Preparation and Characterization of Doped Ceria Nano Particles by Chemical Precipitation
Devi Radhika and A. Samson Nesaraj*,
Department of Chemistry, Karunya University, Coimbatore – 641 114, Tamil Nadu, India.
*Corresponding Author E-mail: drsamson@karunya.edu
ABSTRACT:
In this present research work, chemical precipitation technique was employed to synthesize doped ceria nano particles. The precursor materials, such as, cerium nitrate hexahydrate (basic material) and sodium hydroxide (precipitator material) were used to prepare different compositions of phase pure doped ceria (Ce0.9Gd0.1O2-δ, Ce0.9Y0.1O2-δ, Ce0.8Gd0.2O2-δ and Ce0.8Y0.2O2-δ) with an intention to use in solid oxide fuel cells (SOFCs) as electrolytes. The physico-chemical properties of the resultant oxides were characterized by XRD, FT-IR, particle size analysis and SEM. The XRD results revealed the formation of well-crystalline cubic fluorite structure in all the doped ceria powders after calcination at 750oC. The SEM observation showed an average grain size of about 50 – 100nm for the optimal doped ceria powder and particle size analysis gave a narrow distribution of particle size. These results suggest that doped ceria can further improve the use of SOFC at low temperatures.
KEYWORDS: Ceria, SOFC, chemical precipitation, physical characterization.
INTRODUCTION:
Solid oxide fuel cells (SOFC) are solid state electrochemical devices, which have attracted a great deal of attention as promising systems for electrical power generation because of the high conversion efficiency and low pollution emission1,2. These are two types based on their operating temperatures: high temperature SOFCs and low or intermediate temperature SOFCs (i.e.IT-SOFCs). However, the high operating temperatures have led to many problems, such as electrode sintering, interfacial diffusion between electrolyte and electrodes, mechanical stress due to different thermal expansion coefficients and high cost3. If the operation temperature of the cell can be reduced to the intermediate or low temperature range, (500-700oC), fabrication cost also will also be greatly reduced. Over the past few years, considerable research activities made to lower the operating temperature of SOFCs4-6. To overcome these kinds of problems, the following approaches are normally adopted: decreasing the electrolyte thickness7, developing alternative electrolyte materials with high ionic conductivity at intermediate or low temperature and minimizing electrode polarization resistance7-9.
There have been many researchers conducted research to find alternative electrolyte materials, e.g.CeO210. Ceria-based materials have been extensively studied as the most promising electrolytes for IT-SOFCs. The ionic conductivity of ceria resulting from oxygen vacancies depends on the dopants and their amount7, 9,11. Ceria-based electrolytes easily develop n-type electronic conduction at high temperatures and low oxygen partial pressures12. As one of the most reactive rare earth oxides, ceria and ceria based materials play an important role in various applications as catalysts13. The doping of other rare earth materials such as Gd14 and Y15 into the lattice has been confirmed to be an effective way to improve both the stability and oxygen storage capability. Pure CeO2 is a poor ionic conductor and has the fluorite structure with oxygen vacancies (V0) as the predominant ionic defect7. The substitution of Ce4+ by larger cations such as Gd3+ has been examined in great detail by a number of authors, and Gd-doped ceria is considered for being one of the best ceria based solid electrolytes currently available.
Various synthesis and processing methods have been used to prepare doped ceria with desired properties such as hydrothermal16, co-precipitation10,17-23, combustion24, sol-gel method25, 26 and solid state reaction process27. Among the various synthetic approaches, solid state synthesis requires a high calcinations temperature (above 1000oC) for the formation of a homogeneous phase. It has been normal practice to repeatedly calcine and grind the powder mixture in order to achieve the desired homogeneity. Therefore, the risk of contamination from the grinding media is greater in this process. In general, however, wet chemical processes are capable of producing high purity, homogeneous and ultrafine powders at lower temperature. The synthesis of ceria powder by various wet chemical routes, such as, co-precipitation, hydro thermal solution and the combustion routes has been reported. In many of the methods, the main objective is to reduce the costs of chemical synthesis and to produce pure compounds for technological applications28 -29. The preparation of doped ceria based nano particles by co-precipitation method was not at all discussed elaborately in the literature. Hence, we propose a simple method of making doped ceria particles without any surfactants in presence of a precipitant, sodium hydroxide.
Fig. 1 - Schematic representation of the synthesis of doped ceria nano particles by a chemical precipitation method.
