Structural, Optical, Magnetic and Electrochemical Properties of CeXO2 (X: Fe, and Mn) Nanoparticles

CeXO2 (X: Fe, Mn) nanoparticles, synthesized using the coprecipitation route, were investigated for their structural, morphological, magnetic, and electrochemical properties using X-ray diffraction (XRD), field emission transmission electron microscopy (FE-TEM), dc magnetization, and cyclic voltammetry methods. The single-phase formation of CeO2 nanoparticles with FCC fluorite structure was confirmed by the Rietveld refinement, indicating the successful incorporation of Fe and Mn in the CeO2 matrix with the reduced dimensions and band gap values. The Raman analysis supported the lowest band gap of Fe-doped CeO2 on account of oxygen non-stoichiometry. The samples exhibited weak room temperature ferromagnetism, which was found to be enhanced in the Fe doped CeO2. The NEXAFS analysis supported the results by revealing the oxidation state of Fe to be Fe2+/Fe3+ in Fe-doped CeO2 nanoparticles. Further, the room temperature electrochemical performance of CeXO2 (X: Fe, Mn) nanoparticles was measured with a scan rate of 10 mV s−1 using 1 M KCL electrolyte, which showed that the Ce0.95Fe0.05O2 electrode revealed excellent performance with a specific capacitance of 945 Fּ·g−1 for the application in energy storage devices.


Introduction
Supercapacitors have gained enormous attention due to their highly demanding applications in sustainable energy storage and harvesting [1][2][3]. Therefore, the development of supercapacitors with enhanced performance is one of the main current research areas. Scientists are widely working in search of new materials useful for the fabrication of important components of supercapacitors, such as electrodes. The key points to enhancing the performance of the supercapacitor are to obtain high energy as well as high power densities and a durable life with stable cycles. Ceria (CeO 2 ) is one of the most extensively probed potential candidates due to its fundamental chemical and physical properties [4,5]. However, the properties of CeO 2 can be easily tailored simply by adding suitable dopant ions into the host lattice for various technological applications such as spintronics [6][7][8][9], electrode materials for supercapacitors [10][11][12], a solid oxide fuel cell [13], and antimicrobial agents [4]. From this perspective, cerium oxide (CeO 2 ) appears to be a potential candidate due to its redox properties, which are associated with the reversible oxidation states of cerium (Ce 3+ /Ce 4+ ). Reportedly, Rodrigues et al. have investigated the electrochemical performance of CeO 2, proving it a reversible redox electrochemical system due to the coexistence of Ce 4+ /Ce 3+ states [14]. Other than dynamic electrochemistry, nanoceria has

Experimental
The undoped and CeXO 2 (X: Fe, Mn) nanoparticles have been synthesized via the co-precipitation route using the following raw materials: cerium (III) nitrate hexahydrate (432.22 g/mol); manganese (II) nitrate hydrate (178.95 g/mol); iron (II) nitrate nonahydrate (404.0 g/mol); and NH 4 OH solution of highest purity (99.99%) of CDH. The cerium nitrate hexahydrate was used to synthesize the undoped CeO 2 , while the iron nitrate nonahydrate and manganese nitrate hydrate were used for doping along with the cerium nitrate hexahydrate. The precursors were weighed in stoichiometric amounts and added to the deionized water to make 0.06 M of the solution with continuous stirring. The dropwise ammonia solution was added until the pH of the solution reached 9 and was maintained throughout the experiment. After 2.5 h, the stirring was stopped and the solution was centrifuged to get the precipitate. The precipitates were washed with deionized water and ethanol many times to wash out the impurities and then dried in a hot air oven at 80 • C for 24 h. After grinding, the fine powders were sintered at 500 • C for 5 h. The product was finally ground to characterize for various measurements, viz., XRD, TEM, UV-vis spectroscopy, dc-magnetization, and electrochemical analysis.

