Photocatalytic disinfection efficiency of 2D structure graphitic carbon nitride-based nanocomposites: a review

Abstract

Nanostructured carbon-based photocatalysts have gained attention in photocatalytic disinfection of microbial species. With distinctive features of possessing appropriate electronic band gap structure and high chemical and thermal stability, metal-free polymeric 2D stacked structure graphitic carbon nitride (g-C₃N₄) is an important photocatalytic material for environmental and energy applications. Besides, it has the potential for inactivation of harmful pathogens. Disinfection of microbial species is mainly ascribed to the formation of reactive oxidative species. Further, surface modification of g-C₃N₄ can remarkably improves photocatalytic disinfection efficiency. In this review, we have discussed the recent advances in photocatalytic disinfection using g-C₃N₄-based nanocomposites. An overview of metal-free nanostructure g-C₃N₄, metal (anions) and nonmetal (cations)-doped g-C₃N₄ and g-C₃N₄ hybridized with low band gap semiconductor is also presented. Moreover, we have emphasized on the photocatalytic disinfection mechanism associated with g-C₃N₄-modified composites. Nitrogen-rich g-C₃N₄ polymeric material can serve as an alternative to metal oxide (TiO₂ and ZnO) photocatalysts for photocatalytic disinfection technology. Other applications such as CO₂ photoreduction, H₂ generation, organic pollutant degradation, and sensing using g-C₃N₄-based nanocomposites are also summarized.

Introduction

Elimination of microbial load has been a significant concern owing to threat associated with several infections through food, water, and a range of other mode of infection. In particular, solid surfaces act as a reservoir for surviving gram negative (G⁻) [Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumonia)], gram positive (G⁺) [Enterococcus faecalis (E. faecalis), Staphylococcus aureus (S. aureus)] and fungal pathogen (Candida albicans) [1]. Hence, numerous efforts have been taken for remediating microbial contamination including conventional chemical disinfectants (chlorine, chlorine dioxide, ozone, and alcohol, formaldehyde, or hydrogen peroxide) and ultraviolet irradiation (UV-C, UV-B and UV-A) [2,3,4]. However, such methods have drawbacks of time-limited antimicrobial activity, generation of toxic carcinogenic by-products such as trihalomethanes to the environment, and high operational costs [5]. Hence, it is vital to develop simple, low cost, and effective antimicrobial approaches for reducing harmful microbial load in the environment.

In recent years, nanocomposite-based photocatalytic disinfection processes have gained interest on a commercial scale as an approach to reduce microbial contamination [6, 7]. Photocatalytic disinfection relies on the principle of photoexcitation of a semiconductor material upon absorption of light, the wavelength of light being higher than the band gap energy level of semiconductor material (Fig. 1). In this process of exposure to light, electrons in the valence band (VB) get excited and move to the conduction band (CB) of the semiconductor material. This leaves a photogenerated hole in the VB. Finally, photogenerated holes and electrons pairs are involved in redox reactions. The production of reactive oxygen species (ROS) during photocatalysis can induce leakage of minerals, genetic material, and protein and thus result in cell death and reduction in microbial load.

Figure 1

Schematic illustration of semiconductor-based photocatalytic disinfection mechanism of microbial species

In this aspect, metal oxides like titanium dioxide (TiO₂), zinc oxide (ZnO), magnesium oxide (MgO), and tin oxide (WO₃), and nonmetal oxides such as graphitic carbon nitride (g-C₃N₄), multiwall carbon nanotube (MWCNT), and graphene oxide (GO) semiconductor photocatalysts have been developed and employed for inactivation of microbial species [8,9,10,11,12]. However, intermediate or narrow bandgap semiconductors that have the capability to reduce molecular oxygen (O₂) to cytotoxic ROS show a promising scope for decreasing microbial load in the environment. Of late, Wang et al. [13] reported that metal-free treated rape pollen (TRP) displays remarkable inactivation efficiency toward the different waterborne bacterial species including E. Coli K-12, Pseudomonas aeruginosa (P. aeruginosa), S. aureus, and Bacillus pumilus (B. pumilus). Owing to higher specific surface area, more number of surface-active sites, improved chemical stability, and conductivity, 2D nanomaterial display outstanding antibacterial activity as compare with 1D nanomaterials.

Till date, 2D-structured metal-free polymeric g-C₃N₄ semiconductor materials have been extensively used for antimicrobial applications [14, 15]. At first, Wang and co-workers [16] synthesized g-C₃N₄ for a hydrogen (H₂) evaluation reaction. There has been a significant rise in scientific interest on the development of g-C₃N₄-based materials for energy and environmental applications. Benefiting from its distinctive properties (i.e., appropriate band gap (2.7 eV), high chemical, mechanical, and thermal stability), g-C₃N₄ is popularly used in waste water treatment [17], water splitting [18], NO_x fixation [19], CO₂ conversion [20], and sensing applications [21]. Nevertheless, because of some known drawbacks (i.e., high recombination rate of photogenerated charge carriers and inadequate utilization of visible light) of g-C₃N₄, several surface-modified and improved techniques have been devoted for enhancing its photocatalytic efficiency. This review critically reviews the emerging trends in disinfection using g-C₃N₄-based photocatalysts. A summary of theoretical advances and basic principle for microbial inactivation using g-C₃N₄ are also discussed.

Basic principle of photocatalytic disinfection mechanism on g-C₃N₄

Photocatalytic disinfection mechanism of g-C₃N₄ is explained by the formation of photogenerated charge carriers (electron and hole pair) when exposed to light. Overall electron transfer and formation of ROS species on g-C₃N₄ surface is explained using Eqs. (1–7) [22, 23]. Photogenerated electrons get excited from VB to CB; then, these charge carriers transfer to g-C₃N₄ surface and initiate a series of reactions and generate highly active ROS species. Generally, hydroxyl (·OH) radical, hydrogen peroxide (H₂O₂), and superoxide (·O₂⁻) are key species responsible for photocatalytic disinfection [24,25,26]. It is known that holes (h⁺) in the VB can decompose (oxidize) water into ·OH radicals, thereby contributing to the inactivation of microbial species, especially bacteria. Moreover, other ROS species (H₂O₂ and ·O₂⁻) are mainly formed from reduction of O₂ (${\text{O}}_{2} \to \cdot {\text{O}}_{2}^{ - } \leftrightarrow \cdot {\text{HO}}_{2} \to {\text{H}}_{2} {\text{O}}_{2} \leftrightarrow \cdot {\text{OH}}$), as they effectively react with cellular compounds, membrane leakage of microbial cell walls, and ultimately cause cell death [27, 28].

$$ {\text{g-C}}_{3} {\text{N}}_{4} + h\upsilon \to {\text{g-C}}_{3} {\text{N}}_{4} (e_{\text{CB}}^{ - } + h_{\text{VB}}^{ + } ) $$

(1)

$$ {\text{g-C}}_{3} {\text{N}}_{4 } \left( {h_{\text{VB}}^{ + } } \right) + {\text{H}}_{2} {\text{O}} \to {\text{g-C}}_{3} {\text{N}}_{4} + \cdot {\text{OH}} + {\text{H}}^{ + } $$

(2)

$$ {\text{g-C}}_{3} {\text{N}}_{4 } \left( {h_{\text{VB}}^{ + } } \right) + {\text{OH}}^{ - } \to {\text{g-C}}_{3} {\text{N}}_{4} + \cdot {\text{OH}} $$

(3)

$$ {\text{g-C}}_{3} {\text{N}}_{4 } \left( {e_{\text{VB}}^{ - } } \right) + {\text{O}}_{2} \to {\text{g-C}}_{3} {\text{N}}_{4} + \cdot {\text{O}}_{2}^{ - } $$

(4)

$$ \cdot {\text{O}}_{2}^{ - } + \cdot {\text{HO}}_{2} \to \cdot {\text{OH}} + {\text{O}}_{2} + {\text{H}}_{2} {\text{O}}_{2} $$

(5)

$$ \cdot 2{\text{HO}}_{2}^{ - } \to {\text{O}}_{2} + {\text{H}}_{2} {\text{O}}_{2} $$

