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1 Phys. Status Solidi C 7, No. 3 4, (2010) / DOI /pss Prodution of high quality arbon nanotubes for less than $1 per gram physia pss urrent topis in solid state physis R. Smajda 1, M. Mioni 1, M. Duhamp 1, J. C. Andresen 1, L. Forró 1, and A. Magrez *1,2 1 Laboratoire de Nanostrutures et Nouveaux Matériaux Eletroniques, Eole Polytehnique Fédérale de Lausanne (EPFL), Switzerland 2 Center for Researh on Eletronially Advaned Materials, Eole Polytehnique Fédérale de Lausanne (EPFL), Switzerland Reeived 13 September 2009, revised 12 Otober 2009, aepted 12 Otober 2009 Published online 4 February 2010 PACS Tp, Ka, Db * Corresponding author: arnaud.magrez@epfl.h, Phone: , Fax: In this artile, we report on the mass prodution of arbon nanotubes using a ontinuous prodution system based on a rotary tube furnae. At first, we have optimized the omposition of the metalli nanopartiles. Bimetalli Fe 2 Ni and Fe 2 Co alloys exhibit higher atalyti ativity than pure Fe, Co or Ni. Then, atalyst prodution proess has been modified for the preparation of large quantity of atalyst with low aggregation suitable for large sale synthesis of CNTs. A prodution rate of about 1.2 kg per day has been ahieved. This is yielding to a ost prodution of less than $1 per gram. Finally, we show that the CNTs growth an also be obtained on naturally ourring alite support for further ost redution of the synthesis. Quality of the CNTs produed has been established by measuring their mehanial properties using AFM. Young s modulus of the CNTs an be as high as the ideal value of 1 TPa. 1 Introdution Sine their disovery [1], arbon nanotubes (CNTs) have been at the forefront of nanotehnology beause of their remarkable eletrial, optial, thermal and mehanial properties in ombination with their extraordinary hemial stability. Developing appliations for arbon nanotubes requires the prodution of pure materials in ommerial quantities at affordable pries. Chemial Vapour Deposition (CVD) is onsidered by the industrialist to be the most suitable proess to produe CNTs at low ost. Presently, the market prie of pure arbon nanotubes is higher than $10 per gram for a prodution apaity of few hundreds metri tonnes per year. They remain expensive ompared to arbon blak, with a prie being of the order of $1 per metri tonne. It is expeted that the prie of CNTs will fall as the demand inreases and as the proess improves in terms of prodution yield. However, it is important that the supply of arbon nanotubes remains of high quality. In this paper, we demonstrate that with our ontinuous prodution system based on a rotary tube furnae a prodution rate of more than 1 kg/day an be ahieved with a ost of less than $1 per gram. 75% of the nanotubes produed have a diameter smaller than 10 nm and a young modulus lose to the ideal 1 TPa. 2 Experimental 2.1 Optimisation of the metal atalyst MWCNTs were synthesized by CVD of aetylene and arbon dioxide over supported Fe, Co, Ni atalysts alloys. In a typial atalyst preparation, a stoihiometri amount of metal salts (obalt (II) nitrate hexahydrate-sigma Aldrih %; and/or iron (III) nitrate nonahydrate-sigma Aldrih % and/or nikel(ii) nitrate hexahydrate- Sigma Aldrih %) are dissolved in distilled water. Support (CaCO 3 - Fluka C %) is subsequently dispersed into the solution. The resulting mixture (with Fe 2 M/CaCO 3 = 5%; M= Ni or Co) was dried on a hot plate under ontinuous stirring. Large bathes of asprepared atalyst were produed as well and kept in boxes without speial are. The syntheses of CNTs were arried out in a horizontally mounted quartz tube furnae (stationary onditions) in the temperature range of 350 and 850 o C.

