Structural, Chemical, and Magnetic Investigations of CoreâShell

Article pubs.acs.org/JPCC

Structural, Chemical, and Magnetic Investigations of Core−Shell Zinc Ferrite Nanoparticles J. A. Gomes,† G. M. Azevedo,‡,# J. Depeyrot,*,†,⊥ J. Mestnik-Filho,§ F. L. O. Paula,† F. A. Tourinho,∥ and R. Perzynski⊥ †

Complex Fluids Group, Instituto de Física, Universidade de Brasília, Caixa Postal 04455, 70919-970 Brasília (DF) Brazil Instituto de Física, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, Caixa-Postal 15051, 91501-970 Porto Alegre (RS) Brazil § Complex Fluids Group, Instituto de Química, Universidade de Brasília, Caixa Postal 04478, 70904-970 Brasília (DF), Brazil ∥ Instituto de Pesquisas Energéticas e Nucleares, Av. Prof. Lineu Prestes 2242, 05508-000 São Paulo (SP), Brazil ⊥ PECSA, Université Pierre et Marie Curie, UMR 7195, case 51, 4 place Jussieu, 75005 Paris, France ‡

ABSTRACT: We investigate here the internal structure of zinc ferrite nanoparticles designed and prepared by a soft chemistry method to elaborate magnetic nanocolloids. The strategy used to avoid acid dissolution modifies the chemical composition of the surface of the nanoparticles, which are described as a core of stoichiometric zinc ferrite surrounded by a maghemite shell. Measurements of X-ray absorption nearedge spectroscopy, extended X-ray absorption fine structure, and X-ray diffraction are undertaken to investigate the local structure of nontreated nanocrystals and of surface-treated ones as a function of their sizes. The qualitative analysis of X-ray absorption results indicates a nonequilibrium cation distribution among the interstitial sites of the zinc ferrite nanocrystals core. Ab-initio calculations of theoretical photoelectron backscattering phases and amplitudes give, by fitting Fourier transformed EXAFS data at both Zn and Fe K-edges, an average inversion degree of 0.34. This value well matches the result of Rietveld refinement of X-ray diffraction data. Magnetization measurements performed on dilute aqueous nanocrystal dispersions, liquid at room temperature and frozen at low temperatures, are carried out in order to test the obtained results. With this purpose, the “soft-chemistry” method has been developed to synthesize nanosized ferrites particles dispersible in aqueous media.7−10 It leads to spinel-type nanocrystals within 10 nm in size, with general molecular formula MFe2O4 where M is a divalent transition metal, namely Mn, Co, Ni, Cu, or Zn. Spinel ferrites crystallize in a face-centered cubic lattice (Fd3m) formed by a close-packed arrangement of 32 oxygen anions, creating 64 interstices of tetrahedral symmetry (A) and 32 interstices of octahedral symmetry (B). These sites are partially occupied: only 1/8 of the A-sites and 1/2 of the B-sites are filled by metallic cations. The following crystallographic representation can be used to specify the cation distribution among the tetrahedral and octahedral sites: [M(1−x)2+Fex3+]A[Mx2+Fe(2−x)3+]BO42−. Here x is the inversion degree defined as the fraction of A-sites occupied by Fe3+cations or the fraction of B-sites occupied by M2+ cations. To disperse properly ferrite nanoparticles in acidic media and obtain chemically stable magnetic nanocolloids, it is necessary to avoid their dissolution during the synthesis process. Thus, a core−shell strategy has been developed which consists in

I. INTRODUCTION Magnetic fluids, which are colloidal dispersions of magnetic nanoparticles in a liquid carrier, constitute a very attractive and promising class of nanomaterials. Thanks to the singular combination of their liquid and magnetic properties, these materials develop original responses to a powerful external parameter, the magnetic field. They may thus be confined, displaced, deformed, and controlled. These unique and striking features make them suited for a large number of applications in quite a diverse range of fields from engineering 1 to biomedicals.2,3 Biological applications basically require nanoparticles functionalized with biological effectors to make them interact with or bind to biological targets. This aim is usually achieved promptly, thanks to the high chemical reactivity of the nanocrystals surface. Among the different magnetic materials, spinel ferrite nanoparticles present several advantages that justify their use in biomedical applications. From the chemical point of view, they are not easily oxidized to nonmagnetic species,4 if properly protected. They offer a very reactive surface, which allows to attach ligands5 or to introduce electric charges6 in order to tune their interfacial properties. They also present low sedimentation rates, high colloidal stability, and facility to be functionalized with biological molecules. © 2012 American Chemical Society

Received: June 5, 2012 Revised: October 8, 2012 Published: October 16, 2012 24281

dx.doi.org/10.1021/jp3055069 | J. Phys. Chem. C 2012, 116, 24281−24291

The Journal of Physical Chemistry C

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section IV, taking into account the chemical core−shell model while fitting the experimental EXAFS and XRD data. Then, this crossed analysis provides us with values of the inversion degree that are compared with the value deduced from low temperature magnetization of isolated particles.

