Materials Transactions, Vol. 44, No. 10 (2003) pp. 2124 to 2129
#2003 The Japan Institute of Metals
Photobleaching of Methylene Blue Aqueous Solution Sensitized by
Composite Powders of Titanium Oxide with SrTiO3 , BaTiO3 , and CaTiO3
Shinya Otsuka-Yao-Matsuo*1 , Takahisa Omata, Shin Ueno*2 and Masao Kita*2
Department of Materials Science and Processing, Graduate School of Engineering, Osaka University, Suita 565-0871, Japan
We have examined photocatalytic reactions sensitized by composite particles consisting of titanium oxide, and perovskite-type titanate
containing alkaline earth elements. The photocatalytic activities of perovskite-type oxides SrTiO3 , BaTiO3 , and CaTiO3 are fairly weaker than
that of anatase-type titanium dioxide TiO2 ; however, in photobleaching of methylene blue under irradiation with Xe discharge light, the
composite particles with TiO2 exerted photocatalytic activity several times that of TiO2 alone. Especially, the composite powder containing
30 mass% SrTiO3 exhibited the highest photocatalytic activity. The results imply the flow of photogenerated electrons and holes through the
heterogeneous junctions in the composite particles. The photocatalytic activity of the composite powder was decreased when the SrTiO3 was
doped with Ga and Y, presumably because of recombination of the photogenerated charges via oxygen vacancies created by the doping.
(Received April 21, 2003; Accepted August 18, 2003)
Keywords: strontium titanate, photocatalyst, composite particle, photobleaching, methylene blue
1.
Introduction
Photocatalytic reactions sensitized by TiO2 1,2) and other
semiconductor materials3,4) have attracted extensive interest
as potential solutions to energy and environmental issues.
Most investigations conducted to date have focused on
anatase-type TiO2 ,5) because it exerts relatively high photocatalytic activity under irradiation with light of wavelength
< 390 nm as well as high chemical stability. An important
factor for the efficient induction of photocatalytic reactions is
to restrain recombination of photogenerated electrons and
holes. A well-known approach for spatially separating the
photogenerated charges is to load co-catalysts,4,6) e.g., NiO,
Pt, and RuO2 , on TiO2 . Another approach is to use composite
films and powders consisting of two semiconducting photocatalysts, e.g., TiO2 /GaP,7) TiO2 /Nb2 O5 ,8) TiO2 /SnO2 ,9,10)
and TiO2 /WO3 .11–14)
Very recently, two of the authors15) prepared composite
powders of anatase-type TiO2 with accepter-doped perovskite-type zirconates containing alkaline earth elements; i.e.,
Sr(Zr1x Yx )O3 , Ca(Zr1x Yx )O3 , and Ca(Zr1x Gax )O3 ,
by mixing the components with mortar and pestle and heating
at 973 K. In photobleaching of methylene blue under
irradiation with Xe discharge light, the composite particles
exerted several times that exerted by TiO2 alone.15) No
reaction was observed at the interface between TiO2 and the
zirconates. The authors have inferred that the photo-excited
charges flowed through the heterogeneous junctions. The B
site ion, i.e., Zr4þ in SrZrO3 and CaZrO3 , can be substituted
by the Ti4þ ion.16–18) Accommodation of the Ti4þ ion in the B
site may play a role in establishing a coherent interface,
thereby forming a good electronic junction. Preparation of
SrZrO3 and CaZrO3 required sintering at temperatures as
high as 1773 K. Thus, our attention was directed to titanates
that contain alkaline earth elements, which can be prepared
by sintering at rather low temperatures or by co-precipitation
from aqueous solutions.19) The objective of the present work
*1Corresponding
*2Graduate
author, E-mail: [email protected]
Student, Osaka University
is to examine whether the composite powders of anatase-type
TiO2 with SrTiO3 , BaTiO3 , and CaTiO3 exert photocatalytic
activity higher than that of TiO2 particles alone under
irradiation with Xe discharge light. Another objective is to
examine whether acceptor doping in the titanates influences
on photocatalytic activity of the composites.
2.
