Oxygen Reduction Investigation on Sputtered ZnO Layers with Nano-granular Structure

Zinc oxide (ZnO) films with nano-granular structure were deposited by radio-frequency magnetron sputtering. The absence of preferred orientation was verified by XRD. HAADF-STEM images revealed a nanoporous structure with pore and grain sizes around 10 nm. The narrowness of the pores enabled to approximate that oxygen reduction takes place at the surface of the layers. During cathodic polarization, the surface of the electrodes became rougher due to the development of nodules on the surface. The ZnO electrodes exhibit a high activity towards oxygen reduction in KOH solution (pH=10). Rotating ring-disk electrode measurements coupled with reaction modelling allowed to quantitatively demonstrate that the direct reduction prevailed with the indirect pathway being active as well.