protecting the stoichiometric ferrite nanocore with a thin coat of maghemite.10 As various metallic elements M can be used, it leads to nanosized particles with different magnetic properties. As an example, for magnetic hyperthermia applications, the conversion of electromagnetic energy into heat can be increased by 1 order of magnitude, taking advantage of the exchange coupling between a magnetically hard core and magnetically soft shell.11 However, on the very small spatial scales, new intricate magnetic effects begin to play and modify such properties, like the appearance of superparamagnetism, finite size effects, and surface ones.12 For example, Ni ferrite nanoparticles are well-known to present magnetic properties associated with their disordered surface spins.13,14 In order to study specifically this surface magnetism, one could use nanoparticles with a ZnFe2O4 core, a ferrite that is not magnetic at room temperature. Indeed, in a zinc ferrite crystal with an ideal normal spinel structure, superexchange interactions between Fe3+ cations located at A- and B-sites do not exist since the filled tetrahedral sites are only occupied by nonmagnetic Zn2+ cations. As we will show, the spatial confinement at the nanoscale completely modifies the problem since the stoichiometric ZnFe2O4 cores produced here are magnetic at room temperature. Several works have already shown the existence of a significant magnetic moment of stoichiometric zinc ferrite nanoparticles at room temperature, and these findings are usually attributed to a partially inverted structure.9,10,15−19 Thus, it leads us to suspect that cationic redistribution occurs within the nanocrystals cores, making the synthesized ZnFe2O4 nanoparticles as suited for bioapplications as particles of the other ferrites. The inversion degree in redistributed spinels depends on several factors as, for example, synthesis procedure, spatial confinement on nanoscale, and nature as well as size of the cations.19−24 In stoichiometric zinc ferrite nanocrystals, the cation distribution has been extensively determined using Rietveld refinement of neutrons, diffraction spectra25,26 or Xray ones,27,28 in-field Mö ssbauer spectroscopy measurements,17,19,29,30 nuclear magnetic resonance,30 X-ray absorption,21,22,32−34 and X-ray circular magnetic dichroism.34 The aim of the present work is to understand these finite size effects by means of a systematic study on the local scale of such ZnFe2O4 nanograins. In particular, we investigate nontreated nanocrystals and surface-treated ones with different mean sizes in order to study the influence on their local structure with the surface treatment and the size variation. We check here their core−shell structure by combining synchrotron X-ray absorption near-edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS), and X-ray diffraction (XRD) experiments. These studies, performed on powders obtained at each step of the nanocolloid synthesis, are complemented by SQUID magnetization measurements that are realized with “gas-like” dilute dispersions of independent nanoparticles. For sake of clarity, the paper is organized as follows. In section II, we lay out the details of the experimental methods and the computational procedure we use to characterize the structure of the nanoparticles. In the section III, we present the chemical, structural, and magnetic (room temperature) general characteristics of the nanocrystals combined with qualitative results about their local structure as obtained by XANES and EXAFS. The overall results suggest a nonequilibrium cation distribution among interstitial sites of the structure that enhances the magnetic response of the nanoparticles. These results are subsequently discussed in

II. EXPERIMENTAL METHODS AND COMPUTATIONAL PROCEDURE A. Elaboration of Core−Shell Nanoparticles. Chemical Synthesis. The investigated samples are ZnFe2O4-based nanoparticles of aqueous magnetic fluids, which are prepared as described beforehand.9,10 All the reagents used in this work are products of analytical purity. In a first step, the nanocrystals are chemically synthesized by hydrothermal coprecipitation and are obtained by alkalinizing 1:2 mixtures of M2+ and Fe3+ salts with NaOH, at 100 °C and under vigorous stirring. During this process, one can control the mechanisms of nucleation and crystalline growth that rule their sizes. For example, it has been shown that acting on the velocity of mixing of the reagents, from a quick to a slow dropping procedure, while the temperature, reagent concentrations, and stirring rate are maintained constant, allows monitoring the nanoparticle size. Then, in this work, by varying the addition rate of the reagents, we have obtained a series of precipitates containing nanoparticles with mean size ranging typically between 8 and 14 nm. At the end of this step, the particles are chemically homogeneous and made of stoichiometric zinc ferrite. As they have to be further dispersed in acidic medium, their surface needs to be protected against acid dissolution. Thus, in a second step, the precipitate is submitted to an acid cleaning that consists in washing twice with water and once in a HNO3 solution (2 mol/L) in order to reverse the charge of the nanoparticles and eliminate undesirable less soluble byproducts formed during the first step. Then, each precipitate is hydrothermally treated with a 1 mol/L Fe(NO3)3 solution at 100 °C in order to ensure the thermodynamical stability of the nanocrystals, avoiding their degradation in acidic medium. The surface treatment induces iron enrichment of the nanoparticles, thus creating a superficial layer of maghemite that surrounds the particle core made of a zinc ferrite.32 In the third and last step, the nanoparticles, now of core−shell type, are dispersed in aqueous medium by monitoring the pH (surface potential and density of charge) and the ionic strength (screening of the surface potential) of the solution. The nanoparticles powders analyzed in this work are obtained after evaporation of the aqueous liquid. The produced samples are then labeled Z1 to Z5. Moreover, during the elaboration of sample Z4, some amount (called Z4A) of the precipitate is also collected soon after the coprecipitation step (nontreated nanoparticles) in order to study the influence of the hydrothermal surface treatment on their local structure. Chemical Analyses. The chemical composition of the nanoparticles is determined using suitable chemical titrations. Iron(III) titration is performed by dichromatometry. Zinc(II) titration is quantified using inductively coupled plasma atomic emission spectroscopy (ICP-AES). B. Synchrotron X-ray Studies. Powder Diffraction. Data of X-ray powder diffraction (XPD) are collected at room temperature at the D12A-XRD beamline of the Brazilian Synchrotron Light Laboratory (LNLS), using a double crystal Si(111) monochromator with constant offset, a graphite (002) analyzer, and a scintillation counter as detector. Samples are placed in a cylindrical sample holder that is spun during data 24282



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to the values known from crystallographic data. For Fe K-edge (Zn K-edge) only 7 (7) variables have been used during the fit, while 29 (15) independent parameters are available according to Nyquist’s criterion. Second, the fits of nanostructures spectra are performed with the values of E0 and S02 obtained for the crystalline standard. Debye−Waller factors are modeled as a sum of structural and dynamical terms, the latter being accounted for by a correlated Einstein model with the Einstein temperature as fitting parameter, cation distribution, interatomic distances, and coordination numbers being allowed to vary. The χ2 and the R-factor express here the quality of all the fits.40 C. Magnetization Measurements. Magnetization measurements of dilute solutions of magnetic fluid are performed at 300 and 5 K using a superconducting quantum interference device (SQUID) in the field range 0−4 × 103 kA/m. Their volume fraction of nanoparticles ϕp, their pH, and their ionic strength are adjusted in such a way that the colloidal dispersion can be considered as a “gas of individual particles” at room temperature.14,41 If the freezing process is sufficiently fast, the dispersion state of the magnetic solution is the same at 5 K, allowing therefore the determination of the high-field magnetization of isolated nanoparticles.

