Low Oxidation State and Enhanced Magnetic Properties Induced by

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Low Oxidation State and Enhanced Magnetic Properties Induced by Raspberry Shaped Nanostructures of Iron Oxide Olivier Gerber, Benoit P. Pichon, Corinne Ulhaq-Bouillet, Jean-Marc Greneche, Christophe Lefevre, Ileana Florea, Ovidiu Ersen, Dominique Begin, Sebastien Lemonnier, Elodie Barraud, and Sylvie Begin-Colin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08164 • Publication Date (Web): 05 Oct 2015 Downloaded from http://pubs.acs.org on October 10, 2015

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The Journal of Physical Chemistry

Low Oxidation State and Enhanced Magnetic Properties Induced by Raspberry Shaped Nanostructures of Iron Oxide

Olivier Gerber,1,2 Benoit P. Pichon,1,* Corinne Ulhaq,1 Jean-Marc Grenèche,3 Christophe Lefevre,1 Ileana Florea,1,5 Ovidiu Ersen,1 Dominique Begin,4 Sebastien Lemonnier,2 Elodie Barraud,2,* Sylvie Begin-Colin1,* 1

Institut de Physique et Chimie des Matériaux de Strasbourg, 23 rue du Loess, BP 43, 67037, Strasbourg

2

Institut franco-allemand de recherches de Saint-Louis, 5 rue du Général Cassagnou, 68300, Saint-Louis

3

Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283, Université du Maine ,72085 Le Mans Cedex 9 France

4

Laboratoire des Matériaux, Surfaces et Procédés pour la Catalyse, 25 rue Becquerel 67087 Strasbourg cedex

5 Laboratoire de Physique des Interfaces et des Couches Minces - LPICM, École polytechnique/CNRS, Route de Saclay, 91128 Palaiseau Cedex, France

Abstract

Nanostructures with controlled size, morphology and composition represent a main challenge in materials science because controlling these parameters is fundamental to optimize the subsequent functional properties. Agregated nanostructures, combining both individual and collective properties of nanocrystals, offer interesting perspectives to design new magnetic nanomaterials. In that context, original porous raspberry shaped nanostructures consisting of oriented aggregates of ferrite nanocrystals have been synthesized by an one-pot polyol solvothermal method. Synthesis conditions have been optimized to obtain nanostructures featured by similar sizes of about 250 nm and nanocrystal sizes modulated from 5 to 60 nm, leading to porous structures with tunable specific surface area. Structural and magnetic studies of nanostructures as a function of the nanocrystal size evidenced their low oxidation state and enhanced magnetic properties. Indeed the oriented aggregation of nanocrystals leads to high interface between nanograins reducing significantly their surface oxidation and enhancing their saturation magnetization in comparison to individual nanoparticles of similar sizes. Magnetic moments of each grains are also consequently strongly coupled by dipolar interactions which led to super spin glass effects.

