Preparation of organic compound/g-C₃N₄ composites and their applications in photocatalysis

Abstract

Photocatalysis based on the organic polymer semiconductor, g-C₃N₄, is a green technology, but effective energy conversion is still limited by the small light absorption range and high photogenerated carrier complexation rate of g-C₃N₄ photocatalysts. The introduction of organic molecules into the g-C₃N₄ backbone has become a design hotspot for optimising g-C₃N₄ performance. In this review, recent developments in the morphology of g-C₃N₄-based composites as photocatalysts, strategies for the preparation of organic compound/g-C₃N₄ composites and the applications of organic compound/g-C₃N₄ composites in photocatalysis are introduced. The perspectives on future directions of organic compound/g-C₃N₄ composites are discussed.

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1. Introduction

Photocatalysis is the use of light energy for material transformation and involves the combination of light and a catalyst.¹ g-C₃N₄ has a narrow bandgap structure and absorption of visible light;^2,3 thus, it is capable of performing photocatalytic reactions under visible light irradiation.⁴ The process of g-C₃N₄ photocatalysis is mainly divided into three steps: (1) photon absorption, (2) generation and separation of electron–hole pairs, and (3) the g-C₃N₄ catalyst surface reaction.⁵ This gives g-C₃N₄ great potential for photocatalytic reactions using solar energy, such as hydrogen production,⁶ CO₂ reduction,⁷ pollutant degradation,⁸ sensors,⁹ batteries,¹⁰ and bioimaging.¹¹ The functionalization or modification of the pure g-C₃N₄ material can further improve the catalytic performance of the material and broaden its application range.^12,13

Among the various modification strategies, the construction of g-C₃N₄ based composites by combining g-C₃N₄ with other materials (inorganic compounds and organic compounds) has better prospects of enhanced catalytic properties. (1) Inorganic compounds are usually used to construct g-C₃N₄ based composites, such as MnO,¹⁴ γ-CuI,¹⁵ and SiO₂.¹⁶ They can overcome the disadvantages of small specific surface area, poor light absorption efficiency and fast charge transfer rate.¹⁷ (2) g-C₃N₄ integrated with different organic compounds to construct organic compound/g-C₃N₄ composites has received more and more attention.^18,19 Organic compounds are rich in functional groups and modifiability, and chemical modification can be used to change the properties of organic substances, thus achieving performance modulation. Organic compound/g-C₃N₄ composites can form stable covalent bonds to make the composite materials have better thermal and chemical stabilities.^20,21

The number of studies on photocatalysts based on g-C₃N₄ has been increasing annually (Fig. 1), indicating that this material is an ideal candidate for photocatalysis in various energy environments. Although several reviews have been published on the modification of carbon nitride by organic molecules, this review stands out by specifically discussing three different types of organics complexed with carbon nitride: (1) organic molecule/g-C₃N₄ composites, (2) metal–organic framework/g-C₃N₄ composites, and (3) organic polymer/g-C₃N₄ composites. Photocatalytic efficiency is enhanced by grafting organics into the heptazine structural unit of carbon nitride and changing the forbidden bandwidth of the photocatalyst and surface charge density. This review summarizes preparation methods of g-C₃N₄-based photocatalysts with organic compounds (thermal copolymerisation, in situ polymerisation, hydrothermal method, ball milling method and supramolecular self-assembly method), as well as introduces different morphologies of g-C₃N₄-based composites and applications of these photocatalysts in hydrogen production, CO₂ reduction, and pollutant degradation. Future prospects of organic compound/g-C₃N₄ composite photocatalysts are discussed in the end.

Fig. 1 Statistics on the number of articles published on g-C₃N₄ and g-C₃N₄-based photocatalysts worldwide from 2011 to 2023 (from the Web of Science database).

