A team of researchers from Honda Research Institute USA, Inc., in conjunction with researchers at Purdue University and the University of Louisville, has developed a method for controllably growing carbon nanotubes with metallic conductivity. With further optimization, the researchers say, “direct control over nanotube structure during growth may well be feasible.” A paper on their work appears in the 2 Oct issue of the journal Science.
Carbon nanotubes are grown on the surface of metal nanoparticles, and take the form of rolled graphene sheets. The nanotube bonding configuration is known as its chirality. The chirality determines the conductivity of the nanotube—i.e., either metallic or semiconducting. Nanotubes exhibiting metallic conductivity possess extraordinary strength compared to steel, higher electrical properties than copper, are as efficient in conducting heat as a diamond and are as light as cotton.
The ability either to separate or to synthesize either all semiconducting or all metallic tubes would open new possibilities for fully exploiting carbon nanotubes in a wide range of miniaturization and energy efficiency applications, including much more powerful and compact computers, electrodes for supercapacitors, electrical cables, batteries, solar cells, fuel cells, artificial muscles, composite material for automobiles and planes, energy storage materials and electronics for hybrid vehicles.
Though some methods exist to bias the population of one type of nanotube during synthesis, there is only a limited understanding of exactly what determines chirality during synthesis. There have been important achievements in separating single-walled carbon nanotubes (SWNTs) according to their conductivity and in enriching the distribution of nanotubes with a specific conductivity. Meanwhile, there have been a few reports regarding direct control over nanotube structure during growth. The fact that SWNTs with narrow chiral distributions have been successfully grown indicates that there may be a specific mechanism that controls chirality. The concept of amplifying existing SWNT distributions by seeding growth from another nanotube with well-defined chirality has been proposed; however, evidence for the maintenance of chirality has not yet been reported. The preferential growth of nearly 90 to 96% of semiconducting SWNTs by plasma-enhanced chemical vapor deposition has been reported, but the mechanism that leads to this selectivity remains unclear.—Harutyunyan et al.
The Honda-led team grew SWNTs from Fe nanocatalysts deposited onto a SiO2/Si support and in situ annealed in a He or Ar ambient that contained various ratios of H2 and H2O. They used methane as the carbon source at 860 °C. By varying the noble gas ambient during thermal annealing of the catalyst, and in combination with oxidative and reductive species, they altered the fraction of SWNTs with metallic conductivity from one-third of the population to a maximum of 91%. Past research efforts to control the structural formation of carbon nanotubes with metallic conductivity through conventional methodology resulted in a success rate of approximately 25 - 50%.
This is the first report that shows we can control fairly systematically whether carbon nanotubes achieve a metallic state. Further research is in progress with the ultimate goal to take complete control over grown nanotube configurations to support their real world application.
Our finding shows that the nanotube configuration which defines its conductivity depends not only on the size of the metal nanocatalyst used to nucleate the tube as was previously believed, but importantly also is based on its shape and crystallographic structure, and we learned to control it.—Dr. Avetik Harutyunyan, principal scientist from Honda Research Institute USA, and the leader of the project
Researchers at Purdue, led by Professor Eric Stach, used a transmission electron microscope to observe nanotube formation, revealing that changes in the gaseous environment can vary the shape of the metal catalyst nanoparticles from very sharp faceted to completely round. Researchers at Louisville, led by Professor Gamini Sumanasekera, produced the nanotubes in larger volumes and made careful measurements to determine whether the nanotubes achieve a metallic state.
Avetik R. Harutyunyan, Gugang Chen, Tereza M. Paronyan, Elena M. Pigos, Oleg A. Kuznetsov, Kapila Hewaparakrama, Seung Min Kim, Dmitri Zakharov, Eric A. Stach, Gamini U. Sumanaseker (2009) Preferential Growth of Single-Walled Carbon Nanotubes with Metallic Conductivity. Science Vol. 326. no. 5949, pp. 116 - 120 doi: 10.1126/science.1177599