Continuous Direct Spinning of Fibers of Single‐Walled Carbon Nanotubes with Metallic Chirality

The continuing momentum of carbon nanotube research is driven by a number of unresolved issues, particularly those associated with high-temperature synthesis, but also by the challenge of translating the exciting physical properties of the individual nanotubes into useful materials with new properties and applications. A process by which carbon nanotube fi bers can be spun directly from the chemical vapor deposition (CVD) reaction zone was fi rst reported in 2004. [ 1 ] The fi ber formed consisted of comparatively large diameter ( ≈ 7 nm) double-walled nanotubes that collapse to form stacks of graphene-like layers. [ 2 ] Also shown was the potential for exceptional axial mechanical properties. [ 3 , 4 ] The addition of sulfur in the right proportion is key to the formation of a cloud of entangled nanotubes that have suffi cient mechanical integrity to be drawn out of the reactor as a continuous fi ber. The major role of the sulfur has been seen as a “promoter” as suggested much earlier by Kiang et al. [ 5 , 6 ] In the continuous spinning process, sulfur has been identifi ed as segregating to the surface of the iron catalyst particles and the interface between the particle and the nanotube, [ 7 ] an effect recently simulated using molecular dynamics for Mo–S [ 8 ] and since confi rmed for Fe–S, [ 9 ] yet well known in the literature on cast irons. [ 10 ] The nanotubes produced by the process are exceptionally long, some 100 000 times their diameter, and their growth to this length over a few seconds at 1200 ° C seems associated with the promoting effect of added sulfur. Here we report the observation that if the sulfur is made available soon after the ferrocene is fi rst cracked to nucleate the fl oating iron catalyst particles, then it retards the growth of the particles so they nucleate single-walled nanotubes, which are then extracted as continuous ≈ 10 μ m diameter fi ber. Additionally, the fact that armchair (and thus metallic) nanotubes are dominant in the fi ber is surprising and adds further piquancy to their potential applicability. The process is described in Figure S1 (Supporting Information). We note (Supporting Information, Table S1) that the window of composition ratios for successful spinning is centred on Fe/C 0.008, the same as for successful spinning from thiophene, but the required Fe/S ratio is 0.080 for thiophene compared with 0.006 for the carbon disulfi de. Furthermore, the results provide additional input to the on-going debate as to the source of chirality preference in as-grown SWNTs [ 11–14 ] and is discussed in the context of three specifi c models: imprinting catalyst structure, [ 15 , 16 ] cap control of chirality, [ 17 , 18 ] and dislocation models of nanotube growth and reorientation. [ 18–20 ]