Atomic-Step-Induced Local Nonequilibrium Effects on Surface Oxidation

By temperature-, time-, and pressure-resolved imaging of the dynamics of surface steps on NiAl(100) during its oxidation, we provide direct evidence of the significant effects of atomic steps in controlling the local thermodynamic driving force for oxidation. Our results show that the inherent barriers associated with step crossing by surface species of oxygen cause a heterogeneous oxygen concentration across the crystal surface, giving rise to local nonequilibrium effects governing oxidation even for surfaces that are globally in equilibrium. The asymmetry in the step-crossing barriers for oxygen atoms crossing up or down steps is such that descendant steps exert a local driving force that favors oxidation, whereas ascendant steps locally destabilize the surface oxide in their vicinity. The local differences in the thermodynamic driving force for oxidation due to atomic steps and step bunches give rise to novel phenomena, such as nonmonotonous oxide growth and the net translation motion of surface oxide stripes by growing on one end while receding on the other end