Fig. 3.
Roles of hydrogen and oxygen. (A) SEM of vertical SWNTs grown with CH4/O2 (partial pressure of O2 = 0.8%). (B) Very low yield of SWNTs grown in CH4/H2 (partial pressure of H2 = 7.4%). These control experiments were performed on the same catalyst/substrate and total pressure. (C) A schematic illustration of the proposed role of oxygen species in hydrocarbon-based synthesis of SWNTs. Scavenging of reactive H species by oxygen shuts off (shown by the “X”) the negative H effect to SWNT growth. We suggest that this desirable oxygen effect happens at the catalyst particle site where oxygen blocks H radicals from reacting with sp2 C networks formed on the metal seed particle. (Right) Optical emission spectrum recorded under our optimum V-SWNT growth condition with CH4/H2/O2 showing clear OH emission at 308.9 nm. The measurement was carried out by using a S2000 miniature fiber-optic spectrometer (Ocean Optics, Dunedin, FL). UV-grade quartz fiber was used to guide the plasma emission to the spectrometer.