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

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 ZrOx 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.

Surface Spectroscopy on UHV-Grown and Technological Ni–ZrO2 Reforming Catalysts: From UHV to Operando Conditions

“…The amount of deposited Ni (nominal thickness 3 Å) was controlled by a calibrated quartz microbalance. For further details concerning ZrO 2 film preparation and cluster growth we refer to references [65, 66, 82, 83]. …”

“…Using a laboratory X-ray source the inelastic mean free path (IMFP) of the photoelectrons is about 2 nm (sampling depth ~6 nm), thus the spectrum is still dominated by the alloy substrate. For more surface sensitive (synchrotron-based) XP spectra we refer to our study of the related ultrathin zirconia film on Pt 3 Zr [65]. The major difference of the two films is the higher surface defect density of ZrO 2 /Pd 3 Zr and, while for Pd 3 Zr the interlayer between the alloy and oxide is roughly stoichiometric, for Pt 3 Zr there is an interlayer of pure Pt.…”

“…In order to prepare an ultrathin model zirconia thin film with Ni nanoparticles on top, either a Pd 3 Zr(0001) or Pt 3 Zr(0001) single crystal was cleaned by sputtering (8 × 10 −6 mbar of Ar, 2 kV) and annealing in UHV. After cleaning, the Pd 3 Zr or Pt 3 Zr single crystal was exposed to 1 × 10 −7 mbar of O 2 at 673 K for 30 min, followed by annealing in UHV at 1073 K for 20 min to grow an uniform ultrathin ZrO 2 layer (for details see [65, 66]). On top of this ultrathin layer, an amount of Ni equivalent to a homogeneous layer of nominally 3 Å was deposited at room temperature, utilizing an Omicron electron-beam triple evaporator (EFM 3T) for physical vapor deposition.…”

Epitaxial thin-film growth of Ruddlesden–Popper-type Ba3Zr2O7 from a BaZrO3 target by pulsed laser deposition

“…Figure 4c shows the core level XPS spectrum of Zr 3d. Detailed peak de-convolution shows that the Zr peak is located at 179.34 eV (Zr 3d 5/2 ), shifted by ?0.74 eV relative to pure Zr (178.6 eV) [24].…”

Oxygen self-diffusion mechanisms in monoclinic ZrO2 revealed and quantified by density functional theory, random walk analysis, and kinetic Monte Carlo calculations

“…In particular, such knowledge could guide design by, e.g., aliovalent doping and controlling operating environmental conditions. Experimental [12][13][14] and computational [15][16][17][18][19][20] studies have been carried out to atomistically resolve the structure, valence states, and defect chemistry in ZrO 2 . Zirconium-oxygen system phase stability has been examined by first-principles studies, and a range of suboxide structures with oxygen dissolved into the metal phase have been identified [20].…”