An etchant system, Ag2O-HF-H2O, was used to fabricate vertically aligned Si nanowire (SiNW) arrays on Si wafers. The synthesis was based on catalytic etching and produced large-area brushlike SiNWs on Si wafers. The Ag2O concentration was varied from 0.001 to 0.1 mol/L and various synthesis conditions were optimized. The synthesized SiNWs were investigated by Fourier-transform infrared spectroscopy analysis, optical absorption, and contact-angle measurements. Spectroscopic ellipsometry was also used to assess the surface roughness produced in the early stage of etching. The optical measurements revealed that the SiNWs have high optical absorbance from the far infrared to ultraviolet regions. Passive HF etching of the SiNWs changed their wettability from superhydrophilic (similar to 0 degrees) to highly hydrophobic (similar to 135 degrees). The effect of sulfuric peroxide mixture (SPM) cleaning on the SiNW formation properties was also examined. (C) 2012 Elsevier B.V. All rights reserved.