Influence of Mn Doping on the Crystallinity and Morphology of Cerium Oxide Nanoparticles Synthesized by Co-precipitation

DOI: 10.5281/zenodo.17338324 This work is licensed under a Creative Commons Attribution 4.0 International License. This allows re-distribution and re-use of a licensed work on the condition that the author is appropriately credited and the original work is properly cited. Materials Science: Advances in Synthesis, Characterization and Applications (Vol. 1) - Digambar M. Sapkal, Harshal M. Bachhav, Gaurav Mahadev Lohar, Sanjay P. Khairnar (Eds.) ISBN: 978-93-95369-55-8 (paperback) 978-93-95369-46-6 (electronic) | © 2025 Advent Publishing. https://doi.org/10.5281/zenodo.17338324 CHAPTER 15 Influence of Mn Doping on the Crystallinity and Morphology of Cerium Oxide Nanoparticles Synthesized by Co-precipitation Nandkishor S. Sangle,a* Sangita S. Meshramb* a,bPhysics Department, VPM’S B. N. Bandodkar College of Science (Autonomous), Building 6, Jnanadweepa, Chendani Bunder Road, Thane West, Thane 400601 Maharashtra, India Corresponding author Email: [email protected],a [email protected] Received: 25 August 2025; Accepted: 13 October 2025; Available online: 13 October 2025 Abstract: In this study, we report the synthesis and characterization of pure cerium oxide (CeO₂) and manganese-doped cerium oxide (Mn–CeO₂) nanoparticles using the co-precipitation method. This synthesis route offers a simple and efficient pathway for obtaining high-purity nanomaterials. Structural and optical properties of the synthesized nanoparticles were analysed using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and UV-visible diffuse reflectance spectroscopy (UVDRS). XRD analysis confirmed the formation of crystalline CeO₂ with a fluorite cubic structure, both in pure and Mn-doped samples. The incorporation of Mn did not alter the primary crystal phase but may have induced subtle lattice distortions. FESEM images revealed that the particle size ranges from Nandkishor S. Sangle, Sangita S. Meshram Materials Science: Advances in Synthesis, Characterization and Applications (Vol. 1) - Digambar M. Sapkal, Harshal M. Bachhav, Gaurav Mahadev Lohar, Sanjay P. Khairnar (Eds.) ISBN: 978-93-95369-55-8 (paperback) 978-93-95369-46-6 (electronic) | © 2025 Advent Publishing. 212 approximately 10 nm to 40 nm, indicating successful nanoscale synthesis. UV-DRS results demonstrated enhanced visible light absorption in Mn-doped CeO₂ compared to pure CeO₂, suggesting a narrowed bandgap and potential for improved photocatalytic performance. These findings highlight the influence of Mn doping on the structural and optical behaviour of cerium oxide nanoparticles. Keywords: Cerium Oxide Nanoparticles, Mn-Doped CeO₂, Co-Precipitation Method, Structural Characterization, Optical Properties, XRD, UV-DRS, FESEM 1. Introduction Cerium is a rare earth element and the second member of the lanthanide series in the periodic table. A defining feature of rare earth elements is the shielding of their 4f orbitals by outer 5p and 4d electrons, which imparts unique electronic and catalytic properties.1 Unlike most lanthanides, cerium can exist in both +3 and +4 oxidation states,2 enabling redox flexibility that is central to its catalytic functionality. In bulk form, cerium oxide primarily exists as CeO₂ (cerium(IV) oxide) and Ce₂O₃ (cerium(III) oxide). However, at the nanoscale, cerium oxide nanoparticles (CeO₂ NPs) often contain a mixture of Ce³⁺ and Ce⁴⁺ ions, particularly on the surface, due to oxygen vacancies and surface defects. This mixed valence state is crucial to many of the material’s exceptional properties. Cerium oxide nanoparticles have been widely investigated for a range of technological and biomedical applications. These include solid oxide fuel cells,3 high-temperature oxidation-resistant coatings,4 catalytic systems5,6 solar energy devices7 and potential pharmaceutical agents.8 Among these, their most prominent application lies in catalysis, where their redox behavior and oxygen storage capacity (OSC) make them ideal materials. CeO₂ and CeO₂-based composites have gained significant attention as catalysts and as structural and electronic promoters in heterogeneous catalytic reactions.5 Industrially, cerium oxide is extensively used in threeway catalytic converters for vehicle exhaust treatment,6 the oxidative coupling of methane, and the watergas shift reaction. In this study, we report the synthesis of pure cerium oxide nanoparticles and manganese-doped cerium oxide nanoparticles via the co-precipitation method. The effect of Mn doping on the structural and optical properties of CeO₂ is explored through various characterization techniques. 