collection in order to minimize effects of an eventual preferential orientation. The measurements are performed with monochromatic X-ray beam, λ = 2.0633 Å (6.01 keV), with an approximate area of 4 × 1.5 mm2. Diffraction patterns are obtained typically within 20° ≤ 2θ ≤ 130° interval scanned with an angle step of 0.04°. The simplified integral breadth method, proposed by Williamson and Hall,35 allows for the deconvolution of the size and strain contributions to line broadening, by using β* = (1/DXR) + 2εd*, where β* = (β cos θ)/λ denotes the integral breadth in reciprocal space, β being the line broadening, λ the X-ray wavelength, 2θ the Bragg diffraction angle, and d* the corresponding reciprocal interplane spacing. Using the so-called Williamson−Hall plots, β* versus d*, allows us to determine the values of the diameter DXR and lattice strain ε. In order to complete the analyses, Rietveld structure refinement of the XPD data is performed using the GSAS software package developed by Larson and Von Dreele.36,37 It provides a fitting procedure of experimental results and allows determining here the lattice parameter, the oxygen position, and the cation distribution. The structure refinements are made with a peak shape modeled by a pseudo-Voigt function that includes correction for peak asymmetry and background intensity well accounted by a Chebyshev polynomial function. Each diffractogram is fitted until convergence, the best result being selected on the basis of reliability factor of refinement (Rwp) and quality of fit (χ2). Absorption Spectroscopy Measurements. XAS spectra are collected in transmission mode, at 20, 150, and 300 K, at the LNLS D04B-XAS beamline around the Fe and the Zn K-edges. Each spectrum corresponds to an average over three independent scans. The spectra are calibrated in energy by simultaneous measurements of transmission spectra of Fe and Zn foils. The energies E0 of the first inflection point at the absorption edge of these reference samples are respectively 7112 and 9662 eV. Background removal and isolation of EXAFS oscillations is performed by means of the AUTOBK algorithm.38 Then, the extended fine structure appearing above the absorption edge is isolated and normalized to the edge step height and energy, thus resulting on a spectrum with a “per-atom” basis scale. The raw experimental data in energy space is reduced to photoelectron wave vector and then Fourier transformed to radial coordinates using a Hanning window (between 2.6 and 11 Å−1 for Fe K-edge and 2.5 and 12.0 Å−1 for Zn K-edge). Thus, in this form the data directly reflect the average local environment around the absorbing atoms. Filtered EXAFS spectrum is obtained by back Fourier transformation between 1.0 and 4.6 Å. These processing steps are performed using the IFEFFIT package,38 and theoretical photoelectron backscattering phases and amplitudes are calculated ab initio via the FEFF8 code.39 For each of the cations, theoretical standards are generated with the cation occupying either the octahedral or the tetrahedral sites. Fits of the experimental spectra are performed (in real space) taking into account the weighted sum of all these contributions. The determination of the structural parameters is done as follows. First, the filtered EXAFS spectra of the ZnFe2O4 reference compound taken at different temperatures are fitted simultaneously. Debye−Waller factors σ2, interatomic distances of each path, energy threshold E0, and inelastic losses S02 are allowed to vary while coordination numbers N for all relevant multiple scattering paths and the cations distribution are fixed

III. RESULTS A. Characteristics of Core−Shell Nanoparticles. XRD patterns of samples Z4A and Z4 are presented in Figure 1, and

Figure 1. X-ray powder diffraction patterns of zinc ferrite nanoparticles collected before (Z4A) and after (Z4) the protective surface treatment. The intensity of the diffracted beam is plotted as a function of the scattering angle, 2θ, in degrees. The inset shows the Williamson−Hall plot for both samples; β is the full width at halfmaximum of the diffraction peaks, calculated accounting for additional source of broadening by using a Si standard monocrystal.

the same features are obtained for all others samples. Using Bragg’s law, the diffraction peaks are indexed and associated with only one crystalline phase, which corresponds to the spinel structure. Then, the averaged parameter of the cubic lattice ⟨a⟩ is determined. Table 1 lists the obtained values from the analysis of the eight most intense lines. For all samples, the cubic cell size varies between 8.41 and 8.44 Å, to be compared with the Zn ferrite bulk value of the Joint Committee on Powder Diffraction Standards (JCPDS card No. 82-1049), ⟨a⟩ASTM = 8.441 Å. These results also indicate that nanoparticles of different sizes and resulting from several chemical syntheses crystallize in a structure presenting similar lattice parameters. The inset of Figure 1 shows the Williamson−Hall 24283



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Table 1. Characteristics of ZnFe2O4-Based Nanoparticlesa samples

DXR (nm)

⟨a⟩ (Å)

ε (10−3)

χM

ϕs/ϕp

tsh (nm)

mp (kA/m)

Z4A Z1 Z2 Z3 Z4 Z5

9.6 7.9 8.5 11.7 12.4 13.9

8.42 8.41 8.42 8.42 8.43 8.44

0 2 2 2 2 1

0.33 0.20 0.19 0.23 0.26 0.27

0 0.42 0.45 0.32 0.23 0.20

0 0.71 0.83 0.79 0.58 0.55

208 171 163 188 207 187

The chemical characteristics and the values of the room temperature magnetization of the nanoparticles are also given in Table 1. The content of metallic cations is measured by chemical titrations giving the molar fraction of divalent metal, χM. The chemical core−shell model,10 based on a core of stoichiometric ferrite and a shell of maghemite, allows one to deduce the shell volume fraction ϕs/ϕp and the shell thickness tsh, calculated by using the crystalline size DXR. In Table 1 one can see that χM increases, while ϕs/ϕp decreases, as the particle size increases. Indeed, the surface-to-volume ratio largely increases with decreasing size, therefore offering more contact between the particles and the Fe(NO3)3 solution during the hydrothermal treatment performed to protect them against acid dissolution. The maghemite shell thickness is always of the order of the lattice parameter of the spinel cell. The nanoparticle magnetization mp obtained at 300 K is listed in the last column of Table 1. The values show that our ZnFe2O4-based nanoparticles present a magnetization that is, at room temperature, comparable to the values obtained for the others ferrite nanoparticles9,10 and very different from the zero value expected for the bulk material. It therefore suggests cation inversion in the ZnFe2O4 core of the nanoparticles with the presence of Zn2+ ions at octahedral sites. B. XANES. Figure 2a compares the experimental XANES spectra at Zn K-edge for bulk zinc ferrite and both nanocrystalline samples Z4A and Z4. The spectra exhibit the same three peaks, with the central one (9667 eV) more intense in the nanoparticles spectra than in the bulk spectrum, and the

