INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY Nanotechnology 15 (2004) 1665–1670 PII: S0957-4484(04)85144-4 Synthesis and characterization of nanowires formed by self-assembled iron particles J Knipping 1 , H Wiggers 1 , B F Kock 1 ,TH¨ ulser 2 , B Rellinghaus 1,3 and P Roth 1,4 1 Institute for Combustion and Gasdynamics, University of Duisburg-Essen, 47048 Duisburg, Germany 2 Experimental Physics, University of Duisburg-Essen, 47048 Duisburg, Germany E-mail: knipping@uni-duisburg.de and roth@ivg.uni-duisburg.de Received 19 August 2004, in final form 24 September 2004 Published 22 October 2004 Online at stacks.iop.org/Nano/15/1665 doi:10.1088/0957-4484/15/11/051 Abstract The formation of iron particles without and with carbon coating was studied in a hot wall flow reactor. The precursors ironpentacarbonyl (IPC, Fe(CO) 5 ) and ethylene (C 2 H 4 ) both diluted in N 2 were used in a concentric tubular flow arrangement and were heated to temperatures between 570 and 1170 K at pressures between 50 and 500 mbar. In experiments without C 2 H 4 , either individual iron particles in the size range of 8 nm  d p  15 nm or long iron chains composed of several hundreds of individual iron particles were found depending on the reaction conditions. In experiments with C 2 H 4 addition, these particles or particle chains were covered by a thin carbon/carbide layer. The size of the primary particles was measured in situ by time-resolved laser-induced incandescence (TR-LII) and ex situ by rapid thermophoretic particle probing and TEM imaging. 1. Introduction Iron is one of the most widely used materials in the technological world. In recent years, nanostructured materials and nanoparticles have opened the possibility of new far- reaching usage of old materials. This is mostly a result of size effects which can drastically modify the bulk material properties. Also, iron and iron oxides are in this context of interest because of the size-dependent magnetic, electronic, and catalytic properties. Various gas phase (aerosol) synthesis routes [1–6], are known, by which precursors can be transformed via chemical and physical rate processes into nanoparticles, e.g. flames, laser reactors, plasma reactors, and hot wall reactors. The synthesis of iron particles via the gas phase route is strongly controlled by both the decomposition of the precursor and the single-iron-cluster formation and growth [7–9]. An aerosol model for the formation of iron particles 3 Current address: IFW Dresden eV, 01069 Dresden, Germany. 4 Author to whom any correspondence should be addressed. was introduced by Bilodeau and Proulx [10] and by Giesen et al [11]. As iron particles are very reactive, showing e.g. spontaneous oxidation in air, it is useful to cover the particle surface by a thin oxidic layer or to encapsulate the particles e.g. by a carbon/carbide shell. In the present paper, the synthesis of iron particles by thermal decomposition of ironpentacarbonyl (Fe(CO) 5 ) in a hot wall flow reactor is reported. The reaction has some interesting features: the decomposition starts at very low temperatures and a complete conversion of the precursor without the formation of by-products can be achieved. The lifetime of Fe(CO) 5 at 570 K has been measured to be 5.3 ms [12]. The relative ease of scaling up hot wall reactors qualifies this production route for industrial purposes. Even with the present set-up, production rates in the range of 1– 2gh -1 have been realized. The size of the synthesized particles was determined in situ by time-resolved laser- induced incandescence (TR-LII). Size distribution and particle morphology was also examined by rapid thermophoretic particle sampling and visualization by transmission electron 0957-4484/04/111665+06$30.00 © 2004 IOP Publishing Ltd Printed in the UK 1665