(6)

$$ {\text{g-C}}_{3} {\text{N}}_{4 } \left( {e_{\text{VB}}^{ - } } \right) + {\text{H}}_{2} {\text{O}}_{2} \to {\text{g-C}}_{3} {\text{N}}_{4} + {\text{OH}}^{ - } + \cdot {\text{OH}} $$

(7)

Structure of g-C₃N₄

g-C₃N₄ is a class of π-conjugative polymeric n-type semiconductor and consists primarily of carbon and nitrogen (Fig. 2) [18, 29]. Polymeric g-C₃N₄ was first synthesized by Berzelius and named by Liebig in 1834 [30]. Then, its structure was described by Franklin in 1922 [31]. Importantly, among all allotropes of carbon nitrides at ambient atmosphere, g-C₃N₄ is not only the most stable, but it also has distinctive surface properties (basic surface functionalities, electron-rich properties and H-bonding motifs) that are essential for many photocatalytic applications [32, 33]. Also, the C–N layers in the g-C₃N₄ have a laminar structure with weak van der Waals interactions, which is similar to the structure of graphite [34]. Mainly, it contains of a long pair of nitrogen-based triazine/tri-s-triazine tectonic units and can be easily prepared from low-cost nitrogen-rich feedstocks such as melamine, melamine hydrochloride, urea, thiourea, and dicyandiamide [16, 20, 35,36,37,38,39,40,41,42,43].

Figure 2

a Triazine and b tri-s-triazine (heptazine) structures of g-C₃N₄ [157]

Solvothermal [44], molecular self-assembly [45], microwave irradiation [46], and thermal polycondensation methods [47] are common synthesis approaches to prepare g-C₃N₄-based photocatalysts. Chemical synthesis of g-C₃N₄ has the benefits of formation of uniform and fine particle, limited energy utilization, while the process is economically feasible. Powder form of g-C₃N₄ was synthesized by heating the dicyandiamide with barbituric acid (C₄H₄N₂O₃, BA) mixture at 550 °C in air [30]. Benzene thermal reaction between 1,3,5-trichloromelamine (C₃N₃Cl₃) and sodium amide (NaNH₂) at 180–220 °C for 8–12 h was adapted for preparation of well crystalline g-C₃N₄ nanocrysatllites [48]. Tubular luminescent polymeric networks of g-C₃N₄ has been synthesized using solvothermal route, involving the reaction of 1,3,5-trichlorotriazine (C₃N₃Cl₃) with ammonium chloride (NH₄Cl) in the presence of iron powder at 300 °C [49]. However, it requires several hours for formation of particle and crystallization. Microwave synthesis route has also been explored for synthesizing submicrosphere g-C₃N₄ using cyanuric chloride and sodium azide powder as a precursor for a 30-min time period [50]. Recently, microwave-assisted solvothermal process has been employed for the development of visible light-active tri-s-triazine (C₆N₇) unit-based g-C₃N₄ photocatalysts as shown in Fig. 3 [51]. Nitrogen-rich carbon nitride films were prepared by thermal vapor transport under Ar flow onto the Si and SiO₂ substrates at 250–350 °C [52]. Kang et al. [53] fabricated g-C₃N₄ nanosheets from bulk g-C₃N₄ using facile bacterial liquid exfoliation method (bioetching method) (Fig. 4). The optical and textural properties of g-C₃N₄ may be affected by different preparation conditions and types of precursors used [54, 55]. For instance, urea-derived g-C₃N₄ shows a slightly larger energy band gap as compared with g-C₃N₄ obtained from thiourea [56]. A summary of physicochemical properties and multi-functional application of g-C₃N₄-based composite is presented in Fig. 5.

Figure 3

A Formation mechanism of graphitic carbon nitride obtained from (a) thermal condensation (TC) of melamine and (b) traditional and microwave (MW)-assisted solvothermal methods (ST) using cyanuric chloride and sodium azide as precursors and B SEM images of (a) ST, (b) TC, and (c) MW samples; (d) magnification of MW sample [51]

Figure 4

A Proposed bacterial-inspired synthesis process and mechanism of bacteria-treated (BT) g-C₃N₄ samples and (b) TEM images of (a) g-C₃N₄, (b) bacteria untreated g-C₃N₄ (c) BT- g-C₃N₄-4 h, (d, f) BT- g-C₃N₄ -2d and (e) BT- g-C₃N₄ -5d samples [53]

Figure 5

Schematic representations of physicochemical properties and multi-functional application of g-C₃N₄

Modification strategy of g-C₃N₄

Surface modification and band gap configuration of the pristine g-C₃N₄ can lead to improvement in its photocatalytic performance. Exfoliation of pristine g-C₃N₄ into g-C₃N₄ nanosheets showed larger surface area and effective charge transfer. Structure defect engineering is valuable route for improving the photocatalytic activity of g-C₃N₄. Modification of surface property is favorable for strengthening the adsorption of reactants on its surface and controlling recombination rate of charge carriers, and thus accelerating the surface catalytic reactions. Optimization of crystal structure can efficiently change the electronic property and oxidation and reduction ability of photogenerated charge carriers. Designing of nanostructures (g-C₃N₄ nanotube, g-C₃N₄ nanorods, and mesoporous g-C₃N₄) is an efficient approach for improving the photocatalytic efficiency. g-C₃N₄ nanostructured materials provides large pores, high surface area, improved reactant/product diffusion, and showed multiple light scattering effect. Hybrid structure construction is considered as the most viable method to improve the separation efficiency of charge carriers, which greatly enhances the photocatalytic performance. With respect to the charge separation mechanism of g-C₃N₄-based hybrid structure, it can be categorized into the following types as follows (Fig. 6).

Figure 6

Different types of g-C₃N₄-based heterojunctions: a Type I heterojunction, b Type II heterojunction, cp–n junction, d Schottky junction, e Z-scheme heterojunction (without an electron mediator) and f indirect Z-scheme (with an electronmediator). Note A, D and E_F represent electron acceptor, electron donor and fermi level respectively) [14]

• Type I heterojunction

• Type II heterojunction

• p–n heterojunction

• Schottky junction

• Z-scheme heterojunction

Type I heterojuction, semiconductor I (SC1) has higher VB and higher CB position. During photocatalysis, holes in the VB of SCI get shifted to the VB of semiconductor II (SCII). Meanwhile, photogenerated electrons in SC I migrate toward SCII; thereby, charge carriers are separated. For instance, type I heterojuction morphologies such as SnS₂ (nanoparticles, nanosheets and 3D flower-like) hybridized g-C₃N₄ composites exhibit better photocatalytic activity toward H₂ generation and also show excellent stability [57]. Notably, charge carriers are accumulated on SnS₂ surface, yielding no improvement in charge separation. Type II heterojuction consists of g-C₃N₄ and another low or large band gap semiconductor. After the excitation process, photogenerated electrons move from high CB position to low CB position and holes get transferred from the highest VB potential to the lowest VB potential. Finally, the internal field facilitates separation and movement of photogenerated charge carriers.

For example, Kong et al. [58] established that metal-free type II heterojunction of 0D black phosphorus quantum dots (BPQDs) with 2D g-C₃N₄ composites exhibited improved stability and vigorous photocatalytic efficiency. Type II heterojuction system is not adequate to conquer the high recombination rates in photogenerated systems. The p–n heterojuction photocatalyst obtained by blending p-type and n-type semiconductors materials can provide an additional electric filed to accelerate charge transfer for improving photocatalytic activity. Before light irradiation, holes in the p-type semiconductor material are transferred to the n-type semiconductor, leaving photogenerated electrons. The transfer of electron–hole pairs will refrain when the fermi level system attains equilibrium.

g-C₃N₄ is an n-type semiconductor material because it has NH/NH₂ groups in its structure. Therefore, designing of p–n heterojunction can normally boost the photocatalytic efficiency of g-C₃N₄. Ag₂O is a attractive p-type semiconductor material, which has narrow band gap (1.2 eV) with efficient absorbance of UV–Vis–NIR light. A p–n heteriojunction of Ag₂O/g-C₃N₄ prepared by simple chemical precipitation method and can posses enhanced photocatalytic activity toward organic pollutants degradation. Enhanced photocatalytic efficiency is attributed to strong high separation efficiency of photogenerated electron–hole pairs, UV–Vis–NIR light absorption, and surface plasmon effect of metal Ag [59].

