Synthesis and Characterization of Gadolinium Oxide-Hematite Magnetic Ceramic Nanostructures ()
1. Introduction
Hematite (α-Fe2O3) has been the subject of intense theoretical and experimental investigations due to its use as a magnetic, semiconductor and catalytic material [1] - [6]. Doping hematite with various transition metals and rare earth elements was found to result in an improvement of its electrochemical and photocatalytical properties [7] - [15]. On the other hand, the structural, dielectric, morphological and magnetic properties of Gd2O3 nanorods were investigated, both with and without Co dopants [16] [17] [18] [19]. Gadolinium perovskite, GdFeO3 was found to have interesting low temperature magnetic properties and exhibit an enhancement of the magnetoelectric coupling when substituted with Mg [20] - [25].
The phase sequence in the Fe2O3-Gd2O3 mixed oxide nanoparticles system was discussed in [13] [14], while using co-precipitation as a synthetic method. In the present study, we investigate the structural, magnetic and optical properties of the xGd2O3-(1-x)α-Fe2O3 system with molar concentration x = 0.1, 0.3, 0.5 and 0.7, obtained by mechanochemical activation at different ball-milling times. Our investigation is conducted by Mӧssbauer spectroscopy, X-ray powder diffraction (XRPD), magnetic measurements (hysteresis loops and zero-field-cooling-field-cooling) and optical diffuse reflectance spectroscopy.
2. Materials and Methods
Nanoparticles of xGd2O3-(1-x)α-Fe2O3 (x = 0.1, 0.3, 0.5 and 0.7) were obtained by mechanochemical activation of precursor powders of hematite and gadolinium oxide with particle sizes of 50 and 90 nm, respectively (purity 99.9%, Alfa Aesar). The powders were introduced in a SPEX 8000 mixer mill and ground for time periods ranging from 0 to 12 hours. The powder to ball mass ratio was 1:5. Prior to introduction in the SPEX chamber, the powders were mixed manually using a mortar and pestle.
The room-temperature transmission Mössbauer spectra were recorded using a SeeCo constant accelerator spectrometer equipped with a 25 mCi 57Co gamma-ray source in a Rh matrix. All spectra were analyzed by least-squares fitting using the WINORMOS package of programs in the assumption of Lorentzian lineshapes.
The powder diffraction patterns (XRPD) of the gadolinium oxide-hematite ball-milled samples were recorded with a Malvern Panalytical third generation Empyrean X-ray powder diffractometer system with CuKα radiation (λ = 1.54187 Å). An X’cellerator detector was used, with a diffraction angle 2θ of 10 - 80˚ and a preselected step size of 0.02˚. The diffraction peaks were identified using the powder diffraction file (PDF) cards for hematite, gadolinium oxide and gadolinium perovskite, respectively.
Magnetic property measurements were performed using a superconductor quantum interference device (SQUID)-Quantum Design magnetometer with a 5 T magnetic field for recording the hysteresis loops at 300 and 5 K and a 200 Oe magnetic field for the zero-field-cooling-field-cooling (ZFC-FC) measurements in the 5 - 300 K temperature range.
Optical diffuse reflectance spectra were obtained using a Varian Cary 5000 UV/Vis/NIR spectrometer. The samples were loaded into a Harrick Praying Mantis diffuse reflectance accessory that used elliptical mirrors. BaSO4 was used as a 100% reflectance standard. Scans were performed from 2500 to 200 nm at a rate of 600 nm/min, wavelength data were converted to electron volts, and the percent reflectance data were converted to absorbance units.
3. Results and Discussion
3.1. Mӧssbauer Spectroscopy
Figures 1(a)~(e)-4(a)~(e) show the room-temperature transmission Mӧssbauer spectra of the xGd2O3-(1-x)α-Fe2O3 nanoparticles system for molar concentration x
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Figure 1. Mӧssbauer spectra for x = 0.1 at different milling times.
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Figure 2. Mӧssbauer spectra for x = 0.3 at different milling times.
= 0.1, 0.3, 0.5, 0.7 and ball milling times (BMT) of 0, 2, 4, 8 and 12 hours, respectively. The refined values of the hyperfine parameters extracted from these spectra by nonlinear least-squares fitting are listed in Table 1. The spectra in Figures 1(a)-4(a) were analyzed considering one six-line pattern, with the magnetic hyperfine field (BHF) characteristic to hematite. A small quadrupole-split doublet was added to these fits to account for the presence of ultrafine hematite nanoparticles, with the dimension less than 7 nm.