2 Contributed Artile Phys. Status Solidi C 7, No. 3 4 (2010) 1237 For the CNTs synthesis, the as-prepared atalyst was plaed on a quartz boat and was rapidly introdued into the reation hamber. The furnae was flushed with Ar for 10 min and finally exposed to a mixture of C 2 H 2 and CO 2 with a flux of 1 L/h. After 15 min growth time, the furnae was flushed again with Ar for 10 min. 2.2 Large sale synthesis of CNTs using the rotary tube furnae We elaborated a ontinuous prodution method based on a rotary tube furnae (Fig. 1) to overome the limited apaity and salability of the fixed bed reator [2, 3]. The oven has an 80 mm diameter rotating quartz tube. The atalyst was ontinuously introdued into the reation tube with an endless srew plae at the end of the atalyst ontainer. Aetylene and argon were fluxed at 10 L/h and 80 L/h respetively. After olleting, the material is purified. The atalyst and support are dissolved in a 1.5 M hydrohlori aid during the purifiation proess. MWCNTs are subsequently filtered, washed with distilled water and dried at 120 C overnight. A Figure 1 View of the rotary tube furnae used for large-sale synthesis of CVD. A: Catalyst stok ontinuously driven inside the 3 inh quartz tube. Raw materials (CNTs + Catalyst) are olleted in B 2.2 CNTs preparation for mehanial properties haraterisation Atomi Fore Mirosopy (AFM) samples are prepared by dispersing the CVD grown CNTs in ethanol and suspended by 10 minutes soniation using an ultrasoni finger. One droplet of CNT suspension is deposited on the miro-fabriated Si 3 N 4 membrane. After one minute, the solution was dried off with a nitrogen flux. Individual CNTs were found to bridge holes of the Si 3 N 4 membrane, with their entire length in ontat with the membrane surfae. AFM images are taken with ontat mode and varying loads on eah image. Bending modulus of the CNTs is extrated by measuring the vertial defletion at the middle of the suspended part of the nanotube. 3 Results and disussion 3.1 Optimisation of the atalyst omposition The Chemial Vapor Deposition developped is based on the equimolar reation between aetylene and CO 2 at the surfae of a supported metal atalyst [4]. We have already shown that best supports for the growth of CNTs are alkaline earth arbonate [5]. Here we have foused our study B on the omposition of the metalli nanopartiles of the atalyst. Beside pure Fe, Ni and Co metals, bimetalli alloys with Fe 2 M (M= Ni and Co) omposition have been tested. We have shown that when atalyst is introdued inside the CVD reator, an intermediate oxide (spinel with Fe 2 NiO 4, Fe 2 CoO 4 omposition) of metal transition is formed prior to the redution into metal nanopartiles, proeeding when aetylene and CO 2 gases are introdued. This oxide is single phase when Fe 2 M stoihiometry is applied (as well as for pure Fe, Ni and Co metals). Any deviation from this omposition (Fe/M=2) indue the presene of a seond phase with lower atalyti ativity [5]. We learly see in Table 1 that the mass of CNTs bimetalli alloys exhibits higher atalyti ativity ompared to pure Fe, Co, and Ni transition metals. The reation yield, alulated as the ratio between the mass of nanotubes produed and the mass of arbon introdued (as aetylene) is dramatially enhaned by a fator of more than 4 for Fe 2 Ni and Fe 2 Co ompared to Ni and Co respetively. Catalyst omposition mg of CNTs produed per g of atalyst Reation yield (%) Co Fe Ni Fe 2 Co Fe 2 Ni Table 1 Maximum mass of CNTs produed for different supported metalli nanopartiles. Growth was performed at C. Element Content (wt%) Co 0.15 ± 0.02 Fe 0.47 ± 0.03 Ca 0.21 ± 0.02 Total 0.83 ± 0.03 Table 2 Impurities ontent within the purified CNTs as a result of the ICP MS analysis. 3.2 Large sale prodution of CNTs For the mass prodution of CNTs, larger quantities of atalyst should be produed. Compared to previous report [5, 6], we developed a new proess based on the o-preipitation of the transition metals on the surfae of CaCO 3 by the addition of a weak base. In a typial atalyst preparation, CaCO 3 is dispersed in water by a strong stirring. Transition metal nitrates are added to the slurry. Triethylamine, ammonia, or

physia p s s 1238 R. Smajda et al.: Prodution of high quality arbon nanotubes for less than $1 per gram urea is added while the ph raises 10.

The slurry (solid/liquid ratio is about 1/3) of CaCO 3 oated with metalli nanopartiles is onsequently frozen by dropping in liquid nitrogen and dried afterwards by sublimation.

Therefore, almost all atalyti partiles are exposed [7]. This is suitable for mass prodution and for inreasing the prodution yield ompared to those reported in the Table 1.

