Theoretical Study of Polar Spinel Surfaces: Effect of Termination and Cation Inversion on Structure and Stability

The effect of termination and cation distribution on the stability of low-index MgAl2O4 and ZnFe2O4 surfaces is investigated theoretically at the generalized gradient approximation and self-consistent hybrid density functional theory level. MgAl2O4 is the prototype AB(2)O(4) spinel, and ZnFe2O4 belongs to the spinel-type ferrites that have been proposed as photocatalysts for water splitting (Taffa, D. H.; Dillert, Ulpe, A. C.; Bauerfeind, K. C. L.; Bredow, T.; Bahnemann, D. W.; Wark, M. J. Photonics Energy. 2016, 7, 012009). Because the catalytic activity of different surfaces of the same material can vary significantly, it is of utmost importance to determine the most relevant surface terminations. Spinels can easily undergo an interchange of cations on A and B sites, the so-called inversion. We therefore studied the low-index surfaces (100), (110), and (111) of both normal and fully inverse MgAl(2)O(4 )and ZnFe2O4. For each surface, the surface energy of several possible terminations was calculated with symmetric and stoichiometric slab models. It is found that the surfaces of inverse spinels are less stable than the corresponding normal surfaces. This indicates that full inversion is not facilitated in nanoparticles with a large surface-to-bulk ratio, which are used, for example, in photocatalysis. The Wulff theorem is applied to determine the equilibrium shapes of MgAl2O4 and ZnFe2O4 crystallites. For all compounds, {100} facets dominate the single-crystal surface.