Electrochemical Design of Nanoporous Gold with Tunable Porosity Using Lead Underpotential Deposition

Abstract

Nanoporous gold (NPG) is a metamaterial with unique physicochemical, mechanical, and plasmonic properties of interest for various applications in catalysis and electrocatalysis, energy conversion and storage, (bio)sensing, actuation, and surface-enhanced Raman scattering.1-3 A monolithic structure with the nanoscales size of ligaments and rich surface electrochemistry3 make the NPG materials promising for many energy applications such highly active electrocatalysts for energy conversion, and three-dimensional substrates for energy storage in supercapacitors and lithium ion batteries. The length scale of NPG porosity (ligaments & open pores) and its effect on the properties is an essential aspect for such a broad range of applications. The size of porosity can range from a few nanometres to several microns and can be controlled by parent alloy composition, the method of dealloying (chemical, electrochemical) as well as the temperature of dealloying and annealing. Using the electrochemical surface processes to change and fine-tune size, uniformity and hierarchy of the ligament network has been attracting attention as a possible tool for manipulation and design of new and improved NPG functionalities 4, 5 Here we present an all-electrochemical approach to creating NPG thin films with different size of porosity from AgxAu1-x alloys. AgxAu1-x alloys of various compositions were electrodeposited at the constant potential from a thiosulfate-based solution by controlling the potential and concentration of Ag+ and Au+ ions in solution6. Following the structural (SEM, AFM) and composition (XPS) characterisation of the electrodeposited alloys, the selective dissolution of Ag under electrochemical control was used to create NPG with different size of porosity (5 - 15 nm). A process of Pb monolayer underpotential deposition (UPD) has been shown to be an excellent analytical tool to measure the surface area of as created NPG structures7. In this work, we demonstrate that this UPD process featuring surface stress driven surface allying can also be used as an electrochemical tool to shape and change NPG structure. The in-situ STM and electrochemical studies on Au(111) have shown that Pb ML formation is accompanied by significant Au surface restructuring. The top-surface atoms get displaced by alloying at low Pb ML coverage, followed by dealloying at high Pb-coverage creating a highly mobile Au ad-island and vacancy islands that with repeated cycling can lead to a significant surface morphology changes8. We observed that repeated potential cycling of Pb UPD on NPG resulted in ‘coarsening’ of the nanoporous structure with the effect similar to a so called ‘electrochemical annealing’. Combining the electrochemical and structural characterisation were followed the evolution of porosity and showed that changes of the surface area and ligament/pore size could be controlled by the range of the potentials, scan rate and a number of cycles applied.