Characterization and Functional Improvement of Nanoporous Metals

Dealloying of binary AgAu and ternary AgAuPt alloys has been proven to yield nanoporous metals (NPG and NPG-Pt) that are promising candidates as catalysts and smart materials. The alloy is exposed to electrolytic conditions at which it is favourable for one metal to dissolve, leaving behind a porous network of the other more noble metal(s). The applicability of such materials on a wide scale in the aforementioned fields is influenced by structural stability, and catalytic activity. Such properties are mainly affected by aspects including structural evolution, surface area stability and nanoscale composition. Achieving a deeper realization of the mechanisms that govern the nanoscale behaviour in this material at such small scale is vital for pushing forward this class of materials to industrial applications. Thus, the need for high-resolution characterization emerges, giving rise to the main goals of this research project, in which state-of-the-art characterization techniques are used to improve the already existing insights into the formation and tunability of nanoporous metals. In this thesis, atom probe tomography (APT) of as-dealloyed and thermally coarsened NPG is enabled by inner-pore potentiostatic deposition of Cu. A mechanistic view of the role of Pt during dealloying is achieved by the confirmation of Pt surface segregation during dealloying at unprecedented resolution. Surface enrichment in Pt is uniquely measured and correlated to increase in Ag retention during dealloying, and consequently correlated to increase in ligament sizes. APT analysis also confirmed the further segregation of Pt in NPG-Pt when coarsened in oxidative environments, and the desegregation of Pt in reductive environments. In situ TEM/STEM of thermal coarsening of NPG and NPG-Pt in hydrogen and oxygen has been carried out. Coarsening events such as ligament collapse, ligament coalescence and void annihilation are observed in situ for the first time. The addition of Pt to the precursor increases the thermal coarsening temperature threshold and alters the coarsening mechanism in hydrogen from ligament coalescence, to the Ostwald ripening of ligaments/ligament collapse. Also, the infiltration of NPG and NPG-Pt at significant depths serves to highlight their interesting electroactive properties, and the great prospects that lie in using NPG as a template for growth of 3D nanostructures.Ph.D