CrystEngComm
PAPER
Cite this: CrystEngComm,2015,17,
8463
Received 7th August 2015,
Accepted 30th September 2015
DOI: 10.1039/c5ce01585e
www.rsc.org/crystengcomm
Time-resolved in situ studies on the formation
mechanism of iron oxide nanoparticles using
combined fast-XANES and SAXS†
Anke Kabelitz,
a
Ana Guilherme,
a
Maike Joester,
ab
Uwe Reinholz,
a
Martin Radtke,
a
Ralf Bienert,
a
Katrin Schulz,
c
Roman Schmack,
c
Ralph Kraehnert
c
and
Franziska Emmerling*
a
The reaction of iron chlorides with an alkaline reagent is one of the most prominent methods for the
synthesis of iron oxide nanoparticles. We studied the particle formation mechanism using triethanolamine
as reactant and stabilizing agent. In situ fast-X-ray absorption near edge spectroscopy and small-angle
X-ray scattering provide information on the oxidation state and the structural information at the same time.
In situ data were complemented by ex situ transmission electron microscopy, wide-angle X-ray scattering
and Raman analysis of the formed nanoparticles. The formation of maghemite nanoparticles (γ-Fe
2
O
3
)from
ferric and ferrous chloride was investigated. Prior to the formation of these nanoparticles, the formation
and conversion of intermediate phases (akaganeite, iron(II,III) hydroxides) was observed which undergoes a
morphological and structural collapse. The thus formed small magnetite nanoparticles (Fe
3
O
4
) grow further
and convert to maghemite with increasing reaction time.
Introduction
Iron oxide nanoparticles (FeO
x
NPs) are intensively studied
because of their broad range of applications in different areas
like sensing,
1
catalysis,
2
magnetic storage media,
3
and
biomedicine.
4–6
The most prominent iron oxides are
maghemite (γ-Fe
2
O
3
) and magnetite (Fe
3
O
4
) with a spinel
structure.
7
To synthesize these FeO
x
NPs, the most common
synthesis involves precipitating an iron precursor in an alka-
line, aqueous solution.
8–10
Changing parameters like pH,
iron precursor ratio (Fe
2+
/Fe
3+
), iron concentration, ionic
strength, temperature, alkaline agent, and stabilization agent,
the composition and size of the nanoparticles can be
varied.
11–13
Stabilization agents like phosphates,
14
carboxy-
dextran,
15
and triethanolamine (TREA)
16,17
are used to alter
the magnetic properties and water solubility of FeO
x
NPs and
to achieve a narrow size distribution.
To gain deeper insight into the mechanism of a given NP
synthesis procedure, many studies rely on ex situ methods.
Baumgartner et al. elucidated the formation mechanism of
magnetite nanoparticles based on ex situ cryo-transmission
electron microscopy (TEM).
18
Their study reveals that prelimi-
nary formed nanometric ferrihydrite particles agglomerate
and transform into magnetite nanoparticles. Ahn et al.
observed the formation of different iron oxide hydroxide
intermediates during the formation of magnetite nano-
particles using ex situ X-ray diffraction (XRD) and TEM.
10
Ex
situ analysis of FeO
x
NPs typically requires sample prepara-
tion. Steps like centrifugation and drying could lead to arti-
facts, e.g. a change in the particle size.
19,20
In situ studies can provide information on the particle for-
mation, the formation of intermediates, and phase changes
directly in the reaction media without the need for such a
sample preparation.
21–24
To investigate the formation mecha-
nism in real time, combinations of different analytical tools
like scattering methods (small-angle X-ray scattering (SAXS)
and wide-angle X-ray scattering (WAXS)) are available. In situ
studies on metal oxide systems were reported, coupling scat-
tering and spectroscopic techniques.
25
The formation mecha-
nism of SnO
2
NPs is illustrated by Caetano et al. coupling
Raman spectroscopy and extended X-ray absorption fine
structure (EXAFS), which were supported by SAXS data.
26
Bremholm et al. investigated the formation of magnetite
nanoparticles using in situ SAXS and WAXS in supercritical
water (P
critical
= 221 bar, T
critical
= 374 °C).
27
In their study, an
ammonium ironIJIII) citrate precursor was injected into a
preheated reactor. The authors observed the formation of
amorphous ironIJIII) hydroxide nanoclusters, which dissolve or
CrystEngComm,2015,17,8463–8470 | 8463This journal is © The Royal Society of Chemistry 2015
a
BAM Federal Institute for Materials Research and Testing, Richard-Willstätter
Strasse 11, D-12489 Berlin, Germany. E-mail: franziska.emmerling@bam.de
b
Humboldt-Universität zu Berlin, Department of Chemistry, Brook-Taylor-Strasse
2, D-12489 Berlin, Germany
c
Technische Universität Berlin, Department of Chemistry, Strasse des 17. Juni
124, D-10623 Berlin, Germany
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5ce01585e
Open Access Article. Published on 30 September 2015. Downloaded on 19/05/2017 13:40:16.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
View Journal
| View Issue
8464 |CrystEngComm,2015,17,8463–8470 This journal is © The Royal Society of Chemistry 2015
decompose completely before magnetite nanoparticles crys-
tallize. Jensen et al. described the formation of maghemite
NPs from the ammonium ironIJIII) citrate precursor at high
temperatures and under high pressure by using total scatter-
ing data and atomic pair distribution function (PDF) analy-
sis.
