Controllable Synthesis of Ceria Nanoparticles with Uniform Reactive

Article pubs.acs.org/JPCC

Controllable Synthesis of Ceria Nanoparticles with Uniform Reactive {100} Exposure Planes Yong Chen,†,‡,§ Shuhui Lv,‡,§ Chunlin Chen,*,‡ Changjun Qiu,† Xiangfang Fan,† and Zhongchang Wang*,‡ †

School of Mechanical Engineering, University of South China, Hengyang 421001, China WPI Research Center, Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Japan

‡

ABSTRACT: We investigate, by a systematic first-principles calculation, the surface energies of ceria {100} crystal planes with adsorption of 13 kinds of nonmetallic elements and nine kinds of metallic elements. We predict theoretically that adsorption of nonmetallic B, C, F, Si, P, S, Cl, Br, OH, and I and metallic Sr, La, Mg, Na, K, Y, Ca, and Ba can stabilize the ceria {100} planes through lowering their surface energies. Experimentally, we purposely select KOH as a mineralizing agent by taking into account the calculated surface energies and the factors in hydrothermal synthesis, and demonstrate a successful production of cube-shaped ceria nanoparticles of high purity via carefully optimizing the synthesis parameters. Further comprehensive transmission electron microscopy study identifies all the exposure planes of the cube-shaped ceria nanoparticles as the uniform {100} crystal planes. As a result of this unique morphology, the nanoparticles are found to show markedly enhanced UV-absorption capability as compared to either octahedron-shaped ceria nanoparticles or bulk ceria.

1. INTRODUCTION Ceria (CeO2) nanomaterials find extensive applications, ranging from microcircuit polishing and air purification to solid oxide fuel cells owing to their enhanced properties in comparison to bulk counterpart.1−5 Such property improvement for CeO2 nanomaterials can be attributed not only to the increased surface areas with the downgrade of dimension, but also to specific exposure planes of the CeO2 crystals. A large number of works have already been conducted recently, demonstrating that specifically exposed nanocrystals could indeed play a crucial role in remarkably affecting the chemical activities of nanomaterials.6−8 Perhaps one typical example is CeO2 crystal planes, where the {111} planes represent the most stable low-index ones (i.e., having lowest surface energy), followed by the {110} and {100} crystal planes. Because of this energy origin, octahedron-shaped CeO2 nanoparticles can be readily synthesized since their exposure planes are {111} crystal planes.6 However, it is wellknown that the {100} crystal planes often exhibit significantly improved catalytic functionality as compared to the other two types of low-index planes owing to their highest chemical activity and also for the high concentration of oxygen vacancy.7,8 Previous studies have indeed revealed that oxygen vacancy is more favorable to be formed on the {100} and {110} crystal planes than on the {111} ones. This therefore raises an appealing likelihood to further realize excellent properties of CeO2 nanomaterials by a controllable synthesis of CeO2 nanocrystals with the {100} exposure planes. To date, CeO2 nanomaterials with various morphologies such as nanowires, nanorods, nanocubes, nano-octahedrons, © 2014 American Chemical Society

and nanospheres have been successfully synthesized via a variety of methods.9−19 Among them, the hydrothermal synthesis approach has attracted widespread attention because many of its own merits are ultimately important for one-step low-temperature synthesis, e.g., simple operation, low energy consumption, and large-scale industrialization.20−25 For instance, Wu et al.26 used the hydrothermal method to prepare the CeO2 nanoshapes including cubes and investigated their CO oxidation behaviors. However, a major issue occurs in hydrothermal synthesis process, that is, there is no guiding criterion in the choice of the reaction reagents, and the synthesis process is therefore often conducted in a try-and-error fashion. First-principles calculation is in principle able to solve this problem since it has been demonstrated as one of the most promising ways to predict structures and properties of materials.27,28 Here, we conduct a systematic first-principles calculation to search for the mineralizing agent and report on a successful controllable synthesis of CeO2 nanoparticles with the {100} exposure planes. The nanoparticles are identified to exhibit a cube-shaped morphology, which is associated with enhanced UV-absorption functionality as compared to octahedron-shaped CeO2 nanoparticles and CeO2 bulk.

