Synthesis and characterization of cerium oxide nanoparticles by surfactant-assisted precipitation method

ABSTRACT

Cerium oxide nanoparticles (CeNPs) are one of the most promising metal oxide nanoparticles, with a wide range of applications in the fields of environment, energy, industry, agriculture and biomedical sciences. Cerium oxide nanoparticles (CeNPs) also called as nanoceria was synthesized by precipitation method using two different precursors and characterized by X-ray diffraction (XRD) analysis, FTIR, SEM/EDX and DRS-UV-Visible spectroscopy for the structural, compositional, surface morphological and optical property studies. X-ray diffraction studies revealed that the crystallite structure of the ceria nanoparticles was cubic fluorite like with the crystallite size calculated (using the Debye-sherrer formula) was 5.1 nm for pure ceria nanoparticles and 2.8 nm for surfactant CTAB (cetyltrimethylammonium bromide) assisted CeNPs. As per the results of the EDX compositional analysis, the weight percent content of Ce and O in the formed cerium oxide nanoparticles was 78.7% and 21.3% (pure CeO₂) and 79.3% and 20.7% (surfactant CTAB assisted nanoCeO₂), respectively. SEM analysis confirmed the uniform distribution of CeNPs and the nanocube-like shape of CeNPs formed by the precipitation method. DRS-UV-Visible spectral studies shows that, pure ceria and surfactant assisted ceria exhibit UV-Visible absorption in the range of 320 nm to 346 nm and band gap energy values ranging from 2.4 to 2.6 eV. Surfactant assisted nanoceria shows absorption shifts to the visible region, and the band gap energy values decrease when compared to bulk ceria (Eg = 3.19 eV). The effect of surfactant CTAB on crystallite size and agglomeration, as well as increasing CeO₂ product formation, has been observed.

Keywords:
Cerium oxide nanoparticles; Synthesis; Characterization; Precipitation method; Structural analysis

1. INTRODUCTION

Nanoscience is a multidisciplinary field focused on the investigation and development of size dependent properties of the materials in the nanoscale range (1 to 100 nm). Nanomaterials exhibit unique physicochemical characteristics that differ significantly from their bulk counterparts and have consequently found applications in diverse sectors including, agriculture, pharmaceuticals, energy systems and environmental remediation and many others. Among the various nanomaterials, cerium oxide (CeO₂) nanoparticles have attracted considerable attention in the recent years because of their unique redox ability and rich surface oxygen vacancies, free radical scavenging property, etc., which finds wide range of applications such as a three-way catalytic converter, optics, anti-bacterial agents, solid oxide fuel cells, and polishing materials etc., [¹,²,³,⁴,⁵,⁶,⁷,⁸,⁹,¹⁰,¹¹,¹²,¹³,¹⁴]. Cerium, in general, is a rare-earth element that is the second element in the lanthanide series [¹⁵]. Ceria nanoparticles (CeO₂ NPs) have emerged as a promising candidate for biomedical applications due to their unique features, including antioxidant, anti-inflammatory, angiogenic, and antibacterial properties [⁹, ¹¹].

The structure of CeO₂ is made up of a face-centered cubic unit cell of cations and anions occupying the octahedral interstitial site [¹⁶]. Each cerium cation is coordinated by the eight neighbouring oxygen anions, each oxygen anions is coordinated by four cerium cations. Cerium oxide nanoparticles have both a mix of +3 and +4 state over surface [¹⁷]. The chemical behaviour of rare earth metals is caused by their 4f electrons [¹⁸]. Due to the incredible nature, CeO₂ nanoparticles is increasingly used in the synthesis of composite materials by mixing with metals, metal oxide, and polymer resins to appropriate for more applications. Cerium oxide nanoparticles can be synthesized in a variety of forms using techniques such as co-precipitation, sol-gel, micelle, chemical vapour deposition, sonochemical, microwave and hydrothermal methods [³, ¹⁹]. Among the bottom-up routes, the precipitation/co-precipitation is one of the most commonly used approaches for producing CeO₂ nanoparticles because it is simple, low-cost, scalable, and amenable to control via choice of precursor, pH, aging, precipitant, and post-treatment (calcination) process etc. Recent surveys and reviews continue to identify precipitation as a predominant laboratory and industrial route for ceria nanoparticles synthesis [²⁰–²⁶,²⁷,²⁸]. The main benefits of producing CeO₂ nanoparticles by precipitation method are low cost, mild synthesis conditions, and easy scale up [²⁹].

