Photon Factory Activity Report 2019 #37 (2020)

BL-13B/2018S2-005

Change in Surface Properties across the Crystallographic Edge of Anatase TiO2
Kenichi OZAWA1,2,*, Keita HIROMORI 3, Nobuo NAKAJIMA3, and Kazuhiko MASE 2,4
Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
2
Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK),Tsukuba, Ibaraki 305-0801, Japan
3
Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
4
SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Ibaraki 305-0801, Japan1Binding Energy (eV)2 Experimental
The XPS measurements were carried out at a beam line
13B of the Photon Factory [6], using an SES200
photoelectron energy analyzer (Gamma Data/Sienta). The
photon energies of 400 eV and 753 eV were used to acquire
the spectra. A naturally grown anatase TiO2 single crystal
with mirror-polished (101) and (001) surfaces which forma straight edge in between (SurfaceNet GmbH) was used
(Fig. 1a). The (101) and (001) surfaces were cleaned bycycles of Ar+ sputtering and annealing at 950 K in 5 × 10−4Pa O2 atmosphere. A vapor of acetic acid was introduced
to the loadlock chamber, in which the TiO2 sample wasstored to allow acetic acid to adsorb on the surfaces. Theadsorption temperature was 300 K.

Fig. 1 (a) A natural crystal of anatase TiO2 mounted ona molybdenum sample holder. (b) A line profile of thesurface morphology around the (101)/(001) edge
region. Circles indicate the positions of the TiO 2
surface, and straight lines indicate extrapolation of the(101) and (001) terraces.

37.0

Ti 3p peak position
37.2clean

37.4
37.6

acetic acid−200

C 1s intensity (arb units)

1 Introduction
One of interesting properties that TiO2 photocatalysts
exhibit is that photocatalytic activity depends on thecrystallographic orientation of the TiO2 surfaces. Our
recent X-ray photoelectron spectroscopy (XPS) studies [1,2] has suggested that there is a strong correlation between
photocatalytic activity and the photoexcited carrier
lifetime on the TiO2 surfaces. Another mechanism for theorientation dependent activity is proposed on the basis of
discontinuity of the energy level alignment at thecrystallographic edge where two surfaces with different
orientation come in contact with each other. For example,
the energy levels of the anatase TiO2 has been proposed tolay deeper on the (101) surface than on the (001) surface
so that the photoexcited electrons and holes tend to migrate
to the (101) and (001) surfaces, respectively [3, 4].
However, such energy level discontinuity was only
suggested by the work function, and a direct verification
has not been done so far.
In the present study, using a high precision XYZtranslator for manipulating the sample position [5], weexamined the energy level alignment across the edge
between the (101) and (001) surfaces of anatase TiO 2 byXPS. We also estimated the amounts of adsorbed acetic
acid to compare the adsorption activity among the different
surface sites.

C 1s peak intensity

200

acetic acid1clean0−200
0200Distance from the edge ( m)(101)

3 Results and Discussion
Fig. 1b shows a microscopic view of the surface
morphology in the vicinity of the edge, obtained byadjusting the sample position while monitoring the Ti 2p 3/2
photoemission image. The sample position was adjusted so
as to lie the photoemission image on the microchannel0(001)

Fig. 2 Binding energies of the Ti 3p peaks (a) andintegrated intensities of the C 1s peaks (b) of the clean
and acetic-acid saturated TiO2 surfaces as a function ofthe distance from the edge. Blue circles and red squares
indicate the results of the clean and acetic-acidsaturated surfaces, respectively.

