From three-dimensional flower-like α-Ni(OH)2 nanostructures to hierarchical porous NiO nanoflowers: Microwave-assisted fabrication and supercapacitor properties

Article abstract of DOI:10.1111/j.1551-2916.2010.04090.x

A facile method is presented to prepare flower-like α-Ni(OH) ₂ involving only microwave heating of nickel nitrate hydrate in ethanol solvent without any hard/soft templates or precipitate-controlling agents. After calcination procedure, hierarchical and porous NiO nanoflowers with large BET-specific surface areas and controlled pore-size distributions are readily fabricated. A possible formation mechanism is proposed based on self-assembly under microwave heating. The structure, morphology, and crystalline phase of the products are characterized by transmission electron microscopy (TEM), X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, and N₂ adsorption/desorption isotherm analysis. TEM reveals that the obtained NiO nanoflowers have uniform size and are composed of sheet-like petals that are built up from nanocrystals. The resultant NiO nanostructures present a specific capacitance as high as 277 F/g and good cycling stability.

Full text of DOI:10.1111/j.1551-2916.2010.04090.x

J. Am. Ceram. Soc., 93 [11] 3560–3564 (2010)
DOI: 10.1111/j.1551-2916.2010.04090.x
r 2010 The American Ceramic Society
ournal
J
From Three-Dimensional Flower-Like a-Ni(OH)2 Nanostructures
to Hierarchical Porous NiO Nanoflowers: Microwave-Assisted
Fabrication and Supercapacitor Properties
Yang Ren and Lian Gao*,w
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,
Chinese Academy of Sciences, Shanghai 200050, China
A facile method is presented to prepare flower-like a-Ni(OH)2
involving only microwave heating of nickel nitrate hydrate in
ethanol solvent without any hard/soft templates or precipitate-
controlling agents. After calcination procedure, hierarchical and
porous NiO nanoflowers with large BET-specific surface areas
and controlled pore-size distributions are readily fabricated.
A possible formation mechanism is proposed based on self-
assembly under microwave heating. The structure, morphology,
and crystalline phase of the products are characterized by trans-
mission electron microscopy (TEM), X-ray diffraction, thermo-
gravimetric analysis, Fourier transform infrared spectroscopy,
and N2 adsorption/desorption isotherm analysis. TEM reveals
that the obtained NiO nanoflowers have uniform size and are
composed of sheet-like petals that are built up from nanocrys-
tals. The resultant NiO nanostructures present a specific capac-
itance as high as 277 F/g and good cycling stability.
tion, hydrothermal reaction, and the ‘‘chimie douce’’ tech-
nique.18–22 However, there has been no report on the facile
synthesis of controlled hierarchical a-Ni(OH)2 nanostructures
without any hard/soft templates or precipitate-controlling
agents. Herein, we report a simple route to prepare flower-like
a-Ni(OH)2 involving only one chemical in one solvent. By micro-
wave heating of nickel nitrate hydrate in ethanol solvent at 1501C
for 15 min, high-quality flower-like a-Ni(OH)2 can be obtained
via a self-assembly process. Then, followed with a tunable
calcination of a-Ni(OH)2, hierarchical and porous NiO nano-
structures with large BET-specific surface areas and controlled
pore-size distributions are readily fabricated. Furthermore, elect-
rochemical properties of obtained NiO nanostructures are tested
using cyclic voltammetry (CV) measurements. A high specific
capacitance of 277 F/g and good cycling stability can be achieved
due to the unique architecture. The present simple way for
a-Ni(OH)2 and NiO nanocrystals with controlled architectures
will promote their multifunctional applications and could be
versatile for the preparation of other metal hydroxides/oxides.
