Recent Advances in g-C3N4-Based Materials and Their Application in Energy and Environmental Sustainability

Graphitic carbon nitride (g-C3N4), with facile synthesis, unique structure, high stability, and low cost, has been the hotspot in the field of photocatalysis. However, the photocatalytic performance of g-C3N4 is still unsatisfactory due to insufficient capture of visible light, low surface area, poor electronic conductivity, and fast recombination of photogenerated electron-hole pairs. Thus, different modification strategies have been developed to improve its performance. In this review, the properties and preparation methods of g-C3N4 are systematically introduced, and various modification approaches, including morphology control, elemental doping, heterojunction construction, and modification with nanomaterials, are discussed. Moreover, photocatalytic applications in energy and environmental sustainability are summarized, such as hydrogen generation, CO2 reduction, and degradation of contaminants in recent years. Finally, concluding remarks and perspectives on the challenges, and suggestions for exploiting g-C3N4-based photocatalysts are presented. This review will deepen the understanding of the state of the art of g-C3N4, including the fabrication, modification, and application in energy and environmental sustainability.


Introduction
Along with the rapid growth of the global population and the development of industrialization and urbanization, the demand for fossil energy, such as petroleum, coal, and natural gas, is increasing, as well as the deterioration of environmental pollution [1]. Driven by the ongoing energy and ecological crisis, the development of sustainable energy is a matter of great urgency and is related to the vital interests of people worldwide. As a green and renewable energy, solar energy has become a hot topic [2] and converting it into chemical energy by using photocatalysis is regarded as a potential pathway to supply renewable energy and alleviate environmental issues in the future. The photocatalyst is the most important key in economic photocatalysis application; it should be efficient, stable, low-cost, and capable of harvesting visible light [3]. Many photocatalytic materials have been reported and used in various fields, including hydrogen evolution, contaminant photo-oxidization or photodecomposition, and photoelectrochemical conversion [4,5]. For example, titanium dioxide (TiO 2 ) and related photocatalysts have been used in solar energy conversion, due to their merits of low price, unique optical-electronic properties, great durability, and non-toxicity [6]. However, the large bandgap of TiO 2 (3.2 eV) prevents it from actual solar energy utilization [7,8]. Compared to TiO 2 , the newly emerged graphitic carbon bility under both thermal and photochemical conditions. These merits make g-C3N4 unique material for energy and environmental applications, including photocatalytic H generation, CO2 reduction, and degradation of organic pollutants (dyes, pesticides, pha maceuticals, phenolic compounds) and inorganic pollutants like heavy metals, carbon d oxide reduction, etc. [6,[9][10][11][12][13][14]. In 2009, Wang et al. [15] for the first time proved that g-C3N could be used for photocatalytic hydrogen production upon visible-light irradiatio marking a significant milestone in metal-free photocatalysts. Nevertheless, due to th rapid recombination of photo-induced charges, the photocatalytic performance of g-C3N still possesses a significant possibility to be further enhanced, and the research on g-C3N gives rise to a new upsurge. So far, different strategies have been adopted to increase th photocatalytic efficiency of g-C3N4, such as morphological control, element doping, he erojunction construction, and nanomaterial composition [8,16,17]. In this review, we in troduce the recent advances in the preparation and modification of g-C3N4, and summ rize its photocatalytic application in H2 generation, CO2 reduction, and degradation organic pollutants in recent years. Finally, we conclude the research challenges with C3N4 and suggest perspectives for future research direction.

The Origin and Properties of g-C3N4
Carbon nitride (C3N4), composed of carbon and nitrogen elements, is a kind of o ganic semiconductor material. The history of C3N4 can be traced back to 1834. One type material named "melon" was synthesized by Berzelius and also reported by Liebig, whic is a linear polymer connected triazine and tri-s-triazines ( Figure 1) via secondary nitroge [18,19]. However, this material did not attract much attention due to the lack of compr hensive characterization at the time. Along with the development of characterizatio methods, Franklin [20] probed this material in 1922 and proposed the concept of C3N which indicated that C3N4 could be obtained by polymerizing various ammonia carbon acids. Later, Pauling and Sturdivant [21] demonstrated that tri-s-triazine was the unit melon in 1937. Still, its chemical instability and insolubility in most reagents made it im possible to unveil the structure until 1989, researchers found that when Si in β-Si3N4 wa replaced by C, the derived β-C3N4 was as hard as a diamond [22]. Based on the study, fiv types of C3N4, comprising α, β, pseudocubic, cubic, and graphitic, were predicated in 199 [23]. Among them, the first four materials are hard materials but not favorable to be syn thesized due to their low stability [24] and g-C3N4 is confirmed as the most resistant und surrounding situations; it possesses a similar layered structure to graphene and sp2 h brid π-conjugated electronic band structure. In recent years, large numbers of g-C3N4 m terials have been synthesized by thermal polymerization of urea, melamine, cyanamid dicyandiamide, and thiourea, which indicated that g-C3N4 was composed of melem uni and further confirmed that tri-s-triazine was the basic unit of g-C3N4. The electronic structures of both carbon and nitrogen atoms in g-C 3 N 4 determine their electronic and optical properties. The lone pair electrons of carbon and nitrogen atoms of g-C 3 N 4 interact to create a large π bond, analogous to the benzene ring, and then form a highly delocalized conjugate system. The delocalized conjugate chemical structure contributes to the formation of the stacking of the carbon nitride layer, which connects through amines, and such a structure makes the superior electronic conductivity of g-C 3 N 4 [1]. In addition, the solid covalent bond between carbon and nitrogen atoms leads to the excellent chemical and thermal stability of g-C 3 N 4 . After experimental measurements [26,27] the conduction band (CB) and valence band (VB) of g-C 3 N 4 are −1.3 V and 1.4 V at pH = 7 versus the standard hydrogen electrode (SHE), respectively. Such band positions promote visible light harvesting under oxidation and reduction systems. In short, g-C 3 N 4 possesses unique electronic, structural, physicochemical, and optical properties, sufficient for photocatalytic application in H 2 production, CO 2 photoreduction, and degradation of organic pollutants [9,28].

