Influence of sulfur on yield and morphology of long carbon nanotubes

The effect of a sulfur-containing activator of carbon nanotube growth on the yield and morphology of the synthesis products obtained by aerosol method of gas-phase chemical vapor deposition was studied at the temperature of 1150$^\circ$C. Various contents of the sulfur-containing activator in reaction mixtures were used, where the sulfur content was varied in the interval of 0.1–2.0 wt.%. The synthesis products were studied by electron microscopy and thermogravimetry. It was shown that the sulfur content affected both the yield and the morphology of the synthesis product obtained, as well as the content of residual catalyst. It was revealed that with a sulfur content of 0.1 to 0.5 wt.%, long unidirectional carbon nanotubes prevail in the synthesis products, and with a sulfur content of 0.5 wt.%, curved and Y-shaped nanotubes are synthesized. With further increasing of the sulfur content in the reaction mixture, the proportion of unidirectional carbon nanotubes in the products of synthesis is decreased, and “feathered nanotubes”, including spheroidal particles, are appeared. The use of 1.0 wt.% or more sulfur in the reaction mixture leads to inhibition of continuous growth of carbon nanotubes with the formation of nanosized clusters of amorphous carbon and graphite-like particles. The results obtained allow us to find the optimal reaction mixture contents for the synthesis of carbon nanotubes with a high yield. It is shown that the presence of sulfur affects the nature of the interaction of carbon and the catalyst particles, while the bond of nanotubes with iron weakens, which favorably affects their growth.