Zinc oxide Chemical Bath Deposition on Functionalized organic thin films: Formation of nanorods, nanorockets and nanoflowers

Controlling the morphology of nanostructured materials is critical for their use in technological applications including in sensing, electronics and energy harvesting. In this paper we investigate the reaction pathways involved and their dependence on reactant concentrations in the formation of ZnO nanomaterials on –COOH terminated self-assembled monolayers using a simple chemical bath deposition process which employs zinc acetate, which acts as the Zn source, and ethylenediamine, which acts as both the O source and a complexing agent for Zn²⁺. At a deposition temperature of 318 K (45 °C) our data shows that the concentration of Zn²⁺ as well as the deposition bath pH, which is controlled by the ethylenediamine concentration, is critical in determining the ZnO morphology. Above 0.01 M zinc acetate at low bath pH (~ 7.7–8.5), nanorods and nanorockets are observed to form. The nanorods exhibit a clear interface in the middle indicating that they are composed of two crystals. At lower zinc acetate concentrations over a wide pH range (~ 8.0–10.5) nanoflowers form. The nanorockets and nanoflowers grow via a modified La Mer mechanism in which there are multiple nucleation and crystallization steps. The initial nuclei are sphelurites (nanoflowers) or nanocrystallites (nanorockets). Since the reagent concentrations limit the reaction, for these initial precursor crystallites to increase in size, it is required dissolution and re-precipitation must occur. Thus at later times nanorockets or nanoflowers develop. - Highlights: • Nanorods, nanorockets and nanoflowers form depending on the reaction conditions. • Nanorods grow slowly suggesting low supersaturation conditions are needed. • Nanoflowers and nanorockets form via multiple nucleation and precipitation steps.