Synthesis and properties MFe2O4 (M = Fe, Co) nanoparticles and core–shell structures

Solid State Sciences 46 (2015) 19e26

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Synthesis and properties MFe2O4 (M ¼ Fe, Co) nanoparticles and coreeshell structures O.V. Yelenich a, S.O. Solopan a, *, J.M. Greneche b, A.G. Belous a a b

V. I. Vernadskii Institute of General and Inorganic Chemistry, 32/34 Palladina Avenue, 03680 Kyiv 142, Ukraine LUNAM, Institut des Mol
ecules et Mat
eriaux du Mans (IMMM UMR CNRS 6283), Universit
e du Maine, 72085 Le Mans Cedex, France

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 February 2015 Received in revised form 20 May 2015 Accepted 25 May 2015 Available online 27 May 2015

Individual Fe3xO4 and CoFe2O4 nanoparticles, as well as Fe3xO4/CoFe2O4 core/shell structures were synthesized by the method of co-precipitation from diethylene glycol solutions. Core/shell structure were synthesized with CoFe2O4-shell thickness of 1.0, 2.5 and 3.5 nm. X-ray diffraction patterns of individual nanoparticles and core/shell are similar and indicate that all synthesized samples have a cubic € ssbauer studies of CoFe2O4, Fe3xO4 nanoparticles indicate superspinel structure. Compares Mo paramagnetic properties at 300 K. It was shown that individual magnetite nanoparticles are transformed into maghemite through oxidation during the synthesis procedure, wherein the smallest nanoparticles are completely oxidized while a magnetite core does occur in the case of the largest nanoparticles. The € ssbauer spectra of core/shell nanoparticles with increasing CoFe2O4-shell thickness show a gradual Mo decrease in the relative intensity of the quadrupole doublet and significant decrease of the mean isomer shift value at both RT and 77 K indicating a decrease of the superparamagnetic relaxation phenomena. Specific loss power for the prepared ferrofluids was experimentally calculated and it was determined that under influence of ac-magnetic field magnetic fluid based on individual CoFe2O4 and Fe3xO4 particles are characterized by very low heating temperature, when magnetic fluids based on core/shell nanoparticles demonstrate higher heating effect. © 2015 Elsevier Masson SAS. All rights reserved.

Keywords: Nanostructures Core/shell structures Magnetic materials €ssbauer spectroscopy Mo

1. Introduction In the last decade the study of spinel-type oxides MFe2O4 as crystalline nanoparticles (NPs) or nanostructured powders, where M is a divalent metal, has attracted special attention due to their novel magnetic properties, which are significantly different from those of crystalline ferrites, their bulk counterparts. Indeed, ferrites are technologically important and have been used in many applications including magnetic recording media and magnetic fluids for the storage and retrieval of information, magnetic resonance image enhancement, and others [1e23]. Of particular interest are also wide-range applications of ferrite nanoparticles in medicine, e.g. for drug delivery and hyperthermia in the treatment of cancer diseases [4]. Spinel-type oxides MFe2O4 are often denoted by the formula AB2O4 where A and B refer to tetrahedral and octahedral sites, respectively, in the fcc oxygen lattice [1,3,4]. These compounds

* Corresponding author. E-mail address: [email protected] (S.O. Solopan). http://dx.doi.org/10.1016/j.solidstatesciences.2015.05.011 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.

often form a structure of inverse spinel, where Fe3þ ions occupy A sites, whereas M2þ and remaining Fe3þ ions occupy B sites. It is now well established that the cationic distribution into tetrahedral and octahedral sites strongly influences the magnetic structure combined to the superficial structural state at the nanoscale level [1,3], which make the investigations of nanocrystalline ferrites highly relevant. A large number of publications deals with the co-precipitation of slightly soluble compounds from aqueous solutions [5e8]. The complex and uncontrollable mechanism of such reactions involves crystal nucleation, growth, coarsening or agglomeration processes, which occur simultaneously. This often results in the agglomeration of nanoparticles. In references [9e12], MFe2O4 nanoparticles (M ¼ Mn, Fe, Co, Ni, Zn) with spinel structure have been synthesized from metal chlorides in a diethylene glycol solution. The complex reaction of diethylene glycol with transition-metal cations makes it possible to separate in time the crystal nucleation and growth processes and, thus, to partially control the particles' size and aggregation. At present, there is a strong demand for the magnetic

