The role of sulphur in the synthesis of carbon nanotubes by chemical vapour deposition at high temperatures

Sulphur has been recognised as a growth promoter for carbon fibres and carbon nanotubes for over 30 years. Moreover, the Fe-C-S system, in particular, has been extensively studied for more than half a century in the fields of steelmaking and cast iron. In the present work we examine the role of sulphur in the iron-catalysed growth of carbon nanotubes during the process of direct spinning of fibres from the gas phase. A detailed microstructural characterisation of the reaction products was conducted by high resolution TEM and EELS composition mapping on a dedicated FEG STEM (VG HB 501) equipped with C-s aberration correctors. Our results agree with previous works in classical metallurgy, indicating that sulphur forms a layer on the surface of the catalyst particles that plays a role in encouraging nanotube growth by surface diffusion.