Study On The Optimization Of TiO₂-based Ordered Structural Photoelectrodes And Mechanism Of Improved Photoelectrochemical Cathodic Protection Performance

In recent years,with the proposition of the marine development strategy and the One Belt One Road initiative,our country’s maritime industry has developed vigorously,and the construction of various sea-related projects,ports,ocean transportation,and cross-sea bridges etc.has expanded rapidly.The steel materials served in the marine environment suffer from a harsh environment all the time,and may cause serious economic losses and environmental damage.As an emerging corrosion protection method,the photoelectrochemical cathodic protection technology has received widespread attention.It can directly use the almost endless sunlight energy in nature,and make use of the excellent photoelectric properties of semiconductor materials,to generate photo-electrons and transfer them to the coupled metal to achieve the protection of them.TiO₂ is currently the most widely used semiconductor optoelectronic material,but it has the shortcomings of large band gap energy and high recombination rate of photo-generated electron-hole pairs.Also,in the seawater environment,the photoelectrochemical cathodic protection effect is always not good,and it is often necessary to add additional hole scavenger to achieve the protection of metal.In view of the above technical bottlenecks and practical application limitations of TiO₂,this thesis mainly focuses on the problem of poor photoelectrochemical cathodic protection of metal materials serving in marine environments.Through fabricating ultra-fine nano-based substrates to reduce the charge transfer barrier,and modifying with electronegative polysulfide to optimize the band structure,so that the quasi-Fermi level of the composite photoelectrode is negatively shifted,and the separation of electron-hole pairs is promoted,which is beneficial for the electrons to transport to the coupled metal,and finally realizes the photoelectrochemical cathodic protection effect on the metal materials with different self-corrosion potentials in the simulated marine environment under the simulated sunlight.The specific research content includes:1.Through microscopic morphology control,TiO₂ nano-based materials with various morphologies were prepared on the surface of FTO conductive glass.By adjusting the amounts of solvents DEG and H₂O in the hydrothermal process,the morphology of the TiO₂ nano-substrate can change from nano-tree（NT）to ultra-fine highly branched nano-lawn（UFHBNL）,and then to nano-particle（NP）morphology.At the same time,the crystalline phase changes from amorphous to anatase phase and eventually to anatase-rutile mixed phase.Among them,the anatase phase TiO₂UFHBNL exhibited the best photoelectrochemical performance.When deposite Cd S nanoparticles on it,and photogenerated current density of the photoelectrode can reach 5.6 m A·cm^-2and 3.8 m A·cm^-2under simulated sunlight and visible light,respectively.Such high-efficiency photoelectrochemical performance benefits from the ultra-fine nano-branch which reduces the charge transfer barrier and promotes the rapid transmission of electrons;and the construction of the heterojunction system also greatly promotes the effective separation of photo-generated carriers and significantly expands the light response range.2.Due to the toxic and photo-corrosion prosperities of Cd element,the environmentally friendly ternary chalcogenides（AB_mX_n）have attracred our attention.Zn In₂S₄ is one of the narrow band gap semiconductor materials with high photoelectric activity and negative conduction band potential.The semiconductor material is compounded on the TiO₂ nanobush substrate by in-situ hydrothermal method to prepare the nano-flower-like Zn In₂S₄/TiO₂ nano-bush（NFB）photoelectrode.The interface between the two semiconductors is fully-contacted,which promotes the transport of photogenerated electrons.The constructed three-dimensional Zn In₂S₄/TiO₂ NFB photoelectrode has excellent visible light absorption performance,and the separation of photo-generated carriers is effectively promoted,resulting in high photoelectric activity.The conduction band potential of Zn In₂S₄ is relatively negative,which is conducive to the transport of photogenerated electrons to the coupled metal,thereby realizing the photoelectrochemical cathodic protection of metals with different self-corrosion potentials（such as copper,E40 steel,Q345 and Q235 carbon steel）.Under simulated sunlight,the photoelectrode can provide photoelectrochemical cathodic protection currents of 170,72,63 and 44m A·cm^-2,respectively in Na Cl solution,showing a good protection effect and emplying a huge application potential.3.In order to further improve the electron transport performance and photoelectric activity of the materials,the graphene quantum dots with super-excellent electron mobility are modified through electrodeposition onto the surface of the material.Firstly,a TiO₂ nanowire array（HNAA）with branched structure was prepared by gel-sol method and hydrothermal method,and then r GO QDs were composited on it,and then Cd S particles were also loaded.The branched TiO₂nanowire array reduces the charge transfer barrier and provides a direct and fast channel for photogenerated electrons;the photosensitizer Cd S broadens the spectral response range,and builds a heterojunction electric field at the interface of TiO₂,which improves the separation efficiency of photogenerated carriers;r GO QDs can promote the migration and separation of photogenerated electrons and holes,and further improve the transportation performance of electrons,so that the constructed ternary Cd S/r GO QDs/TiO₂ HNAA composite photoelectrode exhibits high-efficiency photoelectrochemistry and photoelectrochemical cathodic protection performance,under simulated sunlight,it can generate photoelectrochemical cathodic protection current densities of 0.72 m A·cm^-2 and 1.45 m A·cm^-2for 316L SS and Cu,respectively,and also produce cathodic polarization of about 900 m V for both two metals.4.TiO₂ nanosheet arrays with high activity（001）facets were also prepared through one-step hydrothermal method using the microscopic morphology control and facet control.The vertically ordered nanosheet arrays can provide effective channel for the transportation of photogenerated carriers.The high-exposed（001）facet nanosheets can provide more redox active sites,and its defect state density is small,which reduces the secondary recombination of photogenerated electrons and holes at the grain boundary.The SILAR method is also used to deposit Ag In Se₂ nanoparticles on the nanosheets,which broadens the light response range and promotes the separation of carriers.In order to optimize the energy band structure,the TiO₂NSA/Ag In Se₂/In₂Se₃ three-phase heterojunction system was constructed using the In₂Se₃ composite layer to further improve the carrier transmission efficiency at the interface.Under AM 1.5 light illumination,the photoelectrode in Na Cl solution can negatively shift the potential of the coupled 316L SS by about 236 m V,resulting in a photogenerated current density of about 7μA·cm^-2.In summary,in view of the poor photoelectrochemical cathodic protection effect of TiO₂ materials in Na Cl neutral solution,the optimization of ultra-fine nano-based substrates and the modification of polysulfide were employed to improve the migration and separation efficiency the photo-generated carriers,to reduce the charge transfer barrier,to pull the energy charge potential negatively,and to enhance its photoelectrochemical and photoelectrochemical cathodic protection performance,and eventually to realize the cathodic protection effect for metal materials with different self-corrosion potentials in the marine environment,demonstrating huge practical application potential and vigorously promoting the practical process of photoelectrochemical cathodic protection.