Journal of Colloid and Interface Science 255, 293–298 (2002) doi:10.1006/jcis.2002.8679 Size-Selective Chemical Synthesis of Tartrate Stabilized Cobalt Ferrite Ionic Magnetic Fluid S. Neveu, 1 A. Bee, M. Robineau, and D. Talbot Laboratoire L.I.2.C. Colloides Inorganiques, UMR CNRS 7612, Universit´ e Pierre et Marie Curie, Case 63, 4 place Jussieu, 75252 Paris cedex 05, France Received November 5, 2001; accepted August 14, 2002 Ionic magnetic fluid (ferrofluid) is a stable suspension of mag- netic nanoparticles in water. Cobalt ferrite nanoparticles are inter- esting in view of high-density recording storage. The size of the magnetic particles strongly influences the physical properties of the ferrofluids. In this study, we describe the synthesis of ionic magnetic fluid in the presence of tartrate ions. By varying the amount of or- ganic ligands, nanoparticles in a large range of size are obtained: the mean diameter varies from 3 to 10 nm. The effect of tartrate ions on the stability of the ionic magnetic fluid is also studied in relation with the size of the magnetic particles and the amount of adsorbed ligand. C  2002 Elsevier Science (USA) Key Words: nanoparticles; magnetic fluid; cobalt ferrite; control of size; adsorption. 1. INTRODUCTION Magnetic fluids are colloidal suspensions of magnetic nanoparticles in a liquid carrier (1). The stability of such a fluid is mainly dependent on the size of the particles. For many ap- plications (magneto-optic properties, phase transition studies, incorporation in complex systems) it is mostly important to dis- pose of varying size particles. Cobalt ferrite particles receive considerable attention because of their ferromagnetic properties. Particles in the nanometric range were first prepared and studied a long time ago by W. J. Schuele and V. D. Deetscreek (2). Since then, many authors have described the synthesis of cobalt ferrite nanoparticles using coprecipitation (3), sol–gel process (4–6), or synthesis in microemulsion (7, 8). For each of these methods, particle size control in the nanometric range is always a tricky problem. In this study, we propose a procedure which leads to a large range of diameters (3–10 nm): particles are synthesized in water in the presence of organic anions (tartrate ions). The effects of anions on the growth of metallic oxide particles have been investigated on different materials such as hematite (9), lithium ferrite (10), oxyhydroxide ferrite (11, 12). Recently, synthesis of cobalt–ferrite ionic magnetic fluid has been described but the particles sizes obtained range from 10 to 15 nm (13). The 1 To whom correspondence should be addressed. Fax: +33-1-44273675. E-mail: neveu@ccr.jussieu.fr. synthesis of very fine maghemite particles has already been re- ported by some of us (14): in this case the mean diameter particle varies with the amount of ligand from 2 to 10 nm. The aim of the present study is to adapt this process to cobalt ferrite particles. A series of experiments is carried out where cobalt ferrite particles were grown in the presence of tartrate ions. The size of the parti- cles thus synthesized are determined by X-ray diffraction, mag- netic measurements, and transmission electronic microscopy. Prevention of particles agglomeration is one of the crucial prob- lem to overcome in producing stable dispersions. Adsorption of ionizable organic ligands on the particle surface can prevent such an agglomeration. The effect of tartrate ions on the mag- netic fluids stability is studied. The influence of the pH value of the solution on the stability of such a fluid is also reported. 2. EXPERIMENTAL SECTION 2.1. Magnetic Fluid Synthesis Tartrate-stabilized cobalt ferrite ionic magnetic fluid was syn- thesized in a two-step procedure. Cobalt ferrite particles were first produced. The particles were then coated with tartrate ions to be dispersed in aqueous medium. The synthesis process was then adapted in order to produce particles with a large range of sizes. 2.1.a. Synthesis of Tartrate Stabilized Cobalt Ferrite Ionic Magnetic Fluid Particles. The cobalt ferrite particles were obtained by pre- cipitating Co(II) and Fe(III) hydroxydes followed by a heating at boiling temperature. Sodium hydroxide (160 ml, 10 mol L -1 ) was abruptly added under vigorous stirring to a solution of cobalt and ferric nitrate (350 ml, [M] = [Fe] + [Co] = 0.44 mol L -1 , [Co]/[Fe] = 0.4). After stirring 30 min, a nonmagnetic amor- phous suspension was obtained and heated at 100 ◦ C for 2 h. The mixture became magnetic was washed three times with distilled water. To favor the dispersion of the magnetic nanoparticles, a solution of nitric acid (200 ml, 2 mol L -1 ) was then incorpo- rated to the suspension. Successive precipitations with acetone (300 ml) were used to remove the excess of acid and distilled water was added. At this point of the synthesis, the particles had 293 0021-9797/02 $35.00 C  2002 Elsevier Science (USA) All rights reserved.