Concentration Dependence of Oxygen Vacancy on the Magnetism of

Article pubs.acs.org/JPCC

Concentration Dependence of Oxygen Vacancy on the Magnetism of CeO2 Nanoparticles Shih-Yun Chen,*,† Chi-Hang Tsai,‡ Mei-Zi Huang,† Der-Chung Yan,§ Tzu-Wen Huang,∥ Alexandre Gloter,⊥ Chi-Liang Chen,¶ Hong-Ji Lin,¶ Chien-Te Chen,¶ and Chung-Li Dong*,¶ †

Department of Material Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan § Department of Physics, National Tsing Hua University, Hsinchu, Taiwan ∥ Laboratory for High Performance Ceramics, Swiss Federal Laboratories for Materials Science and Technology Ü berlandstrasse 129, CH - 8600 Dübendorf, Switzerland ⊥ Laboratoire de Physique des Solides, CNRS-UMR 8502, Université Paris Sud, 91405 Orsay, France ¶ National Synchrotron Radiation Research Center, Hsinchu, Taiwan ‡

ABSTRACT: This investigation demonstrates a strong dependence of the magnetism of CeO2 nanoparticles on the concentration of oxygen vacancies. A strong magnetic signal is over for a narrow range (0.1 < Ce3+/Ce < 0.2) of Ce3+ concentrations in the nanoparticles. Previous studies have determined that most of the vacancies are at the surface of the particle and, given particles of size 3−10 nm, the strong magnetic signal corresponds to an even narrower range of Ce3+ content at the surface (0.40 < Ce3+surface/Cesurface < 0.45). X-ray magnetic circular dichroism (XMCD) measurement reveals that electrons in Ce bear magnetic moments while oxygen atoms do not respond magnetically indicating that the bounded magnetic polarons are more important to magnetism than is oxygen-mediated exchange.

I. INTRODUCTION The field of spintronics requires the development of semiconductors with ferromagnetically polarized carriers at room temperature (RT) such that both the spin and the charge of the carriers can be coupled with an external magnetic field to control devices. Theoretical predictions made by Dietl et al. and Matsumoto et al. about such materials, dilute magnetic semiconductors (DMS), operated at RT have stimulated considerable interest.1,2 Most experimental results have raised doubts about the origin of the ferromagnetism (FM). Moreover, some authors have mentioned that, without doping with a magnetic element, oxides became magnetic after reduction or when the size of their crystallites is of the order of nanometers. Accordingly, recent research has focused not only on the effect of magnetic doping but also on defects, especially oxygen vacancies, and their relationship to ferromagnetism. Oxygen vacancies have been suggested to form a donor impurity band that promotes exchange coupling.3 Ceria (CeO2) and related compounds are known to provide an effective reservoir of oxygen with a large capacity for storage and release of oxygen vacancies. This large storage capacity has been used in solid-oxide fuel cells and catalytic applications. RTFM is also found in ceria and even in undoped samples. Some researchers have proposed that oxygen vacancies contribute to ferromagnetism. Sundaresan et al. attributed the FM of CeO2 nanoparticle (NP) to exchange interactions between unpaired spins that result from surface oxygen vacancies (VO).4 © 2012 American Chemical Society

Ge et al. identified FM in 5.3 nm CeO2 NP; their calculations from first principles revealed that VO, especially at the surface, can induce magnetic moments in CeO2 NP.5 In contrast, some researchers have doubted the effect of oxygen vacancies on FM. For example, Liu et al. observed FM only in sub-20 nm CeO2 powder and argued that FM might not be linked to VO.6 Li et al. suggested that FM is related not to the surface oxygen vacancies but to the Ce3+/Ce4+ pairs.7 An interesting property of CeO2 is that it can have a stable structure far from the stoichiometric proportions of oxygen. The relaxation or reconstruction of materials with defects at a high concentration is unusual. The nature of the oxygen vacancies, including their type, size, and distribution in CeO2, must therefore be addressed, but a detailed analysis of the magnetism of CeO2 NP with various stoichiometries is lacking. This study systematically investigates the microstructure, magnetic properties, and X-ray absorption spectra of CeO2 NP with various degrees of oxygen deficiency. CeO2 NPs with a large oxygen deficiency were synthesized by a thermal decomposition method, which yields well-dispersed NPs with good crystallization.8−10 The degree of oxygen deficiency was controlled by varying the conditions of postannealing of the as-prepared NPs. Received: July 11, 2011 Revised: December 5, 2011 Published: January 10, 2012 8707

