SolâGel Synthesis and Micro-Raman Characterization of Îµ-Fe2O3

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Sol−Gel Synthesis and Micro-Raman Characterization of ε‑Fe2O3 Micro- and Nanoparticles J. López-Sánchez,*,†,‡,§ A. Serrano,†,‡,⊥ A. Del Campo,⊥ M. Abuín,‡,# O. Rodríguez de la Fuente,‡,§ and N. Carmona‡,§ ‡

Departamento de Física de Materiales, Universidad Complutense de Madrid, 28040 Madrid, Spain Unidad Asociada IQFR(CSIC)-UCM, 28040 Madrid, Spain ⊥ Instituto de Cerámica y Vidrio, CSIC, 28049 Madrid, Spain # CEI Campus Moncloa, UCM-UPM, 28040 Madrid, Spain §

S Supporting Information *

ABSTRACT: In this work, we present a sol−gel synthesis of εFe2O3 nano and microparticles stabilized in silica thin films. Thanks to the relatively high size of the synthesized particles, we have been able to discriminate the Raman signal of the ε- and αFe2O3 phases, thus presenting the first Confocal Raman Microscopy study of isolated ε-Fe2O3 particles. The vibrational modes of each phase are identified at room temperature. The phase transition from ε- to α-Fe2O3 and the morphological modifications are analyzed as a function of the in situ output laser power. A complete study of the Raman spectra for ε-Fe2O3 particles has been performed for a wide range of temperatures (80−570 K). The phonon frequencies and line widths show a behavior in which the contributions from lattice thermal expansion and anharmonic interactions have to be considered. We have also identified a two-magnon mode in the ε-Fe2O3 phase. Its intensity increases close to the Néel transition (TN) and persists well above it. This observation could be one of the few experimental examples of a paramagnon, i.e., a magnetic excitation in a paramagnetic state. ε-Fe2O3 is an elusive Fe2O3 polymorph, which mainly exists in nanoparticle form and has been rarely found as a natural material.1,2 The study of ε-Fe2O3 nanoparticles (NPs) has received significant attention in recent years because of their promising properties such as their giant coercive field (around 2 T at room temperature (RT)), their magneto-resistance or their millimeter-wave ferromagnetic resonance (FMR) absorption.3 Applications on photocatalysis to hydrogen production from organic precursors have also been recently found.4 In the literature, there are several studies about nanowires, nanorods and nanoparticles grown by reverse micelle and sol−gel combined methods.5−9 Additionally, thin epitaxial films of εFe2O3 on Nb:STO(111) substrates grown by Pulsed Laser Deposition (PLD) have been recently reported.10 Over the past decade, new experiments have been performed to obtain epsilon NPs embedded in a silica matrix by the sol−gel method.11−13 Since then, this technique, which can be easily transferred to industrial processes because of their lowtemperature reactions, has given us the opportunity to obtain samples with high homogeneity and tunable particle size.14 During the last years, many efforts have been focused in the synthesis of ε-Fe2O3, carrying out new methods. Despite this, much work remains undone, and part of the research around this polymorph (both synthesis and properties) is still in its © 2015 American Chemical Society

exploratory stage. We present here the best conditions to synthesize and stabilize ε-Fe2O3 in a silica film by a simple onepot sol−gel. The growth mechanisms of this phase are not fully understood and the ε-Fe2O3 metastable polymorph is typically accompanied by hematite (α-Fe2O3), the most thermally stable iron oxide phase.15 Thus, it is usually difficult to isolate the epsilon iron phase from others, hindering its study and characterization. In this work, we have synthesized ε-Fe2O3 and α-Fe2O3 particles embedded in a silica matrix grown on Si(100) substrates. With confocal Raman microscopy (CRM), we identify both iron oxide phases and we show the first isolated Raman spectrum of the ε-Fe2O3 phase. A deep characterization with CRM of the ε-Fe2O3 particles is performed, presenting an analysis of the effect of the incident laser power, the temperature, and the identification of a twomagnon band in ε-Fe2O3.

