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Electrochemical and structural analysis of the RE3+:CeO2 nanopowders from combustion synthesis
Journal of Alloys and Compounds 614 (2014) 118–125
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Electrochemical and structural analysis of the RE3+:CeO2 nanopowders from combustion synthesis C. Esther Jeyanthi a,b,⇑, R. Siddheswaran c, Rostislav Medlín c, M. Karl Chinnu d, R. Jayavel d, K. Rajarajan e,⇑ a
Research and Development Centre, Bharathiar University, Coimbatore 641046, TN, India Department of Physics, Panimalar Engineering College, Chennai 600123, TN, India c New Technologies Research Centre, University of West Bohemia, Plzenˇ 30614, Czech Republic d Centre for Nanoscience and Technology, Anna University, Chennai 600025, India e Department of Physics, R.V. Govt. Arts College, Chengalpet 603001, TN, India b
a r t i c l e
i n f o
Article history: Received 7 May 2014 Received in revised form 26 May 2014 Accepted 27 May 2014 Available online 12 June 2014 Keywords: RE3+:CeO2 nanopowders Combustion synthesis Cyclic voltammetry Photoluminescence Microstructure
a b s t r a c t The reported article deals with the synthesis and characterization of rare earth ions doped ceria (RE3+:CeO2) nanopowders from citrate nitrate auto-combustion route. The crystalline nature and lattice planes of the nanocrystalline RE3+:CeO2 powders were analyzed by X-ray diffraction (XRD) and Selected Area Electron Diffraction (SAED) profile fit. The excited state absorption (ESA) and energy transfer up-conversion (ETU) were studied by photoluminescence (PL) measurement. The spectroscopic properties of the powders were studied using Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The surface morphology, average size, distribution and orientation of the lattice planes of the nanoparticles were studied by using scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The size of RE3+:CeO2 nanoparticles were found to be in the range from 15 to 30 nm which has good agreement with the HRTEM results. The changes in current density with increasing sweep scan potential of the doped ceria powders were studied by cyclic voltammetry (CV) analysis. The specific capacitance range of the rare earths doped ceria of Er:CeO2, Pr:CeO2, Yr:CeO2 and Nd:CeO2 were calculated as (12.9–86.5), (20–72.4), (80–375) and (0.92–4.22) respectively. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Cerium dioxide (CeO2), either pure or doped with different rare earth elements is known for industrial materials because of its superior properties of ionic conductivity [1], refractive index, high melting point (2873 K) [2], large dielectric constant, high transparency in the visible-near IR region, band gap between 3.2 and 3.6 eV for direct electron transition [3], high chemical and thermal stability [4] and oxygen storage capacity [5]. These featured properties are leading to the wide-spread use of ceria as components in applications for solid oxide fuel cell (SOFC) electrolytes [6], catalyst supports, carbon monoxide (CO) reduction catalysts [7–9], gas sensors, electro-chromic and UV-protective coatings [10–13]. Ceria nanoparticles are among the most unique and promising nanomaterials being studied today [14,15]. This is mainly due to the diffusion and reactivity of oxygen vacancies in ceria, which contributes to its ⇑ Corresponding authors. Address: Department of Physics, Panimalar Engineering College, Chennai 600123, TN, India. Tel.: +91 44 2649 0404. E-mail addresses: [email protected] (C. Esther Jeyanthi), drkrr2007@ gmail.com (K. Rajarajan). http://dx.doi.org/10.1016/j.jallcom.2014.05.208 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
high oxygen storage capability [16]. Ceria possess strong anti-oxidative activity associated with reversible Ce4+ M Ce3+ reduction/ oxidation processes facilitated by high degree of non-stoichiometry of CeO2 nanocrystals. CeO2 has fluorite-type structure (space group Fm3m) with FCC cubic lattice; each cerium cation is coordinated by eight oxygen anions [17–19]. A high concentration of O-vacancies is associated with a greater conversion of cerium ions from the Ce4+ states to the Ce3+ ones. Excited Ce4+ ions can only follow non-radiative relaxation paths when returning to the ground state. In contrast, the relaxation of excited Ce3+ ions to the ground state includes radiative pathways that result in fluorescence signals in the region 500–580 nm. The initial concentration of O-vacancies has been shown to correlate to the oxygen storage capabilities of ceria and the corresponding conversion of cerium ions between the Ce3+ and Ce4+ states [20,21]. The concentration of oxygen vacancy in ceria can be increased by doping of ceria with trivalent lanthanides or rare-earth ions, such as Sm, Nd, Er and Pr. It has relatively low activation energies between the dopant ions and the O-vacancies, is a viable technique to increase the ionic conductivity and the catalytic activity of ceria [22]. This can lead to an increase in the oxygen diffusion constant
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Table 1 Chemical reactions during the combustion synthesis. 0.9 0.9 0.9 0.9
[Ce(NO3)36H2O] + 0.1 [Ce(NO3)36H2O] + 0.1 [Ce(NO3)36H2O] + 0.1 [Ce(NO3)36H2O] + 0.1
[Er(NO3)35H2O] + 0.783 [C6H8O7] D ? Ce0.9Er0.1O1.95 + gas " (1) [Nd(NO3)36H2O] + 0.783 [C6H8O7] D ? Ce0.9Nd0.1O1.95 + gas " (2) [Pr(NO3)36H2O] + 0.783 [C6H8O7] D ? Ce0.9Pr0.1O1.95 + gas " (3) [Y(NO3)36H2O] + 0.783 [C6H8O7] D ? Ce0.9Y0.1O1.95 + gas " (4)
in the nanomaterial promoted by the high concentration of charged O-vacancies present in doped ceria [23]. The resulting improvement of oxygen storage capability of doped ceria nanoparticles is studied through experimental verifications of both the increase of Ce3+ ionization states, and the associated O-vacancies via photoluminescence studies. Particularly, the doping of ceria with Er3+ improves the sensitivity of the signal more. The present study deals with the synthesis CeO2 with different rare earth (RE) elements (Nd, Er, Pr and Y) by citrate nitrate auto-combustion method, and also reveal the optical, structural and electrochemical properties.
