Synthesis study of size-controlled noble metal nanoparticle systems of Re, Ir, and Os using AOT-reverse microemulsion based methodologies

Abstract

This thesis presents a novel methodology for synthesizing stable, very small nanoparticles (NPs) of rhenium (Re), iridium (Ir), and osmium (Os) using a reverse microemulsion (AOT-ME) system, with AOT as the negatively charged surfactant to form and stabilize the MEs. These noble metals are among the least studied in their NP form, making this research a valuable contribution to the field. An important initial finding was the identification of a sulfite ion impurity in commercially sourced AOT, most likely introduced during industrial preparation. The reason for the apparent ability of AOT to reduce certain metals, such as Re, was previously unknown among researchers. Although this impurity is present in low concentrations, it became significant when larger quantities of AOT were used to stabilize ME systems, acting as an unintended/hidden reducing agent. While purification with activated carbon could reduce the impurity, it came at a considerable cost of the AOT reagent. Using the AOT-ME system, Re, Ir, and Os NPs were successfully synthesized under ambient conditions with precise control over size and size distribution. Systematic investigation of synthesis parameters—including the water-to-surfactant concentration ratio (W factor) and temperature—revealed that these factors had a significant impact on NP size and stability. Across all three metals, the effect of the W factor was consistent, with higher W values generating larger, more polydisperse particles. While the AOT-ME system effectively maintained colloidal stability and controlled particle size for all three metals, it also played a specific role in slowing the tendency of Re NPs to oxidise into perrhenate. A range of advanced characterization techniques, including UV-Vis spectroscopy, ESI-MS, FE-SEM/EDX, and HR-TEM, were employed to thoroughly analyse the synthesized NPs. UV-Vis confirmed the successful reduction of the metal precursors, while FE-SEM/EDX provided valuable information on surface morphology and elemental composition. HR-TEM offered detailed insights into the size and crystallinity of the NPs. Additionally, ESI-MS helped investigate the chemical stability of the colloidal aqueous solutions. This thesis contributes to the field of noble metal NP synthesis by offering a reproducible method for controlling NP size and stability within AOT-ME systems. The findings provide valuable insights for optimizing synthesis protocols and broaden the potential applications of Re, Ir, and Os NPs in catalysis and materials science.