Generalized Low-Temperature Synthesis of Nanocrystalline Rare-Earth Orthoferrites LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) Hua Xu, Xianluo Hu, and Lizhi Zhang* Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, P. R. China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2061–2065
ReceiVed January 6, 2008; ReVised Manuscript ReceiVed May 14, 2008
ABSTRACT: Rare-earth orthoferrite LnFeO3 nanocrystals were traditionally synthesized at temperatures higher than 700 °C. In this study, we developed a general nanosized heterobimetallic precursors approach for the synthesis of nanocrystalline rare-earth orthoferrite LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) at 500 °C. The nanosized heterobimetallic precursors were obtained via the reaction between the Ln and Fe oleates synthesized from their corresponding metal nitrates and sodium oleate. Subsequently, the calcination of the nanosized heterobimetallic precursors at a relatively low temperature (500 °C) produced nanocrystalline rare-earth orthoferrites. The precursors and products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption, thermal analysis (TGA/DSC), and Fourier transform infrared absorption spectroscopy (FT-IR). On the basis of characterization results, we attributed the low temperature formation of nanocrystalline rare-earth orthoferrites to the reduced diffusion distance between the nanosized heterobimetallic precursors. We thought these heterobimetallic precursors ensured the desirable stoichiometry ratio of the orthoferrite products and avoided the formation of garnet. The magnetization features of the orthoferrites were evaluated at room temperature. The M-H curves revealed that EuFeO3 and GdFeO3 exhibit better weak ferromagnetic behavior, corresponding to the antisymmetric-exchange anisotropy. Our method may be extended to prepare other ternary metal oxides at relatively low temperatures. High-quality nanocrystalline rare-earth orthoferrites LnFeO3 (Ln ) lanthanide elements) have drawn great attention because of their unique physical and chemical properties for various applications. These compounds with perovskite structure are promising as catalysts,1 gas separators,2,3 cathodes in solid oxide fuel cells,4 sensor materials,5 magnetooptic materials,6 spin valves,7 and so forth. These rare-earth orthoferrites were usually synthesized by heating the corresponding metal oxides in a stoichiometric ratio at relatively high temperatures because the solid-solid diffusion was the rate-limiting step in their formation.8 For instance, Zanatta et al. synthesized the GdFeO3 by heating mechanically milled mixed powders of R-Fe2O3 and Gd2O3 in the temperature range of 1000-1100 °C.9 However, the phases obtained from the hightemperature reactions are generally thermodynamically stable ones, and there is little control over the kinetics of phase formation.8 Orthoferrite LnFeO3 is considered to be a metastable compound, less favorable than the thermodynamically stable, magnetic garnets (Ln3Fe5O12) in case of high temperature synthesis. Very recently, Mathur and co-workers reported the preparation of nanocrystalline orthoferrite GdFeO3 without any garnet impurity at 700 °C by using a single molecular precursor with Gd-O-Fe linkages and a Gd/ Fe ratio of 1:1. Such a molecular precursor that incorporates the atomic bonds corresponding to those solid-state species provides convenience for the synthesis of nanocrystalline orthoferrite GdFeO3 because of their inherent control of stoichiometry.10,11 Siemons’ group prepared a series of LnFeO3 via a polyol-mediated route. In their preparations, the LnFeO3 powders were obtained by annealing the amorphous precursors at 700 °C for 12 h.12 Henkes and coworkers reported a nanoparticle-directed solid-state route to synthesize the ternary transition metal oxides at a low temperature (700 °C) using the corresponding metal oxide nanoparticles as the precursors. They suggested that the enhanced reactivity in the low temperature was afforded by the nanoparticle precursors, which decreased the diffusion distances by several orders of magnitude and virtually eliminated solid-solid diffusion as the rate-limiting step.8 Although there were a few reports on the synthesis of rareearth orthoferrite at relative low temperatures,13–16 these methods required very long thermal treatment time and/or were not general. * To whom correspondence should be addressed. E-mail: zhanglz@ mail.ccnu.edu.cn. Tel/Fax: +86-27-6786 7535.
