Carbon 43 (2005) 2571–2578 www.elsevier.com/locate/carbon
Continuous deposition of carbon nanotubes on a moving substrate by open-air laser-induced chemical vapor deposition Kinghong Kwok, Wilson K.S. Chiu
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Department of Mechanical Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 06269-3139, USA Received 21 January 2005; accepted 11 May 2005 Available online 5 July 2005
Abstract Continuous deposition of carbon nanotubes under open-air conditions on a moving fused quartz substrate is achieved by pyrolytic laser-induced chemical vapor deposition. A CO2 laser is used to heat a traversing fused quartz rod covered with bimetallic nanoparticles. Pyrolysis of hydrocarbon precursor gas occurs and subsequently gives rise to rapid growth of a multi-wall carbon nanotube forest on the substrate surface. A ‘‘mushroom-like’’ nanotube pillar is observed, where a random orientation of carbon nanotubes is located at the top of the pillars while the growth is more aligned near the base. The typical carbon nanotube deposition rate achieved in this study is approximately 50 lm/s. At high power laser irradiation, various carbon microstructures are formed as a result of excessive formation of amorphous carbon on the substrate. High-resolution transmission and scanning electron microscopy, and X-ray energy-dispersive spectrometry are used to investigate the deposition rate, microstructure, and chemical composition of the deposited carbon nanotubes. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Chemical vapor deposition; Electron microscopy; X-ray energy-dispersive spectrometry; Reaction kinetics
1. Introduction A vast amount of research effort has been devoted to the development of synthesis techniques that would have the capability of growing high quality, by-product free carbon nanotubes. Arc-discharge, laser ablation, and high-pressure carbon monoxide (HiPco) techniques have been widely used for the growth of single- and multi-wall carbon nanotubes [1–4]. However, low production rate and high deposition temperature or pressure (typically between 3000 and 4000 °C) required by these techniques resulted in a scale-up production of carbon nanotubes that is prohibitively expensive. Thermal CVD and various forms of plasma CVD techniques [5–7] have been successful in growing a relatively *
Corresponding author. Tel.: +1 860 486 3647; fax: +1 860 486 5088. E-mail address:
[email protected] (W.K.S. Chiu). 0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.05.016
large quantity (kilogram and even ton level) of carbon nanotubes. However, relatively low deposition temperature in CVD (550–1200 °C) results in carbon nanotubes with high defect densities and a low degree of graphitization [4], thereby restricting their usage for advanced applications. Recently, Kwok and Chiu [8] have successfully demonstrated the feasibility of using pyrolytic LCVD to deposit carbon nanotubes on stationary fused quartz substrates in open-air. It was found that different regions of carbon growth occur as a result of non-uniform temperature distribution and the reactant gas jetÕs crossflow configuration. However, this technique is not suitable for mass production of nanotubes because a significant amount of pyrolytic carbon by-products are deposited as a result of elevated substrate temperature induced by direct laser irradiation. Furthermore, our previous investigation focused primarily on the fundamental aspect of open-air LCVD synthesis, so that the
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effect of experimental parameters on carbon nanotube growth kinetics, microstructure, and chemical composition has not been examined. A scanning laser-induced CVD technique is proposed and investigated in order to reduce the amount of undesirable by-products and increase the yield of carbon nanotubes. In this technique, the laser heated spot moves with respect to the substrate at a prescribed velocity and direction, thereby reducing the amount of temperature rise on the substrate surface and the laser residence time [9]. The formation rate of pyrolytic carbon film decreases as a result of reduced temperature, which will enhance the carbon nanotube growth by allowing the catalyst particles to remain active for a longer period of time. Furthermore, a moving laser beam will irradiate a larger substrate surface area in a continuous manner, which is favorable for mass production of carbon nanotubes. The objective of this work is to study the feasibility of using open-air pyrolytic LCVD to deposit carbon nanotubes on a moving substrate with direct application to scale-up continuous production of nanotubes. In addition, the influence of laser power intensity and substrate movement on nanotube growth is studied. Extensive search in the open literature reveals no documented experimental study on the growth of carbon nanotubes on a moving substrate using an open-air CVD technique. This finding is expected because the existing CVD techniques involve (i) large-area deposition that requires no moving components and (ii) enclosed deposition chamber restricts the movement of the heat source and the substrate. The main scientific justification for the study of carbon nanotube growth on a moving substrate is the potential in discovering new fundamental physics regarding the growth mechanism of carbon nanotubes.
