high yield synthesis of cnt from sp3 hydrocarbons

7/28/2019 High Yield Synthesis of Cnt From Sp3 Hydrocarbons

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DOI: 10.1007/s00339-004-2929-y

Appl. Phys. A 81, 523526 (2005)

Materials Science & Processing

Applied Physics A

g.z. shen1,2,u

d. chen1

k.-b. tang1

y.-t. qian1

c.-j. lee2

High-yield solvo-thermal synthesisof carbon nanotubes from sp3 hydrocarbons

1 Department of Chemistry and Structure Research Laboratory,University of Science and Technology of China, Hefei, 230026, P.R. China2 Department of Nanotechnology, Hanyang University,

17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea

Received: 10 March 2004/Accepted: 6 May 2004

Published online: 9 July 2004 Springer-Verlag 2004

ABSTRACT Carbon nanotubes have been successfully synthe-sized by a high-yield solvo-thermal process by using sp3-hydrocarbons as the carbon sources and solvents. Studies haveshown that sp3-hydrocarbons used in the process not only actas the carbon sources and solvents, but also increase the yieldof carbon nanotubes dramatically. Besides carbon nanotubes,some interesting carbon materials, such as carbon olives, car-bon hollow spheres, carbon microtubes and crossed carbonnanotubes, were also obtained by the present route.

PACS 61.10.Nz; 81.10.Dn

1 Introduction

Research in the field of carbon nanotubes has

undergone an explosive growth since their discovery byIijima [1]. In carbon nanotubes, the parallel layers ofsp2 car-bon atoms are arranged such that the long axis of the rolled-upcylindersisparalleltothelayers.Accordingtotheoreticalpre-dictions, carbon nanotubes may process extraordinary phys-ical and chemical properties. These include extremely highstrength and flexibility [2] as well as strong capillarity prop-erties [3], and they may also be used as nanoscale electronicdevices consisting entirely of carbon [4]. Several techniques,such as electric arc-discharge [5], laser evaporation [6] andcatalytic decomposition of hydrocarbons, [7] have been suc-cessfully developed to synthesize multi-wall nanotubes.

One of the main challenges is to find a way to producenanotubes on a large scale and at low cost. In this respect, thecatalyst method seems to be the best, because of the lowerreaction temperature and the lower cost of production. More-over, the purification step has been optimised to eliminate thecatalyst and the thermal decomposition of hydrocarbons [8].Unfortunately, complex equipments or procedures are neededto produce carbon nanotubes. As another important catalystroute, the solvo-thermal route avoids the above disadvantagesand can be carried out under mild conditions [912].Most ofthe reports on the solvo-thermal process used sp2-structuralcarbon as carbon stocks for the synthesis ofsp2 hybrid carbon

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nanotubes. The yield of carbon nanotubes by using these sp2-hydrocarbons is very low (< 35%). Very recently, our groupsuccessfully synthesized carbon nanotubes using ethanol, ansp3-structural carbon source in an ethanol thermal reductionprocess, and the yield of carbon nanotubes was largely im-

proved(about 80%)byusing sp3

-structural carbons [13].Thisresult encouraged us to develop a more efficient, simpler andhigher yield route to obtain carbon nanotubes and other car-bon nanomaterials.

In this study, we report on the large-scale synthesis ofsp2 carbon nanotubes via a high yield solvo-thermal route byusing sp3-hydrocarbons, such as pentane, hexane, heptane,octane, etc. as the carbon sources. In this process, the yield ofcarbon nanotubes can be increased to higher than 90%. Thesp3 hydrocarbons not only act as the carbon sources and sol-vents to synthesize sp2-carbon nanotubes, but also increasethe yield of carbon nanotubes dramatically (> 90%). Someinteresting carbon materials, such as carbon olives, car-

bon hollow spheres, carbon microtubes and crossed carbonnanotubes, were also obtained by using sp3 hydrocarbons.

