Coalescence of bimetallic clusters

Pulse radiolytic studies of the kinetics of formation of clusters containing two different metals are more readily accessible, for the reasons given above, when both ions may be reduced by a monoelectronic process. This can be achieved with mixed solutions of the monovalent ions Ag and Au, in the form of KAg(CN)2 and KAu(CN)2. The evolution of the optical absorption spectrum with time was followed specifically at 400 and 520 nm, which correspond to the maxima of the surface plasmon bands of the monometallic silver and gold clusters, respectively. The early steps of the mechanism are rapid reductions of Ag and Au into atoms 

The products of the reactions depicted by Eqs. (20) and (21) are identical. These reactions constitute the early binding between the two metals within the same entity. Mixed species including two different metals have already been detected by pulse radiolysis, e.g. (TlAg)+ in solutions of monovalent-monovalent ionic precursors, and (CdAg) + and (CoAg) + in divalent-monovalent precursors. Further coalescence as in Eqs. (22) and (23) of the mixed species formed in Eqs. (20) and (21) yields alloyed clusters of higher nuclearity, (Ag,Au,)cN- this accounts for the increase in absorbance at 400 and 520 nm, at least up to 2 s.  

The relative absorbances at 400 and 520 nm are very different from those in pure solutions of Ag(CN)2 or Au(CN)2. These features suggest alloying between both metal atoms in each cluster, possibly as depicted in Eqs. (20)-(23). 

Beyond 2 s, the absorbance at 520 nm decreases whereas at 400 nm it continues to increase in proportion (Fig. 5). Both absorbances reach a plateau after 20 s. The plateau at 400 nm is nearly the same as for pure silver solutions, as if the reduction equivalents of gold atoms had been all transferred to silver. Therefore, even when formed soon after the pulse, the alloyed metal clusters progressively lose, during the second time period, the zero-valent gold and are enriched in silver until the gold completely disappears, and gold ions are slowly released (Eq. 24). 

The substitution of gold atoms by silver atoms in the cluster implies that the redox potential at a given nuclearity in the presence of cyanide is more positive for silver than for gold, as for the respective electrode potentials. Oxidation, after some time. 

---