g-C3N4 Based Photocatalyst for the Efficient Photodegradation of Toxic Methyl Orange Dye: Recent Modifications and Future Perspectives

Industrial effluents containing dyes are the dominant pollutants, making the drinking water unfit. Among the dyes, methylene orange (MO) dye is mutagenic, carcinogenic and toxic to aquatic organisms. Therefore, its removal from water bodies through effective and economical approach is gaining increased attention in the last decades. Photocatalytic degradation has the ability to convert economically complex dye molecules into non-toxic and smaller species via redox reactions, by using photocatalysts. g-C3N4 is a metal-free n-type semiconductor, typical nonmetallic and non-toxici polymeric photocatalyst. It widely used in photocatalytic materials, due to its easy and simple synthesis, fascinating electronic band structure, high stability and abundant availability. As a photocatalyst, its major drawbacks are its limited efficiency in separating photo-excited electron–hole pairs, high separated charge recombination, low specific surface area, and low absorption coefficient. In this review, we report the recent modification strategies adopted for g-C3N4 for the efficient photodegradation of MO dye. The different modification approaches, such as nanocomposites and heterojunctions, as well as doping and defect introductions, are briefly discussed. The mechanism of the photodegradation of MO dye by g-C3N4 and future perspectives are discussed. This review paper will predict strategies for the fabrication of an efficient g-C3N4-based photocatalyst for the photodegradation of MO dye.


Introduction
Polluted wastewater containing industrial discharge is one of the main causes of the irreversible degradation of ecosystems [1]. Water pollution is considered a serious concern to the global community, since wastewaters from the textile, leather, food and chemical industries discharge hazardous dyes [2]. Organic dyes are the common coloring agents in textile, cosmetics, leather, paper, plastic, printing, rubber and pharmaceutical industries [3], and their use results in severe water pollution and create environmental and esthetic issues [4]. Most of these dyes are toxic, teratogenic, carcinogenic, xenobiotic, and nonbiodegradable, owing to their complex structures and large size, and their accumulation create potential threats and risks to human and aquatic life [5,6]. Their continuous discharge in wastewaters into the natural aquatic environment causes non-aesthetic pollution and eutrophication, and can generate very toxic byproducts via oxidation, hydrolysis, or other chemical reactions occurring during water treatment [7]. Dyes affect the central nervous system, liver, kidney, skin, enzymatic system, chromosomes and reproductive systems MO is extensively used in dyeing, leather, textile, pulp, paper and printing ind and in research laboratories [21,22]. MO is used widely in the textile industry an major constituent of industrial waste discharge, polluting various water bodies [ concentration of MO dye can reach up to 500 ppm in the textile effluent [24]. M mutagenic, carcinogenic and toxic to aquatic organisms [25,26]. MO is one of t extensively used hazardous anionic azo dyes, is a highly recalcitrant and refracto biotic, and causes a significant burden in the ecosystem [27]. Azo dyes show resis the natural environment, and their bio-degradation has serious hurdles [28]. MO i harmful dye, and cause serious water pollution when released into the environm presence at maximum amounts in drinking water produces anemia, abdomin headache, dizziness, mental confusion, excessive sweating and nausea [23]. Acu sure to MO dye can cause shock, vomiting, cyanosis, increased heart rate, quad tissue necrosis and jaundice in humans [29]. MO accidentally enters the body v tion, where intestinal microorganisms metabolize it into aromatic amines, wh MO is extensively used in dyeing, leather, textile, pulp, paper and printing industries, and in research laboratories [21,22]. MO is used widely in the textile industry and is the major constituent of industrial waste discharge, polluting various water bodies [23]. The concentration of MO dye can reach up to 500 ppm in the textile effluent [24]. MO dye is mutagenic, carcinogenic and toxic to aquatic organisms [25,26]. MO is one of the most extensively used hazardous anionic azo dyes, is a highly recalcitrant and refractory xenobiotic, and causes a significant burden in the ecosystem [27]. Azo dyes show resistance in the natural environment, and their bio-degradation has serious hurdles [28]. MO is a more harmful dye, and cause serious water pollution when released into the environment. Its presence at maximum amounts in drinking water produces anemia, abdominal pain, headache, dizziness, mental confusion, excessive sweating and nausea [23]. Acute exposure to MO dye can cause shock, vomiting, cyanosis, increased heart rate, quadriplegia, tissue necrosis and jaundice in humans [29]. MO accidentally enters the body via ingestion, where intestinal microorganisms metabolize it into aromatic amines, which can cause intestinal  [30]. Thus, MO-containing wastewater should be decolorized and detoxified before being discharged into the environment [31].
