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

9A, 10B/2002G249

Structural Analysis of Bimetallic Nanoparticles prepared in Sub- andSupercritical Fluids by means of EXAFS
Yoshifumi KIMURA*1, Masaki UEJI2, and Masafumi HARADA3
1International Innovation Center, Kyoto University, Kyoto 606-8501, Japan2
Department of Chemistry, Graduate School of Science,
Kyoto University, Kyoto 606-8502, Japan3
Department of Textile and Apparel Science, Faculty of Human Life and Environment,
Nara Women’s University, Nara 630-8506, Japan
Introduction
Small size particles of nanoscale have collectedmuch attention from the view points of both science andindustry due to their remarkable properties in catalytic,
magnetic, and optical fields. Among various kinds ofmetals, gold (Au) is one of the most popular species
under investigation. In this work, we will present theresults of synthesizing small size of Au nanoparticles in
the presence of Rh ions using a high-temperature andhigh-pressure reactor. Until now we have demonstrated
that the thermal reduction of mixtures of noble metal ionssuch as platinum and rhodium ions under the hightemperature and high-pressure conditions produced finenano-particles whose structure are different from those
produced under the ambient condition[1,2]. Here we will
demonstrate that this method produces a unique mixtureof fine metal particles composed of gold and rhodium[3].
.
Experimental
The colloidal dispersions of nanoparticles protectedby PVP have been synthesized by flowing the solutions(1/1 volume mixture of water and ethanol) containing
Au(III) ion or Au(III) and Rh(III) ions through the reactor
at high-temperature and high-pressure in a few seconds(25MPa, 150−200 ºC) by an HPLC pump. As sources ofmetal ions, sodium chloroauric acid(III) and rhodiumchloride(III) were used. In the mixture solutions of ions,the mole fraction ratio of Au to Rh was 1. The total ionicconcentration was 1.5 mM, and the concentration of PVP
was 1.5 g dm−3. The obtained samples were condensed
and then poured into cells for EXAFS measurements, andAu-LIII and Rh-K edges’ EXAFS spectra were collected
at room temperature in a transmission mode at BL-10B
and 9A.
Results and Discussion
In the case of the solution containing Au ion onlyparticles of nearly 50 nm were produced by our method
according to the TEM observation. On the other hand, inthe case of mixture solution of Au and Rh ions, two kindsof particles with obviously different sizes were produced.
Relatively small particles (about 2.5 nm) are seen aroundlarger particles (about 7.5 nm) gathering each other.
Figure 1 shows the Fourier transforms of Rh K-edge andAu LIII-edge of EXAFS spectra, respectively. Bothabsorption edges of the samples are almost successfully
fitted to a single atomic species using the reference foils

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(b)

(a)
200 ºC
Fitting

R/ÅR/ÅFigure 1. Fourier transforms of EXAFS oscillations of nanoparticles
produced by the reduction of both Au and Rh ionic solution with PVP at25 MPa and 200 ºC. The results of Rh K-edge and Au LIII edge areshown in (a) and (b), respectively.
Table 1. Coordination numbers from EXAFS analysis of nanoparticles
produced by the reduction of both Au and Rh ions in water and ethanol
1/1 mixture with PVP at 25 MPa and various temperatures.
SampleEdge

BondAu/Rh(1/1)

Au-LIII
Rh-KAu-Au
Rh-Rh

Coordination Number
150 ºC
200 ºC9.4 ± 2.9
8.9 ± 2.2
6.6 ± 2.3
7.3 ± 2.5

while a little gap can be seen in the peak of 2.2 and 2.7 Å
on Au LIII-edge. This result indicates that the cluster unitof each nanoparticle is composed of almost single atomic
species of Au or Rh, respectively. Table 1 shows thecoordination number (C.N.) obtained from the fitting.
From the average C.N. , the particle sizes are around 1.0
nm for the Rh-K edge, while those from Au LIII-edgearound 2.0 nm, using the truncated octahedral structure
model. Average size probed by EXAFS is known to begenerally smaller than that of particles observed in theTEM image because it is attributed to the structure of a
monocrystalline
clusterunit
which
composes
nanoparticles. From these results the smaller one isexpected to be a Rh particle and the larger one to be anAu. Preliminary analysis of the bond-lengths using highresolution TEM images also supports this assignment.
The detail analysis is now under way.
References[1] Y. Kimura et al., Colloid and Surface A.
Physicochem. Eng. Aspects, 231, 131 (2003).
[2] Y. Kimura et al., Proc. of the 14th International.
Conf. on the Properties of Water and Steam, 345 (2004).
[3] M. Ueji, et al., Chem. Lett. 34(2), 200 (2005).
*ykimura@iic.kyoto-u.ac.jp