I. Introduction
ONTROLLING the architecture of inorganic nanocrystals has
C
been of great research interest due to their unique shape-
II. Experimental Procedures
and size-dependent properties.1,2 NiO material, as an important
functional oxide, has great potential applications as energy stor-
age device, catalysis, magnetic semiconductor, etc.3–5 Many
strategies for fabricating NiO nanocrystals have been reported,
including Ni–organic coordination complex intermediates,6,7
carbothermal reduction,8 Ni(OH)2 precursor (hard templates
or self-assembly approaches), etc.9–11 Among these strategies,
the controlling Ni(OH)2 architecture has attracted more atten-
tion not only because the flexibility in formation of complex 3D
architecture as the precursor of NiO nanostructures,12–16 but
also for their excellent electrochemical properties in recharge-
able alkaline battery applications.17
All chemicals were purchased from Shanghai Chemical Regent
Co. (Shanghai, China) with analytical grade and used without
further purification. Typically, 20 mmol Ni(NO3)2 ꢀ 6H2O was
dissolved in 30 mL anhydrous ethanol under magnetic stirring
to get a 0.67 mol/L green transparent solution. The as-formed
green transparent solution was then transferred into a 50 mL
Teflon container. The vessel containing precursor solution was
subjected to microwave heating maintained at 1501C for 15 min
(heating rate: 251C/min) and then cooled down naturally. Green
precipitates were generated and separated by centrifugation and
washed by ethanol several times, and then dried under vacuum
at 601C overnight. The as-prepared green precursors were cal-
cinated at 3001, 3501, and 4001C for 1 h (heating rate: 11C/min)
in air, respectively. The obtained gray black powders were cor-
respondingly noted as C3001, C3501, and C4001.
Ni(OH)2 has two polymorphs, a- and b-Ni(OH)2. Both of
them have a hexagonal-layered structure and tend to form thin
flakes or platelets in shape. a-Ni(OH)2 is a hydroxyl-deficient
phase with interlayer anions and water molecules rather than
stoichiometric composition in b-Ni(OH)2.17 For the distinguish-
ing structure feature, a-Ni(OH)2 show superior electrochemical
properties than b-Ni(OH)2, but hard to be prepared due to its
easy phase transformation to b-form.17 So far, there are several
methods to obtain a-Ni(OH)2, such as sonochemical precipita-
The microwave oven (2.45 GHz, maximum power 300 W)
used was a focused single-mode microwave synthesis system
(Discover, CEM, Matthews, NC). The phase composition of the
samples was characterized by X-ray powder diffraction (XRD)
˚
(Rigaku D/max 2550 V, Tokyo, Japan, CuKa, l 5 1.54178 A).
The morphologies of the products were observed using a trans-
mission electron microscope (TEM) (JEM200CX, JEOL, To-
kyo, Japan) and scanning electron microscopy (SEM) (JSM
6700F, Tokyo, Japan). A Fourier transform infrared (FT-IR)
(Nexus, Nicolet, Madison, WI) spectrometer was used to exam-
ine the chemical composition of the samples. TG analysis was
conducted using a NETZSCH STA 449C apparatus with an air
flow of 20 mL/min and a heating rate of 10 K/min. Nitrogen
adsorption/desorption isotherms were obtained at 77 K with
S. Suib—contributing editor
Manuscript No. 28215. Received June 21, 2010; approved July 20, 2010.
This work was financially supported by the National Key Basic Research Development
Program of China (973 Program, 2005CB623605), the Shanghai Nanotechnology Promo-
tion Center (No. 0852nm01900), and National Natural Science Foundation of China (Grant
No. 50972157, 50672112).
*Member, The American Ceramic Society.
wAuthor to whom correspondence should be addressed. e-mail: liangao@mail.sic.ac.cn
a Micromeritics ASAP 2010 micropore analysis system
3560

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Rapid Communications of the American Ceramic Society
3561
(Micrometitics, Norcross, GA). Specific surface areas were
obtained by the BET method, and average pore diameters
were determined using the Barrett–Joyner–Halenda method.
The electrochemical measurements were carried out using
three-electrode cell system with Pt wire as counter electrode and
saturated calomel electrode as reference electrode. The working
electrode consisted of NiO products, acetylene black conduc-
tors, and polyvinylidene fluoride in an 80:10:10 weight ratio.