TSAI developed a simple and effective method for synthesizing cerium oxide nanoparticles by homogeneous precipitation using cerium ammonium nitrate and urea. The precipitated cubic cerium oxide (CeO₂) had primary particle size ∼8 nm and the polishing performance of these cerium oxide (CeO₂) nanopowders had been discussed in this work [³⁰]. ZHANG et al. synthesized sphere–like in shape of cerium oxide nanoparticles using cerium nitrate as a precursor and ammonium acid carbonate as a precipitant. The reactant concentration, dispersing agent, and supersonic effect impacted on the nanoparticles size on synthesizing cerium oxide nanoparticles. Experimental results proved that dispersant agent played a major role in deciding the particle size of ceria (CeO₂) NPs [²⁹]. CHELLIAH et al. used cerium nitrate (+3) hexahydrate and sodium hydroxide as the precursors and prepared the nanoceria by hydroxide mediated precipitation method. The formation of a spherical shaped with the size range of 18–30 nm was confirmed by SEM and FTIR studies [³¹]. FARAHMANDJOU et al. synthesized CeO₂ nanoparticles from cerium nitrate hexahydrate and potassium carbonate precursors by chemical precipitation method. Cubic flourite structure ceria nanoparticles with average particle size ~20 nm were confirmed by XRD studies [³²].

CHEN and CHANG synthesized nanocrystalline CeO₂ nanoparticles by the precipitation method using cerium nitrate and ammonia water in the presence of O₂/N₂ atmosphere. They concluded the properties of CeO₂ nanoparticles are dependent on the reaction temperature and the oxygen (O₂) content of O₂/N₂ atmosphere [³³]. AROOJ et al. synthesized the CeO₂ nanoparticles by simple, low cost and eco-friendly co-precipitation method. The influence of calcination temperature over the formation of CeO₂ nanoparticles were analyzed [²⁰]. GAOYUAN REN et al. synthesized high crystallinity CeO₂ by simple precipitation method using cerium nitrate and ammonia as precursors with enhanced CMP performance due to surface oxygen vacancies [²¹].

SONAWANE et al. synthesized CeO₂ nanoparticles by sol-gel and hydrothermal methods and the CeO₂ nanoparticles showed enhanced gas sensitivity and dye degradation [²²]. VIDHI PATHAK et al. reported a comprehensive study on synthesis and characterization of cerium oxide nanoparticles by precipitation method and studied the photocatalytic activity of CeO₂ nanoparticles for different catalyst dose. Synthesized CeO₂ NPs showed excellent anti cancer activity [²³]. GOMES-JUNIOR et al. synthesized the ultrasmall cerium oxide nanoparticles by solvothermal method for the application as electrochemical sensor to detect dopamine in biological fluid [²⁴]. NOSRATI et al. discussed the green synthesized Ceria nanoparticles (CeO₂ NPs) and used for wound healing due to their excellent biological characteristics [²⁵].

