APPLIED PHYSICS LETTERS
VOLUME 84, NUMBER 6
9 FEBRUARY 2004
In situ observations of carbon nanotube formation using environmental transmission electron microscopy Renu Sharmaa) Center for Solid State Science, Arizona State University, Tempe, Arizona 85287-1704
Zafar Iqbal Department of Chemistry, New Jersey Institute of Technology, Newark, New Jersey 07102-1982
共Received 28 February 2003; accepted 12 December 2003兲 Environmental transmission electron microscope is used for in situ observations of the growth mechanism and reaction conditions of carbon nanotubes. Chemical vapor deposition was performed by flowing propylene or acetylene gas 共precursor兲 over Ni or Co catalyst heated to 450 °C and 700 °C. We are reporting the in situ observations of the growth process of carbon nanotubes. Multi-wall nanotubes formed at temperatures as low as 450 °C while only single-wall carbon nanotubes formed at higher temperatures 共700 °C and above兲. At lower temperatures, a cubic phase was also observed to form on the walls of the multi-wall carbon nanotubes. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1646465兴
Carbon nanotubes 共CNTs兲 were dramatically novel nanoscale materials when they were first discovered in 1991 by Iijima using a carbon arc discharge process.1 They have since become one of the most sought out materials for nanotechnology due to their remarkable magnetic, electronic and mechanical properties.2– 4 The structure of a CNT can be described in terms of a single graphite layer 共graphene兲 rolled up to form a single cylinder or concentrically arranged cylinders. The former is referred to as a single-wall nanotube 共SWNT兲 and the latter are called multi-wall nanotubes 共MWNTs兲. MWNTs can form starting from double-walled tubes 共DWNTs兲 to tubes with 50 or more walls or layers. During any synthesis, MWNTs of different diameters, mixtures of SWNTs and MWNTs or mixtures of the three types of SWNTs, are formed. Most SWNT synthesis routes also give rise to chiral tubes of varying chirality. Hence, in order to exploit any particular property, it is important to be able to synthesize a single phase of any one of the above forms. Unfortunately to date, only pure MWNTs with a range of outer diameters or a mixture of SWNTs of varying diameters and chirality, can be synthesized. The introduction of transition and rare-earth metal catalysts in carbon arc and laser ablation techniques led to the first production of SWNTs. Recently, catalytic chemical vapor deposition 共CVD兲5,6 has been found to provide a more controlled, near-equilibrium route to both MWNT and SWNT structures. In the CVD process CNTs are formed by heating a transition metal catalyst 共typically Fe, Ni, and Co兲 at 500–1200 °C in either CO, a hydrocarbon precursor like CH4 , C2 H4 , and C2 H2 or an organo-metallic, such as ferrocene and iron pentacarbonyl. In earlier CVD studies, MWNTs were observed to form mixed in with microcrystalline and amorphous carbon.5 Researchers have now been able to qualitatively select catalyst compositions and reaction
conditions that produce pure SWNTs in greater than 90% yields.6 – 8 Detailed understanding of the growth process and the ability to control the type of SWNT produced still remains as substantial challenges. Our motivation for in situ observation is twofold: 共1兲 To follow the growth process and suggest a mechanism and 共2兲 to use this knowledge to optimize the growth conditions for SWNTs. We hereby report the results of our preliminary in situ investigation of the CNT growth process and its mechanism. In our preliminary studies we have used two different catalyst systems (Ni–SiO2 and Co, Mo–MgO兲9,10 and two precursors, propylene 共99.9999%兲 and welder’s grade acetylene, in order to check the viability of the technique used. The catalysts–supports were dry loaded on 400 mesh Ni grids by rolling the grids in the catalyst powder. Particles hanging out from the grid bars were used for observations as they have the best thermal contact with the furnace. The Ni catalyst samples were first heated at 400 °C for 30– 60 minutes in vacuum or in 1 to 2 Torr of H2 using a furnace heating holder with thermocouple placed on the furnace cover. In situ observations were made in a modified Phillips 430 transmission electron microscope, equipped with environmental cell 共ETEM兲 and Gatan imaging filter that was used to record electron energy-loss spectra.11 The microscope was operated at 200 KV 共0.27 nm point resolution兲 and observations were made at low magnifications in order to observe large regions of the sample. The inlet gas pressures were monitored using thermoionic gage. The first set of experiments was performed using 100–1500 mTorr of propylene as precursor gas and Ni–SiO2 as the catalyst–support system. In the second set of experiments, Ni–SiO2 catalyst– support samples were heated at low 共450 °C兲 and high 共750 °C兲 temperatures whereas the Co, Mo–MgO catalyst– support samples were heated only at high temperatures in 100–1500 mTorr of acetylene, in order to compare the ETEM results with the bulk synthesis process using CO.10 Images were recorded using a CCD camera 共1024⫻1024
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© 2004 American Institute of Physics
Appl. Phys. Lett., Vol. 84, No. 6, 9 February 2004
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FIG. 1. Low magnification TEM images recorded at 450 °C. 共a兲 Ni catalyst particles 共marked by arrows兲 and the support in H2 at 450 °C, 共b兲 and 共c兲 progressive growth of CNT in 300 mTorr of H2 – C2 H2 mixture and 共d兲 another part of the same sample after 3 minutes.
