Controlling metal nanotoppings on the tip of silicide nanostructures.

Utilizing the difference in surface tension between SiO2 and metal catalysts (Mn2+, Ni2+), we show how metals form nanoshells, nanodiscs and nanospheres at the tips of the SiO2 nanostructures of nanocones, nanorods and nanowires. For the Mn2+ catalyst (i), SiO2-nanocones are formed with the hemispherical convex cap of the MnO/SiO2 composite. For the Ni2+ catalyst (ii), SiO2 nanowires are grown due to the concave shape of SiO2 surrounding the multi-faceted NiSi particles at their tip. For the Mn2+/Ni2+ catalyst (iii), SiO2 nanorods are formed with large-sized spherical ferromagnetic single Ni nanocrystals (50-200 nm in diameter) surrounded by the concave MnO2/SiO2 composite at the tip of the SiO2 nanorods. This large-sized spherical formation of the single Ni crystal is possible because Ni is able to be chemically reduced by Mn at 950 degrees C, well below the melting point of Ni (1455 degrees C) due to the alloying effect.