Preparation of g-C 3 N 4
The property of a catalyst varies substantially depending upon the preparation protocols. To achieve the application of g-C 3 N 4 in the field of photocatalysis, the synthesis of high-performance g-C 3 N 4 is a prerequisite. Various methods have been proposed, including thermal condensation, hydrothermal and solvothermal approaches, solid-state fabrication, self-hand synthesis, template-supported formation, deposition-precipitation, and ball milling process [1,29].
Among these methods, thermal condensation, a combination of polycondensation and polyaddition, is the most common method to prepare g-C 3 N 4 . Nitrogen-rich chemicals, such as melamine, urea, and dicyandiamide, are usually used as precursors, and after the deamination process, g-C 3 N 4 is generated under high temperatures. For example, Yan et al. [30] synthesized g-C 3 N 4 with a high photodegradation activity toward methyl orange in a semiclosed system with a two-step heat treatment. By investigating the influence of heating temperature upon the thermal condensation of melamine, the optimal reaction condition for g-C 3 N 4 was 520 • C for 2 h. Liu et al. [31] used urea as a precursor and produced g-C 3 N 4 on a large scale by pyrolysis under ambient pressure without additive assistance. The retainable pyrolysis-generated self-supporting atmosphere and the reaction temperature are two necessary conditions.
Hydrothermal synthesis is also one of the most widely used methods, which is beneficial to control the accuracy in influencing reaction molar ratio and physio-chemical properties [1]. Wu et al. [32] prepared oxygen-containing-groups-modified g-C 3 N 4 (OG/g-C 3 N 4 ) through an in situ one-step hydrothermal treatment of bulk g-C 3 N 4 in pure water. Hydrothermal treatment at 180 • C could promote the increase in the specific surface area of the resulting product from 2.3 to 69.8 m 2 g −1 , and oxygen-containing groups (-OH and C=O) were also successfully grafted on the surface of OG/g-C 3 N 4 via the interlayer delamination and intralayer depolymerization. Due to its high surface area and oxygencontaining surface properties, OG/g-C 3 N 4 demonstrated high photocatalytic performance on H 2 evolution. Ahmad et al.
[33] synthesized a highly efficient double Z-scheme g-C 3 N 4 /AgI/β-AgVO 3 (g-CNAB) ternary nanocomposite using a one-pot hydrothermal route. They have characterized the optical properties, phase structure, and morphology of the as-prepared photocatalysts and evaluated their photocatalytic performance toward the photodegradation of different pollutants under visible-light irradiation. Experimental characterization indicated that g-CNAB possessed a dual Z-scheme heterojunction, which had the features of better spatial separation and charge-carrier transfer. As such, reactive species such as superoxide anion radical and hydroxyl radical can be favorably generated for the degradation of various contaminants.
To improve the photocatalytic performance of g-C 3 N 4 , templating strategy has also been applied to synthesize materials with unique appearance, structure, and properties. Due to the high specific surface area and low surface reflection, silica-based materials, including silica spheres, mesoporous silica, and SBA-15, usually serve as hard templates to fabricate g-C 3 N 4 . Sun et al.
[34] used silica as a template to prepare a highly stable, hollow g-C 3 N 4 nanosphere (HCNS) (Figure 2). Adjustable shell thickness performed as a lightharvesting platform for H 2 evolution under visible light irradiation. However, corrosive reagents (e.g., NaOH, NH 4 HF 2 ) are often used to remove silica templates, which are not friendly to the environment. On the contrary, the soft templating method uses ionic liquid as a template or co-template to fabricate g-C 3 N 4 , which is an environmentally friendly and one-step approach to prepare g-C 3 N 4 with excellent performance. Zhao et al. [35] utilized cyanuric acid-melamine complex and an ionic liquid as soft templates to prepare hollow g-C 3 N 4 spheres with a specific surface area as high as 84 m 2 g −1 . The morphology of g-C 3 N 4 could be well controlled by adjusting ionic liquid and solvent. It has been demonstrated that the as-prepared hollow mesoporous carbon nitride exhibits~30 times higher than traditional g-C 3 N 4 in hydrogen production. Although significant progress has been achieved in the preparation and modification of g-C 3 N 4 , some preparation methods are neither environmentally friendly nor time-saving. Thus, it is necessary to develop green and facile synthesis routes.
To improve the photocatalytic performance of g-C3N4, templating strategy has also been applied to synthesize materials with unique appearance, structure, and properties. Due to the high specific surface area and low surface reflection, silica-based materials, including silica spheres, mesoporous silica, and SBA-15, usually serve as hard templates to fabricate g-C3N4. Sun et al. [34] used silica as a template to prepare a highly stable, hollow g-C3N4 nanosphere (HCNS) (Figure 2). Adjustable shell thickness performed as a light-harvesting platform for H2 evolution under visible light irradiation. However, corrosive reagents (e.g., NaOH, NH4HF2) are often used to remove silica templates, which are not friendly to the environment. On the contrary, the soft templating method uses ionic liquid as a template or co-template to fabricate g-C3N4, which is an environmentally friendly and one-step approach to prepare g-C3N4 with excellent performance. Zhao et al.
[35] utilized cyanuric acid-melamine complex and an ionic liquid as soft templates to prepare hollow g-C3N4 spheres with a specific surface area as high as 84 m 2 g −1 . The morphology of g-C3N4 could be well controlled by adjusting ionic liquid and solvent. It has been demonstrated that the as-prepared hollow mesoporous carbon nitride exhibits ~30 times higher than traditional g-C3N4 in hydrogen production. Although significant progress has been achieved in the preparation and modification of g-C3N4, some preparation methods are neither environmentally friendly nor time-saving. Thus, it is necessary to develop green and facile synthesis routes.

Morphology Control
Morphology control of g-C3N4 is one of the practical and effective methods to improve its photocatalytic performance. g-C3N4 can be regulated to different dimensions ( Figure 3). Then, with g-C3N4 having a larger specific surface area, more adequate active sites can be obtained. Moreover, the visible-light response range of g-C3N4 will be expanded, and the carrier diffusion path will be shortened. This section will discuss 0-3D g-C3N4-based materials and their catalytic performance.

Modification of g-C 3 N 4 3.1. Morphology Control
Morphology control of g-C 3 N 4 is one of the practical and effective methods to improve its photocatalytic performance. g-C 3 N 4 can be regulated to different dimensions ( Figure 3). Then, with g-C 3 N 4 having a larger specific surface area, more adequate active sites can be obtained. Moreover, the visible-light response range of g-C 3 N 4 will be expanded, and the carrier diffusion path will be shortened. This section will discuss 0-3D g-C 3 N 4 -based materials and their catalytic performance.
3.1.1. 0D g-C 3 N 4 g-C 3 N 4 dots are new members of the g-C 3 N 4 family, smaller than 10 nm in size, and have quantum size effect, surface effect, and quantum confinement effect [36]. As the size effect causes the reverse motion of CB and VB, g-C 3 N 4 with broad absorption (from ultraviolet to visible light) can be obtained. The photogenerated charge carriers favorably migrate to the particle surface for initiating oxidation or reduction reactions. In recent years, a variety of low-cost and size-controlled methods have been developed to prepare g-C 3 N 4 dots with different physicochemical properties, including hydrothermal treatment [37], ultrasonic exfoliation [38,39], microwave-assisted solvothermal process [40], and solid reaction approach [1]. A hydrothermal and hot-air assisted chemical oxidation method was proposed to prepare g-C 3 N 4 QDs by etching bulk g-C 3 N 4 to graphene-like nanosheets [41]. Concentrated H 2 SO 4 and HNO 3 etched the nanosheets to produce g-C 3 N 4 nanoribbons with sizes below 10 nm. Then, 5-9 nm g-C 3 N 4 QDs can be obtained after the hydrothermal treatment of nanoribbons at 200 • C (pH = 5). The obtained g-C 3 N 4 QDs exhibited a strong blue emission and upconversion behavior, promising visible-light-driven metal-free photocatalytic systems. Liu et al. [38] synthesized g-C 3 N 4 QDs by recrystallization and ultrasonic exfoliation from the precursor of dicyandiamide. g-C 3 N 4 QDs with different sizes (5-200 nm) can be prepared by adjusting ultrasonic time up to 90 min. Chen et al. [40] prepared 0D/2D CNQDs/g-C 3 N 4 isotype heterojunctions by a simple microwave assisted-polymerization method. The obtained product exhibited excellent photocatalytic performance toward norfloxacin degradation, and its reaction rate was as much as two times higher than pristine g-C 3 N 4 . Zero-dimensional QDs structure materials with nanometer size have a large surface/volume ratio, abundant surface atoms, and unsaturated coordination state, which conduce to their high activity for photocatalysis [42,43]. In addition, the absorption spectra of QDs/g-C 3 N4 will appear blue shift, and band gaps will be broadened, affecting their electronic band structures in photocatalysis [44]. Thus, grafting 0D QDs on g-C 3 N 4 is a promising approach to creating great reactive active sites and enhancing photoelectric conversion ability to improve photocatalytic activity.
3.1.1. 0D g-C3N4 g-C3N4 dots are new members of the g-C3N4 family, smaller than 10 nm in size, an have quantum size effect, surface effect, and quantum confinement effect [36]. As the siz effect causes the reverse motion of CB and VB, g-C3N4 with broad absorption (from ultra violet to visible light) can be obtained. The photogenerated charge carriers favorably m grate to the particle surface for initiating oxidation or reduction reactions. In recent year a variety of low-cost and size-controlled methods have been developed to prepare g-C3N dots with different physicochemical properties, including hydrothermal treatment [37 ultrasonic exfoliation [38,39], microwave-assisted solvothermal process [40], and solid r action approach [1]. A hydrothermal and hot-air assisted chemical oxidation method wa proposed to prepare g-C3N4 QDs by etching bulk g-C3N4 to graphene-like nanosheets [41 Concentrated H2SO4 and HNO3 etched the nanosheets to produce g-C3N4 nanoribbon with sizes below 10 nm. Then, 5-9 nm g-C3N4 QDs can be obtained after the hydrotherm treatment of nanoribbons at 200 °C (pH = 5). The obtained g-C3N4 QDs exhibited a stron blue emission and upconversion behavior, promising visible-light-driven metal-free pho tocatalytic systems. Liu   . Schematic illustration of different techniques for the synthesis of (a) 0D g-C 3 N 4 , (b) 1D g-C 3 N 4 , (c) 2D g-C 3 N 4 , and (d) 3D g-C 3 N 4 . Reprinted with permission from Ref. [25]. Copyright 2021 American Chemical Society.

