Magnetic Properties of Doped CeO 2 Nanoparticles and CeO 2 -Fe 2 O 3 Mixed Oxides

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1 WDS'12 Proceedings of Contributed Papers, Part III, 12 17, ISBN MATFYZPRESS Magnetic Properties of Doped CeO 2 Nanoparticles and CeO 2 -Fe 2 O 3 Mixed Oxides A. Mantlikova, 1,2 B. Bittova, 1 S. Burianova, 1 J. Vejpravova, 1 D. Niznansky, 3 P. Holec 3,4 1 Institute of Physics of the ASCR, v.v.i., Department of Functional Materials, Na Slovance 2, Prague 8, Czech Republic. 2 Charles University Prague, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, Prague 2, Czech Republic. 3 Charles University Prague, Faculty of Science, Department of Inorganic Chemistry, Albertov 2030, Prague 2, Czech Republic. 4 Institute of Inorganic Chemistry of the ASCR, v.v.i., Husinec-Rez, Czech Republic. Abstract. We have investigated magnetic properties of CeO 2 nanoparticles and CeO 2 /SiO 2 nanocomposites doped by various magnetic metal oxide ions (Fe, Gd, Sm, Nd). Samples were obtained by co-precipitation and sol-gel method and characterized by the Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDX). Particle size has been obtained as 5 25 nm for nanocomposite and nm for nanoparticles increasing with the increase of the annealing temperature. Detailed measurements of magnetization demonstrate paramagnetic state in the majority of the doped samples with maximum magnetization decreasing with the decrease of effective magnetic moment of the dopant ions from Gd 3+ to Nd 3+. It has been also observed, that the high-coercivity ε-fe 2 O 3 forms in CeO 2 -Fe 2 O 3 /SiO 2 nanocomposite for higher concentration of the Fe ions (Ce/Fe/Si ratio equal to 1/2/9) and for the highest annealing temperature. Introduction Cerium oxide containing materials have been in focus of intensive research during last years due to the diversity of their applications. One of the most important application of this material is in catalysis [Trovareli, 1996] due to two special features of CeO 2 high reactivity mediated by redox couple of Ce IV /Ce III and unique oxygen storage capacity [Bao et al., 2008]. Due to these useful properties, cerium oxide is usually used in industry as a component of commercial catalysis for reduction of CO, hydrocarbons and NO x emissions from gasoline engines [Wang et al., 2011]. Unique oxygen storage capacity is also useful for solid oxide fuel cells [Shiono et al., 2004]. Another perspective application of cerium oxide based materials is in spintronics [Fernandez et al., 2009] due to good compatibility with microelectronic devices mediated by agreement of its crystal structure and lattice parameter with that of Sillicon [Li et al., 2010]. For use in this application field, the room-temperature magnetic ordering is necessary. However, pure bulk CeO 2 without defects exhibits diamagnetic behavior due to the presence of the Ce 4+ ions. Doping the phase by transition metal ions could introduce the required magnetic ordering to this material [Kumar et al., 2011; Chandra Dimri et al., 2011]. In our work, we have focused on doping the nanocrystalline CeO 2 by Fe, Gd, Sm, and Nd to induce unpaired spins with possibility of magnetic ordering in these materials. The first aim of our work was study the influence of the dopant type and preparation conditions on the magnetic state of the doped cerium oxide nanoparticles. Additional improvements of physical properties of the cerium oxide could be also done by creation of mixed oxide. For example mixed oxides combining CeO 2 with hematite, α-fe 2 O 3, have higher catalytic activity than the individual pure oxides [Bao et al., 2008]. We have focused on combining of CeO 2 with the ε-fe 2 O 3, which is one of the two metastable phases of iron trioxide, which exists only in nanosized form. This phase is interesting and important due to its huge room-temperature coercivity (up to 2 T) [Jin et al., 2004] and also due to the presence of coupling of its magnetic and electric properties [Gich et al., 2006]. Creation of nanocomposite system comprising CeO 2 and ε-fe 2 O 3 results into the multifunctional nanocomposite combining suitable catalytic and magnetic properties, which could extend potential applications of this material. However, the preparation of 12

