Effect of FeO x loaded on CoO x /Al 2 O 3 catalyst for the formation of thin-walled carbon nanotubes Siang-Piao Chai a,b , Wei-Wen Liu c , Kim-Yang Lee a , Wei-Ming Yeoh a , VM Sivakumar a , Abdul Rahman Mohamed a, ⁎ , 1 a School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, SPS Pulau Pinang, Malaysia b School of Engineering, Monash University, Jalan Lagoon Selatan, 46150 Bandar Sunway, Selangor, Malaysia c School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, SPS Pulau Pinang, Malaysia abstract article info Article history: Received 22 October 2008 Accepted 14 March 2009 Available online 18 March 2009 Keywords: Carbon nanotubes Catalyst Electron microscopy Methane decomposition Effects of FeO x loaded on CoO x /Al 2 O 3 catalyst on the yield and morphology of the produced carbon nanotubes were studied. The findings showed that the addition of a small amount of FeO x on the CoO x /Al 2 O 3 catalyst provoked the formation of carbon nanotubes with a thin wall structure. The results also revealed that an increase in FeO x content decreased the yield of carbon nanotubes. An optimized weight ratio of CoO x to FeO x was found to be 8:2 (w/w) whereby the catalyst of this composition grew carbon nanotubes with a thin wall structure and not of diminutive carbon yield. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Carbon nanotubes have created an active area of current research because of their unique structural, mechanical, and electrical proper- ties [1]. They are generally considered as promising building blocks for nanoscale devices. Several nanoelectronics devices based on carbon nanotubes such as quantum wires, field effect transistors, field emitters and diodes have been demonstrated [2–5]. It is well accepted that the properties of carbon nanotubes, including electrical and mechanical properties depend strongly on their chirality, diameter, and wall thickness [6–8]. The interaction or coupling between the constituent graphene layers for carbon nanotubes with thick walls results in their physical and chemical properties being more complicated. On the other hand, carbon nanotubes with smaller diameters and thinner walls are much needed in the miniaturization of electronic applications due to their excellent electronic and electrical properties. Our previous results showed that the NiO/TiO 2 catalyst was effective in producing carbon nanotubes from methane and the activation energy for the process was one of the lowest ever reported in the literature [9]. However, the produced carbon nanotubes possessed a larger diameter (~40 nm) and a thick wall morphology. We had also demonstrated that FeO x might induce the formation of carbon nanotubes with a thin wall structure [10]. Nevertheless, no further study was carried out to investigate the influence of FeO x on the morphology of nanotubes grown. Hence, this letter is aimed at reporting the effect of FeO x loaded on the CoO x /Al 2 O 3 catalyst on carbon yield and morphology of the carbon nanotubes synthesized via methane decomposition. 2. Experimental Co(NO 3 ) 2 .6H 2 O (supplied by Aldrich) and Fe(NO 3 ) 2 .9H 2 O (sup- plied by Merck) were used as metal sources for the preparation of CoO x and FeO x . Alumina (supplied by Ajax) was used as a catalyst support. All the catalysts used in this study were prepared using a conventional impregnation method. The experimental setup and the catalyst preparation procedures had been reported previously [10– 12]. The synthesis of carbon nanotubes was carried out at atmospheric pressure in a stainless steel fixed-bed reactor at a temperature of 700 °C. The product gases were analyzed using on-line gas chromatography (Hewlett-Packard Series 6890, USA). Carbon nano- tubes deposited on the catalysts were analyzed using a transmission electron microscope (TEM) (Philips, CM12) and a scanning electron microscopy (SEM) image of the catalyst particles was taken using LEO Supra 50 VP FESEM. An X-ray diffraction (XRD) pattern of the catalyst after reaction was measured by Bruker D8 Advance Powder Diffractometer. Intensity was measured by step scanning in the 2θ range of 20–70° with a step of 0.02° and a measuring time of 2 s/point. Materials Letters 63 (2009) 1428–1430 ⁎ Corresponding author. E-mail address: chrahman@eng.usm.my (A. Rahman Mohamed). 1 Tel.: +6 04 599 6410; fax: +6 04 5941013. 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.03.019 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet