Local Magnetic and Electronic Structure of the Surface Region of

Article pubs.acs.org/JPCC

Local Magnetic and Electronic Structure of the Surface Region of Postsynthesis Oxidized Iron Oxide Nanoparticles for Magnetic Resonance Imaging Christina Graf,*,†,∥ Christian Goroncy,†,∥ Patrick Stumpf,† Eugen Weschke,‡ Christine Boeglin,§ Hendrik Ronneburg,† and Eckart Rühl*,†

Downloaded by TEXAS A&M INTL UNIV on August 28, 2015 | http://pubs.acs.org Publication Date (Web): August 5, 2015 | doi: 10.1021/jp512023z

†

Institut für Chemie und Biochemie−Physikalische und Theoretische Chemie, Freie Universität Berlin, Takustr. 3, 14195 Berlin, Germany ‡ Helmholtz-Zentrum Berlin für Materialien und Energie, Albert Einstein-Str.15, 12489 Berlin, Germany § Institut de Physique et de Chimie de Strasbourg, UMR7504, CNRS et Université de Strasbourg, 23, rue du Loess, 67034 Strasbourg, France S Supporting Information *

ABSTRACT: Iron oxide nanoparticles (FeOx-NP) are applied in medicine as contrast agents in magnetic resonance imaging (MRI) where they reduce the spin−spin relaxation time (T2-time) of absorbing tissue. Hence, control of their magnetic properties is essential for these applications. Magnetic properties strongly depend on the particle size and shape as well as the surface functionalization of the iron oxide nanoparticles. Especially, structural and magnetic disorder in the region close to the surface (1−2 nm) lead usually to a reduced magnetization compared to the corresponding bulk material. Therefore, X-ray magnetic circular dichroism (XMCD) in the total electron yield (TEY) mode is used to investigate local magnetic and electronic properties of the surface region of monodisperse, spherical FeOx-NPs (Fe3O4/γ-Fe2O3) before and after the postsynthetic treatment in oxygen-rich environment. Charge transfer multiplet calculations of the XMCD spectra are performed to analyze the contributions of Fe2+ and Fe3+ at different lattice sites, i.e., either in octahedral or tetrahedral environment. The analysis of the XMCD data reveals that both, the magnetization of the nanoparticle surface region as well as their maghemite to magnetite ratio, are strongly increased after tempering in an oxidative environment, which likely causes rearrangement of their crystalline order. The magnitude and the kinetics of these variables depend strongly on the particle size. In addition, after thermal annealing a reduced spin canting is extrapolated from the lower magnetic coercivity, which confirms that a structural rearrangement takes place.

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refluxing time,7,8,13 the type of precursor,3,8,14 the possible use of preformed iron oxide nanoparticles as seeds,3,9,11,15 and the ratio of the iron precursor to additional oleate.7,9,16 In contrast to this, it is still challenging to exactly determine and control the composition of these iron oxide nanoparticles, whether they are synthesized by thermal decomposition or precipitation reactions. The nanoparticles obtained from these syntheses consist usually of magnetite (Fe3O4) and maghemite (γ-Fe2O3) in variable mixing ratios.3,4 Both compounds have the same crystal structure, the inverse spinel. Magnetite contains in a 1:1:1 ratio Fe2+ in the octahedral sites (Fe2+Oh), Fe3+ in the octahedral sites (Fe3+Oh), and Fe3+ in the tetrahedral sites (Fe3+Td). This composition can be described by the formula Fe3+[Fe3+Fe2+]O42− where the octahedral species are written within the square brackets. In contrary, maghemite contains only Fe3+, which is located in octahedral sites (Fe3+Oh) as well as in tetrahedral sites (Fe3+Td) with an Fe3+Oh/Fe3+Td ratio of

INTRODUCTION Iron oxide nanoparticles (FeOx-NPs) are applied in medicine as contrast agents in magnetic resonance imaging (MRI), where they reduce the spin−spin relaxation time (T2-time) of absorbing tissue. Up to now, only iron oxide nanoparticles from simple precipitation synthesis in aqueous solution have been used in clinical applications.1,2 These are ill-defined, polydisperse, and polymorph particles. In contrast, modern metallo-organic high temperature syntheses yield well-defined nanoparticles of low polydispersity (300 °C), e.g., octadecene or trioctylamine. The shape and size of such particles can be well adjusted by changing the reaction conditions, for example, the refluxing temperature of the solvent,4,6−11 the heating rate,6,12 the © 2015 American Chemical Society

Received: December 2, 2014 Revised: July 21, 2015 Published: July 22, 2015 19404

DOI: 10.1021/jp512023z J. Phys. Chem. C 2015, 119, 19404−19414

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The Journal of Physical Chemistry C 1.67:1, which can be written as Fe3+[Fe5/33+□1/3]O42−. Here □ symbolize vacancies that are required for charge neutrality.17,18 The oxidation from magnetite to maghemite occurs only in oxygen-containing environments at temperatures below 300− 350 °C. This process takes place in minutes to days, depending on the reaction temperature.18,19 Above 300−350 °C the formation of the rhombohedral hematite (α-Fe2O3) occurs.18 The transition from magnetite to maghemite can be described by using the spinel notation where the parameter δ, in the following referred to as degree of oxidation, has a value range between 0 and 0.33, as follows:18