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nanoparticles with controllable size and narrow size distribution, because they may show considerable heating effects if subjected to magnetic AC-fields [1,4]. In particular, in magnetic particle hyperthermia proposed as a tumor therapy, magnetic nanoparticles are injected in tumor tissue and are heated in an alternating magnetic field in order to destruct the tumor (for a review see [1,13]). However, despite the numerous empirical results on magnetic hyperthermia, there is no systematic understanding of the broad scattering of published data on the AC-losses in magnetic nanoparticles, as well as mechanisms of the losses (for a review see [13,14]). There is also scarcity of data on the relation of the AClosses to the magnetic parameters of a separate nanoparticle and its size. Synthesis of ferromagnetic or ferrimagnetic nanoparticles with a coreeshell structure (CSs) on the basis of spinel-AFe2O4 compounds and study their properties have been in recent years the object of particular interest. This is explained by the fact that the coreeshell structures can increase some of the properties of nanoparticles compared to the individual compounds (Fe3O4, CoFe2O4), for example, SLP (specific loss power) [15,16]. According to the recent published review [17], the magnetic properties of core/shell structures are determined by parameters such as a size, a particular order (soft/hard or hard/soft) and geometric shape of core and shell (spherical or planar) [18,19]. In addition, the magnetic properties depend on the difference in magnetization between the core and shell inclusions as well as existence of the dipolar and exchange-coupled interactions that affect the spin reversal processes [20]. No less important factors in determining the magnetic properties of the core/shell structures are their size distribution and microstructure change when processed at high temperatures. Core and shell may coalesce at high temperatures forming a structure of core-nanoparticles embedded in a shellmatrix [21]. Magnetic nanoparticles with core/shell structure often show properties that are not specific to individual core and shell particle. Core/shell nanoparticles may include a combination of core and shell properties, and thus find application in the development of permanent magnets [22], magnetic recording devices [23], microwave absorption devices [24], magnetic fluids [25] and for biomedical use [26], where such bimagnetic systems (core/ shell nanoparticles) can have significant advantages over conventional nanoparticles. However, the synthesis of such structures is of considerable technological problem. At the same time, the peculiarities of the nature of magnetic loss in core/shell structures which can change the specific loss power value remain insufficiently studied in literature. One of the effective methods for the study of magnetic NPs and core/shell structures based on them is 57 €ssbauer spectrometry. Indeed, this non destructive and Fe Mo local probe technique is able to distinguish Fe species with different valence states, to discriminate Fe sites with different structural atomic surroundings, to better understand the magnetic arrangement of Fe moments from in-field approach and finally to follow the superparamagnetic relaxation phenomena versus temperature. €ssbauer specIn that way, it is important to emphasize that Mo trometry is highly suitable to discriminate the presence of either Fe3O4 or g-Fe2O3 or a mixture of both and to detect the presence of other Fe oxide or hydrooxide. It is clear that the mean value lattice parameter estimated from X-ray pattern modeling remains rather ambiguous to distinguish magnetite from maghemite while that of €ssbauer spectra modeling prothe isomer shift estimated from Mo vides crucial information. For this reason, in the following the iron oxide phase will be noted Fe3xO4 with 0 < x < 0.33, as it is suggested elsewhere [27,28]. In light of the above comments, the aim of the present work was to synthesize Fe and Co-based ferrite nanoparticles with spinel structures and nanoparticles with a coreeshell structure from a

