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Preparation of carbon nanotubes from vacuum pyrolysis of polycarbosilane
Materials Science and Engineering B 106 (2004) 275–281
Preparation of carbon nanotubes from vacuum pyrolysis of polycarbosilane Shyankay Jou∗ , Chao Ken Hsu Materials Science and Technology Program, Graduate School of Engineering Technology, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 10672, Taiwan, ROC Received 29 January 2002; accepted 29 September 2003
Abstract Carbon nanotubes (CNTs) were synthesized by vacuum pyrolysis of two types of polycarbosilane (PCS) with iron nano-particles between 800 and 1100 ◦ C. Straight nanotubes were obtained from low molecular weight (990 g/mol) PCS whereas curled nanotubes were derived from medium molecular weight (1290 g/mol) PCS. Diameters of these straight and curled nanotubes were between 5 and 20 nm. The mechansim of condensed phase growth of carbon nanotubes was discussed. Electron emission capability of these carbon nanotubes increased with their pyrolyzing temperature. The electric fields required to emit a current density of 10−2 A/cm2 from the straight nanotubes being pyrolyzed at 800, 900, 1000, and 1100 ◦ C were 1.17, 0.73, 0.67, and 0.33 V/m, respectively. © 2003 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Field emission; Polycarbosilane; Pyrolysis
1. Introduction Since carbon nanotubes (CNTs) were investigated by Iijima [1], their unique properties and potential applications have been studied substantially. Carbon nanotubes can be used as quantum wires [2,3], field emission cathodes [4–6], electrochemical energy storage units [7] and hydrogen storage materials [8,9]. At present, carbon nanotubes have been produced primarily by arc discharge [1,10–12], laser ablation [13,14], catalyzed chemical vapor deposition (CVD) [15–18] and plasma assisted processes [19–21]. In these processes carbon nanotubes are formed through the gas–solid reactions, from which carbon vapor condenses into solid products directly [1,10–14] or gaseous hydrocarbon species crack into solid carbon through catalytic decomposition [15–21]. On the other hand, formation of carbon nanotubes in condensed phase has rarely been reported. Such examples include converting diamond-like carbon [22] and fullerenes [23] into carbon nanotubes by introducing metal catalysts in the reactions. Carbon nanotubes have also been generated
∗ Corresponding author. Tel.: +886-2-27376665; fax: +886-2-27376799. E-mail address: [email protected] (S. Jou).
0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.09.039
using the thermal segregation of carbon from aluminum carbide (Al4 C3 ) in the presence of metal catalysts [24]. Iron, cobalt, nickel, and their alloys show substantial catalyzing effect on CNTs’ growth in the catalyzed CVD [15–18] and plasma assisted processes [19–21]. The catalyzing effect has been observed in the study of the vapor-grown carbon fibers (VGCFs) earlier than the discovery of the CNTs [25–27]. Carbon dissolution and subsequent precipitation on the surface of the catalysts is considered as one possible mechanism for both the VGCFs and the CNT growth in the gas–solid reactions [26–29]. A similar catalyzing effect is also proposed for the CNT generation in condensed phase as mentioned previously [22–24]. On the other hand, carbon nano-structures other than the CNTs have been produced in the condensed phase by the annealing of materials rich in carbon without the presence of catalyst. For example, carbon onions have been formed from precipitation of carbon in carbon-implanted silver [30,31] and copper [32]. Carbon precipitation has also been observed in carbon-rich silicon carbide (SiC) upon high temperature annealing [33]. In comparison to the CNTs’ growth and the carbon precipitation in condensed phase, metal catalysts play an important role governing the microstructure of the resultant carbon products. Therefore, it is possible to grow CNTs from a mixture composed of
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carbon-rich material and nano-sized catalysts in condensed phase. Polycarbosilane (PCS), a commercial silicon carbide precursor, is selected as the source material to grow nanotubes for this study because its pyrolyzed product is rich in carbon and because it is easy to blend with nano-sized catalysts in solutions. The PCS was primarily used for making SiC fibers in industry and fine fibers with diameters as small as 6 m were synthesized by melt-spinning, curing, and pyrolysis [35]. A carbon to silicon ratio as high as 1.56:1 has been obtained in the silicon carbide fiber that was formed from pyrolyzing PCS in Ar [34]. Moreover, carbon clusters have been observed from aggregation of the excess carbon in the PCS-derived SiC at a temperature above 1100 ◦ C [33]. On the other hand, Wang et al. observed crystallization of -SiC inside the SiC fibers by adding Ni or Fe nano-particles in the PCS precursor and pyrolyzing them between 1000 and 1400 ◦ C [36,37]. Otoishi and Tange grew SiC whiskers from pyrolyzing a mixture of the PCS, milled carbon fibers together with iron and nickel between 1100 and 1300 ◦ C [38]. Yet, CNT was not observed in above studies regarding the pyrolysis of PCS [33–37]. However, it is still possible to grow nanotubes from the pyrolysis of PCS. The growth temperatures of the CNTs in the catalytic decomposition of hydrocarbons are found between 800 ◦ C and the eutectic temperature of the metal–carbon mixtures, such as 1153 ◦ C for Fe–C and 1326.5 ◦ C for Ni–C [29]. The temperatures of above SiC fibers and whiskers could be too high for the CNTs’ growth. Therefore, it is of interest to study the pyrolysis of PCS with nano-sized Fe particles below 1153 ◦ C and the analysis of the nanotubes thus formed.
