NANO LETTERS
NanoTeflons: Structure and EELS Characterization of Fluorinated Carbon Nanotubes and Nanofibers
2002 Vol. 2, No. 5 491-496
T. Hayashi,*,† M. Terrones,‡,§ C. Scheu,| Y. A. Kim,† M. Ru1 hle,| T. Nakajima,⊥ and M. Endo† Faculty of Engineering, Shinshu UniVersity, Wakasato 4-17-1, Nagano 380-8553, Japan, Fullerene Science Centre, CPES, UniVersity of Sussex, Brighton BN1 9QJ, United Kingdom, AdVanced Materials Department, IPICYT, AV. Venustiano Carranza 2425-A, San Luis Potosı´ 78210, Me´ xico, Max-Planck-Institut fu¨ r Metallforschung, Seestrasse 92, 70174 Stuttgart, Germany, and Department of Applied Chemistry, Aichi Institute of Technology, Yakusa-cho, Toyota 470-0392, Japan Received March 4, 2002; Revised Manuscript Received April 2, 2002
ABSTRACT For the first time, we report high-resolution electron energy loss spectroscopy (HREELS) studies on fluorinated carbon nanotubes and nanofibers of various diameters. In addition, we have carried out high-resolution transmission electron microscope studies on the material, as well as HREELS elemental line scans, and elemental mappings. From the experimental EEL near edge structure of the F−K edge, we observed that F establishes two different types of bonds with C: covalent and ionic. From the elemental mapping, we noted that F is always uniformly distributed within the fluorinated carbon tubes of diameters >20 nm. EELS calculations using density functional theory confirmed the existence of covalent and ionic F bonded to the carbon within the tubes at ∼685 and 690 eV, respectively. These values are in good agreement with those obtained experimentally.
The fluorination of single1,2 and multiwalled3-5 carbon nanotubes as well as graphitic fibers6 have only been recently developed in order to modify the structural and electronic properties of the tubular structures, which is caused by the introduction of F within the graphene shells.6,7 In this context, it is important to note that the electronic properties of fluorinated carbon nanotubes could in principle vary from metals to semiconductors and even to insulators8-9 depending on the type of C-F bonding and the location of the F atoms within the carbon network. The most important feature of C-F system is that there are two types of bonding nature such as ionic and covalent bonding. Experimental data demonstrate that intercalated graphite with F (CFx, x < 0.1), establishing C-F ionic bonds, possess extremely high electrical conductivity (close to that observed for metals; e.g., 2 × 10 5 S cm-1).6,7 For high F concentrations, the transport properties of CFx compounds decrease due to the formation of covalent bonds between the C and F atoms. Graphite * To whom correspondence should be addressed. E-mail: hayashi@ endomoribu.shinshu-u.ac.jp. † Faculty of Engineering, Shinshu University. ‡ Fullerene Science Centre, CPES, University of Sussex. § Advanced Materials Department, IPICYT. | Max-Planck-Institut fu ¨ r Metallforschung. ⊥ Department of Applied Chemistry, Aichi Institute of Technology. 10.1021/nl025542o CCC: $22.00 Published on Web 04/24/2002
© 2002 American Chemical Society
fluorides have proved to be useful in the development and fabrication of Li primary batteries,10 solid lubricants11 and could also be, in our opinion, useful for conducting paints and composites. It is important to note that CFx compounds are superior solid lubricants at room temperature when compared to WS2 or MoS2.11 Moreover, because the tribological properties of WS2 and MoS2 nanoparticles/nanotubes are superior to the bulk materials, it is expected that the lubrication properties of fluorinated nanotubes and nanofibers be notably enhanced when compared to other CFx systems. To identify the type of bonding within CFx compounds, X-ray photoelectron spectroscopy (XPS),5,12 optical reflectivity studies5,13 and infrared (IR) spectroscopy5 have been carried out on various pristine carbons. However, and to the best of our knowledge, electron energy loss spectroscopy (EELS) studies on these novel fluorinated carbon materials have never been reported hitherto. In the present study, we present, for the first time, detailed high-resolution electron energy loss spectroscopy (HREELS), and energy filtering elemental mapping studies on fluorinated carbon nanotubes and nanofibers. It is also noteworthy that the bonding within fluorinated nanotubes (or any other CFx compound) has only been characterized on the surface using XPS techniques, and
Figure 1. (a) TEM image of the pristine carbon nanotubes (produced by pyrolytic methods) used in the fluorination process. (b) HRTEM micrograph of a representative pristine carbon nanotube. Note the presence of well crystalline graphene sheets within the structures. (c) TEM image of typical fluorinated fiber showing the absence of an inner core. (d) HRTEM image of the a representative fluorinated nanofiber exhibiting “disordered” domains caused by the presence of F atoms bonded to the carbon layers.
our results constitute the first report related to the bonding within both the internal structure and the surface of the tubular CFx nanomaterials. In this work, we noted that EEL spectra and elemental line scans on individual nanotubes/nanofibers, reveal the presence of C-F bonds uniformly distributed within the whole carbon structure, in the form of ionic and covalent connections established between C and F. Elemental mapping on the fibers indicate that the nanofibers of diameters >20 nm exhibit large F concentrations (atomic ratio of C12Fx where x ) 1), whereas the thin tubes (e.g., 20 nm). Figure 5 depicts the bright field image together with the C and F mapping of the same region. The mapping clearly indicates that F is homogeneously distributed (bonded to) the C atoms within the nanotubes of diameters greater than 20 nm. In conclusion, we have uniformly fluorinated nanotubes of diameters larger than 20 nm by passing F : HF mixtures over pyrolytically grown tubes at 500 °C. The fluorination process is notably enhanced for nanotubes of larger diameter (>20 nm). In this context, we found difficult to fluorinate thin multiwalled carbon 495
Figure 5. (a) TEM image of fluorinated carbon nanotubes, (b) elemental C mapping (in blue) of the image shown in (a), and (c) elemental F mapping (in green) of the image shown in (a).
nanotubes of diameters
