6706

J. Phys. Chem. C 2009, 113, 6706–6718

Site Structure and Photocatalytic Role of Sulfur or Nitrogen-Doped Titanium Oxide with
Uniform Mesopores under Visible Light
Yasuo Izumi,*,† Takaomi Itoi,‡ Shuge Peng,† Kazuki Oka,† and Yoshiyuki Shibata†
Department of Chemistry, Graduate School of Science, Chiba UniVersity, and Department of MechanicalEngineering, Graduate School of Engineering, Chiba UniVersity, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
ReceiVed: December 9, 2008; ReVised Manuscript ReceiVed: February 20, 2009

Anion doping to titanium oxide (TiO2) has been extensively studied to explore visible light-responsive
photocatalysts. Among them, sulfur doping was most effective; however, the site structure and the implication
to promoted photocatalysis have not been reported yet, except for valence state information based on X-ray
photoelectron spectroscopy. Sulfur and/or nitrogen was doped to TiO2 with a narrow pore size distribution,
at 3 nm, using thiourea or urea. They were compared to the uniform mesoporous TiO2 modified via thechemical vapor deposition (CVD) of hydrogen sulfide and the titanate/TiO2 nanotube synthesized hydrothermally using TiS2. In ethanol and O2 under visible light (wavelengths of >370, >420, >480, >520, or >580nm), water formation was promoted by a factor of 12, in clear contrast to the only 2.3× enhancement for
acetaldehyde formation by doping sulfur. These anion-doped mesoporous TiO2 catalysts were characterizedusing Ti and S K-edge extended X-ray absorption fine structure for the first time. The Ti-S bonds weredetected at 2.283-2.44 Å for sulfur-doped mesoporous TiO2 using thiourea and sulfur-doped mesoporous
TiO2 via CVD, demonstrating the presence of substitutional anionic S on the O sites. The doped anionicsulfur created impurity level(s) at 0.8-2.2 eV below the conduction band (CB) minimum of mesoporous
TiO2. It enabled the electron excitation to the CB and then to O2 to convert to water under visible light. In
contrast, the impurity level(s) at 0.8-2.2 eV below the CB minimum was/were not effective to (directly or
indirectly) receive electrons from the substrate (ethanol/ethoxyl). Preferential water formation under visible
light was also observed for TiO2 nanotubes that were synthesized hydrothermally from TiS2.
Introduction
The band gap value (Eg) is critical in the performance of solar
cells and photocatalysts. Titanium oxide (TiO2) has been
extensively studied for photocatalysis under UV and/or visible
light.1,2 Because the Eg values for anatase and rutile-type TiO2
are 3.2 and 3.0 eV, respectively, corresponding to the energies
of UV light, modification of the TiO2 is necessary to be
responsive under visible light. Transition-metal cations were
doped on/in TiO2 to reduce the Eg value, but the doped cationsites often enhance the charge carrier recombination seriously
and affect the interfacial charge transfer to the substrate,depending on the doped amount on/in TiO2.2
The density of states (DOS) was evaluated for doped TiO2
by density functional theory (DFT) calculations using the fullpotential linearized-augmented-plane-wave method. Nonmetalanion doping to TiO2 was predicted to reduce the Eg value, forma DOS derived from the anion near valence band (VB)
maximum or deeper level, and maintain the conduction band(CB) minimum enough high to donate electrons to the oxidants
in photocatalysis.3 Since then, visible-light-excited photocatalysis
has been intensively studied over TiO2 doped with B,4 C,5-10
N,3,11-21 S,22-28 F,29-31 Cl and Br,32 and I33-35 for the decomposition of methylene blue, water, 2-propanol, acetaldehyde,
phenol derivatives, nitric oxide, and other volatile organic
chemicals (VOCs) and dyes (see Table 1).
* Author to whom correspondence should be addressed. E-mail:yizumi@faculty.chiba-u.jp.
†
Department of Chemistry, Graduate School of Science.
‡
Department of Mechanical Engineering, Graduate School of Engineering.

Site structure and the photocatalytic role of nitrogen-doped
TiO2 were reported using electron paramagnetic resonance(EPR), and DFT calculations were reported using the planewave-pseudopotential method.16 The impurity level due tosubstitutional N was calculated at a level 0.14-0.59 eV above
the valence band (VB) maximum. The electron of substitutionalN• or N- level is excited to CB and then irreversibly reduced
O2 to O2•- (adsorbed), based on EPR measurements undervisible light. The effects of reduced Ti3+ level to donate anelectron to the substitutional N level16 and of oxygen vacancies(F center)16,36 were also discussed.
The band gap reduction and photocatalytic promotion undervisible light were most pronounced for sulfur-doped TiO2 (seeTable 1).22-28 Both substitutional cationic S4+/S6+ on the Ti sites
starting from thiourea or sulfuric acid22,24-27 and substitutionalanionic S on the O sites starting from TiS2 CS2, or H2S,23,28,37
have been reported; however, the site structure and the implication to promoted photocatalysis have not been reported yet.
In this paper, sulfur and/or nitrogen was doped during
template mesoporous synthesis. The co-doping syntheses successfully formed a TiO2 matrix with mesopores of narrow poresize distribution at 3 nm, similar to reported mesoporous TiO2
that has been doped with Al,38 V,38,39 Fe,38 Zr,38 Nb,38 Mo,38,40
and Ce.38,41 Thus-obtained sulfur-doped mesoporous TiO2 wascompared to mesoporous TiO2 modified via the chemical vapor
deposition (CVD) method and titanate/TiO2 nanotubes synthesized hydrothermally, starting from TiS2. The intentions are tocompare the photocatalytic effects of homogeneously distributed
sulfur in a matrix via co-doping versus sulfur near the TiO2
surface via CVD method and also the effects of randomly

10.1021/jp810817y CCC: $40.75  2009 American Chemical Society
Published on Web 03/26/2009

sol-gelpyrolysis
sol-gelhydrothermal
sol-gelhydrothermal (template)sol-gelTi(O-iPr)4
TiCl4(NH4)2TiF6
TiCl4
Ti(O-iPr)4Ti(O-iPr)4Ti(O-Bu)4NH4+FH2TiF6(NH4+)2TiF62HBr
I crystal
HIO3
HIO3

F
F,
N
F,
N
Cl, Br(I2)nI
Iimpregnatedcalcined
sol-gelsol-gelsol-gelsol-gelsupercriticalTi(O-iPr)4
TiS2
Ti(O-iPr)4Ti(O-iPr)4Ti(O-iPr)4Ti(O-iPr)4/TiS2
Ti(O-Bu)4

thiourea
TiS2
thioureathioureathiourea
H2SO4/TiS2
CS2

S
S
S
S
S
S
S

sol-gelhydrothermal
solvothermal
sputtering
milling
calcined
sol-gelhydrothermal
CVD
sol-gelimpregnatedcalcined

NEt3
NEt3, etc.
N4(CH2)6
N2
N4(CH2)6, urea
TiN
NH4+ClNH3
NH3
NH4+OH
urea
Ti[OC(NH2)2]6Cl3

N
N
N
N
N
N
N
N
N
N
N
N, C

Ti(O-iPr)4Ti(O-Bu)4
TiCl3
anatase/rutile
P25
TiN
Ti(O-iPr)4
titanate NT
anataseTiCl4titanate
Ti[OC(NH2)2]6Cl3

