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

Zhang K, Zhao Z, Wu Z, Zhou Y - Nanoscale Res Lett (2015)

Bottom Line: As the temperature increasing to 800°C, the Co cores were oxidized to Co3O4 or Co3O4/CoO.Generally, the O2 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 N2 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Life Science, Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, 201204 China.

ABSTRACT
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 800°C in air and N2 protection conditions, respectively. It was found that the oxidization of Co cores is undergoing three steps regardless of being in air or in N2 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 SiO2 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 Co3O4 or Co3O4/CoO. Nevertheless, the oxidization kinetics of Co cores is different for the Co@SiO2 in air and N2 gas conditions. Generally, the O2 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 N2 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.

No MeSH data available.


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Schematic illustration of oxidization of Co cores of Co@SiO2nanoparticles in air condition.
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Fig6: Schematic illustration of oxidization of Co cores of Co@SiO2nanoparticles in air condition.

Mentions: Comparing the measurements in Figure 4a,b, we can make a conclusion that the Co@SiO2 core-shell nanoparticles can be oxidized to Co3O4, in spite of the protection of SiO2 shell. In other words, the SiO2 shell cannot protect the Co nanoparticles from being oxidized to Co3O4, but they could exhibit different behaviors in the air and N2 gas conditions. For the nanoparticles in air condition, the O2 in air can get onto the Co cores easily because the SiO2 shell is in mesoporous state. So even at room temperature, the Co core nanoparticles could be oxidized to CoO which were demonstrated by EXAFS and XANES measurements. In the first step, only the surface atoms of Co cores were oxidized by O2. As the temperature increases up to 300°C, the organic ligands leave off the Co core surface, and the Co surface were oxidized to CoO. With further increase in temperature, the CoO layer increased, which was reflected from the k space of XAFS spectra (Figure 5), and Figure 5a to h shows the fitting results of Co K edge k3-weighted EXAFS spectra of Co@SiO2 nanoparticles in air condition. Finally, the Co core nanoparticles were oxidized thoroughly to Co3O4 when temperature reaches 800°C. Figure 6 gives the diagrammatic sketch of this procedure.Figure 6


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

Zhang K, Zhao Z, Wu Z, Zhou Y - Nanoscale Res Lett (2015)

Schematic illustration of oxidization of Co cores of Co@SiO2nanoparticles in air condition.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4384979&req=5

Fig6: Schematic illustration of oxidization of Co cores of Co@SiO2nanoparticles in air condition.
Mentions: Comparing the measurements in Figure 4a,b, we can make a conclusion that the Co@SiO2 core-shell nanoparticles can be oxidized to Co3O4, in spite of the protection of SiO2 shell. In other words, the SiO2 shell cannot protect the Co nanoparticles from being oxidized to Co3O4, but they could exhibit different behaviors in the air and N2 gas conditions. For the nanoparticles in air condition, the O2 in air can get onto the Co cores easily because the SiO2 shell is in mesoporous state. So even at room temperature, the Co core nanoparticles could be oxidized to CoO which were demonstrated by EXAFS and XANES measurements. In the first step, only the surface atoms of Co cores were oxidized by O2. As the temperature increases up to 300°C, the organic ligands leave off the Co core surface, and the Co surface were oxidized to CoO. With further increase in temperature, the CoO layer increased, which was reflected from the k space of XAFS spectra (Figure 5), and Figure 5a to h shows the fitting results of Co K edge k3-weighted EXAFS spectra of Co@SiO2 nanoparticles in air condition. Finally, the Co core nanoparticles were oxidized thoroughly to Co3O4 when temperature reaches 800°C. Figure 6 gives the diagrammatic sketch of this procedure.Figure 6

Bottom Line: As the temperature increasing to 800°C, the Co cores were oxidized to Co3O4 or Co3O4/CoO.Generally, the O2 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 N2 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Life Science, Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, 201204 China.

ABSTRACT
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 800°C in air and N2 protection conditions, respectively. It was found that the oxidization of Co cores is undergoing three steps regardless of being in air or in N2 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 SiO2 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 Co3O4 or Co3O4/CoO. Nevertheless, the oxidization kinetics of Co cores is different for the Co@SiO2 in air and N2 gas conditions. Generally, the O2 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 N2 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.

No MeSH data available.


Related in: MedlinePlus