Oxygen release and structural changes in TiO
2
films during photocatalytic
oxidation
Kenta Yoshida
a͒
and Takahiro Nanbara
Department of Crystalline Materials Science, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan
Jun Yamasaki and Nobuo Tanaka
Ecotopia Science Institute, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan
and Department of Crystalline Materials Sciences, Nagoya University, Chikusa-ku, Nagoya 464-8603,
Japan
͑Received 12 May 2005; accepted 1 March 2006; published online 2 May 2006͒
Changes in the crystal structure and grain modifications in titanium oxide ͑TiO
2
͒ thin films were
observed during the photocatalytic oxidation of hydrocarbons. When the hydrocarbon and collodion
films were irradiated, single crystalline titanium oxide transformed into polycrystals. The titanium
oxide films gradually became network aggregates. These changes were analyzed with a dedicated in
situ transmission electron microscope and observed three dimensionally by electron tomography. A
detailed analysis of electron energy loss spectra of the samples also revealed that the changes were
associated with the loss of oxygen atoms in the TiO
2
crystal lattice. Correlations between the
polycrystalline grain size of TiO
2
and its catalyst activity were discussed based on the measured
data. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2190721͔
I. INTRODUCTION
Titanium dioxide ͑TiO
2
͒ has been investigated for a
wide range of applications such as in dye-sensitized solar
cells,
1,2
photocatalysts,
3,4
optical coatings,
5
and capacitors
for large-scaled integrated ͑LSI͒ devices.
6
In particular, en-
vironmental purification by the use of photocatalytic TiO
2
materials have attracted attention over the last ten years.
7
These approaches are based on the high photocatalytic activ-
ity of TiO
2
.
Previously, we studied the in situ photo-oxidation pro-
cess by using a dedicated transmission electron microscope
͑TEM͒. Ultraviolet ͑UV͒ light was introduced into the mi-
croscope column using an optical guide. This system enabled
us to observe the photocatalytic reactions dynamically in an
atomic resolution. The photocatalytic decomposition of hy-
drocarbons on TiO
2
films was successfully observed using
the in situ high-resolution transmission electron microscope
͑HRTEM͒ system.
8
The catalytic activity in the submicron
areas was also quantified using electron energy loss spectros-
copy ͑EELS͒. The energy loss near edge structure ͑ELNES͒
spectra of the carbon K core-loss edge at 283.8 eV and of the
L
2,3
core-loss edge of titanium at 460 eV were analyzed.
Measurements of the photocatalytic activity in localized ar-
eas of about 1.1, 0.3, and 0.1
m diameters on the samples
were performed using the EELS measurement.
9
In situ ob-
servations during the TiO
2
photocatalysis revealed that the
crystalline titanium oxide films had changed.
In the present paper, we report on the changes in crystal
structures and shapes of grains in TiO
2
thin films during their
photocatalytic process. The relationship between the size of
titanium oxide and the photocatalytic activity was also stud-
ied.
II. EXPERIMENT
In order to observe in situ changes in the TiO
2
films
during the photocatalytic process inside the TEM, we pre-
pared a triple-layer sample. The first layer was a polyhydro-
carbon film of 30 nm thickness and the second was a thin
crystalline film of rutile TiO
2
, both of which were supported
on a copper ͑Cu͒ microscopic grid covered with a collodion
film as the third layer.
The TiO
2
thin films were prepared by a laser-ablation
method using a 10 Hz Nd:YAG ͑yttrium aluminum garnet͒
laser ͑Spectra-Physics INDI-40͒ operating at 150 nm thick-
ness. The laser beam was focused using an optical lens with
a 30 cm focal length onto a rotating target composed of TiO
2
rutile polycrystals. In order to prepare for the target, a special
grade of TiO
2
powder ͑Wako Pure Chem. Ind. Inc., ϳ1
m͒
was pelletized into a disk of 10 mm diameter and about
2 mm thickness using a uniaxial hydrostatic press at 20 MPa
followed by heating at 1473 K in air for 4 h.
10
The deposi-
tion chamber was pumped down to 3 ϫ10
−8
Torr using a
turbo-molecular pump and an ion pump. During the deposi-
tion, the partial oxygen pressure was kept at 50 mTorr. The
deposition time for 300 laser pulses was up to 30 s.. The
substrate was a cleaved ͑001͒ surface of sodium chloride
͑NaCl͒, which was located 3 cm from the target, and the
substrate temperature was 300–350 °C measured by a ther-
mocouple. After the deposition, a thermal treatment for the
crystallization of the titanium oxide films was performed at
350 ° C for 1 h at an oxygen pressure of 50 mTorr.
11,12
The
film was then cooled gradually to room temperature at a rate
of 10 ° C/ min.
13
The titanium oxide thin films were supported on a Cu-
microscopic grid covered with a collodion film following the
dissolution of the NaCl substrate that was performed with
water. The hydrocarbon materials were deposited subse-
quently with 30 nm thickness by a chemical vapor deposition
a͒
Electronic mail: h046105d@mbox.nagoya-u.ac.jp or a41263a@nucc.cc.
nagoya-u.ac.jp
JOURNAL OF APPLIED PHYSICS 99, 084908 ͑2006͒
0021-8979/2006/99͑8͒/084908/8/$23.00 © 2006 American Institute of Physics99, 084908-1