Nevertheless, the general preparation of the rare-earth orthoferrite nanocrystals at a temperature lower than 700 °C is still a challenge. In this communication, we report that nanocrystalline rare-earth orthoferrites LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) can be obtained at a temperature as low as 500 °C. The synthesis was realized with nanosized heterobimetallic precursors. Heterobimetallic colloidal precursors with a Ln/Fe ratio of 1:1 were first prepared by heating Ln-oleate and Fe-oleate in 1-octadecene at 320 °C for 2 h. These nanosized precursors could reduce the diffusion path of the formation of orthoferrite LnFeO3 nanocrystals during the subsequent calcination, making sure the crystallinization of orthoferrite LnFeO3 at low temperature and avoiding the formation of garnet byproducts. The magnetic properties of the resulting orthoferrite LnFeO3 nanocrystals were studied. The synthesis of the metal oleates were performed as the method reported previously.17 Typically, the preparation of Ln-oleate complex was as follows. Four micromoles of rare-earth metal nitrate and 12 mmol of sodium oleate were dissolved in a mixture composed of 8 mL of ethanol, 6 mL of distilled water, and 14 mL of hexane. The resulting solution was heated to 70 °C and kept at that temperature for 4 h. After the reaction was completed, the upper organic layer containing the Lnoleate complex was transferred to a beaker, and after the evaporation of the hexane, the Ln-oleate complex in a waxy solid form was obtained. The Fe-oleate complex was synthesized with the same procedure. Heterobimetallic precursors were prepared by heating equimolar Ln-oleate and Fe-oleate as well as 2 mmol of oleic acid in 1-octadecene at 320 °C for 2 h. The resulting colloidal solutions were cooled to room temperature and washed with ethanol to obtain the precursors. Finally, the precursors were heated in air at 500 °C for 4 h to obtain the LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) products. Figure 1 shows powder X-ray diffraction (XRD) patterns of the final products calcined at 500 °C. These patterns can be well indexed to orthorhombic LaFeO3 (JCPDS file No. 74-2203), PrFeO3 (JCPDS file No. 78-2424), NdFeO3 (JCPDS file No. 74-1473), SmFeO3 (JCPDS file No. 74-1474), EuFeO3 (JSPDS file No. 741475), and GdFeO3 (JCPDS file No. 74-1900), respectively. The XRD peaks of all the powders were broadened, indicating their particle sizes were small. Thermodynamically stable garnets
10.1021/cg800014b CCC: $40.75 2008 American Chemical Society Published on Web 06/10/2008
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Figure 1. The powder X-ray diffraction (XRD) patterns of the final LnFeO3 powders. (A) LaFeO3; (B) PrFeO3; (C) NdFeO3; (D) SmFeO3; (E) EuFeO3; (F) GdFeO3.
(Ln3Fe5O12) phases were not observed on the powder XRD patterns, revealing the high purities of the products. The morphologies of the final LnFeO3 products were analyzed with scanning electron microscopy (Supporting Information). All the annealed samples consist of irregularly shaped nanoparticles with diameters of several tens of nanometers. Moreover, these nanoparticles are highly agglomerated to form porous structures among the samples. EDX analysis confirms that Gd:Fe in the GdFeO3 sample was 1:1, consistent with the XRD result. We further examined the microstructure of the LnFeO3 samples with transmission electron microscopy (TEM) and high-resolution TEM (Figure 2). Similar to the SEM observation, the particles on TEM images are also agglomerated. However, the grain boundaries are clearly distinguishable. The diameters of LaFeO3, PrFeO3, and NdFeO3 are in range of about 30-50 nm, while SmFeO3 had a larger diameter of about 40-100 nm, EuFeO3 and NdFeO3 had smaller diameters of about 20-40 nm. The corresponding highresolution TEM (HRTEM) images of the LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) confirm their orthorhombic phases. The surface areas of the final products were studied with nitrogen sorption (Figure S3, Supporting Information). Their nitrogen adsorption-desorption isotherms can be classified as type III with hysteresis loops, which may be caused by the weak adsorbateadsorbate interactions.18 The hysteresis loops in the isotherms are attributed to the filling of pores by the agglomeration of the nanoparticles, consistent with TEM observations. The BrunauerEmmett-Teller (BET) specific surface areas of LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) are 31.0, 21.3, 21.8, 20.8, 41.8, 25.6 m2/g, respectively. Compared to the thermodynamic stable garnets (Ln3Fe5O12), the rare earth orthoferrites are recognized as metastable phases. The formation of such metastable phases may be inferred by considering the stability of the crystal structure. Goldschmidt discussed the stability of a perovskite structure of the ABO3 type by using the tolerance factor (t):
t ) (RA + RO) ⁄ RB + RO
(1)
Here, RA and RB are the ionic radii of the larger and smaller cations, respectively, and RO is the ionic radius of oxygen. The perovskite structure with t ) 1 is ideal. The stable perovskite structure has a limited value between 0.8 and 1.19 Kosuke Nagashio reported that the LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) is considered to be stable according to the tolerance factor.20 Therefore, the formation of the LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) is feasible if the preparation procedure is well chosen and controlled.
