Synthesis of multiwall carbon nanotubes by electric arc discharge in liquid environments

Carbon 41 (2003) 2393–2401

Synthesis of multiwall carbon nanotubes by electric arc discharge in liquid environments Marco Vittori Antisari*, Renzo Marazzi, Radenka Krsmanovic ENEA, C.R. Casaccia, U.T.S. Materiali e Nuove Tecnologie, C.P.2400, 00100 Rome A.D., Italy Received 18 November 2002; accepted 24 June 2003

Abstract This work reports the experimental results from the production of multiwall carbon nanotubes (MWCN) synthesized by an electric arc discharge performed in liquid environments between pure graphite electrodes. Both liquid nitrogen and deionised water were suitable for a successful synthesis of this form of carbon aggregation. We report a successful synthesis of MWCN by arc discharge submerged in deionised water. Electron microscopy observations of both the reaction products and the surface of the as-synthesized raw material showed the presence of structural degradation of the MWCN, which probably operates after their growth at the cathode. The degradation is tentatively ascribed to a combination of overheating and high current density experienced by the as-synthesized MWNT, which can be caused by the loose structure of the as-deposited material. The damage appeared to be less severe in water environments, probably owing to the better cooling capacity of water relative to liquid nitrogen.  2003 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes; B. Arc discharge; C. Electron microscopy; D. Microstructure

1. Introduction Since their discovery about 10 years ago [1], arc grown multiwall carbon nanotubes (MWCN) have attracted attention owing to their peculiar physical behavior and to their potential technological applications. Despite a large research effort in the last few years, the synthesis route is still holding back the potential of this modern carbon compound. Both single-wall carbon nanotubes (SWCN) and multiwall carbon nanotubes (MWCN) are generally synthesized by assembling, in defined experimental conditions, carbon atoms obtained by the sublimation of graphite or by the decomposition of suitable precursor molecules. Neglecting the synthesis routes based on the assembling of carbon atoms obtained by the decomposition of a precursor gas in a CVD route [2,3] and the pyrolysis of solid precursors [4,5], the arc-discharge [6,7] and the laser vaporization methods [8] are generally used to obtain *Corresponding author. Tel.: 139-06-30483119; fax: 139-0630483176. E-mail address: [email protected] (M. Vittori Antisari).

sublimated carbon atoms that can rearrange themselves in the form of CN, both SWNT or MWNT depending on the synthesis conditions. Among others, the presence of a suitable catalyzer appears necessary for the synthesis of SWNT. The laser vaporization method, and in particular, furnace laser vaporization [9], can give some control on the structural features of the synthesized SWCN, but have production rates that are not suitable for mass applications. The first method to synthesize CN was the electric arc discharge on which a large research effort to find the best experimental conditions is still on-going in the scientific community in view of the simplicity of the method and of the scaling-up possibilities. However, the nature of the process makes it difficult to carefully control the experimental devices [10]. The electric arc discharge is generally performed in an evacuated reactor where, after evacuation, about 500 mTorr of a buffer gas like helium or hydrogen [11], is introduced. A voltage in the range of a few tens of volts is applied between two graphite electrodes, which are brought into contact in order to strike the electric arc. During the arc discharge carbon atoms sublimate from the anode and, in conventional geometry consisting of two

0008-6223 / 03 / $ – see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00297-5

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graphite electrodes facing each-other, they are deposited at the cathode surface where a carbonaceous deposit rich in MWNT, grows up. To produce SWNT the burning anode has to be doped with a suitable catalytic metal. The role of the gas is to stabilize the electric arc and to thermalize the sublimated carbon atoms in order to allow the formation of the CN during the deposition process. Recently it has been shown that the synthesis of MWCN can also be obtained by an electric arc where the discharge occurs under the surface of liquid nitrogen contained in an open vessel [12] without the need for an evacuated reactor. In this case the reaction atmosphere was N 2 gas resulting from the evaporation of liquid nitrogen under the heat input from the discharge itself. The role of the liquid was then to provide an oxygen free atmosphere for the reaction; even if one of the free variables controlling the process (the nature of the buffer gas and its pressure) is no longer under the operator control in this way, this route appears attractive in view of the extreme simplicity of the experimental apparatus, which cannot be operated under vacuum, and of the possibility of easy access to the reaction chamber during the operation; for example, the burned electrode can be quickly substituted in view of a continuous operation mode for mass production. In the last few months, the synthesis of MWCN and polyaromatic carbon shells by arc discharge in deionized water has been reported [13–15]. In this paper we extended these approaches by exploring more deeply the performance of the electric arc discharge in two liquid media: liquid nitrogen and deionised water. The reaction products were studied by different electron microscopy methods; moreover, the raw material was tentatively purified by an air oxidation method in order to get an order of magnitude of the process yield.

