The Dependence of CNT Aerogel Synthesis on Sulfur-driven Catalyst Nucleation Processes and a Critical Catalyst Particle Mass Concentration

Abstract

The floating catalyst chemical vapor deposition (FC-CVD) process permits macro-scale assembly of nanoscale materials, enabling continuous production of carbon nanotube (CNT) aerogels. 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. 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. 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. Furthermore, CNT aerogel formation requires a critical threshold of FexCy > 160 mg/m3, but is surprisingly independent of the initial catalyst diameter or number concentration. 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. 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.

Agglomeration Dynamics of 1D Materials: Gas‐Phase Collision Rates of Nanotubes and Nanorods

“…An application of the collision kernel for rigid 1D materials is the process of self‐assembled aerogel formation within a CNT reactor, which is of significant academic and industrial interest . Despite the large number of studies on the nanotube aerogel process, the lack of known collision rates between the nanotubes has hindered quantitative understanding of the bundle formation process, a precursor to aerogel formation.…”

“…Quantifying the range of collision kernels β ij in gas‐phase systems consisting of 1D materials gives new insights into the spontaneous formation of aerogels from 1D materials. These results are broadly applicable to all natural and anthropogenic 1D materials of similar geometries undergoing gas‐phase collisions . As such, we employ the term nanotube generally to represent 1D materials that may consist of nanorods, nanotubes, or chain‐like agglomerates with an axial length to diameter ratio > 10:1.…”

High-performance transparent conducting films of long single-walled carbon nanotubes synthesized from toluene alone

“…1(c)), since SWCNT diameter is typically proportional to the size of the catalyst nanoparticle on which the SWCNT nucleates [18,19]. It is generally acknowledged that using an appropriate amount of sulfur in CNT synthesis can increase the yield [20], nanotube diameter [21], and even nanotube length [22], by accelerating tube nucleation and by reducing carbon encapsulation of catalyst particles [23]. Nevertheless, an excess of sulfur could induce deactivation of the catalyst due to the encapsulation of small nanoparticles with a thick layer of sulfur [8].…”

Surfactant-free synthesis of copper nanoparticles and gas phase integration in CNT-composite materials

“…Cu-NPs were characterized both independently as well as part of CNT/Cu-NPs composite samples. For the composite samples, multi-wall CNTs were synthesized following a wellestablished process (see 35,36 and see Supplementary Information S2) which produces CNT ribbons ~2 cm wide, 3 -7 µm thick and up to 15 cm long; however here we have used CNT ribbons that were cut to 2 cm long samples (see Supplementary Information S2.2, S2.3). The CNT ribbons were then placed on silicon substrates and to improve adhesion they were wetted with ethanol at the CNT ribbon ends outside the deposition area and dried for 2 h. In order to produce the composite samples, CNT ribbons on silicon were placed inside the chamber and Cu-NPs were deposited from a distance of ~2 cm from the end of the capillary tube as shown in Fig.1.…”