Magnetic Iron Oxide Nanoparticles: Reproducible Tuning of the Size

Article pubs.acs.org/JPCC

Magnetic Iron Oxide Nanoparticles: Reproducible Tuning of the Size and Nanosized-Dependent Composition, Defects, and Spin Canting Walid Baaziz,*,†,‡ Benoit P. Pichon,† Solenne Fleutot,† Yu Liu,† Christophe Lefevre,† Jean-Marc Greneche,§ Mohamed Toumi,‡ Tahar Mhiri,‡ and 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 S Supporting Information *

ABSTRACT: Iron oxide nanoparticles (NPs) with average sizes in the range 4−28 nm have been obtained by varying different synthesis parameters of the thermal decomposition of an iron precursor (iron stearate) in the presence of surfactants in high boiling solvents. The synthesis parameters affect the NPs nucleation and growth steps, by modifying the stability of iron stearate on which depend the monomer formation and concentration, in agreement with the LaMer model. The monomer formation, which is reaction time and/or temperature dependent, is thus found to vary mainly as a function of the nature of solvents and ligands. The structural and magnetic characterizations of NPs with sizes in the range 5−20 nm confirm that the composition of NPs evolves from the maghemite for small sizes (typically 12 nm) via a perturbed oxidized state for intermediate sizes. The values of saturation magnetization lower than those of bulk magnetite and maghemite were found to be related to this composition evolution and to the presence of oxidation defects, surface spin canting and volume spin canting as a function of NPs diameter. Small NPs presented mainly a surface spin canting. NPs with large sizes display Ms which depends on their oxidized shell thickness, defects and surface spin canting. NPs with intermediate sizes display a surface and in particular a volume spin canting due to a disordered structure induced by a perturbed oxidation state in these NPs. in the field of medical imaging as a contrast agent for magnetic resonance imaging.13,16 One of the challenges is the functionalization of NPs to obtain water suspensions of superparamagnetic NPs with high saturation magnetization and average hydrodynamic sizes smaller than 50 nm (to favor a good biodistribution).6,13,15 These examples show that the size of the magnetic NPs and their magnetic properties are key points to consider when it concerns biomedical applications. Regarding other applications, the composition of iron oxide NPs may be also an important point to consider. Indeed, recent studies have demonstrated the interest in magnetite (Fe3O4) structured in films for the development of magnetoelectronic devices with interesting magnetotransport properties.23−27 Fe3O4 is predicted to be half metallic at room temperature and should allow 100% spin polarization of an electric current passing through.28,29 Films of Fe3O4 NPs have been reported to display very interesting magnetoresistance values in comparison to thin films elaborated by physical methods and are expected to lead to the development of magnetoelectronic devices with enhanced magnetotransport properties.24,25,30,31 However, one

1. INTRODUCTION Magnetic nanoparticles (NPs) are extensively studied from a fundamental point of view for the new properties generated by their size, shape, and composition but also because of their broad range of applications, including magnetic fluids, data storage, catalysis, and biomedical applications.1−7 Depending on the intended application, the magnetic properties of NPs must be “adapted” and then NPs with a given size, composition, and shape have to be designed. This is particularly the case for biomedical applications (MRI, cell sorting, hyperthermia, drug delivery, ...).6−19 For instance, in a magnetic particle based bioseparation process, a magnetically activated separation of biological entities is very interesting and easier compared to other methods currently used, but NPs must display a high magnetic moment to enable efficient separation. Another promising biomedical application of magnetic NPs is the treatment of cancer by magnetic hyperthermia. The amount of heat generated by magnetic NPs strongly depends on the NP magnetic properties which have to display high saturation magnetization and in particular high anisotropy energy which depend strongly on the NP size, composition, and shape.20−22 The particle size distribution is also important as size distribution or energy barriers can cause overheating and undesired magnetic thermal ablation. Finally, NPs are also used © 2014 American Chemical Society

Received: November 24, 2013 Revised: January 19, 2014 Published: January 21, 2014 3795

dx.doi.org/10.1021/jp411481p | J. Phys. Chem. C 2014, 118, 3795−3810

The Journal of Physical Chemistry C

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limitation is that, at the nanoscale, Fe 2+ in Fe 3 O 4 ([Fe3+]Td[Fe3+Fe2+]OhO4 with Td, tetrahedral sites and Oh, octahedral sites) becomes very sensitive to oxidation altering the NP composition and properties, particularly at the surface. Due to their wide range of applications, the reproducible synthesis of iron oxide NPs with controlled magnetic properties and compositions is thus currently a great challenge. It is an inducement to be able to control the synthesis conditions leading to reproducible sizes and compositions and then properties of NPs. Furthermore, a very important issue is to understand the synthesis mechanisms to better control the final mean size and to establish magnetic structure−composition− property relationships. Various synthesis methods have been reported for the preparation of iron oxide magnetic NPs with nanograin sizes such as coprecipitation, hydrothermal, sol−gel, or polyol methods.16,32,33 Among these methods, the synthesis by thermal decomposition of metal complexes in a high boiling solvent in the presence of surfactants appears to be the most appropriate because it allows controlling the size and morphology of the NPs and leads to their in situ functionalization limiting their aggregation in suspension.34−36 The thermal decomposition approach described by Hyeon et al.34 has been chosen because the synthesis system is simple with one iron complex, one type of ligand (mainly oleic acid), and a high boiling point organic solvent. However, despite a lot of studies, the influences of some synthesis parameters remain unclear such as the oleic acid/iron precursor ratio.37 As observed with other synthesis methods, the saturation magnetization of iron oxide NPs is lower than that of bulk phase magnetite or its fully oxidized form: maghemite (γ-Fe2O3). The main reported explanation is the presence of a spin canted layer at the surface of NPs due to the breaking of the crystalline symmetry at the surface of nanocrystals,38−48 but other explanations are also the presence of defects and/or of a spin canting in volume.49−57 Recently, Vichery et al.58 reported that, for small NPs, the surface effects are mainly responsible for the lower Ms, while volume defects are expected for larger sizes. Therefore, there is also currently a need for fine structural and magnetic characterizations as a function of the NP size to establish size-dependent properties. The aims of this paper are at first to determine the reproducible synthesis conditions of iron oxide NPs with different sizes and a narrow size distribution and to explain how some synthesis parameters affect the nucleation and growth step by considering the LaMer model. First experiments have been conducted with iron oleate synthesized in the laboratory,59,60 but due to the low stability of this iron complex which is highly sensitive to the synthesis, washing, and storing conditions,61 an iron stearate complex, a commercial product, has been used. A synthesis in standard conditions has been set and led to the reproducible synthesis of 11 nm NPs. The influences of different synthesis parameters (reflux time, nature of solvents, nature of ligands, oleic acid/iron stearate ratio, hold at 250 °C (supposed to correspond to the nucleation temperature)) on the size of NPs have been investigated. Then these NPs have been finely investigated structurally and magnetically, in particular by performing Mössbauer spectrometry under an applied field and field cooled (FC) magnetic measurements in order to correlate the nanosize, structural properties, and magnetic properties.

