Multi-Morphology CeO₂ Synthesis via Synergistic Induction by Solvent and Ammonium Bicarbonate

Abstract

CeO₂ is a crucial functional material in catalysis and energy applications, whose performance is highly morphology-dependent. Traditional synthesis methods often rely on organic templates or surfactants, which complicate the processes and pose environmental concerns. This study introduces an eco-friendly approach utilizing a methanol–water (MeOH-H₂O) mixed solvent system combined with NH₄HCO₃ to achieve controllable synthesis of multi-morphology CeO₂ without surfactants or templates. The effects of different solvent systems (pure H₂O, pure MeOH, and their mixtures) and NH₄HCO₃ as an inexpensive regulator on precursor phase behavior and crystallization were systematically investigated. By optimizing the Ce:N molar ratios (1:1 to 1:7) as well as reaction times (0.5 to 36 h), our findings indicate that H₂O significantly enhances crystallinity (from 40.9% to 61.4% for precursors, reaching 70.3% after calcination) and promotes octahedra formation in the MeOH-H₂O mixed system, while NH₄HCO₃ acts as a structure-directing agent to control size (e.g., ~240 nm octahedra at Ce:N = 1:1, up to 375 nm at Ce:N = 1:2) and partially substitutes for high-temperature calcination in improving crystallinity. Variety morphologies, including plates, dendrites, octahedra, and hollow structures, were successfully synthesized. This work elucidates the synergistic mechanism by which solvents and NH₄HCO₃ influence CeO₂ nucleation and growth, thereby providing an environmentally friendly synthesis route with significant potential applications in catalysis and energy storage.

1. Introduction

Cerium dioxide (CeO₂), a versatile rare-earth metal oxide, demonstrates extensive application potential in various fields such as catalysis [1], energy storage [2] and environmental remediation [3]. In particular, its potential in advanced catalytic systems like single-atom catalysts has attracted growing attention [4]. This is attributed to its unique redox properties, exceptional oxygen storage capacity and structural flexibility [5]. The performance of CeO₂ is largely influenced by its morphological characteristics, including specific surface area, exposed crystal facets, and particle size [6]. These factors directly affect its surface reactivity, mass transfer efficiency, and catalytic stability [7].

To date, numerous methods have been employed to synthesize CeO₂ materials with diverse morphologies. Techniques such as microemulsion [8], hydrothermal treatment [9], sol–gel synthesis [10], Pechini process [11], and precipitation methods [12] have been utilized. However, these approaches often rely heavily on surfactants or templating agents, such as cetyltrimethylammonium bromide (CTAB) [13], amino acids [14], poly(methyl methacrylate) (PMMA) [15], powdered cellulose [16], polyvinyl alcohol (PVA)/polyvinylpyrrolidone (PVP) [17], and polystyrene spheres [18]. For instance, Ho [19] et al. utilized PVP to synthesize uniform CeO₂ nanospheres, microrods, and spindle-like particles. Song [20] et al. prepared hollow CeO₂ spheres via an oleic acid (OLA)-assisted solvothermal method. Kaviyarasu [21] et al. employed CTAB in ethanol to synthesize hexagonal cubic-shaped CeO₂ nanofibers via a solvothermal method. Li [22] et al. fabricated three-dimensional hierarchical flower-like CeO₂ microspheres using sodium dodecyl sulfonate and PEG 600 as structure-directing agents under mild conditions. Such heavy reliance on organic additives not only increases process complexity and production cost (due to the need for high-purity reagents and subsequent removal steps) but also introduces environmental concerns and post-treatment challenges. Furthermore, certain traditional surfactants, such as CTAB, demonstrate limited biodegradability and are susceptible to leaving residues during wastewater treatment processes. These residues may pose toxic effects on aquatic ecosystems [23,24]. Additionally, some ionic surfactants can decompose under high-temperature or extreme pH conditions, resulting in the formation of toxic by-products. For example, degradation intermediates of alkyl benzene sulfonate have been shown to carry genetic toxicity risks [25,26]. In recent years, there has been an increasing focus on developing green surfactants, such as bio-based or biodegradable alternatives [27,28], yet their efficiency and applicability in nanomaterial synthesis still lag behind those of traditional systems [29,30]. This underscores the urgent need for the development of new environmental control strategies.