dx.doi.org/10.1021/jp2065634 | J. Phys. Chem. C 2012, 116, 8707−8713

The Journal of Physical Chemistry C

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Table 1. Parameters of Annealing Process, Particle Size, Concentration of Ce3+, and Saturation Magnetic Moments of CeO2 NP parameters of annealing process sample as-prepared 300-0-2 300-20-2 300-100-2 500-0-2 500-20-2 500-50-2 500-100-2 500-20-4

temperature (°C) 300 300 300 500 500 500 500 500

PO2 (%) 0 (N2) 20 (air) 100 (O2) 0 (N2) 20 (air) 50 100 (O2) 20 (air)

results time (hr)

size (nm)

2 2 2 2 2 2 2 4

2.75 3 4.4 5 6 9.5 9 10 7

Ce

3+

(%), ICe 28 35 17 10 10 10 9 9 10

3+

Ms (emu/g) paramagnetic paramagnetic 0.18 0.006 0.01 0.24 0.1 0.08 0.22

In CeO2, the oxygen vacancies are known to become completely filled under O2-rich conditions. The experimental results indicate that the observed RTFM in CeO2 NP is strongly related to the concentration of Ce3+ at the surface. Furthermore, the role of oxygen vacancies in the formation of RTFM varies with the degree of oxygen deficiency.

II. EXPERIMENTS A mixed solution was first made by loading cerium(III) acetyl acetonate hydrate (0.5 mM), 1,2-dodecandiol (10 mM) as a stabilizer, and 10 mL octyl-ether in that order into a threenecked round-bottom flask and by stirring the mixture using a magnetic stirring bar. All of the precursor was dissolved at 100 °C, and then the surfactants (oleic acid and oleyamine) were added. Then, the mixed solution was further heated to reflux at 300 °C, and the refluxing was continued for 30 min. The resultant solution was cooled slowly in air, and n-hexane was poured into it. Finally, the precipitates were separated by centrifugation at 6000 rpm for 15 min. The final product was collected and dispersed in a nonpolar organic solvent. The asprepared NPs then underwent different annealing processes: the temperature was 300 or 500 °C, the oxygen partial pressure (PO2) ranged from 0% to 100%, and the time ranged from 2 to 4 h as listed in Table 1. The NPs were characterized using an X-ray diffractometer (XRD) with Cu Kα radiation. The size distribution, morphology, and crystalline structure of the NPs were examined using a transmission electron microscope (TEM, FEI-Tecnai) that was equipped with a field emission gun. The magnetization was measured at room temperature using a superconducting quantum interference device magnetometer (MPMSXL, Quantum Design). X-ray absorption was measured at the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The X-ray absorption near-edge fine structure (XANES) at the Ce L3 edge was measured at Wiggler beamline 17C in transmission mode at room temperature. The monochromator Si(111) crystals on wiggler beamline 17C produced a resolving power E/ΔE ∼ 2 × 104. The energy resolution at the Ce L3 edge (5723 eV) was 0.3− 0.4 eV. X-ray magnetic circular dichroism (XMCD) was measured at Dragon beamline 11A in fluorescence yield mode in an applied alternating magnetic field with a strength of ±1 T. The monochromator resolving power was E/ΔE ∼ 10 000, and the extent of circular polarization of the X-rays was ∼55%.

