Growth of an Ultrathin Zirconia Film on PtZr Examined by High-Resolution X-ray Photoelectron Spectroscopy, Temperature-Programmed Desorption, Scanning Tunneling Microscopy, and Density Functional Theory.

Ultrathin (∼3 Å) zirconium oxide films were grown on a single-crystalline PtZr(0001) substrate by oxidation in 1 × 10 mbar of O at 673 K, followed by annealing at temperatures up to 1023 K. The ZrO films are intended to serve as model supports for reforming catalysts and fuel cell anodes. The atomic and electronic structure and composition of the ZrO films were determined by synchrotron-based high-resolution X-ray photoelectron spectroscopy (HR-XPS) (including depth profiling), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. Oxidation mainly leads to ultrathin trilayer (O-Zr-O) films on the alloy; only a small area fraction (10-15%) is covered by ZrO clusters (thickness ∼0.5-10 nm). The amount of clusters decreases with increasing annealing temperature. Temperature-programmed desorption (TPD) of CO was utilized to confirm complete coverage of the PtZr substrate by ZrO, that is, formation of a closed oxide overlayer. Experiments and DFT calculations show that the core level shifts of Zr in the trilayer ZrO films are between those of metallic Zr and thick (bulklike) ZrO. Therefore, the assignment of such XPS core level shifts to substoichiometric ZrO is not necessarily correct, because these XPS signals may equally well arise from ultrathin ZrO films or metal/ZrO interfaces. Furthermore, our results indicate that the common approach of calculating core level shifts by DFT including final-state effects should be taken with care for thicker insulating films, clusters, and bulk insulators.