Kinetics Studies of Ultralong Single-Walled Carbon Nanotubes

May 28, 2009 - growth. * To whom correspondence should be addressed. E-mail: Lxzheng@ ntu.edu.sg. † School of Mechanical and Aerospace Engineering, ...
0 downloads 0 Views 2MB Size
Download PDF
10896

J. Phys. Chem. C 2009, 113, 10896–10900

Kinetics Studies of Ultralong Single-Walled Carbon Nanotubes Lianxi Zheng,*,† B. C. Satishkumar,|,‡ Pingqi Gao,§ and Qing Zhang§ School of Mechanical and Aerospace Engineering, Nanyang Technological UniVersity, Singapore 639798; Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States of America; and School of Electrical and Electronic Engineering, Nanyang Technological UniVersity, Singapore 639798 ReceiVed: February 23, 2009; ReVised Manuscript ReceiVed: May 5, 2009

Single-walled carbon nanotubes (SWCNTs) were synthesized using ethanol CVD to study the nucleation kinetics of nanotube growth. By counting the number density of SWCNTs, i.e., the number of nanotubes per unit area on the substrate, the nucleation process of SWCNT growth was studied extensively within a wide range of growth temperatures. A nucleation energy about 2.8 eV was obtained from the Arrhenius-like temperature dependence of the number density of SWCNTs. The big difference between nucleation energy and diffusion energy implies the growth route for ultralong SWCNTs, and our approach may afford control over nanotube structure. The novel approach of studying the influence of “measure length” on activation energy may open an opportunity to understand the physics behind growth of nanotubes. Introduction Carbon nanotubes possess superior electronic and mechanical properties, which make them ideal materials for next generation electronic devices, integrated-circuits, sensors, ultraconductive cables, and other applications.1 However, the poor understanding of the detailed growth mechanism hinders further progress in controlled nanotube production. Specifically, the control over chirality of single-walled carbon nanotubes (SWCNTs) during the growth process is still a challenge, despite the development of a wide range of synthesis techniques.2-12 An understanding of the formation mechanism of nanotubes is therefore extremely crucial when designing techniques for the controlled production of nanotube material with controlled structure. This is especially true if one tries to produce SWCNTs with same chirality. Although the detailed growth mechanism is still under debate, it is well agreed that the fundamental processes during nanotube growth include: formation of metal nanoparticles; dissociation of the hydrocarbon precursor molecules; adsorption and diffusion of the gaseous carbon on the catalyst surface; and nucleation of ordered carbon structure resulting in the nanotube structure.3 Among these processes, the nucleation process is of the most importance, because the structure (single wall or multiwall, diameter and chirality) of a nanotube is determined at this stage and later growth mainly elongates its length by adding more carbon atoms onto the existing nanotube edge. Extensive research has been carried out studying the growth mechanism,4-13 and some very useful techniques, such as in situ monitoring,2,8,9 and marked growth11 have been developed to study the nucleation of nanotubes. At the same time, quantitative studies on nucleation energy have faced a great challenge due to the fact that all of the above-mentioned * To whom correspondence should be addressed. E-mail: Lxzheng@ ntu.edu.sg. † School of Mechanical and Aerospace Engineering, Nanyang Technological University. ‡ Center for Integrated Nanotechnologies, Los Alamos National Laboratory. § School of Electrical and Electronic Engineering, Nanyang Technological University. | Present address: Department of Mechanical Engineering, University of Michigan-Dearborn, 4901 Evergreen Rd, Dearborn, MI 48128, USA.

