Growth of Highly Compressed and Regular Coiled Carbon Nanotubes

Apr 10, 2008 - All the CCNTs had a uniform shape with a sharp radius of curvature and a small coil pitch. The growth of such CCNTs may be related to t...
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Growth of Highly Compressed and Regular Coiled Carbon Nanotubes by a Spray-Pyrolysis Method Jian N. Wang,* Lian F. Su, and Zi P. Wu School of Materials Science and Engineering, Shanghai Jiao Tong UniVersity, 800 Dong Chuan Road, Shanghai 200240, P. R. China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1741–1747

ReceiVed July 19, 2007; ReVised Manuscript ReceiVed January 14, 2008

ABSTRACT: Cupric acetate dissolved in ethanol was applied for the first time for synthesizing highly compressed and regular coiled carbon nanotubes (CCNTs). This was done by spray pyrolysis of the ethanol solution intermittently supplied into the reaction zone at 850 °C. The grown CCNTs were in high purity and high yield and consisted of well-defined graphitic layers and fine regular coils. All the CCNTs had a uniform shape with a sharp radius of curvature and a small coil pitch. The growth of such CCNTs may be related to the use of Cu as the catalyst and the periodical supply of the carbon source. That is, such catalyst particles may be in a melted or semimelted state and susceptible to the fluctuation of growth conditions at the experimental temperature. Because of the simplicity of the present technique, it may be suitable for large-quantity production of CCNTs to be used in wide areas. 1. Introduction Carbon nanotubes (CNTs) are now known to have wide applications because of their superior mechanical, physical and chemical properties.1–3 Recently, coiled carbon nanotubes (CCNTs) have received intense studies on their special properties and possible unique applications.4,5 As certain types of CCNTs may have comparable mechanical properties with catalytically grown multiwalled CNTs (MWCNTs), the coiled shape can solve one of the most crucial problems of reinforcement by CNTs:6 a coil provides excellent load transfer, without the need to damage the graphitic network of the CNT by covalent functionalization. To fully exploit this advantage, the entangling of coils has to be avoided and a good dispersion in the matrix has to be achieved. Novel devices and sensors can be built using CCNTs, which can have sensitivity as high as femtograms.7 Also, CCNTs may find applications in nanoelectromechanical systems (NEMS) and when the patterned growth of CCNTs is realized, this may constitute the basis for tactile sensors with very high sensitivity and high resolution.8 CCNTs were first predicted to exist in the early 1990s.9 Such nanotubes are generally described by coil diameter (or tube diameter) and coil pitch. The coil pitch is the distance between adjacent corresponding points along the axis of the helix, as shown in Figure 1. On a microscale, periodic incorporation of pentagon and heptagon pairs into the predominantly hexagonal carbon framework for creating positively and negatively curved surfaces, respectively, can generate a carbon nanotube with a regular coiled structure.10 CCNTs were first observed by Zhang et al. in 1994.11 They observed multiwalled CCNTs with inner and outer diameters of 15–20 nm in the sample grown by catalytic chemical vapor deposition (CVD) of acetylene over silica-supported Co catalyst at 700 °C. Since then, three major CVD-based methods have been investigated with the goal of finding the optimal condition for obtaining CCNTs on a large scale: support-based, substratebased, and template-based. The catalyst used in the first technique was supported by silica,12–15 Al2O3,16 or aluminophosphate.17 The mainly used catalysts were Fe, Ni, Co, or their organic compounds.12–17 Grobert et al.18 observed coiled nanotubes and nanofibers in the preparation of aligned carbon * To whom correspondence should be addressed. Tel.: 86-21-62932015, E-mail: [email protected].

Figure 1. Schematic illustration of the coil pitch and coil diameter of a CCNT.

Figure 2. Schematic diagram of the apparatus used for the synthesis of present CCNTs.

nanotube bundles and films by pyrolysis of solid organic precursors on laser patterned catalytic silica substrates. Highyield production of multiwalled CCNTs was attempted on an indium tin oxide (ITO) glass substrate by Nakayama’s group in Japan.19 But the resultant individual carbon coils usually consisted of two or more ordinary nanotubes and each of them grew with its own diameter and pitch. Another advance in the synthesis of CCNTs was made by Hou et al.,20 in which the CVD process was accomplished by pyrolysis of a vapor mixture of Fe(CO)5 and pyridine or toluene on a silicon substrate at a temperature of 1050–1150 °C under H2 flow. Bai21 synthesized CCNTs at 650 °C by using acetylene as the carbon source and porous aluminum oxide as the template. An additional template CVD growth of CNTs was reported by Zhong et al.22 The aligned straight nanotube arrays were used as a template for controlled synthesis of CCNTs. In addition to the dominating CVD growth of multiwalled CCNTs, coils have also been found with other methods that

