Microstructural and electron spectroscopic characterization of carbon nanostructures and nanotubes produced using multimetal catalysts

The microstructure of graphite arc discharge soot catalysed with Fe, Ni, Co and a mixture of Fe-Ni is investigated. It is shown that as well as single shell nanotubes and nanotube bundles, particles containing random networks of tubules and large diameter short tubules are found. Furthermore, the presence of fullerene nanocrystals directly formed during arcing is reported for the first time. The addition of a supplementary inert metallic element (Ag) to the catalysts, introduced with the aim of filling the tubules, reduces the nanotube yield significantly. Particles of Ag of spherical shape are found to be aligned and encapsulated in a carbon envelope. Energy loss spectroscopy carried out in an electron microscope reveals significant variations in the carbon-carbon bonding of the various soot morphologies which can be related to the formation site during arc discharge. Spectra from single shell nanotube bundles show an increase in the density of states on the antibonding π bands, in agreement with electronic structure calculation predicting metallicity. Spectra from spherical soot particles indicate a similarity in the carbon-carbon bond with the C60/C70 fullerenes. Important variations of the carbon bonding in areas of the nanotube bundles where pentagons are introduced during the growth are also detected. Considering the observed morphologies and yield variations due to various catalysts and the presence of Ag, mechanisms contributing to the growth of the observed microstructures are proposed.