SHAPE-PRESERVING CHEMICAL CONVERSION OF SELF-ASSEMBLED 3-DBIOCLASTIC MICRO/NANOSTRUCTURES VIA LOW-TEMPERATUREDISPLACEMENT REACTIONS

摘要

An astounding variety of self-assembled,rigid (bioclastic)microanostructures aregenerated by micro-organisms known as diatoms (single-celled algae).Each diatom speciesassembles 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 ofintricate frustules with identical 3-D shapes and fine features.The massive parallelism andgenetic precision of such 3-D nanoparticle self-assembly are highly attractive for deviceapplications.However,in order to expand the use of such microanostructures into a broadrange of applications,processes need to be developed to change the silica-based chemistry ofdiatom frustules into other compositions,so as to achieve a wider variety of properties.Displacement reactions can be used to convert SiO2-based diatom frustules into other oxideswhile preserving the 3-D morphology of the starting frustule.In this paper,an oxidation-reduction reaction between Mg gas and SiO2 frustules has been used to convert the frustules intoMgO-bearing replicas.The influence of processing parameters (reaction temperatures,times,and reactant ratios)on the nanostructural evolution,and on the final product phases (particularlythe secondary Si-bearing phases),has been examined.Nanocrystalline MgO-bearing replicas ofdiatom frustules have been synthesized at temperatures as low as 650 °C within a few hours.