Schottky effect is evident at metal–semiconductor interface due to difference in fermi levels, and this generates the built-in electric field and drives the charge flow until the system reaches a state of equilibrium. Therefore, fermi energy levels can be aligned and the recombination of charge carriers is controlled to enhance photocatalytic performance. Furthermore, selection of a suitable metallic co-catalyst with superior charge mobility, low overpotential, and large metal–semiconductor interface to enhance the Schottky effect may be advantageous in boosting the photocatalytic behavior of H₂ generation. Remarkable improvement in photocatalytic H₂ evolution activity over 2D–2D CoP/g-C₃N₄ composites can be attributed to effective carrier separation introduced by Schottky effect, low overpotential and good electron mobility of CoP, and shortened transportation distance of charge transfer in the 2D/2D composite photocatalysts [60]. In another study, Schottky catalyst of metallic MoN-coupled n-type g-C₃N₄ displayed boosted efficiency for H₂ generation and rhodamine blue (Rh B) degradation over bare g-C₃N₄ due to improved charge separation and transportation [61].

In a Z-scheme-based heterojunction system, electrons on the lower value CB of SC II directly combine with VB of SC I. Several studies have explored scheme systems with and without electron mediators. Commonly, nanoscale metals such as Ag and Au as electron mediators have been used for constructing Z-scheme systems, owing to good electrical conductivity. Z-scheme heterojunction of binary LaMnO₃/g-C₃N₄ hybrid nanocomposite not only separates the charge carrier but also endows the LaMnO₃/g-C₃N₄ photocatalyst with strong redox ability, thus boosting the degradation efficiency of tetracycline compound. In another case, the formation of Z-scheme g-C₃N₄/Ag/Ag₃PO₄ (rhombic dodecahedrons morphology) heterojunction via simple in situ deposition method possesses advanced efficiency of charge separation and transfer, as well as stronger redox ability [62].

Theoretical advances on the g-C₃N₄-based nanocomposites

Generally, there exists seven different phases of C₃N₄ with different band gap energies (Table 1). The nitride pores’ size and various electronic environments of the N atom connect to various energetic stabilities. On the basis of density functional theory (DFT), Kroke et al. [63] demonstrated that tri-s-triazine-based structure is energetically favored and is the most stable structure among all allotropes of g-C₃N₄. It is flexible for inducing reactions to alter its surface activity without altering its theoretical structure and composition. It exhibits advantages of strong physicochemical stability and distinctive electronic band structure, owing to high degree of condensation and the presence of heptazine ring structure. Due to the presence of the tri-s-triazine ring structure, g-C₃N₄ is stable up to 600 °C, in N₂, O₂, and air atmospheres [34, 64]. It possesses nitrogen-rich, Lewis and Bronsted basic sites, and shows good potential in CO₂ fixation and activation. High thermal stability implies that g-C₃N₄ can suit for both under oxidative and higher temperatures atmospheres. Besides, g-C₃N₄ offers good chemical stability in a varied range of solvents such as toluene, diethyl ether, dimethylformamide, water, and alcohols, making it an effective catalyst for both gas and liquid phase reactions [29, 65, 66].

Table 1 Different phases of g-C₃N₄ [159, 160]

The morphological structure of g-C₃N₄ plays a substantial role in photocatalysis. Fine control and unique oriented structure of g-C₃N₄ is useful for energy harvesting, conversion, and storage application [67]. Chen et al. [68] explained cleavage of in-plane hydrogen bonds between polymeric melon units by the introduction of CO₃²⁻ which promote amorphization of g-C₃N₄ and increase visible light absorption ability [69]. Usually, CN has weak van der Waals (vdW) interlayer interactions. Li et al. [70] demonstrated that forming internal van der Waals heterostructures (IVDWHs) within g-C₃N₄ can augment interlayer Coulomb interaction and assist charge separation efficiency. Moreover, urea-derived g-C₃N₄ possess strong redox ability, effective absorption of contaminants, and efficient charge separation efficiency, due to its large band gap, large surface area, and moderated N-defects [71]. Besides, it has five substitutional sites and two interstitial sites are usually considered as doping sites (Fig. 7). Doping of g-C₃N₄ alters its geometric structure and electronic and optical properties. For instance, Cui et al. [72] demonstrated that introduction of O on the surface of g-C₃N₄ can extend the absorption of visible light absorption and promote delocalization of HOMO and LUMO. Fabrication of modified g-C₃N₄-based heterojunction nanocomposites with enhanced physicochemical properties for better photocatalytic activity is an emerging concept for precise applications. The better photocatalytic mechanism for g-C₃N₄/MoS₂ hybrid nanocomposites by DFT calculations was reported by Wang et al. [73]. DFT calculations explain that these hybrid nanocomposites possess improved separation of photogenerated carriers.

Figure 7

Eleven adsorption sites in tri-s-triazine-based g-C₃N₄ [158]

Adsorption of reactants photocatalysts surface is significant in photocatalysis because it is directly related with several reactions. Adsorption energies of H₂O, CO₂, H₂, N₂, CO, and CH₄ on g-C₃N₄ are reported to be − 0.513, − 0.226, − 0.078, − 0.117, − 0.155 and − 0.163 eV, respectively [74]. Interestingly, adsorption energies of products are more positive than those of reactants, implying strong adsorption of reactants onto g-C₃N₄ and easy desorption of products from g-C₃N₄, which are noticeably beneficial to start the reaction and reappear adsorption sites, respectively.

It is well established that there are two different half reactions involved in photocatalytic water splitting process: hydrogen generation via H⁺ reduction and O₂ generation via H₂O oxidation. As the standard redox potential for formation of both products (O₂/H₂O = 1.23 V vs. NHE and H₂/H⁺ 0 V vs. NHE) lies within the CB and VB potential of g-C₃N₄, it is theoretically evident that g-C₃N₄ can catalyze overall water splitting reactions. Wirth et al. [75] studied water splitting reaction over g-C₃N₄ using DFT calculation. They reported that the overpotential for H₂O oxidation reaction is more than that of H⁺ reduction process, thus implying that O₂ generation process demand oxidation co-catalyst, while H₂ production can be simply attained without a co-catalyst. Nevertheless, its usage for antimicrobial coating applications is not fully explored owing to challenges associated with limited absorption of visible region and higher recombination rate of photogenerated charge carriers [76, 77].

g-C₃N₄-based photocatalysts for disinfection applications

Different strategies have been adopted to resolve problems associated with pristine g-C₃N₄ (Table 1). These include: (1) texture and morphological modification of g-C₃N₄; (2) doping of g-C₃N₄ using of cations [noble metals: gold (Au), platinum (Pt), and silver (Ag) and transition metal: copper (Cu), nickel (Ni), iron (Fe) and strontium (Sn)] and anions [sulfur (S), carbon (C), fluorine (F), boron (B) and phosphorous (P)]; and (3) incorporation of g-C₃N₄ with suitable semiconductor materials [e.g., TiO₂, CuO, AgX (X = Br, Cl and I), ZnO and GO]. These methods focus on reducing the energy band gap and extend visible light absorption, thus promoting photocatalytic disinfection. Table 2 lists notable recent works on g-C₃N₄ composites for disinfection of microbial species.

Table 2 Photocatalytic disinfection efficiency of g-C₃N₄-based composites

Texture and morphological modified g-C₃N₄-based composites

High surface area photocatalysts possess more surface reactive sites, improved charge transfer efficiency, and accelerated charge carrier separation efficiency; this enhances photocatalytic activity. For instance, Huang et al. [78] showed that E. coli K-12 can be efficiently destroyed with the existence of mesoporous g-C₃N₄, reaching 100% inactivation efficiency after 4-h visible light radiation. They also reported that the surface area of mesoporous g-C₃N₄ is 20 times greater than bulk g-C₃N₄ and photogenerated holes on g-C₃N₄ surface can facilitate bacterial inactivation [78]. Simultaneously, visible light active high surface area (190 m² g⁻¹) porous g-C₃N₄ nanosheets (PCNS) could completely kill E. coli within 4 h, while bulk g-C₃N₄ could kill 77.1% of E. coli cells [79]. Recently, Kang et al. [80] fabricated visible light active porous g-C₃N₄ NS by two different approaches: alternated heating and cooling, and bacterial-inspired liquid exfoliation method. Porous g-C₃N₄ NS gave better water disinfection behavior with respect to disinfection of E. coli as a result of its large surface area, low bandgap and, better electron transport ability [80]. Thurston et al. [27] evaluated the biocidal activity of urea and dicyandiamide and derived two different kinds of g-C₃N₄ films against both G⁻ (E. coli) and G⁺ (S. aureus) bacteria [28]. Enhanced activity of u-g-C₃N₄ on pathogenic organisms was explained to occur due to the factors such as high surface area (72.2 m² g⁻¹), reduced band gap energy (2.86 ± 0.14 eV) and efficient separation of photogenerated electron–hole pairs. Notably, no antimicrobial activity was noticed for both g-C₃N₄ films under dark conditions.

