Nanoporous Materials Produced by Electrochemical Processes
last updated 10/22/2007
Nanoporous Materials Produced by Electrochemical Processes
last updated 10/22/2007
Mechanism of Nanoporosity Formation in Dealloying
Nanotubes produced by Electrochemical Oxidation
Selected Publications
Mechanism of Nanoporosity Formation in Dealloying
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.
Dealloying is a common corrosion process during which an alloy is "parted" by the selective dissolution of the electrochemically more active element(s). This process results in the formation of a nanoporous sponge (Fig. 1)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 tailorable pore size makes it interesting as a nano-filter. Additionally, the high surface area of nanoporous gold made by dealloying Ag-Au can be chemically functionalized, making it suitable for engineering high surface area chemical activity - for example, sensor and catalysis applications1, particularly in biomaterials contexts. Nanoporous platinum2 has great potential as a catalyst. It is known that even gold nanoparticles become catalytically active when they get small enough1,3, so sufficiently small-scale nanoporous gold might be very interesting catalytically too.
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. Fig. 2 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.
Nanotubes produced by Electrochemical Oxidation
Electrochemically enhanced oxidation of a metal under some circumstances results in the formation of solid oxide nanotubes. We have used this method to make titania nanotubes.
116.R.C. Newman, S.G. Corcoran, J.D. Erlebacher, M.J. Aziz and K. Sieradzki, "Alloy Corrosion", MRS Bulletin 24 (7), 24 -28 (July 1999).
131. J.D. Erlebacher, M.J. Aziz, A. Karma, N. Dmitrov, and K. Sieradzki, "Evolution of Nanoporosity in Dealloying", Nature 410 , 450-453 (2001).
86. J.D. Erlebacher and M.J. Aziz, "Morphological Equilibration of Rippled and Dimpled Crystal Surfaces: The Role of Terrace-Width Fluctuations", Surface Science 374 , 427 -442 (1997).
96. J.D. Erlebacher and M.J. Aziz, “Surface Relaxation Mechanisms in the Morphological Equilibration of Crystal Surfaces”, Materials Research Society Symposium Proceedings 440 , 59-64 (1997).