to proceed after arc ignition, a carbonaceous cylinder of the same diameter of the anode and up to several mm long grew on the cathode surface in a time of a few minutes. The growth rate depends on the applied voltage and it is larger at higher voltage. The reacted material, which was crushed in a mortar and sprinkled on a carbon coated microscope grid, was observed with a Jeol 4000 FX transmission electron microscope (TEM) operated at 400 kV, while the free surface of the crust deposit was studied by scanning electron microscopy (SEM). Structural damage induced by electron irradiation in typical observation conditions was not evident after several minutes even in the high resolution images, probably owing to the low current density of the electron beam. In order to estimate the yield of the process, a tentative purification was carried out by heating the raw material in air in an open furnace at 500 8C for different times [14]. The occurrence of structural damage to the MWNT was also in this case controlled by TEM observations.

3. Results and discussion

3.1. Arc discharge in liquid nitrogen In agreement with previous results [12], a high density of CN was always observed, as reported in Fig. 1, when the arc discharge was operated at 22–27 V. In Fig. 2 a TEM lattice image showing the organization of the graphene layers at the nanotube tip is reported. Beside CN,

2. Experimental CN was synthesized by using a very simplified arc discharge apparatus where a spectroscopically pure graphite rod 6 mm in diameter was used as anode, while the cathode was made of a 15-mm diameter graphite block. The arc was powered by a power supply able to provide up to 100 A at a maximum voltage of 50 V. The arc discharge occurs in an open vessel, which can be filled by liquid nitrogen or deionised water; this provides the reaction atmosphere. The explored range of applied voltages was between 20 and 30 V, corresponding to an arc current ranging from about 30 to 70 A. The running time of the arc was in the range from less than 1 min to a few minutes. The two electrodes were manually brought into contact to allow the current to flow through and they can be manually retracted to ignite the arc and also to terminate the operation. Under these experimental conditions the carbon material sublimating at the anode was deposited as a hard crust at the cathode. When the process was allowed

Fig. 1. Low resolution TEM image of the as-grown MWCN synthesized in liquid nitrogen.

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Fig. 2. Lattice resolution TEM image of as-grown MWCN synthesized in liquid nitrogen.

a noticeable fraction of the specimen was made of carbonaceous material with a high density of irregularly shaped multishell carbon onions. For an applied voltage lower than 22 V the arc hardly ignited and the deposited material was made mostly of amorphous carbon with very few nanotubes. For voltages higher than 27 V the sublimating carbon was no longer deposited at the cathode, but a very fine carbonaceous powder dispersed into the liquid nitrogen as a result of the reaction. This material showed a microstructure similar to the nanoporous carbon also obtained by the ball milling of graphite [17] as can be seen by the TEM micrograph reported in Fig. 3. In the inset of Fig. 3, where a detail of the particle surface is reported, single wall nanostructures similar to the features called nanohorns by Ijima [18] can be observed. Even if this type of material obtained without the intentional presence of any catalyst shows peculiar features such as weakening, which is often observed in the 002 graphite reflection in the electron diffraction pattern, a detailed study of its structure is beyond the purpose of the present paper. We can report that high sensitivity EELS spectroscopy performed with a parallel detector was not able to detect the presence of nitrogen traces in this material, like in all the materials synthesized in liquid nitrogen; only the presence of C was detected within the experimental sensitivity. This observation is coherent with the experimental results reported by Ishigami et al. [12]. To obtain a better understanding of the process, the surface of the cathodic deposit collected after the operation of the arc discharge in different experimental conditions

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Fig. 3. TEM image of nanoporous carbon synthesized in liquid nitrogen with 30 V applied to the electric arc. A more contrasted detail of the tubular structures observed at the particle surface is reported in the inset.

