Thermal Analysis Study of the Growth Kinetics of Carbon Nanotubes

May 11, 2009 - With the aim of improving the yield and purity of carbon nanotubes (CNTs), we have studied the chemical vapor deposition (CVD) growth o...
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J. Phys. Chem. C 2009, 113, 9623–9631

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Thermal Analysis Study of the Growth Kinetics of Carbon Nanotubes and Epitaxial Graphene Layers on Them Xiaofeng Feng, Kai Liu, Xu Xie, Ruifeng Zhou, Lina Zhang, Qunqing Li, Shoushan Fan, and Kaili Jiang* Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua UniVersity, Beijing 100084, P. R. China ReceiVed: February 11, 2009; ReVised Manuscript ReceiVed: April 4, 2009

With the aim of improving the yield and purity of carbon nanotubes (CNTs), we have studied the chemical vapor deposition (CVD) growth of CNTs with Fe-Mo powder catalyst and acetylene precursor by using the technique of in situ simultaneous thermal analysis (STA). We could distinguish this way the rapid catalytic growth stage of CNTs and the slow epitaxial growth stage of graphitic impurities on the nanotubes, which were discovered to be graphene layers. Both reactions were first order, and the activation energies were determined to be 177 and 189 kJ/mol, respectively. In the rapid-growth stage, the growth of CNTs could be further tuned from kinetically controlled regime to gas-diffusion-controlled regime, where the CNTs exhibit a longer growth time and a higher yield, using appropriately shaped crucibles. These results are helpful for both elucidating the growth mechanism of CNTs and improving the quality of mass-produced CNTs. 1. Introduction Carbon nanotubes (CNTs) are the focus of intense study due to their excellent physical properties that enable promising applications in many fields.1 So far, a variety of methods for the synthesis of CNTs have been developed, including arcdischarge,2 laser ablation,3 and chemical vapor deposition (CVD).4 Because of the mild preparation conditions and the potential for mass production, the CVD method is particularly attractive for the synthesis of CNTs. There are primarily three kinds of morphologies of CNTs synthesized by the CVD method: tangled CNTs grown from powder catalysts,5 isolated CNTs lying on a substrate,6 and CNT arrays standing on a substrate.7 Currently, only CNTs in the first category have been mass-produced and commercialized.8-10 However, these commercially available CNT powders suffer from lower yield and less purity compared with the CNTs in vertically aligned arrays. So, a major challenge for industrial applications is how to improve the yield and purity of the mass-produced CNTs. The solution to this challenge lies in a better understanding of the growth mechanism of CNTs from powder catalysts. To study the growth mechanism of CNTs, a variety of methods have been developed, including in situ observation under high-resolution transmission electron microscope (HRTEM),11 growth mark methods,12-14 optical imaging methods,15-17 and in situ mass spectroscopy methods.18-20 Despite these efforts, some basic questions are still unresolved, in particular whether the growth of CNTs is gas-diffusioncontrolled (the growth rate is controlled by the gas-diffusion step)13 or kinetically controlled (the growth rate is controlled by the chemical reaction, i.e. the decomposition of acetylene and the formation of CNTs)14 and why the growth terminates.21 Furthermore, whereas most of the growth kinetics studies focused on CNT arrays, few efforts have been made to study CNTs grown from powder catalysts. * To whom correspondence should be addressed. Tel.: +86 10 62796017. Fax: +86 10 62792457. E-mail: [email protected].

Because the in situ simultaneous thermal analysis (STA) technique possesses the merits of quantitatively probing the growth kinetics and analyzing the growth products, it is an ideal tool to study the growth mechanism of CNTs. We therefore use this technique to study the CVD growth of CNTs with Fe-Mo powder catalysts and acetylene precursor, aiming at improving the yield and purity of CNTs. Meanwhile, we distinguish the rapid-growth stage of CNTs and the slow-growth stage of graphitic impurities and measure the activation energies for both stages. For the rapid-growth stage, we further study the kinetically controlled and the gas-diffusion-controlled growth of CNTs and the catalyst deactivation due to the catalyst particle becoming fully enclosed within graphene layers. By comparing results with differently shaped crucibles, solutions for improving the yield and purity of CNTs are proposed. These results are helpful for both elucidating the growth mechanism of CNTs and improving the quality of mass-produced CNTs. 2. Experimental Section 2.1. Preparation of Fe-Mo Catalyst. Fe-Mo catalyst was prepared as follows: 11.32 g Fe(NO3)3 · 9H2O, 0.4 g MoC10H14O6 (molybdenum (VI) oxide bis(2,4-pentanedionate), 99%, Alfa Aesar) and 8 g Al2O3 (Dessugar) were first dissolved in 100 mL CH4O; then the mixture was stirred for 24 h, ultrasonicated for 1 h, and evaporated in a distillation bottle. The remaining yellow powder was Fe-Mo catalyst. 2.2. Growth of CNTs in Thermal Analyzer. The experimental setup is schematically illustrated in Figure 1. The growth of CNTs was performed in a simultaneous thermal analyzer (NETZSCH STA 449 C) at atmospheric pressure with about 1 mg Fe-Mo catalyst in a corundum crucible. In a typical growth experiment, the reactor was heated to the growth temperature at a rate of 30 K/min and was then kept at the growth temperature under the protection of an argon flow for 10 min. Acetylene was then introduced into the reactor to initiate the CNT growth. After a reaction of 30 min, the reactor was cooled down to room temperature under the protection of an argon flow. All gases were delivered into the reaction zone by using mass

10.1021/jp901245u CCC: $40.75  2009 American Chemical Society Published on Web 05/11/2009

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Figure 1. Schematic experimental setup for in situ study of the growth of CNTs by using the STA technique: (1) argon tank, (2) acetylene tank, (3) mass flow controllers, (4) thermal analyzer (reactor), (5) capillary tube, (6) mass spectrometer, and (7) workstation.

