Retracted Article: Recent advances in visible-light graphitic carbon nitride (g-C₃N₄) photocatalysts for chemical transformations

Abstract

Graphitic carbon nitride (g-C₃N₄) has emerged as a new research hotspot, attracting broad interdisciplinary attention in the form of metal-free and visible-light-responsive photocatalysts in the field of solar energy conversion and environmental remediation. These photocatalysts have evolved as attractive candidates due to their non-toxicity, chemical stability, efficient light absorption capacity in the visible and near-infrared regions, and adaptability as a platform for the fabrication of hybrid materials. This review mainly describes the latest advances in g-C₃N₄ photocatalysts for chemical transformations. In addition, the typical applications of g-C₃N₄-based photocatalysts involving organic transformation reactions are discussed (synthesis of heterocycles, hydrosulfonylation, hydration, oxygenation, arylation, coupling reactions, etc.).

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Praveen P. Singh

Currently, Praveen P. Singh is an Assistant Professor in the Department of Chemistry at the United College of Engineering and Research, Prayagraj, India. He obtained his BSc and M.Sc. in Organic Chemistry from T. D. P. G. College (V. B. S Purvanchal University) Jaunpur and DPhil. from the Department of Chemistry, University of Allahabad, India. His current research interests include the development of synthetic receptors for the recognition of biological target structures and the application of visible light chemical photocatalysis towards organic synthesis.

Vishal Srivastava

Currently, Vishal Srivastava is an Assistant Professor, Department of Chemistry, C.M.P. College, (Constituent P.G. College of Central University of Allahabad) Prayagraj, India. He completed his undergraduate (BSc) and post-graduate (M.Sc.) studies in Organic Chemistry and Doctoral Degree (DPhil.) in the Department of Chemistry, University of Allahabad, India. His current research involves the design of novel biologically active photoredox-catalysed synthetic organic compounds.

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1 Introduction

Currently, photocatalytic technology is considered as one of the most important approaches to address the global energy and environmental issues.¹ Inspired by photosynthesis in plants² (Fig. 1), synthetic organic chemists are always trying to develop new organic reactions that can be carried out under green, ecofriendly, and sustainable conditions.

Fig. 1 Natural photosynthesis and the photocatalytic process using g-C₃N₄. Adapted with permission from ref. 2. Copyright 2013, Elsevier.

Polymeric g-C₃N₄ contains the earth-abundant carbon and nitrogen elements. Polymeric carbon nitride (PCN) is a highly active photocatalyst. The history of C₃N₄ polymers and their precursors can be traced back to 1834, where the embryonic form of melon, which is a linear polymer of interconnected tri-s-triazine through secondary nitrogen, was first developed by Berzelius and named by Liebig.^3,4 The utilization of g-C₃N₄ in the field of heterogeneous catalysis started in 2006.⁵ Wang and co-workers, in 2009,⁶ reported the discovery of g-C₃N₄ as a metal-free conjugated semiconductor photocatalyst for H₂ evolution, potentially shifting the research focus from inorganic to polymeric conjugated semiconductor photocatalysts.⁷

The photocatalytic activities of g-C₃N₄ are greatly influenced by its structure,⁸ including its electronic structure, crystal structure, nanostructure and/or heterostructure. In recent years, several synthetic techniques and effective alternative processes have been suggested for the optimization of the photoactivity of g-C₃N₄ with respect to nanostructure design, electronic structure modulation, crystal structure engineering and heterostructure construction. Both doping and copolymerization have been considered to be an efficient technique to incorporate external impurities in g-C₃N₄ for the modulation of its electronic structure and photocatalytic activity. In the context of nanostructure design, nanostructures with controllable morphologies not only facilitate mass transfer in catalysis but also accelerate the collection and separation of electron–hole pairs to drive relevant reactions.⁹ Different synthetic pathways have been developed for g-C₃N₄ materials with nanopores and nanoscale geometrical shapes including hard/soft templating strategies, supramolecular preorganization, solvothermal technology and exfoliation strategies. With respect to crystal-structure engineering, ionothermal synthesis reveals great opportunities for the synthesis of highly crystalline, more completely condensed g-C₃N₄ with enhanced photocatalytic activity.^10a The synthesis of hetero-structured composites by coupling g-C₃N₄ with other semiconductors not only suppresses the recombination of photoinduced charges, but also endows the composites with enhanced properties or novel features as a result of their synergistic effects.^10b Accordingly, the enlightened design of its structure at different levels can provide new insights into the creation of high-performance g-C₃N₄ materials^11a–f for the efficient conversion of solar energy.

Polymeric g-C₃N₄ is an inexpensive and high-performance nanomaterial, exhibiting abundant scope for adaptable applications.^11g Polymeric g-C₃N₄ nanocomposites have been extensively explored for photocatalytic degradation,^11h–o environmental remediation,^11p removal of heavy metals from contaminated water,^11q high-dispersed photocatalyst for phenol mineralization and E. coli disinfection,^11r photocatalytic hydrogen production and water detoxification,^11s photocatalytic water splitting,^11t photo-reduction^11u of CO₂ and efficient removal of tetracycline hydrochloride.^11v

Polymeric g-C₃N₄ has been previously applied in the above-mentioned process but its application in the field of organic transformation has been limited. Visible light photocatalysis is a robust emerging field for organic transformation and highly appreciable. The combination of visible light photocatalysis with polymeric g-C₃N₄ has enabled organic chemists to carry out the functionalization of organic compounds. In continuation of our work on photocatalysed organic synthesis,¹² this review mainly describes the latest advances in g-C₃N₄ photocatalysts for chemical transformations and may be particularly useful for those who are already involved in the area of heterocycles and interested in the use of visible light photocatalysis in combination with polymeric g-C₃N₄ for bioactive heterocyclic synthesis.

2 Graphitic carbon nitride photocatalyst

Several methods have been reported to synthesize g-C₃N₄, including chemical vapor deposition,¹³ plasma sputtering reaction deposition,¹⁴ solvothermal method,¹⁵ and solid-state reaction.¹⁶ However, these protocols are hindered by their strict conditions and the limited access to expensive instruments. Thus, due to its simplicity, high yield and cost effectiveness, thermal polycondensation has become the most popular method for the preparation of g-C₃N₄.^17–19 Given that g-C₃N₄ only contains earth-abundant elements (carbon and nitrogen), it can be prepared with accessible raw materials at low cost. Several inexpensive N-rich precursors with a C–N core structure, such as melamine,^20,21 cyanamide,^22,23 dicyandiamide,^24,25 urea,^26,27 thiourea,^28,29 and mixtures thereof,³⁰ have been widely used to prepare g-C₃N₄ via the thermal polycondensation method (Fig. 2a).³¹ In the facile annealing process, the N-rich molecules are polymerized at the temperature of 450–660 °C to form g-C₃N₄. Yan et al. performed a thermogravimetric–differential scanning calorimetry (TGA-DSC) analysis to investigate the condensation processes of melamine.²¹ They observed that the sublimation and condensation of melamine occur at 297–390 °C, and the subsequent deamination and decomposition of the material happen at a higher temperature of 545–630 °C. The g-C₃N₄ synthesized from urea normally has a higher specific surface area than that synthesized from melamine,³² and g-C₃N₄ obtained from urea shows a larger bandgap than that prepared from thiourea.³³ Zhang and co-workers studied the effect of annealing temperature on the properties of the obtained g-C₃N₄.²⁸ They found that an increase of the temperature in the range of 450–600 °C could be favorable for the polycondensation of g-C₃N₄, and thus could improve the electron delocalization in its aromatic structure. However, a further increase in the temperature to over 650 °C could result in the decomposition of g-C₃N₄ into small particles with increased bandgaps. Therefore, the annealing temperature is a key parameter in the synthesis of g-C₃N₄ nanosheets.

