Oxygen Vacancy Dependent Magnetism of CeO2 Nanoparticles

Nov 3, 2010 - In some cases, it was demonstrated that the FM is due to the segregation of ..... of the oxygen vacancies are localized in the first thr...
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Oxygen Vacancy Dependent Magnetism of CeO2 Nanoparticles Prepared by Thermal Decomposition Method Shih-Yun Chen,*,† Yi-Hsing Lu,† Tzu-Wen Huang,‡ Der-Chung Yan,§ and Chung-Li Dong| Department of Materials Science and Engineering, National Taiwan UniVersity of Science and Technology, Taipei, Taiwan, Institute of Physics, Academia Sinica, Taipei, Taiwan, Department of Physics, National Tsing Hua UniVersity, Hsinchu, Taiwan, and National Synchrotron Radiation Research Center, Hsinchu, Taiwan ReceiVed: May 18, 2010; ReVised Manuscript ReceiVed: September 7, 2010

This study reports that highly oxygen-deficient CeO2 nanoparticles (NPs) can be obtained without reduction treatment by using thermal decomposition method. Different amounts of surfactants are used to control the size of the NPs. The X-ray absorption near-edge spectra (XANES) indicate that the concentration of Ce3+ is higher than 20% for all NPs. It is also found that most Ce3+ locates at the surface. Magnetic-measurement results show that room-temperature ferromagnetism (FM) of the CeO2 is closely related to the concentration of Ce3+ at the surface (IsCe3+). Saturation magnetization (Ms) reaches the maximum value with an IsCe3+ of about 40%; however, Ms decreases when IsCe3+ is raised further. The highest Ms in this study is obtained from the sample without surfactant (Ms ) 0.12 emu/g). This is comparable with the results in other reports in which the CeO2 NPs were subjected to the reduction treatment. Notably, NPs become paramagnetic when IsCe3+ reaches 48%. This study suggests that oxygen vacancy is essential for the formation of FM in CeO2 NPs. However, FM will be suppressed with excess oxygen deficiency. The effect of surfactant on the growth and the stoichiometry of the CeO2 particles will also be discussed in this report. I. Introduction In the field of spintronics, it is essential to develop semiconductors with ferromagnetically polarized carriers at room temperature (RT) such that the spin as well as the charge of the carriers can be coupled with an external magnetic field to control devices. The theoretical predictions of such materials, diluted magnetic semiconductor (DMS), operate at RT by Dietl et al. and Sato et al. triggered considerable studies.1,2 For most of the experimental results, doubts arose about the real origin of ferromagnetism (FM). In some cases, it was demonstrated that the FM is due to the segregation of metallic clusters.3,4 In addition to the magnetic doping effect, oxygen vacancies have been proposed to play an important role in the magnetic origin for oxide DMSs.5 For example, theoretical studies suggest that oxygen vacancies can cause a marked change of the band structure of host oxides and make a significant contribution to the FM.6,7 The formation of bound magnetic polarons (BMPs) has also been proposed to explain the origin of RT FM for some insulating oxide DMSs.8 Electrons locally trapped by oxygen vacancies occupy an orbital overlapping with the d shells of transition-metal (TM) neighbors. Moreover, some of the recent reports mentioned that, without any magnetic element doping, oxides become magnetic when the size of their crystallites is at nanometer scales. Sundaresan et al. attributed the FM of CeO2 nanoparticles (NPs) to the exchange interactions between unpaired spins resulting from surface oxygen vacancy (VO).9 Ge et al. reported FM in 5.3 nm CeO2 NPs, and their firstprinciple calculations revealed that VO, especially at the surface, can induce magnetic moments for CeO2 NPs.10 However, some reports doubted the effect of oxygen vacancy on the formation * Corresponding author. E-mail: [email protected]. † National Taiwan University of Science and Technology. ‡ Academia Sinica. § National Tsing Hua University. | National Synchrotron Radiation Research Center.

