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Nanoscale Cubes and SpheresGermany | Posted on January 4th, 2007
Most conventional methods for the production of porous silicon dioxide nanoparticles suffer from the fact that the growing particles tend to aggregate (clump together), making it difficult to achieve a uniform size. The shape of the particles can hardly be influenced at all. Stein and his team chose a backward approach, first building up a lattice structure of silicon dioxide and then disassembling it to get the shape they wanted. The "moulds" used for the lattice were tiny spheres of a plastic called polymethylmethacrylate (PMMA), which assemble themselves through "closest packing of spheres" into a colloidal crystal. Between the spheres in this structure, there are little, nearly tetrahedral and nearly octahedral spaces. The researchers filled these cavities with a solution containing an organosilicon compound, oxalic acid, and a surfactant. This mixture hardens into a solid gel. The plastic spheres and surfactant are then burned off by heating. The surfactant leaves behind tiny pores, and the gelled organosilicon compound slowly converts to a solid silicon oxide. What remains initially is a silicate lattice that is the negative of the packed spheres: tiny tetrahedra and octahedra attached to each other by delicate bridges. As the conversion to silicon dioxide continues, the structure shrinks until it breaks at the weakest points-the bridges. The fragments formed by this process are shaped like octahedra or smaller tetrahedra. These continue to contract until the octahedra become nearly cubic and the tetrahedra become nearly spherical, making highly uniform structures with worm-like pores.
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