Experimental
2.1 Materials
Very fine powders of SrTiO3 and BaTiO3 (Sr:0.353%,
Fe<0.0001%), having nominal particle sizes of 50 nm, were
purchased from TPL Inc.; SEM observation revealed that the
particles aggregates measured approximately 2 mm. CaTiO3
(3N) was purchased from Mitsuwa Chemicals Co., Ltd.
Figures 1 and 2 show the X-ray diffraction patterns and
diffuse reflectance spectra of these reagent powders; the
SrTiO3 contained traces of anatase- and rutile-types TiO2 ,
and the BaTiO3 contained a trace of BaCO3 . As seen from the
diffuse reflectance spectra, photo-excited electronic transitions among impurity levels were not observed. In the case of
CaTiO3 , photo-excited electronic transitions via impurity
defects were observed. Composite powders consisting of
commercially available anatase-type TiO2 powder (Ishihara
Sangyo, ST-01) and these titanate powders were prepared in
a simple manner. After the TiO2 and titanate powders were
mixed at a mass ratio of 70:30, they were fired at 773 K or
873 K for 1 h without pelletizing, and then milled lightly for
5 min by use of a zirconia mortar. In this paper, the mixing
compositions are denoted as, for example, TiO2 -zSrTiO3 ,
where z indicates the mass% of SrTiO3 added. The mixing
composition z ¼ 30 was adopted in the present study,
because in previous studies on the composite system of
TiO2 -Sr(Zr0:90 Y0:10 )O3 , the maximum composite effect
was observed around z ¼ 30 mass% Sr(Zr0:90 Y0:10 )O3 ,15)
where the addition of Sr(Zr0:90 Y0:10 )O3 enhanced the
photocatalytic activity of TiO2 .
As the acceptor-doped titanates containing alkaline
earth elements, we prepared Sr(Ti0:90 Ga0:10 )O3 and
Sr(Ti0:90 Y0:10 )O3 by way of the standard ceramic process.
Photobleaching of Methylene Blue Aqueous Solution Sensitized by Composite Powders of Titanium Oxide with SrTiO3 , BaTiO3 , and CaTiO3 2125
211
111
211
20°
30°
141
40°
311
113
212
231
132
301
210 201
102 211
031
220
131
022
221
111
10°
042
040
101
200
121
(c) CaTiO3
321
240
210
100
111
110
200
210
100
(b) BaTiO3
Intensity
mortar. In addition, composite powder of TiO2 zSr(Zr0:90 Y0:10 )O3 with z ¼ 30 mass% was prepared in
the same manner as in the previous report,15) and the
photocatalytic activity was compared with that for the
composites consisting of TiO2 and the titanates.
200
110
(a) SrTiO3(TPL)
50°
60°
Diffraction Angle, 2 θ
Fig. 1 X-ray diffraction patterns of the perovskite-type SrTiO3 (TPL),
BaTiO3 , and CaTiO3 powders used in this study, (a) SrTiO3 (TPL)
purchased from TPL Inc., (b) BaTiO3 purchased from TPL Inc.,
(c) CaTiO3 purchased from Wako Pure Cemicals Co.Ltd. : anatasetype TiO2 , : rutile-type TiO2 , : BaCO3 .
BaTiO3
Diffuse Reflectance, R d (%)
100
SrTiO3(TPL)
80
CaTiO3
3.
TiO2(ST-01)
60
CaTiO3
BaTiO3
20
SrTiO3(TPL)
300
400
500
Experimental Results and Discussions
Figure 3 shows the X-ray diffraction pattern for the TiO2 zSrTiO3 (TPL) composite powder fired at 873 K, along with
those for TiO2 (ST-01) and SrTiO3 (TPL) as purchased. The
40
0
2.2 Evaluation of photocatalytic activity
Photocatalytic activity of the sample powders was evaluated by the photobleaching of methylene blue aqueous
solutions.15,20) A 2 105 moldm3 methylene blue aqueous solution was prepared; its maximum absorbance around
664 nm lay between 1.50 and 1.55. The aqueous solution
(100 cm3 ) with the sample powder (0.20 g) was loaded in a
glass container (28 cm2 ) and then set in a water cooling bath.
