Modelling study of single walled carbon nanotube formation in a premixed flame† John Z. Wen, a Matthew Celnik, b Henning Richter, c Meri Treska, a John B. Vander Sande a and Markus Kraft * b Received 7th November 2007, Accepted 25th January 2008 First published as an Advance Article on the web 7th February 2008 DOI: 10.1039/b717256g In this study the formation processes of catalyst nanoparticles and single walled carbon nanotubes (SWCNTs) in a premixed flame doped with Fe(CO) 5 were first modelled using a three-step SWCNT growth model including a detailed surface chemistry model. The growth of SWCNTs was experimentally studied by the length measurement of the SWCNT using Raman radial breathing mode (RBM) and size measurements of the iron oxide catalyst particles using XRD and TEM. The flame chemistry and the formation of the catalyst particles were modelled in detail by means of a sectional model. In a post-processing step the SWCNT population balance growth model was numerically solved using a multivariate stochastic population balance solver. The model was able to capture the growth characteristics and revealed the role of the monolayer. The computational study on the adsorption, dissociation, and reactions of CO, H 2 and H 2 O on iron nanoparticles showed that carbon, hydrogen and oxygen atoms form at the surface of the catalyst. Their ratio, which is controlled by the surface reaction pathways, affects the growth of SWCNTs, the formation of monolayers and the phase transformation of catalyst particles. Introduction There has been increasing research interest in the application and production of carbon nanotubes (CNTs) over the last two decades. 1 This is due to their unique electrical, mechanical and chemical properties. 2 CNTs have potential applications in multi- disciplinary areas such as sensors, polymer composites, electron- ics, mechanical actuators, hydrogen storage in fuel cells, and catalysis. The exploration and investigation of applications in such fields require well characterized structure and greater production yield, which only become achievable when the synthesis process can be precisely controlled. Since the discovery of single walled carbon nanotubes (SWCNTs) in a plasma discharge process in the presence of an iron catalyst, 1 a variety of methods including arc-discharge, laser ablation, chemical vapor deposition (CVD) and aerosol reactor techniques have been investigated. 2 The energy supplies are quite different for these methods: while an aerosol synthesis method, such as a HiPCO reactor, 3 requires an external energy source, flame synthesis can produce SWCNT carbon precursors, heat, and catalyst simultaneously in a continuous reacting flow. 4 Synthesis of SWCNTs in pyrolysis flames has been studied extensively by Vander Wal and co-workers. 5,6 In their approaches catalyst particles were generated, for example, by nebulizing iron salt solutions. The reactive mixture was then introduced into a fuel-rich acetylene flame. Variation of mixture composition and flow rates allowed for the optimization of SWCNT forma- tion. The same author also observed the formation of SWCNTs in diffusion flames when a floating catalyst was formed from a metallocene. 7 The alternative flame synthesis approach was based on the addition of a catalyst precursor, such as iron pentacarbonyl (Fe(CO) 5 ), to the inflow gas mixture prior to the stabilization of a premixed flat flame, as reported in the work of Height et al. 4,8,9 Those authors identified a nanotube formation window of fuel-to-oxygen ratios. They found that while, relative to stoichiometric conditions, an excess of the carbon supply is necessary to enable inception of nanotubes, soot-like structures are formed at too high fuel-to-oxygen ratios. They also investigated the effect of Fe(CO) 5 concentration on the particle size distribution and the shape of the metallic catalyst. In their work the quantity of condensed material increased dramatically with the Fe(CO) 5 concentration, whereas nanotubes appeared to be cleaner at lower concentrations. The afore- mentioned experimental studies under a variety of synthesis conditions suggest that there may be more than one set of physical and chemical processes by which the catalyst exhibits its function during the formation and growth of SWCNTs, though the fundamental catalytic step is the same. 10 In order to increase the production yield and further achieve structure-selective production of SWCNTs (for example with desired diameters and chiralities), experimental and numerical investigations of the formation mechanism of SWCNTs are essential. Recently several in situ electron microscopy studies of carbon nanofibre growth have been reported at the atomic a Material Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA, 02139, USA. E-mail: zywen@ mit.edu; Fax: +1 617-253-6933; Tel: +(01) 617-258-6118 b Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge, UK, CB2 3RA. E-mail: mk306@cam.ac.uk; Fax: +44 (0) 1223 334796; Tel: +44 (0) 1223 762784 c Nano-C, Inc., 33 Southwest Park, Westwood, MA, 02090, USA. E-mail: HRichter@nano-c.com; Fax: +1 781-407-9419; Tel: +1 781-407-9417 † This paper is part of a Journal of Materials Chemistry theme issue on carbon nanostructures. 1582 | J. Mater. Chem., 2008, 18, 1582–1591 This journal is ª The Royal Society of Chemistry 2008 PAPER www.rsc.org/materials | Journal of Materials Chemistry This is the Computational Modelling Group’s latest version of the paper. For the published version please refer to doi: 10.1039/b717256g