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Introduction The explosive growth of nanotechnology has brought challenging innovations in the synthesis of magnetic nanoparticles (NPs) which are able to revolutionize their related application fields such as biomedicine,1, 2 mass storage media,3 energy storage and photoelectrochemical water splitting.4 Indeed the current progress in nanoparticle synthesis allows now controlling different sizes with narrow distribution, shapes and chemical compositions which result in the fine tuning of magnetic properties. In this field, iron oxide is one of the most studied component because it combines low cost, low cytotoxicity, high magnetization saturation and good magnetic anisotropy.4, 5 For instance, cubic iron oxide NPs are much more efficient than spheric ones for cancer treatment by hyperthermia thanks to their higher saturation magnetization and anisotropy.6 Bimagnetic core-shell nanoparticles featured by exchange bias coupling are also very promising to shift the superparamagnetic limit toward room temperature and thus to envision the development of higher density magnetic data storage media.7 Besides these NPs with magnetic properties tuned by their size, composition and shapes, a new class of materials is emerging: magnetic porous nanostructures. Owing to their surface specific area and magnetic properties, mesoporous nanostructures became very attractive for a large panel of applications ranging from catalysts for various reactions,8-10 drug/gene/RNA delivery carriers,2, 11, 12 pollutant recovering13, 14 and anode material for supercapacitors or lithium-ion batteries.15-17 Therefore, the synthesis of such particles of iron oxide has generated a tremendous interest during the last ten years since the report of Deng et al.18 Although several synthesis methods have been developed to produce porous nanostructures, the polyol method has been demonstrated to be the easiest to proceed and extremely reliable to synthesize spherical nanostructures which consist in oriented aggregates of ferrite nanocrystals (nanocystals with common crystallographic orientations directly combined together to form larger ones)19,20-27 Agregated nanostructures with sizes ranging from tens of nanometers to 800 nm and grain size between 5 to 20 nm have been synthesized by tuning condition parameters such as solvents,21, 26 reaction time18, pH25 or stabilizing agent.20, 23, 25 Another advantage of these nanostructures is that the assembly of nanograins results in enhanced magnetic properties and in porous structures with high specific surface areas20, 27 which have been extended more recently to hollow structures.20, 26, 28 Up to now, researches mostly aimed at controlling the synthesis of such nanostructures with different particle and nanograin sizes while the origin of their magnetic properties has been scarcely investigated. Indeed rather high values of saturation magnetization (MS) in comparison to individual nanoparticles are simply explained by the aggregation of nanocrystals with a common crystallographic orientation.20-27 More recently, the magnetic dynamics have been investigated by considering agregated nanostructures with different particle sizes while nanocrystal size was similar.29 The increasing number of 13 nm sized nanograins has been demonstrated to enhance saturation magnetization and collective properties through dipolar interactions. In this context, as oriented assemblies of nanocrystals represents a very interesting approach to enhance the magnetic properties of nanostructured materials, we report on the synthesis of iron oxide raspberry-shaped nanostructures (RSN) with the aim to get a better insight in their magnetic properties. RSNs have been synthesized by using a modified polyol solvothermal method. Experimental conditions have been accurately optimized in order to synthesize RSN with similar sizes 2 ACS Paragon Plus Environment

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while their grain size has been modulated. Such a control in the RSNs structure resulted in the systematic and detailed study of their structural and magnetic properties.

Experimental section Materials Hexahydrate iron(III) chloride (97%), urea (99.3%) and ethylene glycol (99%) were purchased from Alfa Aesar. Succinic acid (≥99.5%) was purchased from Sigma-Aldrich. Synthesis of raspberry nanostructures (RSN) In a typical synthesis FeCl3, 6H2O (30 mmol), succinic acid (10 mmol) and urea (300 mmol) were completely dissolved in ethylene glycol (300 mL) by vigorous mechanical stirring and sonication. The solution was sealed in a Teflon lined stainless steel autoclave (600 mL capacity), slowly heated at 200°C at 1.5°C/min and kept at this temperature for several hours (7 h for RSN5 and 13 h for RSN25). The autoclave was cooled down to room temperature afterwards by water circulation during 1 h. The sediments were separated by centrifugation and washed 3 times with ethanol and 3 times with deionized water to eliminate organic and inorganic impurities. Finally, the precipitate was frozen and dried to remove water giving rise to a black powder (1 500 mg). RSN60 were obtained in the same conditions after 7 h of reaction and by increasing the amount of iron chloride 4 times. The synthesis was also scaled up and performed in larger autoclave (2 L) to produce up to 20 g of material. Characterization techniques The samples were characterized by X-Ray diffraction (XRD) using a Bruker D8 Advance equipped with a monochromatic copper radiation source, (Kα = 0.154056) and a Sol-X detector. High purity silicon powder (a = 0.543082 nm) was systematically used as an internal standard to reset the zero 2θ to calculate with accuracy the lattice parameters and crystal domain size of RSNs. Profile matching refinements were performed through the Fullprof program30 using Le Bail’s method with the modified Thompson-Cox-Hasting (TCH) pseudo-Voigt profile function.31 Scanning electron microscopy (SEM) has been performed on a JEOL 6700F with a 2 nm point resolution. Transmission electron microscopy (TEM) has been performed on a JEOL ARM200F microscope operating at 200 kV and on a JEOL 2100F electron microscope working at 200 kV equipped with a GIF Tridiem spectrometer. The 3D structure of RSN5 has been revealed by electron tomography by considering several objects of interest. Thermogravimetric (TG) and thermodifferential (TD) analysis were performed in the 20 to 500 °C range under nitrogen flow with a heating rate of 5 °C/min by using a Texas Instruments SDT Q600. Dried powders were placed in a platinum crucible. Specific surface areas of the different samples were determined by N2 adsorption–desorption measurements at 77 K by using a Micromeritics TriStar 3000 apparatus. Before the measurements, samples were outgassed at 150 °C overnight in order to desorb impurities or moisture from their surface. Elemental analysis has been performed by the analytic central service of the CNRS UMR7504 at La Vernaison by using a QqToF instrument with a precision of 5 ppm. FTIR spectroscopy was performed using a Digilab Excalibur 3000



