Tuning of Synthesis Conditions by Thermal Decomposition toward

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Tuning of synthesis conditions by thermal decomposition towards core-shell CoxFe1-xO@CoyFe3-yO4 and CoFe2O4 nanoparticles with spherical and cubic shapes. Walid Baaziz, Benoit P. Pichon, Yu Liu, Jean-Marc Greneche, Corinne UlhaqBouillet, Erwan Terrier, Nicolas Bergeard, Valerie Halte, Christine Boeglin, Fadi Choueikani, Mohamed Toumi, Tahar Mhiri, and Sylvie Begin-Colin Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm502269s • Publication Date (Web): 11 Aug 2014 Downloaded from http://pubs.acs.org on August 16, 2014

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Chemistry of Materials

Tuning of synthesis conditions by thermal decomposition towards core-shell CoxFe1-xO@CoyFe3-yO4 and CoFe2O4 nanoparticles with spherical and cubic shapes. Walid Baaziz,†,‡* Benoit P. Pichon,† Yu Liu,† Jean-Marc Grenèche,§ Corinne Ulhaq-Bouillet,† Erwan Terrier,† Nicolas Bergeard,† Valérie Halté,† Christine Boeglin,† Fadi Choueikani,∥ Mohamed Toumi,‡ Tahar Mhiri,‡ Sylvie Begin-Colin†*. †

Institut de Physique et de Chimie des Matériaux de Strasbourg, UMR CNRS-UdS 7504, 23 rue du Loess – BP 43, 67034 Strasbourg Cedex 2, France.

‡

Faculté des Sciences de Sfax, Laboratoire de l'Etat Solide, Route de la Soukra km 3.5 - BP 1171, 3000 Sfax, Tunisia.

§

LUNAM Institut des Molécules de Matériaux du Mans IMMM UMR CNRS 6283, Université du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France. ∥

Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, 91192 Gif-sur-Yvette, France.

ABSTRACT: Spherical core-shell CoxFe1-xO@CoyFe3-yO4 nanoparticles (NPs) as well as spherical and cubic shaped CoFe2O4 NPs were synthesized through thermal decomposition method by adjusting parameters as the nature of precursors and ligands. The use of metal (iron and/or cobalt) oleates and stearates as precursors in the presence of oleic acid as ligand leads to core-shell NPs, due to the reducing environment provided by oleate groups from the oleic acid and precursors. By contrast, the use of oleylamine as ligand favoured the decomposition of precursors and a less reducing medium, which allows obtaining NPs with homogeneous composition. In addition, cobalt ferrite cubic-shaped NPs were synthesized using mixed oleate formed insitu from metal iron chloride and cobalt chloride in presence of sodium oleate. The assynthesized NPs were carefully characterized by combining several techniques including TEM, XRD, 57Fe Mössbauer spectrometry, STEM-EELS and XMCD. The correlation between the crystalline structure and the magnetic properties were investigated by carrying out magnetic measurements as function of an applied field and of temperature. The CoFe2O4 NPs were found to display high coercivity due to their homogeneous composition, while the core-shell NPs show higher blocking temperature and exchange bias properties originating from the interaction between the antiferromagnetic (AFM) core and the ferrimagnetic (FIM) layer at the surface.

1. INTRODUCTION Magnetic metal oxide nanoparticles (NPs) have attracted a great interest due to their physical and magnetic properties, which vary dramatically from their bulk counterparts because of their high surface/volume ratio and their size comparable to the magnetic domain.1 Such nanostructures were found to be promising for extensive applications in several fields including magnetic recording media, spintronics, magnetic resonance imaging (MRI), catalysis, magnetically controlled drug delivery, sensors, etc.2-4 Iron-cobalt based NPs, and especially cobalt ferrite (CoFe2O4), becomes very attractive for a variety of applications in electronic devices, catalysis, ferrofluids, high-density information storage, magnetic recording (high-density digital recording disks) and biomedical applications, owing to their high physical and chemical stability, high magnetocrystalline anisotropy and moderate saturation magnetization.5-12 Furthermore since few years, growing interests were devoted to induce additional anisotropy to the NPs in

order to enhance their magnetic properties by strategies such as tuning the NPs morphology to favour shape anisotropy or synthesizing core-shell NPs to induce exchange magnetic properties.13-16 Indeed, core-shell structures with antiferromagnetic and ferrimagnetic (AFM/FIM) materials display exchange bias properties, which lead to large coercive fields and allow to shift the superparamagnetic limit towards room temperature useful to envision recording media applications. Another strategy is the synthesis of core-shell NPs by using mixed ferrites with hard and soft anisotropy inducing exchange coupled properties which have been found very favourable to therapy by hyperthermia.17 To the best of our knowledge, very few papers have dealt with the elaboration of cubic shaped cobalt ferrite NPs as well as cobalt ferrite based core-shell NPs. Similarly core-shell nanoparticles consisting in a FeCo alloy core coated with a CoFe2O4 layer have shown interesting properties for local hyperthermia or thermo ablative cancer therapy.18 In these nanostructures, the CoFe2O4 layer at the surface warrants the protection of the Fe-Co alloy core from oxidation and their magnetic

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properties were found to ensure good heating properties at lower concentrations than iron oxide NPs.

established and is certainly due to the difficulty in characterizing the composition of such mixed ferrite NPs.37

In order to synthesize magnetic oxide NPs, several methods have been developed including microemulsion, sol-gel, microwave and co-precipitation methods,19-22 but the thermal decomposition of metal complexes in presence of ligand (as capping agent) appears currently to be the most promising to ensure the control of NPs size, shape and composition.23-26 In addition, NPs synthesized by this method are insitu coated by an organic layer of ligands, which ensure their good colloidal stability in organic solvents.

The aim of this work is to report on the influence of the metal precursors nature (oleate or stearate) and of ligands (oleic acid, sodium oleate, oleylamine) on the size, shape and composition of iron-cobalt based oxide NPs synthesized by the thermal decomposition approach. While oleic acid is the most used ligand in the thermal decomposition of iron oleate or stearate,20,23,25,26 oleyamine is more and more reported as a ligand favouring the decomposition of precursors and higher oxidation of ferrous cations.25,41 Spherical and cubic shaped NPs with either a homogeneous or a core-shell structure have been synthesized and finely characterized by a combination of several analysis techniques. Such NPs were found to exhibit high coercive fields and blocking temperature according to their composition and structure.

Kovalenko et al.27 have reported the synthesis of cubic iron oxide from the iron oleate decomposition assisted by sodium or potassium oleates as surfactants. The cubic shape of NPs was attributed to a slower growth rate for the {100} facets due to the adhesion of the surfactant on the growing surface. The structure of these cubes was further reported to consist in a core-shell FeO@Fe3O4 structure. The synthesis mechanism accounts for the initial formation of the Fe1-xO phase which turns in the spinel phase through an oxidation process.23,26 Also, Pellegrino et al.28 synthesized iron oxide nanocubes with homogeneous composition by the thermal decomposition of Fe(acac)3 in presence of decanoic acid in dibenzylether by varying the heating rate, which was found to be an important parameter to control the anisotropy of NPs growth. Recently, they further reported that the decomposition products of dibenzylether would be crucial to synthesize cubic shaped nanoparticles with control over the size and shape.29 Indeed, the cubic shape would be induced by the selective adsorption of compounds (oleate or others) on some high surface energy facets ({100}) of iron oxide nuclei inhibiting their growth in this direction.30-33 Several metal complexes were used as precursors to synthesize CoFe2O4 NPs by thermal decomposition, the most common are acetylacetonate (acac), carbonyl, nitrates and oleates, usually as mixture of two complexes (Fe and Co),15,34 moreover, the use of mixed iron-cobalt oleate has also been reported.35,36 Up to now, most works reported on the formation of NPs with a homogenous composition of cobalt ferrite from oletate complexes. However, some recent papers have mentioned the presence of composition non homogenity in NPs with cobalt atoms being not uniformly distributed.37 Such a change of the cationic distribution was found to have significant influence on the final structural and magnetic properties of NPs.38,39 Heiss and al. observed the formation of coreshell FeO@CoFe2O4 NPs from the decomposition of mixed oleates.40 Similarly, nanocubes synthesized by decomposition of iron oleate in presence of a mixture of sodium oleate and oleic acid were found to display a FeO@Fe3O4 core-shell structure even if the synthesis was conducted under an air atmosphere and it was suggested that the presence of excess of oleate and oleic acid would favour reducing conditions limiting the oxidation of Fe2+ cations.26 Thus, the influence of the nature of ligands and precursors on NPs composition and shapes is not clearly

