The growth of CuO nanowires (NWs) through direct oxidation of copper is widely utilized. We give further evidence of a short-circuit, grain boundary diffusion mechanism. First, we show enhanced CuO NW growth through oxidizing nanocrystalline Cu. Second, we show the presence of a bi-crystal structure with a Cu rich (1 1 -2)/(0 0 -1) boundary along the entire length of the NW. Our analysis suggests that the growth of CuO NWs occurs via the short-circuit diffusion of Cu ions across the Cu2O layer, followed by short-circuit diffusion along the CuO NW bi-crystal grain boundary and to the NW tip, where subsequent oxidation occurs. (C) 2011 Elsevier B. V. All rights reserved.