The surface of hydrogen-terminated silicon nanowires was proved to be photo-active and redox-controllable under a visible light by the color alteration of methylene blue: colorless in dark and blue in light in the presence of H-SiNWs. The process of the redox reaction was monitored by a UV-vis spectrophotometry, showing a repeatable interconversion between methylene blue and lueco methylene blue within 2 h under conditions of alternant dark and light environments. During the redox process, oxygen was proved necessary for the reversible color alteration of methylene blue. The high repeatability in dark-reduction and photo-oxidation of methylene blue demonstrates the catalytic activity of ultrafine hydrogen-terminated silicon nanowires. Finally, a possible mechanism is deduced. (C) 2012 Elsevier Ltd. All rights reserved.