Low frequency ultrasound assisted sequential and co-precipitation syntheses of nanoporous RE (Gd and Sm) doped cerium oxide

Abstract

Cerium oxide (CeO₂), Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures were synthesized using 42 kHz ultrasound assisted sequential and co-precipitation techniques. The nanoporous nature of the powders was revealed from the BET analysis which showed that the observed pore size distributions and the H₂ hysteresis loops of the respective nanostructures confirmed the existence of nanopores in various sizes and shapes. The nanoporous nature of the synthesized powders were further confirmed by the high resolution transmission electron microscopy (HRTEM) and high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) analyses. The perceived shift in the characteristic F_2g Raman active band centered at 464 cm⁻¹ indicated the defects were increased during the sequential co-precipitation synthesis of Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures. Moreover, the intensity of the F_2g peaks can be ordered as CeO₂ < Ce_0.9Sm_0.1O_1.95 (CP)¹ < Ce_0.9Gd_0.1O_1.95 (SP)² < Ce_0.9Sm_0.1O_1.95 (SP)² < Ce_0.9Gd_0.1O_1.95 (CP)¹ which indicates the increase in the concentration of the defects like oxygen vacancies enhances the efficiency of the solid oxide fuel cells (SOFCs) as electrolyte materials. The diffuse reflectance UV-vis solid state spectroscopic analysis demonstrated the narrowing optical band gap of CeO₂ during the sequential and co-precipitation of the Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures.

---

1. Introduction

The incessant usage of non-renewable energy resources leads to the initiation of two major threats to the economy of developing countries firstly, the scarcity and the cost of fossil fuels. The second and most important issue is the environmental pollution associated with the usage of fossil fuels which leads to unpredictable perilous effects throughout the world. To avoid the deleterious effects, fuel cells (FCs) are electrochemical devices which are non-polluting, have higher energy densities and efficiencies when compared to any other energy storage devices being produced.^1,2 Among the various categories of FCs, solid oxide fuel cells (SOFCs) have become easier for the portable and stationary applications however the operating temperature needs to be reduced to improve the next generation of SOFCs. The operating principle of SOFCs can be simply explained as follows: the conversion of fuel into ions and electrons at the anode followed by the reaction of ions with the oxidants at the cathode.^2,3 The electrolyte packed between the anode and the cathode generates the transport of the oxygen ions, which endorses the efficiency of the SOFCs.⁴ The enhanced physicochemical characteristics of the electrolytes certainly increase the efficiency of the SOFCs. The large scale preparation of the electrolytes through economically viable methodologies makes the electrolyte materials inexpensive.

The nanomaterials synthesized by the ultrasound assisted technique exhibited various interesting and unconventional physicochemical characteristics. The utilization of low frequency or power ultrasound (20–100 kHz) has been significantly increased in the recent years for various applications.^5–9 The extreme conditions (high temperature and pressure) produced during the ultrasound irradiation generate non-selective free radicals within the aqueous medium which accelerates the chemical reaction among the precursors and enhances the physicochemical characteristics of the subsequent nanomaterials. On the other hand, cerium dioxide (CeO₂) is a highly refractive ceramic material which is utilized for the various applications of day-to-day life.^10–15 The CeO₂ and modified CeO₂ electrolytes possess interesting features for SOFCs. The balanced behaviour of Ce³⁺/Ce⁴⁺ generates the properties which enable the significant contribution of CeO₂ in SOFC applications. The doping of rare earth ions into the CeO₂ significantly increases the oxygen vacancies and oxygen storage properties of the resulting electrolytes.^16,17 Nevertheless, the loading of rare earth oxides (RE₂O₃) into CeO₂ enhances the oxygen ion conductivity and the mechanical properties of the resultant nanomaterials.^18,19

CeO₂ and modified CeO₂ nanomaterials prepared by using various procedures such as precipitation,²⁰ hydrothermal,²¹ spray-pyrolysis,²² sol–gel²³ and combustion²⁴ were reported in the literature. The low frequency ultrasound assisted synthesis of CeO₂ and modified CeO₂ nanomaterials was expected to modify the physicochemical characteristics and to improve the thermal and mechanical properties of the resulting solid electrolytes. In the present work, CeO₂, Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures were synthesized by using 42 kHz ultrasound. The effect of sequential and co-precipitation on the structural, morphological and optical properties of the Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures was investigated. The developed methodology can be extended for the preparation of various nanomaterials.

