Nature Asia – Carbon nanotubes are known for their remarkable mechanical and electrical properties — but can these properties be improved upon? Calculations by Yongjun Tian and colleagues from Yanshan University and Nankai University in China have now shown that three-dimensional (3D) arrays of nanotubes could have strikingly enhanced mechanical and electronic properties.
Tian and his team used a computational approach to predict the properties of the carbon-based materials that could be made if arrays of carbon nanotubes could be bonded side-by-side to form a three-dimensional polymer structure (see image). Their calculations suggest that such materials should be quite possible to make, and that they would have provide some important improvements to the nanotubes’ properties.
Of the eight structures predicted by the simulations, seven would be ‘superhard’ materials. Individual nanotubes are very stiff and strong in the axial direction, but weak in the radial direction — a factor that limits their application as structural materials. However, the superhard 3D carbon nanotube polymers are strong in both directions. “Most of the nanotube polymers combine this superhardness with good ductility, which gives them the capability of resisting large strains without fracturing,” says Tian. “They could be more resilient than diamond.”
Eight fascinating sp2- and sp3-hybridized carbon allotropes have been uncovered using a newly developed ab initio particle-swarm optimization methodology for crystal structure prediction. These crystalline allotropes can be viewed respectively as three-dimensional (3D) polymers of (4,0), (5,0), (7,0), (8,0), (9,0), (3,3), (4,4), and (6,6) carbon nanotubes, termed 3D-(n, 0) or 3D-(n, n) carbons. The ground-state energy calculations show that the carbons all have lower energies than C60 fullerene, and some are energetically more stable than the van der Waals packing configurations of their nanotube parents. Owing to their unique configurations, they have distinctive electronic properties, high Young’s moduli, high tensile strength, ultrahigh hardness, good ductility, and low density, and may be potentially applied to a variety of needs.
In addition to the mechanical enhancements, the 3D nanotube polymers also displayed some surprising electronic characteristics. Five of the polymers simulated only conduct electrons along particular parallel and isolated one- and two-dimensional channels due to the layout of the carbon–carbon double bonds within the material. “Another notable property is their low density,” notes Tian. “These porous nanotube polymers have potential uses as hydrogen-storage materials, shape-selective catalysts, molecular sieves and absorbents.”
In order to tap into the properties of these materials, the next step will be to make them. Tian’s calculations predict that the nanotube polymers should be more stable than carbon ‘buckyballs’, and more stable than their parent nanotubes in some cases. In fact, such structures have probably been created before as unintended by-products in the synthesis of other novel carbon materials, and simply never isolated. “The next step is to explore approaches to synthesize this class of 3D nanotube polymers, including high-pressure polymerization and chemical reactions,” says Tian.