Diameter control of multiwalled carbon nanotubes using experimental strategies Chien-Sheng Kuo a , Allen Bai b , Chien-Ming Huang a , Yuan-Yao Li a, * , Chi-Chang Hu c , Chien-Chong Chen c a Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan, ROC b Department of Biochemical Engineering, Kao Yuan Institute of Technology, Kao-Hsiung 1821, Taiwan, ROC c Center for Nanotechnology Design and Prototyping, National Chung Cheng University, Chia-Yi 621, Taiwan, ROC Received 9 October 2004; accepted 22 May 2005 Available online 5 July 2005 Abstract Multiwalled carbon nanotubes (MWNTs) were synthesized using a chemical vapor deposition floating feed method in a vertical reactor. Effects of the preparation variables on the average diameter of carbon nanotubes were systematically examined using the fractional factorial design (FFD), path of the steepest ascent, and central composite design (CCD) coupled with the response surface methodology. From the FFD study, the main and interactive effects of reaction temperature, methane flow rate, and chamber pres- sure were concluded to be the key factors influencing the diameter of MWNTs. Two empirical models, representing the dependence of the diameter of carbon nanotubes at the vicinities around maximum (420 nm) and minimum (15 nm) on the reaction temperature and methane flow rate, were constructed in two independent CCD studies. These models, shown as contour diagrams, indicated that the diameter of carbon nanotubes generally increased with increasing reaction temperature and methane flow rate. Based on both models, the diameter of MWNTs from 15 to 420 nm can be controlled precisely by using a continuous CVD fabrication method. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Chemical vapor deposition; Electron microscopy 1. Introduction Mass production of carbon nanotubes in various forms has been widely developed and studied by many research teams using various methods. For instance, an arc plasma jet technique with the cathode placed at an oblique angle of about 30° from the anode was pro- posed to produce the single-walled carbon nanotubes by Ando et al. [1]. Lee et al. [2] employed plasma rotating arc discharge method with the anode rotated at speeds from 0 to 10,000 rev/min (rpm) for the large-scale syn- thesis of carbon nanotubes. In addition, the floating catalyst CVD method was employed by several research groups for the mass production of carbon nanotubes [3]. Lee et al. [4] reported the vapor phase growth of aligned carbon nanotubes in a horizontal quartz tube reactor by heating the flowing mixtures of acetylene and iron pentacarbonyl. Satishkumar et al. [5] fabricated single- walled carbon nanotubes from acetylene with organo- metallic mixtures in a two-stage furnace. Nikolaev et al. [6] investigated the gas-phase synthesis of single- walled carbon nanotubes from carbon monoxide with iron pentacarbonyl in a horizontal quartz tube reactor under a high-pressure and elevated-temperature opera- tion. Zhu et al. [7] employed mixtures of n-hexane or benzene with ferrocene and thiophene in a vertical furnace for the production of double-walled carbon nanotubes. Ci et al. [8,9] studied the fabrication of Carbon 43 (2005) 2760–2768 www.elsevier.com/locate/carbon 0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.05.022 * Corresponding author. Tel.: +886 5 2720411x33403; fax: +886 5 2721206. E-mail address: chmyyl@ccu.edu.tw (Y.-Y. Li). 13