1D g-C 3 N 4
As morphology significantly affects materials' photochemical properties and electron transfer rate, the photocatalytic activity of g-C 3 N 4 can be improved by adjusting its size and shape. The polymer characteristics of g-C 3 N 4 make it an excellent flexible structure. Onedimensional g-C 3 N 4 , including nanowires [45], nanorods [46,47], and nanotubes [48], can be prepared by non-metallic hard-template, soft-template, self-template, and template-free methods. Bai et al. [47] found it possible to transform g-C 3 N 4 from nanoplates to nanorods in a simple reflux way. They compared the photocatalytic activity and intensity of both shapes; results achieved in their study demonstrated that the photocatalytic activity and power of nanorods were~1.5 and 2.0 times higher than those of nanoplates under visible light, attributable to an increase in active lattice face and elimination of surface defects. Jiang et al. prepared melamine crystals by a transitional metal derived re-crystalline process and then created g-C 3 N 4 nanotubes with melamine crystals through a thermal polymerizing reaction method. They have applied transitional metal ions (Fe 3+ , Co 2+ , Ni 2+ , and Mn 2+ ) in the growing of melamine crystals and have characterized the obtained ion-modified g-C 3 N 4 nanotubes with XRD, FT-IR spectra, and XPS ( Figure 4). It has been demonstrated that Fe 3+ion-modified g-C 3 N 4 nanotubes (Fe 3+ R-650 CN) exhibited enhanced absorbance at 500 nm and decreased band gap. The hydrogen evolution rate (7538 µmol h −1 g −1 ) is almost 13.5-fold than that of conventional g-C 3 N 4 nanosheets. Mo et al.
[48] synthesized defectengineered g-C 3 N 4 nanotubes through an efficient self-assembled method, and applied them to hydrogen evolution. Around 6.8% external quantum efficiency was achieved at 420 nm. Among 1D g-C 3 N 4 , nanotubes are superior to others because they allow more effective light absorption, offer more active sites, and invent different electron pathways with tube morphology [25]. Generally, due to the nanometer scale of 1D g-C 3 N 4 -based materials in the radial direction, the diffusion distance (from volume to the surface) of photoexcited charges would be reduced, and the charge separation during photocatalytic reactions would be promoted. In addition, if g-C 3 N 4 -based materials are transformed from 2D to 1D structure, polygonal defects may appear to form the active sites and increase the contact surface of reactions, thus improving the photocatalytic performance.
thermal polymerizing reaction method. They have applied transitional metal ions (Fe 3+ , Co 2+ , Ni 2+ , and Mn 2+ ) in the growing of melamine crystals and have characterized the obtained ion-modified g-C3N4 nanotubes with XRD, FT-IR spectra, and XPS ( Figure 4). It has been demonstrated that Fe 3+ -ion-modified g-C3N4 nanotubes (Fe 3+ R-650 CN) exhibited enhanced absorbance at 500 nm and decreased band gap. The hydrogen evolution rate (7538 μmol h −1 g −1 ) is almost 13.5-fold than that of conventional g-C3N4 nanosheets. Mo et al. [48] synthesized defect-engineered g-C3N4 nanotubes through an efficient self-assembled method, and applied them to hydrogen evolution. Around 6.8% external quantum efficiency was achieved at 420 nm. Among 1D g-C3N4, nanotubes are superior to others because they allow more effective light absorption, offer more active sites, and invent different electron pathways with tube morphology [25]. Generally, due to the nanometer scale of 1D g-C3N4-based materials in the radial direction, the diffusion distance (from volume to the surface) of photoexcited charges would be reduced, and the charge separation during photocatalytic reactions would be promoted. In addition, if g-C3N4-based materials are transformed from 2D to 1D structure, polygonal defects may appear to form the active sites and increase the contact surface of reactions, thus improving the photocatalytic performance.

2D g-C3N4
Bulk g-C3N4, prepared by the traditional one-step calcination method, has a small specific surface area (10 m 2 g −1 ), and the photogenerated electrons and holes tend to recombine, which reduces its photocatalytic activity [50]. Since g-C3N4 has a 2D layered structure connected by van der Waals force, it will produce unique chemical and physical properties as below when it is stripped into multilayer or single-layer nanosheets. First, Bulk g-C 3 N 4 , prepared by the traditional one-step calcination method, has a small specific surface area (10 m 2 g −1 ), and the photogenerated electrons and holes tend to recombine, which reduces its photocatalytic activity [50]. Since g-C 3 N 4 has a 2D layered structure connected by van der Waals force, it will produce unique chemical and physical properties as below when it is stripped into multilayer or single-layer nanosheets. First, the large surface area and easy access to active reaction centers facilitate interactions with reactants. Second, nanoscale thickness or even less can minimize the migration distance of charge carriers, ensuring the rapid transport of charge carriers from the bulk phase to the surface of a catalyst. As a result, it would effectively inhibit electron-hole pairs from recombination. Third, the unique two-dimensional flexible planar structure can enhance compatibility with various modification strategies, such as heterojunction construction, cocatalyst modification, and vacancy introduction. This feature further improves the quantum efficiency in a photocatalysis process [51]. Various methods have been developed to prepare g-C 3 N 4 nanosheets, such as thermal oxidation etching, chemical peel etching, supramolecular self-assembly, and ultrasonic treatment of exudation [1,51]. Among these, thermal oxidation etching is the most common method, which can overcome van der Waals force between layers and peel bulk g-C 3 N 4 into 2D nanosheets under high-temperature oxidation conditions. Niu et al.
[52] prepared g-C 3 N 4 nanosheets (~2 nm thickness) by thermal oxidation etching of bulk g-C 3 N 4 in the air. UV-visible absorption ( Figure 5A) exhibits a blue shift of the intrinsic absorption edge in the nanosheets. Compared to the bulk (2.77 eV), the nanosheets possess a higher specific surface area and 0.2 eV larger bandgap ( Figure 5B). These characteristics benefit electron transport along the in-plane direction and increase the lifetime of photoexcited charge carriers because of the quantum confinement effect. Furthermore, the average hydrogen evolution rate of nanosheets is 170.5 µmol h −1 under UV-visible light, which is 5.4 times higher than that of the bulk counterpart. Due to the simplicity of operation, the supramolecular self-assembly method has attracted much attention. For example, conventional melamine-cyanuric acid (MCA) complexes can be obtained by mixing melamine and cyanuric acid in a solution. However, the use of solvent limits the batch preparation of MCA complexes. To address this issue, Liu et al. [53] proposed synthesizing supramolecular precursors through hydrothermal treatment of dicyandiamide and prepared 3D holey g-C 3 N 4 nanosheets with excellent photocatalytic performance. The precursor exhibited a similar structure to that of conventional MCA. In contrast, their thermal decomposition and morphology were different, which led to the distinction of microstructures, optical properties, charge recombination, photoelectrochemical behavior, and photocatalytic activity. The holey g-C 3 N 4 can make up for the shortcomings of recombination of charge carriers, retarded visible light utilization, and the limited surface-active sites in the bulk g-C 3 N 4 catalysts, which could contribute to their outstanding application in photocatalytic hydrogen evolution.
oped to prepare g-C3N4 nanosheets, such as thermal oxidation etching, chemical peel etching, supramolecular self-assembly, and ultrasonic treatment of exudation [1,51]. Among these, thermal oxidation etching is the most common method, which can overcome van der Waals force between layers and peel bulk g-C3N4 into 2D nanosheets under high-temperature oxidation conditions. Niu et al.
[52] prepared g-C3N4 nanosheets (~2 nm thickness) by thermal oxidation etching of bulk g-C3N4 in the air. UV-visible absorption ( Figure  5A) exhibits a blue shift of the intrinsic absorption edge in the nanosheets. Compared to the bulk (2.77 eV), the nanosheets possess a higher specific surface area and 0.2 eV larger bandgap ( Figure 5B). These characteristics benefit electron transport along the in-plane direction and increase the lifetime of photoexcited charge carriers because of the quantum confinement effect. Furthermore, the average hydrogen evolution rate of nanosheets is 170.5 μmol h −1 under UV-visible light, which is 5.4 times higher than that of the bulk counterpart. Due to the simplicity of operation, the supramolecular self-assembly method has attracted much attention. For example, conventional melamine-cyanuric acid (MCA) complexes can be obtained by mixing melamine and cyanuric acid in a solution. However, the use of solvent limits the batch preparation of MCA complexes. To address this issue, Liu et al. [53] proposed synthesizing supramolecular precursors through hydrothermal treatment of dicyandiamide and prepared 3D holey g-C3N4 nanosheets with excellent photocatalytic performance. The precursor exhibited a similar structure to that of conventional MCA. In contrast, their thermal decomposition and morphology were different, which led to the distinction of microstructures, optical properties, charge recombination, photoelectrochemical behavior, and photocatalytic activity. The holey g-C3N4 can make up for the shortcomings of recombination of charge carriers, retarded visible light utilization, and the limited surface-active sites in the bulk g-C3N4 catalysts, which could contribute to their outstanding application in photocatalytic hydrogen evolution.

