Identifying Spinel Phases in Nearly Monodisperse Iron Oxide Colloidal

Sep 30, 2009 - School of Physical Sciences, Ingram Building, University of Kent, Canterbury CT2 7NH, U. K.. Institut Català de ... Nearly monodispers...
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J. Phys. Chem. C 2009, 113, 18667–18675

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Identifying Spinel Phases in Nearly Monodisperse Iron Oxide Colloidal Nanocrystal Anna Corrias,† Gavin Mountjoy,‡ Danilo Loche,† Victor Puntes,§ Andrea Falqui,† Marco Zanella,|,⊥ Wolfgang J. Parak,|,⊥ and Maria F. Casula*,† Dipartimento di Scienze Chimiche, UniVersita’ di Cagliari and INSTM, 09042 Monserrato (CA), Italy, School of Physical Sciences, Ingram Building, UniVersity of Kent, Canterbury CT2 7NH, U. K., Institut Catala` de Nanotecnologia, Campus UAB 08193 Bellaterra, Spain, Center for NanoScience, Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Munich, Germany, and Fachbereich Physik, Philipps UniVersita¨t Marburg, Marburg, Germany ReceiVed: May 21, 2009; ReVised Manuscript ReceiVed: September 11, 2009

Nearly monodisperse iron oxide colloidal nanocrystals prepared by nonhydrolytic high-temperature solution method were obtained with two different sizes and degrees of oxidation. The characterization of the structural features of the nanocrystals was performed by a multitechnique approach including transmission electron microscopy, X-ray diffraction and X-ray absorption spectroscopy, energy filtered electron microscopy imaging, and SQUID magnetometry. The different techniques provided complementary information on the local oxidation state of iron in the iron oxide nanoparticles, the stability of the phases, the exact crystal structure, and the compositional homogeneity. X-ray diffraction, transmission electron microscopy, and extended X-ray absorption spectroscopy show that the addition of oxidizer to the iron precursor gives rise to monodisperse polycrystalline nanoparticles made out of FeO plus a spinel phase. X-ray absorption near-edge structure, which is very sensitive to the oxidation state and local environment of iron in the different iron oxides, was used to distinguish among isostructural spinel phases of iron (II,III) oxide (magnetite) and iron(III) oxide (maghemite). Singlecrystalline spinel nanoparticles are obtained upon sequential oxidation: in smaller nanoparticles a mixture of mainly Fe3O4 and γ-Fe2O3 is present, whereas the larger nanoparticles are made out of γ-Fe2O3, as also supported by SQUID magnetization measurements. The importance of a multitechnique approach for the elucidation of the compositional and structural details in addition to geometrical parameters in the characterization of nanocrystalline iron oxides is pointed out. 1. Introduction The unconventional or enhanced physicochemical properties of inorganic crystals at the nanoscale regime, which are not encountered in the corresponding bulk materials, have motivated an intense effort in the development of synthetic protocols.1 In fact, it is recognized that the preparation of nanocrystals with uniform size, shape, composition, crystal structure, and surface properties is a key requirement both to elucidate the properties of nanomaterials and to develop building blocks for the fabrication of novel functional devices.2 In particular, the preparation of nanocrystals with monodisperse size and shape has been intensively pursued, which can be successfully achieved by decomposition of organometallic precursors into hot organic surfactants or coordinating solvents.3 Such an approach has been effective in obtaining metals, semiconductors as well as magnetic materials,4-6 enabling the study of the dependence of their physicochemical properties on geometrical factors such as size and shape.7-10 By this hot temperature solution-phase synthesis inorganic nanocrystals coated by an organic monolayer are obtained, which can be dispersed in organic media. Colloidal suspensions of magnetic nanocrystals obtained by this route are particularly promising as ferrofluids with controlled * Corresponding Author: [email protected]. † Universita’ di Cagliari and INSTM. ‡ University of Kent. § Institut Catala` de Nanotecnologia. | Ludwig-Maximilians-Universita¨t Mu¨nchen. ⊥ Philipps Universita¨t Marburg.

behavior because the magnetic properties crucially depend on the microstructure of the sample. Moreover, upon suitable surface modification procedures water-based suspensions can be obtained that enable the development of magnetic nanoparticles for biomedical use such as contrast enhancers in magnetic resonance imaging.11-13 In this context, magnetic iron oxides play a key role due to their relative stability and nontoxicity. Magnetic iron oxides include magnetite, the most magnetic among naturally occurring minerals, a ferrimagnet with chemical formula Fe3O4 (mixed Fe2+ and Fe3+ ions), and maghemite (γFe2O3), which retains the same spinel structure as magnetite but is fully oxidized to Fe3+, and is also ferrimagnetic. The magnetic moment arises from the unbalanced number of vacancies in an antiferromagnetic arrangement, which are present to compensate for the increased positive charge. When iron is thermodynamically oxidized a nonmagnetic ferric phase with a rhombohedral crystal structure, hematite (R-Fe2O3) is formed.14 Because of the rich phase diagram together with the broad spectrum of applications in nanomaterials science, the iron oxides represent a relevant case study to point out the importance of elucidating the detailed nanostructure. In addition to geometrical parameters, structure and structural order, oxidation state, and cation distribution, stoichiometry and its homogeneity at the nanoscale are interrelated in a complex way and may be very difficult to investigate due to the occurrence of similar crystal structures. Most of the time, X-ray and electron diffraction cannot provide a definitive picture of the microstructural features of the nanophase, and techniques sensitive to the

