2010) 162.
[9] M. Tadic´, M. Panjan, D. Markovic´, Mater. Lett. 64 (2010) 2129.
[10] K. Karthik, G.K. Selvan, M. Kanagaraj, S. Arumugam, N.V. Jaya, J. Alloys Compd. 509
(2011) 181.
[11] D. Sun, J. Zhang, H. Ren, Z. Cui, D. Sun, J. Phys. Chem. C 114 (2010) 12110.
[12] F.S. Cai, G.Y. Zhang, J. Chen, X.L. Gou, H.K. Liu, S.X. Dou, Angew. Chem. Int. Ed. 43
(2004) 4212.
[13] L. Dong, Y. Chu, W. Sun, J. Chem. Eur. 14 (2008) 5064.
[14] Y. Wang, Z. Hu, H. Wu, Mater. Chem. Phys. 126 (2011) 580.
[15] L. Liu, Y. Li, S.M. Yuan, M. Ge, M.M. Ren, C.S. Sun, Z. Zhou, J. Phys. Chem. C 114
(2010) 251.
hierarchical architectures. Irregular thin nickel hydroxide
nanosheets were obtained in the absence of ammonia. When
ammonia was added, flowerlike
b-Ni(OH)2 spheres composed of
large numbers of thin nanosheets were formed, indicating the
important role of ammonia for the formation of hierarchical
architectures. The addition of citrate could remarkably reduce the
sizes of these spheres and increase their specific surface areas. The
possible formation mechanism has been discussed in detail. The
morphology-retained NiO can be readily obtained by thermal
[16] B. Zhao, X.K. Ke, J.H. Bao, C.L. Wang, L. Dong, Y.W. Chen, H.L. Chen, J. Phys. Chem. C
113 (2009) 14440.
[17] Y. Cui, C. Wang, S. Wu, G. Liu, F. Zhang, T. Wang, CrystEngComm 13 (2011) 4930.
[18] L.P. Zhu, G.H. Liao, Y.Y.H.M. Xiao, J.F. Wang, S.Y. Fu, Nanoscale Res. Lett. 4
(2009) 550.
[19] H. Wang, Y. Qian, Cryst. Res. Technol. 45 (2010) 545.
[20] S. Sarkar, M. Pradhan, A.K. Sinha, M. Basu, Y. Negishi, T. Pal, Inorg. Chem. 49 (2010)
8813.
decomposition of the as-synthesized
cal properties of the synthesized materials are dependent on their
surface areas, morphologies and structures. Mesoporous -Ni(OH)2
and NiO nanospheres prepared with the addition of citrate showed
high specific capacitances and excellent cycling stabilities, suggest-
ing their promising potentials in supercapacitors.
b-Ni(OH)2. The electrochemi-
b
[21] S. Park, J.H. Lim, S.W. Chung, C.A. Mirkin, Science 303 (2004) 348.
[22] T.D. Ewers, A.K. Sra, B.C. Norris, R.E. Cable, C.H. Cheng, D.F. Shantz, R.E. Schaak,
Chem. Mater. 17 (2005) 514.
Acknowledgements
[23] J.H. Jung, Y. Ono, K. Sakurai, M. Sano, S. Shinkai, J. Am. Chem. Soc. 122 (2000) 8648.
[24] M.W. Heaven, G.W.V. Cave, R.M. Mckinlay, J. Antesberger, S.J. Dalgarno, P.K.
Thallapally, J.L. Atwood, Angew. Chem. Int. Ed. 45 (2006) 6221.
[25] H. Jiang, T. Zhao, C. Li, J. Ma, J. Mater. Chem. 21 (2011) 3818.
[26] C.Y. Cao, W. Guo, Z.M. Cui, W.G. Song, W. Cai, J. Mater. Chem. 21 (2011) 3204.
[27] S. Cabanas-Polo, K.S. Suslick, A.J. Sanchez-Herencia, Ultrason. Sonochem. 18
(2011) 901.
This work was supported by the National Natural Science
Foundation of China (No. 20977024), the Education Department
Foundation of Hebei Province (No. Z2010152) and the Science
Foundation of Hebei Normal University (No. L2010Z07).
[28] H. Zhou, Z. Zhou, Solid State Ionics 176 (2005) 1909.
[29] F.A.A.L Sagheer, M.I. Zaki, Colloids Surf. A 173 (2000) 193.
[30] C.Z. Yuan, X.G. Zhang, L.H. Su, B. Gao, L.F. Shen, J. Mater. Chem. 19 (2009) 5772.
[31] C. Guo, Y.H. Tang, E.L. Zhang, J. Mater. Sci. 20 (2009) 1118.
[32] S. Xing, Z. Zhou, Z. Ma, Y. Wu, Mater. Lett. 65 (2011) 517.
[33] R. Takahashi, S. Sato, T. Sodesawa, M. Kawakita, Chem. Lett. 10 (1999) 1107.
[34] W. Zhou, M. Yao, L. Guo, Y. Li, J. Li, S. Yang, J. Am. Chem. Soc. 131 (2009) 2959.
[35] Z. Hu, Y. Xie, Y. Wang, H. Wu, Y. Yang, Z. Zhang, Electrochim. Acta 54 (2009) 2737.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.materresbull.2012.
06.014.