Characterization Techniques
A Philips X-pert X-ray diffractometer was used to record the diffraction patterns using Cu Kα (λ~1.5418 Å). The FE-TEM (JEOL/JEM-2100F version) operated at 200 kV and was used to capture the TEM micrographs and selected area electron diffraction (SAED) pattern. The UV-vis spectroscopy measurements were obtained using Model LAMBDA 35, PerkinElmer (Waltham, MA, USA), and Raman spectra were recorded using a Raman spectrometer (NRS-3100) of SINCO Instrument Co. The magnetic behavior of the samples was studied using the quantum design physical properties measurement setup (PPMS). The Corrtest-CS150 workstation was utilized for the electrochemical measurements of CeO 2 , Ce 0.95 Fe 0.05 O 2 , and Ce 0.95 Mn 0.05 O 2 nanoparticles. All the electrochemical characterizations were performed with a 1 M aqueous solution of KCL as an electrolyte in a conventional three-electrode cell configuration. The working electrodes were designed by mixing 80% active materials (CeO 2 , Ce 0.95 Fe 0.05 O 2 , and Ce 0.95 Mn 0.05 O 2 nanoparticles), 10% carbon black, and 10% polyvinylidene fluoride (PVDF). The weighted electrode materials were homogeneously mixed using n-methyl-2 pyrrolidinone (NMP) as a solvent to form a slurry. The slurry was pasted on the nickel foam substrate (~1.0 cm 2 ) and then dried at 80 • C in the hot air oven for 12 h. All the electrodes were identical with respect to shape and size. The Ag/AgCl and Pt wires were utilized as reference and counter electrodes, respectively.

XRD Analysis
The structural features of the nanoparticles have been investigated using X-ray diffraction patterns. Rietveld refinement of the patterns, performed using open-access Fullprof software, is displayed in Figure 1a. The lattice parameters, peak shape parameters, background, atomic positions, and occupancies are carefully refined using the pseudo-Voigt function. The experimentally observed and theoretically calculated patterns are represented in black and red, respectively, whereas the difference between the two is represented at the bottom by blue, green, and pink colored lines for undoped, Fe-doped, and Mn-doped CeO 2 nanoparticles, respectively. Bragg's positions are shown by the vertical orange-colored lines. The values of reliability factors and χ 2 obtained after refinement are mentioned in Table 1. The value of χ 2 is between 1.3-1.5, which is acceptable for the good quality of refinement. The refined crystal structure showing 4 oxygen atoms bonded to 1 Ce atom and the 5% fraction of dopant ions substituted in Ce are displayed along with the respective refined XRD spectra. The indexing of the peaks associates the peak positions to the face-centered cubic fluorite structure (space group: Fm3m) of CeO 2, which corresponds to the JCPDS number: 75-0158 [34]. The indexing shows that all the samples attain similar crystallite structures, indicating effective incorporation of Fe 2+ and Mn 2+ in place of Ce 4+ . However, the strain is found to be increased in the doped compounds, giving a maximum value for Fe-CeO 2 . As a consequence, the size-strain plot (SSP) calculation is carried out using equation [35], as shown below.
Now, using the aforementioned equation, a plot is made with each diffraction peak's associated (d 2 hkl β hkl cosθ) term along the X-axis and (d hkl β hkl cosθ) 2 along the Y-axis, as shown in Figure 1b. The intercept of the straight line provides the intrinsic strain of the nanoparticles, and the slope gives the average size. We observed that the size of the crystallites in all samples calculated using the size strain plot is comparable to that determined using the Scherrer method [5]. Rietveld refinement further reveals a shifting in the peak position towards a higher 2θ value, which indicates the decrease in the lattice parameter as displayed in Figure 2a. Comparing the XRD diffraction peaks (111) of undoped CeO 2 and Fe-and Mn-doped CeO 2 nanoparticles, it is evident that the replacement of some Ce 4+ (0.097 nm) ions by smaller radius Fe 2+ (0.077 nm) ions and Mn 2+ (0.087 nm) ions results in an increase in FWHM with a decrease in the crystallite size of the particles. The crystallite structure formed with the smallest unit cell volume is in the case of Fe-CeO 2 nanoparticles, as displayed in Figure 2b. This also attains the smallest density of the compound, 6.9 g/cm 3 (see Table 1). Further, Scherrer's formula is used to calculate the crystallite sizes, which are shown in Figure 2c.
Now, using the aforementioned equation, a plot is made with each diffraction peak's associated (d 2 hklβhkl cosθ) term along the X-axis and (dhklβhklcosθ) 2 along the Y-axis, as shown in Figure 1b. The intercept of the straight line provides the intrinsic strain of the nanoparticles, and the slope gives the average size. We observed that the size of the crystallites in all samples calculated using the size strain plot is comparable to that determined using the Scherrer method [5]. Rietveld refinement further reveals a shifting in the peak position towards a higher 2θ value, which indicates the decrease in the lattice parameter as displayed in Figure 2a. Comparing the XRD diffraction peaks (111) of undoped CeO2 and Fe-and Mn-doped CeO2 nanoparticles, it is evident that the replacement of some Ce 4+ (0.097 nm) ions by smaller radius Fe 2+ (0.077 nm) ions and Mn 2+ (0.087 nm) ions results  lite structure formed with the smallest unit cell volume is in the case of Fe-CeO2 nanoparticles, as displayed in Figure 2b. This also attains the smallest density of the compound, 6.9 g/cm 3 (see Table 1). Further, Scherrer's formula is used to calculate the crystallite sizes, which are shown in Figure 2c.

