g-C3N4-Based Nanomaterials for Visible Light-Driven Photocatalysis

Graphitic carbon nitride (g-C3N4) is a promising material for photocatalytic applications such as solar fuels production through CO2 reduction and water splitting, and environmental remediation through the degradation of organic pollutants. This promise reflects the advantageous photophysical properties of g-C3N4 nanostructures, notably high surface area, quantum efficiency, interfacial charge separation and transport, and ease of modification through either composite formation or the incorporation of desirable surface functionalities. Here, we review recent progress in the synthesis and photocatalytic applications of diverse g-C3N4 nanostructured materials, and highlight the physical basis underpinning their performance for each application. Potential new architectures, such as hierarchical or composite g-C3N4 nanostructures, that may offer further performance enhancements in solar energy harvesting and conversion are also outlined.


Background
Future energy production, storage and security, and combating anthropogenic environmental pollution, represent key global challenges for both developed and emerging nations [1,2].Sunlight, an essentially limitless source of clean energy, has the potential to address both these challenges [3,4], and its utilization to this end entered mainstream science following breakthroughs in semiconductor light harvesting for photocatalysis by Honda and Fujishima in the 1970s [5][6][7].This discovery led to extensive research into titania semiconductor photocatalysts, principally for water splitting and the degradation of aqueous or airborne organic pollutants under UV light irradiation [8][9][10][11][12][13].However, efficient harnessing of visible light (the major component of solar radiation that reaches the Earth's surface) by photocatalysts to drive chemical transformations remains problematic [14][15][16] due to identifying suitable materials that possess narrow band gaps, high quantum yields, efficient charge carrier transport, and low rates of charge carrier recombination, and good thermo-, photo-, and chemical stability.The development of such low cost photocatalysts from earth abundant, and ideally non-toxic elements for visible light harvesting would unlock opportunities for their large-scale application to supplement existing renewable energy networks and pollution control systems.

Semiconductor Photocatalysis
Semiconductor photocatalysis refers to the acceleration of chemical transformations (most commonly oxidations and reductions) brought about through the activation of a catalyst, comprising a semiconductor either alone or in combination with metal/organic/organometallic promoters, through light absorption, with subsequent charge and/or energy transfer to adsorbed species.Note that the direct activation of reactants and intermediates through light absorption is the realm of photochemistry; in establishing whether a transformation is truly photocatalytic it is therefore crucial to establish that photons are absorbed by the catalyst rather than adsorbates [17,18].In the photocatalytic production of so-called 'solar fuels', photoexcited charge carriers drive the conversion of water and CO 2 into H 2 , CO, CH 4 , CH 3 OH and related oxygenates and hydrocarbons [19][20][21].Such processes parallel those in nature wherein sunlight absorbed by chlorophyll in plants promotes starch and oxygen production from carbon dioxide and water), and are hence termed artificial photosynthesis (Figure 1).Photoexcited charge carriers can also either induce the total oxidation (mineralization) of organic pollutants such as those encountered in aquatic environments, either directly, or through the creation of potent oxidants such as hydroxyl radicals [22].

Semiconductor Photocatalysis
Semiconductor photocatalysis refers to the acceleration of chemical transformations (most commonly oxidations and reductions) brought about through the activation of a catalyst, comprising a semiconductor either alone or in combination with metal/organic/organometallic promoters, through light absorption, with subsequent charge and/or energy transfer to adsorbed species.Note that the direct activation of reactants and intermediates through light absorption is the realm of photochemistry; in establishing whether a transformation is truly photocatalytic it is therefore crucial to establish that photons are absorbed by the catalyst rather than adsorbates [17,18].In the photocatalytic production of so-called 'solar fuels', photoexcited charge carriers drive the conversion of water and CO2 into H2, CO, CH4, CH3OH and related oxygenates and hydrocarbons [19][20][21].Such processes parallel those in nature wherein sunlight absorbed by chlorophyll in plants promotes starch and oxygen production from carbon dioxide and water), and are hence termed artificial photosynthesis (Figure 1).Photoexcited charge carriers can also either induce the total oxidation (mineralization) of organic pollutants such as those encountered in aquatic environments, either directly, or through the creation of potent oxidants such as hydroxyl radicals [22].

Photocatalytic Mechanisms
Semiconductor photocatalysis is initiated by exciton formation following photon absorption and the excitation of electrons from the valence band into the conduction band (Step I).The resulting electron-hole pairs may recombine in either the bulk of the semiconductor, or at the surface, with the associated energy released through either fluorescence or thermal excitation of the lattice (Step II); recombination is the primary process that limits photocatalyst efficiency after photon capture.Electrons (and holes) that migrate to the surface of the semiconductor and do not undergo rapid recombination may participate in various oxidation and reduction reactions with adsorbates such as water, oxygen, and other organic or inorganic species (Steps III and IV) [9,10,23,24].These steps are summarized below and illustrated in Figure 2: Step I Light absorption SC h → SC * e h .
Step II Recombination e h → h heat.
Step III Reduction Adsorbate e → Adsorbate .

Photocatalytic Mechanisms
Semiconductor photocatalysis is initiated by exciton formation following photon absorption and the excitation of electrons from the valence band into the conduction band (Step I).The resulting electron-hole pairs may recombine in either the bulk of the semiconductor, or at the surface, with the associated energy released through either fluorescence or thermal excitation of the lattice (Step II); recombination is the primary process that limits photocatalyst efficiency after photon capture.Electrons (and holes) that migrate to the surface of the semiconductor and do not undergo rapid recombination may participate in various oxidation and reduction reactions with adsorbates such as water, oxygen, and other organic or inorganic species (Steps III and IV) [9,10,23,24].These steps are summarized below and illustrated in Figure 2: Step I Light absorption SC + hv → SC× e Oxidation and reduction reactions are fundamental to photocatalytic environmental remediation and solar fuel production, and are ultimately limited by the reduction potential of photoexcited electrons in the conduction band and oxidation potential of photogenerated holes in the valence band.The redox potential, band energies and gap of a semiconductor therefore largely determine the likelihood and rate of charge transfer, and hence are key design parameters for photocatalysts [12,25].Although the underlying physics of space charge carriers and surface-electronic structure of photocatalysts varies between materials and applications, in essence, semiconductor photocatalysis represents interfacial reactions between electrons and holes generated through band gap excitation.Oxidation and reduction reactions are fundamental to photocatalytic environmental remediation and solar fuel production, and are ultimately limited by the reduction potential of photoexcited electrons in the conduction band and oxidation potential of photogenerated holes in the valence band.The redox potential, band energies and gap of a semiconductor therefore largely determine the likelihood and rate of charge transfer, and hence are key design parameters for photocatalysts [12,25].Although the underlying physics of space charge carriers and surfaceelectronic structure of photocatalysts varies between materials and applications, in essence, semiconductor photocatalysis represents interfacial reactions between electrons and holes generated through band gap excitation.

Photocatalytic Materials
The discovery of photocatalytic water splitting over titania electrodes under UV irradiation [5] has led to intensive research into explored H2 production through this approach.Similarly, the first report on the photocatalytic oxidation of cyanide ions over TiO2 powder [26] prompted a rapid expansion in environmental purification research and technologies, particularly for aqueous environments.In both cases, recent research has focused on identifying and developing alternative semiconductors to titania, offering superior performance under solar (rather than UV irradiation) [25].Numerous semiconductors, including ZnO [27], Fe2O3 [28], WO3 [29], SrTiO3 [30], NaTaO3 [31], CdS [32], Ag3PO4 [29], BiPO4 [33], and g-C3N4 [34] are known photocatalysts, with their application dependent on their band gap (Figure 3).Despite a large body of literature, the practical utilization of such photocatalysts for solar fuels production or the degradation of organic pollutants remains a huge challenge due to poor visible light harvesting or efficient conversion of light energy to achieve chemical transformations [13,16,35].

Photocatalytic Materials
The discovery of photocatalytic water splitting over titania electrodes under UV irradiation [5] has led to intensive research into explored H 2 production through this approach.Similarly, the first report on the photocatalytic oxidation of cyanide ions over TiO 2 powder [26] prompted a rapid expansion in environmental purification research and technologies, particularly for aqueous environments.In both cases, recent research has focused on identifying and developing alternative semiconductors to titania, offering superior performance under solar (rather than UV irradiation) [25].Numerous semiconductors, including ZnO [27], Fe 2 O 3 [28], WO 3 [29], SrTiO 3 [30], NaTaO 3 [31], CdS [32], Ag 3 PO 4 [29], BiPO 4 [33], and g-C 3 N 4 [34] are known photocatalysts, with their application dependent on their band gap (Figure 3).Despite a large body of literature, the practical utilization of such photocatalysts for solar fuels production or the degradation of organic pollutants remains a huge challenge due to poor visible light harvesting or efficient conversion of light energy to achieve chemical transformations [13,16,35].

