 Solution

In floating-catalyst CNT growth, a controlled sulfur additive acts as a growth promoter for the catalyst, commonly iron. Sulfur modifies catalyst surface composition, carbon uptake and the strength of the catalyst-carbon interaction; it can facilitate cap formation/lift-off and sustained tube growth instead of complete catalyst encapsulation. It also changes catalyst-particle evolution and hence tube yield, diameter and wall-count distributions.

There is an optimum range: excessive sulfur can occupy active sites or form an insufficiently active sulfide-rich catalyst and reduce growth. The additive is not simply a constituent that must be incorporated into every carbon wall. Its detailed action depends on catalyst, carbon source and process conditions; a single universal melting-point explanation is insufficient.^[ref]