2. Experimental details Both low molecular weight (990 g/mol, L-type) and medium molecular weight (1290 g/mol, A-type) polycarbosilane (Nippon Carbon Inc.) with a structural formula –(HSiCH3 –CH2 )n – were used as the source materials. Iron nano-powder (Cerac Inc.) with an average particle diameter of 17 nm was utilized as the catalyst. About 0.2 g PCS powder and 0.08 wt.% iron nano-powder were mixed in 5 cm−3 toluene (HPLC grade) as the starting materials. The mixtures were spun coated onto silicon wafers at a speed of 50 H3 in air and subsequently pyrolyzed in a vacuum furnace. The samples were heated at a rate of 10 ◦ C/min up to temperatures between 700 and 1100 ◦ C, held there for 1 h and then cooled down in the furnace. During the heating step, the samples were kept at 500 ◦ C for 1 h to drive out small organic volatiles. The pressure was kept lower than 1.33 × 10−3 Pa using a diffusion pumping system during the high temperature pyrolysis and cooling down. After removing the samples from the furnace, the peeled films or loose powder were collected on the Formvar-coated copper grids and examined by transmission electron microscopy (TEM) (JOEL JEM2000-FX). In order to calculate the yields of
the CNTs, about 0.1 g of the pure PCS powder or dry mixtures of the PCS and 0.08 wt.% iron were placed inside a platinum crucible and pyrolyzed in the vacuum furnace at 1000 ◦ C for 1 h. The dry mixtures were prepared by baking the PCS–catalyst mixture solutions in the vacuum furnace at 95 ◦ C for 2 h to drive out the solvent. Field emission measurements were performed in a vacuum chamber at a pressure below 6.65 × 10−4 Pa using a turbomolecular pumping system. The Si substrate with continuous film was glued onto an aluminum holder with silver paste then grounded as cathode. An iron anode 2.3 mm in diameter was kept at 30 m away from the surface of the sample and powered by a Keithley 237 source-measurement unit to collect the emission current. The measurements were carried out in a range of 0–150 V and from 0.1 nA to 10 mA.
3. Results and discussion Clusters of nano-rods or nanotubes were found randomly distributed over the pyrolyzed specimens. Fig. 1a shows a TEM image revealing the general view of nano-rod bundles produced from pyrolyzing a mixture of L-type PCS and iron nano-particles at 700 ◦ C. These nano-rods are straight with diameters between 5 and 20 nm. Their lengths vary from a few hundred nanometers to a few micrometers. Hollow structure is not observed in these nano-rods. A few nano-rods are adhered with particles and film-like materials. Fig. 1b is a TEM image of nanotubes from the mixture of L-type PCS and iron nano-powder pyrolyzed at 900 ◦ C. Most of these hollow nanotubes have an outer diameter around 8 nm. These straight nanotubes are densely packed and covered with film-like materials. A TEM image of the nanotubes from the 1100 ◦ C-pyrolyzed product is shown in Fig. 1c. The outer diameters of these straight nanotubes are between 9 and 17 nm. A few nanotubes have smaller outer diameters at their tips as compared to the outer diameters of their uniform tubular bodies. All the nano-rods and nanotubes derived from the L-type PCS are straight and stretch out toward free space. The film-like materials observed in the 700 and 900 ◦ C-pyrolyzed products are not seen in the 1100 ◦ C sample. An enlarged TEM image of the nanotubes from the 1100 ◦ C-pyrolyzed sample is shown in Fig. 1d. A nanotube with a sharp tip and wide body is seen in the middle left of the figure. The inner diameter of this nanotube is about 1.9 nm at the tip and 2.5 nm in the tube body. Outer diameter and wall thickness at the tip are 8.6 and 3.35 nm, respectively, and increased to 15.4 and 6.45 nm in the tube body. An electron diffraction pattern of the 1000 ◦ C-pyrolyzed nanotubes sample consisting of five diffraction rings is shown in Fig. 2a. The first, third, fourth, and fifth rings coincide with graphite(0 0 2), (1 0 0), (0 0 4), and (1 1 0) planes with plane spacing of 0.34, 0.21, 0.17, and 0.12 nm, respectively [39]. Intensity of the second ring is weak if compared to the other four rings. An electron diffraction pattern of the 1100 ◦ C-pyrolyzed sample is shown in Fig. 2b. There are
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Fig. 1. Typical TEM images of nano-rods and nanotubes from pyrolyzing mixtures of L-type PCS and iron nano-particles at (a) 700 ◦ C, (b) 900 ◦ C, and (c) 1100 ◦ C. (d) An enlarged TEM image revealing a nanotube with sharp tip and wide body.
four diffraction rings that match with graphite(0 0 2), (1 0 0), (0 0 4), and (1 1 0) reflections. The second-diffraction ring in Fig. 2a is not seen here. Based on the diffraction results in Fig. 2a and b and TEM images in Fig. 1a–c, the four diffraction rings correlate to the major tubular phase. Therefore, it can be constructed that these tubular materials are the CNTs. The second-diffraction ring in Fig. 1a probably originates from the particles or the film-like materials. According to the process conditions in the literature regarding the pyrolysis of pure PCS, amorphous or crystalline SiCx was produced below or above 1000 ◦ C [33,34] and its crystallinity can be improved by adding Fe or Ni particles [36,37]. The plane spacing of the second ring in Fig. 2a is about 0.26 nm and matches with a reflection plane in several ␣-SiC polymorphs [40]. Because these film-like materials may not came into contact with the catalyst during the pyrloysis, their composition and structure are expected to be similar to the PCS-derived SiC [33,34]. As a result, carbon nano-rods and nanotubes were grown from the PCS with the aid of Fe nano-particles acting as catalysts. The mechanism to the CNTs growth from the pyrolysis of PCS with the aid of catalysts is considered similar with that in the catalyzed chemical vapor deposition [25–29], except
that the carbon feedstock is from the condensed phase here rather than from the gaseous phase. According to the study on the PCS pyrolysis in inert environment, methane and hydrogen are released from breaking the Si–CH3 and Si–H bonds above 490 ◦ C whereas H2 evolves between 880 and 1050 ◦ C, arising from breaking C–H bonds in Si–CH2 –Si skeleton in the intermediate product [34,41]. Since the samples in our study were first cracked at 500 ◦ C which is below the CNTs formation temperatures [29], the small hydrocarbon species released from the PCS may not contribute to the formation of CNTs directly, but will rather be pumped out from the vacuum system. The carbon source for CNTs growth is possibly supplied from the Si–CH2 –Si skeleton, which will decompose into carbon species on the surface of the Fe catalysts at a high temperature. This reaction is similar to the reaction of diamond-like carbon with a metal catalyst [22] or the precipitation of graphite from a solid state reaction between SiC and Fe [42,43]. Therefore, the growth mechanism of the CNT will be similar to the catalyzed CVD, in which carbon diffuses into the catalyst particle then precipitates out as solid fibers or hollow nanotubes [26–29]. The carbon nano-structures are controlled by both the rate of carbon diffusion into the catalyst and the rate
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Fig. 2. Electron diffraction patterns of nanotube bundles from pyrolyzed mixtures of L-type PCS and iron nano-particles. (a) The 1000 ◦ C-pyrolyzed sample with four diffraction rings which coincide with graphite(0 0 2), (1 0 0), (0 0 4), and (1 1 0) reflections. The second ring belongs to ␣-SiC reflection. (b) The 1100 ◦ C-pyrolyzed products with four diffraction rings which coincide with graphite(0 0 2), (1 0 0), (0 0 4), and (1 1 0) reflections.
