Facile synthesis of polycrystalline NiO nanorods assisted by microwave heating

Article abstract of DOI:10.1111/j.1551-2916.2008.02667.x

Polycrystalline NiO nanorods have been synthesized by thermal decomposition of rod-like nickel oxide precursor achieved by microwave-assisted method. The structure, morphology, and crystalline phase of the products are characterized by transmission electron microscopy (TEM), X-ray diffractometry, Fourier-transformed infrared spectroscopy, and UV-visible absorption spectrum. TEM reveals that NiO nanorods consist of numerous NiO nanoparticles. The average size of NiO nanoparticles as a function of the calcination temperature has been investigated. The optical absorption band gap of polycrystalline NiO nanorods is 3.7 eV.

Full text of DOI:10.1111/j.1551-2916.2008.02667.x

J. Am. Ceram. Soc., 91 [10] 3465–3468 (2008)
DOI: 10.1111/j.1551-2916.2008.02667.x
r 2008 The American Ceramic Society
ournal
J
Facile Synthesis of Polycrystalline NiO Nanorods Assisted by
Microwave Heating
Xuefeng Song and Lian Gao*,w
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,
Chinese Academy of Sciences, Shanghai 200050, China
Polycrystalline NiO nanorods have been synthesized by thermal
decomposition of rod-like nickel oxide precursor achieved by
microwave-assisted method. The structure, morphology, and
crystalline phase of the products are characterized by transmis-
sion electron microscopy (TEM), X-ray diffractometry, Fourier-
transformed infrared spectroscopy, and UV-visible absorption
spectrum. TEM reveals that NiO nanorods consist of numerous
NiO nanoparticles. The average size of NiO nanoparticles as a
function of the calcination temperature has been investigated.
The optical absorption band gap of polycrystalline NiO nanorods
is 3.7 eV.
synthesis protocol is fast, energy saving, and low cost for the
preparation of polycrystalline NiO nanorods will promote its
industrial application.
II. Experimental Procedure
(1) Preparation
All the chemicals were purchased from Shanghai Chemical
Regent Co. (Shanghai, China) and used as received without
any purification. In a typical experiment, 0.28 g of NiCl2 ꢀ 6H2O
was dissolved in 10 mL of ethylene glycol in a warm-water bath
at 601C, and then 0.7 g of sodium acetate (NaAc) was added
into the mixture under vigorous stirring. The as-formed green
transparent solution was transferred into a 50-mL round bot-
tomed flask under magnetic stirring. The solution was heated to
1101C and kept at this temperature for 10 min by microwave
heating. The microwave oven was a focused single-mode mi-
crowave synthesis system (Discover, CEM, Matthews, NC)
equipped with a magnetic stirrer and a water-cooled condenser.
The temperature was controlled by automatic adjusting of mi-
crowave power. The green product was separated by centrifu-
gation and washed by ethanol several times, and then dried in a
dynamic vacuum at 601C. Finally, the green precipitate was
heated at different temperature of 3201, 3501, and 4001C for 40
min.
I. Introduction
OW-DIMENSIONAL inorganic nanostructures possess unique
physical and chemical properties, such as the high surface-
L
to-volume ratio and the high fraction of chemically similar sur-
face sites, and have been the extensive research focus in recent
years.1 As a p-type wide-bandgap semiconductor, NiO is a very
promising material and has attracted increasing attention due to
its extensive important applications such as catalyst,2 electrode
materials for lithium ion batteries and fuel cell,3 electrochromic
films,4 and dye-sensitized photo-cathodes.5 Therefore, many
methods have been observed to fabricate nanostructure NiO,
such as thermal decomposition, sol–gel, surfactant-mediated
synthesis, and anodic alumina membrane template synthesis.6–9
However, there are few researches about microwave-assisted
synthesis of rod-shaped NiO. The microwave-assisted synthesis
of rod-shaped metal oxides (CuO and TiO2) have been investi-
gated.10,11 Pol et al. have reported a simple microwave irradia-
tion method for the large-scale synthesis of submicrometer-sized
TiO2 rods with diameters of about 400 nm and lengths up to
5 mm. Compared with the traditional methods, microwave heat-
ing has unique effects and significant merits such as rapid vol-
umetric heating, high reaction rate, short reaction time, and
energy saving.12–15 Xia and colleagues show that the ethylene
glycol-mediated synthesis of metal oxide nanowires requires
heating the solution at above 1701C for 2 h by convective heat-
ing method.16,17 However, in our present work, we obtain the
similar NiO nanorods at 1101C for 10 min under microwave
irradiation, which should be contributed to the very short non-
ionizing electromagnetic energy waves.10 Herein, we report a
facile and efficient microwave-assisted approach for fabricating
rod-shaped nickel oxide precursor that can be transformed into
pure polycrystalline NiO rod after calcination. The fact that the
(2) Characterization
The phase compositions of the powder were identified using
X-ray diffractometry (XRD, Model D/MAX-2550V, Rigaku,
Tokyo, Japan). The morphologies of the powder were examined
using transmission electron microscopy (TEM, Model 2100,
JEOL, Tokyo, Japan). Scanning electron microscopy (SEM)
images were collected on a JSM (Tokyo, Japan) 6700F scanning
electron microscope. Thermogravimetric analysis of the sample
was performed in air using a thermal analyzer (Model
STA449C, Netzsch Inc., Selb, Germany). Fourier-transformed
infrared (FT-IR) spectrometer was used to collect the spectra of
the samples (NEXUS, Nicolet, Madison, WI), and UV-visible
absorption spectra analysis was performed on a PE Lambda 950
spectrophotometer (Waltham, MA).
III. Results and Discussion
The 1D morphology of the nickel oxide precursor and the as-
annealed nickel oxide polycrystalline rods are investigated by
TEM. Figure 1(a) shows that the rod-like nickel oxide precur-
sors with diameter from 500 to 800 nm and length up to 10 mm
are obtained. The surface of these rod-like nickel oxide precur-
sors is smooth. The TEM image of calcined samples obtained at
3501C is presented in Fig. 1(b). It is evident that the rod-
like morphology is preserved in the calcination process. How-
ever, the magnification of single polycrystalline NiO nanorod
(Fig. 1(c)) demonstrates the smooth surfaces of rod-like nickel
Nancy Dudney—contributing editor
Manuscript No. 24644. Received May 8, 2008; approved July 20, 2008.
This work was supported by National Natural Science Foundation of China
(No. 50572116) and Shanghai Nanotechnology Promotion Center (No. 0652nm022).
*Member, The American Ceramic Society.
wAuthor to whom correspondence should be addressed. e-mail: liangaoc@online.sh.cn
3465

