|It's not just because carbon nanotubes usually are empty that researchers try to trap atoms or molecules inside them. Scientists suspect that nanotubes filled with metal atoms could have promising applications as catalysts and semiconductors.
Now, a research group in Japan has devised a method to precisely control the size and location of metal clusters within a single-walled carbon nanohorn--a megaphone-shaped type of nanotube [Proc. Natl. Acad. Sci. USA, 101, 8527 (2004)]. University of Tokyo chemistry professor Eiichi Nakamura and Sumio Iijima of NEC Corp. and Japan's Research Center for Advanced Carbon Materials spearheaded the research.
To construct the metal-encapsulated structures, the group begins by heating the nanohorns at 420 to 580 °C in the presence of O2. Under these conditions, the nanohorns become pierced and the edges of the new openings are oxidized, creating small regions of hydrophilicity in the hydrophobic graphene structure.
When the perforated nanohorns are placed in a methanolic solution with Gd(OAc)3·(H2O)4, the hydrophilic Gd(III) ions accumulate one by one at the oxidized openings. The researchers expect that other metal ions will behave in a similar fashion. By observing this process, they conclude that the size and location of the opening ultimately determine how many metal ions will aggregate there and whether they will be encapsulated by the tube.
|SEEING SPOTS TEM image and corresponding model of a Gd atom (blue) lodged at the oxidized tip of a nanohorn.
PNAS/COURTESY OF EIICHI NAKAMURA
"Thanks to the availability of the first scanning transmission electron microscope (STEM) capable of performing high-resolution electron energy loss spectrometry for a 3- 3-Å area, we could identify, for the first time, metals trapped within the graphene wall or within the interior of the nanohorn," Nakamura says.
The group noted that metal ions won't travel through openings at the tapered tips of a nanohorn, but instead, a lone atom can become lodged there.
However, metal clusters will form inside the structure if the opening is in the nanohorn's side. There, a cluster of metal ions will develop inside the tube until the aggregate extends to the internal hydrophobic wall opposite the opening. The researchers call the technique a "ship-in-bottle" synthesis.
"The impact of such a technique lies in our ability to control defect dynamics in nanotube structures and hence controllably create hybrid nanotube-based structures," comments Pulickel M. Ajayan, a materials engineering professor at Rensselaer Polytechnic Institute, Troy, N.Y. He describes the report as "a very nice demonstration of how in situ microscopy can be used to reveal real-time events and follow the dynamics of reactions, defect creation, and encapsulation. Kudos go to the masterly microscopy and amazingly clear images."