FLASH OF LIGHT IGNITES NANOTUBES
Even in absence of burning, bonds break and carbon atoms rearrange
Single-walled carbon nanotubes (SWNTs) exposed at close range to a conventional photographic flash burst into flame.
This unusual behavior was discovered accidentally in the laboratory of Pulickel M. Ajayan, associate professor of materials engineering at Rensselaer Polytechnic Institute. Undergraduate student Andres de la Guardia was working in the lab, taking photographs of freshly grown nanotubes, when he noticed that the flash was causing the sample to emit a popping sound. Investigating further, he and Ajayan found that at close range, the flash also ignites the nanotubes.
Ajayan and his collaborators in North America and Europe report that SWNTs prepared by vaporizing graphite using a carbon arc or a laser or by chemical vapor deposition exhibit this behavior, as long as the nanotube samples are dry and "fluffy." By contrast, ignition does not occur for multiwalled nanotubes, graphite powder, fluffy carbon soot, or C60 [Science, 296, 705 (2002)].
Why the difference? Ajayan believes that the nanotubes interact strongly with the light, absorbing a substantial amount of energy as heat. If the nanotube samples are densely packed, the nanotube bundles that crisscross each other rapidly dissipate the heat wave. However, when the material is fluffy, the heat is more likely to be confined locally within the nanotube bundles. According to the group's estimates, temperatures in these bundles climb to at least 1,500 °C, which is well above the temperature required for ignition when oxygen is present.
Combustion, however, is not the only process triggered by the photon flash. Electron microscopy studies of SWNT samples flashed in air, in inert atmospheres, or under vacuum reveal that the material's structure undergoes a surprising amount of bond breakage and atomic rearrangement, even in the absence of burning. In many cases, "shorter carbon nanostructures reminiscent of poorly graphitized carbon" are formed, Ajayan tells C&EN. In helium, the flash leaves behind very few nanotube structures and large amounts of "nanohorns"--short, irregular tubes with conical caps.
Ajayan speculates that this newly discovered behavior of nanotubes could perhaps be applied to triggering combustion or explosive reactions remotely using light and some well-placed nanotubes. One could also imagine making nanotube-containing remote sensors that would be actuated by light, he says.
Angel Rubio, a professor in the materials physics department at the University of the Basque Country, in Donostia-San Sebastián, Spain, says the work is "very impressive." He notes that this is the first time that simple carbon-based materials have been shown to ignite in response to a light flash. The effect, he believes, is intimately tied to the confinement of absorbed energy in the nanoscale structures of SWNTs. "There is important physics and chemistry behind the light-induced structural modification that needs to be understood better so as to control the product," Rubio says.
Ajayan agrees, commenting that "the interaction of radiation such as light with nanotubes will be fascinating to study in more detail."