A second sextet was introduced in the analysis of the Mӧssbauer spectra in Figures 1(b)~(e)-4(b)~(e) with the magnetic hyperfine field smaller than that of hematite and assigned to gadolinium-doped hematite nanoparticles. This sextet originates from substitutions of Fe atoms by Gd atoms and is consistent with the model of local atomic environment. Moreover, a third sextet was necessary in the Mӧssbauer spectrum of Figure 1(c) demonstrating an increased level of the substitutions in the sample milled for 4 h. The dependence of the BHF values of these sextets on the ball milling time is depicted in Figure 5 and indicates the formation of a limited solid solution in the studied samples. A higher level of
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Figure 3. Mӧssbauer spectra for x = 0.5 at different milling times.
substitution is not possible due to the difference in the ionic radii of the Gd3+ ions (0.938 Å) and Fe3+ ions (0.645 Å). It may be noted that the formation of a solid solution was not observed in the xNd2O3-(1-x)α-Fe2O3 system reported previously, due to the specific chemistry of the Nd ions [26].
The Mӧssbauer spectra in Figure 1(d), Figure 1(e) and Figures 2(b)~(e)-4(b)~(e) were analyzed by considering an additional quadrupole-split doublet, with the hyperfine parameters (quadrupole splitting QS and isomer shift, IS, relative to Fe) characteristic to the gadolinium perovskite, GdFeO3. Figure 6 displays the dependence of the relative abundance of this doublet as function of the ball milling time for all values of the molarities used. It can be seen that the contribution of this doublet increases with increasing milling time and reaches 41.64% for the sample with x = 0.7 ball-milled for 12 hours. The occurrence of the gadolinium perovskite can be understood by considering the reaction Gd2O3 + Fe2O3 à 2GdFeO3, which is believed to occur during the mechanochemical activation process.
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Figure 4. Mӧssbauer spectra for x = 0.7 at different milling times.
3.2. X-Ray Powder Diffraction (XRPD)
Figures 7(a)-(e) shows the XRPD patterns of the xGd2O3-(1-x)α-Fe2O3 nanoparticles system for x = 0.5 and ball milling times of 0, 2, 4, 8 and 12 hours, respectively. The pattern in Figure 7(a) is consistent with the reflections from α-Fe2O3 (PDF card 01-089-0597) and Gd2O3 (PDF card 03-065-3181), which represent the starting material. It can be observed that after milling, the reflections indicative of the gadolinium perovskite appear (PDF card 01-074-1900). The lines of GdFeO3 dominate the XRPD pattern after 12 h of exposure to mechanochemical activation. These results are in good, qualitative agreement with the Mӧssbauer findings presented previously.
3.3. Magnetic Measurements
Figure 8(a) shows the hysteresis curve for the gadolinium oxide-hematite equimolar composition (x = 0.5) at 0 h of milling, recorded at 300 K with a 5 T applied magnetic field. The plot is consistent with the presence of a paramagnetic component specific to Gd2O3 (known as a perfect paramagnet) and a weak
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Table 1. Hyperfine parameters of the xGd2O3-(1-x)α-Fe2O3 nanoparticles.
ferromagnetic component, due to hematite. The magnetic state of hematite is antiferromagnetic with the Neel temperature TN = 960 K [1] - [5]. Specific to hematite is the fact that its two magnetic sublattices have equal moment and antiparallel orientation below the Morin temperature, TM = 262 K, for pure hematite [7]. At temperatures higher than the Morin temperature, the two sublattices are slightly canted, leading to weak ferromagnetism [27]. This phenomenon explains the presence of the coercive field of Hc = 158 Oe.
Figure 8(b) displays the hysteresis loop of the 0-h equimolar sample recorded at 5 K and 5 T. It can be seen that the magnetization varies linearly with the applied magnetic field, dependence due to the paramagnetic behavior of gadolinium oxide and the antiferromagnetic behavior of hematite. The order-of-magnitude increase in the magnetization at 5 K compared to the magnetization at 300 K can be explained by the temperature dependence of magnetization given in Figure 8(c). The magnetization evolution with temperature was measured via the
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Figure 5. Magnetic hyperfine fields as function of composition and ball-milling time.
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Figure 6. Abundance of doublet as function of ball-milling time.
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Figure 7. XRPD patterns for the equimolar composition (x = 0.5) as function of BMT.
zero-field-cooling-field cooling (ZFC-FC) procedure: first, the sample was cooled in zero field, then a 200 Oe magnetic field was applied, the ZFC dependence on heating was recorded and next, the temperature dependence of the magnetization on the FC was measured. The thermal hysteresis between 145 and 215 K given by the ZFC-FC curves reveals a first-order thermodynamic transition, which is specific to the Morin transition of hematite. In our case, this transition occurs at lower temperatures, due to the smaller grain sizes and the equimolar mixture of hematite and gadolinium oxide.
