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Important parameters for the catalytic nanoparticles formation towards the growth of carbon nanotube aligned arrays
Diamond & Related Materials 16 (2007) 1082 – 1086 www.elsevier.com/locate/diamond
Important parameters for the catalytic nanoparticles formation towards the growth of carbon nanotube aligned arrays E. Terrado ⁎, E. Muñoz, W.K. Maser, A.M. Benito, M.T. Martínez Instituto de Carboquímica (CSIC), Miguel Luesma Castán 4, 50018 Zaragoza, Spain Available online 13 December 2006
Abstract Presented here is a systematic study on the experimental parameters involved in the formation of catalytic nanoparticles from homogeneous Ni films deposited by dc sputtering towards carbon nanotube (CNT) production on Si/SiO2. We have found a critical temperature and time for the thermal and reduction pre-treatment processes to obtain catalyst nanoparticles with the appropriate size and high density suitable for CNT growth. From such nanoparticles, densely-packed aligned CNT arrays were successfully grown at 750–800°C by thermal CVD. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanotubes; Chemical vapor deposition; Sputtering
1. Introduction Carbon nanotubes (CNTs) have attracted the interest of many research groups because of their striking structural and physical properties [1,2] which make them promising candidates for applications ranging from nano-electronics to field emission displays and electrochemical devices [2–4]. Nevertheless, the ability to grow CNTs directly on a substrate at a desired position [5] is the real challenge from a technological point of view. This control would allow the integration of the grown CNTs into microelectronics circuits [6]. In this sense, chemical vapour deposition (CVD) processes seem to be the more appropriate technique for large-scale production at low cost and direct growth on different substrates [7]. Moreover, CNTs with tailored features are desired since their properties and consequently their final applications strongly depend on their structural characteristics such as diameter, number of layers, length or crystallization degree. CVD is a versatile technique in terms of allowed catalysts, carbon feedstock, growth temperature, gas environments or substrate morphologies [8].
⁎ Corresponding author. Tel.: +34 976733977; fax: +34 976733318. E-mail address: [email protected] (E. Terrado). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.11.004
In the CVD approach, CNTs grow from the decomposition of a carbon precursor (typically a hydrocarbon or CO) at relatively high temperatures (650–1100 °C) on a catalyst surface. The role played by the catalyst in the CNT growth is really decisive and much more complex that just the decomposition of the hydrocarbon molecules. In fact, a deep understanding of the catalyst role in CNT growth implies the consideration of many factors such as its composition, morphology, preparation method, interaction with the support and pre-treatment conditions [9]. The most common catalysts towards CNT production are Fe, Co and Ni. They can be utilized as catalyst for multi-walled nanotubes (MWNTs) or single-walled nanotubes (SWNTs) individually [10–14], combined in binary mixtures [15–17] or with other co-catalyst elements which improve their own catalytic activity [18,19]. On the other hand, the catalyst can be deposited onto the substrate by chemical or physical methods, that is, the catalyst can be deposited by solutions containing it or directly by physical techniques. Physical methods, such as electron gun evaporation [20], thermal evaporation [21], ionbeam sputtering [22] and magnetron sputtering [23] allow the deposition of a homogeneous metal film with nanometric size (typically b20 nm) onto the substrate. Then, subsequent steps upon heating and reduction will be required to break the metal film into nanoparticles with the critic size that enables and promotes CNT growth.