approaches, such as thermal decomposition of metallo-organic precursors as well as capping with organic ligands, such as oleic acid, can strongly reduce surface canting and hence largely preserve the surface magnetization.17,31−34 Even for the systems from the latter approaches, which are similar to those in the present work, recent electron magnetic chiral dichroism (EMCD) studies from Salafranca et al. suggest that the magnetic moment within 1 nm of the particle surface is up to 30% smaller than the magnetization of the core, even at room temperature.32 In less optimized systems, such as γ-Fe2O3 nanoparticles from simple precipitation syntheses, this surface region is highly disordered and often described as a magnetically “dead layer”.32,34,35 Brice-Profeta et al. have already shown in 2005 that XMCD can be used to investigate the occurrence and the effect of spin canting22 since measuring the total electron yield (TEY) is a relative surface sensitive method (sampling depths between less than 1 nm36 to a few nm37−40 are given in the literature). Results of such XMCD studies show excellent agreement with recent theoretical models explaining the origin of surface canting within iron oxide nanoparticles by a Td site reorientation effect assuming that surface canting involves competition between magnetocrystalline, dipolar, exchange, and Zeeman energies.31 Here, we present a systematic study on the effect of postsynthetic oxidation on the properties of the surface region of monodisperse, spherical FeOx-NPs. These FeOx-NPs are prepared by a high temperature approach in organic solvents using iron oleate as a precursor. After the synthesis the particles are tempered in an oxygen rich environment to improve their crystalline order and to change their magnetite to maghemite ratio, especially in their defect-riche surface region. Near edge X-ray absorption fine structure (NEXAFS) and XMCD experiments are used to investigate the local structural and electronic properties of the FeOx-NPs before and after postsynthetic oxidative and thermal treatment. The magnetic properties of the highly relevant surface region, as defined by TEY probe with ∼35% of the particle volume of the smaller and ∼20.5% of the larger particles. Charge transfer multiplet calculations of the XMCD spectra are performed to analyze the contributions of Fe2+ and Fe3+ at different lattice sites, i.e., either in octahedral or tetrahedral environment. The magnetic field dependence of the XMCD spectra is systematically studied and correlated with the spin canting and the magnetic properties of nanoparticles.

Fe3 +[Fe3 +Fe 2 +]O4 2 − → Fe3 +[Fe1 + 2δ 3 +Fe1 − 3δ 2 +□δ ]O4 2 −


→ Fe3 +[Fe5/33 +□1/3]O4 2 −

(1)

As maghemite and magnetite have nearly the same crystal structure, it is difficult to distinguish between them. The type of iron oxide strongly influences the physical properties of the resulting nanoparticles, including their magnetic properties and potential toxicity, since Fe2+ reduces molecular oxygen to O2−• and H2O2 to OH•.20,21 Hence, it is important to precisely determine and control the crystal modification of FeOx-NP, especially the surface region, which is in direct contact with the biological environment. Thereby, most of the standard techniques fail to distinguish between magnetite and maghemite, if the moieties have sizes in the nanometer regime. For instance, XRD (X-ray diffraction) cannot distinguish between both materials because of their similar crystalline patterns,14,17,18 and Mössbauer spectroscopy is hampered by the fact that the superparamagnetic nanoparticles show different temperature-dependent Mössbauer spectra compared to well-crystallized bulk materials.17 X-ray absorption spectroscopy (XAS) is a well-suited technique to distinguish between magnetite and maghemite. Especially, X-ray magnetic circular dichroism (XMCD) is a straightforward choice since it can be used to distinguish between the different oxidation states of iron ions, their local environment in the crystal lattice, and provides information on their magnetic properties. In order to be sensitive to the magnetic moment, two X-ray absorption spectra are recorded under an external magnetic field using left- and right-circular polarized light. The difference of a pair of such absorption spectra leads to the XMCD signal.22 Small changes in site occupancies can lead to dramatic changes in the relative peak intensities of the XMCD signal.23,24 Park et al. showed by the comparison of XMCD spectra with reference materials of magnetite and maghemite that small FeOx-NPs (16 nm in diameter) consist mainly of magnetite.3,4 However, it is critical to use bulk model compounds for the identification of mixed oxide NP since it is difficult to prepare pure maghemite, which is free of traces of magnetite or hematite. The saturation magnetization of iron oxide nanoparticles is often significantly lower than that of the corresponding bulk material, which limits their contrast in MRI. This can be explained by the presence of noncollinear (canted) spins showing a spin-glass-like behavior and modified cation distribution, which can be caused by a partial oxidation of the surface.17,22,25−28 For this disordered surface layer a thickness of about 1−2 nm is assumed depending on the temperature, the surface ligands, and the preparation method of the nanoparticles.28−32 It has been shown that modern synthetic

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EXPERIMENTAL AND THEORETICAL METHODS FeOx-NPs are prepared by a modified version of the synthesis of Park et al.4 Details of the synthesis can be found in the Supporting Information. Thermal Annealing of Iron Oxide Nanoparticles. The purified FeOx-NPs (100 mg) were dried under vacuum and subsequently redispersed in 10 mL of 1-octadecene and 1 mL of oleic acid at room temperature in an ultrasonic bath (Bandelin Sonorex RK 512H). After that, an O2/Ar 1:1 mixture was purged through the dispersion at 130 °C. After 15 and 60 min, respectively, samples were taken. The particles were stored in an oxygen-free atmosphere after the annealing process, so that any unintended contact with oxidation can be excluded. Characterization of Iron Oxide Nanoparticles before and after Thermal Annealing. Dynamic light scattering (DLS) measurements were performed using a Delsa Nano C analyzer from Beckman Coulter. In all DLS measurements the scattering angle is fixed at 165°. Dispersions of the nano19405


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The Journal of Physical Chemistry C

