Adsorption of Water on the Fe3O4(111) Surface: Structures, Stabilities, and Vibrational Properties Studied by Density Functional Theory

The majority of the theoretical work that attempted to provide atomic level details on the adsorption of water at the Fe3O4(111) surface is based on conventional density functionals, which suffer from shortcomings such as, for example, self-interaction errors. In an effort to overcome these uncertainties in theoretical results, we use density functional theory (DFT) employing the Perdew, Burke, and Ernzerhof generalized-gradient corrected exchange-correlation functional augmented by a Hubbard-type U parameter. We test for robustness of these results by application of the Heyd, Scuseria, Ernzerhof hybrid functional. For the two relevant metal terminations (Fe-oct2 and Fe-tet1) having ambient conditions in mind, we determined the minimum energy adsorption structures up to relatively high water coverage, that is, one, two, and three H2O molecules on the p(1 x 1) surface unit cells, respectively. Water adsorbs dissociatively and strongly exothermic on the Feoct(2), whereas molecular adsorption occurs on the Fe-tet1 termination. Using D2O, two IR signals at 2720 and 2695 cm(-1) (typical of OD stretching modes) can be observed for a wide range of temperatures and at moderate water vapor pressures. Our calculations reveal that these IR bands originate from a very stable water dimer-like species. However, at lower temperatures the creation of larger aggregations, such as trimers, appears to be thermodynamically favorable.