Figure 3. Typical TG-DTG profiles of the growth of CNTs in the P-crucible (a), Q-crucible (b), and C-crucible (c), respectively. The inserts are images of the corresponding crucibles. The rapid-growth stage and the slow-growth stage are distinguished in (a). All of the growth was performed at 640 °C with a 4 kPa acetylene partial pressure. Figure 2. Typical SEM (a) and TEM (b) images of the CNTs synthesized in the in situ STA system.

flow controllers. According to the results we obtained in an earlier study,22 carbon deposits on the inner walls of the reactor may affect the growth of CNTs. We therefore removed carbon deposits and other impurities by heating the reactor to above 1000 °C in the presence of oxygen. Then a blank run, which was carried out without catalyst under the same conditions, was recorded as a baseline before the growth of CNTs. 2.3. Characterization of the Products. Scanning electron microscope (SEM) images were taken with an FEI Sirion 200 at an acceleration voltage of 7 kV. High-resolution transmission electron microscope (HRTEM) images were taken with an FEI Tecnai 20 at an acceleration voltage of 200 kV. The products were characterized by SEM, TEM, Raman spectroscopy with an excitation wavelength of 514.5 nm (Renishaw 1000), and thermogravimetric analysis (TGA). Typical SEM and TEM images of the products are shown in Figure 2, revealing abundant multiwalled CNTs with a diameter of around 10 nm. Raman spectra and TGA of the products are exhibited in Figures S1 and S2 respectively of the Supporting Information. 3. Results and Discussion To investigate the growth kinetics of CNTs, we use STA to monitor in situ the weight gain during the growth process. According to previous studies,13,14,21 the growth of CNTs can be either gas-diffusion-controlled (the growth rate is controlled by the gas-diffusion step) or kinetically controlled (the growth rate is controlled by the chemical reaction, i.e. the decomposition of acetylene and the formation of CNTs), depending on different gas-diffusion conditions. To further investigate this gas-diffusion

effect, we used three types of crucibles with different shapes: (1) plate-like crucible (P-crucible, part a of Figure 3), (2) quartzcap-covered plate-like crucible (Q-crucible, part b of Figure 3), (3) cup-like crucible (C-crucible, part c of Figure 3). The catalyst was put in the center of P-crucible and Q-crucible or on the bottom of C-crucible during the growth, so the feedstock molecules must diffuse via different paths to reach the catalyst, respectively. Therefore, the three types of crucibles have different restrictions on the gas diffusion: a strong restriction by the C-crucible, but no restriction by the P-crucible, and a medium restriction by the Q-crucible. Thus, the growth of CNTs in different regimes can be studied accordingly. Typical thermogravimetry-derivative thermogravimetry (TGDTG) profiles of the CNT growth in the three types of crucibles are shown in Figure 3. There are three distinct features in these profiles. First, all of the growth can be easily divided into two growth stages: a rapid-growth stage with a varying growth rate and short time span, and a slow-growth stage with an almost fixed but very low growth rate and long time span. Second, the time evolutions of the growth rates during the rapid-growth stages are quite different for the three types of crucibles, indicating different growth kinetics. Finally, the time span of the rapid-growth stage depends significantly on the crucible type, increasing in the order P-, Q-, and C-crucible. This trend reveals different growth termination effects that are crucible-dependent. We now address three key questions: (1) Why does the growth show two distinct stages, and what are their products? (2) Why do the growth rates exhibit different time evolutions in the three types of crucibles? (3) Why does the rapid-growth stage terminate and what is the mechanism? To answer these questions, we carried out systematic studies on the growth

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Figure 4. Kinetically controlled growth of CNTs in the P-crucible. (a) Typical TEM image of the intermediate products, including both grown CNTs and un-nucleated catalysts; (b) Fit of eq 5 to the experimental DTG profile (600 °C, 4 kPa); (c) Fit of eq 7 to the experimental DTG profile (680 °C, 4 kPa), which yielded excellent agreement.

kinetics of the rapid-growth stage and the slow-growth stage respectively, described in the next section. Theoretical models are then proposed to interpret the growth kinetics and termination mechanisms, on the basis of which we make suggestions for improving the quality and yield of mass-produced CNTs. 3.1. Growth Kinetics of CNTs in Different Regimes (the Rapid-Growth Stage). We observe that the products of the rapid-growth stage are mainly CNTs. To explore the growth kinetics of CNTs, we should first interpret the time evolution of the growth rate. As shown in Figure 3, the time evolutions of the growth rate in the three types of crucibles are different due to their different influences on the gas diffusion. The growth rate increases and declines quickly for the growth of CNTs in the P-crucible; in the Q- and C-crucibles, however, the growth rate first increases rapidly to a maximum, and then decreases slowly, but declines faster later. So, the different crucibles appear to exhibit different growth regimes of CNTs. To compare these in more detail, we define a maximum growth rate (MGR) as the maximum instantaneous growth rate in the rapid-growth stage, with the unit of mg/min or %/min (ratio of the growth rate to the catalyst mass). 3.1.1. Kinetically-Controlled Growth of CNTs in the PCrucible. A typical TG-DTG profile of the CNT growth in the P-crucible is shown in part a of Figure 3. After the introduction of acetylene, the growth rate rapidly increases to MGR, and then declines quickly too; the whole growth process ends in about five minutes. Because there is no restriction on the gas diffusion by the P-crucible, the feedstock can transfer fast and directly to the catalyst, so the growth of CNTs should not be controlled by the gas-diffusion step. Previous studies showed that there were nucleation, growth, and termination periods during the growth of CNT arrays.12,14 However, the three periods cannot be distinguished here, though nucleation and termination should be dominant in the beginning and the end of the rapidgrowth stage, respectively. To understand the growth process, products with different growth times were observed under TEM. Both grown CNTs and un-nucleated catalysts were found in the intermediate products (part a of Figure 4), indicating that some catalysts have already decayed whereas others still do not nucleate. Therefore, nucleation, growth, and deactivation of the catalysts may take place together during the growth process. To quantitatively interpret the DTG profile, we established a model to describe the growth of CNTs from powder catalysts. To simplify the problem, we assume that the catalysts show identical activity, and the growth rate of individual CNTs remains the same during the growth. Thus, the total growth rate is proportional to the number of active catalysts, so we only need to consider the time evolution of the number of active

