1.1 Polymeric graphitic carbon nitride (g-C₃N₄) as photocatalyst

Graphitic carbon nitride (g-C₃N₄), a 'golden photocatalyst,' has provoked ripples of excitement in research societies due to its maximum visible light absorption, cost-effectiveness, greater profusion, non-toxicity, л-conjugated assembly and thermal as well as chemical stability. g-C₃N₄ spurred our enormous attention in 1834 due to its tunable bandgap of 2.7 eV with VB and CB potentials positioned at −1.09 and +1.56 eV, respectively. Owing to these properties, g-C₃N₄ acts as a favourable photocatalytic material and has wider applications in electrochemical water splitting, act as a photocatalyst for H₂ production as well as for photodegradation of noxious waste (Ong et al., 2016). Although, photocatalytic efficiency of bulk g-C₃N₄ is bounded due to (i) its inefficiency to absorb visible light (ii) higher photo-induced EHP recombination (iii) smaller surface area and (iv) lower electrical conductivity. Consequently, several modification approaches have been developed to accomplish spatial charge separation of photo-carriers such as; (1) deposition of metals/non-metals (doping), (2) fabricating porous structures and (3) coupling with alternative semiconductor (heterojunction) in order to boost the photoactivity of g-C₃N₄ (Sudhaik et al., 2018b). Hybridizing g-C₃N₄ with another semiconducting material is a notable technique because of its ability to achieve efficient transference/separation of photo-generated EHP and appropriate redox potential for photocatalytic implementations (Raizada et al., 2019d; Raizada et al., 2020).

1.2 Electronic structure of g-C₃N₄

In recent years’ carbon nitrides (C₃N₄) have gained remarkable attention as a polymeric semiconducting material. The structure of carbon nitride (C₃N₄) materials consist of two-dimensional (2D) sheets of triazine (1,3,5-triazine, C₃N₃) and heptazine (1,3,4,6,7,9,9b-heptaazaphenalene, C₆N₇) core (Kroke et al., 2002). These are aromatic molecules where carbon (C) and nitrogen (N) atoms are arranged in alternative positions in the ring. C₃N₄ exists in seven stages, and these are categorized as α-C₃N₄, β-C₃N₄, cubic C₃N₄, pseudocubic C₃N₄, g-h-triazine, go-triazine and g-h-heptatriazine having bandgap potentials of 5.49, 4.85, 4.30, 4.13, 2.97, 0.93 and 2.88 eV, correspondingly (Teter and Hemley, 1996). Several allotropes of C₃N₄ can be fabricated via temperature assisted process such as heating melamine to 317 °C leads to the formation of melam [(H₂N)₂(C₃N₃)₂]NH which on further heating forms melem (tri amino-heptazine) C₆N₇(NH₂)₃ (Jürgens et al., 2003)_. Above 520 °C, melem condenses, and forms amine linked heptazine units, which is referred to as Liebig’s melon (Liebig, 1844). Pyrolysis of cyanamide, dicyanamide, urea, thiourea, and other molecules comprising of C and N above 520 °C are considered as Polymeric carbon nitride (PCN) called melon (Kessler et al., 2017). The condensation of melem under 520 °C forms triazine based graphitic carbon nitride (g-C₃N₄). g-C₃N₄ exists as the most substantial allotrope of carbon nitride, while tri-s-triazine based g-C₃N₄ was the utmost steady stage at ambient environments and was dynamically favourable. Furthermore, density functional theory (DFT) measurements were accomplished by Kroke and his research group to explore its electronic structure. P_z orbitals of N and C were mainly liable for the construction of VB and CB, as investigated by DFT calculations (Ong et al., 2016). As per reports, lone pair of e⁻ on N atom was the reason for the formation of electronic structure and VB. Since 2009, g-C₃N₄, an n-type semiconducting material, has evolved as a prominent visible light active photocatalyst for degradation of organic toxins from aqueous phase (Kumar et al., 2019; Raizada et al., 2019a).

Fig. 2 illustrated various precursors for facile fabrication of g-C₃N₄ photocatalyst. Several physical as well as chemical approaches have been adopted for preparation of g-C₃N₄ photocatalyst. Mainly, top down and bottom up approaches have been embraced. Top down approach implicates breaking up of bigger g-C₃N₄ blocks into smaller units, namely g-C₃N₄ nanosheets, while bottom up approach involves incorporation of smaller molecules to form larger complex molecules. Moreover, thermal polymerization, solvothermal technique, sol-gel process, template assisted method and ionothermal approaches were commonly used for synthesis of g-C₃N₄ photocatalyst (Sudhaik et al., 2018b; Bojdys et al., 2008; Zhang et al., 2018).

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Fig. 2

Schematic representation of synthesis process of g-C₃N₄ by thermal polymerization of various precursors (Reproduced with permission).

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1.3 Silver halides (AgX) as potential visible light assisted photocatalyst

Fabrication of photocatalyst with extended visible light absorption, plasmonic photocatalytic materials have gained enormous research interests. The plasmonic photocatalyst, mainly silver (Ag) and gold (Au), unveil greater absorption coefficients in wide UV–visible-near infrared range (NIR). This can be accredited to their strong surface plasmon resonance (SPR) (Zhang et al., 2009b; Huang et al., 2006). Particularly, Ag NPs were chosen due to their inexpensiveness as compared to Au NPs, and they exhibited efficient plasmon resonance in solar light (Awazu et al., 2008; Georgekutty et al., 2008).

A collective oscillation of e⁻_CB exhibited by noble metal NPs leads to an oscillating electric field. While these oscillations caused a rearrangement of charge density as well as electric field, therefore generating professed localized surface plasmon (LSP) through a coulombic refurbishing potency to repel the effect of positive nuclei. Plasmon resonant frequency of noble metal nano-junctions overlays the solar spectrum thereby, extending the natural light absorption. When oscillations befall at localized surface plasmon resonance (LSPR) wavelengths, i.e., 530–600 nm, the dissipated energy is assumed to be least, and consumption of natural light is maximum. Under exposure to solar light, SPR of noble metal NPs usually occurs when the frequency of stimulating light is equivalent to the rate of oscillating surface e⁻ (Fig. 3a) (Ye et al., 2012; Wang et al., 2012). Noble metal NPs exhibiting LSPR effect, ameliorated the efficacy of semiconductor-based photocatalysis by interacting between free surface e⁻ and photons, moreover tailoring novel plasmonic photocatalysts (Sarina et al., 2013; Ingram et al., 2011; Primo et al., 2011).

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Fig. 3

Schematic illustration of (a) Surface Plasmon Resonance (SPR) in a noble metal particle; reprinted with permission from Elsevier (Licence no. 4742480664982) (Ye et al., 2012), and (b) Band gap positions of AgCl, AgBr and AgI with their respective valence band and conduction band potentials (Ye et al., 2014; Yao et al., 2016; Islam et al., 2016).

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Till date, much attention has been gained by Ag-based photocatalysts (such as Ag₃PO₄, AgVO₃, Ag₂CO₃, AgX (X = Cl, Br, I), Ag@AgX (X = Cl, Br, I), AgIO₃, and Ag₂WO₄) for photocatalytic degradation of organic toxins. Silver halides (AgX, X = Cl, Br, I) being plasmonic photocatalyst with antibacterial properties are the photosensitive semiconducting materials with ameliorated visible light absorption, and their photosensitivity proposes potential utilization in photocatalysis. They have been broadly exerted as source materials in photographic films and employed as photosensitizers. Also, they can be used as metallic silver precursors (Archana et al., 2015). Under exposure to visible light, AgX undergoes a reduction of Ag⁺ into Ag⁰, as depicted in equation Eqs. (3) and (4) (Tate, 2012);

