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 examined. The progressive evolution with dose of the UV-visible absorption spectra of radiation-induced metal clusters is discussed and compared with those calculated by Mie theory. The clusters have been also observed by transmission electron microscopy and analyzed by X-ray microanalysis and diffraction. At low dose rate, reduced silver atoms transfer an electron to gold ions (either free or at the surface of aggregates). Then, when Au-III ions are totally reduced, reduction of the silver ions occurs in a second step at the surface of gold clusters, and silver-seated Sold aggregates are obtained. At high dose rate, the shape of the absorption spectrum does not change with an increase in the absorbed dose and X-ray microdiffraction confirms that bimetallic alloyed Ag/Au clusters are synthesized. These results imply the preponderant influence of kinetics in the competition between the reduction-coalescence processes and intermetallic electron transfer. The segregation 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 equilibrium through electron transfer from being established. A perfectly ordered nanocrystal, as observed by electron microdiffraction, also implies an intimate association of metal atoms from the early steps of reduction and aggregation.