The sample powder was dispersed in the aqueous solution by
stirring with a magnetic stirrer. The maximum absorbance of
the aqueous solution around 664 nm may change slightly
with variations in temperature, which originate from the
cooling system. After elapse of 1 min, the irradiation with
500W Xe discharge light above the aqueous solution was
started. After elapse of a predetermined time, 12 cm3 of the
solution was aspirated and subjected to centrifugation. The
optical absorption spectrum for the supernatant solution was
recorded by a double-beam spectrophotometer (Hitachi
U4000). At an appropriate interval of time, a UV-cut filter
(Suruga Seiki L42) was inserted, and the photobleaching of
methylene blue under visible light ( > 420 nm) was
examined.
600
700
800
900
1000
Wave Length, λ / nm
(a) SrTiO3(TPL)
Fig. 2 Diffuse reflectance spectra of the perovskite-type SrTiO3 (TPL),
BaTiO3 , and CaTiO3 powders used in this study, along with that of TiO2
(anatase-type, ST-01). SrTiO3 (TPL) and BaTiO3 were purchased from
TPL Inc., and CaTiO3 was purchased from Mitsuwa Cemicals Co.Ltd.
105
211
103
004
112
200
101
(b) TiO 2(ST-01)
Intensity
Powdered raw materials SrCO3 (3N), Ga2 O3 (4N), and Y2 O3
(3N) were purchased from High-purity Chemicals Co., Ltd.,
and TiO2 (4N) were purchased from Rare Metallic Co., Ltd.
These powders were weighed in consideration of their
ignition losses, thoroughly mixed by use of a planetary ball
mill, and then calcined at 1073 K for 10 h. After the powders
were ground and pressed into a 17.2 mm-diameter disk under
265 MPa, the disk was loaded in a Pt crucible and annealed at
1773 K for 10 h. The disk was crushed and pulverized by a
zirconia mortar. For comparison, undoped SrTiO3 powder
was prepared in the same manner; the sintering was
conducted at 1773 K for 10 h. The SrTiO3 powder obtained
here is distinguished from the SrTiO3 (TPL) purchased from
TPL Inc. After the anatase-type TiO2 (ST-01) powder was
mixed with the SrTiO3 powder, Sr(Ti0:90 Ga0:10 )O3 , and
Sr(Ti0:90 Y0:10 )O3 powders, respectively, at a mass ratio of
70:30, the mixed powders were fired at 973 K for 1 h without
pelletizing and then milled lightly for 5 min with the zirconia
(c) TiO 2
-z SrTiO3
(TPL)
10°
20°
30°
40°
50°
60°
Diffraction Angle, 2 θ
Fig. 3 X-ray diffraction patterns for the TiO2 -SrTiO3 (TPL) composite
powder fired at 873 K, and those for TiO2 and SrTiO3 (TPL).
(a) SrTiO3 (TPL), (b) TiO2 (ST-01), (c) TiO2 -zSrTiO3 (TPL) composite
powder (z ¼ 30 mass%). : anatase-type TiO2 , : rutile-type TiO2 ,
: perovskite-type SrTiO3 .
2126
S. Otsuka-Yao-Matsuo, T. Omata, S. Ueno and M. Kita
1.6
(a) TiO2-z SrTiO3(TPL) (z = 30 mass%)
1.6
(b) TiO2-z BaTiO3 (z = 30 mass%)
starting solution
starting solution
1.4
1.4
0 min
0 min
1.2
1.2
1.0
Absorbance, α
Absorbance, α
5 min
30 min
5 min
1.0
30 min
0.8
60 min
0.8
60 min
0.6
70 min
0.6
90 min
0.4
0.4
0.2
0.0
400
500
600
700
800
120 min
0.0
300
400
500
600
700
800
Wave Length, λ / nm
Wave Length, λ / nm
1.6
90 min
0.2
120 min
300
70 min
(c) TiO2-z CaTiO3 (z = 30 mass%)
starting solution
1.4
0 min
Absorbance, α
1.2
5 min
1.0
30 min
0.8
60 min
0.6
70 min
90 min
0.4
120 min
0.2
0.0
300
400
500
600
700
800
Wave Length, λ / nm
very small diffraction peaks of rutile-type TiO2 for the TiO2 zSrTiO3 (TPL) composite are attributable to the impurity
phases in the SrTiO3 (TPL) used. The firing proceeded
crystallization of the anatase-type ST-01 TiO2 powder;
however, it did not transform into rutile-type. This result is
consistent with the previous finding15) that firing at 973 K for
1 h in air did not transform the anatase-type ST-01 TiO2 into
rutile-type. The previous SEM and TEM observation showed
that the perovskite-type zirconate powder (10 mm) was
surrounded by an aggregate of very fine TiO2 powders. In
the present study, the particles became small and we could
not observe the junctions between the titanates and TiO2
particles in the composite powders. We have inferred that
SrTiO3 (TPL), BaTiO3 , and CaTiO3 particles were surrounded by an aggregate of very fine TiO2 powders.
