Recent advancements in the fabrication and photocatalytic applications of graphitic carbon nitride-tungsten oxide nanocomposites

Abstract

The present review focuses on the widely used graphitic carbon nitride (g-C₃N₄)-tungsten oxide (WO₃) nanocomposite in photocatalytic applications. These catalysts are widely employed due to their easy preparation, high physicochemical stability, nontoxicity, electron-rich properties, electronic band structure, chemical stability, low cost, earth-abundance, high surface area, and strong absorption capacity in the visible range. These sustainable properties make them predominantly attractive and unique from other photocatalysts. In addition, graphitic carbon nitride (g-C₃N₄) is synthesized from nitrogen-rich precursors; therefore, it is stable in strong acid solutions and has good thermal stability up to 600 °C. This review covers the historical background, crystalline phases, density-functional theory (DFT) study, synthesis method, 0-D, 1-D, 2-D, and 3-D materials, oxides/transition/nontransition metal-doped, characterization, and photocatalytic applications of WO₃/g-C₃N₄. Enhancing the catalytic performance strategies such as composite formation, element-doping, heterojunction construction, and nanostructure design are also summarized. Finally, the future perspectives and challenges for WO₃/g-C₃N₄ composite materials are discussed to motivate young researchers and scientists interested in developing environment-friendly and efficient catalysts.

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Introduction

In the last few years, with the rapid growth in population, textile, pharmaceutical, food, paper, agriculture, pulp, and cosmetic industries have grown more rapidly, and the use of synthetic chemicals to fulfill the population demands have also increased considerably.^1–3 Petroleum product consumption has also increased since all these industries directly or indirectly rely on fossil fuels to produce electricity.⁴ Moreover, marine oil spill pollution risk has also increased due to rapid maritime oil drilling. These petroleum hydrocarbons are toxic to marine biodiversity and indirectly affect human health.^3,5 Similarly, pharmaceuticals' active compound consumption is increasing daily, and many expired, unused, and residual drugs are discharged into the aquatic environment.^6–8 The situation has become severe, and drug residues have been detected in significant amounts in soil, wastewater, and even in drinking water globally.^9,10 Several projects worldwide are looking for strategies to limit this ecotoxicological risk of pharmaceutical products.^7,11

Dyes and pigments are aromatic, nondegradable, toxic, and widely used in pharmaceuticals, textile industries, food colorants, cosmetics products, plastics farms, photographic film, and paper.^12,13 During the finishing and dyeing operations, approximately 0.2 million tons of dyes are released into the environment each year from textile industries.^12–14 Dye removal from water bodies is significant for the environment since a minute quantity (<1 mg L⁻¹) can be highly toxic for aquatic life, decreasing the photosynthetic activity, hindering sunlight passage, affecting the symbiotic process, and reducing water oxygenation.¹⁵

Since the world is facing pollution problems, several methods (ion exchange, aerobic process, membrane separation, coagulation, sorption, photocatalysis, sonolysis, ozonation, electrocatalysis, electro-Fenton, photo-Fenton, etc.) have been used to purify wastewater.^16,17 However, these techniques are complicated, expensive, time-consuming, and require additional operational costs.^15,18,19 Among these, advanced oxidation processes (AOPs) are fast, most favorable, ecofriendly, and efficient for effectively removing pollutants.

AOPs are chemical methods used to remove various organic and inorganic contaminants from wastewater and are employed as alternatives to conventional treatment methods (filtration or biological treatment) to meet the required water quality standards.²⁰ AOPs involve generating and utilizing hydroxyl radicals (·OH) by radiation in the presence of a catalyst. UV irradiation, ozone, and H₂O₂ improve advanced oxidation processes for photodegradation and nonselectively react with most organic pollutants due to the in situ production of highly reactive hydroxyl radicals (·OH). The advantages of AOPs are as follows: (1) AOPs are highly effective in degrading and removing a wide range of pollutants (organic, inorganic, pesticides, pharmaceuticals, dyes, and certain types of metals). They break down these pollutants into simple compounds that are not easily treatable by other methods.²⁰ (2) AOPs have high oxidation potential, which allows them to oxidize and mineralize pollutants into nontoxic substances (carbon dioxide, water), promoting the complete degradation of contaminants and reducing their toxicity.²¹ (3) They generate highly reactive OH radicals, which attack and breakdown a broad spectrum of dyes, drugs, and other related pollutants, making them versatile and applicable to various contaminants. Besides, they can be applied to different water sources (municipal, industrial effluents, groundwater, and contaminated surface waters).²² (4) AOPs operate under mild conditions and achieve high treatment efficiencies in a relatively short time, which makes them suitable for practical applications.

AOPs include both homogeneous and heterogeneous photocatalysis. Fenton's reagent is a type of homogeneous photocatalyst that includes a mixture of Fe²⁺ salt (using FeSO₄ as an iron source) and hydrogen peroxide that produces hydroxyl radicals (·OH) under ultraviolet (UV) irradiation (<320 nm). Heterogeneous photocatalysts include semiconductor materials such as g-C₃N₄, CdS ZnO, TiO₂, SnO₂, WO_x, Fe₂O₃, ZnS, and BiOI.^18,23,24 In heterogeneous photocatalysis, the contaminants are in a different phase (e.g., dissolved in water or adsorbed on the surface) and come in contact with the photocatalyst, where the oxidation reactions occur. (1) The photocatalyst absorbs light energy and creates electron–hole pairs, generating reactive species capable of initiating oxidation reactions.²⁵ (2) Photocatalysis exhibits selectivity toward organic pollutants, targeting their chemical structures and functional groups, and efficiently degrading the pollutants.²⁶ (3) Photocatalysts are widely available and relatively inexpensive compared to other AOPs, and this accessibility makes them a practical option for water treatment.²⁷ (4) The optimization and customization of the process parameters are essential for photocatalysis.²⁸ Photocatalysis can occur at mild reaction conditions, making it energy efficient and suitable for water treatment. Factors such as the choice of catalyst, pH, temperature, light source, and contaminants' nature can influence the catalytic efficiency.

There are various photocatalysts used in AOPs. The most frequently studied materials are g-C₃N₄, TiO₂, ZnO, CdS, SnO₂, WO_x, Fe₂O₃, and ZnS. Due to the strong oxidation potential of these nanomaterials, they have a high activity of (·OH) radical generation. Photocatalysis based on AOP involves the generation of reactive species through the interaction of a photocatalyst with light energy and reactive species, such as hydroxyl radicals (OH·), superoxide radicals (O₂˙⁻), and singlet oxygen (¹O₂), possesses high oxidation potentials, and plays a crucial role in the degradation of the pollutant. The mechanism of photocatalysis using AOP is shown in Fig. 1. The photocatalyst absorbs photons from a light source, usually ultraviolet (UV) or visible light. Electrons in the valence band (VB) gain sufficient energy for transition to the conduction band, leaving behind positively charged holes in the valence band and generating electron–hole pairs (e⁻/h⁺). The photogenerated electrons and holes participate in the redox reactions with adsorbed species on the surface of the photocatalyst or in the surrounding environment, and holes have a strong oxidizing power and can directly react with organic compounds, while the electrons can reduce oxidants or other species. The photogenerated h⁺ reacts with water molecules adsorbed on the photocatalyst surface, producing OH radicals. These hydroxyl radicals are highly reactive and can initiate the degradation of organic pollutants. The organic compounds are broken down into smaller, less harmful molecules, such as water, carbon dioxide, and other byproducts. The electrons from the conduction band recombine with the holes from the valence band, restoring the catalyst's initial state and allowing the photocatalytic process. The drawback of photocatalysis is that the fast recombination of photogenerated electron/hole (e⁻/h⁺) reduces the catalytic efficiency of the catalyst and (·OH) radical generation. Various surface modifications and structural changes have improved the activities of TiO₂, ZnO, CdS, SnO₂, WO_x, Fe₂O₃, ZnS, and BiOI-based photocatalysts.^29–35

Fig. 1 The water treatment method and photocatalysis (schematic diagram of the photoredox mechanism of different water pollutants).

G-C₃N₄ has been a widely used nanocatalyst due to its easy preparation, nontoxicity, electron-rich properties, chemical stability, cost efficiency, and narrow bandgap (2.7 eV).^24,36 In addition, g-C₃N₄ can easily be synthesized from urea, thiourea, melamine, cyanamide, and dicyandiamide, and it is stable in strong acid solutions and up to 600 °C in air.^37,38 These sustainable properties of g-C₃N₄ make it predominantly elegant for solar energy utilization.^39,40 However, the limitation of pristine g-C₃N₄ is the fast recombination of photogenerated electron/hole (e⁻/h⁺) that reduces the photocatalytic efficiency and hydroxyl radical (·OH) generation.^38,41 The efficiency of pristine g-C₃N₄ photocatalysts can be improved by various surface modifications and structural changes.^23,24,40,41 Similarly, the yield of g-C₃N₄ is very low. Hence, using g-C₃N₄ as the support material is preferred over using it alone.^40,42

Metal oxide (MO_x) materials improve the photocatalytic and electronic device efficiency. The role of tungsten oxide (WO_x) is very prominent among many metal oxides.^43,44 WO_x is a semiconductor transition metal oxide with a bandgap of 2.4–2.8 eV, which can absorb the blue region of the solar spectrum; thus, it has attracted the interest of researchers due to its wide applications in several fields, such as semiconductors, solar energy devices, gas sensors, and photocatalysts.^45,46 WO₃/gC₃N₄, Ag₂WO₄@g-C₃N₄, BaWO₄/g-C₃N₄, WO₃/TiO₂/g-C₃N₄, g-C₃N₄/WO₃/ZnO, gC₃N₄/Bi₂WO₆, and graphene-WO₃/TiO₂ have been reported for various applications and perspective of MWO/g-C₃N₄ doped with various elements shown below in Fig. 2.^{40,43,45,47–51} Combining metal WO_x and g-C₃N₄ has resulted in higher photocatalytic activity than WO_x and pure g-C₃N₄.⁴³ Recently, there has been significant progress in developing g-C₃N₄ and WO₃ materials due to their unique features for various applications, including photocatalytic water splitting, pollutant degradation, energy conversion, and remarkable photochemical stability.

Fig. 2 Doping of MWO_x/g-C₃N₄ with various metals, nonmetals, and metalloids.

Tungsten trioxide has excellent photocatalytic activity in the visible light region. By integrating WO_x and g-C₃N₄, the composite can leverage the advantages of both materials, leading to enhanced photocatalytic efficiency and a broader light absorption range. Similarly, after composite formation, the bandgap reduces to 2.5 eV.

Numerous reviews exist on the synthesis, modification, and applications of g-C₃N₄ and WO₃ for photocatalysis and environmental application. For example, Jiang et al.⁵² wrote a review on doped g-C₃N₄ for photocatalysis, including metal doping, nonmetal doping, codoping, and heterojunction formation using g-C₃N₄.⁵³ Similarly, WO₃-based catalysts have been reported for the photocatalytic and photoelectrocatalytic removal of organic pollutants from water.⁵⁴ This article comprehensively reviews WO₃ and focuses on its properties, various synthesis and preparation methods, and strategies to enhance its performance. Similarly, a WO₃-based photocatalyst for environmental remediation has been reported,⁵⁵ presenting a comprehensive analysis of low-cost and environment-friendly methods for the synthesis of WO₃ and investigating the correlation between morphology control and key strategies to enhance the photocatalytic performance, including elemental doping, cocatalyst hybridization, and heterojunction formation. Previous reviews focused only on either WO₃ single materials or g-C₃N₄. In this review, we briefly discuss all the graphitic carbon nitride (g-C₃N₄)-tungsten oxide (WO₃)-based nanocomposites, their derivatives (0D, 1D, 2D, 3D, transition metal/nonmetal doping, and ternary composite) and their photocatalytic applications. We have specifically focused on the versatile properties and rational design to precisely adjust the optical, electronic, luminescent, structural morphology, Z-scheme, and other physical properties of graphitic carbon nitride (g-C₃N₄)-tungsten oxide (WO₃)-based nanocomposite. In photocatalysis, incorporating metals, transition metals, non metals, oxides, and their codoping has shown excellent potential for bandgap engineering and nanocomposite formation, leading to enhanced light absorption and redox band potentials tailored for specific photocatalytic applications. Moreover, this review also provides a summary of recent progress in the development of efficient and cost-effective graphitic carbon nitride (g-C₃N₄)-tungsten oxide (WO₃)-based nanocomposite in photocatalysis and the removal of contaminants from the environment.

Graphitic carbon nitride-tungsten oxide composites have shown promising properties and applications in photocatalysis. However, some disadvantages are associated with their fabrication and use. Fabricating g-C₃N₄/WO₃ nanocomposites is challenging, and achieving a uniform distribution and strong interaction between WO₃ and g-C₃N₄ requires precise control over the synthesis conditions (temperature, time, precursor ratios). Large-scale synthesis with consistent properties requires complex and costly fabrication processes. The stability of g-C₃N₄/WO₃ is another concern, especially under harsh photocatalytic conditions, and can lead to aggregation during prolonged use, resulting in a decline in the photocatalytic activity over time. This review highlights the composite's physical and chemical properties, shape, DFT structure, optical bandgap, and photocatalytic properties under UV/visible light. This review also focuses on the methods for composite formation to enhance the photocatalytic properties of MWO_x/g-C₃N₄ by reducing the bandgap. Our survey shows that no previous review has been written on fabricated graphitic carbon nitride-based tungstate nanostructured materials (MWO_x/g-C₃N₄) and their photocatalytic applications. Therefore, we hope that this review will provide the researcher with ideas to construct new materials and elaborate their multifunctional applications.

Applications of WO₃/g-C₃N₄

Nanotechnology is a growing field of research due to its properties linked with small particle sizes. Many properties of nanomaterials, such as electrical, optical, and mechanical, can be explained as a function of composition, size, and structural order. The synthesis of nanostructured materials is a complicated process since a small change in the parameters can affect the properties of the end products. However, the nanostructured materials synthesized by various methods under controlled conditions have a high surface-to-volume ratio and more atoms at the grain boundary. These nanostructured materials are used for many applications such as biomedical research, engineering applications, drug delivery, cancer therapy, environment-related application, sensing (biomolecules, pesticides, gas molecules, inorganic anions, and organic molecules), and cell imaging. Nanotechnology replaced old studies and technology to improve the lifestyle and environment for life, resolving many issues and miniaturizing the devices. Tungsten oxide-based graphitic carbon nitride nanostructured materials are semiconductors synthesized by numerous methods. Doping with other materials can adjust their electrical, magnetic, and optical properties. WO₃/g-C₃N₄-based doped materials can be designed in dimensions such as 0D, 1D, 2D, and 3D. WO₃/g-C₃N₄-based pure and doped materials are multifunctional due to their inherent properties.

Below is a short view of the overall applications of doped WO₃/g-C₃N₄ (Fig. 3) and photocatalytic applications against organic pollutants.

Fig. 3 Different applications of WO₃/g-C₃N₄-based materials.

WO₃/g-C₃N₄ materials purify air and water and efficiently degrade pollutants and harmful gases. This application can contribute to cleaner air and water resources.⁵⁶

These materials are used in energy storage devices such as batteries, supercapacitors, and electrochemical performance. Their unique properties and high surface area make them suitable for energy storage and sensitive and selective sensing applications.⁵⁷

These materials are also applied to produce chemicals through selective catalytic reactions. For example, the selective oxidation of alcohol into a carboxylic acid can enable environment-friendly synthesis routes throughout photocatalysis.⁵⁸

WO₃/g-C₃N₄ make them suitable for harnessing solar energy and are utilized in solar fuel production systems (photocatalytic water splitting) to generate clean energy. These materials can also produce solar fuels such as methane, methanol, or even hydrocarbon and are used as alternatives to traditional fossil fuels.⁵⁹

Structure of graphitic carbon nitride and its different phases

Historical background. Graphitic carbon nitride is an n-type semiconductor with a 2D conjugated polymer structure. g-C₃N₄ is an artificial oldest documented polymer reported in 1834, with the general formula of (C₃N₃H)_n, although typically with nonzero amounts of hydrogen. Based on the literature, carbon nitride (CN) was first studied by Berzelius and Liebig in 1834,⁶⁰ when he obtained the “melon” linear polymer of carbon nitride (CN). Then, Franklin 1922 prepared g-C₃N₄ from mercuric thiocyanate by thermal decomposition.⁶¹

There are seven allotropes of graphitic carbon nitride, including α-C₃N₄ (bandgap 5.5 eV), β-C₃N₄ (bandgap 4.85 eV), pseudocubic-C₃N₄ (bandgap 4.13 eV), cubic-C₃N₄ (bandgap 4.3 eV), g-o-triazine (bandgap 0.93 eV), g-h-triazine (bandgap 2.97 eV), and g-h-heptazine (bandgap 2.88 eV) and g-C₃N₄, predicted by Teter and Hemley in 1996, as shown in Fig. 2a.^62–66 All these are hard crystalline forms of carbon nitride except g-C₃N₄. Among these, beta carbon nitride (β-C₃N₄) is a super hard material (pure carbon nitride bulk modulus 4.27 ± 0.15 Mbar) predicted to be harder than diamond (bulk modulus 4.43 Mbar). It has the same crystal structure as β-Si₃N₄.^62,67 Corkill and Cohen calculated the bandgap of β-C₃N₄ to be 6.4 ± 0.5 eV in 1993.⁶⁸

In 1993, Chen et al. synthesized thin C₃N₄ films (by magnetron snorting) on Si medium and polycrystalline zirconium substrate in purely nitrogen medium and investigated the structure of C₃N₄ using electron microscopy and Raman spectroscopy.⁶⁷ Except for g-C₃N₄, all other phases of CN are super hard.⁷⁰ In 2009, Wang and their coworker first reported applying g-C₃N₄ in photocatalysis. These carbon materials (g-C₃N₄) gained importance in the scientific area when breakthrough research was published in 2009, describing the water splitting to hydrogen gas.⁷¹

Moreover, g-C₃N₄ can be obtained from cheap precursors such as nitrogen-rich compounds cyanamide, dicyandiamide, thiourea, guanidinium chloride, guanidine thiocyanate urea, and melamine, and its large-scale production is easy by low-cost synthesis.^66,67 It is easy to modify the structures and morphology of the g-C₃N₄; thus, it has become a hot topic for researchers.⁷²

Crystalline structure and DFT calculations of graphitic carbon nitride. Heptazine units (hexatomic ring) polymerize to form the graphitic phase (g-C₃N₄); its geometry matches with graphite in which a nitrogen atom replaces a carbon atom. Due to this, replacing graphite (crystalline) is 97% less dense than g-C₃N₄ (crystalline).⁶⁴ Heptazine units (hexatomic ring) are composed of triazine units, which have two chemical structures, s-triazine (C₃N₃) and tri-s-triazine (C₆N₇) units (Fig. 4(b)), belonging to R3m space group. At ambient conditions, tri-s-triazine-based activity favors a stable phase of g-C₃N₄.^62,64,66,73 In g-C₃N₄ carbon, hydrogen atoms are interconnected through sp² hybridization, attached by a sigma bond-forming hexagonal layer geometry connected by van der Waals forces (π–π interaction).⁶⁶ The experimental results revealed by the XRD patterns suggest that the unit cell of tri-s-triazine-based g-C₃N₄ is a layered structure having an interlayer distance of 0.319 nm (Fig. 4(c)). The lone pair electrons on N and self-linkage property of C in each layer, tri-s-triazine repeated N atoms engage units, and the calculated distance is 0.712 nm between two N pores. The six-fold cavity is formed by encircling 3 adjacent heptazin units. Based on the symmetry of the tri-s-triazine structure, 3 nonequivalent N atoms and 2 nonequivalent C atoms are present, indicated by (N1–N2–N3/C1–C2, respectively) with N1–C1, C1–N2, N2–C2, and N2–C3 bond lengths of 0.147 nm, 0.134 nm, 0.133 nm, and 0.139 nm, respectively.^69,74,75

Fig. 4 (a) Cubic-C₃N₄ structures down the [00l] axis, showing grey spheres (C atoms) and blue (N atoms), (b) s-triazine, heptazine structures of g-C₃N₄ (reproduced with permission from ref. 66 Copyright 2017, Springer Nature). (c) Geometric structures of g-C₃N₄ bulk and g-C₃N₄ monolayer, DFT structure (reproduced with permission from ref. 75 Copyright 2019, Wiley Online Library). (d) Calculated HOMO, (e) LUMO, (f) band structure and corresponding DOS of monolayer g-C₃N₄ (G, F, Q, and Z are high symmetry points in the reciprocal space) (reproduced with permission from ref. 76 Copyright 2017, Elsevier).

Moreover, the bulk structure of g-C₃N₄ is formed through multilayer stacking, known as ‘AB-stacking’. The interaction energy between adjacent layers measures 0.036 eV Å⁻², indicating the presence of van der Waals forces (weak π–π interactions) between the layers. The semiconductors' energy bandgap (E_g) was calculated using the GGA-PBE (generalized gradient approximation Perdew–Burke–Ernzerhof) function, predicting an E_g of 1.2 eV for monolayer g-C₃N₄ lower than the experimental value of 2.7 eV. The HSE06 functional provides a more accurate prediction, suggesting an E_g of 2.7 eV. In both cases, g-C₃N₄ exhibits an indirect bandgap, with the valence band maximum (VBM) and conduction band minimum (CBM) located at different K points in the Brillouin zone. The calculated band structure and corresponding DOS of monolayer g-C₃N₄ (G, F, Q, and Z are high symmetry points in the reciprocal space).⁷⁷ The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of monolayer g-C₃N₄ were calculated, as shown in Fig. 4(d–f). Following the electronic density of states (DOS), the HOMO covers all N2 atoms, while the LUMO is primarily found on C1 and C2 atoms with some N1 and N2 atoms. The tri-coordinated bridge N (N3) atoms do not contribute to the VB or CB edge, and in essence, N3 atoms do not produce excited electrons when exposed to light, and they do not allow electron migration through N3 atoms or transfer between heptazine (C₆N₇) units. Consequently, the photogenerated electron–hole pairs remain localized within each heptazine unit, resulting in inefficient separation and poor photocatalytic performance.