2. Experimental

2.1. Materials and methods

Nitrates of cerium, gadolinium and samarium, and sodium hydroxide were purchased from Sigma-Aldrich and used as starting materials for the syntheses of CeO₂, Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures without further purification. Unless otherwise specified, all reagents used were of analytical grade and the solutions were prepared using double distilled water. The crystallite sizes of the synthesized nanomaterials were calculated from the X-ray diffraction data (XRD, Philips PW1710 diffractometer, CuK_α radiation, Holland) using the Scherrer equation. The surface morphologies and microstructures of the nanostructures were analyzed by transmission electron microscopy (HRTEM, FEI TITAN G2 80-300) operated at 300 keV. Diffuse reflectance UV-vis spectra of the nanostructures were recorded using a Shimadzu 2550 spectrophotometer equipped with an integrating sphere accessory employing BaSO₄ as a reference material. Raman spectra were recorded using a Dilor LabRam-1B spectrometer, with a 633 nm line of He–Ne laser source with 5.5 mW power. The surface areas, pore volumes and pore diameters of the nanostructures were measured with the assistance of Flowsorb II 2300 of Micrometrics, Inc. The sonochemical reactions in this study were carried out using a commercially available sonicator (8890, Cole-Parmer, USA) producing 42 kHz ultrasonic waves.

2.2. Preparation of CeO₂, Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures

The co-precipitation of Ce_0.9Gd_0.1O_1.95 was carried out according to the procedure that we have reported earlier.²⁵ For sequential precipitation, a slight modification in the synthesis procedure was carried out as follows: the appropriate quantity of the nitrate precursor of gadolinium and CeO₂ was added to 200 mL of double distilled water under vigorous stirring for 15 min. 50 mL of 1 M NaOH was prepared separately. The sonicator was turned on and the time was taken as “time zero” for ultrasound irradiation at the same time as the NaOH was added drop-wise to the nitrate precursor under vigorous stirring. The ultrasound irradiation of the suspension was continued for 30 min. The solid solution was collected by subsequent filtration (0.45 μm nylon membrane filters). The solid solution was dried at 110 °C for 12 h followed by the calcination at 700 °C for 2 h. Similar procedure was adopted for the preparation of the Ce_0.9Sm_0.1O_1.95, CeO₂, Gd₂O₃ and Sm₂O₃ nanostructures. The stoichiometric concentration of CeO₂ and nitrate precursors of gadolinium and samarium was used for the sequential precipitation of Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanocomposites.

3. Results and discussion

The X-ray diffraction analysis of the nanomaterials synthesized using low frequency ultrasound is presented in Fig. 1. The formation of cubic structured Gd₂O₃ and Sm₂O₃ were identified according to the JCPDS numbers 74-1807 and 11-0608, respectively. The bare Sm₂O₃ demonstrated a poor crystalline nature when compared to the bare Gd₂O₃. The bare CeO₂ exhibited a cubic fluorite crystal structure (JCPDS no. 34-0394) as evidenced by Fig. 1. The sequential and co-precipitation syntheses of Ce_0.9Gd_0.1O_1.95, Ce_0.9Sm_0.1O_1.95 revealed that the cubic fluorite crystal structure of CeO₂ was not altered during the synthesis of the nanomaterials. The CeO₂ diffraction pattern was dominant in the doped nanomaterials resulting from the sequentially and co-precipitated cerium (Ce⁴⁺) and dopant rare earth (Gd³⁺ and Sm³⁺) precursors. The observed strategy confirmed that the Gd³⁺ and Sm³⁺ entered into the crystal lattice of CeO₂ which can be evidenced by the absence of the corresponding diffraction patterns of dopant rare earth oxides. However, the penetration of Gd³⁺ and Sm³⁺ in the sequentially precipitated nanostructures was likely to happen during the calcination and there was no core shell structure observed. Besides, further analysis is needed to confirm the morphology and crystal structure of the sequentially and co-precipitated Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures.

