Aligned carbon nanotubes grown on alumina and quartz substrates by a simple thermal CVD process

Diamond & Related Materials 15 (2006) 1059 – 1063 www.elsevier.com/locate/diamond

Aligned carbon nanotubes grown on alumina and quartz substrates by a simple thermal CVD process E. Terrado, M. Redrado, E. Muñoz, W.K. Maser, A.M. Benito, M.T. Martínez ⁎ Instituto de Carboquímica (CSIC), Miguel Luesma Castán 4, 50018 Zaragoza, Spain Available online 27 January 2006

Abstract A very simple spray-coating method is presented here for depositing iron salt and iron sol-gel solutions on quartz and alumina substrates that can be efficiently used to produce carbon nanotubes (CNTs) by thermal chemical vapor deposition (CVD). Aligned and curly and randomly oriented multi-walled carbon nanotubes (MWNTs) have been efficiently grown from iron catalyst over large surfaces using mixtures of acetylene and ammonia under different experimental conditions. The effect of substrate, catalyst precursor, thermal and reductive catalyst pretreatment, and CNT growth temperature on the CNT diameter and morphology have been here studied and discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanotubes; Chemical vapour deposition; Nanotechnology; Catalytic processes

1. Introduction The amazing carbon nanotube (CNTs) properties [1] and, consequently, their promising applications [2–4] strongly depend on the CNTs structural characteristics such as diameter, number of layers, length, or presence of defects. Thus, a very strict control of the influence of experimental production parameters on the CNT structure is required. CNTs are mainly produced by arc discharge [5,6], laser ablation [6,7], plasma-enhanced [8,9], and thermal [6,10,11] chemical vapor deposition (CVD). CVD methods are nowadays the most widely utilized due to their versatility, low cost, controlled growth, and industrial scalability [12–14]. Depending on the final application, thermal CVD could be even more desirable than plasma CVD because thermal CVD processes are more economical [15], suitable for large-area, irregular-shaped substrates, and multiple-substrate coatings [16]. Thus, a large variety of substrates of any size and shape that are compatible with the furnace dimensions, growth temperature, and gas environment can be coated with CNTs. Fe-containing catalysts efficiently promote the CNT growth in CVD processes. The peculiar ability of Fe to promote the CNT growth, together with Ni and Co, is related to its catalytic ⁎ Corresponding author. Tel.: +34 976733977; fax: +34 976733318. E-mail address: [email protected] (M.T. Martínez). 0925-9635/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.10.071

activity for the decomposition of the carbon feedstock (typically a hydrocarbon), the formation of meta-stable carbides, the carbon diffusion, and the formation of graphitic sheets [17]. Fe catalysts are commonly deposited on the desired substrate by pulsed laser deposition [18], thermal [19] or e-beam evaporation [20], sputtering [21,22], or spin-coating [23]. We here show how multi-walled carbon nanotubes (MWNTs) can alternatively be easily grown from Fe-salts and Fe-containing sol-gel spraycoated quartz and alumina substrates. This spray-coating method is very versatile and economical, and significantly simplifies the catalyst preparation procedures for CVD processes. Additionally, the effect of type of substrate, thermal pretreament in vacuum, nitrogen or in the presence of hydrogen, ammonia etching, as well as of the temperature of nucleation and growth on the MWNTs structural characteristics is also studied. 2. Experimental Quartz and alumina substrates (10 mm × 10 mm) were washed with acetone and isopropyl alcohol by sonication and then dried prior to the Fe catalyst coating. Iron nitrate solutions (0.005 and 0.0075 M) in ethanol were sprayed onto the substrates with an air pressure of 1 bar and keeping a constant distance (10 cm) between the employed aerograph and the substrate.

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Table 1 Experimental parameters employed in the described CNT production experiments Exp.

1 2 3

Substrate

quartz alumina quartz alumina quartz alumina

Catalyst

Fe(NO3)3 Fe(NO3)3 Fe(NO3)3 Fe(NO3)3 Fe(NO3)3 Fe(NO3)3

Exp.

Substrate

Catalyst

4

quartz alumina

Fe(NO3)3 Fe(NO3)3

Thermal pretreatment

Ammonia

T (°C)

t (min)

T (°C)

t (min)

Acetylene T (°C)

t (min)

(nm)

800 800 800 800 800 800

30 30 30 30 30 30

800 800 850 850 900 900

5 5 5 5 5 5

800 800 850 850 900 900

15 15 15 15 15 15

40–70 40–50 60–90 60–90 100 100–140

Thermal Pretreatment

CNTs diam.

