In situ electron microscopy of oxygen ion transport kinetics

[ Instrument Network Instrument R & D ] Ion regulation is an important means to generate new physical states and properties. The micro-mechanism of structural phase transition accompanied by ion transport is the key to determine the material properties and device function. The in-situ real-time observation of the dynamic behavior of ion transport at the atomic scale, revealing the atomic mechanism of new properties of materials, is of great significance for material design and device applications.

Bai Xuedong, a researcher at the State Key Laboratory of Surface Physics at the Institute of Physics, Chinese Academy of Sciences / Beijing National Research Center for Condensed Matter Physics, has used in-situ electron microscope technology developed by himself to observe and characterize oxygen ions In the process of transmission kinetics, a series of results have been obtained in the study of phase transitions in ion-regulated structures.

For example, they aimed at cerium dioxide (CeO2), a multifunctional oxide material system, by in-situ external field control methods, and found that the applied electric field can greatly reduce the barrier of oxygen atoms in the cerium dioxide film from the lattice (JACS, 132, 4197 (2010), and systematically characterizes the dynamics of the phase transition of the oxygen ion-regulated structure (APL, 107,211902 (2015); ChemCatChem 8, 3326 (2016); Sci China Chem 62, 1704 (2019)).

Recently, based on the development of in-situ technology and aberration-corrected electron microscopy analysis technology, Wang Lifen, an associate researcher of the research group, and Zhang Yuyang, an associate professor at the University of Chinese Academy of Sciences, have made new progress in the study of atomic mechanisms of oxygen diffusion in CeO2. The research team used aberration-corrected transmission electron microscopy real-time atomic imaging technology and molecular dynamics simulation methods to reveal the atomic mechanism of anisotropic diffusion of oxygen atoms under CeO activation conditions.

The work was published in Physical Review Letters under the title Visualizing Anisotropic Oxygen Diffusion in Ceria under Activated Conditions. Zhu Liang, a graduate student in the surface room SF1 group, and Jin Xin, a graduate student in the nano room group N04 group, are co-first authors.

In this study, Aberration-corrected TEM was used to characterize CeO2 nanoparticles, and direct atom-resolved imaging of Ce and O atoms was achieved. At the same time, a high-energy electron beam was transmitted to the oxygen atoms in cerium oxide. This results in the precipitation of oxygen atoms and the phase transition of CeO2 with cerium oxide and Ce2O3 with Fe-Mn phase (Fig. 1). The electron beam was used for dynamic observation and characterization. At the same time, as a means of inducing oxygen ion migration, the oxygen atoms in the reaction and its real-time diffusion path were captured (Figure 2). The preferential path of oxygen atom diffusion in cerium oxide was observed in situ in real time. Through experimental observation and molecular dynamics simulation, it was found that the oxygen atom in the cerium oxide with fluorite structure takes the <001> direction as a preferential transmission channel.

Combined with the first-principles calculations, it was revealed that the physical reason is that the redistribution of electrons accompanying the diffusion of oxygen atoms changes the local Coulomb force, resulting in lattice disturbances. The diffusion path of the oxygen atoms selects the direction of the lowest disturbance energy (Figure 3) ). The change in the coordination valence state accompanying this oxygen atom diffusion process is also supported by the in-situ electron energy loss spectrum analysis results (Figure 4). The anisotropic transport mechanism of oxygen atoms in cerium dioxide with fluorite structure revealed in this study has a guiding effect on its anisotropy-related properties and functional regulation.

The above work was supported by the Chinese Academy of Sciences, the Ministry of Science and Technology, the National Natural Science Foundation of China, the Beijing Natural Science Foundation of China, and the Youth Promotion Committee of the Chinese Academy of Sciences.