Delamination of pristine g-C₃N₄ can improve photodisinfection activity. Bacteria-etched g-C₃N₄ photocatalysts demonstrated four times enhanced photocatalytic water disinfection activity as compared with pristine g-C₃N₄ and showed outstanding stability under visible light exposure [53]. Functionalization of carboxyl (−COOH) and carbonyl (C=O) groups at the edge of g-C₃N₄ can significantly promote E. coli inactivation efficiency of log₁₀(C/C_o) = 6 (> 99.9999%) under visible light exposure over a period of 30 min with lower catalyst consumption. Also, edge-functionalized g-C₃N₄ nanosheets can effectively promote separation of charge carriers, stimulate upward surface band bending and assist production of H₂O₂, thus improving disinfection efficiency [58].

Also, urea-derived g-C₃N₄ showed sporicidal activity against Bacillus anthracis (B. anthracis) endospores upon exposure to visible light. Nearly, 2 × 10⁷ CFU mL⁻¹ of E. coli was destroyed totally using a visible active single-layered g-C₃N₄ in a period of 4 h. g-C₃N₄ nanosheet and g-C₃N₄ (g-C₃N₄ NS) reduced ~ 5 log and ~ 3 log of E. coli, respectively under similar experimental conditions [81]. Inactivation of MS2 phage using g-C₃N₄ was first studied by Li and et al. [82]. Almost all viral species (1 × 10⁸ PFU mL⁻¹) could be completely killed in a period of around 6 h. Then, Zhang et al. [83] optimized the effect of different operating parameters such as light intensity, metal loading and, reaction temperature for inactivation of bacteriophage MS2 using response surface methodology (RSM) (Fig. 8). Up to 6.58 log PFU mL⁻¹ of viruses were inactivated under the optimized conditions of light intensity (199.8 mW cm⁻²), metal loading (135.4 mg L⁻¹) and reaction temperature (24.05 °C), which is well matched with the experimental value (6.51 log PFU mL⁻¹) [83].

Figure 8

a The 3D response surface plots of the photocatalytic viral inactivation efficiency by g-C₃N₄ for interaction between a light intensity X1 and photocatalyst loading X2, b light intensity X1 and reaction temperature X3 and c photocatalyst loading X2 and reaction temperature X3 [83]

Anions (nonmetal) and cation (metal)-doped g-C₃N₄-based nanocomposites

Transition metal ions and noble metals are commonly employed as dopants to enhance photocatalytic disinfection efficiency. A transition metal can create an extra energy level within the semiconductor material, assisting the formation of electron–hole pairs and broadening spectral absorption toward the visible region. Electrons moving from one of these levels to CB require lesser photon energy than unmodified semiconductors. Numerous transition metal-doped g-C₃N₄ have been reported for photocatalytic applications. But, none of them report its potential for photocatalytic disinfection of microbes. Noble metals incorporated g-C₃N₄ photocatalysts can enhance the photocatalytic performance by creating charge carriers and extending its spectral absorption into visible region. In addition, surface plasmon resonance (SPR) effect of metal species can induce generation of charge carriers in g-C₃N₄ [84,85,86]. Also, noble metals can behave as an electron sink for tapping free electrons, promoting the separation of photogenerated charge carriers and enhancing the photocatalytic efficiency of g-C₃N₄ [87, 88]. i.e., Photogenerated electrons move from the CB of g-C₃N₄ to the metal nanoparticle deposited on g-C₃N₄ surface, while the photogenerated hole remains on g-C₃N₄. This leads to effective separation of photogenerated charge carriers and associated improvement of photocatalytic disinfection performance by the generation of ROS species.

Ag is a well-known bactericidal agent under dark condition. Sorption of Ag on negatively charged cell wall deactivates cellular enzymes, disturbs bacterial cell wall (membrane), and eventually causes cell lysis and cell death. Enhanced generation of ROS species on Ag/g-C₃N₄ nanohybrids surface exhibited higher efficiency to inactivate bacterial species and better ability to destruct biofilms into proteins, nucleic acids, and polysaccharides, when matched with g-C₃N₄ nanosheets, in the presence of visible light [89]. Further, the hybrid effect of Ag/g-C₃N₄ composite could extend visible light utilization efficiency, reduce recombination of charge carriers, promote fast separation and shifting of photogenerated charge carriers, and provide longer lifetime to charge carriers. A hybrid Ag/g-C₃N₄ composite was fabricated by photoreduction approach, exhibiting acceptable disinfection efficiency toward E. coli. It was confirmed through experiments with different chemical scavengers that e⁻, h⁺, and ·O₂⁻ species play a key role in demolition of bacterial cell wall [90]. Xu et al. [91] reported efficient antimicrobial activity against S. aureus by Ag-doped g-C₃N₄, synthesized by a two different approach (hydrothermal treatment and photoassisted reduction) (Fig. 9). In 3 h exposure, nearly, 29.6% and 99.4% and of bacterial cells were inactivated by g-C₃N₄ and Ag/g-C₃N₄ composites, respectively [91]. Besides, h⁺ and O₂⁻ species were observed to be significant species for bacterial inactivation with Ag/g-C₃N₄ (Ag/PCNO) in the photocatalysis process. Also, high efficiency of Ag/PCNO was explained to occur because of SPR effect of AgNPs and the synergistic influence from PCNO molecules.

Figure 9

a Synthesis of Ag/g-C₃N₄ composite and b generation and transfer of photogenerated charge carriers at the interface of Ag/g-C₃N₄ under visible light irradiation [91]

As compared to pure AgNP, one-pot green synthesis of g-C₃N₄/AgNPs nanocomposite using grape seed extract as the stabilizing agent displayed better bactericidal activity against E. coli, P. aeruginosa, S. aureus, and B. subtilis in the absence of light. This can be ascribed to many combinational factors. First, the existence of AgNPs on g-C₃N₄ surface leads to bacterial species inactivation. Second, the incorporated g-C₃N₄ and ligand molecules from grape extract interact with the surface of silver to make AgNPs highly stable. Also, g-C₃N₄/AgNPs nanocomposite is more compatible with G⁻ bacteria than G⁺ types [92]. Munoz-Batista et al. [93] demonstrated that Ag-doped g-C₃N₄(Ag-g-C₃N₄) nanocomposite significantly enhances antimicrobial performance against E. Coli under both UV as well as in the visible region. Nanocomposites exhibit improved bactericidal activity than AgNPs without g-C₃N₄ because of better distribution and stability characteristics of AgNPs on g-C₃N₄ surface.

Apart from Ag, integration of Au NPs with g-C₃N₄ (CNA) can deliver outstanding peroxidase activity toward the breakdown of H₂O₂ to ·OH radicals and can efficiently kill both G⁺and G⁻ bacteria. Also, it shows good efficacy in breaking down existing DR biofilms and in suppressing the creation of fresh biofilms in vitro. Additionally, CNA has high toxicity on cancer cells [94]. Importantly, the amount of metal loading can significantly enhance bactericidal activity. For instance, Bing et al. [89] observed enhanced antimicrobial activity at an optimum mass ratio of 1:10 Ag/g-C₃N₄ nanohybrid with low concentration and with short illumination duration. Limited work on photocatalytic disinfection activity over nonmetal (N, S, C, etc.) doped g-C₃N₄ is available. In one study, anchoring of red phosphorous (r-P) nanoparticle on g-C₃N₄ nanosheets was observed to extend visible light absorption up to 700 nm, exhibiting higher photodisinfection activity than pure g-C₃N₄ and r-P. Improved disinfection activity is mainly ascribed to type I band alignment between g-C₃N₄ and r-P, which facilitates effective charge separation [95].