catalysts. The total number of catalysts is defined as N, including un-nucleated, active, and deactivated catalysts, of which the numbers at time t are expressed as nu(t), na(t), and nd(t), respectively. More assumptions we make are as follows: (1) the number of catalysts that nucleate is proportional to the number of un-nucleated catalysts, with a coefficient R; (2) the number of catalysts that decay is proportional to the number of active catalysts, with a coefficient β. On the basis of the assumptions above, we derive the following equations:

N ) nu(t) + na(t) + nd(t) dna(t) ) Rnu(t) - βna(t) dt dnd(t) ) βna(t) dt

(1) (2) (3)

Taking into account the initial conditions na(0) ) nd(0) ) 0, we can obtain the time evolution of the number of active catalysts by solving the differential eqs 2 and 3:

na(t) )

RN (e-βt - e-Rt) R-β

(4)

Considering the growth rate of an individual CNT (r), the total growth rate of the CNTs (R) can be expressed by:

R(t) ) na(t)r )

RNr -βt (e - e-Rt) R-β

(5)

Furthermore, the average lifetime of the catalysts is calculated by integration:

τ)

∫0∞ na(t)dt N

)

1 β

(6)

which is only related to the decay probability β. To test the validity of this model, we have tried to fit eq 5 to the DTG profiles, which reflect the time evolution of the growth rate. However, the fitting did not yield satisfactory agreement at first because the initial growth rate in the experiment did not increase as sharply as that in the theory, as illustrated in the blue box of part b of Figure 4. We believe this is because the feedstock could not reach the catalysts immediately after the acetylene valve was opened, and thus the initial growth rate was controlled by the gradual increase of acetylene, not the growth kinetics. Nevertheless, after the initial stage (∼1 min), the growth was totally controlled by the intrinsic growth kinetics, so we can still fit eq 5 to the DTG profiles. As demonstrated by the red line in part b of Figure 4, the fitting yielded excellent

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Figure 5. Gas-diffusion-controlled growth of CNTs. (a) MGR remains unchanged for different amounts of catalysts; (b) MGR at different growth temperatures as a function of the acetylene partial pressure; (c) Fits of eq 9 and eq 7 to the DTG profile of the CNT growth (640 °C, 4 kPa) in the C-crucible; (d) Fits of eq 9 and eq 7 to the DTG profile of the CNT growth (640 °C, 4 kPa) in the Q-crucible.

agreement, supporting the soundness of the model, and the fitting parameters for the specific growth condition (600 °C, 4 kPa) were R of 1.9443 min-1, β of 1.9427 min-1, and Nr of 187.3%/ min, respectively. It is interesting that the nucleation probability R is very close to the decay probability β; this point will be discussed later. In addition, the average catalyst lifetime of 0.516 min was obtained using eq 6 for the specific growth condition mentioned above. Thus, the catalyst lifetimes under different growth conditions can be derived, which is of significance to the study of the catalyst activity. This will be discussed in more detail in section 3.3. In addition, to investigate the decay of the catalysts, we have studied the end of the rapid-growth stage. In this period, the nucleation of the catalysts was completed and the growth of CNTs was dominated by the decay of catalysts. Thus, a simpler formula can be derived for describing the decay process of the catalysts. Similar to the model above, by assuming that the active catalysts decay with a probability β, we can straightforwardly derive that the growth rate decays exponentially:

R(t) ) R(t0) exp[-β(t - t0)]

(7)

where R(t) is the CNT growth rate at time t and t0 is the time when the growth begins to be dominated by the decay of catalysts. As illustrated by the red line in part c of Figure 4, fitting of eq 7 to the experimental DTG profile (680 °C, 4 kPa) yielded excellent agreement. This result is consistent with previous studies on the decay of the growth rate of single-walled CNT arrays,23,24 revealing a common trend of exponential decay of growth rate for these catalysts. 3.1.2. Gas-Diffusion-Controlled Growth of CNTs in the C-Crucible. A typical TG-DTG profile of the CNT growth in the C-crucible is shown in part c of Figure 3. When acetylene was introduced, the growth rate increased to MGR, then declined slowly first and rapidly later; the two declining periods reflect two different regimes. In addition, the growth rate is much lower than that in the P-crucible, indicating that it may be restricted by the gas-diffusion effect. For CNT growth to occur, acetylene must reach the catalyst at the bottom of the C-crucible, thus

requiring diffusion through the crucible depth and an increasing CNT thickness. We first investigated the change of MGR with different growth conditions. First, no matter how many catalysts we added, MGR remains almost unchanged (part a of Figure 5), so it appears to be restricted by the gas-diffusion rate. Second, for the growth of CNTs under different growth conditions, MGR is related to the acetylene partial pressure rather than the growth temperature (part b of Figure 5), because the diffusion rate is determined by the concentration gradient of acetylene, but less sensitive to the temperature. These results all indicate that MGR was determined by the diffusion of acetylene through the C-crucible. Furthermore, our estimation showed that the flux of carbon atoms diffusing to the bottom of the crucible is equal to MGR, indicating that all carbon atoms reaching the catalysts are absorbed. Next, we consider the time evolution of the CNT growth rate. As the growth continued, the growth rate declined slowly because the grown CNTs acted as densely packed nanoporous layers presenting a diffusion barrier to the acetylene precursor. Thus, the growth rate should also be restricted by acetylene diffusion through an increasing thickness of CNTs. When the growth continued, the feedstock was reduced, and thus the growth rate declined. Therefore, the time evolution of the CNT growth rate could be deduced (Supporting Text 1 of the Supporting Information). The CNT mass as a function of time (t) can be expressed as:

1 m(t) ) (√A2 + 4B(t - t0) - A) 2

(8)

where A and B are two parameters related to the diffusion coefficient, and t0 is the time when the CNT growth begins. By differentiation we can get the time evolution of the growth rate:

R(t) )

dm(t) ) dt

B

√A

2

(9)

+ 4B(t - t0)

This growth process is similar to the gas-diffusion-controlled growth of CNT arrays.13,17,25

Thermal Analysis Study of CNT Growth

Figure 6. Growth kinetics of CNTs in the P-crucible. (a) MGR at 640 °C as a function of the acetylene partial pressure. (b) MGR with a 4 kPa acetylene partial pressure as a function of the growth temperature. The insert is a plot of the natural logarithm of MGR against the reciprocal of absolute temperature. Solid squares are experimental data, and the red line is the linear fit curve.

Afterward, the growth rate of CNTs declined rapidly in the end of the rapid-growth stage, which was dominated by the decay of the catalysts. In this period, the feedstock was relatively abundant because there were far fewer active catalysts. As a result, the growth rate was found to decay exponentially as well.23,24 Thus, the DTG profile can be partly fitted using eq 9 and eq 7, as illustrated in part c of Figure 5. Note that we have omitted the inverse reaction step in our model, which might play a role in the case of the C-crucible. The growth of CNTs was performed in the Q-crucible as well, which was designed with a medium restriction on the gas diffusion. The DTG profile of the CNT growth in the Q-crucible is similar to that in the C-crucible but with a higher growth rate for the less restriction on the gas diffusion. Fits of eq 9 and eq 7 to the DTG profile are shown in part d of Figure 5, which yielded excellent agreement. The results with this crucible suggested ways to improve CNT yield, which will be discussed in a later section. 3.1.3. Growth Kinetics of CNTs: The Reaction Order and the ActiWation Energy. Whether the growth rate of CNTs is limited by the surface reaction at the gas-catalyst interface or the diffusion of carbon atoms through the catalyst particle is still unclear. Clarifying this point is crucial to elucidating the growth mechanism of CNTs. To determine the rate-limiting step of the CNT growth, it is necessary to know the reaction order and the activation energy, so we studied the growth of CNTs under different acetylene partial pressures and growth temperatures. In the following experiments, the growth of CNTs was performed in the P-crucible to ensure that the CNT growth rate is controlled by the chemical-reaction kinetics. To study the effect of acetylene partial pressure, CNT growth with the acetylene flow tuned from 4 to 9 sccm was performed (the total gas flow was fixed at 100 sccm). At low acetylene concentration, MGR increases linearly with the acetylene partial pressure, as shown in part a of Figure 6, indicating that the reaction is first-order. However, when the acetylene partial pressure is relatively high, MGR arrives at a maximum and then

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9627 saturates. This is due to the catalysts being fully covered with acetylene molecules, while the surface reaction at the gas-catalyst interface is limited, so MGR becomes saturated. Therefore, we can conclude that the CNT growth here is a first-order reaction at low acetylene partial pressure, in agreement with other studies.12,14 However, it will become a reaction of zero order at high acetylene concentration. As illustrated in part b of Figure 6, the growth rates were plotted against the growth temperature in the range of 590 to 720 °C. The acetylene partial pressure was fixed at 4 kPa, revealing that the CNT growth was a first-order reaction. Along with the growth temperature increasing, MGR increases from 590 to 640 °C, then saturates between 640 and 680 °C, but declines when the temperature is higher than 680 °C. It is easy to understand the increasing of MGR from 590 to 640 °C, and the almost unchanged MGR between 640 and 680 °C can be explained by the restriction of feedstock. At the time of MGR, about 40% of the acetylene flow was consumed, which corresponds to the maximum consumption of the gas.26 In addition, MGR declines when the temperature is higher than 680 °C. This decline of CNT growth rate at high temperature has been observed in many studies.16,27-29 It may be due to the softening of the catalyst at high temperature, allowing the catalyst to be easily encapsulated by graphene layers. For a chemical system, the activation energy is an important parameter to determine the rate-limiting step. According to the Arrhenius equation, the rate constant can be expressed as k ) A exp(-Ea/RT),30 where A is the frequency factor, Ea is the activation energy, R is the gas constant, and T is the absolute temperature. Because the growth rate r is proportional to the rate constant, a plot of ln(r) versus 1/T should give a straight line with a slope of -Ea/R. Thus, by plotting the natural logarithm of MGR against the reciprocal of absolute temperature, we obtained an activation energy of 177 kJ/mol (part b of Figure 6), which is consistent with some previous studies.12,14,29,31 According to our previous studies,12,32 this activation energy may be attributed to the heterogeneous decomposition of acetylene.12,29,32 This differs somewhat from other reports,33,34 which claimed that the rate-limiting step of the CNT growth is the diffusion of carbon atoms through the catalyst particle. 3.2. Products and Growth Kinetics of the Slow-Growth Stage. According to the TG-DTG profiles of the CNT growth (Figure 3), following the rapid-growth stage is a slow-growth stage, exhibiting a slow enduring weight gain of the products. Similar results have been observed in some other studies,35,36 but this slow-growth stage has not received much detailed attention, and its products are still unclear. In the following section, we address aspects of the slow-growth stage, in particular whether the products are amorphous carbon or graphitic materials, and what the growth kinetics of this stage are. Answers to these questions help further elucidate the growth mechanism and provide insight into how to improve the purity of CNTs. 3.2.1. What Are the Products of the Slow-Growth Stage? TGA was used to analyze the products of the slow-growth stage. DTG is a common method for quantitative analysis, and the content of each species corresponds to its peak area in DTG. A previous study showed that amorphous carbon and graphitic materials burn at different temperatures,37 so we can measure their proportions in the products from the DTG curve (Figure S2 of the Supporting Information). Once the components of the rapid-growth stage products and those of the final products of both stages are known, we can derive the components of the slow-growth stage products by comparison. The products, grown

