Hydrothermal synthesis and electrochemical performance of NiO microspheres with different nanoscale building blocks

Article abstract of DOI:10.1016/j.jssc.2010.09.006

NiO microspheres were successfully obtained by calcining the Ni(OH) ₂ precursor, which were synthesized via the hydrothermal reaction of nickel chloride, glucose and ammonia. The products were characterized by TGA, XRD and SEM. The influences o

Full text of DOI:10.1016/j.jssc.2010.09.006

Journal of Solid State Chemistry 183 (2010) 2576–2581
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Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Hydrothermal synthesis and electrochemical performance of NiO
microspheres with different nanoscale building blocks
n
Ling Wang, Yanjing Hao, Yan Zhao, Qiongyu Lai , Xiaoyun Xu
College of Chemistry, Sichuan University, Chengdu 610064, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
NiO microspheres were successfully obtained by calcining the Ni(OH)2 precursor, which were
synthesized via the hydrothermal reaction of nickel chloride, glucose and ammonia. The products
were characterized by TGA, XRD and SEM. The influences of glucose and reaction temperature on the
morphologies of NiO samples were investigated. Moreover, the possible growth mechanism for the
spherical morphology was proposed. The charge/discharge test showed that the as-prepared NiO
microspheres composed of nanoparticles can serve as an ideal electrode material for supercapacitor due
to the spherical hollow structure.
Received 1 June 2010
Received in revised form
2
0 August 2010
Accepted 1 September 2010
Available online 8 September 2010
Keywords:
NiO microsphere
Morphology
&
2010 Elsevier Inc. All rights reserved.
Growth mechanism
Electrochemical performance
1
. Introduction
the NiO microspheres with different nanoscale building blocks
was also investigated. In comparison with microspheres com-
posed of nanoplates and microspheres with smooth surface, the
NiO hollow microspheres composed of nanoparticles showed the
best electrochemical performance.
The synthesis of inorganic nanomaterials with well-controlled
morphologies has attracted considerable interest in recent years
because the properties of these materials extensively depend on
their size and shape [1–5]. NiO as an important transition metal
oxide has received extensive attention because of its specific
structure and potential applications in various fields, such as
catalyst [6–8], battery cathodes [9], gas sensor materials [10],
electrochromic films [11] and fuel cell electrodes [12,13]. It is well
known that the properties and applications of NiO are closely
related to its morphology, therefore, the synthesis of NiO with
different size and shape has an important significance in
fundamental science and practical application. Up to now, NiO
with various morphologies has been prepared, such as nanopar-
ticles [14], nanorods/nanowires/nanofibers [15–18], nanosheets
2. Experimental section
2.1. Preparation of NiO microspheres
All the chemicals were purchased and used as received
without any purification. In a typical synthesis of NiO hollow
spheres experiment, 0.5348 g of NiCl ꢀ 6H O and 0.2 mL of
2
2
[
[
19], nanotubes [20], flower-like structures [21], hollow spheres
22] and nanosheet-based NiO microspheres [23]. However, there
ammonia were dissolved in 19.8 mL of distilled water. Then,
2.0000 g of glucose was added and the mixed solution was stirring
for 5 min. The resulting solution was transferred to a 25 mL
Teflon-lined stainless-steel autoclave and heated at 140 1C for
12 h, and then cooled naturally to the room temperature. The
product was collected by filtration, washed with distilled water
and absolute ethyl alcohol. After the product was dried at 60 1C
for 6 h, the black powder precursor was obtained. NiO hollow
spheres could be acquired by calcining the obtained precursor at
450 1C for 2 h.
are few reports about NiO microspheres with different building
blocks used in electrochemical capacitors.
In this paper, a facile method is used to synthesize NiO
microspheres with different nanoscale building blocks via a
hydrothermal route. The influences of glucose and reaction
temperature on the morphologies of NiO samples were investi-
gated. Moreover, the possible growth mechanism for the spherical
morphology was proposed. The electrochemical performance of
NiO microspheres with smooth surface can be prepared when
the hydrothermal reaction temperature is up to 180 1C. NiO
microspheres composed of nanoplates can also be obtained under
the same reaction conditions and without glucose.
n
Corresponding author. Fax: +86 28 85416969.
E-mail address: laiqy5@hotmail.com (Q. Lai).
0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.09.006