2 ε-fe 2 O 3 is rather difficult due to its high thermal instabilities. Therefore, the second aim of our work was study of stabilization of high-coercivity ε-fe 2 O 3 phase in the CeO 2 -Fe 2 O 3 /SiO 2 nanocomposites. Experimental details For purpose of this work, two different series of the Ce 0.8 X 0.2 O 2 (X = Fe, Gd, Sm, Nd) samples were prepared individual matrix-free nanoparticles obtained by co-precipitation method [Gosavi et al., 2010] and nanoparticles embedded in silica matrix the nanocomposites obtained by sol-gel method [Niznansky et al., 1997] with nanoparticles/matrix ratio equal to 1/5. Samples from both series were annealed at two different temperatures 830 and 1130 C and labeled as X_830 respective X_1130 for matrix-free nanoparticles and X_SiO2_830 respective X_SiO2_1130 for nanocomposite. In addition, two series of CeO 2 -Fe 2 O 3 /SiO 2 nanocomposites with different concentration of Fe, e.g. Ce/Fe/Si molar ratio equal to 1/1/20 (L series) and 1/2/9 (H series) were also prepared by the solgel method. The samples were annealed at three different temperatures 950 C, 1050 C and 1100 C using special annealing regime (slow heating rate and waiting at some definite temperatures before next heating [Mantlikova et al., 2012]) and labeled with respect to the Ce/Fe/Si ratio (e.g. to the concentration of iron ions) and final annealing temperature as L950 L1100 and H950 H1100. The samples were characterized by Powder X-ray Diffraction (PXRD). Measurements were performed by Bruker diffractometer AXS GmbH with the Cu-K α radiation at room temperature for mixed oxides and by diffractometer PANalytical X Pert PRO with the Cu-K α radiation at room temperature for the doped CeO 2 nanoparticles. The particle sizes, d were determined using the Rietveld refinement implemented within the FullProf program [Rodriguez-Carvajal, 2000]. High Resolution Scanning Electron Microscopy (HRSEM) Tescan Mira I LMH equipped with an Energy Dispersive X-ray Analysis (EDX) Bruker AXS was used for verification of the samples composition and direct observation of their morphology. The Mössbauer spectroscopy was used for determination of the iron oxide phase composition in the mixed oxide samples. The measurement was done in the transmission mode with 57 Co diffused into a Rh matrix as the source moving with constant acceleration. The spectrometer (Wissel, Germany) was calibrated by standard α Fe foil and the isomer shift is related to this standard at 293 K. The resulting parameters were determined using the NORMOS program. The magnetic measurements were performed using MPMS 7XL (SQUID) device (Quantum Design, San Diego). The zero-field-cooled (ZFC) and field-cooled (FC) temperature dependencies of magnetization were measured in the external magnetic field equal to 1 T. The magnetization isotherms were measured at selected temperatures in magnetic fields varying up to 7 T in both polarities. Results and Discussion Doped CeO 2 nanoparticles The PXRD of all the doped samples verified presence of the CeO 2 nanoparticles. For the nanocomposites, presence of the amorphous SiO 2 matrix was manifested by a broad peak in low diffractions angles (Figure 1 left). The majority of the matrix-free samples were single-phase except the Fe_830 and Fe_1130 samples, which exhibited also presence of hematite. However, the reflections corresponding to the CeO 2 of all single phases matrix-free samples were quite asymmetric and more split (especially in higher angles) indicating presence of two different fractions of cerium oxide nanoparticles with different particle sizes. Also the lattice parameters of the two fractions appear to be different, caused probably by difference in doping level between these two fractions. In contrast, all nanocomposite samples contain only one fraction of CeO 2 all particles have approximately the same particle size and lattice parameters, respectively. However, some of these samples annealed at higher temperature (Sm_SiO2_1130 and Nd_SiO2_1130) exhibit also presence of additional Si-containing phases such as silicates. Lattice parameters and particles size (d) obtained for all samples from the Rietveld refinement are summarized in Table 1. From the two-fraction fit of the matrix-free samples, the correct particle sizes of both fractions could not be determined, but the average d values obtained from the one-fraction fit are sufficient for observation of general trends in d. The d increases with increasing annealing 13

