Synthesis of nanoparticles from submerged metallic rod
applying microwave plasma in liquid
Yoshiaki Hattori1, Shinobu Mukasa1, Hiromichi Toyota1, Toru Inoue2, Shinfuku Nomura1
1
Graduate School of Science and Engineering, Ehime University, Matsuyama, Japan
2
Geodynamics Research Cener, Ehime University, Matsuyama, Japan
Abstract: Metallic nanoparticles can be synthesized by feeding a metallic rod as a
source material into microwave-enhanced plasma in liquid at 20kPa. This method
raise the expectation of rapid synthesis, energy saving, easy collection of the
nanoparticles and selectivity of the characteristics of the nanoparticles by
selecting the kind of the liquid. The nanoparticles are observed by a transmission
electron microscope and analyzed by X-ray diffraction. Zinc nanoparticles in a
hexagonal shape with a diameter of 100nm are synthesized utilizing methanol as
the liquid, while zinc-oxide nanoparticles assembled radially from the sharp
sticks with diameters of 50nm and lengths of 150–200nm are synthesized.
Furthermore, magnesium-hydroxide nanoparticles with a diameter of
approximately 100nm can be synthesized in pure water shaped as triangles,
truncated triangles or hexagons. Silver nanoparticles with a diameter of 20nm can
be synthesized by utilizing pure water as well. The excitation temperatures were
calculated by the Boltzmann plot method using the spectral lines from the water
and the electrode. The excitation temperatures obtained by hydrogen, zinc and
magnesium lines were 3300±100K, 3200±500K and 4000±500K, respectively.
Keywords: microwave plasma in liquid, nanoparticles, excitation temperature,
Boltman plot method
1. Introduction
The characteristics of electrical discharge in water
have been investigated by many researchers and
there are many reports concerning discharge
properties and spectroscopic analysis. Recent
investigations have demonstrated preparation of
nanoparticles by plasma in liquid. Nanoparticles
have been studied in many fields due to their unique
optical, magnetic and electronic properties. Various
techniques including flame spray pyrolysis, laser
ablation, vacuum evaporation, arc discharge and solgel methods have been proposed for the synthesis of
nanoparticles.
Because
collection
of
the
nanoparticles is easier, some techniques are
conducted in liquid. One characteristic of synthesis
by plasma in liquid is that the ionized gas containing
for nanoparticles is rapidly cooled by the
surrounding liquid.
Some bulk material or ionic liquid containing
metal elements such as Zn(CH3COO)2, HAuCl4 and
AgNO3 is used as the precursor for the synthesis of
nanoparticles by generation of plasma in liquid. The
preparation of nanoparticles from the bulk material
can be adjusted by selecting the kind of liquid.
However, an ionic liquid requires a special disposal
procedure for the waste solution which contains
metal elements after synthesis of the nanoparticles.
Plasma in liquid is generated by applying direct
voltage or high frequency voltage to a submerged
electrode. Properties of the plasma and behavior of
the bubbles related to generation of plasma depend
on the method of voltage application to the electrode.
Plasma generated by a DC power supply is used for
the preparation of nanoparticles such as ZnO, Ag,
Au, CuO and WO3. Whereas, there are reports of
synthesis of ZnO and Au nanoparticles by radio
frequency wave or microwave using an ionic liquid.
One of the advantages of synthesis of nanoparticles
using microwave plasma in liquid is the rapid rate of
synthesis. However, synthesis of nanoparticles by
microwave plasma in a liquid using a bulk metal has
only just begun to be studied.
In this study, nanoparticles are synthesized from
bulk metal by microwave plasma in pure water and
alcohol. Nanoparticles are continuously synthesized
by feeding a metallic rod as the source material into
microwave plasma.
2. Experimental Setup
The setup of the equipment is shown in Fig. 1. The
experimental procedure for generating microwave
plasma in liquid are following our previously reports
[1]. The coaxial electrode fixed in the bottom of
reactor vessel is composed of end-rounded copper
inner electrode with an outer diameter of 10mm and
a Teflon, and a copper outer electrode. The
aluminum plate of 30mm in diameter with a tiny
hole in the middle is placed 4mm from the tip of the
coaxial electrode. The role of the plate is
enhancement of production rate [2]. The rod with
diameter in the range of 1-2mm is inserted
verticality through the top of reactor vessel. The tip
of rod is placed in the gap between the plate and the
coaxial electrode through the hole of the plate.
