Michael R. Weatherspoon

Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas

The carbothermal reduction of silica into silicon requires the use of temperatures well above the silicon melting point (≥2,000 °C). Solid silicon has recently been generated directly from silica at much lower temperatures (≤850 °C) via electrochemical reduction in molten salts. However, the silicon products of such electrochemical reduction did not retain the microscale morphology of the starting silica reactants. Here we demonstrate a low-temperature (650 °C) magnesiothermic reduction process for converting three-dimensional nanostructured silica micro-assemblies into microporous nanocrystalline silicon replicas. The intricate nanostructured silica microshells (frustules) of diatoms (unicellular algae) were converted into co-continuous, nanocrystalline mixtures of silicon and magnesia by reaction with magnesium gas. Selective magnesia dissolution then yielded an interconnected network of silicon nanocrystals that retained the starting three-dimensional frustule morphology. The silicon replicas possessed a high specific surface area (>500 m2 g-1), and contained a significant population of micropores (≤20 Å). The silicon replicas were photoluminescent, and exhibited rapid changes in impedance upon exposure to gaseous nitric oxide (suggesting a possible application in microscale gas sensing). This process enables the syntheses of microporous nanocrystalline silicon micro-assemblies with multifarious three-dimensional shapes inherited from biological or synthetic silica templates for sensor, electronic, optical or biomedical applications.

3D Rutile Titania-Based Structures with Morpho Butterfly Wing Scale Morphologies†

The attractive optical, chemical, biochemical, and mechanical properties of the rutile polymorph of titanium dioxide (titania) have led to its use in powder or film form in paints, plastics, cosmetics, sunscreens, interference coatings, separation membranes, gas sensors, and as a food additive. Threedimensional (3D) porous networks of rutile titania are also of considerable interest for separation/sorption, optical, and biomedical applications. The assembly of 3D porous networks of rutile with well-controlled solid and pore morphologies is an active area of research. A number of groups have synthesized titania structures that have wellorganized 3D arrays of macropores/voids by applying coatings of the anatase polymorph of titania on organic templates and then removing the templates. However, attempts to completely convert organized 3D anatase/pore structures into rutile titania replicas, by heat treatment at 800 8C, have resulted in appreciable grain growth and distortion of the solid/pore structures. Herein, we demonstrate for the first time how an intricate, 3D, nanocrystalline rutile structure may be generated with the morphology and nanoscale features inherited from a bioorganic, chitin-based template. Chitin is a natural polysaccharide that is formed into well-organized structures by a variety of organisms (for example, fungi, yeast, arthropods, cephalopods, mollusks, insects). The chitin-based templates in this work are the scales present on the wings of a Morpho butterfly. As revealed by the secondary electron (SE) image in Figure 1a-1, these overlapping scales possess an

Sol-gel synthesis on self-replicating single-cell scaffolds: applying complex chemistries to nature's 3-D nanostructured templates

A sol-gel process was used, for the first time, to apply a multi-component, nanocrystalline, functional ceramic compound (BaTiO3) to a three-dimensional, self-replicating scaffold derived from a single-celled micro-organism (a diatom).

Phosphor Microparticles of Controlled Three-Dimensional Shape from Phytoplankton

We demonstrate how the precise three-dimensional 3D assembly characteristics of biomineralizing micro-organisms may be combined with synthetic chemical processing to generate photoluminescent microparticles with specific 3D shapes and tailored chemistries. Silica-based microshells with a rich variety of controlled shapes are assembled by a type of unicellular algal phytoplankton known as diatoms Bacillariophyceae. Each of the tens of thousands of diatom species generates a microshell with a particular 3D morphology that can be used as a shape-dictating particle template. In this demonstration, the microshells of Aulacoseira diatoms were converted into Eu 3+ -doped BaTiO3-bearing microparticles. The silica-based microshells were first converted into magnesia-based replicas via a gas/solid displacement reaction the silica of native diatom microshells is not chemically compatible with barium titanate. A conformal, sol-gel-derived coating of europium-doped barium titanate was then applied to the chemically compatible magnesia replicas to yield photoluminescent particles that retained the starting microshell shape. Upon stimulation with 337 nm UV light, the 3D microparticle replicas exhibited a bright red emission associated with the 5 D 0 → 7 F 2 transition of Eu+3.