Heterostructure-based g-C₃N₄ nanocomposites

Hybridization of g-C₃N₄ with other semiconductor materials is an attractive approach for enhancing its photocatalytic performance. The key advantages include: broadening absorption into visible region, effective separation of charge carriers by shifting electrons from higher CB to lower CB, and holes from higher VB to lower VB, preventing photocorrosion of semiconductor materials. Micron-sized TiO₂ spheres were enfolded with g-C₃N₄ hybrid structure [(g-C₃N₄)/TiO₂] by hydrothermal calcination approach and (g-C₃N₄)/TiO₂ hybrid material could completely inactivate E. coli within 180 min under visible light exposure. Further, increase or decrease in hydrothermal temperature can alter photocatalytic inactivation efficiencies [96]. Xu et al. [97] described visible light activity of thin g-C₃N₄ loaded on aligned surfaces of TiO₂ nanotube arrays (TiNT) for the removal of E. coli. Bacterial survival ratio of g-C₃N₄/TiNT and g-C₃N₄ composites was ~ 16% and ~ 86%, respectively. Bactericidal results showed that 15 mg of melamine is the optimal precursor load for fabricating highest efficiency g-C₃N₄/TiNT layers. Also, bactericidal activity decreases under anaerobic conditions and no inactivation was observed in catalysts without exposure to light. Zhang and co-workers [98] demonstrated that g-C₃N₄ quantum dots (QDs)-immobilized TiO₂ nanotube array (g-C₃N₄ QDs/TNA) membrane showed an impressive performance for improved antifouling capacity during filtering water containing E. coli under visible light exposure. A vertical heterojunction with Z-scheme feature was achieved by conjoining g-C₃N₄ and anatase TiO₂ with {001} facets by a hydrothermal process. The coupled band structure trigger enhanced photocatalytic antibacterial activity in contrast with physically prepared composites and pure g-C₃N₄ and TiO₂ [99].

Of late, visible light active Bi-based semiconductor materials have gained extensive attention in photocatalytics owing to their unique crystalline structures and absorption characteristics in visible light. Photocatalytic disinfection activities of Bi₂MoO₆/g-C₃N₄ NS_s (BM/CNNs) composites are higher under visible light exposure. Further, photocatalytic disinfection activity of BM/CNNs composite was higher at optimum loading BM content of 20% (by weight) [100]. Z-scheme-type monoclinic dibismuth tetraoxide (m-g-C₃N₄/Bi₂O₄) heterojuction was fabricated through a facile hydrothermal approach by Xia et al. (2017). The optimal ratio (1:0.5) of m-g-C₃N₄/Bi₂O₄ of composite could inactivated 6 log₁₀ CFU mL⁻¹ of E. coli K-12 for a period of 1.5 h of visible light exposure was more efficient than g-C₃N₄ (1.5 log) and m-Bi₂O₄ (4 log) [101]. Particularly, disinfection efficiency did not increase noticeably with increasing m-Bi₂O₄ content, whereas excessive m-Bi₂O₄ can act as the recombination center of photogenerated charge carrier thereby diminishing the separation efficiency of electron–hole pairs.

By modifying g-C₃N₄ with plasmonic photocatalysts, the photocatalytic disinfection activity can be improved to a significant extent. For instance, AgBr (photosensitive material)-coupled g-C₃N₄ nanocomposite could inactivate 3 × 10⁶ CFU mL⁻¹ of E. coli and S. aureus species in 60 min and 150 min, respectively. Generated h⁺ and surface founded. OH on g-C₃N₄-AgBr surface are dominant ROS species that are responsible for bacterial inactivation. Moreover, bacterial disinfection efficiency is higher in alkaline environment owing to the generation of more surface found ·OH [102]. Photocatalytic disinfection activity of Ag₂WO₄/g-C₃N₄ composite against E. coli was investigated by Li et al. [103]. Synthesized photocatalyst [Ag₂WO₄ (5%)/g-C₃N₄ composite] exhibited higher bactericidal efficiency than pure g-C₃N₄ and Ag₂WO₄ under visible light. Superior disinfection efficiency with reaction rate of 0.39 min⁻¹ was ascribed to the dispersion of Ag₂WO₄ particles on g-C₃N₄ surface. Enhanced charge separation rates in Ag₂WO₄(5%)/g-C₃N₄ composite is also an important factor as indicated in Fig. 10 [103]. Due to unique benefits of small size, tunable optical property, and outstanding electron transfer rates, vanadate QDs have proven capabilities as photocatalysts for solar light harvesting. Incorporation of vanadate QDs (AgVO₃ and BiVO₄) on g-C₃N₄ surface can effectively promote its charge separation efficiency and thus result in improved photocatalytic disinfection. AgVO₃/g-C₃N₄ composite (0.75 mg mL⁻¹) showed the superior bactericidal efficiency of 96.4% in 10 min due to the abundant generation of ROS in water [104]. In contrast, g-C₃N₄ sample could inactivate only 54.13% of Salmonella spp. after 10 min treatment. Moreover, ·O₂⁻ was the major ROS of themicrobial inactivation process. Also, the photocatalytic disinfection efficiency on Salmonella spp. was observed to be more effective as matched to S. aureus under same conditions. Particularly, photocatalysts made up of earth-abundant elements are preferred for microbial inactivation. Figure 11 shows the destruction Salmonella bacterial cell in AgVO₃ QDs/g-C₃N₄ at different exposure time under visible light.

Figure 10

Mechanism of photocatalytic disinfection treated with Ag₂WO₄/g-C₃N₄ composite under visible light irradiation [103]

Figure 11

SEM images of Salmonella (10⁷ CFU/mL) treated with AgVO₃/g-C₃N₄ (0.75 mg/mL) under irradiation for a 0, b 5, c 10, d 20, e 30, f 30 min (magnified SEM image) [104]

Z-scheme type 10%Ni₂P/g-C₃N₄ lamellar nanohybrids have been fabricated by hydrothermal synthesis using low-cost red phosphorous precursor and displayed 10 times faster inactivation under visible light exposure as compared with g-C₃N₄. Ni₂P could successfully trap photogenerated electrons and also facilitated h⁺ accumulation, leading to improved disinfection efficiency in case of E. coli K-12 [105]. Correspondingly, microwave in situ hybridization of organic g-C₃N₄ with inorganic CdCO₃ hybrid composite fully hindered the growth of E. Coli bacteria within 45 min of visible light exposure at an optimum concentration of 60 μg mL⁻¹. Nonetheless, higher catalyst loading restricted penetration of light into the suspension, thereby decreasing inactivation efficiency [106]. Sundaram et al. [107] synthesized ZnO/g-C₃N₄ composite using single-step thermal condensation approach and tested its photocatalytic antimicrobial effect against E. coli and S. aureus. They demonstrated that ZnO/g-C₃N₄ catalysts have good antimicrobial efficiency against S. aureus as compared to E. coli.

