# Photocatalytic Hydrogen or Oxygen Evolution from Water  over S- or N-DopedTiO2 under Visible Light

Authors: Kazumoto Nishijima; Takaaki Kamai; Naoya Murakami; Toshiki Tsubota; Teruhisa Ohno
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

S- or N-doping ofTiO2 powder having an anatase or rutile phase extended the
photocatalytic activity for water oxidation and reduction under UV light and visible light
irradiation. For the reduction of water, anatase-doped TiO2 showed higher level of
activity than that of doped TiO2 having a rutile phase using ethanol as an electron donor.
Furthermore, the activity level of S-doped TiO2 for hydrogen evolution was higher than
that of N-doped TiO2 photocatalysts under visible light. Photocatalytic oxidation of water
on doped TiO2 having a rutile phase proceeded with fairly high efficiency when Fe3+ ions
were used as electron acceptors compared to that on doped TiO2 having an anatase phase.
In addition, water splitting under visible light irradiation was achieved by construction of
a Z-scheme photocatalysis system employing the doped TiO2 having anatase and rutile
phases for H2 and O2 evolution and the I−/IO3− redox couple as an electron relay.


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## Body

## 1. Introduction

Photocatalytic reactions on the surfaces ofTiO2 particles have been attracting much attention
because of their possible application to the conversion of solar energy into
chemical energy (water splitting) [1, 2] and to water purification from pollutants [3–5]. TiO2 is chemically stable and nontoxic and it has a
high oxidation power under photoirradiation. However, a high oxidation power of TiO2 is only shown under UV light, which is a great
disadvantage for practical application. Therefore, the development of a TiO2 photocatalyst that shows a high level of
activity under visible light is needed.N, S, or C anion-dopedTiO2 photocatalysts having an anatase phase that
shows a relatively high level of activity under visible light irradiation have
been reported [6–9]. Sakthivel and Kisch succeeded in preparing TiO2 photocatalysts containing some carbon species
that show photocatalytic activity under visible light [10]. We have also reported the
preparation of S cation-doped TiO2 having anatase and rutile phases and its
photocatalytic activity for oxidation of organic compounds [11, 12]. The absorbance of S
cation-doped TiO2 in the visible region is larger than that of
N, C, or S anion-doped TiO2,
and the photocatalytic activity level of S cation-doped TiO2 for oxidation of organic compounds is
relatively high under visible light compared to that of N, C, or S anion-doped TiO2.
It should be noted that S atoms were incorporated into the lattice of TiO2 as cations and were thought to be replaced by
Ti ions in the case of S cation-doped TiO2.Among the chemical reactions suitable for energy
conversion is the decomposition of water, because the change in Gibbs energy is
large. Hence, there have been many attempts to achieve photodecomposition of
water using semiconductor particles such asTiO2 [13], SrTiO3 [14], and K4Nb6O17 [15].Abe et al. developed a
water-splitting system by combining two photocatalytic systems including
oxidation of water and reduction of water systems under visible light [16]. However, the efficiency of
each system (hydrogen-evolving and oxygen-evolving systems) is not sufficient
for application to a practical system. The aim of our study was to establish
effective oxygen- and hydrogen-evolving systems. In this paper, we report the
photocatalytic activities of S-doped or N-doped TiO2 for reduction of water into molecular hydrogen
and oxidation of water into molecular oxygen under visible light using ethanol
and Fe3+ ions as electron donors and electron
acceptors, respectively.


## 2. Experimental

### 2.1. Materials and Instruments

Various kinds ofTiO2 powders having anatase and rutile crystal
structures were obtained from the Catalysis Society of Japan (TIO-3), Ishihara
Sangyo (ST-01), TAYCA Corporation (MT-150A), and Toho Titaniumu Co. (NS-51).
The contents of anatase and the surface areas of these powders were as follows:
ST-01: 100%, 320.5m2/g; MT-150A: 0%, 81.5m2/g; TIO-3: 0%, 48.1m2/g; NS-51: 1.0%, 6.5m2/g. Ethanol, FeCl3,
thiourea, and urea were obtained from Wako Pure Chemical Industry. Other
chemicals were obtained from commercial sources as guaranteed reagents and were
used without further purification. The crystal structures of TiO2 powders were determined from X-ray diffraction
(XRD) patterns measured with an X-ray diffractometer (Philips, X'Pert-MRD) with
a Cu target Kα-ray (λ=1.5405Å).
The relative surface areas of the powders were determined by N2 adsorption/desorption isotherm analysis (BET
method). The measurements were performed using Micromeritics FlowSorb II 2300.
The absorption and diffuse reflection spectra were measured using a Shimadzu
UV-2500PC spectrophotometer. X-ray photoelectron spectra (XPS) of the TiO2 powders were measured using a JEOL JPS90SX
photoelectron spectrometer with an Mg Kα source (1253.6 eV). The shift of binding energy
due to relative surface charging was corrected using the C 1s level at 284.6 eV
as an internal standard. The XPS peaks were assumed to have Gaussian line
shapes and were resolved into components by a nonlinear least-squares procedure
after proper subtraction of the baseline.


