Growth of an Ultrathin Zirconia Film on Pt3Zr Examined by High-Resolution X-ray Photoelectron Spectroscopy, Temperature-Programmed Desorption, Scanning Tunneling Microscopy, and Density Fun

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• 作者：Hao Li ; Joong-Il Jake Choi ; Wernfried Mayr-Schm枚lzer ; Christian Weilach ; Christoph Rameshan ; Florian Mittendorfer ; Josef Redinger ; Michael Schmid ; G眉nther Rupprechter
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：February 5, 2015
• 年：2015
• 卷：119
• 期：5
• 页码：2462-2470
• 全文大小：457K
• ISSN：1932-7455

文摘

Ultrathin (鈭? 脜) zirconium oxide films were grown on a single-crystalline Pt₃Zr(0001) substrate by oxidation in 1 脳 10^鈥? mbar of O₂ at 673 K, followed by annealing at temperatures up to 1023 K. The ZrO₂ films are intended to serve as model supports for reforming catalysts and fuel cell anodes. The atomic and electronic structure and composition of the ZrO₂ 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鈥揨r鈥揙) films on the alloy; only a small area fraction (10鈥?5%) is covered by ZrO₂ clusters (thickness 鈭?.5鈥?0 nm). The amount of clusters decreases with increasing annealing temperature. Temperature-programmed desorption (TPD) of CO was utilized to confirm complete coverage of the Pt₃Zr substrate by ZrO₂, that is, formation of a closed oxide overlayer. Experiments and DFT calculations show that the core level shifts of Zr in the trilayer ZrO₂ films are between those of metallic Zr and thick (bulklike) ZrO₂. Therefore, the assignment of such XPS core level shifts to substoichiometric ZrO_x is not necessarily correct, because these XPS signals may equally well arise from ultrathin ZrO₂ films or metal/ZrO₂ 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.