J. Phys. Chem. B 2001, 105, 11937-11940
11937
Macroscopic Three-Dimensional Arrays of Fe Nanoparticles Supported in Aligned Carbon Nanotubes Anyuan Cao,* Xianfeng Zhang, Jinquan Wei, Yanhui Li, Cailu Xu, Ji Liang, and Dehai Wu Department of Mechanical Engineering, State Key Laboratory of AutomotiVe Safety and Energy, Tsinghua UniVersity, Beijing 100084, China
Bingqing Wei Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 ReceiVed: July 17, 2001; In Final Form: September 26, 2001
A novel route to regularly arrange Fe nanoparticles in three-dimensions over large areas, just like a crystalline cubic lattice of atoms, is reported. The cylindrical Fe particles, acting as the catalyst for carbon nanotube formation by conventional chemical vapor deposition method, are located in the nanotube cavities. The inner nanotube cavities protect and support the Fe particles, which are periodically spaced in the aligned nanotubes. The interparticle spacings can be well-controlled simply by adjusting the feeding interval of ferrocene during the nanotube growth process. Carbon nanotubes, which can carry foreign particles because of capillarity, show potential use in manipulating nanoparticles that otherwise have a strong tendency to aggregate and merge.
Introduction Fe, Co, and Ni are effective catalysts for the growth of carbon nanotubes (CNTs),1-4 especially in the large-scale synthesis of CNTs by chemical vapor deposition (CVD).5,6 Their catalytic behavior and the growth mechanism of CNTs have been studied extensively.7-12 Carbon atoms diffuse through the catalyst particle surface to form a tubular structure. Eventually, when the tube growth stops, catalyst particles will be covered by graphite sheets and remain inside the tube cavity. The catalyst particle diameter typically equals the inner tube diameter, ranging from several to tens of nanometers. These captured metallic particles, although extremely small, are well-protected by the multishells of nanotubes and cannot move freely along the inner tube wall, which is in close contact with them. In this letter, we report that by aligning CNTs using the CVD method catalyst particles inside the tube cavities can be arranged into a regular three-dimensional (3D) pattern, which is difficult to realize through direct manipulation of the nanoparticles because of their strong tendency to aggregate together using typical preparation methods. An intermittent feeding of catalyst precursor was adopted, and each nanotube contains a series of catalyst particles distributed uniformly along its length. The evenly distributed Fe particles in the aligned CNTs are analogous to a macroscopic crystal lattice, in which each Fe particle corresponds to a specially defined position. Such macroscopic 3D arrays, consisting of numerous regularly placed nanoparticles, may exhibit entirely new electronic or magnetic properties. Opening and filling of carbon nanotubes with foreign particles due to capillarity is available,13,14 so aligned CNTs can also be a macroscopic template to arrange and store many kinds of metallic microparticles besides Fe, Co, and Ni catalysts. This technique provides a novel route to spatially manipulate * To whom correspondence should be addressed. E-mail: cayuan96@ mails.tsinghua.edu.cn.
nanoparticles that tend to self aggregate, using macroscopic template carrierssCNTs. Experimental Section Our technique involves the synthesis of self-oriented largearea CNTs, which was first realized by Ren et al.,15 except with the intermittent feeding of the catalyst precursor during the nanotube growth process. Ferrocene has proved to be a good catalyst precursor for the growth of aligned nanotubes,16,17 which would decompose into numerous micro-Fe particles at a temperature above 190 °C. We have produced vertically aligned CNTs on a quartz substrate18,19 by catalytic pyrolysis of ferrocene and xylene. The synthesis of CNTs was carried out in a horizontal quartz tube housed in a muffle furnace, and the ferrocene feeding process is briefly described as follows: Ferrocene was dissolved in xylene to obtain a brown solution of 0.02 g/mol. After the quartz tube was gradually heated to 800 °C in flowing Ar/H2 (1000/150 sccm), the prepared solution was fed into the reaction tube by a syringe pump drop by drop. The sequential drops were carried by the flowing Ar/H2 and arrived at the quartz sheets placed in the middle of the reaction tube, acting as the nanotube growth substrate. The volume of each drop is 0.1 mL, and there is a fixed time interval between sequential drops, which can be varied systematically. Results and Discussion Figure 1a shows the scanning electron microscope (SEM) image of the as-grown CNTs peeled off the quartz substrate. These CNT blocks were grown with a fixed 2-min interval for the ferrocene solution with a reaction time of 40 min. That is, 20 total drops of ferrocene solution were fed into the reaction tube, with one drop fed 2 min after another. The nanotubes are highly aligned and their total length, or block height, is 200 µm. The calculated nanotube growth rate is 5 µm/min. Horizontal parallel lines can be seen from the block side wall with
10.1021/jp0127521 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/01/2001
11938 J. Phys. Chem. B, Vol. 105, No. 48, 2001
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Figure 1. SEM images of as-grown aligned CNT arrays with parallel catalyst lines present across the array section.
