“To make use of the outstanding properties of carbon nanotubes, it is important to devise techniques for assembling these nanoscale materials into macroscopic objects,” Ravi Kane told nanotechweb.org. “We have developed a simple method that provides exquisite control over nanotube assembly.”
First Kane and colleagues made arrays of vertically aligned multiwalled carbon nanotubes on silica substrates by a chemical vapour deposition technique. Then they oxidized the nanotube arrays in an oxygen plasma and immersed them in a liquid. Allowing the liquid to evaporate at room temperature produced capillary forces in the gaps between the nanotubes. This induced microcracks in the nanotube array and caused some of the nanotubes to bend and collapse, creating a cellular foam structure.
The scientists used acetone, toluene, dimethylformamide, tetrahydrofuran and methanol as the liquid, and all had a similar effect. The cellular structures were stable and unaffected by annealing at 800°C for one hour or by submersion in water followed by evaporation. The scientists were also able to remove the foam structures from the substrate using hydrofluoric acid.
The team found that they could alter the structure of the foam by changing the height of the nanotubes, altering the rate of evaporation of the liquid and using patterned arrays of nanotubes. For example, cylindrical arrays of nanotubes formed “wine glass” structures, narrow nanotube stripes resulted in aligned cells and using an array of nanotubes containing circular holes made a regularly patterned foam.
“The assembly process provides a simple and rapid technique for fabricating nanocomposites,” said Kane. “The ability to control the length scale, orientation and shape of the cellular structures, and the simplicity of the assembly process, makes this an attractive system for studying pattern formation in ordered media.”
Now the scientists plan to enhance the strength and stability of the cellular structures and to measure the mechanical properties of the nanotube foams and related nanocomposites. They reported their work in the Proceedings of the National Academy of Sciences.