Polyoxometalate-Based Metal-Organic Framework Fractal Crystals

Assembling complex hierarchical structures facilely to improve the accessibility of catalytic sites inside bulk catalysts has long been desired but remains challenging. Herein, we fabricate the hierarchical polyoxometalate-based metalorganic framework (POM@MOF) cuboctahedrons by solvent-assisted assembly, and propose a fractal growth mechanism. The great disparity in polarity of the dual solvents used endows them a different strength interaction with the POM. As a result, the polar enriched areas induced by the hydrophilic POM cause diffusion-limited aggregation along with the fractal growth. The obtained POM@MOF fractal crystallites possess an ordered layered surface and mesopores developed from the fractal growth and microscopic separation of the system. The cuboctahedron morphology is directed by the anisotropy of the Keggin units. This is of great significance in constructing hierarchical structures via fractal growth caused by diffusion limit by choosing some solvents with proper polarity disparity, which can be extended to other systems.