Hierarchical Structure and Catalytic Activity of Flower-Like CeO2 Spheres Prepared Via a Hydrothermal Method

Hierarchical CeO2 particles were synthesized by a hydrothermal method based on the reaction between CeCl3·7H2O and PVP at 270 °C. The flower-like CeO2 with an average diameter of about 1 μm is composed of compact nanosheets with thicknesses of about 15 nm and have a surface area of 36.8 m2/g, a large pore volume of 0.109 cm3/g, and a narrow pore size distribution (14.9 nm in diameter). The possible formation mechanism of the hierarchical CeO2 nanoparticles has been illustrated. The 3D hierarchical structured CeO2 exhibited a higher catalytic activity toward CO oxidation compared with commercial CeO2.


Introduction
CeO 2 is playing important roles in various fields such as promoters for three-way catalysts [1], fuel cells [2], hydrogen storage materials [3], and oxygen sensors [4]. Although the utilization of ceria is based on its intrinsic chemical properties, the structures and morphologies of CeO 2 also have a significant influence on its properties and applications [5,6].
So far, CeO 2 with different sizes and morphologies such as nanorods [7], nanospheres [8], nanotubes [9], and nanocubes [10] have been synthesized in the last decade. It was proved that CeO 2 nanoparticles with different sizes and morphologies have better properties than general CeO 2 does. CeO 2 nanoparticles afford more active sites because of their high specific surface areas and novel structures [11].
Preparation of CeO 2 with different structures and morphologies provides the basic groundwork for its advanced applications. Hierarchical structured CeO 2 with unique properties and novel functionalities has attracted the attention of many researchers in recent years.
Zhong et al. synthesized a three-dimensional (3D) flower-like CeO 2 micro/nanocomposite structure using cerium chloride as a reactant by a simple and economical route based on an ethylene glycol-mediated process [12]. Li et al. synthesized mesoporous Ce(OH)CO 3 microspheres with flower-like 3D hierarchical structures via different hydrothermal systems, including glucose/acylic acid, fructose/acrylic acid, glucose/propanoic acid, and glucose/n-butylamine systems. Calcination of the Ce(OH)CO 3 microspheres yielded mesoporous CeO 2 microspheres with the same flower-like morphology as that of Ce(OH)CO 3 microspheres [13]. Ouyang et al. reported a facile electrochemical method to prepare hierarchical porous CeO 2 nanospheres and applied them as highly efficient absorbents to remove organic dyes [14]. However, 3D hierarchical structured CeO 2 is commonly synthesized with relatively miscellaneous process, which limited the extensive usage of the prepared ceria. In this paper, we report a facile one-pot hydrothermal route to synthesize 3D hierarchical structured CeO 2 . The present hydrothermal route is low cost and can be easily scaled-up. The fabricated 3D hierarchical structured CeO 2 could be used as a catalyst for CO oxidation and a support for noble metal catalysts.

Preparation of Hierarchical Structured CeO 2
Cerium (III) chloride heptahydrate (CeCl 3 ·7H 2 O), polyvinyl pyrrolidone (PVP), and ethanol were purchased from Beijing Yili Chemical Reagent Co. Ltd. (Beijing, China). All materials were used without any further purification. In a typical synthetic procedure of the hierarchical structured CeO 2 , 0.5 mmol CeCl 3 ·7H 2 O was dissolved in 30 mL deionized water, and then 1 mmol PVP and 20 mL deionized water were added to the solution. After 15 min of magnetic stirring, the homogenous solution was transferred into the Teflon vessel of a hydrothermal bomb, which was then placed in an oven and maintained at 270 • C for 24 h. Then, the solution was cooled to room temperature, and the products were separated by centrifugation and washed with absolute ethanol and distilled water.

Characterization Techniques
The crystal phases of the products were characterized by X-ray diffraction (XRD) using Philips X'pert PRO analyzer (Philips, Amsterdam, The Netherlands) equipped with a Cu K α radiation source (λ = 0.154187 nm) and operated at an X-ray tube (Philips, Amsterdam, The Netherlands) voltage and current of 40 KV and 30 mA, respectively. The morphology of the products was examined by scanning electron microscopy (SEM) using a JEOL JSM 67OOF system (JEOL, Tokyo, Japan) and transmission electron microscopy (TEM) using a JEM-2100 system (JEOL, Tokyo, Japan) operated at 200 kV. Surface composition was determined by X-ray photoelectron spectroscopy (XPS) using an ESCALab220i-XL electron spectrometer (VG Scientific, Waltham, MA, USA) with monochromatic Al K α radiation. Nitrogen adsorption-desorption isotherms were analyzed using an automatic adsorption system (Autosorb-1, Quantachrome, Boynton Beach, FL, USA) at the temperature of liquid nitrogen.

