(Endo)Fullerenes: From Production to Isolation

DELPHINE FELDER-FLESCH

Institut de Physique et Chimie des Matériaux de Strasbourg, UMR CNRS-UdS 7504 – 23 rue du Loess BP 43, 67034 Strasbourg, France

1.1 Introduction

If the existence of buckminsterfullerenes had been predicted in 1970, they were eventually synthesised in the laboratory 25 years ago, but only today have they just been detected where everyone thought that they were naturally: in space. It is worth noting, however, that the geometry of the molecule is that of a regular truncated icosahedron, a shape already known to Leonardo da Vinci and Albrecht Dürer in the 16th century.

Coming back to the 20th century, exactly twenty five years ago, when Sir Harry Kroto made a success, together with Robert Curl and Richard Smalley, to synthesise a fullerene C60, the structure of this molecule, in the shape of a soccer ball, had struck the imagination. Harry Kroto thought that this kind of structures later named "buckminsterfullerenes" in reference to the geodesic domes of the American architect Richard Buckminster Fuller, had to exist in space. In fact, they had been predicted in 1970 by a Japanese chemist, Eiji Osawa, but Kroto, who had dashed at the same time into a search for carbon chains in the interstellar space, thought that they could form in the carbon stars" atmospheres. Having heard about the work of Richard Smalley and Robert Curl at Rice University, he joined these researchers to simulate these atmospheres in the laboratory and tried to detect the presence of C60 molecules. Their success, announced by a Nature article in 1985,3 won these researchers the Nobel Prize in Chemistry in 1996.

However, no solid proof of the existence of buckminsterfullerenes in space had been obtained up to now, while they were already found in certain meteorites. This has just changed, thanks to the observations made in the infrared light from Tc 1, a planetary nebula consisting of material shed by a dying star. An astronomer, Jan Cami, by means of the space telescope Spitzer, discovered the presence of the fullerene C60 and C70: indeed, buckyballs vibrate in a variety of ways – 174 ways to be exact. Four of these vibration modes cause the molecules to either absorb or emit infrared light. All four modes were detected by Spitzer. As these buckyballs are maintained at the ambient tem- perature of the nebula, they vibrate gently and thus their infrared spectrum becomes particularly easy to identify.

On Earth, fullerenes can be found in soot and in certain rocks. They are most studied in the fields of nanotechnology, hydrogen storage, superconductive materials, and in nanomedicine (endohedrals) as magnetic or nuclear probes.

1.2 Production Methods

1.2.1 The Krätschmer–Huffman Arc-Discharge Apparatus

At the time of the C60 discovery there was just one problem: the new carbon allotrope could only be detected through spectroscopic analysis; therefore, researchers could not visualise fullerenes and see the soccer-ball shape with their own eyes, so to speak. Fullerenes remained a matter of theory.

Things changed in 1990 when W. Krätschmer and R. D. Huffman invented a method for producing fullerenes in large amounts. The invention not only provided practical evidence of the 1985 discovery – and the soccer-ball shape – but it also essentially created a new field of scientific study. During the course of his studies on interstellar dust particles at the University of Arizona in the United States, Krätschmer noticed elements with strange properties among the carbon and graphite samples. What was going on? The answer arrived in 1985, when scientists Harry Kroto, Richard Smalley and Bob Curl created C60 in their laboratory while simulating the high-pressure formation of stars in the universe. Knowing that he had encountered C60 molecules in his laboratory, Krätschmer re-examined his findings. Together with Donald Huffman at the University of Arizona they worked out the "Krätschmer–Huffman method" for producing gram-sized samples of fullerenes (Figure 1.1).

This method consisted in evaporating graphite electrodes via resistive heating in an atmosphere of ~ 100 torr of helium. Although the soot contained only a few per cent by weight of C60, it could be conveniently extracted using benzene as solvent. The red-brown benzene solution could be decanted from the black insoluble soot and then dried using gentle heat, leaving a residue of dark brown to black crystalline material. Mass spectral analysis of this material showed peaks at 720 (C60) and 840 (C70) in an approximate ratio of 10:1.

Shortly after the Krätschmer–Huffman method was reported, Smalley's group at Rice University published a modified design for the "C60 generator". In the Smalley apparatus (Figure 1.2), an electric arc is established between two graphite electrodes. Hence, most of the power is dissipated in the arc and not in resistive heating of the rod. The entire electrode assembly is enclosed in a reaction chamber under a reduced pressure (~ 100 torr) of helium: black soot is produced and extraction with organic solvents yields fullerenes.

Within just three years after these methods were elaborated, scientists filed nearly 300 applications for new patent...