MATERIAL AND METHODS:
Preparation of Ce0.9Gd0.1O2-δ , Ce0.9Y0.1O2-δ, Ce0.8Gd0.2O2-δ and Ce0.8Y0.2O2-δ nanoparticles by chemical precipitation:
High purity Ce(NO3)3.6H2O, Gd2O3 /Y2O3, nitric acid and sodium hydroxide were used in the preparation of Ce0.9Gd0.1O2-δ / Ce0.9Y0.1O2-δ / Ce0.8Gd0.2O2-δ / Ce0.8Y0.2O2-δ nanoparticles. Appropriate concentrations of 100 ml of cerium nitrate hexahydrate (Ce(NO3)3).6H2O), 100 ml of gadolinium nitrate (Gd(NO3)3) / 100 ml yttrium nitrate (Y(NO3)3) and sodium hydroxide were made by dissolving them in distilled water. The corresponding nitrate salt solutions were mixed in a container. To which sodium hydroxide was added drop-wise and the solution was stirred continuously to complete precipitation under controlled pH (> 9). Afterwards, the mixture was stirred for 2 h and the resultant yellow coloured precipitate [(Ce(OH)4 + Gd(OH)3) or (Ce(OH)4 + Y(OH)3)] was filtered. The resultant precipitate was washed thoroughly by distilled water and ethanol mixture (volume ratio 9:1) to remove the impurities. Na+ ions which might have physically adsorbed on the precipitate were removed when the precipitate was washed with distilled water and ethanol mixture. The washed precipitate was dried in an oven at 50-100o C for 2-3 hours for the complete removal of the absorbed water. After evaporating the water, the dried precipitate was heat treated at four different temperatures, such as 300, 450, 600 and 750o C for 2 hours each to get a phase pure doped ceria nanoparticle. Fig. 1 shows the schematic illustration of the synthesis of Ce0.9Gd0.1O2-δ / Ce0.9Y0.1O2-δ / Ce0.8Gd0.2O2-δ / Ce0.8Y0.2O2-δ by the chemical precipitation process. Main reactions occur during the experimental procedure can be written briefly as follows:
Reaction mechanism for Ce0.9Gd0.1O2-δ / Ce0.9Y0.1O2-δ :
0.39 NaOH → 0.39 Na+(aq) + 0.39 OH-(aq),
0.09 Ce(NO3)3.6H2O(s) → 0.09 Ce3+(aq) + 0.27 NO3- (aq) + 6H2O(aq),
0.01 Gd/Y(NO3)3 → 0.01Gd3+//Y3+(aq)+ 0.03 NO3-(aq),
0.09 Ce3+(aq) +0.01 Gd3+/Y3++0.39OH-(aq)+ xH2O (aq → 0.09 Ce(OH)4 .x H2O(s)↓ + 0.01 Gd/Y(OH)3.x H2O(s)↓,
50-100o C
0.09 Ce(OH)4.xH2O(s) + 0.01 Gd/Y(OH)3.x H2O(s) → 0.09 Ce(OH)4(s) + 0.01 Gd/Y(OH)3(s)+ x H2O(g) ↑,
300/ 450/ 600 / 750o C for 2 hours
0.09 Ce(OH)4(s) + 0.01 Gd/Y(OH)3 → 0.1Ce0.9Gd0.1O2-δ /0.1Ce0.9Y0.1O2-δ + x H2O(g) ↑
Reaction mechanism for Ce0.8Gd0.2O2-δ / Ce0.8Y0.2O2-δ :
0.38 NaOH → 0.38 Na+(aq) + 0.38 OH-(aq),
0.08 Ce(NO3)3.6H2O(s) → 0.08 Ce3+(aq) + 0.24 NO3- (aq) + 6H2O(aq),
0.02 Gd/Y(NO3)3 → 0.02Gd3+//Y3+(aq)+ 0.06 NO3-(aq),
0.08 Ce3+(aq) +0.02 Gd3+/Y3++0.38OH-(aq)+ xH2O (aq → 0.08 Ce(OH)4 .x H2O(s)↓ + 0.02 Gd/Y(OH)3.x H2O(s)↓,
50-100o C
0.08 Ce(OH)4.xH2O(s) + 0.02 Gd/Y(OH)3.x H2O(s) → 0.08 Ce(OH)4(s) + 0.02 Gd/Y(OH)3(s)+ x H2O(g) ↑,
300/ 450 / 600 / 750o C for 2 hours
0.08 Ce(OH)4(s) + 0.02 Gd/Y(OH)3 → 0.1Ce0.8Gd0.2O2-δ / 0.1Ce0.8Y0.2O2-δ + x H2O(g) ↑
Characterization of the samples:
The powder XRD studies were carried out using a Shimadzu XRD6000 X-ray diffractometer at a scan speed of 5 degrees minute-1 using CuKα radiation. The lattice parameters were calculated by least square fitting method using DOS computer programming. The theoretical density of the powders was calculated with the obtained XRD data. The crystallite sizes of the powder were calculated by Scherrer’s formula. Bruker IFS 66V FT-IR spectrometer was employed to record the FTIR spectra of doped CeO2 powders in the range of 4000 – 400 cm-1. The crystallite sizes of the ceramic powders were calculated by Scherrer’s formula. The particle size of the powder was measured using Malvern Particle Size Analyzer using triple distilled water as medium. The surface morphology of the particles was studied by means of JEOL Model JSM-6360 scanning electron microscope.
