Graphitization thermal treatment of carbon nanofibers

Carbon nanofibers (CNFs) and carbon nanotubes have revolutionized the world of the nanotechnology due to their excellent mechanical, electrical and thermal properties. CNFs are graphitic fibers made of stacks of graphene layers aligned perpendicular, tilted or parallel to the fiber axis, thus resulting in different microstructures. Post-production treatments can be applied to CNFs to improve their performance in several applications. Among them, the heat treatment at high temperature to achieve the transformation of the CNFs into graphite (graphitization) or graphitized CNFs (graphitization heat treatment) has been studied in detail. This review covers the literature on this topic for the last 20 years, analyzing the structural and textural changes shown by the CNFs during graphitization, and how these changes influence their mechanical and electrical properties. Different techniques, particularly, high-resolution transmission electron microscopy, have allowed to determine the microstructure of these nanofilaments. A survey of the applications of graphitized CNFs is provided, these including additives for polymer reinforcement in composites, anodes in lithium-ion batteries, catalyst supports in fuel cells, hydrogen storage and others such as potential biosensors and catalysts in diverse reactions. In this regard, special emphasis is placed on the advantages (or disadvantages) of using graphitized CNFs instead of as-grown CNFs.