Filling carbon nanotubes with metals by the arc-discharge method: the key role of sulfur

Various filled carbon nanotubes have recently been successfully produced by the are-discharge method by doping a 99.4% graphite anode with a transition metal like Cr, Ni, a rare earth like Yb, Dy, or a covalent element like S, Ge. In this work, the structural characteristics of these encapsulated nanowires were studied by High Resolution Transmission Electron Microscopy and their chemical composition was investigated using Electron Energy-Loss Spectroscopy with high spatial resolution: this analysis mode provides elemental concentration profiles across or along the filled nanotubes. Except in the case of Ge for which only pure Ge fillings were identified, surprising amounts of sulfur, which was present as an impurity (approximate to 0.25%) in the graphite rods, were found within numerous filling materials. When using high purity carbon rods, no filled nanotube was obtained. We chose the case of Cr to clearly evidence that the addition of sulfur in catalytic quantity is responsible for the formation of filled nanotubes, including sulfur free encapsulated nanowires. A growth mechanism based on a catalytic process involving three elements, i.e. carbon, a metal and sulfur, and taking into account the experimental results is proposed.