Fig. 2 (a) Schematic illustration of the process for the synthesis of g-C₃N₄. Adapted with permission from ref. 31. (b) Tri-s-triazine-based structure of g-C₃N₄. Adapted with permission from ref. 38. (c) Band structure of g-C₃N₄. Adapted with permission from ref. 45a. (d) Photocatalysis of g-C₃N₄. Adapted with permission from ref. 45b.

With a graphite-like layered structure, g-C₃N₄ exists in the form of 2D nanosheets based on s-triazine (Fig. 2b) or tri-s-triazine (Fig. 2b) tectonic units interconnected by tertiary amines.³⁴ In this case, given that the nitride pore size and the electronic environment of the N atom are different, the energetic stability of the allotropes is different. Among the allotropes of C₃N₄, tri-s-triazine-based g-C₃N₄ is the most stable under ambient conditions.^35–37 Benefitting from the aromatic C–N heterocycles and their high condensation, g-C₃N₄ with tri-s-triazine rings is thermally stable at a temperature of up to 600 °C in the air, as confirmed by thermogravimetric analysis.³⁸ Furthermore, the strong interlayer van der Waals interactions render g-C₃N₄ chemically stable, which can resist many organic solvents, as well as acid and alkaline solutions.^38,39 The 2D g-C₃N₄ nanosheets also provide a large specific surface area due to their graphite-like layered structure, with a theoretical value for monolayer g-C₃N₄ of up to 2500 m² g⁻¹,⁴⁰ which is favorable for the adsorption of reactants in solution, and thus can facilitate surface reactions, including photocatalysis.

The presence of sp²-hybridized C and N results in the unique established π-conjugated electronic structures of g-C₃N₄. Wang et al. performed density functional theory (DFT) calculations,⁴¹ which revealed that the valence band (VB) maximum and the conduction band (CB) minimum consist predominantly of the N and C pz orbitals of g-C₃N₄, respectively, which has a direct bandgap of ∼2.7 eV. As depicted in Fig. 2c, the bandgap of g-C₃N₄ is around 2.7 eV, enabling the harvesting of visible light up to 460 nm. The conduction band (CB) position of g-C₃N₄ is approximately −1.1 eV and its valence band (VB) position is about +1.6 eV with reference to the normal hydrogen electrode, thereby meeting the requirements for photocatalytic water splitting to produce O₂ and H₂, respectively.^41,42 Compared with TiO₂, which has a bandgap of 3.0–3.2 eV and can only respond to UV light,^43,44 g-C₃N₄ possesses more suitable band positions to enable water splitting and shows photocatalytic activity in the visible-light region. For the first time, in 2009,⁴¹ g-C₃N₄ was used as a metal-free photocatalyst for water splitting to generate H₂ under visible-light irradiation. Since this pioneering work and because of its aforementioned intriguing properties, g-C₃N₄ has been attracting significant interest from the scientific community and has drawn worldwide attention in recent years. Furthermore, g-C₃N₄ exhibits photocatalytic activity for CO₂ reduction,^11u pollutant degradation,^11p and photocatalytic degradation^11h–o via photoredox processes (Fig. 2d).

The design of g-C₃N₄-based materials includes zero-dimensional quantum dots (QDs);^46,47 one-dimensional (1D) nanowires,^48,49 nanorods,^50–52 nanotubes,^53,54 and nanobelts;^55,56 two-dimensional (2D) nanosheets^57,58 and nanoplates;⁵⁹ and 3D microspheres,⁶⁰ micro-flowers,⁶¹ sandwich-like structures,⁶² and porous structures.^63–65 For instance, combining theoretical calculation results with practical experimental findings, the structure and electronic and optical properties of modified g-C₃N₄ QDs were systematically and comprehensively explored by Bandyopadhyay et al.⁴⁷ The preparation method is relatively simple for single-component 3D g-C₃N₄. For example, Chen et al. designed a bottom-up supramolecular self-assembly pathway to synthesize 3D porous g-C₃N₄ assembled from highly crystalline and ultrathin nanosheets, which achieved both stability and overall photocatalytic water splitting activity.⁶⁶ The unique electronic structure of g-C₃N₄-based photocatalysts has prompted their applications in selective organic transformations,⁶⁷ including oxidation, reduction, and coupling reactions. Given that these organic transformation reactions play a key role in industrial and fine chemical synthesis, it is very attractive for nontoxic g-C₃N₄-based photocatalysts driven by visible light to accomplish the corresponding reactions under mild conditions.

3 Functionalization of g-C₃N₄

3.1 Elemental doping of g-C₃N₄

Elemental doping involves the incorporation of heteroatoms or impurities (metal or nonmetal elements) in g-C₃N₄ to regulate its absorption, electronic, and physicochemical properties. There are two main types of elemental doping, i.e., nonmetal doping and metal doping. In the field of photocatalysis, bandgap engineering of g-C₃N₄ via the incorporation of anions and cations or the co-doping of both plays a predominant role in modulating its light absorption and redox band potentials for targeted photocatalytic applications. Hitherto, numerous studies on the doping of anions such as O,^68–72 C,^73,74 P,^75–77 S,^78–80 B,^81–85 I,^86,87 F⁸⁸ and some combinations thereof^89–91 in g-C₃N₄ have been extensively reported.

Besides metal elements, nonmetal elemental doping is also a promising way to govern the electronic structure of g-C₃N₄. Jiang and co-workers⁹² fabricated defective oxygen-doped porous C₃N₄ via the polymerization of ammonium persulfate and dicyandiamide. Compared to g-C₃N₄, the bandgap of the oxygen-doped g-C₃N₄ increased from 2.70 to 2.75 eV, and its CB decreased from −0.60 to −0.81 V. The more negative CB position indicates more powerful reduction ability, and hence the photocatalytic performance will increase significantly.