Methyl orange is difficult to be decomposed using conventional methods at ambient conditions, as these methods can accumulate pollutants instead of decomposing them [32]. MO dye is nonbiodegradable, due to its complex aromatic structure and xenobiotic properties [33]. Several methods are described for MO removal, such as adsorption [34][35][36], biodegradation [37], phytoremediation [38] ozonation [39], coagulation/flocculation [40] and heterogenous photodegradation [41][42][43][44]. It has complex molecular structures, which make their removal from wastewater difficult when using conventional treatment methods [45]. Heterogeneous photodegradation has advantages over the other reported conventional techniques due to its process simplicity, complete pollutant mineralization, costeffectiveness, no/fewer harmful byproduct production and its ability to be carried out at ambient temperature and pressure [46]. The heterogeneous photodegradation (PD) process usually requires a photocatalyst or semiconductor, light source, reactor system and the pollutant and oxygen [47].
Various photocatalysts are used for the PD of MO dye e.g., ZnO-rGO nanorods [48], CuO nanoparticles [29], Au/TiO 2 nanoparticles [49], Ag-Ni and Al-Ni nanoparticles [50], silver nanoparticles/amidoxime-modified polyacrylonitrile nanofibers [51], g-C 3 N 4 and B-doped g-C 3 N 4 [52], SGCN/Fe 3 O 4 /PVIs/Pd [53], etc. g-C 3 N 4 is a metal free n-type semiconductor polymer, having several exceptional properties, such as unique structural, optical, electric and physiochemical properties, making g-C 3 N 4 -based materials an emerging class of materials that can be used in various photocatalytic applications [54]. The g-C 3 N 4 can respond to the visible light (up to 460 nm), due to its suitable band gap of about 2.7 eV. Its conduction band potential (ECB) is about −1.1 V (vs. NHE), and, thus, its photogenerated electrons have a very strong reduction ability [55]. g-C 3 N 4 is a typical, nonmetallic and non-toxic polymeric photocatalyst, which has attracted tremendous attention for environmental protection because of its easy synthesis, low cost, excellent photochemical stability, favorable band positions [56][57][58], etc. The use of g-C 3 N 4 , from an economical point of view, is considered as a viable choice in the photocatalysis field, because of its intrinsic properties and favorable internal qualities, such as its electrical conductive, photonic, distinctive 2Dstacked hierarchical features, high physical and chemical durability and non-toxicity [59]. Figure 2 illustrates the triazine (C 3 N 3 ) and tri-striazine/heptazine (C 6 N 7 ) rings, which are the basic structural tectonic units of g-C 3 N 4 [60]. g-C 3 N 4 is a layered structure, having a large specific surface area, electron-rich surface, and suitable band gap energy of 2.7 eV, which promotes its photocatalytic efficiency [61]. g-C 3 N 4 composite-based materials are utilized widely in photocatalytic applications, owing to the low prices of their raw material, easy and simple synthesis, fascinating electronic band structure, high stability, as well as abundant availability [62,63]. g-C 3 N 4 was synthesized, utilizing inexpensive nitrogen-rich substances (cyanamide, melamine, dicyandiamide, urea and thiourea) as precursors, via self-condensation by deamination, through a thermal reaction [64].