Fe2+Oh/Fe3+Oh/Fe3+Td the XMCD of an in situ cleaved highquality single crystal of magnetite (Fe3O4).36

particles in chloroform were diluted to a concentration of 1 mmol/mL and homogenized in an ultrasonic bath for 15 min. All measurements were carried out in 1 cm quartz glass Suprasil cuvettes from HELMA. TEM images were recorded using an EM 902A TEM from Philips with an acceleration voltage of 80 kV. The TEM samples were prepared by dipping 400 mesh copper grids coated by a ∼15 nm carbon film (Quantifoil) into a dispersion of the nanoparticle samples in chloroform. The average diameter and the polydispersity of the nanoparticles were analyzed by using the software Simple PCI from C-Images. XMCD and NEXAFS Experiments. Iron oxide nanoparticles were dried from organic dispersion on a copper substrate in an argon atmosphere. After preparation, the samples were stored in an inert atmosphere and then briefly (∼10 s) exposed to air upon transferring to the spectrometer. XMCD measurements were carried out at the 7 T high-field endstation at the UE46-PGM1 beamline at the BESSY II synchrotron radiation facility (Helmholtz Zentrum Berlin, Germany). The substrate was placed in the experimental chamber perpendicular to the circularly polarized synchrotron beam. At 10 K, a magnetic field of 0.5−4 T was applied. NEXAFS spectra were recorded in TEY at the Fe L2,3 edge at 700−740 eV in ultrahigh vacuum of p < 10−7 Pa. The size of the X-ray beam was 0.92 and 1.00 mm, respectively. All measurements were performed as a series of positive, negative, negative, and positive circularly polarized spectra in order to compensate small drifts as a function of time. Some exemplary measurements were carried out at 300 K (see Supporting Information). Analysis of X-ray Data. All spectra were normalized by dividing the intensity of the incident photon beam recorded by using the drain current of the last mirror. A linear background was subtracted from each NEXAFS spectrum. The half sum of the left- and right-circularly polarized light spectra was determined in order to derive the isotropic X-ray absorption spectrum (XASiso). To calculate the XMCD spectrum the difference between the spectra recorded by using left- and rightcircularly polarized light was determined. The XMCD spectra were normalized to the integrals of XASiso after subtraction of a step function. XMCD spectra in terms of % asymmetry were calculated as XMCDasym [%] =

I+ − I− × 100 I+ + I−

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RESULTS AND DISCUSSION Oxidation of FeOx-NP. Monodisperse and spherical nanoparticles of different sizes (diameter: 11.1 ± 0.6 nm (FeOx-NP-small) and 20.3 ± 0.9 nm (FeOx-NP-large)) were prepared by thermal decomposition in different solvents (see TEM images in Figure 1a,b). The hydrophobic particles were

Figure 1. TEM images of FeOx nanoparticles of (a,c) 11.1 ± 0.6 nm (FeOx-NP-small) and (b,d) 20.3 ± 0.9 nm diameter (FeOx-NP-large) (a,b) before and (c,d) after annealing at 130 °C for 60 min in an O2rich atmosphere.

stabilized by oleate groups on their surface and they are colloidally stable in chloroform for over one month. The hydrodynamic diameters of these particles in chloroform were 17 ± 3 and 22.3 ± 0.2 nm, respectively. These values are only slightly larger than those obtained from TEM studies. The polydispersity index was in both cases below 0.001, which confirms the high colloidal stability of these samples and the absence of aggregates. After purification, the particles were heated in a 10:1 mixture of 1-octadecene and oleic acid to improve their crystalline order and to change their magnetite to maghemite ratio at 130 °C for up to 60 min. An O2/Ar 1:1 mixture is used as a purging gas in this process to investigate the influence of oxygen on the thermal annealing of iron oxide nanoparticles. Under these conditions it is known that maghemite is more stable than hematite, and no further transition is expected.44 The size and shape of the nanoparticles remained unchanged within the experimental error after this annealing process, independently from the specific annealing conditions. Thus, any oxygen-induced degradation process can be excluded. TEM images of the same samples displayed in Figure 1a,b are shown after annealing at 130 °C for 60 min in an O2-rich atmosphere in Figure 1c,d. The hydrodynamic diameters and the polydispersity index of the small nanoparticles remain unchanged within the experimental error after the annealing process. The hydrodynamic diameter of the FeOx-NP-large increased slightly to 26 ± 2 nm after 15 min and to 33.0 ± 0.5 nm after 60 min of annealing at 130 °C in the presence of O2. This slight increase indicated that likely some minor aggregation takes place even though no aggregates were

(2)

for each point of the spectrum, where I is the intensity of the left (−)- or right (+)-circularly polarized light spectrum at this data point, respectively. In this case (only data in Figure 6 labeled as “assym. XMCD [%]”) no linear background was subtracted. Simulation of XMCD Spectra. The ligand field was determined by using multiplet simulations from the program CTM4XAS.41 For all three iron species a d−d Slater integral reduction of 0.7 and a p−d Slater integral reduction of 0.8 was assumed to be valid at a temperature of 1 K. A crystal field of 10 Dq = 1.2 eV and an exchange field μBH = 0.01 eV were taken for octahedral species, whereas 10 Dq = −0.6 eV and an exchange field μBH = −0.01 eV were used for tetrahedral species. Afterward, all calculated X-ray absorption (XAS) spectra were broadened by a Lorentzian of Γ = 0.3 (0.5) eV for the L3 (L2) edge and a Gaussian of σ = 0.25 eV.42,43 The resulting spectra were shifted by −0.6 eV in the case of Fe3+Oh and −0.75 eV in case of Fe3+Td to reproduce for a 1:1:1 ratio of 19406


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Figure 2. Isotropic XAS spectra of samples (a) FeOx-NP-small and (b) FeOx-NP-large before (0 min), during (15 min), and after (60 min) oxidation, measured at T = 10 K at the Fe L2,3 edges. For comparison, XAS spectra of pure magnetite and pure maghemite from Pellegrin et al. (taken from ref 47) are also shown.

