Porous aggregates of unidirectionally oriented microcrystallites of heteropoly compounds

The microstructures of aggregates of heteropoly compounds Cs3PW12O40 and (NH4)(3)PW12O40 precipitated at various temperatures were characterized by SEM, XRD, TEM, ED, and N-2 adsorption. The microstructures were remarkably different according to the precipitation temperature and the kind of cation. Cs3PW12O40 precipitated at 298 K formed dispersed or loosely aggregated fine particles of ca. 10 nm with random crystal orientation. As the temperature increased to 368 K, Cs3PW12O40 self-assembled to form microporous aggregates in which the crystal planes of microcrystallites were oriented unidirectionally, but were not much connected epitaxially with each other. (NH4)(3)PW12O40 precipitated at 273 K formed round aggregates similar to Cs3PW12O40 precipitated at 368 K, while the NH4 salt precipitated at 368 K formed dodecahedral aggregates in which microcrystallites were epitaxially connected with each other (epitaxial self-assembly). These observations indicate that microstructure is controlled by the solubility at the precipitating conditions; with increasing precipitation temperature or decreasing ionic radii of cation, the microstructure of aggregates varies from the random aggregates to unidirectionally oriented microcrystallites and the epitaxial connection between microcrystallites develops. (C) 1998 Elsevier Science B.V. All rights reserved.