Several reviews are reported in terms of the various aspects of g-C 3 N 4 -based photocatalysts, such as their design strategies, properties, photocatalytic applications [65], and artificial photosynthesis, as well as their environmental remediation [66], modification with MXenes, the use of their derivatives for various photocatalytic applications [67], recent advances for photocatalytic CO 2 reduction [68], their use in the disinfection of water and microbial control [69], quantum dot-modified g-C 3 N 4 -based photo-catalysts for different photo-catalytic applications [70], g-C 3 N 4 -metal oxide based nano-composites for photo-catalysis and sensing applications [60], the design and application of active sites in g-C 3 N 4 -based photo-catalysts [71], synthesis and structural developments in visiblelight-active g-C 3 N 4 -based photo-catalysts [72], nonmetal modulation of morphology and composition of g-C 3 N 4 -based photo-catalysts [73], etc. There are no clear reviews reported specifically on recent modifications of g-C 3 N 4 for dye photodegradation. However, a lot of work has been done on dye degradation using g-C 3 N 4 , hence, the present review is further specified to the recent modification in g-C 3 N 4 for the efficient photodegradation of MO dye. This review will help those researchers who want to study the photodegradation of MO, by applying g-C 3 N 4 . The Scopus database indicates that limited literature is available on the photodegradation of MO dye using g-C 3 N 4 . Figure 3 shows the number of articles published per year on the photodegradation of MO dye by g-C 3 N 4 .
etc. The use of g-C3N4, from an economical point of view, is considered as a viable choice in the photocatalysis field, because of its intrinsic properties and favorable internal qualities, such as its electrical conductive, photonic, distinctive 2D-stacked hierarchical features, high physical and chemical durability and non-toxicity [59]. Figure 2 illustrates the triazine (C3N3) and tri-striazine/heptazine (C6N7) rings, which are the basic structural tectonic units of g-C3N4 [60]. g-C3N4 is a layered structure, having a large specific surface area, electron-rich surface, and suitable band gap energy of 2.7 eV, which promotes its photocatalytic efficiency [61]. g-C3N4 composite-based materials are utilized widely in photocatalytic applications, owing to the low prices of their raw material, easy and simple synthesis, fascinating electronic band structure, high stability, as well as abundant availability [62,63]. g-C3N4 was synthesized, utilizing inexpensive nitrogen-rich substances (cyanamide, melamine, dicyandiamide, urea and thiourea) as precursors, via self-condensation by deamination, through a thermal reaction [64]. Several reviews are reported in terms of the various aspects of g-C3N4-based photocatalysts, such as their design strategies, properties, photocatalytic applications [65], and artificial photosynthesis, as well as their environmental remediation [66], modification with MXenes, the use of their derivatives for various photocatalytic applications [67], recent advances for photocatalytic CO2 reduction [68], their use in the disinfection of water and microbial control [69], quantum dot-modified g-C3N4-based photo-catalysts for different photo-catalytic applications [70], g-C3N4-metal oxide based nano-composites for photo-catalysis and sensing applications [60], the design and application of active sites in g-C3N4-based photo-catalysts [71], synthesis and structural developments in visible-lightactive g-C3N4-based photo-catalysts [72], nonmetal modulation of morphology and composition of g-C3N4-based photo-catalysts [73], etc. There are no clear reviews reported specifically on recent modifications of g-C3N4 for dye photodegradation. However, a lot of work has been done on dye degradation using g-C3N4, hence, the present review is further specified to the recent modification in g-C3N4 for the efficient photodegradation of MO dye. This review will help those researchers who want to study the photodegradation of MO, by applying g-C3N4. The Scopus database indicates that limited literature is available on the photodegradation of MO dye using g-C3N4. Figure 3 shows the number of articles published per year on the photodegradation of MO dye by g-C3N4. In photodegradation, major drawbacks of g-C3N4 molecules are the limited efficiency of separating photo-excited electron-hole pairs, a narrow spectral absorption range, highcharge carrier recombination, insufficient sunlight absorption, low specific surface area, as well as a low absorption coefficient [74][75][76][77]. Several modifications have been attempted for improving the photo-catalytic activity of pure g-C3N4 for the efficient photodegradation of MO dye. Some of them are summarized below.