3D g-C3N4
Three-dimensional nanostructures are considered an effective way to improve the properties of photocatalytic materials because they can provide a larger specific surface area and more reactive active sites. Although soft and hard template methods have been Three-dimensional nanostructures are considered an effective way to improve the properties of photocatalytic materials because they can provide a larger specific surface area and more reactive active sites. Although soft and hard template methods have been widely used in the preparation of 3D g-C 3 N 4 , they generally employ toxic substances and organic solvents to remove the template, which limits their application in actual green production. Salt templates have replaced some traditional templates (silica, sodium dodecyl sulfate, anodic alumina) for safe synthesis processes. Qian et al. [54] introduced a simple and effective sodium-chloride-assisted ball milling method to prepare 3D porous g-C 3 N 4 . Three-dimensional cubic sodium chloride particles can be used as an easily removable template to design 3D porous structures and as a limiting structure to prevent aggregation of g-C 3 N 4 during calcination. The modified 3D interconnecting network structure of g-C 3 N 4 has a large specific surface area, significantly improving the photocatalytic performance. The hydrogen production rate can be as high as 598 µmol g −1 h −1 with 3.31% quantum efficiency at 420 nm. In addition, ionic liquids are widely used in many fields due to their excellent fluidity and solubility. In the process of nanomaterial preparation, the ionic liquid can self-assemble into micelles, which impacts the size and morphology of the nanomaterials. For example, Zhao et al. [35] controlled the morphology of hollow mesoporous g-C 3 N 4 spheres by changing ionic liquid concentration. At a low ionic liquid concentration, the prepared mesoporous g-C 3 N 4 showed a hollow spherical structure while, at a high ionic liquid concentration, the cyanate-melamine (CM) complex rearranged, which could induce the formation of a flower-like structure with ultrathin nanosheet ( Figure 6). Moreover, the hollow as-prepared g-C 3 N 4 exhibits higher light absorption in the visible range and a faster separation rate of photogenerated hole-electron pairs than bulk C 3 N 4 . In addition, the hydrogen production of as-prepared hollow mesoporous g-C 3 N 4 exhibits 30 times higher than traditional g-C 3 N 4 , because of its high surface area. with 3.31% quantum efficiency at 420 nm. In addition, ionic liquids are widely used in many fields due to their excellent fluidity and solubility. In the process of nanomateria preparation, the ionic liquid can self-assemble into micelles, which impacts the size and morphology of the nanomaterials. For example, Zhao et al. [35] controlled the morpholog of hollow mesoporous g-C3N4 spheres by changing ionic liquid concentration. At a low ionic liquid concentration, the prepared mesoporous g-C3N4 showed a hollow spherica structure while, at a high ionic liquid concentration, the cyanate-melamine (CM) comple rearranged, which could induce the formation of a flower-like structure with ultrathin nanosheet ( Figure 6). Moreover, the hollow as-prepared g-C3N4 exhibits higher light ab sorption in the visible range and a faster separation rate of photogenerated hole-electro pairs than bulk C3N4. In addition, the hydrogen production of as-prepared hollow meso porous g-C3N4 exhibits ~30 times higher than traditional g-C3N4, because of its high sur face area. In addition, some unique g-C3N4-based structures, like the "seaweed" network, spira rod, and hollow fusiform, have also been reported [55][56][57][58][59], and a superior photocatalyti activity to bulk samples was observed for higher specific surface areas. Among these mor phologies of g-C3N4, 1D nanoribbons, with a large specific surface area, more active sites a short diffusion distance, and preferential directions for photo-generated electron-hol carriers, effectively accelerate the catalytic performance of g-C3N4. Better yet, the 3D mor phology assembled from 1D units have depressed agglomeration, increased exposure o active sites, and decreased mass transportation resistance.

Elemental Doping
Heteroatom doping effectively regulates the electrical, optical, and structural prop erties of semiconductors by introducing active impurities [60]. In general, non-metalli heteroatoms can participate in the C3N4 lattice and partially replace C or N atoms, whil metal atoms can insert into the triangular gap cavity of the g-C3N4 lattice. Doped meta and non-metal atoms generate intermediate band gaps near CB and VB to regulate th In addition, some unique g-C 3 N 4 -based structures, like the "seaweed" network, spiral rod, and hollow fusiform, have also been reported [55][56][57][58][59], and a superior photocatalytic activity to bulk samples was observed for higher specific surface areas. Among these morphologies of g-C 3 N 4 , 1D nanoribbons, with a large specific surface area, more active sites, a short diffusion distance, and preferential directions for photo-generated electronhole carriers, effectively accelerate the catalytic performance of g-C 3 N 4 . Better yet, the 3D morphology assembled from 1D units have depressed agglomeration, increased exposure of active sites, and decreased mass transportation resistance.

Elemental Doping
Heteroatom doping effectively regulates the electrical, optical, and structural properties of semiconductors by introducing active impurities [60]. In general, non-metallic heteroatoms can participate in the C 3 N 4 lattice and partially replace C or N atoms, while metal atoms can insert into the triangular gap cavity of the g-C 3 N 4 lattice. Doped metal and non-metal atoms generate intermediate band gaps near CB and VB to regulate the band structure of g-C 3 N 4 , which can effectively realize the separation and transmission of electron-hole pairs and broaden their optical response range [55,60]. Therefore, doping is a prevalent method to improve the photocatalytic performance of semiconductors.

Non-Metal Doping
Non-metal element doping could maintain the metal-free character of g-C 3 N 4 . Additionally, due to the high ionization energies and electronegativity of non-metals, they can quickly form covalent bonds with other compounds by gaining electrons during the reaction process [61,62]. The introduction of non-metals will break the symmetry of g-C 3 N 4 and result in a faster separation speed of electron-hole pairs [63]. The ordinary non-metallic doping atoms include O, P, S, B, C, N, and halogens (F, Cl, Br, and I). Among them, the O atom, one of the most typical non-metallic doping elements, has shown extraordinary potential in improving the photocatalytic performance of g-C 3 N 4 . Zhang et al. [64] presented a hydrothermal method and fabricated a porous and oxygen-doped g-C 3 N 4 photocatalyst for efficient photocatalytic hydrogen production by forming homogeneous supramolecular complexes (Figure 7). They introduced porous structure and heteroatom doping in g-C 3 N 4 to adjust its active sites and electronic structure for enhanced light harvesting, charge separation, and transfer. Compared with bulk g-C 3 N 4 , the hydrogen evolution activity of the g-C 3 N 4 photocatalysts is 11.3-fold higher than bulk g-C 3 N 4 . She et al. [65] introduced oxygen in g-C 3 N 4 and prepared 2D porous ultrathin oxygen-doped g-C 3 N 4 nanosheets. It has been demonstrated that the band gap was enlarged (~0.20 eV), and the transport ability of photogenerated electrons and the redox ability were improved, which is caused by the quantum confinement effect. Besides, the specific surface area of non-metal doped g-C 3 N 4 is larger (~20 times) than that of the bulk, which will supply more active sites with adequate quality and offer more adsorption sites. In short, due to the increased bandgap, the introduction of the electrophilic groups and the morphology structure, the electron-hole recombination probability is inhibited and the redox ability will be improved, which contribute to the enhanced photocatalytic activity. and result in a faster separation speed of electron-hole pairs [63]. The ordinary non tallic doping atoms include O, P, S, B, C, N, and halogens (F, Cl, Br, and I). Among t the O atom, one of the most typical non-metallic doping elements, has shown extra nary potential in improving the photocatalytic performance of g-C3N4. Zhang et al presented a hydrothermal method and fabricated a porous and oxygen-doped gphotocatalyst for efficient photocatalytic hydrogen production by forming homogen supramolecular complexes (Figure 7). They introduced porous structure and hetero doping in g-C3N4 to adjust its active sites and electronic structure for enhanced light vesting, charge separation, and transfer. Compared with bulk g-C3N4, the hydrogen lution activity of the g-C3N4 photocatalysts is 11.3-fold higher than bulk g-C3N4. She [65] introduced oxygen in g-C3N4 and prepared 2D porous ultrathin oxygen-doped gnanosheets. It has been demonstrated that the band gap was enlarged (~0.20 eV), an transport ability of photogenerated electrons and the redox ability were improved, w is caused by the quantum confinement effect. Besides, the specific surface area of metal doped g-C3N4 is larger (~20 times) than that of the bulk, which will supply active sites with adequate quality and offer more adsorption sites. In short, due t increased bandgap, the introduction of the electrophilic groups and the morpho structure, the electron-hole recombination probability is inhibited and the redox ab will be improved, which contribute to the enhanced photocatalytic activity.

