Self-Organization ofAll-InorganicDodecatungstophosphate Nanocrystallites

Abstract

The crystallinity and porosity of all-inorganic dodecatungstophosphate M3PW12O40 (M=Cs, NH4, Ag, denoted as MPW) particles are controlled by the changes in the synthetic temperatures and countercations. The MPW particles can be classified into three groups by the crystallinity and porosity: (i) mesoporous "disordered" aggregates, (ii) microporous "self-organized" aggregates, and (iii) nonporous single crystals. The formation and growth mechanism of MPW particles is expressed by three steps: formation of nanocrystallites, assembly of the nanocrystallites to form aggregates, and the growth of aggregates by the attachment of nanocrystallites. The time courses of the turbidity of the synthetic solution, the concentration of the nanocrystallites, and the average particle sizes of MPW particles are well reproduced by the calculation based on the mechanism.

Pickering Emulsion Stabilized by Catalytic Polyoxometalate Nanoparticles: A New Effective Medium for Oxidation Reactions

“…However, to obtain monodisperse nanoparticles, the concentration of H 3 A C H T U N G T R E N N U N G [PW 12 O 40 ] must be lower than 10 À3 m. At higher concentrations, the nanoparticles self-assemble into very compact and polydisperse aggregates (see the Supporting Information, Figure S2). The structure is very different from that obtained with [NH 4 [16,17] in which the [PW 12 O 40 ] 3À units are connected by the acidic protons; in this case, each POM is maintained by van der Waals interactions between the alkyl tails of the ammonium cations. In other words, we suppose that the alkyl chains of the cations are overlapped to generate a lamellar-packing arrangement of the POM units.…”

A Simple Approach To Develop Tailored Mesoporosity in Nanostructured Heteropolysalts

“…The TEM images evidence that (i) the mass losses proceed gradually and homogeneously within the nanoparticles; and (ii) they involve the constituent nanocrystals undergoing progressive degradation until complete dissolution. The TEM images of the POM sample are in agreement with the SEM results and with the characterization of analogous samples in the literature, [19,28] as they show that the precursor is composed of nanoparticles with dodecahedral shape and a heterogeneous size distribution in the 50 to 2000…”

“…These features are fully consistent with the model proposed by Mizuno and Misono, [6a] in which nanostructured heteropolysalt particles consist in aggregates of nanocrystals with sizes in the 5-20 nm range that would lead to the dark areas in the TEM images. [28] In the case of the POM-40 sample, the nanoparticles closely resemble those observed for the POM precursor in spite of the material having undergone a mass loss of 40%. The dodecahedral morphology and the lack of transparency is fully preserved.…”

“…[6a,11] The former implicit model considers the porosity present in hetropolysalts as originating from the crystallographic structure and being a direct consequence of the existence of accessible voids in the crystal packing in close analogy with certain openframework crystalline materials such as zeolites or clays. The latter explicit model considers the porosity as external to the crystal packing, and hence, originating from a hierarchical structure organized in the following three levels: primary, that corresponds to the framework of the Keggin clusters; secondary, that is equivalent to the crystallographic structure and corresponds to the formation of ionic nanocrystals of Keggintype anions and bulky cations packed in a body-centered cubic lattice; and tertiary, that corresponds to the self-assembly of nanocrystals with sizes in the 5-20 nm range [28] that aggregate epitaxially following a regular pattern to grow nanoparticles with well-defined polyhedral geometry similar to those shown in Figure 3. According to Misono's explicit model, the porosity originates from the accessible voids left amongst nanocrystals upon epitaxial aggregation.…”

Síntese de materiais híbridos baseados em polioxometalatos : estudo das propriedades estruturais e catalíticas

“…Nesta condição, as moléculas de surfactantes não estão dissociadas como íons e a agregação é significante 91. Desse modo, a síntese da série CTAxH3-XPW não deve seguir o mecanismo relatado na literatura para cátions inorgânicos (como, Cs + , NH4 + e Ag + )37,39,92,93 e o caráter anfifílico das espécies CTA deve desempenhar um papel importante na formação dos materiais. De fato, a adição de excesso de CTAC (i.e., uma quantidade maior que a proporção estequiométrica de 3 moléculas de CTA para uma de HPW), resultou na precipitação de materiais com composições singulares do tipo CTAxPW, onde x > 3,0 (Tabela 3).…”