CO2
HO(CH2)2OH
NBu4+OHTiC
TiC
TiC

C
C
C
C
C
C

pyrolysis
anodization (NT)sol-gelcalcined
calcined
calcined

BH3

Ti metal
Ti metal
TiCl4
TiC
TiC
TiC

synthesis route
sol-gelprecursor

TiO2

TiCl4

precursor

Belement

heteroanion

2.34

2.90

2.6

390-700
400-520390-550
380-550
370-500
425-770
400-600
380-650

2.4-2.3

2.8
2.46
1.87

3.17

2.94

2.95-2.70
2.48-2.29

2.32
2.2
3.17-3.02
2.9

Eg (eV)

410-600
380-580
400-600
400-600
380-600

380-690380-600
390-600
390-600
400-600
420-600
400-580
400-500
380-500
390-700
380-500
380-550

400-700
380-600
390-600
390-600

410-560

400-800

visible light absorption(shoulder, tail) (nm)acetoneacetaldehyde
water
water
Methylene Blue
Methylene Bluephenolwater (solar cell)
Methylene Blue
Methylene Blue
2-propanol
Methylene Blue
Rhodamine 6G
Methylene Blue

Methylene Blue
Rhodamine B, 2,4-dichlorophenol
nitric oxide
Methylene Bluenitric oxideacetaldehyde
Methylene Blue
2-propanol
formic acid
Methyl Orange
4-chlorophenol
Methyl Orange

water (electrochemical)
water (electrochemical)
4-chlorophenol
CCl3CO2H
2-propanol
methylene blue

MTBE

reactant of photocatalysis

TABLE 1: Precursors, Wave Number of Visible Light Absorption, Band Gap, and Catalysis under Visible Light over Anion-Doped TiO2

3× that of P25
10× that of P25
7× of P25
6× of P25

1.3× that of P25
7× that of P25

1.0-10× that of TiO2
0.9-10× that of TiO2
10× that of TiO2
10× that of TiO2
7.9× that of TiO2

8× that of TiO2
1.7× that of P25

4× that of anatase1.1-2.8× that of TiO2
5× that of P25
8× that of anatase/rutile
1-4× that of P25
20× that of P25
1.3× that of TiO2

76× that of TiO2
14× that of TiO2
0.1× that of N-TiO2

7.7× that of TiO2

6× that of anatasepromoted ratio

29
30
31
32
33
34
35

22
23
24
25
26
27
28

11
12
13
3
14
15
16
17
18
20
21
19

5
6
7
8
9
9

4

reference

Doped TiO2 for O2 Reduction under Visible Light
J. Phys. Chem. C, Vol. 113, No. 16, 2009 6707

6708 J. Phys. Chem. C, Vol. 113, No. 16, 2009
distributed sulfur via co-doping versus sulfur distributed along
the nanotubes.
The energy level(s) created between the VB maximum andthe CB minimum due to doping must be critical to control thephotocatalysis of anion-doped TiO2. The chemical states of
dopants were reported based on B 1s,4 C 1s,6,7,9,10 N 1s,3,11-13,18,20,21
S 2p3/2,24,25,27,28,37 and I 3d5/234,35 X-ray photoelectron spectroscopy (XPS). To judge the dopant sites (substitutional orinterstitial), valence (cationic, neutral, or anionic), and geometric
distribution (at surface or in bulk) on/in the TiO2 matrix,
synthesized sulfur- or nitrogen-doped TiO2 were investigatedusing Ti K-edge extended X-ray absorption fine structure(EXAFS) for the first time to nonmetal-doped TiO2. S K-edge
XAFS spectra were also applied to evaluate the amount of doped
sulfur and the valence state.
The photo-oxidation reactions of aliphatic alcohols werereported over carbon-doped,9 nitrogen-doped,17 sulfur-doped,25
or vanadium-doped TiO242 from 2-propanol to acetone and thento CO/CO29,17,25 or from ethanol to acetaldehyde, then to formic
acid, and finally to CO/CO2.42 The effects of dopants to facilitate
band-gap exitation were discussed under visible light. The
reaction mechanism of photo-oxidation of methanol, ethanol,and 1-propanol was already discussed in detail, including theformation of •OH radicals and hole oxidation for adsorbed
intermediates but over undoped TiO2 under UV light.43,44
Therefore, in this work, the difference of ethanol oxidation
reactivity under visible light among synthesized sulfur- or
nitrogen-doped mesoporous TiO2 (or TiO2 nanotubes) was
discussed based on analyzed site structures of doped sulfur or
nirogen and their associated band structures. The role of sulfur
or nitrogen in the reduction of O2 and the oxidation of alcohol
is presented.
Experimental Section
Reagents and Catalyst Syntheses. Rutile-type TiO2 (<5 µm
in size, >99.9% purity, Wako Pure Chemicals), anatase-type
TiO2 (<5 µm in size, >99.9% purity, Wako Pure Chemicals),and FeTiO3 (>99.8% purity, Alfa Aesar) were used as received.
Water (40 mL) was added dropwise to a mixture of titanium
tetraisopropoxide (28.0 mmol; >95% purity, Wako Pure Chemicals), thiourea (14.0 mmol; >98% purity, Wako Pure Chemicals),and dodecylamine (14.0 mmol; >95% purity, Wako Pure
Chemicals) at 273 K. A measured quantity (1.6 mL) of hydrogenchloride (0.1 M; Precious Analytical grade, Wako Pure Chemicals) was added, and the mixture was left still overnight. The
mixture then was transferred to a Teflon container and maintained at a temperature of 333 K for 4 days. The solid wasfiltered and washed with methanol (>99.8%, Wako Pure
Chemicals) and diethyl ether (>99.5%, Wako Pure Chemicals).
The obtained powder was dried at 373 K for 1 day and thenevacuated at 453 K in a Pyrex glass tube that was connected toa vacuum system (10-6 Pa) for 2 h. The tube was sealed by fireand kept at 453 K for 10 days. The resulting solid was washedwith carbon disulfide (>99%, Wako Pure Chemicals), an ethanol(>99.5%, Wako Pure Chemicals) solution of p-toluenesulfonicacid (>99%, Wako Pure Chemicals), and finally ethanol (seeTable 2A). The washing was repeated until no dodecylamine
or its decomposed compounds was detected for the filtrate in
the UV-visible spectra. The powder was evacuated (10-6 Pa)
at 373 K for 2 h. After the heating, no organic compounds (<10nmol) were detected in gas chromatography (GC) measurements.
The thus-synthesized sample is denoted as SN-doped mesoporous TiO2.