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spectrophotometer (CsI beamsplitter) in the wavenumber range 4000−400 cm−1 on samples diluted in KBr pellets. 57

Fe Mössbauer spectra were performed at 300 and 77 K using a conventional constant acceleration transmission spectrometer with a 57Co radioactive source diffused into a Rh matrix. The spectra were fitted by means of the MOSFIT program32 involving discrete distribution of magnetic sextets and/or quadrupolar doublets based on lines with Lorentzian profiles; 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 with Superconducting Quantum Interface Device (SQUID) magnetometer (Quantum Design MPMS-XL 5). 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 cooled to 5 K with no applied field. A magnetic field of 75 Oe was then applied, and the magnetization was recorded upon heating from 5 to 400 K (ZFC). The sample was then cooled down to 5 K under the same applied field, and the magnetization was recorded upon heating from 5 to 400 K (FC). 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 having cooled the sample in zero field. Magnetization (M) curves as a function of the applied magnetic field (H) have been measured at 400K and 5K. Samples were demagnetized at 400 K by applying a degauss field and cooled down to 5 K. The magnetization was then measured at constant temperature by sweeping the magnetic field from +7T to –7T, and then from –7T to +7T. Magnetization to saturation values were given as a function of the mass of the iron oxide. The mass contribution of organic has been removed according to the results obtained by thermogravimetric analysis.

Results and discussions Synthesis of raspberry shaped nanostructures (RSN) Iron oxide raspberry shaped nanostructures which consist in aggregates of iron oxide nanocrystals have been synthesized by a polyol solvothermal approach. A mixture of iron chloride hexahydrate, urea, ethylene glycol, and succinic acid was slowly heated to 200°C in a Teflon lined autoclave and then kept at this temperature for 7 or 13 hours. Such a typical synthesis gives rise to the formation of raspberry shaped nanostructures (RSN) presenting a roughly spherical morphology with a size distribution centered to about 250 nm with dispersion from 50 to 70 nm (Figure 1). The nanocrystal size constituting the RSNs has been modulated to 5, 25 and 60 nm by controlling properly the synthesis parameters (Figure 1). Nanostructures named RSN5 have been synthesized after heating for 7 h at 200 °C, RSN25 were obtained by increasing the heating reaction time up to 13 h. The nanostrustures named RSN60 were obtained by increasing four times the iron chloride concentration.33


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Figure 1. SEM micrographs and size distributions of a) RSN5, b) RSN25 and c) RSN60. The arrow in a) shows a broken RSN exhibiting a hollow structure. SEM micrograph of RSN5 shows some cavity in broken RSNs which corresponds to a hollow structure (Figure 1a). It was confirmed by electron tomography which was performed on several representative RSN5 (Figure 2a and Figure S1 in SI). A visual analysis of the slices extracted at different depths from the reconstructed volumes of the considered RSN5 confirms the presence of a cavity at the center of the aggregate. Such a cavity has been also observed by imaging RSN 25 while it is less visible in RSN60 due to their larger nanocrystal size (Figure 1c). The formation of such hollow nanostructure is generally explained by a inside-out or inverse Ostwald ripening process.22, 34, 35 Long reaction times favors the dissolution of nanograins in the center of aggregates and the growth of nanograins located at their surface by recristalization. Additional information concerning the porous structure of all samples have been obtained by nitrogen absorption-desorption measurements leading to specific surface area of 57.3, 27.3 and 12.9 m²/g for RSN5, RSN25 and RSN60, respectively (Figure S2 in SI). The largest specific surface area of RSN5 is correlated unambiguously to their smallest nanocrystal size.