28
In both mentioned FeO
x
NPs syntheses only Fe
3+
pre-
cursors are used in the reaction. For detailed knowledge
about the formation mechanisms, methods for determining
the changes in the oxidation state during the reaction are
required.
Here, we present the first in situ study of the formation
process of maghemite NPs in aqueous solution coupling
X-ray absorption near edge spectroscopy (XANES) and SAXS
investigation. We followed the occurrence of intermediates
during the co-precipitation reaction by combining these
methods. XANES analysis provides information on the oxida-
tion state and the local structure of the iron atoms. SAXS data
provide information about the size of the particles. We study
the formation of maghemite nanoparticles from ferric and
ferrous chloride using TREA in situ supplemented by comple-
mentary XRD, TEM, Raman and selected area electron dif-
fraction (SAED). The advantage of the chosen synthesis is the
small number of reaction partners and the favorable proper-
ties of TREA, which react as stabilizing and alkaline agent. In
the following, the experimental part, the characterization of
the final products, and intermediates is shown, followed by
the results of the in situ characterization. In addition, we
studied the influence of HCl solution of the reaction mecha-
nism of maghemite NPs to propose a formation mechanism.
Experimental
Synthesis of iron oxide nanoparticles
The synthesis used for the in situ reactions was modified
based on the synthesis procedure described by Peng et al.
16
Peng synthesized γ-Fe
2
O
3
NPs with a diameter of 8 nm
starting from a solution of ferrous and ferric chloride and
adding TREA. The solution was heated and refluxed for 3 h.
In our synthesis, we fixed the temperature to 115 °C to slow
down the reaction.
A typical synthesis is depicted in Fig. 1a. The iron precur-
sor solution was prepared by dissolving ferric chloride hexa-
hydrate (FeCl
3
·6H
2
O, Aldrich, >97% purity) and ferrous chlo-
ride tetrahydrate (FeCl
2
·4H
2
O, Aldrich, ≥99% purity) in a
molar ratio of 1 : 1 in water (Millipore, 18.2 MΩcm, TOC ≤5
ppb, flushed with nitrogen) to reach a total iron concentra-
tion of 0.31 M. All reagents were used without further purifi-
cation. TREA (Acros Oganics, 99+% purity) was mixed with
water in a ratio of 3 :1 (v/v) (TREA solution). Both solutions
were filled into separate vials, sealed, and preheated to 115
°C (oil bath) for 10 min. 16 mL of TREA solution was added
to 4 mL of iron precursor solution. The synthesis was carried
out under continuous magnetic stirring after addition of the
TREA solution to the iron precursor solution. The solution
was treated at 115 °C (oil bath) for 90 min. In a second type
of experiment, HCl was added to the iron precursor solution
(final concentration 0.125 M) (see Table 1).
Sample environment for synchrotron experiments
An acoustic levitator was used as a sample holder for the
SAXS and XANES experiments (see Fig. 1b).
29
In this device a
stationary ultrasonic field allows to levitate liquid samples
and solids in a contact-free environment.
30
The acoustic
levitator was integrated in the μSpot beamline BESSY II syn-
chrotron (Helmholtz Centre Berlin for Materials and Energy,
Germany) as described elsewhere.
31
Time-resolved SAXS and
XANES data were obtained simultaneously.
32,33
Time-resolved in situ XANES and SAXS experiments
The experimental setup is depicted in Fig. 1b. During the syn-
thesis, samples (4 μL) were extracted from the reaction solu-
tion at different reaction times, placed immediately in the
acoustic levitator, and investigated by time-resolved XANES
and SAXS experiments. The X-ray beam was mono-
chromatized using a Double Crystal Monochromator (DCM)
Fig. 1 a) Procedure for the synthesis of γ-Fe
2
O
3
NPs. The two
solutions were preheated to 115 °C and mixed afterwards. The heating
of the iron chloride solutions (iron precursor solution) leads to an
increased turbidity, which is marked in dark yellow. The iron oxide
nanoparticles were obtained after heating the mixed solution for 90
min. b) Experimental setup for coupled XANES and SAXS investigations
using an acoustic levitator as sample holder.
Table 1 Experimental conditions. The ratio of ferric and ferrous chloride
is given by the fraction F of Fe
3+
ions F
Fe3+
(F
Fe3+
+F
Fe2+
=1)inthe
solution
Experiment type Iron precursor solution TREA solution
Iron precursor F
Fe3+
1 FeCl
2
, FeCl
3
0.5 TREA
2 FeCl
2
, FeCl
3
, HCl 0.5 TREA
CrystEngCommPaper
Open Access Article. Published on 30 September 2015. Downloaded on 19/05/2017 13:40:16.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
CrystEngComm,2015,17,8463–8470 | 8465This journal is © The Royal Society of Chemistry 2015
Si (111). A beam size of 100 μm was used for the XANES and
SAXS experiments.
For XANES analysis, the monochromatized beam has an
energy resolution of E/ΔE= 5000, which corresponds to an
energy resolution of about 1.4 eV for Fe-K edge (7112 eV).