2. Morphology of g-C₃N₄-based composites

The development of g-C₃N₄ with a unique morphology represents an effective approach for enhancing the specific surface area and accelerating the carrier transfer rate.²² Significant efforts have been devoted to the design of catalysts' morphological characteristics. Various morphologies of g-C₃N₄-based composites have been successfully synthesized (Fig. 2). (1) Sheet:^23,24 the thin thickness of the sheet is much smaller than the electron wavelength, so it shows an obvious quantum confinement effect, optimizes the band structure, can significantly promote the separation of the photon generated carrier, and shortens the migration path of the photogenerated electrons on the surface of the material.²⁵ (2) Rod:^26,27 the rod shape promotes the transfer of photogenerated electrons along the axial direction to form a unidirectional flow, which suppresses the compounding rate of photogenerated electron–hole pairs and has a larger penetration surface area.²⁸ (3) Tubular:^29,30 tubes have a large layer spacing and an abundance of functional groups, making it easy to introduce defects or doping. The longer axial length leads to multiple refraction and scattering of the incident light, which can effectively increase the utilisation of light.³¹ (4) Sphericity:^32,33 spherical shape can be very good for maximising light harvesting through internal reflection and photonic effects.³⁴ (5) Porous:^35,36 porosity increases the number of surface active sites and specific surface areas and extends the range of light collection, diffusion and adsorption.³⁷ (6) Other shapes.^38,39

Fig. 2 A schematic diagram of the morphologies of g-C₃N₄-based composites.^{23,24,26,27,29,30,32,33,35,36,38,39}

3. Synthesis of organic compound/g-C₃N₄ composites

Using urea, melamine, cyanuric acid, thiourea and dicyandiamide as precursors, the performance and application potential of carbon nitride can be significantly enhanced by the addition of organic molecules containing hydroxyl, amine and other reactive groups that are capable of interacting with carbon nitride. The following methods are commonly used: thermal copolymerisation, in situ polymerisation, hydrothermal method, ball milling method and supramolecular self-assembly method. The method can be selected according to the type of organic molecules, and the doping degree and position of the organic molecules can be precisely regulated, so as to realise the effective regulation of the properties of carbon nitride (Table 1).

Table 1 Methods for the preparation of organic matter/carbon nitride

Method	Advantage	Disadvantage	Ref.
Hydrothermal	The reaction is carried out in the liquid phase, which can effectively and uniformly dope the organic molecules into the carbon nitride skeleton, resulting in the homogeneity of the physical phase of the product	The products of hydrothermal synthesis are usually multiphase mixtures containing unreacted raw materials, by-products and target products. Separation and purification for post-processing are more complex	40–42
Thermopolymerisation	Nitrogen-containing organic precursors are mixed with organic molecular dopants and then calcined at high temperatures. This method does not require complicated equipment and operation steps, so the preparation process is relatively simple	Uneven doping may lead to fluctuations or degradation in the properties of the product	43–47
Supramolecular self-assembly	The polymerisation of organic molecules of precursors is achieved by modulating intermolecular interactions, such as hydrogen bonding, to precisely control the nanostructure and morphology of carbon nitride	During supramolecular self-assembly, the doping uniformity of organic molecules may be affected by intermolecular interaction forces, solvent effects, and heat treatment conditions, leading to fluctuations or degradation in the properties of carbon nitride materials	48–50
In situ polymerisation	The conditions of the polymerisation reaction enable the precise control of the structure and composition of the carbon nitride material. It helps to obtain carbon nitride materials with a specific morphology, pore structure and chemical composition	Some by-products or unreacted raw materials may affect the purity and properties of the final product	51–53
Ball-milling	Mixing and refining of organic molecules with the carbon nitride powder by means of physical mechanics effectively promotes the uniform distribution of organic molecules in carbon nitride	Ball-milling media (grinding balls and grinding jars) may rub and collide with the raw material, resulting in some impurities being mixed into the carbon nitride material	54 and 55

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4. Preparation of organic compound/g-C₃N₄ composites

The composite method of g-C₃N₄ and organic compounds is generally simple, resulting in excellent light absorption performance. The addition of organic compounds with a unique skeleton to the g-C₃N₄ matrix induces a delocalization effect.⁵⁶ This effect effectively promotes the directional transfer of photogenerated electrons, leading to improved photocatalytic performance. Furthermore, it reduces processing costs and minimizes environmental pollution. Among the organic compounds complexed with g-C₃N₄, there is a major division into three types: organic molecule/g-C₃N₄ composites, metal–organic framework/g-C₃N₄ composites, and organic polymer/g-C₃N₄ composites.⁵⁷

4.1. Organic molecule/g-C₃N₄ composites

Organic molecules are mainly composed of elements such as C, H, O, N and S.⁵⁸ 2-Thiobarbituric acid,⁴⁶ 2-aminopyridine,⁴⁴ 1,4-dicarboxybenzene,⁵⁹ 2,6-diaminopurine⁶⁰ and other organic molecular doping in g-C₃N₄ can effectively generate free charge carriers, thereby inhibiting the recombination rate of photogenerated charge carriers.