Izumi et al.
Mesoporous TiO2 was synthesized based on the literature45(see Table 2A). The synthesis route of nitrogen-doped mesoporous TiO2 was similar to that for mesoporous TiO2. Urea (14.0mmol, >99% purity, Wako Pure Chemicals) was used as anitrogen dopant. Note that the carbon from the urea or thiourea
may remain in the catalysts, but identification of the source wasdifficult: it was either from urea/thiourea or dodecylamine/ethanol. Hence, these samples are simply denoted as SN-doped/
N-doped mesoporous TiO2. The filtered powder after heating
at 333 K for 4 days was washed only with diethyl ether (seeTable 2A). The washing was repeated until no dodecylamine,
nor its decomposed compounds, was detected for the filtrate in
the UV-visible spectra.
After intact mesoporous TiO2 was evacuated (10-6 Pa) at 290
K for 3 h, 101 kPa of H2S was introduced and the temperature
was elevated from 290 K to 583 K or 623 K, with a ramping
rate of 6 K/min. Thus-synthesized samples are denoted as
S-doped mesoporous TiO2-583 or TiO2-623 (see Table 2B).
Titanate nanotubes were synthesized based on the literature
method,46-49 as a reference of two-dimensional reaction spaceof a few nanometers. Two grams of TiO2 (Degussa P25, anatase/rutile ) 7/3) and 10 mL of sodium hydroxide (>97%, Wako)
solution (10 M) were mixed and heated in an autoclave (TVS1-30, Taiatsu Techno) at a temperature of 423 K for 1 day. The
precipitate was filtered and washed with HCl solution (0.1 M)
until the filtrate became neutral. The powder was finally washedwith distilled water and dried at 353 K for 2 days. The obtained
titanate nanotubes were calcined at 673 K for 2 h. The thussynthesized sample is denoted as “TiO2 nanotube”.47 A nanotube
sample that contained tetravalent titanium (TiIV) was alsosynthesized using 0.3 g of TiS2 powder (>99% purity, Alfa
Aesar) in a route similar to that for the titanate nanotube. The
thus-synthesized titanate nanotube-S was calcined at 673 K for
2 h (referenced hereafter as TiO2 nanotube-S).
Characterization. Optical spectra were recorded on a UVvisible light spectrometer (Jasco, Model V-650), using an integrating sphere (Jasco, Model ISV-469) for diffuse-reflectance measurements and D2 and halogen lamps for wavelengths below andabove 340 nm, respectively. Measurements were performed at 290
K in the wavelength range of 200-900 nm. The absorbance wasobtained by the transformation based on the Kubelka-Munkequation. The band-gap values were evaluated based on the Taucequation for the allowed indirect transition, R × hν ∝ (hν - Eg)2,
in which R, h, and ν are absorption coefficient, Planck constant,
and wavenumber, respectively.7,19-22,28,50,51
Nitrogen adsorption measurements were performed at atemperature of 77 K and pressures of 1.0-90 kPa in a vacuum
system that was connected to diffusion and rotary pumps (10-6Pa) and equipped with a capacitance manometer (ULVAC,
Models CCMT-1000A and GM-2001). The samples were
evacuated at a temperature of 393 K for 2 h before measurements. X-ray diffraction (XRD) data were obtained using a
Rigaku MiniFlex II diffractometer at a Bragg angle of 2θBragg) 1.5°-50°. Conditions involved 30 kV, Cu KR emission, anda nickel filter. Transmission electron microscopy (TEM) images
were taken using field-emission-type TEM equipment (JEOL,
Model JEM-4000FX) with an accelerating voltage of 400 kV.
Samples were dispersed in ethanol and mounted on amorphous
carbon-coated copper mesh.
Ti K-edge X-ray absorption fine structure (XAFS) spectra
were measured in transmission mode in the Photon Factory (PF)at the High-Energy Accelerator Research Organization (Tsukuba) on beamlines 9A, 9C, and 12C. The storage-ring energy
was 2.5 GeV, and the ring current was between 451 mA andTiO2 (P25)

TiS2titanate/TiO2 nanotube

titanate/TiO2 nanotube-Saamorphous
amorphous

1/37

1/4

Ti/NaOH molar ratio

583
623

TiO2 phasemethanol, diethyl ether
methanol, diethyl ether
diethyl ether

amorphous
amorphous
amorphous

S content (wt %)b

7.9

amorphousd
anataseeamorphousd
anatasee

3.9

dTEM (nm)TiO2 phase(C) Hydrothermal Synthesis

0.48
0.77

at 453 K

0.83

2.3

tTEM (nm)Nanotube Dimensionsd

not detected
not detected

mesopore dXRD (nm)c

62

29

1.9
2.6

3.0(1.7)
0.8

Eg (eV)

110

334

2.7
2.9

mesoporedXRD (nm)c

SBET (m2/g)d

SBET (m2/g)

1.7

S content (wt %)blTEM (nm)p-toluenesulfonic acid/ethanol
CS2, p-toluenesulfonic acid/ethanolp-toluenesulfonic acid/ethanol(B) CVD Dopingat 333 K

TiO2 phase

3.1

3.1

Eg (eV)e

608
229

SBET
(m2/g)

2.9
2.2
2.8

Eg
(eV)

a
Introduced amounts during catalyst synthesis. b Based on the edge-jump values of S K-edge XAFS. c Average diameter, as determined via XRD. d Using an as-synthesized titanate nanotube.
Dimensions are given as dTEM (average diameter, as determined by TEM analysis), tTEM (average wall thickness, as determined by TEM analysis), and lTEM (average length, as determined by TEM
analysis). e After calcining at 673 K.

sourcesampleH2S
H2S

S-doped mesoporous TiO2-583
S-doped mesoporous TiO2-623

T (K)S/Ti ) 1/2
N/Ti ) 1/1

additive(H2N)2CdS(H2N)2CdO

mesoporous TiO2
SN-doped mesoporous TiO2
N-doped mesoporous TiO2

molar ratioa

sampleadditive

SampleWashing Medium after Heating(A) Doping during Template Synthesis

TABLE 2: Synthetic Conditions and Physicochemical Properties of Mesoporous TiO2-, S-, or N-Doped Mesoporous TiO2, and Titanate/TiO2 Nanotubes, Created via (A) Doping
during Template Synthesis, (B) CVD Doping, or (C) Hydrothermal Synthesis

Doped TiO2 for O2 Reduction under Visible Light
J. Phys. Chem. C, Vol. 113, No. 16, 2009 6709

2.9 × 104
0.0043 ((0.0045)-0.0013 ((0.022)
5.5 ((1.3)
5.3 ((1.2)
3.07 ((0.05)
3.290 ((0.014)(e) S-Doped Mesoporous TiO2-623 Sample
0.5 ((0.1)
0.00009 ((0.0008)
2.40 ((0.03)
6.2 ((0.3)
1.881 ((0.001)

0.0095 ((0.0004)

5.8 × 105
-0.0064 ((0.0012)
0.00015 ((0.0011)
2.2 ((0.4)
7.3 ((0.6)
2.905 ((0.010)
3.177 ((0.006)
2.44 ((0.03)
2.0 ((0.6)
2.2 ((0.9)
1.74 ((0.02)
2.01 ((0.03)-0.0030 ((0.0019)-0.0030 ((0.0044)(d) S-Doped Mesoporous TiO2-583 Sample
0.2 ((0.2)-0.0078 ((0.0008)

3.6 × 1040.00063 ((0.0017)-0.0039 ((0.0010)
5.2 ((1.0)
3.2 ((0.5)
2.283 ((0.003)
4.4 ((0.5)
1.879 ((0.003)

0.0052 ((0.0018)
0.0039 ((0.0025)
3.2 ((0.8)
3.0 ((0.6)
1.73 ((0.02)
2.00 ((0.02)

0.0059 ((0.0011)(b) N-Doped Mesoporous TiO2 Sample(c) SN-Doped Mesoporous TiO2 Sample
0.7 ((0.04)-0.00002 ((0.0004)

3.05 ((0.04)
3.24 ((0.03)

0.8 ((0.3)
8.8 ((0.8)
2.89 ((0.02)
3.181 ((0.008)

2.918 ((0.011)
3.191 ((0.005)(a) Mesoporous TiO2 Sample

N-0.0046 ((0.0005)
0.0092 ((0.0009)-0.0054 ((0.0018)
0.0058 ((0.0002)
1.6 ((0.3)
7.1 ((0.9)∆σ2 (Å2)N∆σ (Å )R (Å)

-0.00025 ((0.0019)-0.0017 ((0.0020)
2.3 ((0.6)
1.9 ((0.5)