Ammonia + Acetylene

CNTs diam.

T (°C)

t (min)

T (°C)

t (min)

(nm)

800 800

30 30

900 900

20 20

75–140 100–150

Exp.

Substrate

Catalyst

Reduction

Ammonia

T (°C)

t (min)

T (°C)

t (min)

T (°C)

t (min)

(nm)

5

quartz alumina

sol-gel sol-gel

500 500

60 60

900 900

5 5

900 900

15 15

95–100 60–90

The catalyst was also prepared spraying Fe-containing solgels. [24,25]. A tetraethoxysilane (10 mL), 1.5 M iron nitrate aqueous solution (15 mL), and ethanol (15 mL) mixture was sprayed onto a quartz and alumina substrates to form a film. After gelation of the mixture, the gel was dried overnight at 80 °C to remove the excess water and other solvent. During this process, the gel cracks into small 5–20 mm2 pieces. The Fe-catalyst-coated substrates were then placed on a ceramic boat in the centre of a tubular furnace and two different

Acetylene

CNTs diam.

pretreatments were carried out before the CNTs growth. The experimental conditions for the different experiments are summarized in Table 1. For the iron nitrate sprayed films, a thermal pretreatment process under nitrogen atmosphere during 30 min was required to obtain Fe-catalyst nanoparticles of the desired size for the nanotube nucleation. In the case of the films prepared by sol-gel, the substrates were initially calcined at 750 °C for 7 h under vacuum (b10− 3

Fig. 1. CNTs grown from sprayed Fe-catalyst salt solution on alumina substrates (a) at 800 °C, (b) 850 °C and (c) 900 °C.

E. Terrado et al. / Diamond & Related Materials 15 (2006) 1059–1063

Torr), and then reduced at 500 °C for 1 h in a 9% hydrogen containing nitrogen flow at atmospheric pressure, resulting in the formation of large quantities of catalytic nanoparticles. After these pretreatment processes, both kinds of sprayed catalyst films were etched with a 100 sccm ammonia flow for 5 min at the employed CNT growth temperature (800–900 °C). CNTs were then grown using acetylene (20 sccm) at atmospheric pressure for 15 min at 800 °C (Exp. 1), 850 °C (Exp. 2), and 900 °C (Exp. 3) for the iron nitrate solutions sprayed films. Other experiments were carried out using a simultaneous mixture of ammonia and acetylene at 900°C (Exp. 4) and for the sol-gel sprayed films on quartz and alumina substrates at 900 °C (Exp. 5). Finally, the reactor was slowly cooled down to room temperature under nitrogen. The resulting CNTs were characterized by scanning electron microscopy (SEM, JEOL JSM-6400 microscope), transmission electron microscopy (TEM, Philips CM-30, 300 kV) and Raman spectroscopy (Jobin Yvon T-64000, λ = 514.5 nm).

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3. Results and discussion As SEM (Figs. 1 and 2) and TEM (Fig. 3) studies reveal, MWNTs were produced from the iron nitrate solutions sprayed films under the experimental conditions cited above. The CNT growth temperature clearly affects the MWNT diameter: thus, the MWNT average diameter increases from ∼ 55 nm at 800 °C to ∼ 100 nm at 900 °C for the quartz substrates (Table 1). These values are similar to those obtained on alumina substrates. The growth temperature and the employed substrate not only affect the MWNT diameter, but also the MWNT morphology. MWNT randomly grown on alumina substrates are observed at all tested temperatures (Fig. 1), whereas MWNT grown on quartz at 850 °C and 900 °C are straighter and even well aligned (Fig. 2). It is worth mentioning the important role played by ammonia in the described CNT production process. Efficient MWNT production is only achieved when CVD processes occurred after the ammonia or other reductive pre-treatment is used. We

Fig. 2. CNTs grown from sprayed Fe-catalyst salt solutions on quartz substrates (a) at 800 °C, (b) at 850 °C and (c) at 900 °C.

