Periodic density functional theory study of maghemite (001) surface. Structure and electronic properties

The structure of the iron oxide gamma-Fe2O3 surfaces along (001) direction has been modeled for the first time using periodic DFT. Four different types of surfaces were analyzed depending on the upper layer composition (presence of tetrahedral or octahedral Fe) and the presence of iron vacancy locations. The inclusion of a Hubbard (DFT + U) correction allows a good agreement between calculated and experimental bulk properties. Relaxation of surface layers results in important surface reconstructions, electronic charge transfer with respect to the bulk, and a decrease in the dipole moment. Surfaces with tetrahedral Fe atoms on the top layer resulted to be the most stable ones. An almost linear correlation between the work function and the electronic charge transfer to O atoms was found. An analysis of the density of states shows that the Fermi level in tetrahedral Fe-terminated surfaces is far from the valence band whereas the reverse occurs with octahedral Fe-terminated atoms. The tetrahedral Fe-terminated surface has a more basic character than the octahedral ones. In addition, the presence of a subsurface Fe vacancy stabilizes the system and seems to be adequate for bond activation by electron transfer from the surface to the adsorbate, such as H-2.