Shape-Preserving Chemical Conversion of Self-Assembled 3-D Bioclastic Micro/Nanostructures Via Low-Temperature Displacement Reactions

An astounding variety of self-assembled, rigid (bioclastic) micro/nanostructures are generated by micro-organisms known as diatoms (single-celled algae). Each diatom species assembles an intricate silica nanoparticle-based microshell (frustule) with a particular three-dimensional (3-D) shape and with specific patterns of nanoscale features (pores, channels, ridges, protuberances, etc.). Sustained reproduction of a single diatom can yield enormous numbers of intricate frustules with identical 3-D shapes and fine features. The massive parallelism and genetic precision of such 3-D nanoparticle self-assembly are highly attractive for device applications. However, in order to expand the use of such micro/nanostructures into a broad range of applications, processes need to be developed to change the silica-based chemistry of diatom frustules into other compositions, so as to achieve a wider variety of properties. Displacement reactions can be used to convert SiO 2 -based diatom frustules into other oxides while preserving the 3-D morphology of the starting frustule. In this paper, an oxidation-reduction reaction between Mg gas and SiO 2 frustules has been used to convert the frustules into MgO-bearing replicas. The influence of processing parameters (reaction temperatures, times, and reactant ratios) on the nanostructural evolution, and on the final product phases (particularly the secondary Si-bearing phases), has been examined. Nanocrystalline MgO-bearing replicas of diatom frustules have been synthesized at temperatures as low as 650°C within a few hours.