Metal Doping
In terms of metal-doped g-C3N4, the porous structure of heptamine and electron sp2 nitrogen atoms can provide sites for metal coordination, and doped metal can e bind to the three neighboring N atoms in the form of g-C3N4. Metal element doping, e cially alkali, has been demonstrated to reduce the energy gap, supply more active rea

Metal Doping
In terms of metal-doped g-C 3 N 4 , the porous structure of heptamine and electron-rich sp2 nitrogen atoms can provide sites for metal coordination, and doped metal can easily bind to the three neighboring N atoms in the form of g-C 3 N 4 . Metal element doping, especially alkali, has been demonstrated to reduce the energy gap, supply more active reaction sites, adjust VB position and improve photocarrier separation to enhance the visible light absorption [66]. Due to the uneven distribution of semiconductor charge space, the improvement of photogenerated carrier separation efficiency is increased. In addition, the alkali metal doping can increase the π-conjugated systems and reduce the recombination rate of the semiconductor electron-hole pairs, which will be beneficial to improve the efficiency of H 2 evolution by photolysis [67]. The ordinary doping metal atoms are K, Na, Ag, Au, Fe, Ni, Pt, etc. Gao et al. [68] designed a simple one-step pyrolysis process to synthesize Fe-doped g-C 3 N 4 nanosheets with NH 4 Cl as a "dynamic gas template" and FeCl 3 as a Fe source, respectively ( Figure 8). The experimental results show that Fe species may exist at the state of Fe 3+ and form Fe-N bonds in g-C 3 N 4 , thereby expanding visible light absorption regions and reducing the band gap of g-C 3 N 4 nanosheets. Moreover, doping specific amounts of Fe could promote exfoliation and increase the specific surface area of g-C 3 N 4 , while excessive Fe might break the sheeting structure. The specific surface area of optimized Fe-doped g-C 3 N 4 nanosheets reached 236.52 m 2 g −1 , which was 2.5 times higher than g-C 3 N 4 nanosheets. In addition, Deng et al. [69] prepared K + and cyano-group co-doped crystalline polymeric carbon nitride (KC-CCN) by a one-step thermo-polymerization approach. They applied thiourea and potassium thiocyanate as precursors, and the resulting KC-CCN demonstrated a highly crystal structure, stronger light harvesting, and a higher electron-hole separation ratio.
light absorption regions and reducing the band gap of g-C3N4 nanosheets. Moreover, doping specific amounts of Fe could promote exfoliation and increase the specific surface area of g-C3N4, while excessive Fe might break the sheeting structure. The specific surface area of optimized Fe-doped g-C3N4 nanosheets reached 236.52 m 2 g −1 , which was 2.5 times higher than g-C3N4 nanosheets. In addition, Deng et al. [69] prepared K + and cyano-group co-doped crystalline polymeric carbon nitride (KC-CCN) by a one-step thermo-polymerization approach. They applied thiourea and potassium thiocyanate as precursors, and the resulting KC-CCN demonstrated a highly crystal structure, stronger light harvesting and a higher electron-hole separation ratio.

Heterojunction Construction
The key factor that restricts the activity of semiconductor photocatalysts is improving the separation and transport efficiency of electron-hole pairs. Thus, the construction of g-C3N4-based heterojunction photocatalysts is one of the most common and effective methods. When two semiconductors with different band structures combine to form a heterojunction, effective charge transfer will be included at the interface, improving charge separation and transport efficiency [70][71][72][73][74]. In addition, the light absorption capacity of a photocatalytic system can be enhanced by combining it with narrow-band gap semiconductors. For example, Xu et al. [75] proposed a wet chemical method to fabricate CdS/g-C3N4 (CSCN) heterojunctions in situ. Through the result of XRD, FTIR, TEM, and optica band gap for CSCN, the formation of heterojunctions was confirmed. The CdS nanoparticles dispersed uniformly on the surface of g-C3N4 nanosheets and the interfaces between g-C3N4 and CdS in composites is very close, which can efficiently enhance the electron transfer between the two semiconductors. Comparing the UV-vis DRS spectra of g-C3N (474.9 nm) and CdS (685.7 nm), the absorption thresholds of all CSCN composites locate

Heterojunction Construction
The key factor that restricts the activity of semiconductor photocatalysts is improving the separation and transport efficiency of electron-hole pairs. Thus, the construction of g-C 3 N 4 -based heterojunction photocatalysts is one of the most common and effective methods. When two semiconductors with different band structures combine to form a heterojunction, effective charge transfer will be included at the interface, improving charge separation and transport efficiency [70][71][72][73][74]. In addition, the light absorption capacity of a photocatalytic system can be enhanced by combining it with narrow-band gap semiconductors. For example, Xu et al. [75] proposed a wet chemical method to fabricate CdS/g-C 3 N 4 (CSCN) heterojunctions in situ. Through the result of XRD, FTIR, TEM, and optical band gap for CSCN, the formation of heterojunctions was confirmed. The CdS nanoparticles dispersed uniformly on the surface of g-C 3 N 4 nanosheets and the interfaces between g-C 3 N 4 and CdS in composites is very close, which can efficiently enhance the electron transfer between the two semiconductors. Comparing the UV-vis DRS spectra of g-C 3 N 4 (474.9 nm) and CdS (685.7 nm), the absorption thresholds of all CSCN composites locate between that of g-C 3 N 4 and CdS, which indicates strong visible-light absorption. Among the materials, including the individual CdS, g-C 3 N 4 , and different CSCN composites, CSCN733 possesses the highest adsorption capacity and exhibits the highest methyl orange degradation efficiency, 100% with 40 min adsorption. Liu et al. [76] embedded nanorod-like CoP nanoparticles into g-C 3 N 4 nanosheets to form CoP-CN heterostructure. The XRD data indicated that the 0.5% CoP-CN hybrid incorporates the representative peaks of g-C 3 N 4 and CoP with g-C 3 N 4 demonstrating the main phase. The TEM of 0.5% CoP-CN composites displays a porous and fluffy structure. The binding energy of P 2p 3/2 is lower than that of P 2p 3/2 , while the binding energy of Co 2p 3/2 eV is a little higher than that of metallic Co 2p 3/2 . This result indicates that the electron transfer from Co to P to form Co-P covalent bonds results in a small positive charge of Co and negative charge of P. This finding would account for the excellent activity of CoP-based photocatalysts in the HER process, in which Co serves as active center while P performs as the proton acceptor. The flat band potentials of 0.5% CoP-CN were decreased to −0.28 V and the CB tuned upward for more negative potential, achieving more efficient interfacial charge transportation and separation by establishing a certain inner electric field. Furthermore, among the evaluation of photocatalytic water half-splitting for H 2 production, 0.5% CoP-CN exhibit excellent activity and reached 959.4 µmol h −1 g −1 , which is almost 3.1-fold than that of pristine g-C 3 N 4 nanosheets. This can be attributed to its decreased over-potentials, more negative photo-reductive potentials, increased interfacial charge transfer efficiency, and higher solar-to-hydrogen efficiency. Ma et al. [77] combined TiO 2 with g-C 3 N 4 to form a Z-type heterojunction, which can effectively separate photogenerated electrons and holes and improve the photocatalytic activity. However, the utilization of visible light of TiO 2 is very low because of its large band gap. Thus, to improve this weakness, the Eg of TiO 2 should be reduced, followed by its potential change of valence band and the conduction band. Such a move would change the heterojunction type, which is not consistent with improving the catalytic efficiency of composite materials to visible light. Therefore, various kinds of g-C 3 N 4 -based heterojunction photocatalysts have emerged to enhance the efficiency of photogenerated carrier separation further and light absorption capacity, including ternary [78][79][80][81][82][83] and other type II [84][85][86][87][88] and S-type heterojunction [89,90], Schottky junction [91,92], and van der Waals heterojunction [93,94].
The S-type heterojunction is composed of reduction photocatalysts (RPs) and oxidation photocatalysts (OPs) with staggered band structures, similar to the type-II heterojunction but with an entirely different charge-transfer route [95]. S-type photocatalysis is an effective way to control charge separation in various photocatalytic reactions. Since Yu et al. first proposed the concept of S-type heterojunction in 2019 [96], a large number of studies on the g-C 3 N 4 -based S-type heterojunction have been reported, such as Sb 2 WO 6 /g-C 3 N 4 [97], S-doped g-C 3 N 4 /TiO 2 [98], ZnFe 2 O 4 /g-C 3 N 4 [99], g-C 3 N 4 /Zn 0.2 Cd 0.8 S-DETA [100], g-C 3 N 4 /Bi/BiVO 4 [101], WO 3 /g-C 3 N 4 [102], etc. The Schottky junction photocatalyst has also attracted much attention. Generally, conductor-semiconductor heterojunction has two main combined modes: Schottky junction and ohmic contact. Conductors and semiconductors with different Fermi levels will generate Schottky effects at their contact interfaces to induce internal electric fields to drive charge flow until the system reaches equilibrium. The proper orientation of built-in electric fields will promote the directional separation of charge carriers, leading to the practical generation of photogenerated charge carriers and improving the photocatalytic activity. A large number of g-C 3 N 4 Schottky junction photocatalysts have been reported, such as CoP/g-C 3 N 4 [103], CuS/g-C 3 N 4 [104], Ti 3 C 2 /g-C 3 N 4 [105], carbon/g-C 3 N 4 [106], MoO 2 /g-C 3 N 4 [107], Cu-NPs/g-C 3 N 4 [108], etc. Recently, van der Waals (vdW) heterojunction has been proposed to regulate the electrical and optical properties of 2D materials accurately. vdW heterojunction not only overcomes the lattice matching limitation for enhancing interfacial charge separation and transfer but also leads to strong electronic coupling between layers to improve catalytic activity [109]. The research on g-C 3 N 4 -based vdW heterojunctions is currently enjoying a boom, and different types have been reported, including phosphorene/g-C 3 N 4 [110], g-C 3 N 4 /Zn-Ti LDH [111], g-C 3 N 4 /C-doped BN [112], g-C 3 N 4 /COF package-TD [113], etc.
Moreover, metal-organic framework (MOF) materials have exhibited excellent photocatalytic performances due to their unique porous structures and favorable transfer of e − and h + [114][115][116]. However, most MOFs have low stability and weak light response. Therefore, the heterojunction of MOF and g-C 3 N 4 materials has become popular in recent years. For example, Zhang et al. [117] synthesized a novel hybrid of Zr-based metal-organic framework with g-C 3 N 4 (UiO-66/g-C 3 N 4 ) nanosheets (10:10) by annealing their mixture. The photoelectron can transfer efficiently from the CB of g-C 3 N 4 to that of UiO-66 through the inner electric field generated by the heterojunction, which is beneficial to decrease the recombination of electron/hole. Together with their porous structures, much more organic dye molecules can absorb on the surface of the heterojunction catalyst, thus facilitating the electron/hole transfer and enhanced photocatalytic activity. Han et al. [114] prepared TPVT (tridentate ligand 2,4,6-tris(2-(pyridin-4-yl)vinyl)-1,3,5-triazine)-MOFs and combined them with g-C 3 N 4 . It has been demonstrated that the TPVT-MOFs@g-C 3 N 4 -10 can reach 56.4 µmol·g −1 ·h −1 in CO 2 reduction, which is 3.2-fold higher than that of g-C 3 N 4 . All these researches have provided a new insight into the design of g-C 3 N 4 based photocatalysts to deal with the organic dyes in environment.

