X-ray photoelectron spectroscopy study of the passivation of NiAl(100) by water vapor

Atomistic mechanisms of water vapor-induced surface passivation

The microscopic mechanisms underpinning the spontaneous surface passivation of metals from ubiquitous water have remained largely elusive. Here, using in situ environmental electron microscopy to atomically monitor the reaction dynamics between aluminum surfaces and water vapor, we provide direct experimental evidence that the surface passivation results in a bilayer oxide film consisting of a crystalline-like Al(OH)3 top layer and an inner layer of amorphous Al2O3. The Al(OH)3 layer maintains a constant thickness of ~5.0 Å, while the inner Al2O3 layer grows at the Al2O3/Al interface to a limiting thickness. On the basis of experimental data and atomistic modeling, we show the tunability of the dissociation pathways of H2O molecules with the Al, Al2O3, and Al(OH)3 surface terminations. The fundamental insights may have practical significance for the design of materials and reactions for two seemingly disparate but fundamentally related disciplines of surface passivation and catalytic H2 production from water.

Structural characteristics of Al2O3 ultra-thin films supported on the NiAl(100) substrate from DFTB-aided global optimization

Surfaces of aluminum alloys are often coated with ultra-thin alumina films which form by self-limited selective oxidation. Although the presence of such films is of paramount importance in various applications, their structural and stability characteristics remain far from being known. In particular, on the NiAl(100) substrate, the observed structure has been tentatively assigned to a distorted θ-alumina polymorph, but the film stoichiometry, the nature of its surface and interface terminations, as well as the mechanisms that stabilize the θ phase remain unknown. Using a combined tight-binding/DFT genetic algorithm approach, we explicitly demonstrate that ultra-thin θ(100)-type films correspond to the structural ground state of alumina supported on the (2 × 1)-NiAl(100) substrate. Thus, experimentally observed θ-alumina films correspond to thermodynamic equilibrium, rather than being the result of kinetic effects involved in the alloy oxidation and film growth. They are favoured over other Al₂O₃ phases of dehydrated boehmite, pseudo-CaIrO₃, γ, or bixbyite structures, which have recently been identified among the most stable free-standing ultra-thin alumina polymorphs. Moreover, our results prove that nonstoichiometry can be easily accommodated by the supported θ(100) film structure via an excess or deficiency of oxygen atoms at the very interface with the metal substrate. Dedicated DFT analysis reveals that the oxide-metal interaction at stoichiometric interfaces depends surprisingly little on the composition of the NiAl surface. Conversely, at oxygen-rich/poor interfaces, the number of additional/missing Al-O bonds is directly responsible for their relative stability. Finally the comparison between the experimental and theoretical electronic characteristics (STM and XPS) of supported θ(100)-type films provides clues on the detailed structure of the experimentally observed films.

Comparative Study of the Oxidation of NiAl(100) by Molecular Oxygen and Water Vapor Using Ambient-Pressure X-ray Photoelectron Spectroscopy

The oxidation behavior of NiAl(100) by molecular oxygen and water vapor under a near-ambient pressure of 0.2 Torr is monitored using ambient-pressure X-ray photoelectron spectroscopy. O₂ exposure leads to the selective oxidation of Al at temperatures ranging from 40 to 500 °C. By contrast, H₂O exposure results in the selective oxidation of Al at 40 and 200 °C, and increasing the oxidation temperature above 300 °C leads to simultaneous formation of both Al and Ni oxides. These results demonstrate that the O₂ oxidation forms a nearly stoichiometric Al₂O₃ structure that provides improved protection to the metallic substrate by barring the outward diffusion of metals. By contrast, the H₂O oxidation results in the formation of a defective oxide layer that allows outward diffusion of Ni at elevated temperatures for simultaneous NiO formation.

H₂O Dissociation-Induced Aluminum Oxide Growth on Oxidized Al(111) Surfaces

The interaction of water vapor with amorphous aluminum oxide films on Al(111) is studied using X-ray photoelectron spectroscopy to elucidate the passivation mechanism of the oxidized Al(111) surfaces. Exposure of the aluminum oxide film to water vapor results in self-limiting Al2O3/Al(OH)3 bilayer film growth via counter-diffusion of both ions, Al outward and OH inward, where a thinner starting aluminum oxide film is more reactive toward H2O dissociation-induced oxide growth because of the thickness-dependent ionic transport in the aluminum oxide film. The aluminum oxide film exhibits reactivity toward H2O dissociation in both low-vapor pressure [p(H2O) = 1 × 10(-6) Torr] and intermediate-vapor pressure [p(H2O) = 5 Torr] regimes. Compared to the oxide film growth by exposure to a p(H2O) of 1 × 10(-6) Torr, the exposure to a p(H2O) of 5 Torr results in the formation of a more open structure of the inner Al(OH)3 layer and a more compact outer Al2O3 layer, demonstrating the vapor-pressure-dependent atomic structure in the passivating layer.