Currently, the surfactant- and template-free synthesis of CeO₂ has garnered increasing attention as a promising alternative strategy. For instance, Yu [31] et al. developed a template- and surfactant-free method to synthesize ultrathin CeO₂ nanowires by refluxing at 140 °C for 12 h in a mixed ethanol–water solvent, utilizing 30% NH₃·H₂O as the precipitant. Chen [32] et al. successfully prepared single-crystalline-like CeO₂ hollow nanocubes through a solvothermal approach at 160 °C for 9 h, employing peroxyacetic acid as an oxidizing agent without any templates. In our previous work, we established an oxidation-induced strategy for the template-free hydrothermal synthesis of mesoporous CeO₂ [33,34,35,36]. It was observed that organic solvents such as ethylene glycol significantly influence the morphological evolution of CeO₂ in template-free systems. Specifically, within a simple hydrothermal system comprising only water, ethylene glycol, and Ce(NO₃)₃·6H₂O, the morphology can be tailored to solid spheres, hollow spheres, or multilayered structures by adjusting various synthesis parameters [37]. Nevertheless, the interplay between key synthesis parameters (such as precursor ratio, solvent composition and reaction time) and the dynamic phase evolution during template-free synthesis remains inadequately understood [38]. This knowledge gap considerably limits the rational design of CeO₂ with specific morphologies.

Based on this, the present study proposes a novel synthesis strategy: by introducing environmentally benign and readily available ammonium bicarbonate (NH₄HCO₃) into a MeOH–H₂O mixed solvent system, we achieve the controllable preparation of CeO₂ with diverse morphologies without the use of any surfactants or templates. The synergistic effects of solvent composition, Ce/N molar ratio (1:1–1:7), and reaction time (0.5–36 h) on the crystal phase evolution and morphological control of CeO₂ were systematically investigated. This work not only provides a new pathway for the environmentally friendly and efficient synthesis of CeO₂ but also offers theoretical insights into crystal growth and morphology control under template-free conditions.

2. Results and Discussion

2.1. Influence of Solvothermal Reaction Media

The mineralization behavior of Ce(NO₃)₃·6H₂O, as well as the phase composition and morphology of the resultant products, were first examined in pure H₂O, MeOH, and a mixed solvent system of MeOH-H₂O without the addition of NH₄HCO₃. Experimental results revealed no precipitate formation after solvothermal reactions in the pure aqueous medium, indicating that H₂O exhibits limited efficacy in mineralizing Ce³⁺ ions and is therefore unsuitable as a reaction medium for this synthesis.

Figure 1a presents the XRD patterns of precursors obtained through solvothermal processes in 20 mL pure MeOH and in a mixed solvent comprising 15 mL MeOH combined with 5 mL H₂O. In the pure MeOH system, the precursor displayed four distinct diffraction peaks at 28.57°, 32.11°, 47.30° and 56.25°, which correspond to the (111), (200), (220) and (311) crystal planes of cubic fluorite-structured CeO₂ (JCPDS No. 43-1002). The diffraction intensity observed for the (111) plane was measured at 360. Although other peaks were relatively weak, they remained identifiable and aligned closely with standard patterns for CeO₂; thus confirming that the product consisted predominantly of single-phase CeO₂, albeit featuring relatively low crystallinity. In contrast, when prepared within the MeOH–H₂O mixed solvent system, the precursor exhibited eight clear diffraction peaks that fully matched those outlined in standard cubic CeO₂ patterns corresponding to crystal planes (111), (200), (220), (311), (222), (400), (331), and (420). No impurity phases were detected under these conditions. Notably, there was an increase in diffraction intensity for the (111) plane up to 837, a significant enhancement of approximately 132.5%, which suggests improved crystallinity attributed to the facilitating role of H₂O in promoting Ostwald ripening processes along with better crystal development.

Figure 1. XRD patterns of samples synthesized solvothermally at 180 °C for 12 h (without NH₄HCO₃) in different solvent systems: (a) as-synthesized precursor and (b) after calcination in air at 500 °C for 1 h. Solvent systems: pure MeOH (20 mL) and MeOH–H₂O mixed solvent (15 mL MeOH + 5 mL H₂O).

After calcination at 500 °C for 1 h (Figure 1b), all samples retained the characteristic diffusion patterns of pure cubic CeO₂; however, both the intensity and sharpness of the peaks increased significantly. For the CeO₂ derived from the pure MeOH system, the diffraction intensity of the (111) plane rose from 360 to 438, representing an increase of 21.7%. In contrast, for the sample obtained from a MeOH–H₂O mixed solvent, it escalated from 837 to 1274, marking a substantial increase of 52.2%. These results indicate that incorporating H₂O not only facilitates the formation of highly crystalline CeO₂ during the solvothermal process but also enhances its crystallinity further through subsequent calcination. This finding is crucial for achieving the controlled synthesis of CeO₂ crystallinity and has important implications for its applications in catalysis and other fields. The crystallinity and exposed facets are known to significantly affect the electronic structure and surface reactivity of metal oxides, as demonstrated in doped nickel oxide systems for electrocatalysis [39].