which coincides well with the standard data of CeO2 (JCPDS: 0043-1002). The diffraction lines correspond to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) and (4 0 0) of the pure ceria were observed for the RE3+:CeO2 with small strain. It was observed that, there are no traces of Er, Pr, Y and Nd oxides in the diffraction pattern. Therefore, it substantiates that the RE3+ ions doping in CeO2 is substitutional impurity, which is regarded as single phase formation. The changes in ion size of the substitutional elements influence the very small lattice strain than the pure Ceria (in comparison with the JCPDS file: 00-43-1002). Moreover, the diffraction peaks at 2h = 28° of (1 1 1) were used to estimate the average isotropic crystallite size of the nanoparticles using Scherrer’s formula:
2. Experimental and characterization
DðhklÞ ¼ The main host precursor of cerium nitrate hexahydrate Ce(NO3)36H2O (Aldrich, 99.999%) with different dopants such as Er(NO3)35H2O (Aldrich, 99.9%), Nd(NO3)36H2O (Aldrich, 99.9%), Pr(NO3)36H2O (Aldrich, 99.99%), and Y(NO3)36H2O (Aldrich, 99.9%) were used for the synthesis of each combination of doping in the ratio of 90:10 mol%. For the chemical reaction (combustion), the citric acid anhydrous C6H8O7 (Sigma–Aldrich, 99.5%) of suitable quantities were used according to the balanced calculation from the concepts of propellant chemistry [24]. Stoichiometric amounts of cerium nitrate with respective rare earth chemical were mixed together and dissolved in suitable quantity of de-ionized water. Then the calculated amount of citric acid was also mixed in the solution and stirred mechanically for 2 h at 60 °C to get clear and homogeneous solution. The reaction mixture was put in a platinum or alumina crucible, and it was kept inside a preheated furnace at a temperature of 500 °C. Once the reaction mixture reached the point of spontaneous combustion, it starts burning vigorously. The gaseous volatile molecules such as N2, CO2, H2O and other forms of gaseous products were released during the combustion reaction. Finally, porous solid foam of nanoparticles was obtained within 10 min. The assumed combustion reactions involved among cerium nitrate and fuel with different rare earth nitrate chemicals (nitrates of Er, Nd, Pr and Y, viz.) are given in Table 1. The as-combusted foams were collected and converted to powders by smooth grinding. The same process was repeated for different dopants. The as-prepared ceria powders of different rare earth elements were calcined at 700 °C for 2 h (at a heating rate of 5 °C/min). The calcined RE3+:CeO2 powders were converted into cylindrical compacts with subsequent sintering at 1200 °C for 6 h. The powder X-ray diffraction (XRD) studies were carried for the RE3+:CeO2 powders using Rich-Seifert diffractometer with Cu Ka (k = 1.5405 Å) radiation over the range of 10–70° at a scanning rate of 1°/min. The average particle size of the particles were calculated from full width at half-maximum (FWHM) using Debye–Scherrer’s equation [25] for (1 1 1) crystal face of the nanoparticles. FTIR spectra of the nanopowders were recorded for wavenumber regions 4000–500 cm1 using Perkin–Elmer FTIR spectrometer. Raman spectra in the range of 0–3500 cm1 were carried out using BRUKER-RFS27 FT-Raman spectrometer. Photoluminescence measurements were performed on confocal micro-Raman microscope MonoVista 750 CRS. He–Cd ultraviolet laser was used for excitation emitting on 325 nm wavelength. The measurement system includes Olympus BX51 optical microscope, monochromator with focal distance of 750 mm and cooled back-illuminated CCD camera ProEM: 1600 200. LMU-15X-UVB objective was used for focusing of the beam and collecting signals at ambient room temperature. The structure, surface morphology, shape, size and crystalline planes of the nanoparticles were examined by scanning electron microscope (FEI, Nova NanoSEM 200) and transmission electron microscope (TEM-JEOL JEM 2200 FS). Cyclic voltammetry (CV) was employed to investigate the effectiveness of ceria and doped ceria. The nanopowders were dispersed into colloidal suspensions and further dip-coated on a glassy carbon (GC) plate as working electrode [26]. A platinum wire and Ag/AgCl containing transparent glass were used as counter electrode and reference-electrode, respectively.
kk b cosðhÞ
where k is the X-ray wavelength (1.5405 Å), b is the full width at half-maximum, h is the diffraction angle, k is a constant (0.9), and Dhkl represents the size along (hkl) direction. The size of the rare earths doped ceria particles were found to be in the range from 15 to 30 nm, which is comparable with the size observed from TEM micrographs. 3.2. Up-conversion luminescence in RE3+:CeO2 powders Fig. 2 shows the visible up-conversion luminescence spectra of the RE3+:CeO2 of different dopant ions dependent excitation into different levels of RE3+ using He–Cd laser with excitation wavelength of 325 nm. The photoluminescence spectra of the RE3+:CeO2 nanoparticles exhibit a broad band in blue–green region (from 480 nm to 560 nm). The emission bands in the region 370– 480 nm is generally attributed to charge transfer from O2 to Ce4+, that is, electron transition or charge transfer occurs from oxygen vacancies [27–29]. The region from 370 to 480 nm corresponds to the transition 4G11/2 ? 4I15/2. The region from 500 to 650 nm corresponds to the transition 2H11/2, 4S3/2, F9/2 ? 4I15/2. The pointgroup symmetry of Ce sites in the fluorite CeO2 structure is Oh with
3. Results and discussions 3.1. Powder X-ray diffraction XRD patterns of the rare earths (Er, Nd, Pr and Y) doped CeO2 nanoparticles show the typical cubic fluorite structure (Fig. 1)
Fig. 1. XRD patterns of the RE3+:CeO2 nanoparticles.
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Fig. 2. Up-conversion luminescence spectra of the rare earths doped ceria (RE3+:CeO2) nanoparticles with exciting wavelength of kexc = 325 nm.
Fig. 3. Partial energy level structure of the RE3+:CeO2.
Fig. 5. Raman spectra of the RE3+:CeO2 nanoparticles.
eight-fold oxygen coordination, thereby providing inversion symmetry [30]. Among the other rare earth ions, the Er3+ ion is one of the most efficient ions for up-conversion because the metastable levels of Er3+ can be conveniently populated by ordinary diode lasers or any kind of visible lasers [31]. The up-conversion luminescence intensity of Er3+:CeO2 powders is occurred due to excited state absorption (ESA) and energy transfer up-conversion (ETU) process, respectively, are discussed as the possible mechanisms for the observed up-conversion luminescence. The schematic energy level diagram of the RE3+:CeO2 is shown in Fig. 3 [32,33]. The observed up-conversion luminescence arises due to ESA mechanism which populates thermalized levels (2H11/2, 4S3/2) leads to high efficient green emission and explained as follows. Upon excitation at 325 nm, the ground state Er3+ ions will excite into the 4I11/2 level in singly doped material. After first level excitation, the same pumping source pumps the excited Er3+ ion from 4I11/2 to 4F7/2 level. Subsequent non-radiative relaxation populates the 4S3/2 or 2 H11/2 and 4F9/2 levels. Bright green (546 nm) and weak red (679 nm) emissions were observed due to the transitions (2H11/2, 4 S3/2 ? 4I15/2) and 4F9/2 ? 4I15/2, respectively. 3.3. Fourier transform infrared (FTIR) spectroscopy
Fig. 4. FTIR spectra of the RE3+:CeO2 nanoparticles.
