- Email: [email protected]
Carbon nanotube growth from titanium–cobalt bimetallic particles as a catalyst
Chemical Physics Letters 402 (2005) 149–154 www.elsevier.com/locate/cplett
Carbon nanotube growth from titanium–cobalt bimetallic particles as a catalyst Shintaro Sato *, Akio Kawabata, Daiyu Kondo, Mizuhisa Nihei, Yuji Awano Fujitsu Limited, 10-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0197, Japan Received 6 October 2004; in final form 2 December 2004 Available online 22 December 2004
Abstract Bimetallic particles of Ti and Co have been used to grow multi-walled carbon nanotubes (MWNTs) to study the effects of Ti on the growth. The particles were generated by laser ablation and size-classified with a differential mobility analyzer. Diameter-controlled MWNTs were grown on a silicon substrate from 5-nm particles by thermal chemical vapor deposition using acetylene at 510 and 610 °C. Titanium has been found to enhance the MWNT growth significantly. There is an optimum Ti fraction for the growth, as confirmed by evaluation of the growth probabilities of MWNTs. A possible mechanism of growth enhancement by Ti is discussed. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) are known to have unique electrical, mechanical, thermal, and optical properties that can bring breakthrough in various engineering fields. In spite of numerous studies on their applications [1–5], however, details of the CNT growth mechanism are still under discussion (e.g. [6]). Currently, metal catalyst particles are generally believed to work as nuclei for the growth of CNTs [7,8]. In addition to pure cobalt, iron, and nickel catalysts, often used for the growth by chemical vapor deposition (CVD), bimetallic catalysts are also employed, especially for the growth of single-walled carbon nanotubes (SWNTs). Such catalysts include Fe–Mo [9–12], Co–Mo [13,14], and Fe–Co [15,16]. The first two catalysts contain molybdenum, whose catalytic ability for CNT growth is weak [17], and the last catalyst is a combination of metals whose catalytic abilities are stronger. Alvarez et al. [18] and Murakami et al. [17] stated that *
Corresponding author. Fax: +81 46 250 8844. E-mail address: [email protected] (S. Sato).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.12.018
Mo in the Co–Mo catalyst worked as a stabilizer preventing agglomeration of Co during the SWNT growth. However, the roles of metals with weaker catalytic abilities in bimetallic catalysts are generally not clearly understood. As for bimetallic catalysts to grow MWNTs, ShinoharaÕs group used Co–V, Co–Fe, Co–Ni, Co–Pt, Co–Y, Co–Cu, and Co–Sn metals supported in zeolite [19,20]. They found that Co–V and Co–Fe catalysts gave the best yields under their growth conditions with acetylene as the source gas. They speculated that the better yields obtained with bimetallic catalysts than with pure Co might be attributed to their lower melting temperatures, but this argument is still inconclusive. We have recently found in our application of MWNTs to LSI interconnects that a Ti layer under a Co layer greatly enhances MWNT growth by CVD [21]. Scanning electron microscopy (SEM) revealed particles on the substrate after the MWNT growth. This observation suggests the following hypothesis: the Ti layer somehow assisted the Co layer in forming particles appropriate for the MWNT growth. This hypothesis is similar to the argument by Alvares et al. [18] and
150
S. Sato et al. / Chemical Physics Letters 402 (2005) 149–154
Murakami et al. [17] on systems involving a Co–Mo catalyst. However, titanium may play an important role in the growth of CNTs, a role other than merely forming and stabilizing cobalt particles. Such a role may be clarified by employing Ti–Co bimetallic particles as a catalyst. On this background, we have formed size-controlled Ti–Co particles in the present study with different atomic fractions of Ti [22] to grow CNTs. This procedure has enabled a detailed examination on the role of Ti, because the process of particle formation is independent of the presence of Ti. This Letter reports on the growth of MWNTs and the role of Ti in this process by a detailed analysis of the Ti–Co catalyst particles.
2. Experimental Catalyst particles were generated using the method and apparatus reported in [22]. Fig. 1 shows a schematic diagram of the particle generation and deposition system. Catalyst particles were generated by laser ablation of a Ti–Co alloy target in a low-pressure helium environment (1.5 kPa). Ti–Co alloy targets with atomic Ti fractions of 1%, 5%, 10%, 20%, 50% and 80% were used. The particles were then brought into a tube furnace for annealing at 1000 °C with 1 slpm (standard liter per minute) of He carrier gas. After the annealing, the particles were transported to a differential mobility analyzer (DMA) for size separation by classification based on electrical mobility [23]. The DMA was operated under the conditions, where the size distribution of the classified particles had a geometrical standard deviation of 1.1. Size-classified 5-nm particles were transported through a nozzle to a deposition chamber, where their inertia caused them to collide with a substrate under the nozzle. The particles with a thickness of about one layer were deposited onto the substrate. The substrate was a piece of a (1 0 0) Si wafer.
