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Chemical Vapor Deposition of Carbon Nanotubes An Experiment in Materials Chemistry Bradley D. Fahlman Department of Chemistry, University of California, Irvine, Irvine, CA 92697-2025;
[email protected] Carbon nanotubes are unique structures that may behave as semiconductors or metallic conductors depending on their diameter, chirality, and purity (1). The physical characteristics of nanotubes allow for intriguing applications related to field emission displays (FEDs) (2), hydrogen storage (3), and nanoscale electronic devices (4 ). However, since nanotube growth is dependent on a variety of parameters, further progress toward practical materials will require the elimination of defects and other reaction products (such as amorphous carbon and catalyst particles), as well as gaining synthetic control over tube diameter, length, chirality, and number of concentric shells (5). This paper describes a module that was recently designed to introduce first-year graduate students in Chemical and Materials Physics (ChaMP) to an exciting area of nanotechnology—carbon nanotube growth and characterization. Though the experiment was introduced in the graduate curriculum, it could also be incorporated into an advanced undergraduate physical or materials chemistry course with no modification. Experimental Section
Catalyst Synthesis Iron-doped alumina catalyst particles were synthesized using the method of Kong et al. (6 ). Various amounts of Fe(NO3)3⭈9H2O were dissolved in 30.0 mL of methanol with gentle swirling. Four groups of 4–5 students were assembled and each student in the group prepared a different iron(III) nitrate solution in the range 0.020 M to 0.100 M (Table 1). The iron(III) nitrate solutions were added to 1.0-g portions of alumina nanoparticles (Degussa, Aluminiumoxid C ) and the suspensions were stirred at room temperature for 90 min. After this time, the methanol was removed using a rotary evaporator fitted to a water bath held constant at 50 °C. The resultant brown solids were placed in an oven at 150 °C overnight. The following day, the solids were finely ground using a mortar and pestle and placed in vials for the deposition experiment. Carbon Nanotube Growth For chemical vapor deposition, ca. 0.5 g of the catalyst particles was placed on a flat combustion crucible lid (38 mm i.d.) and placed inside a quartz tube (122 cm × 7.6 cm i.d.) within a three-zone tube furnace (Fig. 1). To ensure that identical conditions were used, all of the four catalysts of varying concentrations were placed in the furnace at the same time. After purging the system for 15 min with argon, the furnace temperature was increased to 1000 °C and methane was introduced at a flow rate of 50 mL min᎑1. Owing to the extreme heat generated by the high-temperature pyrolysis, a nitrogen
stream was necessary to cool the Tygon tubing connecting the oil bubbler to the quartz reaction tube. A Phillips XL30 field-emission SEM (FESEM) fitted with an energy dispersive X-ray spectrometer was used for imaging and quantitative analysis. Hazards Mass spectrometry measurements indicate that only a small amount hydrogen gas is evolved during the catalyzed decomposition of methane (H. Dai, unpublished results). Most likely, this is an artifact of short reaction times (ca. 10 min) and low methane flow rates. Nevertheless, a completely leak-free system must be maintained to prevent the formation of explosive mixtures with air. In addition, the evolved gases should be removed through a bubbler system assembled within a fume hood away from all ignition sources (see Fig. 1). The high temperature that is used for CVD growth dictates the use of heatproof gloves and tongs and personal protective equipment such as goggles and laboratory coats. The alumina nanoparticles are extremely fine powders and
Figure 1. Schematic of the CVD apparatus used to catalytically deposit carbon nanotubes.
Table 1. Iron Concentrations during Preparation of the Catalyst Particles Impregnating Solution/(mol L᎑1)
Impregnating Solution/ (mmol Fe/g support)
Catalyst Particles/ atom % a
0.010
0.30
1.6 ± 0.4b
0.020
0.60
2.4 ± 0.3
0.050
1.50
4.9 ± 0.5
0.075
2.25
6.4 ± 0.3
0.100
3.00
7.5 ± 0.4
a Iron
concentrations determined by EDX for catalyst particles before CVD (±1σ). bThis catalyst concentration was utilized during the development of the experiment.