Fourier transform infrared (FTIR) spectra of the RE3+:CeO2 powders are shown in Fig. 4. The spectrum shows some strong intense bands at around 3430, 1630, 1050 cm1 and 540 cm1. In addition, some weaker absorption peaks were also observed at around 2920, 1380 and 730 cm1. The strong absorption band at 3430 cm1 is attributed to the m(O–H) vibration modes of the physically adsorbed water molecules. The peaks observed at 2920 and 1630 cm1 are assigned to the m(C–H) and d(CH2) vibration modes, respectively. These are due to the presence of atmospheric organic moieties in the exposed samples during the sample preparation and handling [34]. The band observed at 1050 cm1 is generally attributed to the cerium–oxygen groups having a larger double bond character. The bands observed in the lower frequency region at 730 and 540 cm1 are typical of cerium–oxygen groups having a lower double bond character and of Ce–O–Ce chains or Ce–O–RE3+ symmetric stretching of the metal oxide network [35,36]. The broadness of the bands for Pr3+:CeO2 in the region from 3430–1000 cm1 by means that the material captures the organic counterparts, which is assumed as a material’s adsorption nature.
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Fig. 6. SEM micrographs of the RE3+:CeO2 nanoparticles.
Fig. 7. EDX spectra from SEM for the RE3+:CeO2 nanoparticles.
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Fig. 8. HR-TEM micrographs of the (a) Er3+:CeO2, (b) Nd3+:CeO2, (c) Pr3+:CeO2, and (d) Y3+:CeO2 nanoparticles.
3.4. Raman spectroscopy Raman spectra of the RE3+:CeO2 nanopowders are shown in Fig. 5. The spectrum is dominated by the triply degenerate F2g band (Raman active) at 465 cm1 assigned to the CeO2 cubic fluorite structure [37]. This Raman active mode is attributed for the symmetric breathing mode of oxygen atoms around cerium/rare-earth (RE3+) ions [38]. Therefore, they are very sensitive to any disorder in the oxygen sub-lattice resulting from thermal, doping, or grain size. In addition, a broad weaker absorption band near F2g in the region 520–630 cm1 is assigned to a band that originates in the oxygen vacancies in metal dioxides [39]. In addition, the Raman spectrum contains smaller absorption at 2073 and 2570 cm1 are due to symmetric C–H stretching of methyl groups from the reaction of hydrocarbon contamination during the sample preparation or handling [40]. Also a broad region from 2900 to 3400 cm1 arises from the O–H stretching vibration of adsorbed H-bonded water [41]. The appearance of weak bands around 158 cm1 is
attributed to tetragonal displacement of the oxygen atoms from their ideal fluorite lattice positions of the RE3+:CeO2 [42]. 3.5. Surface morphology analysis Fig. 6 shows the SEM micrographs RE3+:CeO2 nanopowders calcined at 700 °C. The powders are formed as foam of agglomerated particles. The SEM micrographs of the cluster powders show platelets and flaky structures with large aggregates. Since the RE3+ dopant concentration is a part of tenth, the morphologies of the powders are almost similar to CeO2. The solid foam has porous structure which is due to the evaporation of gases during their decomposition. The energy dispersive X-ray (EDX) spectrum in Fig. 7 confirms the presence of Ce, O and RE elements such as Er, Pr, Nd and Y. The additional undefined peaks corresponding to C and Al are due to the signals from the sample holder. The surface morphology, particles size, distribution, crystallinity and lattice planes of the RE3+:CeO2 nanoparticles were studied
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nanoparticles were obtained and measured by Gatan CCD camera using the Digital Micrograph software and treated by Process Diffraction software [43]. Data treatment included circular integration of SAED intensities from TEM and calibration of the integrated data intensity peaks by means of camera length (CL) for comparison and indexation purposes with the standard PDF (00-43-1002). The SAED spectra in Fig. 9 confirmed the diffraction of electrons from the lattice planes. The SAED profile fit has a good agreement with the XRD patterns. The EDX spectra from TEM were recorded in order to confirm the elements in the composition. The spectra shown in Fig. 10 confirmed the presence of Ce, O, and rare-earth elements (Er, Pr, Nd and Y) of respective combinations. Apart from these elements, the spectra exhibited some elements such as Cu (copper grid), Ni (nickel grid), C and Si (membrane in the grid) which were present in the carbon coated copper grid/nickel grid used as sample holders.
3.6. Cyclic voltammetry studies The CV patterns for 1–5 cycles of CeO2, Er3+:CeO2, Pr3+:CeO2, Nd :CeO2 and Y3+:CeO2 were recorded with different scanning potential using Na2SO4 electrolyte (but it was compared pure and RE doped ceria of fifth cycle of all samples) as shown in Fig. 11. The oxidation peak of Na2SO4 dissolved in distilled water (1.0 mol/L) for the 1st cycle was observed at 2 V for a sweep potential rate of 10 mV/s and 50 mV/s and 100 mV/s. After 5th cycle, the current density increases and anodic wave was slightly shifted towards higher positive potential and the maximum peak was observed at 1.8 V for a sweep potential rate of 10 mV. Due to doping, the peak density of current was gradually increased from 1 to 5 cycles and the charge density ratios were measured for CeO2 and RE3+:CeO2 for different sweep potential voltages of 10 mV/s and 50 mV/s and 100 mV/s. After the 5th cycle, the shape variations of voltammograms were observed both for pure and doped CeO2 as shown in Fig. 11. The CV curve indicates that the charge density of the CeO2 and RE3+:CeO2 lies in between 2 V (for reduction) and +2 V (for oxidation) as a function of number of cycles. After the 5th cycle of oxidation/reduction, loss in the charge density was practically zero. There can be a possible reason for the better cycling performance of the Y3+:CeO2 nanocrystalline electrode such as the 3+
Fig. 9. Selected Area Electron Diffraction (SAED) profile fit for Pr3+:CeO2 and Nd3+:CeO2 nanoparticles.
by using high resolution transmission electron microscopy as shown in Fig. 8. The size of the rare earths doped ceria nanoparticles are varied in the range from 10 to 30 nm. Also, it was found that the cluster of nanoparticles consist of some of the bigger nanoparticles. The larger particles are assumed to be early formed nanocrystals which further grew during the combustion and the calcination. The high resolution images exhibited the crystalline structure and lattice planes arrangement with atomic resolutions. It was found that the nanocrystals are well defined and crystalline. The inter-planar distances of the CeO2 were calculated using Digital Micrograph software, and indexed with reference to the Powder Diffraction File (PDF) No.: 00-43-1002. The Selected Area Electron Diffraction (SAED) patterns from the RE3+:CeO2
Fig. 10. EDX spectra of the Er3+:CeO2 and Nd3+:CeO2 nanoparticles from TEM.