Figures 4(a), (b), and (c) show variation in the absorption
spectra of methylene blue aqueous solution under irradiation
Fig. 4 Variation in the absorption spectra of methylene blue aqueous
solution sensitized by composite powders under irradiation with 500W Xe
discharge light. The concentration of methylene blue in the starting
solution was 2:0 105 moldm3 . (a) TiO2 -zSrTiO3 (TPL) (z ¼
30 mass%), (b) TiO2 -zBaTiO3 (z ¼ 30 mass%), (c) TiO2 -zCaTiO3 (z ¼
30 mass%) 0–5 min: unfiltered irradiation, 5–30 min: without irradiation,
30–60 min: irradiation using UV-cut filter (L42), 60–120 min: unfiltered
irradiation.
with Xe discharge light, when the composite powders of
TiO2 -zSrTiO3 (TPL), TiO2 -zBaTiO3 , and TiO2 -zCaTiO3
(z ¼ 30 mass%), respectively, were used as photocatalyst.
In all cases, the characteristic absorption peak of methylene
blue around 664 nm decreased and shifted slightly toward a
shorter wavelength, and the solution eventually became
colorless. In Fig. 5, variations in maximum absorbance in the
wavelength range between 600 and 664 nm with time are
compared for various composite powders containing
z ¼ 30 mass% of alkaline earth titanate, as well as for
anatase-type TiO2 (ST-01) powders alone. The commercially
available ST-01 powder of anatase-type TiO2 is known to
have a relatively high photocatalytic activity; the preliminary
treatment, i.e., drying at 413 K and firing at 973 K, had no
significant effect on the results for TiO2 powder alone. When
SrTiO3 (TPL) powder alone was used, the photobleaching
rate was several times smaller than that with TiO2 alone; the
Photobleaching of Methylene Blue Aqueous Solution Sensitized by Composite Powders of Titanium Oxide with SrTiO3 , BaTiO3 , and CaTiO3 2127
1.6
1.4
Absorbance, α
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
30
60
90
120
Irradiation Time, t / min
Fig. 5 Change in maximum absorbance in the wavelength range of 600 to
664 nm sensitized by composite powders of TiO2 with SrTiO3 (TPL),
BaTiO3 , and CaTiO3 , respectively, under irradiation with 500W Xe
discharge light. Solid and chain lines indicate, respectively, unfiltered
irradiation and irradiation with visible light of > 420 nm with use of a
UV-cut filter (L42, Suruga Seiki Co. Ltd.). Dotted line indicates
interruption of irradiation.
: TiO2 -zSrTiO3 (TPL) composite, (z ¼
30 mass%) fired at 873 K, u: TiO2 -zSrTiO3 (TPL) composite, (z ¼
30 mass%) fired at 773 K, t: TiO2 -zBaTiO3 composite, (z ¼ 30 mass%)
fired at 873 K, n: TiO2 -zCaTiO3 composite, (z ¼ 30 mass%) fired at
873 K, : TiO2 powder dried at 413 K, 4: TiO2 powder fired at 973 K,
: SrTiO3 (TPL) purchased from TPL Inc., : TiO2 -zSrZr0:90 Y0:10 O3
composite, (z ¼ 30 mass%) fired at 973 K,15) ^: without sample powders
(blank test).
alkaline earth titanate powders may have fairly weak
photocatalytic activity. Surprisingly, when the TiO2 zSrTiO3 (TPL) composite powder was used as a sample, the
photobleaching proceeds several times faster than with TiO2
alone. Before the irradiation, the composite powder was
white. As the photobleaching proceeded, the sample powder
became colored dark blue, and then returned to the original
white as the bleaching was completed. The coloring of the
sample powders indicates the strong adsorption of methylene
blue ions on the sample. Similar results were obtained for
composite powders consisting of TiO2 , and BaTiO3 or
CaTiO3 .