### 2.2. Preparation of S-or N-DopedTiO2 Powders

S- or N-dopedTiO2 powders were synthesized by previously reported
methods [11, 12, 17–19].S- or N-dopedTiO2 powders having a rutile or anatase phase were
prepared as follows. Thiourea (for S doping) or urea (for N doping) was mixed
with various kinds of TiO2 powders having an anatase or rutile phase in
an agate mortar. The mixed powder was packed in an alumina crucible and
calcined at 400–600°C under air atmosphere for 3 hours. After
calcination, the powder was washed with distilled water. The resulting samples
were yellow in color and were found to have a homogenous anatase or rutile
phase from XRD analysis. For TiO2 powder calcined at a temperature higher than
600°C, no absorbance in the visible region was
observed. The surface areas of S-doped and N-doped TiO2 powders having an anatase phase (ST-01)
calcined at 400°C were 120m2/g and 90m2/g, respectively. The surface areas of S-doped TiO2 powders having a rutile phase (MT-150A, TIO-3,
and NS-51) calcined at 500°C were 52.5m2/g, 32.2m2/g, and 5.4m2/g, respectively. The surface areas of N-doped TiO2 powders having a rutile phase (TIO-3 and
NS-51) calcined at 600°C were 26.3m2/g and 4.9m2/g, respectively. The relative surface area
decreased with increase in the calcination temperature.


### 2.3. Preparation of Pt-LoadedTiO2 Photocatalysts for Hydrogen Evolution

Pt-loadedTiO2 powders were prepared by a photochemical
deposition method. The doped TiO2 photocatalyst was stirred in a 5% ethanol
aqueous solution containing H2PtCl6 and irradiated by a 500 W high-pressure Hg lamp
(2 mW/cm2) for 12 hours. Photoreduction of H2PtCl6 took place, and highly dispersed Pt metal
particles were deposited on the surface of the photocatalyst. After filtrating
and washing by deionized water, the powder was dried at 100°C for 2 hours under reduced pressure to remove
the ethanol adsorbed on the surface of the photocatalyst.


### 2.4. Photocatalytic Reaction

The photocatalytic reactions were carried out using a
closed gas-circulating system (Figure1). The reaction under a wide range of
incident light including UV and visible light was performed using a quartz
glass-made outer irradiation type reactor. Irradiation to the aqueous
suspension of a catalyst was carried out from outside the reactor using a 500 W
xenon lamp (USHIO Co. Ltd., SXUI-501XQ) for H2 evolution and O2 evolution. To limit the irradiation
wavelength, the light beam was passed through a UV-35, L-39, or L-42 filter
(Kenko Co.) to cut off wavelengths shorter than 350, 390, or 420 nm,
respectively. The irradiance of irradiation light was adjusted to 340 mW/cm2.
The photocatalyst powder (0.3 g) was suspended in distilled water using a
magnetic stirrer. S- or N-doped TiO2 having anatase phase (ST-01) was used for hydrogen evolution,
and S- or N-dopedTiO2 having rutile
phase (NS-51) was used for oxygen evolution. The required amount of sacrificial
reagent ethanol for hydrogen evolution and Fe3+ for oxygen evolution was added to the
suspension. The pH of the solution containing Fe3+ ions was adjusted to 2.4 using 1 M HCl aqueous
solution in order to prevent formation of Fe(OH)3.Figure 1
Schematic
diagram of the closed gas circulating system.In the case of water splitting in the two-step redox
system, a 500 W high-pressure mercury lamp (USHIO Co. Ltd., SX-U1501HQ) was used
as a light source. The Hg lamp has emission line spectrum with peaks at 314,
365, 405, and 436 nm. To limit the irradiation wavelength, the light beam was
passed through a UV-35 or L-39 filter (Kenko Co.) to cut off wavelengths
shorter than 350 or 390 nm, respectively. A mixture (0.3 g) of S-dopedTiO2 powder having anatase phase (ST-01) and
S-doped TiO2 powder rutile phase (NS-51) which weight ratio
was 1:1 was suspended in distilled water. The required
amount of solute, such as NaI, was added to the suspension. I−/IO3− ions
were used as a redox couple [16]. The pH of the solutions was adjusted to 9 using 1 M
NaOH aqueous solution in order to prevent generation of I3− ions [16].Finally, the suspension was thoroughly degassed and
then argon gas (35 Torr) was introduced into the system and the system was
exposed to irradiation. EvolvedH2 or O2 gases were analyzed by online gas
chromatography (TCD, molecular sieve 5 A).


