The growth of ultra-thin zirconia films on Pd₃Zr(0 0 0 1)

Abstract

Despite its importance in many areas of industry, such as catalysis, fuel cell technology and microelectronics, the surface structure and physical properties of ZrO2 are not well understood. Following the successful growth of ultra-thin zirconia on Pt3Zr(0 0 0 1) (Antlanger et al 2012 Phys. Rev. B 86 035451), we report on recent progress into ZrO2 thin films, which were prepared by oxidation of a Pd3Zr(0 0 0 1) crystal. Results from scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS) as well as density-functional theory (DFT) are presented. Many sputter-annealing cycles are required for preparation of the clean Pd3Zr alloy surface, because oxygen easily dissolves in the bulk. By oxidation and post-annealing, a homogeneous ultra-thin ZrO2 film was obtained. This is an O-Zr-O trilayer based on cubic ZrO2(1 1 1). Using STM images corrected for distortion and creep of the piezo scanner the in-plane lattice parameter was determined as (351.2 ± 0.4) pm, slightly contracted with respect to the cubic ZrO2 bulk phase. The oxide forms an overlayer that is either incommensurate or has a very large superstructure cell (a = 8.3 nm); nevertheless its rotational orientation is always the same. In contrast to ultra-thin zirconia on Pt3Zr(0 0 0 1), where the uppermost substrate layer is pure (but reconstructed) Pt, STM and XPS suggest a stoichiometric Pd3Zr below the oxide. The oxide film binds to the substrate mainly via bonds between oxygen and the Zr atoms in the substrate. The ultra-thin oxide shows large buckling in STM, confirmed by DFT calculations, where the buckling of the Zr layer can exceed 100 pm. Compared to the ZrO2 film on Pt3Zr(0 0 0 1), the oxide on Pd3Zr(0 0 0 1) has the advantage that the substrate below does not reconstruct, leading to a homogeneous oxide film.

Substoichiometric ultrathin zirconia films cause strong metal–support interaction

“…A closer look at the ultrathin film reveals a hexagonal lattice with interatomic distances of 0.35 nm, as is typical for ultrathin zirconia films, 21,26 see the Fourier transform (FFT) in Figure 1e. When comparing two domains rotated by a multiple of ≈ 60 • , their lattices agree within 1%, demonstrating that the deviations from an exactly hexagonal structure are small.…”

“…Atomically resolved STM images in this work are corrected for piezo drift as described in Ref. 21 which gives accurate distance measurements. We used inverted model systems of supported catalysts on ZrO 2 .…”

“…The ultrathin zirconia film shows a Zr-Zr distance of 0.350 ± 0.003 nm, as also observed for Pt 3 Zr(0001) 26 and Pd 3 Zr(0001). 21 The moiré pattern shown in the inset of Figure 2a 26 The creation of the ultrathin film can be reversed by annealing at 640 • C in 5 × 10 −7 mbar O 2 , see Figure 2b.…”

Ordered hydroxyls on Ca3Ru2O7(001)

“…The bias voltage was applied to the sample; positive or negative bias voltages result in STM images of the unoccupied or occupied states, respectively. All STM images shown were corrected for distortions as described elsewhere 34 . High-quality calcium ruthenate single crystals were grown by the floating zone technique using a mirror-focused furnace 35 .…”

Atomic-Scale Structure of the Hematite α-Fe2O3(11̅02) “R-Cut” Surface

“…All the nc-AFM data presented here were taken in the constant-height mode, where the tuning-fork resonance frequency shift is recorded as the primary signal. All nc-AFM and STM images presented in this work were corrected for distortion and creep of the piezo scanner, as described in ref (36). …”