sample FeOx-NP-large before annealing is significantly different from the XMCD spectra of pure magnetite measured by Pellegrin et al.47 or Goering et al.36 (see Figure 5a), i.e., the B1 peak (see Figure 3a) is much less pronounced indicating the

found in TEM. The polydispersity index remained still below 0.001. Investigation of Oxidation Behavior of the Surface Region of FeOx-NP by XAS Measurements. For a detailed investigation of the local structural and electronic properties of the surface region of iron oxide nanoparticles before and after postsynthetic oxidative via thermal treatment, XAS and XMCD experiments were carried out. All studies were performed with FeOx-NPs dried from organic dispersion on a copper substrate in an argon atmosphere. The FeOx-NPs were stored in an oxygen-free atmosphere after the annealing process, so that any unintended oxidation can be excluded. The X-ray experiments are performed at T = 10 K and magnetic field strengths ranging from 0.5 to 4 T around the Fe L2,3 edges. This rather low temperature was chosen since it turned out that XAS measurements of the same particles at room temperature are much more prone to radiation damage than measurements carried out at 10 K, which were for all samples under study highly reproducible. It is known that magnetite performs a socalled Verwey transition (phase transition) at 117 K, which influences the electronic structure of pure magnetite but not of maghemite.45,46 A comparison of selected XMCD spectra of the same samples measured at 300 and 10 K indicates that these differences are not too large (see Figure S1 in the Supporting Information). Left- and right-circularly polarized light XAS spectra measured at a field strength of 4 T at 10 K of the samples FeOx-NP-small and FeOx-NP-large before and after oxidation are shown in Figures S2 and S3 in the Supporting Information. The corresponding XASiso spectra are shown in Figure 2. Note that a large background signal from the substrate and ligands was subtracted from all XAS data. In both samples, the shoulder at the L3 peak decreases with increasing oxidation, which is typical for the transition from magnetite to maghemite.47 This change of the spectral shape can in principle be used for a quantitative analysis of the composition of the sample if suitable reference compounds exist. Such data have been measured by different groups, e.g., by Pellegrin et al. from epitaxially grown thin magnetite and maghemite films at 300 K47,48 and by Goering et al. from an in situ cleaved high-quality single crystal of magnetite at 150 °C.36 Since the XAS spectra of magnetite are because of the Verwey transition at 117 K slightly temperature dependent,46,49 which causes a slight decrease of the shoulder at the L3 signal, XAS spectra of sample FeOx-NPlarge before annealing measured at 300 K were compared with both reference spectra. This comparison suggests that sample FeOx-NP-large is pure magnetite (see Figure S4 in the Supporting Information). However, the XMCD spectrum of

Figure 3. Normalized XMCD spectra of the samples (a) FeOx-NPsmall and (b) FeOx-NP-large before (0 min), during (15 min), and after (60 min) oxidation, measured at 10 K and 4 T at the Fe L2,3 edges. Panels (c) and (d) correspond to enlarged views of the same spectra at the Fe-L3-edge (c): sample FeOx-NP-small and (d) sample FeOx-NP-large.

presence of a significant amount of maghemite (see Figure 5d and discussion below). In the case of sample FeOx-NP-large before annealing, this comparison indicates that the sample consists of about 84% magnetite (data not shown). Considering only the XMCD data may lead to an overestimation of the percentage of maghemite if its magnetite percentage is not fully magnetized and hence underestimated, but if sample FeOx-NPlarge before annealing would be perfect magnetite it should also show an XMCD spectrum of pure magnetite, which is obviously not the case. Park et al. also observed that larger (14−22 nm) oleate-stabilized iron oxide particles from the same synthesis route that was used for preparing the iron oxide particles in the present study have a more “magnetite-like” XAS spectra (i.e., a more pronounced shoulder of the L3 peak) than pure magnetite (= bulk material used as reference substance).4 It has to be taken into account that TEY is a rather surface sensitive method and that a significant portion of the iron atoms under investigation are surface atoms (see below), which 19407


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from Fe3+-ions in octahedral (Fe3+Oh), and the A-signal occurs due to Fe3+-ions in tetrahedral surrounding (Fe3+Td).52,53 Also some pre-edge structures can be found below the B1-signal (P in Figure 3a), which originates also from Fe2+Oh.22 Consequently, changes of these peaks give specific information on changes in the composition and magnetization of the surface region of the nanoparticles upon thermal treatment. Please note that minor contributions from the other two species also contribute to each of these signals (see Figure 5a and discussion below). Both samples show a strong increase in the normalized XMCD signal with ongoing oxidation. The FeOx-NP-small sample reaches maximum intensity already after 15 min; the intensity of the FeOx-NP-large sample increases further after 15 min and reaches approximately the same intensity as the FeOxNP-small sample after 60 min. A comparison between the normalized XMCD spectra of the small and large particles reveals that before oxidative treatment the intensity of the spectrum of the sample FeOx-NP-small is slightly higher (see Figure 3). This is unexpected since it is reported in the literature that small iron oxide nanoparticles have a lower saturation magnetization than larger ones, which is due to increased surface spin canting.9,10 Therefore, a smaller normalized XMCD signal is expected for this sample. After 60 min of oxidation the shape and intensity of both spectra are practically identical (see also Figure 4 below). Sun et al.

