On the Structure of Bimetallic Nanoparticles Synthesized in Microemulsions

Monte Carlo simulations were carried out to explain experimental results concerning the different structures showed by bimetallic nanoparticles synthesized in microemulsions. It is observed that the difference in reduction rates of both metals is not the only parameter to determine the metal segregation; the interdroplet channel size plays a relevant role. The reduction rate difference determines nanoparticle structure only in two extreme cases: when both reactions take place at the same rate, a nanoalloy structure is always obtained. On the contrary, if both reactions have very different rates, the nanoparticle shows a core−shell structure. However, in the large interval between both extreme cases, the nanoparticle structure is strongly dependent on the intermicellar exchange, which is mainly determined by the flexibility of the surfactant film around the microemulsion droplets. In agreement with experimental results, we demonstrate that just by changing the film flexibility a given bimetallic nanoparticle can be synthesized as a nanoalloy (using a flexible film) or as a core−shell (using a rigid one). This result is very promising for the preparation of bimetallic nanoparticles with a given structure.