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Vol. 91, No. 10
(a)
(b)
(c)
140 (d)
adsorption
desorption
120
100
80
60
40
20
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P )
0
Fig. 1. Transmission electron microscopic images of (a) the nickel oxide precursor; (b) polycrystalline NiO nanorods obtained at 3501C for 40 min;
(c) magnified picture of single polycrystalline NiO nanorod, the inset is the corresponding selected area electron diffraction pattern; (d) nitrogen
adsorption/desorption isotherm of the polycrystalline NiO nanorod.
oxide precursors have transformed to rough structure with
more pores, which should be a result of the degradation of
organic groups and phase transformation. The selected area
electron diffraction pattern shows several Debye–Scherrer
rings (the inset of Fig. 1(c)), corresponding to the reflections
of the NiO polycrystals. Figure 1(d) shows the nitrogen
adsorption/desorption isotherms of polycrystalline NiO
nanorod, indicating the isotherms are of type IV. The sharp
decline in the desorption curve at 0.7oP/P0o0.8 suggests
characteristics of mesoporous materials.
Figure 2 shows the XRD patterns of the nonheated nickel
oxide precursor and as-calcined nanorods at different tempera-
ture. The pattern of the rod-like precursor (Fig. 2(a)) shows that
the diffraction spectra are strikingly similar to those of other
metal oxide precursors reported by other groups,16,17 especially
the strong peak locates in the low-angle region. To determine
exact structure of such a precursor is usually difficult.18,19 Sharp
reflections in the range of small angles can be assigned to the
layered stacking order of brucite-like structures composed of
linear coordination complexes. Chakroune et al.18 detected that
the main lines in the XRD pattern are indexed as the (00l) re-
flections. The strong peak around 101 is characteristic for or-
ganometallic precursors synthesized by the polyol-mediated
process. Jiang et al.17 proposed that ethylene glycol can serve
as a ligand that can form linear coordination complexes. The
linear complexes can then aggregate into ordered nanobundles
to generate nanowires through van der Waals interactions.
Based on the similarities between our reaction and those re-
ported in the literature, we believed that this mechanism might
be applied to explain the formation of rod-like precursors. All
diffraction peaks of as-annealed samples are perfectly consistent
with the cubic structure NiO (JCPDS 47-1049) and can be in-
dexed as (111), (200), and (220) crystal planes. It is clear from
XRD patterns that the peaks are broadened at relatively lower
temperature, suggesting that the particle size of as-obtained NiO
200
111
220
d
c
b
001
d=8.3
002
a
d=4.14
20
10
30
40
/deg
50
60
70
2
θ
Fig. 2. X-ray diffraction patterns of (a) unheated rod-like nickel
oxide precursor and NiO nanorods obtained at different temperature:
(b) 3201C, (c) 3501C, (d) 4001C.

