Thermodynamic analysis and chemical vapor deposition of multi-walled carbon nanotubes from pre-heated CH 4 using Fe 2 O 3 particles as catalyst precursor Melek Cumbul Altay, Serafettin Eroglu n Department of Metallurgical and Materials Engineering, Faculty of Engineering, Istanbul University, Avcilar, 34320 Istanbul, Turkey article info Article history: Received 13 May 2012 Received in revised form 13 November 2012 Accepted 28 November 2012 Communicated by: G.B. Stringfellow Available online 5 December 2012 Keywords: A1. Thermodynamic analysis A3. Chemical vapor deposition processes B1. Carbon nanotubes B1. Iron oxide B1. Nanomaterials abstract The present study aims to investigate influence of pre-heating of CH 4 on the growth of multi-walled carbon nanotubes (MWCNTs) using Fe 2 O 3 particles as catalyst precursor. Equilibrium thermodynamic analyses in the systems of Fe–C–O–H and C–H were performed to better understand the reduction of Fe 2 O 3 by CH 4 and to identify intermediate species which might promote growth of MWCNTs at the temperature range of 1000–1300 K. It was found that CH 4 acted as reducing agent for Fe 2 O 3 catalyst precursor which transformed to Fe 3 O 4 , FeO, Fe and Fe 3 C phases. This result was found to be in agreement with the thermodynamic prediction at 1200 and 1300 K. SEM-EDS analysis revealed that un-preheated CH 4 yielded MWCNTs at 1200 K and a dense C coating at 1300 K. It was also observed that carbon was not formed at temperatures in the range of 1050–1150 K. Whereas, MWCNTs were grown at this temperature range from CH 4 pre-heated at 1200 K. This result was attributed to the intermediate hydrocarbons, especially to C 6 H 6 , formed during pre-heating stage as thermodynamic analysis suggested. Mean diameter of the synthesized tubes was found to increase with growth time and temperature. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Multi-walled carbon nanotubes (MWCNTs) are novel materials with unique properties (e.g. high stiffness, strong axial strength, large surface area) for potential applications such as chemical sensors, catalyst supports, high performance composites. MWCNTs are commonly grown by laser ablation, arc discharging and chemical vapor deposition (CVD) techniques in the presence of transition metal catalysts such as Fe, Ni, Co [1]. Among these techniques, chemical vapor deposition (CVD) has received con- siderable attention because it has inherent advantages including higher scalability, better control of process parameters and product purity. Growth of MWCNTs by CVD technique essentially involves decomposition of carbon-bearing gas, diffusion and precipitation of C atoms on the catalyst particles [2]. Iron compounds (e.g. oxides, nitrates) are often used as catalyst precursors for the synthesis of CNTs. A pre-reduction step is generally taken using H 2 or NH 3 as reducing agent prior to introducing C-containing gas such as CH 4 , CO [3–5]. CNT growth from CH 4 commonly diluted by H 2 takes place at elevated temperatures (e.g. 1273 K) owing to the strong tetrahedral C–H bonds (435 kJ/mol). Our earlier work [6] showed that chemical vapor deposition of pure CH 4 pre-heated at 1200 K yielded MWCNTs in the presence of Fe–Ni (70 wt %) catalyst, whereas no carbon formation was observed without pre-heating at the temperature range of 1050– 1150 K. The present study aims to investigate and discuss chemical vapor deposition of MWCNTs from pre-heated CH 4 using Fe 2 O 3 particles as catalyst precursor. Reduction behavior of Fe 2 O 3 in a flowing undiluted CH 4 atmosphere was also elucidated to gain insight into the MWCNT synthesis. Equilibrium thermodynamic analyses of the Fe–O–C–H and C–H systems (excluding graphite) were carried out to better understand the thermochemistry of the reduction and MWCNT growth processes. 2. Materials and methods The reactor used for the synthesis experiments consists of SiC heating elements, a quartz tube (20 mm in diameter, 500 mm in length) and gas flow meters. g-Fe 2 O 3 (Maghemite) powder with mean particle size 38 718 nm was used as catalyst precursor. Its mean particle size was calculated from at least 50 measurements using the SEM images. The oxide powder (160 mg) was placed in an alumina (Al 2 O 3 ) boat and allowed to react with undiluted CH 4 Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.11.062 n Corresponding author. Tel.: þ90 212 473 7065; fax: þ90 212 473 7180. E-mail address: seref@istanbul.edu.tr (S. Eroglu). Journal of Crystal Growth 364 (2013) 40–45