Helium Plasma Implanted Metallic Nanostructures : growth process,fractality, and its applications
S. Kajita, N. Ohnoa, T. Yoshida, N. Yoshidab
aEcoTopia Science of Institute, Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8603, Japan
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8603, Japanb
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
kajita@ees.nagoya-u.ac.jp

Tungsten (W) will be used for plasma facing components in future fusion devices including ITER. It
has been revealed that helium (He) plasma irradiation to W leads to the formation of fiberform
nanostructures on the surface when certain conditions are satisfied [1,2]. The width of fiber isapproximately 10-30 nm, and the layer thickness increases in proportional to the square root of He
fluence. In addition to W, the nanostructurization was identified on various metals such as molybdenum,titanium, iron, and nickel [3]. The nanostructures change various physical and optical properties: thethermal conductivity decreases, the sputtering yield decreases, the particle reflection coefficientincreases, and the optical reflectance significantly decreases. Although the changes in the physicalproperty may provide concerns for divertor material in fusion devices [4], they can have a potential to be
used for application materials.
In this paper, the growth process and fractal features are discussed based on the detailed surface
analysis, and the potential for the application is shown mainly as a visible light response photocatalysis.
The fractal property of the nanostructures was investigated from the scanning electron microscope(SEM) micrographs, gas adsorption isotherms, and transmission electron microscope (TEM)micrographs. From the cross sectional SEM micrographs, the fractal dimension was deduced andcompared with the one obtained from the adsorption; they were consistent with each other. Fractal
features are identified on various aspects. Pinholes are observed on the surface when the He fluence waslower than 1025 m-2. The numbers and the area of the pinholes on the surface have a fractal relation,
similar as the craters on the moon.
Photocatalytic experiments were conducted using the nanostructured W to decolorize methylene blue.
Significant decolorization occurred on the nanostructure W for visible light. The decolorization
proceeded even for the light of which the wavelength was >700 nm. Although the mechanism of theprocess is yet to be fully understood, it is a surprising fact that the photon energy was significantly lower
than the normal WO3 band gap energy of 2.5 eV. From the experiments using various degree ofoxidization from 20-100%, it was shown that the sample with 60% of the oxidization degree had themost significant photocatalytic activity. It was suggested that the boundary of W(0)-WO3 is the active
site, which is more active compared to WO3[5].
[1] S. Takamura, N. Ohno, et al., Plasma Fus. Res., 1 (2006) 051.
[2] S. Kajita, W. Sakaguchi, et al., Nucl. Fusion 49 (2009) 095005.
[3] S. Kajita, T. Yoshida, et al., J. Appl. Phys. 113 (2013) 134301.
[4] S. Kajita, G. De Temmerman, T. Morgan, et al., Nucl. Fusion 54 (2014) 033005.
[5] K. Komori, T. Yoshida, et al., e-J. Surf. Sci. Nanotech. 12 (2014) 343-348.