Delivered by Publishing Technology to: McMaster University IP: 116.58.253.108 On: Tue, 01 Mar 2016 17:39:12 Copyright: American Scientific Publishers Copyright © 2007 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 7, 3313–3317, 2007 Annealing Effects on 5 nm Iron Oxide Nanoparticles J. M. Vargas 1 , E. Lima, Jr. 1 , L. M. Socolovsky 2 , M. Knobel 2 ∗ , D. Zanchet 3 , and R. D. Zysler 1 ∗ 1 Centro Atómico Bariloche and Comisión Nacional de Energía Atómica, 8400 San Carlos de Bariloche, Rio Negro, Argentina 2 Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas, Campinas (SP) 13083-970, S.P., Brazil 3 Laboratório Nacional de Luz Síncrotron, Av. Giuseppe M. Scolfaro 10000, Campinas (SP) 13084-971, Brazil Morphological, structural and magnetic properties of 4.8 nm iron oxide nanoparticles have been investigated after annealing under inert atmosphere at different temperatures. The as-prepared iron oxide nanoparticles have been synthesized by chemical route from high temperature reaction of Fe(acac) 3 solution in presence of oleic acid and oleylamine surfactant. Annealing the particles at low temperatures (T ann = 573 K) produces an increment of the mean size from 4.8 nm to 6.0 nm, preserving the same morphology. The coercive field of the annealed sample has a small increas- ing with respect to the as-prepared sample in agreement with the mean particle volume change. Annealing at higher temperature (T ann = 823 K) leads to a bimodal size distribution of the iron oxide nanoparticles with 6.0 nm and 17 nm mean sizes respectively, where the bigger particles dominate the observed magnetic properties. Keywords: Iron Oxide Nanoparticles, Annealing Effect. 1. INTRODUCTION Nanoparticles with high-quality morphology, narrow size distribution, crystallinity, and air stability are some of the desirable demands for potential technological appli- cations. 1 2 Many structural and magnetic studies have been published regarding ferrimagnetic magnetite or maghemite nanoparticles because their relevance for recording media, magnetic fluids, and potential use in medicine. 2–6 There- fore, it is important to establish the synthesis conditions to control the size and shape of nanoparticles to possibly tune their properties to a specific technological application. In this way, it is well known that, after the synthesis of the samples, thermal treatments provide a powerful method to optimize the morphological and magnetic properties of the nanoparticles. 7–10 On the other hand, these thermal treatments can mod- ify the nature of the particles. Briefly, under the appro- priate annealed conditions, almost every iron oxide can be converted into at least two others. For example, in the dry state, ferrimagnetic magnetite is readily oxidized to ferrimagnetic maghemite under air atmosphere. At tem- peratures above 300  C the transformation proceeds fur- ther to antiferromagnetic hematite, -Fe 2 O 3 , which is the most stable iron oxide compound (under oxidant ∗ Authors to whom correspondence should be addressed. conditions). 11 Interestingly, heating iron and magnetite in sealed vessels leads to antiferromagnetic W¨ ustite, Fe x O, which is known to be stable only above 560–570  C. Below this temperature, the W¨ ustite may be oxidized to magnetite. 11 12 In the magnetite case, the most common method of fabrication of nanoparticles is the coprecipitation from a solution of Fe(II) and Fe(III) salts using alkali metal hydroxide. 13 14 This method usually produces large parti- cles with a broad size distribution. In the present work, our aim is to study magnetite nanoparticles of approximately 5 nm of diameter obtained by an alternative chemical syn- thesis and the subsequent effect of annealing (under inert gas flux) in the morphology, crystalline structure, and mag- netic properties. 2. EXPERIMENTAL DETAILS Magnetite nanoparticles were prepared by chemical pro- cedure by the reduction of Fe(acac) 3 , in the presence of oleic acid and oleylamine ligand molecules. The surfac- tant:precursor ratio is fixed in 40:1 in order to synthe- size ∼5 nm magnetite nanoparticles (coated by surfactant molecules). 15 The annealing treatments were performed in a tubular furnace, under continuous argon flux (pre- venting the hematite formation) with a 10 K/min ramp. J. Nanosci. Nanotechnol. 2007, Vol. 7, No. 9 1533-4880/2007/7/3313/005 doi:10.1166/jnn.2007.688 3313