2. Experimental apparatus The laser-induced CVD system used for the continuous growth of carbon nanotubes on a moving substrate is shown in Fig. 1. The overall experimental setup is similar to that used for laser deposition on a stationary substrate [8], and only a brief description is given here. A 30-W CO2 laser with a wavelength of 10.6 lm and a Gaussian beam profile is used to heat the substrate. A plano/convex zinc selenide (ZnSe) lense is used in order to focus the laser beam to a desired diameter on the substrate without altering the beam profile. An integrated computer data acquisition and control system is used to control the laser power output. Fused quartz (SiO2) rods of 3-mm diameter are used as the substrate, and the procedure for preparing catalyst particles on the substrate was outlined in [8]. The catalytic substrate is attached to a supporting arm as shown in Fig. 1. The substrate supporting apparatus sits on a linear traverse mechanism, which is connect to a high-torque electric motor that can move the substrate at a constant velocity across the laser targeted spot. The mechanism is designed to slide on linear ball bearings to minimize the effect of both static and dynamical friction. During an experimental run, the substrate is first mounted onto the traverse mechanism, which is positioned away from the laser heating spot. The CO2 laser, the coaxial nozzle and the exhaust hood are turned on and set to the appropriate laser power and flow rates. Then the electrical motor is initialized and begins driving the linear stage along with the substrate through the reactor at a constant velocity. Deposition occurs when the substrate intercepts the laser beam and the reactant gas jet. In this study, the laser power is varied systematically in order to study its effect on the growth kinetics and
Fig. 1. Open-air laser-induced chemical vapor deposition (LCVD) system with a substrate moving mechanism.
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microstructure of the deposited carbon nanotubes while the substrate moving velocity and the catalyst particle size are fixed at 0.14 mm/s and 350 nm, respectively. The hydrocarbon precursor gas used is propane (C3H8) with 99.95% purity and is mixed with ultra-high purity grade hydrogen gas. The flow rates of propane, hydrogen and nitrogen are 0.2, 1.0 and 10 SLPM, respectively. A transmission electron microscope (JEOL 2010 FasTEM) equipped with an X-ray energy-dispersion spectrometry (EDS) system, and an environmental scanning electron microscope (Philips 2020 ESEM) are used to examine the microstructure, reaction kinetics, and chemical composition of the carbon materials deposited in this study.
3. Results and discussion 3.1. Continuous synthesis of carbon nanotube forest by open-air LCVD In the pyrolytic LCVD technique, the effect of moving the substrate in one dimension perpendicular to the focused CO2 laser beam is the same as scanning the laser beam along the substrate. The scanning laser beam can be described as a moving heat source that induces a large temperature gradient on the glass surface, which subsequently leads to thermal decomposition of gas-phase precursors and results in the deposition of a continuous stripe of desired material. Fig. 2a shows laser heating at the beginning section of the fused quartz substrate where no catalyst is presented. Rapid temperature raise becomes obvious by observing the bright spot on
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the substrate surface. In this case, the laser power used for heating is 13.5 W (5.25 MW/m2) and the substrate velocity is 0.14 mm/s. As the laser beam enters the section covered with catalyst, growth of bulk carbon nanotube material could be immediately observed by a magnifying camera as shown in Fig. 2b. The accumulation of carbon nanotubes occur very rapidly at the trailing edge of the laser heating zone and also within the laser heating spot. This observation suggests that both sufficient temperature and residence time is required for the nucleation and subsequent growth of carbon nanotubes since no observable nanotube growth occurs near the front of the moving laser beam. The direct observation of the nanotube growing process within the laser-heating zone has demonstrated the rapidness of this laser driven technique. The side view of the bulk nanotube material, which provides an estimation of its height, is shown in Fig. 2c. Investigation of the deposited material by an optical microscope reveals no detailed microstructural information except for the fact that they appear to be grayish and chunky. Fig. 3a and b shows two ESEM images of the bulk carbon material shown in Fig. 2c. ESEM investigation reveals pillars of ‘‘mushroom-like’’ carbon trees that form a carbon nanotube forest. Higher magnification ESEM studies reveal individual pillars consisting of carbon nanotubes in tangled form with the primary growth direction perpendicular to the substrate surface. The ‘‘mushroom-like’’ geometry could be due to random orientation of the carbon nanotubes locate at the top of the pillars while the growth is more aligned near the base. The height of the carbon nanotube pillars
Fig. 2. (a) Laser heating of an uncoated section on the fused quartz substrate moving at 0.14 mm/s. (b) Laser-induced growth of bulk carbon nanotube material on the moving substrate with the same velocity as in (a), and (c) a side view of the resulting bulk nanotube material.