TEM Analysis
The morphology of the nanoparticles is analyzed through TEM micrographs, as demonstrated in Figure 3a-c. The spherical particles with aggregation can be seen in the micrographs with unaffected morphology for Fe 3+ and Mn 2+ ion incorporation in the CeO2 nanolattice. The particle sizes are calculated using open-access ImageJ software and are represented through the fitted size distribution histograms shown in the insets of respective Figure 3a-c. The histograms show a narrow particle size distribution, indicating uniformity in the particle sizes. Mn-CeO2 nanoparticles have the smallest particle size of 13 nm as compared to CeO2. The indexing through selected area electron diffraction (SAED) ring patterns, shown in Figure 3a′

TEM Analysis
The morphology of the nanoparticles is analyzed through TEM micrographs, as demonstrated in Figure 3a-c. The spherical particles with aggregation can be seen in the micrographs with unaffected morphology for Fe 3+ and Mn 2+ ion incorporation in the CeO 2 nanolattice. The particle sizes are calculated using open-access ImageJ software and are represented through the fitted size distribution histograms shown in the insets of respective Figure 3a-c. The histograms show a narrow particle size distribution, indicating uniformity in the particle sizes. Mn-CeO 2 nanoparticles have the smallest particle size of 13 nm as compared to CeO 2 . The indexing through selected area electron diffraction (SAED) ring patterns, shown in Figure 3a

UV-Vis Absorption Spectra
Figure 4a displays the absorption spectra in the wavelength range of 400-800 nm. The spectra show the maximum absorption at 400 nm, which decreases with the increase in wavelength. The highest absorption has been obtained for undoped CeO 2 . The electronic band gap of nanoparticles is calculated with Tauc's plots, which are displayed in Figure 4b-d for undoped CeO 2 , Fe-CeO 2, and Mn-CeO 2, respectively. The band gap energy of undoped CeO 2 nanomaterials (2.9 eV) is smaller in comparison to its bulk counterpart, CeO 2 (3.35 eV), which may be the outcome of a shift in the 4f electronic states from Ce 4+ (4f 0 ) to Ce 3+ (4f 1 ), which indicates the introduction of an extra electron in the 4f orbital and decreases the band gap energy of undoped CeO 2 nanomaterials. The highest band gap is found to be 2.9 eV for the undoped CeO 2, while the lowest band gap (eV) is exhibited by Fe-doped CeO 2 nanoparticles. The lowering of the band gap may be associated with the smallest ionic radii and oxidation state of Fe ions. When Fe 2+ is doped in the CeO 2 lattice, it substitutes in place of the host cation, Ce 3+ . There is a difference between the oxidation states and ionic radii of Fe ions and Ce ions that leads to the creation of the defect states in the lattice. These defect states are very likely to be the oxygen vacancies in such cases of diluted magnetic semiconductors. These oxygen vacancies create intermediate states via exchange interactions with neighboring electrons, which reduces the band gap of the material. Therefore, the decrease in band gap energy in our doped nanoparticles may be induced by the development of localized impurity defect levels brought on by Fe 2+ and Mn 2+ ion doping, which manifests in oxygen vacancies and Ce 3+ defects.