Several factors influence the photocatalytic activity: the light absorption range, the electron donor in D–A systems: selection of organic molecules, the amount of catalysts (Fig. 3), pyrazines as acceptor co-monomers in donor–acceptor semiconductor polymeric materials, the PCN photocatalyst (PCN-DP) modified by organic molecular pyrazine changed from the original flake shape to the coral shape.⁶¹ The photocatalyst (PCN-DP) resulted in the inhibition of photogenerated electron–hole complexation and charge transfer with higher mobility. The degradation effect of PCN-DP-0.2 on sulfamethazine reached 99% in 60 min (Fig. 4A). Organic molecular carbazole was introduced into the g-C₃N₄ structure to form an intramolecular donor–acceptor (D–A) system. Carbazole served as the electron donor.⁶² The surface of the pure g-C₃N₄ material was smooth and layered, while the introduction of carbazole molecules made the surface of the composite material rough (Fig. 4B). The suitable organic molecules were embedded into the g-C₃N₄ backbone and connected through stable covalent bonds to construct g-C₃N₄-based donor–acceptor systems, accelerating intramolecular charge transfer to promote charge separation.

Fig. 3 Impact of several processes in organic molecular photocatalytic systems.

Fig. 4 (A) (a) SEM images of PCN and (b) PCN-DP-0.2. (c) A schematic band structure of PCN and PCN-DP-0.2.⁶¹ (B) (a) Graphical illustration of the various preparation strategies of g-CN modified with carbazole. (b) SEM images of g-CN and (c) g-CN-0.01Dbc.⁶²

Organic molecule/g-C₃N₄ composites have a highly crystalline network, excellent water dispersion, and photoelectric effect characteristics.⁶³ Organic molecular doping can intentionally introduce some specific elements, functional groups or molecules into the triazine skeleton of g-C₃N₄ to intrinsically optimise the energy band structure and the intrinsic configuration of g-C₃N₄ to achieve excellent and stable photocatalytic effects. Additional literature studies on organic molecule/g-C₃N₄ composites are listed in Table 2.

Table 2 Methods for preparing organic compound/g-C₃N₄ composites and their applications

Catalyst	Surface morphology of organic compound/g-C3N4 composites	Applications	Stability	Ref.
CNU-DAP4.5 (organic molecule)		Photocatalytic H2 production was carried out using Pt as a co-catalyst and TEOA as a sacrificial agent. The H2 production rate of CNU-DAP4.5 at an incident wavelength of 360 nm was about 158.4 μmol h^−1, which is three times higher than CNU	No obvious decrease/16 h	64
g-C3N4@PD-1% (organic molecule)		The photocatalytic activities of the anionic dye of sulforhodamine B (SRB) degradation over the g-C3N4@PD-1% photocatalyst could reach 96.4% after 3 h visible-light irradiation	98.2%/5 cycles	65
CN-25CYS (organic molecule)		CN-25CYS showed the highest photocatalytic H2 production rate of 3861.8 μmol h^−1 g^−1, which was 9.6 times higher than CN-0CYS, with 10 vol% TEOA as the sacrificial agent and 3 wt % Pt as the co-catalyst. In addition, the CN-25CYS photocatalyst also showed significantly enhanced photocatalytic activity in the degradation of rhodamine B (RhB) by visible light irradiation, reaching 100% within 60 min	No obvious decrease/18 h	66
AMCN (organic molecule)		The hydrogen precipitation rate of AMCN-3 was 730.3 μmol g^−1 h^−1, which was about 1.6 times higher than the original MCN	No obvious decrease/20 h	67
ACN-20 (organic molecule)		The degradation rate of RhB by ACN-20 was 100% within 30 min, which was higher than that pure CN (only 44%)	100%/5 cycles	68
ICA (organic molecule)		The degradation efficiency of fluconazole (FCZ) by ICA was 94.2% within 120 min of visible light irradiation, while pure g-C3N4 was only around 50%	—	69
ZPUCN-3 (metal–organic framework)		The ZPUCN-3 heterojunction photocatalyst showed a high CO2 reduction to a CO conversion of 5.05 mmol h^−1 g^−1, which was about 3.2 times and 2.2 times higher than pure UCN and Zr-PMOFs, respectively	No obvious decrease/4 cycles	70
CN-T3 (organic molecule)		The photocatalytic activity of the CN-T3 photocatalyst for the degradation of tetracycline (TC) under visible light irradiation was significantly enhanced, and the photocatalytic degradation efficiency of 2-hydroxynaphthalene was 2.31 times higher than pure g-C3N4	—	71
g-C3N4/NH2-MIL-125(Ti) (metal–organic framework)		Compared to the pure g-C3N4, MOF doping enhances the rate of H2 evolution. At 20wt% MOF doping, the maximum hydrogen production reaches 480 μmol g^−1, and the CO yield is also significantly enhanced to 338 μmol g^−1, which is 4.2 times higher than pure g-C3N4	No obvious decrease/3 cycles	72
PPB/g-C3N4-0.4 (organic polymer)		PPB/g-C3N4-0.4 composites have good photo-Fenton-like catalytic properties. Using PPB/g-C3N4 + H2O2 as composites, 96% of RhB was removed in 50 min, which was 9.4 times higher than pure g-C3N4	>90%/3 cycles	73