The fluorescence of the Fe-K line was detected with a silicon
drift detector (AXA, KETEK, Munich, Germany) at a working
distance of 10 mm. A Fe foil (12.5 μm thick) was used to pro-
vide an accurate internal calibration of the monochromator
for all spectra. The associated uncertainty was experimentally
determined by measuring the foil 10 times. A value of ±0.3 eV
was obtained.
The XANES analysis was adjusted to meet the require-
ments of the time-resolved experiments. The excitation
energy was tuned between 7107 eV and 7134 eV. An energy
step size of 0.5 eV was applied between 7017 eV and 7125 eV
and 1.5 eV between 7125 eV and 7134 eV. The time per step
was four seconds, resulting in acquisition time of 270 s for a
XANES spectrum. XANES data were processed by ATHENA.
34
This GUI program belongs to the main package IFEFFIT (v.
1.2.11). The AutoBK background subtraction procedure was
used with the Rbkg parameter set to 1.0 Å. All spectra were
normalized to 1. The position of the absorption edge can be
derived from the maximum of the first derivative of the
XANES data. Simulations of the XANES spectra for the mole-
cule FeCl
3
·6H
2
O were performed using FEFF9 software.
35
Fur-
ther information is found in the ESI†(Fig. S1).
For SAXS analysis, a two-dimensional MarMosaic CCD
X-ray detector with 3072 ×3072 pixels was used to record the
scattering intensity at a sample-detector distance of 819.8
mm. In a typical experiment, the corresponding SAXS data
were collected for 300 s every seven minutes. The obtained
scattering images were processed and converted into dia-
grams of scattered intensities versus the scattering vector q
employing an algorithm from the FIT2D software.
36
qis
defined by q=4π/λ·sin θwith θbeing half of the scattering
and λbeing the wavelength. (The small shift of the wave-
length required for the XANES measurement can be
neglected in the evaluation of the SAXS data.) All SAXS data
were evaluated using a global scattering function described
by Beaucage et al.,
37
to achieve the particle size in terms of
the radius of gyration and the power law contribution at
higher qvalues.
The curve fitting was done using IRENA
38
implemented in
the software package IGOR PRO. Further information of the
background correction and fitting procedure is given in the
ESI†(S2). A separate SAXS experiment with a higher time res-
olution was performed with a wavelength of 1.0000 Å (12398
eV). The SAXS data were collected for 20 s every three
minutes.
Samples for the Raman measurements were taken from
the reaction solution. Raman spectra were measured on a
LabRam HR800 (Horiba Jobin Yvon, Bensheim, Germany)
equipped with a 633 nm HeNe laser (Horiba Jobin Yvon,
Bensheim, Germany). The instrument was coupled to a BX41
microscope (Olympus, Hamburg, Germany) with a 60×
immersion objective. The backscattered Raman light was
detected by a liquid nitrogen-cooled CCD detector (1024 ×
256 pixels, Horiba). The laser intensity was 5.09·10
5
Wcm
−2
at the liquid sample.
Further characterization
TEM samples were prepared by adding acetone to the reac-
tion solution. The resulting precipitate was dispersed in etha-
nol. 15 μL of the solution were placed onto carbon-coated
200 mesh copper grids (EMS) and dried at room temperature.
TEM and high resolution TEM (HR-TEM) images of the nano-
particles were acquired using a FEI Tecnai G
2
20S-TWIN (FEI,
USA) with an acceleration voltage of 200 keV, equipped with
a LaB
6
electron source. The particle diameter was evaluated
by averaging over 100 measured nanoparticles. Environmen-
tal scanning electron microscopy (ESEM) analysis of the dried
samples was performed on an XL30 ESEM-FG instrument
(FEI Company, Hillsboro, USA) operating at 15 kV.
For XRD analysis, the colloidal solutions were cooled to
room temperature. After the addition of acetone, the precipi-
tate was centrifuged (2500 rpm, 4 min), washed three times
with acetone, and dried for 10 h at room temperature. XRD
patterns were collected using a wavelength of 10 000 Å (12
398 eV). The dried final FeO
x
NPs were analyzed using a stan-
dard powder sample holder. In a typical experiment, XRD
data were collected for 120–180 s. The crystallite size D is
estimated based on the Scherrer equation D=K·λ/β·cosθ,
with λbeing the wavelength of the X-ray radiation, βbeing
the full width at half maximum, Kbeing the Scherrer shape
factor, and θhalf of the scattering angle.
39
Results and discussion
In the following sections, we present the characterization of
the final products and first intermediate followed by the
results of the in situ characterization. Further experiments
with HCl solution were performed and a formation mecha-
nism is proposed based on the resulting data.
Characterization of the final particles without adding HCl
Fig. 2 shows the result of the characterization of the final par-
ticles for the synthesis with ferricIJII) and ferrousIJIII) chloride
in the presence of TREA. The final nanoparticles were charac-
terized using XRD, TEM, and SAXS. Fig. 2a displays the XRD
data of the dried final nanoparticles. All detected reflections
are consistent with data base entry for maghemite (JCPDS,
PDF: 39-1346, red). The diffraction pattern of the product
shows broad reflections, which reflect the small crystallite
size of the nanoparticles around 7.7 nm.