Crystalline structure and DFT calculation of tungsten oxide. Robert Oxland 1841 reported the first preparation method of sodium tungstate (Na₂WO₄) and tungsten trioxide (WO₃). Soon after, he was considered the founder of systematic tungsten chemistry after they were granted a patent for their work. The density of tungsten, equal to that of gold (19.3 g cm⁻³), is among the highest of all metals. Due to its high melting point, it is used as a refractory metal. Tungsten's applications include water splitting, gas sensing, solar cells, disinfection, and degradation of pollutants via photocatalysis and photoelectrocatalysis.^45,78 WO₃ is an ideal candidate out of the many visible active photocatalysts due to the ideal bandgap (2.4–2.8 eV), covering a large range of the solar spectrum. It has high oxidation power (valence band holes +3.1–3.2 V NHE), stability, and nontoxicity.⁷⁹ WO₃ exists in different crystalline phase at different temperatures (−180 and 900 °C), displaying several phase transitions such as monoclinic epsilon (ε-WO₃ phase <−43 °C), triclinic delta (δ WO₃ phase −43 to 17 °C), monoclinic gamma (γ-WO₃ phase 17 to 330 °C), orthorhombic beta (β-WO₃ phase 330–740 °C), and tetragonal alpha (α-WO₃ phase above 740 °C). In addition, other orthorhombic and hexagonal crystalline phases have been reported (Fig. 5(a)). Among these, monoclinic γ-WO₃ phase (space group P2₁/n, 17 to 330 °C) is the most stable, with a bandgap of 2.4–2.8 eV.^79,80 The band structure and the DOS of monoclinic WO₃ were calculated using DFT, Perdew–Wang (PW91), and hybrid functionals. All three functional types predict a direct bandgap for monoclinic WO₃ at room temperature, contradicting experimental results and previous revised Perdue Burke Ernzerhof (PBE) calculations. However, a generalized gradient approximation (GGA) study also reported a direct bandgap and argued that the discrepancy observed in the experimental data could be artificial. Under specific conditions, a direct bandgap may manifest experimental properties that can be mistakenly interpreted as originating from an indirect bandgap. All three hybrid functionals yield a significantly larger bandgap than the standard GGA, at the same time maintaining the same nature of the bandgap (direct or indirect). Among them, the HSE06 hybrid functional with a plane wave basis achieved the best agreement with the experimental value, yielding a bandgap of 2.80 eV. The B3LYP hybrid functional with a localized basis set follows closely with a bandgap of 3.13 eV. The PBE₀ functional overestimates the bandgap, yielding a value of 3.67 eV,⁸¹ as shown in Fig. 5(b–d). Fu et al.⁸² performed DFT computational calculations to investigate further interfacial charge transfer (for WO₃ and g-C₃N₄). The electrostatic potentials and work functions for WO₃ (001) and g-C₃N₄ (001) were calculated to be 6.23 and 4.18 eV, respectively. The difference in the work function diagram exhibits the charge transfer (at the interface of WO₃ and g-C₃N₄). WO₃ has a greater work function, requiring more energy to remove an electron from its surface than g-C₃N₄. As a result, there is a tendency for electrons to transfer from g-C₃N₄ to WO₃ until the Fermi level equilibrium is reached. This charge transfer occurs to equalize the Fermi levels of two materials. The charge transfer process forms an interfacial built-in electric field from the charge separation between the WO₃ and g-C₃N₄ interface. At the interface, the WO₃ surface is −ve while the surface of g-C₃N₄ is +ve, as shown in Fig. 5(e). The intensity of the built-in electric field is significantly enhanced due to the thin layered structures (g-C₃N₄, WO₃) as well as strong Coulomb force, which is beneficial for the transfer and separation of interfacial charge carriers than the bulk counterparts. When materials are in thin layers or have a high surface area, the interfacial contact between them increases, allowing for more efficient charge transfer and separation. In addition, the strong Coulomb force between the charged particles (electrons and holes) within the thin layered structures further enhances the built-in electric field.

Fig. 5 (a) The unit cells of WO3 at different temperatures show different phases (reproduced with permission from ref. 79 Copyright 2010, AIP Publishing). (b) PW91, (c) B3LYP, (d) PBE₀ (band structure and density of states (DOS) of room-temperature monoclinic WO₃ by different kinds of functionals) (reproduced with permission from ref. 81 Copyright 2011, American Chemical Society). (e) Electrostatic potentials WO₃ (001) surface and g-C₃N₄ (001) surface (reproduced with permission from ref. 82 Copyright 2018, Elsevier).

Fabrication strategies for graphitic carbon nitride–tungsten oxide

Various techniques, such as in situ growth, impregnation, thermal treatment, precipitation, ultrasonication, and calcination, have been explored to combine the two components effectively. The design principles of fabrication and strategies involve the meticulous control of synthesis parameters, interfacial engineering, morphology and composition control, and performance optimization through various strategies. By advancing these design principles and strategies, nanocomposites hold great promise for applications in photocatalysis, environmental remediation, and other fields where efficient and sustainable materials are required. Here, we describe a few methods of synthesis and fabrication of WO₃/g-C₃N₄ and their doped composites.

Fu and coworkers⁸² designed 2D–2D ultrathin WO₃/g-C₃N₄ from electrostatic-assisted ultrasonic exfoliation of WO₃ and two-step thermal etching of g-C₃N₄ (Fig. 6(a)). Fig. 6(b) indicates the zeta potentials at pH 4 of bulk WO₃ (−9.7 mV) and WO₃ nanosheets (−22.8 mV), indicating that the higher zeta potential value for WO₃ nanosheets relative to the bulk WO₃ and nanosheet of g-C₃N₄ show positive zeta potential (+10.3 mV) at the same pH. The opposite value of zeta potential results in strong electrostatic attraction, which is beneficial for charge transfer in WO₃ and g-C₃N₄. The work function of WO₃ is greater than the g-C₃N₄ value, inducing a charge transfer from g-C₃N₄ to WO₃. Three factors, namely, Coulomb interaction, band edges, and internal electric field, play a major role in stopping the recombination of electrons from the conduction band of WO₃ and holes (h⁺) from the valence band (VB) of g-C₃N₄. These factors also stop the recombination power of electrons from the conduction band of the g-C₃N₄ and holes (h⁺) from the valence band of WO₃. Praus et al.⁸³ first synthesized the WO₃/g-C₃N₄ using zeta potential values by dispersing both components (exfoliated g-C₃N₄ and WO₃) in an aqueous solution at pH = 2. In the pH range of 1.4–4.0, WO₃ shows negative zeta potentials, while exfoliated g-C₃N₄ exhibited positive zeta potential. X. Han et al.⁸⁴ developed the WO₃/g-C₃N₄ (2D) named CNW composite through the hydrothermal process for H₂ generation using the following precursors: melamine, CTAB, hexadecyl-trimethyl-ammonium-bromide, and WCl₆. 2D WO₃/g-C₃N₄ (10 wt% WO₃) caused 1853 (mmol h⁻¹ g⁻¹) H₂ generation, and the BET of CNW-10 was 86 m² g⁻¹. This value is 20 and 2.5 times greater than that of pristine WO₃ and g-C₃N₄ NSs, respectively. Similarly, the average pore diameter and volumes for g-C₃N₄ NSs (2.0 nm, 0.24 cm³ g⁻¹), CNW-10 (2.0 nm, 0.44 cm³ g⁻¹), and WO₃ (8.1 nm, 0.02 cm³ g⁻¹) were determined. Similarly, Zhuang et al.⁸⁵ synthesized the 2D–2D-WO₃/g-C₃N₄ 2D–2D heterostructure by an in situ preparation strategy from dicyandiamide, Na₂WO₄·2H₂O, and NH₄F for RhB degradation. Sample CNW-13 (13%, WO₃) completely degraded RhB within 40 min. The photocatalyst showed efficient photocatalytic properties and good stability after several cycles. However, synthesizing 2D–2D WO₃/g-C₃N₄ can be complex and challenging. Achieving the desired morphology, composition control, and interfacial properties in these hybrid structures requires precise synthesis techniques. This complexity can limit these materials' scalability and real-life implementation. Similarly, 2D–2D structures may have time-limited light absorption capabilities (particularly in certain range wavelengths). Thus, they might not efficiently utilize the whole solar spectrum of visible light, leading to lower photocatalytic efficiency.

Fig. 6 (a) Systematic synthesis scheme of 2D–2D heterojunction WO₃/g-C₃N₄ through electrostatic interaction (Coulomb interaction), (b) pH = 4 value of zeta potentials of bulk and nanoscale sheets of WO₃ and g-C₃N₄ (reproduced with permission from ref. 82 Copyright 2018, Elsevier).

The elegant WO₃/g-C₃N₄ composite hollow microspheres (CHMs), inorganic–organic 2D–2DWO₃/g-C₃N₄ nanosheets (NS), and WO₃ nanosheets NS/g-C₃N₄ nanosheets (NS) were synthesized by various researchers (Fig. 7(a–e)).⁸⁶ A strategy was developed to enhance the heterostructure system by modulating the electronic and surface properties. A thin 2D/2D WO₃/g-C₃N₄ heterojunction with carbon doping and the bridge were constructed with anionic polyacrylamide (APAM act as functions as the assistant template and carbon source). WO₃ and APAM were inserted into the g-C₃N₄ nanosheet, as shown in Fig. 7(a). APAM is a hydrophilic polymer used for its carbonization potential, and the amide group forms hydrogen bonds on the g-C₃N₄ surface. In the above study, APAM was utilized as a template to fabricate a carbon-decorated 2D/2D WO₃/g-C₃N₄ heterostructure based on Z-scheme photocatalysts and compared to individual WO₃, g-C₃N₄, and their binary composites. The synthesized composite demonstrated significantly enhanced the degradation activity for tetracycline under visible light irradiation, attributed to the reduced bandgap, fast charge transfer rate, and enhanced quantum efficiency.

Fig. 7 (a–e) Synthesis processes for the 2D–2DWO₃/g-C₃N₄ heterojunction through different methods ((a) reproduced with permission from ref. 86 Copyright 2020, Elsevier, (b) reproduced with permission from ref. 87 Copyright 2018, Elsevier, (c) reproduced with permission from ref. 88 Copyright 2018, Royal Society of Chemistry, (d) reproduced with permission from ref. 89 Copyright 2017, IOP Publishing Ltd (e) reproduced with permission from ref. 90 Copyright 2017, Elsevier).

Chen et al.⁸⁷ synthesized a novel hierarchical sheet-on-sheet composite, successfully fabricated by a simple calcination method using acid-treated SrWO₄/g-C₃N₄ as the precursors, as shown in Fig. 7(b). The synthesized WOCN composites demonstrated significantly enhanced photocatalytic activity in RhB degradation under simulated sunlight irradiation. The improved photocatalytic performance was attributed to forming a hierarchical heterostructure, which provided a larger specific surface area, improved visible-light absorption capability, reduced recombination of electron–hole pairs, and enhanced charge carrier separation efficiency. Li et al.⁸⁸ synthesized the WO₃ nanosheet arrays (NSAs) using a hydrothermal growth method, and g-C₃N₄ NSAs on an FTO substrate were prepared through a deposition–annealing process. Specifically, the WO₃ NSAs grown on FTO were immersed in g-C₃N₄ NS dispersions for 1 h and then dried using a nitrogen stream. After repeating this cycle three times, the resulting sample was annealed at 400 °C for 1 h to enhance the adhesion between g-C₃N₄ NS and WO₃ NSAs, facilitating the formation of WO₃/g-C₃N₄ heterojunction arrays (Fig. 7(c)).

Cheng et al.⁸⁹ synthesized the WO₄/g-C₃N₄ by the one-pot pyrolysis method (Fig. 7(d)). The resulting product was labeled as WO₄/g-C₃N₄. During the high-temperature synthesis, the interaction between WO₃ and g-C₃N₄ modified the composite textural structures, thus improving the photocatalytic activity.⁹⁰ The elegant Z-scheme hollow microspheres WO₃/g-C₃N₄ composite was synthesized by precisely controlling the in situ hydrolysis, followed by the polymerization process consecutively, and the synthesis mechanism is shown in Fig. 7(e). The hollow structure gained significant attention due to its unique structural, physical, and chemical properties. These structures possessed characteristics such as low density, high specific surface area, and efficient mass transfer and photon utilization, and were used for photocatalytic reduction purposes. Also, the specific hollow structure enabled prolonged light trapping and enhanced photon utilization efficiency.^91,92

Similarly, a few methods reported to synthesize transition metal-doped WO₄/g-C₃N₄ are given in Fig. 8(a–c) to fabricate codoped WO₄/g-C₃N₄, flowerlike MnWO₄ loading g-C₃N₄ surface, and FeWO₄/g-C₃N₄ nanosheet composites, respectively. Prabavathi et al.⁹³ successfully synthesized CoWO₄ nanoparticles assembled on g-C₃N₄ nanosheets by the hydrothermal method, followed by ultrasonication. The photocatalytic activity of fabricated CoWO₄/g-C₃N₄ was investigated by the degradation of norfloxacin under visible light, demonstrating higher photocatalytic activity than pristine materials (Fig. 8(a)). The excellent photocatalytic activity of these materials was due to a bivalent metal tungstate semiconductor with excellent thermal and chemical stability; combining this metal tungstate with g-C₃N₄ showed enhanced photocatalytic activity. The binary metal oxide cobalt tungstate (CoWO₄) demonstrated remarkable catalytic performance due to its feasible redox couple states (Co²⁺/Co³⁺) and high electrical conductivity ranging from 10⁻⁷ to 10⁻³ S cm⁻². Incorporating tungsten (W) atoms in cobalt oxide significantly improved its conductivity compared to pristine cobalt oxide materials.⁹⁴ Liu et al.⁹⁵ reported the direct Z-scheme MnWO₄/g-C₃N₄ composite fabricated by a facile hydrothermal method. Forming a direct Z-scheme photocatalytic system is a promising and effective approach to minimize the recombination of photogenerated electrons and holes while maintaining the redox ability of each component.⁹⁶ The energy level and band structure of MnWO₄ (narrow bandgap (2.7 eV)) were well-matched with those of g-C₃N₄, making it suitable for the formation of MnWO₄/g-C₃N₄ heterojunction with excellent photocatalytic activity under visible light.

Fig. 8 (a) Fabrication of codoped WO₄/g-C₃N₄-based photocatalytic material (reproduced with permission from ref. 93 Copyright 2019, Elsevier). Schematic representation (b) of the FeWO₄/g-C₃N₄ nanosheet composites preparation (reproduced with permission from ref. 97. Copyright 2018, Royal Society of Chemistry).

Dadigala et al.⁹⁷ synthesized a novel, stable, highly efficient, and visible light active hybrid photocatalytic system for FeWO₄/g-C₃N₄ nanosheets by the in situ self-assembly solvothermal approach. These photocatalysts exhibited superior photocatalytic activity for RhB and TC compared with the pure FeWO₄, g-C₃N₄ nanosheets (Fig. 8(b)). The FeWO₄/g-C₃N₄ composite showed excellent photocatalytic activity because the band level alignment between FeWO₄ and g-C₃N₄ allowed the formation of a direct Z-scheme photocatalytic system, enabling the efficient separation and transfer of photogenerated electron–hole pairs. Besides, FeWO₄ and g-C₃N₄ are visible light-driven photocatalysts, and their combination as the FeWO₄/g-C₃N₄ composite holds promise for achieving high photocatalytic activity.

The design of graphitic carbon nitride–tungsten oxide involves the careful consideration of several key factors. Firstly, the choice of synthesis method is crucial in achieving a well-controlled and homogeneous dispersion of tungsten species. Another important design principle is the tuning of the interface and the interfacial interactions. The interface facilitates efficient charge transfer and promotes synergistic effects between the two materials. Surface modification, functionalization, and heterojunction formation can enhance the interfacial contact and optimize the interfacial charge transfer kinetics. Bandgap engineering to expand the light absorption range, surface modification to enhance the catalytic activity, and cocatalyst deposition to improve charge separation and transfer. The morphology and composition composites are essential to design aspects and can be tailored by controlling the reaction conditions and precursor compositions to obtain desired structures, such as nanoparticles, nanosheets, or hierarchical architectures. Furthermore, efforts can be directed toward optimizing the composition ratios, improving the crystallinity, and exploring novel hybridization to unlock additional functionalities and applications.

The fabrication and photocatalytic applications of g-C₃N₄/WO₃ pose several scientific challenges. One of the challenges is the effective synthesis of well-defined nanocomposite structures with controlled morphology and composition of g-C₃N₄/WO₃. Achieving a uniform dispersion of WO₃ on g-C₃N₄ and maintaining their intimate contact is crucial for maximizing the synergistic effects between the two materials.^56,59 Strategies such as metal doping, nonmetal doping, codoping, and heterojunction formation have been explored to modulate the crystallographic, textural, and electronic properties of nanocomposites to enhance their photocatalytic efficiency. However, the cost of these metals is very high; thus, they are not used on a larger scale.^98,99 Developing efficient catalysts for the degradation of pollutants, H₂ evolution, and CO₂ using g-C₃N₄/WO₃ requires a comprehensive understanding of photocatalytic mechanisms and reaction pathways. Exploring the fundamental principles governing the photocatalytic mechanism and elucidating the roles of each component is essential for designing and optimizing a photocatalyst. The fabrication and photocatalytic applications of g-C₃N₄/WO₃ nanocomposites face synthesis, performance optimization, and mechanistic understanding challenges. Addressing these challenges will contribute to advancing g-C₃N₄/WO₃ nanocomposites for various environmental applications.

Graphitic carbon nitride-tungsten oxide (MWO_x/g-C₃N₄) composites. A photocatalyst should have a special characteristic requirement. The most prominent is narrow bandgap energy that helps establish coupling hybridization in visible light and a suitable band position to carry out the redox reaction. Similarly, the potential chemical difference offers the band-bending junction interface between the coupled semiconductors. Single semiconductors such as g-C₃N₄ or WO₃ cannot fulfill these requirements alone. The construction of the graphitic carbon nitride-based tungsten oxide (MWO_x/g-C₃N₄) design nanostructured materials can overcome these issues. This review deeply analyzed all the research directions of the MWO_x/g-C₃N₄ nanostructured materials prepared and their organic pollutants efficiency in the last 6 years.

WO₃/g-C₃N₄ composite. WO₃/g-C₃N₄ exhibits improved photocatalytic activity compared to pristine components. Introducing WO₃ into g-C₃N₄ induces changes in the electronic structure, leading to red-shifted optical absorptions and upward absorption tails in all the hybrids. The synergistic effect between WO₃ and g-C₃N₄ enhances visible light absorption and promotes efficient charge carrier separation, thus enhancing the photocatalytic performance. WO₃/g-C₃N₄ demonstrates an excellent response to visible light irradiation, enabling it to harness a broader range of solar energy for photocatalytic reactions. It also utilizes a larger portion of the solar spectrum and increases the potential for energy conversion and environmental remediation. WO₃ provides structural stability and prevents the aggregation of g-C₃N₄, resulting in improved long-term performance. Kadi et al.¹⁰⁰ synthesized g-C₃N₄ nanosheets with high surface area via a calcination approach using the high mesoporous MCM-41. It was observed that incorporating WO₃ nanoparticles on the g-C₃N₄ surface increases the charge separation, lowers the bandgap value, and results in higher efficiency.

Electron-rich graphitic nitride materials transfer partial electrons to WO₃ due to the lone pair electrons on nitrogen, thus increasing the electron density of WO₃. A stable g-C₃N₄/WO₃ composite structure formed by strong interactions leads to stable activity. For strong interaction between g-C₃N₄ and WO₃, the interfacial charge transfer reduces the recombination of the e⁻/h⁺ pair, leading to improved efficiency. When weak interactions are formed between two components, decreased photocatalytic activity is observed since the g-C₃N₄/WO₃ composite can split into a mixture of the g-C₃N₄ and WO₃.^101,102 Huang et al.¹⁰¹ first synthesized g-C₃N₄/WO₃ nanomaterial from precursor dicyandiamide and (NH₄)₅H₅[H₂(WO₄)₆]·H₂O for the degradation of 4-chlorophenol (4-CP) and methylene blue (MB), as shown in Fig. 9(e and f). The result indicates that under visible light irradiation, the activity of WO₃/g-C₃N₄ nanomaterial was 4.2 and 2.9 times higher than that of pristine WO₃ and g-C₃N₄, respectively. The TGA results in Fig. 9(a) reveal that the weight loss of WO₃/g-C₃N₄ occurs in the 510–670 °C temperature range. WO₃/g-C₃N₄ composites show stability before 510 °C (Fig. 9(b)). The SEM images of WO₃ indicate that it has obvious edges with particle sizes of approximately 50–200 nm. The TEM images in Fig. 9(e and f) show bright particles of about 100–200 nm diameters (red circles) attached to the g-C₃N₄ surface. WO₃/g-C₃N₄ showed 97% photocatalytic degradation efficiency for MB degradation within 2 h and 43% for 4-CP within 6 h under visible light irradiation (heterojunction formation due to interparticle electron transfer between component semiconductors). In the 4-CP structure, Cl and oxygen atom are strongly covalently bonded to the aryl or phenyl group, and there is no attacking side for the catalyst, while methylene blue has many atoms in the ring such as sulfur and nitrogen available for attack. These bonds easily break down in methylene blue. This issue can be resolved if the pH is optimized for this reaction.

Fig. 9 (a–f) (a) TGA of WO₃/g-C₃N₄, (b) SEM of WO₃, (c) TEM of WO₃/g-C₃N₄, (d) HRTEM of the WO₃/g-C₃N₄ (9.7%) composite, (e) degradation rate of MB by g-C₃N₄, WO₃, and WO₃/g-C₃N₄, and (f) degradation rate of 4-CP by g-C₃N₄ and WO₃/g-C₃N₄ (reproduced with permission from ref. 103 Copyright 2013, Royal Society of Chemistry).

