Dark nanostructured ZnO films formed by anodic oxidation as photoanodes in photoelectrochemical water splitting

•Nanostructured dark ZnO layers were synthesized by anodic oxidation of Zn in NaOH.•During long anodizations nanowire arrays are formed on the surface of porous ZnO.•The obtained films exhibit highly defective nature.•The thermal treatment in air at 200 °C improves PEC activity of the photoanodes.•The best PEC performance was achieved for the 1.6 µm thick anodic films.