Microwave-assisted hydrothermal synthesis of coralloid nanostructured nickel hydroxide hydrate and thermal conversion to nickel oxide

Article abstract of DOI:10.1016/j.materresbull.2009.04.017

Coralloid nanostructured nickel hydroxide hydrate has been successfully synthesized by a simple microwave-assisted hydrothermal process using nickel sulfate hexahydrate as precursor and urea as hydrolysis-controlling agent. A pure coralloid nanostructured nickel oxide can be obtained from the nickel hydroxide hydrate after calcination at 400 °C. The thermal property, structure and morphology of samples were characterized by thermogravimetry (TG), temperature-programmed reduction (TPR), X-ray (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Full text of DOI:10.1016/j.materresbull.2009.04.017

Materials Research Bulletin 44 (2009) 2040–2044
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Microwave-assisted hydrothermal synthesis of coralloid nanostructured nickel
hydroxide hydrate and thermal conversion to nickel oxide
Teh-Long Lai a, Yuan-Lung Lai b, Jen-Wei Yu a, Youn-Yuen Shu a, , Chen-Bin Wang
*
c,**
a Environmental Analysis Laboratory, Department of Chemistry, National Kaohsiung Normal University, Kaohsiung 802, Taiwan, ROC
b Department of Mechanical and Automation Engineering, Da-Yeh University, Changhua 515, Taiwan, ROC
c Department of Applied Chemistry and Materials Science, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taoyuan 335, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history:
Coralloid nanostructured nickel hydroxide hydrate has been successfully synthesized by a simple
microwave-assisted hydrothermal process using nickel sulfate hexahydrate as precursor and urea as
hydrolysis-controlling agent. A pure coralloid nanostructured nickel oxide can be obtained from the
nickel hydroxide hydrate after calcination at 400 8C. The thermal property, structure and morphology of
samples were characterized by thermogravimetry (TG), temperature-programmed reduction (TPR), X-
ray (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM) and transmission electron
microscopy (TEM).
Received 16 February 2009
Received in revised form 20 April 2009
Accepted 30 April 2009
Available online 9 May 2009
Keywords:
A. Nanostructures
B. Chemical synthesis
C. Electron microscopy
ß 2009 Elsevier Ltd. All rights reserved.
1. Introduction
microwave heating has shown higher energy efficiency and
reaction rates to shorten the reaction time [11,17–19]. The use
Metal oxides are widely used in the field of heterogeneous
catalysis. Nickel oxide (NiO) is one of the most versatile materials
among transition metal oxides [1]. Nickel oxide with a variety of
morphologies, i.e., nanoparticles, nanotubes, nanowires, nanofi-
brous, mesoporous, hollow spheres and urchin-like nanostructures
has been reported [2–9]. Liu et al. [9] choose urea as a hydrolysis-
controlling agent and polyethylene glycol as a surfactant to obtain
an urchin-like nickel oxide. Microwave-assisted hydrothermal
synthesis is a convenient and easily controlled technique to
prepare nanocrystalline metal oxides within 2–6 min [10,11]. The
advantages of this process over the conventional hydrothermal
method are: (a) very rapid heating to the required temperature, (b)
extremely rapid kinetics of crystallization and (c) possible
formation of new meta-stable phases [12]. Microwave irradiation
technology has already been applied to industry, family, medical
science and extraction or abatement of environmental organic
pollution for polycyclic aromatic hydrocarbons (PAHs) [13–15],
polychlorinated biphenyls (PCBs) [16], etc. Due to the properties of
internal and volumetric heating (dipole rotation and/or ionic
conduction), thermal gradients during microwave processing are
avoided, providing a uniform environment for reaction. Therefore,
of microwaves as a source of energy is rapidly becoming more
economical and convenient.
Xu et al. [20] synthesize of novel coralloid polyaniline/
BaFe12O19 nanocomposites with outward extending branches
having an average diameter in the range of 20–50 nm. The
coralloid nanostructures compared to else show specific electrical,
magnetic and electromagnetic properties. The coralloid nanos-
tructures may have potential applications in chemical sensors, gas
separation, catalysis, microwave absorbing, and magnetoelectric
devices. In the application of microwave on the synthesis of
nanoporous materials, Tompsett et al. [21] indicate that the
precursor of the species can be nucleated to form tiny single
crystals under microwave irradiation.
In this study, we report on the application of microwave heating
with an autoclave to fabricate coralloid nanostructured nickel
oxide. The thermal property, structure and morphology of samples
are discussed.
2. Experimental
2.1. Preparation of coralloid nanostructured materials
In order to prepare coralloid nanostructured nickel hydroxide
hydrate as material medium, both nickel sulfate (NiSO4ꢀ6H2O) and
urea had been chosen. A mixed solution of 50 ml of 0.6 M
NiSO4ꢀ6H2O and 2.4 g of urea was transferred to the Teflon container
of the double-walled vessel of a microwave system (Milston
* Corresponding author.
** Corresponding author. Tel.: +886 33891716; fax: +886 33892494.
E-mail addresses: shuyy@nknucc.nknu.edu.tw (Y.-Y. Shu), chenbin@ccit.edu.tw,
chenbinwang@gmail.com (C.-B. Wang).
0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.04.017

