Enhanced Room-Temperature Ferromagnetism on Co-Doped CeO2

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Enhanced Room Temperature Ferromagnetism on Co Doped CeO2 Nanoparticles: Mechanism, Electronic and Optical Properties Kugalur Shanmugam Ranjith, Padmanapan Saravanan, Shih-Hsien Chen, Chung-Li Dong, Chi Liang Chen, Shih-Yun Chen, Kandasami Asokan, and Ramasamy Thangavelu Rajendra Kumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp505175t • Publication Date (Web): 24 Oct 2014 Downloaded from http://pubs.acs.org on October 25, 2014

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The Journal of Physical Chemistry

Enhanced Room Temperature Ferromagnetism on Co Doped CeO2 Nanoparticles: Mechanism, Electronic and Optical Properties

Kugalur Shanmugam Ranjith,a Padmanapan Saravanan,b Shih-Hsien Chen,c,d Chung-Li Dong,c Chih Liang Chen,c Shih-Yun Chen,d Kandasami Asokan,e and Ramasamy Thangavelu Rajendrakumar*a, f a

Advanced Materials and Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore, India.

b

Defence Metallurgical Research Laboratory, Hyderabad - 500 058, Andhra Pradesh, India.

c

National Synchrotron Radiation Research Center, Hsinchu, 30076, Taiwan.

d e f

Department of Material Science and Engineering, National Taiwan University of Science and Tecchnology,

Materials Science Division, Inter University Accelerator centre,New Delhi-110067, India.

Department of Nanoscience and Technology, Bharathiar University, Coimbatore, India.

Fax: (91)422-2422387; Tel: 9789757888; E-mail: [email protected]

Abstract: Present study reports the effect of Co doping on the structural, optical, magnetic and electronic properties of CeO2 nanoparticles (NPs) synthesized by simple low temperature coprecipitation method. Co-doping was introduced by adding CoCl3with different mole percentages (0%, 2%, 4% and 6%) to cerium nitrate, which resulted in room-temperature ferromagnetism (RTFM). TEM and XRD analysis showed that the Co doped CeO2 NPs are monodispersed with face centred cubic structure. The 6 % Co doped CeO2 NPs showed coercivity value of 155 Oe and saturation magnetization of 0.028 emu/g at room temperature. The electronic structure of the as-prepared CeO2 and Co-doped CeO2 NPs were investigated by X-ray absorption near-edge structure (XANES) spectroscopy. The XANES spectra at Ce M-, and L-edges clearly indicated decrease in the valency state of Ce ions from Ce4+ to Ce3+ upon Codoping. This causes redistribution of oxygen ions and Co-Co bonding. The XANES study revealed that Co-doping plays a prominent role in improving the ferromagnetism, as Co replaces the Ce site in CeO2 cubic lattice and the concentration of oxygen vacancies may not be high enough to form a delocalized impurity band for enhancing the magnetic percolation of Co-doped

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samples. The XANES spectra at Co L-edges indicate that the direct Co-Co bond formation in the CeO2 lattices and also a weak bond with O ions. This is in agreement with the magnetic measurements which indicate that Co atoms induce enhancement in magnetic behaviour in CeO2 nanostructures.