39
The TEM image of the maghemite nanoparticles (see
Fig. 2b and c) shows spherical particles with an average parti-
cle diameter of 6.8 ± 1.3 nm. The HR-TEM and the corre-
sponding Fourier transformations of the maghemite NPs doc-
ument that the whole particles are crystalline. The lattice
CrystEngComm Paper
Open Access Article. Published on 30 September 2015. Downloaded on 19/05/2017 13:40:16.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
8466 |CrystEngComm,2015,17, 8463–8470 This journal is © The Royal Society of Chemistry 2015
fringes correspond well to the crystal structure obtained from
XRD.
The small-angle X-ray scattering curve of the maghemite
NPs shows a Gaussian decay in the high q-region and a pla-
teau in the low q-region (see Fig. 2d). The power-law decay of
the scattering curve is equal to q
−4
, indicating a smooth sur-
face of the nanoparticles. The scattering curve was fitted by a
unified fit model (Fig. 2d, red line). The evaluation of the
SAXS data results in a radius of gyration of 4.2 nm, corre-
sponding to a particle diameter D
P
of 10.8 nm. The particle
diameter D
P
is related to the radius of gyration R
G
by
. The differences in the determined diameter can
be attributed to a small amount of agglomerates.
Characterization of the first intermediate without adding HCl
Simultaneous in situ XANES and SAXS analyses were
performed on the iron chloride solution and the first inter-
mediate. In Fig. 3a, the XANES spectra (Fe K-edge) are
presented as a function of time. Fig. 3b shows the evaluation
of the integrated pre-edge peak area (IPA) and the position of
the absorption edge during the reaction. The XANES spec-
trum of the yellow iron precursor solution, containing ferrous
and ferric chloride with a fraction of Fe
3+
ions (F
Fe3+
) = 0.5, is
shown as grey curve (Fig. 3a). The spectrum exhibits an
absorption edge at 7124.1 eV and a low IPA (see Fig. 3b). Dur-
ing the reaction at 115 °C, the iron chloride solution turns
into a dark yellow, turbid solution (experiment type 1). The
position of the absorption edge and the IPA show no signifi-
cant change during heating of the iron chloride solution,
which indicates that both Fe
2+
and Fe
3+
ions are still present.
Fig. 4a displays the corresponding series of SAXS data
obtained during the same synthesis. By heating the iron chlo-
ride solution, the scattering intensity increases in the low
q-range (grey line). This indicates the formation of large par-
ticles or aggregates above the size limit of the SAXS experi-
ment (see ESI†Fig. S3).
Fig. 2 a) XRD analysis of the final FeO
x
NPs in comparison to the JCPDS database entry (PDF: 39-1346) for maghemite (red) and a simulated
pattern for maghemite with a crystallite size of 7 nm (grey). The XRD pattern are calculated for Cu Kαradiation. b) TEM image of the maghemite
NPs (scale bar 20 nm). c) HR-TEM image of maghemite NPs with parallel lines marking the observed lattice fringes as well as FFT images corre-
sponding to the two marked regions of the image (scale bar 5 nm). The nanoparticles have a spherical shape and appear to be dominantly mono-
crystalline, since lattice fringes (marked with red lines) extend through the full particle diameter. The Fourier transformations show individual spots
corresponding to the periodical distances of the lattice planes of 0.21 nm and 0.25 nm. d) SAXS data of the final particles in comparison to a theo-
retical fit of spherical particles with a gyration radius of 4.2 nm.
Fig. 3 a) In situ XANES data (Fe K-edge) as a function of time. Spectra
of the iron chloride solution and those obtained during heating are
marked in grey. The spectrum taken one minute after the addition of
TREA is depicted in black. Spectra taken shortly after the addition of
TREA are indicated by a colour change from dark red (7 min) to light
green (91 min). b) Evaluation of the normalized integrated pre-edge
peak area (black squares) and absorption edge position (grey circles) of
the XANES data as a function of time. The arrow at 0 min visualizes the
addition of TREA to the ferric and ferrous solution. The light and dark
grey shaded backgrounds illustrate different stages during the
reaction.
CrystEngCommPaper
Open Access Article. Published on 30 September 2015. Downloaded on 19/05/2017 13:40:16.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
CrystEngComm,2015,17,8463–8470 | 8467This journal is © The Royal Society of Chemistry 2015
The first intermediate formed during heating the precur-
sor solution was identified by XRD as the iron oxide hydrox-
ide akaganeite (β-FeIJIII)OIJOH, Cl) with a crystallite size of 20
nm (Fig. S4a†).
39
The obtained results are backed by Raman
(Fig. S5†) and ESEM (Fig. S4b†) data.
Time-resolved in situ investigation of the reaction of γ-Fe
2
O
3
NPs without adding HCl
To elucidate the formation process of the maghemite nano-
particles, time-resolved investigations combining XANES and
SAXS were performed. This investigation starts with the addi-
tion of TREA to the akaganeite solution and corresponds to
the reaction time “0”. After adding TREA, the dark yellow
akaganeite solution turns dark green immediately and the
absorption edge position shifts slightly to higher energies
(Fig. 3a, black line). The integrated pre-edge peak area does
not change significantly. During the next 21 min the absorp-
tion edge position shifts to higher energies and the IPA
increases by a factor of three (light grey area in Fig. 3b).
