In situ mass spectroscopic analysis for chemical vapor deposition synthesis of single-walled carbon nanotubes

Chemical Physics Letters 533 (2012) 56–59

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In situ mass spectroscopic analysis for chemical vapor deposition synthesis of single-walled carbon nanotubes Takashi Tomie a, Shuhei Inoue b,⇑, Yukihiko Matsumura b a b

Department of Mechanical Science Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan Energy and Environmental Division, Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan

a r t i c l e

i n f o

Article history: Received 6 January 2012 In final form 1 March 2012 Available online 8 March 2012

a b s t r a c t To elucidate the growth mechanisms of carbon nanotubes by chemical vapor deposition, we performed in situ mass spectroscopy for methane CVD. Methane is considered stable at temperature below 1000 °C; however, using appropriate catalysts, it can be decomposed. In previous studies, acetylene and ethane were thought to be key intermediates in the synthesis of carbon nanotubes, but in this study, ethylene was found to be the key building block. The yield of ethylene was low but this is consistent with the fact that the production rate of carbon nanotubes by methane CVD was lower than that by ethylene CVD synthesis. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Since the discovery of single-walled carbon nanotube (SWCNT) [1], they have attracted considerable attention as an advanced material that is potentially applicable in many fields, especially in the field of nano-engineering. Chemical vapor deposition (CVD) is the most appropriate method for preparing SWCNTs because of its cost effectiveness and scalability for mass production. Clarification of the growth mechanisms of SWCNTs will help in finding methods to better control the CVD process and improve yields. Presently, no comprehensive model for their CVD growth is well characterized, but there are some models [2–5] regarding the nucleation process on metal catalysts and how growth depends on the carbon mixture. These models are most commonly applied to the laser ablation [6] method, and arc discharge method [7], which supplies isolated carbon to the catalysts and some theoretical works [8,9] on isolated carbon models have been reported. However, these models do not cover pyrolysis, which is the most important part of a CVD process. Numerous CVD methods have been developed; among these, the most industrially useful methods may be the high-pressure carbon monoxide process [10], alcohol catalytic CVD [11], and the super growth process [12]. In this study, we focused on the methane CVD process, which is considered to be the simplest method of CVD especially for analyzing growth and reaction mechanism because methane is one of the smallest molecules for synthesizing CNTs in CVD methods, and we investigated the reaction products by in situ observation of the products in the reactor. In a previous study on methane CVD, Nozaki and Okazaki [13] suggested a tentative reaction pathway ⇑ Corresponding author. Fax: +81 82 424 5923. E-mail address: [email protected] (S. Inoue). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.03.002

in plasma enhanced CVD (PECVD). In their study, methane is dissociated into CH3 and H by electrons or other charged particles. Then, via a third body collision of CH3, CH3, and X, ethane is formed as the major product. Here, X refers to a third body partner such as helium, methane, or hydrogen. In addition, if the pressure is low, ethylene and acetylene can also be obtained as products. Zhong et al. [14] also carried out a mass spectroscopic analysis of PECVD, and they concluded that acetylene was the key precursor in the growth of SWCNT forests and there was no contribution from methane. In contrast, we found ethylene to be the main product during thermal CVD. Noda et al. [15] mentioned hydrocarbon reforming by aluminum oxide, which was used as a catalyst support; however, we could not confirm its effect.

2. Experimental Figure 1 shows a schematic of our experimental setup [16]. After the catalysts were fully reduced by an Ar/H2 (5%) gas mixture at a rate of 400 sccm while increasing the temperature, methane was supplied at 400 sccm. Normally, argon was also supplied at 400 sccm to normalize the signals. Iron and molybdenum were prepared on aluminum oxide as the catalysts by the following procedure [17]. One gram of the catalyst powder was placed in the designated place in the reactor. The reactor length was 300 mm, and the pressure during synthesis was 1 kPa. After passing through the reactor, gasses were skimmed into the mass spectroscopic area through two pinholes, each 0.5 mm in diameter, to the differential pumping room. Through these two gas expansions (differential pumping room, 1 Pa and mass spectroscopic area, 5  104 Pa), the sample gas cooled. Finally, sample gasses were analyzed by quadrupole mass spectrometry (Q-mass).