2. Materials and Synthesis Method A. Method and Materials The precursor utilized to make pure ceria NPs and Mn-doped ceria NPs are Cerium nitrate hexahydrate [Ce(NO3)3∙6H2O] and Manganese acetate [(CH3.COO)2Mn.4H2O]. The synthesis was carried out by coprecipitation method. Co-precipitation method gives high yield of end product. All of the experimental solutions were prepared with double distilled water containing 10% ethanol. Influence of Mn doping on the crystallinity and morphology of cerium oxide nanoparticles .. Materials Science: Advances in Synthesis, Characterization and Applications (Vol. 1) - Digambar M. Sapkal, Harshal M. Bachhav, Gaurav Mahadev Lohar, Sanjay P. Khairnar (Eds.) ISBN: 978-93-95369-55-8 (paperback) 978-93-95369-46-6 (electronic) | © 2025 Advent Publishing. 213 Materials Used • Cerium Nitrate Hexahydrate – Ce(NO₃)₃·6H₂O Acts as the cerium precursor. It provides Ce³⁺ ions necessary for the formation of cerium oxide (CeO₂). It is a highly soluble salt, making it ideal for aqueous synthesis routes. • Manganese Acetate Tetrahydrate – (CH₃COO)₂Mn·4H₂O Used as the dopant precursor to introduce Mn into the ceria lattice. Provides Mn²⁺ ions which can partially substitute Ce⁴⁺ in the ceria crystal structure, potentially modifying its electronic, structural, and catalytic properties. • Solvent – Double distilled water containing 10% ethanol ensures high purity of the reaction medium. Ethanol may aid in controlling the particle size and morphology by influencing the nucleation and growth rates. B. Synthesis Method: Co-precipitation A Co-precipitation method widely used and straightforward method for synthesizing nanoparticles. This method enables uniform distribution of dopant ions. It Offers a high yield of the end product. Operates under mild conditions, typically at ambient temperature or with moderate heating. Synthesis Procedure a) Preparation of Precursor Solutions • For pure CeO₂ nanoparticles: 0.3 M Ce(NO₃)₃·6H₂O was dissolved in 50 mL of double distilled water containing 10% ethanol. The solution was stirred thoroughly using a magnetic stirrer. • For Mn-doped CeO₂ nanoparticles: Mn-doping solutions were prepared by dissolving either 0.2 M or 0.3 M of (CH₃COO)₂Mn·4H₂O in the same solvent system. These were added to the Ce solution for doping, ensuring a homogeneous mixture. b) Precipitation Process • NaOH solution (0.9 M) was added gradually to the Ce-only solution under constant stirring to initiate precipitation. The 0.9 M concentration corresponds to three times the molarity of the Ce precursor, reflecting the stoichiometry of the reaction and the presence of three nitrate ions per Ce³⁺ ion. • For Mn-doped systems, both NaOH (0.9 M) and the prepared Mn solution were added dropwise and simultaneously to the cerium nitrate solution under continuous stirring. This ensured coprecipitation of cerium and manganese hydroxides. c) Reaction Chemistry The reaction occurring in the Ce-only system is: Ce(NO₃)₃ 6H₂O+3NaOH→Ce(OH)₃+3NaNO₃+6H₂O Nandkishor S. Sangle, Sangita S. Meshram Materials Science: Advances in Synthesis, Characterization and Applications (Vol. 1) - Digambar M. Sapkal, Harshal M. Bachhav, Gaurav Mahadev Lohar, Sanjay P. Khairnar (Eds.) ISBN: 978-93-95369-55-8 (paperback) 978-93-95369-46-6 (electronic) | © 2025 Advent Publishing. 214 • A similar co-precipitation reaction occurs for Mn-doped systems, with Mn²⁺ forming Mn(OH)₂ in the process. d) Aging and Separation • The resulting suspension was left undisturbed for 24 hours to allow complete precipitation and formation of a supernatant layer. • The clear supernatant liquid was carefully removed using a dropper. e) Drying • The remaining precipitate was transferred to a hot air oven and dried at 150°C for 3–4 hours to remove moisture, resulting in dry hydroxide precursors. f) Grinding • The dried solid was ground manually in a mortar and pestle for several hours until a fine, uniform powder was obtained. g) Calcination • The fine powder was calcined at 300–400°C for 3–4 hours in a muffle furnace. • During this thermal treatment: o The hydroxides were converted to their respective oxides (CeO₂ and Mn-doped CeO₂). o The oxidation state of Ce³⁺ was transformed to