Classroom-Scale Demonstrations Using Flash Ignition of Carbon

In the Classroom edited by

JCE DigiDemos: Tested Demonstrations

Ed Vitz Kutztown University Kutztown, PA 19530

Classroom-Scale Demonstrations Using Flash Ignition of Carbon Nanotubes submitted by:

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Dean J. Campbell* and Kylee E. Korte Department of Chemistry and Biochemistry, Bradley University, Peoria, IL 61625; *[email protected] Jesse T. McCann and Younan Xia Department of Chemistry, University of Washington, Seattle, WA 98195

checked by:

Daniel T. Haworth and Mark R. Bartelt Department of Chemistry, Marquette University, Milwaukee, WI 53201-1881

Recently, an undergraduate student and a professor photographing a sample of single-walled carbon nanotubes noticed that the camera flash triggered the ignition of the sample (1, 2). It appears that the extraordinary light absorption properties of the tubes and their remarkably small sizes lead to their rapid heating under the brief but intense illumination of the camera flash. This phenomenon is not observed in solid graphite or in tightly-packed carbon nanotubes. Instead, it is only observed when the single-walled nanotubes are loosely packed and the energy of the flash that is absorbed by the tubes cannot easily dissipate. If the tubes are flashed in an inert atmosphere, bonds within the nanotubes rearrange, indicating that their internal temperatures reach at least 1500 C (1, 2). If the tubes are flashed in air, their high surface areas enable their rapid combustion. The flash-driven combustion is often accompanied by a popping sound, referred to as a photoacoustic effect. Since this discovery, a few other very finely divided carbon structures have been shown to ignite or at least exhibit a photoacoustic effect when exposed to a camera flash (3, 4). Some evidence suggests that this ignition is assisted by the presence of metals in the material (3, 4). Microwaves have also been shown to ignite carbon nanotubes (5). Besides elemental carbon, other types of nanostructures such as silicon nanowires (6), polyaniline nanofibers (7, 8), and hollow gold nanocages (9) have been found to respond to photographic light flashes by melting, fusing, or oxidation. Gold nanostructures such as rods (10–12) and shells (13) have been melted with lasers. The flash ignition of nanotubes has at least two educational applications. This effect relies on the very small dimensions of the carbon nanotubes and therefore is an interesting way to illustrate that nanotechnology can take advantage of unique (and sometimes unexpected) properties of nanoscale materials. (For perspective, a SEM image of a carbon nanotube mat draped over a human hair is available in the Supplemental MaterialW ) From a general chemistry standpoint, the combustion of nanotubes illustrates the importance of surface area in chemical reactions. Extremely finely divided carbon structures such as carbon nanotubes can trap sufficient energy from a camera flash to ignite, while more bulky carbon structures such as charcoal briquettes will not. This is reminiscent of the rapid combustion of iron or lycopodium powders, in which the rate of the combustion of these powdered materials is significantly enhanced by their high surface area in contact with air (14). www.JCE.DivCHED.org

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Although simply observing the effect of the camera flash over a heap of nanotubes is interesting, it is only readily observed close to the nanotubes themselves. Movies of flashinitiated (15) and microwave-initiated (5) combustion have been available on the Internet. A way to make the nanotube combustion more readily observable by a larger group of people in a live setting is to use that combustion to somehow change another substance, for example, by melting a material or igniting a larger fire. In fact, one of the potential applications of nanotube combustion is to use it to ignite other materials (4). Three demonstrations are outlined in this article. Two are based on the melting or combustion of stretched polymer films. These films include Parafilm, latex (e.g., from an examination glove), clear adhesive tape, and plastic food wrap. One of these demonstrations can be visualized on an overhead projector. A third demonstration is based on the combustion of flash paper. Flash paper is paper containing nitrated cellulose fibers. This very flammable paper is available from magic supply companies or can be produced in a laboratory setting with nitric and sulfuric acids (16). Materials • Single-walled carbon nanotubes1 • External camera flasher2 • Lab goggles and lab gloves • Ring stand with fireproof clamp or ring • Scissors or razor blade • Parafilm laboratory film3 • Crystallization dish: 50-mm × 70-mm • Clear adhesive tape4 • Spatula • Forceps • Permanent marker • Foldback binder clips5 • Aluminum foil or disposable metal tray • Newspaper • Flash paper6 • Spray adhesive7