Chen et al.¹⁰⁴ prepared the WO₃/g-C₃N₄ composite by heat treatment and ball milling procedures. They prepared different samples by varying the g-C₃N₄ and WO₃ to check the methylene blue (MB) and fuchsin (BF) degradation efficiency. The photocatalytic efficiencies of pure g- and g-C₃N₄ and WO₃ for MB (35.6%, 40.0%) and BF (30.9%, 42.4%) were determined. When the quantity of WO₃ is 5.0 wt% in g-C₃N₄, the photocatalyst displays the greatest photocatalytic activity, with the photocatalytic efficiencies for MB and BF of 87.9% and 75.6%, respectively. Yang et al.⁴³ manufactured the WO₃/g-C₃N₄ materials through three different methods such as ultrasonic dispersion, calcination, and hydrothermal method, and the prepared samples by these processes were named CNW(U), CNW(C), and CNW(H), respectively. They used the synthesized materials for Rhodamine B degradation. The rate constant for CNW(U), CNW(C), and CNW (H) was 0.034 min⁻¹, 0.022 min⁻¹, and 0.048 min⁻¹, respectively. CNW(H) showed a rate constant 4 and 2.6 times higher than that of WO₃ (0.012 min⁻¹) and g-C₃N₄ (0.018 min⁻¹), respectively. The BET surface area of WO₃, g-C₃N₄, CNW(U), CNW(C), and CNW (H) was 60.6, 11.2, 14.7, 14.3, and 15.1 m² g⁻¹, respectively. The degradation efficiency decreased in the order CNWH (95.3%) > CNWU (85.7%) > CNWC (77.2%). To examine the stability, efficiency, and morphological changes of g-C₃N₄/WO₃ with respect to temperature, Doan et al.¹⁰⁵ prepared the g-C₃N₄/WO₃ composite at different temperatures (500, 600, and 700 °C) and named them WM(500), WM(600), and WM(700), respectively. At 600 °C and 700 °C, the decomposition of g-C₃N₄ occurred, and N-doped WO₃ formation was realized. The maximum degradation was shown by WM(500) for 80% MB compared to other materials synthesized at different temperature ranges. The above experimental data show that the synthesis method affects the interaction between WO₃ and g-C₃N₄. For example, the hydrothermal method shows the maximum efficiency since it can disperse WO₃ on the g-C₃N₄ surface better than other methods. This strong interaction enhanced interfacial charge transfer, efficient separation of electron–hole pairs (photogenerated), and the highest photocatalytic activity. In addition, the stable composite structure (WO₃ and g-C₃N₄) formed by the strong interaction in CNW(H) contributed to its superior stability compared to other photocatalyst composites.

Similar photocatalytic degradation was performed by Gondal et al.⁵² They synthesized g-C₃N₄/WO₃ with dissimilar ratios and degraded Rhodamine B (RhB) and methyl tertiary-butyl (MTBE). Almost all WO₃/g-C₃N₄ composites exhibited higher photocatalytic efficiency than pure WO₃ or g-C₃N₄. The photocatalyst g-C₃N₄/WO_3−x−0.1 (combination ratio of WO₃ of 0.1) exhibited the highest photocatalytic activity (62%) of RhB under visible light irradiation in 90 min, while under the same conditions, pristine WO₃ and g-C₃N₄ only showed a 42% and 27% photocatalytic efficiency, respectively. Similarly, the highest photodegradation efficiency of WO₃/g-C₃N₄-0.2 (combination ratio of WO₃ 0.2) for MTBE (96.7%) was reported, while under the same conditions, the photodegradation proficiency attained on the pure phase of WO₃ and g-C₃N₄ was 65.9% and 45.3%, respectively. The photocatalytic activity gradually reduced when the combination ratio of WO₃ from 0.1 to 0.9 increased in g-C₃N₄. The reason could be the accumulation of WO₃ on pure g-C₃N₄. Increasing WO₃ on the graphitic carbon nitride decreased the separation efficiency of photoinduced electron–hole pairs and the photocatalytic activity effect.

As reported by Karimi and coworkers,⁴⁸ the WO₃/g-C₃N₄ (1 : 3) nanocomposite (W1G3) showed the best photocatalytic activity at pH 2.2 for the degradation of RB5 (99%) under solar light irradiation in 90 min. A greater amount of catalyst results in light scattering due to the reduction and aggregation of the irradiation field since higher concentration causes the solution to be more turbid, thus decreasing the light penetration ability and ultimately reducing the photocatalytic performance. Under proper light exposure, a maximum number of electrons were excited, and their efficiency increased. The acidic medium is most favorable for dye degradation instead of the basic medium. By changing the pH of the solution, the surface properties of the WO₃/g-C₃N₄ nanocomposite changed, thus reducing the number of positively charged species. In the basic solution, excess OH ions decreased the photodegradation of RhB5.

Zhang et al.¹⁰⁶ synthesized the same photocatalyst through direct calcination method from raw materials (ammonium tungstate, melamine) at 550 °C and applied it for the selective oxidation (photocatalytic) of 5-hydroxymethyl furfural (HMF) to 2,5-diformylfuran (DFF) in UV and visible light. Compared to visible light, UV light irradiation easily caused overoxidation and resulted in a lower DFF selectivity, and the highest DFF selectivity (85.6%) was achieved with HMF conversion (31.2%) in the presence of oxygen. They also studied the charge transfer/separation efficiency of photogenerated e⁻/h⁺ pairs of WO₃/g-C₃N₄, further confirmed by the EIS Nyquist circle. The Nyquist circle of WO₃ (4.7%)/g-C₃N₄ was smaller than that of the pristine. The larger diameters indicate greater interfacial resistance, resulting in a higher current loss. Due to its low conductivity, the g-C₃N₄ electrode exhibited a large ohmic serial resistance (R_s). The doping of g-C₃N₄ with WO₃ decreased the R_s value, demonstrating a faster interfacial charge transfer, reduced recombination, and enhanced photocatalytic efficiency. In addition, the photocatalytic oxidation of HMF to DFF on the g-C₃N₄ catalyst was higher in the presence of ACN and PhCF₃ as solvents. The choice of the reaction solvent commonly influences the efficiency of a chemical reaction. Each solvent possesses distinct characteristics such as steric hindrance effect, dielectric constant, polarity, and acid–base properties. This enhanced performance could be attributed to its lower polarity and superior oxygen dissipation. Conversely, polar solvents may lead to reduced conversion rates due to their potential competition with the reactant for the active catalytic site.

Yan et al.¹⁰⁷ reported the in situ coupling of WO₃ with g-C₃N₄ (WO₃/g-C₃N₄) using single-source precursor melamine and tungstate for MO and TC degradation under visible light in 120 min. MO degradation using the pristine catalyst g-C₃N₄ and WO₃ was 38% and 26%, respectively, while the coupling of WO₃ with g-C₃N₄ (W-CN-5) improved the best photocatalytic performance up to 93% for MO and 97% for TC. The mineralization (TOC) of MO (at 120 min was 52.9%) and TC (at 120 min was 56.6%) over the photocatalysts (W-CN-5) was much larger than that of pristine WO₃ (15.7%) and g-C₃N₄ (19.3%) for MO and 3 times more than that (18.2%) for g-C₃N₄ for TC, with the highest reaction rate (0.0213 min⁻¹), 5.6 times greater than that of pristine g-C₃N₄ (0.0038 min⁻¹) for MO using W-CN-5. These results favor the coupling of g-C₃N₄ and WO₃, which promote the photocatalytic activity and mineralization to degrade organic pollutants. MO degradation compared to TC using the same catalyst may be due to the different molecular structures of MO (organic dye) and TC (antibiotic compound) and can influence their degradation kinetics rates. The presence of specific functional groups, aromatic rings, or chemical bonds in TC may make it more susceptible to degradation than MO.

Chang F.¹⁰⁸ also synthesized various combinations of WO₃/g-C₃N₄ binary composites by a facile one-step calcination procedure and applied them for RhB and MB photocatalytic degradation. The UV DRS spectra of the pristine and composite are shown in Fig. 10(a). The band energies of WO₃/g-C₃N₄-90% and g-C₃N₄ were estimated to be 2.48 eV and 2.78 eV, respectively. As shown in Fig. 10(b), the slopes of pristine g-C₃N₄ and WO₃/g-C₃N₄-90% were calculated as 1 and 0.07, respectively, suggesting indirect and direct transitions. The photoluminescence (PL) spectra in Fig. 10(d) demonstrate the suppressed recombination rate of charge carriers in WO₃/g-C₃N₄ compared to the pristine form. Suppressed recombination rate indicates higher degradation efficiency. Significantly, all binary hybrids composite showed higher photocatalytic efficiencies over RhB and MB than bare WO₃ and g-C₃N₄, as shown in Fig. 10(e and f) under visible light (λ ≥ 420 nm). The photocatalytic performance of composites increased with the WO₃ contents by up to 15% but decreased beyond that. This behavior was attributed to the optical competition between WO₃ and g-C₃N₄ components or the potential disruption of heterojunction structures. WO₃/g-C₃N₄ (15%) can degrade about 91.3% of RhB after 150 min and exhibits the largest k value of about 0.0127 min⁻¹. Similarly, MB degradation under visible light irradiation was also checked by WO₃/g-C₃N₄, demonstrating significantly improved photocatalytic efficiency and relatively large k value; the trapping agent detection was also performed, as shown in Fig. 10(g and h). The first limitation of this synthesized photocatalyst is its 150 min degradation time. The second limitation is that this catalyst completely degrades RhB but does not completely degrade MB simultaneously. The possible reason because the degradation of dyes (Rhodamine B and Methylene Blue) depends on various factors (chemical structures, environmental conditions, degradation mechanisms). RhB has a more complex structure with multiple functional groups, making it more susceptible to degradation by environmental factors (reactive chemicals, light, heat). At the same time, MB has a relatively simpler structure (which may contribute to its higher stability). RhB is sensitive to light, especially in the presence of oxygen, and can undergo photobleaching (results in a loss of color). MB is also susceptible to photodegradation; however, its degradation rate may be slower than RhB.

Fig. 10 (a) UV-Vis DRS spectrum of bare WO₃, g-C₃N₄, and binary series, (b) color variation in samples in the inset, ln(αhν) vs. ln(hν − E_g) plot, (c) (αhν)² or (αhν)^1/2versus E_g plot, (d) photoluminescence spectra of pristine WO₃, g-C₃N₄, and composite WO₃/g-C₃N₄ (e) photocatalytic degradation of RhB, (f) apparent rate constants (k) for RhB, (g) photocatalytic degradation of MB, and (h) corresponding k values (reproduced with permission from ref. 108 Copyright 2018, Elsevier).

Researchers also used WO₃/g-C₃N₄ for the degradation of other drugs and dyes, desulfuration, and selective oxidation: Ma et al.¹⁰⁹ designed a WO₃/few-layer g-C₃N₄ photocatalyst through the in situ calcination method for the oxidative desulfurization activity of Model oil (DBT, 4,6-DMDBT, 4-MDBT, and 3-MBT) at different temperatures from 0 °C to 60 °C. The sulfur removal efficiency was 100% at 50 °C at different intervals. The order of desulfurization activity can be listed as DBT > 4-MDBT > 4,6-DMDBT. Priya et al.¹¹⁰ successfully applied the same catalyst, 3 wt% WO₃/g-C₃N₄, for acid orange 7 (AO7) degradation. The degradation efficiency was 100% using 3 wt% WO₃/g-C₃N₄. Reactive species were detected by introducing the radical scavengers into the reaction solution. Ammonium oxalate (AO) employs holes (h⁺) detection, and to remove benzoquinone (BQ) and hydroxyl radicals, isopropanol was used to reduce the level of oxygen radicals (O₂⁻). The result shows that the holes (h⁺) do not significantly affect photodegradation. However, IPA and AO exhibited a quick decrease in the degradation efficiency, revealing the role of superoxide (O₂⁻) and hydroxyl radicals in the degradation using 3 wt% WO₃/g-C₃N₄. Zhu Ma et al.⁵⁶ developed and employed WO₃/g-C₃N₄ using a facile hydrothermal method. (WCN) heterogeneous antibiotics degradation of sulfamethoxazole (SMX) increased to 91.7% when the optimized material WCN8 was applied at a concentration of 1.0 g L⁻¹. Moreover, the photocatalytic performance inhibited both low pH (pH at 3) and higher pH (pH at 7 and 9), while better photocatalytic activity was obtained without pH adjustment. Another study was successfully carried out by Zhao et al.¹¹¹ using WO₃/g-C₃N₄ to desulfurize model oil thiophene and dibenzothiophene using an ionic liquid with the assistance of oxidizing agent H₂O₂ and observed that the 36% WO₃/g-C₃N₄ composite showed improved performance and crystallinity. The removal rate of sulfur for model oil comprising DBT reached 91.2% with H₂O₂ at 60 °C for 180 min by the 36% WO₃/g-C₃N₄ composite. Navarro et al.¹¹² used WO₃/g-C₃N₄ for the degradation of Orange G (OG) dye and ciprofloxacin (CF). 5% WO₃/g-C₃N₄ degraded 98% (OG) and 100% (CF) as compared to bare g-C₃N₄ (40% degradation of OG, 60% degradation of CF) and bare WO₃ (14% degradation of OG, 19% degradation of CF).

The g-C₃N₄/WO₃ composite degrades when synthesized and combined in appropriate amounts since an optimum content of g-C₃N₄ into WO₃ can facilitate the partition of the electron-opening pair. On the other hand, the doping of g-C₃N₄ ends up higher than the limits, the space-charge region winds up thin, and the irradiation of light on WO₃ greatly surpasses the space-charge layer. This impact may induce prompt and less-demanding recombination of electron openings, bringing down photocatalytic degradation. Despite the improved photocatalytic activity of WO₃/g-C₃N₄, this photocatalyst may still have limitations regarding quantum efficiency, synthesis, and cost. During charge transfer processes or recombination of photoinduced electrons and holes, some energy loss may occur, reducing the overall efficiency of WO₃/g-C₃N₄. Synthesis of the WO₃/g-C₃N₄ composite may involve complex procedures and sometimes requires specific conditions, making its production challenging and time-consuming. This type of complexity can limit the scalability and widespread application of the WO₃/g-C₃N₄ composite. The cost of WO₃/g-C₃N₄ and raw materials used for this composite can be relatively high, and this cost factor can hinder its large-scale commercial applications.

Generalized mechanism for photoexcited e⁻/h⁺ separation in WO₃/g-C₃N₄. Two types of mechanisms are described in the literature for the photoexcited e⁻/h⁺ separation of WO₃/g-C₃N₄; (a) Z scheme and (b) S scheme heterojunctions. For the first time, Chen et al. studied the separation of photogenerated e⁻/h⁺ pair for WO₃/g-C₃N₄ photocatalysts. Fig. 11(a and b) shows the bandgap mechanism for WO₃ and g-C₃N₄ and the separation of the photoexcited e⁻/h⁺ process in two different ways. Charge carriers such as holes of WO₃ migrate from the valence band (VB) to the valence band of g-C₃N₄. Similarly, electrons from the conduction band (CB) of g-C₃N₄ migrate from the conduction band of WO₃; they cannot reduce O₂ into ·O₂⁻ radicals. Conversely, the holes in the VB of g-C₃N₄ cannot oxidize OH into ·OH radicals. These results show that the mechanism is unsuitable for active species. ·O₂ and ·OH. According to this mechanism, there is a quick combination among the photoexcited electrons of WO₃ from the conduction band and photoexcited holes of g-C₃N₄ from the valence band. The electrons from the g-C₃N₄ conduction band contain a high negative potential, reducing molecular oxygen into ·O₂⁻, while the holes from the valence band of WO₃ produce OH radicals. It is confirmed that the Z-scheme photocatalyst mechanism is suitable for generating ·O₂⁻ and ·OH radical reactive species. Aslam et al.¹¹³ also reported the degradation of rhodamine B (RhB) and described the formation of radicals for pollutant degradation. They reported that pure g-C₃N₄ takes 80 min while WO₃ takes 110 min to degrade RhB. Similarly, the WO₃/g-C₃N₄ composite takes 30 min for RhB degradation, and ·OH/·O₂⁻ free radicals play an important role in RhB degradation under visible light. Water molecules react with photogenerated holes h⁺ or ·O₂⁻ radicals at the surface of the photocatalyst and change to hydroxyl groups (OH⁻) group. These holes generated in light are utilized by OH⁻ on the photocatalyst surface and generate· OH radicals. O₂ reacts with photoexcited electrons and is converted to hydrogen peroxide, a strong photodegradation oxidant. ·O₂⁻ radicals can directly react with RhB or produce OH radicals by reacting with photoinduced electrons and hydrogen ions (H⁺). ·OH radicals are strong oxidants and finally degrade the RhB, confirming the Z-scheme photocatalyst mechanism. The overall reaction is shown in eqn (i), (ii), and (iii).¹¹³ The pristine structure does not show good efficiency, while the composite exhibits good efficiency. The main reason behind this is the heterostructure formation, facilitating an easy path to the composite, and promoting the separation of photoinduced charge carriers, ultimately enhancing the photocatalytic activity. Furthermore, the 1D composite can provide a high surface area-to-volume ratio with fewer defects, which is helpful for photodegradation.

h⁺ + H₂O → *OH + H⁺ (i)

O₂ + H⁺ + 2e⁻ → H₂O₂ (iii)

Fig. 11 (a and b) Generalized mechanism of the photoexcited e⁻/h⁺ separation process of WO₃/g-C₃N₄ (reproduced with permission from ref. 116 Copyright 2014, Elsevier). (c) Before contact with the work functions, g-C₃N₄ and WO₃; (d) after contact at the internal electric field/band edge bending at the interface of WO₃/g-C₃N₄. (e) Under light irradiation, the S-scheme charge transfer mechanism between WO₃ and g-C₃N₄ (reproduced with permission from ref. 82 Copyright 2018, Elsevier).

Katsumata et al.¹¹⁴ also proposed a photocatalytic mechanism for WO₃/g-C₃N₄ composite, as shown in Fig. 11(a and b). In pristine g-C₃N₄, the photogenerated electrons/holes tend to recombine, which results in low activity because only a small amount (fraction) of them participate in the photocatalytic degradation reaction. Contrary to the pristine from, when the composite of WO₃/g-C₃N₄ contains an optimized content of WO₃ (10 wt%), WO₃ covered a portion of the g-C₃N₄ surface, thus reducing electrons/holes recombination and forming a Z-scheme photocatalytic system.

Aslam et al.¹¹³ and Gondal et al.¹¹⁵ favored generic band–band transfer over Z-scheme mechanism. The bandgap value for the conduction and valence band of g-C₃N₄ is 1.57 eV and −1.12 eV, and the bandgap value of VB and CB for WO₃ is 3.18 eV and 0.41 eV, respectively. The potential of the g-C₃N₄ conduction band is lower than the WO₃ CB, which results from the migration of photoexcited electrons from the CB of WO₃ to the CB of g-C₃N₄. In addition, the band structure of the WO₃/g-C₃N₄ composite used in this study can be estimated using the following empirical equations.

E_VB = χ − E^e + 0.5E_g (iv)

E_CB = E_VB − E_g (v)

where E_VB and E_CB are the valence and conduction band potentials, χ represents the electronegativity (geometric mean of electronegativity) of the semiconductor, and E^e denotes the energy of free electrons on the hydrogen scale, which is approximately 4.5 eV vs. NHE. Similarly, the WO₃ valence band has a higher potential than the g-C₃N₄ valence band, which causes the transfer of photogenerated h⁺ from the valence band of WO₃ to g-C₃N₄. The transfer of photoinduced e⁻/h⁺ pair was carried out through the consecutive reduction of W⁶⁺ to W⁵⁺ by capturing the photoinduced e⁻ at the trapping sites of WO₃. At the same time, W⁵⁺ is re-oxidized to W⁶⁺ through O₂ reduction into ·O₂⁻ radicals, which result in the sufficient separation of the photogenerated e⁻/h⁺ pair and causes a slow-down in the recombination rate.

Nidheesh et al.⁸² also investigated the photocatalytic mechanism of WO₃/g-C₃N₄ by S-scheme heterojunction. Fig. 11 shows the mechanism (a) before contact with the work functions g-C₃N₄ and WO₃, (b) after contact at the interface of WO₃/g-C₃N₄, internal electric field, and band edge bending, and (c) under light irradiation S-scheme charge transfer mechanism between WO₃ and g-C₃N₄. The band edge of g-C₃N₄ bends upward (loss of electrons), and WO₃ bends the edge downward (accumulation of electrons). After light irradiation, electrons are excited from both photocatalysis (VB to CB). Due to the internal electric field, Coulomb interaction, and band edge (bending), the recombination of a few electrons from the conduction band of WO₃ and holes (h⁺) from the valence band (VB) of g-C₃N₄ occurs and also inhibits the recombination of few electrons from the conduction band of g-C₃N₄ and holes (h⁺) from the valence band of WO₃. This charge carrier transfer provides supreme redox capacity and helps to carry out the splitting of organic pollutants by providing strong force (S-scheme heterojunction).

Based on the above discussion, it can be inferred that the photocatalytic mechanism of the WO₃/g-C₃N₄ composite does not align with the conventional charge separation process. Specifically, a conventional Z-scheme photocatalyst is preferred for the photocatalytic degradation of organic pollutants. Other researchers have also documented the Z-scheme photocatalytic mechanism of WO₃/g-C₃N₄.^114,117Table 1 shows a summary of the tungsten-based graphitic carbon nitride composites for several different applications.