2. EXPERIMENTAL SECTION 2+

2+

2.1. Synthesis using Fe and Co stearates as precursors Using metal stearates as precursors, two experiments were carried out by using either oleic acid or oleylamine as ligands. In a typical synthesis, a two necked round bottom flask was charged with 1.245 g (2 mmol) of iron stearate (9% Fe, Strem Chemicals), 0.625 g (1mmol) of cobalt stearate (9-10%Co, Strem Chemicals), 1.4 mL (4.44 mmol) of oleic acid (99%, Alfa-Aesar) and 20 mL of octadecene (90%, Alfa Aesar, bp318 °C). Using octadecene as solvent is explained by the higher decomposition temperature of cobalt stearate by comparison with iron stearate (TGA analysis, Supporting Information Figure S1A). The mixture was sonicated and stirred at 120°C for 30 min, in the absence of a reflux condenser, to remove water traces and dissolve the reactants until a clear solution was obtained, then heated to 270°C with a heating rate of 1°C/min and kept at this temperature for 1 hour to favour the formation of a mixed Fe-Co complex. Finally, the solution was heated up to boiling point (~320°C) with a rate of 5°C/min and refluxed for 120 min under air. After cooling at room temperature, the NPs were washed three times by addition of acetone and hexane (3:1 in volume) and centrifugation (14000 rpm, 10 min) and were easily suspended in chloroform for conservation. The asobtained NPs were found to display a spherical shape and a size of about 14 nm, so named NS14. Similar preparation was performed using a mixture of 1.24 g (2.22mmol) Fe(stearate)2, 0.627 g (4.44 mmol) of Co(stearate)2 and 1.24 g ( 4.6mmol) oleylamine (70% Technical grade, Sigma-Aldrich) as ligand. The same purification approach was used and the synthesized NPs were named NS8. 3+ 2+ 2. 2. Synthesis using mixed (Fe ,Co ) oleate as precursor Oleate synthesis. At first step, a mixed oleate (Fe3+,Co2+) with a 2:1 molar ratio of Fe:Co was prepared by ligands exchange between corresponding metal chlorides and sodium oleate. In more details, 7.2 g (26.66 mmol) of


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FeCl3.6H2O (99%, Merck) and 2.76 g (13.33mmol) of CoCl2.6H2O (99%, Merck) were dissolved in 60 mL of distilled H2O and 80 mL of ethanol. Separately, 32.5 g (106.66mmol) of sodium oleate was dissolved in 140 mL of hexane and was slowly added to the above solution under vigorous stirring. The resulting biphasic mixture was then refluxed at 70 °C for 4h under stirring. The organic phase containing the oleate complex was separated and washed three times with warm distilled water (50°C) and conserved at 4 °C. The TGA curve of the as-obtained oleate complex is shown in the Supporting Information Figure S1B. NPs synthesis. To synthesize the NPs, 5.37g (2.22mmol) of the prepared mixed oleate, 1.4 mL (4.44 mmol) of oleic acid were added to 20 mL of octylether (97%, Fluka, bp 287 °C). The mixture was heated at 120 °C in the absence of a reflux condenser for 30 min and then to its boiling temperature(~287) with a heating rate of 5 °C/min and refluxed for 120 min at this temperature under air. The resultant black solution was cooled down to room temperature and the NPs, named NS10, were washed as described before.

2.3. Synthesis using Fe and Co chlorides as precursors For this experiment, a mixed Fe-Co oleate were synthesized from the metal chlorides and sodium oleate as described in paragraph 2.2, but differently it was decomposed immediately in the same reaction medium(insitu approach). Such method was already applied to synthesize iron oxide NPs and cobalt ferrite with cubic shape.42 In details, 0.18 g (0.66 mmol) of FeCl3.6H2O and 0.07 g (0.33 mmol) of CoCl2.6H2O were mixed with 0.8 g (2.66 mmol) of sodium oleate and 0.84 g (2.98mmol) of oleic acid in 15 mL of octadecene. The mixture was sonicated, heated to 130°C and kept at this temperature for 1hundervigorous stirring to synthesize the oleate precursor. Then, the solution was immediately heated to the boiling temperature of octadecene (~318°C) with a heating rate of 5 °C/min and refluxed for 120 min under air. After cooling at room temperature, the residue was washed three times with warm distilled water (50°C) to remove NaCl formed during the oleate synthesis, and the NPs were precipitated in the organic phase, washed and finally suspended in chloroform. It is worthy to note that in this experiment, 1.5 mmol of sodium oleate was used as ligand in the first step of mixed oleate formation, the excess was be used with oleic acid as surfactants for NPs synthesis. The as-synthesized NPs, having a cubic morphology and a mean size of about 15 nm, were named NC15. 2.4. Characterization Techniques Transmission Electronic Microscopy (TEM) analysis were carried out on a TOPCON 002B microscope operating at 200 kV (point resolution 0.18 nm) and on a JEOL 2100F electron microscope working with a 200 kV accelerating voltage equipped with a GATAN GIF 200 electron imaging filter. The size statistics were calculated from the size measurements of NPs using "ImageJ" program.43 The structure of the NPs was investigated in the high resolu-

tion mode (HR-TEM), high angle annular dark field scanning TEM (STEM-HAADF) with contrast related to the chemical nature of elements and thus very appropriate for the study of multiphase nanomaterials, such as core-shell nano-objects. To study the distribution of iron and cobalt in core-shell structured NPs, further analyses by Electron Energy Loss Spectroscopy (EELS) were performed. Dynamic Light Scattering (DLS) measurements were carried out using a Zetasizernano-SZ (Malvern Instruments) to study the NPs size distribution and their stability in chloroform. Thermogravimetric measurements were performed on dried powder samples by using a SETARAM TGA 92 from 20 °C to 600 °C with a heating rate of 5°C/min under air. The X-ray diffraction (XRD) patterns were recorded at room temperature with a Bruker D8 Advance diffractometer equipped with a monochromatic copper radiation source, (Kα = 0.154056 nm) and a Sol-X detector in the 2765°(2θ) range with a scan step of 0.03. High purity silicon powder (a = 0.543082 nm) was systematically used as an internal standard to determine the 0 shift. Profile matching refinements were performed through the Full-prof program44 using the modified Thompson-Cox-Hasting (TCH) pseudo-Voigt profile function.45 57

