Asuka
Namai
a,
Marie
Yoshikiyo
a,
Sayaka
Umeda
a,
Takayuki
Yoshida
b,
Tatsuro
Miyazaki
b,
Makoto
Nakajima
c,
Keita
Yamaguchi
d,
Tohru
Suemoto
d and
Shin-ichi
Ohkoshi
*ae
aDepartment of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: ohkoshi@chem.s.u-tokyo.ac.jp
bDowa Electronics Materials Co., Ltd., 1-3-1 Kaigandori, Minami-ku, Okayama 702-8506, Japan
cDepartment of Physics, Chiba University, 1-33, Yayoicho, Inage Ward, Chiba-shi, Chiba 263-8522, Japan
dInstitute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
eCREST, JST, K's Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan
First published on 17th July 2013
In this study, we demonstrate a synthesis of rhodium substituted ε-iron oxide, ε-RhxFe2−xO3 (0 ≤ x ≤ 0.19), nanoparticles in silica. The synthesis features a sol–gel method to coat the metal hydroxide sol containing Fe3+ and Rh3+ ions with a silica sol via hydrolysis of alkoxysilane to form a composite gel. The obtained samples are barrel-shaped nanoparticles with average long- and short-axial lengths of approximately 30 nm and 20 nm, respectively. The crystallographic structure study using X-ray diffraction shows that ε-RhxFe2−xO3 has an orthorhombic crystal structure in the Pna21 space group. Among the four non-equivalent substitution sites (A–D sites), Rh3+ ions mainly substitute into the C sites. The formation mechanism of ε-RhxFe2−xO3 nanoparticles is considered to be that the large surface area of the nanoparticles increases the contribution from the surface energy to Gibbs free energy, resulting in a different phase, ε-phase, becoming the most stable phase compared to that of bulk or single crystal. The measured electromagnetic wave absorption characteristics due to natural resonance (zero-field ferromagnetic resonance) using terahertz time domain spectroscopy reveal that the natural resonance frequency shifts from 182 GHz (ε-Fe2O3) to 222 GHz (ε-Rh0.19Fe1.81O3) upon rhodium substitution. This is the highest natural resonance frequency of a magnetic material, and is attributed to the large magnetic anisotropy due to rhodium substitution. The estimated coercive field for ε-Rh0.19Fe1.81O3 is as large as 28 kOe.
Our research group has recently demonstrated electromagnetic wave absorption of ε-iron oxide (ε-Fe2O3), which exhibits a large coercive field (Hc) of 20 kOe,18–28 due to magnetic loss at 182 GHz.29 Furthermore, we found that substituting ε-iron oxide with non-magnetic ions, such as Al3+ and Ga3+, can control the absorption frequency from 182 GHz to 35 GHz,29–31 and in 2012, we achieved absorption frequencies above 182 GHz up to 209 GHz with rhodium-substituted ε-iron oxide (ε-RhxFe2−xO3).32
Herein we successfully develop a synthesis method of ε-RhxFe2−xO3 suitable for large-scale synthesis, and achieve an electromagnetic wave absorption frequency of 222 GHz, which exceeds the previously reported frequency and corresponds to the highest frequency for windows of air in millimeter waves (220 GHz).
