Hydrothermal Synthesis of CeO2 Nanocrystals: Ostwald Ripening or

Article pubs.acs.org/crystal

Hydrothermal Synthesis of CeO2 Nanocrystals: Ostwald Ripening or Oriented Attachment? Ming Lin,*,† Zi Yuan Fu,‡ Hui Ru Tan,† Joyce Pei Ying Tan,† Seng Chee Ng,‡ and Eric Teo‡ †

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, S117602, Singapore ‡ School of Engineering, Republic Polytechnic, 9 Woodlands Avenue 9, S738964, Singapore S Supporting Information *

ABSTRACT: The growth mechanism of CeO2 nanocrystals prepared by the hydrothermal method has been studied in this article. The synthesis of CeO2 nanocrystals follows two stages, the initial nucleation of CeO2 nuclei and the subsequent ripening of nuclei in the hydrothermal process. The nucleation involves the precipitation of Ce3+ cations by OH− ions to form Ce(OH)3 nanoparticles and the transition from Ce(OH)3 to 2−3 nm CeO2 nuclei through an oxidation and rapid dehydration process. In the hydrothermal process, the oriented attachment of nuclei through a lattice matched surface and subsequent Ostwald ripening results in the growth of CeO2 nanocrystals. The dominant mechanism for the ripening of nuclei in hydrothermal reactions is the oriented attachment. The addition of polyvinylpyrrolidone surfactant and adjustment of solution acidity can promote the dispersion of the nuclei and enhance the effective collision among them in the hydrothermal stage, resulting in the oriented aggregation of particles and further growth into larger CeO2 nanocrystals (∼15 nm). Because of the low solubility of CeO2 crystals in water, the Ostwald ripening process (dissolution/reprecipitation) only plays the second important role in the hydrothermal reactions, which converts the assembly clusters into nanocrystals with/without well-defined edges or contributes to the further growth of nuclei from 2−3 nm to 3−5 nm. higher concentration of [OH−],8,9,18 or reductive reagents19 could suppress the oxidation of Ce(OH)3 during Ce(OH)3 crystal growth. Such one-dimensional Ce(OH)3 structures can be converted into the CeO2 counterparts without shape change during the oxidation and dehydration process. Meanwhile, twodimensional CeO2 nanosheets have been synthesized through the assembly of CeO2 nanocrystals.22 Recently, two review papers have summarized the controlled synthesis and assembly of CeO2 nanomaterials in solution,10,23 which have provided insightful and comprehensive overviews regarding the growth of CeO2 nanostructured materials. A complete understanding of CeO2 nanocrystals growth in solution is essential for us to optimize the synthesis methods and provide guidance for the design and fabrication of CeO2 nanocrystals in solution. In general, kinetically controlled crystal growth in solution is through two major approaches: Ostwald ripening and/or oriented attachment.24−27 The “oriented attachment”, instead of “oriented aggregation”, was used in this manuscript because it could better describe the

1. INTRODUCTION Ceria (CeO2) nanocrystals have wide applications in catalysts, fuel cells, UV absorbers, and sensors because of the high mobility of the oxygen species and the reversible transition between Ce4+ and Ce3+ oxidation states in CeO2 crystals.1−10 CeO2 nanocrystals were usually synthesized through solutionbased methods due to the advantages of low-cost, environmental friendliness and the potential to control the sizes and morphologies of the crystals. There are numerous reports on the synthesis of CeO2 nanocrystals using solution-based methods. By comprehensively studying those literature reports, we have concluded that most synthesis methods involved preparation of solutions containing Ce salts, precipitation of Ce salts by reaction with the OH− group either from reactants or from the hydrolysis of water, and transformation to the oxide.7−21 Surfactants were usually applied to control the size and shape of the nanocrystals and to prevent agglomeration of the as-synthesized particles. For instance, CeO2 nanocrystals could be obtained by precipitation of Ce(III) or Ce(IV) salt with hydroxide, mixed hydroxide, urea, ammonia, etc.7,11−17 Moreover, CeO2 nanocubes, nanowires, and nanorods could be fabricated from Ce(OH)3 intermediates.8,9,18−21 The addition of surfactants,19 © 2012 American Chemical Society

