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Low-cost Production and Applications of High Purity Carbon Nanotubes

Stephanos F. Nitodas1,2, and Theodoros K. Karachalios1,3

Nanothinx S.A. Stadiou Street, Platani, Rio Achaias GR-26504, Hellas 2 Foundation for Research and Technology Hellas Institute of Chemical Engineering and High Temperature Chemical Processes Stadiou Str., Platani, P.O.Box 1414, GR-26504 Patras, Hellas 3 Department of Chemical Engineering, University of Patras Rio Patras, GR-26500, Hellas e-mail : [email protected] e-mail: [email protected]

1

Abstract- The development of low-cost carbon nanotubes is presented in this work. In the present work, various types of carbon nanotubes are synthesized in hot-wall CVD reactors using different temperatures and carbon sources (hydrocarbons). Growth rates were continuously monitored using an electronic microbalance coupled to the reactors. The obtained products are characterized by Raman Spectroscopy, Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The data from the deposition process and the characteristics of the developed materials are discussed in detail. The kinetic data are compared with those attained by employing conventional catalysts. Furthermore, techniques of semi-industrial CNT functionalization are developed in this study and results on their effectiveness on the synthesis of resin-based nanocomposites are presented. I. INTRODUCTION The production of carbon filaments, using the decomposition of various gaseous carbon-containing compounds, in presence or absence of catalyst, is already known since the 1970's [1, 2]. Interest in the production of carbon nanotubes (CNT) was revived following the observation of Iijima [3] in 1991 that filamentous carbon, which is produced during the evaporation of carbon electrodes using the arc-discharge method, has nanometer size tube structure. These tubes consist of two or more tubular walls of carbon atoms in hexagonal order, which are open at the edge or close with a hemispheric structure, such as the spherical structure of carbon (fullerene). Since then carbon nanotubes continue to draw much attention for many potential applications, which derive from their unique structure as well as their electronic and mechanical properties [4, 5]. For this purpose, several methods of producing carbon nanotubes have been reported [6-11]. Among these methods, chemical vapor deposition (CVD) can achieve a controllable route for the selective

production of nanotubes with defined properties. The thermal catalytic CVD, which is the method used in the present study, is considered to be the best process for lowcost and large-scale synthesis of high-quality carbon nanotubes, employing various hydrocarbons as carbon gaseous source. The growth mechanism of the CVD synthesis of carbon nanotubes involves decomposition of the carbon source, followed by dissolution of the carbon phase into metal catalytic nanoparticles and re-deposition of carbon on the catalyst surfaces [7]. The majority of the applications of carbon nanotubes cannot be commercialized because of the high CNT cost hitherto. Development of low-cost carbon nanotubes of high purity has been attained by the authors. The competitive advantages of the offered technology stem from the group's proprietary production methods with catalytic chemical vapor deposition (CCVD) that allow the synthesis of carbon nanotubes at lower cost than that of other CNT materials, which are commercially available at present. Furthermore, novel proprietary nanostructured catalysts are used for the production of carbon nanotubes. The main features of the developed technology are: 1. The low cost of the catalysts, which are produced in house. Moreover, due to the sophistication of the production techniques, use of low cost hydrocarbon mixtures as deposition precursors is possible without compromising the high purity and quality of the obtained nanotubes. 2. The high yield of carbon nanotubes deposition, which is based on the proprietary design of the CVD equipment. 3. Minimal levels of amorphous carbon or other impurities that degrade CNT properties. A brief kinetic investigation of the CNT synthesis is presented in this paper, followed by a detailed characterization of the obtained nanotubes. In addition, functionalization schemes of multi-wall carbon nanotubes (MWNT) are proposed aiming at the effective development

