Comparison of the adsorption properties of krypton on multi-walled carbon nanotubes and on graphite.
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Comparison of the adsorption properties of krypton on multi-walled carbon nanotubes and on graphite. Abdelhafid Bougrine1, Karine Varlot1, Nicole Dupont-Pavlovsky1, Jaafar Ghanbaja1, Denis Billaud1 and François Béguin2 1 Laboratoire de Chimie du Solide Minéral, Université Henri Poincaré-Nancy I, BP 239, 54506 Vandoeuvre-les-Nancy cedex, France 2 Centre de Recherches sur la Matière Divisée, 1B, rue de la Férollerie, 45071 Orléans cedex 2, France ABSTRACT Krypton adsorption has been investigated on multi-walled carbon nanotubes prepared by catalytic decomposition of acetylene. Isotherms measured between 77.5 and 83.7K from the first stages of adsorption to the adsorbate saturation vapor pressure are compared to those obtained in the same conditions on graphite. The results are discussed in the light of the nanotube morphology, as determined by transmission electron microscopy. Krypton adsorption proceeds, as on graphite, by successive monomolecular layer condensations on a same surface. From the morphology of the tubes, adsorption probably occurs only on their external surfaces. The first adsorbed layer is commensurate with the substrate. At its completion, it undergoes a transition into an incommensurate solid of higher density. The curvature of the graphene sheets, with respect to those of graphite, produces a stabilization of the commensurate film. The isosteric heat of adsorption Qst was measured to be 11.6 kJ/mol (∆Qst/Qst = 6%) and is lower than that of krypton on graphite. Moreover, the monolayer condensation pressures are higher than those observed with graphite, which probably leads to an incomplete wetting of the surface by limiting the number of krypton monomolecular layers adsorbed before its bulk condensation. INTRODUCTION Physical adsorption on powdered carbon-based materials offers the opportunity of studying a large variety of adsorbent-adsorbate pair properties, owing to the numerous forms of these solids. It is currently applied to the determination of the specific surface area and the characterization of the porous texture of disordered substrates, such as activated carbons for instance [1]. On uniform surfaces like that of graphite, approaching the ideal case of a crystalline plane without defects, it is applied to the physisorbed film characterization. The adsorption of simple molecules on graphite has been shown to proceed by monomolecular layer condensations and evidence has been given of successive two-dimensional (2D) phase transitions within a single monolayer [2]. Nanotubes constitute a novel form of carbon, made of one (single-walled nanotubes) or several coaxial (multi-walled nanotubes) graphene sheets forming seamless cylinders of nanometer diameter and micrometer length. These materials immediately attracted much attention, owing to their fundamental interest and potential applications. They are particularly attractive for physisorption studies because they correspond to an intermediate situation between the ordered and disordered substrates mentioned above. Stepwise adsorption isotherms
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