J. Phys. Chem. C 2007, 111, 14293-14298
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ARTICLES Growth of Carbon Nanotubes by Pyrolysis of Thiophene G. H. Du and W. Z. Li* Department of Physics, Florida International UniVersity, Miami, Florida 33199
Y. Q. Liu AdVanced Materials and Engineering Research Institute, Florida International UniVersity, Miami, Florida 33174
Y. Ding and Z. L. Wang School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: May 24, 2007; In Final Form: July 24, 2007
The synthesis of carbon nanotubes (CNTs) by cobalt-catalyzed pyrolysis of thiophene has been investigated and optimized. The yield and the morphology of the synthesized product are highly sensitive to the experimental conditions. A synthesis temperature of 1000 °C is required for the decomposition of thiophene. The morphology and microstructure of the product are controlled by the thiophene vapor concentration and flow rate. High concentration of thiophene vapor deactivates the catalyst and results in the formation of carbon nanofibers or short CNTs with bad graphitization; the optimum thiophene concentration for CNTs growth is around 0.761%. Thiophene vapor at high flow rate produces Y-junction and multibranched CNTs. Three formation routes are proposed to illuminate the growth mechanism of the branched CNTs.
Introduction Carbon nanotubes (CNTs) are one of the most promising materials for hydrogen storage,1,2 sensors,3 probes,4,5 composites,6 and nanoelectronic applications7,8 due to their unique physical, mechanical, and electronic properties. Intensive research activities to improve the synthesis, quality, and productivity of CNTs have received great rewards.9,10 So far, many methods, including electric arc discharge,11,12 laser evaporation,13,14 chemical vapor deposition (CVD),15,16 and so on, have been developed to produce CNTs. Among these, the CVD method seems to be the potential candidate for a large-scale production. CVD is the catalytic decomposition of hydrocarbon or carbon monoxide feedstock with the aid of supported transition metal catalysts. The most widely used carbon precursors are acetylene,17,18 methane,19,20 ethanol,21 2-propanol,22 ethylene,23 and toluene.24 Thiophene is a heterocyclic compound consisting of four carbon atoms and one sulfur atom in a five-membered ring. It has been used to grow CNTs in the CVD method. For example, it was reported that thiophene can serve as a growth promoter for producing long single-walled carbon nanotube (SWNT) bundles.25 By pyrolyzing organometallics in the presence of thiophene, Y-junction nanotubes were obtained in large quantities.26 Recently, CNT Y-junctions were prepared by decomposing thiophene on a Co/Mg catalyst.27 These results demonstrate that thiophene is a versatile ingredient as a promoter or precursor to produce CNTs. To our best knowledge, there is no detailed * Corresponding author. E-mail:
[email protected].
report so far about the optimum condition for CNT growth by pyrolysis of thiophene. In this paper, we present the synthesis of CNTs using thiophene as carbon precursor and investigate the various effects on the growth and microstructure of the CNTs. Carbon nanofibers, CNTs, and branched CNTs were obtained at different condition in our experiments. The synthesis parameters of CNTs were optimized, and the formation mechanism of the branched CNTs is discussed in detail. Experimental Procedures The preparation procedure of our catalyst is described as follows. Co(NO3)2‚6H2O and Mg(NO3)2‚6H2O were mechanically mixed and ground and then calcined at 600 °C for 1 h in air to decompose the precursor and yield the cluster made of Co and Mg oxides. The resulted powder was then reduced in hydrogen (H2, 100 sccm) and argon (Ar, 200 sccm) for 30 min at 600 °C to form Co nanoclusters supported on MgO substrate, which was collected and used as catalyst. The weight ratio of Co in the catalyst powder is 25%. For the production of CNTs, the Co/MgO catalyst was placed in a quartz boat or on a Si wafer, which was inserted into a horizontal quartz tubular reaction chamber, heated up to the reaction temperature with a flow of Ar and H2. H2 was allowed to bubble through thiophene (C4H4S, liquid) to initiate the nanotubes growth. After growth for 15 min, H2 was closed and the chamber was cooled down to room temperature. The yields of the total carbon after growth were calculated by the following equation,
