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A Study of Defect Structure in Ferromagnetic Nanocrystalline CeO: Effect of Ionic Radius 2

William Lee, Shih-Yun Chen, Eric N Tseng, Alexandre Gloter, and Chi Liang Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02817 • Publication Date (Web): 21 Jun 2016 Downloaded from http://pubs.acs.org on June 25, 2016

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The Journal of Physical Chemistry

A Study of Defect Structure in Ferromagnetic Nanocrystalline CeO2: Effect of Ionic Radius William Lee1, Shih-Yun Chen1,*, Eric Tseng1, Alexandre Gloter2, Chi-Liang Chen3 1

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan 2 Laboratoire de Physique des Solides, Université Paris Sud 11, CNRS UMR 8502, F-91405 Orsay, France 3 National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan To whom correspondence should be addressed. E-mail : *[email protected], telephone number : +886-227376517

Abstract In this study, the relationship between defect structure and magnetic behavior of La-doped CeO2 nanoparticles (NPs) was investigated systematically. The doping level ranges from 0% to 15%. X-ray Absorption Spectroscopy (XAS) and Raman spectroscopy were utilized to investigate the electronic structure of these NPs. It is demonstrated that oxygen vacancy was increased upon doping with La. The major oxygen vacancy defect structure was M3+- VO- M3+ (M: Ce or La) in lightly doped NPs while it changes to La3+- VO- La3+ as the doping level reaching 7%. Scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM/EELS) analysis showed that in La-doped NPs, both the dopant (La3+) and Ce3+ distributed rather homogenously within the NPs, which is different to other doped ceria including Y, Sm and Cr doped ones where a strong interaction between surface, trivalent cerium and dopant were reported. The distinct distribution of defect was attributed to the larger ion radius of La and the nature of La related oxygen vacancy. Moreover, room temperature ferromagnetism (FM) is observed in these La doped ceria but with a weaker intensity when compared to the magnetism obtained for other doped ceria NPs with similar dopant concentration. It indicates that high concentration of defects and dopant at surface is critical to obtain larger FM.

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I. Introduction Since Matsumoto et al. and Dietl et al. theoretically predicted that room temperature ferromagnetism (RTFM) can be observed in semiconductors, such as ZnO and GaN, the field of diluted magnetic semiconductor (DMS) has been widely studied1-2. Notably, oxides, such as TiO2, SnO2, CeO2, etc. doped with transition metal were also found to possess RTFM in recent years3-5. It is believed that the origin of ferromagnetism in these metal oxides is contributed from defects, and in particular oxygen vacancies were suggested to play an important role in the magnetic origin for oxide DMSs. Many theoretical efforts have been done on the oxygen vacancy formation mechanism and its influence on the magnetism. Coey et al. build F-center exchange mechanism (FCE) model6, the origin of FM is related to the presence of oxygen vacancies that trap one electron and provide FM ordering through FCE7-8. Shah et al. confirmed that different F centers such as F0, F+ and F2+ exist in CeO2, but only the F+ centers can contribute to ferromagnetism9. Among oxides which have been reported to exhibit RTFM, ceria (CeO2) is one of the proper systems to investigate oxygen vacancy related magnetism, since an interesting property of CeO2 is that it can have a stable structure far from the stoichiometric proportions of oxygen.10 This large storage capacity has been used widely in catalytic applications and solid-oxide fuel cells (SOFC). The relaxation or reconstruction of materials with defects at such a high concentration is indeed unusual. In addition, with doping trivalent cations into ceria, some of the Ce4+ ions will be replaced by dopant cations to reduce partially Ce4+ to Ce3+, which give rise to oxygen vacancies11. Studies emphasized the role of oxygen vacancies play in the formation of ferromagnetism in CeO2 rises in these recent years12-13. On the other hand, the interactions between dopants and oxygen vacancies have also been stressed recently. Some researchers showed the trapping effect of oxygen vacancies is due to local structure, such as local distortion or strain owing to the size mismatch between Ce4+ ions and the dopants. Wei et al. calculated the association energy of defect structures in various ionic radius dopants doped CeO2, a clear preference for small dopants attracting an oxygen vacancy, while strong association occurring between Ce4+ ions and oxygen vacancy for large dopants14. Yoshida et al. demonstrated that the migration energy of Y, La and Sm doped CeO2 is related to ionic radius and suggested a relationship between the ionic conductivity and lattice deformation15. Those results implied that as utilizing dopants to alter the defect structure of ceria, not only the doping level but also the radius of dopant should be regarded. However, up to now, a clear relation between dopant radii and defect structure has not been built up experimentally. In order to unravel the effect of dopant ionic radius on defect structure and the relevant magnetism in CeO2, in this study, La3+ was chosen as dopant. The ionic radius of La3+ is 1.16 Å,


