Zinc oxide nanowires in chemical bath on seeded substrates: Role of hexamine

We report a study of the role of hexamine in the seeded growth of zinc oxide (ZnO) nanowires by hydrothermal mat route. We show that the growth can be made highly amsotropic (aspect ratio > 150) with almost no detectable increase in diameter (with time of hydrothermal growth) of the obtained nanowires. Results indicate that hexamine acts as a shape inducing molecule, by selectively capping the non-polar crystallographic planes of the zincite crystal. We observe nanowires with typical diameters of similar to 30 nm and lengths exceeding several microns after a 24 h growth period at 60-95 degrees C. Our observations, and reports from the literature, suggest that the concentration of the precursors in the chemical bath can significantly affect the growth rate of the wires. By keeping the concentration of the precursors in the bath at 1 mM, we have observed an extremely slow, but highly anisotropic growth of ZnO nanowires.