Functionalized Graphitic Carbon Nitrides for Environmental and Sensing Applications

g-C

colloidal, seemingly sheet-like, but fully reversible dissolution. [5]he surface area of exfoliated g-C 3 N 4 nanosheets (CNNSs) is much larger than the bulk C 3 N 4 , which offers numerous advantages like better mass transfer during the reaction, a higher number of active sites, and improved material performance.The morphology and chemical composition of g-C 3 N 4 can be modified by controlling the thermal treatment time. [6]The elemental build-up, in-plane holes, and small pore size of g-C 3 N 4 predominantly improve its optical properties and enhance its photocatalytic activity. [7]Electron delocalization occurs throughout π-network of g-C 3 N 4 and leads to the merger of electrons and holes, which will be consumed in reduction and oxidation reactions, respectively.As a result, the photocatalytic activity experiences a boost.As the typical absorption edge for g-C 3 N 4 is around 450 nm, only limited solar energy can be utilized during photocatalytic reactions driven by g-C 3 N 4 .Better optical properties can be obtained by fine-tuning the electronic structure of g-C 3 N 4 through functionalization of ─NH─, ¼N─, ─NH 2 groups existing on the surface.The ion-doped lattice functionalization of g-C 3 N 4 with metal ions is found to bring a significant improvement in the properties, [8] which is extremely beneficial for applications like hydrogen production.
Structure and surface functionalization fundamentally enhance the electronic properties, effective charge transfer, optical absorption, and photocatalytic behaviors.Elemental doping with metals and nonmetals, combining with particles, makes C 3 N 4 an efficient candidate, especially for environmental applications such as water splitting and CO 2 reduction.The copolymerization and compositing with metal oxides are applied to the C 3 N 4. to provide longer cycle life and electrochemical efficiency for sensing and energy storage, Different surface functionalization techniques are used to alter the surface area, enhance charge separation, and increase the photocatalytic efficiency of C 3 N 4 .A significant amount of research work has been dedicated to the functionalization of g-C 3 N 4 with a range of functional materials, like carbon nanotubes (CNTs), [9] graphene, [10] and metal atoms, [11] There are many review articles on this emerging polymer's application, mainly in the energy area. [12]Complementary to these, the critical review focuses on materials domain, including structural modification of C 3 N 4 by covalent functionalization methods (e.g., reductions, oxidations, cycloadditions, crosscoupling reactions, and copolymerizations) and noncovalent ones (e.g., the interaction of organic molecules and nanoparticles with g-C 3 N 4 surface).In addition, elemental doping to the g-C 3 N 4 was also described.Processing bulk and functionalized g-C 3 N 4 s to prepare thin films, hierarchical assemblies, and hybrid structures with carbonaceous materials (e.g., nanotube/g-C 3 N 4 , graphene/g-C 3 N 4 ) has been detailed to give a postsynthesis perspective.Finally, typical examples of environmental and sensing applications of the functionalized g-C 3 N 4 s have been discussed to emphasize the effect of functionalization.

Synthesis of g-C 3 N 4
The most common method to synthesize g-C 3 N 4 involves heating precursors with high nitrogen content, such as melamine, [13] cyanamide, [14] dicyandiamide (DCDA), [15] urea, [16] and thiourea [17] at elevated temperatures (usually >500 C).The key synthetic parameters that can fine-tune the optical and electronic properties of g-C 3 N 4 are: heating rate, precursor type, thermal treatment temperature, time, and atmosphere.Moreover, powdering the sample between heat treatment and washing procedure helps remove impurities.It should also be noted that the extraordinary chemical resistance of g-C 3 N 4 makes the washing procedure easier.It is essential to select the temperature for synthesis carefully as it can directly affect the porosity, surface area, Figure 1.The structure of g-C 3 N 4 and its functional moieties.Adapted with permission. [3]Copyright 2014, the Royal Society of Chemistry.
composition, degree of crystallinity and polymerization, photoluminescence, and the number of active groups in the sample. [18]rea is one of the most commonly used precursors due to its chemical composition (contains C, N, O, and H only), low price, and scalability.For instance, g-C 3 N 4 powders were prepared by Yao et al. via direct pyrolysis of urea. [19]A known amount of precursor was kept in four covered corundum boats and heated at 520, 550, 580, and 610 C for 4 h, respectively, to understand the effect of the temperature.A similar route was followed by Wang et al. for the preparation of highly porous g-C 3 N 4 with melamine. [20]They heated 5 g of melamine in a muffle furnace at 550 C for 4 h under air at a rate of 10 C min À1 and collected the yellow-colored product after cooling.They also carried out thermal copolymerization by mixing 2 g of melamine with different amounts of nucleobases, i.e., uracil, adenine, cytosine, guanine, and thymine.Zhao et al. used a fractional thermal polymerization method and produced g-C 3 N 4 from melamine, DCDA, and guanidine carbonate. [21]ther synthetic methods discussed in the literature include one-step electrochemical synthesis [2] and synthesis of porous g-C 3 N 4 via direct polymerization. [22]The former proposed a single-step electrochemical synthesis method to produce ultrathin g-C 3 N 4 sheets.It involved the electrolysis of melamine and NaOH mixture solution for 40 min.This method was more straightforward and faster than the multistep thermal treatment method as the reaction time was highly reduced.The CNNSs produced through this method were successfully used for H 2 O 2 and uric acid detection.Direct polymerization route reported a convenient route to obtain g-C 3 N 4 from urea with self-supported gas.The generated gas played an essential role in the formation of pores during condensation.The products of this synthetic method were also compared with those obtained using urea-polymerized thiourea and DCDA as a source.The pyrolysis with urea produced less condensed and more porous g-C 3 N 4 with better hydrogen evolution capability.As the catalytic efficiency of g-C 3 N 4 depends on its porosity and surface area, the developed products were found as efficient catalysts.
While being widely used, conventional synthesis methods produce dense bulk g-C 3 N 4 , which has a low surface area.To increase the accessible surface area, g-C 3 N 4 can be exfoliated by breaking the weak interactions between the layers of polymerized tri-s-triazine structures.Bulk g-C 3 N 4 can be exfoliated into 2D nanosheets (NSs) thermally, [23] ultrasonically, [24] and chemically. [25]For instance, Hatamie et al. produced exfoliated NSs by dissolving 0.05 g of bulk g-C 3 N 4 (prepared via pyrolysis of melamine at 600 C for 2 h) in 50 mL of water and carrying out ultrasonication for 10 h to obtain a clear solution and, the obtained suspension was then subjected to centrifugation at 5000 rpm to get rid of unexfoliated material. [26]In addition, some studies [27] reported in situ fabrication of g-C 3 N 4 -based composites by premixing the starting materials and then heating them in a furnace at around 550 C.
Moreover, to counter the limitation of high density and introduce porosity into g-C 3 N 4 materials, soft-templating and hardtemplating were used. [28]Soft templating [29] involves selfassembling amphiphilic molecules that facilitate the formation of intricate structures by combining with precursors.In softtemplating, solvent plays a vital role in defining key characteristics of the fabricated materials, such as size and shape.On the other hand, hard templating, also known as nanocasting method, involves deposition of precursor on a hard template. [30]The precursor is treated to flow into the microchannels of the template and the template is removed in the end to reveal the final porous product.Hard template methods are easier to control than the soft template ones. [28]Recently, a template-free method has also been proposed by Zhou et al. for the large-scale synthesis of 1D g-C 3 N 4 microtubes. [31]This simple method involved hydrothermal treatment of melamine precursor, which yielded white melamine-cyanuric acid microrods.The obtained microrods were pyrolyzed at 550 C for 4 h under N 2 flow.This method was found to produce g-C 3 N 4 microtubes with superior photocatalytic H 2 production.24b]

Characterization Methods
The performance of the g-C 3 N 4 in various applications relies on its intrinsic properties.Hence, it is extremely crucial to enlighten the structure and electronic properties of the g-C3N4 fully.Various spectroscopic and microscopic techniques are available for the characterization of g-C 3 N 4 .Ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) is a useful tool to evaluate the light absorption and estimate the bandgap energy of the g-C 3 N 4 through the absorption edge.Raman spectroscopy is an informative technique to evaluate the primary peaks of g-C 3 N 4 corresponding with the dominant C─N stretching vibrations.X-ray photoelectron spectroscopy (XPS) facilitates the examination of the chemical states at the surface of g-C 3 N 4 .X-ray diffraction (XRD) is an acknowledged technique to investigate the structure and the distance between the layers of the produced g-C 3 N 4 .For accurate surface area measurement, Brunauer-Emmett-Teller (BET) surface area analysis is a favorable technique.Scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) is preferred to study the surface topography and composition of g-C 3 N 4 .

