Page 255 - Adsorbents fundamentals and applications

240 CARBON NANOTUBES, PILLARED CLAYS, AND POLYMERIC RESINS

while Mo was gradually converted into Mo 2 C (Alvarez et al., 2001). The growth
ceased when all Mo converted to carbide and all Co was reduced. The small
fraction of the reduced Co was apparently in the form of highly dispersed clusters
(of the size of 1 nm), which were responsible for the most active SWNT growth
period (i.e., at the beginning). In another work, Cassell et al. (1999) added Mo to
Fe and showed a significantly increased yield of SWNTs by the addition of Mo.
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High yields of SWNTs were obtained from decomposition of CH 4 at 900 Con
these bimetallic catalysts. The optimal catalyst composition was: Fe/Mo = 1/0.17
supported on 1/1 ratio of mixed SiO 2 and Al 2 O 3 . Cassell et al. (1999) attributed
the effect of Mo as a promoter for aromatization of methane, which in turn forms
aromatic intermediates that facilitated the growth of nanotubes. However, from
the discussion above, it is likely that Mo played a similar role in aiding dispersion
of metallic Fe in the Fe/Mo system. TEM images of the SWNTs grown by Cassell
et al. (1999) are shown in Figure 9.8. Using the catalytic route, SWNT strands
as long as 20 cm have been grown (Zhu et al., 2002).
Aligned nanotubes (in the form of a carpet) are of interest for applications.
This has been accomplished by a number of groups by using vapor phase catalysts
such as ferrocene, Fe(C 5 H 5 ) 2 . In a recent example, Kamalakaran et al. (2000)
prepared arrays of large (100–250 nm diameter) aligned MWNTs by pyrolyzing
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a spray solution of ferrocene and benzene in Ar at 850 C.
The purification of nanotubes has been studied. It is relatively easier to purify
the catalytically grown nanotubes than that from graphite vaporization. The metal
catalysts and the inorganic support can be removed with HCl and HF solution.
The transition metals can be dissolved in HNO 3 solution. The amorphous car-
bon can be removed with HNO 3 solution, and also by permanganate solution.
Oxidation with air at mild temperatures is also effective in removing the amor-
phous carbon.
A closely related type of material to MWNT is graphite nanofibers (GNF),
developed by Baker and Rodriguez (Rodriguez et al., 1995). GNF are prepared by
catalytic decomposition of hydrocarbons on metals or metal alloys at temperatures

20nm

10nm

Figure 9.8. TEM images of SWNTs grown from methane decomposition on Fe-Ru/Al 2 O 3
◦
catalyst at 900 C. Also shown is a typical image of the tip of a SWNT capped with a metal
particle. SWNTs grown on Fe-Mo/Al 2 O 3 are similar (from Cassell et al., 1999, with permission).