key: cord-0047363-xftck0gl
authors: Wang, Xiutong; Xu, Hui; Nan, Youbo; Sun, Xin; Duan, Jizhou; Huang, Yanliang; Hou, Baorong
title: Research progress of TiO(2) photocathodic protection to metals in marine environment
date: 2020-07-10
journal: J Oceanol Limnol
DOI: 10.1007/s00343-020-0110-x
sha: f33c3023b71cba16ba82317c99378ab8034334ef
doc_id: 47363
cord_uid: xftck0gl

Corrosion protection has become an important issue as the amount of infrastructure construction in marine environment increased. Photocathodic protection is a promising method to reduce the corrosion of metals, and titanium dioxide (TiO(2)) is the most widely used photoanode. This review summarizes the progress in TiO(2) photogenerated protection in recent years. Different types of semiconductors, including sulfides, metals, metal oxides, polymers, and other materials, are used to design and modify TiO(2). The strategy to dramatically improve the efficiency of photoactivity is proposed, and the mechanism is investigated in detail. Characterization methods are also introduced, including morphology testing, light absorption, photoelectrochemistry, and protected metal observation. This review aims to provide a comprehensive overview of TiO(2) development and guide photocathodic protection.

As need for the marine economy increases, an increasing number of structures, including off shore platforms, marine pipelines, and marine wharfs, have been built with large quantities of steel and other metals. Steel has been the most popular construction material because of its low cost, high strength and good machinability. In marine environments, the corrosion of steel is very severe because of the presence of oxygen, sunshine, microorganisms, Cland other ions.

In a survey project led by Hou from the Institute of Oceanology, Chinese Academy of Sciences (IOCAS), it was found that the cost of corrosion in China was approximately 310 billion USD in 2014, equivalent to 3.34% of the GDP (Hou et al., 2017; Hou, 2019) . In the shipbuilding industry, approximately 9% of income is spent on corrosion protection and repair. However, many factors can play roles in the corrosion of metals in seawater, such as temperature, dissolved oxygen, salinity, and microorganisms, which cause deterioration and failure of metal materials with long exposure times (Melchers, 2003) . Among these factors, the eff ect of Clis especially remarkable; even metals with high corrosion-resistance performance, such as stainless steel, can be aff ected by the high concentration of Cl -, which induces pitting-type corrosion (Tsutsumi et al., 2006 (Tsutsumi et al., , 2007 .

To reduce the corrosion of metals in marine environments, many methods have been proposed, such as coating, corrosion inhibitors and cathodic protection (CP). Among these methods, CP has been widely used for metals with high effi ciency, and it has * Supported by the CAS Strategic Priority Project (No. XDA13040404), the National Natural Science Foundation of China for Exploring Key Scientifi c Instrument (No. 41827805) , and the Shandong Key Laboratory of Corrosion Science a long research history of almost 200 years since its fi rst application in the protection of copper ship sheeting (Davy, 1824) .

Corrosion is a process in which electrons fl ow from the metal, and CP technology is a method to decrease the metal potential; thus, redox reactions on metal surfaces are inhibited, and the corrosion rate is reduced. For traditional CP, two main methods have been designed for the marine industry. The fi rst is sacrifi cial anode cathodic protection (SACP), which uses an active metal with a negative corrosion potential such as magnesium, zinc and aluminum to provide electrons, and the anodes are consumed during the protection. The second method is impressed current CP (ICCP), which uses a rectifi er or potentiostat as an external power source to provide current to keep the metal in the proper potential range. For an SACP system, the quantity of sacrifi cial anodes depends on the dimensions and service life of the structure; however, the production of the anodes requires a considerable amount of energy consumption, and the anodes dissolve in seawater and thus cause pollution and harm to the marine ecology. For the ICCP system, stable power supplies and complicated maintenance devices are needed, which would be diffi cult in some off shore conditions, and large amounts of energy are also necessary during the duty life of the structure.

In recent years, photocathodic protection has been proposed as a new type of technology that utilizes green and sustainable solar energy to provide the current to protect metals from corrosion. A semiconductor can utilize light and produce electrons that are transferred to the metal to reduce the corrosion rate. Photocathodic protection is similar to sacrifi cial anode CP (Park et al., 2001) , and the mechanism is illustrated in Fig.1 . Yuan and Tsujikawa (1995) investigated the photoeff ect of TiO 2 coatings on copper substrates under ultraviolet illumination for the fi rst time. In the last three decades, a considerable amount of eff ort has been devoted to photocathodic protection for metals by using many types of semiconductors, such as TiO 2 , SrTiO 3 , g-C 3 N 4 , In 2 O 3 and ZnO (Yuan et al., 1994; Huang et al., 2000; Ohko et al., 2001; Liu et al., 2007; Bu et al., 2011 Bu et al., , 2013 Li et al., 2014 Li et al., , 2015b Sun et al., 2014 Zhang et al., 2015a; Yang and Cheng, 2018) . Among these materials, TiO 2 is the most widely used material and serves as a photoanode due to its low cost, high stability, nontoxicity and easy availability.

Since Fujishima and Honda (1972) , established a method of water photolysis in which TiO 2 is subjected to light irradiation, the photocatalytic performance of TiO 2 and other semiconductors has been investigated in many fi elds, such as wastewater treatment (Nagaveni et al., 2004) , sterilization (Matsunaga et al., 1988) , dye-sensitized solar cells (O'Regan and Grätzel, 1991) , self-cleaning (Wang et al., 1998; Balaur et al., 2005) , and biomedical fi elds (Oh et al., 2005) . When the energy of the irradiation light is higher than the bandgap (Eg), there are three steps in the photocatalysis: fi rst, the electrons and holes separate, where the electrons are excited from the valence band (VB) to the conduction band (CB) and the holes remain in the VB; second, the excited carriers, including electrons and holes, migrate to the surface; third, the carriers react with electron donors (D) and electron acceptors (A) . This process is shown in Fig.2 . The carrier transmission process under light irradiation includes the generation and transportation of electrons and holes.

However, TiO 2 , with a relatively wide bandgap (approximately 3.2 eV), can absorb only ultraviolet light, which makes up approximately 5% of sunlight. On the other hand, during migration, recombination occurs on the surface and in the bulk. These two drawbacks have limited the applications of TiO 2 in photocatalysis and energy harvesting (Cai et al., 2019) .

There are three types of TiO 2 often used in photoelectrochemistry: rutile, anatase, and brookite. Diff erent TiO 2 types can aff ect the charge transfer and bandgap levels and rutile and anatase are most widely used. Many methods have been used to increase the visible light response and reduce the recombination of TiO 2 , such as energy band engineering, morphology control, nanoassembly, electronic structure calculations and molecular dynamics simulations (Linsebigler et al., 1995; Tong et al., 2012) . Among these methods, material doping is the most commonly used, and it includes metal doping, such as V, Cr, Mn, Fe and Ni (Paramasivam et al., 2012) ; nonmetal doping, such as N, C, S, B, P and F (Fujishima et al., 2008) ; metal oxide doping, such as niobates, tantalates, vanadates and germanates (Tong et al., 2012) ; and narrow bandgap semiconductor coupling . In addition to the strong catalytic property and high charge mobility, cost and stability are also important parameters of the new type of material for the photoanode (Kudo and Miseki, 2009) .

Structures with diff erent dimensionalities (0D, 1D, 2D, 3D) and diff erent energy facets (001, 101) can exhibit special properties (Li and Wu, 2015) , as proposed in Fig.3 . The appropriate nanostructure has high solar energy conversion and results in better photocatalytic activities. Nanoparticles, as 0D structures, usually have a large surface area and are used as photocatalysts in the suspended state, and the small particle size can increase the number of active sites. 1D structures such as nanotubes, nanobelts and nanowires mostly have large length-diameter ratios, which can enhance charge transport along the longitudinal direction. 2D nanomaterials such as nanosheets and fi lms can reduce the distance between photoexcited carriers and increase photon absorption due to the large area. 3D nanostructures can adopt unique arrangements using precise and self-assembly strategies, and specifi c performance can be obtained.

