lunes, 12 de marzo de 2012

Carbon Nanotubes: From Nano Test Tube to Nano-Reactor

One of the most powerful methodologies for manipulation of individual molecules and atoms is based on entrapment and concinement and utilizes the principles of supramolecular host-guest chemistry. Over the past three decades, a wide range of hollow nano-containers have been developed including calixarenes, cyclodextrins, cucurbiturils, supramolecular/coordination cages, and bilayer vesicles. Even though carbon nanotubes (CNTs) have made their entrance as nano-containers much more recently their potential advantages over these established systems are already clear. Being built of sp2-carbon atoms held together by strong covalent bonds, CNTs are significantly more thermally stable and mechanically more robust than any other molecular or supramolecular nanocontainers. The concave side of the nanotube has very low chemical reactivity, so that some aggressive chemical process can be contained within the CNT. Furthermore, single-walled carbon nanotubes (SWNTs) are atomically thin, which means that guest molecules can be studied directly, in real space and at the atomic level, by high-resolution transmission electron microscopy (HRTEM); an imaging tool that can “see” through the nanotube wall.

The nanotube is not just a passive container: In many cases, it acts as an effective template to control the structures of molecular and atomic arrays. In this issue of ACS Nano, Allen et al. explore the behavior of fullerene molecules inside an atypically wide SWNT (d = 2.1 nm), where the guest molecules spontaneously arrange themselves into a zigzag array. The enhanced surface area of the intermolecular contacts significantly stabilizes the molecular array, so that the entire zigzag superstructure turns around its axis without breaking up. In response to the corkscrew motion of the guest molecules, the host nanotube becomes elliptically distorted.

The observed diameter of the SWNT fluctuates by as much as 30% over time. The observation that the SWNT can deviate so significantly from the idealized cylindrical shape has important implications for further development of nanotubes as templates since the elastic distortions of nanotubes (particularly wider SWNTs) must be taken into consideration when predicting the structure or rationalizing the dynamic behavior of the encapsulated guest molecules.

It is logical to extend applications of carbon nanotubes one step further, beyond simple nanocontainers, and toward nanoscale reaction vessels. Shortly after the discovery that molecules can be inserted into SWNTs, it was demonstrated that under irradiation by the electron beam or at high temperature the fullerene guest molecules can oligomerize, coalesce, and merge into corugated tubular structures nested inside the host SWNT. At elevated temperatures, structures formed in this way can anneal further into perfect double-walled nanotube.

Find more information on this work by Allen et al. at ACS:

Khlobystov, A. (2011). Carbon nanotubes: from nano test tube to nano-reactor. ACS Nano, 5(12), 9306-9312.

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