Plasma is generated in liquid at the tip the target by
microwave irradiation of 2.45GHz at 20kPa. After
plasma generation, nanoparticles are continually
synthesized by feeding the target manually. The
produced liquid are dried to a powder at room
temperature by evaporation at 4kPa, and analyzed by
a transmission electron microscope (TEM, JEOL,
JEM-2100) and X-ray diffraction (XRD, MAC
Science, M21X) to determine the characteristics of
target
pump
pressure gauge
reactor vessel
liquid
electrode
Teflon
stub tuner
power
plunger
microwave
generator
waveguide
Figure 1. Experimental apparatus
the
nanoparticles
structure.
In
addition,
spectroscopic measurement (Hamamatsu Photonics,
PMA-11) is conducted during producing of
nanoparticles.
3. Result and discussion
3.1 Synthesis of nanoparticles
Plasma was generated in ethanol by microwave of
180W with the zinc rod of 2.0mm in diameter.
Plasma was generated at the tip of the target. The
bubbles are generated simultaneously with the
plasma ignition. The gap between the coaxial
electrode and the plate was the gap filled with
bubble. The color of the liquid turned gradually gray.
The rod shortened by 9mm during the plasma
generation in 60 seconds. The TEM image of the
produced liquid is shown in figure 2(a). The image
shows Zn nanoparticles of 10-200 nm in diameter.
Because the particles are not only circular, but also
hexagonal and a quadrangular, some nanoparticles
seem to be hexagonal cylinders. The typical XRD
pattern (Cu Kα radiation) of the sample is shown in
Fig. 2(b). The diffraction peaks can be indexed to
the hexagonal structure of Zn. The synthesis rate of
Zn nanoparticles is 12g/hour.
Plasma was generated in pure water using a zinc
rod of 2.0mm in diameter by microwave of 135W.
The zinc rod shortened by 7mm in 45 seconds. The
TEM image of the obtained gray liquid is shown in
Fig. 3(a). A flower-like configuration composed of
many sharpened rods with diameters of 50nm and
lengths of 150 to 200nm was observed. The XRD
pattern of the sample is shown in Fig. 3(b). The
diffraction peaks were indexed to Zn and ZnO. The
synthesis rate of Zn/ZnO nanoparticles is estimated
to be approximately 14g/hour. The synthesis rate is
higher than that of the preparation of nanoparticles
from a zinc electrode [2].
Plasma was generated in pure water by
microwave of 160W at 20kPa using a magnesium
rod of 1.6mm in diameter. The color of the liquid
turned gradually white. The rod shortened by
110mm in 50 seconds. The slight sediment of solid
magnesium particles could be perceived at the
bottom of the reactor. The TEM image of the
obtained whitish liquid is shown in Fig. 4(a).
(a)
(b)
Intensity (a.u.)
50nm
ZnO
Zn
(101) (002)
Zn
(101)
ZnO
(210)
ZnO
(002)
ZnO
(100)
20
Zn
(100)
ZnO
(102)
ZnO
Zn
Zn
(110)
Zn
ZnO ZnO (103) (110)
(102)
(103) (112)
40
60
2θ (degree)
80
Figure 3. The TEM image (a) and the XRD patterns (b) of
Zn/ZnO nanoparticles
(a)
100nm
(b)
Intensity (a.u.)
Triangular, truncated triangular and hexagonal
nanoparticles with a diameter of approximately
100nm were observed. Because no square
nanoparticles could be observed, it is believed that
these nanoparticles are formed across a fairly thin
board. Nanoparticles in the shape regular triangles
might be created by the growth of truncated
equilateral hexagon nanoparticles during rapid
cooling. However, the mechanism of particle
development has not been clarified yet. The typical
XRD pattern of the sample is shown in Fig. 4(b).
The diffraction peaks can be indexed to the
hexagonal structure of Mg(OH)2. The synthesis rate
of Mg(OH)2 nanoparticles is 60g/hour.
Plasma was generated using a silver rod of 1.0mm
in diameter as a source material by microwave of
220W. The liquid turns yellow and the target rod
shortened by 2mm in 70 seconds. The TEM image
of the obtained product is shown in Fig. 5(a).
Spherical nanoparticles with a diameter of
approximately 20nm were observed. The size of the
particles is much smaller than other nanoparticles.
The XRD pattern of the sample is shown in Fig. 5(b).
The diffraction peaks were indexed to Ag. The
production rate of Ag nanoparticles is estimated to
be 0.8g/hour.