2. COMPUTATIONAL AND EXPERIMENTAL DETAILS Calculations were conducted using Vienna ab initio simulation package (VASP) within the framework of density functional Received: October 28, 2013 Revised: January 13, 2014 Published: February 8, 2014 4437

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Figure 1. (a,b) Atomic model of pristine (100) plane of ceria. Schematic model showing the adsorption of a kind of (c) nonmetallic and (d) metallic elements on the ceria (100) plane. Calculated surface energies for the ceria (100) plane with the adsorption of (e) nonmetallic and (f) metallic elements from both GGA and GGA+U methods. The dashed lines give surface energies of the pristine (100) plane of ceria.

theory (DFT).29,30 The projector augmented-wave method (PAW)31 was employed for electron−ion interactions and GGA-PBE32 (Generalized Gradient Approximation of Perdew, Burke, and Ernzerhof) was employed to describe the exchangecorrelation functional. The GGA plus-U method was also applied with Uf = 6.0 eV and Jf = 1.0 eV for Ce f states. A cutoff energy of 500 eV and a regular Monkhorst−Pack grid of 10 × 10 × 1 k points were used for surface calculations. A vacuum region of 10 Å was embedded to avoid the unwanted interaction between the slabs and its period images. Since the (100) surfaces are charged with a net dipole normal to surface, half of the surface atoms are moved from one termination to the other in order to prevent the dipole. In addition, the stoichiometry of CeO2 is maintained in the supercell. All atoms

were fully relaxed until the magnitude of forces on every atom fell below 0.05 eVÅ−1, yielding optimized structures. All chemical reagents were of analytical grade (purity of 99.9 wt.%, Sigma-Aldrich Co., Ltd.) and used with no further purification. The hydrothermal synthesis approach was applied to produce the nanomaterials. The cerium nitrate hexahydrate (1 mmol) was first dissolved into distilled water, which was stirred for 15 min with a magnetic stirrer. The 10 mL/L KOH was then added to the solution in a drop-like way. The mixed solution was next transferred into autoclaves and treated at 230 °C for 36 h under the autogenous pressure. The white products were eventually harvested by centrifuging fast, washing with distilled water and ethanol to remove unexpected ions, and drying at 60 °C in air. 4438

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Figure 2. Total DOS and partial DOSs (PDOS) projected on selected atoms near the surface for the adsorption of (a) nonmetallic and (b) metallic elements on the ceria (100) plane. The DOS and PDOS of bulk atoms are shown as well for comparison. The vertical lines indicate the position of Fermi level.

Microstructures were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) (JSM-7800F, JEOL Ltd.), and transmission electron microscopy (TEM) (JEM-2010F, JEOL Ltd.) operated at an accelerating voltage of 200 kV. For the XRD, a Rigaku D/Max-1200X diffractometry with the Cu Kα radiation operated at 30 kV and 100 mA was used. Optical properties were measured with the UV−vis spectrometer at room temperature.

The surface energies for the adsorbed systems can be expressed as σA0 =

1 ⎡ total bulk ⎤ ⎣Eslab − NCeO2ECeO2 ⎦ 2S

(2)