Figure 1. XRD results of CeO2 NPs with different annealing processes. The black lines represent the pattern of bulk CeO2 (JCPDS 34-0394) with major reflections assigned.

phase with space group Fm31m. It also shows that the broadening of the XRD peaks gradually decreased as either the temperature or the oxygen partial pressure in the annealing atmosphere increased. According to the Scherrer equation,11 the average particle size, estimated from the full width at halfmaximum of the XRD peaks, ranged from 2.75 to 10 nm. Figures 2 displays the high-resolution transmission electron microscopic (HRTEM) images of the (a) as-prepared CeO2 NPs and those annealed under various conditions for 2 h: (b) in O2 at 300 °C, (c) in air at 500 °C, and (d) in O2 at 500 °C. These particles are clearly crystalline, which agrees with XRD results. The dominant fringes of the NPs are (111). The particle sizes, determined from the TEM images, are (a) 2.7 ± 0.5 nm, (b) 5.6 ± 0.6 nm, (c) 9.6 ± 1.1 nm, and (d) 13.4 ± 2.2 nm, respectively. The average particle sizes in the micrographs are of the same order as those obtained from XRD line-broadening analyses. XANES Analysis of Ce L Edge. The electronic structure of Ce was investigated at the Ce L edge. Figure 3 presents the normalized XANES spectra of the Ce L3 edge for CeO2 NP that had been annealed under various conditions. The annealed samples (except for N2-300-2 h) resembled each other closely but substantially differed from the as-prepared NPs. The spectral profiles indicate that these annealed NPs have an oxidation state that differs from that of the as-prepared NPs, but on close inspection, the variation in the near-edge feature is clearly seen to be associated with the annealing temperature and the partial pressure of oxygen in the annealing atmosphere. The information on the Ce oxidation states that is obtained from the Ce L3 is well developed and can be extracted from the

III. RESULTS AND DISCUSSION Microstructure. Figure 1 shows the powder X-ray diffraction (XRD) patterns of CeO2 NPs. The XRD patterns reveal that all of the cerium oxide remained in the nanocrystalline 8708



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Figure 2. HRTEM images of CeO2 NPs with different annealing processes: (a) as-prepared, (b) in O2 at 300 °C for 2 h, (c) in air at 500 °C for 2 h, and (d) in O2 at 500 °C for 2 h.

transitions of states, an arctangent function was subtracted from all of the spectra to exclude the edge jump; they were then fitted with Gaussian functions12−14 as in the representative example in the inset. The method for subtracting out the background using an arctangent function and the implication of each component can be found elsewhere.15 Component C represents the trivalent Ce (in a final 4f1(5d6s)4 state). The concentration of Ce3+ impurities in the CeO2 matrix is then expressed as the ratio IC/ITotal . Table 1 presents the estimated 3+ concentration of Ce3+ (ICe ). In the as-prepared NPs, the concentration of Ce3+ is ∼28%, which is much greater than that 5 For NPs that were of samples prepared by other methods. Ce3+ annealed at 300 °C, the value of I was clearly related to the partial pressure of oxygen in the annealing atmosphere (left panel). The NPs contained 17% Ce3+ when they were annealed in air, 35% when they were annealed in N2, and only 10% when they were annealed in O2. For NPs that were annealed at 3+ 500 °C, the value of ICe , about 9 to 10% (central panel), was similar over a wide range of PO2. Magnetic Measurements. The dc magnetization measurements were made at RT. After the diamagnetic component had been subtracted, RTFM was observed in all NPs except for the as-prepared and 300-0-2 NPs as shown in Figure 4. For CeO2 NPs with RTFM, the variation in saturation magnetization (Ms) is over 1 order of magnitude ranging from 0.006 to 0.24 emu/g. The largest Ms was obtained for NPs that were annealed in air at 500 °C for 2 h, which had an

Figure 3. XANES of Ce L edge of CeO2 NPs with different annealing processes. The inset shows the theoretical fit of XANES spectra of the 500-20-2 sample as a representative example.

deconvoluted spectral components that represent the transitions to the various electronic states. To describe further these 8709



