Engineered g-C3N5-Based Nanomaterials for Photocatalytic Energy Conversion and Environmental Remediation

Photocatalysis plays a vital role in sustainable energy conversion and environmental remediation because of its economic, eco-friendly, and effective characteristics. Nitrogen-rich graphitic carbon nitride (g-C3N5) has received worldwide interest owing to its facile accessibility, metal-free nature, and appealing electronic band structure. This review summarizes the latest progress for g-C3N5-based photocatalysts in energy and environmental applications. It begins with the synthesis of pristine g-C3N5 materials with various topologies, followed by several engineering strategies for g-C3N5, such as elemental doping, defect engineering, and heterojunction creation. In addition, the applications in energy conversion (H2 evolution, CO2 reduction, and N2 fixation) and environmental remediation (NO purification and aqueous pollutant degradation) are discussed. Finally, a summary and some inspiring perspectives on the challenges and possibilities of g-C3N5-based materials are presented. It is believed that this review will promote the development of emerging g-C3N5-based photocatalysts for more efficiency in energy conversion and environmental remediation.


Introduction
As civilization has developed, energy crises and environmental contaminations have become major obstacles to the further development of human society [1][2][3][4]. Semiconductorbased photocatalysis is regarded as one of the most efficient technologies to address serious energy and environmental issues [5][6][7]. The mass production of highly efficient, dependable, and reasonably priced photocatalysts with strong charge carrier segregation, plenty of active sites, a broad optical absorption range, and high redox potentials is crucial for the commercialization of photocatalytic technology [8][9][10]. As a result, much effort has been devoted to designing more robust and efficient photocatalysts.
Graphitic carbon nitride (g-C 3 N 4 ) has sparked extensive interest because of its metalfree nature, facile accessibility, excellent physicochemical stability, and appealing electronic band structure [5,6,11]. However, the inherent disadvantage of carrier recombination and the restricted light absorption range dramatically hampered the practical application of g-C 3 N 4 [12,13]. To elevate the photocatalytic performance of g-C 3 N 4 , versatile modification procedures, such as doping, heterojunctions, and chemical structural regulation (tuning the C/N ratio), have been implemented [14,15]. Compared with g-C 3 N 4 , g-C 3 N 5 has piqued particular interest because of its nitrogen-rich moiety and sp 2 hybridized atoms, which assist in the optimization of the electronic band structure [16].
This review summarizes the latest progress of g-C3N5-based photocatalysts for energy and environmental applications. Various synthesis topologies of pristine g-C3N5 and several engineering strategies for g-C3N5 (such as elemental doping, defect engineering, and heterojunction creation) are presented. In addition, the photocatalytic applications in H2 evolution, CO2 reduction, N2 fixation, NO purification, and aqueous pollutant degradation are discussed. To the best of our knowledge, this is the first review of g-C3N5-based photocatalysts. It is anticipated that this review will provide readers with a deep understanding of emerging g-C3N5-based photocatalysts, which might promote the design of more robust metal-free photocatalysts.

The Template-Free Approach
A template-free approach was reported by Zhang et al. [28]. Two-dimension metalfree g-C 3 N 5 nanosheets were acquired by the polymerization of 3-AT at 500°C for 3 h under a half-cover air using the heating rate of 5 • C min −1 . The prepared g-C 3 N 5 exhibits excellent crystal properties, which can be confirmed by the peaks of 100 (in-plane structural ordering) and 002 (interlayer structural ordering) in XRD characterization (Figure 3a). XPS spectra (Figure 3b-d) confirmed the presence of one-triazole and two-triazine. Compared with g-C 3 N 4 , g-C 3 N 5 exhibits a better visible light response (Figure 3e), a lower band gap (Figure 3f), and a more effective separation of photogenerated electrons and holes ( Figure 3g). It may be due to the abundance of unpaired electrons on g-C 3 N 5 , confirmed by the ESR spectra ( Figure 3h). Meanwhile, the orbital calculations (Figure 3i,j) also confirm that g-C 3 N 5 has better photo-generated carrier separation efficiency than g-C 3 N 4 . Therefore, the prepared g-C 3 N 5 displays better photocatalytic performance than g-C 3 N 4 . This approach is simple, time-saving, energy-saving, and environment-friendly, which makes it conducive to large-scale production and application of materials. A template-free approach was reported by Zhang et al. [28]. Two-dimension metalfree g-C3N5 nanosheets were acquired by the polymerization of 3-AT at 500℃ for 3 h under a half-cover air using the heating rate of 5 °C min −1 . The prepared g-C3N5 exhibits excellent crystal properties, which can be confirmed by the peaks of 100 (in-plane structural ordering) and 002 (interlayer structural ordering) in XRD characterization (Figure 3a). XPS spectra (Figure 3b-d) confirmed the presence of one-triazole and two-triazine. Compared with g-C3N4, g-C3N5 exhibits a better visible light response (Figure 3e), a lower band gap (Figure 3f), and a more effective separation of photogenerated electrons and holes (Figure 3g). It may be due to the abundance of unpaired electrons on g-C3N5, confirmed by the ESR spectra ( Figure 3h). Meanwhile, the orbital calculations (Figure 3i,j) also confirm that g-C3N5 has better photo-generated carrier separation efficiency than g-C3N4. Therefore, the prepared g-C3N5 displays better photocatalytic performance than g-C3N4. This approach is simple, time-saving, energysaving, and environment-friendly, which makes it conducive to large-scale production and application of materials. (c) high-resolution C1s spectra; (d) high-resolution N1s XPS spectra for g-C3N5; (e) UV-vis DRS; (f) Tauc's plot; (g) PL spectra; (h) ESR spectra for g-C3N5 and g-C3N4; (i) and (j) The orbitals for g-C3N4 and g-C3N5. Adapted with permission from [28]. Copyright Elsevier B.V., 2021.

g-C3N5 with Terminal Triazole
The structure of g-C3N5 with terminal triazoles is constructed by 1,2,4-triazoles and 1H-1,2,3-triazoles, which replace two hydrogens on the terminal amino moiety of one triazine ring in g-C3N4. This replacement both enhanced conjugation and increased nitrogen content (-NH2, -NH), which was beneficial to expand the photo-absorption range and enhancing the selectivity and adsorption of acidic gases, respectively. In this section, we will summarize the methods for producing terminal triazole-based g-C3N5 using KIT-6 and KBr templates. (c) high-resolution C1s spectra; (d) high-resolution N1s XPS spectra for g-C 3 N 5 ; (e) UV-vis DRS; (f) Tauc's plot; (g) PL spectra; (h) ESR spectra for g-C 3 N 5 and g-C 3 N 4 ; (i, j) The orbitals for g-C 3 N 4 and g-C 3 N 5 . Adapted with permission from [28]. Copyright Elsevier B.V., 2021.

g-C 3 N 5 with Terminal Triazole
The structure of g-C 3 N 5 with terminal triazoles is constructed by 1,2,4-triazoles and 1H-1,2,3-triazoles, which replace two hydrogens on the terminal amino moiety of one triazine ring in g-C 3 N 4 . This replacement both enhanced conjugation and increased nitrogen content (-NH 2 , -NH), which was beneficial to expand the photo-absorption range and enhancing the selectivity and adsorption of acidic gases, respectively. In this section, we will summarize the methods for producing terminal triazole-based g-C 3 N 5 using KIT-6 and KBr templates.