Ultraviolet-Visible Diffuse Reflectance Spectroscopy
Bandgap, being a predictor of the wavelength of light that a material can absorb, is a critically important property for materials in energy and environment-related applications.A decrease in bandgap energy is often associated with the red shift in the optical response of a material.UV-vis DRS is widely used to detect the bandgap of g-C 3 N 4 .The graph obtained from the UV-vis DRS, known as the absorption spectrum, indicates the range of wavelengths of light absorbed by a semiconductor.The spectral data, including the cut off wavelength (with the minimum absorbance value), is used to work out the bandgap energy.Moreover, the absorption capability of g-C 3 N 4 is affected by the composition, defects, morphology, structure, and thickness (in the case of films) of g-C 3 N 4 .g-C 3 N 4 film with a greater thickness (126 nm) absorbs light with longer wavelength and has a narrower bandgap than the g-C 3 N 4 with a lower thickness (55 nm) (Figure 2a). [33]oreover, N-defects formed at higher temperatures during calcination, lead to an extension in the UVÀvis absorption

Thermal condensation method
Bulk g-C 3 N 4 Urea, melamine, thiourea, cyanamide, and DCDA A weighed amount of precursor was kept in a covered crucible (preferably alumina) and subjected to furnace heating at a temperature within 500-600 C for up to 4 h.The recovered yellow residue was cooled to room temperature and ground into powder.The temperatures for different precursors are urea (550 C), melamine (500-580 C), thiourea (450-650 C), cyanamide (550 C), and DCDA (550 C).
[ 18-20.26.32a]Fractional thermal polymerization method g-C 3 N 4 powder Melamine, guanidine carbonate, and DCDA A covered crucible containing the precursors was heated from 20 to 360 C at a rate of 5 C min À1 .The temperature was kept at 360 C for 30 min and then increased again to a predetermined value within 30 min and kept there for 3 h.This was followed by cooling. [21]

Pyrolysis-assisted chemical vapor deposition method
Free-standing g-C 3 N 4 films

DCDA
The precursor was contained in a quartz tube with its inlet attached to Ar flow and outlet attached to vacuum to maintain a low pressure of about 3 Torr in the tube.This arrangement caused the precursor vapors to enter the furnace (at 600 C).Upon thermal condensation, the vapors coated the walls of the tube.The tube was then cooled to room temperature, the deposition was scraped off and a uniform freestanding film was obtained. [32b] Soft-template method with thermal polymerization Exfoliated g-C 3 N 4 Urea About 10 g of urea was dissolved in ethanol and water taken in 1:1 ratio, heated up to 110 C with constant stirring.The solvent evaporated and a white precipitate was obtained.It was then transferred into a crucible and covered with aluminum foil.The crucible was heated at 550 C for 3 h.The obtained bulk material was then dissolved in ethanol/water again and sonicated for 8 h.After centrifugation, the product was oven dried. [24b] Template-free solvothermal approach with postcalcination Hierarchical nanoporous g-C 3 N 4 microspheres

Cyanuric chloride and melamine
The precursors were mixed in a fixed ratio and dissolved in ACN, which served as a solvent.This mixture was stirred for hours at room temperature and then placed into a Teflon-lined autoclave with a 100 mL capacity.The system was sealed and heated at a particular temperature (120-200 C) for 12-24 h.Upon cooling, the products were centrifuged, washed with ionized water and absolute ethanol, and dried.The postcalcination treatment involved heating the sample at 550 C in air for 4 h or Ar for 2 h.

[32c-e]
Template-free hydrothermalassisted thermal polymerization method 1D g-C 3 N 4 microtubes Melamine Melamine precursor (0.5 g) was mixed into pure water (30 mL).The mixture was heated at 90 C for 10 min with continuous stirring until a clear solution was obtained.The as-obtained melamine clear solution 180 C for a day in a 50 mL Teflon stainless autoclave.The white residue was dried and calcined at 550 C for 4 h under nitrogen.The yellowish product was collected after cooling. [31] Solvent-free solid reaction g-C 3 N 4 powder Cyanuric chloride and melamine Weighed quantities of precursors (20.0 mmol cyanuric chloride and 10.0 mmol melamine) were mixed and placed into a 50 mL stainless steel autoclave.About 1 g Nickel powder was added to the precursors and the autoclave was sealed and heated up to 400 C for 6 h.After cooling to ambient temperature, the product was washed and dried. [32f]

Chemical vapor deposition method
Ordered cubic mesoporous g-C 3 N 4

Melamine
About 6 g of precursor was kept inside a 50 mL alumina crucible and KIT-6 template (2 g) was uniformly dispersed onto the precursor layer.
The closed crucible was heated in a muffle furnace at 320 C for 4 h.Upon evaporation, the precursor will be deposited into the template's pores.Next, the temperature was elevated and maintained at 550 C for 3 h.The silica template was removed by HF acid wash and ordered cubic mesoporous g-C 3 N 4 were obtained. [32g] Microwave-assisted polymerization g-C 3 N 4 sub-microspheres Cyanuric chloride and sodium azide The precursors were weighed (cyanuric chloride: 1.84 g and sodium azide powder: 1.96 g) and manually mixed and ground using agate mortar.The mixture was added to 40 mL of ACN solvent and stirred for 15 min.The prepared suspension was filled into a TFM vessel and placed into the cavity of a microwave system with was heated at a power of 350 W, up to 180 C and held there for 30 min.The irradiated mixture was cooled and passed from a microfiltration membrane (pore size: 0.1 μm).The precipitate was collected and sequentially washed with ethanol, acetone, and distilled water.The sample was oven-dried at 80 C overnight under vacuum.
[32h] spectra of g-C 3 N 4 . [34]The absorption capability of the defective g-C 3 N 4 with different N-defect concentrations, which are produced using various amounts of alkali precursor during the polymerization reaction can be increased by maintaining the bandgap energy within the range of 2.3-2.7 eV (Figure 2b).The spectra show that the g-C 3 N 4 produced by this synthetic method has strong band-to-band absorption.Although, based UV-vis DRS characterizations, it is deduced that the advanced one-step in situ calcination suggests a change in the structure of g-C 3 N 4 , other advanced techniques such as XPS are necessary to reveal the presence of N-defects in the bulk as well as on the surface. [35]

Raman Spectroscopy
Raman spectroscopy is a useful tool to detect vibrational, rotational, and other states in a molecular system.This technique is also applied to study the chemical structures of g-C 3 N 4 .
Praus and coworkers discussed the Raman spectra of the g-C 3 N 4 produced via the thermal treatment of melamine at a range of temperatures.The spectra observed within 1250-400 cm À1 correspond to the skeletal vibrations of nitrogen aromatic rings.The triazine skeletal vibration is visible at 980 cm À1, and it is common for all samples.The typical melem bands can be seen at 1153 cm À1 and 548 cm À1 .The highest peak was observed in samples polymerized at 550 C and 600 C, which signifies the complete condensation of melem given to g-C 3 N 4 and its resulting decomposition at 700 C (Figure 3). [6]

X-Ray Diffraction
XRD is a powerful tool to study the phase structure of crystalline solids.This technique enables the identification of the crystalline

One-step electrochemical method
Ultrathin CNNSs Melamine An electrolyte solution of 3 g melamine and 1 g NaOH was prepared in 50 mL of water.Two 4 Â 4 cm 2 platinum sheets were taken as electrodes and kept 0.5 cm apart.Using a DC power supply, 5 V potential was applied and the electrolysis was carried out with constant stirring for 40 min.As the mixture turned yellow, centrifugation was conducted at 10 000 rpm for 15 min.Finally, it was dialyzed for 3 days to obtain the final product. [2] One-step pyrolysis method BC/g-C 3 N 4 composites Urea and Hickory chips Urea and biomass were mixed in different ratios (25:1, 50:1, and 100:1) and placed into alumina crucibles, which were covered and heated in a muffle furnace at a rate of 5 C min À1 under air until it reached 520 C. The temperature was maintained for 2 h and then cooled to room temperature.The products were washed with distilled water and ethanol and dried at 80 C.About 2 g of HCl-treated melamine was mixed with 1 g of pre-prepared Cu 3 [Fe(CN) 6 ] 2 powder.The mixture was dissolved in 50 mL absolute ethanol with continuous stirring for a couple of hours.The solution was heated up to 60 C to remove ethanol and then placed in a crucible.This was followed by pyrolysis under nitrogen in a tube furnace at a 550 C for 4 h.Upon cooling, the product was washed and dried. [27b] Figure 2. UV-vis spectra of g-C 3 N 4 with different a) thickness and b) N-defect composition.a) Reproduced with permission. [33]Copyright 2016, Wiley-VCH GmbH.b) Adapted with permission. [35]Copyright 2017, Wiley-VCH GmbH.
phase and interlayer stacking planes in g-C 3 N 4 .The governing equations given below, namely Bragg's Law (Equation 1), and Scherrer equation (Equation 2) are utilized to calculate interplanar distance (d) and crystallites size (L), respectively. [36]n where θ stands for the peak position (radian), n is a positive integer, λ signifies the wavelength of the source X-ray (nm), and d is the interplanar distance (nm).L represents the crystallites size (nm), K is Scherrer constant, and β is the full width of half maximum (FWHM).For the g-C 3 N 4 , the weak diffraction peak at %13.1 signifies (1 0 0) lattice planes of triazine units, whereas the sharp diffraction peak at %27.6 is caused by the (0 0 2) lattice planes of interlayer stacking of aromatic segments.However, the shape, intensity, and position of these peaks can vary depending on the synthetic procedures and the surface functionalization.For example, the g-C 3 N 4 samples produced using oxygen-containing urea showed low crystallinity compared with the ones produced by DCDA and melamine under air and nitrogen atmosphere (Figure 4a). [37]In addition, changing the gaseous environment (air or nitrogen) had no significant effects on the XRD results.XRD was also used to investigate the defects in the structure of g-C 3 N 4 .In general, strong oxidizers can create defects in the g-C 3 N 4 structure.A particular example was the use of H 2 O 2 (a mild oxidizer) to generate hydroxyl groups on the surface. [38]In general, strong oxidizers can create defects in the g-C 3 N 4 structure.A particular example was the use of H 2 O 2 (a mild oxidizer) to generate hydroxyl groups on the surface. [38]As a result of hydroxylation, the characteristic (0 0 2) peak at %27.2 shifted to higher 2θ values and the intensity of the peak increased.The observed shift and peak enhancement were dependent on the concentration of H 2 O 2 (0, 10, 20 and 30 wt%) (Figure 4b).

