# Preparation and Characterization of NiO Nanoparticles by  Anodic Arc Plasma Method

Authors: Hongxia Qiao; Zhiqiang Wei; Hua Yang; Lin Zhu; Xiaoyan Yan
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

NiO nanoparticles with average particle size of 25 nm were successfully prepared by anodic arc plasma method.
The composition, morphology, crystal microstructure, specific surface area, infrared spectra, and particle size distribution of product were analyzed by using X-ray diffraction (XRD), transmission electron microscopy (TEM) and the corresponding
selected area electron diffraction (SAED), Fourier transform infrared (FTIR)  spectrum, and Brunauer-Emmett-Teller (BET)N2 adsorption.
The experiment results show that the NiO nanoparticles are bcc structure with spherical shape and well dispersed, the particle size
distribution ranging from 15 to 45 nm with the average particle size is about 25 nm, and the specific surface area is 33 m2/g. The
infrared absorption band of NiO nanoparticles shows blue shifts compared with that of bulk NiO.


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## Body

## 1. Introduction

Nanoparticles exhibit novel properties that significantly
differ from
those of corresponding bulk solid state owing to the different effects in terms of small size effect, surface
effect, quantum size effect, and macroscopic quantum tunnel effect [1, 2]. In
recent years, NiO nanoparticles as a kind of functional material has attracted
extensive interests due to its novel optical, electronic, magnetic, thermal,
and mechanical properties and potential application in catalyst,  battery
electrodes, gas sensors, electrochemic films, photoelectronic devices, and so
on [3–9]. In
these applications, it is still needed for synthesizing high-quality and
ultrafine powders with required characteristics in terms of their size,
morphology, microstructure, composition purity, crystallizability, and so on,
which are the most essential factors which eventually determine the
microstructure and performance of the final products. Therefore, it is very
important to control the powder properties during the preparation process.There
are many different methods reported for the synthesis of NiO nanoparticles,
such as ultrasonic radiation, hydrothermal synthesis, carbonyl method, laser
chemical method, pyrolysis by microwave, sol-gel method, precipitation-calcination,
microemulsion method, and so on [10–15]. However, to
the best of our knowledge, most of the reported experimental techniques for the
synthesis of nanopowders are still limited in laboratory scale due to some
unresolved problems, such as special conditions, tedious procedures, complex
apparatus, low yield, and high cost. From a practical viewpoint, it is vital to
develop a way to manufacture high-quality nanopowders at high throughput
with low cost.Anodic
plasma method is
an effective feasible process to prepare monodisperse metal nanopowders [16].
Compared with the conventional methods, anodic arc plasma method has many advantages:
metal nanopowders prepared by this process with ultrafine particle size, higher
purity, and narrow size distribution, well dispersed and
spherical shape. Furthermore, the physical and chemical properties of the
nanopowders can be easily controlled by varying the processing parameters and
no need for expensive
agents or special equipment. In addition, it is a convenient, inexpensive
process, has
high productive capacity and the potential for further mass production in the
industry.In this paper, NiO nanoparticles were
successfully prepared by anodic arc plasma technique. In addition, the
composition,  morphology, microstructure, specific surface
area, infrared
spectra, the
particle size, and distribution of product by this process were characterized via X-ray
diffraction (XRD), transmission electron microscopy (TEM) and the corresponding selected
area electron
diffraction (SAED), Fourier transform infrared spectrum (FTIR), and BET  N2  adsorption. The
knowledge obtained would enhance a better understanding to the microstructure
of NiO nanoparticles, and finally assist in the further development of novel properties and
also benefit the practical industrial application in the future.