Table 1 Summary of WO₃/g-C₃N₄-based material for the photocatalytic degradation of organic pollutants

Precursor	Synthesis method	Parameters	Pollutants	Degradation efficiency	Time	Ref.
(NH4)5H5[H2(WO4)6]·H2O and dicyandiamide	Calcination, 450 °C	300 W Xe lamp	Methylene blue (MB), 4-chlorophenol (4-CP)	97% (MB), 43% (4-CP)	2 h for MB and 6 h for 4-CP	101
Melamine/Na2WO4·2H2O	Hydrothermal method	250 W Na lamp	Rhodamine B (RhB)	95.3%	2 h	43
Melamine powder, ammonium tungstate	Ball milling, heat treatment	500 W xenon lamp	Methylene blue (MB), fuchsin (BF)	87.9% (MB), 75.6% (BF)	—	104
Melamine, NaWO4·2H2O	Hydrothermal	500 W xenon lamp	Rhodamine B (RhB)	99%	30 min	113
Melamine, WCl6	Calcination	300 W xenon lamp	Methyl orange (MO)	60%	180 min	118
Melamine, WO3	Thermal decomposing	Fluorescent lamp	Methylene blue (MB)	80%	—	105
Melamine and H2WO3	Pyrolysis method	500 W xenon lamp	Rh B and MTBE	62% (RhB), 96.7% (MTBE)	90 min	115
Melamine, H3PW12O40	Calcination	60 °C	Dibenzothiophene (DBT)	91.2%	180 min	111
Melamine, urea, Na2WO4·2H2O	Ultrasonic method	Solar light/400 W xenon lamp	Reactive black 5 (RB5)	98%		48
[(C16H33)2N(CH3)2]2 W2O11, urea	In situ calcination	Thermal (heating)	Dibenzothiophene (DBT)	100%	30 min	109
Urea, H2WO3	Wet-impregnation	450 W xenon lamps	Acid orange 7 (AO7)	99%	75 min	110
Melamine (NH4)10W12O41·xH2O	Calcination method	300 W xenon lamp	5-Hydroxymethyl furfural, 2,5-diformylfuran (DFF)	85.6%	10 h	106

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Modified WO₃/g-C₃N₄ composites

0D, 1D, 2D, 3D modified WO₃/g-C₃N₄ composites. 1D, 2D, and 3D modified WO₃/g-C₃N₄ nanomaterials are largely used in different applications. Singh et al.¹¹⁹ synthesized the novel coral-like WO₃/g-C₃N₄ nanocomposites by a wet chemical process using WO₃ and g-C₃N₄ for the degradation of Methylene blue (MB) and Remazol brilliant red X-3BS (RbX). SEM analysis was done for the morphology of coral-like WO₃/g-C₃N₄, and Fig. 12(a) presents that the composite WO₃/g-C₃N₄ (1 : 1) structure is similar to coral and has crystallographic particle spacing (0.20 nm and 0.39 nm) and close interface (g-C₃N₄ and WO₃) that allows accelerating the separation of photoexcited carriers. The coral-like structure (irregular and rougher surface) of the composite enhanced its adsorption capacity for pollutant degradation (MB (98%) and RbX (92%)). Fig. 12(b) shows the UV DRS of WO₃/g-C₃N₄ nanocomposites, pure g-C₃N₄, WO₃, and WO₃/g-C₃N₄-1 : 1, showing the absorption range up to 500 nm, 504 nm, and 575 nm, respectively. The WO₃/g-C₃N₄-1 : 1 composite extended the absorption range, appreciably improving the visible light adsorption assisting effective degradation and showing that WO₃/g-C₃N₄-1 : 1 absorbs visible light more effectively and generates more electron–hole pairs, thus effectively enhancing its efficiency. The possible reason could be the combination ratio of 1 : 1, which may result in a synergistic effect arising from improved charge transfer and optimized band structure, where two materials work together more effectively to enhance the degradation efficiency. Deviating from this optimal stoichiometry ratio, such as in the cases of WO₃/g-C₃N₄ (1 : 3) and WO₃/g-C₃N₄ (3 : 1), could disrupt the balance of active sites or alter the electronic properties, leading to a decrease in the degradation efficiency. The WO₃/g-C₃N₄-(1 : 1) composite may have a higher surface area and more favorable porosity than other compositions, allowing for more efficient adsorption and reaction of the target pollutants, as shown in Fig. 12(c–f).

Fig. 12 (a) SEM image of WO₃/g-C₃N₄ (−1 : 1), (b) DRS of WO₃/g-C₃N₄ using various photocatalysts (c) C/C₀ MB and (d) C/C₀ RbX. (e) UV-Vis spectra at different time intervals under visible light for the degradation of MB and (f) RbX using WO₃/g-C₃N₄ (−1 : 1) (reproduced with permission from ref. 120 Copyright 2019, Elsevier).

Lou et al.¹²¹ prepared 1D WO_3−x nanowires on 2D g-C₃N₄-NS through a simple solvothermal process. WO_3−x nanowires and g-C₃N₄-NS facilitated the charge transfer in WO_3−x and g-C₃N₄ (beyond WO_3−x/g-C₃N₄-30). Free WO_3−x nanowires were created when W(CO)₆ concentration in the precursor solution was too high, aggregating the nanowires with no contact with g-C₃N₄ surfaces and lowering the photocatalytic activity. The highest efficiency photocatalyst was applied for the methyl orange (MO) degradation with 90% efficiency in 30 min. 1D/2D g-C₃N₄-NS does not completely degrade MO due to certain structural characteristics or functional groups that make it more resistant or may be due to the presence of electron-withdrawing groups in the molecular structure, which can contribute to its photostability. The intermediate byproducts had lower reactivity and required different reaction conditions for complete degradation, leading to incomplete degradation. Firstly, the abundant oxygen vacancies on WO_3−x nanowires contributed to a variation in free electrons and improved the conductivity of WO_3−x. Secondly, the WO_3−x nanowires exhibited a higher capacity for O₂ adsorption compared to g-C₃N₄ due to the presence of surface oxygen vacancies, thereby promoting charge separation on g-C₃N₄.

Pan and coworker¹²² prepared for the first time the novel 2D/2D WO₃/g-C₃N₄ thin photocatalyst through the S-scheme combining carbon (C) with anionic polyacrylamide (APAM) through bridge construction and assistant template, which was named as C-W/N and applied for tetracycline (TC) photodegradation. pH slightly affected the degradation efficiency (82.96%, 84.69%, 87.52%, and 86.33% at pH 3, 5, 7, and 9, respectively). The most efficient photocatalyst C-W/N exhibited 91% photodegradation compared to the other synthesized photocatalysts after 1 h irradiation. WO₃ exhibited the lowest TC degradation, up to 12.14%, due the fast recombination of electron–hole pairs. By adjusting APAM, the photocatalytic efficiency of WO₃ and g-C₃N₄ was promoted. As expected, when WO₃ was combined with g-C₃N₄, it showed better photocatalytic activities than the pristine one. Although constructing 2D–2D WO₃/g-C₃N₄ is an effective approach for photocatalytic degradation, its practical application is still limited due to its stability and durability. Fei Z. et al.¹²³ prepared the photocatalyst WO_3−x/2D g-C₃N₄ heterostructure by a single-step calcination process and studied its photocatalytic activity against the degradation and removal of tetracycline (TC), Rhodamine B (RhB), and S. aureus under an LED light. Doping with nonmetal oxygen atoms increased the surface area, reduced the bandgap, increased the light-harvesting power, and promoted the transport of charged ions.

Antoniadou and a coworker¹²⁴ prepared the novel g-C₃N₄/WO₃ thin films for methylene blue dye removal and hexavalent chromium (Cr⁶⁺) reduction. After 2 h under UV-A, almost 70% degradation of MB, 92.87% Cr⁶⁺ reduction was achieved by g-C₃N₄/WO₃, and 65% degradation of MB, Cr⁶⁺ reduction was 6.17% realized by WO₃, respectively. C. Xu et al.¹²⁵ synthesized leaf-like WO₃ nanoflakes decorated on g-C₃N₄ by the facile impregnation and annealing method for benzyl alcohol (BzOH) to benzaldehyde (BzH) conversion, suggesting that no significant conversion was obtained when the experiment was performed at a low temperature (30–50 °C); however, on increasing the temperature up to 80 °C, 98.6% BzOH conversion was observed. Xiao et al.¹²⁶ constructed WO₃/g-C₃N₄ composite hollow microspheres (CHMs) through the hydrothermal method for the degradation of ceftiofur sodium (CFS) and tetracycline hydrochloride (TC-HCl). The degradation efficiency was 82% for TC-HCl and 70% for CFS within 2 h.

0D, 1D, 2D, and 3D are important in photocatalytic applications.¹²⁷ 0D modifications can improve the stability of materials by creating additional active sites, resulting in enhanced catalytic activity. One-dimensional nanomaterials (1D) can enhance the electron transport within the material, leading to improved conductivity and performance, increasing the material's surface area, providing more contact sites for chemical reactions, and enhancing the catalytic activity and efficiency. In two-dimensional nanomaterials (2D), these two dimensions are outside the nanoscale,¹²⁸ and 2D modifications (such as graphene-based materials) have a high aspect ratio, which allows for improved mechanical strength and electrical conductivity. Three-dimensional nanomaterials (3D) include bulk powders, dispersions of nanoparticles, bundles of nanowires, and nanotubes, as well as multinano layers that do not have any dimension to the nanoscale and can provide structural support and integrity to the material, making it more robust and resistant to degradation. The 3D structure allows for the efficient mass transport of reactants and products, enabling the better utilization of active sites and improved overall performance. A few disadvantages of these materials include the following: the synthesis techniques may require more complex procedures to obtain well-defined 1D, 2D, and 3D structures, and the synthesis methods for the production of large-scale uniform 2D and 3D structures (control over the morphology and interconnected pore structure in bulk materials) have complex procedures and higher costs. Aggregation in 2D/3D sheets may reduce the active surface area, thus limiting the photocatalytic efficiency. The separation and recovery of 1D, 2D/3D materials from the reaction medium can be difficult, thus impacting the recyclability (Table 2).

Table 2 Summary of 0D, 1D, 2D, and 3D modified WO₃/g-C₃N₄ materials against photocatalytic applications with prospective parameters

Precursor	Synthesis method	Parameters	Pollutant/application	Degradation efficiency	Time	Trapping agent	Ref.
Urea, W(CO)6	Solvothermal method	300 W Xe lamp	Orange (MO) degradation	90%	35 min	·OH, ·O2^−	121
Urea, Na2WO3, APAM	Calcination	300 W xenon lamp	Tetracycline (TC)	91%	60 min	·O2^−, h^+	122
Dicyandiamide, Na2WO4·2H2O, NH4F	In situ strategy	70 W metal halide lamp	RhB	100%	40 min	·O2^−	85
Urea, Pluronic P123, WCl6	Calcination process	1000 W xenon lamp	Rhodamine B	75%	100 min	·OH, ·O2^−	129
Melamine, H4Na2O6W	Thermally exfoliated	450 W xenon lamp	Phenol	100%	35 min	—	83
Melamine tungsten powder	Calcination	UVA lamps 15 W/BLB	Methylene blue (MB)	70%	120 min	·O2^−, h^+	124
Urea, Na2WO4	Wet chemical process	65 W CFL lamp	MB, BR	98% (MB), 92% (BR)	160 min 210 min	OH, ·O2^−	119
Melamine, Na2WO4·2H2O	Impregnation and annealing	—	Benzyl alcohol (BA)	89%	7 h	—	125
Melamine, (NH4)10(H2W12O42)·4H2O	Thermal polymerization	300 W Xe lamp	MO, TC	93% (MO), 97% (TC)	120 min	·O2^−, h^+	107
Melamine, Na2WO4·2H2O, Sr(NO3)2	Calcination method	XG500 xenon long-arc lamp	Rhodamine B (RhB)	96%	100 min	OH, ·O2^−	130
Dicyandiamide, Na2WO4·2H2O	Hydrothermal method	300 W xenon lamp	Tetracycline hydrochloride	70%	120 min	·OH, h^+	126

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Transition metal-doped WO₃/g-C₃N₄ composites. Transition metal has broadly been applied for doping nanomaterials, bringing selectivity and efficiency to nanomaterials. Transition metals such as Ti, Pt, Co, Zn, Pd, Fe, Cu, W, and Zr have been applied to modify the nanomaterials' optical and electronic properties. Doping the transition metal into WO₃/g-C₃N₄ can endow the nanomaterials increased light absorption, low photocorrosion, efficient charge transfer, increased surface area, strong stopping power, chemical stability, and short decay line. The following properties that are mandatory for nanomaterials for photocatalytic applications are optical properties, higher transfer charge/separation, good interfacial contact, larger surface, higher crystallinity, good efficiency, and tandem photooxidation (Fig. 13).

Fig. 13 Properties of WO₃/g-C₃N₄ after doping with transition metals.

Generally, introducing transition metals can form new energy levels and extend the visible light response from UV to visible region, suppressing the recombination rate of electron–hole (e⁻/h⁺) charges. In this section, we discuss the transition metal (Ag, Cd, Co, Fe, Cu, Mn, Ni, Zn)-doped WO₄/g-C₃N₄ and their photocatalytic application, and also investigate the binding energies, bandgap reduction, porosity, changes in nanomaterials appearance, and degradation rate after doping transition metals.

Ag-doped WO₃/g-C₃N₄. Vignesh et al.⁴¹ first prepared the silver-supported Ag₂WO₄@g-C₃N₄ by the sonochemical impregnation method for methylene blue (MB) degradation, as shown in Fig. 14(a–d). The addition of g-C₃N₄ greatly impacts the stability, surface area, and degradation efficiency of Ag₂WO₄. Ag₂WO₄@g-C₃N₄ (40%) displays 100% MB dye degradation within 120 min. The Ag₂WO₄@g-C₃N₄ (40%) composite amplified the optical property, which gives the advantage for producing reactive oxygen species. With the addition of NaI and BQ scavengers, the photocatalytic activity is strongly suppressed. This shows that the holes (h⁺) and oxygen radicals (O₂˙⁻) are the principal reactive species for degradation. S_BET was determined for g-C₃N₄ (132.48 m² g⁻¹), Ag₂WO₄ (1.08 m² g⁻¹), Ag₂WO₄@g-C₃N₄-5% (1.68 m² g⁻¹), Ag₂WO₄@g-C₃N₄-10% (2.73 m² g⁻¹), Ag₂WO₄@g-C₃N₄-20% (4.07 m² g⁻¹), and Ag₂WO₄@g-C₃N₄-40% (6.73 m² g⁻¹). The S_BET of the composite is less that of pristine g-C₃N₄, which could affect the photocatalytic activity, as shown in Fig. 14(e–h). The introduction of Ag₂WO₄ nanoparticles onto the g-C₃N₄ surface changes the surface morphology and structure, which results in a decrease in S_BET compared to pure g-C₃N₄, which implies a reduction in the available surface area for photocatalytic reactions. However, it is important to note that other factors, such as the interfacial interactions and synergistic effects between Ag₂WO₄ and g-C₃N₄, can also influence the photocatalytic performance of the composite, even with a lower S_BET value. Similarly, the stability of Ag₂WO₄@g-C₃N₄ under prolonged exposure to light and chemical environments can be a concern, affecting its long-term performance and durability of the materials. However, the photocatalytic efficiency depends not only on the S_BET but also on other factors, such as interfacial interactions and synergistic effects, which can influence the photocatalytic performance even with a lower S_BET value.

Fig. 14 (a) C/C₀ MB (10 ppm); (b) schematic diagram of electron–hole transfer. (c) Influence of radical scavengers on MB. (d) Cycling runs. (e–h) The N₂ adsorption/desorption isotherms of the pristine Ag₂WO₄, g-C₃N₄, and Ag₂WO₄@g-C₃N₄ composite (reproduced with permission from ref. 41 Copyright 2015, Elsevier).

Chen et al.^131,132 reported 2D–2D AgWO₃/g-C₃N₄ by solvent evaporation, followed by the in situ calcination method for the degradation of dye rhodamine B (RhB) as well as the drug tetracycline (TC). The S_BET was measured for g-C₃N₄ (70.13 m² g⁻¹), WO₃ (11.95 m² g⁻¹), and AgWO₃/g-C₃N₄ (50.62 m² g⁻¹). The XPS survey spectrum for the AgWO₃/g-C₃N₄ composite gives the value of C/N/O/W/Ag. In the spectrum of C 1s, the peaks 284.7 eV and 287.9 eV are related to the coordination of N–C=N, and N 1s show three peaks at 398.4 eV, 399.7 eV, and 400.9 eV, indicating the C=N–C, N–(C)₃, and amino (NH₂) groups, respectively. For WO₃, the XPS peaks appear at 35.2 eV and 37.3 eV, and the O 1s peak at 529.6 eV indicates O²⁻ in WO₃. The Ag 3d peaks (368.1 eV, 374.2 eV) are attributed to Ag⁰. These values indicated that AgWO₃/g-C₃N₄ was successfully synthesized and applied for RhB and TC degradation, which were degraded to 97% and ≈90%, respectively. This data shows that the reported catalyst is most efficient for one dye (97% RhB degradation takes only 40 min), while it is not efficient for the drug (TC ≈ 90% degradation in 140 min). This catalyst is selective only for one pollutant and less efficient for others. This may be because the photocatalysts' efficiency often depends upon the adsorption of pollutants on the catalyst's surface for degradation, and the adsorption capacity depends on its surface area, surface chemistry, and pore structure. Suppose a specific photocatalyst has a high adsorption capacity for a specific pollutant. It can effectively concentrate the pollutant near active sites and enhance the degradation rate of the pollutants for this catalyst, while if the same catalyst has low adsorption capacity for another pollutant, the pollutant may not be efficiently captured by the photocatalyst and is less degraded.

Li et al.¹³³ synthesized the same photocatalyst with some modifications. He prepared ultrathin Ag₂WO₄/g-C₃N₄-NS by a simple deposition–precipitation method for the degradation of methyl orange and Rhodamine B. After irradiation of 20 min in RhB, self-degradation is almost negligible, and Ag₂WO₄/g-C₃N₄-NS, NS-20 (RhB 100%) show higher photocatalytic activities than Ag₂WO₄ and g-C₃N₄-NS, NS-10, NS-30. In NS-30 (30%), further increasing Ag₂WO₄ reduces the photocatalytic efficacy and shows that during the photocatalytic reaction, the mass ratio of Ag₂WO₄ and g-C₃N₄-NS is crucial to the synergistic effects. For comparison, P25, g-C₃N₄-NS, and bulk g-C₃N₄-20 composite were also investigated, and less than 20% RhB could be removed under the same conditions, which has much lower photocatalytic activities than the NS-20 composite. In the absence of photocatalysts, the direct photolysis of MO is almost ignored. After 50 min in visible light, MO (6%) can be removed by Ag₂WO₄, g-C₃N₄-B, and P25 but only 15–30% by B-20 and g-C₃N₄-NS. Under the same conditions, about 85% MO is degraded by NS-20. Under the same experimental conditions, the corresponding kinetic constants (k) for RhB and MO by NS-20 were calculated and displayed. The reaction rate constant for RhB degradation is 0.2180 min⁻¹, 18.9, 53.6, 10.9, and 26.5 times higher than that of Ag₂WO₄, g-C₃N₄-B, g-C₃N₄-NS, and B-20 samples, respectively. Similarly, the MO degradation sample is 0.0394 min⁻¹, 28.4, 30.6, 4.8, and 9.8 times higher than that of Ag₂WO₄, g-C₃N₄-B, g-C₃N₄-NS, and B-20 samples, respectively. The results showed that the novel Ag₂WO₄/g-C₃N₄-NS heterostructures are excellent photocatalytic and can have great photocatalytic activity.

Huang et al.¹³⁴ synthesized a novel catalyst Ag₂WO₄/P doped g-C₃N₄ (PCN) by single-step thermal polymerization for indomethacin (IDM) degradation. The experimental data shows that 30% (weight percentage) Ag₂WO₄ in the P-doped g-C₃N₄ by composites exhibits excellent photocatalyst efficiency. The BET surface of 30% Ag₂WO₄/PCN was determined to be 61.43 m² g⁻¹, and the photocatalytic system's total organic carbon (TOC) removal rate was 51.9%. The IDM degradation rate constants of 30% Ag₂WO₄/PCN were −0.233 per min, 4.59 times greater than that of 30% Ag₂WO₄/CN (−0.0508 per min). There are already a number of the Ag₂WO₄/CN composite used for the degradation of the drugs, but here the purpose of the doping of P into the graphitic carbon nitride may be due to the following reason: P doping in graphitic carbon nitride increases the negative surface charge and can also modify the surface chemistry and morphology of g-C₃N₄. It may enhance the adhesion of Ag⁺, which ultimately improves the stability of the phosphorus-doped Ag₂WO₄/CN composite, promotes efficient charge separation, and exposes more active sites. These effects contribute to the composite material's overall photocatalytic activity and performance in pollutant degradation. The overall summary is that the unique properties of Ag₂WO₄ (narrow bandgap, excellent photocatalytic capability) and complement characteristics of g-C₃N₄ offers a stable π-conjugated structure (promotes efficient charge separation), which enables the composite for the effective utilization of solar energy and enhancing the overall photocatalytic activity.

Cd-doped WO₃/g-C₃N₄. Tian et al.¹³⁵ first reported a novel CdWO₄/g-C₃N₄ composite prepared by calcination for Rhodamine B (RhB) degradation. The degradation efficiency of 1 : 10 CdWO₄/g-C₃N₄ nanomaterials for RhB is maximum, with the highest rate constant value (k = 0.164 h⁻¹) in the visible spectrum region. These degradation efficiency values are 1.6 and 54.6 times greater than that of pure g-C₃N₄ and CdWO₄, respectively. SEM analysis data of CdWO₄ shows that it consists of nanorods with 50–100 nm length and 20–50 nm width. The TEM analysis for pure CdWO₄ shows a clear fringe with 0.301 nm interval, which forms the (1 1 1) lattice plane (CdWO₄ monoclinic). Active species trapping experiments were carried out using EDTA and IPA, showing the existence of h⁺/·OH species, which plays a major role in the photocatalytic mechanism. The PL emissions of CdWO₄ and Cd/g-C₃N₄ composites were observed in the 480–500 nm visible range. The PL CdWO₄/g-C₃N₄ exhibited the lowest intensity, showing its lowest recombination. The possible reason behind this may be due to the reason that g-C₃N₄ and CdWO₄ have different bandgap energies, and the energy levels of CdWO₄ and g-C₃N₄ (after being combined in the composite) can align in a way that favors charge transfer and inhibits recombination. This alignment will facilitate the efficient migration of the photoexcited e⁻/h⁺ toward the respective energy levels (in CdWO₄, g-C₃N₄) and minimize the recombination of the photoexcited e⁻/h⁺, resulting in a reduced PL signal.

Maavia et al.¹³⁶ synthesized the same catalyst (g-C₃N₄/CdWO₄) through the hydrothermal method and used it for minocycline (MC) degradation with bandgap energies calculated for CdWO₄ (3.31 eV), g-C₃N₄ (2.67 eV), and g-C₃N₄/CdWO₄ (2.71 eV). The FTIR spectra of CdWO₄ provide information about three types of peaks. The two peaks at 522 cm⁻¹ and 594 cm⁻¹ are the bond stretching vibrations modes of Cd–O, and the peaks at 711 cm⁻¹ and 820 cm⁻¹ indicate W–O and Cd–O–W, respectively. The UV-visible spectra results show that g-C₃N₄ has an absorption at 460 nm and CdWO₄ at about 380 nm. But the g-C₃N₄/CdWO₄ composite shows an increase in the absorption peak intensity from 455 to 470 nm, and this g-C₃N₄/CdWO₄ composite shows better degradation than pristine materials, may be due to the reason that g-C₃N₄ has good absorption capacity in the visible range, while CdWO₄ absorbing UV light and by combining, g-C₃N₄/CdWO₄ can effectively utilize both UV and visible light; this broad spectrum range allows g-C₃N₄/CdWO₄ to degrade the pollutants efficiently. In short, combining the two materials (g-C₃N₄ and CdWO₄) creates a unique interface, promoting the surface reactions or photocatalytic activity. These prominent factors contribute to its superior photocatalytic performance and make it a promising material for various applications in photocatalysis.