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Fig. 1 (a) Schematic diagram of ε-RhxFe2−xO3 nanoparticle synthesis using the sol–gel method. (i) Aqueous ammonia is added to an aqueous solution containing both rhodium nitrate and ferric nitrate, yielding the sol of rhodium and iron hydroxide, (RhxFe2−x)(OH)6 (shown in brown). (ii) TEOS (Si(OC2H5)4) is added to the reaction solution to yield a complex gel, (RhxFe2−x)(OH)6/SiO2, coated with a sol of silica (shown in light blue) via hydrolysis. (iii) Sintering the obtained gel in air yields RhxFe2−xO3 in silica. Sintering at 1080 °C, which is higher than the glass transition temperature, causes nanoparticle aggregation and accelerates crystallization. (iv) SiO2 coated material is added to a NaOH solution to react and remove the SiO2 matrix as a Na2SiO3 solution. (b) TEM images of the final products after removing the silica matrix (samples 1, 3, 5, 7, and 9). |
Fig. 1b shows the transmission electron microscope (TEM) images of the obtained samples. The TEM images indicate that the samples are composed of barrel-shaped nanoparticles. The average long- and short-axial lengths of each sample are: (33 ± 15) × (24 ± 11) nm (1), (35 ± 16) × (25 ± 11) nm (2), (36 ± 17) × (26 ± 12) nm (3), (35 ± 16) × (27 ± 12) nm (4), (36 ± 18) × (27 ± 14) nm (5), (29 ± 17) × (21 ± 13) nm (6), (27 ± 18) × (19 ± 12) nm (7), (23 ± 14) × (16 ± 9) nm (8), and (22 ± 13) × (16 ± 9) nm (9), respectively. The average aspect ratio (i.e., [long axis length]/[short axis length]) of each sample is approximately 1.4 (Fig. S2 and Table S2†).
The crystallographic structures of the obtained samples were studied using powder X-ray diffraction (XRD). As shown in Fig. 2a, the XRD patterns of the obtained samples indicated that the ε-phase was formed in all the samples. Besides the ε-phase, each sample contains the α-phase or γ-phase. Fig. 2b shows the phase ratios of the samples. The compositions for the ε-phase are ε-Fe2O3 (1), ε-Rh0.03Fe1.97O3 (2), ε-Rh0.05Fe1.95O3 (3), ε-Rh0.08Fe1.92O3 (4), ε-Rh0.10Fe1.90O3 (5), ε-Rh0.14Fe1.86O3 (6), ε-Rh0.15Fe1.85O3 (7), ε-Rh0.18Fe1.82O3 (8), and ε-Rh0.19Fe1.81O3 (9) (Table 1).35 The lattice constants of the ε-phase are roughly the same regardless of the increases in the concentration of rhodium substitution, i.e., a-axis: 5.0917 ± 0.0003 Å (1) → 5.1110 ± 0.0009 Å (9), b-axis: 8.7857 ± 0.0005 Å (1) → 8.7980 ± 0.0015 Å (9), and c-axis: 9.4783 ± 0.0008 Å (1) → 9.4722 ± 0.0031 Å (9) (Table 1), because the ionic radius of Rh3+ (0.67 Å) is approximately the same as that of Fe3+ (0.65 Å).36 Among the four non-equivalent substitution sites (A–D sites), Rh3+ ions mainly substitute into the C sites (Fig. 3). Additionally, as the amount of Rh substitution increases, the A- and B-sites also begin to be substituted. The details of the crystal structure analysis are described in the ESI.†
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Fig. 2 (a) Powder X-ray diffraction pattern and Rietveld analysis result of sample 4. Red dots, black line, and blue line represent the observed values, calculated values, and the residual error, respectively. Bars denote Bragg peak positions of the ε- and α-phases. (b) Phase ratios of the samples obtained from the Rietveld analyses. Red, blue, and green represent ε-, α-, and γ-phases, respectively. |
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
---|---|---|---|---|---|---|---|---|---|---|
x(ε) | 0 | 0.03 | 0.05 | 0.08 | 0.10 | 0.14 | 0.15 | 0.18 | 0.19 | |
W(ε)/wt% | 64 | 68 | 67 | 64 | 64 | 59 | 49 | 37 | 24 | |
a/Å | 5.0917 (3) | 5.0945 (3) | 5.0966 (3) | 5.0999 (3) | 5.1020 (3) | 5.1055 (3) | 5.1067 (4) | 5.1095 (5) | 5.1110 (9) | |
b/Å | 8.7857 (5) | 8.7867 (5) | 8.7881 (5) | 8.7900 (5) | 8.7906 (5) | 8.7941 (6) | 8.7932 (7) | 8.7951 (9) | 8.798 (2) | |
c/Å | 9.4783 (8) | 9.4773 (7) | 9.4761 (8) | 9.4745 (7) | 9.4735 (8) | 9.4734 (9) | 9.4699 (11) | 9.471 (2) | 9.472 (3) | |
