Extreme-length carbon nanofilaments with single-walled nanotube cores grown by pyrolysis of methane or acetylene

We describe a process of catalytic hydrocarbon pyrolysis which yields a new class of carbon nanotubes/nanofilaments (NT/NFs). It is characterized by high temperature (1000–1050 °C), and higher concentrations of gaseous species normally active in coke formation. Usually coke growth quickly encapsulates a catalyst particle in such environments, which in turn blocks further NT/NF growth. In our case the NT/NF lengths can reach 3–5 mm at a virtually constant filament diameter of 50–130 nm. HRTEM studies revealed a novel filament structure consisting of a SWNT core 2–4.5 nm in diameter covered by an outer shell of disordered carbon with a soot-like structure. The growing tip of a NT/NF is conical without any visible evidence of a catalyst particle at its end. The growth rate of the observed NT/NFs reaches 5–10 μm/s, far exceeding that of common catalytically grown NTs and filaments. Variation of the catalyst metal (iron, nickel, cobalt) and in the feedstock gas (acetylene, methane) did not result in any significant changes in the NT/NFs structure. These features all point to a non-catalytic growth mechanism of NT/NFs. The process described here can be a basis for development of a technology for production of very long carbon NTs with the aspect ratio as high as several tens of thousands.