Preparation of Titanium-Silicon Binary Oxide Thin Film Photocatalysts by an Ionized Cluster Beam Deposition Method. Their Photocatalytic Activity and Photoinduced Super-Hydrophilicity

Article abstract of DOI:10.1021/jp0308514

Titanium-silicon binary oxide thin films of differing TiO₂ content were prepared by using an ionized cluster beam (ICB) deposition method using multi-ion sources under a dry preparation process. From the results of UV-vis absorption measurements, binary oxide thin films with low TiO ₂ content showed much higher transmittance as compared with the pure TiO₂ thin film, indicating that the Ti-oxide species of these Ti-Si binary oxide thin films exist in a highly dispersed state within the SiO ₂ matrices. Ti K-edge XAFS measurements revealed that such binary oxide thin films with low TiO₂ content (less than 20%) showed only a sharp preedge peak attributed to the highly dispersed Ti-oxide species which are different from the peaks of anatase TiO₂ thin films. Moreover, they were found to exhibit much higher and more efficient photocatalytic reactivity for the decomposition of NO into N₂ and O₂ under UV light irradiation than the pure TiO₂ thin films.

Full text of DOI:10.1021/jp0308514

14278
J. Phys. Chem. B 2003, 107, 14278-14282
Preparation of Titanium-Silicon Binary Oxide Thin Film Photocatalysts by an Ionized
Cluster Beam Deposition Method. Their Photocatalytic Activity and Photoinduced
Super-Hydrophilicity
Masato Takeuchi,† Satoru Dohshi,† Takashi Eura,‡ and Masakazu Anpo*,†
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture UniVersity,
1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan, and Ion Engineering Research Institute Corporation,
2-8-1, Tsuda, Hirakata, Osaka 573-0128, Japan
ReceiVed: July 16, 2003; In Final Form: September 16, 2003
Titanium-silicon binary oxide thin films of differing TiO2 content were prepared by using an ionized cluster
beam (ICB) deposition method using multi-ion sources under a dry preparation process. From the results of
UV-vis absorption measurements, binary oxide thin films with low TiO2 content showed much higher
transmittance as compared with the pure TiO2 thin film, indicating that the Ti-oxide species of these Ti-Si
binary oxide thin films exist in a highly dispersed state within the SiO2 matrices. Ti K-edge XAFS measurements
revealed that such binary oxide thin films with low TiO2 content (less than 20%) showed only a sharp preedge
peak attributed to the highly dispersed Ti-oxide species which are different from the peaks of anatase TiO2
thin films. Moreover, they were found to exhibit much higher and more efficient photocatalytic reactivity for
the decomposition of NO into N2 and O2 under UV light irradiation than the pure TiO2 thin films.
Introduction
matrices, showing a unique and characteristic photocatalytic
performance for the hydrogenation of unsaturated hydrocarbons
with H2O, the decomposition of NO into N2, O2, and N2O, as
well as the reduction of CO2 with H2O to produce CH3OH and
CH4 under UV light irradiation.10-18 Ti-B binary oxide thin
films prepared by using an ionized cluster beam (ICB) deposi-
tion apparatus equipped with a multi-ion source have also been
reported to show much higher photocatalytic performance for
the decomposition of NO and photoinduced super-hydrophilic
property under UV light irradiation.19
In this paper, we will deal with the preparation of Ti-Si
binary oxide thin films including the highly dispersed 4-fold
coordinated Ti-oxide species within the SiO2 matrices by
applying an ICB deposition method using multi-ion sources.
The relationship between the photocatalytic performance for the
decomposition reaction of NO under UV (λ > 270 nm) light
irradiation and the local structures of the Ti-oxide species in
these binary oxide thin films having different TiO2 contents will
also be discussed by analyses of UV-vis absorption and Ti
K-edge XAFS measurements. Furthermore, to discuss the
surface wettability of these Ti-Si binary oxide thin films, the
contact angles of the water droplets were measured in ambient
conditions using a contact angle meter.