3D Hierarchical Structured CeO 2 Prepared via Hydrothermal Method
The powder XRD pattern of the as-prepared sample is shown in Figure 1. As can be seen, the as-prepared sample can be indexed to the cubic phase of CeO 2 (JCPDS No. 34-0394). The average crystallite size calculated by the Scherrer equation is 26.8 nm.
The SEM images of the as-synthesized CeO 2 particles are shown in Figure 2. It can be seen from Figure 2a         The nitrogen adsorption and desorption isotherms of the as-prepared samples and the corresponding pore size distribution curve calculated by the Barret-Joyner-Halenda (BJH) method are shown in Figure 3. The nitrogen adsorption and desorption isotherms exhibit a slim hysteresis loop at a relative pressure of >0.2, which is the type-II curve. The calculated Brunauer-Emmett-Teller (BET) surface area of the as-synthesized CeO 2 is about 36.8 m 2 g -1 . The average pore size calculated by the BJH method is 14.9 nm. corresponding pore size distribution curve calculated by the Barret-Joyner-Halenda (BJH) method are shown in Figure 3. The nitrogen adsorption and desorption isotherms exhibit a slim hysteresis loop at a relative pressure of >0.2, which is the type-Ⅱcurve. The calculated Brunauer-Emmett-Teller (BET) surface area of the as-synthesized CeO2 is about 36.8 m 2 g -1 . The average pore size calculated by the BJH method is 14.9 nm.

Effects of Synthesis Conditions on the Formation of 3D Hierarchical Structured CeO2 and the Possible Formation Mechanism
To investigate the evolution of flower-like CeO2 particles, the samples obtained after different reaction times were characterized by SEM (Figure 4). The reaction temperature and the dosages of CeCl3·7H2O and PVP were kept constant (270 °C, 0.01 M, and 0.02 M, respectively). As we can see in Figure 4a, spherical particles were obtained in the early stage. After 12 h of hydrothermal treatment, the sample (Figure 4b) evolved into spheres on which many scrappy grains grew. We speculate that PVP at the surface of the spheres decomposed gradually at such a high temperature and pressure, and simultaneously, tiny nanoparticles on the surface of the spheres began to grow into nanosheets. As seen in Figure 4c, all spheres have transformed into flower-like CeO2 particles. Based on these observations, the possible formation mechanism of the 3D hierarchical structured CeO2 can be speculated. The schematic mechanism for the 3D hierarchical structured CeO2 obtained during different hydrothermal stages is illustrated in Figure 5. At an early stage, Ce 3+ ions were oxidized gradually by O2 present in the aqueous solution to form small CeO2 nanocrystals. Then, the small CeO2 nanocrystals interacted with PVP and self-assembled as building blocks into spherical particles. As the temperature of the hydrothermal system increased, the PVP at the surface of the spherical particles began to decompose and small nanoparticles began to grow into nanosheets via Ostwald ripening. Due to Ostwald ripening, more were nanosheets formed, and after 24 h of hydrothermal treatment, the PVP completely decomposed and 3D hierarchical structured CeO2 particles were formed.

Effects of Synthesis Conditions on the Formation of 3D Hierarchical Structured CeO 2 and the Possible Formation Mechanism
To investigate the evolution of flower-like CeO 2 particles, the samples obtained after different reaction times were characterized by SEM (Figure 4). The reaction temperature and the dosages of CeCl 3 ·7H 2 O and PVP were kept constant (270 • C, 0.01 M, and 0.02 M, respectively). As we can see in Figure 4a, spherical particles were obtained in the early stage. After 12 h of hydrothermal treatment, the sample (Figure 4b) evolved into spheres on which many scrappy grains grew. We speculate that PVP at the surface of the spheres decomposed gradually at such a high temperature and pressure, and simultaneously, tiny nanoparticles on the surface of the spheres began to grow into nanosheets. As seen in Figure 4c, all spheres have transformed into flower-like CeO 2 particles. Based on these observations, the possible formation mechanism of the 3D hierarchical structured CeO 2 can be speculated. The schematic mechanism for the 3D hierarchical structured CeO 2 obtained during different hydrothermal stages is illustrated in Figure 5. At an early stage, Ce 3+ ions were oxidized gradually by O 2 present in the aqueous solution to form small CeO 2 nanocrystals. Then, the small CeO 2 nanocrystals interacted with PVP and self-assembled as building blocks into spherical particles. As the temperature of the hydrothermal system increased, the PVP at the surface of the spherical particles began to decompose and small nanoparticles began to grow into nanosheets via Ostwald ripening. Due to Ostwald ripening, more were nanosheets formed, and after 24 h of hydrothermal treatment, the PVP completely decomposed and 3D hierarchical structured CeO 2 particles were formed. 5 of 8

Catalytic Performance of 3D Hierarchical Structured CeO2 for CO Combustion
Catalytic application is an important direction for CeO2 researches because the oxygen storage capacity of CeO2 is associated with its ability to undergo a facile conversion between Ce(Ⅲ) and Ce(Ⅳ). Therefore, the catalytic activity of the as-prepared 3D hierarchical structured CeO2 was tested by CO oxidation. As shown in Figure 6, the 3D hierarchical structured CeO2 exhibits better activity toward CO oxidation than commercial CeO2 (purchased from Beijing Yili Chemical Reagent Co. Ltd., Beijing, China) does. The 50% conversion temperature of the 3D hierarchical structured CeO2 is about 320 °C, while that of the commercial CeO2 is higher than 380 °C.

