Study of Defect Structure in Ferromagnetic Nanocrystalline CeO2: Effect of Ionic Radius
2016
Lee, William | Chen, Shih-Yun | Tseng, Eric | Gloter, Alexandre | Chen, Chi-Liang
In this study, the relationship between the defect structure and the magnetic behavior of La-doped CeO₂ nanoparticles (NPs) was investigated systematically. The doping level ranged from 0% to 15%. X-ray absorption spectroscopy (XAS) and Raman spectroscopy were utilized to investigate the electronic structure of these NPs. It was found that the content of oxygen vacancies increased upon doping with La. The major oxygen vacancy defect structure was M³⁺–VO–M³⁺ (M = Ce or La) in lightly doped NPs, whereas it changed to La³⁺–VO–La³⁺ 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, including Y-, Sm-, and Cr-doped cerias, for which strong interactions among the surface, trivalent cerium, and dopant have been reported. 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.
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