Krypton adsorption as a suitable tool for surface characterization of multi-walled CNTs

Abstract

Multi-walled carbon nanotubes (CNTs), pristine and subjected to treatments, are comparatively characterized from N2 and Kr (77 K) adsorption measurements. The CNTs are lab-synthesized by in situ chemical vapour deposition of an iron-based organometallic compound at 895 °C. The treatments applied to the CNTs include low temperature gas-phase oxidation, mild temperature annealing and ultrasonic dispersion in ethanol, in an attempt to examine possible changes in adsorption characteristics. N2 and Kr adsorption measurements give rise to steadily increasing and stepped isotherms, respectively. The former are representative of a multilayer adsorption phenomenon, while the latter indicate successive monolayer condensation. The treatments affect differently gas adsorption capacities of the CNTs. Oxidation leads to CNTs with higher BET specific surface area and increased adsorption capacity, though the effect is more pronounced for Kr adsorption. Ultrasonic dispersion of the CNT brings about a significant reduction only in N2 adsorption capacity. Modifications in the characteristic steps in Kr adsorption isotherms of the CNTs subjected to annealing can be appreciated, although no remarkable changes are observed in N2 adsorption isotherms. Present results demonstrate that determination of Kr adsorption isotherms represents a more suitable tool to obtain a more reliable textural characterization of CNTs than does N2 adsorption.