Preparation of Visible-Light-Active Photocatalysts
Having Cu-Contained Layer by Plasma CVD
Kenji Yamada, Kazuya Yamamoto and Tatsuhiko Sonoda
Kitakyushu National College of Technology, Kitakyushu 802-0985, Japan
Abstract: TiO2 film covered with Cu-contained layer is prepared by plasma CVD
using copper(Ⅱ) acetylacetonate gas and then heat-treated at 723 K in air. A surface
layer of plasma CVD-treated TiO2 film is composed of Cu, C, and O atoms and shows
visible light absorption. The plasma CVD-treated TiO2 film can decompose
isopropanol into acetone and CO2 under visible light irradiation to appear visible
light activity. It is assumed that photo-excited electron reduces directly Cu contained
in the surface layer and the reduced Cu can reduce O2 with two and four electrons to
form H2O2 and H2O, respectively, since it is confirmed from XPS analysis that
C-doping does not take place with the plasma CVD and the origin of the visible light
activity is not C-doping.
Keywords: photocatalyst, visible light, plasma CVD, Cu, interfacial charge transfer
1. Introduction
Anatase and rutile TiO2 photocatalysts mainly
absorb ultraviolet photons. However, solar light only
contains a small amount of ultraviolet photons
(about 5 %), and room lamps emit mainly visible
photons. Therefore efforts have been devoted to
extending the spectral response of pure TiO2
material (mainly anatase TiO2) to visible light [1].
Non-metal doping into TiO2 has become much
attractive for band-gap narrowing and high
photocatalytic activity under visible light.
Visible-light-active TiO2 films were prepared with
plasma surface modification methods [2-4].
Cu(Ⅱ)-grafted TiO2 prepared by impregnation
using CuCl2/2H2O let to the production of
visible-light-active photocatalysts, triggered by the
photo-induced interfacial-charge-transfer from the
valence band of TiO2 to Cu(Ⅱ) ions [5]. In the
photocatalyst systems, the Cu( Ⅰ ) produced by
Cu(Ⅱ) reduction acts as a multi-electron oxygen
reduction catalyst. In addition, the strong oxidation
power of holes produced in the valence band of TiO2
is utilized [5]. In this work, TiO2 film covered with
Cu-contained layer is prepared by plasma CVD
using copper( Ⅱ )
acetylacetonate gas. It is
investigated whether the Cu-contained layer can
cause the photo-induced interfacial-charge-transfer
from the valence band of TiO2 to Cu(Ⅱ) ions,
similar to the Cu(Ⅱ)-grafted TiO2, or not.
2. Experimental
2.1. Preparation of TiO2 film
Anatase TiO2 particles (ST01, Ishihara Sangyo
Co., Tokyo, Japan) were used in this work. The
average diameter of the primary particles was ca. 7
nm. The TiO2 particles were dispersed in 1 M acetic
acid solution at a concentration of 30 wt% under
ultrasonic agitation, with polyethylene glycol of 10
wt% of TiO2 used to prepare the TiO2 suspension.
The TiO2 suspension was screen-printed on a glass
substrate and then sintered in air at 723 K for 30 min
to form TiO2 film.
2.2. Plasma CVD treatment of TiO2 film
An inductively-coupled plasma reactor was used
for plasma CVD treatment of TiO2 film. The films
were treated with plasma CVD using copper(Ⅱ)
acetylacetonate gas. The discharge power was 50 W,
the discharge durations were 10 min and 20 min, and
the gas pressure was 25 Pa. The plasma CVD-treated
films were heat-treated at 473 K for 30 min in air.