After heating the reaction mixture for 90 min, the colloidal
solution turns dark brown. During that period the absorption
edge shifts to higher energies until reaching a value of 7126.9
eV for the final particles (see Fig. 3a, light green line). Fur-
thermore, the IPA changes as the reaction time increases.
Between 21 and 91 minutes after the addition of TREA (dark
grey area), the change in both features is smaller in contrast
to the first 21 min. The IPA increases by a factor of four com-
pared to the broad pre-edge peak of the iron chloride solution.
All XANES spectra reveal a relatively low pre-edge peak
intensity. The change of the pre-edge peak indicates a distor-
tion of a small amount of the centrosymmetric octahedral
coordination geometry of the iron to a tetrahedral coordina-
tion geometry. A pure tetrahedral coordination of the iron
atoms would be characterized by an intense pre-edge peak.
40
Most iron oxides are coordinated octahedrally, resulting in a
small integrated pre-edge peak area. Only magnetite and
maghemite exhibit tetrahedral coordinated Fe in addition to
octahedral coordinated Fe. Fig. 3b shows the shift of the
position of the absorption edge with increasing reaction
time. This change in the pre-edge peak indicates a structural
transition of the geometry of the iron during the reaction
and the oxidation of Fe
2+
to Fe
3+
. During the reaction, the
increase in the integrated pre-edge peak area is accompanied
by a shift of the absorption edge position. These changes
indicate that the variation in the oxidation state and the
structural changes in the iron species are correlated.
Pre-edge peak XANES spectra of iron chloride mixtures for
different fractions of ferric chloride (F
Fe3+
= 0 to 1) in solution
are shown in Fig. S6a and b.†The position of the absorption
edge and the pre-edge peak shift to higher energies with
increasing Fe
3+
content in the solution.
The difference between the centroid of the pre-edge peak
for pure Fe
2+
and Fe
3+
is 1.5 eV, in accordance with the
literature.
40–42
The centroid position of the iron chloride
solution pre-edge peak is shifted to higher energies com-
pared to the iron oxides pre-edge peak (see Fig. S6a†). This
can be explained by a stronger binding of the Cl
−
anion to
the iron cation compared to oxygen.
40,43
Simulations of the
XANES spectra of FeCl
3
·6H
2
O support this result (see
Fig. S1†).
The corresponding series of SAXS data shows that a high
intensity in the low q-range of the SAXS curve is still detect-
able, directly after adding TREA (Fig. 4a, black line). Within
the first 7 min of the reaction, the scattering intensity
increases in the high q-range. Small particles with a radius of
gyration of 3.0 nm are formed (Fig. 4b). SAXS experiments
performed with a higher time resolution (without simulta-
neous acquisition of XANES data) proof that particles with a
gyration radius of 1.6 nm are already formed 4 min after the
addition of TREA (Fig. S3†). Small and large particles are
present at the same time. Raman spectroscopic investigations
hint at the presence of magnetite as dominant phase, besides
the signals for TREA (Fig. S5†). SAED data in Fig. S7†of the
small nanoparticles supports the formation of iron oxides
with a spinel structure (maghemite or magnetite). The large
particles could be described by the already formed akaganeite
or by the formation of iron(II,III) hydroxides.
44
The
Fig. 4 a) SAXS scattering curves as a function of time. Scattering
curves of the iron chlorides precursor solutions obtained during
heating at 115 °C are marked in grey. The scattering pattern taken one
minute after the addition of TREA is marked in black. Scattering
patterns collected shortly after the addition of TREA are marked by a
colour change from dark red (7 min) to light green (91 min). b)
Evaluation of the radius of gyration as a function of time. No
evaluation of a radius of gyration was possible until a reaction time of
7 min. The dashed line at 0 min visualizes the addition of TREA to the
heated iron chlorides. The light and dark grey shaded backgrounds
illustrate different stages during the reaction.
CrystEngComm Paper
Open Access Article. Published on 30 September 2015. Downloaded on 19/05/2017 13:40:16.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
8468 |CrystEngComm,2015,17,8463–8470 This journal is © The Royal Society of Chemistry 2015
corresponding XANES spectrum is characterized by an
increased pre-edge peak. Within the first 7 min the position
of the adsorption edge does not change significantly, indicat-
ing that Fe
2+
cations are still present in the solution.
After 14 min the Gaussian decay shifts to a lower q-range
in the SAXS curve, indicating a growth of the nanoparticles
(light grey area, Fig. 4a). At the same time, the strong inten-
sity in the very low q-range vanishes, indicating the complete
conversion of the first formed large particles to nanoparticles
with a gyration radius of 4.1 nm. The corresponding XANES
data suggest that iron oxide nanoparticles are already present
in this period (14–21 min). Raman spectroscopic investiga-
tions indicate magnetite as dominant phase.
With increasing reaction time, the shape of the scattering
curves shows no significant changes (dark grey area, see
Fig. 4b). The final radius of gyration amounts to 4.2 nm. The
Raman spectra indicate the absence of magnetite Fe
3
O
4
as
dominant phase (669 cm
−1
,A
1g
mode) after a reaction time of
80 min.