Multiple shell analyses were performed for the Fourier-filteredk3-weighted EXAFS data in k- and R-space using empiricalparameters extracted from EXAFS for rutile-type TiO2 and TiS2
for Ti-O, Ti(-O-)Ti, and Ti-S bonds based on reported
crystal structures (see Table 3).57-59 The σ2 values listed in Table
4 were relative to those of the references. The many-body
reduction factor S02 was assumed to be equal for the sample
and reference. The goodness of fit was given as requested by
the Committee on Standards and Criteria in X-ray Absorption
Spectroscopy.
Ethanol Photo-oxidation Tests. The reaction was performed
in a closed circulating system (loop volume of 108 mL). One
hundred milligrams of catalyst was spread homogeneously in aquartz reaction cell60 (bottom plate area is 23.8 cm2) connectedto the circulation loop. The sample was under vacuum (10-6Pa) at 290 K and illuminated for 10 h with a xenon arc lamp
operated at 480 W (Ushio, Model UXL-500D-O) filtered using
L37 (>370 nm, Kenko). After the pretreatment, no gas desorption was detected (<10 nmol) for all the catalysts based on the
online GC equipped with thermal conductivity detector (Shimazu, Model GC-8AT). The pretreatment conditions were1.75 ((0.02)
2.00 ((0.03)

i

R (Å)

i)1

∆σ (Å )(µx - BG )2

N

N

i
i
e smoothing factor∑ exp(-0.075k
2
)

R (Å)

255 mA. A Si(111) double-crystal monochromator and cylindrical double mirror were inserted into the X-ray beam path. The
parallelness of double crystals was adjusted using a piezotranslator and set to 65% intensity to maximum flux. The slit
opening size was 1.0 mm × 1.0 mm in front of the I0 ionizationchamber. The I0 and It ionization chambers were purged withthe mixture of helium and N2 (7:3) and N2, respectively. The
powder samples were ground as fine as possible and mounted
on tape. The Ti K-edge energy for titanium metal was calibrated
to 4964.5 eV.52,53
S K-edge XAFS spectra were measured also in PF on
beamline 9A. The setup was basically similar to that for Ti
K-edge, but the Si(111) monochromator was fully tuned using
a piezo translator.54 The I0 and It ionization chambers were
purged with helium and N2, respectively. Standard samples wereprepared by thoroughly mixing TiS2 with mesoporous TiO2 to
1.6 and 4.5 wt % of sulfur. A small amount (1.4 mg) of powder
samples was ground as fine as possible and mounted onto tape.
The sulfur contents in SN-doped samples were evaluated basedon the comparison of edge-jump values to those for standard
samples previously mentioned. The maximum of the first preedge feature in the spectrum of Na2S2O3 · 5H2O (>99% purity,
Wako Pure Chemicals) was assigned to 2472.02 eV.55
The XAFS data were analyzed with XDAP (XAFS Services
International).56 The pre-edge background was approximated by
the modified Victoreen function, C2/E2 + C1/E + C0. The
background of post-edge oscillation was approximated by asmoothing-spline function, calculated using an equation for thenumber of data points (N).

Ti(-O-)Ti [or Ti(-N-)Ti]

57, 5857, 5859

2

6 1.92-15.00 1.27-1.75
12 1.92-15.00 2.34-2.87
6 2.85-14.12 1.54-2.39

reference

2

1.959
3.058
2.430

R (Å)

Ti-S

k (Å-1)

2

Data Ranges Used

2

Ti-O
rutile-type TiO2
Ti(-O-)Ti rutile-type TiO2
Ti-S
TiS2

Model
R (Å) N

Ti-O (or Ti-N)

compoundTABLE 4: Best Fit Results of Ti K-Edge EXAFS Spectra for Mesoporous TiO2 and Doped Mesoporous TiO2

shellgoodness of fit

TABLE 3: Empirical Fit Parameters for Ti K-Edge EXAFS
Analyses

2.1 × 105

Izumi et al.

2.6 × 105

6710 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Doped TiO2 for O2 Reduction under Visible Light

Figure 1. Diffuse-reflectance UV-visible absorption spectra formesoporous TiO2 (spectrum a), N-doped mesoporous TiO2 (spectrumb), SN-doped mesoporous TiO2 (spectrum c), S-doped mesoporous
TiO2-583 (spectrum d), S-doped mesoporous TiO2-623 (spectrum e),
TiO2 nanotube (spectrum f), TiO2 nanotube-S (spectrum g), and anatasetype TiO2 (spectrum h). (Inset) Tauc plots for mesoporous TiO2 (curvea), SN-doped mesoporous TiO2 (curve c), S-doped mesoporous TiO2583 (curve d), S-doped mesoporous TiO2-623 (curve e), and anatasetype TiO2 (curve h).

chosen and fixed to dehydrate catalyst surface for fair comparison of the dependence tests on a cutoff wavelength of
370-580 nm.
To the reaction system, 1.33 kPa of ethanol and 2.67 kPa ofoxygen were introduced. The light exit of the xenon arc lamp
house (Ushio, Model UI-502Q) was set 2 mm apart from thebottom of the quartz reaction cell. One of the UV-cut filters
L37, L42 (>420 nm), Y48 (>480 nm), Y52 (>520 nm), and O58(>580 nm) (Kenko) was set at the light exit. Products andreactants were analyzed using online GC using Porapak-Q and
Active Carbon columns (GL Sciences).
Results
UV-Visible Spectra. Optical spectra were measured for
doped/undoped mesoporous TiO2, TiO2 nanotubes, and anatasetype TiO2 (see Figure 1). The spectrum for anatase-type TiO2
(spectrum h) exhibited negligible absorption in the wavelength
region of >400 nm. The Eg value was evaluated to 3.2 eV (seecurve h in the inset of Figure 1), which was consistent withvalues given in the literature.1 The absorption edge for mesoporous TiO2 shifted toward greater wavelength (spectrum a)compared to anatase-type TiO2. The Eg value was evaluated to
2.9 eV (see curve a in the inset of Figure 1, and Table 2A).
The spectrum for N-doped mesoporous TiO2 (spectrum b in
Figure 1) was similar to that for mesoporous TiO2. The Eg value
was evaluated to 2.8 eV (see Table 2A). The spectrum for SNdoped mesoporous TiO2 exhibited significant absorption tailingin the wavelength region between 370 nm and 600 nm (seespectrum c in Figure 1). The Eg value decreased to 2.2 eV (seecurve c in the inset of Figure 1, and Table 2A).
The tailing in the visible-light region for S-doped mesoporous
TiO2-583 was similar/less than that for mesoporous TiO2 (seespectrum d in Figure 1, and curve a in the inset of Figure 1).
The Eg value was evaluated to 3.0 eV (see Table 2B).
Extrapolation to the x-axis was also possible to 1.7 eV, becauseof the excitation from a separate impurity S 3p level to the CB(see curve d in the inset of Figure 1).61 As the CVD temperature
with H2S was elevated from 583 K to 623 K, the tailing in thevisible-light region became significant, starting from 370 nm
to as much as 900 nm (see spectrum e in Figure 1). Its associatedEg value was 0.8 eV (see curve e in the inset of Figure 1, andTable 2B).
The absorption tailing for TiO2 nanotube and TiO2 nanotube-S was negligible and very weak, respectively, in the

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6711

Figure 2. High-resolution TEM image for titanate nanotube-S.

