Novel photoanodes based on (Mo-Cr) co-doped titania nanotube for highly efficient photocathodic protection performance of stainless steel

Mo-doped, Cr-doped, and Mo-Cr co-doped titania nanotube arrays were synthesized through the single step anodization of titanium. Mo-Cr co-doped titania nanotubes prepared showed great improvement on photo -catalytic activity in the protection of stainless steel (403SS) under visible light irradiation compared with Mo and Cr mono-doped titania. The open-circuit potential (OCP) of 403SS (in 3.5 wt% aqueous NaCl) coupled with the best co-doped sample (Cr50:Mo50-TNT) was reduced from-240 to-690 mV, while the electrode potential was maintained at-320 mV even upon turning the light off. This study also identified the effect of the addition of sulfur compounds as hole scavengers on the photoelectrochemical (PEC) properties and photoelectrochemical cathodic protection (PCP) by the best Mo-Cr co-doped titania nanotube. Sodium sulfate and sulfite, sodium thiosulfate, sodium sulfide, potassium metabisulfite and thiourea have been used as hole scavengers. The order of photocurrent is as follows: Na2S > K2S2O5 > CS(NH2)2 > Na2S2O3 > no hole scavengers > Na2SO4 > Na2SO3. Mo-Cr co-doped titania nanotube show much more favorable PCP impact on 403SS than the pure titania sample, according to the results of the changes in the potentials and current densities. Furthermore, the results finally proved that sodium sulfide is the most appropriate hole scavenger.