Modification with Carbon Nanocomposites
The weak van der Waals forces between layers and the abundant hydrogen bonds in the molecular structure make g-C 3 N 4 exhibit slow charge transfer kinetics and poor electrical conductivity. Carbon materials have been widely used in photocatalysis due to their low price, good conductivity, high stability, non-toxicity, and harmlessness. Many composite photocatalysts with excellent photocatalytic performance have been prepared by combining carbon nanomaterials (such as fullerene [118], graphene [119], carbon nanotubes [120], etc.) with g-C 3 N 4 , which have unique nanostructures and excellent electron-optical properties ( Figure 9). The introduction of carbon material reduces the electron-hole pair recombination rate of photocatalysts and improves the photo absorption, thus improving the photocatalytic performance of g-C 3 N 4 -based materials. g-C 3 N 4 photocatalysts modified by carbon materials can promote photocatalytic reaction through heterojunction interaction, cocatalyst effect, surface recombination, local charge modification, and other ways [121]. For example, Yuan et al. [122] prepared a graphene-g-C 3 N 4 composite photocatalyst by calcining graphene with melamine, and excellent photocatalytic degradation performance toward RhB was observed under acidic conditions. Ge et al. [123] prepared multi-walled carbon nanotubes (MWNTs)/g-C 3 N 4 composite photocatalyst by heating MWNTs and g-C 3 N 4 , in which MWNTs favored the efficient separation of photo-generated charge carriers. As a result, this material exhibited unique performance in photocatalytic H 2 production under visible light conditions. facilitating the electron/hole transfer and enhanced photocatalytic activity. Han et al. [114 prepared TPVT (tridentate ligand 2,4,6-tris(2-(pyridin-4-yl)vinyl)-1,3,5-triazine)-MOF and combined them with g-C3N4. It has been demonstrated that the TPVT-MOFs@g-C3N4 10 can reach 56.4 μmol·g −1 ·h −1 in CO2 reduction, which is 3.2-fold higher than that of g C3N4. All these researches have provided a new insight into the design of g-C3N4 based photocatalysts to deal with the organic dyes in environment.

Modification with Carbon Nanocomposites
The weak van der Waals forces between layers and the abundant hydrogen bonds in the molecular structure make g-C3N4 exhibit slow charge transfer kinetics and poor elec trical conductivity. Carbon materials have been widely used in photocatalysis due to thei low price, good conductivity, high stability, non-toxicity, and harmlessness. Many com posite photocatalysts with excellent photocatalytic performance have been prepared by combining carbon nanomaterials (such as fullerene [118], graphene [119], carbon nano tubes [120], etc.) with g-C3N4, which have unique nanostructures and excellent electron optical properties (Figure 9). The introduction of carbon material reduces the electron hole pair recombination rate of photocatalysts and improves the photo absorption, thu improving the photocatalytic performance of g-C3N4-based materials. g-C3N4 photocata lysts modified by carbon materials can promote photocatalytic reaction through hetero junction interaction, cocatalyst effect, surface recombination, local charge modification and other ways [121]. For example, Yuan et al. [122] prepared a graphene-g-C3N4 compo site photocatalyst by calcining graphene with melamine, and excellent photocatalytic deg radation performance toward RhB was observed under acidic conditions. Ge et al. [123 prepared multi-walled carbon nanotubes (MWNTs)/g-C3N4 composite photocatalyst by heating MWNTs and g-C3N4, in which MWNTs favored the efficient separation of photo generated charge carriers. As a result, this material exhibited unique performance in pho tocatalytic H2 production under visible light conditions.  As a new carbon-based nanomaterial, carbon dots (CDs) exhibit excellent up-conversion photoluminescence and remarkable photogenerated charge-carrier transfer and reservoir [124][125][126]. They can also modify g-C 3 N 4 and broaden optical absorption by reducing the electron-hole pair recombination rate [127,128]. Such features have caused extensive attention to CDs-modified g-C 3 N 4 . For instance, Fang et al. [129] prepared a CDsmodified g-C 3 N 4 hybrid by dicyandiamide and CDs obtained from the combustion soot of alcohol. Based on the investigation of CDs modification on the structure and photocatalytic activity of g-C 3 N 4 , they found that CDs modification caused the lattice distortion of g-C 3 N 4 , and CDs performed as an electron sink, which could prevent the recombination of photo-generated electron-hole pairs. Ai et al. [130] reviewed the combination methods of g-C 3 N 4 and CDs ( Figure 10) for enhancing photocatalytic performance and indicated that g-C 3 N 4 /CDs hybridization has strong practicability in efficient photocatalytic hydrogen generation, photocatalytic carbon dioxide reduction, and organic pollutant degradation. However, it is still in the early stage. Much effort should be made to develop green and facile synthesis routes and solve the insufficient utilization of visible and near-infrared light. of alcohol. Based on the investigation of CDs modification on the structure and photocata lytic activity of g-C3N4, they found that CDs modification caused the lattice distortion o g-C3N4, and CDs performed as an electron sink, which could prevent the recombination of photo-generated electron-hole pairs. Ai et al. [130] reviewed the combination methods of g-C3N4 and CDs ( Figure 10) for enhancing photocatalytic performance and indicated that g-C3N4/CDs hybridization has strong practicability in efficient photocatalytic hydro gen generation, photocatalytic carbon dioxide reduction, and organic pollutant degrada tion. However, it is still in the early stage. Much effort should be made to develop green and facile synthesis routes and solve the insufficient utilization of visible and near-infra red light.