E. Terrado et al. / Diamond & Related Materials 16 (2007) 1082–1086 Table 1 Series of experiments including thermal pre-treatments (series 1–2), ammonia etching (series 3) and CNT growth (series 4) Series Thermal pre-treatment
1 2 3 4
Reduction
Gas T (°C)
T Gas (min)
N2 N2 N2 N2
5–45 30 30 25
800 750–850 750 750
– – NH3 NH3
Growth
T T Gas (°C) (min)
T (°C)
T (min)
– – 750 750
– – – 750–800
– – – 20
– – 5–30 5
– – – C2H2
1083
3. Results and discussion SEM studies of the samples corresponding to series 1 and 2 reveal the influence of the thermal pre-treatment in the Ni catalyst nanoparticles formation. In series 1, the temperature is kept constant at 800 °C to study the influence of time in the
Presented here is a systematic study of the experimental parameters involved in the formation of Ni catalytic nanoparticles from a homogeneous Ni film deposited by dc sputtering. The aim of this work is to get reproducibly the catalyst nanoparticles not only with the appropriate size and distribution but also with the high density suitable to promote the growth of a densely-packed carbon nanotube array suitable for future applications. 2. Experimental Silicon substrates (10 mm × 10 mm) with an oxide layer of 300 nm were washed with acetone and isopropyl alcohol in a bath sonicator and then dried prior to Ni catalyst coating. Nickel films with a 5 nm-thickness were deposited on the substrates by dc sputtering (BALTEC SCD-050, 150 mA, argon pressure = 5 × 10− 2 mbar). The Ni-coated substrates were then placed on a ceramic boat in the centre of a tubular furnace. Three series of experiments were performed with the aim of optimizing the catalyst size and density towards the growth of aligned CNT arrays. The experimental conditions of series 1–3 are summarized in Table 1. For series 1, thermal pre-treatment processes of the Nicovered samples were carried out at 800 °C under nitrogen atmosphere during 5, 15, 30 and 45 min respectively, in order to study the effect of time in the resulting broken film into nanoparticles. For series 2, thermal pre-treatment processes of the Nicovered samples were carried out in the same flow conditions than series 1 for 30 min at temperatures of 750, 800 and 850 °C to study the influence of temperature in the process. For series 3, thermal treated at 750 °C for 30 min samples were etched with a 100 sccm ammonia flow for 5, 15 and 30 min respectively after the previous pre-treatment processes, with the aim to determine the effect of ammonia etching time. The effects of the experimental parameters on the characteristics of the Ni catalytic layer were studied by scanning electron microscopy (SEM, Hitachi S3400-N) and atomic force microscopy (MFD-3D). From the optimal thermal and reduction conditions, CNTs were grown using acetylene (20 sccm) at atmospheric pressure of 750–800 °C for 20 min (series 4), from the optimized synthesis conditions reported in our previous work [24,25]. The resulting CNTs were characterized by scanning electron microscopy (SEM, Hitachi S3400-N) and transmission electron microscopy (TEM, JEOL-2000 FXII).
Fig. 1. SEM image of the Ni-covered sample after a thermal pre-treatment (a) at 800 °C for 15 min, (b) at 800 °C for 30 min and (c) at 850 °C for 30 min.
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Fig. 2. AFM characterization of the Ni-covered sample after the optimal pre-treatment process. (a) Tapping-mode topography image with the corresponding crosssection height measurements and (b) 3D-imaging surface.
thermal pre-treatment. It was observed that the sputtered film began to be broken into nanoparticles for times higher than or equal to 30 min. In Fig. 1a no particles formation is still observed for the Ni-sputtered film exposed at 800 °C during 15 min. On the contrary, Fig. 1b shows nanoparticles distribution on the Ni-covered sample after the thermal pretreatment at 800 °C for 30 min. The average nanoparticle diameter is around 70 nm and the estimated Ni nanoparticles density is ∼ 4.109/cm2. Increasing the pre-treatment time results in increased average diameters and, therefore, in a decrease in the nanoparticle density. This effect is due to the coalescence of neighbouring particles as much probable as the film is exposed longer to high temperatures. In series 2, the effect of temperature in the thermal pre-treatment is studied by keeping a constant time (30 min). The highest density and the smallest Ni nanoparticles are observed in the Ni-covered samples exposed to a thermal pre-treatment process at 750 °C. In this case, the average diameter is around 40 nm and the Ni nanoparticles density is ∼ 8.109/cm2. At 850 °C (Fig. 1c) only big catalyst nanoparticles are observed (∼ 2–5 μm). These thicker particles are the result of a coalescence phenomenon due to the increase of mobility of nanoparticles on the substrate surface at higher temperatures [26]. From the results obtained in series 1 and 2, it can be concluded that a thermal pre-treatment at 750 °C for 30 min seems to be the optimum condition to obtain a dense distribution of Ni nanoparticles with small diameter and homogeneous size distribution from a sputtered film with nanometric thickness. It is worth mentioning the important role played by ammonia towards CNT production that has been widely discussed in the literature [27,28]. We have observed that efficient CNT growth is only achieved when CVD processes occurred after the ammonia or other reductive pre-treatment (e.g. H2). The
Fig. 3. SEM micrographs of the CNT aligned array grown on Si/SiO2 substrates from the pre-formed Ni nanoparticles. (a) Top view and (b) cross-section detail of a fragment which came off.