of H2 reduction to eliminate carbon species on the surface of the catalyst. The H2 reduction rate might be low in the sample prepared from the L-type PCS at 700 ◦ C. Since the Si–CH2 –Si skeleton is not readily decomposed and H2 concentration is low at this temperature [34,41], the removal rate of carbon species on the catalyst surface is slower than the diffusion rate of carbon through the bulk catalyst. Therefore, a rod-like structure, similar to the VGCFs obtained at low temperature [28,44], is generated as Fig. 1a. The CNTs obtained from the sample pyrolyzed at 1100 ◦ C, as shown in Fig. 1c, is owing to a good balance between the two rates here, just as the mechanism indicated by Cui et al. on the study of the CNTs growth from hydrocarbons [28]. Nanotubes from the A-type PCS have a different morphology as compared with the CNTs from the L-type PCS. A typical electron micrograph of the 1000 ◦ C-pyrolyzed product from a mixture of A-type PCS and iron nano-particles
Fig. 3. (a) Typical TEM image of nanotubes grown at 1000 ◦ C by pyrolyzing mixture of A-type PCS and iron nano-particles. (b) An enlarged TEM image showing V-shape CNTs and adhered films. (c) The electron diffraction pattern of the 1000 ◦ C-pyrolyzed sample from mixture of A-type PCS and iron nano-particles.
is shown in Fig. 3a. The outer diameters and thicknesses of most nanotubes are around 20 and 7 nm, respectively. A few of the tubes have larger diameters. The lengths of the tubes are much larger than their diameters and some nanotubes are longer than 1 m. These nanotubes are curled and entangled
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together just like the morphologies of the CNTs from catalytic decomposition of hydrocarbons [15–18,28]. A few of the nanotubes stretch out into free space from the densely clustered areas. There are several distinct CNTs morphologies as shown in Fig. 3a. A few nanotubes meet other nanotubes at acute angles and produce V-shaped junctions at the lower left corner and other places in Fig. 3a. A Y-shaped nanotube is seen in the upper left corner of Fig. 3a. Catalyst materials are found encapsulated inside several nanotubes. This encapsulation could be due to capillary force and has also been seen in other CNTs fabrication processes [18,45] and the experiments on capillary induced filling of CNTs [46]. An enlarged TEM image of the CNTs containing the V-shaped junctions is shown in Fig. 3b. A nanotube with flat end and uniform diameter is seen near the upper right corner of the figure. It is different from the sharp tips in the CNTs from the L-type PCS, as shown in Fig. 1d. A small area of featureless films is seen between the two V-shaped nanotubes. These thin films are considered as SiC that is the same as the product of pure PCS as discussed in the pyrolyzed L-type PCS samples. The electron diffraction pattern of this sample, as shown in Fig. 3c, also contains graphite(0 0 2), (1 0 0), (0 0 4) and (1 1 0) planes, same as those rings from the L-type sample as indicated in Fig. 2b. Therefore, these curled tubular materials are the CNTs as well. However, the causes of different morphologies in the CNTs derived from the L-type PCS and the A-type PCS are not yet clear. Fig. 4a and b shows the TEM image and the electron diffraction pattern of the nanotubes prepared from the mixture of the A-type PCS and iron nano-particles at 800 ◦ C. These nanotubes are immersed inside featureless materials. The diffraction pattern of the sample shows four graphite rings, same as those ones observed in the CNTs generated from the L-type PCS at 1100 ◦ C (see Fig. 2b) and the A-type PCS at 1000 ◦ C (see Fig. 3c). There is no reflection representing other crystalline material in the sample. Therefore, the featureless film could be amorphous material. Since the source material was the PCS, the featureless film could be composed of SiCx [33,34]. It is suggested that the CNTs grow from inside the bulk PCS and catalyst mixture. When the CNTs grow longer and protrude out toward free space, some SiCx or residual PCS still adhere to the CNTs surface. The raw yields of the CNTs from the pyrloysis of the Aand L-type PCS with 0.08 wt.% Fe are 24 and 2.2%, respectively. They are smaller than raw yields of SiC from the pyrolysis of pure PCS, ranging from 55% for the A-type and 23% for the L-type PCS in this study. It was reported that most weight loss of the PCS during the pyrolysis happened between 500 and 800 ◦ C by the release of CH4 and H2 [34,41]. According to the yield data, more hydrocarbon gas was released but less CNTs were produced from the L-type PCS as compared with the A-type PCS during the pyrolysis. Therefore, the CNTs were not produced from the gas–solid reactions involving hydrocarbon gas and solid CNTs together with the catalysts. Instead, the catalysts provide reac-
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Fig. 4. (a) TEM image and (b) electron diffraction pattern of the 800 ◦ C-pyrolyzed mixture of A-type PCS and iron nano-particles.