Keywords: Co doped CeO2 nanostructures, Dilute magnetic oxide, X-ray absorption, Oxygen vacancies, Valence states, Photoluminescence Introduction Recently, ceria-based nanostructures have attracted greater attention due to its widespread applicability in microelectronics, catalysts, fuel cell technologies, gas sensors, solid state electrolytes and oxygen storage devices.1-7 Since the discovery of room temperature ferromagnetism (RTFM) in dilute magnetic semiconductor oxide (DMO) systems, CeO2has been the subject of intense research for their applications in spintronics. Ferromagnetism (FM) in CeO2 nanoparticles (NPs) is governed by the exchange interactions between the unpaired spins resulting from surface oxygen defect states8-10 and changes in the surface chemical states.11Their optical transparency nature enables them to be a promising candidate for magneto-optoelectronic devices besides their applications in spintronics. Among the various strategies in improving the performance of CeO2 nanostructures, transition-metal (TM) doping has been recognized as a prominent method in improving the magnetic functionality, as well as increasing the carrier transport in the CeO2 crystal lattice.12-14. RTFM in thin films and powders of TM-doped CeO2 nanostructures13,14 generated significant interest in the past several years. However, among the studies Fe, Co and Ni doped CeO2 NPs, Co doping has gained special interest mainly because of its extreme low magnetic susceptibility through different oxidation states, which open up venues for understanding the origin of magnetization in these nanostructures. Numerous techniques have been proposed to synthesis nano-sized undoped and Co doped CeO2 under controlled conditions, viz., chemical reduction15,combustion synthesis16, sonochemical17, sol-gel synthesis18, coprecipitation19,20, self-assembly processes21, hydrothermal22, solid state reaction23, thermal decomposition24 and Pulsed laser deposition.25Generally, as far as the magnetic properties of CeO2 based DMOs are concerned, the reported results are quite divergent. Some authors showed that Co-doped CeO2 nanostructures provide intrinsic ferromagnetic properties at room


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temperature due to the oxygen vacancies8,26, F-center exchange (FCE)22and metallic cluster substitution in host lattice23as possible origin for the enhanced ferromagnetism at ambient conditions. But a very few of them, suggested that the magnetic properties of ceria based nanostructures depends on the surface chemical states and the Ce3+/Ce4+pairs of the nanostructures.11, 27

While the authors often reported that the FM properties are strongly related to the oxygen vacancies, an interesting property of CeO2 is that it can have a stable structure far from the stoichiometric proportions of oxygen. It is therefore important to investigate the reason behind the enhanced magnetization in the CeO2 nanostructures on TM-doping. Doping of Co ions in CeO2 NPs are results in oxygen redistribution due to variation of Co and Ce valencies and hence O ion bonding. Bouaine et al23 reported that the as-prepared Co doped CeO2 samples are paramagnetic and the ferromagnetic behavior appeared on annealing due to the presence of Co magnetic clusters residing entirely at substitutional locations within the host lattice. A strong ferromagnetism in the Co-doped CeO2 was attributed to ferromagnetic coupling between Co ions mediated oxygen vacancies though detailed characterization of local structure around the magnetic dopant ions was not carried out intensively.23

In the present work, we investigate the structural, optical, magnetic and electronic properties of Co-doped monodispersed CeO2 NPs, synthesized nearly at the room temperature by coprecipitation method. The method herein, we describes an effort to understand the enhancement of ferromagnetism in CeO2 DMS upon doping Co ions in host lattice. The doping concentrations of Co-ions were controlled by the molar ratios of Ce and Co-ions added in the growth precursor. The electronic structures of these materials were studied by X-ray absorption near-edge structure (XANES) spectroscopy which is one of the sensitive experimental tools to probe the density of states (DOS) in the vicinity of the Fermi level. XANES measurements were performed to study the local bonding of cobalt, cerium and oxygen ions in Co doped CeO2NPs at Co L-, Ce M-, Ce L-, and O K-edges. The influence of oxygen vacancies and Co ion incorporation in ceria lattices were studied to understand their optical, magnetic and electronic properties by analyzing the charge transfer behavior in the ceria lattices. Through this study, it is suggested that the



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enhancement of FM is attributed to the formation of Co-Co bonding in the Co doped CeO2, without significantly affecting the oxygen vacancies.

