Recent advances in graphite carbon nitride-based nanocomposites: structure, antibacterial properties and synergies

Abstract

Bacterial infections and transmission threaten human health and well-being. Graphite carbon nitride (g-C₃N₄), a promising photocatalytic antibacterial nanomaterial, has attracted increasing attention to combat bacterial transmission, due to the outstanding stability, high efficiency and environmental sustainability of this material. However, the antibacterial efficiency of g-C₃N₄ is affected by several factors, including its specific surface area, rapid electron/hole recombination processes and optical absorption properties. To improve the efficiency of the antibacterial properties of g-C₃N₄ and extend its range of applications, various nanocomposites have been prepared and evaluated. In this review, the advances in amplifying the photocatalytic antibacterial efficiency of g-C₃N₄-based nanocomposites is discussed, including different topologies, noble metal decoration, non-noble metal doping and heterojunction construction. The enhancement mechanisms and synergistic effects in g-C₃N₄-based nanocomposites are highlighted. The remaining challenges and future perspectives of antibacterial g-C₃N₄-based nanocomposites are also discussed.

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Kai Yan

Kai Yan is a lecturer at the College of Bioresources Chemical and Materials Engineering in Shaanxi University of Science and Technology. He received his PhD degree from Fudan University in 2017. His current research interest is mainly focused on multifunctional nanomaterials for biological applications.

Lingjie Meng

Lingjie Meng is a professor at the School of Chemistry in Xi'an Jiaotong University. He received his PhD degree from Xi'an Jiaotong University in 2005 and was a lecturer in Shanghai Jiaotong University from 2005 to 2010. He became an associate professor in 2011. He was a visiting scholar at the University of Texas at Austin from 2011 to 2012. In 2013, he was appointed as a Professor in Xi'an Jiaotong University. His research interests are focused on the nanomaterials for theranostics.

Zhaofu Fei

Zhaofu Fei is a senior research scientist in the Institute of Chemical Sciences and Engineering at the Ecole Polytechnique Fédérale de Lausanne (EPFL). He received his PhD degree from the Technical University of Braunschweig, Germany in 1999. After postdocs in the UK and Germany he joined EPFL in 2002. His research interests are focused on design and synthesis of ionic liquids and related nanomaterials.

Paul J. Dyson

Paul J. Dyson is a professor in the Institute of Chemical Sciences and Engineering at the EPFL. He received his PhD degree from the University of Edinburgh in 1993 and subsequently held positions at Imperial College of Science, Technology and Medicine and the University of York. His research interests are focused on synthesis and properties of compounds and materials with applications in catalysis and medicine.

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1. Introduction

Bacterial infections and their transmission pose a considerable threat to human health, usually leading to delayed wound healing and chronic intestinal diseases.^1,2 Furthermore, pathogenic bacteria frequently contaminate water supplies and the soil, resulting in the death of animals and plants through environmental contamination.³ To address these problems, antibiotics are widely used to kill bacteria, but over-utilization of antibiotics can bring about resistance and secondary contamination.^4–6 Therefore, the development of new strategies to inactivate bacteria without using antibiotics is urgently required as is the inactivation of drug-resistant bacteria. In recent years, semiconductor photocatalysis has attracted significant interest for applications in pollutant degradation⁷ and antimicrobial applications.⁸ Under sunlight irradiation, semiconductor photocatalysts react with water and oxygen to form reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂), hydroxyl radicals (˙OH) and superoxide (O₂˙⁻), which are able to inactivate bacteria by oxidizing the phospholipid membrane, proteins and nucleic acids.^9–13 Common photocatalysts include metallic oxides, sulfides, nitrides, and phosphides, and graphene and its derivatives,^14,15 which show bactericidal activity against both Gram-positive and Gram-negative bacteria, as demonstrated in many studies. However, relatively narrow spectral absorption ranges result in low efficiencies.¹⁶ Together with other problems such as facile aggregation, potential toxicity, and low biocompatibility, practical application of these common photocatalysts is limited. As an emerging non-metallic photocatalyst, graphitic carbon nitride (g-C₃N₄) is easy to prepare and has an appropriate band structure and good biocompatibility,¹⁷ thus showing considerable potential as antibacterial materials.

Similar to the layered structure of graphene, g-C₃N₄ is a polymeric layered material which consists of carbon and nitrogen with some hydrogen (impurity). The conduction band (CB) and valence band (VB) position of g-C₃N₄ are ∼1.13 and 1.57 eV, respectively. The appropriate band gap is about 2.70 eV, i.e. it is a medium band gap semiconductor, readily obtained from the pyrolysis of melamine, dicyandiamide or urea. Benefiting from the connection of tri-s-triazine units through tertiary amines (Fig. 1), g-C₃N₄ has a two-dimensional flake structure. The adjacent g-C₃N₄ flakes interact weakly with each other via van der Waals forces, displaying a layer gap of about 0.33 nm.^18–22 Thus, when such forces between the bulk g-C₃N₄ layers are broken, paper-like g-C₃N₄ nanosheets are obtained.^23–27 From a molecular prospective, C₃N₄ is considered to have two main molecular structures. One comprises a triazine with a tertiary nitrogen atom in the center of a planar triangle (Fig. 1a) that connects three separate triazine rings in an infinitely repeating pattern.²⁸ Another is a tri-s-triazine structurally connected to form g-C₃N₄ (Fig. 1b). This more stable tri-s-triazine structure is commonly used as the structural unit of g-C₃N₄.²⁹ These excellent structural features and properties make C₃N₄ a promising photocatalytic material. Since the first study of the photocatalytic activity of g-C₃N₄ by Wang et al. in 2009, it has become a prevalent photocatalytic material.³⁰ Due to the merits of the non-metallic g-C₃N₄ material, such as a wide visible light absorption range, excellent chemical stability and low toxicity, it has been widely studied to tackle environmental and energy related problems.^31–35 Specifically, g-C₃N₄ has been used as a catalyst for photocatalytic water reduction and oxidation, contaminant degradation and carbon dioxide reduction.^36–41 For photocatalytic degradation, the photo-produced electrons (e⁻) and holes (h⁺) can accelerate reduction and oxidation degradation reactions.^42–47 In addition, the e⁻ and h⁺ can react with surrounding H₂O and O₂ to generate ROS such as ˙O²⁻ and ˙OH⁻. The generated ROS can further degrade pollutants, combining to achieve the removal of contaminants.^48–51

Fig. 1 (a) Tri-s-triazine and (b) s-triazine structure of g-C₃N₄.

