Vibrational behavior of adsorbed CO2 on single-walled carbon nanotubes

We present theoretical and experimental evidence for CO2 adsorption on different sites of single walled carbon nanotube (SWNT) bundles. We use local density approximation density functional theory (LDA-DFF) calculations to compute the adsorption energies and vibrational frequencies for CO2 adsorbed on SWNT bundles. The LDA-DFT calculations give a. range of shifts for the asymmetric stretching mode from about -6 to -20 cm(-1) for internally bound CO2, and a range from - 4 to - 16 cm(-1) for externally bound CO2 at low densities. The magnitude of the shift is larger for CO2 adsorbed parallel to the SWNT surface; various perpendicular configurations yield much smaller theoretical shifts. The asymmetric stretching mode for CO2 adsorbed in groove sites and interstitial sites exhibits calculated shifts of -22.2 and -23.8 cm(-1), respectively. The calculations show that vibrational mode softening is due to, three effects: (1) dynamic image charges in the nanotube; (2) the confining effect of the adsorption potential; (3) dynamic dipole coupling with other adsorbate molecules. Infrared measurements indicate that two families of CO2 adsorption sites are present. One family, exhibiting a shift of about - 20 cm(-1) is assigned to internally bound CO2 molecules in a parallel configuration. This type of CO2 is readily displaced by Xe, a test for densely populated adsorbed species, which are expected to be present on the highest adsorption energy sites in the interior of the nanotubes. The second family exhibits a shift of about -7 cm(-1) and the site location and configuration for these species is ambiguous, based on comparison with the theoretical shifts. The population of the internally bound CO2 may be enhanced by established etching procedures that open the entry ports for adsorption, namely, ozone oxidation followed by annealing in vacuum at 873 K. Xenon displacement experiments indicate that internally bound CO2 is preferentially displaced relative to the -7 cm(-1) shifted species. The -7 cm(-1) shifted species is assigned to M adsorbed on the external surface based on results from etching and Xe displacement experiments. (C) 2004 American Institute of Physics.