The Method Using the KIT-6 Template
Vinu et al. [18] have achieved a remarkable breakthrough in the synthesis of mesoporous g-C 3 N 5 , which was named MCN-8. It was synthesized by a thermal condensation Nanomaterials 2023, 13, 499 5 of 31 process of the 3-amino-1,2,4-triazole precursor in a KIT-6 silica template using a nanocasting technique. In order to detect the structures and properties, advanced characterization techniques are used. XRD measurements reveal the structural ordering and graphitic character of MCN-8 ( Figure 4a). According to the low-angle powder XRD pattern, the MCN-8 materials have a highly organized three-dimensional porous mesostructure. The diffraction peak at 2θ = 27.28 (002) verifies the CN framework's strong crystallinity and interlayer turbostratic ordering, as does a faint peak at 2θ = 13.48 (100), which confirms the in-plane structural ordering. The TEM ( Figure 4d) and SEM (Figure 4e) images clearly demonstrate MCN-8's lattice stripes and porous structure. The N 2 adsorption-desorption measurement (Figure 4b) also reveals that the MCN-8 has organized mesopores and a large specific surface area, both of which contribute to improved adsorption. The near-edge X-ray absorption fine structure (NEXAFS) (Figure 4c), high-resolution XPS (Figure 4f), and energy loss (EEL) spectra (Figure 4g) demonstrate the existence of the terminal triazole unit and tris-triazine ring in the CN framework. This chemical structure not only reduces the bandgap to 2.2 eV (Figure 4h), which is crucial for extending visible light absorption for a faster rate of H 2 evolution (Figure 4i), but it also demonstrates improved selective sensing of toxic acid molecules ( Figure 4j). However, the fabrication of the nano-template (KIT-6) requires a high temperature (540 • C) and a long time, which consumes both energy and time. In order to save energy, Vinu and co-workers [19] used ethanol washing instead of high-temperature calcination to obtain the KIT-6 for synthesizing 3D mesoporous g-C 3 N 5 (MCN-8E). It was found that the porous structure and graphite essence of MCN-8E are strikingly similar to those of MCN-8. Considering the use of a hazardous etching chemical (HF) for template removal causes the loss of active sites and degrades photocatalytic efficacy as well as the cumbersome preparation of the template KIT-6, the search for an energy-saving, non-toxic, and environment-friendly synthesis route has attracted the attention of researchers.
2.2.1. The Method Using the KIT-6 Template Vinu et al. [18] have achieved a remarkable breakthrough in the synthesis of mesoporous g-C3N5, which was named MCN-8. It was synthesized by a thermal condensation process of the 3-amino-1,2,4-triazole precursor in a KIT-6 silica template using a nanocasting technique. In order to detect the structures and properties, advanced characterization techniques are used. XRD measurements reveal the structural ordering and graphitic character of MCN-8 ( Figure 4a). According to the low-angle powder XRD pattern, the MCN-8 materials have a highly organized three-dimensional porous mesostructure. The diffraction peak at 2θ = 27.28 (002) verifies the CN framework's strong crystallinity and interlayer turbostratic ordering, as does a faint peak at 2θ = 13.48 (100), which confirms the in-plane structural ordering. The TEM ( Figure 4d) and SEM (Figure 4e) images clearly demonstrate MCN-8's lattice stripes and porous structure. The N2 adsorption-desorption measurement (Figure 4b) also reveals that the MCN-8 has organized mesopores and a large specific surface area, both of which contribute to improved adsorption. The near-edge X-ray absorption fine structure (NEXAFS) (Figure 4c), high-resolution XPS (Figure 4f), and energy loss (EEL) spectra (Figure 4g) demonstrate the existence of the terminal triazole unit and tri-s-triazine ring in the CN framework. This chemical structure not only reduces the bandgap to 2.2 eV (Figure 4h), which is crucial for extending visible light absorption for a faster rate of H2 evolution (Figure 4i), but it also demonstrates improved selective sensing of toxic acid molecules ( Figure 4j). However, the fabrication of the nano-template (KIT-6) requires a high temperature (540 °C) and a long time, which consumes both energy and time. In order to save energy, Vinu and co-workers [19] used ethanol washing instead of high-temperature calcination to obtain the KIT-6 for synthesizing 3D mesoporous g-C3N5 (MCN-8E). It was found that the porous structure and graphite essence of MCN-8E are strikingly similar to those of MCN-8. Considering the use of a hazardous etching chemical (HF) for template removal causes the loss of active sites and degrades photocatalytic efficacy as well as the cumbersome preparation of the template KIT-6, the search for an energy-saving, non-toxic, and environment-friendly synthesis route has attracted the attention of researchers.   Greener synthesis approaches are urgently needed to eliminate the use of the toxic HF etchant in g-C 3 N 5 production. Wang et al. [20] fabricated g-C 3 N 5 with mesoporous and rod-like morphology (RN-g-C 3 N 5 ) via thermal condensation of the 3-amino-1,2,4-triazole with KBr as the template. Figure 5a,b reveal that the prepared RN-g-C 3 N 5 has a rod-like porous structure due to the guiding effect of KBr. This special structure expands the visible adsorption range (Figure 5c), reduces the bandgap to 1.90 eV (Figure 5d), and improves the separation of electron-hole pairs (Figure 5e,f). As a result, the RN-g-C 3 N 5 displays superior photocatalytic performance and reusability in MB degradation (Figure 5g,h). It shows greater potential for applications in the field of environmental remediation. Compared with the traditional silica templates, the KBr template is an environmentally friendly strategy for avoiding structural damage, the loss of active sites, and significant environmental contamination caused by HF. In short, this procedure is straightforward, environmentally friendly, and inexpensive. Nanomaterials 2023, 13, x FOR PEER REVIEW 6 of 32 edge and nitrogen K-edge NEXAFS spectra (red line for MCN-8, blue line for g-C3N4, and the inset for the proposed molecular structure); (d) TEM image; (e) FE-SEM images; (f) High-resolution XPS spectra: C1s and N1s; (g) EEL spectrum; (h) UV-Vis absorption spectrum; (i) H2 evolution amount in visible light; (j) Gas adsorption detected by the QCM sensor. Adapted with permission from [18]. Copyright Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim, 2017.

The Method Using the KBr Template
Greener synthesis approaches are urgently needed to eliminate the use of the toxic HF etchant in g-C3N5 production. Wang et al. [20] fabricated g-C3N5 with mesoporous and rodlike morphology (RN-g-C3N5) via thermal condensation of the 3-amino-1,2,4-triazole with KBr as the template. Figures 5a and b reveal that the prepared RN-g-C3N5 has a rod-like porous structure due to the guiding effect of KBr. This special structure expands the visible adsorption range (Figure 5c), reduces the bandgap to 1.90 eV (Figure 5d), and improves the separation of electron-hole pairs (Figure 5e,f). As a result, the RN-g-C3N5 displays superior photocatalytic performance and reusability in MB degradation (Figure 5g,h). It shows greater potential for applications in the field of environmental remediation. Compared with the traditional silica templates, the KBr template is an environmentally friendly strategy for avoiding structural damage, the loss of active sites, and significant environmental contamination caused by HF. In short, this procedure is straightforward, environmentally friendly, and inexpensive. Adapted with permission from [20]. Copyright American Chemical Society, 2019.

g-C3N5 Consisted of Heptazine Units Bridged by Azo
g-C3N5, consisting of heptazine units (tri−s−triazine) bridged by azo, is an analogue of g-C3N4, which is derived by replacing the tertiary nitrogen in the g-C3N4 structure with azo. The extended π−conjugation formed by coupling the orbitals of the azo-linkage with the π-conjugated structures of the heptazine narrowed its bandgap, which expanded the optical absorption range. The theoretical design and experimental fabrication of g-C3N5 (−N = N−) will be summarized in this section.

Theoretical Design
Cao et al. [29] designed a 2D crystalline graphitic g-C3N5 material by employing the heptazine structure as the node and the azo group as the linker. Density functional theory (DFT) was employed to investigate the material's optical, electrical, and catalytic characteristics. It was found that 2D g-C3N5 has an excellent performance in non-metal oxygen reduction reactions, gas adsorption, separation, and conversion. Adapted with permission from [20]. Copyright American Chemical Society, 2019.
2.3. g-C 3 N 5 Consisted of Heptazine Units Bridged by Azo g-C 3 N 5 , consisting of heptazine units (tri−s−triazine) bridged by azo, is an analogue of g-C 3 N 4 , which is derived by replacing the tertiary nitrogen in the g-C 3 N 4 structure with azo. The extended π−conjugation formed by coupling the orbitals of the azo-linkage with the π-conjugated structures of the heptazine narrowed its bandgap, which expanded the optical absorption range. The theoretical design and experimental fabrication of g-C 3 N 5 (−N = N−) will be summarized in this section.

Theoretical Design
Cao et al. [29] designed a 2D crystalline graphitic g-C 3 N 5 material by employing the heptazine structure as the node and the azo group as the linker. Density functional theory (DFT) was employed to investigate the material's optical, electrical, and catalytic characteristics. It was found that 2D g-C 3 N 5 has an excellent performance in non-metal oxygen reduction reactions, gas adsorption, separation, and conversion.

Experimental Fabrication
In terms of experimental fabrications, two methods for synthesizing the g-C 3 N 5 framework are presented. As shown in Figure 6a, Kumar et al. [17] synthesized this g-C 3 N 5 via thermal deamination of melem hydrazine (MH), also named 2,5,8-trihydrazinos-heptazine, which was obtained by polymerization of melamine at 425 • C for overnight, followed by a hydrothermal process with hydrazine hydrate. Characterization techniques such as XPS ( Figure 6c) and NMR (Figure 6d) indicated that the structure is made up of two s-heptazine units connected by an azo linkage. The overlap of azo atoms with the aromatic network of heptazine units extends conjugation and increases electron density on the heptazine nucleus ( Figure 6b). The lower EPR signal strength of g-C 3 N 5 compared to g-C 3 N 4 indicates that the number of lone pair electrons in g-C 3 N 5 drops, confirming that extra N atoms bond with tertiary nitrogen atoms outside the heptazine nucleus to form azo rather than substitute C atoms in the heptazine motif ( Figure 6h). This structure increases visible light responsiveness and photogenerated electron-hole separation, as seen by increased visible light absorption in Figure 6e, lower PL intensity in Figure 6f, and a shorter average lifetime of the photogenerated charge carrier in Figure 6g. The 2D g-C 3 N 5 containing s-heptazine units and azo was successfully prepared by this approach, although the process was complicated and time-consuming. In another work, Liu et al. synthesized g-C 3 N 5 nanosheets by heating a mixture of NH 4 Cl and 3-amino-1,2,4-triazole (3-AT), followed by protonation exfoliation. It was found that the morphology of the nanosheet increases the specific surface area, facilitates charge separation, and modifies the band structure [30].

Experimental Fabrication
In terms of experimental fabrications, two methods for synthesizing the g-C3N5 framework are presented. As shown in Figure 6a, Kumar et al. [17] synthesized this g-C3N5 via thermal deamination of melem hydrazine (MH), also named 2,5,8-trihydrazinos-heptazine, which was obtained by polymerization of melamine at 425 °C for overnight, followed by a hydrothermal process with hydrazine hydrate. Characterization techniques such as XPS ( Figure 6c) and NMR (Figure 6d) indicated that the structure is made up of two s-heptazine units connected by an azo linkage. The overlap of azo atoms with the aromatic network of heptazine units extends conjugation and increases electron density on the heptazine nucleus ( Figure 6b). The lower EPR signal strength of g-C3N5 compared to g-C3N4 indicates that the number of lone pair electrons in g-C3N5 drops, confirming that extra N atoms bond with tertiary nitrogen atoms outside the heptazine nucleus to form azo rather than substitute C atoms in the heptazine motif ( Figure 6h). This structure increases visible light responsiveness and photogenerated electron-hole separation, as seen by increased visible light absorption in Figure 6e, lower PL intensity in Figure 6f, and a shorter average lifetime of the photogenerated charge carrier in Figure 6g. The 2D g-C3N5 containing s-heptazine units and azo was successfully prepared by this approach, although the process was complicated and time-consuming. In another work, Liu et al. synthesized g-C3N5 nanosheets by heating a mixture of NH4Cl and 3-amino-1,2,4-triazole (3-AT), followed by protonation exfoliation. It was found that the morphology of the nanosheet increases the specific surface area, facilitates charge separation, and modifies the band structure [30]. In summary, the preparation methods of g-C3N5 with three different structures, namely, g-C3N5 with a terminal triazole, g-C3N5 containing s-heptazine linked by azo, and g-C3N5 containing 1 triazole and 2 triazines, are presented. It has been witnessed that In summary, the preparation methods of g-C 3 N 5 with three different structures, namely, g-C 3 N 5 with a terminal triazole, g-C 3 N 5 containing s-heptazine linked by azo, and g-C 3 N 5 containing 1 triazole and 2 triazines, are presented. It has been witnessed that breakthroughs and progress have been made in the field of g-C 3 N 5 synthesis, from scratch and complexity to simplicity. It is necessary to further explore more environmentally friendly, energy-saving, time-saving, and labor-saving preparation methods.