Received: March 29, 2012 Revised: April 30, 2012 Published: May 7, 2012 3296

dx.doi.org/10.1021/cg300421x | Cryst. Growth Des. 2012, 12, 3296−3303

Crystal Growth & Design

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coated with a layer of carbon film. Structural analysis of nanocrystals was performed on an FEI Titan 80/300 Scanning/TEM (200 kV) and an X-ray diffractometer with Cu Kα1 radiation (λ = 1.5406 Å). Scherrer equation was employed to calculate the average crystallite size6

particle formation process  through active collision and then fusing via sharing the same plane. This is also a general term used by other research groups for this type of particle formation process.25−27 Because of the lack of proper in situ methods to directly track the growth of the nanocrystals in solution, most of the crystal growth mechanisms were elucidated by the analysis of the crystal structures and morphologies after reaction. For example, Wu et al. proposed that the hydrothermal growth of CeO2 nanoparticles followed the classical Ostwald ripening process, and the acidity of the hydrothermal solution played a key role in the dissolution of smaller CeO2 grains.13 This mechanism has been adopted by many other researchers to explain the growth of CeO2 in the hydrothermal process.16,28 However, with a lack of detailed analysis on the obtained crystal structures, it is challenging to assign the growth pathway of the nanocrystals to Ostwald ripening or oriented attachment, especially for those single-crystalline nanoparticles without obvious evidence of self-assembly structures or agglomerated mesocrystal intermediates.24−26 Therefore, a more careful examination of the nanocrystal structures is required, where the pore structures, twin boundaries, defects, and dislocations in the crystal can be revealed to provide additional information to the growth mechanism.25 Recently, with the assistance of electron tomography, we have successfully demonstrated the three-dimensional shape of CeO2 nanocrystals and proposed that oriented attachment of as-synthesized small CeO2 particles during the hydrothermal process would result in a characteristic irregular octahedral shape.29 Some spaces were left between the primary building block crystals (or called mesocrystals) after self-assembly, leading to the formation of porous CeO2 single crystals with irregular octahedral shape.30 In this article, we fully explore the growth of CeO2 nanocrystals in the hydrothermal process and investigate if the crystal ripening in solution occurs via Ostwald ripening or oriented attachment. The details about the growth process, such as initial nucleation of CeO2 nuclei, influence of solution basicity/acidity, the effect of surfactants, and the particle interaction, are discussed here. Strong experimental evidences suggest that the ripening of CeO2 nanocrystals in the hydrothermal process cannot be simply described as an Ostwald ripening process.13 We believe that the comprehensive understanding of the growth mechanism of CeO2 in hydrothermal process in this paper would provide an accurate description for the synthesis of CeO2 in solution.

τ = Cλ /(β cos θ ) where τ is the average size of the crystalline domains, C is the dimensionless shape factor (0.941 used in this work), β is the full width at half-maximum (FWHM), line broadening at full width halfmaximum intensity (fwhm), and θ is the Bragg angle. The particle size statistics for each sample was further examined by a random selection of 100 particles from TEM images where the longest distances in the particles were measured.

3. RESULTS AND DISCUSSION 3.1. Synthesis of CeO2 Nanocrystals with Different Ce3+/OH− Molar Ratios. Figure 1 shows the XRD patterns of

Figure 1. XRD patterns of CeO2 nanocrystals synthesized with (a) 1:1, (b) 1:3, (c) 1:4, (d) 1:5, and (e) 1:10 Ce3+/OH− molar ratios.

the CeO2 nanocrystals obtained at different Ce3+/OH− molar ratios (1:1, 1:3, 1:4, 1:5, 1:10) with PVP. All the diffraction peaks can be assigned to the fcc fluorite structure of ceria (a = 0.5411 nm, JCPDS No. 34-0394), with no other crystalline impurities being detected. It is worth noting that the size and crystalline quality of the CeO2 particles vary with the Ce3+/ OH− molar ratio. The corresponding crystal sizes measured from the CeO2{111} peaks using Scherrer equation were 8.74, 11.89, 13.38, 8.23, 7.25 nm, respectively. The particle size increases with increasing amount of OH− added and then decreases with more alkali added into the solution. The sizes and morphologies of the CeO2 nanocrystals were further examined through TEM images, as shown in Figure 2. The experimental conditions, crystal sizes, and morphologies are summarized in Table 1. Figure 2a,b depicts TEM images of CeO2 nanocrystals prepared with a Ce3+/OH− molar ratio of 1:1, with a pH of 4 measured after the hydrothermal reaction. The synthesized nanocrystals were in near spherical shape with an average size of 8.4 nm which was calculated from TEM images and was in good agreement with 8.7 nm measured by XRD. Each of the particles was rather uniform in shape and quite well-dispersed. HRTEM images show that most particles have regular truncated octahedral shape, with eight {111} surfaces and six {100} surfaces exposed. Figure 2b demonstrates a typical CeO2 particle orientated along the [110] zone axis, where two truncated {100} surfaces and four {111}

