Effects of silica-coating on the photoinduced hole formation and decomposition activity of titanium dioxide photocatalysts under uv irradiation

Article abstract of DOI:10.1007/s10562-012-0914-1

Contrary to the general knowledge in practical use of TiO₂ photocatalyst, that silica coating improves weather resistance by inhibiting chemical activity against resin, the present study revealed that the number of photoinduced holes was increased and the decomposition activity to gaseous reactants was persistent as the effect of silica coating. Graphical Abstract: Silica-coating did not block the hole formation, but prevent the adsorption of the reaction intermediates to sustain the photocatalytic activity.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-012-0914-1

Catal Lett (2012) 142:1474–1481
DOI 10.1007/s10562-012-0914-1
Effects of Silica-Coating on the Photoinduced Hole Formation
and Decomposition Activity of Titanium Dioxide Photocatalysts
Under UV Irradiation
•
Junichi Oguma Yusuke Kakuma
•
•
Masami Nishikawa Yoshio Nosaka
Received: 21 June 2012 / Accepted: 22 September 2012 / Published online: 6 October 2012
ꢀ Springer Science+Business Media New York 2012
Abstract Contrary to the general knowledge in practical
use of TiO2 photocatalyst, that silica coating improves
weather resistance by inhibiting chemical activity against
resin, the present study revealed that the number of photoin-
duced holes was increased and the decomposition activity to
gaseous reactants was persistent as the effect of silica coating.
some reports which showed the improvement for practical
performance by coating photocatalyst. For example, the
increase in the weather resistance was evidenced by mea-
suring the decrease in thickness of paint film [4] and the
number of cracks in resin was decreased with coated TiO2
which increased the weather resistance [5]. Another study
showed that excellent hydrophilic properties could be
obtained for a paint film made by baking a mixture of TiO2
and silica particles [6]. Recently, a photocatalyst composed
of a titanium dioxide/silica complex formed by the sol–gel
method, but not by coating, has been reported to exhibit
high activity in photocatalytic dye decomposition [7–9].
Thus, the effect of silica coating on the photocatalytic
activity has not been clearly understood yet. Since silica-
coated TiO2 photocatalysts are well-known materials for
practical use, it is necessary to explore the effect of coated
silica on the photocatalytic reaction mechanism.
Keywords TiO2 ꢀ Silica coating ꢀ Photocatalytic
reaction ꢀ ESR
1 Introduction
Photocatalytic materials, such as titanium dioxide (TiO2),
are known to have the capability to decompose substances
and create hydrophilic surfaces by the action of light
energy [1–3]. When photocatalysis is applied to antifouling
of house, photocatalytic materials have to be incorporated
in a paint, which can be painted on resin sheets or the
exterior walls. Photocatalytic reactions, however, often
cause chalking in a long term use, that is, the surface of the
paint film becomes white powder. Therefore to decrease
the photocatalytic activity, TiO2 is coated with silica or
alumina, which is the method that has been practically used
for a long time to prevent chalking [1]. Actually, there are
Generally the mechanism of photo-induced reaction in
TiO2 photocatalysis proceeds as following processes.
Photo-absorption in titanium oxide produces conduction
band electrons (e-) and valence band holes (h?) Eq. (1).
The conduction band electrons reduce oxygen molecules in
air to produce superoxide radicals (O2•-) and H2O2 (Eqs. 2
and 3). The valence band holes oxidize a reactant, such as
organic pollutants, to decompose into products and finally
into CO2 (Eq. 4). In the decomposition process, O2 and its
reduction products are involved as complex pathways [10].
J. Oguma ꢀ Y. Kakuma ꢀ M. Nishikawa ꢀ Y. Nosaka (&)
Department of Materials Science and Technology, Nagaoka
University of Technology, 1603-1, Kamitomioka,
Nagaoka 940-2188, Japan
TiO2 þ hm ! TiO2ðeꢁ þ hþÞ
ð1Þ
ð2Þ
ð3Þ
O2 þ eꢁ ! Oꢂ2ꢁ
e-mail: nosaka@nagaokaut.ac.jp
J. Oguma
Department of Latex Research and Development,
Asahi Kasei Chemicals Corporation, 1-3-1, Yakoh,
Kawasaki 210-0863, Japan
Oꢁ2 ꢂ þ eꢁ þ 2Hþ ! H2O2
123

Effects of Silica-coating on TiO2 photocatalysts
1475
hþ þ reactant ! product ! CO2
ð4Þ
(thermogravimetry) analyses were carried out with a TG/
DTA 220 (Seiko Instruments Inc).
