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
aBAM Federal Institute for Materials Research and Testing, Richard-Willstätter Strasse 11, D-12489 Berlin, Germany. E-mail: franziska.emmerling@bam.de
bHumboldt-Universität zu Berlin, Department of Chemistry, Brook-Taylor-Strasse 2, D-12489 Berlin, Germany
cTechnische Universität Berlin, Department of Chemistry, Strasse des 17. Juni 124, D-10623 Berlin, Germany
First published on 30th September 2015
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-Fe2O3) 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 (Fe3O4) grow further and convert to maghemite with increasing reaction time.
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 preliminary 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 nanoparticles using ex situ X-ray diffraction (XRD) and TEM.10Ex situ analysis of FeOx NPs typically requires sample preparation. Steps like centrifugation and drying could lead to artifacts, e.g. a change in the particle size.19,20
In situ studies can provide information on the particle formation, 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 mechanism 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 scattering and spectroscopic techniques.25 The formation mechanism of SnO2 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 (Pcritical = 221 bar, Tcritical = 374 °C).27 In their study, an ammonium iron(III) citrate precursor was injected into a preheated reactor. The authors observed the formation of amorphous iron(III) hydroxide nanoclusters, which dissolve or decompose completely before magnetite nanoparticles crystallize. Jensen et al. described the formation of maghemite NPs from the ammonium iron(III) citrate precursor at high temperatures and under high pressure by using total scattering data and atomic pair distribution function (PDF) analysis.28 In both mentioned FeOx NPs syntheses only Fe3+ precursors 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 oxidation 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 complementary XRD, TEM, Raman and selected area electron diffraction (SAED). The advantage of the chosen synthesis is the small number of reaction partners and the favorable properties 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 mechanism of maghemite NPs to propose a formation mechanism.
A typical synthesis is depicted in Fig. 1a. The iron precursor solution was prepared by dissolving ferric chloride hexahydrate (FeCl3·6H2O, Aldrich, >97% purity) and ferrous chloride tetrahydrate (FeCl2·4H2O, Aldrich, ≥99% purity) in a molar ratio of 1:
1 in water (Millipore, 18.2 MΩ cm, TOC ≤ 5 ppb, flushed with nitrogen) to reach a total iron concentration of 0.31 M. All reagents were used without further purification. 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).
Experiment type | Iron precursor solution | TREA solution | |
---|---|---|---|
Iron precursor | FFe3+ | ||
1 | FeCl2, FeCl3 | 0.5 | TREA |
2 | FeCl2, FeCl3, HCl | 0.5 | TREA |
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 provide 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 requirements 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 molecule FeCl3·6H2O were performed using FEFF9 software.35 Further 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 diagrams of scattered intensities versus the scattering vector q employing an algorithm from the FIT2D software.36q is defined by q = 4π/λ·sinθ with θ being half of the scattering and λ being the wavelength. (The small shift of the wavelength 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 q values.
The curve fitting was done using IRENA38 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 resolution 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 5.09·105 W cm−2 at the liquid sample.
For XRD analysis, the colloidal solutions were cooled to room temperature. After the addition of acetone, the precipitate 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 10000 Å (12
398 eV). The dried final FeOx NPs were analyzed using a standard 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·cos
θ, with λ being the wavelength of the X-ray radiation, β being the full width at half maximum, K being the Scherrer shape factor, and θ half of the scattering angle.39
The TEM image of the maghemite nanoparticles (see Fig. 2b and c) shows spherical particles with an average particle diameter of 6.8 ± 1.3 nm. The HR-TEM and the corresponding Fourier transformations of the maghemite NPs document that the whole particles are crystalline. The lattice 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 plateau in the low q-region (see Fig. 2d). The power-law decay of the scattering curve is equal to q−4, indicating a smooth surface 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, corresponding to a particle diameter DP of 10.8 nm. The particle diameter DP is related to the radius of gyration RG by . The differences in the determined diameter can be attributed to a small amount of agglomerates.
Fig. 4a displays the corresponding series of SAXS data obtained during the same synthesis. By heating the iron chloride solution, the scattering intensity increases in the low q-range (grey line). This indicates the formation of large particles or aggregates above the size limit of the SAXS experiment (see ESI† Fig. S3).
The first intermediate formed during heating the precursor solution was identified by XRD as the iron oxide hydroxide akaganeite (β-Fe(III)O(OH, 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.
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). Furthermore, 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 compared 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 distortion of a small amount of the centrosymmetric octahedral coordination geometry of the iron to a tetrahedral coordination 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 Fe2+ to Fe3+. 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 (FFe3+ = 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 Fe3+ content in the solution.
The difference between the centroid of the pre-edge peak for pure Fe2+ and Fe3+ 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 compared 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 FeCl3·6H2O 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 detectable, 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 simultaneous 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 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, indicating that Fe2+ 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 intensity 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 investigations 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 Fe3O4 as dominant phase (669 cm−1, A1g mode) after a reaction time of 80 min.
After adding TREA, the intensity of the SAXS curve increases in the low q-range (Fig. 5a, black line). This scattering contribution can be related to the formation of larger particles or aggregates above the size limit of the SAXS experiment. With increasing reaction time, final nanoparticles with a radius of gyration of 3.3 nm can be obtained, corresponding 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-Fe2O3 NPs. The difference in particle size can be explained by the higher ionic strength after adding HCl to the iron chloride precursor.13
The formation of akaganeite is not required for the formation 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 investigations. 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 Particle 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 stabilization and alkaline agent.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ce01585e |
This journal is © The Royal Society of Chemistry 2015 |