diethylene glycol solution and to study their structural properties €ssbauer spectrometry. by means of Mo 2. Experimental section For the synthesis of Fe3xO4, CoFe2O4 and Fe3xO4/CoFe2O4 NPs iron (III) chloride nonahydrate (97% FeCl3$9H2O, Sigma Aldrich), cobalt (II) nitrate hexahydrate (98% Co(NO3)2$6H2O, Sigma Aldrich), iron (II) sulfate heptahydrate (99% FeSO4$7H2O, Sigma Aldrich), sodium hydroxide (98% NaOH), diethylene glycol (99% DEG, Sigma Aldrich) were used as a starting reagents. All stages of synthesis were carried out in a three-neck flask in argon atmosphere according to the method described in [29]. Synthesis of Fe3¡xO4: FeSO4$7H2O and FeCl3$9H2O in molar ratio (1:2) were dissolved in DEG. At the same time NaOH in DEG was prepared. The alkali solution was added to the mixture of the salts FeSO4$7H2O and FeCl3$9H2O, and the resulting mixture was stirred for 2 h. The resulting solution was heat-treated at 200e220  C (60 min). Oleic acid than was added to the diethylene glycol solution, and the mixture was stirred for 10e20 min. The resulting precipitate after cooling was centrifuged and redispersed in ethanol and dried under opened air. Synthesis of CoFe2O4: Co(NO3)2$6H2O and FeCl3$9H2O in molar ratio (1:2) were dissolved in DEG. At the same time NaOH in DEG was prepared. The alkali solution was added to the mixture of the salts Co(NO3)2$6H2O and FeCl3$9H2O, and the resulting mixture was stirred for 2 h. The resulting solution was heat-treated at 200e220  C (60 min). Oleic acid than was added to the DEG solution, and the mixture was stirred for 10e20 min. The resulting colloidal solution after cooling was centrifuged and redispersed in ethanol and dried under opened air. Synthesis of Fe3¡xO4/CoFe2O4: as a core were used Fe3xO4 nanoparticles synthesized by the method described above without oleic acid adding procedure. Fe3xO4/CoFe2O4 nanoparticles with a coreeshell structure were synthesized in a three-neck flask in argon atmosphere. Co(NO3)2$6H2O and FeCl3$9H2O were dissolved in DEG, and the solution was stirred for 10e20 min. At the same time NaOH in DEG was prepared. The alkali solution was added to the mixture of the salts Co(NO3)2$6H2O and FeCl3$9H2O. After 2 h of stirring, the Fe3xO4 nanoparticles in DEG were added to reaction medium. The resulting solution was heat-treated at 200e220  C (90 min). In the next stage, oleic acid was added to the reaction medium, and the mixture was stirred for 10e20 min. After cooling the solution was centrifuged and mixed with ethanol. The precipitate was separated by centrifugation and dried under opened air. Nanostructured powders were investigated by PANalytical's Xray diffraction system on X'Pert Powder diffractometer (Cu-Ka radiation, tension 45 kV, current 35 mA, Ni filter). Particle sizes for the magnetic samples were determined using Scherrer's formula: d ¼ 0.9l/bcosq, where l is the wavelength of X-ray (0.154 nm), b is

Fig. 1. Schematic representation of Fe3xO4/CoFe2O4 nanoparticle with a core/shell structure.

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Fig. 2. XRD patterns: (a) Fe3xO4, (b) CoFe2O4, (c) Core/shell structures with shell~1 nm; (d) Core/shell structures with shell~3.5 nm, (e) mix of Fe3xO4 and CoFe2O4.

the full-width at half-maximum of the peak (in radian), and the q is the Bragg's angle of the X-ray diffraction peaks. Calculations of the intensity redistribution and angels of X-ray peaks for individual compounds, their mixtures and core/shell structures were performed by PeakFit 4.12 software using diffractgrams of individual