E. Terrado et al. / Diamond & Related Materials 16 (2007) 1082–1086
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characterization reveals how average nanoparticles diameter increases for the Ni-coated samples from 40 nm (without ammonia time) to 60 nm after the 30 min-thermal pre-treatment and 15 min at 750 °C under ammonia flow (100 sccm). The compromise is to minimize the reduction time that should be add on the optimum pre-treatment time. Fig. 2 include AFM tapping-mode characterization of the Ni-covered sample surface after 25 min-thermal pre-treatment under nitrogen followed by 5 min-ammonia etching. The size observed by AFM of the nanoparticles ranges from 20 to 40 nm. Finally, aligned CNT arrays were successfully grown at 750– 800 °C (series 4) from the pre-formed Ni nanoparticles on the silicon substrates. The observed alignment is a consequence of the high nanoparticles density which induces steric impediments between neighbouring filaments promoting the aligned growth. The lack of van der Waals interactions between neighbouring CNTs results in an entangled CNT growth [30] as it is observed when the nanoparticles density is not high enough. In Fig. 3a a top view of the CNT aligned array is shown. Fig. 3b reveals a detail of a fragment which came off from the CNT array with a length of around 4 μm. The morphology of the produced MWNTs was characterized by TEM (Fig. 4). The presence of catalytic nanoparticles at the tips of the nanotubes (Fig. 4a) suggests that the CNT production occurred via a tip-growth mechanism [31]. A bamboo-like structure [32] is observed in many of the carbon nanostructures (Fig. 4b) with an average diameter of ∼ 40 nm which correlates with the average diameter of the Ni nanoparticles obtained from the above described optimum pre-treatment process, that is, 25 min at 750 °C under nitrogen followed by a 5 min-ammonia etching. On the other hand, TEM images reveal the low graphitization degree of the CNT structure that it is also confirmed by Raman spectra of the grown samples with wide D and G bands and a intensity ratio close to 1 [33]. These spectra features are similar to other reported results on CNT production by thermal CVD processes using Ni as catalyst [34]. 4. Conclusions Fig. 4. TEM representative image of the grown under optimal conditions samples. (a) CNT array exhibiting catalyst nanoparticles at the tips of the tubes (see narrows). (b) Detail of the bamboo-like structure and CNT parallel walls.
observed results in our previous work [24,25] lead us to believe that the use of ammonia might improve the CNT production by keeping the catalyst nanoparticles active for nucleation. The hydrogen that results from the ammonia decomposition at the catalytic sites might react with amorphous carbonaceous materials deposited on the catalyst surface, thus forming volatile products that are easily removed from the catalyst and, in such way, keeping the metal surface clean and active [29]. Nevertheless, it should be taken into account that longer reduction times imply longer exposure of samples to high temperature and consequently more probable coalescence phenomena. Therefore, these reductive pre-treatments need to be also optimized to avoid catalyst coalescence. AFM
It has been stablished that temperature and time are critical parameters which determine the particle catalyst size from a sputtered metal film with a nanometric thickness. There is an optimal temperature for the catalyst nanoparticles formation. Above such optimal temperature the formed particles begin to coalesce. Likewise a compromise between time and temperature must be achieved to obtain a small nanoparticle size and to prevent their coalescence. Reduction process has been optimized to avoid also this coalescence phenomenon. From our study, we have concluded that a thermal pretreatment at 750 °C for 25 min under nitrogen flow followed by a 5 min-ammonia reductive step seems to be the optimum condition to obtain a dense distribution of activated nanoparticles from the 5 nm-sputtered Ni film. Finally, from such nanoparticles with the appropriate density and size denselypacked aligned CNT arrays were successfully grown on Si/SiO2 substrates following a tip-growth mechanism.
