After the ligand-coated nanoparticles and polymers assemble in regular patterns, the material is heated to high temperatures in the absence of air to convert the polymers to a carbon scaffold. The scaffold is then allowed to cool. Because the metal nanoparticles have a very low melting point, without the carbon scaffold they would stubbornly fuse together in an uncontrolled fashion. Using this process, the carbon scaffold can be etched away with an acid, leaving behind a structured solid metal.
The Cornell group used the new method to create a platinum structure (see illustration above) with uniform hexagonal pores, each on the order of 10 nm across--a much larger diameter than previous attempts have been able to produce. Platinum is, so far, the best available catalyst for fuel cells, and a spacious pore structure allows fuel to flow through and react over a larger surface area.
"It opens a completely novel playground because no one has been able to structure metals in bulk ways using polymers," Wiesner explained. "In principle, if you can do it with one metal you can do it with others or even mixtures of metals."
In addition to making porous materials for catalysis, the researchers said, the technique could be used to create finely structured metals on surfaces, a key to transform the field of plasmonics, which studies the interactions among metal surfaces, light, and density waves of electrons, known as plasmons. Currently, researchers are investigating the use of
|Contact: Lisa-Joy Zgorski|
National Science Foundation