tion interfaces to extract carbon from the Si–CH2 –Si skeleton or the SiCx in the intermediate product and initiate the CNTs growth in the condensed phase. Thus, when the CNTs grow longer inside the bulk PSC or SiCx , they might be mechanically blocked by surrounding featureless materials. The CNTs generated from the L-type PCS have straight shapes as compared with the curled shapes from the A-type PSC derived CNTs. The ease of release of small fragments from the L-type PCS during high temperature treatment may form a less dense substance and help straighten CNT growth. The curled CNTs from the A-type PCS might experience an obstacle during their growth inside a dense substance. However, other mechanisms could also affect the morphologies of the CNTs from PCS and need to be realized in the future. The featureless SiCx films are observed in the CNTs samples generated at a temperature below 1100 ◦ C as shown in Figs. 1a and b, 3a and 4a. The amount of the featureless film reduced as the growth temperature increased and became insignificant in the CNTs sample grown at 1100 ◦ C. These films are considered to decompose in a similar way to the process when SiC is subjected to a low vacuum at high temperature, yielding volatile SiO [47]. The remaining
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carbon could also be removed at high temperature by forming CO molecule during the oxidation reaction as reported in the purification of the CNTs [18,48]. Therefore, the CNTs can remain but the SiCx phase will be eliminated and the CNTs samples grown from PCS at high temperature can be purified. The field emission characteristics of the CNTs samples obtained by pyrolyzing mixtures of the L-type PCS and 0.08 wt.% iron nano-powder at five different temperatures are shown in Fig. 5a. The nano-rod sample prepared at
700 ◦ C has insignificant electron emission as compared to the other samples grown at higher temperatures. The emission current of the CNTs sample generated at 800 ◦ C is a few orders of magnitude higher than the nano-rod sample grown at 700 ◦ C. The increase of emission current is dramatic from 900 to 1100 ◦ C. The turn-on field to reach a current density of 10−2 A/cm2 is 1.17 V/m from the CNTs obtained at 800◦ C. Turn-on fields as low as 0.73 and 0.67 V/m are observed from the CNTs grown at 900 and 1000 ◦ C, respectively. The nanotubes prepared at 1100 ◦ C required a low field of 0.33 V/m to reach the same current density. The emission efficiencies of these L-type PCS-derived CNTs are comparable to the efficiencies of the CNTs grown from gaseous phase [4–6]. The increase of emission efficiency with growth temperature in these L-type PCS-derived CNT samples might be due to the improvement of CNTs quality and the elimination of the covered featureless films. The sharp tip at the end of the CNT could also enhance electron emission by concentrating electron flux from the wide tube body to the thin tip. Two emission curves of the A-type PCS-derived CNTs are shown in Fig. 5b. The turn-on fields of the CNT grown at 800 and 1000 ◦ C are 2.2 and 1.35 V/m, respectively. Meanwhile, the emission curve of the 800 ◦ C-grown CNTs is rugged and jumps up to a level same as the 1000 ◦ C-grown CNTs above an electric filed of 2.4 V/m. The unstable emission properties in the 800 ◦ C-grown CNTs might be caused by being burning off the featureless films during the field emission measurement. Field emission capabilities of the CNTs from the A-type PCS are less efficient than from the L-type PCS. It might be due to the different morphologies of the CNTs from the L-type PCS and the A-type PCS. In accordance with the Fowler–Nordheim equation for field emission, electron emission is influenced by the field amplification factor β, which is proportional to the radius of curvature of the emitter [49,50]. The CNTs from the L-type PCS have a large field amplification factor owing to their sharp tips (see Fig. 1d). The CNTs from the A-type PCS have flat ends (see Fig. 3b) and smaller β factor. Moreover, the CNTs from the A-type PCS are curled and fewer tips stretch out to become emission sites during the field emission measurements.
4. Conclusion
Fig. 5. (a) Current density with applied field curves of the electron field emission across CNT samples from pyrolyzed mixtures of the L-type PCS and Fe nano-particles. Upper four curves represent emission from the CNTs samples being prepared at 1100 ◦ C (䊊), 1000 ◦ C (䊐), 900 ◦ C (), and 800 ◦ C (䉫). Bottom curve represents emission from nano-rods sample being prepared at 700 ◦ C (×). (b) Current density with applied field curves of the electron field emission across CNTs samples from the 1000 ◦ C-pyrolyzed (䊐) and 800 ◦ C-pyrolyzed (䉫) mixtures of the A-type PCS and Fe nano-particles.
In summary, a process for preparing carbon nanotubes by the pyrolysis of solid mixtures of the PCS and iron nano-particles at temperatures between 800 and 1100 ◦ C was demonstrated. The pyrolyzed products contain carbon nanotubes and other film-like materials. Straight nanotubes were obtained from the L-type PCS with low molecular weight whereas curled nanotubes were generated from the A-type PCS with medium molecular weight. These nanotubes have same graphitic structure as those CNTs produced by the gas–solid reactions using carbon vapor or hydrocarbon gases. These samples show comparable
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electron emission efficiency to the efficiencies of the CNTs prepared from typical gas–solid reactions. The emission efficiency increased with process temperature and an emission current density of 10−2 A/cm2 was obtained from the 1100 ◦ C-grown CNT sample at a low field of about 0.33 V/m.
