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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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EXAFSk3χ(k) function of Co K edge of Co@SiO2nanoparticles in air (a) and in N2gas condition (b).
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Fig8: EXAFSk3χ(k) function of Co K edge of Co@SiO2nanoparticles in air (a) and in N2gas condition (b).

Mentions: Figure 7 gives the diagrammatic sketch of the oxidation procedure in the N2 gas protection condition. The oxidization of Co core is much different from that in air condition. No exotic O atoms come into the Co@SiO2 during the heating process. Thus, the O atoms could only be from the SiO2 shells. At low temperature, there is no or seldom Co-O bond existing in the system and the Co-Co bond is dominant. When the temperature was above 300°C, the diffusion effect of O at the Co core surface becomes obvious, and a Co-O band layer will be formed at the interface between Co cores and SiO2 shells, which is demonstrated by XAFS in k space (cf. Figure 8b). With further increase in temperature, a lot of O atoms in SiO2 shell could diffuse into the Co cores and resulting in the increase of the Co-O layer. In the Figure 4b, a peak around 1.5 appeared corresponding to the Co-O bond. The m-SiO2 shell makes phase transition to β-SiO2 around 600°C; it is well known that the O becomes active during the phase transition process, so the diffusion of O into Co core is much faster, and leading further oxidization of the Co core. According to Figures 4b and 8b, the Co nanoparticles are likely oxidized to CoO/Co3O4 composite because the O and Si are in stoichiometric equal (Si:O = 1:2) in SiO2 shell.Figure 7


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)

EXAFSk3χ(k) function of Co K edge of Co@SiO2nanoparticles in air (a) and in N2gas condition (b).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Fig8: EXAFSk3χ(k) function of Co K edge of Co@SiO2nanoparticles in air (a) and in N2gas condition (b).
Mentions: Figure 7 gives the diagrammatic sketch of the oxidation procedure in the N2 gas protection condition. The oxidization of Co core is much different from that in air condition. No exotic O atoms come into the Co@SiO2 during the heating process. Thus, the O atoms could only be from the SiO2 shells. At low temperature, there is no or seldom Co-O bond existing in the system and the Co-Co bond is dominant. When the temperature was above 300°C, the diffusion effect of O at the Co core surface becomes obvious, and a Co-O band layer will be formed at the interface between Co cores and SiO2 shells, which is demonstrated by XAFS in k space (cf. Figure 8b). With further increase in temperature, a lot of O atoms in SiO2 shell could diffuse into the Co cores and resulting in the increase of the Co-O layer. In the Figure 4b, a peak around 1.5 appeared corresponding to the Co-O bond. The m-SiO2 shell makes phase transition to β-SiO2 around 600°C; it is well known that the O becomes active during the phase transition process, so the diffusion of O into Co core is much faster, and leading further oxidization of the Co core. According to Figures 4b and 8b, the Co nanoparticles are likely oxidized to CoO/Co3O4 composite because the O and Si are in stoichiometric equal (Si:O = 1:2) in SiO2 shell.Figure 7

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