Photon Factory Activity Report 2016 #34 (2017)

BL-9A/2016G654

PTRF-XAFS study of single Cu site formed on a TiO2(110) surface
covered with a nitrogen-containing molecule
Satoru Takakusagi*, Yuto Nakamura, Yoshihiro Honda, Satoshi Seki, Daiki Kido,
Qiuyi Yuan, Yuki Wakisaka, Kiyotaka Asakura
Hokkaido Univ., Sapporo 001-0021, Japan
1 Introduction
Precise size control of metal species on an oxidesurfaces especially in the range of <1 nm, is now highly
important to develop the next-generation catalysts,sensors, and electronic devices. However this is not easy
since metal atoms are easily aggregated to form large
particles on oxide surfaces. Our group has developed the“premodified surface method” to obtain a highly
dispersed metal species [1-4]. In the premodified surface
method, an oxide surface is first precovered with afunctional organic molecule possessing functional groups
which can strongly coordinate to a metal atom, and thenmetals are deposited on the surface. We have alreadysucceeded in preparing atomically dispersed Cu, Ni, andAu species on the TiO2(110) surfaces premodified withMBA (mercaptobenzoic acid, HS-C6H4-COOH), where
mercapto group acts as a coordinating site to the metalatoms and carboxylic group as a binding part to thesubstrate TiO2(110) surface [2-4].
In this study, we attempted to obtain atomicallydispersed Cu on a TiO2(110) surface by premodification
with a different functional organic molecule (orthoanthranilic acid, H2N-C6H4-COOH) to modify and control
the local Cu structure for catalytic applications. 3D
structure of the Cu species was examined by polarizationdependent total reflection fluorescence (PTRF)-EXAFS.
2 ExperimentNb-doped (0.05 wt%) TiO2(110) surface was cleaned
by HF etching and annealing in air at 973 K for 1h. Thecleaned surface was immersed in 2 mM o-anthranilic acid(o-AA) ethanol solution to modify the TiO2(110) surface
with o-AA monolayer. Cu was vacuum-deposited on theo-AA-modified surface and the coverage was estimated tobe 0.11 ML by XPS measurements (1 ML=5.2×1014/cm2).
PTRF-XAFS measurements were carried out at BL9A
with in-situ PTRF-XAFS chamber (base pressure 8 × 10-8Pa). XAFS analysis was carried out using REX2000 andFEFF8.20.
3 Results and Discussion
Fig. 1 shows the observed Cu K-edge PTRF-EXAFS
spectra of Cu/o-AA/TiO2(110) in three orientations. The
envelopes of the PTRF-EXAFS oscillations damped
quickly compared with the EXAFS spectrum of Cu foil.
This means that the nearest neighbor atom of Cu is not Cu,
but a lighter atom like nitrogen or oxygen. The amplitudeof EXAFS oscillations in the [110] orientation (E//[110])
was slightly stronger than that for the other two parallel
directions, which originates from the Cu-substrateinteraction because the [110] orientation corresponds tothe electric vector pointing to the TiO2 substrate direction.
Curve fitting analysis indicated contribution from CuN(O) interaction (2.15±0.03 Å) and Cu-N(O) interaction(1.93±0.03 Å)
To determine a detailed structure of the Cu species, aniteration method using a FEFF code and a real spacemodel structure was employed. Fig. 2 shows theproposed model structure, and polarization dependent
FEFF simulation using this model structure is illustrated
in Fig. 1 for comparison with the experimental spectra.
The calculated EXAFS oscillations originating from theproposed model showed a good agreement with theobserved spectra.

Fig.1 PTRF-EXAFS spectraof Cu/o-AA/TiO2(110) (blackcurves). The calculated
EXAFS oscillations using thestructure model in Fig. 2 arealso shown as red curves.

Fig.2 A proposed model
structure of Cu/o-AA/TiO2(110).
Cu-N: 2.15 Å, Cu-O:
1.93 Å.
References[1] W.J. Chun et al., Chem. Phys. Lett. 433, 345 (2007).
[2] S. Takakusagi et al., Phys. Chem. Chem. Phys. 15,14080 (2013).
[3] K. Asakura et al., Faraday Discuss. 162, 165 (2013).
[4] S. Takakusagi et al., J. Phys. Chem. C 120, 15785(2016).
* takakusa@cat.hokudai.ac.jp