Enhancement of Ferromagnetism in CeO2 Nanoparticles by

Nov 29, 2012 - XMCD was measured at Dragon beamline 11A using the fluorescence yield mode with an alternating magnetic field of strength ±1 T applied...
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Enhancement of Ferromagnetism in CeO2 Nanoparticles by Nonmagnetic Cr3+ Doping Shih-Yun Chen,*,† Kong-Wei Fong,† Tung-Tse Peng,† Chung-Li Dong,‡ Alexandre Gloter,§ Der-Chung Yan,∥,⊥ Chi-Liang Chen,# Hong-Ji Lin,‡ and Chien-Te Chen‡ †

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan National Synchrotron Radiation Research Center, Hsinchu, Taiwan § Laboratoire de Physique des Solides, Université Paris Sud, CNRS UMR 8502, F-91405 Orsay, France ∥ Department of Physics, National Tsing Hua University, Hsinchu, Taiwan ⊥ Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu, Taiwan # Institute of Physics, Academia Sinica, Taipei, Taiwan ‡

ABSTRACT: This study presents the effect of Cr3+ on the formation of ferromagnetism in Cr-doped CeO2 nanoparticles (NPs). Systematic synchrotron X-ray spectroscopy analysis was utilized to investigate the defects in CeO2 NPs. X-ray absorption spectrum (XAS) revealed that the magnetic properties are correlated to the type and structure of defects. Both concentration of Ce3+ and the oxygen vacancy increased with increasing the content of Cr3+. The magnetism raises as the Cr increases and reaches maximum at Cr3+ ≈ 11% and, in turn, diminishes beyond this value. Interestingly, X-ray magnetic circular dichroism (XMCD) results indicate that ferromagnetism was contributed mainly by Ce3+ ions but not Cr3+. With comparing to the concentration dependence of Ce3+ in reduced undoped CeO2 nanoparticles, the relationships between the defects and magnetism were unraveled. The magnetism of CeO2 NPs was closely related to the oxygen deficiency. The major effect of doping Cr3+ on the formation of ferromagnetism in CeO2 nanoparticles is introducing the high degree of oxygen deficiency caused by the concomitant formation of Cr3+−VO−Ce3+ bondings. At last, it is suggested that the higher Ms value may be attributed to lattice distortion caused by the smaller size of Cr3+.

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suggested.12−14 According to earlier studies, both in experimental and in theoretical viewpoints, FM depends highly on the effect of defects, which can change markedly the band structure of host oxides.15,16 Derivative models describing the formation of FM in nanomaterials include defect-induced,17 core−shell,18 and charge transfer.19 Nonmagnetic elements were doped into oxides advisedly to investigate the defect-dependent FM in oxides. However, the results are conflicting. In ZnO doped with Cu, Thakur et al. supposed that Cu is in mixed valence state. Cu2+,3+ ions are magnetically polarized and then contribute to magnetism.20 Kumar et al., in contrast, found Cu ions exist in Cu2+ state and indicated that defect density increases with Cu doping.21 Herng et al. further predicted that a sufficient amount of both oxygen vacancies and Cu impurities is essential to FM.22 In oxides, it should be borne in mind that, experimentally, oxygen vacancies might easily be generated during growth processes. The concentration of surfactant, type of precursor, and doping of TM elements have all been found to result in a change of the

INTRODUCTION In the field of spintronics, the theoretical predictions of such materials, diluted magnetic semiconductor (DMS), operational at room temperature by Dietl et al. and Sato et al. triggered considerable studies.1,2 Numerous experimental results then confirmed the predictions. Room temperature ferromagnetism (RTFM) was observed in Co-doped, Fe-doped, and Ni-doped oxides, including ZnO,3 TiO2,4 SnO2,5 etc. The origin of ferromagnetism (FM) in these oxides were explained by the formation of bound magnetic polarons (BMPs), which include electrons locally trapped by oxygen vacancies, with the trapped electron occupying an orbital overlapping with the d shells of transition metal (TM) neighbors.6 However, doubts arose for most of the experimental results that the actual origin of FM might be the segregation of metallic clusters or the formation of ferromagnetic TM oxides.7 In addition to the doping effect, RTFM has also been observed in various pristine oxides, such as TiO2,8 SnO2,9 ZnO,10 and CeO2.11 Defects, such as oxygen vacancies in particular, were then suggested to play an important role in the magnetic origin for oxide DMSs. In nanoscaled oxides, the exchange interactions between unpaired spins resulting from surface oxygen vacancy (VO) as the origin of FM is also © 2012 American Chemical Society