In this review, several tactics for enhancing the antimicrobial efficiency of g-C₃N₄-based nanocomposites are discussed (Fig. 2), including the design of different g-C₃N₄ topologies,^52–55 noble metal decoration,^56–58 non-noble metal doping and heterojunction construction.^59–61 These approaches have been shown to effectively boost the antibacterial activity of g-C₃N₄.^62–67 The enhancement mechanisms and synergistic effects of g-C₃N₄-based nanocomposites is highlighted. Additionally, photocatalytic mechanisms have been elucidated by analyzing the interactions between the nanomaterials and bacteria. Lastly, this review concludes by defining future prospects, opportunities and challenges in this exciting field.

Fig. 2 Schematic illustration of antimicrobial enhancement of g-C₃N₄-nanomaterials.

2. g-C₃N₄-based materials and their photocatalytic antibacterial

Both the nitrogen and carbon atoms in g-C₃N₄ are sp² hybridized to form a conjugated structure with delocalized π-electrons, giving g-C₃N₄ the lowest bandgap compared to other phases of C₃N₄.⁶⁸ Compared to other conventional photocatalytic materials, g-C₃N₄ has a narrow band gap (2.7 eV), resulting in a wider spectral absorption range of up to 460 nm, and improving the photocatalytic window.⁶⁹ The photocatalytic reactions of g-C₃N₄ affect its antibacterial performance. When the energy of the visible light illuminating g-C₃N₄ is larger than the band gap energy of g-C₃N₄, e⁻ are promoted from the valence band (VB) to the conduction band (CB), producing active e⁻ and h⁺. However, the e⁻/h⁺ can recombine on the g-C₃N₄ surface. Alternatively, the e⁻/h⁺ pairs diffuse or are transported to the g-C₃N₄ interface by an electric field and undergo redox reactions with the surroundings. As shown in Fig. 3, under visible light illumination, g-C₃N₄ produces ROS that can destroy the bacterial membranes, causing cell membrane permeability, structural degradation and ultimately killing the bacterial.⁷⁰

Fig. 3 Plausible reactions between bacterial and g-C₃N₄-based nanocomposite generated ROS.

2.1 Influence of topology on g-C₃N₄-based antibacterial photocatalysts

The antibacterial activity of g-C₃N₄ is influenced by its topology, in particular, the efficiency of bulk g-C₃N₄ is restricted by its small superficial area and rapid rate of recombination of photogenerated carriers.^71–75 In contrast, mesoporous g-C₃N₄, g-C₃N₄ nanotubes and nanosheets absorb visible light more effectively and provide a larger contact area for reactants because of their larger specific surface areas and a larger number of actives sites.^76,77 In addition, these structures decrease the distance required for the transfer of the charge to the surface of the material, decreasing charge recombination.^78,79 Moreover, due to the quantum size effects, g-C₃N₄ nanosheets are suited to charge transfer and separation processes.^80,81 Therefore, g-C₃N₄ nanosheets and their composite materials show promising photocatalytic antibacterial properties. Li et al. developed a self-cleaning antibacterial membrane by simply filtering g-C₃N₄ nanosheets into polyacrylonitrile porous substrates (Fig. 4a), then forming a stable coating by cross-linking polyvinyl alcohol and glutaraldehyde.⁸² In contrast to membranes without g-C₃N₄ nanosheets, the membranes containing the g-C₃N₄ nanosheets (0.45 wt%) completely inactivated 1 × 10⁶ cfu mL⁻¹E. coli under irradiation with visible light (Fig. 4b and c). The high superficial area of the g-C₃N₄ nanosheets in the membrane provides more active sites that produce ROS for sterilization. Meanwhile, the membranes with g-C₃N₄ nanosheets also showed good permeability to water and degraded dyes. Compared to g-C₃N₄ nanosheets, nanotubes have high aspect ratios, a topology that favors the migration of e⁻ along the axial direction and inhibits the lateral transfer of e⁻, thereby inhibiting the recombination of photogenerated carriers. Moreover, nanotubes usually have large specific surface areas, providing a higher density of actives sites at their surface, which improves photocatalytic antibacterial performance.^83,84 Xu et al. successfully synthesized microtubular nanoporous g-C₃N₄ with a layered structure and nitrogen defects using molecular self-assembly methods.⁸⁵ The hexagonal tubular structure promotes the multiple use of light, and provides a larger density active sites and a directional transfer channel for e⁻. Moreover, the nanoporosity of the material increases the specific surface area to provide rich charge transport paths. In addition, the nitrogen vacancies improve the light harvesting properties of the material (λ > 450 nm) and promote charge separation by trapping charge. Hence, microtubular nanoporous g-C₃N₄ completely inactivated 5 × 10⁶ cfu mL⁻¹E. coli after 4 h of light illumination, compared to only 74% of E. coli sterilized by bulk g-C₃N₄. Hollow porous microspheres not only promote light penetration within the material and light absorption at the pore edges, but also provide sufficient contact area to accelerate interfacial charge transfer. In addition, the thinner pore wall structure reduces the distance (and time) required for charge transfer within the material, decreasing the recombination of photogenerated carriers.⁸⁶ Yang et al. successfully fabricated a self-cleaning, antimicrobial and antifouling membrane by integrating mesoporous g-C₃N₄ (MCN) into polyvinylidene fluoride (PVDF).⁸⁷ The mesoporous structure promotes multiple reflections of incident light and enhances the capacity of the material for light capture, leading to an enhancement in the generation of h⁺ and ROS. The MCN-PVDF membrane showed a significant reduction in the number of E. coli colonies under illumination with visible light over 4 hours, with approximately 3 log deactivation of E. coli.^88,89 In contrast, an analogous experiment using a membrane-free material showed no significant decrease in E. coli communities over the same time period. Additionally, under visible light, MCN-PVDF degrades the antibiotic cefotaxime (CFX) with a degradation rate of over 97% over five cycles.

Fig. 4 (a) Preparation of g-C₃N₄ nanosheet-functionalized composite membranes. The concentration of g-C₃N₄ nanosheets in the membrane are 0, 0.15 wt%, 0.3 wt%, 0.45 wt%, 0.6 wt% in M0, M1, M2, M3, M4, respectively. (b) Antimicrobial activities against E. coli of (b1) control, (b2) M0, (b3) M1, (b4) M2, (b5) M3, (b6) M4. (c) Antibacterial rate of membranes M0, M1, M2, M3, M4. Reproduced from ref. 82 with permission from Royal Society of Chemistry, copyright 2017.