Graphitic Carbon Nitride (g-C3N4)
Solar energy output reaching the Earth's surface is dominated by three regions (Figure 4) of the electromagnetic spectrum, UV (~5%), visible (~45%), and IR (~50%) [36]; visible light photocatalysis therefore offers the best opportunity to obtain maximum solar energy.However, most photocatalysts possess relatively wide band gaps, such as TiO2 (3.0-3.2 eV) and are hence primarily active under UV irradiation (<385 nm) [8].The quest for high performance visible light counterparts is reflected in the

Graphitic Carbon Nitride (g-C 3 N 4 )
Solar energy output reaching the Earth's surface is dominated by three regions (Figure 4) of the electromagnetic spectrum, UV (~5%), visible (~45%), and IR (~50%) [36]; visible light photocatalysis therefore offers the best opportunity to obtain maximum solar energy.However, most photocatalysts possess relatively wide band gaps, such as TiO 2 (3.0-3.2 eV) and are hence primarily active under UV irradiation (<385 nm) [8].The quest for high performance visible light counterparts is reflected in the rapid growth of associated scientific papers and patents [8,10,15] for water splitting, CO 2 reduction and pollutants degradation [35].Graphitic carbon nitride (g-C 3 N 4 ) is a promising metal-free, polymeric semiconductor (Figure 5a) with a narrow band gap suited to visible light absorption (Figure 5b) [34], and amenable to large-scale synthesis.g-C 3 N 4 may also be readily doped or chemically functionalized, permitting tuning of its photophysical properties, and in contrast to many other organic semiconductors, graphitic carbon nitride also exhibits high thermal and chemical stability to oxidation, even at temperatures of 500 • C.There is an extensive literature describing the synthesis of g-C 3 N 4 and its derivatives for various applications [37][38][39][40][41][42][43].This Review focuses on applications in photocatalytic environmental remediation and solar fuel generation, with an emphasis on emerging synthetic strategies to improve the photoactivity of g-C 3 N 4 -based nanostructures through controlling size, morphology, light absorption, charge separation, and ultimately surface reactions.Future research directions are also highlighted.

g-C3N4 Nanostructures: Size and Shape
Engineering materials at the nanoscale is critical to the development of devices for the electronics [44], catalysis [45], biomedical [46], sensing [47], and smart materials [48] sectors, with nanoparticles now in widespread use across science and engineering [48][49][50][51].A number of key aspects differentiate nanomaterials from their bulk analogues.Nanomaterials possess a high surface: bulk atom ratio, which heavily influences their thermodynamic properties resulting in, e.g., melting temperature depression, and elevated solid-solid phase transition temperature.Quantum confinement effects, which influence the electrical and optical properties of nanomaterials, arise from their evolving band structure and the emergence of atomistic like behaviour.Many heterogeneous catalysts exhibit strong size-dependencies due to quantum confinement [52], notably gold [53,54], high surface areas, and the

g-C 3 N 4 Nanostructures: Size and Shape
Engineering materials at the nanoscale is critical to the development of devices for the electronics [44], catalysis [45], biomedical [46], sensing [47], and smart materials [48] sectors, with nanoparticles now in widespread use across science and engineering [48][49][50][51].A number of key aspects differentiate nanomaterials from their bulk analogues.Nanomaterials possess a high surface: bulk atom ratio, which heavily influences their thermodynamic properties resulting in, e.g., melting temperature depression, and elevated solid-solid phase transition temperature.Quantum confinement effects, which influence the electrical and optical properties of nanomaterials, arise from their evolving band structure and the emergence of atomistic like behaviour.Many heterogeneous catalysts exhibit strong size-dependencies due to quantum confinement [52], notably gold [53,54], high surface areas, and the exposure of low-coordination, high energy sites [45,[55][56][57].In concert, these aspects may enhance the rate of interfacial charge transfer from a photocatalyst surface to an adsorbate [58,59].The use of nanostructured g-C 3 N 4 is a fast growing area of photocatalysis research, with nanoparticles, nanorods, nanowires, nanotubes, nanospheres, and particularly nanosheets, demonstrating unique features as components of photocatalyst systems [39].
2-dimensional g-C 3 N 4 : 2D-based materials offer an exceptionally high specific surface area, good crystallinity, rich options for host-guest interactions, maximal light absorption, and improved charge-carrier separation over their 3D analogues [60].Numerous 2D nanomaterials have been reported as heterogeneous catalysts in recent years, with g-C 3 N 4 emerging as one of the most promising photocatalysts.Ping and co-workers developed a facile method to prepare g-C 3 N 4 nanosheets by direct thermal oxidative 'etching' of bulk g-C 3 N 4 under air (Figure 6) [61].In this method, the hydrogen-bond strands of polymeric melon units which form the interlayers, are gradually removed by oxidation such that the thickness of bulk g-C 3 N 4 can be reduced to the desired nanoscale by controlling the etch time, and hence represents a simple, low-cost, and scalable synthesis.The resultant nanosheets exhibit a blue shift of the intrinsic absorption edge in their UV-vis spectra relative to the bulk.The increase in band gap of nanosheets (2.97 eV; Figure 7A) relative to their bulk counterpart (2.77 eV) is further confirmed by a blue shift in the fluorescence emission spectrum of 20 nm (Figure 7B).This widening of the band gap reflects quantum confinement which raises and lowers the conduction and valence band edges respectively [62].Electronic properties of the nanosheets were determined from the corresponding I−V curve, semiconducting characteristics observed for single g-C 3 N 4 nanosheets, suggesting electron transport within the nanosheet plane.In contrast, no current was detected for bulk particle under an applied bias spanning −10-+10 V, evidencing extremely poor electronic conductivity for bulk g-C 3 N 4 .The lifetime of charge carriers in the nanosheets from time-resolved fluorescence decay spectra also exceeded that of bulk g-C 3 N 4 .
to their bulk counterpart (2.77 eV) is further confirmed by a blue shift in the fluorescence emission spectrum of 20 nm (Figure 7B).This widening of the band gap reflects quantum confinement which raises and lowers the conduction and valence band edges respectively [62].Electronic properties of the nanosheets were determined from the corresponding I−V curve, semiconducting characteristics observed for single g-C3N4 nanosheets, suggesting electron transport within the nanosheet plane.In contrast, no current was detected for bulk particle under an applied bias spanning −10-+10 V, evidencing extremely poor electronic conductivity for bulk g-C3N4.The lifetime of charge carriers in the nanosheets from time-resolved fluorescence decay spectra also exceeded that of bulk g-C3N4.Xiaodong and co-workers developed a different liquid exfoliation strategy as a low-cost and green route to ultrathin g-C3N4 nanosheets from bulk g-C3N4 in water, illustrated in Figure 8 [63].From a range of solvents, water effectively exfoliated the g-C3N4 into ultrathin nanosheets, possibly reflecting its high polarity.The morphology of the exfoliated g-C3N4 showed free-standing nanosheets 120 nm across that were almost transparent, and displayed a well-defined Tyndall effect in solution (Figure 8 inset) indicating the presence of monodisperses ultrathin nanosheets.These g-C3N4 nanosheets were very stable in acidic and alkaline environments, but exhibited pH-dependent photoluminescence.The g-C3N4 nanosheets show superior photoabsorption to the bulk counterpart, resulting in an extremely high PL quantum yield of up to 19.6%.Liquid exfoliation of g-C3N4 in isopropanol [64] and methanol [65] resulted in nanosheets with improved photocatalytic performance for the degradation of organic pollutants relative to bulk g-C3N4.Xiaodong and co-workers developed a different liquid exfoliation strategy as a low-cost and green route to ultrathin g-C 3 N 4 nanosheets from bulk g-C 3 N 4 in water, illustrated in Figure 8 [63].From a range of solvents, water effectively exfoliated the g-C 3 N 4 into ultrathin nanosheets, possibly reflecting its high polarity.The morphology of the exfoliated g-C 3 N 4 showed free-standing nanosheets 120 nm across that were almost transparent, and displayed a well-defined Tyndall effect in solution (Figure 8 inset) indicating the presence of monodisperses ultrathin nanosheets.These g-C 3 N 4 nanosheets were very stable in acidic and alkaline environments, but exhibited pH-dependent photoluminescence.
The g-C 3 N 4 nanosheets show superior photoabsorption to the bulk counterpart, resulting in an extremely high PL quantum yield of up to 19.6%.Liquid exfoliation of g-C 3 N 4 in isopropanol [64] and methanol [65] resulted in nanosheets with improved photocatalytic performance for the degradation of organic pollutants relative to bulk g-C 3 N 4 .
Xiaodong and co-workers developed a different liquid exfoliation strategy as a low-cost and green route to ultrathin g-C3N4 nanosheets from bulk g-C3N4 in water, illustrated in Figure 8 [63].From a range of solvents, water effectively exfoliated the g-C3N4 into ultrathin nanosheets, possibly reflecting its high polarity.The morphology of the exfoliated g-C3N4 showed free-standing nanosheets 120 nm across that were almost transparent, and displayed a well-defined Tyndall effect in solution (Figure 8 inset) indicating the presence of monodisperses ultrathin nanosheets.These g-C3N4 nanosheets were very stable in acidic and alkaline environments, but exhibited pH-dependent photoluminescence.The g-C3N4 nanosheets show superior photoabsorption to the bulk counterpart, resulting in an extremely high PL quantum yield of up to 19.6%.Liquid exfoliation of g-C3N4 in isopropanol [64] and methanol [65] resulted in nanosheets with improved photocatalytic performance for the degradation of organic pollutants relative to bulk g-C3N4.