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4.2. Metal–organic framework/g-C₃N₄ composites

Metal–organic frameworks (MOFs), metal and organic ligands, form a specific framework structure, which not only has the activity of metals, but also obtains the flexibility of organic ligands and the choice of functional groups.⁷⁴ g-C₃N₄ and the MOF are both visible light-active photocatalysts. Various MOF materials have been compounded with g-C₃N₄,⁷⁵ and the metal–organic framework/g-C₃N₄ composite photocatalysts significantly improved the quantum efficiencies and reduced the electron–hole recombination rates. Stronger π–π interactions exist between the aromatic ring of the organic ligand in the MOF and the triazine ring of g-C₃N₄.⁷⁶

Compared with other kinds of MOFs, ZIFs exhibit better thermal and chemical stabilities. Among the ZIFs, the most commonly synthesized ones are ZIF-9, ZIF-8 and ZIF-67. These ZIFs are easy to synthesize and can even be crystallized at room temperature. The ZIF-9 precursor was prepared with the assistance of ammonia, and PCN@ZIF-9 (CoWS) was prepared by a one-step hydrothermal method.⁷⁷ ZIF-9 square sheet structures could be seen stacked on PCN nanosheets with some folding at the edges of the monolayer (Fig. 5A).

Fig. 5 (A) (a) SEM images of PCN nanosheets, (b) ZIF-9 and (c) PCN@ZIF-9(CoWS)-3 semiconductor junction nanowires.⁷⁷ (B) (a) Schematic illustration of the preparation of PDI-g-C₃N₄ and PMDI-g-C₃N₄. (b) SEM images of PDI-g-C₃N₄/MIL and (c) PDI-g-C₃N₄.⁷⁸

MIL materials are prepared from trivalent metals such as chromium, iron, aluminium and vanadium with carboxylic acids such as terephthalic acid or homotrimellitic acid. Between the aromatic ring in the MIL and the triazine ring in g-C₃N₄, the abundant surface electrostatic interaction and strong π–π interaction exist. Energetic polymers with donor–acceptor structures (PMDI-g-C₃N₄) were constructed by introducing electron-deficient perylene diimide into the framework of g-C₃N₄.⁷⁸ The square block-shaped MIL spear fixed to the surface of the g-C₃N₄ layered structure could be seen in the SEM image (Fig. 5B).

MOFs are highly porous crystalline and surface area materials, and metal–organic framework/g-C₃N₄ composites can increase the specific surface area and provide more photocatalytic active sites. The composites exhibit better catalytic performance with better photogenerated carrier separation efficiency. Table 2 lists additional literature on metal–organic framework/g-C₃N₄ composites.

4.3. Organic polymer/g-C₃N₄ composites

Organic polymers are a group of compounds with a particularly high relative molecular mass. Organic polymer/g-C₃N₄ composites were formed by coupling target organic monomers and precursors with different chemical compositions and electronic structures. The clustered organic polymer, POP, was attached to the surface of the stacked lamellar structure of g-C₃N₄ by a simple one-pot hydrothermal synthesis method, and additionally, silver nanoparticles were anchored to graphitic carbon nitride modified with porous organic polymers.⁷⁹ When Ag/POP@g-C₃N₄ was used as a catalyst for the carboxylative cyclization reaction of alkyne alcohols with CO₂, the yields of the compounds obtained from their isolation were all higher than 90% (Fig. 6A), indicating that the material had good catalytic CO₂ conversion properties.

