Atomic-scale observation of electron-beam-induced reduction and reversible phase transformation in ceria nanoparticles

초록

Ceria is a promising material for various applications owing to its oxygen transport properties originating from oxygen vacancies. Although electron beam-induced reduction of ceria has been observed, the mechanism of the chemistry involving the electron beam remains unclear. In this study, we report the direct observation of reversible redox reaction in ceria nanoparticles using scanning transmission electron microscopy (STEM). The transition to the bixbyite C-Ce2O3 phase under irradiation of electron beam with the current above the threshold (0.1nA), characterized by alternating bright and dark contrasts in atomic-scale high-angle annular dark-field STEM images, confirms the electron beam-induced reduction of ceria. When the electron beam is turned off or irradiated with the current below 0.1nA, spontaneous re-oxidation occurs owing to the presence of an oxygen source. Electron energy loss spectroscopy results of the Ce-M and O-K edges corroborate the redox reaction of ceria. Our findings reveal that electron beam irradiation induces ionization in CeO2-x, generating oxygen vacancies that heal when the electron beam irradiation is stopped. This observation experimentally demonstrates that ceria tends to maintain its cubic fluorite structure even under oxygen-deficient conditions, which is supported by density functional theory calculations.

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