1-dimensional g-C3N4:
In recent years, 1D nanostructures have attracted interest as photocatalysts due to their unique morphology and photophysical properties [66,67], and hence there is interest in preparing 1D g-C3N4.1D g-C3N4 nanorods with different aspect ratios were prepared by the reflux of g-C3N4 nanoplates as a function of solvent and reflux time [68].The transformation from g-C3N4 nanoplates to nanorods reflects an exfoliation and subsequent re-growth process, which results in 'rolling-up' of individual nanosheets into rods (Figure 9a).The photocatalytic activity of the as-prepared nanorods for methylene blue (MB) degradation in water was explored under visible

1-dimensional g-C 3 N 4 :
In recent years, 1D nanostructures have attracted interest as photocatalysts due to their unique morphology and photophysical properties [66,67], and hence there is interest in preparing 1D g-C 3 N 4 .1D g-C 3 N 4 nanorods with different aspect ratios were prepared by the reflux of g-C 3 N 4 nanoplates as a function of solvent and reflux time [68].The transformation from g-C 3 N 4 nanoplates to nanorods reflects an exfoliation and subsequent re-growth process, which results in 'rolling-up' of individual nanosheets into rods (Figure 9a).The photocatalytic activity of the as-prepared nanorods for methylene blue (MB) degradation in water was explored under visible light (λ > 420 nm) and simulated solar irradiation (λ > 290 nm).The resulting photocatalytic activity and photocurrent response of g-C 3 N 4 nanorods under visible and solar light were about 50-100% greater than the g-C 3 N 4 nanoplates.Reprinted with permission from [68].Copyright American Chemical Society, 2013.
Zhihong and co-workers demonstrated a large-scale synthesis of well-aligned g-C3N4 nanorods via the reactive thermolysis of mechanically activated molecular precursors, C3N6H6 and C3N3Cl3, under heat treatment [69].These nanorods exhibit peculiar optical properties, evidenced by PL emission and UV-vis absorption.Uniform g-C3N4 nanorods were also synthesized via a template of monodispersed, chiral, mesostructured silica nanorods, which were easily prepared via ammoniacatalyzed hydrolysis of tetraethyl orthosilicate with F127 and cetyltrimethylammonium bromide (CTAB) surfactants [70].The one-dimensional, hexagonal mesostructure of the porous silica Zhihong and co-workers demonstrated a large-scale synthesis of well-aligned g-C 3 N 4 nanorods via the reactive thermolysis of mechanically activated molecular precursors, C 3 N 6 H 6 and C 3 N 3 C l3 , under heat treatment [69].These nanorods exhibit peculiar optical properties, evidenced by PL emission and UV-vis absorption.Uniform g-C 3 N 4 nanorods were also synthesized via a template of monodispersed, chiral, mesostructured silica nanorods, which were easily prepared via ammonia-catalyzed hydrolysis of tetraethyl orthosilicate with F127 and cetyltrimethylammonium bromide (CTAB) surfactants [70].The one-dimensional, hexagonal mesostructure of the porous silica nanorods enabled carbon nitride condensation within the pores.The resulting g-C 3 N 4 nanorods demonstrated a high photocatalytic activity in hydrogen evolution from water in the presence of triethanolamine and 1 wt % Pt as a co-catalyst compared to that obtained with a conventional g-C 3 N 4 [71].Porous g-C 3 N 4 nanorods were also prepared by direct calcination of hydrous melamine nanofibers, precipitated from an aqueous solution of melamine [72].Porosity provided an enhanced interfacial area for catalysis.Oxygen atoms doped into the g-C 3 N 4 matrix altered the band structure, resulting in more effective separation of electron/hole pairs and a corresponding excellent visible light photocatalytic activity for hydrogen evolution in the presence of triethanolamine as a hole quencher.A simple wet-chemical route was also reported for the preparation of nanofiber-like g-C 3 N 4 structures with an average diameter of several nm and 100 nm in length [73].The g-C 3 N 4 nanofibers exhibited a high surface area, and low density of crystalline defects, with a slight blue shift of 0.13 eV compared to bulk g-C 3 N 4 , possibly due to more perfect packing, electronic coupling, and quantum confinement effects.The catalytic activity of g-C 3 N 4 nanofibers for Rhodamine B photodegradation was much higher than that of bulk g-C 3 N 4 , with the nanofibers also exhibiting superior stability.An alternative approach to the synthesis of g-C 3 N 4 nanotubes adopted the direct heating of melamine, packed into a compact configuration to favour tubular structures (Figure 10a-d) [74].This route was advantageous since it required no additional organic templates, facilitating commercial, low-cost and large-scale application.The resulting g-C 3 N 4 showed intense fluorescence around 460 nm, and hence has potential application as a blue light fluorescence material.These g-C 3 N 4 nanotubes exhibited better visible light photocatalytic activity for MB degradation than either bulk g-C 3 N 4 or a p25 TiO 2 reference (the latter is unsurprising since pure titania is a UV band gap material).Muhammad and co-workers also prepared tubular g-C 3 N 4 by pre-treating melamine with HNO 3 before thermal processing [75].The g-C 3 N 4 nanotubes were again active for MB and methylene orange (MO) degradation under visible light, and were more stable than bulk g-C 3 N 4 ; the superior activity attributed to the higher surface area (182 m 2 •g −1 ) of the tubes and improved light absorption and charge separation/transfer of electron-hole pairs.g-C 3 N 4 nanotubes can also be obtained through rolling-up nanosheets via a simple water-induced morphological transformation [76], avoiding the use of organic solvents and hence promoting green chemical principles.
Catalysts 2018, 8, x FOR PEER REVIEW 9 of 47 and charge separation/transfer of electron-hole pairs.g-C3N4 nanotubes can also be obtained through rolling-up nanosheets via a simple water-induced morphological transformation [76], avoiding the use of organic solvents and hence promoting green chemical principles.Ribbon-like g-C3N4 nanostructures have been prepared employing dicyandiamide (DCDA) and NaCl crystals as structure-directing agents [77], with a possible mechanism shown in Figure 11.These ribbon-like g-C3N4 nanostructures exhibit interesting optical and electronic properties, including a large blue shift in their absorption spectrum corresponding to an increased band gap from 2.7 eV to 3.0 eV.The latter may reflect the incorporation of some Na + ions within the nitride pores, and functionalization by cyano groups.The ribbon-like g-C3N4 emits blue light at around 440 nm under 365 nm excitation, whereas bulk g-C3N4 exhibited a broad emission spanning 460-520 nm, i.e., yellowgreen light.Unfortunately, these ribbon-like g-C3N4 nanostructures have not yet been tested for as Ribbon-like g-C 3 N 4 nanostructures have been prepared employing dicyandiamide (DCDA) and NaCl crystals as structure-directing agents [77], with a possible mechanism shown in Figure 11.These ribbon-like g-C 3 N 4 nanostructures exhibit interesting optical and electronic properties, including a large blue shift in their absorption spectrum corresponding to an increased band gap from 2.7 eV to 3.0 eV.The latter may reflect the incorporation of some Na + ions within the nitride pores, and functionalization by cyano groups.The ribbon-like g-C 3 N 4 emits blue light at around 440 nm under 365 nm excitation, whereas bulk g-C 3 N 4 exhibited a broad emission spanning 460-520 nm, i.e., yellow-green light.Unfortunately, these ribbon-like g-C 3 N 4 nanostructures have not yet been tested for as photocatalysts.Ribbon-like g-C3N4 nanostructures have been prepared employing dicyandiamide (DCDA) and NaCl crystals as structure-directing agents [77], with a possible mechanism shown in Figure 11.These ribbon-like g-C3N4 nanostructures exhibit interesting optical and electronic properties, including a large blue shift in their absorption spectrum corresponding to an increased band gap from 2.7 eV to 3.0 eV.The latter may reflect the incorporation of some Na + ions within the nitride pores, and functionalization by cyano groups.The ribbon-like g-C3N4 emits blue light at around 440 nm under 365 nm excitation, whereas bulk g-C3N4 exhibited a broad emission spanning 460-520 nm, i.e., yellowgreen light.Unfortunately, these ribbon-like g-C3N4 nanostructures have not yet been tested for as photocatalysts.

