Preparation of 3D rose-like NiO complex structure and its electrochemical property

Article abstract of DOI:10.1016/j.jallcom.2010.01.091

The reaction of nickel chloride and sodium acetate under solvothermal conditions produced Ni(OH)₂ nanoplatelets, which are self-assembled to form a 3D rose-like morphology. After calcination, NiO nanoplatelets were obtained and inherited the rose-like morphology of Ni(OH)₂ precursor. The influences of PEG and reaction temperature on the size and morphology of NiO were investigated. The functions of PEG on the formation of the NiO nanoplatelets and the self-assembly process were discussed. Moreover, the possible mechanism for the formation of rose-like morphology was proposed. The cyclic voltammetry (CV) measurement showed that as-prepared NiO with a rose-like morphology exhibited a good pseudo-capacitance behavior.

Full text of DOI:10.1016/j.jallcom.2010.01.091

Journal of Alloys and Compounds 495 (2010) 82–87
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Preparation of 3D rose-like NiO complex structure and its electrochemical
property
∗
Ling Wang, Yan Zhao, Qiongyu Lai , Yanjing Hao
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:
The reaction of nickel chloride and sodium acetate under solvothermal conditions produced Ni(OH)2
nanoplatelets, which are self-assembled to form a 3D rose-like morphology. After calcination, NiO
nanoplatelets were obtained and inherited the rose-like morphology of Ni(OH)2 precursor. The influences
of PEG and reaction temperature on the size and morphology of NiO were investigated. The functions of
PEG on the formation of the NiO nanoplatelets and the self-assembly process were discussed. Moreover,
the possible mechanism for the formation of rose-like morphology was proposed. The cyclic voltam-
metry (CV) measurement showed that as-prepared NiO with a rose-like morphology exhibited a good
pseudo-capacitance behavior.
Received 30 November 2009
Received in revised form 14 January 2010
Accepted 19 January 2010
Available online 25 January 2010
Keywords:
Electrode material
Oxide material
Chemical synthesis
Crystal growth
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
nia and template [12]. Kuang et al. demonstrated that hierarchical
-Ni(OH)2 microspheres self-assembled from nanosheets build-
␤
NiO nanomaterials have received intensive attention due to
ing blocks were successfully fabricated via a hydrothermal process
their attractive applications in diverse fields, such as electrode
materials [1], magnetic materials [2], optical materials [3], catalysts
[13]. Cao et al. indicated that self-assembled 3D nanostructures ␣-
and ␤-Ni(OH) were prepared in a water-in-oil reverse microemul-
2
[
4] and gas sensors [5]. Of fundamental importance is the prepara-
sion system [14]. However, to the best of our knowledge, few
reports have concerned the synthesis of 3D rose-like NiO via chem-
ical regulation under solvothermal conditions.
tion of NiO nanomaterials with novel morphologies because the
size and shape are intimately related to their properties and appli-
cations. It has been established that size and morphology of NiO
products can be controlled by using the different preparation meth-
ods. For examples, NiO nanoparticles were prepared by anodic arc
plasma method [6]. NiO nanowires were successfully prepared via
a wet chemical method [7]; NiO nanorolls could be selectively
synthesized through a facile hydrothermal synthetic method [8];
NiO nanowalls were obtained by using a plasma assisted oxidation
method [9]; NiO nanotubes had been produced for the first time
via an anodic aluminium oxide (AAO) template processing method
Herein, we report the preparation of the self-assembled 3D
rose-like microstructures of NiO by chemical regulation under
solvothermal conditions, involving complexation–dissociation
equilibrium, hydrolysis equilibrium and precipitation–dissolution
equilibrium. The effects of PEG and reaction temperature on the
size and morphology of samples were also investigated. Moreover,
the possible growth mechanism of the rose-like NiO was proposed
and discussed. As an example of applications, the obtained 3D rose-
like NiO as electrode material showed a good pseudo-capacitance
behavior.
[
10].
In nanomaterials science, 3D complex structure self-assembled
by single nanoparticles has become an attention focus because of its
unique properties and potential applications. Wang et al. reported
that Ni(OH)2 hollow microspheres with Ni(OH)2 nanosheets as the
in situ formed building units were prepared via a novel template-
free approach [11]. Coudun and Hochepied obtained Ni(OH)2 with
a “stack of pancakes” morphology by the dual function of ammo-
2. Experimental
2
.1. Preparation
All the chemicals were purchased and used as received without further purifica-
tion. In a typical synthesis, 0.5348 g of NiCl ·6H O was dissolved in 17 mL of ethylene
2
2
glycol (EG) to form a clear light green solution, then 1.62 g of sodium acetate (NaAc),
.45 g of PEG4000 and 1 mL of deionized water were sequentially added into the
0
above solution under vigorous stirring and heated to get a clear light green solution.
The resulting solution was transferred to a 25 mL Teflon-lined stainless-steel auto-
◦
clave and heated at 190 C for 8 h, and then cooled naturally to room temperature.
∗ Corresponding author. Tel.: +86 28 85416969; fax: +86 28 85416969.
E-mail address: laiqy5@hotmail.com (Q.Y. Lai).
◦
The green product was collected by filtration, washed by ethanol and dried at 60
C
for 3 h. The green product was then immersed in a 6 mol/L KOH solution for 10 h,
0
925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.01.091