The substrate with the catalyst particles was brought into a thermal CVD chamber of a cold-wall type with a heating stage, on which the substrate was placed. The substrate was pretreated for 10 min with 0.1 slpm of hydrogen at 1 kPa and at temperature equal to the growth temperature. Next, a 1:9 mixture of acetylene and argon gases at 1 kPa was introduced with a flow rate of 0.2 slpm for 10 min for the growth of CNTs. The growth temperature was 510 or 610 °C. Ti–Co particles and CNTs were observed by SEM (Hitachi, S-5000), and transmission electron microscopy (TEM; JEOL, JEM-2010F). The particles were also analyzed using scanning transmission electron microscopy (STEM), electron diffraction, energy dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS).
3. Results and discussion 3.1. Analyses of Ti–Co particles before CNT growth An STEM image of 5-nm Ti–Co particles with an atomic Ti fraction of 5%, as displayed in Fig. 2a, shows that the particles are uniform in size. The appearances of particles with other Ti fractions were similar. An EDS analysis of the particles obtained from each Ti–Co target confirmed that the average ratios of the Ti fraction to the Co fraction in the particles agree with those in the original target. TEM images and selected area electron diffraction (SAD) patterns of the particles with Ti fractions of 1% and 5% look similar, as shown in Figs. 2b and c. In particular, both SAD patterns indicate that the Ti–Co particles consist of crystals of a-cobalt and cobalt oxide (CoO). This means that Ti atoms are incorporated in the Co and CoO crystals, forming a primary solid solution. The particles with a Ti fraction
Fig. 1. Schematic diagram of the particle generation and deposition system [22]. The inset shows a cross section of a differential mobility analyzer used to classify Ti–Co bimetallic particles.
S. Sato et al. / Chemical Physics Letters 402 (2005) 149–154
151
Fig. 2. (a) STEM image of 5-nm Ti–Co particles with a titanium fraction of 5%. (b) TEM (left) and SAD (right) images of Ti–Co particles with a titanium fraction of 1%. Assignments of SAD images are: Ring A to CoO, Ring C to a-Co, and Ring B, D, and E to both a-Co and CoO. (c) TEM (left) and SAD (right) images of Ti–Co particles with a titanium fraction of 5%. (d) TEM image of Ti–Co particles with a titanium fraction of 50%. (e) TEM image of a particle attached to an MWNT grown from Ti–Co particles with a 50% Ti fraction. (f) EDS spectrum of the particle shown in (e). (g) Electron diffraction pattern of the particle shown in (e).
of 50% look amorphous, as displayed in Fig. 2d. EDS analyses of the same particles with a focused electron beam (1 nm diameter) revealed that Ti and Co were well mixed and that there was no obvious phase separation between Ti and Co. 3.2. CNTs grown from Ti–Co particles 3.2.1. Optimum Ti fraction for CNT growth Fig. 3 shows SEM images of CNTs grown from Ti– Co particles with various Ti fractions. No aligned CNT was obtained at a Ti fraction of 1%, but an increase in the Ti fraction from 1% to 5% greatly enhanced the CNT growth. Aligned CNTs were obtained with Ti fractions ranging from 5% to 50%.
The maximum CNT height of 30 lm was achieved at 20% and at 610 °C. In order to study the effects of titanium more quantitatively, the growth probability of CNTs at 610 °C was estimated as a function of the Ti fraction by sparsely depositing the particles onto a substrate, growing CNTs, and counting the particles and CNTs. The growth probability was calculated as the number of particles divided by that of CNTs. The results are shown in Fig. 4a along with a representative SEM image used for this estimation (Fig. 4b). The growth probabilities increased drastically when the Ti fraction was varied from 1% to 5% and had a peak at 20%. Our on-going experiments suggest that the growth probabilities are somewhat enhanced with the growth time.
152
S. Sato et al. / Chemical Physics Letters 402 (2005) 149–154
Fig. 3. SEM images of MWNTs grown from Ti–Co particles with Ti fractions varied from 1% to 80% at 510 and 610 °C. Pictures were taken at 40° from the normal of the substrate.
