β-nickel hydroxide nanosheets and their thermal decomposition to nickel oxide nanosheets

Article abstract of DOI:10.1021/jp037513n

Powders of single-crystalline β-nickel hydroxide (β-Ni(OH)₂) nanosheets with the hexagonal structure have been successfully synthesized by the hydrothermal method at 200°C using nickel acetate as the nickel source and aqueous ammonia as both an

Full text of DOI:10.1021/jp037513n

3488
J. Phys. Chem. B 2004, 108, 3488-3491
â-Nickel Hydroxide Nanosheets and Their Thermal Decomposition to Nickel Oxide
Nanosheets
Zhen-Hua Liang, Ying-Jie Zhu,* and Xian-Luo Hu
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of
Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China
ReceiVed: NoVember 18, 2003; In Final Form: January 18, 2004
Powders of single-crystalline â-nickel hydroxide (â-Ni(OH)2) nanosheets with the hexagonal structure have
been successfully synthesized by the hydrothermal method at 200 °C using nickel acetate as the nickel source
and aqueous ammonia as both an alkaline and complexing reagent. The yields of â-Ni(OH)2 nanosheet powders
were higher than 92.4%. This method is simple and low-cost for large-scale production of powders of single-
crystalline â-Ni(OH)2 nanosheets. Single-crystalline nickel oxide (NiO) nanosheets have been synthesized
by thermal decomposition method at 400 °C for 2 h using single-crystalline â-Ni(OH)2 nanosheets as the
precursor. The sheet shape of â-Ni(OH)2 was sustained after thermal decomposition to NiO. The as-prepared
products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM),
differential scanning calorimetric analysis (DSC), and thermogravimetric analysis (TG).
Introduction
electrodes.17,18 Therefore, to explore new synthesis methods for
NiO nanosheets will find its new applications or improve
existing performances. To our knowledge, there has been no
report on the synthesis of NiO nanosheets.
In the past decade, one-dimensional (1-D) nanostructures such
as nanorods, nanowires, and nanotubes have been intensively
studied due to their novel properties and potential applications
as components and interconnects in nanodevices.1 However,
nanosheets have not been widely studied due to the lack of
knowledge on their synthesis. Recently, nickel hydroxide
(Ni(OH)2) has aroused increasing attention due to its applications
in alkaline rechargeable batteries which are most widely used
in many applications ranging from power tools to portable
electronics and electric vehicles. The electrochemical utilization
and practical capacity of Ni(OH)2 cathodes are directly affected
by their morphology and size. The development of today’s
electronic industry needs much higher energy density batteries.
Han et al.2 reported that the capacity of the positive electrode
could be significantly increased when nanophase Ni(OH)2 was
added to micrometer-size spherical Ni(OH)2. It is expected that
Ni(OH)2 nanostructures may have potential applications in high-
energy-density batteries. â-Ni(OH)2 crystallizes with a hexago-
nal brucite-type structure with Ni(OH)2 layers stacked along
the c-axis and an interlayer distance of 4.6 Å. â-Ni(OH)2 is
often selected as the discharged-state material in the electrodes
due to its stability in strong alkaline electrolyte and good
reversibility when charged to â-NiOOH.3
However, there have been only a few reports on the synthesis
of Ni(OH)2 nanostructures. Gedanken et al.4 synthesized scaly
R-Ni(OH)2 nanostructures using the sonochemical method. Li
et al.5 reported the synthesis of â-Ni(OH)2 nanostructures by
the NiC2O4 conversion method, and the as-prepared products
consisted of a mixture of nanosheets and nanorods.
NiO is a semiconductor and an antiferromagnetic material.
NiO has received considerable attention recently due to its
attractive applications in diverse fields, such as catalysis,6,7
battery cathode,8,9 gas sensors,10,11 electrochromic films,12,13
magnetic materials,14,15 active optical fibers,16 and fuel cell
In this paper, we demonstrate that powders of â-Ni(OH)2
single-crystalline nanosheets can be synthesized using nickel
acetate and aqueous ammonia by the hydrothermal method at
200 °C. The exclusive nanosheet morphology of single-
crystalline â-Ni(OH)2 has been achieved by this method. The
preparation was carried out without using any template or seed,
which avoided the subsequent complicated workup procedure
for removal of the template or seed. Furthermore, NiO single-
crystalline nanosheets have been successfully synthesized by
the thermal decomposition method at 400 °C for 2 h using
â-Ni(OH)2 single-crystalline nanosheets as the precursor. The
sheet shape of single-crystalline â-Ni(OH)2 was sustained after
thermal decomposition to NiO.
Experimental Section
Nickel acetate (Ni(CH3COO)2‚4H2O) (Shanghai Chemical
Reagents Company) and aqueous ammonia (NH3‚H2O) (Shang-
hai Lingfeng Chemical Reagents Co., Ltd.) were of analytical
grade and were used as purchased without further purification.
Deionized water was used as the solvent in all experiments. In
a typical experiment (sample 1), 0.7 g of Ni(CH3COO)2‚4H2O
was dissolved in 30 mL of deionized water and 1 mL of 6 M
NH3‚H2O was added dropwise during magnetic stirring. The
solution was sealed into Teflon-lined autoclaves and heated at
200 °C for 5 h. After hydrothermal treatment, a green suspension
was obtained. The product was separated from solution by
centrifugation, washed with deionized water three times, and
dried at 60 °C in a vacuum. Finally, green powder was obtained.
Other samples were prepared by the procedure similar to that
for sample 1, but under different conditions.
X-ray powder diffraction (XRD) patterns were recorded using
a Rigaku D/max 2550V X-ray diffractometer with high-intensity
Cu KR radiation (λ ) 1.54178 Å) and a graphite monochro-
* Corresponding author. Fax: +86-21-52413122. E-mail: y.j.zhu@
mail.sic.ac.cn.
10.1021/jp037513n CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/25/2004