Co-doped WO₃/g-C₃N₄. Xing et al.¹³⁷ reported for the first time Co-doped WO₃/g-C₃N₄ by the precipitation and calcination method at 400 °C for the deep sulfurization of fuel oil (dibenzothiophene (DBT), benzothiophene (BT), and thiophene (TH)), suggesting that the temperature for calcination is an important factor for catalytic performance. They prepared the desired photocatalyst at various temperatures such as 250 °C, 300 °C, 350 °C, and 400 °C. CoWO₃/g-C₃N₄ calcined at 400 °C has the highest desulfurization rate (92% DBT degradation) than the other prepared catalysts. Layered g-C₃N₄ can split into fragments (pieces) at 400 °C, resulting in a larger surface and higher crystallinity. The synthesis temperature is crucial in determining the photocatalyst's structural, physical, and chemical properties, especially in graphitic carbon nitride. At higher temperatures, such as 400 °C, the g-C₃N₄ material tends to have a more ordered and crystalline structure. Similarly, the temperature during the synthesis affects the degree of polymerization of g-C₃N₄, and higher temperatures promote higher degrees of polymerization, resulting in larger g-C₃N₄ domains and improved interfacial interactions between the layers. This can improve the charge carrier mobility, efficient charge separation, and reduced charge recombination during the photocatalytic processes. This can provide a favorable environment for charge transport and facilitate the efficient utilization of photoexcited electrons and holes, thus enhancing the overall photocatalytic performance. These factors contribute to enhanced photocatalytic efficiency. If we look at the same time, the reaction at lower temperatures, WO₃ and g-C₃N₄, do not fully react at lower calcination temperatures; thus, the catalytic performance is low. Increasing the temperature for the synthesis beyond the limit results in the crystal phase transition of the active constituent and decomposition of g-C₃N₄ into the carbon.

Prabavathi et al.¹³⁸ reported that CoWO₄/g-C₃N₄ nanomaterials were successfully fabricated by the hydrothermal method, followed by ultrasonication, and used for norfloxacin degradation. For the chemical bond information (Fig. 15(a)), the XPS analysis of a high-resolution spectrum of the Co 2p shows two peaks at 780.4 and 793 eV, which are related to Co 2p_3/2 and Co 2p_1/2, respectively, or it could be attributed to a +2 state and bandgap energy of CoWO₄ (2.2 eV), g-C₃N₄ (2.71 eV), and CoWO₄/g-C₃N₄ (1.85 eV), as shown in Fig. 15(b). The electron transport resistance (R_cr kΩ) values are CoWO₄/g-C₃N₄ (9.34), CoWO₄ (42.50), and g-C₃N₄ (47.11), which shows that CoWO₄/g-C₃N₄ has less charge transfer resistance and good interfacial contact than the other two (Fig. 14(c and d)). The norfloxacin degradation rate/first-order rate constant k at 80 min was observed for g-C₃N₄ nanosheets (∼51.2%, 0.0089 S⁻¹), CoWO₄ (∼57%), and CoWO₄/g-C₃N₄ (91%, 0.0283 S⁻¹, 0.0105 S⁻¹). The efficiency of CoWO₄/g-C₃N₄ is about 3.181, 2.691 times greater than that of g-C₃N₄ nanosheets and CoWO₄ nanorods, as shown in Fig. 14(e and f).

Fig. 15 (a) XPS spectrum of Co 2p, (b) the corresponding Tauc plot. (c) Electrical impedance, (d) transient photocurrent studies, (e) photocatalytic degradation of norfloxacin, (f) corresponding kinetics curves of CoWO₄/g-C₃N₄, CoWO₄, g-C₃N₄, and CoWO₄/g-C₃N₄ (reproduced with permission from ref. 93 Copyright 2019, Elsevier).

Sahoo et al.¹³⁹ designed CoWO₄-modified g-C₃N₄ nanocomposite by the reflux-calculation method for ciprofloxacin (CIP) degradation. UV-Vis diffuse reflectance analysis (UV-DRS) was conducted for the optical response of CoWO₄-modified g-C₃N₄ nanocomposite, and a characteristic absorption band was observed at 580 nm. The photoluminescence spectrum (PL) emission spectra of g-C₃N₄ and CoWO₄/g-C₃N₄ at 440 nm (strong emission band) because due to the recombination of the band–band charge carrier. Similarly, the degradation rate of 57% for CIP by CoWO₄ and 81% for CoWO₄/g-C₃N₄ in 60 min was observed. During the trapping experiment, superoxide and hydroxide radicals were the main factors for the degradation of CIP. The above summary and results are in favor of the CoWO₄/g-C₃N₄ composite, which can offer enhanced surface reactivity compared to the pristine catalyst because the combination of CoWO₄ and g-C₃N₄ creates a unique interface between them and promotes the surface reactions and catalytic activity. Besides these, the CoWO₄ nanoparticles on the surface of g-C₃N₄ increase the active sites available for photocatalysis, leading to improved photocatalytic efficiency and higher degradation rates of the pollutants.

Fe-doped WO₃/g-C₃N₄. According to our knowledge, the first synthesis of the g-C₃N₄/FeWO₄ nanorods was done by the hydrothermal method.¹⁴⁰ Dadigala et al.¹⁴¹ also successfully prepared FeWO₄/g-C₃N₄ through a solvothermal in situ self-assembly method for the catalytic degradation of tetracycline (TC) and Rhodamine B (RhB). SEM analysis of pure FeWO₄ exhibited spherical shapes (diameter 10–20 nm). HRTEM analysis also shows the lattice spacing (0.326 nm of g-C₃N₄) and lattice fringe (0.37 nm FeWO₄). The peaks of XPS at 723.9 eV and 710.4 eV are attributed to the binding energies of 2p_1/2 and 2p_3/2. Pure FeWO₄ showed absorption over a large UV-visible spectral region in UV-Vis DRS analysis. After the doping of g-C₃N₄ nanosheets with FeWO₄, the nanosheets showed good absorption in the visible region relative to pure g-C₃N₄ nanosheets, and with an increase in the FeWO₄ content, the optical properties of nanosheets was further enhanced. Two methods for synthesizing the g-C₃N₄/FeWO₄ are hydrothermal method and solvothermal in situ self-assembly. In the hydrothermal method, g-C₃N₄/FeWO₄ nanorods were prepared by subjecting the precursors to high temperature and pressure in an aqueous solution, and this method allows for the controlled growth of nanorods and facilitates the integration of g-C₃N₄ with FeWO₄ during the synthesis process, while the solvothermal in situ self-assembly method involves the simultaneous formation of FeWO₄ and g-C₃N₄ in a solvent at high temperature. The self-assembly process leads to the integration of the two components at the nanoscale, and the solvothermal in situ self-assembly method may have the advantage of a more intimate and distributed arrangement and also allows for enhanced interfacial interactions, leading to superior photocatalytic performance. However, controlling the composite particles' size, shape, and uniformity during the self-assembly process, the disadvantage could be challenging. Besides this, FeWO₄/g-C₃N₄ is selective for one pollutant, not for another. For example, it degrades 95% RhB, but tetracycline only degrades 83% due to the different nature of the dyes and drugs and their interaction with FeWO₄/g-C₃N₄ with a different condition.

Rashidizadeh et al.¹⁴² also synthesized a fabricated heterojunction g-C₃N₄ nanosheet/FeWO₄ nanoparticle for tandem photooxidation/Knoevenagel condensation reaction under visible light and O₂ as a green oxidant. Similarly, Choi et al.¹⁴³ reported the heterostructure FeWO₄/g-C₃N₄(FWO/CN) functionalized with the N-doped graphene quantum dots (NGQD) synthesized under the numerous conditions of sonication for the effectual removal of harmful vapors such as 2-butoxyethanol (2BE) and ethylbenzene (EB). C₃N₄/FeWO₄ photocatalyst was also synthesized¹⁴⁴via the facile sonochemical method with post-annealing treatment for the reduction of CO₂. Hexangular flowerlike FeWO₄ heterojunctions containing g-C₃N₄ photocatalyst were fabricated using the in situ growth method.¹⁴⁵ In Fig. 16(a and b), the X-ray diffraction (XRD) patterns of each (g-C₃N₄ and FeWO₄) of the composite exhibit the typical diffraction peaks. For pure FeWO₄, the peaks at 15.60 (010), 18.81 (100), 23.72 (011), 24.50 (110), 30.50 (111), 31.41 (020), 36.42 (002), 38.30 (200), 41.11(200), 44.22 (102), 45.50 (112), 48.60 (211), 51.81(022), 53.90 (130), 61.50 (221), 65 (113), 68.70 (132), and 71.50 (041) planes of monoclinic FeWO₄ (JCPDS card file no. 71-2390). g-C₃N₄ peaks at 12.8 and 27.5° were indexed to the (100) and (002) planes, respectively. The composite g-C₃N₄/FeWO₄ (CNU–FWx) contains both pristine peaks in the XRD patterns. The FTIR peaks at 834 cm⁻¹, 877 cm⁻¹, and 566 cm⁻¹ correspond to the characteristic peaks of the Fe–O–W and Fe–O bond. For thermostability (TGA), bulk g-C₃N₄ (CNU) exhibited rapid weight loss at 520 °C and at about 700 °C; its degradation was completed (Fig. 16(c)). Similarly, the rapid weight loss of g-C₃N₄/FeWO_4's began at 430 °C and reached 500 °C. The photocurrent response of bulk and optimized materials during five periodical-off cycles is shown in Fig. 16(d) and enhanced the photocurrent response generated for optimized materials (CNU–FW90). Furthermore, a small arc radius of CNU–FWx was found in the EIS Nyquist plots, which means that there is a lesser recombination of electron–hole pairs among the interface of g-C₃N₄ and FeWO₄, as shown in Fig. 16(e). In Fig. 16(f), FeWO₄ exhibited poor activity (0.79%) for RhB and FeWO₄ coated on g-C₃N₄. An enhancement in the photodegradation of RhB up to 97.2% was achieved in only 30 min. This is because of the arrangement of FeWO₄/g-C₃N₄ after composite formation. The close integration between the two materials allows for effective photoexcited (e⁻/h⁺) migration, reducing the recombination and enhancing photocatalytic activity, as shown in Fig. 16(h). A schematic illustration of the photocatalytic mechanism of g-C₃N₄/FeWO₄ is shown in Fig. 16(g). The g-C₃N₄/FeWO₄ heterojunction demonstrates a lower valence band (VB) level compared to the pristine by 0.41 V, and a significant difference confirms the enhanced photooxidation ability of the heterojunction since the lower VB level suggests that the heterojunction has a higher capability for accepting and transferring photoexcited holes, which are crucial for driving photooxidation reactions. This unique property of the g-C₃N₄/FeWO₄ heterojunction contributes to its stronger photooxidation ability, making it a promising material for various photocatalytic applications.

Fig. 16 (a) XRD patterns, (b) FTIR spectrum, (c) TGA profile, (d) photocurrent response, (e) EIS Nyquist plots, (f) photocatalytic activity toward the degradation of RhB, (g) bandgap structures, (h) schematic illustration of the photocatalytic mechanism of g-C₃N₄/FeWO₄ (reproduced with permission from ref. 145 Copyright 2018, American Chemical Society).

Cu-doped WO₃/g-C₃N₄. Huang et al.¹⁴⁶ synthesized a new Cu⁺ self-doped CuWO₄/g-C₃N₄ composites (CWO/CN) by in situ hydrothermal methods, and the composite with 30 wt% CuWO₄/g-C₃N₄ displayed good results for the activation of H₂O₂ to degrade the organic dyes products. The FTIR absorption peaks at 578 cm⁻¹, 700 cm⁻¹, and 800 cm⁻¹ display the stretching vibrations of WO₄ and Cu–O. The XPS peaks were obtained at 935.6 eV (Cu 2p_3/2), 955.4 eV (Cu 2p_1/2), and shake-up satellite peaks at 943.7 and 963.0 eV (Cu²⁺). Similarly, peaks 932.7 and 952.5 eV represent 30 CWO/CN, indicating the Cu⁺ oxidation state. The surface area of 30 CWO/CN is 65.1 m³ g⁻¹, as calculated by BET analysis. The degradation efficiency of RhB was 63.0% in 5 min, nearly 100% after 90 min, and only 7.3% in the absence of H₂O₂ in 90 min. Fenton-like catalytic results showed that H₂O₂ significantly improved the degradation rate of RhB and degraded 91.3% of it in 30 min, with the addition of 40 mmol L⁻¹ H₂O₂; at a H₂O₂ concentration of 80 mmol L⁻¹, the degradation rate was decreased. Besides this, methyl orange (81.1%), methylene blue (86.1%), methyl violet (70.9%), and malachite green (82.0%) were also utilized to assess the Fenton-like reaction activity and were degraded within 10 min. The Fenton-like catalytic results demonstrate that the addition of H₂O₂ significantly improves the degradation rate may be due to the reason that H₂O₂ acts as a precursor (generation of highly reactive ·OH radicals) and in the Fenton-like process, H₂O₂ reacts with Fe species to produce ·OH radicals. The combination of Fe and H₂O₂ creates a synergistic effect in the Fenton-like catalytic system, and H₂O₂ continuously supplies ·OH radicals, while Fe acts as a catalyst promoter for the generation and regeneration of ·OH radicals. These ·OH radicals exhibit strong oxidative ability and can effectively degrade organic pollutants because the ·OH radicals have a high oxidation potential and can oxidize a wide range of organic compounds. This synergistic effect enhances the overall degradation efficiency and accelerates the degradation rate of pollutants. Moreover, the catalyst worked at a very high pH, and a very high dose of catalyst was used only to degrade the few ppm solutions in this research work.

Zhou et al.¹⁴⁷ synthesized the same photocatalyst CuWO₄/g-C₃N₄ by hydrothermal reaction for Rhodamine B (RhB) degradation. The 10%CuWO₄/g-C₃N₄ composite displayed the highest degradation efficiency of 93% for RhB within 150 min, and the photodegradation efficiency remains 80% after four cycles, and the reaction mechanism of CuWO₄/g-C₃N₄ photocatalyst was fully discussed to according to the formation of the Z-scheme system. The Z-scheme formed may be because CuWO₄ and g-C₃N₄ have different energy band structures, and CuWO₄ has a narrower bandgap than g-C₃N₄, resulting in the different positions of their VB and CB energy levels. The energy levels of CuWO₄ and g-C₃N₄ align in such a way (after the combination of these two composites) that the CB of CuWO₄ is higher than the CB of g-C₃N₄, while the VB of g-C₃N₄ is higher than the VB of CuWO₄, and this energy level alignment forms the basis for efficient charge transfer and Z-scheme formation. Under the irradiation of light, g-C₃N₄ absorbs light and generates the e⁻/h⁺ pair. As discussed above, due to energy level alignment, the photoexcited electrons in the CB (g-C₃N₄) is readily transferred to the CB of CuWO₄. Here, they are trapped and utilized for redox reactions. Similarly, the h⁺ generated in the VB of g-C₃N₄ migrated to the VB of CuWO₄, creating a strong oxidation environment. This efficient charge separation and transfer between CuWO₄ and g-C₃N₄ form the Z-scheme system.

Mn-doped WO₃/g-C₃N₄. Manganese (Mn)-doped WO₃/g-C₃N₄ heterojunctions were employed for Rhodamine B degradation.¹⁴⁸ The SEM results showed the uniform distribution of flower-like MnWO₄ on the g-C₃N₄ surface. HRTEM exhibited the lattice spaces (0.369 nm) of the MnWO₄ crystallite. The bandgap value of g-C₃N₄ and MnWO₄ is 2.68 eV and 2.46 eV, respectively. The FTIR data of MnWO₄ shows that the band at 3400 cm⁻¹ and 1600 cm⁻¹ correspond to the v(O–H) and δ(O–H) surface hydroxyl group, respectively. Similarly, a strong band was observed at 455 cm⁻¹, 592 cm⁻¹, 735 cm⁻¹, and 885 cm⁻¹ (vibration modes MnWO₄). The XPS spectrum of the MnWO₄ indicates that peaks at 641.9 eV, 35.17 eV, and 531.9 eV are due to Mn 2p, W 4f, and O 1s. The RhB degradation is 44% using g-C₃N₄ and 10% using MnWO₄ (10%) after a 4 h reaction under visible light irradiation. The limitation of this work is that it takes too much time for degradation, and it only degrades 73% after 4 h of irradiation. The reason for poor catalytical activity might be because the hydrothermal method might not provide the ideal conditions (temperature, reaction time, precursor concentration, stirring rate) for the formation of a highly active and well-dispersed MnWO₄/g-C₃N₄ composite during the synthesis process, and the lack of these parameters can result in incomplete crystallization, limited interfacial contact, inadequate particle size, and can lower the catalytic activity. By optimizing these factors, the photocatalytic performance of MnWO₄/g-C₃N₄ can be improved.

The same material was synthesized with some modifications. 1D/2D MnWO₄ nanorods were anchored on g-C₃N₄ nanosheets via a one-pot hydrothermal approach for the photocatalytic degradation of ofloxacin (OFX 90.4%), and the calculated bandgap for this material was 2.58 eV.¹⁴⁹ It was experimentally observed that different combinations of the photocatalyst show different photocatalytic activity, for e.g., MnWO₄@g-C₃N₄ 1 : 0.25 (45%), MnWO₄@g-C₃N₄ 1 : 0.5 (50.2%), MnWO₄@g-C₃N₄ 1 : 1 (90.4%), MnWO₄@g-C₃N₄ 1 : 1.5 (56.8%), and MnWO₄@g-C₃N₄ 1 : 2 (45%), and when the g-C₃N₄ ratio was increased to 1, maximum degradation was observed due to sufficient interaction, leading to synergy toward OFX degradation. After that, a further increase in the g-C₃N₄ ratio results in lesser degradation efficiency due to the shielding effect among MnWO₄ nanorods and OFX molecules. As we know, increasing the ratio of one material to the other material beyond a limit can cause the aggregation of particles within the composite and can also cause a mismatch in electronic and energy levels between the materials. This aggregation reduces the available surface area for catalytic reactions, hinders efficient charge transfer, limits the active sites, impedes the utilization of photoexcited electrons/holes, and hinders the accessibility of the reactants to the catalyst. Consequently, the photocatalyst may experience higher recombination rates and reduce the overall efficiency. Inorganic anions such as SO₄²⁻, CO₃²⁻, and Cl⁻ can also affect the degradation ability of the photocatalysts because of the scavenging properties created by the formation's weak oxidative species. MnWO₄/g-C₃N₄ (MWG) hybrid catalysts were synthesized by a facile hydrothermal approach for the degradation efficiency of RhB and 4-CP dyes.¹⁵⁰ MWG3 (3 : 1) displayed the high photocatalytic activity of RhB (98%), high first-order kinetics of 0.0923 min⁻¹, long-term stability, and loss of only 2.8%. The J–V characteristics reveal that the MnWO₄/g-C₃N₄ photoanode shows high photoconversion efficiency (PCE%) at 7.15% and pure MnWO₄ at 2.34%.

When mixed in the proper amount and method, each composite material has optimal stoichiometry, desired synergistic effects, and photocatalytic activity. Deviating from this optimal ratio can disrupt the balance of interactions and hinder the desired functionalities. Similarly, optimizing various experimental parameters (such as catalyst loading, irradiation intensity, and reaction temperature) can enhance the photocatalytic activity.

Ni-doped WO₃/g-C₃N₄. A novel NiWO₄/g-C₃N₄ photocatalyst was first synthesized for norfloxacin (NRF) photodegradation under visible light.¹⁵¹ The designed photocatalyst was synthesized by the hydrothermal method, followed by sonication. Fig. 16(a and b) shows the SEM images of the NiWO₄@g-C₃N₄ nanocomposite, consisting of two phases. NiWO₄ nanorods were decorated on the surface of the g-C₃N₄ nanosheet. The HRTEM images show the clear lattice fringes (interlayer distance 0.28 and 0.31 nm) of NiWO₄ (111) and g-C₃N₄ (110), respectively (Fig. 17(c, d and f)). In the FT-IR spectrum, the strong peaks at 830 cm⁻¹, 615 cm⁻¹, 450 cm⁻¹, and 711 cm⁻¹ show the vibration mode of O–W, Ni–O, NiO, and W–O–W, respectively. After deconvolution, the XPS peaks of Ni 2p were observed at two different positions, which were assigned to Ni 2p_1/2 (873.4 eV) and Ni 2p_3/2 (855.6 eV). The Tauc's formula was used to calculate the direct bandgap energy of g-C₃N₄, NiWO₄, and NiWO₄@g-C₃N₄, which was found to be 2.74, 2.44 eV, and 2.26 eV, respectively. Fig. 17(e) shows that the mean diameter and pore size distribution of NiWO₄@g-C₃N₄ nanocomposite was 9.08 m² g⁻¹ and 1.8 nm, respectively. The calculated rate constant value for NiWO₄/g-C₃N₄, g-C₃N₄, and NiWO₄ was 0.0547, 0.0152, and 0.0119, respectively, which revealed pseudo-first-order kinetics. The photocatalytic activity of the catalyst was gradually increased up to 97% within 60 min as the catalyst dosage increased from 10 to 50 mg. But the further increase in the dosage decreases the light penetration ability, resulting in a lesser degradation ability because the higher concentration of the catalyst blocks the incoming light, and no sufficient e⁻/h⁺ produced resulted in decreased photocatalysts. NRF concentration is also an important factor for degradation efficiency. Using 10 to 30 mg L⁻¹ NRF, the degradation rate slowed, possibly due to a lower number of hydroxyl radicals at the photocatalyst surface. IPA, AO, and BQ were used as a scavenger, which retards the photocatalytic activity up to 56%, 65%, and 85%, respectively, which clarified that h⁺ and ·OH played the main role in the degradation relative to ·O²⁻. The BET for NiWO₄@g-C₃N₄ was estimated to be 9.08 m² g⁻¹, and BET is not a comparison for other pristine materials. All the points discussing the mechanism of photodegradation above are shown in Fig. 17(g–j).