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Cold Cathode Pirani gauge

QMS

Diffusion pump

Pinhole (500 m) Rotary vacuum pump Pirani gauge

Source gas Mass flow controller Electric furnaces

Ar

Catalyst

Rotary vacuum pump Ar+H2

Figure 1. Schematics of experimental setup. The quadrupole mass spectrometer directly connected to the CVD reactor via two skimmers with 0.5 mm in diameter each. After the reactor pressure of each room (after the first skimmer, and the second skimmer) is 1 Pa and 5  10–4 Pa, respectively. Assuming the diameter of products is comparable to nitrogen, collision number is roughly estimated less than four times until arriving the Q-mass.

3. Results and discussion

Table 1 Mass number of each molecule and its fraction.

Figure 2 shows the mass spectra of methane CVD without catalysts. There seemed to be no remarkable differences at the different temperatures. It appeared from this data that there was little methane pyrolysis, as will be discussed later. The results were in good agreement with the Nernst approximation [18] and theoretical estimations [19]. The peak at m/z = 2 must have been hydrogen, but this hydrogen was not the product of pyrolysis because the signal was also found at room temperature. The mass resolution of Q-mass was not good because there were about 10 signals within 1 m/z. Furthermore, the isotopic distributions of products measured in this experiment were quite distinctive; in other words, each product is shown at their major mass number and it does not shift from an integral mass number, as listed in Table 1.

Product

First mass number (fraction)

Second mass number (fraction)

CH4 C2H4 Ar H2 C2H2

16(0.988) 28(0.977) 40(0.996) 2(1.000) 26(0.978)

17(0.012) 29(0.022) – – 27(0.02)

Thus, we accumulated the data and converted it to intensity of each mass number using the equation below:

IN ¼

Z

Nþ0:25

f ðxÞdx;

N0:25

where IN is the accumulated intensity of each mass number, N is the mass number, and f(x) refers to the data value. The accumulated spectra obtained by applying the above equation are shown in Figure 3. It should be noted that Figures 2 and 3 were derived from the same data. Referring to Figure 3, the signal ratios of hydrogen/ methane were 0.30, 0.21, 0.25, 0.19, and 0.24 at temperatures of 600, 700, 800, 900, and 1000 °C, respectively. The numbers had little fluctuation and no dependency on temperature; thus, we did not consider pyrolysis to have taken place. The ionization technique of our Q-mass was electron ionization (EI) [20], so the hydrogen was considered to be a fragment from methane. For example,

Intensity (arb.unit)

1000 °C

900 °C 800 °C

700 °C RT 0

10

20 30 mass (m/z)

40

50

Figure 2. The mass spectra of methane via reactor without catalysts. The argon gas (m/z = 40) was supplied at the same rate for normalization. Some fragments made by EI are shown at m/z = 15, 14, and 13. The spectrum shown at m/z = 18 is water molecule involved in the reactor.

8 þ   Inonization > < CH4 þ e ! CH4 þ 2e þ þ CH4 ! CH3 þ H or CHþ3 þ H Fragmentation > : þ H þ H ! Hþ2 With respect to the mass number m/z = 28, we considered the same mechanism. This signal intensity, by comparison with that of oxygen (m/z = 32), was too great to be simply considered as residual nitrogen from the air. It was also difficult to assign this

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T. Tomie et al. / Chemical Physics Letters 533 (2012) 56–59

Intensity (arb. unit)

1000 °C

900 °C

Intensity (arb. unit)

0

10

20 30 mass (m/z)

40

50

Figure 4. The mass spectrum of methane pyrolysis with catalysts at 700 °C. Compared with the spectrum without catalysts, the amount of hydrogen is drastically increased.