Figure 2. Three different slices extracted at different depths from the reconstructed volume of the RSN25 object considered for the electron tomographic study. b) and c) show the hollow structure. TEM micrographs evidenced mean nanocrystal sizes of 5, 25 and 60 nm for RSN5, RSN25 and RSN60 respectively (Figure 3, Table 1). The crystallographic structure of all samples was investigated by HRTEM (Figure 3). Lattice fringes corresponding to the (200) or (311) reflections of the iron oxide spinel (magnetite JCPDS file 19-629 or maghemite JCPDS file 39-1346) were identified. Selected area electron diffraction (SAED) patterns also confirmed the formation of the spinel phase (Figure 3c, f, i). Nevertheless, SAED patterns recorded from single RSNs show bright spots instead of rings which are consistent with single crystal structures. These observations show that RSNs are constituted of agregated grains which are featured by similar crystallographic orientations. Although spots are slightly extended due to a small misalignment of nanocrystals, they become sharper for larger grain 5 ACS Paragon Plus Environment


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sizes.27 Such nanostructures which tend to single crystal have been reported to result from an oriented aggregation process.21, 36

Figure 3. Structural data for (a,b,c) RSN5, (d,e,f) RSN25 and (g,h,i) RSN60. a, d, g) TEM micrographs of single RSNs, b, e, h) high resolution TEM micrographs, and the c, f, i) corresponding SAED patterns.

Table 1. Grain size measured from TEM micrographs. Crystal size and cell parameters calculated by refinement of the XRD pattern. Sample

Grain size by TEM (nm)

Crystal size by XRD (nm)


Cell parameter (Å)

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RSN5

5±2

14 ± 1

8.39(4)

RSN25

25 ± 3

15 ± 1

8.39(7)

RSN60

60 ± 4

22 ± 1

8.39(3)

XRD patterns of RSNs display characteristic peaks of the iron oxide spinel structure (Figure S3). However, their refinement by using the Rietveld method led to crystal domain sizes which disagree with grain sizes measured from TEM micrographs (Table 1). This mismatch may be related to the unique crystal structure of RSNs. In the case of small nanograins of 5 nm, the formation process may undergo coalescence of grains which results in larger crystal domain. In contrast, in the case of larger nonograins, the recrystalization process generates defects or dislocations which are confirmed by the slightly extended spots in SAED patterns (Figure 3f, i). Moreover, the refinement of XRD patterns using Rietveld analysis enabled the calculation of lattice parameters which are very close to bulk magnetite (8.396 Å). It is worth notting that for such small nanograin size, magnetite based nanoparticles are well-known to be oxidized at their surface.37, 38 5 nm magnetite nanoparticles are generally fully oxidized in maghemite while the larger sizes nanoparticles convert into magnetite shelled by an oxidized layer. Therefore such values of lattice parameters for RSNs suggest that the RSN structure prevents Fe2+ from oxidation. However, X-ray diffraction is not sufficient to conclude unambiguously on the composition of RSNs and stresses and defects induced by the nanostructuration with common crystallographic orientation between nanocrystals may also alter the lattice parameters values.39 Additional techniques such as FTIR s and 57Fe Mössbauer spectroscopies are required to assess with certainty the chemical composition and crystal structure of RSNs.38, 40 FTIR spectroscopy allows discriminating the magnetite and maghemite phase (Figure S4). The stoechiometric magnetite is featured bya characteristic Fe-O band at around 570 cm-1 while the maghemite phase displays several bands between 800-400 cm-1 which number and resolution depend on the structural order of vacancies in maghemite.41 The FTIR reference spectrum of maghemite phase in Figure S4 is characteristic of a partially ordered maghemite and that of magnetite to a magnetite phase slighty oxidized on the surface as evidenced by the shoulder at about 700 cm−1. A careful analysis of νFe-O vibration modes between 800 to 400 cm-1 shows that RSNs exhibit a main broad Fe−O band which become broader and slightly shifted for RSN5 featured by a smaller cristallite size. The Fe-O bands of RSN25 and RSN60 are consistent with a magnetite phase slightly oxidized in surface while RSN5 appear more oxidized with a composition intermediate between those of magnetite and maghemite. In addition, FTIR spectra (Figure S5) exhibit similar bands in the 4 000 to 800 cm-1 region with the characteristic bands of ethylene glycol (νOH and νC-O) and of succinic acid (νCOO) which remains grafted at the RSN surface after washing with ethanol and water. The precise composition of RSNs was further investigated by 57Fe Mössbauer spectrometry which allows determining the oxidation state of iron species (Figure 4). Transmission spectra recorded at 300 K and 77 K for RSN5 and RSN25 display a magnetic hyperfine structure which consists in sextets with broad and asymmetric lines. Spectra at 300 K exhibiting magnetic sextets show a magnetic ordering which, regarding the chemical composition of RSNs, can be attributed to a ferrimagnetic state. However RSN5 and RSN25 display a mean cristallite size which is much smaller than the 7 ACS Paragon Plus Environment


