Epitaxially self assembled aggregates of polyoxotungstate nanocrystallites, (NH4)(3)PW12O40: 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 elec tron microscopy, Na and Ar adsorption, X-ray and electron diffraction, and solid-state NMR. The formation process is also discussed. Each of these agg regates (0.3-5 mum in size) consisted of 5-10 nm nanocrystallites of (NH4)( 3)PW12O40, which connected epitaxially (the same orientation of crystal pla nes), leaving pores between the nanocrystallites. With an increase in the r eaction time, the initially formed spherical aggregates having micro- and m esopores gradually turned in to dodecahedral aggregates having only micropo res 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 titr ation method using NH4HCO3 aqueous solution. The origin of the micropores o f the dodecahedra is believed to be the narrow spaces between the nanocryst allites (primary particles) of each aggregate.