where NA and EA are the number and total energy of adsorbed atoms (A = X or Y), respectively. The EX is calculated by putting an X2 dimer in a large box with a dimension of 20 × 20 × 20, and the EY is considered to be equal to total energy of the metallic bulk Y. On the basis of eq 2, we calculate surface energies for all the adsorbates, as shown in Figure 1e,f, where the horizontal dashed line indicates the surface energy of the clean (100) surface with no adsorption. To address the electron correlation effect, we present both the GGA and GGA + U data. From Figure 1e, we identify that that adsorption of B, C, F, Si, P, S, Cl, Br, OH, and I atoms can lower surface energy, indicating that these nonmetallic atoms stabilize the (100) surface of ceria, while the adsorbates N, O, and H show the opposite trend. However, the adsorption of all metallic atoms can decrease the surface energy and hence stabilize the ceria {100} surfaces (Figure 1f). It is noteworthy that the calculated surface energies using the GGA (GGA + U) for the adsorbed (100) systems with the OH and K are 0.30 (0.42) and −0.48 (0.06) J/m2, respectively, which are even lower than those of pristine (111) (0.67 J/m2 for GGA and 0.69 J/m2 for GGA + U) and (110) (0.99 J/m2 for GGA and 1.08 J/m2 for GGA + U) surfaces. To shed light on how the nonmetallic and metallic elements stabilize the (100) surface of ceria, we present in Figure 2 total density of states (DOS) and projected DOS (PDOS) plots of several selected surface atoms of the (100) surface together with those of bulk atoms as a reference. From Figure 2a, total DOSs of clean and OH-adsorbed (100) surface show almost a similar feature. The minor difference is that there appear new electron states around 5.0 eV below the Fermi level (EF). By

3. RESULTS AND DISCUSSION Figure 1a,b shows atomic models of clean {100} surfaces of CeO2 terminated with either Ce or O atoms. Their relative stabilities can be evaluated in terms of surface energies. Under the condition that the chemical potential of respective element equals its bulk total energy, surface energies of clean stoichiometric {100} surface can be expressed as σC0 =

1 ⎡ total bulk ⎤ ⎣Eslab − NCeO2ECeO2 − NAEA ⎦ 2S

(1)

bulk where Etotal slab and ECeo2 are total energies of a slab and a bulk unit cell of CeO2; NCeO2 and S are the number of unit cells and surface area, respectively. Using eq 1, surface energies of the Oand Ce-terminated CeO2 {100} surfaces are calculated to be 1.38 (1.47) J/m2 and 1.63 (1.83) J/m2, respectively, using the GGA (GGA + U) method, suggesting that the O-terminated {100} surface is relatively more stable. To investigate how adsorbed atoms impact stability of ceria {100} surfaces, we carried out a systematic energy calculation for the surfaces adsorbed with 13 nonmetallic X atoms (X = H, B, C, N, O, F, Si, P, S, Cl, Br, I, or OH) and nine metallic Y atoms (Y = Sr, La, Mg, Na, Be, K, Y, Ca, or Ba). Figure 1c,d illustrates the atomic models with adsorption of nonmetallic X and metallic Y atoms on the ceria {100} surfaces, respectively.

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indexed as that of a standard CeO2 with fluorite structure, and the diffraction peaks are identified as (111), (200), (220), (311), (222), (400), (331), and (420) lattice planes. Figure 4