Recently, carbon-based composites have also been used as co-catalyst to facilitate transfer of electrons on g-C₃N₄. Also, 2D/2D nanocomposites facilitate better mobility of charge carrier across the heterojunction boundary, which in turn rise the charge carrier transfer and separation efficiency [108]. For example, coupling of 2D-structured g-C₃N₄ with 2D-structured graphene oxide (GO) can improve photogenerated charge separation. Nearly 97.9% E. coli was killed by GO/g-C₃N₄ composite with the concentration of 100 μg mL⁻¹ after 120 min of visible light exposure [109]. Moreover, Wang et al. (2013) fabricated g-C₃N₄ (CN) sheets and reduced graphene oxide (RGO) wrapped on cyclo octasulfur (α-S₈) crystal (CNRGOS₈ and RGOCNS₈) in two different orders and proved its antibacterial activity under both aerobic and anaerobic conditions. CNRGOS₈ displayed superior photocatalytic disinfection activity than RGOCNS₈ under aerobic condition; RGOCNS₈ showed better photocatalytic performance under aerobic conditions as compared to CNRGOS₈ composites [110]. Fullerene (C60 and C70) wrapped g-C₃N₄ nanocomposites have been examined for bactericidal activity against E. coli O157:H7 bacteria under visible light exposure. C70/C₃N₄ composite demonstrated higher photocatalytic inactivation activity than C60/C₃N₄ composite (~ 86%) within 4 h exposure to visible light [111]. On the contrary, bare g-C₃N₄ inhibited only 68% bacterial cells after 4 h of visible light exposure. Correspondingly, g-C₃N₄/EP (porous expanded perlite) composites exhibited excellent antibacterial activity against E. coli (8-log reduction) and antiviral activity (MS2 phage) in the absence of mechanical stirring under visible light exposure of 180 and 240 min, respectively (Fig. 12). Also, water quality parameters such as salinity (NaCl), hardness (Ca²⁺), dissolved oxygen (DO), and proton concentration can enhance the efficiency of g-C₃N₄/EP-520 for MS2 inactivation [112].

Figure 12

Proposed mechanism of bacterial and viral inactivation by g-C₃N₄/EP-520 under visible light irradiation [112]

Loading of noble metals on g-C₃N₄ hybrid composites can considerably improve its photocatalytic disinfection activity. For instance, Adhikari et al. (2016) recently developed ternary Ag-decorated TiO₂ NFs on g-C₃N₄ sheet (Ag-TCN-5) using a two-nozzle electrospinning–calcination approach and showed high disinfection activity against both G⁻ (E. coli) and G⁺ (S. aureus) bacteria. The focus was on the released Ag ions that show enhanced antibacterial effects [113]. Similarly, distribution of Ag on ZnO/g-C₃N₄ hybrid composites by one-pot hydrothermal method gave superior photocatalytic activity toward the degradation of MB and the antibacterial activity against E. coli was reported by Joo et al. [114]. Recently, Pant et al. [115] developed a novel, magnetically separable Ag-Fe₃O₄/g-C₃N₄ composite by hydrothermal treatment. This showed superior photocatalytic activity toward E. coli bacteria. Then, facile thermal heating method fabrication of novel Ag/AgO-modified g-C₃N₄ microspheres (Ag/AgO/g-CNMS) was used. The improved photocatalytic performance of Ag/AgO/g-CNMS is attributed to the collective effects of broadened light absorption and enhanced charge carrier separation efficiency. Further, 5 mg of 10% of Ag/AgO/g-CNMS resulted in complete inactivation of E. coli within 0.5 h under visible light exposure [116]. Also, Zhang et al. [117] introduced Ag/g-C₃N₄ nanosheets on polyethersulfone (PES) membrane by phase-inversion method. They observed that addition of Ag/g-C₃N₄ nanosheets improved photocatalytic activity (degradation of methyl orange), antibacterial activity (P. aeruginosa and E. coli), and antifouling property. Likewise, g-C₃N₄-Bi₂MoO₆-Ag nanocomposite showed a higher antibacterial effect against both G⁺ and G⁻ bacterial species as shown in Fig. 13 [26]. Recently, Z-scheme BiVO₄/Ag/g-C₃N₄ ternary composites have been studied for improved photodisinfection activity against E. coli as compared with BiVO₄/g-C₃N₄ composite. Moreover, ternary composite showed lowered disinfection efficiency in case of sewage samples in contrast with that in synthetic saline solution due to the presence of natural organic matter (NOM) [118].

Figure 13

Charge separation mechanism of the MB dye degradation and disinfection of bacteria over g-C₃N₄-Bi₂MoO₆-Ag tertiary composite [26]

Graphitic carbon nitride-functionalized boron and phenyl (B/phenyl/g-C₃N₄) exhibited huge absorption range from ultraviolet light to near-infrared light. Nearly 99% E. coli were inhibited by B/phenyl/g-C₃N₄ composites under visible light exposure for a duration of 180 min [119]. More interestingly, the photocatalytic antibacterial performance of g-C₃N₄/BiOI/BiOBr composite is obviously enhanced as compared to g-C₃N₄ BiOI and BiOBr photocatalysts. Almost E. coli cells are completely killed under exposure of visible light for more than 3 h, whereas g-C₃N₄, BiOI, and BiOBr photocatalysts inactivated 83.7%, 34.2%, and 29.4% of E. coli cells within 5 h of treatment [120].

Younis et al. [121] examined the removal of different dyes [methyl orange (MO), methylene blue (MB), and crystal violet (CV)] and antimicrobial activity of different pathogens (E. coli, C. albicans and P. aeruginosa) using CaO incorporated g-C₃N₄-based nanocomposites with 4,5-diphenyl-2-thioxo-2,5-dihydro-1H-pyrrole-3-cabonitrile (P₃C@CaO-HCN) composite. They demonstrated that copolymer exhibited more affinity toward MB adsorption (1915.8 µmol g⁻¹), and antibacterial activity toward E. coli (93.5%), C.albicans (85.8%) and P. aeruginosa (61.54%) species. Superior antimicrobial effect was ascribed to the presence pyrrole-3-cabonitrile functionalized with cyanide (C≡N) and protonated g-C₃N₄ (HCN) sheet [121].

In addition, integration of a membrane with the photocatalyst can efficiently mitigate membrane fouling as a result of efficient inactivation of microbial pollutants by photocatalysis. Assembling of g-C₃N₄NS/rGO photocatalyst on inexpensive cellulose acetate membrane via vacuum filtration method (namely g-C₃N₄NS/rGO/CA) could inactivate all bacteria (6.5 log reduction) under visible light in 2 h [122]. Also, incorporation of g-C₃N₄/Bi₂MoO₆ into fluorocarbon resin (PEVE) can enhance photocatalytic sterilization performance of PEVE. The sterilization performance of composite coatings was prominent, when the amount of g-C₃N₄ in Bi₂MoO₆ was about 7%, under visible light irradiation for 4 h. Besides, g-C₃N₄ enhanced charge transfer efficiency of Bi₂MoO₆ in the presence of visible light, and participated to the generation of more strong oxidizing species (O₂⁻ and h⁺) [123]. Correspondingly, alteration of TiO₂ nanotubes/Ti plates with g-C₃N₄-SnO₂composites lead to improved photocatalytic activity in two different systems (chemical and microbial). Better photocatalytic and bactericidal activity of g-C₃N₄-SnO₂/TiO₂ nanotubes/Ti plate was ascribed to the efficient separation of electron–hole between g-C₃N₄-SnO₂ and TiO₂ nanotubes/Ti in the ternary composite. Further, these researchers studied the generation of CO₂ during bacterial species mineralization under visible light exposure (Fig. 14). Initially, the evolved CO₂ levels was unchanged and then gradually elevated with time, showing the restraint of bacteria breathing, followed by bacterial death [124].

Figure 14

SEM images of a bare TiO₂ nanotubes/Ti plate, b g-C₃N₄-SnO₂/TiO₂ nanotubes/Ti plate and c antibacterial activity of fabricated plates for E. coli degradation under visible light irradiation and d CO₂ evolution caused by mineralization of E. coli cells under visible light illumination and in the dark for 32 h [124]

The sandwich structure of g-C₃N₄/TiO₂/kaolinite composite showed higher bactericidal efficiency against S. aureus in comparison with g-C₃N₄, TiO₂, and kaolinite [125]. Song et al. [126, 127] fabricated two different nanocomposites: g-C₃N₄ immobilized with Al₂O₃/EP (g-C₃N₄/Al₂O₃/EP) and both g-C₃N₄ and TiO₂ immobilized Al₂O₃/EP (g-C₃N₄/TiO₂/Al₂O₃/EP). Removal efficiency of Microcystis aeruginosa algal species (2.7 × 10⁶ CFU mL⁻¹) were 74.4% and 88.1% for g-C₃N₄/Al₂O₃/EP and g-C₃N₄/TiO₂/Al₂O₃/EP, respectively after 6 h reaction [126, 127]. g-C₃N₄ and nitrogen–phosphorus co-doped TiO₂ (named as AP-EGC-CT composite) hybridized with functional expanded graphite covered carbon layer composites (AP-EGC) could remove 98.2% algal cells (M. aeruginosa) following 2 h treatment [128]. Algal cell inactivation was significantly facilitated by the photocatalytic oxidation process as shown in Fig. 15.