When g-C₃N₄ has a large specific surface area its photocatalytic performance is enhanced, e.g. in g-C₃N₄ nanosheets and nanotubes, which have a high density active sites. Furthermore, g-C₃N₄ nanocomposites inhibit e⁻/h⁺ pair recombination and have high charge transfer efficiency due to enhanced visible light absorption. Hence, discrepant topologies of g-C₃N₄ should have outstanding antibacterial performance and the discrepant topologies of g-C₃N₄-based materials and their corresponding antibacterial properties are summarized in Table 1.

Table 1 Antibacterial properties of g-C₃N₄ based materials with different topologies

Material	Preparation	Bacteria	Effect	Ref.
g-C3N4 nanosheets	Acid etching and ultrasound	E. coli	∼100%	82
Microtubular nanoporous g-C3N4	Molecular self-assembly	E. coli	99.2%	85
g-C3N4 nanosheets	Freezing and microwave-assisted	E. coli	100%	90
Porous g-C3N4 nanosheets	Template-free	E. coli	100%	91
g-C3N4 nanosheets	Bacterial etching	E. coli	3.65 log	92
g-C3N4 nanosheets	Ultrasound	E. coli O157:H7	0.82 log	93
		S. aureus	0.85 log
g-C3N4 nanosheets	Chemical exfoliation	E. coli K-12	6.5 log	94
g-C3N4 nanosheets	Ultrasound	E. coli	99%	95
bare g-C3N4	Calcination	MS2	100%	96
Mesoporous g-C3N4	Immersion-precipitation phase transformation	E. coli	3 log	87
Mesoporous g-C3N4	Template method	E. coli K-12	99%	97
Mesoporous g-C3N4	Thermal polymerization and selective dialysis approach	E. coli	99%	98
		S. aureus	90%
Ag2WO4/mesoporous g-C3N4	Polymerization	E. coli	100%	99
GO quantum dots/oxidized nanoporous g-C3N4	Self-assembly	E. coli	99.6%	100
Nanomesh g-C3N4	Template method	E. coli K-12	85%	101
CuInSe2:Zn/g-C3N4/TiO2 nanowire	In situ growth	S. aureus	90%	102
Mesoporous g-C3N4	Thermal, polycondensation	E. coli	Effective	103
Porous g-C3N4	Calcination	S. aureus	99%	104
Mesoporous g-C3N4	Calcination	E. coli	Effective	105
Nanostructured g-C3N4	Calcination	E. coli O157:H	97.1%	106
		S. aureus	93.7%
Mesoporous g-C3N4	Hydrothermal	E. coli	Effective	107
g-C3N4 powder	Calcination	E. coli	Effective	108
		S. epidermidis

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2.2 Noble metal decorated g-C₃N₄ nanocomposites

Another strategy used to increase the antibacterial performance of g-C₃N₄ involves modification with noble metal nanoparticles, including silver and gold nanoparticles.^109,110 Surface Plasmon Resonance (SPR) of nanoparticles enhances antibacterial efficiency, by extending the spectral absorption range and promoting the formation of photogenerated carriers in g-C₃N₄.^111–114 Additionally, noble metal nanoparticles act as electron traps, capturing free e⁻ and thus inhibiting the recombination of photogenerated carriers.^115–118 Ma et al. developed Ag/g-C₃N₄ nanocomposites by combining thermal polymerization with photo-assisted reduction.¹¹⁹ A synergistic antibacterial efficiency was achieved with superior sterilization activity of the Ag/g-C₃N₄ nanocomposite compared to pure g-C₃N₄ (Fig. 5f). Ag(0.3 wt%)/g-C₃N₄ exhibited prominent antibacterial performance and suppressed E. coli replication (7.41 log) with only 1.25 h of visible light illumination. In contrast, pure g-C₃N₄ displayed very low inactivation, with only about (0.4 log) E. coli killed following 1.5 h of illumination by visible light (Fig. 5g). Notably, the loading of noble Ag nanoparticles on the g-C₃N₄ nanosheets significantly increases the visible light absorption region due to the SPR effect of the Ag nanoparticles and the charge transfer between the Ag and the g-C₃N₄ nanosheets.¹²⁰ Similarly, the strong and unique surface plasmon resonance (SPR) absorption of gold nanoparticles covers a wide range of spectra, including the visible and near-infrared light (NIR).^121,122 As shown in Fig. 5a, Dai et al. utilized 5–10 nm sized Au nanoparticles to modify g-C₃N₄via liquid-phase exfoliation of g-C₃N₄ combined with the photo-deposition of Au nanoparticles.¹²³ When a mixture of E. coli and the Au/g-C₃N₄ nanocomposite were irradiated at 670 nm the resulting ROS effectively kill the bacteria. The viability of the bacteria continually diminishes over the illumination period (Fig. 5b–e). The incorporation of Au nanoparticles into the g-C₃N₄ nanosheets strikingly improves photocatalytic ROS generation, due to the application of 670 nm light.¹²⁴ In general, noble metal/g-C₃N₄ nanocomposites significantly outperform unmodified g-C₃N₄ in antimicrobial experiments, and provide a viable photocatalytic disinfection method, see Table 2 for a summary.

Fig. 5 (a) Preparation of g-C₃N₄-Au nanoparticle nanocomposties. (b) E. coli bacteria + control sample. (c) E. coli bacteria + g-C₃N₄-1.0% Au. (d) E. coli bacteria + g-C₃N₄-1.0% Au + 10 min irradiation. (e) E. coli bacteria + g-C₃N₄-1.0% Au + 20 min irradiation. Reproduced from ref. 123 with permission from American Chemical Society, copyright 2019. (f) The mechanism of E. coli inactivation in the presence of Ag/g-C₃N₄ under visible light. (g) Disinfection efficiencies of E. coli by the samples under visible light irradiation. Reproduced from ref. 119 with permission from Elsevier, copyright 2019.