Although single-phase cubic CeO₂ can be synthesized via a one-step solvothermal reaction (180 °C, 12 h) in both the pure MeOH and MeOH–H₂O mixed solvent systems, the phase remains unchanged after calcination. However, the composition of the solvent significantly influences the mineralization and crystallization behavior of Ce³⁺, as well as the crystal quality of the final product. Therefore, the crystallinity and grain size of both the solvothermally derived precursors and the calcined CeO₂ were further evaluated, as depicted in Figure 2. In the pure MeOH system (Figure 2a), the crystallinity of both the precursor and calcined CeO₂ was measured at 40.9% and 45.2%, respectively, with corresponding grain sizes of 6.6 nm and 6.9 nm. This indicates that thermodynamically stable nanocrystals were formed during the solvothermal process; moreover, calcination did not induce significant grain growth. In contrast, within the MeOH–H₂O mixed solvent system (Figure 2b), the crystallinity of the precursor registered at 61.4%, which increased to 70.3% after calcination; concurrently, there was a slight decrease in grain size from 16.5 nm to 15.1 nm. This reduction may be attributed to either the removal of amorphous components or repair of structural defects during calcination, both contributing to further enhancement in crystal integrity. A comprehensive summary of these structural parameters, including the calculated strain derived from XRD peak broadening analysis, is provided in Table 1. The notably lower strain value for the CeO₂ sample from the MeOH-H₂O system quantitatively supports the role of H₂O in promoting the growth of more relaxed and defect-annealed crystals.

Figure 2. (a) Crystallinity and (b) grain size of CeO₂ samples synthesized solvothermally at 180 °C for 12 h (without NH₄HCO₃) in different reaction media, before and after calcination in air at 500 °C for 1 h. Solvent systems: pure MeOH (20 mL) and MeOH–H₂O mixed solvent (15 mL MeOH + 5 mL H₂O).

Table 1. Crystallite size and structural parameters of CeO₂ synthesized in different solvent systems.

Sample Description	Condition	Crystallite Size (nm)	Relative Crystallinity (%)	Strain (%)
Precursor (Pure MeOH)	180 °C, 12 h, no NH4HCO3	6.6	40.9	1.678
CeO2 (Pure MeOH)	Calcined at 500 °C for 1 h	6.9	45.2	1.639
Precursor (MeOH–H2O)	180 °C, 12 h, no NH4HCO3	16.5	61.4	0.337
CeO2 (MeOH–H2O)	Calcined at 500 °C for 1 h	15.1	70.3	0.550

Note: Crystallite size (nm) was calculated from the (111) diffraction peak using the Scherrer equation.

Figure 3 illustrates the micro-morphologies of CeO₂ produced after calcination in two different solvent systems. In the pure MeOH system (Figure 3a), CeO₂ particles exhibited spherical hard aggregates due to incomplete Ostwald ripening. Conversely, in the MeOH–H₂O mixed solvent system (Figure 3b), the addition of H₂O effectively inhibited hard aggregation, enhanced particle monodispersity, and facilitated the formation of regular micron-sized octahedral structures with edge lengths less than 3 μm. This observation suggests that H₂O plays a crucial role in mediating chemical reactions between Ce³⁺ ions and MeOH, guiding the transformation of agglomerates into a thermodynamically more stable octahedral morphology.

Figure 3. SEM images of CeO₂ samples synthesized solvothermally at 180 °C for 12 h (without NH₄HCO₃) in different solvent systems after calcination in air at 500 °C for 1 h: (a) pure MeOH (20 mL) and (b) MeOH–H₂O mixed solvent (15 mL MeOH + 5 mL H₂O).

2.2. Influence of a Small Amount of NH₄HCO₃

Based on the aforementioned results, a mixed solvent system comprising 15 mL MeOH and 5 mL H₂O was selected to further examine the impact of adding a small quantity of NH₄HCO₃ on the products. The molar ratios of Ce:N were established at 1:1 and 1:2. As illustrated in Figure 4a, following the incorporation of NH₄HCO₃, the solvothermally obtained precursors retained a phase-pure cubic CeO₂ structure (JCPDS No. 43-1002), indicating that the introduction of NH₄HCO₃ did not alter the crystal structure. However, a significant increase in diffraction peak intensities was observed (reaching values of 2039 and 3309, respectively), suggesting notably enhanced crystallinity, likely attributable to optimized crystal growth within an alkaline environment.

Figure 4. XRD patterns of samples synthesized solvothermally at 180 °C for 12 h in a mixed solvent of 15 mL MeOH and 5 mL H₂O with Ce:N molar ratios of 1:1 and 1:2, (a) before and (b) after calcination in air at 500 °C for 1 h.