Fig. 4. (a) Growth probabilities of CNTs as a function of the Ti fraction of the particles. (b) A representative SEM image used to obtain the probabilities.
3.2.2. TEM analyses of CNTs The TEM images of CNTs obtained from the particles with a titanium fraction of 50% at 610 °C, displayed in Fig. 5a, give evidence for production of MWNTs. Similar TEM images were obtained for other Ti fractions at 610 °C. However, fiber-like structures were sometimes observed at the growth temperature of 510 °C. The outer diameters of the MWNTs grown at 610 °C from the particles with a 50% Ti fraction were measured using the TEM images. The diameter distributions of the original particles and the MWNTs are shown in Fig. 5b. Their geometric means are 5.8 are 5.7 nm, and their geometric standard deviations are 1.09 and 1.13, respectively. The agreement of their distributions confirms that the CNT
outer diameter can be controlled by the particle diameter [22]. Incidentally, the mean particle diameter is found to be larger by 0.8 nm than the diameter of 5 nm set by the DMA, probably because of particle oxidation after the size separation. 3.3. Analyses of Ti–Co particles after CNT growth EDS analyses of the particles attached to the basegrown MWNTs, removed from the sample substrate, showed that the particles were essentially composed of Co. For example, particles attached to the MWNTs grown from Ti–Co particles with a 50% Ti fraction had an average ratio of 1:20 for the Ti and Co fractions.
Fig. 5. (a) TEM images of MWNTs grown from Ti–Co particles with a Ti fraction of 50%. (b) Distribution of the outer diameters, D, of MWNTs grown from Ti–Co particles (Ti fraction of 50%), along with the distribution of particle diameters. The vertical axis shows the number fraction divided by the logarithmic range width.
S. Sato et al. / Chemical Physics Letters 402 (2005) 149–154
Fig. 2e shows a TEM image of such a particle. The EDS spectrum of the same particle, displayed in Fig. 2f, shows distinct Co peaks, but no clear Ti peak is found. A few spots shown in the electron diffraction patterns (Fig. 2g) can be assigned to those for a-cobalt. Furthermore, AES and XPS analyses identified higher fractions of Ti on the substrate than in the original particles (not shown). The series of these analyses indicate that cobalt and titanium are segregated during or after the growth period and that cobalt is somehow localized closer to the MWNT side. 3.4. Possible mechanism of growth enhancement by Ti We here discuss a possible role of Ti in the CNT growth by using the phase diagrams of Ti–Co and Ti–Co–C systems [24]. Firstly, these phase diagrams clearly show that the Ti–Co system can incorporate more carbon as compounds than cobalt, in such a form as TiC1 x. Thus, Ti–Co particles possibly become ÔhotterÕ as a result of the heat of reaction with carbon. Secondly, the melting temperatures of the Ti–Co–C system are generally lower than those of the Co–C system. These features suggest that Ti–Co–C is a suitable system for the growth of CNTs within the framework of the vapor–liquid–solid (VLS) model [7]. As one can easily predict, however, an excessive fraction of Ti would not be effective for the CNT growth because Ti by itself has only weak catalytic ability. The phase diagram of the Ti–Co–C system also suggests that, if the system is cooled from a liquid phase with a relatively high carbon fraction, C, Co, and TiC1 x tend to segregate themselves as solids. Phase separation in post-growth particles, mentioned in Section 3.3, may be understood in this context. It is still an open question whether or not the possible growth-enhancement mechanisms discussed above can be applied to other bimetallic systems. As a preliminary experiment to investigate this issue, we have generated Mo–Co particles and attempted MWNT growth under the same experimental conditions as above. The MWNT growth has not been improved in comparison with the pure-Co case. On the other hand, it has been reported that SWNT growth is improved with Mo–Co particles using alcohol [25]. These results are probably too complicated to be explained by such a simple model as that discussed above. More sophisticated studies are needed for the solution of this problem.
4. Conclusions Titanium–cobalt bimetallic particles with a diameter of 5 nm were utilized as a catalyst for CNT growth. Aligned MWNTs with an outer diameter of about 5 nm were obtained by thermal CVD at a growth tem-
153
perature of 610 °C using acetylene as the source gas. It is confirmed that there is an optimum range of atomic Ti fractions for the MWNT growth, and the peak is found at 20% under the present experimental conditions. By using the Ti–Co particles sparsely deposited on a silicon substrate, the maximum growth probability of MWNTs is estimated to be 20% at a Ti fraction of 20%. Based on these and other findings, enhancement of CNT growth by adding titanium is likely to be linked with the capability of carbon intake and lower melting temperature of the Ti–Co system. This hypothesis should be tested by further experiments to be undertaken in the future.
