PNNL

Abstract

Using a simple ionic model with polarizable oxygen ions and dissociating water molecules, we have calculated the energetics governing the distribution of Fe(II)/Fe(III) ions at the reduced (2 · 1) surface of a-Fe2O3 (hematite) (012) under dry and hydrated conditions. The results show that systems with Fe(II) ions located in the near-surface region have lower potential energy for both dry and hydrated surfaces. The distribution is governed by coupling of the ferrous iron centers to positive charge associated with missing oxygen atoms on the dry reduced (2 · 1) (021) surface. As the surface is hydroxylated, the missing oxygen rows are filled and protons from dissociated water molecules become the positive charge centers, which couple more weakly to the ferrous iron centers. At the same time, the first-layer iron centers change from fourfold or fivefold coordination to sixfold coordination lowering the potential energy of ferric iron in the first layer and favoring migration of ferrous iron from the immediate surface sites. This effect can also be understood as reflecting stronger solvation of Fe(III) by the adsorbed water molecules and by hydrolysis reactions favoring Fe(III) ions at the immediate surface. The balance between these two driving forces, which changes as a function of hydration, provides a compelling explanation for the anomalous coverage dependence of water desorption in ultra-high vacuum experiments.