EXAFS and SAXS analysis in forming of metal nanoparticles in water-in-scCO2 microemulsions

Masafumi Harada 

Nara Women's University, Kita-Uoya Nishimachi, Nara 630-8506, Japan

Abstract

The environmentally benign, nontoxic, and nonflammable supercritical fluids such as water and carbon dioxide (sc-CO2) are the two most abundant and inexpensive solvents. Water-in-scCO2 dispersions in the form of microemulsions offer new possibilities in waste minimization for the replacement of organic solvents in separations, reactions, and materials formation processes. In this study, we have tried to synthesize silver (Ag) nanoparticles by the photo-reduction of AgClO4 and cupper (Cu) nanoparticles by the chemical reduction of Cu(ClO4)2 using reducing reagent such as NaBH4 in the presence of fluorinated surfactants in water-in-scCO2 microemulsions. Furthermore, we have investigated the reduction process of metal ions and the average particle size of the particles by means of in-situ UV-Vis and EXAFS as well as their aggregates in the microemulsions by means of in-situ SAXS measurements.

Water-in-scCO2 microemulsions containing AgClO4 and Cu(ClO4)2 were synthesized in a high-pressure SUS 316 cell which contained fluorinated surfactant FAR-2 or PFNA aqueous solution, as well as AgClO4 (4.3 x 10-4 mol), Cu(ClO4)2•6H2O (1.58 x 10-3 mol) and NaBH4 aqueous solution, respectively. The water-to-surfactant molar ratios (w) were ranging from w=5 to w=1000. The cell was then kept at 35 C and 25MPa for 120 min with continuous stirring to form a single-phase microemulsions. In the case of Ag, after stirring, the microemulsions were irradiated with UV light to perform the reduction of Ag ions to form Ag particles. On the other hand, in the case of Cu, during the stirring, the reduction of Cu ions to form Cu particles by NaBH4 occurred. The EXAFS and SAXS measurements were performed using BL-9A and BL-15A, respectively, at the Photon Factory, High Energy Accelerator Research Organization (KEK-PF), Japan. EXAFS spectra of Ag-K and Cu-K edge were collected to estimate the particle size of Ag and Cu during the reduction process, respectively.

Presentation: oral at E-MRS Fall Meeting 2005, Symposium I, by Masafumi Harada
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-06-23 13:01
Revised:   2009-06-07 00:44
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