22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Improvement in the photocatalytic performance of Cu x O/TiO 2 film due to
pre-plasma-surface-treatment
S. Kogoshi, T. Yasuda, T. Nakano, K. Aoki and N. Katayama
Tokyo University of Science, Faculty of Science and Technology, JP-278-8510 Noda, Chiba, Japan
Abstract: It has been shown that the performance of a Cu x O/TiO 2 film as a visible light
responsible photocatalyst has been improved by two times due to pre-plasma-surfacetreatment. The performance was estimated by measuring the time variation of a contact
angle of a water droplet on a Cu x O/TiO 2 film during a white LED irradiation.
Keywords: plasma surface treatment, photocatalyst, hydrophilicity, Cu x O/TiO 2
1. Introduction
A TiO 2 has been widely used as a photocatalyst [1-4].
However, a TiO 2 photocatalyst is responsible to only
ultraviolet (UV) light with a wavelength less than 380 nm.
If a photocatalyst can be activated by both visible and UV
light, then its performance will be improved. Therefore
visible-light responsible photocatalysts (VLRPs) have
been intensively studied. An oxygen-deficient TiO 2
(TiO 2-x ) [5] and a Cu x O supported TiO 2 (Cu x O/TiO 2 ) [6]
are known as VLRPs. Expecting their synergy effect we
prepared a Cu x O-supported TiO 2-x (Cu x O/TiO 2-x ) and
examined its performance as a VLRP. The performance
was estimated by measuring the time variation of a
contact angle of a water droplet on a photocatalyst film
during a white LED on. Since the surface of a
photocatalyst film was coated with an ethanol solution of
an oleic acid, an initial contact angle was large, about
140 degree and then reducing because an oleic acid was
oxidized and disappearing and a photocatalyst becoming
hydrophilic with time. The faster velocity of a contact
angle reducing means the better performance as a
photocatalyst.
2. Experimental
2.1 Sample preparation
A TiO 2 thin film was prepared by coating ethanol
solution containing TiO 2 nanoparticles (Ishihara-sangyo
ST01 (anatase), weight fraction 10 %) on a glass plate
(2.5×2.5×1.1 mmt, Tempax). The weight of a TiO 2 was
about 8 mg. After drying, it was sintered at 723 K for 2 h.
A TiO 2-x film was prepared by processing a TiO 2 film
with Ar-H 2 mixed microwave plasma-surface-treatment.
The plasma-surface-treatment system is illustrated in
Fig. 1. Parameters of plasma processing are summarized
in Table 1. Cu x O/TiO 2 and Cu x O/TiO 2-x films were
prepared by dripping copper acetate monohydrate (6 wt%,
0.500 ml) on TiO 2 or TiO 2-x films respectively and
precipitated for 6 hours under UV light irradiation.
2.2 Measurements
The reflectance of spectra of the photocatalysts was
measured using a UV-vis spectrometer (Shimadzu
UV-2550). The performances of TiO 2-x , Cu x O/TiO 2 , and
P-I-1-7
Fig. 1. Schematic of the plasma surface treatment system.
Table 1. Plasma processing conditions and others.
Gas species and flow rate
Gas pressure
Microwave frequency
Microwave input power
Plasma container [cube]
Dielectric window
material /thickness
Air gap region [cube]
Processing time
H 2 : 35 sccm, Ar: 35 sccm
0.6 Torr
2.45 GHz
200 W
120 x 120 x 200 mm
Quartz glass/10 mm
156 x 156 x 2 mm
10 min
Cu x O/TiO 2-x are estimated from the time variation of a
contact angle of a water droplet on photocatalytic films
during a white LED on. The surface of a photocatalyst
film was coated with an ethanol solution of an oleic acid.
The conditions of the contact angle measurement are
shown in Table 2. The spectral distribution of the white
LED, which does not include light less than 420 nm, is
shown in Fig. 2. The contact angle was estimated from a
photograph of a droplet taken with a microscope. An
initial contact angle was large, about 140 degree (see
Fig. 3).
1
Table 2. Contact angle measurement conditions.
An ethanol solution of an oleic
acid, concentration
Volume of a water droplet
Cross section of a photocatalyst
film
The time variation of the contact angle of a water
droplet on photocatalytic films are shown in Fig. 5. The
films of TiO 2-x , Cu x O/TiO 2 , and Cu x O/TiO 2-x , all show
visible-light responsible photocatalytic effect, judging
from the contact angles of them became less than
20 degree. The fastest reducing velocity of the contact
angle, which means the best performance as a visible light
responsible photocatalyst, was obtained for a Cu x O/TiO 2x film. The time when the contact angle for Cu x /TiO 2-x
reached 20 degree was about a half of that for Cu x O/TiO 2 .
Although the reason of the improvement is unknown, we
think that the effect of oxygen deficient TiO 2-x on the
precipitation of Cu x O would improve the performance.
10 % (Vol)
10 μl
25 x 25 mm
50
45
Intensity (arb. unit)
40
35
30
25
20
160
15
CuxO/TiO2-x
140
10
TiO2
5
Contact Angle [degree]
120
0
380
430
480
530
580
Wavelength　（ｎｍ）
630
680
Fig. 2. Spectral distribution of white LED.
CuxO/TiO2
100
TiO2-x
80
60
40
20
0
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Time[min]
Fig. 5. Time variation of the contact angle of water
droplet on several photocatalytic films.
Fig. 3. Photograph of water droplet on photocatalyst film.
3. Results and discussion
Fig. 4 shows the reflectance spectra of the
photocatalysts. One can see that TiO 2-x , Cu x O/TiO 2 , and
Cu x O/TiO 2-x can absorb visible-light although TiO 2
cannot (about 90 % reflectance does not mean 10 %
absorption, we think that 10 % of light was not captured
by a detector because TiO 2 cannot absorb visible light).
100
4. Conclusion
The performance of TiO 2-x , Cu x O/TiO 2 and
Cu x O/TiO 2-x as a visible light responsible photocatalyst
are estimated from the time variation of a contact angle of
a water droplet on them during a white LED on. The
fastest reducing velocity of a contact angle, which means
the best performance as a visible light responsible
photocatalyst, was obtained for a Cu x O/TiO 2-x film. It is
faster than that of a Cu x O/TiO 2 by about two times. The
reason of the improvement is unknown and under study.
90
Acknowledgment
This study was supported in part by a Grant-in-Aid for
Scientific Research (No. 2534007) from the Japan Society
for the Promotion of Science (JSPS).
80
Reflectance [%]
70
60
50
40
TiO2
30
TiO2-x
20
CuxO/TiO2
10
CuxO/TiO2-x
0
200
300
400
500
600
700
800
Wavelength [nm]
Fig. 4.
The reflectance spectra of TiO 2 , TiO 2-x ,
Cu x O/TiO 2 , and CuxO/TiO 2-x .
2
5. References
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[3] K. Maeda and K. Domen. J. Phys. Chem. C, 111,
7851 (2007)
[4] A.L. Linsebigler, G. Lu and J.T. Yates. Chem. Rev.,
95, 735 (1995)
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[5]
[6]
T. Nakano, S. Yazawa, S. Araki, S. Kogoshi,
N. Katayama, Y. Kudo and T. Nakanishi. Jap. J.
Appl. Phys., 54, 01AE04 (2015)
H. Irie, S. Miura, K. Kamiya and K. Hashimoto.
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P-I-1-7
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