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Fig. 12. Specific capacities of RE3+:CeO2 for the different scanning potential rate.
Table 2 Specific capacities of RE3+:CeO2 for the different scanning potential rate. Specific capacity (F g1)/scan voltage
10 mV
50 mV
100 mV
CeO2 Er:CeO2 Pr:CeO2 Y:CeO2 Nd: CeO2
1.94 86.5 72.4 375.0 4.22
0.61 23.4 37.4 96.8 0.94
0.56 12.9 20 80.4 0.92
the results obtained for the Ce ion occupation factor that it can be attributed to changes in the anion vacancy radius or to a decrease in the cation coordination number which influenced the correctness of the fit that might be responsible for the observed deviation. The results showed that there was very good agreement between experimental and calculated values [44]. The specific capacitance of the ceria based composites is determined and quantitatively evaluated by the expression voltammetric charge/(potential window composite loading). The integral area of CV curve/scan rate represent the sum of anodic and cathodic voltammetric charges [45]. The specific capacity was calculated using the formula;
C¼
Fig. 11. Cyclic voltammograms of RE3+:CeO2 for the scanning potential at 10 mV/s, 50 mV/s and 100 mV/s.
calcined nanoparticles has a high degree of crystallization and reduced defects, which facilitates the Na+ ions insertion and extraction processes. Synthesized nanoparticle/composite electrode provides impressive performance and unique behavior with increasing values of the discharge/charge capacity and cycle number. The current density increased to 0.068 mA/cm2, 0.33 mA/cm2, 0.63 mA/cm2 and 1.4 mA/cm2 for 100 mV of sweep potential rate for the CeO2, Er3+:CeO2, Pr3+:CeO2 and Y3+:CeO2 respectively, but the Nd3+:CeO2 (electrode) was decreased comparing with the rest, which is not considered as a doping effect (oxidization) because of larger atomic radius of Nd3+ ions. Matovic´ et al reported that, for the prepared Nd doped CeO2 solid solution, there was a 4% of discrepancy and the occupation factors decreased to 72%. So that (Nd:CeO2) may decreases its ionic conductivity. They reported
1 2M v ðV a V b Þ
Z
Vb
iðEÞdV
Va
where C is the specific capacitance of individual sample, Va and Vb are the cut-off potentials in cyclic voltammetry, i(E) is the instantaR neous current, i(E)dV is the total voltammetric charge obtained by integration of positive and negative sweep. (Va Vb) is the potential window width, and ‘M’ is the mass of individual sample, which is the mass difference of the working electrode before and after dipcoating, accuracy range of 0.005–0.007 mg (CeO2–5 g, Er3+:CeO2– 6.5 g, Pr3+:CeO2–7 g, Y3+:CeO2–7 g and Nd3+:CeO2–5 g) to weigh the mass of the samples and ‘m’ is the potential scan rate. Electrostatically the storing charge is determined by surface area. While, for the latter, charge is stored in virtue of highly reversible redox reactions between Ce4+/3+ species. The specific capacitances of the RE3+:CeO2 were calculated and compared with the pure CeO2 nanoparticle. It was found that, the specific capacitance of the REs doped ceria are greater than pure ceria as shown in Fig. 12. The specific capacitance based on CeO2 normalized capacitance is obtained from the CV curves by deducting the capacitance of RE doped CeO2 composites, which are tabulated (Table 2). It was found that, the Y3+:CeO2 has a high specific capacitance of 375 F/g comparing other RE doped CeO2 nanocomposites [46].
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4. Conclusions The combustion synthesized rare earths (Er, Nd, Pr and Y) doped CeO2 (RE3+:CeO2) nanoparticles reveals the cubic fluorite structure of the pure CeO2 with small lattice strains. The photoluminescence spectra of the nanoparticles exhibit a broad band in blue–green region from 480 nm to 560 nm. The conversion efficiency of the Er3+:CeO2 is more predominant than other dopants. The red to green up-conversion emissions were observed at 540 nm and 690 nm, respectively. The Raman spectra are dominated by the triply degenerate F2g Raman active mode at 465 cm1 which is assigned to the CeO2 cubic fluorite structure. The weaker absorption band near F2g in the region 520–630 cm1 has been assigned to a band that originates in the oxygen vacancies in metal dioxides, which confirms the doping of the rare earth elements. The high resolution TEM microscopy showed that the RE3+:CeO2 has well defined particle size (15–30 nm) and distribution. The lattice planes, orientation and crystalline structure were confirmed by HRTEM micrographs and SAED profile fit, respectively. The increasing current density with sweep scan potential in the cyclic voltammograms confirmed that the RE3+:CeO2 can be used as an electrolyte for solid oxide fuel cells. From the calculation of specific heat capacitance, it was found that the Y3+:CeO2 has higher value of capacitance 375 F/g comparing with the other RE doped CeO2 nanocomposites. Acknowledgments The authors R.S and R.M wish to acknowledge the ‘‘Centre of New Technologies and Materials (CENTEM)’’ Project No. CZ.1.05/ 2.1.00/03.0088 for the partial development of the results. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2014. 05.208. References [1] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563–588. [2] G. Balakrishnan, S. Tripura Sundari, P. Kuppusami, P. Chandra Mohan, M.P. Srinivasan, E. Mohandas, V. Ganesan, D. Sastikumar, Thin Solid Films 519 (2011) 2520–2526. [3] T. Wiktorczyk, P. Bieganski, E. Zielony, Opt. Mater. 34 (2012) 2101–2107. [4] R.N. Blumenthal, F.S. Brugner, J.E. Garnier, J. Electrochem. Soc. 120 (1973) 1230–1237. [5] G. Chen, F. Zhu, X. Sun, S. Sun, R. Chen, Cryst. Eng. Commun. 13 (2011) 2904– 2908. [6] C. Laberty-Robert, J.W. Long, K.A. Pettigrew, R.M. Stroud, D.R. Rolison, Adv. Mater. 19 (2007) 1734–1739. [7] J. Kaspar, P. Fornasiero, M. Graziani, Catal. Today 50 (1999) 285–298.