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domain size limit between the superparamagnetic-ferrimagnetic states (the superparamagnetic limit at room temperature is about 25 nm for iron oxide)42 and should exhibit a hyperfine structure corresponding to a quadrupolar doublet at 300 K, correlated to the superparamagnetic relaxation.40 Therefore, the observation of a magnetic sextet at 300 K for RSN5 and RSN25 is due to strong dipolar interactions and may be related to the oriented aggregation of nanograins in RSN. Magnetic measurements detailed below will allow to better ascertain the magnetic state of these RSNs. From the point of view of their composition, one may observe qualitatively that the spectrum of RSN25 at 300 K displays two sextuplets in agreement with a magnetite phase. In contrast, it is not the case for RSN5 which confirms at first their higher oxidation state. The fitting of Mossbauer spectra at 300 K and 77 K requires several magnetic components with Lorentzian lines which are attributed to Fe3+ and Fe2+ as well as Fen+ with intermediate valence states (2 < n < 3) resulting from the incomplete oxidation of Fe2+. The refinement of these spectra has been performed by means of a distribution of a static magnetic hyperfine field which is slightly correlated to the isomer shift (Table 2). The values of the isomer shift which strongly depend on the mean valence state of Fe atoms are consistent with the expected average compositions of RSN5 and RSN25 which were estimated to be Fe2.78O4 and Fe2.90O4, respectively. To schematically describe the oxidation state of RSNs, we have considered a spherical core-shell model system which consists in a magnetite core shelled by maghemite, both featured by the same Lamb-Mossbauer factor f. We have thus estimated the composition of RSNs in magnetite and maghemite. Such a model allows us to compare the mean composition of RSNs as a function of their size, but it is preferable to consider a gradient of oxidation from the surface of RSNs. From these results, the ratio between magnetite and maghemite was calculated following the procedure we have reported on iron oxide nanoparticles.37, 38, 40 The results obtained are in good agreement with the partial oxidation of RSN5 with an estimation of a composition of about 70 % in maghemite. In contrast, the composition of larger grains in RSN25 gets closer to that of magnetite (70 %). These Mössbauer results indicate a lower oxidation degree of the nanograins composing RSNs in comparison with the individual nanoparticles reported in the literature.38, 40, 43 Indeed 5 nm isolated NPs are generally considered to be fully oxidized in maghemite with a composition close to Fe2.67O4 while 20 nm sized nanoparticles synthesized by thermal decomposition contain 55 % of magnetite40 which is even larger (70 %) in 25 nm sized nanoparticles synthesized by coprecipitation.38 Therefore, these results show unambiguously that the raspberry structure allows reducing the oxidation of Fe2+.


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b

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c

0,97

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-5

d

0 5 V (mm/s)

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0,95

0 5 V (mm/s)

10

Figure 4. 57Fe Mössbauer spectra corresponding to a,c) RSN5 and b,d) RSN25 at a,b) 300K and c,d) 77K.