UV-Vis Absorption Spectra
Figure 4a displays the absorption spectra in the wavelength range of 400-800 nm. The spectra show the maximum absorption at 400 nm, which decreases with the increase in wavelength. The highest absorption has been obtained for undoped CeO2. The electronic band gap of nanoparticles is calculated with Tauc's plots, which are displayed in Figure 4b-d for undoped CeO2, Fe-CeO2, and Mn-CeO2, respectively. The band gap energy of undoped CeO2 nanomaterials (2.9 eV) is smaller in comparison to its bulk counterpart, CeO2 (3.35 eV), which may be the outcome of a shift in the 4f electronic states from Ce 4+ (4f 0 ) to Ce 3+ (4f 1 ), which indicates the introduction of an extra electron in the 4f orbital and decreases the band gap energy of undoped CeO2 nanomaterials. The highest band gap is found to be 2.9 eV for the undoped CeO2, while the lowest band gap (eV) is exhibited by Fe-doped CeO2 nanoparticles. The lowering of the band gap may be associated with the smallest ionic radii and oxidation state of Fe ions. When Fe 2+ is doped in the CeO2 lattice, it substitutes in place of the host cation, Ce 3+ . There is a difference between the oxidation states and ionic radii of Fe ions and Ce ions that leads to the creation of the defect states in the lattice. These defect states are very likely to be the oxygen vacancies in such cases of diluted magnetic semiconductors. These oxygen vacancies create intermediate states via exchange interactions with neighboring electrons, which reduces the band gap of the material. Therefore, the decrease in band gap energy in our doped nanoparticles may be induced by the development of localized impurity defect levels brought on by Fe 2+ and Mn 2+ ion doping, which manifests in oxygen vacancies and Ce 3+ defects.

Raman Spectroscopy
The influence of the dopant ion is investigated using the molecular vibrations of the Ce-O8 vibrational unit of the CeO2 matrix [37]. The substitution of TM in place of Ce affects the symmetrical stretching of O-ions around Ce-ions, and the resulting vibrations have been detected through Raman spectra, as represented in Figure 5a-c. The spectra

Raman Spectroscopy
The influence of the dopant ion is investigated using the molecular vibrations of the Ce-O8 vibrational unit of the CeO 2 matrix [36]. The substitution of TM in place of Ce affects the symmetrical stretching of O-ions around Ce-ions, and the resulting vibrations have been detected through Raman spectra, as represented in Figure 5a-c. The spectra show the F 2g Raman active modes corresponding to CeO 2 , which are sensitive to the molecular disorder around Ce ions. The characteristic symmetrical breathing Raman active mode F 2g of cubic fluorite CeO 2 is obtained at~460 cm −1 , which corresponds to the oxygen ions around Ce 4+ ions (O-Ce-O) [37]. This Raman peak for undoped CeO 2 nanomaterials is caused by the growth of oxygen vacancies at the Ce 3+ site as a consequence of the change of the valence state of Ce 4+ ions to Ce 3+ ions. In the present case, the bands are obtained at~462 cm −1 , 453 cm −1 , and 455 cm −1 for undoped, Fe doped, and Mn-doped CeO 2 , respectively, which are closer to the characteristic band indicative of the effective substitution of dopant ions in place of the host cation. However, a decrease in the Raman frequency is clearly observed, indicating the occurrence of oxygen non-stoichiometry with O/Ce < 2. The reduction in oxygen content has been found by calculating the value of oxygen deficit (δ) using the formula: δ = 2.66 (1 − ω n /ω b ), where ω n is the position of the Raman active bands of the samples and ω b is the frequency of the bulk CeO 2 (465 cm −1 ) [29]. The values of δ are indicated in the respective Figure 5a-c, which reveal that even undoped CeO 2 shows a slight oxygen deficiency. It is noteworthy here that when the particle size reduces from bulk to nm scale, noticeable changes take place in the host lattice, including size confinement effects that may lead to oxygen non-stoichiometry. Further, the substitution of Fe and Mn in place of Ce creates more oxygen non-stoichiometry. The introduction of oxygen vacancies in the CeO 2 nanolattice leads to a change in the oxidation states of Ce from +4 to +3, which is favorable for the redox properties and therefore influences the density of states, which further affects the important properties of the material. The reduction in oxygen content has been found by calculating the value of oxygen deficit (δ) using the formula: δ = 2.66 (1-ωn/ωb), where ωn is the position of the Raman active bands of the samples and ωb is the frequency of the bulk CeO2 (465 cm −1 ) [29]. The values of δ are indicated in the respective Figure 5a-c, which reveal that even undoped CeO2 shows a slight oxygen deficiency. It is noteworthy here that when the particle size reduces from bulk to nm scale, noticeable changes take place in the host lattice, including size confinement effects that may lead to oxygen non-stoichiometry. Further, the substitution of Fe and Mn in place of Ce creates more oxygen non-stoichiometry. The introduction of oxygen vacancies in the CeO2 nanolattice leads to a change in the oxidation states of Ce from +4 to +3, which is favorable for the redox properties and therefore influences the density of states, which further affects the important properties of the material.