Fe Mossbauer spectra were performed at 300 K and 77 K using a conventional constant acceleration transmission spectrometer with a 57Co(Rh) source and a bath cryostat. The spectra were fitted by means of the MOSFIT46 program with magnetic components composed of Lorentzian lines while an α-Fe foil was used as calibration sample. The values of isomer shift are quoted relative to that of αFe at 300 K. Infrared spectra were recorded between 4000 cm-1 and 400 cm-1 with a Fourier transform infrared (FT-IR) spectrometer (Digilab FTS 3000). NPs were gently ground and diluted in non-absorbent KBr matrixes. Magnetic measurements were performed with a Quantum Design MPMS SQUID-VSM magnetometer. To avoid moving of NPs during the measurements, the samples were “blocked” with eicosane (solid at ambient temperature). Magnetization curves as a function of the temperature, zero-field cooled (ZFC) and field cooled (FC) curves, were recorded as follows: the sample was introduced into the SQUID at room temperature, demagnetized by applying a degauss field, and zero-field cooled to 4 K with no applied field. A magnetic field of 7.5 mT was then applied, and the magnetization was recorded upon heating from 4 K to 300 K (ZFC). Then, the sample was cooled down to 4 K under the same applied field, and the magnetization was recorded upon heating from 4 K to 300 K (FC). Magnetization curves as a function of the applied magnetic field have been measured at 300 K and 5 K. The sample was introduced in the SQUID at high temperature, demagnetized by applying a degauss field, and cooled to 5 K with no applied field (ZFC sample). The magnetization was measured at constant temperature by sweeping the magnetic field between + 5 T to -5 T, and then from - 5 T to + 5 T (ZFC hysteresis loop). After, the sample was heat-



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ed to room temperature with no applied field and subsequently cooled to 5 K under a field of 4 T (FC sample). The FC hysteresis loop was measured by applying the same field sweep as for the ZFC loop. Magnetization to saturation values were given as a function of the mass of pure oxide, which was determined by thermogravimetric analysis after eliminating the mass of organic layer at the NPs surface. X-ray magnetic circular dichroism (XMCD) experiments consist in the difference between X-ray absorption spectra (XAS) obtained from left and right circularly polarized X-rays of a saturated magnetic materials. In the soft X-ray region, a transition occurs from deeply bound states to unoccupied states which are highly sensitive to the local electronic structure. Consequently, XMCD measurement allows probing the local environment of each ion and is an element specific probe sensitive to the magnetic properties. In case of inverse spinel such as Fe3O4 or CoFe2O4, it allows us to disentangle the multiplet at the Fe L3 edge with the respective amplitude of each peak corresponding to divalent and trivalent iron cations and at cobalt L3 edge to determine the site occupancy of cobalt ions. The experiments have been performed at the DEIMOS beam line of the French synchrotron facility SOLEIL. The magnetic field is applied along the normal of the sample in order to magnetically saturate the samples. The samples consist in monolayer assemblies of nanoparticles deposited by Langmuir-Blodgett technique on 200 nm Si3N4 membranes on a silicon frame to allow transmission measurements.47 Different kinds of nanoparticles have been studied: 20 nm Fe3O4, 8 nm (NS8) and 14 nm (NS14) CoFe2O4 nanoparticles. The XMCD spectra have been measured at room temperature for all samples.

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14 nm, which display a heterogeneous composition with the identification of both wüstite-like (M1-xO) (with M = Fe or Co)and spinel ferrite (MFe2O4) phases by TEM (phase contrast) and X-Ray diffraction (XRD).26 At first, this heterogeneous structure was supposed to be due to the higher decomposition temperature of cobalt stearate by comparison to that of iron stearate involving at first the formation of an iron oxide and then a mixed ironcobalt oxide. Thus, an intermediate step at 270°C, (temperature at which iron stearate starts to decompose) for one hour was added to favour the in situ formation of a mixed iron-cobalt complex and the homogeneous germination of mixed cobalt ferrite nuclei. Spherical NPs were obtained and named NS14. A composition contrast was also observed as detailed below and these NPs are not uniform in composition with the presence of wüstite-like and ferrite phases. Other experiments were conducted in order to obtain NPs with homogeneous composition using a mixed cobalt-iron oleate (Supporting Information Figure S2), which was at first synthesized and then decomposed in presence of oleic acid in similar conditions than before. One may notice that this complex is usually reported to synthesize cobalt ferrite NPs with a homogeneous composition.35 In our experimental conditions, this synthesis led to NPs with again a heterogeneous composition.

3. RESULTS AND DISCUSSIONS 3.1. Synthesis using either cobalt and iron stearates or mixed oleate precursors: core-shell CoxFe1-xO@CoyFe3-yO4 NPs 3.1.1. Structural characterizations As we reported earlier, spinel iron oxide NPs, with spherical morphology and sizes in the range 4-28 nm, have been easily synthesized by thermal decomposition of iron stearate or iron oleate in organic solvents using oleic acid as ligand.23,25 However for reproducibility purposes, the iron stearate complex, a commercial product, was preferred as iron oleate was found to be less stable due to its high sensitivity to the synthesis, washing and storing conditions.25,26,48 Thus, the synthesis of cobalt ferrite NPs was, at first, conducted using a mixture of cobalt and iron stearates in stoichiometric proportions and in presence of oleic acid under the same conditions as those used for the synthesis of iron oxide NPs except the nature of the solvant.25 The choice of octadecene as solvent (boiling point of 318 °C) is well explained by the higher temperature of cobalt stearate decomposition, in comparison with that of iron stearate, as confirmed by TGA experiments (Supporting Information Figure S 1A) and an earlier study.49 The thermal decomposition of the mixture of iron and cobalt stearates in octadecene led to NPs with an average size of

Figure 1. (A,B) TEM micrographs, (C,D) size distributions by TEM (red histograms) and granulometric measurements in chloroform (green curves) of NS14(A,C) and NS10(B,D).

TEM images of both NPs synthesized from a mixture of iron stearate and cobalt stearate (NS14, Figure 1A) and from a mixed iron-cobalt oleate complex (NS10, Figure 1B) confirm the synthesis of NPs with a spherical shape and a very narrow size distribution. The average diameters determined from TEM are 14.6 ± 1.4 nm (σ = 10%) and 10.7 ± 0.5 nm (σ = 5%) for NS14 and NS10, respectively. Such values are in good agreement with the mean hydrodynamic diameter determined by granulometric measurements (Figure 1C and 1D) taking into account the oleic acid coating and evidence the good suspension stability of these NPs in organic solvent ensured by the organic layer at their surface.


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* NS14

*

(200)

(220)

(111)

NS10

(220)

30

(311) (222)

35

40

(422) (511)

(400)

45

50

55

(440)

60

agreement with those of Fe1-xO (4.326 Å,JCPDS file 01-89-687) and of Co1-xO (4.261 Å,JCPDS file 43-1004). The crystallite sizes of the monoxide phase were found to be 11.6 ± 1.0 nm and 10.5 ± 1.0 nm, for NS14 and NS10, respectively, while the crystallite size of the entire NPs was around 12.5 ± 1.0 nm and 10.4 ± 1.0 nm, respectively. Earlier studies26,50 and the larger diameter of the spinel phase by comparison with that of the M1-xO phase strongly support that both NS14 and NS10 nanoparticles display a core-shell structure with a M1-xO core and a spinel shell of 1-2 nm.

65

2 Theta (°)

Figure 2. Experimental (black) and refined (red) XRD patterns of NS14 and NS10. Difference between experimental and refined patterns (Blue line). Positions of the Bragg reflections are represented by vertical bars, (black) MFe2O4 and (red) MO. The reflections of silicon, which was used as reference, are indicated by stars.

The crystalline structure of the as-synthesized NS14 and NS10 was further investigated by X-ray diffraction. In both cases, the XRD patterns in the 27-65° (2θ) range display the characteristic peaks of a spinel phase and a wüstitelike phase, which may be assigned to Fe1-xO and Co1-xO according to the precursors used in this synthesis or to a mixture (Figure 2). A good refinement of XRD patterns, using Fullprof program, was achieved by considering these two components: a spinel-type CoFe2O4 (space group Fd3m, number 227, JCPDS n° 22-1086) and Fe1-xO (space group Fm3m, number 225, JCPDS n°06-0615).26,50 It is worthy to note that some peaks of MFe2O4 and MO phases are overlapped such as (311)/(111), (400)/(200) and (440)/(220) peaks respectively. The crystallite sizes and the values of lattice parameters obtained from the refinement of XRD patterns are summarized in Table 1. Table 1. Lattice parameters calculated from the refined XRD pattern using Fullprof program for NS14 and NS10.