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Figure 8. Growth kinetics of the slow-growth stage. (a) SGR at 640 °C as a function of the acetylene partial pressure; (b) SGR with a 4 kPa acetylene partial pressure as a function of the growth temperature. The insert is a plot of the natural logarithm of SGR against the reciprocal of absolute temperature. Solid squares are experimental data, and the red line is the linear fit curve.

Figure 7. Products of the slow-growth stage. (a) TG profile of the CNT growth at 640 °C with a 4 kPa acetylene partial pressure for 3 h. (b) Diameter distribution of the CNTs grown for 10 min; to the right is a typical HRTEM image of a CNT. (c) Diameter distribution of the CNTs grown for 3 h; to the right is a typical HRTEM image of a CNT with graphene layers on it (indicated by arrow).

at 640 °C with a 4 kPa acetylene partial pressure in the P-crucible, were analyzed by this method (Supporting Text 2 of the Supporting Information), revealing that 82% of the slowgrowth stage products is graphitic materials, and the remaining 18% is amorphous carbon. Analyses of the products grown under other conditions showed similar results. These results raise the question of what the graphitic material is that can grow steadily for a long time? To clarify the problem, CNT growth was performed for 3 h to increase the slow-growth stage products. As shown in part a of Figure 7, the slow-growth stage can keep steady for 3 h, and the products of this stage were increased relatively. Then the products were carefully investigated under HRTEM to reveal the slow-growth stage products. There were no other obvious forms of graphitic materials except CNTs, but these CNTs were covered to varying degrees with graphene layers (part c of Figure 7, and Figure S3 of the Supporting Information), which could be distinguished from amorphous carbon due to their layer structure. Thus, it appears very likely that the epitaxial growth of graphene layers on CNTs is the main product of the slow-growth stage. Further, we obtained the diameter distributions of CNTs grown for 10 min and 3 h, respectively. The two groups of CNTs have similar diameter distributions, but different mean values (Figure 7), due to the increase of graphene layers on CNTs. The average diameter of the CNTs increases from 11.36 to 12.77 nm as the reaction time extends from 10 min to 3 h, indicating that about two graphene layers on average were deposited on CNTs.

To confirm this conclusion, a comparative experiment on CNT arrays was performed. Multiwalled CNT arrays,38 pulled out directly from the substrate, were put in the reactor under the same condition for 3 h. There was about 20% weight gain for the 3 h reaction (part a of Figure S4 of the Supporting Information), which was similar with the slow-growth stage aforementioned. TGA showed that the increased products were graphitic materials (part b of Figure S4 of the Supporting Information), and TEM observations exhibited graphene layers on the CNTs. These results further support the view that the epitaxial growth of graphene layers on CNTs is the main product of the slow-growth stage. In fact, this result is consistent with the observation of epitaxial growth of graphene layers during the growth of carbon fibers, as the growth of CNTs and carbon fibers is intrinsically similar.39,40 To sum up, the slow-growth stage is dominated by the epitaxial growth of graphene layers on CNTs. 3.2.2. Kinetics of the Epitaxial Growth of Graphene Layers on CNTs. Having ascertained that the slow-growth stage is dominated by the epitaxial growth of graphene layers, we have studied in further detail the growth kinetics of this stage, which is important to understanding and optimizing the growth of CNTs. All of the following experiments were performed in the P-crucible to explore the intrinsic growth kinetics of the slowgrowth stage. To analyze the results, a slow-growth rate (SGR) is defined as the growth rate of the slow-growth stage, which is steady during the whole stage, with the unit of %/min. The slow-growth stages at 640 °C with the acetylene partial pressure tuned from 4 to 9 kPa have been studied. As shown in part a of Figure 8, along with the acetylene partial pressure increasing, SGR first increases linearly, but reaches a maximum at 7 kPa, and then saturates. This behavior is similar to that observed during CNT growth (part a of Figure 6), indicating that the epitaxial growth of graphene layers on CNTs is also a first-order reaction at low acetylene partial pressure. However, at high acetylene concentration, SGR becomes saturated due to

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Figure 9. (a) Typical TEM image of the deactivated catalyst, which is coated with graphene layers; (b) total growth time and average lifetime of the catalysts at 640 °C as a function of the acetylene partial pressure; (c) total growth time and average lifetime of the catalysts with a 4 kPa acetylene partial pressure as a function of the growth temperature. Solid squares and circles are experimental data.