2. EXPERIMENTAL SECTION Nanosized CeO2 particles were synthesized by the conventional hydrothermal method. Typically, 0.9 g of PVP (polyvinylpyrrolidone, molecular weight 30 000, final concentration 30 g/L) and 1 mmol of cerium nitrate hexahydrate (Ce(NO3)3·6H2O) were dissolved in 30 mL of deionized (DI) water. Different amounts of sodium hydroxide (NaOH), ranging from 1 to 10 mmol, were added into the mixture to precipitate the salt in order to investigate its role played in the synthesis process. Each mixture was stirred for 30 min under room temperature and transferred into a 50 mL Teflon lined autoclave (Fisher Scientific). The autoclave was heated at 180 °C for 24 h. The precipitate was collected by centrifuge and then washed several times with ethanol and DI water. Finally, the final CeO2 powders were obtained by drying the precipitates in an oven at 80 °C for 2 h. The pH of solution was measured using litmus paper before and after the hydrothermal reaction. The synthesized CeO2 nanocrystals were dispersed in ethanol and dropped onto transmission electron microscopy (TEM) copper grids 3297



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Figure 2. TEM images of CeO2 nanocrystals synthesized at different Ce3+/OH− molar ratios, (a, b) 1:1; (c, d) 1:3; (e, f) 1:4; (g, h) 1:5; and (i, j) 1:10. White arrows indicate pores inside the crystal structure.

Table 1. Size and Morphology of CeO2 Nanocrystals Synthesized with PVP (30 g/L) at Different Experimental Conditions Ce3+/OH− size (nm, XRD) size (nm, TEM) pH (before hydrothermal) pH (after hydrothermal) shape level of agglomeration

1:1

3:3

1:3

1:4

1:5

1:10

8.74 8.4 4−5

16.5 5

11.89 14.3 9

13.38 17.2 12

8.23 15.0 12

7.25 7.8 14

4

5

6−7

10

12

14

regular octahedron well dispersed

regular octahedron with pores dispersed

irregular octahedron with pores dispersed

irregular octahedron with pores dispersed

irregular shape with pores agglomerated

small particles with more surface steps heavily agglomerated

surfaces can be clearly observed. In the 2D TEM images the octahedron with large {100} truncations appears to be of a spherical shape when viewed along the ⟨110⟩ direction. Some crystals with pores inside the structure were also found in the images, which were indicated by arrows. With the change of Ce3+/OH− molar ratio to 1:3, the pH of the solution was raised to 7 after reaction. The synthesized CeO2 nanocrystals have irregular octahedral shape with sharp corners and edges, and more pores were observed in the nanocrystals. The average size increased to 14.3 nm by TEM measurement, which was bigger than that measured from the XRD pattern (11.89 nm). This is because the Scherrer equation was only accurate to count the particles with regular shapes, such as spheres or cubes, and it is not suitable to elucidate the size of elongated CeO2 crystals. The peak broadening was contributed by the width of the elongated particles, not the length of the particles. Thus, TEM measurement is more reliable to reflect the size of asymmetric crystals. CeO2 crystals synthesized with 1:4 Ce3+/OH− molar ratios were generally the same as those fabricated with 1:3 Ce3+/OH−

molar ratio. The particles were larger with an average size of 17.2 nm and most of them appeared to have formed irregular octahedral shapes. When the molar ratio was further adjusted to 1:5 and the pH of solution rose to 12, more voids and pores were observed in the CeO2 nanocrystals. The edges of these nanocrystals were not as sharp as those prepared in low pH medium and the particles tended to agglomerate together. With the Ce3+/OH− molar ratio increasing to 1:10, heavily agglomerated CeO2 crystals with an average size of 7.8 nm were obtained. The accurate determination of particle size through TEM images is difficult due to the severe agglomeration of the particles and the existence of some elongated nanocrystals introduces a larger standard deviation to the size analysis. The HRTEM image demonstrates that elongated nanocrystals were the fusion of some neighboring nuclei through sharing the common crystallographic orientation, resulting in the elongated or assembled single-crystalline particles. A typical example was shown in Figure 2j. Close observation of those particles demonstrated that the width of most nanocrystals was around 4−6 nm. 3298