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RESULTS AND DISCUSSION We have designed two one-pot sol−gel synthetic methods for both powder and thin film samples. To prepare thin films, we Received: September 14, 2015 Revised: December 21, 2015 Published: December 23, 2015 511

DOI: 10.1021/acs.chemmater.5b03566 Chem. Mater. 2016, 28, 511−518

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Chemistry of Materials have selected the acid hydrolysis route with a Si: Fe molar ratio of 1:1 to prevent the aggregation of the nanoparticles. The employed iron oxide precursor is Fe(NO3)3·9H2O (see more details in Experimental Section). To the best of our knowledge, this is the first recipe for the fabrication of ε-Fe2O3 particles embedded in SiO2 sol−gel films. Regarding the powder samples, we have employed an alkaline hydrolysis by adding tetramethylammonium hydroxide. This strong base, which acts also as surfactant, promotes the epsilon stability and averts the aggregation of the nanoparticles. Comparing the synthesis concentration of the recipes mostly used to fabricate epsilon powders,11,16 previous works manage higher concentrations of Fe ions in the sols undergoing to acid and alkaline pH in the same synthesis11 or merging the reverse-micelle and sol−gel processes.5,16 In this way, we are able to obtain large amounts of material facilitating a future transfer to industrial fabrication of ε-Fe2O3 powder. Figure 1A, B show the morphology of the grown films with the iron oxide particles on top. On large amorphous silica patches, with a size of a few tens of micrometers, faceted particles emerging from the surface are clearly visible with scanning electron microscopy (SEM). Their sizes range from 200 nm to 2.2 μm, approximately, as measured with SEM. The particle size distribution follows a log-normal distribution with an average particle size of 0.747 μm (see more details in the Supporting Information). In previous works, more frequently, the size of ε-Fe2O3 ranges between a few nanometers to tens of nanometers.12,17−20 It is worth to point out that the size of the particles synthesized in this work is one of the biggest reported in the scientific literature. These results do not show the presence of particle agglomerates. Besides, well-crystallized particles are observed (see Figure 1B). To analyze the nature of the particles, micro-Raman mappings were carried out on the films. Figure 1C displays an in-plane Raman intensity image on the selected region with a yellow square at the bottom of Figure 1A. Two types of Raman spectra were clearly identified on two different sets of particles (see Figure 1D). One of the Raman spectra clearly corresponds to the α-Fe2O3 phase.21 To unveil the origin of the second Raman spectrum, it must be compared with that obtained from samples in the powder form because these NPs embedded in SiO2 sol−gel films present a low X-ray diffraction (XRD) signal what hinders its characterization. A set of powder samples with a sufficiently high XRD signal (see Figure 2A) was synthesized in order to compare the Raman spectrum obtained in each case. These samples show a diffraction pattern which corresponds to the ε- and α-Fe2O3 phase (see Figure 2B), and a Raman spectrum that can be thus related to those phases. By comparing both Raman spectra (see Figure 2C), we can unambiguously assign the micro-Raman spectrum of the particles in the thin film to ε-Fe2O3 particles (see Figure 1D). Micro-Raman mappings performed on the films show a relatively homogeneous spatial distribution of both α-Fe2O3 and ε-Fe2O3 identified phases, which are isolated from each other. The analysis of the Raman spectra, averaged over the two sets of particles, show that the two phases are not intermixed. No bands from the α-Fe2O3 phase are detected (within our experimental sensitivity) in the ε-Fe2O3 phase, and vice versa. Also, the morphological analysis of the SEM images permits us to conclude that the particles are indeed single particles rather than agglomerates. In-depth Raman intensity images were also obtained (not shown here) and we found that ε-Fe2O3 and α-

Figure 1. (A, B) SEM images of the sample synthesized at 960 °C. Image B shows a detail of the top orange square marked in image A. The damage of two different laser spots (marked with white arrows) can be observed when focusing on ε-Fe2O3 single particles. (C) Inplane Raman intensity image obtained from mapping the region marked with a yellow square at the bottom of panel A, measuring different single Raman spectra taken each 200 nm with an integration time of 0.5 s. Spectral ranges from 1204 to 1356 cm−1 for α-Fe2O3 particles (red color) and from 1398 to 1566 cm−1 for ε-Fe2O3 particles (green color) were integrated to obtain the Raman intensity image. Blue and purple crosses designate the particle where the laser power experiment is carried out (see Figure 4). White and purple crosses indicate the epsilon particle employed in the laser polarization study and the temperature study, respectively, (see the Supporting Information). Yellow cross illustrates the hematite particle used in the temperature study (see the Supporting Information). (D) Average Raman spectra obtained from in-plane Raman image, where it is possible to distinguish 31 and 11 vibrational modes related to ε-Fe2O3 and α-Fe2O3 respectively (see image C). Red and green marked dashed areas are associated with integration range for each polymorph.

