中国科学院海洋研究所机构知识库

近年来，随着海洋强国战略和一带一路倡议的提出，我国海洋事业蓬勃发展，各种涉海工程、港口码头、跨海桥梁等建设迅猛扩张。这些服役于海洋环境的钢铁材料身处严苛的环境，无时无刻都遭受着腐蚀的破坏，造成严重的经济损失和环境破坏。作为新兴的腐蚀防护方法，光电化学阴极保护技术得到了广泛关注，它可以直接利用自然界可持续的太阳能，通过半导体材料的光电转换性能，产生光生电子并转移到被保护的金属表面，实现对金属材料的保护。TiO₂是目前应用最广泛的半导体光电材料，但其存在禁带宽度较大、光生电子-空穴对易复合的缺点；并且其在真实海洋环境中的光电化学阴极保护效果欠佳，常需要额外加入空穴捕获剂才能对金属进行保护。针对TiO₂的上述技术瓶颈以及实际应用限制，本论文主要聚焦于在海洋环境下服役的金属材料的光电化学阴极保护效果欠佳的问题，通过构筑超细化的纳米基层来降低电荷转移势垒，再通过修饰电负性较强的多元硫化物来优化能带结构，使复合光电极的准费米能级发生负移，促进电子-空穴对的分离，有利于电子向耦联金属材料的传输，最终在模拟太阳光照下，对模拟海洋环境中的不同自腐蚀电位的金属材料均实现了光电化学阴极保护效果。具体研究内容包括：

1. 通过微观形貌调控在FTO导电玻璃表面水热生长了具有不同形貌的TiO₂纳米薄膜材料。随着水热过程中溶剂DEG用量的减少，发现TiO₂纳米基层的形貌从纳米树变为超细高度支化纳米草坪(UFHBNL)，再变为纳米颗粒。同时，伴随着晶相从无定形态变为锐钛矿相，最后变为锐钛矿-金红石混相。其中，DEG用量为30 mL时得到的TiO₂ UFHBNL光电极的光电化学性能最佳。通过连续离子层吸附反应（SILAR）在其上沉积CdS纳米颗粒后，在模拟太阳光和可见光下，光电极产生的光生电流密度分别可达5.6 mA×cm^-2和3.8 mA×cm^-2。如此高效的光电化学性能得益于超细的纳米分支降低了电荷转移势垒，促进电子的快速传输；异质结体系的构建也极大的促进了光生载流子的有效分离，并显著拓宽了光响应范围。

2. 由于Cd元素具有毒性，且容易发生光腐蚀，我们继而采用绿色环保的三元硫族化合物（AB_mX_n）来复合TiO₂基层，ZnIn₂S₄是其中一种光电活性高且导带电位较负的窄带隙半导体材料，将其通过原位水热法复合到TiO₂纳米草丛基层上制得了纳米花球ZnIn₂S₄/TiO₂纳米草丛光电极，两者界面充分接触，促进了光生电子的传输。构建的三维ZnIn₂S₄/TiO₂ NFB光电极具有优异的可见光吸收性能，且光生载流子得到了有效的分离。同时，ZnIn₂S₄较负的导带电位，有利于光生电子向耦联金属的传输，进而在模拟太阳光下，实现了对多种不同自腐蚀电位的金属（如铜、E40钢、Q345和Q235碳钢）的光电化学阴极保护，在NaCl溶液中可以分别提供170、72、63和44 μA×cm^-2的光电化学阴极保护电流，展现了巨大的应用潜力。

3. 为进一步提高电子的传输性能来提高材料的光电活性，我们采用电沉积法将具有优异电子迁移率的石墨烯量子点颗粒修饰在材料表面。首先采用凝胶-溶胶法和水热法制备了具有分枝结构的TiO₂纳米线阵列（TiO₂ HNAA），然后在其上复合上rGO QDs，再负载上可见光响应的CdS颗粒。分支结构的TiO₂纳米线阵列降低了电荷转移势垒，为光生电子提供直接的快速通道；光敏剂CdS拓宽了光谱响应范围，并与TiO₂界面处构建了异质结电场，提高了光生载流子的分离和传输效率；rGO QDs可以促进光生电子-空穴的迁移和分离，进一步提高电子的传输性能，从而使构建的三元CdS/rGO QDs/TiO₂ HNAA 复合光电极展现了高效的光电化学和光电化学阴极保护性能，在模拟太阳光照射下，可分别对316L SS和Cu产生0.72 mA×cm^-2和1.45 mA×cm^-2的光电化学阴极保护电流密度，并使两种金属产生约900 mV的阴极极化。

4. 通过一步水热法在FTO表面直接生长了高能（001）晶面的TiO₂纳米片阵列，垂直有序生长的纳米片阵列为光生电子提供了良好的传输通道，高能（001）晶面可以提供更多的氧化还原活性位点，且其较小的缺陷态密度降低了光生电子-空穴在晶界处的二次复合。进而采用连续离子层吸附反应在纳米片上沉积窄带隙的三元硫族化合物AgInSe₂，拓宽了光吸收范围，促进了载流子的分离。为了优化能带结构，采用In₂Se₃进一步修饰并构建了TiO₂ NSA/AgInSe₂/In₂Se₃三相异质结体系，提高了界面处的载流子分离和传输效率。在AM 1.5光照下，光电极在3.5 wt%的NaCl溶液中使耦联的316L不锈钢产生约7 μA×cm^-2的光电化学阴极保护电流密度，同时使电位负移约236 mV。

综上所述，针对真实海水环境下TiO₂基材料的光电化学阴极保护效果欠佳的问题，我们通过超细纳米基层的优化以及多元硫族化合物的修饰等改性策略，来协同提升光生载流子的迁移和分离效率，降低电荷转移势垒，拉负能带电位，进而增强其光电化学和光电化学阴极保护性能，对海洋环境中的不同自腐蚀电位的金属材料实现了阴极保护效果，展现了巨大的实际应用潜力，有力推进了光电化学阴极保护实用化的进程。

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 TiO₂ UFHBNL exhibited the best photoelectrochemical performance. When deposite CdS nanoparticles on it, and photogenerated current density of the photoelectrode can reach 5.6 mA×cm^-2 and 3.8 mA×cm^-2 under 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. ZnIn₂S₄ 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 ZnIn₂S₄/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 ZnIn₂S₄/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 ZnIn₂S₄ 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 44 mA×cm^-2, respectively in NaCl 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 rGO QDs were composited on it, and then CdS particles were also loaded. The branched TiO₂ nanowire array reduces the charge transfer barrier and provides a direct and fast channel for photogenerated electrons; the photosensitizer CdS broadens the spectral response range, and builds a heterojunction electric field at the interface of TiO₂, which improves the separation efficiency of photogenerated carriers; rGO 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 CdS/rGO 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 mA×cm^-2 and 1.45 mA×cm^-2 for 316L SS and Cu, respectively, and also produce cathodic polarization of about 900 mV 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 AgInSe₂ 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 TiO₂ NSA/AgInSe₂/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 NaCl solution can negatively shift the potential of the coupled 316L SS by about 236 mV, resulting in a photogenerated current density of about 7 μA×cm^-2.

In summary, in view of the poor photoelectrochemical cathodic protection effect of TiO₂ materials in NaCl 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.