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Fig. 3. (a) ESEM image of the pillars of ‘‘mushroom-like’’ carbon trees deposited by scanning LCVD. (b) ESEM image of carbon nanotube bundles. (c) TEM image of carbon nanotubes in tangled form with diameter ranging from 5 to 20 nm. (d) HRTEM image of a pair of carbon nanotubes overlapping each other.
deposited in this study ranges from 0.5 to 2.5 mm, but the length of individual nanotubes within the pillars is very difficult to measure because of the tangling of nanotubes and resolution limit of the ESEM. However, by following several relatively straight carbon nanotubes among the bundles, it is found that their lengths exceed 50 lm. The length measurement terminates at a point where further tracing becomes impossible. High-resolution TEM (HRTEM) study was performed in order to examine the detailed microstructural features of LCVD-grown carbon nanotubes. A stripe of carbon nanotube forest that is approximately 3 cm long and 0.3 cm wide is dispersed in an ethanol solution by an ultrasonic process. Small fragments of the carbon nanotube bundle are transferred onto a TEM grid. Fig. 3c shows a TEM image of carbon nanotubes in tangled form with diameter ranging from 5 to 20 nm with the majority of the nanotubes having a diameter of 15 nm. A small amount amorphous carbon is observed among the nanotube bundle, which indicates that amorphous carbon is also deposited during the nanotube growing process. Fig. 3d shows a HRTEM image of a pair of carbon nanotubes overlapping each other. It clearly reveals the tubular graphite lattice structure of carbon nanotubes observed by Iijima [10]. In addition, the hollow core and the graphitic walls can also be directly
seen in the image. In this case, the diameter of the nanotube and its core are approximately 3 and 10 nm, respectively. The distance between graphitic walls is 0.34 nm, which matches the separation distance in bulk graphite. X-ray energy dispersive spectrometry was performed on a LCVD-grown carbon nanotube sample in order to determine its chemical composition. Fig. 4 shows the EDS spectrum taken directly from the carbon nanotube bundle shown in Fig. 3c. In the spectrum, the shaded area represents the characteristic X-ray signal generated from the nanotube sample, while the area under the line represents those from the TEM grid which is composed of amorphous carbon and copper. Carbon nanotube bundles within the sample gives rise to the carbon peak, and oxides forming on the nanotube surface after laser deposition process gives rise to the oxygen peak observed in the EDS spectrum. No palladium or gold signal is detected, which implies the absence of nanotube contamination by catalyst particle intrusion. The absence of catalyst particles observed by TEM and EDS suggests that base-growth is the primary nanotube growth mode in this study. In base-growth mode, catalyst particles remain attached to the substrate surface as the carbon nanotubes lengthen, as opposed to tipgrowth mode where catalyst particles are lifted off from
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Fig. 4. EDS spectrum taken directly from the LCVD-grown multiwall carbon nanotubes. The shaded area in the EDS spectrum represents the characteristic X-ray signal derived from the nanotube sample, while the area under the line represents those from the TEM grid.
