Sol-gel processes involve dispersing solid nanoparticles in a liquid (the “sol”) where they then agglomerate together to form a continuous 3D network that extends throughout the liquid. The resulting mixture is the “gel”. These processes, which often use silica precursors, have the advantage of producing materials whose structures can be controlled with high precision. However, the problem is that the materials produced are typically insulators, which make them useless for high-current-density applications.

Now, Ulrich Wiesner of Cornell University in the US together with colleagues at the EPFL in Lausanne, Switzerland, and Dalhousie University in Canada have developed a simple, yet highly versatile silica sol-gel process built around a multifunctional sol-gel precursor made from amino acids, hyroxy acids or peptides, a silicon alkoxide and a metal acetate. The process produces nanostructured silica/metal/carbon composites with electrical conductivities as high as those of pure metals.

All of the precursors are commercially available, explains Wiesner, and because there are 20 natural (and even more non-natural) amino acids, the chemistry is extremely versatile. “Moreover, we have shown that (almost) every metal of the periodic table can be used, as its acetate,” he told “Finally, since different amino acids have different chemical properties (hydrophilic versus hydrophobic, for example), the approach is compatible with a number of very important self-assembly processes, including block copolymer and colloidal crystal self-assembly, thereby allowing hierarchically structured material to be synthesized.”

High conductivities

By mixing in various amounts of metal precursors, the conductivities of the resulting materials can be tailored to be as high as more than 1000 S/cm. The materials are also porous, which makes them interesting for catalysis applications. They resist elevated temperatures too, so allowing for high-temperature applications. And the fact that the materials are actually four-phase nanocomposites (silica/metal/carbon/pore) provides additional opportunities for tailoring their properties because one or more of the different phases can be retained or removed, adds Wiesner.

“We foresee applications in high-current-density devices, including fuel cells or batteries, where the porous metal percolation networks may prove to be very powerful,” he said. “Simple catalysis or bio-catalysis applications (remember the use of amino acids or peptides) could also benefit.”

The team is now working with other research groups around the world to test these new materials and develop applications.

The sol-gel process is detailed in Nature Materials.