Large-area, vertically aligned silicon nanowire arrays have been successfully fabricated on (001)Si substrates via the redox reactions between silicon and silver ions in the aqueous solution of AgNO3/HF. From SEM and TEM observations, the typical widths of the synthesized SiNWs are in the range of 30-200 nm. The lengths of SiNWs could be tuned from several to tens of micrometers by adjusting the reaction temperatures and time. For the Ni thin films on Si nanowires samples, the growth of epitaxial NiSi2 was found to be more favorable for the samples with smaller nanowire widths. The epitaxial orientation relationship between the NiSi2 and Si nanowires was identified to be NiSi2// Si and (002)NiSi 2//(004)Si. The observed results present the exciting prospect that with appropriate controls, this approach could be applied to investigate the phase transformation and growth kinetics of other metal contacts or interconnects on Si substrates at the nanometer-scale.