Functional Engineering of g-C 3 N 5
Pristine g-C 3 N 5 performs better in the applications of CO 2 adsorption and conversion, pollutant degradation, and H 2 evolution. However, pristine g-C 3 N 5 still suffers electron- hole recombination and limited use of visible light. To address the aforementioned issues, g-C 3 N 5 functional materials have been designed and fabricated to broaden their uses in a variety of domains. We will go over the preparations of g-C 3 N 5 functionalized materials in this section, including defect engineering and heterojunction engineering.

Defect Engineering
One strategy for improving the photocatalytic properties of g-C 3 N 5 is to introduce structural defects into the surface. Defects are produced by the dislocation of an atom, which breaks the periodic arrangement. Doping and vacancy are the two types of engineering defects. Doping usually involves adding atoms to the original structure, while vacancies occur when a host atom is losing its position in the original crystal structure. The defects can cause electronic redistribution in the material, which can regulate spectral absorption and affect the excitation, migration, and recombination of photogenerated electron holes.

Doping Design
Doping is a good strategy because it not only changes the electronic structure, resulting in the band gap and the visible light absorption variation, but it also changes the surface properties, which affect the photocatalytic performance. Next, we will discuss metal doping, non-metal doping, and both metal and non-metal doping in the functional engineering of g-C 3 N 5 .
Metal doping can create defect states in the band structure of g-C 3 N 5 , which trap holes or electrons to change interface charge transfer, inhibiting the recombination of photogenerated holes and electrons. As a new type of N-rich material, the N-rich sites of g-C 3 N 5 with stronger electronegativity are easy to combine with positive metal ions. Transition metals with unoccupied d orbitals, which may function as electron donors or acceptors, can easily form complexes with g-C 3 N 5 through electron transfer. The feasibility of transition metal doping g-C 3 N 5 is verified by Niu's team [31] through first-principles calculations. They designed a set of models for depositing transition metal (TM) atoms (V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Ru, Pd, W, and Pt) on g-C 3 N 5 to construct transition metal (TM) doped g-C 3 N 5 single-atom catalysts (SACs), TM-g-C 3 N 5 . The adsorption energies calculated by DFT are all negative, which demonstrates that the structures of all 12 TM-g-C 3 N 5 are stable. This study has significant implications for future research into g-C 3 N 5 -based stable photocatalytic materials.
In terms of material fabrication, Fang et al. [32] prepared pristine g-C 3 N 5 according to Zhang's method [28] and pyrolyzed a mixture of it with potassium chloride, lithium chloride, and cobalt chloride dihydrate to prepare cobalt-doped g-C 3 N 5 , whose doping pattern is shown in Figure 7a. HR-TEM images (Figure 7b), HAADF-STEM images (Figure 7c), and elemental mapping (Figure 7d-f) all show that individual nitrogen atoms are uniformly doped into g-C 3 N 5 . In addition, it was also confirmed that the configuration of Co-N 4 was formed by the combination of Co with pyridine nitrogen in g-C 3 N 5 , as confirmed by the XANES spectra ( Figure 7g) and R-space of XANES ( Figure 7h). The prepared Co-C 3 N 5 has stable catalytic performance for PCB28 degradation, which is attributed to sulfate radicals from PMS activated by Co-N 4 . Similar work has also been done by Gan et al. [33]. They prepared pristine g-C 3 N 5 according to Vinu's method [18] and pyrolyzed the mixture of pristine g-C 3 N 5 with 1,10-phenanthroline monohydrate and (CH 3 COO) 2 Co·4H 2 O to prepare another cobalt-doped g-C 3 N 5 . The structure of Co-N binding was confirmed by advanced characterization methods, such as a peak at 2170 cm −1 in FT-IR spectra ( Figure 7i) and a Co-Nx peak in the XPS spectra of Co 2p (Figure 7j). Cobalt doping significantly enhances optical properties, such as reducing the band gap from 1.80 to 1.63 eV in Tauc's plot (Figure 7k), which helps to activate PMS for the generation of active species, including free radicals (·O 2 − , SO − 4 ·, and ·OH) and non-free radicals ( 1 O 2 and tetravalent cobalt oxide). In SMX degradation experiments, it can be degraded with a removal rate of 99.57% in 20 min. The tetravalent cobalt oxide performed a vital role, whereas the other species made a minor contribution. In addition, Vinu's team [34] synthesized mesoporous titanium carbonitride, named MTiCN. It was obtained by carbonizing the mixture of mesoporous C 3 N 5 and titanium tetrachloride at 900 • C for 5 h. The resulting MTiCN samples have a rod-like morphology, a mesoporous structure, and a large specific area, as well as a high carbon content. It shows excellent H 2 evolution performance similar to commercial platinum/carbon. Moreover, Vivek Polshettiwar [35] synthesized K-doped g-C 3 N 5 (K-g-C 3 N 5 ) via the thermal polymerization of 3-AT and KBr. A dipole interaction is generated between K and N atoms as opposed to K atoms replacing N atoms, hence altering the distribution of electrons and narrowing the band gap to increase light absorption. As an electron trap, K also facilitates the transfer and separation of photogenerated carriers.
vanced characterization methods, such as a peak at 2170 cm −1 in FT-IR spectra ( Figure 7i) and a Co-Nx peak in the XPS spectra of Co2p (Figure 7j). Cobalt doping significantly enhances optical properties, such as reducing the band gap from 1.80 to 1.63 eV in Tauc's plot (Figure 7k), which helps to activate PMS for the generation of active species, including free radicals (·O2 − , SO − 4·, and ·OH) and non-free radicals ( 1 O2 and tetravalent cobalt oxide). In SMX degradation experiments, it can be degraded with a removal rate of 99.57% in 20 min. The tetravalent cobalt oxide performed a vital role, whereas the other species made a minor contribution. In addition, Vinu's team [34] synthesized mesoporous titanium carbonitride, named MTiCN. It was obtained by carbonizing the mixture of mesoporous C3N5 and titanium tetrachloride at 900 °C for 5 h. The resulting MTiCN samples have a rod-like morphology, a mesoporous structure, and a large specific area, as well as a high carbon content. It shows excellent H2 evolution performance similar to commercial platinum/carbon. Moreover, Vivek Polshettiwar [35] synthesized K-doped g-C3N5 (K-g-C3N5) via the thermal polymerization of 3-AT and KBr. A dipole interaction is generated between K and N atoms as opposed to K atoms replacing N atoms, hence altering the distribution of electrons and narrowing the band gap to increase light absorption. As an electron trap, K also facilitates the transfer and separation of photogenerated carriers. Non-metallic doping is another effective method of modifying g-C3N5. Since it has a high electron affinity and electronegativity, it may easily substitute the N atom in g-C3N5 and establish covalent bonds with the C atom. Non-metallic ions penetrate the lattice and act as impurity energy levels in the valence band, therefore reducing the band gap, separating electrons and holes, and enhancing the catalytic properties. To date, non-metallic components such as N, B, P, etc. have been employed to modify g-C3N5.
Hu et al. [36] investigated the phosphorus doping of g-C3N5 (P-g-C3N5). P-doped triazole-based g-C3N5 was produced by thermal condensation at 500 °C for 4 h, followed by thermal copolymerization at 500 °C for another 4 h, using 3-AT and HCCP as precursors.  Non-metallic doping is another effective method of modifying g-C 3 N 5 . Since it has a high electron affinity and electronegativity, it may easily substitute the N atom in g-C 3 N 5 and establish covalent bonds with the C atom. Non-metallic ions penetrate the lattice and act as impurity energy levels in the valence band, therefore reducing the band gap, separating electrons and holes, and enhancing the catalytic properties. To date, non-metallic components such as N, B, P, etc. have been employed to modify g-C 3 N 5 .
Hu et al. [36] investigated the phosphorus doping of g-C 3 N 5 (P-g-C 3 N 5 ). P-doped triazole-based g-C 3 N 5 was produced by thermal condensation at 500 • C for 4 h, followed by thermal copolymerization at 500 • C for another 4 h, using 3-AT and HCCP as precursors. Figure 8a depicts its doping structures: P−N/P = N structures were formed between P and N, which were derived from the XAFS spectra ( Figure 8b) and EXAFS spectra ( Figure 8c). P 2P orbitals provide donor levels in the forbidden band to donate more electrons, narrowing the band gap and increasing the visible light usage rate, as confirmed by UV-vis spectra ( Figure 8d). Compared with the undoped g-C 3 N 5 specimen, P-g-C 3 N 5 demonstrated a higher separation efficiency of photogenerated charges, which was confirmed by PL spectra (Figure 8e). As a result, the P-g-C 3 N 5 displays an excellent rhodamine B and tetracycline degradation rate. N, which were derived from the XAFS spectra ( Figure 8b) and EXAFS spectra (Figure 8c). P2P orbitals provide donor levels in the forbidden band to donate more electrons, narrowing the band gap and increasing the visible light usage rate, as confirmed by UV-vis spectra (Figure 8d). Compared with the undoped g-C3N5 specimen, P-g-C3N5 demonstrated a higher separation efficiency of photogenerated charges, which was confirmed by PL spectra ( Figure 8e). As a result, the P-g-C3N5 displays an excellent rhodamine B and tetracycline degradation rate. Li et al. [38] constructed 18 varieties of B-doped g-C3N5, including substitution doping and pore doping, followed by investigating the impact of B-doping on the material's photo-absorption range, band structure, and charge separation. The results revealed that the optimal impurity levels increased light absorption, minimized the recombination of photogenerated charge, and improved the performance of the catalyst. In terms of material preparation, Qu et al. [37] synthesized boron-doped g-C3N5 (B-C3N5) by a one-step pyrolysis process employing ammonium borate and 3-AT, whose structure is shown in Figure 8f. The increased ID/IG ( Figure 8h) shows that the graphite-like structure was distorted, confirming the B as the heteroatom doped into the structure of the CN. Indeed, the decreased C/N ratio calculated by element analysis confirmed the loss of carbon atoms. The doping of B makes up for the loss of the C atom, as confirmed by the decreased ESR spectra intensity of B-C3N5 (Figure 8g). That is to say, B tends to occupy the C vacancy sites in C3N5 to form the B-O-H functional group, which can expand the light absorption range (Figure 8i), enhance the charge transfer ability (Figure 8j), and optimize the structural stability. In the photocatalytic nitrogen fixation experiments, it can achieve a photocatalytic nitrogen fixation rate of 421.18 μmol·h −1 ·g −1 , demonstrating its superior performance. The B-O-H groups play the most important role; B can absorb and activate nitrogen using its Lewis acid characteristics, and O-H can provide H protonate to activated nitrogen for NH3 formation using its Bronsted acidic characteristics.
Ding et al. [39] synthesized an N-doped carbon catalyst (PDA-g-CN) by pyrolyzing g-C3N5@PDA hybrid, which was created by the polymerization reaction of dopamine hydrochloride and g-C3N5 under mild alkaline conditions, at 800 °C for 2 h. The developed PDA-g-CN catalyst has shown good degradation efficiency for organic pollutants (SMX) via the non-radical mechanism. Nitrogen with a higher electronegativity makes the surface potential of carbon more positive, thereby enhancing the adsorption capacity of PMS Li et al. [38] constructed 18 varieties of B-doped g-C 3 N 5 , including substitution doping and pore doping, followed by investigating the impact of B-doping on the material's photo-absorption range, band structure, and charge separation. The results revealed that the optimal impurity levels increased light absorption, minimized the recombination of photogenerated charge, and improved the performance of the catalyst. In terms of material preparation, Qu et al. [37] synthesized boron-doped g-C 3 N 5 (B-C 3 N 5 ) by a onestep pyrolysis process employing ammonium borate and 3-AT, whose structure is shown in Figure 8f. The increased I D /I G (Figure 8h) shows that the graphite-like structure was distorted, confirming the B as the heteroatom doped into the structure of the CN. Indeed, the decreased C/N ratio calculated by element analysis confirmed the loss of carbon atoms. The doping of B makes up for the loss of the C atom, as confirmed by the decreased ESR spectra intensity of B-C 3 N 5 (Figure 8g). That is to say, B tends to occupy the C vacancy sites in C 3 N 5 to form the B-O-H functional group, which can expand the light absorption range (Figure 8i), enhance the charge transfer ability (Figure 8j), and optimize the structural stability. In the photocatalytic nitrogen fixation experiments, it can achieve a photocatalytic nitrogen fixation rate of 421.18 µmol·h −1 ·g −1 , demonstrating its superior performance. The B-O-H groups play the most important role; B can absorb and activate nitrogen using its Lewis acid characteristics, and O-H can provide H protonate to activated nitrogen for NH 3 formation using its Bronsted acidic characteristics.
Ding et al. [39] synthesized an N-doped carbon catalyst (PDA-g-CN) by pyrolyzing g-C 3 N 5 @PDA hybrid, which was created by the polymerization reaction of dopamine hydrochloride and g-C 3 N 5 under mild alkaline conditions, at 800 • C for 2 h. The developed PDA-g-CN catalyst has shown good degradation efficiency for organic pollutants (SMX) via the non-radical mechanism. Nitrogen with a higher electronegativity makes the surface potential of carbon more positive, thereby enhancing the adsorption capacity of PMS and promoting the formation of high-redox potential C-PMS* complexes, which can oxidize organic contaminants (SMX) via electron transfer from SMX to C-PMS*.
Metal and nonmetal doping play a significant role in the modification of g-C 3 N 5 , which can alter the band gap, light absorption, and photogenerated charge separation. Most metal elements may enter the lattice by pore doping, whereas most non-metal elements may enter via substitution doping. On top of that, Ao et al. [40] synthesized the metal K and nonmetal I co-doped g-C 3 N 5 (K, I-g-C 3 N 5 ) via pyrolyzing 3-AT and KI. K may enter the lattice by pore doping, and I may enter via substitution doping. The band gap of K and I co-doped g-C 3 N 5 is larger than that of pristine g-C 3 N 5 , which is unfavorable for light absorption.
Fortunately, the more negative conduction band potential has a strong reduction ability and can reduce O 2 to produce H 2 O 2 (O 2 + 2e -+ 2H + → H 2 O 2 ). It was also confirmed that the co-doping of metal and non-metal is an effective way to improve the properties of g-C 3 N 5 .

