The formation of photolytic silver clusters in water/supercritical CO2 microemulsions

We have synthesized small silver clusters in aqueous microemulsion droplets in supercritical carbon dioxide (sc-CO2) solutions with fluorinated surfactants; ammonium carboxylate perfluoropolyether, PFPE-1, PFPE-2 and PFPE-3. Irradiation of water-in-sc-CO2 (w/c) microemulsions of PFPE-1 containing concentrated AgClO4 Solution within the water core (local concentration [AgClO4] = 5.0 M) at P = 36.3 MPa and T = 311 K with weak UV light resulted in the appearance of a broad absorption band in the lambda = 320-400 nm range with a small peak around X = 330 nm and a shoulder around lambda = 370 nm in the UV-vis absorption spectra. Irradiation of the w/c microemulsions of PFPE-2 containing [AgClO4] = 5.0 M with intense UV light brought about the development of almost the same absorption band at the early stage. Then a single broad peak bearing the absorbance as large as about 3.0 around X = 340 nm developed after prolonged irradiation. Those spectral features would demonstrate the formation of small Ag clusters, Ag-n, due to photoreduction of Ag+ ions inside of water core of the w/c microemulsions. Although the absorption spectrum gradually changed during dark storage at P = 30 MPa and T = 311 K, there was no indication of the surface plasmon band even after 20 h storage, indicating that the growth of colloidal Ag nanoparticles, Ag-c, by agglomeration of small Ag clusters does not proceed because microemulsions are separated each other in the sc-CO2. These observations would suggest that the w/c microemulsion is an excellent nanoreactor to confine the reaction field on the nanometer scale. (C) 2008 Elsevier B.V. All rights reserved.