E. Terrado et al. / Diamond & Related Materials 15 (2006) 1059–1063

Fig. 3. TEM images of the CNTs grown from Fe catalyst at 850 °C.

believe the use of ammonia might improve the CNT production by keeping the Fe 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, therefore, keeping the metal surface clean [26]. Fe catalytic nanoparticles are formed on the quartz substrates after the thermal pretreatment at 800 °C and ammonia etching at 850 °C. By employing iron nitrate concentrations higher than 0.01 M, the coating on the substrates was unchanged after the thermal and ammonia pretreatments and no catalytic nanoparticles were observed at any of the employed temperatures. When employing lower concentrations, however, the formation of Fe nanoparticles was observed at temperatures higher than 750 °C. The average diameter of the catalytic particles is ∼ 30– 70 nm after pretreatments at 800 °C. We have also observed that the highest nanoparticle densities result from pretreatments at 850–900 °C, therefore favoring the growth of aligned CNTs. The density of those Fe catalytic particles is about 1 × 1010/cm2. Aligned CNTs were successfully grown at 850 and 900 °C on quartz substrates but no alignment was observed in samples grown at the same experimental conditions on alumina substrates. Thus, the substrate morphology and surface chemistry may play an important role in the final CNT

orientation. On the other hand, high densities of catalytic nanoparticles are required in order to achieve well-aligned CNT growth. At high temperatures, the small catalyst metal particles, with size in the range of tens of nanometers, may rapidly diffuse and coalesce on the substrate forming bigger catalyst nanoparticles. Consequently, the resulting CNT diameter increase and there are more steric impediments between neighboring CNTs, that promote the aligned growth. For this reason, alignment is only observed in the samples grown at 850 and 900 °C. The CNT alignment observed when using quartz substrates enables us to estimate the temperature effect on the CNT growth rate: 0.6 μm/min at 850 °C, and 2 μm/min at 900 °C. The lack of van der Waals interactions between neighbouring CNTs results in a random CNT growth [4,27] as it is observed when using alumina substrates. The structure of the produced MWNTs was characterized by TEM (Fig. 3). The produced CNTs were first removed from the substrates by sonication in methanol and deposited on a TEM grid. The presence of catalytic nanoparticles at the tip of the produced CNTs suggests that the CNT production occurred via a tip-growth mechanism. It is also worth mentioning that some of those nanoparticles were trapped along the CNT structure, and even several nanoparticles can be observed in the same nanotube. The images also reveal the low degree of graphitization of the CNT. Fig. 4 shows representative Raman spectra of MWNTs grown on Fe catalyst at 850 and 900 °C on quartz substrates. Two main Raman features are observed in the spectra: a broad band at ∼ 1335 cm− 1 (D band), and a band at ∼ 1600 cm− 1 (G band). The G band indicates the presence of original graphite features, whereas the D band is characteristics for defects in the graphitic structure and disordered carbon. The intensity ratio of D and G bands (I(D)/I(G)) for the Fe catalyst at 850 °C and at 900 °C, indicates that the degree of crystalline perfection of the grown MWNTs is poor, which is consistent with the described TEM results, and similar to other reported results on CNT production by CVD processes using Fe as catalyst [20]. On the other hand, CNTs were also grown using ammonia and acetylene at 900 °C during 20 min on iron salt sprayed films

1595 cm-1 [G band]

1355 cm-1 [D band] Raman intensity [arb. units]

1062

850°C

1345 cm-1 1580 cm-1

900°C

1200

1300

1500

1400

1600

-1]

Raman shift [cm

Fig. 4. Raman spectra of the CNTs grown on Fe catalyst at 850 and 900 °C.

E. Terrado et al. / Diamond & Related Materials 15 (2006) 1059–1063

on quartz and alumina substrates (Exp. 4, Table 1). SEM studies reveal that the CNT grown under these experimental conditions have similar average diameter (∼100 nm on quartz and ∼125 nm on alumina) than those obtained using ammonia pretreatments at the same temperature (Exp. 3, Table 1). However, the CNTs produced in Exp. 5 are longer, and in some cases exhibit coiled morphologies. Finally, CNTs were also grown from sol-gel films (Exp. 5, Table 1). The average diameter of the produced CNTs was ∼ 95 nm on quartz substrates, and ∼ 75 nm on alumina substrates. Coiled CNTs are observed when employing alumina substrates, therefore indicating that both the catalyst preparation and the substrate characteristics affect the CNT growth and morphology. 4. Conclusions We have here reported a very simple and versatile method for deposition catalyst films prepared with iron salt and iron sol-gel solutions that can be efficiently employed in the production of MWNTs by CVD. Fe catalytic nanoparticles formation is observed on alumina and quartz substrates at temperatures higher than 750 °C and catalyst concentrations lower than 0.01 M. The employed substrate affects the characteristics of the produced MWNTs: while straight and aligned MWNTs were produced at 850–900 °C just on quartz substrates, curly, randomly oriented, and even coiled CNTs were grown at 800–900 °C on alumina substrates.