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which is larger than Y3+ (1.019 Å), Sm3+ (1.079 Å) and Ce3+ (1.14Å). The evolution of defect structure induced by La was investigated first by utilizing a combination of spectroscopic techniques including X-ray Absorption Spectroscopy (XAS), Raman spectroscopy and Scanning Transmission Electron Microscopy-Electron Energy Loss Spectroscopy (STEM-EELS) with atomic sensitivity and then compared to both experimental observations (Y and Sm doped ones) and theoretical calculations. At last, combined the defect- FM relation among CeO2 NPs doped with various nonmagnetic ions, including La, Y and Sm, the mechanism how defect induce FM in nanoparticles was discussed. II. Experimental Procedures The starting precursors are Ce(NO3)3·6H2O (Alfa Aesar, 99.5%) and various amounts of La(NO3)3·6H2O (Alfa Aesar, 99.9%) mixed with 80% Ethylene Glycol (EG)/Water into a three-necked round bottom stirring 600 rpm at room temperature. 3 Molar NH4OH was added when the precursor was completely dissolved. The solution was kept at 60℃ for 21 hours. The precipitates were subsequently separated by centrifugation at 6000 rpm for 15 minutes and washed using DI water and alcohol several times. After drying for 24 hours, CeO2 NPs were obtained. Raman spectra of the CeO2 NPs were recorded using a micro-Raman system (Uni-RAM system) and a diode laser at an excitation wavelength of 532 nm. The X-ray absorption near-edge fine structure (XANES) measurements at Ce L-edge were measured at Wigger beamline 17C by using the fluorescence yield mode at room temperature. The monochrmator Si (1 1 1) crystals were used in Wiggler beamline 17C with an energy resolution ΔE/E better than 2 × 104. The energy resolutions were about 0.3-0.4 eV. The XANES spectra at O K, Ce M, and La M-edge were recorded at HSGM beamline 20A by using the total electron-yield mode with 0.2 eV energy resolutions. STEM/EELS was done using a USTEM-NION microscope and a GATAN EELS modified spectrometer operated at 60 keV in order to limit the beam damage. The UV-vis reflection was measured by UV-Vis/NIR spectrophotometer (Jasco V-670).The magnetization was measured at room temperature by Vibrating Sample Magnetometer (VSM) (Lake Shore 7400 Series). III. Results and discussions Figure 1 shows the results of XRD patterns of Ce1-xLaxO2 with x ranges from 0.03 to 0.15. All the observed peaks from each sample can be identified as the cubic fluorite structure with space group Fm3m of CeO2 (JCPDS 34-0394). No characteristic peaks of impurities like lanthanum oxide were observed. Nevertheless, by enhancing the concentration of La, all peaks were gradually



