Seedless hydrothermal synthesis has been improved by introducing an adequate content of
ammonia into the nutrient solution, allowing the fabrication of dense and ultralong ZnO
nanowire arrays over large areas on a substrate. The presence of ammonia in the nutrient
solution facilitates the high density nucleation of ZnO on the substrate which is
critical for the nanowire growth. In order to achieve an optimal growth, the growth
conditions 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 growth
conditions, showing no fused structure at the foot of the nanowire arrays. Due to
different reaction kinetics, four growth regimes could be attributed, including
the first fast growth, equilibrium phase, second fast growth and final erosion.
Combining this simple method with optical lithography, ZnO nanowires could be grown
selectively on patterned areas. In addition, the as-grown ZnO nanowires could be used
for the fabrication of a piezoelectric nanogenerator. Compared to the device of
ZnO nanowires made by other methods, a more than twice voltage output has
been obtained, thereby proving an improved performance of our growth method.