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

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.