Evaluation of a New Method to Estimate the Micropore Volume and External Surface Area of Single-walled Carbon Nanotubes

Since the discovery of carbon nanotubes in 1991, significant progress has been made to understand their electrical, mechanical, magnetic and optical properties. However, as the ideal adsorbent for adsorption of gases and vapors, nanotubes, especially the single-walled carbon nanotubes (SWNTs), and their adsorption properties remain not been investigated enough. The unique microporous structure of SWNTs gives them remarkable adsorption properties, i.e. surface area and porosity. Several methods exist to determine the surface area and pore volume of adsorbents: t-plot method, Dubinin-Radushkevich (DR) method, and Dubinin-Astakhov (DA) method. The results are usually specific to a certain method and to the segment of the adsorption isotherm which is fitted to the method used. Therefore, the main objective of this study is to find the most appropriate and suitable method to characterize micropore volume and external surface area of SWNTs.

The surface area and porosity results are often used to evaluate the effectiveness of an adsorbent for removal of odors and trace concentrations of organic vapors from air and process streams. The objective is accomplished by evaluating the results of t-plot method, DR method, DA method and a method that was recently developed. Some of the commonly used methods were developed as far back as 1940s. Additionally, the methods were developed for various adsorbents such as carbon black, zeolites and activated carbon. However, nanotubes are fairly recent adsorbent materials. All of these can render the characterization results open to the interpretation by a specific user.

In this study standard N2 adsorption was carried out at 77 K for four commerciallyavailable SWNT sample. The impurities of SWNTs were able to be incorporated into the adsorption isotherm, the results of which could be confirmed by thermogravimetric analysis (TGA) and X-ray photo spectroscopy (XPS) experiments. To make the new developed method applicable to impure SWNT samples, new definition of statistical thickness has been proposed in this study. The consistent results showed the advantages of the modified new method, which is applicable and specific to SWNT samples. Multiple adsorbates should be used in the future analysis, such as water vapor, hexane, and other organic vapors.