Other Characterization Techniques
In addition to the aforementioned characterization techniques, XPS is usually used to characterize the surface chemistry, composition, and bonding states of both the g-C 3 N 4 and its composites. [39]The results obtained through this spectroscopic analysis shed light on the elemental composition of a compound as well as the chemical environment of each component.However, when possible bonding configurations of N and C combine with the unavoidable interaction with the atmospheric gases (e.g., oxygen and hydrogen), interpretation of the acquired spectra become challenging. [40]EM is another prominent technique that utilizes a focused electron beam to scan the surface of a sample and produce an image that provides invaluable information about the microstructure, surface properties, topography, and composition of the sample.In most cases, the surface of g-C 3 N 4 is decorated with other materials (e.g., metal/metal oxide particles) and the information regarding the distribution, uniformity, and composition of these components can be acquired when SEM is coupled with EDX. [41]BET surface area analysis is useful to measure the specific surface area of material by investigation of the physical adsorption of gas molecules on the g-C 3 N 4 surface.It also provides the information of surface porosity which is highly related to the precursor type and synthesis conditions. [42]Although the molecular formula of as-synthesized carbon nitride is represented by C 3 N 4 (as a ratio of C to N), the presence of uncondensed terminal groups bearing O and/or H moieties should not be disregarded.However, due to the limitation of the characterization techniques available, it is difficult to get the real bulk structure of the so-called carbon nitrides.On the other hand, creating the characterization roadmap and progressing by considering the obtained results will be quite productive.Also, to ensure the consistency of the results would improve their accuracy.The effect of thermal treatment on the structure can be observed using the Raman spectra, XRD patterns, SEM and transmission electron microscopy (TEM) images of g-C 3 N 4 .
The in situ characterization methods are essential to analyze the growth mechanism of the g-C 3 N 4 as it is difficult to control the final product.Various in situ characterization techniques, such as in situ heating TEM [43] and thermogravimetric analysis coupled with Fourier-transform infrared spectroscopy (FTIR) [44] , can be used to investigate the calcination of supramolecular precursors.Also, structural characterization as well as direct imaging after calcination can be carried out via in situ heating SEM measurement. [45]In addition, developing in-depth characterization approaches for highly active single-atom catalysts to understand the dynamic evolution during catalytic reactions at the atomic level is so important. [46]In a recent study by Zhang et al., [47] charge transfer, changes in oxidation states, and dynamic evolution of chemical bond was observed via in situ characterization in single-atom Pt-C 3 N 4 catalysts used for the photocatalytic splitting of water.By utilizing synchronous illumination X-ray photoelectron spectroscopy (SI-XPS), it was reported that the cleavage of Pt─N bond led to the formation of a Pt 0 species and the subsequent rearrangement of C─N into C¼N, when excited under light.Moreover, SEM, high-resolution transmission electron microscopy, and elemental mapping of S-Pt-C 3 N 4 revealed the structure and distribution of Pt on C 3 N 4 nanorods.Techniques such as high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure spectroscopy was also applied for in-depth examination of structural and coordination properties.In situ X-ray absorption spectroscopy is a handy technique used to characterize the vacant states and structural properties of excited atoms as well as to examine the transformation in catalysts.It can also be coupled with other tools to investigate local structural behavior in C 3 N 4 -based nanomaterials. [48]o analyze the conductivity of C 3 N 4 -based materials, two contact methods can be used.Two contact method has been applied to perform conductivity comparison of C 3 N 4 -based materials synthesized at various temperatures [49] and having different shapes. [50]The electronic conductivity is calculated using Ohm's Law.Furthermore, electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy has also emerged as a highly sophisticated and nondestructive tool for the detection of free radicals, metallic ions, defects, photoelectrons, and conjugated structure.This characterization technique has been readily used to confirm the presence of vacancies in the structure [51] and to inspect the electronic band structure of g-C 3 N 4 . [52]Shen et al. [53] showed the use of ESR to confirm the enrichment of g-C 3 N 4 with carbon vacancies.A strong ESR signal corresponds to a greater number of unpaired electrons, which can be attributed to the C defects in g-C 3 N 4 , which led to a reduced exciton-effect and enhanced production of charge carriers.Moreover, in a recent study by Xia et al., [54] EPR technique was applied to analyze the hybrid structure and π-conjugated structure of tri-s-triazine unit of g-C 3 N 4 under illuminated and dark conditions.As photocatalytic activity of g-C 3 N 4 is related to the formation of π-conjugated structure, EPR provided invaluable information.Under light conditions, electron transfer takes place from σ-type bonds to the π-conjugated structure, hence photoelectrons are produced.Moreover, using this technique, the hydroxyl radicals (•OH) and superoxide radicals (•O 2 À) were observed during the photocatalysis of g-C 3 N 4 .Using the proposed EPR-based method, the concentration of photoelectrons in g-C 3 N 4 under light can be investigated, which would in turn aid in understanding the photocatalytic performance of g-C 3 N 4 in hydrogen production.Zhao et al. [55] utilized a system which coupled highly rapid spectroscopy with in situ ESR to study the dynamic changes in charge and energy transfer in 2D CNNSs.This technique helped in direct detection of photoelectrons and holes in the produced supramolecular complexes.Copyright 2017, Elsevier.b) Adapted with permission. [38]Copyright 2016, Elsevier.

Surface Functionalization of g-C 3 N 4
Surface functionalization of graphitic materials is a versatile method to create active anchoring sites and to tune the chemical and electronic properties of the material.So far, many strategies have been used to prepare carbon-based advanced functional materials successfully. [56]These strategies can be divided into two main types, namely, covalent, and noncovalent.The covalent strategies can cause permanent modifications and require wellcontrolled chemistries.They usually create defects by damaging the sp 2 framework of the material in subject, which has considerable effect on the electrical properties.However, these defects can be highly useful in many applications (e.g., catalysis, composites), if the defect density is tuned carefully.On the other hand, noncovalent strategies have simpler application processes but no adverse effect on the original structure.In this section, the adaptation of covalent and noncovalent strategies for the surface functionalization of the g-C 3 N 4 are discussed extensively.

Reductions
The framework of g-C 3 N 4 is mainly composed of sp 2 -hybridized carbon and nitrogen atoms, which modulates electronic properties as well as the surface chemistry.56b] Among the known chemistries, the framework charging appears as one of the best reduction approach due to its tunability and it was shown to be effective on CNTs [57] and graphene. [58]This approach uses a one-pot reaction through the use of sodium naphthalide (NaNp) in suitable solvents (e.g., N,N-dimethylacetamide [DMAc], THF) allowing direct synthesis of negatively charged carbon nanomaterials.Recently, the framework charging method has been shown to be effective for the exfoliation and functionalization of polytriazine imide (PTI). [59]In this study, highly reactive NaNp dissolved in DMAc reacted with the g-C 3 N 4, and desired exfoliation of negatively charged PTI was accomplished by the framework charging (Figure 5a).The generated negative charges were further utilized to add surface functionalities such as long alkyl chain on the PTI.In principle, this method offers a tunability over the degree of negative charges as well as the functional groups created on the g-C 3 N 4 surface, and thus it may be used for the fine-tuning of chemical and electronic properties of the g-C 3 N 4 before target applications.The study found the optimum PTI/Na ratio for the best exfoliation of PTI as 7:1, although the fine-tuning of the charge density was also shown (Figure 5b).The color change of the PTI dissolution was evident for the alteration of the electronic properties.Moreover, exceptional stability and a 35 wt% yield was reported for 2D few-layered PTI (FL-PTI) solutions when the PTI/Na ratio of 7:1 was used (Figure 5c).
The simultaneous exfoliation and modification of g-C 3 N 4 via reductive alkylation was also demonstrated in anhydrous THF.In contrast to the aforementioned method, g-C 3 N 4 was reduced by lithium in the presence of naphthalene. [60]The dodecyl functionalized g-C 3 N 4 had a reduced bandgap which enhanced the Adapted with permission. [59]Copyright 2018, Wiley-VCH GmbH.
photoresponse and the visible-light harvesting ability, suggestive of more charge carriers generated under visible-light irradiation.

Oxidations
Oxidation can be effectively used to prepare highly stable suspensions of carbon nanomaterials under highly oxidizing conditions.This results in the insertion oxygen functional groups on the g-C 3 N 4 surface.Even though the g-C 3 N 4 exhibits considerable resistance against acids and bases (e.g., HCl, NaOH), strong oxidants (e.g., H 2 SO 4 , KMnO 4 , and H 2 O 2 ) can provide the chemical oxidation of g-C 3 N 4 to form highly dispersible, surface-functionalized C 3 N 4 -based materials.Oxidation of the carbon nitride using H 2 SO 4 /KMnO 4 was shown to produce polar functional groups, such as amine, carboxyl, and ketone groups, on the surface.Functionalized material, which was used in a cell-imaging study by confocal fluorescence microscopy, showed enhanced dispersibility in water without surfactants or stabilizers. [61]The pristine g-C 3 N 4 was thermally oxidized by heating in pure oxygen atmosphere at varying temperatures and heating periods. [62]As a result, the specific surface area increased, the light absorption range became broader, and separation of photogenerated charge carriers accelerated.Also, this modification boosted the photocatalytic hydrogen production by 4.3 times compared with the pristine one.In another study, oxidation of the g-C 3 N 4 using H 2 O 2 was found to be effective to create oxygen-containing functional groups on the surface and the functionalized material showed enhanced activity for rhodamine B degradation. [38]In addition to the use of strong oxidizers to create oxygen-containing moieties on the g-C 3 N 4 surface, a bottom-up approach which uses a novel nitrogen-rich precursor bearing hydrochloride groups, so-called semicarbazide hydrochloride (ONLH), was shown to increase the amount of oxidized content in the structure of g-C 3 N 4 . [63]The oxygen concentration was controlled thermally by carrying out the synthesis at different temperatures.

Cycloadditions
Cycloadditions are one of the most versatile routes to the covalent chemical modification of sp 2 -hybridized carbon nanomaterials such as CNTs [64] and graphene. [65]Similar to the CNT chemistry, the surface of g-C 3 N 4 polymer was modified using the 1,3-dipolar cycloaddition of in situ generated azomethine yields prepared by thermal condensation of the α-amino acid N-methylglycine and an aldehyde (Figure 6). [66]The surface-modified g-C 3 N 4 polymers with diverse organic groups, including aryl, aryl acid, pyridine, ionic liquids, and ferrocene showed varying electronic properties compared with the pristine g-C 3 N 4 and were used as catalyst for the selective oxidation of 3,5,5-trimethylcyclohex-3-en-1-one using atmospheric oxygen.
A ball-milling assisted solid-state mechanochemical method, which exfoliated the g-C 3 N 4 and generated the reactive radicals and ions on the surface of g-C 3 N 4 , was used to covalently graft fullerene (C 60 ) on the edge planes of the g-C 3 N 4 in the presence of lithium hydroxide (LiOH). [67]Spectroscopic characterizations (e.g., Raman and XPS) revealed the covalent bonding of C 60 onto the edges of CNNSs via a four-membered ring of azetidine.

