Low temperature growth of single-walled carbon nanotubes: Small diameters with narrow distribution

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1 Chemical Physics Letters 419 (2006) Low temperature growth of single-walled carbon nanotubes: Small diameters with narrow distribution W.L. Wang, X.D. Bai, Zhi Xu, S. Liu, E.G. Wang * Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Zhongguancun, Hai-Dian, Beijing , PeopleÕs Republic of China Received 11 October 2005 Available online 1 December 2005 Abstract Low temperature ( C) synthesis of high-quality single-walled carbon nanotubes (SWNTs) via microwave plasma assisted chemical vapor deposition (MPCVD) was demonstrated by pyrolysis of methane over the Fe Mo bimetallic catalyst nanoparticles supported on porous MgO powders. In comparison with higher temperature (800 C) growth by the conventional thermal CVD using the same batch of catalyst, the SWNTs grown by the low temperature MPCVD show small diameters with narrow distribution. Moreover, it was found that the lower the growth temperature, the smaller and more homogenous the tube diameter, showing the feasibility of controlling the SWNTs diameters by tuning the growth temperature. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction * Corresponding author. Fax: address: egwang@aphy.iphy.ac.cn (E.G. Wang). With the unique electronic and mechanical properties, single-walled carbon nanotubes (SWNTs) have attracted a great deal of theoretical, experimental, and technological interest. Over the past 10 years, controlled synthesis involving the catalytic chemical vapor deposition (CVD) of the carbon-bearing compounds over transition metal nanoparticles has been emerging as an elegant strategy to grow high-quality SWNTs [1 3]. For the CVD synthesis of SWNTs, pursuing lower growth temperature has been a long-standing goal. Not only should low temperature synthesis promise more feasibility of up-scaling, it might also enable SWNT synthesis to be compatible with current CMOS technology for hybrid microelectronics [4,5]. Moreover, with decreasing growth temperature, some problems that currently plague SWNT synthesis, e.g., thermal sintering of the metal catalyst nanoparticles at the high-temperature for SWNTs synthesis, could be largely avoided, which in principle would facilitate fine control over the distribution of tube diameters, and hopefully, the chirality as well. In comparison with conventional thermal CVD, plasma enhanced CVD (PCVD) is known to possess advantages in the low temperature synthesis of multi-walled carbon nanotubes and fibers, as dissociation of the carbon feedstock molecules can be significantly promoted in plasma [6 9]. However, albeit a few existing examples that reported SWNTs synthesis by the radio frequency PCVD method, thus far studies on the PECVD growth of SWNTs are very limited [5,10]. Herein, we first report the low temperature synthesis ( C) of SWNTs with small diameters and narrow distribution by using a microwave plasma enhanced CVD (MPCVD) method. The same MPCVD system has previously been employed in our lab to grow a variety of carbon-based nanostructures, such as polymerized carbon nitride nanobells [9], coiled carbon nanotubes [11], and tubular graphite cones [12]. Our low temperature growth of SWNTs by MPCVD was achieved over the Fe Mo bimetallic catalyst nanoparticles supported on porous MgO powders (denoted as Fe Mo/ MgO hereafter). Using the same batch of Fe Mo/MgO catalyst, comparative studies on the growth of SWNTs by the conventional thermal CVD were also carried out, /$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi: /j.cplett

2 82 W.L. Wang et al. / Chemical Physics Letters 419 (2006) where a relatively higher growth temperature (P800 C) was needed. The SWNTs produced by the thermal CVD at higher temperature yielded a significantly broader distribution of the tube diameters than that by lower temperature MPCVD synthesis. Meanwhile, it was found that the lower the growth temperature, the smaller the average SWNT diameters. It shows the potential in controlling the nanotube diameters by low temperature MPCVD synthesis. 2. Experimental The Fe Mo/MgO catalyst with a formulation of Fe:Mo:MgO = 1:0.1:20 (in molar ratio) was prepared by a modified low temperature combustion synthesis process. Briefly, the required amount Mg(NO 3 ) 2 Æ 6H 2 O, Fe 2 (S- O 4 ) 3 Æ 5H 2 O (NH 4 ) 6 Mo 7 O 24 Æ 4H 2 O, citric acid, and NH 4 NO 3 were dissolved in about 15 ml deionized water to obtain a homogenous, clear aqueous solution. The water solvent was then evaporated by a rotary evaporator at 90 C, and the resultant cake was ground with an agate mortar. The so-obtained fine powder was then placed in a furnace preheated to 600 for 5 h in flowing air, and re-ground after being cooled to the room temperature. The MPCVD system (ASTeX 2115) used in our experiments is equipped with a 2.45 GHz, 1.5 kw microwave source. A ratio-frequency heater that allows control of the substrate temperature independent of the