# Particle Size and Pore Structure Characterization of Silver Nanoparticles Prepared by Confined Arc Plasma

Authors: Mingru Zhou; Zhiqiang Wei; Hongxia Qiao; Lin Zhu; Hua Yang; Tiandong  Xia
Journal: Journal of Nanomaterials
 (2009)

---

## Abstract

In the protecting inert gas, silver nanoparticles were successfully prepared by confined arc plasma method. The particle size, microstructure, and morphology of the particles by this process were characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and the corresponding selected area electron diffraction (SAED). TheN2 absorption-desorption isotherms of the samples were measured by using the static volumetric absorption analyzer, the pore structure of the sample was calculated by Barrett-Joyner-Halenda (BJH) academic model, and the specific surface area was calculated from Brunauer-Emmett-Teller (BET) adsorption equation. The experiment results indicate that the crystal structure of the samples is face-centered cubic (FCC) structure the same as the bulk materials, the particle size distribution ranging from 5 to 65 nm, with an average particle size about 26 nm obtained by TEM and confirmed by XRD and BET results. The specific surface area is 23.81 m2/g, pore volumes are 0.09 cm3/g, and average pore diameter is 18.7 nm.


---

## Body

## 1. Introduction

Nanoparticles  exhibit novel properties
that significantly
different from those of corresponding bulk solid-state owing to the small size effect,
surface effect, quantum size effect and quanta tunnel effect [1–5]. During the past years, the investigation for metal nanoparticles has attracted considerable attention due to
their novel physical and chemical properties and potential application in
diverse areas, such as catalyst, microelectronic elements, photoelectronic
devices, lubricants, conductive
materials, activation, and sintering materials [6–11]. All these application
aspects require the powders consisting of monodisperse particles with desired
physical and chemical properties. Many unique properties of nanocrystalline
materials are mainly related to the particle size and pore structure [12–16]. The
investigation on the particle size and pore structure for nanoparticles is
essential to fully
understand the structure of a nanocrystalline material as well as to the
explanation of the intriguing properties, and it offers the possibility to obtain nanoparticles with
desired physical and chemical properties.Arc plasma method is a mature and
advanced materials processing technique, and many metal nanoparticles have
successfully been prepared by this method in the past [16–18]. Compared
with the conventional methods, confined arc plasma method
has been proven to be suitable for production of metal nanopowders with
ultrafine particle size, higher purity, narrow size distribution, well dispersed, and spherical shape. In
addition, the physical and chemical properties of the nanopowders by this
method can be easily improved by varying the processing parameters. Especially,
it is a convenient, inexpensive process, and suitable for mass production in the industry [19].
However, to the
best of our knowledge, there is no report on the preparation
of uniform and monodisperse Ag
nanoparticles in high yield by arc plasma
method. In this
paper, silver nanoparticles were successfully prepared by confined arc plasma
technique in inert atmosphere. In addition, the particle size, lattice
parameter, microstructure,
morphology, specific surface area, and pore parameters of the samples by this
process were characterized via X-ray powder diffraction (XRD), transmission
electron microscopy (TEM), the corresponding selected area electron diffraction
(SAED), and static volumetric absorption analyzer, and the results were
discussed.


