Iron–iron oxide core–shell nanoparticles synthesized by laser pyrolysis followed by superficial oxidation F. Dumitrache a, * , I. Morjan a , R. Alexandrescu a , V. Ciupina b , G. Prodan b , I. Voicu a , C. Fleaca a , L. Albu a , M. Savoiu a , I. Sandu a , E. Popovici a , I. Soare a a National Institute for Lasers, Plasma and Radiation Physics, P.O. Box MG-36, 077125 Bucharest, Romania b ‘‘Ovidius" University of Constanta, Bd. Mamaia 124, Constanta, Romania Abstract Iron-based core–shell nanostructures were synthesized by laser pyrolysis in a two-steps procedure. In a first step, using a cross-flow configuration, the laser radiation was heating a gas phase mixture containing iron pentacarbonyl (vapors) entrained by an ethylene flow, which plays also the role of an energy transfer agent. Secondly, a carefully controlled in situ passivation of the freshly formed pyrophoric iron nanoparticles created a protective iron oxide shell. The produced nanoparticles (22 nm size diameters) with core–shell features were analyzed by TEM, XRD, SAED and Raman spectroscopy. Majoritary iron and gamma iron oxide/magnetite and minoritary carbon phases were identified. In laser pyrolysis experiments in which the reaction temperature was increased, the catalyzed homogeneous nucleation and growth of carbon nanotubes in the gas phase was observed and is presented here for the first time. # 2005 Elsevier B.V. All rights reserved. Keywords: Laser pyrolysis; Passivation; Iron core–oxide shell nanocomposite; Carbon nanotube 1. Introduction Iron as nanopowder exhibit outstanding properties and can be widely used as electronic, electric, and magnetic materials [1], oxidation/reduction catalyst, adsorbents and contrast agents in resonant magnetic imaging. It is an established fact that nanoparticles show novel properties, but the ideal response is associated with isolated particles with narrow dis- tribution and uniform and well-known structure. [2] The main problem for nano-iron systems is the environmental instability actually without any protec- tion iron nanopowder instantaneously burn forming a- Fe 2 O 3 . This process could be avoided by the encapsulation of metal nanoparticles [3,4] which not only protects the nano-crystals from environ- mental degradation, but also could offer new opportunities for the catalytic growth of carbon nanotubes or, optionally, for the study of their magnetic properties. Several studies about nano-iron oxidation behavior have been reported [5–7] and reveal that morphology, phase transformation and phase structure depend on the preparation method. www.elsevier.com/locate/apsusc Applied Surface Science 247 (2005) 25–31 * Corresponding author. E-mail address: fdumit@inflpr.ro (F. Dumitrache). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.037