Controlled growth of core-shell Si-SiO_x and amorphous SiO_2 nanowires directly from NiO/Si

摘要

A simple, direct synthesis method is used to grow core-shell Si-SiO_x and amorphous SiO_2 nanowires by heating a NiO-catalyzed silicon substrate. The morphology of the nanowires was controlled by carbothermal reduction of WO_3, which provides a reductive environment to synthesize crystalline Si nanowires covered with a SiO_x sheath at the growth temperature of 1000-1100 deg C. Only amorphous SiO_2 nanowires were produced when the substrate was annealed without using WO_3/C. Transmission electron microscopy shows that the Si core is 20-50 nm in diameter and the SiO_x shell layer is 40-60 nm thick. After hydrofluoric acid (HF) treatment of core-shell Si-SiO_x, the single-crystalline silicon nanowires (SiNWs) were obtained in large quantities. The HF-treated SiNWs were 20-50 nm in diameter. The main crystal growth direction of the SiNWs was [111]. The nanowires grown were highly pure (no metal catalyst contamination) and very long (hundreds of micrometres). A solid-liquid-solid (SLS) mechanism is proposed for the growth of both core-shell Si-SiO_x and amorphous SiO_2 nanowires.