3 physia p s s 1238 R. Smajda et al.: Prodution of high quality arbon nanotubes for less than $1 per gram urea is added while the ph raises 10. This indues the deposition of Fe, Co or Ni mixed hydroxides. The slurry (solid/liquid ratio is about 1/3) of CaCO 3 oated with metalli nanopartiles is onsequently frozen by dropping in liquid nitrogen and dried afterwards by sublimation. With our equipment, about 3 kg of ie an be olleted in 24 hours. It has been found that freeze drying proess (also known as lyophilisation) produes less aggregated powder. Therefore, almost all atalyti partiles are exposed [7]. This is suitable for mass prodution and for inreasing the prodution yield ompared to those reported in the Table 1. It has to be notied that despite being widely used in food industry, lyophilisation remains Figure 2 Diameter distribution of CNTs produed in the rotary tube furnae. Diameters of 1000 CNTs have been measured from TEM mirographs. relatively expensive. Other drying proesses like spay drying or roller drying proesses, widely used in powder industry, an be used as well and are probably more suitable for large sale prodution of atalyst at low ost. To evaluate the yield of the synthesis, we have introdued 2.1 g of atalyst in the quartz tube of our rotary tube furnae. The growth was performed in onditions desribed in the experimental part of the paper but in stationary onditions (quartz tube not rotating). The mass of CNTs produed is 13.1 g. This orresponds to about 4300mg of CNTs per gram of atalyst and a reation yield of about 85%. In ontrast to the values reported in Table 1, no notieable differene has been observed between Fe 2 Co and Fe 2 Ni transition metal alloys. However, the reation yield is enhaned and the mass of CNTs produed per gram of atalyst dramatially inreased. Large quantities of atalyst have been produed and the furnae was run for ouple of hours. Catalyst is introdued in the rotating quartz tube at a rate of about 10 g per hour. A prodution rate of about 1200 g of CNTs per day has been ahieved. 5 μm Figure 3 SEM mirographs of arbon nanotubes grown on a sea urhin shell. The growth was performed at 500 C to preserve the porous struture of the shell. Mass prodution experiments with the rotary tube furnae are performed at 640 C. The diameter distribution, Fig. 2, shows that CNTs have a mean diameter of about 11 nm. 75% of CNTs have a diameter below 15 nm (Fig. 2). After purifiation with HCl, most of the supported atalyst has been removed suh as the C ontent is higher than 99 % (Table 2). We alulated the ost of the CNTs prodution using our rotary tube furnae over one year. The ost of the hemials (for atalyst preparation (support, preursor salts ) and CNTs purifiation (distilled water, aids )) is estimated from the prie of the produts provided by Sigma Aldrih. Aetylene and argon are purhased from Air liquid. The prie of the furnae as well as the salary for one tehniian for one year is inluded in the alulations. The eletriity required to run the rotary tube furnae is aounted as well. This leads to a prodution ost of $0.5 per gram of purified CNTs. This represents a drasti ost redution sine only 1.2 kg per day an be produed in our reator, as ompared to a prie of $10 per gram ahieved

4 Contributed Artile Phys. Status Solidi C 7, No. 3 4 (2010) 1239 for few hundreds metri tons prodution apaity of industrial reators. Beside the possibility of inreasing the size of the reator of our rotary tube furnae to proess more atalyst simultaneously, prodution ost ould be further redued by using less expensive hemials as ompared to high grade materials used for the mass prodution of CNTs, previously reported in this paper. Hene, CaCO 3 is a sedimentary rok being the third most ommon onstituent of the earth s rust. We have performed the growth of CNTs using the shell, made in alite, of a sea urhin as support (Fig. 3). The sea urhin shell was olleted in Bodrum (Mediterranean Sea, Turkey) and did not experiene any surfae treatment before or after the Fe film deposition. After exposing the Fe oated shell to aetylene at 600 C, the shell surfae is entirely overed with CNTs while the prodution yield is preserved as ompared to CNTs growth proess performed over atalyst produed from high grade material. It shows that CNTs an be grown on naturally ourring minerals. This paves the way towards further ost redution for CNTs prodution. On the other hand, purifiation of CNTs produed from natural alite will be ahieved with oxidizing aids (HNO 3, H 2 SO 4 ) and/or basi solutions (NH 3, NaOH ) to ensure a omplete dissolution of impurities present in natural alite support. However, further study of the influene of the impurities on the CNTs quality will have to be arried out. 3.3 Mehanial properties of CNTs The mehanial properties of CNTs have been measured by the swiss heese method [8, 9]. CNTs an be easily dispersed in a solvent. One deposited on the alumina porous membrane, part of the CNTs an lie over a hole. The adhesion of a CNT on the flat parts of the substrate is usually muh stronger than the normal fore appliable by the AFM antilever, so that we modelled the suspended nanotube as a doubly lamped beam (see Fig. 4). We also assumed Figure 4 Typial AFM image of CNT bridging the hole of a Si 3 N 4 membrane. Defletion [nm] Applied Fore [nn] Figure 5 F-δ urve obtained for a CNTs of a diameter of 10(± 1) nm and of a suspended length of 500(± 10) nm. The alulated Young s modulus of the CNT is 1013 (± 354) GPa. that the beam has a uniform and irular ross setion. The midpoint of a doubly lamped beam deflets by δ when loaded with a fore F, and the bending modulus E b an be derived from the F-δ graph using the equation 3 L df Eb = (1) 192 I dδ where L is the suspended length and I is the seond moment of area of the beam, whih for a filled ylinder is δd 4 /64. D is the outer diameter of the nanotube. We onsider shear to be negligible for the suspended CNTs, as in the ase of long thin beams, and take the bending modulus to be the Young s modulus. To measure the stiffness of a suspended nanotube, we applied a fore at its midpoint with an AFM antilever, aquiring fore-displaement data. The F-δ urves were then obtained by subtrating a referene fore-displaement urve taken on a flat substrate. Young s moduli are obtained by fitting lines through these urves (see Fig. 5). CNTs are known to exhibit exeptional mehanial properties. Very high Young s moduli of about 1TPa have been obtained for CNTs produed by high temperature proess like ar disharge or laser ablation [8, 9]. On the other hand, the graphiti struture of CNTs produed by CVD, usually below 1000 C, ontains a large defet density. Therefore, CVD grown CNTs are regarded as materials with very poor mehanial properties. However, we have reently shown that the Young s modulus of CNTs with diameter below 20nm exhibit lear diameter dependene [10, 11]. CNTs with smaller diameter have better struture quality and onsequently mehanial properties similar to high temperature grown CNTs. In this study, we measured the mehanial properties of CNTs produed in the rotary tube furnae at 640 C. They exhibit exeptional mehanial properties. When CNTs diameter is lower than 10 nm, the Young s modulus an be as high as 1 TPa (Fig. 5). In the materials produed in mass, 53% of CNTs have a diameter lower than 10 nm and 75%