2.3. Surface analyses of TiO2 films
An x-ray photoelectron spectroscopy (XPS)
measurement was carried out with a Shimadzu
ESCA3400 x-ray photoelectron spectrometer
(Shimadzu Co., Kyoto, Japan) to measure changes in
the surface structure after the plasma CVD treatment
and the heat treatment. XPS spectra were collected
by exciting the film without pre-treatment with a
MgK α x-ray source. MgK α radiation was
2.4. Photocatalytic property of TiO2 film
As a method of evaluation of photocatalytic
activity in the plasma CVD-treated TiO2 film,
concentrations of decomposition gases of organic
material were measured under visible illumination.
The film was introduced into a glass vessel and then
isopropanol was introduced. The film was kept in
the dark till the concentration of isopropanol gas in
the vessel became constant. A Bunkoukeiki
OTENTOSUN-VIS20
visible-light
irradiator
(Bunkoukeiki Co., Ltd., Tokyo, Japan) was used
with two cutoff filters for the UV (wavelength λ
<420 nm) and IR region (λ>750 nm) to obtain only
visible light. The film was irradiated with the visible
light and the concentration of isopropanol and those
of acetone and CO2 as decomposition gases were
measured with a Shimadzu GC-8AT gas
chromatograph (Shimadzu Co., Kyoto, Japan).
Figures 2(a) and (b) show Cu2p and C1s XPS spectra,
respectively, in the plasma CVD-treated TiO2 films
after the heat treatment at 723 K. Neither of the
spectra was almost changed by the heat treatment.
C-doping did not take place during the heat
treatment, since C1s peak around 282 eV did not also
appear after the heat treatment, as shown in Figure
2(b).
Figures 3(a) and (b) show Cu/Ti and Cu/C atomic
ratios, respectively, in plasma CVD-treated TiO2
films before and after the heat treatment as a
function of plasma CVD treatment time. The Cu/Ti
and Cu/C atomic ratios were almost increased with
increasing plasma CVD treatment time and the
amounts of the ratios were always higher after the
heat treatment, compared with those before the heat
treatment. Carbon content in the CVD layer was
decreased by the heat treatment, whereas Cu content
was increased.
3.1.3. Oxygen vacancies in plasma CVD treated
TiO2 film
(a)
40000
after pasma CVD (50 W, 20 min)
35000
after plasma CVD (50 W, 10 min)
30000
Intensity/cps
generated with a voltage of 8 kV and current of 30
mA. The spectrometer was calibrated using the
Ag3d5/2 core line. The binding energy values reported
in the present work were corrected by using C1s peak
at 284.8 eV for taking charging effects into accounts.
A UV-vis diffuse reflectance spectrum of the TiO2
film was measured with a JASCO V-500 UV-vis
spectrophotometer (JASCO International Co., Tokyo,
Japan) equipped with an integral-sphere attachment
to measure changes in light absorbance behavior
after the plasma CVD treatment and the heat
treatment.
before plasma CVD
25000
20000
15000
10000
5000
3. Results and Discussion
3.1.2. TiO2 films before and after heat-treatment
0
970
960
950
940
930
920
910
Binding Energy/eV
14000
(b)
12000
Intensity/cps
3.1. Analyses of XPS spectra
3.1.1. TiO2 films before and after plasma CVD
treatment
Figures 1(a) and (b) show Cu2p and C1s XPS spectra,
respectively, in the TiO2 films before and after
plasma CVD treatment. The peaks appeared around
933 eV and 953 eV are associated with Cu2p3/2 and
Cu2p1/2, respectively, in Figure 1(a). On the other
hand, the peak around 944 eV little appeared.
Therefore Cu species are almost in a state of neutral
Cu0 or/and Cu(Ⅰ), but Cu(Ⅱ). The C1s peak around
282 eV did not appear after the plasma CVD
treatment in Figure 1(b), whereas the shoulder
around 287 eV did. It becomes apparent that
C-doping into TiO2 is not realized and C-O and C=O
bonds are formed by the plasma CVD treatment. It
can be confirmed that CVD layer composed of Cu, C,
and O atoms is formed on a surface of TiO2 film.
