Corinne Chanéac

Jean-Pierre Jolivet

Élisabeth Tronc

Comptes Rendus Chimie

Synthesis of iron oxide-based magneticnanomaterials and compositesJean-Pierre Jolivet*, Élisabeth Tronc, Corinne Chanéac‘Chimie de la matière condensée’, UMR CNRS 7574, université Pierre-et-Marie-Curie, 4, place Jussieu, 75252 Paris cedex 05,FranceReceived 3 June 2002; accepted 9 October 2002This article corresponds to a lecture presented at the meetingSynthetic Strategies for New Spin Topologies, organised byD. Gatteschi, H. Ratajczak, A. Müller, M. Verdaguer, and V. Marvaud on 7–9 March 2002, at the Scientific Centre of thePolish Academy of Sciences in Paris.Abstract – Various properties of spinel iron oxide nanoparticles are reviewed in relation to the control of the particle size andthe control of their dispersion in a variety of matrices. The deep knowledge of the different synthesis parameters and theinterfacial oxide–solution characteristics allow us to prepare a large variety of materials like calibrated nanoparticles andinorganic or hybrid nanocomposites.To cite this article: J.-P. Jolivet et al., C. R. Chimie 5 (2002) 659–664 © 2002 Académiedes sciences / Éditions scientifiques et médicales Elsevier SASspinel iron oxide nanoparticles / oxide–solution interface / nanocompositesRésumé– Différentes propriétés de nanoparticules d’oxyde de fer de structure spinelle ont été examinées du point de vue ducontrôle de la taille des particules et de leur dispersion dans diverses matrices. La connaissance approfondie des différentsparamètres de synthèse et des caractéristiques de l’interface oxyde–solution nous permet de préparer une large variété dematériaux, comme des nanoparticules calibrées et des nanocomposites inorganiques ou hybrides.Pour citer cet article :J.-P. Jolivet et al., C. R. Chimie 5 (2002) 659–664 © 2002 Académie des sciences / Éditions scientifiques et médicales ElsevierSASnanoparticules d’oxyde de fer de structure spinelle / interface oxyde–solution / nanocomposites1. IntroductionFerro- and ferrimagnetic materials have been receiv-ing a growing interest for both technological andtheoretical reasons, especially in the context of mag-netic recording. However, the properties of finelydivided magnetic materials closely depend on the sizeof the particles and their state of dispersion andaggregation[1]. It is therefore very important to care-fully control the synthesis of particles and their sur-face state.Various chemical techniques[2] for controlling theparticle size consist in limiting the space available forthe particle growth, by precipitating ions in micro-emulsions, vesicles or polymer solutions. However,such methods raise difficulties in getting out particlesfree from polymer, surfactant or ligands mostly induc-ing surface effects influencing the magnetic behaviourof the particles. Other techniques often used are gelimpregnation[3] and sol–gel methods[4], involvingaqueous metal ions and subsequent thermal treatment.In these conditions, well-defined materials regardingthe crystal structure, the mean particle size, the sizedistribution, the morphology and the state of disper-sion appear very difficult to obtain, because it is diffi-cult to control the reactions.* Correspondence and reprints.E-mail address: jpj@ccr.jussieu.fr (J.-P. Jolivet).COMMUNICATION/PRELIMINARYCOMMUNICATION659C. R. Chimie 5 (2002) 659–664© 2002 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS. Tous droits réservésS1631074802014224/FLAPrecipitation or coprecipitation of cations in aque-ous solutions is an easy and cheap route of synthesisof metal oxide particles [5]. Thus, spinel iron oxides(magnetite Fe3O4, maghemite γ-Fe2O3 and substitutedmagnetites, MFe2O4 with M = Fe, Co, Ni...) are easilyformed by coprecipitating Fe3+ and M2+ ions. How-ever, the physicochemistry of the systems have to becarefully controlled to adjust the characteristics of par-ticles. Our studies showed that the particle size canactually be calibrated over one order of magnitude inthe nanometer range, and that the aqueous dispersionsof maghemite particles can be used to elaborate avariety of nanocomposites where the particle disper-sion is controlled.2. Synthesis and tailoring of iron oxidenanoparticlesMagnetite is easily obtained by coprecipitatingaqueous Fe3+ and Fe2+ ions. At the stoichiometryFe2+/Fe3+ = 0.5, crystallisation of spinel phase isquasi-immediate at room temperature. The mean par-ticle size is monitored by the conditions of themedium: pH and ionic strength, I, imposed