A Study on the Effects of Carrier Gases on the Structure and Morphology of Carbon Nanotubes Prepared

Joumalet Naterl Ga CremistyJournal of Natural Gas Chemistry 14(2005)151-155SCIENCE PRESSA Study on the Effects of Carrier Gases on the Structure andMorphology of Carbon Nanotubes Prepared by Pyrolysisof Ferrocene and C2H2 MixtureWanliang Mi'，Jerry Yuesheng Lin*1,2，Qian Mao'，Yongdan Li',Baoquan Zhang'1. School of Chernical Engineering and Technology, Tianjin Unitversity, Tianjin, 300072, China;2. Department of Chemical and Materials Engineering, Arizona State University, Tempe, AZ 85287- 6006, United States[Manuscript received June 15, 2005; revised July 15, 2005]Abstract: Carbon nanotubes (CNTs) were prepared using different carier gases, with ferrocene as thecatalyst precusor and acetylene as the carbon source. The effects of ammonia and nitrogen a8 carrier gaseson the structure and morphology of CNTs were investigated. Transmission electron microscope (TEM),high-resolution electron microscope (HRTEM), scanning electron microscope (SEM) and X-ray diffraction(XRD) were employed to characterize the products and the catalyst. Experiment results show that theCNTs grown in N2 gas exhibited cylindrical and tubular structure, while a bamboo-like structure wasobserved for the CNTs grown in NH3 gas. Moreover, vertically aligned CNTs were obtained on an Al2O3disk when NH3 was used as the carrier gas. The carrier gas also exerted influence on the shape of thecatalyst. Based on the theory of active centers of catalysis and combined with the particle shape of thecatalyst, a growth model for the vertically aligned CNTs on the substrate is given.Key words: carbon nanotube, ferrocene, vertically aligned, bamboo-like, carrier gas, growth model1. Introductionetc. CVD is a relatively easier method for preparingvertically aligned CNT arrays than the other meth-Unique electrical, magnetic, non- linear opticalods. And CVD is also an efficient method whichand mechanical properties make carbon nanotubescan be easily scaled up and is widely used for nan-to be regarded as fascinating and attractive nanos-otube synthesis. However, the produced CNTs cantructures for a variety of potential applications. In-adopt various irregular morphologies and, moreover,deed, CNTs have been proposed as new materials foroften have a carbon amorphous coating as well as haveelectron- field emitters in panel displays [1], molecular-catalyst particles embedded within their structures.filtration membranes [2], and single-rmolecular tran-These might be due to the effects of the catalyst pre-sistors [3)]. For the above applications, it is highlycursor, the carbon source, the carrier gases used dur-desirable to prepare well- aligned or patterned carboning the preparation and the reaction conditions. Thenanotubes.effect of the carrier gases on the structure and mor-Carbon nanotubes have been synthesized by allphology of the CNTs has not been studied systemat-kinds of possible methods since their discovery, suchically under similar reaction condition yet.as the arc-discharge technique [4], the laser techniqueVertically aligned CNT films, if supported on ad-[5] and by chemical vapor decomposition (CVD) [6)],equatl中国煤化工fer potential appli-●Corresponding author. Tel: 086-022-27402062, Fax: 086-022-2740524HCNMHGFirst author. Email: wlmi@tju.edu.cn.This work was supported by the National Natural Science Foundation of China (No. 50228203 and 20425619) and CheungKong Scholar Program.152Wanliang Mi et al./ Journal of Natural Gas Chemistry Vol. 14 No. 3 2005cations as membranes in a number of new separationplaced in the second zone. Carbon also deposited onand reaction processes. Macroporous a-Al2O3 withthe wall of the reactor. After the reaction was carriedvarious geometries and prepared by press sintering orout for some time, the