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broadened which can be attributed to the shrinking of particles sizes. According to Scherrer’s equation, the particle sizes decreases from 3.5 to 2.1 nm. It is expected that doping with trivalent elements in CeO2 NPs will induce vary kinds of defect. Raman spectroscopy was then carried out to understand the amount and structure of defects in La-doped CeO2 NPs and all spectra were shown in Figure 2(a). CeO2 is a cubic fluorite structure which Ce atom is surrounded by eight O atoms. Raman active modes of fluorite lattice are A1g, Eg and F2g. With unpolarized laser and the conditions, only F2g mode can be seen, which is assigned to the symmetric stretching vibration mode of oxygen atoms and Ce ion around 456 cm-1. 16 This Raman mode is really sensitive to any disorder happening in the structure. 17 In this figure, it is seen that as increasing doping level, F2g band shifts strongly to lower wavenumber (at about 456 cm-1 for the undoped one) and broadens asymmetrically.18 The shift of F2g mode was displayed in Figure 2(b). Above variation of F2g mode demonstrated the increasing of defects in the system.19 Other than F2g mode, a broad band starts from 520 cm-1 to 670 cm-1 was observed. It can be deconvoluted into two additional structures, two peaks center at 560 cm-1 (D1) and 600 cm-1 (D2). According to Nakajima et al.’s theoretical calculations, these bands are assigned to defect species including an oxygen vacancy which breaking the Oh symmetry and defect species with Oh symmetry including a dopant cation (M3+) without any oxygen vacancy, M8, respectively.20 As seen in Figure 2 (c) and (d), as enhancing the content of La doping, the ratio of ID1/ IF2g and ID2/ IF2g increase, showing that both the abovementioned defects increase, especially in heavily doped ones (doping level > 7%). Then the intensity ratio of D1 and D2 (noted as ID1/ ID2) was utilized to demonstrate the evolution of those two types of defects as increasing the doping level. It is seen that the ratio increases rapidly as doping level increased from 5% to 7%, suggesting that the increasing rate of the first type of defect is larger than the other one. In addition to the variation in intensity, it is observed that D1 band starts to shift to higher wavenumber, especially when La contents reach to high doping level, as seen in Figure 2(f). The shift of D1 band has been observed in Gd6, Pr7 and Y21 doped CeO2 NPs recently. The energy shift of D1 band with increased M3+ concentration was attributed to the increase of the number of 2M’Ce: VO complex than M’Ce: VO. Consequently, above Raman results not only demonstrated the type of defect introduced by La doping, but also predicted the variation trend of the component as increasing doping level. Then the hybridization strength between oxygen and surrounding ions was investigated by XANES of O K-edge. The results were plotted in Figure 3 (a). In CeO2, there are three main absorption peaks in the O K-edge, observed at 530, 532 and 537 eV (note as Peak A, B and C). They are ascribed to electronic transitions from O 1s core level to empty O 2p hole states hybridized with the cerium dominated 4f, 5d-eg, and 5d-t2g levels, respectively.22-23As for the


