Optimization of Xe adsorption kinetics in single walled carbon nanotubes

Abstract

Closed end (10, 10) single walled carbon nanotubes (SWNTs) have been opened by oxidation at their ends and at wall defect sites, using ozone. Oxidation with ozone, followed by heating to 973 K to liberate CO and ${\mathrm{CO}}_{\mathrm{2}},$ causes etching of the nanotube surface at carbon atom vacancy defect sites. The rate of adsorption of Xe has been carefully measured as a function of the degree of nanotube etching by ozone. It is found that a level of etching corresponding to wall openings of about 5–7 Å radius is optimal for maximizing the rate of Xe adsorption. Beyond this level of etching, the rate of Xe adsorption decreases as the surface area of the SWNTs decreases due to further carbon atom removal. Both experiment and modeling show that the presence of polar oxidized groups, such as –COOH or –COR groups, with dipole moments in the range 1.5–3.0 D at the perimeter of the defect sites, causes a retardation of the rate of Xe adsorption due to dipole-induced dipole interactions. This effect is larger for smaller radius defect sites and decreases as the defect sites increase in size beyond about 7 Å radius. At large defect radii, the energetic profile of the adsorption pathway controls the physisorption rate. Modeling shows that after Xe adsorption has been completed inside the nanotubes, then Xe clusters begin to form on the outer surface of the nanotubes at the defect sites where polar groups are present. The Xe clustering effect also occurs to a smaller degree when the defect sites are not decorated by polar groups. The experiments and modeling demonstrate how one may optimize the rate of adsorption of a gas into nanotubes by the adjustment of the size and polar character of the vacancy-site entry ports in the walls of the nanotubes.