Fe2O3 particles are homogeneously distributed both along the surface and within the silica matrix. α-Fe2O3 average Raman spectrum exhibits the seven active phonon modes (2A1g + 5Eg) allowed in Raman, apart from other bands (see Table 1).21 At 667 cm−1, the infrared (IR) active longitudinal optical (LO) Eu mode can be identified, which is forbidden in Raman scattering but is activated by disorder in the α-Fe2O3 crystalline lattice.22,23 The vibrational mode at 826 cm−1 is a magnon mode. 24 For larger 512


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Chemistry of Materials

Figure 2. (A) TEM image and (B) XRD pattern of Fe2O3 nanoparticle agglomerates embedded in a SiO2 matrix obtained from powder sample treated at 960 °C. In panel B, reflections corresponding to ε- and α-Fe2O3 are identified;29,30 (C) Raman spectra from nanoparticle agglomerates with a size between 4 and 35 nm (orange spectrum) and from a single microparticle (blue spectrum).

Table 1. Raman Modes and Wavenumbers for α-Fe2O3 Average Raman Spectrum Raman modes A1g (1) Eg(1) Eg(2) Eg(3) Eg(4) A1g (2)

wavenumber (cm−1) 228.08 248.16 294.08 301.90 414.97 504.78

± ± ± ± ± ±

0.24 0.56 0.63 1.27 0.14 1.31

Raman modes Eg(5) Eu (LO) Magnon 2-Eu(LO) Two-magnon

All these factors are crucial to observe each vibrational mode. Comparing our Raman spectrum with that obtained by C. Dejoie et al.20 (only work, to the best of our knowledge, that shows an ε-Fe2O3 Raman spectrum), most of our band positions are shifted toward lower wavenumbers, except the M29 mode that is at larger wavenumbers. This may be due to a less compressed crystal lattice and/or phonon confinement effects in our samples. Besides, the epsilon overtone (M29) from reference20 is shifted toward the hematite overtone. Thus, the authors might have also collected some signal from α-Fe2O3 and so they might have a mixture of both iron oxide phases. According to our assignation of Raman modes for the ε-Fe2O3 particles, Table 2 shows the wavenumbers obtained from a

wavenumber (cm−1) 617.76 667.03 826.35 1326.07 1547.69

± ± ± ± ±

0.08 0.16 0.39 0.50 2.79

wavenumbers, overtones or second order scattering processes of the first-order scattering can be observed. The band around 1326 cm−1 can be assigned to an overtone of the band around 667 cm−1, which is known to be strongly resonantly enhanced.25 Finally, Raman scattering of two-magnon mode may be distinguished at 1548 cm−1.26 Regarding the Raman spectrum of the ε-Fe2O3 phase, its vibrational modes are not identified in the literature, since no spectrum from an isolated phase has been ever measured. The ε-Fe2O3 has an orthorhombic crystal structure with the Pna21 space group and, according to the Bilbao Crystallographic Server,27 we can estimate the number of vibrational modes theoretically. Taking into account the Pna21 (C2 V(mm2)) space group, the mechanical representation is M = 30A1 + 30A2 + 30B1 + 30B2, with Raman actives modes 29A1 + 30A2 + 29B1 + 29B2. Thus, a total of 117 vibrational modes are allowed in Raman scattering, where acoustic modes are not included. In our ε-Fe2O3 average Raman spectrum, we can distinguish 24 modes which have been named M1−M24 in the spectral range of 100−850 cm−1 at RT, which correspond to first-order phonon modes (Figure 1D). The rest of the detected vibrational modes correspond to overtones or second order scattering processes. Likely, many of the allowed Raman modes are located below the detection limit of our system. The Raman spectrum of a ε-Fe2O3 particle was also measured using an 1800 g/mm grating and we were able to identify a larger number of vibrational Raman modes (see details in the Supporting Information). Also, other effects can occur as the role of the orientation of particles due to selection rules and/or the source of excitation (λ = 532 nm).28 In our work, we have collected Raman spectra of particles varying the polarization of the incident laser light. We have not observed significant changes in the relative intensity of Raman modes (see details in the Supporting Information), because particles are dispersed throughout the whole sample with a rounded shape on average.

