Fabrication Of TiO₂ Heterojunction Materials And Properties Of Photocathodic Protection Performance

The countries all around the world are making great efforts to develop the oceanographic engineering,such as marine equipment and marine infrastructure.But the ocean is a harsh corrosive environment,the metals are subjected to rapid corrosion at all times in the marine environment.As a common metal anti-corrosion technology in marine environment,the traditional cathodic protection technology has gradually exposed the inevitable deficiency,include impressed current cathodic protection heavily dependent on energy sources,sacrificial anode cathodic protection releasing metal ions into the environment during its use.The distinguishing feature of photogenerated cathodic protection is no energy consumption and no release of metal ions to the environment.Photogenerated cathodic protection is a new anti-corrosion technology which combines semiconductor photoelectric conversion mechanism and cathodic protection mechanism.As a very ideal photoelectric conversion material,TiO₂is the most studied semiconductor materials in photogenerated cathodic protection.But after decades of research,there are still many key deficiency that need to be overcome.TiO₂ can only respond to ultraviolet light owe to the influence of the wider band gap,which seriously limits its utilizing visible light.The result is difficult for pure TiO₂ to cathodically cathodic polarize metal and achieve adequate cathodic protection under natural illumination.The pure TiO₂ is unable to provide a good photocathodic protection effect for metals in the dark due to the photogenerated carriers recombination in pure TiO₂.It is an effective method to construct heterojunction electric field by combining narrow bandgap semiconductor with pure TiO₂.Narrow bandgap semiconductors can effectively change the energy band of pure TiO₂ and change the position of Fermi energy level,which can extend the visible light response range of pure TiO₂.The results is to restrain the recombination of photogenerated carriers,to promote the separation of photogenerated electron-hole pairs and to improve the effect of photocathodic protection.In this paper,narrow band gap semiconductors such as Bi₂Se₃,Bi₂Te₃@Bi₂O₃,MgIn₂S₄/In₂S₃ and MgIn₂S₄/In₂S₃ were combined with pure TiO₂ photoanode and result in the changing of the energy level and fermi energy level of TiO₂.The photocathodic protection of metals was realized in 3.5 wt%NaCl.The details are as follows:?1?As photoanode,the TiO₂ nanotube array film was successfully fabricated on the surface of titanium by one-step anodic oxidation?20 V,1 h?in an ethylene glycol solution containing NH₄F,H₂O and?CH₂OH?₂.?2?Bi₂Se₃ nanoflowers were successfully obtained on the surface of TiO₂photoanode by chemical bath deposition at 80°C.The narrow bandgap Bi₂Se₃ can effectively adjust the forbidden band width of the TiO₂ photoanode and increase the response range of the TiO₂ photoanode to visible light.The Bi₂Se₃ content on the surface of the TiO₂ photoanode was adjusted by optimizing the Bi³⁺ion concentration.Under visible light,the Bi₂Se₃/TiO₂ photoanode can negatively shift the electrode potential of 304 stainless steel soaked in 3.5 wt%NaCl aqueous solution to-996 mV,which has a good photocatalytic protection effect.In the dark state,the electrode potential of 304 stainless steel can still be maintained at about-400 mV,which still has a certain cathodic protection effect.?3?By controlling the oxygen content in the electrodeposition solution,the drying temperature and time of the sample,a Bi₂Te₃@Bi₂O₃ nanoflower was obtained on the surface of the TiO₂ photoanode by a simple potentiostatic deposition method.Bi₂Te₃acts as a core of nanoflowers,which plays a good role in conduction,light absorption and surface plasmon.Bi₂O₃ acts as a heterojunction electric field between the shell of nanoflowers and TiO₂ photoanode,inhibits the recombination of photogenerated carriers,and promotes photogenerated electron-hole pairs separation.Q235 carbon steel is prone to uniform corrosion in simulated seawater environment,and obvious corrosion products appear when immersed for 1h.Thanks to the excellent photoelectric conversion performance of Bi₂Te₃@Bi₂O₃/TiO₂ ternary heterojunction system,the cathodic protection potential of Bi₂Te₃@Bi₂O₃/TiO₂ photoanode coupled with Q235carbon steel immersed in simulated seawater reaches-976 mV continuously.The cathodic protection potential after 25 hours of illumination remained at-900mV,showing good photochemical stability,and there was still no obvious corrosion product on the surface of Q235 carbon steel,showing good photocatalytic protection effect.?4?The MgIn₂S₄/In₂S₃ microspheres were obtained on the surface of TiO₂photoanode by one-step hydrothermal method.The obtained MgIn₂S₄/In₂S₃/TiO₂photoanodes have the characteristics of uniform material distribution,order and structural stability.The narrow bandgap nano-MgIn₂S₄ and nano-In₂S₃ jointly adjusted the bandgap width and Fermi level of the TiO₂ photoanode,and successfully extended the photoresponse range of the TiO₂ photoanode to the visible region.The composition of MgIn₂S₄/In₂S₃/TiO₂ ternary heterojunction system greatly improves the separation efficiency of photogenerated electron-hole pairs and promotes the spontaneous transfer of photogenerated electrons to the surface of 316L stainless steel.The stable photocurrent density of MgIn₂S₄/In₂S₃/TiO₂ photoanode is about 50?A/cm² under visible light,and the cathodic protection potential of 316L stainless steel is-790 mV,which shows good photocatalytic protection performance.?5?By adjusting the pH of the hydrothermal reaction solution,CaIn₂S₄/In₂S₃micro/nano particles were successfully obtained on the surface of TiO₂ photoanode,and uniformly distributed on the surface of TiO₂ photoanode to obtain a structurally stable CaIn₂S₄/In₂S₃/TiO₂ photoanode.nano-CaIn₂S₄ and nano-In₂S₃ jointly adjust the bandgap width and Fermi level of TiO₂ photoanode,and the ternary heterojunction system greatly enhances the separation efficiency of photogenerated carriers.At the same time,the effect of different hole trapping agents on photocatalytic cathodic protection was studied.It was found that the reduction performance of hole trapping agent has a great influence on the OCP value and IT value of photo-anode coupled 316L stainless steel under visible light.The reduction type of hole trapping agent The stronger the OCP and IT values change.When Na₂SO₃ is used as a hole trapping agent,the stable photocurrent density of CaIn₂S₄/In₂S₃/TiO₂ photoanode is about 40?A/cm² under visible light,and the cathodic protection potential of 316L stainless steel is-770 mV.When 3.5 wt%NaCl was selected as the hole trapping agent,the stable photocurrent density was about 30?A/cm² and the OCP value was-553 mV.Still showing good photo-generated cathodic protection.In summary,the modification of TiO₂ photoanode by four narrow bandgap semiconductor composites fore-mentioned is a very effective method.The combination with narrow bandgap semiconductor helps to adjust the bandgap width of TiO₂photoanode and enhance its response range to visible light.By constructing a two or three heterojunction system,it helps to suppress the recombination of photogenerated carriers and promote the separation of photogenerated electron-hole pairs.By adjusting the Fermi level of the TiO₂ photoanode,the photogenerated electrons spontaneously transfer and enrich to the protected metal,and a more photo-induced cathodic protection effect is obtained.Through the above exploration,not only can the photo-cathode protection performance of the TiO₂ photoanode be improved,but also the method and the internal mechanism for improving the photo-cathode protection effect can be further understood.In particular,a heterojunction is constructed by a narrow bandgap semiconductor to adjust the forbidden band width and the Fermi level position of the TiO₂ photoanode.It will provide reference and reference for the later application of TiO₂ photoanode modification in photocathode protection.