T.-L. Lai et al. / Materials Research Bulletin 44 (2009) 2040–2044
2041
MSD1000, 2.45 GHz, Italy) and irradiated under 500 W microwave
for 3 min. The precipitate was then filtered, washed with deionized
distilled water, and dried in an oven at 100 8C for 20 h to obtain the
as-prepared sample. Then, the as-prepared sample was further
calcined separately under air at 300, 400 and 800 8C for 3 h to obtain
the black species (assigned as C300, C400 and C800, respectively).
2.2. Characterization techniques
X-ray diffraction (XRD) measurements were performed using a
MAC Science MXP18 diffractometer with Cu
˚
(l = 1.5405 A) at 40 kV and 30 mA with a scanning speed in 2u
K
radiation
1
a
of 48 minꢁ1. The crystallite sizes of nickel hydroxide and nickel
oxide were estimated using the Scherrer equation.
The infrared spectra were obtained with
a Perkin-Elmer
spectrum GX spectrometer in the range of 400–4000 cmꢁ1. One
milligram of each powder sample was diluted with 200 mg of
vacuum-dried IR-grade KBr and subjected to a pressure of 8 tonnes.
Reduction behavior of coralloid nanostructured nickel hydroxide
hydrate and nickel oxide was studied by temperature-programmed
reduction (TPR). About 50 mg of the sample was heated in a flow of
10% H2/N2 gas mixture at a flow rate of 10 ml minꢁ1. During the TPR
procedure, the temperature was programmed by 7 8C minꢁ1 heating
rate from room temperature to 600 8C.
Thermal gravimetric analysis (TG/DTG) was carried out using a
Seiko SSC5000 TG system. The mass of the sample was 10 mg and
the heating rate was maintained by 10 8C minꢁ1. The temperature
was raised from room temperature to 800 8C under an airflow with
a rate of 100 ml minꢁ1
.
Transmission electron micrographs (TEM) were taken on a
PHILIPS (CM-200) microscope at an accelerating voltage of 200 kV.
The samples for TEM were prepared by ultrasonic dispersing of the
powder catalysts in ethanol, which was then deposited and dried
on a holey carbon film on a copper grid.
The surface morphology of coralloid nanostructured nickel
hydroxide hydrate and nickel oxide were observed by means of a
scanning electron microscope (JSM-6330TF) operated at 10 kV.
3. Results and discussion
The SEM and TEM images of the as-prepared sample are shown
in Fig. 1. From our observation of the present SEM image (Fig. 1(a)),
the as-prepared sample possesses coralloid nanostructures with
the length varying from 2 to 3
mm. The TEM images (Fig. 1(b) and
(c)) show the assemblies of nanostructures. Detailed observation
finds that the nanostructures consist of sheets or plates with
nanoscale dimensions. The plates with high aspect ratio are
ascertained to grow radial from the core particles. Fig. 2 presents a
schematic diagram for the formation of coralloid architecture
using the microwave-assisted hydrothermal method mentioned
above. The proposed mechanism involves a sequence of steps:
nucleation of nickel precursor and urea to form tiny particles,
growth of tiny particles by aggregation to form sheets or plates,
self-assembly and overlap of sheets or plates to form coralloid
Fig. 1. SEM and TEM images of the as-prepared: (a) 5000ꢂ, (b) 20,000ꢂ and (c)
40,000ꢂ.
nanostructure. Because of the existence of NCO, CO3 and OHꢁ
2ꢁ
250 8C, the gradual weight loss should come from the desorbed
water on the as-prepared sample surface in the heating process.
The weight loss of 14% in step D2 is mainly caused by the
dehydroxylation of nickel hydroxide hydrate, 3Ni(OH)2ꢀxH2O, to
3NiOꢀxH2O (comparison with the theoretical weight loss, the
stoichiometry of x approaches 2.0):
groups, we believe that electrostatic and hydrogen bonds is the
main driving forces for self-assembly to form coralloid nickel
hydroxide hydrate.
The SEM of NiO nanostructures (Fig. 3(a)) can be obtained by
the heat treatment of the coralloid nanostructured nickel
hydroxide hydrate. The TEM images (Fig. 3(b) and (c)) show that
the NiO nanostructures are stable and remain sheets or plates with
3NiðOHÞ2ꢀ2H2O ! 3NiO ꢀ 2H2O þ 3H2O
(1)
The weight loss of 3% and 6% in steps D3 and D4 exhibit only a
tardy weight loss probably due to the further removal of
chemisorbed and occluded water through the conversion of
Ni(OH)2 to NiO.
a diameter of about 2–3
mm.
Fig. 4 shows the TG curve for the thermal decomposition of as-
prepared sample under airflow (100 ml minꢁ1). Four weight loss
regions (assigned as D1, D2, D3 and D4) are observed. Prior to

