Influence of Gas-Phase Reactions on the Growth of Carbon

Jan 22, 2010 - Osaka University. ... These unsaturated carbon chains, in addition to the acetylene gas, with a high temperature should be important ca...
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J. Phys. Chem. C 2010, 114, 2398–2402

Influence of Gas-Phase Reactions on the Growth of Carbon Nanotubes He Ma,† Lujun Pan,*,† and Yoshikazu Nakayama‡ School of Physics and Optoelectronic Technology, Dalian UniVersity of Technology, Dalian, Liaoning, 116024, People’s Republic of China, and Department of Mechanical Engineering, Osaka UniVersity, 2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan ReceiVed: June 7, 2009; ReVised Manuscript ReceiVed: December 25, 2009

The effect of gas-phase reactions on the growth of carbon nanotubes is investigated by a new synthesis method of infrared light local heating chemical vapor deposition where only the substrate can be heated. The simulation of temperature distributions surrounding the heated substrate shows that the region with high temperatures over 600 °C is enlarged with the increase of the substrate temperature. The expansion of the high-temperature region is speculated to result in the growth of carbon nanotubes at a substrate temperature of 770 °C and the successful growth of carbon nanotubes with a higher density at a higher temperature of 900 °C. It is analyzed that polymerization reactions of acetylene gas that occurred in the high-temperature region generate unsaturated carbon chains mainly comprising C4H4. These unsaturated carbon chains, in addition to the acetylene gas, with a high temperature should be important carbon sources for the growth of carbon nanotubes. 1. Introduction Carbon nanotubes (CNTs) have attracted much attention because of their wide application prospect in the fields of nanoelectronic devices, nano- or microelectromechanical systems, functional composites, etc. For realizing these applications, the large-scale, controllable, selective synthesis of CNTs and understanding of their growth mechanism are indispensable. Thermal chemical vapor deposition (CVD) has been proven to become one of the most important methods to synthesize CNTs in different conditions for the satisfaction of their various usages. In recent years, big progress in the synthesis of CNTs by CVD methods, such as superlong and vertically aligned CNTs, diameter control, and selective growth of CNTs, has been achieved by many research groups.1-3 However, the growth mechanism of CNTs is still not understood completely. Most of the studies were focused on the structural and morphological changes of catalyst particles that are certainly important factors for the growth of CNTs. The common knowledge contains the following sequence of reactions:4-6 (1) adsorption and dissociation of hydrocarbons on the surfaces of catalyst particles, (2) formation of a carbon-metal solid-state solution or a metal carbide compound, and (3) carbon atoms’ precipitation from the supersaturated catalyst particle, leading to CNT growth under the continuous hydrocarbon supply. The last two stages have been studied in depth due to the important effect of the catalyst and have been in situ observed clearly by high-resolution TEM7-10 and XRD methods.11 Nevertheless, the first stage is usually simply considered to be the process of dehydrogenation from the provided hydrocarbon gas and the adoption of carbon atoms in catalyst particles in the most suggested models for CNT growth. However, the reactions of supplied hydrocarbon in gas phase during a CVD process are generally complex and non-negligible. The products from these reactions would also flow with the unreacted hydrocarbon to the catalyst particles * To whom correspondence should be addressed. E-mail: [email protected]. † Dalian University of Technology. ‡ Osaka University.

Figure 1. Scheme of the LHCVD system.

and definitely affect the catalytic reactions that occurred at the surfaces of these particles. For the floating CVD method, the mechanism of CNT growth has been investigated by numerous simulations12-14 and experiments.15 It was reported that the products of gas-phase reactions, such as soot precursors, play an important role in CNT growth.16 We have developed a method called local heating (LH) CVD17 to distinguish the roles played by the gas-phase reactions and surface catalytic reactions on the growth of CNTs in a CVD process. Furthermore, a numerical simulation has been performed to explain the influence of the gas-phase reactions on CNT growth. 2. Experimental Section 2.1. Experiment. The LHCVD system is illustrated in Figure 1. Iron films with a thickness of 4 nm in a striped shape prepared by an electron-beam evaporation on Si substrates were used as the catalyst. The substrates were put into a transparent quartz tube with a diameter of 10 mm and were heated by a beam of infrared light introduced to the sample surface from a lamp under the flow of high-purity helium gas. The light beam was limited to irradiate only the area where the substrate was placed without the ability to heat the gas. A high-purity acetylene gas with a concentration of 15% was introduced to synthesize CNTs under the flow of helium gas, and the total gas flow rate was maintained at 20 sccm. The temperatures of substrates were changed from 700 to 950 °C by variation of the lamp power. It is noted that the temperature of the quartz tube was measured to be only 70 °C when the power of the infrared light reached

