Development Kinetics of Bimetal Clusters in Solution Jacqueline Belloni, Mehran Mostafavi, Jamal Khatouri and Chantal de Cointet, Laboratoire de Physico-Chimie des Rayonnements Universite Paris-Sud, Orsay, France Abstract Pulse radiolysis of mixed solutions of two monovalent metal ions KAg(CN) 2 and KAu(CN) 2 has permitted to follow from nanosecond to second time-range the dynamics of the alloying process between gold and silver and of the reactions of the alloyed cluster. The pulse induces the simultaneous ion reduction into silver and gold atoms. Then, through coalescence and association with excess ions, homolog or different, these coexisting atoms give rise to transient bimetallic alloyed clusters. However, in the interval 2-20 sec the clusters are enriched in silver to the expenses of gold, the process being assigned to a slow intermetal electron transfer form gold atoms (less noble in the presence of CN - ) to silver ions adsorbed on clusters. In the presence of the electron acceptor/donor couple MV 2+ /MV + , gold atoms generated in monometallic KAu(CN) 2 solutions act as an electron relay and transfer readily electrons to MV 2+ while they are oxidized. In contrast, when silver and gold atoms are generated simultaneously, the transient bimetallic clusters that they form behave as catalytic electron acceptor towards MV + acting as a developer beyond a critical nuclearity as for pure silver clusters. The clusters grow as large alloyed particles. From the analysis of kinetics signals, it is concluded that even in low proportion the silver increases the redox potential of mixed clusters relative to pure gold clusters in the presence of cyanide. Consequences for gold-sensitization of photograpic emulsions are discussed. Introduction In the field of photographic processes, the role of gold as a catalytic sensitizer for the development of silver clusters in exposed emulsions has been demonstrated for long. The dynamics of metal ion reduction, of the growth and of the reactivity of silver clusters in solution have been studied already by the pulse radiolysis technique for different ligands. 1-4 The aim of this work is to extent this type of investigation to the couple of silver and gold ions in order to follow by time-resolved techniques for the first time the progressive building and the developability of bimetallic clusters and to examine whether the metals are intimately alloyed in the cluster or successively reduced so that the metals are segregated in a core-shell structure. 4 The system selected for the study is the couple Ag (CN) 2 - /Au(CN) 2 - where both metal cations are monovalent and are complexed with the same ligand. First, the growth of mono and bimetallic clusters formed in the radiation-induced reduction of Ag(CN) 2 - or Au(CN) 2 - together or separately will be examined. Then, the perturbation introduced by the presence of the developer on the growth dynamics of pure or alloyed clusters will be studied. Experimental All reagents were pure chemical: gold and silver salts, KAu(CN) 2 and KAg(CN) 2 were from Comptoir Lyon Alemand Louyot, 2-propanol and acetone from Prolabo, methyl-viologen chloride MVCI 2 from Aldrich. Mixed solutions of gold and silver cyanide were stable for hours without change of the UV spectrum with the specific bands of the complex Au(CN) 2 - at 232 and 242 nm. Solutions were thoroughly deaerated by flushing nitrogen gas or under vacuum and stored in the dark. Pulse radiolysis equipment (0.15-1.2 kGy per 3ns- pulse) and γ -irradiation facilities (dose-rate 10-30 kGy h -1 ) were already described. 1 The Ag I and Au I ions are reduced by hydrated electrons, and OH • and H • radicals are scavenged by 2-propanol or and replaced by (CH 3 ) 2 C • OH radicals with strong reducing properties. The developer MV +• is formed during the pulse. Results Cluster Formation in Mixed Ag(CN) 2 - /Au(CN) 2 - Solutions The mixed solutions to be irradiated contain KAg(CN) 2 and KAu(CN) 2 in the 50/50 ratio. To follow the optical absorption evolution we selected the same wavelengths as in previous studies on monometallic solutions of Ag(CN) 2 - and Au(CN) 2 - to facilitate the comparisons, that is at 400 and 520 nm, which correspond to the maximum of the surface plasmon bands of silver and gold clusters, respectively (Figure 1). The early steps of the mechanism are the Ag I and Au I reductions by solvated electrons with the rate constants 1.5 × 10 9 [5] and (9.5 ± 1.5) × 10 9 1 mol -l s -l , 6,7 respectively. IS&T's 50th Annual Conference 6 IS&T's 50th Annual Conference 84 IS&T's 50th Annual Conference Copyright 1997, IS&T