TiO2 thin films coated on various substrates such as glasses,
tiles, and various architectural materials have been reported to
show a super-hydrophilic property in which the contact angle
of water droplets on the surface of the TiO2 thin films can attain
zero degree during UV light irradiation, so that TiO2 thin films
have attracted much attention as photofunctional materials for
an anti-fogging, antibacterial, and stain-proofing agents.1-5 In
fact, we can now see many products utilizing these TiO2 thin
film photocatalysts in practical applications. However, various
improvements are necessary, for example, the addition of SiO2
fine particles6 or the deposition of the SiO2 thin layer7 onto the
surface of the TiO2 thin films, to retain super-hydrophilicity
even under dark conditions for an extended period of time.
Ti-Si sol-gel films prepared by using a mixed solution of
tetraethoxysilane (TEOS) and titanium tetraisopropoxide (TTIP)
have been reported to include the 4-fold coordinated Ti
species.8,9 However, TTIP is easily hydrolyzed by water vapor
in air to form TiO2 aggregates much faster than TEOS forms
SiO2 aggregates, so that it is very difficult to control the gelation
process to prepare binary oxide thin films in which SiO2 and
TiO2 domains mix well with each other. The addition of SiO2
to the TiO2 thin film using a dip coating process6 or a very thin
and porous SiO2 layer coating onto the TiO2 thin films7 have
been also reported as efficient ways to enhance the super-
hydrophilic properties because the added SiO2 particles or a
SiO2 layer coating on the TiO2 thin films work as a means to
store water molecules even under dark condition.
Experimental Section
Ti-Si binary oxide thin films were prepared by an ionized
cluster beam (ICB) deposition apparatus equipped with a multi-
ion source. A schematic diagram of this procedure is shown in
Figure 1. For the ion-source materials of TiO2 and SiO2, metallic
titanium grains (High Purity Chemicals Corp.; grade, 99.99%;
size, 5-10 mm) and silicon monoxide (SiO) grains (Furu-uchi
Chemicals Corp.; grade, 99.99%; size, 5-10 mm) were used,
respectively. The quartz substrates (size, 10 mm × 10 mm × 1
mm) were ultrasonically cleansed in acetone for 15 min, dried
at 373 K for half a day, and then calcined in air at 723 K for 5
Ti-Si binary oxide powder having low TiO2 content prepared
by a sol-gel method has been reported to include the 4-fold
coordinated Ti-oxide species highly dispersed within the SiO2
* To whom correspondence should be addressed. E-mail: anpo@
ok.chem.osakafu-u.ac.jp.
† Osaka Prefecture University.
‡ Ion Engineering Research Institute Corporation.
10.1021/jp0308514 CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/27/2003

Ti-Si Binary Oxide Thin Film Photocatalysts
J. Phys. Chem. B, Vol. 107, No. 51, 2003 14279
Figure 2. UV-vis transmittance spectra of Ti-Si binary oxide thin
films prepared by an ICB deposition method using Ti and SiO ion
sources. The amount of TiO2 (left to right): (a) 6.6, (b) 9.5, (c) 50.1,
and (d) 100%.
Results and Discussion
Figure 1. Schematic diagram of an ionized cluster beam (ICB)
deposition method using Ti and SiO ion sources.
Figure 2 shows the UV-vis absorption spectra of the Ti-Si
binary oxide thin films (film thickness, 700 nm) with different
TiO2 contents measured by the transmittance method. The SiO2
thin film deposited on the quartz substrate did not show any
significant absorption in measurable ranges (200-800 nm) due
to a similar refractive index of the SiO2 thin film and quartz
substrate, so that the absorption spectra of these binary oxide
thin films could be attributed to the Ti-oxide species. The TiO2
thin films were found to show clear interference fringes in
visible light regions typical of the absorption spectra of thin
films, indicating that highly transparent and uniform films could
be formed on the substrate. On the other hand, as the TiO2
content decreased, the intensity of the interference fringes was
found to decrease as well. From these results, it could be
expected that the Ti-oxide species do not have a long-range
order and exist as small clusters highly dispersed within the
SiO2 matrixes. In the XRD patterns of the Ti-Si binary oxide
thin films with lower Ti content, no peaks attributed to anatase
and/or rutile TiO2 could be observed. These XRD results were
in good correspondence with that of the UV-vis absorption
measurements. A remarkable shift in the absorption edge toward
shorter wavelength regions could be seen along with the
decrease in the TiO2 content. The absorption peak of the binary
oxide thin film having a TiO2 content of 6.6% was observed at
around 220-250 nm. The absorption peak of the highly
dispersed and 4-fold coordinated Ti-oxide species incorporated
within the zeolite frameworks has been reported to be observed
at around 220-230 nm.22-26 Therefore, these binary oxide thin
films can be also expected to include the highly dispersed and
4-fold coordinated Ti-oxide species. These shifts in the
absorption edge are explained by the quantum size effect arising
from the presence of ultrafine Ti-oxide species dispersed within
the SiO2 matrixes.