Vacancy Engineering
Vacancy defects also play an important role in enhancing photocatalytic capacity, which can be summarized as having three important aspects: light absorption ability, carrier separation efficiency, and surface reaction ability. In the field of materials, nitrogen vacancies and oxygen vacancies are commonly used to regulate photocatalytic performance. For example, Wang et al. [25] fabricated a triazole-based g-C 3 N 5 material with N vacancies (Nv-g-C 3 N 5 ) through the thermal polymerization of 3-AT assisted by NaOH. Later, Zhang et al. [41] used this method to fabricate the same Nv-g-C 3 N 5 for constructing the N V -g-C 3 N 5 /BiOBr heterojunction. The structure is shown in Figure 9a. A strong Lorentz signal peak observed in the ESR curve of Nv-g-C 3 N 5 confirmed the existence of nitrogen vacancies (Figure 9c). The peak at 2170 cm −1 observed in the FT-IR spectrum suggests the formation of cyano groups (Figure 9b). The cyano-groups extend the photo-absorption range (Figure 9d), reduce the band gaps from 2.08 to 1.5 eV (Figure 9e), and use N vacancies as electron traps to capture and store excited electrons, which facilitate the separation of photogenerated charge carriers. The Nyquist radius of N V -g-C 3 N 5 decreases with the increase of N vacancies in Figure 9f, which is conducive to electron and hole migration and separation. In addition, the PL intensity of Nv-g-C 3 N 5 decreases with the increase of N vacancies in Figure 9g, also reflecting the enhanced separation rate of charge carriers. Both nitrogen vacancies and cyano groups affect the photocatalytic and photoelectrochemical properties of materials. We believe that this simple, effective, and low-cost vacancy engineering is beneficial to the wide application of g-C 3 N 5 in energy conversion and environmental remediation.
which can alter the band gap, light absorption, and photogenerated charge separation. Most metal elements may enter the lattice by pore doping, whereas most non-metal elements may enter via substitution doping. On top of that, Ao et al. [40] synthesized the metal K and nonmetal I co-doped g-C3N5 (K, I-g-C3N5) via pyrolyzing 3-AT and KI. K may enter the lattice by pore doping, and I may enter via substitution doping. The band gap of K and I co-doped g-C3N5 is larger than that of pristine g-C3N5, which is unfavorable for light absorption. Fortunately, the more negative conduction band potential has a strong reduction ability and can reduce O2 to produce H2O2 (O2 + 2e -+ 2H + → H2O2). It was also confirmed that the co-doping of metal and non-metal is an effective way to improve the properties of g-C3N5.

Vacancy Engineering
Vacancy defects also play an important role in enhancing photocatalytic capacity, which can be summarized as having three important aspects: light absorption ability, carrier separation efficiency, and surface reaction ability. In the field of materials, nitrogen vacancies and oxygen vacancies are commonly used to regulate photocatalytic performance. For example, Wang et al. [25] fabricated a triazole-based g-C3N5 material with N vacancies (Nv-g-C3N5) through the thermal polymerization of 3-AT assisted by NaOH. Later, Zhang et al. [41] used this method to fabricate the same Nv-g-C3N5 for constructing the NV-g-C3N5/BiOBr heterojunction. The structure is shown in Figure 9a. A strong Lorentz signal peak observed in the ESR curve of Nv-g-C3N5 confirmed the existence of nitrogen vacancies (Figure 9c). The peak at 2170 cm −1 observed in the FT-IR spectrum suggests the formation of cyano groups (Figure 9b). The cyano-groups extend the photo-absorption range (Figure 9d), reduce the band gaps from 2.08 to 1.5 eV (Figure 9e), and use N vacancies as electron traps to capture and store excited electrons, which facilitate the separation of photogenerated charge carriers. The Nyquist radius of NV-g-C3N5 decreases with the increase of N vacancies in Figure 9f, which is conducive to electron and hole migration and separation. In addition, the PL intensity of Nv-g-C3N5 decreases with the increase of N vacancies in Figure 9g, also reflecting the enhanced separation rate of charge carriers. Both nitrogen vacancies and cyano groups affect the photocatalytic and photoelectrochemical properties of materials. We believe that this simple, effective, and low-cost vacancy engineering is beneficial to the wide application of g-C3N5 in energy conversion and environmental remediation.   [41]. Copyright American Chemical Society, 2020.

Type-I Heterojunction
For a type-I heterojunction (Figure 10a), the energy band structure is nested, with the bottom of the conduction band and the top of the valence band of the narrow band material situated inside the forbidden band of the wide band material. Both holes and electrons are transmitted from semiconductor 1 to semiconductor 2, where they may combine. Consequently, this heterojunction restricts the separation of electrons and holes. There are few materials about type-I heterojunction among the g-C3N5-based heterojunctions. It is worth noting that the azo and heptazine rings expand the conjugation of g-C3N5, increasing the migration properties and separation efficiency of electrons and holes. Thus, g-C3N5 can be used to improve another semiconductor property via type-I heterojunction. For example, Alam et al. [53] fabricated CdS/C3N5 heterojunctions with a core-shell structure by wrapping CdS nanowires with g-C3N5 shells obtained via melamine thermal deamination. It greatly outperformed CdS nanorods alone in the removal of 4-nitrophenol and rhodamine B (RhB) (Figure 11a), which was attributed to the passivating effect generated by the g-