The electrolyte used was 2M KOH aqueous solution. CV mea-
surements were carried out using electrochemical workstation
(Parstat 2273, Princeton Applied Research Co. Ltd., Oak Ridge,
TN) in the scan range of ꢁ0.2 to 0.5 V.
reveals its presence by a set of strong peaks in the range of 1503–
980 cm .
ꢁ1 19 Two peaks, which appear between 630–650 and 460–
490 cmꢁ1, correspond to d-OH and u-NiO vibrations, respec-
tively.20 From the TG analysis (the inset of Fig. 1(c)), the thermal
decomposition behavior is generally in good agreement with
previous reports of a-Ni(OH)2.20 The onset of phase transfor-
mation from obtained flower-like a-Ni(OH)2 to NiO is indicated
with the abrupt weight loss at around 2701C.
The XRD patterns of calcination samples, namely C3001,
C3501, and C4001, are shown in Fig. 2(a). After calcination at
3001C for 1 h, cubic phase NiO can be well indexed (JCPDS 47-
1049). Nevertheless, the appearance of an unknown amorphous
peak, located at 10–201, indicates that a trace amount of impu-
rity still exists after calcination at 3001C for 1 h. While the ca-
lcination temperature is higher than 3001C (C3501 and C4001 in
our case), the impurity disappears and pure cubic phase NiO
samples can be obtained. The average crystalline sizes of sam-
ples C3001, C3501, and C4001 calculated from (200) diffraction
peak by Scherrer’s formula are 9.2, 10.5, and 18.4 nm, respec-
tively. The above results suggest that the phase purity and par-
ticle size of NiO nanostructures can be readily controlled by
calcination procedures. The morphology of calcination sample
C3501 is showed in Fig. 2(b). The shape of calcination particles
well inherits the flower-like architecture of the a-Ni(OH)2 pre-
cursor. However, the surface texture of NiO nanoflowers, as
exhibited in Fig. 2(c), seems to be rough and grainy, which is
quite different from that of a-Ni(OH)2 precursor. SAED pattern
(inset in Fig. 2(c)) in the circle region of a petal manifests the
polycrystalline nature of NiO nanoflowers. The high-magnifica-
tion TEM image (Fig. 2(d)) reveals that the petals are made of
well-defined nanoparticles of ca. 10 nm in size (agree with the
calculated results from Scherrer’s formula), and pores nearly
homogeneously distributed among these nanoparticles. A rep-
resentative high-resolution TEM image is given to examine the
III. Results and Discussion
The morphologies of as-prepared green powder are exhibited in
Figs. 1(a)–(b). SEM image (Fig. 1(a)) clearly shows that the as-
prepared precursor is composed of well-defined particles with
flower-like shape. The particles are uniform in size of ca. 1 mm.
The inset of Fig. 1(a) exhibits that the flower-like architectures
are assembled by several interconnected sheet-like petals with
smooth surface and the thickness ca. 20 nm. The XRD pattern of
the precursor (Fig. 1(c)) exhibits typical features of a-Ni(OH)2
evidenced by the strongest reflection at the low 2y of 12.71. All
of the reflections are matched well with the standard pattern
for nickel nitrate hydroxide (JCPDS 22-0752), indicating the ob-
tained sample can be defined as nitrated a-Ni(OH)2. The
lattice parameters, refined with the MDI Jade 5.0 program (re-
liability factors: R 56.59%), are a 5 0.312 nm and c 5 0.692 nm.
The FT-IR spectrum (Fig. 1(d)) further verifies the chemical
species of a-Ni(OH)2 precursor. The broad bonds at (3580–3400
cmꢁ1) indicate the presence of hydroxyl groups and water mol-
ecules.20 The peak around 1600–1640 cmꢁ1 is due to d-H2O
vibration of the water molecule.20 The interlayer nitrate anion
Fig. 1. (a) Scanning electron microscopic (SEM) image (the inset shows the close-up SEM image of an individual particle), (b) transmission electron
microscopic image, (c) X-ray diffraction pattern (the inset shows the thermogravimetric curve of a-Ni(OH)2 precursor), and (d) Fourier transform
infrared spectrum of the obtained flower-like a-Ni(OH)2 precursor.

