Morphology, stresses, and surface reactivity of nanoporous gold synthesized from nanostructured precursor alloys

Nanoporous metallic materials (NMMs) are generally synthesized using dealloying, whereby the more reactive component is dissolved from a homogeneous alloy in a suitable electrolyte, and the more noble metal atoms simultaneously diffuse into 3-D clusters, forming a bi-continuous network of interconnected ligaments. Nanoporous gold (NPG) in particular is a well-known NMM; it is inert, bio-compatible, and capable of developing large surface areas with 1--100nm pores. While several studies have demonstrated its potential usefulness in fuel cell and sensing devices, its structural, mechanical, and electrocatalytic properties still require further investigation, particularly if NPG is synthesized from precursor alloy films exhibiting metastable nanostructures.;In this dissertation, the electrodeposition (ECD) process, microstrucural characteristics, and metatstability of Au-Ni precursor alloys are investigated. The stresses evolved during Au-Ni alloy nucleation and growth are investigated in situ and correlated with microstructural and electrochemical data in order to identify the various stress-inducing mechanisms. In situ stresses generated during Au-Ni and Au-Ag dealloying were investigated, and additionally correlated with the growth stresses. Finally, the surface area and electrocatalytic properties of NPG are characterized using a variety of electrochemical techniques.;Potentiostatically electrodeposited Au1-x-Nix (x: 0--90at%) films form a continuous series of metastable solid solutions and exhibit a nanocrystalline morphology, with ∼10--20 nm grains, the size of which decreases with increasing Ni content. The formation of a metastable structure was interpreted in terms of the limited surface diffusivities of adatoms at the growing interface and atomic volume differences (∼15%). Internal stresses generated during ECD of Ni-rich films can be explained assuming a 3-D Volmer-Weber growth mode, where the stress is initially compressive, then transitions into tension, and finally remains tensile over longer times. An increase in Ni content overall resulted in (i) an increase in maximum compressive stress, (ii) a decrease in compressive-to-tensile transition thickness, and (iii) an increase in steady-state tensile stress.;The stress profile recorded in situ during Au-Ni and Au-Ag dealloying exhibits an initial rise in tensile stress followed by a steady-state compressive stress over longer times. The former is due to void formation, while the latter is indicative of a stress relaxation mechanism, which may occur either via cracking, a consequence tensile stress build-up from the dissolution of the more reactive alloy component, and/or Au atom clustering, which reduces the curvature of Au ligaments and hence coarsens the overall NPG surface. Up to 55% and 71% stress relaxation were measured for dealloyed Au-Ni and Au-Ag, respectively. Au oxidation can additionally inhibit tensile stress relaxation by kinetically hindering Au atom diffusivity, further contributing to a structurally unstable NPG film.;The surface area (SA) of NPG films was quantified using Cu underpotential deposition (UPD), Au oxidation, electrochemical impedance spectroscopy (EIS), and Ag UPD. While the SA was found to increase linearly with NPG thickness, the EIS-based values were larger than those determined from Cu UPD and Au oxidation by ∼60%. This discrepancy was ascribed partly to the presence of residual Ag atoms on the NPG surface, which act to inhibit Cu reduction and Au oxidation. The EIS probes the entire NPG surface irrespective of surface chemistry, as supported by the negligible change in SA when NPG was additionally modified with UPD Ag.;An electrocatalysis study revealed a de-polarization in the ORR, and therefore an enhancement in the electrocatalytic activity of NPG relative to planar Au. This is consistent with NPG's activity towards the reduction of a H2O2 intermediate, whereas planar Au only partially reduces O2 to H2O2. Modification of NPG with Pt sub-monolayers resulted in an additional depolarization effect with respect to planar Pt.