Enhancement of Adsorption Inside Single-Walled Carbon Nanotubes: Li Doping Effect on n-Heptane van der Waals Bonding

Doping of opened single-walled carbon nanotubes (SWNTs) with metallic Li has been carried out under ultrahigh vacuum. Li atoms enter into the interior sites of the SWNTs by surface migration with an activation energy of 0.3 ± 0.04 eV. Density functional theory calculations indicate that the Li atoms ionize inside the SWNTs to produce Li+ ions. The Li-doped SWNTs exhibit an ∼10% enhancement of the van der Waals bonding for n-heptane molecules, a model for volatile organic compounds, within the interior due to polarization of the alkane molecules in the strong electrostatic field around the Li+ ions. The investigations reveal the utility of temperature-programmed desorption from SWNTs for understanding the energetics of confined molecule adsorption in pristine and doped SWNTs. Statistical mechanical simulations using a polarizable potential for alkanes give increases in isosteric heats of adsorption at high alkane loading that are in excellent agreement with experiments. Our simulations predict that the binding energy of n-heptane in the Li-doped nanotubes can be increased by as much as a factor of 3 at very low alkane loading.