fundamental processes are in co-play during nanotube growth. This in turn makes it difficult for efforts to investigate each process in isolation. The experimental data so far are a collective effect of dissociation, diffusion, and nucleation processes,13 and accordingly so-called “effective” activation energy diverges largely in the range, 1.2-2.7 eV9-13 depending on the synthesis method and nature of the carbon precursor. In addition, different carbon products such as SWCNTs, MWCNTs, carbon fibers, and amorphous carbon have different nucleation energies, which make such a study even more difficult. The purpose of the present work is to study the nucleation kinetics separately from other growth kinetics, and give nucleation energy estimates for extended nanotube growth. Particularly, we use ethanol chemical vapor deposition (CVD) to grow countable individual SWCNTs, and measure the temperature dependence of their number density (number of nanotubes per unit area) to study the nucleation kinetics. Ethanol is found to be a good carbon source to produce relatively clean nanotubes,14,15 so ethanol CVD at high temperature in the range, 850-1000 °C is utilized to produce nanotubes free from extraneous carbon impurities. In our studies, SWCNTs are targeted because, their nucleation involves only one layer of graphene and all of SWCNTs have very narrow diameter distribution.14,15 This way, the nucleation energy is expected to lie in a narrow range for all SWCNTs. The high temperature used here is also helpful to minimize the influence of dissociation of ethanol. The higher the growth temperature, the easier the decomposition of ethanol will be during the CVD growth, and thus it is less likely to be inefficient for nanotube growth. As the number of nanotubes is directly related to nucleation rate, one could measure the number density of nanotubes during the CVD and deduce the kinetics parameters. It should be kept in mind that nucleation contributes to number density but the diffusion process mainly contributes to the growth rate following the nucleation. By measuring the number density instead of the growth rate, the nucleation kinetics is thus isolated from the diffusion kinetics. Therefore, the results of our approach are expected to be illustrative of the nucleation process in CVD growth.

10.1021/jp901640d CCC: $40.75  2009 American Chemical Society Published on Web 05/28/2009

Kinetics Studies of Ultralong SWCNTs

J. Phys. Chem. C, Vol. 113, No. 25, 2009 10897

Figure 1. (a) SEM image of CVD grown sample showing long and well-separated single walled carbon nanotubes (inset is a SEM image at higher magnification), (b) Raman spectrum showing the radial breathing mode of a nanotube, and (c) AFM height profile of the same nanotube as in (b).

Experimental Methods We have previously synthesized SWCNTs as long as 4 cm using ethanol CVD.15 The same growth approach was utilized to grow individual SWCNTs for the current studies. The catalyst was applied on an Si substrate (∼10 × 40 mm) by dip-pen method using 0.1 M FeCl3/ethanol solution. Ethanol was introduced through bubbling, and Ar and H2 mixture was used as carrier gas during the CVD growth. For the purpose of comparison, all of the growth parameters were kept the same, except temperature, for all samples. The growth temperature was varied from 850 to 1000 °C to study the temperature dependence of growth parameters. After the CVD growth, all samples were characterized using scanning electron microscopy (SEM) for the purpose of number density measurements. Atomic Force Microscopy (AFM) was used to measure the height of nanotubes, and Raman spectroscopy measurements were carried out on the samples using WITec CRM200 (Nd:YAG laser, 2.33 eV) and RENISHAW SP-1000 (He:Ne laser, 1.96 eV) confocal Raman system through a 100× air objective, respectively. The radial breathing mode (RBM) spectra were analyzed to deduce diameter information for SWCNTs. Results and Discussion Figure 1a shows a typical SEM image of CVD grown nanotube samples using ethanol. A lot of long nanotubes are shown growing from the catalyst area (upper bright region). The nanotubes are grown with distinct separation between each other. Most of the nanotubes are much longer than 100 µm, and many extend in length up to a centimeter. AFM and Raman spectroscopy have been used at first to characterize the nanotubes outside the catalyst area. Raman G-peak splitting profiles and the radial breathing mode (RBM) frequencies are consistent with those of SWCNTs,16 and no observable D-peak has been found, indicating high quality of these SWCNTs. One typical Raman spectrum of radial breathing mode for those SWCNTs

