Hydrothermal synthesis of nickel hydroxide nanostructures in mixed solvents of water and alcohol

Article abstract of DOI:10.1016/j.jssc.2007.05.009

Nickel hydroxide nanosheets and flowers have been hydrothermally synthesized using Ni(CH₃COO)₂·4H₂O in mixed solvents of ethylene glycol (EG) or ethanol and deionized water at 200 °C for different time. The phase and morph

Full text of DOI:10.1016/j.jssc.2007.05.009

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Journal of Solid State Chemistry 180 (2007) 2095–2101
www.elsevier.com/locate/jssc
Hydrothermal synthesis of nickel hydroxide nanostructures in mixed
solvents of water and alcohol
a,b
Li-Xia Yang , Ying-Jie Zhu
a,b,
Ã
a,b a,b
, Hua Tong , Zhen-Hua Liang ,
a,b
Liang Li , Ling Zhang
a,b
a
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,
Chinese Academy of Sciences, Shanghai 200050, PR China
b
Graduate School of Chinese Academy of Sciences, PR China
Received 18 December 2006; received in revised form 23 April 2007; accepted 3 May 2007
Available online 18 May 2007
Abstract
Nickel hydroxide nanosheets and flowers have been hydrothermally synthesized using Ni(CH COO) ꢀ 4H O in mixed solvents of
3
2
2
ethylene glycol (EG) or ethanol and deionized water at 200 1C for different time. The phase and morphology of the obtained products
can be controlled by adjusting the experimental parameters, including the hydrothermal time and the volume ratio of water to EG or
ethanol. The possible reaction mechanism and growth of the nanosheets and nanoflowers are discussed based on the experimental results.
Porous nickel oxide nanosheets are obtained by heating nickel hydroxide nanosheets in air at 400 1C. The products were characterized by
using various methods including X-ray diffraction (XRD), fourier transform infrared (FTIR), transmission electron microscopy (TEM),
selected-area electron diffraction (SAED), field emission scanning electron microscopy (FESEM). The electrochemical property
of b-Ni(OH) nanosheets was investigated through the cyclic voltammogram (CV) measurement.
2
r 2007 Elsevier Inc. All rights reserved.
Keywords: Nickel hydroxide; Nickel oxide; Nanostructures; Hydrothermal
1
. Introduction
As an active material of the positive electrode of alkaline
rechargeable batteries, the performance of such batteries
highly depends on the size, morphology and phase of
Nickel hydroxide (Ni(OH) ) is widely used in many
2
applications from power tools to portable electronics and
electric vehicles. There are two polymorphs of Ni(OH)2,
which are designated as a-Ni(OH)2 and b-Ni(OH)2,
respectively. The a-Ni(OH)2 phase displays a more
Ni(OH) . Considerable work has been focused on the
2
morphology-controllable preparation of Ni(OH)2 and
related composite materials [5–8]. Recently, Matsui et al.
reported the synthesis of single-crystalline Ni(OH) nanor-
2
˚
disorderly and larger interlayer spacing (47.5 A) as the
ods by the hydrothermal reaction in the cavity of carbon
nanotubes prepared in anodic aluminum oxide template
[9]. Ni(OH) tubes were prepared by a template method
interlamellar space contains anions, e.g., nitrate, carbo-
nate, sulfate and water molecules. The hexagonal b-
Ni(OH) phase is brucite-like and well-oriented Ni(OH)
2
and tested as the positive electrode material of alkaline
rechargeable batteries [10,11]. Liang et al. prepared single-
crystalline Ni(OH) nanosheets using Ni(CH COO) and
2
2
layers are perfectly stacked along the c-axis with an
˚
interlamellar distance of 4.60 A [1–4].
2
3
2
aqueous ammonia [12]. Ribbon- and board-like Ni(OH)2
were also reported and the board-like nanostructures were
helpful in improving the electrochemical performance
Ã
Corresponding author. State Key Laboratory of High Performance
Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,
Chinese Academy of Sciences, Shanghai 200050, PR China.