reactor was cooled down toextrusion-sintering method is regarded as the mostroom temperature under an NH3 or N2 gas.commonly used support for inorganic membranes uti-Scanning electron microscope (SEM) images werelized in commercial processes. Generally, supports arerecorded on the as deposited samples using a Philipsdefined by compacting of submicron-sized aluminaXL 30ESEM instrument for checking the nature andparticles. This work attempted to prepare verticallythe morphological characteristics of the aligned nan-aligned CNTs on substrates by the CVD method, us-otube bundles.Transmission electron microscopeing different carrier gases. Ammonia and 'nitrogen(TEM) images were obtained with a JEOL-TEM-gases were selected as the carrier gases for the study.100CX II electron microscope. A D/max-2500 X-rayPossible formation mechanism of vertically aligneddiffractometer (XRD) produced by Rigaku Interna-CNTs on the disk was also discussed from the pointtional Corp. of Japan was employed to analyze theof the shape the catalyst particles.CNTs and carbon, using Cu Ka radiation. A PhilipsTecnai G2 F20 high- resolution transmission electron2. Experimentalmicroscope was employed to characterize the CNTs.2.1. Preparation of Al2O3 substrate3. Results and discussionThe macroporous a-Al2O3 disk was prepared bythe following proces: a- Al2O3 powder (particle di-3.1. Morphology of CNTs on the substrateameter≤2 μm, Gaoda Materials Factory, Kaifeng,China) was first pressed at high pressure (ca. 242-Figure 1 is the TEM micrograph of the CNTs pre-484 MPa for a disk) in a mould and then sinteredpared in gaseous NH3 (a) and N2 (b) at 800 °C andat high temperatures (at a maximum temperature ofscraped from the Al2O3 substrate, which shows that1280 °C for 2 h) in order to enbance its mechani-the diameter of the CNTs prepared in the NH3 gas iscal strength. After sintering, the a-Al2O3 disks werelarger than that prepared in the N2 gas. The CNTspolished with #1000 sand papers and cleaned in anultrasonic acetone bath for 30 min.2.2. Preparation of vertically aligned CNTsfilm on a-Al2Og diskA two-stage furnace tubular quartz reactor wasemployed to prepare the aligned CNTs. Ferrocenehas been shown to be a good precursor for produc-ing Fe catalyst particles, which can seed for nanotubegrowth. Acetylene was selected as the carbon source.The substrate in the deposition zone was heated toa desired CVD temperature (800 °C) at a rampingrate of 10 °C/min. The ferrocene vapor from sublima-tion was fed into the second furnace by NH3 or N2 gasand decomposed into iron nanoparticles or cementiteswhich deposited on the surface of the alumina sub-strate. A mixture of acetylene and ammonia/ nitrogenof 1:10 was subsequently introduced into the CVD re-actor at a total flow rate of 110 ml/min. The pressure中国煤化工inside the quartz tube was held constant at about 1atm in all the experiments. After the introduction ofMHCNMHGthe acetylene for several minutes, lots of black car-Figure 1. TEM images of CNTs grown in gaseousbon were deposited on the surface of the Al2O3 diskNHs(a)andNz(b)at800°Cfor10minJournal of Natural Gas Chermistry Vol. 14 No.3 2005153prepared in NH3 are about 100 nm in outer diame-3.2. Catalyst structure enwraped in the CNTter, and only a small part of them are about 50 nm.tipAll the CNTs have a bamboo like structure, as shownin the Figure. The bamboo- like compartment of theCNTs prepared in NH3 and N2 have differentCNTs is about 100 nm in length. On the other hand,spaces in the graphite sheet even at the same reactionmost of the CNTs prepared in N2 gas are about 80 nmtemperature and time. The graphite sheet space wasin outer diameter and 10 nm in inner diameter, to-about 0.348 and 0.384 nm for the CNTs grown in N2gether with a small amount of CNTs in diametersand NH3, respectively, as calculated from