10.1021/jp9047677 CCC: $40.75  2009 American Chemical Society Published on Web 09/30/2009

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TABLE 1: Sample Acronyms, Preparation Conditions, and Structural Parameters Arising from TEM and XRD Characterization for the Iron Oxide Colloidal Samples sample name

growth temperature (°C)

iron/oxidizer molar ratio 1st injection

IO_1 IO_2 IO_3 IO_4

300 290 300 290

1:1.5 1:1.5 1:1.5 1:1.5

oxidation state and to short-range order are required as well. Mossbauer, IR, and Raman spectroscopy have been successfully used to differentiate among the different iron oxides15-17 and to monitor the oxidation of magnetite nanoparticles.18 X-ray absorption spectroscopy techniques such as X-ray absorption near-edge structure (XANES) at the K-edge of Fe has proven to be a powerful technique to study the structure of Fe-containing oxides19-21 as it gives definitive information on oxidation states and by the fingerprint approach can enable identification of crystal phases. Iron atoms in the oxides possess partially filled 3d bands that give rise to the characteristic preedge features in XANES spectra, which are sensitive to the metal coordination geometry and site symmetry of excited atoms. Extended X-ray absorption fine structure (EXAFS) gives information about bond distances and coordination numbers of shells surrounding the absorbing atom, revealing the structure even in poorly ordered and very small nanocrystals. For these reasons, X-ray absorption spectroscopies are regarded as powerful tools for the structural study of nanocrystalline materials22-24 and for providing accurate feedback on the effect of synthetic parameters. In this work, we have prepared iron oxide samples of two different sizes with different oxidation degrees by sequential addition of an oxidizer. The local oxidation state of iron in iron oxide nanoparticles, its stability, the crystal structure, and the compositional homogeneity were investigated by the use of a multitechnique approach involving conventional X-ray diffraction (XRD) and transmission electron microscopy (TEM), energy-filtered TEM, high-resolution TEM, EXAFS and XANES, and SQUID magnetic measurements. 2. Experimental Section 2.1. Materials. For the synthesis of iron oxide nanocrystals, iron pentacarbonyl (Fe(CO)5, 99.5%, Alfa Aesar) and 3-chloro peroxybenzoic acid (or mCPBA, meta-chloroperoxybenzoic acid, C7H5O2Cl, 77% Aldrich) were used as the iron source and as the oxidizer, respectively. Prior to use, the latter was dehydrated by extraction with dichloromethane and dehydration with phosphorus anhydride, followed by vacuum-drying. Tridecanoic acid (C13H26O2, 98%, Alfa Aesar) was used as surfactant and dioctyl ether (C16H34O, 99%, Aldrich) was used as solvent. Water-free toluene and ethanol were used to disperse and precipitate respectively the resulting hydrophobic iron oxide nanocrystals. The following standards were used as reference compounds for X-ray absorption investigation: wustite FeO (99.9% Aldrich), magnetite Fe3O4 (99.997% Alfa Aesar), maghemite γ-Fe2O3 (99+%, Alfa Aesar), hematite R-Fe2O3 (99.99%, Alfa Aesar). 2.2. Sample Preparation. The synthesis of iron oxide nanocrystals was carried according to a procedure previously reported16 and gave rise to stable colloidal suspensions of capped hydrophobic iron oxide nanocrystals in nonpolar solvents such as toluene. Briefly, sample preparation was carried out inside a drybox using airless procedures. In a typical synthesis, a solution of tridecanoic acid in octyl ether in a 25 mL three-necked flask

iron/oxidizer molar ratio 2nd injection

〈d〉TEM (nm)