wavelength region of 400-600 nm (see spectra f and g in
Figure 1). The Eg values were evaluated to 3.1 eV (see Table
2C). Predominant formation of anatase-type TiO2 wasreported46,47 at ∼673 K, starting from titanate. The spectra f
and g for TiO2 nanotube and TiO2 nanotube-S consisted ofa major anatase phase and, therefore, exhibited negligible
absorption at 400-600 nm, similar to that for anatase-type
TiO2 (see spectrum h in Figure 1).
Specific Surface Area, XRD, High-Resolution TEM, andS K-Edge XAFS. The Brunauer, Emmett, and Teller (BET)
specific surface area (SA) obtained based on N2 adsorption was
greatest for mesoporous TiO2 (608 m2/g). The specific SA
decreased to 229 m2/g for the SN-doped mesoporous TiO2 (seeTable 2A). For S-doped mesoporous TiO2 via CVD, the specific
SA decreased to 1.9-2.6 m2/g (see Table 2B). The specific SA
for titanate nanotubes was 110-334 m2/g on a level similar to
SN-doped mesoporous TiO2 (see Tables 2C and 2A).
Narrow pore size distribution centered at 2.7 nm (2θBragg )3.34°) for mesoporous TiO2 was preserved for SN-doped
mesoporous TiO2 centered at 2.9 nm (2θBragg ) 3.09°; see Table
2A). The lower angle XRD peak disappeared for S-doped
mesoporous TiO2-583/623 (Table 2B). High-resolution TEM
images were taken to observe the nanotube structure. The
average tube diameter, wall thickness, and length, which were
7.9, 2.3, and 29 nm, respectively, for titanate nanotube changed
to 3.9, 0.83, and 62 nm, respectively, for the thinner and longer
titanate nanotube-S (see Figure 2). This is the first report oftitanate/TiO2 nanotube synthesis starting from TiS2.
The sulfur contents in SN-doped mesoporous TiO2 andS-doped mesoporous TiO2-583/623 samples were evaluated to
1.7, 0.48, and 0.77 wt %, respectively, based on the S K-edge
XAFS spectra. The three near-edge spectra all consisted of an
intense peak at 2482.2-2482.4 eV, accompanied by peaks at2473.9-2474.4eV(w),2489.3-2489.4eV(w),and2498.2-2499.0
eV (w,br). The peak positions were similar to those for TiS2:
2482.4 eV (s), 2472.8 eV (m), 2489.2 eV (w), and 2498.9 eV(w,br).
Ti K Pre-edge Region. The Ti K-edge spectra in the preedge region for mesoporous TiO2, doped mesoporous TiO2, andtitanate nanotubes are depicted in Figure 3A. The spectra forstandard Ti compounds were also shown (see Figure 3B).
The pre-edge region for mesoporous TiO2 consisted of an
intense peak at 4969.3 eV, accompanied by weaker peaks at4967.2 and 4972.6 eV (see spectrum a in Figure 3A). The
spectral pattern in this energy region resembled that for FeTiO3(see spectrum c in Figure 3B), except that the peak at 4972.6
eV did not appear for FeTiO3. The pre-edge (spectrum a in
Figure 3A) and post-edge peak patterns well-resembled that foramorphous TiO2 that has been reported elsewhere.39,45,62
All the pre-edge spectral patterns for N-doped and SN-doped
mesoporous TiO2 and S-doped mesoporous TiO2-583 and 623

6712 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Figure 3. (A) Ti 1s-3d pre-edge region spectra for mesoporous TiO2
(spectrum a), N-doped mesoporous TiO2 (spectrum b), SN-doped
mesoporous TiO2 (spectrum c), S-doped mesoporous TiO2-583 (spectrum d), S-doped mesoporous TiO2-623 (spectrum e), titanate nanotube(spectrum f), and titanate nanotube-S (spectrum g). (B) Reference
spectra for rutile-type TiO2 (spectrum a), anatase-type TiO2 (spectrumb), FeTiO3 (spectrum c), and TiS2 (spectrum d).

(see spectra b-e in Figure 3A) resembled that for mesoporous
TiO2 (spectrum a), suggesting a predominantly amorphousmatrix structure (see Tables 2A and 2B). The weak peak at4972.6 eV appeared for mesoporous TiO2-derivative samples(see spectra a-e in Figure 3A) shifted to 4972.3 eV for titanate
nanotube and titanate nanotube-S (spectra f, g). The lattersamples may consist of amorphous matrix structure distorted
from titanate1,48,49 (see Table 2C).
Ti K-edge EXAFS. The k3-weighted EXAFS function
measured for mesoporous TiO2 was depicted in spectrum a inpanel (1) of Figure 4. In its associated Fourier transform(spectrum a in panel (2) of Figure 4), two intense peaks appeared
at 1.2 and 1.7 Å (phase shift uncorrected), and those wereassigned to Ti-O bonds. Two relatively weaker peaks appeared
at 2.4 and 2.9 Å (phase shift uncorrected). The data in regionof 0.94-3.03 Å were fit with two Ti-O shells and two
Ti(-O-)Ti shells. The best-fit result was Ti-O bonds at 1.75
and 2.00 Å with the total coordination number (N) 4.2 andTi(-O-)Ti bonds at 2.918 and 3.191 Å with the total N ) 9.5(see Table 4a and spectrum a in panels (3) and (4) of Figure 4).
Note that site structure evaluated from EXAFS is the statistical
average for all the element sites in the sample,63,64 andunsaturated and bulk saturated Ti sites were suggested. The weakshoulder feature at 2.1 Å was not fit as either Ti-O orTi(-O-)Ti (see spectrum a in panel (4) of Figure 4).
Both the k3-weighted EXAFS function and its associatedFourier transform for S-doped mesoporous TiO2-583 wellresembled the corresponding data for mesoporous TiO2 (seespectra d and a in Figure 4). The fit result for data in the regionof 0.93-3.00 Å with four waves [two Ti-O and two Ti(-O-)Ti]
was Ti-O bonds at 1.74 and 2.01 Å with the total N ) 4.2 andTi(-O-)Ti bonds at 2.905 and 3.177 Å with the total N ) 9.5(see entry d in Table 4 and spectrum d in panels (3) and (4) inFigure 4). When a Ti-S shell was added in the fit, the fit result
for two Ti-O and two Ti(-O-)Ti shells negligibly changed
and Ti-S bonds at 2.44 Å with the N ) 0.2 were given (seespectrum d in Table 4). The goodness-of-fit value negligiblychanged at 5.8 × 105 with the addition of one Ti-S shell.
The amplitude of k3-weighted EXAFS function for N-doped
mesoporous TiO2 was suppressed to <1.5 in the k-region of >8