DXR is the crystalline size and ε the strain, both deduced from the Williamson−Hall method, excepted for sample Z4A, deduced from Rietveld analysis;44 ⟨a⟩ is the average lattice parameter deduced using the Bragg’s law for the eight most intense diffracted line; χM is the molar fraction of divalent metal calculated using the molar concentrations of each metal measured by chemical titrations; ϕs/ϕp is the volume fraction of the maghemite shell and tsh its thickness, both determined10 using the values of DXR; mp is the particle magnetization determined9 for samples Z1 to Z5 from the Langevin analysis of magnetization curves, obtained at 300 K with dilute liquid dispersions, as a function of the applied field. For sample Z4A, the magnetization is measured on powder sample, and mp is the value of the magnetization at 4 × 103 kA/m. a

plot where β cos θ is plotted against sin θ for all the indexed lines of the diffractograms. Here, β is calculated accounting for additional source of broadening by using a Si standard monocrystal. A linear extrapolation is applied to this plot, and then the intercept gives λ/DXR and the slope gives 4ε. The values of the nanocrystals mean size are collected in Table 1. They show that as the velocity of mixing of the reagents varies from a quick to a slow dropping procedure, the nanoparticles average diameter continuously increases from 7.9 nm for sample Z1 to 13.9 nm for sample Z5. The size difference between nontreated nanoparticles (sample Z4A, DXR = 9.6 nm) and surface-treated ones (sample Z4, DXR = 12.4 nm) is due to the acid cleaning step performed soon after the coprecipitation, which leads to small particles dissolution and therefore shifts the average size toward larger value. Recently, such a method has been successfully employed on the X-ray peak profile to determine the lattice strain in maganites 42 and zinc ferrite43nanocrystals. The inset of Figure 1 shows that particles obtained soon after coprecipitation step (sample Z4A) do not present any strain. On the contrary, for samples Z1 to Z5 (only shown for sample Z4), a nonzero slope is observed, indicating the existence of internal stresses. The obtained strain values are collected in Table 1. Then, one can conclude here that the enrichment of the surface with iron induced by the surface treatment is accompanied by strain of the nanocrystal lattice, this effect being more pronounced as the nanocrystals size decreases. One would wonder if any nanosized particles do not suffer from the same kind of stress and always present nonzero strain. However, several studies, based on the Williamson−Hall plot to separate the line broadenings relative to the grains size effect and the strain effects have indicated that this is not always the case. In cerium oxide nanoparticles, it suggests no heterogeneous strain in nanoparticles smaller than 15 nm.44 In cobalt ferrite nanoparticles, the milling-induced high coercivity has been associated with the microstructural strain deduced from the Williamson−Hall plot.45 In this same study, the coprecipitated particle submitted to a low temperature annealing and mechanical milling remains in the 10−20 nm size and does not present strain before and after mechanical milling.

Figure 2. (a) Experimental XANES spectra, obtained at Zn K-edge, of zinc ferrite bulk material and nanoparticles samples Z4A and Z4. (b) FEFF calculations of the Zn K-edge in ZnFe2O4 nanoparticles as a function of the degree of site substitution. The solid black and red lines correspond to Fe absorbers at the Fe and Zn sites, respectively. The dashed lines correspond to the Zn Ferrite presenting and increasingly higher site inversion. The raw FEFF energy scale was offset by 10 eV for better comparison with the experimental results. 24284



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shoulder, centered in the bulk spectrum at 9675 eV, absent in the nanoparticles spectra. Figure 2b shows the absorption spectra calculated for several values of the inversion degree using the FEFF 8.4 code39 for Zn ferrite structures. Because XAFS probes the average environment around the absorbing atom, the calculations described in more detail elsewhere46 here include contributions due to Zn atoms located in both tetrahedral and octahedral sites (solid, black, and red curves, respectively). The evolution of the XANES spectra with the degree of site inversion is depicted in Figure 2b as dashed lines. They are obtained from weighted averages of the spectra corresponding to Zn in tetrahedral and octahedral sites (solid lines). Our calculations show that as the inversion degree increases, the intensities of the central peak, located at ∼9667 eV, increases and the intensity of the shoulder, located at 9675 eV, diminishes, in good qualitative agreement with our experimental results. Two comments are worth to be made. First, our observations are in good agreement with both the experimental and theoretical results reported in ref 22, in which similar FEFF calculations of bulk ZnFe2O4 were performed with the same SCF and FMS parameters, in the so-called “Z + 1 approximation”. Second, this qualitative behavior with increasing inversion degrees is the same as the one observed in the experimental spectra of Z4A and Z4 samples. Such results show unambiguously the presence of Zn2+ ions at octahedral sites of nanoparticles both before and after the surface treatment step of the synthesis. Room temperature XANES spectra at Fe K-edge, of references compounds, of sample Z4A and sample Z4 are presented in Figure 3. The absorption edge energy is taken as the maximum of the first derivative of the normalized absorbance. The values obtained for nanoparticles and bulk ZnFe2O4 samples are equal, whereas the absorption edge of other reference compounds, such as Fe3O4, FeO, and Fe, is shifted by a few electronvolts to lower energy due to their smaller average iron valence. The energy edges deduced from the spectra of Figure 3 shows, as expected, that the average valence of iron ions in the synthesized nanoparticles and in bulk samples is always equal to +3. Similar observations hold for the spectra taken at the K-edge of the divalent metal and indicate that the average oxidation state of zinc cations is equal to +2. Moreover, we observe that neither the surface treatment with ferric nitrate nor the nanograins size influences the valence state significantly. The inset of Figure 3 exhibits a magnification of the pre-edge region, which shows a small peak associated with the electronic 1s → 3d quadrupole and 1s → 3d/4p dipole transition. The prepeak observed for magnetite spectrum is more intense than for bulk zinc ferrite as its intensity is larger for cations localized at tetrahedral sites than for cations at octahedral sites.22,46 Both Z4A and Z4 nanoparticles samples have intermediate prepeak intensities, then corroborating the existence of iron ions both in tetrahedral and octahedral sites, differently from bulk zinc ferrite, where no Fe3+ ions occupy tetrahedral sites. Our Fe Kedge XANES experimental results are in good qualitative agreement with Fe K-edge FEFF calculations presented in Figure 3b, where the XANES spectra due to Fe ions located at the Fe and Zn crystallographic sites (solid red and black lines, respectively) are singled out. The dashed, dotted, and dotdashed lines correspond to the Zn Ferrite presenting increasingly higher degree of inversion and are obtained, as in Figure 2b, by suitable weighted average of the solid lines. As in Figure 3, the inset shows a magnification of the pre-edge