Received: July 4, 2012 Revised: October 17, 2012 Published: November 29, 2012 26570

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degree of oxygen deficiency.23,24 In light of these discoveries, there has been an emerging consensus that defects in DMS systems play critical roles in inducing or mediating FM despite the lack of direct experimental accesses on the nature and positions of these defects. In this article, we present a detailed characterization of the magnetic properties of pristine and Cr-doped CeO2 nanoparticles (NPs) to investigate the role of Cr doping in tailoring FM. Unlike many other metallic transition elements, Cr and its main oxidation product Cr2O3 are antiferromagnetic and would not induce extrinsic FM even if Cr clustering occurred. Moreover, 3d3 high-spin configuration of trivalent Cr3+ ions was supposed to be in favor of generating magnetic moments in the host semiconductors.25 The structure and magnetism of CeO2 NPs were investigated systematically to clarify the effect of defects on the origin of FM in CeO2 NPs. Our experimental findings suggest that magnetism of oxides depends strongly on the defect structure, which can be tuned by the doping characteristics.

Figure 1. XRD results of CeO2 NPs with different concentration of Cr doping as well as the reference, JCPD 34-0394. Inset shows the concentration dependence of Cr on lattice parameter, a.

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The broadening of the peak can be attributed to both the size and doping effect. According to the Scherrer equation, the average particle size is estimated to range from 2 to 3 nm. Moreover, it is noted that the diffraction peak shifts toward a higher angle as the Cr content increases, revealing the change in lattice parameters. Such a change is quantified in the inset of Figure 1a, where the lattice parameter a decreases monotonically with increasing Cr content. The reduction in the lattice constant by doping can be attributed to the substitution of the smaller Cr3+ ions (0.65 Å) onto the Ce4+ (1.04 Å) lattice sites, which indicates that the Cr ions are incorporated into the CeO2 matrix. Figure 2 shows the scanning transmission electron microscope (STEM) high-angle annular dark -ield (HAADF) images

EXPERIMENTAL PROCEDURES The CeO2 NPs were prepared by precipitation method. Ce(NO3)3·6H2O and various content of Cr(NO3)3·6H2O were mixed with 80% EG/water under stirring at 600 rpm at room temperature. When the precursor was totally dissolved, NH4OH (3 mol/L) was added. The solution was kept at 60 °C for 21 h. The precipitates were separated by centrifugation at 6000 rpm for 15 min and then washed by using DI water and alcohol several times. After drying for 24 h, CeO2 NPs were obtained. The NPs were characterized by an X-ray diffractometer (XRD) with Cu Kα radiation and beamline 01C2 at the National Synchrotron Radiation Research Center (NSRRC), Taiwan. Particle size distribution, particle morphology, and crystal structure of the NPs were examined by a transmission electron microscope (TEM, FEI-Tecnai) equipped with a field emission gun. The magnetization was measured by using a superconducting quantum interference device magnetometer (MPMS2, Quantum Design). X-ray absorption measurements were carried out at the NSRRC. The X-ray absorption near-edge fine structure (XANES) measurements at Ce L3-edge and Cr K-edge were performed at Wigger beamline 17C at room temperature. The monochromator Si(111) crystals were used in Wiggler beamline 17C. The energy resolution at the Ce L3-edge (5723 eV) was about 0.4 eV. The XANES spectra at the O K-edge were recorded at the HSGM beamline 20A using total electron yield mode. The energy resolution was set to 0.2 eV. XMCD was measured at Dragon beamline 11A using the fluorescence yield mode with an alternating magnetic field of strength ±1 T applied. The monochromator resolving power was E/ΔE ≈ 10 000, and the extent of circular polarization of X-rays was ∼55%.