800 °C

diameter—in the case of a linear molecule, its major axis is considered—the actual number of each atom could be roughly estimated by the following equation [21]:

700 °C

Ni ¼

X

C0  m 

k¼0

RT

0

5

10

15 20 mass (m /z)

25

30

Figure 3. The accumulated mass spectra of Figure 2. All of these figures are normalized by the intensity of argon signal. There is not remarkable differences among them. Though the slight increase of the signal of m/z = 28, which is considered to be ethylene is shown, pyrolysis of methane is considered to be little.

signal to ethylene synthesized by pyrolysis because it was also present at room temperature. Therefore, it was proposed that this ethylene was synthesized by the two body reaction shown below:

CH4 ! CH2 þ 2H CH2 þ CH2 ! C2 H4 In addition, other fragments such as CH (13) and C (12) were also detected. The balance of hydrogen does not seem to have been conserved; however, the quantitative analysis required great attention to detail. Usually, the ionization energy of the EI MS method is much higher than a molecule’s ionization potential, so the probability of ionization depends on its cross section. Here, because the ionization potential of these products are about 10 eV, assuming the cross section is proportional to its van der Waals

Ik 2

dk

;

where Ni is the number of an atom, C0 is a constant, m is number of atoms in the molecule, Ik is the intensity of a molecule, and dk denotes the molecule’s diameter. According to the above equation, the number ratio of H/C becomes nearly 4, and the associated parameters are listed in Table 2. Because there appeared to be nothing but contamination in the reactor, and methane was only the reactant, this number ratio was quite consistent. As a result, methane was decomposed a little under 1000 °C. In the actual CVD process, catalysts are used for synthesis, and these catalysts play quite important roles. Figure 4 shows the accumulated mass spectra of CVD at 700 °C with catalysts. Compared with Figure 3, the ratio of H2/CH4 was clearly higher in Figure 4. This was considered to be evidence of catalytic pyrolysis. On the basis of our results and those presented in the Letter referred to previously, we infer that methane was not cracked thermally under 1000 °C; however, with catalysts such as molybdenum and iron, methane can dissociate at lower temperatures. Even at 600 °C, this pyrolysis was observed. To remove the background signals owing to fragmentation and contamination, we subtracted the room-temperature spectrum from that taken at 700 °C. Methane and also CH3 decreased significantly, as shown in Figure 5. This methyl radical was a possible product of pyrolysis, but as we mentioned before, it was reasonable to consider it to be a fragment from EI. Thus, it was considered that a decrease in methane by pyrolysis resulted in fewer methyl radicals. On the other hand, the ethylene peak clearly increased, and a small amount of acetylene (m/z = 26) was found. Besides these main CVD products, a small amount of water (m/z = 18) and methanol (m/z = 41), seen as a fragment (methanol normally has m/z = 42), were observed. Water contamination must have led to the formation of methanol. It should be noted that too much hydrogen was observed when we considered the atom balance. Following the above-mentioned estimation, in this experiment, the carbon balance was preserved but

Table 2 The balance of H/C. The amount of each molecule is roughly estimated from its intensity assuming van der Waals diameter. Molecule

vdW Dia

d2

Intensity

Intensity/d2

C-m

H-m

C

H

H/C

CH4 CH3 CH2 CH H2 C2H4

2.2 2.2 2.2 2.2 1.4 3.77

4.8 4.8 4.8 4.8 2.0 14.2

9.3 7 1 0.4 2.3 1.6

1.92 1.45 0.21 0.08 1.17 0.11

1 1 1 1 0 2

4 3 2 1 2 4

1.92 1.45 0.21 0.08 0.00 0.23 3.88

7.69 4.34 0.41 0.08 2.35 0.45 15.32

3.95

difference (arb. unit)

T. Tomie et al. / Chemical Physics Letters 533 (2012) 56–59

+0.1

±0

−0.1 0

10

20 30 mass (m / z)

40

50

Figure 5. The differential mass spectrum of with and without catalysts at 700 °C. Regarding to this spectrum, methane is clearly decreased; on the contrary, the amounts of hydrogen and ethylene are increased. This result indicates that the pyrolysis of methane is promoted and ethylene and acetylene (m/z = 26) are thought to be the key products of SWCNT.

