Studying Of The Oxygen Vacancies In The Ceo₂-Based Solid Solutions By Raman Spectroscopy

When Ce4+ions were replaced by other metal ions, there are oxygen vacancies sites. And the presence of oxygen vacancies with CeO2-based solid solutions affects the structural, electronic and chemical properties of these materials, such as increasing the oxygen storage/release capacity (OSC), transport rate of bulk oxygen, the surface oxygen adsorb/release capacity, resisting sintering ability of the catalysis, etc. Therefore, CeO2-based mixed oxides have been widely used in catalytic reaction and solid oxide fuel cell and so on. It is well known that Raman spectroscopy is an efficient and rapid analysis technique for obtaining information about the defect sites of CeO2-based solid solutions, especially the oxygen vacancies sites. For the CeO2-based solid solutions, three feature Raman peaks at460,560and600cm-1could be observed, which are ascribed to F2g symmetrical stretching vibration mode of CeO2in fluorite structure, oxygen vacancy and MO8-type complex, respectively. The presence of oxygen vacancies was related to the ionic valence states of the dopant, while the MO8-type complex was associated with the ionic radii of the dopant. The observed value of oxygen vacancy concentration was related to the absorbance of the samples and the surface enrichment of oxygen vacancy. The in-situ Raman spectroscopic investigations show that the atmospheres and temperatures had great influence on the absorbance of the samples, which alters the detection depth of the laser to the samples and consequently the observed value of oxygen vacancy concentration.In this work, the CeO2-based mixed oxides were prepared by a sol-gel method. Raman spectroscopy, combined with X-ray diffraction(XRD), transmission electron microscopy(TEM), optic microscopy, ultraviolet visible diffuse reflectance spectrum(UV-Vis DRS) and so on, were used to investigate the evolution and the change of defect sites, the effect of sample ordering level on Raman signal, as well as the change of oxygen vacancies concentration and microstructure of CeO2by doping different metal ions (Ln=Gd、Y、Lu、La、Zr、Sm、Pr and Tb), the detailed results are as follows:1. Raman spectroscopic study of defect sites in Ce1-xGdxO2-δ solid solutionsA series of Ce1-xGdxO2-δ solid solutions were prepared using a sol-gel method, Raman spectroscopy with325and514nm excitation wavelengths, X-ray diffraction (XRD), transmission electron microscopy images (TEM) and UV-Vis diffuse reflectance spectra (UV-Vis DRS) were used to analyze the distributions of defect sites in Ce1-xGdxO2-δ solid solutions and the effect of Gd content on their concentration. There were two defect sites of oxygen vacancies and GdO8-type complex corresponding to the Raman peaks at560cm-1and600cm-1, respectively. The UV Raman spectroscopy could provide the surface information, while the Visible Raman spectroscopy could get the whole information of the sample. The concentration of oxygen vacancies and GdO8-type complex were higher on the surface than in the bulk due to the surface enrichment. However, comparing with the concentration of GdO8-type complex, the increase in the concentration of oxygen vacancies in the bulk was more obvious than that of in the surface.2. Raman spectroscopic study of defect sites in Ce0.8M0.1O2-d solid solutionsA series of Ce0.8M0.2O2-δ (M=Gd, Zr, La, Sm, Y, Lu, and Pr) samples were characterized by Raman spectroscopy to investigate the evolution of defect sites (oxygen vacancies and MO8-type complex) and their distributions in the samples. It was found that the evolution of oxygen vacancies was due to the different ionic valence state of dopant from that of Ce4+, while the evolution of MO8-type complex was due to the different ionic radius of dopant from that of Ce4+. The distributions of defect sites were investigated using325and514nm excitation laser lines, indicating that the defect sites were surface enriched. Moreover, increasing ordering level of the sample led to a decline in the concentration of the MO8-type complex in the sample but the constant concentration of oxygen vacancies, implying that the metastable MO8-type complex species were more disordered compared to the oxygen vacancies.3. Influencing factor on the observed concentration of oxygen vacancies The observed concentration of oxygen vacancies were influenced by dopant content, the optical absorbance and the ordering level of the samples. In other words, with the increase of dopant content, there many Ce4+ions were replaced by other mental ions in CeO2lattice, leading to more oxygen vacancies sites, so the concentration of oxygen vacancies increased; with the increase of optical absorbance of the sample, the information from the Raman scattering could be obtained from the bulk to surface, and the oxygen vacancies sites were enriched on the surface of the samples, so the concentration of oxygen vacancies increased; with the increase of ordering level, compared with the MO8-type complex, the oxygen vacancies were more ordered, so the constant concentration of oxygen vacancies but the decline in the concentration of the MO8-type complex in the sample as the calcination temperatures increased.