SVI-2
プラズマ気液界面における物理・化学現象と新規ナノバイオ物質創製
Physical and Chemical Phenomena on Plasma Gas-Liquid Interface andSynthesis of Novel Nano-Bio Materials
金子俊郎
Toshiro KANEKO
東北大院工
Dept. of Electronic Eng., Tohoku Univ.
1. Introduction
Plasmas generated in contact with liquid [1]have attracted much attention as a novel reactive
field in creation of functional nanoparticles [2]because the brand-new chemical reactions areyielded at the gas-liquid interface, which areinduced by the physical actions of thenon-equilibrium plasmas. Especially, the patternedgold nanoparticle (AuNP) structure can besynthesized by the gas-liquid interfacial plasma andis expected to be applied to novel bio-imaging
device, optoelectronic device, photoelectric device,and so on, using a principle of the surface plasmon
resonance of the AuNPs.
To realize the periodic structure of thenanoparticles, we adopt a novel plasma technique
combined with introduction of ionic liquids understrong magnetic fields up to 4 tesla (T) [3]. Since
the plasma generated under the strong magnetic
field keeps its structure due to confinement along
the magnetic field lines, the plasma structure can betranscribed to the liquid surface, resulting in thesynthesis of the structured nanoparticles at thegas-liquid interface when the plasma reduces themetal chlorides in the liquid.
2. Experimental Apparatus
Figure 1(a) shows the schematic of anexperimental setup for nanoparticle structureformation using a gas-liquid interfacial discharge
plasma (GLIDP) under strong magnetic fields,
which has a glass cell with 15 mm inner diameterand 10 mm depth in a cylindrical glass chamberwith 75 mm diameter and 200 mm length. A DCvoltage VD is supplied to an upper cathode electrodecomposed of a stainless steel (SUS) plate and aSUS mesh grid is used as an anode electrode topromote a spatial diffusion of the plasma. Typicaldischarge current is ID=1~3 mA. Nitrogen gas isadopted as a discharge gas in view of its reactivity,
and the gas pressure Pgas is varied from 20 to 100
Pa.

The new kind of the ionic liquid(N.N.N.-Trimethyl-N-propyl-ammoniumBis(trifluoro methane sulfonyl) imide) put in the glass
cell is placed on a peltier element which is locatedat a distance of 50 mm from the anode electrode.
Since this ionic liquid does not become supercooled
state, we can make the ionic liquid solid state bycooling the ionic liquid using the peltier element
located under the glass cell. When the strong
magnetic fields are applied along the machine axis,
the generated plasma is strongly magnetized, andthen, the periodic plasma structure formed by themesh anode is maintained just above the ionicliquid as shown in Fig. 1(b). Typical plasma
parameters for ID = 3 mA, Pgas = 30 Pa, z = 10 mmare as follows: electron density ne~1010 cm-3,electron temperature Te~0.35 eV, plasma spacepotential ϕ ~ 1 V.
The AuNPs are synthesized in the ionic liquidby the plasma reduction of Au chloride (HAuCl4).
The ionic liquid can be cooled by the peltierelement and becomes the solid state as mentioned
above, with keeping the AuNP structure formed by
the plasma irradiation at the liquid interface.

Fig. 1. (a) Schematic of experimantal setup for
nanoparticle structure formation and (b) photo of
the formed periodic plasma structure. Pgas = 30
Pa, ID = 3 mA, B = 1 T.

3. Experimental Results
Figure 2 shows photos of the temporal
evolution of the periodic AuNP structure which isformed at the gas-liquid interface in accordancewith the periodic plasma structure under thecondition of the strong magnetic field of B = 1 T for(a) ID = 3 mA and (b) ID = 1 mA. The density of theperiodic structured AuNPs increases with plasma
irradiation time tpi, and the obvious structure isformed typically within tpi = 2 min. Surprisingly,
the synthesized AuNPs have the mesh shape for ID= 3 mA, namely, the AuNPs are synthesized in theshadow region of the wire of the mesh anodeelectrode as shown in Fig. 2(a). For ID=1 mA, on
the other hand, the AuNPs are oppositely
synthesized in the hole regions of the mesh anodeelectrode.

larger than that of the C-H bond (D=4.29 eV), canbe dissociated by the high-energy ions. Therefore,
the AuNPs are destroyed by the oxidation effect of
the fluorine radicals which come from thedissociation of the ionic liquid by the collision withthe high energy charged particle. Here, Larmor
radius of the nitrogen molecular ion is calculated to
be about 0.1 mm based on the assumption that theion temperature is room temperature and B = 1 T.
Since the wire diameter of the ring electrode is 0.6mm, the ring shaped AuNP structure can remain
even if the ions slightly irradiate inside the shadow
region of the wire due to the Lamor motion.

Fig. 3. (a) Model of the synthesis mechanism of
the ring shaped Au nanoparticles and (b)chemical formula of the ionic liquid used in thisexperiment.

Fig. 2. Temporal evolution of the periodic
nanoparticle structure formed by controlled
gas-liquid interfacial plasmas for (a) ID = 3 mAand (b) ID = 1 mA. Pgas = 30 Pa, B = 1 T..
To explain the phenomena, we use the modelof the AuNP synthesis as shown in Fig. 3(a). As
mentioned above, the charged particles such as theelectrons and the positive ions cannot reach to theshadow region of the ring electrode, namely, only
neutral radicals can arrive at the shadow region.
The ionic liquid used in this experiment is
described in Fig. 3(b), which has C-H bond incation (positive ion) and C-F bond in anion(negative ion). When the radicals in the plasma areirradiated to the ionic liquid, the C-H bond of theionic liquid is considered to be dissociated, and thegenerated hydrogen radicals reduce the Au ions [3],resulting in the synthesis of AuNPs in the shadow
region of the ring electrode. On the other hand, inthe plasma irradiation region, relatively high-energy
ions are irradiated to the ionic liquid, and the C-F
bond whose dissociation energy (D=5.07 eV) is

4. Conclusions
The periodic AuNP structures are formed,
which correspond to the shape of thestrongly-magnetized plasmas generated using themesh electrode on the ionic liquid. It is interestingthat the structure of the AuNPs depends on thedischarge current, namely, the AuNPs aresynthesized in the plasma ion irradiation region forsmall discharge current, while the AuNPs aresynthesized in the shadow region of the mesh for
relatively large discharge current. These phenomena
are well explained by the reduction and oxidation
effects of the radicals which are generated by theplasma irradiation to the ionic liquid and resultant
dissociation of the ionic liquid.
References[1] T. Kaneko, K. Baba and R. Hatakeyama, J.
Appl. Phys. 105 (2009) 103306.
[2] T. Kaneko, Q. Chen, T. Harada, and R.
Hatakeyama, Plasma Sources Sci. Technol. 20(2011) 034014.
[3] T. Kaneko, S. Takahashi, and R. Hatakeyama,Plasma Phys. Control. Fusion (2012) in press.