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electronic transitions from O 1s to La 5d–O 2p states of La2O3, the main absorption peak presents between 533 and 537 eV.24 Upon La doping, the intensity of pre-peak at 530 eV is declined, as shown in the inset, which indicates the diminution of hybridization between Ce 4f0 and O 2p states. To further clarify the change of XANES between ceria and La2O3, differential curves of each spectrum was plotted together in Figure 3 (b). It is observed that the trend of peak B and C was similar among La doped CeO2 NPs, which is apparently different from La2O3. In addition, the intensity ratio of peak B and C (note as IB/IC) decreases gradually as doping level increases. The variation of the ratio, IB/IC, has been attributed to the change of oxygen vacancy ordering.25-26 Accordingly, it is implied that oxygen vacancies were introduced by La doping. Also, the hybridization between O and La was enhanced while it was declined between O and Ce. Since the increment of oxygen vacancies would result in the change of valence, the valence state of cations was determined by XANES. Figure 4 shows XANES La M-edge spectra of CeO2 NPs doping with different contents of La along with reference sample, La2O3. All the peaks of spectra not shifted, implying the valence of La in these doped CeO2 NPs is in trivalent state. The increment of peak intensity can be attributed to concentration effect. XANES of Ce L-edge of La doped CeO2 NPs were plotted in Figure 5. It is agreed that the valence state of Ce varies between 4+ and 3+, depending mostly on the degree of oxygen deficiency. The change of valence of Ce can be expressed by the concentration of Ce3+ which could be estimated by spectra fitting. As shown in the figure, all the spectra were subtracted by an arctangent function to exclude the electronic transition to continuum states and then fitted by five Gaussian functions. Interpretations of each peak have been given in elsewhere.27 Among these five peaks, component C at 5727 eV corresponds to a transition from the initial electron configuration 2p4f15d0 to the final configuration 2p4f15d*. Therefore, the concentration of Ce+3 impurities in a CeO2 matrix can then be expressed as the ratio IC/ITotal, where IC is the intensity of peak C and Itotal refers to the sum of deconvoluted peaks of A, B and C. The calculated results are showed in the inset of Figure 5. The value of CCe3+ of La doped CeO2 NPs increases gradually upon increasing the doping level of La. It reaches to the maximum as the content of La was increased to 7%. With further increasing the doping level, CCe3+ starts to decrease. Together above XANES analysis with Raman spectra, evolution of defect type induced by La doping was unraveled. In the beginning, defect in the form of M3+- VO - M3+ (including La3+- VO La3+ and La3+- VO - Ce3+) and M8 were induced, both result in the enhancing of CCe3+. Notably, the increasing rate is much higher of the former type. As doping level above 7%, the major type defect becomes M3+- VO - M3+ and the preferred substitution position of La is in the complex La3+- VO La3+. The latter one resulted in the decreasing of CCe3+. Abovementioned evolution of the defect of



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La doped NPs is similar to what have been observed in Y-doped ones.21 Notably, spatial distribution of defects was found to be strongly dependent on doping level of Y-doped NPs. Therefore, the distribution of defect in La-doped CeO2 NPs was investigated by STEM/EELS with high spatial resolution. Figure 6 and 7 show the results of 3% and 15% La-doped CeO2. The chemical maps obtained from EELS Ce M and La M-edge were present in Figures 6(b, c, d) and 7(b, c, d). The Ce3+/Ce4+ ratios estimated by choosing different energy windows and some corresponding spectra, can be seen in Figures 6(e,f) and 7(e,f). At low dopant (Figure 6d), it is very clear that La-rich clusters in the size of several Å were detected throughout the whole particle with no particular enrichment of surface. The Figure 7d shows the La/Ce ratio in a La doped 15% sample. Even in such case, a surface enrichment of the dopant or more generally a clustering of the dopant at the defective area (surface, but also grain boundaries) is not obvious. The La signal is observed throughout the NPs with some brightest contrast corresponding to very small clusters. This is distinct from previous studies about Y21, Cr28, Sm29, Gd30 and Mn31 doped CeO2 NPs. In those systems, dopant-rich domains were observed in heavily doped NPs with a strong trend to aggregate at surfaces. Two basic phenomena have been used to explain surface enrichment in doped metal oxide materials. The first is the formation of a space charge layer32, and the second is the elastic strain energy. The later one depends on the ionic size mismatch between dopant and host cations33 and association energy. In this study, the mismatch between dopant and host cation is much smaller than other doped NPs. The radius of La3+ (1.16 Å) is larger than that of Y3+ (1.04 Å).34 Moreover, it has been suggested by M. Nolan theoretically that defect structure in CeO2 depends on the respective radii.35 Also, M. Nakayama et al.36 used first-principles DFT to calculate the defect association effects between RE3+ ions and oxygen vacancies. Their results showed that smaller RE3+ ions strongly trap oxygen vacancies while larger RE3+ ions repel them. They found that doping with larger RE3+ ions decreases the trapping effect of oxygen vacancies but increases the energy barrier for oxide ion hopping. Consequently, different distribution of dopant among La, Y and Sm doped CeO2 NPs can be attributed to the radii of dopants. The distribution of trivalent cerium in these particles is also different to previous reports which showed a strong reduction of the last cerium oxide slab (that can be modulated as a function of thermal treatment) and a strong interaction with the dopant.21, 28-29 Figure 6(e, f) shows the cerium valence distribution for a low La-doping level. It is possible to see more Ce3+ nearby the surface of the particle in the map (Figure 6f) or by comparing the spectra for thin area or thick area (Figure 6e). Nevertheless, the valence change is rather small and as compared to previous report the Ce valence is more homogeneous. It is in particular very difficult to establish a clear correlation between the