Ag/AgX photocatalyst can be attained from AgX by exploiting their photosensitivity. These photocatalysts are highly efficacious as well as stable under visible light radiation. Despite their broad bandgap, greater than the edge of natural light, AgX has been developed as visible light assisted photocatalytic material (Grzelczak and Liz-Marzán, 2014). However, the photo-corrosive nature of Ag-based compounds limits the practical implementations of AgX as photocatalysts. Therefore, silver chloride (AgCl) having bandgap of 3.25 eV with VB and CB potentials positioned at +3.16 eV and −0.09 eV, respectively (Ye et al., 2014), while silver bromide (AgBr) has a bandgap of 2.60 eV with VB and CB potential located at +2.30 eV and −0.30 eV, respectively (Yao et al., 2016). Also, silver iodide (AgI) possesses a band gap of 2.73 eV with VB potential at +2.40 eV and CB potential at −0.33 eV (Islam et al., 2016). Fig. 3b depicted the band gap positioning of AgCl, AgBr and AgI, respectively. Therefore, coupling of a semiconductor with another semiconducting material has emerged as a new technology for boosted photocatalytic performance (Hasija et al., 2019a). Precisely, coupling of Ag-based photocatalysts with another semiconductor material having appropriate band edge positioning can effectively migrate and separate charge carriers which further participate in photocatalytic reactions. As a consequence, the availability of electrons which participate in the photo-reduction of Ag⁺ into Ag⁰ reduced significantly providing superior stability to the system (Chen et al., 2015b; Liu et al., 2011). Chen et al. tailored plasmonic Ag/AgBr/g-C₃N₄ photocatalyst via a superficial means for mineralization of methylene blue (MB) under solar light irradiation. Ag/AgBr with 40% content exhibited superior photocatalytic activity, which was about 2.4 and 1.9 times greater than bare g-C₃N₄ and Ag/AgBr hybrid. Scanning electron microscopy (SEM) analysis confirmed the deposition of sphere-like Ag/AgBr particles on aggregated g-C₃N₄ nanosheets (NS) (Fig. 4a). Moreover, particle size of Ag/AgBr particles ranging between 50 nm and 200 nm was observed in transmission electron microscopy (TEM) images. The proposed mechanism (Fig. 4b) for Ag/AgBr/g-C₃N₄ nanocomposite, suggesting the boosted photocatalytic efficacy was attributable to the synergetic effect of Ag, AgBr, and g-C₃N₄. Under solar light illumination, both g-C₃N₄ (2.7 eV) and AgBr (2.6 eV) get excited, succeeding in the transference of photo-generated e⁻ from g-C₃N₄ to CB of AgBr, thereby reacting with adsorbed O₂ to produce CO₂. Furthermore, metallic Ag produced photo-induced e⁻ on the AgBr surface because of a strong SPR effect. AgBr possesses inferior fermi level than that of Ag ensuing transfer of e⁻ from Ag to AgBr and lastly transference to adsorbed O₂ to generate ^•O₂⁻ (Chen et al., 2015a, 2015b). Fig. 5(a–c) depicted facile routes for preparation of AgCl, AgBr and AgI, respectively, employing various precursors.

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Fig. 4

(a) SEM imageries of Ag/AgBr/g-C₃N₄ nanohybrid depicting deposition of Ag/AgBr particles on aggregated g-C₃N₄ NS, and (b) Mechanistic view of Ag/AgBr/g-C₃N₄ composite explicating boosted photocatalytic efficiency; reproduced with permission from Elsevier (Licence no. 4746461343390) (Chen et al., 2015).

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Fig. 5

Synthesis scheme for facile fabrication of (a) AgCl, (b) AgBr and (c) AgI.

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Recently, g-C₃N₄ grafted AgX heterojunctions have witnessed surge interest of the research community owing to its potential to minimalize the recombination of photo-excited charge carriers, thereby augmenting the solar light absorption. For example, Lan et al. delineated an integral report on highly efficient solar light assisted AgX@g-C₃N₄ (X = Cl, Br, I) photocatalyst for photodegradation of pollutants (Lan et al., 2013). In this research article, they discussed AgX and g-C₃N₄, preparation of AgX@g-C₃N₄ nanocomposite, briefly discussed the results along with the degradation mechanism for mineralization of methyl orange (MO) dye. In another report, Xu et al. explored AgX/g-C₃N₄ (X = Br, I) nanohybrid with synergistic photocatalytic efficiency (Xu et al., 2013a). Keeping in mind the studies mentioned above, we have tried to present an extensive report on cutting-edge trends in AgX/g-C₃N₄ hybrids emphasizing pollutant mineralization accompanied by energy applications. To best of our knowledge, no review article has been published to date, as per Scopus data. Therefore, in focus of this review, we have discussed comprehensively g-C₃N₄ and AgX nanostructures, synthetic routes for facile fabrication of AgX/g-C₃N₄ nanocomposite, engineering AgX/g-C₃N₄ nano-junctions involving type-ΙΙ and Z-scheme based heterostructures along with their practical implementations in photocatalytic degradation processes and energy conversion followed by bacterial disinfection. To get an overall idea of literature being reported on AgX/g-C₃N₄ composite “Scopus” database has been carried out to acquire effective outcomes. Consistent with Scopus data, we got 970, 165, 85 and 89 results employing keywords such as “g-C₃N₄ + AgCl photocatalyst”, “g-C₃N₄ + AgBr photocatalyst”, “g-C₃N₄ + AgI photocatalyst” and “g-C₃N₄ + AgX photocatalyst”, respectively, from 2009 to December 2019 (Fig. 6a–c). With such an escalating amount of reports on g-C₃N₄ tailored AgX, it is the apt and high time to impart an extensive review of AgX/g-C₃N₄ nanomaterials. Herein we presented a substantial and efficient review of this evolving hot research area.

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Fig. 6

Scopus data analysis (a) Pie diagram depicting multifunctional applications of AgX/g-C₃N₄ photocatalyst, (b) Bar graph portraying annual publications, and (c) Citations of g-C₃N₄, AgX as well as AgX/g-C₃N₄ photocatalyst from year 2009 to December 2019.

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2.1 Precipitation method

Precipitation method is often employed for AgX/g-C₃N₄ nanocomposite fabrication_. Mohamed et al. explored AgCl@pg-C₃N₄ nano photocatalyst, where mesoporous graphitic carbon nitride (pg-C₃N₄) was fabricated via pyrolysis of dicyandiamide and urea at an atmospheric environment. Silver nitrate (AgNO₃) and potassium chloride (KCl) were employed as starting materials, and the AgCl@pg-C₃N₄ composite was prepared in-situ. The photocatalytic ability of prepared samples was estimated for photodegradation of atrazine. No peak for AgCl was observed during X-ray diffraction (XRD) analysis due to lower AgCl content or successful dispersal of AgCl onto a g-C₃N₄ surface (Fig. 7a). UV–Visible diffuse reflectance spectroscopy (UV-DRS) spectra revealed that increased weight percentage of AgCl above 3% shifted the absorption edges of pg-C₃N₄ towards extended wavelengths because higher weight percentage increases the agglomeration and size of AgCl, which reduces the absorption edge (Mohamed, 2015). Yao et al. investigated novel Ag/AgCl/g-C₃N₄ nanocomposite by precipitation in geothermal water at ambient temperature. Geothermal water is a sort of natural heat energy and renewable energy resource, which provides green energy. Here, geothermal water was employed as a chlorine source for the preparation of AgCl NPs. 50% Ag/AgCl/g-C₃N₄ revealed more excellent photocatalytic capability for the elimination of organic dyes under illumination of natural light. SEM studies explained that the deposition of Ag/AgCl NPs onto the surface of g-C₃N₄ lead to an irregular surface and a close interface was formed among g-C₃N₄ and Ag/AgCl. Also, TEM analysis illustrated the decoration of several shady Ag/AgCl NPs on lamellar g-C₃N₄ nanosheets, with greater dispersion (Yao et al., 2014).

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Fig. 7

(a) XRD analysis of as-prepared AgCl@pg-C₃N₄; reprinted with permission from Elsevier (Licence no. 4746510793950) (Mohamed, 2015), (b) XRD spectra of Ag/AgBr/g-C₃N₄ photocatalyst showing face-centered cubic assembly of AgBr, (c) TEM studies confirming the formation of as-synthesized nanocomposite; reprinted with permission from Elsevier (Licence no. 4746550937986) (Cao et al., 2013), TEM images demonstrating (d) platelet-like morphology of bulk g-C₃N_4, and (e) particle size of Ag@AgBr/g-C₃N₄ composite; reprinted with permission from Elsevier (Licence no. 4746561201233) (Yang et al., 2014).

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The advantages increasing the utilization of precipitation method for facile preparation of NPs are the relatively low reaction temperatures, fine and uniform particle size with weakly agglomerated particles, low cost and homogeneity of component distribution. Moreover, the precipitation method is not considered as a controlled process in terms of reaction kinetics. Also, the as-prepared solids have a wide particle size distribution, uncontrolled agglomerated morphology and incomplete precipitation of metal ions (Deraz, 2018).