Table 2. Raman Modes and Wavenumbers for ε-Fe2O3 Average Raman Spectrum Raman modes M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15 M16

wavenumber (cm−1) 115.86 146.42 165.46 194.61 213.97 266.86 298.94 329.77 346.45 362.09 377.77 396.90 419.04 439.14 460.61 488.20

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.27 0.78 1.05 0.39 0.74 0.77 0.58 0.84 0.56 0.98 1.04 1.07 5.21 0.24 1.06 1.66

Raman modes M17 M18 M19 M20 M21 M22 M23 M24 M25 M26 M27 M28 M29 M30 M31 (two-magnon)

wavenumber (cm−1) 559.16 578.93 596.91 643.44 668.93 703.56 731.36 829.14 1187.52 1276.06 1328.84 1378.29 1434.65 1473.49 1640.79

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.68 0.78 0.98 0.19 0.18 0.65 0.50 0.40 1.03 2.00 2.69 3.14 1.09 0.66 3.93

Lorentzian fitting of the Raman spectrum bands. This assignation has been possible because of the relatively large particle size and the separation between particles. Taking into account these factors, we can rule out any significant α-Fe2O3 signal in the ε-Fe2O3 spectra. Additional evidence of the existence of ε-Fe2O3 comes from electron diffraction measurements, namely with selected area electron diffraction (SAED) patterns with transmission electron microscopy (TEM). Figure 3A presents a single ε-Fe2O3 nanoparticle embedded in SiO2 matrix which is projected 513


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excitation source and as the probe, we can monitor in situ the possible structural changes in the particles. Figure 4A shows the Raman spectra of an ε-Fe2O3 microparticle (marked with a blue cross in Figure 1C) recorded at different output laser powers, ranging from 0.2 to 30 mW. The position of the vibrational modes for the epsilon phase shifts toward lower wavenumbers and their intensity gradually increases as the laser power increases up to 2.8 mW. At 3.2 mW, the modes of the ε-Fe2O3 phase are almost vanished, whereas others resembling the shape of the α-Fe2O3 Raman spectrum have emerged. Both phases coexist between 3.2 and 3.6 mW (see Figure 4C). The intensity of the α-Fe2O3 modes becomes stronger as the laser power increases up to 3.6 mW. However, from 3.6 to 20 mW, the position of the α-Fe2O3 Raman modes shift toward lower wavenumbers and gradually decrease in intensity while the spectrum background signal increases. The shift toward lower wavenumbers can be basically related to strain effects due to a local increase of the temperature induced by the laser irradiation, which is accompanied by band broadening, favoring the anharmonic interactions. From 20 to 30 mW, the overall behavior of vibrational modes is the same, but the spectrum background signal decreases. This may be explained by the recrystallization or morphological modification associated with a damage induced by laser irradiation. In fact, it can be observed that the irradiated particle has undergone a morphological change (see Figure 1B at region marked with a yellow rectangle at the bottom and inset in Figure 4C). After irradiating with the laser up to a power of 30 mW, the sample region was kept at RT without laser incidence for some minutes. A final spectrum was recorded at the same region with a laser power of 0.2 mW, and well-defined Raman bands of αFe2O3 were clearly distinguished. The vibrational modes are shifted toward high wavenumbers showing a structural

Figure 3. TEM results of a single ε-Fe2O3 nanoparticle embedded in a SiO2 matrix, from a thin film. (A) TEM image; (B) SAED pattern. For this ε-Fe2O3 nanoparticle, taking into account its orthorhombic crystal structure with the Pna21 space group, forbidden reflections of the type h00: h ≠ 2n and h0l: h ≠ 2n appear with remarkable intensity caused by Umweganregung (multiple reflections).

with an extent of around 100 nm. Samples were scratched from the coating and deposited onto a TEM grid. Consequently, this value is not comparable with the data provided by SEM images. Figure 3B shows the SAED pattern taken at the nanoparticle. The indexation associated with ε-Fe2O3 can be checked. Electron diffraction patterns show the symmetry expected for εFe2O3.