## 2. Experimental

The
detailed experimental apparatus was fully illustrated elsewhere [16], it mainly includes the stainless
steel vacuum chamber, the gas supply device, the DC power supply, the plasma
generator with a high frequency initiator, the vacuum pump, the water-cooled
collection cylinder, and the water-cooled copper crucible. The
bulk pure material to be evaporated was laid on the
water-cooled copper crucible, which served as the anode. Ni rod with 10 mm diameter is mounted in
an insulated and axial manner, which was also water cooled and
served as the cathode. In the process of preparation, the vacuum chamber was pumped
to 10−3 Pa and then a
mixture of  O2 and Ar (1 : 4 by volume ratio)  was backfilled as a
reactant gas to reach the desired pressure. The electric arc in the inert environment was
automatically ignited between the Ni electrode and the nozzle by high-frequency initiator,
which was maintained by the current source at the
preestablished values of the voltage and current. Under argon pressure and
electric discharge current, the ionized gases were driven through the nozzle
outlet and form the plasma jet. The bulk metal nickel (purity 99.99%) was heated and melted by the high temperature, and
metal atom detached from the metal surface when the plasma jet kinetic energy
exceeds the metal superficial energy, and evaporated into atom soot. Above
the evaporation source was a region of supersaturated metal vapor, where metal
atoms diffused around and collided with O atom to form NiO at high temperature
due to the oxidation reaction. When the vapor was supersaturated, a new phase
was nucleated homogeneously out of the aerosol systems. The
droplets were rapidly cooled and combined to form primary particles by an
aggregation growth mechanism. The particles were transported from the
nucleation and growth region to the inner walls of the cylinder by the free
inert gas convection between the hot evaporation source and the cooled
collection cylinder, the loose nanoparticles could be obtained.The
crystalline structures of the products were characterized by a
rotating-target X-ray diffractometer (Japan Rigaku D/Max-2400) equipped  with a monochromatic high-intensity
Cu Ka radiation (λ = 1.54056 Å, 40 kV, 100 mA). The sample was scanned in a range from 30 to  90° (2θ) with scanning rate was 0.005°/s and step size 0.02°. The average
crystalline
grain size of the NiO nanoparticles was estimated from the
half maximum width and the peak position of an XRD line broadened according the Scherrer
formula, which was further confirmed by the transmission electron
microscopy (TEM) results and BET results.The particle size and
morphology of the sample and the corresponding selected area electron
diffraction (SAED) were examined by JEOL JEM-1200EX transmission electron
microscopy (TEM) with an accelerating voltage of 80 kv. In the process of preparation
of the TEM specimen, a small amount of the powders was dispersed in a few
milliliters of ethanol in an ultrasonic bath and ultrasonic treated for 30 minutes, and a
drop from an
eye dropper of the resulting suspension was placed onto a carbon-coated copper grid. The samples  were placed in
a vacuum oven to dry at ambient temperature before examining. The sample is
scanned in all zones before the picture is taken.The specific  surface area of
the sample was calculated from the BET adsorption equation and measured by the accelerated
surface area porosimetry (ASAP 2010) instrument produced by  Micromerities Corp. at liquid nitrogen
temperature (78 K).The
absorption spectroscopy spectra of the product were recorded from 400 to 4000cm−1 on a Nicolet Nexus 670
Fourier  transform
infrared (FT-IR) spectrometer using the KBr pellet technique to determine the
chemical structure of the sample ．