Composites and Heterojunctions
g-C3N4 composite materials were prepared with different materials for enhancing their photodegradation efficiency. Heterojunction systems were developed for improving the photo-catalytic activity of g-C3N4, enhancing its oxidation and reduction capabilities by utilizing the negative conduction band (CB) of one component, with the more positive In photodegradation, major drawbacks of g-C 3 N 4 molecules are the limited efficiency of separating photo-excited electron-hole pairs, a narrow spectral absorption range, highcharge carrier recombination, insufficient sunlight absorption, low specific surface area, as well as a low absorption coefficient [74][75][76][77]. Several modifications have been attempted for improving the photo-catalytic activity of pure g-C 3 N 4 for the efficient photodegradation of MO dye. Some of them are summarized below.

Composites and Heterojunctions
g-C 3 N 4 composite materials were prepared with different materials for enhancing their photodegradation efficiency. Heterojunction systems were developed for improving the photo-catalytic activity of g-C 3 N 4 , enhancing its oxidation and reduction capabilities by utilizing the negative conduction band (CB) of one component, with the more positive valence band (VB) of the other component [78]. The ZnO/g-C 3 N 4 composite (ZnO/g-C 3 N 4 -20 wt%) exhibited the highest photo-catalytic activity and good recyclability when compared to a pure ZnO catalyst and g-C 3 N 4 catalyst [62]. The 5 wt% CuO/g-C 3 N 4 nanolayer composites exhibited very efficient activity and removed 99.7% of the MO dye in 4 min, owing to their nano-size and porous nature [79]. MoS 2 /TiO 2 heterostructure were decorated on g-C 3 N 4 nanosheets to obtain a g-C 3 N 4 @MoS 2 /TiO 2 nanocomposite photocatalyst, which shows a highly efficient removal of (98%) of MO dye in visible light and long-term stability when compared to neat g-C 3 N 4 and MoS 2 /TiO 2 . The improved photo-catalytic activity of the g-C 3 N 4 @MoS 2 /TiO 2 nanocomposite could be attributed to its strong response to visible light, the effective separation of photogenerated electron-hole pairs and the narrowed band gap. The catalyst caused a reduction of the C/C0 value of just 2.5%, from the first to the fifth cycle experiment [80]. Similarly, the AgBr/g-C 3 N 4 composite system represents one with enhanced photo-catalytic activity when compared to mono-component systems under visible light irradiation. In this study, the composite AgBr:g-C 3 N 4 , having a mole ratio of 2:1, displayed efficient activity, and this ratio confirmed that the recombination of e − /h + is the rate-limiting step in the coupled system. The re-used catalyst demonstrated good activities by being used in four successive runs without any significant loss in its activity [81]. A ternary ZnO/Fe 3 O 4 /g-C 3 N 4 composite (magnetic recyclable) efficiently degraded MO dye. Among these composites, the ZnO/Fe 3 O 4 /g-C 3 N 4 -50% composite was found to be very effective, and degraded MO dye more effectively than pure ZnO and pure g-C 3 N 4 . There were three reasons discussed in terms of their enhanced photodegradation efficiency. Their UV-Vis spectra confirmed the strongest visible light response intensity of this composite. The electrochemical and PL results revealed that the rate of recombination of e − /h + pair was the lowest. The electrochemical results showed that the photoelectron transfer rate was fastest in this composite than that observed in single components. The ZnO/Fe 3 O 4 /g-C 3 N 4 -50% composite exhibited a higher photocatalytic activity after five recycles [82]. Some g-C 3 N 4 composites reported for the efficient PD of MO dye are AgBr:g-C 3 N 4 [83], g-C 3 N 4 /TNTs heterojunctions [84], ternary g-C 3 N 4 /ZnO-W/M nanocomposites [85], g-C 3 N 4 -TiO 2 nanocomposite [86], CdS/g-C 3 N 4 hybrid nano-photocatalysts [87], MoO 3 -g-C 3 N 4 composites [88], g-C 3 N 4 /ZnO composites [89], CoFe 2 O 4 /g-C 3 N 4 nanocomposites [90], g-C 3 N 4 /Bi 4 O 5 I 2 [91], ternary g-C 3 N 4 /Ag/γ-FeOOH photocatalysts [92], etc. Some g-C 3 N 4 -based composites/nanocomposites used for the efficient PD of MO dye are consolidated in Table 1.  [110], and MnOx [111], were coupled to form g-C 3 N 4 composite/heterojunctions for enhanced photodegradation of MO dyes.