NS14

NS10

Crystalline phase

Lattice parameter (Å)

Spinel (MFe2O4)

8.456(1)

Monoxide (MO)

4.239(1)

Spinel (MFe2O4)

8.406(1)

Monoxide (MO)

4.255(1)

The lattice parameter values of the spinel phase of NS14 and NS10 are slightly larger than those of bulk Fe3O4 (8.396 Å, JCPDS file 19-629), and CoFe2O4 (8.391 Å, JCPDS file 22-1086). Such results have been already observed in FeO@Fe3O4 core-shell nanocubes synthesized by thermal decomposition of iron oleate and were attributed to the epitaxial growth of the spinel phase and to oxidation mechanisms of the Fe1-xO core with the diffusion of cations and vacancies generating high strains at the wüstite/spinel interface and in the spinel shell.26 The values of lattice parameter of the M1-xO phase are in

Figure 3. (A,D) HR-TEM micrographs, (B,E) electronic diffraction (ED) pattern and (C,F) S-TEM images of NS14 (A,B,C) and NS10 (D,E,F).

To get more insight about the NPs crystalline structure, NS14 and NS10 were characterized by High Resolution TEM (HR-TEM), Electronic Diffraction (ED) and Scanning Transmission Electron Microscopy (S-TEM). The HR-TEM micrographs of NS14 and NS10 (Figure 3A and 3D) show NPs with lattice fringes characteristic of well crystallized structures. For NS14, the inter-planar distances dhkl of 0.210 nm and 0.240 nm may correspond to the (400) and (311) plans of the spinel structure, but also to the (200) and (111) plans of the M1-xO structure. The ED patterns (Figure 3B and 3E) of both NS14 and NS10 show diffraction rings, which may be indexed with (311),(400) and (440) planes of the spinel structure, and/or with the (111), (200) and (220) reflections of the monoxide structure (Supporting Information Table S1). Indeed the main XRD peaks of both phases being very close, HR-TEM and ED do not allow identifying and clearly discriminating between the two phases.26 A slight contrast difference between the core and the outer layer of NPs is visible by STEM (Figure 3C,D) confirming at first a core-shell structure with two different compositions in the inner and the outer parts of NPs. Therefore, to better localize iron and cobalt elements in NPs, further analyses by STEM-HAADF in dark field and by Electron Energy Loss Spectroscopy (EELS) were carried out (Figure 4). For both NPs, the analyses were realized along one particle according the "spectrum line" (green line), and they show the simultaneous presence of both iron and cobalt on throughout the entire volume of the nanoparticle. The Fe/Co ratio is quite homogenous over


Chemistry of Materials the whole diameter of both NS14 (Figure 4C) and NS10 (Figure 4D). Such results suggest that NPs consist in a mixed iron and cobalt monoxide core (CoxFe1-xO) coated with a shell of cobalt ferrite (CoyFe3-yO4).

with a discrete distribution of hyperfine field linearly correlated to that of isomer shift to reproduce the asymmetry of outer lines, the refined mean value of which is close to that of a ferric species and (ii) a broadened line sextet with an isomer shift typical of a ferrous species (1.01 mm/s). Such a modelling approach is fairly consistent with that involved at 300K. In addition, a quantitative agreement is observed for the contents of each component assumed to be proportional to their respective absorption area (assuming also the same recoil-less f factors): the ferrous contents are estimated at approximately 20 and 30 % for NS14 and NS10, respectively. It is also important to emphasize that (i) the mean values of the isomer shift corresponding to the first component suggest a composition close to that of CoFe2O4 and (ii) one cannot exclude the presence of Co within the FeO, but the lack of resolution of the hyperfine structure prevents from an estimation of Co content. So, the decomposition of the hyperfine structure into 2 components allows us to confirm the core-shell structure of NS10 and NS14 with a CoxFe1-xO core surrounded by a cobalt ferrite shell.

At 77 K (as illustrated in Figure 5C and D), the hyperfine structure consists of sextets with asymmetrical broadened lines in agreement with the spin fluctuations which are slowed by thermal effect by comparison to 300K. It is important to emphasize first that a single discrete distribution of hyperfine fields linearly correlated to that of isomer shift cannot successfully well reproduce the experimental spectra. Indeed, to describe the asymmetry of the 77K Mössbauer spectra, the fitting model should involve two independent magnetic components: (i) one

Relative transmission

0,99

C 1,00

-10

-5

0

V (mm/s)

5

10

0,99 0,98 0,97

-10

-5

0 V (mm/s)

5

10

1,00

0,99

D1,00 Relative transmission

As the most adapted method to investigate the composition of NPs being 57Fe Mössbauer spectrometry, Mössbauer spectra have been recorded at 300 K and 77 K to gain further insight into the phase composition of NPs. The Mössbauer spectra of NS14 and NS10, recorded at 300 K (Figure 5A and 5B), exhibit a complex hyperfine structure which consists of an asymmetrical central line superimposed to a strongly broadened line, attributed to superparamagnetic relaxation phenomena in agreement with the small size of NPs. The refinement of the spectra at 300 K requires the presence of two main different components: (i) a first component with a mean isomer shift value (0.38-0.40 mm/s) rather close to that typical to ferric species (red line) resulting from both a quadrupolar doublet due to very fast relaxation phenomena (smaller particles) and a broad line imaging a continuous distribution of hyperfine field (NS10 Figure 5B) or a broad and discrete distribution of hyperfine field (NS14 Figure 5A) illustrating the presence of Fe moments in fast relaxation and (ii) a quadrupolar doublet with slightly broad lines (blue) describing the asymmetry of the central part, which is clearly attributed to the presence of Fe2+species, as Fe1-xO (with stoichiometry close to the ideal one).

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Whatever the starting complex either a mixed Fe-Co oleate or a mixture of cobalt and iron stearates precursors, a core-shell structure was obtained with a mixed CoFe wüstite core and a cobalt ferrite shell. The observed results suggest strongly the formation of a mixed Fe-Co wüstite core which was oxidized upon exposure to air as already noticed during the synthesis of cubic shaped NPs by thermal decomposition of iron oleate.26 It is worthy to note that similar core-sell structures were obtained whatever the synthesis atmosphere (under air or argon). Thus, in the case of the stearate precursor (NS14), the formation of a CoxFe1-xO core may be explained by the synthesis conditions which would be strongly reducing. Indeed, Zhang et al. reported that oleates can serve as a source of electron donor for the reduction of uranium.51 On the other hand, oleic acid have been reported to act as a reducing agent: the formation of metallic iron NPs via the decomposition of an iron-stearate complex with oleic


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acid and/or sodium oleate was reported to be favoured by the reducing environment created by the decomposition of oleic acid.52 Therefore, oleate and oleic acid molecules are suggested to contribute to the reducing environment leading to mixed cobalt-iron monoxide which is further oxidized when exposed to air, as metal monoxides are not stable in air.26

FeO@CoFe2O4 and FeO@MnFe2O4 core-shell NPs40 due to the strong interaction of the FiM spins of the spinel phase with those of the core. In contrast, the magnetization was saturated in the FeO@Fe3O4 system, which could be explained by the lower anisotropy of the spinel shell and/or the presence of gradient of concentration between the two crystalline phases.26