Magnetisation
The magnetization behavior of CeO2, Fe-CeO2, and Mn-CeO2 nanoparticles has been studied using VSM at room temperature. The hysteresis loops showing magnetization versus magnetic field (M-H) are displayed in Figure 6a

Magnetisation
The magnetization behavior of CeO 2 , Fe-CeO 2 , and Mn-CeO 2 nanoparticles has been studied using VSM at room temperature. The hysteresis loops showing magnetization versus magnetic field (M-H) are displayed in Figure 6a-c. The respective insets show the M-H curve at a low field and infer that all the samples demonstrate weak ferromagnetic ordering at room temperature. The various magnetic parameters such as saturation magnetization (M S ), remnant magnetization (M R ), and coercivity (H C ) are calculated for the undoped and X (Fe, Mn) doped CeO 2 nanoparticles (see Table 2). The undoped CeO 2 has the lowest Ms value~1.5 × 10 −4 emu/g which changes after doping and shows the maximum value for Fe-doped CeO 2 nanoparticles. Although there have been numerous theoretical and experimental investigations on RTFM in these oxides [7,8], there is still great controversy regarding the ferromagnetic ordering in these oxides with rare earth and transition metal cation doping and its correlation to the formation of defects and oxygen vacancies. The main condition in CeO 2 for the observation of ferromagnetic behavior is its tendency towards oxygen non-stoichiometry. When transition metal ions are doped in the CeO 2 lattice, they create defects such as oxygen vacancies, which interact with the neighboring electron [38]. The oxygen vacancies entrap the electrons, which undergo exchange interactions and create bound magnetic polarons (BMP), which induce ferromagnetic ordering [39]. curve at a low field and infer that all the samples demonstrate weak ferromagnetic ordering at room temperature. The various magnetic parameters such as saturation magnetization (MS), remnant magnetization (MR), and coercivity (HC) are calculated for the undoped and X (Fe, Mn) doped CeO2 nanoparticles (see Table 2). The undoped CeO2 has the lowest Ms value ~1.5 × 10 −4 emu/g which changes after doping and shows the maximum value for Fedoped CeO2 nanoparticles. Although there have been numerous theoretical and experimental investigations on RTFM in these oxides [7,8], there is still great controversy regarding the ferromagnetic ordering in these oxides with rare earth and transition metal cation doping and its correlation to the formation of defects and oxygen vacancies. The main condition in CeO2 for the observation of ferromagnetic behavior is its tendency towards oxygen non-stoichiometry. When transition metal ions are doped in the CeO2 lattice, they create defects such as oxygen vacancies, which interact with the neighboring electron [38]. The oxygen vacancies entrap the electrons, which undergo exchange interactions and create bound magnetic polarons (BMP), which induce ferromagnetic ordering [39].   In order to get more insights into the contribution of BMP to the ferromagnetic behavior of the samples, the M-H loops are fitted with the Langevin function (L(x)). The Langevin function has been employed as described in the literature [40][41][42][43].  Table 2. The value of M o is found to be highest for the Ce 0.95 Fe 0.05 O 2 and lowest for pure CeO 2 , even though the true spontaneous magnetization (m eff ) is observed to be in reverse order. A similar case has been reported by Mohanty et al., indicating m eff varying inversely from the spontaneous magnetization, which has been attributed to the competing exchange interactions between BMPs and the matrix [43]. Further, the values of N and χ m are also found to be highest for Ce 0.95 Fe 0.05 O 2 . Thus, the values of M o , N, and χ m are observed to be lowest for pure CeO 2 and highest for Ce 0.95 Fe 0.05 O 2 , indicating enhanced ferromagnetic behavior in Ce 0.95 Fe 0.05 O 2 , which confirms the formation of BMPs as a consequence of doping as well as the contribution of the matrix. Since the dopant concentration is the same (5%) in both the doped samples, the enhanced ferromagnetic ordering can not only be associated with the dopant concentration; however, the nature of the elements, i.e., Fe and Mn, also plays a part. Although individual Mn atoms have a higher magnetic moment than Fe, Mn is likely to dwell in the matrix antiferromagnetically, which may be the possible reason for the higher magnetic behavior induced in Fe doped CeO 2 as compared to Mn doped CeO 2 . In addition, the oxidation state may significantly affect the exchange interactions as Mn is possibly incorporated in the host matrix in the Mn 2+ oxidation state (see Section 3.6) and Fe is in Fe 2+ /Fe 3+ mixed valence states, which leaves Fe with more electrons and/or induces a higher number of oxygen vacancies in the matrix. Thus, the doping of Fe in the CeO 2 matrix enhances the ferromagnetic behavior of the host system.