Formation studies of γ-Fe
2
O
3
NPs with HCl
To probe whether the formation of large akaganeite particles
is crucial for the nanoparticle particle formation, an experi-
ment in 0.125 M HCl solution was carried out (experiment
type 2). The SAXS data are shown in Fig. 5a. In contrast to
the experiment without HCl solution, no scattering contribu-
tion of the heated ferric and ferrous chloride solution can be
detected. This indicates the absence of akaganeite as
particles.
After adding TREA, the intensity of the SAXS curve
increases in the low q-range (Fig. 5a, black line). This scatter-
ing contribution can be related to the formation of larger par-
ticles or aggregates above the size limit of the SAXS experi-
ment. With increasing reaction time, final nanoparticles with
a radius of gyration of 3.3 nm can be obtained, correspond-
ing to a diameter of 8.5 nm.
The use of a 0.125 M HCl solution for the iron precursor
solution shows a moderate effect on the XANES data (see
Fig. 5b, c and S8†). The position of the absorption edge and
the IPA for the maghemite NPs obtained with and without
adding HCl solution are comparable. Thus in comparison
with experiment type 1, the detection of large particles after
the addition of TREA can be explained by the formation of
iron(II,III) hydroxides.
44
The formation of akaganeite can be
excluded due to the knowledge that akaganeite only form at a
pH lower than 6.
45
This indicates that the first intermediate
phase akaganeite is not required for the formation of the
γ-Fe
2
O
3
NPs. The difference in particle size can be explained
by the higher ionic strength after adding HCl to the iron
chloride precursor.
13
Proposed formation mechanism for the γ-Fe
2
O
3
NPs from all
experiments
Considering the obtained results, a formation mechanism
can be proposed (Fig. 6). The iron chlorides dissolve in
aqueous solution to ferric and ferrous chloride. After heating
the iron chloride salts at 115 °C, akaganeite is formed. The
formation of akaganeite results from the hydrolysis of FeCl
3
in aqueous solution under thermal treatment.
46
The incorpo-
ration of the Cl
−
ions stabilizes the tunnel structure of
akaganeite. At the same time, Fe
2+
ions are still present in
the solution. The dissolution of TREA in water release OH
−
Fig. 5 a) SAXS data as a function of time in 0.125 M HCl solution. The
scattering curve 1 min after the addition of TREA is marked in black.
The scattering curves obtained shortly after the addition of TREA are
indicated by a colour change from dark red (7 min) to light green (91
min). b, c) Evaluation of the normalized integrated pre-edge peak areas
and absorption edge positions of the XANES data as a function of time
in 0.125 M HCl solution. Data for the experiment, without adding HCl
(black) and with adding HCl to the iron chloride solution (blue) are
depicted. After the collection of the data for iron chlorides at room
temperature, the solution was treated at 115 °C (black arrow). The
arrow at 0 min visualizes the addition of TREA to the ferric and ferrous
solution. The light and dark grey shaded backgrounds illustrate differ-
ent stages during the reaction.
CrystEngCommPaper
Open Access Article. Published on 30 September 2015. Downloaded on 19/05/2017 13:40:16.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
CrystEngComm, 2015, 17,8463–8470 | 8469This journal is © The Royal Society of Chemistry 2015
ions, which can replace the Cl
−
ions in the akaganeite struc-
ture at higher pH.
10
The high temperature and the basic
media can lead to a collapse of the akaganeite.
45
The formation of akaganeite is not required for the forma-
tion of maghemite nanoparticles as seen in experiment type
2. Dissolving the iron precursor in a 0.125 M HCl solution
suppresses the formation of akaganeite and simplifies the
reaction mechanism.
47
The green solution, seen after the
addition of TREA, is usually explained by the formation of
iron(II,III) hydroxides.
44
The formed phases were studied using Raman investiga-
tions. Raman analysis indicates the absence of iron oxides
like maghemite and magnetite as dominant phase during the
first minute. The formation of small nanoparticles with a
radius of gyration of 3 nm can be detected after a reaction
time of 7 min. Raman analysis shows a strong indication for
magnetite (signal at 669 cm
−1
) (see Fig. S5†). In literature the
transformation of the iron(II,III) hydroxides to magnetite is
described by a dissolution/recrystallization process.
48,49
Parti-
cle growth is detected up to a radius of gyration of 4.2 nm
during the reaction. The absence of the magnetite signal in
the Raman spectrum for the final particles can be explained
by the oxidation of the magnetite nanoparticles to
maghemite during the last reaction period (20 minutes).
These results show that the formation process of maghemite
nanoparticles can be studied in situ using TREA as stabiliza-
tion and alkaline agent.
Conclusions
The combination of time-resolved in situ SAXS and XANES
experiments allows investigating the formation of γ-Fe
2
O
3
nanoparticles in aqueous solution on a structural and chemi-
cal level. Based on the data, a formation mechanism
consisting of four phases is proposed. In the first stage
akaganeite is formed as an intermediate after heating the
iron chloride solutions. Fe
2+
ions are still present during this
process. After addition of TREA, iron(II,III) hydroxides are
formed. During the next 7 min the formation of magnetite
particles in the low nm range could be detected (R
G
= 3 nm).