uniform spacing between lines. The average interline distance is measured to be about 10 µm. Although the parallel lines near the block bottom are not as clear as those near the block roof, it can be calculated that there are a total of 20 such lines present in the block side wall. The number of parallel lines equals the number of ferrocene solution drops. We show by X-ray energy analysis that these parallel lines are rich in catalyst (Fe) particles, and the appearance of every line is due to the feeding of a drop of ferrocene.19 But here, the ferrocene solution concentration is much lower, and the Fe particles in the area of the parallel lines exist mainly inside the nanotubes, whereas a certain amount of them adhere to the nanotube outer walls as shown in ref 19. Figure 1c is a close view of the observed lines in the block side wall. Four parallel lines are indicated (arrows) with interline distances of 10, 6, and 11 µm, respectively. The distance difference is due to the fluctuation of the ferrocene feeding interval during the nanotube growth. The lines are perpendicular to the nanotube axis (the alignment direction) and perfectly straight, indicating that Fe particles inside the nanotubes exist with the same period (Figure 1d, a schematic profile of Figure 1c). Thus, the whole CNT array contains numerous Fe particles, regularly arranged into a macroscopic three-dimensional matrix analogous to a crystalline cubic lattice in which one Fe particle corresponds to an atom. Figure 1b is the roof morphology of the CNT array, showing the vertical alignment of nanotubes. The space between the parallel catalyst lines should depend on the feeding interval of ferrocene solution, because their repetition comes from the successively fed drops. CNT samples were prepared with three feeding intervals of 0.5, 2, and 4 min (Figure 2). All of the samples show parallel lines across the section just like those shown in Figure 1a but with different interline spacings. The average distance in the sample with a short interval of 0.5 min is only 5 µm (Figure 2a). But for an interval as long as 4 min, the interline distance reaches 15 µm (Figure 2c). It seems that the space between catalyst lines increases with longer catalyst feeding intervals. However, the growth rate of nanotubes is 10, 5, and 4 µm/min for the 0.5, 2, and 4 min intervals, respectively, showing a tendency to monotonically decrease. For a feeding interval of more than 5 min, the nanotube growth slows down rapidly and tube alignment lowers. This is because the drop of ferrocene and xylene will be exhausted after such a long interval so that nanotubes can hardly continue their growth because of a lack of carbon source.
Figure 2. SEM images of CNT arrays with different interline spacings (L): (a) L ) 5 µm; (b) L ) 10 µm; (c) L ) 15 µm; (d) L ) 5 µm and 20 µm; (e) a schematic profile of the CNT array in panel d.
We further demonstrated the availability of different catalyst line frequencies simply by varying the ferrocene feeding interval. A sample was prepared with a 1-min interval for half an hour followed by a 5-min interval for another hour. An obvious boundary is observed in the middle of the as-grown CNT array (line A in Figure 2d), which is the transfer point from the 1-min to the 5-min interval. The catalyst lines in the portion below line A are dense, with a constant interline distance of 5 µm, as expected with the 1-min interval. The space between lines above line A, however, increases to nearly 20 µm (Figure 2e). So, when the catalyst feeding interval changes, the frequency of the resulting parallel lines varies correspondingly. The height of the two parts divided by A line is nearly the same, 180 µm each. However, it took 30 min to grow the first (lower) part and 60 min for the upper part. This is because of the two feeding intervals adopted for the two parts, which result in different growth rates during the two feeding stages. The existence of Fe particles inside carbon nanotubes was further examined by transmission electron microscopy (TEM). Figure 3a is a typical TEM image of the Fe particles confined in nanotube cavities, including two short cylinders (each has a length of about 40 nm) and a longer one (nearly 200 nm). Generally, the observed Fe particles by TEM are cylindrical, in correspondence to the tube shape, with their widths equal to the inner tube diameters (about 10-20 nm in our sample). The two short cylinders staying inside one nanotube should come from one ferrocene drop rather than two drops. Because a drop of ferrocene would decompose into numerous micro-Fe particles, it is probable that several Fe particles arrive at one nanotube end sequentially. Furthermore, they are very close with a
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Figure 3. TEM images of CNTs with Fe particles inside their cavities: (a) cylindrical Fe particles; (b) Fe particles with different shapes; (c) four Fe particles staying at the same height of CNTs; (d) HRTEM image of a cylindrical Fe particle, showing good contact between the particle and the inner tube wall.
