Atomic layer reversal on CeO2 (100) surface

Abstract

The structure and properties of CeO 2 surfaces have been intensively studied due to their importance in a lot of surface-related applications. Since most of surface techniques probe the structure information inside the outermost surface plane, the subsurface structure information has been elusive in many studies. Using the profile imaging with aberration-corrected transmission electron microscopy, the structure information in both the outermost layer and the sublayers of the CeO 2 (100) surface has been obtained. In addition to the normal structures that have been reported before, where the surface is Ce-or O-terminated, a metastable surface has been discovered. In the new structure, there is an atomic layer reversal between the outermost layer and the sublayer, giving a structure with O as the outermost layer for the stoichiometry of normal Ce-terminated surface. The charge redistribution for the polarity compensation has also been changed relative to the normal surface.Keywords: surface structure, ceria, atomic layer reversal, aberration-corrected TEM, first-principles calculations Materials based on CeO 2 , owing to their outstanding physical and chemical properties, have played important roles in solar cells [1], solid-oxide fuel cells [2,3], UV blockers [4-6] and a variety of catalytic reactions like three-way automotive catalytic converters, the production and purification of hydrogen [7][8][9][10]. Many of these applications are closely related to the specific local structures on CeO 2 surface such as islands, steps, oxygen vacancies and Ce 3+ ions. For instance, the high capacity of CeO 2 in oxygen storage-release is largely contributed by the oxygen vacancies that can be quickly formed and eliminated [11]; the abundant oxygen vacancies in Ru/CeO 2 catalysts serve as the active site for CO 2 activation [12] and also behave as the active sites for many catalytic reactions with the accompanied reduction of Ce 4+ ions [13]; the high activity of Au/ceria catalysts in the water-gas shift reaction has been traced back to the ionic Au species that only strongly associated with cerium-oxygen surface [14,15]. Hence, CeO 2 surfaces have been attracting large amount of investigation. In previous studies on the surface of CeO 2 , single crystals were usually chosen as the main research objects. Scanning tunneling microscope (STM) and atomic force microscope (AFM) were commonly used in structural characterization [16][17][18]. Most of research findings are focused on the (111) surface of ceria single crystals or epitaxial thin films. Namai et al. [16,17] observed the structure and dynamic behavior of CeO 2 (111) surfaces in STM, and visualized the surface oxygen atoms, oxygen point defects, and multiple oxygen defects on oxygenterminated CeO 2 (111) surface through noncontact AFM. Esch et al. [18] unraveled the local structure of oxygen vacancies on the (111) surface and subsurface combining high resolution STM with density functional calculations. They found that the electrons left behind by released oxyge...

Understanding CeO2‐Based Nanostructures through Advanced Electron Microscopy in 2D and 3D

“…Recently, a metastable surface was described by Huang et al on the CeO 2 {100} surfaces ( Figure ) . The atomic relaxation is less than 0.2 Å for the normal Ce‐ and O‐terminated {100} surfaces.…”

“…[19] Recently, a metastable surface was described by Huang et al on the CeO 2 {100} surfaces (Figure 4). [20] The atomic relaxation is less than 0.2 Å for the normal Ce-and O-terminated {100} surfaces. However, a surprisingly huge surface relaxation was revealed for the outermost Ce layer and the Ce sublayer.…”

“…d,e) Simulated images relaxed using first‐principles calculations together with the corresponding models. Reproduced with permission . Copyright 2017, Springer Nature.…”

Synthesis, Structure and Catalytic Properties of Faceted Oxide Crystals

“…For example, one {100} area (blue box in Figure 11a) includes both Ce‐terminated (yellow bar) and O‐terminated (red bar) regions as shown by the magnified image in Figure 11b; while another {100} area (white box in Figure 11a) shows ( ) R45° reconstructed (Figure 11f) and Ce‐terminated {100} regions (Figure 11g–i). Overall, the lack of a single kind of surface termination on cube‐shaped CeO 2 reemphasizes the importance of considering factors such as reconstruction and the existence of multiple surface terminations and surface defect densities when assessing and/or interpreting catalytic properties of CeO 2 materials [170–172] . Confirming the actual surface termination under synthesis or reaction conditions is key to developing a better understanding of the principles underlying their synthesis, stability, and application as heterogeneous catalysts.…”

“…Overall, the lack of a single kind of surface termination on cube-shaped CeO 2 reemphasizes the importance of considering factors such as reconstruction and the existence of multiple surface terminations and surface defect densities when assessing and/or interpreting catalytic properties of CeO 2 materials. [170][171][172] Confirming the actual surface termination under synthesis or reaction conditions is key to developing a better understanding of the principles underlying their synthesis, stability, and application as heterogeneous catalysts. An example of this is the facet-dependent methanol adsorption and dehydrogenation behavior of CeO 2 reported by Wu et al, an aspect that is discussed in greater detail in section 3.3.5.…”