LETTER pubs.acs.org/NanoLett
Increasing the Efficiency of Single Walled Carbon Nanotube Amplification by Fe Co Catalysts Through the Optimization of CH4/H2 Partial Pressures Alvin W. Orbaek, Andrew C. Owens, and Andrew R. Barron* Richard E. Smalley Institute for Nanoscale Science and Technology, Department of Chemistry, and Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas 77005, United States
bS Supporting Information ABSTRACT: Single walled carbon nanotubes (SWNTs) seeds are grown using Fe Co nanoparticles on spin-on-glass. The relative efficiency of nucleation and amplification (versus etching) was investigated as a function of the CH4/H2 feedstock ratio and growth temperature. At 900 °C, maximum amplification is obtained with CH4/H2 ratio of 80:20 but 60:40 for nucleation. Amplification is further enhanced at 800 °C, compared with etching dominating at 1000 °C. Amplification of SWNTs is in equilibrium with etching; higher carbon feedstock pressure and decreased temperature increase the rate of amplification; the converse increases etching. KEYWORDS: Amplification, colbalt, carbon nanotube, etch, iron, nucleation
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ingle-walled carbon nanotubes (SWNTs) resemble a graphene sheet that is rolled into a tube based on a specific roll vector, or chiral angle, that determines the tube diameter and importantly the electronic properties. Nanotubes having armchair chirality are metallic in nature and can be useful in electronic applications due to the potential for ballistic electron transport along the tube axis.1 The potential of a wire with a current carrying capacity of 109 A/cm2 as compared to the best material, copper, of 106 A/cm2 but without the associated weight, thermal expansion, or thermal-induced power loss has prompted the concept of the armchair quantum wire (AQW) for the efficient energy distribution over large (potentially global) distances.2 Unfortunately, while wires or fibers of SWNTs have been successfully produced,3,4 this concept has a significant hurdle. Namely, irrespective of the synthetic routes any sample of SWNTs contains a broad range of chiralities of which a low fraction (∼8%) is of the desired conductive structure. Thus, if low cost, efficient, energy transport is to be a reality it will be necessary to prepare samples exclusively of armchair tubes. The most obvious approaches to this problem involve either type specific growth or separation of types. Although several theoretical studies have suggested that there is a potential preference for armchair growth5,6 and while there have been reports of increased percentage metallic growth,7 there is no repeatable protocol for growth of purely metallic SWNTs. In contrast, separation by type can prepare armchair enriched samples as high as 98% purity,8 and even enrich left- and righthanded nanotubes of identical chirality.9 However, these separation processes are limited to very small scales without a clear path to scale-up. We have previously suggested an alternative but r 2011 American Chemical Society
complementary approach to either specific growth or separation, and this is amplification.10 In a process related to PCR of DNA, amplification of SWNTs would involve the isolation of a small amount of metallic SWNTs (through separation), to these would be attached a pro-catalyst (metal oxide) nanoparticle, creating a SWNT-catalyst conjugate that would allow the growth of an additional length of SWNT, with the same chirality as the progenitor SWNT template.10 In pioneering the concept of SWNT amplification, we have demonstrated that with an iron catalyst the length of an individual SWNT can be dramatically extended without alteration in the diameter;10 however, the yield of amplification was