Fax: +86 21 52413122.
[13,14]. Ni(OH)2 hollow spheres [15,16] and stacks of
pancakes [1] were also reported. These preparation
E-mail address: y.j.zhu@mail.sic.ac.cn (Y.-J. Zhu).
methods usually involve the precipitation of Ni(OH) by
2
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.05.009

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L.-X. Yang et al. / Journal of Solid State Chemistry 180 (2007) 2095–2101
2096
employing an alkaline reagent, such as urea, ammonia and
NaOH [9–19]. In some methods the template is needed. It is
highly desirable to explore simple and low-cost methods for
X-ray diffractometer with a high-intensity Cu Ka radiation
(l ¼ 1.54178 A) and a graphite monochromator. Fourier
˚
transform infrared (FTIR) spectra were obtained with a
spectrophotometer (Thermo Niocolet, NEXUS). The
morphology was studied by a JEOL JSM-6700F field
emission scanning electron microscopy (FESEM). The
transmission electron microscopy (TEM) micrographs, and
selected-area electron diffraction (SAED) patterns were
taken on a JEOL JEM-2100F field emission transmission
electron microscope with an accelerating voltage of 200 kV.
The cyclic voltammetric measurements were performed in a
three-compartment electrolysis cell at 25 1C using platinum
powder microelectrode with the diameter of 100 mm as the
working electrode.
the large-scale production of Ni(OH) nanostructures cater-
2
ing to the large demand of this kind of battery material.
Nickel oxide (NiO) has many applications in various
fields, such as catalysis [20], gas sensors [21], battery
cathode [22], magnetic materials [23,24] and fuel cell
electrodes [25]. Although NiO nanoparticles [26], nano-
rods/nanowires/nanofibers [27–29], nanosheets [12], nano-
rings [30], hollow spheres [15,16,31] and worm-like
nanoporous NiO [32] were reported, the synthesis of
nanoporous nanosheets of NiO is still a big challenge to
chemists with an aim to improve the specific surface area
and enhance the corresponding performances.
Herein, we report a facile strategy to the synthesis of
Ni(OH) nanosheets and nanoflowers through the hydrolysis
3. Results and discussion
2
of nickel acetate in the mixed solvents of water and ethylene
glycol (EG) or ethanol, which may be scaled up to synthesize
Ni(OH) without the use of the template and other additives.
3.1. Phase evolution
2
The crystal phase and structure information on the
products were obtained by XRD measurements. Fig. 1
shows the XRD patterns of samples prepared using nickel
Both a-Ni(OH) and b-Ni(OH) can be produced by simply
2
2
adjusting the hydrothermal time. The morphology transfor-
mation process from a-Ni(OH) to b-Ni(OH) in the solution
2
2
acetate in 1 mL EG and 23 mL H O at 200 1C for different
2
were systematically investigated. Moreover, NiO nanoporous
nanosheets were successfully synthesized by thermal decom-
hydrothermal time. The XRD pattern of the product
prepared at 200 1C for 1 h (sample A1, Fig. 1a) shows a
mixture of typical layered products. One of the phases is
indexed to the (0 0 3), (0 0 6), (0 0 9), (1 0 1) and (1 1 0)
position of the as-synthesized Ni(OH) at 400 1C in air.