the substrate and carried along by the nanotube as it grows in length so that nanoparticle traces will be found within the bundle. The ESEM and the TEM results have clearly demonstrated the feasibility of using scanning pyrolytic laserinduced chemical vapor deposition to grow a continuous stripe of carbon nanotube forest on a moving glass surface. The deposition rate of multi-wall carbon nanotubes achieved in this study is determined based on the height of the pillars since each pillar consists of densely packed carbon nanotubes. The nanotube growth time is defined as the duration of a given substrate section residing within the laser beam, which is equal to the diameter of the laser beam, divide by the substrate moving velocity. Since this study used 1.8 mm as the laser beam diameter and 0.14 mm/s as the substrate velocity, the growth time is calculated to be 12.5 s. The average height of the pillars deposited with a laser power of 13.5 W is measured by ESEM to be 0.65 mm. The resulting deposition rate is determined to be 50 lm/s, which is relatively high compared to rates reported by thermal and plasmaenhanced chemical vapor deposition [11–14]. 3.2. Effect of laser power on carbon nanotube forest morphology A second set of experiments were performed in order to study the effect of intense laser irradiation on the microstructure and the growth kinetics of carbon nanotubes by pyrolytic LCVD. The same experimental parameters are used in this case but with the laser power increased to 15.5 W (5.75 MW/m2). Fig. 5a shows a schematic of a continuous stripe of nanotube forest with two distinctive regions of growth. The ‘‘mushroom-like’’ carbon nanotube pillars are found in Region A as shown
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Fig. 5a and b. However, a layer of amorphous carbon film forms beneath the pillars as a result of increased laser power. No amorphous carbon layer is observed to deposit on the substrate surface when a lower laser power of 13.5 W was used. The formation of a carbon layer on the substrate surface covers the catalyst particles, thereby terminating carbon nanotube growth. Region B forms at the perimeter of Region A, and consists of loosely packed multi-wall carbon nanotubes growing in random orientation as shown in Fig. 5c. Carbon nanotubes with diameter ranging from 30 to 50 nm and length ranging from 5 to 50 lm are observed in Region B. This region is located at the edge of the Gaussian laser beam, and therefore the surface temperature is lower compared to Region A. Since less hydrocarbon molecules would undergo thermal decomposition at a lower substrate temperature, it is expected that less carbon nanotubes will form in Region B. In addition, the rapid formation of nanotube pillars in Region A interferes with the transport of reactants to Region B, which may cause further reduction in growth rate of carbon nanotubes observed in the region. A third set of experiments are performed with the laser power increased to 21.5 W (8.35 MW/m2). In this case, deposition of individual bulk carbon chunks was observed by visual inspection of the resulting sample as shown in Fig. 5d. The appearance of this bulk carbon material resembles the nanotube forest observed in the previous sample except that they are discontinuous islands rather than a continuous stripe. ESEM investigation also reveals two distinctive regions of carbon growth, and the morphology of the carbon deposit within each region is shown in Fig. 5e and f. In this case, the bulk carbon islands are labeled as Region C, and the remaining area within the path of the laser beam is labeled as Region D. Fig. 5e shows that Region C consists of densely packed carbon fibers grown in random orientation with length reaching several hundred microns. It is important to point out that carbon fibers and multi-wall carbon nanotubes have a similar structure, and look similar under SEM. However, carbon nanotubes have a much smaller diameter than the fiber and have well-graphitized walls [4,15]. Beside carbon fibers, carbon nanotube bundles are occasionally observed branching out from near the tip of the carbon fibers as shown in Fig. 5e. ESEM investigation of Region D shown in Fig. 5f reveals columns of fiber-like carbon structures sprouting perpendicular to the substrate surface with an average diameter of 10 lm and height ranging from 30 to 100 lm. In addition, a significant amount of amorphous carbon is deposited on the surface. Carbon nanotube bundles are not found in this region as a result of excessive amorphous carbon deposition, which covered up potential nanotube precipitation sites on the catalyst surface.
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Fig. 5. (a) A schematic of a continuous stripe of nanotube forest showing two distinct regions of growth. (b) ‘‘Mushroom-like’’ carbon nanotube pillars were found in Region A. (c) Loosely packed multi-wall carbon nanotubes grew in random orientation found in Region B. (d) Bulk carbon chunks deposited at high laser power irradiation with two distinctive regions of growth. (e) Densely packed carbon fibers grown in random orientation and (f) columns of fiber-like carbon structures sprouting perpendicular to the substrate surface with a significant amount of amorphous carbon.