2 Experimental

The catalyzer precursors are prepared according tothe literature [9]. In a typical process, 0.15 g of catalyzer pre-cursor was put into a 25-mL stainless steel tank, and then thetank was filled with hydrocarbons (pentane, hexane, heptane,octane, and so on) to about 70% of its total volume. The tankwas sonificated forabout 10 minto make the catalyzer precur-sor disperse well. After sonification, 2 g of Na was added andthe tank was maintained at 450 C for about 10 h. After cool-ing to room temperature, the resulting black precipitate was

washed several times with alcohol, dilute acid, and distilledwater. After washing the precipitate was then vacuum dried at60 C for 4 h.

3 Results and discussion

The purity and phase structure of the as-preparedproducts were obtained by XRD (X-ray diffraction), whichwasrecorded by a Philips Xpert ProSuper X-ray diffractome-ter with Cu K radiation ( = 1.5418). Two intense peakswere observed in the XRD pattern (figure not shown here),which can be indexed to hexagonal graphite with cell con-stants comparable to the reported values (JCPDS 41-1487).


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The panoramic morphologies of the carbon nanotubes ob-tained were examined by FE-SEM, which was performed ona field-emission microscope (JEOL, 7500B) operated at anacceleration voltage of10kV. The FE-SEM image obtained(Fig. 1a) by mounting the solid samples on a copper meshwithout any dispersion treatment, reveals that most of theprepared products (> 90%) are wire-like nanostructures withtypical lengths in the range of0.55m and widths of about

40nm. Further characterization about the prepared productswas performed on a Hitachi H-800 transmission electron mi-croscope with an accelerating voltage of 200kV. The typ-ical TEM images of the as-prepared products showed thatthe wire-like nanostructures under FE-SEM observation areactually carbon nanotubes (Fig. 1b and c). These images re-veal that the carbon nanotubes produced in the present solvo-thermal process are with an inner diameter of1040nm andouter diameter of4060nm, respectively. These carbon nano-tubes can grow as long as 4m. From the TEM images, wecan see that most of the carbon nanotubes produced from thesolvo-thermal process are curved and some carbon nanotubesmay even curve into ring-like structure (Fig. 1d).

Besides carbon nanotubes, some interesting carbon ma-terials, such as micrometer carbon olives, carbon hollow

FIGURE 1 a FE-SEM image of a prepared sample, which shows that large-scale rod-like crystals are obtained from the present route; b TEM image ofprepared sample, which shows the rod-like crystals under FE-SEM obser-vation are actually carbon nanotubes; c TEM image of several long curvedcarbon nanotubes; d TEM image of ring-like carbon nanotubes (the insetTEM image shows a catalyst particle present in the tip of the carbon nano-tubes)

Hydrocarbons Pentane Hexane Heptane Octane

Products Carbon Carbon Carbon Carbonnanotubes nanotubes nanotubes nanotubes

Carbon Hollow spheres Crossedolives and Microtubes nanotubes

TABLE 1 Relationship between the carbon sources and the morphologyof the products

spheres, carbon microtubes, and crossed carbon nanotubes,have also been obtained by the present solvo-thermal route.Table 1 lists the relationship between different carbon sourcesand the morphologies of the final products. Figure 2a showsthe SEM image of a sample made by using pentane as thecarbon source. It can be seen that besides carbon nanotubes(marked with arrows), some interesting carbon olives arealso obtained. The grain size of a typical olive is about5m. Further TEM images (Fig. 2b) reveals the hollowstruc-ture of the carbon olive. When hexane was used, carbonhollow spheres and carbon microtubes were also observed in

addition to carbon nanotubes. Figure 2c shows the obtainedcarbon hollow spheres and carbon microtubes. The sizes ofthe carbon hollow spheres range from 100 to 1000 nm and thediameter of the carbon microtubes is about 1m. Figure 2dshows the TEM image of the crossed carbon nanotubes ob-tained, that have a diameter of about 200 nm. All these inter-

FIGURE 2 a SEM image of obtained carbon olives; b TEM image of car-bon olives, which reveals their hollow structures; c TEM images of carbonhollow spheres (shown with arrows) and carbon microtubes; d TEM imageof crossed carbon nanotubes


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esting carbon materials may have some interesting propertiesand may have great potential applications in various fields,and further studies of them are under way. By carefully opti-mizing the experimental factors, such as temperature, reactiontime, catalyst, etc., the yields of these novel carbon materialscan be greatly increased and this work (and the characteriza-tion of these carbon materials) is still under way in our group.

