Uli Wiesner, Cornell professor of materials science and engineering, led the research team that developed a method to overcome the tendency of metals to cluster into uncontrolled structures.

After etching away the carbon material left from the use of intermediary polymers to organize the metal nanoparticles, the platinum structure features large (0.01 micrometers) hexagonal pores. The illustration depicts the completed porous platinum structure. Click to enlarge. Image courtesy of Scott Warren & Uli Wiesner, Cornell University

First, metal nanoparticles measuring about 2 nanometers (nm) or 10-20 atoms in diameter, are coated with an organic material known as a ligand. The ligands form thin jackets around the metal atoms, changing their surface chemistry. Keeping the ligand jackets thinly tailored is a key factor that permits the volume of metal in the final structure to be large enough to hold its shape when the organic materials are eventually removed.

The jacketed metal atoms are then put in a solution containing block co-polymers, a kind of nano-scaffolding material. The innovative use of the ligands allows for the metal nanoparticles to be dissolved—even at high concentrations—in such a solution. A block co-polymer is made up of two different long chains, or blocks, of molecules linked together to form a predictable pattern.

After the ligand-coated nanoparticles and polymers assemble in regular patterns, the material is pyrolized (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 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.

The platinum-carbon nanocomposite had very high electrical conductivity (400 siemens per centimeter) for an ordered mesoporous material fabricated from block copolymer self-assembly.

It opens a completely novel playground because no one has been able to structure metals in bulk ways using polymers. In principle, if you can do it with one metal you can do it with others or even mixtures of metals.

—Uli Wiesner

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 plasmons to transmit more information across metal wires in microchips and to improve optics applications, like lasers, displays, and lenses.

The research team was led by Uli Wiesner at Cornell University and included Francis DiSalvo, the J.A. Newman Professor of Chemistry and Chemical Biology, and Sol Gruner, the John L. Wetherill Professor of Physics, both at Cornell, and other undergraduate and graduate students.

The research was funded by the National Science Foundation and the Cornell Fuel Cell Institute.

Resources

• Scott C. Warren, Lauren C. Messina, Liane S. Slaughter, Marleen Kamperman, Qin Zhou, Sol M. Gruner, Francis J. DiSalvo, Ulrich Wiesner (2008) Ordered Mesoporous Materials from Metal Nanoparticle–Block Copolymer Self-Assembly, Science Vol. 320. no. 5884, pp. 1748 - 1752 DOI: 10.1126/science.1159950