pixels兲 and a digital video recording system before, after and during the reaction. Ex-situ HREM images were recorded using a JEOL 4000EX operated at 400 KV with 0.17 nm point resolution. Although filamentous growth was observed when propylene was leaked over Ni–SiO2 at 400 °C in an ETEM, the tubular character of nanotubes could not be observed in these filaments. High-resolution electron micrographs 共HREM兲 confirmed these filaments to be purely graphitic in nature similar to those reported by Baker when Ni–Fe catalysts samples were heated in methane.12 Acetylene (C2 H2 ), on the other hand provided the growth of well-defined nanotube structure. Low magnification images, recorded three minutes apart showed the growth of nanotubes at 450 °C in 300 mTorr of C2 H2 关Figs. 1共a兲– 1共c兲兴. Ni particles at times could be observed not only at the apex of these tubes as reported earlier, but also in the middle of the tubes 关marked by an arrow in Figs. 1共b兲 and 1共c兲兴. These tubes grew in a zigzag manner, sometimes making a 360° turn. In order to evaluate the effect of the electron beam on the growth rates, the sample was surveyed after cooling down to room temperature and after evacuating the hydrocarbons from the microscope chamber. CNTs were observed to have formed all over the sample area 关Fig. 1共d兲兴. Thus it can be concluded that the observed growth process is not electron beam induced and the in situ observations can be directly compared to bulk synthesis conditions. The HREM images 共Fig. 2兲 confirmed that growth of MWNT is favored at 450 °C. The presence of a few graphene
sheets forming a wall perpendicular to the length of a tube and a ‘‘bubble’’ within the wall of the tube indicates that this CNT is in the process of development 关Fig. 2共a兲兴. The inner and the outer diameters of these tubes were found to range from 2 to 8 nm and 8 –17 nm, respectively. Some variation in both the number of graphene sheets constituting the wall as well as in the inner tube was found to be present within the tubes 关Fig. 2共b兲兴. Again, these observations can be attributed to the fact that the reaction had not completed and we were not observing growth under equilibrium conditions. Frequently, a two-dimensional lattice was also observed to form on the outer walls of the MWNTs 关Fig. 2共b兲兴 under these conditions. As only carbon was present in the electron energy-loss spectra recorded from this region, it is safe to assume that it is a form of carbon. The d-spacings of this structure were accurately measured using a diffractogram 关Fig. 2共c兲兴 of the area marked B in Fig. 2共b兲. The measured spacing may correspond to cubic phase with a⫽4.5 Å, and have similarity to one of the low energy forms of carbon predicted by O’Keeffe et al.13 They describe this structure to be a body centered cubic (I4 1 32) with a⫽4.065 Å and carbon atoms in 8 共a兲 at 1/8, 1/8, 1/8, etc. positions. We have used their parameters to simulate an image along the 具110典 zone axis for comparison 关Fig. 2共c兲兴. The overall structure matches well but the errors of measured and predicted cell parameters are quite large; we are, therefore, still investigating other possibilities. At higher temperatures 共700°– 800 °C兲, single and double walled CNTs were observed to form predominantly,
FIG. 2. HREM images of typical MWNT observed to form at 450 °C. 共a兲 A MWNT with inner and outer diameters of 7.4 and 16.8 nm, respectively, with a few graphene layers 共marked by the arrow兲 and a ‘‘bubble’’ 共marked A兲 within one of the walls. 共b兲 Another MWNT with varying inner diameter and a new form of carbon structure formed on the walls, 共c兲 high magnification image of the area marked B in 共b兲. The simulated image and digital diffractogram from the particle are inset at the center and the bottom right respectively. The zone axis used for simulations is 具100典, defocus⫽⫺450 Å and thickness⫽81.3 Å.
