Au/Rh nanoparticles in microemulsions: Conucleation rate and microemulsion dynamics effects on nanostructure

Improving the catalytic efficiency of bimetallic nanoparticles depends on the ability to control the distribution of the metals within the final nanoparticle. This distribution is expected to be determined by the reduction and nucleation rates of the specific metal pair. Metal pairs such as Au-Rh, exhibit contrary trends, whereby the faster reducing metal (Au) nucleates slower, and the slower-reducing metal nucleates faster (Rh). A kinetic computer simulation study was carried out to investigate the structural arrangement of Au-Rh nanoparticles synthesized in microemulsions. Results indicate that Au, due to its rapid reduction, nucleates earlier and tends to accumulate in the core, while Rh is deposited in the outer layers. The findings provide mechanistic insights into the nucleation process within micelles. A systematic variation of the conucleation rate of Au/Rh pair is carried out. The results show that the slower the conucleation rate, the later the Au deposition, resulting in greater mixing of the two metals in the final structure. In addition, a detailed study is made of the impact of microemulsion dynamics on nanoparticle architecture. A slow intermicellar exchange rate delays the reduction of Au and the exchange of Au atoms between micelles. Consequently, nucleation is hindered because it takes longer for enough free Au atoms to be found within the same micelle to initiate nucleation. The influence of the intermicellar exchange rate is particularly significant when conucleation is slow. To localize Rh at the surface, the optimal conditions include flexible surfactant films and high reactant concentrations.