plasma power is directly coupled to a moveable graphite substrate stage in the growth chamber, and the temperature was measured with a thermocouple mounted under the stage [9,11,12]. Prior to SWNT growth, a quartz boat containing a carefully weighed amount (typically 100 mg) of the catalyst powder was placed onto the graphite stage. The growth chamber was firstly pumped down to less than 0.5 Pa, and then filled with hydrogen with a pressure of KPa. When the sample was heated to a desired temperature in the hydrogen ambience, a microwave plasma of 850 W excitation was ignited. Methane was then introduced as carbon feedstock to start SWNT growth for 30 min. During the growth, the H 2 and CH 4 flow rates were fixed to 200 and 5 sccm, respectively, with a total pressure of 2.5 KPa. At the end of growth, a black powder was obtained, which contains both the SWNTs and the catalysts, i.e., the MgO support powders, and the Fe and Mo species. To remove these impurities, the as-grown black powder was suspended in a stirred 36% HCl acid solution overnight at room temperature. The suspension was then filtered through a PTFE 0.2 lm membrane, washed to neutral ph with deionized water, and dried in vacuum at room temperature. 3. Results and discussion Fig. 1a shows a typical scanning electron microscopy (SEM) image of the unpurified, as-grown black powder by MPCVD at 700 C. The relatively high density of SWNT bundles can be clearly observed with a web-like Fig. 1. (a) SEM image of the unpurified, as-grown SWNT products by MPCVD at 700 C. (b) SEM image of the purified SWNTs by HCl acid treatment. (c) HRTEM image of the as-grown SWNTs. (d) The diameter distribution of the SWNTs determined by HRTEM.

3 W.L. Wang et al. / Chemical Physics Letters 419 (2006) structure covering on the Fe Mo/MgO catalysts. The SWNT bundles have smooth and clean surfaces. The total carbon yield (defined as the weight increase relative to the initial catalyst weight) is around 20 wt% for the as-grown sample, as determined by thermogravimetric analysis (TGA). The as-grown SWNTs sample can be further purified by mild HCl acid treatment by the removal of the MgO supports and most of the Fe and Mo species without damaging the SWNTs. A typical SEM image of the purified sample is shown in Fig. 1b, in which one can find that the catalyst materials have been effectively removed. Structural details of the as-grown SWNT samples were further investigated by the high-resolution transmission electron microcopy (HRTEM). As shown in Fig. 1c, bundles of SWNTs with clean and straight tube walls are apparent in the HRTEM images, revealing high quality of the asgrown SWNTs. The diameter distribution statistics derived by HRTEM analysis further reveals that the as-grown SWNTs have a mean diameter of 1.00 nm with a standard deviation of 0.23 nm, as displayed in Fig. 1d. For a comparative study, a conventional thermal CVD system was also used to grow SWNTs, where the same batch of Fe Mo/MgO catalyst and the same carbon feedstock of methane were adopted. It was found that efficient growth of SWNTs by the thermal CVD could not be achieved at a temperature below 800 C. In Fig. 2, we display the Raman spectra (532 nm excitation) of the asgrown SWNTs by MPCVD at different growth temperatures of 700 C, 600 C, and 500 C, respectively. For comparison, the spectrum of SWNTs grown by the thermal CVD at 800 C was also shown at the bottom of the same figure. As expected for scattering from resonantly enhanced SWNTs [13], all the Raman spectra clearly revealed the well-known sharp feature of G-band at 1593 cm 1 and the low-frequency radial breathing mode (RBM) in the range cm 1. Also seen is the D-band with a maximum at 1345 cm 1, which is associated with disordered carbon impurities and SWNT defects. For the SWNT samples grown by MPCVD at 700 C, the amplitude of D-band is relatively weak, indicating high quality and purity of the as-grown SWNTs. With decreasing growth temperature, the D-band intensity increases gradually, which is due to the increase of the non-swnt carbon impurities in the as-grown products, as confirmed by microscopic observations. As for the sample grown at 500 C, the D-band has become appreciable. Nevertheless, it is clear that there still exists a substantial amount of SWNTs in the as-grown products. But with further lowering growth temperature down to 450 C, both SEM and TEM observations revealed the predominance of the amorphous and nanocrystalline carbon and thick nanofibers, while SWNTs are hardly seen. Our further detailed exper- Fig. 2. Resonant Raman scattering spectra of the as-grown SWNT samples by thermal CVD at 800 C (a), and by MPCVD at 700 C (b), 600 C (c), and 500 C (d), respectively, by using the same batch of catalyst. The right panel is a magnified view of the low-frequency RBM region. The excitation laser line was 532 nm. The spectra in the left panel are normalized to the G-band at ca cm 1, while the magnified RBM signals in the right panel are normalized to the strongest peak.

