NANO LETTERS
Oxidation of Fe Nanoparticles Embedded in Single-Walled Carbon Nanotubes by Exposure to a Bright Flash of White Light
2002 Vol. 2, No. 11 1277-1280
Nadi Braidy,§ Gianluigi A. Botton,*,† and Alex Adronov*,‡ Departments of Chemistry and Materials Science and Engineering, McMaster UniVersity, 1280 Main St. W., Hamilton, Ontario, Canada, L8S 4M1 Received July 23, 2002
ABSTRACT Single-walled carbon nanotubes (SWNTs), upon exposure to the flash from a regular 35 mm camera, were found to ignite and produce small amounts of a bright orange solid. X-ray powder diffraction analysis revealed this solid to be composed mostly of Fe2O3, along with trace amounts of Fe3O4, originating from the Fe catalyst used in SWNT production. Upon TEM investigation, this material was further found to be composed of two different oxide morphologies, including nanoparticles entrapped within SWNT bundles and larger aggregates of fused grains.
Introduction. The truly remarkable structural and electronic properties of single-walled carbon nanotubes (SWNTs)1 have attracted a great deal of attention from researchers around the world.2-5 SWNTs are generally believed to be extremely stable materials that have a nearly uniform diameter on the order of 1-1.5 nm and are up to several microns in length.5 These structures have extremely high tensile strength and elasticity,6,7 are thermally conducting, and can be electrically conducting or semiconducting, depending on their chirality.8 Indeed, they are the only carbon-based structures that behave as metallic conductors without requiring any additional dopants. Some of the more recent advances in carbon nanotube research include their chemical functionalization by various methods,9-22 their utilization in the development of single-molecule field effect transistors (FETs),23-26 their organization into circuits capable of performing logic operations,27,28 and the growth of continuous SWNT bundles up to 20 cm in length.29 This progress is paving the way for the utilization of SWNTs in a variety of areas, such as nanoscale molecular electronics, strengthening agents in composite materials, or sensing and actuating devices.22 Very recently, Ajayan and co-workers reported a phenomenon which indicates that carbon nanotubes might not be as stable as once thought.30 These researchers discovered that exposure of SWNT samples to an ordinary photographic * Corresponding authors. † Department of Materials Science and Engineering. Tel. (905) 525-9140 ext. 24767. Fax. (905) 521-2773. E-mail:
[email protected] ‡ Department of Chemistry. Tel. (905) 525-9140 ext. 23514. Fax. (905) 521-2773. E-mail:
[email protected] § Department of Materials Science and Engineering. 10.1021/nl025718m CCC: $22.00 Published on Web 10/01/2002
© 2002 American Chemical Society
flash results in their ignition under ambient conditions. The reported combustion and structural reorganization of nanotubes implies that this material is capable of harnessing light energy and converting it into heat. Ajayan and co-workers estimated that the temperature inside the sample reaches between 1500 and 2000 °C at the very localized points of ignition. Although this spectacular effect occurs only upon exposure to a bright flash at a very short distance and is not likely to pose a safety threat in the general preparation and utilization of SWNTs, the impact of this discovery on future applications of this material may be significant and warrants further investigation. In our own attempts to reproduce the nanotube flashing experiment with a sample of raw tubes purchased from Carbon Nanotechnologies Inc., we indeed observed the ignition of the samples accompanied by the previously reported “popping” sound.30 However, to our surprise, we also noticed that at the end of the burning process, a bright orange solid material was produced in the precise points where the burning had occurred. This orange solid did not dissolve or swell in organic solvents. X-ray diffraction and TEM analysis of the orange product revealed it to be composed of iron oxide features with nanoscale dimensions. The formation of oxidized catalyst material as a result of nanotube flashing was briefly mentioned by Ajayan and coworkers,30 but the production of these materials in isolatable quantities and the investigation of their morphology has not been reported. Here, we describe the formation and morphology of these iron oxide structures, which result from the exposure of the raw SWNTs to a bright flash of light.
Figure 1. TEM micrograph of as-received, raw SWNTs produced by the HiPCO process. The nanotubes are organized in 15-100 nm diameter bundles, with the individual tubes having a diameter of 1.2-1.4 nm. Large clusters of amorphous carbon, along with a number of Fe nanoparticles, can be seen throughout the sample.
