The kinetics of the hydrothermal growth of ZnO nanostructures

The immersion of a material, seeded with ZnO nanoparticles, in an aqueous solution of Zn(NO₃)₂ and hexamethylenetetramine (HMT) at 90 °C yields an extended array of one-dimensional ZnO on the substrate surface. The structure of the ZnO evolves with reaction time. Initially nanorods are formed. At longer times the rods are tipped with nanotubes. Here we report a series of experiments in which both the composition of the reaction solution; concentrations of H⁺, Zn²⁺ and HMT; and the structure of ZnO deposited on the substrate are monitored as a function of reaction time. It was found that the change from ZnO rod to tube growth arises when the solution composition is such that it is no longer thermodynamically favorable to precipitate Zn(OH)₂.