22G10
Structure-Controlled Synthesis of Gold-Nanoparticles Using
Strongly-Magnetized Gas-Liquid Interfacial Plasma
気液界面強磁化プラズマによる金ナノ粒子の構造制御合成
Toshiro Kaneko and Rikizo Hatakeyma
金子 俊郎，畠山 力三
Department of Electronic Engineering, Tohoku University
6-6-05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
東北大学 大学院工学研究科 電子工学専攻 〒980-8579 仙台市青葉区荒巻字青葉6-6-05
A periodic structure of gold nanoparticles is formed by reducing an aqueous solution of gold chloride using
a spatio-periodically generated plasma under the strong magnetic field. In addition, it is found that aring-shaped nanoparticle structure is formed corresponding to the shape of a ring electrode inserted into theplasma, where the nanoparticles are synthesized at the position without plasma irradiation due to theshielding by the ring electrode.

1. Introduction
Recently, highly-ordered periodic structures ofmetal nanoparticles have attracted much attention
due to their high catalytic activity, uniquephotosensitive reactivity, and so on. Especially,
the synthesis of various kinds of nanoparticles
using the plasma-liquid interfaces [1] isadvantageous in that a reducing agent is theplasma itself, and then, the synthesis iscontinuous during the plasma irradiation. To
realize the periodic structure of the nanoparticles,
we adopt a novel plasma technique combinedwith introduction of ionic liquids [2] under strong
magnetic fields up to 4 tesla (T).
Since the plasma generated under the strong
magnetic field keeps its structure due toconfinement along the magnetic field lines, theplasma structure can be transcribed to the liquidsurface, resulting in the synthesis of thestructured nanoparticles at the gas-liquid interface
when the plasma reduces the metal chlorides in
the liquid.
This method could contribute to supplying aconsiderable amount of spatially-periodic
nanoparticles available for the development of
unique optoelectronic devices.
2. Experimental Apparatus
Figure 1(a) shows the schematic of anexperimental setup for the discharge plasma incontact with an ionic liquid, which has a glass cellwith 15 mm inner diameter and 10 mm depth in acylindrical glass chamber with 75 mm diameter and
200 mm length.

A DC voltage VD is supplied to an upper cathodeelectrode composed of a stainless steel (SUS) plateand a SUS mesh grid is used as an anode electrodeto promote a spatial diffusion of the plasma.
The ionic liquid (N.N.N.-Trimethyl-N-propylammonium Bis(trifluoromethanesulfonyl) imide)
put in the glass cell is placed on a peltier element
which is located at a distance of 50 mm from theanode electrode. When the strong magnetic field isapplied along the machine axis, the generated
plasma is strongly magnetized, and then, theperiodic plasma structure formed by the mesh
anode is maintained just above the ionic liquid asshown in Fig. 1(b).
Gold nanoparticles (AuNPs) are synthesized in
the ionic liquid by the plasma reduction of Au
chloride such as HAuCl4·3H2O [3,4].
(a)(b)

Fig. 1. (a) Schematic of the experimental setup, and(b) photo of the synthesized periodic plasmastructure.

The ionic liquid can be cooled by the peltierelement and becomes a solid state, with keeping thenanoparticle structure formed by the plasma
irradiation at the liquid interface.
3. Results and Discussion
Figure 2 shows pictures of the temporal evolutionof the periodic nanoparticle structure which isformed at the gas-liquid interface in accordancewith the plasma structure under the condition of astrong magnetic field of B = 1 T. The density of theperiodic structured nanoparticles increases withtime, and the obvious structure is formed typicallywithin 5 min. When the plasma is irradiated for
longer than 5 min, the nanoparticles diffuse and thestructure is broadened.
In order to suppress the diffusion of thenanoparticles, the temperature of the ionic liquid isreduced using the peltier element, resulting in theincrease in viscosity of the ionic liquid. Although
the diffusion of the nanoparticles is suppressed, thenanoparticle synthesis rate becomes low. Therefore,
it is necessary to precisely control the temperatureof the ionic liquid for the fine periodic nanoparticlestructure.
As the next step, we attempt to form more finely
periodic structures of the nanoparticles based on theself-organizing behavior of turbulent plasmas
generated by the nonlinear development of plasma
fluctuations. For this purpose, a ring electrode isinserted in the plasma column as shown in Fig. 3(a),and a positive DC bias voltage VC is applied to theelectrode. It is found that the high frequency
fluctuation is excited by the positive bias voltage,
however the self-organized plasma structure is notobserved at present.
Using this configuration, the nanoparticles are

(a)

(b)

Fig. 3. (a) Experimental apparatus and (b) the ring
shaped nanoparticle structure. B=1 T.

synthesized by reducing the Au chloride as shownin Fig. 3(b). It is found that the ring shaped
nanoparticle structure is formed corresponding to
the shape of the inserted ring electrode. This resultmeans that the nanoparticles are synthesized at theposition without plasma irradiation due to theshielding by the ring electrode.
Since the gold nanoparticles are usuallysynthesized by the reduction effect of the hydrogenradical in the plasma, the charged particles (positiveions or electrons) are considered to inhibit thesynthesis of the nanoparticles in the plasma
irradiation region.
4. Conclusions
Gold nanoparticles with periodic structure or ring
shaped structure are synthesized using the strongly
magnetized plasmas. The ring shaped nanoparticle

structure is corresponding to the shape of the ring
electrode inserted in the plasma.
These results imply that the pattered nanoparticle
structure can be freely formed and is expected to be
applied to the wavelength-selective wave guideusing a principle of the surface plasmon resonanceof the nanoparticles.
Acknowledgments
This work is supported by Grant-in-Aid for
Scientific Research from the Ministry of Education,Culture, Sports, Science and Technology, Japan.

Fig. 2. Temporal evolution of the periodic
nanoparticle structure formed by controlled gasliquid interfacial plasmas.

References[1] T. Kaneko, Q. Chen, T. Harada, and R. Hatakeyama:
Plasma Sources Sci. Technol. 20 (2011) 034014.
[2] T. Kaneko, K. Baba, and R. Hatakeyama: J. Appl.
Phys. 105 (2009) 103306.
[3] T. Kaneko, K. Baba, T. Harada, and R. Hatakeyama:Plasma Proc. Polym. 6 (2009) 713.
[4] Y. B. Xie and C. J. Liu: Plasma Proc. Polym. 5(2008) 239.