Ce⁴⁺, forming the stable fluorite-structured CeO₂. o Residual organic compounds (e.g., acetate) and moisture were fully eliminated. o Calcination temperature was kept well below the melting point of the material to preserve nano-sized features and avoid sintering. h) Observations Color Change o The pure CeO₂ nanoparticles exhibited a light-yellow color after calcination. o As the Mn doping concentration increased, the nanoparticles' color turned progressively darker, indicating successful incorporation of Mn into the ceria matrix and possible changes in oxidation states (e.g., Mn²⁺, Mn³⁺, Mn⁴⁺) and oxygen vacancy formation. Influence of Mn doping on the crystallinity and morphology of cerium oxide nanoparticles .. Materials Science: Advances in Synthesis, Characterization and Applications (Vol. 1) - Digambar M. Sapkal, Harshal M. Bachhav, Gaurav Mahadev Lohar, Sanjay P. Khairnar (Eds.) ISBN: 978-93-95369-55-8 (paperback) 978-93-95369-46-6 (electronic) | © 2025 Advent Publishing. 215 3. Nanoparticles Characterization a) Powder X-ray diffraction Pure Cerium Doped Cerium Fig. 1. XRD graph of pure and doped ceria Powder X-ray diffraction patterns of nanomaterials were recorded using an X-ray diffractometer employing CuKα radiation. The CuKα source was operated at 20kV40kV. Diffraction data were recorded in a 2θ range from 20° to 80° in 0.02° steps for 0.6 s The X-ray diffraction (XRD) pattern shown above corresponds to pure cerium oxide (CeO₂) nanoparticles synthesized via the co-precipitation method. The diffraction peaks observed at 2θ values of approximately 28.5°, 33.1°, 47.5°, 56.3°, 59.1°, and 69.4° are indexed to the crystallographic planes (111), (200), (220), (311), (400), and (331) respectively. These peaks are in good agreement with the standard data from the JCPDS card, confirming that the synthesized nanoparticles possess a fluoritetype cubic crystal structure, characteristic of CeO₂. The most intense peak at 28.5° corresponding to the (111) plane indicates that this is the preferred growth orientation in the synthesized sample. The sharp and well-defined peaks suggest that the material is highly crystalline, with minimal structural defects or amorphous content. Furthermore, the absence of any additional or impurity peaks confirms the phase purity of the synthesized CeO₂ nanoparticles. This result confirms the successful formation of pure cerium oxide with desirable crystalline properties, suitable for further modification or application in catalytic and optical materials. The X-ray diffraction (XRD) pattern shown illustrates the crystalline structure of Mn-doped cerium oxide (CeO₂) nanoparticles at two different doping concentrations: 0.1 M (blue curve) and 0.2 M (red curve). In both samples, prominent diffraction peaks appear at 2θ values of approximately 28.5°, Nandkishor S. Sangle, Sangita S. Meshram Materials Science: Advances in Synthesis, Characterization and Applications (Vol. 1) - Digambar M. Sapkal, Harshal M. Bachhav, Gaurav Mahadev Lohar, Sanjay P. Khairnar (Eds.) ISBN: 978-93-95369-55-8 (paperback) 978-93-95369-46-6 (electronic) | © 2025 Advent Publishing. 216 47.5°, 56.3°, 59.1°, and 69.4°, which correspond to the (111), (220), (311), (400), and (331) planes, respectively. These peaks match well with the standard JCPDS card for fluorite-type cubic CeO₂, indicating that manganese doping does not alter the fundamental crystal structure of cerium oxide. The absence of secondary or impurity peaks confirms that Mn ions are successfully incorporated into the CeO₂ lattice without forming separate manganese oxide phases. A comparison between the two doping levels shows a slight reduction in peak intensity and broadening with increasing Mn content, particularly in the 0.2 M doped sample. This suggests a reduction in crystallite size and possible lattice strain or defect formation, which are typical effects of dopant substitution in the host lattice. The slight broadening of peaks may also indicate increased disorder within the crystal structure due to Mn incorporation. Overall, the XRD analysis confirms that both 0.1 M and 0.2 M Mn-doped CeO₂ nanoparticles maintain a crystalline fluorite structure, with Mn doping subtly influencing the crystallinity and particle size depending on the dopant concentration. b) Ultraviolet Diffuse Reflectance Spectroscopy (UV-DRS) UV-diffuse reflectance spectroscopy (DRS) is a type of surface analysis. As a probing medium, it employs