Experimental Section. Raw carbon nanotubes were purchased from Carbon Nanotechnologies Inc. and were used as received. Irradiation of approximately 5 mg samples of these tubes was performed with the flash of a Kodak Advantix 4700ix camera at a distance of 2-5 cm. After irradiation, the resulting reddish material was separated manually using tweezers and a magnifying glass. X-ray diffraction measurements were carried out using a Bruker D8 Advance X-ray diffractometer with Cu KR radiation. TEM analysis of the nanotubes before and after irradiation was performed using a Philips CM12 and a JEOL 2010F microscope, with an accelerating voltage of 120 and 200 KV, respectively. In addition, the JEOL 2010F was equipped with a parallel electron energy loss spectrometer (Gatan model 666) and an ultrathin window X-ray detector (Oxford Instruments’ Pentafet) for electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX), respectively. Sample preparation for TEM involved dispersing ca. 1 mg of material in 5 mL of methanol using ultrasonic agitation. A drop of this suspension was placed on a holey carbon coated copper grid and left to air-dry. Results and Discussion. Figure 1 illustrates a transmission electron microscopy (TEM) image of the as-received sample of raw, unpurified, single-walled carbon nanotubes (SWNTs). From this image, a network of bundles having diameters of 15 to 100 nm is clearly visible, along with clusters of amorphous carbon (a-C) particles (10-50 nm diameter). In addition, Fe nanoparticles, used as the catalyst during production of the SWNTs, having a diameter on the order of 1-5 nm, appear embedded within the bundles and the a-C particles. This particular sample contained ca. 23% Fe (wt/wt). Significantly, the EELS measurements performed on the Fe nanoparticles revealed their composition to be bulk Fe, with no detectable amounts of any oxidized material, such as Fe2O3, being present. These features are all typical of the raw material manufactured by the HiPCO process. Figure 2, curve (a), displays the powder diffraction pattern of the raw nanotube sample. Judging from the positions and 1278
Figure 2. Powder diffraction patterns of nanotube sample before (a) and after (b) the flashing experiment. The sample used for collection of spectrum (b) was manually isolated from unreacted carbon nanotubes. Asterisks indicate visible (nonoverlapping) reflections originating from Fe3O4. Angle and intensity of standard Bragg reflections of standard Fe2O3 are displayed below the patterns.
relative intensities of the 2D triangular rope lattice peaks, the material is likely composed of a broad distribution of, on average, low-diameter nanotubes.31 Moreover, the large peak width is due to the low crystallinity (small coherence length) provided by the small diameter bundles. Again, these characteristics are typical of the raw SWNT material produced by the HiPCO process.32 After exposure of this sample to the flash of our 35 mm camera, the resulting bright orange material was carefully collected and its powder diffraction pattern was measured. In this sample, it was found that the diffraction peaks corresponding to SWNTs were considerably reduced and replaced by new, sharper peaks (Figure 2, curve b). These new peaks have positions and relative intensities that are in excellent agreement with the powder pattern of polycrystalline Fe2O3 (the JCPDF reflections for the standard are depicted at the bottom of Figure 2). Weak reflections, indicated by stars in Figure 2b, reveal the presence of a small amount of Fe3O4 in the sample, which is likely formed as a kinetic product due to the fast oxidation reaction. These observations indicate that flashing the sample results in the previously reported burning or transformation of carbon nanotubes30 and the simultaneous oxidation of the remaining iron clusters into the bright orange colored Fe2O3. It should be noted that Fe nanoparticles are themselves pyrophoric, but are apparently stabilized when inside nanotube bundles. As the nanotubes burn, the nanoparticles are released and the combination of a temperature rise and exposure of the particles to air results in rapid oxidation. TEM investigations revealed the coexistence of two distinct Fe2O3 nanostructures (Figure 3): (i) small nanoparticles trapped within a network of residual SWNT bundles (Figure 3, top-left) and (ii) large randomly interconnected or fused grains (Figure 3, bottom left). EDX and EELS measurements confirmed the coexistence of Fe and O in both types of nanoparticles (Figure 4). The energy loss near edge structure of the Fe L23 white line (at 704 eV) and the shape of the O K edge (at 528 eV, Figure 4) are both typical of the bonding between Fe and O within Fe2O3.33,34 Moreover, the Debye-Scherrer ring pattern obtained by electron selected area diffraction (SAD) confirmed the polycrystalline Fe2O3 nature of both structures. In the case of the fine Nano Lett., Vol. 2, No. 11, 2002
Figure 3. TEM images of the two different morphologies found within the oxidized material. The inset in upper left shows Fe2O3 nanoparticles embedded within nanotube bundles, and the inset in the lower left shows the free-standing fused grains of Fe2O3.
