Photon Factory Activity Report 2017 #35 (2018)

BL-13B/2015S2-008

Photocatalytic Activity of Anatase and Rutile TiO2 Surfacesand Its Correlation with Lifetime of Photoexcited Carriers
Kenichi OZAWA1,*, Susumu YAMAMOTO2, Ryu YUKAWA3, Ro-Ya Liu4, Naoya TERASHIMA5,Yuto NATSUI5, Hiroo KATO5, Kazuhiko MASE3,6, and Iwao MATSUDA2
1
Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
2
Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3
Institute of Materials Structure Science, KEK, Tsukuba, Ibaraki 305-0801, Japan
4
Institute of Physics, Academia Sinica, Taipei City 11574, Taiwan
5Department of Advanced Physics, Hirosaki University, Hirosaki, Aomori 036-8561, Japan
6SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Ibaraki 305-0801, Japan

Fig. 1 (A) C 1s spectra of adsorbed acetic acid on r-TiO2(110) at various
UV-irradiation times. (B) Plot of the C 1s intensity against the UVirradiation time.
−1min )

Regarding the carrier life3time, we examined fourTiO2 surfaces [r-TiO2(110),r-TiO2(011), a-TiO2(001),
2and sputtered r-TiO2(110)]
without adsorbed acetate [2].
The determined lifetimes
1
were converted to barrierheight-corrected lifetimes so
as to reproduce the lifetimes
on the acetate-saturated
0surfaces; they are 450 ns,
0200
400
600
Carrier Lifetime (ns)360 ns, and 7 ns for rTiO2(110), a-TiO2(001), andFig. 2 Correlation between ther-TiO2(011), respectively.
carrier lifetime and thephotocatalytic activity.
The lifetime is found tobe proportional to kph. A
plot of kph against the lifetime gives a linear and positive
straight line (Fig. 2). This correlation indicates that thecarrier lifetime should be a key factor to understandvarious properties of TiO2 photocatalysis, including theorientation-dependent activity.

2 Experimental
The photocatalytic activity was evaluated by a rate
constant of a photocatalytic decomposition of acetic acid.
Acetic acid adsorbs in the form of acetate, and the rate of
the photocatalytic decomposition of acetate was estimatedfrom a decreasing rate of the C 1s XPS peak intensity
while the ultraviolet (UV) laser with an energy of 3.31 eV
was irradiated. The carrier lifetime was determined froma time evaluation of the UV-induced surface photovoltage(SPV). The SPV resulted in the shift of the valence andcore levels of TiO2. We examined the SPV via a shift ofthe Ti 2p level. The measurements were done at BL-13Bof PF (XPS) and BL07LSU of SPring-8 (TRXPS).
3 Results and Discussion
The intensity of the C 1s peak of adsorbed acetate
decreases exponentially upon irradiation of the UV lightwith a time constant τ ph of 370 min on r-TiO2(110) (Fig.
1). Since the reciprocal of τ ph is a rate constant kph, kph on
r-TiO2(110) is evaluated to be 2.7×10-3 min-1. This is thelargest rate constant among all surfaces examined in thepresent study. kph on a-TiO2(101) and (001) are 2.1×10-3
min-1 and 1.7× 10-3 min-1, respectively, while r-TiO2(011) and (001) are inactive with kph = 0 min-1.

Photocatalytic Activity kph (x 10−31 Introduction
One of many interesting aspects that TiO2 photocatalysts exhibit is a morphology dependence of thephotocatalytic activity; anatase TiO2 (a-TiO2) generallyshows a higher activity than rutile TiO2 (r-TiO2). On theother hand, it has also been known that the TiO2
photocatalytic activity depends on the orientation of thecrystal surfaces. Although several mechanisms such as asurface-dependent redox potential, anisotropic diffusionof the carriers, etc. have been proposed as a possible
origin of the orientation dependence, none of them
explains comprehensively the phenomenon. In the presentstudy, we systematically assessed the photocatalytic
activity and photoexcited carrier lifetime on singlecrystal a-TiO2(101) and (001) surfaces and r-TiO2(110),(001), (011), and (100) surfaces by X-ray photoelectron
spectroscopy (XPS) and time-resolved XPS (TRXPS) andverified the activity-lifetime relation [1].

[1] K. Ozawa et al., J. Phys. Chem. C 122, 9562 (2018).
[2] K. Ozawa et al., J. Phys. Chem. C 120, 29283 (2016).
* ozawa.k.ab@m.titech.ac.jp