L. Wang et al. / Journal of Solid State Chemistry 183 (2010) 2576–2581
2577
2.2. Characterization
TGA was performed on a NETZSCH TG 209 F1 thermogravi-
metric analyzer with 50 mL/min of air flow from 50 to 600 1C at a
heating rate 10 1C/min. Powder X-ray diffraction data were
collected on a Rigaku D/MAX-rA diffractometer with Cu K
a
radiation (
l
¼0.15418 nm) being operated at 40 kV and 100 mA.
The morphology and particle size were examined by the scanning
electron microscope (SEM, JSM-5900LV, Japan).
2.3. Electrochemical measurements
Nickel oxide electrodes were prepared by pressing an active
paste onto a nickel foam substrate. The paste contained 70 wt%
as-obtained nickel oxide, 25 wt% acetylene black (AB) and 5 wt%
polyvinylidene fluoride (PVDF), which were dissolved by
N-methy-l-2-pyrrolidine (NMP). First, the paste was dried in
vacuum at 60 1C for 12 h. Second, the active paste was pressed
onto a nickel foam substrate that served as a current collector
2
(
surface was 1 cm ) under a pressure of 20 MPa. Then the nickel
oxide electrodes were dried in vacuum at 60 1C for 3 h.
The charge/discharge test was performed with a two-electrode
cell in which the NiO electrode was used as positive electrode and
the AC electrode was used as negative electrode in 6 mol/L KOH
aqueous solutions, and it was carried out with Neware battery
program-control testing system. All the charge/discharge tests
were carried out at room temperature.
Fig. 2. (a) XRD pattern of Ni(OH)
2
precursor prepared in the present of glucose at
140 1C for 12 h and (b) XRD pattern of NiO sample after calcination Ni(OH)
precursor at 450 1C for 2 h.
2
on the thermal analysis, NiO can be prepared by calcining the
Ni(OH) precursor at 450 1C for 2 h.
2
3.2. XRD analysis
The XRD pattern of the Ni(OH)
Fig. 2a. The broad diffraction peak indicates that the Ni(OH)
precursor has a poor crystallinity. Fig. 2b shows the XRD pattern
of NiO sample after calcining Ni(OH) precursor at 450 1C for 2 h.
The diffraction peaks at 37.21, 43.31 and 62.91 of are
corresponding to the (1 1 1), (2 0 0) and (2 2 0) planes, respec-
tively, which can be indexed perfectly to cubic NiO phase. These
results are in good agreement with the data of JCPDS file No.
2
precursor is presented in
3
. Results and discussion
2
3.1. Thermal analysis
2
2y
2
TGA curve of the Ni(OH) precursor is shown in Fig. 1. The
initial weight loss of about 8% between 50 and 200 1C was due to
the removal of physically adsorbed water molecules and carbo-
naceous polysaccharide. The rapid weight loss of about 90%
between 200 and 450 1C corresponded to the removal of the
intercalated water molecules and the decomposition of the
Ni(OH)
and organic species were completely removed from the interslab
space of Ni(OH) , and NiO was formed as the final product. Based
47-1049. It is clear that the resulting sample possesses very good
crystallinity after it was annealed at 450 1C for 2 h.
Fig. 3 shows the XRD patterns of NiO samples obtained by
2
. Beyond 450 1C, all the intercalated water molecules
calcining Ni(OH)
thermal conditions: in the presence of glucose at 140 1C for 12 h
Fig. 3a), without glucose at 140 1C for 12 h (Fig. 3b) and in the
2
precursors prepared under the different hydro-
2
(
presence of glucose at 180 1C for 12 h (Fig. 3c). All the peaks in the
XRD patterns can be indexed as the cubic pure phase of NiO
(
JCPDS No. 47-1049). No other peaks can be observed, indicating
the high purity of the as-prepared NiO samples.
3.3. SEM analysis
Fig. 4a shows the SEM image of Ni(OH)
the presence of glucose at 140 1C for 12 h. The Ni(OH)
2
precursor obtained in
precursor
exhibits a spherical morphology with the diameter of 4–5 m and
the sphere surface is relatively smooth. SEM image of NiO hollow
microspheres obtained by calcining Ni(OH) precursor at 450 1C
for 2 h is shown in Fig. 4b. The NiO hollow microsphere has an
average diameter of 2 m and an average shell thickness around
00 nm. The obvious holes found on NiO hollow spheres were
2
m
2
m
2
produced by the rapid combustion process. Under the fast
calcination process, the gas produced by burning carbonaceous
polysaccharide was rapidly released, leading to the appearance of
holes. Such porous structure had much more specific surface area
than that of compact structure, which was expected to exhibit
better properties in electrochemistry.
2
Fig. 1. TGA curve of Ni(OH) precursor obtained at 140 1C for 12 h.

