(403f) Novel Technique of Estimating the Fraction of Open-Ended Carbon Nanotubes

We have developed an adsorption based technique to estimate the fraction of open-ended nanotubes in samples of single-walled carbon nanotubes (SWNTs). We discovered that SWNTs that already have a high surface area of 100's m2/g had only 45 ? 50% open-ended nanotubes. Our technique involves a practical approach to modeling heterogeneous bundles of SWNTs by sub-dividing the total adsorption on a bundle into two contributions: inside the nanotube in a bundle and on the external surface of the outermost nanotubes of the bundles. The technique integrates sample morphology, i.e., purity and sample characterization by Raman spectroscopy, into grand canonical Monte Carlo (GCMC) simulation of a small probe molecule, such as N2 adsorption at 77 K, in nanotube bundles. Simulated adsorption isotherm is then compared with standard N2 adsorption isotherms (77 K) of the sample. We are able to generate the characteristic type II isotherm for N2 in nanotube samples. We introduced a scaling parameter for endohedral adsorption which resulted in a near-perfect reproduction of experimental isotherms that are typically measured for 20 ? 30 mg of samples. We refer to this parameter as the fraction of open-ended nanotubes. We have tested samples procured from several commercial suppliers, such as MER Corp., Carbon Nanotechnologies Inc., Carbon Solutions Inc., Carbolex Inc., and BuckyUSA. We have found that the fraction of open-ended nanotubes in these samples ranged between 0 to 60%. Our methodology resulted in a perfect reproduction of experimental N2 (77 K) adsorption isotherms for each of the tested samples, elucidating that our experimental-theoretical methodology is not specific to a particular sample and can be successfully extended to most SWNTs.

The fraction of open-ended SWNTs cannot otherwise be determined by visual characterization of samples due to a large aspect ratio of the nanotubes and the spaghetti-like arrangement of the bundles. Therefore, gas adsorption could become an appropriate tool to quantify this structural property of nanotube samples: much like estimating the surface area of a material using gas adsorption. This study should also convey that many advances are still needed in synthesis and processing of carbon nanotubes and a large fraction of maximum achievable adsorption capacities of carbon nanotubes remains untapped.