4 84 W.L. Wang et al. / Chemical Physics Letters 419 (2006) iments show that the lowest temperature limit for SWNT growth by MPCVD is around 500 C. To our knowledge, this temperature is one of the lowest records for SWNT synthesis [4,5,10]. The Raman shift of RBM is known to be inversely proportional to the SWNT. By an analysis of the magnified RBM signals shown in the right panel of Fig. 2, it is evident that the SWNTs grown by MPCVD at lower temperature have smaller and more narrowly distributed tube diameters in comparison with the SWNTs grown by the thermal CVD at 800 C. Note that the same batch of Fe Mo/ MgO catalyst was employed for all cases. Meanwhile, with decreasing growth temperature, the diameter distribution shifts to thinner side. Since the Raman scattering of SWNTs occurs through a resonant process, the RMB peaks obtained at a fixed laser wavelength do not reflect the entire tube abundance. So different laser lines other than the 532 nm were also used for each SWNT sample, and, as expected, the same trend has been obtained as that of the 532 nm excitation. The temperature-dependent variations of the SWNTs diameter were further confirmed by detailed HRTEM measurements of the diameter statistics for each sample, as shown in Fig. 3. The SWNTs grown by MPCVD at 700 C have a mean diameter of 1.00 nm with standard deviation of 0.23 nm (also seen in Fig. 1d), whereas the SWNTs grown by thermal CVD at 800 C averages the diameter of 1.60 nm with a standard deviation larger than 0.55 nm. The average diameters of the SWNTs by MPCVD at 600 C and 500 C are 0.91 nm (with standard deviation of 0.15 nm) and 0.88 nm (with standard deviation of 0.12 nm), respectively. The diameter statistics derived by HRTEM show a good agreement with the Raman spectra analysis. Tube diameter (nm) By MPCVD By thermal CVD at 800 o C Growth temperature ( o C) Fig. 3. Plot of the average diameters ± standard deviation for SWNTs samples grown under different temperatures. Showing the temperaturedependence of the tube diameters distribution variations. The trend of SWNTs diameters reduction with decreasing growth temperature has also reported previously for SWNTs synthesis by the conventional thermal CVD, e.g., the ethanol CVD method developed by Maruyma et al. and the CoMoCat process by Resasco group [4,14]. For the catalytic CVD growth of SWNTs, it is well-established that the tube diameter correlates strongly with the size of the metal catalyst nanoparticles initiating their growth. Thus by controlling sizes of catalyst nanoparticles, one should be able to control the SWNT diameters. However, under the harsh condition for SWNT synthesis at relatively high temperatures, it is hard to get a fine control over the catalyst particles sizes due to the inevitable thermal-sintering. With lower growth temperature, in contrast, thermalsintering of the metal particles can be effectively avoided. At the same time, lower growth temperature also prevents the primary metal particles from growing too rapidly. These factors would benefit the generation of more homogenous catalyst nanoparticles with smaller sizes, and thus result in SWNTs with smaller and more homogenous diameters. 