Monitoring of Sulfur Sites Doped in/on Titanium Oxide to Enable Photocatalysis
under Visible Light Using S K-edge XANES
Yasuo Izumi1 and Yoshiyuki Shibata2
Department of Chemistry, Graduate School of Science, Chiba University,
1-33 Yayoi, Inage-ku, Chiba 263-8522
2
Department of Nanomaterial Science, Graduate School of Advanced Integration Science, Chiba University,
1-33 Yayoi, Inage-ku, Chiba 263-8522
1(Received June 9, 2009; CL-090549; E-mail: yizumi@faculty.chiba-u.jp)

S substituting on the O atoms of TiO2 responsible for visible light-responsive catalysis.

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Vol.38 No.9 2009 p:912{913
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September 5, 2009

The Chemical Society of Japan
Published on the web (Advance View) August 22, 2009; doi:10.1246/cl.2009.912

912

Chemistry Letters Vol.38, No.9 (2009)

Monitoring of Sulfur Sites Doped in/on Titanium Oxide to Enable Photocatalysis
under Visible Light Using S K-edge XANES
Yasuo Izumi1 and Yoshiyuki Shibata2
Department of Chemistry, Graduate School of Science, Chiba University,
1-33 Yayoi, Inage-ku, Chiba 263-8522
2Department of Nanomaterial Science, Graduate School of Advanced Integration Science, Chiba University,
1-33 Yayoi, Inage-ku, Chiba 263-8522
1(Received June 9, 2009; CL-090549; E-mail: yizumi@faculty.chiba-u.jp)
The electronic state and site structure of doped S to enable
photocatalysis of TiO2 under visible light were investigatedusing S K-edge XANES compared to theoretically generated
spectra. S sites substituting on the O atoms of TiO2 were predominant and essential in photocatalysis based on S K absorption edge peak energy and post-edge pattern. Minor cationic/
elemental S sites were detected as peaks due to the transition
to vacant level of S3p, but most were lost during photocatalysis.

Photocatalysis has been investigated to decompose nitrogenoxides, sulfur oxides, and volatile organic compounds1 and reduce water and carbon dioxide into hydrogen2 and methanol/
formic acid,3 respectively, over semiconductors, e.g., TiO2 .
TiO2 needs UV light to perform photocatalysis due to the band
gap of 3.0–3.2 eV, but dopants eﬀectively reduce the band gap
to the visible light region.4
Electronic excitation from the valence band of TiO2 to theimpurity cationic V level and subsequent photoreduction overV–TiO2 were monitored using V K-edge XAFS.5–7 Charge recombination occurs less frequently in anion-doped TiO2 . Doped
anions eﬀectively reduce the band gap and promote photooxidation under visible light.8 In this letter, the electronic structure ofdoped S sites in/on TiO2 and the change during ethanol photooxidation were monitored using S K-edge XANES.
Two types of S-doped TiO2 photocatalysts were prepared.
One was synthesized by mixing thiourea with Ti tetraisopropoxide in a molar ratio of 1:2 during mesostructure formation using
dodecylamine.8 The S content in the sample after template removal and evacuation at 373 K was 1.7 wt %. The obtained catalysts were denoted SN-doped mesoporous TiO2 . Another wassynthesized via CVD under 101 kPa of hydrogen sulﬁde withmesoporous TiO2 8 separately synthesized using dodecylamine.
The S contents in the samples were 0.48 and 0.77 wt % after
CVD up to 583 and 623 K, respectively. Obtained catalysts weredenoted as CVD-S-doped mesoporous TiO2 -583 or -623.
S K-edge XAFS spectra were measured at 290 K at the Photon Factory in High-Energy Accelerator Research Organization
on beamline 9A (Proposal No. 2007G576). A Si(111) double
crystal monochromator was fully tuned. Except for the S content
evaluation in the sample based on the edge jump value in transmission mode, spectra were measured in ﬂuorescence detection
mode. A 1.4-mg portion of sample powder was ground as ﬁne aspossible and mounted on tape. The maximum of the ﬁrst preedge feature in the spectrum of Na2 S2 O3 5H2 O was assigned
as 2472.02 eV.9
The S K-edge XANES spectra were theoretically generatedusing ab initio calculation code FEFF 8.410 operated in self-con-.

sistent ﬁeld and fully multiple scattering modes. The exchangecorrelation potential of the Dirac–Hara and Hedin–Lundqvist
imaginary part was chosen and corrected by adding a constantshift of 2:5 eV to the Fermi level. The energy of the theoretically generated spectrum was shifted by +2.6 eV to compare
to the experimental data.
Twin peaks appeared at 2470.0 and 2471.9 eV for TiS2(Figure 1A-d). The energy diﬀerence between S3p and Ti3d isca. 5 eV11 and the energy levels interact in TiS2 (Scheme 1a).
The twin peaks were assigned to transition to ð1  2 Þ1=2 jTi3d >jS3p > ðÞ levels. The intensity of this transition is proportional to the S3p character 2 mixed into the wave function .9,12
Peak energies for S K post-edge spectrum for fresh SN-doped mesoporous TiO2 (Figure 1A-a) were 2481.8(s), 2488.5(w),Figure 1. (A) Normalized S K-edge XANES spectra for SNdoped mesoporous TiO2 before (a) and after ethanol oxidation
under visible light (b), CVD-S-doped mesoporous TiO2 -623
after ethanol oxidation under visible light (c), and TiS2 (d).
(B) Theoretical S K-edge XANES spectra for S site substituting
on the O atom (a) or Ti atom (b) of anatase-type TiO2 or on the O
atom at TiO2 (001) surface (a0 ), S site of SO2 adsorbed on the Ti
atom (c) or O atom (d) at TiO2 (001) surface, and S site of TiS2(e). (Inset) S substitution model on the O atom of TiO2 .

Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.9 (2009)

Scheme 1. Proposed energy diagram for TiS2 (a) and S-doped
mesoporous TiO2 (b).
and 2497.4 eV(w,br) very similar to 2481.7, 2488.5, and2498.1 eV for TiS2 (Figure 1A-d). In contrast, the pre-edge feature was signiﬁcantly suppressed at 2472.1 (sh) and 2473.5 eVfor fresh SN-doped mesoporous TiO2 compared to twin peaksfor TiS2 . After catalysis in ethanol and O2 gas under light>420 nm,13 the pre-edge peak at 2473.1 eV and the shoulder
peak at 2479.4 eV became even weaker (Figure 1A-b). The S
K-edge XANES spectrum for CVD-S-doped mesoporous TiO2
was essentially identical to that for SN-doped mesoporous TiO2
both after photocatalysis (Figures 1A-b and 1A-c). Spectra for
the former negligibly changed during photocatalysis.
We assumed anatase-type TiO2 14,15 for theoretical calculations. Doped S substituted on O or Ti of the TiO2 matrix (substitution model) or SO2 /SO3 H adsorbed on Ti or O at theTiO2 (001) surface.
The S K-edge XANES spectrum calculated for doped Smodel (total 261 atoms; Figure 1B, inset) substituting on the Oof TiO2 is depicted in Figure 1B-a. The S–Ti bond distance(R) was 2.283 Å (coordination number N ¼ 2) based on Ti Kedge EXAFS for SN-doped mesoporous TiO2 .8 The absorption
edge peak at 2481.8 eV, shoulder peak(s) at 2478.0 eV, andbroad post-edge peak at 2496.4 eV nicely reproducedFigure 1A-a. The post-edge pattern was somewhat similar to
those of sulfates16,17 probably because two O atoms were at2.011 Å to doped S in the substitution model due to the strainof long Ti–S bonds in TiO2 (Figure 1B, inset). When the RS{Tiwas elongated to 2.44 Å (CVD-S-doped mesoporous TiO2 ),8peaks in Figure 1B-a shifted within 1.2 eV toward lower energy
but the spectral pattern was similar.
In the substitution model on Ti atom (no S site relaxationconsidered, total 261 atoms; Figure 1B-b) and SO2 adsorption
models on Ti (RS{Ti ¼ 2:44 Å, total 150 atoms; Figure 1B-c)
or O atom (RS{O ¼ 1:574 Å, total 150 atoms; Figure 1B-d),the peaks at 2475.1–2475.6 eV became more intense as the Ssites became more positive (Figure 1B-c ! Figure 1B-d !
Figure 1B-b). This peak was also observed for SO2 -adsorbed
metal cluster.9 Thus, weak pre-edge peaks appearing at
2471.9–2473.5 eV (Figures 1A-a–1A-c) were due to transition
to S3p of minor cationic and/or elemental S sites9 in S-doped
mesoporous TiO2 (Scheme 1b). The peaks at 2471.9–2473.5 eV became weaker after photocatalysis probably because
cationic/elemental S species desorbed (Figures 1A-a and 1A-b).
Calculated spectra of adsorbed SO3 H either on Ti or O consisted of peaks at 2481.7 and 2504.0 eV, but the former peak waseven broader than Figure 1B-d and did not match Figures 1A-a–
1A-c. Intense peaks were reported to appear between 2482.4 and2483.4 eV for Na, Mg, Al, K, Ca, Mn, Fe, and Ba sulfates16,18
inconsistent with 2481.5–2481.8 eV for Figures 1A-a–1A-c. Because the catalysts could be used repeatedly for photocatalysis,8
the substitutional S sites on the O atoms were concluded to be913
photocatalytically active rather than cationic and/or elementalS sites.
The S sites substituting on the O atoms were not necessarily
at the surface because the spectrum for S substituting on the O
atom at the TiO2 (001) surface (RS{Ti ¼ 2:283 Å, N ¼ 2, total
201 atoms; Figure 1B-a0 ) did not resemble any experimental
spectra.
The twin pre-edge peaks and main peak in experimental datafor TiS2 (Figure 1A-d) were nicely reproduced at 2476.4,2478.6, and 2488.1 eV in calculated Figure 1B-e.19 However,the peaks shifted by 6.4–6.7 eV toward lower energy in the experiment. The reason for this discrepancy is not known, but limited model cluster size of TiS2 (88 atoms) may be critical.
The twin peaks at 2470.0 and 2471.9 eV for TiS2 due to transition to ð1  2 Þ1=2 jTi3d > jS3p > (Scheme 1a) were notobserved for S-doped TiO2 mostly because latter Ti3d antibonding levels consist of greater O2p character than S3p (S contentsin catalysts: 0.48–1.7 wt %). Smaller S3p character 02 in the antibonding levels led to the pre-edge peaks weaker. Additionally,
the interaction between Ti3d and O2p and S3p may destabilize
the antibonding levels more in S-doped TiO2 near to S4p level(Scheme 1b) than in the case between Ti3d and S3p in TiS2 .
The authors thank ﬁnancial support from the Grant-in-Aidfor Scientiﬁc Research C (No. 19550134) from Mext.
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Published on the web (Advance View) August 22, 2009; doi:10.1246/cl.2009.912