of CNT-reinforced resin composites. Functionalization of the CNT surface cannot only lead to increased dispersibility of carbon nanotubes in various organic solvents and polymers [12, 13], but also to increase the strength of the interface between the CNT and the polymer matrix [12]. Significant toughening of polymer matrices through the incorporation of CNT has been reported [13-15]. A loading of 1 wt.% multi-wall carbon nanotubes (MWNT), randomly distributed in an ultra-high molecular weight polyethylene film, was reported to increase the strain energy density by 150% and increase the ductility by 140%. The present study focuses on CNT functionalization with carboxylic and amine groups. II. MATERIALS AND METHODS The method of multi-wall carbon nanotubes production used here is based on the synthesis of carbon nanostructures by catalytic chemical vapor deposition of hydrocarbon sources on substrates of metal oxides impregnated with metal catalysts (Fe and Al) [9]. The experimental system (R&D unit) consisted of a vertical quartz tube with inner diameter of 15mm, heated with a resistance furnace, in a length of 22 cm. The isothermal zone of the reactor was approximately 17 cm. Temperature was controlled by a controller with two Pt/Pt­Rh thermocouples. The catalyst was placed in a flat vessel made from platinum, and it was positioned in the middle of the isothermal zone of the reactor. The vessel was coupled to a digital microbalance (CAHN D-101) with 1g sensitivity able to monitor the weight change of the sample with time. The experimental device was completed by mass flow controllers and flow read-out units. After stabilization of the system at the operating temperature, which varied between 650oC and 800oC, the gaseous feed stream was supplied to the reactor. The feed consisted of ethylene (C2H4) or acetylene (C2H2) and helium (He) or nitrogen (N2) as carrier gas. Methane was also employed as feed gas, but the initially obtained deposition rates were rather low and it was thus decided not to carry out any further experiments with this hydrocarbon. Besides the R&D unit, experiments were also conducted in a horizontal quartz reactor of larger diameter (pilot unit), without the use of a sensitive microbalance. The catalytic substrate is inserted in suitable quartz plates, which are placed throughout the isothermally heated length of the quartz tube. The two proprietary catalysts which were used in these experiments consist of Al2O3 and Fe2O3, with different concentrations and particle sizes of the precursor materials.Catalyst C1 was used for the deposition from ethylene and catalyst C2 for the deposition from acetylene. C2 also contains traces of transition metals. More specifically, its active component is bimetallic and consists of Fe and Mo.

III. RESULTS AND DISCUSSION Fig. 1 shows the relative weight change of the deposit with time at various temperatures during the deposition of carbon nanotubes using catalyst C1 at the group's R&D unit. The feed contains 30% ethylene and 70% helium (carrier gas). It can be deduced from the results that the higher the temperature the faster the deposition during the first minutes of the reaction. The early deceleration of the rate increase at 800oC may be attributed to the fact that rapid growth of nanotubes takes place around the catalyst during the first minutes of deposition. As a consequence, the rate of diffusion of the gaseous precursors towards the surface of the catalyst decreases, and diffusion becomes then the controlling step of the process. After 1 h of deposition, the purity of the nanotubes is close to 94% at 700oC and 93.5% at 650oC, whereas the purity at 800oC is 90%.

24 22 20

Catalyst C1 T( C) 700 650 800

o

a b c

% C2H4 30 30 30

Qtotal(sccm) 200 200 200

Dep.Time (min) 60 60 60

Relative Weight Gain, g/g

18 16 14 12 10 8 6 4 2 0 0 500 1000 1500 2000 2500 3000 3500 4000

c b a

Time, sec

Fig. 1. Relative weight gain of carbon nanotubes with respect to time at 30% C2H4 and various temperatures (catalyst C1).

Fig. 2 shows the weight change of carbon nanotubes deposited from ethylene on catalyst C2 at 600oC. This temperature has been chosen as the optimum one for obtaining thin multi-wall carbon nanotubes (see Fig. 6b below). Catalyst C1 initially demonstrates a relative stable rate of deposition, followed by a substantial decline in the rate and attainment of a plateau. On the other hand, catalyst C2 exhibits a stable reaction rate even after two hours of deposition. The two catalysts have been thoroughly designed and manufactured in order to be used exclusively with C2H4 (C1) and C2H2 (C2) for leading to deposits of high carbon yield. C1 was also employed in experiments with acetylene and led to higher deposition rates. However, the obtained products were characterized by much larger diameters than those typically known for carbon nanotubes. As it can be seen in Fig. 3, the material resembles more carbon fibers since its average diameter is around 200 nm. On the other hand, the use of C2 with C2H2 led to the synthesis of even thinner MWNT in comparison with the MWNT resulted from tests with C2H4. Therefore, it was decided to conduct acetylene deposition experiments using catalyst C2.