Acknowledgements This work was supported by Grant No. NSC 89-2215-E011-001 from the ROC National Science Council.
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Preparation of carbon nanotubes from vacuum pyrolysis of polycarbosilane Shyankay Jou∗ , Chao Ken Hsu Materials Science and Technology Program, Graduate School of Engineering Technology, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 10672, Taiwan, ROC Received 29 January 2002; accepted 29 September 2003
Abstract Carbon nanotubes (CNTs) were synthesized by vacuum pyrolysis of two types of polycarbosilane (PCS) with iron nano-particles between 800 and 1100 ◦ C. Straight nanotubes were obtained from low molecular weight (990 g/mol) PCS whereas curled nanotubes were derived from medium molecular weight (1290 g/mol) PCS. Diameters of these straight and curled nanotubes were between 5 and 20 nm. The mechansim of condensed phase growth of carbon nanotubes was discussed. Electron emission capability of these carbon nanotubes increased with their pyrolyzing temperature. The electric fields required to emit a current density of 10−2 A/cm2 from the straight nanotubes being pyrolyzed at 800, 900, 1000, and 1100 ◦ C were 1.17, 0.73, 0.67, and 0.33 V/m, respectively. © 2003 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Field emission; Polycarbosilane; Pyrolysis
1. Introduction Since carbon nanotubes (CNTs) were investigated by Iijima [1], their unique properties and potential applications have been studied substantially. Carbon nanotubes can be used as quantum wires [2,3], field emission cathodes [4–6], electrochemical energy storage units [7] and hydrogen storage materials [8,9]. At present, carbon nanotubes have been produced primarily by arc discharge [1,10–12], laser ablation [13,14], catalyzed chemical vapor deposition (CVD) [15–18] and plasma assisted processes [19–21]. In these processes carbon nanotubes are formed through the gas–solid reactions, from which carbon vapor condenses into solid products directly [1,10–14] or gaseous hydrocarbon species crack into solid carbon through catalytic decomposition [15–21]. On the other hand, formation of carbon nanotubes in condensed phase has rarely been reported. Such examples include converting diamond-like carbon [22] and fullerenes [23] into carbon nanotubes by introducing metal catalysts in the reactions. Carbon nanotubes have also been generated
∗ Corresponding author. Tel.: +886-2-27376665; fax: +886-2-27376799. E-mail address: [email protected] (S. Jou).
0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.09.039
using the thermal segregation of carbon from aluminum carbide (Al4 C3 ) in the presence of metal catalysts [24]. Iron, cobalt, nickel, and their alloys show substantial catalyzing effect on CNTs’ growth in the catalyzed CVD [15–18] and plasma assisted processes [19–21]. The catalyzing effect has been observed in the study of the vapor-grown carbon fibers (VGCFs) earlier than the discovery of the CNTs [25–27]. Carbon dissolution and subsequent precipitation on the surface of the catalysts is considered as one possible mechanism for both the VGCFs and the CNT growth in the gas–solid reactions [26–29]. A similar catalyzing effect is also proposed for the CNT generation in condensed phase as mentioned previously [22–24]. On the other hand, carbon nano-structures other than the CNTs have been produced in the condensed phase by the annealing of materials rich in carbon without the presence of catalyst. For example, carbon onions have been formed from precipitation of carbon in carbon-implanted silver [30,31] and copper [32]. Carbon precipitation has also been observed in carbon-rich silicon carbide (SiC) upon high temperature annealing [33]. In comparison to the CNTs’ growth and the carbon precipitation in condensed phase, metal catalysts play an important role governing the microstructure of the resultant carbon products. Therefore, it is possible to grow CNTs from a mixture composed of
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carbon-rich material and nano-sized catalysts in condensed phase. Polycarbosilane (PCS), a commercial silicon carbide precursor, is selected as the source material to grow nanotubes for this study because its pyrolyzed product is rich in carbon and because it is easy to blend with nano-sized catalysts in solutions. The PCS was primarily used for making SiC fibers in industry and fine fibers with diameters as small as 6 m were synthesized by melt-spinning, curing, and pyrolysis [35]. A carbon to silicon ratio as high as 1.56:1 has been obtained in the silicon carbide fiber that was formed from pyrolyzing PCS in Ar [34]. Moreover, carbon clusters have been observed from aggregation of the excess carbon in the PCS-derived SiC at a temperature above 1100 ◦ C [33]. On the other hand, Wang et al. observed crystallization of -SiC inside the SiC fibers by adding Ni or Fe nano-particles in the PCS precursor and pyrolyzing them between 1000 and 1400 ◦ C [36,37]. Otoishi and Tange grew SiC whiskers from pyrolyzing a mixture of the PCS, milled carbon fibers together with iron and nickel between 1100 and 1300 ◦ C [38]. Yet, CNT was not observed in above studies regarding the pyrolysis of PCS [33–37]. However, it is still possible to grow nanotubes from the pyrolysis of PCS. The growth temperatures of the CNTs in the catalytic decomposition of hydrocarbons are found between 800 ◦ C and the eutectic temperature of the metal–carbon mixtures, such as 1153 ◦ C for Fe–C and 1326.5 ◦ C for Ni–C [29]. The temperatures of above SiC fibers and whiskers could be too high for the CNTs’ growth. Therefore, it is of interest to study the pyrolysis of PCS with nano-sized Fe particles below 1153 ◦ C and the analysis of the nanotubes thus formed.