by a salt.Typical experimental conditions [6, 7] are: 2.5 ml of asolution containing a mixture of FeCl2 (1 mol l–1) andFe(NO3)3 (2 mol l–1) were slowly, automaticallyinjected into 25 ml of a solution of NaNO3 at a con-centration in between 0.5 and 3 mol l–1. The pH ofthe medium was fixed and kept constant (∆pH ≈ 0.1)by addition of NaOH, using an automatic potentiomet-ric device. All solutions were carefully de-aeratedwith nitrogen, which was continuously bubbled duringthe precipitation. Ageing of the suspensions wasachieved under argon atmosphere. Various sampleswere prepared in the range 8.5 ≤ pH ≤ 12 and0.5 ≤ I ≤ 3 mol l–1).Electron micrographs (Fig. 1) show spheroidal par-ticles whatever the synthesis conditions, but with verydifferent mean sizes. X-ray Diffraction (XRD) patterns(Fig. 2) are typical of the spinel phase. The line posi-tions are the same for all samples, indicating no sig-nificant structural variation between the samples.Fig. 3 shows the influence of the precipitation con-ditions on the mean size of the particles after 8-dayFig. 1. Transmission Electron Micrographs (TEM) of particles synthesised under various conditions: (a) pH = 9, I = 0.5 mol l–1 [D = 12.5 nm;σ(D) = 2.5 nm]; (b) pH = 12, I = 0.5 mol l–1 [D = 6.5 nm; σ(D) = 1 nm]; (c) pH = 12, I = 3 mol l–1 [D = 1.6 nm; σ(D) = 0.2 nm] (T = 25 °C ; 8-dayaged suspensions).Fig. 2. X-ray diffraction patterns (λCu Kα = 1.5406 Å) of particles synthesised (a) at different pHs with I = 1 mol l–1 and (b) at pH = 12 withdifferent ionic strengths (T = 25 °C; 8-day aged suspensions).660J.-P. Jolivet et al. / C. R. Chimie 5 (2002) 659–664ageing. The mean size is strongly dependent on theacidity and the ionic strength of the precipitationmedium, and clearly decreases with increasing pHand I.It is interesting to observe that such an influence ofacidity on the particle size is relevant to thermody-namics rather than kinetics (nucleation and growthprocesses) [7, 8]. The pH acts on protonation–deprotonation equilibriums of surface hydroxylatedgroups and, hence, on the electrostatic surface charge.The ionic strength carries out more or less electro-static screening between charged groups and, then, atfixed pH, the electrical charge increases with I. Con-sequently, a change in the surface electrical chargedue to pH leads to a change in the chemical composi-tion of the interface, inducing a decrease of the inter-facial tension, γ, as stated by Gibbs’s law,dγ = –Γi dµi, where Γi is the density of adsorbed spe-cies i with chemical potential µi. Finally, the surfacecontribution, dG = γ dA (A is the surface area of thesystem) to the free enthalpy of the formation of par-ticles is lowered, allowing the increase in surface areaof the system [9].Maghemite γ-Fe2O3 does not form directly in solu-tion by precipitation of ferric ions, but a small propor-tion of Fe2+ (x ≥ 0.1) induces the crystallisation of allthe iron to spinel. Studies of the early precipitaterevealed that all Fe2+ ions were incorporated into aFe2+-ferrihydrite and gave rise to high electron mobil-ity, as evidenced by Mössbauer spectroscopy [10, 11].This mixed valence material transforms with time intospinel by two simultaneous competing pathways: (i)solid state reaction with dehydration and spinel order-ing at short range without particle size variation; (ii)dissolution of Fe2+–Fe3+ complexes from the surface,followed by crystallisation of spinel oxide (Ostwaldripening), this process taking place with considerableparticle growth. The endproducts of the coprecipita-tion are single phase only for x ≥ 0.33, process (ii)seeming dominant during the evolution from the ini-Fig. 3. Mean diameter of particles after 8-day ageing (a) as a function of pH (NaNO3 1 mol l–1) and (b) as a function of ionic strength(pH = 12) (T = 25 °C).Fig. 4. Mean diameter of particles of (a) CoFe2O4, (b) MnFe2O4, (c) NiFe2O4 after 24-h ageing at 100 °C of suspensions as a function of pH.661Pour citer cet article : J.