the limited effective deposition area for epitaxial growth, and thus it becomes a reaction of zero order. To study the effect of the growth temperature, experiments were carried out at different temperatures ranging from 590 to 720 °C with a 4 kPa acetylene partial pressure. As the growth temperature increased, SGR also increased. By plotting the natural logarithm of SGR against the reciprocal of absolute temperature, we can obtain an activation energy of 189 kJ/mol from the slope of the fitted line (part b of Figure 8). This activation energy of the epitaxial growth is close to that of CNT growth, suggesting similar reaction kinetics. Also, the growth of CNTs and epitaxial growth of graphene layers both include the heterogeneous decomposition of acetylene, so the two similar activation energies suggest the same chemical-reaction barrier. Therefore, at relatively low partial pressure of acetylene, the rate-limiting step for both the catalytic growth of CNTs and the epitaxial growth of graphene layers on CNTs is the heterogeneous decomposition of acetylene.12,29,32 3.3. Deactivation and Lifetime of the Catalysts. One crucial problem in the synthesis of CNTs is automatic termination of the growth, which prevents the synthesis of arbitrarily long CNTs. So, the question is why does the growth stop, or how do the catalysts become deactivated? Various reasons have been proposed.41-46 One explanation is that the catalyst poisoning is caused by the accumulation of amorphous carbon over it. However, recent study shows that amorphous carbon can be consumed by Ni catalyst and graphene layers are precipitated.47 So, the deposition of amorphous carbon may not be the reason for catalyst deactivation. Under TEM observations, we found that the deactivated catalysts here were coated with graphene layers (part a of Figure 9), which may prevent the absorption of carbon atoms. This effect provides a plausible explanation for the catalyst deactivation. According to the model mentioned in section 3.1.1, the decay probability is close to the nucleation probability, suggesting a similarity between the nucleation and deactivation. During the nucleation process of the growth, carbon concentration of the catalyst is supersaturated and graphene layers will be precipitated.32 Moreover, precipitation of graphene layers also occurs during the deactivation process, thus encapsulating the catalyst. This could be due to the fluctuation of carbon concentration in the catalysts as well as the growth environment. It was found that the growth of CNTs under low acetylene partial pressure (part b of Figure 9) and in the C-crucible (part c of Figure 3) had a longer growth time due to the low carbon concentration and the stable growth environment, respectively. To quantitatively study the catalyst deactivation, the average lifetime of the catalysts was obtained from fits of the growth equation, and the resulting catalyst lifetimes under different conditions are shown in Figure 9. To further test our model,

the total growth time, defined as the time when the growth rate is above 5% of MGR in the rapid-growth stage, was deduced from the measurements. This total growth time should be comparable to the average catalyst lifetime. As shown in Figure 9, the changing trends of the average lifetime and the total growth time are consistent, revealing the validity of the model. As the acetylene partial pressure increased, the average lifetime of the catalysts declined (part b of Figure 9), which can be attributed to the larger fluctuation at higher carbon concentration. However, the change of catalyst lifetime with the growth temperature is quite complex: the catalyst lifetime remains short at low temperature (∼590-620 °C), then increases from 630 to 680 °C, but declines when the temperature is higher than 680 °C (part c of Figure 9). The reason for short lifetime of the catalysts at low temperature is unclear. At high temperature, the catalyst will be softened and can be easily encapsulated by graphene layers, leading to the decline of the lifetime. Thus, we achieve an optimal catalyst lifetime at around 680 °C in our experiments. These results provide useful clues to how to extend the lifetime and improve the catalytic activity of the catalysts. 3.4. Suggestions for Improving the Yield and Purity of CNTs. At present, only CNTs grown from powder catalyst have been mass-produced and commercialized,8-10 but these CNTs suffer from lower yield and less purity compared with CNT arrays. So, one important challenge is how to improve the yield and purity of the mass-produced CNTs. On the basis of the above studies of the growth mechanism, we make several suggestions for how to improve the yield and purity of CNTs. To improve the CNT yield (defined as the weight ratio of CNTs versus catalyst), optimization of the growth conditions is always important.8-10 In our experiments, low acetylene partial pressure and an appropriate temperature (680 °C) can create a high yield (Figure 10). Further, it is of vital importance to control the local carbon concentration around the catalysts. However, just varying the acetylene partial pressure is not a very effective way of controlling the local carbon concentration. This can be better achieved through the use of an appropriately shaped crucible, which will affect the gas diffusion, and thus have a crucial impact on the local carbon concentration. This we have demonstrated by the growth of CNTs in the three types of crucibles. As shown in Figure 3, a yield of 400% was obtained when the growth was performed in P-crucible, with a short growth time. The CNT growth in the C-crucible was much slower, but a yield of 430% was achieved due to the low carbon concentration. Furthermore, the Q-crucible was designed as a medium case, and a higher yield of 490% was obtained with yet a shorter growth time, which should be attributed to the medium restriction on the gas diffusion, providing an appropriate carbon concentration. Hence, it is more effective to control

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Feng et al. Furthermore, there are always impurities in the as-grown CNTs, and we have shown in this study that one important example is graphitic impurities on CNTs. The epitaxial growth of graphitic impurities can take place steadily after the growth of CNTs. Therefore, to improve the purity of CNTs, we can control the reaction time to a proper value (no slow-growth stage), and thus reduce the graphitic impurities. These suggestions will be helpful for improving the yield and purity of the mass-produced CNTs. 4. Conclusions

Figure 10. (a) The CNT yield at 640 °C as a function of the acetylene partial pressure; (b) the CNT yield with a 4 kPa acetylene partial pressure as a function of the growth temperature. Solid squares are experimental data; (c) the quality of CNTs with different growth temperatures. The black square is the intensity ratio of D to G bands in Raman spectra, and the red circle is the initial burning temperature.

