Electrochemical synthesis of carbon nanotubes and microtubes from molten salts G. Kaptay al, I. Sytchev a, J. Miklósi b, P. Nagy b, P. Póczik b, K. Papp b, E. Kálmán b a University of Miskolc, Hungary 3515 Miskolc, Egyetemváros b Chemical Research Center of the HAS, Hungary 1025, Budapest, Pusztaszeri út 59-67 Keywords: electrochemical synthesis, intercalation, carbon nanotubes, carbon microtubes, LiCl, NaCl, KC1, NaCl-MgCl 2, NaCl-CaCl 2 ABSTRACT In this paper the possibility to produce carbon nanotubes and microtubes is presented by electrochemical synthesis from molten chlorides of alkali and alkali earth metals. It has been experimentally proven, that the deposition of Sn or Ni on a graphite cathode does not lead to the formation of new carbon structures, while the deposition of Li, Na, K, Mg or Ca on a graphite cathode leads to the formation of carbon nano-micro tubes and other nano-micro structures. The diameter of carbon tubes synthesised vary from 1.5 nm to 15 μm, with an average size of the order of 100 nm. INTRODUCTION Since the discovery of carbon nanotubes in 1991 [1], several methods of their synthesis have been described [2]. One of the most perspective ways to produce carbon nanotubes in bulk quantities is their production by electrochemical deposition of some liquid alkali metal from molten alkali halides on graphite cathodes discovered in 1995 by Hsu et al. [3]. Synthesis of carbon nanotubes was described by Hsu et al in details from molten LiCl, LiBr and CaCl 2 [4]. Synthesis of β-sn filled carbon nanotubes from LiCl-SnCl 2 molten salt was demonstrated in [5]. The synthesis of nanotubes was performed by Fray et al in molten LiCl, NaCl and KC1 [6-9]. In works of Fray the phenomenon of electrochemical synthesis of carbon nanotubes from molten alkali chlorides was connected for the first time with intercalation of alkali metals in graphite [6-9]. The goal of our research work was to investigate experimentally the possibility to convert normal graphite cathodes into carbon nanotubes in molten salts by electrochemical synthesis, reproducing data for alkali chlorides, and examining for the first time alkali-earth chlorides. EXPERIMENTAL PROCEDURE Experiments were performed in a vertical tube furnace, in a stainless steel vessel, under argon atmosphere. Electrolyte was kept in a glassy carbon crucible, which also served as anode during electrolysis experiments. Cylindrical graphite cathode was used with diameter of 6 mm. Solid salts of various compositions (see Table 1), and the cathode above it were kept for 24 hours at e-mail: fkmkap@gold.uni-miskolc.hu 257
250 C at a 1 mbar vacuum for drying. The salt was then melted in an argon atmosphere, and the experiments were carried out at temperatures given in Table 1. Table 1. The list of experiments performed Molten salt LiCl NaCl KC1 NaCl - 5w% MgCl 2 NaCl - 5w% CaCl 2 NaCl - 5w% SnCl 2 NaCl - 5 w% NiCl 2 T, C 700 nano-micro tubes found Yes Yes Yes (little) Yes* Yes* No* No* *cathodic current density was kept below the limiting current density of the bi-valent metal Before the electrolysis cyclic voltammetric curves were obtained in each system. As an example, the polarization curve obtained at a scan rate of 0.1 V/s in molten pure LiCl on the graphite electrode before the electrolysis experiments is shown in Fig. 1. Electrolysis experiments were performed with a current density of 0.2 A/cm 2 with duration of 10 minutes. The salt was kept to cool to room temperature in Ar atmosphere after the electrolysis. After electrolysis erosion of the cathode was observed, and therefore the originally white salt turned into partly black. The electrolyte was washed by distilled water and as a result an aqueous suspension of carbonaceous material was obtained. This solution was shaken with an equal volume of toluene. After several hours of standing the carbonaceous material appeared partly in the toluene phase, partly at the water/toluene, partly at the toluene/air interface. The water was removed and toluene was dried. The remaining carbonaceous material was then sonicated with acetone, the acetone was evaporated, and the sample was transferred for SEM (Scanning Electron Microscope) or AFM (Atomic Force Microscope) analysis. EXPERIMENTAL RESULTS As a result of experiments tubes and also some other carbon structures have been obtained. Although it is practically impossible to characterize fully a macroscopic sample at a nanometer scale, the following general conclusions could be drawn (see Fig. 2 - Fig. 6): i. the deposition of Sn and Ni did not lead to cathode erosion, and therefore no carbon nanomicro structures were found in these experiments, at all, ii. the deposition of alkali and alkali-earth metals lead to cathode erosion and different nanomicro structures (mainly tubes) were formed. The synthesis of carbon nano-micro tubes from pure LiCl, NaCl and KC1 salts have been only reproduced by us (see [3-9]). However, for the first time the synthesis of carbon nano-micro tubes have been successfully performed with deposition of Mg and Ca from chloride melts. Based on the limited number of experiments performed so far, no specific features of carbon nano-micro tubes can be stated depending on whether Li, Na, K, Mg or Ca was deposited on graphite cathode, iii. the diameter of carbon tubes varied from 1.5 nm to 15 μm, with an average size of the order of 100 nm. The wall thickness was measured only in several cases, but it seems to be by 1 order of magnitude below the tube diameter (see Fig. 5). Except the thinnest nanotubes (Fig. 3) the majority of tubes are multi-wall nanotubes (MWNT), obviously consisting of more than one graphite layers. iv. according to their length, carbon tubes can be divided into "short" (Fig. 2) and "long" (Fig-s 3-6) tubes. The short tubes were just by several times longer than their diameter, while the 258