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Therefore, a systematic study of the influence of each parameter involved in CNT production is essential to control and reproduce the growth process bearing in mind any potential application. Acknowledgements Funded by MEC (MAT2002-04630-C02-01 and TEC200405098-C02-02). E.T. sincerely acknowledges I. Tacchini for his effort in the SEM characterization. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
V.N. Popov, Mater. Sci. Eng. R 43 (2004) 61. M. Terrones, Int. Mater. Rev. 49 (2004) 325. R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Science 297 (2002) 787. J. Robertson, Mater. (Today October 2004) 46. Z.J. Zhang, B.Q. Wei, G. Ramanath, P.M. Ajayan, Appl. Phys. Lett. 77 (2000) 3764. W.B. Choi, E. Bae, D. Kang, S. Chae, B.-H. Cheong, J.-H. Ko, E. Lee, W. Park, Nanotechnology 15 (2004) S512. C. Liu, A.-J. Cheng, M. Clark, Y. Tzeng, Diamond Relat. Mater. 14 (2005) 835. A. Huczko, Appl. Phys. A 74 (2002) 617. A.-C. Dupuis, Prog. Mater. Sci. 50 (2005) 929. V. Ivanov, J.B. Nagy, P. Lambin, A. Lucas, X.F. Zhang, D. Bernaerts, Chem. Phys. Lett. 223 (1994) 329. Y. Li, W. Kim, Y. Zhang, M. Rolandi, D. Wang, H. Dai, J. Phys. Chem. B 105 (2001) 11424. C. Klinke, J.-M. Bonard, K. Kern, Surf. Sci. 492 (2001) 195. J.-F. Colomer, C. Stephan, S. Lefrant, G. Van Tendeloo, I. Willems, Z. Kònya, Chem. Phys. Lett. 317 (2000) 83. R. Seidel, G.S. Duesberg, E. Unger, A.P. Graham, M. Liebau, F. Kreupl, J. Phys. Chem. B 108 (2004) 1888. Y. Yang, Z. Hu, Y.J. Tian, Y.N. Lü, X.Z. Wang, Y. Cheng, Nanotechnology 14 (2003) 733.
[16] A.M. Cassel, G.C. McCool, Ng.H. Tee, J.E. Koehne, B. Chen, J. Li, Appl. Phys. Lett. 82 (2003) 817. [17] H. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 80 (1996) 471. [18] Y. Li, J. Liu, Y. Wang, Z.L. Wang, Chem. Mater. 13 (2001) 1008. [19] S. Tang, Z. Zhong, Z. Xiong, L. Sunu, L. Liu, J. Lin, Chem. Phys. Lett. 350 (2001) 19. [20] V.I. Merkulov, A.V. Melechko, M.A. Guillorn, D.H. Lowndes, M.L. Simpson, Appl. Phys. Lett. 79 (2001) 1178. [21] M. Chhovalla, K.B.K. Teo, C. Ducati, N.L. Rupersinghe, G.A.J. Amaratunga, A.C. Ferrari, D. Roy, J. Robertson, W.I. Milne, J. Appl. Phys. 90 (2001) 5308. [22] L. Delzeit, C.V. Nguyen, B. Chen, R. Stevens, A. Cassell, J. Han, M. Meyyappan, J. Phys. Chem. B 106 (2002) 5629. [23] J. Han, J.-B. Yoo, C.-Y. Park, H.-J. Kim, G.S. Park, M. Yang, I.T. Han, N. Lee, W. Yi, S.G. Yu, J.M. Kim, J. Appl. Phys. 91 (2002) 483. [24] E. Terrado, M. Redrado, E. Muñoz, W.K. Maser, A.M. Benito, M.T. Martínez, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 26 (2006) 1185. [25] E. Terrado, M. Redrado, E. Muñoz, W.K. Maser, A.M. Benito, M.T. Martínez, Diamond Relat. Mater. 15 (2006) 1059. [26] V.I. Merkulov, D.H. Lowndes, Y.Y. Wei, G. Eres, E. Voelke, Appl. Phys. Lett. 76 (2000) 3555. [27] K.S. Choi, Y.S. Cho, S.Y. Hong, J.B. Park, D.J. Kim, J. Eur. Ceram. Soc. 21 (2001) 2095. [28] Z.Y. Juang, I.P. Chien, J.F. Lai, T.S. Lai, C.H. Tsai, Diamond Relat. Mater. 13 (2004) 1203. [29] K.S. Choi, Y.S. Cho, S.Y. Hong, J.B. Park, K.H. Son, D.J. Kim, J. Appl. Phys. 91 (2002) 3847. [30] C. Liu, A.-J. Cheng, M. Clark, Y. Tzeng, Diamond Relat. Mater. 14 (2005) 835 BUSCARLO. [31] K.B.K. Teo, C. Singh, M. Chhowalla, W.I. Milne, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, vol. 1, 2004, p. 665. [32] J.W. Jang, C.E. Lee, T.J. Lee, S.C. Lyu, C.J. Lee, Curr. Appl. Phys. 6 (2006) 141. [33] Final report MEC project NANOENER (MAT2002–04630-C02–01). [34] C.J. Lee, J. Park, J.A. Yu, Chem. Phys. Lett. 360 (2002) 250.
