Study On Constructing Heterojunction Modified Titanium Dioxide For Photogenerated Cathodic Protection

The ocean covers 71%of the earth’s surface,has vast space and rich resources,but the marine environment is a complex corrosive environment.Seawater is a multi-component aqueous solution that dissolves a variety of inorganic salts,which makes it a natural strong electrolyte with conductive properties.The chemical or electrochemical interaction between metal materials in the ocean and environmental media will cause material damage or change and metal corrosion.In order to limit or avoid the onset of corrosion,people have investigated 304 stainless steel（304SS）with corrosion resistance.However,in the marine environment,because of its small radius and strong penetration,chloride ions are easy to be absorbed by metal and destroy the passivation film,resulting in pitting corrosion,endangering the metal components,and reducing the safety of the entire equipment.Photogenerated cathodic protection method can protect steel by transferring electrons generated by semiconductor materials to steel and lowering the potential.TiO₂ is applied to protect 304SS due to its high dielectric constant,good stability and higher photocatalytic activity.However,TiO₂ has broadband gap and low utilization of visible light can be improved by combining with semiconductor to form heterojunction.Z-scheme heterojunction,with strong oxidation-reduction ability,high charge separation efficiency,high stability,and wide spectral sensitivity,may produce effective spatial separation of photogenerated electron-hole pairs on various semiconductors.In this study,anodic oxidation was used to create TiO₂ nanotubes,and designed Mn Fe₂O₄@SiO₂/TiO₂,Mn Co₂O₄/TiO₂ and MXene@Fe₂O₃/TiO₂ three kinds of Z-scheme heterojunction.The mechanism of photocathode protection is discussed through photoelectrochemical performance test.Preparation of MnFe₂O₄/TiO₂ and Mn Fe₂O₄@SiO₂/TiO₂ on TiO₂ nanotubes by hydrothermal method.To explore the effects of Mn Fe₂O₄ loading with different concentrations on the protective properties of Mn Fe₂O₄/TiO₂.Also,it was looked at how stable the materials were both before and after SiO₂ coating.The SEM test results demonstrate that as Mn Fe₂O₄ loading rises,the size of nanoparticles on the top of nanotubes steadily increases;The OCP test results show that when the loading amount is 50 m M/L of Mn²⁺,the photoanode has the best protection performance for 304SS under visible light irradiation,and the Z-scheme heterojunction can rapidly separate the photogenerated carriers.When the SiO₂ is coated,it still has good performance after 150days of use,improving the stability of the material.MnCo₂O₄ nano-sheets were formed by hydrothermal method,and then sintered to make Mn Co₂O₄ nano-sheets agglomerate into nano-flowers.The photocathodic protection performance of Mn Co₂O₄ with large specific surface area on the modified TiO₂ was investigated.The UV-visible diffuse reflection results show that flower-like Mn Co₂O₄ can improve the utilization of visible light.Photovoltaic characteristic curve test（i-V）and electrochemical impedance spectroscopy（EIS）showed that photogenerated carrier recombination was inhibited.According to the band structure measured by ultraviolet photoelectron spectroscopy（UPS）,the photogenerated electrons between Mn Co₂O₄ and TiO₂ are transported according to the Z-scheme heterojunction path to improve the photoelectric properties of the composite.Fe₂O₃/TiO₂ composite was prepared by loading Fe₂O₃ nanoparticles by hydrothermal method,and then Ti₃C₂T_x suspension was evenly coated on Fe₂O₃/TiO₂to prepare MXene@Fe₂O₃/TiO₂.The protective properties of different concentrations of Fe₂O₃ and Ti₃C₂T_x on the composite were investigated.OCP test results show that when Fe₂O₃ concentration is 5 m M/L and 1μL Ti₃C₂T_x has the best protection effect on 304SS.This is because when Ti₃C₂T_x contacts with semiconductors,it can generate internal electric field,generate energy band bending,form Schottky barrier,and prevent the recombination of electrons and holes in TiO₂.Through photoluminescence（PL）spectra and EIS tests,the separation rate of photogenerated electrons and holes was investigated,and the redox ability of Z-scheme heterostructures was improved.