Control of ZnO nanorod array alignment synthesized via seeded solution growth

The critical factors that determine the degree of alignment of dense ZnO nanorod (NRAs) arrays synthesized via a two-step seeding and solution growth process were systematically examined, with a goal of optimizing the array density and surface area for hybrid organic–inorganic photovoltaic (PV) devices. Unexpectedly, we found that the degree of alignment of ZnO nanorod arrays depended strongly on the ambient humidity level during the seeding step. Close-packed ZnO nanorod arrays with [0 0 1] axis perpendicular to the substrate were obtained only when seeded at >20% relative humidity (RH) at room temperature (RT), according to data from scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) spectroscopy. In addition, when the seeding RH was raised to >60%, the polydispersity in the diameter and length of the ZnO nanorods increased, although good alignment was maintained. As a result, we determined an optimal seeding humidity of 30–40% for the synthesis of ordered ZnO nanorod arrays. Atomic force microscopy and contact angle measurements revealed that the density of ZnO seeds was significantly lower below the 20% relative humidity threshold, indicating that water vapor plays a critical role in generating insoluble zinc hydrolysis products that act as nucleation sites for the subsequent growth of ZnO nanorods. Finally, we found that the alignment of the ZnO nanorod arrays was strongly disrupted as the roughness of the underlying fluorinated tin oxide substrate increased. The improved control of the morphology of ZnO nanorod arrays may lead to improved carrier collection of conducting polymer-ZnO nanorod array hybrid PV devices.