L. Wang et al. / Journal of Alloys and Compounds 495 (2010) 82–87
83
◦
Fig. 1. XRD pattern of Ni(OH)2 precursor sample.
Fig. 2. XRD patterns of NiO samples obtained at 190 C: (a) without adding PEG; (b)
adding PEG400; (c) adding PEG4000; (d) adding PEG10000.
◦
and washed till to be neutral. After the green product was dried at 60 C for 6 h,
rose-like Ni(OH)2 was obtained. The rose-like NiO could be acquired by calcining
the obtained Ni(OH)2 powder at 400 C for 2 h.
SEM was used to observe the grain morphology and particle
size of prepared NiO samples. From Fig. 3a and b, we can see that
NiO samples without PEG exhibit lamellar morphology and each
slice has small surface area. The prepared NiO nanoplatelets are so
thick that they cannot curve and aggregate into flower-like struc-
ture. Fig. 3c and d shows that NiO nanoplatelets become large and
thin in the presence of PEG400, which tend to incurvate and form
flower-like complex structure. Fig. 3e and g shows the overall mor-
phologies of the as-obtained rose-like NiO samples by the addition
of PEG4000 and PEG10000, respectively, indicating that the indi-
vidual flakes are curved and connect to each other through the
center to form rose-like architectures with the size of about 2.5 ␮m
in diameter. From the magnification images of the rose-like NiO
in Fig. 3f and h, it is revealed that the entire architecture is built
from nanoplatelets with smooth surfaces. The thickness of these
nanoplatelets is about 70 nm and they connect to each other to form
◦
2.2. Characterization
Powder XRD data were collected on a Rigaku D/MAX-rA diffractometer with Cu
K␣ radiation (ꢀ = 0.15418 nm) being operated at 40 kV and 100 mA. The SEM images
were taken on a Hitachi S-450 electron microscope.
2.3. Cyclic voltammetry (CV) measurements
Nickel oxide electrodes were prepared by pressing an active paste into a nickel
foam substrate. The paste contained 70 wt% rose-like nickel oxide, 25 wt% acetylene
black 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 C for 12 h. Second,
the active paste was pressed into a nickel foam substrate that served as a current
2
collector (surface was 1 cm ) under a pressure of 20 Mpa. Then the nickel oxide
◦
electrodes were dried in a vacuum at 60 C for 3 h. The CV tests were performed with
a three-electrode cell equipped with a working electrode, a platinum foil counter
electrode and a saturated calomel reference electrode (SCE), and carried out with
LK2005 electrochemical workstation system. The CV measurements were carried
out in 6 mol/L KOH solution at room temperature.
3
D rose-like structures. Based on these results, it is clear that PEG
has a significant influence on the morphologies of the products.
Without PEG only patch product of NiO was produced. However,
when PEG with the appropriate molecular weight was chosen as
the surfactant, the rose-like morphology could be obtained.
3
. Results and discussion
3.1. XRD of Ni(OH)2 precursor
3.3. Influences of reaction temperature
Fig. 1 shows the XRD pattern of Ni(OH)2 precursor which was
◦
synthesized in the presence of PEG4000 at 190 C for 8 h and has a
The samples prepared under the solvothermal condition of
◦
◦
◦
◦
◦
◦
◦
◦
◦
hexagonal structure. The peaks at 19.3 , 33.1 , 38.5 , 52.1 , 59.1 ,
100 C, 140 C and 190 C were respectively calcined at 400 C for
2 h. Fig. 4 shows the XRD patterns of NiO samples treated at differ-
ent reaction temperatures. All the NiO samples were synthesized in
◦
◦
6
2.7 and 69.3 could be attributed to the (0 0 1), (1 0 0), (1 0 1),
1 0 2), (1 1 0), (1 1 1) and (2 0 0) planes, respectively [JCPDS Card No.
4-0117]. No diffraction peaks of any impurity phase are detected
in the XRD pattern.
(
1
◦
◦
the presence of PEG4000. The diffraction peaks at 37.2 , 43.3 and
◦
62.9 could be assigned to the (1 1 1), (2 0 0) and (2 2 0) planes of
cubic-phased NiO [JCPDS Card No. 47-1049], respectively. No other
phase has been observed in the diffraction patterns, indicating that
Ni(OH)2 precursor has completely transformed into cubic-phased
3
.2. Influences of PEG
◦
All the NiO samples were obtained by calcining the Ni(OH) pre-
NiO after calcination at 400 C for 2 h. It should be noted that among
2
◦
cursor at 400 C for 2 h. Fig. 2 shows the XRD patterns of different
NiO samples prepared without adding PEG (Fig. 2a), with PEG400
the three NiO samples, the diffraction intensity of the sample syn-
◦
thesized at 190 C is the strongest and the half-peak width is the
(
Fig. 2b), PEG4000 (Fig. 2c) and PEG10000 (Fig. 2d). The diffraction
narrowest, indicating the product synthesized at this temperature
has the highest degree of crystalline character.
◦
◦
◦
peaks at the positions of 37.2 , 43.3 and 62.9 are corresponding
to the (1 1 1), (2 0 0) and (2 2 0) planes, respectively. These peaks
can be indexed perfectly to cubic NiO phase and are in good agree-
ment with the data of JCPDS file No. 47-1049. Among the four NiO
samples, the diffraction peaks in Fig. 2c have the strongest intensity
and the sharpest shape, indicating that the sample prepared in the
presence of PEG4000 has the best crystallization.
Reaction temperature can also affect the morphology of NiO par-
ticles. Fig. 5 shows the SEM images of NiO samples synthesized
with PEG4000 as the surfactant at different reaction tempera-
tures. Fig. 5b, d and f is the magnification images of Fig. 5a, c
and e, respectively. We can see that the rod-like NiO sample
◦
can be obtained at the reaction temperature of 100 C. When the