performing PDOS analysis, the new states are identified to mainly come from the adsorbed O p and H s orbitals, which hybridize with the surface O p and Ce f states. Moreover, a larger degree of hybridization is observed near EF between electron states of surface atoms and those of adsorbed O atom. This demonstrates that there forms covalent bonding between the adsorbed OH and the surface atoms on (100) surface, which stabilizes the (100) surface, in consistence with the previous study revealing that covalent bonds are strong to prevent further chemical evolution of surfaces.33 However, for the system adsorbed with K, a large difference is visible between the clean and K-adsorbed surfaces (Figure 2b). The electronic states of K-adsorbed surface are shifted to lower energy region, which is attributed to the higher EF of metallic K than that of CeO2. Further PDOS analyses reveal a large degree of hybridization between the adsorbed K and surface atoms, demonstrating the existence of covalent bonding, which stabilizes the (100) surface. In view of the above theoretical data, the compounds containing nonmetallic B, C, F, Si, P, S, Cl, Br, OH, and I and metallic Sr, La, Mg, Na, K, Y, Ca, and Ba atoms could be adopted as the mineralizing agents. However, on closer inspection, the metallic elements Sr, La, Y, Mg, and Si are not eligible because they form insoluble oxides. Likewise, the nonmetallic F, P, and S elements cannot be adopted either in that they may react with Ce3+ ions, forming the insoluble compounds too. Although the alkaline-earth Ca metal can in principle satisfy the requirements as a mineralizing agent, it is not qualified either because the concentration of calcium hydroxide solution is too low. In addition, although the remaining K and Na metals show a very similar surface energy for the adsorbed systems, the radius of K+ ions is larger than that of Na+ ions, indicating that the K+ ions can have a smaller solubility in the crystal of ceria as compared to Na+ ions. This means that the as-synthesized ceria nanocrystals are chemically more pure if K+ ions are used. Moreover, the K+ ions can inhibit the dendritic growth owing to the lower hydrated coefficient of K+ than Na+ ions. For these reasons, we hereafter select the KOH as the mineralizing agent to prepare ceria nanocubes. To determine phase structure of the synthesized particles, we performed XRD analyses, as shown in Figure 3, where textual orientations of the detected matter are shown as well for easy reference. From the figure, the typical XRD spectrum can be

Figure 4. FE-SEM images of the as-synthesized CeO2 nanoparticles. The as-synthesized CeO2 nanoparticles are of cube shape with sharp corners and well-defined edges.

shows the SEM images of the as-synthesized nanoparticles, from which one can clearly note that the nanoparticles have a cubic shape with sharp corners and well-defined edges. Further, their surfaces are neat and smooth with no other particles adsorbed. We also analyze size distribution of the nanoparticles using the NANO MEASURER software, as shown in Figure 5. Evidently, the size of the as-synthesized nanoparticles is in principle uniform, ranging slightly from 20 to 44 nm, which gives an average value of ∼32 nm.

Figure 5. Size distribution of the as-synthesized CeO2 nanoparticles. Size of the nanoparticles spans the range from 20 to 44 nm with an average size of ∼32 nm.

To extract microstructure and morphology information, we further conduct TEM study of the synthesized CeO 2 nanoparticles. Figure 6a,b shows two typical bright-field images, which confirm that the nanoparticles are of cubic shape and high crystallinity, and that the surfaces are flat and clean. However, corners of the nanocubes are somewhat round with a

Figure 3. XRD spectrum of the powder synthesized via the hydrothermal method. The texture orientation is shown for easy reference. The spectrum can be indexed as that of the standard CeO2 with the fluorite structure. 4440

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In light of the aforementioned microstructural characterization, we propose a growth mechanism for the cube-shaped CeO2 nanoparticles. First, the hydrated Ce3+ ions react with OH− ions that are produced from the KOH in solution, resulting in the formation of milky Ce(OH)3·nH2O precipitates. The Ce(OH)3·nH2O precursors are subsequently oxidized by the oxygen already dissolved in solution at high temperature and high pressure conditions, giving rise to the nucleation of CeO2 crystals. These crystal nuclei then grow up, forming well-defined cube-shaped CeO2 nanoparticles terminated by six {100} planes. The reaction mechanism can be expressed by the following formulas: Ce3 + + 3OH− + nH 2O → Ce(OH)3 ·nH 2O

(3)

4Ce(OH)3 ·nH 2O + O2 → 4CeO2 ·nH 2O + 4H3O+ + 2O2 −

(4)

It is well-known that ceria acts as one of the most important materials for blocking UV rays in a wide UV-ray absorption region. To investigate how the exposure planes impact the absorption properties of CeO2 nanoparticles, we measure UV− vis absorption spectra of the cube-shaped (i.e., exposed with the {100} crystal planes) CeO2 nanoparticles together with the octahedron-shaped (i.e., exposed with the {111} crystal planes) ones as a reference. The synthesis process and structural characterization for the octahedron-shaped nanoparticles have been reported in our previous work.34 Figure 7 shows UV−vis

Figure 6. (a) Bright-field TEM image of CeO2 nanoparticles. (b) TEM image of an individual CeO2 nanocube. (c) SEAD pattern of the cube-shaped CeO2 nanoparticles along the [100] direction. (d) HRTEM image showing a corner of a nanocube. (e) EDS spectrum taken from the CeO2 nanocubes.

