New insight into the mechanism of cathodic electrodeposition of zinc oxide thin films onto vitreous carbon

In this study, the mechanism of zinc oxide (ZnO) electrodeposition from aqueous zinc nitrate solution at 70 °C was investigated on vitreous carbon and bulk zinc electrodes using cyclic voltammetry experiments. The first widely accepted route corresponds to ZnO precipitation from Zn2+ and OH− produced by NO3− reduction; the second route, which is discussed in this article, is due to Zn2+ reduction into metallic Zn followed by its oxidation by nitrate ions. For this case, we demonstrate why Zn formation cannot be observed while Zn2+ reduction was detected. Structure and morphology of bulk deposits were investigated using X-ray diffraction and scanning electron microscopy. Thin films of well-oriented ZnO nanorods were grown on vitreous carbon from Zn(NO3)2 solution at low cathodic potential corresponding to that of zinc metal deposition. X-ray diffraction (XRD) measurements showed that the nanorods are crystalline and preferably grow along the (0 0 2) direction.

► A new mechanism for ZnO formation on vitreous carbon is proposed in nitrate medium.
► At low potential, a ZnO layer is obtained by direct oxidation of bulk zinc by nitrate ions.
► At higher cathodic potential, ZnO is obtained by two parallel mechanisms.
► The nanostructured film obtained at −1.1 V has a (0 0 2) texturation.