Fig. 1 X-ray diffraction patterns of Gd₂O₃ (a), Sm₂O₃ (b), CeO₂ (c), sequentially precipitated Ce_0.9Gd_0.1O_1.95 (d), Ce_0.9Sm_0.1O_1.95 (e) and co-precipitated Ce_0.9Gd_0.1O_1.95 (f), Ce_0.9Sm_0.1O_1.95 (g).

The Raman spectra observed for the (co-precipitation and sequential precipitation) prepared nanostructures are shown in Fig. 2. They suggest that the vibrational changes occurred due to the introduction of rare earth oxides into the crystal structure of CeO₂. The cubic fluorite crystal structures of CeO₂, Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 can be identified from the strong and intense Raman active (F_2g) bands at 455–470 cm⁻¹. The oxygen vacancies in the crystal structure of CeO₂ can be illustrated by the two less intensive bands which appear at the range of 250–290 and 605–625 cm⁻¹.²⁶ The CeO₂ showed its characteristic F_2g Raman active band centered at 464 cm⁻¹, the sequentially and co-precipitated Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures showed ∼± 2 nm shift in the F_2g Raman active band for the cubic fluorite structure (inset of Fig. 2). The observed decrease in the intensity of the F_2g band showed the increase in the concentration of defects like oxygen vacancies.²⁹ The intensive Raman bands observed at 361 (B_g) and 345 cm⁻¹ (A_g and F_g modes) suggested the cubic crystal structures of Gd₂O₃ and Sm₂O₃.^27,28 The absence of the Raman active modes of Gd₂O₃ (361 cm⁻¹), Sm₂O₃ (345 cm⁻¹) and the shift observed at the F_2g Raman active band for the cubic fluorite crystal structure (464 cm⁻¹) clearly designated the incorporation of the rare earths into the crystal structure of CeO₂. On the other hand, the intensity of the F_2g peaks can be ordered as follows: CeO₂ < Ce_0.9Sm_0.1O_1.95 (CP)³ < Ce_0.9Gd_0.1O_1.95 (SP)⁴ < Ce_0.9Sm_0.1O_1.95 (SP)² < Ce_0.9Gd_0.1O_1.95 (CP)¹.

Fig. 2 Raman spectra of various nanostructures (SP and CP denotes the sequential and co-precipitation syntheses).

The HRTEM micrographs observed for the bare CeO₂, Gd₂O₃ and Sm₂O₃ are presented in Fig. 3. The CeO₂ exhibited nanoparticle morphology and the average grain size was ∼20 nm (Fig. 3a). The finger print lattice fringe distance was calculated as 0.31 nm (Fig. 3b) which corresponds to the (1 1 1) crystal plane of the cubic fluorite structure of CeO₂. The micrographs of Gd₂O₃ and Sm₂O₃ revealed nanorod morphologies of several nanometers in length and thickness along with approximately 15% nanoparticle morphology (Fig. 3c and e). The finger print lattice fringe distances were calculated as 0.31 nm and 0.32 for the corresponding (2 2 2) crystal planes of Gd₂O₃ (Fig. 3d) and Sm₂O₃ (Fig. 3f), respectively. The HRTEM micrographs of the sequentially and co-precipitated Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures demonstrated the formation of clear internal crystal lattice structures (Fig. 4 and 5). This confirmed that high crystallinity was achieved during the synthesis of the nanomaterials. The HRTEM micrographs (Fig. 4) of the sequentially (Fig. 4 a–c) and co-precipitated (Fig. 4 d–f) Ce_0.9Gd_0.1O_1.95 nanostructures demonstrated that the significant number of bare Gd₂O₃ nanorods (Fig. 3c) were transformed to the nanoparticle morphology.

Fig. 3 HRTEM micrographs of CeO₂ (a and b), Gd₂O₃ (c and d) and Sm₂O₃ (e and f).

Fig. 4 HRTEM micrographs of sequentially precipitated Ce_0.9Gd_0.1O_1.95 (a–c) and co-precipitated Ce_0.9Gd_0.1O_1.95 (d–f).

Fig. 5 HRTEM micrographs of sequentially precipitated Ce_0.9Sm_0.1O_1.95 (a–c) and co-precipitated Ce_0.9Sm_0.1O_1.95 (d–f).