The photocatalytic enhancement, achieved by coupling g-C 3 N 4 with other materials, can be easily understand from the mechanism of photodegradation of MO by BiOI/AgI/g-C 3 N 4 composites. The photocatalyst exhibited a sandwich-type structure, with AgI sandwiched between g-C 3 N 4 and BiOI. The CB levels of AgI, BiOI and g-C 3 N 4 are about − 0.55 eV, 0.58 eV and − 1.12 eV (vs NHE), respectively, and the VB levels of AgI, BiOI and g-C 3 N 4 are about 2.25 eV, 2.31 eV and 1.57 eV, respectively. Irradiating under visible light, AgI, BiOI and g-C 3 N 4 become excited and generate e − and h + . The photogenerated e − present in the CB of g-C 3 N 4 will freely transfer to the CB of AgI, and then transfer into the CB of BiOI. Similarly, h + present in the VB of BiOI migrates to the VB of g-C 3 N 4 , using the pathway of AgI. Some photo-induced e − present in CB of g-C 3 N 4 directly transfers to the CB of BiOI via contacted interfaces, however, a higher barrier height between the g-C 3 N 4 and BiOI would hinder the process partially. Thus, the stepwise transfer of charge occurres, which results in the efficient transfer of e − and the enhanced separation of created charges. The e − reacts with O 2 and generates • O 2 − that degrades MO dye efficiently, while the h + can oxidize, directly, the MO dye molecules through the hole oxidation pathway. The VB edge potentials of BiOI, AgI, and g-C 3 N 4 in the BiOI/AgI/g-C 3 N 4 composites are insufficient for the formation of • OH radicals. The proposed possible MO degradation reaction mechanisms are represented schematically in the Figure 4 [115]. The photogenerated e − present in the CB of g-C3N4 will freely transfer to the CB of AgI, and then transfer into the CB of BiOI. Similarly, h + present in the VB of BiOI migrates to the VB of g-C3N4, using the pathway of AgI. Some photo-induced e − present in CB of g-C3N4 directly transfers to the CB of BiOI via contacted interfaces, however, a higher barrier height between the g-C3N4 and BiOI would hinder the process partially. Thus, the stepwise transfer of charge occurres, which results in the efficient transfer of e − and the enhanced separation of created charges. The e − reacts with O2 and generates • O2 − that degrades MO dye efficiently, while the h + can oxidize, directly, the MO dye molecules through the hole oxidation pathway. The VB edge potentials of BiOI, AgI, and g-C3N4 in the BiOI/AgI/g-C3N4 composites are insufficient for the formation of • OH radicals. The proposed possible MO degradation reaction mechanisms are represented schematically in the Figure 4 [115].

Doping of g-C3N4
As g-C3N4 demonstrated poor catalytic efficiency due to low light adsorption and high recombination of the separated charges, which can be overcome by doping, in order to increase their conductivity. The LUMO and HOMO of the g-C3N4 are controllable. Owing to their tunable bandgap, the g-C3N4 photocatalytic efficiency is affected. Therefore, g-C3N4 can be modified by doping with elements. Doping is the intentional inserting of impurities into the certain semiconductor in order to tune its bandgap [116]. Sensitized doping can improve the catalytic activity of g-C3N4 photocatalysts [117]. For example, sulfur fluoride-doped g-C3N4 (F-SCN) shows enhanced MO degradation when compared to sulfur-doped g-C3N4 (SCN) and bulk g-C3N4 (BCN), as concluded from the Figure 5. Figure  5a shows that the F-SCN fluorescence intensity is the lowest, indicating a reduction in the created charges' recombination rate to a certain extent, an increase in their numbers, and an improvement in its photocatalytic performance. The impedance spectrum (EIS) in the Figure 5b revealed that the radius of F-SCN is smaller than that of SCN and BCN, which revealed that F-SCN demonstrated the highest rate of electron transport. Similarly in the transient photocurrent response study, as represented in the Figure 5c, F-SCN displayed

Doping of g-C 3 N 4