are not inside a fully saturated magnetite or maghemite crystal but bound to oleate molecules and hence contribute in a different way to the XAS signal. Moreover, small nanoparticles are more distorted, which might also influence the XAS spectrum. In conclusion, the XAS spectra indicate that with increasing annealing the investigated surface region of the particles becomes more maghemite-like, but a quantization of this process is not possible. Comparing the data in Figure 2a,b reveals that sample FeOx-NP-small has initially a slightly less pronounced shoulder at the L3 peak and is thus slightly more maghemite-like than sample FeOx-NP-large. The XAS spectrum of these smaller particles after 15 min is already strongly changed toward maghemite, whereas a further treatment with O2 for another 45 min only slightly reduces the shoulder of the L3 signal. Thus, the oxidation process is already after 15 min nearly complete. In the case of sample FeOx-NP-large the transition of the XAS spectrum toward maghemite is significantly slower; however, after 60 min of thermal oxidation the XAS spectra of sample FeOx-NP-small and sample FeOxNP-large are rather similar. Investigation of Oxidation Behavior of the Surface Region of FeOx-NP by XMCD Measurements. To compare the XMCD spectra with each other, the XMCD spectra were normalized to the corresponding XASiso spectra, i.e., they were divided by the integral over the spectrum at the Fe L2,3 edges (700−734 eV). For a sample that contains only one magnetic ion species and where the intra-atomic dipole moment can be neglected the intensity of the XMCD spectrum is proportional to the average value of the magnetic moment; thus, the magnetization is proportional to the number of atoms, and the normalized spectrum is proportional to the total magnetization per atom.50 In the case of a compound containing three different magnetic iron species, this is only true, when we assume that the magnetization of all three species is constant or is changing evenly for all three species. The latter is approximately correct, when we assume that the annealing process induces a general decrease of the nanocrystal’s lattice disorder and reduces spin canting of all iron species. Under this assumption an analysis of the normalized XMCD spectrum allows conclusions about the contributions of the different iron species to the total magnetization of the sample23 and thus the extent of the oxidation process. In order to verify this assumption, additional XMCD spectra in terms of % asymmetry were calculated and discussed below. Normalized XMCD spectra of iron oxide nanoparticles with different sizes at the Fe L2,3 edges before and after oxidation are shown in Figure 3. As shown in Figure 3, a normalized XMCD-spectrum consists at the Fe L2,3 edges of two parts: intense signals at the Fe L3 edge region from 707 to 714 eV and much weaker signals in the Fe L2 edge region from 720 eV up to 727 eV. In the following only the intense signals at the Fe L3 edge will be discussed in greater detail. Both signals consist of three peaks, at the Fe L3 edge, iron species in octahedral surrounding mainly contribute to negative signals (marked as B1 and B2 in Figure 3a), whereas iron species in tetrahedral surrounding mainly contribute to a positive signal (A in Figure 3a). At the Fe L2 edge it is the other way around. Also the oxidation state can be distinguished in XMCD spectra. Fe2+-ions absorb at lower photon energy than Fe3+-ions so that they appear at lower energies in the normalized XMCD spectra.51 Therefore, the B1signal around 709 eV arises mainly from the Fe2+ in octahedral surrounding (Fe2+Oh), the B2-signal around 711 eV originates

Figure 4. XMCD spectra of samples (a) FeOx-NP-small and (b) FeOx-NP-large before (0 min), during (15 min), and after (60 min) oxidation, where the intensity of the A-signal was set to unity. All spectra were measured at a magnetic field of B = 4 T at T = 10 K.

described that annealing of magnetite particles under argon atmosphere at 600 °C leads to a dramatic increase in saturation magnetization, whereas oxidation at 250 °C in oxygen-rich atmosphere reduces the saturation magnetization.11 In the present work, a lower temperature and shorter reaction time were used, which might explain the higher signal intensity in XMCD due to postsynthetic ordering effects, which occurred under oxidative treatment. For a better comparison of the changes of the relative intensities, the XMCD spectra were normalized by setting the intensity of feature A to unity (see Figure 4), as the total amount of Fe3+Td should remain unchanged (see eq 1). Before oxidation, the signals of the octahedral iron species (B1- and B2-signal) show high intensity relative to that of the tetrahedral species (A-signal) in the XMCD spectrum of the 19408


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The Journal of Physical Chemistry C sample FeOx-NP-small (black dashed line in Figure 4a). After 15 min of treatment in an oxygen-rich atmosphere the spectrum changes significantly (see red line in Figure 4a). The relative intensity of the B1-signal strongly decreases, whereas the relative intensity of the B2-signal slightly increases. Further oxidation for up to 60 min has nearly no influence on the shape and intensity of the XMCD spectra (see blue dashdotted line in Figure 4a). Compared to the FeOx-NP-small the oxidation behavior of the FeOx-NP-large is different. Within the first 15 min the relative intensity of the B1- and B2-signals in the XMCD spectrum decrease, this implies that the relative contributions of both octahedral iron species decrease. After 60 min the relative intensity of the B1-signal is further reduced, whereas the relative intensity B2-signal increases and reaches almost the intensity of the untreated sample. The XMCD spectra of both samples have in the beginning of the oxidation a significant pre-edge-structure (marked as region P in Figure 3a), which originates from the Fe2+-ions.22 This structure nearly completely disappears with ongoing oxidation in the spectrum of sample FeOx-NP-small, whereas it can still be found in the spectrum of sample FeOx-NP-large after 60 min, albeit with significantly reduced intensity. This observation suggests that in the case of FeOx-NP-small no or only small amounts of Fe2+Oh are present after the oxidation process, whereas the FeOx-NPlarge sample still contains a significant amount of Fe2+Oh even after 60 min of oxidation and thus a higher fraction of magnetite. The reason for the different durations of the oxidation process is likely due to the fact that smaller particles have a higher curvature radius and therefore higher enthalpies and free energies due to a positive surface energy.44 The smaller the particles, the more near surface atoms dominate the XMCD spectrum. Nevertheless, if one assumes a sampling depth of 1.5 nm, the contribution of the outermost atomic layer of the nanoparticles to the XMCD spectrum is 12.9 ± 0.2% for the FeOx-NP-small and 11.8 ± 0.1% for FeOx-NP-large, so that this effect is rather negligible. However, significant differences between the XMCD spectra and their temporal changes are observed, which reflect the significantly different oxidation behavior of both species. Simulation of the XMCD Spectra. Ligand field multiplet calculations were performed using the program CMT4XAS41 to determine the composition of the surface region of the iron oxide nanoparticles. This program allows for the calculation of the individual XAS spectra of the Fe2+Oh, Fe3+Oh, and Fe3+Td species. Hence, after calculating the corresponding XMCD spectra a quantification of the contributions of these ions contributing to the total XMCD spectrum can be made. A XMCD spectrum of an in situ cleaved high-quality single crystal of magnetite from Goering et al. measured at 150 K was taken as reference for setting the fitting parameters (see Methods).54 The XMCD spectra of such bulk magnetite material do not change in a large temperature range (20−150 K) above and below the Verwey transition (123.8 K).55 It has to be mentioned that there is a huge variation in literature spectra of magnetite and maghemite. This is likely due to difficulties in preparation of pure modifications and prevention of oxidation during measurements as well as differences in signal corrections and different materials used as sample holders.4,19,22,23,36,53,56−59 Figure 5a shows the simulated XMCD spectra of the three iron species and the resulting 1:1:1 superposition of Fe2+Oh, Fe3+Td, and Fe3+Oh, thus a theoretical XMCD spectrum of magnetite. Only the spectrum at the Fe-L3 edge was used to fit