Izumi et al.
Å-1 (see spectrum b in panel (1) of Figure 4). Accordingly, thepeak appeared at 2.4 Å in its associated Fourier transform
became weaker (see spectrum b in panel (2) of Figure 4) thanthe peaks appeared at similar bond distances for other samples.
The data in the region of 0.90-3.00 Å were first fit with threeshells [two Ti-O (or Ti-N) and one Ti(-O-)Ti {orTi(-N-)Ti}], but the goodness-of-fit value dramatically decreased from 7.8 × 105 in the three-shell fit to 2.1 × 105 in
four-shell fit (see spectrum b in panels (3) and (4) of Figure 4).
The best fit with four shells was Ti-O (or Ti-N) bonds at 1.73
and 2.00 Å with the total N ) 6.2 and Ti(-O-)Ti {orTi(-N-)Ti} bonds at 2.89 and 3.181 Å with a total of N ) 9.6(see entry b in Table 4).
The k3-weighted EXAFS function for SN-doped mesoporous
TiO2 (see spectrum c in panel (1) of Figure 4) was apparently
different from the data in spectrum a, b, or d in panel (1) of
Figure 4 in the k-region of >8 Å. In its associated Fouriertransform (spectrum c in panel (2) of Figure 4), two peaks due
to Ti-O bonds centered between 1 and 2 Å merged into one
centered at 1.5 Å and a new shoulder peak appeared at 1.9 Å.
The new peak was not fit with parameters of Ti-O bond, but
nicely fit with those of Ti-S bond. The best-fit result for datain the region of 1.13-3.17 Å was Ti-O bonds at 1.879 Å with
N ) 4.4, Ti-S bonds at 2.283 Å with N ) 0.7, and Ti(-O-)Ti
bonds at 3.05 and 3.24 Å with a total of N ) 8.4 (see spectrum
c in Table 4).
The k3-weighted EXAFS function and its associated Fouriertransform for S-doped mesoporous TiO2-623 resembled thecorresponding data for SN-doped one (see spectra e and c in
Figure 4). The best-fit result for the former in the region of
1.03-3.18 Å with four shells of Ti-O, Ti-S, and two
Ti(-O-)Ti was similar to that for the latter, but the Ti-S bonddistance was longer at 2.40 Å and Ti-O and Ti(-O-)Ti
coordination was greater (6.2 and 10.8, respectively), comparedto values obtained for the SN-doped one (see entries e and c in
Tables 4).
Ethanol Photo-oxidation Reactions under Visible Light.
Over mesoporous TiO2, acetaldehyde (2.1 µmol h-1 gcat-1) andwater (3.4 µmol h-1 gcat-1) were formed in the ethanol oxidation
reaction under visible light (>420 nm) (see Figure 5a and entry
a in Table 5). A similar acetaldehyde formation rate (2.3 µmol
h-1 gcat-1) was reported,65,66 but the water formation rate in theliterature (16 µmol h-1 gcat-1) decreased to 21% (see entry a
versus entry a′ in Table 5). Because the pretreatment of catalyst
was extended from 2 h under vacuum (from ref 65) to 10 hunder vacuum, under UV-visible light (>370 nm) in this work,
the difference of surface hydroxyl group density over TiO2 may
be critical for the photo-oxidation of O2. Pretreated under the
more-severe condition, the products were exclusively acetaldehyde and water and no CO2, acetic acid, or formic acid weredetected.
The product formation rates over N-doped and SN-doped ones
were 1.1-1.8 times and 2.3-6.8 times greater, respectively,
than corresponding values over mesoporous TiO2 (see spectra
a-c in Table 5). The promotion of water formation was greaterby S-doping than N-doping.
Next, the SN-doping effect was compared to S-doping effectvia CVD on mesoporous TiO2. The acetaldehyde formation was
not promoted, but the water formation rate was 11 times greaterover S-doped mesoporous TiO2-583 compared to over undopedone (see spectra d and a in Table 5). The promotion of
acetaldehyde formation over S-doped mesoporous TiO2-623 was
1.9 times on similar extent to SN-doped and N-doped ones
(1.8-2.3 times). Water formation was dramatically promoted

Doped TiO2 for O2 Reduction under Visible Light

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6713

Figure 4. Ti K-edge EXAFS spectra for mesoporous TiO2 (spectrum a), N-doped mesoporous TiO2 (spectrum b), SN-doped mesoporous TiO2
(spectrum c), S-doped mesoporous TiO2-583 (spectrum d), and S-doped mesoporous TiO2-623 (spectrum e). Panel (1) shows the k3-weighted
EXAFS χ-function, panel (2) shows its associated Fourier transform, and panels (3, 4) best-fit the results in k-space (3) and R-space (4). (The thin
red line represents the experimental values, and the thick blue line represents calculated values). (The dashed line (---) represents the magnitude andthe dotted line (•••••) represents the imaginary part (in panels (2) and (4).)

by a factor of 12 times over S-doped mesoporous TiO2-623 thanthat over mesporous TiO2 (see spectra e and a in Table 5). The
photocatalytic result over TiO2 nanotube-S was similar to thatfor S-doped mesoporous TiO2-583 (see Table 5f).
In summary, S-doping was effective to mesporous TiO2 for
the O2 reduction to water under visible light (>420 nm). Evensmall amount of S in TiO2 nanotube-S was as effective as
relatively greater amounts of S in SN-doped mesoporous TiO2
and S-doped mesoporous TiO2-623. In contrast, the promotionof acetaldehyde formation was only 0.90-2.3 times greater,
upon doping with either sulfur or nitrogen.
The formation rate dependences on the cut-off wavelength(370-580 nm) were plotted for water and acetaldehyde (seeFigure 5B). The S-doped mesoporous TiO2-623 catalyst thatexhibited extended visible-light absorption tailing (see spectrum
e in Figure 1) was used. The water formation rate under visible
light with a wavelength of >580 nm) was still 33% of that undervisible light with a wavelength of >420 nm in Figure 5B. The
water formation rate decreased similar to the decomposition rates
of Methylene Blue24 and 2-propanol25 over S-doped TiO2 as
the cutoff wavelength progressively increased. These dependences were effectively less steep, compared to the ethanoloxidation over V-doped mesoporous TiO2,60 the Methylene Blue
decomposition over TiO2-xNy,3 and the water decomposition
over Ti-N-O-F31 and Ga-Zn-N-O.67
Discussion
N-Doped Mesoporous TiO2. Nitrogen was doped in/on TiO2
with uniform mesopores. Nitrogen doping using urea during
template synthesis negligibly affected the optical spectra (seespectra a and b in Figure 1). The Eg value was 2.8 eV (seeTable 2A). The Eg values were 2.46-3.17 eV for N-doped

TiO2,12,15,17,20,21 except for that synthesized from hexamethylenetetraamine and TiCl3 solvo-thermally, which was 2.29 eV(Table 1).13 The Ti 1s-3d pre-edge peak pattern did not change
because of the nitrogen doping (see spectra a and b in Figure
3A), demonstrating an amorphous TiO2 matrix.
Based on Ti K-edge EXAFS analyses, the bond distances
for two Ti-O (or Ti-N) and two Ti(-O-)Ti {or Ti(-N-)Ti}
shells changed only within 0.03 Å, but the total N for Ti-O
(or Ti-N) bonds effectively increased, starting from 4.2 to 6.2,
because of the nitrogen doping (see spectra a and b in Table4). This suggests a transition from unsaturated TiOy (y ≈ 4) tosaturated TiO6-xNx octahedra. Thus, N atoms substituted on theO sites of the TiO2 matrix with uniform mesopores to form
O-Ti-N bonds, in accord with EPR and DFT study.16 The
matrix was not distorted much, based on similar Ti-O (orTi-N) and Ti(-O-)Ti {or Ti(-N-)Ti} bond distances (seespectra a and b in Table 4), because the atomic sizes of the O
and N atoms are similar. The doping effect of nitrogen was apromotion of acetaldehyde formation by only 1.8 times undervisible light (see spectra a and c in Table 5).
S-Doped Mesoporous TiO2. Sulfur was doped in/on TiO2
with uniform mesopores via co-doping or CVD. The absorption
tailing in the visible-light region grew progressively in thefollowing order: TiO2 nanotube-S < S-doped mesoporous TiO2583 ≈ mesoporous TiO2 < SN-doped mesoporous TiO2 <S-doped mesoporous TiO2-623 (see spectra g, d, a, c, and e in
Figure 1). Corresponding Eg values were 3.1, 3.0 (1.7), 2.9, 2.2,
and 0.8 eV, respectively (see Table 2). The Eg values of 2.3-2.6
eV were reported for S-doped TiO2 (see Table 1).22,28 The sulfur
doping via CVD (see spectrum e in Figure 1) at 623 K may be
excessive.

