Gas adsorption as a tool for surface characterization of multi-walled carbon nanotubes assemblies

D. ZILLI; S. BLACHER; C. J. GOMMES; J. P. PIRARD; A. L. CUKIERMAN

Edimburgo, Reino Unido

Congreso; 8th International Symposium on the Characterisation of Porous Solids (COPS VIII); 2008

The University of Edimburgh

Knowledge of adsorption characteristics of carbon nanotubes (CNTs) is essential to develop them into useful gas storage media. Because of interaction between adsorbent and adsorbate, it is possible to obtain much information about textural properties by adsorption employing different gases. Adsorption on CNTs arrays can occur inside the individual nanotubes, in the interstitial spaces between nanotubes, in the outer grooves present on the peripheral surface of the arrays where neighbouringnanotubes meet, and on the remaining peripheral surface of the arrays. When nitrogen is used as an adsorbate, adsorption reportedly occurs preferentially in the interstitial spaces, so alignment of the individual nanotubes forming the array plays an important role. In this context,the present work aims at examining characteristics of the surface of vertically self assembled multi walled CNTs by adsorption of nitrogen and krypton at 77 K.CNTs arrays were lab-synthesized by in situ catalytically chemical vapour deposition (CCVD) of iron (II) phthalocyanine for different reaction temperatures, keeping constant otherwise operating conditions. Metal nanoparticles, amorphous carbon and/or carbonaceous impurities were removed applying a two-step purification process, consisting of mild gas phase oxidation followed by mineral acid treatment and sonication. Pristine CNTs were also subjected to a sonication process. The pristine and purified CNTs arrays were characterized by N2 and Kr (77 K) adsorption measurements. N2- and Kr-adsorption isotherms of Type II and Type VI (stepped curves), according to IUPAC classification, were determined, respectively. In Kr-adsorption isotherms two steps can be easily distinguished, indicating a layer by layer adsorption process. Comparison of N2- and Kr-adsorption measurements pointed out that the former should provide information about surface area of the CNTs arrays as a whole entity, while the latter appears as a more reliable tool to characterize the surface area of the individual CNTs making up the self oriented arrays.