Cross-Coupling Reactions
Covalent functionalization of carbon nitride frameworks through cross-coupling reactions were studied using a carbon nitride framework with a halogen (Cl, Br, and I)-substituted phenyl (Figure 7). [68]In a cross-coupling reaction, the halogenated phenyl-group-modified carbon nitrides were subjected to a reaction with a phenylboronic acid and a tert-butyl acrylate through Suzuki and reductive-Heck cross-coupling reactions, respectively.The final covalently modified carbon nitrides displayed enhanced dispersibility in many solvents, such as dimethyl sulfoxide, THF, n-hexane, and diethyl ether, suggestive of surface group addition.The potential photocatalytic activity of the prepared catalysts was tested by the efficient photo-oxidation of benzyl alcohol as a model reaction in an organic solvent and they showed enhanced photocatalytic activity compared with the unmodified carbon nitrides.
g-C 3 N 4 with polymers leads to more advanced functionalities.In addition, the photoinitiation, photocatalysis, or photoluminescence properties of g-C 3 N 4 have been effectively developed and optimized in polymer hybrids, due to enhanced delamination and synergy between the materials.Polymer-modified g-C 3 N 4 and their applications are shown in Table 2. [69] Fan et al. reported usage of terephthalaldehyde as an inexpensive aromatic comonomer to create aromatics-grafted g-C 3 N 4 -based copolymer with better electrical conductivity and charge separation performance for hydrogen production. [70]With this strategy, most of the aromatics can be grafted into the network of C 3 N 4 .Adding an optimized quantity of polymer to the precursor of C 3 N 4 during the synthetic process is one of the most cost-effective copolymerization methods.In biosensor applications, better sensitivity and selectivity are offered by polymerfunctionalized g-C 3 N 4 due to dispersion enhancement and the sensing site's access.The polymeric matrix doped with g-C 3 N 4 not only improves the efficiency of the electrode materials used for energy storage application but also makes them more stable in comparison with the pure polymer matrix.Also, g-C 3 N 4 can serve as an effective photoinitiator during the synthesis of polymer particles.Better dispersibility and processability of polymermodified g-C 3 N 4 can also be useful for thin-film formation. [71]upramolecular preorganization of g-C 3 N 4 is one of the important methods to functionalize and tune g-C 3 N 4 .Significant parameters that influence the structure of the product are: sequence of the monomer, type of the solvent, and reaction times. [72]Zhou et al. [73] produced supramolecular C 3 N 4 nanocomposite using melamine-cyanuric acid and barbituric acid as monomers.The surface area so obtained was superior to that of pure g-C 3 N 4 .Moreover, to regulate heteroatom doping and tune structure of g-C 3 N 4 , a supramolecular precursor selfassembly method was developed by Wang et al. [74] In addition, the density functional theory (DFT) simulations in this study, provides a unique model for g-C 3 N 4 functionalization.In another study, phosphorus-doped hexagonal tubular g-C 3 N 4 was obtained by applying the phosphorous acid-assisted hydrothermal method and thermal treatment, respectively. [75]Consequently, a large surface area was achieved, which enabled the number of reactive sites to increase.

Miscellaneous Covalent Surface Functionalizations of g-C 3 N 4
On the structure of the g-C 3 N 4 , there are free NH 2 groups and these primary amine groups are highly reactive to many reagents including anhydrides, acyl halides, aldehydes, and carboxylic acids.The electronic structure of carbon nitride was modified by introducing imide functional groups on its framework through amidation reaction between free NH 2 groups and pyromellitic dianhydride. [76]The resultant new polymeric photocatalyst responded to visible-light and showed enhanced photocatalytic methyl orange degradation, attributed to the separation of the photoinduced electrons and holes.In another example, CNTs were covalently bound to the g-C 3 N 4 surface. [77]In this covalent addition, CNTs were initially oxidized to produce carboxylic acid groups on the surface and these groups were chlorinated using SOCl 2 to yield acyl chloride group on the nanotube surface.The reaction between free NH 2 groups on the g-C 3 N 4 and the acyl chloride groups on CNTs yielded CNT-functionalized g-C 3 N 4 .The obtained covalently bonded hybrid photocatalyst showed enhanced activity for O 2 reduction reaction to H 2 O 2 in presence of formic acid.Observed activity Figure 7.The schematic for the postcovalent functionalization of CN-Ph-Br via Suzuki and reductive-Heck reaction.Adapted with permission. [68]opyright 2018, Wiley-VCH GmbH.was attributed to enhancement of the photoinduced electron generation.Similarly, another amidation reaction between zinc tetracarboxyphthalocyanine (ZnTcPc) and g-C 3 N 4 yielded the covalently functionalized g-C 3 N 4 -ZnTcPc (g-C 3 N 4 /ZnTcPc) hybrid photocatalyst. [78]Efficient electron transfer from the lowest unoccupied molecular orbital of ZnTcPc to the conduction band of g-C 3 N 4 mitigated the charge recombination probability and enhanced the photocatalytic activity for the degradation of rhodamine B and 4-chlorophenol under visible irradiation.
In a recent study, Windle and coworkers synthesized a novel photocatalyst for hydrogen production, which involved the covalent attachment of nickel bis-amino thiophenol catalyst onto four types of 2D g-C 3 N 4 , including three of them prepared from urea, DCDA, and ONLH, and an oxygen-doped one produced by treating the DCDA-based C 3 N 4 with formic acid (FAT).These CNcatalyst hybrid materials, namely CN urea -Ni(abt) 2 , CN DCDA -Ni(abt) 2 , FAT-Ni(abt) 2 , and ONLH-Ni(abt) 2 , were formed as a result of the coupling of aryl amines and aryl chlorides in the presence of potassium tert-butoxide catalyst (Figure 8). [79]The CN-catalyst materials exhibited over 8 days of hydrogen evolution, rapid and lasting charge separation, and photoactivity under irradiation of 475 nm light.Although the surface area was found to be related to catalyst loading, the lowest loading was observed for CN DCDA that had a surface area higher than FAT.This confirms that the chemical composition of the surface also played a role in defining the loading capacity.CN urea was the most efficient due to its impressively high quantum yield.Moreover, the oxygen-doped CNs can utilize a broader region of the visible spectrum than the nondoped ones.On the other hand, CN urea and CN DCDA were the only materials to undergo substantial reduction for electron transmission to Ni(abt) 2 as well as Ni(bdt) 2 ; however, this transfer was higher for the former catalyst than the later one and this affected their respective hydrogen production capability.

Photoinduced Functionalization of g-C 3 N 4
Photoinduced functionalization is lately used as a method to obtain better dispersibility especially producing the preferable g-C 3 N 4 thin films.Cao et al. present a unique polymer g-C 3 N 4 combining method as polymer-grafted g-C 3 N 4 . [80]Methyl methacrylate, glycidyl methacrylate, and isobornyl acrylate polymerized separately by adding Cu(I)Br, ligand, and initiator at 45 C for 30 min.Using graft polymer brushes these polymers grafted on a g-C 3 N 4 surface.The poor dispersibility of g-C 3 N 4 in organic solvents is one of the challenges to produce thin films.However, the polymer/g-C 3 N 4 structure enhanced dispersibility and enable smooth surface spin coating.
In another study, cyanuric acid-melamine hydroxyl ethyl methacrylate (CM-HEMA) precursor which has good dispersibility, was synthesized using visible-light irradiation.Heat treatment curing provides a gentle surface and stable thermoset coating.CM-HEMA films modified with poly-(styrene) and poly(N,N-dimethylacrylamide), so the tuned surface features and selectivity enabled the applications such as photocatalytic and photoelectrode.Both FTIR and elemental analysis were used for identifying grafting. [82]

Noncovalent Strategies
Noncovalent modifications include van der Waals forces, p-p interaction, hydrophobic interaction, electrostatic attraction, and hydrogen bonding.Such noncovalent strategies are mainly involved in noncovalent interaction of small organic molecules and nanoparticles.

Noncovalent Interaction of Organic Molecules
One of the methods for enhancing the electronic and optical properties of C 3 N 4 is the physical adsorption of organic molecules on g-C 3 N 4 surface.The redox band edge positions may be modified with the organic molecules on the surface of the C 3 N 4 .The DFT is one of the computational quantum mechanical modeling methods that are used to reveal the characteristics of the valence and conduction bands, the effective mass of carriers, band edge positions, and optical properties.Accuracy of DFT depends on whether the simulated crystal is a good approximation of reality or not, and various other factors, including the material type and studied property.Zhang et al. [83] proposed a theoretical model that shows photocatalytic activity enhancement of small organic molecule absorbed g-C 3 N 4 .They modified g-C 3 N 4 photocatalyst with different heterocyclic small molecules as a ligand (thiophene, furan, pyrrole, or pyrryl) and investigated the properties of modified g-C 3 N 4 , including their electronic structure, effective carrier mass, position of band edge, and optical properties, by DFT calculations.Considering the fast transfer of electrons in the pyrrole-modified g-C 3 N 4 , it was acknowledged to be a praiseworthy photocatalyst.A study on the noncovalent interaction between the 2,4-dinitrophenylhydrazone (2,4 graphitic DNP) derivatives and carbon nitride was also conducted. [84]Derivatives of 2,4-DNP hydrazine were produced and combined with g-C 3 N 4 to increase the light absorption of the resulted photocatalyst.Some of the spectroscopic techniques, such as the FTIR and UV-vis, were used for characterization in different polar solvents like N,N-dimethylformamide (DMF) and acetonitrile (ACN).Doping g-C 3 N 4 with 5% of dinitrophenylhydrazone derivatives led to absorption in the visible region and facile charge immobilization, which indicates reduced optical bandgap.Experimental results were found to be in line with DFT.

