Research progress of g-C₃N₄-based photocatalytic anticorrosion coatings

Abstract

Metal corrosion remains a long-standing challenge for industrial facilities, marine engineering and other fields. In recent years, photocatalytic anticorrosion coatings have become a current research hotspot due to their environmentally friendly nature, low-toxicity, active anticorrosion properties and potential for long-lasting protection. In particular, graphite nitride (g-C₃N₄) has been widely used as a photocatalytic anti-corrosion filler to enhance the barrier protection capability and thus inhibit corrosive ions due to its unique two-dimensional layered structure and excellent physicochemical properties. This review comprehensively describes the latest research progress in carbon nitride-based photocatalytic anti-corrosion materials, including the development history, synthesis pathway, and intrinsic mechanism of g-C₃N₄ materials, and discusses in detail the modification strategies of g-C₃N₄-based photocatalytic anti-corrosion systems. Therein, emphasis is placed on the modification approaches for precise modulation of morphology, heterogeneous structure building, novel photothermal synergistic effects, and corrosion inhibitor loading, aiming to prolong the corrosion ion transport paths and enhance the photogenerated carrier transfer rate to strengthen the cathodic polarization effect and achieve highly efficient anticorrosion performances. This review also provides a comprehensive overview of the current research progress and future directions of g-C₃N₄-based photocatalytic materials as anticorrosive fillers, as well as an ideal reference for researchers in the field of anticorrosive coatings.

---

1. Introduction

Metal materials are widely used in production and construction in various industries; nevertheless, the problem of metal corrosion, as a common phenomenon, is affecting social and economic development, and will lead to immeasurable material waste and economic losses.^1–3 As a result, various types of corrosion protection for metals have been developed to extend the service life of metals, such as coatings, electroplating, and metal alloying.^4–7 Coating protection has become one of the most common anti-corrosion measures due to its convenience.^8–10 Compared with traditional solvent-based anti-corrosion coatings, photocatalytic anti-corrosion coatings have both the insulation and protection of traditional coatings, while their energy-saving, green and sustainable advantages and corrosion inhibition properties have made them a research hotspot in the field of anti-corrosion.^11–15

After the discovery of the Honda–Fujishima effect in the 1960s, photocatalytic technology has displayed remarkable promise and has triggered extensive research in several fields.^16–18 In 1976, Carey et al. discovered that TiO₂ was effective at decomposing PCBs under UV light conditions, which is considered to be the pioneering work of photocatalytic technology in the elimination of environmental pollutants.¹⁹ In the following decades of research, photocatalytic technology has disclosed extraordinary potential in the fields of pollutant degradation and air purification, and antibacterial and anticorrosion properties.^20–23

Photocatalytic materials are usually dissolved in the anticorrosive matrix as active fillers, preventing the penetration of corrosive ions by covering the small cracks and pores formed during the curing process of the coating, and contributing to environmental protection while realizing physical protection compared with organic solvents.^24–28 Besides this, under visible light conditions, photocatalytic materials can generate photogenerated electron–hole pairs with strong redox capacity, which helps to decompose organic pollutants on the surface of coatings and has a positive effect on corrosion caused by microorganisms.^29–31 Furthermore, the photogenerated electrons can jump to the metal surface and enhance the cathodic polarization, efficiently improving the corrosion resistance.^32–34 In recent years, two-dimensional photocatalytic semiconductor materials with excellent spatial isolation capability, such as graphene-based nanomaterials, metal oxides, and graphite carbon nitride (g-C₃N₄), have been used extensively in the field of anticorrosive coatings, where their superior structure can effectively fill the cracks during the curing process of the coatings and form a labyrinth effect to enhance shielding and protection, thus prolonging the life of the anticorrosive coating.^35–37 Among them, g-C₃N₄ stands out because of its unique energy band structure, environmental protection ability, non-toxicity and cheap and accessible raw materials, and has undergone considerable development in the field of anticorrosive fillers.^38–40 As a new type of non-metallic semiconductor material, g-C₃N₄ has a forbidden bandwidth of ∼2.7 eV, hence it has a wider absorption spectral range and a superior response performance under visible light conditions.^41–43 Meanwhile, g-C₃N₄ has excellent thermal and chemical stability and suggests superior reaction performance below 600 °C, which is promising for practical applications.^44–46 Although g-C₃N₄ has many advantages, it also faces several drawbacks and challenges that prevent it from being extensively used. The broad forbidden bandwidth of g-C₃N₄ enhances its visible light response range while limiting its light absorption efficiency, in addition to the fact that photoexcited electron–hole pairs are prone to complexation, which greatly affects the quantum efficiency of the photocatalytic reaction.^47–49 Pure g-C₃N₄ tends to exhibit inferior crystallinity, inhibiting charge separation and transport properties. Herein, g-C₃N₄ is prone to clustering, which decreases the photocatalytic active sites and considerably affects the reaction efficiency and catalytic activity.^46,50–52 Therefore, in an effort to fully realize the application value of g-C₃N₄, modifications such as morphology modification, heterostructure construction, and photothermal-assisted photocatalytic corrosion prevention are carried out.^53–55

There has been a continuous stream of studies related to g-C₃N₄ photocatalysis for a long time, and most of the studies are on the modification strategies of g-C₃N₄ to enhance its efficiency in degrading water pollution and photolysis of water for hydrogen production.^56–58 For instance, Zhang et al. used cellulose nanofibers (CNFs) as templates for the preparation of nanosheets of C₃N₄ and attached the sheets of C₃N₄ to the surface of the prepared CNF membranes by vacuum filtration, and the prepared filtration membranes possessed well-organized multilayered structures while achieving high efficiency, high throughput, and strong stability of the degradation process.⁵⁹ Ding et al. composited g-C₃N₄ with mesoporous carbon with high electrical conductivity, and the introduction of a small amount of mesoporous carbon was beneficial for improving the electron delocalization in order to enhance the separation efficiency of the photogenerated carriers, and the maximum hydrogen production rate of the mesoporous carbon/C₃N₄ sample reached 102 μmol h⁻¹.⁶⁰ However, the comprehensive reports and discussions on the application of g-C₃N₄ photocatalysis in anticorrosive coatings are not sufficient. In this review, we summarize the research on the application of g-C₃N₄ anticorrosive coatings in recent years and make a systematic summary of our findings.

To provide a comprehensive overview of the research progress of g-C₃N₄ photocatalysts in the field of anticorrosive coatings, we firstly describe the theoretical mechanism of photocatalytic anticorrosion. Secondly, the physical properties, crystal structure, and energy band structure of g-C₃N₄, as well as its morphological features and synthetic pathways for anticorrosion applications are outlined. Immediately after that, the modification strategies of g-C₃N₄ corrosion protection materials are elaborated, mainly from four aspects: morphology modulation, photothermal-assisted photocatalytic corrosion protection, heterojunction construction and corrosion inhibitor loading (Scheme 1). Finally, the application of g-C₃N₄ in the field of photocatalytic anticorrosion coatings is summarized and predicted with a focus on current research hotspots. This review provides a summary of the research progress of g-C₃N₄ in anticorrosive coatings, which can contribute to the design and synthesis of efficient and practical g-C₃N₄ anticorrosive materials.

Scheme 1 A brief overview of modification strategies for g-C₃N₄-based photocatalytic anti-corrosion coatings.

2. Mechanisms of photocatalytic anti-corrosion coatings

The application of g-C₃N₄ in the field of anticorrosion coatings is a comprehensive system that combines its physical structure and chemical stability (Scheme 2), which is mainly attributed to the following three aspects of understanding: (i) After mixing g-C₃N₄ with a large anticorrosive substrate, the two-dimensional structure of g-C₃N₄ can form a zig-zagging diffusion path inside the substrate, which slows down the rate of penetration of the electrolyte and impedes the contact between the aqueous solution and the metallic substrate, a phenomenon known as the ‘labyrinth effect’ that can significantly improve the corrosion inhibition performance.^61–63 The g-C₃N₄, in addition to being a physical barrier to the coating, also fills the pore defects of organic coatings to reduce the penetration of external solvents.^39,64 (ii) g-C₃N₄ has a large specific surface area and its structure is rich in residual functional groups. When it is used as a nanofiller, a large number of surface-active sites can form integrated nanostructures between the nanofiller and the polymer matrix, and through the chemical–physical interactions between molecules, such as chemical bonding, van der Waals forces, p–p interactions (i.e., non-covalent interactions) or electrostatic interactions, a strong interfacial bonding or grafting of other functional groups can be constructed.^65–68 The construction of strong interfacial bonding or grafting of other functional groups improves the corrosion resistance of the composite coatings. (iii) As a visible light photocatalytic material with suitable bandgap width and redox potential, g-C₃N₄ generates photogenerated electron–hole pairs under excitation, which in turn generates reactive oxygen species (ROS) such as hydroxyl radicals (˙OH), superoxide radicals (˙O₂⁻), and hydrogen peroxide (H₂O₂); the ROS have a strong redox capacity which can effectively destroy biological structures, thus inhibiting microbial activity and enhancing the passivation process of the metal surface.^69–72 Hence, as a light-electron conversion center, the photogenerated electrons generated by light excitation of g-C₃N₄ can migrate and accumulate on the cathode surface, resulting in the polarization of the cathodic potential of the metal, thus inhibiting the dissolution reaction at the anode and protecting the metal substrate from corrosion.

Scheme 2 Mechanism diagram of g-C₃N₄-based photocatalytic anti-corrosion coating.

Despite its advantages as an anticorrosive nanofiller, g-C₃N₄ exhibits poor aqueous dispersion due to strong interlayer van der Waals forces, demonstrates insufficient compatibility with polymer matrices, and is prone to surface agglomeration, which significantly limits access to photocatalytic active sites.^49,73,74 Therefore, from the kinetic point of view, the recombination efficiency of g-C₃N₄ photogenerated carriers is much higher than the rate of redox reaction, and the photogenerated carriers are easily compounded, leading to their low utilization, which seriously affects the photocatalytic reaction activity.^75–78 Consequently, researchers have sought different modification strategies to prepare g-C₃N₄ composites with high reactive sites and high photoenergy utilization, such as altering the morphology and structure of g-C₃N₄ to enhance the utilization of surface active sites, or using heterostructure construction to enhance the carrier separation efficiency and enhance the reaction rate.^79,80 Meanwhile, in order to solve the problem of low overall solar energy utilization of single-component photocatalysts, the photocatalytic performance is enhanced by modification strategies such as loading co-catalytic systems, or selecting photothermal materials for photothermal-assisted photocatalysis.^81,82

3. Introduction to g-C₃N₄ materials

3.1. Morphological structure of the C₃N₄

C₃N₄ as a non-metallic material is a compound composed entirely of the elements carbon and nitrogen. The first polymer derivatives of C₃N₄ were synthesised artificially by Berzelius and Liebig in the 1830s, and the related research has continued to gain momentum as a result of the unique electronic structure attributed to C₃N₄.^83,84 In 1997, Teter et al. carried out first-principles calculations on the relative stability, structure and physical properties of carbon nitride polymorphs, proved the five main structural forms of carbon nitride, including alpha, beta, cubic (c-C₃N₄), quasi-cubic (p-C₃N₄), and graphitic phases (g-C₃N₄), and predicted the cubic form of C₃N₄.^85–87 Among these five phases the first four are superhard materials, which are not easy to prepare, and thus it is difficult to realize their application value, while g-C₃N₄, as a two-dimensional layered polymer material, has attracted wide attention due to its unique energy band structure and electrochemical stability.^53,88 Besides, g-C₃N₄, as a non-metallic semiconductor photocatalyst, combines unique photovoltaic properties with a bandgap width of about 2.7 eV, a valence band maximum of about 1.6 eV, and a conduction band minimum of about 1.1 eV,^89,90 which enables g-C₃N₄ to have a broad range of light absorption and a strong ability to adapt to the environment, and thus accelerates the performance of the separation of photogenerated carriers.⁹¹ g-C₃N₄ has two basic structural units, the s-triazine core and the tri-s-triazine units (Fig. 1a and b), since the carbon and nitrogen atoms in the π–π conjugated six-membered ring structure are hybridized by sp² orbitals and connected by covalent bonds.^92,93 The terminal N atoms in the conjugated unit of the structure link each unit ring together to form a two-dimensional layered structure with conjugated surfaces, which is also known as a triazine or heptazine ring conjugated ring system, and which can be extended in planar surfaces to form a single planar layer, and it can be stacked to form a graphite-like layered structure by the effect of van der Waals’ forces in the layers.^54,94–97 Since g-C₃N₄ is constructed entirely from C and N atoms, its raw materials can be selected from carbon and nitrogen-rich precursors. During the preparation process, the selection of different materials can determine the characteristics of g-C₃N₄ such as morphology tuning, bandgap width, crystallinity and active site coverage, which greatly affect the photocatalytic reaction activity.^98–100 To target the formation of g-C₃N₄ with different morphological features, synthetic routes such as high-temperature pyrolysis, solvothermal method, template method, and microwave heating method have been fully considered to construct the ideal morphological characterization and bandgap widths to achieve the maximum benefit of the photocatalytic reactivity.

Fig. 1 Primary building units of g-C₃N₄: (a) s-triazine and (b) tri-s-triazine.

3.2. Synthesis pathway of g-C₃N₄

3.2.1 Pyrolysis. High-temperature calcination is one of the most common and convenient routes for the preparation of g-C₃N₄, mainly using organic materials rich in C=N bonds as precursors, such as melamine, cyanuric acid and dicyandiamide, which are prone to form s-triazine and tri-triazine structures after high-temperature cleavage, constituting the basic units of g-C₃N₄.^101–103 Due to the obvious differences in the chemical structures of the precursors, the mechanisms and polymerization pathways for the pyrolysis synthesis of g-C₃N₄ are inconsistent, which in turn affects the morphological properties and reactivity of the derivatives. For example, Li et al. prepared pure g-C₃N₄ by direct calcination of melamine and examined the photocatalytic activation of PMS.¹⁰⁴ Yang et al. obtained graphitic carbon nitride by direct pyrolysis of urea in a muffle furnace at 600 °C and discussed the effects of pyrolysis temperature (Fig. 2), thermal time and thermal rate on the structure and properties of g-C₃N₄.¹⁰¹ The results implied that rapid heating caused g-C₃N₄ to behave as a loose thin layer with more amino defects, while slow heating resulted in a higher degree of polymerization and a more complete layer structure of the product; a long heating period and high pyrolysis temperature affected the photocatalytic activity of g-C₃N₄.^105,106 Synchronously, besides the direct calcination of single-phase precursors, high-performance g-C₃N₄ photocatalysts can also be synthesized by a simple calcination process after mixing precursors proportionally with each other. It has been reported and confirmed that mixing different kinds of nitrogen-containing precursors to prepare g-C₃N₄ can construct g-C₃N₄/g-C₃N₄ homo-p–n junctions at the contact interface to enhance the photocatalyst reactivity.^107,108 For example, Niu et al. obtained g-C₃N₄ photocatalysts with high catalytic performance by mixing dual precursors such as melamine/thiourea, melamine/urea and urea/thiourea and calcining them.¹⁰⁹ Characterization studies showed that the g-C₃N₄ after the mixing and calcination of the dual precursors had a larger specific surface area, and there was a heterogeneous structure inside the samples, whose photogenerated electric field accelerated the separation and transport of the carriers.^110,111

Fig. 2 Schematic diagram of formation of g-C₃N₄ by urea pyrolysis at different heating rates and structure–activity relationship of the products.¹⁰¹ Copyright 2021, Elsevier.

During the pyrolysis process, the precursors are typically subjected to a nitrogen or air atmosphere and are continuously calcined in a closed vessel at 500 °C–600 °C.¹¹² When cyanamide-based materials are selected as precursors, the polycondensation reaction results in the conversion of cyanamide-based materials to melamine when the calcination temperature reaches 230 °C.^113–115 Until 335 °C, the precursor materials that have been completely converted to melamine continue to reorganize to form s-triazine and tri-s-triazine units, and then completely polymerize into pure g-C₃N₄ after 520 °C.^114,116 Nevertheless, high-temperature processing above 600 °C typically results in the decomposition of the polymer framework of g-C₃N₄, leading to structural degradation and reduced yields. Besides, some of the precursors, which contain either a C=O bond (urea) or a C=S bond (thiourea), in order to achieve the theoretical carbon to nitrogen ratio of 0.75, require additional activation energy links to form chemical bonds.^117–119 Meanwhile, g-C₃N₄ prepared by calcination normally forms defective pores on the surface, thus limiting the rate of carrier generation and migration.

3.2.2 Solvothermal method. Solvothermal methods typically use water or organic solvents as the reaction medium to prepare raw materials with unique physical forms and superior chemical properties by heating a closed autoclave carrier.^120,121 Compared with pyrolysis, solvothermal methods are less energy intensive and more economical. The use of solvothermal technology can efficiently achieve the dispersion of materials, and the strong pressure and high temperature inside the autoclave can help to enhance the reaction force between molecules and accelerate the reaction process of materials.¹²² The selection of precursors is crucial when g-C₃N₄ is synthesized by solvent thermal method, and high-nitrogen compounds such as urea, melamine, and dicyandiamide are commonly used as raw materials. During the pyrolysis process, the pyrolysis of nitrogen-rich materials releases ammonia and its small molecules as a source of nitrogen and promotes the process of nitrogen doping, and at the same time, high-temperature solvent heat treatment helps to promote the cross-linking and reorganization of nitrogen-doped carbon chains, and ultimately, the formation of a stable graphite-like structure.^122–124 Choosing water or alcohol organic solvents as the reaction medium can enhance the solubility and dispersibility of precursors while homogenizing the pressure and heat during the high temperature and high-pressure reaction process, reducing the occurrence and adverse effects of side reactions.¹²³ Cao et al. synthesized Cl-doped g-C₃N₄ with controllable doping sites by a stirring-assisted solvothermal method. Compared with g-C₃N₄ synthesized by thermal polycondensation, the product has larger specific surface area, stronger photooxidation ability, smaller band gap and suppressed electron–hole complex.¹²⁵ Zhang et al. chose thiourea as the precursor and water or ethanol as the solvents, and heated them at 550 °C for 2 h to generate two types of g-C₃N₄ with widely differing properties, respectively, as shown in Fig. 3a. Firstly, a comparison with g-C₃N₄ obtained by calcination illustrates that the synthesis from the solvent-heated method produces larger surface area and pore sizes,¹²⁶ which further exposes more active sites. Comparing the use of water and ethanol solvents, the latter leads to the synthesis of carbon self-doped g-C₃N₄, offering more possibilities for designing electronic and energy band structures.¹²⁷

Fig. 3 (a) Schematic diagram of water/ethanol solvent-assisted synthesis of porous g-C₃N₄.¹²⁷ Copyright 2017, Elsevier; (b) H₂ evolution reaction mechanism of UCN/β-AgVO₃ system. (c) Synthetic route of UCAx.¹²⁸ Copyright 2021, Elsevier.

Compared with the single pyrolysis or hydrothermal method to prepare g-C₃N₄ in lamellar form, the method of calcining the precursor after hydrothermal pretreatment can effectively improve the heterostructure of planar interlayers and modulate the microstructure of g-C₃N₄ to form a highly crystalline material with a porous and high specific surface area, and at the same time,¹²⁹ the hydrothermal pretreatment can help to introduce functional groups and modulate the energy band structure in order to obtain photocatalysts with high stability and strong electrical conductivity.¹³⁰ Liang et al. prepared a new carbon and nitrogen precursor polymer, porous g-C₃N₄, by hydrothermal pretreatment of cyanamide and calcination, and its high specific surface area enabled it to exhibit unique photocatalytic activity as well as good stability.¹³¹ Yang et al. successfully prepared 3D mesoporous ultrathin g-C₃N₄ (UCN) using hydrothermal pretreatment and annealing steps, while the prepared β-AgVO₃ nanorods were anchored in the g-C₃N₄ lattice to synthesize a novel UCN/β-AgVO₃ p–n heterojunction photocatalyst (Fig. 3b and c). The porous structure provides more surface active sites while the heterogeneous structure of the contact surface accelerates the propagation of photogenerated carriers, and the modified UCN broadens the spectral absorption range and extends the carrier lifetime.^128,132,133

3.2.3 Template method. Morphology control is one of the main means to improve the activity of photocatalysts, and the template method has been used as one of the most common ways to prepare g-C₃N₄ with a special conformal structure.^134,135 This process is commonly used to prepare g-C₃N₄ with porous, hollow core–shell, and multidimensional layered structures to increase the surface area of the photocatalyst and thus expose more active sites.^97,136 Commonly used template methods include hard and soft template methods. The hard template method often uses SiO₂ nanorods, SBA-15, etc. as templates, which can precisely control the mesopore size of the synthesized compounds with templates to prepare uniform and adjustable void morphology, although the process needs the removal of the templates by using strong acidic etching solutions such as hydrofluoric acid (HF) and sodium borohydride (NaBH₄), which may inevitably cause environmental pollution.¹³⁷ Similarly, the soft template method is prominent in the construction of ideal g-C₃N₄. Surfactants and amphiphilic polymers are often used as templating agents during the preparation process, and the solvent can be naturally decomposed at high temperatures, which avoids the use of toxic substances, although the residual carbon may affect the activity of the photocatalyst, and the soft template method is unable to accurately control the size of pore size and morphological characteristics.¹³⁸ Nevertheless, the template method is still used as one of the most common synthesis methods, and in recent years, the synergistic preparation of soft–hard templates has been proposed as a new route combining the advantages of both soft and hard templates and enabling the controllable preparation of carbon materials with multistage pore structures with high void ratios.¹³⁹ Zhao et al. prepared the 3DOM g-C₃N₄ structure using SiO₂ nanospheres as a template and dicyandiamide as a precursor, and its morphology manifested a well-dispersed nanosphere structure with a diameter of 400 nm, and the open structure provided more active sites for surface adsorption and higher carrier mobility (Fig. 4a).¹⁴⁰ Furthermore, porous g-C₃N₄ was prepared by using acidified melamine and green bubble template (urea) as shown by Wang et al. The improved photocatalytic performance was attributed to the formation of a porous structure, which significantly facilitates the separation of charge carriers and promotes electron transfer.¹⁴¹ Gao et al. prepared mesoporous g-C₃N₄ by a facile soft–hard template synergistic method, using a mixture of silica sol and F127 as the mesoporous template. The nitrogen-deficient mesoporous g-C₃N₄ samples were obtained by modifying the addition of F127 (Fig. 4b). Furthermore, the samples prepared using a combination of templates (FMCN-x) were compared with those prepared using only soft (FCN) and hard (MCN) templates, and the results indicated a significant increase in the specific surface area and photocatalytic properties.^142,143 The presence of F127 enhanced the pore-forming effect of the silica-sol, leading to a higher specific surface, with a smaller silica-sol addition. On the other hand, the soft–hard template synergistic organization method introduced nitrogen defects in the g-C₃N₄ material, and the C/N ratio of FMCN-4 was increased to 1.09, improving the photocatalytic performance.¹⁴⁴

Fig. 4 (a) The synthetic strategy of 3DOM SnO₂/CN composites.¹⁴⁰ Copyright 2022, Elsevier; (b) the effect of the facile soft–hard template cooperative organization on the generation of g-C₃N₄.¹⁴⁴ Copyright 2024, Elsevier.