Materials and Methods

CeO2 NPs were prepared by co-precipitation method. 2 M of Ce(NO3)3.6H2O was dissolved in 100 mL of ethanol solvent and the contents were heated up to 50 ºC under stirring. After 30 minutes of stirring, 10 mL of 30% of ammonia solution was added into the reaction mixture which resulted in a pale yellow coloured solution and the contents were maintained at 50 ºC for 4 h under stirring. After this reaction time, the precipitates were collected and centrifuged, washed with water and ethanol thrice and then dried at 60 ºC and annealed at 200 ºC for 2 h. For the Codoped CeO2 NPs, 0.04, 0.08 and 0.12 mole of cobalt chloride was added in the Ce(NO3)3.6H2O growth solution for different percentages for doping and the above mentioned growth procedure was repeated. The crystalline structure of the as-prepared samples were studied using a Phillip X’pert Pro Advance Powder X-ray diffractometer (Cu-Kα radiation; λ=1.5418 Aº). The crystallite size of the nanopowders, dXRD was calculated using the Scherrer equation. The morphology of the as-prepared CeO2 NPs was studied using a Tecnai F-12 transmission electron microscope (TEM) operating at an acceleration voltage of 15 kV. UV-Vis absorption spectra of both the pure and Co-doped CeO2 NPs were recorded in the wavelength range of 200−800 nm at room temperature using a JASCO V620 UV-Vis spectrophotometer. The photoluminescence (PL) measurements were performed on a Fluorolog-3 luminescence spectrometer (JASCO-FP6600) under excitation at 350 nm at room temperature. Magnetic measurements were performed with a vibrating sample magnetometer (VSM) (ADE make, Model EV9) up to a maximum applied field of 2 T. All XANES measurements were performed at the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. The XANES spectra at O K-, CeM- and Co L-edges were measured at soft-x-ray beam line BL20A. Ce L-edge was recorded at hard-xray beam line BL17C. The resolutions for soft-x-ray and hard-x-ray spectroscopic measurements were set at 0.2 and 0.5 eV, respectively.

Results and discussion


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Structural characterization

Figure 1a shows the XRD patterns of (a) standard data of cubic phase CeO2, (b) undoped CeO2, (c) 2 % Co doped CeO2, (d) 4 % Co doped CeO2 and (e) 6 % Co doped CeO2. These results are in good agreement with that of the bulk cubicCeO2 (JCPDS card No. 43-1002) with no evidences for the presence of Ce2O3 phase. The strong and broad diffraction peaks observed in the XRD patterns indicate that the CeO2 particles are crystalline with their sizes in the nanometre range. The XRD pattern of undoped CeO2 showed diffraction peaks corresponding to (111), (200), (220), (311) and (222) planes located at 2θ = 28.7, 33.3, 47.5, 56.4 and 58.9°respectively, attributed to the face centred cubic structure. Using the Scherrer equation, the crystallite size of the CeO2 NPs was estimated to be 5- 8 nm using the characteristic peak at 28.7 corresponding to (111). The calculated lattice parameter constant (a) for the undoped CeO2 is 0.538 nm and this value is consistent with that of the bulk CeO2 (0.5411 nm, JCPDS card No. 43-1002). On comparing the XRD peaks of the pure and Co-doped CeO2 particles, the full width of half maximum (FWHM) of the diffraction peak values increased with the Co-dopant concentration. This indicates that on increasing the Co-dopant, the crystallite size of the particles was reduced (see Figure 1b). The reduction in crystallite size may be due to the replacement of Co-dopant ion in the crystal lattices, which has lower ionic radius as compared to the Ce ion. On increasing the doping concentration of the Co ions, the lattice parameter values of the cubic structured CeO2 were reduced.

Morphological characterization

The representative TEM images of pure and Co doped CeO2NPs are shown in Figure 2 (a-d). These TEM images reveal that the individual mono dispersed NPs (Figure 2 a-d) are having the particle size of 6-12nm. The selected area electron diffraction (SAED) of the CeO2 and the Co doped CeO2 are consistent with the cubic fluorite structure of bulk CeO2 with ring patterns corresponding to the (111), (200), (220), (311) planes. The crystallite size was found to be decreased in the case of Co doped CeO2 NPs compared to that of the pure CeO2 NPs. The particle size values in the range of 5 – 12 nm were estimated for the CeO2 and Co-doped CeO2 NPs and these values are in agreement with the XRD results.



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Optical characterization Optical absorption spectra of the pure and Co-doped CeO2 NPs are shown in Figure 3. The absorption peaks of the pure, 2% Co doped, 4% Co doped and 6% Co doped CeO2 NPs are found at 303, 305, 308 and 317 nm respectively. There is a strong absorption below 400 nm in the spectra, which is due to the charge transfer from O 2p to Ce 4f states in CeO2.28. 29The optical band gap (Eg) was calculated from the absorption spectrum of the NPs using the equation.