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Fig. 2. (a) X-ray diffraction patterns of NiO products of C3001, C3501, and C4001, respectively, (b) scanning electron microscopic image, (c) low-
magnification transmission electron microscopic (TEM) image (the inset shows the SAED pattern of circle region), (d) high-magnification TEM image,
and (e) high-resolution TEM image of C3501 product.
interplanar distance of such nanoparticles. The lattice fringes
with a spacing of 0.206 and 0.241 nm are well accord with the d-
spacing of (111) and (200) plane of cubic phase NiO, and the
dihedral angle of 54.71 further confirms that the resultant ar-
chitectures are composed of NiO nanocrystals.
heating and generates OHꢁ. Then, the released OHꢁ further
reacts with Ni21 and NO3ꢁ to create a-Ni(OH)2 nuclei.26 A large
amount of nuclei burst and further grow into particles in larger
sizes. At this stage, 2D nanosheets are readily to be formed as
dominant morphology because of the layered structure feature
of a-Ni(OH)2 and proper monomer concentration. Subse-
quently, the produced primary nanosheets could assemble
into 3D flower-like architecture due to crystal-face attraction,
van der waals forces and hydrogen bonds.27,28 Simultaneously,
the particles could also heterogeneously nucleate and grow onto
the petals of flower-like structure because of the continuous
mass supply. Significantly, microwave heating helps to more
quickly chemical reactions and facilitates the diffusion of
the mass, which favors the stable growth of an flower-like
a-Ni(OH)2 with nanosheets as building blocks.22 Finally, after
calcination, the flower-like a-Ni(OH)2 transfer into highly
porous NiO nanostructures with maintained morphology by
the removal of interlayer anions.
Figure 4 shows the CV curves of pure NiO samples of C3501
and C4001. A couple of redox peaks in each curve clearly
testifies the faradaic pseudocapacitive behavior of both NiO
electrodes.29 The specific capacitances (Cs) of C3501 and C4001
calculated from the CV curves according to the reported equa-
tion30 are 277 and 182 F/g, respectively. Compared with the
reported Cs value of NiO nanostructures prepared by molten
salt, chemical precipitation, organic complex templates, oxalates
precursor approaches, etc.,7,29,31–34 the obtained sample C3501
exhibits improved capacitance property. This notable improve-
ment should be associated with the obtained hierarchical and
porous nanostructures. The high BET surface area and appro-
priate pore channel are beneficial for the accessibility of elec-
trolyte OHꢁ to the electrode active surface and a fast diffusion
rate within the redox phase, which results in high specific
capacitances.7 Besides, sample C3501 with large BET area
and pore size at 3.6 nm obtained under lower calcination
Figure 3(a) shows the nitrogen adsorption/desorption
isotherms of pure-phase NiO samples C3501 and C4001. The
hysteresis loop, appearing in the relative pressure (p/po) range
of 0.45–1, indicates that the pore sizes in both NiO products are of
mesoscale.23 The specific surface area of the sample C3501 is 197
m2/g and a narrow pore-size distribution can be clearly observed
with peak size at ca. 3.6 nm (Fig. 3(b)). On the other hand, as the
annealing temperature increases to 4001C, the specific surface area
of sample C4001 decreases to 97.2 m2/g and the peak size of pore
distribution shifts to ca. 7.2 nm. This phenomenon should be at-
tributed that the primary nanoparticles grow larger via thermal
recrystallination of Ostwald-ripening mechanism. During this
process, intercrystallited mesopores evolve into larger sized pores
as the primary nanoparticles grow.24,25
To explain the formation of hierarchical and porous NiO
nanostructures, a possible formation mechanism based on ex-
perimental results is proposed. The thermodynamic process
could be formulated as follows26:
4 CH3CH2OH ðlÞ þ NOꢁ3 ðlÞ ¼4CH3CHO ðlÞ þ NH3 ðgÞ
(1)
þ OHꢁ ðlÞ þ 2H2O ðlÞ
Ni2þ ðlÞ þ ð2 ꢁ xÞ OHꢁ ðlÞ þ xNOꢁ3 ðlÞ
(2)
! NiðOHÞ2ꢁxðNO3Þx ðsÞ
With microwave heating, the precursor solution goes through
nucleation, aggregation, and ripening process. Namely, at the
beginning, NOꢁ3 is quickly reduced by ethanol under microwave

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Fig. 5. Cyclic voltammetry curves of the C3501 obtained at different
scan rates (the inset shows the variation of the specific capacitance with
cycle number of C3501 at scan rate of 2.5 mV/s).