Fig. 17 (a) SEM images, (b) TEM images, (c) FT-IR spectra, (d) XRD, (e) BET, (f) XRD of the NiWO₄@g-C₃N₄ nanocomposite, (g) C/C₀, (h) kinetic study, (i) trapping agent, and (j) mechanism of photocatalytic degradation of NRF by the NiWO₄@g-C₃N₄ nanocomposite (reproduced with permission from ref. 152 Copyright 2019, Elsevier).

g-C₃N₄ and NiWO₄ were fabricated to establish g-C₃N₄/NiWO₄ for the photocatalytic degradation of toluene.¹⁵³ The combination of 1C/1N exhibited the highest photocatalytic degradation activity: removal efficiency (95.3%) and mineralization degree (99.1%). The g-C₃N₄/NiWO₄ heterojunction absorbs visible light for electron excitation from the CB to VB, and due to the heterojunction, the photoexcited electrons in the CB of NiWO₄ combine with the photoexcited holes in the VB of the g-C₃N₄, which provide the maximum amounts of accessible electrons and holes for toluene degradation under visible light. This interface-driven synergy enhances the overall performance of the composite and contributes to its superior catalytic activity in various photocatalytic applications. NiWO₄@g-C₃N₄ may face challenges related to stability and durability because the nanocomposite's prolonged usage may undergo structural degradation, chemical leaching, or material loss, leading to reduced catalytic performance and hindering its practical applications.

Zn-doped WO₃/g-C₃N₄. Similarly, a novel heterostructured g-C₃N₄/ZnWO₄ (CNZW) nanocomposite was fabricated by hydrothermal methods for Rhodamine degradation.¹⁵⁴ Raman spectroscopy was also performed to understand the binding between g-C₃N₄ and ZnWO₄. The peaks at 1345 cm⁻¹ and 1581 cm⁻¹ correspond to pure g-C₃N₄, and the internal and external vibration modes of the ZnO₆ octahedron and WO₆ octahedron show the Raman modes (275, 356, 415, 702, and 915 cm⁻¹) for pristine ZnWO₄. Similarly, the A_1g symmetric stretching (∼915 cm⁻¹), E_g asymmetric stretching (∼702 cm⁻¹), and T_2g bending deformation (2A_g + B_g, ∼415, 356, and 275 cm⁻¹) are correlated to the monoclinic structure of ZnWO₄. This result conforms to the formation of the interaction of g-C₃N₄ and ZnWO₄. It is observed that the g-C₃N₄/ZnWO₄ nanocomposites exhibited lower PL emission intensities than that of pristine ZnWO₄ and g-C₃N₄, which is attributed to improved interfacial charge transfer due to superior charge separation, which in turn significantly enhances the photocatalytic activity. The photocatalytic ability of the photocatalysts at 80 min follows the order CNZW20 99% > CNZW10 76% > CNZW30 63% > gC₃N₄ 44% > ZnWO₄ (18%). After recycling, the degradation efficiency of the composite continuously decreases, which means that it is efficient only up to a limit. During the recycling of g-C₃N₄/ZnWO₄, nanocomposites may experience surface fouling (due to the deposition of organic/inorganic species) from the reaction mixture, and this fouling creates a barrier that inhibits the reactant adsorption and hinders the photocatalytic performance.

Recently, ZnWO₃/g-C₃N₄ (CZW) was reported for phenol degradation under UV irradiation.¹⁵⁵ Their characterization is checked by different techniques such as XRD data, which shows the diffraction angles at 15.30° (0 1 0), 23.78° (0 1 1), 24.25° (0 2 1), 30.56° (1 1 1), and 36.32° (0 0 2). These peaks could be attributed to the structure of the ZnWO₄. In the FT-IR spectra, two peaks at 3447 cm⁻¹ and 1620 cm⁻¹ indicate that the basic hydroxyl groups existed in ZnWO₄, and the surface of ZnWO₄ was hydroxylated. Similarly, in the XPS data, the binding energies at 1045.1 eV and 1021.7 eV are assigned to Zn²⁺ 2p_1/2 and Zn²⁺ 2p_3/2, respectively. The binding energy values of Zn 2p_3/2 and Zn 2p_1/2 were observed at 1021.7 and 1045.1 eV, respectively, which can be assigned to Zn²⁺ ions in ZnWO₄. The BET-specific surface areas were determined for g-C₃N₄ (4.698 m² g⁻¹), ZnWO₄ (4.698 m² g⁻¹), and 5-CZW (21.315 m² g⁻¹). The different combinations of photocatalysts show the different degradation efficiency of ZnWO₄ (16.8%), 1-CZW (21.8%), 3-CZW (25.4%), 5-CZW (30.4%), 7-CZW (23.2%), and g-C₃N₄ (3.9%).

In transition metal-doped WO₃/g-C₃N₄, the transition metal can improve the photocatalytic efficiency of the composites. Some transition metals can enhance the stability, ensure long-term performance, and extend the composite's lifespan. Moreover, transition metal dopants can modify the electronic structure and band alignment, and this modification facilitates efficient charge separation, reducing electron–hole recombination and enhancing the overall photocatalytic performance. Transition metal-doped WO₃/g-C₃N₄ has potential in various fields such as photocatalytic water splitting to produce hydrogen, solar cell, and CO₂ reduction. Furthermore, it can be employed in energy storage devices, sensors, and optoelectronic devices. However, some disadvantages of this composite are briefly described below: transition metal-doped WO₃/g-C₃N₄ can only be degraded by a limited number of dyes, drugs, and other pollutants because different pollutants possess varying chemical structures and properties, and if a composite is well-suited to the chemical nature of a pollutant, it can form strong bonds, promote reactive species generation, and facilitate degradation. However, pollutants with complex chemical structures may be more challenging for the composite to break down effectively. Moreover, the composite possesses specific active sites that promote photodegradation and activity, and the selectivity of these active sites determines how effectively the composite can interact with pollutants. The composite with high activity can facilitate the conversion of maximum pollutants into less harmful byproducts, but the composite with selectivity can specifically target certain pollutant species. Therefore, the different composite may be designed to target specific pollutants or related pollutant classes, and the degradation depends on a combination of composite properties and pollutant characteristics.

Table 3 presents the transition metal-doped WO₃/g-C₃N₄-based materials for the degradation and removal of various materials with their efficiency.

Table 3 Summary of transition metal-doped WO₃/g-C₃N₄ materials against photocatalytic applications with prospective parameters

Catalyst	Synthesis method	Parameters	Pollutant/application	Degradation efficiency	Time	Trapping agents	Ref.
Ag2WO4@g-C3N4	Sono-chemical impregnation	150 W xenon arc lamp	MB	100%	120 min	h^+, O2˙^−	41
AgWO3/g-C3N4 2D/2D heterostructure	Solvent evaporation and in situ calcination	XG500 xenon long-arc lamp	RhB	96% (RhB), ≈90% (TC)	40 min, 140 min	O2˙^−, ·OH	131
			TC
Ag2WO4/P doped g-C3N4	Single-step thermal polymerization	350 W xenon lamp	Indometacin orange (MO)	52%	15 min	O2˙^−	134
CdWO4/g-C3N4	Mixed-calcination method	500 W xenon lamp	Rhodamine B (RhB)	≈45	4 h	·OH, h^+	135
g-C3N4/CdWO4	Hydrothermal process	250 W xenon lamp	Minocycline (MC)	86%	5 h	·OH	136
CoWO4/g-C3N4	Hydrothermal method	Tungsten–halogen	Norfloxacin (NF)	91%	80 min	·OH	138
g-C3N4/FeWO4	Solvothermal method	Sunlight	RhB, TC	99% (RhB), 88% (TC)	90 min	O2˙^−, ·OH	141
CuWO4/g-C3N4 heterojunction	Hydrothermal method	500 W xenon lamps	Rhodamine B (RhB)	93%	150 min	O2˙^−, ·OH	147
MnWO3/g-C3N4 heterojunction	Facile hydrothermal method	250 W xenon lamp	Rhodamine B (RhB)	73%	4 h	O2˙^−, ·OH	148
1D/2D MnWO4 nanorods on g-C3N4	One-pot hydrothermal	150 mW cm^−2 tungsten lamp	Ofloxacin (OFX)	90%	70 min	h^+, O2˙^−	149
MnWO4/g-C3N4 hybrid	Hydrothermal method	Xenon lamp XQ-500 W	RhB, 4-CP	98% (RhB), 91% (4-CP)	100 min	·OH	150
NiWO4 nanorods anchored on g-C3N4	Hydrothermal method	Tungsten lamp (150 mW)	Norfloxacin (NRF)	97%	60 min	·OH, h^+	151
g-C3N4/NiWO4	Hydrothermal	1000 W Xe	Toluene	95%	180 min	·OH	153
g-C3N4/ZnWO4 films	Hydrothermal	UV lamp 12 W	Phenol	—	4 h	O2˙^−	155
g-C3N4/ZnWO4	In situ hydrothermal technique	100 W solar simulator	Rhodamine (RhB)	99%	180 min	·OH, h^+	154
CoWO4/g-C3N4	Reflux-calcinations method	Sunlight	Ciprofloxacin (CIP)	81%	60 min	O2˙^−, ·OH	139

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Ternary-doped modified materials of WO₃/g-C₃N₄ composites. Ternary-doped modified materials of WO₃/g-C₃N₄ enable the exploitation of synergistic effects between the different dopant elements and WO₃/g-C₃N₄ host materials. The presence of multiple dopants can in WO₃/g-C₃N₄ creates a cooperative interaction, and this synergistic effect can include improved charge transfer, enhanced light absorption, and efficient separation of electron–hole pairs, which enhances the overall photocatalytic performance, making them more attractive for various energy conversion and environmental remediation applications. Similarly, various natures of metal oxide nanoparticles and graphene oxide are widely used for many applications due to their good intrinsic properties at domestic and commercial levels. To enhance the properties of g-C₃N₄, its combination with metal oxides such as WO₃, ZnO, TiO₂, and graphene seems more suitable. These composites would lead to new hybrid nanostructured materials and have good photocatalytic properties against various natural constituents such as dyes, drugs, pharmaceutical waste, and pesticides. Photoelectrochemical and electrochemical mechanism-based semiconducting catalysis can oxidize organic pollutants or degrade inorganic pollutants in wastewater.¹⁵⁶

Currently, dual Z-scheme-based various ternary composites of g-C₃N₄ were synthesized, stimulating the complete transformation of photoexcited electrons and giving high charge separation properties.^157,158 Tang et al.¹⁵⁹ synthesized a ternary g-C₃N₄/WO₃/AgI photocatalytic system with a double Z-scheme for neonicotinoid degradation. The developed photocatalyst has an exclusive transmission path for photoinduced charge carriers, thus increasing the photocatalyst's activity. Guo et al.¹⁶⁰ prepared a dual Z-scheme Bi₂S₃/BiVO₄/MgIn₂S₄ nanocomposite heterojunction for the photocatalytic degradation of carbamazepine by the in situ growth method. Photoexcited charge carriers have a Z-scheme transformation path, which leads to enhanced photocatalytic activity. These ternary structures contain a better charge transformation path, thus improving the catalyst performance. Hongfei Y. et al.¹⁶¹ fabricated AgBr/β-Ag₂WO₄/g-C₃N₄ dual Z-scheme heterostructure by the precipitation method for tetracycline(TCH) and rhodamine B degradation. The synthesized ternary composite AgBr/β-Ag₂WO₄/g-C₃N₄ (CNAWAB7) degraded the rhodamine by almost 99.2% within just 25 min respective to CN, β-Ag₂WO₄, and composite of both CNAW9. The ternary composite also showed little better results for the degradation of tetracycline (60% within 25 min). The fabricated photocatalyst has better charge separation properties due to the alignment of a band, which mainly improves the photocatalytic performance of the proposed catalyst against TCH and rhodamine. However, catalytic efficiency decreases after every cycle. Mitra M. et al.¹⁶¹ synthesized a new ternary Z-scheme material by incorporating the nanoparticles of CoWO₄ and Fe₃O₄ over g-C₃N₄ by the simple refluxing calcination method. The ternary nanocomposite with 10% g-C₃N₄/Fe₃O₄/CoWO₄ degraded almost 100% Rhodamine B in 240 min. The proposed catalyst has high catalytic efficiency, but the time duration is very high to degrade Rhodamine B completely. There is a further need to improve to minimize the time factor.

Longbo J. et al.¹⁶² synthesized a new composite material (WO₃/g-C₃N₄/Bi₂O₃) by single-step cocalcination strategy utilizing simple melamine, bismuth(III) nitrate pentahydrate, and tungstic acid as the starting material. The photocatalytic capability of the catalyst against TC increased by up to 80% (Fig. 10(b)). The pore volume, BET surface area, and pore diameter of the catalyst improved by preparing the composite of g-C₃N₄ with WO₃ and Bi₂O₃. To effectively remove these drugs via photocatalysis, Nguyen T. et al.¹⁶³ introduced another ternary Z-scheme dual Cu-NiWO₄/g-C₃N₄ heterojunction for the photocatalytic breakdown of n-hexane under visible light. The designed catalyst gives better removal and degradation efficiency up to 5 cycles and has a better surface area (84.6 m² g⁻¹), which is a sign of the stability of the catalyst, but there is a need to test it for more than five cycles. Y. O. Ibrahim et al.¹⁶⁴ synthesized a ternary nanocomposite-based TiO₂/WO₃/g-C₃N₄ photocatalyst to degrade methylene blue as a pollutant under visible light. The photocatalyst TiO₂/WO₃/g-C₃N₄ with 15 wt% g-C₃N₄ displayed the maximum photodegradation action with 91.5% effectiveness. The stability and reusability of the designed catalyst are not good enough, but by seeing this work, the potential photocatalyst can be formed for pollutant degradation under visible light.

M. B. Tahir et al.⁵⁰ designed a photocatalyst to degrade aspirin and caffeine under visible light. They synthesized WO₃-TiO₂-g-C₃N₄ by a simple hydrothermal process. Tingting Y. et al.¹⁶⁵ synthesized a self-sustained catalytic fuel cell system based on anodic TiO₂/g-C₃N₄ heterojunction and cathodic WO₃/W for the oxidation of triclosan or rhodamine B and reduction of NO³⁻-N to N₂ instantaneously. When these two electrodes are combined, a microscopic current was produced without harvesting light that caused nitrate reduction into N₂ at the cathode. The electrons generated by the system activate the oxygen molecules to generate reactive oxygen species, which cause the oxidation of triclosan and rhodamine B by h⁺ in the anode chamber. The electron produced at the anode is transferred to the cathode through an external conducting system where the reduction of nitrates occurs. The designed self-biased system removed almost ∼90% RhB and reduced ∼95% NO₃–N under UV light after 4 h. Along with the generation of photo-excited electrons ∼98%, triclosan was degraded, and ∼81% NO³⁻-N in the cathode chamber was removed in 60 min. The catalytic mechanism of the designed photoelectrochemical and the degradation rate discussed above are shown in Fig. 18(a–c).

Fig. 18 (a) Degradation mechanism of the self-sustained system. (b) RhB and NO₃⁻-N degradation rate in anode/cathode chamber and spontaneous current. (c) TCS and NO₃⁻-N degradation rate in the anode/cathode chamber and spontaneous current (reproduced with permission from ref. 166 Copyright 2017, Elsevier).

Na Lu et al.¹⁶⁷ synthesized a Z-scheme photocatalyst for the photodegradation of antibiotic ciprofloxacin, and RGO was used as an electron mediator. They synthesized the photocatalyst g-C₃N₄/RGO/WO₃ by simple self-assembly and photoreduction method. The light-harvesting and charge transfer properties became high with the assimilation of RGO as an electron mediator in place of conductive material within C₃N₄/WO₃. The charge mobility properties were studied using photoluminescence (PL) and electrochemical impedance spectroscopy (EIS), and the photocatalytic degradation efficiency of the synthesized material against ciprofloxacin is still 85%. Yongchao B.¹⁶⁸ synthesized WO₃/RGO/protonated g-C₃N₄ (PCN) composites by the microwave-assisted hydrothermal method for the degradation of tetracycline (TC-HCL). The proposed photocatalyst by this method showed a shorter path of charge migration and a wide interface contact area. The photodegradation efficiency of the 20% WO₃/RGO/PCN composite is maximum. This photocatalyst shows more than 90% catalytic efficiency relative to the previously designed one. By analyzing this work, it is possible to do more work in this field and near future synthesis of a photocatalyst with 100% efficiency, that requires minimum time for the photodegradation of multiple pollutants.

The above discussion highlights the potential of ternary-doped modified WO₃/g-C₃N₄ for advanced photocatalysis. However, it is important to note that the specific scientific advantages may vary depending on the dopant elements, synthesis methods, and intended applications. Ternary photocatalysts typically consist of three different materials, which can increase the complexity of their synthesis, characterization, and application for optimization. Achieving precise control over the composition, morphology, and size of three components in the ternary composite can be challenging, with batch-to-batch variation potentially resulting in low reproducibility. The synthesis of ternary photocatalyst composites can be more demanding than binary or single-component photocatalysts since the longer reaction times and harsher conditions increase energy consumption and create complexity. The synthesis of ternary materials often involves more cost, leading to higher production costs; ternary photocatalysts composite often involves complex interactions, and their performance may depend on intricate mechanisms. Multiple components in the ternary composite can introduce additional chemical reactions and phase separation, leading to decreased photocatalytic activity.

Nonmetal-doped modified WO₃/g-C₃N₄ composites. At present, modification and variations in the electronic properties of g-C₃N₄ by doping with heteroatom emerge with time. Heteroatom doping typically includes nonmetal element doping. Some favorable changes occur after doping with heteroatoms, such as expanding the crystal cell, broadening the ions transfer path, distributing the crystal cell, and promoting electron transport. In addition, doping with the heteroatom can modify the intrinsic crystal structure, bandgap, crystal structure stability, and affect the charge state. Controllable doping can modify the reaction kinetics with nonmetal atoms. However, well-regulated doping methodologies are crucial for optimized the performance, and further development is needed.

Nonmetal-doped g-C₃N₄/WO₃ materials are environment friendly and utilizing them as dopants can reduce the toxicity of heavy metals, making materials more sustainable and cost-effective. Introducing nonmetal elements into g-C₃N₄/WO₃ can improve the material's resistance to photocorrosion, structural changes, and efficient use in photocatalytic processes. Huinan Che et al.¹⁶⁹ synthesized nitrogen-doped graphene quantum dots (NGQDs)-modified Z-scheme g-C₃N₄/Bi₂WO₆ heterojunctions through the hydrothermal method for the photocatalytic degradation of multiple antibiotic drugs such as ciprofloxacin (CIP), tetracycline (TC), and oxytetracycline (OTC) under near-infrared and visible light. Relative to pure g-C₃N₄, Bi₂WO₆, and 60% C₃N₄/Bi₂WO₆, the visible light absorption intensity of 3% NGQDs-60% C₃N₄/Bi₂WO₆ heterojunction increased from 500 nm to 700 nm, suggesting that the incorporation of 3% NGQDs enhanced the photogenerated electron–hole pair production, which results in enhanced photocatalytic activity. The photocatalytic activity of 3% NGQDs–60% C₃N₄/Bi₂WO₆ is higher than that of other pure g-C₃N₄, Bi₂WO₆, and 60% C₃N₄/Bi₂WO₆ photocatalysts. By varying the weight of NGQDs between 1 to 7 wt%, the photocatalytic activity of the photocatalyst against TC increased. A further increase decreased the catalyst's performance, indicating that a high concentration of NGQDs reduced the light absorption capability. NGQDs are more spread over the surface of g-C₃N₄/Bi₂WO₆. The photocatalytic efficiency of 3% NGQDs-60% C₃N₄/Bi₂WO₆ within 60 min against TC was 85.2%, 89.1%, 80.8%, and 75.0%. The photocatalytic performance of the catalyst is lower against other mentioned drugs, only degrading specific pollutants. These catalysts should be designed to degrade multiple drugs under visible and UV light regions within a minimum time and with maximum reusability.

Mi G. et al.¹⁷⁰ designed a novel photocatalyst to degrade organic solvent vapors, which are toxic. They fabricated nitrogen-doped carbon quantum dots NCQDs, g-C₃N₄ (CN), and Bi₂WO₆ (BWO) heterostructure (NCQDs/CN/BWO) by a simple sonication method. The TEM and SEM images in Fig. 18(a and b) show that the average size range of 2.5–9.4 nm and lattice distance of NCQDs was 0.320 nm; the corresponding (002) plane of graphitic carbon and the SAED pattern in the SEM image of NCQDs (inset) confirmed the diffraction ring of the (002) crystal plane. NCQDs exhibited a light blue color for suspension when subjected to UV irradiation (Fig. 18(c)). In dark, unmodified, nearly transparent CQDs solution, the color difference between unmodified CQDs and NCQDs is attributed to the absence or presence of N in the CQDs, indicating that the NCQDs sample contained N. Fig. 18(d) shows that the removal efficiency of nUD by NCQDs/CN/BWO-1 was 57.8 (average) ± 0.8%, while that observed for CN/BWO-0.5, NCQDs/BWO, NCQDs/CN, CN, and BWO was 35.6 ± 0.9%, 29.2 ± 0.4%, 26.1 ± 0.3%, 17.6 ± 0.9%, 13.4 ± 1.2%, respectively. The catalytic performance was still good after 5 cycles; however, the performance efficiency was lower. This work established a landmark for designing a more effective photocatalyst to remove these hazardous organic-based vapors from the atmosphere. Jong U. et al.¹⁴³ synthesized another ternary composite NGQD/FWO/CN material to remove harmful vapors. Firstly, the FeWO₄/g-C₃N₄ heterostructure was synthesized, and then N-doped graphene quantum dots (NGQDs) were decorated on it. The normal elimination efficiency of vaporous ethylbenzene (EBZ) and 2-butoxyethanol (2BE) over a typical NGQD/FWO/CN sample were 97.3% (±0.7%) and 55.1% (±0.6%), as shown in Fig. 19(e and f).

Fig. 19 (a) TEM image and size distribution and (b) HRTEM image with the SAED pattern of NCQDs. (c) UV-Vis spectra of CQDs and NCQDs suspended in deionized water. (d) Average efficiencies ± standard error for nUD and mXYL removal.¹⁷¹ (e) Efficiencies of the photocatalytic removal of 2BE and (f) EBZ (reproduced with permission from ref. 172 Copyright 2020, Elsevier).