14

Catalyst: C2 T( C) 600

o

12

% C2H2 30

Qtotal(sccm) 200

Dep.Time (min) 120

Relative Weight Gain, g/g

10

8

6

4

2

0 0 1000 2000 3000 4000 5000 6000 7000 8000

Time, sec

Fig. 2. Relative weight gain of carbon nanotubes with respect to time using acetylene and catalyst C2.

Experiments carried out at longer deposition times led to the conclusion that the purity level increases at any reaction temperature. Deposition of carbon nanotubes also took place at the group's pilot unit. The experiments at 700oC were tailored for the deposition of multi-wall carbon nanotubes with 97% purity, which can reach 99% at high reaction times.

Fig. 4. Variation of the relative weight gain of the system "catalyst/deposit" with time during the deposition of carbon nanotubes in the group's R&D unit.

The "as-produced" carbon nanotubes were fully characterized via Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Raman spectroscopy and Thermogravimetric analysis (TGA). SEM and TEM were employed for the examination of the morphology and the structure of the nanotubes. SEM was performed at 20 kV using JEOL JSM-5200 system. To study the sample using SEM, a small quantity of the carbon bulk material was placed on an aluminium base with carbon paint. High Resolution TEM (HRTEM) JEOL 2010 of CPERI/CERTH (Thessaloniki, Greece) was also performed. Raman spectroscopy was employed for the determination of the type of CNT, as well as the presence of defects and amorphous carbon. The characterization of the material was carried out in a Confocal Nikon Modified Raman Microprobe with laser source at 514.5 nm (2.41 eV), readily obtainable from an argon ion laser. CNT purity was estimated with a post-deposition TGA treatment, which also accounts for the percentage of residual catalyst. Fig. 5 shows representative SEM micrographs of CNT material produced from ethylene at 700oC (MWNT1 and MWNT2) on catalyst C1 (Fig. 5a and 5b, respectively) and from acetylene on catalyst C2 (MWNT3) (Fig. 5c) in the group's pilot unit. (The difference between MWNT1 and MWNT2 is the deposition time.) In all images, there is no sign of any impurities (e.g., amorphous carbon) and only bundles of multi-wall carbon nanotubes are observed. The only exception is MWNT2 where particles of carbon soot are occasionally present. SEM and TEM analysis revealed a diameter range of MWNT1 and MWNT2 of 20-40 nm (Fig. 6a) and >40 nm, respectively. The diameter of MWNT3 varies from 6 to 10 nm (Fig. 6b) and therefore, this type of multi-wall carbon nanotubes falls under the category of thin MWNT.

Fig. 3. SEM image of product deposited from acetylene using catalyst C1.

Fig. 4 presents the variation of the relative weight gain of the system "catalyst/deposit" with time during the deposition of carbon nanotubes at 700oC in the authors' small laboratory reactor (R&D unit), using catalyst C1 (novel catalyst), a conventional catalyst similar to the ones being employed in other CVD methods, and a catalyst based on a metallurgical byproduct. This series of experiments was conducted in order to optimize catalyst C1. The same operating conditions were employed for the three catalysts. The employed ethylene percentage was chosen based on the results of Fig. 1. At 30% C2H4 and 700oC, the reaction curve tends to attain a plateau and this renders diffusion as the controlling step of the deposition process. In order to restrict the diffusion effect and operate at the kinetic region of the process, the optimum percentage of ethylene was tailored at 10%. While the conventional catalyst shows a maximum yield of 30%, the novel catalyst attains a yield of 2000% after a few minutes of deposition. High deposition yield (1800%) is also achieved by the catalyst that was based on the metallurgical byproduct, but only after a longer time of deposition.

a

percentages were determined using TGA. (The combustion of multi-wall carbon nanotubes takes place at temperatures above 550oC, whereas that of amorphous carbon at 300370oC.) TGA analysis of MWNT2 revealed a carbon content of 99.2% (i.e. metal particle content 0.8%) which contains carbon impurities (carbon soot) of ~1%.