E g = 1240

λ Absorp . Edge

Doping Co ions changes the optical properties. It increases the absorption edge towards higher wavelength, indicating the incorporation of Co in CeO2 NPs. For undoped CeO2, a sharp absorption peak appears at about 303 nm. However, for x % Co doped CeO2(x = 2%, 4% and 6%) NPs, the sharp absorption peak shows an obvious red-shift and increased up to 317 nm. Onset of the absorption edges of the pure, 2 % Co doped, 4 % Co doped and 6 % Co doped CeO2 NPs were at ~401, 472, 453 and 428 respectively. The doping concentration makes the absorption shift in the visible region, which is in contrast to the absorption threshold of CeO2 NPs. From the band edge absorption slope of the pure, 2% Co doped CeO2, 4% Co doped CeO2 and 6% Co doped CeO2 NPs the band gap was calculated as 3.09, 2.63, 2.74 and 2.89 eV, respectively. The doped samples show more efficiency for the absorption of visible region.

From the

absorbance in the UV region indicated that the band gap energy of Co-doped CeO2was lower than that of the pure CeO2NPs; consequently revealing that the doping effect decreased the band gap. The 2% doped samples showed lower band gap when compared with the rest of the samples. The absorption is caused by the induced dopant level near the conduction band of CeO2 as shown in Figure 4a. The reduction in the band gap may be due to the impurity defect level created by the influence of Co doping. The maximum UV absorption peak and the band edge absorption with respect to the doping percentages were plotted in Figure 4b.

The photoluminescence (PL) spectra (figure. 5) of the CeO2 and Co doped CeO2 NPs was obtained at the excitation wavelength of 350 nm. The PL emission around 400-500 nm is


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associated with the charge transfer from Ce4f level to O2p level (valence band). Localization of the energy levels between Ce4f band and O2p band are due to the defects, which could lead to wider emission bands.30A strong blue emission around 402 nm corresponds to the transition from the Ce 4f band to the valence band of CeO2.31. The suppression of emission intensity of 402 nm peak is easily understood in the present case. Colis et al31 have reported that emission intensity around 402 nm increased upon Co doping -suggesting the existence of large amount of oxygen vacancies. But in our case, the emission intensity around 402 nm is suppressed, which probably suggests decrease of oxygen vacancies or the absence of 4f band in the valence band of CeO2. The defect levels localized between the Ce 4f band and O2p band can lead the emission band below 3 eV and the localization of the Ce 4f band lie on the valence band O 2p will lead the emission at high energy levels more than 3eV. PL spectra shows that the emission of the undoped CeO2 around 400-420 nm region is associated with the charge transfer from Ce 4f to O 2p level.22 The bluish green emission at 469 nm is also attributed due to the surface defects and few authors reported that these peaks are related to the dislocations or oxygen defects.32, 33 On comparing with the undoped nano ceria the emission peaks of the Co doped CeO2 samples was found to be red shifted. This could be due to the formation of more impurity levels. The defect states are localized in between the Ce 4f and O 2p levels, leading to red shift. This clearly indicated that the defects arose on Co doping. On further increasing the Co doping to 6%, it might take the Ce 4f states and increased the surface defect states leading to increased red shift of the CeO2 NPs by suppressing the Ce 4f to O 2p energy transfer level emission.

Magnetic characterisation

The magnetization hysteresis loops obtained with a maximum field of ±20 kOe, for the undoped and Co doped CeO2 NPs are shown in figure 6, which display the RTFM properties. The figure 6 a-c shows the magnified view of the RTFM properties of undoped and Co doped CeO2 NPs. The saturation magnetization, Ms and remanent magnetization, Mr were estimated from the M vsH curves. It can be noticed that the Ms values tend to increase with the Co doping concentration (x). The influence of Co doping causes an increase in remanent magnetization from 0.0003emu/g to 0.01emu/g. The observed changes in the magnetic properties with the increase in Co concentration can be correlated with decrease in the lattice constant (fig. 1) due to the Co ion



