Computational Study on Surface Structure and Crystal Morphology of γ-Fe₂O₃: Toward Deterministic Synthesis of Nanocrystals

This work is the first application of classical atomistic theory to a comprehensive treatment of γ-Fe₂O₃ surfaces. The surface energy and attachment energy of several low-index surfaces of γ-Fe₂O₃ have been calculated by the classical atomistic simulation methods. A mean-field approximation involving scaling the short-range potentials, charges, and force constants of the iron ions by a fraction corresponding to the partial occupation of octahedral iron sites in the spinel structure was employed. Reconstruction of the polar surfaces was required to remove the dipole perpendicular to the surface and to achieve structural stability. This was accomplished through the addition of vacancies. The two-dimensional periodic calculations were carried out with the MARVIN code. The (112) surface consisting of iron and oxygen ions had the smallest relaxed surface energy of 1.86 J/m², while several others including (001), (011), and (012) were within 0.1 J/m² of this value. The calculated surface energies of these planes were used in a Wulff plot to predict a polyhedral crystal habit for these crystals at thermodynamic equilibrium. The (111) surfaces terminated with iron ions had the smallest attachment energies, and an octahedral crystal with (001) facets is predicted for the growth morphology of γ-Fe²O³. Such surfaces possess iron ions in 3-fold coordination sites at 0.5 A above the plane of oxygen ions, making them accessible to interact with adsorbed molecules. This information is relevant to understanding the growth of nanocrystals of γ-Fe ₂O₃.