2. EXPERIMENTAL DETAILS 2.1. Synthesis of Nanoparticles. In a typical synthesis named in the text as “standard conditions”, a two-necked round-bottom flask was charged with 1.38 g (2.22 mmol) of iron stearate (9.47% Fe, Strem Chemicals), 1.254 g (4.44 mmol) of oleic acid (99%, Alfa-Aesar) as ligand, and 20 mL of octyl ether (97%, Fluka, boiling point (bp) of 287 °C) as solvent. The mixture was sonicated, stirred, and heated to 120 °C for 30 min without solvent/water cooler to dissolve the reactants until a clear solution was obtained and refluxed at this temperature to remove water residues. The solution was then heated to boiling temperature (∼287 °C) with a heating rate of 5 °C/min and refluxed at this temperature for 2 h under air. The resultant black solution was then cooled and the NPs were precipitated by the addition of an excess of acetone the first time and washed three times by a mixture of hexane/acetone (1/3) followed by centrifugation (14 000 rpm, 10 min). Finally, the iron oxide NPs were easily suspended in chloroform with a precise concentration (10 mg/mL) to prevent aggregation and under argon to preserve the surface state. From these standard conditions, the parameters which have been varied are the following: the reaction duration at the octyl ether boiling temperature, the oleic acid/iron stearate ratio, the nature of ligands, the introduction and the influence of a hold at 250 °C (corresponding to iron stearate decomposition and approached by thermogravimetric analysis (TGA)) to induce the nucleation at lower temperature and then favor the growth step, and the nature of solvent (hexadecene (bp 274 °C), octyl ether (bp 287 °C), octadecene (bp 318 °C), eicosene (bp 330 °C), hexadecanol (bp 344 °C), and docosene (bp 365 °C)). 2.2. Characterization of Nanoparticles. NPs were characterized by transmission electron microscopy (TEM) with a TOPCON 002B microscope operating at 200 kV (point resolution 0.18 nm). The size distribution of NPs was calculated from the size measurements of more than 300 nanoparticles using ImageJ software. The X-ray diffraction (XRD) pattern was recorded at room temperature with a Bruker D8 Advance diffractometer equipped with a monochromatic copper radiation source (Cu Kα = 0.154 056 nm) and a Sol-X detector in the 27−65° (2θ) range with a scan step of 0.03°. High purity silicon powder (a = 0.543 082 nm) was systematically used as an internal standard. Profile matching refinements were performed through the Fullprof program62 using Le Bail’s method63 with the modified Thompson−Cox−Hasting (TCH) pseudo-Voigt profile function. Infrared spectra were recorded between 4000 and 400 cm−1 with a Fourier transform infrared (FTIR) spectrometer, Digilab Excalibur FTS 3000 series. Samples were gently ground and diluted in nonabsorbent KBr matrixes. 57 Fe Mössbauer spectra were performed at 300 and 77 K using a conventional constant acceleration transmission spectrometer with a 57Co(Rh) source and a bath cryostat. Then, an in-field Mössbauer spectrum was collected at 11 K using a cryomagnetic device that generates an external magnetic field parallel to the γ-beam. The spectra were fitted by means of the MOSFIT program64 involving asymmetrical lines and lines with Lorentzian profiles, and an α-Fe foil was used as the calibration sample. The values of isomer shift are quoted relative to that of α-Fe at 300 K. Magnetic measurements were recorded on samples in the powder state with a superconducting quantum interference 3796



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Figure 1. Average diameter of NPs as a function of (a) the reflux time and (b) the boiling point of solvent.

nucleation at a lower temperature and with a low yield in nuclei to favor then grain growth. 3.1.1. Influence of the Reaction Time at the Reflux Temperature. To study the influence of the reaction time at the reflux temperature, NP synthesis was performed in standard conditions but up to a reaction time of 24 h at reflux temperature. Some drops of suspensions were taken at different reaction times from 0 h up to 24 h (Figure 1a). When the mixture reaches the boiling temperature (t = 0 h), NPs with a size centered around 4 nm are observed, although they are not homogeneous in size. The presence of NPs at t = 0 h and 287 °C shows that the nucleation step has certainly occurred before the reflux temperature is reached. With the oleate complex, which has been widely studied,34,59−61,65 the nucleation step was reported to occur from monomers resulting from the dissociation of iron oleate.34 Monomers, the minimum building units of iron oxide nanocrystals, are supposed to be intermediate species such as polyironoxo clusters, whose concentration increases as the reaction proceeds.65 The TGA curve of iron stearate is very similar to that of iron oleate with a first weight loss at about 250 °C, suggesting a similar decomposition process in the same temperature range (Figure 2 SI in the Supporting Information). After reflux for 1 h, the mean size is around 10 nm, continues to increase up to 6 h, and stabilizes around 14.5 nm (Figure 1a). Therefore, grain growth of NPs occurs mainly during this reflux time at 287 °C. That supports that germination would occur at around 250 °C, as already reported for iron oleate, but with a low yield in nuclei favoring therefore the growth step when the temperature further increases and/or during reflux time at higher temperatures. The occurrence of this grain growth during reflux time suggests that the monomer formation continues after the germination step and is time and certainly temperature dependent. Indeed, Kwon et al.65 observed the formation of monomers up to 310 °C. That was confirmed by performing syntheses in solvents with different and higher boiling temperatures. 3.1.2. Influence of the Nature of the Solvent. The synthesis has been conducted in solvents with different boiling temperatures and also in solvents of different natures, either apolar (alkenes) or polar (alcohol or ether) solvents. With alkene solvents, the size of NPs increases quite linearly with the boiling temperature of solvents (Figure 1b) as already and usually reported.35,59 The average size obtained with octadecene in these synthesis conditions deviates from the line and two NP populations are observed (Figure 3 SI in the Supporting Information): that particular point will be discussed

