ABSTRACT Transition metal sulfides have been widely investigated as electrocatalysts for both the oxygen and hydrogen evolution reactions. Here, we synthesized copper, nickel, cobalt, and iron sulfides using a facile successive ionic layer adsorption reaction (SILAR) occurring in porous transparent titanium dioxide nanotubes. Nanotubes are fabricated by anodization of a titanium layer sputtered onto indium tin oxide‐coated glass slides. X‐ray photoelectron spectroscopy measurements confirmed the presence of copper oxides and sulfides, cobalt oxides and sulfides, nickel oxides and sulfides as well as iron oxides. Although the walls of the titania nanotubes are modified using 5 m m aqueous solutions containing the metal and sulfide ions, the initial transparency has been preserved. According to microscopic studies and elemental analysis, the sulfides are uniformly distributed on the walls forming a metal oxide/metal sulfide heterojunction. Among all investigated materials, titania overgrown by cobalt oxide and sulfide exhibits the highest current density of 28 mA cm −2 recorded at +2.1 V vs. RHE during oxygen evolution, while the non‐modified electrode reached only 1.5 mA cm −2 . Taking into account both the high transparency and activity toward oxygen evolution, the investigated electrodes are an important element for a semitransparent tandem device for overall water splitting.
Lipińska et al. (2025) studied this question.