Separation and Diameter-Sorting of Empty (End-Capped) and Water-Filled (Open) Carbon Nanotubes by Density Gradient Ultracentrifugation

摘要

Ever since the discovery of single-wall carbon nanotubes (SWCNTs), their unique and remarkably diverse electronic and optical properties, which depend critically on their exact diameter and chiral structure, have proven to be both a blessing and a curse, as synthetic methods invariably produce mixtures of structures, while applications demand more uniform properties. The sorting of SWCNTs by buoyant density through solubilization as individually isolated tubes using bile salt surfactants, followed by ultracentrifugation to equilibrium in a density gradient (DGU), is definitely the most promising technique for separating SWCNTs that have different diameters and chiral structures. However, the diameter-sorting is still not well understood, and the isolation of few or even individual SWCNT species has been limited to SWCNTs with relatively small diameters. In the present work, we have obtained an unprecedented insight into the sorting mechanism by showing that each individual SWCNT chirality actually does not concentrate at one, but rather at two different densities, which correspond to empty and water-filled nanotubes (Figure 1): the intact (end-capped, and therefore empty) SWCNTs, which are present mainly in carefully solubilized samples, can be separated from water-filled SWCNTs by DGU. Remarkably, these empty tubes possess far superior properties than the water-filled tubes used in previous nanotube research, and the overall reversed trend of buoyant density with increasing diameter enhances sorting of large-diameter tubes.