Photocatalytic Application in Energy and Environmental Sustainability
The energy and environmental crises have been an ongoing challenge, which is re lated to the vital interests of people around the globe. How to solve this problem through sustainable development strategies is considered deeply by scientific researchers. Photo catalysis provides a powerful technique for fully utilizing solar in the field of energy con version [28,131]. Here, we will mainly introduce the photocatalytic application of g-C3N

Photocatalytic Application in Energy and Environmental Sustainability
The energy and environmental crises have been an ongoing challenge, which is related to the vital interests of people around the globe. How to solve this problem through sustainable development strategies is considered deeply by scientific researchers. Photocatalysis provides a powerful technique for fully utilizing solar in the field of energy conversion [28,131]. Here, we will mainly introduce the photocatalytic application of g-C 3 N 4 in energy and environmental remediation, including H 2 production, CO 2 photoreduction, and pollutant degradation.

H 2 Production
Hydrogen is gathering strong momentum as a pivotal energy transition pillar driven by the global shift toward decarbonization. Nevertheless, 85% of H 2 is produced from fossil fuel combustion, which generates roughly 500 metric tons of carbon dioxide every year and proffers a challenge and obstacle toward the sustainable living of future generations [132]. Solar-driven photocatalytic H 2 generation as a promising technology has received extensive attention in addressing the global energy crisis [133,134]. Photocatalytic water splitting for the energy transformation from solar to eco-friendly fuels has been studied for decades with various semiconductor photocatalysts. As a type of semicon-ductor photocatalyst, g-C 3 N 4 is simple and inexpensive to fabricate, and has an adequate bandgap (≈2.7 eV) for activation upon sunlight irradiation. Wang's group first utilized g-C 3 N 4 in photocatalytic H 2 evolution [15,135]. Nonetheless, pristine g-C 3 N 4 is far from satisfactory energy conversion because of its low light energy utilization, low density active sites, and ineffective isolation of the photogenerated excitons. Thus, researchers have proposed numerous strategies to boost the photocatalytic activity of g-C 3 N 4 -based materials for H 2 production. For example, the g-C 3 N 4 /carbon-dot-based nanocomposites, which possess enormous visible light absorption and applicable energy structures, have been prepared and serve as efficacious photocatalysts in photocatalytic water splitting for H 2 generation under light illumination [128,134,136,137]. Gao et al. reported hexagonal tubular g-C 3 N 4 /CD-based nanocomposites which exhibited nine times higher than bulk g-C 3 N 4 in H 2 production rate [134] and related results indicated that CDs performed as both photosensitizer and electron acceptor. CDs could absorb long wavelength light to extend the visible-light response region and suppress the recombination of electron-hole pairs. Hussien et al. [138] combined four different strategies (non-metal doping, porosity generation, functionalization with amino groups, and thermal oxidation etching) in a onepot thermal reaction and successfully prepared amino-functionalized ultrathin nanoporous B-doped g-C 3 N 4 by using NH 4 Cl as a gas bubble template, together with a thermal exfoliation process to produce ultrathin sheets ( Figure 11). According to the process, the surface area, adsorption capacity, and charge migration of the as-prepared photocatalyst have been improved, and a 3800 µmol g −1 h −1 H 2 generation rate and 10.6% prominent quantum yield were recorded. Li et al. [139] decorated carbon self-doping g-C 3 N 4 nanosheets with gold-platinum (AuPt) nanocrystals through a photo-deposition route and compared the photocatalytic H 2 evolution performance of Pt/CCN, Au/CCN, Au/Pt/CCN, and Pt/Au/CCN, in which AuPt/CCN stood out and gave the highest H 2 generation rate (1135 µmol h −1 ). The excellent performance can be ascribed to the non-plasmon-related synergistic effect of Au and Pt atoms in AuPt nanocrystals. Sun et al. [140] assessed the arrangements of metal-and non-metal-modified g-C 3 N 4 composites in hydrogen evolution and found that the contribution of dye conjugation in non-metallic g-C 3 N 4 composites favored their performance ( Figure 12). However, the co-catalyst doping strategy was recommended for metallic g-C 3 N 4 composites. In addition, the hybrid of MOF materials and g-C 3 N 4 is also a good approach to develop novel photocatalysts. For example, Devarayapalli et al. [141] reported a g-C 3 N 4 /ZIF-67 nanocomposite and obtained a 2084 µmol g −1 H 2 production, which is 3.84-fold greater than that of bare g-C 3 N 4 .
Molecules 2023, 27, x FOR PEER REVIEW Figure 11. Scheme for a one-pot thermal preparation of amino-functionalized ultrathin rous B-doped g-C3N4. Reprinted with permission from Ref. [138]. Copyright 2021 Else Figure 11. Scheme for a one-pot thermal preparation of amino-functionalized ultrathin nanoporous B-doped g-C 3 N 4 . Reprinted with permission from Ref. [138]. Copyright 2021 Elsevier. Figure 11. Scheme for a one-pot thermal preparation of amino-functionalized ultra rous B-doped g-C3N4. Reprinted with permission from Ref. [138]. Copyright 2021 Based on the descriptions mentioned above, Table 1 compares the p different g-C3N4-based materials for photocatalytic H2 generation reported three years.  Based on the descriptions mentioned above, Table 1 compares the performance of different g-C 3 N 4 -based materials for photocatalytic H 2 generation reported within the last three years.   Rising atmospheric levels of CO 2 and the consumption of fossil fuels raise a concern about the continued reliance on the utilization of fossil fuels for both energy and chemical production [173]. Photocatalytic reduction of CO 2 is a promising strategy to meet increasing energy needs and reduce the greenhouse effect [174]. Through photocatalytic reduction, CO 2 can be converted to light oxygenates and hydrocarbons. Photocatalytic CO 2 reduction is a multielectron transfer process. Fu et al. [175] have listed the possible reaction and corresponding redox potentials and stated that CO 2 was complicated to reduce at room temperature due to its stable chemical structure. For the complex reaction, five factors, comprising the matching of band energy, separation of charge carrier, kinetic of e-and hole transfer to CO 2 and reductant, the basicity of photocatalyst, and the strength and coverage of CO 2 adsorption, are considered to be crucial [176]. As a hot member of photocatalysts, g-C 3 N 4 has been applied to CO 2 photo-reduction in recent years because the CB of g-C 3 N 4 is sufficient to reduce CO 2 to various hydrocarbons, such as CH 3 OH, CH 4 , HCHO, and HCOOH, etc. (Figure 13) [177]. However, metal-free g-C3N4 is limited for CO2 reduction activity due to its po ity to activate the C-O bond of CO2. To improve the photocatalytic movement of C version, different metal units have been composited with g-C3N4 for broadening sorption response range, and accelerating the charge separation and transfer, such C3N4 [178], Co 2+ /g-C3N4 [179,180], Au/g-C3N4 [181], etc. Metal nanoparticles actin catalysts could effectively improve the photocatalytic activity and selectivity of duction. In addition, other methods, including doping, loading cocatalysts and n bons, constructing Z-scheme, and heterojunction, have also been employed [16,1 For example, Fu et al. [190] prepared hierarchical porous O-doped g-C3N4 na (OCN-Tube) through continuing thermal oxidation exfoliation and curling conde of bulk g-C3N4. Due to the higher specific surface area, better light harvesting, hig However, metal-free g-C 3 N 4 is limited for CO 2 reduction activity due to its poor ability to activate the C-O bond of CO 2 . To improve the photocatalytic movement of CO 2 conversion, different metal units have been composited with g-C 3 N 4 for broadening the absorption response range, and accelerating the charge separation and transfer, such as Pt/g-C 3 N 4 [178], Co 2+ /g-C 3 N 4 [179,180], Au/g-C 3 N 4 [181], etc. Metal nanoparticles acting as cocatalysts could effectively improve the photocatalytic activity and selectivity of CO 2 reduction. In addition, other methods, including doping, loading cocatalysts and nanocarbons, constructing Z-scheme, and heterojunction, have also been employed [16,[182][183][184][185][186][187][188][189]. For example, Fu et al. [190] prepared hierarchical porous O-doped g-C 3 N 4 nanotubes (OCN-Tube) through continuing thermal oxidation exfoliation and curling condensation of bulk g-C 3 N 4 . Due to the higher specific surface area, better light harvesting, higher CO 2 uptake capacity, and superior separation efficiency of photogenerated charge carriers, the OCN-Tube exhibits excellent photocatalytic CO 2 reduction performance into CH 3 OH. The CH 3 OH evolution rate was as high as 0.88 µmol g −1 h −1 , five times higher than the bulk (0.17 µmol g −1 h −1 ). Huo et al. [191] fabricated amine-modified step-scheme (S-scheme) porous g-C 3 N 4 /CdSe-diethylenetriamine (A-PCN/CdSe-DETA) by a one-step microwave hydrothermal method. The modification by amine and formation of S-scheme heterojunction contributed to the remarkable photocatalytic performance of A-PCN/CdSe-DETA composite in CO 2 reduction and a CO production rate of 25.87 µmol/(h g) was achieved under visible-light irradiation. Wang et al. [174] reviewed different modification methods of g-C 3 N 4 -based photocatalysts for CO 2 reduction. They discussed each method (including morphology adjustment, co-catalysts, heterostructures, and doping) and compared the theoretical calculations and experimental results. By morphology adjustment, g-C 3 N 4 with various shapes can be fabricated, such as rods, tubes, nanosheets, hollow spheres, and honeycomb-like structures. Due to the advantage of cocatalysts (e.g., Au, Ag, Pt, Pd, MXene, AuCu alloy, Pd-Ag), g-C 3 N 4 with co-catalysts can be widely applied to activate CO 2 on the surface. Heterojunction with different types is also an effective method to improve the properties of g-C 3 N 4 -based materials. In addition, elemental doping is considered a common method to enhance photocatalytic quantum efficiency by changing the energy band, surface electronic property, and electrical conductivity. Table 2 compares the performance of different g-C 3 N 4 -based materials for photocatalytic CO 2 reduction reported within the last three years.

