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

Li H, Choi JI, Mayr-Schmölzer W, Weilach C, Rameshan C, Mittendorfer F, Redinger J, Schmid M, Rupprechter G - J Phys Chem C Nanomater Interfaces (2014)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Institute of Materials Chemistry, Vienna University of Technology , 1060 Vienna, Austria.

ABSTRACT

Ultrathin (∼3 Å) zirconium oxide films were grown on a single-crystalline Pt3Zr(0001) substrate by oxidation in 1 × 10(-7) 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-Zr-O) films on the alloy; only a small area fraction (10-15%) is covered by ZrO2 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 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.

No MeSH data available.


STM images of the oxide annealed at (a) 880K, (b) 923 K, (c) 1023K, and (d) 1073 K. The brightness of terraces represents the height,and the bright patches are the oxide clusters.
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fig10: STM images of the oxide annealed at (a) 880K, (b) 923 K, (c) 1023K, and (d) 1073 K. The brightness of terraces represents the height,and the bright patches are the oxide clusters.

Mentions: Inorder to directly prove the coexistence of the trilayer filmand oxide clusters, as well as to examine the influence of annealingtemperature on oxide morphology, corresponding STM images of oxideannealed at different temperatures were recorded. The STM images inFigure 10 show that the surfaces annealed at880 and 923 K exhibit many clusters with 5–20 nm diameter.Their mottled or scratchy appearance indicates interaction with thetip, probably because they are poorly conducting. It is known thatthe ultrathin trilayer oxide already shows a large band gap,10 and tunneling is only possible due to the smallthickness. Thus, it is not unexpected that large ZrO2 clusters,which should be chemically similar to bulk ZrO2, are insulatingwith a large band gap.


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

Li H, Choi JI, Mayr-Schmölzer W, Weilach C, Rameshan C, Mittendorfer F, Redinger J, Schmid M, Rupprechter G - J Phys Chem C Nanomater Interfaces (2014)

STM images of the oxide annealed at (a) 880K, (b) 923 K, (c) 1023K, and (d) 1073 K. The brightness of terraces represents the height,and the bright patches are the oxide clusters.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4321327&req=5

fig10: STM images of the oxide annealed at (a) 880K, (b) 923 K, (c) 1023K, and (d) 1073 K. The brightness of terraces represents the height,and the bright patches are the oxide clusters.
Mentions: Inorder to directly prove the coexistence of the trilayer filmand oxide clusters, as well as to examine the influence of annealingtemperature on oxide morphology, corresponding STM images of oxideannealed at different temperatures were recorded. The STM images inFigure 10 show that the surfaces annealed at880 and 923 K exhibit many clusters with 5–20 nm diameter.Their mottled or scratchy appearance indicates interaction with thetip, probably because they are poorly conducting. It is known thatthe ultrathin trilayer oxide already shows a large band gap,10 and tunneling is only possible due to the smallthickness. Thus, it is not unexpected that large ZrO2 clusters,which should be chemically similar to bulk ZrO2, are insulatingwith a large band gap.

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Institute of Materials Chemistry, Vienna University of Technology , 1060 Vienna, Austria.

ABSTRACT

Ultrathin (∼3 Å) zirconium oxide films were grown on a single-crystalline Pt3Zr(0001) substrate by oxidation in 1 × 10(-7) 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-Zr-O) films on the alloy; only a small area fraction (10-15%) is covered by ZrO2 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 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.

No MeSH data available.