Figure 3. (a) Experimental XANES spectra, at Fe K-edge, of zinc ferrite and Fe3O4 bulk materials and nanoparticles samples Z4A and Z4. The inset displays the distinctive pre-edge resonances. (b) FEFF calculations of the Fe K-edge in ZnFe2O4 nanoparticles as a function of the degree of site substitution. The solid black and red lines correspond to Fe absorbers at the Fe and Zn sites, respectively. The dashed, dotted, and dot-dashed lines correspond to the Zn ferrite presenting and increasingly higher site inversion. The inset exhibits a magnification of the pre-edge region.

region. It is readily noticeable that the same trends observed for the pre-edge peaks in the experimental data are reproduced by the FEFF calculations. It thus corroborates the existence of cation inversion in our nanoscaled samples, with iron ions in both tetrahedral and octahedral sites, differently from bulk zinc ferrite, where Fe3+ ions are not present in tetrahedral sites. C. EXAFS. Figure 4 presents Fourier-transformed EXAFS data taken at 20 K at Zn K-edge, for samples of nanoparticles Z4A and Z4 and of bulk material. In all the cases, the Fourier

Figure 4. Magnitude of the k3-weighted Fourier-transformed EXAFS signals taken at 20 K, at Zn K-edge, for zinc ferrite bulk material and nanoparticles samples Z4A and Z4. 24285



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transform amplitude is extracted from EXAFS signal using the same parameters. Then the data can be represented in the same coordinate system and allow a direct comparison. All spectra exhibit two major contributions always located around the same radial distances, indicating that both nanocrystals and bulk compound present very similar local structures, a result consistent with XRD patterns of Figure 1. For radial coordinates larger than 4 Å, the observed peaks have small amplitudes showing that EXAFS data are overall mainly sensitive to the first coordination shells. The amplitudes of the spectra of nanoparticles are systematically lower than bulk counterpart due to structural disorder and/or reduced coordination number. Indeed, the atoms localized in the surface shell are likely under-coordinated, and their proportion largely increases with the size reduction. The first peak of bulk spectrum, centered close to 1.5 Å, is associated with the first coordination shell of oxygen atoms around the zinc cations. The second one, located around 3 Å, is assigned mainly to the second shell of neighboring atoms and includes simple and multiple scattering paths involving zinc ions located at tetrahedral sites. The nanocrystals spectra present the same two main contributions. Nevertheless, one peak develops around 2.6 Å as a consequence of additional scattering paths involving zinc ions located at octahedral sites. This behavior therefore confirms the previous analyses of XANES data, and the whole of these results strongly support a cationic distribution of metallic cations inside the nanocrystals different from that of ideal ZnFe2O4 material. Moreover, this cation inversion occurs during the coprecipitation step of the synthesis as indicated by the spectra obtained for sample Z4A from Figures 2−4.

An analogous behavior is observed for Fe ions. Thus, these calculations demonstrate that, in our samples of nanoparticles, which are all well larger than 5 nm in diameter, our hypothesis on the variations of the coordination number is sound. Figure 6a shows, for samples Z4A and Z4, the Fouriertransformed EXAFS data taken at 20 K at Zn K-edge (open symbols) and their respective best fit (solid line) obtained as described in section II. Filtered contributions of the first

IV. DISCUSSION In the following we propose a crystalline core−shell model of nanoparticles, probed here by combining the quantitative analyses of EXAFS spectra with Rietveld refinement of XRD data. In order to account for the modifications of nanocrystal structure induced by the surface treatment during the synthesis procedure, we first examine sample Z4A. In that sample, the nanoparticles have not been surface-treated and thus do not present any maghemite shell. Then, the associated surfacetreated sample Z4 is examined using the core−shell scheme for Rietveld analysis of XRD spectra, and the same method is employed for nanoparticles with different sizes. Finally, we show that magnetization results obtained at low temperature corroborate our structural analysis. A. EXAFS. Fitted Fourier-transformed EXAFS data are obtained as detailed in part B of section II. Measurements performed at various temperatures are fitted simultaneously in order to reduce the correlation between the Debye−Waller factor and the coordination number. Moreover, we impose that the variations of the coordination numbers occur in the same proportion for all coordination shells. To check the validity of this hypothesis, we calculated using a FORTRAN code, and considering an ideal zinc ferrite structure, the average coordination number of each coordination shell as a function of the nanoparticle diameter. The variations of NNi, the average coordination number of the ith shell normalized to its bulk value, are obtained for Zn ions (1 ≤ i ≤ 9) and presented in Figure 5. Its inset depicts the size dependence of the ratio NN2/NN1, and the whole Figure 5 typically shows that above a diameter of 5 nm, the relative variations of the NNi are approximately the same whatever i.

Figure 6. (a) Fourier transform amplitude of the experimental EXAFS spectrum (dots) and its fit (line) obtained for samples Z4A and Z4at Zn K-edge at 20 K. (b) Filtered contributions of the first shells of coordination for samples Z4A and Z4.

Figure 5. Average coordination number of zinc ions in the ith coordination shell normalized to its bulk value NNi (i = 1−9), as a function of the nanoparticles diameter. The inset shows the size dependence of the ratio between the coordination number of the second and first shells. The vertical line around 5 nm indicates the threshold at which the variations of the coordination numbers occur in the same proportion.

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Table 2. Interatomic Distances, in Å, Obtained by Fitting the Fourier-Transformed EXAFS Data, at Both Fe and Zn K-Edgesa sample bulk Z4A Z4 Z5 Z1 a

Fe(A)−O

Fe(B)−O

Fe(B)−MC

1.85 1.85 1.85 1.85

2.05 2.02 2.01 2.01 2.01

2.96 2.91 2.90 2.90 2.90

Fe(A)−MC

Zn(A)−O

Zn(B)−O

Zn(B)−MC

Zn(A)−MC

3.41 3.40 3.40 3.40

1.92 1.92 1.92 1.92 1.92

2.09 2.09 2.09 2.09

3.01 3.01 3.01 3.01

3.54 3.53 3.53 3.53 3.53

The associated error is ±0.01 Å.

