Adsorption studies of methane films on catalytic carbon nanotubes and on carbon filaments

Adsorption measurements of CH4 on catalytically produced carbon nanotubes were used to determine the wetting behavior of the films, the presence of capillary condensation, and the specific surface area of the tubes. Two groups of carbon tubes were produced. The set of larger tubes had inner diameters on the order of 1 mu m, while the set of narrower tubes had inner diameters on the order of 10-100 nm. The narrower carbon tubes were either oxidized in nitric acid, or subjected to high-temperature treatment at 2400 degrees C in vacuum, or left in as-produced condition. The activation processes were used to open the ends of the tubes. Surface area determinations for untreated and treated tubes established that the specific surface area increased as a result of activation. Isothermal adsorption-desorption cycles were measured on treated and untreated tubes. Hysteresis loops, indicative of the formation of a capillary condensate on the substrate, were present only for the open-ended treated tubes; no hysteresis loops were present on the untreated tubes. The wetting behavior of methane films above and below the bulk triple point was determined. Solid films incompletely wet the tubes. This behavior contrasts with the complete wetting exhibited by CH4 on exfoliated graphite, in spite of the fact that both the tubes and the exfoliate have essentially the same composition. Liquid methane completely wets the nanotubes.