Noncovalent Interaction of Nanoparticles
Preparation of semiconductor-noble metal composites can enhance photocatalytic efficiency as well as photogenerated electron-hole separation.Noble metals with localized surface plasmon resonance (SPR) demonstrate strong absorption of visible light.In addition, the adsorption efficiency of the C 3 N 4 improves due to the existence of metal nanoparticles.Qin et al. increased the visible-light absorption, by the decoration of g-C 3 N 4 surface by Ag nanoparticles via chemical reduction. [85]he improvement in visible-light absorption of Ag-deposited g-C 3 N 4 was supported by the localized SPR effect, which caused a reduction in photogenerated electron-hole generation and improved photocatalytic performance.The photocatalytic activity development is primarily dependent upon both the synergistic effect between the components and the proper load amount of Ag.The 3 wt% Ag-loaded Ag/g-C 3 N 4 composite offered the best photoactivity, which was about 3.6 times higher than that of pure g-C 3 N 4 .Yuan et al. produced ZIF-8/g-C 3 N 4 composites via the ultrasonic in situ method. [86]Visible-light absorption, electron transfer, and the photocatalytic reaction rate are improved due to the unique hybrid structure.Upgraded photocatalytic performance and excellent stability were observed in ZIF-8/g-C 3 N 4 composites.
In another assay, Wang et al. [87] revealed the distinctive holeaccepting feature of microwave-synthesized carbon dot ( m CD) using transient absorption spectroscopy (TAS).The m CD/CN composite supports electron accumulation on the surface of CN as a result of hole transfer to m CD.It was also observed that sonication-based carbon dot ( s CD) serves as an electron acceptor in s CD/CN, whereas m CD acts as a distinguished hole acceptor in m CD/CN after excitation.

Surface Functionalization via Elemental Doping
Doping, the process of addition of structural impurities, has been known as an efficient method to change the bandgap and band structures of g-C 3 N 4 .It also expands the light absorption and increases the separation of the electron-hole pairs.In elemental doping, impurities can be metallic or nonmetallic.The effect of doping with metalloid group elements was also reported in a recent study. [88]Doping with anions is reported to result in hybridization of p-orbitals of the dopants with those of g-C 3 N 4 , whereas cation doping produces a discrete band through 3 d orbitals of transition metals leading to fine-tuning of the valence band.However, when codoped, anions and cations aids in bandgap adjustment as well as keeps the balance within charges. [89]

Metallic Doping
Metallic impurities are commonly incorporated into the g-C 3 N 4 structure using a homogenous solution of g-C 3 N 4 precursor and metallic salt.This mixture when subjected to thermal condensation produces g-C 3 N 4 decorated with metallic ions. [11]Generally, elements from the lithium family (K þ , Na þ ) and transition metals (Pt, Pd) are used.But in recent years, this method has also been utilized for other transition metals, such as Cu, W, and Mo. [90]he overall effect of K and Na doping on g-C 3 N 4 is quite similar; however, K-doped materials perform better in photocatalytic applications. [91]On the other hand, noble metal doping boosts movement of carrier, improves electron-hole separation, and narrows the bandgap.Transition metals, including Fe, Cu, W, Zn, Mo, and Zr, have mainly gained the attention of researchers as doping elements. [11]he work by Das et al. [92] highlighted the role of cobalt (Co) doping on the photoluminescence characteristics, the photocatalytic activity, and stability of the g-C 3 N 4 structure.The doped material demonstrated an increased surface area, decreased PL intensity, better stability, and better photocatalytic behavior under light than the pristine g-C 3 N 4 .Electrons were trapped due to the addition of Co and this helped in separation of carriers of charge.Doping of g-C 3 N 4 with S-block elements, such as lithium, sodium, potassium, magnesium, calcium, strontium, and barium, was investigated by Fronczak et al. [93] Cyanamide was chosen as the precursor while corresponding metal chloride was used to produce the samples via polycondensation.The elemental doping was found to have positive effects on the surface, texture, morphology, as well as structural characteristics of g-C 3 N 4 .S-block dopants promoted the adsorption of g-C 3 N 4 , which triggered the elimination of methyl blue and copper ions from aqueous media.
While metals promote the electron-hole separation, high cost, defects found in them, and poor thermal stability of ions are some limitations. [11,88]

Nonmetallic Doping
Nonmetals have high ionization energy and electronegativity and can form a covalent bond by reacting with other components.Nonmetallic doping facilitates in tuning the absorbance, redox potentials, and movement of photoinduced charge carriers. [11]ifferent nonmetallic dopants such as phosphorus, [94] sulfur, [95] carbon, [96] nitrogen, [97] oxygen, [98] boron, [99] and halogens (e.g., fluorine [100] and iodine [101] ) have been used.Elements with low electronegativity, such as F, B, S, and P, narrow down the bandgap. [88]Oxygen doping boosts adsorption ability, promotes the production of photogenerated holes, increases the surface area, and improves the separation of photogenerated charge carriers. [11]he theoretical study by Ghashghaee et al. reported the effect of B, O, S, and P doping on the siting and electronic characteristics of triazine and heptazine structures of g-C 3 N 4 . [102]O and S preferred to substitute bridging N atoms, whereas B occupied the places of internal carbon atoms in the bigger rings at the center.P replaced bridging and interstitial two-coordinated N atoms in heptazine and triazine structures, respectively.Moreover, in contrast to the O, S, and P doping, B doping was driven by negative charges.
Tian et al. reported the fabrication of 3D ultrathin porous Ndoped g-C 3 N 4 from melamine using a simple hydrothermal pretreatment method along with calcination. [103]Hydrothermal pretreatment makes urea a richer nitrogen source and porogen.Ndoping refines the electronic structure of g-C 3 N 4 and prevents the problem of incoming foreign atoms.This is one of the most impressive surface functionalization techniques to enhance the response of visible-light and photocatalytic activity of g-C 3 N 4 . [103]ai et al. proposed a one-step modification scheme to produce highly porous, S-doped g-C 3 N 4 . [104]The impressive enhancement in the photocatalytic behavior of the doped material was attributed to the synergy between high porosity and doped structure.

Codoping
Codoping conveys the individual benefits of each of the dopants and enhances the structural and optical properties of g-C 3 N 4 .K─Na, [105] Cu─Fe, [106] P─O, [107] and S─P [108] are some of the significant doping combinations.Regarding the effect of codoping, K─Na doping enlarges the surface area and enhances the rate of separation in photogenerated changes.Babu et al. studied the effect of bielemental doping, i.e., with B and S on g-C 3 N 4 . [109]he g-C 3 N 4 decorated with B and S showed improved absorption of light, superior separation of charges, better electron transfer, and greater effective surface area than undoped or singly doped materials.Moreover, codoping caused defects in which photoelectrons were stuck and photogenerated charge carrier could not recombine.Dou et al. codoped g-C 3 N 4 with metallic Fe(II) and nonmetallic S. [89] The mixture of melamine and ferrous chloride in trithiocyanuric acid was used to synthesize the codoped product, which demonstrated excellent photodegradation for RhB under illumination.According to the theoretical estimations by DFT, codoping effectively reduced the bandgap and provided more active sites, which in turn enhanced electron-hole transfer.Yang et al. [110] also reached a similar conclusion by synthesizing a nitrogen-deficient g-C 3 N 4 with melamine and doping it with ammonium phosphate and ammonium chloride.The structure codoped with P and Cl showed a more negative conduction band potential.For more insight into the effect of different dopants on the structure and properties of g-C 3 N 4 , readers are directed toward the review by Jiang et al. [11] The aforementioned surface modification techniques improve the electronic properties of the surface and accelerate photocarrier separation.Moreover, polymer-modified g-C 3 N 4 benefits from enhanced dispersibility and processability, which makes it suitable for film formation.Using the most suitable surface modification for a particular application is of great significance.Elemental doping is very useful for tuning the bandgap of semiconductive g-C 3 N 4 .In a nutshell, K─N, O, N, and C─Fe doping result in larger surface area, better absorption ability, a modified electronic structure, and reduced bandgap, respectively.

Processing of C 3 N 4
Processing of C 3 N 4 leads to better mechanical properties, rapid hydrogen evolution under visible light, and suppressed recombination of photogenerated charges.It is mainly classified into C 3 N 4 -based structures and their hybrids with other nanomaterials.Due to the combination of favorable properties of both materials, C 3 N 4 hybrid structure performs comparatively well.In this part, the C 3 N 4 -based thin films and hierarchical structures, C 3 N 4 -CNTs, and C 3 N 4 -graphene hybrid structures are discussed.