0-dimensional g-C3N4:
0D materials such as quantum dots are of great interest in photocatalysis [78].g-C3N4 quantum dots have been prepared from bulk g-C3N4 by thermochemical etching [74].This tunable multi-step preparation involves thermal exfoliation of 3D bulk g-C3N4 into 2D nanosheets, followed by acid etching with concentrated H2SO4 and HNO3 to produce 1D nanoribbons.In this second step, some C-N bonds which connect the tri-s-triazine units are oxidized, resulting in the introduction of oxygenate functional groups, such as carboxylates, at edges and on the basal plane.Cleavage of the nanosheets along preferential orientations yields nanoribbons with diameters <10 nm and several tens of nm in length.In a final step, nanoribbons are converted to 0D quantum dots of 5-9 nm across by hydrothermal treatment (Figure 12) that are highly soluble in water, and stable in solution under ambient conditions for almost eight months.These quantum dots exhibited light 'up-conversion' when excited by long wavelength light, for example, irradiation with 0-dimensional g-C 3 N 4 : 0D materials such as quantum dots are of great interest in photocatalysis [78].g-C 3 N 4 quantum dots have been prepared from bulk g-C 3 N 4 by thermochemical etching [74].This tunable multi-step preparation involves thermal exfoliation of 3D bulk g-C 3 N 4 into 2D nanosheets, followed by acid etching with concentrated H 2 SO 4 and HNO 3 to produce 1D nanoribbons.In this second step, some C-N bonds which connect the tri-s-triazine units are oxidized, resulting in the introduction of oxygenate functional groups, such as carboxylates, at edges and on the basal plane.Cleavage of the nanosheets along preferential orientations yields nanoribbons with diameters <10 nm and several tens of nm in length.In a final step, nanoribbons are converted to 0D quantum dots of 5-9 nm across by hydrothermal treatment (Figure 12) that are highly soluble in water, and stable in solution under ambient conditions for almost eight months.These quantum dots exhibited light 'up-conversion' when excited by long wavelength light, for example, irradiation with 705-862 nm light resulted in 350-600 nm emission, encompassing a large portion of the visible-light spectrum.This up-conversion was proposed to occur via a multiphoton process involving anti-Stokes photoluminescence.The ability of g-C 3 N 4 quantum dots to convert NIR to visible light renders them a promising universal energy transfer component in a photocatalytic system, able to harness long wavelength solar energy.This was demonstrated for water splitting, in which quantum dots were added to promote photocatalytic H 2 production by platinized bulk g-C 3 N 4 and P25, with dramatic rate-enhancements (up to 52-fold) observed for the latter under visible light irradiation in the presence of a methanol sacrificial hole scavenger.Single layered g-C 3 N 4 quantum dots were also prepared by Guoping and co-workers, although in this instance for two-photon fluorescence imaging of cellular nucleus [42].They again adopted a multi-step synthesis involving acid treatment of bulk g-C 3 N 4 to form a porous material and subsequently ultrathin nanosheets, with subsequent ammonia addition, hydrothermal treatment, and ultrasonication of the porous g-C 3 N 4 nanosheets liberating aqueous suspensions of g-C 3 N 4 quantum dots.
presence of a methanol sacrificial hole scavenger.Single layered g-C3N4 quantum dots were also prepared by Guoping and co-workers, although in this instance for two-photon fluorescence imaging of cellular nucleus [42].They again adopted a multi-step synthesis involving acid treatment of bulk g-C3N4 to form a porous material and subsequently ultrathin nanosheets, with subsequent ammonia addition, hydrothermal treatment, and ultrasonication of the porous g-C3N4 nanosheets liberating aqueous suspensions of g-C3N4 quantum dots.3D-dimensional g-C3N4: 3D nanomaterials unlock a vast and complex design space for constructing novel and efficient photocatalytic systems [79], such as hierarchical 3D nanoporous g-C3N4 microspheres using a template-free solvothermal synthesis [80].In this example, a two-step synthesis was adopted: (i) amorphous and nanoporous g-C3N4 microspheres were prepared from melamine and cyanuric chloride in acetonitrile; and (ii) subsequently subjected to thermal processing at 550 °C under argon to transform the amorphous microspheres into hierarchical g-C3N4 microspheres (Figure 13).Surprisingly, the hierarchical g-C3N4 microspheres exhibited a red-shift relative to the bulk counterpart, and uncalcined microspheres, attributed to the high degree of condensation and packing between the layers within the microspheres.The photoluminescence emission intensity of hierarchical g-C3N4 microspheres was low compared to bulk and uncalcined g-C3N4 microspheres indicating that calcination suppresses radiative charge recombination in the hierarchical structure.These porous g-C3N4 microspheres also exhibit a narrowed band gap (2.42 eV), lower electrical resistance and a higher photoresponse than the bulk material, facilitating visible-light harvesting and more efficient transport and separation of photo-induced charge carriers.Hierarchical g-C3N4 nanospheres, comprised of nanosheet assemblies, were also prepared by Jinshui and coworkers, but employing high area silica nanospheres as sacrificial templates [81].The silica template offered efficient cyanamide adsorption, and a framework for the formation of interconnected 2D g-C3N4 nanosheets during self-polymerization on heating.The excellent thermal and mechanical stability of silica spheres enabled high temperature construction of the hierarchical g-C3N4 nanospheres, and could subsequently be removed through etching by NH4HF2 solution, with the hierarchical g-C3N4 retaining a spherical morphology.These hierarchal nanospheres are constructed of flat nanosheets emanating from the center (sphere surface) and then interconnecting to form a mesoporous shell (Figure 14), this structure may favour both charge separation and mass transport 3D-dimensional g-C 3 N 4 : 3D nanomaterials unlock a vast and complex design space for constructing novel and efficient photocatalytic systems [79], such as hierarchical 3D nanoporous g-C 3 N 4 microspheres using a template-free solvothermal synthesis [80].In this example, a two-step synthesis was adopted: (i) amorphous and nanoporous g-C 3 N 4 microspheres were prepared from melamine and cyanuric chloride in acetonitrile; and (ii) subsequently subjected to thermal processing at 550 • C under argon to transform the amorphous microspheres into hierarchical g-C 3 N 4 microspheres (Figure 13).Surprisingly, the hierarchical g-C 3 N 4 microspheres exhibited a red-shift relative to the bulk counterpart, and uncalcined microspheres, attributed to the high degree of condensation and packing between the layers within the microspheres.The photoluminescence emission intensity of hierarchical g-C 3 N 4 microspheres was low compared to bulk and uncalcined g-C 3 N 4 microspheres indicating that calcination suppresses radiative charge recombination in the hierarchical structure.These porous g-C 3 N 4 microspheres also exhibit a narrowed band gap (2.42 eV), lower electrical resistance and a higher photoresponse than the bulk material, facilitating visible-light harvesting and more efficient transport and separation of photo-induced charge carriers.Hierarchical g-C 3 N 4 nanospheres, comprised of nanosheet assemblies, were also prepared by Jinshui and co-workers, but employing high area silica nanospheres as sacrificial templates [81].The silica template offered efficient cyanamide adsorption, and a framework for the formation of interconnected 2D g-C 3 N 4 nanosheets during self-polymerization on heating.The excellent thermal and mechanical stability of silica spheres enabled high temperature construction of the hierarchical g-C 3 N 4 nanospheres, and could subsequently be removed through etching by NH 4 HF 2 solution, with the hierarchical g-C 3 N 4 retaining a spherical morphology.These hierarchal nanospheres are constructed of flat nanosheets emanating from the center (sphere surface) and then interconnecting to form a mesoporous shell (Figure 14), this structure may favour both charge separation and mass transport in photocatalysis.The nanospheres had a wider band gap than bulk g-C 3 N 4 , possibly due to quantum size effects, but superior light harvesting across the optical spectrum, especially between 430-590 nm.This may arise from multiple reflections (and hence opportunities for absorption) of incident light within the hierarchical architectures, and/or presence of a high density of defective sites associated with exposure of low-coordination sites at the 'sharp' edges of the constituent nanosheets.
Hollow g-C 3 N 4 : Hollow nanostructures are another promising morphology for energy storage and conversion applications, with significant research efforts devoted to the design and synthesis of hollow nanostructures with high complexity through manipulating their geometry, chemical composition, building blocks, and interior architecture to, e.g., enhance their electrochemical performance [82,83].Hollow g-C 3 N 4 nanospheres have been synthesized using silica nanoparticles as templates [83,84].Careful control over the shell thickness of such polymeric g-C 3 N 4 hollow nanospheres prevents deformation of the core-shell arrangement (Figure 15), even after 400 • C processing.Although a blue shift in the band gap accompanying their synthesis is undesirable, and attributed to either quantum effects or enhanced H-type interlayer packing, further chemical methods, such as extending the pi system by anchoring aromatic motifs, exist to improve visible light absorption, for example, through co-polymerization; such chemical modification and extended p-conjugation can red-shift optical absorption, and improve charge separation in the shell, without damaging the hollow polymeric architectures [85].This strategy has been adopted to tune the semiconductor properties of the shell in the hollow g-C 3 N 4 nanospheres to enhance photocatalytic activity for hydrogen evolution under visible light.A simple, molecular cooperative assembly of low cost triazine molecules into hollow g-C 3 N 4 is also reported by Young-Si et al. [86], with this precursor enabling simultaneous optimization of the textural and photophysical properties of g-C 3 N 4 .
Catalysts 2018, 8, x FOR PEER REVIEW 11 of 47 in photocatalysis.The nanospheres had a wider band gap than bulk g-C3N4, possibly due to quantum size effects, but superior light harvesting across the optical spectrum, especially between 430-590 nm.This may arise from multiple reflections (and hence opportunities for absorption) of incident light within the hierarchical architectures, and/or presence of a high density of defective sites associated with exposure of low-coordination sites at the 'sharp' edges of the constituent nanosheets.conjugation can red-shift optical absorption, and improve charge separation in the shell, without damaging the hollow polymeric architectures [85].This strategy has been adopted to tune the semiconductor properties of the shell in the hollow g-C3N4 nanospheres to enhance photocatalytic activity for hydrogen evolution under visible light.A simple, molecular cooperative assembly of low cost triazine molecules into hollow g-C3N4 is also reported by Young-Si et al. [86], with this precursor enabling simultaneous optimization of the textural and photophysical properties of g-C3N4.