Table 2 Antibacterial properties of noble metal decoration and non-noble metal doped g-C₃N₄ nanocomposites

Main component	Material	Preparation	Bacteria	Effect	Ref.
Noble metal decoration	Ag/g-C3N4	Thermal polymerization and photo-assisted reduction	E. coli	7.41 log	119
	Ag/g-C3N4	Single-pot, microemulsion	E. coli	9.95 log	166
	Ag/P/g-C3N4	Pyrolysis and green reduction.	E. coli	7 log	167
	Ag/polydopamine/g-C3N4	Ultrasound and freeze-drying	E. coli	Effective	168
	Ag/oxidized porous g-C3N4	Photo-assisted, reduction	S. aureus	99%	169
	Ag/g-C3N4	Biogenic, methodology	E. coli	Effective	174
			S. aureus
			P. aeruginosa
	Ag/g-C3N4	Calcination	E. coli	Effective	175
			S. aureus
			B. subtili
			P. aeruginosa
	Ag/g-C3N4	Photo-deposition method	E. coli	Effective	177
			P. aeruginosa
	Au/g-C3N4	Liquid-phase, exfoliation and photodeposition	E. coli	Effective	123
	Au/g-C3N4	Deposition–precipitation	E. coli	82%	170
			S. aureus	79%
	Au/g-C3N4	Calcination	E. coli	99%	176
Non-noble metal doping	g-C3N4-Zn^2+@graphene	Chemical vapor deposition and ultrasonic dispersion	S. aureus	99%	158
	CQDs/g-C3N4	Impregnation	S. aureus	7 log	165
	Fullerene/g-C3N4	Hydrothermal method	E. coli O157:H7	86%	171
	S-CQD/hollow tubular g-C3N4	Self-assembly and ultrasound	E. coli	6.88 log	172
	Ag/P/Co/S/g-C3N4	Calcination	E. coli	7 log	173

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2.3 Non-noble metal doped g-C₃N₄ nanocomposites

While g-C₃N₄ nanocomposites with noble metals have been shown to improve the photocatalytic antibacterial properties of the material, the high cost of noble metals prohibits widespread applications.^125–129 Thus, g-C₃N₄ nanocomposites derived from inexpensive and abundant elements that are non-toxic would be advantageous.^130–134 In such materials the g-C₃N₄ band gap may even be reduced to improve the separation efficiency of photogenerated carriers and the photoabsorption region may even be expanded to further improve the photocatalytic antibacterial performance.^135–142 Surface engineering of carbon-based materials has been an effective tool for construction of materials with special functions.^143–149 Advantageously, Lewis basic N-sites on the surface g-C₃N₄ allow strong interactions with Lewis acids, i.e. zinc ions, similar to that observed for other materials.^150–157 For example, g-C₃N₄–Zn²⁺@graphene oxide (SCN–Zn²⁺@GO) were prepared using chemical vapor deposition (CVD).¹⁵⁸ The bidentate ligand, SCN, may coordinate to the Zn²⁺ ions to form cross-links with GO, and additionally changing the crystal structure of g-C₃N₄ and introducing defect sites (Fig. 6a). The resulting SCN–Zn²⁺@GO nanocomposite possessed excellent antibacterial activity. Irradiation at 808 nm (NIR) led to heating and irradiation at 660 nm resulted in the generation of ROS and the combination of photothermal and photodynamic processes effectively killed bacteria within a short time (almost quantitatively under the conditions employed). In Fig. 6b and d, the E. coli and S. aureus blank groups possess unbroken topologies, with glossy bacteria membranes and intact intra-cell structures. The membrane structures of both E. coli and S. aureus are ruptured under 808 nm and 660 nm light illumination. The intra-cell density decreases and part of cytoplasm overflows (Fig. 6c and e, red arrows indicate protein or intra-cell material leakage and the black arrows indicate bacterial membrane distortion). In comparison, when SCN–Zn²⁺@GO was exposed to either 808 nm or 660 nm illumination, the inactivation rate obtained was only 20–66%. Based on the above results, the antibacterial mechanism is proposed in Fig. 6f. The ROS pass through the cell membrane of the bacteria to oxidize intracellular proteins and interfere homeostasis, while hyperthermia weakens the activities of the proteins and reduces adenosine triphosphate synthesis, inactiving E. coli and S. aureus within 10 min. Similarly, novel CoB/g-C₃N₄ nanosheets were successfully prepared by an electrostatic self-assembly process coupled with calcination.¹⁵⁹ The interfacial Co–N bond could act as an e⁻ transport channel by accelerating the e⁻ transfer from g-C₃N₄ to CoB, as supported by density functional theory (DFT) calculations and indirectly evidenced from antibacterial experiments (Fig. 6g). Consequently, the e⁻ induced O₂ reduction process is promoted in CoB/g-C₃N₄, which boosts the generation of ROS (Fig. 6h). Notably, CoB/g-C₃N₄ exhibited superior disinfection efficacy of 100% against S. aureus with 125 min under visible light irradiation.

Fig. 6 (a) Preparation of SCN–Zn²⁺@GO. TEM topology of S. aureus (b) and E. coli (c) as control, (d) and (e) following treatment with SCN–Zn²⁺@GO 20% after 10 min irradiation with visible light (the red arrows indicate protein leakage and the dark arrows indicate rupture or ruffling of the bacterial membrane). (f) Antibacterial mechanism of SCN–Zn²⁺@GO 20% under 808 or 660 nm irradiation of ROS and hyperthermia. Reproduced from ref. 158 with permission from WILEY-VCH, copyright 2018. (g) Structural models of the g-C₃N₄ (001) surface, CoB-(010) surface and CoB/g-C₃N₄ after geometry optimization. (h) The mechanism of S. aureus bacteria inactivation in the presence of CoB/g-C₃N₄ under visible light. Reproduced from ref. 159 with permission from the American Chemical Society, copyright 2019.

Quantum dots (QDs) are an important low-dimensional semiconductor materials. Because of their high reactivity and strong charge transfer abilities, QDs have been applied in photocatalytic sterilization.¹⁶⁰ Carbon quantum dots (CQDs) were combined with g-C₃N₄ to enhance charge transfer and store e⁻.^161–164 Tang et al. constructed a CQD/g-C₃N₄ photocatalyst by impregnation.¹⁶⁵ The addition of CQDs dramatically increased the disinfection performance, which was attributed to the increased ROS levels. The CQD/g-C₃N₄ nanocomposites exhibit a greatly enhanced bactericidal efficiency under illumination with visible light. In contrast, the CQDs alone showed no catalytic activity against S. aureus under comparable conditions. Hence, the interaction between the CQDs and g-C₃N₄ plays a significant role in increasing the bacterial inactivation efficiency.

2.4 g-C₃N₄ heterojunction nanocomposites

Heterojunctions could enable the directional migration of photoinduced charges, allowing the charge to be enriched in specific direction, a process that should recue or even inhibit the recombination of photogenerated carriers.^178,179 The antimicrobial properties of different g-C₃N₄ heterojunctions are discussed, including type I and type II heterojunctions, p–n heterojunctions, and Z-scheme heterojunctions.^180–184 The photocatalytic antibacterial properties of g-C₃N₄ heterojunction nanocomposites are listed in Table 3.