10.1021/jp905345p  2010 American Chemical Society Published on Web 01/22/2010

Influence of Gas-Phase Reactions on the Growth of CNTs

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Figure 3. Geometric model of the conventional CVD system.

CVD was also simulated. The scheme of the geometric model is shown in Figure 3. The tube was 1200 mm long. The furnace enwrapping the middle part of the quartz tube was 700 mm long. The inner diameter of the quartz tube was 28 mm. A mixture gas of helium and acetylene with a flow rate of 260 sccm was also used as the feedstock gas. In both LHCVD and conventional CVD models, buoyancy effect was considered by adding a source term of Ffluid · g · β · (T - Tref) in the momentum equation in the y direction, where Ffluid was the density of the mixture gas, g was the acceleration of gravity, β was the volumetric thermal expansion coefficient, T was the local temperature, and Tref was equal to 27 °C. The validity of the simulation was checked by comparing the simulated wall temperature with the measured data. Figure 4 shows the measured temperature of the outer wall right under the substrate and the simulation data at the same point under the substrate temperatures of 600, 700, 800, and 900 °C. It is clearly observed that the experimental data are well-fitted by the calculated data, suggesting the validity of our simulation. In our simulation, the mass conservation was satisfied well because the difference between the inlet flow rate and outlet flow rate was only 1.43 × 10-5%. The sum residual error of the temperature in the whole field was 0.55% of the inflowing energy, and the value of temperature at each point was not changed any more, suggesting that the temperature field was converged.

Figure 4. Comparison between the measured temperature (circle) and the simulation result (square) at different substrate temperatures of 600, 700, 800, and 900 °C.

3. Results and Discussion

Figure 2. Geometric model of the LHCVD system.

the level of heating the substrate to 900 °C. Therefore, it was reasonable to consider that the quartz tube was transparent to the infrared light. The deposition time was 10 min. The deposits were then observed by a scanning electron microscope (SEM). 2.2. Numerical Simulation. To understand the growth mechanism of CNTs synthesized by the LHCVD method, temperature distributions in the CVD chamber were simulated by the computational fluid dynamics software (PHOENICS, Concentration Heat and Momentum Ltd.) based on the 3D, steady-state and laminar flow mode. Figure 2 shows the geometric model for the simplification of the LHCVD system. The parameters used in the model were the following: (1) The diameter of the quartz tube was 10 mm, and the thickness of the tube wall was 1 mm. An air layer of 0.25 mm was set outside the quartz tube to ensure that the tube could dissipate heat sufficiently. (2) The mixture gas of helium and acetylene was used in the model. The inlet flow rate was 20 sccm. (3) A Si substrate (5 × 10 mm2), whose temperature was changed from 700 to 1000 °C, was placed at the center of the quartz tube. For comparison, the temperature distribution in a conventional

At the substrate temperature of 700 °C, there is no CNT grown on the whole substrate in the LHCVD. Figure 5 shows the SEM micrographs of the deposits grown at the temperatures of (a) 770, (b) 850, and (c) 910 °C. Straight lines drawn in the images (a) and (b) indicate the boundaries of the patterned iron films and bare Si substrate. At the substrate temperature of 770 °C, only a few CNTs are grown at the edge of the iron film. When the temperature is raised to 850 °C, the region where CNTs are grown is enlarged to the whole area of the patterned iron film. The inset in Figure 5b, a magnified image of an arbitrary region, shows that the grown CNTs are low in density and short in length. When the substrate temperature is raised to 910 °C, a large number of CNTs with an average diameter of 20 nm are synthesized on the substrate, illustrated in Figure 5c, which is similar to the CNTs grown by conventional thermal CVD using acetylene as reaction gas at temperatures below 650 °C.18 Notice that, in all substrate temperatures, no CNT is grown on the substrate where no iron film is precoated, indicating that the growth of CNTs is catalytic activated. For conventional thermal CVD, 700 °C is a suitable temperature for the growth of high-density CNTs by using the same conditions.3,19 The only difference between the two systems is