Important parameters for the catalytic nanoparticles formation towards the growth of carbon nanotube aligned arrays E. Terrado ⁎, E. Muñoz, W.K. Maser, A.M. Benito, M.T. Martínez Instituto de Carboquímica (CSIC), Miguel Luesma Castán 4, 50018 Zaragoza, Spain Available online 13 December 2006
Abstract Presented here is a systematic study on the experimental parameters involved in the formation of catalytic nanoparticles from homogeneous Ni films deposited by dc sputtering towards carbon nanotube (CNT) production on Si/SiO2. We have found a critical temperature and time for the thermal and reduction pre-treatment processes to obtain catalyst nanoparticles with the appropriate size and high density suitable for CNT growth. From such nanoparticles, densely-packed aligned CNT arrays were successfully grown at 750–800°C by thermal CVD. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanotubes; Chemical vapor deposition; Sputtering
1. Introduction Carbon nanotubes (CNTs) have attracted the interest of many research groups because of their striking structural and physical properties [1,2] which make them promising candidates for applications ranging from nano-electronics to field emission displays and electrochemical devices [2–4]. Nevertheless, the ability to grow CNTs directly on a substrate at a desired position [5] is the real challenge from a technological point of view. This control would allow the integration of the grown CNTs into microelectronics circuits [6]. In this sense, chemical vapour deposition (CVD) processes seem to be the more appropriate technique for large-scale production at low cost and direct growth on different substrates [7]. Moreover, CNTs with tailored features are desired since their properties and consequently their final applications strongly depend on their structural characteristics such as diameter, number of layers, length or crystallization degree. CVD is a versatile technique in terms of allowed catalysts, carbon feedstock, growth temperature, gas environments or substrate morphologies [8].
⁎ Corresponding author. Tel.: +34 976733977; fax: +34 976733318. E-mail address: [email protected] (E. Terrado). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.11.004
In the CVD approach, CNTs grow from the decomposition of a carbon precursor (typically a hydrocarbon or CO) at relatively high temperatures (650–1100 °C) on a catalyst surface. The role played by the catalyst in the CNT growth is really decisive and much more complex that just the decomposition of the hydrocarbon molecules. In fact, a deep understanding of the catalyst role in CNT growth implies the consideration of many factors such as its composition, morphology, preparation method, interaction with the support and pre-treatment conditions [9]. The most common catalysts towards CNT production are Fe, Co and Ni. They can be utilized as catalyst for multi-walled nanotubes (MWNTs) or single-walled nanotubes (SWNTs) individually [10–14], combined in binary mixtures [15–17] or with other co-catalyst elements which improve their own catalytic activity [18,19]. On the other hand, the catalyst can be deposited onto the substrate by chemical or physical methods, that is, the catalyst can be deposited by solutions containing it or directly by physical techniques. Physical methods, such as electron gun evaporation [20], thermal evaporation [21], ionbeam sputtering [22] and magnetron sputtering [23] allow the deposition of a homogeneous metal film with nanometric size (typically b20 nm) onto the substrate. Then, subsequent steps upon heating and reduction will be required to break the metal film into nanoparticles with the critic size that enables and promotes CNT growth.
