How Single-Walled Carbon Nanotubes are Transformed into Multiwalled Carbon Nanotubes during Heat Treatment

High-pressure carbon monoxide (HiPCO) single-walled carbon nanotubes (SWCNTs) were heat treated at high temperatures from 1700 to 3000 °C. During the heating below 2500 °C, the diameters of the SWCNTs gradually increase from ∼1.0 to >1.5 nm, and at the temperatures higher than 2500 °C, double-, triple-, multiwalled carbon nanotubes (MWCNTs) appear as a consequence of the coalescence of SWCNT bundles. It is surprising that most MWCNTs have odd number of walls, such as 3 or 5. The even–odd number effect agrees well with the mechanism of SWCNT bundle coalescence proposed by López, M. J. [ Phys. Rev. Lett. 2002, 89, 255501], in which an SWCNT that templated the layer by layer coalescence of surrounding SWCNTs is responsible for the enrichment of MWCNTs with odd number of walls. This study confirms the mechanism of SWCNT bundle coalescence, discovers an interesting odd–even number of walls effect in the consequent MWCNTs, and suggests that it is possible to obtain structure-controllable MWCNTs via SWCNT bundle coalescence.