A conceptual model for the structure of catalytically grown carbon nano-fibers

Typical platelet-type, herringbone-type, and tubular-type carbon nano-fibers (CNF) were catalytically prepared through three distinctly different routes. By comprehensive observations using SEM, TEM, STM, and XRD, these CNFs were found to have common sub-structures, which we call carbon nano-rods (CNR) and carbon nano-plates (CNP). A CNR was a carbon cluster of 8-10 graphene layers with unique diameters of about 2.5 nm and variable lengths in the range of 15-100 nm. CNPs appeared to be sets of 5-25 graphene stacks, probably formed by association of several CNRs. The faceted catalyst surfaces determine the particular ordered arrangements of the CNRs or CNPs in the final fiber form that result in the production of platelet, herringbone, or tubular-type CNFs. The diameters of the CNFs were defined by the length of the CNR and CNP sub-units (or a series of these) and their angles of association relative to the fiber axis. Graphitization at high temperatures closed the ends (edges) of carbon hexagons in CNRs to form concentrically layered dome-like caps on the surface of CNFs. Such sub-structure units can be separated by grinding. (C) 2005 Elsevier Ltd. All rights reserved.