Jul 18, 2008
Nanoporous gold nanowire/AAO composite membrane for flow-through reactions
Nanoporous metals are of technical importance in high-performance catalysis, sensing and fuel cells, due to their large specific surface areas. In general, nanoporous metal skeletons are formed by selective dissolution (i.e. dealloying) of noble metal components from a binary alloy.
Nanoporous gold (NPG), one of the typical porous metals, has received considerable attention recently. Of particular interest is its excellent catalytic activity that still remains at a low temperature (room temperature or even lower temperatures). Moreover, it has a good thermal stability and is resistant to oxidation. The catalytic applications of NPG, however, are hindered by the diffusion limitation of reactant molecules in bulk NPG or chronic mechanical failure in thin NPG films.
In a recent article published in Nanotechnology Liu et al. present a flow-through type composite membrane that can overcome the technical issues associated with conventional NPG-based catalysts. The composite membrane, in which NPG nanowires are periodically embedded in an anodic aluminium oxide (AAO) matrix with ordered hexagonal arrays of uniform parallel nanopores, was prepared by the electrodeposition of Au-Ag alloy nanowires into an AAO membrane, followed by a selective etching of less noble Ag. Upon selective removal of Ag from the Au-Ag nanowires, a significant shrinkage in volume of metal nanowires occurred, leaving behind a gap between the outer surfaces of the NPG nanowires and the pore walls of AAO membrane (see figure).
The resulting NPG nanowire/AAO composite membrane offers several distinct advantages over conventional bulk NPG foams and thin films. First, the new composite membrane allows flow-through type catalysis fully utilizing the entire surface of NPG in a catalytic reaction, unlike conventional NPG in which the effective surface area is limited by the diffusion of reactants. Second, the unique geometric configuration of the composite membrane together with good thermal and mechanical stability of AAO support prevents NPG nanowires from mechanical failures, even under harsh reaction conditions (for example, high temperature and large flux pressures).
The morphology evolution of NPG nanowires was also investigated as a function of pore diameter of AAO membrane. The study will shed light on the formation mechanism of porous nanostructures during the dealloying process in confined geometries.
About the author
L Liu and Z Huang are postdoctoral researchers at the Max Planck Institute of Microstructure Physics (MPI-Halle), working on the template synthesis and characterization of nanomaterials. W Lee, who was a team leader at MPI-Halle, is working at the Korea Research Institute of Standards and Science (KRISS). R Scholz, who specializes in transmission electron microscopy, is a research staff member at MPI-Halle. U Gösele is a director at MPI-Halle and also adjunct professor at Martin-Luther University Halle-Wittenberg and at Duke University, North Carolina, US.