Figure 5. SEM micrographs of deposits grown on the iron film at the temperatures of (a) 770, (b) 850, and (c) 910 °C. The straight lines drawn in the images (a) and (b) show the edges of the patterned iron films. The insets in (b) and (c) are magnified images of an arbitrary region in each micrograph.

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Figure 6. Simulated temperature change along the centerline of the chamber for conventional CVD at 700 °C.

the heating modes that are heating both sample and gases in a conventional CVD chamber but only heating the sample in our LHCVD chamber. Figure 6 shows the simulated temperature distribution in the conventional CVD chamber along the centerline when the furnace temperature is set to be 700 °C, which is a good condition for the growth of well-aligned CNTs. Feedstock gas is heated to the furnace temperature as soon as it flows into the heating zone, and this temperature is maintained until it flows out of the heating zone. The residence time above the substrate is approximately 1 s in the conventional CVD system, which is almost the same as the result reported by Suekane et al.,20 who use a similar system and reaction conditions with us. On the other hand, the simulated contours of temperatures higher than 600 °C in the cross section of the middle part of the LHCVD chamber with substrate temperatures of (a) 700, (b) 800, (c) 900, and (d) 1000 °C, are shown in Figure 7. Furthermore, to figure out the details of these variations, the temperature changes above the center of the substrate along the dotted lines drawn in Figure 7 are plotted in Figure 8. The region surrounding the substrate with temperatures higher than 600 °C is enlarged with the increase of substrate temperature. It is observed from Figure 8 that the gas temperatures are lower than 600 °C in the region of 0.5 mm apart from the substrate when the substrate temperature is 700 °C, and the boundary line of 600 °C is expanded to 1.0, 1.5, and 1.9 mm above the center of the substrate at temperatures of 800, 900, and 1000 °C, respectively. The growth results obtained from the acetylene LHCVD show that CNTs cannot be grown until the substrate temperature is raised to 750 °C, that is, high enough for surface catalytic reactions, suggesting that CNT growth cannot be determined by only surface catalytic activities. Acetylene gas with a high temperature and the gas-phase reactions that inevitably occurred at this temperature also play key roles for CNT growth. It is reported that acetylene shows a strong tendency to polymerize at the temperature of 600-700

Ma et al.

Figure 8. Temperature changes above the substrate along the dotted lines drawn in Figure 7, corresponding to the substrate temperatures of 700 (triangle), 800 (diamond), 900 (square), and 1000 °C (circle).

TABLE 1: Typical Reactions of Acetylene Pyrolysis and Their Reaction Rate Constants at 1000 K

no.

reactionsa,b

forward reaction rate constanta (cm3 mol-1 s-1)

1 2 3 4 5 6 7

C2H2 + C2H2 ) •C4H3 + •H •C4H3 + •C4H3 ) C4H4 + C4H2 C2H2 + •H ) •C2H3 C2H2 + •H ) •C2H + H2 C2H2 + •C2H ) C4H2 + •H C2H2 + •C2H3 ) C4H4 + •H C 2H 4 + C 2 H 2 ) C 4 H 6

2.2 × 101 1.00 × 1011 1.63 × 1012 8.28 × 108 1.04 × 1014 1.61 × 1011 5.26 × 1010

a

The reactions are selected in ref 26, and the rate constants are obtained from the NIST Chemistry WebBook.28 b The reactions listed in Table 1 are reversible. The backward reaction rate constants are not listed in Table 1 because the forward rate constants are much larger than the backward ones, except for reactions 1 and 4.

