Adsorption of CF4 on the Internal and External Surfaces of Opened Single-Walled Carbon Nanotubes: A Vibrational Spectroscopy Study

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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.