These reactive species, h?, O2•-, and H2O2, are funda-
mental reaction intermediates involved in photocatalysis
and closely related with the photocatalytic activity.
Therefore the detection of these species is important to
understand the details of the reaction mechanism. The
authors used electron spin resonance (ESR) spectroscopy
to detect h? and the chemiluminescence method to ana-
lyze the photocatalytic reactions [11–13]. Although pho-
tocatalytic paints for practical use contain silica-coated
TiO2, the effect of silica treatments has not been quanti-
tatively reported with respect to the reactive species so
far. Therefore, in the present report, the fundamental
change in the reaction mechanism has been examined as
the effect of silica-coating on TiO2 photocatalyst by
measuring the amount of photoinduced holes with ESR
spectroscopy and the time profile of CO2 evolution on
organic substance decomposition under UV irradiation.
As a result, it was discovered that silica coating does not
inhibit the formation of holes for photocatalytic reaction
and enhances a long-term decomposition activity for
gaseous reactants.
2.4 ESR Measurements
Electron spin resonance spectra were measured by using an
ESP-300E spectrometer (Bruker with an X-band cavity). A
sample of 20 mg in vacuum (0.1 Torr) at 77 K was mea-
sured in dark at first and then irradiated with UV light from
a high-pressure mercury lamp (250 W, Ushio Optical
Modulex BA-H250) through a Hoya U360 band-pass filter.
The conditions for ESR measurements are microwave
power of 10 mW, modulation amplitude of 0.2 mT, time
constant of 327.68 ms, and scanning time of 335.54 s. The
each ESR spectrum was integrated twice, and the differ-
ences between those measured in dark and under irradia-
tion were employed to calculate the amount of holes
generated by the irradiation.
2.5 Decomposition of Organic Substances
On a petri dish of 40 mm in diameter, 150 mg of silica-
coated or un-coated TiO2 was dispersed in a suitable
amount of distilled water then dried at 100 ꢁC for 1 h in an
oven. After setting the petri dish in a circulation photore-
actor equipped with a CO2 monitor (LI-840, LI-COR Co.),
the air in the reactor was replaced with synthetic air (N2/O2
mixture free from CO2). After the measurement system
became stable, 50 lL of organic gas and 10 lL of H2O
were introduced into the reactor and circulated for 2–3 h
before the irradiation to allow the adsorption of the reac-
tants on the photocatalyst surface. Following this and while
the sample was irradiated with a 365 nm LED (0.2 mW/
cm2, NCCU033A, Nichia Co.), the increase in the amount
of CO2 was continuously measured. Toluene and acetal-
dehyde were selected as the organic target compounds in
this experiment, because they are representative pollutants
in air.
2 Experimental
2.1 Silica-Coated TiO2
A sample of silica-coated anatase TiO2 powder was
obtained by freeze-drying a 12 wt%-silica-coated anatase
TiO2 sol (MPT-422, developed by Ishihara Sangyo, Ltd.).
For comparison, un-coated anatase TiO2 powder (ST-01,
Ishihara Sangyo, Ltd.) was also employed. The BET sur-
face areas of the particles for silica-coated TiO2 and un-
coated TiO2 were 240 and 320 m2/g, respectively.
2.2 High-Resolution Scanning Transmission Electron
Microscope (STEM-EDX) Observation
3 Results and Discussion
The morphology of silica-coated and un-coated TiO2 par-
ticles were observed with a high-resolution scanning
transmission electron microscope (HR-STEM: HD-2300A,
Hitachi, Ltd.) in the Analysis & Simulation Center, Asahi
Kasei Corporation. An energy dispersive X-ray spectros-
copy (EDX) attached was used for analyzing the elements,
Ti, Si, and O, in the powders.
3.1 STEM-EDX Observations
Figures 1 and 2 show the results of STEM images and
STEM-EDX analysis for the silica-coated TiO2 and the
uncoated TiO2, respectively. For both silica-coated and
uncoated TiO2, the primary particles of ca. 10 nm gathered
together to form particle aggregates of approximately
60 nm in diameter as secondary particles. In the case of
silica-coated TiO2, in the EDX mapping pictures of Ti, Si,
and O, the high-contrast points are located in the same
place of the STEM image. The observation revealed that
2.3 Analyses of XRD and TG
XRD (X-ray diffraction) analyses were carried out with a
RAD-3A X-ray diffractometer (Rigaku Co., Ltd.) and TG
123

1476
J. Oguma et al.
Fig. 1 STEM-EDX analysis for
silica-coated titanium dioxide:
STEM image (upper left panel)
and EDX element mapping
images of Si, Ti, and O (other
panels)
silica was efficiently dispersed on the surface of the pri-
mary particle of TiO2.On the other hand, in the case of
uncoated TiO2, in mapping picture of Si, no contrast points
were observed though small background signals was
observed. The TEM picture conforms that the size of
uncoated TiO2 particles used in the present research is
similar to that of the coated TiO2.
