A R T I C L E S
Zhou et al.
nickel hydroxide.16,17 So far, nickel hydroxide with various
morphologies has been synthesized including nanoplates,18
hollow spheres,19 ribbonlike and boardlike structures,20 flower-
like structures,10,21 and tubes.17 However, three-dimensional
organizations of higher-level structures of nickel hydroxide so
far have not been reported.
Our work presented here creates a brand-new hierarchical
structure in a bottom-up fashion. Benefiting from the inherent
spatial asymmetry of Ni(OH)2 and the hydrogen bonds linked
with hydrazine molecules, hierarchical concave polyhedron
achieves its intelligent growth with the 60° rotation between
two parts of quasi-triangular-pyramids. This simple method for
building up structures from nanoscale building blocks with the
assistance of molecular linkage may be utilized for fabricating
a wide variety of nanoarchitectures.
Figure 1. XRD pattern of the as-prepared sample.
Experimental Section
All chemicals were of analytical grade and used without further
purification. In a typical experiment, 0.128 g of nickel(II) 2,4-
pentanedionate (NiC10H14O4, 95%, Alfa Aesar) and 0.666 g of
polyvinyl pyrrolidone (PVP, average MW 58000, K29-32, Acros)
were first dissolved in 100 mL of deionized water at room
temperature. Then the solution was heated to 75 °C in an oil bath
under magnetic stirring. Afterward, 3 mL of hydrazine monohydrate
liquid (N2H4 ·H2O, 80%) was added into the mixture dropwise. In
30 min, 1.2 mmol sodium hydroxide solution (in 10 mL deionized
water) was added. The reaction was maintained for 2 h at 75 °C
(the precursor was obtained), and then 5 mL of N2H4 ·H2O was
introduced. Then the solution was kept still for another 2 h.
Afterward, the green precipitate was obtained and rinsed with
ethanol and deionized water several times.
Figure 2. SEM images of the as-grown ꢀ-Ni(OH)2. (a) Overhead view
and (b) top and side views of typical polyhedrons. Each polyhedron is
composed of two staggered truncated triangular pyramids.
The nickel electrodes were prepared as follows. The nickel
hydroxide powder was mixed with black carbon and carboxymethyl
cellulose as a binder in a weight ratio of 85:10:5. And then the
mixture was incorporated into a nickel foam (1.2 × 1.2 cm) with
spatula. The pasted nickel electrodes were dried at 80 °C and then
pressed at a pressure of 5 MPa for 1 min. The cyclic voltammetry
(CV) measurements were carried out on a Versa STAT 3
electrochemical workstation (Princeton Applied Research, USA).
The cyclic voltammetry scan rate was 1 mV/s within a potential
range from 0.1 to 0.64 V (vs Hg/HgO) at room temperature. The
experiments were carried out in a three-electrode glass cell in which
contained the nickel electrode as the working electrode, Pt foil as
the counter electrode, Hg/HgO as the reference electrode, and 6 M
KOH aqueous solution as the electrolyte.
The as-prepared nickel hydroxide was transformed to nickel
oxide by thermal decomposition in nitrogen at 500 °C for 1 h.
The structures and compositions of the as-prepared products were
characterized by X-ray powder diffraction (XRD) using a Rigaku
Dmax2200 X-ray diffractometer with Cu KR radiation (λ ) 1.5416
Å). The XRD specimens were prepared by means of flattening the
powder on the small slides. The morphologies of the synthesized
samples were studied by a field-emission gun (FEG) scanning
electron microscope (Hitachi S-4800, 5 kV) with the samples
obtained from the thick suspension dropping on the silicon slice.
Transmission electron microscopy (TEM) and high-resolution TEM
(HRTEM) investigations were carried out by a JEOL JEM-2100F
microscope. The as-grown samples were dispersed in ethanol and
dropped onto a carbon film supported on a copper grid for the drying
process in air. Fourier transform infrared (FTIR) spectrum was
recorded for a KBr diluted sample using a Nicolet Avatar 360 FT-
IR spectrometer.
Results and Discussion
Figure 1 presents the powder XRD pattern of the as-prepared
products. All the diffraction peaks can be indexed as ꢀ-phase
hexagonal nickel hydroxide (JCPDS 14-0117). No peaks due
to R-Ni(OH)2 were observed in the XRD pattern, which confirms
the purity of the samples.
(16) (a) Che, G.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998,
393, 346–349. (b) Han, X.; Xie, X.; Xu, C.; Zhou, D.; Ma, Y. Opt.
Mater. 2003, 23, 465–470.
Typical SEM images of the nickel hydroxide are shown in
Figure 2. Figure 2a is an overview of the samples with the
concave polyhedron morphology. A peculiar morphology of
elongated concave polyhedrons is evident with a length from
300 to 500 nm and with side lengths of the top triangles from
300 to 500 nm. A magnified image shown in Figure 2b provides
top and side views of the structures. Obviously, the ꢀ-Ni(OH)2
concave polyhedrons are composed of two staggered triangular
pyramids truncated on the joined planes and rotated by an angle
of 60° from each other. Each polyhedron has two regular
triangles at the ends and six quasi-triangles at the sides. To join
the two truncated pyramids together, the side quasi-triangles
tend to develop curved surfaces instead of flat surfaces. Hence
the novel nanostructure looks like a regularly twisted triangular
prism wringed smoothly in the middle.
(17) Cai, F.-S.; Zhang, G.-Y.; Chen, J.; Gou, X.-L.; Liu, H.-K.; Dou, S.-X.
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Y. Mater. Chem. Phys. 2007, 106, 375–378. (c) Li, Y.; Tan, B.; Wu,
Y. Chem. Mater. 2008, 20, 567–576. (d) Zhu, J.; Gui, Z.; Ding, Y.;
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1125–1129. (b) Duan, G.; Cai, W.; Luo, Y.; Sun, F. AdV. Funct. Mater.
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2960 J. AM. CHEM. SOC. VOL. 131, NO. 8, 2009