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T.-L. Lai et al. / Materials Research Bulletin 44 (2009) 2040–2044
Fig. 2. Schematic explanation of the formation of coralloid nanostructure.
Fig. 5 shows the XRD patterns of the as-prepared and calcined
(Fig. 5(a)) indicates that all the diffraction peaks can be perfectly
indexed [ICCD card, no. 22-0444, 3Ni(OH)2ꢀ2H2O], consisting of a
nickel hydroxide hydrate. The XRD pattern of the C300 sample
(Fig. 5(b)) presents a weak peak around 138 (comes from (0 0 1)
plane of 3Ni(OH)2ꢀ2H2O phase). As the calcined temperature
reaches above 400 8C, the (0 0 1) plane of 3Ni(OH)2ꢀ2H2O is not
observed (Fig. 5(c)). The results show that the 3Ni(OH)2ꢀ2H2O is
converted completely into nickel oxide [ICCD card, no. 47-1049,
NiO] after calcination above 400 8C.
sample. All of the detectable peak positions and relative intensities
are consisted from those of the 3Ni(OH)2ꢀ2H2O (Fig. 5(a)) and
nickel oxide (Fig. 5(b) and (c)) phases. The as-prepared sample
Fig. 6 shows the TPR profiles of the as-prepared and calcined
sample. Aside from the slight reduction peak at 297 8C (this peak
perhaps is attributable to the reduction of nickel oxide that comes
from the dehydration of the as-prepared sample, see Fig. 4), the
Fig. 4. TG curve for the thermal decomposition of as-prepared sample under airflow.
Fig. 3. SEM and TEM images of the C400: (a) 5000ꢂ, (b) 20,000ꢂ and (c) 100,000ꢂ.
Fig. 5. XRD patterns of the samples: (a) as-prepared, (b) C300 and (c) C400.

