Short-circuit diffusion growth of long bi-crystal CuO nanowires

► Dense, vertically-aligned arrays of ultra-long CuO nanowires (NWs) were grown. ► Surface mechanical attrition treatment of Cu facilitated the CuO NW growth. ► A bi-crystal structure along the entire length of the NW was observed. ► Evidence of a short-circuit, grain boundary diffusion mechanism was revealed.