Periodic DFT and HR-STEM Studies of Surface Structure and Morphology of Cobalt Spinel Nanocrystals. Retrieving 3D Shapes from 2D Images

In this study, we investigated the morphology of Co₃O₄ nanocrystals using periodic PW91 and PW91+U density functional calculations and Wulff construction. Theoretical investigations were combined with high resolution scanning transmission electron microscopy, which revealed that the examined spinel samples, obtained by pH-controlled precipitation, exhibit well-developed faceted crystallites. The results of slab calculations enabled to characterize the surface structure and reconstruction of the low index (100), (110), and (111) planes that were observed in HR-STEM pictures. The surface energies of the relaxed facets increased in the following order 1.39 J m⁻² (100) < 1. 48 J m⁻² (111) < 1.65 J m⁻² (110). These ab initio calculated values were used to predict a rhombicuboctaedral equilibrium habit of the cobalt spinel crystallites, by means of the Wulff plot. The obtained results were in remarkable agreement with the experimental shapes retrieved by matching 3D Wulff polyhedra to 2D STEM images observed along three different [100], [110], and [111] directions. Thus, the joined theoretical and experimental approach applied successfully herein, allowed not only for univocal shape determination and surface structure reconstruction of the faceted Co₃O₄ nanocrystals, but also provided the in-depth understanding of the observed features at the atomic level.