Type-I Heterojunction
For a type-I heterojunction (Figure 10a), the energy band structure is nested, with the bottom of the conduction band and the top of the valence band of the narrow band material situated inside the forbidden band of the wide band material. Both holes and electrons are transmitted from semiconductor 1 to semiconductor 2, where they may combine. Consequently, this heterojunction restricts the separation of electrons and holes. There are few materials about type-I heterojunction among the g-C 3 N 5 -based heterojunctions. It is worth noting that the azo and heptazine rings expand the conjugation of g-C 3 N 5 , increasing the migration properties and separation efficiency of electrons and holes. Thus, g-C 3 N 5 can be used to improve another semiconductor property via type-I heterojunction. For example, Alam et al. [53] fabricated CdS/C 3 N 5 heterojunctions with a core-shell structure by wrapping CdS nanowires with g-C 3 N 5 shells obtained via melamine thermal deamination. It greatly outperformed CdS nanorods alone in the removal of 4-nitrophenol and rhodamine B (RhB) (Figure 11a), which was attributed to the passivating effect generated by the g-C 3 N 5 shell for CdS and the effective charge separation produced by the type-I heterojunctions between CdS and g-C 3 N 5 . Through type-I heterojunction, the electrons transfer from CdS to g-C 3 N 5 , and the holes also transfer from CdS to g-C 3 N 5 , thus promoting hole extraction on CdS. Furthermore, photoinduced electron migration and separation are facilitated by the expanded conjugation in g-C 3 N 5 . In addition, Wang et al. [54] fabricated another kind of CdS/C 3 N 5 for enhanced H 2 production. It is fabricated by the hydrothermal reaction of the uniform suspension solution obtained by adding g-C 3 N 5 to an aqueous solution of NH 2 CSNH 2 and Cd(NO 3 ) 2 ·4H 2 O at 180 • C for 12 h. The synthesized CdS/C 3 N 5 greatly decreased the electron-hole recombination rate via type-I heterojunction, which can produce hydrogen more than four times faster than pristine g-C 3 N 5 (Figure 11b). Furthermore, Wang et al. [55] successfully fabricated the g-C 3 N 4 /g-C 3 N 5 VDWs junction via a facile calcination technique with the precursors 3-AT for g-C 3 N 4 and melamine for g-C 3 N 5 . It exhibited outstanding RhB and TC-HCl degradation performance, which was attributable to the charge transfer from g-C 3 N 4 nanolayers to g-C 3 N 5 nanorods through a type-I heterojunction interface and then effectively activating molecular oxygen to generate ROS (Figure 11c). tated by the expanded conjugation in g-C3N5. In addition, Wang et al. [54] fabricated another kind of CdS/C3N5 for enhanced H2 production. It is fabricated by the hydrothermal reaction of the uniform suspension solution obtained by adding g-C3N5 to an aqueous solution of NH2CSNH2 and Cd(NO3)2·4H2O at 180 °C for 12 h. The synthesized CdS/C3N5 greatly decreased the electron-hole recombination rate via type-I heterojunction, which can produce hydrogen more than four times faster than pristine g-C3N5 (Figure 11b). Furthermore, Wang et al. [55] successfully fabricated the g-C3N4/g-C3N5 VDWs junction via a facile calcination technique with the precursors 3-AT for g-C3N4 and melamine for g-C3N5. It exhibited outstanding RhB and TC-HCl degradation performance, which was attributable to the charge transfer from g-C3N4 nanolayers to g-C3N5 nanorods through a type-I heterojunction interface and then effectively activating molecular oxygen to generate ROS (Figure 11c).

Type-II Heterojunction
Type-II heterojunction ( Figure 10b) has a staggered arrangement of energy bands; the CB and VB potentials of semiconductor 1 are higher than those of semiconductor 2. Under visible light irradiation, photoinduced electrons may migrate from the CB of semiconductor 1 to that of semiconductor 2, while photoinduced holes are transferred in the opposite direction, from semiconductor 2 to semiconductor 1. Photoinduced electrons and holes accumulated on semiconductors 2 and 1, respectively. Consequently, type-II heterojunctions seem to achieve charge carrier separations.
In this section, g-C3N5-based type-II heterojunction materials and their synthesis methods are briefly reviewed. The basic process is to prepare g-C3N5 material, then mix it with other materials through precipitation or hydrothermal reaction to obtain the

Type-II Heterojunction
Type-II heterojunction ( Figure 10b) has a staggered arrangement of energy bands; the CB and VB potentials of semiconductor 1 are higher than those of semiconductor 2. Under visible light irradiation, photoinduced electrons may migrate from the CB of semiconductor 1 to that of semiconductor 2, while photoinduced holes are transferred in the opposite direction, from semiconductor 2 to semiconductor 1. Photoinduced electrons and holes accumulated on semiconductors 2 and 1, respectively. Consequently, type-II heterojunctions seem to achieve charge carrier separations.
In this section, g-C 3 N 5 -based type-II heterojunction materials and their synthesis methods are briefly reviewed. The basic process is to prepare g-C 3 N 5 material, then mix it with other materials through precipitation or hydrothermal reaction to obtain the corresponding heterojunction materials. For example, Sun et al. [42] constructed a g-C 3 N 5 /Bi 4 O 5 Br 2 type-II heterojunction by in-situ growth of Bi 4 O 5 Br 2 on g-C 3 N 5 nanosheets through the hydrothermal method (Figure 12a). The interlayer morphology and N-rich structure enhanced visible light utilization, the bismuth-rich property of Bi 4 O 5 Br 2 increased the excited charge carrier lifetime, and the type-II surface heterojunction (g-C 3 N 5 /Bi 4 O 5 Br 2 ) promoted charge transfer and separation. As shown in Figure 12c, the reactive species ·O 2 − , produced by the reaction between separated electrons on the CB of Bi 4 O 5 Br 2 and the adsorbed O 2 , together with the separated holes on the VB of g-C 3 N 5 , improve the degradation efficiency of sulfathiazole (Figure 12b). Indeed, Meng et al. [56] constructed a 2D/2D C 3 N 5 /GO type-II heterojunction using a sonochemical self-assembly method, achieving a U(VI) removal rate of 96.1% (Figure 12d,e). The photoreduction ability of the electrons on the GO conduction band and the physical adsorption ability of GO itself play an important role (Figure 12f). In addition, p-n heterojunction, a special type-II heterojunction, is composed of p-type and n-type semiconductors. Under visible light irradiation, the photoexcited electrons in the conduction band of the p-type semiconductor move to the n-type semiconductor, driven by the built-in electric field, while the holes move in the opposite direction, forming a p-n heterojunction. For example, Zhang et al. [41] used a hydrothermal process to make N V -g-C 3 N 5 /BiOBr. Nitrogen vacancies and P-N heterojunction promote N 2 adsorption and activation. However, the theory of type-II heterojunctions has also been questioned. Yu et al. [57,58] have presented the inadequacies of the theory of type-II heterojunction from thermodynamic, kinetic, and energy viewpoints. First, the carrier migration to lower CB or higher VB reduces the redox ability of the material accompanying the energy loss. Sec- However, the theory of type-II heterojunctions has also been questioned. Yu et al. [57,58] have presented the inadequacies of the theory of type-II heterojunction from thermodynamic, kinetic, and energy viewpoints. First, the carrier migration to lower CB or higher VB reduces the redox ability of the material accompanying the energy loss. Secondly, this transfer is also kinetically unfavorable because of the repulsive forces between electrons and electrons and holes and holes.
Z-scheme or S-scheme heterojunction (Figure 10c) has the same interleaved energy band structure as a Type-II heterojunction but a completely different mechanism for charge transfer. Electrons migrate from a high Fermi energy level to a low Fermi energy level when two semiconductors come into contact, creating an internal electric field (IEF) and energy band edge bending. Additionally, there are also Coulomb interactions between electrons and holes. As a result of the three aforementioned actions, electrons and holes with low redox potential merge, leaving electrons and holes with high redox potential in the conduction band of semiconducting 1 and the valence band of semiconductor 2. As a result, Z-or S-scheme heterojunctions can achieve excellent separation of photogenerated carriers while retaining a high redox capacity.
Z-scheme heterojunction is developed on the basis of liquid-phase and solid-state Z-scheme heterojunction. Yu et al. [57,58] demonstrated that the charge transfer hypothesis of liquid-phase and all-solid Z-scheme heterostructures is unfavorable in thermodynamics and dynamics, proposing that the "Z-scheme" name be abandoned and substituted with a new name: S-scheme heterostructure. In accordance with the literature, we will discuss the S-and Z-scheme heterostructures, respectively.

Z-Scheme Photocatalyst
Z-scheme heterojunction (Figure 10c) has been shown to be a promising technique for improving a single semiconductor's photocatalytic efficiency. It can also be employed to modify g-C 3 N 5 materials without doubt. To enhance the photocatalytic performance of g-C 3 N 5 , C 3 N 5 -based Z-scheme heterojunctions composed of g-C 3 N 5 and the other semiconductor have been investigated. Herein, g-C 3 N 5 -based Z-scheme heterojunction materials and their synthesis methods are briefly reviewed. The basic process is to prepare the g-C 3 N 5 material first, then mix it with other materials through liquid phase mixing, in-situ precipitation, hydrothermal reaction, and calcination to obtain the corresponding heterojunction materials. For example, Shi et al. [52] fabricated NH 2 -UiO-66/N-CN Z-scheme heterojunction photocatalysts via assembly in liquid mixing (Figure 13a). Taking Pt as a cocatalyst, the fabricated NH 2 -UiO-66/N-CN-2 displays a high hydrogen evolution rate (Figure 13b), which is attributed to the separation of photo-generated carriers facilitated by the close contact interface of Z-scheme heterojunction. This charge transfer mechanism was also verified by the density functional theory calculation (Figure 13c). This provides an effective method to improve the performance of the g-C 3 N 5 photocatalyst by adjusting the electron distribution to promote charge separation through heterojunction interface engineering. tion photocatalysts via assembly in liquid mixing (Figure 13a). Taking Pt as a cocatalyst, the fabricated NH2-UiO-66/N-CN-2 displays a high hydrogen evolution rate (Figure 13b), which is attributed to the separation of photo-generated carriers facilitated by the close contact interface of Z-scheme heterojunction. This charge transfer mechanism was also verified by the density functional theory calculation (Figure 13c). This provides an effective method to improve the performance of the g-C3N5 photocatalyst by adjusting the electron distribution to promote charge separation through heterojunction interface engineering. Figure 13. NH 2 -UiO-66/N-CN Z-scheme Heterojunction: (a) synthetic routes; (b) photocatalytic H 2 production amounts; (c) charge density differences and energy-level diagrams; (d) the proposed Z-scheme charge transfer mechanism. Adapted with permission from [52]. Copyright American Chemical Society, 2022.