-P. Jolivet et al. / C. R. Chimie 5 (2002) 659–664tial stage of precipitation, as noticeable growth ofparticles is observed.The comparison with the cases where M2+ is differ-ent from Fe2+ emphasises the role of electron mobilitybetween Fe2+ and Fe3+ ions in the crystallisation pro-cess. Mobility in the bulk presumably drives a localcubic close-packed ordering, made possible by theloose structure of the early material. This local order-ing gradually extends to the whole fine particles (pro-cess (i)) and to the growing particles by process (ii).With the other divalent cations, intervalence transfersare generally negligible and a spinel ferrite forms onlyby process (ii) (dissolution–crystallisation) [11]. Nev-ertheless, a control of particle size is possible on awide range, because the solubility of the solid isstrongly dependent on the pH of the medium [12](Fig. 4).3. Oxidation of magnetite particles.Formation of aqueous solsof maghemiteDue to the high electron mobility in the bulk, themagnetite nanoparticles are very unstable against oxi-dation, leading, at term, to maghemite. Differentmechanisms [13] involving ionic and/or electronicinterfacial transfers are implicated, depending on thepH of the suspension. In basic medium, the oxidationof Fe2+ ions proceeds by oxygen reduction at thesurface of particles (electron transfer only) and coordi-nation of oxide ions, while in acidic medium andanaerobic conditions, surface Fe2+ ions are desorbedas hexaaquo complexes in solution (electron and iontransfer) according to:Fe3+ A Fe22.5+ BO4 + 2 H+→ 0.75 Fe3+ A Fe5/33+ L1/3  BO4 + Fe2+ + H2Owhere A and B stand for tetrahedral and octahedralsites, respectively, and L represents a vacancy.Maghemite nanoparticles, bearing a high positivecharge density (σ ≈ 0.3 C m–2 at pH 2, and low ionicstrength, 10–2/5.10–2 mol l–1), can easily be dispersedin acidic water, forming cationic sols practically freefrom aggregation [14–16]. On raising the pH, σdecreases, the repulsions between particles weaken,the degree of agglomeration increases. Aggregationremains limited provided that pH < PZC. The aggre-gate configuration, dependent on the pH, ranges fromsmall chain-like clusters of a few particles to branchychains of more than ca 50 particles. In every case, thestructure is loose and the particles are not in closecontact. At pH ∼ PZC, σ ∼ 0, a floc forms. AtpH > PZC, the particle re-disperse, but aggregation isgenerally more significant than at pH < PZC, becausethe surface is less solvated.4. Dispersion of maghemitenanoparticles in polymers and silicaglasses. NanocompositesThe particles synthesised and dispersed in aqueousmedium can be trapped in different solid matriceskeeping their dispersion state.By adding a hydrosoluble polymer in the sol anddrying of the mixture, solid samples were obtained.The oxide/polymer mass ratio determines the averagedistance between the objects existing in the sol, iso-lated particles or aggregates. This technique was usedto prepare series of composites made up ofmaghemite particles with the same size distributionand different aggregation/dispersion states in polyvi-nylic alcohol, typically isolated particles with volumeconcentration ranging from ∼ 1 to 20%, clusters ofvarying length and flocs, in order to study the mag-netic properties of noninteracting particles and theinterparticle interaction effects [1, 15, 17–19]. Suchmaterials allowed us to give the first demonstrativeexperimental verification of the Néel–Brown modelfor superparamagnetic relaxation [17]Composites made up of well-dispersed maghemiteparticles in an epoxy resin were obtained by poly-merisation of the resin inside an organosol of particles[20]. The transfer of particles from aqueous mediumto an organic solvent (sulfolane, propylene carbonate,hexamethylphosphorus-triamide) needs the adsorptionof a coupling agent such as a phosphonic acid (phe-nylphosphonic acid, PPA). The head of the molecule,the phosphate group, strongly adsorbs onto particles,as evidenced by infrared and Mössbauer spec-troscopies. The room temperature Mössbauer spectraof uncoated and coated particles are shown in Fig. 5.The quadrupole doublet observed for the coated par-ticles demonstrates the formation of a paramagneticsurface complex, due to the strong complexing abilityof PPA for iron [21].The hydrophobic tail (phenyl group) of the couplingagent allows the dispersion in the organic solvent andprevents aggregation of the particles forming a kineti-cally stable organosol, as indicated by light scattering.The components of CIBA–GEIGY epoxy resin(araldite LY556, hardener HY917 and acceleratorDY070) introduced in the sol form a homogeneoushybrid composite material after heating at 60 °C for24 h. The materials were examined as thin slices bytransmission electron microscopy (Fig. 6).662J.