3.2.4 Microwave heating method. The microwave heating method utilizes the interaction between microwaves and polar molecules, vibrating the molecules while generating heat by friction. Compared with traditional external heating, microwave heating can realize the uniform heating of the reactants.¹⁴⁵ The frequency of microwave is usually controlled between 300 MHz and 300 GHz, which matches the vibration frequency of polar molecules, such as water molecules, and thus can directly act on the molecules, shorten the heating time, and then improve the reaction rate and realize the high efficiency of product preparation.^146,147 The use of microwave irradiation for heating precursors, such as melamine, can be controlled to synthesize at the ideal time and temperature, and the morphology and structure of the products can be changed by adjusting the microwave power and irradiation time. The g-C₃N₄ prepared by the microwave heating method often exhibits a lamellar or flaky structure, and the rapid temperature rise performance accelerates the cleavage and polymerization of the precursor, which can effectively improve the crystallinity of the product and reduce the defective voids.¹⁴⁸ Herein, microwaves can directly act on the bonding between molecules, which can realize the interfacial adjustment between reactants and build stable composite structures, which is quite effective for heterojunction construction. Liu et al. synthesized novel g-C₃N₄ nanobelt-assembled algal structures by the microwave method using melamine supramolecules as precursors in Fig. 5a and b; the products possessed strong stability and high specific surface area. The photocatalytic performance was significantly improved due to the synergistic effect of its excellent structure, O doping and appropriate N deficiency.¹⁴⁹ Chen et al. used microwave-assisted construction of Bi₂MoO₆/g-C₃N₄ heterostructure with enhanced two-dimensional and two-dimensional (2D/2D) structures. The type II heterojunction construction exhibited excellent charge separation efficiency and high redox capacity, and the work functional properties of the photocatalysts were calculated by DFT, demonstrating the positive role played by 2D/2D structures in charge transport.¹⁵⁰ Liu et al. synergistically prepared porous ultrathin g-C₃N₄ nanosheets with high crystallinity and extended visible light absorption by precursor-hydrothermal pretreatment and microwave heating (Fig. 5c). The hydrothermal pretreatment promoted the generation of micro/mesoporous thin nanosheet structures, while the microwave polymerization process endowed the products with enhanced visible light absorption regions, narrow bandgap widths, high crystallinity and carrier-delayed complexes.¹⁵¹

Fig. 5 (a) Schematic illustration for the fabrication of CNNR through a microwave synthesis. (b) SEM images (inset is a seaweed) of CNNR.¹⁴⁹ Copyright 2020, Elsevier; (c) TEM images of HCN-25 min.¹⁵¹ Copyright 2018, Elsevier.

3.2.5 Other means of preparation. Besides the several common ways mentioned above, there are also special synthetic forms for preparing different forms of g-C₃N₄, such as the gas–solid phase deposition method that utilizes gaseous or vaporous precursors to generate g-C₃N₄ thin films by atomic bonding and product accumulation directly on a solid substrate,^152,153 and the exfoliation method, which utilizes mechanical forces to prepare large pieces of g-C₃N₄ into nanoscale sheets in order to enhance the material's specific surface area; similarly, the chemical cleavage method can also be used to adjust the morphological structure of bulk g-C₃N₄; the reaction is carried out by hydrolysis under acid or alkali conditions, and the ideal morphology can be achieved by controlling the reaction conditions.^154,155 In order to achieve recyclability and durability for the photocatalyst, Lei et al. proposed a thermal steam condensation method, which can directly grow 2D g-C₃N₄ nanosheets with a large, uniformly thin layer on carbon fiber cloth (Fig. 6a). The carbon fiber functionalized by oxygen-containing groups has a strong interaction with the evaporated melamine gas molecules, and at the same time, with the tight stacking of the g-C₃N₄ interlayer coupled with the high efficiency of carbon fiber's electron-absorbing capacity as well as Schottky junction, the composite membrane reveals significant advantages in promoting photostimulated carrier separation and transport.¹⁵⁶ Yuan et al. prepared g-C₃N₄ nanosheets by probe ultrasound-assisted liquid stripping and constructed 2D–2D MoS₂/g-C₃N₄ photocatalysts in Fig. 6b and c, in which light-induced charge transfer was accelerated due to the presence of a large surface area and the formation of a two-dimensional interface between the MoS₂ and the g-C₃N₄ nanosheets, and the photocatalytic rate of hydrogen production reached 1155 μmol h⁻¹ g⁻¹.¹⁵⁷ Meanwhile, Shi et al. designed ultrathin oxygen-doped g-C₃N₄ nanosheets using a facile hot gas impact stripping effect, where the ultrathin thickness led to a reduced charge transfer distance, a larger specific surface area exposing a more active reaction site, and O doping to form an electron-rich center facilitating carrier segregation and negatively transferring conduction-band-enhanced reduced electrons, as shown in Fig. 6d, which provides a novel strategy to achieve highly efficient photocatalytic activity.¹⁵⁸ Papailias et al. used chemical stripping and thermal treatment to prepare ultrathin g-C₃N₄, respectively, and demonstrated that both types of corrosion prevention resulted in g-C₃N₄ with high pore volume and specific surface area, while chemical stripping led to a wider bandgap and more aggressive VB edges. Furthermore, in the case of chemical stripping, electron paramagnetic resonance (EPR) measurements demonstrated the formation of superoxide radicals and an increase in the reactivity of photogenerated electrons.¹⁵⁹ In summary, Table 1 comparatively demonstrates the common synthesis pathways of g-C₃N₄ and reveals the effects of different synthesis methods on the photocatalytic anticorrosion properties.

Fig. 6 (a) Schematic illustration of growing CN sheet on carbon fiber via polymerization of evaporative precursors. Exciton dissociation at the interface between CN sheet containing compact interlayer spacing and graphene coated on the surface of carbon fiber. Right aside shows the corresponding charge transfer within CN sheet, graphene and carbon fiber.¹⁵⁶ Copyright 2022, Elsevier; (b and c) SEM and TEM image of bulk g-C₃N₄.¹⁵⁷ Copyright 2019, Elsevier; (d) formation mechanism of UOCN-2 nanosheets.¹⁵⁸ Copyright 2022, Elsevier.

Table 1 Summary of the synthesis of g-C₃N₄-based photocatalysts

Precursor	Synthesis methods	Reaction temperature, time & atmosphere	Topography	Band gap	Ref.
Melamine	Pyrolysis	550 °C for 3 h	Bulk particles	2.75 eV	104
Urea	Pyrolysis	580 °C for 3 h	—	2.42 eV	101
Melamine, urea and thiourea (mixed precursor)	Pyrolysis	550 °C for 2 h	Nanosheets	—	109
Thiourea	Solvent-assisted	550 °C for 2 h	Nanosheets	—	127
Cyanuric chloride and dicyandiamide	Solvothermal	180 °C for 48 h	—	1.77–1.78 eV	125
Cyanamide solution (50 wt%)	Solvothermal and calcination	150 °C for 6 h and 550 °C for 2 h	Porous	2.72 eV	131
Urea and melamine	Solvothermal	180 for 24 h	Ultra-thin porous	2.71 eV	128
Dicyandiamide	Hard template	550 °C for 4 h	Sphere	3.34 eV	140
Melamine and urea	Soft template	550 °C for 4 h	Porous	2.63 eV	141
Melamine	Soft template	550 °C for 4 h	Mesoporous	—	144
Melamine and cyanuric acid	Microwave-assisted	700 W for 0.5 h	Seaweed-like	2.5 eV	149
Urea	pyrolysis	550 °C for 4 h	Nanosheets	2.78 eV	150
Dicyandiamide	Microwave-assisted	800 W for 25 min	High crystallinity and porous nanosheets	2.46 eV	151
Melamine	Vapor deposition	550 °C for 2 h	Sheet	2.71 eV	156
Urea	Calcination and solvothermal	550 °C for 2 h	Nanosheets	2.86 eV	157
Urea	Vapor Phase Stripping	550 °C for 2 h (3 times)	Ultra-thin nanosheets	2.77 eV	158
Melamine	Chemical peeling	Acid treatment at room temperature	Porous sponge-like	2.95–2.97 eV	159

---

The synthesis methods of g-C₃N₄ and its structure regulation mechanisms are systematically described above, which provide key support for the optimization of material properties. Through diverse preparation strategies such as high-temperature pyrolysis and solvothermal synthesis, the precise regulation of the crystal structure, specific surface area and active sites of g-C₃N₄ has been achieved. However, a single synthetic approach still has limitations in terms of photogenerated carrier separation efficiency and long-term anticorrosion stability.^160,161 In this regard, a modification strategy based on ‘structure–property’ synergistic optimization has broadened the direction of the research, and the modification engineering of g-C₃N₄ to achieve efficient charge migration,¹⁶² coupling the intrinsic properties of the material with the protective needs of the coating in depth, has provided the theoretical basis and technological pathway for the development of a new generation of intelligent anticorrosion systems.

4. Effect of g-C₃N₄-based modification on anticorrosive coatings

4.1. Topographical control

The performance of photocatalytic anticorrosive coatings relies on the strong redox capacity of the photocatalysts generated during the separation of photogenerated carriers and the unique interlayer arrangement structure in the matrix to impede the penetration of water molecules, and thus, the nanostructures, sizes, and morphologies of the photocatalysts have a significant impact on the exposure of the surface active sites, which can directly affect the photogenerated conversion rate and anticorrosive performance of the photocatalysts.^87,97 Therefore, much research has been devoted to enhancing the specific surface area of photocatalysts, which covers the targeted design of nanostructures, such as ultrathin nanosheets,¹⁶³ flower-like structures,¹⁶⁴ and thin-film states,¹⁶⁵ etc., and the morphology adjustments can directly work to enhance the performance of photocatalytic anticorrosive coatings. Herein, we co-ordinate the introduction of several mainstream morphological modulation methods of g-C₃N₄ in order to work towards obtaining more efficient anti-corrosion coatings.

4.1.1 Photocatalytic anti-corrosion coating of g-C₃N₄ ultrathin sheets. The high exposure of active sites on the surface of ultrathin g-C₃N₄ nanosheets provides the basis for a highly efficient photothermal conversion, while at the same time increasing the coverage per unit area in the substrate to prolong the penetration path of the water molecules and enhance the ‘labyrinth effect’.^166,167 Ultrathin lamellae of g-C₃N₄ can be prepared using strategies such as ultrasound/physical stripping method/chemical cleavage. The research group of Sun et al. used the thermal stripping technique to segment the bulk g-C₃N₄ into g-C₃N₄ nanosheets with high aspect ratios.¹⁶⁸ Observing the SEM images before and after the secondary calcination (Fig. 7a and b), the compact and continuous interlayer stacking structure of the bulk g-C₃N₄ was stripped off efficiently, which indicates that the thermal stripping method can destroy the van der Waals forces between the layers and disperse them to form ultrathin sheets.^169–171 And the side of the sheet shows the laminar flow structure of the material with an aspect ratio of 10 : 1 (Fig. 7c). Meanwhile, comparing the phase analysis of the XRD patterns before and after stripping, the results are exhibited in Fig. 7d. The two strong diffraction peaks at 12.6° and 27.5° correspond to the crystal face diffraction peaks (100) and (002) formed by the interlayer structure, whereas only a relatively weak diffraction peak (002) can be observed at 27.7°, and the decrease in intensity indicates the size reduction of g-C₃N₄ after stripping,¹⁷² further demonstrating the effective stripping of the interlayer structure. Electrochemical impedance spectrum (EIS) combines long-term immersion experiments and equivalent circuit fitting to resolve physical barrier coating attenuation in multiple dimensions,¹⁷³ and is the primary method for assessing the service life of photocatalytic anti-corrosion coatings.¹⁷⁴ In this study, the long-term corrosion resistance of bulk g-C₃N₄ coatings and flake g-C₃N₄ coatings in 3.5 wt% NaCl solution was compared by EIS; for the bulk g-C₃N₄-EP coatings, the impedance of the coatings remained flat after 5 h of immersion, and their low-frequency capacitance behaviour has been extended to the mid-frequency region, which suggests that water and O₂ within the coatings are prone to penetrate the substrate, leading to erosion behaviour (Fig. 7e). In comparison, the g-C₃N₄ after cyclic sintering was a nano-type structure, which improved the ability to hinder the corrosive medium inside the coating due to the presence of uniformly dispersed nano-type structures in the coating, and |Z|_{0.01 Hz} of g-C₃N₄-EP remained at 10¹⁰ cm² after 120 days in Fig. 7f. In another work, Li et al. prepared two-dimensional g-C₃N₄ with an ultrathin porous structure (thickness between 0.1–0.3 nm), which provided sufficient mosaic active sites for the precipitation of CuO and Cu,¹⁴⁵ and electrodeposited it into a two-dimensional g-C₃N₄/Cu₂O/Cu coating using sulfate electrolyte, which was named CG. Meanwhile, as a control sample, Cu₂O/Cu coatings were electrodeposited from a sulfate electrolyte, and the obtained coating was named C0. Comparing the SEM images and elemental distribution profiles of g-C₃N₄ and CG (Fig. 7g–i), it is clearly observed that spherical Cu₂O nanoparticles and 2D g-C₃N₄ nanosheets were embedded in a homogeneous and dense Cu layer, whereas the TEM showed a rich pore structure of 2D g-C₃N₄, which increased the specific surface area of the nanosheets and the binding sites,¹⁷⁵ thus facilitating the deposition of Cu and Cu₂O (Fig. 7j). The separation, migration trends and transport properties of photogenerated carriers in g-C₃N₄, C0 and CG were investigated using photoluminescence (PL) spectroscopy.¹⁷⁶ As shown in Fig. 7k, compared with 2D g-C₃N₄, both C0 and CG exhibited weaker PL intensities, indicating that the complexation of photogenerated carriers was lower in both coatings, and the heterojunction formed by the combination of g-C₃N₄ and Cu oxide with Cu effectively inhibited the complexation of photogenerated electron–hole pairs. The EIS results of the coatings demonstrated the magnitude of impedance radii in the order of 2D g-C₃N₄ > C0 > CG, which indicated that CG exhibited higher photogenerated electron transfer efficiency and enhanced photocatalytic activity (Fig. 7l).¹⁷⁷ Numerous demonstrations have disclosed that ultrathin g-C₃N₄ prepared using routes such as the thermal stripping method has excellent advantages for improving photocatalytic properties as well as hindering ion permeation paths, which provides an even greater incentive to develop synthetic methods with special morphologies.

Fig. 7 SEM images of (a and a₁) bulk g-C₃N₄ and (b, c and c₁) g-C₃N₄; (d) XRD of bulk g-C₃N₄ and g-C₃N₄; Bode plot of (e) bulk g-C₃N₄-EP and (f) g-C₃N₄-EP coating.¹⁶⁸ Copyright 2023, Elsevier; SEM image of (g) 2D g-C₃N₄ and (h) CG; (i) mapping images of CG; (j) TEM of CG; (k) PL spectra and (l) Nyquist plots of 2D g-C₃N₄, C0 and CG.¹⁷⁷ Copyright 2024, Elsevier.

Similarly, Li et al. also constructed a benzotriazole (BTA) corrosion inhibitor nanocontainer with barrier/controlled-release/corrosion inhibition properties by assembling g-C₃N₄ sheets,¹⁷⁸ hollow polyaniline capsules and outer polydopamine walls. Hollow polyaniline and g-C₃N₄ sheets with adsorption properties provide an effective space and place for BTA corrosion inhibitor loading. The outer polydopamine wall not only enables the controlled release of the corrosion inhibitor, but also ensures good compatibility between the composite container and the water-based epoxy resin.^179,180 Furthermore, polydopamine (PDA) and polyaniline (PANI), as the container framework, can also act as active inhibitors after corrosion inhibitor release and enhance the self-healing protection of aqueous coatings. Analyzing the microscopic morphology of the hybrids using transmission electron microscopy, it was observed that hollow polyaniline capsules (MP) were successfully prepared and uniformly loaded on the surface of g-C₃N₄ flakes, as shown in Fig. 8a. Comparing it with Fig. 8b, it was verified that the BTA inhibitor was successfully filled in the capsule structure, while some BTA molecules were also adsorbed on the surface of g-C₃N₄ flakes, and PDA was used in the encapsulation process of the whole CPAA (g-C₃N₄@MP@BTA@PDA) material. By comparing the Nyquist and Bode plots of pure WEP and CPAA-modified (Fig. 8c–f), it can be seen that the impedance of pure WEP decreased significantly with the prolongation of the immersion time, whereas the impedance of the modified composite coating was still higher than 10⁸ Ω cm² after the coating had been immersed for 80 days, which reflected the good corrosion resistance of the coating.¹⁸¹ It is worth mentioning that the change of pH in the localized microregion of the CPAA/WEP coating during corrosion triggered the release of the inhibitor to the interface and the formation of a precipitated film on the metal surface to repair the damaged coating.

Fig. 8 HR-TEM images of (a) CP and (b) CPAA; Nyquist plots of (c) blank WEP and (d) CPAA/WEP after exposure for different times in 3.5 wt% NaCl electrolyte; Bode plots of (e) blank WEP and (f) CPAA/WEP after exposure for different times in 3.5 wt% NaCl electrolyte.¹⁷⁸ Copyright 2023, Elsevier.

4.1.2 Photocatalytic anticorrosive coating of g-C₃N₄ in thin-film state. Other than employing 2D g-C₃N₄ nanosheets as anticorrosive matrix fillers to obtain extended diffusion length and high specific surface area, another efficient method is to cover the metal surface with a thin film of precursor and prepare g-C₃N₄ films directly by using the thermal condensation reaction during electrochemical deposition or high-temperature spraying.^182,183 The prepared thin-film state coating can provide additional physical protection for the substrate metal while realizing strong resistance to acid and alkali corrosion, abrasion, and high temperature.^11,184 A study on the photocurrent conversion performance and photoelectrochemical cathodic protection capability of g-C₃N₄ thin films on 304 steel under light illumination was presented by Bu et al. Observation of SEM images and truncated views of the film surface revealed that the g-C₃N₄ powder was uniformly dispersed and crack-free in the surface layer as shown in Fig. 9a–c, and its loose structure facilitated the penetration of the electrolyte to ensure the full exposure of the powder particles,¹⁸⁵ which in turn enhanced the photoelectric conversion rate. As an organic polymer, the energy band structure of g-C₃N₄ consists of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). When the film receives light, the electrons in the HOMO are excited to the LUMO and form an excited state, and the electron–hole pairs are effectively separated by the electric field formed at the electrolyte interface (Fig. 9d).^186,187 Since the LUMO potential of g-C₃N₄ is more negative than the corrosion potential of 304 SS, the electrons are then transferred to the surface of 304 SS to form a cathodic polarization, while the photogenerated holes can move to the surface of the thin film coating to oxidize water and organic/inorganic pollutants. Meanwhile, Yang et al. deposited g-C₃N₄ thin films on the surface of magnesium alloy by one-step chemical vapor deposition using urea as a precursor, as shown in Fig. 9e. Observation of the surface morphology of the coating by Atomic Force Microscopy (AFM) illustrated that the film texture was uniformly dense and smooth, and the surface layer had fine particles with little variation (Fig. 9f). To verify the anticorrosive property, the Nyquist EIS curve was examined, and it was found that the interfacial electrode/electrolyte charge transfer resistance R_ct of the optimum doping amount could reach ∼562.80 KΩ cm⁻², which is approximately 100 times of that of the bare coating (Fig. 9g). This may be because during the deposition process, N in g-C₃N₄ can provide lone pair electron pairs, which form empty tracks with Mg ions in the magnesium alloy coating, so that g-C₃N₄ is firmly fixed on the alloy surface. Meanwhile, the strong intermolecular attraction within g-C₃N₄ forms π–π stacking to enhance the densification of the coating.¹⁸⁸ Based on the results presented in this review, the design of a photocatalytic anticorrosive coating in a thin-film state helps to form a dense structure on the surface of the substrate, effectively insulates the corrosive medium, and is compatible with different morphologies while ensuring the mechanical strength of the substrate, which is a rich application prospect.

Fig. 9 SEM images of (a and b) the top and (c) cross-sectional views of the g-C₃N₄ thin-film photoelectrode; (d) schematic illustrations of the mechanism of the photoelectrochemical cathodic protection for 304 SS using g-C₃N₄ thin-film photoelectrode.¹⁸⁵ Copyright 2013, Elsevier; (e) schematic diagram for preparation of carbon nitride films on the surface of magnesium alloy; (f–g) Nyquist EIS spectra of blank AZ31B and coated AZ31B samples in PBS solution at 37 ± 0.5 °C.¹⁸⁸ Copyright 2019, Elsevier.

4.1.3 Photocatalytic anti-corrosion coating of g-C₃N₄ attached to a special morphology substrate. Apart from the common ultra-thin flakes of g-C₃N₄ that can greatly enhance the anticorrosive performance of the coating when used as a substrate filler, g-C₃N₄ with a special morphological structure also provides a large specific surface area, enhances the dispersion of the filler in the layer, and improves the shielding effect.¹⁸⁹ One of the most used morphology control methods is to construct a substrate material with special morphology, such as flower-like,¹⁶⁴ rod-like,¹⁹⁰ etc., and then attach g-C₃N₄ to the surface of the substrate material to construct a composite photocatalytic system, which then realizes efficient cathodic protection. Kong et al. obtained a SrTiO₃ photoanode with a flower-like structure by the direct solvothermal method and directly attached graphitic carbon nitride to its surface to construct a STO/g-CN heterojunction system with high active sites and photorefractive index (Fig. 10a), whose charge transfer mechanism effectively inhibited the recombination of photogenerated charges, which in turn enhanced the corrosion resistance of the composite. Analyzing the Tafel polarization curves of the composites in Fig. 10b, it can be found that the equilibrium potential of the 304 SS connected to the semiconductor photoanode was negatively shifted to a certain extent compared with the bare 304 SS.¹⁹¹ In addition, the modified composites had a more negative equilibrium potential compared with pure g-CN and STO, and exhibited superior corrosion resistance.¹⁹² Meanwhile, comparing the photocurrent density curves before and after the modification (Fig. 10c), it was found that the photocurrent density produced by (STO/g-CN)-x was 2–4 times higher than that produced by STO, with (STO/g-CN)-30% representing the strongest photocurrent density; no significant change in the photocurrent density was found after four intermittent light cycles,¹⁹³ and the result was in agreement with that of the Tafel curve. The experimental conclusions proved that the flower-like STO/g-CN possessed higher specific surface area and more reaction sites, which could generate more electron–hole pairs under visible light illumination, and the heterogeneous structure between them also provided a fast transfer channel for the negative electrons to flow into lower energy potentials and could always keep the metal potential lower than the metal self-corrosion potential,¹⁹⁴ as shown in Fig. 10d, which in turn greatly improved the anodic protection ability of the composites.¹⁹⁵

Fig. 10 (a) The preparation of STO/g-CN composites; (b) the Tafel polarization curve of SrTiO₃, g-C₃N₄ and (STO/g-CN)-x% composites under light; (c) the photocurrent densities generated by STO, g-CN, and (STO/g-CN)-x after the coupling of the semiconductors with 304 SS; (d) the energy-level distribution and charge-transfer mechanism of the STO/g-CN composite.¹⁹⁵ Copyright 2022, Elsevier.