E. Terrado et al. / Diamond & Related Materials 16 (2007) 1082–1086 Table 1 Series of experiments including thermal pre-treatments (series 1–2), ammonia etching (series 3) and CNT growth (series 4) Series Thermal pre-treatment
1 2 3 4
Reduction
Gas T (°C)
T Gas (min)
N2 N2 N2 N2
5–45 30 30 25
800 750–850 750 750
– – NH3 NH3
Growth
T T Gas (°C) (min)
T (°C)
T (min)
– – 750 750
– – – 750–800
– – – 20
– – 5–30 5
– – – C2H2
1083
3. Results and discussion SEM studies of the samples corresponding to series 1 and 2 reveal the influence of the thermal pre-treatment in the Ni catalyst nanoparticles formation. In series 1, the temperature is kept constant at 800 °C to study the influence of time in the
Presented here is a systematic study of the experimental parameters involved in the formation of Ni catalytic nanoparticles from a homogeneous Ni film deposited by dc sputtering. The aim of this work is to get reproducibly the catalyst nanoparticles not only with the appropriate size and distribution but also with the high density suitable to promote the growth of a densely-packed carbon nanotube array suitable for future applications. 2. Experimental Silicon substrates (10 mm × 10 mm) with an oxide layer of 300 nm were washed with acetone and isopropyl alcohol in a bath sonicator and then dried prior to Ni catalyst coating. Nickel films with a 5 nm-thickness were deposited on the substrates by dc sputtering (BALTEC SCD-050, 150 mA, argon pressure = 5 × 10− 2 mbar). The Ni-coated substrates were then placed on a ceramic boat in the centre of a tubular furnace. Three series of experiments were performed with the aim of optimizing the catalyst size and density towards the growth of aligned CNT arrays. The experimental conditions of series 1–3 are summarized in Table 1. For series 1, thermal pre-treatment processes of the Nicovered samples were carried out at 800 °C under nitrogen atmosphere during 5, 15, 30 and 45 min respectively, in order to study the effect of time in the resulting broken film into nanoparticles. For series 2, thermal pre-treatment processes of the Nicovered samples were carried out in the same flow conditions than series 1 for 30 min at temperatures of 750, 800 and 850 °C to study the influence of temperature in the process. For series 3, thermal treated at 750 °C for 30 min samples were etched with a 100 sccm ammonia flow for 5, 15 and 30 min respectively after the previous pre-treatment processes, with the aim to determine the effect of ammonia etching time. The effects of the experimental parameters on the characteristics of the Ni catalytic layer were studied by scanning electron microscopy (SEM, Hitachi S3400-N) and atomic force microscopy (MFD-3D). From the optimal thermal and reduction conditions, CNTs were grown using acetylene (20 sccm) at atmospheric pressure of 750–800 °C for 20 min (series 4), from the optimized synthesis conditions reported in our previous work [24,25]. The resulting CNTs were characterized by scanning electron microscopy (SEM, Hitachi S3400-N) and transmission electron microscopy (TEM, JEOL-2000 FXII).
Fig. 1. SEM image of the Ni-covered sample after a thermal pre-treatment (a) at 800 °C for 15 min, (b) at 800 °C for 30 min and (c) at 850 °C for 30 min.
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Fig. 2. AFM characterization of the Ni-covered sample after the optimal pre-treatment process. (a) Tapping-mode topography image with the corresponding crosssection height measurements and (b) 3D-imaging surface.
thermal pre-treatment. It was observed that the sputtered film began to be broken into nanoparticles for times higher than or equal to 30 min. In Fig. 1a no particles formation is still observed for the Ni-sputtered film exposed at 800 °C during 15 min. On the contrary, Fig. 1b shows nanoparticles distribution on the Ni-covered sample after the thermal pretreatment at 800 °C for 30 min. The average nanoparticle diameter is around 70 nm and the estimated Ni nanoparticles density is ∼ 4.109/cm2. Increasing the pre-treatment time results in increased average diameters and, therefore, in a decrease in the nanoparticle density. This effect is due to the coalescence of neighbouring particles as much probable as the film is exposed longer to high temperatures. In series 2, the effect of temperature in the thermal pre-treatment is studied by keeping a constant time (30 min). The highest density and the smallest Ni nanoparticles are observed in the Ni-covered samples exposed to a thermal pre-treatment process at 750 °C. In this case, the average diameter is around 40 nm and the Ni nanoparticles density is ∼ 8.109/cm2. At 850 °C (Fig. 1c) only big catalyst nanoparticles are observed (∼ 2–5 μm). These thicker particles are the result of a coalescence phenomenon due to the increase of mobility of nanoparticles on the substrate surface at higher temperatures [26]. From the results obtained in series 1 and 2, it can be concluded that a thermal pre-treatment at 750 °C for 30 min seems to be the optimum condition to obtain a dense distribution of Ni nanoparticles with small diameter and homogeneous size distribution from a sputtered film with nanometric thickness. It is worth mentioning the important role played by ammonia towards CNT production that has been widely discussed in the literature [27,28]. We have observed that efficient CNT growth is only achieved when CVD processes occurred after the ammonia or other reductive pre-treatment (e.g. H2). The
Fig. 3. SEM micrographs of the CNT aligned array grown on Si/SiO2 substrates from the pre-formed Ni nanoparticles. (a) Top view and (b) cross-section detail of a fragment which came off.