After calcination at 500 °C (Figure 4b), the phase composition of all samples remained unchanged. When the molar ratio of Ce:N was set at 1:1, the diffraction intensity increased to 2762, indicating an enhancement in crystallinity. In contrast, at a Ce:N molar ratio of 1:2, the intensity remained essentially stable (3382), suggesting that an appropriate amount of NH₄HCO₃ can partially substitute for the crystallinity-enhancing effect provided by high-temperature calcination.

Following the introduction of NH₄HCO₃, the CeO₂ products preserved their octahedral morphology but underwent a significant reduction in size from the micron-scale to the nano-scale, as revealed by SEM analysis (Figure 5). Specifically, at a Ce:N molar ratio of 1:1, the resulting octahedra were relatively uniform, with an average edge length of approximately 240 nm (Figure 5a. Conversely, at a Ce:N molar ratio of 1:2, the particle size distribution exhibited slight broadening, with maximum edge lengths reaching up to 375 nm (Figure 5b). These findings suggest that NH₄HCO₃ functions as an effective structure-directing agent, and that its dosage can further modulate both the particle size and the uniformity of the resulting CeO₂ nanocrystals.

Figure 5. SEM images of CeO₂ samples synthesized solvothermally at 180 °C for 12 h in a mixed solvent of 15 mL MeOH and 5 mL H₂O with Ce:N molar ratios of (a) 1:1 and (b) 1:2, after calcination in air at 500 °C for 1 h.

2.3. Influence of Ce:N Molar Ratios

To further investigate the impact of NH₄HCO₃ dosage on the phase composition and morphology of the products, we increased the Ce:N molar ratio to 1:3–1:7 within a fixed mixed solvent system (15 mL MeOH + 5 mL H₂O). Figure 6a presents the XRD patterns of the solvothermal precursors at varying ratios. At a Ce:N ratio of 1:3, alongside cubic CeO₂ (JCPDS No. 43-1002), diffraction peaks corresponding to hexagonal CeCO₃OH (JCPDS No. 32-0189) emerged and were predominant. As the Ce:N ratio was raised to 1:5, there was an intensified presence of CeCO₃OH diffraction peaks accompanied by a reduction in intensity for those associated with CeO₂. This trend indicates that NH₄HCO₃ facilitates both the dissolution of CeO₂ and promotes the formation of the CeCO₃OH phase. When further increasing the Ce:N ratio to 1:7, stabilizing diffraction intensities for both phases suggested that an equilibrium between dissolution and recrystallization had been attained. Following calcination (Figure 6b), all samples completely converted into pure cubic CeO₂, thus confirming that the intermediate phase of CeCO₃OH can be entirely decomposed into CeO₂ via thermal treatment.

Figure 6. XRD patterns of samples synthesized solvothermally at 180 °C for 12 h with different Ce:N molar ratios of 1:3, 1:4, 1:5, 1:6, and 1:7 (a) before and (b) after calcination in air at 500 °C for 1 h.

The morphologies of CeO₂ obtained after calcination at various Ce:N molar ratios are illustrated in Figure 7. At a Ce:N ratio of 1:3 (Figure 7a), the product predominantly comprised nanodisks with diameters approximately 500 nm (with an aspect ratio, thickness/diameter, estimated from representative areas to be about 0.1–0.2), accompanied by a limited number of nano-octahedra (edge length < 350 nm). Increasing the ratio to 1:4 (Figure 7b) resulted in a transformation into aggregates of nanoparticles displaying discernible porous structures, likely attributed to Ostwald ripening or dissolution–recrystallization processes. The primary nanoparticles within these aggregates ranged from ~20 to 80 nm in size. At a Ce:N ratio of 1:5 (Figure 7c), dendritic structures formed from nanoparticles emerged alongside some aggregated particles; this may be due to the excessive presence of NH₄HCO₃, which potentially heightened the supersaturation level of the solution and facilitated oriented attachment among nanoparticles. The dendritic branches are typically 50–80 nm in width and several micrometers in length. Similar morphological evolution from plates to dendrites driven by additive concentration has been observed in other metal oxide systems [40]. As we progressed to a Ce:N ratio of 1:6 (Figure 7d), the aggregates were no longer present, while dendritic structures continued to develop further, indicating that NH₄HCO₃ promotes the establishment of a new dissolution–recrystallization equilibrium within the precursor phase. However, when increasing the Ce:N ratio to 1:7 (Figure 7e), there was an observed reduction in size for dendritic structures along with the re-emergence of irregular plate-like forms and aggregates composed of nano-octahedra, suggesting that an excess amount of NH₄HCO₃ might disrupt the original crystallization kinetics by introducing multiple competing growth pathways during morphological evolution of the precursor phase. To correlate these morphological changes with structural characteristics, key parameters derived from XRD for all Ce:N ratios are compiled in Table 2. Of particular note is the trend in lattice strain, which increased significantly with higher NH₄HCO₃ content, providing a structural rationale for the observed transition from well-defined octahedra to complex dendritic and aggregated morphologies.