un-coated TiO2 are 9.4 and 7.5 nm2, respectively. At
temperatures higher than 150 ꢁC, the difference between
the silica-coated and un-coated TiO2 increases. Physi-
sorbed water molecules are eliminated up to around
150 ꢁC, and surface hydroxyl groups (or chemisorbed
water molecules) desorbed at higher temperatures. We can
therefore say that silica coating has increased the amount of
surface hydroxyl groups.
3.2 Analyses of XRD and TG
3.3 ESR Measurements
As the results of XRD analysis in Fig. 3, there exist no
differences in the peak of the crystal between the silica-
coated and uncoated TiO2. Furthermore, the peak width
was not changed by silica-coating, indicating the crystallite
size of TiO2 was almost no changed. It has been pointed at
that the particle size is a crucial factor in the dynamics of
photocatalytic reactions [14], these observations suggest
the samples used in the present research differs only in the
silica-coating and the properties of TiO2 crystallite is
almost same.
Figure 5 shows the ESR spectra for silica-coated and un-
coated TiO2. It is probable that signal appeared at
g = 2.007–2.019 represents internal- or surface-trapped
holes captured by oxygen atoms, and that at g = 2.062
represents adsorbed superoxide radicals (O2•-) [15]. As
summarized in our previous paper [15], the signal for
2.004–2.018 (g1 = 2.004, g2 = 2.014 and g3 = 2.018) is
for internal-trapped holes, and that for 2.004–2.030
(g1 = 2.004, g2 = 2.018 and g3 = 2.030) is for surface-
trapped holes. Therefore, the peak at 2.004–2.018 observed
in the present study is attributable to internal-trapped holes
captured by oxygen atoms. The signals for trapped elec-
trons (Ti3?), which is normally observed on the higher
magnetic field, could not be clearly recognized for both
samples. This observation agrees with previously reported
Figure 4 shows the results of TG analysis. The weight
loss on heating up to 900 ꢁC was 10.8 % for the silica-
coated TiO2, while it was 10.2 %, for the un-coated TiO2.
Increase of adsorbed water by SiO2 seems small, but when
the weight losses are normalized by the surface area, the
eliminated water molecules for the silica-coated and
123

Effects of Silica-coating on TiO2 photocatalysts
1477
Fig. 2 STEM-EDX analysis for
uncoated titanium dioxide:
STEM image (upper left panel)
and EDX element mapping
images of Si, Ti, and O (other
panels)
Fig. 4 Thermogravimetry curve of silica-coated titanium dioxide
(SiO2/TiO2) and uncoated TiO2
peak is also observed before the irradiation as shown in the
figure by broken curve. The defect signal for silica-coated
TiO2 (Fig. 5a) was less conspicuous by overlapping a lar-
ger signal of trapped holes.
A comparison of the two spectra in Fig. 5a, b indicates
that the silica-coated TiO2 has a slightly larger number of
holes. An apparent small difference at g = 2.013–2.019
between the UV irradiated spectra in Fig. 5a, b stems from
the difference in the spectra before the irradiation. That is,
the photoinduced ESR signal of holes was not altered by
the silica coating, suggesting that the local structure of
holes trapped in the silica-coated TiO2 at 77 K may be
similar to that of the un-coated TiO2. The absolute number
of holes was measured with a standard pitch sample. The
numbers of photoinduced holes for the un-coated TiO2 and
Fig. 3 XRD patterns of silica-coated titanium dioxide (SiO2/TiO2)
and uncoated TiO2
ESR data for a similar anatase sample [16]. It is probably
due to that the electrons are localized over several centers
and their signals were dispersed in a wide spectral region,
resulting in the difficulty of the observation.
