Reduction and Oxidation of Maghemite (001) Surfaces: The Role of Iron Vacancies

The knowledge of surface reduction and oxidation energetics in reducible oxides is essential for the design of improved catalysts for oxidation reactions. This is particularly true for iron oxides, a very attractive material system, because of the availability and biocompatibility of its constituents. In this work, by means of the density functional theory, we have thoroughly studied the gamma-Fe2O3(001) maghemite surfaces, taking full account of iron vacancies beyond a mean field approach. Despite the structural similarity with the more studied magnetite Fe3O4 surfaces from which maghemite differs only for the presence of iron vacancies in the octahedral sites and for the absence of reduced Fe2+ cations, the surface properties are quite different. Our investigation shows that the presence of Fe vacancies is responsible for an increase in the surface reducibility. Also, it favors surface oxidation. The main reason is that the Fe vacancies cause a decrease of electronic charge of the surface oxygen atoms, which then become more reactive. We have considered different surface terminations and found that the reduced surfaces are more stable than the simple bulk-truncated ones. The reduction leads to a new reconstruction of the surface, which is the most stable surface termination among those investigated. We have examined the charge transfers and the modifications in the electronic structure caused by the surface reduction.