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2007, 111, 1907-1910 Published on Web 02/06/2007
Growth Kinetics of 0.5 cm Vertically Aligned Single-Walled Carbon Nanotubes Guofang Zhong,*,†,‡ Takayuki Iwasaki,† John Robertson,‡ and Hiroshi Kawarada† School of Science and Engineering, Waseda UniVersity, Tokyo 169-8555, Japan, and Engineering Department, UniVersity of Cambridge, U.K. ReceiVed: NoVember 22, 2006; In Final Form: January 19, 2007
Half-centimeter-high mats of vertically aligned single-walled carbon nanotubes were grown at 600 °C by point-arc microwave plasma chemical vapor deposition. The mats were produced from 0.5 nm of an Fe catalyst layer, thus showing one of the highest catalytic yields of ∼105 times. The growth process shows a lack of poisoning of the catalyst, in contrast to other reports. The experimental results confirm that the growth rate is ultimately limited by the gas phase diffusion of hydrocarbon radicals.
Single-walled carbon nanotubes (SWNTs) have many potential applications due to their unique physical properties.1 However, their bulk production and purity still limit these applications. SWNTs are particularly useful when grown by catalytic chemical vapor deposition (CVD) to form mats of vertically aligned single-walled nanotubes (VA-SWNTs).2-8 Recently, VA-SWNT and multi-walled carbon nanotube (MWNT) mats have been grown up to macroscopic sizes of millimeters in height.7,9 It is therefore of interest to understand what controls their growth rate and their ultimate height. Catalytic CVD of SWNTs is believed to occur by the dissociation of a hydrocarbon molecule on the surface of a nanosized catalyst droplet, the diffusion of the carbon atoms through or over the catalyst surface, and extrusion of the carbon nanotube in the root growth mode.10,11 Plasma-enhanced CVD (PECVD) aids this process by providing a source of predissociating growth species from the plasma.12 Catalyst poisoning could occur by the coating of the catalyst droplet by some amorphous or platelet carbon which blocks access of the growth species to the catalyst surface. Clearly, catalyst poisoning could be reduced if an etching process somehow maintains a clear catalyst surface. Hata et al.7 reported that a small content of water vapor could significantly prolong the catalyst lifetime in their water-assisted ethylenebased CVD, by protecting the catalyst against amorphous carbon coating. However, eventually, the catalyst activity decayed exponentially with time, so the nanotube length saturated at ∼1 mm.13 Geohegan et al.9,14 have produced thick MWNT mats, but often by a resupply of the ferrocene catalyst. Puretzky et al. used this data to create a detailed kinetic model of growth rates, which included growth saturation/poisoning by the blocking of the catalyst surface by a graphitic layer.15 Previously, PECVD has been used to provide a source of growth species and to extend the growth regime to lower temperatures. Deposition and etching can be balanced to avoid the deposition of amorphous carbon around a CNT array.16 In * Corresponding author. E-mail:
[email protected]. † Waseda University. ‡ University of Cambridge.
10.1021/jp067776s CCC: $37.00
this paper, using a point-arc-plasma and a remote-plasma condition where ion bombardment of the substrate can be completely eliminated, we control radicals necessary for the SWNT growth and a very efficient catalyst formulation to grow extremely thick VA-SWNT mats. In our case, it is possible that the balance between growth and etching allows us to prolong the catalyst life. We then provide evidence that gas phase diffusion limits nanotube growth rates in this case. We developed a sandwichlike catalyst to grow thick VASWNT mats.8,17-19 The catalyst consists of ultrathin sputtered layers of Al2O3 (top, 0.5 nm), Fe (0.5 nm) on Al2O3 (g5 nm). Here, Fe is the catalyst and Al2O3 acts as a support layer and antisintering agent. The radical CVD system consists of a pointarc microwave plasma which is located at the edge of antenna. The system was used to grow diamond and carbon nanofibers as described previously.20,21 After loading, the catalyst receives a short, 5 min pretreatment with no plasma at a substrate temperature of about 600 °C, a chamber pressure of 20 Torr, and gas flows of H2 (over 99.9999% pure, 45 sccm) and CH4 (over 99.9999% pure, 5 sccm). The pretreatment restructures the thin Fe layer into a series of separate nanoparticles.19,22 The 60 W microwave plasma is then ignited to start nanotube growth. The growth process can last from a few minutes to tens of hours. We previously found an almost linear growth rate of VASWNTs.8,17 The SWNTs are densely packed, with a density of 1-3 × 1016 m-2, or 3-10 nm spacing.19 In order to study what controls the growth kinetics and termination, we used three types of patterned-catalyst substrates. Type I substrates are Si wafers fully coated with no patterning. Type II substrates are type I substrates onto which an array of 0.7 mm diameter gold dots has been evaporated, leaving a honeycomb pattern of exposed catalyst. Type III substrates are Si wafers directly patterned with dots or lines of sandwich catalyst, for growing rod arrays or wall-shaped SWNTs. Figure 1 shows digital and field emission scanning electron microscopy (FE-SEM) (Hitachi S-4800) images of halfcentimeter-high VA-SWNT samples of a honeycomb mat, a rod array, and an individual thin wall which grow from patterned substrates. Our SWNT mats can be easily removed from the substrates by tweezers or a razor blade, as in the work of Hata © 2007 American Chemical Society
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Figure 1. Digital (a) and FE-SEM (b-d) images of half-centimeterhigh VA-SWNTs samples grown on type II and III substrates: (a) a honeycomb mat (pitch size, 900 µm; hole diameter, 700 µm); (b) a cross-sectional morphology of the mat; (c) a rod array (pitch size, 1 mm; rod diameter, 600 µm); (d) an individual thin wall (300 µm × 4 mm). The thin SWNT strands grown at the edge of honeycomb holes in part b buckle due to the fast local growth rate and greater availability of carbon radicals.
