Synthesis study of size-controlled rhenium nanoparticle systems using reverse micelle-based methodologies

Abstract

In this study, rhenium nanoparticles were successfully synthesized with precise size control using a water-in-oil microemulsion method under an ambient atmosphere. Spectroscopic characterization using UV-Vis and ESI-MS techniques was used to study the stability of the rhenium(IV) precursor and verify its reduction into rhenium-based nanoparticles. The morphology and size of the rhenium nanoparticles were examined using HR-TEM. The results indicate formation of amorphous rhenium nanoparticles with an average size of 2.2 nm at a water-to-surfactant concentration ratio (W factor) of 10. These rhenium nanoparticles also have good size-stability compared to rhenium colloids generated in water alone. After one week, the particle diameter increased by only 0.6 nm with no evidence of aggregation. An investigation into the effect of various factors on the size and size distribution of the rhenium nanoparticles indicate that the W factor is the most influential parameter (among the considered factors) for size tuning, given that increases in W led to bigger and more polydisperse particles. Additionally, the microemulsion surfactant (AOT) protects the rhenium nanoparticles from their natural tendency to oxidize to perrhenate ion under an air atmosphere. However, a significant consideration in the use of AOT is the previously unreported presence of a chemical impurity which is able to reduce the rhenium(IV) precursor salt and generate rhenium nanoparticles within these microemulsion systems. Our investigation found that this impurity is the sulfite ion which is most likely introduced during manufacturing.