2
578
L. Wang et al. / Journal of Solid State Chemistry 183 (2010) 2576–2581
We found that the reaction temperature played a crucial role
in the formation of the products. Fig. 5 shows the SEM images of
Ni(OH) and NiO samples synthesized at different reaction
(Fig. 6c and d). However, the tightness of NiO spherical
aggregation was increased.
2
temperatures. At the reaction temperature of 80 1C (Fig. 5a), only
nanoparticles could be prepared. After calcining at 450 1C for 2 h,
the nanoparticles were aggregated and formed large particles
3
.4. Formation mechanism of NiO microspheres
Taking into account the contributions of Sun and Li [24], Li
et al. [25] and Titirici et al. [26], the possible growth mechanism
for NiO hollow sphere was proposed. When the mixture of
(
Fig. 5b). When the reaction temperature was up to 140 1C, the
sample morphologies changed and spherical Ni(OH) samples
with a diameter of 4–5 m were prepared (Fig. 5c). The NiO
hollow spheres with an average diameter of 2 m were obtained
2
m
NiCl
2
2
ꢀ 6H O, ammonia and glucose were treated at 140 1C under
m
hydrothermal conditions, carbonaceous polysaccharide micro-
spheres were formed and their surface had a large number of
hydroxyl and carbonyl groups with good hydrophilicity. Ni ions
would adsorb on the surface of carbonaceous polysaccharide
after fast calcination at 450 1C for 2 h (Fig. 5d). The holes existed
on the rough surface of hollow spheres can be obviously observed.
2
+
2
At the higher temperature of 180 1C, Ni(OH) microspheres with
smooth surface appeared as the main products (Fig. 5e). From
Fig. 5f we can see that NiO microspheres with smooth surface
were obtained, indicating the spherical morphology can be well
sustained after the heat treatment process.
In addition, glucose also played an important role in the
formation of the hollow microsphere composed of nanoparticles.
ꢁ
microspheres and simultaneously react with OH ions provided
by the hydrolysis of the ammonia, which resulted in the
formation of an incompact nickel hydroxide shell. NiO hollow
sphere could be easily achieved by a rapid combustion process.
When the hydrothermal reaction temperature increased up to
1
ꢁ
80 1C, the hydrolysis of the ammonia provided more OH ions.
Fig. 6a and c shows the SEM images of Ni(OH)
2
samples prepared
2
+
ꢁ
Ni
cations reacted with more OH
ions to form nickel
without glucose. The Ni(OH) samples have sphere-like morphol-
2
hydroxide shell. The thickness of the nickel hydroxide was larger
than that of nickel hydroxide obtained at 140 1C. The spherical
morphology with a smooth surface could be sustained by a rapid
heat treatment process. From the XRD patterns of Fig. 3a and c, it
should be noted that the sample synthesized at 180 1C has
stronger diffraction intensity and narrower half-peak width,
indicating the product synthesized at this temperature has a
higher crystallinity. Hence, the complete and smooth surface
microsphere was obtained at 180 1C.
ogies, which are composed of many nanoplates. After calcining
at 450 1C for 2 h, the spherical morphology was still retained
It is well known that Ni(OH)
Under a certain hydrothermal condition (without glucose, at
40 1C for 12 h), Ni(OH) nanoplatelets are easily formed because
2
has a hexagonal layer structure.
1
2
of their intrinsic lamellar structure. The nanoplatelets can
aggregate into the spherical morphology through self-assembly
process by the driving forces, such as electrostatic and dipolar
fields associated with the aggregation, hydrophobic interactions,
hydrogen bonds, crystal-face attraction and van der Waals forces
[27]. The spherical morphology composed of nanoplates can be
sustained by a rapid heat treatment process.
3.5. Electrochemical performance of the products
To investigate the effect of different morphologies of NiO on
the electrochemical properties, their charge/discharge perfor-
mances were measured.
The NiO microsphere samples prepared by calcining the
2
Fig. 3. XRD patterns of NiO obtained by calcining Ni(OH) precursors prepared in
Ni(OH)
2
precursor at 450 1C for 2 h were used as electrode
the different hydrothermal conditions: (a) adding glucose, 140 1C 12 h (b) without
glucose, 140 1C 12 h and (c) adding glucose, 180 1C 12 h.
material for charge/discharge test. Figs. 7 and 8 show the
Fig. 4. (a) SEM image of Ni(OH)
2
precursors obtained in the present of glucose at 140 1C for 12 h and (b) SEM image of NiO hollow microspheres obtained by calcining
Ni(OH) precursors at 450 1C for 2 h.
2

L. Wang et al. / Journal of Solid State Chemistry 183 (2010) 2576–2581
2579
Fig. 5. SEM images of Ni(OH)
2
(left-hand) and NiO (right-hand) samples obtained at various reaction temperatures: (a, b) 80 1C, (c, d) 140 1C and (e, f) 180 1C.
Fig. 6. (a) SEM image of Ni(OH)
2
samples obtained without glucose at 140 1C for 12 h, (c) NiO samples obtained after calcining at 450 1C for 2 h, (b) and (d) the
magnification images of the photographs of (a) and (c), respectively.