The ESR peak appeared between the peaks marked
g = 2.004 and 2.014 in Fig. 5b is attributable to lattice
defects which are stable at room temperature, because this
123

1478
J. Oguma et al.
toluene by the amount of CO2 generated as the terminal
product. The relevant reactions are expressed by the fol-
lowing formulas:
(a)
CH3CHO þ 2:5O2 ! 2CO2 þ 2H2O
ð5Þ
C7H8 þ 9O2 ! 7CO2 þ 4H2O
ð6Þ
Acetaldehyde undergoes photocatalytic decomposition to
CO2 without producing intermediates; in contrast, toluene,
which is not strongly adsorbed, is known to decompose via
relatively stable intermediates such as benzyl-alcohol and
benzaldehyde [10].
Figures 6a and 7a are time profiles of evolved CO2
under the UV irradiation up to 10 h, in which the dashed
lines indicate the theoretical amounts of carbon dioxide at
full decomposition, i.e. [CO2]max. The numbers in the fig-
ures are the degrees of decomposition calculated from the
amounts of CO2 generated at 10 h of the irradiation. Fig-
ures 6b and 7b show magnified views corresponding to
irradiation up to 30 min, and Figs. 6c and 7c show the
logarithm of the relative increment in CO2 concentration
up to 10 h. As Fig. 6a shows, the un-coated TiO2 exceeded
the silica-coated TiO2 in the decomposition of acetalde-
hyde up to 3 h after the start of the irradiation, however
after 10 h the degree of decomposition became 61.6 % for
silica-coated and 40.2 % for un-coated TiO2. In the
decomposition of toluene, Fig. 7a indicates that silica-
coated TiO2 exceeds un-coated TiO2; the degree of
decomposition after 10 h of irradiation is 15.5 % for silica-
coated and 9.6 % for un-coated TiO2. Hence, the silica-
coated TiO2 is superior to the un-coated TiO2 at long-term
irradiation, especially for decomposition of toluene, which
(b)
Fig. 5 ESR spectra of silica-coated titanium dioxide (SiO2/TiO2)
(a) and un-coated TiO2 (b), measured at 77 K under ultraviolet
irradiation in vacuum. Dashed curves show the spectra measured
before ultraviolet irradiation
is
a
substance difficult to be photocatalytically
the silica-coated TiO2 were 5.3 9 1013 and 7.0 9 1013
spins at 77 K, respectively. Therefore, the amount of
photoinduced holes for coated TiO2 was 1.32 times larger
in spite of the amount of TiO2 was smaller by the silica
component of 12 %. Several reasons for the increase in
holes with silica coating may be invoked. The modification
with silica increases the surface hydration as suggested by
the TG observation and the hydration was reported to be
sensitive to the hole trap [17].
decomposed.
When the reaction proceeds limitedly by the light
absorption, the rate of excitation (Eq. 1) determines whole
reaction processes, and so the rate of CO2 generation would
be constant (Eq. 7) during the irradiation. Therefore, the
time dependence of CO2 concentration could be expressed
by Eq. (8).
d½CO2ꢃ
r ¼
¼ k0
ð7Þ
ð8Þ
dt
Though coating silica on TiO2 is a well-known technique
for industrial use, the present study found that the holes gen-
erated in photocatalysts do not disappear by the coating.
Therefore, the photocatalytic activity of silica-coated TiO2 is
expected to be similar to that of uncoated TiO2.
½CO2ꢃ ¼ k0t
where k0 is a zero-order reaction rate constant correlated
with the light intensity. As shown in Figs. 6b and 7b, this
equation held immediately after starting the irradiation.
When the reaction rate is limited by mass transfer, the
amount of a reactant, A, decreases in accordance with first-
order reaction kinetics (Eq. 9), because the reaction rate
decreases in proportion to the concentration [A] of the
substance being decomposed.
3.4 Decomposition of Organic Substances
Figures 6 and 7 show the relationship between the light
irradiation time for decompositions of acetaldehyde and
123

Effects of Silica-coating on TiO2 photocatalysts
1479
(a)
(a)
(b)
(b)
(c)
(c)
Fig. 7 Time dependence of the concentration of evolved CO2 in
photocatalytic decomposition of gaseous toluene with un-coated TiO2
and silica-coated (SiO2/TiO2). a Time profile during irradiation up to
10 h. The numbers in the figure represent the degree of decomposition
of acetaldehyde. b Magnified view showing irradiation up to 30 min.
