Photon Factory Activity Report 2005 #23 Part B (2006)
High Pressure Science

10B, NW2A/2005G190

Time-resolved EXAFS measurements in Forming of Ag Nanoparticles
in Water-in-scCO2 Microemulsions
Masafumi HARADA*1, Yasuhiro INADA2, and Masaharu NOMURA2
Department of Clothing Environmental Science, Faculty of Human Life and Environment,
Nara Women’s University, Nara 630-8506, Japan
2
KEK-PF, Tsukuba 305-0801, Japan

1Introduction
Supercritical fluids provide several processing
advantages over conventional solvents in materials
chemistry and especially in nanocrystal synthesis [1].
Water-in-scCO2 microemulsions have been applied as"microreactors" for the synthesis of silver (Ag)nanoparticles [2, 3]. In this study, the time-resolved insitu DXAFS measurements have been carried out toinvestigate the reduction process in ms-order time scale+of Ag ions to Ag nanoparticles by 1) the photo-reductionof AgClO4 using UV-light and 2) the chemical reduction
using a reducing reagent, hydrazine monohydrate(H2NNH2•H2O), in the presence of Aerosol-OT (AOT) inwater-in-scCO2 microemulsions.
ExperimentalAg 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 DXAFS measurements and the othertwo were quartz windows for the irradiation of UV-lightfrom a 500W high-pressure Hg lamp. Water-in-scCO2
microemulsions containing AgClO4 were prepared byadding CO2 into the cell which contained a mixture of-3
AOT and AgClO4 (3.16 × 10 mol) aqueous solution. The
water-to-surfactant molar ratios (w) was equal to 2.3, and+water content and metal concentration [Ag ] were 3.87
wt% and 226 mM, respectively. The cell was then kept at35°C and 25MPa for 2-3 hours with continuous stirring toform microemulsions. In the case of the photo-reduction,
the microemulsions were irradiated with UV-light for the+designated time to achieve the reduction of Ag . In thecase of the chemical reduction, on the other hand, thesmall amount of hydrazine solution was injected into thecell under high-pressure conditions by a high-pressuresyringe pump.
The time-resolved in-situ DXAFS measurements werecarried out at NW2A beamline in PF-AR using an Si(311)bent crystal (Laue-type) and a self-scanning photodiodearray coupled by a Gd2O2S(Tb)-embrocated fiber opticalplate. DXAFS spectra were recorded with the acquisition
time of 60 ms. DXAFS spectra for Ag-K edge werecollected to evaluate the electronic state as well as thecoordination number of Ag nanoparticles in the presenceof AOT in water-in-scCO2 microemulsions.

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Users' Report

Fig. 1. Time-resolved DXAFS spectra at the Ag-K edge+during the reduction process of Ag ionic solutions after theinjection of reducing agent in the presence of AOT in waterin-scCO2 microemulsions.

Results and Discussion+Figure 1 shows the time evolution of µ(E) vs E for Ag
ions in the water-in-scCO2 microemulsions after theinjection of the hydrazine aqueous solution. Before theinjection, the spectrum is quite similar to that of thereactant (AgClO4 aqueous solution, not shown here),+resulting in the Ag ions dissolved homogeneously in the+water-in-scCO2 microemulsions. The reduction of Ag to00
Ag and the association of Ag into Ag nanoparticles
proceed concurrently with the reduction time increasing.
After the reduction of 120 min, the XAFS spectrum at theAg-K edge energy region is the same as that of Ag foil.
+This means the reduction of Ag ions completed within 23 hours in continuous stirring by the addition of hydrazine.
On the other hand, in the photo-reduction of the above+
microemulsions containing Ag ions, the reduction does
not occur remarkably, so that the XAFS spectrum before
the reduction does not change even after the irradiation ofUV-light more than 2 hours. The precise in-situ DXAFS
experiments will be carried out in near future.
References[1] J. A. Darr, M. Poliakoff, Chem. Rev. 99, 495 (1999).
[2] N. Kometani et al., Chem. Lett. 682 (2000).
[3] J. Liu et al., Chem. Eur. J. 11, 1854 (2005).
* harada@cc.nara-wu.ac.jp