10.1021/jp0740292 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/12/2007
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carbon yield (wt %) ) [(Mf - Mi)/Mi] × 100 where Mf is the total weight of the powder after synthesis and Mi is the initial weight of the catalyst powder before placing into the reaction chamber. The morphology and microstructure of the synthesized products were examined using field emission scanning electron microscope (FESEM, JEOL JSM-6330F) and transmission electron microscope (TEM, Philips CM 200 and Hitachi HF-2000 FEG). Chemical composition analysis was performed by an energy dispersive X-ray spectrometer (EDS) equipped to a Hitachi HF-2000 FEG TEM. Results and Discussion Effect of Reaction Temperature on Carbon Nanotube Growth. The reaction temperature plays an important role in influencing the growth process of CNTs. Figure 1 shows the SEM images of the product grown at three synthesis temperatures: (Figure 1a) 800 °C, (Figure 1b) 900 °C and (Figure 1c) 1000 °C. Other synthesis parameters are the same for the three samples. Specifically, the Co/MgO powder was placed in a quartz boat, which was inserted into the reaction chamber for growth; the H2 and Ar flow rate was fixed to 100 and 500 sccm, respectively, during the CNT growth, and the growth time was 15 min. As shown in Figure 1, the products obtained at 800 and 900 °C have fibrous morphology with short length. Careful inspection revealed that they are not CNTs but amorphous carbon fibers. The CNTs were produced when the reaction temperature was 1000 °C (Figure 1c). These CNTs have a diameter distribution from 100 to 600 nm, and their length is about 10 µm. A catalyst particle can be seen on the tip of each CNT, indicating a tip-growth mechanism. Figure 1d shows the dependence of carbon yield on the reaction temperature. The carbon yield is around 13% at the reaction temperatures of 800 and 900 °C. There is a rapid increase for the carbon yield when the reaction temperature is 1000 °C, and the carbon yield reaches 32%. The experimental result indicates that thiophene has a very low decomposition on Co/MgO catalyst if the temperature is below 1000 °C. We selected 1000 °C as the reaction temperature for our subsequent experiments. Effect of Ar Flow Rate on Carbon Nanotube Growth. At the optimized temperature of 1000 °C, we carried out a set of experiments to investigate the influence of Ar flow rate on CNT growth. H2 flow rate was kept constant at 100 sccm in all runs, while Ar flow rate was varied from 0 to 1700 sccm. When Ar flow rate was low, e.g., 0 and 200 sccm (Figure 2, parts a and b), few CNTs were formed, whereas the carbon yield was as high as 38% (Figure 2f). The product was mainly carbon particles and some carbon fibers. A few short nanotubes with the length of 10 µm were obtained at an Ar flow rate of 500 sccm (Figure 1c). The quantity of CNTs increased and their length reached 20-30 µm when the Ar flow rate was 800 sccm (Figure 2c). Ar works as dilute gas in our experiments. When the Ar flow rate is low, the thiophene concentration is very high; too many carbon atoms precipitate on the catalyst surface and deactivate the catalyst very soon. Since high Ar flow rate can dilute the thiophene vapor, the Co catalysts remain active for a long time and many long CNTs can form. For example, the CNTs grown at Ar flow rate of 1100 sccm (Figure 2d) are about 40 µm in length. However, the length of CNTs decreased when the Ar flow rate increased to 1400 sccm (Figure 2e). This length decrease of the CNTs is attributed to the overdilution of the thiophene vapor. As shown in Figure 2f, the carbon (including CNTs and other kind of carbon products) yield reduces with the increase of the Ar flow rate, because the carbon source
Figure 1. FESEM images of the samples grown at different temperatures: (a) 800, (b) 900, and (c) 1000 °C. (d) A graph shows the dependence of carbon yield on the temperature. H2 and Ar flow rates were fixed to 100 and 500 sccm, respectively, during the reaction.
