Surface structures of ZrO2 films on Rh(111): From two layers to bulk termination

• Published in: Surface science 2019-01, Vol.679, p.180-187
• Main Authors: Lackner, Peter, Zou, Zhiyu, Mayr, Sabrina, Choi, Joong-Il Jake, Diebold, Ulrike, Schmid, Michael
• Format: Article
• Language: English
• Subjects: Annealing; Crystal defects; Grain boundaries; High temperature; Ions; Low energy electron diffraction; Molten salt electrolytes; Monoclinic and tetragonal zirconia; Phase transformation; Rhodium; Scanning tunneling microscopy; Solid electrolytes; Substrates; Surface structure; Thick films; Thickness; Thin film growth; Thin films; Zirconium dioxide; Zirconium oxide
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/j.susc.2018.09.004

•UHV-compatible sputter deposition of ZrO2 leads to clean and flat ZrO2 films.•The creation of a thin monoclinic ZrO2 film is reported for the first time.•Tetragonal ZrO2 films can be prepared and atomically resolved with STM.•Below 5 ML, the structures are influenced by the Rh(111) substrate.•STM indicates positive charge at grain boundaries, which can impede oxygen transport. We have studied zirconia films on a Rh(111) substrate with thicknesses in the range of 2–10 monolayers (ML) using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Zirconia was deposited using a UHV-compatible sputter source, resulting in layer-by-layer growth and good uniformity of the films. For thicknesses of 2–4 ML, a layer-dependent influence of the substrate on the structure of the thin films is observed. Above this thickness, films show a (2 × 1) or a distorted (2 × 2) surface structure with respect to cubic ZrO2(111); these structures correspond to tetragonal and monoclinic zirconia, respectively. The tetragonal phase occurs for annealing temperatures of up to 730 °C; transformation to the thermodynamically stable monoclinic phase occurs after annealing at 850 °C or above. High-temperature annealing also breaks up the films and exposes the Rh(111) substrate. We argue that the tetragonal films are stabilized by the interface to the substrate and possibly oxygen deficiency, while the monoclinic films are only weakly defective and show band bending at defects and grain boundaries. This observation is in agreement with positive charge being responsible for the grain-boundary blocking effect in zirconia-based solid electrolytes. Our work introduces the tetragonal and monoclinic 5 ML-thick ZrO2 films on Rh(111) as a well-suited model system for surface-science studies on ZrO2, as they do not exhibit the charging problems of thicker films or the bulk material and show better homogeneity and stability than the previously-studied ZrO2/Pt(111) system. [Display omitted]