T.-L. Lai et al. / Materials Research Bulletin 44 (2009) 2040–2044
2043
Fig. 8. SEM image of the C800 (50,000ꢂ).
Fig. 6. TPR profiles of the samples: (a) as-prepared, (b) C300 and (c) C400.
strong band at 2200 cmꢁ1 (attributed to the
band appearing at 1396 cmꢁ1 (due to the ns
n
a (NCO) mode), a weak
reductive signal of nickel hydroxide hydrate in TPR (Fig. 6(a))
shows an overlap around 336 and 370 8C according to Eq. (2):
mode) and
(NCO)
broader band around 645 cmꢁ1 (due to the d(NCO) mode) [23,24].
The prominent band at 1033–1123 cmꢁ1 is typical of SO4 ions.
2ꢁ
3NiðOHÞ2ꢀ2H2O þ 3H2 ! 3Ni þ 8H2O
(2)
The absorption band at 490 cmꢁ1 and 600–645 cmꢁ1 indicate that
the as-prepared sample is Ni–O stretching vibration mode and
deformational Ni–OH mode. The C300 spectrum (Fig. 7(b)) displays
a series of weak peaks around 2200, 1396, 1123, 1033 and
645 cmꢁ1 that confirms the mixed phased existing in C300 sample.
The broad peak around 3500 cmꢁ1, corresponding to –OH, is also
Fig. 6(c) shows the reductive signal of the C400 sample that
presents one-step reduction around 329 8C. According to our
previous report [22], the pure coralloid nanostructured nickel
oxide is reduced directly into Ni according to Eq. (3):
2ꢁ
NiO þ H2 ! Ni þ H2O
(3)
decreased in intensity. This means that the NCO and CO3 are
almost removed while still containing some hydroxyl group that
comes from the adsorbed water after calcinations at 400 8C
(Fig. 7(c)). The NCO and CO32ꢁ seem assisted sheets or plates with
nanoscale dimensions growth on 3Ni(OH)2ꢀ2H2O nanoparticles
and the formation of coralloid nanostructured.
The plates with nanostructured morphology are completely
destroyed after heat treatment approaching 800 8C (Fig. 8). The
water of crystallization on the plate nanostructure compound
influences the particle morphology of the final nickel oxides. In
comparison with the thermal analysis of TG (Fig. 4), the water of
crystallization can escape as the temperature reaches above
700 8C. The results further confirm the various morphologies
between C400 and C800 samples.
The different shape of the TPR profile obtained for C300
(Fig. 6(b)) is probably due to the mixture of two solid phases (the
XRD measurement reveals the existence of 3Ni(OH)2ꢀ2H2O and
NiO) that affect the shape of the reduction profile. The peak around
312 8C is the reduction of nickel oxide and the peak at 330 8C comes
from the reduction of nickel hydroxide.
In order to understand the crystallographic sites occupation and
the cationic jumps along the phase transition of nickel oxide, the IR
absorption spectra are measured for nickel hydroxide hydrate and
nickel oxide. Fig. 7 shows the IR absorption spectra of the as-
prepared and calcined sample. The spectrum of coralloid
nanostructured nickel hydroxide hydrate (Fig. 7(a)) presents a
The coralloid nanostructured nickel oxide is similar to the
urchin-like nanostructures that might exhibit some novel proper-
ties, which in turn can lead to new and important applications in
cell (high electric capacitance) and catalysis (hydrogenation
reaction) [9,25,26].
4. Conclusion
We have successfully synthesized the coralloid nanostructured
nickel hydroxide hydrate through a rapid microwave-hydrother-
mal process. The pure coralloid-like nanostructured nickel oxide
can be obtained from the calcination of nickel hydroxide hydrate at
400 8C. XRD, SEM, TEM, IR and TPR analysis shows that nickel oxide
has apparent coralloid nanostructures and remains sheets or plates
with nanoscale dimensions with length 2–3
mm.
Acknowledgements
We are pleased to acknowledge financial support for this study
from the National Science Council of the Republic of China under
contract numbers NSC 97-2811-M-017-001 and NSC 96-2113-M-
606-001-MY3.
Fig. 7. IR spectra of the samples: (a) as-prepared, (b) C300 and (c) C400.

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