Table 2. Refined values of hyperfine parameters obtained from the RSN5 and RSN25 Mössbauer spectra at given temperatures. a Standard deviation equal to ±0.01.

b

Standard deviation equal to

±0.2. c Standard deviation equal to ±0.5. Temperature Average (K) isomer shifta (mm/s)

Average Hyperfine fieldb (T)

Average Chemical Quadrupolar composition shift c (%)

RSN 5

300

0.41

43.2

0.00

Fe2.79O4

Ratio Fe3O4/ γ-Fe2O3 (%) 33/67

RSN 25

300

0.48

47.2

0.00

Fe2.90O4

68/32

RSN 5

77

0.50

50.1

0.04

Fe2.77O4

31/69

RSN 25

77

0.58

50.1

-0.03

Fe2.90O4

71/29

Magnetic properties of RSNs The magnetic properties of RSN5, RSNS25 and RSN60 have been studied as a function of their nanocrystal size by SQUID magnetometry (Figure 5 and Table 3). Temperature dependent measurements of magnetization (ZFC/FC curves) are not characteristic of superparamagnetic NPs. Indeed zero field cooled (ZFC) curves do not show any maximum (even with RSN5 which are constituted of 5 nm sized nanograins) which is usually related to the blocking temperature at which 9 ACS Paragon Plus Environment


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occurs the crossover between superparamagnetic and ferrimagnetic states (Figure 5c). However, ZFC/FC measurements are very sensitive to dipolar interactions. The highest is the strength of dipolar interaction, the highest is the blocking temperature (the largest shift of the ZFC maximum curves to higher temperature). This behavior is also correlated to FC curves which do not show any increase of magnetization below the blocking temperature.44, 45 Therefore, the ZFC/FC curves of RSNs may be correlated to strong dipolar interactions. These results are in agreement with Mössbauer spectra which support RSNs in a ferrimagnetic state at room temperature. Both measurements point out that the oriented aggregation of nanocrystals favors strong dipolar interactions between nanograins. Such collective properties may result in a superspin glass, or possibly a super ferromagnetic state which has already observed in assemblies of nanoparticles with similar sizes.40, 43 However the decrease of the magnetization between 50 and 350 K in the FC M(T) curves (Figure 5 c) is often attributed to spin glass effects as reported in nanoparticle assemblies46, 47 or at the interface of AFM/F(i)M core-shell particles.39, 48 Furthermore a contribution is observed around 30 K in all ZFC curves which should not correspond to blocking temperatures (TB) for such grain sizes.37, 40, 43 In the case of RSN60, another contribution at around 120 K could be attributed to the Verwey transition.37, 49 It agrees with the lower oxidation state of magnetite in this sample which is correlated to larger nanograin size as it has been reported previously for iron oxide nanoparticles of 40 nm.37 The magnetic behavior of RSNs has been investigated more deeply by measuring the imaginary part of the magnetic susceptibility (X’’) under an alternative magnetic field (Figure 5 d). X’’(T) curves clearly show a peak centered at low temperature (Tf) for all three samples. This Tf peak is often related to the contribution observed on ZFC curves below 50 K and does not correspond to the blocking temperature of nanocrystals.40, 43, 50, 51 Kostopoulos et al showed that this contribution is shifted to higher temperature by the increasing the number of nanograins, i.e. dipolar interactions and is ascribed to surface spin glass freezing.29 Furthermore, the absence of a second X’’ maximum related to the blocking temperature of RSNs was expected at higher temperatures. These observation evidenced again strong dipolar interactions in RSNs resulting in blocking temperatures higher than room temperature. The orientated agregation of nanograins with a rather good alignment of crystal domains (Table 3) induces collective effects and stronger dipolar interactions.52 Therefore such measurements confirm a superspin glass behavior of our RSNs which is independant of the nanograin size. The M(H) curves recorded at 300 K are characteristic of superparamagnetic NPs (Figure 5a). In contrast, M(H) curves recorded at 5 K display hysteresis loops and are characteristic of blocked magnetic domains (Figure 5b). The fact that RSN60 exhibit no hysteresis at 300K agrees with the crystal size (22 nm) calculated by refinement of the XRD pattern which is correlated to the size of magnetic domains (table 3). The observation of no hysteresis loops at 300 K with all RSNs while the blocking temperature of all RSNs is higher than 300 K confirms again that M(H) measurements are less sensitive to dipolar interactions than magnetization temperature dependent ZFC/FC measurements.44, 45, 53 Nevertheless, one observes an increase of coercive fields (HC) and remanent magnetization (MR) with the grain size, which is ascribed to the effect of dipolar interactions and magnetic anisotropy of nanocrystals (Table 3).45 On the other hand, RSN5 and RSN25 are featured by high magnetization saturation (MS) values at 300 K which increase with grain size. In contrast, RSN60 displays a MS which is close to that of RSN5 whereas it was expected to be higher than the one of RSN25 (Table 3). The RSN5 and RSN25 10 ACS Paragon Plus Environment