21

peaks with maximum intensity. X-ray diffraction analysis of individual peaks were carried out in the range of 2q-angels from 32 to 42 , in steps of 0.01 and X-ray exposure time per point of 30 s. Individual as-prepared nanoparticles and core/shell structures were investigated by 57Fe Mossbauer spectrometry: spectra were recorded at 300 and 77 K in a transmission geometry using 57Co/Rh g-ray source mounted on an electromagnetic drive with a triangular velocity form. The samples consist of a thin powdered layer containing 5 mg Fe/cm2. The obtained spectra were analyzed by a least-square fitting method to Lorentzian functions. The isomer shift values (d) are referred to that of a-Fe at 300 K. The size and morphology of powder particles have been determined by means of a JEM-1230 scanning electron microscope. When calculation particles size distribution, TEM images were analyzed according to the procedure used by Peddis et al. [30]. Magnetic measurements were performed in the 2e300 K temperature range using commercial Quantum Design Magnetic Property Measurements System equipped with superconducting quantum interference device (SQUID). Magnetic moment was measured upon heating for both zero-field-cooled (ZFC) and field-cooled (FC) conditions. Isothermal magnetic hysteresis loops were measured at 10 and 300 K in the magnetic field interval of 3eþ3 T. For the calorimetric determination of specific loss power the ferrofluids based on synthesized nanoparticles were prepared using 0.1% aqueous agarose solutions [31] and placed into a coil (5 turns, 30 mm diameter) that provides the alternating magnetic field (frequency 300 kHz, amplitudes up to 7.7 kA/m). All measurements of temperature heating were performed according to the procedure used by Veverka et al. [32] and

Fig. 3. XRD patterns of separate nanopowders Fe3xO4 and CoFe2O4, their mechanical mixture (mix Fe3xO4 and CoFe2O4, 1:1), and core/shell structure with the shell thickness ~ 3.5 nm. Experimental data are shown by dots, the baseline (straight line under the experimental curve) and curves calculated by means of pseudo-Voigt function (those presented separately and with the experimental curve) are shown by solid lines.

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specific loss power (SLP) values were calculated using equation (1)

SLP ¼

CFluid ,VS dT , mpowder dt

(1)

where dT/dt is the initial slope of the graph of the change in temperature versus time, CFluid is the volumetric specific heat capacity of the sample solution, Vs is the sample volume and mpowder is the mass of magnetic material in the fluid. 3. Results and discussion In the study, individual Fe3xO4 and CoFe2O4 nanoparticles, as well as Fe3xO4/CoFe2O4 core/shell structures (core/shell scheme are shown in Fig. 1) have been synthesized by the method of coprecipitation from diethylene glycol solutions. In the synthesis of core/shell structures as the core and shell Fe3xO4 and CoFe2O4 were used, respectively. Individual Fe3xO4 nanoparticles had a spherical shape and an average diameter of 4 nm (Fig. 5b). Core/ shell structures were synthesized with shell thickness of 1.0, 2.5 and 3.5 nm by using different amount of initial salt solutions. Fig. 2 presents the X-ray diffraction patterns for deposited from diethylene glycol solutions Fe3xO4 and CoFe2O4 nanoparticles, as well as core/shell structures (Fe3xO4/CoFe2O4) with CoFe2O4-shell thickness of 1.0, 2.5 and 3.5 nm. X-ray diffraction patterns of nanoparticles are similar and indicate that all synthesized samples have a cubic spinel structure (JCPDS Card Number 19-0629 [33]). With an increase of shell thickness, the increase in intensity and decrease the width of the diffraction peaks are observed in the Xray diffraction patterns. The refinement of these patterns provides clearly the mean lattice parameter which does not allow the crystalline phase to either magnetite or maghemite or a mixture of both

