Dose Rate Effects on Radiolytic Synthesis of Gold−Silver Bimetallic Clusters in Solution

Radiolysis of mixed AuIII/AgI 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 AuIII ions are totally reduced, reduction of the silver ions occurs in a second step at the surface of gold clusters, and silver-coated gold 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.