â-Nickel Hydroxide Nanosheets
J. Phys. Chem. B, Vol. 108, No. 11, 2004 3489
deionized water at 200 °C for 5 h. The product was a single
phase of well-crystallized â-Ni(OH)2 with the hexagonal
structure (JCPDS 74-2075). No peaks due to R-Ni(OH)2 or
impurities were observed by XRD. The yield of â-Ni(OH)2
powder was 92.4%. Other samples prepared by the hydrothermal
method under different conditions have XRD patterns similar
to Figure 1.
Figure 2 shows TEM micrographs of four samples prepared
by the hydrothermal method. Figures 2a and 2b show TEM
micrographs of the same sample (sample 1) as described in
Figure 1. One can see â-Ni(OH)2 nanosheets as well as nanorods
with diameters of 5-20 nm and lengths up to ∼100 nm.
â-Ni(OH)2 nanosheets have irregularly shaped morphologies
with sizes in the range of 25-160 nm. Many of nanosheets are
irregularly hexagonal with the angles of adjacent edges of 120°
as indicated by arrows in Figure 2b. We suggest that the surface
of the nanosheets is the {0001} planes of the hexagonal
â-Ni(OH)2 phase, and the angles of 120° may be those of the
{10-10} and {01-10} planes, and the edges should correspond
to the {10-10} and {01-10} planes. The suggestion is
supported by electron diffraction (ED) shown in Figure 2e. In
addition to nanosheets, nanorods were present. The coexistence
of nanosheets and nanorods was also observed by Li et al.5 for
â-Ni(OH)2 synthesized by the NiC2O4 conversion method.
Figure 1. XRD pattern of sample 1 prepared by hydrothermally treating
a solution containing 0.7 g of Ni(CH3COO)2‚4H2O, 1 mL of 6 M NH3‚
H2O, and 30 mL of H2O (pH ≈ 7.5) at 200 °C for 5 h. Ni(CH3COO)2‚
4H2O was dissolved in deionized water and then aqueous ammonia
was added dropwise to the Ni(CH3COO)2 solution during magnetic
stirring before the hydrothermal treatment.
mator. TEM images were taken with JEOL JEM-2010 and JEOL
JEM-200CX transmission electron microscopes. Differential
scanning calorimetric analysis (DSC) and thermogravimetric
analysis (TG) were carried out with a STA-409PC/4/H Luxx
simultaneous TG-DTA/DSC apparatus (Germany) with a heating
rate of 10 °C min-1 in flowing air.
Results and Discussion
To obtain exclusive â-Ni(OH)2 nanosheets, we explored the
influence of hydrothermal time on the product. Figure 2c shows
TEM micrograph of sample 2 prepared under the same condition
as sample 1 except that hydrothermal treatment time was 30 h
Figure 1 shows the XRD pattern of a typical sample (sample
1) prepared by hydrothermally treating a solution of 0.7 g of
Ni(CH3COO)2‚4H2O, 1 mL of 6 M NH3‚H2O, and 30 mL of
Figure 2. TEM micrographs of four samples prepared by the hydrothermal method. (a) The same sample (sample 1) as described in Figure 1; (b)
the magnified TEM image for sample 1; (c) sample 2 prepared by hydrothermally treating a solution prepared as the following procedure: 0.7 g
of Ni(CH3COO)2‚4H2O was dissolved in 30 mL of H2O, and 6 M NH3‚H2O was added dropwise to adjust the pH to 7.5. The resulting solution was
hydrothermally treated at 200 °C for 30 h; (d) sample 3 prepared under the same condition as sample 2 except that pH was adjusted to 9.6; (e)
sample 4 prepared by hydrothermally treating a solution prepared according to the following procedure: 0.07 g of Ni(CH3COO)2‚4H2O and 0.016
g of poly(ethylene glycol) (PEG) was dissolved in 20 mL of H2O, and 6 M NH3‚H2O was added dropwise to adjust the pH to 7.8. The resulting
solution was hydrothermally treated at 210 °C for 33 h. The inset shows the electron diffraction pattern of a single nanosheet; (f) standing nanosheets
from sample 4 on the TEM sample holder. The inset shows the electron diffraction (ED) pattern collected from the spot labeled by the circle.