However, in addition to the dimensions of the nanostructure, the facet design is also important; three-dimensional hierarchical spheres of anatase TiO 2 with exposed TiO 2 (001) facets and single-crystal nanosheets have superior photoactivity (Yang et al., 2009; Chen et al., 2010) , as illustrated in Fig.4 .

In this review, we summarize the recent progress in the application of TiO 2 in the fi eld of photocathodic protection. The modifi cation and mechanism are discussed, and characterization and analysis techniques are also introduced.

TiO 2 MODIFICATION AND APPLICATION TiO 2 tends to be modifi ed by semiconductors in the following three ways: increasing the separation effi ciency of photogenerated electron-hole pairs, decreasing the conduction band potential of the semiconductor and changing the redox potential of the electrolyte. A suffi ciently negative CB potential is required for photoelectrochemical CP. Hitherto, based on these strategies, researchers have synthesized TiO 2 nanotube photoanodes and modifi ed them by increasing the surface area, doping, sensitizing quantum dots, and combining diff erent semiconductors to extend their functionalities in the visible range. Coupling with narrow-bandgap semiconductors, metals and nonmetals are used for photocathodic protection, and the protected metal tends to be polarized into a safe range under visible light (Li et al., 2017a; Bu et al., 2018) .

There are several methods to synthesize TiO 2 , such as the hydrothermal process, the sol-gel method, liquid phase deposition, and anodization. During the hydrothermal process of TiO 2 , pH, and temperature are the key factors determining morphology, and diff erent materials lead to distinct properties. employed a TiCl 4 solution as the Ti source with a pH of 5-6 at 180°C and obtained TiO 2 nanoparticles. Zhang et al. (2010) selected titanium foil with a porous TiO 2 fi lm as the source of Ti to obtain a three-dimensional TiO 2 NT network under hydrothermal conditions, and the structure could be useful in photocatalysis and other fi elds. Lan et al. (2018) assembled a uniform and ordered TiO 2 nanosheet by the hydrothermal-induced solventconfi ned method, and the assembly is illustrated in Fig.5 .

Through the sol-gel method, TiO 2 fi lm can be synthesized by aging of a TiO 2 sol liquid, coating this liquid on the metal surface and then heating. Shen et al. (2005) used tetra-n-butyl titanate and ethyl acetoacetate as raw materials to synthesize a TiO 2 sol and then distributed the sol on the steel substrate using the dip-coating method. The coatings showed good corrosion resistance. In the sol-gel method, it is easy to control the coating thickness; however, the fi lms usually crack during heating. To avoid this, Shen et al. (2005) soaked the TiO 2 fi lm in boiling water for 10 min, and the cracks decreased with less heating time. Zhu et al. (2010) also developed a combined sol-gel and hydrothermal method and prepared a 3D titanium nanowire network. Liquidphase deposition is a useful method in which a TiO 2 fi lm can be prepared on an ITO substrate surface, and the substrate is placed vertically at a low temperature (less than 100°C). Lei et al. (2012) used (NH 4 ) 2 TiF 6 and H 3 BO 3 as precursor bath solutions. After drying naturally and annealing in air, TiO 2 is obtained. Anodization is an eff ective method to prepare nanotubes, and it has been widely used in the preparation of TiO 2 . In general, titanium foil materials need to be cleaned in polishing solutions containing acetone, ethanol, and deionized water. During anodization, titanium foil and platinum sheets are connected to the positive and negative poles of the DC power supply, respectively. The structure of the nanotubes depends on the electrolyte, temperature, potential, etc. Recently, two-step anodization has been proposed and applied, including two processes with diff erent applied powers and times. The fi rst step is to obtain a glossy surface that could provide a better growth environment for the nanotubes in the next anodization step. Finally, annealing is used to increase the crystallinity, and the time and temperature can vary based on the proportion of rutile and anatase TiO 2 (Roy et al., 2011) .

As mentioned above, modifi cation is the most widely used method to improve the photocatalytic performance of TiO 2 , and many materials are used to modify and dope TiO 2 . Among these materials, sulfi de compounds are very popular because of their superior performance.

CdS is a type II-VI semiconductor with a bandgap of 2.4 eV, and its photoelectron transmission capacity is preeminent. Li et al. (2011) coated CdS nanoparticles onto TiO 2 nanotube arrays, and the composite could protect the metals under UV and visible light. However, CdS is sensitive to photocorrosion, and Boonserm et al. (2017) investigated the reaction during the measurement and found that corrosion of the nanocomposite fi lm decreased the photocurrent. However, it is diffi cult to simultaneously achieve both high electron transmission effi ciency and superior redox capacity.

ZnS has some properties similar to those of CdS, such as nontoxicity and chemically stable performance. It has been reported that a ZnS coating can protect CdS and CdTe from photocorrosion Zhang et al., 2015a) . Construction of Z-scheme photocatalytic systems consisting of two semiconductors as photoexcitation systems, with the constructor located at the interface to reduce the contact resistance, is increasing in popularity. Zhu et al. (2019) employed CdS to modify TiO 2 nanowires, prepared a ZnS shell to protect CdS from photocorrosion and added Ag as an electron transporter between CdS and TiO 2 . Upon coupling with the composite, the potential of 403SS decreased to -1 174 mV (vs. SCE), and the photocurrent reached 19.97 mA/cm 2 . The absorption edge was nearly 700 nm, and the visible absorption range increased because the energy of the incident ray was absorbed by the noble metal. Moreover, surface plasmon resonance might be triggered. CdTe (Eg=1.47 eV) is an ideal photoelectric conversion material that has a very high optical absorption coeffi cient (>104/cm ) and good overlap with the solar spectrum (Li et al., 2015a) . The light absorption range of the CdTe/TiO 2 composite fi lm can be expanded to the visible region by adjusting the size of the quantum dots, which can capture electrons and generate multiple electron-hole pairs (e --h + ) under illumination. The photoelectric conversion effi ciency was very high after CdTe quantum dots were used to modify TiO 2 . However, the stability of CdTe is the main problem hindering practical application. To improve the stability of CdTe QDs, ZnS shells were deposited on CdTe/TiO 2 composite fi lms. The results showed that the ZnS/ CdTe/TiO 2 composite fi lm had excellent photoelectrochemical properties , and the mechanism is illustrated in Fig.6 . Sb 2 S 3 is also used to modify TiO 2 because of its narrow bandgap. Some experiments have been performed, and the results show that the bandgaps of Sb 2 S 3 /Sb 2 O 3 /TiO 2 and Sb 2 O 3 /TiO 2 are approximately 2.02 eV and 3.32 eV, respectively, revealing that Sb 2 S 3 reduces the bandgap. It is also known that Sb 2 S 3 is a p-type semiconductor, and when it combines with an n-type semiconductor, for example, TiO 2 and Sb 2 O 3 , the Fermi level of diff erent materials fl attens; at the same time, a built-in electric fi eld forms, which can drive electron transfer (Li et al., 2018b) . Similarly, the bandgap of Ni 3 S 2 is 2.5 eV, making this material also appropriate to modify TiO 2 . Nan et al. (2019) found that the photocurrent increased to 53.1 μA/cm 2 , which is higher than that of pure TiO 2 . In a double electrolytic cell working with a three-electrode system coupled with Ni 3 S 2 , the potential of 304SS decreased to a minimum of -720 mV (vs. SCE) with 0.1 mol/L Na 2 S and 0.2 mol/L NaOH as the hole scavenger solution and 3.5 wt.% NaCl as the corrosive solution, simulating a marine environment. Compared with TiO 2 , the composite has stronger absorption in the visible region. In addition, the UV-Vis spectroscopy showed that the absorption is redshifted, indicating that the forbidden band energy decreases; the mechanism is shown in Fig.7 .

The direct bandgap of SnS is approximately 1.4 eV, and the optical absorption coeffi cient α is larger than 10 4 /cm. SnS is nontoxic, inexpensive and environmentally compatible. Sn is one of the most promising materials because of its good photoconductivity and nonlinear optical response. SnS has been proven to have good photocatalytic performance in the application of photocatalysis. It has been reported that SnS can be used as a sensitizer of TiO 2 . Therefore, it is of great signifi cance to apply SnS/TiO 2 composites to photocathode protection (Shao et al., 2018) .