is shown in Figure 1b, and the AFM height profile of the same CNT is shown in Figure 1c. From observed RBM frequency (132 cm-1), the diameter of this SWCNT is calculated using the expression of 248 cm-1/ω (ω is RBM frequency),16 and is found to be 1.88 nm, which is almost the same as the height obtained from AFM height profile. For all long SWCNTs that show RBM modes in Raman spectra, the diameter is found to be in the range of 1.3 to 2.2 nm, with the average value around 1.7 nm. This is a very typical diameter distribution for CVD grown SWCNTs that we studied earlier.17 In the area that is very close to catalyst area, SWCNTs with diameter as small as 0.7 nm have also been observed. All calculated diameters from Raman measurements are very consistent with AFM height profiles, further confirming the nature of SWCNTs. Inside the thin painted catalyst regions, short and defective nanotubes were observed in addition to above-mentioned long SWCNTs. However, for the present studies, we only consider these long SWCNTs that have grown out of catalyst region, and such short and defective CNTs are not counted for nucleation studies. The long length and proper separation space make the each of the individual SWCNTs very clearly identifiable (under SEM), and this in turn allowed us to count their number densities without ambiguity. Although, sometimes two or more SWCNTs may join together, they could be distinguished by tracking them over longer distances along the length during SEM observation. Because all SWCNTs lie on the surface of the Si substrate, we calculated the number density as the number of SWCNTs within the unit length of a growth width, which is the length along the direction perpendicular to the nanotube growth direction. Thus, what we measure is the equivalent 2D number density, the unit of which will be counts/cm2. We show the variation of number density for nanotubes with CVD growth temperature in Figure 2a. As the temperature increases, the nanotube number density also increases. This is an expected trend, because, as the growth temperature increases carbon atoms have more energy to nucleate into nanotubes, resulting in a higher number density

10898

J. Phys. Chem. C, Vol. 113, No. 25, 2009

Zheng et al.

Figure 3. Temperature dependence of number density for nanotubes grown by ethanol CVD method.

Figure 2. (a) Distribution of nanotube number density at various growth temperatures during CVD growth, (b) number density of nanotubes as a function of length at various growth temperatures, and (c) length distribution of nanotubes (squares represent growth time of 1 h, while circles represent 2 h growth time).

for nanotubes. At higher temperature, the number density drops off, due to the fact that ethanol decomposition at high enough temperatures leads to deposits of amorphous carbon and graphite. Since almost all SWCNTs are aligned pretty well along the growth direction, the distance from the measurement point to the catalyst edge can substantially represent the length of the nanotubes. Then a length distribution of SWCNTs at various growth temperatures has been obtained and shown in Figure 2b. It is obvious that a greater number of nanotubes of given length are grown as the growth temperature increases. Thus, it is obvious that both number density and length of nanotubes are parameters that show temperature dependence. In the next step, we sought to investigate the effect of growth duration on number density and nanotube length. In Figure 2c, we show

the graph depicting number density as a function of nanotube length at two different growth durations. In this case, the number density changes dramatically within the first 2 mm of growthlength, but becomes flat thereafter, indicating that most nanotubes attain a length in the wide range from 100 µm to 2 mm. Fewer of the nanotubes grow out to be longer than 5 mm. A nanotube is more likely to grow to centimeter length once they pass the 2 mm critical length. We have also found that longer growth time yields a much flatter distribution (data represented by blue squares in Figure 2c). These data suggest that the nucleation becomes kind of saturated at longer growth times, and most short nanotubes nucleated much earlier in time and a smaller number of them grew much longer. We now proceed to deduce the kinetics parameters of our CVD growth process. Because of the difficulty of counting all SWCNTs at once, we initially considered only those SWCNTs that are longer than 100 µm. We refer to this length (i.e., 100 µm) as the “measure length” hereafter, and the influence of it on our results will be discussed later. In Figure 3, we show a plot illustrating the temperature dependence of the nanotube number density at a “measure length” of 100 µm. From this semilogarithmic plot, we observe that the nanotube number density increases as temperature increases in the low temperature region, but then decreases at high temperature. This later trend is due to the competition between nanotube growth and catalyst deactivation10 during CVD growth. At a low temperature range (850-950 °C), the catalyst particles are active and the nanotube growth dominates. As the temperature increases, more and more nanotubes are nucleated and grown and thus the number density increases with the temperature. However, when the temperature is relatively high, other carbon products, such as amorphous carbon and graphene sheets, start to grow around and wrap up the catalyst nanoparticles, making the growth of SWCNTs less favorable. Thus, high enough temperatures during CVD growth result in the decrease in number density for nanotubes. This is evidenced by the fact that samples grown at high temperature show the coverage of amorphous carbon and graphite nanoparticles. Nevertheless, in the wide range of temperature, 850-950 °C, the logarithmic value of nanotube number density is linear to 1/T. This in turn implies that the number density of nanotubes is an exponential function of growth temperature. This approach has been followed in the literature to deduce the activation energy9-13 of nanotube growth, wherein the nanotube growth rate is related in similar way to the temperature. From the Arrhenius form relation, N ) A exp (-Eact/kT) (where N is the number density, Eact is the activation energy, T is the temperature, k is the Boltzmann constant, and A is a constant), an activation energy (Eact(L)) estimate of 3.5 eV could be deduced.

