Investigation of AOT-based microemulsions for the controlled synthesis of MoSx nanoparticles : an electron microscopy study

Molybdenum sulfide nanoparticles have been synthesized using microreactors made of Bis(2-ethylhexyl)sulfosuccinate (AOT)/water/n-heptane microemulsions. The reverse microemulsion phase was characterized by transmission electron microscopy (TEM) after freeze fracture (FF) and replication. Freezing was achieved using a high pressure cooling device. It was found to be very respectful of the sample structure, and allowed the study of various parameters such as the water-to-surfactant molar ratio and the ionic strength of the water cores. The FF mean diameters were compared to and found larger than results obtained by dynamic light scattering, which suggests that a more stable aggregated state of the micelles has been reached during cooling. It was found that the addition of NP-5 as a nonionic cosurfactant at a small concentration compared to that of AOT, resulted in a significant decrease of the molybdenum sulfide particle size. The FF study led us to the conclusion that this decrease could be attributed not only to a higher fluidity of the microemulsion interface, but also to a significantly higher mean curvature of the interfaces of the micelles. MoSx nanoparticles have been studied by EDS and direct TEM imaging. They have a mean size of 8 nm in the AOT/water/n-heptane microemulsion and as observed for the micelle mean diameter, the mean size of the MoSx nanoparticles was smaller (4 nm) in the presence of the cosurfactant NP-5.