8
4
L. Wang et al. / Journal of Alloys and Compounds 495 (2010) 82–87
◦
Fig. 3. SEM images of NiO samples obtained at 190 C (a, b) without adding PEG; (c, d) adding PEG400; (e, f) adding PEG4000; (g, h) adding PEG10000.
◦
solvothermal reaction temperature was up to 140 C, the morphol-
ogy of sample changed, and the rod-like and rose-like NiO products
were obtained simultaneously. When the reaction temperature
1D or other complex structures by controlling reaction conditions
such as temperature, reaction time, concentration, etc. [16–19]. As
shown in Fig. 6, the morphology of NiO product is almost the same
◦
further increased to 190 C, rose-like NiO sample composed of
as that of Ni(OH) precursor, indicating that the rose-like morphol-
2
nanoplatelets appeared as the main product. It seems that lower
synthetic temperature favors the formation of rod-like NiO prod-
uct and the higher synthetic temperature favors the formation of
well-crystallized NiO phase with rose-like morphology.
ogy can remain well during the calcination process.
It is well known that Ni(OH)2 has a hexagonal layer structure
with each Ni atom coordinates with six oxygen atoms. In such lay-
ered structure, three oxygen atoms are above the crystal plane, and
the other three oxygen atoms are below the crystal plane. Hydro-
gen atoms solely form “hydrogen layer” and the distance of the
adjacent hydrogen atoms is about 1.1 nm. In a certain hydrother-
mal condition, Ni(OH)2 nanoplatelets are easily formed because
of their intrinsic lamellar structure [20]. The nanoplatelets can
aggregate into the flower-like morphology through self-assembly
process by the driving forces, such as electrostatic and dipolar fields
associated with the aggregate, hydrophobic interactions, hydro-
gen bonds, crystal-face attraction, and van der Waals forces [21].
Based on the experimental results and above discussion, the pos-
3.4. Growth mechanism of the rose-like NiO
In general, the crystal formation process in a solution is mainly
divided into two stages: nucleation and crystal growth. In the
nucleation process, the special crystal structure and symmetry
determine its inherent morphology, which is a key factor to control
the crystal morphology [15]. In contrast, crystal growth is a ther-
modynamically and dynamically controlled process. Crystal seeds,
which maintain their inherent shape in some extent, can grow into