2. Experimental details Both low molecular weight (990 g/mol, L-type) and medium molecular weight (1290 g/mol, A-type) polycarbosilane (Nippon Carbon Inc.) with a structural formula –(HSiCH3 –CH2 )n – were used as the source materials. Iron nano-powder (Cerac Inc.) with an average particle diameter of 17 nm was utilized as the catalyst. About 0.2 g PCS powder and 0.08 wt.% iron nano-powder were mixed in 5 cm−3 toluene (HPLC grade) as the starting materials. The mixtures were spun coated onto silicon wafers at a speed of 50 H3 in air and subsequently pyrolyzed in a vacuum furnace. The samples were heated at a rate of 10 ◦ C/min up to temperatures between 700 and 1100 ◦ C, held there for 1 h and then cooled down in the furnace. During the heating step, the samples were kept at 500 ◦ C for 1 h to drive out small organic volatiles. The pressure was kept lower than 1.33 × 10−3 Pa using a diffusion pumping system during the high temperature pyrolysis and cooling down. After removing the samples from the furnace, the peeled films or loose powder were collected on the Formvar-coated copper grids and examined by transmission electron microscopy (TEM) (JOEL JEM2000-FX). In order to calculate the yields of
the CNTs, about 0.1 g of the pure PCS powder or dry mixtures of the PCS and 0.08 wt.% iron were placed inside a platinum crucible and pyrolyzed in the vacuum furnace at 1000 ◦ C for 1 h. The dry mixtures were prepared by baking the PCS–catalyst mixture solutions in the vacuum furnace at 95 ◦ C for 2 h to drive out the solvent. Field emission measurements were performed in a vacuum chamber at a pressure below 6.65 × 10−4 Pa using a turbomolecular pumping system. The Si substrate with continuous film was glued onto an aluminum holder with silver paste then grounded as cathode. An iron anode 2.3 mm in diameter was kept at 30 m away from the surface of the sample and powered by a Keithley 237 source-measurement unit to collect the emission current. The measurements were carried out in a range of 0–150 V and from 0.1 nA to 10 mA.
3. Results and discussion Clusters of nano-rods or nanotubes were found randomly distributed over the pyrolyzed specimens. Fig. 1a shows a TEM image revealing the general view of nano-rod bundles produced from pyrolyzing a mixture of L-type PCS and iron nano-particles at 700 ◦ C. These nano-rods are straight with diameters between 5 and 20 nm. Their lengths vary from a few hundred nanometers to a few micrometers. Hollow structure is not observed in these nano-rods. A few nano-rods are adhered with particles and film-like materials. Fig. 1b is a TEM image of nanotubes from the mixture of L-type PCS and iron nano-powder pyrolyzed at 900 ◦ C. Most of these hollow nanotubes have an outer diameter around 8 nm. These straight nanotubes are densely packed and covered with film-like materials. A TEM image of the nanotubes from the 1100 ◦ C-pyrolyzed product is shown in Fig. 1c. The outer diameters of these straight nanotubes are between 9 and 17 nm. A few nanotubes have smaller outer diameters at their tips as compared to the outer diameters of their uniform tubular bodies. All the nano-rods and nanotubes derived from the L-type PCS are straight and stretch out toward free space. The film-like materials observed in the 700 and 900 ◦ C-pyrolyzed products are not seen in the 1100 ◦ C sample. An enlarged TEM image of the nanotubes from the 1100 ◦ C-pyrolyzed sample is shown in Fig. 1d. A nanotube with a sharp tip and wide body is seen in the middle left of the figure. The inner diameter of this nanotube is about 1.9 nm at the tip and 2.5 nm in the tube body. Outer diameter and wall thickness at the tip are 8.6 and 3.35 nm, respectively, and increased to 15.4 and 6.45 nm in the tube body. An electron diffraction pattern of the 1000 ◦ C-pyrolyzed nanotubes sample consisting of five diffraction rings is shown in Fig. 2a. The first, third, fourth, and fifth rings coincide with graphite(0 0 2), (1 0 0), (0 0 4), and (1 1 0) planes with plane spacing of 0.34, 0.21, 0.17, and 0.12 nm, respectively [39]. Intensity of the second ring is weak if compared to the other four rings. An electron diffraction pattern of the 1100 ◦ C-pyrolyzed sample is shown in Fig. 2b. There are
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Fig. 1. Typical TEM images of nano-rods and nanotubes from pyrolyzing mixtures of L-type PCS and iron nano-particles at (a) 700 ◦ C, (b) 900 ◦ C, and (c) 1100 ◦ C. (d) An enlarged TEM image revealing a nanotube with sharp tip and wide body.
four diffraction rings that match with graphite(0 0 2), (1 0 0), (0 0 4), and (1 1 0) reflections. The second-diffraction ring in Fig. 2a is not seen here. Based on the diffraction results in Fig. 2a and b and TEM images in Fig. 1a–c, the four diffraction rings correlate to the major tubular phase. Therefore, it can be constructed that these tubular materials are the CNTs. The second-diffraction ring in Fig. 1a probably originates from the particles or the film-like materials. According to the process conditions in the literature regarding the pyrolysis of pure PCS, amorphous or crystalline SiCx was produced below or above 1000 ◦ C [33,34] and its crystallinity can be improved by adding Fe or Ni particles [36,37]. The plane spacing of the second ring in Fig. 2a is about 0.26 nm and matches with a reflection plane in several ␣-SiC polymorphs [40]. Because these film-like materials may not came into contact with the catalyst during the pyrloysis, their composition and structure are expected to be similar to the PCS-derived SiC [33,34]. As a result, carbon nano-rods and nanotubes were grown from the PCS with the aid of Fe nano-particles acting as catalysts. The mechanism to the CNTs growth from the pyrolysis of PCS with the aid of catalysts is considered similar with that in the catalyzed chemical vapor deposition [25–29], except
that the carbon feedstock is from the condensed phase here rather than from the gaseous phase. According to the study on the PCS pyrolysis in inert environment, methane and hydrogen are released from breaking the Si–CH3 and Si–H bonds above 490 ◦ C whereas H2 evolves between 880 and 1050 ◦ C, arising from breaking C–H bonds in Si–CH2 –Si skeleton in the intermediate product [34,41]. Since the samples in our study were first cracked at 500 ◦ C which is below the CNTs formation temperatures [29], the small hydrocarbon species released from the PCS may not contribute to the formation of CNTs directly, but will rather be pumped out from the vacuum system. The carbon source for CNTs growth is possibly supplied from the Si–CH2 –Si skeleton, which will decompose into carbon species on the surface of the Fe catalysts at a high temperature. This reaction is similar to the reaction of diamond-like carbon with a metal catalyst [22] or the precipitation of graphite from a solid state reaction between SiC and Fe [42,43]. Therefore, the growth mechanism of the CNT will be similar to the catalyzed CVD, in which carbon diffuses into the catalyst particle then precipitates out as solid fibers or hollow nanotubes [26–29]. The carbon nano-structures are controlled by both the rate of carbon diffusion into the catalyst and the rate
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Fig. 2. Electron diffraction patterns of nanotube bundles from pyrolyzed mixtures of L-type PCS and iron nano-particles. (a) The 1000 ◦ C-pyrolyzed sample with four diffraction rings which coincide with graphite(0 0 2), (1 0 0), (0 0 4), and (1 1 0) reflections. The second ring belongs to ␣-SiC reflection. (b) The 1100 ◦ C-pyrolyzed products with four diffraction rings which coincide with graphite(0 0 2), (1 0 0), (0 0 4), and (1 1 0) reflections.
