Practical modeling of heterogeneous bundles of single-walled carbon nanotubes for adsorption applications: Estimating the fraction of open-ended nanotubes in samples

A practical approach for adsorption modeling of heterogeneity of single-walled carbon nanotube (SWNT) bundles has been developed. The method integrates experimental analysis with grand canonical Monte Carlo (GCMC) simulation of a small probe molecule, such as nitrogen at 77 K. Using this method, it is possible for one to separately estimate adsorption inside the nanotubes, adsorption on the external surface of the bundles, and adsorptive contributions from the impurities present in samples. By introducing a scaling parameter for adsorption in the internal porous volume of the bundles, the predicted adsorption isotherm results in a near replication of the experimental N₂ adsorption isotherm. We refer to this parameter as the volume fraction of open-ended nanotubes. Our GCMC-assisted experimental characterization method has been applied successfully to several commercial samples obtained from different suppliers, such as MER Corp., Carbon Nanotechnologies Inc., Carbon Solutions Inc., Carbolex Inc., and BuckyUSA. It was found that the volume fraction of openended SWNTs in these samples ranged between 0 and 55%. The majority of the samples were subjected to some purification treatment by the manufacturer and exhibited an already high BET surface area of hundreds of square meters per gram. The near-perfect reproduction of the experimental N₂ (77 K) adsorption isotherm for each of the tested samples shows that our characterization method is not specific to a particular sample and can be extended to most SWNTs successfully. The fraction of open-ended SWNTs cannot otherwise be estimated by visual characterization of the samples because of the large aspect ratio of nanotubes and the spaghetti-like arrangement of the bundles. Our method has the potential to become a standard technique to quantify this structural property of SWNT samples.