3.1.2. Magnetic properties Due to the combination of CoxFe1-xO and CoyFe3-yO4, which are antiferromagnetic (AFM) and ferrimagnetic (FIM) respectively, the core-shell structures of NS14 and NS10 are expected to exhibit interfacial exchange-bias (EB) properties resulting from the coupling between the AFM core and FIM shell. Such exchange bias phenomena should induce a shift of the blocking temperature towards room temperature and enhancement of the coercive field below the Néel temperature TN (200 K for FeO and 298 K for CoO).53-56

To get more insight about the exchange coupling at the FIM/AFM interface, FC magnetization curves were carried out after cooling down the samples under an applied field of 4T (Figure 6C and 6D). The FC curves are found to display a horizontal shift confirming that the spins which do reverse experience a strong unidirectional anisotropic energy barrier created by the exchange coupling at the FIM/AFM interface. In addition, one observes a vertical shift attributed to the presence of pinned FIM spins, which do not follow the direction of the reversal applied field.40,57 Such a vertical shift was already observed in core-shell NPs of FeO@CoFe2O4 and was assigned to a percentage of magnetization due to uncompensated spins.40

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Figure 6. (A,B) Magnetization curves as function of an applied field at 300 K and 5 K with inset a zoom of the coercive field, (C,D) at 5 K after cooling without (ZFC) and with (FC) an applied field of 4 T of nanoparticles (A,C) NS14 and (B,D) NS10. (E,F) Magnetization curves as function of temperature and zoom on the maximum temperature of ZFC curve.

Magnetization curves of NS14 and NS10as a function of an applied field measured at 300 K and 5 K are given in Figure 6A and 6B, respectively. At 300 K, no hysteresis loops are observed, that is consistent with superparamagnetic state. In contrast, at 5 K the hysteresis curves of NS14 and NS10 are typical of magnetic NPs below Néel temperature and characteristic of first magnetization curves without reaching the saturation magnetization. Such a behaviour was observed in literature in the case of

3.2. Syntheses using either metal chlorides as precursors or oleylamine as ligand with stearate precursors towards homogeneous CoFe2O4 NPs 3.2.1. Structural characterizations To synthesize homogenous cobalt ferrite NPs (i.e. without a core-shell structure), a process involving the insitu synthesis of a mixed cobalt-iron precursor by heating a mixture of iron and cobalt chlorides, sodium oleate and oleic acid in octadecene has been carried out, at first. In a second approach, the previous synthesis with iron and cobalt stearates has been reproduced by replacing oleic acid by oleylamine. The amine ligands have been reported to enhance the decomposition of iron precursor due to acido-basic reaction between amine ligands and carboxylate ligands of iron complex.25,41 The as-synthesized NPs in the first approach (NC15, Figure 7A) are found to display a cubic-shape with an average size of 15.7 ± 2.6 nm (σ =15 %). The cubic morphology may be attributed to the presence of an excess of sodium oleate which should favour the cubic growth by adsorbing preferentially on specific facets and also the in situ formed oleate precursor which decomposes over a wide range of temperatures providing with the temperature rate a continuous but low flow of monomer, essential condition for such anisotropic growth.25,58,59 When oleic acid is replaced by oleylamine in the synthesis with stearate precursors, NPs with a spherical shape and a diameter of 8 ± 1.6 nm (σ =10 %) were obtained (NS8, Figure 7B). The smaller size of NS8, in comparison with NS14 synthesized in the same conditions but with oleic acid as ligand, is due to the fact that amine



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group favours the decomposition of stearates and thus the increase in the concentration of monomers leading to the formation of a larger amount of nuclei with a smaller size according to the La Mer theory.25

Figure 7. (A,B) TEM micrographs and (C,D) size distributions (red histograms) and granulometric measurements in chloroform ( green curves) of NPs (A,C) NC15 and NS8 (B,D).

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results were confirmed by ED pattern (Figure 8C), which exhibits clearly the rings characteristic of a spinel structure. The dhkl distances measured from the SAED pattern are indexed with those of CoFe2O4 (JCPDS n° 22-1086) in Supporting Information Table S2. Unlike the core-shell NPs previously described, the STEM micrograph realized on NC15 (Figure 8D) presents clearly a homogeneous contrast throughout the total surface of NPs, suggesting that the NPs have a unique crystalline phase and uniform composition. The nanoparticles NC15 and NS8 were characterized by XRD (Figure 9). The corresponding patterns were found to display the sole peaks of cobalt ferrite phase (JCPDS n° 22-1086, space group Fd3m, number 227) and their refinement lead to lattice parameters a = 8.394(1) Å and a= 8.397(2) Å for NC15 and NS8, respectively. These values are slightly higher than the one of bulk CoFe2O4 (8.391 Å), which can be assigned either to a lower concentration in cobalt or to a different cationic distribution. The crystallite sizes calculated from the refinement are 15.1 nm and 9 nm for NC15 and NS8, respectively, and are in good accordance with the mean size of NPs observed by TEM.

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Indeed, oleic acid, as ligand, was reported to stabilize the metal complex and the formation of a low flow of monomer, with the temperature increase or hold, inducing a low yield in nuclei and favouring the grain growth. The average hydrodynamic sizes of NC15 and NS8 determined by granulometric measurements are respectively 19.8 ± 0.1 nm and 10.5 ± 0.1 nm, in agreement with NPs coated by an oleate layer (Figure 7C and 7D). Representative HR-TEM micrographs of NC15 and NS8 (Figure 8A and 8B) show that NPs have a very well crystallized structure. For NC15, the measured distances of 0.301 nm and 0.272 nm can be attributed to the dhkl of (311) and (200) plans of the spinel structure of CoFe2O4.Such

Figure 9. Experimental (black) and refined (red) XRD patterns of NC15 and NS8. Positions of the Bragg reflections of CoFe2O4 (JCPDS n° 22-1086) are represented by blue vertical bars. The reflections of silicon, which was used as reference, are indicated by stars.

3.2.2. NPs Composition XMCD experiments have been performed on NS8 cobalt ferrite NPs, NS14 core-shell NPs and magnetiteFe3O4 NPs of 20 nm which were used as a reference to compare them and evidence some differences. In case of these inverse spinel structures, three different peaks should appear at the Fe L3 edge (~716 eV ± 2 eV) corresponding to each iron cations: octahedral (Od) Fe2+ and Fe3+ sites and tetrahedral (Td) Fe3+ sites. Figure 10A illustrates the X-ray absorption spectrum at Fe L3 edge for the NS8 assembly at room temperature while Figure 10B represents the XMCD spectra at room temperature for each sample with an external applied magnetic field of 4T for the NS8 cobalt ferrite and NS14 core-shell NPs and 0.5T for the magnetite samples. They all demonstrate the expected shape with three main peaks at the Fe L3 edge.60 The two negative


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ones at 715 eV and 717 eV correspond to (Od) Fe2+ and Fe3+ sites respectively. The positive one at 716 eV is associated to Fe3+ (Td). The relative amplitudes of the peaks agree well with previous results from the literature on Fe3O4 thin films61 and on biogenically produced ferrite nanoparticles.62 As one compares the XMCD spectra for the cobalt ferrite and core-shell nanoparticles, one can observe a decrease of the low energy negative peak for both samples corresponding to a decrease of Fe2+ cations on octahedral sites. This indicates that as the cobalt amount increases, the Co2+ cations replace predominantly the Fe2+.

tion with oleylamine of CoFe2O4 nanoparticles enables to suitably insert cobalt in the ferrite structure. This is another indication of a more homogeneous structure of the NS8. The increase of the low energy peak of Fe3+ in octahedral sites, larger for NS14 than NS8, suggests a larger amount of Fe3+ in Od sites. This would be in agreement with the presence of wüstite in this sample as wüstite has metallic ions in Od sites. Figure 11A represents the XAS spectrum at the L3 edges of Co for the 8nm CoFe2O4NPs while Figure 11B compares XMCD spectra at Co L3 edge for theCoFe2O4and core-shell NPs submitted to an applied magnetic field of 4T. One retrieves the fine structures observed previously in CoFe2O4 thin films.63 In addition, according to calculations of the spectra for each site, it has been demonstrated that the XAS and XMCD amplitudes at cobalt edges are most probably associated to a Co2+ octahedral occupancy.64 Moreover, the octahedral and tetrahedral sites correspond to opposite sign of the XMCD amplitudes due to their antiferromagnetic coupling. Finally, one can notice that the XMCD sign of the Co2+ is the same as the one of Fe2+ on octahedral sites. These results confirm that cobalt mainly resides in octahedral sites. 1,00

Figure 10. (A) XAS spectrum for NS8 CoFe2O4 nanoparticles at 300K. (B) XMCD spectra measured at 300 K for three kinds of ferrite based nanoparticles monolayer assemblies. The applied magnetic field is different for Fe3O4 nanoparticles (0.5T), and CoFe2O4 and core-shell (4T) to ensure their magnetic saturation.