Fig. 6 shows a series of HRTEM images of carbon nanotubes and carbon fibers found in Region C. The multi-wall carbon nanotubes observed in Region C, shown in Fig. 6a, have similar diameter and microstructure compared to those deposited in the previous set of experiments where a lower laser power is used. Detailed examination under TEM reveals that a multi-wall carbon nanotube is embedded into each carbon fiber core as indicated in Fig. 6b. This observation clearly indicates that the formation of carbon fibers begin with the growth of carbon nanotubes since it is unlikely for car-
bon nanotubes to grow inside the solid carbon fiber. The formation of carbon fiber could be explained by the following proposed mechanism. Initially, carbon atoms precipitate from the carbon-saturated metal particle surface and form a carbon nanotube. However, rapid pyrolysis of hydrocarbon molecules due to the intense laser irradiation results in excessive amorphous carbon deposited onto the carbon nanotube. The thickness of the amorphous carbon layer increases as the nanotube lengthens and eventually becomes the carbon fiber observed in Fig. 5e.
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Fig. 6. (a) HRTEM image of carbon nanotubes found in Region C. (b) Multi-wall carbon nanotube coated with a thick layer of amorphous carbon with metal particles scattered along the nanotube. (c, d) are TEM images of large diameter carbon fibers revealing polycrystalline structure. (Inserts are selected area electron diffraction patterns.)
HRTEM investigation of carbon fibers with diameter ranging from 75 to 250 nm reveals a polycrystalline structure as shown in Fig. 6c and d. The selected area electron diffraction patterns (inserted in Fig. 6c and d) obtained from the fibers confirm the formation of a polycrystalline carbon layer [15,16] on the fiber surface. The build-up of a polycrystalline carbon layer forms yet another layer on the fiber. This three-layer carbon fiber structure is very interesting because it combines three distinctive forms of solid carbon with sp2 bonding state into a single structure. Notice that it is difficult to see the carbon nanotube within the large diameter carbon fiber because the thick layer of carbon prevents the penetration of the electron beam used in TEM imaging, but occasionally a carbon nanotube core can be seen on the fiber as shown in Fig. 6c. Fig. 6b–d shows that metal particles are scattered along the carbon fiber, which is not observed in the previous case where a lower laser power was used. This result indicates that high power laser irradiation can induce a sufficiently high surface temperature to melt the catalyst particles during the initial nanotube growth
stage. The adhesion force between metal particles and the substrate surface reduces dramatically [17] when the metal particles start to melt, which allows the subsequent growth of carbon nanotubes to lift the unbounded metal particles from the surface and carry them along on the nanotube as it lengthens. Notice that only a portion of the original metal particle seed is being lifted since the size of metal particles found along the nanotube is approximately 1/7 of the original particle. The lifted metal particle acts as the catalyst for the growth of carbon nanotube bundles that branch out from the tip of the carbon fibers as shown in Fig. 5e. Similar nanotube branching phenomenon have been reported in microwave plasma-enhanced CVD of multi-wall carbon nanotubes by Tsai et al. [18].
4. Conclusions In this study, the feasibility of using open-air laser-induced chemical vapor deposition to grow carbon nanotubes on a moving substrate is successfully
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demonstrated with direct application to high efficient mass production of carbon nanotubes. Scanning electron microscopy reveals pillars of densely packed carbon nanotubes deposited on the substrate with height reach several hundred microns. The typical growth rate of multi-wall carbon nanotubes obtained in this study is 50 lm/s, which is relatively high compared to the maximum growth rates achievable in other chemical vapor deposition processes. High-resolution transmission electron microscopy reveals the tubular graphite lattice structure of multi-wall carbon nanotubes with 0.34 nm separation distance between the graphite walls. Continuous growth of carbon nanotube forest with minimum amorphous carbon and catalyst particle contamination is obtained when a laser power density of 5.25 MW/m2 is used for deposition. At higher laser power irradiation, excessive deposition of amorphous carbon results in the formation of relatively large diameter carbon fibers with metal particles scattered along the length of the fiber.
Acknowledgments Financial support by the National Science Foundation is gratefully acknowledged. The authors thank Roger Ristau and Mary Anton at the Institute of Materials Science at the University of Connecticut for their assistance with transmission and scanning electron microscopy.
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