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Electrochemical and structural analysis of the RE3+:CeO2 nanopowders from combustion synthesis C. Esther Jeyanthi a,b,⇑, R. Siddheswaran c, Rostislav Medlín c, M. Karl Chinnu d, R. Jayavel d, K. Rajarajan e,⇑ a
Research and Development Centre, Bharathiar University, Coimbatore 641046, TN, India Department of Physics, Panimalar Engineering College, Chennai 600123, TN, India c New Technologies Research Centre, University of West Bohemia, Plzenˇ 30614, Czech Republic d Centre for Nanoscience and Technology, Anna University, Chennai 600025, India e Department of Physics, R.V. Govt. Arts College, Chengalpet 603001, TN, India b
a r t i c l e
i n f o
Article history: Received 7 May 2014 Received in revised form 26 May 2014 Accepted 27 May 2014 Available online 12 June 2014 Keywords: RE3+:CeO2 nanopowders Combustion synthesis Cyclic voltammetry Photoluminescence Microstructure
a b s t r a c t The reported article deals with the synthesis and characterization of rare earth ions doped ceria (RE3+:CeO2) nanopowders from citrate nitrate auto-combustion route. The crystalline nature and lattice planes of the nanocrystalline RE3+:CeO2 powders were analyzed by X-ray diffraction (XRD) and Selected Area Electron Diffraction (SAED) profile fit. The excited state absorption (ESA) and energy transfer up-conversion (ETU) were studied by photoluminescence (PL) measurement. The spectroscopic properties of the powders were studied using Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The surface morphology, average size, distribution and orientation of the lattice planes of the nanoparticles were studied by using scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The size of RE3+:CeO2 nanoparticles were found to be in the range from 15 to 30 nm which has good agreement with the HRTEM results. The changes in current density with increasing sweep scan potential of the doped ceria powders were studied by cyclic voltammetry (CV) analysis. The specific capacitance range of the rare earths doped ceria of Er:CeO2, Pr:CeO2, Yr:CeO2 and Nd:CeO2 were calculated as (12.9–86.5), (20–72.4), (80–375) and (0.92–4.22) respectively. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Cerium dioxide (CeO2), either pure or doped with different rare earth elements is known for industrial materials because of its superior properties of ionic conductivity [1], refractive index, high melting point (2873 K) [2], large dielectric constant, high transparency in the visible-near IR region, band gap between 3.2 and 3.6 eV for direct electron transition [3], high chemical and thermal stability [4] and oxygen storage capacity [5]. These featured properties are leading to the wide-spread use of ceria as components in applications for solid oxide fuel cell (SOFC) electrolytes [6], catalyst supports, carbon monoxide (CO) reduction catalysts [7–9], gas sensors, electro-chromic and UV-protective coatings [10–13]. Ceria nanoparticles are among the most unique and promising nanomaterials being studied today [14,15]. This is mainly due to the diffusion and reactivity of oxygen vacancies in ceria, which contributes to its ⇑ Corresponding authors. Address: Department of Physics, Panimalar Engineering College, Chennai 600123, TN, India. Tel.: +91 44 2649 0404. E-mail addresses: [email protected] (C. Esther Jeyanthi), drkrr2007@ gmail.com (K. Rajarajan). http://dx.doi.org/10.1016/j.jallcom.2014.05.208 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
high oxygen storage capability [16]. Ceria possess strong anti-oxidative activity associated with reversible Ce4+ M Ce3+ reduction/ oxidation processes facilitated by high degree of non-stoichiometry of CeO2 nanocrystals. CeO2 has fluorite-type structure (space group Fm3m) with FCC cubic lattice; each cerium cation is coordinated by eight oxygen anions [17–19]. A high concentration of O-vacancies is associated with a greater conversion of cerium ions from the Ce4+ states to the Ce3+ ones. Excited Ce4+ ions can only follow non-radiative relaxation paths when returning to the ground state. In contrast, the relaxation of excited Ce3+ ions to the ground state includes radiative pathways that result in fluorescence signals in the region 500–580 nm. The initial concentration of O-vacancies has been shown to correlate to the oxygen storage capabilities of ceria and the corresponding conversion of cerium ions between the Ce3+ and Ce4+ states [20,21]. The concentration of oxygen vacancy in ceria can be increased by doping of ceria with trivalent lanthanides or rare-earth ions, such as Sm, Nd, Er and Pr. It has relatively low activation energies between the dopant ions and the O-vacancies, is a viable technique to increase the ionic conductivity and the catalytic activity of ceria [22]. This can lead to an increase in the oxygen diffusion constant
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Table 1 Chemical reactions during the combustion synthesis. 0.9 0.9 0.9 0.9
[Ce(NO3)36H2O] + 0.1 [Ce(NO3)36H2O] + 0.1 [Ce(NO3)36H2O] + 0.1 [Ce(NO3)36H2O] + 0.1
[Er(NO3)35H2O] + 0.783 [C6H8O7] D ? Ce0.9Er0.1O1.95 + gas " (1) [Nd(NO3)36H2O] + 0.783 [C6H8O7] D ? Ce0.9Nd0.1O1.95 + gas " (2) [Pr(NO3)36H2O] + 0.783 [C6H8O7] D ? Ce0.9Pr0.1O1.95 + gas " (3) [Y(NO3)36H2O] + 0.783 [C6H8O7] D ? Ce0.9Y0.1O1.95 + gas " (4)
in the nanomaterial promoted by the high concentration of charged O-vacancies present in doped ceria [23]. The resulting improvement of oxygen storage capability of doped ceria nanoparticles is studied through experimental verifications of both the increase of Ce3+ ionization states, and the associated O-vacancies via photoluminescence studies. Particularly, the doping of ceria with Er3+ improves the sensitivity of the signal more. The present study deals with the synthesis CeO2 with different rare earth (RE) elements (Nd, Er, Pr and Y) by citrate nitrate auto-combustion method, and also reveal the optical, structural and electrochemical properties.
