As the energy crisis and global warming are emerging as major issues, the development of renewable and green energy based on alternative energy resources such as solar, wind, hydrogen or geothermal sources, has attracted considerable interest.[1–5] The energy harvesting technologies based on these natural resources have been well established, and their use has gradually increased. Yet there are still many forms of energy sources in our living environment, which are not being utilized. Mechanical energy is one of the most representative sources that can be artificially generated from vibration, human walking, movement of automobiles, etc., which are abundant in the living environment, but all of which are usually wasted. In recent years, we have developed the nanogenerators (NG) that harvest this abandoned mechanical energy with variable frequencies and amplitudes via the piezoelectric effect.[6–14] The fundamental mechanism of a NG is related to a piezoelectric potential generated in nanowires (NWs) when they are dynamically strained under an external force, and the corresponding transient current that flows to balance the Fermi levels at two contacts. Further improvement in their performance has been achieved by pretreatment of ZnO NW arrays, which could drive an electrical watch for a few minute after an accumulation of the resulting charge.[15] Despite its advantages, the NG is still difficult to apply to energy harvesting device to scavenge the mechanical energy from the environment owing to high cost, low-throughput process, and weak durability. Thus, it is critically necessary to develop innovative strategies toward achieving cost-effective, large-area and robust NG in order to consistently harvest the mechanical energy from the environmental sources for an extended period.

In this work, we report a robust and large-area NG-based on cost-effective Al electrodes which could enable energy harvest from the walking motions. ZnO NW arrays were uniformly grown on a large-area Al foil surface with the size of 5 cm× 6 cm, which was also used as an electrode. In order to prevent the detachment of ZnO NWs from the substrate at the boundary interface under an applied strain, a micro-scale rough surface of the Al foil was produced to increase the surface contact area by sandblasting prior to the growth of ZnO NW. Multiple NGs were easily integrated in parallel and serial connections for increasing the output voltage and current. The maximum output voltage from the three-layer stacked NG with serial connections reached 0.43 V, and the maximum output current density from the parallel integration with three units of them approached the 54 nA/cm 2. Furthermore, three-dimensionally integrated NG, which was respectively composed of three units in width, length, and height, demonstrated the potential to work as an energy harvesting device under the human walking; the maximum output voltage exceeded 3 V and the maximum output current reached 195 nA. This is a key step towards a development of robust energy harvesting devices for practical use in an environment where dynamic stress/strain is available, such as the road where people can not only walk but also drive their cars.