V/Å3 | 424.00 (5) | 424.24 (5) | 424.43 (5) | 424.72 (5) | 424.88 (5) | 425.34 (6) | 425.24 (7) | 425.59 (10) | 425.9 (2) | |
Rh occupancy/% | A | 0 | 0 | 0 | 0 | 0 | 0 (1) | 0 (1) | 2 (2) | 3 (3) |
B | 0 | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 2 (1) | 2 (1) | 2 (3) | 5 (4) | |
C | 0 | 6 (0) | 11 (0) | 17 (0) | 21 (0) | 26 (1) | 29 (1) | 32 (2) | 31 (3) | |
D | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
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Fig. 3 (a) Crystallographic structure of ε-RhxFe2−xO3. (b) Rh occupancies versus substitution ratios of x(ε) at the A–D sites. Dotted lines are a guide to the eye. |
Next we discuss the formation mechanism of ε-RhxFe2−xO3 nanoparticles in the present synthesis. In general, the bulk or single-crystal Fe2O3 transforms from γ-Fe2O3 to α-Fe2O3 when the temperature is elevated.37 However, the large surface area of the nanoparticles increases the contribution from the surface energy to the Gibbs free energy (G),38 which is responsible for a different phase, ε-phase, becoming the most stable phase compared to that of bulk or single crystals.39–41Fig. 4 depicts the particle size (d) dependence of the free energy per molar volume (G/Vm) for the γ-phase, ε-phase, and α-phase.42,43 The G/Vmversus d curve of the ε-phase intercepts the G/Vmversus d curves of the γ-phase and α-phase, indicating that particle sizes in between these intercept points make the ε-phase as the most stable phase. In our method, synthesizing RhxFe2−xO3 as nanoparticles yields particle sizes that make the ε-phase most stable.
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Fig. 4 Schematic curves of Gibbs free energy per unit volume for each phase of Fe2O3 (solid lines) and RhxFe2−xO3 (dotted lines), (Gi/Vm,i), versus particle size (d) where i = γ, ε, and α.42,43 Green, red, and blue lines represent the γ-, ε-, and α-phases, respectively. If rhodium substitution elevates the chemical potential of RhxFe2−xO3, and the increase of the chemical potential of the ε-phase is greater than those of the other two phases, the particle size range where the ε-phase is most stable becomes narrower. This explains why the α-phase and γ-phase co-exist. |
The reason for the difficulty to obtain a single ε-phase in the Rh-substituted system can be explained by the fact that the chemical potential of RhxFe2−xO3 increases due to rhodium substitution. If the chemical potential of the ε-phase increases largely compared to the other two phases, the G/Vmversus d curve of the ε-phase becomes relatively higher, narrowing the particle size range where the ε-phase is most stable. A narrower range explains the co-existence of the α- and γ-phases.
The following possibility can be cited as the reason that the particles have a barrel-shaped morphology. In (RhxFe2−x)(OH)6/SiO2, which is a precursor, a small amount of Fe3+ and Rh3+ ions is included in silica. Their presence reduces the glass transition temperature compared to pure silica.44 Thus, aggregation of nanoparticles progresses in the silica matrix, which increases the crystallization degree and forms the barrel-shaped nanoparticles.
A terahertz time domain spectroscopy (THz-TDS) system was set up to measure electromagnetic wave absorption properties in the millimeter wave region (Fig. 5). Fig. 6 shows the measured millimeter wave absorption spectra of ε-RhxFe2−xO3. As we previously reported,29 sample 1, which is ε-Fe2O3, shows an absorption peak centered at 182 GHz. Rhodium substitution increases the resonance absorption frequency: 187 GHz (2), 192 GHz (3), 197 GHz (4), 202 GHz (5), 207 GHz (6), 213 GHz (7), 217 GHz (8), and 222 GHz (9). Sample 9 breaks our previously reported record of 209 GHz for ε-Rh0.14Fe1.86O3,32 and currently has the highest electromagnetic wave resonance frequency for a magnetic material.