As g-C 3 N 4 demonstrated poor catalytic efficiency due to low light adsorption and high recombination of the separated charges, which can be overcome by doping, in order to increase their conductivity. The LUMO and HOMO of the g-C 3 N 4 are controllable. Owing to their tunable bandgap, the g-C 3 N 4 photocatalytic efficiency is affected. Therefore, g-C 3 N 4 can be modified by doping with elements. Doping is the intentional inserting of impurities into the certain semiconductor in order to tune its bandgap [116]. Sensitized doping can improve the catalytic activity of g-C 3 N 4 photocatalysts [117]. For example, sulfur fluoride-doped g-C 3 N 4 (F-SCN) shows enhanced MO degradation when compared to sulfur-doped g-C 3 N 4 (SCN) and bulk g-C 3 N 4 (BCN), as concluded from the Figure 5. Figure 5a shows that the F-SCN fluorescence intensity is the lowest, indicating a reduction in the created charges' recombination rate to a certain extent, an increase in their numbers, and an improvement in its photocatalytic performance. The impedance spectrum (EIS) in the Figure 5b  an enhanced photocurrent response, which suggests that the e − h + photoexcitation in F-SCN materials requires a longer time, and their utilization time can also be extended as compared to SCN and BCN [118]. an enhanced photocurrent response, which suggests that the e − h + photoexcitation in F-SCN materials requires a longer time, and their utilization time can also be extended as compared to SCN and BCN [118]. Doping of g-C3N4 with W improved its photocatalytic efficiency and degraded 99.6% within 60 min, using visible light [119]. Introducing boron into the structure of g-C3N4 will narrow its band gap, in order to absorb more visible light [52]. Boron-doped rGO/g-C3N4 nanocomposites (B-5%rGO/g-C3N4) approximately degraded 100% of the MO dyes within 240 min [120]. The P and S-codoped g-C3N4 enhanced the light absorption capability, surface area, and the charge separation efficiency, and showed higher catalytic activity. The pure g-C3N4, P-doped g-C3N4 and S-doped g-C3N4 degraded 15.26%, 22.67% and 24.86%, while the P and S codoped g-C3N4 (PSCN-50) degraded 73.25% of MO dye in 60 min under visible light irradiation. The PSCN-50 sample had no obvious activity even after five cycling runs, indicating high stability. [121]. BiVO4/pyridine-doped g-C3N4 shows enhanced performance when compared to the individual components, and photodegraded 97% of MO molecules within 150 min under visible light irradiation. The BiVO4/pyridine-doped g-C3N4 photocatalyst did not show an obvious reduction of activity, even after five cycling runs, implying the stability of photocatalysts [122]. A g-C3N4 molecule was reconstructed as a g-C3N4 nanoring by adding natural pollen, which results in abundant heteroatom (C)doping, increase surface area and the formation of a porous hollow structure. The Cdoped g-C3N4 demonstrated 2.8 times higher MO degradation activity than that of bulk g-C3N4, and the sample H-CN200-C degraded 95.5% dye after 120 min [123].
In some studies, both doping and coupling are applied to form doped g-C3N4 heterojunctions, in order to enhance MO dye degradation. For example, g-C3N4 was doped with Doping of g-C 3 N 4 with W improved its photocatalytic efficiency and degraded 99.6% within 60 min, using visible light [119]. Introducing boron into the structure of g-C 3 N 4 will narrow its band gap, in order to absorb more visible light [52]. Boron-doped rGO/g-C 3 N 4 nanocomposites (B-5%rGO/g-C 3 N 4 ) approximately degraded 100% of the MO dyes within 240 min [120]. The P and S-codoped g-C 3 N 4 enhanced the light absorption capability, surface area, and the charge separation efficiency, and showed higher catalytic activity. The pure g-C 3 N 4 , P-doped g-C 3 N 4 and S-doped g-C 3 N 4 degraded 15.26%, 22.67% and 24.86%, while the P and S codoped g-C 3 N 4 (PSCN-50) degraded 73.25% of MO dye in 60 min under visible light irradiation. The PSCN-50 sample had no obvious activity even after five cycling runs, indicating high stability. [121]. BiVO 4 /pyridine-doped g-C 3 N 4 shows enhanced performance when compared to the individual components, and photodegraded 97% of MO molecules within 150 min under visible light irradiation. The BiVO 4 /pyridine-doped g-C 3 N 4 photocatalyst did not show an obvious reduction of activity, even after five cycling runs, implying the stability of photocatalysts [122]. A g-C 3 N 4 molecule was reconstructed as a g-C 3 N 4 nanoring by adding natural pollen, which results in abundant heteroatom (C)-doping, increase surface area and the formation of a porous hollow structure. The C-doped g-C 3 N 4 demonstrated 2.8 times higher MO degradation activity than that of bulk g-C 3 N 4 , and the sample H-CN200-C degraded 95.5% dye after 120 min [123].
