The role of polarity and surface energy in the growth mechanism of ZnO from nanorods to nanotubes

Abstract

The polarity of zinc oxide nanostructures is crucial to modern electronic devices in terms of electrical and optical properties. However, it is still unclear whether the growth direction which affects the polarity of zinc oxide nanorods in hydrothermal processes is Zn-, O- or mix-polar. Earlier studies suggested that it should be Zn-polar based on thermodynamic calculations. Later studies proposed that the nanorods are O-polar, i.e. less stable than Zn-polar, resulting in the formation of nanotubes by KOH etching. Recently, the possibility of the co-existence of both Zn- and O-polar has been demonstrated. Therefore, we investigated the polarity issue by fabricating two types of ZnO nanorods under acidic and alkaline growth conditions. The as-grown and etched morphologies of these two types of nanorods are obviously different. Valence band X-ray photoemission spectroscopy (VB-XPS) has been employed to determine the polarity. We found that nanorods from both conditions are Zn-polar. This led us to find out that the formation of nanotubes is determined by the surface energy on the Zn-polar face of heterogeneously grown nanorods. The surface energy of ZnO nanorods can be controlled by a second acidic chemical bath, as long as the surface is not annealed. Furthermore, the thermodynamics of the process was studied to investigate the possible growth mechanism after the confirmation of polarity.