Mesoporous g-C3N4:
Mesoporous photocatalysts have attracted attention for their (comparatively) high quantum efficiency associated with high surface areas, superior molecular mass transport in-pore [87], and opportunities for enhanced light harvesting through the internal scattering of incident light.An atomically thin mesoporous mesh of g-C3N4 nanosheets was recently prepared by solvothermal synthesis (Figure 16) which exhibits outstanding photocatalytic activity for H2 production [88].

Solar Fuel Generation
Solar fuels production from CO 2 and water via artificial photosynthesis is one of the promising strategies to deliver H 2 , syngas and hydrocarbons as sustainable energy and chemical feedstocks [19].g-C 3 N 4 offers the promise of metal-free and scalable photocatalysts for visible light use.

H 2 Evolution
Hydrogen is one of the most promising alternative energy sources to fossil fuels; however, the large energy barrier to water splitting still presents a challenge to practical photocatalytic systems [35]: Advanced materials are hence sought that are amenable to harnessing sunlight for either direct photochemical, or photoelectrochemical water splitting.For photocatalytic water splitting, the conduction band (CB) energy must be sufficiently negative (relative to normal hydrogen electrode (NHE)) such that photoexcited electrons are sufficiently energetic to reduce water [94,95]: The redox potential for the overall reaction at pH = 7, E H = −1.23 V (NHE), with the corresponding half-reactions of −0.41 V (Equation ( 4)) and 0.82 V, giving an overall ∆G 0 = +237 kJ•mol −1 .
Most single component photocatalysts exhibit poor activity for visible light-driven H 2 production.However, the combination of g-C 3 N 4 with a metal co-catalyst and hole scavenger can afford high visible-light photoactivity.Shubin and co-workers [64] prepared g-C 3 N 4 nanosheets by thermal exfoliation which demonstrated a superior hydrogen production from a water/triethanolamine solution relative to the bulk nitride; nanosheets with a thickness as low as 2 nm were optimal, achieving rate-enhancements of 5.5-and 3-fold under UV-vis and visible light irradiation respectively.Single atomic layer g-C 3 N 4 nanosheets prepared by a chemical exfoliation [96] also display better photogenerated charge transport and separation than bulk g-C 3 N 4 , presumably due to the improved H 2 evolution.Atomically thin, mesoporous g-C 3 N 4 nanomesh prepared by solvothermal routes exhibits an exceptional photocatalytic activity for H 2 evolution [88] of 8510 µmol•h −1 •g −1 (with an apparent quantum efficiency of 5.1% at 420 nm), far higher than the 1560 µmol•h −1 •g −1 achieved over non-porous 2D g-C 3 N 4 nanostructures or 350 µmol•h −1 •g −1 observed for bulk g-C 3 N 4 (apparent quantum efficiency 3.75% at 420 nm); the porous g-C 3 N 4 nanomesh possessed a high surface area and better alignment of conduction and valence band edges.g-C 3 N 4 nanorods also show high photocatalytic activity for hydrogen production from water in the presence of triethanolamine (TEOA) and a 1 wt % Pt co-catalyst [68] wherein the platinum nanoparticles uniformly decorate the g-C 3 N 4 nanorods.Such materials are also superior to mesoporous analogues [70].TEOA is the most common hole scavenger for g-C 3 N 4 photocatalysts wherein it confers superior activity to methanol (a 14-fold rate enhancement); although the origin of this difference remains poorly understood, Jones and co-workers speculated that the nitrogen lone pair is responsible for the enhanced activity [62].P25 also exhibited superior activity for photocatalytic hydrogen production when TEOA was employed as a hole scavenger (versus methanol), albeit to a lesser extent than for carbon nitride.g-C 3 N 4 nanotubes synthesized through a rolling-up mechanism by water-induced morphological transformation also display superior visible-light H 2 production bulk g-C 3 N 4 or g-C 3 N 4 nanosheets [76].g-C 3 N 4 quantum dots [77] prepared from bulk g-C 3 N 4 by thermochemical etching were three times more active than bulk g-C 3 N 4 under visible irradiation when promoted by 1 wt % Pt and using 10% triethanolamine as a sacrificial agent, possibly due to up-conversion of NIR to visible light and concomitant increased light harvesting.Tuning of the electronic band structure of g-C 3 N 4 quantum dots [97] to optimize their visible or NIR light response, further enhances photocatalytic H 2 evolution.P-doped g-C 3 N 4 nanosheets also exhibit promising visible-light photocatalytic H 2 productivity of 1596 mmol•h −1 •g −1 (apparent quantum efficiency of 3.56% at 420 nm) superior to other metal-free g-C 3 N 4 nanosheet photocatalysts [98].The excellent photocatalytic activity originates from P-doped macroporous analogues arises from empty mid-gap states (−0.16 V vs. NHE) which extend light harvesting up to 557 nm.Macropores also increased the surface area to 123 m 2 •g −1 , and shortened the charge-to-surface migration length to only 5-8 nm.
Hierarchically 3D nanoporous g-C 3 N 4 microspheres [80] have also been exploited for water splitting in aqueous solution with 15 triethanolamine and 3 wt % Pt as a co-catalyst under visible light.These g-C 3 N 4 microspheres showed H 2 productivity 2.5 times higher than that of bulk g-C 3 N 4 , and good stability over five consecutive recycles.Hierarchical g-C 3 N 4 nanospheres [81] comprised of nanosheets with 3 wt % Pt co-catalyst showed significant improvements in H 2 production, with an apparent quantum yield of 9.6% at 420 nm, far superior to that for individual g-C 3 N 4 nanosheets of 3.75%.Monodispersed, hollow g-C 3 N 4 nanospheres are also reported to exhibit high photoactivity for water splitting, and a high apparent quantum yield of 7.5% [84].H 2 evolution over these hollow g-C 3 N 4 spheres was significantly enhanced by addition of a MoS 2 co-catalyst, with the formation of the MoS 2 /g-C 3 N 4 heterojunctions (Figure 17 g-C3N4 has also been coupled with semiconductors and metal nanoparticles that exhibit visible light surface plasmon resonances to extend their spectral range.Such heterojunction materials offer enhanced separation of photoexcited charge carriers, and hence suppressed recombination and energy loss through fluorescence [39,91].Noble metal-promoted g-C3N4 offers improved UV and visible light harvesting, fast molecular diffusion, and a high density of photoactive sites [100-102].TiO2/g-C3N4 heterojunctions have been fabricated by a two-step hydrothermal-calcination route from melamine, followed by an in-situ solid-state reaction [103].The resulting TiO2/g-C3N4 g-C 3 N 4 has also been coupled with semiconductors and metal nanoparticles that exhibit visible light surface plasmon resonances to extend their spectral range.Such heterojunction materials offer enhanced separation of photoexcited charge carriers, and hence suppressed recombination and energy loss through fluorescence [39,91].Noble metal-promoted g-C 3 N 4 offers improved UV and visible light harvesting, fast molecular diffusion, and a high density of photoactive sites [100][101][102].TiO 2 /g-C 3 N 4 heterojunctions have been fabricated by a two-step hydrothermal-calcination route from melamine, followed by an in-situ solid-state reaction [103].The resulting TiO 2 /g-C 3 N 4 heterostructures possess a narrow band gap and good photoactivity (556 µmol −1 •g −1 ) for H 2 evolution under visible-light irradiation compared to pure g-C 3 N 4 (108 µmol•h −1 •g −1 ) or TiO 2 (130 µmol•h −1 •g −1 .Core@shell heterojunction nanocomposites have additional advantages due to a high interfacial contact area between the shell and core components [104].For example, CdS@g-C 3 N 4 core/shell nanowires [104] with different g-C 3 N 4 contents were prepared by a combined solvothermal and chemisorption method (Figure 18) in which g-C 3 N 4 uniformly adsorbs over CdS nanowires resulting in enhanced improved photocatalytic H 2 production of 4152 µmol•h -1 •g -1 for 2 wt % g-C 3 N 4 .A one-step self-assembly route was recently developed to fabricate core-shell architecture comprising carbon spheres decorated by g-C 3 N 4 .These composites showed extended light absorption and high mechanical stability, with enhanced conductivity for charge transport [105], delivering hydrogen evolution rates of 129 mol•h −1 , and 8-fold improvement over pristine g-C 3 N 4 (16 mol•h −1 ).Other g-C 3 N 4 nanocomposites were investigated with a range of materials and morphologies [39,82,91,, to access different charge transfer mechanisms between g-C 3 N 4 and the other components.These include a g-C 3 N 4 -based type II heterojunction [103], g-C 3 N 4 -based p-n heterojunction [91,129], g-C 3 N 4 -based Z-scheme heterojunction [113,130], g-C 3 N 4 /metal heterojunction [100,102], and a g-C 3 N 4 /carbon heterojunction [131].The design of g-C 3 N 4 heterojunction photocatalysts is an attractive strategy to tune the electronic structure and redox potentials for visible-light absorption photocatalytic H 2 generation.Table 1 compares the performance of different g-C 3 N 4 photocatalysts.Reprinted with permission from [104].Copyright 2013 American Chemical Society.

