Opening carbon nanotubes with oxygen and implications for filling

CAPPED hollow carbon nanotubes^1,2 can be modified into nanocomposite fibres by simultaneous opening of the caps (by heating in the presence of air and lead metal) and filling of the interior with an inorganic phase³. To generalize this approach, greater understanding is needed of the reaction mechanism between the tube caps and oxygen. Here we report that the oxidation of carbon nanotubes in air for short durations above about 700 ^°C results in the etching away of the tube caps and the thinning of tubes through layer-by-layer peeling of the outer layers, starting from the cap region. The oxidation reaction follows an Arrhenius-type relation with an activation energy barrier of about 225 kJ mol^-1 in air. Heating of closed nanotubes with an oxide (Pb₃O₄) in an inert atmosphere lowers the activation barrier for the reaction and opening of the tubes occurs at lower temperatures. Contrary to intuition, however, open tubes are much more difficult to fill with inorganic materials than in the one-step filling of tubes reported previously³. But various other experiments might be possible in the inner nano-cavities of the open tubes such as studies of catalysis and of low-dimensional chemistry and physics.