2
580
L. Wang et al. / Journal of Solid State Chemistry 183 (2010) 2576–2581
Table 1
The first cycle and the 1000th cycle discharge specific capacitance for the NiO/AC
supercapacitor with different nanoscale building blocks of NiO: (a) NiO hollow
sphere composed of nanoparticles, (b) NiO sphere composed of nanoplates and
(c) NiO sphere with a smooth surface.
ꢁ
1
Sample
Discharge specific capacitance (C
s
) (F g
)
First cycle
1000th cycle
a
b
c
51.7
45.1
32.9
45.8
38.2
24.3
Fig. 7. The charge/discharge curves of the first cycle for the NiO/AC supercapacitor
at a constant current density of 100 mA/g in 6 mol/L KOH solution: (a) NiO hollow
sphere composed of nanoparticles, (b) NiO sphere composed of nanoplates and
(c) NiO sphere with a smooth surface.
Fig. 9. Cycling performance of the NiO/AC hybrid capacitor at a current density of
00 mA/g in 6 mol/L KOH solution: (a) NiO hollow sphere composed of
1
nanoparticles, (b) NiO sphere composed of nanoplates and (c) NiO sphere with a
smooth surface.
following formula:
i ꢂ
Dt
V ꢂ m
C
s
¼
ð2Þ
D
where i (A),
DV (V),
D
t (s) and m (g) are the discharge current,
discharge potential range, discharge time consumed in the
potential range V, and the active mass of the single electrode,
D
Fig. 8. The charge/discharge curves of the 1000th cycle for the NiO/AC super-
respectively. Based on the discharge curves in Figs. 7 and 8, the
results of discharge specific capacitance for NiO/AC are listed in
Table 1 according to formula (2).
capacitor at a constant current density of 100 mA/g in 6 mol/L KOH solution:
(a) NiO hollow sphere composed of nanoparticles, (b) NiO sphere composed of
nanoplates and (c) NiO sphere with a smooth surface.
Fig. 9 presents the cycling performance of the NiO/AC hybrid
capacitor at a current density of 100 mA/g in 6 mol/L KOH
solution. The data illustrate that these hybrid capacitors exhibited
good capacity retention. As shown in Fig. 9 and Table 1, it is
obvious that sample (a) exhibited better electrochemical perfor-
mance and had the highest discharge specific capacity among the
three samples, which may be attributed to its unique morphology
with hollow sphere composed of nanoparticles and coarse surface.
This kind of morphology should be in favor of the electrolyte
inserting into or expelling out the oxide matrix and increase
contact area between the electrolyte and the electrode material as
a result of improving the utilization of the material [29]. It is also
in favor of electrochemical reaction on the interface. Therefore
NiO/AC supercapacitor with NiO hollow sphere composed of
nanoparticles has a higher pseudo-capacitance. Sample (b) had
the lower discharge specific capacity than that of sample (a), but
higher than that of sample(c). The reason was as followed: In
comparison with sample (c), sample (b) composed of NiO
charge/discharge curves of the first and the 1000th for the NiO/AC
supercapacitor at a constant current density of 100 mA/g in
6
mol/L KOH solution, respectively. As shown, a good linear
potential variation vs. time is observed for all charge/discharge
curves, which is a typical characteristic of an ideal supercapacitor.
The charge-storage mechanism of NiO for a pseudo-capacitor
electrode in alkaline solution has been represented as an
2
+
3+
invertible oxidation–reduction reaction between Ni and Ni
which is described by the following reaction [28]:
,
charge
"
NioþzOHꢁ
zNiOOHþð1ꢁzÞNiOþzeꢁ
ð1Þ
discharge
Based on the discharge curves, the discharge specific capaci-
tance (C ) of these hybrid supercapacitors were calculated by the
s

L. Wang et al. / Journal of Solid State Chemistry 183 (2010) 2576–2581
2581
nanoplates also had larger specific surface area and in favor of the
contact with the electrolyte.
Based on these results, it is believed that the NiO with a hollow
sphere composed of nanoparticles morphology is a good electrode
material in supercapacitor field.
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Charge/discharge test results showed that the as-prepared NiO
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This work is financially supported by the National Natural
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