The solution-based catalyst precursors routes have re-

cently attracted much more attention due to their low-cost andthe advantages of easily tuning the composition and concen-tration of the catalysts [14, 15]. The domain growth mechan-ism in this route is the well-known vapor-liquid-solid (VLS)mechanism. The typical phenomenon of the VLS mechan-ism is the presence of a catalyst particle at the end of eachcarbon nanotube [14]. In our present process, although cata-lyst particles can be detected at the end of the carbon nano-tubes (Fig. 1d inset), it is difficult to say that the presentroute is controlled by the VLS mechanism as the reactionconducted in the closed system is quite complex and the de-tailed growth mechanism undoubtedly needs further study.Previously, Li et al. [16] reported the synthesis of diamond

powder using CCl4 in a similar process. In the synthesis ofdiamond, according to free energy calculations, the reduc-tion ofCCl4 by sodium is highly thermodynamically spon-taneous (G0 (diamond) = 99.2kJmol1), which is fa-vorable for the formation of diamond. But in the presentsynthetic process, according to the free energy calculations(G0 are 8.4kJmol1, 0.25kJmol1, 8.0kJmol1,and16.4kJmol1, respectively for pentane, hexane, heptane, oc-tane), most of the reactions are not thermodynamically spon-taneous, and the formation of diamond is impossible and onlycarbon nanotubes are formed. Based on the above analysis, itis reasonable to assume that the present solvo-thermal processinvolved the first reduction of catalyst precursor by sodium to

produce catalyst particles, followed by the growth of carbonnanotubes from the catalyst particles by pyrolysis of hydro-carbons under self-generated high pressure. Without a cata-lyst, no carbon nanotubes were obtained.

The formation ofsp2-carbon nanotubes was further con-firmed by the Raman procedure. Figure 3 shows the Ramanspectrum, and there are two strong peaks at 1345 cm1 and1590cm1, indicating the graphite structure of the carbonnanotubes [17, 18]. These results are in good agreement withour previous reports [11].

The advantage of the present solvo-thermal process isthe use ofsp3-hydrocarbons. These sp3-hydrocarbons act notonly as the carbon sources, but also as the solvent, which

decreases the cost of large-scale production of carbon nano-tubes and allows the reaction to be performed under mildconditions. The successful production of sp2 carbon nano-tubes from sp3 hydrocarbons provides a good example of thestudy of the transformation from sp3-bonded carbon to sp2-bonded carbon. In the solvo-thermal process, the use ofsp3-hydrocarbons also increases the yield of carbon nanotubes.By FE-SEMand TEMobservations, the yield of carbon nano-tubes was calculated to be as high as 90%, which resolves theshortcoming of previously reported solvo-thermal process, inwhich sp2-hydrocarbons were used as the carbon sources.

Another advantage of the present solvo-thermal process isthe ability to produce catalyst-particle filled carbon nanotubes

FIGURE 3 Raman spectrum of products which shows two graphite peaksat 1345 cm1 and 1590 cm1

and carbon nanorods, which can be produced by adjustingthe amount of catalyst precursor. Figure 4 shows a typicalTEM image of the as-prepared catalyst-particle-filled carbonnanorods. So the present route may provide a novel one-step low-cost method to synthesize metal (alloy)-filled carbonnanotubes or carbon nanorods, simply by an choosing appro-priate catalyst precursor.

4 Conclusions

In summary, a novel high-yield solvo-thermalroute has been used to prepare carbon nanotubes by usingsp3-hydrocarbons as the carbon sources. By choosing an ap-

propriate catalyst precursor, this route can be used to preparemetal- (alloy-) filled carbon nanotubes and carbon nanorods.

FIGURE 4 TEM image of catalyst particles filled carbon nanorods


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Some other interesting types of carbon materials can also beprepared by the present solvo-thermal process.

ACKNOWLEDGEMENTS This work is supported by the Na-tional Natural Science Foundation of China and the 973 Projects of China.One of the authors (Dr. Shen) thanks Dr. Liu in Hanyang University forhelpful discuss.

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high yield synthesis of cnt from sp3 hydrocarbons

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