Afterwards, the growth of magnetite NPs to the final particle
size was observed. Finally, maghemite NPs with a radius of
gyration of 4.2 nm were formed. The addition of HCl solution
leads to γ-Fe
2
O
3
NPs, although akaganeite as large particles
are not formed. This indicates that the intermediate forma-
tion of akaganeite is not mandatory for the formation mecha-
nism of the γ-Fe
2
O
3
NPs. The presence of both, ferric and fer-
rous ions is important for the formation mechanism of FeO
x
NPs. The benefit of TREA is its property to react as alkaline
and stabilization agent. Therefore, the critical point in the
FeO
x
NP synthesis, the addition of the base, can be investi-
gated by the addition of a stabilization agent and intermedi-
ates can be detected. Thus the addition of a stabilization and
alkaline agent results in an enhanced control of the particle
size. The obtained knowledge will allow controlling the for-
mation of the NPs in solution and further to tune the proper-
ties of the final product.
Acknowledgements
The authors acknowledge ZELMI at Technical University Ber-
lin for support in TEM analysis. The authors acknowledge
Ines Feldmann for support in ESEM analysis. RK thanks in
particular Einstein Foundation Berlin for generous support
provided by an Einstein-Junior-Fellowship (EJF-2011-95). FE
und RK thank for the generous support provided by the CRC
1109.
Notes and references
1 V. Urbanova, M. Magro, A. Gedanken, D. Baratella, F.
Vianello and R. Zboril, Chem. Mater., 2014, 26, 6653.
2 P. Li, D. E. Miser, S. Rabiei, R. T. Yadav and M. R. Hajaligol,
Appl. Catal., B, 2003, 43, 151.
3 S. Sun, C. B. Murray, D. Weller, L. Folks and A. Moser,
Science, 2000, 287, 1989.
4 E. Amstad, M. Textor and E. Reimhult, Nanoscale, 2011, 3,
2819.
5 A. Meddahi-Pellé, A. Legrand, A. Marcellan, L. Louedec, D.
Letourneur and L. Leibler, Angew. Chem., Int. Ed., 2014, 53,
6369.
6 S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. V. Elst
and R. N. Muller, Chem. Rev., 2008, 108, 2064.
7 R. M. Cornell and U. Schwertmann, The Iron Oxides, 2003.
8 I. Martínez-Mera, M. E. Espinosa-Pesqueira, R. Pérez-
Hernández and J. Arenas-Alatorre, Mater. Lett.,2007,61, 4447.
9 M. Fang, V. Strom, R. T. Olsson, L. Belova and K. V. Rao,
Nanotechnology, 2012, 23.
10 T. Ahn, J. H. Kim, H.-M. Yang, J. W. Lee and J.-D. Kim,
J. Phys. Chem. C, 2012, 116, 6069.
11 H.-C. Roth, S. P. Schwaminger, M. Schindler, F. E. Wagner
and S. Berensmeier, J. Magn. Magn. Mater., 2015, 377, 81.
12 L. Babes, B. Denizot, G. Tanguy, J. J. Le Jeune and P. Jallet,
J. Colloid Interface Sci., 1999, 212, 474.
Fig. 6 Synthesis procedure and the proposed mechanism for
maghemite nanoparticle synthesis. The dissolved iron chlorides are
depicted as small orange (Fe
2+
) and yellow (Fe
3+
) spheres. Akaganeite
is presented as grey grids. The formation of the iron(II,III) hydroxides is
depicted as green disordered shapes. The nanoparticles with a spinel
structure are presented as black spheres. The radii of the particles are
given.
CrystEngComm Paper
Open Access Article. Published on 30 September 2015. Downloaded on 19/05/2017 13:40:16.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
8470 |CrystEngComm, 2015, 17,8463–8470 This journal is © The Royal Society of Chemistry 2015
13 X.-P. Qiu, Chin. J. Chem., 2000, 18, 834.
14 Y. Sahoo, H. Pizem, T. Fried, D. Golodnitsky, L. Burstein,
C. N. Sukenik and G. Markovich, Langmuir, 2001, 17, 7907.
15 K. Tao, S. Song, J. Ding, H. J. Dou and K. Sun, Colloid Polym.
Sci., 2011, 289, 361.
16 Q. Peng, J. Gan, S. Wang, L. Kong, G. Chen, Y. Yang and G.
Huang, Ind. Eng. Chem. Res., 2013, 52, 7713.
17 T. Xia, J. P. Wang, C. L. Wu, F. C. Meng, Z. Shi, J. Lian, J.
Feng and J. Meng, CrystEngComm, 2012, 14, 5741.
18 J. Baumgartner, A. Dey, P. H. H. Bomans, C. Le Coadou, P.
Fratzl, N. Sommerdijk and D. Faivre, Nat. Mater., 2013, 12,
310.
19 J. Tuoriniemi, A. Johnsson, J. P. Holmberg, S. Gustafsson,
J. A. Gallego-Urrea, E. Olsson, J. B. C. Pettersson and M.
Hassellov, Sci. Technol. Adv. Mater., 2014, 15.
