An approach for the large-scale synthesis of high-purity silicon nanowires (SiNWs) in ultrahigh vacuum is presented. A mixture of Si and SiO2 is evaporated by an electron beam, and the growth temperature is 700 degrees C, which is much lower than those used for other oxide-assisted growths. A new type of single-crystal SiNWs, with [221] orientation, is thus synthesized. Moreover, it is experimentally demonstrated that SiO intermediates are formed in the process, and the nanowires are obtained via a disproportionation reaction of 2SiO -> Si + SiO2. A growth mechanism is proposed and the critical factors for the formation of 1D nanowires are also determined. The approach is particularly compatible with the mature Si-based technology, and is favorable for device integration and practical applications.