The CB of Ag 2 S (0.3 eV) is higher than that of TiO 2 (0.1 eV), and the VB of Ag 2 S (0.7 eV) is lower than that of TiO 2 (3.1 eV). Therefore, nontoxic Ag 2 S cosensitized with TiO 2 can improve the separation of photogenerated electron-hole pairs and accelerate the transfer of carriers. The Ag 2 S/TiO 2 composite has higher visible light utilization than pure TiO 2 . Under visible light, the photochemical properties of the Ag 2 S/TiO 2 composite are better than those of pure TiO 2 , so the composite can be used to protect stainless steel (Ning et al., 2017; Yang et al., 2019) . Bi 2 S 3 is an attractive material due to its narrow bandgap (Eg=1.3 eV) and high photoelectric conversion effi ciency. It can absorb almost all visible light from the solar spectrum. There is a strong interface electron fi eld between Bi 2 S 3 nanoparticles and TiO 2 nanotube arrays. The electric fi eld increases the separation of photogenerated carriers and then reinforces the photoelectrochemical properties (Hu et al., 2017; Guan et al., 2018b) . The narrow bandgap doping mechanism is shown in Fig.8 (Li et al., 2017a; Yang et al., 2019) . MnS has excellent photoelectric performance and is mainly used as a buff er material, photoelectric device and magnetic component of many important diluted magnetic semiconductors. Although MnS has a wide bandgap (3.7 eV), as a p-type semiconductor, it can form a p-n heterojunction with TiO 2 (n-type semiconductor). The inherent fi eld in the p-n heterojunction can reduce the recombination of photogenerated electrons and holes, which is benefi cial to the photoelectric properties of materials (Ge et al., 2015) .

ZnIn 2 S 4 is a ternary sulfur compound with a narrow bandgap of 2.34-2.48 eV, which is signifi cantly narrower than that of TiO 2 . It is widely used as a visible light-responsive photocatalyst. In addition, the potential at the bottom of the CB of ZnIn 2 S 4 is -0.74 eV, and the potential at the top of VB is more positive than the oxygen-producing potential. Therefore, ZnIn 2 S 4 is a promising photoanode material in photoelectrochemistry. As an n-type semiconductor, it can form a heterojunction electric fi eld at the interface when it is combined with TiO 2 . It is obvious that ZnIn 2 S 4 can promote the separation of electrons and holes, and the TiO 2 /ZnIn 2 S 4 composite has enhanced charge transfer and photocatalytic degradation properties . AgInS 2 is a nontoxic and environmentally friendly visible photosensitizer, and it is an ideal photoelectric conversion material to replace toxic cadmium sulfi de. It has potential for application in the fi eld of photoelectrochemical conversion and photocatalysis. The bandgap of AgInS 2 is between 1.87 and 2.03 eV, and the CB and VB of AgInS 2 are 1.08 eV and 0.83 eV, respectively, with unique absorption in the visible and near-infrared regions. Sensitization of the ordered TiO 2 NTs by AgInS 2 QDs can signifi cantly improve the PEC conversion effi ciency in the visible region, and the ternary structure of AgIn x S y -sensitized TiO 2 also enhances visible light photocatalytic activity (Sun et al., 2018a) .

Selenide and telluride are similar to sulfi de, as Se and Te belong to the VIA family, and these compounds manifest semiconductor properties and have already been applied in the fi eld of photoelectrochemistry. For example, ZnSe has attracted great interest due to its small bandgap (2.7 eV) and outstanding photoelectric properties . NiSe 2doped TiO 2 NTs broaden the response range of visible light, and CdSe has a bandgap of 1.6-1.8 eV and can thus absorb visible light Wang et al., 2016a) . It has been reported that Bi 2 Se 3 is an ideal n-type semiconductor with a bandgap of 0.35 eV, and it is a distinctive topological insulator with a conductive surface and dielectric body structure. Hence, the absorption coeffi cient of Bi 2 Se 3 is high in the visible and near-infrared regions . CdTe is also used in the protection of metals, and Wang (Yang et al., 2016b) coated CdTe on TiO 2 nanotubes using the potentiodynamic deposition method. The potential of stainless steel upon coupling decreased to -850 mV (vs. SCE), and the composite promoted the absorption of visible light. 

To improve the photoelectrochemical response, Au, Ag and Ni were doped into TiO 2 to prepare widelight-response composite materials. However, the doping of noble metals may introduce additional defects into the lattice structure of the TiO 2 substrate, increase the recombination rate of photogenerated carriers, and lead to the loss of photoelectrochemical activity. Therefore, an appropriate element content needs to be maintained.

Ag is one of the most important noble metals used to decorate TiO 2 nanotubes. The surface plasmon resonance (SPR) eff ect of Ag nanoparticles on TiO 2 can prolong the visible light response and improve the absorption capacity of TiO 2 . SPR occurs when the oscillation frequency of the incident electromagnetic fi eld matches that of the free electron under visible light (Ma et al., 2020) . The energy of the incident ray is absorbed, and the absorption of visible light extends due to the proper distribution of the Ag nanoparticles , as shown in Fig.9 . In addition, the high Schottky barrier between Ag nanoparticles and TiO 2 materials can prevent the recombination of photogenerated electrons and holes, thus promoting the electron transfer process. Therefore, the combination of Ag nanoparticles on TiO 2 NTs is a promising way to improve the corrosion protection performance of 304SS under visible light (Li et al., 2014) . Another noble metal, Au, is also exploited in the same fi eld. There have been some reports about Au and TiO 2 composites. Zhu et al. (2013) proposed that the Au/TiO 2 compound with ZnS could prevent electrons from fl owing from the substrate/Au surface into the Au/solution owing to the potential barrier, as the conduction band (1.85 eV vs. NHE) is higher than the Fermi level of Ag (+0.5 eV vs. NHE).

Research on Fe doping has been carried out as well; iron can enter the lattice of TiO 2 , thus destroying the integrity of TiO 2 and generating defect dots, which plays an important role in provoking the separation of photoelectrons and holes in promoting photocatalytic activity (Li et al., 2007) . It has been found that iron ions can improve the photocatalytic activity of TiO 2 , and TiO 2 doped with iron can be used as an energy storage agent, which can maintain corrosion resistance for a long time in the dark Momeni et al., 2018) .

When Ni is substituted for Ti atoms, oxygen vacancies are produced in TiO 2 , which promote the transfer of photoinduced electrons, improve the photoelectric conversion effi ciency of TiO 2 under visible light, and improve the corrosion resistance. Sun et al. (2013) found that Ni doping introduced an impurity energy level that was higher than the TiO 2 valence band, thus decreasing the conduction band, which hence remained negative. Therefore, Ni-doped TiO 2 has a visible light response, and Ni doping is conducive to the photoelectrochemical corrosion resistance. With the increase in the Ni doping amount, the main doping mode is gap doping. However, the content of oxygen vacancies decreases during this process. Excessive Ni will form recombination centers of electrons and holes in TiO 2 , which will worsen the visible light response performance.

Vanadium is an inhibitor used in paint. Thus, Chen et al. (2019) prepared V-doped TiO 2 through a sol-gel solution. In the composite, V atoms entered the TiO 2 lattice, which enhanced the anti-corrosion capability. Cerium nitrate is also a corrosion inhibitor applied in aqueous corrosive media. Li et al. (2012) exploited cerium nitrate to synthesize a Ce-doped TiO 2 fi lm. Due to the suppression of Ti 4+ , the transfer capacity of photogenerated electron-hole pairs increased, and recombination decreased. The absorption edge of the composite thus redshifted. However, metal doping increased the effi ciency of electron transfer to the protected metal, and the mechanism is shown in Fig.10 .

Metal-oxides usually have unique structures and appropriate bandgaps to enhance the light absorption of TiO 2 to achieve high photocatalytic performance.