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Table 1. Experimental Conditions (Temperature, Flow Rate of Ar Gas, And Feeding Rate of Spray Solution) and Corresponding Products 30 mL h-1 feeding rate

50 mL h-1 feeding rate

temperature

40 L h-1 flow rate

80 L h-1 flow rate

40 L h-1 flow rate

80 L h-1 flow rate

800 °C 850 °C 900 °C

nothing regular CCNTs irregular CCNTs

nothing regular CCNTs irregular CCNTs

straight CNTs Y-junction and straight CNTs Y-junction and straight CNTs

straight CNTs Y-junction and straight CNTs Y-junction and straight CNTs

had been proven to be effective for producing straight nanotubes: the laser evaporation method23 and the opposed flow flame combustion method.24 It is also worth mentioning that CCNTs were observed by accident in the electrolysis of molten salt (NaCl) at 810 °C in the synthesis of single-walled and multiwalled nanotubes.25 But these techniques need more complicated equipment or higher energy consumption, and just very few CCNTs could be observed in the sample. The CVD method is still dominant for growing CCNTs at present. However, it has many drawbacks that hold back the large-scale production for various applications. The complicated process of catalyst preparation makes the supported CVD difficult to be scaled up. That is, the fact that CCNTs can be collected only on supported catalyst particles or substrate or template is not favorable for continuous production. Some hazardous chemicals were involved in previous CVD methods which may raise environmental issues. A further problem is that the CCNTs were obtained usually as a byproduct of traditional straight multiwalled nanotubes. Therefore, it is still desired to explore a simple, high-yield, and reliable method for the growth of regular CCNTs. Here, we report a spray-pyrolysis method for producing highly compressed and regular CCNTs. That is, cupric acetate was dissolved in ethanol and the solution was sprayed into the reaction zone. The nanotubes consisted of good graphitic layers with fine regular coils. The yield of CCNTs was as high as 90% (by volume). The continual feeding of carbon resource and catalyst is favorable for growing CCNTs continually. 2. Experimental Section

more than 90% of the observed CNTs were CCNTs with slightly different coil diameters and pitches. Figure 3a shows a typical TEM micrograph of this sample. The coil diameters are different from each other from about 30 to 80 nm. Most CCNTs are highly compressed and have very small coil pitches. In addition to CCNTs, there are a few straight CNTs with diameters of about 10–20 nm. The black particles in the as-grown sample are catalyst particles. This will be confirmed by the following HRTEM results. The CCNT has a clear tubular structure, and the coil diameter is so small that it approaches the diameter of the tube. The regular and bright circular spots appearing on the nanotube are coil nodes and are due to the small coil pitch of this particular type of highly compressed CCNTs. Figure 3b shows two crossed CCNTs with a similar coil pitch and diameter. Arrow 1 points at a nanotube tip from which the catalytic particle has fallen off and the tubular structure can be clearly observed. Arrow 2 illustrates the presence of a catalyst particle with a diameter of about 20 nm at the end of a CCNT. Figure 4a shows a segment of a long CCNT which structure can be illustrated by the inset three-dimensional (3D) model. The coil pitch of the CCNT is approximate to the diameter of the tube and the coil diameter is about 2 times the diameter of the tube. The nanotubes with a very sharp radii curvature are called “zigzag shaped nanotubes” in the previous report for their peculiar structure.26 For the small pitch and coil diameter, the CCNT appears to be like two lines of circular spots arranged tightly together. Figure 4b and Figure 4c show TEM images of CCNTs observed at different angles. Their structures are also illustrated by the inset 3D models. Although these CCNTs seem to have different structures under TEM, all of them are highly