Vol. 83 No. 10 October 2006

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Journal of Chemical Education

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In the Classroom

Nano Hair Trigger

Experimental Procedures

Nanotubes on a Drum Stretch a Parafilm sheet (or other plastic film) over the opening of the crystallization dish.8 Parafilm can be stretched to less than one-fifth of its original thickness. A 5-cm × 5cm sheet can be easily stretched to cover a 7-cm-wide dish. To improve the visibility of the thin sheet on an overhead projector, a permanent marker can be used to draw on the film either before or after stretching. Place a small quantity (roughly 1 cm3) of nanotubes on the film (Figure 1A). A spatula is acceptable for moving the tubes, but the tines of an open forceps are also useful for transferring the fluffy material. It is best to do this away from any drafts that could blow the nanotubes away. Make sure that all flammable materials are removed from the vicinity of the demonstration. Hold the camera flasher right over the nanotubes. Fire the flash and move it away quickly. There will likely be an audible popping sound from the photoacoustic response of the nanotubes. The nanotubes will ignite, causing the film to melt or burn as shown in Figure 1B. A film under tension will widen its own holes, making for a more dramatic film failure. (A movie of the demonstration is available in the Supplemental Material.W)

Figure 1. Images captured from a movie of the melting of Parafilm: (A) The nanotubes are placed on a sheet of Parafilm stretched over a crystallizing dish. Marks of 1-cm in spacing have been placed on the film to increase its visibility and to provide a sense of scale. (B) Flash-initiated combustion of the nanotubes burns and melts the polymer film.

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Suspend a hook (wire or a bent paper clip) from a ring or clamp mounted on a ring stand. Cut a 0.3-cm × 10-cm strip of adhesive tape (or other plastic film) using a razor blade or scissors. Attach a foldback paper clamp to each end of the tape strip. Bend the tape strip into a U shape (sticky side out), by bringing the clamps together. Dip the bottom of the U into the container of nanotubes. If the container neck is relatively narrow, place the eraser of a pencil at the bottom of the U (on the non-sticky side) to make the tape loop narrower. The tubes will likely stick to the adhesive-coated side of the tape much better than the uncoated side. Gently lift the strip out of the container, allowing some of the excess, unbound fluff to fall back into the container. The nanotubes remaining on the strip should be allowed to project away from the strip surface to maximize contact with the air. Pressing the tubes down on the tape will reduce their contact with air and drastically reduce their chances of flash-induced combustion. Gently hang one clipped end of the nanotube-laden tape strip from the hook on the ring stand. Place the aluminum foil or disposable metal tray on the counter under the tape to help prevent the slim chance of flames spreading from the burning strip. Make sure that all flammable materials are removed from the vicinity of the demonstration. Hold the camera flasher very close to the nanotube-laden tape. Fire the flash. There may be an audible popping sound from the photoacoustic response of the nanotubes, and the nanotubes will ignite, causing the tape strip to melt or burn. When the strip melts through, the large foldback paper clamp drops to the foil-covered countertop with a dramatic clank.

Flash-Ignited Flash Paper Suspend a hook (wire or a bent paper clip) from a ring or clamp mounted on a ring stand. Suspend a foldback paper clamp from the hook. Cut a 1-cm × 4-cm strip of flash paper using scissors. Combustion of this small quantity of paper appears to be fairly innocuous. Lay the strip of flash paper on newspaper or some other disposable surface. The strips are very light, so tape one end down to avoid blowing them around with the spray adhesive. Spray a light coating of spray adhesive onto the strip. To achieve a thin coating, it may help to direct the bulk of the spray near, but not directly on, the strip. Too much adhesive seems to interfere with the combustion of the strip. Alternatively, a light coating of glue from a glue stick has been used by one of the reviewers of this article as an adhesive. While the adhesive is still sticky, use a forceps to pick up the strip and place it into a container holding the nanotubes. The tubes will likely stick to the adhesive-coated side of the strip much better than the uncoated side. Gently lift the strip over the container, allowing some of the excess, unbound fluff to fall back into the container. The nanotubes remaining on the strip should be allowed to project away from the strip surface to maximize contact with the air. Pressing the tubes down on the paper will reduce their contact with air and drastically reduce their chances of flash-induced combustion.