ZnO has a photovoltaic eff ect and a bandgap similar to that of TiO 2, and the electron mobility in zinc oxide is approximately three times that of TiO 2 . High electron mobility can eff ectively reduce the secondary recombination of photoinduced electronhole pairs, thus signifi cantly improving the lifetime of the carriers. Therefore, ZnO has a higher photoelectric conversion potential, and the potential of ZnO is more negative than that of TiO 2 . Xu et al. (2014) designed a ZnO/TiO 2 composite using a simple hybrid sol-gelpowder method . The OCP of 304SS coupled with the ZnO/TiO 2 fi lm was up to -730 mV (vs. SCE) in 3 wt.% NaCl at an annealing temperature of 500°C, and the absorption of the composite fi lm redshifted. Multilayered ZnO/TiO 2 coatings exhibited anticorrosion performance on 304SS (Boukerche et al., 2019) . However, ZnO has a wide energy gap of 3.37 eV, which still needs to be ameliorated in the synthesis and structure.

As an n-type semiconductor, α -Fe 2 O 3 has been verifi ed to absorb almost 40% of the solar spectrum, but α-Fe 2 O 3 also has a short carrier diff usion length, high electron hole binding rate and poor electron mobility (Xue et al., 2020) . Cui and Pei (2019) prepared TiO 2 nanotubes modifi ed with Fe 2 O 3 particles. Theoretically, the bandgap of the Fe 2 O 3 / TiO 2 composite is 2.2 eV; thus, it has a wider spectrum response than pure TiO 2 . Furthermore, the photocurrents of pure TiO 2 and the Fe 2 O 3 /TiO 2 composite are 90 and 400 μA/cm 2 , respectively. Therefore, nontoxic Fe has good performance in photocathode protection (Deng et al., 2015) . V 2 O 5 can store electrons and cations, and TiO 2 /V 2 O 5 composite materials can serve as photoelectrodes for light energy conversion and storage .

The indirect bandgap energy of In 2 O 3 is 2.8 eV, so In 2 O 3 is an eff ective semiconductor material with a visible light response. In 2 O 3 has a good band structure, with a CB bottom of -0.63 eV, a VB of +2.17 eV and a conduction band potential that is more negative than the corrosion potential of steel. Therefore, In 2 O 3 is a potential anti-corrosion anode under visible light irradiation. In addition, there are many oxygen vacancies in In 2 O 3 that can aff ect the photoelectrochemical conversion. Keeping the structure unchanged, oxygen vacancies can improve the photoelectric conversion performance and eff ectively expand the visible light absorption area .

Bi 2 O 3 (p-type semiconductor) is the simplest Bibased oxide. Due to its excellent properties, such as high dielectric constant, refractive index and photoluminescence, it has been widely used in a wide range of photoelectrochemical applications. There are six polymorphic forms of Bi 2 O 3 : α-Bi 2 O 3 , β-Bi 2 O 3 , γ-Bi 2 O 3 , δ-Bi 2 O 3 , ε-Bi 2 O 3 and ω-Bi 2 O 3 (triclinic). Among these polymorphs, β-Bi 2 O 3 has a unique tetragonal crystal structure, resulting in a narrow bandgap (2.3-2.8 eV) and providing a transport channel for photogenerated electrons and holes. Therefore, β-Bi 2 O 3 is a suitable candidate for TiO 2 nanotube modifi cation. More importantly, if p-Bi 2 O 3 and n-TiO 2 are integrated into composite materials, a p-n heterojunction will be formed at the interface, which can signifi cantly improve the separation effi ciency of photogenerated electrons and holes (Guan et al., 2018a) . The slow discharging of 304SS and the potential variation of 403SS are shown in Fig.11 . After light exposure, it can be seen that the potential of both 304SS coupled with TiO 2 /V 2 O 5 and 403SS coupled TiO 2 /Bi 2 O 3 composites increase slowly in the dark, and both values are lower than the original corrosion potential of metals.

Some inhibitors are now applied for photogenic CP, such as polypyrrole (PPy) and polyaniline (PANI), which are both types of conductive polymers. Conducting polymers are conductive and have conjugated sequences of double and single bonds, and they participate in corrosion protection by forming compact and protective oxide fi lms on the surface of the substrate.

PANI has benzenoid and quinonoid units and has a good ability to transport holes into n-type semiconductors. PANI is one of the most attractive conductive polymer materials and is widely used in the fi eld of photoelectrochemistry (Zhang et al., 2017b) . PANI exhibits the properties of p-type semiconductors. Combining PANI with p-type TiO 2 increases the fi nal photoelectric fi eld. In addition, an appropriate amount of PPy is helpful for photosensitization, heterojunction formation and the electron pool eff ect. Under white light irradiation, PPy provides suffi cient photocathodic protection to Q235 carbon steel. Many researchers have fabricated PPy on TiO 2 composites (Lenz et al., 2003; Cui et al., 2015; Ren et al., 2016) to form synergistic interactions and increase the protective properties relative to those of the individual materials. Moreover, the addition of TiO 2 nanoparticles altered the surface of PPy to introduce more sites of interaction with the corrosion product. Polyacrylate was also applied in TiO 2 coating through the liquid-phase deposition method at 80°C (Lei et al., 2013) and formed hierarchical superstructures due to the interaction between these materials. The composite showed a higher photocurrent and lower potential than the original material; thus, polyacrylate could off er photocathodic protection to the metal. SrTiO 3 is a well-known solid semiconductor material with high photoelectric activity. The bandgaps of SrTiO 3 and TiO 2 are similar, approximately 3.2 eV, and SrTiO 3 is a p-type semiconductor with a perovskite structure. It was found that the SrTiO 3 /TiO 2 composite could increase the charge separation rate and thus improve the photoelectrochemical properties (Zhu et al., 2014; Bu et al., 2018) .

The structure of layered hydroxides (LDHs) consists of cation metal layers and interlayers of a charge-balanced anion. Researchers have been inspired, and some follow-up work is being executed. Based on zinc oxide, LDHs have a strong absorption rate of visible light. In iron-based LDHs, an oxygen bridge decreases the recombination of electron-hole pairs and extends the eff ective diff usion distance of the holes. In addition, after heat treatment, semiconductor properties are exhibited by ZnAlFe-LDH materials . Therefore, LDHs are applicable to TiO 2 modifi cation.

Crystalline tin dioxide (SnO 2 ) is an important n-type semiconductor. Its photoelectrochemical properties are similar to those of TiO 2, and the electron mobility of SnO 2 is higher than that of TiO 2 for single crystals and the corresponding nanostructures. The conduction band edge of SnO 2 is lower than that of TiO 2 , so the conduction band of SnO 2 can be used as an electron pool to preserve electrons in the case of coupling of photogenerated electrons. Thus, the SnO 2 coating can be used as a kind of energy storage material, and the SnO 2 /TiO 2 electrode has a good CP eff ect on 304SS in the dark (Subasri and Shinohara, 2003; Li et al., 2014; Hu et al., 2015a, b) .

Co(OH) 2 /TiO 2 has been reported to have improved electrochemical properties relative to pure TiO 2 because Co(OH) 2 has a good ability to capture photogenic holes and provide additional reaction sites. To accelerate the transfer velocity of electrons, the binary mixture requires other components; for example, Xie et al. (2018) and Lu et al. (2020) added graphene to the mixture.