Table 3 Antibacterial properties of g-C₃N₄ heterojunction nanocomposites

Material	Preparation	Bacteria	Effect	Ref.
Bi2S3/g-C3N4	Ultrasound	E. coli	99.6%	185
		S. aureus	99.2%
Red P/g-C3N4	Sonochemical	E. coli K-12	7 log	186
Perylene diimide/oxygen-doped g-C3N4	In situ electrostatic assembling	S. aureus	99.6%	196
Bi2MoO6/g-C3N4	In situ solvothermal	E. coli	100%	199
MnO2/g-C3N4	Thermal vapor liquid-polymerization and redox	E. coli	99.96%	212
		S. aureus	99.26%
TiO2/kaolinite/g-C3N4	Sol–gel method	S. aureus	2.9 log	220
Ag/AgBr/g-C3N4/nitrogen-doped graphene aerogel	Hydrothermal and freeze-drying	E. coli	∼6 log	252
		S. aureus	∼1.2 log
Ag/AgBr/g-C3N4	In situ deposition–precipitation	E .coli	7.9 log	253
Ag2WO4/g-C3N4	Ultrasound	E. coli	90%	222
Ag2PO4/g-C3N4	Co-precipitation and thermal pyrolysis	E. coli	Effective	254
		S. aureus
Ni2P/g-C3N4	In situ anchoring and hydrothermal	E. coli K-12	7 log	255
m-Bi2O4/g-C3N	Hydrothermal	E. coli K-12	6 log	223
Vanadium modified g-C3N4/TiO2	Calcination and ultrasonic	E. coli	Effective	256
		S. aureus
BiVO4/Ag/g-C3N4	Photodeposition and hydrothermal	E. coli	6.5 log	225
Ag/ZnO/g-C3N4	Thermal polymerization and solvothermal	E. coli	7.4 log	226
Ag/ZnO/g-C3N4	Thermal polymerization and solvothermal	E. coli	6.19 log	257
Ag2WO4/Ag/g-C3N4	Hydrothermal and situ reductive	E. coli	3.05 log	258
Bi2MoO6–Ag/g-C3N4	Hydrothermal method	E. coli	Effective	259
		S. aureus
α-Fe2O3/CeO2/g-C3N4	Hydrothermal method	E. coli	Effective	260
		S. aureus
CuS/g-C3N4	Electrostatic adsorption	E. coli	99%	250
		S. aureus	98%
Vanadate QDs/g-C3N4	Sol–gel method	Salmonella H9812	96%	251
BiOI/g-C3N4	In situ generation	E. coli	96%	261
		S. aureus	98%
Cu2O/g-C3N4	Chemical precipitation	E. coli	7 log	262
Cu2O/g-C3N4	Hydrothermal method	E. coli	Effective	263
		S. aureus
TiO2 nanofibers/Ag/g-C3N4	Ultrasound	E. coli	99%	264
		S. aureus	83%
Ag/AgCl/g-C3N4	In situ implanting and anchoring	Tetracycline-resistant bacteria	100%	265
γ-Fe2O3/Ag/AgCl/g-C3N4	Solvothermal and photodeposition	E. coli	5.59 log	266
RGO/CA/g-C3N4	Ultrasonification	E. coli	6.5 log	267
TiO2/CuBA/g-C3N4	Ultrasound	E. coli	Effective	268
		S. aureus
Ag2ZrO3/g-C3N4	Co-precipitation	E. coli	97%	189
		B. subtilis	99%
AgBr/g-C3N4	Adsorption deposition	E. coli	6.5 log	269
BiFeO3/Cu2O/g-C3N4	Hydrothermally and ultrasonic	E. coli	Effective	270
		S. aureus
ZnO/g-C3N4/cellulose	Ultrasonic irradiation	E. coli	Effective	271
		S. aureus
CdS/g-C3N4	Sonochemical	E. coli	Effective	272
		S. aureus
GO/g-C3N4	Sonication	E. coli	97.9%	70
AgCl/CNTs/g-C3N4	Deposition–precipitation	E. coli	Effective	273
MoO3−x/g-C3N4	Hydrothermal	E. coli	100%	274
AgO/g-C3N4	Chemical oxidation	S. aureus	89%	275
Fe-2,5-thiophenedicarboxylic acid/g-C3N4	Microwave-heating	E. coli	100%	276
Mg1.2Ti1.8O5/g-C3N4	Sol–gel method and calcination	E. coli	100%	277
CuWO4/g-C3N4	Sol–gel method	E. coli	Effective	278
		S. aureus
ZnBi2O4/g-C3N4	Ultrasound-assisted chemical exfoliation	E. coli	Effective	279
		S. aureus
Cr–ZnO/g-C3N4	Chemical coprecipitation method	E. coli	Effective	280
		S. aureus
		B. subtilis
ZnBi2O4/g-C3N4	Thermal polycondensation	E. coli	97%	281
TiO2/Ag/g-C3N4	Vacuum freeze-drying	E. coli	84%	282
AgBr/g-C3N4	Calcination	P. putida	100%	283
TiO2 nanotubes/Ti/g-C3N4/SnO2	Dipping and calcination	E. coli	Effective	284
BiOCl/g-C3N4	Hydrothermal method	E. coli	96%	285
NiFe2O4/g-C3N4	Hydrothermal method	A. flavus	90%	286
Perylene-3,4,9,10-tetracarboxylic diimide/g-C3N4	In situ	E. coli	Effective	287
		S. aureus
TiO2/g-C3N4	In situ	E. coli	65%	288
ZnO/Mn/g-C3N4	Chemical co-precipitation	E. coli	Effective	289
		S. aureus
ZnTiO2/S/g-C3N4	In situ	E. coli	Effective	290
		S. aureus
Ag–ZnO@g-C3N4	Physical mixing method	E. coli	Effective	291
		S. aureus
		B. subtilis
Poly(vinyl alcohol)/g-C3N4	Casting	P. aeruginosa	Effective	292
Polyaniline/g-C3N4	In situ oxidative polymerization methodology	E. coli	Effective	293
		S. pneumoniae
PVA/Starch/Ag@TiO2/g-C3N4	Solution casting	E. coli	Effective	294
		S. aureus
Fe@ZnO/g-C3N4	Chemical co-precipitation	E. coli	Effective	295
		S. aureus
		B. subtilis
		S. salivarius
g-C3N4-based metal-free	Calcination	E. coli	Effective	296
		B. subtilis
Ag2S/g-C3N4	Sonochemical	E. coli	Effective	297
		S. aureus
		B. subtilis
		S. salivarius
TiO2 nanofibers/g-C3N4	Electrospinning-calcination	E. coli	100%	298
		S. aureus
Ag3PO4/g-C3N4	Hydrothermal method	E. coli	Effective	299
Ag2O/g-C3N4	Chemical deposition method	M. aeruginosa	99%	300
TiO2/g-C3N4	Hydrothermal and calcination	E. coli	100%	301
Ag2O/g-C3N4	Physical mixing method	M. aeruginosa	100%	302
AgO/g-C3N4	In situ	E. coli	Effective	303
Ag3PO4/g-C3N4	Hydrothermal method	Bacteriophage f2	100%	304
AgBr/g-C3N4	In situ	E. coli	100%	305
TiO2/g-C3N4	Hydrothermal	E. coli	100%	306
Porphyrin/g-C3N4	In situ	S. aureus	63%	307
Ag/Ag/Ag/g-C3N4/BiVO4	In situ	E. coli	100%	308
ZnO–Cd/g-C3N4	In situ	E. coli	Effective	309
		S. aureus
RGO/S8/g-C3N4	In situ	E. coli K-12	100%	310

