Controlling Carbon Nanotube Type in Macroscopic Fibers Synthesized by the Direct Spinning Process

Abstract

We report on the synthesis of kilometers of continuous macroscopic fibers made up of carbon nanotubes (CNT) of controlled number of layers, ranging from single-walled to multiwalled, tailored by the addition of sulfur as a catalyst promoter during chemical vapor deposition in the direct fiber spinning process. The progressive transition from single-walled through collapsed double-walled to multiwalled is clearly seen by an upshift in the 2D (G′) band and by other Raman spectra features. The increase in number of CNT layers and inner diameter results in a higher fiber macroscopic linear density and greater reaction yield (up to 9%). Through a combination of multiscale characterization techniques (X-ray photoelectron spectroscopy, organic elemental analysis, high-resolution transmission electron microscopy, thermogravimetric analysis, and synchrotron XRD) we establish the composition of the catalyst particles and position in the isothermal section of the C–Fe–S ternary diagram at 1400 °C. This helps explain the unusually low proportion of active catalyst particles in the direct spinning process (<0.1%) and the role of S in limiting C diffusion and resulting in catalyst particles not being in thermodynamic equilibrium with solid carbon, therefore producing graphitic edge growth instead of encapsulation. The increase in CNT layers is a consequence of particle coarsening and the ability of larger catalyst particles to accommodate more layers for the same composition.

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“…The low intensity ratio of D to G bands (ID/IG) around 0.15 indicates the high graphitic nature (G band) of these CNTs with respect to the amount of defects (D band) as, i.e., carbonaceous impurities with sp 3 bonding or broken sp 2 bonds. 2D band position around 2,700 cm −1 also confirms the multi-wall character of the nanotubes within the fiber (Reguero et al, 2014). On the other hand, XPS spectroscopy is a more surface sensitive technique compared to Raman spectroscopy therefore C1s core level emission ( Figure 1C) not only presents the components related to pure graphitic nanotube structure as C sp 2 at 284.5 eV (C = C) and π − π* band at 291.1 eV but also C sp 3 components at 285.0 eV (C-C, C-H) and 286.4 eV (C-O) from acetone condensation process by-products at the cathode surface which does not affect the graphitic nature of the nanotubes.…”

“…The precursors wt.% ratio was adjusted (97.7 wt.% of butanol, 0.8 wt.% of ferrocene, and 1.5 wt.% of thiophene) to synthesize carbon nanotubes with few layers (<5) with an average diameter of 5 nm (Reguero et al, 2014). These large-diameter few-layer CNTs tend to collapse into graphitic ribbons.…”

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“…The fibres were produced by direct spinning method from the gas phase during the synthesis of CNTs by CVD. [Reguero et al 2014] The reaction involves the use of butanol (97.7wt.%) as carbon source, ferrocene (0.8wt.%) as catalyst and tiophene (1.5wt.%) as promoter. The process was carried out in a vertical CVD reactor at 1200ºC in hydrogen atmosphere with a precursor feed rate of 5mL/h.…”

“…In this Chapter, CNT fibre is used as a tough scaffold material that provides high surface area (256m 2 /g) for the inorganic deposition and high electrical conductivity MWCNTs are obtained with a small diameter and very large length. [Reguero et al 2014] Figure 6.1 shows optical and electron microscope images of the CNT fibre material exhibiting its hierarchical structure. The CNT fibre consists in a continuous macroscopic fibre easily transferable to any substrate to form planar electrodes as it can be seen in Figure 6.1a.…”

“…Finally, the CNTf spectra shows a wide band between 288-292eV that is assigned to O-C=O species and importantly, π-π* interactions between the CNT bundles. [Reguero et al 2014] Figure 6.12: C1s region of XPS spectra for a) 20nm_TiO2/CNTf hybrid, b) pristine CNTf and c) pure TiO2 sample.…”