Singling out the Electrochemistry of Individual Single-Walled Carbon Nanotubes in Solution Demis Paolucci,* ,‡ Manuel Melle Franco, ‡ Matteo Iurlo, ‡ Massimo Marcaccio, ‡ Maurizio Prato, † Francesco Zerbetto, ‡ Alain Pe ´ nicaud, § and Francesco Paolucci* ,‡ INSTM, Unit of Bologna, Dipartimento di Chimica, UniVersita ` di Bologna, Via Selmi 2, I-40126 Bologna, Italy, INSTM, Unit of Trieste, Dipartimento di Scienze Farmaceutiche, UniVersita ` di Trieste, Piazzale Europa, 1, I-34127 Trieste, Italy and Centre de Recherche Paul Pascal-CNRS, UniVersite ´ Bordeaux-I, AV. Schweitzer, 33600 Pessac, France Received November 27, 2007; E-mail: francesco.paolucci@unibo.it Abstract: Bandgap fluorescence spectroscopy of aqueous, micelle-like suspensions of SWNTs has given access to the electronic energies of individual semiconducting SWNTs, while substantially lower is the success achieved in the determination of the redox properties of SWNTs as individual entities. Here we report an extensive voltammetric and vis-NIR spectroelectrochemical investigation of true solutions of unfunctionalized SWNTs and determine the standard electrochemical potentials of reduction and oxidation as a function of the tube diameter of a large number of semiconducting SWNTs. We also establish the Fermi energy and the exciton binding energy for individual tubes in solution. The linear correlation found between the potentials and the optical transition energies is quantified in two simple equations that allow one to calculate the redox potentials of SWNTs that are insufficiently abundant or absent in the samples. Introduction The properties of single wall carbon nanotubes (SWNTs) depend markedly on their diameter and helicity. In the bulk phase, the identification of specific quantities relative to individual tubes is notoriously challenging and requires extreme ingenuity, as in the case of bandgap fluorescence spectroscopy of aqueous, micelle-like suspensions of SWNTs that has given access to the electronic energies of individual semiconducting SWNTs. 1 Substantially lower is the success achieved in the determination of the redox properties of SWNTs 2,3 that are necessary to design the fabrication and/or the integration of the tubes into, for example, electroluminescent or other (opto)elec- tronic practical devices. Some data, as a function of the electrode potential, are available. 4–6 The n- and p-doping bleaches the intra-gap transitions (and the Raman radial breathing modes) allowing a determination of the redox levels of a few semicon- ducting SWNTs supported onto solid electrodes. 7–9 However, the strong tube-tube and tube-substrate interactions gave poorly resolved spectra, without proper matching of specific optical transitions to individual SWNTs. The difficulty of performing proper solution spectro-electrochemical experiments able to gain information on individual nanotubes is associated with (i) the high ionic strength typical of electrochemical experiments (due to the addition of a supporting electrolyte) that promotes flocculation of the SWNT suspension, (ii) the interference, with the electrochemical experiment, of the sur- factants used to solvate the SWNT that foul the working electrodes, and (iii) the limited electrochemical stability window of the aqueous medium. These difficulties are minimized by a recently reported, innovative way to form thermodynamically stable solutions of unmodified and uncut SWNTs. 10 Upon reduction with alkali metals, SWNTs produce polyelectrolyte salts (Scheme 1) that are soluble in polar organic solvents without use of sonication, surfactants, or functionalization. Here, we report an extensive vis-NIR spectroelectrochemical investigation of true solutions of unfunctionalized SWNTs that allowed the determination of the standard redox potentials of individual semiconducting SWNTs as a function of the tube structure. Both the reduction and the oxidation of each SWNT were independently investigated, and the Fermi energy and the exciton binding energy of the individual tubes in solution were also established. Voltammetry. Polyelectrolyte SWNT salts were obtained by reacting arc-discharge samples (a-NT) or HiPco samples (h- NT) with different alkali metals (Na, K). 10 Oxygen free solutions were then prepared for electrochemical investigations in ultradry DMSO. To avoid flocculation of the SWNTs, low concentrations ‡ Universita ` di Bologna. † Universita ` di Trieste. § Universite ´ Bordeaux. (1) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley, R. E.; Weisman, R. B. Science 2002, 298, 2361. (2) O’Connell, M. J.; Eibergen, E. E.; Doorn, S. K. Nat. Mater. 2005, 4, 412. (3) Zheng, M.; Diner, B. A. J. Am. Chem. Soc. 2004, 126, 15490. (4) Rafailov, P. M.; Maultzsch, J.; Thomsen, C.; Kataura, H. Phys. ReV. B 2005, 72, 045411. (5) Kavan, L.; Dunsch, L. ChemPhysChem 2007, 8, 974. (6) Kazaoui, S.; Minami, N.; Matsuda, N.; Kataura, H.; Achiba, Y. Appl. Phys. Lett. 2001, 78, 3433. (7) Kavan, L.; Rapta, P.; Dunsch, L.; Bronikowski, M. J.; Willis, P.; Smalley, R. E. J. Phys. Chem. B 2001, 105, 10764. (8) Corio, P.; Jorio, A.; Demir, N.; Dresselhaus, M. S. Chem. Phys. Lett. 2004, 392, 396. (9) Okazaki, K.; Nakato, Y.; Murakoshi, K. Phys. ReV.B 2003, 68, 035434. (10) Pe ´nicaud, A.; Poulin, P.; Derre ´, A.; Anglaret, E.; Petit, P. J. Am. Chem. Soc. 2005, 127,8. Published on Web 05/14/2008 10.1021/ja710625p CCC: $40.75  2008 American Chemical Society J. AM. CHEM. SOC. 2008, 130, 7393–7399 9 7393