to be unambiguously attributed. But, the crystallite sizes (DXRD) of synthesized nanoparticles and core/shell structures were estimated from the (311) diffraction maximum using obtained X-ray patterns. Taking into account that core and the shell have the same density, they can not be distinguished by the contrast of the TEMimage. Therefore, to confirm the formation of core/shell structure, we used a comparative analysis of X-Ray diffraction patterns collected from separate particles Fe3xO4 and CoFe2O4, mechanical mixture composed of these compounds taken in 1:1 ratio, and supposed core/shell structures (Fig. 3). Diffraction patterns were recorded at angles from 32 to 42 with the long exposure time of 300 s/point and step of 0.01. XRD profile peaks were fitted and analyzed by means of PeakFit software using the pseudo-Voigt function [34]. NaCl powders were used as an internal standard, while the total deviation from zero point did not exceed 0.01 for all of XRD patterns. XRD patterns of separate Fe3xO4 nanoparticles exhibit the reflections from planes (311) and (222), which are characterized by the maxima at angles 2q ¼ 35.63 and 2q ¼ 37.42 respectively. Again, separate CoFe2O4 nanoparticles demonstrate only single reflection from plane (311), which is characterized by the maximum at 2q ¼ 35.62 . Accordingly, XRD collected from mechanical mixture demonstrate the superposition (angle, intensity) of the diffraction peaks characteristic for separate compounds Fe3xO4 and CoFe2O4 (Fig. 3). In this case, the reflection from plane (222) is not well distinguished on XRD patterns comparing with the superimposed most intense peaks of Fe3xO4 and CoFe2O4. In contrast, XRD patterns of the supposed core/shell structure do not merely correspond to the mixture of Fe3xO4 and CoFe2O4 exhibiting the reflections from planes (311) and (222) with the maxima at angles 2q ¼ 35.59 and 2q ¼ 37.33 respectively. These results confirm the formation of core/shell structure rather than a

€ssbauer spectra of CoFe2O4 (a), Fe3xO4 (b) nanoparticles and Fe3xO4/CoFe2O4 core/shell structures (shell ~ 1 nm) (c); Fe3xO4/CoFe2O4 core/shell structures Fig. 4. Mo (shell ~ 2.5 nm) (d); Fe3xO4/CoFe2O4 core/shell structures (shell ~ 3.5 nm) (e) at 300 K (left side) and of CoFe2O4 (f), Fe3xO4 (g) nanoparticles and Fe3xO4/CoFe2O4 core/shell structures (shell ~ 1 nm) (h); Fe3xO4/CoFe2O4 core/shell structures (shell ~ 2.5 nm) (i); Fe3xO4/CoFe2O4 core/shell structures (shell ~ 3.5 nm) (j) at 77 K (right side).

O.V. Yelenich et al. / Solid State Sciences 46 (2015) 19e26

mechanical mixture. Moreover, the presence of the reflection from plane (222) e in contrast to a separate CoFe2O4 e may denote adapting of CoFe2O4 shell structure to the parameters of core Fe3xO4. This is again accompanied by the shift of XRD peaks towards expected decrease in the unit cell parameters of core/shell structure comparing with separate phases. In order to study the characteristics of the crystalline structure formation as well as peculiarities of magnetic properties of core/ shell structures in this work Fe3xO4-CoFe2O4 systems depending on the thickness of CoFe2O4-shell was investigated with the use of 57 € ssbauer spectrometry. Fig. 4 compares Mo €ssbauer spectra Fe Mo of CoFe2O4, Fe3xO4 nanoparticles and core/shell structures with a different shell thickness recorded at 300 and 77 K. The 300K hyperfine structures of CoFe2O4 and Fe3xO4 samples (Fig. 4a and b) result from a prevailing quadrupolar doublet (isomer shift d~0.34 mm/s, relative area %Fe~49%) while those at 77K (Fig. 4f and g) are composed of a quadrupolar doublet (isomer shift d~0.47 mm/ s, relative area %Fe~20%) and a magnetic sextet with broadened and asymmetrical lines. They are clearly attributed to the presence of superparamagnetic relaxation phenomena, as typically observed for nanoparticles. It is also important to discuss the mean values of