3.2 Copolymerization of g-C₃N₄

The photocatalytic reactivity of g-C₃N₄ is attributed to its π-conjugated framework because of its sp²-hybridization system. However, the aromatic π system of g-C₃N₄ in its pristine form generally encounters intrinsic drawbacks, namely, the rapid recombination of charge carriers, inadequate absorption and utilization of sunlight and low surface areas as a result of the defective polymerization of its organic framework, considerably restricting its photochemical functions.^93–95 Thus, employing the merits of the polymeric π-conjugated system, it is envisaged that the delocalization of the π electrons can be further extended by modifying and anchoring the existing molecular structure of g-C₃N₄ with other structure-matching aromatic groups or organic additives during the copolymerization process to modulate its intrinsic properties.^96–100 The copolymerization approach is regarded as typical molecular doping to adjust the conventional π systems, electronic properties, optical absorption, band structure and importantly photocatalytic performance of g-C₃N₄.^101–105

Che and co-workers^106a investigated a thermal-conjugate in-plane polymerization approach using glucose as the starting material to integrate C_ring into the triazine units of g-C₃N₄. The synthetic route for (C_ring)-C₃N₄ including the dehydration and polymerization process is shown in FIG. 3A. The successful fusion of C_ring in the C₃N₄ matrix was confirmed by the TEM and HRTRM analysis (Fig. 3b and c). The lattice fringe in orange ellipses (Fig. 3c) is 0.21 nm, in accordance with the (100) plane of graphite. The enrichment of C (elliptic region) can be observed in Fig. 3d, further suggesting the presence of C_ring. In addition, the integration of C_ring in the matrix of g-C₃N₄ can significantly broaden its visible-light absorption, facilitate charge carrier separation, and thus promote its photo-catalytic overall water splitting (OWS) activity. The C_ring-C₃N₄ photocatalyst accelerated the surface reaction kinetics and enhanced the separation of photocarriers in the photoconversion of CO₂ to CH₄ and adsorption and photodegradation of methylene blue^106b (Fig. 3e).

Fig. 3 (a) Synthetic procedure for C_ring-g-C₃N₄. (b) TEM image and (c) HRTEM image of C_ring-g-C₃N₄. The elliptic and rectangular areas represent a carbon ring and g-C₃N₄ matrix, respectively. (d) EDS mapping of C_ring-g-C₃N₄. The inset image is the corresponding mapping area. Adapted with permission from ref. 106a. Copyright 2017, the American Chemical Society. (e) Photoconversion of CO₂ to CH₄ and the adsorption and photodegradation of methylene blue. Adapted with permission from ref. 106b. Copyright 2021, the American Chemical Society.

It is expected that the fusion of conjugated aromatic groups in g-C₃N₄ units is a favourable route to regulate the electronic features of g-C₃N₄ to furnish materials with desirable physical and chemical properties, consequently delivering highly efficient photocatalysts. The current research on the copolymerization of co-monomer-modified g-C₃N₄ is expected to offer a new path for the sustainable conversion of solar energy due to its fascinating novel properties.

3.3 Crystalline g-C₃N₄

g-C₃N₄ derived via direct polymerization exhibits an amorphous or hypocrystalline structure due to the incomplete polymerization of the precursors, resulting a relatively poor photocatalytic performance. Due to kinetic hindrance problem in the solid-state route, the formation of highly crystalline g-C₃N₄ is a major challenge. However, the insertion of suitable metal salts can enhance the mass-transfer efficiency and improve the crystallinity, thus enabling a new route to synthesize highly crystalline g-C₃N₄. Bhunia and co-workers¹⁰⁷ demonstrated that highly crystalline g-C₃N₄ (PTI-0.13) can be synthesized via the polymerization of melamine, 2,4,6- triaminopyrimidine, and a mixture of LiCl and KCl in air at 550 °C.

3.4 Surface-engineered g-C₃N₄

Surface engineering of the interface of g-C₃N₄ can enhance the amount of adsorption sites by increasing the absorption of protons and inhibit carrier recombination, ultimately resulting in an improved photocatalytic performance. Yu and co-workers¹⁰⁸ proposed a surface engineering strategy to gradually dope graphitic carbon rings in g-C₃N₄. By changing the amount of 1,3,5-cyclohexanetriol, graphitic carbon rings were incorporated at different depths in g-C₃N₄. Gradual surface doping furnished g-C₃N₄ with suitable electronic band structures and built-in electric fields, resulting in accelerated carrier separation and improved photocatalytic performance. In another case, Wang's group¹⁰⁹ fabricated proton- and –OH-modified g-C₃N₄ spheres and evaluated the effects of the introduction of protons and –OH on the photocatalytic degradation and HER activity. The results suggested that proton functionalization has a lower influence on the photocatalytic hydrogen process, while –OH can dramatically enhance the photocatalytic hydrogen production rate.

3.5 Single-atom engineering of g-C₃N₄

In the conversion of solar energy, single-atom photocatalysts are of tremendous importance due to their maximum utilization of metal active sites, high catalytic activity, and selectivity. Accordingly, g-C₃N₄ is an excellent support for single-metal atoms (SMAs) because the lone electron pairs present in the N atoms can coordinate with SMAs. To date, various SMAs such as Pt,^110–112 Ni,¹¹³ and Cu^114,115 have been investigated for the photocatalytic HER. Zhou and co-workers¹¹⁰ demonstrated a low-temperature in situ photocatalytic reduction approach to synthesize a high-density Pt single atom (PtSA/g-C₃N₄). Zhang and co-workers¹¹² selected single-atom Pt/C₃N₄ nanorods (S–Pt–C₃N₄) as a model material and employed synchronous illumination X-ray photoelectron spectroscopy to monitor the bond evolution process (Fig. 4).

Fig. 4 (a) Elemental mapping and (b) HAADF-STEM images of the S–Pt–C₃N₄ sample. (c) K-edge XANES spectra of the S–Pt–C₃N₄ catalyst and Pt foil. (d) Fourier transform Pt L3-edge EXAFS spectra for S–Pt–C₃N₄ and Pt foil without phase correction. SI-XPS Pt 4f spectra of (e) S–Pt– C₃N₄ and (f) M-Pt-C₃N₄. Adapted with permission from ref. 112. Copyright 2020, John Wiley & Sons, Inc.

4 g-C₃N₄-based heterojunction photocatalysts

g-C₃N₄ exhibits a bandgap energy, also known as the void region, covering the top of the filled VB to the bottom of the vacant CB. Initially, the absorption of a photon with energy equal to or greater than the bandgap results in the excitation of an electron from the VB to CB, leaving behind an empty state, which constitutes a positive hole.^116,117 Once spatially separated, the charge carriers in the excited states, which then migrate to the surface of g-C₃N₄, are responsible for initiating the reduction and oxidation processes for the photocatalytic conversion of reactant molecules (Fig. 5).

Fig. 5 Schematic illustration of photoexcited electron–hole pairs in g-C₃N₄ with possible decay pathways. A and D denote electron acceptor and electron donor, respectively. Adapted with permission from ref. 31.

