Synthesis and detection the oxidization of Co cores of Co@SiO2 core-shell nanoparticles by in situ XRD and EXAFS

In this paper, the Co@SiO2 core-shell nanoparticles were prepared by the sol-gel method. The oxidization of Co core nanoparticles was studied by the synchrotron radiation-based techniques including in situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) up to 800A degrees C in air and N-2 protection conditions, respectively. It was found that the oxidization of Co cores is undergoing three steps regardless of being in air or in N-2 protection condition. In the first step ranging from room temperature to 200A degrees C, the Co cores were dominated by Co-0 state as well as small amount of Co2+ ions. When temperature was above 300A degrees C, the interface between Co cores and SiO2 shells was gradually oxidized into Co2+, and the CoO layer was observed. As the temperature increasing to 800A degrees C, the Co cores were oxidized to Co3O4 or Co3O4/CoO. Nevertheless, the oxidization kinetics of Co cores is different for the Co@SiO2 in air and N-2 gas conditions. Generally, the O-2 in the air could get through the SiO2 shells easily onto the Co core surface and induce the oxidization of the Co cores due to the mesoporous nature of the SiO2 shells. However, in N-2 gas condition, the O atoms can only be from the SiO2 shells, so the diffusion effect of O atoms in the interface between Co core and SiO2 shell plays a key role.