5 physia p s s 1240 R. Smajda et al.: Prodution of high quality arbon nanotubes for less than $1 per gram of CNTs have a diameter lower than 15 nm (Fig. 2) for whih the Young s modulus is higher than 100 GPa [11]. 4 Conlusion In this work, we have reported the mass prodution of high quality CNTs at low ost by Chemial Vapour Deposition Proess. As a result of atalyst omposition optimisation, we have shown that bimetalli alloys with Fe 2 M (M=Ni or Co) exhibits higher atalyti ativity than pure Fe, Co or Ni. Using our rotary tube furnae, we have produe CNTs at a rate of 1.2 kg per day. 85% of aetylene is transformed into CNTs. They exhibit a mean diameter of XX nm and have good mehanial properties with Young s modulus lose to 1 TPa. The ost for the prodution of suh CNTs is about $0.5 per gram. This an be further redued by using natural alite. Aknowledgements We thank the Centre Interdisiplinaire de Mirosopie Eletronique (CIME) for aess to eletron mirosopes and the tehnial support of A.J. Kulik for the AFM measurements is aknowledged. This work was partially funded by the European Framework. VIACARBON as well as the Swiss National Foundation and its NCCR Nanosale Siene. Referenes [1] M. Monthioux and V.L. Kuznetsov, Carbon 44, (2006). [2] E. Couteau, K. Hernadi, J.W.Seo, L.Thien-Nga, Cs. Mikó, R. Gaál, and L. Forró, Chem. Phys. Lett. 378, 9-17 (2003). [3] J.W. Seo, E. Couteau, P. Umek, K. Hernádi, P. Maroux, B. Lukić, Cs. Mikó, M. Milas, R. Gaál, and L. Forró, New J. Phys. 5, (2003). [4] A. Magrez, J.W. Seo, V.L. Kuznetsov, and L. Forró, Angew. Chem. Int. Ed. 46, (2007). [5] A. Magrez, J.W. Seo, Cs. Mikó, K. Hernádi, and L. Forró, J. Phys. Chem. B 109, (2005). [6] J.W. Seo, A. Magrez, M. Milas, K. Lee, V. Lukova, and L. Forró, J. Phys. D:Appl. Phys. 40, R109-R120 (2007). [7] M. Mionić, D.T.L. Alexander, L. Forró, and A. Magrez, Phys. Status Solidi B 245, (2008). [8] J.P. Salvetat, G.A.D. Briggs, J.M. Bonard, R. R. Basa, A.J. Kulik, T. Stoekli, N.A. Burnham, and L. Forró, Phys. Rev. Lett. 82, (1999). [9] J.P. Salvetat, A.J. Kulik, J.M. Bonard, G.A.D. Briggs, T. Stoekli, K. Méténier, S. Bonnamy, F. Béguin, N.A. Burnham, and L. Forró, Adv. Mater. 11, (1999). [10] B. Lukić, J.W.Seo, R.R. Basa, S. Delpeux, F. Béguin, G. Bister, A. Fonsea, J.B. Nagy, A. Kis, S. Jeney, J.A. Kulik, and L. Forró, Nano Lett. 5, (2005). [11] K. Lee, B. Luki, A. Magrez, J.W. Seo, G.A.D. Briggs, A.J. Kulik, and L. Forró, Nano Lett. 7, (2007).

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