Synthesis and detection the oxidization of Co cores of Co@SiO(2) core-shell nanoparticles by in situ XRD and EXAFS

In this paper, the Co@SiO(2) 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 800°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 200°C, the Co cores were dominated by Co(0) state as well as small amount of Co(2+) ions. When temperature was above 300°C, the interface between Co cores and SiO(2) shells was gradually oxidized into Co(2+), and the CoO layer was observed. As the temperature increasing to 800°C, the Co cores were oxidized to Co(3)O(4) or Co(3)O(4)/CoO. Nevertheless, the oxidization kinetics of Co cores is different for the Co@SiO(2) in air and N(2) gas conditions. Generally, the O(2) in the air could get through the SiO(2) shells easily onto the Co core surface and induce the oxidization of the Co cores due to the mesoporous nature of the SiO(2) shells. However, in N(2) gas condition, the O atoms can only be from the SiO(2) shells, so the diffusion effect of O atoms in the interface between Co core and SiO(2) shell plays a key role.