1. Introduction

Cerium oxide is a major compound in the rare earth family that has attracted considerable attention because of their vast technological applications, such as photocatalysts, [1-3] electrochemical cells, [4] Optical and Magnetic Properties, [5] gates for metal-oxide semiconductor devices and phosphors, [6-7] biomaterials, [8] sorbents for H₂S removal, [9] and ultrviolet absorbents for sunscreens, [10] which are of great interest due to their wide applications, in particular, as redox or oxygen storage promoters in the three-way catalysts, catalysts for H₂ prodution from fuels, and solid state conductors for fuel cells. [11-12]

Cerium (IV) oxide, also known as ceric oxide, ceric dioxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare-earth metal cerium. It is a pale yellow-white powder with the chemical formula CeO₂, which is insoluble in water. It is an important commercial product and an intermediate in the purification of the element from the ores. The distinctive property of this material is its reversible conversion to a nonstoichiometric oxide. Cerium Oxide is fluorite phase structure (Fig. 1), whose molecular weight is 172.115 g/mol and chemical formula is CeO₂. The valence of cerium is +4, so it possesses strong oxidability. Its melting point and boiling point is 2400°C and 3500°C, respectively. CeO₂ can be obtained by the oxidation of Ce₂O₃:

2 Ce₂O₃ +O₂ → 4 CeO₂.

Normally, CeO₂ can not be reacted with the common acid. Furthermore, CeO₂ occur the oxidation-reduction reaction with CO in the presence of high temperature:

4 CeO₂ + 2 CO → 2 Ce₂O₃ + 2 CO₂ ↑

The work presented here aims at the development of an integrated research-education activity on novel controlled synthesis of CeO₂ hollow microspheres.

Figure 1. The structure of cerium dioxide

2. Experiment Description

2.1. Experiment Materials

Cerous nitrate (Bide Chemical Reagent Limited Company, Shanghai, China), sodium hydrate (NaOH) (Hengda Fine Chemical Products Limited Company, Shanghai, China), anhydrous methanol (Tianjin Fuyu Fine Chemical Co. Ltd., Tianjin, China), aqueous NH₄OH solution (Hengda Fine Chemical Products Limited Company, Shanghai, China) and tetraethylorthosilicate (TEOS) (Tianjin Fuyu Fine Chemical Co. Ltd., Tianjin, China) were purchased. All reagents were of analytical grade and used as received without further treatment. Distilled water was used in the experiment.

2.2. Instruments and Characterization

The morphologies and sizes of the resulting CeO₂ products were determined by field-emission scanning electron microscopy (FE-SEM, JSM 6700F), transmission electron microscopy (TEM, JEM-2000EX), The crystal structures of the resulting products were characterized by X-ray powder diffraction (XRD, D/MAX-500 X-ray powder diffractometer with Cu Kα radiation, λ=1.5418 Å at 40 kV and 70 mA) and an UV-vis spectrophotometer (Cary 500), respectively.

3. Experimental Procedure

3.1. Preparation of Silica Spheres

The uniform silica spheres were synthesized using Stöber method. [13] 245 mL of concentrated aqueous NH₄OH solution was added into the aqueous EtOH solution (EtOH 225 mL, H₂O 10 mL) and the resulting solution was stirred vigorously for 30 min. Then 21 mL of tetraethylorthosilicate (TEOS) was added in the solution and was stirred for 4 hrs. The mixture was centrifuged, decanted and dried in 100°C.

3.2. Preparation of CeO₂ Hollow Microspheres

In a typical synthesis of CeO₂ hollow microspheres, 0.868 g Ce(NO₃)₃·6H₂O was dissolved in 60 mL deionized water containing 0.1 g silica sol particles and the resulting mixture was ultrasonically irradiated for 20 min to form uniform solution. The obtained uniform solution was transferred to a Teflon- line stainless steel autoclave, which was sealed and maintained at 190°C for 24 h, and then air-cooled to room temperature. The resulting yellow precipitate was collected, washed with distilled water, and dried in air at room temperature. The yellow precipitate was treated with 0.5 M NaOH solution to remove the silica sol.