Surfactants are amphiphiles that lower surface tension. As a result, it prevents particle agglomeration and thus controls the growth and size of nanoparticles. So far, anionic, cationic, and non-ionic surfactants such as sodium dodecyl sulphate (SDS), cetylpyridinium chloride (CPC), cetyltrimethylammonium bromide (CTAB), and polyethylene glycol (PEG) have been used in the synthesis of cerium oxide nanoparticles. The surfactant used has an effect on the size, shape, and morphology of the cerium oxide nanoparticles [³, ³⁴, ³⁵]. The surfactant provides a suitable site for the growth of particulate assemblies and influences the formation process, which includes nucleation, growth, coagulation, and flocculation. CeO₂ nanoparticles, for example, are produced using the surfactant CTAB, cerium nitrate, and ammonium hydroxide as precipitation agents [³⁶]. CHITSAZ et al. investigated the impact of PVP and CTAB surfactants on the morphology of cerium oxide nanoparticles. PVP surfactant reduced the crystalline size, whereas CTAB increased the amount of CeO₂ obtained. As a result, surfactant is crucial in the formation of nanoparticles [³⁷]. The present study describes the synthesis of CeO₂ nanoparticles using cerium nitrate hexahydrate and cerium ammonium nitrate as starting materials and NaOH and urea as capping agents in conjunction with the surfactant CTAB. The structural, morphological, and optical properties of the as-synthesized CeO₂ nanoparticles were studied using XRD, FTIR, SEM/EDX and DRS-UV-Visible spectral analysis.

2. MATERIALS AND METHODS

The chemicals used in this experimental study were of analytical grade reagents and purchased from Merck. They are cerium nitrate hexahydrate (Ce(NO₃)₃6H₂O), cerium ammonium nitrate (Ce(NH₄)₂(NO₃)₆), Urea, Sodium hydroxide and cetyltrimethylammonium bromide (CTAB). Double distilled water (ddH₂O) is used as the solvent in the synthesis of ceria nanoparticles.

2.1. Synthesis of CeO₂ nanoparticles by precipitation method

The reaction mixture was prepared by mixing cerium nitrate and sodium hydroxide in a molar ratio of 1:3 using the precipitation method, where cerium nitrate serves as the cerium source and sodium hydroxide acts as the precipitating agent. Double distilled water (ddH₂O) was used to get the homogenous reaction mixture. Under constant stirring, a solution of sodium hydroxide (NaOH) was added dropwise to the cerium nitrate solution. First, the purple-colored precipitate was observed and the stirring continued for further 15 minutes. Then the reaction mixture was centrifuged and kept at 80°C in hot air oven for 30 minutes. The dried product was annealed in muffle furnace maintained at 270°C for 5 hrs. The obtained yellow powder CeO₂ nanoparticles (CeNPs) were stored in air tight container for further characterization studies. In the similar manner, CeO₂ nanoparticles (CeNPs) have been prepared by the addition of the surfactant CTAB. The ceria sample obtained by this route is named as sample CeNPs-A (Figure 1(a)).

Figure 1
(a & b) Scheme for the synthesis of cerium oxide nanoparticles with and without surfactants (CeNPs-A & B).

Furthermore, the ceria nanoparticles are prepared by using cerium ammonium nitrate as precursor and urea as the reducing agent. Cerium ammonium nitrate (0.025M) was dissolved in 250 mL of double distilled water with urea (2.4M) in a 500 mL beaker. Then, the solution was heated up to 90°C to decompose the urea. After the completion of the reaction, the reaction mixture was kept for cooling at room temperature. The product was separated, filtered and rinsed with double distilled water. The final product was dried at 60°C in hot air oven. In the similar way, the CeO₂ nanoparticles have been synthesized by the addition of surfactant CTAB. The ceria sample obtained by this route is named as sample CeNPs-B (Figure 1(b)). Both the samples of CeNPs-A & B are characterized by different techniques and the results are presented in this paper.

2.2. Characterization of the prepared CeO₂ nanoparticles

As synthesized ceria nanoparticles (CeNPs) was characterized by powder X-ray diffraction analysis (XRD) method with Bruker X-Ray diffractometer (Model: D8 Advance ECO XRD systems with SSD160 1D Detector) with monochromatic CuKα-radiation of a wavelength of 0.15406 nm. The samples were scanned from the 2θ angle of 0° to 100°. FTIR spectra, in the range of 400–4000 cm^–1, were recorded on IR-Tracer-100 FT-IR Spectrophotometer with KBr pellets. The elemental composition and morphology of the nanoparticles were analyzed by EVO-18, CARL ZEISS scanning electron microscope attached with EDX. The optical properties such as absorption maxima and band gap energy value are examined by DRS-UV-Visible spectroscopy.