2
2
. Experimental section
planes of a-Ni(OH) with well-defined peaks at 0.9421,
2
0.4731, 0.3169, 0.2690 and 0.1552 nm, and the other has a
2
.1. Synthesis procedure of the samples
larger d spacing (1.718 nm) indicated as an asterisk. When
the hydrothermal time was prolonged to 1.5 h (sample A2),
a predominant well-crystalline brucite-like phase of b-
In a typical experiment, 0.625 g Ni(CH COO) ꢀ 4H O
3
2
2
was dissolved in 24 mL mixed solvents of water and EG (or
ethanol) with magnetic stirring to form a homogeneous
solution, then was transferred into a Teflon-lined stainless
steel autoclave (40 mL capacity), sealed and maintained at
Ni(OH) with a hexagonal structure (JCPDS No.74-2075)
was formed (Fig. 1b). However, a-Ni(OH) as a minor
2
2
phase with d spacings of 1.512, and 0.7508 nm (labeled as
#) was also observed. A single phase of well-crystallized
brucite-like b-Ni(OH)2 formed when the hydrothermal
time was 3.5 h (sample A3, Fig. 1c). No peaks from the
200 1C for a fixed time (0.5–10.5 h). The samples were
designated as A1, A2, A3 and A4 when the volume ratio of
EG to H O was 1:23 for 1, 1.5, 3.5 and 10.5 h, respectively;
2
samples B1 and B2 when the volume ratio of EG to H O
2
was 1:1 for 1.5 and 3.5 h, respectively; samples C1, C2 and
C3 when EG was used as the only solvent (24 mL) for 3.5,
5
when the volume ratio of ethanol to H O was 1:1 for 1, 1.5,
.5 and 10.5 h, respectively; samples D1, D2, D3 and D4,
2
3
.5 and 10.5 h, respectively. After the autoclave was cooled
to room temperature naturally, the product was separated
by centrifugation, washed with deionized water and
ethanol and finally dried at 60 1C in air. For the
preparation of NiO, the obtained Ni(OH) was calcined
in a tubal oven at 400 1C in air for 2 h. The calcined
samples were labeled as samples E1 and E2 by heating the
2
corresponding Ni(OH) samples A4 and D4, respectively.
2
2
.2. Characterization of the samples
Fig. 1. XRD patterns of samples hydrothermally prepared using
The as-prepared samples were characterized by X-ray
powder diffraction (XRD) with a Rigaku D/max 2550 V
Ni(CH COO)
3
2
ꢀ 4H
2 2
O in 1 mL EG and 23 mL H O at 200 1C for different
time. (a) sample A1; (b) sample A2; (c) sample A3.

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2097
precursor or intermediates were observed. The expansion
of the c parameter of the unit cell of Ni(OH) was observed
2
owing to the intercalation of anions. Further evidence
for the presence of interlamellar anions and water
molecules comes from the FTIR studies and will be
discussed later.
Fig. 2 shows the FTIR spectra of different samples.
Fig. 2a shows a typical FTIR spectrum of a-Ni(OH) . The
2
ꢁ
1
broad band at around 3429 cm is characteristic of the
stretching vibration of hydroxyl groups hydrogen-bonded
to H O. The band at about 656 cm is due to the dNi–O–H
ꢁ
1
2
vibration [33–36]. The bands observed in the range of 1700-
ꢁ
1
000 cm are attributed to the presence of acetate ions
1
[
37]. Fig. 2c shows a typical FTIR spectrum of b-Ni(OH) .
2
ꢁ
1
Fig. 3. XRD patterns of samples hydrothermally prepared using
A narrow and sharp peak at 3641 cm is owing to the
uO–H stretching vibration, which confirms the brucite
structure of b-Ni(OH) phase. Peaks at 3444 and
Ni(CH COO)
3
2
2 2
ꢀ 4H O in mixed solvents of EG and H O (volume
ratio ¼ 1:1) at 200 1C for different time: (a) sample B1; (b) sample B2.
2
ꢁ
1
1
628 cm are assigned to the u(H O) stretching vibration
2
and d(H O) bending vibration of water molecules adsorbed
2
ꢁ
1
in the product. The strong band at 513 cm corresponds
to the dO–H of hydroxyl groups. A small weak peak
ꢁ
1
centered around 467 cm
stretching mode [33–35]. Fig. 2b shows mixed character-
is assigned to the Ni–O
istics of a-Ni(OH) and b-Ni(OH) , in which the bands at
2
2
ꢁ
1
about 656 and 513 cm are characteristic of a-Ni(OH)2
and b-Ni(OH) , respectively, consistent with the XRD
2
result of Fig. 1b.
