Synthesis And Formation Mechanism Of Cerium Oxide Nanoparticles

Cerium oxide nanoparticles (CeO2) have been widely applied in catalysts, UV absorber, fuel cells and polishing powder based on their unique physical and chemical characteristics which depend on their structure, shape and size. Numerous methods have been developed to synthesize nanosized CeO2particles, such as homogeneous precipitation method, hydrothermal process, sol-gel technique, microemulsion method. Among them, the dispersion of nanoparticles obtained by homogeneous precipitation method and sol-gel technique is often bad. In addition, CeO2nanoparticles prepared by microemulsion method have good dispersity, but the yield is low, limiting their production in a large quantity.In this work, we utilized the oil-water interface method to prepare stable monodispersed hydrophobic CeO2nanoparticles. In comparison to other methods, it has many advantages, such as easy operation, low cost, and narrow size-distribution of particles. Effects of the reactants concentration, oxidation reaction temperature and surfactant on the size of CeO2particles were studied. Based on the results above, we developed the formation mechanism of hydrophobic CeO2. Also, CeO2:Y3+powders were successfully synthesized by the oil-water interface method and their luminescent properties were compared under different synthesizing conditions. Furthermore, CeO2nanoparticles with different sizes were formed with addition of seeds, and the formation mechanism of hydrophilic CeO2was analyzed.The main research results of this paper are briefly described as follows:(1) Uniform hydrophobic CeO2nanoparticles were obtained by a novel oil-water interface method. Effects of reactants concentration, oxidation reaction temperature, and the type of surfactants on the final products were investigated. The X-ray diffraction (XRD) result indicated that the synthesized particles were in a cubic structure. Transmission electron microscopy (TEM) images exhibited that nearly monodispersed CeO2nanoparticles were obtained with an average size of4.60nm. The UV-visible absorbance showed that the nanoparticles had a strong absorption at the wavelength below400nm. The room temperature photoluminescence (PL) showed that the particles had different luminescent properties prepared with different surfactants. Fourier transform infrared (FT-IR) showed that the particles were capped with oleic ions. The value of the contact angle proved that all the particles have a strong hydrophobic property.(2) Effect of NaOH concentration on the size of CeO2nanoparticles was discussed in this paper. When the OH-/Ce3+molar ratio was below4, or a little larger than4, the most important factors influencing the size of CeO2nanoparticles was the concentration of hydro-complex, positively charged Ce(OH)x4-x (x<4). Under this condition, the size of CeO2nanoparticles increased with increasing NaOH concentration. When OH-ions were greatly excessive, more oleic ions were produced and the oleic ions were rapidly adsorbed on CeO2nuclei formed in the oil-water interface to inhibit their growth, and then the final size decreased.(3) The formation mechanism of hydrophobic CeO2nanoparticles with the oil-water interface method was proposed. Once the oil phase and the water phase mix, the reaction occur to produce Ce(OH)4, and the NaOH concentration affects the final size of CeO2nanoparticles. The surfactant adsorption plays the decisive role to the products going into the oil phase. After the CeO2nucleation, the adsorption of surfactants on particles decreases the interfacial tension, and then particles spontaneously enter into the oil phase and stop growing, resulting in the production of monodisperse CeO2nanoparticles.(4) The CeO2:Y3+powders were also successfully prepared by oil-water interface method. The effect of doping concentration, alkaline concentration, calcining temperature and surfactant on the final products were investigated. The XRD profile showed that Y3+was doped in CeO2crystal lattice. The PL spectra showed the particles had different luminescent properties under different reaction conditions. And the optimum reaction conditions were as follows. The doping concentration was8.0%, the alkaline concentration was0.40M, and the calcining temperature was600℃. In this case, the luminescent properties increased with addition of surfactant.(5) CeO2nanoparticles with different sizes were synthesized with addition of seeds and the effects of the amount of seeds and calcining temperature were explored. The results showed that the size of CeO2nanoparticles decreased with increasing amount of seeds. The calcining temperature had little influences on the final size of CeO2nanoparticles. More importantly, the formation mechanism was investigated in detail.During the nucleation process, the nucleation will be accelerated with addition of seed. In the growth stage, CeO2nanoparticles grow with aggregation to reduce the surface energy of particles, which leads to production of the secondary particles consisting of many primary particles. However, the protonation of PVP adsorption on the surface of CeO2nanoparticles, makes the adjacent two primary particles repel each other. As a result, the shape of secondary particles was irregular.