## 2.1. Materials and Instruments

Various kinds ofTiO2 powders having anatase and rutile crystal
structures were obtained from the Catalysis Society of Japan (TIO-3), Ishihara
Sangyo (ST-01), TAYCA Corporation (MT-150A), and Toho Titaniumu Co. (NS-51).
The contents of anatase and the surface areas of these powders were as follows:
ST-01: 100%, 320.5m2/g; MT-150A: 0%, 81.5m2/g; TIO-3: 0%, 48.1m2/g; NS-51: 1.0%, 6.5m2/g. Ethanol, FeCl3,
thiourea, and urea were obtained from Wako Pure Chemical Industry. Other
chemicals were obtained from commercial sources as guaranteed reagents and were
used without further purification. The crystal structures of TiO2 powders were determined from X-ray diffraction
(XRD) patterns measured with an X-ray diffractometer (Philips, X'Pert-MRD) with
a Cu target Kα-ray (λ=1.5405Å).
The relative surface areas of the powders were determined by N2 adsorption/desorption isotherm analysis (BET
method). The measurements were performed using Micromeritics FlowSorb II 2300.
The absorption and diffuse reflection spectra were measured using a Shimadzu
UV-2500PC spectrophotometer. X-ray photoelectron spectra (XPS) of the TiO2 powders were measured using a JEOL JPS90SX
photoelectron spectrometer with an Mg Kα source (1253.6 eV). The shift of binding energy
due to relative surface charging was corrected using the C 1s level at 284.6 eV
as an internal standard. The XPS peaks were assumed to have Gaussian line
shapes and were resolved into components by a nonlinear least-squares procedure
after proper subtraction of the baseline.


## 2.2. Preparation of S-or N-DopedTiO2 Powders

S- or N-dopedTiO2 powders were synthesized by previously reported
methods [11, 12, 17–19].S- or N-dopedTiO2 powders having a rutile or anatase phase were
prepared as follows. Thiourea (for S doping) or urea (for N doping) was mixed
with various kinds of TiO2 powders having an anatase or rutile phase in
an agate mortar. The mixed powder was packed in an alumina crucible and
calcined at 400–600°C under air atmosphere for 3 hours. After
calcination, the powder was washed with distilled water. The resulting samples
were yellow in color and were found to have a homogenous anatase or rutile
phase from XRD analysis. For TiO2 powder calcined at a temperature higher than
600°C, no absorbance in the visible region was
observed. The surface areas of S-doped and N-doped TiO2 powders having an anatase phase (ST-01)
calcined at 400°C were 120m2/g and 90m2/g, respectively. The surface areas of S-doped TiO2 powders having a rutile phase (MT-150A, TIO-3,
and NS-51) calcined at 500°C were 52.5m2/g, 32.2m2/g, and 5.4m2/g, respectively. The surface areas of N-doped TiO2 powders having a rutile phase (TIO-3 and
NS-51) calcined at 600°C were 26.3m2/g and 4.9m2/g, respectively. The relative surface area
decreased with increase in the calcination temperature.


## 2.3. Preparation of Pt-LoadedTiO2 Photocatalysts for Hydrogen Evolution

Pt-loadedTiO2 powders were prepared by a photochemical
deposition method. The doped TiO2 photocatalyst was stirred in a 5% ethanol
aqueous solution containing H2PtCl6 and irradiated by a 500 W high-pressure Hg lamp
(2 mW/cm2) for 12 hours. Photoreduction of H2PtCl6 took place, and highly dispersed Pt metal
particles were deposited on the surface of the photocatalyst. After filtrating
and washing by deionized water, the powder was dried at 100°C for 2 hours under reduced pressure to remove
the ethanol adsorbed on the surface of the photocatalyst.


