Helically shaped multiwalled carbon nanotubes (HCNTs) (diameter 30-80 nm) were obtained as main products in large quantities by co-pyrolysis of Fe(CO) 5 as floating catalyst precursor and pyridine or toluene as carbon source at temperatures above 1000 °C under flow of hydrogen. In contrast, pyrolysis at 700-800 °C yielded only carbon-coated Fe nanoparticles. The HCNTs were characterized by electron microscopy, Raman spectroscopy, and wide-angle X-ray diffraction. The growth mechanism of the HCNTs was discussed on the basis of previously reported models.

The continuous interest in carbon nanotubes (CNTs) and nanotube arrays has led us to discover new routes of high-yield and low-cost synthesis of CNTs with controlled tube structure. Here we show that a dilute solution of FeCl 3 in toluene and N,N-dimethylaminoacetate is a high-efficiency floating catalyst precursor in a chemical vapor deposition process. The growth structure and the self-assembling behavior of the resulting multiwall tubes was characterized using scanning and transmission electron microscopy. Clear indications are provided of the participation of iron carbide in the tube formation at higher temperatures. The characteristic cone-shaped morphology of the tubes is described in detail and discussed in terms of its interesting practical implications.