2.2 Deposition precipitation method

A plasmonic Z-scheme based AgCl/Ag/g-C₃N₄ photocatalytic material was fabricated by Bao and his research group employing melamine (C₃H₆N₆), hexadecyl trimethyl ammonium chloride (CTAC), AgNO₃ as a precursor molecule. The photocatalytic efficiency of prepared nano-sample was explored for photodegradation of MO dye. The results revealed, Ag/AgCl NPs with a particle size of 5–15 nm were evenly distributed onto a g-C₃N₄ network. TEM studies demonstrated a lamellar sheet-like assembly and amorphous nature of pure g-C₃N₄. However, Ag/AgCl exhibits spherical morphology. It was observed that Ag/AgCl particles (dark spots) were evenly dispersed on the g-C₃N₄ surface. A slight redshift was found in the absorption edge by UV-DRS spectra of AgCl/Ag/g-C₃N₄ composite, which may be accredited to the SPR effect of Ag crystal, i.e., formed in-situ on AgCl surface. Also, superior photocatalytic efficacy of AgCl/Ag/g-C₃N₄ nanocomposite was indexed to the SPR of Ag nanocrystal (Bao and Chen, 2016).

Cao and co-workers fabricated a proficient VLT Ag/AgBr/g-C₃N₄ photocatalyst using C₃H₆N₆, sodium bromide (NaBr), and AgNO₃ as predecessors for the exclusion of MO dye. The superior photocatalytic ability was observed for 50% Ag/AgBr/g-C₃N₄ nanocomposite, which may be accredited to metallic Ag and synergistic effects of AgBr/g-C₃N₄ alliance. Fig. 7b explained XRD pattern indicating face-centered cubic assembly of AgBr, where small amounts of Ag⁺ were reduced to metallic Ag, and no prominent peaks for metallic Ag were obtained. TEM imaginings (Fig. 7c) confirmed the formation of as-synthesized nanocomposite possessing random microstructure (Cao et al., 2013). Ag/AgBr tailored graphite-alike carbon nitride was synthesized by Xu and his research group via deposition-precipitation method (Xu et al., 2013b). Yang et al. explored novel Z-scheme Ag@AgBr/g-C₃N₄ plasmonic photocatalyst by photoreduction of AgBr/g-C₃N₄ nanohybrid. Bulk g-C₃N₄ was synthesized by polymerization of urea (NH₂CONH₂), while cetyl methyl ammonium bromide (CTAB) and AgNO₃ were the starting materials employed for the fabrication of Ag@AgBr hybrid. Ag@AgBr/g-C₃N₄ nanocomposite revealed stronger photocatalytic ability towards photo-degradation of MO and RhB. A diffraction peak at 38.1° attributed to metallic Ag was observed during XRD analysis. SEM and TEM analysis evaluated the morphological features of the Ag@AgBr/g-C₃N₄ composite. Fig. 7d displayed a platelet-like morphology of bulk g-C₃N₄, which provides more reactive sites for the growth of Ag@AgBr nanohybrids. Sphere-like Ag@AgBr hybrids with a particle size of 200 nm were formed, and the platelet-like structure remains inviolate after Ag@AgBr particle growth on the g-C₃N₄ surface (Fig. 7e) (Yang et al., 2014).

Zhang al. constructed a novel VLT AgI/g-C₃N₄ photocatalyst using C₃H₆N₆, AgNO₃, potassium iodide (KI) and polyvinylpyrrolidone (PVP) as precursors with enhanced photocatalytic efficiency for diclofenac (DCF) removal (Zhang et al., 2017a). Murugesan et al. synthesized a VLD AgI/g-C₃N₄ nanocomposite via deposition precipitation method (Murugesan et al., 2018a).

2.3 Hydrothermal method

The hydrothermal approach is utilizing the chemical reaction among reagents at a particular temperature and pressure (Ghosh and Pal, 2019). Using a hydrothermal approach for facile preparation of AgX/g-C₃N₄ photocatalyst is an unusual approach. For example,

Akbarzadeh et al. employed bismuth (ΙΙΙ) chloride (BiCl₃), ammonium metavanadate (NH₄VO₃), ethanolamine (C₂H₇NO), C₃H₆N₆ and AgNO₃ as precursors for the fabrication of g-C₃N₄/Ag/AgCl/BiVO₄ micro-flowers viz. one-pot hydrothermal treatment. Photocatalytic performance of as-prepared nanocomposite was examined for photo-removal of ibuprofen (IBF). To BiCl₃ solution, NH₄VO₃ was added to yield orange-coloured precipitates after that ethanolamine was added, and colour of the solution altered. Subsequently, the AgNO₃ solution was added to the above mixture via the precipitation-photodeposition method, and the whole solution was exposed under visible light in a photoreactor. The g-C₃N₄ solution was prepared and was consequently added to the above-prepared mixture by undergoing ultrasonication. The mixture was autoclaved and heated at 180 °C for 12 h, and then the obtained precipitates were dried to obtain g-C₃N₄/Ag/AgCl/BiVO₄ micro-flowers. XRD pattern (Fig. 8a) demonstrated a diffraction peak at 15.11°, which helped distinguish monoclinic scheelite BiVO₄ (m-BiVO₄) from tetragonal BiVO₄ (t-BiVO₄). While, peaks at 38.2° and 44.6° were attributed to metallic Ag, which indicated the photo-reduction of AgNO₃ from Ag⁺ to Ag⁰ (Akbarzadeh et al., 2018). Raizada and research group explored BiOBr/PSCN/Ag/AgCl Z-scheme photocatalytic mechanism for degradation of phenol under visible light, via a facile hydrothermal technique. SEM images (Fig. 8b) depicted the successful loading of BiOBr/Ag/AgCl NPs on phosphorus (P) and sulfur (S) co-doped g-C₃N₄ (PSCN) nanosheet (Raizada et al., 2020b).

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Fig. 8

(a) XRD pattern distinguishing monoclinic scheelite BiVO₄ from tetragonal BiVO₄ in g-C₃N₄/Ag/AgCl/BiVO₄ composite; reproduced with permission from Elsevier (Licence no. 4746601497828) (Akbarzadeh et al., 2018), (b) SEM images depicting successful loading of BiOBr/Ag/AgCl NPs on PSCN nanosheet; reproduced with permission from Elsevier (Licence no. 4754600966798) (Raizada et al., 2019b), (c) Detailed fabrication process of AgBr@g-C₃N₄/BiOBr nano-junction, (d) SEM imaginings revealing dispersal of AgBr particles on g-C₃N₄/BiOBr composite; reproduced with permission from Elsevier (Licence no. 4746611095568) (Tang et al., 2019), and (e) SEM studies confirming formation of PGCN/AgI/ZnO/CQDs heterostructure; reproduced with permission from Elsevier (Licence no. 4746620007509) (Hasija et al., 2019).

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Tang et al. investigated AgBr@g-C₃N₄/BiOBr via hydrothermal route followed by the in-situ ion-exchange method, utilizing NH₂CONH₂, KBr, ethylene glycol (EG) and AgNO₃ predecessors. The photocatalytic experimentations were carried out for RhB and tetracycline (TC) as target contaminants, under irradiation of visible light. Fig. 8c illustrated the detailed fabrication process of AgBr@g-C₃N₄/BiOBr nanoheterojunction. Fig. 8d explained SEM micrographs, which indicated the dispersal of AgBr particles on g-C₃N₄/BiOBr composite (Tang et al., 2019a). Chen et al. studied 3D Ag/AgBr/g-C₃N₄@NGA Z-scheme based photocatalyst for efficient MO removal and antimicrobial activities were also investigated (Chen et al., 2019).

PGCN/AgI/ZnO/CQDs nanocomposite was developed by Hasija and co-workers employing NH₂CONH₂, diammonium hydrogen phosphate (NH₄)₂HPO₄, AgI, zinc nitrate tetrahydrate Zn(NO₃)₂, sodium hydroxide (NaOH), sodium iodide (NaI) and zinc oxide (ZnO) as starting material. While CQDs were prepared using bamboo leaves through facile hydrothermal treatment. SEM images confirmed the formation of PGCN/AgI/ZnO/CQDs nano-junction, as shown in Fig. 8e (Hasija et al., 2019c). Xue et al. studied g-C₃N₄/Bi₂WO₆/AgI nanocomposite following a Z-scheme photocatalytic system for efficient photodegradation of TC through hydrothermal reaction followed by in-situ precipitation method (Xue et al., 2019). Zhang et al. developed a VLD Z-scheme AgI/LaFeO₃/g-C₃N₄ photocatalyst for removal of norfloxacin (NOF) (Zhang et al., 2019).