---

2.4.1 Type I heterojunction nanocomposites. In general, type I heterojunctions are rarely considered as the optimal choice in photocatalysis because the photogenerated carriers can transfer to the interface with other semiconductors, reducing the redox capacity of the charge carriers. Nevertheless, under visible light irradiation, type I heterojunctions have the unique advantage, i.e. the e⁻ and h⁺ can be transferred from one semiconductor to another. If another semiconductor has a wide photoabsorption window, a broad-spectrum-response photocatalyst with minimal charge recombination can be obtained by creating a type I heterojunction (Fig. 7a). Li et al. developed zinc-doped g-C₃N₄ (g-C₃N₄–Zn) with Bi₂S₃ nanorod heterojunctions (g-C₃N₄–Zn/BiS), using ultrasonication.¹⁸⁵ In contrast to the precursors (g-C₃N₄–Zn and BiS), effective charge separation at the photocatalyst interface is achieved by adjusting the band gap, the density of the electronic distribution, and the oxygen adsorption capacity of the g-C₃N₄–Zn/BiS heterojunction. DFT calculations were employed to predict the stable crystal structure and the interface space between CN–Zn and BiS (Fig. 8a). The e⁻ and h⁺ were separated effectively by the energy band offset and the interface electric field, hence the g-C₃N₄–Zn/BiS heterojunction produces abundant ROS and shows excellent photocatalytic efficiency. Near-quantitative bactericidal efficiency towards S. aureus was achieved after only 10 min of NIR irradiation (Fig. 8b). In addition, red P was a novel single-element photocatalyst, and its visible light response range is up to 700 nm. Efficient light harvesting is imperative for photocatalysts, and with this in mind Wang et al. developed a wide-spectral-response g-C₃N₄/red P photocatalyst using ultrasound.^186–188 Ultrasonication was used to obtain nanosheets from bulk g-C₃N₄, and red P particles were anchored to the g-C₃N₄ nanosheets to construct close g-C₃N₄/red P heterojunctions. g-C₃N₄/red P may form a new wide-spectral-responsive photocatalytic system to fully utilize the solar energy. In addition, g-C₃N₄/red P was used as a dual activity center photocatalyst, exhibiting dramatically improved photocatalytic efficiency for sterilization under illumination with visible light. While the g-C₃N₄/red P nanocomposite showed 7 log cfu mL⁻¹ bacterial inactivation after 1.3 h, the photocatalytic bacterial inactivation of pure g-C₃N₄ was limited, with < 1.5 log cfu mL⁻¹E. coli inactivation after 2 h of illumination.

Fig. 7 Various types of heterojunctions. (a) Type I heterojunction model. (b) Type II heterojunction model. (c) Z-type heterojunction model. (d) p–n heterojunction model.

Fig. 8 (a) Structural models of g-C₃N₄, g-C₃N₄–Zn, BiS, and g-C₃N₄–Zn/BiS after geometry optimization. (b) TEM topology of S. aureus after treatment with control or g-C₃N₄–Zn/BiS after 10 min irradiation. The white arrows indicate twisted and broken cell membranes and the blue arrows point to intracellular matrix outflow. Reproduced from ref. 185 with permission from WILEY-VCH, copyright 2019.

2.4.2 Type II heterojunction nanocomposites. Type II heterojunctions g-C₃N₄-based nanocomposites have been widely reported as photocatalysts, e.g. Ag₂ZrO₃/g-C₃N₄,¹⁸⁹ Nb₂O₅/g-C₃N₄ and Bi₂MoO₆/g-C₃N₄.^190,191 These materials have interlaced band gaps and appropriate VB and CB energies. Staggered heterojunctions are the most efficient type of heterojunctions due to highly efficient charge transfer,^192–194 and therefore, type II heterojunctions are widely used.¹⁹⁵ Exposure of a type II heterojunction to visible light results in the transition of an e⁻ from the VB to the CB, generating a corresponding h⁺ in the VB. When the CB of semiconductor A is higher in energy than the CB of semiconductor B, the e⁻ in the CB of semiconductor A is transferred to the CB of semiconductor B. Simultaneously, the h⁺ in the VB of semiconductor B is transferred to the VB of semiconductor A. Finally, they react with O₂ and H₂O in the surrounding media to produce ROS, leading to good antibacterial effects (Fig. 7b). Gao et al. prepared a perylene diimide (PDI)/oxygen-doped g-C₃N₄ nanosheet (PDI/O-g-C₃N₄) nanocomposites using an in situ electrostatic assembly method.¹⁹⁶ The PDI expanded the visible light range of the material, resulting in abundant photogenerated charge carriers and accumulation of ROS, boosting the oxidative capability. As a consequence, PDI/O-g-C₃N₄ demonstrated strong antibacterial oxidation activity under visible light with 96% of the S. aureus fully inactivated by PDI/O-g-C₃N₄ under 3 h of light irradiation, whereas only 62% of the S. aureus cells were inactivated by the control material. Coincidentally, Bi₂MoO₆ not only intersects the g-C₃N₄ band gap, but also has a very similar band gap energy (∼2.7 eV). Consequently, Bi₂MoO₆ combines with g-C₃N₄ to afford neoteric and efficient nanocomposites.^197,198 As shown in Fig. 9a, Li et al. developed Bi₂MoO₆/g-C₃N₄ heterojunctions using an in situ solvothermal approach.¹⁹⁹ The results showed that the photocatalyst completely inactivated 2.5 × 10⁷ cfu mL⁻¹E. coli after 3 h light irradiation (Fig. 9b).

