Synthesis of ε-Keggin-Type Cobaltomolybdate-Based 3D Framework Material and Characterization Using Atomic-Scale HAADF-STEM and XANES

We describe the preparation of e-Keggin-type cobaltomolybdate-based 3D frameworks with sodium cations, NaH9[e-CoIIMoV8MoVI4O40CoII2], and their characterization by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure (XAFS) spectroscopy. Atomic-scale HAADF-STEM images of e-Keggin compounds were obtained for the first time, and positions of Mo and Co were confirmed. Furthermore, clear evidence of the presence of a CoO4 tetrahedron was obtained by X-ray absorption near-edge structure (XANES) analysis. Their characterization clearly revealed that e-Keggin-type cobaltomolybdate units, [e-CoMo12O40]n−, constructed by a central CoIIO4 tetrahedron and 12 surrounding MoO6 octahedra, are linked with CoII to form 3D frameworks.