the HRTEMless than 20 nm. The CNTs have a tubular structure.images. The graphite sheet space of the CNTs in NH3No matter what carrier gas was used, the diameter ofwas larger than that in N2, which indicates that NH3the CNTs was not uniform. There were sorme carbonhas an effect on graphitic degree of the CNTs. NH3nanotubes with smaller diameters, even smaller thancould cause the graphite degree of the CNTs to de-10 nm.crease. The graphite sheet space of the CNTs synthe-Figure 2 shows the SEM micrograph of the cross-sized in both of the above-mentioned gases was largersection of the a- Al2O3 disk coated with a black filmthan that of the graphite (0.335 nm).after the reaction at 800 °C in NH3 (a) and N2In order to see more clearly the shape of the(b) for 10 min, respectively. The upper layer is thecatalyst packed in the CNT tip, we selected someCNTs and the bottom part is the support which con-CNTs with smaller diameters for studying their high-sists of submicron-sized a-Al2O3 crystalline particles.resolution transmission electron microscopy. Figure 3The crystalline particles are about 2 μm in diameter.gives the HRTEM images of the carbon nanotubesThere are almost no CNTs grown in the substrate, butonly on the substrate. As is shown, the CNTs grew .vertically on the surface of the a-Al2O3 disk in NH3.The CNT was about 10 μm in length for a reactiontime of 10 min. But the alignment of CNTs grown inN2 was not as good as that grown in NHs, and the di-ameter of CNTs was smaller than that grown in NH3.(6)中国煤化工MHCNMHGFigure 2. SEM micrographs of CNTs grown on Al2O3disk in gaseous NH3 (a) and N2 (b) at 800Figure 3. HRTEM images of carbon nanotubes pre-°C for 10 minpared in gaseous NH3 (a) and Nz (b)154Wanliang Mi et al./ Journal of Natural Gas Chemnistry Vol. 14 No. 32005prepared in the NH3 (a) and N2 (b) gases. It can be .applicable. The growth of some carbon naotubes inseen that the catalyst grown in NHs has a cone -shapedour experiment can be explained by this base growthstructure, while that grown in N2 is in a column-model, as indicated by the arrow in Figure 1(a).shaped structure. This indicates that the carrier gasLee has explained the growth model of bamboo-has an effect on the shape of the catalyst. Carbon-like structured CNTs [10]. One of the objectives ofcontained Ni particles could change their morphologythis paper is to explain the reason why well-definedfrom a spherical to a cone shape when the ratio ofvertically aligned CNTs were formed in NH3 insteadCH4 to NH3 approached 1, and the CNTs alignmentof in N2 gas. In fact, many growth mechanismsturned better, as reported by Juang [7]. This studyhave made discussions under different reaction condi-confirms that NH3 has an influence on the 'shape oftions[11], such as the density-controlling mechanism.the catalyst.This paper will elucidate the effect of the catalystFigure 4 compares the XRD patterns of the CNTsmorphology on the alignment of the CNTs. Catalyticgrown at 800 °C for 10 min in different carrier gases.reactions occur only at the active sites of the cata-Because of the small thickness of the coated CNTlyst, as stated by the active center theory. And activefilm, the majority of the diffraction peaks were fromcenters of catalysts usually exist at the edges, corthe a-Al2O3 substrate. As indicated by the aster-ners, kinks or bugs of the catalyst surface, becauseisk, it is still possible to see the diffraction peaks atthe chemical bonds of these sites are lack of satura-20=26.6° and 26.3°, respectively, for the samples pre-tion, resulting in their good adsorption abilities. Aspared in NH3 and N2 gases, which are characteristicshown in Figure 3, activation energy on the tip of theof carbon nanotubes (002) and similar to the graphitecone shaped catalyst is lower than on other sites, sincecharacteristic diffraction peak (20=26.50). The char-this site is easier to react with the carbon atoms. Acacteristic diffraction peaks of carbon nanotubes pre-cordingly, reaction rate at this site is higher. The car-pared by Valiente [8] appeared at 20= 25.39, and doo2bon graphitic sheet will first separate from the cone-was equal to 0.352 nm. In our experiment, the doo2shaped tip, then the CNTs will grow in this direction.of the CNTs prepared in N2 gas was about 0.339 nm,Since ferrocene could form cone-shaped nanopar-close to that of graphite (0.335 nm). Therefore, theticles in NH3, when the nanoparticles were depositedgraphitic degree was improved in this study.on the substrate, the site at the cone shaped tip couldexert a steadier directing action to the top. Thus,the initial growth of the CNTs will be pointed to thedirection vertical to the substrate according to thebase growth model.. Even if some catalyst particletips did not point to the upward direction, but dur-ing the growth of the CNTs, they will turn into a ver-tically aligned mode because of the steric hindrancefrom the adjacent nanotubes via the Van der Waalsforce among the carbon nanotubes [12,13]. Schematicdiagrams of the vertically aligned growth model are1)shown in Figure 5. .10 205060702010 )Figure 4. XRD patterns of carbon nanotubes pre-pared in gaseous NHs (1) and N2 (2)3.3. Growth mechanism of vertically alignedCNTs on macroporous substrates中国煤化工of the vertiallyAccording to Fan and Lee [9,10], the curvatureof the compartment layers is directed to the tip forYHCNMHGmodelsome CNTs, irrespective of the encapsulation of theFor the CNTs grown in the N2 gas, ferrocenecatalytic particles, and the base growth model can beformed column-like nanoparticles. They could lie eas-Journal of Natural Gas Chemnistry Vol. 14 No.32005155ily on the substrate, so that the CNTs could growbasing on the active center theory.from one end or both ends of the particles, and theirdirections were not vertically aligned, but were nearlyReferencesparallel to the substrate. Even with the existence ofthe Van der Waals force and the steric hindrance,[1] Rinzler A G, Hafner J H, Nikolaev P et al. Science,and the CNTs had grown slightly vertically aligned1995, 269: 1550on the substrate, their alignment was not as good as[2] Che G, Lakshmi B B, Fisher E R et al. Nature, 1998,that grown in NH3. This can be seen from the com-393: 346parison of Figure 2. The nanocatalyst figuration has[3] Tans s J, Verschueren A R M, Delkker C. Nature,significant influence on the growth and alignment of1998, 393: 49the CNTs. The cone-shaped nanoparticles also con-4] Ebbesen T W, Ajayan P M. Nature, 1992, 358: 220tributed to the growth of the vertically aligned CNTs[5] Thess A, Lee r, Nikolaev P et al. Science, 1996, 273:because of their influence on the initial growth direc-483tion of the CNTs.[6] Jose- Yacaman M, Miki-Yoshida M, Rendon L et al.Appl Phys Lett, 1993, 62: 657[7] JuangZ Y, ChienIP, LaiJ F et al. Diamond Relat4. ConclusionsMater, 2004, 13: 12038] Valiente A M, Lopez P N, Ramos I R et al. Carbon,Experimental results show that carbon nanotubes2000, 38: 2003formed bamboo-like structure when grown in NH3.9] Fan S, Chapline M G, Franklin N R et al. Science,Ihe growth of certain CNTs could be explained by1999, 283: 512the base growth model, including tubular structured[10] Lee C J, Park J. J Phys Chem B, 2001, 105: 2365CNTs grown in N2. The diameter of the CNTs syn-[11] Mi W L, Lin Y S, Zhang B Q et al. Huaxue Jinzhanthesized in NH3 was larger than that in N2. The align-(Prog Chem), 2004, 16(6): 843ment of the CNTs grown in NH3 was better than that[12] Zhu C, Xie Z, Guo K. Diamond Relat Mater, 2004,in N2. A catalyst morphology model is employed to13: 180elucidate the difference on the alignment of the CNTs [13] Tucknott R, Yaliraki S N. Chem Phys, 2002, 281: 455中国煤化工MYHCNMHG