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Although the majority of Ce3+ ions (Ce3+/OH− < 1:3) or the OH− ions (Ce3+/OH− > 1:3) had been precipitated before the hydrothermal reaction, the decrease of pH after the reaction (Table 1) indicated that hydrolytic condensation occurred for unreacted Ce3+ during the hydrothermal process until the ionization equilibrium was achieved in the final solution. Since the solubility of CeO2 in water is very low (2 × 10−48),13 the excess Ce3+ in the solution with a Ce3+/OH− molar ratio of 1:1 or 3:3 continued hydrolyzing to form stable CeO2 nanocrystals and H+ ions, generating an acid environment in the solution. The final yield of CeO2 nanocrystals from these two samples were close to 1 and 3 mmol, respectively, suggesting a nearly completed hydrolysis of the Ce3+ precursors. However, the final pH of both solutions was measured around 4, which was higher than that (1 or 2) from the theoretical calculation with the assumption that all Ce3+ ions were precipitated and the corresponding amount of H+ were generated. To resolve this contradiction, a blank experiment was performed where the pure PVP−water solution was prepared and adjusted to pH of 1 by HNO3. After a 24 h hydrothermal reaction, the pH of solution increased to 4, suggesting most H+ ions were neutralized by PVP. It should be kept in mind that this further hydrolytic reaction was not the Ostwald ripening process as Ostwald ripening involved the growth of large particles at the expense of smaller ones. The source of solute ions should not be from the unreacted precursors but from the dissolution of smaller particles. 3.2. Initial Nucleation of CeO2 before Hydrothermal Reaction. To better understand the growth mechanism of CeO2 nanocrystals during the hydrothermal process, we have investigated the size, structure, and morphology of CeO2 nuclei before the hydrothermal reaction. After mixing the Ce3+ with alkali, brown precipitates were formed instantly. The color of precipitates changed from brown to light purple and then to light yellow after 30 min, suggesting the oxidation of Ce3+ to Ce4+ of the particles.19 In order to probe the initial nucleation of CeO2 nuclei, we purged the DI water with argon to remove as much oxygen as possible before the reactants were added. The precipitates formed from the 1:3 Ce3+/OH− molar ratio were collected instantly and checked with XRD. The diffraction pattern (Figure 3a) confirmed that Ce(OH)3 was generated after precipitation of reactants in the absence of oxygen. An average crystallite size of Ce(OH)3 were measured to be 3.24 nm using the Scherrer equation. Without Ar purge or with the solution exposed to oxygen again, the precipitates could be easily oxidized by O2 dissolved

in the water. Figure 3b depicts the XRD pattern of the precipitate after oxidation, showing a cubic CeO2 crystal structure with an average size of 2.8 nm. The oxidation and subsequent fast dehydration of the Ce(OH)3 particles resulted in the formation of smaller CeO2 nuclei than the corresponding Ce(OH)3 precursors. It has been proposed that PVP could stabilize the Ce(OH)4 nuclei and then the subsequent solvothermal treatment converts Ce(OH)4 into CeO2 nuclei and further into highly crystallized CeO2 nanoparticles.31 In our experiment, the XRD results have confirmed that Ce(OH)4 is not a stable phase, which will undergo rapid dehydration due to its low basicity and give rise to the formation of CeO2 nuclei at room temperature without any hydrothermal treatment. Figure 4 demonstrates the TEM image of obtained CeO2 nuclei before

Figure 4. (a) TEM and (b) HRTEM image of CeO2 nuclei before the hydrothermal reaction with Ce3+/OH− molar ratios of 1:3.