In separate work, Guan et al. prepared an efficient g-C₃N₄/SrTiO₃ co-decorated rutile titanium dioxide nanorod composite film photoanode on a conductive glass substrate using a three-step synthesis process of hydrothermal reaction and chemical vapor deposition. The g-C₃N₄/SrTiO₃/TiO₂ composite film with appropriate cascade energy band structure had a significant improvement, the visible light absorption density was 4.5 times higher than that of the unmodified titanium dioxide film, and the composite film photoanode reduced the coupled 403 stainless steel's potential in 0.5 M NaCl solution by 680 mV compared with its free corrosion potential, which significantly enhanced the photoelectrochemical cathodic protection effect.^196,197 As shown in Fig. 11a–c, the titanium dioxide film exhibited rod-like nanostructures, and the nanorods grew vertically and uniformly on the surface of the FTO substrate, and with the gradual loading of SrTiO₃ and g-C₃N₄ on the surface of the TiO₂ nanorods, the size of the rods changed, and a smooth surface was formed. The UV-vis spectra of the prepared film samples are shown in Fig. 11d; g-C₃N₄/SrTiO₃ decoration led to red-shifted absorption of the titanium dioxide composite film due to the light scattering effect of strontium titanate enhanced by the composite film through a larger surface area and roughened surfaces, which in turn enhanced the TiO₂ to produce a very high absorption efficiency in the visible region.¹⁹⁸ The photoexcited electrons were transferred from high-intensity to low-energy CBs in the composites, and therefore through the CB of strontium titanate to the CB of titanium dioxide, and finally through the FTO substrate and copper wires connected to the composite film photoanode to 403 SS to provide cathodic protection for steel (Fig. 11e and f). Thus, the g-C₃N₄/SrTiO₃/TiO₂ composite film photoanode has high optical and photoelectrochemical properties and exhibits a highly enhanced photocathodic protective effect on steel.^199,200 Attaching g-C₃N₄ to the special morphology of the substrate material can significantly increase its specific surface area, and its unique nanostructure can significantly improve the mechanical properties of the coating and enhance the toughness and wear resistance of the coating. In addition, the structure can also enhance the densification and shielding effect of the coating and reduce the penetration of corrosive media.

Fig. 11 (a–c) SEM and TEM images of g-C₃N₄/SrTiO₃/TiO₂; (d) UV-vis absorption spectra of the prepared nanorod films: (1) TiO₂, (2) SrTiO₃/TiO₂, and (3) g-C₃N₄/SrTiO₃/TiO₂; schematic diagrams of (e) the g-C₃N₄/SrTiO₃/TiO₂ composite film photoanode structure and (f) the energy band structure and electron–hole separation and transfer mechanism in the composite film under white light irradiation.²⁰⁰ Copyright 2023, Elsevier.

4.1.4 Photocatalytic anti-corrosion coating of g-C₃N₄ doped with metal/non-metal ions. To address the low visible light utilization due to the narrow bandgap of g-C₃N₄, additional energy levels or defects can be introduced by doping with metal/non-metal ions, which in turn modulates the bandgap width of the pure-phase carbon nitride and promotes photogenerated electron–hole separation.^201–204 In fact, metal impurity injections can achieve a uniform distribution of metal ions by mixing the g-C₃N₄ precursor with the corresponding metal soluble salts through a thermal condensation process.²⁰⁵ In contrast, non-metallic doping tends to bring about higher electronegativity and ionization, allowing it to gain electrons to form covalent bonding connections during the reaction with other compounds, and the reaction is not affected by thermal changes in the chemical state.^206–209 The team of Li et al. prepared an oxygen-doped g-C₃N₄ (OCNNS) composite epoxy coating, and the synthesized OCNNS nanosheets exhibited high specific surface area and water dispersibility, which enabled efficient metal anodic protection performance.²¹⁰ The observed microscopic morphology of OCNNS exhibited irregular nanosheets, and the thickness of the nanosheets became progressively thinner with multiple calcinations, enhancing the dispersion of OCNNS in the base solution (Fig. 12a). Fig. 12b confirms the successful synthesis of oxygen-doped CNNS as indicated in the XPS spectrum, which can be divided into two peaks at 532.1 and 533.2 eV, representing the formation of C–O bonds, indicating that the N atoms in the triazine ring were replaced by O atoms.^211,212 Fig. 12d utilized the open circuit potential (OCP) to assess the difficulty of corrosion occurrence, and it was observed that all coated samples exhibited positive OCP values compared with the Q235 iron sheet and the EP coating with OCNNS-2 consistently had the largest positive value. As the immersion time increased, corrosive ions eroded the coating in contact with the metal surface, resulting in more negative OCP values, while OCNNS-2 still exhibited the best photocatalytic corrosion resistance.²¹³ This result is consistent with the performance of the impedance arc radius data following 28 days of immersion (Fig. 12c).²¹⁴ From the above data, it can be seen that the addition of OCNNS photocatalyst to the epoxy resin can significantly enhance the corrosion resistance of the carbon steel substrate, and the photocatalytic anti-corrosion mechanism of the system was thus deduced, as shown in Fig. 12e. Under the light condition, OCNNS was photoexcited to generate electrons at CB, which were then injected into the metal and accumulated in the corrosion cathode to participate in the redox reaction, thus reducing the corrosion rate and corrosion current density and protecting the metal from corrosion.²¹⁵

Fig. 12 (a) SEM images of OCNNS-2 photocatalysts; (b) high-resolution spectra of O 1 s over CNNS, OCNNS-1 and OCNNS-2 samples; (c) Nyquist plots of CNNS, OCNNS-1, OCNNS-2, EP and Q235 coatings within 28 days immersion under the sunlight irradiation; (d) OCP values of as-prepared coatings after immersion in 3.5 wt% NaCl solution for 1, 7, 14 and 28 days under the sunlight irradiation; (e) possible photocatalytic anticorrosion mechanism of OCNNS photocatalyst/coating.²¹⁰ Copyright 2022, Elsevier.

Similarly, Jing et al. used a K&I co-doping technique to modulate the functional band structure of g-C₃N₄ from amphoteric semiconductor to N-type semiconductor and used it for photocathodic protection of coupling metals in sodium chloride solution. Compared with the layered nanostructures possessed by conventional g-C₃N₄, n-C₃N₄ presents large-sized flat nanolayers with flatter surfaces and uniform thickness. As shown in Fig. 13a and b, the EDS mapping exhibited successful loading of K and I elements, and a comparison of the N₂ adsorption–desorption isotherms and the corresponding pore size distribution curves of g-C₃N₄ and n-C₃N₄ indicated that compared with the surface pore distribution of g-C₃N₄, the presence of significantly fewer pores and a lower specific surface area in n-C₃N₄,^216,217 which resulted in the formation of a large and flat nano-layer, which was conducive to the photoelectrode in the formation of smooth charge transfer channels, facilitated photogenerated electron–hole pair transfer. Comparing the UV-visible diffuse reflectance spectra and PL spectra of carbon nitride before and after K&I modification (Fig. 13c and d), it can be found that the light absorption performance of n-C₃N₄ has been significantly improved and the band gap width has been shortened to a certain extent,²¹⁸ which contributes to the enhancement of the charge transfer capability. Additionally, the PL intensity of n-C₃N₄ was much smaller than that of g-C₃N₄, indicating that the photoinduced electrons and holes generated by n-C₃N₄ are more easily separated, which could also improve the photoelectric conversion efficiency and obtain higher PECCP performance.²¹⁹ In another work, a similar attempt was made by the team of Anadebe et al. to utilize S-doped graphitic carbon nitride (S-g-C₃N₄) as an effective corrosion inhibitor for steel tubes in CO₂-saturated 3.5 wt% NaCl solutions. S-g-C₃N₄ was firstly synthesized by a simple pyrolysis and polymerization route, and its inhibition efficiency for polarization and impedance methods was verified to be 98% and 94%, respectively, and attributed to the doping effect of the S element, the composite showed a good affinity on the steel surface.²²⁰ In order to verify the nature and type of the reactions in the anodic and cathodic parts, polarization measurements were used, as shown in Fig. 13e, typical polarization curves for different S-g-C₃N₄ concentrations in 3.5 wt% NaCl solution under continuous CO₂ purge. The addition of S-g-C₃N₄ nanomaterials led to a significant decrease in cathodic current density, and the anodic current density also decreased slightly on the whole anodic side, making the Ecorr values move in a more negative direction. It was observed that the radii of the semicircle plots of the impedance spectra all increased systematically after the addition of S-g-C₃N₄ nanomaterials (Fig. 13f and g), which implies that the synthesized S-g-C₃N₄ nanomaterials can build up a dense thin film layer on the surface of the electrodes, and that the radius of each arc or semicircle implied an upward trending change with the increase in the amount of S-g-C₃N₄ content, which can be attributed to an increase in the total surface of the S-g-C₃N₄ molecules on the steel coverage, and the synergistic effect of in-plane structural motifs of the hepatzine frame work and the s-triazine unit of S-g-C₃N₄ nanomaterial.²²¹ From the above research literature, elemental doping can greatly change the energy band structure of g-C₃N₄, resulting in a change in the band gap width to form CB and VB shifts, broadening the spectral absorption range, enhancing the active sites on the surface, and providing a richer site for the adsorption and piggybacking of reactants. Meanwhile, the impurity energy levels and defect states formed by the dopant elements can provide transport paths for electron transfer.^222,223 Therefore, applying the modified g-C₃N₄ material to the anticorrosive substrate can provide more reactive active sites and faster photogenerated electron separation efficiency, which in turn can provide more electrons for the realization of cathodic protection, thus protecting the anode from corrosion.

Fig. 13 (a) EDS mapping of n-C₃N₄; (b) N₂ adsorption–desorption isotherms for g-C₃N₄ and n-C₃N₄ and the inset is the pore size distribution; (c) PL spectra and (d) EIS plots of g-C₃N₄ and n-C₃N₄.²¹⁹ Copyright 2021, Elsevier; (e) polarization plot in blank and at varying S-g-C₃N₄ concentrations in 3.5% NaCl solution under continuous CO₂ purging; (f) Nyquist and (g) log modulus plot in blank and at varying S-g-C₃N₄ concentration in 3.5 wt% NaCl solution under continuous CO₂ purging.²²¹ Copyright 2022, Elsevier.

4.2. Constructing heterojunctions

Heterojunctions based on g-C₃N₄-based photocatalysts can be classified into several types according to their structural compositions and electronic energy bands, including heterojunctions of types I and II, S and Z heterojunctions,^224,225 etc., and their energy band structures are shown in Fig. 14a–e, where different heterojunctions can result in different electron transfer paths. Heterojunctions are usually made by compositing g-C₃N₄ with other semiconductor materials in some form, and can maintain the original unique crystal structure and electrical properties in the composition.²²⁶ When a semiconductor photocatalyst is exposed to light radiation, due to its bandgap structure, the photogenerated electron–hole pairs are excited, and the potential difference in the heterojunction formed by the energy bands of the two connected semiconductor materials causes the photogenerated electron–hole migration on the CB and VB of the two semiconductors,²²⁷ which improves the spatial charge separation, reduces the carrier recombination rate, and induces the generation of the reduction reaction and the oxidation reaction.²²⁸

Fig. 14 Several common types of heterogeneous junctions: (a) I-scheme; (b) II-scheme; (c) S-scheme; (d) liquid-phase Z-scheme; (e) all-solid-state Z-scheme.

The composite photocatalyst with heterogeneous structure is added to the substrate as anti-corrosion filler, and the built-in electric field is utilized to accelerate the transmission of carriers. The strong redox ability of the material can also effectively decompose organic pollutants in the corrosive medium and reduce the corrosion of the coating and substrate. Moreover, the efficiently separated photogenerated electrons can be effectively injected into the metal substrate to realize cathodic protection.²²⁹ Therefore, the construction of heterojunction materials as anticorrosive fillers becomes a reliable way to realize effective coating antifouling and anticorrosion performance.

4.2.1 Modification by creating type-I and type-II heterojunctions. A co-composite of g-C₃N₄ with another semiconductor photocatalyst can form type I or type II heterojunctions and exhibit different electrochemical properties. When the composites are exposed to light, electron–hole pairs can traverse from the VB and CB of one semiconductor to the energy bands of the partner semiconductor, thus enhancing the photocatalytic performance.^230,231 The reaction can be generated on photocatalysts with lower redox potentials, and the positions of the energy bands of different composites will determine different forms of electron transport paths.²³² For example, Zhao et al. constructed photoanodes based on Ni₃S₂/g-C₃N₄ composites using hydrothermal deposition and electrophoretic deposition techniques. The OCP curves were utilized to evaluate the PEC cathodic protection performance of NS/CN-x, and it was observed that the OCP shifted rapidly negatively when the light was on, and positively shifted at different rates when the light was off (Fig. 15a). Therefore, the photogenerated electrons could migrate from the semiconductor to the metal surface and accumulate rapidly under light, forming a cathodic polarization and corrosion protection for the substrate material, and the negative shift potential decreased with the enhancement of the Ni₃S₂ content, in which the OCP of NS/CN-4 decreased to −486 mV, which is much lower than that of the pure-phase CN and other composite samples. The results indicated that the combination of Ni₃S₂ and g-C₃N₄ facilitates light utilization and promotes the rapid photogenerated electron transfer.²³³ In Mott–Schottky plots (Fig. 15b), all the samples illustrate n-type semiconductor properties with the bottom of the conduction band (ECB) approximating a flat band potential corresponding to the horizontal intercept, where the ECB of NS/CN-4 is about −0.61 V and the VB position is 1.96 V. Meanwhile, the slopes of the NS/CN-4 curves exhibit the highest carrier density and concentration, which is favorable for achieving photocathodic protection.²³⁴ Based on the above, the mechanism diagram of the system is reasonably deduced, in which NS/CN generates a large number of photoinduced carriers due to photoexcitation under natural light conditions (Fig. 15c), and at the same time, because the CB of Ni₃S₂ is more negative than that of C₃N₄, the photoelectrons in the CB of Ni₃S₂ are rapidly transferred to the conduction band of C₃N₄,²³⁵ and ultimately migrate to the surface of the metal, which inhibits the oxidation of Fe and realises the cathodic protection. However, as the potential difference still exists, holes in the valence band could migrate from C₃N₄ to the Ni₃S₂ valence band at one time, further accelerating the separation of electron–hole pairs within the heterojunction with a long delay in the dark.²³⁶ However, the type II heterojunction similar to the one constructed above also has certain drawbacks, such as the weak separation strength of the built-in electric field at the heterojunction interface, and some of the carriers will still be compounded at the interface, and furthermore, problems such as performance degradation may occur during long-term use, which prevents the achievement of long-lasting and stable carrier separation.²³⁷ Therefore, Z-scheme and S-scheme hetero-junctions are constructed to achieve continuous and stable photogenerated carrier separation capability.^238,239

Fig. 15 (a) OCP of measured EIS spectra and (b) Mott–Schottky plots of 304SS electrode coupled with (1) CN, (2) NS/CN-8, (3) NS/CN-2, (4) NS/CN-1, (5) NS/CN-4 photoanodes; (c) A schematic illustration for the NS/CN for photocathodic protection of 304SS.²³⁶ Copyright 2021, Elsevier; (d) optical photographs of Q235CS, polyaniline (PANI), g-C₃N₄, and S-scheme g-C₃N₄/PANI coatings after 720 h salt spray test; (e) schematic of S-scheme g-C₃N₄/PANI heterojunction (1) before contact; (2) after contact; (3) light irradiation.²⁴⁰ Copyright 2024, Wiley-VCH GmbH.

4.2.2 Modification by creating Z-scheme and S-scheme heterojunctions. Ma et al. used the secondary forging method to establish a face-to-face structured S-scheme g-C₃N₄/polyaniline (PANI) heterojunction for the protection of steel, and compared the corrosion of the substrate with different additive fillers in the salt spray test within 720 h. Compared with other materials, the g-C₃N₄/PANI coating only resulted in a certain number of rust spots along the direction of the scratches and a small number of corrosion spots (Fig. 15d). This is due to the enhanced barrier effect of the S-scheme heterojunction in the coating.²⁴¹ To further visualize the corrosion resistance of the coatings, the coatings were peeled off from the surface of the carbon steel substrate at the end of the test. It is worth noting that there was almost no accumulation of corrosion products in Q235 under the S-scheme g-C₃N₄/PANI coating, indicating that the coating was still effective at protecting the metal substrate. The anti-corrosion mechanism of the coating was speculated as shown in Fig. 15e. PANI has a higher Fermi energy level than g-C₃N₄, and when PANI and g-C₃N₄ are in close contact under dark conditions, the electrons of PANI will spontaneously flow to g-C₃N₄ until their Fermi energies reach the same level. Due to the loss of electrons, the interfacial region near the PANI coating is positively charged, leading to the formation of an electron depletion layer and the upward bending of the PANI band edge. In contrast, due to electron gain, the interfacial region near the g-C₃N₄ coating is negatively charged, leading to the formation of an electron accumulation layer and downward bending of the band edge of the g-C₃N₄ coating.^242,243 As a result, an internal electric field is built between g-C₃N₄ and PANI, hindering the continuous flow of electrons from PANI to g-C₃N₄. When exposed to light, both PANI and g-C₃N₄ transform into excited states by exciting electrons from their valence band (VB) to conduction band (CB). Subsequently, the photoelectrons accumulated in the CB of g-C₃N₄ tend to recombine with the photogenerated holes in the VB of PANI, driven by the internal electric field and Coulomb interactions, and this heterojunction provides a stronger redox capacity by achieving carrier separation while avoiding as much as possible the complexation between the useful electrons and holes.²⁴⁰

Similar work was developed by the team of Zhang et al. to reduce physical or chemical damage to marine equipment coatings while addressing microbial adhesion contamination. The team used a CuO/g-C₃N₄ (CuO/CN) S-scheme heterojunction filler incorporated into a polydimethylsiloxane (PDMS) matrix to design a multifunctional corrosion-resistant coating with a high degree of autonomy from photothermal and anti-fouling resistance. A lattice stripe spacing of 0.28 nm could be significantly detected in the high-resolution transmission electron microscopy (HRTEM) images (Fig. 16(a)), which was attributed to the (111) crystalline surface of CuO, suggesting the construction of a CuO/CN-1 heterojunction.²⁴⁴ The open circuit voltage (OCP) was utilized to evaluate the metal corrosion (Fig. 16b), and after a period of immersion, it was observed that the OCP values of the coatings all illustrated a certain degree of decrease, in which the OCP values of the CuO/CN-1/PDMS coatings were significantly higher than those of the pure phase and the other composite fillers, indicating that the corrosion resistance of the coatings was effectively improved. In order to further evaluate the long-term corrosion resistance effect, EIS tests were carried out under different immersion times (Fig. 16c), and the low-frequency impedance radius of CuO/CN-1/PDMS was much higher than that of other coatings, and with the prolongation of immersion time, the impedance radius of the composite coatings was slightly reduced while still better than that of other coatings, and the Rc values were more stable,³⁷ indicating that the coatings were structurally intact, and that they could effectively prevent the infiltration of corrosive media. As the S-scheme heterojunction generated by the CuO and CN composite drove the high-speed separation of photogenerated carriers, the electrical signals of hydroxyl radicals (˙OH) and superoxide radicals (˙O₂⁻) generated during the reaction were monitored and analyzed during the process using electron spin resonance (ESR).²⁴⁵ It was observed that the electrical signals of ˙OH and ˙O₂⁻ were not generated under dark conditions, while their signal peaks were clearly observed under visible light conditions (Fig. 16d). Besides, the intensity of the ˙OH signal of the CuO/CN-1 composite was higher than that of CN,^246,247 which could effectively cause oxidative stress to the cells and thus reduce the microbial attachment rate. Combining the above experiments, it is reasonable to speculate that the mechanism of the system, as shown in Fig. 16e, comprehensively utilizes the labyrinth effect of the two-dimensional material CN to achieve barrier protection, and achieves spatial separation of carriers with the assistance of the S-scheme heterojunction, which not only creates a cathodic protection of the substrate, but at the same time enhances the corrosion resistance and antimicrobial properties of the coating.²⁴⁸

Fig. 16 (a) HRTEM of CuO/CN-1; (b) OCP values over as-prepared coatings after immersion in 3.5 wt% NaCl solution for 1, 15, 30, 45 and 60 d; (c) Nyquist plots for as-prepared coatings in 3.5 wt% NaCl solution in 60 d; (c) ESR spectra of (d) DMPO-˙OH over CN and CuO/CN-1 samples; (e) schematic representation of photothermal self-healing and antimicrobial properties over CuO/CN/PDMS composite coating.²⁴⁸ Copyright 2025, Elsevier.

Unlike the staggered band structure of the S-scheme heterojunction, the Z-type heterojunction gets its name from the fact that its carrier migration path resembles the letter “Z”. In contrast to the lack of carrier separation efficiency in conventional type-II heterojunctions, in Z-scheme heterojunctions, the CBs located at lower VB and higher potential sites can convert electrons and holes into reactive oxygen species (ROS), thus maintaining the strong redox properties of the carriers, and thus facilitating efficient separation of electron–hole pairs.^249–251 When constructing Z-scheme heterojunctions, holes with high oxidation potentials and electrons with high reduction potentials are usually retained, thus realizing the strong redox ability of the photocatalyst. Among them, as a strongly reducing photocatalyst, combining g-C₃N₄ with oxidation catalysts can significantly improve the carrier separation efficiency, such as compounding with metal oxide materials, which can ensure the stability and durability of the photocatalysts while enhancing the visible-light absorption ability of g-C₃N₄.^38,252 Therefore, the team of Xie et al. designed a composite photoanode consisting of TiO₂ nanotubes and g-C₃N₄ nanosheets to enhance the photocathodic protection of nickel-plated coatings on Mg alloys. Sufficient interfacial contact and Z-type heterostructure building achieved by direct deposition of g-C₃N₄ on the surface of the nanotubes facilitated carrier transfer and separation. The interfacial structure of CN/TiO₂ was determined by HRTEM (Fig. 17a). The interface in the HRTEM image consists of amorphous regions attributed to g-C₃N₄, and non-countable lattice stripes with a spacing of 0.35 nm, corresponding to the (101) facet of TiO₂. The interfacial contact between g-C₃N₄ and TiO₂ was clear and tight, suggesting formation of a heterojunction.^253,254 Observation of the capacitive arc in Fig. 17b and c indicates the two-time constant behavior of the Mg/Ni electrode, and the capacitive arc diameter of the Mg/Ni electrode coupled to the photoanode was significantly reduced compared with the uncoupled Mg/Ni electrode. The Mg/Ni-CN/TiO₂ system had the smallest arc diameter in the presence of light. The uncoupled Mg/Ni electrode exposed to 3.5 wt% NaCl solution had an impedance modulus of 25.0 kΩ cm² at a frequency of 0.01 Hz (|Z|_{f=0.01 Hz}). In comparison, the Mg/Ni-CN/TiO₂ system was 1.76 kΩ cm² in the dark state and 1.39 kΩ cm² in the light. From the EIS parameters, it could be confirmed that the modification of g-C₃N₄ reduced the charge-transfer resistance of the photoanode and promoted the transfer of photogenerated electrons in the electrode.²⁵⁵ In order to test the long-term stability of the photoanode, the CN/TiO₂ composite was coupled to the Mg/Ni electrode for a long period of time, and the OCP value was about −0.50 V in the dark state, which decreased sharply to −0.82 V after visible light illumination and remained relatively stable within 5.1 h. After switching off the light, the OCP value rebounded and stabilised at around −0.55 V within 11.9 h. The OCP value remained stable for 24 hours under alternating dark and light conditions, and no corrosion pits were observed on the electrode surface, confirming the excellent stability of the coating (Fig. 17d). Two charge transfer path structures were modeled based on the energy band structure in Fig. 17e, which were considered as type-II and type-Z schemes. Since the reaction process exhibited a strong DMPO-˙OH signal, which is not consistent with the redox signal of the type-II heterojunction, it can be shown that the heterojunction tends to favor the Z-scheme, and thus the electrons on the CB of TiO₂ are complexed with the holes on the VB of g-C₃N₄, resulting in the accumulation of the electrons on the CB of g-C₃N₄ and holes on the VB of TiO₂, which exhibits a strong redox capacity.²⁵⁶ Another similar work was presented by Zhang et al. who constructed ternary g-C₃N₄/graphene/TiO₂ composites using a one-step hydrothermal method. The photoelectrochemical corrosion protection of the substrate material was realized by using two-dimensional nanosheets and TiO₂-wrapped structures to form an internal electric field in the heterogeneous semiconductor and to generate a Z-scheme charge transfer mechanism, which pushes the electrons generated by photoexcitation to a high-energy potential and promotes the transfer of the electrons from the photoelectrode to the metal. The coupled Tafel curves and surface corrosion morphology in Fig. 17f confirm the effectiveness of the photocathodic protection, with photoexcited electron transfer leading to an increase in current density and a negative displacement of the potential level. The appearance of the coupled metal presents a tendency to decrease in the number of etching spots as the CGT doping varies.²⁵⁷ The carrier transfer mechanism of the ternary nanocomponent was proposed as a Z-scheme based on the energy band distribution and the segregated transport process of photogenerated charges, as shown in Fig. 17g. Electrons are spontaneously transferred from g-C₃N₄ to titanium dioxide until the Fermi potentials are aligned to the same level in the heterogeneous semiconductor junction. As a result, an electron depletion layer is formed on the g-C₃N₄ surface and an electron accumulation layer is formed on the titanium dioxide surface.²⁵⁸ Under illumination, the internal electric field in the Z-scheme heterojunction localizes the photogenerated electrons in the holes in the CB of g-C₃N₄ and the VB of titanium dioxide, which further achieves carrier separation and transfer. Consequently, the photocathode corrosion resistance of the coating is enhanced, and a comparison of the Nyquist curves of 0CGT and 5CGT in Fig. 17h demonstrates that the effective separation of photogenerated carriers significantly improved the photoelectrochemical performance of the hybrids.²⁵⁹

Fig. 17 (a) HRTEM image showing the interface of the CN/TiO₂ composite; (b and c) EIS spectra of Mg/Ni electrode with and without coupling with TiO₂ and CN/TiO₂ composite photoanode under visible light irradiation; (d) stability test of the CN/TiO₂ composite photoanode coupled with Mg/Ni electrode; (e) mechanism of photocathodic protection of the CN/TiO₂ composite photoanode to the Mg/Ni electrode.²⁵⁶ Copyright 2025, Elsevier; (f) potentiodynamic polarization curves of the prepared samples with the optical appearance of the coupled 304 SS inserted for comparison, the relevant curves is identified as 1: 0CGT; 2: 1CGT; 3: 3CGT; 4: 5CGT; 5: 7CGT; 6: 10CGT; (g) schematic of photoexcited carrier transfer in g-C₃N₄/graphene/TiO₂ Z-scheme system; (h) Nyquist plots with equivalent circuit inserted of the synthesized 0CGT and 5CGT samples.²⁵⁹ Copyright 2023, Elsevier.

