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2007, 111, 6158-6160 Published on Web 04/05/2007
Two-Stage Growth of Single-Walled Carbon Nanotubes Hang Qi, Dongning Yuan, and Jie Liu* Department of Chemistry, Duke UniVersity, Durham, North Carolina 27708 ReceiVed: February 20, 2007; In Final Form: March 13, 2007
The growth of single-walled carbon nanotubes (SWNTs) in a chemical vapor deposition (CVD) system is a complex process. In a typical growth mechanism, the process includes the nucleation stage, the growth stage, and the termination stage. However, most nanotube growth experiments were performed under identical growth conditions for different stages. Here, a two-stage growth process is studied with different growth environments at the nucleation stage and growth stage. Priming the catalysts with carbonaceous species is treated as a wholly separate process from nanotube growth throughout this work. The optimum conditions for these two stages are found to be different. Conditions that give a high yield of active catalysts will cause impurities to cover the catalysts and halt growth. Similarly, conditions that allow prolonged growth of nanotubes will fail to activate all of the catalysts, resulting in low overall yield. The yield of carbon nanotube growth by CVD can be significantly improved by using a higher carbon feeding rate in the nucleation stage followed by lower carbon feeding rate for continued growth.
Carbon nanotubes (CNTs) are a family of materials with unique properties and potential applications.1-3 Among all of the methods of synthesizing CNTs, chemical vapor deposition (CVD) is the most promising method for low cost and large scale production of highly pure nanotubes.4-9 In general, CVD method employs carbon-containing molecules such as hydrocarbons, carbon monoxide, or alcohols as a carbon source. These carbon precursors decompose under high temperature with the help of catalytic metal nanoparticles and provide a source of carbon for CNT growth. Compared with arc discharge10 and laser ablation11 methods, CVD is more versatile, enabling selective growth of different kinds of CNT such as multiwalled carbon nanotubes (MWNTs), double-walled carbon nanotubes (DWNTs), or single-walled carbon nanotubes (SWNTs) either in bulk or directly on a substrate such as Si wafers.7,12-14 Synthesizing SWNTs directly on Si wafers provides a promising route to the fabrication of nanotube field effect transistors,15 chemical sensors,16 and logic circuits.17,18 In recent years, much effort has been made to elucidate the mechanism of SWNT growth.19-24 Although significant progress has been made in the production of SWNTs by CVD, understanding of the growth mechanism is still incomplete. In general, a CVD procedure is composed of the following steps. First, the catalysts are heated to a certain temperature for catalyst pretreatment. After the desired temperature is reached, a gas mixture is fed through the system which includes some kind of carbon source, an inert carrier gas, and possibly a chemical to control the oxidative/reductive nature of the environment. To stop the process to collect the products, the gas mixture is turned off after a certain period of growth and the temperature is returned to room temperature. In most published growth experiments, the growth conditions are held constant throughout * To whom correspondence should be addressed. E-mail: j.liu@ duke.edu.
10.1021/jp071448q CCC: $37.00
the whole experiment, with the possible exception of the catalyst pretreatment step. For parameters such as composition and flow rate of the gas mixture, temperature, and pressure, no distinction is made between CNT nucleation and growth. In a recent report, we have demonstrated that the carbon feeding rate is a key parameter which plays an important role in the growth of SWNTs. We reported that different carbon feeding rates resulted in SWNTs with different diameters. This result has been explained by a theory that for any particular carbon feeding rate, only catalyst particles within a certain range of diameter will be activated for nucleation and growth of SWNTs.25 Thus, we have chosen a change in carbon feeding rate to separate the nucleation stage and growth stage. Additionally, theoretical calculations have helped greatly in elucidating the growth mechanism of carbon nanotubes.26,27 Recent theoretical results showed that the SWNT growth process was indeed a complicated process and there were more than one stage in such a process. Moreover, the roles of the catalyst are different in different stages. Thus, it is predicted that keeping the carbon feeding rate constant throughout the whole process of SWNT growth may limit both yield and purity. In this report, we explored the differences between the nucleation stage and the growth stage by carrying out a novel two-stage experiment to examine the growth of SWNTs. A short pulse of carbon feeding gas with higher carbon feeding rate was added at the beginning of growth to promote greater nucleation of nanotubes. Compared to growth without the added nucleation step, we obtained a much higher yield of SWNTs. The experimental results indicate that the carbon feeding rates desired for nucleation and growth are different. To obtain SWNTs with high yield, the carbon feeding rates should match the requirements of both nucleation stage and growth stage. Carbon feeding rate refers here to the rate at which carbon enters the catalyst particle. It is not an independent variable but depends on a series © 2007 American Chemical Society
Letters
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Figure 1. AFM images of iron oxide particle array on Si wafer (A) before and (B) after exposure to the growth conditions but without carbon source.
