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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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Phase-uncorrected Fourier transform spectra of Co K-edge EXAFS signals with temperature. (a) Air condition and (b) N2 gas protection condition.
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Fig4: Phase-uncorrected Fourier transform spectra of Co K-edge EXAFS signals with temperature. (a) Air condition and (b) N2 gas protection condition.

Mentions: The post-edge background was removed by using a derivative method [28,29]. For the Co@SiO2 core-shell nanoparticles in air condition, the Fourier transforms were performed in the k range of 2.67 to 14.49 , and the first Co-Co and Co-O shells were isolated by Fourier filter with R range of about 1.10 to 2.70 . Figure 4 shows the Fourier-transformed k3-weighted EXAFS spectra of Co@SiO2 samples in air and N2 conditions. The amplitudes and phase shifts of Co-Co and Co-O atom pairs were extracted from theory spectra of CoO which was calculated by FEFF 8.0 [26]. For fitting the EXAFS spectra, we consider the peak around 1.5 to Co-O bonds and the peak around 2.4 to Co-Co bonds respectively. Therefore, the Co-O and Co-Co scattering paths were used to fit the spectra. The amplitude and phase shift of Co-O atom pair were calculated with FEFF 8.0 code, and the amplitude and phase shift of Co-Co were attracted from Co-foil EXAFS measurement. From the Figure 4a, two peaks were observed during the heating process, and Co-O and Co-Co bonds could fit the spectra very well which were shown in Figure 5. It means that in air condition, the Co core nanoparticles were partially oxidized even at room temperature and then were gradually oxidized to Co3O4 with the temperature rising to 800°C. However, only one peak was indicated in the N2 gas condition when the temperature was below 400°C (Figure 4b). With further increase in temperature, the second peak appeared. Consequently, in N2 gas protection condition, the Co core nanoparticles could be oxidized to CoxOy when the temperature was above 400°C, and below that temperature, the Co core nanoparticles are dominated by Co0 state. Unfortunately, the EXAFS spectra of Co@SiO2 nanoparticles could not be fitted well by Co-O and Co-Co scattering paths. Nevertheless, they showed the same trend as in the air condition.Figure 4


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)

Phase-uncorrected Fourier transform spectra of Co K-edge EXAFS signals with temperature. (a) Air condition and (b) N2 gas protection condition.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Fig4: Phase-uncorrected Fourier transform spectra of Co K-edge EXAFS signals with temperature. (a) Air condition and (b) N2 gas protection condition.
Mentions: The post-edge background was removed by using a derivative method [28,29]. For the Co@SiO2 core-shell nanoparticles in air condition, the Fourier transforms were performed in the k range of 2.67 to 14.49 , and the first Co-Co and Co-O shells were isolated by Fourier filter with R range of about 1.10 to 2.70 . Figure 4 shows the Fourier-transformed k3-weighted EXAFS spectra of Co@SiO2 samples in air and N2 conditions. The amplitudes and phase shifts of Co-Co and Co-O atom pairs were extracted from theory spectra of CoO which was calculated by FEFF 8.0 [26]. For fitting the EXAFS spectra, we consider the peak around 1.5 to Co-O bonds and the peak around 2.4 to Co-Co bonds respectively. Therefore, the Co-O and Co-Co scattering paths were used to fit the spectra. The amplitude and phase shift of Co-O atom pair were calculated with FEFF 8.0 code, and the amplitude and phase shift of Co-Co were attracted from Co-foil EXAFS measurement. From the Figure 4a, two peaks were observed during the heating process, and Co-O and Co-Co bonds could fit the spectra very well which were shown in Figure 5. It means that in air condition, the Co core nanoparticles were partially oxidized even at room temperature and then were gradually oxidized to Co3O4 with the temperature rising to 800°C. However, only one peak was indicated in the N2 gas condition when the temperature was below 400°C (Figure 4b). With further increase in temperature, the second peak appeared. Consequently, in N2 gas protection condition, the Co core nanoparticles could be oxidized to CoxOy when the temperature was above 400°C, and below that temperature, the Co core nanoparticles are dominated by Co0 state. Unfortunately, the EXAFS spectra of Co@SiO2 nanoparticles could not be fitted well by Co-O and Co-Co scattering paths. Nevertheless, they showed the same trend as in the air condition.Figure 4

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