Figure 2. STEM/HAADF images of CeO2 NPs with different concentration of Cr doping: (a) 5% and (b) 11%. Scale bar = 5 nm.

of the CeO2 NPs doped with various content of Cr. Only CeO2 particles with crystallographic structures related to fluorite ceria have been observed, and there is not any observed secondary phase or cluster. The STEM-HAADF image shows that NPs exhibit very clear lattice fringes essentially dominated by the CeO2(111) orientation. At lower Cr concentration, each NP is a well crystalline faceted nanocrystal. The crystallinity will be degraded with increasing Cr content. Magnetic Measurements. The results of dc magnetization measured at a temperature of 300 K are shown in Figure 3. The linear component, such as paramagnetism or diamagnetism, has been subtracted from the data. All the samples show clear FM with small coercivities ranging from 80 to 120 Oe. The saturated magnetization (Ms) increases with increasing the

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RESULTS Microstructure Analysis. Figure 1a shows XRD patterns of Ce1−xCrxO2 nanoparticles with x = 0.01, 0.03, 0.05, 0.07, 0.09, 0.11, 0.15, and 0.2. For each sample, all the observed peaks can be indexed with the cubic fluorite structure of CeO2 (JCPDS 34-0394), which is consistent with the standard values for bulk CeO2. No characteristic peaks of impurities, such as other forms of cerium oxides, pure Ce, or Cr oxides, were observed within the detection sensitivity. It is also shown that peaks are gradually broadened as the Cr content is increased. 26571

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O K-edge. It should be noted that, when oxygen vacancy is formed, there are two electrons left behind and that these two electrons would migrate to the Ce site in order to remain charge neutral. As a result, tetravalent Ce (4f0) likely turns into trivalent Ce (4f1). Since the prepeak of O K-edge of CeO2 is originated from the strong hybridization between O 2p and Ce 4f orbitals, any change of the prepeak reflects the change of 4f occupancy, which may associate with the oxygen vacancy. The hybridization between cerium and oxygen was investigated by O K-edge, as plotted in Figure 4b. In stoichiometric CeO2, there are three main absorption peaks in O K-edge, located at 530.7, 533.2, and 537.7 eV, which are ascribed to electronic transitions from the O 1s core level into the empty O 2p hole states hybridized with the cerium-dominated 4f, 5d-eg, and 5dt2g levels, respectively. The most significant change in these spectra is that the intensity of the peak at 530.7 eV is reduced as the content of Cr is increased, as magnified in the inset of Figure 4. The decline of the peak at 530.7 eV indicates the degradation of the Ce 4f0 state, which means the tetravalent Ce is reduced to trivalent Ce by the doping of Cr. Therefore, the changes of O K-edge XAS herein imply that a higher concentration of oxygen vacancies was generated in the Crdoped CeO2 NPs than in the undoped ones. More detail will be provided in the later discussion. Ce L-edge. The valence of Ce was investigated by the Ce L3edge XANES. Figure 5a plotted the normalized XANES spectra of Ce L3-edge of CeO2 NPs doped with different concentration of Cr. The evolution of the spectral profile with Cr concentration is identified. The remarkable change is the shoulder-like feature at about 5727 eV in the undoped CeO2 NPs; as indicated by the arrow, it becomes predominant at high doping level. In order to estimate the charge state of Ce, a standard procedure was employed.26 All the spectra were subtracted by an arctangent function to exclude the edge jump and were then fitted with Gaussian functions, as the representative example of 3% Cr-doped CeO2 NPs shown in the bottom of Figure 5a. Component A splits into A1 and A2 owing to the crystal field splitting,27 which can be assigned to

Figure 3. Magnetic measurements performed at RT of CeO2 NPs with different concentration of Cr doping.

content of Cr doping, when doping concentration is smaller than 11%. With 11% Cr doping, CeO2 NPs exhibits the largest Ms, which is 0.0075 emu/g. However, as seen in the inset of Figure 3, Ms decreases with further increasing the dopant concentration x. XANES Analysis. Cr K-edge. As mentioned, the valence and the type of the dopant (in the form of oxides or metal cluster) in the material host have profound effect on the magnetic properties in this class of samples. Therefore, the valence of Cr was determined by the Cr K-edge XANES. Cr K-edge XANES spectra of CeO2 NPs with different content of Cr doping, along with the reference samples Cr2O3 and CrO3, are shown in Figure 4a. Notably, the energy of the absorption edge and overall spectral profile of the Cr-doped CeO2 NPs resemble those of Cr2O3 and differ significantly to that of CrO3, implying the charge state of chromium in Cr-doped CeO2 is likely to be Cr3+.