of parameters such as the partial pressure of the carbon source, structure of the carbon source species, and growth temperature. We tuned the carbon feeding rate by changing the carbon source species while keeping all other parameters unchanged. Methane and ethane were chosen as carbon source species for the low and high carbon feeding rate, respectively. Ethane is known to have a higher carbon feeding rate than methane because twocarbon species are much more easily absorbed by catalyst particles.28-30 Catalysts for SWNT growth in this report are uniform Fe nanoparticles loaded on SiO2 coated Si wafers. The array of monodisperse Fe nanoparticles was obtained with the help of block-copolymer PS-PVP (poly(styrene-b-4-vinylpyridine)) micelles.31 FeCl3 and PS-PVP were first dissolved in toluene and stirred for 12 h to obtain a clear and uniform solution which was then spin coated onto a Si wafer. Finally, the Si wafer was treated with oxygen plasma and heated to 700 °C in air to remove all organic species, leaving an array of highly uniform iron oxide nanoparticles on the surface of the wafer (Figure 1A). The average size of the nanoparticles is 2.1 ( 0.4 nm. Figure 1B shows the catalyst array after exposure to the growth conditions but without carbon source. It is clear that the catalysts did not aggregate under the growth conditions used in this set of experiments. The growth of SWNTs was performed in a quartz tube in a horizontal furnace under atmospheric pressure. Before SWNT growth, the chamber was sealed and then cycled three times through vacuuming and flushing with Ar. The goal of this step is to remove oxygen and water residues in the growth chamber, which are known to perturb growth of SWNTs.25,32 SWNT growth was started by heating the Si wafer loaded with iron oxide nanoparticles under a H2 flow from room temperature to the growth temperature, which is 800 °C for all experiments. After the system reached 800 °C, the carbon feeding gas, either diluted methane or ethane, was introduced to the system. Methane was diluted to 40 vol % with 60 vol % H2 whereas diluted ethane consisted of 0.3 vol % ethane, 60 vol % H2, and 39.7 vol % Ar. The total gas flow was fixed at 2000 sccm throughout the SWNT growth. After a certain growth time, the carbon feeding gas was cut off and the sample was cooled to room temperature under the protection of a H2 flow. All of the samples were imaged with scanning electron microscopy (SEM, FEI XL30 SEM-FEG) at a voltage of 1 kV and atomic force microscopy (AFM, Digital Instruments Nanoscope IIIa, Vecco) in tapping mode. The yields of SWNTs under our growth condition using either methane or ethane alone are very low. As shown in Figure 2, few SWNTs can be found in SEM images (Figure 2A) of both the one-stage methane growth sample and the one-stage ethane growth sample (Figure 2B). However, when a pulse of ethane
Figure 2. SEM images of samples prepared with (A) without pulse; (B) pulse only; and (C) with pulse. The insertions are corresponding schemes.
was followed by methane, the yield of nanotubes is significantly increased (Figure 2C). From these experiments, it is clear that the pulse of ethane activated significantly more catalysts for subsequent growth of nanotubes. The low yield in Figure 2A can be explained by the mismatch of the nucleation conditions and the growth conditions provided by methane alone. However, even though methane alone does not provide efficient nucleation, it is able to sustain the growth of nanotubes already nucleated on the catalysts, as confirmed by the difference of nanotubes yield between panels A and C in Figure 2. Additionally, nucleation time, which is the length of nucleation stage, is an important parameter. Figure 3 shows SEM images of samples with different nucleation times (Figure 3AC) and their control samples (Figure 3D-F). The control samples were prepared with the nucleation stage only. Both of the samples prepared with nucleation time of 5 (Figure 2B,C) and 10 s (Figure 3B,E) showed considerable increase in yield of SWNTs compared to the corresponding controls. In contrast, the sample with 2 s pulse showed a much smaller increase of SWNT yield (Figure 3A,D) and the yield of 20 s ethane pulse sample was almost the same as the corresponding control sample (Figure 3C,F). These results suggest there is an optimum nucleation time for the nucleation stage. Under the condition of our experiments, the optimum nucleation time is around the range of 5-10 s. A nucleation time shorter than 5 s is not long enough, and only a small proportion of catalyst particles are activated to catalyze growth of SWNTs. When the nucleation time is longer than the optimum time, most of catalyst particles are deactivated due to excessive carbon feeding, and these catalyst particles cannot catalyze growth of SWNTs in the following growth stage. In summary, we have explored the effect of different carbon feeding gases during a two-stage SWNT growth experiment. Based on the results, a two-stage growth model which divides the process of SWNT growth into a nucleation stage and a growth stage is proposed. Nucleation stage is at the beginning of SWNT growth and very short, in which the catalyst particles get ready to catalyze the growth of SWNTs and short nanotubes nucleated on the nanoparticles. In the following growth stage, the nanotubes grow longer. These two stages have different requirements for optimal efficiency. Nucleation requires a higher
6160 J. Phys. Chem. C, Vol. 111, No. 17, 2007
Letters Mr. Michael E. Woodson for proofreading during the preparation of the manuscript. References and Notes
Figure 3. SEM images of samples prepared (A-C) with two-stage and different nucleation time; (D-F) nucleation stage only with different nucleation time. (A and D) 2 s; (B and E) 10 s; (C and F) 20 s.
carbon feeding rate than growth. Nucleation time is also an important factor; exposing the catalysts for too long of a time in a high carbon feeding rate environment deactivates most of the catalysts for growth of nanotubes. Only by matching the optimized conditions for both the nucleation stage and growth stage can a high yield of nanotubes be obtained. These results help us to understand the origin of poor yield in most early nanotube growth experiments and provide insight for us to design the growth conditions to obtain nanotubes with higher yield. Acknowledgment. The work is supported by a grant from DOE (DE-FC36-05GO15103) as part of the Center of Excellence for Carbon-Based Hydrogen Storage. The authors thank
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