Kinetics Studies of Ultralong SWCNTs

J. Phys. Chem. C, Vol. 113, No. 25, 2009 10899

Figure 4. (a) The effect of “measure length” of nanotubes on the activation energy and (b) Linear fit of activation energy. The arrow shows the activation energy for “measure length” of -0.25 mm.

Since CNT growth is very much catalyst-dependent and the catalyst can aggregate together under high temperature treatment, it is then reasonable to think such a temperature dependence is catalyst related. To clear up this doubt, we have carried out AFM studies on a specially designed sample that has been grown at 900 °C for only 5 min. From AFM images (not shown), only short CNTs can be found on the substrate, and no CNT was found to be longer than 1 mm. But to our surprise, a large number of spherical shaped catalyst particles, whose diameters range between 1 and 10 nm (with average size about 5 nm) were seen on the substrate (around catalyst area only). The average separation between these particles is about 60 nm, which is ∼1000× smaller than that for CNTs grown in the even densest growth condition, indicating that only a very small portion of catalyst particles yield long CNTs during CVD growth. This observation, together with the fact that the catalyst number density has a negative temperature dependence (the number decreases with temperature), can confirm that the observed positive temperature dependence of SWCNT density relates to nucleation kinetics, and such a nucleation process is not limited by the catalyst in our growth conditions. Nevertheless, this activation energy (Eact(L)) estimate was obtained by counting the number of nanotubes that are longer than 100 µm (L ) 100 µm), which means that these nanotubes need to overcome such an energy barrier to have nucleated and sustained the growth with length up to 100 µm. Obviously, it does not reflect the real nucleation, simply because there exist many nanotubes that are shorter than 100 µm which we have not taken into account yet. So before making any connection between this activation energy, Eact(L) and the nucleation energy (Enuc), the influence of the “measure length” on such an activation energy has to be studied. We used various “measure length” values in the wide range of 0.05-5.0 mm to calculate the corresponding estimates for activation energy, and the results are presented in Figure 4a. It is found that the activation energy increases as the nanotube “measure length” increases. This is due to fact that, in addition to nucleation, the nanotubes need to sustain further growth, having lengths up to the “measure length”. The activation energy shows saturation at high “measure length” values. We draw two lines in Figure 4a to illustrate the linear and nonlinear trends in the activation energy behavior. The linear region is of interest to us as it enables us to deduce the nucleation energy. We regraph the linear data set for activation energy dependence on “measure length” in Figure 4b. For the purpose of comparison, we retain the same range for “measure length”. One may notice that the data points scale very well and a linear