3490 J. Phys. Chem. B, Vol. 108, No. 11, 2004
Liang et al.
Figure 4. XRD pattern of the product (sample 5) obtained by thermal
decomposition of â-Ni(OH)2 nanosheets at 400 °C for 2 h.
Figure 3. Differential scanning calorimetric analysis (DSC) and
thermogravimetric analysis (TG) curves of â-Ni(OH)2 nanosheets.
(5 h for sample 1). Exclusive â-Ni(OH)2 nanosheets with sizes
ranging from 50 to 160 nm were observed. No nanorods were
observed. Compared with sample 1 prepared at 200 °C for 5 h,
both the average size and thickness of nanosheets increased for
sample 2 prepared at 200 °C for 30 h. This implies that the
growth process of nanosheets occurred by increasing the
hydrothermal treatment time. It is interesting to note that by
increasing hydrothermal treatment time from 5 to 30 h, the shape
of â-Ni(OH)2 nanosheets had changed from irregularly hex-
agonal (Figures 2a and 2b) to quasi-circular (Figure 2c).
We investigated the influence of pH on the product. Samples
2 and 3 were prepared under the same condition except that
pH was different. Figure 2c shows a TEM micrograph of sample
2 prepared at pH ) 7.5. Exclusive nanosheets were observed.
The sizes of nanosheets in sample 2 (pH ) 7.5) are similar to
those of sample 3 prepared at pH ) 9.6. This implies that pH
has limited influence on the sizes of nanosheets. We used
aqueous ammonia to adjust the pH of the solution to be weak
alkaline in order to provide OH- ions for the formation of
Ni(OH)2. NH3 could also act as a complexing reagent for Ni2+
2+
ions to form Ni(NH3)62+. The formation of Ni(NH3)6 may
control the reaction rate of Ni2+ ions with OH- ions, playing a
role in the formation of â-Ni(OH)2 nanosheets.
A TEM micrograph of sample 4 is shown in Figure 2e, which
shows nanosheets with sizes of several hundred nanometers.
The inset of Figure 2e shows the ED pattern of a randomly
selected single nanosheet lying on the TEM sample holder,
which was obtained by focusing the incident electron beam
along the [0001] zone axis. ED pattern can be indexed to the
hexagonal â-Ni(OH)2, consistent with the XRD result. ED
patterns taken on different individual nanosheets were essentially
the same. This indicates the single-crystalline structure of
â-Ni(OH)2 nanosheets. Figure 2f shows a bundle of standing
nanosheets from sample 4 on the TEM sample holder, from
which we estimate that the thickness of nanosheets was in the
range of ∼12 to ∼20 nm. The inset in Figure 2f shows the ED
pattern collected from the spot labeled by the circle on a single
standing nanosheet. The ED pattern was taken with an electron
beam parallel with the nanosheet surface. It shows the perfect
one-dimensional ED pattern, indicating a layered structure of
â-Ni(OH)2 nanosheets. The calculated spacing between Ni(OH)2
layers stacked along the c-axis was about 4.7 Å, consistent with
that reported (4.6 Å).19 This result supports the finding that the
surface of the nanosheets is the {0001} planes of the hexagonal
â-Ni(OH)2 phase.
Figure 5. (a) TEM micrograph of â-Ni(OH)2 nanosheet precursor
before thermal decomposition; (b) TEM micrograph of NiO nanosheets
(sample 5) obtained by thermal decomposition of â-Ni(OH)2 nanosheets
at 400 °C for 2 h; (c) a single NiO nanosheet, showing the angle of
adjacent edges of 120° as indicated by arrows; (d) ED pattern of an
individual NiO nanosheet.
∼ 20%, in good agreement with the theoretical value (19.4%)
calculated from the following reaction:
Ni(OH)2 endothermic8 NiO + H2O
The DSC curve showed an endothermic peak with a maximum
located at 326 °C. The temperature range of the endothermic
peak in the DSC curve fits well with that of weight loss in the
TG curve, corresponding to endothermic behavior during the
decomposition of â-Ni(OH)2 to NiO.
We explored the possibility of using â-Ni(OH)2 nanosheets
as the precursor for synthesis of NiO nanosheets. On the basis
of TG and DSC results, we used 400 °C to ensure the complete
decomposition of â-Ni(OH)2. Figure 4 shows the XRD pattern
of NiO powder (sample 5) prepared by thermal decomposition
of single-crystalline â-Ni(OH)2 nanosheets at 400 °C for 2 h.
The thermal behavior of â-Ni(OH)2 nanosheets was inves-
tigated with TG and DSC measurements (Figure 3). A TG curve
showed that â-Ni(OH)2 started to decompose (weight loss) at
about 285 °C. The major weight loss happened rapidly between
∼298 and ∼342 °C. The total weight loss was measured to be