Fig. 2. XRD patterns obtained on calcined doped CeO2 nanoparticles prepared by the chemical precipitation method (a) Ce0.9Gd0.1O2-δ , (b) Ce0.9Y0.1O2-δ, (c) Ce0.8Gd0.2O2-δ and (d) Ce0.8Y0.2O2-δ
Fig. 3. FTIR spectra obtained on calcined doped CeO2 nanoparticles prepared by the chemical precipitation method (a) Ce0.9Gd0.1O2-5 , (b) Ce0.9Y0.1O2-δ, (c) Ce0.8Gd0.2O2-δ and (d) Ce0.8Y0.2O2-δ
RESULTS AND DISCUSSION:
XRD studies obtained on doped CeO2 powders:
The XRD patterns of the Gd / Y doped CeO2 powder prepared by the chemical precipitation method are shown in Fig. 2 (a-d). The XRD patterns on the heat treated powders reveal the formation of well-crystallined single phase materials. It can be seen from the figures there were no clear impurity phases in the samples. The XRD patterns obtained on the doped CeO2 particles were matched with the standard data for CeO2 (JCPDS card No. 34-0394). The lattice parameters were calculated by DOS computer programming. We observed that Gd / Y doped CeO2 powders exhibited cubic (f.c.) cell geometry as reported10,14. The unit cell volumes calculated for the ceria particles remained similar. The theoretical density, Dth is calculated using the equation30: The formula for measuring theoretical or X-ray density (in gcm-3) is
M
Dth = z --------
N x V
Where ‘M’ (in atomic-weight units) is the mass of atomic ensemble constituting one unit of the chemical formula, ‘z’ is the number of such chemical units in one unit cell of the crystal, N is the Avagadro’s number and ‘V’ (in Å3) is the volume of the crystalline unit cell as determined by X-ray diffraction. These values were also similar to the reported data24.
Table 1. XRD data obtained on doped CeO2 nanoparticles
|
Sample |
Crystal structure |
Unit Cell parameter ‘a’ (Å) |
Unit cell volume (Å3) |
Crystallite Size (nm) |
Theoretical density (g/cc) |
|
CeO2 (JCPDS No. 34-0394) |
cubic (f.c.) |
5.4113 |
158.45 |
-- |
7.22 |
|
Ce0.9Gd0.1O2-δ |
cubic (f.c.) |
5.4244 |
159.60 |
12.603 |
7.20 |
|
Ce0.9Y0.1O2-δ |
cubic (f.c.) |
5.4208 |
159.29 |
11.870 |
6.92 |
|
Ce0.8Gd0.2O2-δ |
cubic (f.c.) |
5.4048 |
157.88 |
11.880 |
7.38 |
|
Ce0.8Y0.2O2-δ |
cubic (f.c.) |
5.4109 |
158.41 |
11.860 |
6.78 |
Fig. 4. Particle size analysis patterns obtained on calcined doped CeO2 nanoparticles prepared by the chemical precipitation method (a) Ce0.9Gd0.1O2-δ , (b) Ce0.9Y0.1O2-δ, (c) Ce0.8Gd0.2O2-δ and (d) Ce0.8Y0.2O2-δ
Fig. 5. SEM photographs obtained on calcined doped CeO2 nanoparticles prepared by the chemical precipitation method (a) Ce0.9Gd0.1O2-δ , (b) Ce0.9Y0.1O2-δ, (c) Ce0.8Gd0.2O2-δ and (d) Ce0.8Y0.2O2-δ
Table 2. Particle characteristics data obtained on doped ceria nano particles powder prepared by chemical precipitation method
|
Sample |
Peak 1 |
Peak 2 |
Average particle size (d.nm) |
||
|
% intensity |
Diameter (nm) |
% intensity |
Diameter (nm) |
||
|
Ce0.9Gd0.1O2-δ |
91.7 |
5.477 |
8.3 |
1459 |
5.209 |
|
Ce0.9Y0.1O2-δ |
100 |
0.6213 |
-- |
-- |
0.4012 |
|
Ce0.8Gd0.2O2-δ |
100 |
386.1 |
-- |
-- |
385.1 |
|
Ce0.8Y0.2O2-δ |
100 |
1372 |
-- |
-- |
193.1 |
The broadening of X-ray diffraction peaks provides a convenient method for measuring small particle sizes. As the crystallite size decreases the width of the diffraction peak (or the size of the diffraction spot) increases. An approximate expression for the peak broadening is given by Scherrer’s equation31:
0.9 λ
D= ---------
β cos θ
