Dark nanostructured ZnO films formed by anodic oxidation as photoanodes in photoelectrochemical water splitting
By Mika, Krystyna; Syrek, Karolina; Uchacz, Tomasz; Sulka, Grzegorz D.; Zaraska, Leszek
Published in Electrochimica Acta
2022
Abstract
Zinc oxide, due to its unique properties is very often considered as an alternative for titanium dioxide commonly used in photoelectrochemical and photocatalytic applications. In this paper, we present, for the first time, a complex characterization of the photoelectrochemical activity of dark ZnO layers formed by anodic oxidation of metallic Zn in alkaline electrolyte. A special emphasis was put on finding any relations between anodizing duration, morphology of the obtained layers, their defective nature, and photoelectrochemical activity. The research strategy included detailed analysis of morphology of the nanostructured ZnO, their physicochemical characterization (e.g., absorption and luminescence properties), and finally, extensive photoelectrochemical measurements. It was found that dark nanoporous ZnO layers are formed during anodization of Zn at the potential of 2 V in 1 M NaOH. However, if the process is longer than 15 min, the surface of the anodically generated film is gradually covered with a layer of ultra-thin nanowires. The highly defective nature of ZnO films was confirmed using photoluminescence measurements, and the number of defects increases significantly with time of anodization. On the other hand, further thermal treatment in air at 200 °C resulted in the gradual elimination of defects, especially those responsible for visible emission. The best photoelectrochemical performance was achieved for the oxide films with a thickness of c.a. 1.6 µm for both as-anodized and annealed samples. The formation of “lawn” nanowires on the surface of the nanoporous film did not cause any significant improvement in the photoelectrochemical properties of the materials.