The correlationship between ferromagnetism and defect structure in doped CeO2 nano-particles

• Main Authors: William - Lee, 李維烈
• Other Authors: none
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/91099024938425546282

博士 === 國立臺灣科技大學 === 材料科學與工程系 === 105 === In this study, the relationship between the defect structure and magnetism of doped CeO2 nanoparticle (NPs) was prepared by precipitation method, and was systematically investigated by using spectroscopy and microscopy. Including X-ray absorption spectroscopy (XAS) and Raman spectroscopy were utilized to investigate the electronic structure of these doped CeO2 NPs. First, different size of ionic radius dopant, La and Y were doped in CeO2. It was found that the content of oxygen vacancies increased upon increasing dopant. The major oxygen vacancy defect structure was M3+−VO−M3+ (M = Ce, La or Y) in lightly doped NPs, whereas it changed to La3+−VO−La3+ or Y3+-Vo-Y3+ as the doping level reached 7%. Scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM/EELS) analysis showed that, in the La-doped NPs, both the dopant (La3+) and Ce3+ were distributed rather homogeneously within the NPs, which is different from the behavior in other doped ceria materials, whereas Y-doped CeO2, for which strong interactions among the surface, trivalent cerium, and dopant. The distinct distribution of defects was attributed to the larger ion radius of La and the nature of the La-related oxygen vacancies. Moreover, room-temperature ferromagnetism (FM) was observed in these La-doped ceria but with a weaker intensity compared to the magnetism obtained for other doped ceria NPs with similar dopant concentrations. This indicates that high concentrations of defects and dopant at the surface are critical for obtaining larger FM. Next, Pr3+(4f2) has different electron configuration from La3+(4f0), it is demonstrated that by raising the content of Pr, the degree of oxygen deficiency is enhanced monotonically, suggesting that oxygen and/or oxygen-related vacancies are induced. XAS analysis indicates the preference of the Pr-ions for the Pr3+ oxidation state. The concentration of Ce3+ stay constant at 9% then raise to 10% when Pr reaches 9%, with further Pr doping, Ce3+ decreases to 8%. From the results of Raman spectroscopy, the defect structures evolved from Ce4+-VO-Ce3+ to Pr3+-VO-Ce3+ as Pr contents increase. Together all spectroscopic results, the structure and evolution of above oxygen related defect was then unraveled. Furthermore, all the Pr-doped CeO2 NPs were found to be ferromagnetic at room temperature. The relationship between defect structure and ferromagnetism is explained by FCE mechanism. Then Pr-doped CeO2 NPs were annealed in O2 atmosphere. From the results of spectroscopy, the defect structures become Pr3+-VO-Pr3+ in high doping level of O2 annealed Pr-doped CeO2 NPs while Ce4+-VO-Ce3+, Pr3+-VO-Ce3+ in Pr-doped CeO2. It is also observed that the distribution of oxygen vacancies after samples annealed is inhomogeneous from microscopy, although the ratio of Ce3+ slightly decreases after annealing, Ce3+ ions aggregate at the surface of particles. The relationship between defect structure and ferromagnetism can be explained by FCE mechanism. Furthermore, the amount of F+ center can be estimated by the results of XANES Ce L-edge and Raman spectroscopy. It is found the saturated magnetization is strongly related to the amount of F+ center in doped CeO2 NPs.