Adsorption to carbon nanotubes
We have probed the adsorption property of single-wall carbon nanotube (SWNT) bundles using the temperature-programmed desorption technique.
The SWNT sample cleanliness effect on the 4He adsorption was investigated. Room air contacting significantly decreased the 4He adsorption capacity.
The 4He adsorption vs. pump-out temperature on SWNT samples and on charcoal was obtained. A two-state binding site model did not fit well to the SWNT data, while it fit well to the charcoal data indicating the 4He binding energy on charcoal to be 400 ± 32 K which agreed with other group's value. Using the desorption rate isotherm analysis technique, we obtained coverage dependant 4He binding energies on SWNT bundles. Our values agreed with other group's results at near 400 K where the coverages overlapped, and our energy value increased to a much higher value at near 900 K at lower coverages beyond the lowest coverage of other group.
The 4He addition temperature was changed from 273 K to lower values in the 8–40 K range for three SWNT samples and a charcoal sample. While the 4He adsorption was not sensitive on the addition temperature on charcoal, it was different on SWNT samples. Some sites were not accessible for 4He atoms at low temperatures. The 4He access to these sites increased as the gas addition temperature increased, and at 35 K and above a full 4He access to a 273 K dosed level was observed. An activated diffusion model fit to the 4He amount, vs. gas addition temperature data yielded the activation energy for diffusion to be 28 ± 14 K and 47 ± 6 K on two samples. One sample showed more restricted 4He access for 4He at 15 K. This sample had more impurities.
Codesorption measurements were done on SWNT samples. Xe in smaller quantity (6% level) than 4He and H2, suppressed the adsorption of other gases to the background level. H2 suppressed 4 He to the background level, when added in equal amount at 273 K. However when 4He was added at 273 K and H2 was added later at 19 K, H2 did not suppress the 4He adsorption. Equal mixture doses of 4He and 3He at 273 K yielded 8.4 times more 4He binding than 3He. This strong isotope selectivity agreed with the predicted quantum sieving effect.