coordination shell for both samples are presented in Figure 6b. At both Fe and Zn K-edges, the fit of the spectra includes contributions of single and multiple scattering. As expected for ferrite nanocrystals and as we observed here, only simple scattering paths are relevant to describe the signal of the first two coordination shells around the absorbing atoms.47 That is not the case for more distant coordination shells, where multiple scattering contributions should necessarily be taken into account. As seen in Figures 6a and 6b, the agreement between the experimental data and the fit is good in the 1−3.5 Å range, for both nontreated and surface treated nanocrystals. Similar agreement is observed for all the other samples analyzed in this work. Table 2 summarizes the results of interatomic distances, in angstroms, obtained at both edges for all the samples. As an example, Fe(A)−O stands for the distance between iron ions in tetrahedral sites and the first layer of oxygen anions, Fe(B)− MC stands for the distance between the iron ions in octahedral sites and the first layer of metallic cation (iron or zinc, depending on the cation distribution in the nanocrystal), and Zn(B)−O corresponds to the distance between the inverted zinc ions in octahedral sites and the first layer of oxygen ions. The values relative to nanoscaled samples present interatomic distances very close to the values obtained for the reference bulk material, therefore confirming that all the nanomaterials investigated here crystallize in a spinel-type arrangement. The comparison of the interatomic distances between samples Z4A and Z4 shows that the ion−ion distances are not affected by the protective surface treatment. Moreover, it can also be seen that the collected values do not depend on the nanocrystal size. The values of the relative average number of coordination NN, obtained now by considering the contributions of all the coordination shells, are presented in Table 3. At Fe K-edge, the average coordination of iron atoms, for nanoparticles obtained soon after the coprecipitation step (sample Z4A), is found to be

65% of its ideal bulk value. A similar reduction of the average coordination of zinc ions is observed after analysis at the Zn Kedge. Such reduced average coordination is ascribed to surface effects, since atoms sitting near the surface of the particles have smaller coordination numbers as compared to atoms sitting within the core. In the case of surface-treated nanoparticles (sample Z4), the average coordination of iron atoms decreases even more and reaches only 48% of the bulk coordination number. This value being much smaller than the one corresponding to the particles obtained soon after the coprecipitation suggests that the iron atoms, incorporated to the nanocrystal shell during the protective surface treatment, are largely under-coordinated. On the contrary, at Zn K-edge, NN varies from 69% in sample Z4A to 78% in sample Z4. The larger value obtained with surface-treated nanoparticles is attributed to zinc ions, which, after the surface treatment, are mainly localized within the nanocrystal core and therefore more coordinated. At both edges, size-sorted samples Z1 and Z5 also present approximately the same reduction of the average coordination when compared to sample Z4. Table 3 also lists the filling rate of tetrahedral sites determined at both Zn and Fe K-edges and the inversion degree x deduced from the analysis at Zn K-edge and associated with the zinc ferrite core of the nanoparticles in surface treated samples. As mentioned in the Introduction, ideal zinc ferrite crystallizes in a normal spinel structure, with zinc ions located at tetrahedral sites and iron ions located at octahedral sites (x = 0). Then, for bulk sample, the percentage of zinc ions at tetrahedral sites is found to be 100, as expected. For sample Z4A, the results obtained at Fe K-edge indicate that 34% of the tetrahedral sites are occupied by iron ions. Furthermore, the fraction of A-site filled by zinc ions, derived from EXAFS fits at Zn K-edge, is 64%. The agreement between both amounts, derived from a collection of different experiments, can be considered as excellent since their sum almost matches 100. It suggests that nanoparticles obtained just after the coprecipitation step already present a different cation distribution as compared to the normal spinel structure. It is characterized by an inversion degree deduced at Zn K-edge equal to 0.36(2), which corresponds to the migration, on average, of 2.88 zinc ions from A-sites to B-sites, per unit cell. In the literature, the occurrence of such a partial inversion appears as strongly dependent on the method used to prepare the ZnFe2O4 nanomaterial. Methods that use high-temperature treatment generally lead to the ideal normal spinel structure,20,25,48 whereas high energy ball milling,17,26 mechanochemistry,49,50 and soft chemical routes15,16,22,27−29 induce a nonequilibrium cation distribution among the interstitial sites. The inversion degree, found here for our coprecipitated nanoparticles, compares well with the values relative to others nanocrystals also obtained by soft chemical routes. After the protective treatment of the nanoparticles surface, the fraction of A-sites occupied by zinc ions remains constant,

Table 3. Results of Fourier-Transformed EXAFS Data at Both Zn and Fe K-Edgesa filling rate at A-sites (%) samples

NN Fe K-edge (%)

NN Zn K-edge (%)

Fe K-edge

Zn K-edge

x

bulk Z4A Z4 Z5 Z1

100 65 48 51 54

100 69 78 82 81

00 34 52 53 55

100 64 64 67 68

0 0.36 0.36 0.33 0.32

a

NN is the relative average number of coordination, in percentage of the ideal bulk value; x is the inversion degree deduced from the analysis at Zn K-edge (associated with the zinc ferrite core of the nanoparticles in surface-treated samples). The associated errors are ±3%, ±5%, and ±5%, respectively. 24287



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distribution in the ZnFe2O4 core is fixed to the value obtained from the Rietveld refinement of sample Z4A, a value which agrees well with EXAFS determinations and is independent of the nanocrystal size. One can wonder if such an analysis would work since the surface layer of maghemite could be too thin in order to allow for a reliable result. However, all of the fits presented better reliability factors whenever the two phases are taken into account. Figure 7a,b well illustrates this result for sample Z4.