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incorporation in CeO2. Any decrease in the lattice constant will imply strong electronic bonding between Co, Ce and O ions. This results in change in the magnetic property which depends on electronic configuration of Co and Ce and how these ions are hybridized with O ions. The undoped CeO2 NPs also showed the RTFM, due to the presence of surface oxygen spin order in the CeO2 NPs.8 The exact origin and enhancement of RTFM in Co doped CeO2 NP is not understood well. Recent studies indicated that oxygen vacancies,8, 26 F centers,22 and defects in DMS systems23 play a major role in the magnetic exchange interactions. Recently, Coey et al.,34 proposed a model to explain the observed ferromagnetism, based on impurity-band exchange and density-functional calculations by considering the oxygen defect induced donor sites. In the Fcenter-mediated mechanism, the radius of the electron orbital isa direct function of the dielectric constant.35 Since CeO2 has a high dielectric constant of 26, the radius of the F-center electrons may be large enough to produce magnetic interactions at low Co concentrations. As x increases to 0.04, a fraction of the Co ions are likely to enter the interstitial positions which require excess of oxygen ions for charge neutrality. At higher concentration of the Co ions, it forms an excellent FM behavior. However, more investigations are required to confirm the exact oxidation state of Co ions, their host locations and to determine the role of oxygen stoichiometry.23, 24 Although, both pure CeO2 and all the Co-doped samples exhibit FM behavior, the enhanced FM may be attributed to the addition of Co. For further understanding, the electronic properties of these NPs were studied by XANES spectroscopy.

Electronic structure studies

The XANES at CeM4,5-edge arises due to the electron transitions from Ce 3d3/2 and 3d5/2core levels into Ce 4f unoccupied electronic states. It thus reflects the occupancy of 4f orbitals. Figure 7 displays the XANES CeM4, 5-edges of Co doped CeO2 NPs. The information provided by Ce M4 and M5 are the same because of the selection rule and these correspond tothe final unoccupied 4f states. As evident from the figure, the spectral features of Ce M4 and M5 –edges are very much similar. Hence only spectral features of Ce M5-edge are discussed36.A predominant peak is observed at 886 eV with a significant satellite structure at 891 eV. These satellite structures originate from transitions to Ce 4f states in the conduction band and thus these are indicative of the contribution of 4f0 states.36The intensity of this satellite peak can also be


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used to determine the amount of 4f0 states and is enlarged in the inset. It is seen that the intensity of this satellite peak is reduced as the doping level is increased, indicating that the Ce valency is decreased upon Co doping. However, the change inthe 4f0-related satellite peak is rather small. To confirm that the Ce valency varies with Co doping, Ce L3-edge XANES measurement is performed and the results are presented in Figure 8. The XANES at Ce L3-edge involves the electronic transition from Ce 2p to outmost shell 4f5d6s level and has been widely used to study the electronic configurations of Ce [Ce3+: 4f1(5d6s)3 and Ce4+: 4f0(5d6s)4]. The evolution of the spectral profile upon Co doping is identified. The change is evident in the pre-peak feature at about 5722 eV and the main double-peak structure at about 5732 eV and 5739 eV, as indicated by arrows. Generally, the valency of Ce ions can be determined from the main double-peak structure in Ce L3-edge.24 The increase in peak intensity observed at 5732 eV compared to that of those noticed at 5739 eV –suggests that the Ce valency tend to decrease as the Co concentration is increased. This variation is clearly seen in the inset (right panel). The reduction of Ce valency is in line with the earlier results from the spectra of Ce M4,5-edges. Pre-peak feature at about 5722 eV is originated from the dipole-forbidden 2p to 4f transition and therefore its intensity is relatively weak as compared to the dipole-allowed 2p to 5d transitions. Although small the variation of this peak upon Co doping is revealed. The intensity of the pre-peak feature decreased with Co doping. The decrease of this feature implies that Ce 4f orbital gains charge upon Co doping, resulting in decrease of the Ce valence state, which is consistent with the results from intensity ratio of double-peaks structure. Owing to the overlap of wave function of O 2p and Ce 4f orbital, the preedge of O K-edge, which arises from the O 2p-Ce 4f states may provide the complementary information to Ce L- and M-edges.