4.3. Photothermal assisted

In recent years, new coatings with both efficient corrosion resistance and intelligent self-healing have become a cutting-edge trend in the field of protective coatings.^260,261 To realize the automatic repair performance of the coating after damage, and then restore the original corrosion resistance, photothermal materials are increasingly used in light-triggered self-healing anti-corrosion coatings. Utilizing light-induced physical and chemical reactions to repair surface defects in coatings due to external factors contributes to the advantages of photothermal self-healing technology, such as precision, speed, and efficiency, which has a long-lasting and wide range of application prospects.^262,263 Commonly used photothermal self-healing materials can be categorized into extrinsic and intrinsic types. Extrinsic self-healing materials utilize nanocontainers wrapped with a coating repair agent, and after the coating is damaged, the nanocontainers release the healing agent under light excitation to repair the defects; however, this method is non-renewable, which will also limit the diversity of its applications. Intrinsic photothermal materials, on the other hand, realize self-healing by constructing reversible dynamic bonds between materials.²⁶⁴ Under external stimulation, the reversible bonds can break and reorganize to repair the damaged area, and this has become the current mainstream trend in photothermal-assisted coating repair.^265,266 The use of photothermal materials in the field of anti-corrosion, not only in the targeted repair of coating crack damage, can also effectively realize the long-lasting protection of the coating to achieve sustainable development. The team of Zhang et al. is dedicated to modified g-C₃N₄-based photothermal-assisted anticorrosion coatings, and they have proposed utilizing tannic acid (TA) to modify graphitic carbon nitride (g-C₃N₄) photocatalytic materials and loading them onto polydimethylsiloxane (PDMS) for photothermal self-healing smart anticorrosion of carbon steel materials. Doping of two-dimensional carbon nitride (CN) nanosheets into PDMS enhanced the labyrinth effect. The composite coatings prepared after TA–CN integration exhibited the best corrosion resistance properties, which could be attributed to the hydrogen bonding association between TA and CN, which further enhanced the densification of the coatings and impeded the penetration of corrosive ions,²⁶⁷ in response to the Tafel curve, in which TA–CN-4/PDMS exhibited the lowest corrosion current density of 3.83 × 10⁻¹² A cm⁻², clearly demonstrating the positive effect of TA–CN doping on enhancing the corrosion resistance of the coating (Fig. 18a). In order to verify the photothermal effect when TA–CN is loaded on the coating, infrared thermography was used to monitor the temperature change of the coating within 5 min, which proved that the temperature of the coating has an obvious tendency to increase under the light radiation, and with the increase of TA content the coating exhibited a stronger photothermal conversion property, a phenomenon that strongly proves the existence of the photothermal effect (Fig. 18b). In addition, in order to evaluate the photothermal self-healing properties of the composite coatings, the morphological changes on the surface of the abraded coatings were monitored by scanning electron microscopy (Fig. 18c), and it was observed that after 180 min of simulated sunlight irradiation, the remaining coatings indicated varying degrees of scratches’ healing compared with PDMS, in which the restoration of the coatings was more pronounced with the increase in the doping amount of TA. This phenomenon suggests that under the influence of light, the hydrogen bonding reorganization between TA–CN occurs, while the activation of the dynamic motion inherent in the PDMS chain is mobilized to achieve the anticorrosion coating.²⁶⁸ Another similar work was likewise used to demonstrate the superiority of the photothermal effect on anticorrosive coatings, and thus a black carbon nitride was synthesized for use as a filler for polydimethylsiloxane (PDMS) to construct BCN/PDMS composite coatings with multifunctional anticorrosive/anti-fouling coatings with photothermal self-healing properties. The photothermal temperature changes of the coatings were monitored using an infrared camera, as shown in Fig. 18d, and it was found that the temperature of the BCN-4/PDMS coatings warmed up to 53.9 °C within 5 min, which can be attributed to the fact that the black carbon nitride has a stronger photothermal absorption conversion property.^269,270 The self-healing property of the coating was verified by a scratch healing test under the photothermal effect, and the morphological changes of the coating scratches within 90 min were recorded by scanning electron microscopy (Fig. 18f). It was demonstrated that the BCN/PDMS coatings exhibited superior self-healing properties after the addition of BCN, and the healing properties increased with the deepening of the BCN color. This is due to the fact that when the composite coating is scratched, the destructive force leads to the temporary change of the chain orientation and local deformation around the scratched area, and when it is exposed to light, the light energy is effectively converted into heat energy, and internal stress is generated in the scratches, which results in the proximity of the two sides of the scratches to each other; the molecular chains diffuse into each other and are rapidly connected to each other to arrange^271,272 so that the scratches are healed. In addition, the corrosion resistance of the coatings was examined using the salt spray test (Fig. 18e), where images of the corrosion changes of the coatings were recorded at 24 h intervals; it was clearly observed that less corroded products appeared on the surface of BCN/PDMS compared with the presence of a large amount of corrosive material at the scratches of the PDMS and CN/PDMS coatings, indicating that the barrier effect of the coatings had been enhanced and that the coatings had good corrosion resistance.²⁷³

Fig. 18 (a) The Tafel curves of as-prepared coatings; (b) Infrared thermography photos of PDMS, CN/PDMS and TA–CN-4/PDMS coatings under AM 1.5G irradiation; (c) SEM images of scratches and healing after 3 h for composite coatings.²⁶⁸ Copyright 2024, Elsevier; (d) temperature change curves of PDMS, CN/PDMS and BCN-4/PDMS coatings under Xenon lamp irradiation; (e) optical photographs of composite coatings after 24, 48 and 72 h salt spray tests; (f) SEM images of scratches and healing for composite coatings.²⁷³ Copyright 2025, Wiley-VCH GmbH.

4.4. Corrosion inhibitor loading

The use of 2D nanomaterials as anti-corrosion fillers is one of the main ways to improve the durability and barrier capacity of coatings, while prolonged use leads to an increase in microscopic defects in the coatings, and a single 2D material is not sufficient to make the coating protective in the long term. One of the strategies to overcome this limitation is the use of smart coatings with self-healing capability, which can retard the corrosion reaction of steel substrates through corrosion inhibitors in addition to the use of photothermal materials that result in coatings with repair properties.²⁶⁰ Therefore, the corrosion inhibitor needs to be piggybacked and sequestered, so that when the coating receives severe rupture or corrosion, the corrosion inhibitor placed in the microcapsule leaks out and forms a dense protective film on the surface of the substrate, which improves the service life of the coating.²⁷⁴ Hence, in an attempt to achieve efficient loading of corrosion inhibitors and give full play to their efficacy, the combination of corrosion inhibitors with two-dimensional layered g-C₃N₄ nanosheets provides an ideal platform for the loading of corrosion inhibitors by taking advantage of the rich functional groups on the surface of g-C₃N₄ and the high specific surface area. Corrosion inhibitors can be effectively immobilized on g-C₃N₄ surfaces and within its interlayer spaces via physisorption, chemical bonding, and interlayer intercalation. This facilitates the formation of a stable composite system, enhancing the barrier properties of the anti-corrosion coating and enabling long-term protective performance.²⁷⁵ For illustration, Samadianfard et al. stabilized losartan potassium (LP) as a corrosion inhibitor on the surface of g-C₃N₄ nanoplates on AM60B magnesium alloy with 100 ppm g-C₃N₄, and due to chemical interactions between the free amine groups of the nanoplates and the epoxide groups of the coating g-C₃N₄, a dense sol–gel coating that was free of microcracks and strongly sensitive to pH was obtained. To confirm this conclusion, FTIR spectra of the powdered sol–gel material before and after the addition of the g-C₃N₄ nanoplates were recorded (Fig. 19a), and it can be seen that the intensity of the epoxide group infrared bands at 854 and 908 cm⁻¹ decreased in the presence of g-C₃N₄, suggesting that the epoxy ring is opened by the amine group. The other characteristic IR bands at 760 (Si–C bending), 1054 (Si–O stretching), 1460 (C–H alkane stretching), 1636 (O–H bending), 2876, 2932, 2992 (asymmetric and symmetric stretching vibrations of methyl and methylene), and 3400 cm⁻¹ (SiOH) remained constant. This result suggests the possibility of chemical interaction between g-C₃N₄ nanoplates and sol–gel coatings.^276,277 As shown in Fig. 19b, the polarization curves of different coatings were simulated after immersion in acid rain solution for 24 h. The anodic and cathodic currents of the sol–gel coating containing g-C₃N₄@LP were significantly lower than those of the other coatings, and their corrosion current densities decreased significantly. After finishing the EIS experiment, the coatings were scanned and analyzed, as represented in Fig. 19c; compared with other coatings, the coatings containing g-C₃N₄@LP nanoplates could be protected from corrosion damage to a certain extent in corrosive environments, and the comparison of a large number of cracks existing on the surface of the coatings containing g-C₃N₄ suggests that the intelligent release of LP has a good anti-corrosion effect on repairing the damaged areas.²⁷⁸

Fig. 19 (a) FTIR spectra of neat and g-C₃N₄-containing sol–gel materials; (b) polarization curves of different sol–gel coatings after 24 h immersion in the simulated acid rain solution; (c) morphology of (1) neat sol–gel, (2) sol–gel + g-C₃N₄ and (3) sol–gel + g-C₃N₄@LP coatings after 120 h immersion in the simulated acid rain.²⁷⁸ Copyright 2024, Elsevier; (d) R_T versus immersion time for different scratched epoxy nano-composite coatings; (e) FE-SEM images of the scratch site after 24 h of immersion in salt solution: Ce-BTA@ZI@GN/EP; (f) protection mechanism of composite Ce-BTA@ZI@GN/EP.²⁷⁹ Copyright 2024, Elsevier; (g) Bode plots of ZCN/EP coatings after immersion in 3.5 wt% NaCl solution for 0, 10, 20, 30 and 40 days; (h) time evolution of log|Z|_{0.01 Hz}, f_b for the complete coating; (i) Nyquist plots of ZCN/EP with scratches at different immersion time stages.²⁸⁰ Copyright 2025, Elsevier.

Direct loading of corrosion inhibitors onto the surface of g-C₃N₄ nanosheets is simple and convenient, although it suffers from poor adhesion and easy failure, so microencapsulation and nano-dispensing of active corrosion inhibitors are recommended. In response to this demand for modification, metal–organic frameworks (MOFs) are ideal carriers for loading corrosion inhibitors due to their high porosity, tunable structure, and large surface area. The pores of MOFs can accommodate a wide range of molecules, including organic/inorganic corrosion inhibitors, and release them on demand.^281,282 This also promotes the dispersion of g-C₃N₄ filler in the epoxy matrix, which further improves the barrier properties of the coating against corrosive ions and its own mechanical strength. Accordingly, Hasanzadeh et al. designed ZIF-67 metal–organic framework-modified g-C₃N₄ nanosheets based on benzotriazole (BTA) and cerium (Ce³⁺) corrosion inhibitors built into an epoxy resin for improving the corrosion resistance of mild steel. Comparison of the overall resistance R_T of all the samples (Fig. 19d) suggests that there is some enhancement in the resistance of all the other samples as compared with the blank brine sample, with Ce-BTA@ZI@GN having the highest resistance, indicating the optimum corrosion resistance.²⁸³ FE-SEM/EDS tests were utilized to assess the morphology and elemental weight percentage of the deposited layer in the damaged area, as shown in Fig. 19e; it was observed that the least amount of corrosion products was present in the damaged area of the Ce-BTA@ZI@GN/EP coatings, and the presence of N, Ce, and Co ions at the deposition was also detected, suggesting a successful release of the BTA, Ce, and ZI fractions, and hence the formation of cerium oxide and hydroxide products at the damaged area and the formation of a protective layer of organic/inorganic complexes, which effectively inhibited the corrosion process.²⁸⁴ According to a series of tests, the barrier and self-healing mechanism of the composite system was discussed as shown in Fig. 19f. Based on the existence of micropores and cracks in EP, which would lead to the direct penetration of corrosive substances, the addition of g-C₃N₄ could effectively retard the corrosion path, and at the same time, piggybacking on ZIF could help to increase the specific surface area of the composite material, enhance the dispersibility and the barrier effect, and have a curing reaction with EP to enhance the cross-linking density. And then the introduction of BTA and Ce³⁺ could quickly form a dense protective film in the coating damage, impeding the penetration of corrosive ions, in multi-party mutual synergy to complete the coating for long-term anti-corrosion work.²⁷⁹ In another work, Li et al. modified ZIF-8 on the surface of g-C₃N₄ nanosheets using an in situ loading method to construct ZIF-8@g-C₃N₄ (ZCN) smart nanoparticles, which exhibited an acidic pH response, releasing Zn²⁺ cations and imidazole molecules, and achieving a corrosion inhibition efficiency of 87.2% on carbon steel within 5 h. The composite coatings modified with ZCN revealed superior corrosion protection, while the pH response of ZCN reacted to localized corrosion to form products such as zinc oxide and zinc hydroxide, which in turn hindered the corrosion process. Moreover, ZCN could give the coating the ability to resist ultraviolet rays and ensure the long-term stability of the coating. The electrochemical performance of the composite coatings was measured using Bode plots for different immersion times (Fig. 19g), in which the log|Z|_{0.01 Hz} value of ZCN/EP only decreased to 10.47 Ω cm², a phenomenon based on the epoxy curing reaction involving Zn-MOF, which further increased the cross-linking density of the coatings, and also filled and blocked the cracks and microporous defects in the coatings.^285–287 Compared with pure EP and CN/EP coatings, the ZCN/EP coatings exhibited capacitive behavior over a wider frequency range, indicating a superior barrier effect. The breakpoint frequency (f_b) in the Bode plot can be used as an important indicator for assessing the micro delamination of the coatings (Fig. 19h), and an increase in f_b indicates enhanced separation of the coating from the carbon steel substrate. The f_b values of all coatings moved to higher frequencies with time. f_b increased significantly for EP and CN/EP coatings, indicating a severe loss of coating adhesion. In contrast, the ZCN/EP coatings displayed only a slight increase, indicating minimal peeling. This was mainly attributed to the high barrier properties and activity inhibition of ZCN.²⁸⁸ In order to evaluate the self-healing effect of ZCN/EP, EIS was used to assess the electrochemical performance of the scratched out coating surface (Fig. 19i), in which the impedance arc diameter of the ZCN/EP coating indicated an inverse growth and was always higher than the initial value, indicating that the corrosion resistance of the coating was enhanced, which further proved that Zn-MOF had an active inhibition effect, limiting the diffusion of corrosive substances and mitigating corrosive reactions.²⁸⁰

In summary, we summarize the modification strategies of various g-C₃N₄-based photocatalytic anticorrosive fillers and compare their anticorrosive properties, as provided in Table 2, to exhibit the performance of different modification routes.

Table 2 Summary of modified g-C₃N₄-based photocatalytic anti-corrosion performance

Photocatalyst	Modification route	Substrates	Test environments	|Z|0.01 Hz (Ω cm^2)	OCP (V)	Ecorr (VSCE)	Ref.
g-C3N4-ESO-EP	Topographical control	Q235 carbon steel	CCUS	1.73 × 10^11	—	—	168
g-C3N4/Cu2O/Cu	Topographical control	316L SS	Microbiological corrosion	—	—	−0.32–−0.24	177
g-C3N4	Topographical control	304 SS	3.5 wt% NaCl solution	—	−0.415	—	185
g-C3N4	Topographical control	AZ31B Mg alloy	PBS solution	—	—	−1.285	188
STO/g-CN	Topographical control	304 SS	3.5 wt% NaCl solution	—	−0.480	—	195
CPAA	Topographical control	P110 steel specimen	3.5 wt% NaCl solution	1.773 × 10^8	−0.189	—	178
CN/SrTiO3/TiO2	Topographical control	403 SS	0.5 M NaCl solution	—	—	−0.580	289
OCCNS	Topographical control	Q235 carbon block	3.5 wt% NaCl solution	—	−0.2–−0.15	−0.23	210
n-C3N4	Topographical control	316L SS	3.5 wt% NaCl solution	—	—	−1.42	219
S-g-C3N4	Topographical control	X-65 steel	3.5 wt% NaCl solution enriched with CO2	—	−0.72–−0.715	−0.726	221
Ni3S2/g-C3N4	Constructing heterojunctions	304 SS	3.5 wt% NaCl solution	—	−0.486	—	236
g-C3N4/PANI	Constructing heterojunctions	Q235 carbon block	3.5 wt% NaCl solution	1.12 × 10^9	—	—	240
CuO/CN	Constructing heterojunctions	Q235 carbon block	3.5 wt% NaCl solution	3.49 × 10^9	—	−0.39	248
CN/TiO2	Constructing heterojunctions	Mg/Ni alloy	3.5 wt% NaCl solution	—	−0.84	—	256
CN/graphene/TiO2	Constructing heterojunctions	304 SS	3.5 wt% NaCl solution	—	—	−0.780	259
TA–CN	Photothermal assisted	Q235 carbon block	3.5 wt% NaCl solution	3.45 × 10^8	—	—	268
BCN	Photothermal assisted	Q235 carbon block	3.5 wt% NaCl solution	9.32 × 10^10	—	—	273
Sol–gel@g-C3N4	Corrosion inhibitor	AM60B Mg alloy	Simulated acid rain	—	—	−1.429	278
Ce-BTA@ZI@GN	Corrosion inhibitor	MS	3.5 wt% NaCl solution	—	−0.570	−0.639	279
ZCN	Corrosion inhibitor	Q235 MS	3.5 wt% NaCl solution	3.84 × 10^10	—	−0.69–−0.67	280

---

5. Summary and outlook

Reviewing the whole treatise, we provide an in-depth summary and analysis of the cutting-edge research and applications of g-C₃N₄-based photocatalytic anticorrosion materials. g-C₃N₄ has attracted much attention as a filler material for photocatalytic anticorrosion coatings due to its unique two-dimensional layered structure and excellent physicochemical properties.^290,291 This review elucidates that g-C₃N₄ inhibits corrosive ions through physical barriers, cathodic polarisation and its strong redox activity, thus prolonging the service life of anticorrosive coatings.²⁹² It also provides a comprehensive overview of g-C₃N₄ synthesis methods, including pyrolysis, solvothermal methods, template-assisted synthesis, and microwave heating. These methods allow precise control of morphology, crystallinity and specific surface area of g-C₃N₄, which is essential for optimizing its photocatalytic properties. This review further discusses various modification strategies aimed at addressing g-C₃N₄ limitations, such as high complexation rates of photogenerated carriers and poor light absorption efficiency.^293–296 These strategies include morphological modulation, heterostructure building and photothermal-assisted photocatalysis.²⁹⁷ These modifications improve the separation and transfer efficiency of photogenerated carriers, reduce the complexation rate, and broaden the light absorption range, making g-C₃N₄ more efficient under visible light conditions.

