Physical adsorption of xenon in open single walled carbon nanotubes: Observation of a quasi-one-dimensional confined Xe phase

The adsorption of Xe into carbon single walled nanotubes with both closed and open ends has been investigated using temperature programmed desorption and other surface analytical tools. It has been found that opening the ends of the nanotube by chemical cutting increases both the kinetic rate and the saturation capacity of the nanotubes for Xe at 95 K. Further enhancement in Xe adsorption kinetics and capacity are achieved by treating the nanotubes in vacuum at 1073 K where CO, CO2, CH4, and H-2 are evolved. On this basis it is postulated that surface functionalities such as -COOH block entry ports for adsorption at the nanotube ends and at the defect sites on the walls. The thermal destruction of these functionalities leads to enhanced adsorption. The denser phase of Xe inside the saturated nanotubes desorbs by zero-order kinetics (E-d=26.8 +/- 0.6 kJ/mol). It is postulated that a quasi-one-dimensional Xe confined phase in equilibrium, with a two-dimensional Xe gas phase on the exterior, provides a phase transition governing the zero-order kinetics desorption process. (C) 2000 American Institute of Physics. [S0021-9606(00)71313-4].