L. Wang et al. / Journal of Alloys and Compounds 495 (2010) 82–87
85
sible growth mechanism of the rose-like Ni(OH)2 microstructures
has been proposed.
During the formation of Ni(OH)2 nanoplatelets, the related
reaction equations can be described as follows, which involve
the complexation–dissolution equilibrium, hydrolytsis equilibrium
and precipitation–dissolution equilibrium.
EG + Ni ꢀ EG − Ni2+ (complex)
2
+
(1)
heating
−
−
CH COO + H O
ꢀ
CH COOH + OH
(2)
(3)
3
2
3
Ni2 + OH− ꢀ EG − Ni2+ (complex)
+
2
+
In the primary stage of the reaction, EG molecules react with Ni
2+
ions to form coordinated complexes (EG − Ni ) (Eq. (1)) and the
−
hydrolysis of the acetate group provides OH ions (Eq. (2)). Ni(OH)2
nuclei are formed instantaneously in solution via the reaction
between Ni2+ cations and OH anions, which are released through
−
complexation–dissociation equilibrium and hydrolysis equilibrium
(
Eq. (3)). The ions are released slowly through the above-mentioned
Fig. 4. XRD patterns of NiO samples obtained in the presence of PEG4000 at various
reaction temperatures: (a) 100 C; (b) 140 C; (c) 190 C.
◦
◦
◦
chemical regulation process, wherefore it is beneficial to form
nanoplatelets. From the contrast experiments (Fig. 3), it is found
◦
◦
◦
Fig. 5. SEM images of NiO samples obtained in the presence of PEG4000 at various reaction temperatures: (a, b) 100 C; (c, d) 140 C; (e, f) 190 C.