device (SQUID) magnetometer (Quantum Design MPMS-XL 5). Magnetization curves as a function of the temperature (M(T) curve) (zero field cooled (ZFC)/field cooled (FC) curves) were recorded as follows: the sample was introduced in the SQUID at room temperature and cooled to 5 K with no applied field after application of a careful degaussing procedure. A magnetic field of 7.5 mT was then applied, and the magnetization was recorded upon heating from 5 to 300 K (ZFC). The sample was then cooled to 4 K under the same applied field, and the magnetization was recorded upon heating from 4 to 300 K (FC). Saturation magnetization values were given as a function of the mass in iron oxide measured by TGA or elemental analysis. Thermogravimetric measurements were performed on dried powder samples from 20 to 600 °C at 5 °C/min under air by using a SETARAM TGA 92 apparatus. Magnetization curves as a function of the applied magnetic field (M(H) curve) have been measured at 300 and 5 K. The sample was introduced in the SQUID at high temperature, and cooled to 5 K with no applied field (ZFC sample) after application of a subsequent degaussing procedure. The magnetization was then measured at constant temperature by sweeping the magnetic field from +5 to −5 T, and then from −5 to +5 T (ZFC hysteresis loop). To evidence the exchange bias effect, FC (M(H)) curves have been further recorded after heating to 300 K and cooling to 5 K under a magnetic field of 5 T. The FC hysteresis loop was then measured by applying the same field sweep as for the ZFC loop. The coercive field (Hc) and the Mr/Ms ratio were measured from ZFC M(H) curves. The exchange bias field (He) was measured from FC M(H) curves. The AC measurements were performed to measure the susceptibility under an alternating magnetic field of 3.5 Oe at a frequency of 1 Hz from 5 to 400 K after the sample was cooled in zero field.

3. RESULTS AND DISCUSSION 3.1. Influences of Synthesis Parameters on Nanoparticle Size. In standard conditions with a ligand/precursor ratio of 2, octyl ether as solvent, and a reaction time of 2 h at the reflux temperature (boiling point of the solvent), NPs with a mean size of 11.2 ± 0.9 nm determined by TEM have been synthesized and are named NP11 (Figure 1 SI in the Supporting Information). From these standard conditions, the influences of different parameters on the NP size have been investigated such as the reaction duration at the boiling temperature, the nature of solvents, the oleic acid/iron stearate ratio, the nature of ligands, and the introduction and the influence of duration of a hold at 250 °C to induce the 3797



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Supporting Information). This would explain the increase in the mean NP size when the reaction temperature (e.g., the boiling point of alkene solvent) increases. The slight size deviation from linearity with octadecene may be related to the fact that the boiling point of octadecene corresponds to the nucleation temperature. Indeed, Kwon et al.65 observed that increasing the reaction time at 320 °C affects the size distribution and shape of the synthesized NPs. The formation of NPs is observed with hexadecene due to the hold for 2 h at this temperature which allows generating sufficient monomers for nucleation. With octyl ether, the nucleation occurs at lower temperature and Cmax is certainly smaller than with alkene solvents leading to nuclei with larger size. Then the growth step is mainly controlled by the hold at 287 °C. This is in agreement with the increase of the mean size from 4 to 14.5 nm when the hold time increases (Figure 1a). The small mean size reached with hexadecanol despite its high boiling point (344 °C) suggests also a nucleation step at higher temperature with a higher yield than with octyl ether or eicosene, which displays a close boiling temperature. The iron complex would be more stable in hexadecanol than in alkene than in octyl ether. Hexadecanol displays an alcohol end function which may interact/chelate with the iron complex and contribute to its stabilization, leading to its decomposition (and then germination) at higher temperature (coordinating solvent) with a high Cmax. Such solvent stabilization is lower with octyl ether due to the presence of the long alkyl chains hiding the ether function. Furthermore, the affinity of stearate chains for octyl ether should favor its decomposition by contrast with alkene solvents. In contrast, the lower affinity of oleic acid for alkene solvents should favor their interaction with the iron stearate, increasing its stability. 3.1.3. Influence of the Ligand/Precursor Molar Ratio. To study the influence of this parameter on the average size of NPs, the experiments were carried out either in octyl ether or in octadecene and the oleic acid/iron stearate ratio was varied (Figure 3). With octyl ether, the mean size of NPs increases and then decreases when the amount of ligand increases. By contrast, a slight decrease of the mean size is observed with octadecene when the ratio increases.

below. This increase of the mean size with the boiling temperature in alkene solvents confirms that the growth rate essentially depends on reaction temperature. However, in our experimental conditions, the sizes of NPs obtained with polar solvents deviate from this quite linear curve. In octyl ether, the mean NP size appears slightly larger and is smaller in hexadecanol. This evidences that the nature of the solvent acts also on the NP nucleation and growth steps. This may be related to a stability of the iron complex depending on the nature (functional group) of the solvent. Indeed, nucleation was observed to occur below 287 °C in octyl ether when Kwon et al. reported a nucleation above 320 °C in octadecene.65 Thus, considering these two different nucleation temperatures depending on the nature of the solvents, one can suppose that the solvent modifies the stability of the complex and then its decomposition kinetics. The LaMer model66 combined with the consideration of a nucleation temperature depending on the nature of the solvent may explain the observed nanosize differences. In the LaMer model (Figure 2), the energy barrier of the nucleation process

Figure 2. Variation of monomer concentration during germination and growth steps (Cs the solid solubility) according to LaMer and Dinegar.66

is much higher than that of the growth process. When the supersaturation of the monomer (C > Cmin) is high enough to overcome this energy barrier, burst nucleation will take place, resulting in the formation and accumulation of stable nuclei (Figure 2). The minimum radius of a stable nucleus that can grow spontaneously in the supersaturated solutions is inversely proportional to the supersaturation.65,67 At high supersaturation (high Cmax), a large amount of nuclei are generated but with small radius, while for small supersaturation (small Cmax) a lower amount of nuclei are formed but with larger radius. The higher decomposition temperature of the iron precursor in alkenes49 would be in agreement with a Cmax value higher than that with octyl ether (nucleation at 250 °C) leading to a larger amount of small nuclei with alkene solvents. This is supported by the observations of Kwon et al. that the nucleation occurs at 320 °C and suddenly.65 Then the growth of NPs would depend on the reaction time and/or temperature which control the formation of monomers necessary to this growth step. If the reaction temperature is higher than the nucleation temperature and/or if reaction time is longer, a growth step may occur. Indeed, the TGA curve shows that the iron precursor decomposition occurs in the wide range of temperature between 200 and 400 °C (Figure 2 SI in the

Figure 3. Average diameter of the NPs as a function of molar ratio ligand/precursor in octadecene and octyl ether as solvents. For comparison, data (■) from an earlier study with iron oleate precursor in octyl ether are also given.59 3798