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In the Classroom

Gently hang the nanotube-laden flash paper strip from the foldback paper clamp on the ring stand. Gently hang the second foldback paper clamp from the bottom end of the flash paper to keep the strip fully extended, as shown in Figure 2A. Larger clamps will be more dramatic when they fall. Place the aluminum foil or disposable metal tray on the counter under the flash paper to reduce the slim chance of flames spreading from the burning paper. Make sure that all flammable materials are removed from the vicinity of the demonstration. Hold the camera flasher very close to the nanotube-laden flash paper. Orient the flash bulb parallel to the direction of the strip so that the maximum flash energy is directed to as many nanotubes as possible. Fire the flash. There will likely be an audible popping sound from the photoacoustic response of the nanotubes, and the flash paper should ignite into a bright flare (Figure 2B). As the paper rapidly burns, the large foldback paper clamp drops to the foil-covered countertop with a dramatic clank. (A movie of the demonstration is available in the Supplemental Material.W) Hazards The carbon nanotubes have a very low density and can easily become airborne. The health issues associated with nanostructures are still under scrutiny (17, 18), but skin contact with and inhalation of the nanotubes and their combustion byproducts should be avoided.9 Fume hood use can help minimize exposure. Gloves are recommended to help prevent hands from being covered with nanotubes and adhesive. Flash paper is extremely flammable and should be treated with the respect due to all pyrotechnic materials. Care must be taken to prevent accidental ignition from heat, sparks, and so forth. Do not inhale the contents of the spray adhesive can. After they cool, the burnt materials can be disposed in the trash. Although the light flashes are brief, they can be hard on the eyes. It is recommended that the light flashes are directed away from the audience.

toacoustic effect. Several of the flash paper strips laden with nanotubes can be stored for days in a desiccator. This reduces the immediate preparation time for this demonstration, but bear in mind that the more the strips are bumped, the more nanotubes are lost, reducing the chances of a successful demonstration. As noted earlier, carbon nanotube combustion can be initiated with energy sources other than flashbulbs. One of the reviewers of this article has initiated combustion with a violet laser (406.7 nm, 154 mW) as well as infrared energy sources such as a heated nichrome wire, a soldering gun, and a butane lighter. We have used an incandescent heat lamp to initiate combustion. We have also placed carbon nanotubes between the flat surfaces of two polystyrene Petri dishes and irradiated the assemblage in a microwave oven. A brief period of irradiation is all that is required to heat and burn the nanotubes, deforming and melting the polystyrene and welding the dishes together. This experiment also works with graphite powder. Finally, we have placed carbon nanotubes between the flat surfaces of two polystyrene Petri dishes and placed the assemblage near the Fresnel lens focal point of a 7800 lumen overhead projector. (The mirror and focusing lens were removed and a ring stand and clamp were used to hold the assemblage about 40 cm above the glass top surface of the projector. CAUTION: The focused light from an overhead projector can be dazzlingly bright and can heat some objects dramatically.) The transparent Petri dishes themselves are not

Discussion If the flash does not ignite the nanotubes sufficiently to initiate the demonstrations, make sure the flash is as close to the tubes as possible and try again. Each time the flash fires, some of the nanotubes will likely burn, so too many unsuccessful attempts on a single demonstration may burn away the tubes without initiating the demonstration. As always, practice the demonstrations beforehand to make sure the camera flash is sufficiently intense. These demonstrations have had a high success rate when properly set up, but having spare demonstrations on hand cannot hurt, even if only for an encore. The ability of burning nanotubes to melt or burn through plastic can also burst an inflated balloon. However, in addition to the possibility of scattering burning material, the popping balloon might distract the audience from potentially hearing the rather faint photoacoustic effect. On the other hand, a film stretched across a crystallization dish might act as a sort of drum head to amplify the sound of the pho-

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Figure 2. Images captured from a movie of the flash paper combustion: (A) The nanotube-laden flash paper is suspended from a foldback paper clamp. (B) Combustion of the flash paper.