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presence of La and change of Ce3+. An the best, comparing the Ce M edges as a function of the local fluctuation of the La concentration, a small shift to lower energy of the Ce M edges is observed for higher La content. In the case of heavily doped ceria of the Figure 7(e,f), it is also observed a small Ce3+ enrichment at the surface. No clear correlation between Ce3+ and dopant is observed within the particle. It is only observed a small decrease of the surface Ce3+ when La is also present at the surface. This is in agreement with the XAS observation that indicated a decrease of the Ce3+ for La doing above 7%. Above spectroscopic analysis implying that doping with La will result in the change of defect distribution, in other words,a more homogeneous distribution than for previously reported NPs21,28-31. It should be bear in mind that in nanoparticles, including TiO2 and CeO2, it has been reported that shrinkage of band gap was owed to the high concentration of defects at surface. The concentrated defects were believed to form intermediate defect energy states in the band gap of CeO2. These states retard the transition electrons from O 2p to Ce 4f, resulting in the narrowing of band gaps. Accordingly, it is worthy to investigate the band structure of La-doped NPs. Band gap of La doped NPs was determined based on UV-vis absorption spectra by using the equation [F(R)hν]1/n vs. hν (n=2 for indirect band gap). With fitting a linear line to the straight part of the curves and extrapolating to the x-axis, the value of band gap can be estimated, as shown in Figure 8. Variation of band gap as increasing defect concentration (Ce3+ + M3+) was plotted in the inset. Results of Y and Sm doped NPs were plotted together. It is seen that variation trend of band gap in La doped NPs is slow. Band gap did not change obvious within wide range until defect concentration reached 20%. As comparing to other two systems, decreasing rate from high to low is in the sequence Sm, Y, and La doped NPs. It has been proposed that the faster decreasing rate indicated the higher degree of defect concentration at surface. The results in Figure 8 thus implied that there are less impurity states at surface in La doped NPs, which is consistent with above spectroscopic analysis. The relationship between defect structures and magnetic behavior of La-doped CeO2 NPs was investigated. The mass magnetization curves of all samples as functions of temperature are shown in Figure 9. The curves were fitted by the Curie-Weiss law with an additional constant which stands for a ferromagnetic component or a diamagnetic component, depending on the sign of this constant. All curves can be fitted by the Curie-Weiss law. Results of 15% La doped NPs were used as an example, as shown in the inset. Both experimental and fitting curves of the 15% La-doped sample were plotted together, showing the experimental results can be fitted well. Since all samples have a positive constant term which stands for a ferromagnetic component, it is then clear that all these



