The interaction between O2 and metal surfaces is one of the most fundamental gas-surface interactions. In addition, a thorough knowledge about metal oxidation is of crucial importance for many practical reasons, ranging from prevention of corrosion to the use of oxides as catalysts and insulating layers in electronic devices. One example where an oxide is actually used in order to prevent corrosion is found in the jewelry industry. In order to retain the shine and luster of white gold it is covered by a very thin Rh layer, which after the formation of an ultra-thin oxide layer, will prevent further oxidation.
We have studied the oxidation of a Rh(111) single crystal using under conditions ranging from UHV to pressures of 1 bar at elevated temperatures, concentrating on the transition from a surface to a bulk oxide. At typical sample temperatures of around 500°C, a RhO2 surface oxide is formed at a pressure of 10-3 mbar, while the fully oxidized Rh2O3 phase is found first at oxygen pressures of at least 10 mbar [1]. In between, however, we report the formation of a second bulk oxide phase, exhibiting a spinel-like Rh3O4 structure, which to our knowledge has not been discussed in this context.
These findings will be compared to theoretical predictions based on DFT. At first sight the experiments and calculations seems to disagree. E.g. the Rh2O3 phase is predicted to be thermodynamically stable (and thus should form) at pressures as low as 10-4 mbar. That is 1 magnitude lower than where the oxidation process starts with the formation of the surface oxide, and 5 magnitudes lower than when we actually find this phase. There are, however, intermediate steps of the oxidation process that will work as thresholds hindering the system to reach thermodynamical equilibrium. E.g. O2 molecules will not easily adsorb and dissociate on the oxide surface, as they do on the metallic Rh surface. In this presentation we will describe the details of the apparent discrepancy between experiment and theory.
[1] J. Gustafson et al., Phys. Rev. Lett. 92 (2004) 126102 |