DOSE-RATE EFFECTS ON RADIOLYTIC SYNTHESIS OF GOLD-SILVER BIMETALLIC CLUSTERS IN SOLUTION

Radiolysis of mixed Au-III/Ag-I solutions at different dose rates is e xamined. The progressive evolution with dose of the UV-visible absorpt ion spectra of radiation-induced metal clusters is discussed and compa red with those calculated by Mie theory. The clusters have been also o bserved by transmission electron microscopy and analyzed by X-ray micr oanalysis and diffraction. At low dose rate, reduced silver atoms tran sfer an electron to gold ions (either free or at the surface of aggreg ates). Then, when Au-III ions are totally reduced, reduction of the si lver ions occurs in a second step at the surface of gold clusters, and silver-seated Sold aggregates are obtained. At high dose rate, the sh ape of the absorption spectrum does not change with an increase in the absorbed dose and X-ray microdiffraction confirms that bimetallic all oyed Ag/Au clusters are synthesized. These results imply the preponder ant influence of kinetics in the competition between the reduction-coa lescence processes and intermetallic electron transfer. The segregatio n or the alloying of the metals is controlled by the reduction rate; a fast total reduction of both types of metal ions prevents the redox e quilibrium through electron transfer from being established. A perfect ly ordered nanocrystal, as observed by electron microdiffraction, also implies an intimate association of metal atoms from the early steps o f reduction and aggregation.