From the Labs: Nanotechnology

Better Catalysts
Multifaceted platinum nanoparticles may help explain catalysis and could lead to cheaper fuel cells and alternative fuels

Source: “Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity”
Na Tian et al.
Science 316: 732-735

Results: Researchers at the Georgia Institute of Technology in Atlanta and at Xiamen University in China have made platinum nanoparticles with a new 24-sided shape. The surfaces of the nanoparticles have four times as much catalytic activity as the surfaces of commercial catalysts. This is because of the greater number of unstable atoms at the particles’ edges and the odd angles of the shape’s many facets.

Why it matters: Platinum is a common component of industrial cata­lysts. It’s also used in fuel cells and in experimental methods for producing alternative fuels. But it’s expensive, so researchers are constantly looking for ways to use less of it by making catalysts more active. The new shape and the methods used to make it could also help reveal how catalysts work in general, providing hints for researchers attempting to make better catalysts from cheaper materials.

Methods: The Georgia Tech and Xiamen researchers began with platinum particles scattered on a carbon surface. They then applied an oscillating voltage to the surface, inducing alternating chemical reactions that broke down the platinum particles, releasing platinum atoms. The voltage also influences how the atoms recombine to form new particles. For example, when a positive voltage is applied, oxygen atoms can infiltrate the growing nanoparticles, dislodging platinum atoms from certain areas but not from others. This is the process the researchers exploited to create the 24-sided shape. (The same process also causes a layer of platinum oxide to form on other parts of the nanoparticles, protecting them.)

Next Steps: The new platinum nanoparticles, which are 50 to 200 nanometers in diameter, are still at least 10 times the size of the particles now used in commercial cata­lysts, so they have a larger proportion of expensive platinum locked beneath their surfaces, where it can’t catalyze reactions. As a result, though the new nanoparticles are better catalysts by area, they are, for now, worse by volume, the key parameter when it comes to cost. The researchers are now modifying their fabrication process to produce smaller nanoparticles that still have the novel shape.

Source: “Nanoimprinted Semi­transparent Metal Electrodes and Their Application in Organic Light-Emitting Diodes”
Myung-Gyu Kang and L. Jay Guo
Advanced Materials online, April 13, 2007

Results: University of Michigan researchers have made flexible grids of copper, gold, and aluminum that are almost transparent, so thin and distantly spaced are their wires. The wires are 120 or 200 nano­meters wide and separated by gaps of about 500 nanometers in one direction and 10 micrometers in the other. Used as electrodes, the grids outperformed the indium tin oxide (ITO) electrodes commonly used in displays and ­photovoltaics.

Why it matters: The grids could be particularly useful for organic light-emitting diodes (OLEDs), which make displays that are bright, efficient, and potentially flexible. OLEDs are now limited to use in small displays, such as those in mobile phones. ITO is too brittle for use in larger flexible displays. The metal grids are not brittle and have better electrical properties than ITO.

Methods: The researchers used a technique called nanoimprint lithography to achieve the precise wire width and spacing necessary for the grids, testing different wire configu­rations for their transparency and their electronic properties. They also tested a prototype OLED that used a copper-grid electrode instead of an ITO one.

Next Steps: The proportions of the wires are being optimized to help the grids compete with other potential replacements for ITO, such as films made of carbon nanotubes.