The location of the reactive sites may be either on the g-C₃N₄ surface, where the photoexcitation occurs, or across the interface of another semiconductor or cocatalyst.¹¹⁸ In this case, the main role of g-C₃N₄ is to absorb light, produce electron–hole pairs and transfer them to its surface or to a cocatalyst. Indeed, g-C₃N₄ is chemically active only when the photo-induced electron–hole pair is consumed simultaneously before recombination occurs in a fraction of a nanosecond.^119–121

The excitation of the bandgap upon the absorption of light and the formation of electron–hole pairs can also be rapidly accompanied by a few deexcitation pathways (Fig. 5). It is noteworthy that the successful migration of charge carriers to the acceptor molecules, which results in reduction and oxidation processes (pathways 1 and 2, respectively), also competes with the recombination processes.¹²² For example, the recombination process can take place via two different paths, as follows: (1) on the surface of the particle depicted in pathway 3 (surface recombination) and (2) in the bulk of g-C₃N₄ illustrated in pathway 4 (volume recombination). These recombination processes are disadvantageous to the photocatalytic efficiency. When the recombination process occurs, the excited electrons revert to the VB, dissipating the energy as heat without reacting with the adsorbed species on the surface of g-C₃N₄.¹²³ Therefore, various strategies are employed to prevent the recombination of charge carriers in g-C₃N₄ and upgrade its photocatalytic performance.

The construction of heterojunctions is another feasible approach to enhance the properties of g-C₃N₄. Especially, its charge separation efficiencies can be dramatically accelerated via the construction of heterojunctions. According to the component, the heterojunction can be classified into five types, i.e., type II, Z-scheme, S-scheme, g-C₃N₄/metal, and g-C₃N₄/carbon heterojunctions.

4.1 g-C₃N₄-based type II heterojunction

A g-C3N4-based type II heterojunction can be formed with TiO₂,¹²⁴ ZnO,¹²⁵ WO₃,¹²⁶ CdS,¹²⁷ ZnIn₂S₄,¹²⁸ BiVO₄,¹²⁹ Ag₃PO₄,¹³⁰ and many others. Xiao and co-workers^124a proposed an in situ exfoliation and conversion strategy to synthesize a holey C₃N₄ nanosheet/TiO₂ nanocrystal heterojunction (Fig. 6a–e). In type-II heterojunctions, the CB and VB of semiconductor A are higher than that of semiconductor B and their different chemical potentials show band bending at the junction. Type-II heterojunctions are less effective given that they provide low reduction and oxidation potentials, and the transfer of charges is difficult due to electrostatic repulsion between similar charges^124b and their quantum efficiency has been found to be low. For the type-II heterostructure, reduction and oxidation reactions occur at semiconductor B with lower reduction ability, while semiconductor A has low oxidation ability. Consequently, the redox potential of type-II heterojunctions significantly decreases. The electrons in the CB of semiconductor B and holes in the VB of semiconductor A migrate owing to the electrostatic repulsion between the hole–hole and electron–electron pairs. A novel type of structure named Z-scheme, which has a new charge migration mechanism, has been developed to overcome the drawbacks of type-II heterojunctions.

Fig. 6 (a) PL spectra (360 nm excitation), (b) time-resolved fluorescence spectra, (c) Nyquist plots from electrochemical impedance spectroscopy, and (d) transient photocurrent response of TiO₂, g-C₃N₄, g-C₃N₄/TiO₂, and holey C₃N₄/TiO₂. (e) Proposed synergistic effect of surface reactions and charge separation in holey C₃N₄/TiO₂. Adapted with permission from ref. 124a. Copyright 2020, Elsevier. Z-scheme mechanism for the photocatalytic oxidation of 5-HMF to DFF over WO₃/g-C₃N₄. Adapted with permission from ref. 132b. Copyright 2019, Elsevier.

4.2 g-C₃N₄-based Z-scheme heterojunction

In the traditional type II heterojunctions, electrons tend to inject in the CB with mild reduction ability, while holes tend to inject in the VB with weak oxidation ability because of the thermodynamic effect. Therefore, after their separation and transfer, both the reduction and oxidation ability would be weakened. Accordingly, the exploration of heterojunctions that simultaneously promote carrier separation and maintain the oxidation and reduction ability is of practical importance. In principle, the transport direction of the electrons and holes of green plants are opposite to the traditional type II heterojunction, which is named Z-scheme heterojunction. Z-scheme systems can be divided into three types, i.e., traditional Z-scheme, all-solid-state Z-scheme, and direct Z-scheme. Their detailed differences can be seen in ref. 131a The Z-scheme photocatalytic system, as a new type of heterostructure, can improve the separation of electron–hole pairs and enhance the redox ability compared with the corresponding pristine materials, resulting in wide opportunities in various application areas, especially in terms of tremendous potential in improving the photocatalytic reaction activity.^{124b,131b–d} However, the Z-scheme has many problems associated with electron mediators such as their complex fabrication process, high reverse reaction, and shielding outcome by shuttle redox mediators.^124b Taking an all-solid-state Z-scheme material as an example, Pan et al.^132a engineered a g-C₃N₄/RGO/Fe₂O₃ Z-scheme system with RGO as an electron shuttle. In comparison with g-C₃N₄, g-C₃N₄/RGO/Fe₂O₃ exhibited a dramatically enhanced photocurrent, suggesting an improvement in the charge carrier separation due to the incorporation of RGO. In addition, the lower PL intensity of g-C₃N₄/RGO/Fe₂O₃ suggested the inhibited recombination of carriers. Therefore, g-C₃N₄/RGO/Fe₂O₃ exhibited efficient solar light-induced OWS activity. Wu et al.^132b utilised a WO₃/g-C₃N₄ composite, which was prepared via a directly calcination method with ammonium tungstate hydrate and melamine as raw materials, for the photocatalytic oxidation of 5-hydroxymethylfurfural (5-HMF) to 2,5-diformylfuran (DFF) (Fig. 6f).

4.3 g-C₃N₄-based S-scheme heterojunction

Yu's group proposed the S-scheme heterojunction because of the great fundamental problems of type II heterojunctions and the complexity and misunderstanding of Z-scheme photocatalysts.^131a Similar to the type II heterojunction, the S- scheme system contains an oxidation photocatalyst and a reduction photocatalyst with staggered band structures. However, the charge transfer route of the S-scheme system is completely different from the type II heterojunction.^133a The major advantages of S-scheme photocatalysts are the reservation of photogenerated holes and electrons in the VB and CB of the oxidation and reduction photocatalysts. The simultaneous recombination of useless electrons and holes generated by the internal electrical field imparts strong redox ability.^124b As discussed before, in type II heterojunctions, the redox abilities in both the CB and VB are weakened. In contrast, the strong redox potential is maintained in the S-scheme heterojunction due to the powerful photoinduced electrons and holes accumulated in the CB of the reduction photocatalyst and VB of the oxidation photocatalyst, respectively, with the recombination of the extra carriers. The disadvantages and limitations of S-scheme photocatalysts include the fact that the scope of their application is mainly applicable to powder photocatalysts and they do not apply to photoelectrochemistry and solar cells with external circuits. The two selected semiconductors constructing S-scheme photocatalysts are mainly n-type semiconductors and should have suitable band structures and obvious Fermi level differences.^133b For example, Fu et al.^134a employed an electrostatic self-assembly method (Fig. 7a–e) to design an ultrathin WO₃/g-C₃N₄ S-scheme heterogeneous composite. Cao et al.^134b described that S-scheme Fe₂O₃/g-C₃N₄ photocatalysts can degrade commercial cefalexin and amoxicillin (20 mg L⁻¹) under visible light, with five- and nine-times higher performances than that of g-C₃N₄, respectively. Furthermore, the detailed evidence to propose S-scheme Fe₂O₃/g-C₃N₄ heterojunctions and comparison of their photocatalytic performance in the degradation of antibiotics were also included in this protocol (Fig. 7f).