[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

Acknowledgements

[24]

Funded by MEC (MAT2002-04630-C02-01 and TEC200405098-C02-02).

[25]

References

[27]

[1] M.S. Dresselhaus, G. Dresselhaus, Ph. Avouris (Eds.), Carbon nanotubes: Synthesis, Structure, Properties and Applications, Topics in Applied Physics, vol. 180, Springer, Berlin, Germany, 2001.

[26]

1063

J. Robertson, Mater. Today (October 2004) 46. R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Science 297 (2002) 787. H. Dai, Surf. Sci. 500 (2002) 218. D.S. Bethune, C.H. Kiang, M.S. de Urief, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, Nature 363 (1993) 605. Y. Ando, X. Zhao, T. Sugai, M. Kumar, Mater. Today (October 2004) 22. W.K. Maser, E. Muñoz, A.M. Benito, M.T. Martínez, G.F. de la Fuente, Y. Maniette, E. Anglaret, J.-L. Sauvajol, Chem. Phys. Lett. 292 (1998) 587. Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegel, P.N. Provencio, Science 282 (1998) 1105. J.G. Céspedes, C. Corbella, M. Galán, G. Viera, E. Bertran, Fuller. Nanotub. Carb. N. 13 (2005) 447. S. Gan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassen, H. Dai, Science 283 (1999) 512. M. Pérez, C. Vallés, W.K. Maser, M.T. Martínez, S. Langlois, J.L. Sauvajol, A.M. Benito, Carbon 43 (14) (2005) 3034. P. Nikolaev, M.J. Bronikowski, R. Kelley Bradley, F. Rohmund, D.T. Colbert, K.A. Smith, R.E. Smalley, Chem. Phys. Lett. 313 (1999) 91. Y. Murakami, S. Chiashi, Y. Miyauchi, M. Hu, M. Ogura, T. Okubo, S. Maruyama, Chem. Phys. Lett. 385 (2004) 298. K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Science 306 (2004) 1362. Y.C. Choi, D.W. Kim, T.J. Lee, C.J. Lee, Y.H. Lee, Synth. Mater. 117 (2001) 81. A. Huczko, Appl. Phys., A 74 (2002) 617. C.-H. Kiang, J. Chem. Phys. 113 (2000) 4763. J.I. Sohn, C. Nam, S. Lee, Appl. Surf. Sci. 197–198 (2002) 568. C.J. Lee, J. Park, J.A. Yu, Chem. Phys. Lett. 360 (2002) 250. Y.-M. Shyu, F.C.-N. Hong, Mater. Chem. Phys. 72 (2001) 223. J.-B. Park, G.-S. Choi, Y.-S. Cho, S.-Y. Hong, D. Kim, S.-Y. Choi, J.-H. Lee, K.-I. Cho, J. Cryst. Growth 244 (2002) 211. Y. Huh, J.Y. Lee, J.H. Lee, T.J. Lee, S.C. Lyu, C.J. Lee, Chem. Phys. Lett. 375 (2003) 388. Ph. Mauron, Ch. Emmenegger, A. Züttel, Ch. Nützenadel, P. Sudan, L. Schlapbach, Carbon 40 (2002) 1339. Z.-W. Pan, H.-G. Zhu, Z.-T. Zhang, H.-J. Im, S. Dai, D.B. Beach, D.H. Lowndes, Chem. Phys. Lett. 371 (2003) 433. M. Nath, B.C. Satishkumar, A. Govindaraj, C.P. Vinod, C.N.R. Rao, Chem. Phys. Lett. 322 (2000) 333. K.S. Choi, Y.S. Cho, S.Y. Hong, J.B. Park, K.H. Son, D.J. Kim, J. Appl. Phys. 91 (2002) 3847. C. Liu, A.-J. Cheng, M. Clark, Y. Tzeng, Diamond Relat. Mater. 14 (2005) 835.