Large-area, vertically aligned silicon nanowire (SiNW) arrays have been successfully synthesized in an aqueous solution containing AgNO3 and HF on (001)Si substrates. The as-synthesized SiNWs were determined to be perfectly single crystalline with the axis of the wire parallel to the  direction. The typical widths of the SiNWs were in the range of 30-200 nm. The lengths of SiNWs could be tuned from several to tens of micrometers by adjusting the synthesis temperature and time. We measured the formation rate at different reaction temperatures. The activation energy for linear growth of the SiNWs, as obtained from an Arrhenius plot, was found to be about 0.36 eV. In addition, the Si substrates with highly oriented SiNW arrays were found to exhibit significant hydrophobic properties. The water contact angles of the HF-treated SiNW arrays were measured to be about 120-148°, much greater than that with a flat silicon wafer surface (∼85 to 88°).