tances are as high as 265, 220, and 188 F/g with scan rate at 5,
10, and 20 mV/s, respectively. The inset of Fig. 5 shows the
variation of the specific capacitance with cycle number at scan
rate of 2.5 mV/s. After 100 cycles, a high specific capacitance of
271 F/g can be stabilized. This good cycling quality suggests that
the as-obtained porous NiO nanoflowers have potential appli-
cation in electrochemical supercapacitors.
IV. Conclusions
In summary, we report a facile method to prepare high-quality
flower-like a-Ni(OH)2 via self-assembly achieved by microwave
heating of nickel nitrate hydrate in ethanol solvent at 1501C for
15 min. After calcination procedure, hierarchical and porous
NiO nanocrystals with large specific surface areas of 197 m2/g
and narrow pore distributions of 3.6 nm are readily fabricated.
Electrochemical tests show that a high specific capacitance of
277 F/g and good cycling stability can be achieved due to the
resultant NiO nanostructures. The present simple route for
novel a-Ni(OH)2 and NiO nanostructures should be extendable
for the preparation of other metal hydroxide/oxide.
Fig. 3. (a) N2 adsorption–desorption isotherms, and (b) Barrett–Joy-
ner–Halenda desorption size-distribution plots of C3501 and C4001.
temperature possesses higher specific capacitance than that of
C4001. This phenomenon indicates the increased crystal size,
caused by calcination at higher temperature, leads to the de-
crease of specific surface area, accordingly reducing the capac-
itance properties.30 Figure 5 gives the CV curves of sample
C3501 working at different scanning rates. The shape of the CV
curves well maintains the characteristic of psedocapacitance re-
dox course, indicating the prepared NiO electrode can work well
at increased scanning rate. The corresponding specific capaci-
References
1Y. Cui and C. M. Lieber, ‘‘Functional Nanoscale Electronic Devices Assem-
bled Using Silicon Nanowire Building Blocks,’’ Science, 291, 851–3 (2001).
2A. H. Lu and F. Schuth, ‘‘Nanocasting: A Versatile Strategy for Creating
Nanostructured Porous Materials,’’ Adv. Mater., 18, 1793–805 (2006).
3M. Yoshio, Y. Todorov, K. Yamato, H. Noguchi, J. Itoh, O. Masaki, and
T. Mouri, ‘‘Preparation of LiyMnxNi1ꢁxO2 as
a Cathode for Lithium-Ion
Batteries,’’ J. Power Sources, 74, 46–53 (1998).
4R. Alcantara, P. Lavela, J. I. Tirado, R. Stoyanova, and E. Zhecheva,
‘‘Changes in Structure and Cathode Performance with Composition and Prepa-
ration Temperature of Lithium Cobalt Nickel Oxide,’’ J. Electrochem. Soc., 145,
730–6 (1998).
5E. L. Miller and R. E. Rocheleau, ‘‘Electrochemical Behavior of Reactively
Sputtered Iron-Doped Nickel Oxide,’’ J. Electrochem. Soc., 144, 3072–7 (1997).
6X. F. Song and L. Gao, ‘‘Facile Synthesis of Polycrystalline NiO Nanorods
Assisted by Microwave Heating,’’ J. Am. Ceram. Soc., 91, 3465–8 (2008).