## 2. Experimental

Silver nanoparticles were
prepared by confined arc plasma technique in inert atmosphere with home-made experimental
apparatus described elsewhere [18, 19]. In the process of
preparation, the vacuum chamber was pumped to 10−3Pa and then was
backfilled with inert gas near to 103Pa. The electric arc in the inert
environment was automatically ignited between the wolfram electrode and the
nozzle by high-frequency initiator. Under argon pressure and electric discharge
current, the ionized gases were driven through the nozzle outlet and form the
plasma jet [20]. The bulk metal silver (purity
99.99%) was
heated and melted by the high temperature of the plasma, 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, there is a region filled with supersaturated metal
vapor, where the metal atoms diffused around and collided with each other to
decrease the nuclei forming energy. When the metal vapor was supersaturated, a
new phase was nucleated homogeneously out of the aerosol systems [21]. The
droplets were rapidly cooled and combined to form primary particles by an
aggregation growth mechanism [22]. The free inert gas convection between the
hot evaporation source and the cooled collection cylinder transported the
particles out of this nucleation and growth region to the inner walls of the
cylinder. The
loose nanoparticles could be obtained after a period of passivation and stabilization
with working gas.Table1 shows the
referential technological parameters of preparing silver nanoparticles by
confined arc plasma method. To investigate the structure
of the samples, the as-obtained nanoparticles were analyzed by a
rotating target X-ray diffractometer (Japan Rigaku D/Max-2400) equipped with a monochromatic high-intensity
Cu Ka radiation (λ=1.54056Å,40kV,100mA), The X-ray powder diffraction data were recorded in a
range from 30
to 100°(2θ) with scanning rate 0.005°/s and step size 0.02°. The average
grain size of
the nanoparticles was estimated from the
half-maximum width and the peak position of an XRD line broadened according to the Scherrer
formula.Table 1
Referential technological parameters
of preparing metal nanoparticles by confined arc plasma.
Gas pressureAtmosphereArc voltageArc currentCooled conditionYield rateParticle size0.4~1.4 KPaHe,N2, Ar20~30 V60~160 AWater0.5~1.3 g/min20~100 nmThe particle size
and morphology of the
sample and the corresponding selected area electron diffraction (SAED) were examined by transmission electron
microscopy (TEM) and Japan JEOL JEM-1200EX microscope with an accelerating voltage of 80 kv,
respectively. In the process of preparation of the TEM specimen, a small
amount of the powders was dispersed in a few milliliters of normal butanol in an
ultrasonic bath
and sonicated for 30 minutes, and a drop from an eye dropper of
this dispersion sample was
placed on a copper grid coated with holey carbon film. 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.TheN2 absorption-desorption isotherms
of the samples at liquid nitrogen temperature (78 K) and gas saturation vapor
tension range were measured by using the ASAP 2010 static volumetric absorption analyzer
produced by Micromerities Corp., NY,
USA. Approximately 0.3 to 0.5 g of powder were placed in a test tube and allowed to degas for 2 hours at 175°C in flowing nitrogen. This
removes contaminants such as water vapor and adsorbed gases from the samples. The
static physisorption isotherms were obtained with N2 in liquid nitrogen, the amount of liquid
nitrogen adsorption, or desorption from the material as a function of pressure
(P/P0=0.025−0.999). Data
were obtained by admitting or removing a known quantity of adsorbing gas in or out of a sample
cell containing the solid adsorbent maintained at a constant temperature (77.35 K). As adsorption or desorption occurs, the pressure in the sample cell changes
until equilibrium is established. From the N2 static
physisorption isotherm of the samples to obtain the single
layer adsorption capacity, the specific surface area of the sample was calculated
from the BET adsorption equation. Based on the BJH academic model, the properties of the
cumulative pore specific surface area, cumulative pore volume, average pore
diameter, and BJH desorption pore distribution curves of the samples were estimated by BJH analysis method.