The HRTEM micrographs (Fig. 5) of the sequentially precipitated (Fig. 5 a–c) Ce_0.9Sm_0.1O_1.95 nanostructures showed 25% nanorod morphology whereas the co-precipitated (Fig. 5 d–f) Ce_0.9Sm_0.1O_1.95 nanostructures showed only 5% nanorod morphology. However, the ratio of nanorods versus nanoparticles had significantly decreased compared to the bare Sm₂O₃ (Fig. 3e). A similar kind of surface distribution of samarium was detected on the surface of CeO₂ when compared with the Ce_0.9Gd_0.1O_1.95. This confirmed that the 42 kHz low frequency ultrasound was adequate to stimulate the initiation of Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures from the precursors. The grain size achieved for the Ce_0.9Gd_0.1O_1.95 (Fig. 4a and d) and Ce_0.9Sm_0.1O_1.95 (Fig. 5a and d) nanopowders was calculated to be ∼10 nm from the HRTEM analysis. The calculated grain size from the HRTEM analysis was in good agreement with the grain size calculated from the XRD patterns using the Debye–Scherrer equation. The observed decrease in the grain size suggested an increase in the defects of individual nanomaterials.³⁰

The formation of nanoporous structures is shown in Fig. 4b, c and f, 5c, e and f (circled). The HRTEM micrographs additionally demonstrated the formation of Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 when the precursors underwent the ultrasonic irradiation. The sequentially precipitated nanostructures exhibited a greater number of nanopores when compared to the co-precipitated materials. The formation of the nanoporous structure was not observed for the bare Gd₂O₃ and Sm₂O₃ under the same laboratory conditions. The nanoporous structure of CeO₂ was noticed from Fig. 3a, however the HRTEM analysis evidently suggested that the number of nanopores in CeO₂ was found to increase during the synthesis of the rare earth doped CeO₂.

The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) analysis of the sonochemically synthesized Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 revealed the formation of nanopores which can be easily identified as the black blotches in the micrographs (Fig. 6a and b & Fig. 7a and b). The diameters of the nanopores were measured as ∼1 to 2 nm during the HAADF-STEM analysis. The sequentially precipitated Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 exhibited a large number of nanopores when compared to the co-precipitated nanostructures. The HAADF emission by analysis confirmed the formation of Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 and no other phases belonging to impurities were detected. The EDX analysis of the corresponding nanomaterials is presented in Fig. 6c and d & Fig. 7c and d. The elemental analysis indicated the stoichiometric ratio of the nanomaterials.

Fig. 6 HAADF-STEM micrographs of sequentially precipitated Ce_0.9Gd_0.1O_1.95 (a), Ce_0.9Sm_0.1O_1.95 (b) and the corresponding EDX images (c and d).

Fig. 7 HAADF-STEM micrographs of co-precipitated Ce_0.9Gd_0.1O_1.95 (a), Ce_0.9Sm_0.1O_1.95 (b) and the corresponding EDX images (c and d).

Brunar, Emmett and Teller (BET) analysis was performed for the sonochemically synthesized nanostructures to understand the various structural and textural properties of CeO₂, sequentially and co-precipitated Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95. Fig. 8 shows the type IV adsorption–desorption profiles measured for all the sonochemically synthesized nanopowders. The CeO₂, Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 adsorption–desorption profiles demonstrated the narrow H₂ type hysteresis loop which was not observed for Gd₂O₃ and Sm₂O₃. The observed H₂ type hysteresis loops revealed the complex pore structures which can also be evidenced by the HRTEM and HAADF-STEM analyses. The surface areas and the various parameters associated with the pores are presented in Table 1 which illustrates the decrease in the surface area of the sequentially precipitated Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 compared to the bare CeO₂. Besides, Barrett–Joyner–Halenda (BJH) analysis (Fig. 9) showed that the pore volume distribution attained for the bare CeO₂ was not significantly altered during the sequential precipitation. However, the H₂ hysteresis observed for co-precipitated rare earth doped ceria was broader (Fig. 8) than the bare CeO₂. Also, the surface areas and pore diameters of the co-precipitated Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 were modified compared to the bare CeO₂ (Table 1 and Fig. 9). Considerable changes occurred during co-precipitation indicated by the formation of different sized and shaped nanopores³¹ under the low frequency ultrasound assisted process. Moreover, Table 1 clarifies the various nanoporous properties of the synthesized nanostructures.

Fig. 8 BET analysis of various nanostructures (SP and CP denotes the sequential and co-precipitation).