7C. C. Yu, L. X. Zhang, J. L. Shi, J. J. Zhao, J. H. Gao, and D. S. Yan,
‘‘A Simple Template-Free Strategy to Synthesize Nanoporous Manganese and
Nickel Oxides with Narrow Pore Size Distribution, and their Electrochemical
Properties,’’ Adv. Funct. Mater., 18, 1544–54 (2008).
8X. Wang, L. Yu, P. Hu, and F. Yuan, ‘‘Synthesis of Single-Crystalline Hollow
Octahedral NiO,’’ Cryst. Growth Des., 7, 2415–8 (2007).
9S. A. Needham, G. X. Wang, and H. K. Liu, ‘‘Synthesis of NiO Nanotubes for
Use as Negative Electrodes in Lithium ion Batteries,’’ J. Power Sources, 159, 254–7
(2006).
10D. B. Wang, C. X. Song, Z. S. Hu, and X. Fu, ‘‘Fabrication of Hollow Spheres
and Thin Films of Nickel Hydroxide and Nickel Oxide with Hierarchical
Structures,’’ J. Phys. Chem. B, 109, 1125–9 (2005).
Fig. 4. Cyclic voltammetry curves of C3501 and C4001 products.

3564
Rapid Communications of the American Ceramic Society
Vol. 93, No. 11
11X. F. Song and L. Gao, ‘‘Facile Synthesis and Hierarchical Assembly of Hol-
Activity Synthesized by Microwave-Assisted Hydrothermal Method,’’ Chem. Mater.,
20, 308–16 (2008).
low Nickel Oxide Architectures Bearing Enhanced Photocatalytic Properties,’’
J. Phys. Chem. C, 112, 15299–305 (2008).
23J. H. Huang and L. Gao, ‘‘Synthesis and Characterization of Porous Single-
Crystal-Like In2O3 Nanostructures via a Solvothermal–Annealing Route,’’
12W. Zhou, M. Yao, L. Guo, Y. M. Li, J. H. Li, and S. H. Yang, ‘‘Hydrazine-
Linked Convergent Self-Assembly of Sophisticated Concave Polyhedrons of beta-
Ni(OH)(2) and NiO from Nanoplate Building Blocks,’’ J. Am. Chem. Soc., 131,
2959–64 (2009).
J. Am. Ceram. Soc., 89, 724–7 (2006).
24H. C. Zeng, ‘‘Synthetic Architecture of Interior Space for Inorganic
Nanostructures,’’ J. Mater. Chem., 16, 649–62 (2006).
13J. J. Wang, H. Pang, J. Z. Yin, L. N. Guan, Q. Y. Lu, and F. Gao, ‘‘Con-
trolled Fabrication and Property Studies of Nickel Hydroxide and Nickel Oxide
Nanostructures,’’ Crystengcomm, 12, 1404–9 (2010).
25X. W. Lou, D. Deng, J. Y. Lee, and L. A. Archer, ‘‘Thermal Formation
of Mesoporous Single-Crystal Co3O4 Nano-Needles and their Lithium Storage
Properties,’’ J. Mater. Chem., 18, 4397–401 (2008).
14L. Liu, Y. Li, S. M. Yuan, M. Ge, M. M. Ren, C. S. Sun, and Z. Zhou,
‘‘Nanosheet-Based NiO Microspheres: Controlled Solvothermal Synthesis and
Lithium Storage Performances,’’ J. Phys. Chem. C, 114, 251–5 (2010).
15J. X. Zhu, Z. Gui, Y. Y. Ding, Z. Z. Wang, Y. Hu, and M. Q. Zou, ‘‘A Facile
Route to Oriented Nickel Hydroxide Nanocolumns and Porous Nickel Oxide,’’
J. Phys. Chem. C, 111, 5622–7 (2007).
26C. L. Tang, G. S. Li, and L. P. Li, ‘‘Fabrication of Nickel Nitrate Hydroxide
Microsphere with Inheriting Morphology to beta-Ni(OH)2 and NiO,’’ Chem.
Lett., 37, 1138–9 (2008).
27L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song, and L. J. Wan,
‘‘Self-Assembled 3D Flowerlike Iron Oxide Nanostructures and their Application
in Water Treatment,’’ Adv. Mater., 18, 2426–31 (2006).
