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Morphology of Multi-Walled Carbon Nanotubes Affected by the Thermal Stability of the Catalyst System E. Dervishi,† Z. Li,‡ A. R. Biris,§ D. Lupu,§ S. Trigwell,| and A. S. Biris*,†,‡ Applied Science Department, UniVersity of Arkansas at Little Rock, Arkansas, 72204, UALR Nanotechnology Center, Graduate Institute of Technology, UniVersity of Arkansas at Little Rock, Arkansas, 72204, National Institute for Research and DeVelopment of Isotopic and Molecular Technologies, P.O. Box 700, R-400293 Cluj-Napoca, Romania, and Electrostatic and Surface Physics Laboratory, Mail Code: ASRC-20, Kennedy Space Center, Florida 32899 ReceiVed September 19, 2006. ReVised Manuscript ReceiVed NoVember 9, 2006
High-quality multi-walled carbon nanotubes (MWCNTs) were efficiently synthesized on the CaCO3 supported Fe-Co catalyst with catalytic chemical vapor deposition method using acetylene as carbon source. The relationship between the catalyst structure and the carbon nanotube growth was systematically studied by using multiple techniques including SEM, TEM, TGA, Raman spectroscopy, and XRD. It was found that the MWCNT product demonstrates two groups in the outer diameter distribution, which is associated with the partial decomposition of CaCO3. This thermal decomposition was found to have as a result the coexistence of two catalytic systems throughout the entire synthesis process: Fe-Co/ CaCO3 and Fe-Co/CaO. It was also found that the smaller diameter nanotubes grow on the Fe-Co/CaO system, while Fe-Co/CaCO3 produces larger diameter tubes.
1. Introduction Carbon nanotubes (CNTs) are highly interesting materials because of their intricate chemical and physical properties and their wide range of possible applications. These range from physical applications such as composite fibers1 or field effect transistors,2 chemical applications such as hydrogen storage,3 to possible biomedical applications.4 Over the past decade, different production techniques have been developed, and the most commonly used approaches are arc discharge,5,6 laser ablation,7,8 and chemical vapor deposition.9,10 Among these techniques, the catalytic chemical vapor deposition (CCVD) method attracts a lot of interest by making possible the large-scale and high-quality production of carbon nanotubes at a relatively low cost. Additionally, with the CCVD * Corresponding author. Tel: (501) 683-7458. Fax: (501) 569-8020. Email:
[email protected]. † Applied Science Department, University of Arkansas at Little Rock. ‡ Graduate Institute of Technology, University of Arkansas at Little Rock. § National Institute for Research and Development of Isotopic and Molecular Technologies. | Electrostatic and Surface Physics Laboratory.
(1) Kotov, N. A. Nature (London) 2006, 442 (7100), 254. (2) Javey, A.; Guo, J.; Farmer, D. B.; Wang, Q.; Wang, D.; Gordon, R. G.; Lundstrom, M.; Dai, H. Nano Lett. 2004, 4 (3), 447. (3) Nikitin, A.; Ogasawara, H.; Mann, D., Denecke, R.; Zhang, Z.; Dai, H.; Cho, K.; Nilsson, A. Phys. ReV. Lett. 2005, 95 (22), 225507. (4) Kam, N. W. S.; O’Connell, M.; Wisdom, J. A.; Dai, H. PNAS 2005, 102, 11600. (5) Ebbesen, T. W.; Ajayan, P. M.; Nature 1992, 358 (6383), 220. (6) Ando, Y.; Zhao, X.; Inoue, S.; Suzuki, T.; Kadoya, T. Diamond Relat. Mater. 2005, 14, 729. (7) Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1995, 243 (1-2), 49. (8) Maser, W. K.; Munoz, E.; Benito, A. M.; Martinez, M. T.; de la Fuente, G. F.; Maniette, Y.; Anglaret, E.; Sauvajol, J.-L. Chem. Phys. Lett. 1998, 292, 587. (9) Endo, M.; Takeuchi, K.; Igarashi, S.; Kobori, K.; Shiraishi, M.; Kroto, H. W. J. Phys. Chem. Solids 1993, 54 (12), 1841. (10) Hernadi, K.; Konya, Z.; Siska, A.; Kiss, J.; Oszko, A.; Nagy, J. B.; Kiricsi, I. Mater. Chem. Phys. 2002, 77, 636.