23

isomer shift estimated at both temperatures. For the first sample, one can conclude that magnetite nanoparticles have been strongly transformed into maghemite through oxidation for the synthesis procedure (x ¼ 0.31): a simple coreeshell model allows to estimate the mean diameter of the magnetite nucleus at about 0.3 nm assuming a unique size but the more reasonable scenario would suggest that the smallest nanoparticles are completely oxidized while a magnetite core does occur in the case of the largest nanoparticles. In the case of Co ferrite nanoparticles, the values of isomer shift are fairly consistent with those expected in micro Co ferrite with the presence of pure ferric species [35]. € ssbauer spectra of core/shell On the contrary, the 300K Mo nanoparticles (Fig. 4, spectra c-e) exhibit a quadrupolar component at the center of the spectrum superimposed to a magnetically split component with broadened and asymmetrical lines. One observes a gradual decrease in the relative intensity of the quadrupole doublet at the expense of an increase in the relative intensity of the magnetic sextet with increasing shell thickness. It is important to emphasize the same tendency at 77K (Fig. 4, spectra h-i) with a progressive disappearance of the quadrupolar doublet. Indeed the addition of the external shell of Co-ferrite should contribute to

Fig. 5. TEM images of individual nanoparticles and core/shells structures: (a) CoFe2O4; (b) gFe2O3; (c) gFe2O3/CoFe2O4 (shell ~ 1 nm); (d) gFe2O3/CoFe2O4 (shell ~ 3.5 nm); (e) particle size distributions obtained by TEM analysis.

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enlarge the size of native nanoparticles, favoring thus a decrease of the superparamagnetic relaxation phenomena. A further important feature results from the significant decrease at both RT and 77 K of the mean isomer shift value: for the thicker layer (isomer shift d~0.38 mm/s at 300 K), the values are consistent with the presence of pure ferric species while a significant mean reduction of Fe is noted for the particles with the thinner Co-ferrite layer (isomer shift d~0.41 mm/s at 300 K), suggesting thus a mixture of magnetite €ssbauer spectrum obtained on the coreand Co-ferrite. The 77K Mo thicker shell nanoparticles (illustrated in Fig. 4j) exhibits a hyperfine structure with quite narrow lines, in agreement with a magnetically blocked structure. The spectrum can be well described by two discrete components attributed to ferric species well located in tetrahedral and octahedral sites. But a simulating approach involving magnetic sextets corresponding to ideal gFe2O3 and CoFe2O4 phases does not allow to estimate their respective proportions, because of similar values of their hyperfine parameters. In the case of the other two coreeshell nanoparticles, the mean isomer shift values suggest that they do not exclusively possess ferric species, suggesting thus a mixture of magnetite and maghemite recovered by a Co ferrite layer, preventing thus a quantitative description for the same reasons. Fig. 5 shows the results of the electron microscope investigations of gFe2O3, CoFe2O4 NPs and gFe2O3/CoFe2O4 core/ shell structures. We observe the formation of slightly agglomerated particles with a spherical shape. Individual CoFe2O4 nanoparticles had mean diameter 2e3 nm and were characterized by a narrow distribution in size (Fig. 5a), while the average size of individual

