Abstract
Surface X-ray diffraction has been employed to elucidate the surface structure of α-Cr2O3(0001) as a function of water partial pressure at room temperature. In ultra high vacuum, following exposure to ∼2000 Langmuir of H2O, the surface is found to be terminated by a partially occupied double layer of chromium atoms. No evidence of adsorbed OH/H2O is found, which is likely due to either adsorption at minority sites, or X-ray induced desorption. At a water partial pressure of ∼30 mbar, a single OH/H2O species is found to be bound atop each surface Cr atom. This adsorption geometry does not agree with that predicted by ab initio calculations, which may be a result of some differences between the experimental conditions and those modeled.
Highlights
The presence of a passive surface film is key to the exceptional corrosion resistance of stainless steel alloys.[1,2] much effort has targeted the characterization and enhancement of this protective layer, which is often composed, at least partially, of chromia.[1,2] Such work includes fundamental studies of single crystal surfaces of α-Cr2O3 to gain atomic scale insight into pertinent properties, e.g., refs 3−10
As a starting point for these calculations, a clean surface terminated by a single layer of 3-fold coordinated chromium atoms was assumed, as depicted in Figure 1A; this surface termination is labeled Cr− O3−Cr− on the basis of its first three atomic layers
This comparison shows that there is no appreciable variation in the width of the reflection, indicating that terrace size is not significantly influenced by the presence of H2O. These data suggest that the presence of ∼30 mbar of H2O vapor leads to a modification of the surface structure of αCr2O3(0001). This supposition will be confirmed below, through analysis of the crystal truncation rods (CTRs) data sets acquired from Cr2O3− H2OUHV and Cr2O3−H2O30mbar
Summary
The presence of a passive surface film is key to the exceptional corrosion resistance of stainless steel alloys.[1,2] much effort has targeted the characterization and enhancement of this protective layer, which is often composed, at least partially, of chromia.[1,2] Such work includes fundamental studies of single crystal surfaces of α-Cr2O3 to gain atomic scale insight into pertinent properties, e.g., refs 3−10. Most of these measurements have been conducted in ultra high vacuum (UHV), limiting their relevance with regard to mechanistic understanding of corrosion performance in engineering environments. Targeting this omission, the current study is concerned with determining the surface structure of α-. It was concluded that two other terminations become energetically favorable in the presence of H2O. Increasing the H2O partial pressure resulted in the attachment of an intact H2O molecule to each dihyroxylated Cr to form a new surface termination, i.e. (H2O(OH)2)−Cr−O3−
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