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OUTPUT LASER POWER DEPENDENCE A way to check the metastability of the synthesized ε-Fe2O3 phase is to expose the particles to the increasing power of the Raman laser light. The laser, among other factors, locally increases the temperature of the irradiated area, which is around 0.68 μm2 (smaller than the size of the irradiated particles, see inset Figure 4C). By using the laser, both as the

Figure 4. (A) Evolution of the Raman spectra of an ε-Fe2O3 microparticle (see blue cross in Figure 1C) as laser power increases from 0.2 to 30 mW, showing the ε-Fe2O3 to α-Fe2O3 transformation. A final Raman spectrum taken on the same region after the ε- to α-Fe2O3 transformation at 0.2 mW is presented as well. (B) Evolution of the intensity difference between A and B lines at 425 and 1900 cm−1, respectively. (C) M29 (ε-Fe2O3) and overtone (α-Fe2O3) mode position versus laser power, showing the structural transition from ε-Fe2O3 to α-Fe2O3 and the morphological modification as the laser power increases. Inset in Figure C shows the SEM image of the microparticle after Raman study. 514


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Chemistry of Materials relaxation.31 These results evidence a complete and permanent transformation from ε-Fe2O3 to α-Fe2O3.

For low temperatures around 130 K, subtle variations are detected in the bandwidth and position of ε-Fe2O3 Raman bands. These changes in the slope of the curves might be originated by a structural transition from a collinear ferromagnetic state to a square-wave incommensurate magnetic structure (see details in the Supporting Information).36 For this iron oxide phase, a most interesting observation is related to the anomalous behavior of the M31 mode at 1641 cm−1. Close to the Néel temperature (around 500 K for ε-Fe2O3),1 a great enhancement of its Raman intensity and a shift toward higher wavenumbers are manifest above 450 K. This result is opposite to the observed behavior of the rest of the Raman modes, which show a monotonous decreasing tendency in their position and intensity as temperature increases (see the Supporting Information). We can identify this band as a twomagnon excitation. It is remarkable that the two-magnon mode persists above TN (i.e., in the absence of a magnetic order), but a similar behavior has been indeed reported for other types of antiferromagnetic materials, where two-magnon bands are present well above the TN (magnons above TN are called paramagnons).37 Relatively well studied antiferromagnetic materials such as α-Fe2O3,38 AlFeO3,39and BiFeO340 also show bands attributed to two-magnon interactions. This type of Raman scattering involves a simultaneous excitation of a pair of magnons with equal and opposite momenta k on each of the sublattices.41 However, the representation of the frequency versus temperature of the M31 band shows that the decreasing trend breaks around 450 K. We associate this steep change with the Néel transition. Below TN, the frequency shift of the twomagnon drops off whereas the relative intensity (IM31/IM29) is practically constant. By contrast, above TN the change of the two-magnon mode position steps up while the intensity increases reaching a maximum around 510 K (see Figure 7A and Figure 7B). These observations might be associated with the occurrence of paramagnon excitation for temperatures above TN.37 A similar experiment performed in ε-Fe 2O3 particle agglomerates shows the measurement reproducibility, obtaining the same features characteristic of paramagnon excitations (see Figures 7A and 7B). In this case, the Néel transition is slightly displaced toward lower temperatures than for the microparticle experiment. This effect is in good agreement with the variation in the Néel transition with particle size. Thermal analyses were also performed on a single α-Fe2O3 microparticle marked with a yellow cross in Figure 1C. Hematite has a Morin transition around 260 K.15 Displaying the relative Raman frequency as a function of temperature of the two-magnon mode (see the Supporting Information), we detect a drastic change in the tendency of the curve close to the Morin transition. Accordingly, this result may suggest the determination of this magnetic transition for a single hematite microparticle by CRM.