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Fig. 5 Schematic diagram of the THz-TDS measurement system. |
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Fig. 6 Millimeter wave absorption spectra measured at room temperature. |
The mechanism of electromagnetic wave absorption in magnetic materials is mainly caused by magnetic loss due to magnetic domain wall motion or natural resonance (zero-field ferromagnetic resonance).45 The observed high-frequency millimeter wave absorption in the present material is attributed to the natural resonance phenomenon. The mechanism is described below. When an electromagnetic wave is irradiated into a magnetic material, the magnetic component of the electromagnetic wave tilts the magnetization from the anisotropy field (Ha). Once the magnetization is tilted, it precesses around Ha due to the gyromagnetic effect. A resonance occurs at the frequency matching this precession and causes electromagnetic wave absorption. The resonance frequency fr can be expressed as: fr = νHa/2π, where ν is the gyromagnetic constant. Because a uniaxial magnetic anisotropic sample exhibits a proportional relationship between Ha and the coercive field (Hc), we investigated the Hc values of the samples.
Fig. 7 shows the magnetization versus external magnetic field curve at room temperature (300 K). Although samples 1 to 4 show an increase in Hc as the amount of rhodium substitution increases, i.e., 21.7 kOe (1), 22.5 kOe (2), 22.9 kOe (3), and 23.9 kOe (4), samples 5 to 9 exhibit a distortion in the hysteresis loop with a negative effect on Hc, i.e., 23.9 kOe (5), 22.6 kOe (6), 15.5 kOe (7), 4.2 kOe (8), and 0.9 kOe (9) (Fig. 8a, black open squares). To estimate the true Hc value of the ε-phase, we applied a correction that considered the contributions of the α-phase and the γ-phase. The estimated Hc values for ε-RhxFe2−xO3 were 21.9 kOe (ε-Fe2O3), 22.5 kOe (ε-Rh0.03Fe1.97O3), 22.9 kOe (ε-Rh0.05Fe1.95O3), 24.0 kOe (ε-Rh0.08Fe1.92O3), 25.2 kOe (ε-Rh0.10Fe1.90O3), 26.3 kOe (ε-Rh0.14Fe1.86O3), 26.7 kOe (ε-Rh0.15Fe1.85O3), 27.7 kOe (ε-Rh0.18Fe1.82O3), and 28.1 kOe (ε-Rh0.19Fe1.81O3) (Fig. 8a, red solid circles). The last two Hc values exceed our previously reported value, and are the highest values among metal oxides to date. ESI contains detailed magnetic characteristics with respect to their temperature dependences.†
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Fig. 7 Magnetization versus external magnetic field curves at 300 K. Open circles are the observed values. Dotted lines represent contributions from each phase (red, blue, and gray dotted lines represent the ε-, α-, and γ-phases, respectively). Black solid lines are their sum. Red solid lines denote the estimated value for a single-phase ε-RhxFe2−xO3. |
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Fig. 8 (a) Hcversus x(ε) plots. Solid red circles and black open squares represent the estimated Hc values and observed Hc values, respectively. Red dotted lines are a guide to the eye. (b) frversus Hc plots. |
The correlation between Hc and fr demonstrates a monotonic increase in fr with Hc (Fig. 8b). This means that rhodium substitution increases Hc, which indicates an increase in the magnetic anisotropy, Ha, and achieves a high fr of 222 GHz.
220 GHz is the highest-frequency window of air. Around this frequency, electromagnetic wave absorption by air is small. Consequently, this frequency band is expected as a carrier frequency for wireless communications.46–49 The materials developed herein should be useful as millimeter wave absorbing materials for unnecessary electromagnetic waves, which cause electromagnetic interference.
Footnote |
† Electronic supplementary information (ESI) available: The detailed information of synthesis, investigation of the sintering temperature, TEM study, crystal structure analysis, and magnetic properties. See DOI: 10.1039/c3tc30805g |
This journal is © The Royal Society of Chemistry 2013 |