Synthesis-on and synthesis-off modes of carbon arc operation during synthesis of carbon nanotubes

Arc discharge synthesis of single-walled carbon nanotubes (SWCNTs) remains largely uncontrollable, due to incomplete understanding of the synthetic process itself. We show that synthesis of SWCNTs by a carbon arc may not constitute a single continuous process, but may instead consist of two distinct modes. One of these, a synthesis-on mode, produces the majority of the nanomaterials. During the synthesison mode, proportionally more carbon nanotubes are collected than in another mode, a synthesis-off mode. Both synthesis-on and synthesis-off modes for a typical arc configuration, employing a hollow anode filled with a mixture of powdered metal catalyst and graphite, were characterized by using in situ electrical, imaging, and spectroscopic diagnostics, along with ex situ imaging and spectroscopy. The synthesis-on mode duration is rare compared to the total arc run-time, helping to explain the poor selectivity found in the final collected products, a known inadequacy of arc synthesis. The rarity of the synthesis on mode occurence may be due to the synthesis off mode being more favorable energetically. (C) 2017 Elsevier Ltd. All rights reserved.