The above modifi cation is mainly focused on improving the photoelectric conversion effi ciency, and there are some modifi cations that provide CP without light by introducing semiconductors with charge storage capacity into TiO 2 photoanodes, such as WO 3 (Guan et al., 2018b) . WO 3 has a narrow bandgap of 2.6 eV and is responsive to visible light. In particular, due to the electrochemical reduction of WO 3 , it has the ability to store energy, which makes it possible to protect metals in the dark. When TiO 2 is illuminated, electrons are excited from the VB to the CB. There are two pathways for electrons injection into the metal and acceptance by WO 3, which is reduced to consequently tungsten bronze (M x WO 3 , M=H, Li, Na, etc.; x ≤1) and in this process, the electrons were stored . Zhou et al. (2009) prepared a TiO 2 /WO 3 bilayer coating that provided 6-h photocathodic protection after 1 h of irradiation. In addition to WO 3 /TiO 2 coating, WO 3 can be deposited as nanoparticles on TiO 2 nanotube arrays, resulting in synergistic eff ects between 1D and 3D nanostructures (Sun et al., 2018b) . Yu et al. (2018) combined CP with superhydrophobicity by constructing TiO 2 nanoparticles and WO 3 nanosheet compounds. In the complex of a substrate with a TiO 2 / WO 3 coating, Yu found that some amorphous particles were present and were likely to prevent pitting corrosion. The antisepsis method prevents electron consumption; WO 3 can preserve electrons, and the mechanism is discussed. Therefore, WO 3 is considered to be a good way to modify TiO 2 (Jing et al., 2016) , and the process is illustrated in Fig.12 . CeO 2 has widespread application in organic-dyefree solar cells, and it has been reported that the bandgap of CeO 2 is shifted by 80 nm compared to that of TiO 2 . Some researchers have started to employ it in photocathodic protection. Subasri et al. (2006) found that the CeO 2 /TiO 2 bilayer coating had better photocathodic protection than the coating of CeO 2 alone, although CeO 2 had better conductivity. For the bilayer coating, the outer TiO 2 coating accepts light to generate photoelectrons, and photoelectrons spread to the substrate across CeO 2 . Subasri et al. (2006) also found that the protection of Cu lasted for 40 h after the light was turned off . Zuo et al. (2018) prepared three kinds of nanotubes with diff erent morphologies and used a schematic diagram to simulate and depict the behavior of incident light and refl ected light. The fl ower-like nanostructure has the most voids where light has more probability to refl ect and scatter. For the nanorod structure, the regular array and straight-pipe structure have less obstruction to stop incident rays escaping from the surface. Thus, with suffi cient voids, fl owerlike TiO 2 showed the best photocathodic protection, highest photocurrent, lowest potential and smallest charge transfer resistance. also prepared a fl ower-like N-TiO 2 fi lm with a two-level nanostructure and found that the composite showed good CP even in the dark. The diff erent nanostructures with LDHs , nanofl owers, spheres, and N-doped fl ower-like doping for TiO 2 are illustrated in Fig.13 .

Special methods such as hydrogenation and UV radiation strategies are also used to modify the TiO 2 nanostructure. Wei et al. (2016) tried to improve the photoelectrochemical properties via hydrogenated TiO 2 nanotubes (H-TiO 2 ). After hydrogenation, the surface structure of H-TiO 2 was disordered, and oxygen vacancies and Ti 3+ were introduced. As a result, the photocurrent density was 1.20 mA/cm 2 at 0.7 V under simulated light. Zhang et al. (2017a) irradiated the surface of a TiO 2 nanotube array fi lm with UV light. In subsequent photoelectric tests, the photocurrent increased by 50% compared with that of pristine TiO 2 , and the corrosive potential negatively shifted to -678 mV (vs. SCE) in 3.5 wt.% NaCl and 0.01 mol/L NaOH. The reason for this phenomenon is that UV treatment contributed to the formation of hydroxyl groups on the TiO 2 surface, and the presence of hydroxyl groups accelerated the recombination of photogenerated electrons and holes descend and the water oxidation reaction. The open circuit potential variation of 304SS coupled with TiO 2 fi lm with the treatment of hydrogenation and UV irradiation is shown in Fig.14. 

The photocathodic protection performance of TiO 2 and semiconductor-doped composites on metal surfaces is the most important concern for researchers.

Reprinted with permission (Yu et al., 2018) . Copyright 2018, Elsevier.

Characterization of the materials not only focuses on the physical and chemical properties, such as photocatalytic properties, but also includes the protection eff ect for metals under diff erent conditions, such as the protection potential of a metal during long-term continuous activity.

Observing the morphology is usually the fi rst step when investigating the surface of TiO 2 nanocomposites. The structure is an important factor that determines the properties of materials doped with TiO 2 . From scanning electron microscopy (SEM) images, some characteristics can be known, such as the thickness of the surface fi lm, the size of the nanoparticles and nanotubes, and the interspace of clusters (Lenz et al., 2003) . In Fig.15 , carbon fi bers (CFs) as substrates and TiO 2 nanosheets with the (001) facet can be seen clearly in the FESEM images .

Transmission electron microscopy (TEM) can also be used to observe the surface morphologies of the surface fi lms. Specifi cally, the crystallization of the microcrystal plane and the lattice fringe spacing can be determined from the TEM images. In addition, TEM, including high-resolution TEM (HRTEM) and selected-area electron diff raction (SAED), can distinguish the internal structure of crystals and growth direction, and it has a higher defi nition than SEM. From the TEM and HRTEM images in Fig.16 , the distribution of ZnIn 2 S 4 nanosheets on the TiO 2 nanotube structure is very clear, as are the lattice spacing and the corresponding facet .

X-ray diff raction (XRD) is a technology that satisfi es Prague's law to confi rm whether TiO 2 is crystalline or amorphous, determine the specifi c phase structure qualitatively, and utilize the crystallization direction to verify the element composition (Yuan and Tsujikawa 1995; Ma et al., 2020) . Ding et al. (2019) confi rmed through XRD that TiO 2 was an amorphous phase and caused no obvious essential change in the g-C 3 N 4 spectrum except for increasing the peak intensity and shifting the peaks to the larger degree. Through XRD analysis, Li et al. (2017b) concluded that Ag existed in a cubic phase, and as the TiO 2 peak remained unchanged, the Ag atom did not enter the TiO 2 lattice and replace the Ti. From the typical binding energy of the XPS spectroscopy, it could be concluded that TiO 2 was successfully fabricated. Similarly, Liu et al. (2019) also analyzed the characteristic signal of Ti 4+ and Sn 4+ , which verifi ed that TiO 2 and SnO 2 were successfully fabricated.

In addition to the above measurements, X-ray photoelectron spectroscopy (XPS) is also a common technology to characterize TiO 2 fi lms. XPS uses indefi nite carbon to calibrate the binding energy (Yuan and Tsujikawa, 1995; Cui et al., 2015; Hu et al., 2017; Nan et al., 2019) . By analyzing the binding

a. reprinted with permission (Wei et al., 2016) . Copyright 2016, Elsevier; b. reprinted with permission (Zhang et al., 2017a) . Copyright 2017, Elsevier.

Reprinted with permission . Copyright 2019, Elsevier. energy in the diff raction spectroscopy, the element doping and the cohesion between diff erent materials can be determined, as shown in Fig.17 .

Photoelectric property tests, including UV-Vis DRS, photoluminescence (PL), and Raman spectroscopy, are signifi cant for photogenerated CP.

Ultraviolet-visible diff use refl ectance spectroscopy (UV-Vis DRS) is used to evaluate optical absorption. Through UV-Vis DRS, it was observed that the absorption edge of pure TiO 2 was 380 nm, and the bandgap was calculated to be 3.2 eV. In addition, compared with that of pure TiO 2 , the increasing absorption of CdS/TiO 2 indicated the validity of the modifi cation . Nan et al. (2019) converted the absorption spectrum of the Ni 2 S 3 /TiO 2 composite fi lm into Tauc plots through the Kubelka-Munk function and found that the band edge of the composite redshifted and the bandgap of Ni 2 S 3 decreased to 3.1 eV, as illustrated in Fig.18 . After examined TiO 2 and its composite by UV-Vis DRS, Xie et al. (2019) found that diff erent optical absorption performances for anatase and rutile TiO 2 as showed in Fig.18d .

A PL spectroscopy is also capable of certifi cating the usefulness of modifi cation. For instance, the CQDs/Ag/TiO 2 compound has a lower emission intensity than pure TiO 2 , indicating that fewer photogenerated carriers recombined and generated emission; that is, the conduction of CQDs and Ag contributes to the separation of electrons and holes . The PL spectroscopy can be analyzed with UV-Vis data to determine the light absorption, as shown in Fig.19 .

Similarly, in fl uorescence measurements, the fl uorescence intensity represents the recombination amount of photoelectrons and holes. Therefore, it is usually used to investigate the photocathodic eff ect of doping compounds. Xu et al. (2020) noticed that the highest peak position in the fl uorescence emission spectroscopy among all samples was approximately 425 nm, and the intensity gradually decreased with the introduction of CdSe, graphene and PANI, which greatly promoted photoelectron-hole separation. The TiO 2 /CdSe/PANI/graphene (TCPG) composite has a lower fl uorescence intensity, which suggests a higher electron-hole pair separation effi ciency. Cheng et al. (2017) obtained the fl uorescence spectra of SiO 2 /TiO 2 fi lm, and it was obvious that the compound fi lm had lower fl uorescence emission, and the spectra are shown in Fig.20 .