of H2 reduction to eliminate carbon species on the surface of the catalyst. The H2 reduction rate might be low in the sample prepared from the L-type PCS at 700 ◦ C. Since the Si–CH2 –Si skeleton is not readily decomposed and H2 concentration is low at this temperature [34,41], the removal rate of carbon species on the catalyst surface is slower than the diffusion rate of carbon through the bulk catalyst. Therefore, a rod-like structure, similar to the VGCFs obtained at low temperature [28,44], is generated as Fig. 1a. The CNTs obtained from the sample pyrolyzed at 1100 ◦ C, as shown in Fig. 1c, is owing to a good balance between the two rates here, just as the mechanism indicated by Cui et al. on the study of the CNTs growth from hydrocarbons [28]. Nanotubes from the A-type PCS have a different morphology as compared with the CNTs from the L-type PCS. A typical electron micrograph of the 1000 ◦ C-pyrolyzed product from a mixture of A-type PCS and iron nano-particles
Fig. 3. (a) Typical TEM image of nanotubes grown at 1000 ◦ C by pyrolyzing mixture of A-type PCS and iron nano-particles. (b) An enlarged TEM image showing V-shape CNTs and adhered films. (c) The electron diffraction pattern of the 1000 ◦ C-pyrolyzed sample from mixture of A-type PCS and iron nano-particles.
is shown in Fig. 3a. The outer diameters and thicknesses of most nanotubes are around 20 and 7 nm, respectively. A few of the tubes have larger diameters. The lengths of the tubes are much larger than their diameters and some nanotubes are longer than 1 m. These nanotubes are curled and entangled
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together just like the morphologies of the CNTs from catalytic decomposition of hydrocarbons [15–18,28]. A few of the nanotubes stretch out into free space from the densely clustered areas. There are several distinct CNTs morphologies as shown in Fig. 3a. A few nanotubes meet other nanotubes at acute angles and produce V-shaped junctions at the lower left corner and other places in Fig. 3a. A Y-shaped nanotube is seen in the upper left corner of Fig. 3a. Catalyst materials are found encapsulated inside several nanotubes. This encapsulation could be due to capillary force and has also been seen in other CNTs fabrication processes [18,45] and the experiments on capillary induced filling of CNTs [46]. An enlarged TEM image of the CNTs containing the V-shaped junctions is shown in Fig. 3b. A nanotube with flat end and uniform diameter is seen near the upper right corner of the figure. It is different from the sharp tips in the CNTs from the L-type PCS, as shown in Fig. 1d. A small area of featureless films is seen between the two V-shaped nanotubes. These thin films are considered as SiC that is the same as the product of pure PCS as discussed in the pyrolyzed L-type PCS samples. The electron diffraction pattern of this sample, as shown in Fig. 3c, also contains graphite(0 0 2), (1 0 0), (0 0 4) and (1 1 0) planes, same as those rings from the L-type sample as indicated in Fig. 2b. Therefore, these curled tubular materials are the CNTs as well. However, the causes of different morphologies in the CNTs derived from the L-type PCS and the A-type PCS are not yet clear. Fig. 4a and b shows the TEM image and the electron diffraction pattern of the nanotubes prepared from the mixture of the A-type PCS and iron nano-particles at 800 ◦ C. These nanotubes are immersed inside featureless materials. The diffraction pattern of the sample shows four graphite rings, same as those ones observed in the CNTs generated from the L-type PCS at 1100 ◦ C (see Fig. 2b) and the A-type PCS at 1000 ◦ C (see Fig. 3c). There is no reflection representing other crystalline material in the sample. Therefore, the featureless film could be amorphous material. Since the source material was the PCS, the featureless film could be composed of SiCx [33,34]. It is suggested that the CNTs grow from inside the bulk PCS and catalyst mixture. When the CNTs grow longer and protrude out toward free space, some SiCx or residual PCS still adhere to the CNTs surface. The raw yields of the CNTs from the pyrloysis of the Aand L-type PCS with 0.08 wt.% Fe are 24 and 2.2%, respectively. They are smaller than raw yields of SiC from the pyrolysis of pure PCS, ranging from 55% for the A-type and 23% for the L-type PCS in this study. It was reported that most weight loss of the PCS during the pyrolysis happened between 500 and 800 ◦ C by the release of CH4 and H2 [34,41]. According to the yield data, more hydrocarbon gas was released but less CNTs were produced from the L-type PCS as compared with the A-type PCS during the pyrolysis. Therefore, the CNTs were not produced from the gas–solid reactions involving hydrocarbon gas and solid CNTs together with the catalysts. Instead, the catalysts provide reac-
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Fig. 4. (a) TEM image and (b) electron diffraction pattern of the 800 ◦ C-pyrolyzed mixture of A-type PCS and iron nano-particles.
