Formation And Evolution Of Nanoporous Gold

Porous metals are new engineering materials, which have both functional and structural advantages over traditional metals, in that they have much lower density and higher surface area, while without sacrificing their excellent mechanical and electric properties. Therefore, they are particularly attractive to serve as base materials in research fields such as catalysis, filtration, sensors and biotechnology. In this thesis, we discuss in great detail the fabrication and processing of a new type porous metal, nanoporous gold (NPG), using a very simple yet highly controllable dealloying method.Dealloying is a type of electrochemical corrosion process where the selective dissolution of one or more active components from an alloy occurs in a corrosive environment, leaving behind a porous scaffold made of more noble species. For example, nanoporous gold with homogeneous structures and properties in macroscopic dimension can be made through selective dissolution of Ag from Au/Ag alloys simply by immersing the alloy in concentrated nitric acid. Upon silver dissolution, the inert gold atoms left behind will locally self-organize at the solid-electrolyte interface to form a three-dimensional interconnected spongy structure, with structure units (pore and ligament) tunable at nanometer scale.Under free corrosion conditions, because gold atoms diffuse very quickly in nitric acid, the typical pore and ligament sizes are of order 10 nanometers. Considering that the pore structure formation is the results of the competition between silver dissolution and gold surface diffusion, we investigated the NPG structure formation and evolution under different controlled corrosion conditions. And we found that the pore and ligament sizes of NPG could be easily tuned by varying the dealloying parameters, such as time, temperature or potential applying on the alloy, or by heat treatment in air after dealloying. In this way, NPG with pore size ranging from a few nanometers to several microns can be routinely fabricated in a controllable manner.In particular, we emphasized that in order to obtain a nanoporous metal structure with sub-10 nanometer length scale, one can choose enhancing the silver dissolution rate by for example applying an additional electro-potential on Ag/Au alloy anode; or select suppressing the gold surface diffusion rate by for example employing the dealloying in non-acidic solutions. The smallest NPG we fabricated so far has an average pore/ligament size around 3 nanometers, by anodic etching Ag/Au alloy in KNO₃.By a combination of controlled corrosion and annealing, we further investigated the possibility of making hierarchical porous structures, such as porous bi-layers and tri-layers. And we proved that the structure within each part of the sample could be adjusted easily and independently by a variation of the experimental conditions, such as corrosion time, annealing time or annealing temperature. These functional structures may find wide applications in filtration or sensing.