Surface Orientation and Structure of LaTiO₂N Nanoparticles

Perovskite oxynitrides have attracted much attention as visible light photocatalysts and are primarily used as nanoparticles or in particle-based electrodes for photocatalytic or photoelectrochemical water splitting. The surface termination of these particles will directly impact their catalytic activity, and a fundamental understanding of which facets are likely exposed will help in the further optimization of these catalysts. Based on experimental transmission electron microscopy indexing of facets, we calculate, in this work, surface energies of differently reconstructed surfaces and the resulting Wulff shapes using two different approaches. In the first, we account for an explicit anion order, and in the second, we treat the anions as fully disordered using the virtual crystal approximation. By comparing experiment and theory, we conclude that a partial anion order is likely to occur on LaTiO2N surfaces. Moreover, our results show that the anion reordering from cis in the bulk to trans on the surface, as previously suggested for the {001} surface, is general, applying also to different surface orientations and reconstructions. The detailed knowledge of surface structures developed here is a solid foundation on which to computationally explore their relative activity.