海洋环境腐蚀与生物污损重点实验室知识库

海洋蕴藏着丰富的自然资源，世界各国都在大力发展海洋事业，新增大量海洋开发设备和海洋基础设施等。而海洋却是一种苛刻的腐蚀环境，处于海洋环境中的金属时刻遭受着快速腐蚀的破坏。作为一种海洋环境中常用的金属防腐蚀技术，传统阴极保护技术逐渐暴露出自身的缺点，外加电流阴极保护对能源供给有严重的依赖性，牺牲阳极阴极保护在使用过程中不断向环境释放各种金属离子。光生阴极保护的优点在于既不消耗能源，也不向环境释放金属离子，它是综合了半导体光电转换特性和阴极保护机理的新型防腐蚀技术。纳米二氧化钛（TiO₂）是一种非常理想的光电转换材料，一直是光生阴极保护中研究最多的半导体材料，然而经过数十年的努力研究，却仍然存在许多关键缺陷需要克服。TiO₂的宽禁带严重限制了它对可见光的响应能力，导致其在自然光照下不能为金属提供足够的光生阴极保护效果。另外，纯TiO₂中的光生载流子极易复合，使其无法在暗态下为金属提供很好的光生阴极保护效果。通过窄禁带半导体与纯TiO₂复合，构建异质结电场是解决上述缺点的有效方法之一。窄禁带半导体能够有效调整TiO₂的能带结构，改变费米能级的位置，因此对拓宽TiO₂的可见光响应范围，抑制光生载流子复合，促进光生电子-空穴对分离，提升光生阴极保护效果大有益处。本文通过硒化铋（Bi₂Se₃），碲化铋@氧化铋（Bi₂Te₃@Bi₂O₃），硫化铟镁/硫化铟（MgIn₂S₄/In₂S₃）和硫化铟钙/硫化铟（CaIn₂S₄/In₂S₃）四种窄禁带半导体与纯TiO₂光阳极复合，有效调整了TiO₂的能级结构，拉升了费米能级，实现了在3.5 wt% 氯化钠（NaCl）溶液中对耦连金属的光生阴极保护作用。具体内容如下：

（1）以氟化铵、水和乙二醇的混合液为电解液，钛片为阳极，铂片为阴极，采用一步阳极氧化法（20 V，1 h）成功在钛片表面构建出TiO₂纳米管阵列膜，并以此作为TiO₂光阳极。

（2）采用化学浴沉积法（80℃）成功在TiO₂光阳极表面得到Bi₂Se₃纳米花，其中窄禁带Bi₂Se₃能够有效调整TiO₂的禁带宽度，拓宽TiO₂对可见光的响应范围，并通过优化Bi³⁺离子浓度调整TiO₂表面Bi₂Se₃的负载量。可见光照射下，Bi₂Se₃/TiO₂光阳极可以将浸泡在3.5 wt% NaCl溶液中的304不锈钢的电极电位负移至-996 mV，表现出很好的光生阴极保护效果。暗态下依然可以将304不锈钢的电极电位维持在-400 mV左右，仍具有一定的光生阴极保护效果。

（3）采用一步恒电位沉积法在TiO₂光阳极表面制备Bi₂Te₃@Bi₂O₃纳米花，其中通过调整沉积液中的氧含量、样品烘干温度和时间来促进Bi₂Te₃表面形成Bi₂O₃。Bi₂Te₃作为纳米花的核起到导电、光吸收和表面等离子作用，Bi₂O₃作为纳米花的壳与TiO₂构成异质结电场，抑制光生载流子复合，促进光生电子-空穴对分离。由于Bi₂Te₃@Bi₂O₃/TiO₂三元异质结体系具备优异的光电转换性能，可见光下Bi₂Te₃@Bi₂O₃/TiO₂与浸泡在3.5 wt% NaCl溶液中的Q235碳钢耦连后阴极保护电位达到-976 mV，连续光照25 h后的阴极保护电位依然保持在-900 mV左右，表现出较好的光化学稳定性，并且Q235碳钢表面未出现腐蚀产物，表现出很好的光生阴极保护效果。

（4）通过一步水热法在TiO₂光阳极表面制备MgIn₂S₄/In₂S₃微米花，获得的MgIn₂S₄/In₂S₃/TiO₂光阳极具有物质分布均匀、结构稳定有序的特点。窄禁带MgIn₂S₄和In₂S₃共同调整了TiO₂光阳极的禁带宽度和费米能级，成功将TiO₂的光响应范围拓宽到可见光区。构成的MgIn₂S₄/In₂S₃/TiO₂三元异质结体系大幅提升了光生电子-空穴对的分离效率，同时促进了光生电子自发向316L不锈钢表面转移。可见光下MgIn₂S₄/In₂S₃/TiO₂的稳定光电流密度约50 μA/cm²，对316L不锈钢的阴极保护电位达到-790 mV，表现出良好的光生阴极保护效果。

（5）通过调整水热反应液的pH，成功在TiO₂光阳极表面得到均匀有序分布的CaIn₂S₄/In₂S₃纳米颗粒，获得结构稳定的CaIn₂S₄/In₂S₃/TiO₂光阳极。CaIn₂S₄和In₂S₃共同调整了TiO₂的禁带宽度和费米能级，构成的CaIn₂S₄/In₂S₃/TiO₂三元异质结体系大幅提升了光生载流子的分离效率。同时研究了不同空穴捕获剂对光生阴极保护性能的影响，发现空穴捕获剂的还原性强弱对可见光下光阳极耦连316L不锈钢的开路电位值和光电流密度值有较大影响，其中空穴捕获剂的还原型越强，开路电位值越负、光电流密度值越大。在选用Na₂SO₃作为空穴捕获剂时，可见光下CaIn₂S₄/In₂S₃/TiO₂的稳定光电流密度值约40 μA/cm²，对316L不锈钢的阴极保护电位达到-770 mV。在选用3.5 wt% NaCl溶液作为空穴捕获剂时，稳定光电流密度约30 μA/cm²，开路电位值约-550 mV。仍表现出较好的光生阴极保护效果。

综上所述，采用四种窄带隙半导体复合改性TiO₂光阳极是一种非常有效的方法，均有助于调整TiO₂的禁带宽度，拓宽其在可见光区的响应范围。通过构建二元或三元异质结体系，形成的异质结电场有效抑制了光生载流子复合，促进了光生电子-空穴对分离。通过拉升TiO₂的费米能级，促进了光生电子自发向被保护金属转移并富集，获得了更佳的光生阴极保护效果。通过上述探索，不仅能够提升TiO₂的光生阴极保护性能，而且让本人对提升光生阴极保护效果的方法和内在机理有了更深入的了解，特别是通过窄禁带半导体构建异质结来调整TiO₂的禁带宽度和费米能级。为今后在光生阴极保护方面对TiO₂进行改性提供参考和借鉴。

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, TiO₂ is 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 TiO₂ photoanode 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. Bi₂Te₃ acts 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 Q235 carbon 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 TiO₂ photoanode by one-step hydrothermal method. The obtained MgIn₂S₄/In₂S₃/TiO₂ photoanodes 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 μA/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/In₂S₃ micro/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 μA/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 μA/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 TiO₂ photoanode 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.