Mechanism of Nanoporosity Formation in Dealloying
This is work that was 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 nanoporoussponge (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. 2shows 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.
Titania (TiO2) is of great interest for applications in photovoltaic devices such as solar cells4,5(Fig. 3 ), photo-catalysis6 and photo-electrochemistry7-11. For example, titania is a key component of the dye-sensitized solar cell as shown in Fig. 4 , we have fabricated rafts of TiO2 nanotubes (Fig. 5) by the electrochemical oxidation method popularized by Grimes12 in order to study processing-structure-properties relationships. Nanotube length, diameter, and properties depend on controllable details of electrochemical oxidation process. The first measurements of the electronic conductivity of films of nanotubes is shown in Fig. 6. Our collaborator Shriram Ramanathan 's group made the measurement by putting silver electrodes on both surfaces of the nanotube film and recording the current vs. applied voltage for various temperatures.
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6 A.I. Cardona, R. Candal, B. Sanchez, P. Avila, and M. Rebollar, "TiO2 on Magnesium Silicate Monolith: Effects of Different Preparation Techniques on the Photocatalytic Oxidation of Chlorinated Hydrocarbons", Energy 29 , 845 (2004).
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10 T. Bak, J. Nowotny, M. Rekas, and C.C. Sorrell, "Photo-Electrochemical Hydrogen Generation from Water Using Solar Energy. Materials-Related Aspects", International Journal of Hydrogen Energy 27 , 991 (2002).
11 J. Nowotny, C.C. Sorrell, T. Bak, and L.R. Sheppard, "Solar-Hydrogen: Unresolved Problems in Solid-State Science", Solar Energy 78 , 593 (2005).
12 G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, and C.A. Grimes, "A Review on Highly Ordered, Vertically Oriented TiO2 Nanotube Arrays: Fabrication, Material Properties, and Solar Energy Applications", Solar Energy Materials and Solar Cells 90 , 2011 (2006).