When increasing the volume ratio of EG to water in the
mixed solvents to 1:1 and hydrothermally treated for 1.5 h
(
sample B1), the XRD peaks can be indexed to a single
phase of a-Ni(OH) , as shown in Fig. 3a. However, when
2
the hydrothermal time was increased to 3.5 h, a single
phase of b-Ni(OH) was obtained from the transformation
Fig. 4. XRD patterns of samples hydrothermally prepared using
2
Ni(CH COO)
3
2
2
ꢀ 4H O in a single solvent of EG at 200 1C for different
of metastable a-Ni(OH) intercalated by acetate ions and
2
time: (a) sample C1; (b) sample C2; (c) sample C3 (the peaks labeled with
asterisk (*) can be indexed to nickel).
water (Fig. 3b, sample B2).
The XRD pattern of sample C1 (Fig. 4a) shows
characteristic diffraction peaks of a-Ni(OH) at 8.70,
2
˚
.35, 2.62 and 1.52 A, respectively. Of these, the first two
4
followed by a pronounced asymmetry on the higher angle
side. These ‘‘saw-tooth’’ reflections are typical of turbos-
tratic phases which are ordered in two dimensions, but
whose layers are orientationally disordered. They can be
assigned to the two-dimensional (10) and (11) reflections,
respectively. When the hydrothermal time was increased to
can be assigned to (0 0 l) (l ¼ 1, 2). The reflections at 2.62
˚
and 1.52 A are both broad, with a sharp rise in intensity
5
.5 h (sample C2), a mixture of a-Ni(OH) and nickel was
2
formed (Fig. 4b). The formation of nickel indicates that
further hydrothermal treatment of Ni(CH COO) ꢀ 4H O
3
2
2
2
+
in EG will result in the reduction of Ni to Ni, which is
further evidenced by sample C3 (Fig. 4c) in which nickel
was formed as the main phase.
From the above XRD results, we can conclude that
higher volume ratio of EG in the reaction system will
reduce the interlayer spacing and enhance the stability of
a-Ni(OH) . However, in the pure solution of EG, nickel
2
ꢁ
was found as the reduction product. CH COO is a weak
basic ion which tends to hydrolyze to form CH COOH and
OH in aqueous solution. Therefore, we propose that
–
Ni(OH) is formed via the hydrolysis of CH COO ions
3
3
ꢁ
Fig. 2. FTIR spectra of samples: (a) sample A1; (b) sample A2; (c) sample
A3.
2
3

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2098
ꢁ
2+
followed by the reaction between OH and Ni ions. The
volume ratio of EG or ethanol to water, since they have
different physical properties, and hence different influence
presence of water is favorable for the hydrolysis of
ꢁ
CH COO ions, leading to a more basic condition
3
on the crystal phase. In fact, the formation of a-Ni(OH)
2
ꢁ
comparing with that without water. In this case, the
transformation rate of a-Ni(OH) to b-Ni(OH) is accel-
erated by the more hydrolysis of CH COO ions. That is
3
was stabilized by the intercalated CH COO ions and H O
molecules. Different d value was caused by the different
amount of the intercalated ions and H O molecules, which
3
2
2
2
ꢁ
2
why for the same hydrothermal time, e.g., 1.5 h, a mixture
of a-Ni(OH) and b-Ni(OH) was obtained at EG/H O
was influenced by the solution characteristics. As the
hydrothermal time increased, the amount of intercalated
ions and H O molecules decreased gradually, and finally
2
2
2
ratio of 1:23, while only a single phase of a-Ni(OH) was
2
2
obtained at EG/H O ratio of 1:1. Although a-Ni(OH) can
a-Ni(OH) supported by ions and H O molecules collapsed
2
2
2
2
form with different interlayer spacings, in fact, the previous
literature reported that the amount of water and anions
as a result of the formation of b-Ni(OH)2.