Figure 4. XANES of (a) Cr K-edge and (b) O K-edge of CeO2 NPs with different concentration of Cr doping. 26572

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Figure 5. (a) XANES of Ce L-edge of CeO2 NPs with different concentration of Cr doping. Inset shows the theoretical fit of XANES spectra of 3% Cr doped sample as a representative example. (b) Plots of the energy difference of the bands (ΔE(B,C)) and IC/Itotal in the XANES spectra of CeO2 NPs as a function of concentration of Cr doping.

the ionic Ce4+ configuration, i.e., to the transition from initial electron configuration 2p64f 0 (5d6s)4 to final configuration 2p54f 0(5d6s)5. Component B is attributed to a charge transfer Ce4+ configuration, i.e., mainly to transition from initial electron configuration 2p64f1L(5d6s)3 to final configuration 2p54f 1L(5d6s)4 where L stands for a hole in the anion ligand orbital. The strong contribution of the B peak in CeO2 suggests a strong covalent character in this material and gives rise to the great overlapped Ce 4f−O 2p orbitals.28 The strong hybridization between Ce 4f and O 2p is also evidenced by comparing the O K-edge and Ce L3-edge in which both absorption edges reflect the consistent information of Ce charge state. Component C is assigned to a Ce3+, that is strongly ionic character, and thus corresponds to a transition from initial electron configuration 2p64f 1(5d6s)3 to final configuration 2p54f1(5d6s)4. It should be noted that, whether the valence estimated from Ce L3-edge is equal to the actual chemical valence remains questionable. The origin of the L3-edge resonances of CeO2 is still under debate. However, so far, a great body of studies uses this idea to describe this one-to-one relationship. At least it gives the tendency of the valence variation. Therefore, the approach employed herein to extract the amount of Ce3+ is acceptable and convincible. Moreover, it is worth noting that a novel X-ray spectroscopy at the third generation synchrotron source, i.e., resonant inelastic X-ray scattering (RIXS), is a promising tool to investigate the Ce valence state. Sham et al.29 and Kotani et al.30 have reported that, by tuning the excitation X-ray photon energy properly, the RIXS signal from a specific configuration (Ce3+ or Ce4+) can be greatly enhanced. The change of concentration of Ce3+ and covalency upon the Cr doping can be investigated by the relationship between each transition (components A, B, and C), as shown in Figure 5b. First, the concentration of Ce3+, evaluated by Ic/Itotal, is enhanced with increasing the content of Cr. The Ic and Itotal

refer to the intensity of deconvoluted peak C and the sum of deconvoluted peaks A, B, and C. It is about 7% in undoped CeO2 NPs and reaches 18% with doping 20% of Cr. These extra Ce3+ introduced by doping Cr should have an effect on the change in lattice parameter shown in Figure 1. With 11% Cr doping, the change in lattice parameter in the CeO2 NPs is about 0.02 Å, much less than that in CeO2 NPs doping with 10% Yb3+ and Lu3+, namely, 0.1 Å and 0.12 Å, respectively.31 Since the ionic size of Cr3+ is smaller than that of Yb3+ and Lu3+, the change in lattice parameter in Cr-doped CeO2 is unusual. However, the size of Ce3+ is 1.1 Å, larger than that of both Cr3+ and Ce4+. Therefore, the reduction in lattice parameter caused by the Cr3+ doping might be compensated by the induction of Ce3+, as indicated in undoped CeO2 NPs.32 Second, the energy difference between component B and C (ΔE(B−C)) decreases with increasing the concentration of Cr. This implies a lower covalency between Ce and O ions33 and is supported by the results from the O K-edge spectra. Recently, it was found in undoped CeO2 NPs that the covalency strongly depends on the oxygen deficiency as well as particle size.23,24,26 It is noted that the size of NPs in this study is similar to those in the literature (2−2.3 nm). However, with the same concentration of Ce3+, the covalency is less in the Cr-doped ones. For example, CeO2 NPs doped with 11% of Cr and CeO2 NPs prepared by thermal decomposition with 0.64 mM surfactant have a similar value of ΔE (3.4 eV), yet the concentration of Ce3+ is 10% for the former and it is 23% for the later. As a consequence, in the Cr-doped NPs, the hybridization between Ce and O ions is not only altered by Ce3+, but also by the presence of Cr3+.