of FM. For example, Liu et al. observed FM only in sub 20 nm CeO2 powders and argued that FM might not be linked to VO.11 Recently, Li et al. further suggested that FM is not related to the surface oxygen vacancy but to the Ce3+/Ce4+ pairs.12 On the basis of these experimental results, various models describing the formation of FM in nanomaterials have been proposed, for example, defect-induced mechanism,13,14 coreshell,15 and charge transfer.16 However, until now, a clear connection between magnetic properties and electronic structures has not been established. Besides, the understanding of the degree of oxygen deficiency of NPs with different sizes or synthesized by various methods is still lacking. It should be bear in mind that, experimentally, oxygen vacancies might easily be generated during growth processes. Particles with different crystallize sizes are accompanied with varied concentrations of vacancies. Many controversial results of oxide DMSs could possibly be related to the variation of their oxygen vacancies.17 The thermal decomposition method is one of the chemical methods that can synthesize well-dispersed NPs with good crystallization.18,19 Importantly, the size distribution obtained by using this method is narrow. Therefore, the inconsistence coming from the wide size distribution can be excluded. In this paper, we present systematic studies including the microstructure, magnetic properties, and absorption spectra of CeO2 NPs prepared by the thermal decomposition method. To control the particle size, surfactants with different concentrations were added: (1) 0 mM, (2) 0.32 mM, (3) 0.96 mM, and (4) 1.6 mM. Our experimental results indicate that the concentration of Ce3+ on the surface could be responsible for the observed RTFM in CeO2 NPs. Furthermore, the concentration dependence of oxygen vacancy on the magnetism is also discussed. Apart from the fundamental physics behind the FM of this system, we also demonstrate that, among the various processing techniques used to prepare cerium oxide NPs, the thermal decomposition method

10.1021/jp1045172  2010 American Chemical Society Published on Web 11/03/2010

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Figure 1. XRD results of CeO2 NPs with different concentration of surfactant: 0, 0.32, 0.96, and 1.6 mM. The black lines represent the pattern of bulk CeO2 (JCPDS 34-0394) with major reflections assigned.

is an attractive method to obtain highly oxygen-deficient NPs without substantial reduction treatments. II. Experimental Procedures A mixed solution was first made by loading cerium(III) acetyl acetonate hydrate (0.5 mM), 1,2-dodecandiol (10 mM) as stabilizer, and 10 mL octyl-ether into a three-necked roundbottom flask and stirring by using a magnetic stirring bar, sequentially. All of precursor was solved at 100 °C, and then, the different concentrations of 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 the resultant solution. Finally, the precipitates were separated by centrifugation at 6 000 rpm for 15 min. The final product was collected and dispersed in the nonpolar organic solvent. The NPs were characterized by X-ray diffractmeter (XRD) with Cu KR radiation. Particle size distribution, particle morphology, and crystal structure of the NPs were examined by transmission electron microscope (TEM, FEITecnai) equipped with a field emission gun. The magnetization was measured at RT by using a superconducting quantum interference device magnetometer (MPMSXL, Quantum Design). X-ray absorption measurements were carried out at the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The X-ray absorption near-edge fine structure (XANES) measurements at Ce L3-edge were performed at Wigger beamline 17C by using the transmission mode at RT. The monochromator Si (111) crystals were used in Wiggler beamline 17C with an energy resolution ∆E/E better than 2 × 104. The energy resolution at Ce L3-edge (5723 eV) was about 0.3-0.4 eV. The XANES spectra at Ce M4,5- and O K-edge were recorded at HSGM beamline 20A by using the total electron-yield mode. The energy resolution was set to 0.2 eV. III. Results and Discussions Microstructure Results. The powder X-ray diffraction (XRD) patterns for CeO2 NPs are shown in Figure 1. All the samples can be identified as the cubic fluorite structure with space group Fm31m. The XRD patterns reveal that the cerium oxide prepared by the thermal decomposition method retains the nanocrystalline phase, even when no surfactants were added.

Figure 2. HRTEM images of CeO2 NPs with different concentrations of surfactant (a) 0 mM, (b) 0.32 mM, (c)0.96 mM, (d) 1.6 mM.