CO2 Reduction
Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the continued reliance on the use of fossil fuels for both energy and chemicals production [3,163], to which the photocatalytic reduction of CO2 to light oxygenates and hydrocarbons could provide a sustainable solution.CO2 reduction involves multi-electron transfer and hence the reaction kinetics for, e.g., formic acid, carbon monoxide, formaldehyde, methanol and

CO 2 Reduction
Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the continued reliance on the use of fossil fuels for both energy and chemicals production [3,163], to which the photocatalytic reduction of CO 2 to light oxygenates and hydrocarbons could provide a sustainable solution.CO 2 reduction involves multi-electron transfer and hence the reaction kinetics for, e.g., formic acid, carbon monoxide, formaldehyde, methanol and methane production are intrinsically slower than for H 2 production.CO 2 photoreduction begins with molecular adsorption at the catalyst surface, wherein the anion radical is generated by the transfer of electrons photoexcited across the semiconductor band gap following light absorption.In the case of aqueous phase CO 2 reduction, charge-compensation occurs through concomitant water splitting and the transfer of photoexcited holes in the valence band onto hydrogen atoms, with the resulting protons migrating to the CO 2 anion.The reduction potentials for CO 2 photoreduction with water to various products are described below (relative to NHE at pH = 7) [11,164]: Key factors influencing CO 2 photocatalytic reduction include band energy matching, efficient charge-carrier separation, kinetic of e-and hole transfer to CO 2 and the reductant, and the basicity of the photocatalyst and hence strength and coverage of CO 2 adsorption [164].In recent years, the g-C 3 N 4 nanostructured materials have been studied for CO 2 photoreduction [92,165], due to their excellent stability, sufficiently negative CB energy and narrow band gap.Many strategies are reported to promote g-C 3 N 4 with condensed matter and molecular sensitizers [166,167], such as doping with metals [168,169] and non-metal [170][171][172], heterojunction construction [173][174][175][176] and Z-scheme composites employing co-catalysts [165][166][167]173,175]. Pengfei et al. reported ultrathin C 3 N 4 nanosheets for enhanced photocatalytic CO 2 reduction [177] in which surface functionalization and textural modification by NH 3 -mediated thermal exfoliation enhanced light harvesting, charge-carrier redox potentials, and the surface area for CO 2 adsorption (to 0.2 mmol•g −1 ), resulting in CH 4 and CH 3 OH productivities of 1.39 and 1.87 µmol•h −1 •g −1 respectively, a five-fold increase over bulk g-C 3 N 4 .Jiaguo and co-workers [168] demonstrated that Pt promotion significantly influenced both the activity and selectivity of g-C 3 N 4 for CO 2 photoreduction to CH 4 , CH 3 OH, and HCHO; Pt nanoparticles improved charge separation across the metal/semiconductor interface, and lowered the overpotential for CO 2 reduction.Qingqing et al. reported Pd nanoicosahedrons with twin defects promoted CO 2 reduction into CO and CH 4 over C 3 N 4 nanosheets [126].CO 2 conversion reached 61.4%, with an average CO productivity of 4.3 µmol•g −1 •h −1 and average CH 4 productivity of 0.45 µmol•g −1 •h −1 , indicating the presence of highly reactive sites for CO 2 adsorption and activation.
Hierarchical, porous O-doped g-C 3 N 4 nanotubes prepared via successive thermal oxidation exfoliation and condensation of bulk g-C 3 N 4 also show promise for photocatalytic CO 2 reduction under visible light [171].As-prepared O-doped g-C 3 N 4 nanotubes comprise interconnected, multi-walled nanotubes with uniform diameters of 20-30 nm, which evolve methanol at 0.88 µmol•g −1 •h −1 , five times faster than bulk g-C 3 N 4 (0.17 µmol•g −1 •h −1 ).Heterojunction composites of g-C 3 N 4 /ZnO synthesized by a one-step calcination route [165] are also superior to bulk g-C 3 N 4 (2.5-fold enhancement), ascribed to a direct Z-scheme mechanism reflecting efficient ZnO → g-C 3 N 4 electron transfer occurring the interface.Zhongxing et al. reported that CeO 2 -modified C 3 N 4 photocatalysts produced by a simple hydrothermal route were effective for the selective photocatalytic reduction of CO 2 to CH 4 [178], with a CH 4 productivity of 4.79 mmol•g −1 •h −1 , about 3.44 times that of g-C 3 N 4 .Wang et al. prepared a 2D-2D MnO 2 /g-C 3 N 4 heterojunction photocatalyst by an in-situ redox reaction between KMnO 4 and MnSO 4 adsorbed at the surface of g-C 3 N 4 [179] for photocatalytic CO 2 reduction to CO (9.6 mmol•g −1 ), in which band matching facilitated efficient separation of photogenerated charge-carriers .Photocatalytic CO 2 reduction reaction is also reported over a direct Z-scheme g-C 3 N 4 /SnS 2 catalyst [180] which yielded both CH 3 OH (2.3 µmol•g −1 ) and CH 4 (0.64 µmol•g −1 ), with electrons in SnS 2 combining with holes in g-C 3 N 4 .Another Z-scheme mechanism is invoked for a MoO 3 /g-C 3 N 4 composite [181].Ryo and co-workers adopted a different approach, attaching Ru(bipy)complexes to g-C 3 N 4 nanostructures; these displayed improved activity for CO 2 photoreduction to formic acid, with a high apparent quantum yield of 5.7% at 400 nm under visible light (Figure 19).Anchoring of polyoxometalate clusters to C 3 N 4 also creates active photocatalysts for CO 2 reduction [179].Here, noble-metal-free Co 4 polyoxometallates were used to achieve a staggered band alignment, with the Co4@g-C 3 N 4 hybrid photocatalysts achieving 107 µmol•g −1 •h −1 and 94% selectivity for CO production under visible light (λ ≥ 420 nm); cumulative CO production reached 896•µmol•g −1 after 10 h irradiation, far exceeding that for unpromoted g-C 3 N 4 .A multicomponent heterostructure, termed an intercorrelated superhybrid, comprising AgBr supported on g-C3N4 decorated in turn on N-doped graphene (prepared by wet-chemical synthesis) has also shown excellent activity for the photocatalytic reduction of CO2 to methanol and ethanol (Figure 20) [174].Oluwatobi et al. reported g-C3N4/(Cu/TiO2) [182] nanocomposites prepared by pyrolysis and impregnation for enhanced photoreduction of CO2 to CH3OH and HCOOH under UVvis irradiation wherein maximum productivities of CH3OH and HCOOH under visible light were 2574 and 5069 mmol•g −1 respectively.Enhanced photoactivity was attributed to the location of the metal within the composite and consequent distribution of photoexcited electrons.Hailong et al. also A multicomponent heterostructure, termed an intercorrelated superhybrid, comprising AgBr supported on g-C 3 N 4 decorated in turn on N-doped graphene (prepared by wet-chemical synthesis) has also shown excellent activity for the photocatalytic reduction of CO 2 to methanol and ethanol (Figure 20) [174].Oluwatobi et al. reported g-C 3 N 4 /(Cu/TiO 2 ) [182] nanocomposites prepared by pyrolysis and impregnation for enhanced photoreduction of CO 2 to CH 3 OH and HCOOH under UV-vis irradiation wherein maximum productivities of CH 3 OH and HCOOH under visible light were 2574 and 5069 mmol•g −1 respectively.Enhanced photoactivity was attributed to the location of the metal within the composite and consequent distribution of photoexcited electrons.Hailong et al. also studied g-C 3 N 4 /Ag-TiO 2 hybrid photocatalysts [183], wherein CO and CH 4 were preferentially formed, with a maximum CO 2 conversion of 47 µmol•g −1 , and product yields of 28 µmol•g −1 CH 4 formation and 19 µmol•g −1 CO.Enhanced activity was proposed to arise from the transfer of photoexcited electrons across the g-C 3 N 4 /TiO 2 heterojunction, and subsequently from TiO 2 → Ag nanoparticles due to the lower Fermi level; this spatial separation of charge greatly suppressed the electron-hole recombination, with electrons accumulating on the Ag nanoparticles on the TiO 2 surface.Table 2 compares the performance of different g-C3N4 photocatalysts for photocatalytic CO2 reduction.Table 2. Cont.