Table 1 BET and BJH characteristics of the sonochemically synthesized nanostructures

S. no.	Nanopowder	BET surface area (m^2 g^−1)	Total pore volume (cm^3 g^−1)	Micropore volume (cm^3 g^−1)	Mesopore volume (cm^3 g^−1)	Pore diameter (nm)
a Co-precipitation.b Sequential precipitation.
1.	CeO2	41	0.11	0.02	0.09	11.7
2.	Ce0.9Gd0.1O1.95 (CP)^a	48	0.10	0.02	0.08	8.3
3.	Ce0.9Sm0.1O1.95 (CP)^a	38	0.09	0.02	0.08	9.7
4.	Gd2O3	23	0.04	0.01	0.03	7.1
5.	Sm2O3	30	0.05	0.01	0.04	7.1
6.	Ce0.9Gd0.1O1.95 (SP)^b	36	0.10	0.02	0.08	11.9
7.	Ce0.9Sm0.1O1.95 (SP)^b	37	0.10	0.02	0.08	11.8

---

Fig. 9 BJH pore size distribution curve for the co-precipitated {() Ce_0.9Gd_0.1O_1.95 and () Ce_0.9Sm_0.1O_1.95}, () CeO₂ and sequentially precipitated {() Ce_0.9Gd_0.1O_1.95 and () Ce_0.9Sm_0.1O_1.95} nanostructures.

The diffuse reflectance (DR)-UV-vis spectral analysis of the synthesized nanostructures is presented in Fig. 10. The bare Gd₂O₃ and Sm₂O₃ showed their absorption maxima in the ultraviolet region and the absorption of bare CeO₂ can be indexed in the visible light region. The characteristic visible light absorption of the bare CeO₂ considerably demonstrated that the number of defects (fluorite crystal structure) was increased during the ultrasound irradiations which led to a shift in the characteristic absorption when compared to the commercial CeO₂.³² The absence of the characteristic absorption of Gd₂O₃ and Sm₂O₃ in the DR-UV-vis spectra also supported the formation of Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 in addition to the Raman and HRTEM analyses. The absorption band edge was blue shifted for the sequentially precipitated Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 whereas it was red shifted for the co-precipitated Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 when compared to the same band of the bare CeO₂ (Fig. 10). The Tauc plot derived from the Kubelka–Munk function is shown in Fig. 11. The optical band gap of the bare CeO₂ was 2.228 eV which was significantly lower than the band gap reported for commercial CeO₂.³² The optical band gaps of the sequentially precipitated Ce_0.9Gd_0.1O_1.95 (2.321 eV) and Ce_0.9Sm_0.1O_1.95 (2.410 eV) were higher than the bare CeO₂ (2.228 eV). Besides, the co-precipitated Ce_0.9Gd_0.1O_1.95 (2.214 eV) and Ce_0.9Sm_0.1O_1.95 (2.212 eV) showed narrower optical band gaps than CeO₂. The respective properties of Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95, when compared to the optical band gap of bare Gd₂O₃ (5.05 eV), Sm₂O₃ (4.5 eV) and CeO₂, indicated that the rare earth oxides acted as band gap modifiers during the ultrasound assisted sequential and co-precipitation.

Fig. 10 Diffuse reflectance (DR)-UV-vis spectra of various nanostructures (SP and CP denotes the sequential and co-precipitation).

Fig. 11 Tauc plot of the synthesized nanostructures calculated from the DR-UV-vis spectrum using the Kubelka and Munk function (SP and CP denotes the sequential and co-precipitation).

4. Conclusion

The CeO₂, Gd₂O₃, Sm₂O₃, Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 nanostructures were successfully synthesized using 42 kHz ultrasound assisted sequential and co-precipitation techniques. The nanorod morphologies of the Gd₂O₃ and Sm₂O₃ were transformed into nanoparticle morphologies during the preparation of Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95. The HRTEM analysis significantly indicated the decrease in the grain sizes of the sequential and co-precipitated Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 compared to the bare CeO₂, Gd₂O₃ and Sm₂O₃. In accordance with the grain size, BET and BJH analyses illustrated that the surface areas, nanoporous textures and pore diameters of the rare earth doped nanoceria were modified compared to the bare and commercially available CeO₂. Moreover, the observed shift shown in the Raman spectra and the band gap narrowing during the diffuse reflectance UV-vis investigation illustrated that the defects (oxygen vacancies) increased in the Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 compared to the bare CeO₂, Gd₂O₃ and Sm₂O₃. The increased number of nanopores was identified from the HAADF-STEM analysis and the BET analysis demonstrated that the pore diameter of the sequentially precipitated rare earth doped nanoceria was not significantly modified compared to the bare CeO₂ and co-precipitated rare earth doped nanoceria. Therefore, the ultrasound assisted synthesis of Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 will make a significant contribution to improve the ion conducting properties which enhance the efficiency of solid oxide fuel cells.