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samples have both ferromagnetic and paramagnetic components. Then, M-H curves were carried out by VSM at room temperature. The results were shown in Figure 10. At first, paramagnetic behavior was studied, which was estimated by the slope at high fields. As shown in the inset, paramagnetic signal gradually increased as increasing the doping level of La until doping level reaches 7%, then it decreased. Since there are no unpaired electrons in the trivalent state of La, the contribution to paramagnetism must be solely from Ce3+. The free electron in the outermost shell of the Ce3+ ion may result in paramagnetism. Above results consistent with previous report that in ceria, not all Ce3+ contribute to ferromagnetism. Some of Ce3+ ions contribute to paramagnetism.37 Figure 11 shows M-H curves after subtracting paramagnetic part. All samples show RTFM with Hc ranges from 100 to 150 Oe. As seen in the inset, the saturated magnetization (Ms) increases to a maximum of 0.006 emu/g corresponding to a doping concentration of 9%; it subsequently decreases with further increasing doping concentration. Origin of ferromagnetism in ceria has been discussed both in theoretical and experimental. Fernandes et al.’s DFT calculations showed that both oxygen and cerium vacancies lead to ferromagnetism.38 In nanoparticles, Ge et al. predicted that the oxygen vacancy induced magnetic moment depends on the location of the vacancy.39 They suggested that Vo at surface can induce more magnetic moment. However, Li et al.’s results demonstrated that the higher the number of surface Ce3+/Ce4+ pairs, the more robust the ferromagnetism appears.40 As for the magnetic exchange mechanism in oxides, it can be explained under the scheme of F-center exchange (FCE)5,9. The presence of F+ centers is directly responsible for FM, which explains the variability of Ms in the doped samples. On the other hand, in nanomaterials, it has been proposed that RTFM could be also resulted from charge transfer ferromagnetism (CTF). In this study, according to XANES and Raman results, the type of oxygen vacancy defect induced by La doping is La3+-VO-Ce3+ in lightly doped NPs while it becomes La3+-VO-La3+ at heavily doped ones. At low doping level, magnetism increases with La doping due to the increase of total Ce3+ and the ferromagnetism mediated by F+ center such as La3+-VO-Ce4+. At higher La, the prevalence of La3+-VO- La3+ complexes, reduces the total number of Ce3+ and does not mediate ferromagnetism resulting in a reduce magnetism. Above evolution of oxygen vacancy as well as the F+ centers is similar to that in Y doped NPs. As a consequence, CCe3+- Ms relation, which can be inferred to the the relationship between magnetization and defects of these two systems was compared. As shown in Figure 12 (a). It is seen that within this range of CCe3+, Ms is proportional to the value of CCe3+ of both systems. However, two differences were noted. The first is that CCe3+ is higher in La doped NPs than Y doped ones.


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Second, it is shown that with the same value of CCe3+, Ms is much higher of Y doped NPs than La doped ones. The former one indicated that doping with La will introduce more defects into ceria. This is consistent with both Raman and XRD results. As for the stronger relation between Ce3+ and Ms in Y doped NPs, different evolution of defect between these two systems should be regarded. As mentioned in earlier section, though the type of oxygen vacancy is the same between La and Y doped NPs as increasing doping level, the distribution of defects is different. Both the band structure analysis and STEM/EELS investigations predicted the more homogeneous distribution of dopant as well as Ce3+ in La doped CeO2. Therefore, as shown in Figure 12 (b), though CCe3+ in La doped NPs is higher, it results in stronger paramagnetic signals but not higher Ms. This corroborates previous result that NPs with clustered impurities has a superior Ms value compared to NPs with randomly distributed impurities. Consequently, in this study, we first demonstrated the effect of La doping on the defect structure of CeO2 NPs and then compared the results with Y and Sm doped ones. As regarding the formation of ferromagnetic ordering, it is shown that the incorporation of 4f dopants without unoccupied outermost atomic orbitals, the evolution of defect structure should be noted. Strong correlation between distribution of defect and ionic radii of the dopants was observed, which was attributed to the strain and association energy. This is different to ceria doped with 3d dopants, where Fernandes et al. suggested that the ferromagnetic ordering is independent of ionic radius of dopants.41 Those findings can be utilized in functional materials design. IV. Conclusion The defect structure of La-doped CeO2 NPs was investigated systematically by spectroscopy. It demonstrated that in the beginning, defect in the form of M3+- VO - M3+ (including La3+- VO - La3+ and La3+- VO - Ce3+) and M8 were induced, both result in the enhancing of CCe3+. As doping level above 7%, the major type defect becomes M3+- VO - M3+ and the preferred substitution position of La is in the complex La3+- VO - La3+. Notably, it is found that both the dopant (La3+) and Ce3+ distributed homogeneous throughout the particle even the doping level was above 7%. The distinct evolution of oxygen vacancy was attributed to the disparate ionic radii of the dopants. At last, after comparing CeO2 NPs doped with different dopants, the relationship between magnetic behavior and defect structure was further clarified. Acknowledgement This research is supported by the National Science Council of Taiwan, Republic of China, under the Contract No. MOST 104-2112-M-011 -001 -MY3 and under the ANR contract



