The change in the catalytic surface then lowers the activation energy for the nucleation of a CNT due to the decreased free surface energy of the eutectic 26 and increases carbon diffusion processes, favoring CNT growth 28. The role of S in promoting CNT growth is principally attributed to the formation of an FeS-Fe eutectic either as island-zones on the catalytic nanoparticle surface 25, 26 or the eutectic causing full surface liquefaction of the catalyst nanoparticle 28, 31. In CVD CNT carpet growth where batch reaction times are typically 15–60 minutes 23, S is not vital but the effects of S on increasing CNT diameters, numbers of walls and yield are observed in numerous studies 24, 25, 26, 27, 28, 29 and also apply in continuous aerogel spinning systems 30. Initial hypotheses included S positively influencing the growth kinetics by scavenging blocking groups around the growing edge of CNTs or S species binding to and stabilizing the growing ends of CNTs. Early studies showed that S addition both facilitated CNT synthesis at lower temperatures 21 and increased the diameter range of the CNTs 22. S promotion of CNT growth was first recognized empirically in an arc discharge system, soon after Iijima’s seminal paper giving a detailed characterization of CNTs and CNT growth 20. There are many chemical routes to gas phase synthesized CNT aerogels, in the following referred to as CNT aerogels, based around hydrocarbons, aromatic hydrocarbons or alcohols 18 but all share the common features of a catalyst precursor (typically ferrocene) and a S or elemental group 16 containing compound 19, essential for aerogel synthesis. The dramatic improvements provided by bulk CNT materials in diverse fields such as high strength and toughness composites 4, 5, 6, 7, electrically conductive 8 and thermally conductive materials 9, 10, 11, 12, and electromagnetic shielding 13 coupled with the facile synthesis route, means applications research continues to expand worldwide at an increasing rate with new groups joining the field 14, 15, 16, 17. The innovative step has led to a process for the production of bulk CNT materials which is now under pilot exploration by at least two companies 2, 3. Similar content being viewed by othersĪ continuous and scalable one-step production of a macroscopic material composed of carbon nanotubes (CNTs) only became possible by adding sulfur (S) to existing floating catalyst (FC) techniques 1. This finding provides evidence that low-cost and high throughput CNT aerogel routes may be achieved by decoupled and enhanced catalyst production and control, opening up new possibilities for large-scale CNT synthesis. The robustness of the critical catalyst mass concentration principle is proved further by producing CNTs using alternative catalyst systems Fe nanoparticles from a plasma spark generator and cobaltocene and nickelocene precursors. Furthermore, CNT aerogel formation requires a critical threshold of Fe xC y > 160 mg/m 3, but is surprisingly independent of the initial catalyst diameter or number concentration. This paper presents a paradigm shift in the understanding of the role of S in the process with new experimental studies identifying that S lowers the nucleation barrier of the catalyst nanoparticles. While aerogel formation in FC-CVD reactors requires a catalyst (typically iron, Fe) and a promotor (typically sulfur, S), their synergistic roles are not fully understood. Despite the intensive research in the field, fundamental uncertainties remain regarding how catalyst particle dynamics within the system influence the CNT aerogel formation, thus limiting effective scale-up. The floating catalyst chemical vapor deposition (FC-CVD) process permits macro-scale assembly of nanoscale materials, enabling continuous production of carbon nanotube (CNT) aerogels.
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