Fig. 7 (a) Procedure for the formation of 2D/2D WO₃/g-C₃N₄ heterojunctions by electrostatic interaction. Electrostatic potentials of (b) WO₃ (001) surface and (c) g-C₃N₄ (001) surface. Insets show the structural models of the materials for DFT calculation. EPR spectra of (d) DMPO⁻ ˙OH in aqueous and (e) DMPO⁻˙O₂⁻ in methanol dispersion in the presence of g-C₃N₄, WO₃ nanosheets and 15% WO₃/g-C₃N₄. Adapted with permission from ref. 134. Copyright 2018, Elsevier. S-scheme α-Fe₂O₃/g-C₃N₄ nanocomposite as heterojunction photocatalyst for the degradation of antibiotics. Adapted with permission from ref. 134b. Copyright 2022, the American Chemical Society.

4.4 g-C₃N₄/metal heterojunction

Many studies have demonstrated that the formation of a g-C₃N₄/metal heterojunction can effectively enhance the activity of g-C₃N₄. The mechanism of g-C₃N₄/metal is different from g-C₃N₄. When a metal contacts a semiconductor, an energy barrier difference is produced at the interface due to their different work functions and this energy barrier is denoted as the Mott–Schottky energy barrier. The Mott–Schottky energy barrier can greatly promote the electron transfer, and hence promote charge-carrier separation. Specifically, noble metals such as Ag^135–138 and Au^139,140 exhibit surface plasmon resonance effects upon visible-light irradiation, which can dramatically promote the light absorption and near field enhancement. However, the incorporation of noble metals increases the expense, and thus the exploration of green and low-cost systems is of significant importance.

4.5 g-C₃N₄/carbon

In g-C₃N₄/carbon heterojunction systems, carbon is regarded as a promising component due to its merits of being metal free and conductive, excellent physico-chemical stability, low cost, and good light absorption ability.¹⁴¹ The commonly used carbon materials, including graphene,¹⁴² graphyne,^143–145 CNT,^146–151 C₆₀,¹⁵² and carbon dots,^153–157 can significantly enhance the absorption ability and promote charge separation. Xiang and co-workers¹⁵⁸ synthesized graphene/g-C₃N₄ composites via the pyrolysis of melamine, graphite oxide, and hydrazine hydrate. The obtained sample exhibited dramatically enhanced photocatalytic hydrogen evolution activity in comparison with graphene and g-C₃N₄. After the incorporation of graphene, the photogenerated electrons of g-C₃N₄ can transfer to the surface of graphene, and hence inhibit the recombination of carriers of g-C₃N₄ and promote the photocatalytic water-splitting reaction.

In summary, the above-mentioned studies provide a new pathway for the design of g-C₃N₄-based hybrids for elevated charge transfer in the target photocatalytic reactions. This can undoubtedly promote the smart design of g-C₃N₄-based nanomaterials. It is expected that the novel fabrication procedures can lead to huge application potential in the other layered photocatalysts and multicomponent hybrid photocatalysts.

5 Synthetic applications of g-C₃N₄-based photocatalysts

5.1 g-C₃N₄-catalyzed reactions of quinoxalin-2(1H)-ones with N-aryl glycines under visible light

Yu et al.¹⁵⁹ reported the visible light-induced heterogeneous g- C₃N₄-catalyzed decarboxylative reaction of the low-cost and easily available quinoxalin-2(1H)-ones with N-aryl glycines as starting materials (Scheme 1). The simplicity of this synthesis and excellent regioselectivity make this protocol a promising tool for building molecules with diverse skeletons and discovering bioactive compounds.

Scheme 1 g-C₃N₄-catalyzed reactions of quinoxalin-2(1H)-ones with N-aryl glycines under visible light.

The authors proposed a tentative mechanism for these divergent transformations (Scheme 2). Upon irradiation with visible light, the g-C₃N₄ semiconductor is excited to generate electrons and holes, which play important roles in organic transformations, acting as single-electron acceptors and donors, respectively. Initially, radical intermediate A is formed through a photooxidation process by releasing a CO₂ molecule and proton. Then, radical A is added to the C=N bond of 1 to afford intermediate C. Then, intermediate C undergoes a single-electron photo-reduction to generate anion D. Then intermediate D can be converted into product 3 after protonation. Alternatively, the oxidation of radical A affords iminium species B. The combination of iminium species B and anion D gives intermediate E, followed by intramolecular cyclization to produce product 4 by releasing ArNH₂.

Scheme 2 Proposed reaction mechanism for g-C₃N₄-catalyzed reactions of quinoxalin-2(1H)-ones with N-aryl glycines under visible light.

5.2 Synthesis of thiocyanated heterocycles

B. Yu and co-workers¹⁶⁰ reported a metal-free photocatalytic strategy for the preparation of thiocyanated heterocycles from the inexpensive NH₄SCN using carbon nitride (g-C₃N₄) as a general heterogeneous catalyst in a green solvent under an air atmosphere and irradiation from a blue LED (Scheme 3). This general method showed advantages such as having a metal-free procedure and an external-oxidant-free procedure, utilizing a recyclable catalyst and a green solvent, wide applicability, applied at room temperature, and easy to scale up.

Scheme 3 Synthesis of thiocyanated heterocycles.

They proposed the mechanism for this visible light-promoted reaction, which was catalyzed by g-C₃N₄ (Scheme 4). Firstly, under the irradiation of visible light, g-C₃N₄ absorbs photons and generates holes in its valence band (VB) and electrons in its conduction band (CB). Then, the holes generated in the VB (E_VB = +1.2 V vs. SCE) obtain electrons from NH₄SCN via single-electron transfer (SET) to generate the SCN radical. Simultaneously, the electrons in the CB are transferred to O₂ (air) through SET to generate O₂˙⁻. Then, the SCN radical is added to the C=C bonds of 5a to generate radical intermediate A, which is then subjected to intramolecular cyclization to obtain radical intermediate B. Furthermore, B is oxidized by O₂ (air) to form carbocation C through an SET process. Finally, C is transformed into the target product 9a by deprotonation.