Figure 7. SEM images of CeO₂ samples synthesized solvothermally at 180 °C for 12 h with different Ce:N molar ratios of (a) 1:3, (b) 1:4, (c) 1:5, (d) 1:6, and (e) 1:7 after calcination in air at 500 °C for 1 h.

Table 2. Crystallite size and structural parameters of CeO₂ synthesized with different Ce:N molar ratios.

Ce:N Molar Ratio	Crystallite Size (nm)	Relative Crystallinity (%)	Strain (%)	Dominant Phase (Before Calcination)	Dominant Morphology (After Calcination)
1:1	15.5	79.6	0.152	CeCO3OH + CeO2	Nano-octahedra
1:2	17.2	88.2	0.121		Nano-octahedra
1:3	18.7	69.4	0.614		Nanodisks
1:4	9.7	67.9	1.084		Nanoparticle aggregates
1:5	10.5	61.9	1.110		Dendritic structures
1:6	11.1	66.9	0.854		Dendritic structures
1:7	12.2	68.7	0.830		Irregular plates/aggregates

2.4. Influence of Solvothermal Reaction Time

The influence of reaction time (0.5–36 h) on phase evolution and morphology was systematically investigated in the mixed solvent system (15 mL MeOH + 5 mL H₂O) at a Ce:N ratio of 1:3. After the shortest reaction time of 0.5 h (Figure 8a), the main phase of the precursor obtained through solvothermal synthesis was orthorhombic Ce₂O(CO₃)₂·H₂O (JCPDS No. 44-0617). When the reaction time was extended to 3 h, Ce₂O(CO₃)₂·H₂O remained the dominant phase, but the hexagonal CeCO₃OH phase (JCPDS No. 32-0189) also appeared. At 6 h, the diffraction peaks of Ce₂O(CO₃)₂·H₂O became very weak, while CeCO₃OH became the major phase, accompanied by the emergence of cubic CeO₂. After 18 h, the Ce₂O(CO₃)₂·H₂O phase disappeared, and CeO₂ became dominant, with CeCO₃OH as the secondary phase. Interestingly, at 24 h, the diffraction peaks of CeCO₃OH intensified and became dominant again, possibly due to transient reaction conditions such as local pH fluctuations or incomplete consumption of precursors. After 36 h, the system returned to being dominated by CeO₂, with CeCO₃OH as the minor phase, indicating that prolonged reaction time drives the system toward thermodynamic equilibrium, as CeO₂ was the most stable phase under oxidative conditions. After calcination at 500 °C (Figure 8b), all precursors were completely converted to cubic CeO₂, consistent with their thermal decomposition pathways, with no residual precursor phases observed. Furthermore, variations in XRD peak width and intensity among the CeO₂ samples may originate from differences in inherited grain size or defect density from the precursor compositions.

Figure 8. XRD patterns of samples synthesized solvothermally at 180 °C for different reaction times of 0.5, 3, 6, 18, 24, and 36 h with a Ce:N molar ratio of 1:3 (a) before and (b) after calcination in air at 500 °C for 1 h.

Figure 9 illustrates the morphological evolution of CeO₂ samples following calcination at various solvothermal reaction times. After 0.5 h, the predominant morphology consisted of micron-sized plate-like structures formed from nanoscale sheets, likely resulting from rapid nucleation under conditions of high supersaturation. These primary sheets have a thickness on the order of tens of nanometers and lateral dimensions of 1–10 μm. At 3 h, a coexistence of plate-like and dendritic structures was observed. By 6 h, the morphology transitioned to nanodisk structures. These nanodisks measured ~300–600 nm in diameter, consistent with the dimensions observed in the Ce:N series (Figure 7a). After 18 h, nano-octahedra emerged alongside disk-like structures. Notably, at 24 h, triangular plate-like (with side lengths ~1 μm) and hollow spherical structures (with diameters ~500 nm) appeared while the quantity of disk-like structures diminished; this observation indicates transient dissolution–recrystallization and Ostwald ripening processes were occurring. Following a duration of 36 h, the morphology predominantly reverted to octahedral forms, which is consistent with the XRD results and confirms the thermodynamic stability of CeO₂ under prolonged reaction conditions. The octahedra at this stage are well-defined with edge lengths ranging from 100 to 500 nm. The quantitative evolution of crystallographic parameters with reaction time is detailed in Table 3. The data revealed a systematic decrease in lattice strain over time, which aligns with the progression from kinetically formed metastable phases (e.g., carbonate intermediates) towards the thermodynamically stable, low-strain CeO₂ octahedra.