Fig. 6 (A) (a) Carboxylic cyclization of propargylic alcohols with CO₂. (b) and (c) SEM images of g-C₃N₄ and POP@g-C₃N₄.⁷⁹ (B) (a) Preparation of the g-C₃N₄/PFSO composite. (b) SEM images of pure g-C₃N₄ and (c) the g-C₃N₄/PFSO-1 composite.⁸⁰

Granular PFSO was dispersed on the irregular layered structure of g-C₃N₄.⁸⁰ Composites of g-C₃N₄ with polymers containing dibenzothiophene groups (g-C₃N₄/PFSO) were synthesized by using a wet impregnation method to form polymer heterojunctions (Fig. 6B).

Organic polymers are formed by bridging organic monomers through covalent bonds, which have adjustable surface area, pore size, molecular structure, and high chemical and thermal stability. Organic polymer/g-C₃N₄ composites have unlimited potential in the field of photocatalysis. Current reports offer possibilities for the combination of organic polymers with g-C₃N₄, as listed in Table 2.

5. Applications of organic compound/g-C₃N₄ composites

Organic compound/g-C₃N₄ composites have been applied in photocatalytic fields such as pollutant degradation, hydrogen production, and CO₂ reduction.

5.1. Pollutant degradation

The organic molecule/g-C₃N₄ composites synthesized by Meng et al. had photoelectron catalytic degradation functions in water treatment under visible light irradiation.⁸¹ The porous carbon nitride nanosheets containing cyclopentadiene units with well-modulated energy band structures were synthesised. A large amount of O₂ was reduced by photogenerated electrons (e⁻) to produce ˙O₂⁻, while photogenerated holes (h⁺) oxidised ˙O₂⁻ to generate ¹O₂. The photodegradation of acetaminophen and methyl parahydroxybenzoate occurred at the C_CPD–g-C₃N₄ interface. These actives (h⁺, ˙O₂⁻, and ¹O₂) could oxidise the pollutants to CO₂, NO₃⁻ and H₂O. The degradation pathway of methyl para-hydroxybenzoate as the target pollutant starts with the removal of the hydroxyl group attached to oxygen, then h⁺ and ¹O₂ attack C2 and C6 for the hydroxylation reaction, followed by ˙O₂⁻ attacking the C10 and C4 bonds to produce hydroquinone. Hydroquinone is further oxidised and finally reduced to CO₂ and H₂O (Fig. 7).

Fig. 7 (a) and (b) Degradation pathways and products of C_CPD-g-C₃N₄.⁸¹

Due to the oxytetracycline (OTC) complex molecular structure, high stability and low biodegradability, it is urgent to reduce its environmental hazards. Yellowish-brown BCNNS in the degradation efficiencies of oxytetracycline was 72% for 60 minutes, while pure CNNS was only degradate 50% of oxytetracycline.⁸² The benzene ring, as the CN organic molecular dopant, can reduce the bandgap, resulting in an increase in both the CB and the VB. ¹³C NMR carbon spectra confirmed the presence of BCNNS50, which exhibited a new peak at 120–140 ppm corresponding to aromatic carbons. BCNNS50 also demonstrated good photocatalytic cycling stability (Fig. 8A). Compared with pure CN, the degradation effect on hygromycin in the presence of organic molecules (thiophene ring) was found to be the best, achieving 93% within 60 minutes.⁸³ The new peak observed at 128 ppm in the ¹³C NMR carbon spectrum was attributed to the C=C of the thiophene aromatic ring, indicating successful bonding of the thiophene ring to the CN backbone. The ESR indicated that ˙OH and ˙O₂⁻ played a dominant role in the degradation process (Fig. 8B).

Fig. 8 (A) (a) ¹³C solid-state nuclear magnetic resonance spectra of CNNS and BCNNS50. (b) Photocatalytic degradation efficiency of CNNS, BCNNS25, BCNNS50, and BCNNS75 for OTC. (c) Four cycling tests on BCNNS50 for OTC degradation.⁸² (B) (a) Photocatalyst CN and TCN-x degradation efficiency for OTC. (b) ESR spectra of the DMPO⁻˙OH adduct and (c) DMPO–˙O₂⁻ adduct for TCN-5.⁸³

Photocatalytic degradation, as an environmentally friendly treatment method, can effectively reduce the concentration of pollutants in wastewater and mitigate their negative impact on the ecosystem.^84,85 Typically, h⁺, ˙O₂⁻, ¹O₂, and ˙OH are the active groups in photocatalytic degradation of pollutants,⁸⁶ and organic compounds become electron donors when they bind to the triazine ring of the g-C₃N₄ skeleton, causing a large number of electrons to accumulate around them and changing the electron distribution, thus affecting the activation of molecular oxygen. The recent literature has shown that organic compound/g-C₃N₄ composites exhibit strong catalytic degradation performance for contaminants in real samples when exposed to visible light irradiation (Table 3).