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values are much higher than the ones of individual NPs of 5 nm and 20 nm and get closer to bulk magnetite (92 emu/g).40, 43 Although rather high Ms values have been already reported for RSNs,12-14, 17, 22 its origin is still not clear. These values are particularly surprising in comparison to the ones reported for individual iron oxide NPs with sizes lower than 20 nm. Indeed MS values are usullay lower than that of magnetite (Fe3O4) (92 emu/g) and maghemite (γ-Fe2O3) (76 emu/g) in bulk state, which is ascribed to the presence of defects in crystal structure and spin canting at the surface or in volume dependending on the NP diameter.40 In fact, surface oxidation of iron (II) in Fe3O4 NPs generates vacancies/defects with random distribution and result in disordered γ-Fe2O3 structure. In addition, small nanoparticles with large surface/volume ratio display break of symmetry for spins located at the nanoparticle surface which induces spin random orientations. Therefore, one may advance that the well-defined geometrical arrangement of nanograins in RSNs favors a large interface between magnetic monodomains which reduces surface and volume defects and disordering. Therefore, it results in strong magnetic coupling and harder dynamic reversal of magnetization. Furthermore, the lower oxidation state of RSNs than the corresponding individual nanocrystals also contributes to the enhancement of MS. Nevertheless, the unexpected low value of RSN60 may be related to a higher amount of defects which is supported by cristallite size calculated from XRD patterns which are smaller than the TEM mean size. Moreover, such a large grain size results in the lowest grain interface and favors a closer behavior to invidual nanoparticles.

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RSN 5 RSN 25 RSN 60

50

100

150

200

250

300

350

400

50

100

150

Temperature (K)

200

250

300

350

400

Temperature (K)

Figure 5. Magnetic properties of RSN5, RSN25 and RSN60. Magnetization curves against an applied magnetic field a) at 400 K and b) at 5 K. c) Magnetization curves against temperature recorded under a static field. d) Imaginary part of the magnetic susceptibility against temperature recorded under an alternative field. 11 ACS Paragon Plus Environment


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Table 3. Magnetic properties of RSN5, RSN25 and RSN60. Magnetization saturation (MS) has been measured at 300 K. Coercive field (HC) and magnetization saturation (MS) have been recorded at 5 K.

Average RSN size (nm)

RSN 5 250

RSN 25 245

RSN 60 260

TEM grain size (nm)

5

25

60

XRD cristallite size (nm)

14

15

22

MS (emu/g)

76.2

80.9

75.3

HC (Oe)

126

289

372

MR (emu/g)

15.0

30.6

22.2

Tf (K)

23

22

35

Conclusion In summary, the oriented attachment of nanograins in raspberry shape nanostructure has a strong influence on the structural and magnetic properties in comparison to individual nanoparticles featured by similar sizes to the ones of the constituting nanograins. The orientated agregation state reduces significantly the surface oxidation of nanograins and decreases their structural and magnetic disorder arising commonly from surface oxidation, surface and volume spin canting and/or broken bonds when considering individual nanoparticles. A direct consequence is the significant enhancement of the saturation magnetization. On the other hand, the similar crystallographic orientation of nanograins favors collective properties of these magnetic domains. Very strong dipolar interactions result in the strong coupling of magnetizations which means that magnetic anisotropy energy is over classed and result in super-spin-glass. Such porous nanostructures are very promising, for a large range of applications including waste recovering and catalysis thanks to the combination of high surface area and unusual magnetic properties.

Acknowledgments Financial supports were provided by the Direction Générale de l’Armement.

Supporting information Electronic tomography micrographs, XRD patterns and FTIR spectra. This information is available free of charge via the Internet at http://pubs.acs.org


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Low Oxidation State and Enhanced Magnetic Properties Induced by

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