Synthesis and Microstructural Characterization of Lanthanum Doped Cerium Oxide by Solution Combustion Synthesis

Abstract

Materials based on doped ceria are considered promising elements for applications in solid oxide fuel cells (SOFCs). Cerium oxide can be classified as a mixed conductor. The type of dopant also greatly influences properties of doped ceria. Hence, studying the type of dopant to be used in the synthesis process is of great importance. Recent studies have shown that the lanthanide or alkaline earth ions are the most commonly dopants used in ceria. In order to obtain nanometric powders, which favor the catalytic effect and are more reactive than other powders, a technique of obtaining powders via solution combustion synthesis (SCS) was selected, and the type of fuel used, and its excess (content) were analyzed. The parameters that were varied in this study were related to the dopant (Ce_(1-x) La_(x) O_{(2- δ)}, where x = 0.1, 0.2, 0.3) and the type of fuel used (urea or sucrose) The powders were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the TGA, a significant increase in the remaining mass loss was observed with an increase in the dopant content when both urea and sucrose were used. The SCS method enabled the production of lanthanum oxide doped ceria phase using both fuels. The XRD of the samples obtained using urea as fuel exhibited well-defined, narrow, and intense peaks immediately after synthesis, and this characteristic was maintained after thermal treatment. On the other hand, the use of sucrose as fuel enables the production of the same cristalinity after thermal treatment at 850°C. In addition, these samples had a higher specific surface area and smaller crystallite size compared to those obtained using urea as fuel.

Keywords:
Ce_(1-x) La_(x) O_(2-δ); microstructure; combustion synthesis; dopant and fuel effects

1. Introduction

Fuel cells are continuous electrochemical transducers that convert chemical energy into electrical energy. Their principle of operation can be summarized as combining one oxygen atom with two hydrogen atoms to produce water, electrical energy, and thermal energy¹.

Materials based on doped ceria are considered promising elements for applications in solid oxide fuel cells (SOFCs). Cerium oxide can be classified as a mixed conductor, as it exhibits both ionic and electronic conductivity, and its electrical properties depend on the temperature, oxygen partial pressure, dopant concentration, and impurity content. In such materials, ionic conductivity occurs due to the mobility of oxygen vacancies at sufficiently high temperatures².

The importance of studying the microstructure of powders lies in its considerable influence on the electrical and ionic conductivity in fuel cells. In general, the microstructural characteristics with the strongest influence on ionic conductivity include the electrolyte porosity, the segregation of impurities to grain boundary regions, the grain size, and the formation of space charges at the boundaries. Reducing the average grain size, and hence increasing the grain boundary area, can reduce the impurity concentration per unit area, thus facilitating the flow of oxygen vacancies through these interfaces³.

The type of dopant also greatly influences properties of doped ceria, according to Yahiro et al.⁴. Hence, studying the type of dopant to be used in the synthesis process is of great importance. Recent studies have shown that the lanthanide or alkaline earth ions are the most commonly dopants used in ceria⁵.

The method used to synthesize the powder therefore plays a crucial role in this process, as it directly affects the microstructural characteristics of the material. Obtaining powders on a nanometer scale with controlled distribution of aggregate sizes favors the sintering process, and reduces the temperature required for material densification⁶.

According to the literature, nanometric cerium oxide powders have been obtained through various techniques, such as co-precipitation⁷-⁹, sol-gel¹⁰,¹¹, and solution combustion synthesis (SCS)¹²-¹⁷. In these studies, it was observed that nanometric powders with crystallite sizes of between 20 and 40 nm can be obtained by the sol-gel method¹⁰,¹¹, from 20 to 30 nm by co-precipitation⁹, and from 10 to 40 nm via SCS¹²-¹⁷.

Of the methods listed, SCS is a particularly promising technique, since it allows for rapid synthesis of powders (within a few minutes), with nanometric particles that are well-dispersed and highly homogeneous¹³-¹⁸.

This technique was previously used to synthesize gadolinium-doped cerium oxides using glycine and urea as fuel¹⁹. In another study, the technique was employed to synthesize cerium oxides (CeO₂) doped with gadolinium, samarium, calcium, and yttrium, also using urea as fuel²⁰. In these works, the authors synthesized nanometric and monophasic powders with particles measuring approximately 10 nm. This demonstrates the efficacy of urea as a fuel in the combustion synthesis process, and shows that the type of fuel can alter the final characteristics of the synthesized powders (particle size and homogeneity).

Other authors¹⁰,²¹,²² have studied the synthesis process of CeO₂ using lanthanum as a dopant, via SCS, using citric acid or glycerol as a complexing agent¹⁰,²¹, or by co-precipitation using ammonia as a precursor²². The powders obtained via SCS, of these studies, showed structural properties that were compatible with application in SOFCs. Although the SCS process has been used to obtain CeO₂ doped with lanthanum oxide, no researchers have used sucrose as fuel while varying the dopant (La) content in the powder synthesis process.

