Application of Integrated Micro Solution Plasma
to Decomposition of Organic Substances in Water
T. Shirafuji1, J. Hieda2, N. Saito3,4 and O. Takai2,3,4
1
Dept. Phys. Electron. & Info., Osaka City Univ., Osaka, 558-8585, Japan
2
Dept. Mater. Phys. Energy Eng., Nagoya Univ., Nagoya 464-8603, Japan
3
EcoTopia Sci. Res. Inst., Nagoya Univ., Nagoya 464-8603, Japan
4
JST/CREST, Nagoya Univ., Nagoya 464-8603, Japan
Abstract: should not be more than 200 words.
Keywords: list 3-5 relevant keywords
1. Introduction
In contrast to the conventional solution chemistry,
the solution plasma processing (SPP) involves
accelerated electrons which contribute to generate
active chemical species, such as radicals, ions, UV
photons and metastable excited atoms [1, 2]. Such
active species are expected to enhance through-put
of the solution chemistry and to promote the
reactions which do not proceed without catalysts.
In our previous work, we have successfully
obtained glow discharges in water, and applied this
technique to modify the surface of nano-materials
[3]. Since our solution plasma is ignited in a small
volume between two stylus electrodes, actual
treatment area or volume should be enlarged for
practical industrial application. In the case of gas
phase processes, large area processing is realized by
producing large area plasmas. In the case of SPP,
however, large volume plasmas in liquid are
meaningless, because the most important region is
gas-liquid interface. Thus, preparation of large
number of tiny plasmas (microplasmas) is rather
important in the case of SPP. This should be named
as “integrated micro solution plasma”.
In order to realize the integrated micro-solution
plasmas, we have utilized interfaces between a plane
dielectric plate and porous dielectric material. In this
work, we report that this type of configuration can
ignite the micro solution plasmas at the interface,
and demonstrate that organic substances in aqueous
solution can be decomposed with this technique.
Vapp = 2kV
through dielectric barrier
Gas gap
ε=1
Insulator
frame
Pulse
voltage
source
Insulator shield to avoid
unintentional discharges
ITO (for top-view observation)
5 kV
Insulator
frame
ε=4
ε=4
Quartz ( t = 1mm, diam.= 10 cm)
3.2 kV
10 kHz
2 μs/on
Microplasmas at interface
Melamine foam ( t = 2 cm)
w/ aqueous solution
M.B. 50 mg/L, 120 uS/cm (KCl)
Copper plate
Figure 1. Schematics for realizing integrated micro-solution
plasmas.
Grounded
through conductive liquid
0
1.2 mm
Figure 2. Calculated potential profile around a gas-gap
formed at the interface between the quartz and porous
dielectric
gap / conductive aqueous solution.
2. Experimental
Figure 1 shows the experimental setup for
realizing the micro solution plasmas with porous
dielectric. The porous dielectric material employed
in this work is Basotect® (BASF) [4], which is an
open cell foam made from melamine resin.
Approximate pore size is 200-500 um. It holds
conductivity-controlled aqueous solution to be
subjected to our plasma treatment.
The plane quartz plate is attached on the top of
the porous dielectric foam. If the aqueous solution is
properly filled in the porous dielectric, we can
expect formation of tiny gas gaps at the interface
between the quartz plate and porous dielectric. Each
tiny gas-gap region is now forming a dielectricbarrier-discharge configuration made of quartz / gas-
Thus, if we apply ac high voltage between top of
the quartz plate and bottom of the porous dielectric,
we can expect electrical discharges in each gas-gap.
Figure 2 shows calculated potential profile around
the gas-gap region. Large potential drop is expected
in the gas gap as shown in the figure. Thus, we can
expect generation of microplasmas on the surface of
liquid which is hold in the insulator frameworks of
the porous dielectric material.
The top conductive electrode is made of ITO,
because we need to observe the interface plasmas
through the electrode. The diameter of the ITO is 10
cm. Bottom electrode is copper plate. In this work,
the voltage applied between the electrodes is pulsed
voltage with amplitude of 5 kV, frequency of 10 kHz
and width of 2 μs.