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of amine favors the decomposition of iron stearate and thus the increase in the concentration of monomers, Cmax, leading to the formation of a larger amount of nuclei with a smaller size according to the LaMer theory. 3.1.5. Separation of Germination and Growth Steps. As the formation of monomer depends on temperature, the triggering of nucleation at a low temperature (leading to small Cmax) should lead to nuclei with a low yield and should thus favor the grain growth step in solvents with high boiling points such as octadecene, docosene, and eicosene. Therefore, the effect of a hold at 250 °C with different durations in order to try to better separate the germination and growth steps and enhance the growth step has been investigated (Figures 5 SI and 6 SI in the Supporting Information and Table 1). A hold

Despite the numerous studies on this parameter, the influence of the amount of ligand on the NP size is still not clear and appears quite complex:37,68 either an increase69−72 or the opposite trend73 is observed. It is generally reported that oleic acid molecules react with the precursor and induce the formation of more stable complexes than the initial complex.59,68 This stabilization should slow down the nucleation step. On the other hand, no NP formation was observed for a very high ratio (r = 10) and it is proposed that an excess of oleic acid will cover the growth sites, inhibiting the further growth of the nanoparticles.69,74 Bronstein et al.68 (as well as Salas et al.37) observed an evolution with iron oleate and oleic acid in eicosane similar to ours in octyl ether without proposing an explanation when Salas et al. reported an influence of oleic acid on nucleation and growth rates. Such discrepancies in reported results may originate from reaction parameters such as the reactant concentration, heating rate, refluxing time, and oleic acid/iron complex ratio range which are different as a function of the articles. However, they can be due to the two above-suggested competitive mechanisms:37 higher stabilization of the iron complex which decomposes at high temperature and also stronger stabilization of nuclei by the ligand with the increase in the amount of oleic acid affecting the grain growth step. Whatever the solvent, the increase in the amount of ligand should induce a higher stability of the iron precursor, which thus decomposes at higher temperature. This should lead to higher Cmax and then to a burst of nucleation with the formation of a large amount of nuclei. Thus the concentration in monomer available for the growth step decreases. As the nucleation occurs at higher temperature in octadecene than in octyl ether, this evolution should better fit with that is observed with octadecene. In addition, the formed nuclei should be strongly stabilized by oleic acid in agreement with the fact that oleic acid would have a higher affinity for the surface of nuclei than for the hydrophobic octadecene solvent in contrast to octyl ether. Thus when the ligand concentration increases, the amount of oleic acid at the surface and around nuclei becomes higher, which contributes to slowing down or inhibiting the growth process. In the case of octyl ether, due to the lower decomposition temperature of the iron complex, the nucleation rate is slowed down: Cmax is smaller leading to a smaller amount of nuclei by comparison with octadecene with a similar ratio. Thus more monomers are available for the further grain growth step and a grain growth is observed at first when the amount of oleic acid increases. However, when the amount of ligand further increases, there is competitively an increase in the stabilization of nuclei. These competitive effects can explain why, in a second step, a decrease of the NP size is observed when the amount of ligand further increases. 3.1.4. Influence of the Nature of the Ligands. Several ligands, listed in Table 1 SI in the Supporting Information, differing by the length of the alkyl chain or by the nature of the hydrophilic group (carboxylate or amine functions), have been tested in standard conditions (Figure 4 SI in the Supporting Information). There is no dependence of the average size on the type of carboxylate ligands (oleic acid, octanoic acid, lauric acid, or stearic acid). When ligands bear an amine function, NPs with smaller sizes are observed to form. The amine ligands have been reported to enhance the decomposition of precursor due to acido−basic reaction between amine ligands and carboxylate ligands of the iron complex.75 Thus the presence

Table 1. Average size with Standard Deviation of NPs Synthesized with Different Holding Times at 250 °C in Different Solvents hold at 250 °C 0 min

30 min

60 min

solvent

⟨D⟩ (nm)

σ (%)

⟨D⟩ (nm)

σ (%)

⟨D⟩ (nm)

σ (%)

octadecene eicosene docosene hexadecene octyl ether

7.6 15.5 22.7 5.2 11.2

13.8 11.1 8.6 7 9

8.5 16.7 20.3 5.4 10.8

8.6 10.7 9.4 7 6

14.5 16.9 28 − −

9.9 9.4 15.7 − −

duration of 30 min did not induce an increase of the average size whatever the solvent. Only with octadecene and docosene was a size increase observed after a hold duration for 60 min: NPs with a mean size of 28 nm were obtained in docosene after a hold for 60 min at 250 °C. 3.1.6. Influence of Water Traces. One has noticed that the mean size of NPs was varied depending on weather and in particular when the hygroscopy is high. NPs with a smaller size (≈8 nm instead of 11 nm) were obtained. Such an influence of external parameters such as water traces has already been reported.68,76 An experiment was carried out by suppressing the heating step at 120 °C for 30 min which aims at removing traces of water before starting the experiment. The sosynthesized NPs display an average size of 7.5 ± 0.6 nm (σ = 9%) (Figure 7 SI in the Supporting Information). The presence of water would stabilize the iron complex as hexadecanol which then decomposes at higher temperature, leading to the formation of a large amount of nuclei and thus limiting the grain growth. 3.1.7. Discussion. By tuning synthesis parameters of the thermal decomposition of iron stearate, NPs with an average size in the range 4−28 nm were synthesized without involving a seed-mediated growth step. The thermal decomposition method is interesting as it allows separating the germination and growth steps and thus obtaining NPs with narrow size distribution. The NP size evolution as a function of synthesis parameters is in agreement with the LaMer model and depends strongly on the stability of the iron complex. The higher is its stability, the higher is its decomposition temperature and then the concentration in monomer (Cmax). High Cmax induces the formation of a large amount of nuclei, whose grain growth depends on the final reaction temperature and reaction time. The stability of the precursor has been shown to depend on the nature of the solvent, the amount of ligands, the nature of 3799