S-Scheme Heterojunction
The reported C 3 N 5 -based S-scheme heterostructure consisting of g-C 3 N 5 and another oxidizing semiconductor may improve the photocatalytic activity of g-C 3 N 5 . Through charge transfer at the interface of the S-scheme heterojunction, the holes in g-C 3 N 5 may be coupled with the electrons in another semiconductor through charge transfer at the interface of an S-scheme heterojunction. The electrons in g-C 3 N 5 and holes in another semiconductor were preserved, which successfully separates electrons and holes and maintains a high redox capability. Herein, g-C 3 N 5 -based S-scheme heterojunction materials and their synthesis methods are briefly reviewed. The basic process is to prepare the g-C 3 N 5 material first, then mix it with other materials through wet chemical synthesis, hydrothermal reactions, and a solvothermal approach to obtain the corresponding heterojunction materials. For example, Li et al. [45] constructed a Bi 2 MoO 6 /C 3 N 5 S-scheme heterojunction with oxygen vacancies (OVs) via in-situ solvothermal synthesis (Figure 14a). Benefiting from the efficient separation and transfer of photogenerated charge carriers by S-scheme charge transfer, enriched structural defects, and the close contact interface formed by the in-situ growth, this material remarkably enhanced visible-light photocatalytic degradation efficiencies for TC and Cr(VI) (Figure 14b), whose degradation mechanisms are shown in Figure 14c. The development of S-scheme heterojunctions holds great promise for the design of g-C 3 N 5 -based photocatalysts and the applications of g-C 3 N 5 -based materials in energy conversion and environmental remediation. the efficient separation and transfer of photogenerated charge carriers by S-scheme charge transfer, enriched structural defects, and the close contact interface formed by the in-situ growth, this material remarkably enhanced visible-light photocatalytic degradation efficiencies for TC and Cr(VI) (Figure 14b), whose degradation mechanisms are shown in Figure 14c. The development of S-scheme heterojunctions holds great promise for the design of g-C3N5-based photocatalysts and the applications of g-C3N5-based materials in energy conversion and environmental remediation.

Schottky Junction
A Schottky junction is made from a metal and an n-type semiconductor material, similar to a p-n junction. N-type semiconductors have a higher Fermi energy level than metal, so electrons flow from the semiconductor to the metal. This makes an electron accumulation layer on the metal side and an electron depletion layer on the semiconductor side. This creates an electric field from the semiconductor to the metal, or a "Schottky barrier", permitting electrons to go from the semiconductor to the metal but not return. The metal plays the role of converging electrons and promotes the effective separation of electrons and holes. In this section, we will talk about three types of Schottky materials for g-C 3 N 5 : conventional structures, core-shell structures, and two-dimensional nanosheet structures.
In conventional Schottky connections, metal particles are dispersed or deposited on the semiconductor surface. For example, Zhang et al. [59] fabricated Pt-C 3 N 5 and P-Pt-C 3 N 5 by loading Pt on g-C 3 N 5 through NABH 4 reduction and in-situ photo-deposition, respectively. Chen et al. [60] constructed a Ni-C 3 N 5 material by loading Ni onto g-C 3 N 5 using a simple annealing method. The Schottky barrier allows electrons to pass from the g-C 3 N 5 conductor band to Pt (Ni) but not back, thereby preventing the combination of photogenerated electrons and holes in g-C 3 N 5 and enhancing the photocatalytic performance of Pt-and Ni-C 3 N 5 .
The core-shell structure is formed by wrapping metal nanoparticles with semiconductor materials, thus forming strong contact between two materials at the interface. Li et al. [61] synthesized HCNs@ATTZ by impregnating 5-amino-1H-tetrazole (5-ATTZ) into hollow carbon nanospheres (HCNs), which were subsequently carbonized at 400 • C to produce HCNs@g-C 3 N 5 (Figure 15a). This is a core-shell-structured Schottky heterojunction in which HCNs are the core and g-C 3 N 5 is the shell (Figure 15b). The difference in Fermi levels produced band bending and an internal electric field. They act similarly to a Schottky heterojunction to promote the separation of electrons and holes. Two-dimensional (2D) nanosheet materials with chemically active surfaces or edges have a larger specific surface area, which may provide more active sites for photoelectrochemical reactions compared to pristine particles. Meng et al. [62] fabricated the Schottky heterojunction C3N5/RGO containing 2D-structured g-C3N5 and 2D-structured reduced graphene oxide (Figure 15c). Electrons were transported from g-C3N5 to RGO through a Schottky heterojunction, efficiently separating electrons and holes. Moreover, RGO with high electrical conductivity was favorable for electron migration. As a result, the produced C3N5/RGO exhibits good photoreduction for uranium (Figure 15d,e).

Environmental and Energy Applications
g-C3N5 has been extensively applied in two major areas: energy conversion (H2 evolution and N2 reduction) and environmental remediation (pollutant degradation, CO2 reduction, NO removal, and peroxide activation). They will be discussed in the subsequent chapters. Two-dimensional (2D) nanosheet materials with chemically active surfaces or edges have a larger specific surface area, which may provide more active sites for photoelectrochemical reactions compared to pristine particles. Meng et al. [62] fabricated the Schottky heterojunction C 3 N 5 /RGO containing 2D-structured g-C 3 N 5 and 2D-structured reduced graphene oxide (Figure 15c). Electrons were transported from g-C 3 N 5 to RGO through a Schottky heterojunction, efficiently separating electrons and holes. Moreover, RGO with high electrical conductivity was favorable for electron migration. As a result, the produced C 3 N 5 /RGO exhibits good photoreduction for uranium (Figure 15d,e).

Environmental and Energy Applications
g-C 3 N 5 has been extensively applied in two major areas: energy conversion (H 2 evolution and N 2 reduction) and environmental remediation (pollutant degradation, CO 2 reduction, NO removal, and peroxide activation). They will be discussed in the subsequent chapters.

H 2 Evolution Reaction (HER)
Solar-driven water-splitting technology has become a research hotspot for sustainable hydrogen manufacturing technology. According to the thermodynamic properties, the process of overall H 2 O spitting for hydrogen and oxygen evolution is non-spontaneous with a Gibbs free energy of 237.2 kJ·mol −1 , which is equal to a potential of 1.23 eV. For the hydrogen evolution reaction (HER), the potential at the semiconductor's conduction band bottom must be more negative than the hydrogen evolution potential (H + /H 2 = 0 eV). For the oxygen evolution reaction (OER), the potential at the valence band top must be more positive than the oxygen evolution potential (H 2 O/O 2 = 1.23 eV). If you want to achieve H 2 and O 2 evolutions at the same time, the band gap of the semiconductor photocatalyst must be greater than 1.23 eV. For example, Liu et al. [30] synthesized pristine C 3 N 5 nanosheets with a band gap of 1.5 eV, a CB potential of −0.333 eV, and a VB potential of 1.217 eV. The conduction band bottom is more negative than 0 eV, which satisfies the hydrogen evolution conditions. The hydrogen evolution rate of 28.97 µmol·g −1 ·h −1 was achieved. Although the band gap is greater than 1.23 eV, the valence band potential is less than 1.23 eV (the OER potential), which means OER cannot take place.

Quantum Efficiency
Ref.  3.94 6.8% (at 420 nm) [52] Catalyst (0.05 g L −1 ) TEOA (10 vol%) Pt cocatalyst (2 wt%) C 3 N 4 /rGO/C 3 N 5 300 W Xe-lamp(λ ≥ 400 nm) 6. 38 3.5% (at 420 nm) [68] Catalyst (1 g L −1 ) TEOA (10 vol%) Pt cocatalyst (1.0 wt%) 4.2. CO 2 Reduction Reaction (CO 2 RR) CO 2 , being the primary byproduct of burning fossil fuels, may considerably contribute to global warming and environmental harm. Photocatalytic CO 2 reduction reactions (CO 2 RR) are a promising technique that converts CO 2 to hydrocarbon fuels. It not only lowers CO 2 contaminants but also supplies clean fuels such as methane (CH 4 ), methanol (CH 3 OH), and ethanol (CH 3 CH 2 OH) that can replace fossil fuels. However, because the linear CO 2 molecule has a relatively stable physical chemistry property with a Gib free energy of 394.28 KJ·mol −1 , it is difficult to convert CO 2 into C1 compounds (CO, CH 4 , and CH 3 OH) and C2 compounds (CH 3 CH 2 OH). To break the initial carbon-oxygen bond (750 KJ·mol −1 ), a tremendous amount of energy is required. Due to the remarkable improvement in visible light utilization, g-C 3 N 5 -based materials are expected to be candidates for CO 2 RR. The photogenerated holes can oxidize H 2 O into O 2 and H + , which H + can couple with electrons to reduce CO 2 to C1 products, followed by coupling C and C to obtain the C2 product. In order to accomplish the above reduction process, the potential at VB must be more positive than that of O 2 /H 2 O, and the potential at CB bottom must be more negative than that of C1/CO 2 .
Adsorption is the key step for CO 2 conversion. Cao et al. [29] demonstrated that g-C 3 N 5 (−N = N−) with basicity groups exhibits preferable affinity and notable adsorption selectivity for CO 2 by DFT calculation. At 298 K, the adsorption capacities of CO 2 , CH 4 , and H 2 are 11.8 mmol g −1 at 30 bar, 7.9 mmol g −1 at 50 bar, and 0.75 mmol g −1 at 50 bar, respectively (Figure 16a). It indicated that g-C 3 N 5 is a viable choice for the adsorptive separation of CO 2 /H 2 and CH 4 /H 2 , particularly CO 2 /H 2 (Figure 16b). Vinu et al. [19] prepared the ordered mesoporous g-C 3 N 5 (MCN-8E-T) for CO 2 adsorption with 5.63 mmol g −1 of capacity at 273 K and 30 bar (Figure 16c). Vinu's colleagues [27] also investigated MCN-14 materials with different pore sizes, which exhibit 5.6-9.1 mmol g −1 of CO 2 adsorption abilities at 0 • C and 30 bar and 14-38% of Faraday efficiencies for CO formation. Except for this conversion from CO 2 to CO, Morikawa et al. [69] demonstrated that g-C 3 N 5 (-N = N-) can efficiently catalyze CO 2 to CH 4 and CH 3 CH 2 OH under visible light illumination by DFT calculation, with −0.54 eV (Figure 16d) and −0.61 eV (Figure 16e) of limiting potentials, respectively. Both of them were less than HER free energy (1.18 eV) (Figure 16f), which indicated that g-C 3 N 5 is an effective catalyst for CO 2 RR due to its strong suppressing effect on HER. Bharati Debnath and coworkers [35] reported that 1% K-doped g-C 3 N 5 (-N = N-) could convert CO 2 to CH 4 with H + provided by H 2 O, displaying 100% selectivity (Figure 17g-i). The keys to improving CO 2 absorption and reduction may be a large specific surface area, the suitable size of pores, a more negative conduction potential with strong reduction capacity, and effective charge separation. (d) and (e) Calculated free energy diagram corresponding to the optimal path of the CO2 conversion to CH4 and CH3CH2OH on the g-C3N5, respectively; (f) Free energy diagram of HER (side reaction) on the g-C3N5; (g) and (h) CH4 and H2 evolution rate in photocatalytic CO2 reduction by g-C3N5: K0.01, respectively; (i) Schematic representation of the photocatalytic CO2 reduction using g-C3N5: K0.01. (a−b) Adapted with permission from [29]. Copyright Royal Society of Chemistry, 2018. (c) Adapted with permission from [19]. Copyright Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim, 2017. (d−f) Adapted with permission from [69]. Copyright Elsevier Inc, 2020. (g−i) Adapted with permission from [35]. Copyright American Chemical Society, 2022.

