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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.


(Left) Zr 3d and (right)O 1s XP spectra of clean alloy and ofthe oxide annealed at 1023 K (spectra taken at 300 K, kinetic energy≈140 eV).
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fig4: (Left) Zr 3d and (right)O 1s XP spectra of clean alloy and ofthe oxide annealed at 1023 K (spectra taken at 300 K, kinetic energy≈140 eV).

Mentions: Thegrowth of the oxide on Pt3Zr was also monitored bysynchrotron-based HR-XPS, as shown in Figure 4 and Table 1. The spectra were recorded atnormal emission with photon energies of 320 and 670 eV for the Zr3d and O 1s ranges, respectively, yielding in both cases photoelectronswith a kinetic energy of ∼140 eV. With that kinetic energy,only the photoelectrons from the topmost surface layers escape. Byuse of NIST Standard Reference Database 71,26 the inelastic mean free path (IMFP) for ZrO2 correspondingto 140 eV is 0.54 nm; that is, mainly the first two layers are probed.The Zr 3d spectrum of the clean alloy exhibits a pronounced doubletat 179.6 and 182.0 eV due to Zr 3d5/2 and Zr 3d3/2, respectively. In addition, another doublet with much lower intensitywas observed at 180.7 and 183.1 eV. Almost no signal could be detectedin the O 1s region; thus the doublet with higher intensity is assignedto metallic Zr in Pt3Zr, whereas the small features wereassigned to Zr bound to residual oxygen. For comparison with pureZr, we will use the value of 178.6 eV reported for a Zr(0001) singlecrystal;27 thus the alloyed Zr is shiftedto higher binding energy (BE) by +1.0 eV.


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)

(Left) Zr 3d and (right)O 1s XP spectra of clean alloy and ofthe oxide annealed at 1023 K (spectra taken at 300 K, kinetic energy≈140 eV).
© Copyright Policy
Related In: Results  -  Collection

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

fig4: (Left) Zr 3d and (right)O 1s XP spectra of clean alloy and ofthe oxide annealed at 1023 K (spectra taken at 300 K, kinetic energy≈140 eV).
Mentions: Thegrowth of the oxide on Pt3Zr was also monitored bysynchrotron-based HR-XPS, as shown in Figure 4 and Table 1. The spectra were recorded atnormal emission with photon energies of 320 and 670 eV for the Zr3d and O 1s ranges, respectively, yielding in both cases photoelectronswith a kinetic energy of ∼140 eV. With that kinetic energy,only the photoelectrons from the topmost surface layers escape. Byuse of NIST Standard Reference Database 71,26 the inelastic mean free path (IMFP) for ZrO2 correspondingto 140 eV is 0.54 nm; that is, mainly the first two layers are probed.The Zr 3d spectrum of the clean alloy exhibits a pronounced doubletat 179.6 and 182.0 eV due to Zr 3d5/2 and Zr 3d3/2, respectively. In addition, another doublet with much lower intensitywas observed at 180.7 and 183.1 eV. Almost no signal could be detectedin the O 1s region; thus the doublet with higher intensity is assignedto metallic Zr in Pt3Zr, whereas the small features wereassigned to Zr bound to residual oxygen. For comparison with pureZr, we will use the value of 178.6 eV reported for a Zr(0001) singlecrystal;27 thus the alloyed Zr is shiftedto higher binding energy (BE) by +1.0 eV.

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.