Figure 15

A (a) Changes in conductivity during photocatalytic process; Microscopic images after (b) 15 min and (c) 3 h reaction and (d) SEM image after 9 h reaction; B Schematic diagram of the algal inactivation process [128]

Other applications

CO₂ photoreduction

The energy band structure of g-C₃N₄ is suitable for CO₂ photoreduction to sufficient number of value-added chemical fuels (CO, CH₄, CH₃OH, HCOOH and C₂H₅OH.

$$ {\text{CO}}_{2} + 2h^{ + } + 2e^{ - } \to {\text{CO}} + {\text{H}}_{2} {\text{O}}\quad \left( { - \,0.52\;{\text{Vs}}\;{\text{NHE}}} \right) $$

(8)

$$ {\text{CO}}_{2} + 2h^{ + } + 2e^{ - } \to {\text{HCOOH}}\quad \left( { - \,0.58\;{\text{Vs}}\;{\text{NHE}}} \right) $$

(9)

$$ {\text{CO}}_{2} + 4h^{ + } + 4e^{ - } \to {\text{HCHO}} + {\text{H}}_{2} {\text{O}}\quad \left( { - \,0.48\;{\text{Vs}}\;{\text{NHE}}} \right) $$

(10)

$$ {\text{CO}}_{2} + 6h^{ + } + 6e^{ - } \to {\text{CH}}_{3} {\text{OH}} + {\text{H}}_{2} {\text{O}}\quad \left( { - \,0.38\;{\text{Vs}}\;{\text{NHE}}} \right) $$

(11)

$$ {\text{CO}}_{2} + 8h^{ + } + 8e^{ - } \to {\text{CH}}_{4} + {\text{H}}_{2} {\text{O}}\quad \left( { - 0.24\;{\text{Vs}}\;{\text{NHE}}} \right) $$

(12)

$$ 2{\text{CO}}_{2} + 12h^{ + } + 12e^{ - } \to {\text{C}}_{2} {\text{H}}_{5} {\text{OH}} + 3{\text{H}}_{2} {\text{O}}\quad \left( { - 0.39\;{\text{Vs}}\;{\text{NHE}}} \right) $$

(13)

Difference in microstructure and crystallinity of g-C₃N₄ leads to different kinds of products. For instance, urea-derived g-C₃N₄ produces CH₃OH and C₂H₅OH from CO₂; using melamine-derived g-C₃N₄ only leads to the selective formation of C₂H₅OH (Mao et al. [43]). So far, g-C₃N₄ with various nanostructures such as mesoporous structures, nanosheets, nanowires, and nanocomposites have been synthesized for enhanced CO₂ photoreduction (Niu et al. [129]). Besides, it is explained that the amino groups of g-C₃N₄ play a crucial role for the adsorption and activation of CO₂ on its surface [130, 131]. Modification of electronic band structure and textural property of g-C₃N₄ can improve its photocatalytic activity. Doping of metals (Cu, Pt, Mg and Mo) and nonmetal (S, P, r-P and O) on g-C₃N₄ composites have been studied for CO₂ photoreduction, as an approach to provide a positive effect on reducing the charge carrier recombination and lower band gap energy, thus leading to superior photocatalytic activity toward the reduction of CO₂ [42, 132,133,134,135,136]. Moreover, different Ru complexes, trans(Cl)-[Ru(bpyX₂)(CO)₂Cl₂] (bpyX₂ = 2,2′-bipyridine with substituents ‘X’ in the 4-positions, X = H, CH₃PO₃H₂ or CH₂PO₃H₂), achieved enhanced photocatalytic activities of CO₂ into HCOOH with high turnover number (> 1000) (Kuriki et al. [137]). Integration of zero-dimensional carbon dots on 2D g-C₃N₄ nanocomposites significantly influences the reduction of CO₂ into value-added chemicals (CH₄ and CO). Besides 3 wt% of CND loading showed highest evolutions of CH₄ (29.23 μmol·gcatalyst⁻¹) and CO (58.82 μmol·gcatalyst⁻¹) under visible light exposure after 10 h. The resultant apparent quantum efficiency (AQE) was 0.076% [138]. Increase in photoactivity using CND/pCN-3 was explained to be because of synergistic interaction between pCN and CNDs, allowing effective migration of photoexcited electrons from pCN to CNDs via well-contacted heterojunction interfaces which retard charge recombination. Likewise, several type II and Z-scheme g-C₃N₄-based heterojunctions have been evaluated for CO₂ photoreduction; examples include: TiO₂/g-C₃N₄, LaPO₄/g-C₃N₄, B₄C/g-C₃N₄, In₂O₃/g-C₃N₄, g-C₃N₄/NaNbO₃, g-C₃N₄/ZIF-8, CdIn₂S₄/mpg-C₃N₄, g-C₃N₄/NiAl-LDH, Ag₃PO₄/g-C₃N₄, SnO₂x/g-C₃N₄, ZnO/g-C₃N₄, gC₃N₄/Bi₂WO₆, g-C₃N₄/Bi₄O, g-C₃N₄/Bi₄O₅I₂, carbon dots/g-C₃N₄ and WO₃/g-C₃N₄ [138,139,140].

Hydrogen generation

Semiconductor photocatalyst gC₃N₄ has the proper band edge potential for water splitting applications (Eqs. 14, 15)

$$ {\text{H}}_{2} {\text{O}} + 2h^{ + } \to 2{\text{H}}^{ + } + 0.5{\text{O}}_{2} \quad \left( {1.23\;{\text{V}}\;{\text{vs}}\;{\text{NHE}}} \right) $$

(14)

$$ 2{\text{H}}^{ + } + 2e^{ - } \to {\text{H}}_{2} + 0.5{\text{O}}_{2} \quad \left( { - 0.82\;{\text{V}}\;{\text{vs}}\;{\text{NHE}}} \right) $$

(15)

Also, g-C₃N₄ can be further modified in different routes for improving its H₂ generation ability. So far, g-C₃N₄ nanosheets, mesoporous g-C₃N₄ nanomesh, g-C₃N₄ nanorods, and g-C₃N₄ quantum dots have shown enhanced H₂ production activity than that of bulk g-C₃N₄. Coupling of g-C₃N₄ with metal/nonmetal nanoparticles and other semiconductor materials could extend their spectral range. Generally, F, C, S, I, and P-doped g-C₃N₄ are used for H₂ generation, showing enhanced production of H₂. Rh, Pt, Ag, Au, Zn, and Sn-doped g-C₃N₄ nanocomposites promote H₂ productivity. For instance, FeP/g-C₃N₄ nanocomposites prepared by Zeng et al. (2018) exhibited outstanding H₂ production activity under visible light exposure. When the loading content of FeP was 2.19%, the catalyst exhibited the maximum production yield of H₂ (177.9 μmol g⁻¹ h⁻¹) with an apparent quantum yield (AQY) value of 1.57% at 420 nm. Excellent hydrogen evolution was attributed to active sites formation and heterojunctions [141]. In another research on photocatalytic generation of H₂, the composite structure comprising of 1D/2D Co₂P/g-C₃N₄ heterostructure by solution phase approach showed improved photocatalytic hydrogen generation without the support of noble metals as cocatalysts. Maximum H₂ production (53.3 μmol h⁻¹ g⁻¹) was achieved at an optimum Co₂P nanorods loading of 3 wt% [142]. In addition, porous g-C₃N₄ nanosheets modified with flower-like and network-like MoSe₂ nanostructures generate H₂ amount of 114.5 μmol h⁻¹ g⁻¹ and 136.8 μmol h⁻¹ g⁻¹, respectively. The optimal loading of MoSe₂ nanostructures is 5 wt%, giving maximum H₂ evolution rates of 114.5 μmol h⁻¹ g⁻¹ and 136.8 μmol h⁻¹ g⁻¹, respectively. In addition, more amount of H₂ generation was achieved over network-like MoSe₂ nanostructures decorated than flower-like MoSe₂, ascribed to the effective charge migration and separation based on synergistic effects arising from the unique sheet-on-sheet heterointerface in N-CN [143]. Similarly, TiO₂/g-C₃N₄, InO₃/g-C₃N₄, MoS₂/g-C₃N₄, CeO₂/g-C₃N₄, NiO/g-C₃N₄, CdS/g-C₃N₄, Al₂O₃/g-C₃N₄, and Cu(OH)₂/g-C₃N₄ heterojunctions have been widely studied as photocatalyst for H₂ generation reaction [144,145,146,147,148,149].