## 3. Results and Discussion

The
purity and crystallinity of the as-synthesized NiO nanoparticles were examined
by using powder X-ray diffraction (XRD) as shown in Figure1. It can be seen
from Figure 1 that the diffraction peaks are low and broad due to the small size effect and incomplete
inner structure of the particle. The peaks positions appearing at 2θ  = 37.20°, 43.20,  62.87°,  75.20°,
and  79.38° can be readily indexed as (111),  (200),  (220),  (311),  and (222)
crystal planes of the bulk NiO, respectively. All these diffraction peaks can
be perfectly indexed to the face-centered cubic (FCC) crystalline structure of
NiO, not only in peak position, but also in their relative intensity of the
characteristic peaks, which is in accordance with that of the standard spectrum  (JCPDS, No. 04-0835).  The XRD pattern
shows that the samples are single phase  and no any other impurities distinct diffraction peak except the characteristic
peaks of FCC phase NiO was detected. This
result shows that the physical phases of the NiO nanoparticles have higher
purity prepared in this work. The NiO lattice constant calculated from the XRD
data is   4.1729 Å, which
is in good agreement with the reported data (JCPDS, No. 04-0835).Figure 1
XRD pattern of NiO nanoparticles.The
average crystallite size is calculated by X-ray diffraction line broadening
using the Scherrer formulad=Kλ/Bcosθ, where d represents the grain size; K = 0.89 is the Scherrer constant related to the
shape and index (hkl) of the crystals; λ is the wavelength of the X-ray (Cu Ka,
1.54056 Å); θ is the diffraction angle of the
peak; B stands
for the full width at half-height of the peaks (in radian) given by B2=Bm2−Bs2,
where Bm is the full width at half maximum (FWHM) of the sample and Bs is the
half-width of a standard sample with a known crystal size greater than 100 nm,
the effect of instrumental broadening on the reflection peaks is calibrated.
The crystallite sizes of NiO samples is 23 nm which was calculated
from measured values for the spacing  of the (111) plane.Figure2(a) shows the
representative transmission electron microscopy image of NiO nanoparticles. TEM
analysis of the products provided information on the size and morphology of NiO
nanoparticles and their state of agglomeration. It can
be seen from Figure 2(a) that the NiO nanoparticles had spherical shapes and were
well dispersed with smooth surface and uniform size. Few small particles aggregate
into secondary particles because of their extremely small dimensions and high
surface energy.(a) TEM
micrograph and (b) the selected area electron diffraction pattern of NiO
nanoparticles.
(a)(b)Figure2(b) shows the
corresponding selected area electron diffraction (SAED) pattern. The SAED
pattern consists of five diffraction rings with different  radii and one center. The
diameter of the
diffraction ring in SAD pattern is proportional to h2+k2+l2,
where (hkl) are
the Miller indices of the planes corresponding to the ring, counting from the
center 1st,  2nd, 3rd, 4th, and 5th rings correspond to (111), (200), (220),
(311), and (222) planes, respectively. Tropism of the particles at random and
small particles
cause the widening of diffraction rings that made up of many diffraction spots,
which indicates that the nanoparticles are  polycrystalline structure. The
SAED pattern also confirms that the NiO nanoparticles  are face-centered cubic (FCC) structure NiO in
crystallography, which consistent with the above X-ray  results.From the data obtained by
TEM micrographs, the particle size histograms can be drawn and the mean size of
the particles can be determined. Figure3 shows the particle size distribution
of NiO
nanoparticles. It can be seen that the particle sizes possess
a small and narrow size distribution in a range from 15 to 45 nm, and the mean
diameter (taken as average particle diameter) is about 25 nm. We
notice that the mean particle size determined by TEM is in good agreement with
the average crystallite size calculated by the Scherer formula from the XRD
patterns. According to the TEM image, it
could be concluded that this preparation method had successfully overcome the
problem of agglomeration and appropriate to obtain the NiO nanoparticles with
smaller crystalline size.Figure 3
Particle size distribution of NiO nanoparticles.The
surface area analysis was carried out on NiO nanoparticles by BET method.
Assuming the particles possess solid, spherical shape with smooth surface and
same size, the surface area can be related to the average equivalent particle
size byDBET = 6000/(ρ⋅Sw)  (in
nm), where DBET is the average diameter of a spherical
particle; Sw represents the measured surface area of the powder in m2/g; ρ is the theoretical density in g/cm3. Figure 4
shows BET plots of NiO nanoparticles, the specific surface area of NiO
nanoparticles
calculated using the multipoint BET-equation is 33 m2/g, and the
calculated average equivalent particle size is 28 nm. We noticed that the particle
size obtained from the BET and the TEM methods agree very well with the result
given by X-ray line broadening. The results of TEM observations and BET methods
further confirmed and verified the relevant results obtained by XRD as
mentioned above.Figure 4
BET
plots of NiO nanoparticles.Figure5 shows the FTIR
spectra of NiO nanoparticles, which showed several significant absorption
peaks. The broad absorption band in the region of 600–700 cm−1   is
assigned to Ni–O stretching
vibration mode; the broadness of the absorption band indicates that the NiO
powders are nanocrystals. The size of samples used in this study was much less
than the bulks form NiO, so that NiO nanoparticles had its IR peak of Ni–O
stretching vibration and shifted to blue direction. Due to their quantum size effect and
spherical nanostructures, the FTIR absorption of NiO nanoparticles is blue-shifted
compared to that of the bulk form. Besides the Ni–O  vibration, it could be seen
from Figure 5 that the broad absorption band centered at 3440 cm−1 is attributable to the
band O–H stretching vibrations and the weak band near 1635 cm−1 is assigned to H–O–H bending
vibrations mode were also presented due to the adsorption of water in air when
FTIR sample disks were prepared in an open air. These observations provided the
evidence to the effect of hydration in the structure. Meanwhile, it implied the
presence of hydroxyl in the precursor, and the broad absorption around 767 cm−1 is
assigned to the band C=O stretching vibrations. The serrated absorption bands
in the region of 1000–1500 cm−1 are
assigned to the O–C=O symmetric and asymmetric stretching  vibrations and the C–O
stretching vibration, but the intensity of the band has weakened, which
indicated that the ultrafine powers tend to strong physically absorption to H2O
and CO2.Figure 5
FTIR spectra of
NiO nanoparticles.


## 4. Conclusions

(1) Cubic NiO nanoparticles with spherical
shape higher purity, well dispersed, and narrow size distribution ranges from 15 to 65 nm
were successfully prepared by anodic arc plasma method and
different approaches such as XRD, TEM, SAED, FTIR, and BET were used to characterize.(2) The specific surface area of the sample is 33 m2/g
calculated from the BET adsorption equation, the average particle size about 25 nm, the average
equivalent particle size obtained from the TEM and confirmed by XRD and
BET results. The infrared absorption band of the NiO nanoparticles show blue shifts
compared with that of bulk NiO.



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*Source: 795928-2009-02-25.xml*