°C.21 In the pyrolysis of acetylene, main reactions with significant rate constants result in the generation of C4 species.22-27 The typical reactions and their reaction rate constants at 1000 K are summarized in Table 1. It is considered that these generated C4 species may also be important carbon sources for CNT growth. It should be noted that the gas-phase reactions of acetylene are activated by the hydrogen radicals. The initial hydrogen radicals may be originated mainly from the surface catalytic reactions, such as C2H2 + Fen f C2H - Fen + H - Fen,29 the polymerization reaction 1 (Table 1), and the direct dissociation of acetylene molecules. The reaction rate constant of the catalytic dehydrogenation reaction on the scale of 1012 to 1013 s-1 is much larger than that of reaction 1, leading to mass releasing of hydrogen radicals or molecules to the gas phase above the substrate and then enhancement of gas-phase reactions.

Figure 7. Simulated temperature contours in the region where the gas temperatures are higher than 600 °C at the substrate temperatures of (a) 700, (b) 800, (c) 900, and (d) 1000 °C in the middle of the LHCVD chamber.

Influence of Gas-Phase Reactions on the Growth of CNTs Among these reactions in Table 1, the rate constants imply that reaction 5, which produces C4H2, would be the main reaction. However, hydrogenation reactions with acetylene prefer the generation of •C2H3 radicals to •C2H radicals because the rate constant of reaction 3 is much larger than that of reaction 4, resulting in the lack of •C2H radicals in the gas phase. Sequentially, there would be less concentration of C4H2 than that of C4H4 in the gas phase. For reaction 7, although its rate constant approaches that of reaction 6, the reaction would be limited by the amount of ethene generated mainly from the decomposition of styrene that is a later product of polymerization reactions.24 Sequentially, the proportion of C4H6 would be much lower than that of C4H4 generated from reactions 2 and 6. Moreover, the carbon triple bond in C4H4 can be opened easily by the surface catalytic reaction. The formed structure that consists of two sp2 hybridized carbon bonds could be a very good unit for the assembly of a CNT after dehydrogenation on the surface of the catalyst particle. On the other hand, more carbon atoms in these C4 species than in C2H2 could also be helpful to the high-speed growth of CNTs. Taking into account the above factors, C4H4, therefore, would be the main generation among the polymerization products of acetylene, contributing to CNT growth. It is noted that benzene, polycyclic aromatic hydrocarbons (PAHs), and soot precursors are also important products in acetylene polymerization reactions. Benzene and PAHs would not be important to CNT growth because their structures are very stable in high-temperature environments.30 Soot precursors are possible to play a key role in CNT and soot growth.16 However, the concentration of the soot precursors produced only by the pyrolysis of acetylene in the gas phase is considered to be very low, and the gasphase radical reactions are enhanced by the surface catalytic reactions mentioned above. More soot precursors are produced in the region above the substrate and are drifted downstream along with the gas flow because of the short residence time of only 1 s. On the other hand, the time for generation of C4 species, mainly C4H4, is much shorter than that for soot precursors. Therefore, C4H4 would be more important to the growth of CNTs than the soot precursors. In the conventional CVD chamber with a temperature of 700 °C, sufficient polymerization reactions of acetylene occurs to form C4 species above the substrate. These species as well as the hot unreacted acetylene gas can be supplied to the surfaces of catalyst particles, which would result in the highly efficient growth of CNTs. In the LHCVD, although the temperature of 700 °C is high enough for the surface catalytic reactions, CNTs still cannot be synthesized because the gas is not heated enough or the gas-phase reactions are not active enough to supply effective hydrocarbons for the surface catalytic reactions. However, when the substrate temperature is increased, the high-temperature zones in the gas phase expand. The amount of heated gas as well as the generated unsaturated carbon chains in the gas phase is increased simultaneously so that more effective hydrocarbons could be provided to the surfaces of the catalyst particles, leading to the growth of CNTs. This is also the reason why CNTs are grown in a higher density at a higher temperature. Besides the enhancement of the polymerization reactions, the lack of amorphous carbon formed by the decomposition reactions is also an important reason for the growth of CNTs synthesized at 900 °C. It is generally accepted that the decomposition reaction of acetylene occurs at the temperature of 500 °C and becomes violent with the temperature increased.