100

80

% Relative Weight Loss, g/g

60

b

a MWNT1 (97%) b MWNT2 (99%)

40

a b

20

0 0 100 200 300 400 500

o

600

700

800

Temperature, C

c Fig. 7. Relative weight loss (TGA curve) during combustion of MWNT1 and MWNT2.

Fig. 5. SEM images of MWNT1 (a), MWNT2 (b) & MWNT3 (c).

Fig. 8 shows the combustion peaks ­ i.e. the normalized derivative of the % relative weight loss with temperature ­ of the three types of MWNT. It can be seen in as the tubes diameter and number of layers decrease, their combustion peaks occur at lower temperature: 577oC for Thin MWNT, 608oC for MWNT1 and 638oC for MWNT2. It can be also concluded from TGA and TEM results that the diameters of the produced multi-wall carbon nanotubes increase as their purity increases.

1.0

a

Norm.Deriv.Weight with Temp.

0.8

a b c

MWNT1 (97%) MWNT2 (99%) MWNT3 (Thin MWNT>90%)

a

b

0.6

c

0.4

b

0.2

0.0 540 560 580 600 620 640

o

660

680

700

Temperature, C

Fig. 6. TEM images of MWNT1 (a) and Thin MWNT (MWNT3) (b).

Fig. 8. Combustion peaks for MWNT1, MWNT2 and MWNT3.

The carbon content of MWNT1 is 97.06% (i.e. metal particle content 2.94%) and the amount of amorphous carbon in the carbon content is minimal (Fig. 7). These

The characterization of the as-produced carbon nanotubes was completed with m-Raman measurements. The laser

excitation wavelength was 514.5 nm and the laser's power 1.93 mW. The spectra were acquired using a back ­ scattering geometry at room temperature. Fig. 9 displays Raman spectra for MWNT1 and MWNT2. The two main first - order peaks that correspond to MWNT are present. A strong band (G-band) at ~ 1581 and 1584 cm1 , respectively for MWNT1 and MWNT2, is observed which is the Raman ­ allowed phonon high ­ frequency E2g first - order mode and which is attributed to the movements of carbon atoms in opposite directions along the surface of a tube [16, 17]. A weaker peak (D-band) is seen at ~ 1359 cm-1 (MWNT1) and 1350 cm-1 (MWNT2), which originates from defects in the curved graphene sheets & tube ends or in the presence of carbon coating on the outer surface of the tubes [18, 19].

the tubes. Carboxylated nanotubes have been reported to augment the cure rate of epoxy resins at lower temperatures and enhance reinforcement of the resins [21]. The dried sample was then dispersed in a solution of thionyl chloride (SOCl2) and dimethylformamide (DMF) and stirred at 50oC for 24 h. The filtered product was then stirred in ethylenediamine (C2H4(NH2)2) for 6h, filtered and dried overnight. X-ray photoelectron spectroscopy (XPS) analysis showed that the above treatment introduced nitrogen-based (amino) groups at the CNT surface by 4.9% w/w. The functionalized nanotubes were dispersed in a phenolic resin compound and an epoxy resin by sonication treatment at a CNT weight percentage of 3%.

1584

Relative Intensity

1359

a

a

500

1000

1500

-1

2000

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Raman shift, cm

1581

=514.5 nm

Relative Intensity

2 m

1350

b b

1300

1400

1500

1

1600

1700

Raman Shift (cm- )

Fig. 9. Raman Spectra of bulk MWNT1 (a) and MWNT2 (b) produced by CVD. Spectrum taken at =514.5nm.

c

Fig. 10. SEM images of (a) phenolic resin compound, (b) MWNT1phenolic resin composite and (c) MWNT1-epoxy resin composite.

