Synthesis and the Visible Light Activity of Anodic CoOx-TiO2 Nanocomposites

Photoelectrochemical water splitting is considered one of the most promising methods for generating clean energy. Band gap engineering based on introducing transition metal oxides into the crystalline network of stable nanostructured semiconductors (e.g., TiO₂) is of great interest. In the present work, nanotubular TiO₂ was synthesized via anodization followed by wet impregnation with a cobalt acetate solution (2−10 impregnation cycles) to obtain a heterojunction based on CoO_x-modified TiO₂ upon annealing. To gather information connected to the material morphology, composition, crystallinity, optical, semiconducting, and photoelectrochemical properties, a variety of techniques were used, for example, scanning electron microscopy (SEM)/energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (RS), ultraviolet−visible diffuse reflectance spectroscopy (UV−Vis DRS), Mott−Schottky analysis, and finally photoelectrochemical tests. Deposition of CoO_x onto TiO₂ resulted in a reduction of the optical band gap (from 3.21 to 2.38 eV), which led to a significant improvement of the studied materials’ photoresponse in the visible range (the wavelength range in which the material is active has been extended to 600 nm).