Acknowledgements The authors thank Dr. Naoki Yokoyama, General Manager of Nanotechnology Research Center of Fujitsu Laboratories Ltd. and Prof. Sumio Iijima at National Institute of Advanced Industrial Science and Technology for their support and useful suggestions. Discussions with Dr. Mari Ohfuti are also appreciated. This work was supported by the Advanced Nanocarbon Application Project, which was consigned to Japan Fine Ceramics Center (JFCC) by New Energy and Industrial Technology Development Organization (NEDO) of Japan.
References [1] S.J. Tans, A.R.M. Verschueren, C. Dekker, Nature 393 (1998) 49. [2] S. Akita, Y. Nakayama, S. Mizooka, Y. Takano, T. Okawa, Y. Miyatake, S. Yamanaka, M. Tsuji, T. Nosaka, Appl. Phys. Lett. 79 (2001) 1691. [3] Y. Sakakibara, S. Tatsuura, H. Kataura, M. Tokumoto, Y. Achiba, Jpn. J. Appl. Phys. 42 (2003) L494. [4] M. Nihei, M. Horibe, A. Kawabata, Y. Awano, Jpn. J. Appl. Phys. 43 (2004) 1856. [5] M. Endo, Y.A. Kim, T. Hayashi, K. Nishimura, T. Matsushita, K. Miyashima, M.S. Dresselhaus, Carbon 39 (2001) 1287. [6] F. Ding, A. Rosen, K. Bolton, Chem. Phys. Lett. 393 (2004) 309. [7] G.G. Tibbetts, J. Cryst. Growth 66 (1984) 632. [8] M. Yudasaka, R. Yamada, N. Sensui, T. Wilkins, T. Ichihashi, S. Iijima, J. Phys. Chem. B 103 (1999) 6224. [9] J. Kong, H.T. Soh, A.M. Cassell, C.F. Quate, H. Dai, Nature 395 (1998) 878. [10] A.M. Cassell, J.A. Raymakers, J. Kong, H. Dai, J. Phys. Chem. B 103 (1999) 6484. [11] J.H. Hanfer, M.J. Bronikowski, B.R. Azamian, P. Nokolaev, A.G. Rinzler, D.T. Colbert, K.A. Smith, R.E. Smalley, Chem. Phys. Lett. 296 (1998) 195. [12] M. Su, B. Zheng, J. Liu, Chem. Phys. Lett. 322 (2000) 321. [13] B. Kitiyanan, W.E. Alvarez, J.H. Harwell, D.E. Resasco, Chem. Phys. Lett. 317 (2000) 497. [14] Y. Murakami, Y. Miyauchi, S. Chiashi, S. Maruyama, Chem. Phys. Lett. 377 (2003) 49. [15] S. Maruyama, R. Kojima, Y. Miyauchi, S. Chiashi, M. Kohno, Chem. Phys. Lett. 360 (2002) 229.
154
S. Sato et al. / Chemical Physics Letters 402 (2005) 149–154
[16] Y. Murakami, Y. Miyauchi, S. Chiashi, S. Maruyama, Chem. Phys. Lett. 374 (2003) 53. [17] Y. Murakami, S. Chiashi, Y. Miyauchi, S. Maruyama, Jpn. J. Appl. Phys. 43 (2004) 1221. [18] W.E. Alvarez, B. Kitiyanan, A. Borga, D.E. Resasco, Carbon 39 (2001) 547. [19] K. Mukhopadhyay, A. Koshio, N. Tanaka, H. Shinohara, Jpn. J. Appl. Phys. 37 (1998) L1257. [20] K. Mukhopadhyay, A. Koshio, T. Sugai, N. Tanaka, H. Shinohara, Z. Konya, J.B. Nagy, Chem. Phys. Lett. 303 (1999) 117.
[21] M. Horibe, A. Kawabata, D. Kondo, M. Nihei, Y. Awano, submitted. [22] S. Sato, A. Kawabata, M. Nihei, Y. Awano, Chem. Phys. Lett. 382 (2003) 361. [23] P.A. Baron, K. Willeke, Aerosol Measurements: Principles, Techniques, and Applicatisecond, second ed., Wiley, New York, 2001. [24] D. Bandyopadhyay, R.C. Sharma, N. Chakraborti, J. Phase Equilibria 21 (2000) 179. [25] M. Kohno, T. Orii, M. Hirasawa, T. Seto, Y. Murakami, S. Chiashi, Y. Miyauchi, S. Maruyama, Appl. Phys. A 79 (2004) 787.