Figure 9. SEM images of CeO₂ samples synthesized solvothermally at 180 °C for different reaction times of 0.5, 3, 6, 18, 24, and 36 h with a Ce:N molar ratio of 1:3 after calcination in air at 500 °C for 1 h.

Table 3. Crystallite size and structural parameters of CeO₂ synthesized for different reaction times (Ce:N = 1:3).

Reaction Time (h)	Crystallite Size (nm)	Relative Crystallinity (%)	Strain (%)	Dominant Phase (Before Calcination)	Dominant Morphology (After Calcination)
0.5	8.3	23.2	0.956	Ce2O(CO3)2·H2O	Plate-like structures
3	9.6	60.1	0.804	Ce2O(CO3)2·H2O + CeCO3OH	Plates + Dendrites
6	10.6	68.3	0.605	CeCO3OH + CeO2	Nanodisks
18	9.8	62.1	0.722	CeO2 + CeCO3OH	Nano-octahedra + Disks
24	9.2	65.6	0.725	CeCO3OH	Triangular plates + Hollow spheres
36	9.1	69.5	0.762	CeO2 + CeCO3OH	Irregular octahedra

In summary, the evolution of the final CeO₂ morphology, from plates, dendrites, and hollow structures to octahedra, triangular plates, and hollow architectures, underscores the intricate interplay between dynamic and thermodynamic factors during nanomaterial growth. The ultimate morphology is significantly influenced by both the chemical composition of the precursor and the chemical reactions that take place throughout the solvothermal process. Consequently, potential chemical reactions and processes that may occur during this solvothermal reaction have been proposed and are summarized in Equations (1)–(15).

First, the potential interaction between Ce³⁺ ions and MeOH, as illustrated in Equations (1)–(3), facilitates the crystallization of CeO₂ under the influence of O₂. This oxygen originates from air trapped within the reactor and from oxygen dissolved in the solvent. This observation elucidates why CeO₂ can be synthesized in MeOH or MeOH–H₂O systems without requiring the addition of NH₄HCO₃. It is noted that MeOH can be oxidized on CeO₂ surfaces at elevated temperatures (e.g., ~200 °C) [41], which may influence the local redox environment during synthesis. Furthermore, the H₂O and NO₃⁻ from Ce(NO₃)₃·6H₂O are not merely spectators; they can participate in the reaction network. H₂O can mediate the ion solvation and hydrolysis, while NO₃⁻ can act as an inherent oxidant during thermal decomposition, potentially generating species such as NO₂, N₂O₅, and O₂ at the applied temperatures, thereby contributing to the oxidation of Ce³⁺.

$${\mathrm{Ce}}^{3+}{+3(\mathrm{HOCH}}_3) \rightleftarrows {\mathrm{Ce}(\mathrm{OCH}}_3{)}_3+3{\mathrm{H}}^{+}$$

(1)

$${\mathrm{Ce}(\mathrm{OCH}}_3{)}_3{+3(\mathrm{HOCH}}_3) \rightleftarrows { \mathrm{Ce}\left(\mathrm{OH}\right)}_3{+3(\mathrm{CH}}_3{\mathrm{OCH}}_3)$$

(2)

$${ \mathrm{Ce}\left(\mathrm{OH}\right)}_3 \stackrel{{\mathrm{O}}_2}{\mathtt{\rightleftarrows }}{ \mathrm{CeO}}_2$$

(3)

Subsequently, the initially formed CeO₂ nanocrystals may serve as catalysts for reactions involving MeOH molecules, potentially resulting in chemical transformations as described in Equations (4)–(7). These processes generate derivatives such as HCHO and HCOOH. The resultant CeO₂ may subsequently dissolve under the influence of HCOOH, as indicated in Equations (8) and (9). Furthermore, H₂O serves a mediating role by participating in chemical interactions between Ce³⁺ ions and MeOH, possibly via Equation (8). This process likely facilitates the transformation of hard agglomerates into octahedral structures, which is supported by the findings presented in Figure 1, Figure 2 and Figure 3.

$${2(\mathrm{HOCH}}_3) \stackrel{{\mathrm{CeO}}_2}{\rightleftarrows }{ \mathrm{CH}}_3{\mathrm{OCH}}_3+{\mathrm{H}}_2\mathrm{O}$$

(4)

$${2(\mathrm{HOCH}}_3)+{\mathrm{O}}_2 \stackrel{{\mathrm{CeO}}_2}{\rightleftarrows } 2\mathrm{C}{\mathrm{O}}_2\uparrow +4{\mathrm{H}}_2\mathrm{O}$$