h in order to obtain a clean surface before the deposition process
of these thin films.
The metallic Ti and SiO as the source materials were heated
to approximately 2200 K in each crucible, and Ti and SiO vapors
were each introduced into the high vacuum chamber. At this
time, the Ti metal and SiO clusters reacted with sufficient
amounts of O2 molecules (oxygen pressure, 2 × 10-4 Torr) in
the vacuum chamber to produce stoichiometric TiO2 and SiO2
clusters. These TiO2 and SiO2 clusters ionized by electron beam
irradiation were then accelerated by an electric field (acceleration
voltage, 500 V) and bombarded onto the substrate to form
Ti-Si binary oxide thin films. The Ti/Si ratio, the film thickness,
and the deposition rate (about 0.1 nm/s) could be strictly and
easily controlled by monitoring their parameters with a quartz
film thickness meter (Inficon) during the deposition process.
The temperature of the substrate was kept at 623 K in order to
obtain a good crystallinity and strong adhesion of the thin film
onto the substrates.
The resulting transparent Ti-Si binary oxide thin films were
characterized by various spectroscopic means such as XRD
(Rigaku, RINT-1200) and UV-vis absorption (Shimadzu, UV-
2200A) measurements at room temperature. To investigate the
local structures of the Ti-oxide species in the SiO2 matrices,
the Ti K-edge XAFS spectra of these thin films were obtained
in the fluorescence mode at 295 K at the BL-7C facility of the
Photon Factory at the National Laboratory for High Energy
Physics (KEK-PF) in Tsukuba. A Si(111) monochromator was
used to monochromatize the X-rays from the 2.5 GeV electron
storage ring. The normalized spectra were obtained by a
procedure described in previous publications.20,21
To investigate the local structures of the Ti-oxide species
included in these Ti-Si binary oxide thin films, the Ti K-edge
XAFS spectra were measured at the BL-7C facility of High
Energy Physics in Tsukuba. Figure 3 (left side) shows the
XANES spectra of the binary oxide thin films with different
TiO2 contents. In the spectra with low TiO2 content (less than
10%), only a sharp preedge peak could be observed at 4.97
keV which is attributed to the highly dispersed Ti-oxide species
that differs from that of anatase TiO2 thin films. As the TiO2
content increased, the intensity of the sharp preedge peaks was
found to decrease, indicating that the highly dispersed ultrafine
Ti-oxide species aggregate to form small clusters containing
a part of the TiO6 octahedrons. Figure 3 (right side) also shows
the Fourier transforms of the EXAFS oscillation of these binary
The photocatalytic properties of these Ti-Si binary oxide
thin films were investigated by carrying out the photocatalytic
decomposition of NO under UV light (λ > 270 nm) irradiation.
Pretreatment and photocatalytic reactions for the decomposition
of NO were carried out using the same procedure shown in
previous papers. UV (λ > 270 nm) light irradiation was carried
out with a 100 W high-pressure Hg lamp (Toshiba, SHL-100
UVQ-2) through a color filter (Toshiba Glass, UV-27). The
contact angles of the water droplets on these binary oxide thin
films were measured under ambient conditions using a contact
angle meter (Kyowa Interface Science Co. Ltd., CA-X) at 295
K. The procedures for UV light irradiation were the same as
that for the photoreactions described above.

14280 J. Phys. Chem. B, Vol. 107, No. 51, 2003
Takeuchi et al.
Figure 4. Reaction time profiles of the photocatalytic decomposition
of NO on Ti-Si binary oxide thin film (TiO2 content, 6.6%), under
UV light irradiation (λ > 270 nm) at 275 K.