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Figure 2. FESEM images of catalyst particles (4.9 atom % Fe) (a) before and (b) after chemical vapor deposition.
students have to ensure that gloves, goggles, and respirators are worn to prevent excessive irritation or inhalation. The iron(III) nitrate nonahydrate used in the synthesis is an oxidizer and irritant and should be treated accordingly, with appropriate protective gear such as gloves, goggles and respirators. Proper measures must be taken to prevent exposure to methanol through inhalation or skin absorption. This volatile solvent must be completely isolated from ignition sources. Results Table 1 lists the iron concentrations of the as-prepared catalysts and the ferric nitrate solutions used in the syntheses. The iron concentrations of the catalysts represent the average values for four groups of 4–5 students. Owing to the lack of observable nanotubes grown from catalysts containing 1.6 atom % Fe, this concentration was not included in student CVD experiments. The data show that under the conditions used for catalyst syntheses, there was a nonlinear relationship between the iron concentration in solution and in the final catalyst particles. That is, increasing the iron concentration of the impregnating solution by a factor of 50 only resulted in a threefold increase in the iron concentration of the individual catalyst particles. 204
Figure 3. FESEM images of nanofibers/nanotubes grown from (a) 6.4 atom % Fe, (b) 4.9 atom % Fe and (c) 2.4 atom % Fe catalyst particles. The estimated average diameter of the nanotubes is observed to decrease as the iron concentration of the catalyst particles is decreased.
The overall morphology of the catalyst particles changes drastically following thermolysis. Figure 2 shows FESEM images of catalyst particles before and after the CVD of nanotubes. The as-prepared catalyst particles appear amorphous (Fig. 2a); however, after thermolysis at 1000 °C for 10 min, individual irregular-shaped crystallites become apparent (Fig. 2b).
Journal of Chemical Education • Vol. 79 No. 2 February 2002 • JChemEd.chem.wisc.edu
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had the opportunity to synthesize and observe structures that hitherto have only been discussed in the literature. Such an example of a systematic examination of nanotube processing parameters effectively introduced the students to the following concepts and procedures.
Figure 4. FESEM image of a helical nanotube grown from catalyst particles containing 6.4 atom % Fe.
The yield of carbon nanotubes drastically decreased as the iron concentration of the catalyst was decreased. Similarly, nanotube diameters decreased concomitantly with the iron concentration of the catalyst particles. As seen in Figure 3, the estimated average nanotube diameter range is 70–80 nm for 6.4 atom % Fe catalysts (Fig. 3a), 30–40 nm for 4.9 atom % Fe catalysts (Fig. 3b), and below 10 nm for 2.4 atom % Fe catalysts (Fig. 3c). The average nanotube diameter for the latter structures is in the range reported for nanotubes consisting of a single or double layer of graphitic sheets (single-walled nanotubes [SWNTs] and double-walled nanotubes [DWNTs], respectively) (7). By contrast, the diameter range found for the 6.4 atom % Fe catalysts is outside the range normally reported for carbon nanotubes; hence, these structures are probably best referred to as nanofibers (8). Figure 4 illustrates another feature that was discovered only for nanotubes grown from catalysts containing 4.9 and 6.4 atom % Fe. Two models for helical nanotube growth have been proposed (9, 10). The first assumes that there exist catalyst particles with diameters significantly larger than the carbon nanotubes; the helical structure is thought to arise from the difference in carbon diffusion rates on the crystallographic faces of the particle (10). However, at the level of magnification used in our experiments, such evidence for large catalyst particles was not found. Gao et al. also did not observe relatively large catalyst particles and suggested another mechanism for growth related to reactions occurring adjacent to the individual catalyst particle (9). This model would provide a reasonable explanation for the helical structures found in our system, since these helical structures are only found in areas where there exists a significant quantity of amorphous carbon deposits (Fig. 4), another indication of catalyst-absent decomposition reactions. Conclusions Because of the many parameters that must be optimized for suitable nanotube growth, this experiment represents an extremely practical and interesting undergraduate/graduate laboratory experiment. A high level of student interest and motivation was observed during this laboratory, as students
1. Catalyst synthesis. Although simplistic, the synthetic method used to produce Fe 2O 3/Al2O3 catalysts is important because it illustrates the difference in metal concentrations that may exist in solution relative to individual catalyst particles. The method of mixing alumina nanoparticles with an appropriate metalcontaining solution is very common to create aluminasupported catalyst powders. Since this method may also be used for a host of other support materials or doping agents, students may draw from this synthetic frame of reference for future inorganic/organic investigations. 2. Chemical vapor deposition methodology. This experiment introduces students to the terminology, experimental design, and safety issues related to CVD as well as the many variables that affect deposition. It is important for students to realize that the decomposition of methane into carbon and hydrogen does not simply occur in the gas phase, but rather through a surface-catalyzed pathway similar to the CVD of thin films onto a variety of substrates. 3. Carbon nanotube growth. Students were introduced to nanotube growth models and to the variation in physical characteristics with changes in catalyst compositions. 4. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX). In addition to learning the theoretical concepts related to SEM imaging and EDX measurements, students gained hands-on experience with proper sample-preparation techniques and careful operation of a SEM to obtain high-quality images.