It can be seen in Figure 8(d) for the 4-h sample that the coercive field evidenced at 300 K significantly decreased, while the magnetization increased, without reaching saturation in a field of 5 T. Further increase in the magnetization
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Figure 8. Variation of the hysteresis loops at 300 K (panels (a), (d), (g)) and 5 K (panels (b), (e), (h)) for Gd2O3-Fe2O3 samples at 0 h and after 4 h and 12 h of ball-milling, respectively. Variation of magnetization as function of temperature at ZFC-FC and 200 Oe, for the initial (panel (c)) and the ball-milled samples at 4 h (panel (f)) and 12 h (panel (i)), respectively.
can be observed in the hysteresis loop recorded at 5 K and depicted in Figure 8(e). As can be inferred from Figure 8(f), the thermal hysteresis specific to the Morin transition is drastically affected after the 4-h milling time. Thus, the Morin transition occurs over a broad temperature range and the antiferromagnetic coupling is replaced by the weak ferromagnetism down to 20 K. This is due to the decrease in the dimension of the nanoparticles formed.
The hysteresis curves recorded for the sample milled for 12 h reveal an increase in magnetization and coercivity both at 300 K (Figure 8(g)) and 5 K (Figure 8(h)). As a result of the ball milling performed, Gd substitutions for Fe in the hematite lattice add increasingly higher contributions to magnetization. The ZFC-FC dependence specific to the 12-h milled sample (Figure 8(i)) shows a small Morin transition, between 60 and 100 K, result which is consistent with the consumption of hematite in the mechanochemical activation reaction and the formation of the gadolinium perovskite phase. In GdFeO3, Gd-Gd, Gd-Fe and Fe-Fe interactions result in its antiferromagnetic behavior along with a weak ferromagnetic behavior [22] [23]. Consequently, in the equimolar mixture of Gd2O3-Fe2O3 we deal with a superposition of magnetic behaviors, with multiple magnetic phase transitions for both intrinsic causes and changes induced by mechanical grinding.
3.4. Optical Diffuse Reflectance Spectroscopy
Figure 9(a) shows the optical absorption spectra of the gadolinium oxide-hematite equimolar mixture as function of energy over the UV-VIS-NIR spectral range for all milling times employed. For the starting material, hematite has a band gap of 1.9 - 2.2 eV in the visible region, while gadolinium oxide has a gap value of 5.2 eV in the ultraviolet range. For the ball-milled material, it can be seen that the absorbance is considerably enhanced and broadened, an effect we believe to result from the substitution of Gd ions for Fe.
Since the 2.2 eV transition in hematite is indirect, a plot of (αE)2 as function of energy (eV) is able to yield the band gap of the compound, according to the Tauc plots [28] [29] [30]. Indeed, it can be observed in Figure 9(b)) that the intercepts give a value of ~2.1 eV for the band gap; moreover, this value is independent of
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Figure 9. Optical diffuse reflectance spectra for the equimolar composition as function of BMT.
the milling time employed. These results show that the gadolinium oxide-hematite mixed-oxide nanostructures have semiconductor properties.
4. Conclusion
In this study, we successfully synthesized the compositional series xGd2O3-(1-x)α-Fe2O3 at four different molar ratios (x = 0.1, 0.3, 0.5 and 0.7) by mechanochemical activation and characterized its magnetic and optical properties as function of ball milling time by Mӧssbauer spectroscopy, X-ray powder diffraction, magnetic measurements and optical diffuse reflectance spectroscopy. Both hysteresis loops and zero-field-cooling-field-cooling determinations were employed. It was found that solid solutions were formed and the gadolinium perovskite was the end product of the milling performed. The compounds were semiconductors with band gaps of 2.1 eV. Semiconducting oxides have formed the basis for technological significant advancements in displays, sensors and photovoltaics, and are setting the stage for emerging applications related to mechanically flexible electronics.
Acknowledgements
This work was supported in part by the National Science Foundation, USA under grants number DMR-0854794 and DMR-1002627-1. Funding was also received from the Ministry of Research, Innovation and Digitization (Romania), CNCS/CCCDI-UEFISCDI projects number CPN-III-P2-2.1-PED 493/2020. J.A.A. and A.J.C. acknowledge the support of the National Science Foundation, USA under Grant DMR-1611198. The X-ray powder diffractometer was purchased with funds from the National Science Foundation, USA (DUE-0511444) and an upgrade was financed by the Bayer School of Natural and Environmental Sciences at Duquesne University.