Scheme 4 Proposed reaction mechanism for the synthesis of thiocyanated heterocycles.

5.3 Selective oxidative cleavage of C=C bonds in aryl olefins

S. Das and co-workers¹⁶¹ reported the use of polymeric carbon nitride (PCN) as a photocatalyst for the selective cleavage of C=C bonds in aryl olefins. This method was selective, scalable in the presence of sunlight and the photocatalyst was easily recyclable. Excellent substrate scope with high selectivity is the main advantage of this recyclable catalyst. In addition, they could perform late-stage C=C bond cleavage in aryl olefins and degradation of complex molecules to achieve highly valuable products and pharmaceuticals (Scheme 5).

Scheme 5 Metal-free heterogeneous g-C₃N₄ photocatalyst for the selective oxidative cleavage of C=C bonds in aryl olefins by directly harvesting solar energy.

5.4 Aerobic benzylic C–H oxygenations

Y. Cai and co-workers¹⁶² reported a metal-free heterogeneous photocatalytic system for highly efficient benzylic C–H oxygenations using oxygen as an oxidant. This visible light-mediated oxidation reaction utilized graphitic carbon nitride (g-C₃N₄) as a recyclable, nontoxic and low cost photocatalyst. The mild reaction conditions allowed the generation of synthetically and biologically valuable materials from readily accessible alkyl aromatic precursors in good yields (Scheme 6).

Scheme 6 g-C₃N₄-based heterogeneous photocatalyst for visible light-mediated aerobic benzylic C–H oxygenation.

The probable reaction mechanism was proposed by the authors (Scheme 7). Due to the structural and electronic properties of g-C₃N₄ with a band gap of 2.7 eV, visible-light excitation leads to the efficient separation of photogenerated electron hole pairs. The photoelectrons (CBM of g-C₃N₄ located at −0.88 V vs. RHE) can activate O₂ to the superoxide radical anion O₂˙⁻ (reduction potential at −0.56 V vs. SCE). The photogenerated hole of g-C₃N₄ further undergoes single-electron transfer (SET) with the substrate including isochromans, benzylarenes and N-acyl-derived tetrahydroisoquinolines or isoindolines to afford benzylic radical A. Regarding the substrate N-aryl-derived tetrahydroisoquinolines or isoindolines, SET leads to the formation of the corresponding nitrogen radical cation B, followed by base-promoted deprotonation to generate benzylic radical C. Furthermore, A and C react with the peroxide radical ˙OOH from superoxide anions to generate intermediates E and F. Finally, they transform into the products with the simultaneous formation of water as the side product.

Scheme 7 Proposed reaction mechanism for g-C₃N₄-based heterogeneous photocatalyst for visible light-mediated aerobic benzylic C–H oxygenations. Adapted with permission from ref. 162.

5.5 Aerobic cyclization of thiobenzanilides

C. Y. Zhou and co-workers¹⁶³ reported the use of a metal-free heterogeneous photocatalyst for the synthesis of benzothiazoles via intramolecular C–H functionalization/C–S bond formation of thiobenzanilides from benzothioamide 17 by the inexpensive graphitic carbon nitride (g-C₃N₄) under visible light irradiation. This reaction provides access to a broad range of 2-substituted benzothiazoles 18 in high yields under an air atmosphere at room temperature (Scheme 8).

Scheme 8 Visible-light carbon nitride-catalyzed aerobic cyclization of thiobenzanilides under ambient conditions.

The plausible reaction mechanism was proposed by the authors (Scheme 9). Initially, N-substituted benzothioamide 17 adsorbs on the surface of g-C₃N₄ to form complex A with the formation of a new donor level above the valence band of g-C₃N₄. Visible-light irradiation of A drives the promotion of an electron to the conduction band of g-C₃N₄ with the generation of radical cation B. The photogenerated electron quenching by O₂ and loss of a proton from B produce radical C. The sulfur-centered radical of C intramolecularly attacks the benzene ring to form aryl radical D, which releases an electron to the conduction band of g-C₃N₄ to generate cation E under visible-light irradiation. Then, E undergoes deprotonation and rearomatization to provide the final product 18.

Scheme 9 Proposed reaction mechanism for visible-light carbon nitride-catalyzed aerobic cyclization of thiobenzanilides under ambient air conditions.

5.6 Graphitic carbon nitride for α-aminoalkyl radical additions, allylations, and heteroarylations

M. Rueping and co-workers¹⁶⁴ reported a protocol for the photooxidative activation of α-silylamines and α-amino acids for desilylative and decarboxylative additions, allylations, and heteroarylations in the presence of graphitic carbon nitride (g-C₃N₄). This procedure has broad scope and provides the desired products in high yields (Scheme 10). In this protocol, the heterogeneous nature of the reaction system enables the recovery and reuse of the catalyst without loss in its reactivity. The reaction can also be conducted in a continuous flow fashion, even on the gram-scale using a simple and reusable continuous-flow photoreactor, which illustrates the practicability of this heterogeneous photocatalysis protocol.

Scheme 10 Heterogeneous visible-light photoredox catalysis with graphitic carbon nitride for α-aminoalkyl radical additions, allylations, and heteroarylations.

5.7 Meerwein hydration reaction of alkenes

T. Niu and co-workers¹⁶⁵ reported a green and efficient visible light-induced Meerwein hydration reaction of alkenes in aqueous medium using mpg-C₃N₄ as a recyclable photocatalyst. This protocol provides a direct approach for the preparation of racemic alcohols via a free radical mechanism (Scheme 11). In this reaction, water acts as both a solvent and reagent without any additives or co-solvents.

Scheme 11 Heterogeneous visible-light-induced Meerwein hydration reaction of alkenes in water using mpg-C₃N₄ as a recyclable photocatalyst.

The plausible reaction mechanism proposed by the authors (Scheme 12). Initially, mpg-C₃N₄ is excited upon irradiation with visible light, which undergoes single-electron transfer (SET) to generate aryl radical A from diazonium salt 23. Adding A to alkene 24 generates the corresponding benzyl radical B, which is oxidized by the valence band of mpg-C₃N₄ to form intermediate C. Then, H₂O as a nucleophilic reagent attacks intermediate C, followed by dehydrogenation to afford the corresponding product 25.

Scheme 12 Proposed reaction mechanism for visible-light-induced Meerwein hydration reaction of alkenes in water using mpg-C₃N₄ as a recyclable photocatalyst.

5.8 Multicomponent hydrosulfonylation of alkynes

T. Niu and co-workers¹⁶⁶ reported the synthesis of β-ketosulfones 30 via the facile and efficient graphitic carbon nitride (p-g-C₃N₄)-photocatalyzed hydrosulfonylation of alkynes with the insertion of sulfur dioxide under aerobic conditions at room temperature. In this protocol, isotope experiments confirmed that the oxygen atom of the products comes from H₂O, while the O₂ plays an important role in the reaction by quenching the DABCO radical cation (Scheme 13).