XRD pattern

1:1.5 1:1.5

10.0 ( 0.5 14.4 ( 0.8 8.0 ( 1.0 13.0 ( 1.2

polyphase polyphase spinel phase(s) spinel phase(s)

connected to a Liebig condenser was outgassed for 30 min and then heated under Ar flow at 290 °C. A solution of iron pentacarbonyl in ether and a solution of the oxidizer (mCPBA) in ether were then rapidly coinjected. The overall iron molar concentration was 0.1 M, and the iron/surfactant/oxidizer molar ratios were 1:3:1.5. The solution flask was heated at 290 °C for 5 min to allow particle growth, after which time the solution was cooled to 40 °C, and ethanol was added to precipitate nanoparticles from the solution. The particles were separated by centrifugation and washed twice by redispersing in toluene and precipitating with ethanol. Because of the monodispersity of the nanoparticles, no further size selection procedure was carried out on any samples. Several parameters were shown to affect the particle size, such as the growth time, the temperature of injection and growth effect, and the absolute and relative concentration of the precursors.16 Here, to vary the average nanocrystal size we have performed the synthesis also at higher temperature (300 °C), which produces smaller nanocrystals. To obtain nanocrystals with different oxidation states, a second injection of oxidizer was performed one minute after the solution turned dark. The amount of oxidizer injected was 1.5 times the moles of iron pentacarbonyl. After a growth time of 4 min, the solution was cooled and the nanoparticles were precipitated from the solution by adding ethanol. In Table 1, the sample names and their preparation parameters are summarized. 2.3. Transmission Electron Microscopy. TEM micrographs were recorded on a JEOL 200CX microscope operating at 200 KV. The samples were dispersed in ethanol and dropped on a carbon-coated copper grid. High-resolution electron transmission microscopy (HREM) and energy filtered (EF) images were obtained by using a JEOL JEM-2010 high-resolution transmission electron microscope equipped with a LaB6 cathode and with a Gatan image filter (GIF) spectrometer. The latter allows reconstruction of the image of the sample by collecting only the electrons that have lost energy in a given range of the electron energy loss (EELS) spectrum. In an EF image, the presence of the selected element appears as a bright zone on a dark background. No sign of beam damage was observed on the samples during investigation. 2.4. X-ray Diffraction. XRD patterns were recorded using Cu KR radiation on a X3000 Seifert diffractometer equipped with a graphite monochromator on the diffracted beam. The nanocrystals were isolated from the colloidal suspension by precipitation with ethanol and then deposited on a lowbackground sample holder. Phase identification was performed by comparison with the Powder Diffraction File database.25 2.5. X-ray Absorption Spectroscopy. EXAFS and XANES spectra were recorded in transmission mode at beamline 7.1 at the SRS synchrotron (Daresbury Laboratory, U.K.). Spectra at the Fe (7112 eV) K-edge were acquired at room temperature using a Si(111) monochromator. Samples with a suitable and highly uniform optical thickness were prepared from powders. The samples were first isolated from the colloidal suspension by precipitation, then dispersed in an inert solvent, and finally filtered onto polyethylene supports. The XANES spectra of Fe

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metal foil was recorded simultaneously to calibrate the energy of the monochromator (accurate to (0.2 eV). The program Viper26 was used to sum the data, identify the beginning of the absorption edge, fit pre, and post edge backgrounds, and hence to obtain the normalized absorbance χ as a function of the modulus of the photoelectron wavevector k. Fourier transform (FT) of χ(k) shows peaks that correspond to local atom correlations (shifted with respect to the real distances due to the phase shift). The positions of the peaks (R) correspond to bond distances between the central and the backscatterer atoms, whereas the amplitudes are related to the coordination number (N) and to the static and thermal disorder (σ) of the atoms around the absorber. Quantitative determination of the amount of different phases in the samples was achieved using the program LINCOM.26 This program runs a least-squares routine to minimize the difference between one EXAFS interference function and the combination of up to nine others. The quality of the fit was judged from the normalized sum of residuals

R-factor )

∑ k3n|χexpt(kn) - χlincom(kn)| n

∑ k3n|χexpt(kn))|

× 100

(1)

n

and from the reduced chi-squared27

reduced chi-squared ) Nind 1 Nind - P N

∑ n

k3n(χexpt(kn) - χlincom(kn))2 ε2n

(2)

where N is the number of data points, Nind is the number of independent parameters, P is the number of the fitted parameters, and n are the individual errors of the experimental data points. XANES spectra were processed in the usual way to obtain normalized absorbance.28 XANES at the K-edge of transition metals in oxides involves the excitation of a 1s photoelectron into p-type states.29 The energy of the absorption edge increases by ∼3 eV when oxidation state increases from +2 to +3.19 A pre-edge peak may occur before the absorption edge due to 1s to 3d transitions with 3d-4p mixing. The pre-edge peak is prominent for tetrahedral sites, but not for octahedral sites,29 because of the noncentrosymmetric nature of the former. The main peak and shoulders of the absorption edge reflect 4p continuum states and shape resonances of the excited atom environment, and secondary peaks at ∼30 eV higher energy correspond to multiple scattering from neighboring atoms. In the fingerprint approach, these features can be used to identify crystal phases by comparing samples with XANES spectra of reference compounds. 2.6. Magnetic Characterization. Magnetization measurements were performed in a SQUID magnetometer (MPMS 5S from Quantum Design) in a range between 5 and 300 K and up to 50 kOe. Iron oxide samples (0.3 mL) were introduced in a sealed Teflon capsule for magnetization measurements. The particle concentration in solution was kept at around 0.2-0.3% in mass. Zero-field-cooled (ZFC) magnetizations were measured by cooling the samples in a zero magnetic field and then increasing the temperature in a static field of 50 Oe, whereas field-cooled (FC) curves were obtained by cooling the samples in the same static field. The field dependence of the magnetiza-