hydrothermal treatment, which exhibited poor crystallinity with many surface defects and mainly {111} facets exposed. Although PVP had been intentionally added in the solution to stabilize the particles and prevent agglomeration, agglomeration of the CeO2 nuclei was observed. 3.3. Particle Agglomeration and Roles of PVP. Particle agglomeration is a very common phenomenon during the synthesis of nanocrystals in solution which is avoided by researchers working on nanoparticle synthesis. However, it plays an important role in the particle ripening process through oriented attachment mode. The agglomeration of CeO2 nanoparticles is controlled by the attraction of particles, repulsive forces, concentration of particles in the solution, and other experimental conditions.31 Stable dispersion of small CeO2 nanocrystals can be achieved below pH of 2.5, where some of the hydroxyls were protonated and the long-term electrostatics repulsion was established through the adsorption of nitrate anions surrounding the surface of the CeO2 nuclei.32−34 At higher pH, the bounded nitrate ions were released and replaced by the hydroxyl groups, and the reaction of surface hydroxyls resulted in the aggregation between adjacent particles.32,33 The aggregation among CeO2 nanocrystals would have occurred via the formation of Ce−O−Ce bonds with the combination reaction of Ce−OH bonds present on the particle surface,13,31 where the density of the hydroxyl groups on the CeO2 surface is strongly determined by the pH of the solution. The addition of surfactant can shift the onset of aggregation to a higher pH due to the replacement of nitrate ions with surfactants. The stronger steric forces between surfactants would prevent the coagulation of nanocrystals. Taniguchi et al. had reported the synthesis of highly dispersed CeO2 nanocrystals with a size of 2−3 nm using oleate groups as capping

Figure 3. XRD patterns of precipitation with Ce3+/OH− molar ratios at 1:3, (a) before oxidation (Ce(OH)3), (b) after oxidation (CeO2). 3299



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agents.15 PVP is another widely used polymer in nanocrystal synthesis, where the CeO2 nanoparticles can be stabilized through the interaction between surface hydroxyl groups and the hydroxyl groups/lactim groups of PVP.31 The long carbon chains of PVP with the other hydrophilic group extending into the water can decrease the aggregation of the particles. Thus, the agglomeration of CeO2 nanocrystals can be adjusted by the attractive force (oriented aggregation by sharing a common crystallographic orientation) and repulsive force (steric hindrance effect from PVP or electrostatic repulsion from covalent bound nitrite ions). Figure 5 compares the photo

(Supporting Information). The steric hindrance effect of PVP led to the formation of highly dispersed nuclei in the solution at pH 5, higher than the reported stable dispersion at pH 2.5 without addition of surfactant.32 It is worth noting that the concentration of nuclei in the solution of Ce3+/OH− with a molar ratio of 1:1 was one-third that in the solution with a molar ratio of 1:3 if we assumed that uniform sized CeO2 nuclei were formed before the hydrothermal treatment. The well-dispersed nano nuclei could possibly be induced by the low concentration and low collision probability among nuclei. To prove that this dispersion of nuclei had occurred by the adjustment of solution pH, the density of CeO2 nuclei in the solution was increased three times, while the Ce3+/OH− molar ratio (3:3) and pH value were kept almost the same. A darker and transparent colloid solution was obtained (Figure 5a(III)), indicating the highly dispersed CeO2 nuclei in the solution. TEM image (Figure S1c, Supporting Information) shows that clusters were formed and uniformly dispersed on carbon film, with each cluster comprising several nuclei with the same crystallographic orientation. Figure 5b,c shows a typical cluster constructed from at least six building nuclei without visible misorientations, with five octahedral CeO2 nuclei with exposed {111} and {100} facets and one cubic with {110} and {100} facets. In short, the steric hindrance effect from PVP and a moderate pH can remarkably reduce the attractive forces between the particles, prevent the aggregation, and lead to a nucleation of dispersed nuclei in colloid solution. Dispersed nuclei clusters can also be observed with increasing concentration of nuclei in the solution. Without PVP or in a basic medium, the strong attractive force among the particles gives rise to the agglomerated nuclei, as shown in Figure 5a (II), (IV), and (V). The other ions, such as NO3−, Na+ in the solution, have a negligible influence on the aggregation of the CeO2 nuclei except that they could change the pH value of the solution. For comparison, the CeO2 nanocrystals were synthesized without addition of PVP using the hydrothermal reaction. The results were shown in Figure 6 and described in Table 2. Different from the CeO2 nanocrystals synthesized with the assistance of PVP, single crystalline CeO2 nanocrystals with octahedral shape and internal pores were only synthesized at pH 1 to 2. With an increase in the pH of the solution, only heavily aggregated CeO2 nanoparticles with a size range of 3−5 nm were obtained after hydrothermal reactions, which are similar to those particles grown at a pH of above 12 and with the addition of PVP. Since the solubility of CeO2 was extremely low in the water, the excess Ce ions in the water (1:1, 3:3, and 1:2.5 sample) continued to undergo hydrolytic condensation in the hydrothermal reaction to form CeO2, resulting in the decrease of pH to 1 and 2. 3.4. Discussion on Growth Mechanism of CeO 2 Nanocrystals in Water. Following the undoubted evidence shown above, we suggest that the hydrothermal synthesis of CeO2 nanocrystals from Ce(III) precursor follows two stages, the initial nucleation of CeO2 nuclei and the subsequent ripening of nuclei during hydrothermal process. The nucleation process involves precipitation of Ce3+ cations by OH− ions to form Ce(OH)3 nanoparticles and transition from Ce(OH)3 to 2−3 nm CeO2 nuclei through the oxidation and rapid dehydration process. The oxidation and dehydration process could have occurred simultaneously because no crystalline Ce(OH)4 phase was found in our work. The size and