In addition, Raman spectra can be used to determine whether the components vary under diff erent treatments. For instance, Bamoulid et al. (2008) observed TiO 2 fi lms with diff erent dip times through Raman spectroscopy, and the spectral signature of the surface remained unchanged; thus, the dipping treatment time would not trigger a composition change. Hu et al. (2017) discerned the presence of Bi 2 S 3 on TiO 2 nanotubes through a similar characteristic Raman peak of the specimen with Bi 2 S 3 deposited on TiO 2 and FTO.

To investigate and verify the photogenerated CP eff ect of the photoanodes, electrochemical tests are necessary to examine the potential and current of workstation. The photoelectrochemical cell is usually designed with three electrodes, a working electrode (WE), a counter electrode and SCE as the reference electrode (RE) (Park et al., 2013 , Li et al., 2018b , as depicted in Fig.21. 

The open circuit potential test and i-t are used to examine the potential and current density with time, respectively. If the potential range is safe for stainless steel, photoanode coupling with a metal can provide protection, and testing the potential for a long time can determine the stability of the photoanode. The OCP and i-t represent the photogenerated charge separation effi ciency. In addition, the more negative the OCP is, the higher the i-t, and better the protection.

OCP and i-t are usually tested by a three-electrode system on an electrochemical workstation system, such as PARSTAT 4000, CHI660B and Gamry Reference 600. In this system, generally, to simulate a marine environment, 0.5 mol/L (3.5 wt.%) NaCl is placed in the corrosion cell, and the coated metal, SCE and Pt foil are all placed in the NaCl solution as working, reference and counter electrodes, respectively. If the metal is guarded by an independent photoanode, connection remains unchanged and the metal is coupled with the photoanode. In the threeelectrode system, to further promote the effi ciency of electron and hole separation, Na 2 S and NaOH as holetrapping agents are added to the photoanode cell.

The OCP values of a series of compounds can be compared to determine the composition with optimal photogenerated protection. As shown in the SiO 2 /TiO 2 compound coupled with 304SS, 10% SiO 2 has the most negative potential (-642 mV vs. SCE), 170 mV lower than that of pure TiO 2 (Cheng et al., 2017) . The potential for electron transport indicated that SiO 2 was instrumental in enhancing the electrochemical properties of TiO 2 . Park et al. (2013) tested the OCP of coated stainless steel over a long time, and the restoration time of the compound was approximately 6 h after 3 h of irradiation. Under light irradiation, WO 3 and TiO 2 were excited, electrons were transferred to the metal, and the metal with the WO 3 /TiO 2 coating exhibited a lower potential range of 0.5 V and 0.7 V (vs. SCE) in 3.5 wt.% NaCl. Compared with pure TiO 2 , WO 3 enhanced the photoelectrons and increased the protection time after the simulated light was turned off . After turning off the light, the potential increased slowly to the previous potential in the dark. The transient photocurrent was used to directly evaluate the effi ciency of photoelectron and pair separation. Cui et al. (2015) investigated the photocurrent density of 304SS coupled with a PPy/ TiO 2 nanofi lm photoanode. While exposed to the light, the current of PPy/TiO 2 increased to 60.5 μA/cm 2 , and there was no apparent change in the pure TiO 2 photoanode. Thus, it could be determined which material has a photocathodic protection eff ect (Wang et al., 2016a) , as shown in Fig.22 . In addition to being used for comparison among composites, OCP can monitor the potential under onoff cycles and long-term illumination. However, the long-term stability and continuous protection performance in the dark are two important factors that aff ect photocathodic protection, as shown in Fig.23 . Hence, long-term OCP measurements are necessary to evaluate the photoelectrochemical properties. Liang et al. (2017) coupled WO 3 /TiO 2 with 403SS a. reprinted with permission (Xu et al., 2020) . Copyright 2020, Elsevier; b. reprinted with permission (Cheng et al., 2017) . Copyright 2017, Elsevier. and found that the eff ective CP lasted for 19 h. Li et al. (2014) set the illumination time as 2 h per cycle, and after turning off the light, the potential of 304SS coupled with Ag and SnO 2 cosensitized TiO 2 photoanodes remained below the corrosion potential for more than 8 h, which exhibited good durability.

Electrochemical impedance spectroscopy (EIS) shows the electrochemical properties of an electrode and the reaction process at an interface. The Nyquist plots can refl ect the electrolyte resistance, coating resistance, coating capacitance, charge transfer resistance, and electrical double layer capacitance of a fi lm and refl ect the electron-hole transfer effi ciency and the conductance of the fi lm material. Comparing the resistance of diff erent materials can be used to identify better CP. The diameter of the capacitive arc represents the resistance. A large diameter represents high resistance. The Nyquist plot represents a threeelement equivalent circuit model, in which the charge transfer resistance and the solution resistance can be represented by Rct and Rs. Furthermore, the small Rct represents a quick electron transfer speed. Zhang et al. (2013) and verifi ed that the CQDs/Ag/TiO 2 composite had a small Rct in the Nyquist plots, which means that a large number of excited photoelectrons fl owed to the metal; thus, it was protective for 403SS.

Mott-Schottky measurements can be used to determine the energy band structure and semiconductor type, which provides guidelines for composing materials. Xu et al. (2020) determined the type of TiO 2 composites and calculated the fl at band according to the slope of the Mott-Schottky spectroscopy, and the reason for the transmission change was further verifi ed; that is, materials with diff erent fl at band potentials can be used to construct an internal space electric fi eld that can drive the fl ow of electrons. The positive slope of the spectra of the diff erent materials implied that pure TiO 2 and Ni 2 S 3 /TiO 2 are n-type semiconductors. According to the Mott-Schottky equation, the fl at band E fb can be calculated. The deposition of Ni 2 S 3 is clear. The fl at band of the compound shifted negatively; thus, the Fermi level increased, and the conduction band shifted negatively, which indicated an easier and faster transfer effi ciency (Nan et al., 2019) . The EIS and Mott-Schottky analyses are shown in Fig.24. 

In addition, there are also other analysis techniques, such as i-v and the Tafel method. In photoelectrochemical anticorrosion research, the Tafel polarization curve is used to recognize the corrosion potential and current. Linear sweep voltammetry can be used to measure the threshold bias potential, which is the crucial potential corresponding to the occurrence of photoanodes and related to the reducing ability. Li et al. (2018a) investigated i-v tests for TiO 2 and the Sb 2 S 3 /Sb 2 O 3 / TiO 2 composites and found that the composite had a more negative potential, stronger reduction capacity, higher photocurrent and more effi cient transfer ability.

The Tafel curve refl ects the polarization process, and the corrosion current density can be easily obtained from the curve. Zhou et al. (2009) tested the corrosion current and found that 304SS coupled with WO 3 /TiO 2 had a more negative potential than pure 304SS under illumination. Zuo et al. (2018) verifi ed that the fl ower-like nanostructure had the best corrosion protection performance among nanorods, nanospheres and fl ower-like nanostructures because it had the most negative potential and the highest current in the Tafel curve, as shown in Fig.25 .

Although there are many characterization and test methods to verify the protection eff ect of TiO 2 , it is important to view the real anticorrosion degree in a simulated marine environment and monitor the surface of stainless steel in real time.

Optical microscopy was used to observe the corrosion morphology change of metals after testing (Mahmoud et al., 2005) . Direct observation can be used to verify the credibility of previous outcomes. Xu et al. (2020) analyzed the surface morphology of 304SS through optical microscopy after diff erent corrosion times. Many corrosion pits could be seen on the surface of 304SS without protection. After coupling with TiO 2 photoanodes, the quantity and size of the pits decreased.