It is also shown that, with increasing the surfactant concentration from 0.32 to 1.6 mM, the broadening of the XRD peaks gradually decreases. According to the Scherrer equation,20 the average particle size estimated by the full width at half maximum of XRD peaks is 3.6, 2.2, 2.6, and 3.8 nm for 0, 0.32, 0.96, and 1.6 mM NPs, respectively. Figure 2 shows the high-resolution transmission electron microscopy (HRTEM) images of the CeO2 NPs prepared with various concentrations of surfactants. It is observed clearly that these particles are crystalline. As indicated in Figure 2b, the dominating fringes of NPs are (111). In addition, the TEM images reveal that all the particles have a truncated cubic shape, which is different from the cubic ones synthesized by other methods.12 It is also found that, with the addition of surfactant, the size distribution is narrow and the degree of aggregation is low. The particle size according to the TEM images is 3.4 ( 1.1, 2.7 ( 0.5, 3 ( 0.8, and 3.3 ( 0.5 nm for 0, 0.32, 0.96, and 1.6 mM NPs, respectively. The average particle sizes found in the micrographs are of the same order as those obtained from XRD line-broadening analyses. XANES Analysis. O K-Edge. The hybridization between cerium and oxygen was investigated by O K-edge. The O K-edge features of TM oxides are very sensitive to the chemical environment around the X-ray absorbing atom and are strongly dependent on the oxidation state. Figure 3 displays the O K-edge which probes the O 2p mixed with Ce conduction states. For the stoichmetric CeO2, there are three peaks which locate at 530, 532.5, and 537 eV. These peaks are related to electronic transitions from the O 1s core levels into the empty O 2p hole states hybridized with the cerium-dominated 4f, 5d-eg, and 5dt2g levels, respectively. However, it is seen that, for all samples, the peak at 530 eV is weak, particularly for the 0.32 mM sample. The decreased intensity of the 530 eV peak compared with the CeO2 reflects that the weakening of the Ce 4f0 state results from Ce4+ being reduced to Ce3+ in the nanoscale CeO2. In addition, the two principal peaks shift slightly to 532 and 537.5 eV. These features imply that all NPs of the present study are highly oxygen deficient, particularly the 0.32 mM NPs. Ce M-Edge. The valence of Ce ions can be determined by the Ce M-edge. As shown in Figure 4, two main peaks (M4

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Figure 3. XANES of O K-edge of CeO2 NPs with different concentrations of surfactant.

Figure 4. XANES of Ce M-edge of CeO2 NPs with different concentrations of surfactant.

and M5) with satellites on the high-energy side (Y and Y’) are observed clearly. The main M4 and M5 features of CeO2 are related to the transitions from an atomic-like f0 ground-state configuration.21 Peaks Y and Y’ originate from the transitions to 4f states in the conduction band.22 It is observed that the main peaks M4 and M5 shift to the low photo energy and that shoulder structures appear at the low-energy sides, implying the increases of the Ce3+ contribution. Also, the NPs exhibit a mixture of Ce3+ and Ce4+ configurations in the ground states. For the mixed valence materials, the tendency of the valence and the relative changes can be determined from M-edge.23,24 It then can be estimated that the concentration of Ce3+ is higher than 21%. This is much higher than the values reported.12,25 Moreover, when considering that the relative intensity between the main peaks and the satellite varies, as shown in the inset, the highest value of IY’/IM5 is found in 0 mM NPs, whereas the lowest is in the 0.32 mM ones. The varied intensities of Y and Y’ originate from the delocalization of the f-electrons.26 The higher intensity ratio of IY’/IM5 indicates the stronger hybridization between O 2p and Ce 4f orbitals. These results are coincident with the O K-analysis. Ce L-Egde. The spectral profiles of Ce M-edge are affected slightly by the crystal field and other bonding effects.27 The valence estimation of the Ce-based compounds from Ce L3-

Chen et al.

Figure 5. XANES of Ce L-edge of CeO2 NPs with different concentrations of surfactant. The inset shows the theoretical fit of XANES spectra of 0 mM sample as a representative example.

edge have been established. Therefore, the electronic structure of Ce is further investigated by Ce L-edge which can offer complementary information to M-edges. Normalized XANES spectra of Ce L3-edge for CeO2 NPs with different concentrations of the surfactants are plotted in Figure 5. It is seen that the change in the near -edge feature is related to the concentration of surfactant. To further describe the transition of states, all the spectra were subtracted by an arctangent function to exclude the edge jump and were then fitted with Gaussian functions,28–30 like the representative example in the inset. The method to subtract background by using arctangent function is described in ref 25. Component A arises from a core excited Ce4+ final state with the configuration 2p4 f0 5d*, where 2p denotes a hole in the 2p shell and 5d* refers to the excited electron in the 5d state. The split of component A into two sub peak A1 and A2 was caused by the crystal-field splitting. Component B is attributed mainly to a 2p4 f1 5d* state, where the asterix stands for a hole in the anion ligand orbital. Component C represents a Ce3+. As for the feature D, it is due to the dipole-forbidden 2p-to-4f transition.31–33 The changes induced by the surfactant concentrations on each transition (components A, B, and C) as well as the particle size are plotted in Figure 6. Both the energy difference between component B and C, ∆E(B-C), and the relative intensity ratios of component B to component A, IB/IA, are clearly sizedependent. It can be observed that the larger particle size, the higher the values of ∆E(B-C) and IB/IA. The higher ∆E(B-C) and IB/IA indicate a higher covalency of CeO2 caused by the larger particle size. However, it is also noted that the values of both ∆E(B-C) and IB/IA in our results are smaller than those reported by Chen et al.25 The comparison is made under similar particle-size conditions. The above results implies that the covalency of our NPs is less than those in other reports. As for the variation of the intensity of component C, it is expressed by the ratio, IC/ITotal. This ratio can be an indication of the concentrations of Ce3+ impurity in a CeO2 matrix. It is seen that the concentration of Ce3+ of the NPs prepared by the thermal decomposition method is much higher than those prepared by other methods.10 The estimated value is higher than 20% for all NPs. This is consistent with O K-edge spectra; the NPs are highly oxygen-deficient. According to the above microstructural analysis, it is found that the particle size and degree of oxygen deficiency of NPs

