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 (2,000 °C)¹. Solid silicon has recently been generated directly from silica at much lower temperatures (850 °C) via electrochemical reduction in molten salts^{2, 3}. However, the silicon products of such electrochemical reduction did not retain the microscale morphology of the starting silica reactants^{2, 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 (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^{4, 5, 6} or synthetic silica templates^{7, 8, 9} for sensor, electronic, optical or biomedical applications^{10, 11, 12, 13}.