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the Supporting Information) exhibit the characteristic peaks of the AB2O4 spinel structure (space group Fd3m, number 227). Indeed, the magnetite phase and its corresponding oxidized phase, maghemite, have a similar structure, and they display thus similar XRD patterns. The refinement of the XRD pattern allowed calculating the mean values of lattice parameters of all NPs and to compare them to those of stoichiometric magnetite Fe3O4 (0.8396 nm, JCPDS file 19-629) and maghemite γ-Fe2O3 (0.8346 nm, JCPDS file 39-1346) phases (Figure 6a). Lattice parameters and crystallite sizes of all synthesized NPs are given in Table 4 SI in the Supporting Information. The lattice parameters calculated for different batches with the same NP size are very similar, suggesting a similar composition and confirming a good reproducibility of the synthesis. For the smallest NPs (NP5), the values are very close to those of maghemite, and when the size increases, the lattice parameters get closer to those of stoichiometric magnetite. It is now well-established that the Fe2+ ions located at the surface of magnetite particles are very sensitive to oxidation, and when the size decreases, their sensitivity to oxidation increases.34,77,78 Similar results were noticed with NPs synthesized by coprecipitation.77 For sizes smaller than 20 nm, the composition of NPs was reported to be close to that of maghemite, and for sizes higher than 20 nm, NPs were found to display a core−shell structure with a core of stoichiometric magnetite surrounded by an oxidized layer. Considering these first results, the sensitivity to oxidation also exists in the NPs synthesized by thermal decomposition. However, the lattice parameter becomes very close to that of magnetite for sizes higher than 11 nm, which suggests that the core−shell structure should be observed when the size is higher than 11 nm. Thus the oleic acid coating and the organic solvent should prevent from oxidation NPs synthesized by thermal decomposition, by comparison with those synthesized by coprecipitation which are “naked” and in water. Another characterization method allowing discrimination between the magnetite and maghemite phases is infrared spectroscopy as both phases display characteristic Fe−O IR bands. The IR spectrum of NP11 between 4000 and 400 cm−1 in Figure 7a is representative of the five batches of NPs. Three distinct zones are present: the bands between 3000 and 2800 cm−1 are attributed to alkyl chains at the surface of NPs, those between 1800 and 900 cm−1 are attributed to the asymmetric and symmetric COO− bands of oleate, and those between 800 and 400 cm−1 are attributed to Fe−O bands of iron oxide. Figure 7b compares the Fe−O IR bands of NP5, NP8, NP11, NP15, and NP20 to those of magnetite and maghemite (maghemite was obtained by subsequent annealing of magnetite NPs of 40 nm synthesized by coprecipitation).78,80 The magnetite displays a single broad band located at 580−590 cm−1 and a shoulder at about 700 cm−1 attributed to surface oxidation, while the maghemite phase displays several bands between 800 and 400 cm−1 whose number and resolutions depend on the structural order of vacancies in maghemite.79,80 The IR spectrum of the maghemite phase in Figure 7b is characteristic of a partially ordered maghemite. The IR spectra of NP5, NP8, NP11, NP15, and NP20 exhibit a broad Fe−O band intermediate between those of magnetite and maghemite, which confirms an intermediate composition of these NPs. In a qualitative study of the proportion of magnetite (and maghemite) in synthesized NPs, the position of the maximum of the band has been compared with those of magnetite and maghemite (Figure 6b). For all NPs, the band

ligands, and traces of water which affect the nucleation and growth step as a function of the synthesis conditions. Indeed, the growth of NPs is favored by the continuous decomposition of the iron complex with the increase in the temperature (in the range 200−400 °C from TGA). This temperature-dependent growth rate is supported by the increase of the NP size with the reaction temperature (e.g., the boiling point of solvents) in particular with alkene solvents. 3.2. Microstructural Characterization as a Function of Size. Five batches of NPs with sizes of 5, 8, 11, 15, and 20 nm (called NP5, NP8, NP11, NP15, and NP20, respectively) were selected to study the influence of the NP size on the magnetic and structural properties. The synthesis conditions, which are given in Table 2 SI in the Supporting Information, have been performed several times to demonstrate the reproducibility of the method (Table 2). Figure 4 shows TEM images of NP5, Table 2. Average Diameter Measured by TEM and Standard Size Deviation of the Five Batches of NPs Synthesized Several Times NPs

D TEM (nm)

σ (%)

NP5

5.2 ± 0.3 5.4 ± 0.4 5.5 ± 0.6 5.4 ± 0.5 8.1 ± 0.6 11.2 ± 0.9 12.1 ± 1.1 12.6 ± 1.8 10.5 ± 0.6 10.4 ± 0.6 15.8 ± 1.1 15.5 ± 1.8 14.5 ± 1.2 17.8 ± 1 20.3 ± 1.9 20.5 ± 2.2 20.5 ± 1.6 22.7 ± 1.9

7 7 11 15 7 8 9 14 6 7 7 12 8 5 9 10 8 8

NP8 NP11

NP15

NP20

DmeanTEM (nm)

5.4 8.1

11.3

15.9

21

NP8, NP15, and NP20 and their size distributions after washing and without performing any size selection process. All NPs display a spherical morphology, except for NP20, which are slightly faceted. The synthesis of the latter was carried out with an excess of oleic acid to try to avoid oriented grain growth, but faceting still maintains. For all NPs, the standard deviation is between 7 and 9%, confirming the good monodispersity of NPs. The slight variation in average size and NP size distribution for a given batch is mainly due to variation in hygroscopic conditions induced by the weather. The electronic diffraction patterns of NPs display rings which confirm the formation of well-crystallized NPs. They were indexed with hkl reflections of iron oxide spinel structure (Figure 8 SI in the Supporting Information and Table 3 SI in the Supporting Information). The high-resolution TEM (HRTEM) images (Figure 5) show different crystalline plans depending on the orientation of NPs with the direction of the electron beam. The following dhkl have been mainly measured: 4.79, 2.35, and 1.98 Å corresponding respectively to the (111), (311), and (422) plans of the spinel structure. The composition of NPs was further analyzed by X-ray diffraction (XRD). The XRD patterns of all NPs (Figure 9 SI in 3800



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Figure 4. TEM images of nanoparticles: (a) NP5, (b) NP8, (c) NP15, and (d) NP20. Insets: size distributions.

Figure 5. HR-TEM images of (a) NP5, (b) NP11, (c) NP15, and (d) NP20.

Figure 6. (a) Lattice parameter as a function of NP diameter (for each size, values are given for synthesis in the same conditions), compared with those of the stoichiometric magnetite (red line) and of maghemite (green line) and (b) position of the most intense IR band for NP5, NP8, NP11, NP15, NP20, magnetite (red line) and maghemite (green line) used as references.

position is intermediate between that characteristic of maghemite (638 cm−1) and magnetite (571 cm−1). In addition, when the size of NPs increases, the position of the band gets closer to that of stoichiometric magnetite. These qualitative results confirm the evolution of the composition according to XRD. Nevertheless, the most adapted method to estimate the mean composition of NPs is 57Fe Mössbauer spectrometry by determining the oxidation state of iron species.77,80,81

The Mössbauer spectrum at 300 K exhibits a hyperfine structure with very broadened and overlapped lines consistent with the presence of very fast superparamagnetic relaxation effects. That explains why only the Mössbauer spectra obtained at 77 K have been considered (as illustrated in Figure 10 SI in the Supporting Information). However, the Mösbauer spectrum of NP5 at 77 K exhibits always broad lines with a broad centered doublet due to superparamagnetic relaxation effects because the blocking temperature of these NPs is lower than 77 3801



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Figure 7. (a) IR spectrum of NP11 between 4000 and 400 cm−1, (b) IR spectra of NP5, NP8, NP11, NP15, and NP20, and reference spectra of magnetite and maghemite between 850 and 450 cm−1.