was observed by SEM. The carbonaceous cylinder deposited on the cathode was collected and directly observed without any further preparation. A feature common to all the experimental conditions relative to applied voltages in the range 22–27 V is the extreme heterogeneity of the sample surface. In fact, despite the relatively small surface of the electrode, several morphologies and microstructures were observed on the same sample. A compact material with a globular aspect is always found in an outer surface layer at the cylinder periphery. Since the temperature in this region was expected to be lower owing to the direct contact with the cooling environment, we can infer that probably in this region the temperature was too low for nanotube synthesis. On the contrary, in the inner part of the specimen a less dense material was observed showing a variety of morphologies illustrated in the following. Fig. 4 represents an area where high quality MWNT, forming a fine network are present. The nanotubes appear to nucleate at the underlying surface and to grow independently of each other. However, SEM analysis of different areas of the same sample, even if very close to each other, show different microstructures. In the low magnification picture of Fig. 5 a groove network can be observed. Even if this kind of structure could be the result of a thermal shrinkage related to the high synthesis temperature, it can also indicate compaction of a previously continuous structure. This latter hypothesis is supported by the experimental evidence

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Fig. 4. High quality CN observed by SEM on the cathodic surface, after arc discharge in liquid nitrogen.

Fig. 6. Higher magnification image of the same area imaged in Fig. 5.

that the crack network is present only in selected portions of the specimen surface. However, in this case the material is also constituted by a nanotube network, as evidenced in the higher magnification image (Fig. 6). When the buried material is observed, the structure results are more dense with a globular morphology, as reported in Fig. 7 taken in a specimen region where the top layer was accidentally removed. When selected parts of the sample were used for TEM analysis, this behavior was confirmed by observations at higher spatial resolution. When the black soft material present in the central area of the top surface was observed, only good quality long MWNT, with a few multiwall carbon shells were found. On the contrary, the observation of the raw material collected 1 mm below the surface shows only heavily degraded material with short and irregular nanotubes and

multiwall carbon shells embedded in a carbonaceous matrix. Fig. 8 shows the difference between the top and buried materials when observed in the TEM at low magnification. The nanotubes protruding from the crushed material deposited on the TEM grid have very different morphologies, in particular as far as the length and the straightness are concerned. In Fig. 9 an intermediate magnification picture of the buried material is reported showing nanotubes with a distorted morphology. The lattice resolution image reported in Fig. 10 shows the presence of structural damage in the buried material. It is extremely difficult to obtain high quality lattice resolution pictures from this material owing probably to the poor perfection of the atomic organization. However, this picture shows how the lattice fringes practically disappear toward the tip of the nanotube located at the right side of the picture.

Fig. 5. Low magnification SEM image of a specimen surface area contiguous to Fig. 4, showing a possible densification of the deposited CN network.

Fig. 7. Structure of the buried material observed by SEM on the same sample of Figs. 4–6.

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Fig. 8. Comparison between the microstructure of the raw material collected from the top of the cathodic deposit (right side) and 1 mm below the surface (left side) observed at the same magnification.

Fig. 10. Lattice resolution image of a degraded nanotube collected 1 mm below the free surface of the deposit. The low quality of the image reflects the low intrinsic contrast due probably to a poor perfection of the crystal lattice.

These observations allow us to draw the following hypothesis on the experimental conditions leading to the nucleation and to the degradation of CN in the above experimental conditions. The experimental evidence that good quality nanotubes are present only at the top surface of the sample, where the just deposited material is present, points to the presence of

some degradation mechanism operating on the CN deposited at the cathode. The degradation mechanism should be related to some major modification of the environmental conditions induced by the CN growth itself, since we observe at the same localization in the deposit the CN growth in a first step and their collapse later. In other words, only the CN present at the free surface of the cathodic deposit, which are just synthesized, show a high structural quality, while buried material that experienced a longer permanence in the growing deposit has a degraded structure. In the literature, the degradation of the CN structure has been reported as a consequence of mechanical stress [18– 20], high temperature exposure [21], high current density [22], and electron irradiation [23]. Even if the degradation mechanism can depend on the details of the structure, and in particular can differ from SWNT to MWNT, we can base our considerations on the hypothesis that a combination of high temperature and mainly high current density are able to induce noticeable structural modifications in our MWNT. The average density of the just grown top layer, which appears to be lower than the bulk material, as evidenced from Fig. 4, can qualitatively explain the observed effect. In fact, at a first approximation and neglecting joule heating owing to the low electrical resistivity of graphite, we can consider that both electrodes are heated by a heat flux through their surface facing the plasma region which, in turn, has a maximum density in the gap between the electrodes themselves.

Fig. 9. A detail of the microstructure of the buried material.

