Silicon nanowire (SiNW) arrays are formed by Ag-assisted electroless etching at 25 degrees C in 25% HF solution with AgO having concentration ranging from M = 0.003 to 0.2 mol/l. The maximum length of the SiNWs is similar to 65 mu m at an M of similar to 0.045 mol/l. Optical absorption spectroscopy and Fourier-transform infrared spectroscopy show that the SiNWs have extremely large optical absorbability not only in the UV region but also in the far-infrared region. No photoluminescence is observed from the AgO/HF-formed SiNWs that are in direct contrast with the conventional AgNO3/HF-formed nanowires that generally emit orange light. Post-stain etching in the HF/HNO3 solution of the AgO/HF-formed nanowires enables light emission in the orange region. Passive HF etching of the SiNWs changes their wettability from highly hydrophilic (similar to 0 degrees) to superhydrophobic (similar to 147 degrees). This Ag-assisted electroless etching mechanism can be explained on the basis of an energy-band diagram of the Ag/p-Si interface in the AgO/HF solution. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3583598] All rights reserved.