Figure 5. (a) Simulation of the XMCD spectra of the three different iron species and resulting XMCD spectrum of magnetite. (b) Comparison between the simulated and experimental XMCD spectra of FeOx-NP-small for B = 4 T at 10 K at the Fe-L3 edge. Contributions of the Fe2+Oh and Fe3+Oh ions are shown, where the intensity Fe3+Td is set to unity, as well as the A/B ratio for the samples (c) FeOx-NPsmall and (d) FeOx-NP-large.

the experimental results. A fit over the full spectrum usually leads to less accurate results since the signals at the Fe L2 edge are much weaker and less structured than those at the Fe L3 edge.23,52 To the best of our knowledge a systematic investigation of the transition from magnetite to maghemite during the oxidation of the surface region of iron oxide nanoparticles by simulating their measured XMCD spectra has not been carried out to date. For the simulations of the measured XMCD spectra it was assumed that the contribution of Fe3+Td remains unchanged and was set to unity. The contributions of the two octahedral species were varied until the fit between measured and experimental spectrum was optimized. Figure 5b shows exemplarily a simulation of the XMCD spectrum at the Fe L3 edge of sample FeOx-NP-small before oxidation. Figure 5 b confirms the agreement between the experimental and simulated XMCD spectra at the Fe L3 edge. The simulations of the other spectra can be found in the Supporting Information (see Figure S5). There, the full XMCD spectra at the L2,3 edges are shown. All spectra show a good agreement at the Fe L3 edge and a slightly worse agreement for the much weaker signal at the Fe L2 edge (see Figure S5). It has to be noted that the experimental spectra have a slightly narrower line shape than the simulated ones, especially if oxidation has taken place (see Figure S5b,c,e,f in the Supporting Information). In contrast to the XASiso, XMCD is more site specific due to opposite signs of the spectral contributions of octahedral and tetrahedral species.60,61 The ratio between the contributions of the iron species to tetrahedral and octahedral sites was calculated. This provides us with a systematic comparison of the changes in the experimental spectra during oxidation by affecting the A/B ratio (see Figure 3a). This ratio, as well as the relative contributions of Fe2+Oh and Fe3+Oh ions, are plotted as a function of the oxidation time for the samples FeOx-NP-small in Figure 5c and FeOx-NP-large in Figure 5d, respectively. The contribution of Fe3+Td was set to unity. Before the oxidation, the surface region of sample FeOx-NPsmall has a Fe2+Oh/Fe3+Oh/Fe3+Td ratio of 0.64 ± 0.01:1.05 ± 19409


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The Journal of Physical Chemistry C 0.1:1 ± 0.01 (see Figure 5c) and therefore a reduced Fe2+Oh content compared to pure magnetite.43 After 15 min of oxidation the ratio of Fe2+Oh drops to 0.39 ± 0.01, whereas the ratio of Fe3+Oh increases to 1.21 ± 0.01, and after 60 min of oxidation the contribution of Fe2+Oh reduces further to 0.31 ± 0.01, whereas Fe3+Oh increases to 1.26 ± 0.01. The ongoing oxidation can also be monitored by the A/B ratio. which slightly increases during the oxidation and reaches a value slightly higher than the value of 0.6, which is characteristic for maghemite (see eq 1). For the sample FeOx-NP-large the results are different. Before oxidation the surface region of these particles has a Fe2+Oh/Fe3+Oh/Fe3+Td ratio of 0.67 ± 0.01:1.12 ± 0.01:1 ± 0.01 (see Figure 5d). This changes to 0.61 ± 0.01:1.07 ± 0.01:1 ± 0.01 after 15 min in oxygen rich atmosphere and coincides using the results shown in Figure 4b, where the intensity of both octahedral signals was reduced compared to the tetrahedral one. Thereby, a transition of Fe2+ from octahedral into tetrahedral sites can be excluded since this would lead to the growth of a second positive peak below the B1-signal in XMCD spectra,62 as reported by Pearce et al. for titanomagnetites (Fe3‑xTixO4). As this is not the case here (see Figure 3b,d) it can be concluded that no structural transition without oxidation takes places. After 60 min in oxygen rich atmosphere the FeOx-NP-large have a Fe2+Oh/Fe3+Oh/Fe3+Td ratio of 0.39 ± 0.01:1.2 ± 0.01:1 ± 0.01. This comparison shows the advantages of these simulations to observe the relative changes of each species. A simple comparison of the intensities of the signals A, B1, and B2 (see Figure 4b) would suggest that the fraction of Fe3+Oh decreases for the FeOx-NPlarge during the oxidation process. However, the simulations reveal that at in the end of this process the ratio of Fe3+Oh is higher (1.12 ± 0.01 to 1.2 ± 0.01) than in the beginning. Also for this sample the A/B ratio increases during the entire oxidation process, which is an indicator for the ongoing oxidation. However, the data shown in Figure 5c,d indicate that the surface regions of the samples FeOx-NP-small and FeOxNP-large still contain a significant fraction of Fe2+Oh, i.e., the samples are more maghemite-like but not fully oxidized to maghemite. In most reported simulations of maghemite based on multiplet calculations the main signal of the Fe3+Oh ions (the B1-signal) is only a minor peak at the L3 edge,42,43,63−65 whereas in experimental spectra of maghemite this signal is more pronounced.47 This may lead to a slight overestimate of the remaining Fe2+ content in not completely oxidized FeOx NP samples. The simulations confirm that the surface region of the smaller FeOx-NPs reaches their final oxidation state faster, than the that of the larger ones, which shows a higher amount of Fe2+Oh even after 60 min oxidation. In addition, the data shown in Figure 5c,d suggests that the surface region of the larger FeOx-NPs undergoes an ordering process within the first 15 min of the oxidation process, where the amount of Fe3+Td increases compared to the fraction of Fe3+Oh. This is in contrast to previous work reported by Morral et al. and Schedin et al., where oxidized iron oxide films showed a reduced amount of Fe3+Td after oxidation.23,43 The observed ordering effect in this work goes along with a strong increase in the total magnetization (see Figure 3b,d). Notably, such an effect was not found for the surface region of the smaller FeOx-NPs, likely because it already has before oxidation a smaller Fe3+Oh/Fe3+Td ratio as well as a significantly higher magnetization. Furthermore, the simulations clearly show that even before oxidation the surface regions of FeO x -NPs have no stoichiometric 1:1:1 ratio of the three iron species; thus, they