which coincides well with the standard data of CeO2 (JCPDS: 0043-1002). The diffraction lines correspond to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) and (4 0 0) of the pure ceria were observed for the RE3+:CeO2 with small strain. It was observed that, there are no traces of Er, Pr, Y and Nd oxides in the diffraction pattern. Therefore, it substantiates that the RE3+ ions doping in CeO2 is substitutional impurity, which is regarded as single phase formation. The changes in ion size of the substitutional elements influence the very small lattice strain than the pure Ceria (in comparison with the JCPDS file: 00-43-1002). Moreover, the diffraction peaks at 2h = 28° of (1 1 1) were used to estimate the average isotropic crystallite size of the nanoparticles using Scherrer’s formula:
2. Experimental and characterization
DðhklÞ ¼ The main host precursor of cerium nitrate hexahydrate Ce(NO3)36H2O (Aldrich, 99.999%) with different dopants such as Er(NO3)35H2O (Aldrich, 99.9%), Nd(NO3)36H2O (Aldrich, 99.9%), Pr(NO3)36H2O (Aldrich, 99.99%), and Y(NO3)36H2O (Aldrich, 99.9%) were used for the synthesis of each combination of doping in the ratio of 90:10 mol%. For the chemical reaction (combustion), the citric acid anhydrous C6H8O7 (Sigma–Aldrich, 99.5%) of suitable quantities were used according to the balanced calculation from the concepts of propellant chemistry [24]. Stoichiometric amounts of cerium nitrate with respective rare earth chemical were mixed together and dissolved in suitable quantity of de-ionized water. Then the calculated amount of citric acid was also mixed in the solution and stirred mechanically for 2 h at 60 °C to get clear and homogeneous solution. The reaction mixture was put in a platinum or alumina crucible, and it was kept inside a preheated furnace at a temperature of 500 °C. Once the reaction mixture reached the point of spontaneous combustion, it starts burning vigorously. The gaseous volatile molecules such as N2, CO2, H2O and other forms of gaseous products were released during the combustion reaction. Finally, porous solid foam of nanoparticles was obtained within 10 min. The assumed combustion reactions involved among cerium nitrate and fuel with different rare earth nitrate chemicals (nitrates of Er, Nd, Pr and Y, viz.) are given in Table 1. The as-combusted foams were collected and converted to powders by smooth grinding. The same process was repeated for different dopants. The as-prepared ceria powders of different rare earth elements were calcined at 700 °C for 2 h (at a heating rate of 5 °C/min). The calcined RE3+:CeO2 powders were converted into cylindrical compacts with subsequent sintering at 1200 °C for 6 h. The powder X-ray diffraction (XRD) studies were carried for the RE3+:CeO2 powders using Rich-Seifert diffractometer with Cu Ka (k = 1.5405 Å) radiation over the range of 10–70° at a scanning rate of 1°/min. The average particle size of the particles were calculated from full width at half-maximum (FWHM) using Debye–Scherrer’s equation [25] for (1 1 1) crystal face of the nanoparticles. FTIR spectra of the nanopowders were recorded for wavenumber regions 4000–500 cm1 using Perkin–Elmer FTIR spectrometer. Raman spectra in the range of 0–3500 cm1 were carried out using BRUKER-RFS27 FT-Raman spectrometer. Photoluminescence measurements were performed on confocal micro-Raman microscope MonoVista 750 CRS. He–Cd ultraviolet laser was used for excitation emitting on 325 nm wavelength. The measurement system includes Olympus BX51 optical microscope, monochromator with focal distance of 750 mm and cooled back-illuminated CCD camera ProEM: 1600 200. LMU-15X-UVB objective was used for focusing of the beam and collecting signals at ambient room temperature. The structure, surface morphology, shape, size and crystalline planes of the nanoparticles were examined by scanning electron microscope (FEI, Nova NanoSEM 200) and transmission electron microscope (TEM-JEOL JEM 2200 FS). Cyclic voltammetry (CV) was employed to investigate the effectiveness of ceria and doped ceria. The nanopowders were dispersed into colloidal suspensions and further dip-coated on a glassy carbon (GC) plate as working electrode [26]. A platinum wire and Ag/AgCl containing transparent glass were used as counter electrode and reference-electrode, respectively.
kk b cosðhÞ
where k is the X-ray wavelength (1.5405 Å), b is the full width at half-maximum, h is the diffraction angle, k is a constant (0.9), and Dhkl represents the size along (hkl) direction. The size of the rare earths doped ceria particles were found to be in the range from 15 to 30 nm, which is comparable with the size observed from TEM micrographs. 3.2. Up-conversion luminescence in RE3+:CeO2 powders Fig. 2 shows the visible up-conversion luminescence spectra of the RE3+:CeO2 of different dopant ions dependent excitation into different levels of RE3+ using He–Cd laser with excitation wavelength of 325 nm. The photoluminescence spectra of the RE3+:CeO2 nanoparticles exhibit a broad band in blue–green region (from 480 nm to 560 nm). The emission bands in the region 370– 480 nm is generally attributed to charge transfer from O2 to Ce4+, that is, electron transition or charge transfer occurs from oxygen vacancies [27–29]. The region from 370 to 480 nm corresponds to the transition 4G11/2 ? 4I15/2. The region from 500 to 650 nm corresponds to the transition 2H11/2, 4S3/2, F9/2 ? 4I15/2. The pointgroup symmetry of Ce sites in the fluorite CeO2 structure is Oh with
3. Results and discussions 3.1. Powder X-ray diffraction XRD patterns of the rare earths (Er, Nd, Pr and Y) doped CeO2 nanoparticles show the typical cubic fluorite structure (Fig. 1)
Fig. 1. XRD patterns of the RE3+:CeO2 nanoparticles.
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Fig. 2. Up-conversion luminescence spectra of the rare earths doped ceria (RE3+:CeO2) nanoparticles with exciting wavelength of kexc = 325 nm.
Fig. 3. Partial energy level structure of the RE3+:CeO2.
Fig. 5. Raman spectra of the RE3+:CeO2 nanoparticles.
eight-fold oxygen coordination, thereby providing inversion symmetry [30]. Among the other rare earth ions, the Er3+ ion is one of the most efficient ions for up-conversion because the metastable levels of Er3+ can be conveniently populated by ordinary diode lasers or any kind of visible lasers [31]. The up-conversion luminescence intensity of Er3+:CeO2 powders is occurred due to excited state absorption (ESA) and energy transfer up-conversion (ETU) process, respectively, are discussed as the possible mechanisms for the observed up-conversion luminescence. The schematic energy level diagram of the RE3+:CeO2 is shown in Fig. 3 [32,33]. The observed up-conversion luminescence arises due to ESA mechanism which populates thermalized levels (2H11/2, 4S3/2) leads to high efficient green emission and explained as follows. Upon excitation at 325 nm, the ground state Er3+ ions will excite into the 4I11/2 level in singly doped material. After first level excitation, the same pumping source pumps the excited Er3+ ion from 4I11/2 to 4F7/2 level. Subsequent non-radiative relaxation populates the 4S3/2 or 2 H11/2 and 4F9/2 levels. Bright green (546 nm) and weak red (679 nm) emissions were observed due to the transitions (2H11/2, 4 S3/2 ? 4I15/2) and 4F9/2 ? 4I15/2, respectively. 3.3. Fourier transform infrared (FTIR) spectroscopy
Fig. 4. FTIR spectra of the RE3+:CeO2 nanoparticles.