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Although our XMCD measurements are not completely quantitative, one can observe that the contributions of each peak at the Fe edge are slightly different for NS14 and NS8 monolayers. In particular, the peaks corresponding to Fe2+ tetrahedral or Fe3+ octahedral sites are almost not affected by the increase of cobalt in NS8 as compared to NS14 sample. This last result shows that the elabora-

Figure 12. Fe Mössbauer spectra of nanoparticles NS8 and NC15 measured at 77 K.

Figure 12 shows Mössbauer spectra recorded at 77 K on NC15 and NS8 samples: the pure magnetic hyperfine structure can be perfectly decomposed into 2 magnetic sextets. The values of the hyperfine parameters are fairly consistent with those of Co-ferrite, i.e. isomer shift values (0.42 and 0.55 mm/s) and hyperfine fields of 50.7 and 53.5T typical of HS Fe3+ ions located in tetrahedral and octahedral sites, respectively, excluding thus the presence of ferrous species in both samples (and thus of wüstite). But the present respective Fe3+ (in Od and Td) ratios (69



3.2.3. Magnetic properties The formation of a unique crystalline phase (not a coreshell structure) for NC15 and NS8 was further confirmed by carrying out magnetic measurements as function of an applied magnetic field with or without cooling down under an applied field and as function of temperature (ZFC and FC curves). For both NPs, the magnetization curves at 5K display an opened hysteresis loop, which is characteristic of ferrimagnetic NPs below their blocking temperature (TB). At 300 K, the NC15 and NS8 were found to display ferrimagnetic and superparamagnetic behaviours, respectively (Figure 13A and 13B). The saturation magnetization Ms, at 5 K, of NC15 and NS8 were found to be 68 emu/g and 38 emu/g, respectively. Such values are in good accordance with those reported for CoFe2O4 NPs with similar sizes and prepared by the thermal decomposition method. Song Q. reported Ms values in the range of 60-80 emu/g for spherical and cubic CoFe2O4 of 5-13 nm,15 while Cabrera L.I. found that NPs of 12 nm display a Ms value of 54 emu/g.34 Indeed, the saturation magnetization values of CoFe2O4 at the "nanometer scale" vary with the synthesis method and increases as a function of the size, but still lower than those of bulk cobalt ferrite (80 emu/g)65 due to the presence of defects and a surface and

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The as-synthesized NC15 and NS8 were found to display a more homogeneous CoFe2O4 crystalline structure due to the presence of only Co2+ and Fe3+cations in the reaction medium. For NS8, the use of oleylamine as surfactant makes the reaction medium less reducing and favours the decomposition of the stable cobalt stearate complex. The decrease of the NPs size from 14 nm to 8 nm using oleic acid and oleylamine as surfactants, respectively, can be attributed to the fact that iron and cobalt stearates are less stable with the amine function, which lead to important nucleation step and less growth. Similar results were observed for iron oxide NPs synthesized by thermal decomposition of iron stearate.25 In the case of NC15, starting from metals chlorides to create a mixed ironcobalt oleate precursor, which was decomposed insitu was already reported to synthesize ferrite NPs.42 Such approach limits the reduction of Fe3+ initially present in iron oleate during its storage. In addition, it was reported that the presence of Cl- ions and the use of Na-oleate as surfactant, may effectively stabilize the {100} facets of cubic spinel structure and induce the formation of cubic shaped nanocrystals.42

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volume spin canting.25,66-68 Furthermore the defects or composition gradient suggest by combining Mössbauer spectrometry and XMCD should also contribute to decrease the Ms values. The values of coercive field (HC) at 5K of 20.6 103 Oe for NC15 and 19 103 Oe for NS8 are slightly higher than the values reported for similar sizes of NPs synthesized from the thermal decomposition of oleate and acetylacetonate precursors described in the ref. 34 and the ref. 35. In addition, these values of Hc are much higher than those we reported previously for iron oxide NPs (355 Oe and 269 Oe for spherical NPs of 8 and 15 nm, respectively) synthesized in the same conditions,25 which agree with the high magnetocrystalline anisotropy of CoFe2O4. The higher value for NC15 agrees with the larger size which have the highest magnetocrystalline anisotropy. Magnetization (M/Ms)

% : 31 % for NC15 and 63 % : 37 % for NS8) differ strongly from those usually observed in Co-Ferrite which exhibits a normal spinel structure (50:50). It is clear that this estimation should be improved and refined by means of infield low temperature Mössbauer experiments: indeed, the hyperfine structure splits into two well-resolved magnetic sextets as a consequence of the blocked ferrimagnetic structure.


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Figure 13C shows the FC curve of NC15 after cooling down under an applied field of 4T (FC). The homogenous crystalline structure of the NPs was confirmed by quite no shift of the magnetization curve. Only a very low horizontal shift of 261 Oe for NC15 and 57 Oe for NS8 is observed which correspond to the interaction of FIM spins of CoFe2O4 at the interface with the canted layer localized at the NPs surface. The ferrimagnetic behaviour of NC15 at 300K was confirmed by measuring ZFC/FC curves, which show a maximum of the ZFC curve at 340 K (Figure 13D). For NS8, the value of 212 K advises a superparamagnetic behaviour in agreement with the absence of hysteresis loop at 300 K. The values of TB estimated from the ZFC curves are in good agreement with those reported for CoFe2O4 NPs with similar sizes69 but higher than that of iron oxide NPs.25 The higher value of NC15 in comparison with NS8 is due to their larger size, and the shape of the FC curve could be explained by the presence of strong dipolar interactions, which would agree with the ferrimagnetic behaviour of nanoparticles NC15. While the presence of a


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minimum in the FC curve could be an indication of superspinglass behaviour, as discussed by Bedanta and coworkers.70,71

4. CONCLUSION In conclusion, cubic and spherically-shaped NPs of cobalt and iron mixed oxide have been synthesized with controlled size and very narrow size distribution by adjusting the experimental conditions of thermal decomposition of metal complexes such the nature of precursors, ligands and solvents. Using either stearates or mixed oleate as precursors with oleic acid as surfactant, leads to spherical NPs exhibiting a core-shell structure with a CoxFe1-xO core and a CoxFe3-xO4 spinel layer at the surface. This crystalline structure is due to the reducing environment owing the presence of oleate groups and oleic acid surfactant. The structure was investigated by TEM, XRD, EELS, Mössbauer spectrometry and such NPs were found to be superparamagnetic at room temperature, but showing interesting exchange bias coupling at the AFM/FIM interface for temperatures below Neel temperature (TN) of antiferromagnetic core. The AFM/FIM was confirmed by magnetization curves as function of applied field at 5 K after cooling under 4 T field showing horizontal and vertical shift of the hysteresis loop. Synthesis in the same conditions from cobalt and iron stearates, but with oleylamine as surfactant, lead to spherical NPs without a core-shell structure. Also, an insitu approach of oleate formation from metals chlorides an sodium oleate was conducted to synthesize CoFe2O4 based NPs with cubic morphology. In these two cases, the medium reaction was found to be less reducing. Combined XMCD and Mössbauer analyses suggest that NPs should contains defects or composition gradient which would explain, with spin canting effects, their lower values of saturation magnetizations by comparison with bulk cobalt ferrite. However the as-synthesized NPs display high coercive field confirming the insertion of cobalt.