Nitrogen Reduction Reaction (NRR)
Inspired by the process of biological nitrogen fixation, photocatalytic nitrogen reduction for ammonia synthesis has garnered attention. As the g-C3N5 is excited by photons, the generated holes can oxidize H2O into O2 and H + , and electrons may activate N2 with the aid of H + to generate NH3. Compared with the Haber-Bosch method, this progressive hydrogenation method avoids energy waste and CO2 emissions, which is both energysaving and environmentally friendly. The potential at the CB bottom needs to be more negative compared to the N2/reduction products (N2/N2·, N2/N2H, N2/N2H2, N2/N2H4, N2/N2H5 + , and N2/NH3), while the potential at VB top needs to be more positive compared to that of O2/H2O. Bharati Debnath [70] designed a g-C3N5/NiCr-LDH heterostructure, a hybrid of g-C3N5 and NiCr-layered double hydroxide. DFT calculations showed its ability to adsorb and activate N2 molecules. The possible adsorption configurations and binding sites, the optimized configurations and adsorption energy (Eads), and the N-N bond lengths of the adsorbed N2 molecules are presented in Figure 17a-d, Figure 17e  (d,e) Calculated free energy diagram corresponding to the optimal path of the CO 2 conversion to CH 4 and CH 3 CH 2 OH on the g-C 3 N 5 , respectively; (f) Free energy diagram of HER (side reaction) on the g-C 3 N 5 ; (g,h) CH 4 and H 2 evolution rate in photocatalytic CO 2 reduction by g-C 3 N 5 : K0.01, respectively; (i) Schematic representation of the photocatalytic CO 2 reduction using g-C 3 N 5 : K0.01. (a,b) Adapted with permission from [29]. Copyright Royal Society of Chemistry, 2018. (c) Adapted with permission from [19]. Copyright Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim, 2017. (d−f) Adapted with permission from [69]. Copyright Elsevier Inc, 2020. (g−i) Adapted with permission from [35]. Copyright American Chemical Society, 2022.

Nitrogen Reduction Reaction (NRR)
Inspired by the process of biological nitrogen fixation, photocatalytic nitrogen reduction for ammonia synthesis has garnered attention. As the g-C 3 N 5 is excited by photons, the generated holes can oxidize H 2 O into O 2 and H + , and electrons may activate N 2 with the aid of H + to generate NH 3 . Compared with the Haber-Bosch method, this progressive hydrogenation method avoids energy waste and CO 2 emissions, which is both energy-saving and environmentally friendly. The potential at the CB bottom needs to be more negative compared to the N 2 /reduction products (N 2 /N 2 ·, N 2 /N 2 H, N 2 /N 2 H 2 , N 2 /N 2 H 4 , N 2 /N 2 H 5 + , and N 2 /NH 3 ), while the potential at VB top needs to be more positive compared to that of O 2 /H 2 O.
Bharati Debnath [70] designed a g-C 3 N 5 /NiCr-LDH heterostructure, a hybrid of g-C 3 N 5 and NiCr-layered double hydroxide. DFT calculations showed its ability to adsorb and activate N 2 molecules. The possible adsorption configurations and binding sites, the optimized configurations and adsorption energy (Eads), and the N-N bond lengths of the adsorbed N 2 molecules are presented in Figure 17a-d, Figure 17e-h, and Figure 17i-l, respectively. The lesser absorption energy (−0.64 eV/N 2 ) confirmed that the Cr atom is the best adsorption site of the N 2 molecule. The increased N-N bond length (1.16 Å) compared to the free N 2 molecule (1.078Å) suggests the activation of the N 2 molecule. The experimental results also confirmed that the outstanding NH 3 yields of g-C 3 N 5 /NiCr-LDH were 7.51 and 2.86-folds larger than those of pristine g-C 3 N 5 and NiCr-LDH, respectively (Figure 17m). This is mainly due to the separation of electrons and holes produced by the heterojunction interface between g-C 3 N 5 and NiCr-LDH, which facilitates more electrons for reducing N 2 to NH 3 (Figure 17n). B-C 3 N 5 , produced by doping boron into g-C 3 N 5 , was used to immobilize N 2 for NH 3 synthesis, as confirmed by in-situ DRIFTS spectra (Figure 17o-p), with an NH 3 yield 1.72 times that of pristine g-C 3 N 5 (Figure 17q). B-doping enhances light absorption and electron-hole separation and also provides B-O-H functional groups. The B site is capable of adsorbing and activating nitrogen, while the O-H site supplies the H + to N 2 for NH 3 formation [37]. Moreover, Fe, an essential element of nitrogenase, plays an important role in N 2 fixation; abundant oxygen vacancies (OVs) generated by W 18 O 49 can adsorb and activate N 2 , and the narrow band gap of g-C 3 N 5 expands the use of light. Based on these advantages, Li et al. [71] constructed Fe-W 18 O 49 /g-C 3 N 5 hybrids for NH 3 production at a rate of 131.6 mol·g −1 ·h −1 , which was much superior to that of pristine g-C 3 N 5 or Fe-W 18  confirmed that the outstanding NH3 yields of g-C3N5/NiCr-LDH were 7.51 and 2.86-folds larger than those of pristine g-C3N5 and NiCr-LDH, respectively (Figure 17m). This is mainly due to the separation of electrons and holes produced by the heterojunction interface between g-C3N5 and NiCr-LDH, which facilitates more electrons for reducing N2 to NH3 (Figure 17n). B-C3N5, produced by doping boron into g-C3N5, was used to immobilize N2 for NH3 synthesis, as confirmed by in-situ DRIFTS spectra (Figure 17o-p), with an NH3 yield 1.72 times that of pristine g-C3N5 (Figure 17q). B-doping enhances light absorption and electron-hole separation and also provides B-O-H functional groups. The B site is capable of adsorbing and activating nitrogen, while the O-H site supplies the H + to N2 for NH3 formation [37]. Moreover, Fe, an essential element of nitrogenase, plays an important role in N2 fixation; abundant oxygen vacancies (OVs) generated by W18O49 can adsorb and activate N2, and the narrow band gap of g-C3N5 expands the use of light. Based on these advantages, Li et al. [71] constructed Fe-W18O49/g-C3N5 hybrids for NH3 production at a rate of 131.6 mol·g −1 ·h −1 , which was much superior to that of pristine g-C3N5 or Fe-W18O49.

NO Removal
Nitrogen oxides (NOx), especially NO, present a grave risk to global ecosystems. Photocatalysis is a viable NO removal technique that can convert NO to NO under visible light illumination. For example, Zhang et al. [60] constructed a Ni-C3N5 photocatalyst for NO removal. Under light irradiation, it increased O2 absorption (Figure 18a) and e -/h + separation, promoting O2 activation and the production of more reactive oxygen species (ROS). In-situ DRIFT spectroscopy was used to investigate the conversion of NO to NO3 - (Figure 18b). Ion chromatography was used to identify the formation of NO3 - (Figure 18c). Degradation (m) Photocatalytic ammonia production rates of g-C 3 N 5 /NiCr-LDH; (n) Schematic illustration of a mechanism for NH 3 production using g-C 3 N 5 /NiCr-LDH; (o-p) In-situ DRIFTS spectra of B-C 3 N 5 ; (q) NH 3 yields for g-C 3 N 5 and B-C 3 N 5 x (x = 1, 2, 3). (a-n) Adapted with permission from [70]. Copyright Elsevier B.V., 2022. (o-q) Adapted with permission from [37]. Copyright American Chemical, 2020.

NO Removal
Nitrogen oxides (NOx), especially NO, present a grave risk to global ecosystems. Photocatalysis is a viable NO removal technique that can convert NO to NO under visible light illumination. For example, Zhang et al. [60] constructed a Ni-C 3 N 5 photocatalyst for NO removal. Under light irradiation, it increased O 2 absorption (Figure 18a) and e − /h + separation, promoting O 2 activation and the production of more reactive oxygen species (ROS). In-situ DRIFT spectroscopy was used to investigate the conversion of NO to NO 3 − (Figure 18b). Ion chromatography was used to identify the formation of NO 3 − (Figure 18c). Degradation experimental results show that 0.1-Ni-C 3 N 5 has the largest NO removal efficiency (about 54%) (Figure 18d). A series of quenching experiments confirmed that the ROS (·O 2 − , 1 O 2 , and ·OH) play an important role in NO removal (Figure 18e). The mechanism of NO removal by g-C 3 N 5 -based photocatalyst is further understood, as can be seen in Figure 18f. When photons with sufficient energy irradiate the Ni-C 3 N 5 , the electrons leap to the conduction band, forming holes in the valence band. The electrons can activate O 2 to generate reactive oxygen species (·O 2 − , 1 O 2 , and ·OH). The h + and the generated ·O 2 − , 1 O 2 , and ·OH can convert NO to NO 3 − . Moreover, TiO 2 (P25)-C 3 N 5 [72] Z-scheme heterojunctions displayed a NO removal rate of 67.1%, which is attributed to the effective separation of photogenerated e − and h + and the reserve of strong redox potentials provided by Z-scheme heterojunctions, promoting more ROS formation. Table 2 presents applications of g-C 3 N 5 -based materials for photocatalytic NO removal.  (Figure 18d). A series of quenching experiments confirmed that the ROS (·O2 − , 1 O2, and ·OH) play an important role in NO removal (Figure 18e). The mechanism of NO removal by g-C3N5-based photocatalyst is further understood, as can be seen in Figure 18f. When photons with sufficient energy irradiate the Ni-C3N5, the electrons leap to the conduction band, forming holes in the valence band. The electrons can activate O2 to generate reactive oxygen species (·O2 − , 1 O2, and ·OH). The h + and the generated ·O2 -, 1 O2, and ·OH can convert NO to NO3 − . Moreover, TiO2 (P25)-C3N5 [72] Z-scheme heterojunctions displayed a NO removal rate of 67.1%, which is attributed to the effective separation of photogenerated eand h + and the reserve of strong redox potentials provided by Z-scheme heterojunctions, promoting more ROS formation. Table 2 presents applications of g-C3N5-based materials for photocatalytic NO removal. (f) Proposed mechanism for photocatalytic NO removal on Ni-C3N5 material. Adapted with permission from [35]. Copyright Wiley-VCH GmbH, 2022.

