Photon Factory Activity Report 2014 #32 (2015) B

BL-9A/2013G123

Atomic-scale Structure of a Pt4 Cluster on TiO2(110) Surface
Investigated by the PTRF-XAFS MethodYusaku F. Nishimura1 , Yoshihide Watanabe1,* , Hiromitsu Uehara2 , Atsushi Beniya1 ,Ryo Suzuki1 , Kiyotaka Asakura2 , and Satoru Takakusagi2
1 Toyota Central R&D Labs., Inc., Aichi 480-1192, Japan
2 Hokkaido University, Sapporo, Hokkaido 001-0021, Japan
1 Introduction
Size-selected noble metal clusters are expected toimprove catalytic activit ies and thus to reduce the use ofnoble metals, especially for automotive catalysts. So far,
we have developed size-selected cluster-producing andcluster-characterizing system to clarify the relationships
between the number of constitutive metal ato ms andchemical properties [1]. Atomic-scale structures of Ptn
clusters on TiO2 (110) surface are, however, still unclear.
In this study, we performed polarizat ion-dependent totalreflection fluorescence XAFS (PTRF-XAFS) [2] toevaluate the structure of a Pt4 cluster on TiO2 (110) surface.
2 Experiment
2 × 1013 cm−2 of Pt4 clusters were deposited on acleaned TiO2 (110) single crystal in the manner describedin Refs [1,3]. The Pt4 /TiO2 (110) sample was supplied invacuum to PTRF-XAFS measurements at BL-9A
emp loying
thePTRF-XAFS-dedicatedsystem.
Procedures of PTRF-XAFS measurement and analysis arerepresented in Ref [3]. The goodness of fit between theobserved (χobs (k)) and the calculated (χcal (k)) EXAFS
oscillations, R, was evaluated by using the reduced chisquare expressed as follows���χ cal (k) − χ obs (k)1R=���σ�χ (k)⎷ N − P − 1 30≤k/nm−1≤90

2obs

where N , P, and σχ� stand for the nu mber of dataobs(k 1)points, the number of fitting parameters, and the standard
deviation of average χobs (k) at k = k 1 , respectively.
3 Results and Discussion
In our previous STM study [4], Pt4 clusters depositedon TiO2 (110) surface appeared to have a pseudo-squareshaped planar structure. Preliminary PTRF-XAFS
analysis [3] indicated that a Pt4 cluster on the TiO2 (110)
surface might have a regular tetrahedral structurestabilized by Pt–O bonds formed with the O ato ms at theTiO2 (110) surface. We co mpared three expectedstructures for Pt4 clusters consistent with STM results, as
shown in Fig. 1: (a) a regular tetrahedron (tetrahedron),(b) a square in which a diagonal was parallel to the [001]direction (d iamond shape), and (c) a square in which aside was parallel to the [001] direction (square). Here, Pt–Pt bond length in these model structures was fixed to thevalue estimated by curve fitting analysis (0.2645 n m).
Relative position of a Pt4 cluster to the TiO2 (110) surface
was determined as follows. Thousands of Pt4 positions

against the TiO2 (110) surface(a)were systematically calcu lated.
For each position, FEFF
calculations were performed
to evaluate the values of R.
Finally, the models with(b)minimu m R values wereselectedasthemostappropriate models for theabove three structures, as(c)depicted in Fig. 2.
The R values for threestructures in Fig. 2 are listedin Table 1. Based on the Rvalues, a regular tetrahedral Fig. 1: Model structuresof Pt4 /TiO2 (110).
structure is considered to be
the most likely structure for (a)(b)
Pt 4 clusters on the whole. It
would be possible that aregular tetrahedron (Fig. 1a)looks like a pseudo-square in
2: 2D views of
the 2D view fro m the direction Fig.
a regular tetrahedron.
as shown in Fig. 1b.
Further investigation and analysis will be required toobtain compatible structure model with the STM results.
Table 1: R values for EXAFS oscillation fits.
–E // [001] E // [110] E // [110](a) Tetrahedron
0.97
0.93
0.95(b) Diamond shape1.110.96
1.23(c) Square
1.15
1.41
1.29
Structure

Acknowledgement
Part of this study was financially supported by Grantin-Aid for Scientific Research on Innovative Areas “NanoInformatics” (grant number 25106010) from JSPS.
References[1] Y. Watanabe and N. Isomura, J. Vac. Sci. Technol. A27, 1153 (2009).
[2] K. Asakura, S. Takakusagi, H. A riga, W.-J. Chun, S.
Suzuki, Y. Ko ike, H. Uehara, K. Miyazaki, and Y.
Iwasawa, Faraday Discuss. 162, 165 (2013).
[3] Y. F. Nishimura et al., Photon Factory Activity Report2013 B, 329 (2014).
[4] N. Isomu ra, X. Wu, and Y. Watanabe, J. Chem. Phys.
131, 164707 (2009).
* ywatanabe@mosk.tytlabs.co.jp