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largest Ms. Additionally, CeO2 NPs with the same concentration of Ce3+ exhibited various Ms; for example, for CeO2 NPs with 10% Ce3+, Ms was found to be 0.006, 0.01, 0.22, or 0.24 emu/g. To understand the above contradictions, some characteristics of CeO2 are considered. The first is the effect of size. Wan et al.20 predicted that, as the annealing temperature rises above 130 °C, the increase in the particle size has a significant effect on the concentration of Ce3+. Herein, CeO2 NPs were annealed at either 300 or 500 °C yielding various particle sizes. Broadly, the size of NP was 5 nm following annealing at 500 °C. According to the XRD and XAS results, the size of CeO2 NPs with 10% Ce3+ was in the range 5−10 nm. Next, theoretical calculations indicated that, in CeO2, the formation energy of an oxygen vacancy at the surfaces is less than that in the bulk.21 In nanoparticles, the formation of oxygen vacancies is easier than the extension of their surfaces.22−24 Accordingly, the distribution of Ce3+, equivalent to the distribution of oxygen vacancies, might be inhomogeneous in CeO2 NPs. Third, theoretical calculations also predict that, among low-index planes, the most stable surface is the (111) surface, followed by (110), and then (100). The optimal location of the oxygen vacancy is in the second oxygen atomic layer of the (111) surface.25 Yang et al. suggested that the additional charges that are produced by the formation of the oxygen vacancies are localized in the first three layers of the surface.26 Therefore, they posited a core− shell-like structure, which is stoichiometric CeO2 and whose shell is CeO2−x. Generally, the growth and stability of the materials can be determined by two important physical quantities: surface tension (γ) and cohesive energy (G). Whereas surface tension is caused by the various attractive intermolecular forces and has dimensions of energy per unit area, the bulk cohesive energy is the energy required to separate the atoms of the solid into isolated atomic species. Since γ denotes the surface energy per unit area and G is the attractive energy per unit volume, the surface energy can be expressed as γA and the bulk cohesive energy is GV, where the quantities A and V represent the surface area and volume of the solid, respectively. In nature, materials attempt to balance these two quantities (γA = GV). Hence, the surface-to-bulk ratio is defined as the number of surface atoms to the number of bulk atoms and reflects the relative magnitudes of the surface tension and the bulk cohesive energy of the material that binds the solid. Restated, the surface-to-bulk ratio is directly related to the ratio of surface-tobulk energy (A/V = G/γ) and is inversely proportional to the radius or thickness of the solid. For example, if the object is cubic, the surface area (A = 24r2) to volume (V = 8r3) ratio is (A/V) ∼ 1/r. Similarly, for a spherical object, the surface area (A = 4πr2) to volume (V = (4πr3/3)) ratio is (A/V) ∼ 1/r. The surface-to-bulk ratio can then be obtained as the green curve in Figure 6a. However, the above expression is too simple for small particles whose surfaces are very large relative to their volumes. In the case in our investigation, the surface-to-bulk ratio is estimated as follows. The lattice constant of the unit cell of CeO2 (0.34 nm) (viewed as the thickness of the surface layer) and the dimensions of the nanoparticles (V = (4/3)πr13 where r1 denotes the radius) are considered, and the volume of the surface layer is then estimated to be Vs = V − Vc = (4πr13/3) − (4π(r1 − 0.34)3/3). Therefore, the surface-to-bulk ratio is represented as the blue dashed curve in Figure 6a. This curve is very close to above ∼1/r. The solid symbols on this

Figure 4. Magnetic measurements performed at RT of CeO2 NPs with different annealing processes. The inset shows the M−H curve of the as-prepared NP, which is paramagnetic.

Ms of 0.24 emu/g, followed by NPs that were annealed in air at 500 °C for 4 h, which had an Ms of 0.22 emu/g. These values of Ms are comparable to those in previous reports in which the CeO2 NPs were doped with magnetic elements, such as Co and Fe. Combining the above XAS analysis and magnetic measurements enables the dependence of the magnetism of CeO2 NPs on the concentration of Ce3+ to be determined. Some studies of the magnetic properties of CeO2 with various degrees of oxygen deficiency have been conducted.16,17 Fernandes et al. showed a nearly linear increase in Ms with Ce3+ concentration up to 47.2%.18 Li et al. observed a linear dependence between Ms and the Ce3+/Ce4+ ratio up to 0.8.7 Figure 5 in this study shows that Ms is not monotonically proportional to the concentration of

Figure 5. Relationship between Ms and the concentration of Ce3+ estimated from Ce L edge.