Acknowledgements

The authors would like to thank FONDECYT no. 1130916 Government of Chile, Santiago, for financial assistance.

References

1. Fuel cell technology: reaching towards commercialization, ed. N. Sammes, Springer, London, 2006.
2. A. Hermanna, T. Chaudhuria and P. Spagnol, Bipolar plates for PEM fuel cells: A review, Int. J. Hydrogen Energy, 2005, 30, 1297–1302.
3. S. Srinivasan, Fuel cells: from Fundamentals to applications, New York, Springer, 2006.
4. S. P. Jiang, Development of lanthanum strontium manganite perovskite cathode materials of solid oxide fuel cells: a review, J. Mater. Sci., 2008, 43, 6799–6833.
5. S. Pilli, P. Bhunia, S. Yan, R. J. LeBlanc, R. D. Tyagi and R. Y. Surampalli, Ultrasonic pretreatment of sludge: A review, Ultrason. Sonochem., 2011, 18, 1–18.
6. H. Okkay, M. Bayramoglu and M. F. Oksuzomer, Ce_0.8Sm_0.2O_1.9 synthesis for solid oxide fuel cell electrolyte by ultrasound assisted co-precipitation method, Ultrason. Sonochem., 2013, 20, 978–983.
7. S. Cho, J. Kim, J. Chun and J. Kim, Ultrasonic formation of nanobubbles and their zeta-potentials in aqueous electrolyte and surfactant solutions, Colloids Surf., A, 2005, 269, 28–34.
8. Y. Wang, D. Zhao, W. Ma, C. Chen and J. Zhao, Enhanced sonocatalytic degradation of azo dyes by Au/TiO₂, Environ. Sci. Technol., 2008, 42, 6173–6178.
9. P. Sathishkumar, R. V. Mangalaraja, H. D. Mansilla, M. A. Gracia-Pinilla and S. Anandan, Sonophotocatalytic (42 kHz) degradation of Simazine in the presence of Au/TiO₂ nanocatalysts, Appl. Catal., B, 2014, 160–161, 692–700.
10. P. Trogadas, J. Parrondo and V. Ramani, Platinum supported on CeO₂ effectively scavenges free radicals within the electrolyte of an operating fuel cell, Chem. Commun., 2011, 47, 11549–11551.
11. P. Jasinski, T. Suzuki and H. U. Anderson, Nanocrystalline undoped ceria oxygen sensor, Sens. Actuators, B, 2003, 95, 73–77.
12. T. Mori and J. Drennan, Influence of microstructure on oxide ionic conductivity in doped CeO₂ electrolytes, J. Electroceram., 2012, 17, 749–757.
13. X. Lu, T. Zhai, H. Cui, H. Shi, S. Xie, Y. Huang, C. Liang and Y. Tong, Redox cycles promoting photocatalytic hydrogen evolution of CeO₂nanorods, J. Mater. Chem., 2011, 21, 5569–5572.
14. S. Yabea and T. Satob, Cerium oxide for sunscreen cosmetics, J. Solid State Chem., 2003, 171, 7–11.
15. J. F. de Lima, R. F. Martins, C. R. Neri and O. A. Serra, ZnO:CeO₂-based nanopowders with low catalytic activity as UV absorbers, Appl. Surf. Sci., 2009, 255, 9006–9009.
16. B. Matović, J. Dukić, A. Devečerski, S. Bošković, M. Ninić and Z. Dohčević-Mitrović, Crystal structure analysis of Nd-doped ceria solid solutions, Sci. Sintering, 2008, 40, 63–68.
17. B. Choudhury and A. Choudhury, Lattice distortion and corresponding changes in optical properties of CeO₂ nanoparticles on Nd doping, Curr. Appl. Phys., 2013, 13, 217–223.