CCNTs were prepared by the experimental setup shown in Figure 2. The experimental setup only consisted of an electric furnace, a quartz tube (30 mm inner diameter), a sample collector, and a quartz capillary used for spraying. Cupric acetate (AR, produced by Shanghai Reagent Co.) was dissolved in ethanol at a given concentration of 15 g L–1. In a typical experiment, the quartz tube was flushed with Ar flow first in order to eliminate oxygen from the reaction chamber and then heated to a high temperature (800, 850, or 900 °C). After the tube was held at this temperature for 15 min, Ar flow was initiated at a rate of 40 or 80 L h-1, and the ethanol solution dissolved with cupric acetate was supplied by an electric squirming pump (DDB-320, Shanghai ZhiXin instrument Ltd.). The supplying rate was adjusted to be 30 or 50 mL min-1. After 1–3 h, the supply of ethanol was terminated, and the reaction chamber was cooled gradually to room temperature. All experimental conditions and corresponding results are listed in Table 1. Transmission electron microscopy (TEM, JEM-100CX) was used to study the microstructure and morphology of the as-grown sample. High resolution TEM (HRTEM, JEM-2010F) was used to study the lattice structures of CCNTs and catalyst particles. X-ray diffraction (XRD) experiments were also conducted on the as-grown sample. The X-ray diffractometer (Bruker D8 Advance, Bruker AXS, Germany) was operated at 40 kV and 40 mA. Nickel-filtered Cu KR radiation was used in the incident beam. Thermal Gravimetric Analysis (in air) (Pyrisdiamond TG/DTA) and Raman spectroscopy (JOBLN-YVON T64000) were used to further characterize the purity and graphitic structure of the as-grown sample, respectively.

3. Results The sample prepared at 850 °C and 30 mL min-1 was extensively examined in TEM. Statistic estimation showed that

Figure 3. TEM images of the as-grown sample (a) and two crossed CCNTs with a similar coil pitch and diameter (b). Note the presence of a catalyst particle at the end of tube 2 but not at that of tube 1 in (b).

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Figure 4. (a-c) TEM images of CCNTs and their 3D models (insets) and (d) HRTEM image of the tube wall.

compressed with small coil pitches and coil diameters. Figure 4d reveals that the walls of these CCNTs consist of well-defined graphitic layers. The interlayer spacing was measured to be about 0.34 nm, and thus the graphitic layers may be the graphite planes of (002). Figure 5 illustrates the HRTEM images of the as-grown sample. Figure 5a is for a CCNT with a diameter of about 30 nm and a wall thickness of about 8 nm. The tip of the tube is open, and the catalyst particle may have been removed at a certain stage of sample handling. Figure 5b is the enlarged vision of the tip of the CCNT shown in Figure 5a. From the micrograph, the graphitic layers can be clearly seen, and there is just very little amorphous carbon on the surface of the wall. It should be noted that the graphitic structure observed in the walls of the CNTs appears irregular and even wavy at some places. This is not necessarily caused by the widespread presence of defects but may result from the curvature of the CCNTs. This is particularly true for the present CCNTs, which are highly compressed with small coil pitches. The black particles observed in Figure 5a are enlarged and further shown in images c and d in Figure 5. Because lattice fringes can be observed, these particles could not be amorphous carbon, but may be catalyst particles. Their diameters are 20–30 nm, which correspond to the diameters of CNTs. The interlayer spacing is about 0.21 nm. Thus, the imaged lattice may correspond to the (111) planes of Cu. Figure 6a shows the XRD pattern of the as-grown sample. Three phases are identified: graphite, Cu, and Cu2O. The peak of graphite is very strong and sharp, suggesting that the graphitic structure is dominating in the sample. Except for the strong peak for graphite, the peaks of Cu are also strong, but those for Cu2O are relatively weak. This observation suggests that the catalyst particles observed in the sample may be mainly Cu with a few being Cu2O. The amount of Cu contained in the sample may be estimated from the TGA result presented in Figure 6b. After complete oxidation at temperatures higher than 600 °C, about

12 wt % of the starting material remained. Assuming that the remained material is CuO, the amount of Cu contained in the starting material is estimated to be about 9.6 wt %. Inspection of the TGA curve also shows that the weight loss at temperatures lower than 400 °C is minor. This observation suggests that defects contained in the CCNTs and amorphous carbon included in the as-grown sample should be very limited. Otherwise, significant weight loss would have been observed even at temperatures lower than 400 °C. The graphitic structure of the present CCNTs can be further verified by the Raman spectroscopic result shown in Figure 6c. The Raman spectrum consists of two peaks at high frequencies. The first peak appears at about 1600 cm-1 as the so-called G peak related to E2g graphite mode, and the second peak at around 1400 cm-1 as the D peak induced by a defect-related vibration mode.20 The intensity ratio of the G and D peaks (IG/ID) is about 2.9. This observation is an additional evidence for that the present CCNTs have a good graphitic structure. If the CCNTs contained a large amount of defects or even were made up of disordered carbon atoms, the intensity ratio would be much smaller than 2 as observed for some multiwalled CNTs, coiled CNTs and carbon nanofibers.20 From Table 1, it can be seen that the flow rate of Ar gas has a little influence on CNT structure under the present experimental conditions. But a high flow rate may decrease the yield of CCNTs. The main influencing factors of CNT structure are temperature and the feeding rate of spray solution. Regular CCNTs can be grown only at a feeding rate of 30 mL h-1 and temperature of 850 °C. Figure 7a is a TEM image of the sample grown at 850 °C with a feeding rate of 50 mL h-1. No CCNTs can be found except for straight CNTs and Y-junction CNTs. Figure 7b shows a TEM image of the sample grown at 900 °C with a feeding rate of 30 mL h-1. The nanotubes grown at this temperature have larger diameters and are coiled in disorder. Thus, the feeding rate is a decisive factor to growing CCNTs,