Fig. 9 (a) Preparation of the Bi₂MoO₆/g-C₃N₄ heterojunction. (b) E. coli re-cultured after treatment with 20% Bi₂MoO₆/g-C₃N₄ as a function of time. Reproduced from ref. 199 with permission from Elsevier, copyright 2016.

2.4.3 Z-scheme heterojunction nanocomposites. Recently, Z-scheme heterojunctions have been widely studied as the structure accelerates the separation of photogenerated carriers. The e⁻ in the VB of semiconductor A transfers to the CB of semiconductor B, and the remaining h⁺ and e⁻ undergo redox reactions with the oxygen and water in the surroundings to generate ROS (Fig. 7c). As expected, Z-scheme heterojunctions exhibit excellent photocatalytic disinfection performance.^200–204 MnO₂ is an inexpensive, abundant, biocompatible semiconductor that has a similar bandgap to g-C₃N₄.^205–211 Wu et al. successfully constructed a MnO₂/g-C₃N₄–Ti heterojunction using thermal vapor liquid-polymerization and redox methods²¹² (Fig. 10a). Contact between the g-C₃N₄ and MnO₂ formed a Z-scheme heterojunction. The MnO₂/g-C₃N₄–Ti composite inactivates S. aureus and E. coli in near-quantitative yields (Fig. 10b and c). In addition, TiO₂ is an outstanding photocatalyst that binds with g-C₃N₄ for form a nanocomposite with high thermal stability.^213–219 Li et al. constructed a g-C₃N₄/TiO₂/kaolinite heterojunction using a sol–gel approach combined with self-assembly²²⁰ (Fig. 10d). Compared to bulk g-C₃N₄ or TiO₂, the 3D structured g-C₃N₄/TiO₂/kaolinite nanocomposite displayed increased adsorption-photocatalytic sterilization of S. aureus under light irradiation. The g-C₃N₄/TiO₂/kaolinite composite inactivated 2.9 log S. aureus bacteria after 5 h of illumination, superior to g-C₃N₄ (1.6 log) and TiO₂ (0.8 log) alone (Fig. 10e). In addition, under visible light, the g-C₃N₄/TiO₂/kaolinite nanocomposite exhibits heightened adsorption-photocatalytic degradation of ciprofloxacin, a broad-spectrum antibiotic. The antibacterial efficiency of the g-C₃N₄/TiO₂/kaolinite composite may be attributed to both the improved light utilization and an increase in e⁻ transfer and separation efficiency (Fig. 10f). The visible light activated g-C₃N₄/TiO₂/kaolinite composite is a useful material for pollutant decomposition and bacterial elimination.²²¹

Fig. 10 (a) Schematic showing the preparation of MnO₂/g-C₃N₄–Ti. (b and c) The antibacterial effect of MnO₂/g-C₃N₄–Ti irradiated for 20 minutes against S. aureus and E. coli, respectively. Reproduced from ref. 212 with permission from Elsevier, copyright 2019. (d) Schematic illustration of the preparation of g-C₃N₄/TiO₂/kaolinite. (e) Photocatalytic disinfection efficiency of S. aureus for different samples. (f) Schematic diagram of photocatalytic mechanism of the g-C₃N₄/TiO₂/kaolinite. Reproduced from ref. 220 with permission from Elsevier, copyright 2019.

2.4.4 Dual-path heterojunction nanocomposites. Generally, the charge migration paths observed in g-C₃N₄ heterojunctions are mostly type II and Z-scheme heterojunctions, which expedite the fast separation of photogenerated charges and intensify the antibacterial activity of semiconductor materials. Many type II and Z-scheme heterojunction nanocomposites have been shown to inactivate bacteria under irradiation with visible light, including m-Bi₂O₄/g-C₃N₄, AgWO₄/g-C₃N₄.^222,223 Nevertheless, due to the relatively low redox potentials in type II and Z-scheme heterojunctions, these photocatalysts lack strong redox abilities.²²⁴ It is known that e⁻ accumulate in the CB of semiconductor A, which has a high reduction potential, and h⁺ leave the VB of semiconductor B, which has a high oxidation potential. This not only effectively separates the e⁻/h⁺ pairs, but also produces the optimal redox properties. Therefore, the two models of ternary heterojunctions were also studied to further improve the antibacterial performance of photocatalytic heterojunctions. Zeng et al. constructed a ternary BiVO₄/Ag/g-C₃N₄ heterojunction using photo-deposition and hydrothermal methods.²²⁵ Based on heterojunction band gap energy level and surface chemistry, a dual Z-scheme photogenerated carrier transfer model was applied to BiVO₄/Ag/g-C₃N₄. Notably, the ternary BiVO₄/Ag/g-C₃N₄ heterojunction markedly strengthened the photocatalytic antibacterial capability, completely inactivating 6.5 log E. coli cells after 1 h of light illumination, whereas the binary BiVO₄/g-C₃N₄ heterojunction inactivated only 0.5 log E. coli under comparable conditions. The Ag and BiVO₄ nanoparticles on the g-C₃N₄ nanosheets inhibit recombination of the photogenerated carriers, thus promoting ROS generation. Ma et al. developed ZnO/Ag/g-C₃N₄ heterojunction using a solvothermal reaction.²²⁶ This composite was used to kill E. coli under illumination with visible light. The ZnO/Ag/g-C₃N₄ composite demonstrated significant visible light sterilization efficiency compared to g-C₃N₄, Ag/g-C₃N₄ and ZnO/g-C₃N₄ materials. Specifically, ZnO/Ag/g-C₃N₄ inactivated 7.4 log E. coli after 2 h light illumination. However, only 0.49 log and 2.61 log E. coli were inactivated by g-C₃N₄ and ZnO/g-C₃N₄. The interface of ZnO improves the sterilization performance by increasing the separation rate of charges because of the SPR effect of Ag and the similar band gap energies of ZnO to g-C₃N₄.^227–231

