Growth and morphological study of zinc oxide nanoneedles grown on the annealed titanate nanotubes using hydrothermal method

Hydrothermal growth of ZnO on the annealed titanate nanotube films results in the oriented hexagonal-needlelike structures. The size, shape, density, and alignment of ZnO film are significantly affected by annealing temperature and orientation of the beneath titanate layer. It is believed that oxygen and hydrogen vacancies, generated due to dehydration of interlayered OH groups while annealing of the titanate, are responsible for the changes in the morphology of the ZnO. Microscopic observations clearly resolved nanoneedles with the base diameter of similar to 150 nm and length of similar to 5 mu m with lattice spacing of 0.52 nm, indicating single crystalline ZnO and grown along the (0001) direction. A growth model is presented based on the layer-by-layer growth (three-step growth) as a function of growth time (2-6 h). Thicknesses of these three steps were found increasing with growth time. The second step (II) of growth exhibits the same feature as that of the first step (I), i.e., bounded with six crystallographic, lower surface energy facets of {0111} surfaces and capped with {0111} planes, but with higher step thickness and sharper tip. Finally, the obtained ZnO nanoneedles exhibit {0111} facets, the third step (III), with sharp tip in the [0001] direction.