## 2.4. Photocatalytic Reaction

The photocatalytic reactions were carried out using a
closed gas-circulating system (Figure1). The reaction under a wide range of
incident light including UV and visible light was performed using a quartz
glass-made outer irradiation type reactor. Irradiation to the aqueous
suspension of a catalyst was carried out from outside the reactor using a 500 W
xenon lamp (USHIO Co. Ltd., SXUI-501XQ) for H2 evolution and O2 evolution. To limit the irradiation
wavelength, the light beam was passed through a UV-35, L-39, or L-42 filter
(Kenko Co.) to cut off wavelengths shorter than 350, 390, or 420 nm,
respectively. The irradiance of irradiation light was adjusted to 340 mW/cm2.
The photocatalyst powder (0.3 g) was suspended in distilled water using a
magnetic stirrer. S- or N-doped TiO2 having anatase phase (ST-01) was used for hydrogen evolution,
and S- or N-dopedTiO2 having rutile
phase (NS-51) was used for oxygen evolution. The required amount of sacrificial
reagent ethanol for hydrogen evolution and Fe3+ for oxygen evolution was added to the
suspension. The pH of the solution containing Fe3+ ions was adjusted to 2.4 using 1 M HCl aqueous
solution in order to prevent formation of Fe(OH)3.Figure 1
Schematic
diagram of the closed gas circulating system.In the case of water splitting in the two-step redox
system, a 500 W high-pressure mercury lamp (USHIO Co. Ltd., SX-U1501HQ) was used
as a light source. The Hg lamp has emission line spectrum with peaks at 314,
365, 405, and 436 nm. To limit the irradiation wavelength, the light beam was
passed through a UV-35 or L-39 filter (Kenko Co.) to cut off wavelengths
shorter than 350 or 390 nm, respectively. A mixture (0.3 g) of S-dopedTiO2 powder having anatase phase (ST-01) and
S-doped TiO2 powder rutile phase (NS-51) which weight ratio
was 1:1 was suspended in distilled water. The required
amount of solute, such as NaI, was added to the suspension. I−/IO3− ions
were used as a redox couple [16]. The pH of the solutions was adjusted to 9 using 1 M
NaOH aqueous solution in order to prevent generation of I3− ions [16].Finally, the suspension was thoroughly degassed and
then argon gas (35 Torr) was introduced into the system and the system was
exposed to irradiation. EvolvedH2 or O2 gases were analyzed by online gas
chromatography (TCD, molecular sieve 5 A).


## 3. Results and Discussions

### 3.1. Physical Properties of S-Doped and N-DopedTiO2

Diffuse reflectance spectra for the obtained S-dopedTiO2 and N-doped TiO2 are shown in Figure 2. All doped TiO2 powders showed absorption in the visible
region. Photoabsorption in the visible region of the doped TiO2 having an anatase phase was stronger than that
of the doped TiO2 having a rutile phase. The TiO2 having a rutile phase was hardly doped by S
atoms or N atoms because the rutile phase has a smaller surface area and more
stable crystal structure than those of the anatase phase. The absorption band
edge of S-doped TiO2 shifted to a longer wavelength than that of
N-doped TiO2.
The band gap energies of S-doped TiO2 and N-doped TiO2 can be estimated from the tangent lines in the
plot of the square root of the Kubelka-Munk functions against the photon energy
[20], as shown in the
insert to Figure 2. The band gap energies were estimated to be 2.28 eV
(S-doped TiO2 having an anatase phase), 2.58 eV (N-doped TiO2 having an anatase phase), 2.34 eV (S-doped TiO2 having a rutile phase), and 2.94 eV (N-doped TiO2 having a rutile phase).Diffuse reflectance spectra for the obtained S-dopedTiO2 and N-doped TiO2 having an anatase or rutile phase. (a) TiO2 having an anatase phase (ST-01), (b) TiO2 having a rutile phase (NS-51).
(a)(b)The chemical states of S atoms or N atoms incorporated
intoTiO2 were studied by measuring the XPS spectra of
the S-doped TiO2 or N-doped TiO2.
Figure 3(a) shows the S2p3/2 spectrum of S-doped TiO2.
As shown in Figure 3(a), a strong peak around 168 eV was observed. This peak
is thought to consist of several oxidation states of S atoms such as S6+ and
S4+ states [11, 12].
Figure 3(b) shows the N 1s spectrum of N-doped TiO2.
Two peaks around 396 eV and 399 eV were observed. The peak around 396 eV was
assigned to N atoms forming Ti–N bonds, and the peak around 399 eV was assigned
to N2 molecules adsorbed on the TiO2 surface [17].XPS spectra of S-dopedTiO2 and N-doped TiO2. (a) S-doped TiO2,
(b) N-doped TiO2.
(a)(b)


