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New Approaches to the Synthesis of Porous and/or High Surface Area Transition Metal Oxides
(2009)
- We have explored the applicability of hypothesized approaches to the synthesis of porous and/or high surface area transition metal oxides. In addition, applicability and advantage of charged templates where strong Coulomb interactions favour the supramolecular arrangements/assembly were studied. The problems related with the dynamics of polymeric nanostructures for the synthesis of predesigned mesostructures could be avoided by crosslinking micelles, strictly speaking non-continuous phase in the bulk structure. Thereby, we presented a new approach for the grafting of Keggin POMs around the core-crosslinked PB-P2VP worm-like polymer templates (A 1 and 2). The produced POM-1 exhibits high dispersion, improved surface area and is thus expected to be useful in catalytic, electrochemical and biotechnology related applications. The general applicability of the method to other Keggin POMs and spherical polymer nanostructures were studied. Developed Keggin POMs-1 to 6 showed high dispersion of Keggin POM and surface areas. To the best of our knowledge, our approaches lead to Keggin POM nanocomposites with the highest surface areas reported todate. As-synthesized Keggin POM nanocomposites are amorphous. We have studied the removal of polymer template and crystallization of hybrid to corresponding metal oxides through step-wise calcinations under argon followed by air. We have presented another approach to the synthesis of high surface area and mesoporous keggin POM framework materials using amphiphilic PI-PDMAEMA block copolymers (A 3). The calcined mesoporous materials exhibit Keggin POM hexagonal pore structure with high keggin POM dispersion, improved surface area. These developed materials are expected to be useful in catalytic applications. A fundamental principle involved in this method is that an attractive interaction between the organic block copolymer and the keggin POM precursors is obtained via Coulombic interactions through in situ quaternization (protonation) of PDMAEMA part, which also ensure the formation of a homogeneous hybrid material without any macrophase separation. Further, step-wise calcinations under argon and air lead to evolution of mesoporous keggin POM material. To the best of our knowledge, this is the first hexagonally ordered mesoporous Keggin POM framework material. We have presented a low-temperature, non-hydrothermal synthesis route to rutile nanocrystals. Both rutile and anatase nanocrystals exhibit positive surface charges. In contrary to the above approaches where polymer templates are cationic and inorganic precursors are anionic, in this case, inorganic nanocrystals are cationic and polymer templates are anionic. In this approach, we have demonstrated that crystalline TiO2 nanocomposites with well-defined crystalline form could be directly synthesized at temperatures as low as 40 oC by mesostructuring the positively charged crystalline titania colloids over anionic spherical polyelectrolyte brush particles under aqueous conditions. Stepwise calcinations first under argon followed with a second calcination in air lead to the complete removal of polymer template without collapse and hollow porous spheres with crystalline framework are obtained. Porosity and surface areas increased dramatically after stepwise calcinations. Moreover, the porous rutile nanomaterials are photocatalytically active. We proved that our hypothesis to the synthesis of crystalline TiO2 nanocomposites with well-defined crystalline form and morphologie is feasible.
