Water-induced room-temperature transformation of straight Ge/Si core/shell nanowires into circular silica nanotubes

A facile method for producing circular silica nanotubes starting from straight Ge/Si core/shell nanowires is described. Single-crystalline Ge nanowires were selected as templates and sacrificing materials because they can be easily oxidized and the resulting Ge oxide is water soluble. Si shell layers were deposited via sputtering on the as-grown Ge nanowires, resulting in the formation of Ge/Si core/shell nanowires. The as-synthesized Ge/Si core/shell nanowires were dipped in water at room temperature and amorphous silica nanotubes were produced after 14 days. The characteristics of the transformation from the Ge/Si core/shell nanowires into silica nanotubes were observed and a mechanism is proposed. We also investigated how circular silica nanotubes could be fabricated, and the findings indicate that the strong surface tension of water results in the formation of Ge/Si core/shell nanowires in which the thickness of the shell layers is non-uniform. These were converted into circular silica nanotubes, on the evaporation of the water. We believe that this method can provide fundamental insights into the fabrication, shape control and self-assembly of nanostructures.