### 3.2. Selective Water Oxidation over DopedTiO2 Photocatalysts in an Aqueous Solution Containing Fe3+ Ions

We investigated photocatalytic reduction of water intoH2 or oxidation of water into O2 over various semiconductor photocatalysts in
an aqueous solution containing an electron donor or an electron acceptor. In
most cases, the reactions readily terminated when the concentration of the
product in the solution reached a certain level because of the backward reactions.
However, we found that efficient and selective O2 evolution proceeded over TiO2 photocatalysts having a rutile phase in the
aqueous solution (pH<3) containing Fe3+ ions as electron acceptors with the following
reactions [21]: (1)TiO2+hν⟶e−+h+,2H2O+4h+⟶O2+4H+,Fe3++e−⟶Fe2+.The optimized concentration ofFe3+ ions was 2 mmol/L. The rate of oxygen evolution
was lowered by raising the concentrations of Fe3+ ions owing to a filter effect of the solution
at a high concentration [21]. Excess Fe3+ ions decreased rate of evolved oxygen because Fe3+ aqueous solution has absorption in visible
light region. Figure 4 shows the amounts of evolved O2 over S- or N-doped TiO2 having rutile phase (NS-51) and N-doped TiO2 having anatase phase (ST-01) from water
containing Fe3+ ions as electron acceptors. The photocatalytic
activity level of S-doped TiO2 for water oxidation was higher than that of
N-doped TiO2.
However, the amounts of evolved O2 on S- or N-doped TiO2 did not agree with concentration of Fe3+ ions. The backward reaction, oxidation of
Fe2+ ions with holes, was thought to proceed. S-doped TiO2 showed photoinduced oxidation of water under
visible light irradiation at wavelengths longer than 390 nm. Photocatalytic
oxidation of water on doped TiO2 having a rutile phase proceeded with a fairly
high efficiency compared to that on doped TiO2 having an anatase phase as shown in
Figure 4. This result suggested that the activity for the evolution of O2 from H2O is dependent on the particle size of
doped TiO2 powders. On anatase particles with a large
surface area, decomposition of pollutants in air and water proceeds efficiently
[4, 5]. In these reactions, the
large surface area is especially important, because the concentrations of the
pollutants are usually very low. On the other hand, for splitting water, which
is an important reaction to convert light energy into chemical energy, rutile
particles with a small surface area are efficient [21–23]. In this case, a band
bending should be developed in each particle to oxidize water, and hence large
particles are advantageous.Figure 4
Amount of
evolvedO2 from water containing Fe3+ ions as electron acceptors on S-doped TiO2 having rutile phase (NS-51), N-doped TiO2 having rutile phase (NS-51), and N-doped TiO2 having anatase phase (ST-01).


### 3.3. Selective Water Reduction over DopedTiO2 Photocatalysts Deposited with Pt in an Aqueous Solution Containing Ethanol

H2 evolution from water containing ethanol as a
sacrificial reagent proceeded over platinized TiO2 having an anatase phase under UV and visible
light irradiation. To improve the rate of H2 evolution on doped TiO2,
Pt was deposited on the TiO2 surface. The optimized amount of Pt deposited
on the surface of TiO2 was 0.5 wt%. Deposition of an excess amount of
Pt (1.0 wt%) decreased photocatalytic activity because aggregated Pt particles
on the TiO2 surface prevented the TiO2 photocatalyst from absorbing incident light or
because TiO2 surface active sites were covered with Pt
particles. The reduction of water containing ethanol is as
follows: (2)C2H5OH+3H2O+6h+⟶2CO2+6H+,2H++2e−⟶H2.Figure5 shows the amounts of evolved H2 from water containing ethanol over S- or
N-doped TiO2 having anatase phase (ST-01) and S-doped TiO2 having rutile phase (NS-51) under UV light
irradiation. The amount of evolved H2 on doped TiO2 was smaller than that on pure TiO2 because impurity level was formed by doping
treatment. However, under visible light irradiation at wavelengths longer than
420 nm, the amount of evolved H2 on doped TiO2 was higher than that on pure TiO2 as shown in Figure 5(b). S-doped TiO2 showed a higher level of activity than that of
N-doped TiO2 for water reduction because S-doped TiO2 showed higher photoabsorption than that of
N-doped TiO2 in the visible region as shown in
Figure 2(a). Photocatalytic reduction of water on doped TiO2 having an anatase phase proceeded with a fairly
high efficiency compared to that on doped TiO2 having a rutile phase as shown in Figure 5.
This result suggested that the activity for the evolution of H2 from H2O is dependent on the surface area of doped TiO2 powder. Potential of the conduction band of
anatase phase is at a more negative position than that of rutile phase.
Therefore, the excited electrons easily transfer H+ ions on the surface of TiO2 having an anatase phase.Amount of evolvedH2 from water containing ethanol as an electron
donor on S- or N-doped TiO2 (anatase phase: ST-01, rutile phase: NS-51)
(a) under UV light irradiation at wavelengths longer than 350 nm or (b) under
visible light irradiation at wavelengths longer than 420 nm.
(a)(b)


