Letter

Nature 446, 172-175 (8 March 2007) | doi:10.1038/nature05570; Received 13 November 2006; Accepted 2 January 2007

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

Zhihao Bao1, Michael R. Weatherspoon1, Samuel Shian1, Ye Cai1, Phillip D. Graham1, Shawn M. Allan1, Gul Ahmad1, Matthew B. Dickerson1, Benjamin C. Church1, Zhitao Kang1, Harry W. Abernathy III1, Christopher J. Summers1, Meilin Liu1 and Kenneth H. Sandhage1

  1. School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

Correspondence to: Kenneth H. Sandhage1 Correspondence and requests for materials should be addressed to K.H.S. (Email: ken.sandhage@mse.gatech.edu).

The carbothermal reduction of silica into silicon requires the use of temperatures well above the silicon melting point (greater than or equal to2,000 °C)1. Solid silicon has recently been generated directly from silica at much lower temperatures (less than or equal to850 °C) via electrochemical reduction in molten salts2, 3. However, the silicon products of such electrochemical reduction did not retain the microscale morphology of the starting silica reactants2, 3. 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 (less than or equal to20 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 biological4, 5, 6 or synthetic silica templates7, 8, 9 for sensor, electronic, optical or biomedical applications10, 11, 12, 13.

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