J. Phys. Chem. C 2009, 113, 18667–18675
18667
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 Català de
Nanotecnologia, Campus UAB 08193 Bellaterra, Spain, Center for NanoScience,
Ludwig-Maximilians-UniVersität München, Munich, Germany, and Fachbereich Physik, Philipps UniVersität
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 Català de Nanotecnologia.
|
Ludwig-Maximilians-Universität München.
⊥
Philipps Universität 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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J. Phys. Chem. C, Vol. 113, No. 43, 2009
Corrias et al.
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
Iron Oxide Colloidal Nanocrystal
J. Phys. Chem. C, Vol. 113, No. 43, 2009 18669
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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Corrias et al.
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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Corrias et al.
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
J. Phys. Chem. C, Vol. 113, No. 43, 2009 18673
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 Mössbauer 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
18674
J. Phys. Chem. C, Vol. 113, No. 43, 2009
Corrias et al.
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.
Iron Oxide Colloidal Nanocrystal
Supporting Information Available: XANES of reference
oxides and ZFC-FC curves recorded at different temperatures.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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