During hydrothermal process, conditions of the reaction are very imperative. Solvent type, temperature as well as duration affect the synthesis of the products. Hydrothermal approach offers many advantages such as relatively mild operating conditions (reaction temperatures < 300 °C), one-step synthetic process, environmental friendly, and good dispersion in solution (Sōmiya and Roy, 2000). Besides, use of expensive autoclaves hinders its widespread applicability.

2.4 Ion-exchange method

An ion-exchange method is frequently employed for the synthesis of AgX/g-C₃N₄ composite. The operation circumstance is facile as well as lower in cost while choosing raw material is the crucial step in this scheme. For instance,

Zhang et al. employed C₃H₆N₆, AgNO₃, EG, and hydrochloric acid (HCl) as precursor molecules for the synthesis of plasmonic Ag@AgCl/g-C₃N₄ photocatalyst viz. in-situ ion exchange process. The photocatalytic performance of as-fabricated nanocomposite was estimated for degradation of RhB, MO, MB, and phenols under natural light illumination. The detailed synthesis of Ag@AgCl/g-C₃N₄ nanocomposite was illustrated in Fig. 9a. TEM imaginings explained the black Ag@AgCl hybrid with a particle size of ∼8–12 nm was evenly dispersed on the surface of exfoliated g-C₃N₄ (Fig. 9b). XRD analysis demonstrated the diffraction peaks for Ag@AgCl/g-C₃N₄, revealing face-centered cubic AgCl and peak intensities increased with increasing content of AgNO₃ (Zhang et al., 2014). A novel AgCl/Ag₃PO₄/g-C₃N₄ ternary composite was explored by Zhou and research group, with enhanced photocatalytic efficiency towards sulfamethoxazole (SMX) degradation (Zhou et al., 2017).

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Fig. 9

(a) Fabrication of Ag@AgCl/g-C₃N₄ nanocomposite, (b) TEM imageries showing dispersal of black Ag@AgCl particles onto g-C₃N₄ surface; reprinted with permission from Elsevier (Zhang et al., 2014) (c) SEM analysis representing loading of cubic AgBr particles on rod-like particles of Ag₂CO₃; reprinted with permission from Elsevier (Licence no. 4746900302562) (Tang et al., 2017), (d) Synthesis scheme of g-C₃N₄/Ag₃PO₄/AgI composite; reprinted with permission from Elsevier (Licence no. 4746910427726) (Tang et al., 2019), and (e) Preparation of g-CN/Ag₃VO₄/AgBr nanocomposite (Barzegar et al., 2018).

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Tang et al. fabricated g-C₃N₄/Ag₂CO₃/AgBr heterostructure by surface alteration of g-C₃N₄/Ag₂CO₃ thru AgBr NPs via the ion-exchange process. The synthesis process involved starting materials such as NH₂CONH₂, sodium carbonate (Na₂CO₃), AgNO₃, and NaBr. XRD pattern revealed the coexistence of Ag₂CO₃ and AgBr particles in the g-C₃N₄/Ag₂CO₃/AgBr composite. The Fourier-transform infrared spectroscopy (FTIR) results confirmed the presence of NPs and the formation of heterojunction. Irregular and cubic AgBr particles were observed on polyhedral rod-like particles of Ag₂CO₃, as explicated by SEM analysis (Fig. 9c) (Tang et al., 2017).

Tang and research group reported a Z-scheme based g-C₃N₄/Ag₃PO₄/AgI nano-photocatalyst for evaluating the photocatalytic ability for nitenpyram (NTP) degradation. Fig. 9d explained the preparation method of g-C₃N₄/Ag₃PO₄/AgI nanocomposite through the in-situ ion-exchange route (Tang et al., 2019b).

Ion exchange method is widely employed in industrial scale for waste water treatment. The process is environmental friendly, has low maintenance cost and can provide high flow rate to treated water. Along with these benefits, certain limitations associated with this process are bacterial contamination, chlorine adulteration and adsorption of organic matter.

2.5 Refluxing method

Refluxing is a process involving condensation of vapours and the return of this condensate to the system from which it is originated. g-C₃N₄/Ag₃VO₄/AgBr nanocomposites were studied by Barzegar et al. for the efficient elimination of toxins under visible light. The photocatalysts were synthesized by loading Ag₃VO₄ and AgBr particles onto a g-C₃N₄ surface. Moreover, the detailed fabrication process can be observed in Fig. 9e and the solution was refluxed at 96 °C for 1hr (Barzegar et al., 2018b).

Akhundi et al. reviewed g-C₃N₄/Fe₃O₄/AgI nanocomposite by reflux method at 96 °C (Akhundi and Habibi-Yangjeh, 2016c). A visible-light-driven g-C₃N₄/Ag₂CrO₄/AgI photocatalyst was explored by Barzegar and co-workers via the refluxing process, for efficient photodegradation of RhB (Barzegar et al., 2018a). The typical process involved mixing of AgNO₃ (0.109 g) with a suspension containing 0.35 g of g-C₃N₄/AgCrO (20%) sample. After that the sample was treated with 0.096 g of NaI and refluxed for 60 min at 96 °C.

The method requires the use of relatively cheaper equipment’s that are easy to operate and their setup is facile. Also, these methods are time consuming, involves the use of flammable solvents and sometimes these techniques may be destructive limiting their practical implementations.

Tables 1 and 2 illustrates various other methods for fabrication of AgX coupled g-C₃N₄ nano-hybrids with different photocatalytic applications.

Table 1 Different AgX/g-C₃N₄ nanocomposites for pollutant degradation.

S.No.	Photocatalyst	Fabrication routes	Targeted Pollutant	Light source	Degradation Mechanisms	References
1.	Ag/AgCl/g-C3N4	Facile preparation	MO	300 W Xe lamp with a 400 nm cut-off filter	__	Yang et al. (2014)
2.	g-C3N4/Fe3O4/AgCl	Facile preparation	RhB	50 W LED source		Akhundi and Habibi-Yangjeh (2015a)
3.	PoPD/AgCl/g-C3N4	Photoinitiated polymerization method	TC	250 W Xe arc lamp		Sun et al. (2019)
4.	Ag/AgCl@ZIF-8/g-C3N4	Facile stirring method	LVF	150 W Xe lamp equipped with 420 nm filter		Zhou et al. (2019)
5.	AgCl-Ag/g-C3N4	Three step synthetic method	MO	350 W Xe lamp with UV cut-off filter (λ > 420 nm)		Yu et al. (2019)
6.	g-C3N4/AgCl/CDs	Impregnation method	MB, RhB	40 W LED bulb		Matheswaran et al. (2019)
7.	γ-Fe2O3/Ag/AgCl/g-C3N4	Solvothermal and photodeposition method	Escherichia Coli (E. coli)	500 W tungsten halogen lamp		Ai et al. (2019)
8.	Ag/AgBr/g-C3N4	Solvothermal method	RhB	300 W Xe arc lamp equipped with ultraviolet cut filter		Xu and Zhang (2013)
9.	g-C3N4/GO/AgBr	–	RhB	250 W Xe lamp wiyh a 420 nm cut-off filter		Miao et al. (2017b)
10.	Ag/AgBr/V2O5/PCN	Hydrothermal method	Phenol	35 W LED lamp		Raizada et al. (2019b)
11.	g-C3N4/AgBr/rGO	–	2,4-dichlorophe-nol	250 W Xe lamp		Zhou et al. (2018)
12.	g-C3N4/Fe3O4/AgI/Ag2CrO4	Refluxing Method	RhB	50 W LED lamp		Akhundi and Habibi-Yangjeh (2016a)

Table 2 Different AgX/g-C 3N₄ nanocomposites for CO₂ photo-reduction and H₂ evolution.