6714 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Izumi et al.

Figure 5. Panel (A) shows the time course of ethanol oxidation reactions over (a) mesoporous TiO2, (b) N-doped mesoporous TiO2, (c) SN-doped
mesoporous TiO2, (d) S-doped mesoporous TiO2-583, (e) S-doped mesoporous TiO2-623, and (f) titanate nanotube-S under the illumination of
xenon arc lamp filtered with L42 (>420 nm) in the first 2 h and L37 (>370 nm) in next 2 h. Panel (B) shows the rate dependence of acetaldehyde
and water formation on the cutoff filter wavelength over S-doped mesoporous TiO2-623.

Although the average pore size remained unchanged in therange of 2.7-2.9 nm by SN-doping (see Table 2A), the uniform
mesopore structure was destroyed for S-doped TiO2 via CVD,
because of the high reaction temperature (583-623 K). With
the increase in the amount of doped heteroatom, the bondingstrain of solid progressively increases and uniform mesoporesshould become unstable. In fact, the specific SA decreased from608 m2/g to 229 m2/g (SN-doped mesoporous TiO2) (see Table
2A), similar to the progressive decrease of specific SA for
vanadium-doped mesoporous TiO2.39,68 The retardation of phasetransition from the anatase phase to the rutile phase in thetemperature range of 973-1173 K was reported by doping of

S and N, resulting in the relatively higher specific SA(53.3-10.0 m2/g), compared to that for undoped TiO2.69 In thiswork, the doped S/N partially inhibited uniform mesostructureformation consisted of amorphous Ti oxide/hydroxide anddodecylamine, resulting in a relatively lower SA, compared tothat for undoped mesoporous TiO2. The specific SA decreased
to 1.9-2.6 m2/g for S-doped mesoporous TiO2-583 and 623(Table 2B), which is consistent with the loss of uniform
mesopores.
The Ti 1s-3d pre-edge peak pattern remained unchanged
for mesoporous TiO2, SN-doped mesoporous TiO2, S-doped
mesoporous TiO2-583, and S-doped mesoporous TiO2-623

Doped TiO2 for O2 Reduction under Visible Light

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6715

TABLE 5: Products Formation Rates in the Ethanol
Photo-oxidation (55 µmol of Ethanol, 110 µmol of O2) over
Mesoporous TiO2, S and/or N-Doped Mesoporous TiO2, TiO2
Nanotube-S, Crystalline TiO2, and V-Doped Mesoporous
TiO2 under Visible Light (> 420 nm)

SCHEME 1: Energy Diagram of Anionic S-Doped
Mesoporous TiO2 Wersus Cationic V-Doped Mesoporous
TiO2 and of Reductive and Oxidative Reactions under
Visible Light

Formation Rates(µmol h-1 gcat-1)
entrya
a
a′
b
c
d
e
f
g
h

catalyst
mesoporous TiO2
SN-doped mesoporous TiO2
N-doped mesoporous TiO2
S-doped mesoporous TiO2-583
S-doped mesoporous TiO2-623
TiO2 nanotube-S
TiO2 (P25)
V-doped mesoporous TiO2

CH3CHO H2O CO2 reference(s)
2.1
2.3
4.8
3.7
2.2
3.9
1.9
19
23

3.4
16
23
3.8
39
42
39
4.9
16

0
0.3
0
0
0
0
0
0.2
0.3

this work65, 66this work
this work
this work
this work
this work
65
60, 65, 66aFor entries a′, g, and h, the pre-treatment was evacuation for
2 h at 290 K.For entries a-f, the pretreatment was evacuation for
10 h at 290 K under UV-visible light (>370 nm).

(see spectra a, c, d, and e in Figure 3A). Amorphous TiO2
matrix was retained below the crystallization temperature of
TiO2 (623-673 K)62 and probably also due to the bondingstrain of solid originating from doped sulfur or nitrogen.69
Based on the Ti K-edge EXAFS analyses, Ti-S bonddistances were obtained at 2.40-2.44 Å for S-doped mesoporous
TiO2-583 and TiO2-623 (see Table 4d, e). The distances werenear the typical value for Ti4+-S2- (2.43 Å for TiS2; see Table3). The Ti-S bond distance was smaller at 2.283 Å for SNdoped mesoporous TiO2 (see Table 4c). The presence of
substitutional S was demonstrated on the O sites of TiO2 matrix
for S-doped mesoporous TiO2-583 and TiO2-623. Besides, S
K-edge near-edge spectral pattern for these S-doped mesoporous
TiO2 resembled that for TiS2. Anionic S substitution was
supported. The anionic S sites were more complex because
nitrogen that has been derived from thiourea may compete with
S to substitute on the O sites of TiO2 matrix for SN-doped
mesoporous TiO2.
The substitutional S2- sites should be formed at the surface
in the reductive CVD condition in H2S for S-doped mesoporous
TiO2-583 and TiO2-623. In the synthesis of SN-doped mesoporous TiO2, thiourea was used as a sulfur source, which
normally resulted in substitutional S4+ or S6+ sites after
calcination of the catalyst.22,24-27 Because the SN-doped sample
was not calcined in this work, (at least a part of) the sulfur inthe sample was anionic substitutional sulfur. The relatively
shorter Ti-S bond distance may be due to the strain effect ofheteroatomic sulfur inside the TiO2 matrix or due to co-doped
nitrogen.
Photocatalytic Role of Doped S. The promotion of acetaldehyde formation by doping sulfur was in the order of TiO2
nanotube-S ≈ S-doped mesoporous TiO2-583 (0.48 wt % S) <S-doped mesoporous TiO2-623 (0.77 wt % S) < SN-doped
mesoporous TiO2 (1.7 wt % S) in ethanol + O2 under visible
light (>420 nm) (Table 5) in the same order of sulfur contents(see Table 2) or the N value increase for Ti-S bonds obtainedby EXAFS (see Table 4). The promotion by doped S (or SN)
was only 0.90-2.3 times greater, which is comparable to thefactor of 1.8 observed by doped N.
In contrast, the photoreduction of O2 to water wasenormously enhanced by doping sulfur. The promotion ratio
was in the order of SN-doped mesoporous TiO2 (6.8) < TiO2
nanotube-S (11) ≈ S-doped mesoporous TiO2-583 (11) <S-doped mesoporous TiO2-623 (12), which is much greaterthan the ratio of 1.1 observed for N-doped mesoporous TiO2
(see Table 5). The sulfur contents and the N values for Ti-S
bonds (0.7, 0.2, and 0.5 for SN-doped mesoporous TiO2,
S-doped mesoporous TiO2-583, and S-doped mesoporous
TiO2-623, respectively) were not correlated to the promotion
ratio for water formation. The preferential water formation(reduction side) rather than acetaldehyde (oxidation side)
under visible light was also demonstrated as the less steep
decrease of water formation rate than that for acetaldehyde
as the cutoff wavelength progressively increased (see Figure
5B). Thus, anionic doped sulfur was the key to promote O2
reduction to water under visible light, even in the presenceof small amount of sulfur. The promotion was not enhanced
more with further sulfur doping. The amount of sulfur (0.48
wt%) doped via the CVD method at 583 K was sufficient toeffectively produce electrons and holes under visible light.
Additional amounts of sulfur via CVD at 623 K (0.77 wt %S) or thiourea mixing (1.7 wt % S) were excessive. In
contrast, the doped molar amount of nitrogen by mixing urea
(N/Ti molar ratio ) 1/1) may be fewer, because typical doped
amounts for nitrogen were reported to be 0.1-2.5 at. % in
TiO2.3,12,13,16,20,21 Also, the change in UV-visible data wastrivial upon nitrogen doping in this study (see Figure 1). The
difference in doping amount should be one of the major
reasons for the difference in reactivity on the reduction side
(water formation) between S-doped and N-doped mesoporous
TiO2 (see Table 5).
The formation rate ratio of acetaldehyde and water was 0.62
over mesoporous TiO2 and 1.4 over V-doped mesoporous TiO2
(see entries a and h in Table 5).60,65,66 The reaction mechanism
was listed in reactions 1-5. The reaction mechanism that was
promoted by anionic sulfur is also proposed in reactions 1′-5′(major route) and reactions 1′′- 5′′ (minor route).
The pretreatment condition in this work (10 h undervacuum, in UV-visible light) decreased catalytic water
formation rates, relative to the condition of 2 h undervacuum65 for mesoporous TiO2 (see entries a and a′ in Table5). This difference is related to the surface concentration of
the hydroxyl group. The more-severe condition in this work
is more realistic in practical application, because the remarkably quick water formation in ref 65 (212 µmol h-1 gcat-1)
occurred only in the initial hour of reaction, and the rate
gradually decreased in repeated catalytic tests.60 Under visible
light (>420 nm), reaction 5 (one-to-one formation of acetaldehyde and water) proceeded over V-doped mesoporous
TiO2.65 Reductive reaction 4 of O2, with the excited electrons
formed when the doped cation impurity level was below theCB minimum of TiO2 (see Scheme 1), should be lesseffective, compared to reaction 4′ or 4′′ proceeding at the
CB of S-doped TiO2, based on water formation rates in Table
5. Actually, the four electron-reduction reaction 4, 4′, or 4′′