Carbon nanotube growth from titanium–cobalt bimetallic particles as a catalyst Shintaro Sato *, Akio Kawabata, Daiyu Kondo, Mizuhisa Nihei, Yuji Awano Fujitsu Limited, 10-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0197, Japan Received 6 October 2004; in final form 2 December 2004 Available online 22 December 2004
Abstract Bimetallic particles of Ti and Co have been used to grow multi-walled carbon nanotubes (MWNTs) to study the effects of Ti on the growth. The particles were generated by laser ablation and size-classified with a differential mobility analyzer. Diameter-controlled MWNTs were grown on a silicon substrate from 5-nm particles by thermal chemical vapor deposition using acetylene at 510 and 610 °C. Titanium has been found to enhance the MWNT growth significantly. There is an optimum Ti fraction for the growth, as confirmed by evaluation of the growth probabilities of MWNTs. A possible mechanism of growth enhancement by Ti is discussed. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) are known to have unique electrical, mechanical, thermal, and optical properties that can bring breakthrough in various engineering fields. In spite of numerous studies on their applications [1–5], however, details of the CNT growth mechanism are still under discussion (e.g. [6]). Currently, metal catalyst particles are generally believed to work as nuclei for the growth of CNTs [7,8]. In addition to pure cobalt, iron, and nickel catalysts, often used for the growth by chemical vapor deposition (CVD), bimetallic catalysts are also employed, especially for the growth of single-walled carbon nanotubes (SWNTs). Such catalysts include Fe–Mo [9–12], Co–Mo [13,14], and Fe–Co [15,16]. The first two catalysts contain molybdenum, whose catalytic ability for CNT growth is weak [17], and the last catalyst is a combination of metals whose catalytic abilities are stronger. Alvarez et al. [18] and Murakami et al. [17] stated that *
Corresponding author. Fax: +81 46 250 8844. E-mail address: [email protected] (S. Sato).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.12.018
Mo in the Co–Mo catalyst worked as a stabilizer preventing agglomeration of Co during the SWNT growth. However, the roles of metals with weaker catalytic abilities in bimetallic catalysts are generally not clearly understood. As for bimetallic catalysts to grow MWNTs, ShinoharaÕs group used Co–V, Co–Fe, Co–Ni, Co–Pt, Co–Y, Co–Cu, and Co–Sn metals supported in zeolite [19,20]. They found that Co–V and Co–Fe catalysts gave the best yields under their growth conditions with acetylene as the source gas. They speculated that the better yields obtained with bimetallic catalysts than with pure Co might be attributed to their lower melting temperatures, but this argument is still inconclusive. We have recently found in our application of MWNTs to LSI interconnects that a Ti layer under a Co layer greatly enhances MWNT growth by CVD [21]. Scanning electron microscopy (SEM) revealed particles on the substrate after the MWNT growth. This observation suggests the following hypothesis: the Ti layer somehow assisted the Co layer in forming particles appropriate for the MWNT growth. This hypothesis is similar to the argument by Alvares et al. [18] and
150
S. Sato et al. / Chemical Physics Letters 402 (2005) 149–154
Murakami et al. [17] on systems involving a Co–Mo catalyst. However, titanium may play an important role in the growth of CNTs, a role other than merely forming and stabilizing cobalt particles. Such a role may be clarified by employing Ti–Co bimetallic particles as a catalyst. On this background, we have formed size-controlled Ti–Co particles in the present study with different atomic fractions of Ti [22] to grow CNTs. This procedure has enabled a detailed examination on the role of Ti, because the process of particle formation is independent of the presence of Ti. This Letter reports on the growth of MWNTs and the role of Ti in this process by a detailed analysis of the Ti–Co catalyst particles.
2. Experimental Catalyst particles were generated using the method and apparatus reported in [22]. Fig. 1 shows a schematic diagram of the particle generation and deposition system. Catalyst particles were generated by laser ablation of a Ti–Co alloy target in a low-pressure helium environment (1.5 kPa). Ti–Co alloy targets with atomic Ti fractions of 1%, 5%, 10%, 20%, 50% and 80% were used. The particles were then brought into a tube furnace for annealing at 1000 °C with 1 slpm (standard liter per minute) of He carrier gas. After the annealing, the particles were transported to a differential mobility analyzer (DMA) for size separation by classification based on electrical mobility [23]. The DMA was operated under the conditions, where the size distribution of the classified particles had a geometrical standard deviation of 1.1. Size-classified 5-nm particles were transported through a nozzle to a deposition chamber, where their inertia caused them to collide with a substrate under the nozzle. The particles with a thickness of about one layer were deposited onto the substrate. The substrate was a piece of a (1 0 0) Si wafer.