tion was measured up to 50 kOe, after field and zero field cooling at low T. 3. Results and Discussion Figure 1 shows representative TEM bright field images of the colloidal samples: all the samples are nearly monodisperse and the average size is 10.0 nm, 14.5 nm, 8.0 nm, 13.0 nm going from IO_1 to IO_4, respectively. Part c of Figure 1 shows that the XRD patterns of IO_1 and 2 are very similar and show the presence of more than one polymorph of iron oxide. The XRD patterns of samples IO_3 and 4 reported in part f of Figure 1 are very similar and are consistent with the presence of an iron oxide with a spinel structure, such as Fe3O4 or γ-Fe2O3. The XRD pattern observed for samples IO_1 and IO_2 can be attributed to the formation of polycrystalline nanoparticles in which FeO and spinel phases are simultaneously present.16 The coexistence of FeO and Fe3O4 crystalline domains is not surprising taking into account the compositional and structural affinity of these two compounds. FeO is quite often nonstoichiometric due to partial oxidation of iron to Fe(III) and therefore its composition may approach the magnetite formula. Moreover, both the FeO (rock salt-type) and the spinel structures are based on a cubic close packed lattice, with oxygen closepacked layers on (111) planes. By sequential addition of oxidizer, only a spinel phase is formed, which was ascribed to γ-Fe2O3, that is to full oxidation, according to Raman spectroscopy.16 The second injection of oxidizer does not affect significantly the size and shape of the initial polycrystalline nanoparticles, as observed by comparing samples IO_1 versus IO_3 and IO_2 versus IO_4, respectively. On the other hand, the formation of iron oxides by oxidation of iron nanoparticles can be accompanied by a deep morphology variation: iron oxide hollow spheres were obtained starting from plain Fe nanocrystals because of ion diffusions dictated by the occurrence of the Kirkendall effect.21,32 It is likely that such strong ion gradients at the nanoscale that drive the morphology variation during ion rearrangement do not occur in our synthesis, where a significant amount of oxidant is added together with the iron precursor. The wide range of morphologies as well as oxidation pathways is a consequence of the complex interplay between factors affecting iron oxide formation and crystallization. Noteworthy, similarly to our findings, it was reported elsewhere that iron oxide crystallization is associated to the oxidation process.21 It should be noted however that the energy provided during the oxidation of the nanoparticles into a hot solution may itself promote crystallization. In fact, we have observed that polyphase particles with increasing size and crystallinity (as deduced by XRD peak sharpening) were produced at longer growth times, without performing any sequential injection of oxidizer.16 Although maghemite and magnetite have a slightly different cell parameter (8.351 Å for γ-Fe2O3 and 8.396 Å for Fe3O4) due to the extra line broadening of the peaks, the two nanocrystalline phases cannot easily be distinguished by XRD. Magnetite is an inverse spinel with ferrous ions in octahedral sites and ferric ions equally distributed between octahedral and tetrahedral sites. Maghemite is a ferric oxide with an inverse spinel structure that contains, as in magnetite, cations in tetrahedral and octahedral positions, the only difference being the presence of vacancies, usually in octahedral positions, to compensate for the increased positive charge. XANES spectra were used to examine the average oxidation state of iron in the samples. The main absorption edge shifts to

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Figure 1. TEM representative images of sample IO_1 (a), IO_2 (b), IO_3 (d), IO_4 (e), and corresponding XRD patterns of the polyphase (c) and spinel phase (f) samples. Scale bar in the inserts is 20 nm.

higher energy by ∼3 eV as the oxidation state of iron changes from 2+ to 3+, as seen in the XANES spectra (Figure S1 of the Supporting Information) of iron oxides FeO, Fe3O4, γ-Fe2O3, and R-Fe2O3 (referred to as hematite). Figure 2 shows the spectra of the samples (a) IO_1 and (b) IO_2 compared to FeO and Fe3O4. The average oxidation state of iron in IO_1 is similar to 2.67+ (i.e., similar to that in Fe3O4), and in IO_2 it is slightly less than 2.67+. The value of ∼2.67+ suggests either the samples contain a majority of Fe3O4, or they contain a mixture of FeO and γ-Fe2O3 (noting that the presence of the hexagonal ferric oxide hematite, R-Fe2O3, was ruled out on the basis of XRD results which indicate the presence of a spinel structure). Part a of Figure 2 also shows the same samples after 1 year, denoted IO_1′ and IO_2′, and, from the shift in the absorption edge to higher energy, it is seen that the average oxidation state of iron has increased slightly in both IO_1′ and IO_2′. Figure 3 shows the spectra of the samples (a) IO_3 and (b) IO_4 compared to Fe3O4 and γ-Fe2O3. The average oxidation state of iron in IO_3 is between 2.67+ and 3+ (i.e., between that of iron in Fe3O4 and γ-Fe2O3), and in the sample IO_4 it is 3+. This suggests the sample IO_3 contains a significant amount of γ-Fe2O3, and IO_4 contains mostly γ-Fe2O3. Figure 3 also