method, the growth of carbon nanotubes can be controlled by adjusting the reaction conditions and choosing proper catalysts. In the CCVD method, carbon nanotubes are produced from the thermal decomposition of the carbon-containing molecules on desirable metal catalysts (commonly Fe, Co, and Ni). Many efforts have been put forth to optimize catalyst formulations and operating conditions.11,12 The catalyst composition controlled by the preparation method affects the efficiency and selectivity of the catalytic reaction toward the synthesis of desired CNTs. Considering the strong correlation between the diameter of the nanotubes and the size of the catalytic metal particles, the high dispersity of the catalytically active metal ingredient on the support seems to be a critical point in the synthesis of carbon nanotubes by the CCVD method.13 In this contribution, we produced multi-walled carbon nanotubes (MWCNTs) on a calcium carbonate supported Fe-Co catalyst by using acetylene as a carbon source. The utilization of CaCO3 as a support has many advantages over conventional supports like SiO2 and Al2O3. The removal of the support and the catalysts of the catalytic system FeCo/CaCO3 can be easily performed just by acid treatment, washing with distilled water, and filtration, which greatly reduces the purification cost. It was found that the partial decomposition of CaCO3 supports under the reaction conditions results in two groups in the outer diameter distributions of MWCNTs. The effect of the catalyst’s thermal behavior (11) Kong, J. A.; Cassell, A. M.; Dai, H. Chem. Phys. Lett. 1998, 292, 567. (12) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1999, 313, 91. (13) Dai, H.; Rinzler, A. G.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1996, 260, 471.
10.1021/cm062237l CCC: $37.00 © 2007 American Chemical Society Published on Web 12/29/2006
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on the characteristics of the nanotubes (diameter and crystallinity) was also investigated. 2. Experimental Design The Fe-Co/CaCO3 catalyst was prepared as previously explained by Couteau et al.14 The stoichiometric composition of the catalyst was Fe/Co/CaCO3 ) 2.5:2.5:95 wt %. First, the weighted amount of metal salts Fe(NO3)3‚9H2O and Co(CH3COO)2‚4H2O were dissolved into distilled water with agitation, and CaCO3 was added to the solution after the metal salts were completely dissolved. The pH value of the mixture solution was adjusted to about 7.5 by a dipping ammonia solution, to avoid the release of CO2 occurring when carbonates contact acids.15 Then, the water was evaporated with a steam bath under continuous agitation, and the catalyst was further dried at about 130 °C overnight. Carbon nanotubes were synthesized on the Fe-Co/CaCO3 catalyst with the CCVD approach using acetylene as a carbon source.16 About 100 mg of the catalyst was uniformly spread into a thin layer on a graphite susceptor and placed in the center of a quartz tube with an inner diameter of 1 in., which was positioned horizontally inside a resistive tube furnace. Heating was applied after purging the system with nitrogen at 200 mL/min for 10 min, and acetylene was introduced at 3.3 mL/min for about 30 min when the temperature reached around 720 °C. These flow rates correspond to a linear velocity of the gas mixture inside the reactor of 40 cm/ min. Therefore, it takes approximately 14 s for the acetylene/ nitrogen mixture to travel from one side of the catalyst bed to the other (length of 9 cm). The as-produced CNTs were purified in one easy step using a diluted hydrochloric acid solution and sonication. To understand the relationship between catalyst structure and carbon nanotube growth, the catalyst and CNTs were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Raman scattering spectroscopy, and X-ray diffraction. The morphologies of catalysts before and after 5 min of the growth reaction were monitored with a JEOL 6400F high-resolution scanning electron microscope. TEM pictures of the carbon nanotubes were obtained using a FEI Tecnai F30 transmission electron microscope. For this analysis, carbon nanotubes were dispersed in 2-propanol and ultrasonicated for 10 min. A few drops of the suspension were deposited on the TEM grid, then dried and evacuated before analysis. Thermogravimetric analysis was used to study the thermal behavior of the catalyst system and to determine the overall purity of CNTs. Thermogravimetric analysis was performed under air flow of 150 mL/min using Mettler Toledo TGA/ SDTA 851e. Raman scattering studies of the catalyst and CNTs were performed at room temperature using Horiba Jobin Yvon LabRam HR800 equipped with a charge-coupled detector, a spectrometer with a grating of 600 lines/mm and a He-Ne laser (633 nm) and Ar+ (514 nm) as excitation sources. The laser beam intensity measured at the sample was kept at 5 mW. The microscope focused the incident beam to a spot size of