The substrate with the catalyst particles was brought into a thermal CVD chamber of a cold-wall type with a heating stage, on which the substrate was placed. The substrate was pretreated for 10 min with 0.1 slpm of hydrogen at 1 kPa and at temperature equal to the growth temperature. Next, a 1:9 mixture of acetylene and argon gases at 1 kPa was introduced with a flow rate of 0.2 slpm for 10 min for the growth of CNTs. The growth temperature was 510 or 610 °C. Ti–Co particles and CNTs were observed by SEM (Hitachi, S-5000), and transmission electron microscopy (TEM; JEOL, JEM-2010F). The particles were also analyzed using scanning transmission electron microscopy (STEM), electron diffraction, energy dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS).
3. Results and discussion 3.1. Analyses of Ti–Co particles before CNT growth An STEM image of 5-nm Ti–Co particles with an atomic Ti fraction of 5%, as displayed in Fig. 2a, shows that the particles are uniform in size. The appearances of particles with other Ti fractions were similar. An EDS analysis of the particles obtained from each Ti–Co target confirmed that the average ratios of the Ti fraction to the Co fraction in the particles agree with those in the original target. TEM images and selected area electron diffraction (SAD) patterns of the particles with Ti fractions of 1% and 5% look similar, as shown in Figs. 2b and c. In particular, both SAD patterns indicate that the Ti–Co particles consist of crystals of a-cobalt and cobalt oxide (CoO). This means that Ti atoms are incorporated in the Co and CoO crystals, forming a primary solid solution. The particles with a Ti fraction
Fig. 1. Schematic diagram of the particle generation and deposition system [22]. The inset shows a cross section of a differential mobility analyzer used to classify Ti–Co bimetallic particles.
S. Sato et al. / Chemical Physics Letters 402 (2005) 149–154
151
Fig. 2. (a) STEM image of 5-nm Ti–Co particles with a titanium fraction of 5%. (b) TEM (left) and SAD (right) images of Ti–Co particles with a titanium fraction of 1%. Assignments of SAD images are: Ring A to CoO, Ring C to a-Co, and Ring B, D, and E to both a-Co and CoO. (c) TEM (left) and SAD (right) images of Ti–Co particles with a titanium fraction of 5%. (d) TEM image of Ti–Co particles with a titanium fraction of 50%. (e) TEM image of a particle attached to an MWNT grown from Ti–Co particles with a 50% Ti fraction. (f) EDS spectrum of the particle shown in (e). (g) Electron diffraction pattern of the particle shown in (e).
of 50% look amorphous, as displayed in Fig. 2d. EDS analyses of the same particles with a focused electron beam (1 nm diameter) revealed that Ti and Co were well mixed and that there was no obvious phase separation between Ti and Co. 3.2. CNTs grown from Ti–Co particles 3.2.1. Optimum Ti fraction for CNT growth Fig. 3 shows SEM images of CNTs grown from Ti– Co particles with various Ti fractions. No aligned CNT was obtained at a Ti fraction of 1%, but an increase in the Ti fraction from 1% to 5% greatly enhanced the CNT growth. Aligned CNTs were obtained with Ti fractions ranging from 5% to 50%.
The maximum CNT height of 30 lm was achieved at 20% and at 610 °C. In order to study the effects of titanium more quantitatively, the growth probability of CNTs at 610 °C was estimated as a function of the Ti fraction by sparsely depositing the particles onto a substrate, growing CNTs, and counting the particles and CNTs. The growth probability was calculated as the number of particles divided by that of CNTs. The results are shown in Fig. 4a along with a representative SEM image used for this estimation (Fig. 4b). The growth probabilities increased drastically when the Ti fraction was varied from 1% to 5% and had a peak at 20%. Our on-going experiments suggest that the growth probabilities are somewhat enhanced with the growth time.
152
S. Sato et al. / Chemical Physics Letters 402 (2005) 149–154
Fig. 3. SEM images of MWNTs grown from Ti–Co particles with Ti fractions varied from 1% to 80% at 510 and 610 °C. Pictures were taken at 40° from the normal of the substrate.
Fig. 4. (a) Growth probabilities of CNTs as a function of the Ti fraction of the particles. (b) A representative SEM image used to obtain the probabilities.