ꢁ
(
CH COO ) intercalated into the interlayers of a-Ni(OH)
3.2. Microstructure evolution
3
2
could influence the value of the d spacings, which was
influenced by the different physical and chemical properties
of the solution when adjusting the content of EG. It is
noted that the introduction of EG can adjust the interlayer
The morphology and structure of the samples prepared
for different hydrothermal time was characterized by TEM
and SAED. Fig. 6a reveals that sample A1 consisted of
irregularly shaped or curved nanosheets. Fig. 6b shows a
hollow-core flower-like nanostructure with large-size
nanosheets as the petals. Due to the thin thickness of the
nanosheets, crumpled nanosheets were observed. By
increasing the hydrothermal time to 1.5 h, dense flower-
like nanostructures with sizes of several micrometers were
observed, which were constructed with the organization of
nanosheets. From the FESEM images of Fig. 6d and its
inset, the thickness of nanosheets was about 30 nm
estimated from standing nanosheets. However, further
prolonging the hydrothermal time to 3.5 h, flower-like
nanostructures vanished and only nanosheets were ob-
served (Figs. 6e and f). The SAED pattern (inset of Fig. 6f)
shows that the individual nanosheet was single-crystalline
with thickness along [0 0 1] direction (c-axis of hexagonal
b-Ni(OH) ). The sizes of b-Ni(OH) nanosheets prepared
spacing of unstable a-Ni(OH) , more EG will favor the
2
stability of a-Ni(OH) and at the same time reduce the
2
interlayer spacing of the a-Ni(OH) , as evidenced in our
2
experiments.
We also tried the experiments in which EG was
substituted with ethanol. A single phase of a-Ni(OH)2
was obtained, whose XRD pattern shows a diffraction
peak at low angle around 0.9521 nm and a broad
asymmetric band at about 0.2668 nm (Fig. 5a). When the
hydrothermal time was increased to 1.5 h, the partial
transformation of a-Ni(OH)2 to b-Ni(OH)2 occurred,
indicating that a-Ni(OH)2 is easier to transform to
b-Ni(OH)2 in ethanol aqueous solution than in equal
volume EG aqueous solution, which was due to the dif-
ferent physical properties of ethanol from EG. b-Ni(OH)2
formed when the hydrothermal time was 3.5 h or longer.
It is believed that the physical and chemical properties of
the solvent can influence the solubility, reactivity, and
diffusion behavior of the reagents and the intermediate
2
2
for 10.5 h were similar to those prepared for 3.5 h, as
shown in Figs. 6e and g, indicating that the formation
process was almost complete in 3.5 h. The formation of
nanosheets was a general result of the layered structure of
Ni(OH)2.
[38,39]. In the present reaction system, the properties of the
mixed solvents can be easily adjusted by controlling the
The morphology of Ni(OH) formed in mixed solvents
2
of ethanol and water was investigated, as shown in Fig. 7.
At the early stage (1 h), small nanosheets were obtained
with a poorly crystallized structure (Figs. 7a and b).
Sphere-like structures with sizes of about 1 mm were
observed for the hydrothermal time of 1.5 h. From the
magnified TEM image (Fig. 7d), one can see that the
sphere consisted of organized nanosheets. Flower-like
structures consisting of curved nanosheets were also
obtained (Fig. 7e). The SAED pattern taken from a
randomly selected individual sphere-like structure (inset of
Fig. 7d) exhibits bright elongated dots rather than rings,
indicating that it consisted of Ni(OH) nanosheets with an
2
oriented crystallographic axis along the [0 0 1] direction.
This phenomenon was different from that of using EG as
the solvent, and we suggest that in this process the
formation of the spheres and flowers consisting of
nanosheets was due to the oriented attachment mecha-
nism. It is apparent that the solution environment can
Fig. 5. XRD patterns of samples hydrothermally prepared using
ꢀ 4H
Ni(CH
3
COO)
2
2 2
O in mixed solvents of ethanol and H O (volume
ratio ¼ 1:1) at 200 1C for different time: (a) sample D1; (b) sample D2; (c)
sample D3 (the peaks labeled with asterisk (*) can be indexed to a-
Ni(OH) ).
2

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2099
Fig. 6. TEM and FESEM images of samples hydrothermally prepared using Ni(CH COO)
hydrothermal time: (a), (b) sample A1; (c), (d) sample A2; (e), (f) sample A3; (g), (h) sample A4.
