Epitaxially self assembled aggregates of polyoxotungstate nanocrystallites, (NH4)3PW12O40:: Synthesis by homogeneous precipitation using decomposition of urea

Microporous dodecahedral aggregates of a polyoxotungstate were synthesized by homogeneous precipitation from aqueous solution using hydrolysis of urea at 373 K (H3PW12O40 + 3/2(NH2)(2)CO + 3/2H(2)O --> (NH4)(3)PW(12)O(40)down arrow + 3/2CO(2)). The aggregates isolated after various reaction periods were characterized by several methods such as high-resolution scanning electron microscopy, Na and Ar adsorption, X-ray and electron diffraction, and solid-state NMR. The formation process is also discussed. Each of these aggregates (0.3-5 mum in size) consisted of 5-10 nm nanocrystallites of (NH4)(3)PW12O40, which connected epitaxially (the same orientation of crystal planes), leaving pores between the nanocrystallites. With an increase in the reaction time, the initially formed spherical aggregates having micro- and mesopores gradually turned in to dodecahedral aggregates having only micropores and higher crystallinity. The dodecahedral aggregates obtained after 24 h were almost stoichiometric, (NH4)(3)PW12O40, according to P-31 NMR, and had essentially the same structure as those prepared previously by the titration method using NH4HCO3 aqueous solution. The origin of the micropores of the dodecahedra is believed to be the narrow spaces between the nanocrystallites (primary particles) of each aggregate.