Figure 2. FESEM images of the samples grown at 1000 °C with different Ar flow rates: (a) 0, (b) 200, (c) 800, (d) 1100, and (e) 1400 sccm. (f) A graph shows the relationship between the carbon yield and Ar flow rate. The H2 flow rate was fixed to 100 sccm, and the catalyst powder was placed in a quartz boat for CNT growth.
(thiophene) concentration goes down. In other words, the conversion efficiency of thiophene to carbon is very low at high Ar flow rate. From SEM observation it is evident that the yield of CNTs grown at an Ar flow rate of 800 and 1100 sccm is higher than that of the CNTs obtained at other conditions. Therefore, the optimum Ar flow rate for the CNT growth with high yield is around 800-1100 sccm. In our experiments, H2 bubbles through the liquid thiophene and promotes the evaporation of the thiophene. Thiophene vapor is carried by H2, flows into the reaction chamber, and is further diluted by Ar before reaction. It is reasonable to infer that thiophene vapor is in equilibrium with its liquid phase due to the continuous bubbling. In other words, the H2 gas is saturated
Growth of CNTs by Pyrolysis of Thiophene
Figure 3. FESEM images of the as-grown samples on Si wafers at 1000 °C with different Ar flow rates: (a) 200, (b) 500, (c) 800, (d) 1100, (e) 1400, and (f) 1700 sccm. The H2 flow rate was kept constant at 100 sccm during the growth.
with thiophene vapor. Therefore, we can calculate the vapor concentration of thiophene in the reaction chamber and analyze quantitatively its influence on the CNT growth. The saturated vapor pressure of thiophene at room temperature (22 °C) is about 69.8 torr; thus, we know the vapor concentration of thiophene in the reaction chamber is about 8.4%, 3%, 1.5%, 1%, 0.76%, 0.61%, and 0.51%, respectively, corresponding to the Ar flow rate of 0, 200, 500, 800, 1100, 1400, and 1700 sccm. The
J. Phys. Chem. C, Vol. 111, No. 39, 2007 14295 optimum thiophene concentration for CNTs growth with high yield is around 1-0.76%. Effect of Substrate on Carbon Nanotube Growth. In the above experiments, the catalyst powder was placed in a quartz boat for the CNT growth. Here we placed the Co/MgO catalyst powder on a silicon wafer, which was inserted into the quartz chamber for CNT growth at 1000 °C with H2 at a flow rate of 100 sccm and Ar at a flow rate varied from 200 to 1700 sccm. Figure 3 shows the SEM images of as-prepared product on Si wafer. The product in Figure 3a is carbon nanofibers, prepared with an Ar flow rate of 200 sccm. Their diameters are not uniform and range from 50 to 300 nm. In contrast to Figures 1 and 2, apparently large quantities of CNTs were formed with Ar flow rates of 500, 800, 1100 sccm (Figure 3b-d). The CNTs are very long and cover the entire top surface of the Si wafer. For the CNTs in Figure 3, parts c and d, their lengths can reach 0.5-1 mm. Their diameters are uniform and are around 90 nm. A few branched CNTs were found in these samples. When the Ar flow rate increased to 1400 and 1700 sccm (Figure 3, parts e and f), the CNT yield reduced and the CNTs grew short. The carbon yield for the sample with an Ar flow rate of 800 sccm is 83%, about 3.5 times higher than that obtained by the quartz boat. So the Si wafer greatly enhances the CNT growth. The optimum thiophene concentration for CNT growth is still around 1-0.76%. A higher concentration of thiophene is prone to grow carbon nanofibers, whereas lower concentration results in short CNTs with low yield. Effect of Carbon Source on Carbon Nanotube Growth. Furan (C4H4O) is analogous to thiophene where the sulfur atom is replaced by oxygen. Cyclopentene (C5H8) also has a fivemembered ring, but it does not contain a sulfur atom. Furan and cyclopentene were used as carbon sources, respectively, to prepare CNTs at the same condition as for thiophene. CNTs were not found in the product by pyrolysis of furan and cyclopentene (Figure 4), and the product was carbon particles and fibers. Furan and cyclopentene have similar molecular structures with thiophene, but there is no CNT formed at the
Figure 4. FESEM images of the product by pyrolysis of furan (a) and cyclopentene (b) at 1000 °C with H2 flow rate of 100 sccm and Ar flow rate of 800 sccm.