3
2
ꢀ 4H
2
O in 1 mL EG and 23 mL H
2
O at 200 1C for different
Fig. 7. TEM and FESEM images of samples hydrothermally prepared using Ni(CH
for different time: (a), (b) sample D1; (c)–(e) sample D2; (f), (g) sample D3; (h) sample D4.
3
COO)
2
2
ꢀ 4H O in the mixed solvents of ethanol and water at 200 1C
remarkably influence the growth of the particles. The
nucleation and growth of crystals are strongly dependent
on the properties of the solvent. The different morphology
formed was due to the different physical and chemical
properties of EG and ethanol, such as boiling point,
dielectric constant, viscosity, polarity and surface tension.
Further hydrothermal treatment of this kind of spheres and
flowers, the nanosheets with the same orientation seemed
to recrystallize into individual single-crystalline na-
nosheets, as revealed in Figs. 7f and g. The SAED pattern
(inset of Fig. 7g) shows a single-crystalline nature of the
nanosheet. Further increasing the hydrothermal time
to 10.5 h, hexagonally shaped nanosheets were obtained
(Fig. 7h).

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Fig. 8. (a) XRD patterns of sample E1; (b), (c) TEM images of sample E1; (d)–(f) TEM images of sample E2.
3
.3. Thermal transformation from Ni(OH) to NiO
2
reference electrode. The common feature on the CVs of
b-Ni(OH) nanosheets (Fig. 9) is that there are two partly
2
The b-Ni(OH) nanosheets as a precursor were thermally
overlapped oxidation current peaks of b-Ni(OH)2 na-
nosheets located at the potential of about 450 and 520 mV,
and one reduction current peak at the potential of about
320 mV. But these two oxidation current peaks did not
occur at the first scan, for sample A3, these two oxidation
current peaks appeared at the third scan, and vanished at
the seventh scan. However, for sample D3, these two
oxidation current peaks appeared at the third scan, and
vanished at the fourteenth scan. This phenomenon
indicates that new phase formed during the electrochemical
cycling process, which changes the electrochemical reaction
route, and the electrochemical process involves more
2
treated at 400 1C for 2 h in order to convert Ni(OH) to
2
NiO. The XRD pattern of sample E1 is shown in Fig. 8a,
which can be indexed to a single phase of a cubic NiO
(
Ni(OH) were observed in XRD pattern, indicating the
JCPDS No. 78-0643). No peaks from the precursor
2
complete transformation of Ni(OH) to NiO. The TEM
2
micrographs of two samples (E1 and E2) are presented in
Figs. 8b–f. As for sample E1, the nanosheets were more
irregular than the precursor Ni(OH) nanosheets, and some
2
nanosheet fragments were also observed. From a high-
magnification image (Fig. 8c), one can see the presence of
nanopores in the nanosheet. However, for the sample
prepared using ethanol (sample E2), the size and shape of
obtained NiO nanosheets were similar to those of the
2
+
reactions other than the mere oxidation process of Ni
3
+
to Ni
.
original b-Ni(OH) nanosheets. The NiO sheet-like struc-
4. Conclusion
2
ture had no obvious go-through pores, which can be
explained by the aggregation process of the nanosheets.
Nanosheets and flowers of nickel hydroxide (b-Ni(OH)2
and a-Ni(OH) ) have been synthesized using Ni(CH
2
3
3
.4. Cyclic voltammogram
COO) ꢀ 4H O in mixed solvents of ethylene glycol (EG)
2
2
or ethanol and deionized water by a hydrothermal method
at 200 1C for different time. The phases and morpho-
logies of the obtained nickel hydroxide could be con-
trolled by adjusting the experimental parameters such as
Fig. 9 shows the typical cyclic voltammograms (CVs) of
the b-Ni(OH) nanosheets (samples A3 and D3) recorded
at a scan rate of 10 mV/s in 6 M KOH solution vs. Hg/HgO
2

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