Portland, Ore. - Researchers at the Weizmann Institute of Science (Rehovot, Israel) have demonstrated composite nanotubes of gold, silver, palladium and copper that were created at room temperature-a claimed first-in a three-step process. The composite structures support the scientists' claim that their process enables application-specific formulations exhibiting electrical and optical properties tuned for use in sensors, catalysts and chemistry-on-a-chip devices.
"We've discovered a not-previously-seen process of spontaneous coalescence of nanoparticles to form multilayered, interconnected, rigid nanoparticle nanotubes," said professor Israel Rubinstein at the Weizmann Institute of Science. Rubinstein was assisted in the work by staff scientist Alexander Vaskevich, postdoctoral associate Michal Lahav and doctoral student Tali Sehayek at the institute's department of materials and interfaces.
Carbon nanotubes, discovered in 1991, can have both semiconducting and metallic properties and are 100 times stronger than steel. Single-wall nanotubes, essentially built of an atomically thin, rolled sheet of graphite, are extraordinary conductors of heat and electricity.
In contrast, Rubinstein's multiwall nanotubes are formed by choosing a set of nanoparticle building blocks-semiconducting, metallic or polymeric-for a specific application and then coaxing the particles into forming composite tubes. The structures are not as strong as single-wall tubes; rather, their value is in their ability to directly sense, emit or perform novel optical or electrical functions.
Rubinstein's three-step process begins with an aluminum oxide template with nanoscale pores. The template is treated chemically so that it attracts nanoparticles of the types chosen for the specific application. In initial tests, the template was treated to attract gold, silver, palladium and copper particles.
In step two, the researchers poured a solution of 14-nanometer-diameter particles through the porous template. The particles spontaneously bound themselves to the aluminum oxide pores and to one another, thereby creating hollow, multiwalled, composite nanotubes within the template pores.
In step three, the aluminum oxide template was dissolved, leaving behind just the nanotubes.
"We expected that passing a nanoparticle solution through the pores of a specially treated aluminum oxide membrane would result in covering of the inner walls with a single layer of nonconnected nanoparticles," said Rubinstein. "We did not expect them to bind to each other, creating nanotubes."
The group's initial undertaking had been to study the passage of biological molecules through a nanoporous membrane, but they quickly switched to nanotubes once they discovered the room-temperature process for making them. According to Rubinstein, the approach upends the traditional method of using high-temperature annealing to create nanotubes; indeed, the conventional process actually prevents the spontaneous formation of nanotubes in template pores.
"This exceptional process, of spontaneous room-temperature binding of nanoparticles to form tubes, is not yet fully understood," said Rubinstein. "But we now think that everything interesting happens at room temperature."
Rubinstein's group so far has been able to create various metal and composite nanotubes, including gold, silver, gold/palladium and copper-coated gold tubes. Rubinstein said the nanotubes exhibit novel optical and electrical properties as well as a high surface area-useful for sensors, catalysts and microfluidic labs-on-chip.
The research was supported by the Clore Center for Biological Physics, Philip M. Klutznick Fund for Research, Fritz Haber Center for Physical Chemistry, Minerva Stiftung Gesellschaft fur die Forschung mbH and Angel Faivovich Foundation for Ecological Studies.