Photocatalyst	Methodology	Process Conditions	Major products	Charge transfer mechanism	Quantum efficiency	Reference
AgCl@g-C3N4	Deposition-precipitation method	11 W compact fluorescent lamp	Methane Acetic acid Formic acid		53.03 μmol g−1 6.01 μmol g−1 2.51 μmol g−1	Murugesan et al. (2018b)
g-C3N4/Ag/AgCl/BiVO4 (CAB)	Facile one-pot hydrothermal method	8 fluorescent lamps, 8 W each	Methane		205 μmol h−1 g−1	Rather et al. (2018)
AgBr/g-C3N4/NG (ACNNG)	Green and facile method	Xe arc lamp	Methanol Ethanol	–	105.89 µmol g−1 256.45 µmol g−1	Li et al. (2015)
Ag/AgX(X = Cl, Br, I)/g-C3N4	In-situ solid phase method	300 W Xe lamp	H2		Ag/AgCl/CN-4; 26.39 μmol g−1h−1 Ag/AgBr/CN-4; 18.05 μmol g−1h−1 Ag/AgI/CN-4; 59.22 μmol g−1h−1	Bai et al. (2019)
PCN/GO/Ag/AgBr	Hard template method and selective chemical etching	300 W Xe lamp with a light filter of 420 nm	H2		3.69 mmol g−1h−1	Li et al. (2019)
g-C3N4-Ag/AgBr (CNAA)	Solvothermal method	Xe lamp	H2		47.84 μmol g−1h−1	Li et al. (2020)

3.1 Type-ΙΙ heterojunctions

Combining two semiconducting materials where CB of semiconductor Ι (SC-Ι) is lower than that of semiconductor ΙΙ (SC-ΙΙ) and VB of SC-Ι has a higher value than that of SC-ΙΙ, shown in Fig. 10. In type-II heterostructures, the band potentials are positioned at optimum levels (staggered gap) that offer spatial charge carrier separation with enhanced photocatalytic efficacy. For instance, a novel work reporting deposition of AgCl NPs on the g-C₃N₄ surface was explored by Zhao et al. (2012). Shi and research group investigated AgCl decorated g-C₃N₄ (AgCl-CN), explicating the photodegradation process of oxalic acid (OA) under visible light irradiation. g-C₃N₄ and AgCl possess band gaps of 2.7 eV and 3.25 eV, respectively. Fig. 11a illustrated degradation mechanism explicating the transference of h⁺_VB of AgCl to VB of CN while CN e⁻_CB shift to CB of AgCl. Therefore, photo-generated e⁻_CB of AgCl (E_CB = −0.06 eV) reduces O₂ to produce ^•O₂⁻ due to its more negative reduction potential, while h⁺_VB of CN react with absorbed water to produce ^•OH radicals. These ^•O₂⁻ and ^•OH radicals were responsible for the photocatalytic degradation of OA (Shi et al., 2019).

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Fig. 10

Diagrammatic representation of type- II heterojunction.

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Fig. 11

(a) Type- II degradation mechanism in AgCl-CN nanocomposite; reproduced with permission from Elsevier (Licence no. 4747851498621) (Shi et al., 2019), (b) XRD pattern for g-C₃N₄-NS/CDs/AgCl photocatalyst with no prominent peaks for CDs, (c) EDX spectra of as-prepared nanocomposite, and (d) EDX mapping confirming elemental distribution; reprinted with permission from Elsevier (Licence no. 4747860423948) (Asadzadeh-Khaneghah et al., 2018).

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Asadzadeh-Khaneghah et al. explicated the loading of carbon dots (CDs) and AgCl over the g-C₃N₄ network by a simple approach for the photocatalytic degradation of RhB, MB, MO, and phenol. g-C₃N₄-NS/CDs/AgCl photocatalyst with 20% of AgCl exhibited maximum photocatalytic activity. CDs acted as electron mediator amongst g-C₃N₄-NS and AgCl semiconductors. XRD analysis illustrated (Fig. 11b) that the introduction of CDs does not affect the assembly of g-C₃N₄ and no diffraction peaks were detected for CDs. Due to the existence of several functional groups such as hydroxyl, carbonyl, carboxyl, and epoxy in CDs, excessive amounts of oxygen elements were observed in Energy Dispersive X-ray (EDX) spectra (Fig. 11c). Fig. 11d explicated elemental distribution in g-C₃N₄-NS/CDs/AgCl (20%) nanocomposite by EDX mapping. Further FTIR analysis demonstrated that exfoliation of g-C₃N₄/CDs and integration of CDs and AgCl particles does not have any significant influence on the g-C₃N₄ structure. As well, no peak for the Ag-Cl bond was noticed, as observed in Fig. 12a. The exfoliated nature of g-C₃N₄-NS was confirmed by the UV-DRS spectra, exhibiting a slight blue shift (Fig. 12b) (Asadzadeh-Khaneghah et al., 2018).

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Fig. 12

g-C₃N₄-NS/CDs/AgCl composite showing (a) FTIR analysis, (b) UV–visible spectra; reproduced with permission from Elsevier (Licence no. 4747860423948) (Asadzadeh-Khaneghah et al., 2018), (c) FTIR examinations confirming presence of Zn-O bond in g-C₃N₄/ZnO/AgCl hybrid, and (d) TGA curves explicating the reduced weight of g-C₃N₄ (Akhundi et al., 2015); reproduced with permission from Elsevier (Licence no. 4747860921119).

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Akhundi and co-workers designed g-C₃N₄/ZnO/AgCl ternary nano photocatalyst employing a facile methodology for the photocatalytic degradation process. Typically, g-C₃N₄/ZnO/AgCl (40%) composite unveiled enhanced photocatalytic performance as compared to g-C₃N₄/ZnO and g-C₃N₄/AgCl nanocomposites. Further, FTIR studies (Fig. 12c) confirmed the presence of the ZnO sample with a characteristic peak at 540 cm⁻¹ indexed to the Zn—O bond. Thermal gravimetric analysis (TGA) examinations (Fig. 12d) revealed that bulk g-C₃N₄ lost its 97% of weight when heated up-to 700 °C and decomposed to produce unsteady compounds. g-C₃N₄/ZnO/AgCl nanocomposite loses its weight from about 420 °C and lasts up-to 660 °C, while loading of ZnO/AgCl onto g-C₃N₄ sheet lead to decreased thermal stability. The photodegradation process of RhB was carried out at 25 °C revealing 9% of RhB was photolyzed in 420 min under natural light illumination. 40% g-C₃N₄/ZnO/AgCl composite exhibited maximum photocatalytic efficiency than pure g-C₃N₄, g-C₃N₄/ZnO, and g-C₃N₄/AgCl composites. It was observed that about 39%, 46%, 58%, and 99% RhB was photodegraded in 270 min over g-C₃N₄, g-C₃N₄/ZnO, and g-C₃N₄/AgCl and g-C₃N₄/ZnO/AgCl (40%) samples, respectively. Approximately 6.7% (in dark) RhB molecules were adsorbed over as-synthesized composite after 420 min (Akhundi and Habibi-Yangjeh, 2015c). Lv and research group prepared Ag@AgCl/g-C₃N₄/TiO₂ ceramic films with superior photocatalytic abilities and a self-cleaning effect (Lv et al., 2018).

In another work, Miao and co-workers reported g-C₃N₄/AgBr photocatalyst loaded with CDs for the photocatalytic removal of organic contaminants. g-C₃N₄/CDs/AgBr photocatalyst revealed exceptional photocatalytic ability as comparable to pristine g-C₃N₄, AgBr, and g-C₃N₄/AgBr. Integration of CDs into g-C₃N₄/AgBr amended the photocatalytic efficacy due to light-absorbing and electron sinking nature of CDs amongst g-C₃N₄ and AgBr. TEM analysis clarified the size of CDs which was found to be 5 nm (Fig. 13a) and loading of AgBr NPs on the surface of CDs modified g-C₃N₄ NS was shown in Fig. 13b and c. XRD pattern explained the incorporation of CDs into g-C₃N₄ causing a shift of peaks with a slight distortion of the g-C₃N₄ lattice, as shown in Fig. 13d. Moreover, the FTIR examination described that no prominent peak for CDs was noticed in g-C₃N₄/CDs/AgBr nanocomposite (Fig. 13e). As reported earlier, Brunauer-Emmett-Teller (BET) specific surface area of g-C3N4/CDs (68.20 m²/g) was higher than that of pristine g-C₃N₄ (57.09 m²/g), demonstrating that addition of g-C₃N₄ lead to the boosted surface area. Consequently, it offers more reactive sites for adsorption and catalysis processes. Photoluminescence (PL) examinations (Fig. 13f) revealed that integration of CDs or AgBr onto g-C₃N₄ sheet inhibits the spatial charge separation of photo-induced charge carriers. Moreover, the photocatalytic ability of nanocomposite was enhanced, and insertion of CDs lessened the recombination of photo-induced EHP (Miao et al., 2017a).