Pollutant Degradation
Semiconductor materials based on g-C3N5 have been used extensively for the removal of different pollutants from water. Figure 19 summarizes the application of g-C3N5based materials in the degradation of pollutants, including direct photocatalytic degradation and activated peroxide degradation. We first summarized the direct photocatalytic

Pollutant Degradation
Semiconductor materials based on g-C 3 N 5 have been used extensively for the removal of different pollutants from water. Figure 19 summarizes the application of g-C 3 N 5 -based materials in the degradation of pollutants, including direct photocatalytic degradation and activated peroxide degradation. We first summarized the direct photocatalytic processes of the g-C 3 N 5 -based materials alone. When g-C 3 N 5 is irradiated with photons of sufficient energy, electrons jump to the conduction band and generate holes in the valence band. Electrons oxidize O 2 to generate ROS (·O 2 − , 1 O 2 , ·OH), whereas holes oxidize H 2 O to generate ·OH. The resultant h + , ·O 2 − , 1 O 2 , and ·OH may oxidize organic pollutants to form CO 2 and H 2 O, while electrons can photo-reduce pollutants from high valence to low valence. For example, Cr (VI) and U (VI) can be reduced to Cr (IV) and U (IV), respectively [44,45,62,73]. Meanwhile, the h + can also oxidize Hg 0 to Hg 2+ [43]. processes of the g-C3N5-based materials alone. When g-C3N5 is irradiated with photons of sufficient energy, electrons jump to the conduction band and generate holes in the valence band. Electrons oxidize O2 to generate ROS (·O2 -, 1 O2, ·OH), whereas holes oxidize H2O to generate ·OH. The resultant h + , ·O2 -, 1 O2, and ·OH may oxidize organic pollutants to form CO2 and H2O, while electrons can photo-reduce pollutants from high valence to low valence. For example, Cr (VI) and U (VI) can be reduced to Cr (IV) and U (IV), respectively [44,45,62,73]. Meanwhile, the h + can also oxidize Hg 0 to Hg 2+ [43]. Moreover, persulfate (PMS/PDS) and sodium percarbonate (SPC) can be activated by g-C3N5-based materials to produce ROS for pollutant degradation. There are both free radicals and non-free radicals in the persulfate activation by g-C3N5-based materials. The free radical process refers to the oxidative degradation of pollutants by active species, such as SO4 − ·, ·OH, and ·O2 − , produced by activating persulfate; the non-radical process includes the oxidation of pollutants by 1 O2, electron transfer between persulfate and pollutants, and the direct oxidation of pollutants by persulfate. For example, Zhu et al. [28] demonstrated that g-C3N5 can activate PMS or PDS with the assistance of visible light to degrade emerging micropollutants. PMS can be activated by g-C3N5 to produce SO4 − ‧ and ‧OH. Meanwhile, electrons in the g-C3N5 conduction band can react with O2 to produce ·O2 − and 1 O2. The above species (h + , ·O2 − , SO4 − ·, 1 O2, and ·OH) are jointly involved in the degradation process of EMs. Ding et al. [39] demonstrated PDA-g-CN-1.0 obtained by coating g-C3N5 with polydopamine could effectively activate PMS without light to generate high-potential C-PMS* complexes, which could completely oxidize SMX within 5 min. Fang et al. [32] demonstrated that Co-C3N5 can activate PMS without light to completely degrade PCB28 within 30 min, which was significantly superior to the traditional metal-based activation. It was found that sulfate radical was the main active species for PCB28 degradation, which was generated from the decomposition of PMS activated by Co-N4. In contrast to this conclusion, Zhu et al. [33] discovered that high-valent cobalt oxides play an important role in SMX degradation by another Co-C3N5 activating PMS with the help of visible light. This difference may be caused by the combination of photocatalysis and the transition metal cobalt under visible light irradiation.
Sodium percarbonate (SPC, Na2CO3·1.5H2O2), as a solid carrier of H2O2, not only has the properties of H2O2, but also has the possibility of producing carbonate radicals. Due to its easy transportation, storage, and handling properties, it has been used as a substitute for H2O2 in AOPs. Zhang et al. [67] employed ultrathin-C3N5 and SPC coupling photocatalytic techniques to degrade sulfamethoxazole, arriving at a removal rate of 93.97% in 120 min. The main processes are as follows: (1) the separation of eand h + ; (2) the formation of ROS (·OH, ·O2 − , and 1 O2); and (3) the degradation of pollutants by ROS (·OH, ·O2 − , 1 O2, ·CO3 − , and h + ). This work expands the application of g-C3N5-based materials in AOPs. Table 3 summarizes the recent developments in the photo-degradation and photo-reduction Moreover, persulfate (PMS/PDS) and sodium percarbonate (SPC) can be activated by g-C 3 N 5 -based materials to produce ROS for pollutant degradation. There are both free radicals and non-free radicals in the persulfate activation by g-C 3 N 5 -based materials. The free radical process refers to the oxidative degradation of pollutants by active species, such as SO 4 − ·, ·OH, and ·O 2 − , produced by activating persulfate; the non-radical process includes the oxidation of pollutants by 1 O 2 , electron transfer between persulfate and pollutants, and the direct oxidation of pollutants by persulfate. For example, Zhu et al. [28] demonstrated that g-C 3 N 5 can activate PMS or PDS with the assistance of visible light to degrade emerging micropollutants. PMS can be activated by g-C 3 N 5 to produce SO 4 − · and ·OH. Meanwhile, electrons in the g-C 3 N 5 conduction band can react with O 2 to produce ·O 2 − and 1 O 2 . The above species (h + , ·O 2 − , SO 4 − ·, 1 O 2 , and ·OH) are jointly involved in the degradation process of EMs. Ding et al. [39] demonstrated PDA-g-CN-1.0 obtained by coating g-C 3 N 5 with polydopamine could effectively activate PMS without light to generate high-potential C-PMS* complexes, which could completely oxidize SMX within 5 min. Fang et al. [32] demonstrated that Co-C 3 N 5 can activate PMS without light to completely degrade PCB28 within 30 min, which was significantly superior to the traditional metalbased activation. It was found that sulfate radical was the main active species for PCB28 degradation, which was generated from the decomposition of PMS activated by Co-N4. In contrast to this conclusion, Zhu et al. [33] discovered that high-valent cobalt oxides play an important role in SMX degradation by another Co-C 3 N 5 activating PMS with the help of visible light. This difference may be caused by the combination of photocatalysis and the transition metal cobalt under visible light irradiation.
Sodium percarbonate (SPC, Na 2 CO 3 ·1.5H 2 O 2 ), as a solid carrier of H 2 O 2 , not only has the properties of H 2 O 2 , but also has the possibility of producing carbonate radicals. Due to its easy transportation, storage, and handling properties, it has been used as a substitute for H 2 O 2 in AOPs. Zhang et al. [67] employed ultrathin-C 3 N 5 and SPC coupling photocatalytic techniques to degrade sulfamethoxazole, arriving at a removal rate of 93.97% in 120 min. The main processes are as follows: (1) the separation of e − and h + ; (2) the formation of ROS (·OH, ·O 2 − , and 1 O 2 ); and (3) the degradation of pollutants by ROS (·OH, ·O 2 − , 1 O 2 , ·CO 3 − , and h + ). This work expands the application of g-C 3 N 5 -based materials in AOPs. Table 3 summarizes the recent developments in the photo-degradation and photo-reduction of pollutants by g-C 3 N 5 -based photocatalyst. Table 3. Summary of the application of g-C 3 N 5 -based materials in degrading pollution.   * indicates that C and PMS form a complex (C-PMS*)

Conclusions and Future Outlook
In summary, this review presents an overview of the synthesis, functionalization, and applications of g-C 3 N 5 -based photocatalysts. Tremendous advancements have been witnessed during the past few years. Nevertheless, there are still some challenges and opportunities to be tackled.
Firstly, most reported g-C 3 N 5 -based photocatalysts are limited to the laboratory scale. To reach an industrial scale, time-saving and environmentally friendly processes for the synthesis of g-C 3 N 5 -based materials must be investigated. The new technology for the synthesis of g-C 3 N 5 should be rapid, cost-effective, and time-efficient. Toxic templates, eco-damaging etchants, and complicated procedures should be avoided. Additionally, material recycling and sustainability are crucial elements for implementing industrial applications at scale. Combining g-C 3 N 5 with narrower bandgap semiconductors can increase the efficiency of exploiting the full spectrum of sunlight; constructing suited heterojunctions can facilitate the separation of photoinduced carriers while preserving a high redox capacity; and coupling with magnetic materials can promote quick recycling.
Photocatalytic reactions heavily depend on separated electrons and holes. In order to comprehend the photocatalytic mechanism of the detection and characterization of electrons, holes, and reaction intermediates are vital. However, it is difficult to identify the transiently changing electrons, holes, and reaction intermediates using conventional characterization techniques. Therefore, more progressive technologies, such as in situ XPS, in situ IR spectra, and time-resolved terahertz spectra, may be employed to unveil the transitory processes of reactions.
As model pollutants for assessing photocatalytic activity, MB, MO, and RhB are often used. However, their photosensitivity and adsorption affect degradation efficiency, which is not an independent contribution of the photocatalysts. To accurately determine the photocatalytic efficacy of g-C 3 N 5 -based materials, it is necessary to employ other model pollutants, including medicines, pesticides, and phenols. In addition to elucidating the efficiency and processes of degradation, the biotoxicity and other possible dangers of degradation byproducts should be thoroughly explored.
With the further advancement of experimental research and theoretical calculations, the underlying photocatalysis process will be better understood, and the solar-to-energy conversion bottleneck might be overcome. Massive commercial applications of g-C 3 N 5based photocatalysts will foster renewable revolutions in the fields of energy and environmental engineering.
Author Contributions: Conceptualization, C.Z. and J.Z.; methodology, C.Z. and J.Z.; writingoriginal draft preparation, J.L.; writing-review and editing, J.L. and S.W.; visualization, J.L. and S.W.; supervision, J.L. and S.W. All authors have read and agreed to the published version of the manuscript.