Article pubs.acs.org/cm
Facile Synthesis of Monodisperse Maghemite and Ferrite Nanocrystals from Metal Powder and Octanoic Acid Kifah S. M. Salih,† Patrizia Mamone,† Gunder Dörr,† Thomas O. Bauer,‡ Alexander Brodyanski,§ Christine Wagner,§ Michael Kopnarski,§ Robin N. Klupp Taylor,⊥ Serhiy Demeshko,# Franc Meyer,# Volker Schünemann,‡ Stefan Ernst,† Lukas J. Gooßen,*,† and Werner R. Thiel*,† †
Fachbereich Chemie, Technische Universität Kaiserslautern, Erwin Schrödinger Straße 54, 67663 Kaiserslautern, Germany Fachbereich Physik, Technische Universität Kaiserslautern, Erwin Schrödinger Straße 56, 67663 Kaiserslautern, Germany § Institut für Oberflächen- und Schichtanalytik GmbH, Trippstadter Straße 120, 67663 Kaiserslautern, Germany ⊥ Institut für Partikel Technologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Cauer Straße 4, 91058 Erlangen, Germany # Institut für Anorganische Chemie, Georg-August-Universität Göttingen, Tammann Straße 4, 37077 Göttingen, Germany ‡
S Supporting Information *
ABSTRACT: Extremely small, monodisperse, and spheric maghemite (γ-Fe2O3, 2−3 nm) and manganese (4−7 nm), cobalt (3−5 nm), and zinc ferrite (5−7 nm) nanocrystals are directly accessible on a large scale starting from inexpensive metal powders and octanoic acid by thermolysis in a high-boiling solvent. Bigger particle size is obtainable by prolonged reaction time according to the Ostwald ripening principle. The superparamagnetic nanocrystals and their assembly have been characterized by transmission electron microscopy, powder X-ray diffraction, Mössbauer spectroscopy, magnetic measurements, and energy-dispersive X-ray spectroscopy. KEYWORDS: maghemite, nanoparticles, ferrites, octanoic acid, TEM, Mössbauer spectroscopy
■
INTRODUCTION Magnetic iron oxide nanoparticles are of substantial interest for diverse applications such as high density magnetic storage systems,1 magnetic resonance imaging (MRI), bioseparation, controlled drug delivery, hyperthermia, biosensors,2,3 and catalyst heterogenization.4 In contrast to bulk iron oxide, crystalline nanoparticles possess only a single magnetic domain. Thus, below a certain size, they become superparamagnetic.5 Because the magnetic properties are strongly size-dependent, methods for the selective synthesis of magnetic nanoparticles with a specific size and a narrow size distribution are of constantly high interest.6 Traditionally, iron oxide nanoparticles are synthesized by coprecipitation processes from aqueous solutions in the presence of organic surfactants, in microemulsions, by laser pyrolysis,5−7 or via chemical vapor deposition.8,9 However, these processes often lead to relatively large particles or broad size distributions, or are difficult to scale up. In recent years, the thermal decomposition of iron carboxylates has evolved to a widely applicable method for producing nanocrystalline iron oxide. The carboxylate solubilizes the iron precursor and stabilizes the iron oxide nanoparticles formed during thermal decarboxylation. Iron carboxylates have been generated in-situ from iron pentacarbonyl,10,11 goethite (FeO(OH)),12,13 or iron(III) acetylacetonate14,15 and carboxylic acids. Arguably the most versatile synthetic approach was developed by Hyeon et al. They © XXXX American Chemical Society
reported that monodisperse iron oxide nanocrystals with narrow size distributions can be generated selectively via the thermolysis of iron oleate obtained in an extra synthetic step from FeCl3 and sodium oleate.16 Depending on the solvent and reaction temperature employed, particles with average diameters ranging from 5 to 22 nm are accessible. X-ray spectroscopic studies indicated that the ratio of Fe3O4/Fe2O3 decreases with the particle size. The synthesis of particles with average sizes below 5 nm requires even more elaborate procedures. Hyeon et al. reported that such particles are formed when heating preformed iron oleate in a mixture of oleyl alcohol, oleic acid, and diphenyl ether to 250 °C with a well-defined temperature gradient followed by rapid cooling to room temperature.17 There are only a few examples for the synthesis of nanosized particles directly from metals, and they require rather elaborate techniques. These examples include the synthesis of iron oxide nanoparticles from iron particles via electron beam lithography or from iron anodes via electrochemical methods.18 In view of the manifold applications of these particularly small superparamagnetic particles, e.g. as imaging contrast Special Issue: Synthetic and Mechanistic Advances in Nanocrystal Growth Received: October 16, 2012 Revised: January 16, 2013
A
dx.doi.org/10.1021/cm303344z | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
agents, for the production of core−shell nanoparticles with a functionalized shell of highest possible surface area, or for their embedding in porous supports with small pore sizes, a simpler and easier to perform synthetic protocol is of profound interest.19−21 We herein describe a straightforward and easily scalable process for the production of particularly small (2−3 nm) monodisperse superparamagnetic maghemite nanoparticles in one step from iron metal and octanoic acid. The synthetic concept also allows the preparation of highly magnetic manganese, cobalt, and zinc ferrite nanocrystals.
■
EXPERIMENTAL SECTION
Materials. Octanoic acid, oleic acid, benzoic acids, phenylacetic acid, diphenyl ether, and biphenyl were purchased from Acros Organics. Iron fine powder was obtained from Riedel-de Haën. Manganese powder, APS