c Logarithm of the increment of CO2 concentration as a function of
irradiation time
Fig. 6 Time dependence of the concentration of evolved CO2 in
photocatalytic decomposition of gaseous acetaldehyde with un-coated
TiO2 and silica-coated (SiO2/TiO2). a Time profile during irradiation
up to 10 h. The numbers in the figure represent the degree of
decomposition of acetaldehyde. b Magnified view showing irradiation
up to 30 min. c Logarithm of the increment of CO2 concentration as a
function of irradiation time
ꢀ
ꢁ
½CO2ꢃMaxꢁ½CO2ꢃ
½CO2ꢃMax
ln
¼ k1t
ð10Þ
When the decrease rate of reactant is equal to the genera-
tion rate of CO2, formula (Eq. 90) holds, which in turn
gives formula (Eq. 10), representing an expression that the
logarithm of increase in CO2 concentration is linearly
correlated with irradiation time. Here, k1 is a first-order
d½Aꢃ
r ¼ ꢁ
¼ k1½Aꢃ
ð9Þ
dt
À
Á
d½CO2ꢃ
r0 ¼
¼ k1 ½CO2ꢃMax ꢁ ½CO2ꢃ
ð90Þ
dt
123

1480
J. Oguma et al.
limited by the light absorption in a zero-order reaction
process. As shown in Fig. 6c, for un-coated TiO2 after
200 min the reaction proceeds well in accordance with a
logarithmic relationship, but its first-order reaction rate
constant is 36 9 10-5 min-1 is smaller than that of
176 9 10-5 min-1 for silica-coated TiO2. This is because,
without silica, the catalyst surface will be altered in the first
200 min. In contrast to this, for the catalyst modified with
silica, the reaction appeared to be first-order, and it
remained relatively stable from the start of the irradiation,
so that the coated-silica increased the transfer rate of
acetaldehyde to the TiO2 reactive sites and the reaction site
was not suppressed by the intermediates.
(a)
(b)
Now let us move on the toluene decomposition in Fig. 4
to discuss the effect of silica modification. Figure 7b shows
that, in the first stage of the reaction, the reaction proceeds
determined by light absorption with a zero-order reaction
rate constant of 11.4 ppm/min for silica-coated TiO2,
which is slightly smaller than that of 14.4 ppm/min for
un-coated TiO2. Both the decomposition rates quickly
decreased during the irradiation, but the decrease for
un-coated TiO2 fell faster than the silica-coated counter-
part. After 3 min from the start of the irradiation, silica-
coated TiO2 overtook the un-coated one in the formation of
CO2. The period of the reaction limited by the light
absorption was very short, indicating that the TiO2 surface
adsorbed many kinds of intermediates caused by side
reactions and gradually decreased the reaction rate. Con-
trary to the case of acetaldehyde decomposition, in which
the reaction rate was not decreased with silica-coating, the
rate of toluene decomposition fell off. This is probably
because the intermediates generated by toluene decompo-
sition imposed the load to reach the final product of CO2.
Figure 7c shows that the first-order reaction rate constant
k1 for un-coated TiO2 was 6.5 9 10-5 min-1; while that
Fig. 8 Plausible reaction scheme showing the effect of SiO2 coating
on the decomposition of organic substances. a un-coated titanium
dioxide. b silica-coated titanium dioxide
reaction rate constant, with r0 = 7r for the toluene
decomposition reaction expressed by Eq (6). Hence, the
first-order reaction rate constant, k1, can be obtained from
the logarithm of the relative increase in CO2 concentration.
As shown in Figs. 6c and 7c, after irradiation for a few
hours this equation held for both reactions with the two
differential photocatalysts.
Generally speaking in photocatalytic reactions,
a
decomposition reaction whose rate is limited by the light
absorption (Eq 7) will change to that whose rate is limited
by mass transfer (Eq 90), because the amount of the reac-
tant decreases during the reaction and the concentration
becomes low relative to the amount of the active sites
resulting in a first-order reaction. When the reactant gen-
erates several kinds of stable intermediates, the reaction
process that leads to the final product CO2 may be pro-
longed, resulting in the deviation of the time dependencies
of zero- or first-order reaction kinetics stated above.
for silica-coated TiO2 was as large as 14.5 9 10-5 min-1
.
Hence, the mass transfer of toluene to reactive sites must
be faster for silica-modified TiO2. According to a very
recent report [18], the decomposition rate is reduced due to
silica promoting separation of the generated intermediates
from the catalyst surface. Therefore it is probable that
decomposition intermediates cover the oxidation sites
(surface-trapped holes) as schematically shown in Fig. 8.
Hence, the high photocatalytic activity observed for the
silica modified TiO2 may be due to that the intermediates
were adsorbed on silica and does not block the oxidation
sites of TiO2. It has been reported already that, when the
silica coating layer was incomplete, the photocatalytic
activity of coated nanoparticles was higher than that of
TiO2 nanoparticles, while, when the surface of TiO2 was
coated completely, the activity sharply decreased [9].
