The role of sulfur in the CVD carbon nanotube synthesis

Sulfur and sulfur-containing compounds are widely considered as a poison for numerous technologies vital for our civilization: steam methane reforming, water gas shift reaction, and cracking/reforming of hydrocarbons. Nevertheless, in small concentrations, this poison turns out to be a remedy for such technologies as the Fischer-Tropsch process or synthesis of carbon nanomaterials. Though sulfur has been employed as a promoter for carbon nanotube (CNT) synthesis for many years, a clear understanding of the sulfur effect (essential for efficient nanotube production) is still challenging. In this review, we aim to discuss the complex role of sulfur in a chemical vapor deposition (CVD) CNT synthesis. To unwrap the complex effect of sulfur on the nanotube growth, we assess the system from different points of view: CNT structure originates from the heterogeneous catalytic process, which is subsequently defined by the very changes in the metal nanoparticle. The introduction of sulfur into the iron triad (emphasizing Fe, as the most abundant) nanoparticle is known to have a complex effect: the melting point decreases, surface tension drops due to surface segregation, and the carbon solubility is reduced. These trends manifest through competing processes on a catalyst surface resulting in a bell-shaped dependence of catalytic activity on the sulfur concentration. Though sulfur is not incorporated into the CNT structure, its presence, however, indirectly affects the yield, nanotube thickness, and even defines a transition between single-, double-, and multi-walled CNTs. Finally, we discuss the optimal sulfur concentration and highlight yet open questions to be answered in future.