Formation of ZnO nanowires during anodic oxidation of zinc in bicarbonate electrolytes

•ZnO nanowires were synthesized by anodization of Zn in bicarbonate electrolytes.•Mechanism of nanowires formation was proposed.•Nanowire bundles are initially formed both inside and outside the pits.•Characteristic root-like structures were formed inside the cavities.•Effects of anodizing conditions on the morphology of anodic films are discussed.

Dense arrays of ZnO nanowires were obtained by one-step anodic oxidation of metallic Zn in bicarbonate electrolytes followed by 2 h of annealing in air at 200 °C. A detailed inspection of the anode morphology at the initial stages of anodizations was performed. It was confirmed that nanowire bundles are initially formed both inside and outside the pits created during the anodic dissolution of the metallic substrate. In consequence, root-like structures are formed inside the cavities. However, no roots are observed for nanowire bundles formed outside the pits, directly on the surface of Zn. The effect of anodizing conditions on the growth of nanowires was studied. It was found that applying higher anodizing potentials and more concentrated electrolytes result in not only the significantly faster nanowires growth, but also the formation of denser NW films. In addition, the electrolyte agitation can also increase the rate of nanowires formation. Finally, we proved for the first time that the previous usage of electrolyte and the orientation of electrodes, can significantly affect the morphology of as-formed anodic films. Thermal treatment of as-anodized samples resulted in the successful conversion of the carbonate-containing precursor to crystalline wurtzite ZnO without the loss of nanowire morphology.