As stated earlier, coating TiO2 with silica is a well-
known technique for practical applications. It is also widely
Taking these factors into account, let us discuss the
effect of silica coating on the photocatalytic reactions.
Photocatalytic decomposition of acetaldehyde was shown
in Fig. 6 as the time profile of CO2 evolution. Un-coated
TiO2 in Fig. 6b shows a zero-order reaction rate constant as
large as 14.2 ppm/min at the initial stage, but, after the first
few minutes, the rate is suddenly decreased, indicating the
suppression of the reaction sites by the reaction interme-
diates. In contrast, for silica-coated TiO2 in Fig. 6b,
although its initial reaction rate is as small as 2.0 ppm/min,
there is a linear increase in CO2 concentration for more
than 10 min, which indicates that the reaction rate was
123

Effects of Silica-coating on TiO2 photocatalysts
1481
Acknowledgments The authors wish to thank the Analysis &
Simulation Center, Asahi Kasei Corporation for their cooperation in
the use of STEM-EDX.
known that, when used in paint, silica-coated TiO2 gives a
method to improve the weather resistance, such as the
prevention of chalking [1]. It is true that silica modification
decreases initial reaction rates, as was shown in our
experiments on two different photocatalytic decomposition
processes for gaseous acetaldehyde and toluene. However,
it was also discovered that silica coating does not inhibit
catalytic activity, but rather, enhances long-term decom-
position activity. Therefore, silica coating may suppress the
photocatalytic activity only for solid materials, such as
resins, which should directly contact with the TiO2 surface.
References
1. Fujishima A, Zhang X, Tryk D (2008) Surf Sci Rep 63:515–582
2. Kaneko M, Ohkura I (eds) (2002) Photocatalysis: science and
technology. Springer, Tokyo
3. Anpo M, Kamat PV (eds) (2010) Environmentally benign phot-
ocatalysts: applications of titanium oxide-based materials.
Springer, New York
4. Evan AW, Murley RD (1962) Paint Technol 26:10–16
5. Blakey RR (1980) XV FATIPEC Cong 1:244
6. Machida M, Norimoto K, Watanabe T, Hashimoto K, Fujishima
A (1999) J Mater Sci 34:2569–2574
4 Conclusions
7. Cao CH (2012) Adv Mater Res 221:489
8. Zhang X, Li Y, Li Z, Zhang S (2011) Adv Mater Res
571:239–242
9. Li QY, Chen YF, Zeng DD, Gao WM, Wu ZJ (2005) J Nanopart
Res 7:295–299
10. Henderson MA (2011) Surf Sci Rep 66:185–297
11. Hirakawa T, Yawata K, Nosaka Y (2007) Appl Catal A
325:105–111
12. Hirakawa T, Nosaka Y (2008) J Phys Chem C 112:15818–15823
13. Nosaka Y, Takahashi S, Sakamoto H, Nosaka AY (2011) J Phys
Chem C 115:21283–21290
14. Zhang Z, Wang CC, Zakaria R, Ying JV (1998) J Phys Chem B
102:10871–10878
15. Nakaoka Y, Nosaka Y (1997) J Photochem Photobiol A Chem
110:299–305
16. Hirakawa T, Nakaoka Y, Nishino J, Nosaka Y (1999) J Phys
Chem B 103:4399–4403
17. Micic OI, Zhang Y, Cromack KR, Trifunac AD, Thurnauer MC
(1993) J Phys Chem 97:7277–7283
In order to explore the effect of silica coating onto the
photocatalytic activity of TiO2, ESR measurements under
UV irradiation and experiments on the decomposition of
organic substances were performed for silica-coated and
un-coated TiO2. The ESR measurements revealed that
silica coating did not cause holes to disappear, but rather
increased their number. From the decomposition of acet-
aldehyde and toluene, it was discovered that silica-coated
TiO2 is more active than un-coated TiO2 in terms of gas-
phase decomposition of acetaldehyde and toluene. The
superiority was explained as follows. In case of the
un-coated TiO2, the oxidation sites, or surface-trapped
holes, that develop the decomposition activity were
covered by decomposition intermediates. In the case of
silica-coated TiO2, silica layer probably prevented the
intermediates from covering the active sites at TiO2
surface.
18. Nussbaum M, Paz
14:3392–3399
Y
(2012) Phys Chem Chem Phys
123