Environmental Remediation
Many large-scale processes operated by the petrochemical, textile and food industries discharge polluted water into the aquatic environment [215].Organic dyes are often used in textile, printing, and photographic industries, and a sizable fraction of these are lost during the dying process into effluent wastewater streams.Even low concentrations of such dyes pose serious risks to human and animal health, and their bio-or chemical degradation is challenging [216,217], hence the development advanced oxidation processes (AOPs) to treat contaminated drinking ground and surface waters, and wastewaters containing toxic or non-biodegradable compounds are sought [218,219].Semiconductor photocatalysis offer an effective and economic approach to the treatment of recalcitrant organic compounds at low concentrations in wastewater [220][221][222][223]. Photoexcited holes are the key active species in such photocatalytic environmental remediation, being powerful oxidants in their own right, or reacting with water to produce hydroxyl radicals ( • OH) which are themselves powerful oxidants with an oxidation potential of 2.8 eV (NHE).Reactively-formed • OH can rapidly attack adsorbed pollutants at the surface of photocatalysts or in solution, to achieve their mineralization as CO 2 and water.Mechanisms for the photocatalytic oxidation of organic pollutants in water are widely discussed in the literature [4,221,222].Briefly: Organic Compound + • OH → degradation products (23) Organic Compound + SC h + → degradation products (24) Organic Compound + SC e − → degradation products ( A variety of active radicals, including O 2 2 ) = −0.3eV(NHE) lie well within the band gap.In other words, the redox potential of photoexcited holes must be sufficiently positive to generate • OH radicals, and that of photoexcited electrons sufficiently negative to generate O 2 •− .
Considerable efforts have been devoted to developing photocatalysts for water purification under solar irradiation.g-C 3 N 4 based nanostructures are potential photocatalysts for the degradation of various pollutants [39,42], with photophysical properties of the parent nitride modified through doping with heteroatoms, heterojunction formation with other materials, and textural improvements to enhance surface area and porosity.For example, ultrathin g-C 3 N 4 nanosheets derived from bulk g-C 3 N 4 by exfoliation in methanol exhibit enhanced photocatalytic performance for methylene blue (MB) degradation [65].g-C 3 N 4 nanotubes show superior photoactivity under visible light for MB degradation than bulk g-C 3 N 4 or P25 [74].Tahir and co-workers also employed tubular g-C 3 N 4 for MB and methyl orange (MO) photocatalytic degradation under visible light, observing better stability and activity than bulk g-C 3 N 4 , attributed to the high surface area (182 m 2 •g −1 ) and improved light absorption and charge separation/transfer [75].1D g-C 3 N 4 nanorods with different aspect ratios have been screened for MB degradation under visible light (λ > 420 nm) and simulated solar irradiation (λ > 290 nm) [68].The resulting photocatalytic activity and photocurrent response of g-C 3 N 4 nanorods under visible light were 1.5-2.0times that of g-C 3 N 4 nanoplates.A simple chemical route was reported for preparing nanofiber-like g-C 3 N 4 structures which showed promising activity for Rhodamine B (RhB) photodegradation [73].
g-C 3 N 4 doping is a common strategy to broaden spectral utilization and band alignment to drive separate photogenerated charge carriers.Doping by metals such as Cu and Fe [224][225][226], non-metals such as B, C, O, or S [224,[227][228][229][230][231], and co-doping [232][233][234] have all been employed for environmental depollution applications.For example, S and O co-doped g-C 3 N 4 prepared by melamine polymerization and subsequent H 2 O 2 activation prior to trithiocyanuric acid functionalization (Figure 21a) enhanced the photocatalytic degradation of RhB (Figure 21b) 6-fold relative to the parent g-C 3 N 4 nanosheet [235].Doping resulted in a strongly delocalized HOMO and LUMO that increased the number of active sites and improved the separation of photogenerated electrons and holes.
Catalysts 2018, 8, x FOR PEER REVIEW 28 of 47 Considerable efforts have been devoted to developing photocatalysts for water purification under solar irradiation.g-C3N4 based nanostructures are potential photocatalysts for the degradation of various pollutants [39,42], with photophysical properties of the parent nitride modified through doping with heteroatoms, heterojunction formation with other materials, and textural improvements to enhance surface area and porosity.For example, ultrathin g-C3N4 nanosheets derived from bulk g-C3N4 by exfoliation in methanol exhibit enhanced photocatalytic performance for methylene blue (MB) degradation [65].g-C3N4 nanotubes show superior photoactivity under visible light for MB degradation than bulk g-C3N4 or P25 [74].Tahir and co-workers also employed tubular g-C3N4 for MB and methyl orange (MO) photocatalytic degradation under visible light, observing better stability and activity than bulk g-C3N4, attributed to the high surface area (182 m 2 •g −1 ) and improved light absorption and charge separation/transfer [75].1D g-C3N4 nanorods with different aspect ratios have been screened for MB degradation under visible light (λ > 420 nm) and simulated solar irradiation (λ > 290 nm) [68].The resulting photocatalytic activity and photocurrent response of g-C3N4 nanorods under visible light were 1.5-2.0times that of g-C3N4 nanoplates.A simple chemical route was reported for preparing nanofiber-like g-C3N4 structures which showed promising activity for Rhodamine B (RhB) photodegradation [73].
g-C3N4 doping is a common strategy to broaden spectral utilization and band alignment to drive separate photogenerated charge carriers.Doping by metals such as Cu and Fe [224][225][226], non-metals such as B, C, O, or S [224,[227][228][229][230][231], and co-doping [232][233][234] have all been employed for environmental depollution applications.For example, S and O co-doped g-C3N4 prepared by melamine polymerization and subsequent H2O2 activation prior to trithiocyanuric acid functionalization (Figure 21a) enhanced the photocatalytic degradation of RhB (Figure 21b) 6-fold relative to the parent g-C3N4 nanosheet [235].Doping resulted in a strongly delocalized HOMO and LUMO that increased the number of active sites and improved the separation of photogenerated electrons and holes.Plasmonic photocatalysts have also been exploited for environmental remediation, for example, 7-15 nm Au and Pt nanoparticles photodeposited on g-C3N4 are promising for the photocatalytic degradation of tetracycline chloride as a representative antibiotic whose uncontrolled release is of concern [236].The Au surface plasmon resonance broadens the optical adsorption range, while Pt acts as a sink for photoexcited electrons.The combination of noble metals and g-C3N4 enables tunable heterojunctions with improved charge transport than traditional nanocomposites [237][238][239][240][241][242][243], and such multicomponent heterostructures are a promising solution to environmental depollution [39,40,42], for example g-C3N4/Ag3PO4 systems for MO degradation [242,243].Ag3PO4@g-C3N4 core-shell photocatalysts have also been applied to MB degradation under visible light, achieving 97% conversion in 30 min compared with only 79% for a physical mixture of the Ag3PO4 and g-C3N4 components, and 69% for pure Ag3PO4.The g-C3N4 shell may protect Ag3PO4 from dissolution in the composite, conferring superior stability.Core-shell g-C3N4@TiO2 photocatalysts synthesized by a solgel and in situ re-assembly route and subsequently applied to phenol removal under visible light Plasmonic photocatalysts have also been exploited for environmental remediation, for example, 7-15 nm Au and Pt nanoparticles photodeposited on g-C 3 N 4 are promising for the photocatalytic degradation of tetracycline chloride as a representative antibiotic whose uncontrolled release is of concern [236].The Au surface plasmon resonance broadens the optical adsorption range, while Pt acts as a sink for photoexcited electrons.The combination of noble metals and g-C 3 N 4 enables tunable heterojunctions with improved charge transport than traditional nanocomposites [237][238][239][240][241][242][243], and such multicomponent heterostructures are a promising solution to environmental depollution [39,40,42], for example g-C 3 N 4 /Ag 3 PO 4 systems for MO degradation [242,243].Ag 3 PO 4 @g-C 3 N 4 core-shell photocatalysts have also been applied to MB degradation under visible light, achieving 97% conversion in 30 min compared with only 79% for a physical mixture of the Ag 3 PO 4 and g-C 3 N 4 components, and 69% for pure Ag 3 PO 4 .The g-C 3 N 4 shell may protect Ag 3 PO 4 from dissolution in the composite, conferring superior stability.Core-shell g-C 3 N 4 @TiO 2 photocatalysts synthesized by a sol-gel and in situ re-assembly route and subsequently applied to phenol removal under visible light were seven times more photoactive than bulk g-C 3 N 4 .Increasing the g-C 3 N 4 shell thickness from 0 to 1 nm increased the photodegradation rate constant from 0.0018 to 0.0386 h −1 ; however, thicker shells slowed charge transport to the external photocatalyst surface, lowering activity.Z-scheme N-doped ZnO/g-C 3 N 4 hybrid core-shell nanostructures (Figure 22Aa,b) were successfully prepared via a facile, low-cost, and eco-friendly ultrasonic dispersion method [244].The g-C 3 N 4 shell thickness was tuned by varying the g-C 3 N 4 loading.Direct contact between the N-doped ZnO core and g-C 3 N 4 shell introduced a new energy level into the N-doped ZnO band gap, effectively narrowing the band gap.Consequently, these hybrid core-shell nanostructures showed greatly enhanced visible light photocatalysis for RhB degradation compared to pure N-doped ZnO surface or g-C 3 N 4 components (Figure 22Ac) [240].A facile, reproducible, and template-free synthesis has also been demonstrated to prepare magnetically separable g-C 3 N 4 −Fe 3 O 4 nanocomposites (Figure 22Ba) [37].Monodispersed Fe 3 O 4 nanoparticles with 8 nm diameter were uniformly deposited over g-C 3 N 4 sheets (Figure 22Bb) and exhibited enhanced charge separation and photocatalytic activity for RhB degradation under visible light irradiation (Figure 22Bc).These g-C 3 N 4 −Fe 3 O 4 nanocomposites showed good stability with negligible loss in photocatalytic activity even after six recycles, and facilitated magnetic catalyst recovery (Figure 22Bd).Xiao et al. demonstrated that the excellent stability of g-C 3 N 4 towards photocatalytic oxidation in the presence of organic pollutants reflects strong competition of the latter for • OH radicals under practical working conditions, resulting in preferential decomposition of the pollutants rather than the carbon nitride [245].
Catalysts 2018, 8, x FOR PEER REVIEW 29 of 47 were seven times more photoactive than bulk g-C3N4.Increasing the g-C3N4 shell thickness from 0 to 1 nm increased the photodegradation rate constant from 0.0018 to 0.0386 h −1 ; however, thicker shells slowed charge transport to the external photocatalyst surface, lowering activity.Z-scheme N-doped ZnO/g-C3N4 hybrid core-shell nanostructures (Figure 22Aa,b) were successfully prepared via a facile, low-cost, and eco-friendly ultrasonic dispersion method [244].The g-C3N4 shell thickness was tuned by varying the g-C3N4 loading.Direct contact between the N-doped ZnO core and g-C3N4 shell introduced a new energy level into the N-doped ZnO band gap, effectively narrowing the band gap.Consequently, these hybrid core-shell nanostructures showed greatly enhanced visible light photocatalysis for RhB degradation compared to pure N-doped ZnO surface or g-C3N4 components (Figure 22Ac) [240].A facile, reproducible, and template-free synthesis has also been demonstrated to prepare magnetically separable g-C3N4−Fe3O4 nanocomposites (Figure 22Ba) [37].Monodispersed Fe3O4 nanoparticles with 8 nm diameter were uniformly deposited over g-C3N4 sheets (Figure 22Bb) and exhibited enhanced charge separation and photocatalytic activity for RhB degradation under visible light irradiation (Figure 22Bc).These g-C3N4−Fe3O4 nanocomposites showed good stability with negligible loss in photocatalytic activity even after six recycles, and facilitated magnetic catalyst recovery (Figure 22Bd).Xiao et al. demonstrated that the excellent stability of g-C3N4 towards photocatalytic oxidation in the presence of organic pollutants reflects strong competition of the latter for • OH radicals under practical working conditions, resulting in preferential decomposition of the pollutants rather than the carbon nitride [245].Several multicomponent nanocomposites based on g-C3N4 nanosheets such as Au@g-C3N4-PANI [246], Au-NYF/g-C3N4 [105], g-C3N4/CNTs/Al2O3 [247], AgCl/Ag3PO4/g-C3N4 [248], and g-C3N4/Zn0.11Sn0.12Cd0.88S1.12[249] are also reported; the performance of different g-C3N4 photocatalysts for the photodegradation of representative aqueous pollutants is summarized in Table 3.Several multicomponent nanocomposites based on g-C 3 N 4 nanosheets such as Au@g-C 3 N 4 -PANI [246], Au-NYF/g-C 3 N 4 [105], g-C 3 N 4 /CNTs/Al 2 O 3 [247], AgCl/Ag 3 PO 4 /g-C 3 N 4 [248], and g-C 3 N 4 /Zn 0.11 Sn 0.12 Cd 0.88 S 1.12 [249] are also reported; the performance of different g-C 3 N 4 photocatalysts for the photodegradation of representative aqueous pollutants is summarized in Table 3.    6. Conclusions g-C 3 N 4 nanostructures offer tunable textural, electronic and optical properties that are amenable to tailoring for solar energy harvesting and subsequent photocatalytic transformations for energy and environmental applications.Diverse synthetic methods are available to prepare pure g-C 3 N 4 nanostructures of different dimensionality and porosity, and to integrate these within multi-functional nanocomposites with enhanced solar spectral utilization, apparent quantum yields, charge separation and transport, and ultimately photocatalytic activity and stability.The sustainable production of H 2 as an energy vector from water splitting is perhaps the most promising application, although issues remain regarding the use of sacrificial reagents and a lack of interdisciplinary efforts to improve photoreactor design.Photocatalytic reduction of CO 2 is at a more preliminary stage, with improvements in both activity, and the ability to select specific products for either energy (e.g., CO, CH 4 , methanol, and formic acid) or chemicals (e.g., >C 2 olefins or alkanes) pre-requisites to bench scale demonstrations.Wastewater treatment using g-C 3 N 4 -based photocatalysts appears promising; however, a lack of standardization in either reactor design or experimental protocols hampers quantitative comparisons due to issues such as decoupling adsorption versus reaction, and photocatalysis from direct photochemical activation of chromophores.

