Clustering of Oxygen Vacancies at ${\mathrm{CeO}}_2(111)$: Critical Role of Hydroxyls

By performing density functional theory calculations corrected by an on site Coulomb interaction, we find that the defects at the ${\mathrm{CeO}}_2(111)$ surface observed by the scanning tunneling microscopy (STM) measurements of Esch et al. [Science 309, 752 (2005)] are not mere oxygen vacancies or fluorine impurities as suggested by Kullgren et al. [Phys. Rev. Lett. 112, 156102 (2014)], but actually the hydroxyl-vacancy combined species. Specifically, we show that hydroxyls play a critical role in the formation and propagation of oxygen vacancy clusters (VCs). In the presence of neighboring hydroxyls, the thermodynamically unstable VCs can be significantly stabilized, and the behaviors of oxygen vacancies become largely consistent with the STM observations. In addition to the clarification of the long term controversy on the surface defect structures of ${\mathrm{CeO}}_2(111)$, the “hydroxyl-vacancy model” proposed in this work emphasizes the coexistence of hydroxyls and oxygen vacancies, especially VCs, which is important for understanding the catalytic and other physicochemical properties of reducible metal oxides.

Figure 1

Calculated energetics for the structural evolution of the clustering of fluorine impurities at ${\mathrm{CeO}}_2(111)$. All of the structures are depicted in schematic plan views from the side and top. ${\mathrm{O}}_S$ and ${\mathrm{O}}_{\mathrm{SS}}$ represent the surface and subsurface oxygen, respectively. Clean denotes the clean ${\mathrm{CeO}}_2(111)$ surface.

Figure 2

Illustrative diagrams and calculated averaged oxygen vacancy formation energies of reduced ${\mathrm{CeO}}_2(111)$ surfaces. The simulated filled-state STM image of TSVT 1 is also shown. ${\mathrm{O}}_{\mathrm{SS}}(\text{out})$ represents the subsurface oxygen, which has a nearly identical $z$ coordinate with the surface oxygen ions due to a strong outward relaxation. “Underneath: ${\mathrm{Ce}}^{3+}$“stands for a ${\mathrm{Ce}}^{3+}$ in the second trilayer.

Figure 3

Left panel: Calculated energetics for the structural evolution of the growth of surface VCs at a hydroxylated ${\mathrm{CeO}}_2(111)$. The oxygen vacancy diffusion ($\mathrm{SV}\to \mathrm{SSV}$) of the ${\mathrm{H}}_{\text{sub}}+\mathrm{SVD}$ is also shown. Vacancy spaces are indicated by dashed blue polygons. Hydrogen adsorption energies for the surrounding ${\mathrm{O}}_{\mathrm{SS}}$ ions in the ${\mathrm{H}}_{\text{sub}}+\mathrm{SVD}$ local structure are also labeled. Right panel: Schematic side view for the relaxation of the subsurface hydroxyl after the formation of one or two nearest neighbor SVs. (see Supplemental Material [30] for the detailed structural information, Figs. S2 and S3).

Figure 4

Up panel: Simulated filled-state STM images (bias $-3\mathrm{V}$) of the three most stable SVTs that involve two outward relaxed subsurface hydroxyls. Bottom panel: Calculated local density of states (LDOS) of two typical oxygen ions, i.e., a normal ${\mathrm{O}}_S$ and an ${\mathrm{O}}_{\mathrm{SS}}(\text{out})$ of the subsurface hydroxyl, in the $2{\mathrm{H}}_{\text{sub}}+\mathrm{LSVT}$ structure.