Table 3. Mean Values of Hyperfine Parameters Refined from Mössbauer Spectra Recorded on NPs with Different Sizesa NP size (nm)

T (K)

5

300 77 300 77 77 300 77 77 300 77 300 77

8 11b 12 15 16b 20

IS ± 0.01 (mm·s−1)

QS ± 0.01 (mm·s−1)

HF ± 2.0 (T)

composition ± 0.01

Fe3O4 ± 2 (% atom Fe)

emaghemite ± 0.2 (nm)

0.44

−0.02

25.8

Fe2.67O4

0

2.5

0.45 0.47 0.42 0.50 0.54

−0.00 0.00 0.00 0.00 −0.01

40.4 48.9 − 45.4 48.5

Fe2.68O4 Fe2.72O4 Fe2.79O4 Fe2.77O4 Fe2.83O4

5 15

2.5 2.6

30 50

2.0 1.6

0.50

0.00

45.4

Fe2.82O4

45b

1.9

0.55

0.00

49.7

Fe2.85O4

55

1.8

IS, isomer shift; QS, quadrupolar shift; HF, hyperfine field. bSamples with Mössbauer measurements made more than 1 month after their synthesis, so we think that they are more oxidized.

a

been calculated assuming the presence of spherical particles and the same Lamb−Mössbauer factor f for both phases. Thus, the thickness of the maghemite layer on the surface of NPs has been estimated. Such a model allows us to compare the mean composition of NPs as a function of the size, but it is preferable to consider a gradient of oxidation from the outside to the inside of the nanoparticle.77,80,81 The data given in Table 3 confirm the XRD and IR results that when the size increases, the amount in magnetite increases. Some inconsistencies may be noted for NP16 and NP11 which have been analyzed more than 1 month after the synthesis, which results in a higher oxidation state. Indeed, the composition of NP11 has been determined 1 month after the synthesis while that of NP12 just after the synthesis and similar oxidation with time has been observed. Furthermore, Mössbauer spectra have been recorded several months after the synthesis and the composition was found to evolve no more. From our earlier results and from the current results, it appears that, for NPs with sizes in the range 8−12 nm, the oxidation slows down strongly when the thickness of the oxidized layer reaches 2.5 nm. In contrast, the oxidation state of NPs with sizes higher than 12 nm is lower: the thickness of the maghemite layer of these NPs decreases below 2 nm. These results confirm again that the smaller the size, the higher the sensitivity of NPs to oxidation. To conclude, all the characterizations confirm that NPs with sizes smaller than 12 nm are oxidized while those with sizes larger than 12 nm display a higher amount of magnetite consistent with a core of magnetite surrounded by an oxidized layer, the thickness of which decreases as the size increases. If

K (see section 3.3). In contrast, one observes a pure magnetic structure on the spectra of other NPs at 77 K (Figure 10 SI (a) in the Supporting Information) with weakly broadened and asymmetric lines originating from the occurrence of remaining superparamagnetic relaxation phenomena probably due to the smaller size NPs.81 One can notice that as the size increases this line broadening decreases as the relaxation effects decrease. It is important to emphasize that the comparison of the spectra at 77 K with those obtained with γ-Fe2O3−Fe3O4 solid solutions with similar stoichiometry deviations82 shows that NPs consist not in such a solid solution. Indeed, the Mössbauer spectra are different at 77 K, as the spectrum is deconvoluted with only two sextets for the solid solution. The spectra can be well described using at least five components that are attributed to Fe3+ located in tetrahedral sites, Fe3+ in octahedral sites, Fe with intermediate valence states, and Fe2+ in octahedral sites. From our earlier results on the determination of the composition of iron oxide NPs from Mössbauer spectrometry, these spectra were in fact described by means of distribution of a static magnetic hyperfine field slightly correlated to that of isomer shift.77,80,81 By interpolating between the average isomer shift values of different maghemite and magnetite phases obtained from a large amount of iron oxide NPs synthesized by different methods, the mean value of the isomer shift allowed estimation of the mean composition of the different NPs.77,80,81 The composition of NPs is given in Table 3. To schematize the oxidation rate of NPs, a core−shell structure, which consists of a core of magnetite with an oxidized shell layer being maghemite, has been considered. The composition of NPs in maghemite and magnetite has thus 3802



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Figure 8. (a) Magnetization as a function of temperature, ZFC−FC curves of five batches of NPs measured under a permanent field of 75 Oe. The magnetization is normalized to the maximum of the ZFC curve. (b) Maximum temperature of ZFC curve as a function of NP diameter.

Table 4. Maximum Temperatures of ZFC (TmaxZFC), of the Real Part of Susceptibility χ′ (Tmaxχ′) , and of the Imaginary Part of Susceptibility χ″ (Tmaxχ″) Measured at 1 Hz for the Five Batches of NPs