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Figure 5. HRTEM image of CCNTs. (a) Highly compressed CCNT with an open tip; (b) enlarged vision of the tip in (a); (c) catalyst particle with a diameter of about 35 nm; (d) lattice fringes of the particle.

and temperature can influence the structure of CCNTs, being regular or irregular. 4. Discussion The precursor of catalyst used in the present experiment was cupric acetate which was dissolved in alcohol and supplied to the high-temperate reactor by electric squirming pump. The solution was carried to the reaction zone by Ar gas and decomposed under high temperature. Cu and Cu2O were the decomposition products under the condition of inert gas protection.27 Ethanol can act as cosolution which can reduce the temperature to obtain pure-phase Cu from cupric acetate more easily. In the present experimental condition, Cu and Cu2O were both found in samples with Cu being dominating. Although bulk Cu is considered as a catalytically inactive metal, the results using nanoparticles of Cu as a catalyst to grow carbon nanotubes and nanofibers have been reported in previously studies.28–30 To increase the electron density of Cu catalyst, alkali-element additives are usually needed to dope the catalyst.31 Vander Wal et al.28 reported that the reactive activity of copper for synthesis

of carbon nanofibers could be improved by accepting electron density from the support with Lewis base character. The properties of copper nanoparticles can also be enhanced by their small physical sizes. The electronic density of states associated with surface atoms will be altered, and unusual shapes or facets may exist at the surface. Thus, the catalytic activity of the metal may depend on the size and shape of the particle. This suggests that CNTs can be synthesized over a Cu catalyst by modifying its activity. To date, several growth mechanisms have been proposed for the carbon coil formation. On the basis of the concept of a spatial-velocity hodograph, Amelincks et al.10 proposed that the mismatch between the extrusion velocities of carbon from the catalyst grain results in the helix. Fonseca et al.32 suggested a growth mechanism on a catalyst particle at a molecular level using the heptagon-pentagon construction suggested by Dunlop33 and explained the formation of curved nanotubes, tori, or coils. Motojima and Chen34 postulated a 3D growth mechanism of carbon coils based on the “anisotropy of carbon deposition” theory, which relies on an assumption that the different catalytic activities of crystallographic facets define the geometry of CNTs.

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Figure 7. TEM images of samples synthesized at 900 °C and a feeding rate of (a) 50 mL h-1 (Y-junction CNTs) or (b) 30 mL h-1 (irregular CCNTs). Note the presence of a catalyst particle at the tip of CNTs in (b).

Figure 6. (a) XRD pattern, (b) TGA, and (c) Raman spectrum for the as-grown sample (850 °C).

Also based on the heptagon-pentagon construction theory, Lu et al.14 proposed a helix formation mechanism, which involves carbon core formation centering on a catalytic particle followed by carbon helices growth. However, without altering the hexagonal motifs, Ramachandran and Sathyamurthy35 demonstrated a simple model for the construction of coiled CNTs in terms of a slight shifting of carbon atoms between adjacent layers of carbon atoms running perpendicular to the tube axis. Although the mechanism at the molecular level changes from one model to the other, the process at the nanoscale is generally believed to consist of nucleation and nonuniform growth of carbon tubes from the catalytic nanoparticle as a result of the variation of the growth condition. It is wellknown that the growth of CNTs is influenced by many factors, including the homogeneity of the catalytic activity of the particle, the supply of the carbon source and the distribution of carbon atoms in the catalytic particle. If any of these factors changes, the growth condition and process could be altered and an irregular CNT would form.