3 Gd_SiO2_1130 Gd_1130 Bragg positions for CeO θ ( ) Figure 1. Left panel: PXRD patterns of the Gd-dopped samples annealed at 1130 C. Bragg positions corresponding to the CeO 2 are depicted by vertical bars. Right panel: HRSEM image of the Gd_1130 sample. temperature and it is smaller for the nanocomposite samples in comparison with the corresponding matrix-free samples. The observation is supported by broadening of the reflections due to the smaller particle size. The lattice parameters of samples from both series also increase with increasing of the dopant ion s diameters from Fe 3+ to Nd 3+, which confirms the real introduction of the cations into the crystal structure of the CeO 2. For the selected matrix free-samples the particle appearance was also direct observed using by HRSEM. The obtained image for the Gd_1130 sample is presented in Figure 1 right. There are individual nanoparticles (small objects) with quite similar spherical shape and also their aggregates. Other matrix-free samples have similar particle appearance. The measurement of EDX verified the 20% content of the dopant cations in each sample (obtained Ce/X ratios were close to 4 as was expected) and also the nanoparticles/matrix ratio equal to 1/5 for nanocomposite samples. For the magnetic measurements, we have selected only the single-phase samples. Majority of the measured samples exhibits a typical paramagnetic behavior (Figure 2 left). In contrast, the Fe_SiO2_1130 sample exhibit rather complicated temperature dependence of magnetization (inset of Figure 2) pointing at existence of magnetic ordering in this sample. The temperature dependence exhibit presence of two magnetic transitions at around 100 and 150 K, which are characteristic for ε-fe 2 O 3. Therefore, it is possible that Fe_SiO2_1130 sample contain this phase. Magnetic susceptibility of the Gd_1130 sample was fitted by the Curie-Weiss law (Figure 2 right) [Blundell, 2001]. Table 1. Lattice parameters, a and particle sizes, d obtained from the PXRD measurements. The values of the particle sizes for matrix-free samples doped by Gd, Sm and Nd are average values obtained from the one-fraction fits and the lattice parameters of these samples, a 1 and a 2 are obtained from the two-fraction fits. Matrix-free nanoparticles Nanocomposites Sample d (nm) a 1 (Å) a 2 (Å) Sample d (nm) a (Å) Fe_ (9) Fe_SiO2_ (4) Fe_1130 > (6) Fe_SiO2_ (4) Gd_ (1) 5.415(0) Gd_SiO2_ (4) Gd_ (8) 5.412(5) Gd_SiO2_ (5) Sm_ (3) 5.419(4) Sm_SiO2_ (0) Sm_ (7) 5.419(3) Sm_SiO2_ (6) Nd_ (5) 5.416(2) Nd_SiO2_ (3) Nd_ (5) 5.418(2) Nd_SiO2_ (8) 14

4 Gd_1130 Nd_1130 Gd_SiO2_1130 Nd_SiO2_830 Fe_SiO2_ Fe_SiO2_ χ Μ (mol/m 3 ) Experimental data Fit by Curie-Weiss law Figure 2. Left panel: Temperature dependencies of ZFC magnetization of the matrix-free and nanocomposite samples measured at 10 mt. In the inset is dependence for the Fe_SiO2_1130 sample. Right panel: Reciprocal susceptibility of Gd_1130 sample (grey) with the result of Curie-Weiss fit (black) µ 0 H (T) µ 0 H (T) Gd_1130 Nd_1130 Gd_SiO2_1130 Nd_SiO2_830 Fe_SiO2_ µ 0 H (T) 10 K 300 K Figure 3. Left panel: Magnetization isotherms of the matrix-free and nanocomposite samples measured at 300 K In the inset are details of magnetization isotherms of Nd_1130, Nd_SiO2_830 and Fe_SiO2_1130 samples. Right panel: Magnetization isotherms of the Fe_SiO2_1130 sample measured at 10 and 300 K. The resulting effective magnetic moment per ion µ eff calc = 7.39 µ B is in good agreement with the theoretical value µ eff theor = 7.94 µ B for Gd 3+ ion, which confirms that the paramagnetic behavior of this sample is caused by the presence of Gd 3+ ions. For the