ASSOCIATED CONTENT Supporting Information. TGA curves of iron stearate cobalt stearate and mixed ironcobalt oleate complexes (Figure 1 SI), Distances calculated from electronic diffraction pattern of nanoparticles NS14, NS10 (Table 2 SI) and NC15 (Table 3 SI). This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Tel.: +33 3 68 85 26 76. Fax: +33 3 68 85 27 61. *E-mail: [email protected]. Tel.: +33 3 88 10 71 92. Fax +33 3 88 10 72 50.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

The authors thank A. Derory for SQUID measurements, D. Burger for TGA measurements. and P. Ohresser for help and support during the XMCD measurements at the SOLEIL DEIMOS beam line. The authors thank the "Agence Nationale de la Recherche" in France via the project EQUIPEX UNION: # ANR-10-EQPX-52 and the LABEX NIE: ANR-11-LABX-0058_NIE.

REFERENCES (1) LaConte, L.; Nitin, N.; Bao, G. Mater. Today. 2005, 8, 32-38. (2) Frey, N. A.; Peng, S.; Cheng, K.; Sun, S. Chem. Soc. Rev. 2009, 38, 2532-2542. (3) Tartaj, P.; del Puerto Morales, M.; Veintemillas-Verdaguer, S.; Gonzalez Carreno, T.; Serna, C.J. J. Phys. D.: Appl. Phys. 2003, 36, R182-R197. (4) Srinivasan, B.; Li, Y.; Jing, Y.; Xu, Y.; Yao, X.; Xing, C.; Wang, J. P. Angew.Chem. Int. Ed. 2009, 48, 2764-2767. (5) Pallai, V.; Shah, D.-O. J. Magn. Magn. Mater. 1996, 163, 243-248. (6) Skomski, R. J. Phys.: Condens. Matter. 2003, 15, R1-R56. (7) Senapati, K. K.; Borgohain, C.; Phukan, P. Journal of Molecular Catalysis A. 2011, 339, 24-31. (8) Tang, D. P.; Yuan, R.; Chai, Y. Q.; An, H. Adv. Funct. Mater. 2007, 17, 976-982. (9) Chen, Z.-G.; Tang, D. Y. Bioproc. Biosyst. Eng. 2007, 30, 243-249. (10) Chen, J. J. Inorg. Mater. 2009, 24, 967-972. (11) Bhame, S. D.; Joy, P. A. Sens. Actuators, A, 2007, 137, 256261. (12) Pradhan, P.; Giri, J.; Samanta, G.; Sarma. H. D.; Mishra, K. P.; Bellare, J.; Banerjee, R.; Bahadur, D. J. Biomed. Mater. Res. B. 2007, 81, 12-22. (13) Jones, M.R.; Macfarlane, R. J.; Prigodich A. E.; Patel, P. C.; Mirkin, C. A. J. Am. Chem. Soc. 2011, 133, 18865-18869. (14) Nangia, S.; Sureshkumar, R. Langmuir. 2012, 28, 1766617671. (15) Song, Q.; Zhang, Z. J. J. Am. Chem. Soc. 2004, 126, 61646168. (16) Srikala, D.; Singh, V. N.; Banerjee, A.; Mehta, B. R. Journal of Nanoscience and Nanotechnology, 2010, 10, 8088-8094. (17) Lee, J. H.; Jang, J. T.; Choi, J. S.; Moon, S. H.; Noh, S. H.; Kim, J. W.; Kim, J. G.; Kim, I. S.; Park, K. I.; Cheon, J. Nat. Nanotechnol. 2011, 6, 418-422. (18) Habib, A. H.; Ondeck, C. L.; Chaudhary, P.; Bockstaller, M. R.; McHenry, M. E. J. App. Phy. 2008, 103, 307. (19) Wu, W.; He, Q.; Jiang, C. Nanoscale Res. Lett. 2008, 3, 397-415. (20) Hyeon, T. Chem. Commun. 2003, 927-934. (21) Thanh, N. T. K. Magnetic Nanoparticles: From Fabrication to Clinical Applications CRC Press, 2012. (22) Chen, D.; Zhang, Y. Chen, B.; Kang, Z. Ind. Eng. Chem. Res. 2013, 52, 14179-14184. (23) Demortiere, A.; Panissod, P.; Pichon, B. P.; Pourroy, G.; Guillon,D.; Donnio, B.; Begin-Colin, S. Nanoscale. 2011, 3, 225232. (24) Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.;Park, J. H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891895. (25) Baaziz,W.; Pichon, B. P.; Fleutot, S.; Liu, Y.; Lefevre, C.; Greneche, J. M.; Toumi, M.; Mhiri, T.; Bégin-Colin, S. J. Phys. Chem. C. 2014, 118, 3795-3810. (26) Pichon, B. P.; Gerber, O.; Lefevre, C.; Florea, I.; Fleutot, S.; Baaziz, W.; Pauly, M.; Ohlmann, M.; Ulhaq, C.; Ersen, O.; Pierron-Bohnes, V.; Panissod, P.; Drillon, M.; Begin-Colin, S. Chem. Mater. 2011, 23, 2886-2900.