Table 3 Summary of photocatalytic degradation of multiple pollutants over organic compound/g-C₃N₄ composites

Catalyst	Surface morphology of organic compound/g-C3N4	Initial concentration	Degradation efficiency	Main active species	Ref.
g-C3N4-Nx (organic molecule)		50 mL of cefotaxime (10 mg L^−1)	98.4% (25 min)	˙O2^−	87
				h^+
CCNx (organic molecule)		30 mL of BPA (100 μM)	99.0% (60 min)	^1O2	88
				˙OH
				SO4˙^−
DX (CN) (organic molecule)		150 mL of phenol (10 mg L^−1)	15.4% (120 min)	˙OH	89
		Ofloxacin (20 mg L^−1)	91.3% (120 min)	˙O2^−
		Ampicillin (20 mg L^−1)	22.6% (120 min)
PTBCN-X (organic molecule)		50 mL of RhB (10 mg L^−1)	94.8% (60 min)	˙O2^−	90
PA-CN (organic molecule)		50 mL of sulfamethazine (20 mg L^−1)	99.7% (60 min)	˙O2^−	91
				˙OH
CCPD-g-C3N4 (organic molecule)		50 mL of acetaminophen (10 mg L^−1)	100% (45 min)	˙O2^−	81
		Methylparaben (10 mg L^−1)	100% (90 min)	^1O2

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5.2. Hydrogen production

Under the background of low carbon emission reduction, green hydrogen production technology has received wide attention, and photocatalytic hydrogen production by water decomposition is one of them. The polymer semiconductor, g-C₃N₄, is a visible light responsive hydrogen production material with a suitable band gap, but due to the limitations of its visible light trapping ability and poor photogenerated charge mobility, it is possible to improve the photocatalytic hydrogen production activity by complexing the carbon nitride with an organic compound and introducing the hydrophilic active group, thus improving the dispersion properties in the aqueous solution. By complexing carbon nitride with organic materials and introducing hydrophilic active groups, the dispersion performance in aqueous solution is improved, thus improving the photocatalytic hydrogen generation activity.

The process of hydrogen production catalysed by organic molecule/g-C₃N₄ composites, BS-CN, mainly consists of these steps.⁹² Under visible light irradiation, the electrons in the valence band absorbed enough energy to jump to the conduction band. The photogenerated electrons accumulated in the Pt active sites, which reduced H⁺ to produce H₂ by the Pt co-catalysts. The photogenerated holes oxidised the lactate into organic chemicals. The introduction of benzenesulfonyl chloride into the carbon nitride skeleton extended the π-electron delocalisation properties (Fig. 9).

Fig. 9 The mechanism diagram of photocatalytic production of H₂ and organic chemicals at BS-CN under visible light irradiation.⁹²

The flexible nature of g-C₃N₄ facilitates its binding to metal–organic frameworks (UiO-66-NH₂) and the resulting composite may be a good candidate for photocatalytic hydrogen production with good photocatalytic activity and stability in aqueous solution.⁹³ The octahedral structure UiO-66-NH₂ was loaded on the surface of phosphorus-doped tubular carbon nitrates in situ to obtain the p-TCN@U6-3 composite material. Its hydrogen production performance was 8.19 times higher than p-TCN and UiO-66-NH₂. The composites exhibited excellent photocatalytic hydrogen production performance. ˙O₂⁻ played a crucial role in the photocatalytic hydrogen production process of p-TCN@U6-3 (Fig. 10A).

Fig. 10 (A) (a) SEM image of p-TCN@U6-3. (b) The H₂ evolution rates of p-TCN, UiO-66-NH₂ and p-TCN@U6-3. (c) EPR spectra of p-TCN, UiO-66-NH₂ and p-TCN@U6-3 under visible light irradiation.⁹³ (B) (a) A schematic diagram of preparation steps of BF-g-C₃N₄. (b) Photocatalytic H₂ evolution rates of BFx-g-C₃N₄ samples under visible light irradiation.⁹⁴ (C) (a) Photocatalytic hydrogen evolution over CN and CNSO-20. (b) Schematics of the relative positions of the HOMO and LUMO of pure CN and CNSO-20.⁹⁵

