Breaking the nanostructure mould

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Researchers at the University of Minnesota have developed a process for producing nanoscale cubes and spheres using the controlled disassembly of larger, lattice-like structures. This could make the production of more uniform porous nanostructures simple and controlled.

Most conventional methods for the production of porous silicon dioxide nanoparticles suffer from the fact that the growing particles tend to aggregate, making it difficult to achieve a uniform size. The shape of the particles is also difficult to influence.

The Minnesota 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 the plastic polymethylmethacrylate (PMMA), which assemble themselves through closest packing of spheres into a colloidal crystal. Between the spheres in this structure, there are tiny approximately tetrahedral and 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. These continue to contract until the octahedra become nearly cubic and the tetrahedra become nearly spherical, making highly uniform structures with worm-like pores.

By varying the colloidal crystals used as the mould, the size and shape of the resulting particles can be controlled. Through vapour deposition or polymer grafting, other compounds can be added to the structure. Subsequent etching away of the silicon oxide allows this new technique to be used as a starting point for nanostructures made of other materials.