C3N4-Based Thin Films
Primarily, there are two processing approaches for the preparation of C 3 N 4 -based thin films, namely: bottom-up and top-down.As a bottom-up route, in thermal vapor condensation, [111] melamine was heated to 600 C with heating rates of 3 C min À1 for 3 h in a crucible placed in a muffle furnace (Figure 9a).The growth of highly uniform g-C 3 N 4 films was observed on the fluorine-doped tin oxide (FTO) substrate placed on the top of the crucible.The resulting films exhibited photocurrent density as high as 0.12 mA cm À2 at the bias of 1.55 V (vs reversible hydrogen electrode) with Na 2 S as the sacrificial reagent, attributed to the intimate contact between the film and the substrate, decreased charge transport, and charge transfer resistance.Liquid-based direct growth method [112] used supramolecular complexes (e.g., a mixture of cyanuric acid and 2,4-diamino-6phenyl-1,3,5-triazine [CMp]) liquifying before thermal condensation at high temperatures.Produced porous C 3 N 4 films displayed open-circuit voltage of greater than 1 V when used as the electron acceptor in a polymer solar cell (Figure 9b).The doctor-blade method is also used for producing porous supramolecular films at a large scale.The precursor pastes can be deposited on a range of substrates, such as FTO, porous TiO 2 , silicon wafer, and glass, and the film thickness can be easily adjusted. [113]Microcontact printing [114] appears as an another approach that involves precursor (e.g., cyanamide) to be sandwiched between two substrates; the precursor gets condensed and printed on the substrates upon heating.To produce g-C 3 N 4 on conductive substrates like carbon fiber, nickel foam, and FTO glass, electrophoretic deposition method is commonly used. [115]The C 3 N 4 -toluene suspension was prepared, and a direct current was applied between the conductive substrate and the counter electrode.This technique is especially useful for thermally unstable substrates. [116]In addition, there are other bottom-up methods including solvothermal, [117] sol-gel, [118] and electrodeposition. [119]On the other hand, in topdown processes, the films can be produced from the synthesized g-C 3 N 4 following some traditional methods such as spraying [120] and spin coating. [121]The aforementioned processes have several advantages and disadvantages when compared with each other.Readers interested in comprehensive comparisons are directed to the review article on films of g-C 3 N 4 . [33,116,122]ery recently, an original sputtering method using radio frequency magnetron (RFM) was used for the stoichiometric growth of photoactive C 3 N 4 thin films. [123]This method utilizes g-C 3 N 4 powder in the compact form to serve as the sputtering target.The thin-film deposition was carried out on different substrates, including borosilicate glass and TiO 2 nanotubular array, under two different plasma media, i.e., Ar and N 2 at various time durations.According to their findings, in comparison with N 2 , Ar plasma enabled faster production of thinner and more stoichiometric C 3 N 4 films with a higher current density.The most suitable time duration for RFM sputtering at 100 W, with g-C 3 N 4 as target and Ar as plasma was found to be 3 h.

C3N4-Based Hierarchical Structures
Hydrogels are notable polymeric materials with a crosslinked hydrophilic structure.Due to their unique swelling properties and ability to retain shape, they perfectly imitate human tissue and find extensive use in biomedicine, tissue-engineering, drug-delivery, self-healing, and shape-memory materials.However, these materials are limited by their weak mechanical properties.To improve their mechanical properties, hydrogels are reinforced with particles that dissipate stress and provides extra stabilization through repulsion between charged filler particles.Amidst these bulk soft materials, g-C 3 N 4 acts as a praiseworthy photoinitiator, and reinforcer and g-C 3 N 4 /hydrogel hybrids exhibit excellent mechanical properties.
When g-C 3 N 4 is exposed to light, radical formation occurs on its surface in an aqueous solution due to its photoactive nature. [124]The surface of g-C 3 N 4 works as an initiation platform and g-C 3 N 4 are covalently embedded into the gel network.No transfer of electrons or the presence of an external crosslinker is required.Hence, g-C 3 N 4 , a monomer, and crosslinking agents such as water-soluble acrylamide derivatives, can lead to rapid hydrogenation under visible light irradiation.
g-C 3 N 4 can be used for hydrogel formation through covalent bonds, integrated into hydrogels in the form of supramolecular hydrogel formation, and/or processed into the scaffolds of the gels to induce improved mechanical properties.Furthermore, the diversity of g-C 3 N 4 enables the synthesis of new and improved hydrogels with unique properties like ultralow friction, thermo responsiveness, super stretch ability, and simultaneous toughness and flexibility.Photopolymerization and reinforcement via g-C 3 N 4 help develop gels with superior spatial control.Very recently, a comprehensive review article has reported the role of g-C 3 N 4 in polymerization reactions and the properties of g-C 3 N 4 /polymer composite structures. [125]s shown in Figure 10a, photocatalysis by semiconductors involves the generation of charges under light, which in turn reacts with O 2 or H 2 O to form free radicals. [126]While g-C 3 N 4 itself does not form hydroxyl radicals, it reduces oxygen to superoxide radicals, and the polymerization is initiated (Figure 10b).Using bulky g-C 3 N 4 might not form stable hydrogel with polyacrylamide (PAM) as the chains have weak interaction within them.Moreover, bulky g-C 3 N 4 neither disperses well in water nor distributes evenly in hydrogels.A solution for this was proposed by Dong and coworkers, who designed a composite hydrogel using alkali-exfoliated CNNSs as a crosslinking agent, initiator, and reinforcer in PAM. [126]They proposed a simple hydrogel fabrication method using photoinduced polymerization.The photoactive CNNS initiated polymerization of acrylamide monomers, and a 3D network was formed due to hydrogen bonds.As a result, a hydrogel was formed with CNNS dispersed in it, as shown in Figure 10c.The synthesized product demonstrated extraordinary UV absorption and visible transparency capabilities, making it ideal for UV radiation shielding, fluorescence labeling, and photocatalysis.The produced hydrogels are flexible, photostable, and have self-healing properties.
Another limitation of g-C 3 N 4 -based hierarchical structures is their inadequate structural porosity and recombination of photogenerated charges.Thus, Huang and coworkers [127] fabricated a unique cellulose-derived hierarchical g-C 3 N 4 /TiO 2 -nanotube heterostructure.As shown in Figure 11, the surface sol-gel process was used to fabricate the g-C 3 N 4 /TiO 2 -NT nanocomposite.The natural cellulose extracted from regular filter paper by layer-bylayer self-assembly is used as a structural template for TiO 2 nanotube.Cellulose nanofiber surface was coated with ten layers ultrathin TiO 2 thin films and calcined at 450 C. TiO 2 nanotubes together with urea were crushed, and a mixture was prepared, which was again calcined in a closed crucible to finally form a  [111] Copyright 2015, Elsevier.b) Adapted with permission. [112]Copyright 2014, American Chemical Society.Reproduced with permission. [127]Copyright 2020, American Chemical Society.and c) CNNSs.Reproduced with permission. [126]Copyright 2019, Wiley-VCH GmbH.
g-C 3 N 4 /TiO 2 -NT nanocomposite.The hierarchical network of the g-C 3 N 4 /TiO 2 composite was similar to that of cellulose, whereas close heterojunctions were observed within g-C 3 N 4 and TiO 2 phases.As a result of this processing, the composite was found to have remarkable photocatalytic properties, superior transfer efficiencies of photogenerated charges, and a photoresponse range extending from UV to visible-light region of the spectrum.Both these methods are marred by limitations such as nonuniform mixing and insufficient contact and in some cases decrease in N content due to heat treatment at elevated temperatures.In addition, 2D CNNSs were produced by thermal oxidation etching followed by liquid exfoliation to enhance their interaction with other materials.While this technique faced yield-related problems, the interlayer distance between the layered g-C 3 N 4 was opened up, leading to the enlarged surface area, enhanced physicochemical properties, and improvements in photoactivities.
A versatile method to self-assemble CNNSs and CNTs at low temperatures was shown to form g-C 3 N 4 NS-CNT heterostructures. [128]This method made use of the π-π stacking and electrostatic interactions.Three-dimensional hybrid structures made up of CNNSs and CNTs offered promising catalytic activity for oxygen evolution as well as stability.The g-C 3 N 4 protonated with hydrochloric acid to decrease the exfoliation time and provide a homogeneous distribution of C 3 N 4 layers.Using hydrothermal process, positively charged CNNSs and negatively charged oxidized CNTs created 3D composite (Figure 12).Due to the large work function and excellent electron conductivity of CNTs, a high nitrogen content, and porous structure, these hybrid structures exhibit rapid transfer of photoelectrons and mass, improved H 2 evolution, and stable water-splitting ability. [129]Consequently, the migration rate of photoelectrons, separation of charges, and overall efficiency of charge carriers in C 3 N 4 are enhanced.Also, it should be acknowledged here that when producing this 3D g-C 3 N 4 NS-CNT structure, the inherent structural properties of g-C 3 N 4 and CNTs are not compromised.These hybrid structures could be readily used for the electrocatalytic oxygen evolution reaction.
Chen's group also synthesized g-C 3 N 4 /CNT structures using cyanamide as a precursor.The structural features, porosity, and BET surface area of the original materials were well retained, improved photocatalytic activity for H 2 evolution under visible light was also observed. [9]The photocatalytic activity was monitored using photocurrent measurement techniques and steady/ time-resolved photoluminescence spectroscopy.The findings showed that the superior properties are attributed to the amazing compatibility between g-C 3 N 4 and conjugated CNTs.The CNT conjugates were found to boost the durability of charge carriers, control the population of temporary charge carriers, as well as boost the concentration of permanent ones.Due to this, the overall quantum efficiency is increased.CNT loading Figure 12. Preparation of the 3D g-C 3 N 4 NS-CNT porous composite.Reproduced with permission. [128]Copyright 2014, Wiley-VCH GmbH. of approximately 0.2 wt% revealed the best results, i.e., H 2production rate of 39.4 mmol h À1 .