Figure 5. (a) A photo of CeO2 nuclei solution, (b) HRTEM image of one aggregated cluster from solution 5a(III), (c) schematic outlines showing the possible oriented aggregation of six CeO2 nuclei.

of solutions of CeO2 nuclei formed before hydrothermal reaction with different Ce3+/OH− molar ratios and with/ without addition of PVP. The delicate interaction among the nuclei results in the formation of dispersed or agglomerated colloid solution. In a basic environment, large amounts of precipitates were observed to be suspended in the solution no matter whether PVP was added or not. This is because the stronger attraction of surface hydroxyl groups between adjacent CeO2 particles resulted in coalescence of the nuclei at higher pH. The coalescence of nuclei was also confirmed by TEM images (Figure 4b). However, with the assistance of PVP, the CeO2 nanocrystals after hydrothermal reaction were not agglomerated as severely as nuclei due to the loss of surface areas with particle growth and decrease of solution pH (Figure 2c,e, Table 1). Under an acidic environment, both the number of hydroxyl groups on the particle surface and the attractive forces between CeO2 nuclei were substantially decreased. The CeO2 nuclei solution produced from Ce3+/OH− molar ratio of 1:1 with a pH of 5 demonstrates a transparent colloid solution and the well-dispersed particles were observed by TEM in Figure S1a 3300



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resulting in the formation of only heavily aggregated particles with more surface defects. Recently, using electron tomography to visualize the threedimensional structures of CeO2 nanocrystals, we found that oriented aggregation of 4−5 nm presynthesized CeO2 nanoparticles can result in the growth of CeO2 single-crystalline nanoparticles with an irregular truncated octahedral shape29 and porous octahedral shape,30 where all the pores extend along the ⟨110⟩ directions. These two types of irregular octahedrons, especially those with pores along the ⟨110⟩ directions which coincides with the space orientation created by self-assembly of small particles, are characteristic indications of the CeO2 nanocrystals being formed predominantly through the oriented attachment process. On the basis of the shape and morphological studies here, we suggest that the oriented attachment of 2−3 nm nuclei through a lattice matched surface and subsequent Ostwald ripening results in the growth of CeO2 nanocrystals in the hydrothermal process. Both ripening mechanisms play roles during hydrothermal synthesis of CeO2 nanocrystals, and the oriented attachment is the dominant process in controlling the growth of large particles. In retrospect, these porous structures are not expected to be produced from the gradual growth of nanocrystals through the Ostwald ripening process (Figures 2 and 6). Oriented attachment is achieved by effective collision among nuclei with the same crystallographic orientation in the hydrothermal stage. Promoting the dispersion and collision of nuclei is the key for hydrothermal growth of bigger CeO2 nanocrystals. This is consistent with the fact that the collision of nuclei occurs more frequently and efficiently in dispersed solutions, such as in an acidic medium without addition of PVP or a moderate pH range with PVP. It can be suppressed in a solution where the nuclei are severely coagulated and the active collision among nuclei is prohibited. Even for heavily agglomerated nuclei, the oriented attachment between some neighboring nuclei still occurs, leading to the formation of rodlike nanocrystals. PVP prevents the aggregation of particles in the hydrothermal reaction and promotes the orientated attachment through effective collision, so that the welldispersed and large particles can be formed through the oriented attachment process at a broader pH range. Ostwald ripening process (dissolution/reprecipitation) plays a second important role in the hydrothermal reactions due to the low solubility of CeO2 crystals in water. It can be achieved by the precipitation of solution ions on large crystals from the dissolution of smaller nuclei and the dissolution and precipitation occurred at convex and concave regions on aggregated clusters due to the different chemical potential. Thus, it converts the assembly clusters into nanocrystals with/

