Adsorption of Polar and Nonpolar Molecules on Isolated Cationic C₆₀, C₇₀, and Their Aggregates

Abstract

Physisorption on graphite, graphene, nanotubes, and other graphitic structures has been the subject of numerous studies, partly driven by interest in the nature of order in two‐dimensional systems, their phase transitions, and the use of graphitic scaffolds for reversible storage of hydrogen at high volumetric density and low mass. In contrast, physisorption on individual fullerenes or small aggregates of fullerenes has remained largely unexplored, last but not least, because of technical challenges. A summary of recent progress in identifying specific adsorption sites on positively charged C60, C70, and their aggregates is given in this Minireview. Adsorption energies and storage capacities for helium, hydrogen, methane, oxygen, nitrogen, water, and ammonia are determined. Mass spectrometric data reveal the formation of a commensurate phase in which all hollow sites of C60 or C70 are occupied. This phase is identified for all nonpolar molecules, including oxygen, which does not form a commensurate phase on planar graphite. The polar molecules, on the other hand, do not wet fullerenes and they do not form this commensurate phase. A hierarchy of other distinct adsorption sites are identified for nonpolar molecules, namely, groove sites for fullerene dimers and beyond, and dimple sites for fullerene trimers and beyond. Furthermore, evidence is presented for the preferential adsorption of hydrogen and methane in registered sites on fullerene dimers. The interpretation of experimental data that merely count the number of preferred adsorption sites is aided by molecular dynamics simulations, which utilize interaction potentials derived from ab initio calculations to determine adsorption energies.

Adsorption of helium on isolated C60and C70anions

“…At the same time, curvature increases the distance between atoms adsorbed at hollow sites to an extent that not only helium but even larger particles (including hydrogen, methane, ethylene or nitrogen) can be accommodated in the 1 × 1 phase of C 60 , with one particle each at the centres of the 12 pentagons and 20 hexagons (25 hexagons for C 70 ) [19,22,24,25], for a review see [12]. The anomaly at n = 32 also appeared in mass spectra of C 60 that was allowed to react in a gas aggregation cell with red phosphorous [26] or alkaline earth metals [27].…”

“…In a series of experiments we have recently studied the adsorption of helium [11] and several molecular gases [12] on isolated C + 60 and C + 70 cations. Mass spectra indicate a strong energetic preference for formation of the 1 × 1 phase in which the 12 pentagons plus 20 hexagons of C 60 (25 hexagons for C 70 ) are decorated with one particle each.…”

Adsorption of sodium and cesium on aggregates of C60

“…It is conceivable that large fullerene complexes offer nucleation sites not found for C 60 or small (C 60 ) m complexes that favor formation of a metallic droplet [44]. Dimple sites offer enhanced adsorption energies for physisorbed molecules [56,57]. But dimple sites exist for all (C 60 ) m complexes with m 3; what is special about m = 4?…”

On the Size and Structure of Helium Snowballs Formed around Charged Atoms and Clusters of Noble Gases

“…15,23 Here, n = 12 is a frequently observed anomaly, often interpreted to indicate formation of a solvation shell with icosahedral symmetry, 22,24 but the number of helium atoms in the first solvation shell may be much larger. For example, 18–20 atoms are needed to complete the first solvation shell for Mg + , 20,25 20 for I 2 + , 21 60 for C 60 + , and 62 for C 70 + .…”