TRANSITION-METAL ATOMS INTO CARBON NANOTUBES

Transition-Metal Atoms And Their Small Clusters Into Carbon Nanotubes

Abstract

We have developed a method that enables the efficient insertion of transition-metal atoms and their small clusters into carbon nanotubes. As a model system, Os complexes attached to the exterior of fullerene C60 (exohedral metallofullerenes) were shown to be dragged into the nanotube spontaneously and irreversibly due to strong van der Waals interactions, specific to fullerenes and carbon nanotubes. The size of the metal-containing groups attached to C60 was shown to be critical for successful insertion, as functional groups too bulky to enter the nanotube were stripped off the fullerene during the encapsulation process. Once inside the nanotube, Os atoms catalyse polymerisation and decomposition of fullerene cages, which is related to a much higher catalytic activity of metal atoms situated on the surface of the fullerene cage, as compared to metal atoms in endohedral fullerenes, such as M@C82. Thus, exohedral metallofullerenes show promise for applications in catalysis in carbon “nano” test tubes.

Transition-Metal Atoms And Their Small Clusters Into Carbon Nanotubes

Introduction

Owing to their extremely high mechanical stability, relative chemical inertness, and the availability of different diameters from sub-nanometre to hundreds of nanometres, carbon nanotubes have been widely used as nano-sized containers for molecules and atoms.1 Molecules and atoms encapsulated in nanotubes are often arranged in geometrically regular, 1D arrays, some of which do not exist outside carbon nanotubes and thus can be regarded as products of confinement at the nanoscale.1 The structural and dynamic properties of some materials encapsulated in nanotubes change drastically as a result of this confinement, and there is a growing body of evidence that the chemical reactivity of encapsulated compounds can also be altered inside carbon nanotubes (Iijima, 2001, 11).

In turn, the physicochemical properties of nanotubes can be significantly affected by the species present inside, and this offers a methodology for tuning the functional properties of nanotubes, such as the electronic band gap, and the concentration and mobility of charge carriers. Transition metal3 and lanthanide containing molecules appear to be most effective for this purpose as the metal atoms encapsulated in nanotubes provide magnetically, optically, redox or catalytically active centres within the nanotube structure.

The presence of well-defined metal centres within carbon nanotubes is highly desirable, but the methods of insertion of metals in nanotubes are still far from perfect. Insertion of liquid metals in nanotubes is prohibitive because of their high surface tension, whereas insertion of molten metal salts, which also relies upon capillary forces, is cumbersome because of the high melting temperature of most transition-metal salts. As a result, neither of these methods produces nanotubes efficiently filled with well-defined, metal-containing species.

The most efficient method of insertion of metals into nanotubes relies on specific non-covalent interactions between the nanotube and the guest species. Endohedral metallofullerenes—M@Cn (Figure 1?a)—in which a single metal atom or a small cluster of metal atoms are incarcerated in a cage of n carbon atoms, are strongly attracted to the internal cavity of the nanotube due to the highly effective van der ...