Figure 9 (a) shows the normalized XANES spectra at O K-edge wherein the transition of 1s electrons to the continuum states occurs. In all NPs, there are three peaks located at 530, 532.5 and 537 eV, correspond to the empty O 2p hole states hybridized with the Ce-dominated 4f, 5deg and 5d-t2g states.37 The peak positions of cerium dioxides are quite different from those of cobalt oxides, implying the absence of cobalt oxide in these NPs. The 530 eV peak reflect the hybridized O 2p-Ce 4f states. It is clear that the intensity is enhanced as Co is increased.



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Considering the 4f orbital gains charge upon Co doping (as indicated in earlier Ce M- and Ce Ledges), and the O 2p orbital loses charge (as evidenced in O K-edge), it suggests that the charge transfer occurs O and Ce states. It is noted that the crystal field effects appear when the undoped and Co-doped NPs are compared, as the separation of energy of 5d-eg and 5d-t2g states of Codoped NPs is different from that of undoped ones37, 38. The change of the crystal effect may lead to the less covalent character in Co-doped CeO2 NPs and possibly gives rise to the charge transfer between Ce and O. Further, it is also noted that the rising absorption edge is varied with different doping concentration. The rising absorption edge determines the energy level of conduction band minimum. As Co doping increases, the conduction band minimum shifts to lower energy and then shift back to higher energy side. The shift of the rising edge suggests the band gap variation with respect to the doping level, and it has similar tendency with the result from UV-Vis spectra, shown in Fig. 4(b). Figure 9 (b) displays the Co L-edge, which exhibits the transitions from Co 2p1/2 and 2p3/2 to Co 3d unoccupied states. Peak positions and the spectral line shape of the 3d metal L-edge strongly depend on the local atomic and electronic structure of the metal ions, therefore, providing the information on the electronic configuration and valence states. Basically, all spectra exhibit mainly two single-white-line peaks (L2 and L3) and the ratio of L2/L3 is about 1/2, implying that Co doped CeO2 NPs behave as if Co ions are present along with CoO. It should be pointed out that the Co can be oxidized very easily. Minor trace of CoOx from Co L-edge is inevitable, especially for small amount of isolated Co. However, we note the effect from CoOx on the electronic structure is insignificant since there is no obvious CoOx feature appearance in O K-edge spectra. On increasing the doping concentration, the CoOxrelated peak is absent, suggesting the formation of more Co-Co bond in the sample with higher Co doping level.

There is also some evidence of the interaction between Co and O in Co L-

edge. A weak spectral feature at 782.3 eV in Co L-edge is related to the metal-ligand interaction and implies the Co is bonded with oxygen neighbours. But as synthesized undoped CeO2 NPs are having O deficiency which readily introduces VO into the nanostructures. More over on Co doping, Co ion will bond with one O ion, while on Ce will always bond with two O ions. Accordingly, sufficient Vo will be formed in Co doped CeO2 NPs, which provides sufficient ferromagnetic coupling between Co atoms. We expect that Co2+- Vo-Co2+ group will form in Co doped CeO2 NPs. That may be the reason for the weak response or absence of Co oxide.


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Bouaine et al., predicted that FM exchange interactions might be enhanced by chemical substitutions.23 Ferrai et al., suggested that in CeO2 based materials with oxygen vacancies eventually results in enhancement of ferromagnetic ordering.26 Chen et al., predict that the FM of CeO2 NP is suggested to be intrinsically related to the concentration of Ce3+ on the surface but an excess of oxygen vacancies may eliminate FM24. Moreover, previous investigations were defining the role of oxygen related defects were mediated on magnetic behavior on the CeO2 NPs. This study shows that the poor ferromagnetic behavior of the undoped CeO2 NPs, are due to the possibility of native oxygen vacancies. But the concentration of oxygen vacancies may not be high enough to form a delocalized impurity band for enhancing the magnetic percolation in undoped and Co doped CeO2 samples. The O K-edge XANES results imply that oxygen vacancies do not increase on incorporation of Co atoms within the cubic host lattice. Because of reduction of Ce valence states due to Co doping, it is likely that there is redistribution of O ions without creating additional vacancies. It is clear that the enhancement of RTFM nature on Co doped CeO2 NPs is due to Co ions which definitely plays an important role. Whereas the ferromagnetic sample shows a limited ratio between Co metal and CoO suggesting that direct Co-Co bonds form due to the induction of Co atoms in CeO2 lattice in doping process which enhances the ferromagnetic behavior. Again looking into the spectra of reference samples (CoO and Co metal) reported by Bouaine et al.,23 one can conclude that the increase in the I(L2)/I(L3) ratio is due to Co interaction in the CeO2 lattices. Because of reduction of Ce valence due to Co doping and the chemical substitutions of the metallic ions, are likely to the reason of enhanced the ferromagnetic behavior. This reiterates that the presence of Co incorporation in the host lattice is responsible for the observed enhanced ferromagnetism.