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Appl. Phys. Lett., Vol. 84, No. 6, 9 February 2004
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FIG. 3. HREM images showing 共a兲 SWNT and graphene-like filaments 共marked by arrows兲 共b兲 bundle of SWNTs and 共c兲 uniform SWNT forming a bridge between two catalyst clusters. Figures 共a兲 and 共b兲 are from Ni–SiO2 catalyst and 共c兲 is from Mo–MgO catalyst heated at ⬇750 °C in 300 mTorr of C2 H2 in the ETEM.
occasionally accompanied by extremely thin graphene filaments 关Fig. 3共a兲兴. Often a bundle of SWNT sheets 关Fig. 3共b兲兴 were also observed to form. There are a number of ways SWNTs were found to be different than MWNTs. SWNT grew straight out from the catalyst sample with very uniform inner diameters for individual tubes 关Fig. 3共c兲兴 but varied between 1 and 1.5 nm for different tubes within the sample. Also, there was no catalyst present at the apex or in the middle of the tubes as was the case for MWNTs. It has been proposed that the SWNT grow from the catalyst that is present at the root of the tube.2 In our images, the roots of SWNT’s were embedded deep in the catalyst matrix and it is difficult for us to conclude that an individual catalyst particle is at the root of the tube. They were often observed to make a bridge between two clusters of catalyst particles as shown in Fig. 3共c兲. Raman spectra were recorded from the same grids examined in the electron microscope using confocal micro-Raman spectroscopy, 514.5 nm excitation and spatial resolution from 1 to 2 m. The low frequency radial modes were clearly observed from samples prepared in acetylene at 700 °C confirming that the spectra were from SWNTs. Two sets of strong radial breathing mode lines at 174.5 and 177.4 cm⫺1 , and 238.9 and 255.7 cm⫺1 were observed corresponding to average SWNT diameters from 1.26 to 1.28 nm and 0.875 and 0.936 nm, respectively. The average diameters calculated from the Raman data agreed rather well with individual diameters seen in the electron microscope data. In conclusion: Filamentous growth was clearly seen at low temperature in propylene, confirming the earlier results of Baker and coworkers. However, multiwall tubes were observed to form at temperatures as low as 450 °C in acetylene, with catalyst observed both at the center and at the tips of the tubes. Remarkably, a crystalline cubic carbon phase appears
to form on the walls of the growing MWNTs. It would be of interest to identify this phase and determine its role in the growth of SWNTs at temperatures above 700 °C. MicroRaman spectroscopy and HREM images have independently confirmed the formation SWNTs at these low temperatures. SWNTs form both as individual tubes and bundles, on both Ni and Co, Mo catalyst together with thin filaments that appear to be narrow graphene sheets. Catalyst particles are not observed at the tips or inside the grown SWNTs. It appears that the tubes emerge from the agglomerates of the catalyst– support particles. Ni–SiO2 samples were provided by Professor T. Cale and the use of Center for High Resolution microscopy at ASU is gratefully acknowledged. S. Iijima, Nature 共London兲 354, 56 共1992兲. Carbon Nanotubes: Synthesis, Structure, Properties and Applications, edited by M. Dresselhaus, G. Dresselhaus, and Ph. Avouris 共Springer, Berlin, 2001兲, and chapters therein. 3 Z. Iqbal, in Nanostructured Carbon for Advanced Applications, edited by G. Benedek, P. Milani, and V. G. Ralchenko 共Kluwer Academic, 2001兲, p. 309. 4 M. J. Bronikowski, P. A. Willis, D. T. Colbert, K. A. Smith, and R. A. Smalley, J. Vac. Sci. Technol. A 19, 1800 共2001兲. 5 A. Lan, Z. Iqbal, A. Aitouchen, M. Libera, and H. Grebel, Appl. Phys. Lett. 81, 433 共2002兲. 6 M. J. Yacaman, M. M. Yoshida, L. Rendon, and J. G. Santiesteban, Appl. Phys. Lett. 62, 202 共1993兲. 7 B. Kitiyanan, W. E. Alvarez, J. H. Harwell, and D. E. Resasco, Chem. Phys. Lett. 317, 497 共2000兲. 8 Y. Zhang, Y. Li, W. Kim, D. Wang, and H. Dai, Appl. Phys. A: Mater. Sci. Process. 74, 325 共2002兲. 9 T. Cale and J. T. Richardson, J. Catal. 79, 378 共1983兲. 10 A. Goyal, Z. Iqbal, and R. Sharma, unpublished. 11 R. Sharma and K. Weiss, Microsc. Res. Tech. 42, 270 共1998兲. 12 R. T. K. Baker and J. J. Chludzinski, J. Catal. 64, 464 共1980兲. 13 M. O’Keeffe, G. B. Adams, and O. F. Sankey, Phys. Rev. Lett. 68, 2325 共1992兲. 1 2
