M. Bienfait

Where are the molecules adsorbed on single-walled nanotubes?

Volumetric measurements of Xe, CF4 and SF6 adsorption isotherms on single-walled carbon nanotubes grouped in bundles are reported. Adsorption in the grooves on the outer bundle surface, on the remaining part of the external surface, in the interstitial channels as well as on amorphous carbon impurities is discussed in the light of a comparison of these results with those recorded previously for methane on the same substrate.

Transition bidimensionnelle du premier ordre; cas du xénon adsorbé sur la face (0001) du graphite

Submonolayer adsorption isotherms of xenon condensed on the (0001) face of graphite are measured between 85°K and 102°K by Auger electron spectroscopy. A two-dimensional phase change 2D gas ⇄ 2D solid is emphasized. The solid phase is characterized by low energy electron diffraction. It is a two-dimensional crystal in epitaxy on the graphite. The analysis of the adsorption isotherms measured with a sensivity of 1500 of monolayer, i.e. 1010 atoms, allows to determine the integral heat of adsorption at the two-dimensional phase change (5.5 ± 0.1 kcal mole−1). We also deduced from our measurements, the binding energy of an individual atom of xenon on the (0001) face of graphite, the heat and the entropy of fusion of the two-dimensional crystal.

Adsorption of H2 and D2 on Carbon Nanotube Bundles

Adsorption isotherms of H2 and D2 deposited on single wall, closed end carbon nanotube bundles have been measured. Two temperature ranges were covered, a) T>77 K to study adsorption on high energy binding sites, and b) T<45 K to characterize adsorption on the graphite-like (graphene) outside surface of the bundles. For reference, N2 and Ar isotherms were measured at T>77 K. We observe two well defined steps in the isotherms that correspond to adsorption on at least two distinct locations, grooves/interstitials and the graphene surface. The calculated isosteric heats of adsorption, Qst, in the grooves/interstitials are approximately twice the value on the graphene.

Monolayer adsorption of Kr and Xe on metal surfaces: Structures and uniaxial phase transitions on Cu(110)

Abstract Two-dimensional phase transitions in monolayers of Kr and Xe adsorbed on the (110) face of Cu were studied. As the external gas pressure is raised, a compression of adatoms along the close packed rows is observed. For Xe, first a c(2 × 2) commensurate structure is formed by a continuous transition, then a gradual compression gives rise to a commensurate-incommensurate phase transition, for which the misfit variation is consistent with recent theoretical models. On the other hand, for Kr a first order gas-solid transition is found, followed by a slight compression giving rise to a c(2 × 8) superstructure. A discussion of the results and a comparison between Xe and Kr is presented.

Neutron diffraction and numerical modelling investigation of methane adsorption on bundles of carbon nanotubes

Abstract Neutron diffraction measurements on a powder sample of bundles of single-walled carbon nanotubes are used to gain insight into adsorption sites for methane molecules. In parallel, numerical methods, based on empirical force-fields, are employed to calculate the adsorption of methane in bundles of nanotubes and neutron diffraction patterns based on these structures. Adsorption sites in grooves on the outer surface of bundles and on the curved surfaces of the nanotubes are characterised by their binding energy and relative abundance. Comparison with published adsorption isotherms indicate that the more stable groove sites must also be complemented by interstitial sites. Such sites are found to be energetically unfavourable in homogeneous bundles composed of average diameter nanotubes, but can become favourable in bundles made of bigger diameter tubes and in more realistic, heterogeneous bundles. Nanotube deformation is an important factor, limiting the size of interstitial channels, but also allowing the stabilisation of adsorbed methane molecules. Diffraction patterns calculated for methane–nanotube structures with partially occupied interstitial sites are in best agreement with the experimental diffraction data.

Surface premelting of thin films of methane

Quasi-elastic neutron scattering is used to measure the temperature dependence of the thickness and of the translational mobility of the quasi-liquid phase stable below the bulk melting temperature, at the solid-vapor (111) and (100) interfaces of thin CH 4 films absorbed on graphite and MgO, respectively. It is shown that both the close-packed (111) and the more open (100) surfaces undergo a premelting transition. Both faces have an average mobility of their quasi-liquid layer similar to that of bulk liquid (diffusion coefficient in the 10 −5 cm 2 s −1 range). At the same temperature the quasi-liquid layer is slightly thinner on the (111) surface than on the (100) plane. This nearly complete isotropy with crystallographic orientation is expected from molecular dynamics simulation of van der Waals solids.

Diffusivity of a two-dimensional lattice fluid: CH4 adsorbed on MgO(100)

Abstract The diffusive motions of a 0.8 layer of CH 4 adsorbed on MgO(100) are measured at 72, 88 and 97 K by quasi-elastic neutron scattering. It is shown that at 72 K the methane film is solid and its molecules perform an isotropic rotational motion. At 88 and 97 K, the adsorbed layer is in a two-dimensional fluid state in which the molecules jump between equidistant (4.21 A) lattice sites of the MgO surface. The mean residence time has been determined ( ∼ 1 × 10 −10 s at 88 K and ∼ 4 × 10 −11 s at 97 K). The corresponding translational diffusion coefficients are ∼ 5 × 10 −6 cm 2 s −1 at 88 K and 12 × 10 −6 cm 2 s −1 at 97 K. The diffusivity of this lattice fluid is compared to that of the same molecules adsorbed on graphite (0001) previously reported. The reduced mobility observed in the case of CH 4 /MgO(100) is related to the important depth of the potential wells on the MgO(100) surface.

Surface melting on the close-packed (111) face of methane thin films condensed on graphite

Abstract Quasi-elastic neutron scattering was used to measure the temperature dependence of the thickness and of the translational mobility of the liquid-like phase, stable below the bulk melting temperature, at the solid-vapor interface of thin CH 4 (111) films adsorbed on graphite. It is shown that on the close-packed (111) face of CH 4 surface melting occurs. The average mobility of the liquid-like layer is similar to that of bulk liquid, as well as for the more open (100) face (diffusion coefficient in the 10 −5 cm −2 s −1 range). At the same temperature, the liquid-like layer is slightly thinner on the (111) surface than on the (100) plane. This near isotropy with crystallographic orientation was expected from the molecular dynamics simulation of van der Waals solids.

Interfacial melting of ice in graphite and talc powders

Quasi-elastic neutron scattering of H2O-saturated powders of graphitized carbon black and talc show unfrozen water persisting below the bulk freezing point, to temperatures below -30°C. Several features are consistent with surface melting at the ice-substrate interfaces, but the temperature dependence disagrees with the standard theory. The discrepancies are attributed to size effects, and are supported by model calculations. Similarities with the behavior of unfrozen water in subzero soils suggest that they share the same fundamental mechanisms.

Wetting and multilayer adsorption

Abstract The recent studies on wetting behavior of physisorbed films are reviewed in this article. At very low temperature, it is shown that the wetting of solud films can be complete or incomplete and that incomplete wetting is a wide-spread phenomenon. The rare occurrence of complete wetting results from the absence of strain energy in the corresponding multilayers. On the other hand, incomplete wetting is due, except for the commonplace cases of weak substrate, to the existence of a structural misfit between the lateral crystal structure of the film and its bulk phase. When the temperature is raised, a crossover between incomplete and complete wetting may occur. This wetting transition can be driven by the loss of bulk crystal structure, and hence bulk-film mismatch at melting (wetting transition at the triple point). It can also occur when the film remains solid, at a temperature far from any other two-dimensional or three-dimensional transition. The detailed microscopic reasons for such a transformation are not fully understood.