In terms of applications, g-C₃N₄-based photocatalytic coatings have manifested great potential for protecting metals from corrosion. It has been demonstrated that the incorporation of g-C₃N₄ into the coating can form a strong protective layer that prevents corrosive media from encountering the metal surface, thereby significantly extending the service life of the metal.^298,299 The review provides several examples, including the use of ultrathin nanosheets and films of g-C₃N₄ to enhance the ‘labyrinth effect’ and improve corrosion resistance. Additionally, doping g-C₃N₄ with other semiconductors in heterojunction structures has been revealed to enhance photocatalytic activity and corrosion protection. The development of g-C₃N₄-based photocatalytic anticorrosion coatings is expected to expand significantly as the demand for sustainable and efficient anticorrosion solutions continues to grow.³⁰⁰ Future research should focus on further optimizing the material structure to enhance the dispersion and stability of g-C₃N₄ in coatings, thereby improving their corrosion resistance and photocatalytic efficiency. Advanced synthesis techniques that allow precise control of the morphology and crystallinity of g-C₃N₄ are essential for the development of materials with higher specific surface area and better light absorption properties.^301,302 Innovative modification strategies, such as the construction of more efficient heterojunctions and the doping of photothermal materials, are essential for improving the performance of g-C₃N₄-based coatings. These strategies can improve the separation and transfer of photogenerated carriers, reduce the compounding rate, and extend the light absorption range, making g-C₃N₄ more efficient under visible light conditions.^303,304 In addition, the development of multifunctional composites by integrating and synergizing g-C₃N₄ with other materials, such as photosensitizers and co-catalysts, can further enhance the photothermal conversion efficiency and corrosion resistance of the coatings. At present, the photothermal-assisted photocatalytic system, as a cutting-edge field of anticorrosion coating research,^291,305 should focus on real cases in actual outdoor use environments to truly realize the practical application of photothermal anticorrosion coatings and to meet the demand for industrialization. In addition, future research should also focus on achieving high-efficiency photothermal conversion efficiency, such as optimizing the role played by near-infrared light (NIR) in photothermal-assisted photocatalysis, in order to improve the efficiency and selectivity of the photocatalytic reaction,³⁰⁶ thereby expanding its application in energy conversion and environmental remediation. To match the needs of industrial production, the low cost of raw materials is accompanied by the need to realize large-scale preparation techniques, which require both strong stability and operational reproducibility. At the same time, in order to adapt to the needs of complex environments, the construction of smart-responsive anticorrosive coatings with multi-stage barrier structures and adapted to extreme environmental conditions has become a hot research topic.^307–309 g-C₃N₄-based photocatalytic anti-corrosion coatings are expected to expand their applications to various fields including marine engineering, aerospace and infrastructure construction. In the marine environment, these coatings can protect ships and offshore structures from severe corrosion caused by seawater exposure.^310,311 In aerospace applications, g-C₃N₄-based coatings can improve the durability and safety of aircraft components by providing strong protection against corrosion.³⁰⁸ Additionally, in infrastructure construction, these coatings can extend the life of buildings and bridges by preventing corrosion of steel reinforcement.^312–314

In summary, g-C₃N₄-based photocatalytic anti-corrosion coatings represent a promising solution to the global challenge of metal corrosion. Through continuous optimization and innovation in material synthesis, modification strategies and application development, g-C₃N₄-based coatings will play an even more important role in sustainable corrosion protection, contributing to reduced material wastage, lower maintenance costs and enhanced environmental sustainability.^315–317 Future research and development efforts could further unlock the potential of g-C₃N₄-based materials, paving the way for more efficient and longer-lasting corrosion protection solutions for a wide range of industries.

Data availability

The data that support the findings of this study are available on request from the corresponding author.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 22006057 and 21906072), China Postdoctoral Science Foundation (2023M743178), Jiangsu Province Industry-University-Research Cooperation Project (BY20231482) and the Open Fund of the Key Laboratory of Solar Cell electrode Materials in China Petroleum and Chemical Industry (2024A093).

