Electrochemical oxidation of single wall carbon nanotube bundles in sulfuric acid

Electrochemical doping of bisulfate ions into single wall carbon nanotube ( SWNT) bundles has been studied using coulometry, cyclic voltammetry, mass-u ptake measurements, and Raman scattering experiments. A spontaneous charge- transfer reaction is observed Drier to the application of an electrochemica l driving force, in sharp contrast to previous observations in the graphite -H2SO4 system. A mass increase of the SWNT sample and a concomitant upshift of the Raman-active tangential mode frequency indicate oxidation (i.e., re moval of electrons) of the SWNT bundles. In fact, using Raman scattering, w e were able to separate the spontaneous and electrochemical contributions t o the overall charge transfer, resulting in the value of an upshift of 320 cm(-1) per hole, per C-atom introduced into the carbon pi-band by the bisul fate (HSO4-) dopant. This value may prove to be a universal measure of char ge transfer in acceptor-type SWNT compounds. At a critical electrochemical doping, the SWNT bundles are driven into an "overoxidation" regime, where t hey are irreversibly oxidized with the formation of C-O covalent bonds, ana logous to electrochemical formation of graphite oxides.