Pergamon PII: S0022-3697(97)00233-8 J. Phys. Chem Solids Vol 58, No. 7, pp. 1091-1102, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-3697/97 $17.00 + 0.00 MICROSTRUCTURAL AND ELECTRON SPECTROSCOPIC CHARACTERIZATION OF CARBON NANOSTRUCTURES AND NANOTUBES PRODUCED USING MULTIMETAL CATALYSTS G. A. BOTTON*, G. BURNELL*, C. J. HUMPHREYS*, T. YADAVt and J. C. WITHERSt *Department of Materials Scienceand Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. tMER Corporation, Tucson, AZ 85706, U.S.A. (Received 17 January 1995; accepted in revised form 27 November 1996) Abstract--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 fullerenenanocrystals directlyformed 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 fillingthe 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 singleshellnanotubebundles show an increasein the density of states on the antibonding 7rbands, 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, mechanismscontributing to the growth of the observed microstructures are proposed. © 1997 ElsevierScienceLtd. All rights reserved. Keywords: A. fullerenes,C. electron microscopy,C. electron energy loss spectroscopy(EELS), D. electronic structure, D. microstructure. 1. INTRODUCTION The production of single shell carbon nanotubes on a graphite cathode using metal catalysts [1-4] may have considerable impact on the potential application of fullerene materials. It is therefore important to study the reaction products of the synthesis and to assess whether these products exhibit some of the properties that have been predicted. This paper reports a detailed characterization of the microstructure of the soot obtained in a graphite arc discharge using different catalysts (Fe, Ni, Co and a 50 : 50 mixture of Fe-Ni). We also report on the effect of adding a noble metal, Ag, both on the yield of nanotubes and on the micro- structure observed. Considering the great difficulty of assessing the properties of these structures with con- ventional macroscopic methods due to the size of the structures produced, we present electron energy loss spectroscopy (EELS) data of the various microstruc- tures and correlate spectra with observed morpholo- gies. EELS is sensitive to the electronic structure of materials and our spectra were obtained with high energy resolution and from nanometre-size areas; we discuss the near-edge structure of spectra by comparing the carbon ls spectrum (the K edge) in various forms of carbon taking into account the structural environment of the carbon atoms, and we relate the results to the predicted electronic properties of nanotubes based on recent band structure calcula- tions. This is the first detailed study of the electronic structure of carbon nanostructures using EELS, apart from the plasmon loss work of Lin et al. [4]. From this microstructural study, we suggest possible mechanisms contributing to the catalysis. In the studies reported in the literature [e.g. 1-4], marked differences in the yield of tubules due to the particular metal or oxide used in the synthesis have been reported. For example, Iijima and Ichihashi [1] showed that the synthesis using Fe is successful only when methane is present in the reactor, while Bethune et al. [2] have argued that, in a helium atmosphere, Fe, Ni and a 50 : 50 Ni-Cu mixture do not catalyse the growth of tubules while Co does. Lin et al. [4] have reported the success of large scale synthesis of single shelled tubules when using a Cu-filled anode and a He atmosphere. Various morphologies of growth have been reported where single shell tubules are observed 1091