equal to 64%, as shown from the analysis at Zn K-edge for sample Z4. It suggests that the cation distribution of the ZnFe2O4 nanoparticle core is not affected by the surface treatment. Table 3 also presents the cation distributions obtained for samples Z1 and Z5. Both show an inversion degree different from that of the bulk material but very similar to the value determined for sample Z4, equal to 0.36. At Fe K-edge, the value found for the filling rate of tetrahedral sites by iron ions increases from 34% to 52% and the sum of the filling rates determined at both edges is no more around 100. This is not surprising as the tetrahedral sites now seen at Fe K-edge are no longer related to the zinc ferrite particle core but associated with the whole particle. It therefore means that the iron ions incorporated to the maghemite surface shell mainly localizes at tetrahedral sites. Indeed, the hydrothermal surface treatment induces, first, the release of zinc ions from the nanocrystal shell and, second, the enrichment of this shell by iron ions.10 This is clearly seen with the crystallographic representation deduced from our results. Using the filling rate of tetrahedral site derived at Fe K-edge, the formula that corresponds to the particle structure obtained just after the coprecipitation step writes [Zn5.32+Fe2.73+]A[Zn2.72+Fe13.33+]BO322−. After the surface treatment, this representation does not stand anymore for the whole particle structure but only for that of the particle core. The formula associated with the crystalline structure of the shell is now the maghemite formula that writes [Fe83+]A[Fe13.33+□2.7]BO322−. Thus, one can deduce that at octahedral sites, the vacancies represented by □, correspond to the release of zinc ions from the outer layer and at tetrahedral sites, the released zinc ions are substituted by iron ions. B. Rietveld Refinement of XRD. We have already fitted Xray diffraction data of sample Z4A by using the Rietveld analysis performed in a one-phase (spinel-type) model, and the procedure is detailed elsewhere.46 The nanoparticles mean size deduced from the refinement of the diffracted intensity profile is 9.6 nm. The lattice parameter is found to be equal to 8.430(1) Å, in good agreement with the standard JCPDS value of 8.441 Å (JCPDS card No. 82-1049). Note that the value of Table 1 equal to 8.42 Å, obtained by using Bragg’s law applied to the eight most intense diffracted lines, well agrees with that given by the Rietveld approach. The value found for the inversion degree, x = 0.33(2), matches excellently well our EXAFS determination, x = 0.36(2). Let us now focus on the X-ray diffraction data corresponding to the samples obtained after the surface treatment. Because of the enrichment of the nanocrystal surface with iron atoms, a more realistic modeling is obtained in terms of a crystalline core−shell model. Rietveld refinements of the diffraction patterns of samples Z1 to Z5 are therefore performed using two phases of spinel type, namely, the zinc ferrite core and the protective surface layer of maghemite.10 The percentage of each phase corresponds to the values determined by chemical analysis and collected in Table 1. Both the nanoparticles diameter and the strain parameter are fixed by the results obtained from the Williamson−Hall method presented in section III. Indeed, in our case, there is no way to obtain the full width at half-maximum separately for each phase. However, the Williamson−Hall is a good approximation to determine the average line width. We use it in the refinement with two phases, and we see that the result is better than refining with a single phase. Thus, in our fit, the refined structural parameters are the cubic cell sizes, acore and ashell, and the oxygen positions, ucore and ushell, of each phase. Then, for each sample, the cation

Figure 7. Rietveld refinement of XRD patterns. (a) Sample Z4: the refinement is performed using only one spinel-type phase (χ2 = 7.24, Rwp = 8.34%). (b) Sample Z4: the refinement is performed using two spinel-type phases (χ2 = 4.86, Rwp = 5.25%); X-ray data are shown as plus marks; the solid lines are the best fit to the data and the tic marks show the positions of the allowed reflections of the used phases. The lower curves represent the difference between observed and calculated profiles. The inset of (b) displays the density of the nanoparticles dp deduced from the Rietveld refinement, as a function of the nanoparticle diameter DXR.

Figure 7a presents the Rietveld refinement of sample Z4 obtained using only one spinel-type phase: the reliability factors are χ2 = 7.24 and Rwp = 8.34%. When two spinel-type phases are used, these parameters become χ2 = 4.86 and Rwp = 5.25%. Their values are significantly lower, and Figure 7b also shows for sample Z4 the better agreement between calculated and experimental data. In particular, it can easily be seen that the contribution of the [440] peak is better fitted. All the other samples present similar fitting quality by using the two spineltype phases, which is confirmed by the low values of χ2 and Rwp collected in Table 4. In each sample, we have also tested the refinement results for slight variations of the nanoparticles size and strain. No improvement of the reliability factors is obtained when considering variations of about 10%. The fit is for all the samples better when using two spinel-type phases, and this result can be probably attributed, first, to the large relative volume of the shell, which, for samples Z1 to Z5, varies between 20% and 45% of the whole particle, respectively. 24288



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Table 4. Crystallographic Parameters of Core−Shell Nanoparticles Deduced from Rietveld Analysis of XRD Data Using Two Spinel Ferrite Phasesa samples Z1 Z2 Z3 Z4 Z5

acore (Å)

ashell (Å)

8.445 8.458 8.431 8.431 8.425

8.395 8.413 8.396 8.396 8.432

(1) (1) (1) (1) (1)

(1) (1) (1) (1) (1)

ucore 0.253 0.251 0.255 0.254 0.256

ushell

(1) (1) (1) (1) (1)

0.259 0.258 0.259 0.258 0.253

(1) (1) (1) (1) (1)

dp (g/cm3)

χ2

Rwp (%)

5.09 5.05 5.16 5.21 5.21

2.23 2.93 4.36 4.86 2.77

3.96 5.07 4.95 5.25 5.04

a

acore and ashell are the cubic cell size of the zinc ferrite core and the maghemite shell, ucore and ushell are the oxygen parameter of these two phases, dp is the density of the core−shell nanoparticles, χ2 characterizes the quality of fit, and Rwp is the reliability factor of refinement.

in the case of zinc ferrite, reduces to mS = 10xNdμB/MM. We therefore performed magnetization measurements at low temperature in order to evaluate the inversion degree associated with the ZnFe2O4 core of investigated nanoparticles. Figure 8 shows the magnetization curves performed at 5 K on dilute magnetic solutions of samples Z1, based on smaller

