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

Muhammad Ikram Nabeel a, Dilshad Hussain *a, Naseer Ahmad a, Muhammad Najam-ul-Haq b and Syed Ghulam Musharraf *a
aHEJ Research Institute of Chemistry, International Center for Chemical and Biological Sciences, University of Karachi, Karachi-75270, Pakistan. E-mail: dilshadhussain@iccs.edu; musharraf1977@yahoo.com
bInstitute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800, Pakistan

Received 14th March 2023 , Accepted 18th August 2023

First published on 21st August 2023


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.


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, WOx, 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, WOx, 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, WOx, Fe2O3, ZnS, and BiOI-based photocatalysts.29–35


image file: d3na00159h-f1.tif
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 (MOx) materials improve the photocatalytic and electronic device efficiency. The role of tungsten oxide (WOx) is very prominent among many metal oxides.43,44 WOx 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 WOx and g-C3N4 has resulted in higher photocatalytic activity than WOx 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.


image file: d3na00159h-f2.tif
Fig. 2 Doping of MWOx/g-C3N4 with various metals, nonmetals, and metalloids.

Tungsten trioxide has excellent photocatalytic activity in the visible light region. By integrating WOx 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 MWOx/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 (MWOx/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.


image file: d3na00159h-f3.tif
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
image file: d3na00159h-f4.tif
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 (Eg) was calculated using the GGA-PBE (generalized gradient approximation Perdew–Burke–Ernzerhof) function, predicting an Eg 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 Eg 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.
image file: d3na00159h-f5.tif
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.


image file: d3na00159h-f6.tif
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.


image file: d3na00159h-f7.tif
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.


image file: d3na00159h-f8.tif
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 (MWOx/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 (MWOx/g-C3N4) design nanostructured materials can overcome these issues. This review deeply analyzed all the research directions of the MWOx/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.


image file: d3na00159h-f9.tif
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/WO3−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[thin space (1/6-em)]:[thin space (1/6-em)]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 (Rs). The doping of g-C3N4 with WO3 decreased the Rs 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.


image file: d3na00159h-f10.tif
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(Eg) plot, (c) (αhν)2 or (αhν)1/2versus Eg 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)
 
image file: d3na00159h-t1.tif(ii)
 
O2 + H+ + 2e → H2O2(iii)

image file: d3na00159h-f11.tif
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.

 
EVB = χEe + 0.5Eg(iv)
 
ECB = EVBEg(v)
where EVB and ECB are the valence and conduction band potentials, χ represents the electronegativity (geometric mean of electronegativity) of the semiconductor, and Ee 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


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[thin space (1/6-em)]:[thin space (1/6-em)]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[thin space (1/6-em)]:[thin space (1/6-em)]1, showing the absorption range up to 500 nm, 504 nm, and 575 nm, respectively. The WO3/g-C3N4-1[thin space (1/6-em)]:[thin space (1/6-em)]1 composite extended the absorption range, appreciably improving the visible light adsorption assisting effective degradation and showing that WO3/g-C3N4-1[thin space (1/6-em)]:[thin space (1/6-em)]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[thin space (1/6-em)]:[thin space (1/6-em)]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[thin space (1/6-em)]:[thin space (1/6-em)]3) and WO3/g-C3N4 (3[thin space (1/6-em)]:[thin space (1/6-em)]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[thin space (1/6-em)]:[thin space (1/6-em)]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).
image file: d3na00159h-f12.tif
Fig. 12 (a) SEM image of WO3/g-C3N4 (−1[thin space (1/6-em)]:[thin space (1/6-em)]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[thin space (1/6-em)]:[thin space (1/6-em)]1) (reproduced with permission from ref. 120 Copyright 2019, Elsevier).

Lou et al.121 prepared 1D WO3−x nanowires on 2D g-C3N4-NS through a simple solvothermal process. WO3−x nanowires and g-C3N4-NS facilitated the charge transfer in WO3−x and g-C3N4 (beyond WO3−x/g-C3N4-30). Free WO3−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 WO3−x nanowires contributed to a variation in free electrons and improved the conductivity of WO3−x. Secondly, the WO3−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 WO3−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


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).
image file: d3na00159h-f13.tif
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. SBET 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 SBET 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 SBET 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 SBET 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 SBET but also on other factors, such as interfacial interactions and synergistic effects, which can influence the photocatalytic performance even with a lower SBET value.
image file: d3na00159h-f14.tif
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 SBET 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[double bond, length as m-dash]N, and N 1s show three peaks at 398.4 eV, 399.7 eV, and 400.9 eV, indicating the C[double bond, length as m-dash]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[thin space (1/6-em)]:[thin space (1/6-em)]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 2p3/2 and Co 2p1/2, 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 (Rcr 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).


image file: d3na00159h-f15.tif
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 2p1/2 and 2p3/2. 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/FeWO4'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.


image file: d3na00159h-f16.tif
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 2p3/2), 955.4 eV (Cu 2p1/2), 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[thin space (1/6-em)]:[thin space (1/6-em)]0.25 (45%), MnWO4@g-C3N4 1[thin space (1/6-em)]:[thin space (1/6-em)]0.5 (50.2%), MnWO4@g-C3N4 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (90.4%), MnWO4@g-C3N4 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5 (56.8%), and MnWO4@g-C3N4 1[thin space (1/6-em)]:[thin space (1/6-em)]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[thin space (1/6-em)]:[thin space (1/6-em)]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 JV 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 2p1/2 (873.4 eV) and Ni 2p3/2 (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).
image file: d3na00159h-f17.tif
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 A1g symmetric stretching (∼915 cm−1), Eg asymmetric stretching (∼702 cm−1), and T2g bending deformation (2Ag + Bg, ∼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+ 2p1/2 and Zn2+ 2p3/2, respectively. The binding energy values of Zn 2p3/2 and Zn 2p1/2 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


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).


image file: d3na00159h-f18.tif
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).


image file: d3na00159h-f19.tif
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[thin space (1/6-em)]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 (NOx) 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.

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