shows the same samples after 1 year, denoted IO-3′ and IO-4′, and the oxidation state of iron in both samples is 3+. The pre-edge peak in the XANES spectra indicates whether the sample contains tetrahedrally or octahedrally coordinated iron.20 The pre-edge peak in iron oxides (Figure S1 of the Supporting Information) is quite pronounced for Fe3O4 and γ-Fe2O3, which have a mixture of tetrahedrally and octahedrally coordinated Fe, but is very diminished for structures which have only octahedrally coordinated iron such as FeO, for which the pre-edge peak is barely visible, and R-Fe2O3, for which the preedge peak is less than half the size compared to spinel phases. The pre-edge peaks of the samples are shown in detail in the inserts (c) in Figures 2 and 3. Part c of Figure 2 shows that the pre-edge peaks of samples IO_1 and IO_2 are in the same position as those for Fe3O4 and γ-Fe2O3 but not R-Fe2O3, and they are significantly diminished compared to that of Fe3O4 but still more prominent than that of FeO. This is more consistent with the samples containing a mixture of FeO and γ-Fe2O3 and is less consistent with the samples IO_1 and IO_2 containing a majority of Fe3O4. Part c of Figure 3 shows that the pre-edge peak of sample IO_4 is very similar to that of γ-Fe2O3 and not R-Fe2O3 (which has a very diminished pre-edge peak) in

Iron Oxide Colloidal Nanocrystal

Figure 2. XANES spectra of samples (a) IO_1 (solid black) and (b) IO_2 (dashed black) compared to reference compounds of FeO (dashed gray) and Fe3O4 (solid gray). Also shown are samples after 1 year (a) IO_1′ (dash-dot black) and (b) IO_2′ (dotted black). The insert (c) shows details of the pre-edge peak. (Note that the energy scale is relative to the energy of the pre-edge peak in Fe metal, 7112 eV, and spectra have been shown with vertical offsets for clarity).

Figure 3. XANES spectra of samples (a) IO_3 (solid black) and (b) IO_4 (dashed black) compared to reference compounds of Fe3O4 (solid gray) and γ-Fe2O3 (heavy gray). Also shown are samples after 1 year (a) IO_4′ (dash dot black line) and (b) IO_3′ (dotted black line). The insert (c) shows details of the pre-edge peak. (Note that the energy scale is relative to the energy of the pre-edge peak in Fe metal, 7112 eV, and spectra have been shown with vertical offsets for clarity).

agreement with XRD data. This indicates that sample IO-4 is γ-Fe2O3. The pre-edge peak of sample IO_3 (part c of Figure 3) is slightly lower than that of γ-Fe2O3, being more similar in height to that of Fe3O4. In the fingerprint approach, the shape of the XANES spectra of the samples is compared with those of reference compounds. The shape of the main absorption edge peak, the shoulder at

