Raman Spectroscopic Study Of Oxygen Vacancies In CeO₂-based Mixed Oxides

Rare-earth-doped CeO2-based mixed oxides have been widely used in three way catalysts, solid oxide fule cell and so on. In these applications, the presence of oxygen vacancies with CeO2 affects the structural, electronic and chemical properties of materials, thus playing a vital role in determining the catalytic and conducting properties of these materials. It is well known that Raman spectroscopy is an excellent, nondestructive and rapid analysis technique for obtaining information about the oxygen vacancies of CeO2. Besides, the rule of oxygen vacancy concentration under real reaction status (temperature and atmosphere) can be detected by in situ Raman spectroscopy, combined with different excitation laser lines.In this work, the CeO2-based mixed oxides were prepared by a sol-gel method. Raman spectroscopy, combined with X-ray diffraction (XRD), confocal microscopy, ultraviolet visible diffuse reflectance spectrum (UV-Vis DRS), H2-temperature programmed reduction (H2-TPR) and O2-temperature programmed desorption (O2-TPD) techniques were used to analyze the shift of the F2g vibration mode of CeO2, oxygen vacancy concentration and the effect of sample optical absorbance on Raman information, as well as the microstructure and oxygen vacancy concentration of lanthanides-doped CeO2 (Ln=Sm, Gd, Pr and Tb). The detailed results are as follows:1. Raman spectroscopic study of oxygen vacancies in Ce1-xTbxO2-δmixed oxidesThe dopant of Tb introduced a new Raman band of oxygen vacancies, at about 587 cm-1 in the spectra. Increasing Tb content in the sample resulted in the increasing oxygen vacancy concentration as well as the blue shift of F2g vibration mode of CeO2 because of dominant effect of the shrinking of crystal cell. 2. Effect of optical absorbance on the Raman spectra of CeO2-based mixed oxidesThe change in optical absorbance of sample significantly affected the observed Raman information (peak intensity and the variation rule of oxygen vacancy concentration) under in situ conditions. With increasing temperature, the optical absorption of sample decreased because of the release of oxygen and consequent change in microstructure. The decline in optical absorption enabled the Raman laser increase its sampling depth, therefore, the deeper layer phonons of sample also involved in Raman scattering even if the same excitation laser line was used. This exerted more pronounced effect on 514 nm laser than on 785 nm laser, because both Tb and oxygen vacancies enriched on the surface of material and 514 nm light provided surface information, while 785 nm light provided almost whole information of sample.3. UV and visible Raman studies of oxygen vacancies in Ce0.9Ln0.1O2-δmixed oxidesThe observed oxygen vacancy value (A580/A465) was related to the absorption of Ce0.9Ln0.1O2-δand the wavelength of laser line. Because the oxygen vacancies enriched on the surface of mixed oxides, the whole oxygen vacancy concentration of sample was far lower than that of its outer layer. The visible Raman spectroscopy reflected the whole information of Ce0.9Sm0.1O2-δand Ce0.9Gd0.1O2-δmixed oxides because of its weak absorption in visible laser. However, because the Ce0.9Pr0.1O2-δand Ce0.9Tb0.1O2-δsample absorbed strongly in visible laser, the laser only carried out outer layer information of sample. Besides, the dopant contents decreased rapidly from surface to bulk.