The incorporation of the whole organic molecule (basic fuchsin) into the superstructure of g-C₃N₄ narrowed the bandgap. It had good photocatalytic hydrogen production performance under visible light, and its hydrogen evolution performance was 4.4 times higher than pure g-C₃N₄ (Fig. 10B).⁹⁴

The hydrogen precipitation activity of CNSO-20 was 8.5 times higher than that of pure CN.⁹⁵ Organic molecules with good hydrophilicity: dibenzothiophene analogues, which improved the dispersion of CNSO-20 in water, thus increasing their photocatalytic hydrogen evolution activity. The obtained energy band structure spectra showed that doping of SO led to a downward shift of conduction bands and a narrowing of the band gap value, which was favourable for the transition of the electrons (Fig. 10C).

Therefore, the development of new photocatalytic materials for hydrogen production is imminent. The hydrogen evolution performance of the organic compound/g-C₃N₄-based composites is summarized in Fig. 11 to provide the development of the catalysts in the photolysis system. The efficiency of photocatalytic hydrogen production is affected by the amount of the catalyst, Pt co-catalyst, which needs to be considered and optimised in order to improve its efficiency.

Fig. 11 Hydrogen evolution performance of the organic compound/g-C₃N₄ composites.^96–104

5.3. CO₂ reduction

CO₂ is an important carbon feedstock in chemical synthesis, and the use of solar energy to fix CO₂ to simulate plant photosynthesis to produce chemical fuels is one of the best options for achieving carbon neutrality in green chemistry. The reduction of CO₂ is a complex redox process that involves the formation of various carbon-containing products and the precipitation of hydrogen. Meng and co-workers incorporated ZIF-67, a zinc-based MOF, with g-C₃N₄ nanosheets (g-C₃N₄/ZIF-67) via a facile thermal polymerization method.¹⁰⁵ g-C₃N₄/ZIF-67 exhibits a larger specific surface area, providing more reactive sites for adsorption of CO₂. Increased adsorption of CO₂ in favourable of photocatalytic CO₂ reduction. Under visible light irradiation, CO₂ was adsorbed by the photocatalyst g-C₃N₄/ZIF-67. The holes oxidized H₂O in solution to H⁺, while the electrons reduced CO₂ to CH₃CH₂OH (Fig. 12a).

Fig. 12 (a) The mechanism of the photocatalyst g-C₃N₄/ZIF-67 process.¹⁰⁵ (b) and (c) Distributions of the HOMO and LUMO for CN and EY-PhCN. (d) The mechanism involved for CO₂ reduction by the photocatalyst EY-PhCN under visible light.¹⁰⁶

The benzene ring and eosin Y were covalently grafted onto the carbon nitride photocatalyst by thermal calcination to produce EY-PhCN.¹⁰⁶ According to DFT calculations, the uniform electron distribution in CN can easily lead to electron–hole complexation, and the grafting of the benzene ring and eosin Y can lead to the disruption of the symmetry structure of the carbon nitride homotriazine ring as well as induce the directional transfer of electrons. The HOMO of EY-PhCN is located at the point of attachment to the benzene ring. The HOMO of EY-PhCN is located on the triazine ring attached to the benzene ring and the LUMO is located at the terminal site attached to eosin. The movement of electrons from the edge of the central phase significantly inhibits the rate of photogenerated carrier complexation. The mechanism of CO₂ reduction by EY-PhCN under visible light is as follows. Photogenerated electrons reduce CO₂ to CO and H₂, TEOA acts as an electron donor to consume the photogenerated holes, and the electrons on VB photoluminescence absorb energy to jump to the CB. The electrons in the CB reduce CO₂ to CO and CH₄ (Fig. 12b).

MOLs are a new class of 2D metal–organic framework with an open two-sided planar structure and unique electronic properties.¹⁰⁷ The active sites in MOLs are more accessible to CO₂, which accelerates the reaction. A series of 2D/2D TM-MOL/CN heterojunction composites were assembled through in situ ultrasound-assisted synthesis.¹⁰⁸ Among the composites, Co-MOL/CN(400) with the optimized ratio demonstrated the best photocatalytic CO₂ reduction activity and favourable cycling stability. After 12 hours of illumination, 10.5 μmol CO was produced. The ¹³CO₂ photoreduction experiment further emphasized that CO₂ was the only raw material for CO in this reaction system (Fig. 13A).

