Synthesis and characterization of monodisperse iron and zirconium oxide nanoparticles via thermal decomposition method

Abstract

Understanding of geochemical behaviors of actinides is essential to the safety assessment of a deep geological disposal of radioactive wastes. In nuclear waste repositories which are under reducing condition, actinides such as U, Np, and Pu usually exist as tetravalent oxidation state and can be transported in colloidal forms. However, a comprehensive understanding for colloidal transport mechanism is difficult since various types of An(IV) colloids may exist. Therefore, an experiment using well-defined nanoparticles helps to mechanistically understand and elucidate the transport in the simpler system. In this study, monodisperse oxide nanoparticles of iron and zirconium were synthesized using thermal decomposition method. Zirconium was utilized as inactive homologue of An(IV). Because the size of colloids that impacts various geochemical reactions is very important, various sized nanoparticles were synthesized. In addition, synthesized nanoparticles were investigated in terms of the size distribution and crystal structure using TEM, XRD, and Raman spectroscopy. Thermal decomposition method with metal oleate was employed for synthesizing the nanoparticles. Various sized iron and zirconium oxide nanoparticles were made, and this experiment helped to understand the mechanism of synthesis. Major elements which influence the monodisperse nanoparticles in thermal decomposition method are the thermal decomposition temperature, the reaction time, and the surfactant. Thermal decomposition temperatures of iron and zirconium are $320^\circ C$ and $350^\circ C$, respectively, which were analyzed by TGA and DTGA. Through the TEM images taken at various reaction times, the optimal reaction times were estimated to be 30 minute for iron oxide nanoparticles and 24 hours for zirconium oxide nanoparticles. Surfactants can hinder the agglomeration of nanoparticles and thus control the size of nanoparticles. In case of zirconium oxide nanoparticles, the concentration ratio of oleic acid and oleylamine changed the morphologies (nanowire and sphere). Conclusively, the diameters of synthesized iron oxide nanoparticles were 8.8 $\pm$ 0.5, 12.4 $\pm$ 0.7, and 14.2 $\pm$ 0.8 nm. The crystal structure of iron oxide was analyzed to be maghemite with cubic structure by using XRD and Raman spectroscopy. On the other hand, the zirconium oxide nanoparticles were synthesized with size distribution of 3.7 $\pm$ 0.3, 4.1 $\pm$ 0.4, and 4.5 $\pm$ 0.4 nm. The major phases of zirconium oxide nanoparticles were revealed to be monoclinic with very minor fraction of tetragonal crystal structure. For both iron and zirconium oxide nanoparticles, the size of nanoparticles decreased, as the concentration of surfactant increased through rapid nucleation. In addition, the standard deviation of zirconium oxide nanoparticles in size was larger than that of iron oxide nanoparticles due to the broader range of thermal decomposition.