Structural Dynamics of Al₂O₃/NiAl(110) During Film Growth in NO₂

Abstract

While continuum descriptions of oxide film growth are well established, the local structural dynamics during oxide growth are largely unexplored. Here, we investigate this using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) for the example of alumina film growth on NiAl(110) following NO2 exposure. To maintain a well-defined system, we have adopted a cyclic growth approach of NO2 adsorption and annealing. NO2 adsorption at 693 K results in the formation of a vacancy island pattern in the NiAl(110) substrate, which is filled with AlOx by diffusion of O through the alumina film. The patches of AlOx coalesce to form smooth terraces upon annealing to 1200 K. By repeated cycling, we have grown films of up to 0.9 nm thick. While peak shifts in the XPS spectra indicate that the film maintains its insulating character upon thickening, our STM data show that there is a finite density of states within the band gap. The thickening of the alumina film is accompanied by the formation of trenches in the surface, which we interpret to be the result of film stress relief.

Functionalization and passivation of ultrathin alumina films of defined sub-nanometer thickness with self-assembled monolayers

“…Diimine rhenium(I) tricarbonyl complexes can be incorporated into efficient homogenous photocatalytic systems for CO 2 reduction [40,41] and H 2 production [42][43][44], and their attachment as SAM is a step towards their integration into a heterogeneous system. The growth of highly-crystalline alumina films on NiAl(1 1 0) of up to 1.5 nm thickness was previously demonstrated [30]: either cycles of NO 2 adsorption and subsequent annealing [67], or by the high-temperature oxidation of 2L-alumina/NiAl(1 1 0) with oxygen [30] can be used to increase the thickness in a controlled manner. This can enable high-precision tuning of the electron transfer between 6.…”

Revisiting the Formation of the (√67 × √67)R12.2° Bilayer Oxide on Ni3Al(111) by In Situ STM—Surprises Regarding Oxygen Volume Diffusion

“…Observing bulk oxidation already at T = 765 K questions the applicability of the oxide preparation protocol by dosing oxygen at T = 1000 K for the formation of the (√67 × √67)­R12.2° bilayer aluminum oxide phase that entirely covers the Ni 3 Al­(111) crystal. It also explains the question raised by Mom et al when investigating a similar system why oxidation of NiAl(110) by extended NO 2 exposure did not produce an oxide of a substantial thickness but still led to a surface on which STM could be performed . What has been overlooked was the fact that oxide growth below the surface is a source of metal atoms that diffuse toward the surface and thus constantly provide electrically conductive areas between growing oxide islands.…”

“…Differences in the thermal expansion coefficients of oxide and the support layer become important and decide whether or not these brittle layers form cracks upon temperature variation and whether such layers remain protective and serve as a diffusion barrier for oxygen preventing bulk oxidation of the reactive metal alloy. , In surface science, thin aluminum surface oxides attracted great attention mainly on NiAl and Ni 3 Al as these thin surface oxide layers mimic an alumina surface with preparative control at the atomic level. Here, especially the surface oxide formation on low-index surfaces such as the (100), (110), and (111) surfaces was studied in detail [e.g., NiAl(100), ,− NiAl(110), ,,− NiAl(111), , Ni 3 Al­(100), , Ni 3 Al­(110), , and Ni 3 Al­(111) ,,− ]. Thin alumina films are widely used as model surfaces that serve as oxide supports for catalytically active metal or metal oxide particles as a model of heterogeneous catalysts used in the industry. − In contrast to heterogeneous catalyst powders, such model surfaces do not suffer from charging and can be investigated using electron-based analytical techniques.…”

“…Bulk diffusion comes into play when Ni x Al y alloys are oxidized more deeply. This explains why even thick oxide layers remain electronically conductive so that STM can still be performed–a question raised by Mom et al in a recent study . In addition to the observed onset of volume diffusion and its effect on the surface composition, an annealing experiment in ultrahigh vacuum (UHV) of a Ni 3 Al­(111) surface covered by a pre-prepared (7 × 7) oxide layer was performed.…”