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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.


Side views of (a) Pt3Zr substrate and (b) structureof the trilayer oxide film. (c) Three-trilayer film used for corelevel calculations, together with layer designations.
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fig1: Side views of (a) Pt3Zr substrate and (b) structureof the trilayer oxide film. (c) Three-trilayer film used for corelevel calculations, together with layer designations.

Mentions: The model cells for the calculations are similar to the ones describedbefore,10 using a (√3 × √3)model cell to mimic the experimentally found (√19 × √19)superstructure. The oxide film was placed on a 9-layer-thick alloysubstrate slab with an ABACABACA stacking. At the alloy–oxideinterface, the Zr atoms in the A-type layer were replaced by Pt tosimulate the Pt termination found in the experiments;10 see Figure 1b,c. Concerning theoxide overlayer, one Zr atom of the oxide film was positioned on topof a substrate Pt atom to which it binds.10 The two other Zr atoms of the oxide in the cell buckle up [markedas Zr(high) in Figure 1b] and do not bond tothe substrate. This choice of a unit cell should allow the simulationof the most extreme cases of different Zr sites; other configurationsalso present in the large experimental unit cell will have their Zratoms somewhere in between.


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)

Side views of (a) Pt3Zr substrate and (b) structureof the trilayer oxide film. (c) Three-trilayer film used for corelevel calculations, together with layer designations.
© Copyright Policy
Related In: Results  -  Collection

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

fig1: Side views of (a) Pt3Zr substrate and (b) structureof the trilayer oxide film. (c) Three-trilayer film used for corelevel calculations, together with layer designations.
Mentions: The model cells for the calculations are similar to the ones describedbefore,10 using a (√3 × √3)model cell to mimic the experimentally found (√19 × √19)superstructure. The oxide film was placed on a 9-layer-thick alloysubstrate slab with an ABACABACA stacking. At the alloy–oxideinterface, the Zr atoms in the A-type layer were replaced by Pt tosimulate the Pt termination found in the experiments;10 see Figure 1b,c. Concerning theoxide overlayer, one Zr atom of the oxide film was positioned on topof a substrate Pt atom to which it binds.10 The two other Zr atoms of the oxide in the cell buckle up [markedas Zr(high) in Figure 1b] and do not bond tothe substrate. This choice of a unit cell should allow the simulationof the most extreme cases of different Zr sites; other configurationsalso present in the large experimental unit cell will have their Zratoms somewhere in between.

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.