In some studies, both doping and coupling are applied to form doped g-C 3 N 4 heterojunctions, in order to enhance MO dye degradation. For example, g-C 3 N 4 was doped with Cl and then coupled with Bi 2 WO 6 to obtain ClCN/Bi 2 WO 6 heterojunctions. In this study, Bi 2 WO 6 and ClCN degraded only 20% and 30%, while 10% ClCN/Bi 2 WO 6 degraded 99% MO dye in 40 min. The enhanced photocatalytic efficiency was credited to the creation of S-scheme heterojunctions, which inhibited the separated charges' recombination, but accelerated the recombination of the relatively useless holes and electrons [124]. Other such g-C 3 N 4 -based photocatalysts for MO dye degradation reported are B-doped g-C 3 N 4 /MoO 3 [125], Fe-doped ZrO 2 nanoparticle-supported g-C 3 N 4 hybrids [126], CeO 2 /P-C 3 N 4 composites [127], etc.

Crystal Phase Control and Defects Introduction
Modification of the bandgap for improved photocatalytic efficiency can also be achieved by variations in the crystal phase, as well as facet control [78]. The crystallinity of g-C 3 N 4 has been altered by incorporating Ag with g-C 3 N 4 via calcination for 8 h. The obtained Ag-decorated g-C 3 N 4 achieved the degradation of MO, and degraded 98.7% of dye in 2 h. Calcination broadened the range of visible light responses, as well as conferred a high surface area, high Ag dispersibility, and low recombination rate of photogenerated e − /h + recombination [128]. Porous graphitic carbon nitride (p-C 3 N 4 ) has been prepared via a simple pyrolyzing treatment of g-C 3 N 4 , which introduces structural defects into the g-C 3 N 4 via breaking some bonds, as shown in the Figure 6. The defects were found to be advantageous for the generation of e − /h + charges and the prevention of the recombination of these charges. The p-C 3 N 4 exhibited a narrow band gap for promoting visible light utilization as compared with g-C 3 N 4 . p-C 3 N 4 degraded 90% of the MO dye in 90 min, which was sufficiently higher than the activity of g-C 3 N 4 (only 19% after 90 min) under visible light [129]. Similarly, the results discussed in the doping section about the P and S-co-doped g-C 3 N 4 indicate that P and S co-doping causes defects in the sample structure. The structure defects could trap photo-induced electrons, which can promote the e − /h + separation, inhibiting the recombination of the photogenerated charges and prolonging their lifetime [121]. Cl and then coupled with Bi2WO6 to obtain ClCN/Bi2WO6 heterojunctions. In this study, Bi2WO6 and ClCN degraded only 20% and 30%, while 10% ClCN/Bi2WO6 degraded 99% MO dye in 40 min. The enhanced photocatalytic efficiency was credited to the creation of S-scheme heterojunctions, which inhibited the separated charges' recombination, but accelerated the recombination of the relatively useless holes and electrons [124]. Other such g-C3N4-based photocatalysts for MO dye degradation reported are B-doped g-C3N4/MoO3 [125], Fe-doped ZrO2 nanoparticle-supported g-C3N4 hybrids [126], CeO2/P-C3N4 composites [127], etc.

