Study On Photocathode Protection And Anti-corrosion Mechanism Of Zinc Indium Sulfide/TiO₂

Cathodic protection is an important means to improve metal corrosion resistance and prolong metal service life.Although traditional cathodic protection,including sacrificial anode and impressed current cathodic protection,is one of the most widely used technologies,there are still some problems,such as energy consumption,sacrificial anode loss,environmental pollution,etc.Therefore,it is a challenge to pursue a green and environmentally friendly cathodic protection technology.The photoelectrochemical cathodic protection（PECCP）is based on the photoelectric conversion effect of semiconductors and only utilize the solar energy to protect metals from corrosion,thus PECCP is a promising and pollution-free anticorrosion technology.In the field of PECCP,TiO₂ has been one of the most popular semiconductors due to its excellent photocatalytic properties.However,it still has some deficiencies:（1）TiO₂ is a typical wide band gap semiconductor material（about 3.2 e V）,the absorption threshold of sunlight is about 380nm,which can only use the ultraviolet light in the solar spectrum;（2）In addition,the recombination rate of photogenerated electron hole pairs produced by TiO₂ is high,which leads to low photoelectric conversion efficiency;（3）More importantly,as its conduction band potential is not sufficiently negative,TiO₂ can only provide photocathode protection for stainless steel or copper which are not susceptible to corrosion,while the protection effect for carbon steel is not significant.In this study,the PECCP performance of TiO₂ was improved by heterojunction compound modification based on the combination of the wide band gap semiconductor TiO₂ and the narrow band gap semiconductor ternary zinc indium sulfide（Zn₅In₂S₈,Zn₃In₂S₆and Zn In₂S₄）.In addition,the photocathodic protection mechanism of composite photoanode on metal corrosion resistance was preliminarily explored.The main research contents are as follows:（1）TiO₂ nanowires were first prepared on FTO conductive glass by using the hydrothermal method,and then Zn₅In₂S₈ nanosheets were synthesized on TiO₂ nanowires by using the secondary hydrothermal method,finally,the Zn₅In₂S₈/TiO₂ composite photoanode was obtained.The results of XRD and XPS show that we have successfully prepared the target compound.The SEM images showed that Zn₅In₂S₈ nanosheets were arranged in the microspheric shape,which were randomly distributed in the gaps of TiO₂nanowires.DRS results showed that the optical absorption threshold of Zn₅In₂S₈/TiO₂composite photoanode was red shifted to 481 nm,indicating that the optical absorption property of the composite has been significantly improved.In addition,PL and EIS results showed that photogenerated electron hole pairs of the composite photoanode has significantly lower recombination ratio compared to pure TiO₂ and pure Zn₅In₂S₈,which has higher photoelectric activity.The PECCP performance results showed that the protection effect of Zn₅In₂S₈/TiO₂ composite photocathode on 304 stainless steel was significantly improved,and that its photoinduced open circuit potential and photogenerated current density can reach-500 m V and 14μA/cm² respectively.However,the protection effect of Zn₅In₂S₈/TiO₂ composite photoanode on Q235 carbon steel is not obvious;（2）In order to further improve the PECCP performance of ternary zinc indium sulfide/TiO₂ composite for Q235 carbon steel,Zn₃In₂S₆/TiO₂nanocomposite photoanode were prepared by changing the ratios of three reactants:Zn SO₄·7H₂O,In Cl₃·4H₂O and CH₃CSH₂.The characterization results of microstructures and chemical composition of the prepared materials by XRD,SEM,XPS and HRTEM showed that the target sample was successfully synthesized,which had good microstructure.DRS and PL results showed that the optical absorption threshold of Zn₃In₂S₆/TiO₂ is 496 nm and the band gap is reduced to 2.45 e V.Obviously,the utilization ratio of the composite to sunlight has been significantly improved.EIS results showed that the charge transfer resistance of the composite photoanode is significantly lower compared to pure TiO₂.As a result of improvement of photoelectric conversion efficiency,the photoinduced open circuit potential and photogenerated current density of Zn₃In₂S₆/TiO₂ composite photoanode for304 stainless steel can reach-650 m V and 18μA/cm² respectively.In addition,the potential of Q235 carbon steel negatively shifted about 60 m V by Zn₃In₂S₆/TiO₂photoanode,which realized the photocathode protection for Q235 carbon steel.（3）By comparing the microstructure,chemical composition,light absorption characteristics and photoelectric conversion ability of three composite photoanodes:Zn₅In₂S₈/TiO₂,Zn₃In₂S₆/TiO₂and Zn In₂S₄/TiO₂,the relationships between their structures and their photocathode protection performance were studied.The results showed that three kinds of composite photoanodes significantly improve the photoelectric performance of pure TiO₂,improve the utilization rate of sunlight,reduce the recombination rate of electron hole pairs and enhance the photoelectric conversion ability.Therefore,the PECCP performance of the composite photoanode has been significantly improved.In fact,the improvement of these properties is due to the Z-scheme heterojunction structure formed after the modification of TiO₂ with ternary zinc indium sulfide.Compared with the traditional type-Ⅱheterojunction,the Z-scheme heterojunction electron transfer mechanism simultaneously possess the wide range light absorption,high charge separation efficiency and strong redox ability,thus enhancing the photocathode protection performance of the composite photoanode for the metal.Zn In₂S₄/TiO₂ composite photocathode has the best PECCP effect on 304 stainless steel and Q235 carbon steel in these three kinds of composite photoanodes.After coupling with304 stainless steel and Q235 carbon steel,the photoinduced open circuit potential is-800m V and-750 m V respectively,and the photogenerated current density can reach 47μA/cm² and 35μA/cm² respectively.Therefore,the modification of TiO₂ by ternary zinc indium sulfide not only enhances its PECCP performance,but also extends the range of protection metals for TiO₂ cathodic protection.Compared with type-Ⅱheterojunction,the Z-scheme heterojunction is expected be more suitable for the PECCP,which believed to be an effective strategy for the potential applications of photoelectrochemical cathodic protection.