Effects of high-temperature rapid thermal annealing for seed layers on the crystallographic evolution in hydrothermal ZnO nanostructures

In this work, we investigated effects of high temperature rapid thermal annealing for the zinc oxide (ZnO) seed layers on the growth morphology and crystal orientation of hydrothermal ZnO nanorods (NRs). The seed layers were prepared by sol-gel spin coating and annealed by two-step rapid thermal processes at different peak temperatures ranging from 600 to 900 °C for a short time period of 1 min. The seed layers annealed in a temperature range of 600-800 °C were all polycrystalline; however, they exhibited a highly Zn-deficient amorphous state when annealed at 900 °C as observed by X-ray photoelectron spectroscopy, X-ray diffraction (XRD), and cross-sectional transmission electron microscopy (TEM). The vertical NRs normal to the substrate were grown along [001] direction atop the polycrystalline seeds annealed at 600-800 °C, whereas different growth morphology of flower-like NRs was observed on the seeds annealed at 900 °C with the strongest XRD peak along the [100] orientation. From our cross-sectional TEM analysis, this flower-like architecture was initiated from the pioneer crystals laterally grown along [001] direction guiding the subsequent growth of petal NRs oriented by a slight difference in growth direction.