Physical Adsorption on Single Wall Carbon Nanotubes
The adsorption of molecules on single wall carbon nanotubes in the regime of physical adsorption was investigated by means of temperature programmed desorption (TPD) and infrared spectroscopy under ultrahigh vacuum conditions. Three adsorption sites could be resolved in the TPD spectra of a number of molecules, such as CCl4, normal alkanes with 5-9 carbon atoms in the chain, and an unsaturated hydrocarbon, 1-heptene. The resolution of spectral features in the TPD spectra is especially pronounced in the case of normal alkane molecules. By comparison with theoretical simulations, these three adsorption sites were shown to correspond to: (1) to the nanotube interior; (2) the groove sites between adjacent nanotubes on the outside of the nanotube bundles; and (3) the exterior surface of the nanotubes. The nanotube interior has the highest binding energy, followed by the groove sites and exterior sites. When two different adsorbates, such as CCl4 and n-C9H20, are coadsorbed on nanotubes, the adsorbate with higher polarizability (n-nonane) occupies the more highly-binding adsorption sites, qualitatively displacing the other adsorbate to sites with lower binding energies. By studying the dependence of the capacity of the adsorption sites on the shape and length of the adsorbed molecules, it was shown that linear groove sites behave as a one-dimensional adsorption space. In contrast, the interior adsorption sites, which have diameters substantially larger than the size of the typical adsorbate molecules, behaved as a three-dimensional adsorption space. When 1-heptene confined inside nanotubes was subjected to a flux of an aggressive chemical agent, atomic H, it was seen that the rate of reaction was 3-4 times slower compared to 1-heptene on the exterior of the nanotubes. This demonstrates that the nanotube walls can provide shielding for interior-adsorbed species.
Infrared spectroscopy showed that the í3 vibrational mode of CF4 (the mode with the highest transition dipole moment) shifts to lower frequency upon adsorption on carbon nanotubes. The shift is greatest for the internally-adsorbed species (35 cm-1), while the exterior-adsorbed CF4 shows a shift of 15 cm-1. This points at the stronger van der Waals interaction when the molecule is adsorbed in the interior.
Advisor:Prof. David W. Pratt; Prof. J. Karl Johnson; Prof. Kenneth D. Jordan; Prof. John T. Yates, Jr.
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:09/19/2007