N. Dupont-Pavlovsky

OPENING OF SINGLE-WALLED CARBON NANOTUBES: EVIDENCE GIVEN BY KRYPTON AND XENON ADSORPTION

Adsorption isotherms of krypton between 77 and 93 K and xenon between 110 and 120 K on mechanically-opened single-walled carbon nanotubes (SWNTs) have been measured. The comparison of the results with those obtained under the same conditions on closed nanotubes is discussed. Evidence of adsorption inside the tubes is given through the appearance of an additional adsorbed amount at very low relative pressure on the isotherms. Different orderings are predicted for xenon and krypton, based on their respective adsorbed amounts inside the nanotubes, and on geometrical considerations. Finally, the fraction of opened nanotubes accessible to xenon and krypton was estimated, as well as the impurity content originating from the cutting process.

Influence of high temperature treatments on single-walled carbon nanotubes structure, morphology and surface properties

Single-walled carbon nanotubes produced by the MER Corporation have been treated in an argon atmosphere at temperatures between 1400 and 2400°C. The modifications resulting from the heat treatment have been followed by adsorption volumetry and TEM measurements. 1600°C appears as a turning point for the high temperature treatments. Heating nanotubes up to 1600°C essentially results in the removal of metal particles. At higher temperatures the changes in morphology are predominant, and single-walled nanotubes packed in bundles are transformed into multi-walled nanotubes.

Krypton adsorption on (0001) graphite preplated with carbon tetrachloride

Abstract Thermodynamic and structural characterizations of krypton adsorption at 79 K on (0001) graphite preplated with carbon tetrachloride (CCI 4 ) have been performed by means of classical volumetric measurements and X-ray diffraction. Krypton condensation results in the compression and partial displacement of the preadsorbed CCl 4 film, and in the growth of three-dimensional CCl 4 crystallites. When less than one CCl 4 monolayer is preadsorbed, krypton adsorption occurs preferentially on the bare part of graphite and the krypton two-dimensional solid undergoes a commensurate-incommensurate transition before CCl 4 displacement. There is no significant indication for the formation of a two-dimensional (krypton-CCl 4 ) solid solution.

Comparison of the adsorptive properties of papyex and uncompressed exfoliated graphite

Abstract The adsorption isotherms of krypton at 77.3 and 90 K, methane at 77.3 K and xenon at 105 K have been compared when respectively determined on uncompressed exfoliated graphite and Papyex (compressed exfoliated graphite), in the first monolayer range. An alteration of the isotherms on Papyex has been observed, due to the compression of the initial exfoliated graphite. This compression leads to an increase of the surface heterogeneity and a decrease of the uniform patch size. Nevertheless, the main characteristics of the isotherms are preserved. Moreover, during the first monolayer formation, according to the state of the film, some differences in the kinetics appear, which are not noticeable in the case of uncompressed exfoliated graphite: high speed of adsorption when the film is mobile, and low speed in the case of a 2D solid.

Adsorption on pre-plated graphite: displacement transition for the (krypton-cyclohexane) and (methane-carbon tetrachloride) films

Abstract The displacement, by krypton adsorption, of a cyclohexane monolayer pre-adsorbed on graphite has been studied between 77 and 84 K by means of volumetric measurements. A displacement critical temperature of 80.25 K has been determined. From volumetric measurements of methane adsorption at 77 K on graphite pre-plated with carbon tetrachloride, a displacement of the pre-adsorbed layer has also been shown to occur, according to a continuous process. This displacement has been confirmed by a structural characterization of the film, by means of neutron diffraction. During this transition, two two-dimensional solid phases coexist on the surface of graphite. One of them exhibits a structure close to that of pure adsorbed carbon tetrachloride at the same temperature. The other structure is close to that of pure methane. Its crystal parameter decreases with increasing adsorbed amounts of methane.