consist not purely of magnetite. This can be explained by assuming that the outmost atomic layer of the nanoparticles is always completely oxidized,15 leading to a reduced amount of Fe2+Oh in all samples, especially at the beginning of oxidation. Further, the Fe2+Oh/Fe3+Oh/Fe3+Td ratios obtained by the simulations do not correspond to simple stoichiometric mixtures of maghemite and magnetite neither before nor after the oxidation process. In a stoichiometric mixture of magnetite and maghemite, Fe2+Oh as one according to eq 1 with the parameter δ defined above gives Fe2 +Oh /Fe3 +Oh = (1 − 3δ)/(1 + 2δ)

(3)

In the present case the portion of Fe3+Oh is always too low (8% in the case of sample FeOx-NP-large before oxidation, 14− 15% in all other cases.). This deviation might be explained by the fact that at the surface of the nanoparticles no completely saturated layer of maghemite exists since the Fe atoms at the surface are bound to oleate molecules and thus have a different oxidation state (for detail see the recent work by Salafranca et al.32). Since TEY is a surface sensitive method, about 12.9 ± 0.2% for the FeOx-NP-small and 11.8 ± 0.1% for the FeOx-NPlarge of all investigated iron atoms are surface atoms (see above). As explained above we took a high-quality single crystal of magnetite as reference for setting the fitting parameters, which is free from such surface bound organic molecules. Already the comparison between the XAS spectra of the nanoparticles under investigation and the XAS spectra of pure magnetite and maghemite show that the nanoparticles cannot be simply described as a mixture of both compounds (see discussion above and Figure 2). For the determination of the contributions of each iron ion species to the total iron content it was assumed that the contributions of the calculated spectra of the individual ion species to the total spectrum are proportional to the mass fractions of the three ions species. However, this is only correct when the magnetizations of the three iron species are in a constant ratio and if the intra-atomic dipole moment can be neglected. In order to verify if the former is correct, XMCD spectra in terms of % asymmetry were calculated (see eq 2). The corresponding spectra of FeOx-NP-small and FeOx-NPlarge are shown in Figure 6a,b, respectively. The calculation of

Figure 6. XMCD spectra in terms of % asymmetry of samples (a) FeOx-NP-small and (b) FeOx-NP-large before (0 min), during (15 min), and after (60 min) oxidation measured at a magnetic field of B = 4 T at T = 10 K. 19410


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Figure 7. Field-dependent XMCD spectra at T = 10 K where for better comparison the A-signal of the Fe3+Td was set to unity of (a) sample FeOxNP-small before oxidation, (b) FeOx-NP-small after 15 min of oxidation, and (c) FeOx-NP-small after 60 min of oxidation as well as (d) sample FeOx-NP-large before oxidation, (e) FeOx-NP-large after 15 min of oxidation, and (f) FeOx-NP-large after 60 min of oxidation.

calculated spectra of the individual ion species, the contribution of the Fe3+ ions was slightly underestimated compared to the contribution of the Fe2+ species. Thus, the estimated value for Fe3+Oh after 15 min in Figure 5d (implying the rather surprising decay of the Fe3+Oh concentration with increasing oxidation) is likely not correct and should be significantly higher. It should be further noted, that at 10 K magnetite is in the monoclinic phase. In this phase the XMCD spectra are significantly influenced by the intra-atomic dipole moment of the spin density distribution. It was recently shown that the magnetic dipole moment depends on the oxidation state and the lattice site, and therefore, even the B1-signal in the XMCD spectra in terms of % asymmetry is not completely proportional to the magnetization. Averaging over all Fe sites, the reduction of the Fe spin moment by the magnetic dipole moment is about 10% for magnetite. This reduction is expected to further decrease if the fraction of Fe2+ ions decreases by oxidation.45 Field Dependence of the XMCD Spectra. Spin canting significantly reduces the net magnetization of iron oxide nanoparticles and can be monitored by field-dependent XMCD measurements.22,31 Therefore, XMCD spectra were recorded at different field strength from 0.5 T up to 4 T. An overview of these spectra can be found in the Supporting Information in Figure S7. These data reveal that the intensity of the XMCD spectra of the nonoxidized samples is strongly field dependent (see Figure S6a,d), increasing the signal strength about 2.9 times for the sample FeOx-NP-large and about 1.6 times for the sample FeOx-NP-small. In contrast, the spectra of the oxidized samples show only a weak field-dependent signal increase. The field dependence of XMCD spectra can be explained by an increase of the average projection of the magnetic moments of the particles along the applied field.22,31 The stronger field dependence of the nonoxidized samples can be explained by a higher degree of spin-disorder in the surface regions of these samples, especially for the larger particles, which agrees well with their overall low magnetization even at 4 T compared to the oxidized samples and the smaller particles (see Figure 3). In