Fig. 13 (A) (a) CO₂ photoreduction performance over the CN sample. (b) Control experiments of photocatalytic CO₂ reduction under different conditions. (c) Gas chromatography and mass spectroscopy analysis of the generated gas from the photocatalytic reduction of ¹³CO₂ by Co-MOL/CN(400).¹⁰⁸ (B) (a) SEM image of TpTta COF. (b) Photocatalytic yields of CO for CN, Co@TpTta/CN, NCN and Co@TpTta/NCN.¹⁰⁹

COFs, as organic polymers with a large number of aromatic rings, form heterojunction photocatalysts through π–π interactions with g-C₃N₄. The carbonylimine-containing TpTta COF exhibited flower-like morphology.¹⁰⁹ This framework acted as an electron bridge, connecting the photocatalyst and metal active site, and combined with defective g-C₃N₄ to form a composite material known as Co@TpTta/NCN. The CO formation rate of Co@TpTta/NCN was 37.3 μmol h⁻¹, while the original CN was only 0.3 μmol h⁻¹ (Fig. 13B).

The active sites of CO₂ adsorption and photoreduction on g-C₃N₄ are sparse due to the specific surface area size and pore size distribution. Organic compound/g-C₃N₄ composites form a stable chemical bond, optimizing the electronic structure. Organic compounds can act as an electron acceptor or donor, cooperating with g-C₃N₄ to promote the formation of specific CO₂ reduction products and improve product selectivity. Organic compounds can provide additional reactive sites, enhance mass transfer, stabilize the catalyst, and effectively improve the efficiency of carbon dioxide reduction (Fig. 14).

Fig. 14 Organic compound/g-C₃N₄ composite photocatalysts for photocatalytic reduction of CO₂.^{106,110–116}

5.4. Other applications

In addition to the three photocatalytic applications described above, organic/carbon nitride composites have been actively used in photocatalytic bactericidal applications. Poly(vinylimide) was grafted onto the surface of carbon nitride nanosheets, and bacteria were adsorbed onto the photocatalyst surface by electrostatic attraction, and the active species produced by the photocatalyst effectively eradicated the bacteria. The inactivation efficiency of the photocatalyst reached 6.2 log for Escherichia coli in 45 min and that of Enterococcus faecalis reached 4.2 log in 60 min.¹¹⁷

6. Summary and prospects

In this paper, the morphology of g-C₃N₄, the preparation of organic compound/g-C₃N₄ composites and their applications in photocatalysis are summarized and discussed.

(1) Organic compound/g-C₃N₄ composites adjust the band gap of g-C₃N₄ to facilitate sunlight absorption and charge migration. They have the advantages of synergistic effects, high stability and designability. Therefore, organic compound/g-C₃N₄ composites show greater application prospects in photocatalysis.

(2) Organic compound/g-C₃N₄ composites can improve the properties of carbon nitride materials and show better photocatalytic activity.

Some challenges are still existing in the research of synthetic strategies and performance optimization of organic compound/g-C₃N₄ composite photocatalysts.

(1) Copolymerization strategies are usually limited by the type of active group and limited by the copolymerization temperature, leading to a decrease in the degree of polymerization and inhibiting further improvement of photocatalytic activity.

(2) The copolymerisation of organic monomers with the reactive groups of the carbon nitride precursor can lead to the formation of interfacial defects (more NH₂ exposed on the surface), which leads to the disruption of the reaction centre, resulting in the termination of the growth of the organic polymer at the carbon nitride interface and a decrease in the photocatalytic performance.

(3) Most of the prepared photocatalysts are in powder form and have recycling problems, which need to be further optimised.

(4) By grafting organic molecules into the heptazine structural unit of carbon nitride, the organic molecules act as electron donors and the carbon nitride acts as an electron acceptor, changing the charge density on the surface of the photocatalysts as well as the electron transfer paths during the photocatalytic process, and ultimately enhancing the photocatalytic effect.

(5) Organic molecule isomers can be considered. Under the premise of maintaining the same elements, the carbon frame, position and functional group isomerism and other conditions can occur, which will have a significant impact on their photocatalytic kinetics.

(6) Organic molecules with specific structures and properties are designed by screening organic molecules for potential effects through computer-aided molecular design methods. These molecules may contain specific functional groups (amino group and carboxyl group) to enhance the photocatalytic performance of composites.

Data availability

No data were used for the research described in the article.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (21375117) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions and Postgraduate Research & Practice Innovation Program of Jiangsu Province (Yangzhou University) SJCX24_2203.

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