J. Phys. Chem. C, Vol. 114, No. 6, 2010 2401 In conventional CVD at 900 °C, excess amorphous carbon generated by the acetylene decomposition would cover the surfaces of the catalyst particles and induce their deactivation in a short time, resulting in a failure of the CNT growth at such a high temperature. However, in the LHCVD, almost no amorphous carbon is observed on the substrate surface. This could also be explained by the temperature distribution shown in Figure 8. Even though the substrate temperature arrives to 1000 °C, more than 70% of the region in gas phase still has the temperatures of lower than 700 °C at which polymerization reactions mainly occurred. The high-temperature area (>800 °C) that is attributed to producing amorphous carbon is limited only in the zone very near to the surface of the substrate. Therefore, the amount of amorphous carbon generated is so small and could be neglected. 4. Conclusions The effect of gas-phase reactions, in addition to that of the catalytic reactions on the CNT growth, has been studied. The numerical simulation of the temperature distribution in the LHCVD system has shown that the area of high temperature over 600 °C surrounding a substrate is enlarged obviously with the increase of the substrate temperature, which explains the experimental results of temperature dependence of CNT growth. It is considered that acetylene gas with a high temperature and the unsaturated carbon chains mainly consisting of C4H4 generated from the polymerization reactions in gas phase are all the carbon sources for the growth of CNTs. References and Notes (1) Chakrabarti, S.; Kume, H.; Pan, L.; Nagasaka, T.; Nakayama, Y. J. Phys. Chem. C 2007, 111, 1929. (2) Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Science 2004, 306, 1362. (3) Pan, L. J.; Nakayama, Y.; Shiozaki, H.; Inazumi, C. J. Mater. Res. 2004, 19, 1803. (4) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512. (5) Lee, C. J.; Park, J. H. Appl. Phys. Lett. 2000, 77, 3397. (6) Puretzky, A. A.; Geohegan, D. B.; Jesse, S.; Ivanov, I. N.; Eres, G. Appl. Phys. A: Mater. Sci. Process. 2005, 81, 223. (7) Helveg, S.; Lopez-Cartes, C.; Sehested, J.; Hansen, P. L.; Clausen, B. S.; Rostrup-Nielsen, J. R.; Abild-Pedersen, F.; Norskov, J. K. Nature 2004, 427, 426. (8) Sharma, R.; Moore, E.; Rez, P.; Treacy, M. M. J. Nano Lett. 2009, 9, 689. (9) Yasuda, A.; Kawase, N.; Banhart, F.; Mizutani, W.; Shimizu, T.; Tokumoto, H. J. Phys. Chem. B 2002, 106, 1849. (10) Schaper, A. K.; Hou, H.; Greiner, A.; Phillipp, F. J. Catal. 2004, 222, 250. (11) Nishimura, K.; Okazaki, N.; Pan, L. J.; Nakayama, Y. Jpn. J. Appl. Phys., Part 2 2004, 43, 471. (12) Endo, H.; Kuwana, K.; Saito, K.; Qian, D.; Andrews, R.; Grulke, E. A. Chem. Phys. Lett. 2004, 387, 307. (13) Kuwana, K.; Endo, H.; Saito, K.; Qian, D.; Andrews, R.; Grulke, E. A. Carbon 2005, 43, 253. (14) Kuwana, K.; Saito, K. Carbon 2005, 43, 2088. (15) Wasel, W.; Kuwana, K.; Reilly, P. T. A.; Saito, K. Carbon 2007, 45, 833. (16) Reilly, P. T. A.; Whitten, W. B. Carbon 2006, 44, 1653. (17) Pan, L. J.; Nakayama, Y. Nanotechnol. Precis. Eng. 2009, 7, 102. (18) Qi, H.; Qing, L.; Lian-zeng, Y.; Wei-li, C.; Qi, Z. Chin. J. Chem. Phys. 2007, 20, 207. (19) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Science 1996, 274, 1701. (20) Suekane, O.; Nagasaka, T.; Kiyotaki, K.; Nosaka, T.; Nakayama, Y. Jpn. J. Appl. Phys., Part 2 2004, 43, 1214. (21) Egloff, G.; Lowry, C. D., Jr.; Schaad, R. E. J. Phys. Chem. 1932, 36, 1457. (22) Cullis, C. F.; Franklin, N. H. Proc. R. Soc. London, Ser. A 1964, 280, 139.

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