Figure 3. XANES (A-C) and Fourier transforms of EXAFS (a-c)
of the Ti-Si binary oxide thin films prepared by an ICB deposition
method using Ti and Si ion-sources. The amount of TiO2 (top to
bottom): 6.6, 9.5, and 50.1%.
TABLE 1: Results of the Curve Fitting of Fourier
Transforms of EXAFS Data for the Ti-Si Binary Oxide
Thin Films
TiO2 contents (%)
shell
Ra (Å)
CNb
σc (Å)
6.6
9.5
50.1
100
Ti-O
Ti-O
Ti-O
Ti-O
1.81
1.82
1.85
1.93
4.3
4.4
4.9
6.0
0.0001
0.0001
0.0056
0.0089
a Bond distance. b Coordination number. c Debye-Waller factor.
Figure 5. Effect of the TiO2 content on the production yields of N2
(circle plots) and N2O (square plots) and the selectivity for N2 formation
(triangle plots) in the photocatalytic decomposition of NO on Ti-Si
binary oxide thin films under UV light irradiation (λ > 270 nm) at
275 K.
oxide thin films. In every spectra, a strong peak at around 1.6-
1.7 Å attributed to the neighboring O atoms (Ti-O) can be
seen in the first coordination sphere; however, a peak at around
2.8-3.0 Å attributed to the Ti atoms (Ti-O-Ti) could not be
observed in the second coordination sphere. These results clearly
indicate that no aggregated TiO2 species exist although ultrafine
Ti-oxide clusters including the 4-fold coordinated Ti-oxide
species are highly dispersed within the SiO2 matrixes. Further-
more, the results of the curve fitting in the Fourier transforms
of the EXAFS data for these binary oxide thin films were
collected, as shown in Table 1. The Ti-O bond distance of the
highly dispersed 4-fold coordinated Ti-oxide species incorpo-
rated within the zeolite framework at the atomic level has been
reported to be about 1.78 Å. The Ti-O bond distance in these
Ti-Si binary oxide thin films was found to be slightly longer
at 1.81-1.82 Å. These results indicate that the Ti-oxide species
in these Ti-Si binary oxide thin films exist not as a completely
isolated tetrahedral TiO4 unit incorporated within the SiO2
matrixes at the atomic level but as highly dispersed ultrafine
Ti-oxide clusters including the 4-fold coordinated Ti-oxide
species.
UV light (λ > 270 nm) irradiation of these Ti-Si binary
oxide thin films in the presence of NO were found to lead to
the photocatalytic decomposition of NO with an evolution of
N2 and O2. Figure 4 shows the reaction time profiles of the
photocatalytic decomposition of NO on the Ti-Si binary oxide
thin film having a TiO2 content of 6.6% under UV light
irradiation. This photoreaction to evolve N2 was found to
proceed with a good linearity against the light irradiation time
which ceased under dark conditions. These results clearly
indicate that these Ti-Si binary oxide thin films including the
highly dispersed Ti-oxide species work efficiently as a pho-
tocatalyst under UV light irradiation as is the case with pure
TiO2 thin films.
The photocatalytic reactivity and selectivity of N2 formation
for the decomposition of NO were found to strongly depend
on the local structures of the Ti-oxide species. Figure 5 shows
the effects of the TiO2 content on the production yields of N2
and N2O as well as on the selectivity for N2 formation in the
photocatalytic decomposition of NO. As the TiO2 content in
these binary thin films was lessened, the amount of N2 formation
increased, whereas in contrast, the amount of N2O formation
decreased, resulting in the higher selectivity for N2 formation.
The amount of N2 produced on the Ti-Si binary oxide thin
film prepared by an ICB deposition method and having a TiO2
content of 6.6% for the photocatalytic decomposition of NO
under UV light irradiation for 7 h was 2 times greater as
compared to the pure TiO2 thin films which were also prepared
by an ICB deposition method.