The objective of this work is therefore to study the effects of varying the type of fuel (urea and sucrose) and the concentration of dopant inserted into the cerium oxide lattice (Ce_(1-x) La_(x) O_{(2- δ)}, where x = 0.1, 0.2, 0.3) on the characteristics of the obtained powders, with the aim of synthesizing samples with suitable characteristics for potential use in SOFCs.

2. Materials and Methods

Using the SCS method, lanthanum nitrate (99.0% purity) and cerium nitrate (99.0% purity) from the VETEC brand were used as oxidizing agents, and sucrose (C₁₂H₂₂O₁₁, 99.3% purity) and urea ((NH₂)₂CO, 99.8% purity) were used as reducing agents.

The stoichiometric composition of the precursor solution for combustion synthesis and the molar quantity of fuel to be added to the solution were determined by the method proposed by Jain et al.²³, as shown in Equation 1 below.

$\varnothing =\sum \frac{\left(\mathrm{Coefficient\; of\; the\; oxidizing\; element}\right)*\left(\mathrm{valence}\right)}{\left(\mathrm{Coefficient\; of\; the\; reducing\; element}\right)*\left(\mathrm{valence}\right)}$ (1)

where Φ is the stoichiometric elemental composition. Using this equation, it was also possible to calculate the molar quantity of fuel to be added to the solution.

The nitrates and fuels were mixed, with the proportion of lanthanum nitrate varying from 10% to 30% in a reducers-to-oxidizers stoichiometric ratio of 1:2 for the fuel urea and 1:1 for the fuel sucrose, a fuel excess solution, to give the compositions in Table 1. The mixture was heated on a heating mantle at 350°C, where the reaction occurred. The powders resulting from this synthesis process were calcined at 850°C and characterized before and after calcination. In the calcination process, a LINDBERG electric furnace, model 59545-B, was used. The heating rate was 10°C/min until reaching a temperature of 850°C, remaining at this temperature for 1 hour. The temperature was determined based on thermogravimetric analysis (ATG).

Table 1
Summary of the compositions considered here.

Nomenclature	Fuel	% Lanthanum nitrate (mol)
10LaSAC	Sucrose	10
20LaSAC	Sucrose	20
30LaSAC	Sucrose	30
10LaURE	Urea	10
20LaURE	Urea	20
30LaURE	Urea	30

The X-ray diffraction (XRD) technique was used to identify the resulting crystalline phases using a PHILIPS diffractometer (model X’PERT MPD), equipped with a graphite monochromator and Cu-Kα radiation (λ=1.5406Å). Analyses were conducted within an angular range (2θ) of between 20° and 75°, with a step size of 0.05°, a counting time of 2 s, and ½ slits. The results were processed using the X’pert Highscore program, based on the International Centre for Diffraction Data (ICDD) database.

The crystallite size before and after calcination was obtained by Scherrer’s equation, where D is the crystallite size, λ is the wavelength of the X-ray source (1.5406Å), β is the full width at half maximum (FWHM) in radians, and θ corresponds to the Bragg angle.

$D=\frac{\mathrm{0,95}\lambda }{\beta \cos \theta }$ (2)

To determine the specific surface area of the samples, the QUANTACHROME NOVA 1000 analyzer was used.

The variation in the remaining mass of the samples after combustion synthesis was conducted by thermogravimetric analysis (TGA). This technique involves monitoring the mass variation of a sample as a function of temperature in a controlled temperature and atmosphere environment. The equipment used was the TGA Q50 V20.13 BUILD 39, with nitrogen gas at a flow rate of 90 ml/min over a temperature range from 20°C to 900°C, and a heating rate of 20°C per minute.

Lastly, a microstructure analysis was conducted using scanning electron microscopy (SEM) and TEM. For SEM, a JEOL microscope (model JSM-6060) was utilized and In the TEM analysis, the JEM-1400 Jeol Transmission Electron Microscope was used with an operating capacity of 120 keV.

3. RESULTS

The first analysis carried out was TGA (Figure 1), which allowed us to determine the mass loss of the samples with increasing temperature (Table 2). Through this process, it was possible to identify the temperature at which the greatest loss of material occurred, and thus to define the calcination temperature of the powders after combustion synthesis.

Figure 1
TGA of powders after synthesis, with varying types of fuel and percentage of lanthanum oxide.

Table 2
Mass loss as a function of temperature, with varying dopant content and type of fuel.

SAMPLES	MASS LOSS
10LaSAC	0.8%
20LaSAC	1.2%
30LaSAC	1.6%
10LaURE	2.2%
20LaURE	2.8%
30LaURE	3.2%

Based on the TGA results, we observed that the synthesized powders yielded different outcomes as we increased the dopant content, and as the type of fuel was varied (urea or sucrose).