further inspection of magnetic properties of our samples, the magnetization isotherms at different temperatures were measured (Figure 3). The majority of the samples exhibit paramagnetic behavior with saturation magnetization decreasing with decrease of the effective magnetic moment of the dopant ions from Gd 3+ to Nd 3+ [Blundell, 2001]. The magnetization isotherms of the Fe_SiO2_1130 sample exhibit behavior typical for the magnetically ordered material due to presence of the ε Fe 2 O 3 phase. CeO 2 -Fe 2 O 3 Mixed Oxides The PXRD of all samples verified the presence of CeO 2 nanoparticles and the amorphous SiO 2 matrix. The obtained particles sizes of CeO 2 increase with increasing annealing temperature and they were larger for samples with higher concentration of Fe (the H series). Because of the large overlap of the Bragg positions for all the iron trioxide phases, it was not possible to exact determine the phase composition of Fe 2 O 3 in our samples any coincidence of the presence of α, β, γ and ε phases [Mohapatra et al., 2010; Murad et al., 1987] could not be neither confirmed nor omitted by this 15

5 12 L950 L L H950 H1050 H1100 Figure 4. Temperature dependencies of ZFC and FC magnetization of the L (Left panel) and H (Right panel) mixed oxides samples measured at 10 mt. measurement. Therefore, the Mössbauer spectroscopy was used as the only valuable experiment to resolve individual oxide phase(s) in these samples. It was found, that all samples from H series contain only α and ε with no traces of both superparamagnetic phases β and γ. On the other hand, all samples from the L series contain one of the superparamagnetic phases and L950 sample exhibit no traces of the required ε phase. Magnetic measurements of these samples were in good agreement with the phase composition obtained from the Mössbauer spectroscopy. All samples from H series exhibit typical magnetic properties of ε-fe 2 O 3 phase including temperature dependence of magnetization with characteristic two magnetic transitions around 100 and 150 K (Figure 4 right) and magnetization isotherms with huge value of room-temperature coercivity. On the other hand, all samples from L series exhibit also superparamagnetic behavior (Figure 4 left), which in the case of L_1100 sample totally suppressed large value of room-temperature coercivity. More details with figures could be found in [Mantlikova et al., 2012]. Conclusion In summary, we have studied magnetic properties of doped CeO 2 nanoparticles and CeO 2 -Fe 2 O 3 mixed oxides. It has been found, that better preparation method for the doped CeO 2 nanoparticles is the sol-gel method with lower annealing temperature resulting in the single phases samples with only one size fraction of the CeO 2 with the approximately same lattice parameters and particle size around 6 nm. In contrast with the diamagnetic behavior of the pure CeO 2, majority of the measured samples exhibits paramagnetic behavior caused by presence of the magnetic metal ions, as expected. The only one exception was the Fe_SiO2_1130 sample, which exhibit magnetic ordering due to the formation of the ε-fe 2 O 3 phase. For the CeO 2 -Fe 2 O 3 mixed oxides, creation of different phases of Fe 2 O 3 depending on the preparation conditions, like the particle size was observed. As the best conditions for the stabilization of the ε-fe 2 O 3 phase (with totally suppressed presence of any superparamagnetic phases) were found the higher concentration of the Fe ions and the higher annealing temperature. These preparation conditions result to the creation of nanocomposite system with the highest room-temperature coercivity up to 2 T. Acknowledgments. The work was supported by the Grant Agency of the Czech Republic, project no. P108/10/1250 and by Grant Agency of Charles University, project no The work is a part of the research plan MSM that is financed by the Ministry of Education of the Czech Republic. References Bao, H., X.Chen, J.Fang et al., Structure-activity Relation of Fe 2 O 3 -CeO 2 Composite Catalysts in CO Oxidation, Catal. Lett. 125, 160,

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