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

9A, 9C/2004G102

EXAFS Analysis in Forming of Metal Colloids
in Water-in-scCO2 Microemulsions
Masafumi HARADA*1 and Yoshifumi KIMURA2
Department of Textile and Apparel Science, Faculty of Human Life and Environment,
Nara Women’s University, Nara 630-8506, Japan
2
International Innovation Center, Kyoto University, Kyoto 606-8501, Japan

IntroductionThe
environmentallybenign,nontoxic,andnonflammable supercritical fluids such as water andcarbon dioxide (sc-CO2) are the two most abundant andinexpensive solvents. Water-in-scCO2 dispersions in theform of microemulsions offer new possibilities in waste
minimization for the replacement of organic solvents inseparations, reactions, and materials formation processes[1]. In this study, we have tried to synthesize cupper
nanoparticles (Cu nanoparticles) by the chemical
reduction of Cu(ClO4)2 using reducing reagent such as
NaBH4 in the presence of fluorinated surfactants in waterin-scCO2 microemulsions, and have investigated theaverage particle size of the Cu nanoparticles by means ofEXAFS measurements.
Experimental
Cu nanoparticles were synthesized in a high-pressure
SUS 316 cell (inner volume of 14.0 mL) equipped withfour optical windows: two of them were CVD diamondwindows for in-situ EXAFS measurements and the othertwo were quartz windows for the UV-Vis absorption
microemulsions
measurements.
Water-in-scCO2
containing Cu(ClO4)2 were prepared by adding sc-CO2
into the cell which contained fluorinated surfactant PFNA
aqueous solution (kindly provided by NEOS Co. Ltd.),-3Cu(ClO4)2·6H2O (1.58 × 10 mol) and NaBH4 aqueoussolution. The water-to-surfactant molar ratios (w) werew=126. The cell was then kept at 35 °C and 25MPa for
120 min with continuous stirring to form a single-phasemicroemulsions. During the stirring, the reduction of Cu
ions to form Cu(0) particles in the microemulsions
occurred.
The in-situ EXAFS measurements wereperformed at BL-9A and/or BL-9C in the irradiation with
UV light from a 500W high-pressure Hg lamp. EXAFS
spectra of Cu-K edge were collected in a transmission
mode to estimate the coordination number of Cu
nanoparticles in the water-in-scCO2 microemulsions.
Results and Discussion
2+Figure 1 shows the µ(E) vs E for Cu species in thewater-in-scCO2 microemulsions (water content in thesystem is 17.9 wt%) which are consisted of PFNA andCu(ClO4)2·6H2O aqueous solution with w=126 before
photo-reduction.
The dotted line (a) shows the
background spectrum obtained from the high pressure cellwith CVD diamond windows with sc-CO2 at 35 °C and25MPa. Diffraction spikes from CVD diamonds are notobserved in this experiment, which is different from thecase of single crystal diamonds (not shown in this report).
The dashed line (b) shows the spectrum obtained fromwater/sc-CO2/PFNA emulsions in the same cell equippedwith the CVD diamond windows. The solid line (c) is
obtained by the subtraction of spectrum (a) fromspectrum (b). The present data were obtained from themeasurement during the stirring of the emulsions,
resulting in getting a worse S/N spectrum. It is expectedthat the data quality as for S/N will become better without
the stirring of these emulsions. Based on these results, it
is suggested that the size of water pool in water-in-scCO2
microemulsions would be huge so that the EXAFSspectrum would be disturbed by the stirring. The
reduction procedureswith NaBH4 and thedetailed analysis are inprogress.

2(c)10µ(E)

1(b)
-1(a)

-2-3
8800

9000

9200

9400

9600

9800

E (eV)

Fig. 1. X-ray absorption coefficient µ(E) at the Cu-K
edge for (a) sc-CO2, (b) water/sc-CO2/PFNA
microemulsions, and (c) the subtracted one.
References[1] J. M. DeSimone and W. Tumas, Green Chemistry
using liquid and supercritical carbon dioxide, Oxford
University Press, New York, 2003.
* harada@cc.nara-wu.ac.jp

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