### 3.4. Water Splitting overTiO2 Photocatalysts in an I−/IO3− Shuttle Redox-Mediated System

Photocatalytic water splitting intoH2 and O2 was observed in a two-step redox system using
an iodate/iodide redox mediator. In this system, we used Pt-loaded S-doped TiO2 having an anatase phase (ST-01) for H2 evolution and S-doped TiO2 having a rutile phase (NS-51) for O2 evolution under incident light including UV
and visible light. Figure 6 shows the mechanism of the two-step redox
system. On TiO2 having an anatase phase, water is reduced to H2 by photoexcited electrons and the iodide ions
are oxidized to iodate ions by holes. The iodate ions are reduced to generate
iodide ions by photoexcited electrons at the same time as water is oxidized to O2 by holes on TiO2 having a rutile phase. The optimized
concentration of I− ions was 20 mmol/L. The oxidation products of I− ions were
IO3− and I3−. An excess concentration of I− ions resulted in I3−
generation. The accumulation of I3− caused a light loss due to the strong
absorption of I3− around 350 nm, resulting in a lower efficiency of the
photocatalytic reaction [16]. Reactions over a rutile phase are as
follows: (3)4OH−+4h+⟶O2+2H2O,IO3−+3H2O+6e−⟶I−+6OH−. On the other hand,
reactions over an anatase phase are as follows: (4)2H2O+2e−⟶H2+2OH−,I−+6OH−+6h+⟶IO3−+3H2O.Figure 6
Mechanism for
water splitting onTiO2 with an I−/IO3− redox-mediator system.Amounts of evolvedH2 and O2 in the two-step redox system under incident
light including UV and visible light are shown in Figure 7. Under UV light
irradiation without a cutoff filter and UV light irradiation at wavelengths longer
350 nm, both H2 and O2 evolved. However, the ratio of amount of
evolved H2 to amount of evolved O2 was not 2:1. O2 molecules generated from water oxidation were
thought to adsorb on the TiO2 surface [24]. Under visible light irradiation at wavelengths
longer than 390 nm, no O2 evolution was observed because the number of
holes is not sufficient for O2 generation under low intensity of incident
light. Under these circumstances, the intermediates generated by water
oxidation were thought to be reduced by photoexcited electrons.Figure 7
Amounts of
evolvedH2 and O2 from water containing I−/IO3− redox on
S-doped TiO2 powders having an anatase phase (ST-01) and a
rutile phase (NS-51) under UV or visible light irradiation. (a) under UV light
irradiation without a cutoff filter, (b) under UV light irradiation at
wavelengths longer than 350 nm, (c) under visible light irradiation at
wavelengths longer than 390 nm.


## 3.1. Physical Properties of S-Doped and N-DopedTiO2

Diffuse reflectance spectra for the obtained S-dopedTiO2 and N-doped TiO2 are shown in Figure 2. All doped TiO2 powders showed absorption in the visible
region. Photoabsorption in the visible region of the doped TiO2 having an anatase phase was stronger than that
of the doped TiO2 having a rutile phase. The TiO2 having a rutile phase was hardly doped by S
atoms or N atoms because the rutile phase has a smaller surface area and more
stable crystal structure than those of the anatase phase. The absorption band
edge of S-doped TiO2 shifted to a longer wavelength than that of
N-doped TiO2.
The band gap energies of S-doped TiO2 and N-doped TiO2 can be estimated from the tangent lines in the
plot of the square root of the Kubelka-Munk functions against the photon energy
[20], as shown in the
insert to Figure 2. The band gap energies were estimated to be 2.28 eV
(S-doped TiO2 having an anatase phase), 2.58 eV (N-doped TiO2 having an anatase phase), 2.34 eV (S-doped TiO2 having a rutile phase), and 2.94 eV (N-doped TiO2 having a rutile phase).Diffuse reflectance spectra for the obtained S-dopedTiO2 and N-doped TiO2 having an anatase or rutile phase. (a) TiO2 having an anatase phase (ST-01), (b) TiO2 having a rutile phase (NS-51).
(a)(b)The chemical states of S atoms or N atoms incorporated
intoTiO2 were studied by measuring the XPS spectra of
the S-doped TiO2 or N-doped TiO2.
Figure 3(a) shows the S2p3/2 spectrum of S-doped TiO2.
As shown in Figure 3(a), a strong peak around 168 eV was observed. This peak
is thought to consist of several oxidation states of S atoms such as S6+ and
S4+ states [11, 12].
Figure 3(b) shows the N 1s spectrum of N-doped TiO2.
Two peaks around 396 eV and 399 eV were observed. The peak around 396 eV was
assigned to N atoms forming Ti–N bonds, and the peak around 399 eV was assigned
to N2 molecules adsorbed on the TiO2 surface [17].XPS spectra of S-dopedTiO2 and N-doped TiO2. (a) S-doped TiO2,
(b) N-doped TiO2.
(a)(b)