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DILUMAGOX 12-IS10-0002-01. We would also like to give special thanks to Dr. Der-Chung Yan in magnetic measurements and Prof. Lying-In Chen for provision of Raman spectroscopy and UV-Vis/NIR spectrophotometer. Reference [1]

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Figure 1 XRD patterns of Ce1-xLaxO2 with x ranges from 0.03 to 0.15.



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Figure 2 (a) Raman spectra of Ce1-xLaxO2 with x ranges from 0.03 to 0.15. Evolution of the spectra was studied by comparing the effect of concentration on the shift of D1 band (b), the ratio between the intensity of D1 and F2g band, ID1/IF2g (c), the ratio between the intensity of D2 and F2g band, ID2/IF2g (d), the ratio between the intensity of D1 and D2 band, ID1/ID2 (e), and the shift of F2g band (f).


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Figure 3 (a) XANES of O K-edge and (b) differentials of the spectra of La-doped NPs.



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Figure 4 XANES of La M-edges of La doped NPs.


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Figure 5 XANES of Ce L-edges of La doped NPs.



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Figure 6. STEM-EELS-HAADF of 3% La doped ceria, (a) STEM/HAADF, (b) Ce M-edge, (c) La M-edges, (d) La/Ce ratio, (e) EELS spectra obtained from La-rich and La-poor regions at a surface area (lower part) and a thicker area (upper part), (f) Ce3+ / Ce4+ ratio. In (e), the bars are an eye indication to evaluate small energy shift between spectra. The thicker and La poor area shows the largest Ce4+ contribution.


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Figure 7. STEM-EELS-HAADF of 15% La doped ceria, (a) STEM/HAADF, (b) Ce M-edge, (c) La M-edges, (d) La/Ce ratio, (e) EELS spectra obtained from La-rich and La-poor regions at a surface area, (f) Ce3+ / Ce4+ ratio. In (e), the bars are an eye indication to evaluate small energy shift between spectra. The La rich area shows the largest Ce4+ contribution.



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Figure 8 UV-vis absorption spectra of La doped NPs. Variation of band gap as increasing defect concentration (Ce3+ + M3+) of La, Y and Sm doped NPs was plotted in the inset.


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Figure 9 Magnetization –temperature dependence of CeO2 NPs doped with different content of La. The curves were fitted by the Curie-Weiss law with an additional constant. The inset shows both experimental and fitting curves of the 15% La-doped sample. The constants for 0%, 3%, 5%, 7%, 9%, 11%, and 15% La-doped samples are 0.00236 emu/g, 0.00292 emu/g, 0.002 emu/g, 0.00265 emu/g, 0.00228 emu/g, 0.00244 emu/g, and 0.00153 emu/g respectively.



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Figure 10 M-H curves at RT of CeO2 NPs doped with different content of La.


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Figure 11 M-H curves after subtracting paramagnetic contribution of La doped NPs.



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Figure 12 (a) The relationship between magnetization and concentration of Ce3+, and (b) the concentration effect of Ce3+ on paramagnetism of La and Y doped NPs.


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upper: STEM/EELS analysis of CeO2 NPs doped with 3% and 15% La. Homogeneous distribution of La3+ and Ce3+ were observed. lower: (a) The relationship between magnetization and concentration of Ce3+, and (b) the concentration effect of Ce3+ on paramagnetism of La and Y doped NPs. 257x168mm (150 x 150 DPI)


Effect of Ionic Radius - ACS Publications - American Chemical Society

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