Adsorbed gases on (0001) graphite and boron nitride: specific behaviour of krypton

Abstract Properties of monomolecular films physisorbed on graphite and boron nitride are generally similar in comparable temperature ranges, except for krypton: the incipient triple point and the commensurate-incommensurate 2D solid transition which occur on graphite do not appear on boron nitride. These discrepancies are shown not to be ascribable to the differences in superficial homogeneity of the various samples used. They certainly reflect the properties of the films on perfect (0001) uniform faces of graphite and boron nitride. The adsorption potentials and thermodynamical data are compared for films giving rise to phase transitions which were studied on both substrates. The differences in the krypton film properties probably originate in the particular size of krypton with respect to the distances between potential wells on both substrates. In this case, the differences in adsorption potentials would be sufficient to result in a drastic change in the nature of the transitions.

Structural study of a chloroform film adsorbed on graphite

The structural characterization of a chloroform (CHCl3) film adsorbed on graphite was performed by means of X-ray and neutron diffraction between 40 and 227 K. Results are consistent with those of a previous thermodynamic characterization and can be related to difficulties in the adsorbed film nucleation. The first layer melts at 166 K. At 227 K, the CHCl3 bilayer spectrum is that of a liquid, in agreement with the 225 K melting temperature determined from the isotherm measurements. The diffraction profiles of solid adsorbed CHCl3 are representative of a mixture of two structures on a microcospic scale. One of them is hexagonal with a 5.91 A parameter, and appears to be the most stable, the second is a square unit cell with a 5.77 A parameter.

Adsorption studies of a krypton film adsorbed on catalytically synthesized multiwalled carbon nanotubes: Dependence on the nanotube morphology

Krypton adsorption isotherms on multiwalled carbon nanotubes have been measured at 77 K and compared to those obtained under the same conditions on exfoliated graphite. The results are discussed in the light of the nanotube morphology, as determined by transmission electron microscopy and electron energy loss spectroscopy. The isotherms exhibit two steps representative of successive monomolecular layer condensations on the external walls of nanotubes, instead of five steps on graphite. The monolayer condensation pressures are higher than those observed with graphite, which probably limits their number at krypton bulk condensation pressure. Near the completion of the first condensed layer, a second transition towards a phase of higher density occurs, the nature of which is discussed.

Etude de l'exfoliation des composes d'insertion graphite-trioxyde de soufre

Resume Exfoliated graphite, studied in this paper, is produced by a rapid heating of intercalation compounds obtained from natural graphite flakes. Each flake is strongly expanded along the c axis, and results in a vermicular material. The “exfoliation” phenomenon finds application in the elaboration of flexible graphite sheets of low density. The intercalation compounds were synthetized according to the usual two temperature furnace method. The intercalated species was sulfur trioxyde. The exfoliation was performed in a torch flame. Two characterization methods have been used: first, krypton physisorption enabled us to determine the specific surface area, and to characterize the superficial homogeneity of the sample. Isotherms are presented on Fig. 1. They are all similar, except the curve corresponding to the first stage sample. The dependence of the specific area on the initial stage of the intercalation compound is plotted on Fig. 3; no significant variation is observed. Qualitative observations of the expanded samples have been performed with a stereoscan microscope (Fig. 4). Sulfur agregates were observed on the surface of the sample obtained from a first stage intercalation compound [Fig. 2(a)]. Their identification by spectroscopy is presented in Fig. 2(b). From these results, it has been shown that intercalation compound expansion does not depend on the stage. The exfoliation of high stages leads to exfoliated graphite whose specific area is about 27m 2 g − . The Herold-Daumas model can explain these properties.

Thermodynamic characterization of a dichloromethane-xenon film adsorbed on (0001) graphite

Xenon adsorption between 95 and 115 K on graphite pre-plated with a dichloromethane monolayer results in a partial displacement of the pre-adsorbed layer and in the formation of a two-dimensional (dichloromethane-xenon) solution. When the dichloromethane pre-plating is in the sub-monolayer range, a xenon monolayer is condensed on the bare part of the surface before the composite layer organization. For dichloromethane coverages greater than a monolayer, kinetic problems arise for the formation of xenon bilayer patches, which implies in this case its separation from the two-dimensional solution.