Figure 6. TEM images of CeO2 nanocrystals synthesized without addition of PVP at different Ce3+/OH− molar ratios, (a) 1:1, (b) 3:3, (c) 1:2.5, (d) 1:3, (e) 1:4, and (f) 1:10.

morphology of CeO2 nuclei synthesized in this work is similar to that formed by the precipitation of Ce-oleate precursors using oleate as a surfactant,15 further confirming that this is a typical process for the initial nucleation of Ce3+ precursors via precipitation. Moreover, the coagulation of CeO2 nuclei is controlled by steric effects of PVP and surface hydroxyls modified by the solution acidity, resulting in the formation of dispersed CeO2 nuclei in acidic medium and agglomerated CeO2 nuclei in alkaline medium before hydrothermal reaction. The acidity of the solution plays a critical role in the CeO2 nanocrystals growth in the subsequent hydrothermal reaction. It affects both the density of hydroxyl groups on the surface of CeO2 particles and the solubility of Ce4+ from the CeO2 nuclei,29−32 that is, the dispersion of nanoparticles (oriented attachment) and dissolution−precipitation process (Ostwald ripening). The decrease in pH value of the solution increases the solubility of CeO2. A higher solubility of CeO2 can induce an increased dissolution/precipitation process which leads to the formation of crystals with sharp corners and edges. On the contrary, increasing [OH¯] lowers the solubility of CeO2,

Table 2. Size and Morphology of CeO2 Nanocrystals Synthesized without Addition of PVP Ce3+/OH− size (nm, TEM) pH (before hydrothermal) pH (after hydrothermal) shape level of agglomeration

1:1

3:3

1:2.5

1:3

1:4

1:10

7.3 5

7.9 5

5.4 5

3.4 10

4.0 12

3.7 14

1

1

2

6

12

13

regular octahedron with pores well dispersed

regular octahedron with pores agglomerated

porous octahedron with surface steps agglomerated

small and fused particles heavily agglomerated



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collision among nuclei, the oriented attachment process is substantially prohibited. However, sharing a common crystallographic orientation between neighboring nuclei still occurs and results in the formation of chain-like or rod-like nanocrystals (Figure 2i,j). This further confirms that the solubility of CeO2 is quite low so that the Ostwald ripening is unable to recrystallize the agglomerated particles into a bigger crystal. Instead, only 3−5 nm nanocrystals formed in the agglomerated precipitates. The schematic mechanism for the formation of CeO 2 nanocrystals during the different stages is illustrated in Figure 8.

without well-defined edges, or contributes to the further growth of nuclei to around 3−5 nm at 180 °C. It does not account for the growth of CeO2 nanocrystals larger than 6 nm. Therefore, at a pH of 4 (Ce3+/OH− = 1:1), the welldispersed nuclei and low collision frequency caused by the protection of PVP result in the formation of ∼8 nm CeO2 nanocrystals. Increasing the concentration of nuclei three times (Ce3+/OH− = 3:3) can increase the collision rate, thus generating the assembly clusters through sharing of a common crystallographic orientation among the nuclei. The increased collision rate and oriented attachment process lead to the formation of larger CeO2 nanocrystals (∼16.5 nm). Since more PVP molecules were adsorbed on the surface of CeO2 nanocrystals at pH of 4, active surface sites can be partially blocked by PVP from the attachment of solute ions and result in the formation of steps and defects on the surface. The continuing hydrolytic condensation from unreacted Ce3+ ions and dissolution/precipitation process would then convert the oriented aggregated mesocrystals into regular octahedral shape crystals with fewer pores (Figures 2a and 7).

Figure 8. A schematic diagram showing the formation of CeO2 nanocrystals.