Catalytic Performance of 3D Hierarchical Structured CeO 2 for CO Combustion
Catalytic application is an important direction for CeO 2 researches because the oxygen storage capacity of CeO 2 is associated with its ability to undergo a facile conversion between Ce(III) and Ce(IV). Therefore, the catalytic activity of the as-prepared 3D hierarchical structured CeO 2 was tested by CO oxidation. As shown in Figure 6, the 3D hierarchical structured CeO 2 exhibits better activity toward CO oxidation than commercial CeO 2 (purchased from Beijing Yili Chemical Reagent Co. Ltd., Beijing, China) does. The 50% conversion temperature of the 3D hierarchical structured CeO 2 is about 320 • C, while that of the commercial CeO 2 is higher than 380 • C.

Catalytic Performance of 3D Hierarchical Structured CeO2 for CO Combustion
Catalytic application is an important direction for CeO2 researches because the oxygen storage capacity of CeO2 is associated with its ability to undergo a facile conversion between Ce(Ⅲ) and Ce(Ⅳ). Therefore, the catalytic activity of the as-prepared 3D hierarchical structured CeO2 was tested by CO oxidation. As shown in Figure 6, the 3D hierarchical structured CeO2 exhibits better activity toward CO oxidation than commercial CeO2 (purchased from Beijing Yili Chemical Reagent Co. Ltd., Beijing, China) does. The 50% conversion temperature of the 3D hierarchical structured CeO2 is about 320 °C, while that of the commercial CeO2 is higher than 380 °C.  The sample was further characterized by XPS and the Ce 3d electron core level spectra are shown in Figure 7. The four main 3d 5/2 features at 882.7, 885.2, 888.5, and 898.3 eV correspond to V, V', V", and V"' components, respectively. The 3d 3/2 features at 901.3, 903.4, 907.3, and 916.9 eV correspond to U, U', U", and U"' components [15], respectively. The signals V' and U' are characteristic of Ce(III) [16]. According to the ratio of the area for Ce 3+ peaks to the whole peak area in Ce 3d region, the amount of Ce 3+ of 3D hierarchical structured CeO 2 is 51.8%. The amount of Ce 3+ of commercial CeO 2 is 13.2%. The 3D hierarchical structured CeO 2 has a much higher Ce 3+ concentration, which implies a much higher concentration of oxygen defects compared with commercial CeO 2 . A large amount of oxygen defects enhances the conversion between Ce(III) and Ce(IV), thereby supplying more reactive oxygen. Thus, the special structure of 3D hierarchical structured CeO 2 provides more active sites for CO oxidation.
shown in Figure 7. The four main 3d5/2 features at 882.7, 885.2, 888.5, and 898.3 eV correspond to V, V', V'', and V''' components, respectively. The 3d3/2 features at 901.3, 903.4, 907.3, and 916.9 eV correspond to U, U', U'', and U''' components [15], respectively. The signals V' and U' are characteristic of Ce(III) [16]. According to the ratio of the area for Ce 3+ peaks to the whole peak area in Ce 3d region, the amount of Ce 3+ of 3D hierarchical structured CeO2 is 51.8%. The amount of Ce 3+ of commercial CeO2 is 13.2%. The 3D hierarchical structured CeO2 has a much higher Ce 3+ concentration, which implies a much higher concentration of oxygen defects compared with commercial CeO2. A large amount of oxygen defects enhances the conversion between Ce(III) and Ce(IV), thereby supplying more reactive oxygen. Thus, the special structure of 3D hierarchical structured CeO2 provides more active sites for CO oxidation.

Conclusions
In summary, a simple and economical hydrothermal route was presented to synthesize 3D hierarchical structured CeO2 using CeCl3·7H2O and PVP. The 3D hierarchical structured CeO2 has a beautiful flower-like structure, which consists of many nanosheets. A two-stage growth process was identified for the formation of 3D hierarchical structured CeO2, and Ostwald ripening was found to play an important role in the transformation of the nanoparticles into nanosheets. The 3D

Conclusions
In summary, a simple and economical hydrothermal route was presented to synthesize 3D hierarchical structured CeO 2 using CeCl 3 ·7H 2 O and PVP. The 3D hierarchical structured CeO 2 has a beautiful flower-like structure, which consists of many nanosheets. A two-stage growth process was identified for the formation of 3D hierarchical structured CeO 2 , and Ostwald ripening was found to play an important role in the transformation of the nanoparticles into nanosheets. The 3D hierarchical structured CeO 2 exhibited a higher catalytic activity toward CO oxidation compared with commercial CeO 2 .