Scheme 13 Heterogeneous carbon nitride photocatalyst multicomponent hydrosulfonylation of alkynes to access β-keto sulfones with the insertion of sulfur dioxide in aerobic aqueous medium.

The plausible mechanism was proposed by the authors (Scheme 14). Initially, aryldiazonium cation 26 with DABCO·(SO₂)₂ 29 provides complex A through electrostatic interactions. Subsequent homolytic cleavage of an N–S bond and single-electron transfer generate an aryl radical, sulfur dioxide, and radical cation intermediate B˙⁺. Then, the aryl radical reacts with sulfur dioxide to give sulfonyl radical C. Meanwhile, electrons and holes are produced upon the excitation of p-g-C₃N₄ under visible light irradiation. Consequently, an electron reduces molecular oxygen to produce a superoxide radical, which is quenched immediately with DABCO radical cation B˙⁺. Simultaneously, the obtained sulfonyl radical C reacts with alkynes 27 to form intermediate D. Subsequent SET oxidation of intermediate D forms alkenyl carbocation E, which undergoes hydrolysis the to produce the desired β-keto sulfones 30.

Scheme 14 Proposed reaction mechanism for heterogeneous carbon nitride photocatalyst multicomponent hydrosulfonylation of alkynes to access β-keto sulfones with the insertion of sulfur dioxide in aerobic aqueous medium.

5.9 Mizoroki–Heck-type reactions and synthesis of 1,4-dicarbonyl compounds

B. König and co-workers¹⁶⁷ reported the synthesis of 1,4-dicarbonyl compounds and substituted alkenes (Mizoroki–Heck type coupling) starting from secondary and tertiary alkyl halides and vinyl acetate or styrene derivatives using visible-light photocatalysis (Scheme 15). This protocol uses mesoporous graphitic carbon nitride (mpg-CN) as a heterogeneous organic semiconductor photocatalyst and Ni(II) salts as Lewis acid catalysts.

Scheme 15 Mizoroki–Heck type coupling and synthesis of 1,4-dicarbonyl compounds using mpg-C₃N₄ organic semiconductor visible light photo-redox catalysis.

The plausible mechanism was proposed by the authors (Scheme 16). The exact role of the nickel(II) salt is not clear; however, the authors speculated that the nickel salt acts as a Lewis acid to coordinate with the ester group of alkyl bromide 32, and thereby lowers the reduction potential of alkyl bromide 32. Alternatively, it may be possible that it can activate the double bond by coordinating with olefin 31 given that the formation of the product was observed in the presence of other Lewis acids. However, reactions involving non-activated alkyl bromides did not yield any product, which ruled out the typical cross-coupling mechanism. Based on these observations, they believed that mpg-CN under photochemical illumination generates two-dimensional surface redox centers as electron–hole pairs. The photogenerated electron effectively reduces alkyl bromide 32 and generates alkyl radical A. Then, alkyl radical A adds to the double bond of vinyl acetate B and the resulting radical is oxidized by the photogenerated hole, delivering carbocation C. Successive loss of the acetyl group presumably supported by the nucleophiles present in the reaction medium such as bromide anions or the solvent yields the corresponding 1,4-dicarbonyl compounds 33, whereas proton loss in the case of olefins 34 results in a Mizoroki–Heck type cross-coupling product.

Scheme 16 Proposed reaction mechanism for the reaction involving the mpg-C₃N₄ semiconductor.

5.10 Photocatalytic (het)arylation of C(sp³)–H bonds with carbon nitride

B. König and co-workers¹⁶⁸ reported the use of mesoporous graphitic carbon nitride (mpg-CN) as a heterogeneous organic semiconductor photocatalyst for the direct arylation of C(sp³)–H bonds in combination with nickel catalysis (Scheme 17). This protocol is based on a durable organic semiconductor mpg-CN photocatalyst and a simple nickel catalyst, providing a powerful alternative to conventional homogeneous photoredox catalysts.

Scheme 17 Photocatalytic (het)arylation of C(sp³)–H bonds with carbon nitride.

The authors postulated two plausible mechanistic scenarios (Scheme 18). Initially, the Ni(0) complex I undergoes oxidative addition with an aryl halide, delivering Ni(II) oxidative addition complex II. Concurrently, light absorption by the mpg-CN semiconductor photocatalyst triggers charge separation, producing two-dimensional surface redox centers as electron hole pairs. In pathway A, SET oxidation of complex II by the photogenerated hole (VBM located at +1.2 V vs. SCE, E_1/2 (Ni^II/Ni^III) = +0.85 V vs. SCE) affords species III, which may undergo Ni(III)-X homolysis to give a halogen radical and Ni(II) species IV. The resulting halogen radical can rapidly abstract a hydrogen atom from DMA (H–Br BDE ∼88 kcal mol⁻¹, H–Cl BDE ∼102 kcal mol⁻¹, and α-amino C–H BDE ∼89–94 kcal mol⁻¹), which immediately recombines with species IV to form V. Subsequently, the reductive elimination of V results in the desired product and Ni(I) species VI. Finally, reduction of VI by the electron located on the semiconductor surface (CBM located at −1.5 V vs. SCE, E_1/2 (Ni^I/Ni⁰) = −1.42 V vs. SCE) regenerates Ni(0) and completes the catalytic cycle. In pathway B, mpg-CN serves as a light-absorbing antenna undergoing an energy transfer process (EnT) (singlet–triplet band gap of ca. 0.39 eV) to produce electronically excited Ni(II) species VII. Homolysis of the Ni(II)–X bond and HAT followed by a rebound of the resulting carbon-centered radical with VIII generates Ni(II) species IX. Reductive elimination from the electronically excited species X, promoted by EnT with mpg-CN, provides the final product and regenerates the Ni(0) species, thus completing the catalytic cycle.

Scheme 18 Proposed reaction mechanism for mpg-CN/nickel photocatalytic C(sp3)–H arylation. Adapted with permission from ref. 168.

5.11 Synthesis of phenols from aryl halides

F. Zhang and co-workers¹⁶⁹ reported the synthesis of phenols 39 via the facile and efficient hydroxylation reaction of aryl halides 38 with water through the merger of a heterogeneous organic semiconductor graphitic carbon nitride (g-C₃N₄) photocatalyst and a homogeneous nickel(II) bipyridine organometallic catalyst under visible-light irradiation at room temperature (Scheme 19).

Scheme 19 Visible light-catalytic hydroxylation of aryl halides with water to phenols by carbon nitride and nickel complex cooperative catalysis.