Figure 7. UV−vis absorption spectra of the cube- and octahedronshaped CeO2 nanoparticles. The red spectrum is for the nanocubes, and the blue one is for the nano-octahedra.

radius of ∼2 nm. Further statistical analysis of the nanoparticles in many bright-field images reveals that they have an average diameter of ∼30 nm. Moreover, the selected-area diffraction (SAED) pattern uncovers that the nanoparticles are [100] oriented with fluorite structure (Figure 6c). In addition, the (002) and (02̅0) diffraction spots are also detected, which arise from other surfaces of a nanocube. This confirms that surfaces of the nanocubes are composed of {100} crystal planes. Figure 6d shows a high-resolution TEM (HRTEM) image taken around the corner of a nanocube along [100] zone axis. Lattice spacing of two groups of perpendicular lattice fringes is determined to be ∼0.27 nm, in accord with that of the CeO2 {002} planes, in further support of the conclusion that the CeO2 nanocubes have six {100} planes. We further perform chemical analysis using an energy dispersive X-ray spectroscopy (EDS), as shown in Figure 6e, where only the oxygen and cerium signal are detected, suggesting that the CeO2 nanocubes are of high purity.

absorption spectra of the two types of samples, where one can see that there appears an absorbance peak at the wavelength of ∼373 nm in the cube-shaped case, whereas the peak is at the wavelength of ∼365 nm in the octahedron-shaped case (dashed lines). The absorbance peak of the cube-shaped CeO 2 nanoparticles shifts by 27 nm from that of bulk CeO2, which is at a wavelength of 400 nm,35 while that of the octahedronshaped ones shifts by 35 nm. Upon closer comparison of the two absorption spectra, we find that the absorbance peak in the cube-shaped case shows a steeper slope, indicative of a higher efficiency of the nanocubes than the nano-octahedra.

4. CONCLUSIONS We demonstrate, by a comprehensive first-principles calculation, that the adsorption of nonmetallic B, C, F, Si, P, S, Cl, 4441

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Br, OH, and I and metallic Sr, La, Mg, Na, K, Y, Ca, and Ba could stabilize the CeO2 {100} planes via lowering their surface energies. In consideration of the practical preparation processes, we adopt the KOH as a mineralizing agent and successfully synthesize cube-shaped CeO2 nanoparticles by a simple yet efficient hydrothermal method. Microstructural analysis reveals that all exposure planes of the nanoparticles are uniform {100} crystal planes. As a consequence of this unique morphology, the cube-shaped CeO2 nanoparticles are found to show significantly improved UV-absorption ability as compared to both the octahedron-shaped CeO2 nanoparticles and bulk CeO2. Such a combined study of theoretical prediction based on the first-principles calculations and deliberate design of the mineralizing agent in the hydrothermal synthesis shall serve as a paradigm for tailoring facets of nanoparticles of interest for a wide range of technological applications.

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AUTHOR INFORMATION

Corresponding Authors

*(C.C.) E-mail: [email protected]. Tel: +8122-217-5933. Fax: +81-22-217-5930. *(Z.W.) E-mail: [email protected]. Author Contributions §

Y.C. and S.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Z.W. thanks the financial supports from the Grant-in-Aid for Young Scientists (A) (grant no. 24686069), the Challenging Exploratory Research (grant no. 24656376), the Sasakawa Scientific Research Grant, and the JGC-S Foundation. S.L. appreciates the Grant-in-Aid for JSPS fellows (grant no. 23· 01799). Y.C. thanks the financial support by the construct program of the key discipline in HuNan province.

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