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characterization reveals how average nanoparticles diameter increases for the Ni-coated samples from 40 nm (without ammonia time) to 60 nm after the 30 min-thermal pre-treatment and 15 min at 750 °C under ammonia flow (100 sccm). The compromise is to minimize the reduction time that should be add on the optimum pre-treatment time. Fig. 2 include AFM tapping-mode characterization of the Ni-covered sample surface after 25 min-thermal pre-treatment under nitrogen followed by 5 min-ammonia etching. The size observed by AFM of the nanoparticles ranges from 20 to 40 nm. Finally, aligned CNT arrays were successfully grown at 750– 800 °C (series 4) from the pre-formed Ni nanoparticles on the silicon substrates. The observed alignment is a consequence of the high nanoparticles density which induces steric impediments between neighbouring filaments promoting the aligned growth. The lack of van der Waals interactions between neighbouring CNTs results in an entangled CNT growth [30] as it is observed when the nanoparticles density is not high enough. In Fig. 3a a top view of the CNT aligned array is shown. Fig. 3b reveals a detail of a fragment which came off from the CNT array with a length of around 4 μm. The morphology of the produced MWNTs was characterized by TEM (Fig. 4). The presence of catalytic nanoparticles at the tips of the nanotubes (Fig. 4a) suggests that the CNT production occurred via a tip-growth mechanism [31]. A bamboo-like structure [32] is observed in many of the carbon nanostructures (Fig. 4b) with an average diameter of ∼ 40 nm which correlates with the average diameter of the Ni nanoparticles obtained from the above described optimum pre-treatment process, that is, 25 min at 750 °C under nitrogen followed by a 5 min-ammonia etching. On the other hand, TEM images reveal the low graphitization degree of the CNT structure that it is also confirmed by Raman spectra of the grown samples with wide D and G bands and a intensity ratio close to 1 [33]. These spectra features are similar to other reported results on CNT production by thermal CVD processes using Ni as catalyst [34]. 4. Conclusions Fig. 4. TEM representative image of the grown under optimal conditions samples. (a) CNT array exhibiting catalyst nanoparticles at the tips of the tubes (see narrows). (b) Detail of the bamboo-like structure and CNT parallel walls.
observed results in our previous work [24,25] lead us to believe that the use of ammonia might improve the CNT production by keeping the catalyst nanoparticles active for nucleation. The hydrogen that results from the ammonia decomposition at the catalytic sites might react with amorphous carbonaceous materials deposited on the catalyst surface, thus forming volatile products that are easily removed from the catalyst and, in such way, keeping the metal surface clean and active [29]. Nevertheless, it should be taken into account that longer reduction times imply longer exposure of samples to high temperature and consequently more probable coalescence phenomena. Therefore, these reductive pre-treatments need to be also optimized to avoid catalyst coalescence. AFM
It has been stablished that temperature and time are critical parameters which determine the particle catalyst size from a sputtered metal film with a nanometric thickness. There is an optimal temperature for the catalyst nanoparticles formation. Above such optimal temperature the formed particles begin to coalesce. Likewise a compromise between time and temperature must be achieved to obtain a small nanoparticle size and to prevent their coalescence. Reduction process has been optimized to avoid also this coalescence phenomenon. From our study, we have concluded that a thermal pretreatment at 750 °C for 25 min under nitrogen flow followed by a 5 min-ammonia reductive step seems to be the optimum condition to obtain a dense distribution of activated nanoparticles from the 5 nm-sputtered Ni film. Finally, from such nanoparticles with the appropriate density and size denselypacked aligned CNT arrays were successfully grown on Si/SiO2 substrates following a tip-growth mechanism.
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Therefore, a systematic study of the influence of each parameter involved in CNT production is essential to control and reproduce the growth process bearing in mind any potential application. Acknowledgements Funded by MEC (MAT2002-04630-C02-01 and TEC200405098-C02-02). E.T. sincerely acknowledges I. Tacchini for his effort in the SEM characterization. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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