PAPKOV ET AL . VOL. 7 ’ NO. 1 ’ 126 –142 ’ 2013 www.acsnano.org 126 December 18, 2012 C 2012 American Chemical Society Extraordinary Improvement of the Graphitic Structure of Continuous Carbon Nanofibers Templated with Double Wall Carbon Nanotubes Dimitry Papkov, † Allison M. Beese, ‡ Alexander Goponenko, † Yan Zou, † Mohammad Naraghi, ‡ Horacio D. Espinosa, ‡ Biswajit Saha, § George C. Schatz, § Alexander Moravsky, ^ Raouf Loutfy, ^ Sonbinh T. Nguyen, § and Yuris Dzenis †, * † Department of Mechanical and Materials Engineering, Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0526, United States, ‡ Department of Mechanical Engineering and § Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States, and ^ MER Corporation, 7960 South Kolb Road, Tucson, Arizona 85706, United States R ecent discoveries of new nanocarbon allotropes 1 and subsequent demon- strations of superior mechanical prop- erties for two of these allotropes, carbon nanotubes (NT), and graphene, have raised hopes for new structural materials with extraordinary mechanical performance. However, after two decades of intensive research, a viable carbon nanotube-rein- forced supernanocomposite, defined as a bulk composite with properties exceeding the properties of existing advanced com- posites, 2,3 is yet to be demonstrated. The problems are well-documented and include difficulties with NT dispersion, NT alignment, achieving a high volume fraction of NT, and optimizing stress transfer to NTs. It is still difficult to make a high-quality NTÀpolymer composite with a volume fraction of nano- tubes higher than a few percent. 1 As a result, most current and projected near-term appli- cations of nanotube composites appear to rely on the thermal and electrical proper- ties of NTs rather than their mechanical superiority. 1,4 The situation is only slightly better for NT- reinforced fibers. Fibers are ideally suited for nanoreinforcement because of their small * Address correspondence to ydzenis@unl.edu. Received for review July 30, 2012 and accepted November 24, 2012. Published online 10.1021/nn303423x ABSTRACT Carbon nanotubes are being widely studied as a reinforcing element in high-performance composites and fibers at high volume fractions. However, problems with nanotube proces- sing, alignment, and non-optimal stress transfer between the nanotubes and surrounding matrix have so far prevented full utilization of their superb mechanical properties in composites. Here, we present an alternative use of carbon nanotubes, at a very small concentration, as a templating agent for the formation of graphitic structure in fibers. Continuous carbon nanofibers (CNF) were manufactured by electrospinning from polyacrylonitrile (PAN) with 1.2% of double wall nanotubes (DWNT). Nanofibers were oxidized and carbonized at temperatures from 600 °C to 1850 °C. Structural analyses revealed significant improvements in graphitic structure and crystal orientation in the templated CNFs, with the largest improvements observed at lower carbonization temperatures. In situ pull-out experiments showed good interfacial bonding between the DWNT bundles and the surrounding templated carbon matrix. Molecular Dynamics (MD) simulations of templated carbonization confirmed oriented graphitic growth and provided insight into mechanisms of carbonization initiation. The obtained results indicate that global templating of the graphitic structure in fine CNFs can be achieved at very small concentrations of well-dispersed DWNTs. The outcomes reveal a simple and inexpensive route to manufacture continuous CNFs with improved structure and properties for a variety of mechanical and functional applications. The demonstrated improvement of graphitic order at low carbonization temperatures in the absence of stretch shows potential as a promising new manufacturing technology for next generation carbon fibers. KEYWORDS: continuous carbon nanofibers . double-wall carbon nanotubes . carbonization templating . graphitic structure . molecular dynamics simulations ARTICLE