Near Edge X-ray Absorption Fine Spectroscopy (NEXAFS)
The oxidation states of the ions are investigated using the NEXAFS spectra shown in Figure 7a-d. Figure 7a displays the L 3,2 edge spectra of Fe-doped CeO 2 at~705 and 709 eV together with the reference spectra of FeO, Fe 2 O 3 , and Fe 3 O 4 . The L 3,2 spectra arise due to the electronic transitions between the core levels of Fe 2p 3/2 and 2p 1/2 states and outer Fe 3d states. These transitions result in the formation of holes that take part in charge transfer processes [14]. The L 3 edge of Fe-doped CeO 2 shows no splitting and shows more resemblance to that of Fe 3 O 4 , indicating that Fe is dissolved in mixed valence states (Fe 2+ /Fe 3+ ). Similarly, Figure 7b displayed the L 3,2 edge spectra of Mn-doped CeO 2 along with the reference spectra of MnO, MnO 2 , and Mn 2 O 3 . The spectra of Mn-doped CeO 2 resemble more closely those of MnO, which show the presence of Mn 2+ states in the host matrix. Thus, the various enhanced properties in Fe-doped CeO 2 can be associated with the mixed valence states of Fe, which are responsible for the reduced oxygen stoichiometry in the lattice and the formation of oxygen vacancies. We conclude that the electronic structure of CeO 2 changes due to a change in the vacant number of 4f orbitals and hybridization with the lattice oxygen, although TM does not show any secondary phases in the CeO 2 lattice.

Electrochemical Study
The CV measurements are performed to study the capacitance performance of C Ce0.95Mn0.05O2, and Ce0.95Fe0.05O2 electrodes using the three-electrode system in 1 M K electrolyte. The CV curves of CeO2, Ce0.95Mn0.05O2, and Ce0.95Fe0.05O2 nanoparticles, as h lighted in Figure 8a-d, were measured at different potential scan rates of 10, 20, 50, 100 mV s −1 . The CV measurement was done in the potential window of −0.9 V and 0. It is worth noticing that the features of the CV curves for all electrodes are analogous. comparison of the CV profiles of CeO2, Ce0.95Mn0.05O2, and Ce0.95Fe0.05O2 electrodes m ured at a scan rate of 10 mV s −1 has been displayed in Figure 8d. One can notice f Figure 8d that the Ce0.95Fe0.05O2 electrode has a higher area under the curve compare other electrodes. The specific capacitance (CS) calculated using the CV profiles of the C Ce0.95Mn0.05O2, and Ce0.95Fe0.05O2 electrodes has been shown in Figure 9a-c. The specific pacitance (CS) values are determined using the following relation: ∆ where v denotes the scan rates (V/s), m (g) is the mass of the active material deposited the electrode, ∆ represents the potential window, and n = 1 is used for the th electrode cell. It is observed that the Cs values decrease with an increase in scan rates mV s −1 -100 mV s −1 ) from 205 Fּ ּ g −1 to 120 Fּ ּ g −1 , 805 Fּ ּ g −1 to 199 Fּ ּ g −1 , and 945 Fּ ּ g −1 to Fּ ּ g −1 for CeO2, Ce0.95Mn0.05O2, and Ce0.95Fe0.05O2, respectively, which indicates the usual formance of supercapacitors. Figure 9d describes the comparison of the Cs for diffe electrodes and highlights that the Ce0.95Fe0.05O2 electrode has the highest specific cap tance value of 945 F•g −1 .

