Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

12C/2003G088

XAFS study on sonochemically synthesized Au-Pd composite nanoparticles
supported on γ-Fe2O3 nanoparticle
12
Hiroaki NITANI*1, Takashi
NAKAGAWA, Masato YUYA1, Takahiro
ONO1, Shuichi EMURA,
2
3
4
1
Satoshi
SEINO,Kenji
OKITSU,
Yoshiteru
MIZUKOSHI,TakaoA.
YAMAMOTO
1
Graduate School of Engineering, Osaka Univ., 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
2TheInstitute of Scientific and Industrial Research, Osaka Univ., Ibaraki, Osaka 567-0047, Japan3Graduate
School of Engineering, Osaka Pref. Univ., 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan
4
Faculty of Engineering, Nagasaki Univ., 1-14 Bunkyo-machi, Nagasaki, Nagasaki 852-8521, Japan

Results and Discussions
A typical XANES spectrum of the sample is shown inFig. 1, together with those of reference materials, Au foil(metal) and Au2O3. It is clear that the spectrum features ofthe sample well agree with those of the metal reference.
We conclude that Au exists in the metallic state in thesample. Figure 2 shows obtained RDF of the presentsample, in which a split peak is noticed around 0.25 nm.
Considering bond-lengths, this peak must reflect twobonds, Au-Au bond and Au-Pd bond. Then the theoretical
fit was performed with the “FEFFIT” and “FEFF7” code
assuming the single scattering path. The coordination

The present sample
Au foil
Au2O 311910 11940 11970
Energy [eV]F.T. magnitude |FT|

Experimental
An aqueous solution of 50 mL containing HAuCl4,
Na2PdCl4 (Au/Pd atomic ratio = 1:1) and commercial γFe2O3 nanoparticle powder was prepared. The solutionowas irradiated at 20 C for 1 hour by the ultrasonic waveof the frequency and power of 200 kHz and 200 W,
respectively. The detailed explanation of the mechanism
for the reduction of metal ions with the ultrasonic wave is
described in the previous report [1]. Au-LIII XAFS
measurements were performed. X-ray absorption spectra
were obtained at the BL-12C beam line of KEK-PF. All
the measurements were performed in air at room
temperature in the transmission mode. Synchrotron
radiation was monochromated by using an Si(111) crystal.
Unwanted high harmonics were eliminated with afocusing Rh mirror. The intensities of incident andtransmitted X-rays were measured with ionizationchambers. The photon energy was calibrated with the firstinflection point on the edge jumps of Au foil.

numbers determined around Au atoms, NAu-Au = 10.2 and
NAu-Pd = 1.4, significantly differ from each other. Theinteratomic distance between neighboring Au atoms, RAuAu = 0.287 nm, well agrees with the Au-Au bond of bulk
Au, 0.288 nm, whilst that from Au to Pd RAu-Pd = 0.281 nm
differs from RAu-Au. These results indicate that each Au
atom is surrounded mainly by Au and a little by Pd. If thecomposite spherical nanoparticle of Au-Pd = 1:1 with 8.3nm diameter (which determined from TEM observation)
has a core-shell substructure, it is calculated to have an
Au core with 6.6-nm diameter and a Pd shell with 0.86nm thickness. Since the Au atoms at the core-shellinterface are faced to coordinate not only to Au atoms but
also to Pd atoms, the average coordination number, NAu-Au,is smaller than the value, 12, in the fcc structure.
Assuming the present size and composition of the present
bimetallic nanoparticle, it was calculated as NAu-Au = 11.2,
which reasonably agrees with NAu-Au = 10.2+2.6,
determined by the present EXAFS analysis. The results ofthese structural analyses indicated that the nanoparticles
supported on γ-Fe2O3 nanoparticles had a core-shellstructure in which a thin Pd layer envelopes an Au
particle.

Absorption coefficient [arb. unit]

Introduction
The nano-sized catalyst plays an important role in thefuel cell and many important processes such as those inthe petrochemical industry because of its higher specificsurface area, and therefore lower weight and cost. For
developing more advanced catalytic material, a composite
nanoparticle involving two or more material phases has
been studied. Au-Pd bimetallic nanoparticle supported on
γ-Fe2O3 nanoparticle is one of the most promisingmaterials for new type of catalyst which has high catalytic
activity and collectability by the use of magnet. We
examined the substructure of the Au-Pd nanoparticles by
the techniques of XAFS.

Fitting range

00.2
0.4
0.6
Distance [nm]Fig. 1. Au-LIII edge XANES Fig. 2. RDF of the sample.
spectra of the sample and The broken line shows thereference materials
range of EXAFS fitting.
References[1] Y. Mizukoshi et al., J. Phys. Chem. B 101, 7033
(1997).
* h-nitani@mit.eng.osaka-u.ac.jp

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