Reactivities of ultrathin alumina films exposed to intermediate pressures of H2O: Substrate-mediated mechanism for growth and loss of surface order

Kelber, Jeffry A.; Magtoto, N.P.; Vamala, Chiranjeevi; Jain, Manjari; Jennison, D. R.; Schultz, Peter A.

doi:10.1016/j.susc.2007.06.036

Cited by 14 publications

(27 citation statements)

References 38 publications

(71 reference statements)

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“…Hence, the fraction of Ni atoms in the growing oxide film is significantly larger even for a 5% alloy. The existence of differences in the structure of the oxide film formed on Pure Al and Ni-Al alloy substrates was also observed in the ab initio studies by Jennison and co-workers, 78,79 and Kresse et al 80 for ultrathin aluminum oxide scale formed on Ni-Al alloy surfaces. Firstprinciples density-functional calculations by Jennison et al 79 on the structure of ultrathin films ͑with self-limiting thickness less than 20 Å͒ on Ni-Al surfaces predicted a different Al 2 O 3 structure, which has no bulk counterpart.…”

Section: Discussionmentioning

confidence: 57%

Molecular dynamics simulation study of nanoscale passive oxide growth on Ni-Al alloy surfaces at low temperatures

Sankaranarayanan

Ramanathan

2008

Phys. Rev. B

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Oxidation kinetics of Ni-Al ͑100͒ alloy surface is investigated at low temperatures ͑300-600 K͒ and at different gas pressures using molecular dynamics ͑MD͒ simulations with dynamic charge transfer between atoms. Monte Carlo simulations employing the bond order simulation model are used to generate the surface segregated minimum energy initial alloy configurations for use in the MD simulations. In the simulated temperature-pressure-composition regime for Ni-Al alloys, we find that the oxide growth curves follow a logarithmic law beyond an initial transient regime. The oxidation rates for Ni-Al alloys were found to decrease with increasing Ni composition. Structure and dynamical correlations in the metal/oxide/gas environments are used to gain insights into the evolution and morphology of the growing oxide film. Oxidation of Ni-Al alloys is characterized by the absence of Ni-O bond formation. Oxide films formed on the various simulated metal surfaces are amorphous in nature and have a limiting thickness ranging from ϳ1.7 nm for pure Al to 1.1 nm for 15% Ni-Al surfaces. Oxide scale analysis indicates significant charge transfer as well as variation in the morphology and structure of the oxide film formed on pure Al and 5% Ni-Al alloy. For oxide scales thicker than 1 nm, the oxide structure in case of pure Al exhibits a mixed tetrahedral ͑AlO 4 ϳ 37%͒ and octahedral ͑AlO 6 ϳ 19%͒ environment, whereas the oxide scale on Ni-Al alloy surface is almost entirely composed of tetrahedral environment ͑AlO 4 ϳ 60%͒ with very little AlO 6 ͑Ͻ1%͒. The oxide growth kinetic curves are fitted to Arrhenius-type plots to get an estimate of the activation energy barriers for metal oxidation. The activation energy barrier for oxidation on pure Al was found to be 0.3 eV lower than that on 5% Ni-Al surface. Atomistic observations as well as calculated dynamical correlation functions indicate a layer by layer growth on pure Al, whereas a transition from an initial island growth mode ͑Ͻ75 ps͒ to a layer by layer mode ͑Ͼ100 ps͒ occurs in case of 5% Ni-Al alloy. The oxide growth on both pure Al and Ni-Al alloy surfaces occurs by inward anion and outward cation diffusions. The cation diffusion in both the cases is similar, whereas the anion diffusion in case of 5% Ni-Al is 25% lower than pure Al, thereby resulting in reduced self-limiting thickness of oxide scale on the alloy surface. The simulation findings agree well with previously reported experimental observations of oxidation on Ni-Al alloy surface.