J. Phys. Chem. C, Vol. 113, No. 43, 2009 18671 ∼20-25 eV, and the secondary peaks at ∼30-35 eV, are quite different for each of the iron oxides FeO, Fe3O4, γ-Fe2O3 and R-Fe2O3 (Figure S1 of the Supporting Information). Figure 2 shows the spectra of the samples (a) IO_1 and (b) IO_2 compared to FeO and Fe3O4. The spectra of IO_1 has a shape quite similar to that of Fe3O4 but this shape could also arise due to a mixture of FeO and γ-Fe2O3. The spectra of IO_2 also has a shape similar to that of Fe3O4 but has a more prominent secondary peak at ∼30 eV like that of FeO, which clearly indicates that IO_2 contains a significant proportion of FeO. Figure 3 shows the spectra of samples (a) IO_3 and (b) IO_4 compared to those of Fe3O4 and γ-Fe2O3. The spectra of IO_4 is very similar to that of γ-Fe2O3 in all respects, which confirms this sample contains γ-Fe2O3. The spectra of IO_3 is similar to γ-Fe2O3 but has a more rounded main absorption edge peak and a less pronounced shoulder at ∼25 eV and secondary scattering peak at ∼35 eV, compared to γ-Fe2O3. This indicates that IO_3 contains another phase, either a significant amount of Fe3O4 (as the spectra of IO_3 has a shape intermediate between Fe3O4 and γ-Fe2O3), or a small amount of FeO. The same sample after one year, denoted IO_3′, has a spectra which is very similar to that of γ-Fe2O3. These data indicate that the nanocrystals in the colloidal suspension are exposed to further oxidation even at room temperature, as previously observed in water suspensions of magnetite nanocrystals that oxidize within a couple of months.18 The EXAFS k3χ(k) and FT of the iron oxide reference compounds (FeO, Fe3O4, and γ-Fe2O3), shown in Figure 4, indicate a close similarity between Fe3O4 and γ-Fe2O3 in agreement with their very similar structures. The frequencies of the oscillations are the same in the two compounds and peaks in the FT are present at about the same distance values. The main difference between Fe3O4 and γ-Fe2O3 is in the amplitude of the oscillations of the EXAFS k3χ(k) and in the height of the FT peaks, consistent with the presence of vacancies in γ-Fe2O3 possibly accompanied with a more disordered structure. On the other hand, the EXAFS k3χ(k) of FeO present significant differences with those of other two reference compounds, especially in the regions 2-6 Å-1 and 9-14 Å-1. The comparison of the FT of FeO with those of Fe3O4 and γ-Fe2O3 show the latter have a split second peak, whereas in FeO there is no splitting of the second peak. The second peak is mainly due to Fe-Fe distances in all reference compounds. However, in FeO, where only the octahedral sites are occupied, a single second peak occurs due to a single Fe-Fe distance between two octahedral sites. In contrast, the spinel phases have both octahedral and tetrahedral sites occupied. The first component of the split second peak is due again to the Fe-Fe distance between two octahedral sites, whereas the second component of the split second peak is due to both Fe-Fe distances between two tetrahedral sites and between one octahedral and one tetrahedral site. Moreover, significant multiple scattering effects due to collinear arrangement of atoms are present in the highly symmetric FeO structure.30 The EXAFS interference functions of the samples, which are reported in Figure 5, are much noisier than those of standards. Nevertheless, a careful comparison of the second peak of the FTs (and also of the EXAFS k3χ(k) in the region 2-5 A-1) allows one to gather information on the phases present in the different samples. Because of the close similarity between the Fe3O4 and γ-Fe2O3 structures, EXAFS is able to distinguish FeO from the spinel phases but cannot distinguish the two spinel phases. To distinguish between γ-Fe2O3 and Fe3O4, only XANES results can be used.

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Figure 4. EXAFS k3χ(k) (A) and corresponding Fourier transforms (B) for γ-Fe2O3 (a), Fe3O4 (b), and FeO (c).

Figure 5. EXAFS k3χ(k) (A) and corresponding Fourier transforms for IO_1 (a), IO_2 (b), IO_3, IO_4. Experiment (-), LINCOM fit results ( · · · ).

The sample that seems most like FeO is IO_2. However, the second FT peak present a small shoulder on the right-hand side, which suggests the presence of some spinel phase together with FeO. In IO_1 sample, the shoulder is more evidently consistent with a larger fraction of spinel phase, which accompanies the FeO. In IO_3, a single large second peak is present in the FT consistent with a fairly large amount of spinel phase. Similar comments can be made for IO_4 sample. To evaluate in a quantitative way the amount of FeO and spinel phase in the samples, the program LINCOM was used running a least-squares routine to minimize the difference between the EXAFS k3χ(k) of the samples and the combination of the EXAFS k3χ(k) of FeO and γ-Fe2O3. γ-Fe2O3 was chosen between the two reference compounds with spinel structure, γ-Fe2O3 and Fe3O4, because the amplitude of the oscillations for γ-Fe2O3 is more similar to those of the samples. The results of the linear combination are shown in Figure 5 and the results are summarized in Table 2. It should be pointed

TABLE 2: Quantitative Evaluation of the Relative Amount of FeO and Spinel Phase in the Samples as Obtained by the Program LINCOM and Related Least Square Error sample name

% FeO phase

% spinel phase

R-factor

reduced chi-squared

IO_1 IO_2 IO_3 IO_4

24(3) 42(3) 24(5) 9(5)

76(3) 58(3) 76(5) 91(5)

35 28 50 34

157 131 257 143

out that because only two reference compounds were used to fit the data of the samples using LINCOM, only one parameter is actually fitted, that is the amount of one of the two reference compounds, the amount of the second reference compounds being 100% minus the amount of the first one. The fit results confirm the qualitative observations made on the FTs. The sample with the higher amount of FeO is IO_2 containing 42(3)% FeO and 58(3)% spinel, IO_1 contains 24(3)% FeO and

Iron Oxide Colloidal Nanocrystal

Figure 6. Magnetic susceptibility of IO_4 sample: ZFC curve measured at 50 Oe. The solid line corresponds to the theoretical SPM relaxation of magnetic nanoparticles (Curie-Weiss law).