K. M. et al.¹⁷³ synthesized a porous oxygen-doped-g-C₃N₄/WO₃ photocatalyst for H₂ production. The optimal production of hydrogen was 15 142 μmol g⁻¹, obtained using 10 mg photocatalyst (O-g-CNW-4). However, loading more than 10 mg catalysts reduced the photocatalytic H₂ evaluation. The heteroatom doping of WO₃/g-C₃N₄ set a new site for developing a new photocatalyst for various applications. There is a further need to work in this field to synthesize photocatalysts that can be applied commercially. However, nonmetal doping in photocatalysis can have a few disadvantages, for instance, it may not have the same photocatalytic activity as the metal doping photocatalyst since they can not provide active sites for the desired chemical transformations, thus lowering the photocatalytic efficiency. Nonmetal dopants may decrease the stability of the photocatalytic material by introducing defects in the structure, reducing the material's durability over time and limiting the lifespan of the photocatalyst. Some nonmetals may have narrow absorption ranges, may not effectively utilize the visible light spectrum, and can restrict the photocatalyst's ability.

Limitations of WO₃/g-C₃N₄-based materials

The stability of WO₃/g-C₃N₄ limits its practical applications since some studies reported issues with WO₃/g-C₃N₄ photocorrosion under prolonged irradiation or harsh reaction conditions. Therefore, strategies such as 0D, 1D, 2D, 3D,^{122,126,130,174,175} metallic and bimetallic doping, or structural modifications have been explored to enhance the stability of these photocatalysts.^{40,43,45,47–51}

The preparation method for the WO₃/g-C₃N₄ can impact the performance efficiency and stability of the photocatalyst.¹⁰⁰ Different synthesis methods (hydrothermal, coprecipitation, calcination, in situ synthesis, thermal condensation, acid treatment, etc.) have been employed to fabricate the WO₃/g-C₃N₄ composite; however, the optimized synthesis parameters and achieving uniform dispersion can be challenging.^{122,126,130,174,175}

The efficiency of WO₃/g-C₃N₄ can vary depending on several factors (morphology, composition, and pH conditions). However, there is still room for improvement to enhance the WO₃/g-C₃N₄ efficiency.^82,176

Different studies have reported different bandgap values, even for the same doped materials of the WO₃/g-C₃N₄-based nanocomposite.^56,58,59 Advanced spectroscopic characterization can be useful to evaluate the bandgap energy of the doped WO₃/g-C₃N₄ nanocomposite.

A deep understanding of the photoexcited electron–hole separation process is unclear on the WO₃/g-C₃N₄-based nanocomposite. Different advanced characterization techniques should verify it for better understanding.

Efficient recycling and catalyst recovery of WO₃/g-C₃N₄ is an important parameter in practical applications, and some techniques such as filtration, centrifugation, or magnetic separation are used to recover photocatalysts for reuse. After the separation, the catalyst shows less photocatalytic activities. The separation from the system is not fruitful; therefore, further research is needed to develop more effective and scalable methods for photocatalyst separation and recycling. It is a drawback for real applications, which could be resolved with the development of science and technology. More attention should be paid to overcoming the separation and recyclability issue of the WO₃/g-C₃N₄-based nanocomposite.

Future perspectives

The combination of WO₃ and g-C₃N₄ as photocatalysts has significant advances in recent research. Here are some of the most important advancements for WO₃/g-C₃N₄ photocatalysts based on the above data.

• The synergistic effect between WO₃ and g-C₃N₄ has shown improved photocatalytic performance compared to individual components, and this advancement opens avenues for the further optimization and exploration of the WO₃/g-C₃N₄ system.

• Researchers have made progress in understanding the mechanisms behind the improved photocatalytic activity of WO₃/g-C₃N₄. Current research further elucidates the charge separation, transfer processes, and surface reactions contributing to enhanced performance.

• Modifying the band structure of WO₃/g-C₃N₄ through element doping and heterojunction formation offers opportunities to tailor the photocatalytic properties. Future research can explore novel strategies to fine-tune the band structure for specific applications and improved efficiency.

• While significant progress has been made in the laboratory, scaling up the synthesis and application of WO₃/g-C₃N₄ photocatalysts is an important area for future research. Efforts can be directed toward developing scalable synthesis methods and exploring real-world applications.

• In the future, this material can solve global energy challenges and minimize water pollution by providing green and economical cheap materials for designing different modification techniques. This material can be used in water splitting (the process that converts water into hydrogen and oxygen gases). Hydrogen gas can serve as a clean and renewable energy source soon.

• Researchers should apply these materials in environmental studies to remove volatile organic compounds for air purification. This application can improve the air quality in environments. The photocatalytic properties of the WO₃/g-C₃N₄-based nanocomposite make them more effective in volatile organic compounds (VOCs) and nitrogen oxides (NO_x) detection from air. This material can detect bimolecular, toxic gases, and environmental pollutants shortly for designing such sensing devices.

• The surface area of the WO₃/g-C₃N₄-based nanocomposite is small. To enhance the specific surface area of the WO₃/g-C₃N₄-based nanocomposite, further studies should be focused on, such as synthesizing mesoporous WO₃/g-C₃N₄-based nanocomposite using different methods.

• The antimicrobial properties of the WO₃/g-C₃N₄-based nanocomposite can be utilized for developing antibacterial coatings on surfaces, preventing the growth and spread of bacteria and infections, and medical equipment. Drug delivery in the present era is the most popular medical field. Researchers should pay attention to these materials for drug delivery. Further investigations should focus on this application in in vitro and in vivo studies and living organisms.

• Little data is available on these materials (WO₃/g-C₃N₄) for artificial photosynthesis and nitrogen fixation. Researchers should apply this catalyst in this recent hot field.

• Transition metals are important in nanotechnology since have variable oxidation states. They adsorb on another surface and activate them. Several transition metals are not reported for doping in WO₃/g-C₃N₄-based nanocomposite and need to be further investigated in detail. In addition, s-block and p-block elements can significantly change the properties of WO₃/g-C₃N₄-based nanocomposites.

• Computational approaches for theoretical calculations to evaluate the degradation pathways for understanding the photocatalytic mechanism can also be helpful in WO₃/g-C₃N₄-based materials.

• Similarly, future research work must focus on the development of superconductors. Superconductors are used mostly in levitating trains, engines, generators, highly sensitive optical sensors, detectors of magnetic fields, low-noise amplifiers, transformers, magnetic resonance imaging (MRI), and areas that apply strong magnetic fields. A limited number of publications are available on this material for a superconductor.

Conclusion

In this review, we described the history, crystalline structure, synthesis, precursor, method of fabrication, modification (element-doping and structure modification), characterization, and application of WO₃/g-C₃N₄-based nanocomposite in photocatalysis. It is observed that the pristine g-C₃N₄ and WO₃ show unacceptable efficiency due to their low surface area, insufficient sunlight absorption, and fast recombination of photoinduced electron–hole pairs. Strategies to improve the efficiency of WO₃/g-C₃N₄ highlighted the various synthetic routes such as sol–gel, ultrasonic dispersion, microwave irradiation, precipitation, hydrothermal, pyrolysis method, and calcination methods are discussed. A wide range of composites, including WO₃/g-C₃N₄, modified WO₃/g-C₃N₄, (0D, 1D, 2D, 3D) light metal-doped WO₃/g-C₃N₄, transition metal-doped WO₃/g-C₃N₄, ZnO-WO₃/g-C₃N₄, and TiO₂-WO₃/g-C₃N₄ nanomaterials, are investigated. These modifications provide fascinating properties such as a wide range of visible light responses, good redox ability, suitable bandgap, nontoxicity, environmental friendliness, high physicochemical stability, and thermal stability. A brief overview of the recent advanced publications from 2009 to 2023 is discussed for the photocatalytic degradation of organic pollutants. In future studies, WO₃/g-C₃N₄-based nanocomposites can be applied for the betterment of human beings.

Data availability

All the data is available in the manuscript.

Conflicts of interest

Authors declare no competing interests.

Acknowledgements

There is no funding to report for this article. Authors acknowledge the Higher Education Commission (HEC) of Pakistan for this research.