One may point out two important phenomenological
features of the composite powders, which appear clearly in
Fig. 5. First, the photobleaching rate for the composite is
higher than those for the respective constituent oxides.
Second, the absorbance of the methylene blue aqueous
solution at 0 min; i.e., after dispersal of the composite
powders, is lower than that as prepared. For all the
composites examined in this study, the maximum absorbance
values at 0 min for the composite powders were rather
smaller than that for TiO2 alone. Especially, the absorbance
value at 0 min for the TiO2 -zSrTiO3 (TPL) powder prepared
at 773 K was considerably smaller. The flow of charges
through the heterogeneous junctions in the composites may
lead to the accumulation of charges on the particle surface,
which in turn must cause the adsorption of methylene blue
ions;15) in turn, the adsorption must result in a slight decrease
in the methylene blue concentration in the solution. The
decrease in absorbance thereafter by the irradiation could be
attributed to the photocatalytic bleaching of the methylene
blue, because the decrease in absorbance almost stopped with
the interruption of irradiation, and further, with successive
irradiation, the aqueous solution finally became colorless.
One may recognize a clear correlation between the two
phenomenological features. As the decrease in the absorbance at 0 min became large, the photobleaching rate under the
irradiation increased. Thus, one can believe a model that the
flow of the photogenerated electrons and holes through the
heterogeneous-junctions in the composites brought a spatial
separation of the charges; this spatial separation caused the
adsorption of methylene blue ions and proceeded the photocatalytic bleaching of the aqueous solutions, because recombination of the photogenerated charges was depressed.
When the firing temperature was held constant as 873 K,
the photocatalytic activity of the TiO2 -zSrTiO3 (TPL) composite was greater than those of the TiO2 -zBaTiO3 , and TiO2 zCaTiO3 composites. As is well known, the magnitude and
slope of the diffusion potential appearing around heterogeneous junctions can be related to Fermi levels, impurities
contents, and permittivities of the two constituent oxides.21)
We may try to estimate the effect of these factors on the basis
of the present results; however, to derive a strict conclusion,
many additional experiments must be conducted. One point
to emphasize is that the addition of SrTiO3 exerted the largest
effect, with a magnitude similar to that of Sr(Zr0:90 Y0:10 )O3
reported previously.15) Preparation of SrTiO3 by the conventional ceramic method is fairly easier than preparation of the
zirconates, because the Sr2þ and Ti4þ ions diffuse fast.
SrTiO3 can be prepared by the reaction between TiO2 and
SrCO3 at temperatures as low as 1173 K, or by coprecipitation from aqueous solutions.19) This indicates that
the composite of TiO2 -zSrTiO3 can also be prepared at lower
temperatures, thereby depressing grain growth. The results
for TiO2 -zSrTiO3 (TPL) fired at 773 K, shown in Fig. 5, imply
expanded applicability of TiO2 -zSrTiO3 composite as photocatalyst.
Figures 6 and 7 show the X-ray diffraction patterns
and diffuse reflectance spectra of the SrTiO3 ,
Sr(Ti0:90 Ga0:10 )O3 , and Sr(Ti0:90 Y0:10 )O3 powders prepared by the conventional ceramic method. The XRD results
for the Sr(Ti0:90 Y0:10 )O3 confirm the precipitation of trace
SrY2 O4 phase;22) that is, some of the added yttrium was
depleted as SrY2 O4 without doping in the SrTiO3 phase. The
XRD results also show that impurity phases in the SrTiO3
and Sr(Ti0:90 Ga0:10 )O3 are negligible. The SrTiO3 prepared
by the conventional ceramic method may contain defects
related to oxygen vacancies; i.e., VO , VO , and VO , oxygen
at interstitial sites; i.e., Oi0 , Oi00 , and Oi , and holes on the
lattice oxygen; i.e., OO . Especially, the SrTiO3 prepared by
sintering at temperatures as high as 1773 K must contain
defects related to oxygen vacancies. Diffuse reflectance,
shown in Fig. 7, is equivalent to transmittance of the sample.