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Fig. 13

g-C₃N₄/CDs/AgBr nanocomposite presenting (a) TEM analysis of CDs, (b and c) TEM studies exhibiting loading of AgBr NPs on the surface of CDs modified g-C₃N₄ NS, (d) XRD pattern revealing shift of peaks by anchoring CDs into g-C₃N₄ sheet, (e) FTIR spectra describing no prominent peak for CDs, and (f) PL explorations of as-synthesized nanocomposite; reproduced with permission from Elsevier (Licence no. 4750670232021) (Miao et al., 2017).

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Chen et al. explored Ag/AgBr/g-C₃N₄ photocatalyst by a facile technique with greater photocatalytic ability for elimination of MB. The optimum Ag/AgBr content with maximum photocatalytic efficiency was determined to be 40%. The efficacious photocatalytic ability could be allocated to extended visible light absorption and effective spatial charge separation due to the SPR effect of Ag⁰. Ag/AgBr particles own sphere-like morphology with a particle size of 500 nm (Fig. 14a), and the intercalation of AgBr particles into g-C₃N₄ layers was explained in SEM images. With increased AgBr content, bigger AgBr particles were obtained and assembled instantaneously. Ag/AgBr NPs ranging from 50 nm to 200 nm were well distributed onto g-C₃N₄ sheet, deprived of noticeable accumulation. Fig. 14b explained that several Ag crystals with a particle size of 10 nm appeared along with AgBr particle. X-ray photoelectron spectroscopy (XPS) studies determined the elemental composition of as-synthesized composite. XPS peaks confirm that Ag/AgBr/g-C₃N₄ powders comprised of Ag, Br, O, N, and C elements and with the element content of 2.81%, 1.8%, 3.77%, 42.68%, and 48.93%, respectively. The diffraction peaks at 366.8 eV and 372.8 eV were allocated to Ag⁺ in AgBr, and the peaks at 374.5 eV and 368.5 eV were indexed to Ag⁰, as illustrated in Fig. 14c. Ag/AgBr/g-C₃N₄ nanocomposite displayed significant improvement of light absorption at wavelengths 500–900 nm (Fig. 14d), and upon aggregating the Ag/AgBr content absorption intensities improved, which could be indexed to the SPR effect of Ag nanocrystals. The improved photocatalytic ability of Ag/AgBr/g-C₃N₄ composite may be due to the synergetic effect of Ag, AgBr, and g-C₃N₄, as shown in. At the same time, metallic Ag on AgBr surface plays a crucial role in e⁻ transformation (Cao et al., 2013). The band potentials of g-C₃N₄ and AgBr were 2.7 eV and 2.6 eV, respectively0, with more negative CB energies of g-C₃N₄ (−1.12 eV), than that of AgBr (−0.3 eV) and more positive VB energies of AgBr (+2.3 eV), than that of g-C₃N₄ (+1.58 eV). Therefore, photo-induced e⁻_CB easily gets transferred from g-C₃N₄ to CB of AgBr, thereby producing CO₂ by reacting with adsorbed O₂. Meanwhile, photo-induced h⁺_VB of AgBr relocates to the surface of g-C₃N₄. Furthermore, metallic Ag can provide photo-induced e⁻ accredited to SPR effect, and Fermi level of AgBr lies inferior to that of Ag, consequently e⁻ transferal from Ag to AgBr and lastly transferred to adsorbed O₂ to generate ^•O₂⁻ (Chen et al., 2015a). Another work was explored by Akhundi and co-workers reporting the construction of g-C₃N₄/AgBr/Fe₃O₄ nano-composite with enhanced photocatalytic performance (Akhundi and Habibi-Yangjeh, 2015b). Mousavi et al. investigated a VLT magnetically recoverable g-C₃N₄/Fe₃O₄/Ag₂WO₄/AgBr quaternary photocatalyst for photodegradation of contaminants (Mousavi and Habibi-Yangjeh, 2018b).

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Fig. 14

(a, b) SEM images of Ag/AgBr/g-C₃N₄ photocatalyst, (c) XPS examination revealing existence of Ag⁺ in AgBr, and (d) UV–Visible analysis of as-fabricated photocatalyst; reprinted with permission from Elsevier (Licence no. 4746461343390) (Chen et al., 2015).

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Liu et al. reported AgI@g-C₃N₄ core-shell structures for efficient degradation of MB dye under the illumination of visible light (Liu et al., 2015). AgI/g-C₃N₄ nanocomposite was prepared by Lei and group, while TEM images (Fig. 15a) explicated that AgI NPs were stacked on g-C₃N₄ surface with increasing AgI content and particle size of AgI was observed to be about 30 nm (Lei et al., 2016). Akhundi and his research group explored recyclable g-C₃N₄/Fe₃O₄/AgI photocatalyst with superior photocatalytic activity for removal of RhB via the reflux method at 96^°. The g-C₃N₄/Fe₃O₄/AgI (20%) composite illustrated more exceptional photocatalytic ability than bulk g-C₃N₄ and g-C₃N₄/Fe₃O₄ samples. XRD spectra for g-C₃N₄/Fe₃O₄/AgI nanocomposites revealed that diffraction peaks for AgI were associated to hexagonal crystal phase (Fig. 15b). SEM experimentations explained the loading of Fe₃O₄ and AgI particles on g-C₃N₄ sheet. Also, particle size was observed to be about 30 nm, as shown in Fig. 15c. Moreover, no peak for the Ag-I bond was observed, as illustrated by FTIR studies. Fig. 15d depicted two typical peaks at 430 and 620 cm⁻¹ attributed to the Fe-O bond. The mechanistic view for g-C₃N₄/Fe₃O₄/AgI nanocomposite explicated the excitation of e⁻ from VB to CB under visible light illumination. Attributable to confined band gaps of g-C₃N₄ and AgI, the e⁻_CB reacts with adsorbed O₂ to generate H₂O₂, and these H₂O₂ molecules produced ^•OH radicals by e⁻ capturing. Though VB of g-C₃N₄ does not produce ^•OH radicals owing to lower VB energies of g-C₃N₄ (+1.58 eV) than adsorbed —OH (E°_-OH/^•_OH = +2.38 eV) and H₂O (E°_H2O/^•_OH) = +2.72 eV). The O₂, ^•O₂⁻ and h⁺ produced were the reactive species (RSs) accountable for photodegradation of RhB, as shown in Fig. 15e (Akhundi and Habibi-Yangjeh, 2016c).

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Fig. 15

(a) TEM imaginings of AgI/g-C₃N₄ nanocomposite; reproduced with permission from Elsevier (Licence no. 4750100779496) (Lei et al., 2016), (b) XRD pattern of g-C₃N₄/Fe₃O₄/AgI nano-hybrid, (c) SEM analysis explained loading of Fe₃O₄ and AgI particles on g-C₃N₄ sheet, (d) FTIR studies confirming Fe-O bond, and (e) Photocatalytic degradation mechanism of as-prepared photocatalyst; reproduced with permission from Elsevier (Licence no. 4750110390251) (Akhundi and Habibi-Yangjeh, 2016).

