Role of magnetism on transition metal oxide surfaces in vacuum and solvent

Abstract

The transition metal oxides MnO, FeO, NiO, and CoO are essential materials systems for catalysis applications and energy technologies. These materials exhibit a magnetic phase transition from paramagnetism to antiferromagnetism upon cooling. In this work, we show that an accurate treatment of the surfaces requires a description of the disordered local magnetic moments of the paramagnetic phase. We determine how the magnetic phase transition and the presence of solvent affect the surface energies using density-functional theory and a solvation model. To accurately account for the correlations in the $d$-electron system, and to match the observed magnetic order and band gaps of the room-temperature phases, we include the Hubbard-$U$ correction. In the case of MnO, FeO, and CoO, which are paramagnetic semiconductors or insulators at room temperature, we demonstrate the importance of the local magnetic moments and model the materials using special quasirandom structures. To determine the equilibrium shape of MnO, FeO, NiO, and CoO nanocrystals, we calculate the surface energies of their low-energy (100), (110), and (111) facets and perform the Wulff construction. For the (111) facet, we consider various reconstructions that remove the polar nature of the unreconstructed surface. The processing conditions of these oxide nanoparticles, in most cases, involve a solvent. To analyze the influence of the solvent environment on the surface energies of different facets and thereby the crystal shape, we calculate the surface energies of these oxides in water using the continuum solvation model VASPsol. We find that the surface energies decrease due to the dielectric screening. However, as the ratio of surface energies remains sufficiently similar, we predict that the equilibrium crystal shape is only weakly affected by the presence of the solvent.