Figure 5. Low-magnification (a) and high-magnification (b) FESEM images of branched CNTs grown at 1000 °C with H2 flow rate of 200 sccm and Ar flow rate of 800 sccm.
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Figure 6. Typical TEM images of the synthesized nanotubes. (a) Carbon nanofibers grown from Ar flow rate of 200 sccm. (b) Carbon nanotubes and (c) Y-junction CNT grown with Ar flow rate of 500 sccm. (d) Carbon nanotubes prepared with Ar flow rate of 800 sccm. (e, f, g and h) Yjunction and multibranched CNTs. (i) TEM image of a catalyst encapsulated in CNT. (j) EDS spectrum and (k) SAED pattern taken from this catalyst.
same condition. Therefore, the sulfur atom in thiophene is playing a crucial role in promoting the CNT growth. It was reported that sulfur can promote the graphitization of carbon materials by acting as cross-linker and being then released from the graphitic structure.28,29 So the growth of CNTs is not attributed to the specific ring structure of thiophene but to the presence of sulfur. Effect of H2 (Thiophene) Flow Rate on Carbon Nanotubes Growth. Thiophene vapor is carried into the reaction chamber by H2 in our experiments. Thus, the H2 flow rate determines the amount of thiophene that goes into the reaction chamber. Figure 5a is an SEM image of the sample prepared with an Ar flow rate of 1700 sccm and H2 flow rate of 200 sccm. In comparison with the sample grown at a H2 flow rate of 100 sccm (Figure 3f), more CNTs were formed due to the increased thiophene vapor concentration. The vapor concentration of thiophene in the reaction chamber is 0.96%, similar to the
concentration for the sample shown in Figure 3c. However, the thiophene flow rate for the former is twice as much as that for the latter. The difference in thiophene flow rate leads to the difference in the microstructure of the CNT product. Figure 5b is a high-magnification SEM image revealing that these CNTs are branched carbon nanotubes (BCNTs). Therefore, the high thiophene flow rate facilitates the growth of BCNTs. TEM Characterization of Carbon Nanotubes. To determine the microstructure of the CNTs, some samples were examined by TEM. Figure 6a shows a TEM image of carbon nanofibers produced with Ar flow rate of 200 sccm. These nanofibers are curly with a diameter of 60 nm. Shown in Figure 6b and 6c are some CNTs and a Y-junction CNT produced with an Ar flow rate of 500 sccm. These CNTs have a big diameter and coarse surface, indicating they are not well graphitized. The well-graphitized CNTs were observed in the sample obtained with an Ar flow rate of 800 sccm (Figure 6d). These results
Growth of CNTs by Pyrolysis of Thiophene
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Figure 7. Schematic showing three basic routes to form a branched CNT. TEM images of a (a) bent CNT, (b) Y-junction CNT with a catalyst at its junction, and (c) Y-junction CNT without a catalyst at its junction to support the proposed mechanism.