3.2.2. TEM analyses of CNTs The TEM images of CNTs obtained from the particles with a titanium fraction of 50% at 610 °C, displayed in Fig. 5a, give evidence for production of MWNTs. Similar TEM images were obtained for other Ti fractions at 610 °C. However, fiber-like structures were sometimes observed at the growth temperature of 510 °C. The outer diameters of the MWNTs grown at 610 °C from the particles with a 50% Ti fraction were measured using the TEM images. The diameter distributions of the original particles and the MWNTs are shown in Fig. 5b. Their geometric means are 5.8 are 5.7 nm, and their geometric standard deviations are 1.09 and 1.13, respectively. The agreement of their distributions confirms that the CNT
outer diameter can be controlled by the particle diameter [22]. Incidentally, the mean particle diameter is found to be larger by 0.8 nm than the diameter of 5 nm set by the DMA, probably because of particle oxidation after the size separation. 3.3. Analyses of Ti–Co particles after CNT growth EDS analyses of the particles attached to the basegrown MWNTs, removed from the sample substrate, showed that the particles were essentially composed of Co. For example, particles attached to the MWNTs grown from Ti–Co particles with a 50% Ti fraction had an average ratio of 1:20 for the Ti and Co fractions.
Fig. 5. (a) TEM images of MWNTs grown from Ti–Co particles with a Ti fraction of 50%. (b) Distribution of the outer diameters, D, of MWNTs grown from Ti–Co particles (Ti fraction of 50%), along with the distribution of particle diameters. The vertical axis shows the number fraction divided by the logarithmic range width.
S. Sato et al. / Chemical Physics Letters 402 (2005) 149–154
Fig. 2e shows a TEM image of such a particle. The EDS spectrum of the same particle, displayed in Fig. 2f, shows distinct Co peaks, but no clear Ti peak is found. A few spots shown in the electron diffraction patterns (Fig. 2g) can be assigned to those for a-cobalt. Furthermore, AES and XPS analyses identified higher fractions of Ti on the substrate than in the original particles (not shown). The series of these analyses indicate that cobalt and titanium are segregated during or after the growth period and that cobalt is somehow localized closer to the MWNT side. 3.4. Possible mechanism of growth enhancement by Ti We here discuss a possible role of Ti in the CNT growth by using the phase diagrams of Ti–Co and Ti–Co–C systems [24]. Firstly, these phase diagrams clearly show that the Ti–Co system can incorporate more carbon as compounds than cobalt, in such a form as TiC1 x. Thus, Ti–Co particles possibly become ÔhotterÕ as a result of the heat of reaction with carbon. Secondly, the melting temperatures of the Ti–Co–C system are generally lower than those of the Co–C system. These features suggest that Ti–Co–C is a suitable system for the growth of CNTs within the framework of the vapor–liquid–solid (VLS) model [7]. As one can easily predict, however, an excessive fraction of Ti would not be effective for the CNT growth because Ti by itself has only weak catalytic ability. The phase diagram of the Ti–Co–C system also suggests that, if the system is cooled from a liquid phase with a relatively high carbon fraction, C, Co, and TiC1 x tend to segregate themselves as solids. Phase separation in post-growth particles, mentioned in Section 3.3, may be understood in this context. It is still an open question whether or not the possible growth-enhancement mechanisms discussed above can be applied to other bimetallic systems. As a preliminary experiment to investigate this issue, we have generated Mo–Co particles and attempted MWNT growth under the same experimental conditions as above. The MWNT growth has not been improved in comparison with the pure-Co case. On the other hand, it has been reported that SWNT growth is improved with Mo–Co particles using alcohol [25]. These results are probably too complicated to be explained by such a simple model as that discussed above. More sophisticated studies are needed for the solution of this problem.
4. Conclusions Titanium–cobalt bimetallic particles with a diameter of 5 nm were utilized as a catalyst for CNT growth. Aligned MWNTs with an outer diameter of about 5 nm were obtained by thermal CVD at a growth tem-
153
perature of 610 °C using acetylene as the source gas. It is confirmed that there is an optimum range of atomic Ti fractions for the MWNT growth, and the peak is found at 20% under the present experimental conditions. By using the Ti–Co particles sparsely deposited on a silicon substrate, the maximum growth probability of MWNTs is estimated to be 20% at a Ti fraction of 20%. Based on these and other findings, enhancement of CNT growth by adding titanium is likely to be linked with the capability of carbon intake and lower melting temperature of the Ti–Co system. This hypothesis should be tested by further experiments to be undertaken in the future.
