Thermal Conversion of Bundled Carbon Nanotubes into Graphitic Ribbons

High temperature heat treatment (HTT) of bundled single-walled carbon nanotubes (SWNTs) in vacuum (∼10-5 Torr) has been found to lead to the formation of two types of graphitic nanoribbons (GNRs), as observed by high-resolution transmission electron microscopy. Purified SWNT bundles were first found to follow two evolutionary steps, as reported previously, that is, tube coalescence (HTT ≈ 1400 °C) and then massive bond rearrangement (HTT ≈ 1600 °C), leading to the formation of bundled multiwall nanotubes (MWNTs) with 3−12 shells. At HTT > 1800 °C, we find that these MWNTs collapse into multishell GNRs. The first type of GNR we observed is driven by the collapse of diameter-doubled single-wall nanotubes, and their production is terminated at HTT ≈ 1600 °C when the MWNTs also start to form. We propose that the collapse is driven by van der Waals forces between adjacent tubes in the same bundle. For HTT > 2000 °C, the heat-treated material is found to be almost completely in the multishell GNR form.