Fourier transform infrared (FTIR) spectra of the RE3+:CeO2 powders are shown in Fig. 4. The spectrum shows some strong intense bands at around 3430, 1630, 1050 cm1 and 540 cm1. In addition, some weaker absorption peaks were also observed at around 2920, 1380 and 730 cm1. The strong absorption band at 3430 cm1 is attributed to the m(O–H) vibration modes of the physically adsorbed water molecules. The peaks observed at 2920 and 1630 cm1 are assigned to the m(C–H) and d(CH2) vibration modes, respectively. These are due to the presence of atmospheric organic moieties in the exposed samples during the sample preparation and handling [34]. The band observed at 1050 cm1 is generally attributed to the cerium–oxygen groups having a larger double bond character. The bands observed in the lower frequency region at 730 and 540 cm1 are typical of cerium–oxygen groups having a lower double bond character and of Ce–O–Ce chains or Ce–O–RE3+ symmetric stretching of the metal oxide network [35,36]. The broadness of the bands for Pr3+:CeO2 in the region from 3430–1000 cm1 by means that the material captures the organic counterparts, which is assumed as a material’s adsorption nature.
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Fig. 6. SEM micrographs of the RE3+:CeO2 nanoparticles.
Fig. 7. EDX spectra from SEM for the RE3+:CeO2 nanoparticles.
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Fig. 8. HR-TEM micrographs of the (a) Er3+:CeO2, (b) Nd3+:CeO2, (c) Pr3+:CeO2, and (d) Y3+:CeO2 nanoparticles.
3.4. Raman spectroscopy Raman spectra of the RE3+:CeO2 nanopowders are shown in Fig. 5. The spectrum is dominated by the triply degenerate F2g band (Raman active) at 465 cm1 assigned to the CeO2 cubic fluorite structure [37]. This Raman active mode is attributed for the symmetric breathing mode of oxygen atoms around cerium/rare-earth (RE3+) ions [38]. Therefore, they are very sensitive to any disorder in the oxygen sub-lattice resulting from thermal, doping, or grain size. In addition, a broad weaker absorption band near F2g in the region 520–630 cm1 is assigned to a band that originates in the oxygen vacancies in metal dioxides [39]. In addition, the Raman spectrum contains smaller absorption at 2073 and 2570 cm1 are due to symmetric C–H stretching of methyl groups from the reaction of hydrocarbon contamination during the sample preparation or handling [40]. Also a broad region from 2900 to 3400 cm1 arises from the O–H stretching vibration of adsorbed H-bonded water [41]. The appearance of weak bands around 158 cm1 is
attributed to tetragonal displacement of the oxygen atoms from their ideal fluorite lattice positions of the RE3+:CeO2 [42]. 3.5. Surface morphology analysis Fig. 6 shows the SEM micrographs RE3+:CeO2 nanopowders calcined at 700 °C. The powders are formed as foam of agglomerated particles. The SEM micrographs of the cluster powders show platelets and flaky structures with large aggregates. Since the RE3+ dopant concentration is a part of tenth, the morphologies of the powders are almost similar to CeO2. The solid foam has porous structure which is due to the evaporation of gases during their decomposition. The energy dispersive X-ray (EDX) spectrum in Fig. 7 confirms the presence of Ce, O and RE elements such as Er, Pr, Nd and Y. The additional undefined peaks corresponding to C and Al are due to the signals from the sample holder. The surface morphology, particles size, distribution, crystallinity and lattice planes of the RE3+:CeO2 nanoparticles were studied
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nanoparticles were obtained and measured by Gatan CCD camera using the Digital Micrograph software and treated by Process Diffraction software [43]. Data treatment included circular integration of SAED intensities from TEM and calibration of the integrated data intensity peaks by means of camera length (CL) for comparison and indexation purposes with the standard PDF (00-43-1002). The SAED spectra in Fig. 9 confirmed the diffraction of electrons from the lattice planes. The SAED profile fit has a good agreement with the XRD patterns. The EDX spectra from TEM were recorded in order to confirm the elements in the composition. The spectra shown in Fig. 10 confirmed the presence of Ce, O, and rare-earth elements (Er, Pr, Nd and Y) of respective combinations. Apart from these elements, the spectra exhibited some elements such as Cu (copper grid), Ni (nickel grid), C and Si (membrane in the grid) which were present in the carbon coated copper grid/nickel grid used as sample holders.
3.6. Cyclic voltammetry studies The CV patterns for 1–5 cycles of CeO2, Er3+:CeO2, Pr3+:CeO2, Nd :CeO2 and Y3+:CeO2 were recorded with different scanning potential using Na2SO4 electrolyte (but it was compared pure and RE doped ceria of fifth cycle of all samples) as shown in Fig. 11. The oxidation peak of Na2SO4 dissolved in distilled water (1.0 mol/L) for the 1st cycle was observed at 2 V for a sweep potential rate of 10 mV/s and 50 mV/s and 100 mV/s. After 5th cycle, the current density increases and anodic wave was slightly shifted towards higher positive potential and the maximum peak was observed at 1.8 V for a sweep potential rate of 10 mV. Due to doping, the peak density of current was gradually increased from 1 to 5 cycles and the charge density ratios were measured for CeO2 and RE3+:CeO2 for different sweep potential voltages of 10 mV/s and 50 mV/s and 100 mV/s. After the 5th cycle, the shape variations of voltammograms were observed both for pure and doped CeO2 as shown in Fig. 11. The CV curve indicates that the charge density of the CeO2 and RE3+:CeO2 lies in between 2 V (for reduction) and +2 V (for oxidation) as a function of number of cycles. After the 5th cycle of oxidation/reduction, loss in the charge density was practically zero. There can be a possible reason for the better cycling performance of the Y3+:CeO2 nanocrystalline electrode such as the 3+
Fig. 9. Selected Area Electron Diffraction (SAED) profile fit for Pr3+:CeO2 and Nd3+:CeO2 nanoparticles.
by using high resolution transmission electron microscopy as shown in Fig. 8. The size of the rare earths doped ceria nanoparticles are varied in the range from 10 to 30 nm. Also, it was found that the cluster of nanoparticles consist of some of the bigger nanoparticles. The larger particles are assumed to be early formed nanocrystals which further grew during the combustion and the calcination. The high resolution images exhibited the crystalline structure and lattice planes arrangement with atomic resolutions. It was found that the nanocrystals are well defined and crystalline. The inter-planar distances of the CeO2 were calculated using Digital Micrograph software, and indexed with reference to the Powder Diffraction File (PDF) No.: 00-43-1002. The Selected Area Electron Diffraction (SAED) patterns from the RE3+:CeO2
Fig. 10. EDX spectra of the Er3+:CeO2 and Nd3+:CeO2 nanoparticles from TEM.