4. Conclusion remark In summary, MPCVD growth of high-quality SWNTs at low temperature was demonstrated by catalytic decomposition of methane over the Fe Mo/MgO catalyst. Efficient growth of SWNTs by MPCVD can be achieved at a temperature as low as 500 C. The as-grown SWNTs by the low temperature MPCVD have small diameters with fairly narrow distributions, whereas the SWNTs produced by a comparative thermal CVD process at higher temperature (800 C) using the same batch catalyst yielded a significantly broader diameter distribution with larger mean diameter. Within the tested temperature range of C, a clear trend is observed that the lower the growth temperature, the smaller and more homogenous the tube diameters, showing the feasibility of controlling the SWNTs diameters by tuning the growth temperature. Controlling the distribution of tube diameter and the chirality has been a major goal for SWNT synthesis. Recently, it was reported that one dominant factor determining the chirality distribution of SWNTs is the tube diameter, and the small-diameter SWNTs tend to have a small number of chiral vector indices [15 17]. For instance, the small-diameter SWNT samples produced by both the CoMoCat and ethanol CVD methods at relatively low temperature were demonstrated to have a strong preference for the near-armchair structures, with a predominance abundance of the (6,5) and (7,5) chiralites [15,16]. As inspired by these results, it can be expected that our small diameter SWNTs produced by MPCVD at low temperature might also have a narrow chirality distribution. Further studies on determining the (n,m) distribution of these SWNT samples are now underway.

5 W.L. Wang et al. / Chemical Physics Letters 419 (2006) Acknowledgments Financial supports from the NSF (No and ), MOST, and CAS of China were acknowledged. W.L. Wang also acknowledges the financial supports from the China Postdoctoral Science Foundation and the K.C. Wong Education Foundation, Hong Kong. References [1] H. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 260 (1996) 471. [2] J. Kong, H. Soh, A. Cassell, C.F. Quate, H. Dai, Nature 395 (1998) 878. [3] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Science 306 (2004) [4] S. Maruyama, R. Kojima, Y. Miyauchi, S. Chiashi, M. Kohno, Chem. Phys. Lett. 360 (2002) 229. [5] Y. Li, D. Mann, M. Roland, W. Kim, A. Ural, S. Hung, A. Javey, J. Cao, D. Wang, E. Yenlimez, Q. Wang, J.F. Gibbons, Y. Nishi, H. Dai, Nano Lett. 4 (2004) 317. [6] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, P.N. Provencio, Science 282 (1998) [7] S. Hofmann, C. Ducati, J. Robertson, B. Kleinsorge, Appl. Phys. Lett. 835 (2003) 135. [8] J.M. Ting, K.H. Liao, Chem. Phys. Lett. 396 (2004) 469. [9] X.C. Ma, E.G. Wang, W. Zhou, D.A. Jefferson, J. Chen, S. Deng, N. Xu, J. Yuan, Appl. Phys. Lett. 75 (1999) [10] T. Kato, G.H. Jeong, T. Hirata, R. Hatakeyama, K. Tohji, K. Motomiya, Chem. Phys. Lett. 381 (2003) 422. [11] D.Y. Zhong, S. Liu, E.G. Wang, Appl. Phys. Lett. 83 (2003) [12] G. Zhang, X. Jiang, E.G. Wang, Science 300 (2003) 472. [13] H. Kataura, Y. Kumazawa, Y. Maniwa, I. Umezu, S. Suzuki, Y. Ohtsuka, Y. Achiba, Synth. Met. 103 (1999) [14] W.E. Alvarez, F. Pompeo, J.E. Herrera, L. Balzano, D.E. Resasco, Chem. Mater. 14 (2002) [15] S.M. Bachilo, L. Balzano, J.E. Herrera, F. Pompeo, D.E. Resasco, R.B. Weisman, J. Am. Chem. Soc. 125 (2003) [16] Y. Miyauchi, S. Chiashi, Y. Murakami, Y. Hayashida, S. Maruyama, Chem. Phys. Lett. 387 (2004) 198. [17] H. Ago, S. Imamura, T. Okazaki, T. saito, M. Yumura, M. Tsuji, J. Phys. Chem. B 109 (2005)