22P012-O
Generation of Plasmas in Porous Materials
with A Gas-Liquid Mixed Phase Medium

Tatsuru Shirafuji
Dept. Physical Electronics and Informatics, Osaka City University
3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan558-8585
3-3-138
Generation of plasmas in porous materials with a gas-liquid mixed phase medium has been performed,
where the liquid and gaseous medium is conductivity-controlled water and nitrogen gas. In comparison toconventional one-point electrical discharge between two stylus electrodes, this technique enables us to treat
large volume liquid medium flowing in a fluid channel. The generation of plasmas in the porous materials
has been confirmed by observing optical emission. Application capability of this technique has been
demonstrated by performing synthesis of Au nano-particles.

2. Experimental
Figure 1 shows schematic illustration of theexperimental setup for generating 3D integrated
micro solution plasmas. The reactor is constructedwith the EVAC glass components. The main porous
material, in which the plasma is generated, isporous SiO2 (pumice) containing random pores.
Most of the pores are connected topologically,
although some of them are isolated. Averaged sizeof the pores is approximately 0.5 mm (diameter).
The second small-size porous material is used forsupplying gas (N2, 2 L/min) from the bottom side of
the main porous material. In future, both of the gas
and liquid will be fed through the electrode in orderto have gas-liquid mixed phase medium efficiently.
A metal electrode, where the high voltage pulse
is applied, is inserted in the center axis of theporous material. The grounded electrode is attachedon the outer surface of the glass components asshown in Fig.1. The liquid medium employed inthis work is water with NaCl (16 mS/m) for
observation of plasma. The liquid used for Au

30 mm

Grounded
Metal mesh

Glass

Main
porousdielectricmaterial

LiquidPlasmaInsulator

Front crosssectional view

HV metalelectrodeSide crosssectional view

Grounded
Metal mesh

1. Introduction
In contrast to the conventional solution
chemistry, the solution plasma processing (SPP),
which has been invented by O. Takai and N. Saito[1,2], involves accelerated electrons which
contribute to generate active chemical species,
such as radicals, ions, UV photons and metastable
excited atoms. Such active species are expected
to enhance through-put of the solution chemistry
and to promote the reactions which do notproceed without catalysts.
In our previous work, we have successfully
obtained glow discharges in water, and applied thistechnique 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 forpractical industrial application. In the case of gasphase processes, large area processing is realized byproducing large area plasmas. In the case of SPP,
however, large volume plasmas in liquid aremeaningless, because the most important region isgas-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 recently utilized interfaces
between a plane dielectric plate and porous
dielectric material, and successfully obtained large
area integrated micro solution plasma [4]. In thiswork, we report that 3-dimensionally integrated
plasmas can be obtained in a porous dielectricmaterial, and demonstrate that Au nano-particles
can be synthesized by using this technique.

PlasmaGas+Liquid(in future)
Sub smallporousdielectricGas (at present)

Fig.1 Schematic illustration of the experimental setup for
generating 3D integrated micro solution plasmas.

(a)

6

Absorbance (arb. units)

30 mm

5SPP in porous
dielectric (Flow-type)

4Residual solution
in porous dielectric
1-day after the SPP

321(b)
0300400500600700800

Wavelength (nm)Fig.3 Absorbance spectrum of the residual wine-red coloredliquid substance after the 3D integrated micro solution plasma
process (10 min.) of the H(AuCl4) aqueous solution.
30 mm

Fig.2 (a) Photograph of the experimental setup without
discharge, and (b) observed optical emission from the plasma
generated in the porous material.

nano-particle synthesis is 0.15-mM H(AuCl4)-4H2O(hydrogen tetrachloro- aurate (III) tetrahydrate)aqueous solution (150 uS/cm) with 10 wt.% gelatin.
In both cases, voltage is supplied from a bipolar
high voltage power source (Haiden, SBP-5K-HF2),
from which square-wave pulse voltage with 5-kV
amplitude is supplied. Pulse frequency is 20 kHz
and pulse width is 20 us.
3. Results and Discussion
Figure 2(a) shows actual experimental setup
corresponding to the Fig.1. Figure 2(b) shows
optical emission from inside of the porous materials.
We can see volume-plasma generation in the porous
material, although its size is not distributed
uniformly. The parasitic discharges are also
observed at the edges of metal mesh, but they canbe avoided by utilizing conductive thin films (ITO,for example) evaporated on the outer surface of theglass component. This result indicates that ourconcept is appropriate for obtaining volume 3D
integrated micro solution plasmas.
In order to confirm capability of material
processing by using this technique, we have
performed Au nano particle synthesis using
H(AuCl4) aqueous solution. Only the liquidmaterial has been replaced form water to theH(AuCl4) aqueous solution for this experiment.

Figure 3 shows optical absorbance spectrum of
the residual wine-red colored liquid substance in thereactor after 10-min treatment with the 3D
integrated micro solution plasmas. We can see a
broad absorption peak around the 550 nm, whichcorresponds to plasmon resonance absorption by
Au nano particles [1,2]. This result indicates thatour 3D integrated micro solution plasma hascapability of Au nano particle synthesis.
Because of the basic concept of this technique,we can expect that this technique can be applied tocontinuous
large
volumeprocessingfor
synthesizing Au nano particles by appropriately
designing the process reactor.
Acknowledgments
This work has been partly supported by theCREST/JST, the Knowledge Cluster Initiative
Tokai Region Nanotechnology Manufacturing
Cluster, Grant-in-Aid for Scientific Research on
Innovative Areas, and Grant-in-Aid for Scientific
Research (C) by Ministry of Education, Culture,
Sports, Science and Technology, Japan. The author
thanks to the following students for theirexperimental help; Mr. Y. Himeno, Mr. M. Iwaki,
Mr. A. Nakamura and Mr. J. Ueda.
References[1] N. Saito, J. Hieda, C. Miron, O. Takai: Surf. Finish.
58 (2007) 810.
[2] O. Takai: Pure Appl. Chem. 80 (2008) 2003.
[3] J. Hieda, T. Shirafuji, Y. Noguchi, N. Saito, O. Takai:
J. Jpn. Inst. Metals 73 (2009) 938.
[4] T. Shirafuji, J. Hieda, N. Saito, O. Takai: 20th Int.
Symp. Plasma Chem., (2011) in CDROM.