Furthermore, the relationship between the selectivity for N2
formation in the photocatalytic decomposition of NO under UV
light irradiation (circle plots) and the coordination number of
the Ti-oxide species determined by XAFS measurements
(square plots) as well as the wavelength of the absorption edge
determined by the UV-vis absorption spectra (diamond plots)
can be seen in Figure 6. The tendency toward selectivity for
N2 formation in the photocatalytic decomposition of NO was

Ti-Si Binary Oxide Thin Film Photocatalysts
J. Phys. Chem. B, Vol. 107, No. 51, 2003 14281
Figure 6. Relationship between the selectivity for N2 formation (circle
plots) in the photocatalytic decomposition of NO and the coordination
number of the Ti-oxide species (square plots) as well as the wavelength
of the absorption edge (diamond plots).
Figure 8. (a) Model of Ti-Si binary oxide thin films having highly
dispersed Ti-oxide species. (b) The reaction mechanism of the
photocatalytic decomposition of NO into N2 and O2 on the 4-fold Ti-
oxide species.
figure also shows the detailed time courses under UV light
irradiation. The initial contact angle of the water droplets of
about 40-50° on the TiO2 thin films before light irradiation
could reach less than 10° by UV light irradiation of only about
30 min, while retaining its super-hydrophilicity during irradia-
tion. On the other hand, the initial contact angles of the water
were found to be slightly smaller than those on pure TiO2 thin
films, gradually lessening with the UV irradiation time and
saturating at around 10°. However, because only the SiO2 thin
films did not show any response to UV light irradiation, these
changing in the surface wettability on these Ti-Si binary oxide
thin films under UV light irradiation could be associated with
the existence of the highly dispersed Ti-oxide species. Thus,
these results indicate that the highly dispersed Ti-oxide clusters
including the tetrahedral Ti-oxide species exhibit photoinduced
super-hydrophilic properties under UV light irradiation as in
the surface of TiO2 thin films. However, the slower photo-
responses of these Ti-Si binary oxide thin films are explained
by the much longer distances between each Ti-oxide clusters
because the Ti-oxide species are highly dispersed within the
SiO2 matrixes. Furthermore, because the surface of SiO2
originally exhibits the hydrophilic property, the contact angles
of water droplets before light irradiation and after saturation in
dark conditions were found to be small when compared to the
pure TiO2 thin films.
Figure 7. Time courses of the changing in the contact angles of the
water droplets on Ti-Si binary oxide thin films having different TiO2
contents.
found to be in good correspondence with the coordination
numbers of the Ti-oxide species. In other words, Ti-Si binary
oxide thin films having low TiO2 content contain a tetrahedral
TiO4 unit incorporated within the SiO4 matrixes. These results
clearly indicate that the highly dispersed ultrafine Ti-oxide
species included with the tetrahedral TiO4 units play a very
important role in the formation of N2 and O2 in the photocata-
lytic decomposition of NO under UV light irradiation. However,
as the absorption edge of these Ti-Si binary oxide thin films
with low TiO2 content shifts toward shorter wavelength regions,
it is necessary for these thin films with highly dispersed Ti-
oxide species to be irradiated by UV light having much shorter
wavelengths.
To discuss the surface wettability of these Ti-Si binary oxide
thin films, the contact angles of water droplets were measured
under UV light irradiation under ambient conditions. Figure 7
shows the time courses of the contact angles of water droplets
on these thin films when the samples were irradiated by UV
light for 3 h and then kept in the dark for 19 h. The inserted
Conclusions
Ti-Si binary oxide thin films having different TiO2 contents
were successfully prepared by applying an ionized cluster beam
(ICB) deposition method using multi-ion sources as a dry
process. From the results of UV-vis absorption measurements,
the transmittance of these binary oxide thin films with low Ti
content was found to be much higher than that of the pure TiO2
thin film, indicating that the Ti-oxide species in the Ti-Si
binary oxide thin films are highly dispersed within the SiO2
matrixes, as shown in Figure 8a. The results of Ti K-edge XAFS
measurements showed the existence of the highly dispersed Ti-
oxide species having a tetrahedral local structure which is
different from the octahedral structure of anatase TiO2. These
characteristic tetrahedral Ti-oxide species were also found to
exhibit more efficient photocatalytic reactivity and higher
selectivity for N2 formation in the decomposition of NO under

14282 J. Phys. Chem. B, Vol. 107, No. 51, 2003
Takeuchi et al.
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