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ASSIGNMENT OF THE TWO-MAGNON MODE The evolution of the Raman spectrum as a function of temperature from 80 to 540 K was analyzed for both single αand ε-Fe2O3 microparticle (marked with yellow and purple crosses in Figure 1C, respectively) and for agglomerations of εFe2O3 particles with smaller size (marked with a yellow cross in Figures 5A, B). Evident effects of temperature, such as

Figure 5. (A) Atomic force microscopy (AFM) image and (B) inplane Raman intensity image obtained from mapping the region marked with a yellow square in Figure A, of a sample prepared at 960 °C. The yellow cross on Figures A and B marks the ε-Fe2O3 nanoparticle agglomerate studied in this experiment. Spectral range from 1200 to 1376 cm−1 for α-Fe2O3 particles (in red) and from 1400 to 1576 cm−1 for ε-Fe2O3particles (in green) were integrated to obtain the Raman intensity image.

softening and shifting toward lower wavenumbers with increasing temperature, are observed in each Raman mode (see Figure 6 and the Supporting Information).32,33 Besides, the resolution of Raman spectrum is gradually enhanced for lower temperatures. These effects are attributed to a lattice expansion and phonon−phonon interactions.28,34,35

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CONCLUSIONS In summary, we have designed two one-pot sol−gel synthetic methods for obtaining α-Fe2 O3 and ε-Fe 2O 3 particles embedded in a SiO2 matrix. The particle sizes for both thin film and powder samples comprised between 0.2−2.2 μm and 4−35 nm, respectively. A complete CRM study of the microand nanoparticles has been performed, identifying and discriminating the active Raman modes for isolated ε-Fe2O3 particles, without traces of other iron oxides. The ε-Fe2O3 to αFe2O3 transformation as a function of the output laser power

Figure 6. Evolution of the Raman spectra from 80 to 570 K of an εFe2O3 single microparticle. At temperatures over 450 K, the growth of the M31 mode, which we attribute to a two-magnon mode, is clearly evident. 515


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Growth of Powder Samples. The recipe for powder samples is slightly different. An hydroethanolic solution of iron nitrate nonahydrate (Fe(NO3)3·9H2O, Sigma-Aldrich >98%), barium nitrate (Ba(NO3)2, Sigma-Aldrich >98%) and cetyltrimethylammonium bromide (C19H42BrN, Sigma-Aldrich >99%) of 8:0.5:7 molar ratio was prepared. A solution of tetramethylammonium hydroxide was then added to the first one. Finally, tetraethoxysilane (TEOS, SigmaAldrich 98%) was added dropwise to the mixture under mild stirring. The resulting precipitate was collected by centrifugation and left for drying for several days. Afterward, a thermal treatment at 960−990 °C in air resulted in a sintered fine brown powder. Material Characterization. The morphological structure was described by SEM (Figures 1A, B), performed with a CRESTEC CABL-9500C instrument. TEM data were obtained with a JEOL 2100 LaB6 TEM operating at 200 kV, equipped with a CCD camera ORIUS SC1000 (model 832). TEM and SAED data were analyzed using the ImageJ software, which was employed to calculate the interplanar distance, zone axis and angles formed between reflections from experimental data. Otherwise, we can build the epsilon iron oxide structure and create its reciprocal lattice for a specific zone axis by using CaRIne software. Results obtained with ImageJ software were checked and correlated with the angles between different reflections and interplanar distances calculated using CaRIne. Crystalline structure of ε-Fe2O3 NPs was identified by SAED patterns. Raman measurements were carried out using a Confocal Raman Microscope (Witec ALPHA 300RA) with a Nd:YAG laser light source (532 nm) in p-polarization. This polarization is parallel to the x direction of the images shown in this work. The optical resolution of this confocal microscope is, approximately, 200 and 500 nm in the lateral and vertical directions, respectively. The spectral resolution of the system is 0.02 cm−1, under the best measurement conditions. Raman spectra were recorded in the spectral range of 0−3600 cm−1 by using a 600-g/ mm grating. Samples were mounted on a piezo-driven scan platform with a positioning accuracy of 4 nm in lateral and 0.5 nm in vertical directions. Raman measurements at RT were performed using an objective with a numerical aperture (NA) of 0.95. Measured Raman spectra using different laser output powers, between 0.2 and 30 mW, were consecutively carried out on the same region of the sample, and the laser power was controlled by a Thorlabspotentiostat. On the other hand, in situ temperature Raman spectra varying the temperature from 80 to 570 K using a LNP95 heating and cooling system from Limkam were carried out with an objective of NA 0.75, for which argon gas was used in order to purge the sample chamber and avoid condensation in the upper lid of the window surface. The spot size of the laser was estimated to be between 0.68 and 0.86 μm, using the objective with NA of 0.95 and 0.75, respectively. Collected spectra were analyzed by using Witec Control Plus Software and the exact band positions, linewidths, and intensity were obtained from fitting the peaks by Lorentzian function. AFM images were obtained with the Witecsystem (Witec ALPHA 300RA) where the AFM is coupled to the confocal microscope. AFM images were taken at RT using silicon tips (with a resonant frequency of 200−300 kHz) in noncontact mode.