2.4.5 p–n heterojunction nanocomposites. The construction of p–n type heterojunctions can increase the spectral response range of photocatalytic semiconductors.^232–234 The p–n type heterojunction needs to form at the interface of the space charge region and these heterojunctions form an internal potential that guides the e⁻ and h⁺ in opposite directions.^235–239 The e⁻ transfers to the CB of the n-type semiconductor, whereas the h⁺ transfers to the VB of the p-type semiconductor (Fig. 7d). The separation effect of charges in p–n heterojunctions is higher than other heterojunctions leading to superior photocatalytic antibacterial activity.^240–245 CuS is a p-type semiconductor and is the material of choice for photocatalysis due to its narrow band gap and excellent physicochemical stability.^246–249 Ding et al. synthesized CuS/g-C₃N₄ heterojunction using a hydrothermal approach harnessing electrostatic adhesion.²⁵⁰ In the CuS/g-C₃N₄ heterojunction (Fig. 11a), the e⁻ and h⁺ transfer in reverse directions between g-C₃N₄ and CuS, accelerating the separation of charges, thus producing high levels of ROS and increasing the photocatalytic antibacterial activity. In addition, the CuS/g-C₃N₄ heterojunction can transform visible light to heat (Fig. 11b). Hence, due to the synergistic influence of the ROS and thermal effects, the CuS/g-C₃N₄ composite inactivated E. coli and S. aureus bacteria near-quantitatively after 20 min of light irradiation. In contrast, g-C₃N₄ only inactivated 30% of the E. coli cells and 25% of the S. aureus cells (Fig. 11c).

Fig. 11 (a) Preparation of CuS/g-C₃N₄. (b) Photothermal images following irradiation as a function of concentration and time. (c) In vitro antibacterial activity for S. aureus and E. coli. Reproduced from ref. 250 with permission from Elsevier, copyright 2020. (d) Formation mechanism of vanadate QDs/g-C₃N₄. (e) Photocatalytic disinfection efficiency of Salmonella with different samples. (f) Photocatalytic disinfection efficiency of Salmonella with AgVO₃/g-C₃N₄ at different concentrations. (g) Bacteria colony growth in the presence of AgVO₃/g-C₃N₄ with Salmonella. (h) Bacteria colony growth in the presence of AgVO₃/g-C₃N₄ with Salmonella at different concentrations. The corresponding thermal images of AgVO₃/g-C₃N₄ following irradiation for (i) 5 min and (j) 10 min. Bacteria colony growth with (k) AgVO₃/g-C₃N₄ and (l) BiVO₄/g-C3N4 in the dark. Reproduced from ref. 251 with permission from Elsevier, copyright 2017.

Wang et al. fabricated vanadate (AgVO₃ and BiVO₄) QD/g-C₃N₄ nanocomposites using urea²⁵¹ (Fig. 11d). Due to the abundant production of ROS by the vanadate QDs and g-C₃N₄, the vanadate QDs/g-C₃N₄ composites exhibited high bactericidal efficiency, with 96% inactivation (AgVO₃ QDs/g-C₃N₄) and 87% inactivation (BiVO₄ QDs/g-C₃N₄) of Salmonella after only 10 min of light illumination (Fig. 11e and g). As shown in Fig. 11f and h, the photocatalytic disinfection efficiency increases with increasing photocatalyst concentration. Only 22% of Salmonella were killed with a AgVO₃/g-C₃N₄ composite concentration of 0.5 mg mL⁻¹. However, at the same period, the photocatalytic inactivation of Salmonella increased to 58% when the photocatalyst concentration reach 0.75 mg mL⁻¹. It is apparent from Fig. 11i and j that there is no significant change in temperature during the antibacterial tests. Furthermore, the bacteria grew well on the LB plate, meaning that photocatalyst does not kill the Salmonella (Fig. 11k and l). Considering the simplicity of the synthetic process, the chemical durability and the sterilization results, vanadate QDs/g-C₃N₄ are ideal photocatalysts for applications in environmental settings.

3. Conclusions and perspectives

Materials based on g-C₃N₄ are promising photocatalysts with excellent physico-chemical properties and have considerable promise in antibacterial applications. Nevertheless, the antibacterial applications of bulk g-C₃N₄ are limited by its narrow absorption of visible light and facile recombination of charges. Consequently, a variety of g-C₃N₄-based nanocomposites have been developed with high superficial areas, improved e⁻/h⁺ separation efficiencies and expanded visible light absorption ranges, combining to enhance their antibacterial activity. In this review, we highlighted the main strategies used to amplify the photocatalytic efficiency of g-C₃N₄-based nanocomposites and their antimicrobial properties, including different topologies, noble metal decoration, non-noble metal doping and heterojunction construction. The enhancement mechanisms and synergistic effects of g-C₃N₄-based nanocomposites was also discussed. Although g-C₃N₄ is an ideal photocatalyst for the construction of nanocomposites for antibacterial applications, there are still some issues to be solved and opportunities for further research:

(1) The antibacterial mechanism of g-C₃N₄-based nanocomposites include destroying cell membranes and cell walls, producing endotoxins, causing protein mutations, interfering with protein synthesis and oxidizing organics. However, the role of each process in the antibacterial activity has not yet been clearly defined, suggesting that future research on the antibacterial mechanisms of g-C₃N₄-based nanocomposites would be meaningful.

(2) Although g-C₃N₄-based nanocomposites have been extensively studied, the photocatalytic properties are not always predictable, and the performance between g-C₃N₄ and co-composites is often found to be additive and not synergistic. Therefore, molecular models that allow better composite design would be useful.

(3) When constructing g-C₃N₄ heterojunctions, a single heterojunction has many limitations in terms of light absorption and e⁻ separation. Thus, the construction of dual heterojunctions, such as dual Z-type or combined Z-type and type II heterojunctions could enhance the photocatalytic effect of g-C₃N₄ nanocomposites and is a key topic for future research and development.

(4) The antibacterial efficiency of g-C₃N₄-based nanocomposite photocatalysts relies largely on ultraviolet and blue light. Extending the range to longer wavelengths would be advantageous.

(5) Most studies were carried at laboratory scales and synthetic strategies for large-scale production are challenging. The development of simple and large-scale green and sustainable synthetic methods that can be automated are required to facilitate commercial applications.

(6) ROS are also able to destroy viruses and therefore further research exploring the antiviral properties of g-C₃N₄ nanocomposites would be valuable.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51803112, 216740859), China Postdoctoral Science Foundation Grant (2018M633503), Key Laboratory Construction Program of Xi'an Municipal Bureau of Science and Technology (201805056ZD7CG40), and Innovation Capability Support Program of Shaanxi (No. 2018PT-28, 2019PT-05).

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