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The main heat source can be direct electron bombardment for the anode, and either irradiation from the anode, heat transfer from the plasma or the latent heat of the condensing carbon for the cathode [24]. Looking at the structure of the cathodic deposit and considering that both the effective thermal conductivity and the average heat capacity are related to the density of the material, we can expect that such a loose structure can suffer a noticeable temperature increase owing to the difficulty of dissipating the heat flux coming from its surface toward the bulk. According to this hypothesis, as soon as the MWCN begins to grow at the cathode surface their temperature increases, since they act as an effective thermal barrier to the heat flux coming from the plasma; moreover, the small heat capacity further enhances the temperature increase. Similarly, the current density flowing in this structure should also increase since the total current remains roughly unaffected while we observe a reduction of the effective cross-section of the conductor. The coupling of the temperature increase and of the increase of current density can be the reason for the degradation of the deposited nanotubes, which hence begin to collapse, gradually increasing the material compaction. This can give rise to a shrinkage of the structure, which can originate from the groove network observed in the top layer (Fig. 5); the ongoing process can further increase the compactness of the structure until finally a dense structure like the one observed in Fig. 7 results. This mechanism can find some support considering that the buried material, which completely suffered the degradation process, shows a fully dense structure where only a few CN with a distorted structure are still present. It should be noticed that all the above morphologies corresponding to the images reported from Figs. 4 to 7 have been observed on different surface areas of the same cathodic deposit and this can be related to the local nature of the discharge in the electric arc. In fact, the electric current in this type of experiment does not flow uniformly through the whole electrode surface, however, the arc connects the most favorable points between the cathode and the anode surfaces, and it moves from one place to another on the electrode surface, following the modification of the gap geometry caused by the sublimation of material at the anode and its deposition at the cathode [25]. Consequently, on the free surface we can observe material deposited at different times, and showing a different degradation situation. Once the deposited material is buried by further condensation, the degradation process is almost complete and, probably, no further degradation occurs owing to the compactness of the structure resulting from the collapse process. It has often been observed that the cross-section of the deposited material appears to be layered in most cases and this can be a consequence of the above effect [24]. We can thus give a schematic picture of what can happen during the deposition of carbon at the cathode in a

simplified hypothesis that the cathode is indirectly heated by heat transfer from the anode, which in turn is directly heated by electron bombardment. At the beginning of the experiment, when the anode temperature is high enough for carbon sublimation, the cathode temperature is still too low for CN nucleation. This point has been experimentally checked by performing very short runs, from which only amorphous carbon is found in the deposit. Later on, the temperature at the cathode increases until it reaches the threshold for CN nucleation, beginning from the center of the deposit. The microstructure of the deposited material, made essentially of well separated MWCN with a low value of the average density, induces a strong increase of both temperature and current density causing the gradual collapse of the structure to a hard and dense layer. This restores the initial condition as far as the thermal conductivity and heat capacity are concerned, so that the process can be iterated cyclically in time giving rise to the observed layered structure of the deposit. On the basis of these considerations a key role in the synthesis of CN is played by the stability of the surface temperature of the cathodic deposit, which can be affected among other factors by the ability of the surroundings in favoring an efficient heat exchange with the deposited material and in particular with the free surface where the growth occurs. Liquid nitrogen, despite the extremely low temperature, is subjected to a fast and violent evaporation when the arc discharge is operating; this feature does not allow direct heat exchange between the growing material and the liquid, but only a less efficient cooling by the nitrogen gas. We have thus attempted to find other liquid media for the nanotube synthesis, able to host an arc discharge and showing better cooling characteristics relative to liquid nitrogen. Deionised water appears to be a reasonable solution, even considering the possible dissociation of water under the effect of the arc discharge and the possibilities of reaction with the hot carbon. We have worked under the hypothesis that the material transfer from the anode to the cathode is fast enough to reduce the contribution of these effects. On the other hand, water has good electrical insulation properties, excellent cooling capabilities, is less strongly evaporating than liquid nitrogen and it is able to insulate the reaction from the atmospheric oxygen.

3.2. Arc discharge in deionised water The arc discharge apparatus was operated in the same way as previously described with the only difference being that deionised water was used instead of liquid nitrogen. The structure of the raw material obtained from a discharge performed at 28 V is reported in the low magnification TEM micrograph of Fig. 11 and in the lattice resolved image of Fig. 12. The deposited material is mainly

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Fig. 11. TEM image of the as-grown MWCN synthesized in water.