Ce3+: it increases to a maximum and then decreases with a further increase in the concentration of Ce3+. The value of Ms was highest in NPs of which the concentration of Ce3+ was about 10%. This value is much smaller than the value predicted from the experimental results or theoretically indicating that a large concentration of oxygen vacancies is essential to producing large Ms.19 Moreover, the magnetism varied greatly 3+ with only a slightly increase in ICe from 9% to 10%: The NPs changed from paramagnetic to ferromagnetic and had the 8710



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it occurs mostly in metallic systems. The other exchange mechanisms can occur in an insulator or a semiconductor. One possibility is F-center exchange (FCE), which is a subcategory of the bound magnetic polaron (BMP) model.33 The concept of FCE coupling is based on the BMP model and is interpreted with reference to the presence of oxygen vacancies; the magnetic ions via the vacancies (F-center) constitute a BMP and produce ferromagnetism. In undoped CeO2, the origin of magnetism is supposed to result from the decreased oxidation state of Ce ions. Singhal et al.34 suggested that oxygen vacancies form a significant density of F centers that facilitate the observed mediation of the ferromagnetic coupling. Notably, the presence of oxygen vacancies gives rise to the formation of the reduction of the Ce3+ ions; when an oxygen vacancy is formed, two electrons are left behind; these may therefore be transferred to a Ce4+ ion converting Ce4+ into Ce3+. This process yields mixed Ce3+ and Ce4+ states in these NPs. The FM might consequently arise from a nearest-neighbor interaction: double exchange (Ce3+−O−Ce4+) or superexchange (Ce3+−O−Ce3+), both of which are mediated by oxygen ions. The elementary selectivity tool must therefore be utilized to clarify this issue. XMCD is a powerful spectrometric tool that has the elemental and orbital selectivity to investigate the magnetic properties of materials. XMCD. The XMCD (i.e., I+ − I−) spectrum is obtained from the difference between the XAS with opposite sample magnetizations: parallel (I+) and antiparallel (I−) to the orientation of the photon helicity. The spectra of four samples were compared. The samples are the CeO2 standard, which is paramagnetic; as-prepared CeO2 NP, which is paramagnetic; 30020-2 NP, which is weakly ferromagnetic, and 500-20-2 NP, which is strongly ferromagnetic. In Figure 7a and c, the solid line (open circles) represents the I+ (I−) and the black and red solid lines represent the corresponding XMCD signal (Figure 7b). The XMCD signal was weak consistent with the weak magnetic response determined from the SQUID data.

Figure 6. (a) Size dependence of the surface-to-bulk ratio as 3+well as the concentration of Ce3+ and (b) the relationship between IsCe and Ms.

dashed curve represent actual, experimentally measured nanoparticle sizes, and they facilitate comparison with corresponding Ce3+ concentrations. The change in the Ce3+ concentration in the particles varies similarly to the surface-tobulk ratio. The physical and chemical properties of solids are bulk properties, but for nanoparticles, they are determined primarily by the surface. Consequently, the similarity of variations implies that the Ce3+ concentration in the particles is influenced mostly by the surface atoms. This result may explain the above discrepancies. In this work, the concentration of Ce3+ is estimated from the XAS, whereas elsewhere, it is obtained from the XPS. The latter is believed to be more sensitive to the surface. The average oxidation number of Ce throughout the particles might thus be overestimated. From the results in Figure 6a, the concentration of Ce3+ at 3+ the surface (IsCe ) can 3+ be obtained. Figure 6b plots the relationship between IsCe and Ms. This figure can be divided 3+ slowly from3+ into four parts: (1) for IsCe < 38%, Ms increases 3+ 0.006 to 0.015 emu/g; (2) for 38% < IsCe < 40%, as IsCe increases, Ms increases greatly from 0.015 to 0.24 emu/g, which3+ is an increase of over 1 order of magnitude; (3) for 41% < IsCe < 47%, Ms decreases rapidly from 0.24 emu/g to 0.06 emu/g, 3+ and (4) for IsCe > 48%, the FM of CeO2 NPs is absent and the material becomes paramagnetic. Stoichiometric CeO2 is known to be paramagnetic. The key issue herein is to understand the possible origin of the ferromagnetism in deficient CeO2 nanoparticles. The origin of the FM has been discussed in terms of double exchange (DE),27,28 superexchange (SE),29,30 the Ruderman−Kittel− Kasuya−Yosida (RKKY) interaction,31,32 or bound magnetic polarons (BMP).3 RKKY is unlikely because it requires a localized 4f electron that can interact with the itinerant delocalized spin;