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Figure 7. Magnetic measurements performed at RT of CeO2 NPs with different concentrations of surfactant.

Figure 6. Plots of the particle size, energy difference of the bands (∆E(B,C)), and intensity ratios of the bands (IB/IA) and IC/Itotal in the XANES spectra of CeO2 NPs as a function of the concentration of surfactant.

do not vary monotonically with the concentration of surfactant. Both the concentration dependence of the feature and the stoichiometry of NPs can be explained by the process of the thermal decomposition method as follows. During the thermal decomposition method, the surfactant molecules act as spatial separation between nuclei. These spatial separations will prevent the nuclei from aggregation and then result in the small particle size as well as the narrow size distribution. This is consistent with our observations that both the particle size and the size distribution of NPs with surfactants are smaller than those without surfactants. However, as also shown in our observations, the particle size does not decrease monotonously with increasing the concentration of surfactant. This can be attributed to the enhanced disorder when increasing the surfactant, which may give rise to a reduced nucleation rate and therefore a larger particle size. In addition to the variation of particle sizes, it should be bear in mind that, during the thermal decomposition process, the oxygen atoms support the substantial oxidization, and growth comes from the solution. Thus, it is anticipated that both the oxidation and growth will be slowed down when there are spatial separations in the solution. As a consequence, the highest degree of oxidation of NPs will be the 0 mM NPs because there is no separation. With the addition of 0.32 mM surfactant, the diffusion of oxygen atoms was suppressed by the surfactants which thus results in the high degree of oxygen deficiency. With further increasing the concentration to 0.96 mM and even 1.6 mM, as mentioned in the previous section, the coarsened grain size decreases the degree of oxygen deficiency. Magnetic Measurements. The results of dc magnetization measurements performed at RT are shown in Figure 7. The diamagnetic component has been subtracted from the data.

RTFM could be observed for all NPs except for the 0.32 mM sample. The 0.32 mM sample is paramagnetic, as shown in the inset. The saturation magnetization (Ms) is largest for the 0 mM sample (0.12 emu/g), which is almost ten times larger than the others (0.012 emu/g for 0.96 mM and 0.015 emu/g for 1.6 mM NPs). It is noted that, even for the later two samples, the values of Ms are still comparable to those in previous reports in which the CeO2 NPs were subjected to the reduction treatment.10 According to the XANES results, it is found that the smaller particle size, the less covalency as well as the higher concentration of Ce3+ will be obtained, which implies that the degree of oxygen deficiency is proportional to the inverse of the particle size. Figure 8a shows the size dependence of Ms, however, it is seen that Ms is enhanced when increasing the particle size. This varies from the prediction that Ms will be enhanced with increasing the surface fraction.9,34 Another discrepancy could be found in the 0.32 mM NPs. The concentration of Ce3+ is 24% in the 0.32 mM NPs, which is less than the value of 27.8% reported by Li et al.12 However, the 0.32 mM NPs in the present study is paramagnetic whereas the NPs show FM in the report of Li. et al. Their results were calculated on the basis of XPS, which is more sensitive to surface, whereas ours were based on XAS, where it is well accepted that the information comes from the overall region. This might imply that the distribution of Ce3+ is inhomogeneous throughout the particle. Moreover, theoretical calculations of cerium oxides have pointed out that the O vacancies formed on the (110) and (111) surfaces are more steady than those in the bulk.35 Yang et al. further suggested that the additional charges from the formation of the oxygen vacancies are localized in the first three layers of the surface.36 We thus suppose a core-shell-like structure, of which the core is stoichiometric CeO2 and the shell is CeO2-x. We assume that the thickness of the surface shell is one monolayer, which is 3.4 Å. Figure 8b compares the size dependence of the surface-to-bulk ratio as well as the concentration of Ce3+ (IaCe3+). It is seen clearly that the variation trend of these two are almost coincident, which implies that Ce3+ tends to locate at the surface. The concentration of Ce3+ at the surface (IsCe3+) can then be obtained on the basis of the content of oxygen. The relationship between IsCe3+ and Ms is plotted in Figure 8c. Importantly, it is found that (1) Ms increases with IsCe3+ and (2) when IsCe3+ is around 43%, Ms increases drastically and reaches a maximum, but after that, it soon decreases. However, a further increase of IsCe3+ to about 48% results in the disappearance of FM.

