Carbon microtubes: tuning internal diameters and conical angles.

In this paper, we report a synthesis strategy for a new class of hollow, curved carbon morphologies, 'carbon microtubes' (CMTs), with absolute control over their conical angles and internal diameters. Our synthesis methodology employs nitrogen or oxygen dosing to change the wetting behaviour of gallium metal with the growing carbon walls to tune the conical angles. Increasing N(2) concentrations in the gas phase during growth increases the conical angles of CMTs from +25° to about -20°. A methodology using the timing of oxygen or nitrogen dosing during CMT growth is shown to tune the internal diameters anywhere from a few nanometres to a few microns. The walls of the carbon microtubes are characterized using transmission electron microscopy (TEM) and Raman spectroscopy and are found to consist of aligned graphite nanocrystals (2-5 nm in size). Furthermore, dark field images of CMTs showed that the graphite nanocrystals are aligned with their c-axes perpendicular to the wall surface and that the crystals themselves are oriented with respect to the wall surface depending upon the conical angle of the CMT.