20 M. Baalousha, Y. Ju-Nam, P. A. Cole, B. Gaiser, T. F.
Fernandes, J. A. Hriljac, M. A. Jepson, V. Stone, C. R. Tyler
and J. R. Lead, Environ. Toxicol. Chem., 2012, 31, 983.
21 J. Polte, M. Herder, R. Erler, S. Rolf, A. Fischer, C. Wurth,
A. F. Thunemann, R. Kraehnert and F. Emmerling,
Nanoscale, 2010, 2, 2463.
22 J. Polte, X. Tuaev, M. Wuithschick, A. Fischer, A. F.
Thuenemann, K. Rademann, R. Kraehnert and F.
Emmerling, ACS Nano, 2012, 6, 5791.
23 M. Wuithschick, B. Paul, R. Bienert, A. Sarfraz, U. Vainio, M.
Sztucki, R. Kraehnert, P. Strasser, K. Rademann, F.
Emmerling and J. Polte, Chem. Mater., 2013, 25, 4679.
24 J. F. Kettemann, M. Wuithschick, G. Caputo, R. Krahnert, N.
Pinna, K. Rademann and J. Polte, CrystEngComm, 2015, 17,
1865.
25 M. Staniuk, O. Hirsch, N. Kränzlin, R. Böhlen, W. van Beek,
P. M. Abdala and D. Koziej, Chem. Mater., 2014, 26, 2086.
26 B. L. Caetano, F. Meneau, C. V. Santilli, S. H. Pulcinelli, M.
Magnani and V. Briois, Chem. Mater., 2014, 26, 6777.
27 M. Bremholm, M. Felicissimo and B. B. Iversen, Angew.
Chem., Int. Ed., 2009, 48, 4788.
28 K. M. O. Jensen, H. L. Andersen, C. Tyrsted, E. D. Bojesen,
A. C. Dippel, N. Lock, S. J. L. Billinge, B. B. Iversen and M.
Christensen, ACS Nano, 2014, 8, 10704.
29 M. C. Schlegel, K.-J. Wenzel, A. Sarfraz, U. Panne and F.
Emmerling, Rev. Sci. Instrum., 2012, 83, 055101.
30 J. Leiterer, W. Leitenberger, F. Emmerling, A. F. Thunemann
and U. Panne, J. Appl. Crystallogr., 2006, 39, 771.
31 S. E. Wolf, J. Leiterer, M. Kappl, F. Emmerling and W.
Tremel, J. Am. Chem. Soc., 2008, 130, 12342.
32 O. Paris, C. Li, S. Siegel, G. Weseloh, F. Emmerling, H.
Riesemeier, A. Erko and P. Fratzl, J. Appl. Crystallogr.,
2007, 40, s466.
33 J. Polte, T. T. Ahner, F. Delissen, S. Sokolov, F. Emmerling,
A. F. Thunemann and R. Kraehnert, J. Am. Chem. Soc.,
2010, 132, 1296.
34 B. Ravel and M. Newville, J. Synchrotron Radiat., 2005, 12,
537.
35 J. J. Rehr, J. J. Kas, F. D. Vila, M. P. Prange and K. Jorissen,
Phys. Chem. Chem. Phys., 2010, 12, 5503.
36 A. P. Hammersley, K. Brown, W. Burmeister, L. Claustre, A.
Gonzalez, S. McSweeney, E. Mitchell, J.-P. Moy, S. O.
Svensson and A. W. Thompson, J. Synchrotron Radiat.,
1997, 4, 67.
37 G. Beaucage, H. K. Kammler and S. E. Pratsinis, J. Appl.
Crystallogr., 2004, 37, 523.
38 J. Ilavsky and P. R. Jemian, J. Appl. Crystallogr., 2009, 42,
347.
39 P. Scherrer, Nachr. Ges. Wiss. G6ttingen, 1918, 98.
40 M. Wilke, F. Farges, P. E. Petit, G. E. Brown and F. Martin,
Am. Mineral., 2001, 86, 714.
41 S. Fujieda, A. Yoshino, K. Shinoda, Y. Takahashi and S.
Suzuki, ISIJ Int., 2014, 54, 125.
42 D. Testemale, J. Brugger, W. Liu, B. Etschmann and J.-L.
Hazemann, Chem. Geol., 2009, 264, 295.
43 M. J. Apted, G. A. Waychunas and G. E. Brown, Geochim.
Cosmochim. Acta, 1985, 49, 2081.
44 P. Refait and J. M. R. Génin, Corros. Sci., 1993, 34, 797.
45 J. Cai, J. Liu, Z. Gao, A. Navrotsky and S. L. Suib, Chem.
Mater., 2001, 13, 4595.
46 J. D. Bernal, Clay Miner., 1959, 4, 15.
47 R. M. Cornell and R. Giovanoli, Clays Clay Miner., 1988, 36,
385.
48 A. Sumoondur, S. Shaw, I. Ahmed and L. G. Benning,
Mineral. Mag., 2008, 72, 201.
49 P. H. Refait, M. Abdelmoula and J. M. R. GÉnin, Corros. Sci.,
1998, 40, 1547.
CrystEngCommPaper
Open Access Article. Published on 30 September 2015. Downloaded on 19/05/2017 13:40:16.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