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Chen et al. vacancy sites.40 They supposed that these electrons remaining on the oxygen vacancy sites will be polarized by the reduced ions, eventually leading to the enhancement of ferromagnetic ordering. Their hypothesis tallies with our observations; that is, when increasing the concentration of Ce3+ within a narrow range, the values of Ms change dramatically over an order of magnitude. When combining the above results with the analysis of Ce3+ concentration and crystallization from the TEM and XANES, the relationship between magnetic properties and the concentration of Ce3+ (the degree of oxygen deficiency) can be built up as follows. (1) Low concentration of Ce3+ (or low degree of oxygen deficiency): the existence of Ce3+ gives rise to the net spin and thus induces magnetism. The spin is coupled by superexchange model. (2) High concentration of Ce3+ (or high degree of oxygen deficiency): the relaxation of crystal is noticeable, which enhances the electron density to remain at the oxygen vacancy sites because of the delocalization of f electrons and thus promotes the polarization of electrons. (3) When further increasing the concentration of Ce3+, the change of crystal structure results in the decrease of the delocalization of f electrons. In other words, the excess of oxygen vacancies results in the absence of media to transfer the spin between two cerium ions; therefore, the NPs become paramagnetic. Consequently, this study predicts that highly oxygen-deficient CeO2 NPs can be obtained by using the thermal decomposition method without reduction treatment. The degree of oxygen deficiency can be controlled by the concentration of surfactant. Moreover, our analysis suggests that the FM of CeO2 NPs is intrinsically related to the concentration of Ce3+ on the surface. Ms increases with IsCe3+. However, excess oxygen vacancies may cause the disappearance of FM. The detailed relaxation or reconstruction around the oxygen vacancy with various degrees of oxygen deficiency and the observations of the distribution of oxygen vacancy through the particle are in need of further investigations. IV. Conclusions

Figure 8. (a) Size dependence of Ms, (b) size dependence of the surface-to-bulk ratio as well as the concentration of Ce3+, and (c) relationship between IsCe3+ and Ms.

The origin of magnetism in CeO2 is supposed to result from the reduction of valence in Ce ions. It is well-known that the stoichiometrical CeO2 is paramagnetic. When the valence of Ce changes from +4 to +3, an unpaired spin in the Ce f orbitals is generated, and thus, magnetism is induced. In insulating oxides, the electronic spins of cations are generally coupled by the superexchange interactions between neighboring magnetic ions via an anion. Some works have been done to study the magnetic properties of CeO2 with various degrees of oxygen deficiency.37,38 However, most of those studies focus on samples of low oxygen deficiency. The relaxation or reconstruction of materials with high concentration of defect might be different. Narcizo et al. has predicted that FM exchange interactions may be enhanced through strain or chemical substitutions.39 They have shown direct spectroscopic evidence of electronic mixing enhanced under physical pressure. Very recently, Han et al. suggested that, in CeO2 materials with high concentration of oxygen vacancies, the localized states induced by oxygen removal not only contain Ce 4f electrons but also receive substantial contribution from the electrons remaining on the

Crystallized and uniform-sized CeO2 NPs were synthesized by using the thermal decomposition method with different surfactant concentrations. The variation of features and structures of NPs are both explained by the process of thermal decomposition. The relationships between particle size, degree of oxygen deficiency, and magnetism were revealed. The observations point out that the magnetism of NPs was closely related to the concentration of Ce3+ at the surface. The high Ms value was attributed to the combined contributions from superexchange and polarization caused by the heavy oxygen-vacancy doping. In addition, according to the XANES and magnetic measurement results, it is proposed that Mwhen increases when increasing the concentration of Ce3+ on the surface. However, FM disappears with excess Ce3+. Synopsis Highly oxygen-deficient CeO2 nanoparticles are synthesized by the thermal decomposition method without reduction treatment. RT FM of the CeO2 nanoparticles is demonstrated and has a close relationaship with the concentration of Ce3+ which mostly locates at the surface. However, RT FM disappears with excess oxygen deficiency. Acknowledgment. This research is supported by the National Science Council of Taiwan, Republic of China, under the

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