References

1. P. V. Nidheesh, M. Zhou and M. A. Oturan, An overview on the removal of synthetic dyes from water by electrochemical advanced oxidation processes, Chemosphere, 2018, 197, 210–227.
2. T. Shindhal, et al., A critical review on advances in the practices and perspectives for the treatment of dye industry wastewater, Bioengineered, 2021, 12(1), 70–87.
3. X. Yang, et al., Visible-near-infrared-responsive g-C3N4Hx+ reduced decatungstate with excellent performance for photocatalytic removal of petroleum hydrocarbon, J. Hazard. Mater., 2020, 381, 120994.
4. P. G. Kougias and I. Angelidaki, Biogas and its opportunities—A review, Front. Environ. Sci. Eng., 2018, 12, 1–12.
5. Administration, U.E.I., Annual Energy Outlook 2011: With Projections to 2035, Government Printing Office, 2011.
6. P. Gao, et al., A critical review on bismuth oxyhalide based photocatalysis for pharmaceutical active compounds degradation: Modifications, reactive sites, and challenges, J. Hazard. Mater., 2021, 412, 125186.
7. M. M. Moura, et al., Degradation of the mixture of the ketoprofen, meloxicam and tenoxicam drugs using TiO2/metal photocatalysers supported in polystyrene packaging waste, Water Sci. Technol., 2021, 83(4), 863–876.
8. H. Wang, et al., Visible-light-driven removal of tetracycline antibiotics and reclamation of hydrogen energy from natural water matrices and wastewater by polymeric carbon nitride foam, Water Res., 2018, 144, 215–225.
9. L. M. Bexfield, et al., Hormones and pharmaceuticals in groundwater used as a source of drinking water across the United States, Environ. Sci. Technol., 2019, 53(6), 2950–2960.
10. B. P. Gumbi, et al., Detection and quantification of acidic drug residues in South African surface water using gas chromatography-mass spectrometry, Chemosphere, 2017, 168, 1042–1050.
11. Z. Zhao, et al., In Situ Preparation of Mn0. 2Cd0. 8S-Diethylenetriamine/Porous g-C3N4 S-Scheme Heterojunction with Enhanced Photocatalytic Hydrogen Production, Adv. Sustainable Syst., 2023, 7(1), 2100498.
12. C. J. Ogugbue and T. J. Sawidis, Bioremediation and detoxification of synthetic wastewater containing triarylmethane dyes by Aeromonas hydrophila isolated from industrial effluent, Biotechnol. Res. Int., 2011, 2011, 967925.
13. P. A. Carneiro, et al., Homogeneous photodegradation of CI Reactive Blue 4 using a photo-Fenton process under artificial and solar irradiation, Dyes Pigm., 2007, 74(1), 127–132.
14. A. Aziz, et al., Chitosan-zinc sulfide nanoparticles, characterization and their photocatalytic degradation efficiency for azo dyes, Int. J. Biol. Macromol., 2020, 153, 502–512.
15. S. Khan, et al., Electrochemical sensors based on biomimetic magnetic molecularly imprinted polymer for selective quantification of methyl green in environmental samples, Mater. Sci. Eng., C, 2019, 103, 109825.
16. N. H. Tran, M. Reinhard and K. Y.-H. Gin, Occurrence and fate of emerging contaminants in municipal wastewater treatment plants from different geographical regions-a review, Water Res., 2018, 133, 182–207.
17. K. He, et al., Advances in nanostructured silicon carbide photocatalysts, Acta Phys.-Chim. Sin., 2022, 38, 2201021.
18. F. C. Moreira, et al., Electrochemical advanced oxidation processes: a review on their application to synthetic and real wastewaters, Appl. Catal., B, 2017, 202, 217–261.
19. N. Li, et al., Recent Developments in Functional Nanocomposite Photocatalysts for Wastewater Treatment: A Review, Adv. Sustainable Syst., 2022, 6(7), 2200106.
20. N. Taoufik, et al., Comparative overview of advanced oxidation processes and biological approaches for the removal pharmaceuticals, J. Environ. Manage., 2021, 288, 112404.
21. K. Ayoub, et al., Application of advanced oxidation processes for TNT removal: a review, J. Hazard. Mater., 2010, 178(1–3), 10–28.
22. P. K. Pandis, et al., Key points of advanced oxidation processes (AOPs) for wastewater, organic pollutants and pharmaceutical waste treatment: A mini review, ChemEngineering, 2022, 6(1), 8.
23. H. Sun, et al., One-pot thermal polymerization route to prepare N-deficient modified g-C3N4 for the degradation of tetracycline by the synergistic effect of photocatalysis and persulfate-based advanced oxidation process, Chem. Eng. J., 2021, 406, 126844.
24. D. Zhu and Q. Zhou, Monitoring, and Management, Action and mechanism of semiconductor photocatalysis on degradation of organic pollutants in water treatment: A review, Environ. Nanotechnol., Monit. Manage., 2019, 12, 100255.
25. R. Dewil, et al., New perspectives for advanced oxidation processes, J. Environ. Manage., 2017, 195, 93–99.
26. M. H. Mahdi, T. J. Mohammed, and J. A. Al-Najar, Advanced Oxidation Processes (AOPs) for treatment of antibiotics in wastewater: a review, in IOP Conference Series: Earth and Environmental Science, IOP Publishing, 2021.
27. C. Comninellis, et al., Advanced oxidation processes for water treatment: advances and trends for R&D, J. Chem. Technol. Biotechnol., 2008, 83(6), 769–776.
28. S. A. Fast, et al., A critical evaluation of advanced oxidation processes for emerging contaminants removal, Environ. Processes, 2017, 4, 283–302.
29. J. Y. Hwang, et al., Crystal phase-dependent generation of mobile OH radicals on TiO2: Revisiting the photocatalytic oxidation mechanism of anatase and rutile, Appl. Catal., B, 2021, 286, 119905.
30. E. S. Elmolla and M. Chaudhuri, Degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution by the UV/ZnO photocatalytic process, J. Hazard. Mater., 2010, 173(1–3), 445–449.
31. X. Feng, et al., High performance, recoverable Fe3O4ZnO nanoparticles for enhanced photocatalytic degradation of phenol, Chem. Eng. J., 2014, 244, 327–334.
32. Q. Li, et al., CdS/graphene nanocomposite photocatalysts, Adv. Energy Mater., 2015, 5(14), 1500010.
33. M. T. Uddin, et al., Nanostructured SnO2–ZnO heterojunction photocatalysts showing enhanced photocatalytic activity for the degradation of organic dyes, Inorg. Chem., 2012, 51(14), 7764–7773.
34. D. Zhu and Q. Zhou, Action and mechanism of semiconductor photocatalysis on degradation of organic pollutants in water treatment: A review, Environ. Nanotechnol., Monit. Manage., 2019, 12, 100255.
35. I. Ani, et al., Photocatalytic degradation of pollutants in petroleum refinery wastewater by TiO2-and ZnO-based photocatalysts: recent development, J. Cleaner Prod., 2018, 205, 930–954.
36. X. Gu, et al., Self-assembly synthesis of S-scheme g-C3N4/Bi8 (CrO4) O11 for photocatalytic degradation of norfloxacin and bisphenol A, Chin. J. Catal., 2022, 43(10), 2569–2580.
37. J. Jia, et al., Recent advances on g–C3N4–based Z-scheme photocatalysts: Structural design and photocatalytic applications, Int. J. Hydrogen Energy, 2023, 48(1), 196–231.
38. H. Wang, et al., Synthesis and applications of novel graphitic carbon nitride/metal-organic frameworks mesoporous photocatalyst for dyes removal, Appl. Catal., B, 2015, 174, 445–454.
39. T. Song, et al., Preparation, structure and application of g-C3N4/BiOX composite photocatalyst, Int. J. Hydrogen Energy, 2021, 46(2), 1857–1878.
40. Y. Gong, M. Li and Y. J. C. Wang, Carbon nitride in energy conversion and storage: recent advances and future prospects, ChemSusChem, 2015, 8(6), 931–946.
41. K. Vignesh and M. Kang, Facile synthesis, characterization and recyclable photocatalytic activity of Ag2WO4@ g-C3N4, Mater. Sci. Eng., B, 2015, 199, 30–36.
42. Z. Fan, et al., Bridging effect of S–C bond for boosting electron transfer over cubic hollow CoS/g-C3N4 heterojunction toward photocatalytic hydrogen production, Langmuir, 2022, 38(10), 3244–3256.
43. M. Yang, et al., The influence of preparation method on the photocatalytic performance of g-C3N4/WO3 composite photocatalyst, Ceram. Int., 2014, 40(8), 11963–11969.
44. J. Bai, et al., Integration of 2D layered CdS/WO3 S-scheme heterojunctions and metallic Ti3C2 MXene-based Ohmic junctions for effective photocatalytic H2 generation, Chin. J. Catal., 2022, 43(2), 359–369.
45. M. G. Peleyeju and E. Viljoen, WO₃-based catalysts for photocatalytic and photoelectrocatalytic removal of organic pollutants from water A review, J. Water Process Eng., 2021, 40, 101930.
46. H. Wang, et al., Structure–performance correlation guided applications of covalent organic frameworks, Mater. Today, 2022, 106–133.
47. V. Alman, et al., Sunlight Assisted improved photocatalytic degradation of rhodamine B using Pd-loaded gC 3 N 4/WO 3 nanocomposite, Appl. Phys. A: Mater. Sci. Process., 2020, 126(9), 1–9.
48. M. Karimi-Nazarabad, et al., Highly efficient photocatalytic and photoelectrocatalytic activity of solar light driven WO3/g-C3N4 nanocomposite, Sol. Energy Mater. Sol. Cells, 2017, 160, 484–493.
49. C. Wang, et al., BaWO4/g-C3N4 heterostructure with excellent bifunctional photocatalytic performance, Chem. Eng. J., 2020, 385, 123833.
50. M. B. Tahir, M. Sagir and K. Shahzad, Removal of acetylsalicylate and methyl-theobromine from aqueous environment using nano-photocatalyst WO3-TiO2@ g-C3N4 composite, J. Hazard. Mater., 2019, 363, 205–213.
51. S. Li, et al., In situ construction of a C 3 N 5 nanosheet/Bi 2 WO 6 nanodot S-scheme heterojunction with enhanced structural defects for the efficient photocatalytic removal of tetracycline and Cr (vi), Inorg. Chem. Front., 2022, 9(11), 2479–2497.
52. L. Jiang, et al., Doping of graphitic carbon nitride for photocatalysis: A reveiw, Appl. Catal., B, 2017, 217, 388–406.
53. C. Dong, et al., A review on WO3 based gas sensors: Morphology control and enhanced sensing properties, J. Alloys Compd., 2020, 820, 153194.
54. M. G. Peleyeju and E. L. Viljoen, WO3-based catalysts for photocatalytic and photoelectrocatalytic removal of organic pollutants from water–A review, J. Water Process. Eng., 2021, 40, 101930.
55. B. S. Kalanoor, H. Seo and S. S. Kalanur, Recent developments in photoelectrochemical water-splitting using WO3/BiVO4 heterojunction photoanode: A review, Mater. Sci. Energy Technol., 2018, 1(1), 49–62.
56. J. Yao, et al., Improved photocatalytic activity of WO3/C3N4: By constructing an anchoring morphology with a Z-scheme band structure, Solid State Sci., 2019, 95, 105926.
57. O. Ola, et al., Pseudocapacitive Performance of WO 3/gC 3 N 4 Nanocomposites, in 2021 IEEE 11th International Conference Nanomaterials: Applications & Properties (NAP), IEEE, 2021.
58. H. Qian, et al., One-Pot Construction of Porous WO3/g-C3N4 Nanotubes of Photocatalyst for Fast and Boosted Photodegradation of Rhodamine B and Tetracycline, J. Electron. Mater., 2023, 1–16.
59. X. Zhang and S. He, WOx/g-C3N4 layered heterostructures with controlled crystallinity towards superior photocatalytic degradation and H2 generation, Carbon, 2020, 156, 488–498.
60. J. Liebig, Annalen, 1834, 10, 10.
61. E. C. Franklin, The ammono-carbonic acids, J. Am. Chem. Soc., 1922, 44(3), 486–509.
62. W. K. Darkwah, et al, Mini review on the structure and properties (photocatalysis), and preparation techniques of graphitic carbon nitride nano-based particle, and its applications, Nanoscale Res. Lett., 2018, 13(1), 1–15.
63. K. Qi, S.-y. Liu and A. Zada, Graphitic carbon nitride, a polymer photocatalyst, J. Taiwan Inst. Chem. Eng., 2020, 109, 111–123.
64. A. O. Idris, et al., Graphitic carbon nitride: a highly electroactive nanomaterial for environmental and clinical sensing, Sensors, 2020, 20(20), 5743.
65. V. Battula, et al., A true oxygen-linked heptazine based polymer for efficient hydrogen evolution, Appl. Catal., B, 2019, 244, 313–319.
66. A. Wang, et al., Recent advances of graphitic carbon nitride-based structures and applications in catalyst, sensing, imaging, and LEDs, Nano-Micro Lett., 2017, 9(4), 1–21.
67. M. Y. Chen, D. Li, X. Lin, V. P. Dravid, Y. W. Chung, M. S. Wong and W. D. Sproul, Analytical electron microscopy and Raman spectroscopy studies of carbon nitride thin films, J. Vac. Sci. Technol., A, 1993, 11(3), 521–524.
68. J. L. Corkill and M. L. Cohen, Calculated quasiparticle band gap of β-C₃N₄, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 48(23), 17622.
69. M. F. R. Samsudin, N. Bacho, and S. Sufian, Recent development of graphitic carbon nitride-based photocatalyst for environmental pollution remediation, in Nanocatalysts, IntechOpen. 2018.
70. D. Wei, et al., In situ construction of interconnected SnO2/nitrogen-doped Carbon@ TiO2 networks for lithium-ion half/full cells, Electrochim. Acta, 2018, 290, 312–321.
71. Q. Hao, et al., Graphitic carbon nitride with different dimensionalities for energy and environmental applications, Nano Res., 2020, 13(1), 18–37.
72. B. Zhu, et al., First-principle calculation study of tri-s-triazine-based g-C3N4: a review, Appl. Catal., B, 2018, 224, 983–999.
73. W.-J. Ong, et al., Graphitic carbon nitride (g-C3N4)-based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability?, Chem. Rev., 2016, 116(12), 7159–7329.
74. H.-W. ZUO, et al., Pt4 clusters supported on monolayer graphitic carbon nitride sheets for oxygen adsorption: a first-principles study, Acta Phys.-Chim. Sin., 2016, 32(5), 1183–1190.
75. B. Zhu, Review on DFT calculation of s-triazine-based carbon nitride, Carbon Energy, 2019, 1(1), 32–56.
76. B. Zhu, et al., First principle investigation of halogen-doped monolayer g-C3N4 photocatalyst, Appl. Catal., B, 2017, 207, 27–34.
77. B. Zhu, et al., First-principle calculation study of tri-s-triazine-based g-C3N4: a review, Appl. Catal., B, 2018, 224, 983–999.
78. J. Kim, et al., Platinized WO3 as an environmental photocatalyst that generates OH radicals under visible light, Environ. Sci. Technol., 2010, 44(17), 6849–6854.
79. D. Migas, et al., Tungsten oxides. I. Effects of oxygen vacancies and doping on electronic and optical properties of different phases of WO 3, J. Appl. Phys., 2010, 108(9), 093713.
80. S. S. Kalanur, L. T. Duy and H. Seo, Recent progress in photoelectrochemical water splitting activity of WO3 photoanodes, Top. Catal., 2018, 61(9), 1043–1076.
81. F. Wang, C. Di Valentin and G. Pacchioni, Electronic and structural properties of WO3: a systematic hybrid DFT study, J. Phys. Chem. C, 2011, 115(16), 8345–8353.
82. J. Fu, et al., Ultrathin 2D/2D WO3/g-C3N4 step-scheme H2-production photocatalyst, Appl. Catal., B, 2019, 243, 556–565.
83. P. Praus, et al., Synthesis and properties of nanocomposites of WO3 and exfoliated g-C3N4, Ceram. Int., 2017, 43(16), 13581–13591.
84. X. Han, et al., WO3/g-C3N4 two-dimensional composites for visible-light driven photocatalytic hydrogen production, Int. J. Hydrogen Energy, 2018, 43(10), 4845–4855.
85. H. Zhuang, et al., In situ construction of WO 3/gC 3 N 4 composite photocatalyst with 2D–2D heterostructure for enhanced visible light photocatalytic performance, New J. Chem., 2019, 43(44), 17416–17422.
86. T. Pan, et al., Anionic polyacrylamide-assisted construction of thin 2D-2D WO3/g-C3N4 Step-scheme heterojunction for enhanced tetracycline degradation under visible light irradiation, J. Hazard. Mater., 2020, 393, 122366.
87. J. Chen, et al., Fabrication of hierarchical sheet-on-sheet WO3/g-C3N4 composites with enhanced photocatalytic activity, J. Alloys Compd., 2019, 777, 325–334.
88. Y. Li, et al., Construction of inorganic–organic 2D/2D WO 3/gC 3 N 4 nanosheet arrays toward efficient photoelectrochemical splitting of natural seawater, Phys. Chem. Chem. Phys., 2016, 18(15), 10255–10261.
89. C. Cheng, et al., WO3/g-C3N4 composites: one-pot preparation and enhanced photocatalytic H2 production under visible-light irradiation, Nanotechnology, 2017, 28(16), 164002.
90. T. Xiao, et al., In situ construction of hierarchical WO3/g-C3N4 composite hollow microspheres as a Z-scheme photocatalyst for the degradation of antibiotics, Appl. Catal., B, 2018, 220, 417–428.
91. M.-H. Pham, et al., Visible light induced hydrogen generation using a hollow photocatalyst with two cocatalysts separated on two surface sides, Phys. Chem. Chem. Phys., 2014, 16(13), 5937–5941.
92. J. Yuan, et al., TiO2/SnO2 double-shelled hollow spheres-highly efficient photocatalyst for the degradation of rhodamine B, Catal. Commun., 2015, 60, 129–133.
93. S. L. Prabavathi, et al., Construction of heterostructure CoWO4/g-C3N4 nanocomposite as an efficient visible-light photocatalyst for norfloxacin degradation, J. Ind. Eng. Chem., 2019, 80, 558–567.
94. X. Xing, et al., CoWO4 nanoparticles prepared by two methods displaying different structures and supercapacitive performances, Electrochim. Acta, 2015, 157, 15–22.
95. K. Liu, et al., Synthesis of direct Z-scheme MnWO4/g-C3N4 photocatalyst with enhanced visible light photocatalytic activity, Nano, 2017, 12(10), 1750129.
96. J. Low, et al., A review of direct Z-scheme photocatalysts, Small Methods, 2017, 1(5), 1700080.
97. R. Dadigala, et al., Construction of in situ self-assembled FeWO 4/gC 3 N 4 nanosheet heterostructured Z-scheme photocatalysts for enhanced photocatalytic degradation of rhodamine B and tetracycline, Nanoscale Adv., 2019, 1(1), 322–333.
98. X. Zhang, X. Zhang and P. Yang, Transition metals decorated g-C3N4/N-doped carbon nanotube catalysts for water splitting: A review, J. Electroanal. Chem., 2021, 895, 115510.
99. H. Jiang, et al., Recent advances in heteroatom doped graphitic carbon nitride (g-C3N4) and g-C3N4/metal oxide composite photocatalysts, Curr. Org. Chem., 2020, 24(6), 673–693.
100. M. W. Kadi and R. M. Mohamed, Increasing visible light water splitting efficiency through synthesis route and charge separation in measoporous g-C3N4 decorated with WO3 nanoparticles, Ceram. Int., 2019, 45(3), 3886–3893.
101. L. Huang, et al., Visible-light-induced WO 3/gC 3 N 4 composites with enhanced photocatalytic activity, Dalton Trans., 2013, 42(24), 8606–8616.
102. W. Yang, et al., Hierarchical NiCo 2 O 4@ NiO core–shell hetero-structured nanowire arrays on carbon cloth for a high-performance flexible all-solid-state electrochemical capacitor, J. Mater. Chem. A, 2014, 2(5), 1448–1457.
103. L. Huang, et al., Visible-light-induced WO 3/gC 3 N 4 composites with enhanced photocatalytic activity, Dalton Trans., 2013, 42(24), 8606–8616.
104. S. Chen, et al., Study on the separation mechanisms of photogenerated electrons and holes for composite photocatalysts g-C3N4-WO3, Appl. Catal., B, 2014, 150, 564–573.
105. A. T. Doan, et al., Graphitic gC 3 N 4-WO 3 composite: synthesis and photocatalytic properties, Bull. Korean Chem. Soc., 2014, 35(6), 1794–1798.
106. H. Zhang, et al., Photocatalytic selective oxidation of biomass-derived 5-hydroxymethylfurfural to 2, 5-diformylfuran on WO3/g-C3N4 composite under irradiation of visible light, J. Photochem. Photobiol., A, 2019, 371, 1–9.
107. H. Yan, et al., Single-source-precursor-assisted synthesis of porous WO3/g-C3N4 with enhanced photocatalytic property, Colloids Surf., A, 2019, 582, 123857.
108. F. Chang, et al., Binary composites WO3/g-C3N4 in porous morphology: Facile construction, characterization, and reinforced visible light photocatalytic activity, Colloids Surf., A, 2019, 563, 11–21.
109. R. Ma, Preparation of highly dispersed WO₃/few layer g-C₃N₄ and its enhancement of catalytic oxidative desulfurization activity, Colloids Surf., A, 2019, 572, 250–258.
110. A. Priya, et al., A study of photocatalytic and photoelectrochemical activity of as-synthesized WO3/g-C3N4 composite photocatalysts for AO7 degradation, Mater. Sci. Energy Technol., 2020, 3, 43–50.
111. R. Zhao, et al., Preparation of WO3/g-C3N4 composites and their application in oxidative desulfurization, Appl. Surf. Sci., 2017, 392, 810–816.
112. A. Navarro-Aguilar, et al., An efficient and stable WO3/g-C3N4 photocatalyst for ciprofloxacin and orange G degradation, J. Photochem. Photobiol., A, 2019, 384, 112010.
113. I. Aslam, et al., The synergistic effect between WO 3 and gC 3 N 4 towards efficient visible-light-driven photocatalytic performance, New J. Chem., 2014, 38(11), 5462–5469.
114. H. Katsumata, et al., Z-scheme photocatalytic hydrogen production over WO 3/gC 3 N 4 composite photocatalysts, RSC Adv., 2014, 4(41), 21405–21409.
115. M. Gondal, et al., Preparation of WO 3/gC 3 N 4 composites and their enhanced photodegradation of contaminants in aqueous solution under visible light irradiation, React. Kinet., Mech. Catal., 2015, 114(1), 357–367.
116. S. Chen, et al., Study on the separation mechanisms of photogenerated electrons and holes for composite photocatalysts g-C3N4-WO3, Appl. Catal., B, 2014, 150, 564–573.
117. K.-i. Katsumata, et al., Preparation of graphitic carbon nitride (g-C3N4)/WO3 composites and enhanced visible-light-driven photodegradation of acetaldehyde gas, J. Hazard. Mater., 2013, 260, 475–482.
118. Y. Zang, et al., Facile synthesis of composite g-C 3 N 4/WO 3: a nontoxic photocatalyst with excellent catalytic activity under visible light, RSC Adv., 2013, 3(33), 13646–13650.
119. J. Singh, et al., Synthesis of coral like WO3/g-C3N4 nanocomposites for the removal of hazardous dyes under visible light, J. Alloys Compd., 2019, 808, 151734.
120. J. Singh, A. Arora and S. Basu, Synthesis of coral like WO3/g-C3N4 nanocomposites for the removal of hazardous dyes under visible light, J. Alloys Compd., 2019, 808, 151734.
121. Z. Lou and C. J. C. Xue, In situ growth of WO 3− x nanowires on gC 3 N 4 nanosheets: 1D/2D heterostructures with enhanced photocatalytic activity, CrystEngComm, 2016, 18(43), 8406–8410.
122. T. Pan, et al., Anionic polyacrylamide-assisted construction of thin 2D-2D WO3/g-C3N4 Step-scheme heterojunction for enhanced tetracycline degradation under visible light irradiation, J. Hazard. Mater., 2020, 393, 122366.
123. F. Zhang, et al., One-step oxygen vacancy engineering of WO3-x/2D g-C3N4 heterostructure: Triple effects for sustaining photoactivity, J. Alloys Compd., 2019, 795, 426–435.
124. M. Antoniadou, et al., Bifunctional g-C3N4/WO3 thin films for photocatalytic water purification, Water, 2019, 11(12), 2439.
125. C. Xu, et al., Facile construction of leaf-like WO 3 nanoflakes decorated on gC 3 N 4 towards efficient oxidation of alcohols under mild conditions, New J. Chem., 2018, 42(20), 16523–16532.
126. T. Xiao, et al., In situ construction of hierarchical WO3/g-C3N4 composite hollow microspheres as a Z-scheme photocatalyst for the degradation of antibiotics, Appl. Catal., B, 2018, 220, 417–428.
127. F. Xu, et al., Step-by-step mechanism insights into the TiO2/Ce2S3 S-scheme photocatalyst for enhanced aniline production with water as a proton source, ACS Catal., 2021, 12(1), 164–172.
128. W. Wang, et al., Efficient degradation of tetracycline via coupling of photocatalysis and photo-fenton processes over a 2D/2D α-Fe2O3/g-C3N4 S-scheme heterojunction catalyst, Acta Phys.-Chim. Sin., 2022, 38(7), 2201008.
129. S. Deng, et al., WO 3 nanosheets/gC 3 N 4 nanosheets' nanocomposite as an effective photocatalyst for degradation of rhodamine B, Appl. Phys. A: Mater. Sci. Process., 2019, 125(1), 44.
130. J. Chen, et al., Fabrication of hierarchical sheet-on-sheet WO3/g-C3N4 composites with enhanced photocatalytic activity, J. Alloys Compd., 2019, 777, 325–334.
131. J. Chen, et al., Ag nanoparticles decorated WO3/g-C3N4 2D/2D heterostructure with enhanced photocatalytic activity for organic pollutants degradation, Appl. Surf. Sci., 2019, 467, 1000–1010.
132. S. M. Hashmi, et al., Enhancement of mechanical properties of epoxy/halloysite nanotube (HNT) nanocomposites, SN Appl. Sci., 2019, 1(4), 296.
133. Y. Li, et al., In situ loading of Ag2WO4 on ultrathin g-C3N4 nanosheets with highly enhanced photocatalytic performance, J. Hazard. Mater., 2016, 313, 219–228.
134. J. Huang, et al., Ultrathin Ag2WO4-coated P-doped g-C3N4 nanosheets with remarkable photocatalytic performance for indomethacin degradation, J. Hazard. Mater., 2020, 392, 122355.
135. N. Tian, H. Huang and Y. Zhang, Mixed-calcination synthesis of CdWO4/g-C3N4 heterojunction with enhanced visible-light-driven photocatalytic activity, Appl. Surf. Sci., 2015, 358, 343–349.
136. A. Maavia, et al., Facile synthesis of g-C3N4/CdWO4 with excellent photocatalytic performance for the degradation of Minocycline, Mater. Sci. Energy Technol., 2019, 2(2), 258–266.
137. P. Xing, et al., Preparation of CoWO4/g-C3N4 and its Ultra-Deep Desulfurization Property, Aust. J. Chem., 2016, 70(3), 271–279.
138. S. L. Prabavathi, et al., Construction of heterostructure CoWO4/g-C3N4 nanocomposite as an efficient visible-light photocatalyst for norfloxacin degradation, J. Ind. Eng. Chem., 2019, 80, 558–567.
139. S. Sahoo, et al., Facile construction of CoWO4 modified g-C3N4 nanocomposites with enhanced photocatalytic activity under visible light irradiation, Mater. Today: Proc., 2021, 35, 193–197.
140. D. P. Ojha, et al., Decoration of g-C3N4 with hydrothermally synthesized FeWO4 nanorods as the high-performance supercapacitors, Chem. Phys. Lett., 2018, 712, 83–88.
141. R. Dadigala, et al., Construction of in situ self-assembled FeWO 4/gC 3 N 4 nanosheet heterostructured Z-scheme photocatalysts for enhanced photocatalytic degradation of rhodamine B and tetracycline, Nanoscale Adv., 2019, 1(1), 322–333.
142. A. Rashidizadeh, et al., Graphitic Carbon Nitride Nanosheet/FeWO4 Nanoparticle Composite for Tandem Photooxidation/Knoevenagel Condensation, ACS Appl. Nano Mater., 2020, 3(7), 7057–7065.
143. J. U. Choi and W.-K. Jo, FeWO4/g-C3N4 heterostructures decorated with N-doped graphene quantum dots prepared under various sonication conditions for efficient removal of noxious vapors, Ceram. Int., 2020, 46(8), 11346–11356.
144. R. Bhosale, et al., Direct Z-scheme g-C3N4/FeWO4 nanocomposite for enhanced and selective photocatalytic CO2 reduction under visible light, ACS Appl. Mater. Interfaces, 2019, 11(6), 6174–6183.
145. L. Zhang, et al., In situ growth of g-C3N4 on hexangular flowerlike FeWO4 microcrystals: highly efficient catalyst and the crucial roles of Fe3+/Fe2+ couple in the photoassisted oxidation and reduction reactions, J. Phys. Chem. C, 2018, 122(24), 12900–12912.
146. R. Huang, et al., In situ synthesis of Cu+ self-doped CuWO4/g-C3N4 heterogeneous Fenton-like catalysts: the key role of Cu+ in enhancing catalytic performance, Sep. Purif. Technol., 2020, 250, 117174.
147. S. Zhou, et al., Enhanced visible light photocatalytic degradation of rhodamine B by Z-scheme CuWO 4/gC 3 N 4 heterojunction, J. Mater. Sci.: Mater. Electron., 2021, 32(3), 2731–2743.
148. K. Liu, et al., Synthesis of direct Z-scheme MnWO4/g-C3N4 photocatalyst with enhanced visible light photocatalytic activity, Nano, 2017, 12(10), 1750129.
149. K. Saravanakumar, et al., 1D/2D MnWO4 nanorods anchored on g-C3N4 nanosheets for enhanced photocatalytic degradation ofloxacin under visible light irradiation, Colloids Surf., A, 2019, 581, 123845.
150. V. Rathi, A. Panneerselvam and R. J. Sathiyapriya, Surface decoration of MnWO 4 nanoparticles with gC 3 N 4 nanosheets to build hetero-structure and its structural, optical and enhanced visible light photocatalytic activity, J. Mater. Sci.: Mater. Electron., 2020, 31(17), 14823–14837.
151. V. J. C. Muthuraj, et al, Superior visible light driven photocatalytic degradation of fluoroquinolone drug norfloxacin over novel NiWO4 nanorods anchored on g-C3N4 nanosheets, Colloids Surf., A, 2019, 567, 43–54.
152. V. Muthuraj, Superior visible light driven photocatalytic degradation of fluoroquinolone drug norfloxacin over novel NiWO4 nanorods anchored on g-C3N4 nanosheets, Colloids Surf., A, 2019, 567, 43–54.
153. V.-D. Dao, et al., Superior visible light photocatalytic activity of g-C3N4/NiWO4 direct Z system for degradation of gaseous toluene, J. Solid State Chem., 2019, 272, 62–68.
154. R. Koutavarapu, et al., A novel one-pot approach of ZnWO 4 nanorods decorated onto gC 3 N 4 nanosheets: 1D/2D heterojunction for enhanced solar-light-driven photocatalytic activity, J. Mater. Sci., 2020, 55(3), 1170–1183.
155. S. Zhan, et al., g-C3N4/ZnWO4 films: preparation and its enhanced photocatalytic decomposition of phenol in UV, Appl. Surf. Sci., 2015, 358, 328–335.
156. Z. Bian, et al., Plant uptake-assisted round-the-clock photocatalysis for complete purification of aquaculture wastewater using sunlight, Environ. Sci. Technol., 2015, 49(4), 2418–2424.
157. J. Zheng and L. J. C. E. J. Zhang, Designing 3D magnetic peony flower-like cobalt oxides/g-C3N4 dual Z-scheme photocatalyst for remarkably enhanced sunlight driven photocatalytic redox activity, Chem. Eng. J., 2019, 369, 947–956.
158. M. Tang, Facile synthesis of dual Z-scheme g-C₃N₄/Ag₃PO₄/AgI composite photocatalysts with enhanced performance for the degradation of a typical neonicotinoid pesticide, Appl. Catal., B, 2020, 268, 118395.
159. M. Tang, et al., Rationally constructing of a novel dual Z-scheme composite photocatalyst with significantly enhanced performance for neonicotinoid degradation under visible light irradiation, Appl. Catal., B, 2020, 270, 118918.
160. Y. Guo, et al., Mediator-free direct dual-Z-scheme Bi2S3/BiVO4/MgIn2S4 composite photocatalysts with enhanced visible-light-driven performance towards carbamazepine degradation, Appl. Catal., B, 2019, 254, 479–490.
161. H. Yin, et al., Construction of AgBr/β-Ag2WO4/g-C3N4 ternary composites with dual Z-scheme band alignment for efficient organic pollutants removal, Sep. Purif. Technol., 2021, 272, 118251.
162. L. Jiang, et al., In-situ synthesis of direct solid-state dual Z-scheme WO3/g-C3N4/Bi2O3 photocatalyst for the degradation of refractory pollutant, Appl. Catal., B, 2018, 227, 376–385.
163. N. T. T. Truc, et al., Superior activity of Cu-NiWO4/g-C3N4 Z direct system for photocatalytic decomposition of VOCs in aerosol under visible light, J. Alloys Compd., 2019, 798, 12–18.
164. Y. O. Ibrahim and M. Gondal, Visible-light-driven photocatalytic performance of a Z-scheme based TiO2/WO3/g-C3N4 ternary heterojunctions, Mol. Catal., 2021, 505, 111494.
165. T. Yu, L. Liu and F. Yang, Heterojunction between anodic TiO2/g-C3N4 and cathodic WO3/W nano-catalysts for coupled pollutant removal in a self-biased system, Chin. J. Catal., 2017, 38(2), 270–277.
166. T. Yu, L. Liu and F. Yang, Heterojunction between anodic TiO2/g-C3N4 and cathodic WO3/W nano-catalysts for coupled pollutant removal in a self-biased system, Chin. J. Catal., 2017, 38(2), 270–277.
167. N. Lu, et al., Construction of Z-Scheme g-C3N4/RGO/WO3 with in situ photoreduced graphene oxide as electron mediator for efficient photocatalytic degradation of ciprofloxacin, Chemosphere, 2019, 215, 444–453.
168. Y. Bao, et al., Heterostructured WO3/RGO/protonated g-C3N4 three-layer nanosheets for enhanced visible-light photocatalytic activity, Appl. Surf. Sci., 2019, 496, 143639.
169. H. Che, et al., NGQD active sites as effective collectors of charge carriers for improving the photocatalytic performance of Z-scheme gC 3 N 4/Bi 2 WO 6 heterojunctions, Catal. Sci. Technol., 2018, 8(2), 622–631.
170. M. G. Kim and W.-K. Jo, Visible-light-activated N-doped CQDs/g-C3N4/Bi2WO6 nanocomposites with different component arrangements for the promoted degradation of hazardous vapors, J. Mater. Sci. Technol., 2020, 40, 168–175.
171. M. G. Kim and W.-K. Jo, Visible-light-activated N-doped CQDs/g-C3N4/Bi2WO6 nanocomposites with different component arrangements for the promoted degradation of hazardous vapors, J. Mater. Sci. Technol., 2020, 40, 168–175.
172. J. U. Choi and W.-K. Jo, FeWO4/g-C3N4 heterostructures decorated with N-doped graphene quantum dots prepared under various sonication conditions for efficient removal of noxious vapors, Ceram. Int., 2020, 46(8), 11346–11356.
173. K. Mallikarjuna, et al., Synthesis of oxygen-doped-g-C3N4/WO3 porous structures for visible driven photocatalytic H2 production, Phys. E, 2021, 126, 114428.
174. C. Cheng, et al., WO3/g-C3N4 composites: one-pot preparation and enhanced photocatalytic H2 production under visible-light irradiation, Nanotechnology, 2017, 28(16), 164002.
175. Y. Li, et al., Construction of inorganic–organic 2D/2D WO 3/gC 3 N 4 nanosheet arrays toward efficient photoelectrochemical splitting of natural seawater, Phys. Chem. Chem. Phys., 2016, 18(15), 10255–10261.
176. B.-C. CHEN, et al., Research progress on g-C3N4-based Z-scheme photocatalytic system, Acta Phys.-Chim. Sin., 2016, 32(6), 1371–1382.

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