(5)

$${2(\mathrm{HOCH}}_3)+{\mathrm{O}}_2 \stackrel{{\mathrm{CeO}}_2}{\rightleftarrows } 2\mathrm{H}\mathrm{C}\mathrm{H}\mathrm{O}+2{\mathrm{H}}_{2}O$$

(6)

$${\mathrm{HOCH}}_3+{\mathrm{O}}_2 \stackrel{{\mathrm{CeO}}_2}{\rightleftarrows } \mathrm{HCOOH}+{\mathrm{H}}_2\mathrm{O}$$

(7)

$$\mathrm{HCOOH}\underset{{\mathrm{H}}_2\mathrm{O}}{\overset{\mathrm{Ionization}}{\rightleftarrows }}{\mathrm{COOH}}^{-}+{\mathrm{H}}^{+}$$

(8)

$${4\mathrm{CeO}}_2+12{\mathrm{H}}^{+}\rightleftarrows {4\mathrm{Ce}}^{3+}+6{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$$

(9)

After the addition of NH₄HCO₃ to the MeOH–H₂O mixed solvent system, three processes involving NH₄HCO₃ occur: thermal decomposition (Equation (10)), ionization (Equations (10) and (11)), and hydrolysis (Equations (12) and (13)). On the one hand, these processes lead to the generation of gases such as NH₃ and CO₂, which further increases the pressure within the solvothermal reaction system. This elevation in pressure facilitates product precipitation or dissolution–recrystallization processes along with Ostwald ripening mechanisms, while significantly enhancing the crystallinity of precipitated crystals. A comparison of the XRD shown in Figure 4a,b supports this hypothesis: under Ce:N molar ratios of 1:1 and 1:2, CeO₂ produced via the solvothermal process exhibited no significant changes before and after calcination, indicating that introducing an appropriate amount of NH₄HCO₃ can substitute for high-energy-consuming calcination process. On the other hand, these processes also yield species including CO₃²⁻, OH⁻, H⁺, H₂CO₃, and NH₄OH, thus creating favorable conditions for crystallizing initial grains such as Ce₂O(CO₃)₂·H₂O (Equation (14)) alongside the subsequent formation of CeCO₃OH intermediates (Equation (15)). Notably, it is important to recognize that the thermodynamic processes described in Equations (3), (14) and (15) exist in a delicate yet complex dynamic equilibrium. Herein dissolution and recrystallization occur simultaneously while influenced by various factors such as pH level of the system, concentration of CO₃²⁻ ions, pressure conditions, and reaction duration, potentially leading to coexistence among three phases at certain stages, as depicted in Figure 8a at 180 °C for 6 h. Among the proposed pathways, the oxidation of Ce³⁺ (Equations (1)–(3)), the decomposition and hydrolysis of NH₄HCO₃ (Equations (10)–(13)), and the formation/decomposition of the intermediate carbonate phases (Equations (14)–(15)) are considered the key steps governing phase evolution and morphology control.

$${\mathrm{NH}}_3\uparrow +\mathrm{C}{\mathrm{O}}_2\uparrow +{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Thermolysis}}{\rightleftarrows }{ \mathrm{NH}}_4{\mathrm{HCO}}_3\stackrel{\mathrm{Ionization}}{\rightleftarrows }{\mathrm{NH}}_4^{+}+\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}$$

(10)

$${\mathrm{HCO}}_3^{-}\stackrel{\mathrm{Ionization}}{\rightleftarrows } {\mathrm{CO}}_3^{2-}+{\mathrm{H}}^{+}$$

(11)

$${\mathrm{HCO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Hydrolysis}}{\rightleftarrows }{\mathrm{H}}_2{\mathrm{CO}}_3+{\mathrm{OH}}^{-}\hspace{1em}({\mathrm{H}}_2{\mathrm{CO}}_3\rightleftarrows {\mathrm{CO}}_2\uparrow +{\mathrm{H}}_2\mathrm{O})$$

(12)

$${\mathrm{NH}}_4^{+}+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Hydrolysis}}{\rightleftarrows }{\mathrm{NH}}_4\mathrm{OH}+{\mathrm{H}}^{+}\hspace{1em}({\mathrm{NH}}_4\mathrm{OH}\rightleftarrows {\mathrm{NH}}_3\uparrow +{\mathrm{H}}_2\mathrm{O})$$

(13)

$${2\mathrm{Ce}}^{3+}+2{\mathrm{OH}}^{-}+2{\mathrm{CO}}_3^{2-}\rightleftarrows {\mathrm{Ce}}_2{\mathrm{O}(\mathrm{CO}}_3{)}_2·{\mathrm{H}}_2\mathrm{O}\downarrow$$

