Slowing Down Kinetics in Microemulsions for Nanosegregation Control: A Simulation Study

The synthesis of bimetallic nanoparticles in microemulsions has been studied by simulation. The aim of this study was to understand the principles underlying kinetics in colloidal systems, which allowed us to further investigate the interaction between the intermicellar exchange rate and chemical reduction and ultimately to discover how these nanostructures may become. We present here results for three couples of metals with different reduction rates, the reduction rate of the faster metal being 10 times faster than the rate of the slower one in all cases. Under these conditions, the impact of slowing down chemical reductions on the final nanoarrangement can be studied. Our results show that a bimetallic nanostructure can be tuned by changing the microemulsion composition only when both metal reduction rates are quite fast in relation to the intermicellar exchange rate. If this is not the case, the compartmentalization of the reaction medium does not play any role because of chemically controlled kinetics, and the result is a fixed bimetallic arrangement that depends only on the two reduction rates. Furthermore, the general belief that the difference in reduction potentials determines the metal segregation in a bimetallic nanoparticle should be redefined because metal segregation depends also on how fast the reductions take place in relation to the intermicellar exchange rate. Last, it should be pointed out that the reduction rate of a given metal is actively affected by the reduction rate of the other metal in the couple. Thus, Pt reduction is faster in the presence of Rh than Au, and this acceleration is more pronounced for high concentrations and rigid films.