NO and N₂ fixation

Photocatalytic technology is a promising route in air purification technology, particularly because it does not associate with increased secondary pollution. Self-structure-modified graphene-like g-C₃N₄ nanosheets enlarge the band gap, inducing strong oxidation of NO to NO₂⁻ or NO₃ under visible light exposure [19].

$$ e^{ - } + {\text{O}}_{2 } \to \cdot {\text{O}}_{2}^{ - } $$

(16)

$$ \cdot {\text{O}}_{2}^{ - } + {\text{NO}} \to {\text{NO}}_{3}^{ - } $$

(17)

$$ h^{ + } + {\text{OH}}^{ - } \to \cdot {\text{OH}} $$

(18)

$$ {\text{NO}} + 2 \cdot {\text{OH}} \to {\text{NO}}_{2} + {\text{H}}_{2} {\text{O}} $$

(19)

$$ {\text{NO}}_{2} + \cdot {\text{OH}} \to {\text{NO}}_{3}^{ - } + {\text{H}}^{ + } $$

(20)

Co-functionalization O/La, O/Ba, and SrO on the surface of amorphous carbon nitride promotes the formation of localized electrons, thus boosting photocatalytic NO removal efficiency [68, 69, 150]. Cui et al. [69] proposed the conversion pathwayfor NO adsorption and photocatalytic NO oxidation processes on SrO-clusters@amorphous carbon nitride composites [68, 69, 150]. Nearly 100% improved NO purification efficiency was achieved over O, K-functionalized g-C₃N₄ with IVDWHs [70]. Li et al. [151] demonstrated that photocatalytic efficiency and selectivity of Ca intercalated g-C₃N₄ for NO removal can be considerably enhanced because of the functionality of the localized excess electrons around Ca. In another case, incorporation of Sr caused uneven electron distribution on the surface of g-C₃N₄, accelerating light absorption ability, separation, and transfer of photogenerated charge carriers. As a result, NO_x is efficiently oxidized by active species and transformed into target products of NO₂⁻ and NO₃⁻ rather than other toxic by-products [152]. Coexisting of K and Cl ions in the interlayer of g-C₃N₄ not only suppresses charge transfer barrier but also acts as the dual channel for electrons and holes transfer to extend their lifetime, thereby improving NO_x removal efficiency under visible light exposure as compared with pristine g-C₃N₄ and K-g-C₃N₄. Moreover, CN-KCl (3%) showed best photocatalytic activity with NO_x removal ratio of 38.4%, even more than K doped g-C₃N₄ (31.4%) [71]. Moreover, Chen et al. [153] prepared MnOx/g‐C₃N₄ catalyst that were relatively stable and had synergistic photothermal catalytic activity toward NO purification under UV–visible light irradiation. These researchers also proposed the corresponding conversion pathway and mechanism of NO oxidation at 60 °C [153].

NVs (nitrogen vacancy) endow g-C₃N₄ with photocatalytic N₂ fixation ability for three reasons. NVs exhibits the same size and shape of as the nitrogen atom, as a result it selectively adsorb and activate N₂. Second, NVs effectively progress the charge separation efficiency of photogenerated carriers and generate more photoelectrons. Third, NVs promote photogenerated electron transfer from g-C₃N₄to adsorbed N₂. All three reasons make photocatalytic N₂ fixation on the surface of V-g-C₃N₄ easier. Chen et al. [154] systematically investigated single transition metal atoms decorated on the g-C₃N₄ with nitrogen vacancies (TM@NVs-g-C₃N₄), performing as electrocatalysts for conversion of N₂ into NH₃.

Organic pollutant degradation

Due to the band position, g-C₃N₄ is among the best known photocatalysts for the remediation of pollutants present in aqueous solutions. Generally, O₂⁻, ·OH, and h⁺ play a substantial role in degrading different organic pollutants into CO₂, H₂O, and fractions of organic acids as explained in Eqs. (21, 22).

$$ {\text{Organic}}\;{\text{pollutants}} + {\text{g-C}}_{3} {\text{N}}_{4} - h^{ + } \to {\text{g-C}}_{3} {\text{N}}_{4} + {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}} + {\text{intermediates}} $$

(21)

$$ {\text{Organic}}\;{\text{pollutants}} + {\text{O}}_{2} \to {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}} + {\text{intermediates}} $$

(22)

Initially, the organic pollutant is transferred into the interphase of g-C₃N₄ and aqueous solution. Then, the adsorbed particles get oxidized or decomposed by ROS species. After that, degradation products (intermediates) are desorbed from g-C₃N₄ surface to its interface. Lastly, reaction products get released to the bulk solution. The structure and preparation methodologies of g-C₃N₄ influence the degradation efficiency of organic pollutants. As compared with the bulk g-C₃N₄, exfoliated g-C₃N₄ nanosheets display improved methylene blue degradation efficiency. Several studies have been available for surface modification of g-C₃N₄ by depositing metal nanoparticles (e.g., Ag, Au, Pt, Pd, and Ni) on its surface to degrade organic contaminants from wastewater under visible light irradiation. Correspondingly, hybrid nanostructures built by depositing g-C₃N₄ with other semiconductor materials (TiO₂, ZnO, AgX (X = Br, Cl, and I), Ag₂O, Ag₂CO₃, Ag₃VO₄, Fe₃O₄, and Bi₂MoO₆, CaIn₂S₄, C-dots, V₂O₅, CdS, BiOBr, etc.) also revealed better visible light photodegradation for pollutants than pure g-C₃N₄.

Sensing and bioimaging application

g-C₃N₄ nanosheet offers more surface area-to-volume ratio and have strong response for detection of various metal ions like Cu²⁺, Fe^3+, Hg²⁺, and Cr²⁺ [32]. Also, Fe/g-C₃N₄, g-C₃N₄ nanosheet/MnO₂ and proton functionalized ultrathin g-C₃N₄ nanosheets can detect glucose, glutathione (GSH), and heparin molecules, respectively [155]. Additionally, g-C₃N₄ can also be used for temperature sensing as reported by Debanjan et al. [21]. They found that the photoluminescence PL intensity is decreased as the temperature increased. Owing to its non-toxicity, metal-free nature, and high stability, g-C₃N₄ nanosheets and nanodots are promising candidates for cell imaging. For example, g-C₃N₄ QDs have been utilized as biomarkers for the labeling of the cell membranes [156].

Conclusion

Photocatalytic disinfection process offers good prospects for biomedical, water treatment, and food applications. In comparison with metal oxides, g-C₃N₄ can offer a safe and reliable disinfection technology. Visible light-driven g-C₃N₄-based materials are a promising and innovative approach for antimicrobial applications. It is clear from several findings that physical and chemical properties of g-C₃N₄-based composites can offer acceptable levels of microbial inactivation. The fundamental properties of g-C₃N₄ and their action against microbial species inactivation have been discussed. Inactivation of the wide range of microorganisms is attributed to the formation of ROS species on the surface of g-C₃N₄ during the photocatalysis process. Various kinds of g-C₃N₄ modification techniques (cationic and anionic doping, coupling with another semiconductor material, and surface sensitization) are effective approaches. Furthermore, the photocatalytic disinfection activity of protozoa is not yet well known. Currently, most of the g-C₃N₄-based photodisinfection application are developed and tested in under laboratory conditions. Issues on efficiency due to faster recombination of photogenerated electron–hole pairs need to be addressed. However, it is necessary to investigate the experimental and theoretical aspect of tri-s-triazine-based g-C₃N₄ for photocatalysis process so as to helps to predict reaction pathways as well as band structure changes of g-C₃N₄ after being hybridizing with certain materials. In addition to the charge separation mechanism, thermodynamics and kinetics study of photocatalytic reaction thorough investigation.