fit describes the expected behavior for activation energy. From Figure 4b, we now extract the activation energy at zero “measure length”, which is ∼3.2 eV. This is not the estimate for nucleation energy yet, because there exist a lot of nanotubes inside the catalyst area that we could not take into account. In the present studies, the catalyst was applied by dip-pen method, and the width of catalyst line is ∼0.5 mm. If we assume that nanotubes uniformly grow out of the whole catalyst area, then counting number density at a “measure length” of -0.25 mm (the center of the catalyst strip) will include all of the nanotubes in the catalyst area. In this way, we obtain an activation energy value, which is ∼2.8 eV, at “measure length” of -0.25 mm. This particular data point is shown in Figure 4b by an arrow, and is extracted from the experimental data by extrapolation. Since most of the nanotubes have been accounted at this “measure length”, we may consider this value for activation energy as an estimate for the nucleation energy (Enuc) for nanotube growth. This estimate for nucleation energy is at the higher end of previously reported values for effective activation energy.9-13 Several possible reasons could explain this large difference. First, our data are deduced following the assumption that the activation energy depends on the “measure length” for nanotubes, In other words, we accounted for all of the nanotubes which had potential to grow very long. The other nanotube products (for example, very large diameter nanotubes), which are not stable and cannot grow very long due to intrinsic defects, were ignored. This is indeed evidenced by the fact that we observed many large diameter and short length nanotubes inside the catalyst area. Second, our nanotubes are all single-walled, which are expected to have higher nucleation energy than that for multi-walled nanotubes or other carbonaceous products. Third, since we measure temperature dependence of number density instead of growth rate, our data may reflect the pure nucleation process, so a higher energy is expected. This can also explain the large variation in previously reported values for the activation energy, as literature data are actually the weighted average values of nucleation energy and diffusion energy, depending on the degree of involvement of the diffusion process in a particular growth method. Our data agrees very well with the recent in situ observation,9 in which a slow growth regime was observed and nucleation energy of ∼2.7 eV was estimated. Therefore, the current mean estimate of ∼2.8 eV could be considered as the nucleation energy for stable and sustainable single walled carbon nanotube growth. It may be recalled that diffusion plays an important role during CVD growth and estimates for the same are ∼1.5 eV for bulk diffusion18,19 and ∼0.4 eV3,13 for surface diffusion. Thus,

10900

J. Phys. Chem. C, Vol. 113, No. 25, 2009

it is obvious that there is a large difference between the nucleation energy (Enuc) and the diffusion energy. This further suggests that elongating an existing nanotube is energetically much easier than nucleating a new structure. It is known that ethanol CVD can produce long nanotubes without extraneous carbon impurities even at very low temperature due to the etching effect of hydroxyl radicals.20 Hence, it could be a very promising technique for the seeded growth21 of single-walled nanotubes to obtain a controlled structure. It is also worth mentioning that from Figure 4a, we find that the dependence of activation energy Eact(L) (obtained under particular “measure length”) is spread into two regimes; at short “measure length” (for short nanotubes) the activation energy increases very quickly as “measure length” increases, while at longer “measure length” (for long nanotubes), the dependence becomes saturated. From the definition, we know that the activation energy Eact(L) means the energy barrier for a nanotube for being nucleated and grown up to “measure length” L. Then the dependence of Eact(L) on L observed here reflects the survival capability of a nucleated nanotube after certain growth length. This could be interesting as one could control growth conditions to tailor the preferential growth of long nanotubes. Whether the present ethanol CVD approach affords control over chirality control is to be investigated further by seeded regrowth approach.21 Also, the existence of different regimes suggests the possibility of different termination mechanisms during CVD growth. In our previous studies, we have reported a skidding termination mode for our ultralong nanotube growth.15 During such growth, the catalyst at the nanotube tip is initially floating over the substrate, and the growth would finally be terminated once the catalyst is deactivated due to skidding and contacting on the substrate surface. Although further studies are needed, we believe such a skidding termination mechanism reflects the growth of long nanotubes. In the case of shorter nanotubes, the catalyst encapsulation by amorphous carbon or graphene sheets remains the main reason for the termination of growth. Conclusions In summary, we have carried out detailed studies on singlewalled carbon nanotube growth kinetics. By counting the number density of nanotubes, details of the nucleation and growth process were studied. We use the approach of counting the number density of nanotubes having “measure length” (L) and deduce the activation energy. From the Arrhenius form of temperature dependence for the number density, an estimate for activation energy of ∼2.8 eV was obtained. This energy can serve as the nucleation energy for stable and sustainable single-walled carbon nanotube growth. Perhaps, further studies