Where ‘D’ is crystallite size in nm, ‘λ’ is the radiation wavelength (for CuKα radiation, λ = 1.5418 Å), ‘θ’ is the diffraction peak angle and ‘β’ is the broadening of the line (“half width”) measured at half its maximum intensity (in radians). The crystallite size of the particles, determined with Scherrer’s formula, was found to be in the range of 11.86 – 12.60 nm . It was reported that the average crystallite size of pure CeO2 prepared by thermal decomposition of cerous nitrate was found to be in the range of 18.8 – 22.1 nm32. Therefore, our values are less than the reported data. The crystallographic parameters obtained on the doped CeO2 nanoparticles are given in Table 1.
FTIR studies obtained on doped CeO2 powders:
Figs. 3 (a-d) show the FTIR spectra obtained on doped CeO2 nanoparticles prepared by the chemical precipitation method. FTIR measurements were done using the KBr method at RT. According to the standard IR spectra, peaks appeared at around 1400 and 500 cm-1 as well as the shoulder peaks that appear around 1500 – 1700 cm-1 are attributed to Ce-O vibration mode.33. The samples showed a peak at 2400 cm-1 which is due to the presence of dissolved or atmospheric CO2 in the samples34. The peaks which are appeared at around 3400 cm-1 are related to the O-H stretching vibration of H2O in the samples35. The peaks appeared in all the samples are similar to each other, which indicate the structural similarities in Gd / Y doped CeO2 samples.
Particle size measurements obtained on doped CeO2 powders:
The prepared doped ceria particles were subjected to particle size measurements using Malvern particle size analyzer with triple distilled water as medium. For all the measurements, the sample is sonicated in triple distilled water for about 5 minutes and after that the sample is subjected for particle size analysis. The particle size distribution curves of doped CeO2 are shown in Fig. 4 (a-d). The particle characteristics obtained on doped ceria power are indicated in Table 2.The results revealed that Ce0.9Y0.1O2- δ has less particle size (0.4 nm) when compared with other samples. Large particles (~385 nm) are present in Ce0.8Gd0.2O2- δ. Hence, it was noticed that both nano and micro particles are present in all the samples. The agglomeration of particles may be due to the high temperature treatment36.
Scanning Electron Microscopic(SEM) studies of doped CeO2 powders:
From the micrographs (Fig. 5 (a–d)), it was understood that nanosized particles are present in all the samples. The grain size is not uniform in all the samples. The grain size is varied in the range of 50 to 100 nm. The presence of bigger grains in the sample may be due to the combination of few particles together. The average grain size is found to be around 80 nm.
CONCLUSIONS:
Chemical precipitation process can be effectively used for the preparation of phase pure Ce0.9Gd0.1O2-δ / Ce0.9Y0.1O2-δ / Ce0.8Gd0.2O2-δ / Ce0.8Y0.2O2-δ nanoparticles using cheap chemicals such as cerium nitrate hexa hydrate, Gd2O3, Y2O3, nitric acid and sodium hydroxide. The powder XRD data obtained on doped CeO2 nanoparticles is in good agreement with the standard reported JCPDS data. All the samples have cubic (f.c.) crystal structure. From the FTIR data, it is understood that the characteristic peak of Ce-O is present all the samples. The particulate properties of all the samples suggest that the particles are present in nanometer and micrometer range. The presence of microparticles may be due to the agglomeration of smaller particles at high temperature.
ACKNOWLEDGMENTS:
ASN thanks Karunya University for promoting nanomaterials based research activity in the Department of Chemistry.
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Received on 05.08.2011 Modified on 17.08.2011
Accepted on 25.08.2011 © AJRC All right reserved
Asian J. Research Chem. 4(9): Sept, 2011; Page 1447-1452