Since Cu has a melting point (∼1085 °C for bulk Cu) lower than that for Fe (∼1538 °C for bulk Fe), Cu nanoparticles may be in a melted or semimelted state at the present experimental temperature. Such melted particles may be favorable for dissolution of carbon atoms and the resultant Cu-C alloy may be susceptible to the fluctuation of external conditions.36 That is, any changes in temperature and carbon supply could change the local growth conditions such as the concentration gradient of carbon atoms and catalytic homogeneity in the particle and thus the uniformity of the growth of CNTs. This may be the reason why CCNTs rather than conventional straight ones easily formed in the present experiments when compared with previous ones using Fe or other transitional metals as the catalyst. The catalyst particle serves as a nucleation center to dissolve the gaseous carbon atoms decomposed from ethanol (Figure 8a). Oversaturation will result in an extrusion of a tube in the form of a hexagonal carbon network from one surface of the catalyst particle (Figure 8b). When the local growth condition is altered, the extrusion as a hexagonal network may be violated by introducing nonhexagons. It is known that incorporation of nonhexagons such as pentagons and heptagons (P-H) into the hexagonal network can induce curvature into the tube.14 This is seen as the change in local growth rate or direction (Figure 8c). Continuous change in local growth rate would result in a coiled tube (Figure 8d). In practice, the nucleation rate of carbon rings can vary with the fluctuation of experimental conditions, such as the flow rate of carbon source and reaction temperature. The continuous growth of the CCNTs shown in Figures 3 and 4 requires a continuous supply of P-H pairs at a constant rate in order to maintain the coiled shape. In the present experiment, the spray solution was pumped into the reaction zone regularly through a quartz capillary (Figure 2). At the feeding rate of 30 mL h-1, it was observed that the solution flowed slowly in the fine quartz tube. When the head of the solution reached a high enough temperature, it vaporized abruptly and expanded into the reaction

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curvature and a small coil pitch. The external diameter of the CCNTs was about 30–50 nm with the inner diameter being about 10–20 nm. The growth of such CCNTs may be related to the use of Cu as the catalyst. That is, such catalyst particles may be in a melted or semimelted state and susceptible to the fluctuation of growth conditions induced by the periodical supply of the carbon source. Because the CCNTs were produced continuously with a high yield in the sample and no hazardous chemicals were involved, the present technique may be suitable for large-quantity production. Acknowledgment. J.N.W. thanks The Outstanding Youth Fund from The National Natural Science Foundation of China and the National 863 Project of 2007AA05Z128 from the Ministry of Science and Technology of China. Figure 8. Schematic diagrams for growth of CCNTs. (a) Dissolution of C atoms into Cu; (b) precipitation of C from Cu and growth of a straight CNT; (c) curving of CNT due to different growth rates at locations 1 and 2; (d) further curving and formation of a coiling node; (e) final CCNT with a catalyst at the tip.

chamber. Then, the evacuated region in the fine quartz tube was to be filled and the vaporization would take place again. Thus, the supply of the carbon source was actually intermittent, leading to a periodical variation of the experimental condition. This may be the other essence, in addition to the use of Cu catalyst, for the growth of the present regular CCNTs. Because the Cu particle has a low melting temperature and may be in melted or semisolid state at the experimental temperature, the gradient of carbon concentration and homogeneity of catalytic activity can be easily and rapidly influenced. Thus, it can be envisaged that the creation rate of P-H pairs is very fast and the interpair distance is very small. It may be under such circumstances that the present highly compressed CCNTs with circular nodes result (Figures 3 and 4). If the creation rate of P-H pairs is very slow and they are distributed periodically, slightly curved nanotubes would be formed as observed in many previous studied where transition metals, which are less susceptible to the fluctuation of external conditions, are used as the catalyst.9–22 The observation of CCNTs with irregular coil diameter and coil pitch shown in Figure 7b indicates that the periodical variation of the growth condition might be destroyed, and the creation rate of the P-H pairs was fluctuant at 900 °C. When the feeding rate was raised to 50 mL h-1, however, the flow of the solution in the quartz capillary was found to be continuous, and an intermittent supply of the carbon source was not observed. This is the case of the growth of the observed straight CNTs (Table 1) as the local growth condition was stable and no P-H pairs were created. When there is a slight change in growth condition, a small number of P-H pairs may be introduced, leading to nucleation of a new branch on the straight CNT. This is the mechanism proposed for the growth Y-junction CNTs.36 For the growth of a new branch, however, there must be enough supply of carbon atoms to ensure that the new nanotube can form and grow continually. This may interpret the formation of Y-junction CNTs (Figure 7a) when the feeding rate was relatively high (50 mL h-1).

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(19) (20) (21) (22) (23) (24) (25) (26) (27)

5. Conclusions In conclusion, highly compressed CCNTs can be prepared by spray pyrolysis of an ethanol solution containing cupric acetate as the catalyst precursor at a temperature of 850 °C. All CCNTs have a uniform shape with a sharp radius of

(28) (29) (30)

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