3. RESULTS AND DISCUSSION

3.1. X-ray diffraction (XRD) analysis

Figure 2 depicts the typical XRD pattern of the synthesized CeO₂ nanoparticles, which are identical to the cubic fluorite type crystal structure and fit harmoniously with CeO₂ standard JCPDS reference No.81-0792. The diffraction peaks appear at 28.58°, 33.00°, 47.70°, 56.75°, 59.28°, 69.97°, 76.79°, 79.79° and 88.80° which corresponds to the crystal planes of (111), (200), (220), (311), (222), (400), (331), (420) and (422) respectively. There is no impurity peak observed in the XRD pattern other than CeO₂, indicating the high purity of the synthesized CeO₂ nanoparticles. Debye-Scherrer equation was used to calculate the crystallite size of the synthesized CeO₂ nanoparticles [³¹, ³⁸, ³⁹].

Figure 2
XRD Pattern of the synthesized pure CeO2 and surfactant CTAB assisted CeO2 for the samples of CeNPs-A (a & b) & CeNPs-B (c & d).

(1) $$\text{D =}\left(\text{K λ}\right)\text{/}\left(\text{β cos θ}\right)$$

Where D is the crystallite size (nm), K is the Scherrer constant (0.9), λ is the X-ray source wavelength (0.15406 nm), θ is the Bragg angle, and β is the line broadening at half the maximum intensity (FWHM). The mean crystallite size, calculated using the Debye–Scherrer equation for pure CeO₂ and surfactant CTAB assisted nanoceria was 5.1 nm (a) & 3.97 nm (b) (CeNPs–A), and 3.40 nm (c) & 2.79 nm (d) (CeNPs–B). Based on the results, the crystallite size was reduced for the surfactant CTAB assisted ceria nanoparticles compared to the pure ceria, indicating that the synthesized ceria nanoparticles are suitable for photocatalysis and other related field of applications.

3.2. Compositional analysis by EDS technique

Figures 3 and 4 shows a typical EDS spectrum of pure CeO₂ and surfactant CTAB assisted nanoceria, as well as elemental mapping analysis images. The presence of Ce and O in the synthesized cerium oxide nanoparticles is confirmed by the EDS spectrum. Figures 3 and 4 shows that the weight percentages of Ce and O are 78.7% and 21.3% (for pure CeO₂) and 79.3% and 20.7% (for surfactant CTAB assisted CeO₂) respectively. The elemental mapping images of synthesized CeO₂ nanoparticles confirm the presence of Ce and O, which are also uniformly distributed throughout the sample.

Figure 3
(a & b) EDS spectrum and mapping images (a-cerium (Ce)-oxygen (O), b-cerium (Ce) & c-oxygen (O)) of pure CeO2 nanoparticles (CeNPs-A).

Figure 4
(a & b) EDS spectrum and mapping images (a-cerium (Ce)-oxygen (O), b-cerium (Ce) & c-oxygen (O)) of surfactant CTAB assisted CeO2 NPs (CeNPs-A).

3.3. Surface morphology by SEM analysis

Figure 5(a–d) depicts SEM images of pure CeO₂ and surfactant CTAB assisted CeO₂ NPs prepared by the simple precipitation method. The SEM images clearly show that no agglomeration occurred during the crystallization of CeNPs, and the nanocube-like shape of ceria nanoparticles formed by the precipitation method with a size of ~3 nm was confirmed by XRD measurements.

Figure 5
(a-d) SEM images of pure CeO2 (a & c) and CTAB assisted CeO2 NPs (b & d).

