M. Chorro (PhD thesis), J. Cambedouzou and P. Launois, LPS.
Collaboration with the team of M. Monthioux (CEMES, Toulouse), S. Rols (ILL, Grenoble) and the team of B. Sundqvist (Umeå university, Sweden).
EuroPhysics Letters 79, 56003 (2007).
Carbon science has known an increase of interest since the discovery of two new nano-scale materials: fullerenes in 1985 and carbon nanotubes (nano-cylinders) in 1993. In 1998 the observation of fullerenes inside nanotubes offered to chemists and physicists a new ‘all carbon’ object of study. Such new nanostructures obtained by the insertion of fullerenes in the hollow cavity of the nanotubes are called ‘peapods’, the structure reminding pods (nanotubes) full of peas (fullerenes). Different varieties of peapods are synthesized according to the kind of molecule inserted, for instance the C60 molecule, which looks like a soccer ball, or the C70 molecule, which has a rugby ball shape.
Does the confinement have an effect on the physico-chemical properties of the fullerenes inside the nanotube?
We tried to induce a reaction well-known in the three dimensional C60 and C70 crystals, namely, the fullerene polymerization. During this reaction, covalent bonds are formed in between fullerenes. We considered the pressure and temperature range leading to the formation of one-dimensional chains of polymerized fullerenes in the bulk crystal. We have shown using X-ray scattering that C60 molecules polymerize, while C70 ones do not.
Why? In order to polymerize, fullerenes have to present one of their double-bonds in front of one belonging to a neighboring molecule in a perfect face to face. This implies a particular orientation of the C60 and C70 molecules, with more geometrical constrains for C70 because only the polar double-bonds are involved in the polymerization. Our results are explained according to the following simple considerations. Under the combined effects of temperature and pressure, C60 molecules can rotate and come close together. This increases the probability for the formation of covalent bonds between them. On the other hand, C70 molecules do not have enough space in the tube to translate and rotate in order to reach a configuration compatible with their polymerization.
This simple example shows that the confinement induced by the carbon nanotube wall affects the chemistry of inserted fullerenes.