Conclusion Monodispersed pure CeO2 and Co doped CeO2 NPs with uniform size distributions were successfully synthesized by simple co-precipitation method. XRD analysis show that these samples possess face centered cubic structure. The UV-Vis and PL studies indicate that the Co doping causes the change in band gap and red shift when compared to the undoped CeO2NPs.Doping of Co results in oxygen ion redistribution. The RT-FM properties of CeO2 NPs enhanced with increasing Co doping level. XANES measurements reveal that there is no evidence for increase in oxygen vacancies on Co doping and Co replaces the Ce sites. This



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results in change of valency of Ce 4+ to Ce 3+, which is likely to be accompanied by redistribution of oxygen with Co impurities. All these factors should be considered in understanding the enhancement of ferromagnetism.

Acknowledgements

The authors KSR and RTR would like to thank the Department of Science and Technology, Government of India, for financial support under the Nano mission project (SR/NM/NS113/2010- BU (G)) and the author RTR would like to thank Department of Science and Technology, Government of India, for financial support under the SERB project (SR/FTP/PS099/2011).

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Ge, H. Y.; Wang, H.; Liu, E. Z.; Liu, J. F.; Jiang, J. Z.; Li. Y. K.; Xu. Z. A; Li, H. Y. On the Origin of Ferromagnetism in CeO2 Nanocubes. Appl. Phys Lett. 2008, 93, 062505.

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Liu, Y.; Lockman, Z.; Aziz, A.; Driscoll, J. M. Size Dependent Ferromagnetism in Cerium Oxide (CeO2) Nanostructures Independent of Oxygen Vacancies. J. Phys: Condens. Matter. 2008, 20, 165201.

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Li, M.; Ge, S.; Qiao, W.; Zhang, L.; Zuo, Y.; Yan, S. Relationship Between the Surface Chemical States and Magnetic Properties of CeO2 Nanoparticles. Appl. Phys. Lett. 2009, 94, 152511.

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Yue, L.; Zhang, X. M. Structural Characterization and Photocatalytic Behaviors of Doped CeO2. J. Alloys. Compd, 2009, 475,702-705.

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Brito, P. C. A.; Santos, D. A. A.; Duque, J. G. S.; Macedo, M. A. M. Structural and Magnetic Study of Fe-Doped CeO2. Physica B. 2010, 405, 1821-1825.

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Saranya, J.; Ranjith, K. S.; Saravanan, P.; Mangalaraj, D.; Rajendra Kumar, R. T. Co doped CeO2 nanoparticles: Enhanced Photocatalytic Activity Under UV and Visible light Irradiation. Mat. Res. Semicon. Proc. 2014, 26, 218-224.

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Figure Caption Figure 1. (a) XRD pattern of the undoped and Co doped CeO2 NPs, (a) standard data of cubic phase CeO2, (b) undoped CeO2, (c) 2 % Co doped CeO2, (d) 4 % Co doped CeO2 and (e) 6 % Co doped CeO2.

(b) The graph of change in the grain size and lattice parameter vs doping

concentration.

Figure 2. TEM images of undoped and Co doped CeO2 NPs, (a) undoped CeO2, (b) 2 % Co doped, (c) 4 % Co doped and (d) 6 % Co doped CeO2 NPs. Insets show the corresponding SAED patterns.