16Y. Wang, Q. S. Zhu, and H. G. Zhang, ‘‘Fabrication of Beta-Ni(OH)(2)
and NiO Hollow Spheres by a Facile Template-Free Process,’’ Chem. Commun.,
41, 5231–3 (2005).
28S. Y. Zeng, K. B. Tang, T. W. Li, Z. H. Liang, D. Wang, Y. K. Wang, Y. X.
Qi, and W. W. Zhou, ‘‘Facile Route for the Fabrication of Porous Hematite
Nanoflowers: Its Synthesis, Growth Mechanism, Application in the Lithium ion
Battery, and Magnetic and Photocatalytic Properties,’’ J. Phys. Chem. C, 112,
4836–43 (2008).
17B. Mavis and M. Akinc, ‘‘Three-Component Layer Double Hydroxides by
Urea Precipitation: Structural Stability and Electrochemistry,’’ J. Power Sources,
134, 308–17 (2004).
29Y. G. Wang and Y. Y. Xia, ‘‘Electrochemical Capacitance Characterization of
NiO with Ordered Mesoporous Structure Synthesized by Template SBA-15,’’
Electrochim. Acta, 51, 3223–7 (2006).
18H. J. Liu, T. Y. Peng, D. E. Zhao, K. Dai, and Z. H. Peng, ‘‘Fabrication of
Nickel Oxide Nanotubules by Anionic Surfactant-Mediated Templating Method,’’
Mater. Chem. Phys., 87, 81–6 (2004).
30F. B. Zhang, Y. K. Zhou, and H. L. Li, ‘‘Nanocrystalline NiO as an Electrode
Material for Electrochemical Capacitor,’’ Mater. Chem. Phys., 83, 260–4 (2004).
31Y. Z. Zheng and M. L. Zhang, ‘‘Preparation and Electrochemical Properties
of Nickel Oxide by Molten-Salt Synthesis,’’ Mater. Lett., 61, 3967–9 (2007).
32W. Xing, F. Li, Z. F. Yan, and G. Q. Lu, ‘‘Synthesis and Electrochemical
Properties of Mesoporous Nickel Oxide,’’ J. Power Sources, 134, 324–30 (2004).
33Y. Z. Zheng, H. Y. Ding, and M. L. Zhang, ‘‘Preparation and Electrochemical
Properties of Nickel Oxide as a Supercapacitor Electrode Material,’’ Mater. Res.
Bull., 44, 403–7 (2009).
19B. H. Liu, S. H. Yu, S. F. Chen, and C. Y. Wu, ‘‘Hexamethylenetetramine
Directed Synthesis and Properties of a New Family of Alpha-Nickel Hydroxide
Organic–Inorganic Hybrid Materials with High Chemical Stability,’’ J. Phys.
Chem. B, 110, 4039–46 (2006).
20M. Taibi, S. Ammar, N. Jouini, F. Fievet, P. Molinie, and M. Drillon, ‘‘Lay-
ered Nickel Hydroxide Salts: Synthesis, Characterization and Magnetic Behaviour
in Relation to the Basal Spacing,’’ J. Mater. Chem., 12, 3238–44 (2002).
21P. Jeevanandam, Y. Koltypin, and A. Gedanken, ‘‘Synthesis of Nanosized
a-Nickel Hydroxide by a Sonochemical Method,’’ Nano Lett., 1, 263–6 (2001).
22L. P. Xu, Y. S. Ding, C. H. Chen, L. L. Zhao, C. Rimkus, R. Joesten, and
S. L. Suib, ‘‘3D Flowerlike Alpha-Nickel Hydroxide with Enhanced Electrochemical
34W. B. Ni, D. C. Wang, Z. Z. Zhao, Z. J. Huang, and J. W. Zhao, ‘‘A Novel
Synthesis of Mesoporous Nickel Oxide with Excellent Cycling Stability Used in
Supercapacitors,’’ Chem. Lett., 39, 98–9 (2010).
&