et al.7 Figure 2 displays the time evolution of the heights of VA-SWNTs for unpatterned (type I) and patterned (type II and III) substrates. These produce SWNT mats 2.5 mm and halfcentimeter high, respectively, within 20-30 h. Figure 2a indicates a parabolic growth behavior, while Figure 2b illustrates a linear growth trend. The continuous increase of SWNT length with time shows that the catalyst activity is highly durable in both cases. The growth rate of VA-SWNTs shown in Figure 2a does decrease with time, but it does not show the sharp, exponential saturation due to catalyst poisoning found in other reports.13,15 We now consider the underlying growth mechanism. First, there is no growth under the present conditions if the plasma is off, that is, under thermal CVD conditions, at this gas ratio of H2 and CH4, catalyst, and pressure. Thus, the growth species must be a carbon or hydrocarbon radical, not the methane itself. Second, the present VA-SWNT mats are known by marker measurements to grow by the root growth mechanism.18 Third, the SWNT height shows both parabolic and linear dependences on time. Also, the patterned growth (Figure 2a) shows a factor 2 higher growth rate than unpatterned growth. This and the root growth suggest that growth of VA-SWNT mats is limited by diffusion of growth species through the porous mat to the growth site. Growth will follow a “Deal-Grove” type diffusion law:
h2 + Ah ) B(t - τ0)
(1)
where h is the nanotube length, t is time, B is the parabolic rate constant related to diffusion-limited growth, B/A is the reaction rate at the catalyst surface, and τ0 is any delay time on growth. Two diffusion processes could be limiting in eq 1: either gaseous diffusion of radicals through the porous mat, as shown in Figure 3a, or surface diffusion of growth species along the nanotube walls to the root, as suggested by Louchev.23 Now,
Figure 2. Time evolution of the growth of VA-SWNTs on different types of substrates: (a) parabolic growth of SWNT rod arrays (triangles), SWNT honeycomb mats (circles), and without patterns (squares); (b) linear growth of thin SWNT walls (stars).
Figure 3. (a) Illustration of the model for diffusion-limited growth of the VA-SWNT mat via a root growth mode. (b) Schematic diagraph of patterned growth, in which lateral diffusion of carbon radicals from the holes can enhance the growth rate.
the sp2 C atoms of nanotube walls are unsaturated, and they are likely to react with any growth species moving over them. This would give side-wall thickening and create “nanocones”. This has indeed been observed by Merkulov24 but is not seen here. We therefore rule out diffusion over the CNT surface. This leaves diffusion of growth species through the porous mat. Further experiments support this idea. First, the patterned honeycomb mat and rod array grew about twice as quickly as the unpatterned mat, Figure 2a. At a pressure of 20 Torr and 600 °C, the mean free path of carbon radicals is estimated to be 30-50 µm.23 This is 3 orders of magnitude more than the mean spacing between SWNTs (10 nm), but it is about 10 times less than the hole diameter (700 µm) in the honeycomb pattern. Hence, transport of carbon radicals through the unpatterned mat is by Knudsen diffusion, while inside the holes it is transition-diffusion-controlled. As mass transfer of carbon
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Figure 4. Characterization of VA-SWNTs: (a) HRTEM images of clean SWNT bundles; (b) TGA results of the raw VA-SWNT mats (ramping rate at 5 °C/min under fluxing air). The solid line represents the weight-temperature curve; the dot line is the derivative of weight.
