Surface structure of coherently strained ceria ultrathin films

Cerium oxide, or ceria, is an important material for solid oxide fuel cells and water splitting devices. Although the ceria surface is active in catalytic and electrochemical reactions, how its catalytic properties are affected by the surface structure under operating conditions is far from understood. We investigate the structure of the coherently strained $\mathrm{Ce}{\mathrm{O}}_2$ ultrathin films on yttria-stabilized zirconia (001) single crystals by specular synchrotron x-ray diffraction (XRD) under oxidizing conditions as a first step to study the surface structure in situ. An excellent agreement between the experiment data and the model is achieved by using a “stacks and islands” model that has a two-component roughness. One component is due to the tiny clusters of nanometer scale in lateral dimensions on each terrace, while the other component is due to slightly different $\mathrm{Ce}{\mathrm{O}}_2$ thickness that span over hundreds of nanometers on neighboring terraces. We attribute the nonuniform thickness to step depairing during the thin film deposition that is supported by the surface morphology results on the microscopic level. Importantly, our model also shows that the polarity of the ceria surface is removed by a half monolayer surface coverage of oxygen. The successful resolution of the ceria surface structure using in situ specular synchrotron XRD paves the way to study the structural evolution of ceria as a fuel cell electrode under catalytically relevant temperatures and gas pressures.

Figure 1

Top view of the atomic arrangement of the first ML (oxygen anions, grey) and second ML (cerium cations, blue) of a $\mathrm{Ce}{\mathrm{O}}_2$ (001) surface. The black square marks the conventional fluorite unit cell, whereas the yellow square marks the primitive unit cell on the surface. (a) Bulk terminated surface with 100% occupancy of oxygen on the topmost layer. (b) $(1\times 1)$ relaxation with 50% occupancy of oxygen. This is one way of removing surface dipole consistent with a previous report [33]. The periodicity and size of the surface lattice remains unchanged, and there is no reconstruction.

Figure 2

Schematics of the two structural models in the CTR fitting. The grey and blue stripes near the surface represent the ${\mathrm{O}}^{2-}$ and $\mathrm{C}{\mathrm{e}}^{4+}$ MLs, respectively. (a) Stack only model: each stack consists of both the bulk YSZ, the interfacial YSZ and the ceria overlayer. The stack surface is oxygen terminated, with a fixed 50% oxygen occupancy. The thicknesses of neighboring stacks differ by one bilayer consisting of oxygen anions and cerium cations. (b) Stacks and islands model: an additional bilayer (of one Ce ML and one O ML) with low occupancies is introduced on top of each stack, representing islands on the thin film surface manifested as incomplete coverage. In both models, the fractions of each stack are variables. In the stacks and islands model, the occupancies of the island layer are also fitting parameters. In both models all layers of $\mathrm{Ce}{\mathrm{O}}_2$, whether they represent layers or islands, and the YSZ substrate are coherent. The grey region labeled “surface YSZ” is a region where some intermixing of Ce and Zr(Y) is introduced into the CTR modeling.

Figure 3

Experimental data of the specular rod (00L) (blue circles) and the best of fits based on stack only model (solid blue curve) and the stacks and islands model (dashed red curve). The structure factors are plotted in log scale against the momentum transfer in the out-of-plane direction. The agreement is visually improved when the island layer are introduced on top of each terrace and the FoM is reduced.

Figure 4

Hypothesis of step depairing behind the observed nonuniform film coverage. (a) Step pairing occurs on the YSZ surface due to high-temperature annealing before deposition. The step height can be either 0.5 or 1.0 UC. (b) Due to the Schwoebel effect, the step of the film advances by incorporating adatoms primarily from the lower terrace (the terrace below the step on its left side). (c) As the first step advances on terrace 1, it leaves behind a second step between the film and the terrace to its right. The second step eventually advances as well by incorporating adatoms landed on the grown film. The original 1.0 UC step between the substrate terraces is broken into two 0.5 UC steps in the film. As a result, part of the $\mathrm{Ce}{\mathrm{O}}_2$ film (to the right of the second step) grown on terrace 1 is one bilayer thicker than those grown on neighboring terrace 2 and 3.

Figure 5

AFM images (a) and (b) and line profiles (c) and (d) of the annealed YSZ (left column) and as-deposited $\mathrm{Ce}{\mathrm{O}}_2$ surfaces (right column). The line profiles are averaged over the region included in the white window in the AFM images. The smallest, unpaired step in the fluorite structure is 0.5 UC (one bilayer of cerium cations and oxygen anions). Black arrows in the line profiles point at paired steps that are 1.0 UC tall where the rest of the steps are only 0.5 unit cells. Similar images were taken at more than 30 locations for both the bare substrate and the deposited film. Statistics of the steps can be found in Table 1.