The conductivity of the aqueous solution hold in
the porous foam is controlled to be 120 μS/cm by
using KCl, otherwise we cannot obtain electrical
discharge at the interface.
When we constructed simple sandwich structure
with the quartz plate / porous foam / copper
electrode, we had unintentional discharge between
the ITO and the copper electrode. Thus, we have
Figure 3. The integrated micro-solution plasmas observed
through the top quartz electrode.
Figure 4. Optical emission spectrum of the plasma observed
at the interface between the quartz plate and porous foam
containing methylene blue aqueous solution.
Figure 5. Structure of methylene blue molecules subjected to
the microplasma treatment.
installed a glass ring as an insulator shield as found
in Figure 1.
3. Results and discussion
Figure 3 shows optical images observed when we
apply the pulse voltage between the electrodes. As
can be seen in the figure, many microplasmas are
formed at the interface between the quartz plate and
porous foam. Actual size of the microplasma is
approximately 300 μm, which is corresponding to
the pore size of porous melamine foam employed in
this work. In our previous work, the plasma in
contact with liquid is only one tiny plasma generated
between two stylus electrodes. In this work, the size
of each microplasma is as small as 300 μm, but their
integrated size is as large as 10-cm diameter.
Furthermore, scaling toward larger diameter is
possible if the voltage source has enough current
capacity. This type of micro-solution plasma,
therefore, can be a good candidate for large volume
plasma processing of liquid substances.
Figure 4 shows optical emission spectrum of the
plasma taken through the quartz plate with ITO
electrode. Since the gas-gaps formed in this work is
not generated via Joule heating of the aqueous
solution, the major component of the gas-gaps is air.
Thus, the spectral profile of the emission spectrum
also looks like that of air, which composed of the
second positive system band of nitrogen molecules.
Since the gas-gaps are in contact with aqueous
solution, we can expect H2O vapor in the air in the
gas-gaps, and OH generation through electron
impact dissociation of the H2O molecules. However,
within the present experimental condition, we cannot
observe explicit emission peak corresponding to the
OH radicals. Further investigation is required.
Figure 6. Schematics for realizing integrated micro-solution
plasmas.
Although we cannot observe OH radicals, we
have examined possibility of decomposition of
organic contamination in water with this microsolution plasma. In this work, we have employed
methylene blue as the organic contaminant in water,
because its decomposition is easily identified
through its decolorization. Concentration of the
methylene blue is 50 mg/L. One-run duration of the
plasma treatment is10 minutes.
Figure 5 shows the results of the plasma
treatment of the methylene blue aqueous solution.
The treatment was repeated until the color of
aqueous solution became transparent. Total duration
of the treatment is 60 minutes. As can be seen in the
figure, the aqueous solution became almost
completely transparent after the treatment for 60
minutes.
The methylene blue decolorization can also occur
through reduction reaction. In this case, however,
blue color comes back via oxidation reaction. Since
the oxidation can occur even with the air, we can
check it by exposing our sample to the air. Although
we exposed our sample to the air for 1 week (and
more, until now), the color change was not observed.
Thus, we can conclude that the methylene blue
molecules are decomposed through our plasma
treatment.
Summary
In order to improve application capability of
liquid-related plasmas, we have tried to generate socalled “integrated micro solution plasmas”, which
consist of micro plasmas in or in contact with liquid.
In this work, we have employed a piece of porous
dielectric foam and a dielectric plate to form a kind
of tiny DBD at the interface between them, and
successfully obtained many tiny micro plasmas at
the interface between them. Actual diameter of the
discharge region is 10 cm, which is much larger than
our previous point discharge of a few mm.
Although OES spectrum did not contain OH
emission, methylene blue molecules can be
decomposed via plasma treatment using our
technique. Thus, this type of micro-solution plasma
can be a good candidate for large volume plasma
processing of liquid substances.
References
[1] N. Saito, J. Hieda, C. Miron, O. Takai: Surf.
Finish., 58, 810 (2007).
[2] O. Takai: Pure Appl. Chem., 80, 2003 (2008).
[3] J. Hieda, T. Shirafuji, Y. Noguchi, N. Saito and
O. Takai: J. Jpn. Inst. Metals, 73, 938 (2009).