the hydrogen balance was not. The reason for this was not completely clear, but it was reasonable to consider the excess hydrogen to have been outgas from the reactor because the spectrum taken at room temperature was used for the background signal to clear the reaction products. Consequently, in this experiment, ethylene was the main product along with a small amount of acetylene formed in pyrolysis. This result seemed to conflict entirely with the previous studies, but Nozaki mentioned that ethylene might be a major product under low pressure; therefore, we consider our result to be consistent. As the spectra show, the amount of ethylene synthesized in this catalytic pyrolysis was still small even under catalysis; however, when we took into account the production yield of SWCNTs, this low yield of ethylene appropriately explained the results. To conclude the results of all experiments, the synthesis of ethylene was a key factor for SWCNT growth; when ethylene was produced, the growth of SWCNT was confirmed by Raman spectroscopy. This hypothesis is also supported by reports of CVD synthesis using ethylene. In those studies, SWCNT growth was confirmed at slightly lower temperatures than those required for methane CVD. To achieve better yields, ethylene might be employed as a source gas. 4. Conclusion Using quadrupole mass spectrometry, we carried out an in situ mass spectroscopic analysis of SWCNT growth in methane CVD system. Without catalysts, methane was stable below 1000 °C and very little decomposition was observed. Using the appropriate

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catalysts, pyrolysis was observed at temperatures as low as 600 °C. Comparing the spectra taken at 700 °C with those taken at room temperature, we confirmed the pyrolysis of methane and the synthesis of ethylene and a small amount of acetylene. Consequently, for the growth of SWCNTs, ethylene was one of the most important ingredients. This result is consistent with the fact that the production rate of carbon nanotubes by methane CVD synthesis is lower than that by ethylene CVD synthesis [22]. Acknowledgments The measurement of Raman spectroscopy was made using HORIBA-JY T64000 at the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University. Part of this research was supported by JSPS the Grant-in-Aid for Young Scientists (B) (No. 23760188) and Electric Technology Research Foundation of Chugoku. References [1] S. Iijima, T. Ichihashi, Nature 363 (1993) 603. [2] M. Yudasaka, R. Yamada, S. Iijima, J. Phys. Chem. B 103 (1999) 6224. [3] H. Kataura, Y. Kumazawa, Y. Maniwa, Y. Ohtsuka, R. Sen, S. Suzuki, Carbon 38 (2000) 1691. [4] H. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 260 (1996) 471. [5] S. Inoue, Y. Kikuchi, Chem. Phys. Lett. 410 (2005) 209. [6] A. Thess et al., Science 273 (1996) 483. [7] C. Journet et al., Nature 388 (1997) 756. [8] Y. Shibuta, S. Maruyama, Chem. Phys. Lett. 382 (2003) 381. [9] F. Ding, A. Rose˙n, K. Bolton, J. Chem. Phys. 121 (2004) 2775. [10] P. Nikolaev, M.J. Bronikowski, R.K. Brafley, F. Rohmnd, D.T. Colbert, K.A. Smith, R.E. Smalley, Chem. Phys. Lett. 313 (199) 91. [11] S. Maruyama, R. Kojima, Y. Miyauchi, S. Chiashi, M. Kohno, Chem. Phys. Lett. 360 (2002) 229. [12] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Science 306 (2004) 1362. [13] T. Nozaki, K. Okazaki, Plasma Process Polym. 5 (2008) 301. [14] G. Zhong, S. Hofmann, F. Yan, J. Phys. Chem. 133 (2009) 17321. [15] S. Noda, K. Hasegawa, H. Sugime, K. Kakehi, Z.Y. Zhang, S. Maruyama, Y. Yamaguchi, Jpn. J. Appl. Phys. 46 (2007) 399. [16] T. Tomie, S. Inoue, M. Kohno, Y. Matsumura, Diam. Relat. Mater. 19 (2010) 1401. [17] K. Mukhopadhyay, A. Koshio, N. Tanaka, H. Shinohara, Jpn. J. Appl. Phys. 37 (1998) L1257. [18] R.C. Cantelo, J. Phys. Chem. 30 (1926) 1641. [19] C Guéret, M. Daroux, F. Billaud, Chem. Eng. Sci. 52 (1997) 815. [20] M.L. Vestal, Chem. Rev. 101 (2001) 361. [21] G.H. Wannier, Phys. Rev. 90 (1953) 817. [22] S.C. Lyu, B.C. Liu, S.H. Lee, C.Y. Park, H.K. Kang, C.W. Yang, C.J. Lee, J. Phys. Chem. B 108 (2004) 1613.