The Effect of Calcination Temperature on the Characteristics of CeO 2 Synthesized Using the Precipitation Method

was synthesized using the precipitation


Introduction
Cerium oxide (CeO2) is a metal oxide that has a fluorite cubic structure (FCC) [1].With a redox potential of 1.7 V, it possesses the unique property of being able to undergo redox cycling between Ce 3+ ⇌ Ce 4+ [2,3].The redox cycle allows CeO2 to be applied as an energy storage device [4], materials for making supercapacitors [5], catalysts [6], nanozymes in plants [7], and antioxidants [8,9].As antioxidants, CeO2 are capable of degrading •OH radicals, with their activity being influenced by several factors such as particle size and the presence of crystal defects [10].
The synthesis of CeO2 can be carried out using several methods, such as reverse microemulsion [6], hydrothermal [11], sol-gel [12,13,14], green synthesis [15,16,17] combustion [18,19], ball-milling [20,21], precipitation and coprecipitation [22,23,24,25].The precipitation method is the most commonly used because it is easy to perform, cost-effective, has high reaction rates, and can be conducted on both large and small scales.Synthesis of CeO2 using the precipitation method can be carried out by adding a solution of cerium (III) nitrate to a base solution, such as ammonium hydroxide, to form a precipitate.The precipitate is then dried and calcined at temperatures ranging from 300-700°C [26].
The characteristics of CeO2 produced, such as particle shape and size, crystalline properties, and morphology, are influenced by the temperature and duration of calcination [26].The characteristics of CeO2 significantly impact its performance when used as an antioxidant [27].Literature studies indicate that the crystallite and particle size tend to increase with the rise in calcination temperature from 350 to 700°C, while the surface area tends to decrease.The best radioprotective and antioxidant activities occur at a calcination temperature of 500°C [22,26,27,28].
The synthesis of inorganic materials by the precipitation method typically results in particles with non-uniform shapes and sizes.Surfactants can be introduced to address this issue, as they are capable of forming micelles that regulate particle growth, resulting in particles with uniform shapes and sizes [29].Cetyltrimethylammonium bromide (CTAB) surfactant has been proven to reduce particle size in the synthesis of CeO2 using the coprecipitation method [30].
This study aims to synthesize CeO2 using the precipitation method, with cerium (III) nitrate hexahydrate serving as the cerium precursor and CTAB surfactant acting as the morphology-directing agent.Based on literature studies, it can be hypothesized that varying the temperature will affect the morphology of CeO2.Therefore, this study varies calcination temperatures at 500°C, 600°C, and 700°C to examine its impact on the characteristics of CeO2, including product purity, crystallinity, particle size and shape, and fluorescence properties.

Method
A 50 mL cerium precursor solution was produced by dissolving 21.71 grams of cerium (III) nitrate hexahydrate and 0.0164 grams of CTAB in distilled water.Subsequently, the solution was treated with 32% NH4OH until a precipitate formed or the solution reached a pH of 9. Afterward, the solution was allowed to stand for 48 hours, and the precipitate was separated from the solution.The precipitate was then dried in an oven at 60°C for three hours and subsequently subjected to calcination at varying temperatures of 500, 600, and 700°C for 2 hours each.The resulting samples were respectively labeled Ce500, Ce600, and Ce700.

Characterization
Functional group analysis was performed using FTIR with the KBr pellet method within the wavenumber range of 500-4000 cm -1 .Crystallinity and crystal size were assessed utilizing XRD with a 2θ angle range of 25-100°.Surface morphology profiles were acquired through imaging with SEM-EDS.The effect of temperature on the fluorescence properties of the resulting CeO2 was investigated using a fluorescence spectrophotometer at a wavelength range of 200-600 nm.

