Solution-phase surface reconstruction and structural transformation in MWNTs

We report on structural changes induced in arc-grown multiwalled carbon nanotubes (MWNTs) due to ultrasonic agitation in an aqueous iron nitrate solution. The surface reconstructions were observed to follow three different pathways: (1) transformation from a tubular to a spherical / polyhedral onion-like geometry that involves a lower surface energy, (2) surface reconstruction of shells resulting in NT shrinkage from a larger diameter tube with a hollow core to a smaller diameter one with a solid core, and (3) the creation of highly reactive atomic-scale vacancies on carbon shells with low activation energies in the case of remaining NTs with a hollow core. Upon electron irradiation at extremely low beam intensities (1-2 A/cm²), the NTs with a hollow core have been observed to undergo structural reconstructions that result in shell shrinkage. The demonstrated techniques provide the capability to produce carbon onions in large-scale quantities and allow precision engineering of the geometry of condensed carbon allotropes. In addition, these results offer a route towards creating NTs with increased current densities/specific strength.