4. CONCLUSION In conclusion, the hydrothermal synthesis of CeO2 nanocrystals from Ce(III) precursor includes the initial nucleation of CeO2 nuclei and the subsequent ripening during the hydrothermal process. The nucleation process involves the precipitation of Ce3+ to form Ce(OH)3 nanoparticles and the transition from Ce(OH)3 to 2−3 nm CeO2 nuclei through the oxidation and rapid dehydration processes. In the hydrothermal process, the oriented attachment of nuclei through a lattice matched surface and subsequent Ostwald ripening results in the growth of CeO2 nanocrystals. The oriented attachment process is the dominant mechanism for the ripening of nuclei in the hydrothermal reactions, and it relies on the effective collision among nuclei during the reaction. Ostwald ripening process (dissolution/reprecipitation) plays the second important role in the hydrothermal reactions due to the low solubility of CeO2 crystals and slow dissolution of small particles in water. It can convert the assembly clusters into nanocrystals with/without well-defined edges or contribute to the further growth of nuclei to around 3−5 nm at 180 °C. PVP prevents the aggregation of particles in hydrothermal reaction and gives rise to the growth of welldispersed and large particles through oriented attachment at a broader pH range. The results and conclusions in this work are clear and convincing. We believe this is a representative pathway for the initial nucleation and growth of CeO2 in the hydrothermal process. We have observed similar phenomenon in all our

Figure 7. TEM images of CeO2 nanocrystals synthesized with addition of PVP at a different Ce3+/OH− molar ratio of 3:3.

With increasing attractive forces between particles by adsorption of more hydroxyl groups on the surface at the moderate pH range (6−10), large and asymmetric crystals are preferably formed through the oriented attachment process. The low dissolution/precipitation process only leads to formation of particles with sharp edges. With the pH further increased to 12, the higher oriented aggregation rate and lower solubility of CeO2 nuclei lead to formation of even larger CeO2 particles but with more surface defects (Figure 2h). Without adding PVP and in acidic medium (pH < 2), the porous octahedral structures indicate the growth mechanism still follows the oriented attachment process, followed by dissolution/precipitation or continuing hydrolytic condensation to form particles with sharp edges. This is consistent with the fact that stable dispersion of small CeO2 nanocrystals can be achieved below pH of 2.5. The long-term electrostatics repulsion was established through the adsorption of nitrate ions on the surface of CeO2 nuclei;32 hence the increased collision among the dispersed nuclei significantly improves the oriented attachment growth of the larger nanocrystals. At a pH of 14 with PVP or pH larger than 2 without PVP, due to the presence of a high concentration of the hydroxyl group on the surface and attraction among the nuclei, particles tend to be strongly agglomerated together. Without effective 3302



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synthesis under different conditions, such as at different temperatures (Figure S2 in the Supporting Information), in ethanol medium (Figure S3 in the Supporting Information), with different sized CeO2 nuclei precursors (Figure S4 in the Supporting Information), and with different starting pH (Figure S5 in the Supporting Information). After checking other research works, we found that the mechanism proposed here can also explain the growth of CeO2 nanoparticles in many other literature reports.11−17 However, this mechanism is only applied to solution with pH between 1 to 14 for the formation of octahedral CeO2 nanocrystals. For hydrothermal synthesis in higher concentrated basic medium, CeO2 nanocubes or nanowires are synthesized from the Ce(OH)3 precursor instead, which is different from the octahedral CeO2 nuclei used here.8,9,18−21 The growth mechanism of CeO2 nanocrystals in the hydrothermal process proposed in this work is similar to the two-step nucleation mechanism of crystals.35,36 Agglomeration of nanoparticles is a common phenomenon during materials synthesis, and the findings in this work strongly indicate that oriented attachment growth of the particles cannot be neglected during reactions in many other systems, especially when the solubility of product is very low in the solution (metal oxide, pure metals). The growth of large nanocrystals and crystals with specific shapes should not be explained by the classical Ostwald ripening process only, while oriented attachment play an important role in the chemical synthesis.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of CeO2 nuclei synthesized with/without addition of PVP at different Ce3+/OH− molar ratios; TEM images of CeO2 nanocrystals synthesized at 100 °C; TEM images of CeO2 nanocrystals synthesized at 180 °C in ethanol; TEM images of CeO2 nanocrystals synthesized with different pH values. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 65-6874 5374. Fax: 656874 4778. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of IMRE (IMRE/10-1C0424) is gratefully acknowledged.

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REFERENCES

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Hydrothermal Synthesis of CeO2 Nanocrystals: Ostwald Ripening or

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