Figure 7. Raman characterization of the two-magnon mode in the εFe2O3 phase as a function of temperature. (A) Evolution of ω(T)/ ω(80 K) and (B) IM31/IM29 for the two-magnon mode in two different experiments: for a ε-Fe2O3 micropaticle (red color) and for a nanoparticle agglomerate (blue color). A continuous decrease in the two-magnon position is observed up to around 450 K, where it shifts to higher frequencies. A great enhancement of the two-magnon intensity is observed above 450 K.

has been tracked in situ for an isolated particle. The transformation, which takes place at a given laser power threshold, is not reversible. We have carried out a systematic in situ temperature (80−570 K) Raman study for α- and ε-Fe2O3 particles, and changes in the spectra can be attributed to a lattice thermal expansion and anharmonic effects. The evolution of the M31 mode in Raman spectra with temperature exhibits clear signs of a two-magnon excitation in the ε-Fe2O3 phase, whose intensity grows close to the Néel transition and persists well above it. As the paramagnetic state governs in this temperature range, a two-magnon/paramagnon transition is strongly suggested.

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EXPERIMENTAL SECTION

ASSOCIATED CONTENT

S Supporting Information *

Growth of Film Samples. Film samples were reproducibly prepared by the sol−gel method. Tetraethyl orthosilicate (TEOS, Sigma-Aldrich) was used as silica matrix precursor; Iron(III) nitrate nonahydrate (Fe(NO3 )3 ·9H 2O, Sigma-Aldrich) as iron oxide precursor; barium nitrate (Ba(NO3)2, Sigma-Aldrich) to stabilize εFe2O3 against transformation to α-Fe2O3;3 and cetyltrimethylamonium bromide (C19H42BrN, Sigma-Aldrich) as capping agent.42 Samples were prepared with a 1:1:0.002:7.5 TEOS:Fe(NO3)3·9H2O:Ba(NO3)2:H2O molar ratio. The resulting sol was stirred 24 h at RT and then deposited by dip-coating on Si(100) wafers. The obtained coatings were dried at 60 °C for 3−7 days. A thermal treatment at 960 °C for 30 min was finally carried out to achieve the gelification of the films and obtain the iron oxide particles.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03566. Macroscopic aspect of the samples, effect of CTAB addition, particle size distribution for both thin film and powder samples, laser polarization study, Raman spectrum for an ε-Fe2O3 microparticle measured with a 1800 g/mm grating, temperature dependence of ε-Fe2O3 NPs agglomerates, and temperature dependence of a αFe2O3 single microparticle on SiO2 sol−gel films (PDF) 516


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

J.L.-S. and A.S. contributed equally in this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C. Vasquez-Villanueva is acknowledged for the characterization of the first spectra recorded and for fruitful discussions about Raman spectroscopy. This work has been supported by the MICINN through projects MAT2012-38045-C04-03 and MAT2013-48009-C04-01-P. J.L.-S. and M.A. thank the FPI fellowship and the UCM Campus of International Excellence (PICATA Program), respectively, for predoctoral fellowships.

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