we compare the composition of NPs synthesized by thermal decomposition to that of NPs synthesized by coprecipitation, NPs synthesized by thermal decomposition seem less oxidized for similar sizes. Such resistance to oxidation can be due to the coating with oleic acid and their dispersion in organic solvent. 3.3. Magnetic Characterizations as a Function of Size. 3.3.1. Magnetic Measurements. The magnetization of NPs as function of the temperature and under a static field of 75 Oe is shown in Figure 11 SI in the Supporting Information. When the NP size increases, the maximum of the ZFC curve (usually assimilated as the blocking temperature) is shifted to higher temperatures as is often reported (Figure 8a).83−85 The increase in the blocking temperature is quasi-linear as a function of the size of the nanoparticles (Figure 8b), which can be well explained by an increase in magnetocrystalline energy and the need of more important thermal energy to unblock the magnetic moment. However, one cannot exclude a contribution of dipolar interactions to the observed temperature as NPs cannot be considered as isolated (they are only coated by oleic acid). Nevertheless, this evolution was further confirmed by ac measurements. The real part χ′ and imaginary part χ″ of susceptibility of NPs as a function of the temperature and under an alternating field of 3.5 Oe at varying frequencies between 0.3 and 300 Hz for NP5, and at frequency of 1 Hz for different sizes, are shown in Figure 12 SI in the Supporting Information. The maximum of the imaginary part of the susceptibility as a function of the temperature gives the blocking temperature of NPs. Most NPs present a single peak except NP15 and NP20, which display several peaks certainly due to a distribution of dipolar interactions in these powders (some nanoparticles may have diffused into eicosane, which was used to seal the SQUID capsules containing NPs). Only the most intense peak of this curve was considered. The maximum temperatures of the ZFC curve (TmaxZFC), of the real part of susceptibility χ′ (Tmaxχ′), and of the imaginary part of susceptibility χ″ (Tmaxχ″) are summarized in Table 4. They increase with the size of NPs as expected and are in agreement with reported values for NPs with similar sizes.86,87 Magnetization curves as a function of an applied field at 300 and 5 K are presented in Figure 9. The curves at 300 K (Figure 9a) display an hysteretic behavior consistent with superparamagnetic NPs above their blocking temperature, whereas curves at 5 K present hysteresis loops characteristic of magnetic NPs below the blocking temperature (Figure 9b). The values of the coercive field Hc, the Mr/Ms ratio, and the saturation magnetization Ms are given in Table 5 SI in the Supporting

NP5 NP8 NP11 NP15 NP20

TmaxZFC (K)

Tmaxχ′ (K)

Tmaxχ″ (K)

25 75 160 202 270

36 ± 1.7 99 ± 0.4 180 ± 0.5 220 ± 0.9 264 ± 0.8

21 ± 0.3 62 ± 0.4 142 ± 1 150 ± 0.7 248 ± 1.2

Information and depicted in Figure 10 according to the diameter of NPs. The coercive field increases up to a size of 10 nm and then decreases slightly when the size increases. Such behavior has already been observed34,85,88,89 and would be due to the transition from the monodomain to the multidomain regime. The Mr/Ms ratio at 5 K is observed here to increase with the NP size, and this is certainly due to an increase in dipolar interactions between NPs displaying higher magnetization.85,90 Theoretically, in a set of isolated nanoparticles (without interaction between them) with easy axes of magnetization randomly oriented, the residual magnetization is equal to half the saturation magnetization (Mr/Ms = 0.5).91 The saturation magnetization increases as the NP size increases, in agreement with reported results.34,38,49,92−94 This evolution may also be correlated to the increase in the fraction of magnetite in NPs. However, whatever the size, their values are lower than that of maghemite, the oxidized phase of magnetite. These lower values are often being ascribed to defects and/or spin canting within the volume and/or at the surface of the particles.39−57,88 These noncollinear spin arrangements are known to be related to local changes of magnetic anisotropy at the surface or in the volume.39−58,92 The origin of misaligned spins at the particle surface is the presence of broken exchange bonds, which result from a reduced coordination of Fe surface ions and leads to magnetic frustration.39−58,92 However, canted spins can also be present in the volume of the particles because of the presence of Fe vacancies.39,58 Morales et al.39 showed that the level of vacancy ordering has also a large influence on spin canting. Maghemite NPs with random distribution of Fe vacancies display mean canting angles of the magnetic moments for Fe atoms in tetrahedral sites and in octahedral sites which are almost twice those of particles that present partial or full vacancy ordering.39,58,92 Recently, Vichery et al.58 reported that, for 7 nm NPs, defect annealing and vacancy ordering have a weak impact on the saturation magnetization values of NPs and the 3803



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Figure 9. Magnetization curves of NP5, NP8, NP11, NP15, and NP20 as a function of an applied magnetic field at (a) 300 and (b) 5 K.

Figure 10. (a) Coercive field Hc, (b) Mr/Ms ratio, (c) saturation magnetization Ms measured at 5 K, and (d) % Fe3O4 calculated from Mössbauer spectrometry as a function of NP diameter.

surface effects are mainly responsible for the lower Ms; however, volume defects are expected for larger sizes. Depending on the synthesis method, the magnetic properties are often quite different even for similar sizes. Indeed, they depend strongly on intrinsic phenomena but also on extrinsic effects based not only on geometric size distribution but also on nonuniform chemical composition and lack of crystallinity due to extended defects or local atomic rearrangements.49,50,58 3.3.2. Magnetic and Mössbauer Measurements under an Applied Magnetic Field. The presence of spin canting in NPs as a function of the size was thus further investigated by performing Mö ssbauer measurements under an applied magnetic field and magnetization measurements after cooling under a magnetic field (FC curves). To evaluate possible exchange interactions between the core of NPs and a canted layer at the surface, magnetization measurements were performed at temperatures between 5 and 50 K after cooling the powders under a field of 5 T (Figure 13

SI in the Supporting Information). Surface defects due to symmetry breaking of atoms at the NP surface and/or a disordered layer resulting from heterogeneous surface oxidation can lead to a destabilization of the ferrimagnetic order of spins at the surface, which leads to a core−shell system similar to that of metal NPs oxidized at the surface. In the case of ferromagnetic or ferrimagnetic NPs and at low temperatures, the disordered spins at the surface can lead to the formation of a spin glass like layer which can be assimilated to an antiferromagnetic layer. Cooling with applying an external field will orient the spins in its direction, except those canted, which leads to a shift of the hysteresis loop to the negative fields, allowing calculation of an exchange field He. A shift of the magnetization curves toward negative magnetic field values is observed after measurements after cooling the powders under an applied field, which shows the existence of exchange interaction in NPs (Figure 13 SI in the Supporting Information). The NPs would be constituted of a magnetically 3804



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Table 5. Values of Coercive Field Hc (Oe) and Exchange Field He (Oe) Calculated from FC Magnetization Curves at 5 K, % Magnetite, and Values of Saturation Magnetization Given as a Function of NP Diameter NP5

NP8

NP11

T (K)

Hc

He

Hc

He

Hc

5 10 20 30 40 50

185 37 11 18.5 24 27

70 42 19 8.5 5 3

354 216 65 18 0 8

101 80 50 27 14 8

356.5 301.5 184.5 103 56 28

blocking temp (K) % Fe3O4 Ms (emu/g)

NP15 He

NP5

NP8

81.5 65.5 43.5 33 24 17 NP11

21 ± 0.3 0 51 ± 5

62 ± 0.4 5±2 58 ± 5

142 ± 1 15 ± 2 61 ± 5

Hc 219 176 114 68.5 29 6.5

NP20 He

17 15 10 8.5 7 6.5 NP15 150 ± 0.7 50 ± 2 71 ± 5

Hc

He

325 297.5 227 176 135.5 107.5

33 29.5 19 17 12.5 10.5 NP20 248 ± 1.2 55 ± 2 82 ± 5

Figure 11. (a) Coercive field Hc and (b) exchange field He values for NP5, NP8, NP11, NP15, and NP20 between 5 and 50 K after cooling under an applied field of 5 T.

