Synthesis Of Zirconia Nanoparticles And Formation Mechanism

Owing to the excellent physical and chemical properties, zirconia (ZrO2) can be applied as ceramics, catalysis, sensors, wear-resistant refractory materials and also in other fields. Zirconia nanoparticles (NPs) capped with lipophilic groups will greatly extend its application range. For example, oil-soluble zirconia NPs can be used as catalyst or catalyst carrier in oil phase reaction to improve the diversity of the catalytic reaction, also as lubricating oil additives to enhance the wear resistance.Among the three crystal types of zirconia, tetragonal zirconia (t-ZrO2) NPs and monoclinic zirconia (m-ZrO2) NPs have been more widely applied. The formation mechanisms of ZrO2NPs of different types are not the same. And the phase type of ZrO2NPs can been regulated and controlled through adjusting synthesis methods and conditions. Numerous methods have been developed to fabricate ZrO2NPs, such as co-precipitation, hydrothermal method and microemulsion processes. The products synthesized by co-precipitation and the hydrothermal method are commonly hydrophilic, limiting their application in oil phase. The lipophilic ZrO2NPs can been prepared by microemulsion process, but high temperature was needed for calcination of the products. Due to the simple operation, mild reaction conditions, lower cost and no roasting, the oil-water interface method newly reported has been one of the most appropriate processes for preparing lipophilic ZrO2NPs.In this paper, uniform oil-soluble ZrO2NPs with controlled crystal type are synthesized by the oil-water interface method. Firstly, effects of various reaction conditions, including reaction temperature, NaOH concentration and reaction time, have been discussed detailly, on the formation and transformation of ZrO2NPs. Secondly, addition of corresponding seeds can contribute to studying on the transformation of crystals and size change of ZrO2NPs. Furthermore, the formation and transformation mechanisms of zirconia have been proposed based on the combination of oil-water interface method and seeding technology.The main research achoevements of this paper are briefly described as following:(1) Oil-soluble t-ZrO2NPs have been synthesized successfully by the oil-water interface method. Aging at100℃for24h with3.0M NaOH were the optimum reaction conditions to prepare uniform spherical t-ZrO2NPs, with an average size of4.0nm. The particle size was increased with the rise of temperature and the decline of NaOH concentration.(2) Oil-soluble m-ZrO2NPs have been also formed by the oil-water interface method. Aging at180℃for24h with3.0M NaOH were optimized to be the raction conditions to prepare the nearly monodispersed spindle-like m-ZrO2NPs, with an average size of491.8×154.3nm. With the rise of temperature and concentration of NaOH, the particle size was increased.(3) The shape, size of ZrO2NPs and their phase transformation were controlled by changing conditions like NaOH concentration, temperature and aging time by the oil-water interface method. The role of the reaction temperature was dominant among those conditions. The products were m-ZrO2NPs when the temperature was above140℃; And when the temperature was lower than140℃, it is a mixture of t-ZrO2and m-ZrO2NPs with the lower NaOH concentration than1.0M, and it is t-ZrO2NPs with1.0M NaOH or even higher concentrations.(4) It is the first time to combine the seeding technology with the synthesis of ZrO2NPs. The corresponding crystal NPs can be obtained under the lower temperature and NaOH concentration just by adding seeds of different crystal types. The addition of seeds can alter not only the crystal type of the products, but also the size of particles.(5) The formation and transformation mechansims of ZrO2NPs prepared by the oil-water interface method are deduced in this paper. After zirconium oleate and OH-react in the oil/water interface, the most possible products, the precursory complexes of Zr(OH)x(x-4)-(x=5) in water phase and oleate-capped zirconium hydroxide in oil phase, gather and react in the interface to form the ZrO2monomers. Then, under proper conditions, the nucleation of meta-stable t-ZrO2takes place. As for the active nuclei of ZrO2, there is a strong competition among the surface deposition of ZrO2monomers, surface adsorption of oleate ions, and spontaneous aggregation of the nuclei And compared to other factors, the surface deposition can reduce more surface energy. Under a lower temperature and higher NaOH concentration, the surface deposition of ZrO2monomers can reduce the surface energy, then the products tend to be t-ZrO2NPs. Conversely, under a higher temperature or lower NaOH concentration, the deposition does not reduce the surface energy, some of the t-ZrO2NPs aggregate together to transform into m-ZrO2NPs for minimizing their surface energy. Surface adsorption of oleate ions are throughout the entire process. Once the proper surface adsorption makes the certain hydrophobic ZrO2NPs, the particles enter into the oil phase to minimize their surface energy. At this point the particles stop growing or aggregation, and reach a stable state.