Within the wavelength range of ¼ 450{600 nm, absorption
increased in the order of SrTiO3 (TPL), SrTiO3 , Sr(Ti0:90 Y0:10 )O3 .and Sr(Ti0:90 Ga0:10 )O3 . Annealing at high
temperature and acceptor doping are known to induce oxygen
vacancies in the bulk oxide; the latter produces greater
numbers of oxygen vacancies. Almost all the added gallium
was doped in the titanate. Thus, from the absorption results,
we may conclude that the coloration was induced by photoexcited electronic transitions via oxygen vacancies, and Ga
and Y were doped in the Ti sites in the strontium titanates.
Figure 8 shows the variations in the maximum absorbance
2128
S. Otsuka-Yao-Matsuo, T. Omata, S. Ueno and M. Kita
200
111
110
211
1.6
(a) SrTiO3
1.4
Absorbance, α
210
100
1.2
(b) Sr(Ti0.9 Ga0.1 )O3- δ
1.0
0.8
0.6
0.4
0.2
Intensity
0.0
(c) Sr(Ti0.9 Y0.1 )O3- δ
10°
20°
30°
40°
50°
Diffraction Angle, 2θ
60°
Diffuse Reflectance, Rd (%)
Fig. 6 X-ray diffraction patterns for perovskite-type SrTiO3 ,
Sr(Ti0:90 Ga0:10 )O3 , and Sr(Ti0:90 Y0:10 )O3 , prepared by a conventional ceramic method. (a) SrTiO3 , (b) Sr(Ti0:90 Ga0:10 )O3 ,
(c) Sr(Ti0:90 Y0:10 )O3 . : SrY2 O4 , [: unknown peak.
100
80
SrTiO 3(TPL)
TiO2(ST-01)
SrTiO 3
60
Sr(Ti 0.9 Ga0.1 )O3- δ
40
Sr(Ti 0.9 Y0.1 )O3- δ
20
0
300
400
500
600
700
800
Wave Length, λ / nm
900
1000
Fig. 7 Diffuse reflectance spectra of perovskite-type SrTiO3 ,
Sr(Ti0:90 Ga0:10 )O3 , and Sr(Ti0:90 Y0:10 )O3 , prepared by a conventional
ceramic method, and those of SrTiO3 (TPL) and TiO2 (anatase-type
ST-01).
of the methylene blue aqueous solution within the wavelength range of 600 to 664 nm with time, where the solution
was sensitized by SrTiO3 , TiO2 -zSr(Ti0:90 Y0:10 )O3 , and
TiO2 -zSr(Ti0:90 Ga0:10 )O3 with z ¼ 30 mass%. The rate of
decrease in the absorbance for SrTiO3 was lower than that for
SrTiO3 (TPL) purchased from TPL Inc. This result may be
attributed to the SrTiO3 (TPL) containing trace amounts of
anatase- and rutile-type TiO2 . As seen in Fig. 8, the
composite effect was observed for TiO2 -zSr(Ti0:90 Y0:10 )O3
and TiO2 -zSr(Ti0:90 Ga0:10 )O3 , as well as for the undoped
composite powders. Further, a trend was observed such that
doping with gallium and yttrium slightly reduced the photocatalytic activity of the composite with the titanate. One may
infer that the oxygen vacancies produced by the doping
0
30
60
90
Irradiation Time, t / min
120
Fig. 8 Change in maximum absorbance within the wavelength range of
600 to 664 nm of methylene blue sensitized by composite powders of TiO2
with Sr(Ti0:90 Ga0:10 )O3 , and Sr(Ti0:90 Y0:10 )O3 , respectively, along
with that for TiO2 -SrTiO3 composite powder, under irradiation with 500W
Xe discharge light. Solid and chain lines indicate, respectively, unfiltered
irradiation and irradiation with visible light of > 420 nm with use of a
UV-cut filter (L42, Suruga Seiki Co. Ltd.). Dotted line indicates
interruption of irradiation.