Crystal Phase Control and Defects Introduction
Modification of the bandgap for improved photocatalytic efficiency can also be achieved by variations in the crystal phase, as well as facet control [78]. The crystallinity of g-C3N4 has been altered by incorporating Ag with g-C3N4 via calcination for 8 h. The obtained Ag-decorated g-C3N4 achieved the degradation of MO, and degraded 98.7% of dye in 2 h. Calcination broadened the range of visible light responses, as well as conferred a high surface area, high Ag dispersibility, and low recombination rate of photogenerated e -/h + recombination [128]. Porous graphitic carbon nitride (p-C3N4) has been prepared via a simple pyrolyzing treatment of g-C3N4, which introduces structural defects into the g-C3N4 via breaking some bonds, as shown in the Figure 6. The defects were found to be advantageous for the generation of e -/h + charges and the prevention of the recombination of these charges. The p-C3N4 exhibited a narrow band gap for promoting visible light utilization as compared with g-C3N4. p-C3N4 degraded 90% of the MO dye in 90 min, which was sufficiently higher than the activity of g-C3N4 (only 19% after 90 min) under visible light [129]. Similarly, the results discussed in the doping section about the P and S-codoped g-C3N4 indicate that P and S co-doping causes defects in the sample structure. The structure defects could trap photo-induced electrons, which can promote the e − /h + separation, inhibiting the recombination of the photogenerated charges and prolonging their lifetime [121]. Similarly g-C3N4 molecules were also used for enhancing the photocatalytic activity of other materials, such as graphene aerogels [130], for the efficient photodegradation of MO dye.

Conclusion and Future Perspectives
The photodegradation of MO is the most effective method for its complete mineralization. g-C3N4 based materials are widely utilized in photo-catalytic materials owing to their low raw material prices, high physicochemical stability, easy and simple synthesis, earth-abundant nature and fascinating electronic band structure. The lower degradation efficiency of g-C3N4 can be enhanced via coupling with other materials to form composites and heterojunctions. Efficiency can also be enhanced through introducing structural de- Similarly g-C 3 N 4 molecules were also used for enhancing the photocatalytic activity of other materials, such as graphene aerogels [130], for the efficient photodegradation of MO dye.

Conclusions and Future Perspectives
The photodegradation of MO is the most effective method for its complete mineralization. g-C 3 N 4 based materials are widely utilized in photo-catalytic materials owing to their low raw material prices, high physicochemical stability, easy and simple synthesis, earth-abundant nature and fascinating electronic band structure. The lower degradation efficiency of g-C 3 N 4 can be enhanced via coupling with other materials to form composites and heterojunctions. Efficiency can also be enhanced through introducing structural defects, crystal phase control and doping with metals and non-metals. Preparation of composites and heterojunctions is the most followed approach, because the resulted materials exhibit improved photocatalytic efficiency, owing to their synergistic effects. Similarly, the structural defects were found to be beneficial for the e − /h + generation and inhibition of the charge recombination.
There are a few dimensions that still need thorough investigations. Some of them are the following.
The majority of the g-C 3 N 4 based photocatalysts are not economical and their preparation methods are quite complex. Therefore, exploring the unprecedented g-C 3 N 4-based photocatalysts, which are economical, easy to prepare and have superior photocatalytic characteristics to cope with the industrial needs, is very important.
Each individual process of photodegradation of MO dye through g-C 3 N 4 based photocatalysts needs to be visualized specifically through in situ characterization techniques. Through in situ characterizations, one can fabricate highly robust, cost-effective and perfectly photon energy-matched catalysts with commercial feasibility and effective photostability.
Monitoring of the photodegradation of dyes in aqueous solutions by recording the changes in UV-Vis absorption may not allow the accurate detection of the full degradation of organic pollutants. Therefore, it is suggested to use other monitoring techniques.
Mechanistic understanding of the PD of MO by g-C 3 N 4 -based photocatalysts has been examined, but researchers need to further investigate this with theoretical and computational and support. Through computational approaches, researchers are able to design g-C 3 N 4 -based photocatalysts for the efficient adsorption and photodegradation of MO dyes. Through this approach, researchers can also choose effective doping and coupling materials for g-C 3 N 4 , for the efficient adsorption and photodegradation of MO dyes. DFT calculations can suggest dye degradation mechanisms and are helpful for supporting the obtained experimental results.