long tubes had a length with dimension at least 2 magnitudes higher compared to their diameter. DISCUSSION - THE MECHANISM OF GRAPHITE NANOTUBE SYNTHESIS The commonly accepted way to produce carbon nanotubes is not in the molten salt, but rather in the gas phase. Using laser or electric arc, carbon clusters (consisting of 1 to 10 carbon atoms) are broken from bulk graphite, and those clusters recombine in the gas phase to form fullerenes or nanotubes [1]. However, when carbon nanotubes are synthesized by such a low-energy process as electrochemistry of molten salts below 1,000 C, carbon atoms or clusters cannot be primarily broken from the bulk graphite cathode. The role of the formation of intercalation compounds in the mechanism of formation of carbon nanotubes was first suggested by Fray [7-9]. This idea has been proven by our experiments, showing no nanotube formation with Sn and Ni deposition, as these metals do not form intercalation compounds with graphite. On the other hand, all studied alkali and alkali-earth metals are known for their intercalation compounds, and all of them have been found to be effective agents for synthesis of carbon nano-micro tubes. Hence, the mechanism of nanotube formation can be written shortly as follows. The primary process is the deposition of alkali or alkali-earth metals at the graphite/molten salt interface. The second step is diffusion of these metals into the space between the graphite layers. The driving force of fast diffusion is the formation of stable intercalation compounds. The intercalated atoms cause mechanical stresses and lead to the ablation of one or several graphite layers from the bulk graphite. These graphite layers will be suspended in the molten salt and soon or later will be turned to form tubes. The driving force for this tube-formation is the attractive interfacial forces between the two edges of the suspended graphite planes having broken carbon bonds. This process can be assisted (especially for tubes with larger diameter) by small chlorine bubbles rising through the molten salt and bending the horizontally situated graphite sheets due to buoyancy force. After the recombination of the carbon-carbon bonds the morphology of the carbon structure is stabilized in the form of nano-micro tubes. SUMMARY Stimulated by the recent papers of Hsu et al [3-5] and Chen et al [6-9] the possibility to produce carbon nanotubes by depositing alkali and alkali-earth metals from their molten chlorides on the surface of graphite cathodes has been confirmed. For the first time carbon nano-micro tubes were produced depositing Mg and Ca on graphite cathodes. The diameter of carbon tubes vary from 1.5 nm to 15 μm, with an average size of the order of 100 nm. The idea of Fray [7, 9] on the role of intercalation of alkali metals into the graphite cathode during the formation of carbon nano-micro tubes has been confirmed and extended to alkali earth metals. Further research is needed to find more exact relationship between production parameters (i.e. composition of the molten salt, current density, temperature, type of graphite cathode, time of treatment) and morphology, structure and properties of carbon nanotubes synthesized. LITERATURE 1. S. Iijima: Nature vol. 354 (1991) p. 56 2. Th.W. Ebbesen (Ed.): Carbon nanotubes - Preparation and Properties, CRC Press, Boca raton, FL, 1997 3. W.K.Hsu, J.P. Hare, M. Terrones, H.W. Kroto, D.R.M. Walton, P.J.F. Harris: Nature, vol. 259
667 (1995) p. 687 4. W.K. Hsu, M. Terrones, J.P. Hare, H. Terrones, H.W. Kroto, D.R.M. Walton: Chemical Physical Letters, vol. 262 (1996) p. 161 5. W.K. Hsu, M. Terrones, H. Terrones, N. Grobert, A.I. Kirkland, J.P. Hare, K. Prassides, P.D. Townsend, H.W. Kroto, D.R.M. Walton: Chemical Physical Letters, vol. 284 (1998) p. 177 6. G.Z. Chen, X. Fan, A. Ludget, M.S.P. Shaffer, D.J. Fray, A.H. Windle: J. of Electroanalytical Chemistry, vol. 446 (1998) p. 1 7. D.J. Fray: Molten Salts Bulletin, vol. 66 (1999) p. 2 8. G.Z. Chen, I. Kinloch, M.S.P. Shaffer, D.J. Fray, A.H. Windle. in:,,advances in Molten Salts - From Structural Aspects to Waste Processing", ed. by M. Gaune-Escard, begell house inc., New-York, 1999, p. 97 9. D.J. Fray: in:,,advances in Molten Salts - From Structural Aspects to Waste Processing", ed. by M. Gaune-Escard, begell house inc., New-York, 1999, p. 196 Fig. 1. Cyclic voltamogram of molten LiCl on graphite cathode at 700 C measured with a scan rate of 0.1 V/s Fig. 2. SEM image of some 200 nm diameter, short carbon nanotubes 260
Fig. 3. The AFM image of a 1.57 nm diameter, long nanotube Fig. 4. SEM images of a 180 nm diameter (across) and a 75 nm diameter (top right) long nanotubes 261
Fig. 5. SEM image of a 3 μm diameter, long micro-tube, with a wall thickness of about 200 nm Fig. 6. SEM image of a 15 μm diameter, long micro-tube 262