76(3)% spinel, IO_4 is mostly spinel, having 9(5)% FeO and 91(5)% spinel. The fit for IO_3 is less good especially in the region 7.5-9 Å-1 and therefore the estimate of 24(5)% FeO and 76(5)% spinel is less reliable. The EXAFS and XANES data suggest that the samples with larger diameter have a slightly higher oxidation degree. These results may seem counterintuitive, as one would expect that smaller particles are more easily oxidized due to the larger surface-to-volume ratio. However, an extra stability has been observed in different systems for the smallest nanocrystals, which can be attributed to the fact that in larger crystals it is less costly to produce dislocations and defects that facilitate oxygen diffusion.31,32 Among the broad range of potential applications, magnetic properties of iron oxides are of particular interest, with special reference to the development of nanomaterials for biomedical purposes. Magnetic characterization was performed by collecting the magnetic response (M) of the IO_4 sample as a function of the applied field (H) and of the temperature. Figure 6 shows the temperature dependence of the low field magnetization in a zero field cool (ZFC)-field cool (FC) process. When the sample is cooled in the absence of magnetic field (ZFC), a random distribution of cluster magnetizations freezes, the total magnetization of the system being equal to zero. If the magnetization is then measured by increasing the temperature and applying simultaneously a small field (50 Oe), magnetization first increases as the cluster moments are progressively unblocked and able to align toward the applied field. After reaching a maximum (blocking temperature, TB), the magnetization decreases due to thermal oscillation of the magnetic moment. Because each cluster behaves as a single magnetic moment, this is referred to as superparamagnetism (SPM). Besides, when the sample is cooled in the presence of the small magnetic field (field cooling, FC), the cluster magnetizations become progressively blocked favoring the direction of the applied field and a remanence is found, which increases monotonously as the temperature decreases. The observed behavior of the FC above TB corresponds to a Curie-Weiss SPM behavior (magnetization is proportional to 1/T). The coincidence of the maximum of the ZFC magnetization curve (TB) with the merging of the ZFC and FC curves (Tirr) rules out the occurrence of significant particle aggregation or large size distributions. The magnetic anisotropy was estimated from the value of the blocking temperature, TB. The mean blocking temperature of an assembly of fine magnetic particles depends on the energy barrier distribution and the experimental time window. The mean energy barrier and the mean blocking temperatures are related

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Figure 7. Hysteresis loops at different temperatures of IO_4 sample: magnetization curves as a function of the applied field at 50, 175, and 300 K. Insert: a detail at low field of the hysteresis loop at 50 K, showing a coercivity field of 80 Oe.

as KV ∝ kBT, where K is the anisotropy constant (erg/cm3), V the average particle volume (cm3), and kB the Boltzmann constant (1.38 · 1016 erg/K). For a given measuring time tm and a given relaxation time τ0 in the Arrhenious law τ ) τ0 exp (E/kBT), the energy of the anisotropy barrier (E ) KV) and the thermal energy (kBT) are related through KV ) ln (tm/τ0)kBT. For SQUID measurements, tm is about 50 s and in the present case τ0 ∼ 10-11 s, then ln tm/τ0 ∼ 27. Thus, in our experiment, the mean energy barrier and the mean blocking temperatures are related as:

〈KV〉 = 27kB〈T〉 For a 13 nm nanoparticle with TB ) 55 K, the anisotropy value is found to be 1.8 × 105 erg/cm3, compared to the bulk values of 5 × 104 erg/cm3 for maghemite and 1.1 × 105 erg/ cm3 for magnetite. Anisotropy values for magnetite nanoparticles are always much larger than for bulk magnetite, and, because the observed value of 1.8 × 105 erg/cm3 is not much larger compared to 1.1 × 105 erg/cm3 for bulk magnetite, this suggests the nanoparticles are maghemite.32 The reported anisotropy constants for solid maghemite nanoparticles, with magnetic volumes similar to those analyzed here, cover a broad range of values on the order of ∼105-106 erg/cm3, depending on sample morphology, the extent of the interaction between nanoparticles, and the calculation method.32 The hysteresis loops indicate that the sample is far from saturation even at low T and 30 kOe, in agreement with the large anisotropy, as shown in Figure 7 for fields up to 10 KOe. At higher T, the diamagnetic contribution from the solvent and the sample holder dominates the magnetization at high fields, showing a negative magnetization versus field loop in the measurement at 300 K. Such magnetization curves have been observed in maghemite nanoparticles where there exists a high contribution to the magnetic behavior from the surface. At the surface, the broken translational symmetry of the crystal and the lower coordination generates randomness in the exchange interactions and thus a spin frustration leading to larger anisotropies and reduced magnetic saturation. Surface spin disorder and surface spin canting have been evidenced by Mo¨ssbauer spectroscopy,33 inelastic neutron scattering,34 X-ray absorption spectroscopy and dichroism,35 and polarized neutron diffraction.36 The existence of such a disordered spin layer leads to the reduction of the number of spins aligning with the external field and hence to the decrease of the saturation magnetization.37-39 On the basis