References

1. C. W. Yang, Y. Zhu, T. Wang, X. Wang and Y. B. Wang, Research progress of metal organic framework materials in anti-corrosion coating, J. Polym. Eng., 2024, 44, 1–12.
2. W. W. Zhao, F. X. Li, X. H. Lv, J. X. Chang, S. C. Shen, P. Dai, Y. Xia and Z. Y. Cao, Research Progress of Organic Corrosion Inhibitors in Metal Corrosion Protection, Crystallographica, 2023, 13, 1329.
3. Y. P. Liu, Y. Y. Zhou, W. Z. Wang, L. M. Tian, J. Zhao and J. Y. Sun, Synergistic damage mechanisms of high-temperature metal corrosion in marine environments: A review, Prog. Org. Coat., 2024, 197, 108765.
4. Q. Ma, L. D. Wang, W. Sun, Z. Q. Yang, S. L. Wang and G. C. Liu, Effect of chemical conversion induced by self-corrosion of zinc powders on enhancing corrosion protection performance of zinc-rich coatings, Corros. Sci., 2022, 194, 109942.
5. E. K. Saathoff, M. J. Bishop, I. C. Levitsky, J. D. Skimmons, A. W. Langham and S. B. Leeb, Cathodic Protection Measurement and Modeling, IEEE Trans. Instrum. Meas., 2025, 74, 2000712.
6. M. K. Zadeh, M. Yeganeh, M. T. Shoushtari and A. Esmaeilkhanian, Corrosion performance of polypyrrole-coated metals: A review of perspectives and recent advances, Synth. Met., 2021, 274, 116723.
7. I. A. W. Ma, S. Ammar, S. S. A. Kumar, K. Ramesh and S. Ramesh, A concise review on corrosion inhibitors: types, mechanisms and electrochemical evaluation studies, J. Coat. Technol. Res., 2022, 19, 241–268.
8. A. Królikowska, L. Komorowski, E. Langer and M. Zubielewicz, Promising Results of the Comparison of Coatings on Aged Bridges and of Same Coatings in Laboratory, Materials, 2022, 15, 3064.
9. F. T. Cao, G. Y. Gao, Y. M. Feng, B. A. Getachew and T. G. Wang, Electrochemical Properties of Metallic Coatings, Metals, 2022, 12, 2125.
10. M.-S. Gang, S.-H. Eom, Y.-C. Cho, J. Ahn, K. S. Soo and J.-B. Lee, An Electrochemical Evaluation of the Corrosion Properties of the Steel with the Type and the Thickness of Metallizing Coatings, J. Korean Inst. Resour. Recycl., 2016, 25, 55–62.
11. Y. N. Ma, R. Chen, G. Q. Fei, M. Y. Guo, Y. Y. Li, Y. H. Duan, X. J. Wu and H. H. Wang, Enhanced anti-aging and anti-corrosion performance of waterborne epoxy coating layers over the dual effects of g-C₃N₄ photocatalysis, J. Appl. Polym. Sci., 2022, 139, 52356.
12. T. R. Ovari, B. Trufán, G. Katona, G. Szabó and L. M. Muresan, Correlations between the anti-corrosion properties and the photocatalytic behavior of epoxy coatings incorporating modified graphene oxide deposited on a zinc substrate, RSC Adv., 2024, 14, 10826–10841.
13. Y. N. Ma, H. H. Wang, L. Y. Sun, E. Z. Liu, G. Q. Fei, J. Fan and Y. M. Kan, Unidirectional electron transport from graphitic-C₃N₄ for novel remote and long-term photocatalytic anti-corrosion on Q235 carbon steel, Chem. Eng. J., 2022, 429, 132520.
14. S. A. Z. Estekhraji and S. Amiri, Synthesis and Characterization of Anti-fungus, Anti-corrosion and Self-cleaning Hybrid Nanocomposite Coatings Based on Sol-Gel Process, J. Inorg. Organomet. Polym. Mater., 2017, 27, 883–891.
15. H. Li, L. Xin, K. Zhang, X. L. Yin and S. R. Yu, Fluorine-free fabrication of robust self-cleaning and anti-corrosion superhydrophobic coating with photocatalytic function for enhanced anti-biofouling property, Surf. Coat. Technol., 2022, 438, 128406.
16. A. Fujishima and K. Honda, Electrochemical Photolysis of Water at a Semiconductor Electrode, Nature, 1972, 238, 37–38.
17. B. Ohtani, Revisiting the Original Works Related to Titania Photocatalysis: A Review of Papers in the Early Stage of Photocatalysis Studies, Electrochemistry, 2014, 82, 414–425.
18. Y. Wan, Q. Liu, Z. H. Xu, J. Z. Li, H. J. Wang, M. Y. Xu, C. L. Yan, X. H. Song, X. Liu, H. Q. Wang, W. Q. Zhou and P. W. Huo, Interface engineering enhanced g-C₃N₄/rGO/Pd composites synergetic localized surface plasmon resonance effect for boosting photocatalytic CO₂ reduction, Carbon Lett., 2024, 34, 1143–1154.
19. J. H. Carey, J. Lawrence and H. M. Tosine, Photodechlorination of PCB's in the presence of titanium dioxide in aqueous suspensions, Bull. Environ. Contam. Toxicol., 1976, 16, 697–701.
20. N. J. Ismail, M. H. D. Othman, H. S. Zakria, S. Borhamdin, M. S. Moslan, M. H. Puteh, J. Jaafar, N. Hashim, N. D. Kerisnan and E. Yahaya, Improved visible light-responsive bisphenol A photodegradation utilizing TiO₂/WS₂ photocatalytic membranes with energy storage ability, J. Mater. Sci., 2024, 59, 12361–12383.
21. P. Murugesan, J. A. Moses and C. Anandharamakrishnan, Photocatalytic disinfection efficiency of 2D structure graphitic carbon nitride-based nanocomposites: a review, J. Mater. Sci., 2019, 54, 12206–12235.
22. X. Guan, Y. B. Ren, S. F. Chen, J. F. Yan, G. Wang, H. Y. Zhao, W. Zhao, Z. Y. Zhang, Z. H. Deng, Y. Y. Zhang, Y. Dai, L. D. Zou, R. Y. Chen and C. L. Liu, Charge separation and strong adsorption-enhanced MoO₃ visible light photocatalytic performance, J. Mater. Sci., 2020, 55, 5808–5822.
23. T. Yu, Z. M. Hu, H. M. Wang and X. Tan, Enhanced visible-light-driven hydrogen evolution of ultrathin narrow-band-gap g-C₃N₄ nanosheets, J. Mater. Sci., 2020, 55, 2118–2128.
24. S. M. Chen, B. Li, R. G. Xiao, H. H. Luo, S. W. Yu, J. H. He and X. Liao, Design an Epoxy Coating with TiO₂/GO/PANI Nanocomposites for Enhancing Corrosion Resistance of Q235 Carbon Steel, Materials, 2021, 14, 2629.
25. C. J. Wang, S. Zhang, S. Q. Feng, S. Y. Liu, H. N. Liu, Q. Q. Yin, Z. H. Wang, Z. J. Liu and H. Y. Wang, UV-catalytic fillers with hydrophilic groups encapsulating hydrophobic groups enhance the corrosion resistance of epoxy coatings in harsh environments, Prog. Org. Coat., 2025, 200, 109066.
26. A. Kumar, K. Kumar, P. K. Ghosh and K. L. Yadav, MWCNT/TiO₂ hybrid nano filler toward high-performance epoxy composite, Ultrason. Sonochem., 2018, 41, 37–46.
27. Y. Chen, R. Liu and J. Luo, Improvement of anti-aging property of UV-curable coatings with silica-coated TiO₂, Prog. Org. Coat., 2023, 179, 107479.
28. S. T. Cai, X. Chen, S. L. Wang, X. Q. Liao, Z. Chen and Y. Lin, Silica coating of quantum dots and their applications in optoelectronic fields, Chin. Chem. Lett., 2025, 36, 110798.
29. X. L. Liu, L. G. Tang, G. S. Zhou, J. Q. Wang, M. S. Song, Y. Hang, C. C. Ma, S. Han, M. Yan and Z. Y. Lu, In situ formation of BiVO₄/MoS₂ heterojunction: Enhanced photogenerated carrier transfer rate through electron transport channels constructed by graphene oxide, Mater. Res. Bull., 2023, 157, 112040.
30. B. F. Shan, J. Deng and Z. Y. Zhao, Density Functional Theory Study on the Interfacial Properties of CuS/Bi₂S₃ Heterostructure, Phys. Status Solidi B, 2021, 258, 2100268.
31. B. F. Li, Y. X. Wang, Y. Liu, Z. Han, T. T. Xing, Y. M. Zhang and C. M. Chen, Design and engineering strategies of porous carbonaceous catalysts toward activation of peroxides for aqueous organic pollutants oxidation, Chin. Chem. Lett., 2025, 36, 110374.
32. X. Yang, X. J. Guo, Y. X. Chen, Z. L. Chai, X. X. Sheng, J. K. Liu, X. G. Zeng and X. D. Chen, Preparation and electrochemical inhibition properties of Ce³⁺-photomodified zinc phosphate materials, New J. Chem., 2022, 46, 2068–2080.
33. Y. F. Zhu, R. G. Du, J. Li, H. Q. Qi and C. J. Lin, Photogenerated Cathodic Protection Properties of a TiO₂ Nanowire Film Prepared by a Hydrothermal Method, Acta Phys.-Chim. Sin., 2010, 26, 2349–2353.
34. Y. H. Zhu, H. Liu, D. Y. Zhang, J. Z. Wang and F. Y. Yan, Effect of polarization potentials on tribocorrosion behavior of Monel 400 alloy in seawater environment, Tribol. Int., 2022, 168, 107445.
35. H. Wang, J. H. Xu, X. S. Du, H. B. Wang, X. Cheng and Z. L. Du, Stretchable and self-healing polyurethane coating with synergistic anticorrosion effect for the corrosion protection of stainless steels, Prog. Org. Coat., 2022, 164, 106672.
36. W. L. Shi, Z. Xu, Y. X. Shi, L. L. Li, J. L. Lu, X. H. Sun, X. Du, F. Guo and C. Y. Lu, Constructing S-scheme charge separation in cobalt phthalocyanine/oxygen-doped g-C₃N₄ heterojunction with enhanced photothermal-assisted photocatalytic H₂ evolution, Rare Met., 2024, 43, 198–211.
37. S. Y. Zhou, P. Li, C. B. Zhang, Y. Wang, J. T. Hu and R. K. Jia, Synthesis of O,F Co-Modified g-C₃N₄ for Photocatalytic H₂ Evolution Activity Improvement and Corrosion Protection, Crystallographica, 2024, 14, 1063.
38. S. X. Zuo, Y. Chen, W. J. Liu, C. Yao, Y. R. Li, J. Q. Ma, Y. Kong, H. H. Mao, Z. Y. Li and Y. S. Fu, Polyaniline/g-C₃N₄ composites as novel media for anticorrosion coatings, J. Coat. Technol. Res., 2017, 14, 1307–1314.
39. Y. Q. Xia, N. G. Zhang, Z. P. Zhou, C. L. Chen, Y. Q. Wu, F. Zhong, Y. M. Lv and Y. He, Incorporating SiO₂ functionalized g-C₃N₄ sheets to enhance anticorrosion performance of waterborne epoxy, Prog. Org. Coat., 2020, 147, 105768.
40. P. Praus, A brief review of s-triazine graphitic carbon nitride, Carbon Lett., 2022, 32, 703–712.
41. R. Peymanfar, A. Mohammadi and S. Javanshir, Preparation of graphite-like carbon nitride/polythiophene nanocomposite and investigation of its optical and microwave absorbing characteristics, Compos. Commun., 2020, 21, 100421.
42. X. F. Wang, J. J. Cheng, H. G. Yu and J. G. Yu, A facile hydrothermal synthesis of carbon dots modified g-C₃N₄ for enhanced photocatalytic H₂-evolution performance, Dalton Trans., 2017, 46, 6417–6424.
43. X. H. Wu, X. F. Wang, F. Z. Wang and H. G. Yu, Soluble g-C₃N₄ nanosheets: Facile synthesis and application in photocatalytic hydrogen evolution, Appl. Catal., B, 2019, 247, 70–77.
44. P. P. Singh and V. Srivastava, Recent advances in visible-light graphitic carbon nitride (g-C₃N₄) photocatalysts for chemical transformations, RSC Adv., 2022, 12, 18245–18265.
45. X. D. Zhao, Q. Liu, X. L. Li, H. M. Ji and Z. R. Shen, Two-dimensional g-C₃N₄ nanosheets-based photo-catalysts for typical sustainable processes, Chin. Chem. Lett., 2023, 34, 108306.
46. B. R. Bhagat and A. Dashora, Understanding the synergistic effect of Co-loading and B-doping in g-C₃N₄ for enhanced photocatalytic activity for overall solar water splitting, Carbon, 2021, 178, 666–677.
47. J. Chen, S. H. Shen, P. H. Guo, M. Wang, J. Z. Su, D. M. Zhao and L. J. Guo, Plasmonic Ag@SiO₂ core/shell structure modified g-C₃N₄ with enhanced visible light photocatalytic activity, J. Mater. Res., 2014, 29, 64–70.
48. D. L. Jiang, J. Li, C. S. Xing, Z. Y. Zhang, S. Meng and M. Chen, Two-Dimensional Caln₂S₄/g-C₃N₄ Heterojunction Nanocomposite with Enhanced Visible-Light Photocatalytic Activities: Interfacial Engineering and Mechanism Insight, ACS Appl. Mater. Interfaces, 2015, 7, 19234–19242.
49. R. Zhang, X. M. Zhang, S. W. Liu, J. W. Tong, F. Kong, N. K. Sun, X. L. Han and Y. L. Zhang, Enhanced photocatalytic activity and optical response mechanism of porous graphitic carbon nitride (g-C₃N₄) nanosheets, Mater. Res. Bull., 2021, 140, 111263.
50. X. Y. Song, Y. Y. Duan, S. X. Li, P. Ouyang, L. Chen, H. Ma, W. J. Wang, Y. H. Li and F. Dong, Tailored synthesis of metal clusters modified g-C₃N₄ photocatalysts for energy and environmental applications, Coord. Chem. Rev., 2025, 526, 216351.
51. T. J. Chen, C. J. Song, M. S. Fan, Y. Z. Hong, B. Hu, L. B. Yu and W. D. Shi, In situ fabrication of CuS/g-C₃N₄ nanocomposites with enhanced photocatalytic H₂-production activity via photoinduced interfacial charge transfer, Int. J. Hydrogen Energy, 2017, 42, 12210–12219.
52. X. Zhou, C. H. Zhao, J. H. Chen and L. Y. Chen, Influence of B, Zn, and B-Zn doping on electronic structure and optical properties of g-C₃N₄ photocatalyst: A first-principles study, Results Phys., 2021, 26, 104338.
53. J. L. Wang and S. Z. Wang, A critical review on graphitic carbon nitride (g-C₃N₄)-based materials: Preparation, modification and environmental application, Coord. Chem. Rev., 2022, 453, 214338.
54. Q. Wang, Y. F. Li, F. L. Huang, S. F. Song, G. G. Ai, X. Xin, B. Zhao, Y. J. Zheng and Z. P. Zhang, Recent Advances in g-C₃N₄-Based Materials and Their Application in Energy and Environmental Sustainability, Molecules, 2023, 28, 432.
55. C. M. Xu, H. He, Z. Xu, C. D. Qi, S. Y. Li, L. L. Ma, P. X. Qiu and S. G. Yang, Modification of graphitic carbon nitride by elemental boron cocatalyst with high-efficient charge transfer and photothermal conversion, Chem. Eng. J., 2021, 417, 129203.
56. Q. Liu, Z. G. Zhai, J. Y. Sun, Y. Y. He, Z. B. Yuan and S. J. Chen, Synthesis of Graphitic Carbon Nitride and Polypyrrole Nanocomposite (PPy/g-C₃N₄) as Efficient Photocatalysts for Dibenzothiophene Degradation in Oilfield Produced Wastewater, Int. J. Electrochem. Sci., 2022, 17, 221264.
57. E. Prabakaran, T. Velempini, M. Molefe and K. Pillay, Comparative study of KF, KCl and KBr doped with graphitic carbon nitride for superior photocatalytic degradation of methylene blue under visible light, J. Mater. Res. Technol., 2021, 15, 6340–6355.
58. R. C. Yang, X. G. Teng, X. J. Lu, X. Y. Li, L. Kuai, R. L. Zhang, C. G. Zhang and Z. C. Wu, Effect of Interface Contact Between C and C₃N₄ on Photocatalytic Water Splitting, Catal. Lett., 2018, 148, 1435–1444.
59. L. Zhang, G. Meng, G. Fan, K. Chen, Y. Wu and J. Liu, High flux photocatalytic self-cleaning nanosheet C₃N₄ membrane supported by cellulose nanofibers for dye wastewater purification, Nano Res., 2020, 14, 2568–2573.
60. N. Ding, L. Zhang, M. Hashimoto, K. Iwasaki, N. Chikamori, K. Nakata, Y. Xu, J. Shi, H. Wu, Y. Luo, D. Li, A. Fujishima and Q. Meng, Enhanced photocatalytic activity of mesoporous carbon/C₃N₄ composite photocatalysts, J. Colloid Interface Sci., 2018, 512, 474–479.
61. D. Dutta, A. N. F. Ganda, J. K. Chih, C. C. Huang, C. J. Tseng and C. Y. Su, Revisiting graphene-polymer nanocomposite for enhancing anticorrosion performance: a new insight into interface chemistry and diffusion model, Nanoscale, 2018, 10, 12612–12624.
62. F. R. Wang, X. X. Sheng, M. Zhang, M. Miao, J. K. Liu, J. C. Liu, Y. S. Ma and P. P. Liu, Design and enhanced anticorrosion performance of a Zn₅Mo₂O₁₁·5H₂O/h-BN nanocomposite with labyrinth of nanopores, Nanoscale, 2023, 15, 3199–3211.
63. X. J. Li, Z. Y. Xue, W. T. Sun, J. H. Chu, Q. J. Wang, L. B. Tong and K. S. Wang, Bio-inspired self-healing MXene/polyurethane coating with superior active/passive anticorrosion performance for Mg alloy, Chem. Eng. J., 2023, 454, 140187.
64. C. Ye, Y. G. Zhu, H. Y. Sun, F. Y. Chen, H. F. Sun, W. Dai, Q. P. Wei, L. Fu, A. M. Yu, S. Y. Du, M. H. Yang, L. F. Huang, J. H. Yu, N. Jiang and C. T. Lin, Layer-by-layer stacked graphene nanocoatings by Marangoni self-assembly for corrosion protection of stainless steel, Chin. Chem. Lett., 2021, 32, 501–505.
65. Y. Zhu and Z. C. Zhang, Investigation of the anticorrosion layer of reinforced steel based on graphene oxide in simulated concrete pore solution with 3 wt% NaCl, J. Build. Eng., 2021, 44, 103302.
66. Q. Li, S. X. Zheng, J. B. Pu, J. H. Sun, L. F. Huang, L. P. Wang and Q. J. Xue, Thermodynamics and kinetics of an oxygen adatom on pristine and functionalized graphene: insight gained into their anticorrosion properties, Phys. Chem. Chem. Phys., 2019, 21, 12121–12129.
67. J. Wu, X. C. Ding and X. S. Zhu, Preparation of organic compound/g-C₃N₄ composites and their applications in photocatalysis, Mater. Chem. Front., 2024, 8, 3859–3876.
68. G. Zhao, H. C. Yang, M. Q. Liu and X. J. Xu, Metal-Free Graphitic Carbon Nitride Photocatalyst Goes Into Two-Dimensional Time, Front. Chem., 2018, 6, 551.
69. V. A. Mooss, A. A. Bhopale, P. P. Deshpande and A. A. Athawale, Graphene oxide-modified polyaniline pigment for epoxy based anti-corrosion coatings, Chem. Pap., 2017, 71, 1515–1528.
70. X. Y. Zhang, Y. H. Chen, Z. Q. Jin, H. S. Jiang, Z. Li, Y. Liu, Z. Gao and X. Q. Wang, Direct Z-scheme In₂O₃/g-C₃N₄ nanocomposites: Breaking through the performance limitations of conventional type II heterojunctions for photocathodic protection of Q235 CS in 3.5 wt% NaCl solution, J. Alloys Compd., 2024, 970, 172558.
71. X. S. Cao, L. Yue, F. Lian, C. X. Wang, B. X. Cheng, J. Z. Lv, Z. Y. Wang and B. S. Xing, CuO nanoparticles doping recovered the photocatalytic antialgal activity of graphitic carbon nitride, J. Hazard. Mater., 2021, 403, 123621.
72. Y. Yang, J. F. Zhu, Y. F. He, M. Li, Y. Liu, M. M. Chen and D. W. Cao, Charge transfer in photocatalysis of direct Z-scheme g-C₃N₄-based ferroelectric heterojunction, J. Alloys Compd., 2022, 893, 162270.
73. T. Zhong, W. B. Huang, Z. N. Yao, X. H. Long, W. Qu, H. N. Zhao, S. H. Tian, D. Shu and C. He, Engineering of Graphitic Carbon Nitride (g-C₃N₄) Based Photocatalysts for Atmospheric Protection: Modification Strategies, Recent Progress, and Application Challenges, Small, 2024, 20, 2404696.
74. J. W. Fu, J. G. Yu, C. J. Jiang and B. Cheng, g-C₃N₄-Based Heterostructured Photocatalysts, Adv. Energy Mater., 2018, 8, 1701503.
75. B. Y. Liu, J. Y. Du, G. L. Ke, B. Jia, Y. J. Huang, H. C. He, Y. Zhou and Z. G. Zou, Boosting O₂ Reduction and H₂O Dehydrogenation Kinetics: Surface N-Hydroxymethylation of g-C₃N₄ Photocatalysts for the Efficient Production of H₂O₂, Adv. Funct. Mater., 2022, 32, 2111125.
76. Y. D. Luo, X. Q. Wei, B. Gao, W. X. Zou, Y. L. Zheng, Y. C. Yang, Y. Zhang, Q. Tong and L. Dong, Synergistic adsorption-photocatalysis processes of graphitic carbon nitrate (g-C₃N₄) for contaminant removal: Kinetics, models, and mechanisms, Chem. Eng. J., 2019, 375, 122019.
77. A. Hussain, N. Ali, S. S. Ali, J. H. Hou, I. Aslam, H. Naeem, M. Boota, M. Ul-Hussan, J. Yin and X. Z. Wang, Diverse morphological study for nonmetal-doped g-C₃N₄ composites with narrow bandgap for improved photocatalytic activity, Res. Chem. Intermed., 2022, 48, 2857–2870.
78. T. H. N. Thi, H. T. Huu, H. N. Phi, V. P. Nguyen, Q. D. Le, L. N. Thi, T. T. T. Phan and V. Vo, A facile synthesis of SnS₂/g-C₃N₄ S-scheme heterojunction photocatalyst with enhanced photocatalytic performance, J. Sci.:Adv. Mater. Devices, 2022, 7, 100402.
79. Q. Y. Du, H. D. Zhang, Z. Q. Jiang, K. Xiong, Q. Yang, N. Yang, Y. T. Song and J. Chen, Structural and chemical approaches to enhance the photo/electrocatalytic performance of carbon/g-C₃N₄ composite materials, Chem. Commun., 2023, 59, 8476–8487.
80. L. A. Du, B. Gao, S. Xu and Q. Xu, Strong ferromagnetism of g-C₃N₄ achieved by atomic manipulation, Nat. Commun., 2023, 14, 2278.
81. H. Wang, W. J. He, X. A. Dong, G. M. Jiang, Y. X. Zhang, Y. J. Sun and F. Dong, In situ DRIFT investigation on the photocatalytic NO oxidation mechanism with thermally exfoliated porous g-C₃N₄ nanosheets, RSC Adv., 2017, 7, 19280–19287.
82. F. He, G. Chen, Y. G. Yu, S. Hao, Y. S. Zhou and Y. Zheng, Facile Approach to Synthesize g-PAN/g-C₃N₄ Composites with Enhanced Photocatalytic H₂ Evolution Activity, ACS Appl. Mater. Interfaces, 2014, 6, 7171–7179.
83. A. A. Sivkov and A. Y. Pak, On possible synthesis and crystalline structure of nanodisperse C₃N₄ carbon nitride, Tech. Phys. Lett., 2011, 37, 654–656.
84. L. Li, Z. Y. Huang, Y. F. Yang, Y. Y. Wei, G. K. Liu, Q. Y. Xia and H. L. Wang, Potentials and reaction mechanisms of metal-free B/g-C₃N₄/graphene catalyst for reducing carbon monoxide to ethylene, Mol. Catal., 2025, 578, 114992.
85. D. M. Teter and R. J. Hemley, Low-Compressibility Carbon Nitrides, Science, 1996, 271, 53–55.
86. Y. G. Wang, F. S. Liu, Q. J. Liu, X. Y. Ling, W. P. Wang and M. Zhong, Recover of C₃N₄ nanoparticles under high-pressure by shock wave loading, Ceram. Int., 2018, 44, 19290–19294.
87. D. D. Ma, Z. M. Zhang, Y. J. Zou, J. T. Chen and J. W. Shi, The progress of g-C₃N₄ in photocatalytic H₂ evolution: From fabrication to modification, Coord. Chem. Rev., 2024, 500, 215489.
88. X. Zhang and P. Yang, g-C₃N₄ Nanosheet Nanoarchitectonics: H₂ Generation and CO₂ Reduction, ChemNanoMat, 2023, 9, e202300041.
89. D. Zhang, G. Q. Tan, M. Wang, B. Li, M. Y. Dang, H. J. Ren and A. Xia, The modulation of g-C₃N₄ energy band structure by excitons capture and dissociation, Mater. Res. Bull., 2020, 122, 110685.
90. C. Prasad, H. Tang, Q. Q. Liu, I. Bahadur, S. Karlapudi and Y. J. Jiang, A latest overview on photocatalytic application of g-C₃N₄ based nanostructured materials for hydrogen production, Int. J. Hydrogen Energy, 2020, 45, 337–379.
91. S. W. Cao, J. X. Low, J. G. Yu and M. Jaroniec, Polymeric Photocatalysts Based on Graphitic Carbon Nitride, Adv. Mater., 2015, 27, 2150–2176.
92. H. H. Liu, D. L. Chen, Z. Q. Wang, H. J. Jing and R. Zhang, Microwave-assisted molten-salt rapid synthesis of isotype triazine-/heptazine based g-C₃N₄ heterojunctions with highly enhanced photocatalytic hydrogen evolution performance, Appl. Catal., B, 2017, 203, 300–313.
93. Y. Li, F. Gong, Q. Zhou, X. H. Feng, J. J. Fan and Q. J. Xiang, Crystalline isotype heptazine-/triazine-based carbon nitride heterojunctions for an improved hydrogen evolution, Appl. Catal., B, 2020, 268, 118381.
94. Z. X. Zeng, H. T. Yu, X. Quan, S. Chen and S. S. Zhang, Structuring phase junction between tri-s-triazine and triazine crystalline C₃N₄ for efficient photocatalytic hydrogen evolution, Appl. Catal., B, 2018, 227, 153–160.
95. J. Ortega and O. F. Sankey, Relative stability of hexagonal and planar structures of hypothetical C₃N₄ solids, Phys. Rev. B: Condens. Matter Mater. Phys., 1995, 51, 2624–2627.
96. C. J. Pickard, A. Salamat, M. J. Bojdys, R. J. Needs and P. F. McMillan, Carbon nitride frameworks and dense crystalline polymorphs, Phys. Rev. B, 2016, 94, 094104.
97. M. A. Ahmed, S. A. Mahmoud and A. A. Mohamed, Unveiling the photocatalytic potential of graphitic carbon nitride (g-C₃N₄): a state-of-the-art review, RSC Adv., 2024, 14, 25629–25662.
98. B. C. Zhu, L. Y. Zhang, B. Cheng and J. G. Yu, First-principle calculation study of tri-s-triazine-based g-C₃N₄: A review, Appl. Catal., B, 2018, 224, 983–999.
99. E. O. Oseghe, S. O. Akpotu, E. T. Mombeshora, A. O. Oladipo, L. M. Ombaka, B. B. Maria, A. O. Idris, G. Mamba, L. Ndlwana, O. S. Ayanda, A. E. Ofomaja, V. O. Nyamori, U. Feleni, T. T. I. Nkambule, T. A. M. Msagati, B. B. Mamba and D. W. Bahnemann, Multi-dimensional applications of graphitic carbon nitride nanomaterials - A review, J. Mol. Liq., 2021, 344, 117820.
100. N. S. N. Hasnan, M. A. Mohamed and Z. A. M. Hir, Surface Physicochemistry Modification and Structural Nanoarchitectures of g-C₃N₄ for Wastewater Remediation and Solar Fuel Generation, Adv. Mater. Technol., 2022, 7, 2100993.
101. W. B. Yang, L. H. Jia, P. Wu, H. B. Zhai, J. He, C. J. Liu and W. Jiang, Effect of thermal program on structure-activity relationship of g-C₃N₄ prepared by urea pyrolysis and its application for controllable production of g-C₃N₄, J. Solid State Chem., 2021, 304, 122545.
102. J. H. Liu, T. K. Zhang, Z. C. Wang, G. Dawson and W. Chen, Simple pyrolysis of urea into graphitic carbon nitride with recyclable adsorption and photocatalytic activity, J. Mater. Chem., 2011, 21, 14398–14401.
103. W. Miao, Y. Liu, X. Y. Chen, Y. X. Zhao and S. Mao, Tuning layered Fe-doped g-C₃N₄ structure through pyrolysis for enhanced Fenton and photo-Fenton activities, Carbon, 2020, 159, 461–470.
104. R. Li, Z. Wu, Y. Yang, S. Sun, R. Ma and H. Ding, Photocatalytic persulfate activation by silica microsphere-supported g-C₃N₄ for efficient carbamazepine degradation, Mater. Sci. Semicond. Process., 2024, 184, 108792.
105. H. L. Lee, Z. Sofer, V. Mazánek, J. Luxa, C. K. Chua and M. Pumera, Graphitic carbon nitride: Effects of various precursors on the structural, morphological and electrochemical sensing properties, Appl. Mater. Today, 2017, 8, 150–162.
106. X. H. An, W. M. Xu, S. Zhang, Z. T. Li, J. T. Zheng, J. Zhang, W. T. Wu and M. B. Wu, Further activation of g-C₃N₄ with less N-H defects for enhancing photocatalytic hydrogen evolution, Catal. Commun., 2019, 125, 114–117.
107. J. H. Kim, M. Ji, C. H. Ryu and Y. I. Lee, Effect of pyrolysis conditions on the physicochemical properties of graphitic carbon nitride for visible-light-driven photocatalytic degradation, Arch. Metall. Mater., 2020, 65, 1111–1116.
