Vertical Growth of Two-Dimensional Zinc Oxide Nanostructures on ITO-Coated Glass: Effects of Deposition Temperature and Deposition Time

Two-dimensional (2D) zinc oxide (ZnO) nanostructures of thickness 40−100 nm and length up to several micrometers are vertically grown on indium−tin oxide-coated glass substrates by using electrochemical deposition in the temperature range of 22−90 °C. The deposition temperature is found to play a significant role in controlling the morphology of the nanostructures, from randomly oriented, poorly crystalline nanosheets at 22 °C to well-aligned nanowalls with wurtzite structure at 70 °C. At 80 °C, nanowalls are found to merge with one another, and they become patchy fibrillar structures at 90 °C. An optical direct band gap of 3.3 eV is measured for all the samples deposited above 50 °C, while their transmittance varies from 40 to 75%, depending on their morphologies. X-ray photoelectron spectroscopy (XPS) shows that the ZnO nanostructures are covered by a Zn(OH) 2 overlayer, in good accord with the general growth model that involves Zn 2+ hydroxylation followed by dehydration to ZnO. Furthermore, our XPS data also reveals the presence of chlorine throughout the ZnO nanostructures, which verifies for the first time the 2D growth mechanism that involves Cl - capping of the (0001) plane of ZnO and thereby redirecting growth in the (101̄0) plane. Early growth of nanowalls at 70 °C arising from spherical nanoparticles to elongated nanobars is demonstrated by varying the deposition time.