Photon Factory Activity Report 2016 #34 (2017)

BL-13B/2015S2-008

Ambient Pressure Soft X-ray Photoelectron Spectroscopy Study on Chemical
Reactivity of TiO2 Surfaces towards CO and O2
Kenichi Ozawa1,*, Kensuke Tobishima2, Kazuhisa Isegawa3, Tetsuya Miyazawa4, Hiroshi Kondoh3,
Hiroo Kato2 and Kazuhiko Mase4,5
1
Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
2
Hirosaki University, Hirosaki, Aomori 036-8561, Japan
3
Keio University, Kohoku, Yokohama 223-8522, Japan
4
SOKENDAI, Tsukuba, Ibaraki 305-0801, Japan
5
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

2 EXPERIMENTAL
Commercially available r-TiO2 single crystals with(110), (011), (100) and (001) orientation were used. The
crystal surfaces were cleaned in the ultrahigh vacuum bycycles of Ar+ sputtering and annealing in O2 atmosphere.
APXPS measurements were carried out at BL-13B [5]. C
1s, O 1s and Ti 2p core-level spectra were acquired in theenvironment of CO, O2 and CO + O2 mixture gases. All
the experiments were carried out at room temperature.
3 RESULTS and DISCUSSION
Chemical species on the r-TiO2 surfaces under the CO
atmosphere (13 Pa) are identified from the C 1s spectra (aleft panel in Fig. 1) as molecularly adsorbed CO(285−286 eV) and carboxylate (which also includes

COgas

Intensity (arb. units)

1 INTRODUCTION
Photocatalytic activity of TiO2 is superior to any othersemiconductor materials. This lead to TiO2 as a photocatalyst that is used in many commercially available
photocatalytic products. It has been recognized that thephotocatalytic activity depends on not only the crystal
structure of TiO2 but also the orientation of the crystal
surface. For example, the (101) surface of anatase TiO2 ismore active than the (001) surface in various photocatalytic reactions like photooxidation to generate OH
radicals [1], which initiate decomposition of chemical
species on the surface. Regarding the rutile TiO2 (r-TiO2)
surfaces, an activity order of (001) > (100) > (110) has
been found for photooxidation of methanol andhydroxylation of terephthalic acid [2]. Contrastingly, rTiO2(011) exhibits the highest activity for methyl orange
decomposition, followed by (110), (001) and (100)surfaces [3]. A face-dependent redox potential [1], a facespecific electron-trapping probability [4], anisotropic
diffusion of the charge carriers in the bulk [3], etc. havebeen proposed to explain the surface-dependence of thephotocatalytic activity. In the present study, we haveverified another possible origin of the face-dependence,i.e., the surface chemical activity. The chemical activityof the r-TiO2 surfaces is comparatively assessed in a CO
atmosphere by ambient pressure X-ray photoelectron
spectroscopy (AP-XPS). It is found that r-TiO2(110) is themost active surface, while the least active one is rTiO2(011). R-TiO2(100) and (001) are in-between them.

13 Pa CO & 13 Pa O2

13 Pa COcarboxylate
or CO 2(ad)C 1sCOadh = 753 eV

COgas

r−TiO 2(011)

carboxylate
or CO 2(ad)COadr−TiO 2(100)r−TiO 2(001)r−TiO 2(110)290
285
Binding Energy (eV)290285
Binding Energy (eV)Figure 1 C 1s spectra of the r-TiO2 surfaces under the CO (13 Pa)
and CO + O2 (26 Pa) atmosphere. The photon energy used was753 eV. An intense peak at 291−292 eV is gaseous CO.

adsorbed CO2) (~289 eV). The carboxylate should beformed between adsorbed CO and lattice O of the r-TiO2surface. Involvement of the O atoms that are formed as aresult of CO dissociation is safely denied because atomic
C is absent on the surface. The fact that the amount of thecarboxylate is larger than that of adsorbed COirrespective of the surface orientation suggests that theactivation barrier for CO oxidation must not be high.
Judging from the peak intensities, the order of thechemical reactivity for CO adsorption (and, thus, COoxidation) is (110) > (001) ≈ (100) > (011). This order isunchanged when O2 coexists with CO, though the activity
is enhanced on the (001), (110) and (011) surfaces. The
activity order towards CO adsorption and oxidation is thesame as that for acetic acid adsorption. This indicates thatthe stability of the adsorbed carboxylate should determine
the activity of the r-TiO2 surfaces.
The experimental results obtained in the present study
will help to assess the contribution of the surface
chemical activity to the face-dependent photocatalytic
activity once the photocatalytic activity is determined
under the same ambient condition. This will be the next
step in our research.
References[1] J. Pan et al., Angew. Chem. Int. Ed. 50, 2133 (2011).
[2] A.Y. Ahmed et al., J. Phys. Chem. Lett. 2, 2461 (2011).
[3] T. Luttrel et al., Sci. Rep. 4, 4043 (2014).
[4] T. Tachikawa et al., J. Am. Chem. Soc. 113, 7197
(2011).
[5] H. Kondoh et al., Catal. Today 260, 14 (2016).
* ozawa.k.ab@m.titech.ac.jp