4.2. Titanium Dioxide 4947 4.3. Germanium Oxide 4949 4.4. Gallium Oxide and Zinc Oxide 4950 4.5. Mixed-Valence Oxides of Iron and Cobalt (Fe 3 O 4 and Co 3 O 4 ) 4951 4.6. Multicomponent Oxides: BaTiO 3 , BaTiOF 4 , CaMoO 4 , and (Fe,Co) 3 O 4 4952 5. Protein-and Peptide-Directed Syntheses of Non-Oxide Semiconductors 4954 5.1. Introduction 4954 5.2. Biogenic and Biologically-Derived Production of Semiconductor Nanoparticles 4954

The carbothermal reduction of silica into silicon requires the use of temperatures well above the silicon melting point (> or =2,000 degrees C). Solid silicon has recently been generated directly from silica at much lower temperatures (< or =850 degrees 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 degrees 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 m(2) g(-1)), and contained a significant population of micropores (< or =20 A). 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.

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Titania (TiO2) nanoparticles are widely used, or are under active development, for a range of applications in (photo)catalysis, photovoltaics, enzyme support, energy storage, and photonics. The peptide-directed room-temperature formation of titania nanoparticles can be an attractive alternative to higher-temperature synthetic methods. However, the influence of the peptide primary structure on the titania precipitation activity at room temperature is not well understood. Through the selective binding of phage-displayed 12-mer peptides to TiO2 substrates, we have identified 20 peptides with an affinity for titania. The average numbers of arginine, lysine, and histidine residues present in these 20 peptides were distinctly higher than for the overall peptide-bearing phage library. Synthetic 16-mer versions of four of these peptides (i.e., 12-mer peptides with C-terminal tetrapeptide tags for quantitative spectrophotometry) induced the formation of 8.1–38.7 mol TiO2/mol peptide after exposure for only 10 min to an otherwise water-stable Ti(IV) complex at room temperature and a pH of 6.3. X-ray diffraction analyses, electron diffraction analyses, and high-resolution transmission electron microscopy revealed that the peptide-induced titania contained fine (<10 nm) anatase and monoclinic β-TiO2 nanocrystals, along with an amorphous phase. The titania yield increased with the number of positive charges carried by these peptides. On the basis of these results, a peptide was designed that exhibited the highest titania formation activity reported to date for a peptide (82.9 mol TiO2/mol peptide), as well as a reduced pH dependence for such titania formation.