the local carbon concentration around the catalysts through the choice of crucible shape. This way, a more suitable carbon concentration can be obtained and the CNT yield can be further improved. Another feature of CNTs for practical applications is their quality, which is related to the growth temperature. In our studies, Raman spectra and TGA were combined to study the quality of CNTs. We employed first-order Raman spectroscopy to obtain the information about the crystallinity of CNTs (Figure S1 of the Supporting Information). Two Raman bands at ∼1350 cm-1 (D band) and ∼1580 cm-1 (G band) are originated from disorder-induced features due to the finite particle size effect or lattice distortion and the Raman active in-plane atomic displacement E2g mode, respectively.48 The intensity ratio of D to G band (ID/IG) has a linear relation with the inverse of the in-plane crystallite dimension. As the growth temperature increases from 600 to 700 °C, the value of ID/IG declines from 1.02 to 0.82, indicating that the degree of long-range ordered crystalline perfection of the CNTs increases with the temperature. In addition, if the crystallization of the graphene layers of CNTs is better, the initial burning temperature of the CNTs, obtained from the TGA (Figure S2 of the Supporting Information), will be higher.49 Thus, the quality of CNTs can be expressed by both the value of ID/IG in Raman spectra and the initial burning temperature. As demonstrated in part c of Figure 10, the quality of CNTs becomes better as the growth temperature increases.33,49,50 This result suggests a straightforward approach to improve the quality of CNTs.

In conclusion, we have used in situ STA as well as other experimental techniques to study the growth mechanism of CNTs. CNT growth with Fe-Mo powder catalysts and acetylene precursor was performed and monitored by in situ thermal analysis. The rapid-growth stage as well as the slow-growth stage was distinguished, of which the products were CNTs and epitaxial graphene layers on them, respectively. In the rapidgrowth stage, the growth of CNTs could be tuned from kinetically controlled regime to gas-diffusion-controlled regime through the crucible. By varying the acetylene partial pressure and growth temperature, the growth of CNTs was shown to be first-order reaction with an activation energy of 177 kJ/mol, whereas the epitaxial growth of graphene layers was also firstorder reaction with an activation energy of 189 kJ/mol. We suggest that both the rate-limiting steps for the growth of CNTs and graphene layers are the heterogeneous decomposition of acetylene. Further studies on the deactivation and lifetime of the catalysts were provided. On the basis of these studies of the growth mechanism, we have made several suggestions for ways to improve the yield and purity of CNTs. These results contribute to a better understanding of the growth mechanism of CNTs and should enable improvements in the yield and quality of the mass-produced CNTs. Acknowledgment. This work was financially supported by the National Basic Research Program of China (2005CB623606, 2007CB935301), NSFC (10704044, 50825201, 10721404), and Fok Ying Tung Education Foundation (111049). We thank Qingyu Zhao for the help in operations of the STA system, Chen Feng for helpful discussions, and Francois Grey for polishing the language. Supporting Information Available: Raman spectra and TGA of the as-grown CNTs, TEM images of CNTs with epitaxial graphene layers, TG profile of the epitaxial growth of graphene layers on CNT arrays for 3 h and TGA of the whole products, derivation of the time evolution of CNT growth rate in the C-Crucible, and the TGA method used to analyze the products of the slow-growth stage. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; Dresselhaus, M. S., Dresselhaus, G., Avouris, Ph., Eds.; Springer: Heidelberg, 2001. (2) Iijima, S. Nature 1991, 354, 56. (3) Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1995, 243, 49. (4) Tibbetts, G. G. J. Cryst. Growth 1984, 66, 632. (5) Dai, H. J.; Rinzler, A. G.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1996, 260, 471. (6) Kong, J.; Soh, H. T.; Cassell, A. M.; Quate, C. F.; Dai, H. J. Nature 1998, 395, 878. (7) Fan, S. S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. J. Science 1999, 283, 512.

Thermal Analysis Study of CNT Growth (8) Venegoni, D.; Serp, P.; Feurer, R.; Kihn, Y.; Vahlas, C.; Kalck, P. Carbon 2002, 40, 1799. (9) Morancais, A.; Caussat, B.; Kihn, Y.; Kalck, P.; Plee, D.; Gaillard, P.; Bernard, D.; Serp, P. Carbon 2007, 45, 624. (10) Wei, F.; Zhang, Q.; Qian, W. Z.; Yu, H.; Wang, Y.; Luo, G. H.; Xu, G. H.; Wang, D. Z. Powder Technol. 2008, 183, 10. (11) Helveg, S.; Lo´pez-Cartes, C.; Sehested, J.; Hansen, P. L.; Clausen, B. S.; Rostrup-Nielsen, J. R.; Abild-Pedersen, F.; Norskov, J. K. Nature 2004, 427, 426. (12) Liu, K.; Jiang, K. L.; Feng, C.; Chen, Z.; Fan, S. S. Carbon 2005, 43, 2850. (13) Zhu, L. B.; Hess, D. W.; Wong, C. P. J. Phys. Chem. B 2006, 110, 5445. (14) Zhu, L. B.; Xu, J. W.; Xiao, F.; Jiang, H. J.; Hess, D. W.; Wong, C. P. Carbon 2007, 45, 344. (15) Kim, D. H.; Jang, H. S.; Kim, C. D.; Cho, D. S.; Yang, H. S.; Kang, H. D.; Min, B. K.; Lee, H. R. Nano Lett. 2003, 3, 863. (16) Geohegan, D. B.; Puretzky, A. A.; Ivanov, I. N.; Jesse, S.; Eres, G.; Howe, J. Y. Appl. Phys. Lett. 2003, 83, 1581. (17) Hart, A. J.; van Laake, L.; Slocum, A. H. Small 2007, 3, 772. (18) Tian, Y. J.; Hu, Z.; Yang, Y.; Wang, X. Z.; Chen, X.; Xu, H.; Wu, Q.; Ji, W. J.; Chen, Y. J. Am. Chem. Soc. 2004, 126, 1180. (19) Tian, Y. J.; Hu, Z.; Yang, Y.; Chen, X.; Ji, W. J.; Chen, Y. Chem. Phys. Lett. 2004, 388, 259. (20) Liu, J. X.; Ren, Z.; Duan, L. Y.; Xie, Y. C. Acta Chim. Sinica 2004, 62, 775. (21) Xiang, R.; Yang, Z.; Zhang, Q.; Luo, G. H.; Qian, W. Z.; Wei, F.; Kadowaki, M.; Einarsson, E.; Maruyama, S. J. Phys. Chem. C 2008, 112, 4892. (22) Liu, K.; Liu, P.; Jiang, K. L.; Fan, S. S. Carbon 2007, 45, 2379. (23) Futaba, D. N.; Hata, K.; Yamada, T.; Mizuno, K.; Yumura, M.; Iijima, S. Phys. ReV. Lett. 2005, 95, 056104. (24) Einarsson, E.; Murakami, Y.; Kadowaki, M.; Maruyama, S. Carbon 2008, 46, 923. (25) Puretzky, A. A.; Eres, G.; Rouleau, C. M.; Ivanov, I. N.; Geohegan, D. B. Nanotechnology 2008, 19, 055605. (26) Ago, H.; Uehara, N.; Yoshihara, N.; Tsuji, M.; Yumura, M.; Tomonaga, N.; Setoguchi, T. Carbon 2006, 44, 2912. (27) Chhowalla, M.; Teo, K. B. K.; Ducati, C.; Rupesinghe, N. L.; Amaratunga, G. A. J.; Ferrari, A. C.; Roy, D.; Robertson, J.; Milne, W. I. J. Appl. Phys. 2001, 90, 5308. (28) Puretzky, A. A.; Geohegan, D. B.; Jesse, S.; Ivanov, I. N.; Eres, G. Appl. Phys. A: Mater. Sci. Process. 2005, 81, 223.