gFe2O3 nanoparticles was 3.5e4.5 nm (Fig. 5b). For the nanoparticles of core/shell structure with increasing thickness of the shell there is a growing mid-size as well as size distribution (Fig. 5c, d, e). As seen in table, the average particle diameter of nanoparticles and core/shell structures calculated from XRD well agrees with the results of electron microscopy investigations. Hysteresis loop magnetization measurements as function of the applied magnetic field were performed for the synthesized CoFe2O4 and gFe2O3 nanoparticles (Fig. 6A and B). The values of magnetic parameters are listed in Table 1. The measurements were carried out at T ¼ 300 K and 10 K (squares and open circles, respectively). The saturation magnetization in the field of 30 kOe field for CoFe2O4 nanoparticles is 28.1 and 36.4 emu/g at 300 and 10 K, respectively. At the same time, the saturation magnetization in the field of 30 kOe at 300 and 10 K for Fe3O4 nanoparticles is 33.3 and 44.2 emu/g, respectively. The magnetization curves M(H) are hysteretic at 10 K for CoFe2O4 and gFe2O3 nanoparticles (coercivity is near 9080 and 217 Oe, respectively) and almost completely anhysteretic at 300 K (insets of Fig. 6 A and B). Fig. 6C and D illustrate the temperature dependences of the magnetization of the CoFe2O4 and gFe2O3 powders, measured in field of Н measur ¼ 20 Oe after cooling the powder in zero field (ZFC) and in Н ¼ Н measur (FC). For the nanoparticles under study, the MFC(Т ) and MZFC(Т ) curves coincide in the vicinity of room temperature, but diverge in the low-temperature region. In addition, the kink corresponding to the Verwey transition temperature of stoichiometric magnetite is not observed at about 120K in the ZFC curve for gFe2O3 powders, allowing the occurrence of mainly

Fig. 6. Hysteresis loops for the CoFe2O4 (A) and gFe2O3 (B) nanoparticles at 300 K and 10 K (squares and open circles, respectively); Temperature dependences of MZFC and MFC in 20 Oe probing field for the CoFe2O4 (C) and gFe2O3 nanoparticles (D).

O.V. Yelenich et al. / Solid State Sciences 46 (2015) 19e26

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Table 1 Structural and magnetic characteristic of individual NPs and CSs. Composition parameter D, nma DScherer DTEM

sTEMb Hc, Oec At 300 K At 10 K Ms, emu/gd At 300 K At 10 K SLP, W/g a b c d

CoF2O4

g-Fe2O3

gFe2O3/CoF2O4 (shell ~ 1 nm)

gFe2O3/CoF2O4 (shell ~ 2.5 nm)

gFe2O3/CoF2O4 (shell ~ 3.5 nm)

2.8 2e4 ±0.5

3.9 3e5 ±0.5

4.3 5e8 ±1.1

7.1 6e11 ±2.5

10 8e18 ±3.9

9.9 9082

12.6 217.9

18.0 6601.0

12.1 6304.3

28.1 36.4 0.13

33.3 44.2 0.14

50.1 64.1 5.1

30.6 41.6 3.285

1.285

Average particle size. Standard deviation. Coercivity. Saturation magnetization.

maghemite to be confirmed. The MZFC(Т ) curves display a maximum at a temperature Т В z 140 and 80 К for CoFe2O4 and gFe2O3, respectively, which is called a blocking temperature [36]. Below TB the difference between MFC(Т ) and MZFC(Т ) becomes especially noticeable. The nature of the MFC(Т ) and MZFC(Т ) curves €ssbauer spectra (pres(presence of blocking temperature) and Mo ence of doublets instead sextet) indicate that CoFe2O4 and gFe2O3 nanoparticles synthesized from diethylene glycol solutions are characterized by superparamagnetic properties at room temperature. Fig. 7A shows the magnetization M vs magnetic field H dependences for the gFe2O3/CoFe2O4 core/shell structure synthesized with shell thickness of 2.5 nm. Estimated values of saturation magnetization and coercive field for core/shell structures with shell 1, 2.5 and 3.5 nm are listed in Table 1. The measurements were carried out at T ¼ 300 K and 10 K (squares and open circles, respectively). As follows from the results of research, the magnetization curves M(H) for nanoparticles with core/shell structure (shell 1, 2.5 and 3.5 nm) are weakly hysteretic at 300 K (inset of Fig. 7A) and hysteretic at 10 K (below the blocking temperature). Fig. 7B shows the temperature dependences of the magnetization of the core/shell structures (gFe2O3/CoF2O4), measured in field of Н measur ¼ 20 Oe after cooling the powder in zero field (ZFC) and in Н ¼ Н measur (FC). For the core/shell structures (shell ~ 1, 2.5 and 3.5 nm) under study, the MFC(Т ) and MZFC(Т ) curves display a blocking temperature TB z 250e300 K.