g-C 3 N 4 /Graphene Hybrid Structures
The application of pristine g-C 3 N 4 is limited due to its unimpressive electronic conductivity and photocatalytic activity.As a solution to this problem, g-C 3 N 4 can be combined with graphene-a material having a similar 2D planar structure.Graphene is a 2D sheet of sp 2 -hybridized conjugated carbons, which acts as a building block for all significant graphitic materials (0D-3D).The desired properties of graphene include high thermal and electrical conductivity, large surface area, commendable strength, and high surface and optical properties.In g-C 3 N 4 /graphene hybrids, graphene can either be used as a functional component or as a support for immobilized g-C 3 N 4 .A unique blend of properties can be achieved by processing g-C 3 N 4 with graphene into hybrid structures.With its large specific surface area and high surface conductivity, graphene promotes the transfer of charges and separation of electrons and holes, making the hybrid an attractive material for energy storage and conversion applications, such as photo-oxidation, photo-and electrocatalytic evolution of hydrogen, electrocatalytic oxygen evolution reaction, fuel cells, and Li-ion batteries. [10]everal methodologies have been adopted for the preparation of g-C 3 N 4 /graphene hybrids, including thermal, solvothermal, and other physiochemical reactions.Zhao et al. [10] presented a brief review of the synthetic strategies for these materials.Furthermore, a dual-mesh structure that involved selective growth of 2D mesoporous g-C 3 N 4 meshes on 2D mesoporous graphene meshes (g-CN@G MMs) locally was synthesized. [130]he continuous mesh-on-mesh network was full of edges and defects, which facilitated rapid electron and mass transfer via multiple channels and provided active sites for hydrogen bonds.Due to these advantageous features, g-CN@G MMs exhibited extraordinary hydrogen evolution reactivity and durability, which rivalled most nonmetal and even some metallic catalysts.
Similarly, a unique metal-free hybrid catalyst by combining g-C 3 N 4 with nitrogen-doped graphene quantum dots (N-GQDs) was prepared. [131]In comparison with pure g-C 3 N 4 , this N-GQDs/g-C 3 N 4 catalyst exhibited superior photocatalytic activity for hydrogen generation from the splitting of H 2 O.
As it is mentioned in the section about g-C 3 N 4 -based hierarchical structures, g-C 3 N 4 can be formed and combined with various materials such as CNTs and graphene.The large work function and enhanced electronic conductivity of g-C 3 N 4 / CNTs structure improve the H 2 evolution ability of g-C 3 N 4 .Although various g-C 3 N 4 -based composite structures have shown improved properties and structures, there are some features that require attention.For instance, while g-C 3 N 4 enhances the mechanical properties of the hydrogel, the porosity and photogenerated charge recombination are two important issues that need to be further studied.
6. Applications of g-C 3 N 4 g-C 3 N 4 with distinctive semiconducting properties have become the center of attention for their performance in applications related to energy as well as the environment. [132]However, instant electron-hole recombination in these materials decreases their efficiency and restricts their use.This recombination is delayed in carbon nitride (C 3 N 4 )-based nanocomposites, thus improving the efficiency. [133]Being sustainable, inexpensive, stable, and metal-free makes C 3 N 4 materials extremely attractive for photocatalytic applications, however, it is worth mentioning that combining g-C 3 N 4 with other sorts of nanoparticles widen their usage field from photocatalysis to sensing, bioimaging, novel solar energy exploitation including photocatalytic coenzyme regeneration, templating, and carbon nitride-based devices. [134]lthough g-C 3 N 4 has been extensively used in solar-to-energy applications, herein, we would like to take the attention of the readers to less-studied areas such as environmental and sensing applications, and some of these applications are concisely discussed in this part.

Environmental Applications
Wide-ranging water pollution, mostly caused by clothing, foodservice, and automobile industries, has worsened water body contamination around the world. [135]Therefore, innovative methods are required for treatment of water and detection and filtration of nonbiodegradable materials.
Due to the fact that g-C 3 N 4 compounds are nature-friendly, scientists are interested in exploring their photocatalytic properties for water treatment and contaminant degradation or adsorption.However, low surface area, high recombination rate of charge carriers, and inadequate light absorption of pure g-C 3 N 4 make it insufficient for photocatalytic degradation.As a consequence, most of the studies are focused on producing modified C 3 N 4 photocatalysts and investigating new methods such as noble metal-free doping, nonmetallic doping, and codoping of g-C 3 N 4 photocatalysts for filtration of pollutants from water.As an example, the adsorption efficiency of Na-doped g-C 3 N 4 synthesized by cyanamide and NaCl in removing methyl blue was studied. [136]he maximum absorption for methyl blue of the doped material was considerably higher than the undoped one.In fact, the adsorption reached 367 mg g À1 , while equilibrium was reached within 1 min.It is worth mentioning that this remarkable adsorption was due to the high porosity of synthesized adsorbent.Analyses carried out illustrated that methyl blue was adsorbed on Na-doped C 3 N 4 through electrostatic interactions.
Deprotonation of g-C 3 N 4 using Na ions considerably improves photo reactivity as well as absorption of light. [137]This process was conducted by heating g-C 3 N 4 to 550 C in the presence of NaCl.An increase in photocatalytic activity was attributed to the improved rate of charge transfer and optical absorption.The performed analysis revealed textural features such as the substitution of Na ions with the protons of amino functional groups and binding to atoms of nitrogen through a polymerization reaction.In another assay, an inexpensive and ecological Cg-C 3 N 4 /Fe 3 O 4 nanocomposite was synthesized for magnetic solid-phase extraction (MSPE) of pollutants such as brominated flame retardants (BFRs) from water. [138]The high surface area of this Taiji (an equilibrium between hydrophobicity and hydrophilicity)-principle-designed nanocomposite resulted in a maximum extraction rate of BFRs.For the aforementioned extraction technique, the consumption rate of the solvent was found to be low, and the adsorbent was reusable.The results, such as low detection limit (limits of detection [S/N ¼ 3]: 0.1-0.2μg L À1 ) and high recovery percentage (over the range of 92.4-99.8%)obtained from the analyses, revealed that the synthesized nanocomposite could be a great candidate for water treatment.
Metal-free black phosphorus/polymeric carbon nitride photocatalysts was prepared in N-methyl-2-pyrrolidone using liquid exfoliation technique displayed suitable photoactivity in the activation of oxygen reactions owing to effective charge transfer due to the desirable energy configuration. [139]In other words, 98% of RhB was decomposed within 15 min of irradiation.The rate constant for the degradation of rhodamine B (RhB) pollutants using the 10% BP/CN composite was four times greater than that of pure carbon nitride.
A two-step method was used to synthesize phosphorus-doped g-C 3 N 4 (P(x%)-g-C 3 N 4 ) by using melamine and phosphonitrilic chloride trimer (Cl 6 N 3 P 3 ) as precursors [140] and the prepared P(x%)-g-C 3 N 4 was coated on TiO 2 nanoparticles using a solvent-thermal method to synthesize P(x%)-g-C 3 N 4 /TiO 2 photocatalyst.Extended absorption of light along with high photocatalytic activity were some of the advantages related to the produced P(x%)-g-C 3 N 4 /TiO 2 composite.
The results obtained by evaluating the photocatalytic activity of synthesized composite in the degradation of methyl blue under visible-light radiation revealed that P(0.1%)-g-C 3 N 4 /TiO 2 composite offers the highest activity, which is approximately 5 times higher than that of pristine g-C 3 N 4 . [140]roduction of phosphorus-doped porous ultrathin g-C 3 N 4 nanosheets (PCN-S) was carried out using P element doping as well as a thermal exfoliation technique. [141]High absorption of visible light, enhanced chemical resistance, porous surface, along with a large surface area are some advantages of PCN-S revealed by performed characterization analyses including FTIR, photocurrent response (I-t), XRD, TEM, and UV-vis DRS.P doping and in-plane pores of the NSs were found to have a positive impact on the photocatalytic efficiency of PCN-S on Cr (VI) reduction, as it widened the response region to light as well as increased the surface area with more active sites.The techniques utilized in this assay can motivate the preparation of g-C 3 N 4 -based compounds having better photocatalytic performance for the degradation of other pollutants, such as 2,4diclorophenol.
Another technique to undertake the restrictions related to utilizing bulky g-C 3 N 4 as photocatalysts and improving the photocatalytic activity was doping with two or more elements.In this study, melamine, ferrous chloride, and trithiocyanuric acid were calcified to synthesize Fe þ2 and S co-doped g-C 3 N 4 . [89]The patterns obtained by XRD analysis illustrated the similar crystal structures of the Fe (II)-doped, Fe (II)-S codoped, and pristine g-C 3 N 4 .Furthermore, the energy dispersive X-ray analysis related to Fe (II)-S codoped g-C 3 N 4 , demonstrated the dispersion of Fe and S in the high percentage of carbon and nitrogen.This study managed to achieve 91% photocatalytic degradation for the RhB dye and the degradation was enhanced by a factor of 5.4 times for the Fe(II)-S codoped g-C 3 N 4 compared with pristine g-C 3 N 4 .In another study, Deng et al. [142] proposed a highly optimized and proficient g-C 3 N 4 -based photocatalyst, which was doped by O, functionalized with carbon QDs, and loaded onto reduced graphene oxide (rGO).The photocatalyst proved to be a promising candidate for the degradation of a characteristic antibiotic, lincomycin.Due to the synergistic effect of dopant, CQDs, and rGO, the rate of degradation as well as the extent of decomposition was enhanced.
Moreover, doping of g-C 3 N 4 by KCl was investigated and it was shown to overcome the limitations related to quick recombination of charge carriers and improves the photocatalytic efficiency in NO removal. [143]K and Cl ions in g-C 3 N 4 contribute to providing a double channel for electron and hole transfer.In case of KCl-doped g-C 3 N 4 , the holes in the valance band have a high capability of oxidation and the charge carriers have extended life-time due to the dual electronic channel.These unique properties increase the NO x removal efficiency.
Introduction of potassium and phosphorus into g-C 3 N 4 for photocatalytic H 2 O 2 production under visible-light irradiation was also studied. [144]With regards to the prepared catalyst, integrated heteroatoms increased the photogenerated charge carriers by reducing the bandgap to the visible light area.As a result, the transmission rate was increased and the recombination of the charge carriers delayed, leading to considerably higher photoactivity of catalysts.The surface-functionalized g-C 3 N 4 was also used for phenol degradation, [145] methyl orange degradation, [145] oxytetracycline removal, and 2-chloroethyl ethyl sulfide degradation.
Recently, Xiao et al. [146] presented a low-cost, high-efficiency method to produce g-C 3 N 4 planes decorated with single Cu atoms.First, intercalation of chlorophyll sodium copper was carried out, and a supramolecular precursor was obtained.This precursor was then heated to obtain the final product.The bond between Cu atoms and compositional N (Cu─N x ) opened active channels for charge transfer, which considerably boosted the transfers of photogenerated charge carriers, both in-plane and interlayer.The proposed g-C 3 N 4 -based catalyst was found to be highly suitable for prolonged photocatalytic evolution of hydrogen as well as for production of phenol through benzene.90a,b] To learn more about recent advancements in energy and environmental applications of g-C 3 N 4 , the review article by Barrio et al. is highly recommended. [148]