Thermal decomposition takes place in two or three steps, depending on the fuel amount and type, and complete decomposition is observed above 850 ºC. At around 200 ºC, the loss can be assigned to moisture. At 300 ºC and 400 ºC, there is a possible decomposition of reactant residues, and starting at 800 ºC, the phase final formation. The samples synthesized with sucrose presents lower mass loss than samples synthesized with urea. Therefore, the synthesis with sucrose presents a lower amount of organic residues after synthesis. It probably must have reached a higher temperature during the synthesis, which resulted in the burning of organic matter.

From the data in the graph and Table 2, it can be seen that increasing the dopant content and using urea tended to give a greater residual mass loss of the samples with increasing temperature. Based on the results obtained through thermal analyses, it was defined that the calcination temperature would be 850°C, as an intermediate value for all samples.

The results obtained indicate that the samples that used urea as fuel showed a greater loss of mass when compared to sucrose. It is likely that the solutions containing sucrose, during synthesis, have become more viscous, retaining more of the gases generated in combustion and, consequently, causing a greater expansion of the gel formed, shows less organic material after combustion synthesis in solution that was not eliminated during synthesis. Regarding the lanthanum content in the sucrose samples, reflecting directly as we will see (Table 3) on the surface area of the samples.

Table 3
Specific surface area (BET), Crystallite sizes and Lattice Parameter of the samples before and after calcination.

SAMPLES	Specific surface area before calcination (m2/g)	Specific surface area after calcination (m2/g)	Crystallite size before calcination (nm)	Crystallite size after calcination (nm)	Lattice Parameter Before Calcination (nm)	Lattice Parameter After Calcination (nm)
CeO2 [JCPDS file no. 81-0792]	-	-	-	-	5,4124	-
10LaSAC	23.26	19.23	4.82	20.51	5,4371	5,4359
20LaSAC	55.50	26.52	3.12	13.67	5,4391	5,4730
30LaSAC	35.33	29.26	3.66	12.62	5,4747	5,5022
10LaURE	9.78	8.73	18.23	23.58	5,4171	5,5056
20LaURE	10.39	9.78	16.41	21.22	5,4587	5,4359
30LaURE	11.07	10.52	14.65	19.29	5,4956	5,4663

The X-ray diffractograms of the powders obtained before and after calcination, using sucrose and urea as fuels and with varying lanthanum oxide contents in the cerium oxide lattice, are presented in Figures 2.

Figure 2
X-ray diffractograms of the synthesized samples, with varying lanthanum oxide concentration and two types of fuel (a) before and (b) after calcination at 850°C.

Figure 2 (a) and (b) show X-ray diffractograms of the synthesized samples varying the lanthanum oxide concentration and the type of fuel used before and after calcination at 850°C, respectively. All peaks (*) were identified as cerium oxide with fluorite-like structure (JCPDS no. 81-0792) and the shifts of the positions relative to ceria may indicate lanthanum doping. Since the ionic radius of La⁺³ (0.122nm) is larger than that of Ce⁺⁴ (0.102nm), the dislocations to small positions 2q and the increase in the lattice parameter (Table 3) suggest the replacement of Ce⁺⁴ by La⁺³ (La_Ce) as well the increase in point defects.

Before calcination, the diffractograms for the sample obtained using sucrose as fuel (Figure 3 d,e,f) showed broader peaks, indicating a smaller crystallite size and low-intensity peaks. However, after the thermal treatment, the diffractogram aspects of these samples demonstrated better crystallinity, with well-defined, narrow and intense peaks. The samples obtained using urea as fuel exhibited well-defined, narrow, and intense peaks immediately after synthesis, and this characteristic was maintained after thermal treatment (calcination). This finding is consistent with observations made by other authors¹⁰,¹⁹,²⁰ who synthesized the same system (doped CeO) via sol-gel¹⁰ or SCS, using urea as fuel¹⁹,²⁰.

Figure 3
X-ray diffractograms of the synthesized sample, before and after calcination at 850°C, varying lanthanum oxide concentration and the types of fuel, urea (a, b, c) and sucrose (d, e, f).

In the samples obtained using sucrose as fuel with a lower concentration of Lanthanum (10%LaSac), the presence of another phase (2θ=45°) (#) is observed, before calcination (Figure 2a). After calcination at 850°C, the same phase is observed with less intensity (Figure 2b). The same phase was also observed, but at a lower intensity in the sample with 10% Lanthanum (10% LaUre) using urea as fuel. This phase was not identified through the JCPDS.