Second, it has to be considered the relatively high contrast between the Zn and Fe X-ray form factors. Table 4 also lists the cubic lattice parameter determined for the ferrite core, which matches well with the value of standard bulk material. Note that the cell size obtained for the maghemite shell is larger than the maghemite reference value, equal to 8.346 Å (JCPDS card No. 39-1346), and smaller than the lattice parameter of the core phase. Indeed, the maghemite layer can be seen as an epitaxial continuation of the core structure, and this last result illustrates the reduction of the cell mismatch at the core−shell interface. Moreover, as already pointed out for sample Z4A, the values of the oxygen positions, determined for both the spinel phases of the core−shell model, do not correspond to a perfect compact packing. The value obtained for the maghemite shell is larger than the value corresponding to the zinc ferrite core. It is probably related to the protective surface treatment used to avoid the acid dissolution of the particles that enhances the distortions of oxygen atoms mainly at the surface layer. Finally, the density dp of the core−shell nanoparticles can be determined by using a weighted sum of core and shell contributions where dcore = 5.33 ± 0.04 g/cm3 and dshell = 4.76 ± 0.03 g/cm3 are the densities, directly obtained from the Rietveld refinement, of the core and of the shell, respectively. Both values well compare with generally used ones around 4.9 g/cm3 for maghemite51,52 and 5.3 g/cm3 for ZnFe2O4.53,54 As shown in the inset of Figure 7b, the density of the nanoparticles increases as the nanocrystal size increases, since the proportion of the maghemite shell becomes smaller. A linear extrapolation of the nanoparticles density as a function of the nanoparticles diameter DXR would lead to the density of ZnFe2O4 bulk material for particle sizes larger than 20 nm. C. Magnetization Measurements. In spinel ferrites, the metallic cations located at A- and B-sites occupy the nodes of two subnetworks of spins and the superexchange interaction between them favors antiparallel alignment of spins, leading to antiferromagnetic ordering. However, thanks to the difference between the numbers of A- and B-sites and to the way they are occupied, the overall behavior is ferrimagnetic. Then, if the distribution of the metallic cations inside A- and B-sites and the magnetic moment of each ion are known, the saturation magnetization of the ferrite at T = 0 K writes55 mS = (Nd/ MM)[∑BnB,B − ∑AnB,A]μB, where nB,i is the number of Böhr magnetons, μB, associated with the i site of the unit cell, MM is the molar mass of ferrite, d its density, and N is Avogadro’s number. The summation extends over the A and B sites of the unit cell. In nanocrystals based on spinel ferrites, cation redistribution can modify significantly the properties of saturation magnetization when compared to bulk material. However, it would be theoretically possible to deduce the inversion degree x from the value of the magnetization, which,

Figure 8. Magnetization curves recorded at 5 K for samples Z1, Z3 and Z4.

particles, and Z3 and Z4, based on larger ones. All show a good saturation performance and the high field magnetization that can be measured is 368, 391, and 404 kA/m for particles of 7.9, 11.7, and 12.4 nm, respectively. These large and unexpected values are related to the presence of Fe3+ at A-sites of the spinel structure associated with the zinc ferrite core. Indeed, strong superexchange interactions between Fe3+ ions located at both sites of the spinel nanostructure can develop. From this high field values at low temperature, the cation inversion parameter x related to the redistributed zinc ferrite core was determined, considering the saturation magnetization of the nanocrystal as a sum of weighted contributions of a zinc ferrite core and a maghemite shell. Then, using, the volume fraction of core and shell of Table 1 and a magnetization of the maghemite shell equal to 350 kA/m,56 it is found x = 0.295 for sample Z1, x = 0.33 for sample Z3, and x = 0.34 for sample Z4. These values are in excellent agreement with those deduced fitting EXAFS spectra (see Table 3) and with the value found by Rietveld analysis of X-ray diffractogram for nontreated nanoparticles, which is also relative to the ZnFe2O4 core of surface-treated nanoparticles.

V. CONCLUSION We have investigated at the atomic level the structure of ferrite nanoparticles especially designed to chemically synthesize aqueous magnetic nanocolloids. These particles are made of a zinc ferrite core surrounded by a protective maghemite shell. Because of the close connection between magnetic properties, 24289


The Journal of Physical Chemistry C nanocrystal chemistry and structure, we have shown that a clear understanding of the internal nanoparticles atomic arrangement at each step of the chemical synthesis is essential to account for their magnetic properties. Although the heterogeneity of the chemical composition modifies the problem, it has been possible to study the nonequilibrium site occupancy in the zinc ferrite core of the particles, by crossing the analyses of Fe and Zn K-edge X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure measurements, Rietveld refinement of X-ray diffraction patterns, and magnetization curves. The very good agreement between the qualitative features of experimental Zn K-edge XANES spectra and their ab initio calculated counterparts clearly shows the large transference of zinc ions from tetrahedral sites to octahedral ones. Moreover, the intensity profile of the Fe K-edge prepeak associated with the electronic 1s → 3d quadrupole and 1s → 3d/4p dipole transition indicates the consequent Fe3+ inversion from B-sites to A-ones. This cation redistribution takes place during the coprecipitation step without altering the long-range structural order of the whole nanocrystals. These results are also confirmed by the qualitative analysis of Fourier-transformed EXAFS data obtained at Zn−K edge. Fourier-transformed EXAFS data obtained at both edges are fitted using ab initio calculations of theoretical photoelectron backscattering phases and amplitudes. The hypothesis made on the variations of the coordination numbers, occurring in the same proportion for all coordination shells, is carefully checked. The good agreement between experimental and calculated spectra allows an accurate quantitative analysis. Our results show that the interatomic distances are very close to those of standard bulk ferrites and affected neither by the protective surface treatment nor by the size reduction. As a consequence of interface effects, the average coordination number of surface atoms decreases and this reduction is enhanced after the surface treatment. It is also concluded that the averaged percentage of Zn2+ ions located at B sites of the zinc ferrite core is around 34%, a value that does not change after the surface treatment and is independent of the size of the particles. This degree of cationic exchange is also found by Rietveld refinement of X-ray diffractogram using a one-phase modeling for nontreated particles. Finally, high field magnetization measurements performed at low temperatures are strongly consistent with the presence of Fe3+ at A sites of the spinel nanostructure cores and the consequent development of superexchange interactions between iron ions at both interstitial sites. In the future, it would be interesting to perform in-field Mössbauer experiments which allow resolving the sextet contributions corresponding to Fe3+ spins at A and B sites due to their antiparallel ordering. We also plan to realize neutron diffraction in order to get a better contrast between the contributions of both metallic cations. This would probably be very helpful in order to develop an actual core−shell model as well for the diffraction analysis as for the theoretical EXAFS calculations.

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ACKNOWLEDGMENTS

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REFERENCES

Article

The authors thank the Brazilian agencies CNPq, FAPESP, FAPDF, and FINATEC and are greatly indebted to LNLS for beam-time obtained on D12A-XRD1 beamline (X-ray diffraction) and on D04B-XAFS1 beamline (X-ray absorption). J. Depeyrot is very grateful for the CAPES grant contract BEX 1439/11-1, and we acknowledge the exchange program CAPES-COFECUB no. 714/11 and PICS no. 5939. We also thank M. H. Sousa for technical assistance.

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AUTHOR INFORMATION

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*E-mail depeyrot@fis.unb.br. Notes

The authors declare no competing financial interest. # ́ Formerly at Laboratório Nacional de Luz Sincrotron - LNLS, Caixa Postal 6192, 13084-971 Campinas (SP), Brazil. 24290



Article

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Structural, Chemical, and Magnetic Investigations of CoreâShell

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