Surfactant-Templated Nanoporous Metal Films and Powders

Abstract

At sufficiently high concentrations, many surfactants can direct electrodeposited or chemically reduced metals to form nanoporous structures. The surfactants can be small molecules, block copolymers, or polymers with functional endgroups. Under some conditions, the pores exhibit uniform, tunable sizes and long-range ordering. We will present a brief overview of demonstrations of these phenomena from the literature, and then focus on our own efforts to produce and characterize various forms of nanoporous palladium, a material of interest for energy storage, hydrogen storage, gas separation and sensing, and catalysis applications. We have used liquid-cell transmission electron microscopy and small-angle neutron scattering to study the mechanisms of particle growth in the presence of surfactants. Our results suggest that Pd nanoparticles form and sinter around micelles present in the aqueous phase. Changing the size of the surfactant molecules affects the size of these micelles, and ultimately determines the pore dimensions. Careful control of surfactant self-assembly and particle growth conditions is required to obtain long-range order. However, porosity that is more disordered, but still of quite uniform size distribution, can be obtained under much more relaxed conditions, and lower surfactant concentrations. We believe that interactions between the surfactants and the metal surfaces are more influential in these cases. The flexible range of conditions under which surfactants are able to induce nanoporosity suggests that this technology could prove to be a valuable synthetic method for energy materials with high power densities, as well as chemical engineering materials with high surface reaction rates. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. The Lujan Neutron Scattering Center at LANSCE is funded by the DOE Office of Basic Energy Sciences and Los Alamos National Laboratory under DOE Contract DE-AC52-06NA25396. The work was funded in part by Sandia’s Laboratory-Directed Research and Development program.