radicals in the SWNT mat is much slower than in the honeycomb holes, the holes act as fast channels for mass transfer of carbon radicals (see Figure 3b), and hence, patterned growth is faster. Nevertheless, it is still diffusion-limited. This is in agreement with the data of Figure 2a. Two other experiments further confirm this diffusion-limited mode. First, note that Resasco et al. found that the nanotubes on the top of a mat are knotted together laterally.25 This maintains a flat top surface on the mat. Any nanotube which attempts to grow faster than its neighbors is held back, and buckles slightly along its length. This behavior is indeed seen in the SEM image in Figure 1b. On the other hand, for the type III catalyst pattern which grows into isolated towers, the towers can bend over to allow nanotubes to grow faster on one side which has better gas access, as in Figure 1c. A second experiment is to form an isolated catalyst line, 300 µm wide and 4 mm long. This grows into an even higher wall mat, Figure 1d. Gas has access to both sides of this. This grows at a linear rate, as seen in Figure 2b. It grows without a diffusion limit, limited purely by the reaction rate, B/A in eq 1. The growth was stopped in this case because the nanotube quality eventually deteriorated according to Raman spectroscopy not due to catalyst poisoning. Why is the catalyst highly durable against poisoning? Here, we use CH4 which is a high-purity and an endothermic source of carbon, unlike acetylene. It is less likely to coat catalytic nanoparticles by pyrolitic amorphous carbon. Second, radical CVD has some advantages over other methods for controlling carbon activity at the catalyst surface, and balancing deposition
J. Phys. Chem. B, Vol. 111, No. 8, 2007 1909 and etching. The carbon radical concentration at the substrate can be controlled below a critical level not to form amorphous carbon on the catalyst by the gas ratio of CH4 to H2, the microwave power, the chamber pressure, and the distance between the microwave plasma and the substrate. Third, once SWNTs start to grow, the diffusion-limited kinetics further lowers the carbon radical concentration at the SWNT/substrate interface, a negative feedback to inhibit catalyst poisoning. In contrast, Hata suggests that water vapor may act as a gentle etchant, limiting the overgrowth of carbon on the catalyst, but eventually a strong poisoning effect is seen in their growth kinetics.13 Dai notes that O will balance the concentrations of C and H radicals.26 The catalyst yield can be defined as the mass of nanotubes produced divided by the initial mass of catalyst. Our 5 mm nanotube mat grows from a 0.5 nm Fe catalyst layer. The measured mass density of the SWNT mat is 0.066 g/cm3, corresponding to a nanotube areal density of the order 1012 cm-2, as measured previously. Thus, the catalyst yield is estimated to be ∼105 times, larger than that of Hata et al.7 This implies that our SWNT mats are nearly catalyst-free, so that there is no need for further purification and realignment. High-resolution transmission electron microscopy (HRTEM, JEOL 2010), thermogravimetric analysis (TGA, Shimadzu TGA50), and Raman spectroscopy (Renishaw inVia Raman microscope) were also used to evaluate the purity of the free standing millimeter-high SWNT mats. Figure 4a shows a typical HRTEM image of a SWNT sample. The TEM specimen preparation is reported elsewhere.17 It confirms that the nanotubes are no doubt SWNTs in bundles and free from catalytic nanoparticles. The individual SWNTs usually show relatively large average diameters around 3 nm. The TGA spectrum of the raw millimeter-high SWNT mats is shown in Figure 4b. The derivative data show only one sharp peak in the vicinity of 675 °C, corresponding to the combustion of SWNTs from 550 to 750 °C. The slight weight loss up to 550 °C indicates there is a small percentage of amorphous carbon codeposited with SWNTs, but there is no associated derivative peak. There is no measurable residue remaining after heating above 800 °C, which agrees with the extremely high production yield, suggesting that the VA-SWNT mats are essentially catalyst-free. The purity (over 90 wt %) of our millimeter-high SWNT mats is thus very close to purified SWNTs produced by different methods.27,28 Figure 5a shows a series of Raman spectra measured along the length of the SWNT mat using the 633 nm excitation wavelength. Figure 5b plots the details of the low-frequency radial breathing modes (RBMs). The corresponding SWNT diameter in Figure 5b was calculated via ω (cm-1) ) 224/d (nm) + 14.29 Except for a few Raman spectra from the SWNT tip portion, the RBMs span from about 70 to 400 cm-1, and all of the profiles of RBMs and the high-frequency tangential mode bands (G bands at ∼1590 cm-1) remain almost unchanged along the length. This suggests that the long VA-SWNTs have a range of diameters from 0.6 to 3.0 nm and chiralities, and also retain their distributions along the length. However, it was found that the intensity of disorder-induced bands (D bands at ∼1314 cm-1) increases slightly from root to tip, which implies some degradation of VA-SWNTs in quality. It was found that quality degrades over very long growth times, probably due to deposition from the plasma, as revealed by TGA test. Although the low overall intensity ratios of D bands to G bands in Figure 5a suggest that 2-mm-long SWNTs are still fairly high quality, the half-centimeter-long SWNTs showed poorer Raman spectra.
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Figure 5. (a) Series of Raman spectra measured at a constant increment from tip to root of a 2-mm-long SWNT sample; each spectrum has been normalized with regard to its G band. (b) Details of RBM peaks.
This implies that, to obtain high-quality centimeter-long SWNTs, some optimization is still required. In conclusion, growth of high-quality VA-SWNTs at 600 °C and a low pressure by remote-plasma-assisted CVD exhibits a very promising future for applied research of SWNTs. A nanotube yield of ∼105 was found. Diffusion-limited growth kinetics lacking catalyst poisoning was proposed for unpatterned growth of VA-SWNTs, and it was proven by the analyzing time evolution of unpatterned and patterned growth. This method gives reasonable process control and scale-up. Acknowledgment. This work is supported in part by a Grant-in-Aid for Center of Excellence (COE) Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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