Hex nut shaped mesocrystals from hierarchical organization of hexagonal WO ₃ microcrystals

In this work, we report on the synthesis and characterization of hex nut shaped mesocrystals built from the hierarchical organization of hexagonal tungsten trioxide particles. The hexagonal particles were stacked in an oriented fashion, with the (001) facet corners at an angle of 120°. The synthesis is based on the hydrolysis of tungsten hexachloride (WCI6) in ethanol in the presence of urea, assisted by solvothermal conditions. The role of urea appears to be crucial in determining crystal structure and the hierarchical organization, and without urea monoclinic tungsten trioxide is obtained. The samples were characterized by X-ray diffraction, SEM, HRTEM, TGA and FT-IR spectroscopy. The results suggest that hexagonal particles form by a topotactic transformation of cubic tungsten oxide nanoparticles. SEM characterization after heat treatment allowed to establish that hex nut shaped mesocrystals conserved their hierarchical organization up to 450 °C.