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melted by the focused light, but dark objects such as carbon nanotubes and even dark permanent marker ink can absorb sufficient energy to melt the polystyrene and weld the dishes together. Visual observations and observations made using a scanning electron microscope seem to indicate that the carbon nanotubes are not damaged by the intense light. Lasers have been used to heat carbon nanofibers imbedded in polyethylene, melting the polymer (19). Whereas all of these experiments demonstrate the energy absorbing properties of carbon nanotubes, people are not necessarily surprised by lasers, focused light, infrared, and microwave sources as being capable of igniting fires. The more counterintuitive camera flashes may add a bit more of the element of surprise to the demonstrations. Flash-driven nanotube combustion illustrates the connection between the surface area and the rate of a chemical reaction and opens an educational window into the world of nanoscale carbon structures (5, 20).10 One of the authors plans to use these demonstrations in science shows following demonstrations that burn finely divided powders and also plans to regale the audience with puns about “flash” paper. Acknowledgments We would like to acknowledge Marianne Cavelti for use of resources in the chemistry lecture demonstration lab at the University of Washington (UW). Work was performed in part at the University of Washington Nanotech User Facility (NTUF), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the NSF. DC acknowledges sabbatical support from Bradley University. JM acknowledges the MLSC at the UW for a student fellowship award. YX has been supported in part by a Career Award from the NSF (DMR-9983893) and a fellowship from the David and Lucile Packard Foundation. We also thank the reviewers for additional suggestions. WSupplemental

Material

A SEM image of a carbon nanotube mat draped over a human hair and movies of the demonstrations are available in this issue of JCE Online. Notes 1. Single-walled nanotubes are available from the SigmaAldrich Company (Milwaukee, WI) and other sources. Each individual tube is impossible to see, but many tubes together appear as very low density black fluff (one sample was roughly 0.002 g兾cm3). To maximize the surface area of the nanotubes, AP-grade (AP stands for “as prepared”) tubes are recommended. Purifying the nanotubes beyond this grade can pack them together more tightly, making them less likely to ignite. Suppliers of AP-grade, single-walled nanotubes include CarboLex Inc., 234 McCarty Court, Lexington, KY 40508 ($100兾g, minimum order $200). Smaller quantities of the CarboLex material may be purchased through the Sigma-Aldrich Company, P.O. Box 2060, Milwaukee, WI (catalog number 51930, a 250-mg quantity costs $69.50 and should last for many demonstrations). The single-walled nanotubes used in this work were obtained from Rice University, PO Box 1892, Houston, Texas

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77251-1892, home of the Carbon Nanotechnology Laboratory. An excellent Web page listing nanotube suppliers is the nanotube yellow pages (21) maintained by Tomanek. 2. A secondhand flasher from the Vivitar Corporation (Newbury Park, CA) was used. The coaxial connector at the end of the flasher cable was modified with a push button for use as a trigger. To be further removed from the combustion and avoid blocking the view of spectators, attach the flasher to some sort of stick or pole. To amplify the popping sound of the nanotube combustion, it might be helpful to attach a microphone to the stick as well. Keep in mind that a camera flash also makes a faint popping sound, so it would be instructive to compare the flash sound by itself with that of the flash ignition of the nanotubes. 3. Parafilm can be purchased from Pechiney Plastic Packaging (Chicago, IL). 4. Clear adhesive tape such as Scotch Magic Tape can be purchased from the 3M Corporation (St. Paul, MN). 5. Foldback binder clips can be purchased from Boise Cascade Office Products (Itasca, IL). 6. The flash paper that we used for this demonstration was Fantastic Flash from the NASSAU Chemical Corporation, P.O. Box 547, North Olmsted, OH 44070-0547. Booklets of twenty 5.7cm × 7.6-cm sheets were purchased from a local magic supply store for about $4 each. There are many magic supply Web sites that also sell flash paper. 7. Spray adhesive such as Spra-Ment Craft and Display Adhesive can be purchased from the 3M Corporation (St. Paul, MN). 8. This demonstration may be performed on an overhead projector. 9. Airborne nanotubes can be minimized by covering the nanotubes-on-a-drum setup with a slightly larger, inverted beaker. Similarly, an empty, clear, 2-L bottle with a slit cut into it may be used to enclose the ring and clamps of the nano-hair-trigger and flash-ignited-flash-paper setups. 10. Directions for using LEGO bricks to build a cylindrical section of a carbon nanotube can be found on the Web (22). This file is associated with the online book Exploring the Nanoworld with LEGO Bricks (23).