The reaction mechanism was proposed by the authors (Scheme 20). In the proposed mechanism for the Nidabpy/g-C₃N₄ dual catalytic system, g-C₃N₄ acts as a photocatalyst and Et₃N provides electrons. After visible light illumination, g-C₃N₄ is excited to enable the effective separation of the photogenerated electron–hole pairs. The photogenerated holes can cause the oxidation of Et₃N to form a cationic radical. Then, the Et₃N radical undergoes a hydrogen atom transfer process with water to generate s hydroxyl radical. At this stage, the concurrent oxidative addition of aromatic halides to Ni(0) species should deliver the Ni(II) species, which would be trapped by the hydroxyl radical to form an Ni(III) organometallic adduct. Thereafter, reductive elimination would deliver the desired phenols product and Ni(I) species. Finally, the photogenerated electron by semiconductor excitation is utilized for the reduction of the Ni(I) species to Ni(0) species via single-electron transfer. Then, concurrent oxidative addition of aromatic halides to Ni(0) species delivers the Ni(II) species to complete the Ni catalytic cycle.

Scheme 20 Proposed mechanism of Nidabpy/g-C₃N₄-catalyzed visible-light-driven hydroxylation of aromatic halides with water. Adapted with permission from ref. 169.

5.12 Sonogashira coupling synthesis of ynones

H. Kaur and co-workers¹⁷⁰ reported the use of copper nanoparticles supported on graphitic carbon nitride (Cu₂O NPs@ g-C₃N₄) as a photoactive catalyst for the synthesis of ynones under mild and sustainable conditions (Scheme 21). This methodology is simple and efficient and allows the reaction to progress more efficiently under visible light at room temperature.

Scheme 21 Sonogashira coupling between benzoyl chloride and phenyl acetylene catalyzed by Cu₂O NPs@g-C₃N₄.

The authors proposed a plausible reaction mechanism for Cu₂O NPs/g-C₃N₄ (Scheme 22). In this mechanism, they proposed that the reaction proceeds via the Csp-H activation of alkyne molecules under visible light irradiation given that Cu₂O NP/g-C₃N₄ has the ability to promote the formation of the copper acetylide complex more effectively. The copper acetylide complex reacts with the preformed iminium ion to form a 2-pyridinyl propargyl amine intermediate. Subsequently, the cycloisomerization of the intermediate affords the desired product.

Scheme 22 Proposed reaction mechanism for the synthesis of ynones catalyzed by Cu₂O NP/g-C₃N₄ catalyst. Adapted with permission from ref. 170.

5.13 Anaerobic oxidation of alcohols to ketones

J. Liu and co-workers¹⁷¹ reported the use of a platinum nanocluster/graphitic carbon nitride (Pt/g-C₃N₄) composite solid catalyst with photocatalytic anaerobic oxidation function for the highly active and selective transformation of alcohols 43 to ketones 44. The desired products were successfully obtained in good to excellent yield from various functionalized alcohols at room temperature (Scheme 23).

Scheme 23 . Photocatalytic dehydrogenation of alcohols under anaerobic conditions.

The authors proposed a possible two-electron oxidation mechanism for the photocatalytic anaerobic dehydrogenation of alcohols (Scheme 24). In this mechanism, firstly, holes (h⁺) on the valence band (VB) and electrons (e⁻) on the conduction band (CB) are generated under the irradiation of solar light. Then, the Pt-mediated h⁺ extracts hydrogen from substrate 43 to release α-alcohol radicals. Subsequently, the α-alcohol radical intermediate is oxidized to afford the corresponding product 44. Simultaneously, the electrons in the CB band transfer to the Pt nanoclusters through the Schottky-junction to drive the formation of H₂ molecules. Furthermore, both byproduct 45 and H₂ provide evidence for the reaction mechanism for the photocatalytic anaerobic dehydrogenation of alcohols.

Scheme 24 Proposed two-electron oxidation mechanism of photocatalytic anaerobic dehydrogenation of alcohols initiated by Pt/g-C₃N₄.

5.14 Photocatalytic decarboxylative alkyl/acyl radical addition and reductive dimerization of para-quinone methides

Y. Cai and co-workers¹⁷² reported a modified carbon nitride-based heterogeneous photocatalytic system for both the decarboxylative addition and reductive dimerization of para-quinone methides (Scheme 25). The potassium-intercalated carbon nitride (CN–K) was facilely prepared via the direct KCl-induced structure remodeling of bulk g-C₃N₄, exhibiting remarkable catalytic activity. The heterogeneous nature and mild reaction conditions of this protocol allow good catalyst recyclability, broad substrate scope, scale-up in a continuous flow, and applications for the target synthesis.

Scheme 25 Potassium-modified carbon nitride for heterogeneous photocatalytic decarboxylative alkyl/acyl radical addition and reductive dimerization of para-quinone methides.

The authors proposed the mechanism (Scheme 26), in which, firstly, visible-light absorption by CN–K leads to efficient charge separation, yielding two-dimensional surface redox centers as electron–hole pairs. The photo-electrons reduce p-QMs to the corresponding diaryl methyl radical A via the PCET process. For the reductive dimerization reaction, radical 1,6-addition of A to p-QMs generates radical B. The photogenerated hole of CN–K with a valence band potential of +0.84 eV undergoes single-electron oxidation with HCOONa to form HCOO˙, followed by hydrogen atom transfer (HAT) to B, affording the target dimerization products. For decarboxylative 1,6-addition reaction, the photogenerated hole of CN–K undergoes single-electron transfer (SET) with carboxylic acid anions (+0.91 to +1.25 V vs. SCE for benzylic acid anions) to deliver the corresponding alkyl radical C or acyl radical D. The direct coupling of radical A with radical C or D affords the target 1,6-addition adducts.

Scheme 26 Proposed reaction mechanism for potassium-modified carbon nitride for heterogeneous photocatalytic decarboxylative alkyl/acyl radical addition and reductive dimerization of para-quinone methides.

6 Conclusions and perspectives

In the field of photocatalysis, g-C₃N₄-based photocatalytic materials are considered one of the best candidates in terms of their properties, preparation, and applications. g-C₃N₄-based photocatalysts have significant advantages and simple strategies can be used to change the properties of these photocatalysts, while their overall structure composition remains unchanged. The outstanding properties of g-C₃N₄-based photocatalysts make them the preferred form of visible light photocatalysts for a variety of applications compared to other photocatalysts. This review includes the application of g-C₃N₄ via numerous synthetic strategies in an expanded manner. The application of g-C₃N₄ in chemical functionalization via the covalent or noncovalent approach using a wide range of chemical modifiers has been proven to be a useful strategy to amplify and introduce new properties in this photocatalyst. The properties and utilization of g-C₃N₄ will give researchers some new insights, ideas, and momentum, thus enhancing the production of clean energy and their application in efficient visible light-driven catalysts in sustainable development to next-level perspectives and encourage researchers for the future development in this research field. Although significant efforts have already been devoted to the modification of g-C₃N₄ materials and optimizing their photocatalytic activity, the potential of g-C₃N₄ materials is yet to be fully exploited. In summary, we believe that with the great efforts of researchers in synthesis, characterization, and mechanistic understanding, we can finally obtain an ideal photocatalyst that achieves the aim of utilizing g-C₃N₄ to carry out various chemical transformations.

Conflicts of interest

“There are no conflicts to declare”.

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