6716 J. Phys. Chem. C, Vol. 113, No. 16, 2009

consists of several elementary steps, probably including theformation of reactive hydroxyl radicals.2,14,60,72-75 Anotherpossibility of the minor route is that the S--Ti that forms
oxidizes surface OH to •OH radical. The •OH radical oxidizes
ethanol and O2 is reduced similarly to reaction 4.
On the other hand, oxidative reaction 3 to produce
acetaldehyde proceeded at the VB of V-doped mesoporous
TiO2 (see entry h in Table 5), but the reaction 3′′ wasseriously inhibited at the S2- impurity level (see Scheme 1).
Therefore, major route reaction 5′ proceeded in a manner inwhich photocreated holes were accumulated in the sulfur.
The energy interval between the CB minimum and (highest)
S impurity level was 0.8-2.2 eV, based on the UV-visible
spectra (see Table 2). Thus, the major reason seems energetic;
the S2- impurity level would be enough high above theVB3,26,61 than the oxidation potential between ethanol andacetaldehyde and may not receive electron from the substrate(see Scheme 1). Alternative explanation based on kinetics isthat HS-Ti should participate in the O2 reduction in reaction
4′′, but the reactivity may be lower than that for HO-Ti andreaction 4′′ may proceed slower than reaction 4′.
The coordinative unsaturation of Ti was demonstrated by
the decrease of first and second coordination Ti-O andTi(-O-)Ti from 6 and 12, respectively, for standard
compound (see Table 3) to 4.2-6.2 and 8.4-10.8, respectively, in addition to 0.2-0.7 of Ti-S coordination for
S-doped mesoporous TiO2 samples (see Table 4). Therefore,
it is consistent that adsorbed ethoxyl group in reaction 2′ or
2 has a tendency to remain stable on the surface63 and only
O2 reduction to water proceeded catalytically over S-doped
catalysts. The amount of hydrogen dissociated from ethanol(reaction 2′) that was supplied for catalytic tests corresponds towater formed for 6.5 h over most active S-doped mesoporous TiO2623 (see reaction 5′ and Table 5) and was not totally consumed inreaction tests for 4-5 h (see Figure 5).

Izumi et al.

In summary, sulfur that was doped on/in mesoporous TiO2
formed filled impurity level(s) located 0.8-2.2 eV below theCB minimum. The electron excitation from the S level(s) to
CB and further to O2 was enabled under visible light. In contrast,
the sufficiently high potential of the S level(s) than VB ofmesoporous TiO2 was disadvantageous to receive electron(directly or indirectly) from ethanol/ethoxyl. Thus, the reductionof O2 to water (and oxidation from ethanol to adsorbed ethoxyl
species) proceeded 6.8-12 times faster than mesoporous TiO2.
The high potential nature of S-doped mesoporous TiO2 would
be applied for the decomposition of volatile organic compounds(VOCs) or dyes of higher oxidation potential than the impurity
S level or for the reverse reaction of water oxidation to O2 andproton reduction to H2 (water splitting).76
Conclusions
SN-doped TiO2 with uniform mesopores centered at 2.9
nm was synthesized using thiourea. The specific surface area(SA) decreased from 608 m2/g (mesoporous TiO2) to 229
m2/g, because of bonding strain at the doped sulfur andnitrogen in the solid. S-doped TiO2 was also synthesized via
chemical vapor deposition (CVD) of H2S over mesoporous
TiO2 at 583 or 623 K. The specific SA seriously decreased
to 1.9-2.6 m2/g, in accord with the loss of uniform mesoporesof TiO2. The absorption tailing in the visible-light region was
ranked in the following order: S-doped mesoporous TiO2583 ≈ mesoporous TiO2 < SN-doped mesoporous TiO2 <S-doped mesoporous TiO2-623. The ultraviolet-visible(UV-vis) absorption data indicated the formation of S
impurity energy level(s) located 0.8-2.2 eV below theconduction band (CB) minimum of mesoporous TiO2. The
Ti-S bonds were detected at 2.283-2.44 Å for these samples
by extended X-ray absorption fine structure (EXAFS),
demonstrating the presence of substitutional anionic sulfur
on the O sites of the TiO2 matrix.

Doped TiO2 for O2 Reduction under Visible Light
The anionic sulfur doping enabled electron excitation fromthe impurity level(s) to the CB and then to O2 to form water(12× promotion, compared to mesoporous TiO2) under visible
light, but ethanol oxidation was promoted by only 2.3 times,
because the sulfur impurity level(s) with relatively highelectrochemical potential made receiving (directly or indirectly)
the electron from ethanol/ethoxyl difficult. The kinetic effectsof Ti-OH, Ti-SH, or •OH radicals were also suggested.
Nitrogen doping to mesoporous TiO2 was not effective.
Although the amount of doped sulfur was minimal for TiO2
nanotube synthesized hydrothermally using TiS2, based on the
optical spectrum, the photocatalytic result was similar to thatover S-doped mesoporous TiO2-583. Even in the presence of asmall amount of anionic sulfur on/in TiO2, the O2 reduction towater was promoted.
Acknowledgment. The X-ray absorption experiments wereconducted under the approval of the Photon Factory Proposal
Review Committee (No. 2007G576). The authors are thankful
for financial support from the Grant-in-Aid for Scientific
Research C (19550134) from JSPS and the Grant-in-Aid in thepriority area “Molecular Nano-dynamics” (No. 432-17034013)
from MEXT.
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