“Sponge Crystal”: a novel class of microporous single crystals formed by self-assembly of polyoxometalate (NH4)3PW12O40 nanocrystallites

Abstract

In this account, a new concept of ''sponge crystals'' is presented on the basis of new interpretation of our previous results of porous heteropolyacids, that is, porous aggregates of self-assembled (NH 4 ) 3 PW 12 O 40 nanocrystallites (Ito, Inumaru, and Misono, J. Phys. Chem. B 101 (1997) 9958; Chem. Mater. 13 (2001) 824) ''Sponge crystals'' are defined as single crystals having continuous voids within them, but unlike zeolites, no intrinsic structural pores. This new category includes molecular single crystals having continuous voids originating from series of neighboring vacancies ( ‡1 nm) of the constituent large molecules, affording nanospaces in the crystals. A typical example of ''sponge crystals'' is (NH 4 ) 3 PW 12 O 40 , which is formed via the dropwise addition of ammonium hydrogen carbonate into an H 3 PW 12 O 40 aqueous solution (titration method) at 368 K. The resulting (NH 4 ) 3 PW 12 O 40 nanocrystallites (ca. 6-8 nm) then self-assemble with the same crystal orientation to form porous dodecahedral aggregates in the solution. During the formation process, necks grow epitaxially between the surfaces of the nanocrystallites (''Epitaxial SelfAssembly'') to form aggregates of which each aggregate has an ordered structure as a whole single crystal. Although the crystal structure of (NH 4 ) 3 PW 12 O 40 has no intrinsic structural(''built-in'') pores, X-ray diffraction, electron diffraction and gas adsorption experiments all reveal that each (NH 4 ) 3 PW 12 O 40 aggregate is comprised of a single crystal bearing many micropores. These pores are considered to be continuous spaces as neighboring vacancies of the molecules (anions and cations) originating from the residual spaces between the self-assembled nanocrystallites. The porous (NH 4 ) 3 PW 12 O 40 single crystals are considered a special case of ''mesocrystals,'' as was recently discussed by Co¨lfen and Antonietti (Angew. Chem. Int. Ed. 44 (2005) 5576). In contrast to most ''mesocrystals,'' which are generally polycrystalline in nature, each aggregate of (NH 4 ) 3 PW 12 O 40 is a characteristic porous single crystal. Furthermore, the micropores of (NH 4 ) 3 PW 12 O 40 are totally different from those found in other microporous crystals: zeolites have ''built-in'' pores defined by their crystal structure, while the (NH 4 ) 3 PW 12 O 40 nanocrystallites have none. Since (NH 4 ) 3 PW 12 O 40 micropores are continuous spaces as neighboring vacancies of the molecules, the shapes of the (NH 4 ) 3 PW 12 O 40 pores can in principle, assume various connectivities or networks within the crystal, and as such, are subsequently termed: ''sponge crystals.''

Mesocrystals—Ordered Nanoparticle Superstructures

“…In most cases, the mesocrystal intermediate cannot be directly observed but can only be concluded from the final porous single crystal structure as observed for BaSO 4 [135] or (NH 4 ) 3 PW 12 O 40 . [42] A second analytical challenge is the multi step process of mesocrystal formation consisting at least of two elementary steps: (i) nanoparticle nucleation and (ii) mesocrystal assembly. Usually, there are more steps involved like the formation of amorphous precursors, the formation of mineral bridges between the nanoparticles in a mesocrystal or the crystallographic fusion of the crystallographically aligned nanocrystals to single crystalline domains.…”

“…Although the mineral bridge concept can easily explain the crystallographic orientation of the nanocrystals in a mesocrystal, mineral bridges are very hard to detect with an electron microscope in a microtome thin cut and have to be assumed to be only a few nanometer in diameter. Nevertheless, these bridges could be observed in some cases [41][42][43] and the regions between the nanocrystals were shown to be stainable with a dye. [41] If a mesocrystal grows via mineral bridges as proposed above, the kinetics of mesocrystal formation should be rather slow as compared to free crystallization, which disagrees with the fast mesocrystal formation, which is often observed.…”

“…This is nicely illustrated for the example of (NH 4 ) 3 PW 12 O 40 mesocrystals, which finally form so-called sponge crystals. [42] The cubic (NH 4 ) 3 PW 12 O 40 first forms rounded mesostructures from ca. 6 nm nanocrystallites, which then slowly transforms to a dodecahedral shape of a mesocrystal with rough surfaces and single crystal scattering behavior.…”

A nanocomposite superstructure of metal oxides with effective charge transfer interfaces

“…The resulting precipitates are then collected and washed to remove impurities, and, in some cases, the precipitates also require further annealing. In recent years, new methods such as microwave-assisted hydrothermal 15 , bioinspired 16 and titration 17 methods have been developed to simplify the synthesis procedure and improve its efficiency. For instance, Zhou et al 6 prepared anatase TiO 2 mesocrystals by topotactically transforming NH 4 TiOF 3 crystals without changing their morphology.…”

Roles of interfaces in nanostructured composites: nanocatalysts, sponge crystals and thin films

“…This yields a "sponge crystal" [ Fig. 1(e)], 5) as we discovered in 1996, and we named this process "epitaxial self-assembly". 6) Hetero-epitaxial growth of thin films on crystalline substrates can also significantly affect the structure and physical properties of the crystalline thin film.…”