Zheng et al. are needed to address the ultimate challenge of control of chirality in CVD growth. The studies of nanotube growth kinetics demonstrated here may spur further experimental and theoretic research. Considering the homogeneous and impurity free growth of long nanotubes, our approach has the potential to tailor the growth of nanotubes of desired electronic properties. Acknowledgment. One of the authors, L.Z. would like to thank the Singapore MOE tier 1 RG26/08 research fund for financial support. References and Notes (1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787–792. (2) Hofmann, S.; Sharma, R.; Ducati, C.; Du, G.; Mattevi, C.; Cepek, C.; Cantoro, M.; Pisana, S.; Parvez, A.; Cervantes-Sodi, F.; Ferrari, A. C.; Dunin-Borkowski, R.; Lizzit, S.; Petaccia, L.; Goldoni, A.; Robertson, J. Nano Lett. 2007, 7, 602–608. (3) Hofmann, S.; Csanyi, G.; Ferrari, A. C.; Payne, M. C.; Robertson, J. Phys. ReV. Lett. 2005, 95, 036101 (1-4). (4) Kuznetsov, V. L.; Usoltsev, A. N.; Chuvilin, A. L.; Obraztsova, E. D.; Bonard, J.-M. Phys. ReV. B 2001, 64, 235401 (1-7). (5) Crespi, V. H. Phys. ReV. Lett. 1999, 82, 2908–2910. (6) Gavillet, J.; Loiseau, A.; Journet, C.; Willaime, F.; Ducastelle, F.; Charlier, J. C. Phys. ReV. Lett. 2001, 87, 275504 (1-4). (7) Fan, X.; Buczko, R.; Puretzky, A. A.; Geohegan, D. B.; Howe, J. Y.; Pantelides, S. T.; Pennycook, S. J. Phys. ReV. Lett. 2003, 90, 145501 (1-4). (8) Puretzky, A. A.; Geohegan, D. B.; Jesse, S.; Ivanov, I. N.; Eres, G. Appl. Phys. A: Mater. Sci. Process. 2005, 81, 223–240. (9) Lin, M.; Tan, J. P. Y.; Boothroyd, C.; Loh, K. P.; Tok, E. S.; Foo, Y. L. Nano Lett. 2006, 3, 449–452. (10) Kim, K. E.; Kim, K. J.; Jung, W. S.; Bae, S. Y.; Park, J.; Choi, J.; Choo, J. Chem. Phys. Lett. 2005, 401, 459–464. (11) Liu, K.; Jiang, K. L.; Feng, C.; Chen, Z.; Fan, S. S. Carbon 2005, 43, 2850–2856. (12) Ducati, C.; Alexandrou, I.; Chhowalla, M.; Amaratunga, G. A. J.; Robertson, J. Appl. Phys. 2002, 92, 3299–3303. (13) Bronikowski, M. J. J. Phys. Chem. C 2007, 111, 17705–17712. (14) Zheng, L. X.; Liao, X. Z.; Zhu, Y. T. Mater. Lett. 2006, 60, 1968– 1972. (15) Zheng, L. X.; O’Connell, M. J.; Doorn, S. K.; Liao, X. Z.; Zhao, Y. H.; Akhadov, E. A.; Hoffbauer, M. A.; Roop, B. J.; Jia, Q. X.; Dye, R. C.; Peterson, D. E.; Huang, S. M.; Liu, J.; Zhu, Y. T. Nat. Mater. 2004, 3, 673–676. (16) Dresselhaus, M. S.; Dresselhaus, G.; Jorio, A.; Souza Filho, A. G.; Saito, R. Carbon 2002, 40, 2043–2061. (17) Doorn, S. K.; Zheng, L. X.; O’Connell, M. J.; Zhu, Y. T.; Huang, S. M.; Liu, J. J. Phys. Chem. B. 2005, 109, 3751–3758. (18) Smithells, C. J. Smithells Metals Reference Book, 7th ed.; Elsevier Butterworth-Heinemann: New York, 2004. (19) Baker, R. T. K. Carbon 1989, 27, 315–323. (20) Maruyama, S.; Kojima, R.; Miyauchi, Y.; Chiashi, S.; Kohno, M. Chem. Phys. Lett. 2002, 360, 229–234. (21) Wang, Y. H.; Kim, M. J.; Shan, H. W.; Kittrell, C.; Fan, H.; Ericson, L. M.; Hwang, W- F.; Arepalli, S.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2005, 5, 997–1002.

JP901640D