(14)

$${\mathrm{Ce}}^{3+}+{\mathrm{CO}}_3^{2-}+{\mathrm{OH}}^{-}\rightleftarrows {\mathrm{CeCO}}_3\mathrm{OH}\downarrow$$

(15)

3. Materials and Methods

3.1. Materials

Cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.95%) and ammonium bicarbonate (NH₄HCO₃, 99.995%) were purchased from Aladdin Co. Ltd. (Shanghai, China). Methanol (MeOH, analytical grade, 99.5%) was supplied by Chengdu Kelong Chemical Co. Ltd. (Chengdu, China). Deionized water (H₂O) was obtained from an HRO-402 ultrapure water system (Chengdu Zhonghan Water Treatment Equipment Co., Ltd., Chengdu, China) and used throughout all experiments.

3.2. Synthesis of CeO₂ Powders

The synthesis procedure is illustrated in Figure 10. In a typical process (Route a), 4 mmol of Ce(NO₃)₃·6H₂O and varying amounts of NH₄HCO₃ (corresponding to Ce:N molar ratios from 1:1 to 1:7) were added to a mixed solvent of 15 mL MeOH and 5 mL H₂O. After stirring for 15 min, the solution was transferred into a Teflon-lined autoclave and reacted at 180 °C for different durations (0.5–36 h). The resulting precipitate (namely, “Precursor” in Figure 10) was collected, washed with deionized water until neutral pH, dried at 60 °C for 24 h, and finally calcined at 500 °C for 1 h in air (with a ramp rate of 10 °C/min) to obtain light-yellow CeO₂ powders.

Figure 10. Schematic illustration of the synthesis procedure and control experiments: (a) the primary route with NH₄HCO₃ in a mixed solvent of 15 mL MeOH and 5 mL H₂O; control experiments (b), (c), and (d) conducted without NH₄HCO₃ in (b) 20 mL pure MeOH, (c) 15 mL MeOH + 5 mL H₂O, and (d) 20 mL pure H₂O (yielded no precipitate).

For comparison, three control experiments were performed without NH₄HCO₃: in 20 mL MeOH (Route b), 15 mL MeOH + 5 mL H₂O (Route c), and 20 mL H₂O (Route d). In contrast, no precipitate formed in the pure aqueous system (Route d). It should be noted that in the pure aqueous system (Route d), no precipitate was obtained under the given conditions, confirming the necessity of MeOH for precursor mineralization in this system.

3.3. Characterization

The crystalline phases of the precursors and final products were analyzed by X-ray diffraction (XRD, DX-2700, Dandong Haoyuan Instrument Co., Ltd., Dandong, China) using Cu Kα radiation. The morphologies of the samples were observed by scanning electron microscopy (SEM, SEM5000, CIQTEK Co., Ltd., Hefei, China).

4. Conclusions

CeO₂ with diverse morphologies was controllably synthesized by adjusting the MeOH-H₂O solvent composition, introducing NH₄HCO₃ (Ce:N = 1:1–1:7), and varying solvothermal reaction times (0.5–36 h). A single-phase cubic CeO₂ can be produced via a one-step solvothermal reaction at 180 °C for 12 h in both pure MeOH and MeOH–H₂O mixed systems; however, no precipitate forms in purely aqueous solutions, highlighting the critical role of solvent choice. The addition of H₂O to MeOH significantly improved CeO₂ crystallinity from 40.9% to 70.3% after calcination and transformed the morphology from spherical agglomerates into well-dispersed micron-sized octahedra (<3 μm). NH₄HCO₃ acted as both a structure-directing agent and a crystallization promoter: even at low concentrations (Ce:N = 1:1–1:2), it enabled precise size control, yielding nano-octahedra (~240 nm) and demonstrating potential to partly substitute high-temperature calcination for crystallinity optimization. At higher Ce:N ratios, morphology evolved from nanoplates (~500 nm at 1:3) to dendritic structures (1:4–1:6), revealing the role of NH₄HCO₃ in directing phase transformation via intermediate CeCO₃OH. With extended reaction time, the CeO₂ morphology progressed from plates and dendrites to hollow architectures and finally to thermodynamically stable octahedra, reflecting a complex interplay of kinetic and thermodynamic factors. This work clarifies the synergistic mechanism between solvent-mediated kinetics and NH₄HCO₃ pyrolysis chemistry in regulating CeO₂ morphology, providing an eco-friendly, template-free strategy for designing morphology-tunable CeO₂ materials with potential applications in catalysis and energy storage. To fully realize their potential in catalysis and energy storage, future efforts will focus on correlating the specific morphologies with functional properties and extending this synergistic strategy to other metal oxide systems.