The crystallite size was calculated from the Scherrer equation and based on the FWHM of the (111) plane. The results (Table 3) indicated that the samples exhibited different characteristics after thermal treatment and Lanthanum addition. After thermal treatment, there was an increase in the crystallite size due to coalescence, thus providing an increase in sample crystallinity following the thermal treatment. The samples obtained using sucrose tended to show a significant increase in crystallite size after thermal treatment compared to those obtained using urea. However, the crystallite sizes are smaller than those obtained using urea as fuel, even after thermal treatment. These values are in the same order of magnitude as those observed by other authors¹⁰,¹⁹,²⁰. In these works, a crystallite size in the range of 10 nm was observed when urea was used as a complexing agent.

An analysis of the specific surface area shows that the sucrose samples gave superior results compared to those of urea, both before and after calcination; this finding is consistent with the calculations performed using the Scherrer method.

The thermal treatment resulted in a decrease in the specific surface area, and hence an increase in crystallite size, as observed in Table 3. After calcination, the samples showed a gradual increase in specific surface area with increasing dopant content, especially when sucrose was used. On the other hand, calcination after synthesis using urea as fuel, did not influence the final properties of the powders. This demonstrates that the use of urea as fuel does not require subsequent calcination to obtain the same phases as when using sucrose.

The lanthanum addition tends to reduce the crystallites size and increase the specific surface area and lattice parameters, before and after calcination, regardless of the type of fuel used.

Figures 4 and 5 show SEM micrographs of the resulting powders after thermal treatment at 850°C.

Figure 4
SEM micrographs of the synthesized powders obtained using sucrose, after calcination: (a) 10%La, (b) 20%La, and (c) 30%La, at 5,000x magnification.

Figure 5
SEM micrographs of the synthesized powders obtained using urea, after calcination: (a) 10%La, (b) 20%La, and (c) 30%La, at 5,000x magnification.

From the micrographs obtained, the size, shape and state of agglomeration of the analyzed samples can be observed. The type of dopant inserted in the cerium oxide network exerts little influence on the microstructural aspect of the synthesized powders. Therefore, the type of fuel has more influence on the microstructural characteristics of the resulting powders. This behavior is probably due to the generation of gases observed in the type of fuel used, favoring the formation of a more viscous solution that tends to reduce the heat during synthesis, which may have led to greater agglomeration of the particles, as observed in Figure 5 (using urea) retaining gases and causing the expansion of the gel. With the formation of this more viscous solution, the generation of gases during synthesis may have removed heat from the reaction and decreased the available energy, giving rise to a more agglomerated powder.

It can be observed that sucrose favors the formation of powder clusters and finer particles. This result confirms the observations reported by Tarragó et al.²⁴, where sucrose was used as a fuel to obtain lanthanum manganite.

In the Figure 6 can be observed the TEM micrograph of the sample synthesized with 20% lanthanum oxide using sucrose as fuel (20LaSac). The results show that the use of sucrose as fuel in the synthesis by combustion in solution favors the production of nanometric powders, presenting particles with an average diameter below 50nm.

Figure 6
TEM micrograph of post-synthesized samples, after calcination, of 20LaSAC samples. with scale size of 100nm (A) and 50nm (B).

4. Conclusion

From this work, it is possible to conclude that the SCS method enabled the obtention of lanthanum oxide doped ceria with control over the lanthanum oxide concentration. This synthesis method is also an effective technique for obtaining nanometric powders. Furthermore, a simple change of fuel tends to alter the resulting microstructure. The dopant content has influence on the final characteristics of the samples, in terms of crystallinity, crystallite size, and specific surface area, was also observed.

Based on the results obtained here, it can be concluded that the use of sucrose as fuel favors the production of smaller crystallites size, especially when 20 mol% dopant (La) is used. The specific surface area of these samples was also higher than that observed when urea was used as fuel. On the other hand, the use of urea as fuel makes it possible to obtain the same phases obtained when using sucrose as fuel without the need for calcination of the powders after synthesis, except in the sample with the lowest lanthanum content. The microstructure obtained through SEM indicated the production of powders with a foam-like and homogeneous appearance with both fuels.

The addition of dopants tended to decrease the crystallites size and increase the specific surface area regardless of the type of fuel used. Also increase the lattice parameters, demonstrated the replace of Ce⁺⁴ by La⁺³ and obtention of lanthanum oxide doped ceria.

In general, the SCS method enabled the production of lanthanum oxide doped ceria without thermal treatment when urea is used as fuel, and after thermal treatment, when sucrose as used as fuel. Furthermore, it was possible to obtain nanometric cristallites, with high specific surface area, especially when sucrose was used as fuel. These characteristics favors its use as electrode in SOFC.

5. Acknowledgements

The authors are grateful to CAPES and CNPQ for their financial support for this work, and CNPQ and CME-UFRGS for the SEM-EDS analysis.

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Publication Dates

• Publication in this collection
  24 Feb 2025
• Date of issue
  2025

History

• Received
  24 Nov 2024
• Reviewed
  10 Jan 2025
• Accepted
  19 Jan 2025