Figure 3. UV-Vis absorbance spectra of undoped and Co doped CeO2 NPs, (a) undoped CeO2, (b) 2 % Co doped, (c) 4 % Co doped and (d) 6 % Co doped CeO2 NPs. Inset shows the maximum absorbance of Co doped CeO2 NPs. Figure 4. (a) Band diagram of CeO2 and Co doped CeO2 NPs (b) Band gap and grain size variation with respective to the doping concentration.

Figure 5. Photoluminescence spectra of undoped and Co doped CeO2 NPs. Figure 6. M-H plots of the undoped and Co doped CeO2NPs Figure 7. XANES spectra at Ce M-edges of the undoped and Co doped CeO2 NPs. Figure 8. XANES of Ce L-edge spectra of the undoped and Co doped CeO2 NPs. Figure 9. XANES spectra undoped and Co doped CeO2 NPs at (a) O K-edge, (b) Co L-edges.


Intensity (a.u)

(b)

30

40

50

60

70

(c)

30

40

50

60

70

(d)

30

40

50

60

70

(e)

30

40

50

60

70

30

40

50

60

70

20

(311)

(400)

(311) (222)

(220)

(a)

(200)

a

80

2theta (deg)

b

9.5 5.380 9.0 5.375

8.5 8.0

5.370

7.5 5.365 7.0 5.360

6.5 6.0

Lattice Constant (a)

Particle Crystaline Size (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(111)

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5.355

5.5 5.350 5.0 0%

2%

4%

6%

Co doping Concentration

\

Figure 1



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2


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c

b

M axim um absorbance (nm )

318

A bsorbance (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


316 314 312 310 308 306 304 302

d

0%

2% 4% Doping Concentration

6%

a

200

300

400

500

Wave length (nm)

Figure 3


600

700

800


b

3.2

8.0

Grain size (nm )

7.5 3.0 7.0 2.8

6.5

6.0

2.6

5.5 0

1

2

3

4

5

Co doping percentage (%)

Figure 4


6

Band gap (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4 1 9 .9

4 4 1 .2

4 6 8 .5

Pure CeO2

2% Co doped CeO2

5 4 6 .8

P L in te n s ity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 0 2 .9

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4% Co doped CeO2

6% Co doped CeO2

400

450

500

550

Wavelength (nm)

Figure 5


600

650


a

0.04

Magnetisation (em u/g)

0.03

Undoped CeO2 2% Co doped CeO2 6% Co doped CeO2

0.02 0.01 0.00 -0.01 -0.02 -0.03 -0.04 -20000

-10000

0

10000

20000

Applied Voltage (Oe)

b Magnetisation (emu/g)

0.04

Un doped CeO2 2% Co doped CeO2 6% Co doped CeO2

0.02

0.00

-0.02

-0.04 -1000 -800 -600 -400 -200

0

200

400

600

800 1000

Applied Field (Oe)

c

0.005 0.004 0.003

Magnetisation (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Undoped CeO2 2% Co doped CeO2 6% Co doped CeO2

0.002 0.001 0.000 -0.001 -0.002 -0.003 -0.004 -0.005 -1000 -800 -600 -400 -200

0

200 400 600 800 1000

Applied Field (Oe)

Figure 6


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Page 23 of 27

0%

Ce M4,5-edges

Co-doped CeO2 Nanoparticles

6%

Intenstiy (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6%

4%

2%

0%

880

890

900

Photon Energy (eV)

Figure 7


910


Ce L3-edge

Co-doped CeO2 Nanoparticles

6%

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4%

2%

0%

6% 0%

0%

6% 5710

5720

5730

5740

Photon Energy (eV) Figure 8


5750

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a

Co-doped CeO2 NPs

O K-edge


6% 4% 2% 0% 0% 2% 4% 6%

520

b

530

4%

0%

6%

540 Photon Energy (eV)

L3

2%

550

560

Co L3,2-edges


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


L2

6% 4% 2%

770

780

790

800

Photon Energy (eV)

Figure 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Content


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207x156mm (150 x 150 DPI)


Enhanced Room-Temperature Ferromagnetism on Co-Doped CeO2

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