Sensing Applications
Although the g-C 3 N 4 nanostructures with high surface-area-tovolume ratio are essential candidates for bioimaging and sensing, their efficiency in these fields can be improved by applying some changes in their structures through modification. [149]134a,149b] As an example, highly fluorescent g-C 3 N 4 / quantum dots conjugation (g-C 3 N 4 QDs) was produced through the uncomplicated ethanol-thermal treatment of bulk g-C 3 N 4 using KOH and used in the cell membrane labeling.The synthesized g-C 3 N 4 QDs make a great contribution to bioimaging due to its properties, such as being nontoxic, water-soluble, stable, and biocompatible along with possessing narrow size distribution. [150]s subgroup J of avian leukosis viruses (ALVs-J) is of great concern in guarantying food safety, Zhou et al. designed a unique electrochemical immunosensor using mesoporous graphitic carbon nitride (mpg-C 3 N 4 ) which could be utilized as the sensor platform to connect to the primary antibodies (Ab1). [151]Thionine-mpg-C 3 N 4 has also been recommended to be a potential electroactive probe as well as the carrier of secondary antibodies (Ab2).As compared with bulk g-C 3 N 4 , synthesized platform offered benefits like greater surface area and plentiful active sites.Immunosensing tests of TCID 50 (TCID 50 : 50% tissue culture infective dose) were studied by differential pulse voltammetry.An increase in the reduction current was observed when the concentration of ALVs-J increased and the detection limit was estimated as 120 TCID 50 mL À1 (Figure 13a).The effect of incubation time was studied, and the findings were used to optimize electrochemical response and current-temperature diagram.The current increased along with the increasing temperature, which was followed by a sharp decline after 37 C (Figure 13b).Consequently, 37 C was proposed as a suitable incubation temperature.
In another work, Prussian blue (PB) and melamine undergo a one-step pyrolysis reaction forming the graphitic carbon nitride/ iron oxide in which Fe 2 O 3 nanoparticles act as a protecting layer of g-C 3 N 4 .The features including exhibiting a longer cycle life (up to 1000 cycles) along with greater surface area, lead to greater electrochemical efficiency of synthesized composite in enzymefree detection of glucose with a response time less than 3 s and a linear range of 2.0 Â 10 À6 À 2.4 Â 10 À3 mol L À1 . [137]t/ZnO/g-C 3 N 4 composite structure was found to be a potential sensor for the detection of gases such as ethanol and NO 2 . [152]n this microwave-assisted fabrication method, exfoliated CNNSs were utilized as a substrate for the growth of ZnO nanorods through bonding of Zn and N components.This was followed by Pt nanoparticle deposition, which results in the formation of Pt/ZnO/g-C 3 N 4 nanostructures.This composite material was able to detect ethanol and NO 2 at 250 and 150 C, respectively.The sensor was found to be dependent on the analyte concentration and the detection limit for ethanol and NO 2 was found as 0.072 and 0.05 ppm, respectively (Figure 14a).The Pt/ZnO/g-C 3 N 4 sensor showed a temperature-dependent selectivity toward ethanol and NO 2 compared with other gases such as Cl 2 , NH 3 , acetone, and formaldehyde (Figure 14b).A pyrolysis method has also been utilized to produce graphitic carbon nitride (g-C 3 N 4 ) by melamine. [153]In this research, the carbon paste electrode was modified by casting the g-C 3 N 4 /chitosan solution on the electrode.Due to the interaction between Hg (II) and CNNSs, the sensor with high selectivity for the Hg (II) detection with a detection limit of 1.0 Â 10 À8 mol L À1 was fabricated.It is worth mentioning that selective Hg (II) detection was achieved in the existence of other elements such as Fe (II) as well as Cu (II).

Conclusion
Due to its highly stable, nontoxic, metal-free, and 2D layered structure, g-C 3 N 4 has emerged as a commercially significant material, especially in environmental and energy-related applications.Its impressive photocatalytic activity and ease of fabrication have attracted the attention of researchers in recent years.Various studies have explored cost-effective and green synthetic routes for g-C 3 N 4 as well as methodologies to fine-tune its properties.This Review has provided a glimpse into the current developments in the synthesis, characterization, surface functionalization, and processing of g-C 3 N 4 .
Although g-C 3 N 4 is recognized for its substantial benefits, there is still a room for improvement to utilize it to the full potential.Efforts are needed for the preparation of g-C 3 N 4 with larger surface area, higher porosity, greater number of active sites, extended visible-light absorption, improved charge transport and separation, and prolonged π-conjugated build-up.The surface characteristics, which directly determines the properties of g-C 3 N 4 , is heavily dependent on its synthetic process, including precursors chosen, adopted polymerization temperature, time, and atmosphere.To achieve g-C 3 N 4 with a superior set of qualities, it is incredibly crucial to choose the proper reaction parameters.As a precursor, urea, which can provide g-C 3 N 4 with a higher rate of hydrogen evolution and better porosity, is preferred over thiourea and DCDA precursors, whereas air treatment offered the best atmosphere.As most precursors of g-C 3 N 4 are earth-abundant, the production procedure is affordable.However, their low yields (<10%) is still an issue and alternative experimental procedures are in need to increase their yield.
Pristine g-C 3 N 4 is not highly efficient for photocatalytic applications due to its limited physicochemical properties and needs enhancement through surface modification.However, processing techniques without using toxic reagents and precious metals would be beneficial for both cost and sustainability.
Moreover, modification at an atomic/molecular level via different covalent and noncovalent strategies detailed in this article was found to be equally fruitful.Doping and copolymerization were identified as the most common techniques in the literature, whereas framework charging of g-C 3 N 4 is a rarely used technique.In fact, the tendency of covalent strategies to react with defects has shifted the focus toward noncovalent modification techniques.
For the characterization of g-C 3 N 4 , most researchers used XRD and SEM as tools to study g-C 3 N 4 structure and morphology, respectively.Although the chemical formula of g-C 3 N 4 has been widely adopted, and many crystalline carbon nitride structures are synthesized, no carbon nitride with ideal graphitic structure has been fabricated so far.Light absorption and bandgap were obtained by UV-vis analysis.The BET analysis is also critical to specify porosity and surface area information, which directly relates to active sites in photo-reactions.UV-vis and two contact methods are useful tools to characterize the conductivity and electronic structure of C 3 N 4 .Also, in situ characterization methods are important to investigative growth mechanism and dynamic changes.Furthermore, processing methods for g-C 3 N 4 were found to have a profound effect on its overall performance.This discussion encompassed the fabrication of C 3 N 4 -based thin films, hydrogels, and C 3 N 4 -CNT and C 3 N 4 -graphene hybrid structures.There is still room for hybrid structures, including hybrid 2D materials with sulfides.
Designing intricate g-C 3 N 4 based nanocomposites and examining the effect of semiconductors on the electronic properties of g-C 3 N 4 would be two of the appealing research areas.Also, filling the existing gap in the numerical modeling of g-C 3 N 4 -based photocatalysts such as prediction of real electronic structure would not only enable exhaustive investigation of the performance of these photocatalysts to reduce the waste of time and resources but also make a way for new and improved photoreaction systems.Considering the widespread usage of g-C 3 N 4 -based materials in energy storage, water splitting for H 2 evolution, filtration, degradation of pollutants, sensing, and bio-imaging, it has become the need of the hour to increase the quantity and quality of the research work focusing on the advancement of photocatalytic performance, enrichment of properties, and optimization of the synthetic and processing methods of these materials.

Figure 4 .
Figure 4. a) XRD patterns of the prepared g-C 3 N 4 samples which are denoted X-CN-Y, the X presents the precursor material used (D: DCDA; M: melamine; U: urea) and Y presents the applied reaction conditions (A: air and N: nitrogen gas), b) XRD spectrum of g-C 3 N 4 and hydroxylation modified samples with 0, 10, 20, and 30 wt% concentration H 2 O 2 , respectively.a) Adapted with permission.[37]Copyright 2017, Elsevier.b) Adapted with permission.[38]Copyright 2016, Elsevier.

Figure 5 .
Figure 5. a) The schematic for surface charging and exfoliation of PTI.b) The effect of PTI/Na ratio on the degree of exfoliation and the electronic properties of PTI dissolution.Yellow, green, and red rectangles correspond to low-to-high [Na], respectively, with the inset photographs showing resultant negatively charged PTI dissolution.c) Effect of the initial PTI amount on the concentration/yield of negatively charged PTI dissolution (7:1 PTI/Na ratio).Adapted with permission.[59]Copyright 2018, Wiley-VCH GmbH.

Figure 9 .
Figure 9.The deposition of g-C 3 N 4 film via a) thermal vapor condensation and b) liquid-based direct growth.a) Adapted with permission.[111]Copyright 2015, Elsevier.b) Adapted with permission.[112]Copyright 2014, American Chemical Society.

Figure 11 .
Figure 11.Schematic diagram showing the preparation process of the natural cellulose substance-derived g-C 3 N 4 /TiO 2 -NT nanocomposite.Reproduced with permission.[127]Copyright 2020, American Chemical Society.

Figure 10 .
Figure 10.Illustration for the a) photoinitiated polymerization of acrylamide by g-C 3 N 4 and the PAM gelation with b) g-C 3 N 4 ,and c) CNNSs.Reproduced with permission.[126]Copyright 2019, Wiley-VCH GmbH.

5. 3
. g-C 3 N 4 /CNT Hybrid Structures g-C 3 N 4 /CNT structures are traditionally produced either by physical mixing of the two or through in situ immobilization of g-C 3 N 4 on CNTs.

Figure 14 .
Figure14.a) The gas response of the Pt/ZnO/g-C 3 N 4 sensor at different gas concentrations and the detection limit for ethanol and NO 2 .Inset: duallogarithm of gas response (S) and gas concentration (C) for the Pt/ZnO/g-C 3 N 4 sensor.b) The selectivity of the Pt/ZnO/g-C 3 N 4 sensor in a gas concentration of 10 ppm at different temperatures.Adapted with permission.[152]Copyright 2018, Elsevier.

Table 2 .
Polymer modified g-C 3 N 4 and their applications.

Table 3 .
g-C 3 N 4 -based catalysts for environmental applications.