Enhanced photoelectrochemical cathodic protection of stainless steel under visible light using Co₃O₄–ZnO-modified TiO₂ nanotubes

Developing solar energy conversion strategies is crucial to overcome stainless steel corrosion protection challenges. Through the chemical bath deposition (CBD) technique, zinc and cobalt nanoparticles were deposited onto TiO₂ nanotubes (TNTs) to use as photoanodes in the photoelectrochemical cathodic protection (PECP) of AISI 304 stainless steel. The composition of the chemical bath affected the structure and properties of the deposited TNT layers, which were analyzed using field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), elemental mapping, X-ray diffraction (XRD), and ultraviolet–visible (UV–Vis) diffuse reflectance spectroscopy. Considering both illuminated and dark conditions, the photocathodic protection performance of the composite film was evaluated in a 3.5 wt% NaCl solution. The Co₃O₄–ZnO/TNT photoanodes demonstrated enhanced light absorption, charge separation, and improvement of photoelectrochemical properties, attributed to the synergistic effect of the ternary system, n–p junction formation at the Co₃O₄–ZnO interface, and TiO₂–ZnO heterojunction. The optimal photoanode achieved a 1.6% higher photocurrent density than the original TNT photoelectrode. Moreover, the Co₃O₄–ZnO/TNT photoanodes demonstrated corrosion protection for stainless steel by exhibiting a negative shift in corrosion potential of the steel (− 614 mV vs. Ag/AgCl for the optimal photoanode) following light exposure and showing delayed cathodic protection when the light was turned off. This study suggests that Co₃O₄–ZnO/TNTs have a promising potential for use in photocathodic protection. It also highlights how the photoelectrochemical properties of deposited layers are affected by the chemical bath composition.