Molecular characterization of macroscopic aerogels of single-walled carbon nanotubes

Abstract

Single-walled carbon nanotubes (SWCNT) can be assembled into various macroscopic architectures, most notably continuous fibers and films, produced currently on a kilometer-per-day scale by floating catalyst chemical vapor deposition and spinning from an aerogel of CNTs. An attractive challenge is to produce continuous fibers with controlled molecular structure with respect to the diameter, chiral angle and ultimately (n,m) indices of the constituent SWCNT "molecules". This work presents an extensive Raman spectroscopy and high-resolution transmission electron microscopy study of SWCNT aerogels produced by the direct spinning method. By retaining the open structure of the SWCNT aerogel, we reveal the presence of both semiconducting and metallic SWCNTs and determine a full distribution of families of SWCNT grouped by optical transitions. The resulting distribution matches the chiral angle distribution obtained by electron microscopy and electron diffraction. The effect of SWCNT bundling on the Raman spectra, such as the G − line shape due to plasmons activated in the far-infrared and semiconductor quenching, are also discussed. By avoiding full aggregation of the aerogel and applying the methodology introduced, rapid screening of molecular features can be achieved in large samples, making this protocol a useful analysis tool for engineered SWCNT fibers and related systems.

Simultaneous improvements in conversion and properties of molecularly controlled CNT fibres

“…At lower sulfur content in the precursor mixture the spectra are characteristic of SWCNT, with the presence of radial breathing modes (RBMs) and the superposition of G + and Gcomponents of the G band from both metallic and semiconducting SWCNT. The RBMs observed under 532 nm irradiation correspond to diameters of 0.9 to 1.7 nm, although the actual diameter distribution is significantly broader [20].…”

Identification of Collapsed Carbon Nanotubes in High-Strength Fibers Spun from Compositionally Polydisperse Aerogels

“…Collecting the material in the aerogel form and depositing it directly onto TEM grids preserves the open network structure of CNTs in the material, enabling direct observation of low-density areas of the sample by conventional TEM. 20 Collapsed CNTs can be identified by their planar cross section. Figure 1a shows an example of a large number of collapsed CNTs in the sample, with obvious collapse features.…”

“…Large areas can be probed with Raman spectroscopy, for example, but it provides inconclusive evidence of CNT collapse: the general lack of radial breathing modes (RBM) 4 can be due to multiple reasons other than collapse, and even the observation of resolved transverse and longitudinal optical tangential modes (G -, G + ) in the absence of RBS may be due to round double-walled CNTs. This difficulty is compounded by the fact that the process used to synthesize fibers with evidence of collapsed CNTs, floating catalyst chemical vapor deposition (FCCVD), 19 produces inherently very polydisperse samples in terms of the CNT diameter and number of layers 20 as a consequence of a wide catalyst particle size distribution 21 . Such polydispersity means the spun aerogels are not formed exclusively of single-or multi-layer CNTs and are instead comprised of a population of CNTs with different diameter, number of layers, and different cross-sectional morphology (e.g.…”

Initial competing chemical pathways during floating catalyst chemical vapor deposition carbon nanotube growth

“…24 Sulfurous compounds have seen success as growth promoters in the formation of SWCNT films, SWCNT aerogels, and CNT fibers. [25][26][27] In the absence of additional growth promoters and precursors, multiwalled CNTS (MWCNTs) and MWCNT arrays 28 often include embedded Fe nanoparticles, 29 a problem that can be mitigated via the use of CNT templates. 30 In the case of tuning the catalyst, Fe nanoparticle catalysts derived from Fc result in longer SWCNTs 24 than pre-made Fe catalysts.…”