Multi-wall carbon nanotubes of 97% purity (MWNT1) underwent further treatment in order to functionalize them with amine groups for use in the preparation of CNTreinforced resin composites. The functionalized sample was prepared by treating MWNT1 with nitric acid (HNO3) [20]. The material was then ultra-sonicated for 10 min in HNO3 65% (100 ml per 1 g of CNT in "fine powder" form); the solution was mechanically stirred at 220°C for 20 min, cooled down to room temperature, filtered and finally dried at room temperature for a few hours. This procedure enables the attachment of COOH-groups on the surface of

The morphology of the obtained composites was examined with SEM. The SEM image of the phenolic resin compound is presented in Fig. 10a, whereas Fig. 10b depicts the CNTphenolic resin composite. A fairly homogeneous dispersion of carbon nanotubes is clearly seen. The same applies to the CNT-epoxy resin composite, the micrograph of which is shown in Fig. 10c.

IV. SUMMARY The deposition kinetics and the characteristics of the obtained CNT product on two catalysts were investigated. The results show that the rate of deposition and the final yield depend on the temperature of the process, the carbon precursor (ethylene or acetylene), the composition of catalyst, as well as the extent of the reaction. The experimental results show that the deposition yield is remarkably high when ethylene and catalyst C1 are employed. More specifically, the carbon yield approaches 900% at 600oC (Thin MWNT), 3500% at 700oC (MWNT 97%) and 9900% at 800oC (MWNT 99%). Furthermore, MWNT 97% were functionalized with amino-groups and an effective method is proposed for the development of MWNT-based resin composites. All products were characterized using a variety of microscopic and spectroscopic techniques. V. REFERENCES [1] R. T. K. Baker, M. A. Barber, P. S. Harris, F. S. Feates and R. J. Waite, "Nucleation and growth of carbon deposits from the nickel catalyzed decomposition of acetylene", J. Catalysis, vol .26, July 1972, 51-62. [2] R. T. K. Baker, P. S. Harris, R. B. Thomas and R. J. Waite, "Formation of filamentous carbon from iron, cobalt and chromium catalyzed decomposition of acetylene", J. Catalysis, vol 30, July 1973, 86-95. [3] S. Iijima , "Helical Microtubules of Graphitic Carbon", Nature, vol. 354, November 1991, 56-68. [4] D. S. Bethune, C. H. Klang, M. S. de Vries, G. Gorman, R. Savoy, J. Vasquez and R. Beyers , "Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls", Nature, vol. 363, June 1993, 605-607. [5] S. B. Sinnott and R. Andrews, "Carbon Nanotubes: Synthesis, Properties, and Applications", Crit. Rev. Solid State Mater. Sci., vol. 26, September 2001, 145-249. [6] B. I. Yakobson and P. Avouris, "Carbon Nanotubes" (Topics in Applied Physics), Editors: M. S. Dresselhaus, G. Dresselhaus, and P. Avouris, Berlin / Heidelberg, Vol. 80, 2001, 287­329. [7] H. Dai , "Carbon Nanotubes" (Topics in Applied Physics), Editors: M. S. Dresselhaus, G. Dresselhaus, and P. Avouris, Berlin / Heidelberg, vol. 80, 2001, 29-53. [8] S. Maruyama, R. Kojima, Y. Miyauchi, S. Chiashi, M. Kohno, "Low-temperature synthesis of high-purity singlewalled carbon nanotubes from alcohol", Chem. Phys. Lett., vol. 360, July 2002, 229-234. [9] K.B. Kouravelou, S.V. Sotirchos and X.E. Verykios, "Catalytic effects of production of carbon nanotubes in a thermogravimetric CVD reactor.," Surf Coat Tech, vol. 201, 2007, 9226-9231. [10] K. Mukhopadhyay, A. Koshio, T. Sugai, N. Tanaka, H. Shinohara, Z. Konya, J. B. Nagy, "Bulk production of quasi-aligned carbon nanotube bundles by the catalytic chemical vapour deposition (CCVD) method", Chem. Phys. Lett., vol. 303, April 1999, 117-124. [11] L. Huang, X. Cui, B. White and S. P. O'Brien, "Long and oriented single-walled carbon nanotubes grown by ethanol chemical vapor deposition", J. Phys. Chem. B, vol. 108, September 2004, 16451-16456.

