P008-0
2323P008-0
Surface modification of titanium by the exposure to helium plasma
ヘリウムプラズマ照射によるチタンの表面構造変化
1

Daiki Kitaoka, 2Shin Kajita, 1Noriyasu Ohno, and 2Tomoko Yoshida
北岡大輝, 梶田 信，大野哲靖，吉田朋子

1Graduate School of Engineering, Nagoya University, Fro-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan
名古屋大学工学研究科，〒464-8603 名古屋市千種区不老町
2EcoTopia Science Institute, Nagoya University, Fro-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan
名古屋大学エコトピア科学研究所，〒464-8603 名古屋市千種区不老町

The formation of micron-sized helium bubbles/holes and fibreform nanostructure on metals due to helium
plasma exposure has been investigated. Because these nanostructures have very large surface areas, they
have potential to increase the effective surface area and improve catalytic properties. Thus, the applicationof these nanostructure to catalytic materials is suggested. In this study, we investigated the surface
modification of titanium under helium plasma exposure using the liner divertor simulator NAGDIS-I. And
we explore the necessary condition, such as the incident ion energy and surface temperature, for thefibreform nanostructure formation in titanium by helium irradiation.

1. Introduction
Titanium dioxide (TiO2) has attracted muchinterest from the scientific and industrial fields,such as for light-induced water splitting,dye-sensitized solar cells, and self-cleaning surfaces[1]. Due to its unique electronic and catalytic
properties, the photo-degradation of organic
contaminant becomes one of the most important
application of TiO2 [2]. Because these catalytic
reaction occurs on the material surface, fabricationof large effective surface areas are necessary for
improving the photo-catalytic properties.
The irradiation of helium to metals leads tomicrostructural evolution on the surface region,such as helium holes/bubbles and fibreform
nanostructure [3]. These structures have potential toincrease the effective surface area and improve
photo-catalytic properties [4]. However, thenecessary
conditions
forthefibreform
nanostructure formation of metals, except fortungsten, have not been investigated yet.
In this study, the change of the titanium surface
nature under helium plasma exposure isinvestigated by the measurement of the relativereflectance and the surface analysis using ascanning electron microscope (SEM). Furthermore,
we explore the condition for the fibreform
nanostructure formation in titanium by heliumirradiation.
2. Experiments
The substrate materials were titanium provided
by Nilaco Co. with 99.5 % purity and 0.5 mm inthickness. Samples were polished to a mirror finish.
Irradiation of helium plasma was carried out using

the liner divertor plasma simulator NAGDIS-I.
Steady state high-density plasma is produced by adc arc discharge using a LaB6 cathode heated with afilament. The magnetic field strength was 0.05 T. In
order to control the incident ion energy to thesamples, they were electrically biased in someexperiments. Fig. 1 shows the schematic of theexperimental setup in the NAGDIS-I. The
reflectivity of samples was measured by using aHe-Ne laser, whose wavelength is 632.8 nm, and aphotodiode with a band-pass optical filter toexclude the emission from the plasma. The surface
temperature of the samples was measured byradiation thermometer. After the helium plasma
irradiation, the surface modification of samples wasobserved by SEM.

Fig. 1. A schematic of relative reflectance measurement
setup3. Results and discussion
The exposure conditions of helium plasma aresummarized in Table 1. The electron density ne andtemperature Te were from 2.5 × 1018 to 4.3 × 1018
m−3 and 8 eV, respectively. These plasma
parameters were measured with a Langmuirprobe. The samples were biased to -70 V (Ti1), -60
V (Ti2), -75 V (Ti3), and -85 V (Ti4), respectively.
Fig. 2 shows the relative reflectance of samples
exposed to the helium plasma. For Ti1 and Ti2, therelative reflectance was kept high. On the otherhand, the reflectance decreased to around zero for
Ti3 and Ti4; further the reflectance decreased
during shorter period of time for Ti4. Even though
the ion flux did not change so much in Ti3 and Ti4,
the reduction time in reflectance decreases
significantly with increasing the incident ion energy.
Although the surface temperature slightly increases
with the incident ion energy, the surface damages
should strongly depend on the incident ion energy.
Fig. 3(a)–(d) shows the SEM micrographs of thesamples after the helium plasma exposure. On theTi1 surface, neither hole nor fibreform
nanostructure is observed (Fig. 3(a)). On the otherhand, helium bubbles and holes are formed on theTi2 surface (Fig. 3(b)). For Ti1 and Ti2, incident ion
energy was about the same. Thus, the helium babble
formation would depends on the surface
temperature. Concerning Ti3, and Ti4, the surface
color was totally black visually after the helium
irradiation. The surfaces of Ti3 and Ti4 are coveredwith a micrometer fine structured material shown in
Fig. 3(c) and (d). Compared with these fibreform
nanostructures, the size of nanostructure of Ti4 islarger than that of Ti3. Since the surface
temperature of Ti3 and Ti4 were almost the same, it
is likely that the size of fibreform nanostructure
depends on the incident ion energy.
4. Conclusion
We have investigated the necessary conditions
for the fibreform nanostructure formation intitanium by helium irradiation. We have found thatthe incident ion energy and surface temperature areimportantparameterstoform
fibreform
nanostructured titanium.

Fig. 2. Measured relative reflectance of titanium underhelium plasma exposure; the detail exposure conditions
are summarized in Table 1.

Fig. 3. SEM micrographs of titanium exposed to helium
plasma under the conditions shown in Table 1.
References[1] M. Bozorgtabar, M. Rahimipour, M. Selehi, and M.
Jafarpour: Surf. Coat. Technol. 205 (2011) S229.
[2] S.C. Lee, C.W. Lee, S.C. Lee, and J.S. Lee: Mater.
Lett. 62 (2008) 564.
[3] S. Kajita, W. Sakaguchi, N. Ohno, N. Yoshida, and T.
Saeki: Nucl. Fusion Res. 49 (2009) 095005.
[4] S. Yamamoto, S. Nagata, A. Takayama, and M.
Yoshikawa: Nucl. Instr. and Meth. B 249 (2006) 374.

Table 1. Experimental parameters of the helium irradiation experiment for Ti1, Ti2, Ti3, and Ti4
Ti1
Ti2
Ti3
Ti4
Incident ion enrgy
62 eV
56 eV
73 eV
83 eV
Fluence
8.4×1025 m-2
7.6×1025 m-2
4.7×1025 m-2
2.1×1025 m-2
22 -2 -1
22 -2 -1
22 -2 -1
Ion flux
2.3×10 m s
2.1×10 m s
2.6×10 m s
2.3×1022 m-2s-1
Time
3600 s3600 s1800 s
900 s
Temperature
580 K
660 K
750 K
760 K
Surfacemodification He bubble/holes
Fibreform nostructure Fibreform nanostructure