Filling of Carbon Nanotubes with Compounds in Solution or Melted Phase

Since their discovery, carbon nanotubes (CNT) have been found to exhibit remarkable structural, mechanical and electronic properties. One such property is the ability to encapsulate foreign materials inside their cylindrical cavity, for application in different fields. The procedures to fill CNT may be classified into two main groups: (a) filling in solution, using the wet chemistry route and (b) filling with a melted phase. In both cases, the filling is induced by the capillary forces. It is also possible to fill CNT in the vapour phase, although there are only few very specific examples available in the literature to date. After filling, oxides and metallic particles can be obtained by a subsequent thermal annealing in the required atmosphere. In the wet chemistry route, the nanotubes are usually treated by an oxidizing agent in order to open their tips. The filling is then performed by placing the opened tubes in a solution of the selected compound (or a precursor). When the compound is dissolved in an oxidizing acid such as nitric acid (HNO₃), it is possible to combine opening and filling in a single step. Although this method allows the introduction of heat-sensitive species inside carbon nanotubes, the yield varies strongly with the diameter of the carbon nanotubes and is generally rather low in the case of CNT with a small inner diameter. This filling route mainly leads to isolated nanoparticles or short nanowires. Filling with melted compounds is a solvent-free route. The CNT are directly immersed in the melted material and capillary forces drive the compound into the CNT. Although this route is more restrictive in terms of materials, it allows for the continuous filling of CNT with long nanocrystals (up to a few micrometers), with a higher filling yield in the available CNT (up to ca. 70%). This chapter will describe these two different methods for filling CNT and illustrate them with a few selected examples.