X-ray Photoelectron Spectroscopy Study of the Passivation of NiAl(100) by Water Vapor

Abstract

The oxidation of NiAl(100) surfaces by water vapor is studied using X-ray photoelectron spectroscopy (XPS) to elucidate the effect of temperature and vapor pressure on the surface passivation mechanism of the NiAl alloy. The water-vapor oxidation at ambient temperature (25 °C) results in self-limiting Al(OH)3/Al2O3 bilayer film growth to a less extent of the limiting thickness regimes, in which the growth of the inner Al2O3 layer occurs via dehydration of the outer Al(OH)3 layer. The growth of the passivating overlayer at the ambient temperature depletes Al and forms a Ni-rich layer at the oxide/alloy interface that impedes supply of Al atoms to the outer surface for Al(OH)3 formation via the hydration reaction, whereby resulting in a more Al-deficient structure of the outer Al(OH)3 layer upon increasing the vapor pressure. In contrast, the water-vapor oxidation at 300 °C results in Al2O3 single-layer film growth to a larger limiting thickness without involving the transient hydroxide phase of Al(OH)3. It is shown that increasing the oxidation temperatures results in the formation of a more compact Al2O3 film owning to the enhanced bulk diffusion rate that maintains an adequate supply of Al atoms to the oxide/alloy interface to sustain the oxide film growth to the full extent of the limiting thickness.

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

“…the formation of aluminum oxide is energetically strongly favored over the NiO formation. The selective oxidation of Al in the NiAl alloy results in the enrichment of Ni in the oxide/substrate interface region, which has been observed for the oxidation of the NiAl(100) under UHV conditions 12,13. In view of the much higher gas pressure in the present work, significant Ni enrichment can occur in the subsurface region because of the growth of the much thicker aluminum oxide film compared to the oxidation under the UHV conditions.…”

“…However, the OH peak disappears at 300 °C and above, suggesting the aluminum hydroxide is not stable at the temperature above 300 °C and decomposes into the aluminum oxide, consistent with the previous studies. 12,34 27,37,38 The Ni 2p spectra was not collected to further confirm this conclusion due to the limitation of the beamline, the highest photon energy we can reach was around 700 eV while the BE range for Ni 2p is from 840 eV to 880 eV. However, the O 1s peak at the lower binding energy and the existence of Ni 3p at 300 ⁰C and above provide clear evidence for NiO formation.…”

“…The thin aluminum oxide film grown by selective oxidation of Al in the NiAl alloy is widely used as one of the most important supports for dispersed metal catalysts. [1][2][3][4][5] Due to these important industrial applications and also the fundamental importance in understanding the degradation mechanism of intermetallic alloys, the initial-stage oxidation of single-crystal NiAl(100) has been extensively studied to address the atomistic mechanism of the formation of ultrathin aluminum oxide films using a wide range of surface science tools including low-energy electron diffraction (LEED) [6][7][8][9][10][11] , X-ray photoelectron spectroscopy (XPS) [11][12][13] , electron energy loss spectroscopy 6,7 , scanning tunneling microscopy (STM) [8][9][10][11][14][15][16] , and lowenergy electron microscopy (LEEM) 10,16 . These surface science studies of the oxidation of NiAl are performed mostly under ultrahigh vacuum (UHV) conditions with carefully dosing small amounts of oxygen gas.…”

“…NiAl is a technologically important intermetallic material possessing a high melting point, low density, good oxidation/corrosion resistance, and metal-like electrical and thermal conductivities. Thin aluminum oxide films grown by selective oxidation of Al in the NiAl alloy are widely used as one of the most important supports for dispersed metal catalysts. − Because of this important industrial application and also the fundamental importance in understanding the degradation mechanism of intermetallic alloys, the initial-stage oxidation of single-crystal NiAl(100) has been extensively studied to address the atomistic mechanism of the formation of ultrathin aluminum oxide films using a wide range of surface science tools including low-energy electron diffraction (LEED), − X-ray photoelectron spectroscopy (XPS), − electron energy loss spectroscopy, , scanning tunneling microscopy (STM), − ,− and low-energy electron microscopy (LEEM). , These surface science studies of the oxidation of NiAl are performed mostly under ultrahigh vacuum (UHV) conditions by carefully dosing small amounts of oxygen gas. Although the surface science approach has been very successful in many cases as shown in the aforementioned studies, the pressure gap between the surface science experiments and the technologically relevant oxidation processes that often take place within high-temperature and high-pressure regimes prevents the direct extrapolation of the knowledge obtained from the idealized condition.…”

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

“…8, middle), which arises from the difference of their surface work functions and is further enhanced The large valence band offset between the oxide film and the NiAl substrate also results in very different binding energies of the Al core electrons. Indeed, XPS experiments systematically reveal Al 2p core level shifts (CLS) of 2.5-3.0 eV, 61,[61][62][63] with a much stronger binding of electrons in the oxide. Our results show that the initial state contribution represents about half of this effect and is further enhanced by the final state effects which systematically increase the calculated CLS by an additional 1.5 eV.…”