10000
after plasma CVD (50 W, 20 min)
after plasma CVD (50 W, 10 min)
before plasma CVD
8000
6000
4000
2000
0
310
305
300
295
290
285
280
275
Binding Energy/eV
Figure 1. (a) Cu2p and (b) C1s XPS spectra in the
TiO2 films before and after plasma CVD treatment.
In Ti2p spectra in the TiO2 films before and after
plasma CVD treatment and the plasma CVD-treated
TiO2 film after heat treatment, a shoulder in the
Ti2p3/2 peak around 458 eV did not appear around a
binding energy lower than that of the peak. The
shoulder is associated with the formation of Ti3+
originating from the oxygen vacancies 〔 4 〕 .
Therefore the oxygen vacancies are not formed by
the plasma CVD and heat treatments. It is also
confirmed from the Ti2p spectra that the plasma CVD
and heat treatments are not brought into C-doping. It
is assumed from the appearance of Ti2p peak in the
plasma CVD-treated TiO2 film that the thickness of
the plasma CVD layer is below a few nm.
3.2. Analyses of UV-vis spectra
Figure 4 shows UV-vis diffuse reflectance spectra
in TiO2 films before and after plasma CVD treatment
and the plasma CVD-treated TiO2 film after the heat
treatment. The TiO2 film showed visible light
absorption after the plasma CVD treatment. The
visible light absorption was increased after the heat
0.5
(a)
Heat Treatment
35000
after plasma CVD (50 W, 20 min)
30000
after plasma CVD (50 W, 10 min)
(a)
0.45
2
0.4
0.35
Cu/ Ti
Intensity/cps
40000
treatment.
Valence band electrons in TiO2 can be directly
transferred to Cu(Ⅱ) via interfacial charge transfer
to produce Cu(Ⅰ) under visible-light irradiation [5].
The Cu(Ⅰ) ion is remarkably versatile regarding the
oxygen reduction. The reaction of 2Cu(Ⅰ) with
oxygen proceeds through a two-electron reduction,
producing H2O2 and that of either 3Cu( Ⅰ ) or
4Cu( Ⅰ ) with oxygen proceeds through a
fore-electron reduction, producing H2O. Since
potential of valence band of TiO2 and redox potential
of Cu(Ⅱ)/Cu(Ⅰ) are 3.04 V (vs. SHE, pH=0) and
0.16 V (vs. SHE, pH=0), respectively, Cu-grafted
TiO2 shows photocatalytic activity under irradiation
of photons above 2.88 eV. Photo-excited electrons
will be transferred from valence band of TiO2 to
Cu(Ⅱ)/Cu(Ⅰ) containing in the plasma CVD layer,
since the wavelength of absorption edge is around
500 nm in the plasma CVD-treated TiO2 film, as
shown in Figure 4.
25000
0.3
0.25
1
0.2
20000
0.15
0.1
15000
1 after plasma CVD
2 after plasma CVD and heat treatment
0.05
10000
0
5000
0
5
0
970
960
950
940
930
920
10
15
20
Plasma CVD Treatment Time / min
910
Binding Energy/eV
0.14
after plasma CVD (50 W, 20 min)
Intensity/cps
12000
8000
0.08
1
0.06
6000
0.04
4000
0.02
2000
0
0
310
2
0.1
after plasma CVD (50 W, 10 min)
10000
(b)
0.12
Cu/ C
14000
(b)
Heat Treatment
1 after plasma CVD
2 after plasma CVD and heat treatment
0
305
300
295
290
285
280
275
5
10
15
20
Plasma CVD Treatment Time / min
Binding Energy/eV
Figure 2. (a) Cu2p and (b) C1s XPS spectra in the
plasma CVD-treated TiO2 films after the heat
treatment at 723 K.
Figure 3. (a) Cu/Ti and (b) Cu/C atomic ratios in the
plasma CVD-treated TiO2 films before and after
the heat treatment as a function of plasma CVD
treatment time.
