This is ongoing work being done in collaboration with Jonah Erlebacher (now at Johns Hopkins), Karl Sieradzki (Arizona State) and Alain Karma (Northeastern), started when Jonah was a postdoc here.
To see a really cool (but huge, about 93 megabytes) movie (.AVI format) of an atomistic simulation of the early stages of this process, click here to go to our downloadable-data page. You might want to read a bit about it first, to train your eye about what to look for.

Figure 1:  SEM micrographs of nanoporous gold made by selective dissolution of silver from Ag-Au alloys immersed in nitric acid.  (A) cross-section of dealloyed Au32%Ag68% thin film.  (B) Plan-view of dealloyed Au26%Ag74% bulk material.  The porosity is open and ligament spacings as small as 5 nm have been observed.  Measurements of the surface area are of order 2 m²/g, comparable to commercial supported catalysts.

Dealloying is a common corrosion process during which an alloy is "parted" by the selective dissolution of the electrochemically more active elements. This process results in the formation of a nanoporous sponge composed almost entirely of the more noble alloy constituent. Even though this morphology evolution problem has attracted considerable attention, the physics responsible for porosity evolution have remained a mystery. We have discovered that nanoporosity is apparently due to an intrinsic dynamical pattern formation process - pores form because the more noble atoms are chemically driven to aggregate into two-dimensional clusters via a spinodal decomposition process at the solid-electrolyte interface. The applications potential of nanoporous metals is enormous. For instance, the high surface area of nanoporous gold made by dealloying Ag-Au can be chemically tailored, making it suitable for sensor applications, particularly in biomaterials contexts. If we can make nanoporous Pt, it might be highly useful as a catalyst.

Figure 2.  Simulated nanoporous gold.  Please see our article in Nature (publication #131) on this topic.  A slightly outdated but broader review can be found in our review article in MRS Bulletin (publication #116) .  See also two early papers on morphological relaxation (without etching) using these simulation techniques (publication #86)(publication #96).

We hypothesized that the morphology is determined solely by diffusion and dissolution processes occurring solely at the metal/electrolyte interface. To test this, we developed a kinetic Monte Carlo (KMC) model to simulate Ag-Au alloy dissolution as a prototypical system exhibiting selective dissolution. Only two things can happen in this simulation: exposed Ag can dissolve, and exposed Au can diffuse on the surface. The above figure shows a simulated porous structure with 2-5 nm ligament widths. The simulations seem to be successful not only in modeling the nanoporous morphology, but also in modeling the dynamic behavior of the dissolution current vs. overpotential.  We are learning how to model this process analytically as an instability of a planar interface in a continuum model.

Last chance to see the cool but huge (93 megabytes) movie (.AVI format); click here to go to our downloadable-data page!