Literature Cited 1. Dagani, R. Chem. Eng. News 2002, 80 (17), 9. 2. Ajayan, P. M.; Terrones, M.; de la Guardia, A.; Huc, V.; Grobert, N.; Wei, B. Q.; Lezec, H.; Ramanath, G.; Ebbesen, T. W. Science 2002, 296, 705. 3. Bockrath, B.; Johnson, J. K; Sholl, D. S.; Howard, B.; Matranga, C.; Shi, W.; Sorescu, D. Science 2002, 297, 192– 193. 4. Desilets, S.; Brousseau, P.; Gagnon, N.; Cote, S. C.; Trudel, S. Flash-ignitable Energetic Material. U.S. Patent Application 20040040637, March 4, 2004. 5. Lisensky, G. C. Nanotubes and Other Forms of Carbon. http:/ /mrsec.wisc.edu/Edetc/cineplex/nanotube/text.html (accessed Jun 2006). 6. Wang, N.; Yao, B. D.; Chan, Y. F.; Zhang, X. Y. Nano Lett. 2003, 3, 475–477. 7. Huang, J.; Kaner; R. B. Nat. Mater. 2004, 3, 783–786. 8. Li, D.; Xia, Y. Nat. Mater. 2004, 3, 753–754. 9. Chen, J.; Wiley, B.; Li, Z.-Y.; Campbell, D.; Saeki, F.; Cang, H.; Au, L.; Lee, J.; Li, X.; Xia, Y. Adv. Mater. 2005, 17, 2255–2261.


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In the Classroom 10. Chang, S.-S.; Shih, C.-W.; Chen, C.-D.; Lai, W.-C.; Wang, C. R. C. Langmuir 1999, 15, 701–709. 11. Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 6152–6163. 12. Link, S.; El-Sayed, M. A. J. Chem. Phys. 2001, 114, 2362– 2368. 13. Aguirre, C. M.; Moran, C. E.; Young, N. J. J. Phys. Chem. B 2004, 108, 7040–7045. 14. Tested Demonstrations in Chemistry; Alyea, H. N., Dutton, F. B., Eds.; Journal of Chemical Education: Easton, PA, 1965; p 8. 15. Nanotubes in a Flash–Ignition and Reconstruction. [Supplementary Material for Ajayan et al. Science 2002, 296 (5568), 705]. http://www.sciencemag.org/cgi/content/full/296/5568/705/ DC1 (accessed Jun 2006).

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16. Solomon, S.; Hur, C.; Lee, A.; Smith, K. J. Chem. Educ. 1995, 72, 1133–1134. 17. NIOSH Safety and Health Topic: Nanotechnology. http:// www.cdc.gov/niosh/topics/nanotech (accessed Jun 2006). 18. King, A. G. J. Chem. Educ. 2005, 82, 666–670. 19. Lu, Y.; Shao, D. B.; Chen, S. C. App. Phys. Lett. 2004, 85, 1604–1606. 20. Murphy, C. J.; Campbell, D. J. Chem. Educator 2000, 5, 1– 2. 21. The Nanotube Site. http://www.pa.msu.edu/cmp/csc/ nanotube.html (accessed Jun 2006). 22. Buckytubes. http://mrsec.wisc.edu/Edetc/LEGO/PDFfiles/ UCinstruct/buckytube.PDF (accessed Jun 2006). 23. Exploring the Nanoworld with LEGO Bricks. http:// mrsec.wisc.edu/Edetc/LEGO/index.html (accessed Jun 2006).


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Classroom-Scale Demonstrations Using Flash Ignition of Carbon

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