Mesoporous Metals

CORNELL UNIVERSITY researchers have developed a synthesis for well-ordered metallic materials that feature pores in the previously unattainable tens-of-nanometers size range (Science 2008, 320, 1748). Known as "mesoporous" metals, these kinds of materials can mediate a substantial flow of molecules through their large pores and may be useful as fuel-cell electrodes or in other applications such as catalysis and photonics.

The variety and number of porous materials, including zeolite-type compounds and metal-organic frameworks, have grown substantially in recent years due to advances in synthesis methods. Yet those procedures, which often involve organic compounds as structure-directing agents, have not made it easy to prepare ordered metals with pores larger than roughly 2 nm.

"The challenge with metals is that their high surface energies cause the particles to cluster," explains Ulrich Wiesner, the Cornell material scientist who led the team. This tendency to aggregate makes it difficult to coax metal particles into lining up in an orderly fashion, which is a critical step in forming ordered materials.

Rather than following what Wiesner calls "the traditional 'heat it and beat it' approach" to structuring metals, Wiesner, Scott C. Warren, and their coworkers prepared their materials through self-assembly of block copolymers and stabilized platinum nanoparticles.

Specifically, the team used ionic-liquid ligands (methylammonium chloride compounds) to render the 1.8-nm platinum particles soluble and to prevent their agglomeration. When combined with isoprene-methacrylate block copolymers, the metal particles self-assemble to form ordered metal-organic hybrid structures. Finally, by applying various heat treatments and etching procedures, the researchers removed the organic components, leaving a metallic platinum material with an orderly array of pores measuring 10–20 nm.

Describing the work as "a significant advance," Edward J. Kramer, a professor of materials and chemical engineering at the University of California, Santa Barbara, remarks that the novel material may lead to a host of applications. He notes, for example, that in addition to potential use in battery and fuel-cell electrodes, the new material may also be useful for chemical separations and fabrication of mesoporous crystals for catalysis.