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The Surprising Appearance of Nanotubular Fullerene D5h(1)-C90 Print
Wednesday, 24 November 2010 00:00

The previously undetected fullerene D5h(1)-C90—with a distinct nanotubular shape—has been isolated as the major C90 isomer produced from Sm2O3-doped graphite rods and structurally identified by single-crystal x-ray diffraction. Fullerenes are well-defined molecules that consist of closed cages of carbon atoms and distinct inside and outside surfaces. They tend to form very small crystals; consequently, high-resolution data was collected using small-molecule crystallography at ALS Beamline 11.3.1. The discovery of nanotubular D5h(1)-C90, which is a fullerene with 90 carbon atoms and D5h symmetry, opens a bridge between molecular fullerenes and carbon nanotubes.

Fullerenes Mimicking Nanotubes

The nanotubular fullerene D5h(1)-C90 has demonstrated some interesting properties that straddle the line between molecular fullerenes and carbon nanotubes. It has the same cylindrical shape as nanotubes and a belt of carbon bonds around its diameter.

Since they form discrete units of even-numbered carbon atoms, fullerenes are easier to separate and solubilize than are nanotubes. The class of nanotube-like fullerenes to which D5h(1)-C90 belongs is even more useful because the molecules of that class share the electrical properties of nanotubes. The favorable electronic and optical properties of fullerenes and carbon nanotubes make them useful in applications such as light-harvesting photo cells and related technologies. Scientists will now be able to make use of the duality of this fullerene class, working with them as models with which to study carbon nanotubes.

 

A comparison of the structures of C60 and D5h(1)-C90 as determined by single-crystal x-ray diffraction.

In recent years, the well-known solid allotropes diamond and graphite have been joined by new allotropes: fullerenes, carbon nanotubes, and graphene. Diamond consists of four-coordinate carbon atoms with tetrahedral geometry, while the other allotropes involve three-coordinate carbon atoms. In graphite, these carbon atoms are arranged in hexagonal sheets that are stacked upon one another. Graphene is simply a single hexagonal graphitic sheet with a thickness of only one atom.

Carbon nanotubes can be conceived as hexagonal graphene sheets rolled into cylindrical shapes. These tubes may consist of a single wall of carbon atoms (single-walled carbon nanotubes) or may consist of multiple layers of tubes nested inside one another (multi-wall carbon nanotubes). Carbon nanotubes are produced as mixtures in which the individual tubes can vary in length, width, precise alignment of the component hexagons, and the chemical nature of the unique carbon atoms at the two ends of the tube. Graphene is likewise produced as sheets of varying size with generally less well-defined structures for those carbon atoms at the outer edges.

 

A depiction of two of the motifs of carbon nanotubes, (10,0) "zigzag" and (5,5) "armchair."

Fullerenes of varying sizes (from 60 to more than 500 carbon atoms) have also been observed, and individual molecules such as C60 and C70 have been isolated in pure form. Each fullerene is constructed of 12 pentagonal rings of carbon atoms and a number of hexagonal rings. For example, the prototypical C60, the most readily prepared fullerene, has 20 hexagonal rings in addition to the 12 pentagons.

Isolating higher fullerenes in an isomerically pure form is challenging, especially since the number of isomers increases as the size of the fullerene cage expands, as per the isolated pentagon rule (IPR). The IPR requires that each pentagon be surrounded by five hexagons to avoid strain-inducing pentagon–pentagon contact. There are 46 isomers of C90 that obey the IPR, but none of these isomers had previously been obtained in pure form. Indeed, in the absence of Sm2O3, no D5h(1)-C90 has ever been detected.

The structures of nanotubular fullerenes derived from C60 by successive addition of 10 carbon atoms in a plane through the center of the preceding fullerene. Only the three molecules in yellow have been isolated in pure form. These fullerene molecules are capped versions of (5,5) single-walled carbon nanotubes.

The oblong fullerene D5h(1)-C90 belongs to a set of nanotube-like fullerenes with the formula C60+10n, which have alternating D5h symmetry (when n is odd and the end caps are eclipsed) or D5d symmetry (when n is even and the end caps are staggered). The structure of D5h(1)-C90 (n = 3) is thus closely related to that of C70 (n = 1). However, within this family only C60, C70, and D5h(1)-C90 have been isolated in pure form and characterized crystallographically.

The isolation of D5h(1)-C90 provides a unique molecular model for carbon nanotubes that will allow scientists to explore the chemical and physical properties of a distinctly cylindrical fullerene. The armchair-style belts that are found at the waist of D5h(1)-C90 are a unique feature of this particular fullerene, but are the fundamental building block of carbon nanotubes.

 


 

Research conducted by H. Yang, A. Jiang, Z. Wang, and Z. Liu (Zhejiang University, P.R. China); H. Jin (Jiliang University,  P.R. China); B.Q. Mercado, M.M. Olmstead, and A.L. Balch (University of California, Davis); and C.M. Beavers (Berkeley Lab).

Research funding: National Science Foundation and the Natural Science Foundation of China. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: H. Yang, C.M. Beavers, Z. Wang, A. Jiang, Z. Liu, H. Jin, B.Q. Mercado, M.M. Olmstead, and A.L. Balch, "Isolation of a small carbon nanotube: The surprising appearance of D5h(1)-C90," Angew. Chem. Int. Ed. 49, 886 (2010).

ALS Science Highlight #217

 

ALSNews Vol. 315