Acknowledgements The authors thank Dr. Naoki Yokoyama, General Manager of Nanotechnology Research Center of Fujitsu Laboratories Ltd. and Prof. Sumio Iijima at National Institute of Advanced Industrial Science and Technology for their support and useful suggestions. Discussions with Dr. Mari Ohfuti are also appreciated. This work was supported by the Advanced Nanocarbon Application Project, which was consigned to Japan Fine Ceramics Center (JFCC) by New Energy and Industrial Technology Development Organization (NEDO) of Japan.
References [1] S.J. Tans, A.R.M. Verschueren, C. Dekker, Nature 393 (1998) 49. [2] S. Akita, Y. Nakayama, S. Mizooka, Y. Takano, T. Okawa, Y. Miyatake, S. Yamanaka, M. Tsuji, T. Nosaka, Appl. Phys. Lett. 79 (2001) 1691. [3] Y. Sakakibara, S. Tatsuura, H. Kataura, M. Tokumoto, Y. Achiba, Jpn. J. Appl. Phys. 42 (2003) L494. [4] M. Nihei, M. Horibe, A. Kawabata, Y. Awano, Jpn. J. Appl. Phys. 43 (2004) 1856. [5] M. Endo, Y.A. Kim, T. Hayashi, K. Nishimura, T. Matsushita, K. Miyashima, M.S. Dresselhaus, Carbon 39 (2001) 1287. [6] F. Ding, A. Rosen, K. Bolton, Chem. Phys. Lett. 393 (2004) 309. [7] G.G. Tibbetts, J. Cryst. Growth 66 (1984) 632. [8] M. Yudasaka, R. Yamada, N. Sensui, T. Wilkins, T. Ichihashi, S. Iijima, J. Phys. Chem. B 103 (1999) 6224. [9] J. Kong, H.T. Soh, A.M. Cassell, C.F. Quate, H. Dai, Nature 395 (1998) 878. [10] A.M. Cassell, J.A. Raymakers, J. Kong, H. Dai, J. Phys. Chem. B 103 (1999) 6484. [11] J.H. Hanfer, M.J. Bronikowski, B.R. Azamian, P. Nokolaev, A.G. Rinzler, D.T. Colbert, K.A. Smith, R.E. Smalley, Chem. Phys. Lett. 296 (1998) 195. [12] M. Su, B. Zheng, J. Liu, Chem. Phys. Lett. 322 (2000) 321. [13] B. Kitiyanan, W.E. Alvarez, J.H. Harwell, D.E. Resasco, Chem. Phys. Lett. 317 (2000) 497. [14] Y. Murakami, Y. Miyauchi, S. Chiashi, S. Maruyama, Chem. Phys. Lett. 377 (2003) 49. [15] S. Maruyama, R. Kojima, Y. Miyauchi, S. Chiashi, M. Kohno, Chem. Phys. Lett. 360 (2002) 229.
154
S. Sato et al. / Chemical Physics Letters 402 (2005) 149–154
[16] Y. Murakami, Y. Miyauchi, S. Chiashi, S. Maruyama, Chem. Phys. Lett. 374 (2003) 53. [17] Y. Murakami, S. Chiashi, Y. Miyauchi, S. Maruyama, Jpn. J. Appl. Phys. 43 (2004) 1221. [18] W.E. Alvarez, B. Kitiyanan, A. Borga, D.E. Resasco, Carbon 39 (2001) 547. [19] K. Mukhopadhyay, A. Koshio, N. Tanaka, H. Shinohara, Jpn. J. Appl. Phys. 37 (1998) L1257. [20] K. Mukhopadhyay, A. Koshio, T. Sugai, N. Tanaka, H. Shinohara, Z. Konya, J.B. Nagy, Chem. Phys. Lett. 303 (1999) 117.
[21] M. Horibe, A. Kawabata, D. Kondo, M. Nihei, Y. Awano, submitted. [22] S. Sato, A. Kawabata, M. Nihei, Y. Awano, Chem. Phys. Lett. 382 (2003) 361. [23] P.A. Baron, K. Willeke, Aerosol Measurements: Principles, Techniques, and Applicatisecond, second ed., Wiley, New York, 2001. [24] D. Bandyopadhyay, R.C. Sharma, N. Chakraborti, J. Phase Equilibria 21 (2000) 179. [25] M. Kohno, T. Orii, M. Hirasawa, T. Seto, Y. Murakami, S. Chiashi, Y. Miyauchi, S. Maruyama, Appl. Phys. A 79 (2004) 787.