Oxidation History
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| Figure 1 |
Incorporation of oxygen into transition metals en route to metallic oxidation, especially nickel, has been heavily studied. Extensive efforts have been undertaken toward developing a more complete description of the initial mechanistic stages in the growth of oxide layers. Interest in understanding the formation and properties of oxides ranges from metallic corrosion mitigation to employment of thin oxide films as catalysts or catalytic supports. For metals that oxidize, incorporation can proceed via one of two processes: when oxygen exposure sufficiently exceeds the limit to form a chemisorbed surface overlayer, or when the system is heated to the point where it is energetically favorable to absorb oxygen by incorporation into the underlying metal lattice. However, unlike analogous sub-surface hydrogen systems, not much is understood regarding the role of oxidation history or the presence of sub-surface oxygen in the selvedge region of a crystal in further oxygen dissolution. Oxygen-induced reconstruction behavior of Ni(977) has been thoroughly investigated in our group using both real- and reciprocal- space techniques. In the presence of a small amount of adsorbed oxygen, less than 2% of a monolayer, the surface doubles over a thermal range that is limited on the low end by thermally promoted mobility of atoms including step meandering and at the high end by dissolution of the step-edge adsorbed oxygen. At temperatures where oxygen dissolves, the surface re-singles indicating the need for oxygen to stabilize merged steps at these relatively elevated temperatures. We are highlighting the variance at elevated temperature where double steps become unstable with respect to reentrant singling. We believe that the temperature at which double steps become unstable due to oxygen dissolution can be understood in light of the previous dissolution history of oxygen into the crystal. We present a montage of data from experiments on two different Ni(977) crystals that involved the interaction with oxygen at elevated temperatures.
| Figure 2 |
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HAS was used to track oxygen-induced structural evolution of the Ni(977)-heavy surface from single steps to double steps, back to single steps, and eventually to oxide in the presence of oxygen at 500 K (Figure 1). Scattering in this experiment was performed across the step edges along the azimuth (downstairs direction), revealing information regarding terrace widths. The distance between the two diffraction peaks is inversely proportional to the average terrace width on the surface. At the inception of the experiment, the two peaks correspond to diffraction from single steps. The center peak that appears at approximately 0.12 L O2 exposure signals diffraction from double steps. During the course of these experiments, doubling was found to proceed at temperatures as high as 550 K. This temperature is remarkably higher than the temperatures associated with oxygen dissolution for other O/Ni systems. Resingling of this crystal was never observed. In order to study the reconstruction dynamics with HAS in the limit of small amounts of adsorbed oxygen the second Ni(977) sample was acquired. The chemical potential of the selvedge region can be modified by the presence of oxygen, which in turn influences the thermodynamics of partitioning between surface adsorbed and sub-surface absorbed oxygen. Variations in oxygen segregation can account for changes in the stabilization regime of the double step phase. The nature of the dissolution history for Ni(977)-heavy made it possible to actually have double step phase stability at higher temperatures compared to Ni(977)-light. Exploration of the rich reconstruction behavior was further probed with LEED and STM after the Ni(977)-light sample had undergone oxidation experiments short of inducing irreversible faceting to produce Ni(977)-moderate. In the course of mapping out the phase diagram for doubling and singling of the Ni(977)-moderate crystal, LEED revealed a significant temperature shift, from 475 to 570 K, for re-singling of the surface. This is the same trend in modification of the dynamic range for surface reconstruction observed between Ni(977)-heavy and Ni(977)-light. A related study on the behavior of the p(2x2)-O system on this crystal showed that this temperature coincides with AES measurements of where oxygen completely dissolves. Note, too, that dissolution temperature varies with oxidation history of the crystal for Ni(111). Figure 2 summarizes the widening temperature windows following cumulative oxygen absorption; Figure 3 shows illustrative STM images of singled and doubled Ni(977) surfaces. The large shift in the temperature associated with re-singling of the Ni(977)-moderate sample is attributed to cumulative absorption of oxygen from previous experiments including oxidation experiments performed after the low coverages studies using Ni(977)-light were completed. While the sub-surface concentration of oxygen was not measured for the crystals studied, the observed shifts in reconstruction dynamics are monotonically increasing as a fucntion of integrated oxygen exposure and are concomitant with our understanding of the strong effect oxygen plays as both an adsorbate and an absorbate in influencing surface energetics.
| Figure 3 |
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These data show that the dissolution temperature for the step-edge adsorbed oxygen has been modified by the previous oxidation and oxygen incorporation. In the previous experiment using HAS on Ni(977)-light, the dissolution temperature was lower than 500 K.
We have shown that the thermal range over which the doubling occurs can be extended as the oxygen concentration
in the crystal selvedge region increases with multiple dissolutions. Dissolution history directly influences structural
phase stability by altering the driving forces governing selvedge and bulk oxygen absorption. Interestingly, the
modifications in dissolution thermodynamics and kinetics are irreversible. Oxygen absorption into the stepped crystals
has been modified permanently even though the crystal structure has not been altered by faceting. Cross-talk between
bulk, i.e. selvedge, and surface oxygen is remarkably influential in determining phase stability with ramifications
extending to equilibrium morphology and rates of metallic oxidation.
96. "Influence of oxygen dissolution history on reconstruction behavior of a stepped metal surface"