8
6
L. Wang et al. / Journal of Alloys and Compounds 495 (2010) 82–87
◦
Fig. 6. SEM images of samples prepared in the presence of PEG4000: (a) Ni(OH)2 precursor samples obtained at 190 C for 8 h; (b) NiO samples obtained from Ni(OH)2
◦
precursor after 400 C heat treatment. The insets are the magnification images of the corresponding photographs.
that PEG plays a critical role in the formation of the rose-like
structure. It is well known that PEG is an excellent surfactant in
preparation of shape-controlled nanostructures, since it can selec-
tively adsorb on a certain face of the crystal to minimize the surface
energy and accelerate the growth rate of other faces [22]. The
molecular weight of PEG has a significant effect on the formation of
rose-like structure. The different molecular weight of PEG has dif-
ferent adsorption capacity due to the different chain length and
the number of reactive oxygen. Thus, the size and thickness of
sheets are not the same, resulting in different degree of curve and
aggregation.
itably. At the same time, from the viewpoint of crystal growth
kinetics, the high temperature is propitious to the growth of large
nanoplatelets. These large nanoplatelets are easy to incurvate
and aggregate into self-assembled rose-like Ni(OH)2 microstruc-
tures.
3.5. Cyclic voltammetry
The rose-like NiO sample, which was prepared in the presence
◦
of PEG4000 at 190 C, was used as the electrode material for cyclic
voltammetry measurement. Fig. 7 shows the CV curve of NiO elec-
trode measured in 6 mol/L KOH electrolyte at a scan rate of 10 mV/s.
In the potential range of −0.2 to 0.4 V, two prominent redox peaks
appearance and the oxidation and the reduction potential have a
shift of around 0.15 V in CV curve, indicating that this process is
pseudo-reversible and NiO electrode has good pseudo-capacitance
performance. The charge-storage mechanism of NiO for a pseudo-
capacitor electrode in alkaline solution has been represented as
Reaction temperature also has a significant influence on the
formation of the flower-like structure. When the reaction temper-
◦
2+
ature is low as 100 C, less Ni ions can be released because of the
strong coordination between EG and Ni2 ions, leading to the small
size nanoplatelets. At the same time, PEG strongly adsorbs on the
surface of the nanoplatelets, which prevents the growth of larger
sheets. From the viewpoint of crystal growth kinetics, lower tem-
perature is against the nanoplatelets growing into larger sheets.
Thus, to decrease their surface energy, the small nanoplatelets
spontaneously attached together to generate the rod-like morphol-
+
2
+
3+
an invertible oxidation–reduction reaction between Ni and Ni
which is described by following reaction [23].
,
charge
◦
NiO + zOH− zNiOOH + (1 − z)NiO + ze−
ꢀ
discharge
ogy. When the reaction temperature increases up to 190 C, more
(4)
2+
Ni cations are released due to the weak coordination between
2
+
−
EG and Ni ions and more OH species are generated from the
acetate hydrolysis process. As a result, large nanoplatelets are pro-
duced. In addition, with the increase of temperature, PEG weakly
adsorbs on the crystal surface, making nanoplatelets grow prof-
The discharge specific capacitance can be estimated from the
voltammetric charge surrounded by the CV curve according to the
following formula [24]
ꢀ
Q
1
2ꢁV
I
C = ꢁ
=
dI
(5)
V × w
vw
where C is the specific capacitance of the electrode based on the
mass of active materials (F/g), Q is the sum of anodic and cathodic
voltammetric charges on positive and negative sweeps (C), I is the
current during discharge process (A), ꢁV is the potential window of
CV (V), v is the scanning rate (V/s) and w is the mass of active elec-
trode materials (g). According to formula (5), the discharge specific
capacitance of NiO electrode can be calculated as 87.2 F/g based on
the CV curve in Fig. 7. Further investigations of the electrochemical
properties are in progress.
4. Conclusions
3
D rose-like microstructure of Ni(OH)2 self-assembled from
nanoplatelets have been successfully prepared by chem-
ical regulation under solvothermal conditions, involving
complexation–dissolution equilibrium, hydrolysis equilibrium
and precipitation–dissolution equilibrium. Subsequently, the
rose-like NiO nanomaterial was obtained by calcining Ni(OH)2
precursor at 400 C for 2 h. The results indicated that the molecular
Fig. 7. CV curve of NiO electrode in the potential range of −0.2 to 0.4 V in 6 M KOH
◦
solution at a scan rate of 10 mV/s.

L. Wang et al. / Journal of Alloys and Compounds 495 (2010) 82–87
87
weight of PEG and the reaction temperature would affect the size
and morphology of the product. Rose-like NiO is easy to form
by using PEG4000 as the surfactant and keeping the reaction
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◦
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