108. T. K. A. Nguyen, T. T. Pham, N. P. Huy and E. W. Shin, The effect of graphitic carbon nitride precursors on the photocatalytic dye degradation of water-dispersible graphitic carbon nitride photocatalysts, Appl. Surf. Sci., 2021, 537, 148027.
109. Y. Niu, F. Hu, H. Xu, S. Zhang, B. Song, H. Wang, M. Li, G. Shao, H. Wang and H. Lu, Exploration for high performance g-C₃N₄ photocatalyst from different precursors, Mater. Today Commun., 2023, 34, 105040.
110. Y. Y. Ma, Y. Yang, C. X. Lu, X. D. Wen, X. C. Liu, S. J. Wu, K. Lu and J. Q. Yin, Enhanced thermal resistance of carbon/phenolic composites by addition of novel nano-g-C₃N₄, Compos. Sci. Technol., 2019, 180, 60–70.
111. M. Ismael, Y. Wu, D. H. Taffa, P. Bottke and M. Wark, Graphitic carbon nitride synthesized by simple pyrolysis: role of precursor in photocatalytic hydrogen production, New J. Chem., 2019, 43, 6909–6920.
112. H. G. Yu, H. Q. Ma, X. H. Wu, X. F. Wang, J. J. Fan and J. G. Yu, One-Step Realization of Crystallization and Cyano-Group Generation for g-C₃N₄ Photocatalysts with Improved H₂ Production, Sol. RRL, 2021, 5, 2000372.
113. F. Saman, C. H. Se Ling, A. Ayub, N. H. B. Rafeny, A. H. Mahadi, R. Subagyo, R. E. Nugraha, D. Prasetyoko and H. Bahruji, Review on synthesis and modification of g-C₃N₄ for photocatalytic H₂ production, Int. J. Hydrogen Energy, 2024, 77, 1090–1116.
114. J. Bi, Z. Zhu, T. Li and Z. Lv, Research progress on g-C₃N₄-based materials for efficient tetracyclines photodegradation in wastewater: A review, J. Water Process Eng., 2024, 66, 105941.
115. F. Dong, Z. Y. Wang, Y. J. Sun, W. K. Ho and H. D. Zhang, Engineering the nanoarchitecture and texture of polymeric carbon nitride semiconductor for enhanced visible light photocatalytic activity, J. Colloid Interface Sci., 2013, 401, 70–79.
116. D. Yue, S. N. M. Raj, J. V. Kumar, M. W. Alam, P. Rosaiah, M. Selvaraj, I. N. Reddy and C. Bai, History of metal free g-C₃N₄ photocatalysts for hydrogen production: A comprehensive review, Diamond Relat. Mater., 2024, 146, 111228.
117. H. F. Dang, S. H. Mao, Q. Li, M. Y. Li, M. M. Shao, W. L. Wang and Q. B. Liu, Synergy of nitrogen vacancies and partially broken hydrogen bonds in graphitic carbon nitride for superior photocatalytic hydrogen evolution under visible light, Catal. Sci. Technol., 2022, 12, 5032–5044.
118. S. Wang, D. D. Lou, Z. J. Wang, N. Yu, H. F. Wang, Z. G. Chen and L. S. Zhang, Synthesis of ultrathin g-C₃N₄/graphene nanocomposites with excellent visible-light photocatalytic performances, Funct. Mater. Lett., 2019, 12, 1950025.
119. G. Xin and Y. L. Meng, Pyrolysis Synthesized g-C₃N₄ for Photocatalytic Degradation of Methylene Blue, J. Chem., 2013, 2013, 187912.
120. J. Feng, M. M. Gao, Z. Q. Zhang, M. Z. Gu, J. X. Wang, W. J. Zeng and Y. M. Ren, Comparing the photocatalytic properties of g-C₃N₄ treated by thermal decomposition, solvothermal and protonation, Results Phys., 2018, 11, 331–334.
121. B. Q. Chen, X. J. Sun, Y. Z. Hong, Y. W. Tian, E. L. Liu, J. Y. Shi, X. Lin and F. C. Xia, A facile solvothermal recrystallization strategy engineering ultrathin g-C₃N₄ nanosheets for efficient boosting photocatalytic H₂ evolution, Renewable Energy, 2024, 237, 121747.
122. P. A. Nguyen, T. K. A. Nguyen, D. Q. Dao and E. W. Shin, Ethanol Solvothermal Treatment on Graphitic Carbon Nitride Materials for Enhancing Photocatalytic Hydrogen Evolution Performance, Nanomaterials, 2022, 12, 179.
123. C. C. Hu, M. S. Wang and W. Z. Hung, Influence of solvothermal synthesis on the photocatalytic degradation activity of carbon nitride under visible light irradiation, Chem. Eng. Sci., 2017, 167, 1–9.
124. M. T. Abdullahi, M. Ali, W. Farooq, M. Khan, M. Younas and M. N. Tahir, Solvothermal synthesis of carbon nitride (g-C₃N₄): bandgap engineering for improved photocatalytic performance, Sustainable Energy Fuels, 2025, 9, 1109–1119.
125. M. Cao, K. Wang, I. Tudela and X. Fan, Improve photocatalytic performance of g-C₃N₄ through balancing the interstitial and substitutional chlorine doping, Appl. Surf. Sci., 2021, 536, 147784.
126. J. Xu, H. T. Wu, X. Wang, B. Xue, Y. X. Li and Y. Cao, A new and environmentally benign precursor for the synthesis of mesoporous g-C₃N₄ with tunable surface area, Phys. Chem. Chem. Phys., 2013, 15, 4510–4517.
127. W. Zhang, Z. Zhao, F. Dong and Y. Zhang, Solvent-assisted synthesis of porous g-C₃N₄ with efficient visible-light photocatalytic performance for NO removal, Chin. J. Catal., 2017, 38, 372–378.
128. K. Yang, T. Liu and Z. Jin, 3D mesoporous ultra-thin g-C₃N₄ coupled with monoclinic β-AgVO₃ as p-n heterojunction for photocatalytic hydrogen evolution, Mol. Catal., 2021, 513, 111828.
129. F. Y. Rao, J. B. Zhong and J. Z. Li, Improved visible light responsive photocatalytic hydrogen production over g-C₃N₄ with rich carbon vacancies, Ceram. Int., 2022, 48, 1439–1445.
130. W. Iqbal, C. Y. Dong, M. Y. Xing, X. J. Tan and J. L. Zhang, Eco-friendly one-pot synthesis of well-adorned mesoporous g-C₃N₄ with efficiently enhanced visible light photocatalytic activity, Catal. Sci. Technol., 2017, 7, 1726–1734.
131. L. Liang, Y. Cong, F. Wang, L. Yao and L. Shi, Hydrothermal pre-treatment induced cyanamide to prepare porous g-C₃N₄ with boosted photocatalytic performance, Diamond Relat. Mater., 2019, 98, 107499.
132. B. L. Zhao, X. Z. Zou, J. H. Liang, Y. L. Luo, X. X. Liang, Y. W. Zhang and L. Niu, A Unique, Porous C₃N₄ Nanotube for Electrochemiluminescence with High Emission Intensity and Long-Term Stability: The Role of Calcination Atmosphere, Molecules, 2022, 27, 6863.
133. R. R. Zhao, J. P. Gao, S. K. Mei, Y. L. Wu, X. X. Wang, X. G. Zhai, J. B. Yang, C. Y. Hao and J. Yan, Facile synthesis of graphitic C₃N₄ nanoporous-tube with high enhancement of visible-light photocatalytic activity, Nanotechnology, 2017, 28, 495710.
134. Y. Sari, P. L. Gareso, B. Armynah and D. Tahir, A review of TiO₂ photocatalyst for organic degradation and sustainable hydrogen energy production, Int. J. Hydrogen Energy, 2024, 55, 984–996.
135. B. M. Namoos, A. R. Mohamed and K. A. Ali, Methods and strategies for producing porous photocatalysts: Review, J. Solid State Chem., 2023, 320, 123834.
136. S. J. Wang, J. Q. Zhang, B. Li, H. Q. Sun and S. B. Wang, Engineered Graphitic Carbon Nitride-Based Photocatalysts for Visible-Light-Driven Water Splitting: A Review, Energy Fuels, 2021, 35, 6504–6526.
137. Y. Shen, X. J. Guo, X. K. Bo, Y. Z. Wang, X. K. Guo, M. J. Xie and X. F. Guo, Effect of template-induced surface species on electronic structure and photocatalytic activity of g-C₃N₄, Appl. Surf. Sci., 2017, 396, 933–938.
138. Y. S. Chen, B. Yang, W. Y. Xie, X. Y. Zhao, Z. A. Wang, X. T. Su and C. Yang, Combined soft templating with thermal exfoliation toward synthesis of porous g-C₃N₄ nanosheets for improved photocatalytic hydrogen evolution, J. Water Process Eng., 2021, 13, 301–310.
139. K. S. Al-Namshah and R. M. Mohamed, Development of mesoporous Bi₂WO₆/g-C₃N₄ heterojunctions via soft- and hard-template-assisted procedures for accelerated and reinforced photocatalytic reduction of mercuric cations under vis light irradiation, Ceram. Int., 2021, 47, 19268–19268.
140. Y. Zhao, X. Wang, T. Wang, X. Li, Y. Fu, G. Zhao and X. Xu, g-C₃N₄ templated synthesis of 3DOM SnO₂/CN enriched with oxygen vacancies for superior NO₂ gas sensing, Appl. Surf. Sci., 2022, 604, 154618.
141. Y. Wang, F. He, L. Chen, J. Shang, J. Wang, S. Wang, H. Song, J. Zhang, C. Zhao, S. Wang and H. Sun, Acidification and bubble template derived porous g-C₃N₄ for efficient photodegradation and hydrogen evolution, Chin. Chem. Lett., 2020, 31, 2668–2672.
142. A. Kumar, S. Singh and M. Khanuja, A comparative photocatalytic study of pure and acid-etched template free graphitic C₃N₄ on different dyes: An investigation on the influence of surface modifications, Mater. Chem. Phys., 2020, 243, 122402.
143. M. Cheng, P. F. Lv, X. Zhang, R. J. Xiong, Z. B. Guo, Z. Q. Wang, Z. Zhou and M. H. Zhang, A new active species of Pd-Nx synthesized by hard-template method for efficiently catalytic hydrogenation of nitroarenes, J. Catal., 2021, 399, 182–191.
144. X. Gao, Q. Y. Li, Y. L. Wang, Q. Wei, S. P. Cui and Z. R. Nie, A facile soft-hard template cooperative organization approach for mesoporous g-C₃N₄ with high photocatalytic performance, Appl. Surf. Sci., 2024, 657, 159574.
145. Y. Zhang, K. D. Li, J. Liao, X. F. Wei and L. Zhang, Microwave-assisted synthesis of graphitic carbon nitride/CuO nanocomposites and the enhancement of catalytic activities in the thermal decomposition of ammonium perchlorate, Appl. Surf. Sci., 2020, 499, 143875.
146. N. Plubphon, S. Thongtem, A. Phuruangrat, C. Randorn, S. Kaowphong, S. Narksitipan and T. Thongtem, Direct microwave heating synthesis and characterization of highly efficient g-C₃N₄ photocatalyst, Inorg. Chem. Commun., 2022, 139, 109386.
147. N. An, Y. Zhao, Z. Y. Mao, D. K. Agrawal and D. J. Wang, Microwave modification of surface hydroxyl density for g-C₃N₄ with enhanced photocatalytic activity, Mater. Res. Express, 2018, 5, 035502.
148. Y. P. Yuan, L. S. Yin, S. W. Cao, L. N. Gu, G. S. Xu, P. W. Du, H. Chai, Y. S. Liao and C. Xue, Microwave-assisted heating synthesis: a general and rapid strategy for large-scale production of highly crystalline g-C₃N₄ with enhanced photocatalytic H₂ production, Green Chem., 2014, 16, 4663–4668.
149. Y. Liu, X. Guo, Z. Chen, W. Zhang, Y. Wang, Y. Zheng, X. Tang, M. Zhang, Z. Peng, R. Li and Y. Huang, Microwave-synthesis of g-C₃N₄ nanoribbons assembled seaweed-like architecture with enhanced photocatalytic property, Appl. Catal., B, 2020, 266, 118624.
150. R. Chen, L. Wang, J. Ding, J. Zhang, H. Wan and G. Guan, Microwave-assisted construction of Bi₂MoO₆/g-C₃N₄ heterostructure for boosting photocatalytic CO₂ conversion, J. Alloys Compd., 2023, 960, 170605.
151. Q. Liu, X. Wang, Q. Yang, Z. Zhang and X. Fang, A novel route combined precursor-hydrothermal pretreatment with microwave heating for preparing holey g-C₃N₄ nanosheets with high crystalline quality and extended visible light absorption, Appl. Catal., B, 2018, 225, 22–29.
152. K. Schutjajew, P. Giusto, E. Härk and M. Oschatz, Preparation of hard carbon/carbon nitride nanocomposites by chemical vapor deposition to reveal the impact of open and closed porosity on sodium storage, Carbon, 2021, 185, 697–708.
153. B. Y. Liang, D. H. Han, C. H. Sun, W. X. Zhang and Q. Qin, Deposition of ZnO flowers on the surface of g-C₃N₄ by solid phase reaction, Funct. Mater. Lett., 2018, 11, 1850020.
154. M. L. Zhang, Y. Yang, X. Q. An, J. J. Zhao, Y. P. Bao and L. A. Hou, Exfoliation method matters: The microstructure-dependent photoactivity of g-C₃N₄ nanosheets for water purification, J. Hazard. Mater., 2022, 424, 127424.
155. J. Wei, X. X. Fu, J. H. Dong and W. Ke, Corrosion Evolution of Reinforcing Steel in Concrete under Dry/Wet Cyclic Conditions Contaminated with Chloride, J. Mater. Sci. Technol., 2012, 28, 905–912.
156. L. Lei, H. Fan, Y. Jia, X. Wu, Q. Zhong and W. Wang, Ultrafast charge-transfer at interfaces between 2D graphitic carbon nitride thin film and carbon fiber towards enhanced photocatalytic hydrogen evolution, Appl. Surf. Sci., 2022, 606, 154938.
157. Y.-J. Yuan, Z. Shen, S. Wu, Y. Su, L. Pei, Z. Ji, M. Ding, W. Bai, Y. Chen, Z. T. Yu and Z. Zou, Liquid exfoliation of g-C₃N₄ nanosheets to construct 2D-2D MoS₂/g-C₃N₄ photocatalyst for enhanced photocatalytic H2 production activity, Appl. Catal., B, 2019, 246, 120–128.
158. Y. Shi, L. Li, H. Sun, Z. Xu, Y. Cai, W. Shi, F. Guo and X. Du, Engineering ultrathin oxygen-doped g-C₃N₄ nanosheet for boosted photoredox catalytic activity based on a facile thermal gas-shocking exfoliation effect, Sep. Purif. Technol., 2022, 292, 121038.
159. I. Papailias, N. Todorova, T. Giannakopoulou, N. Ioannidis, N. Boukos, C. P. Athanasekou, D. Dimotikali and C. Trapalis, Chemical vs thermal exfoliation of g-C₃N₄ for NO_x removal under visible light irradiation, Appl. Catal., B, 2018, 239, 16–26.
160. X. F. Lu, Q. L. Wang and D. L. Cui, Preparation and Photocatalytic Properties of g-C₃N₄/TiO₂ Hybrid Composite, J. Mater. Sci. Technol., 2010, 26, 925–930.
161. C. Feng, Z. Y. Chen, J. P. Jing, M. M. Sun, J. Tian, G. Y. Lu, L. Ma, X. B. Li and J. Hou, Significantly enhanced photocatalytic hydrogen production performance of g-C₃N₄/CNTs/CdZnS with carbon nanotubes as the electron mediators, J. Mater. Sci. Technol., 2021, 80, 75–83.
162. Q. Yan, G. F. Huang, D. F. Li, M. Zhang, A. L. Pan and W. Q. Huang, Facile synthesis and superior photocatalytic and electrocatalytic performances of porous B-doped g-C₃N₄ nanosheets, J. Mater. Sci. Technol., 2018, 34, 2515–2520.
163. W. Guo, S. J. Ming, Z. Chen, J. J. Bi, Y. J. Ma, J. Y. Wang and T. Li, A Novel CVD Growth of g-C₃N₄ Ultrathin Film on NiCo₂O₄ Nanoneedles/Carbon Cloth as Integrated Electrodes for Supercapacitors, ChemElectroChem, 2018, 5, 3383–3390.
164. M. Sun, Q. Yan, T. Yan, M. M. Li, D. Wei, Z. P. Wang, Q. Wei and B. Du, Facile fabrication of 3D flower-like heterostructured g-C₃N₄/SnS₂ composite with efficient photocatalytic activity under visible light, RSC Adv., 2014, 4, 31019–31027.
165. W. Q. Cai, D. F. Zhang, F. J. Zhang and W. C. Oh, Preparation and photocatalytic activity of a novel BiOCl/g-C₃N₄ thin film prepared via spin coating, J. Korean Ceram. Soc., 2020, 57, 331–337.
166. Z. X. Liu, J. J. Tian, P. Liu, J. C. Liu and J. K. Liu, Design and Synthesis of Mosaic ZnO/g-C₃N₄ Heterojunction Materials with Excellent Anticorrosion Performance, Ind. Eng. Chem. Res., 2024, 63, 13218–13229.
167. M. Tabish, J. M. Zhao, A. Kumar, J. T. Yan, J. B. Wang, F. Shi, J. Zhang, L. J. Peng, M. A. Mushtaq and G. Yasin, Developing epoxy-based anti-corrosion functional nanocomposite coating with CaFe-Tolyl-triazole layered double hydroxide@g-C₃N₄ as nanofillers on Q235 steel substrate against NaCl corrosive environment, Chem. Eng. J., 2022, 450, 137624.
168. Y. Sun, S. Yuan, Z. Bai, B. Liang, D. Fu, H. Hu, L. Pei, R. Wang, Y. Zhu and H. Wang, A unique anti-corrosion composite coating with CO₂ gas barrier and acid resistance suitable for CCUS environment, Chem. Eng. J., 2023, 472, 144879.
169. B. Lin, H. Li, H. An, W. B. Hao, J. J. Wei, Y. Z. Dai, C. S. Ma and G. D. Yang, Preparation of 2D/2D g-C₃N₄ nanosheet@ZnIn₂S₄ nanoleaf heterojunctions with well -designed high-speed charge transfer nanochannels towards high efficiency photocatalytic hydrogen evolution, Appl. Catal., B, 2018, 220, 542–552.
170. H. Zhang, L. H. Guo, L. X. Zhao, B. Wan and Y. Yang, Switching Oxygen Reduction Pathway by Exfoliating Graphitic Carbon Nitride for Enhanced Photocatalytic Phenol Degradation, J. Phys. Chem. Lett., 2015, 6, 958–963.
171. F. Guo, X. Huang, Z. Chen, H. Sun and L. Chen, Prominent co-catalytic effect of CoP nanoparticles anchored on high-crystalline g-C₃N₄ nanosheets for enhanced visible-light photocatalytic degradation of tetracycline in wastewater, Chem. Eng. J., 2020, 395, 125118.
172. M. Wu, J. M. Yan, X. W. Zhang and M. Zhao, Synthesis of g-C₃N₄ with heating acetic acid treated melamine and its photocatalytic activity for hydrogen evolution, Appl. Surf. Sci., 2015, 354, 196–200.
173. B. Y. Chang, A novel analysis method for electrochemical impedance spectra using deep learning, Electrochim. Acta, 2023, 462, 142741.
174. A. Thapa and H. W. Gao, Low-frequency Inductive Loop and Its Origin in the Impedance Spectrum of a Graphite Anode, J. Electrochem. Soc., 2022, 169, 110535.
175. S. Surender, M. N. Kavipriyah and S. Balakumar, Synergistic effect in g-C₃N₄/CuO nanohybrid structures as efficient electrode material for supercapacitor applications, Inorg. Chem. Commun., 2023, 150, 110557.
176. D. Tuschel, Photoluminescence Spectroscopy Using a Raman Spectrometer, Spectroscopy, 2016, 31, 14–21.
177. M. Li, Z. Hu, D. Liu, Y. Liang, S. Liu, B. Wang, C. Niu, D. Xu, J. Li and B. Han, Efficient antibacterial and microbial corrosion resistant photocatalytic coating: Enhancing performance with S-type heterojunction and Cu synergy, Chem. Eng. J., 2024, 495, 153519.
178. C. Li, C. Zhang, Y. He, H. Li, Y. Zhao, Z. Li, D. Sun and X. Yin, Benzotriazole corrosion inhibitor loaded nanocontainer based on g-C₃N₄ and hollow polyaniline spheres towards enhancing anticorrosion performance of waterborne epoxy coatings, Prog. Org. Coat., 2023, 174, 107276.
179. M. Ghaderi, H. C. Bi and K. Dam-Johansen, Ultra-stable metal-organic framework-derived carbon nanocontainers with defect-induced pore enlargement for anti-corrosive epoxy coatings, J. Colloid Interface Sci., 2025, 681, 130–147.
180. J. F. Wu, W. H. Pu, C. Z. Yang, M. Zhang and J. D. Zhang, Removal of benzotriazole by heterogeneous photoelectro-Fenton like process using ZnFe₂O₄ nanoparticles as catalyst, J. Environ. Sci., 2013, 25, 801–807.
181. Y. H. Lei, N. Sheng, A. Hyono, M. Ueda and T. Ohtsuka, Effect of benzotriazole (BTA) addition on Polypyrrole film formation on copper and its corrosion protection, Prog. Org. Coat., 2014, 77, 339–346.
182. J. H. Di, Y. Lu, W. W. Wang, X. Y. Wang, C. L. Yu, J. Zhao, F. Zhang and S. P. Gao, Transparent g-C₃N₄ thin film: Enhanced photocatalytic performance and convenient recycling, J. Phys. Chem. Solids, 2021, 155, 110114.
183. S. Minato, H. Nagata, M. Matsuda and N. Ohtani, Preparation of dispersible carbon nitride powder and fabrication of its thin films by wet-process, Mol. Cryst. Liq. Cryst., 2024, 768, 868–882.
184. D. N. Sutar, A. K. Pramod, H. Islam, A. V. S. Sainath, U. Pal and S. K. Batabyal, Cs₃Bi₂Cl₃Br₆:g-C₃N₄ Nanostructure-Based Thin Film Photocatalysts for Hydrogen Production under Daylight and Simulated Light, ACS Appl. Nano Mater., 2024, 7, 25556–25568.
185. Y. Bu, Z. Chen, J. Yu and W. Li, A novel application of g-C₃N₄ thin film in photoelectrochemical anticorrosion, Electrochim. Acta, 2013, 88, 294–300.
186. R. Tejasvi and S. Basu, Formation of C₃N₄ thin films through the stoichiometric transfer of the bulk synthesized g-C₃N₄ using RFM sputtering, Vacuum, 2020, 171, 108937.
187. Q. S. Yao, M. Lu, Y. P. Du, F. Wu, K. M. Deng and E. J. Kan, Designing half-metallic ferromagnetism by a new strategy: an example of superhalogen modified graphitic C₃N₄, J. Mater. Chem. C, 2018, 6, 1709–1714.
188. G. Yang, T. Chen, B. Feng, J. Weng, K. Duan, J. Wang and X. Lu, Improved corrosion resistance and biocompatibility of biodegradable magnesium alloy by coating graphite carbon nitride (g-C₃N₄), J. Alloys Compd., 2019, 770, 823–830.
189. X. Zhu, F. Guo, J. Pan, H. Sun, L. Gao, J. Deng, X. Zhu and W. Shi, Fabrication of visible-light-response face-contact ZnSnO₃@g-C₃N₄ core-shell heterojunction for highly efficient photocatalytic degradation of tetracycline contaminant and mechanism insight, J. Mater. Sci., 2020, 56, 4366–4379.
190. N. I. M. Rosli, S. M. Lam, J. C. Sin and A. R. Mohamed, Fabrication of Z-scheme rod-like Ag₂Mo₂O₇/g-C₃N₄ for phenol degradation under IO₄⁻/visible light system, Mater. Lett., 2021, 294, 129791.
191. K. V. Rybalka, L. A. Beketaeva and A. D. Davydov, Estimation of corrosion current by the analysis of polarization curves: Electrochemical kinetics mode, Russ. J. Electrochem., 2014, 50, 108–113.
192. P. Khadke, T. Tichter, T. Boettcher, F. Muench, W. Ensinger and C. Roth, A simple and effective method for the accurate extraction of kinetic parameters using differential Tafel plots, Sci. Rep., 2021, 11, 8974.
193. Y. Liu, Q. L. Yu, M. R. Cai, B. Yu and F. Zhou, Effects of Nanoadditives on the Anticorrosion Performance of Nanocomposite Coatings: A Review, ACS Appl. Nano Mater., 2024, 7, 12249–12272.
194. L. Gu, J. H. Ding and H. B. Yu, Research in Graphene-Based Anticorrosion Coatings, Prog. Chem., 2016, 28, 737–743.
195. C. Kong, D. Qing, X. Su, Y. Zhao, J. Wang and X. Zeng, Improved photoelectrochemical cathodic protection properties of a flower-like SrTiO₃ photoanode decorated With g-C₃N₄, J. Alloys Compd., 2022, 924, 166629.
196. Y. Y. Bu and J. P. Ao, A review on photoelectrochemical cathodic protection semiconductor thin films for metals, Green Energy Environ., 2017, 2, 331–362.
197. P. C. Deng, J. Y. Shangguan, J. Z. Hu, H. Huang and L. B. Zhou, Anticorrosion Method Combining Impressed Current Cathodic Protection and Coatings in Marine Atmospheric Environment, Coatings, 2024, 14, 524.
198. A. V. Poshakinskiy, A. N. Poddubny and N. A. Gippius, Doppler-Raman crossover in resonant scattering by a moving layer, Phys. Rev. A, 2020, 102, 043523.
199. I. A. Metwally and A. Al-Badi, Factors affecting pulsed-cathodic protection effectiveness for deep well casings, Anti-Corros. Methods Mater., 2009, 56, 196–205.
200. Z. C. Guan, J. Hu, H. H. Wang, H. Y. Shi, H. P. Wang, X. Wang, P. Jin, G. L. Song and R. G. Du, Decoration of rutile TiO₂ nanorod film with g-C₃N₄/SrTiO₃ for efficient photoelectrochemical cathodic protection, J. Photochem. Photobiol., A, 2023, 443, 114825.
201. L. P. Wang, M. M. Niu, Y. Liu, Y. K. Xie, Z. C. Ma, M. Y. Zhang and C. T. Hou, The Ovs surface defecting of an S-scheme g-C₃N₄/H₂Ti₃O₇ nanoheterostructures with accelerated spatial charge transfer, J. Colloid Interface Sci., 2023, 645, 639–653.
202. Q. Y. Guo, Y. H. Zhang, J. R. Qiu and G. P. Dong, Engineering the electronic structure and optical properties of g-C₃N₄ by non-metal ion doping, J. Mater. Chem. C, 2016, 4, 6839–6847.
203. Q. J. Zhang, J. P. Jing, Z. Y. Chen, M. M. Sun, J. R. Li, Y. Li and L. K. Xu, Enhanced photoelectrochemical cathodic protection performance of g-C₃N₄ caused by the co-modification with N defects and C deposition, J. Mater. Sci.:Mater., 2019, 30, 15267–15276.
204. J. Z. Kong, H. F. Zhai, W. Zhang, S. S. Wang, X. R. Zhao, M. Li, H. Li, A. D. Li and D. Wu, Visible Light-Driven Photocatalytic Performance of N-Doped ZnO/g-C₃N₄ Nanocomposites, Nanoscale Res. Lett., 2017, 12, 114825.
205. Y. Shen, Y. Wang, P. Shan, R. Xu, X. Sun, J. Hou, F. Guo, C. Li and W. Shi, Boosted photo-self-Fenton degradation activity by Fe-doped carbon dots as dual-function active sites for in situ H₂O₂ generation and activation, Sep. Purif. Technol., 2025, 353, 128529.
206. R. Singh, M. Chauhan, P. Garg, B. Sharma, P. Attri, R. K. Sharma, D. Sharma and G. R. Chaudhary, A critical review on visible light active graphitic carbon nitride (g-CN) based photocatalyst for environment remediation application: A sustainable approach, J. Cleaner Prod., 2023, 427, 138855.
207. B. Y. Liu, X. Q. Nie, Y. Tang, S. Yang, L. Bian, F. Q. Dong, H. C. He, Y. Zhou and K. Liu, Objective Findings on the K-Doped g-C₃N₄ Photocatalysts: The Presence and Influence of Organic Byproducts on K-Doped g-C₃N₄ Photocatalysis, Langmuir, 2021, 37, 4859–4868.
208. H. Sun, F. Guo, J. Pan, W. Huang, K. Wang and W. Shi, One-pot thermal polymerization route to prepare N-deficient modified g-C₃N₄ for the degradation of tetracycline by the synergistic effect of photocatalysis and persulfate-based advanced oxidation process, Chem. Eng. J., 2021, 406, 126844.
209. W. Shi, S. Yang, H. Sun, J. Wang, X. Lin, F. Guo and J. Shi, Carbon dots anchored high-crystalline g-C₃N₄ as a metal-free composite photocatalyst for boosted photocatalytic degradation of tetracycline under visible light, J. Mater. Sci., 2020, 56, 2226–2240.
210. L. Li, Y. Shi, Z. Xu, H. Sun, M. D. S. Amin, X. Yang, F. Guo and W. Shi, Environmentally friendly synthesis of oxygen-doped g-C₃N₄ nanosheets for enhancing photocatalytic corrosion resistance of carbon steel, Prog. Org. Coat., 2022, 163, 106628.
211. F. R. Sultanov, C. Daulbayev, B. Bakbolat, Z. A. Mansurov, A. A. Urazgaliyeva, R. Ebrahim, S. S. Pei and K. P. Huang, Microwave-enhanced chemical vapor deposition graphene nanoplatelets-derived 3D porous materials for oil/water separation, Carbon Lett., 2020, 30, 81–92.