3.4. FTIR analysis

Figures 6 and 7 shows the FT-IR spectra of CeNPs-A & B samples, respectively. The FTIR spectra revealed the following characteristic peaks in the 2000 cm^–1–3700 cm^–1, 1100 cm^–1–1700 cm^–1 and 880 cm^–1–400cm^–1 ranges respectively. The broad absorption peaks observed in the FTIR spectra above 2000 cm^–1 to 3400 cm^–1 is due to the -OH stretching vibration of absorbed water molecules in the prepared sample. The strong absorption peaks observed between 1100 cm^–1 and 1700 cm^–1 are caused by the bending vibration of H₂O molecules that could be adsorbed on the molecules surfaces. The strong absorption bands observed in the range of below 880 cm^–1 to 400 cm^–1 which is the characteristic peak are due to the Ce-O stretching vibrations produced by the formed CeO₂ nanoparticles. The bands observed between 2800 cm^–1 and 3020 cm^–1 was due to surfactant CTAB C-H stretching. The bands 2849 cm^–1 and 2922 cm^–1 depicted the symmetric and asymmetric stretching of CTAB, C-H hydrocarbons, respectively [³⁷–⁴⁰,⁴¹,⁴²,⁴³].

Figure 6
FT-IR spectra of (a) pure CeO2 nanoparticles (b) surfactant CTAB assisted CeO2 nanoparticles (CeNPs sample-A).

Figure 7
FT-IR spectra of (c) pure CeO2 nanoparticles (d) surfactant CTAB assisted CeO2 nanoparticles (CeNPs sample-B).

3.5. UV-Visible spectral studies

Figures 8 and 9 shows the DRS UV-Visible absorption spectra for pure ceria nanoparticles and CTAB-assisted ceria nanoparticles. Diffuse reflectance Spectroscopy is used to determine the surface electronic states and band gap energy of a nanomaterial [¹⁴]. For pure ceria nanoparticles and CTAB-assisted ceria nanoparticles, a well-defined absorbance peak was observed in the range of 320–346 nm. The absorption shifts (red shifts) observed for CTAB assisted CeNPs indicate that absorption shifts to the visible region. The band gap of the synthesized CeO₂ nanoparticles is determined through Tauc’s plot by plotting (αhυ)² versus energy (eV). The band gap of the CeO₂ nanoparticles is in the range between 2.4 eV and 2.6 eV observed depending upon the preparation methods [⁴⁴, ⁴⁵].

Figure 8
DRS-UV-Visible spectrum (left) and corresponding Tauc plots (right) for the synthesized pure ceria (a) and CTAB assisted nanoceria (b) (CeNPs sample-A).

Figure 9
DRS-UV-Visible spectrum (left) and corresponding Tauc plots (right) for the synthesized pure ceria (c) and CTAB assisted nanoceria (d) (CeNPs sample-B).

4. CONCLUSIONS

In conclusion, the cerium oxide nanoparticles (CeO₂ NPs) were successfully synthesized by the precipitation method using two different precursors with the addition of surfactant CTAB and characterized by XRD, SEM/EDX, FT-IR, and DRS-UV-Visible spectroscopy for the structural and morphological studies. The results confirmed the formation of cubic fluorite-type nanoceria, with the crystallite sizes reduced from 5.1 nm (for pure CeO₂ NP’s) to 2.8 nm in the presence of CTAB surfactant. EDX analysis confirmed the elemental composition of Ce and O, with atomic percentages of 78.7% and 21.3% for pure CeO₂, and 79.3% and 20.7% for CTAB-assisted CeO₂ NP’s, respectively. SEM images revealed that surfactant CTAB addition effectively minimized the agglomeration and reduced the particle size of CeO₂ NP’s. DRS-UV-Visible studies showed absorption in the range of 320–346 nm with corresponding band gap energies of 2.4–2.6 eV for the pure CeO₂ and surfactant CTAB assisted CeO₂ NP’s. In summary, the findings demonstrate that CTAB-assisted precipitation is a simple and cost effective route for producing nanoceria with enhanced structural and optical properties, making them promising candidates for photocatalysis and other advanced applications.

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Publication Dates

• Publication in this collection
  16 Mar 2026
• Date of issue
  2026

History

• Received
  25 July 2025
• Accepted
  18 Dec 2025