When testing the protection of ZnO and TiO 2 layers on 304SS, Boukerche et al. (2019) found through optical microscopy that ZnO and TiO 2 fi lms with diff erent proportions had diff erent colors, which provided the ability to discern the features of fi lms. Li and Fu (2013) observed the morphology after accelerated corrosion tests, which were conducted for 316L stainless steel substrates with chromium-doped TiO 2 coatings, and found that the photocathodic protection was increased by using chromium doping. Xie et al. (2019) compared the protection of Q235 steel with Co(OH) 2 -modifi ed TiO 2 to that of traditional sacrifi cial anodes in 3.5 wt.% NaCl solution for 15 days and investigated the macrocorrosion a. reprinted with permission . Copyright 2017, Elsevier; b. reprinted with permission (Li et al., 2014) . Copyright 2014, Elsevier. morphologies of the steel, as shown in Fig.26 . The experiments proved that the photoanode composites provided eff ective supplemental protection for steel.

To explore the detailed morphology of the metal under the protection of photoanodes with light irradiation, SEM was employed for further investigation. Liu et al. (2014) found that the SEM images of 304SS showed that some corrosion pits emerged on the surface after dipping the samples in 0.5 mol/L NaCl solutions without photocathodic protection and that no pits were observed on the surface of 304SS coupled with an Fe-doped TiO 2 photoanode, as shown in Fig.27 .

Photocathodic protection is a promising technology for metal corrosion in marine environment. The present review shows the application of TiO 2 and its modifi cation to prevent corrosion. Nevertheless, two defects limit the widespread application of TiO 2 : low absorption of visible light and quick recombination of photogenerated carriers. In the past 25 years, much progress has been achieved in this fi eld, and diff erent nanostructures with doping, modifi cation and band construction have been developed to improve the photocatalytic performance of TiO 2 . There is still a large potential to promote the utilization of sunlight.

More characterization and analysis are needed to increase the understanding of the fundamental and kinetic processes of photocathodic protection. For example, density functional theory calculations can help design composite materials and determine the mechanism of the process. However, crucial problems still exist in the application of photocathodic protection. To eff ectively promote the separation of electrons and holes, sacrifi cial agents such as Na 2 S, NaOH and Na 2 SO 4 are usually added to the solution, and much eff ort has been made to substitute the sacrifi cial agents. Another challenge is achieving continuous protection in a dark environment. Although energy storage materials have increased protection without visible light irradiation, the development of new green energy materials with low cost is still needed. TiO 2 -based composite materials will play an important role in the photocathodic protection fi eld.

However, to improve the photocathodic protection performance, a wider range of new materials and new technologies should be investigated. For example, MXenes and MOFs can be constructed into 2D nanosheets (Guan and Han, 2019) , and physical vapor deposition and chemical vapor deposition (Muratore et al., 2019) can also be used. Combining photocathodic protection with sacrifi cial anode protection or new energy harvesting technology, such as triboelectric nanogenerators, can harvest wave energy (Feng et al., 2016) and mechanical energy and provide more eff ective protection for metals. Thus, it is expected that the combination of photocathodic protection and triboelectric nanogenerators will be of use in anticorrosion technologies.

Data sharing not applicable to this article as no datasets were generated or analysed during the current study. a. reprinted with permission (Li et al., 2018b) . Copyright 2018, Elsevier; b. reprinted with permission (Liang et al., 2018) . Copyright 2018, Elsevier.

Tailoring the wettability of TiO 2 nanotube layers

An effi cient protection of stainless steel against corrosion: combination of a conversion layer and titanium dioxide deposit

Photoelectrochemical response and corrosion behavior of CdS/TiO 2 nanocomposite fi lms in an aerated 0.5 M NaCl solution

Study of the photoelectrochemical cathodic protection mechanism for steel based on the SrTiO 3 -TiO 2 composite

A novel application of g-C 3 N 4 thin fi lm in photoelectrochemical anticorrosion

Fabrication of SrTiO 3 nanocrystalline fi lm photoelectrode and its photoelectrochemical anticorrosion properties for stainless steel

Light-driven sustainable hydrogen production utilizing TiO 2 nanostructures: a review

Constructing hierarchical spheres from large ultrathin anatase TiO 2 Nanosheets with nearly 100% exposed (001) facets for fast reversible lithium storage

Nanomaterials for renewable energy production and storage

Corrosion protection of carbon steels by electrochemically synthesized V-TiO 2 / polypyrrole composite coatings in 0.1 M HCl solution

26 Corrosion morphologies of Q235 unprotected (a), protected (b) by SACP and protected by Al photoanode composite (c) a, b, c. reprinted with permission

27 SEM images of 304SS blank (a); no protection (b); and coupled with Fe-doped TiO 2 (c) Reprinted with permission

Eff ect of SiO 2 -doping on photogenerated cathodic protection of nano-TiO 2 fi lms on 304 stainless steel

Enhanced photocathodic protection performance of Fe 2 O 3 /TiO 2 heterojunction for carbon steel under simulated solar light

Polypyrrole nanowire/TiO 2 nanotube nanocomposites as photoanodes for photocathodic protection of Ti substrate and 304 stainless steel under visible light

On the corrosion of copper sheeting by sea water, and on methods of preventing this eff ect, and on their application to ships of war and other ships

Photocathodic protection of iron oxide nanotube arrays fabricated on carbon steel

2019. g-C 3 N 4 /TiO 2 hybrid fi lm on the metal surface, a cheap and effi cient sunlight active photoelectrochemical anticorrosion coating

Paper-based triboelectric nanogenerators and their application in self-powered anticorrosion and antifouling

Electrochemical photolysis of water at a semiconductor electrode

TiO 2 photocatalysis and related surface phenomena

Photocathodic protection of 304 stainless steel by MnS/ TiO 2 nanotube fi lms under simulated solar light

Functionalized hybridization of 2D nanomaterials

Carbon quantum dots/Ag sensitized TiO 2 nanotube fi lm for applications in photocathodic protection

Fabrication of heterostructured β-Bi 2 O 3 -TiO 2 nanotube array composite fi lm for photoelectrochemical cathodic protection applications

Enhanced photoelectrochemical performances of ZnS-Bi 2 S 3 /TiO 2 /WO 3 composite fi lm for photocathodic protection

Fundamentals of TiO 2 photocatalysis: concepts, mechanisms, and challenges

The cost of corrosion in China

Introduction to a study on corrosion status and control strategies in China

Bi 2 S 3 modifi ed single crystalline rutile TiO 2 nanorod array fi lms for photoelectrochemical cathodic protection

Facile ultrasonic deposition of SnO 2 nanoparticles on TiO 2 nanotube fi lms for enhanced photoelectrochemical performances

SnO 2 nanoparticle fi lms prepared by pulse current deposition for photocathodic protection of stainless steel

Photoeff ect on corrosion behavior of SrTiO 3 -coated galvanized steel. Zairyo -to -Kankyo

Photoelectrochemical cathodic protection induced from nanofl ower-structured WO 3 sensitized with CdS nanoparticles

Heterogeneous photocatalyst materials for water splitting

Uniform ordered two-dimensional mesoporous TiO 2 nanosheets from hydrothermal-induced solvent-confi ned monomicelle assembly

Photogenerated cathodic protection of stainless steel by liquid-phase-deposited sodium polyacrylate/TiO 2 hybrid fi lms

Liquid phase deposition (LPD) of TiO 2 thin fi lms as photoanodes for cathodic protection of stainless steel

Application of polypyrrole/TiO 2 composite fi lms as corrosion protection of mild steel

Preparation and photocathodic protection property of ZnIn 2 S 4 /RGO/TiO 2 composites for Q235 carbon steel under visible light

3D ZnIn 2 S 4 nanosheets/TiO 2 nanotubes as photoanodes for photocathodic protection of Q235 CS with high effi ciency under visible light

Ag and SnO 2 cosensitized TiO 2 photoanodes for protection of 304SS under visible light

Photocathodic protection of 304 stainless steel by Bi 2 S 3 /TiO 2 nanotube fi lms under visible light

Enhanced photocathodic protection performance of Ag/ graphene/TiO 2 composite for 304SS under visible light

CdTe and graphene co-sensitized TiO 2 nanotube array photoanodes for protection of 304SS under visible light

Preparation and photocathodic protection performance of CdSe/reduced graphene oxide/TiO 2 composite

Semiconductor-based photocatalysts and photoelectrochemical cells for solar fuel generation: a review

Photogenerated cathodic protection of fl ower-like, nanostructured, N-doped TiO 2 fi lm on stainless steel