3.3. Schematic Procedure Used to Synthesize CeO₂ Hollow Microspheres

Scheme 1 shows the overall procedure used to synthesize CeO₂ hollow microspheres. Before hydrothermal treating of the reaction solution, silica microspheres are dispersed in Ce(NO₃)₃ solution by ultrasonic vibration. Under hydrothermal conditions, Ce(NO₃)₃ were decomposed to form CeO₂ nanocrystals on the surfaces of silica microspheres. The basic reaction may be expressed as follows:

Scheme 1. Schematic procedure used to synthesize CeO2 hollow microspheres

4. Result and Discussion

4.1. X-ray Powder Diffraction (XRD) Analysis

X-ray powder diffraction (XRD) analysis was carried out to investigate the phases of the as–synthesized products. A typical XRD pattern of the as-synthesized products is shown in Figure 1. All of the diffraction peaks in Fig. 2 can be exactly indexed to the face-centered CeO₂ with lattice constants a = 5.410 Å, which are in good agreement with the literature values (JCPDS 34-0394). [14]

Figure 2. XRD patterns of as-synthesized CeO2 hollow spheres

4.2. SEM Analysis of as-synthesized CeO₂ Hollow Spheres

Figure 3. SEM image of as-synthesized CeO2 hollow spheres synthesized at the concentrations of 0.1M Ce(NO3)3

Fig. 3 shows SEM image of CeO₂ hollow spheres synthesized at the concentrations of 0.1M Ce(NO₃)₃. The SEM photograph reveals that the as-synthesized products consist of uniform monodisperse spheres. Furthermore, it is clear that CeO₂ hollow spheres are composed of CeO₂ nanoparticles. Several individual broken hollow microspheres are present in the products as indicated by arrows in Figure 2, indicating that CeO₂ spheres possess hollow structures. The shell thickness of CeO₂ hollow spheres is 20 nm.

4.3. TEM Analysis of as-synthesized CeO₂ Hollow Spheres

Figure 4. TEM images of as-synthesized CeO2 hollow microspheres: the thickness of shell is (a) 20 nm (b) 35 nm, (c) 45 nm, and (d) 50 nm, respectively

Fig. 4 shows TEM images of CeO₂ hollow microspheres synthesized with different concentrations of Ce(NO₃)₃. The strong contrast between the core and edge indicates that CeO₂ microspheres are hollow. The shell thickness of CeO₂ hollow microspheres can be controlled by adjusting the concentrations of Ce(NO₃)₃ in the reaction solution. As the concentrations of Ce(NO₃)₃ are 0.050 M, 0.075 M, 0.100 M, and 0.125 M, the shell thickness are 20, 35, 45, and 50 nm, respectively (Table 1). The inner diameters of CeO₂ hollow microspheres are similar to that of silica microspheres.

Table 1. The relation between the concentrations of Ce(NO3)3 and the shell thickness of CeO2 hollow spheres

4.4. UV/vis Analysis of as-synthesized CeO₂ Hollow Spheres

As ultraviolet blocking materials, CeO₂ has strong absorption properties in the ultraviolet range. Fig. 5 shows the UV/vis absorption spectra of the CeO₂ powders. As the shell thickness increases from 20 nm to 50 nm, the absorbing boundary of CeO₂ hollow microspheres is blue-shifted from 450 nm to 430 nm. The strong absorption in the spectra is due to the charge-transfer transition from O^2- (2p) to Ce⁴⁺ (4f) orbitals in CeO₂, which overruns the well-known f to f spin-orbit splitting of the Ce 4f state. [15-17] To the best of our knowledge, there was no report on them. The optical properties of CeO₂ hollow microspheres will prevent some damage from ultraviolet rays. Compared with some previous work on ultrafine CeO₂ nanoparticles, their strong absorption band is below 400 nm in the spectrum. [18] As a result, the high absorption in the UV region for CeO₂ hollow microspheres indicates that this material was suitable as a UV-blocking material.

Figure 5. UV–vis absorbance spectra of CeO2 hollow microsphere powder: the thickness of shell is (a) 20 nm, (b) 45 nm and (c) 50 nm

In summary, CeO₂ hollow microspheres have been synthesized by hydrothermal decomposition reaction of Ce(NO₃)₃ on the surfaces of silica microspheres. The shell thickness of CeO₂ hollow microspheres can be controlled from 20 nm to 50 nm by adjusting the concentration of Ce(NO₃)₃ in the reaction solution. The high absorption in the UV region for CeO₂ hollow microspheres indicates that this material was suitable as a UV-blocking material. There experiments were performed at atmospheric pressure conveniently, making them suitable for materials physics and materials chemistry laboratories.

ACKNOWLEDGEMENTS

This work was financially supported by High quality curriculum project of graduate education in Shandong China (SDYKC17036), Upgraded Project of Shandong Province for Guidance Ability of Graduate Tutors (SDYY16014, SDYY15035), and National Training Programs of Innovation and Entrepreneurship for Undergraduates, China (201710426061).