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Section: Discussionmentioning

confidence: 57%

Molecular dynamics simulation study of nanoscale passive oxide growth on Ni-Al alloy surfaces at low temperatures

Sankaranarayanan

Ramanathan

2008

Phys. Rev. B

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“…16,36 This also contrasts with the thickness increase reported for the 0.7 nm thick transitional films on NiAl(110) and Ni 3 Al(110) reported in the 10 -6 Torr water vapor pressure range. 15 The atomic surface structure of the oxide film was not determined in the present study. However, it seems quite unlikely that the thermal oxide film grown has a well-ordered defect-free termination.…”

Section: Resultsmentioning

confidence: 73%

Modifications and Growth Mechanisms of Ultrathin Aluminum Oxide Films on NiAl in Water

et al. 2010

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X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) have been combined to study the modifications and growth of alumina thin films on NiAl(001) induced by exposure to water vapor at low pressure (10 -6 mbar) and by immersion in ultrapure liquid water at room temperature. The film thickness and stoichiometry, as well as the alloy composition changes, were analyzed by angle-resolved XPS measurements. Ultrathin (∼0.7 nm) hydroxylated native oxide films were compared to thicker (∼4 nm) anhydrous and oxygendeficient thermal oxide films grown at 900°C under low air pressure (10 -4 mbar). The results show that immersion in liquid water causes the formation of one to two equivalent additional monolayers of aluminum oxide at the interface between the native oxide film and the alloy, suggesting anion transport along the oxide/ alloy interface after entry at intergranular oxide sites. The hydroxylated oxide surface remains unchanged. In contrast, immersion of the thicker thermal oxide films in liquid water causes the formation of ∼3 equivalent additional aluminum oxide monolayers at the oxide/water interface, indicating cation transport through the film. AFM suggests a preferential growth at the boundaries between the oxide grains due to faster ion transport. The ∼4 nm thick thermal oxide film is inert when exposed to water at low vapor pressure (10 -6 mbar) confirming the pressure gap for the water-induced modifications of transitional alumina films. Exposure to ambient air partially annihilates the oxygen deficiency observed after formation as shown by the variation of the O-Al stoichiometry and core level shift effects.

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“…Which of these scenarios prevails is system-dependent . In the oxidation studies on Al 2 O 3 /NiAl(110), it is sometimes assumed that Al 3+ diffuses to the surface. , However, this assumption seems incompatible with the fine etching pattern observed here. If the Al 3+ ions formed during oxidation of the NiAl substrate diffuse through the film, the preexisting alumina film would have to be broken up in order to fill the vacancies that the Al 3+ ions leave behind.…”

Section: Resultsmentioning

confidence: 86%

“…Apart from its use in catalysis, the Al 2 O 3 /NiAl(110) system has also been discussed as a model system for metal–insulator–metal devices 35 , 42 and nanoelectronics. 32 In such cases, the most essential properties are the film’s insulating characteristics. The shifts in our XPS peaks clearly establish that the insulating character of the alumina film is maintained upon thickening.…”

Section: Resultsmentioning

confidence: 99%

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

et al. 2017

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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.

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Reactivities of ultrathin alumina films exposed to intermediate pressures of H2O: Substrate-mediated mechanism for growth and loss of surface order

Cited by 14 publications

References 38 publications

Molecular dynamics simulation study of nanoscale passive oxide growth on Ni-Al alloy surfaces at low temperatures

Molecular dynamics simulation study of nanoscale passive oxide growth on Ni-Al alloy surfaces at low temperatures

Modifications and Growth Mechanisms of Ultrathin Aluminum Oxide Films on NiAl in Water

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

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Reactivities of ultrathin alumina films exposed to intermediate pressures of H2O: Substrate-mediated mechanism for growth and loss of surface order

Cited by 14 publications

References 38 publications

Molecular dynamics simulation study of nanoscale passive oxide growth on Ni-Al alloy surfaces at low temperatures

Molecular dynamics simulation study of nanoscale passive oxide growth on Ni-Al alloy surfaces at low temperatures

Modifications and Growth Mechanisms of Ultrathin Aluminum Oxide Films on NiAl in Water

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

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