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Figure 8. HREM image of a self-assembled ensemble of IO_4 nanocrystals (center) and energy filtered images selectively showing the iron (left) and oxygen (right) distribution within the nanocrystals.

of these observations, solid oxide magnetic nanoparticles are usually modeled as a magnetic core surrounded by a shell of canted spins. Thus, the magnetization of nanoparticles is not saturated at the highest measuring fields. At large fields, the magnetization versus field shows a monotone increasing behavior, which is consistent with a paramagnetic behavior of the disordered spins at the particle surface and the small saturation magnetization observed.40 Moreover, magnetization curves below the freezing point of the solvent allow us to rule out the occurrence of a significant contribution to the magnetic behavior arising from particle shape effects (Supporting Information). Magnetic characterization indicates that the nanocrystals show the expected behavior of small oxide nanoparticles with a large disorder of spins, leading to weakly coupled paramagnetic atoms, which are hard to saturate even at low T. Although maghemite or magnetite are hard to distinguish by magnetometry at the nanoscale, the results are consistent with the IO_4 nanoparticles being maghemite, in agreement with X-ray absorption results. To gain insight into the iron and oxygen distribution within the nanocrystals at the nanometer scale, energy filtered (EF) TEM images were collected. A representative image of sample IO_4 is reported in Figure 8, which compares the HREM image of a self-assembly of nanocrystals with the corresponding Fefiltered (left) and O-filtered (right) images, that is the same area reconstructed by collecting only the electrons losing energy selectively in the spectral ranges corresponding to the Fe M-edge (94 eV, ∆E ) 7 eV), and to the O K-edge (535 eV, ∆E ) 30 eV). In the EF image, the presence of the selected element is indicated by a bright zone on a dark background. The HREM image shown in the center indicates the presence of nearly monodisperse 13 nm nanocrystals, which self-assemble into a close packing thanks to the monodispersion, in agreement with low-resolution TEM data. The nanocrystals appear as dark areas and the lattice planes are also visible suggesting that the nanocrystals are single crystalline. The bright area between the nanocrystals is representative of the organic capping layer, which is transparent to the electron beam. The bright area in the EF images shows that both iron and oxygen are present in the whole nanocrystal area, indicating a homogeneous distribution of the two elements throughout the nanocrystal. It should be pointed out that the more pronounced background noise in the oxygenfiltered image is due to the reduced intensity of the O K-edge EELS, which lies at higher energies compared to the Fe M-edge EELS and to the presence of oxygen in the fatty acid on the surface of the nanocrystals.

4. Conclusions Nearly monodisperse iron oxide nanocrystals were obtained by a nonhydrolytic high-temperature solution method based on the decomposition of iron pentacarbonyl. The nanocrystal size and oxidation degree were varied respectively by adjusting the reaction temperature and by performing sequential injection of an oxidizer. The different samples were compared using the information provided by different characterization techniques such as TEM, XRD, and X-ray absorption spectroscopy. XRD and TEM point out that polycrystalline nanoparticles made out of FeO and a spinel phase are formed at low oxidant amounts, whereas single crystalline spinel nanoparticles are obtained by increasing the oxidation extent. No relevant variation in particle morphology was observed upon full oxidation of the polycrystalline nanoparticles, in agreement with similarity among the different crystal structures. EXAFS investigation enables quantitative evaluation of the relative amount of the two phases in the polycrystalline samples, the relative amount of the spinel phase being 76% and 58% in the 10 and 14.5 nm nanoparticles, respectively. In addition, XANES pointed out that the polycrystalline colloids spontaneously undergo slow oxidation and it was found that the larger nanocrystals oxidize more readily. We found that the spinel structure in the 8 nm particles is a mixture of mainly Fe3O4 and γ-Fe2O3, whereas the 13 nm crystals are made out of γ-Fe2O3, as also supported by SQUID magnetization measurements. Energy-filtered TEM images support the compositional homogeneity of the single crystalline 13 nm nanoparticles. These results point out the importance of XANES for the elucidation of subtle changes in the oxidation state of iron in iron oxide and shows that a multitechnique approach helps in understanding the complex properties of iron oxide nanocrystals widely used from biomedicine to catalysis. The precise control of the microstructure of the nanocrystals and of the related magnetic features is most crucial for the improvement of technological application of iron oxides properties, which strictly rely on quantitative determination of their properties, such as nuclear magnetic resonance imaging41 as well micro- and spintronics.42 Acknowledgment. This work was supported by the INSTM under the PRISMA project. M.F.C. thanks the CNR under the Short-term mobility programme; STFC Daresbury Laboratory is acknowledged for the provision of synchrotron radiation, which was supported by the European Community-Access of Research Infrastructure action of the Improving Human Potential Program.

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