Interplay of Different Reaction Pathways in the Pulsed Galvanostatic Deposition of Zinc Oxide

Abstract

The electrochemical deposition of crystalline zinc oxide (ZnO) out of aqueous Zn(NO3)2 solutions under pulses of controlled current were studied on gold substrates in micrometer dimensions. Three significant stages were observed for the potential established during depositions in all potential-time curves. Scanning electron and confocal laser microscopy analysis revealed a correlation between a successively completed coverage of the gold electrode by ZnO and an abrupt transition of the deposition and rest potential to less negative values. The deposition time at which such a transition was detected depended not only on the amount of deposited ZnO but also on the current density during electrodeposition, pointing at a different growth mode of ZnO, and on the electrode geometry, pointing at the influence of different diffusion profiles of the reacting ions. In order to elucidate the role of different participating redox reactions in the course of electrodepositions of ZnO, measurements were also carried out in various reference electrolytes. Additionally, pulsating times were varied which revealed that the three stages in the potential were only seen for pauses which were equal or longer in time than the current pulse. The significance of each stage and the corresponding reaction is discussed for the preparation of ZnO films completely covering an electrode surface to optimize the deposition for future applications in electronic devices.