A photoelectrochemical study of CdS modifi ed TiO 2 nanotube arrays as photoanodes for cathodic protection of stainless steel

The Fe-doped TiO 2 nanotube arrays as a photoanode for cathodic protection of stainless steel

Improvement in corrosion protection properties of TiO 2 coatings by chromium doping

Study on cerium-doped nano-TiO 2 coatings for corrosion protection of 316 L stainless steel

Sb 2 S 3 /Sb 2 O 3 modifi ed TiO 2 photoanode for photocathodic protection of 304 stainless steel under visible light

Enhanced photoelectrochemical anticorrosion performance of WO 3 / TiO 2 nanotube composite fi lms formed by anodization and electrodeposition

A highly effi cient ZnS/CdS@TiO 2 photoelectrode for photogenerated cathodic protection of metals

Photocatalysis on TiO 2 surfaces: principles, mechanisms, and selected results

Novel bis-silane/TiO 2 bifunctional hybrid fi lms for metal corrosion protection both under ultraviolet irradiation and in the dark

Preparation of graphene/WO 3 /TiO 2 composite and its photocathodic protection performance for 304 stainless steel

A highly effi cient reduced graphene oxide/SnO 2 /TiO 2 composite as photoanode for photocathodic protection of 304 stainless steel

Characteristics and anticorrosion performance of Fe-doped TiO 2 fi lms by liquid phase deposition method

Synergetic eff ect of graphene and Co(OH) 2 as cocatalysts of TiO 2 nanotubes for enhanced photogenerated cathodic protection

Study on the photocathodic protection of 304 stainless steel by Ag and In 2 S 3 co-sensitized TiO 2 composite

Infl uence of ultraviolet light irradiation on corrosion behavior of weathering steel with and without TiO 2 -coating in 3 mass% NaCl solution

Continuous-sterilization system that uses photosemiconductor powders

Modeling of marine immersion corrosion for mild and low-alloy steels -Part 1: phenomenological model

Preparation of Ni-Pt/Fe-TiO 2 nanotube fi lms for photoelectrochemical cathodic protection of 403 stainless steel

Physical vapor deposition of 2D Van der Waals materials: a review

Solar photocatalytic degradation of dyes: high activity of combustion synthesized nano TiO 2

Fabrication of Ni 3 S 2 /TiO 2 photoanode material for 304 stainless steel photocathodic protection under visible light

Preparation and photocathodic protection property of Ag 2 S-TiO 2 composites

Growth of Nano-scale hydroxyapatite using chemically treated titanium oxide nanotubes

Photoelectrochemical anticorrosion and self-cleaning eff ects of a TiO 2 coating for type 304 stainless steel

A low-cost, high-effi ciency solar cell based on dye-sensitized colloidal TiO 2 fi lms

A review of photocatalysis using self-organized TiO 2 nanotubes and other ordered oxide nanostructures

A novel photoelectrochemical method of metal corrosion prevention using a TiO 2 solar panel

Electrophoretic deposition of nano-ceramics for the photo-generated cathodic corrosion protection of steel substrates

Highly effi cient polypyrrole sensitized TiO 2 nanotube fi lms for photocathodic protection of Q235 carbon steel

TiO 2 nanotubes: synthesis and applications

Synthesis and photocathodic protection properties of nanostructured SnS/TiO 2 composites

Corrosion protection of 316 L stainless steel by a TiO 2 nanoparticle coating prepared by sol-gel method

Evaluation of the performance of TiO 2 -CeO 2 bilayer coatings as photoanodes for corrosion protection of copper

Investigations on SnO 2 -TiO 2 composite photoelectrodes for corrosion protection

Eff ect of ZnO on the corrosion of zinc, Q235 carbon steel and 304 stainless steel under white light illumination

Enhanced photoelectrochemical cathodic protection performance of the C 3 N 4 @In 2 O 3 nanocomposite with quasi-shell-core structure under visible light

Enhanced visible light-driven activity of TiO 2 nanotube array photoanode co-sensitized by

Highly effi cient visible light induced photoelectrochemical anticorrosion for 304 SS by Ni-doped TiO 2

Enhanced photoelectrochemical cathodic protection performance of the In 2 O 3 /TiO 2 composite

Hierarchical WO 3 /TiO 2 nanotube nanocomposites for effi cient photocathodic protection of 304 stainless steel under visible light

TiO 2 -WO 3 photoelectrochemical anticorrosion system with an energy storage ability

Nano-photocatalytic materials: possibilities and challenges

Monitoring of rusting of stainless steels in marine atmospheres using electrochemical impedance technique

Pitting corrosion mechanism of Type 304 stainless steel under a droplet of chloride solutions

Photogeneration of highly amphiphilic TiO 2 surfaces

Bi 2 Se 3 sensitized TiO 2 nanotube fi lms for photogenerated cathodic protection of 304 stainless steel under visible light

Preparation of ZnWO 4 /TiO 2 composite fi lm and its photocathodic protection for 304 stainless steel under visible light

Preparation of NiSe 2 /TiO 2 nanocomposite for photocathodic protection of stainless steel

CdTe/TiO 2 nanocomposite material for photogenerated cathodic protection of 304 stainless steel

Hydrogenated TiO 2 nanotube arrays with enhanced photoelectrochemical property for photocathodic protection under visible light

Longterm photoelectrochemical cathodic protection by Co(OH) 2 -modifi ed TiO 2 on 304 stainless steel in marine environment

Design of new al photoanode composite for cathodic protection based on photocatalytic material and sacrifi cial anode

Fabrication of an innovative designed TiO 2 nanosheets/CdSe/polyaniline/graphene quaternary composite and its application as in-situ photocathodic protection coatings on 304SS

Preparation of porous TiO 2 /ZnO composite fi lm and its photocathodic protection properties for 304 stainless steel

Performance of photocatalytic cathodic protection of 20 steel by α-Fe 2 O 3 /TiO 2 system

Solvothermal synthesis and photoreactivity of anatase TiO 2 nanosheets with dominant {001} facets

Polydirectional microvibration energy collection for self-powered multifunctional systems based on hybridized nanogenerators

Ag 2 S decorated TiO 2 nanosheets grown on carbon fi bers for photoelectrochemical protection of 304 stainless steel

One-step facile preparation of ZnO nanorods as high-performance photoanodes for photoelectrochemical cathodic protection

Constructing superhydrophobic WO 3 @TiO 2 nanofl ake surface beyond amorphous alloy against electrochemical corrosion on iron steel

Photopotentials of copper coated with TiO 2 by sol-gel method. Zairyo -to -Kankyo

Characterization of sol-gelderived TiO 2 coatings and their photoeff ects on copper substrates

Facile formation of branched titanate nanotubes to grow a three-dimensional nanotubular network directly on a solid substrate

Fabrication of CdTe/ZnS core/shell quantum dots sensitized TiO 2 nanotube fi lms for photocathodic protection of stainless steel

Photogenerated cathodic protection of 304ss by ZnSe/TiO 2 NTs under visible light

Enhancement of photoelectrochemical and photocathodic protection properties of TiO 2 nanotube arrays by simple surface UV treatment

Constructing ternary polyaniline-graphene-TiO 2 hybrids with enhanced photoelectrochemical performance in photo-generated cathodic protection

Highly effi cient photoelectrochemical performance of SrTiO 3 /TiO 2 heterojunction nanotube array thin fi lm

Photogenerated cathode protection properties of nano-sized TiO 2 /WO 3 coating

Photocathodic protection properties of TiO 2 -V 2 O 5 composite coatings. Materials and Corrosion -Werkstoff e Und Korrosion

Photocathodic protection properties of three-dimensional titanate nanowire network fi lms prepared by a combined sol-gel and hydrothermal method

Highly effi cient photoinduced cathodic protection of 403SS by the all-solidstate Z-scheme ZnS-CdS-Ag@TiO 2 Nanoheterojunctions

Fabrication of heterostructured SrTiO 3 /TiO 2 nanotube array fi lms and their use in photocathodic protection of stainless steel

Fabrication and photoelectrochemical properties of ZnS/Au/TiO 2 nanotube array fi lms

Shape-dependent photogenerated cathodic protection by hierarchically nanostructured TiO 2 fi lms