ZnPc/TiO2 composites for photocathodic protection of 304 stainless steel

ZnPc/TiO2 composites were synthesized by a drop-coating method and used as photoanodes for photocathodic protection to slow the corrosion of 304 stainless steel (304SS). TiO2 is a wide-band-gap (3.2 eV) conductor that only absorbs UV light with a wavelength <380 nm, and the recombination rate of photogenerated carriers in TiO2 is high. Zinc phthalocyanine (ZnPc) is a material with strong visible-light absorption. The combination of ZnPc and TiO2 can increase the absorption of visible light. Furthermore, the lowest unoccupied molecular orbital (LUMO) conduction-band level of ZnPc is higher than that of TiO2; thus, the electrons in ZnPc excited by the light can migrate to the conduction band of TiO2, thereby accelerating the separation of electrons and holes. The results show that the visible-light absorption of the ZnPc/TiO2 composite is markedly improved compared with that of TiO2 alone. The open-circuit potential (OCP) of 304SS coupled with the ZnPc/TiO2 composite decreased substantially compared with that of 304SS coupled with pure TiO2, reaching approximately −700 mV. The proposed ZnPc/TiO2 composites offer superior photocathodic protection for 304SS.