Results and Discussion
Figure 1 shows the physical appearance of the CeO2 powder produced at different calcination temperatures.CeO2 has a pale yellow powder form at all calcination temperatures with a weight of about 2.7 grams or a yield of about 93%.The formation of CeO2 is believed to follow Reaction (1).
The addition of ammonia solution (NH4OH) as an OH -source in the solution will lead to precipitation according to Reaction (2).
The FTIR spectra of synthesized CeO2 (Figure 2) show similarities with the FTIR spectra of CeO2 in the study conducted by Jayakumar et al. [34].The peak around the wavenumber 850 cm -1 indicates Ce-O vibrations.The broad peak around 3600-3400 cm -1 represents O-H vibrations from trapped water molecules, evidenced by the peak at 1625 cm -1 [29,30,31].The peaks around 1320 cm -1 and 1060 cm -1 are vibrations from "carbonate-like" species formed due to the interaction between CO2 and CO on the surface of CeO2 [34,35].The transmittance of the O-H vibration peak in the Ce700 sample is smaller than in the Ce500 and Ce600 samples.This phenomenon is consistent with the research results of Sheena et al. [36], which indicate that the intensity of the O-H vibrations decreases with increasing calcination temperature.The XRD analysis results of the CeO2 samples with varying temperatures can be seen in Figure 3.All samples show the same diffraction pattern, with peaks appearing at 2θ = 28.5°(111), 33.09° (200), 47.5° (220), 56.3° (311), 59.08° (222), 69.4° (400), 77.1° (331), 79.07° (420), 88.4° (422), and 95.3° (511).According to JCPDS No. 00-034-0394, these peaks are characteristic of CeO2 with a fluorite cubic crystal lattice (FCC).The absence of other peaks indicates that the synthesis at all temperatures produced pure CeO2.As the temperature increases, the diffractogram peaks tend to shift to the left (Table 1).The peak shift is caused by changes in the lattice parameters induced by the elevated calcination temperatures [37].The Ce500 and Ce600 samples have relative intensities that match the data from JCPDS No. 00-034-0394, indicating that the crystal growth direction in these samples aligns with the JCPDS standard.The Ce700 sample exhibits an increased diffraction intensity in the ( 220) and (311) planes, indicating that crystal growth in the sample tends to be oriented toward these planes [38].
The FWHM values of the samples decrease with increasing calcination temperature, indicating improved crystallinity.Higher temperatures enhance atomic arrangement, increasing the number of crystalline regions [26].This increase in crystallinity boosts X-ray diffraction intensity [39].The crystallite size of CeO2, calculated using the Scherrer equation, also increases with rising calcination temperature, as higher temperatures cause pore interconnections [26].Using Bragg's law, the interplanar spacing (d) and lattice parameter (a) for the (111) plane in Ce500, Ce600, and Ce700 samples were 3.120, 3.120, and 3.124 Å, and 5.406, 5.406, and 5.410 Å, respectively.SEM-EDS surface morphology analysis was conducted for Ce600 and Ce700 samples due to similar characterization results for Ce500 and Ce600 (Figures 4  and 5).The Ce700 sample shows more uniform and smaller particle sizes than Ce600.Using ImageJ, Ce600 particle sizes ranged from 0.2 to 5.6 μm, while Ce700 ranged from 0.12 to 2.9 μm.These findings contrast with Singh et al. [28], who reported increased particle size with higher calcination temperature.Ce and O element mapping show different surface profiles, with Ce700 having a more uneven surface than Ce600.
The EDS analysis results show that both samples have similar peaks.The distribution of Ce and O atoms on the surface of the CeO2 samples can be seen in Table 2.In the Ce600 sample, the ratio of Ce/O atom counts is approximately 0.48, while in the Ce700 sample, it is 0.57.This indicates that in the Ce700 sample, more Ce atoms are exposed on the surface.This data is consistent with the SEM results, where Ce700 has smaller particle sizes.The decrease in the number of O atoms may also be related to the increase in catalytic sites caused by the increase in the number of oxygen vacancies on the surface [10].
The fluorescence properties of CeO2 are shown in Figure 6.The sample was excited at λ = 247 nm and emitted light at λ = 496 nm, consistent with Windarti et al. [40].The excitation spectrum of CeO2 results from electrons moving from the 4f to the 5d orbital of Ce 3+ ions.Instability in the 5d orbital causes electrons to return to the 4f orbital, emitting visible light [40].The Ce600 sample has the highest excitation and emission intensity, likely due to fewer oxygen vacancies [41].EDS results confirm that Ce600 has more O atoms than Ce700.The antioxidant and catalytic activity of CeO2 increase with more oxygen vacancies and crystal defects [40], suggesting Ce700 has higher antioxidant activity than Ce500 and Ce600.

Conclusion
Pure cerium oxide can be synthesized using the precipitation method at calcination temperatures of 500-700°C.The calcination temperature affects the characteristics of the resulting CeO2, such as crystallinity, crystallite size, crystal parameters, surface morphology, and fluorescence properties.The crystallinity, crystallite size, and crystal parameter (a) of CeO2 increase with the rising calcination temperature.Surface morphology shows that the shape of CeO2 particles is irregular, with the size decreasing as the calcination temperature increases.The Ce/O ratio on the surface increases with higher calcination temperatures, indicating that cerium becomes more exposed on the surface.The fluorescence emission intensity does not correlate with the calcination temperature, with calcination at 600°C showing the highest fluorescence emission intensity (λ = 496 nm).Therefore, the selection of calcination temperature should be adjusted according to the intended application of CeO2.

Table 1 .
Analysis results based on XRD data of CeO2 samples

Table 2 .
Analysis results based on XRD data of CeO2 samples

Table 3 .
The ratio of cerium atoms to oxygen atoms and CeO2 particle size