Synthesis, properties, and applications of carbon nanotubes filled with foreign materials: a review

Carbon nanotubes (CNTs)—hollow microtubules of graphitic carbon—possess unique physical properties by themselves; meanwhile, their empty cylindrical cores also offer the storage of foreign materials, giving rise to a new set of hybrid nanostructures. Filling CNT cores not only reinforces nanotubes in the radial direction preventing from buckling but also integrates the attributes of both the host CNTs and guest fillers simultaneously. A low dimensionality, higher anisotropy, quantum confinement, and enlarged surface-to-volume ratio owned by these nanohybrids can generate distinctive magnetic, electrochemical, optical, electromagnetic, and other physicochemical properties. Encapsulation of a wide variety of inorganic and organic materials inside CNTs in different structural forms such as nanoparticles, nanorods, and atomically thin nanowires has documented many interesting and useful applications in devices such as lithium-ion batteries, microwave absorbers, biosensors/chemosensors, data storage units, drug delivery systems, nanothermometers, nanofilters, memory devices, etc. Thus, research studies have affirmed that filled CNTs-based devices can lead the world to a new technological level; however, there are some serious barriers perturbing their commercialization. Till date, numerous parameters have been identified to influence the filling process during both in situ and ex situ filling methods, but there is still a lack of complete understanding in the filling mechanism which governs the filling yield, size, and structure of CNTs, as well as those of filling materials. This article presents an overview of recent progress made in the synthesis of filled CNTs and the study of their properties and functional applications. We also outline the existing challenges in the field and communicate their possible resolutions.