ultraviolet (UV) light. Light interacts with "strongly absorbing materials" such as metals, alloys, semiconductors, and so on in the first 10-20 nm. Fig. 2. Absorbance of UV-DRS of pure and doped ceria The UV–Visible Diffuse Reflectance Spectroscopy (UV-DRS) graph (Fig. 2) presented illustrates the optical absorption behavior of pure cerium oxide (CeO₂) and Mn-doped CeO₂ nanoparticles at two different doping concentrations: 0.1 M and 0.2 M. The absorbance spectra reveal a notable shift in the optical properties as a result of Mn doping. Pure CeO₂ nanoparticles (black curve) show a distinct Influence of Mn doping on the crystallinity and morphology of cerium oxide nanoparticles .. Materials Science: Advances in Synthesis, Characterization and Applications (Vol. 1) - Digambar M. Sapkal, Harshal M. Bachhav, Gaurav Mahadev Lohar, Sanjay P. Khairnar (Eds.) ISBN: 978-93-95369-55-8 (paperback) 978-93-95369-46-6 (electronic) | © 2025 Advent Publishing. 217 absorption edge in the ultraviolet region, around 380–400 nm, which is typical for CeO₂ with a wide band gap of approximately 3.2 eV. Upon doping with Mn, a significant enhancement in visible light absorption is observed. The 0.1 M Mn-doped sample (blue curve) exhibits a moderate red shift in the absorption edge, indicating a slight narrowing of the band gap and improved absorption into the visible region. This effect becomes more pronounced in the 0.2 M Mn-doped sample (red curve), which shows a broader and higher absorbance extending further into the visible range (up to ~700 nm), confirming a more substantial reduction in the band gap. This progressive red shift and increase in absorbance with higher Mn content can be attributed to the introduction of impurity energy levels within the band gap and the formation of oxygen vacancies or defect states, which promote electronic transitions under lower energy (longer wavelength) light. Overall, the UV-DRS results demonstrate that Mn doping effectively enhances the visible light absorption capability of CeO₂ nanoparticles, making them more suitable for applications in photocatalysis, solar energy conversion, and other optoelectronic fields. Fig. 3. Reflectance of UV-DRS of pure and doped ceria The above graph (Fig. 3) shows the UV–Visible Diffuse Reflectance Spectra (UV-DRS) of pure CeO₂ nanoparticles and Mn-doped CeO₂ nanoparticles at 0.1 M and 0.2 M doping concentrations, plotted as % reflectance (%R) vs. wavelength. This reflectance data provides insight into the optical band gap and light interaction behavior of the materials. The pure CeO₂ sample (black curve) exhibits high reflectance in the visible region, particularly between 400 nm and 800 nm, indicating that it absorbs primarily in the UV region and reflects most visible light. This corresponds to its large band gap (~3.2 eV) and limited utility in visible light-driven applications. However, a significant decrease in reflectance is observed in the Mn-doped samples, especially the 0.2 M Mn-doped CeO₂ (red curve), which shows the lowest reflectance Nandkishor S. Sangle, Sangita S. Meshram Materials Science: Advances in Synthesis, Characterization and Applications (Vol. 1) - Digambar M. Sapkal, Harshal M. Bachhav, Gaurav Mahadev Lohar, Sanjay P. Khairnar (Eds.) ISBN: 978-93-95369-55-8 (paperback) 978-93-95369-46-6 (electronic) | © 2025 Advent Publishing. 218 across the entire spectrum. This indicates a higher absorption of light, particularly in the visible region, as Mn doping introduces defect levels or intermediate states within the band gap. The 0.1M Mn-doped sample (blue curve) shows an intermediate behavior, with reflectance lower than that of pure CeO₂ but higher than the 0.2M doped sample. The decrease in reflectance with increasing Mn concentration confirms the enhanced optical absorption due to Mn incorporation, which results in a narrowing of the effective band gap. This enhancement in visible light absorption is consistent with the absorbance data and makes Mn-doped CeO₂ more suitable for photocatalytic and optoelectronic applications, where efficient light harvesting is essential. c) Field Emission Scanning Electron Microscopy Field Emission Scanning Electron Microscopy FESEM of nanoparticles were performed by using “NOVA NANOSEM NPEP450”. This instrument offers high resolution and excellent contrast, extended accelerating voltage ranging from 50 eV to 30 kV, at wide magnification range. The imaging used various acceleration