Adsorption of CF 4 on the Internal and External Surfaces of Opened Single-Walled Carbon Nanotubes: A Vibrational Spectroscopy Study Oleg Byl, † Petro Kondratyuk, † Scott T. Forth, ‡ Stephen A. FitzGerald, ‡ Liang Chen, § J. Karl Johnson, § and John T. Yates, Jr.* ,† Contribution from the Department of Chemistry, Surface Science Center, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260, Department of Chemical and Petroleum Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261, National Energy Technology Laboratory, Pittsburgh, PennsylVania 15236, and Department of Physics, Oberlin College, Oberlin, Ohio 44074 Received July 11, 2002; E-mail: jyates@pitt.edu Abstract: Infrared spectroscopy has been used to make the first experimental discrimination between molecules bound by physisorption on the exterior surface of carbon single-walled nanotubes (SWNTs) and molecules bound in the interior. In addition, the selective displacement of the internally bound molecules has been observed as a second adsorbate is added. SWNTs were opened by oxidative treatment with O3 at room temperature, followed by heating in a vacuum to 873 K. It was found that, at 133 K and 0.033 Torr, CF4 adsorbs on closed SWNTs, exhibiting its ν3 asymmetric stretching mode at 1267 cm -1 (red shift relative to the gas phase, 15 cm -1 ). Adsorption on the nanotube exterior is accompanied by adsorption in the interior in the case of opened SWNTs. Internally bound CF4 exhibits its ν3 mode at 1247 cm -1 (red shift relative to the gas phase, 35 cm -1 ). It was shown that, at 133 K, Xe preferentially displaces internally bound CF4 species, and this counterintuitive observation was confirmed by molecular simulations. The confinement of CF4 inside (10,10) single-walled carbon nanotubes does not result in the production of lattice modes that are observed in large 3D ensembles of CF4. I. Introduction Since the discovery of single-walled carbon nanotubes (SWNTs) by Iijima 1 and by Bethune et al. 2 in 1993, there has been a large interest in their application as sorbents. 3-10 This is due to the deep potential energy well for adsorption in the interior of the nanotube. 11-13 The synthesis of SWNTs normally produces closed structures where each tube is terminated by an end cap, which prevents adsorption within the interior. 14,15 Oxidative chemical treatments 16,17 must be applied to the closed SWNTs to open the end caps to access the interior of the nanotubes. 18 While oxidation in solution [HNO 3 + H 2 O 2 + H 2 - SO 4 ] has been found to be effective for opening closed SWNTs, we have developed a gas-phase ozone oxidation process, which is more easily controlled. This O 3 oxidation procedure has been extensively investigated by IR spectroscopy in previous stud- ies. 19,20 Oxidation can remove the end caps of individual SWNTs as well as produce or enlarge vacancy defects on the nanotube walls. Both carbonyl groups and C-O-C functional groups are known to form at the rims and at defective wall sites by oxidation. 18,19,21 Heating to 773-1073 K removes these blocking groups (by evolution of CO and CO 2 22 ), leaving open entry ports for gas adsorption into the interior. 23 * To whom correspondence should be addressed. † Surface Science Center, University of Pittsburgh. ‡ Oberlin College. § Department of Chemical and Petroleum Engineering, University of Pittsburgh, and National Energy Technology Laboratory. (1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (2) Bethune, D. S.; Kiang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605. (3) Muris, M.; Dupont-Pavlovsky, N.; Beinfait, M.; Zeppenfeld, P. 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