Since in Wurtzite structure of hexagonal ZnO nanorods, side
planes are non-polar and stable; while top (Zn-terminated) and
bottom (O-terminated) planes are polar and quasi-stable; in the
hydrothermal growth process, polar planes can attract opposite
charges (Zn2þ or OH) from the solution and form nuclei. Liu et al.
[23] showed that, with increase of annealing temperature of the
seed layer and improvement of crystallinity, the growth rate of
nanorods will increase.