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3.2 Z-scheme based heterojunctions

Z-scheme heterojunction possesses more negative CB potentials and more positive VB potentials, thereby lowering the bandgap of the semiconductor. In Z-scheme arrangements, under natural light illumination, e⁻_CB of SC-II merge with photo-generated h⁺_VB of SC-I, as illustrated in Fig. 16. This caused the formation of sufficiently strong oxidative h⁺_VB in SC-II and reductive e⁻_CB of SC-I. This induced interface electric field accelerates the separation of EHP. For example, Zhou and co-workers investigated a plasmonic Ag/AgCl/g-C₃N₄ Z-scheme nano photocatalyst by an in-situ oxidation process. Ag/AgCl/g-C₃N₄ with an Ag/AgCl amount of 2.7 at % attained a more exceptional photocatalytic ability for mineralization of MO. TEM study demonstrated the dispersal of Ag NPs on the flake-like structure of g-C₃N₄, with a particle size of 10–50 nm, as shown in Fig. 17a. Consistent with Fig. 17b g-C₃N₄ acted as template for dispersion of Ag/AgCl crystals due to which agglomeration reduced and interface among Ag/AgCl augmented. No covalent bond was noticed amongst Ag/AgCl and g-C₃N₄, and the deposition of Ag/AgCl on the g-C₃N₄ surface was demonstrated by FTIR analysis, Fig. 17c. As per photocurrent examinations, the transient photocurrent response of Ag/AgCl/g-C₃N₄ composite is greater than that of bare g-C₃N₄, which means higher separation efficacy of EHP could be achieved, as illustrated in Fig. 17d (Zhou et al., 2014). Li et al. Ag@AgCl QDs decorated g-C3N4 nano-plates viz. oil-in-water self-assembly process (Li et al., 2017). Bao et al. explored a plasmonic AgCl/Ag/g-C₃N₄ composite ascribed to Z-scheme based mechanism, presented in Fig. 17e. Therefore, under exposure to solar light, both Ag and g-C₃N₄ get excited, thus producing photo-induced EHP. The photo-illuminated e⁻ of Ag NPs travel to CB of AgCl to reduce O₂. In the meantime, photo-induced e⁻ of g-C₃N₄ go to Ag NPs to recombine with photo-generated h⁺ that were created by plasmonic absorption of Ag NPs. While the plasmon-induced h⁺_VB of g-C₃N₄ reduces the organic pollutant (Bao and Chen, 2016).

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Fig. 16

Diagrammatic representation of Z-scheme based heterojunction.

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Fig. 17

(a and b) TEM imageries of Ag/AgCl/g-C₃N₄ photocatalyst, (c) FTIR investigations of as-synthesized composite, (d) Photocurrent analysis depicting higher separation efficacy of EHP; reprinted with permission from Elsevier (Licence no. 4750660226117) (Zhou et al., 2014), and (e) Z-scheme photocatalytic mechanism of AgCl/Ag/g-C₃N₄ composite; reprinted under licence from CC BY 4.0 (Bao and Chen, 2016).

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A Z-scheme Ag@AgBr grafted graphitic carbon nitride (Ag@AgBr/g-C₃N₄) by photo reducing AgBr/g-C₃N₄ nanohybrids was fabricated by deposition–precipitation process. The photocatalytic proficiency of the prepared sample was explored for the photo-degradation of MO and RhB dye. Under solar light irradiation, e⁻_VB of both g-C₃N₄ and AgBr were excited to vacant CB producing EHP, respectively. As a result of more negative CB edges of AgBr (−0.3 V) comparable to fermi level of Ag (0.4 V), e⁻ in CB of AgBr can simply migrate into Ag metal through Schottky barrier. Since the VB edge of g-C₃N₄ (1.4 V) is more favourable than the fermi level of Ag, therefore, e⁻ from Ag travel to VB of g-C₃N₄. Subsequently, a Z-scheme mechanism was proposed, as shown in Fig. 18a. Accordingly, e⁻_CB of g-C₃N₄ reacts with O₂ forming O₂^−• radicals while h⁺_VB of AgBr travel onto the surface forming Br⁰ atoms. ^•O₂⁻ and Br⁰ were the RSs accountable for the photodegradation of MO (Yang et al., 2014). In another work, Feng et al. constructed a g-C₃N₄/AgBr photocatalyst and concluded that integration of AgBr hybrid on g-C₃N₄ NS results in the formation of heterostructures, as elucidated by electrochemical impedance spectroscopy (EIS) Nyquist plots. A reduced arc radius of the EIS Nyquist plot reflects greater charge transference befalling at the interface. Owing to a smaller arc radius of as-prepared samples, these revealed more efficient spatial separation of plasmon-induced charge carriers (Feng et al., 2015). Azimi et al. proposed a ternary g-C₃N₄/AgBr/ZnO nanocomposite for active degradation of MB dye (Azimi et al., 2018).

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Fig. 18

(a) Mechanistic view of Ag@AgBr/g-C₃N₄ nanocomposite; reproduced with permission from Elsevier (Licence no. 4751150026281) (Yang et al., 2014), (b) TEM studies explicating uneven dispersal of AgI NPs onto g-C₃N₄ surface, and (c) Z-scheme charge transfer mechanism in g-C₃N₄/Bi₂WO₆/AgI nanohybrid; reproduced with permission from Elsevier (Licence no. 4751171004447) (Xue et al., 2019).

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Xue et al. explored a ternary g-C₃N₄/Bi₂WO₆/AgI Z-scheme photocatalytic system with improved photocatalytic activity towards the exclusion of TC. TEM analysis (Fig. 18b) depicted the microspheres of Bi₂WO₆, and it was observed that AgI NPs were unevenly deposited on g-C₃N₄ surface, with the existence of an active link among g-C₃N₄, AgI as well as Bi₂WO₆. Fig. 18c illustrated the proposed mechanism for photodegradation of organic pollutants. Under visible light excitation, the photo-illuminated e⁻_CB of Bi₂WO₆ recombines with h⁺_VB of g-C₃N₄ and AgI, resulting in spatial charge separation of photo-induced EHP. The transference of photo-charge carriers leads to retaining of e⁻ at CB of g-C₃N₄ and AgI, while h⁺ remains accumulated at the VB of Bi₂WO₆, respectively. Owing to more negative CB energies of g-C₃N₄ (−0.913 eV) and AgI (−0.503 eV) than O₂/^•O₂⁻ (−0.33 eV vs. NHE), the photo-induced e⁻_CB of g-C₃N₄ and AgI could reduce O₂ to ^•O₂⁻. Moreover, photo-induced h⁺ of Bi₂WO₆ can activate H₂O molecules to produce ^•OH due to less negative VB potential of Bi₂WO₆ (3.147 eV) than that of H₂O/^•OH (+2.72 eV) (Xue et al., 2019). A dual Z-scheme g-C₃N₄/Ag₃PO₄/AgI photocatalyst with improved photocatalytic ability for removal of neonicotinoid pesticide, was explored (Tang et al., 2019b). In other work, Zhang et al. fabricated a ternary photocatalyst with greater photocatalytic activities for the elimination of NOF (Zhang et al., 2019).

4.1 Reduction of carbon dioxide (CO₂) to useful hydrocarbon fuels

A novel Z-scheme AgCl@g-C₃N₄ nanocomposite was fabricated via deposition-precipitation routes, thereby, loading different ratios of AgCl on g-C₃N₄ surface. The photocatalytic ability of as-prepared sample was investigated under visible light eradication for CO₂ reduction in an aqueous medium (Fig. 19a). The VB and CB potentials for g-C₃N₄ were located at +1.58 eV and −1.12 eV, correspondingly, while for AgCl, these are located at 3.2 eV and −0.06 eV, respectively. As a result of a broad bandgap, AgCl (∼3.26 eV) could not absorb visible light. Attributable to the LSPR effect, Ag⁰ NPs absorb visible light, thereby producing EHP. Consequently, photo-generated e⁻ can migrate from Ag⁰ NPs to AgCl hybrid. Thus the accumulated particles in AgCl CB cannot photo-reduce CO2 to methane (CH₄), acetic acid (CH₃COOH), and formic acid (HCOOH). Since, CB potentials of AgCl (−0.06 eV vs. NHE) is more favourable than the typical redox potential of CO2 to CH₄ (−0.24 V Vs. NHE), CH₃COOH (−0.31 V) and HCOOH (−0.58 V vs. NHE). The accumulated e⁻ on the surface of AgCl can directly associate with the h⁺_VB of g-C3N₄. The spatial charge separation of photo-illuminated EHP does not follow a conservative double-transferal approach. Therefore, a possible Z-scheme mechanism for the reduction process has been suggested. The surface e⁻ of AgCl directly associates with h⁺_VB of g-C₃N₄. Meanwhile, the photo-generated e⁻_CB of g-C3N4 (−1.12 eV) could react with adsorbed CO2 to reduce CH₄, CH₃COOH, and HCOOH. It can be inferred that the Z-scheme charge transference path in AgCl@g-C3N4 nanocomposite can effectually lower the recombination of photo-charge carriers (Murugesan et al., 2018b).

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Fig. 19

(a) Schematic representation of Z-scheme charge transferal mechanism in AgCl@g-C₃N₄ nanocomposite; reproduced with permission from Elsevier (Licence no. 4751201294275) (Murugesan et al., 2018a, 2018b), photocurrent density of photocatalyst in (b) Absence of light, Mott-Schottky slopes of (c) BiVO₄, (d) g-C₃N₄, and (e) Nyquist plot of as-fabricated photocatalyst; reprinted with permission from Elsevier (Licence no. 4752980086692) (Rather et al., 2018).