tion interfaces to extract carbon from the Si–CH2 –Si skeleton or the SiCx in the intermediate product and initiate the CNTs growth in the condensed phase. Thus, when the CNTs grow longer inside the bulk PSC or SiCx , they might be mechanically blocked by surrounding featureless materials. The CNTs generated from the L-type PCS have straight shapes as compared with the curled shapes from the A-type PSC derived CNTs. The ease of release of small fragments from the L-type PCS during high temperature treatment may form a less dense substance and help straighten CNT growth. The curled CNTs from the A-type PCS might experience an obstacle during their growth inside a dense substance. However, other mechanisms could also affect the morphologies of the CNTs from PCS and need to be realized in the future. The featureless SiCx films are observed in the CNTs samples generated at a temperature below 1100 ◦ C as shown in Figs. 1a and b, 3a and 4a. The amount of the featureless film reduced as the growth temperature increased and became insignificant in the CNTs sample grown at 1100 ◦ C. These films are considered to decompose in a similar way to the process when SiC is subjected to a low vacuum at high temperature, yielding volatile SiO [47]. The remaining
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carbon could also be removed at high temperature by forming CO molecule during the oxidation reaction as reported in the purification of the CNTs [18,48]. Therefore, the CNTs can remain but the SiCx phase will be eliminated and the CNTs samples grown from PCS at high temperature can be purified. The field emission characteristics of the CNTs samples obtained by pyrolyzing mixtures of the L-type PCS and 0.08 wt.% iron nano-powder at five different temperatures are shown in Fig. 5a. The nano-rod sample prepared at
700 ◦ C has insignificant electron emission as compared to the other samples grown at higher temperatures. The emission current of the CNTs sample generated at 800 ◦ C is a few orders of magnitude higher than the nano-rod sample grown at 700 ◦ C. The increase of emission current is dramatic from 900 to 1100 ◦ C. The turn-on field to reach a current density of 10−2 A/cm2 is 1.17 V/m from the CNTs obtained at 800◦ C. Turn-on fields as low as 0.73 and 0.67 V/m are observed from the CNTs grown at 900 and 1000 ◦ C, respectively. The nanotubes prepared at 1100 ◦ C required a low field of 0.33 V/m to reach the same current density. The emission efficiencies of these L-type PCS-derived CNTs are comparable to the efficiencies of the CNTs grown from gaseous phase [4–6]. The increase of emission efficiency with growth temperature in these L-type PCS-derived CNT samples might be due to the improvement of CNTs quality and the elimination of the covered featureless films. The sharp tip at the end of the CNT could also enhance electron emission by concentrating electron flux from the wide tube body to the thin tip. Two emission curves of the A-type PCS-derived CNTs are shown in Fig. 5b. The turn-on fields of the CNT grown at 800 and 1000 ◦ C are 2.2 and 1.35 V/m, respectively. Meanwhile, the emission curve of the 800 ◦ C-grown CNTs is rugged and jumps up to a level same as the 1000 ◦ C-grown CNTs above an electric filed of 2.4 V/m. The unstable emission properties in the 800 ◦ C-grown CNTs might be caused by being burning off the featureless films during the field emission measurement. Field emission capabilities of the CNTs from the A-type PCS are less efficient than from the L-type PCS. It might be due to the different morphologies of the CNTs from the L-type PCS and the A-type PCS. In accordance with the Fowler–Nordheim equation for field emission, electron emission is influenced by the field amplification factor β, which is proportional to the radius of curvature of the emitter [49,50]. The CNTs from the L-type PCS have a large field amplification factor owing to their sharp tips (see Fig. 1d). The CNTs from the A-type PCS have flat ends (see Fig. 3b) and smaller β factor. Moreover, the CNTs from the A-type PCS are curled and fewer tips stretch out to become emission sites during the field emission measurements.
4. Conclusion
Fig. 5. (a) Current density with applied field curves of the electron field emission across CNT samples from pyrolyzed mixtures of the L-type PCS and Fe nano-particles. Upper four curves represent emission from the CNTs samples being prepared at 1100 ◦ C (䊊), 1000 ◦ C (䊐), 900 ◦ C (), and 800 ◦ C (䉫). Bottom curve represents emission from nano-rods sample being prepared at 700 ◦ C (×). (b) Current density with applied field curves of the electron field emission across CNTs samples from the 1000 ◦ C-pyrolyzed (䊐) and 800 ◦ C-pyrolyzed (䉫) mixtures of the A-type PCS and Fe nano-particles.
In summary, a process for preparing carbon nanotubes by the pyrolysis of solid mixtures of the PCS and iron nano-particles at temperatures between 800 and 1100 ◦ C was demonstrated. The pyrolyzed products contain carbon nanotubes and other film-like materials. Straight nanotubes were obtained from the L-type PCS with low molecular weight whereas curled nanotubes were generated from the A-type PCS with medium molecular weight. These nanotubes have same graphitic structure as those CNTs produced by the gas–solid reactions using carbon vapor or hydrocarbon gases. These samples show comparable
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electron emission efficiency to the efficiencies of the CNTs prepared from typical gas–solid reactions. The emission efficiency increased with process temperature and an emission current density of 10−2 A/cm2 was obtained from the 1100 ◦ C-grown CNT sample at a low field of about 0.33 V/m.
Acknowledgements This work was supported by Grant No. NSC 89-2215-E011-001 from the ROC National Science Council.
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