Igniting Nanotubes with a Flash (2002)

The astonishing results of P. M. Ajayan et al. (“Nanotubes in a flash—ignition and reconstruction,” Brevia, 26 April, p. [705][1]) demonstrating ignition of carbon nanotubes after exposure to a photographic flash inspired us to make further explorations along the same lines. We found that similar effects can be obtained with other carbons that bear noble metal catalysts, for example, Pd on carbon.

We first reproduced both the photoacoustic effect and the ignition of single-walled carbon nanotubes. Exposing commercially available nanotubes (HiPco or as-prepared samples from Tubes@Rice) to a flash from an ordinary photoflash unit held 1 to 3 cm from the surface of the samples resulted in easily heard retorts. Sound intensity increased as the flash was moved closer to the samples. On very close approach (<1 cm), both nanotube samples ignited with a dull red glow. These two materials contain catalytic particles of iron or nickel/cobalt alloy, respectively, in addition to carbon. Other carbons (Norit-activated carbon and graphite powder) produced a weaker photoacoustic effect but did not ignite. However, several commercial Pd catalysts supported on carbon (Pd loadings of 5, 10, and 30 weight %) all produced marked photoacoustic effects and ignition. Simple physical mixtures of Pd powder, iron carbide powder, or copper powder with graphite or Norit produced photoacoustic effects but not ignition. Similarly, graphite powder that was sputter-coated with very small amounts of Pd did not ignite.

Our simple survey indicates that the photoacoustic effect and ignition are not peculiar to carbon nanotubes. The common features of materials that ignite are the combination of a well-dispersed metal catalyst in intimate contact with a high-surface area carbon. Although the mechanism of this ignition process is unclear, we hypothesize that it arises from photophysical effects associated with metal and carbon in chemical contact.

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Bockrath et al. Confirm our experimental observation about the photoignition of single-walled carbon nanotubes (SWNTs) but suggest that the catalytic particles present in the sample play a key role in the ignition process. Although this might be the case, the ignition process is far more complicated than that. Indeed, the lack of observable photo-induced sound and ignition in our experiments with chemical vapor deposition-grown multiwalled nanotubes (MWNTs) and compacted SWNTs, both of which have catalyst particles of a similar fraction, size, and chemical state to those of uncompacted SWNTs, indicates that the unique structure of the carbon and the density of the nanotubes are the primary factors that give rise to photoacoustic response and ignition. In other words, as we indicated in our Brevia, the heat confinement in the carbon structures (i.e., avoiding dissipation into the bulk) is necessary to achieve the temperatures required for ignition. At that point, the catalytic particles most likely help ignition, as Bockrath et al. 's results indicate.

The structural reconstruction of the SWNTs (during exposure to light flash in vacuum or inert gas atmospheres) in the absence of ignition (in air) occurs throughout the sample, not just near the relatively sparse catalytic particles; the high temperatures required for such reconstructions will naturally oxidize and burn carbon in air whether catalysts are present or not.

Although the results of Bockrath et al. show that the catalytic particles play a favorable role in the ignition process, the exact mechanism is not understood. In particular, the role of the density, nature, and dimensions of the carbon surfaces; the nature and size of catalytic particles; and the carbon-catalytic interface need to be determined. Finally, it would also be crucial to test metal-free SWNTs, but, unfortunately, such samples can only be prepared by purification procedures that effectively densify the samples and, hence, remove the photo-induced effects.

 [1]: /lookup/doi/10.1126/science.296.5568.705