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9631 (29) Li, Q. W.; Zhang, X. F.; DePaula, R. F.; Zheng, L. X.; Zhao, Y. H.; Stan, L.; Holesinger, T. G.; Arendt, P. N.; Peterson, D. E.; Zhu, Y. T. AdV. Mater. 2006, 18, 3160. (30) Atkins, P.; de Paula, J. Atkins Physical Chemistry, 7th ed.; New York: Oxford, 2002; p 879. (31) Zhang, Q.; Huang, J. Q.; Zhao, M. Q.; Qian, W. Z.; Wang, Y.; Wei, F. Carbon 2008, 46, 1152. (32) Jiang, K. L.; Feng, C.; Liu, K.; Fan, S. S. J. Nanosci. Nanotechnol. 2007, 7, 1494. (33) Lee, Y. T.; Park, J.; Choi, Y. S.; Ryu, H.; Lee, H. J. J. Phys. Chem. B 2002, 106, 7614. (34) Bartsch, K.; Biedermann, K.; Gemming, T.; Leonhardt, A. J. Appl. Phys. 2005, 97, 114301. (35) Cassell, A. M.; Raymakers, J. A.; Kong, J.; Dai, H. J. J. Phys. Chem. B 1999, 103, 6484. (36) Vinciguerra, V.; Buonocore, F.; Panzera, G.; Occhipinti, L. Nanotechnology 2003, 14, 655. (37) Shi, Z. J.; Lian, Y. F.; Liao, F. H.; Zhou, X. H.; Gu, Z. N.; Zhang, Y. G.; Iijima, S. Solid State Commun. 1999, 112, 35. (38) Zhang, X. B.; Jiang, K. L.; Feng, C.; Liu, P.; Zhang, L. N.; Kong, J.; Zhang, T. H.; Li, Q. Q.; Fan, S. S. AdV. Mater. 2006, 18, 1505. (39) Endo, M.; Takeuchi, K.; Kobori, K.; Takahashi, K.; Kroto, H. W.; Sarkar, A. Carbon 1995, 33, 873. (40) Carbon Filaments and Nanotubes: Common Origins, Differing Applications?; Biro´, L. P., Bernardo, C. A., Tibbetts, G. G., Lambin, P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2001. (41) Bower, C.; Zhou, O.; Zhu, W.; Werder, D. J.; Jin, S. H. Appl. Phys. Lett. 2000, 77, 2767. (42) Emmenegger, C.; Bonard, J. M.; Mauron, P.; Sudan, P.; Lepora, A.; Grobety, B.; Zuttel, A.; Schlapbach, L. Carbon 2003, 41, 539. (43) Schaper, A. K.; Hou, H. Q.; Greiner, A.; Phillipp, F. J. Catal. 2004, 222, 250. (44) Kuwana, K.; Endo, H.; Saito, K.; Qian, D. L.; Andrews, R.; Grulke, E. A. Carbon 2005, 43, 253. (45) Reina, A.; Hofmann, M.; Zhu, D.; Kong, J. J. Phys. Chem. C 2007, 111, 7292. (46) Wen, J. Z.; Richter, H.; Green, W. H.; Howard, J. B.; Treska, M.; Jardim, P. M.; Vander Sande, J. B. J. Mater. Chem. 2008, 18, 1561. (47) Anton, R. Carbon 2008, 46, 656. (48) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53, 1126. (49) Lee, C. J.; Park, J.; Huh, Y.; Lee, J. Y. Chem. Phys. Lett. 2001, 343, 33. (50) Kim, K. E.; Kim, K. J.; Jung, W. S.; Bae, S. Y.; Park, J.; Choi, J.; Choo, J. Chem. Phys. Lett. 2005, 401, 459.

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