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DISCUSSION Above experimental results demonstrate that doping of Cr3+ in CeO2 NPs results in an increase of Ce3+, a lower covalency, and an enhancement of saturated magnetic moment. XMCD is then 26573

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this peak indicates that the concentration of Ce3+ is increased. The concentration of Ce3+ of these undoped NPs ranges from 7% (sample i) to 19% (sample vi). The correlations between FM and the concentration of Ce3+ of the Cr-doped and undoped CeO2 NPs was then plotted together in Figure 8. It is seen that Ms is strongly dependent on

utilized to unravel whether the origin of FM should be attributed to the Ce3+ or the Cr3+. XMCD results reveal that, while there is no magnetic moment on the Cr site, there is a net magnetic moment on the Ce site (Figure 6). In other words, the presence of Ce3+ and the oxygen defect are essential for the occurrence of FM in Cr-doped CeO2 NPs.

Figure 6. XMCD spectra of Ce M4,5- and Cr L2,3-edges.

Interestingly, although the dopant Cr3+ does not contribute to FM directly, as indicated by XMCD result, it can enhance the magnetism, as shown in Figure 3. To look into the effect of Cr3+ doping on the formation of FM, the concentration dependence of Ce3+ on Ms among CeO2 NPs with and without Cr3+ doping were compared. All these CeO2 NPs were prepared by using the same synthesis method. The concentration of Ce3+ of undoped CeO2 NPs is altered either by changing the pH values from 11.7 to 12.8 of the solution or by annealing in a reducing atmosphere at 300 °C for 1 to 5 h. XRD results of these NPs and the assigned sample numbers were shown in Figure 7a, all the CeO2 NPs exhibit the same crystal structure. The corresponding Ce L3-edges XANES of these NPs are plotted in Figure 7b. The peak around 5727 eV gains intensity with respect to the peak around 5731 eV by increasing the pH value or the annealing time. The growth of

Figure 8. (a) Room temperature saturation magnetizations of the Crdoped and annealed CeO2 NPs versus their corresponding levels of Ce3+. Different concentration dependence of Ce3+ on Ms was correlated to the various distributions of defects caused by Cr3+. (b,c) Distribution of defects on the (111) plane of undoped and Crdoped samples, respectively. Notably, the amount of Ce3+ is the same between these two figures. It is shown that, with the existence of Cr3+, the number of clusters large enough to result in magnetism is higher in Cr-doped CeO2.

the concentration of Ce3+ in both systems. The Ms values of both systems are enhanced as the Ce3+ is increased and then declined when Ce3+ exceeds certain values of Ce3+. Notably, the concentration dependence of Ce3+ on Ms is more significant in Cr-doped CeO2 NPs than in undoped ones. In other words, similar M s value can be obtained by different Ce 3+ concentration in Cr-doped and undoped NPs. For example, while the Ms value of 0.0025 emu/g is obtained in a lower Ce3+ concentration (7.5%) in Cr-doped NPs, the same magnitude of Ms can be obtained in a higher Ce3+ concentration (13%) in undoped ones. In undoped CeO2 material, the concentration dependence of Ce3+ on Ms has been proposed by several groups.24,34,35 An unpaired spin in the Ce f orbitals is generated when the valence