Figure 4. EELS spectra of the two different morphologies observed within the oxidized material.
nanoparticles trapped between the SWNT bundles, smooth and continuous rings were observed, whereas spotty rings appeared in the case of the SAD pattern of the larger, fused grains. The presence of these two types of features may give an indication that different temperatures were reached in different portions of the sample. To produce the large interconnected grains, it is likely that melting of the individual Fe or Fe2O3 nanoparticles occurred, which would require temperatures in excess of 1500 °C (the melting points of Fe and Fe2O3 are 1535 °C and 1565 °C, respectively). This would be in line with the previous observation that, after exposure to the flash, the nanotubes become structurally reorganized, which requires temperatures between 1500 and 2000 °C. In other parts of the sample, temperatures may have reached more modest values, precluding the melting or fusion of nanoparticles but still allowing for fast and efficient oxidation to Fe2O3. Typical temperatures for the oxidation of iron to Fe2O3 in air range between 400 and 700 °C.35 A closer inspection of the regions containing trapped nanoparticles in a network of bundles reveals a high density of nanoparticles located between ropes of SWNTs (Figure Nano Lett., Vol. 2, No. 11, 2002
Figure 5. TEM image of nanotube bundles with embedded Fe2O3 nanoparticles formed after exposure of the sample to the flash. The inset shows a close-up of a few bundles and corresponds to a 300 nm wide area.
5), whereas the Fe nanoparticles of the pristine sample appeared trapped within bundles. Here, the average bundle size is in the 5-15 nm range, while the particle diameter is between 15 and 20 nm. These sizes differ from the pristine sample, where nanoparticles having a diameter of 1 to 5 nm were embedded within much thicker bundles (15-100 nm). It is likely that individual iron nanoparticles were completely oxidized, resulting in the observed and expected volume increase of the particles. The resulting larger particles may have migrated outside the SWNT bundles as a result of their volume change. Closer investigation of the fused grains revealed a self-supported randomly interconnected network (Figure 6a) having diameters ranging from 30 to 50 nm. Moreover, the fused grain morphology differs from the largely oval shape of the trapped nanoparticles, being composed essentially of irregular shapes. Each grain appears to be formed of single crystalline domains, as indicated by the presence of Moire´ fringes (Figure 6a, arrows) resulting from the superposition of two crystals having different orientations. Regions containing both structures, often constituting the boundary between areas of exclusive fused grain or entrapped nanoparticle structures, can be seen from Figure 6b where a strand of nanoparticle structures stretches through a region of grains. Conclusions and Outlook. The oxidation of iron nanoparticles embedded within a sample of single-walled carbon nanotubes can be efficiently achieved through exposure of the sample to the flash from an ordinary 35 mm camera. Based on the powder diffraction data, the majority of the oxidized material was found to be composed of Fe2O3, with a small amount of Fe3O4. TEM analysis of the oxidized structures revealed two different morphologies, one consisting of individual nanoparticles and the other consisting of irregular fused grains. The latter structures indicate that temperatures in excess of 1500 °C were reached at localized points within the sample. In other areas, where the temperature remained lower, individual particles were oxidized but did not fuse together. The irradiation wavelength and 1279
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Figure 6. Two different areas showing fused grains of Fe2O3. Moire´ fringes, indicated by arrows, are clearly visible in (a). A bundle of nanotubes with embedded Fe2O3 particles passing through an area of fused grains is shown in (b).
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intensity dependence of this phenomenon will be the focus of future investigations. These findings may have implications for the utilization of carbon nanotubes as “nanoreactors” capable of efficiently performing chemical transformations that require high temperatures simply by exposing the mixture of nanotubes and starting materials to a bright flash of light. Acknowledgment. We thank W. Gong for XRD analysis and the Natural Sciences and Engineering Council of Canada (NSERC) for financial support of this work.
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