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Rather et al. explored g-C₃N₄/Ag/AgCl/BiVO₄ (CAB) composite for photo-reduction of CO₂ to CH₄ under alkali activation. The morphology of the CAB microstructure was reliant on anchoring of AgNO₃ and g-C₃N₄, as explained by SEM studies. Pristine BiVO₄ and g-C₃N₄ own sheet-like structure, while Ag/AgCl nanocrystals were formed by deposition of Ag⁰ and precipitation of AgCl onto BiVO₄. The XPS spectra revealed the existence of all probable elements in the g-C₃N₄/Ag/AgCl/BiVO₄ microstructure. The chemical shift near reduced binding energies was perceived after achieving the photocatalytic CO₂ reduction, and downshift was observed because of the integration of sodium (Na) after alkali treatment. The interfacial possessions of the nanocomposite were scrutinized by observing their photoelectrochemical cell (PEC) response employing linear sweep voltammetry (LSV) technique mutually in the existence and nonexistence of light. CAB-1 (5.03 mA cm⁻¹) and g-C₃N₄-BiVO₄ (4.1 mA cm⁻²) witnessed small currents in the absence of light, succeeded by g-C₃N₄ (0.97 mA cm⁻²), BiVO₄ (0.03 mA cm⁻²), and Ag/AgCl/BiVO₄ (0.03 mA cm⁻²) at a potential ranging between −0.6 V and 0.5 V, as depicted in Fig. 19b. After exposure to light (150 W) the photocurrent density improved instantaneously in CAB-1 (17.5 mA cm⁻²) proceeded by g-C₃N₄-BiVO₄ (12.67 mA cm⁻²), Ag/AgCl-BiVO4 (5.24 mA cm⁻²), g-C₃N₄ (4.25 mA cm⁻²) and BiVO₄ (0.91 mA cm⁻²). Also, the positive Mott-Schottky (MS) slopes (Fig. 19c and d) specified that both BiVO₄ and g-C₃N₄ were n-type semiconducting materials, as revealed by LSV measurements (Tachikawa et al., 2016) and retained the similar properties after the construction of flower-shaped microstructure. The PEC outcomes are in arrangement with EIS calculations. Nyquist studies (Fig. 19e) signified reduced charge transferal resistance (Rct) and segregation of photo-induced charge haulers in the as-prepared microstructure. g-C₃N₄/Ag/AgCl/BiVO₄ composite follows the Z-scheme mechanism for the photocatalytic alteration of CO₂ to CH₄, where Ag/AgCl act as an e⁻ sink. Both BiVO₄ and g-C₃N₄ were excited under visible light resulting in the generation of h⁺ and e⁻ in their particular E_VB and E_CB. Ag and AgCl were present in vicinity; the polarization effect of AgCl inclines to alter the charge density of Ag, followed by generation of various partial positive as well as negatively charged regions on its surface. Moreover, the Schottky barrier formed among BiVO₄ and Ag/AgCl leads to the smooth flow of e⁻_CB of BiVO₄ onto Ag/AgCl surface. The h⁺_VB of g-C₃N₄ transferred towards Ag/AgCl to eliminate the inward e⁻_CB of BiVO₄ and to counteract the polarization effect of AgCl. This mechanistic insight helped in the dissociation of h⁺ in E_VB of BiVO₄ and e⁻ in E_CB of g-C₃N₄ to accomplish the photocatalytic reaction. The water oxidation takes place at VB of BiVO₄ to generate active protons and these protons couples through e⁻ from E_CB of g-C₃N₄ to reduce CO₂ into CH₄ (Rather et al., 2018).

4.2 Hydrogen (H₂) evolution by OWS

Li et al. tailored g-C₃N₄-Ag/AgBr (CNAA) heterostructure via solvothermal technique. N₂ adsorption-desorption isotherms confirmed that g-C₃N₄ and CNAA own H₃ kind of hysteretic hoop, which specified that porous arrangements were initiated from an accumulation of lamellar assemblies. Also, BET surface areas of g-C₃N₄ and CNAA were found to be 69.71 m²/g and 17.16 m²/g, correspondingly. The reduced BET surface area was because of the presence of AgBr NPs, which may obstruct the pore structure of g-C₃N₄ NS for apt particle size, shown in Fig. 20a. EDX mappings were employed to explore the elemental configuration of CNAA surface and confirmed the existence of C, N, Ag, and Br elements. UV–Vis DRS spectra (Fig. 20b) demonstrated that AgBr was in-situ deposited on the surface of g-C₃N₄ to enhance the light response. The detailed mechanism (Fig. 20c) explicated the transference of e⁻_CB of g-C₃N₄ to more negative e⁻_CB of AgBr, followed by the migration of photo-excited e⁻ to Ag plasma. The hydrogen evolution rate (HER) (H₂O + e⁻ → H₂) for high-density e⁻_CB of AgBr, as well as Ag plasmas, can easily induce greater negative potential than hydrogen reduction potentials (φH₂/H⁺). Instantaneously, h⁺_VB of g-C₃N₄ and AgBr were straightaway apprehended and quenched by additional TEOA, thus obstructing the recombination of photo-carriers, thereby producing oxidative active radicals (Li et al., 2019). Li et al. explored construction g-C₃N₄/graphene oxide-Ag/AgBr Z-scheme composite for effective H₂ evolution (Li et al., 2019). Another work by Bai and the research group reported Ag/AgX (X = Cl, Br, I)/g-C₃N₄ nanocomposite for photocatalytic H₂ evolution (Bai et al., 2019).

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Fig. 20

(a) BET surface area of g-C₃N₄-Ag/AgBr nano-heterostructure, (b) UV–Visible spectra revealing deposition of AgBr onto g-C₃N₄ NS, and (c) Mechanistic insight of charge transfer and generation of reactive radical species in g-C₃N₄-Ag/AgBr nanohybrid; reproduced with permission from Elsevier (Licence no. 4752990867947) (Li et al., 2019).

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4.3 Microbial inactivation

Deng et al. synthesized g-C₃N₄-AgBr composite by anchoring AgBr NPs on the g-C₃N₄ surface for photocatalytic bacterial disinfection. The antibacterial activities for Gramnegative strain Escherichia coli (E. coli) and Gram-positive strain Staphylococcus aureus (S. aureus) were analyzed under illumination of solar light. Comprehensive deactivation of 3 × 106 CFU/mL feasible cell density was fulfilled in 60 min for E. coli and 150 min for S. aureus, correspondingly. During photocatalytic process, Ag⁺ released does not take part in disinfection activities. The photocatalytic microbial inactivation of g-C₃N₄-xAgBr (x = 0.5, 1, 2, 4) headed for E. coli was explored. g-C₃N₄-AgBr wholly deactivated 6.5 log E. coli in 60 min, whereas 1.2 log, 2.0 log, and 1.67 log reduction of feasible cell density were detected for g-C₃N₄-0.5 AgBr, g-C₃N₄-2 AgBr, and g-C₃N₄-4 AgBr within the elongated reaction time of 70 min, respectively. No considerable modification of viable cell density of Gram-positive S. aureus was perceived for the shady as well as blank control experimentations. The results suggested that the feasibility of S. aureus would be affected by either visible light emission or the direct contact with g-C₃N₄-AgBr NPs under dark surroundings. Minimal inactivation activities toward S. aureus were perceived for bulk g-C₃N₄ owing to its elevated rate of recombination. Comparable to E. coli, g-C₃N₄-AgBr photocatalyst unveiled exceptional germicidal effects to S. aureus. Deactivation kinetics of g-C₃N₄-AgBr for S. aureus was somewhat slower than that of E. coli (Deng et al., 2017).

Murugesan and research group investigated AgI@g-C3N4 photocatalyst exhibiting enhanced antimicrobial properties prepared by the deposition-precipitation approach. The photocatalytic performance of AgI@g-C₃N₄ nanocomposite for bacterial disinfection of E. Coli was studied employing the disc diffusion process (Murugesan et al., 2018a, 2018b). In another work, Raizada et al. explored highly efficient photocatalyst with superior visible light activity for pollutant exclusion and bacterial disinfection (Raizada et al., 2019d).