## 3.2. Selective Water Oxidation over DopedTiO2 Photocatalysts in an Aqueous Solution Containing Fe3+ Ions

We investigated photocatalytic reduction of water intoH2 or oxidation of water into O2 over various semiconductor photocatalysts in
an aqueous solution containing an electron donor or an electron acceptor. In
most cases, the reactions readily terminated when the concentration of the
product in the solution reached a certain level because of the backward reactions.
However, we found that efficient and selective O2 evolution proceeded over TiO2 photocatalysts having a rutile phase in the
aqueous solution (pH<3) containing Fe3+ ions as electron acceptors with the following
reactions [21]: (1)TiO2+hν⟶e−+h+,2H2O+4h+⟶O2+4H+,Fe3++e−⟶Fe2+.The optimized concentration ofFe3+ ions was 2 mmol/L. The rate of oxygen evolution
was lowered by raising the concentrations of Fe3+ ions owing to a filter effect of the solution
at a high concentration [21]. Excess Fe3+ ions decreased rate of evolved oxygen because Fe3+ aqueous solution has absorption in visible
light region. Figure 4 shows the amounts of evolved O2 over S- or N-doped TiO2 having rutile phase (NS-51) and N-doped TiO2 having anatase phase (ST-01) from water
containing Fe3+ ions as electron acceptors. The photocatalytic
activity level of S-doped TiO2 for water oxidation was higher than that of
N-doped TiO2.
However, the amounts of evolved O2 on S- or N-doped TiO2 did not agree with concentration of Fe3+ ions. The backward reaction, oxidation of
Fe2+ ions with holes, was thought to proceed. S-doped TiO2 showed photoinduced oxidation of water under
visible light irradiation at wavelengths longer than 390 nm. Photocatalytic
oxidation of water on doped TiO2 having a rutile phase proceeded with a fairly
high efficiency compared to that on doped TiO2 having an anatase phase as shown in
Figure 4. This result suggested that the activity for the evolution of O2 from H2O is dependent on the particle size of
doped TiO2 powders. On anatase particles with a large
surface area, decomposition of pollutants in air and water proceeds efficiently
[4, 5]. In these reactions, the
large surface area is especially important, because the concentrations of the
pollutants are usually very low. On the other hand, for splitting water, which
is an important reaction to convert light energy into chemical energy, rutile
particles with a small surface area are efficient [21–23]. In this case, a band
bending should be developed in each particle to oxidize water, and hence large
particles are advantageous.Figure 4
Amount of
evolvedO2 from water containing Fe3+ ions as electron acceptors on S-doped TiO2 having rutile phase (NS-51), N-doped TiO2 having rutile phase (NS-51), and N-doped TiO2 having anatase phase (ST-01).


## 3.3. Selective Water Reduction over DopedTiO2 Photocatalysts Deposited with Pt in an Aqueous Solution Containing Ethanol

