A. Thomy

Two-dimensional phase transitions as displayed by adsorption isotherms on graphite and other lamellar solids

Abstract An introductory review is given to the study of two-dimensional phase transitions, as most easily observed on lamellar solids. Two examples have been chosen, namely the xenon-graphite and krypton-graphite couples, which have been the subject of the most extensive and detailed studies. These illustrate the development of research in the field, providing significant and typical results leading to fundamental questions on the nature of two-dimensional phases and their transitions.

Comparative study of the first adsorbed layer of xenon and krypton on boron nitride and graphite

Abstract The thermodynamic properties of the first adsorbed layer of xenon and krypton on crystalline boron nitride have been studied between 95 and 125°K and between 77 and 93°K, respectively. They are compared to those previously determined for the first layer of the same gases adsorbed on graphite. With both xenon and krypton the final state of the layer is probably the same as on graphite: a self-determined two-dimensional (2D) solid state having approximately the same density and structure as a (111) plane of the 3D crystal which forms at saturation. As on graphite, the xenon layer also undergoes two successive first-order transitions above a definite temperature: a 2D gas → 2D liquid transition and a 2D liquid → 2D solid transition. But with krypton, only one phase transition has been observed whereas three occur on graphite: two first-order transitions (2D gas → 2D liquid and 2D liquid → 2D localized solid) and a localized → delocalized transition which is of higher order. The only transition observed with krypton on boron nitride is first order, but the nature of this transition has not yet been determined.

Propriétés thermodynamiques du film de tétrafluorométhane adsorbé sur graphite

Abstract A set of adsorption isotherms of tetrafluoromethan (CF 4 ) on a homogeneous graphite surface has been determined by volumetric method between 77 and 115 K. The following information can be deduced: the 2D critical point temperature of the different adsorbed monolayers is located above the triple point temperature characterizing the 3D adsorbate (89.5 K). No two-dimensional triple point has been observed, the corresponding temperatures being probably appreciably lower than 77 K. Moreover, the triple points of 2nd, 3rd,… layers are presumably situated in the supersaturation domain; accordingly, the number of the film layers would be limited and would increase with temperature.

Étude comparée de la première couche d'adsorption de méthane sur nitrure de bore et graphite

Abstract The adsorption of methane on crystalline boron nitride has been studied between 77 and 90 K by the volumetric method, especially in the first monolayer domain. It is shown that the thermodynamical properties of the layer are similar to those previously obtained on graphite. It can be inferred from the results that below 77 K, the layer undergoes two first-order phase transitions: a 2D gas-2D liquid transition and a 2D liquid-2D solid transition. In its solid state the localization of the layer is certainly the same on both substrates.

Critical and triple point temperatures of the first adsorbed layer of nitric oxide on graphite, boron nitride and lamellar halides

Abstract The two-dimensional critical temperature T c (2D) of the first layer of nitric oxide adsorbed on homogeneous surfaces of graphite, boron nitride, cadmium halides (CdI 2 , CdBr 2 , CdCl 2 ) and magnesium bromide has been determined as accurately as possible. A correlation curve between T c (2D) and the adsorbent crystalline parameter is compared to the correlation curve of this parameter with the two-dimensional triple point temperature T t (2D) of the same layer on the above substrates and on those previously investigated by Enault and Larher. It appears from this comparison that T c (2D) is less sensitive than T t (2D) to the nature of the substrate: the difference between the extreme values of T c (2D) is 25 K, whereas it is larger than 70 K for T t (2D). Moreover the lowest values of T c (2D) correspond to the substrates for which the lowest values of T t (2D) have also been observed, namely graphite and boron nitride, both standing out by the smoothness of the adsorption potential of their cleavage faces. Furthermore we are led to reconsider the nature of the less dense phase (α phase) which hitherto has been inferred to be a 2D solid essentially composed of dimers lying flat on the surface.

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.

Photoemission of physisorbed Xe as a probe of Ni(111) oxidation under low pressure

Abstract 5p levels of physisorbed xenon have been used to sample the “local” work function on Ni(111) submitted to low pressure oxygen at 330 K. In the chemisorption region, changes in structure are revealed by the appearance of multiple peaks. During oxide growth, results are in agreement with an island growth mechanism, starting from around 10 13 defects/cm 2 .

Adsorption de krypton sur membranes et fibres de carbone

Abstract Surface properties of carbon membranes heattreated at 1000 and 2800°C (made from graphitic oxide) and carbon fibres made from acrylic (AG 12 – 2500°C) and rayon fibres (WYB 2500–3000°C) are studied by physical adsorption. The surface properties reflect the degree of organization in the bulk material: surface homogeneity of graphitized membranes (2800°C) compares favorably with that of the best graphitic surfaces yet known; the surface of the non-graphitized rayon fibres are markedly more heterogeneous than the ones of also non-graphitized but better oriented acrylic fibres.

Adsorption de krypton sur Papyex dans le domaine de la monocouche

Among the different samples used for studying physisorbed films on graphitic surfaces, compressed exfoliated graphite, such as Papyex or Grafoil, plays an outstanding role, as it is particularly suitable for scattering experiments. But the compression performed to prepare this kind of sample has been shown to increase the heterogeneity of the surface, which results, as was observed in some cases [6,7,9] in an alteration of the shapes of isotherms, and a slowing down of the adsorption kinetics, with respect to more homogeneous graphitic surfaces. In this work, thermodynamical data characteristic for a physisorbed monolayer of krypton on Papyex have been determined from volumetric measurements performed either at constant temperature or at constant fractional coverage and compared to results obtained on more homogeneous surfaces. The isotherm set between 78 and 90 K is given in Figs. 2 and 3. It is on the whole compatible with the phase diagrams obtained on more homogeneous graphites (Fig. 1). But a slight smoothing of the steps, as well as a slowing down of the adsorption kinetics, which appears mainly for low temperatures and solid two-dimensional (2D) films, are observed as they were before [6, 7,9]. Consistency between these results and those obtained by measurements at constant fractional coverage enables us to assume that adsorption equilibrium is reached in the temperature range common to both kinds of experiments. The constants A and B of the equation logP = −(A/T) + B, representing the dependence of the equilibrium pressure on temperature at constant fractional coverage, had been determined from volumetric measurement results, and also compared to values previously obtained in similar conditions on more homogeneous substrates, for different stages of the film formation (Table 1). Values for both types of samples are in good agreement, except for the 2D gas-solid transition (θ = 0.4 − T < 85 K), as could be foreseen from the shapes of isotherms in this domain, namely the bending of the linear part towards higher pressures than on more homogeneous graphitic substrates. The 2D triple point and critical temperatures with very similar values have been proposed before, which were determined in using graphite SPl type[10], while from other results it has been concluded that these two characteristic temperatures actually do not exist[11]. Owing to the lack of precision of our determinations the existence or the non existence of a 2D liquid domain cannot be asserted. As a conclusion, the main features of the krypton phase diagram being preserved, it can be assumed that the structural properties of the films determined on Papyex are, on the whole, characteristic of adsorption on a uniform surface.