Photon Factory Activity Report 2019 #37 (2020)

plate (MCP) of the analyzer in the center position of theMCP. The edge is observed to be rounded with a width of~50 µm. However, the size of the synchrotron radiationwas ~30 µm in the vertical direction (the Z direction) [5],the edge should be sharpener than that shown in Fig. 1b.
To evaluate the energy level alignment across the edge,
peak positions of the Ti 3p spectra were determined. Fig.
2a shows that, on the clean TiO2 surface, the binding
energy of the Ti 3p peak is at 37.34−37.35 eV on both(101) and (001) terraces, while the peak position is 37.47
eV at the edge (blue circles). This suggests that the energy
level of TiO2 is nearly the same on both (101) and (001)terraces and is deeper by ~100 meV at the edge in between
the two terraces. Namely, the TiO2 band bends
downwardly when going from the terraces to the edge, andthe energy levels of the (101) and (001) are not smoothly
connected at the edge. As the TiO2 surface is being
covered with acetic acid, a dip structure of the energy levelis preserved, while the adsorption induces a slightdownward band bending (squares in Fig. 2a).
Such a detailed band structure around the edge region
has not been reported so far. The dynamic of thephotoexcited carriers has been discussed on the basis of asimple and smooth energy level alignment at the edge
between the (101) and (001) surfaces [3, 4]. Thus, it isneeded to reconsider the carrier dynamics around the edge.
The deeper lying of the TiO2 level at the edge suggests anaccumulation of the photoexcited electrons so that thephotocatalytic oxidation reaction could be facilitated moreat the edge than on the terraces.
Fig. 2b shows a variation of the C 1s peak intensity
along the line from one terrace to the other terrace. Even
after the extensive surface cleaning, carbon contaminantsremain on the surface (blue circles). Despite the low
concentration, a higher amount of the contaminants isfound at the edge than on the terraces. This may reflect ahigher number of coordinatively unsaturated atoms in theedge region.
As the surface is being covered with a saturation
amount of acetic acid, the C 1s peak intensity is enhancedby more than 20 time (red squares in Fig. 2b). The
amount of adsorbed acetic acid is higher on the (001)terrace than the (101) terrace. This means that the (001)surface of anatase TiO2 is more active for acetic acidadsorption than the (101) surface. Interestingly, theedge, which is active for contaminants, is not an active
site for acetic acid adsorption.
Although not shown here, the line profile of the Ti 2p/O
1s intensity ratio is found to be similar to the C 1s intensity
profile; namely, the Ti 2p/O 1s ratio is higher on the (001)terrace than on the (101) terrace. This means that thesurface Ti density is higher on (001) than on (101). Acetic
acid is known to adsorb on the TiO2 surfaces in the formof acetate, and each acetate is bonded to the surface
Ti atom(s) via its O atom(s). Thus, the line profile of the C
1s intensity (the squares in Fig. 2b) can be understood by
the proposed adsorption mechanism of acetic acid on theTiO2 surface.
If the photoexcited carriers are drifted along thepotential gradient (band bending), the electrons and theholes are assumed to be accumulated at the edge region
and the terraces, respectively. Comparison ofphotocatalytic

activity among the edge and the (101) and (001) terracesis necessary to prove the carrier dynamics along the surface
parallel direction in the vicinity of the crystallographicedge.
Acknowledgements
The XPS measurement was performed under theapproval of the Photon Factory Advisory Committee(Proposal No. 2018S2-005). This work was financially
supported by a Grant-in-Aid for Scientific Research (GrantNo. 16H03867) from Ministry of Education, Culture,Sports, Science, and Technology of Japan.
References[1] K. Ozawa et al., J. Phys. Chem. 122, 9562 (2018).
[2] K. Ozawa et al., e-J. Surf. Sci. Nanotechnol. 17, 130(2019).
[3] S. Kashiwaya et al., Adv. Energy Mater. 8, 1802195(2018).
[4] T.-Y. Lee et al., ACS Omega 3, 10225 (2018).
[5] Y. Aiura et al., J. Synchrotron Radiat. 27 (2020), DOI:10.1107/S1600577520006712.
[6] A. Toyoshima et al., J. Phys.: Conf. Ser. 425, 152019(2013).
* ozawa.k.ab@m.titech.ac.jp