these spectra makes it possible to determine the magnetization per ion species, if energy values exist where the intensity of both, the XAS and the XMCD signal, solely originates from one or at least one subgroup of ion species. According to Figure S6, this applies approximately to the position B1, for the Fe2+Oh ions, whereas the signals at the positions A and B2 cannot be assigned purely to one iron species. At least the B2-signals are dominated by the contributions of the Fe3+ ions, whereas the contribution of the Fe2+ ions is negligible. In the case of the B1signal, it has to be taken into account that experimental spectra of pure maghemite have a small but still visible B1 edge,22,47 so that also the B1-signal in the XMCD spectra in terms of % asymmetry is not solely produced by Fe2+. From Figure 6 it can be seen that also in the XMCD spectra in terms of % asymmetry the intensity increases with ongoing oxidation, which shows that the magnetization per ion increases as discussed above. In the case of sample FeOx-NP-small (see Figure 6a) the main increase in intensity as well as the structural changes of the XMCD spectra takes place within the first 15 min. Afterward only minor changes occur. The intensities of the B1 peak and the B2 peak increase rather evenly, which suggests that also the magnetization per ion increases evenly, supporting the assumptions taken above for the analysis of the XMCD spectra. In contrast, the sample FeOx-NP-large shows a different behavior (see Figure 6b). Within the first 15 min all signal intensities increase. However, the B1-signal increases significantly stronger than the B2-signal. After 60 min of oxidative treatment, the intensity of the B1 peak remains almost constant compared to the value after 15 min, whereas the intensity at the B2 peak significantly increases. These observations suggest that the magnetization of the Fe2+ ions increases faster during the thermally and oxidatively induced ordering process than that of the Fe3+ ions, but at the end of this process all ion species have a significantly increased magnetization. This results implies that in the analysis of the normalized XMCD spectra of the surface region of sample FeOx-NP-large after 15 min of oxidative treatment by using the 19411


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The Journal of Physical Chemistry C order to investigate the site specific field-dependence, all spectra were normalized for better comparison by setting the Asignal to unity and were compared to each other as a function of the applied magnetic field (see Figure 7). It can be seen that in the case of sample FeOx-NP-small (see Figure 7a) the octahedral signals of both Fe2+ (∼709.5 eV) and Fe3+ (∼711 eV) increase with increasing field strength if no oxidation has taken place. After a field of 2 T was reached, the shape of the spectrum does not change anymore, but at lower magnetic field strengths the relative intensity of both B-signals increases by nearly 18%. With ongoing oxidation (see Figure 7b for 15 min and Figure 7c for 60 min of treatment in an oxygenrich atmosphere), this effect vanishes, and after 60 min the spectral shape is practically field independent. In the case of sample FeOx-NP-large it can be seen that before oxidation significant spin canting occurs up to a field strength of 2 T (see Figure 7d), where the relative intensities of the B1- and B2signals increase up to 39%. Also here the field dependence on the spectral shape vanishes with ongoing oxidation. It has to be mentioned that for the sample FeOx-NP-large after 60 min in oxygen rich atmosphere the spectrum measured at 1 T (see red line in Figure 7f) shows a different behavior than what is observed for all other spectra. We assume that here some errors in the acquisition occurred, which were seen after the beam time and motivates to neglect these spectra. Thus, the present data show that oxidation of the surface region of FeOx-NP induces, besides the transition to a mainly maghemite-like structure, an ordering process, where spin canting is reduced and the spin misalignment is compensated if the magnetic field exceeds 2 T.

biocompatible ligands on these oxidation processes will be investigated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jp512023z. Additional XMCD and XAS spectra as well as details on the synthesis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +49 30 838 55304. Fax: +49 30 838 455304. E-mail: [email protected]. *Tel: +49 30 838 52396. Fax: +49 30 838 452396. E-mail: [email protected]. Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support for this research by the priority program of the Freie Universität Berlin “Nanoscale Functional Materials”. We thank Dr. Detlef Schmitz from Helmholtz-Zentrum Berlin für Materialien und Energie for his support during the beam time and valuable discussions.

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REFERENCES

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CONCLUSIONS Nearly monodisperse iron oxide nanoparticles of two different sizes (11 ± 0.6 and 20.3 ± 0.9 nm), synthesized by thermal decomposition of iron oleate, were annealed in an oxidative atmosphere. It could be shown that this postsynthetic treatment induces no changes in the size and hydrodynamic diameter, so that aggregation as well as oxygen-induced degradation processes can be excluded. To investigate the influence of the oxidative thermal treatment on the composition of the surface region of these particles, XMCD was used to identify chemical modifications (maghemite/ magnetite ratio) as a function of the postsynthetic treatments. We show that XMCD measurements combined with charge transfer multiplet calculations evidence that the postsynthetic oxidative treatment induces an incomplete transition to maghemite. We also explain the strongly increased total magnetization of the surface regions after such treatment. Hereby, the kinetics of this transition is strongly size dependent, which emphasizes the importance to use particles with narrow size distribution. A systematic study of the field dependence of the XMCD spectra reveals that before the oxidation process especially in the surface region of the larger nanoparticles significant spin canting occurs, which is strongly reduced by thermal annealing. Thus, postsynthetic oxidation of iron oxide nanoparticles by purging nanoparticle dispersions at elevated temperatures is a simple and highly efficient approach for improving the magnetic and structural properties of such magnetically less ordered surface regions of iron oxide nanoparticles. This region is usually the particle area that determines the low magnetization and the contact with biological systems. In ongoing work the influence of 19412


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Local Magnetic and Electronic Structure of the Surface Region of

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