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Communications of the American Ceramic Society
3467
nanoparticle increases with the increase of heating temperature.
The average crystalline sizes of the obtained NiO nanoparticles
are calculated using the Debye–Sherrer equation to be 6.7, 8.8,
and 17.6 nm for 3201, 3501, and 4001C, respectively.
1.2
1.0
0.8
0.6
0.4
0.2
0.0
25
20
15
10
5
In the FT-IR spectra of rod-like precursor (Fig. 3(a-I)), the
vibrational bands of CH2 at 2946 and 2869 cmꢁ1 as well as the
stretching band of C–OH at 1061 cmꢁ1 from ethylene glycol unit
are observed; simultaneously, the strong absorption bands of
C5O in acetate ion at 1569 and 1445 cmꢁ1 are also detected.
The three bands appearing around 1034, 883, and 682 cmꢁ1
should be assigned to the stretching and bending vibrations of the
C–O species in the precursor, also further confirming the presence
of acetate ions.20 Therefore, the chemical structure of rod-like
precursor is likely CH3COO–Ni–OC2H4OH. The curve (Fig. 3(a-
II)) of NiO nanorod obtained after 3501C shows no such bands
of rod-like precursor, indicating the complete decomposition of
rod-like precursor. Furthermore, the peak at about 475 cmꢁ1 as-
signed to Ni–O stretching vibration is clearly observed.
The thermogravimetric and differential thermogravimetry
analysis results (Fig. 3(b)) further confirm that the rod-like pre-
cursor formula is CH3COO–Ni–OC2H4OH. The TG curve of
rod-like precursor shows two weight loss (21.2% and 45.6%),
corresponding to the departure of water or excess polyol mol-
ecules and the decomposition of the precursor, respectively. The
formula weight calculated from the weight loss of 45.6% is 178
g/mol, which is very close to the formula weight of CH3COO–
Ni–OC2H4OH (179 g/mol).
0
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Ep/eV
300
400
500
600
700
800
Wavelength (nm)
Fig. 4. UV-vis spectrum and (aEp)2 vs Ep curve (inset) for polycrystal-
line NiO nanorod calcined at 3501C.
1/2 for an indirect transition.21 Hence, the optical band gap for
the absorption peak can be obtained by extrapolating the linear
portion of the (aEp)2 vs Ep curve to zero (the inset of Fig. 4). The
value of band gap of the polycrystalline NiO nanorod is about
3.7 eV, which is smaller than the value of bulk material
(4.0 eV).22 It is well known that semiconductors with nanoscale
size show a blue shift in their spectra due to the quantum con-
finement effects. However, the as-obtained samples have an Eg
smaller than the bulk one. This effect is likely due to the chem-
ical defects or vacancies present in the intergranular regions
generating new energy level to reduce the band gap energy.23,24
UV-vis spectrum of polycrystalline NiO nanorods is shown in
Fig. 4. A strong absorption in the UV region is observed at
wavelength smaller than 375 nm, which should be attributed to
band gap absorption of NiO nanoparticles assembling the poly-
crystalline NiO nanorods. The optical transition type and the
optical band gap (Eg) can be calculated by using the following
equation:
n
IV. Conclusions
ðaEpÞ / KðEp ꢁ EgÞ
The microwave-assisted method has been successfully used for
fast synthesis of rod-shaped CH3COO–Ni–OC2H4OH that can
be transformed into polycrystalline NiO nanorods at different
temperature of 3201, 3501, and 4001C. While increasing the
heating temperature, the size of NiO nanoparticles becomes
larger. The optical absorption band gap of the polycrystalline
NiO nanorod is determined to be 3.7 eV. This method is simple,
fast, and low cost for the preparation of polycrystalline NiO
nanorods.
where K is a constant, Ep is the discrete photon energy, a is the
absorbance coefficient, and n is either 2 for direct transition or
II
Ni-O
I
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