An extremely porous and lightweight material, known as an aerogel, has been created from partially unrolled multiwalled carbon nanotubes, according to researchers [J. Zhong et al., Carbon 77 (2014) 637–644, DOI: 10.1016/j.carbon.2014.05.068].
The team from Rensselaer Polytechnic Institute and Harbin Institute of Technology, Sun Yat-sen University, and Nanchang University in China synthesized aerogels from suspensions of unfurled – or ‘unzipped’ – carbon nanotubes. The outer layers of the nanotubes are exfoliated to form leaf-like structures attached to stem-like inner tubes.
“Inspired by the structure of the leaf, which is constituted by veins, midribs and laminas, we synthesized fully unzipped but partially exfoliated carbon nanotubes to mimic the leaf structure,” explains first author Jing Zhong of Harbin Institute of Technology. “[The resulting] nanocarbon aerogel is a kind of very porous bulk material.”
The combination of leaf- and stem- or vein-like structures creates a unique material. The leaf-like parts provide a large surface area and functional groups, while the inner unexfoliated stem-like carbon nanotubes retain their electrical conductivity and mechanical integrity. The aerogel can be functionalized to be highly soluble, without sacrificing the porosity or uniformity, while the density and mechanical properties can be tuned.
Nanocarbon aerogels can be formed in various ways, but here the researchers used a simple combination of oxidation to unzip the nanotubes, which were prepared by chemical vapor deposition, and freeze-drying. Applying the latter process to a dispersion of unzipped nanotubes creates an ordered structure of vertical tubes with horizontal leaf-like attachments, together forming a honeycomb-like structure.
The team was hoping to create an aerogel with both good mechanical and electrical properties, says Zhong, and the results look promising. In compression tests, the researchers report the aerogel can withstand strains as high as 80% and still recover its original shape. The aerogel also exhibits high damping capabilities over multiple cycles. Meanwhile, the honeycomb-like structure provides a pathway for electron transport, with aerogels of density 5 mg/cm3 showing conductivity of 0.005 S/cm.
“[The] material can be used as piezoresistive material with very stable performance under very long compression cycles, and it could be a very good sensor,” Zhong told Materials Today. “Compared to [previously reported] carbon nanotubes aerogels, our aerogels have much larger surface area, which is very important for catalytic-related applications.”
However, Zhong admits that the electrical conductivity of the leaf-like aerogels is not as good as the best carbon nanotube aerogels, and better control of the structure is needed.
“We are now trying to understand how to control the microstructures of these aerogels by a combination of choosing the proper nanocarbon structure and freezing method,” he says.
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