Figure 7. (a) Ce M4,5-edges XAS taken with opposite sample magnetizations parallel (I+) and antiparallel (I−) to the orientation of the photon helicity. (b) XMCD spectra of Ce M4,5-edges (black lines) and O K edge (red lines). The inset shows the XMCD asymmetry ratio. (c) O K edge XAS taken with I+ and I−. 8711



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As a consequence, the structure and arrangement of Ce3+ (oxygen vacancies) should be considered. The complicated concentration dependence of Ms in CeO2 NPs can be described as follows. First, on the slightly reduced surface, single vacancies are observed. When the concentration of Ce3+ is low, as shown in Figure 8a, the spacing between Ce3+ and the F center is too large to result in ferromagnetism. As the proportion of Ce3+ increases, the orbitals of the F center begin to overlap each

Although the XMCD sum rule demonstrates that integrated intensities of the absorption and XMCD spectra can yield quantitative information about the orbital and spin moments, the weak XMCD signal and difficulties with the background subtraction make extracting such information impracticable in the present work. Thus, the spin and orbital moments of Ce and O atoms were not extracted herein; instead, this study focuses on the dependence of their overall magnetic properties on these NPs. The XMCD asymmetry ratio, (I+ − I−)/(I++ I−), is proportional to the magnetic moment averaged over the various sites; an estimate of the asymmetry ratio can directly yield information on the relative average moment of Ce. Thus, the variation of the XMCD asymmetry ratio is estimated to represent approximately the change of the magnetic moments of Ce atoms in the NPs. As shown in the inset of Figure 7b, the asymmetry ratio of 500-20-2 NP exceeds that of 300-20-2 NP indicating that the average magnetic moment of 500-20-2 NP is larger than that of 300-20-2 NP. Figure 7c shows the O K edge XMCD spectra; neither 50020-2 NP nor 300-20-2 NP yields an XMCD signal implying that the oxygen sites do not contribute to FM. The mixed valence character is one criterion for identifying DE and was revealed by XAS. It might have led to the nearest-neighbor interaction under the Ce3+−O−Ce4+ framework. The other criterion for identifying DE is that a delocalized electron must hop between cations of various oxidation states via an anion. The lack of an XMCD signal at the O site is evidence that FM is probably not produced by double exchange. From the Ce M edge, the as-prepared sample yields no XMCD signal, but the signal in 500-20-2 NP, though small, is real and larger than that in 300-20-2 NP indicating that the magnetism of 500-20-2 NP exceeds that of 300-20-2 NP, which is consistent with the SQUID measurement. The 4f electrons hence contribute to FM, and the superexchange might be responsible for this phenomenon. However, this model alone cannot explain the complicated variation of Ms with the concentration of Ce3+. As mentioned above, CeO2 has a large capacity for the storage and release of oxygen vacancies. Partially reduced CeO2−x is reportedly stable in a cubic structure with x up to ∼0.4.35 Notably, neutron diffraction measurements on single crystals of CeO1.765 and CeO1.8 have indicated the existence of four phases of CeOy in the range 1.79 < y < 1.80836 and an ordered arrangement of oxygen vacancies.37 Moreover, analysis of the total scattering (Bragg plus diffuse components) using reverse Monte Carlo (RMC) modeling also indicates that the oxygen vacancies preferentially align as pairs in the cubic directions as the degree of nonstoichiometry increases.38 These experimental results demonstrate that oxygen vacancies are not randomly distributed in the structure. Furthermore, scanning tunneling microscopy (STM),39 dynamic force microscopy (DFM), and noncontact atomic force microscopy (NCAFM)40 have all been utilized to investigate more directly the defect structure on the CeO2(111) surface with various degrees of oxygen deficiency. The results reveal the presence of single vacancies on a slightly reduced surface. With further reduction, linear oxygen vacancy clusters become the dominant defect structures. Torbrügge et al. indicated that, at a sufficient concentration of subsurface oxygen vacancies, these defects form linear patterns leaving defect-free areas between them.41 Varied oxygen vacancy structures in nanomaterials have also been identified from positron annihilation spectra.42

