Three-dimensional, porous scaffolds made from noble metal nanoparticles are highly desirable for use in energy storage, catalysis, piezoelectronics, and electro-optical sensors. Until now, methods for synthesizing such scaffolds from gold, palladium, and platinum have been slow and cumbersome. Now, researchers have developed a quick and simple alternative synthesis method.

Metallic aerogels combine the high surface areas of a nanoporous scaffold with the exemplary catalytic properties of certain metals. These novel materials were first described 10 years ago and offer an inert, ultralightweight, ductile, malleable, and strong scaffold that serves as an ideal substrate for chemical reactions.

Initial approaches to making metallic aerogels started with combustion synthesis, which involves igniting a pellet of an organometallic complex that then self-assembles into a nanofoam. But controlling shape and structure is difficult and involves energetic materials and costly pressure vessels. Next, scientists created hybrid oxide aerogels infiltrated with an aromatic polymer that can be smelted into metallic aerogels. Again, control over the resulting nanoscale structure is limited due to the high temperatures required to smelt the oxide, and the process only works for a few metals such as iron and nickel. More recently, researchers have assembled metal nanoparticles from the bottom up into gels which they can then dry to make aerogels. This technique gives high surface area materials, but scalability is challenging due to the quantity of nanoparticles required, and it involves organic ligands to stabilize the particles.

“We wanted a way to make noble metal aerogels faster and more directly,” says John Burpo, professor and head of the Department of Chemistry and Life Sciences at the United States Military Academy in West Point, New York, who led the work. In previous laboratory work, he and his colleagues had observed metal aerogels forming when noble metal salt solutions came into contact with common chemical reducing agents. “We were pretty excited about that, and wanted to explore it further.”

In a new study, they selected a range of noble metal salts—precursors to the noble metals gold (such as HAuCl₄ 3H₂O), palladium (such as Na₂PdCl₄), and platinum (such as K₂PtCl₄)—and reduced them with several common chemical agents including dimethylamine borane and sodium borohydride. When the metal salt solutions were combined in a one-to-one ratio with the reducing agent, they formed gels within seconds to minutes. The researchers then freeze-dried the gels to produce metallic aerogels. They also tested different salt solution concentrations to identify the threshold at which a gel forms.

“We found that certain combinations of ions and reducing agents worked better than others,” Burpo says. The aerogels achieved densities 25 to 400 times less than their bulk material counterparts, commensurate with other aerogels that “take much longer to synthesize.”  Additionally, Burpo measured the aerogels’ specific surface areas—that is, the total area exposed within the porous matrix—by determining how much nitrogen gas the structures adsorbed. Specific surface areas were 3.06, 15.43, and 20.56 m²/g for Au, Pd, and Pt aerogels. Perhaps most importantly, these measurements correlated with the material’s capacitance as estimated by electrochemical impedance spectroscopy.

“This [method] lets you do direct synthesis of a metal aerogel network from molecular precursors, instead of having to synthesize nanoparticles in advance and eliminating high temperature steps,” says Stephen Steiner, President and CEO of Aerogel Technologies, LLC, who was not involved in the study.  “It opens the door to some really powerful nanoarchitectural possibilities!”

The new technique may be fast, but it still lacks control over the shape of the resulting aerogels. To improve shape control, Burpo plans to use biopolymer gels as a “shape template” on which to carry out the same reduction technique. The work facilitates rapid production of aerogels with tunable porosity and structural geometry for use in any material application requiring high control over reactivity, conductivity, and mass transport.

Read the abstract in the Journal of Materials Research.