Ordered phases of ethylene adsorbed on charged fullerenes and their aggregates

Abstract

In spite of extensive investigations of ethylene adsorbed on graphite, bundles of nanotubes, and crystals of fullerenes, little is known about the existence of commensurate phases; they have escaped detection in almost all previous work. Here we present a combined experimental and theoretical study of ethylene adsorbed on free C₆₀ and its aggregates. The ion yield of [Formula: see text] measured by mass spectrometry reveals a propensity to form a structurally ordered phase on monomers, dimers and trimers of C₆₀ in which all sterically accessible hollow sites over carbon rings are occupied. Presumably the enhancement of the corrugation by the curvature of the fullerene surface favors this phase which is akin to a hypothetical 1 × 1 phase on graphite. Experimental data also reveal the number of molecules in groove sites of the C₆₀ dimer through tetramer. The identity of the sites, adsorption energies and orientations of the adsorbed molecules are determined by molecular dynamics calculations based on quantum chemical potentials, as well as density functional theory. The decrease in orientational order with increasing temperature is also explored in the simulations whereas in the experiment it is impossible to vary the temperature.

Fig. 1

Mass spectrum of helium nanodroplets doped with C₆₀ and ethylene (Et). Shown are the mass regions around the bare C₆₀ monomer through trimer (panels a through c). The most prominent ion series are due to ${({\text{C}}_{60})}_m{\text{Et}}_n^{+}$ (m = 0, 1, 2, 3). Each of these ions forms a group of mass peaks that differ in the number of ¹³C (natural abundance 1.07%). Ions that contain no ¹³C are marked by symbols; ions belonging to the same series (same value of m) are connected by lines. Also marked in panels a and b are isotopically pure ions (no. ¹³C) that contain a water impurity or a CH₂ unit (open symbols). (A colour version of this figure can be viewed online.)

Fig. 2

Mass spectrum of helium nanodroplets doped with C₆₀ and ethylene (Et), displayed in three overlapping sections from 1400 to 3630 u. For each ${({\text{C}}_{60})}_m{\text{Et}}_n^{+})$ ion the most abundant isotopologue is marked, ions with the same value of m are connected by lines. Several significant anomalies in the ion yield versus n are observed; the corresponding (m, n) values are indicated. (A colour version of this figure can be viewed online.)

Fig. 3

Ion yield of C₆₀–ethylene complexes containing up to 4 C₆₀ plotted versus the number n of adsorbed C₂H₄. Significant anomalies are marked; they indicate enhanced adsorption energies. (A colour version of this figure can be viewed online.)

Fig. 4

MD optimized structures of a hollow site in C₆₀C₂H₄ in top view (a) and side view (b), a groove site in (C₆₀)₂C₂H₄ (c) and a dimple site in (C60)₃C₂H₄. The tilt angle β is shown together with the definition of the Et–C₆₀ distance d_Et in panel b.

Fig. 5

Panel a: The cumulative sums of ethylene molecules versus their distance min{d_i} from the nearest C₆₀ for the monomer, dimer, and trimer. Plateaus signal completion of the first adsorption layer. Panel b: The cumulative sums of molecules that lie in the midplane between any pair of fullerenes; plateaus indicate completion of groove sites for the dimer through tetramer. Panel c: The cumulative sums of molecules versus their energy E in the complex, computed as the sum of pairwise interactions (see text for details). A clear plateau is seen only for the C₆₀ monomer. (A colour version of this figure can be viewed online.)

Fig. 6

Adiabatic adsorption energies D_n of ethylene for the dimer (upper panel) and trimer (lower panel) versus the total number n of adsorbed molecules. Blue solid curves represent MD-values and red dashed curves re-optimized PL-DFT values. Abrupt drops in D_n occur when all groove sites are filled. (A colour version of this figure can be viewed online.)

Fig. 7

Temperature dependence of C₂H₄ orientation for the 1 × 1 phase on C₆₀. The CC axis orientation (α, left panel) is measured using the valence angle between the center of mass (COM) of C₆₀, the COM of C₂H₄ and C of C₂H₄. The orientation of the C₂H₄ plane (β, right panel) is measured relative to the plane enclosed by α as dihedral angle (COM-C₆₀, COM-C₂H₄, C, and H). (A colour version of this figure can be viewed online.)

Fig. 8

Relative orientation of neighboring C₂H₄ molecules for the 1 × 1 phase on C₆₀. The angle γ measures the orientation between the two CC axis of two C₂H₄ molecules, corrected for a tilt due to the curvature. γ is produced by projection of the second CC– axis on the second fullerene face and further projection on the first fullerene face. A detailed definition of γ is given in the text. (A colour version of this figure can be viewed online.)