In order to understand the formation mechanism of the nanocrycratalline orthoferrites LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) at such a low temperature, we studied the heterobimetallic precursor of GdFeO3 in detail. Figure 3 shows a TEM image of the GdFeO3 precursor. The precursor consists of nearly monodispersed and irregularly shaped nanoparticles with diameters of about 8 nm. Meanwhile, energy dispersive X-ray spectroscopy (EDX) analysis revealed the individual nanoparticles were of homogeneous distribution of Gd and Fe with a 1:1 ratio at a nanometer scale. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of GdFeO3 precursor were shown in Figure 4. Its DSC curve exhibits an exothermic peak in the temperature of 311 °C, which is associated with the elimination of organic solvent (1-octadecene) present in the GdFeO3 precursor. The decomposition process is evident in the TGA curve as supported by a continuous decrease (about 75%) in the sample weight. Moreover, there are also some small exothermic peaks in the temperature range 370-435 °C and a strong exothermic peak at 458 °C observed in the DSC curve. According to a minor weight loss in the TGA curve, the former small exothermic peaks are ascribed to the gradually decomposition of oleate, which is chemically adsorbed to the nanoparticle surface as a surfactant ligand, and this decomposition process may generally occur at the oxide crystallization temperature.21 The later strong exothermic peak at 458 °C is attributed to the crystallization of the gadolinium orthoferrite phase. The Fourier transform infrared spectra of the GdFeO3 precursor calcined at different temperatures are provided in Supporting Information (Figure S4). The spectrum shows a CdO stretching peak at 1700 cm-1, which is a characteristic peak for a metal-oleate complex.17 This peak disappeared when the precursor heated to 450 °C, suggesting that the oleate ligands in the GdFeO3 precursor were removed in the temperature range 330-450 °C. This confirms the result of thermal analysis. In contrast to a much higher temperature required in the traditional methods for the synthesis of LnFeO3, our nanosized heterobimetallic precursor based method can crystallize LnFeO3 nanocrystals with orthorhombic phase at a temperature as low as 500 °C. This clearly shows the enhanced reactivity afforded by these nanosized heterobimetallic precursors. Ahn and co-workers reported that in the case of yttrium iron garnet, the coprecipitation of Y and Fe hydroxides, or complexation of Y3+ and Fe3+ ions by a polymeric agent, significantly lowered the synthesis temperature. They attributed this temperature lowering to the reduction of the diffusion path.22 On the basis of these previous studies and our characterization results, we think the nanosized heterobimetallic precursors in this study not only ensure the desirable stoichoimetry ratio between rare earth metals and Fe to avoid the formation of garnets, but also decrease the diffusion path between the precursor particles, and thus virtually lower or eliminate the rate-limiting solid-solid diffusion barrier existing in traditional solid-state synthesis. Rare-earth orthoferrite crystallizes in a distorted perovskite structure with an orthorhombic unit. The distortion from the ideal perovskite is mainly in the position of the rare earth ions, whereas the Fe3+ iron is surrounded by six oxygen ions giving octahedral coordination.23,24 Because the alignment of Fe moments is not strictly antiparallel but slightly canted, this results in a small net magnetization, giving rise to a weak ferromagnetic behavior.11 For example, GdFeO3, belonging to the class of orthoferrite (G-type magnet), possesses a strong uniaxial anisotropy.10 Recently, it has attracted much interest because of their high coercivity25 and Faraday rotation,6 and thus is an interesting material for magnetooptical data storage devices. Figure 5 shows the magnetic hysteresis loops of the nanocrystalline rare-earth orthoferrites LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) samples. The hysteresis plot for the SmFeO3 shows a linear decrease with field, with no sign of saturation even at -1.7 T, resembling a paramagnet. However, the large coercivity (Hc ) 1.9 kOe) observed indicates that the sample is not superparamagnetic. Mather and co-workers
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Figure 2. The TEM images of LnFeO3 samples. (A) LaFeO3; (B) PrFeO3; (C) NdFeO3; (D) SmFeO3; (E) EuFeO3; (F) GdFeO3; the HRTEM images of LnFeO3 samples. (G) LaFeO3; (H) PrFeO3; (I) NdFeO3; (J) SmFeO3; (K) EuFeO3; (L) GdFeO3.