18. T. S. Zhang, J. Ma, L. B. Kong, P. Hing and J. A. Kilner, Preparation and mechanical properties of dense Ce_0.8Gd_0.2O_2−δ, ceramics, Solid State Ionics, 2004, 167, 191–196.
19. A. Akbari-Fakhrabadi, R. V. Mangalaraja, F. A. Sanhueza, R. E. Avila, S. Ananthakumar and S. H. Chan, Nanostructured Gd–CeO₂ electrolyte for solid oxide fuel cell by aqueous tape casting, J. Power Sources, 2012, 218, 307–312.
20. A. I. Y. Tok, L. H. Luo and F. Y. C. Boey, Carbonate co-precipitationof Gd₂O₃-doped CeO₂ solid solution nano-particles, Mater. Sci. Eng., A, 2004, 383, 229–234.
21. F. Zhang, S. P. Yang, H. M. Chen and X. B. Yu, Preparation of discrete nano size ceria powder, Ceram. Int., 2004, 30, 997–1002.
22. C. Goulart and E. Djurado, Synthesis and sintering of Gd-doped CeO₂ nanopowders prepared by ultrasonic spray pyrolysis, J. Eur. Ceram. Soc., 2013, 33, 769–778.
23. G. S. Wua, T. Xiea, X. Y. Yuana, B. C. Cheng and L. D. Zhang, An improved sol–gel template synthetic route to large-scale CeO₂ nanowires, Mater. Res. Bull., 2004, 39, 1023–1028.
24. L. D. Jadhava, M. G. Chourashiya, K. M. Subhedar, A. K. Tyagi and J. Y. Patil, Synthesis of nanocrystalline Gd doped ceria by combustion technique, J. Alloys Compd., 2009, 470, 383–386.
25. P. Sathishkumar, R. V. Mangalaraja, O. Rozas, H. D. Mansilla, M. A. Gracia-Pinilla and S. Anandan, Low frequency ultrasound (42 kHz) assisted degradation of Acid Blue 113 in the presence of visible light driven rare earth nanoclusters loaded TiO₂ nanophotocatalysts, Ultrason. Sonochem., 2014, 21, 1675–1681.
26. R. Kydd, W. Y. Teoh, K. Wong, Y. Wang, J. Scott, Q. H. Zeng, A. B. Yu, J. Zou and R. Amal, Flame-synthesized ceria-supported copper dimers for preferential oxidation of CO, Adv. Funct. Mater., 2009, 19, 369–377.
27. C. Le Luyer, A. Garcia-Murillo, E. Bernstein and J. Mugnier, Waveguide Raman spectroscopy of sol–gel Gd₂O₃ thin films, J. Raman Spectrosc., 2003, 34, 234–239.
28. S. Jiang, J. Liu, C. Lin, X. Li and Y. Li, High-pressure X-ray diffraction and Raman spectroscopy of phase transitions in Sm₂O₃, J. Appl. Phys., 2013, 113, 113502–113506.
29. X. Yao, C. Tang, Z. Ji, Y. Dai, Y. Cao, F. Gao, L. Dong and Y. Chen, Investigation of the physicochemical properties and catalytic activities of Ce_0.67M_0.33O₂ (M = Zr⁴⁺, Ti⁴⁺, Sn⁴⁺) solid solutions for NO removal by CO, Catal. Sci. Technol., 2013, 3, 688–698.
30. L. Ilieva, G. Pantaleo, I. Ivanov, A. M. Venezia and D. Andreeva, Gold catalysts supported on CeO₂ and CeO₂–Al₂O₃ for NO_x reduction by CO, Appl. Catal., B, 2006, 65, 101–109.
31. A. Grosman and C. Ortega, Capillary condensation in porous materials. Hysteresis and interaction mechanism without pore blocking/percolation process, Langmuir, 2008, 24, 3977–3986.
32. S. A. Ansari, M. M. Khan, M. O. Ansari, S. Kalathil, J. Lee and M. H. Cho, Band gap engineering of CeO₂ nanostructure using an electrochemically active biofilm for visible light applications, RSC Adv., 2014, 4, 16782–16791.

---