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Fig. 12. Specific capacities of RE3+:CeO2 for the different scanning potential rate.
Table 2 Specific capacities of RE3+:CeO2 for the different scanning potential rate. Specific capacity (F g1)/scan voltage
10 mV
50 mV
100 mV
CeO2 Er:CeO2 Pr:CeO2 Y:CeO2 Nd: CeO2
1.94 86.5 72.4 375.0 4.22
0.61 23.4 37.4 96.8 0.94
0.56 12.9 20 80.4 0.92
the results obtained for the Ce ion occupation factor that it can be attributed to changes in the anion vacancy radius or to a decrease in the cation coordination number which influenced the correctness of the fit that might be responsible for the observed deviation. The results showed that there was very good agreement between experimental and calculated values [44]. The specific capacitance of the ceria based composites is determined and quantitatively evaluated by the expression voltammetric charge/(potential window composite loading). The integral area of CV curve/scan rate represent the sum of anodic and cathodic voltammetric charges [45]. The specific capacity was calculated using the formula;
C¼
Fig. 11. Cyclic voltammograms of RE3+:CeO2 for the scanning potential at 10 mV/s, 50 mV/s and 100 mV/s.
calcined nanoparticles has a high degree of crystallization and reduced defects, which facilitates the Na+ ions insertion and extraction processes. Synthesized nanoparticle/composite electrode provides impressive performance and unique behavior with increasing values of the discharge/charge capacity and cycle number. The current density increased to 0.068 mA/cm2, 0.33 mA/cm2, 0.63 mA/cm2 and 1.4 mA/cm2 for 100 mV of sweep potential rate for the CeO2, Er3+:CeO2, Pr3+:CeO2 and Y3+:CeO2 respectively, but the Nd3+:CeO2 (electrode) was decreased comparing with the rest, which is not considered as a doping effect (oxidization) because of larger atomic radius of Nd3+ ions. Matovic´ et al reported that, for the prepared Nd doped CeO2 solid solution, there was a 4% of discrepancy and the occupation factors decreased to 72%. So that (Nd:CeO2) may decreases its ionic conductivity. They reported
1 2M v ðV a V b Þ
Z
Vb
iðEÞdV
Va
where C is the specific capacitance of individual sample, Va and Vb are the cut-off potentials in cyclic voltammetry, i(E) is the instantaR neous current, i(E)dV is the total voltammetric charge obtained by integration of positive and negative sweep. (Va Vb) is the potential window width, and ‘M’ is the mass of individual sample, which is the mass difference of the working electrode before and after dipcoating, accuracy range of 0.005–0.007 mg (CeO2–5 g, Er3+:CeO2– 6.5 g, Pr3+:CeO2–7 g, Y3+:CeO2–7 g and Nd3+:CeO2–5 g) to weigh the mass of the samples and ‘m’ is the potential scan rate. Electrostatically the storing charge is determined by surface area. While, for the latter, charge is stored in virtue of highly reversible redox reactions between Ce4+/3+ species. The specific capacitances of the RE3+:CeO2 were calculated and compared with the pure CeO2 nanoparticle. It was found that, the specific capacitance of the REs doped ceria are greater than pure ceria as shown in Fig. 12. The specific capacitance based on CeO2 normalized capacitance is obtained from the CV curves by deducting the capacitance of RE doped CeO2 composites, which are tabulated (Table 2). It was found that, the Y3+:CeO2 has a high specific capacitance of 375 F/g comparing other RE doped CeO2 nanocomposites [46].
C. Esther Jeyanthi et al. / Journal of Alloys and Compounds 614 (2014) 118–125
4. Conclusions The combustion synthesized rare earths (Er, Nd, Pr and Y) doped CeO2 (RE3+:CeO2) nanoparticles reveals the cubic fluorite structure of the pure CeO2 with small lattice strains. The photoluminescence spectra of the nanoparticles exhibit a broad band in blue–green region from 480 nm to 560 nm. The conversion efficiency of the Er3+:CeO2 is more predominant than other dopants. The red to green up-conversion emissions were observed at 540 nm and 690 nm, respectively. The Raman spectra are dominated by the triply degenerate F2g Raman active mode at 465 cm1 which is assigned to the CeO2 cubic fluorite structure. The weaker absorption band near F2g in the region 520–630 cm1 has been assigned to a band that originates in the oxygen vacancies in metal dioxides, which confirms the doping of the rare earth elements. The high resolution TEM microscopy showed that the RE3+:CeO2 has well defined particle size (15–30 nm) and distribution. The lattice planes, orientation and crystalline structure were confirmed by HRTEM micrographs and SAED profile fit, respectively. The increasing current density with sweep scan potential in the cyclic voltammograms confirmed that the RE3+:CeO2 can be used as an electrolyte for solid oxide fuel cells. From the calculation of specific heat capacitance, it was found that the Y3+:CeO2 has higher value of capacitance 375 F/g comparing with the other RE doped CeO2 nanocomposites. Acknowledgments The authors R.S and R.M wish to acknowledge the ‘‘Centre of New Technologies and Materials (CENTEM)’’ Project No. CZ.1.05/ 2.1.00/03.0088 for the partial development of the results. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2014. 05.208. References [1] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563–588. [2] G. Balakrishnan, S. Tripura Sundari, P. Kuppusami, P. Chandra Mohan, M.P. Srinivasan, E. Mohandas, V. Ganesan, D. Sastikumar, Thin Solid Films 519 (2011) 2520–2526. [3] T. Wiktorczyk, P. Bieganski, E. Zielony, Opt. Mater. 34 (2012) 2101–2107. [4] R.N. Blumenthal, F.S. Brugner, J.E. Garnier, J. Electrochem. Soc. 120 (1973) 1230–1237. [5] G. Chen, F. Zhu, X. Sun, S. Sun, R. Chen, Cryst. Eng. Commun. 13 (2011) 2904– 2908. [6] C. Laberty-Robert, J.W. Long, K.A. Pettigrew, R.M. Stroud, D.R. Rolison, Adv. Mater. 19 (2007) 1734–1739. [7] J. Kaspar, P. Fornasiero, M. Graziani, Catal. Today 50 (1999) 285–298.
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