Conclusions
The undoped and Ce0.95X0.05O2 (X: Fe, Mn) nanoparticles, synthesized using the coprecipitation route, are studied for their structural, morphological, optical, magnetic, electronic, and electrochemical properties. The Rietveld refinement has revealed the singlephase formation of the face-centered fluorite structure of CeO2. The Fe and Mn have been successfully incorporated into the CeO2 matrix. The lattice parameters and crystallite dimensions are found to be lowest for Fe-doped CeO2, mainly because of the lowest ionic radii of Fe as compared to Mn and Ce. The reduced dimensions led to the enhanced strain in Fe-doped CeO2 nanoparticles. The particle size obtained from the TEM micrographs also favored the XRD results. The band gap is also found to be minimal for Fe-doped CeO2 nanoparticles. The Raman spectra revealed the maximum oxygen non-stoichiometry in the Fe-doped CeO2 nanoparticles. The ferromagnetism can be seen for all the nanoparticles with a small hysteresis at room temperature. The smallest value of saturation magnetization and the magnetic moment has been found for pure CeO2 nanoparticles and is observed to be enhanced as a consequence of doping, with the highest value for Ce0.95F0.05O2 nanoparticles. The presence of oxygen vacancies is confirmed by Raman and NEXAFS analyses, which also exhibit a mixed valence state for Fe-ions (Fe 3+ and Fe 2+ ) and Ce-ions (Ce 3+ and Ce 4+ ). The cyclic voltammetry results demonstrate that the Ce0.95F0.05O2 electrode displayed the maximum value of specific capacitance (945 F g −1 ) recorded at 10 mVs −1 scan rate.

Conclusions
The undoped and Ce 0.95 X 0.05 O 2 (X: Fe, Mn) nanoparticles, synthesized using the coprecipitation route, are studied for their structural, morphological, optical, magnetic, electronic, and electrochemical properties. The Rietveld refinement has revealed the singlephase formation of the face-centered fluorite structure of CeO 2 . The Fe and Mn have been successfully incorporated into the CeO 2 matrix. The lattice parameters and crystallite dimensions are found to be lowest for Fe-doped CeO 2 , mainly because of the lowest ionic radii of Fe as compared to Mn and Ce. The reduced dimensions led to the enhanced strain in Fe-doped CeO 2 nanoparticles. The particle size obtained from the TEM micrographs also favored the XRD results. The band gap is also found to be minimal for Fe-doped CeO 2 nanoparticles. The Raman spectra revealed the maximum oxygen non-stoichiometry in the Fe-doped CeO 2 nanoparticles. The ferromagnetism can be seen for all the nanoparticles with a small hysteresis at room temperature. The smallest value of saturation magnetization and the magnetic moment has been found for pure CeO 2 nanoparticles and is observed to be enhanced as a consequence of doping, with the highest value for Ce 0.95 F 0.05 O 2 nanoparticles. The presence of oxygen vacancies is confirmed by Raman and NEXAFS analyses, which also exhibit a mixed valence state for Fe-ions (Fe 3+ and Fe 2+ ) and Ce-ions (Ce 3+ and Ce 4+ ). The cyclic voltammetry results demonstrate that the Ce 0.95 F 0.05 O 2 electrode displayed the maximum value of specific capacitance (945 F g −1 ) recorded at 10 mVs −1 scan rate.