As more chemistry departments become interdisciplinary, undergraduate experiments that feature current research interests in chemistry must also borrow resources from other departments. Hence, collaboration with engineering, materials science, or other departments with suitable instrumentation could be established to meet the instrumental needs of this experiment. For example, since the chemistry department at UCI does not have a SEM, a suitable instrument was utilized through collaboration with the Department of Chemical and Biochemical Engineering and Materials Science. Another option for smaller schools may be to collaborate with local industries possessing a SEM for the characterization portion of this experiment. This latter option would afford the added benefit of supplying an industrial contact for students that may become valuable upon graduation. Acknowledgments I wish to thank Peter Taborek (Physics) and V. Ara Apkarian (Chemistry), ChaMP founding co-directors, for the opportunity to work with the ChaMP program and for their
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superb organization, helpful discussions, and support. Financial support for the ChaMP summer laboratory class is from The Camille and Henry Dreyfus Foundation (SG-99-038) and the National Science Foundation (CHE-9725462). More information related to this unique graduate program may be obtained from the Web site http://www.champ.uci.edu. W
Supplemental Material
Further lab documentation, notes, and questions for students are available in this issue of JCE Online. Literature Cited 1. Hamada, N.; Sawada, S.; Oshiyama, A. Phys. Rev. Lett. 1992, 68, 1579. 2. Fan, S.; Chapline, M. G.; Franklin, N. F.; Tombler, T. W.; Cassel, A. M.; Dai, H. Science 1999, 283, 512. Collins, P. G.; Zettl, A. Phys. Rev. B 1997, 55, 9391. Bonard, J.-M.; Salvetat, J. P.; Stockli, T.; de Heer, W. A.; Forro, L.; Chatelain, A. Appl. Phys. Lett. 1998, 73, 918.
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3. Nutzenadel, C.; Zuttel, A.; Chartouni, D.; Schlapbach, L. Electrochem. Solid-State Lett. 1999, 2, 30. Dillon, A. C.; Jones, K. M.; Bekedahl, T. A.; Kang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. 4. Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, Ph. Appl. Phys. Lett. 1998, 73, 2447. 5. Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483. 6. Kong, J.; Cassell, A. M.; Dai, H. Chem. Phys. Lett. 1998, 292, 567. 7. Colomer, J.-F.; Stephan, C.; Lefrant, S.; Van Tendeloo, G.; Willems, I.; Konya, Z.; Fonseca, A.; Laurent, Ch.; Nagy, J. B. Chem. Phys. Lett. 2000, 317, 83, and references therein. 8. Tomanek, D. The Nanotube Site; http://www.pa.msu.edu/cmp/ csc/NTSite/nanopage.html#top (accessed Aug 2001). 9. Gao, R.; Wang, Z. L.; Fan, S. J. Phys. Chem. B 2000, 104, 1227. 10. Amelinckx, S.; Zhang, X. B.; Bernaerts, D.; Zhang, X. F.; Ivanov, V.; Nagy, J. B. Science 1994, 265, 635.
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