Electrochemical synthesis and characterization of dark nanoporous zinc oxide films

Abstract

Dark nanoporous layers of zinc oxide were obtained by simple anodic oxidation of metallic zinc and characterized by various physicochemical methods. Anodization was performed in 1 M NaOH electrolyte at the potentials of 2 V, or 4 V. All processes were carried out for 30 min at room temperature without stirring of the electrolyte. Selected samples were annealed in air at 200 °C for 2 h. The characterization of morphology and composition of as obtained materials was performed. It was confirmed that such kind of anodic films were porous in nature and exhibited a sponge-like morphology, rather than a channel-like structure typically observed in other types of anodic oxides. XRD, XPS and Raman spectroscopy confirmed that ZnO was obtained directly during anodization. Moreover, it was found out that the material was highly disordered with a high amount of oxygen vacancies and zinc interstitials, despite the applied anodizing potential. However, a significant improvement in the crystallinity of ZnO was achieved due to the annealing process for the samples anodized at higher potential. The UV–Vis spectroscopy confirmed the low reflectance of the obtained layers related to their black color. The hydrophilic character of the layers was confirmed by contact angle measurements. The characterization of porous ZnO films confirmed both a high surface area and strong absorption of the visible light that can be utilized in photoelectrochemical and photocatalytic applications.