attributed this situation to the contribution from the paramagnetic susceptibility of Sm ions and a minor ferrimagnetic contribution due to the canting of the Fe sublattice.10 The other LnFeO3 (Ln ) La, Pr, Nd, Eu, Gd) samples reach saturation at the offered magnetic field. It indicates that these samples are weak ferromagnets. Among these nanocrystalline rare-earth orthoferrites, EuFeO3 and GdFeO3 exhibit better weak ferromagnetic behavior from the M-H curves, which have saturation magnetization intensities of 3.99 and 3.89 emu/g, respectively. We compared the ferromagnetic properties of EuFeO3 and GdFeO3 samples with some previous reports. For example, nanocrystalline orthoferrite GdFeO3 was prepared by the calcination of a novel heterobimetallic precursor at 800 °C in Mathur’s study. The M(H) curve of the resulting nanocrystalline orthoferrite GdFeO3 showed a nearly linear increase with field, without signs of saturation up to 5 T, resembling a paramagnet.10 While Pathak and co-workers synthesized nanocrystalline GdFeO3 powders through different solution-based chemical routes followed with high tem-
perature calcination, including the triethylammounium-carbonate coprecipitation method with subsequent calcination at 800 °C, the polyvinyl alcohol-urea precipitation method with subsequent calcination at 650 °C, and potassium ferricyanide precursor method with subsequent calcination at 730 °C. The saturation magnetization values of GdFeO3 samples prepared by the three methods were 1.335, 1.271, 1.328 emu/g, respectively.16 In the current study, the synthesized nanocrystalline orthoferrite GdFeO3 exhibited a saturation magnetization intensity of 3.89 emu/g, higher than the previous reports. Moreover, we prepared some samples with the traditional methods and compared their ferromagnetic behaviors with those of the EuFeO3 and GdFeO3 samples prepared by our method. For example, we synthesized EuFeO3 by a sol-gel method using citric acid as a complexing reagent as reported in Li’s work26 and GdFeO3 by calcining the Gd[Fe(CN)6] precursor at 600 °C for 12 h.15 The magnetic hysteresis loops of the different EuFeO3 and GdFeO3 samples were provided in Supporting Information. Figure S5 shows that the EuFeO3 powder prepared by our method has a saturation
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Figure 3. The TEM image of GdFeO3 precursor.
Communications Supporting Information).15 So we conclude that our method is able to prepare nanocrystalline orthoferrite powders with attractive weak ferromagnetic properties. In summary, we reported that nanosized heterobimetallic precursors could serve as a novel building block to produce nanocrystalline rare-earth orthoferrites at a temperature as low as 500 °C. This generalized low-temperature approach possesses two advantages. First, the heterobimetallic precursors with a desirable stoichiometry ratio (1:1) of Ln and Fe favor the production of orthoferrites without generating any garnet byproducts during the formation of the LnFeO3 nanocrystals. Second, the nanoscale nature of the precursors significantly reduces the diffusion path between the precursor particles, and thus virtually lowers or eliminates the rate-limiting solid-solid diffusion barrier existing in traditional solid-state synthesis. Among these nanocrystalline rare-earth orthoferrites, EuFeO3 and GdFeO3 exhibit better weak ferromagnetic behavior. This nanosized heterobimetallic precursors method may be extended to prepare other ternary metal oxides at relatively low temperatures.
Acknowledgment. This work was supported by National Basic Research Program of China (973 Program) (Grant 2007CB613301), National Science Foundation of China (Grants 20673041, 20503009 and 20777026), Program for New Century Excellent Talents in University (Grant NCET-07-0352), and the Key Project of Ministry of Education of China (Grant 108097). Supporting Information Available: The powder XRD patterns of the GdFeO3 precursor; the SEM images of LnFeO3 samples; nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves of LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd); FT-IR spectra of the GdFeO3 precursor calcined at different temperatures; the magnetic hysteresis loops of the EuFeO3 powders prepared by our method and traditional sol-gel method; the magnetic hysteresis loops of the GdFeO3 powders prepared by our method and the Gd[Fe(CN)6] precursor method. The material is available free of charge via the Internet at http://pubs.acs.org.
Figure 4. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the GdFeO3 precursor.
Figure 5. The full hysteresis curves of the rare-earth orthoferrite LnFeO3 (Ln ) La, Pr, Nd, Sm, Eu, Gd) powders measured at 300 K up to 2 T.
magnetization intensity of 3.99 emu/g, much higher than that (0.325 emu/g) of the EuFeO3 sample prepared by the traditional sol-gel method.26 Meanwhile, the GdFeO3 synthesized in this work possessed saturation magnetization intensity of about 3.89 emu/g, significantly higher than that of the sample prepared with the Gd[Fe(CN)6] precursor method reported by Navarro (Figure S6,
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CG800014B