distance of only about 250 nm, whereas those particles coming from the next drop of ferrocene solution are 10 µm apart. Figure 3b is a TEM image showing Fe particles with different shapes. The upper nanotube has a 100-nm-long cylinder; however, the lower nanotube has a triangular (left) particle and an ellipsoid (right) particle. There are very few Fe particles adhering to the outside of the CNTs, indicating that most of the Fe particles become confined in the open tube ends during growth and then become wrapped by the growing nanotube walls. The length of the cylindrical particle varies greatly, ranging from 10 nm up to nearly 1 µm, depending on the size of ferrocene decomposed Fe particles received by the nanotube ends. Figure 3c shows four aligned nanotubes with an inner diameter of about 10 nm. The Fe particle inside the middle nanotube is a 150 nm long cylinder, whereas the one in the right tube is a small sphere (size 20 nm). Each of the left two nanotubes contains a long cylindrical Fe particle with a length of more than 300 nm. All of the particles stay at exactly the same height in their host, corresponding to a horizontal straight line across the section of aligned CNTs shown in Figure 1. Figure 3d is the enlarged view of the Fe particle inside the tubes, showing straight graphite layers and good contact between the nanotube and the cylindrical particle. The formation of the three-dimensional Fe particle matrix can be explained by the schematic model presented in Figure 4. When the first drop of ferroncene in xylene was fed into the reaction tube, Fe nanoparticles and carbon atoms (clusters) decomposed from the drop were carried by the flowing Ar and arrived at the substrate surface (quartz). Carbon nanotubes thus grew upward from these catalyst particles with carbon atoms continuously adding to their open ends. Once a second drop was fed, Fe particles coming from this drop would also arrive at the open tube ends to promote nanotube growth (Figure 4a). When graphite layers grew along the particle surface, the particles would be thinned so that most of them are cylindrical.
Figure 4. A schematic model of the formation of uniformly distributed catalyst lines during the growth of nanotube arrays.
After the graphite layers grew around the Fe particles for a period of time (an interval between sequential drops), the third group of Fe particles arrived, and the cycle continued (Figure 4b). The spacing between particles in one tube is increased by the duration of the drop interval. Finally, the growing nanotube array formed on the substrate directs the three-dimensional assembly of distributed catalyst particles simultaneously. Conclusions A novel route for regularly arranging Fe nanoparticles into three-dimensional arrays by aligned growth of CNTs using the CVD method was demonstrated. The spacing between adjacent Fe particles along the nanotube can be controlled by the catalyst feeding interval during the nanotube growth process. The configuration may exhibit new electronic and magnetic properties due to the spatial distribution of numerous metallic nanoparticles confined within the nanotubes. The mechanical strength and chemical stability of CNTs make them a potential candidate for storing and protecting high-activity nanoparticles in a regular pattern. The methodology developed in this study could be regarded as a general approach for the fabrication and manipulation of nanostructures consisting of various nanoparticles and CNTs.
11940 J. Phys. Chem. B, Vol. 105, No. 48, 2001 Acknowledgment. The authors thank Mrs. D. R. Ou and Mr. Y. J. Yan for SEM studies on the samples. This work was supported under the State Key Program for Fundament Research of MOST, China (Grant No. G20000264-04) and the Doctor Dissertation Foundation of Tsinghua University. References and Notes (1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (2) Bethune, D. S.; Klang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605. (3) Colomer, J.-F.; Bister, G.; Willems, I.; Ko´nya, Z.; Fonseca, A.; Van Tendeloo, G.; Nagy, J. B. Chem. Commun. 1999, 1343. (4) Fonseca, A.; Hernadi, K.; Piedigrosso, P.; Colomer, J.-F.; Mukhopadhyay, K.; Doome, R.; Lazarescu, S.; Biro, L. P.; Lambin, Ph.; Thiry, P. A.; Bernaerts, D.; Nagy, J. B. Appl. Phys. A 1998, 67, 11. (5) Journet, C.; Maser, W. K.; Bernier, P.; Loiseau, A.; Lamy de la Chapelle, M.; Lefrant, S.; Deniard, P.; Lee, R.; Fischer, J. E. Nature 1997, 388, 756. (6) Cassell, A. M.; Raymakers, J. A.; Kong, J.; Dai, H. J. Phys. Chem. B 1999, 103, 6484.
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