It is notable that whether the synthesized
nanoparticles are hydroxide, oxide or metal can be
determined by the kind of source metal. Oxide
nanoparticles were synthesized from aluminum and
iron by a similar procedure.
Mg(OH)2
(101)
Mg(OH)2
(001)
Mg(OH)2
(102)
Mg(OH)2
(100)
20
Mg(OH)2
(110) Mg(OH)2
(111)
Mg(OH)2
(201)
40
60
2θ (degree)
80
Figure 4. The TEM image (a) and the XRD patterns (b) of
Mg(OH)2 nanoparticles
(a)
(a)
(b)
(b)
Intensity (a.u.)
100nm
Zn
(101)
Intensity (a.u.)
20nm
Ag
(111)
Ag
(20 0)
20
Zn
(002) Zn
(100)
20
Zn
(102)
40
60
2θ (degree)
Zn
(103)
Zn
(110)
80
Figure 2. The TEM image (a) and the XRD patterns (b) of Zn
nanoparticles
40
60
2 θ (degree)
Ag
(220)
Ag
(311)
80
Figure 5. The TEM image (a) and the XRD patterns (b) of Ag
nanoparticles
1
Hα
(a)
OH
0.5
intensity
The system described here is associated strongly
with the preparation of nanoparticles from a bulk
material using pulsed laser ablation in liquid (PLAL).
Mg(OH)2, ZnO and Ag nanoparticles are
synthesized in water from the respective pure metal
by PLAL. Because the lasing efficiency is generally
10%, the amount of nanoparticles per energy
conception in the system described here will be
larger than for PLAL.
O
0.1
Hβ
0.05
Hγ
0
400
600
wavelength (nm)
800
1
ZnΙ
(b)
ZnΙ
intensity
0.5
ZnΙ
0.1
0
400
ZnΙΙ
Hα
600
wavelength (nm)
1
(c)
ZnΙΙ
ZnΙΙ
0.05
800
Mg
Mg
intensity
0.5
Mg
0.1
Mg
Mg
Na
Mg
0.05
Hα
0
400
600
wavelength (nm)
1
(d)
intensity
800
Ag
Ag
Ag
Ag
0.5
Ag
Ag
0.1
OH
Ag
Hα
0.05
O
O
Na
0
400
600
wavelength (nm)
800
Figure 6. Emission spectrum of the plasma generated in water
without rod (a) and with a rod of Zn(b), Mg(c) and
Ag(d).
-22
hydrogen
zinc
magnesium
-24
ln(Iλ/Ag)
3.2 excitation temperature
The excitation temperature was estimated using the
Boltzmann plot method under the assumption of
local thermodynamic equilibrium. The spectral
analysis of plasma in water for the excitation
temperature was conducted at 20kPa. When the
plasma was generated without a metallic target,
typical emission lines of plasma in water such as OH,
H and O were detected as shown in Fig. 6(a). The
scale of emission intensity in Fig. 6 differs between
the lower-half and the upper-half. Typical plots
applied to Boltzmann plot method are shown in
figure 7. The excitation temperature obtained by
three hydrogen peaks at 656nm, 486nm, and 434nm
is 3300±100K. These plots are fitted by least-squares
regressions. When the plasma was generated with a
metallic target, the spectral line from metallic rod
was detected. Figure 6(b) shows the spectrum of the
plasma generated with zinc rod. The excitation
temperature obtained from the zinc lines of 330nm,
334nm, 468nm, 472nm and 481nm is 3200±500K.
Figure 6(c) shows the spectrum of the plasma
generated with magnesium rod. The excitation
temperature obtained from the magnesium lines of
416nm, 435nm, 470nm, 552nm, 571nm and 880nm
is 4000±500K. Figure 6(d) shows the spectrum of
the plasma generated with silver rod. The silver
spectra of 328nm, 338nm, 521nm, 547nm, 768nm,
and 827nm were observed. Because the intensity
ratio of each peak changed considerably with time,
the excitation temperature could not be estimated.
To summarize, the excitation temperatures obtained
from hydrogen, magnesium and zinc lines are in the
range of 3000 to 4000K.
O
-26
-28
-30
6
8
10
E, eV
12
14
Figure 7. Boltzmann plot of microwave plasma in water
References
[1] Y. Hattori, S. Mukasa, S. Nomura, and H.
Toyota, J. Appli. Phys. 107 (6) 063305 (2010)
[2] Y. Hattori, S. Mukasa, H. Toyota, T. Inoue, and
S. Nomura, Mater. Lett. 65 188 (2011)