Figure 1 .
Figure 1.(a) Natural, and (b) artificial photosynthesis through water splitting and CO2 reduction, and (c) photodegradation of aqueous organic pollutants.

Figure 1 .
Figure 1.(a) Natural, and (b) artificial photosynthesis through water splitting and CO 2 reduction, and (c) photodegradation of aqueous organic pollutants.

Figure 2 .
Figure 2. Principal photophysical processes for a semiconductor (SC) under light irradiation.

Figure 2 .
Figure 2. Principal photophysical processes for a semiconductor (SC) under light irradiation.

47 Figure 3 .
Figure 3. Band gap energy and band edge energies of different semiconductors.

Figure 3 .
Figure 3. Band gap energy and band edge energies of different semiconductors.

Figure 6 .
Figure 6.Thermal exfoliation as a low-cost and green method to prepare ultrathin g-C3N4 nanosheets from bulk g-C3N4 in water.Reproduced with permission from [61].Copyright John Wiley & Sons Inc., 2012.

Figure 6 .
Figure 6.Thermal exfoliation as a low-cost and green method to prepare ultrathin g-C 3 N 4 nanosheets from bulk g-C 3 N 4 in water.Reproduced with permission from [61].Copyright John Wiley & Sons Inc., 2012.Catalysts 2018, 8, x FOR PEER REVIEW 7 of 47

Figure 8 .
Figure 8. Liquid exfoliation route as a low-cost and green method to prepare the ultrathin g-C3N4 nanosheets from bulk g-C3N4 in water.Reprinted with permission from [63].Copyright American Chemical Society, 2013.

Figure 8 .
Figure 8. Liquid exfoliation route as a low-cost and green method to prepare the ultrathin g-C 3 N 4 nanosheets from bulk g-C 3 N 4 in water.Reprinted with permission from [63].Copyright American Chemical Society, 2013.

Catalysts 2018, 8 ,
x FOR PEER REVIEW 22 of 47 electron-hole recombination, with electrons accumulating on the Ag nanoparticles on the TiO2 surface.

Figure 21 .
Figure 21.(a) Synthetic strategy, and (b) photocatalytic activity of S and O co-doped g-C3N4 for RhB degradation.Reproduced with permission from [235].Copyright Royal Society of Chemistry, 2017.

Figure 21 .
Figure 21.(a) Synthetic strategy, and (b) photocatalytic activity of S and O co-doped g-C 3 N 4 for RhB degradation.Reproduced with permission from [235].Copyright Royal Society of Chemistry, 2017.

Table 2
compares the performance of different g-C 3 N 4 photocatalysts for photocatalytic CO 2 reduction.
•− , • OH, HO 2 • , in addition to H 2 O 2 have been invoked as the oxidants responsible for mineralization, with • OH the most likely candidate Equation (23).

Table 3 .
Photocatalytic degradation of aqueous pollutants over g-C 3 N 4 nanostructured catalysts.