: TiO2 -zSr(Ti0:90 Ga0:10 )O3 composite,
(z ¼ 30 mass%) fired at 973 K, : TiO2 -zSr(Ti0:90 Y0:10 )O3 composite,
(z ¼ 30 mass%) fired at 973 K, u: TiO2 -zSrTiO3 composite, (z ¼
30 mass%) fired at 973 K.
: TiO2 powder dried at 413 K, : TiO2
powder fired at 973 K, : SrTiO3 prepared by ceramic method, : TiO2 zSrZr0:90 Y0:10 O3 composite, (z ¼ 30 mass%) fired at 973 K,15) ^: without
sample powders (blank test).
served as sites for the recombination of the photogenerated
charges.
A recent study reported that a composite consisting of
TiO2 and Sr(Zr0:90 Y0:10 )O3 induced photocatalytic activity
under visible light of > 420 nm.15) The strontium titanate
absorbed light of wavelength < 800 nm, by virtue of the
photo-excited electronic transitions among the impurity
levels related to oxygen, e.g., VO , VO , and VO , oxygen
at interstitial sites such as Oi0 , Oi00 , and Oi , and holes on the
lattice oxygen such as OO . As seen from Figs. 5 and 8, under
irradiation with visible light of > 420 nm, a small but clear
decrease in absorbance was observed for TiO2 zSr(Zr0:90 Y0:10 )O3 . As seen in Fig. 8, the decreases in
absorbance for TiO2 -zSr(Ti0:90 Y0:10 )O3 and TiO2 zSr(Ti0:90 Ga0:10 )O3 are smaller than that for TiO2 zSr(Zr0:90 Y0:10 )O3 . Rather, a clear decrease in the absorbance by the visible light irradiation was observed in Fig. 5 for
TiO2 -zBaTiO3 containing undoped titanate. Causes for these
results under the visible light irradiation remain unknown,
and advanced discussion requires further detailed study.
4.
Conclusions
We have examined photocatalytic activity of composite
particles in the photobleaching of methylene blue aqueous
solution under irradiation with Xe discharge light. Composite
powders were prepared by mixing titanium dioxide and
commercially available alkaline earth titanates. The effect of
acceptor doping in the titanates was also examined. The
obtained results are as follows,
(1) Under irradiation with un-filtered Xe discharged light,
composite particles consisting of anatase-type TiO2 , and
SrTiO3 , BaTiO3 or CaTiO3 exerted photocatalytic activity in
photobleaching of methylene blue several times that exerted
Photobleaching of Methylene Blue Aqueous Solution Sensitized by Composite Powders of Titanium Oxide with SrTiO3 , BaTiO3 , and CaTiO3 2129
by TiO2 alone. That is, the addition of SrTiO3 , BaTiO3 , or
CaTiO3 enhanced the photocatalytic activity of TiO2 .
(2) Before light irradiation, the maximum absorbance of
methylene blue around 664 nm decreased upon dispersal of
the composite powders. This was attributable to the adsorption of the methylene blue ions on the surface of composite
particles.
(3) A clear relationship was recognized between the rate of
decrease in the absorbance due to the adsorption of
methylene blue ion before irradiation and the photobleaching
of methylene blue after irradiation; the former was larger, and
the latter degradation rate increased. The phenomena can be
explained by a model for the flow of the charges through
heterogeneous junctions in the composites.
(4) Among SrTiO3 , BaTiO3 , and CaTiO3 , the addition of
SrTiO3 had the greatest effect in increasing the photocatalytic
activity of TiO2 . The addition of BaTiO3 induced slightly
photocatalytic activity for visible light of > 420 nm.
(5) Strontium titanates doped with Ga and Y absorbed visible
light of wavelength > 420 nm; however, the doping of the
titanate decreased the photocatalytic activity of the TiO2 SrTiO3 composite, and did not induce a clear activity for
visible light of > 420 nm. One may infer that oxygen
vacancies produced by the doping behaved as sites for
recombination of the photogenerated charges.
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