Fig. 8 shows variation of temperature of magnetic liquid containing nanoparticles (gFe2O3, CoFe2O4 or gFe2O3/CoFe2O4) versus residence time in AC-magnetic field (frequency of 300 kHz and magnetic field strength of 7.7 kA/m). It was determined that under long-term influence of acmagnetic field magnetic fluid based on individual CoFe2O4 and gFe2O3 particles are characterized by very low heating temperature. At the same time, magnetic fluids based on core/shell structures demonstrate higher heating effect and therefore characterized by much higher values of SLP (Table 1). Increase in SLP values when forming core/shell structures, where core e Fe3O4 and shell e CoFe2O4, is related to the increase of anisotropy of magnetic properties and, due to the appearance of magnetic exchange interaction between the magnetic core and shell [15]. However, when increase in the thickness of shell (CoFe2O4) is significant, the magnetic properties are mainly determined by the contribution from the CoFe2O4, which, as follows from the obtained results (see above) exhibits low SLP values. Therefore, there is a certain optimum ratio of core and shell size, when such core/shell structures show the maximum of SLP values. To the best of our knowledge, there is only one study on hyperthermic effects and the calculation of SLP values for similar core/shell structures in the literature [15]. This study was carried out under a magnetic field of 37.3 kA/m, a frequency of 500 kHz and magnetic fluids based on toluene and thus straightforward comparison with our data is not possible. Thus, possibility of synthesized gFe2O3/CoFe2O4 core/

Fig. 7. Hysteresis loops for the gFe2O3/CoF2O4 (shell ~ 2.5 nm) (A) at 300 K and 10 K (squares and open circles, respectively); Temperature dependences of MZFC and MFC in 20 Oe probing field for the gFe2O3/CoF2O4 (shell ~ 2.5 nm) (B).

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[9]

[10] [11]

[12]

[13]

[14]

[15] Fig. 8. Dependence of the temperature fluid heating versus residence time in applied ac-magnetic field. Magnetic fluid were prepared on the basis of different nanoparticles: a e CoFe2O4; b e gFe2O3; c - gFe2O3/CoFe2O4 (shell ~ 3.5 nm); d e gFe2O3/CoFe2O4 (shell ~ 2.5 nm); e  gFe2O3/CoFe2O4 (shell ~ 1 nm).

[16] [17]

shell structures to generate heat in ac-magnetic field (high values of specific loss power) and have superparamagnetic properties allow to consider it's as a potential mean for tumor hyperthermia.

[18] [19]

4. Conclusion In summary, crystalline CoFe2O4, Fe3xO3 nanoparticles and Fe3xO3/CoFe2O4 core/shell structures with a shell thickness of 1.0, 2.5 and 3.5 nm have been synthesized by precipitation from a diethylene glycol solution. The survey findings showed that the synthesized nanoparticles are characterized by spinel crystalline structure and have narrow size distribution. Investigations of €ssbauer spectra recorded at both 300K and 77 K which unamMo biguously reveal the main occurrence of Fe3þ ions, allow to conclude that magnetite has been nearby transformed into maghemite during the synthesis procedure. In addition, magnetic measurements showed that individual CoFe2O4 and gFe2O3 nanoparticles and core/shell structures are characterized by superparamagnetic behavior. Based on the fact that the gFe2O3/ CoFe2O4 core/shell structures are characterized by superparamagnetic properties and exhibit high values of specific loss power, one can conclude that they may be of significant interest for hyperthermia in the treatment of malignant tumors.

[20]

[21]

[22] [23] [24]

[25] [26] [27]

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