Figure 7. (a) XRD and (b) normalized Ce L-edge results of undoped CeO2 NPs prepared by altering the synthesis parameters of the precipitation method. 26574

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the effect of Cr3+ doping reduces the distance between magnetic Ce3+ by increasing the density of defects, which could promote the formation of magnetism. Moreover, additional contributions on the formation of FM were owed to the much smaller ion size of Cr3+ than that of Ce4+.

of Ce changes from +4 to +3, and thus, the magnetism is induced, but it should be borne in mind that not all the Ce3+ contributes to the magnetism. Recently, Ackland et al. suggested that only about 0.1−0.5% of the volume of the CeO2 samples are magnetic according to the anhysteretic magnetization curves.36 Herng et al. also predicted that sufficient amount of both oxygen vacancies and Cu impurities is essential to the observed FM in the Cu-doped ZnO system.23 Furthermore, our recent systematic studies implied that the arrangement of defects is important in the formation of FM in undoped CeO2 NPs.33 Accordingly, the effect of Cr3+ doping on the magnetism in CeO2 NPs can be explained as follows. Since the doping of Cr3+ induces the oxygen vacancies and, in turn, the formation of Ce3+, reducing the hybridization between Ce 4f and O 2p orbitals, defects induced by Cr3+ are likely to be in the form of Ce3+−VO−Cr3+ rather than Cr3+−VO−Cr3+. Then, Ce3+−VO− Cr3+ and Ce3+−VO−Ce3+ coexist in the Cr-doped CeO2 NPs, while in the undoped CeO2 NPs, there is only Ce3+−VO−Ce3+. Thus, it is expected that, with the same concentration of Ce3+, distance between Ce3+ will be reduced in the Cr-doped NPs owing to the higher density of defects, as shown in the inset of Figure 8. Notably, XMCD results indicate that, in Cr-doped CeO 2 NPs, only Ce 3+ contributes to the magnetism. Consequently, the faster aggregation of Ce3+ by the formation of Ce3+−VO−Cr3+ could promote the formation of FM. Therefore, in Cr-doped CeO2 NPs, less Ce3+ concentration can produce the same magnetism as in undoped ones with higher Ce3+ concentration. With further increasing the Cr3+, the more rapid suppression of Ms than that in undoped ones is attributed to the formation of abundant oxygen vacancies, which give rise to the pair ion interaction that favors antiferromagnetism.37 Another effect on the magnetism by the Cr3+ doping may not be ignored, that is, the distortion of crystal structure caused by the different size of dopant Cr3+ and host Ce4+ or Ce3+. In recent years, in addition to the oxygen vacancy, the effects of crystal relaxation or reconstruction caused by the high concentration of defect or the different size of dopant on the formation of magnetism have been proposed. A direct structure−magnetism relationship has been suggested in the Sn1−xMxO2 system, where M is Cr, Fe, and Co.38 The occurrence of spin polarization promoted by the change in crystal structure was also proposed in a highly oxygen deficient CeO2 system.39 Direct spectroscopic evidence of electronic mixing enhanced under physical pressure has been reported by Narcizo et al. They predicted that FM exchange interactions may be enhanced through strain or chemical substitutions.40 Thus, the distortion introduced by doping may also contribute to the enhancement of average magnetic moment on Ce ions. Related experiments including XMCD and high-pressure XANES are in progress.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +886 227376517. Fax: +886 227376544. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the National Science Council of Taiwan, Republic of China, under the Contract No. NSC 1002112-M-011-002-MY3 and No. NSC 100-2911-I-213-501MY2.

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REFERENCES

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CONCLUSIONS Crystallized and uniform CeO2 NPs with different concentration of Cr were synthesized by using precipitation method. The charge state of the dopant was determined as Cr3+. XANES analysis revealed that, as the content of Cr increases, the concentration of Ce3+ increases, while the hybridization between Ce and O ions depressed. The structure of defect induced by Cr3+ was supposed to be Ce3+−VO−Cr3+. Together with the magnetic measurement results, XMCD analysis, and Ms dependence on the concentration of Ce3+, it is implied that 26575

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp3066376 | J. Phys. Chem. C 2012, 116, 26570−26576