ordered core surrounded by a magnetically disorderd shell at the surface. The values of Hc and He as a function of the NPs size and the temperature are given in Table 5 and presented in Figure 11. Both fields decrease when the temperature increases. The decrease of He with temperature is ascribed to a decrease of the exchange coupling up to its disappearance at the blocking temperature. The He values are much smaller than those reported for FeO@Fe3O4 core−shell NPs displaying exchange bias, and this may support that no FeO is present in NPs. This point is fairly consistent with Mössbauer data; indeed, the isomer shift values are typical of ferric species, excluding thus the significant presence of ferrous ones located in FeO. The evolution of Hc is related to the decrease of the anisotropy energy of NPs with the increase of the thermal energy. The evolutions of both the coercive field Hc and the exchange coupling field He measured at 5 K after cooling under an applied field of 5 T, as a function of the size of NPs, are illustrated in Figure 12. The He value is higher for NPs with small size, and then it decreases for larger NPs. However, a lower He value is observed for NP5 than for NP8 and NP11; this suggests that the exchange interactions are lower in NP5 (Figures 11b and 12). For larger sizes (size higher than 11 nm), the He values became lower and negligible. The observed evolution may be partly related to the composition of NPs as a function of the size. The composition of NP5 is very close to that of the oxidized form of magnetitemaghemiteand is thus more homogeneous than NP8 and NP11 that may be considered as NPs displaying a high oxidation gradient, which should favor the presence of defects and thus spin canting in volume in addition to surface spin canting, resulting from

Figure 12. Values for coercive field Hc (black curve) and exchange field He (red curve) measured at 5 K as a function of NP diameter.

symmetry breaking. For larger sizes, the oxidation layer is thinner; surface spin canting may remain but, according to the lower volume of the shell, volume canting should not exist. The presence of spin canting and the oxidation state of NPs were further investigated by recording Mössbauer spectra at 10 K under an applied field of 8 T applied parallel to the γ-beam (Figure 13). In this part, additional NPs with a size of 9 nm (NP9) were characterized to investigate NPs with “intermediate” sizes. The presence and the intensity of the second and fifth lines of the magnetic sextet give clear evidence for 3805



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NP5 and NP8, the mean value of the angle is higher and in the range 30−37°. By contrast, the intermediate lines characteristic of the spin canting do not occur for NP15 (see particularly in the 4−6 mm/s range) but the characteristics lines of Fe2+ are observed (see the range −5 to −7 mm/s): such a feature is consistent with the higher amount of magnetite in these NPs. The spin canting in these NPs is negligible. From the mean values of angles, the thickness of the spin canted layer may be calculated (e = r/2 sin 2θ) where r is the mean radius of NPs and θ is the mean canted angle. The thickness of the spin canted layer is lower with NP5 (e ≈ 0.4 ± 0.1 nm, corresponding to about one atomic layer) than with NP8 and NP9 (e ≈ 0.7 ± 0.1 nm, about two atomic layers). Furthermore, the ratio of iron in tetrahedral/octahedral sites of these last NPs shows a slight excess of iron in tetrahedral sites which leads to cationic inversion for these NPs. This induces thus a volumetric noncollinear magnetic structure, in agreement with the results deduced from magnetic measurements detailed above that the spin canting should originate from the presence of an oxidation gradient in these NPs. The weak excess of iron in Td sites would be due to the oxidation state of the NPs and would be responsible in part for the observed spin canting. For NP11, a similar spin canted layer thickness (e ≈ 0.77 ± 0.10 nm, two atomic layers) is noticed but the ratio of iron in tetrahedral/octahedral sites is in agreement with that of maghemite, which is the major phase of these NPs. This thickness smaller than 0.03 nm for NP16 is thus negligible and the ratio of iron in tetrahedral/octahedral sites is in agreement with the composition of these NPs. Quite a good correlation is noticed between the He values and the thickness of the spin canted layer. However, for NPs with a composition close to that of maghemite as NP5 and with a composition closer to that of magnetite as NP12 and NP15, the proportion of iron on the different sites is close to that expected. In contrast, with NPs with an intermediate composition but close to that of maghemite as NP8 and NP9, the proportion of iron at different sites deviated and would be responsible for a spin canting in volume. This may be related to defects induced by the oxidation process which has been demonstrated to induce the presence of antiphase boundaries in oxidized FeO NPs.96 These defects are reported to be also responsible for the lower saturation magnetization of NPs. All these results suggest that, for NPs with small sizes which are quite fully oxidized, they display a spin canting due to surface effects. For NPs with larger sizes which are constituted of a core of magnetite and an oxidized shell, they display also a surface spin canting whose contribution decreases as the size

Figure 13. Mössbauer spectra of NP5, NP8, NP9, NP12, and NP15 at 10 K and under an external magnetic field of 8 T applied parallel to the γ-beam.

misaligned Fe magnetic moments with respect to the applied field and thus for the presence of spin canting. The average canting angles can be derived independently for both Fe moments from the ratios of the line intensities of A2,5 to A1,6 (external lines) according to the following relation: β = arcsin[(3A2,5/2)A1,6/(1 + 3A2,5/4A1,6)]1/2.95 The mean angle values for both tetrahedral and octahedral Fe magnetic moments and the average thickness of canted layer are given in Table 6. For NP9 and NP11, the canting is clearly evident in Mössbauer spectra and the mean angle of iron moments is about 30−35° for both octahedral and tetrahedral sites. For

Table 6. Average Angle of Canting, Thickness of the Canted Layer, and Proportions of Tetraedral/Octaedral Sites from Mössbauer Spectra under an Applied Field average canting angle (deg) ecanted layer (nm) tetrahedral/octahedral site proportions % magnetite emaghemite % canted vol Ms (emu/g) He (Oe) a

NP5

NP8

NP9

NP12

NP16

30−37 0.4 ± 0.1 37:63 0 2.5 22 51 ± 5 70

34−37 0.7 ± 0.1 39:61 5 2.5 24 58 ± 5 101

30−35 0.7 ± 0.1 41:59

30−35 0.8 ± 0.1 38:62 30 2 18 55 ± 5 a

∼0

Magnetic Iron Oxide Nanoparticles: Reproducible Tuning of the Size

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