Neutron Diffraction from Ethylene Adsorbed on Graphite

Abstract

A neutron-diffraction study of deuterated-ethylene overlayers on graphite basal planes reveals the existence of three distinct adsorbed solid phases as well as the coexistence of adsorbed and bulk phases at higher coverages. Evidence of a surface wetting transition involving transfer of molecules from the bulk to adsorbed phase is also obtained.PACS numbers: 68.10. Gw, 64.60.Cn, 68.40.+ e, 68.45.Da In striking contrast to other gases studied to date, ethylene layer formation on graphite basalplane surfaces exhibits a pronounced dependence on temperature. Adsorption isotherms 1 show evidence of an increasing number of layers as the temperature is raised, exactly opposite to what is usually observed. Low-coverage NMR 2 scans made at low temperatures first indicate an adsorbed solid monolayer; then, when the coverage is increased, signs of a second, bulklike component appear. This, on heating, reverts to an adsorbed phase. Evidently raising the temperature switches the system from a configuration in which a second adsorbed layer cannot form to one in which it can. What appears to be taking place is an unusual type of bulk-to-surface phase transition known as the "prewetting" transition. 3 ' 4 We are currently in the process of investigating ethylene overlayers on graphite with neutron diffraction and would like to report here some of what we have learned about the structural relationships underlying this curious competition between two-and three-dimensional ordering. Concurrent with our measurements, Sutton, Mochrie, and Birgeneau 5 have made an x-ray study of the same system. Comparison of the experiments has been of invaluable help in interpreting both sets of data.All of our work was done with a triple-axis neutron spectrometer operated at a fixed energy of 14.4 meV and with the analyzer section of the instrument set to accept only elastically scattered neutrons. A pyrolytic graphite filter was used to remove higher-order contamination. Our sample was deuterated ethylene (C 2 D 4 ) on either Grafoil or slightly recompressed, exfoliated graphite. Substrate surface areas were determined by a separate nitrogen-adsorption isotherm measurement. Thus we define coverage on a scale in which a complete V~3x/~3 commensurate monolayer [nearest-neighbor (nn) distance 4.26 A, area 0.0636 molecule/A 2 ] is assigned unit value even though ethylene seems not to form a commensurate overlayer in any coverage regime. As is usual in this type of experiment, the diffracted signal was obtained by taking the difference between counts with and without the adsorbate in the sample cell.Plotted in Fig. 1 are the low-temperature diffraction patterns observed at a number of repre-2000 C2D4 ON GRAFOIL \H 1000 2.85 FIG. 1. Low-temperature diffraction patterns from ethylene overlayers at various coverages. Warren lineshape fits to the data are represented by the solid lines. The arrows indicate positions of the diffraction peaks of 3D, monoclinic-solid ethylene.

Orientational ordering of ethylene on graphite

“…Ethylene, C2H%, is a planar molecule with carboqcarbon distance 1.338 A, carbon-hydrogen distance 1.093 A, and hydrogen-carbon-hydrogen angle 1 17.2". Both steric arguments and configuration energy calculations suggest that the C=C axes of the molecules in the LD phase are aligned parallel to the substrate surface (8,9). It is also known from Larese and Rollefson's nmr study of the spin-lattice relaxation time, T I , that some type of orientational ordering occurs near 37 K, although the structure of the ordered phase could not be identified (10).…”

“…inconclusive (8). Encouraged by the nmr and heat capacity ~h~ structures and dynamics of rare gas and molecular films studies, however, and by the success of the MD simulations in physisorbed on the basal plane of graphite have been intensively describing the melting behavior of the films, we set out once investigated during the past decade.…”

The low temperature structure of ethylene monolayers physisorbed on the graphite basal plane

“…This is a higher surface density than was used in the neutron measurements of ref. 9. The discrepancy between the calculated and measured density for the LD phase may be due to the fact that the Williams intermolecular potential was too attractive.…”

“…If we define a coverage of rz = 1.0 to be a registered triangular a x fi R30° monolayer, then at coverages below n = 0.8 ethylene forms the so-called low density (LD) phase. Published diffraction data suggest a triangular lattice for this phase, with a nearest neighbor (n-n) distance of 4.65 A (9). For n > 1.05, ethylene forms the high-density (HD) pha!e, which is a triangular lattice with n-n distance of 4.22 A.…”

“…Calculations suggest that in the HD phase the C=C bonds are aligned perpendicular to the plane of the surface, while in the LD phase the C=C bonds are aligned parallel to the plane of the surface (2). Between n = 0.8 and rt = 1.05 a two-phase (HD + LD) region exists (9,15). A variety of experiments indicate that both the LD and HD phases have a transition at about 35 K from an ordered herringbone to a phase in which the molecules are rotationally disordered (1 3-15).…”