Degradation of Organic Pollutants
Along with rapid population growth and significant industrialization development, large numbers of toxic, hazardous, and endless contaminants invade the environment, threatening to human life, especially a variety of pollutants present in water that are difficult to eliminate or degrade naturally. Photocatalytic degradation of contaminants is a green and efficient technology for coping with sewage [128,223]. Different kinds of g-C 3 N 4 -based materials (Table 3) have been exploited to increase the photodecomposition efficiency of pollutants, such as the constructed heterojunction, loading O 2 -reduction cocatalysts, g-C 3 N 4 /CDs-based nanocomposites, and so on [182,[224][225][226]. Generally, under the irradiation of visible light, the photogenerated electrons (e − ) on the g-C 3 N 4 catalyst will be excited from VB to CB, leaving holes (h + ) in the VB. The holes can oxidize pollutants directly or react with H 2 O/OH − to form hydroxyl radicals [227]. When the REDOX potential of g-C 3 N 4 composites is more negative than O 2 /O 2 − , the photogenerated electrons in the material can react with O 2 to produce O 2 − with strong oxidation capacity [228]. In addition, the resulting O 2 − could be protonated to produce OH [229]. Finally, the RhB dye is degraded to CO 2 and H 2 O under the action of these free radicals ( Figure 14). Chen et al. [230] fabricated a BiFeO 3 /g-C 3 N 4 heterostructure through mixing-calcining and compared its performance with BiFeO 3 . Around 30% higher photocatalytic efficiency toward RhB dye was observed for the BiFeO 3 /10% g-C 3 N 4 heterostructure, which was assigned to the contribution of a higher concentration of O 2 − . Zhang et al. [231] studied the selective reduction of molecular oxygen on g-C 3 N 4 and probed its effect on the photocatalytic phenol degradation process. Compared with bulk g-C 3 N 4 , the exfoliated nanosheet yielded a three times improvement in photocatalytic phenol degradation. It has been demonstrated that bulk g-C 3 N 4 prefers to reduce O 2 to O 2 − via one-electron reduction. At the same time, the photoexcited g-C 3 N 4 nanosheet facilitates the two-electron reduction of O 2 to yield H 2 O 2 because of the formation of 1,4-endoperoxide species. The two-electron reduction of O 2 on the nanosheet surface boosts hole generation and thus accelerates phenol oxidation degradation [231,232]. Thus, to improve the photocatalytic performance of g-C 3 N 4 , more effort should be devoted to strengthening the solid O 2 -reduction reactions. For example, Liu et al. [83] reported a heterojunction material of K-doped g-C 3 N4 nanosheet -CdS and degraded tetracycline with 94% degradation under visible light in 30 min. In addition, due to the electronegativities, ionic radius differences, and impurity states, element doping is also an effective method to manipulate the electronic structure and physicochemical performance of g-C 3 N 4 -based materials. Gao et al. [68] synthesized Fe-doped g-C 3 N 4 nanosheets and obtained 1.4-and 1.7-fold higher degradation rates of MB than that of pure g-C 3 N 4 nanosheets and bulk g-C 3 N 4 , which indicated that the exploitation of efficient g-C 3 N 4based photocatalysts with high stabilization and degradation under visible light irradiation would significantly contribute to sewage disposal. Zhang et al. [117] synthesized a novel hybrid of Zr-based metal-organic framework with g-C 3 N 4 (UiO-66/g-C 3 N 4 ) nanosheets and applied a photodegradation of methylene blue, by which a 100% photodegradation was achieved within 4 h under visible light. This research has provided a new insight into the design of g-C 3 N 4 -based photocatalysts to deal with organic dyes in the environment. irradiation would significantly contribute to sewage disposal. Zhang et al. [117] sy sized a novel hybrid of Zr-based metal-organic framework with g-C3N4 (UiO-66/g nanosheets and applied a photodegradation of methylene blue, by which a 100% p degradation was achieved within 4 h under visible light. This research has provi new insight into the design of g-C3N4-based photocatalysts to deal with organic d the environment. Figure 14. Photocatalytic mechanism of g-C3N4/rGO (reduced graphene oxide) for the degra of Rhodamine B. Reprinted with permission from Ref. [122]. Copyright 2015 Elsevier.

Conclusions and Future
Perspective g-C 3 N 4 -based materials are still a research hotspot in photocatalysis, especially their application in energy and environmental sustainability. Although significant progress has been achieved in the preparation and modification of g-C 3 N 4 , several issues remain to be resolved in future research: (1) Some preparation methods are neither environmentally friendly nor time-saving. Thus, it is necessary to develop green and facile synthesis routes. For example, it should be encouraged to use plant leaves, natural halloysite, and some natural raw materials in the preparation of g-C 3 N 4 -based materials. (2) The absorption ability of available g-C 3 N 4 -based materials to visible and near-infrared light is still low, which is not beneficial to improving solar energy utilization. Coupling g-C 3 N 4 with visible and near-infrared CDs might be an effective strategy. It would efficaciously improve the e − /h + pair separation capability and visible light harnessing capability, thus enhancing the related photocatalytic performance. (3) Some structures of modified g-C 3 N 4base materials are complex, and the corresponding photocatalytic reaction mechanisms is not clear yet. Introducing density functional theory could provide insights into the photocatalytic mechanisms via disclosing the materials' structural, electronic, optical, and other properties. Detailed reaction processes can be performed by using in situ monitoring techniques (e.g., in situ infrared spectroscopy and mass spectrometry) to capture the reactive intermediates. (4) Although microscopic techniques and time-resolved spectroscopy have achieved the study of the steady-state charge distribution and charge transfer dynamics of photocatalysts, tracking the spatiotemporally evolving charge transfer processes in single photocatalyst particles and elaborating their exact mechanism is still a great challenge. Thus, it is significant to develop techniques to map holistic charge transfer processes at the single-particle level, identify where charges go and reveal how long they live on different sites. (5) Finally, the integration of artificial intelligence (AI) and other interdisciplinary techniques will play a tremendous driving role in precisely designing g-C 3 N 4 -based photocatalysts with excellent performance. For example, AI models could be developed to correlate photocatalytic performance with experimental conditions, which may help predict the photocatalytic performance of g-C 3 N 4 -based materials, improve the trial-and-error paradigm, and design new composite structures.