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(27) Kovalenko, M. V.; Bodnarchuk, M. I.; Lechner, R. T.; Hesser, G. N.; Schaffler, F.; Heiss, W. J. Am. Chem. Soc. 2007, 129, 6352-6353. (28) Guardia, P.; Di Corato, R.; Lartigue, L.; Wilhelm, C.; Espinosa, A.; Garcia-Hernandez, M.; Gazeau, F.; Manna, L.; Pellegrino, T. ACS Nano, 2012, 6, 3080-3091. (29) Guardia, P.; Riedinger, A.; Nitti, S.; Pugliese, G.; Marras, S.; Genovese, A.; Materia, M.A.; Lefevre, C.; Manna L.; Pellegrino, T. J. Mater. Chem. B, 2014, 2, 4426-4434. (30) Nguyen, T. D.; Do, T. O. Nanocrystals book edited by Yoshitake Masuda, ISBN 978-953-307-199-2, Published: June 28, 2011 under CC BY-NC-SA 3.0 license DOI: 10.5772/17054. (31) Cushing, B. L.; Kolesnichenko, V. L.; O'Connor, C. J.; Chemical Reviews. 2004, 104, 3893-3946. (32) Kwon, S.G.; Hyeon, T. 2009. Kinetics of Colloidal Chemical Synthesis of Monodisperse Spherical Nanocrystals. John Wiley & Sons, Inc. (33) Liu, R. S. Controlled Nanofabrication: Advances and Applications CRC Press, 17 oct. 2012, 355. (34) Cabrera, L. I.; Somoza, Á.; Marco, J. F.; Serna, C. J.; Morales, M.P. J. Nanopart. Res. 2012, 14, 873. (35) Bao, N.; Shen, L.; Wang, Y.; Padhan, P.; Gupta, A. J. Am. Chem. Soc. 2007, 129, 12374-12375. (36) Bao, N.; Shen, L.; Wang, Y.-H. A. Ma, J.; Mazumdar, D.; Gupta, A. J. Am. Chem. Soc. 2009, 131, 12900-12901. (37) Peddis, D.; Yaacoub, N.; Ferretti, M.; Martinelli, A.; Piccaluga, G.; Musinu, A.; Cannas, C.; Navarra, G.; Greneche, J.M.; Fiorani, D. J. Physics: Condens. Matter. 2011, 23, 426004. (38) Ayyappan, S.; Mahadevan, S.; Chandramohan, P.; Srinivasan, M. P.; Philip, J.; Raj, B. J. Phys. Chem. C. 2010, 114, 6334-6341. (39) Deatsch, A., E.; Evans, B. A. J. Magn. Magn. Mater. 2014, 354, 163-172. (40) Bodnarchuk, M. I.; Kovalenko, M. V.; Groiss, H.; Resel, R.; Reissner, M.; Hesser, G.; Lechner, R. T.; Steiner, W.; Schäffler, F.; Heiss, W. Small 2009, 5, 2247-2252. (41) Mourdikoudis, S.; Liz-Marzan, L. M. Chem. Mater. 2013, 25, 1465-1476. (42) Xu, Z.; Shen, C.; Tian, Y.; Shi, X.; Gao, H. J. Nanoscale 2010, 2, 1027-1032. (43) Wayne Rasband (NIH), http://rsb.info.nih.gov/ij/ (44) Rodríguez-Carvajal, J. Phys. B (Amsterdam, Neth.) 1993, 192, 55-69. (45) Thompson, P.; Cox, D. E.; Hastings, J. B. J. Appl. Crystallogr. 1987, 20, 79-83. (46) Teillet, J.; Varret, F. Homemade MOSFIT program. Université du Maine. (47) Lopez-Flores, V.; Bergeard, N.; Halte, V.; Stamm, C.; Pontius, N.; Hehn, M.; Otero, E.; Beaurepaire, E.; Boeglin, C. Phys. Rev.B. 2013, 87, 214412. (48) Bronstein, L.M.; Huang, X.; Retrum, J.; Schmucker, A.; Pink, M.; Stein, B.D.; Dragnea, B. Chem. Mater. 2007, 19, 36243632. (49) Baaziz, W.; Begin-Colin, S.; Pichon, B.P.; Florea, I.; Ersen, O.; Zafeiratos, S.; Barbosa, R.; Begin, D.; Pham-Huu, C. Chem. Mater. 2012, 24, 1549-1551. (50) Baaziz, W.; Pichon, B.P.; Lefevre, C.; Ulhaq-Bouillet, C.; Toumi, M.; Mhiri, T.; Bégin-Colin, S. J. Phys. Chem. C. 2013, 117, 11436-11443. (51) Zhang, F.; Wu, W.; Parker, J.; Mehlhorn, T.; Kelly, S.; Kemner, K.; Zhang, G.; Schadt, C.; Brooks, S.; Criddle, C.; Watson, D.; Jardine, P. Journal of Hazardous Materials. 2010, 183, 482-489. (52) Kim, D.; Park, J.; An, K.; Yang, N.K.; Park, J.G.; Hyeon, T. J. Am. Chem. Soc. 2007, 129, 5812-5813. (53) Cornell, R. M.; Schwertman, U. The Iron Oxides; WileyVCH: Weinheim, Germany, 2003.

Page 12 of 14

(54) Kavich, D. W.; Dickerson, J. H.; Mahajan, S. V.; Hasan, S. A.; Park, J. H. Phys. Rev. B 2008, 78, 174414. (55) Nogues, J.; Sort, J.; Langlais, V.; Skumryev, V.; Surinach, S.; Munoz, J. S.; Baro, M. D. Phys. Rep. 2005, 422, 65-117. (56) Hai, H. T.; Yang, H. T.; Kura, H.; Hasegawa, D.; Ogata, Y.; Takahashi, M.; Ogawa, T. J. Colloid Interface Sci. 2010, 346, 37-42. (57) Maaz, K.; Mumtaz, A.; Hasanain, S.K.; Ceylan, A. J. Magn. Magn. Mater. 2007, 308, 289-295. (58) Shevchenko, E.V.; Talapin, D.V.; Schnablegger, H.; Kornowski, A.; Festin, Ö.; Svedlindh, P.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2003, 125, 9090-9101. (59) Kwon, S. G.; Piao, Y.; Park, J.; Angappane, S.; Jo, Y.; Hwang, N. M.; Park, J. G.; Hyeon, T. J. Am. Chem. Soc. 2007, 129, 12571-12584. (60) Pilard, M.; Ersen, O.; Cherifi, S.; Carvello, B.; Roiban, L.; Muller, B.; Scheurer, F.; Ranno, L.; Boeglin, C. Phys. Rev. B 2007, 76, 214436. (61) Chen, J.; Huang, D.J.; Tanaka, A.; Chang, C.F.; Chung, S.C.; Wu, W.B.; Chen, C.T.; Phys. Rev. B 2004, 69, 085107. (62) Coker, V.; Telling, N.; van der Laan, G.; Pattrick, R.; Pearce, C.; Arenholz, E.; Tuna, F.; Winpenny, R.; Lloyd, J. ACS Nano. 2009, 3, 1922-1928. (63) Van der Laan, G.; Arenholz, E.; Chopdekar, R. V.; Suzuki, Y. Phys. Rev. B 2008, 77, 064407. (64) Van der Laan, G.; Kirkman, I. W. J. Phys.: Condens. Mater. 1992, 4, 4189-4204. (65) Smit, J.; Wijn, H.P.J. "Ferrites", London, 1959. (66) Daou, T.J.; Grenèche, J.M.; Pourroy, G.; Buathong, S.; Derory, A.; Ulhaq-Bouillet, C.; Donnio, B.; Guillon, D.; BéginColin, S. Chem. Mater. 2008, 20, 5869-5875. (67) Morales,M.P.; Veintemillas-Verdaguer,S.; Montero,M.I.; Serna,C.J.; Roig,A.; Casas,L.; Martinez, B.; Sandiumenge, F. Chem. Mater. 1999, 11, 3058-3064. (68) Dayen, J. F.; Faramarzi, V.; Pauly, M.; Kemp, N. T.; Barbero, M.; Pichon, B.P.; Majjad, H.; Begin-Colin, S.; Doudin, B. Nanotechnology 2010, 21, 335303. (69) Song, Q.; Zhang, Z.J. J. Phys.Chem. B 2006, 110, 1120511209. (70) Bedanta, S.; Kleeman, W. J. Phys. D: Appl. Phys. 2009, 42, 013001. (71) Petracic, O.; Chen, X.; Bedanta, S.; Kleeman, W.; Sahoo, S.; Cardoso, S.; Freitas, P. P. J. Magn. Magn. Mater. 2006, 300, 192-197.


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Table of contents

Tuning of synthesis conditions by thermal decomposition towards spherical and cubic CoFe2O4 NPs and CoxFe1-xO@CoyFe3-yO4 core-shell NPs

Walid Baaziz,†,‡* Benoit P. Pichon,† Yu Liu,† Jean-Marc Grenèche,§ Corinne Ulhaq-Bouillet,† Erwan Terrier,† Nicolas Bergeard,† Valérie Halté,† Christine Boeglin,† Fadi Choueikani,∥ Mohamed Toumi,‡ Tahar Mhiri,‡ Sylvie Begin-Colin†*.

(Fe+Co) Stearates + Oleylamine

(Fe+Co) Stearates + Oleic acid

Spinel CoxFe3-xO4

Core-shelled CoyFe1-yO@CoxFe3-xO4

(Fe+Co) Chlorides + Sodium oleate


(Fe-Co) Oleate + Oleic acid

Tuning of Synthesis Conditions by Thermal Decomposition toward

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