[12] A. Eitan, K. Jiang, D. Dukes, R. Andrews and L. S. Schadler, "Surface Modification of Multiwalled Carbon Nanotubes: Toward the Tailoring of the Interface in Polymer Composites", Chem. Mater. vol. 15, November 2003, 3198-3201. [13] M. C. Weisenberger, E..A. Grulke, D. Jacques, T. Rantell, and R. Andrews, "Enhanced mechanical properties of polyacrylonitrile/multiwall carbon nanotube composite fibers" J Nanosci Nanotech, vol. 3, December 2003, 535539. [14] A. B. Dalton AB, S. Collins, E. Munoz E, J. M. Razal, V. H. Ebron, J. P. Ferraris, J. N. Coleman, B. G. Kim and Ray H. Baughman, "Super-tough carbon-nanotube fibres", Nature, vol. 423: 703, June 2003, doi:10.1038/423703a. [15] S L. Ruan, P. Gao, X. G. Yang, T.X. Yu, "Toughening high performance ultrahigh molecular weight polyethylene using multiwalled carbon nanotubes", Polymer, vol. 44, September 2003, 5643­5654, [16] Y. Li, X. B. Zhang, X. Y. Tao, J. M. Xu, W. Z. Huang, J. H. Luo, Z. Q. Luo, T. Li, F. Liu, Y. Bao and H. J. Geise, "Mass production of high-quality multi-walled carbon nanotube bundles on a Ni/Mo/MgO catalyst", Carbon, vol. 43, 2005, 295-301. [17] P. Tan, L. An, L. Liu, Z. Guo, R. Czerw, D. L. Carroll, P. M. Ajayan, N. Zhang and H. Guo, "Probing the phonon dispersion relations of graphite from the double-resonance process of Stokes and anti-Stokes Raman scatterings in multiwalled carbon nanotubes", Phys. Rev. B, vol. 66, December 2002, 245410-245417. [18] C. Singh, M. S. P. Shaffer and A. H. Windle, "Production of Controlled Architectures of Aligned Carbon Nanotubes by an Injection Chemical Vapor Deposition Method", Carbon, vol. 41, 2003, 359-368. [19] L. Liu, Y. Qin, Z. X. Guo, D. Zhu, "Reduction of solubilized multi-walled carbon nanotubes", Carbon, vol. 41, 2003, 331-335. [20] V. Raffa, G. Ciofani, S. Nitodas, T. Karachalios, D. D'Alessandro, M. Masini and A. Cuschieri "Can the properties of carbon nanotubes influence their internalization by living cells?" Carbon, vol. 46, 2008, 1600-1610. [21] J. Bae, J. Jang and S. H. Yoon, "Cure behavior of the liquid-crystalline epoxy/carbon nanotube system and the effect of surface treatment of carbon fillers on cure reaction", Macromol. Chem. Phys., vol 203, October 2002, 2196­2204. VI. ACKNOWLEDGEMENTS The authors would like to thank Dr. Vassilis Drakopoulos and Dr. Amaia Soto Beobide for their assistance with the SEM and Raman analysis. This work has been partially performed in the framework of project NINIVE (Non Invasive Nanotransducer for In vivo gene thErapy) project funded by the European Commission (contract n° 033378) and partially in the framework of the national project PENED 516 (Production and study of nanostructured carbon materials (nanotubes and nanoporous membranes) for production, storage and use of hydrogen).

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