## 3. Results and Discussion

Figure1(a) shows the representative
transmission electron microscopy image of Ag nanoparticles. The TEM
observation shows that the morphology of Ag nanoparticles is monodisperse; most of them
are quasispherical
shapes with smooth surface and uniform size. Some 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
Ag nanoparticles.
(a)(b)Figure1(b) shows the corresponding selected area electron diffraction
(SAED) pattern. It can be indexed to the reflection of face-centered cubic
(FCC) structure in crystallography, and this result was also investigated by means of
X-ray diffraction. Tropism of the particles at random and small
particles causes the widening of
diffraction rings that made up of many diffraction spots, which indicate that the
nanoparticles are polycrystalline structure. Electron diffraction
reveals that each particle is composed of many small crystal nuclei, which is a
convincing proof that the particles grow in an aggregation model.From the data obtained by TEM micrographs, the particle size histograms
can be drawn, and the mean size of the particles can be determined. Figure2
shows the particle size distribution of Ag nanoparticles. It can be seen that the particle sizes
range is from 5 nm
to 65 nm, and the
median diameter (taken as average particle
diameter) is about 26 nm, being
deduced from the images, which shows a relatively broad size distribution.Figure 2
Particle size distribution of Ag
nanoparticles.Figure3 shows the typical X-ray diffraction pattern for the specimen.
The diffraction peaks are broad, suggesting that the sample consists of very
small particles. The major peaks of the pure Ag powders are observed. Five
broad peaks with 2θ values of 38.14°,44.70°,64.57°,77.37°, and 81.69° corresponding to the (111),
(200), (220), (311), and (222) planes of the bulk Ag, respectively, which
can be assigned to Ag FCC structure. The XRD pattern shows that the samples are
single phase, and no other distinct diffraction peak, except the characteristic
peaks of FCC phase Ag, was found.Figure 3
XRD patterns of Ag
nanoparticles.From the full width at half maximum, the grain size for the sample can be calculated from half widths of the major diffraction
peak (111) according to Scherrer formula,d=Kλ/Bcosθ,
where d is 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, 1.54056Å), θ is the diffraction angle, and B is the
corrected full width at half maximum (in radian). The average crystallite size was found
to be about 24 nm, which is well consistent with the average
particle diameter obtained from TEM images of Figure 2(a).According to the electron diffraction formulaRdhkl=λL and the X-ray diffraction λ=2dhklcosθ,
the values of interplaner spacings dhkl were calculated from
the diameters of the diffraction rings as well as from
the results of the XRD analysis.  For FCC structure, dhkl=a/h2+k2+l2, the lattice parameter (a) can be calculated from measured values
for the spacing of the (111) plane, respectively. Table 2 presents the results of
the lattice parameter and the interplaner spacings measured in the TEM-SAED and
XRD analyses, and compares them to standard ASTM data (a=4.088Å), the
lattice constriction was found.Table 2
Comparison of interplaner spacings(dhkl) and the lattice
parameter (a) with standard ASTM data.
MethodTEM (Å)XRD (Å)ASTM
standard value (Å)Interplaner
spacings(dhkl)2.9782.9772.980Lattice
parameter (a)4.0864.0844.088TheN2 absorption-desorption isotherms of the samples were
measured by using
the static volumetric absorption analyzer. Figure 4 shows the typical nitrogen sorption isotherms of Ag nanoparticles. It shows
the sample presents typical IV adsorption, in the low-pressure region (P/P0<0.7). It can be seen from
the graph that the isotherms relative flat, namely, the adsorption
and desorption isotherms
completely superposition, owing to the adsorption of the samples mostly
occurs in the micropores. At the relative high pressure region (P/P0>0.7), due to the capillary
agglomeration phenomenon, the isotherms increase rapidly and form a lag loop.Figure 4
N2 adsorption-desorption isotherms of Ag nanoparticles.Figure5 shows  BET plots of  Ag nanoparticles. The specific surface area of Ag nanopowder calculated using the multipoint BET-equation
is 23.81 m2/g. Assuming that the particles have solid, spherical
shape with smooth surface, and same size, the surface area can be related to
the average equivalent particle size by the equation DBET=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, and ρ is the theoretical density in g/cm3. Table 3 presents the BET experimental results of Ag nanoparticles. We noticed that the particle
size obtained from the BET and the TEM methods agrees 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.Table 3
BET experimental
results of Ag nanoparticles.
ConstantCMonolayer adsorption volumeVmBET surface areaSBETAverage particle sizeDBET32.77542.4258cm3/g23.81m2/g28 nmFigure 5
BET plots of Ag
nanoparticles.Figure6 shows the typical BJH desorption pore size distribution curves
of Ag
nanopowder. From the
curves, we can see that most of the micropores with a size
smaller than 40 nm, the probability pore size of which estimated from the peak
position is about 23 nm, and possesses
a relatively narrow pore size distribution. Moreover, such micropores have not been observed
within particles by TEM (see Figure 2). Therefore, these particles are actually
grain clusters, that is, small polycrystals. Based on the BJH academic
model, the property of the cumulative pore specific surface area, cumulative
pore volume, average pore diameter, and the probability pore size of pores were calculated by
BJH analysis method and summarized in Table 4.Table 4
Pore structure parameters of Ag
nanoparticles.
Cumulative
surface
area of
poresSBJHCumulative pore
volume of poresVBJHAverage pore
diameterdBJHProbability pore sizeD18.91m2/g0.0882cm3g−118.7 nm23 nmFigure 6
BJH pore size distribution
curves of Ag nanoparticles.


## 4. Conclusions

(1)
Silver nanoparticles were successfully prepared by
confined arc plasma method in the protecting inert atmosphere. The
nanoparticles prepared by this method achieved uniform particle size,
higher purity, well-dispersed and quasispherical shape.(2)
The crystalline structure of the
particles is FCC structure the same as that of the bulk materials, the particle size distribution
ranges from 5 to 65 nm with average particle size about 26 nm, the average
equivalent particle size obtained from the TEM and confirmed by XRD and
BET results.(3)
The specific surface area of the sample is 23.81m2/g calculated from
the BET adsorption equation. Based on the BJH academic model, the cumulative
pore specific surface area, the cumulative pore volume, the average pore
diameter, and the probability pore
size of the samples are 18.91m2/g, 0.0882cm3/g, 18.7 nm, and 23 nm, respectively.



---

*Source: 968058-2009-02-08.xml*