Seedless hydrothermal synthesis has been improved by introducing an adequate content ofammonia into the nutrient solution, allowing the fabrication of dense and ultralong ZnOnanowire arrays over large areas on a substrate. The presence of ammonia in the nutrientsolution facilitates the high density nucleation of ZnO on the substrate which iscritical for the nanowire growth. In order to achieve an optimal growth, the growthconditions have been studied systematically as a function of ammonia content,growth temperature and incubation time. The effect of polyethyleneimine (PEI)has also been studied but shown to be of no benefit to the nucleation of ZnO.Ultradense and ultralong ZnO nanowires could be obtained under optimal growthconditions, showing no fused structure at the foot of the nanowire arrays. Due todifferent reaction kinetics, four growth regimes could be attributed, includingthe first fast growth, equilibrium phase, second fast growth and final erosion.Combining this simple method with optical lithography, ZnO nanowires could be grownselectively on patterned areas. In addition, the as-grown ZnO nanowires could be usedfor the fabrication of a piezoelectric nanogenerator. Compared to the device ofZnO nanowires made by other methods, a more than twice voltage output hasbeen obtained, thereby proving an improved performance of our growth method.