voltages, beam currents, working distance and aperture setting to obtain the highest resolution possible. The below shows FESEM images of pure and doped ceria. Fig. 4. FESEM of pure cerium The morphology of pure CeO₂ nanoparticles, as revealed by the FESEM images, shows a highly aggregated and densely packed structure, typical for nanoparticles synthesized via the co-precipitation method. In the low-magnification image (left), the surface appears rough and granular, suggesting the presence of nanoparticle clusters. As the magnification increases (middle image), more distinct particle boundaries become visible. The particles appear to be irregular in shape, with rough surfaces and occasional signs of porous regions or voids between aggregates. This suggests that while primary particles are nanoscale, they tend to agglomerate, likely due to high surface energy and Van der Waals forces. In the high-magnification image (right), where particle size measurements are provided, the nanoparticles exhibit sizes ranging from approximately 20 to 30 nm. This nanoscale dimension is consistent with the crystallite size derived from XRD analysis and confirms the nanostructured nature of the material. The presence of well-defined but closely packed grains indicates good crystallinity but also suggests some level of particle coarsening or sintering, possibly introduced during the drying or calcination steps. Overall, the FESEM images confirm that pure CeO₂ nanoparticles are polycrystalline, irregularly shaped, and within the nanoscale range, with a tendency toward agglomeration. This morphology is Influence of Mn doping on the crystallinity and morphology of cerium oxide nanoparticles .. Materials Science: Advances in Synthesis, Characterization and Applications (Vol. 1) - Digambar M. Sapkal, Harshal M. Bachhav, Gaurav Mahadev Lohar, Sanjay P. Khairnar (Eds.) ISBN: 978-93-95369-55-8 (paperback) 978-93-95369-46-6 (electronic) | © 2025 Advent Publishing. 219 suitable for applications requiring high surface area, such as catalysis, although surface modification or doping (e.g., with Mn) may be required to optimize properties further. Fig. 5. FESEM of manganese doped cerium The FESEM micrographs reveal that the Mn-doped CeO₂ nanoparticles exhibit a distinct morphological evolution and surface structure compared to pure ceria. In the low-magnification image (top), the particles are aggregated and densely packed, with a somewhat granular surface texture. This aggregation is typical for nanoparticles and can be attributed to their high surface energy and strong interparticle interactions. As the magnification increases (middle image), the particles begin to show more defined and flower-like or layered structures, indicating a potential influence of Mn doping on crystal growth patterns and surface energy distribution during synthesis. At the highest magnification (bottom image), the individual particles are clearly visible, and size measurements show nanoparticle dimensions ranging between 20 to 37 nm. The particles appear plate-like and closely packed, with relatively sharp edges, which may suggest a degree of anisotropic growth. The observed size range aligns well with the crystallite sizes estimated from XRD analysis, supporting the nanocrystalline nature of the samples. The incorporation of Mn appears to influence both particle shape and surface roughness, potentially enhancing surface area and active sites for photocatalytic or sensor applications. Overall, the FESEM images confirm that Mn doping alters the surface morphology of CeO₂, leading to more defined, smaller, and rougher particles compared to pure CeO₂. These structural modifications could significantly impact the optical and catalytic behavior of the nanoparticles. Applications of Pure and Mn-Doped CeO₂ Nanoparticles o Photocatalysis: Enhanced visible light absorption due to Mn doping makes them suitable for photocatalytic degradation of pollutants in water and air purification systems. o Solar Energy Conversion: Improved light absorption properties support their use in solar cells and photo-electrochemical cells for energy harvesting. o Catalysis: CeO₂ is widely used in automotive catalytic converters (three-way catalysts). Mn doping increases surface area and active sites, enhancing redox and oxidation reactions. o Sensors: Their high surface reactivity and conductivity make them ideal for gas sensing and environmental monitoring.