Fig. 13. A detail of the specimen imaged in Fig. 9 showing an open CN tip.

constituted of MWCN embedded in an amorphous carbon matrix. A few nanotubes show an open tip (Fig. 13), which can indicate a possible effect of the hydrogen resulting from thermal decomposition of water in stabilizing the open tip structure. Similarly to what was observed on the nanotubes synthesized under liquid N 2 , high sensitivity EELS does not provide evidence for the presence of elements different from carbon. The main differences

relative to the discharge performed in liquid nitrogen can be summarized as follows:

Fig. 12. Lattice resolved image of a MWNT tip synthesized in water.

(a) The current flowing in the water submerged electric arc is smaller than in liquid nitrogen, so that generally slightly higher voltages are required to keep the current at a fixed value. The useful range for the nanotube synthesis is then shifted between 25 and 30 V, with the same general features of the process previously observed. For lower voltages, mainly amorphous carbon is found, while for higher voltages, when the evaporating material is no longer deposited at the cathode, nanoporous carbon is the main result of the process. (b) The raw material is softer suggesting a less dense structure that can result from a more efficient cooling of the growing cathodic deposit. Moreover, the spurious phase is in this case constituted mainly by amorphous carbon with a minor presence of multiwall polyaromatic carbon shells. This feature can still be related to the different synthesis temperature and to a more efficient cooling of the free surface. In fact, amorphous carbon is the main reaction product when the cathode temperature is too low, while multiwall carbon shells can be one of the final or intermediate structures during the nanotube degradation process. In fact, they constitute one of the dominant phases found in the buried layer of the raw material synthesized in liquid nitrogen. This feature can play an essential role in the purification process. In fact, all the purification routes are based on the

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selective oxidation in air or in a suitable chemical reagent, of this carbonaceous material under the hypothesis that the closed network structure of CN can be more resistant to oxidation than an open structure like amorphous carbon or nanocrystalline graphite [16]. It is quite evident that the amorphous carbon can be considered less resistant to oxidation than the carbon shells in this frame, which show, at a first approximation, a closed structure similar to the CN tips.

3.3. Raw material purification The raw materials resulting from both types of experiments have been submitted to selective air oxidation in an open furnace at 500 8C in order to set up an upper limit to the nanotube yield of the processes. The temperature chosen was a reasonably efficient but not too severe oxidation temperature for polyaromatic carbon. The material synthesized under water has been more closely investigated considering the above arguments relative to the expected lower oxidation resistance of the spurious phases. Microstructural TEM investigations showed the onset of the nanotube structural damage after 1 h of treatment as evidenced in Fig. 14. The structural damage begins generally at the nanotube tips or at some structural defect and the final result is the transformation of the ordered CN structure into a disordered one. For longer times the degradation process is still more evident and many CN without the tips and with a strongly deformed shape can be found. Therefore, we arbitrarily decided to limit the oxidation time to 0.5 h.

With the purpose of making the oxidation process more efficient by reducing the particle size in order to increase the specific surface area and to enhance the diffusion of air, we submitted the raw material to low energy ballmilling in a device equipped with an agate vial and one agate ball for a few hours. The weight loss for 0.5 h at 500 8C was then used as an indicator of the nanotube yield of the process. When the arc discharge is operated at 28 V the weight loss was about 50 wt.%. We want to stress that this figure has to be considered just an upper limit for the nanotube yield while a correct estimate of the process yield requires a careful setting up of the purification procedure, a subject which is beyond the purpose of this paper.

4. Conclusions We carefully analyzed the synthesis of MWCN by arc discharge operated in liquid media and in particular in liquid nitrogen and deionised water. Liquid nitrogen provides a good environment for the MWCN synthesis, but the strong evaporation caused by the operation of the arc discharge does not allow a good thermal exchange between the synthesized material and its surroundings. The loose structure resulting from the growth process is then modified in a short time, probably by the combined effect of high temperature and high current density. The resulting material is constituted by a hard compact phase where most of the MWCN have a distorted morphology and a degraded structure. On the contrary, liquid water besides providing a suitable environment also provides the thermal conditions necessary to retain good quality nanotubes in the raw material, while the reactivity of the water with hot carbon does not appear to have any major effects on the reaction. Moreover, the spurious phase is mostly constituted of amorphous carbon, which is probably more easily eliminated by thermal oxidation, potentially enhancing the possibility of setting up a suitable purification process by selective oxidation. We want to stress that at the time when this paper was submitted for publication no previous results on successful synthesis of MWCN by water submerged arc discharge were reported in the literature.

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Fig. 14. High resolution TEM image showing the beginning of the structural damage at the tip of a CN oxidized for 1 h at 500 8C.

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