H2 evolution from water containing ethanol as a
sacrificial reagent proceeded over platinized TiO2 having an anatase phase under UV and visible
light irradiation. To improve the rate of H2 evolution on doped TiO2,
Pt was deposited on the TiO2 surface. The optimized amount of Pt deposited
on the surface of TiO2 was 0.5 wt%. Deposition of an excess amount of
Pt (1.0 wt%) decreased photocatalytic activity because aggregated Pt particles
on the TiO2 surface prevented the TiO2 photocatalyst from absorbing incident light or
because TiO2 surface active sites were covered with Pt
particles. The reduction of water containing ethanol is as
follows: (2)C2H5OH+3H2O+6h+⟶2CO2+6H+,2H++2e−⟶H2.Figure5 shows the amounts of evolved H2 from water containing ethanol over S- or
N-doped TiO2 having anatase phase (ST-01) and S-doped TiO2 having rutile phase (NS-51) under UV light
irradiation. The amount of evolved H2 on doped TiO2 was smaller than that on pure TiO2 because impurity level was formed by doping
treatment. However, under visible light irradiation at wavelengths longer than
420 nm, the amount of evolved H2 on doped TiO2 was higher than that on pure TiO2 as shown in Figure 5(b). S-doped TiO2 showed a higher level of activity than that of
N-doped TiO2 for water reduction because S-doped TiO2 showed higher photoabsorption than that of
N-doped TiO2 in the visible region as shown in
Figure 2(a). Photocatalytic reduction of water on doped TiO2 having an anatase phase proceeded with a fairly
high efficiency compared to that on doped TiO2 having a rutile phase as shown in Figure 5.
This result suggested that the activity for the evolution of H2 from H2O is dependent on the surface area of doped TiO2 powder. Potential of the conduction band of
anatase phase is at a more negative position than that of rutile phase.
Therefore, the excited electrons easily transfer H+ ions on the surface of TiO2 having an anatase phase.Amount of evolvedH2 from water containing ethanol as an electron
donor on S- or N-doped TiO2 (anatase phase: ST-01, rutile phase: NS-51)
(a) under UV light irradiation at wavelengths longer than 350 nm or (b) under
visible light irradiation at wavelengths longer than 420 nm.
(a)(b)


## 3.4. Water Splitting overTiO2 Photocatalysts in an I−/IO3− Shuttle Redox-Mediated System

Photocatalytic water splitting intoH2 and O2 was observed in a two-step redox system using
an iodate/iodide redox mediator. In this system, we used Pt-loaded S-doped TiO2 having an anatase phase (ST-01) for H2 evolution and S-doped TiO2 having a rutile phase (NS-51) for O2 evolution under incident light including UV
and visible light. Figure 6 shows the mechanism of the two-step redox
system. On TiO2 having an anatase phase, water is reduced to H2 by photoexcited electrons and the iodide ions
are oxidized to iodate ions by holes. The iodate ions are reduced to generate
iodide ions by photoexcited electrons at the same time as water is oxidized to O2 by holes on TiO2 having a rutile phase. The optimized
concentration of I− ions was 20 mmol/L. The oxidation products of I− ions were
IO3− and I3−. An excess concentration of I− ions resulted in I3−
generation. The accumulation of I3− caused a light loss due to the strong
absorption of I3− around 350 nm, resulting in a lower efficiency of the
photocatalytic reaction [16]. Reactions over a rutile phase are as
follows: (3)4OH−+4h+⟶O2+2H2O,IO3−+3H2O+6e−⟶I−+6OH−. On the other hand,
reactions over an anatase phase are as follows: (4)2H2O+2e−⟶H2+2OH−,I−+6OH−+6h+⟶IO3−+3H2O.Figure 6
Mechanism for
water splitting onTiO2 with an I−/IO3− redox-mediator system.Amounts of evolvedH2 and O2 in the two-step redox system under incident
light including UV and visible light are shown in Figure 7. Under UV light
irradiation without a cutoff filter and UV light irradiation at wavelengths longer
350 nm, both H2 and O2 evolved. However, the ratio of amount of
evolved H2 to amount of evolved O2 was not 2:1. O2 molecules generated from water oxidation were
thought to adsorb on the TiO2 surface [24]. Under visible light irradiation at wavelengths
longer than 390 nm, no O2 evolution was observed because the number of
holes is not sufficient for O2 generation under low intensity of incident
light. Under these circumstances, the intermediates generated by water
oxidation were thought to be reduced by photoexcited electrons.Figure 7
Amounts of
evolvedH2 and O2 from water containing I−/IO3− redox on
S-doped TiO2 powders having an anatase phase (ST-01) and a
rutile phase (NS-51) under UV or visible light irradiation. (a) under UV light
irradiation without a cutoff filter, (b) under UV light irradiation at
wavelengths longer than 350 nm, (c) under visible light irradiation at
wavelengths longer than 390 nm.


## 4. Conclusion

H2 evolution from water containing ethanol as an
electron donor was performed on N- or S-doped TiO2 under visible light irradiation. An anatase TiO2 powder showed a higher level of activity than
that of a rutile TiO2 powder. Oxidation of water containing Fe3+ ions as electron acceptors proceeded on doped TiO2 under UV and visible light irradiation. Water
splitting was performed in an I−/IO3− redox system on TiO2 having an anatase phase for H2 evolution and TiO2 having a rutile phase for O2 evolution. Under UV irradiation, both H2 and O2 were evolved, though the ratio of amount of
evolved H2 to amount of evolved O2 was not 2:1.



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*Source: 173943-2007-11-28.xml*