212. F. A. Stevie and C. L. Donley, Introduction to X-ray photoelectron spectroscopy, J. Vac. Sci. Technol., A, 2020, 38, 063204.
213. Q. Q. Li, J. H. Huang, S. Q. Liu, L. X. Guan and K. L. Huang, Synthesis and characterization of polypyrrole doped by p-tolyl sulfonic acid and its anticorrosion property for magnesium, Acta Chim. Sin., 2008, 66, 571–575.
214. M. Suryamathi, K. Ramachandran, P. Viswanathamurthi and R. Ramesh, Hematite nanofibers based photoanode for effective photoelectrochemical water oxidation, J. Mater. Sci.:Mater., 2022, 33, 9180–9193.
215. F. Guo, L. Wang, H. Sun, M. Li, W. Shi and X. Lin, A one-pot sealed ammonia self-etching strategy to synthesis of N-defective g-C₃N₄ for enhanced visible-light photocatalytic hydrogen, Int. J. Hydrogen Energy, 2020, 45, 30521–30532.
216. I. Iribarren, G. Sánchez-Sanz, I. Alkorta, J. Elguero and C. Trujillo, Evaluation of Electron Density Shifts in Noncovalent Interactions, J. Phys. Chem. A, 2021, 125, 4741–4749.
217. N. R. Lugg, G. Kothleitner, N. Shibata and Y. Ikuhara, On the quantitativeness of EDS STEM, Ultramicroscopy, 2015, 151, 150–159.
218. L. Y. Yang, C. P. Liu, D. L. Zhang, Z. T. Yang, Z. Y. Wei, H. M. Fan, X. N. Huang, S. N. Zhao and C. He, Chitosan-functionalized graphitic carbon nitride for enhanced corrosion resistance in epoxy coatings, Colloids Surf., A, 2025, 707, 135924.
219. J. Jing, Z. Chen, C. Feng, M. Sun and J. Hou, Transforming g-C₃N₄ from amphoteric to n-type semiconductor: The important role of p/n type on photoelectrochemical cathodic protection, J. Alloys Compd., 2021, 851, 156820.
220. J. Wei, X. X. Fu, J. H. Dong and W. Ke, Corrosion Evolution of Reinforcing Steel in Concrete under Dry/Wet Cyclic Conditions Contaminated with Chloride, J. Mater. Sci. Technol., 2012, 28, 905–912.
221. V. C. Anadebe, V. I. Chukwuike, V. Selvaraj, A. Pandikumar and R. C. Barik, Sulfur-doped graphitic carbon nitride (S-g-C₃N₄) as an efficient corrosion inhibitor for X65 pipeline steel in CO₂- saturated 3.5% NaCl solution: Electrochemical, XPS and Nanoindentation Studies, Process Saf. Environ. Prot., 2022, 164, 715–728.
222. S. Iqbal, A. Bahadur, S. Ali, Z. Ahmad, M. Javed, R. M. Irfan, N. Ahmad, M. A. Qamar, G. C. Liu, M. B. Akbar and M. Nawaz, Critical role of the heterojunction interface of silver decorated ZnO nanocomposite with sulfurized graphitic carbon nitride heterostructure materials for photocatalytic applications, J. Alloys Compd., 2021, 858, 158338.
223. Y. Shen, R. Xu, P. Shan, S. Zhang, L. Sun, H. Xie, F. Guo, C. Li and W. Shi, Abundant Edge Active Sites-Modified High-Crystalline g-C₃N₅ for Hydrogen Peroxide Production from Pure-Water via a Quasi-Homogeneous Photocatalytic Process, Small, 2024, 20, 2401566.
224. D. L. Huang, X. L. Yan, M. Yan, G. M. Zeng, C. Y. Zhou, J. Wan, M. Cheng and W. J. Xue, Graphitic Carbon Nitride-Based Heterojunction Photoactive Nanocomposites: Applications and Mechanism Insight, ACS Appl. Mater. Interfaces, 2018, 10, 21035–21055.
225. F. Guo, X. Huang, Z. Chen, L. Cao, X. Cheng, L. Chen and W. Shi, Construction of Cu₃P-ZnSnO₃-g-C₃N₄ p-n-n heterojunction with multiple built-in electric fields for effectively boosting visible-light photocatalytic degradation of broad-spectrum antibiotics, Sep. Purif. Technol., 2021, 265, 118477.
226. Y. M. Feng, Y. Z. Wang, M. Y. Li, S. S. Lv, W. Li and Z. C. Li, Novel visible light induced Ag₂S/g-C₃N₄/ZnO nanoarrays heterojunction for efficient photocatalytic performance, Appl. Surf. Sci., 2018, 462, 896–903.
227. W. Zhang, W. Shi, H. Sun, Y. Shi, H. Luo, S. Jing, Y. Fan, F. Guo and C. Lu, Fabrication of ternary CoO/g-C₃N₄/Co₃O₄ nanocomposite with p-n-p type heterojunction for boosted visible-light photocatalytic performance, J. Chem. Technol. Biotechnol., 2021, 96, 1854–1863.
228. J. J. Li, W. Wei, C. Mu, B. B. Huang and Y. Dai, Electronic properties of g-C₃N₄/CdS heterojunction from the first-principles, Phys. E, 2018, 103, 459–463.
229. S. W. Xu, Z. W. Li, J. H. Wen, P. Qiu, A. J. Xie and H. P. Peng, Review of TiO₂-Based Heterojunction Coatings in Photocathodic Protection, ACS Appl. Nano Mater., 2024, 7, 8464–8488.
230. J. L. Mo, L. Li, X. D. Li, X. Y. Yao, X. Xiang and X. T. Zu, First-principles study of a novel type-II As₂C₃/MoSi₂N₄ heterostructure with high carrier mobility in photocatalytic water splitting, Int. J. Hydrogen Energy, 2025, 100, 982–993.
231. X. Q. Cao, Y. Y. Huang, Y. Y. Xi, Z. Lei, J. Wang, H. N. Liu, M. J. Shi, T. T. Han, M. E. Zhang and X. L. Xu, Interfacial photoconductivity effect of type-I and type-II Sb₂Se₃/Si heterojunctions for THz wave modulation, Chin. Phys. B, 2023, 32, 116701.
232. P. Hao, P. Shan, J. Lu, L. Sun, H. Qin, F. Guo, C. Li and W. Shi, Magnetic-field-induced activation of S-scheme heterojunction with core-shell structure for boosted photothermal-assisted photocatalytic H₂ production, Fuel, 2024, 373, 132394.
233. H. W. Huang, C. Y. Liu, H. L. Ou, T. Y. Ma and Y. H. Zhang, Self-sacrifice transformation for fabrication of type-I and type-II heterojunctions in hierarchical Bi_xO_yI_z/g-C₃N₄ for efficient visible-light photocatalysis, Appl. Surf. Sci., 2019, 470, 1101–1110.
234. D. D. Zhang, G. Q. Xu and F. Chen, Hollow spheric Ag-Ag₂S/TiO₂ composite and its application for photocatalytic reduction of Cr(VI), Appl. Surf. Sci., 2015, 351, 962–968.
235. K. Geng, P. Shan, J. Shi, Y. Shen, S. Yan, M. Zhao, F. Guo, G. Wang and W. Shi, Broad spectral absorption cooperates with local plasma resonance for promoted photothermal-assisted photocatalytic hydrogen production, Chem. Eng. J., 2025, 507, 160561.
236. M. Zhao, X. Yang, X. Li, Z. Tang and Z. Song, Photocathodic protection performance of Ni₃S₂/g-C₃N₄ photoanode for 304 stainless steel, J. Electroanal. Chem., 2021, 893, 115324.
237. P. Shan, P. Hao, K. Geng, W. Xue, B. Xiong, J. Hou, F. Guo, Y. Sun and W. Shi, Hierarchical confinement effect of Co-Co Prussian blue analogues for photothermal-assisted photocatalytic H₂ production, Chem. Eng. J., 2025, 509, 161502.
238. H. M. Zeng, Y. L. Liu, Z. G. Xu, Y. Wang, Y. Q. Chai, R. Yuan and H. Y. Liu, Construction of a Z-scheme g-C₃N₄/Ag/AgI heterojunction for highly selective photoelectrochemical detection of hydrogen sulfide, Chem. Commun., 2019, 55, 11940–11943.
239. J. Gao, G. Xu, Q. F. Liang, X. X. Xu, H. L. Wang, S. Y. Liu, J. Ren, Z. B. Fang and S. H. Wei, Solar-driven overall water splitting over a S-scheme semiconductor heterojunction of 2D g-C₃N₄/1D LaNbON₂, Appl. Catal., A, 2023, 666, 119420.
240. Y. Ma, P. Li, P. Wu, M. Guo, Y. Yang and X. Li, A g-C₃N₄/PANI S-scheme heterojunction with face-to-face structure to improve the anticorrosion property on Q235 steel and enhanced mechanism study, J. Appl. Polym. Sci., 2024, 141, e55296.
241. Q. Yang, X. T. Wang, J. Shi, J. Q. Wei and Y. Q. He, Constructed a novel of Znln₂S₄/S-C₃N₄ heterogeneous catalyst for efficient photodegradation of tetracycline, Environ. Sci. Pollut. Res., 2023, 30, 111152–111164.
242. N. Thangavel, K. Pandi, A. R. M. Shaheer and B. Neppolian, Surface-state-induced upward band bending in P doped g-C₃N₄ for the formation of an isotype heterojunction between bulk g-C₃N₄ and P doped g-C₃N₄: photocatalytic hydrogen production, Catal. Sci. Technol., 2020, 10, 8015–8025.
243. L. L. Bi, D. D. Xu, L. J. Zhang, Y. H. Lin, D. J. Wang and T. F. Xie, Metal Ni-loaded g-C₃N₄ for enhanced photocatalytic H₂ evolution activity: the change in surface band bending, Phys. Chem. Chem. Phys., 2015, 17, 29899–29905.
244. H. J. Zhao, Y. Zhou, R. J. Wu, Z. B. Han, X. Li and Z. Yu, A 2D/2D BiPO₄/g-C₃N₄-B Z-type heterojunction for enhanced photocatalytic degradation of dye pollutants, Korean J. Chem. Eng., 2023, 40, 3068–3078.
245. T. Van Nguyen, M. Tekalgne, T. P. Nguyen, Q. Van Le, S. H. Ahn and S. Y. Kim, Electrocatalysts based on MoS₂ and WS₂ for hydrogen evolution reaction: An overview, Battery Energy, 2023, 2, 20220057.
246. Z. J. Lan, Y. L. Yu, J. H. Yao and Y. A. Cao, The band structure and photocatalytic mechanism of MoS₂-modified C₃N₄ photocatalysts with improved visible photocatalytic activity, Mater. Res. Bull., 2018, 102, 433–439.
247. J. J. Liu, C. B. Xiong, S. J. Jiang, X. Wu and S. Q. Song, Efficient evolution of reactive oxygen species over the coordinated π-delocalization g-C₃N₄ with favorable charge transfer for sustainable pollutant elimination, Appl. Catal., B, 2019, 249, 282–291.
248. S. Zhang, Y. Shen, Y. Yan, F. Guo and W. Shi, Integrated CuO/g-C₃N₄ S-scheme heterojunction self-healing coatings: A synergistic approach for advanced anti-corrosion and anti-biofouling performance, J. Mater. Sci. Technol., 2025, 223, 22–33.
249. Y. X. Ge, X. J. Guo, D. Zhou and J. K. Liu, Construction and excellent photoelectric synergistic anticorrosion performance of Z-scheme carbon nitride/tungsten oxide heterojunctions, Nanoscale, 2022, 14, 12358–12376.
250. S. J. Liu, F. T. Li, Y. L. Li, Y. J. Hao, X. J. Wang, B. Li and R. H. Liu, Fabrication of ternary g-C₃N₄/Al₂IO₃/ZnO heterojunctions based on cascade electron transfer toward molecular oxygen activation, Appl. Catal., B, 2017, 212, 115–128.
251. K. Sun, X. Wang, H. Yuan, J. Hou, W. Shi, C. Li and F. Guo, Magnetically separable and recyclable ZnFe₂O₄ nanoparticles as an effective activator in resorcinol-formaldehyde resins-based photocatalysis-self-Fenton system, Sep. Purif. Technol., 2024, 351, 128044.
252. R. Cheng, J. Y. Wen, J. C. Xia, Z. Y. Li, W. Z. Sun, L. J. Shen, L. Shi and X. Zheng, Visible-light photocatalytic activity and photo-corrosion mechanism of Ag₃PO₄/g-C₃N₄/PVA composite film in degrading gaseous toluene, Catal. Today, 2019, 335, 565–573.
253. Y. F. Chen, W. X. Huang, D. L. He, S. T. Yue and H. Huang, Construction of Heterostructured g-C₃N₄/Ag/TiO₂ Micro-spheres with Enhanced Photocatalysis Performance under Visible-Light Irradiation, ACS Appl. Mater. Interfaces, 2014, 6, 14405–14414.
254. D. Yan, X. Wu, J. Y. Pei, C. C. Wu, X. M. Wang and H. Y. Zhao, Construction of g-C₃N₄/TiO₂/Ag composites with enhanced visible-light photocatalytic activity and antibacterial properties, Ceram. Int., 2020, 46, 696–702.
255. S. H. Zhou, S. K. Liu, K. Su and K. L. Jia, Graphite carbon nitride coupled S-doped hydrogenated TiO₂ nanotube arrays with improved photoelectrochemical performance, J. Electroanal. Chem., 2020, 862, 114008.
256. Z. H. Xie, Y. Wen, H. Yao, Y. Liu, G. Yu and C. J. Zhong, A Z scheme g-C₃N₄/TiO₂ heterojunction for enhanced performance in protecting magnesium and nickel couple from galvanic corrosion, J. Alloys Compd., 2025, 1010, 177463.
257. S. Pourhashem, J. Z. Duan, F. Guan, N. Wang, Y. Gao and B. R. Hou, Neweffects of TiO₂ nanotube/g-C₃N₄ hybrids on the corrosion protection performance of epoxy coatings, J. Mol. Liq., 2020, 317, 114214.
258. Z. H. Xie, Y. Wen, H. Yao, Y. Liu, G. Yu and C. J. Zhong, A Z-scheme g-C₃N₄/TiO₂ heterojunction for enhanced performance in protecting magnesium and nickel couple from galvanic corrosion, J. Alloys Compd., 2025, 1010, 177463.
259. W. Zhang, X. Tao, H. Guo, J. Ding, H. Sun and X. Zhan, Enhanced photoelectrochemical protection of 304 stainless steel induced by the internal electric field effect of a g-C₃N₄/graphene/TiO₂ Z-scheme system, Mater. Res. Bull., 2023, 164, 112283.
260. H. Feng, T. Wang, L. Cao, Y. Pu, Z. Zhao and S. Chen, Recent achievements and applications of photothermal self-healing coatings: A review, Prog. Org. Coat., 2024, 187, 108153.
261. Y. Wang, J. Wang, L. Huang, X. Ding, Z. Chen, C. Ren, W. Hao, L. Ma and D. Zhang, Photothermally activated self-healing coatings for corrosion protection: A review, Prog. Org. Coat., 2023, 185, 107886.
262. T. Wang, W. Wang, H. M. Feng, T. X. Sun, C. C. Ma, L. Cao, X. D. Qin, Y. Lei, J. M. Piao, C. Feng, Q. L. Cheng and S. G. Chen, Photothermal nanofiller-based polydimethylsiloxane anticorrosion coating with multiple cyclic self-healing and long-term self-healing performance, Chem. Eng. J., 2022, 446, 137077.
263. P. Shan, K. Geng, L. Guo, L. Kuang, Y. Shen, B. Xiong, J. Hou, F. Guo, G. Wang and W. Shi, Synergistic effects of photothermal response and Schottky junction for enhanced photothermal-assisted photocatalytic hydrogen production, Chem. Eng. J., 2025, 513, 162801.
264. J. Lu, Z. Chen, Y. Shen, H. Yuan, X. Sun, J. Hou, F. Guo, C. Li and W. Shi, Boosting photothermal-assisted photocatalytic H₂ production over black g-C₃N₄ nanosheet photocatalyst via incorporation with carbon dots, J. Colloid Interface Sci., 2024, 670, 428–438.
265. L. Cheng, C. B. Liu, H. C. Zhao and L. P. Wang, Hierarchically self-reporting and self-healing photothermal responsive coatings towards smart corrosion protection, Chem. Eng. J., 2023, 467, 143463.
266. C. B. Guo, C. Huang, Y. B. Lian and Z. Y. Chen, pH-responsive and NIR photothermal self-healing coating for metal protection, J. Taiwan Inst. Chem. Eng., 2024, 164, 105703.
267. G. M. Liu, F. Yu, L. Yang, J. H. Tian and N. Du, Cerium-tannic acid passivation treatment on galvanized steel, Rare Met., 2009, 28, 284–288.
268. S. Zhang, Y. Shen, J. Lu, Z. Chen, L. Li, F. Guo and W. Shi, Tannic acid-modified g-C₃N₄ nanosheets/polydimethylsiloxane as a photothermal-responsive self-healing composite coating for smart corrosion protection, Chem. Eng. J., 2024, 483, 149232.
269. S. H. Yang, Y. X. Huang, P. S. Li, B. Y. Xiao, G. W. Liu, X. D. Chen, Y. Hu and Z. H. Yang, Tannin-based modified graphene oxide anti-corrosion composite coating with favourable corrosion inhibition, self-healing and photothermal conversion properties, Corros. Sci., 2024, 231, 111956.
270. X. Xu, F. Qiao, Y. Liu and W. Liu, Preparation of Cu(OH)₂/Cu₂S arrays for enhanced hydrogen evolution reaction, Battery Energy, 2024, 3, 20230060.
271. T. Shen, Z. H. Liang, H. C. Yang and W. H. Li, Anti-corrosion coating within a polymer network: Enabling photothermal repairing underwater, Chem. Eng. J., 2021, 412, 128640.
272. X. C. Wang, Y. K. Tang, S. R. Cheng, C. B. Liu, Q. M. Gao, W. P. Lian, Y. X. Yuan, A. Q. Li, C. B. Li and S. S. Guan, Polydimethylsiloxane Composite Sponge Decorated with Graphene/Carbon Nanotube via Polydopamine for Multifunctional Applications, ACS Appl. Polym. Mater., 2023, 5, 6022–6033.
273. S. Zhang, Y. Tang, X. Wang, Y. Shen, T. Zhou, C. Lu, F. Guo and W. Shi, Photothermal Self-Healing Black g-C₃N₄ Nanosheet-Based Coatings: A Novel Approach for Enhanced Anticorrosion and Antibiofouling Protection, Small, 2025, 2411729.
274. W. Q. Huang, S. R. Zhao, X. G. Zeng and J. K. Liu, Two-Dimensional Zinc Silicate/Polyaniline Nanocomposites Enhance the Anticorrosion Properties of Epoxy Resin Coatings, ACS Appl. Nano Mater., 2024, 7, 13489–13500.
275. Y. Q. Xia, Y. He, C. L. Chen, Y. Q. Wu, F. Zhong and J. Y. Chen, Co-modification of polydopamine and KH560 on g-C₃N₄ nanosheets for enhancing the corrosion protection property of waterborne epoxy coating, React. Funct. Polym., 2020, 146, 104405.
276. T. N. Myasoedova, R. Kalusulingam and T. S. Mikhailova, Sol-Gel Materials for Electrochemical Applications: Recent Advances, Coatings, 2022, 12, 1625.
277. R. V. Lakshmi, S. Sampath and S. T. Aruna, Silica-alumina based sol-gel coating containing cerium oxide nanofibers as a potent alternative to conversion coating for AA2024 alloy, Surf. Coat. Technol., 2021, 411, 127007.
278. R. Samadianfard, D. Seifzadeh and B. Dikici, Smart sol-gel nanocomposite containing inhibitor-stabilized g-C₃N₄ nanoplates for corrosion protection of magnesium alloy, Surf. Coat. Technol., 2024, 484, 130764.
279. A. Hasanzadeh and A. R. SaadatAbadi, ZIF-67 metal-organic framework decorated g-C₃N₄ nanosheets with inbuilt organic-inorganic corrosion inhibitor for self-healing epoxy anti-corrosion coating, Surf. Coat. Technol., 2024, 492, 131218.
280. X. Li, H. Liu, S. Meng and L. Wang, Three birds with one stone: ZIF-8@g-C₃N₄ nanosheets simultaneously enhance barrier property, active protection and UV aging resistance of epoxy coatings, Sep. Purif. Technol., 2025, 355, 129722.
281. C. L. Zhou, M. F. Pan, S. J. Li, Y. X. Sun, H. J. Zhang, X. H. Luo, Y. L. Liu and H. B. Zeng, Metal organic frameworks (MOFs) as multifunctional nanoplatform for anticorrosion surfaces and coatings, Adv. Colloid Interface Sci., 2022, 305, 102707.
282. L. Jiang, Y. M. Dong, Y. Yuan, X. Zhou, Y. R. Liu and X. K. Meng, Recent advances of metal-organic frameworks in corrosion protection: From synthesis to applications, Chem. Eng. J., 2022, 430, 132823.
283. M. H. Ashfaq, M. Imran, A. Haider, A. Shahzadi, M. Mustajab, A. Ul-Hamid, W. Nabgan, F. Medina and M. Ikram, Antimicrobial potential and rhodamine B dye degradation using graphitic carbon nitride and polyvinylpyrrolidone doped bismuth tungstate supported with in silico molecular docking studies, Sci. Rep., 2023, 13, 17847.
284. Y. H. Zhang, Application of water-soluble polymer inhibitor in metal corrosion protection: Progress and challenges, Front. Energy Res., 2022, 10, 997107.
285. S. Zafari, M. N. Shahrak and M. Ghahramaninezhad, New MOF-Based Corrosion Inhibitor for Carbon Steel in Acidic Media, Met. Mater. Int., 2020, 26, 25–38.
286. F. Zhong, Y. He, P. Q. Wang, C. L. Chen and Y. Q. Wu, Novel pH-responsive self-healing anti-corrosion coating with high barrier and corrosion inhibitor loading based on reduced graphene oxide loaded zeolite imidazole framework, Colloids Surf., A, 2022, 642, 128641.
287. J. J. Cao, C. B. Guo, X. P. Guo and Z. Y. Chen, Inhibition behavior of synthesized ZIF-8 derivative for copper in sodium chloride solution, J. Mol. Liq., 2020, 311, 113277.
288. K. Cao, L. L. Yu, X. L. Liu, Y. Q. Yang and F. B. Ma, Synthesis of ZIF-67@ZIF-8 with Core-shell Structure for Enhancing Epoxy Coating Corrosion Protection Property on Magnesium Alloy, Int. J. Electrochem. Sci., 2021, 16, 210328.
289. Z. C. Guan, J. Hu, H. H. Wang, H. Y. Shi, H. P. Wang, X. Wang, P. Jin, G. L. Song and R. G. Du, Decoration of rutile TiO₂ nanorod film with g-C₃N₄/SrTiO₃ for efficient photoelectrochemical cathodic protection, J. Photochem. Photobiol., A, 2023, 443, 114825.
290. Y. M. Liu, J. P. Wang and P. Yang, Photochemical reactions of g-C₃N₄-based heterostructured composites in Rhodamine B degradation under visible light, RSC Adv., 2016, 6, 34334–34341.
291. L. Q. Kuang, Z. Z. Chen, Y. J. Yan, F. Guo and W. L. Shi, Research progress of g-C₃N₄-based materials for photothermal-assisted photocatalysis, Int. J. Hydrogen Energy, 2024, 87, 20–49.
292. Y. Y. Bu and Z. Y. Chen, Highly efficient photoelectrochemical anticorrosion performance of C₃N₄@ZnO composite with quasi-shell-core structure on 304 stainless steel, RSC Adv., 2014, 4, 45397–45406.
293. J. H. Wen, L. Zhou, Q. X. Tang, X. Z. Xiao and S. Q. Sun, Photocatalytic degradation of organic pollutants by carbon quantum dots functionalized g-C₃N₄: A review, Ecotoxicol. Environ. Saf., 2023, 262, 115133.
294. B. Hu, F. P. Cai, T. J. Chen, M. S. Fan, C. J. Song, X. Yan and W. D. Shi, Hydrothermal Synthesis g-C₃N₄/Nano-InVO₄ Nanocomposites and Enhanced Photocatalytic Activity for Hydrogen Production under Visible Light Irradiation, ACS Appl. Mater. Interfaces, 2015, 7, 18247–18256.
295. F. Shaik, R. Milan and L. Amirav, Gold@Carbon Nitride Yolk and Core-Shell Nanohybrids, ACS Appl. Mater. Interfaces, 2022, 14, 21340–21347.
296. Y. Shen, J. Shi, Y. Wang, Y. Shi, P. Shan, S. Zhang, J. Hou, F. Guo, C. Li and W. Shi, Incorporation of hydroxyl groups and π-rich electron domains into g-C₃N₄ framework for boosted sacrificial agent-free photocatalytic H₂O₂ production, Chem. Eng. J., 2024, 498, 155774.
297. A. G. Akerdi, M. Mohsenzadeh, K. Mahmoudian and S. H. Bahrami, A review on heterogeneous g-C₃N₄ for efficient treatment of contaminants: fabrication, morphology control and environmental application, Int. J. Environ. Sci. Technol., 2025, 22, 7317–7352.
298. E. M. Fayyad, F. Nabhan and A. M. Abdullah, Focused Review on Graphitic Carbon Nitride (g-C₃N₄) in Corrosion and Erosion Applications, Coatings, 2024, 14, 1596.
299. M. Mirzaee, A. Rashidi, A. Zolriasatein and M. R. Abadchi, Corrosion properties of organic polymer coating reinforced two-dimensional nitride nanostructures: a comprehensive review, J. Polym. Res., 2021, 28, 62.
300. X. J. Xu, Y. S. Xu, Y. H. Liang, H. Y. Long, D. C. Chen, H. W. Hu and J. Z. Ou, Vacancy-modified g-C₃N₄ and its photocatalytic applications, Mater. Chem. Front., 2022, 6, 3143–3173.
301. A. H. Navidpour, D. R. Hao, X. W. Li, D. H. Li, Z. G. Huang and J. L. Zhou, Key factors in improving the synthesis and properties of visible-light activated g-C₃N₄ for photocatalytic hydrogen production and organic pollutant decomposition, Catal. Rev. - Sci. Eng., 2023, 66, 1665–1736.
302. S. Y. Kuchmiy and O. L. Stroyuk, Photocatalytic Fixation of Molecular Nitrogen in Systems Based on Graphite-Like Carbon Nitride: a Review, Theor. Exp. Chem., 2021, 57, 85–112.
303. A. K. M. Kalyani, R. Rajeev, L. Benny, A. R. Cherian and A. Varghese, Surface tuning of nanostructured graphitic carbon nitrides for enhanced electrocatalytic applications: a review, Mater. Today Chem., 2023, 30, 101523.
304. Z. Tian, X. Yang, Y. F. Chen, H. Huang, J. Hu and B. Wen, Fabrication of alveolate g-C₃N₄ with nitrogen vacancies via cobalt introduction for efficient photocatalytic hydrogen evolution, Int. J. Hydrogen Energy, 2020, 45, 24792–24806.
305. J. Zhang, Z. W. Hu, J. L. Zheng, Y. Q. Xiao, J. Song, X. T. Li, C. X. Cheng and Z. Y. Zhang, Photothermal-assisted solar hydrogen production: A review, Energy Convers. Manage., 2024, 318, 118901.
306. J. H. Shen, J. L. Liu, X. Y. Fan, H. Liu, Y. Bao, A. P. Hui and H. A. Munir, Unveiling the antibacterial strategies and mechanisms of MoS₂: a comprehensive analysis and future directions, Biomater. Sci., 2024, 12, 596–620.
307. S. D. Liu, C. Y. Zhao and H. L. Huang, Research Progress of Intelligent Anti-Corrosion Coatings and Their Healing Agents, Adv. Mater. Technol., 2025, 2401669.
308. Y. J. Chen, Z. H. Sui and J. Du, Review on aviation intelligent self-healing anti-corrosion coating, Anti-Corros. Methods Mater., 2025, 72, 170–177.
309. L. Y. Sun, Z. L. Liu, Z. G. Lv, Y. Ding, J. W. Sun and C. H. Gao, “Kill two birds with one stone” strategy to attain intelligent early warning anti-corrosion coating with a dual response mechanism, Prog. Org. Coat., 2025, 200, 109030.
310. Y. H. Su, Y. J. Xu, H. Wang, S. H. Dong, X. Cheng and H. B. Wang, A hydrophobic and self-healing polyurethane coating based on CeO₂@SiO₂/Phen for corrosion detection and protection, Prog. Org. Coat., 2024, 195, 108639.
311. M. Z. Hua, R. J. Jiang, Y. Q. Lu, Y. Su and Y. Zhao, Self-powered electrochemical protection for marine corrosion and fouling control: Principles, advances, and prospects, Nano Energy, 2025, 135, 110676.
312. J. Wang, M. Du, G. N. Li and P. Y. Shi, Research progress on microbiological inhibition of corrosion: A review, J. Cleaner Prod., 2022, 373, 133658.
313. J. Z. Jiang, L. L. Yu, J. H. Peng, W. P. Gong and W. Sun, Advance in the modification of g-C₃N₄-based composite for photocatalytic H₂ production, Carbon Lett., 2025, 35, 417–440.
314. Y. Yang, H. L. Hu, J. X. Ai, H. Wang and H. J. Du, Pt-functionalized S-doped g-C₃N₄ nanosheet for sensitive electrochemical determination of sulfamonomethoxine, Carbon Lett., 2024, 34, 917–927.
315. Y. P. Liu, Y. Y. Zhou, L. M. Tian, J. Zhao and J. Y. Sun, Intelligent anti-corrosion coating with self-healing capability and superior mechanical properties, J. Mater. Sci., 2024, 59, 16749–16767.
316. X. Qin, J. B. Wan, Q. Zhang, Y. J. Zhang, H. Z. Yu and S. W. Shi, Polyaniline-modified graphitic carbon nitride as electrode materials for high-performance supercapacitors, Carbon Lett., 2023, 33, 781–790.
317. Y. Li, X. Li, H. W. Zhang, J. J. Fan and Q. J. Xiang, Design and application of active sites in g-C₃N₄-based photocatalysts, J. Mater. Sci. Technol., 2020, 56, 69–88.

---