Figure 8. (111) surface plane with (a) 5% oxygen vacancies and (b) 23% oxygen vacancies. According to ref 32, the oxygen vacancy is line or triangle ordered. The oxygen at the surface is shown in purple, and its triangular lattice is made more visible.

other and gradually begin to increase Ms. On the basis of the BMP model, the vacancies can constitute the magnetic polaron and favor FM when it exceeds a certain value. Oxygen vacancies then tend to appear in the form of clusters as the proportion of Ce3+ exceeds a particular value, which according to our work is about 40% as shown in Figure 8b. The formation of such clusters is expected to facilitate the ferromagnetic coupling. Second, in the ideal CeO2 crystal, the bond angle Ce−O−Ce is 109°. The presence of an oxygen vacancy would reduce the bond angle Ce3+−O−Ce3+. According to the GKA (Goodenough−Kanamori−Anderson) rules, the 90 degree superexchange interaction tends to produce ferromagnetic behavior.19 Additionally, the size of Ce3+, 0.97 Å, differs from that of Ce4+, 1.14 Å. In the cluster region, the relaxation of the crystal structure should therefore be considered. Souza-Neto et al. predicted that FM exchange interactions might be enhanced by strain or chemical substitutions.43 Han et al. suggested that in CeO2 materials with oxygen vacancies at a large concentration, the localized states that are induced by oxygen removal contain not only Ce 4f electrons but also a substantial number of the electrons that remain at the vacancy sites.19 These authors posited that these electrons that remain at the oxygen vacancy sites become polarized by the reduced ions eventually enhancing ferromagnetic ordering. These mechanisms explain the very large enhancement of Ms in the narrow range between 8712



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38% and 40%. To elucidate, the FM in this stage arises from magnetic polarons that are constituted by oxygen vacancies and from the 90 degree superexchange under the Ce3+−O−Ce3+ framework. Third, as the concentration of Ce3+ is increased above 41%, the structure of the ceria is modified so greatly that Ms begins to decrease and finally disappears. The suppression is most likely caused by the formation of abundant oxygen vacancies and gives rise to the pair ion interaction that favors antiferromagnetism.

IV. CONCLUSION In this work, CeO2 NPs with various degrees of oxygen deficiency were obtained from a thermal decomposition method and then were annealed using various treatments. Our results reveal a quantitative relationship between the induced RTFM and VO. The FM of CeO2 NP is suggested to be intrinsically related to the concentration of Ce3+ on the Ce3+ surface. Ms increases with Is , but an excess of oxygen vacancies may eliminate FM. The details of the relaxation or reconstruction around the oxygen vacancies with various degrees of oxygen deficiency and the observed distribution of oxygen vacancies throughout the particles require further investigation.

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ACKNOWLEDGMENTS

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REFERENCES

This research is supported by the National Science Council of Taiwan, Republic of China, under the Contract No. NSC 982112-M-011-002-MY2 and NSC 100-2911-I-213-501-MY2.

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Concentration Dependence of Oxygen Vacancy on the Magnetism of

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