3.3. Analyses of photocatalytic property
Figure 5 shows peak area ratio of
acetone/isopropanol as a function of visible-light
irradiation time in TiO2 films before and after
plasma CVD treatment and the plasma CVD-treated
TiO2 film after the heat treatment. An amount of
produced acetone was increased with increasing
irradiation time and its increase was remarkable after
the heat treatment. Figure 6 shows peak area ratio of
CO2/N2 as a function of visible-light irradiation time
in TiO2 films before and after plasma CVD treatment
and the plasma CVD-treated TiO2 film after the heat
treatment. An amount of produced CO2 was
increased with increasing irradiation time and its
increase was remarkable after the heat treatment. It
is elucidated from Figures 5 and 6 that visible light
activity of the TiO2 film is developed by the plasma
CVD and becomes remarkable after the heat
treatment.
References
[1] A. Fujishima, X. Zhang, DA Tryk, Surface
Science Reports, 63 (2008) 515.
[2] K. Yamada, H. Nakamura, S. Matsushima, H.
Yamane, T. Haishi, K. Ohira, K. Kumada, CR.
Chimie, 9 (2006) 788.
[3] K. Yamada, H. Yamane, S. Matsushima, H.
Nakamura, K. Ohira, M. Kouya, K. Kumada,
Thin Solid Films, 516 (2008) 7482-7487.
[4] K. Yamada, H. Yamane, S. Matsushima, H.
Nakamura, T. Sonoda, S. Miura K. Kumada,
Thin Solid Films, 516 (2008) 7560-7564.
[5] H. Irie, S. Miura, K. Kamiya, K. Hashimoto,
Chem. Phys. Lett., 457 (2008) 202-205.
TiO2 film covered with Cu-contained layer can be
prepared by plasma CVD using copper( Ⅱ )
acetylacetonate gas. C-doping into TiO2 does not
realized in the plasma CVD-treated TiO2 film before
and after the heat treatment at 473 K in air. The
content of Cu in the plasma CVD layer is increased
by the heat treatment. Visible light activity of the
film is increased by the heat treatment. It will result
in absorption edge around 500 nm and the visible
light activity that photo-excited electrons are
transferred from the valence band of TiO2 to
Cu(Ⅱ)/Cu(Ⅰ).
Peak Area Ratio of
Acetone/Isopropanol
0.035
4. Conclusion
2
0.02
0.015
0.01
0.005
1
0
1
2
3
4
5
6
7
Irradiation Time/h
Figure 5. Peak area ratio of acetone/isopropanol as
a function of visible-light irradiation time in TiO2
films before and after plasma CVD treatment and
the plasma CVD-treated TiO2 film after the heat
treatment.
0.5
3 after Plasma CVD (50 W, 20 min)
and heat treatment
0.4
3
0.3
2
0.2
1
400
500
600
700
800
Wavelength/nm
Peak Area Ratio of CO2/N2
Absorbance
2 after plasma CVD (50 W, 20 min)
0
300
0.025
1 before plasma CVD
0.6
0.1
0.03
0
0.8
0.7
3
1 before plasma CVD
2 after plasma CVD (50 W, 20 min)
3 after plasma CVD (50 W, 20 min)
and heat treatment
1.80
3
1 before plasma CVD
2 after plasma CVD (50 W, 20 min)
3 after plasma CVD (50 W, 20 min)
and heat treatment
1.70
1.60
1.50
1.40
1.30
1.20
2
1.10
1
1.00
0
1
2
3
4
5
6
Irradiation Time/h
Figure 4. UV-vis diffuse reflectance spectra in TiO2
films before and after plasma CVD treatment and
the plasma CVD-treated TiO2 film after the heat
treatment.
Figure 6. Peak area ratio of CO2/N2 as a function of
visible-light irradiation time in TiO2 films before
and after plasma CVD treatment and the plasma
CVD-treated TiO2 film after the heat treatment.
7