Electrochemical Oxidation of Single Wall Carbon Nanotube Bundles in Sulfuric Acid G. U. Sumanasekera, ² J. L. Allen, ²,‡ S. L. Fang, ²,‡ A. L. Loper, ² A. M. Rao, ‡ and P. C. Eklund* ,²,‡ Department of Physics and Astronomy and Center for Applied Energy Research, UniVersity of Kentucky, Lexington, Kentucky 40506 ReceiVed: NoVember 9, 1998 Electrochemical doping of bisulfate ions into single wall carbon nanotube (SWNT) bundles has been studied using coulometry, cyclic voltammetry, mass-uptake measurements, and Raman scattering experiments. A spontaneous charge-transfer reaction is observed prior to the application of an electrochemical driving force, in sharp contrast to previous observations in the graphite-H 2 SO 4 system. A mass increase of the SWNT sample and a concomitant upshift of the Raman-active tangential mode frequency indicate oxidation (i.e., removal of electrons) of the SWNT bundles. In fact, using Raman scattering, we were able to separate the spontaneous and electrochemical contributions to the overall charge transfer, resulting in the value of an upshift of 320 cm -1 per hole, per C-atom introduced into the carbon π-band by the bisulfate (HSO 4 - ) dopant. This value may prove to be a universal measure of charge transfer in acceptor-type SWNT compounds. At a critical electrochemical doping, the SWNT bundles are driven into an “overoxidation” regime, where they are irreversibly oxidized with the formation of C-O covalent bonds, analogous to electrochemical formation of graphite oxides. Introduction Graphite’s lamellar structure with weak van der Waals interlayer bonding between the graphene sheets, and its am- photeric nature, have allowed the synthesis of a large number of donor and acceptor graphite intercalation compounds (GICs) in which sheets of intercalated atoms or molecules are inserted between the host graphene planes. 1 Of particular interest to the present work here on single wall carbon nanotubes (SWNTs), graphite-bisulfate intercalation compounds C p + HSO 4 - (xH 2 SO 4 ) have been prepared by both chemical 2 and electrochemical oxidation of graphite in sulfuric acid. 3,4 The electrochemical oxidation of graphite (i.e., removal of electrons) in sulfuric acid, first reported by Ru ¨dorff, 3 has been shown to provide an exact determination of charge transfer (f ) 1/p) between the host carbon and guest intercalate layers through coulometry. Thus these compounds provided a convenient means to study the effect of charge transfer on the physical properties of acceptor GICs. A large number of studies devoted to graphite-bisulfate intercalation compounds then followed. 5-27 A single wall carbon nanotube (SWNT) can be envisioned as a long, rolled up graphene sheet with a seamless joint. 28 The SWNT exhibits a well-defined, periodic structure defined by the “roll up” vector C(n,m) ) na + mb, where a and b are primitive translation vectors of the graphene sheet, and n and m are integers. The symmetry, unit cell dimension, and tube diameter are uniquely determined by the integers (n, m) and the carbon interatomic distance. SWNTs prepared by pulsed laser vaporization (PLV) 30 and the arc discharge (AD) 29 methods have been shown by transmission electron microscopy (TEM) to self-organize into bundles containing tens to hundreds of tubes held in a triangular “rope” lattice by the van der Waals force. 30 Thus, by analogy to GICs, it was expected that bundles of SWNTs could be oxidatively (reductively) intercalated, where the guest anions (cations) occupy sites in the interstitial channels in the rope lattice. Evidence for the existence of such compounds has been demonstrated recently for alkali metal and halogen vapor-phase-doped SWNTs. 31-34 Doping provides an attractive means of controlling the electronic properties of SWNTs. Here, we present a study of the electrochemical anodic oxidation of SWNTs in sulfuric acid. In situ Raman scattering and mass uptake were used to study the guest-host charge-transfer chemistry. Interesting comparisons can be made for the anodic oxidation of graphite and SWNT hosts in sulfuric acid, and they will be discussed below. Experimental Section ACS reagent grade sulfuric acid (H 2 SO 4 95%, Fisher Scien- tific, as received) was used and diluted with distilled water as needed. Pressed pellet mats of SWNT bundles containing coproduced carbon nanoparticles synthesized by the electric arc discharge (AD) and pulsed laser vaporization (PLV) process were studied, giving very similar electrochemical results. The SWNTs obtained from both these processes exhibited a narrow diameter distribution with a most probable diameter in the range 1.3-1.4 nm. The diameter of a (10, 10) armchair nanotube is 1.37 nm. In a separate experiment, the coproduced carbon nanoparticles were found not to be electrochemically active. Electrochemical reactions were carried out in a closed quartz cell with the standard three-electrode configuration. The quartz cell allowed in situ Raman spectroscopy of the SWNTs during anodic oxidation, and this cell has been described previously. 23 Typically, a mat of SWNTs (4 × 4 × 0.1 mm 3 ) was pressed onto a platinum (Pt) plate and used as the “working” electrode. A Pt wire served as the “counter” electrode and the cell potential was measured between a “reference” electrode (saturated calomel electrode, or SCE) and the working electrode. * To whom correspondence should be addressed. ² Department of Physics and Astronomy. ‡ Center for Applied Energy Research. 4292 J. Phys. Chem. B 1999, 103, 4292-4297 10.1021/jp984362t CCC: $18.00 © 1999 American Chemical Society Published on Web 04/30/1999