Controlled growth of core–shell Si–SiOxand amorphous SiO2nanowires directly from NiO/Si

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 °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.