â-Nickel Hydroxide Nanosheets
J. Phys. Chem. B, Vol. 108, No. 11, 2004 3491
All reflections in Figure 4 can be indexed to the face-centered
cubic (fcc) NiO phase (space group: Fm3m [No. 225]). The
lattice constant (a ) 4.173 Å) is in good agreement with the
reported data (JCPDS, No. 78-0643). No peaks due to â-Ni(OH)2
were observed by XRD, indicating that â-Ni(OH)2 was com-
pletely decomposed to NiO at 400 °C for 2 h.
Overseas Chinese (Hundred Talents Program) is gratefully
acknowledged. We thank the Fund for Innovation Research from
Shanghai Institute of Ceramics, Chinese Academy of Sciences
and the Fund from Shanghai Natural Science Foundation. We
also thank Professor Meiling Ruan for assistance with TEM
experiments.
The morphology of as-prepared NiO sample (sample 5) was
investigated by TEM, as shown in Figure 5. Figure 5a shows
the precursor single-crystalline â-Ni(OH)2 nanosheets with sizes
of ∼200- ∼300 nm before thermal decomposition. Figure 5b
shows NiO nanosheets with sizes up to ∼1 µm. Figure 5c shows
a single nanosheet with the angles of adjacent edges of 120° as
indicated by arrows. The corresponding ED pattern is shown
in Figure 5d, which was obtained by focusing the incident
electron beam along the [1-1-1] zone axis. It can be indexed
to the fcc NiO, consistent with the XRD result. ED patterns on
different individual nanosheets were essentially the same,
indicating that NiO nanosheets are single-crystalline in structure.
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Acknowledgment. Financial support from Chinese Academy
of Sciences under the Program for Recruiting Outstanding
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