demonstrate that high Ar flow rate (or properly diluted thiophene vapor) improves the graphitization and makes the product transform from carbon nanofibers to CNTs. A Y-junction CNT is shown in Figure 6e, revealing the long and straight branches with hollow cylinder structure. The outer diameter of these branches is the same and is about 100 nm, and the thickness of the wall is 30 nm. The angles between the branches of this Y-junction tube are 145° and 70°. The angles between the branches vary for different BCNTs. Figure 6f shows a Yjunction CNT viewed along such a direction that two branches are overlapped in their projected image. Two typical multibranched CNTs are shown in Figure 6, parts g and h. A multibranched CNT is composed of several Y-junctions. Different branches in one BCNT have the same diameter, indicating that one catalyst is responsible for the growth of all branches. Figure 6i is a TEM image of a catalyst particle attached to a tip of a CNT. This result indicates that the growth mechanism for the CNTs is tip-growth. The chemical composition of the catalyst was analyzed by the EDS spectrum, and it reveals that the catalyst is composed of Co and S (Figure 6j). In this spectrum, the C signal originates from the CNTs. A selected area electron diffraction (SAED) pattern shown in Figure 6k was taken on this catalyst, and it revealed that the catalyst particle is cubic Co9S8 with a unit cell of a ) 0.99 nm (JCPDS card 86-2273). The diffraction spots can be indexed as the (220) and (11h3) planes; the [110] direction of the catalyst is parallel to the CNT growth direction. Growth Mechanism of Branched Carbon Nanotubes. TEM observations on the microstructure of the BCNTs offer a possibility to understand their growth mechanism. In our experiments, the initial catalyst is Co, which transforms to Co9S8 after reaction. It reveals a strong reaction between Co and S during CNT growth. The trait of this CNT growth by pyrolysis of thiophene is that the carbon atoms and sulfur atoms adsorb and coexist on the surface of the catalyst. We suggest that the interaction between sulfur and Co changes the active sites of the catalyst, thus change the growth direction, resulting in the formation of branched CNTs. Based on our TEM observation, we propose three routes to grow branched CNTs, which are schematically presented in Figure 7. For route 1, a straight CNT grows first and then changes its growth direction due to the change of the active site on the catalyst. As a result, a bent CNT is formed as shown in Figure 7a. For route 2, the catalyst
loses its mobility when the active site changes; two branches subsequently grow on the catalyst to form a Y-junction CNT with a catalyst located at its junction. This is supported by the CNT shape shown in Figure 7b. For route 3, the catalyst loses its mobility when the first CNT branch grows; however it moves forward again with the growth of the second CNT branch. Figure 7c is a TEM image showing a Y-junction without a catalyst at its junction. These three basic routes can combine each other to form a multibranched CNT. Conclusions We have investigated the influence of several synthesis parameters on the CNT growth by the cobalt-catalyzed pyrolysis of thiophene. The yield and the morphology of the product are highly sensitive to the experimental conditions. A synthesis temperature of 1000 °C is required for the decomposition of thiophene. In comparison to the quartz boat, the Si wafer greatly enhances the yield of CNTs. No CNT was found when furan and cyclopentene were used instead of thiophene, revealing that the sulfur in thiophene plays a crucial role in promoting the formation of CNTs. The morphology and the microstructures of the product are determined by the thiophene vapor concentration and flow rate. High concentration of thiophene vapor deactivates the catalyst and results in the formation of carbon nanofibers or short CNTs with bad graphitization; the optimum thiophene concentration for CNTs growth ranges from 1-0.76%. High flow rate of thiophene vapor produces Y-junction CNTs and multibranched CNTs. These Y-junction CNTs have long branches, showing a potential application in a nanodevice. The growth mechanism has been discussed, and three formation routes have been suggested. Acknowledgment. This work was supported by the NSF Career Grant DMR-0548061. References and Notes (1) Jang, J. W.; Lee, C. E.; Oh, C. I.; Lee, C. J. J. Appl. Phys. 2005, 98, 074316. (2) Ye, Y.; Ahn, C. C.; Witham, C.; Fultz, B.; Liu, J.; Rinzler, A. G.; Colbert, D.; Smith, K. A.; Smalley, R. E. Appl. Phys. Lett. 1999, 74, 2307. (3) Chopra, S.; McGuire, K.; Gothard, N.; Rao, A. M.; Pham, A. Appl. Phys. Lett. 2003, 83, 2280. (4) Dai, H. J.; Hafner, J. H.; Ringler, A. G.; Colbert, D. T.; Smalley, R. E. Nature 1996, 384, 147.
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