-P. Jolivet et al. / C. R. Chimie 5 (2002) 659–664Silica composites were prepared by polymerisingsilicic acid or alkoxysilanes inside the aqueous sol[22, 23]. Hydrolysis and condensation of the precur-sors take place in situ and the reactions yield a gelthat leads, after drying at room temperature, to atransparent monolithic glass. Such a procedure leadsto homogeneous solid composites (Fig. 7), containingup to 40 wt% Fe2O3. In both cases, infrared, near-infrared and Mössbauer spectroscopies indicate nodetectable Fe–O–Si bond. In fact, the ‘dispersability’of particles into the silica matrices results from solva-tion of silanol groups of the matrix by associated-water layers surrounding the particles, without otherchemical surface interactions. The silica matrix acts asan antisintering agent, which stabilises the maghemiteparticles against the thermal transformation into hema-tite. Whereas with powdered uncoated particles, theγ → α-Fe2O3 transformation starts below 300 °C andis generally complete at 400 to 500 °C, in compositeswith sufficiently low particle concentration, no trans-formation occurs as long as the matrix prevents themigration and coalescence of particles that is, up toaround 1000 °C, the temperature at which the glasssoftens or starts crystallising [24].5. ConclusionThe aqueous chemistry of spinel iron oxide nano-particles offers broad possibilities for tailoring materi-Fig. 5. Mössbauer spectra (at room temperature) of uncoated andPPA coated maghemite particles (9.0 nm).Fig. 6. Transmission electron micrographs (a) and quasi-elastic light scattering observation (b) of maghemite particle (4.7 nm) dispersed inhexamethylphosphorus triamide; micrographs of magnetic composites (4.7 nm maghemite particle-epoxy resin, thin slice) (c).Fig. 7. TEM micrograph of a maghemite–silica composite (18 wt%Fe2O3).663To cite this article: J.-P. Jolivet et al. / C. R. Chimie 5 (2002) 659–664als for a wide range of utilisation. The careful controlof the size and degree of dispersion of the particles incomposite materials can allow one to reveal unex-pected phenomena.Acknowledgements. We are deeply grateful to our magnetician colleagues, M. Noguès, LMOV, University of Versailles, France, D. Fiorani,IC-Mat, CNR, Monterotondo Staz., Italy, F. Lucari and F. D’Orazio, Università L’Aquila, Italy, for appreciating our materials and toM. Lavergne, UPMC, Paris, France, for electron microscopy observations.References[1] J.-L. Dormann, D. Fiorani, E. Tronc, Adv. Chem. Phys. 98 (1997)283.[2] A.J.I. Ward, S.E. Friberg, Mater. Res. Sci. Bull. 14 (1989) 41.[3] C.R.F. Lund, J.A. Dumesic, J. Phys. Chem. 85 (1981) 3175.[4] G. Ennas, A. Musinu, G. Piccaluga, D. Zedda, D. Gatteschi,C. Sangregorio, J.L. Stanger, G. Concas, G. Spano, Chem. Mater.10 (1998) 495.[5] J.-P. Jolivet, Metal Oxide Chemistry and Synthesis: from Solutionto Solid State, Wiley, Chichester, 2000.[6] J.-P. Jolivet, L. Vayssières, C. Chanéac, E. Tronc, Mater. Res.Symp. Proc. 432 (1997) 145.[7] L. Vayssières, C. Chanéac, E. Tronc, J.-P. Jolivet, J. Colloid Inter-face Sci. 205 (1998) 205.[8] R. Aveyard, Chem. Ind. 474 (1987).[9] C. Froidefond, C. Chanéac, E. Tronc, J.-P. Jolivet (to be published).[10] J.-P. Jolivet, P. Belleville, E. Tronc, Clays Clay Miner. 40 (1992)531.[11] E. Tronc, P. Belleville, J.-P. Jolivet, J. Livage, Langmuir 8 (1992)313.[12] A. Pottier, C. Chanéac, E. Tronc, J.-P. Jolivet (to be published).[13] J.-P. Jolivet, E. Tronc, J. Colloid Interface Sci. 125 (1988) 688.[14] J.-P. Jolivet, R. Massart, J.-M. Fruchart, Nouv. J. Chim. 7 (1983)325.[15] P. Prené, et al., IEEE Trans. Magn. 29 (1993) 2658.[16] J.-P. Jolivet, C. Chanéac, P. Prené, L. Vayssières, E. Tronc, J. Phys.IV France 7 C (1997) 1–573.[17] E. Tronc, et al., Hyperf. Interact. 95 (1995) 129.[18] J.L. Dormann, F. d’Orazio, F. Lucari, E. Tronc, P. Prené, J.-P. Joli-vet, D. Fiorani, R. Cherkaoui, M. Noguès, Phys. Rev. B 53 (1996)14291.[19] J.-L. Dormann, L. Spinu, E. Tronc, J.-P. Jolivet, F. Lucari,F. d’Orazio, D. Fiorani, J. Magn. Magn. Mater. 202 (1999) 251.[20] C. Chanéac, E. Tronc, J.-P. Jolivet, Mat. Res. Soc. Symp. Proc. 628(2000) CC641.[21] E. Tronc, J.-P. Jolivet, Hyperf. Interact. 525 (1986) 28.[22] C. Chanéac, E. Tronc, J.-P. Jolivet, Nanostruct. Mater. 6 (1995)715.[23] C. Chanéac, E. Tronc, J.-P. Jolivet, J. Mater. Chem. 6 (1996) 1905.[24] E. Tronc, C. Chanéac, J.-P. Jolivet, J. Solid State Chem. 139 (1998)93.664J.-P. Jolivet et al. / C. R. Chimie 5 (2002) 659–664

Synthesis of iron oxide-based magnetic nanomaterials and composites

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Synthesis of iron oxide-based magnetic nanomaterials and composites

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