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

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.

Adducing Knowledge Capabilities of Instrumental Techniques Through the Exploration of Heterostructures' Modification Methods

The ongoing evolution of technology has facilitated the global research community to rapidly escalate the constant development of novel advancements in science. At the forefront of such achievements in the field of photocatalysis is the utilisation, and in oftentimes, the adaptation of modern instrumentation to understand photo-physical properties of complex heterostructures. For example, coupling in-situ X-ray Raman scattering spectroscopy for real-time degradation of catalytic materials.

Nano-zirconia supported by graphitic carbon nitride for enhanced visible light photocatalytic activity

Graphitic carbon nitride (g-C₃N₄) was prepared by high-temperature calcination of urea. A mixture of g-C₃N₄ and nano-ZrO₂ precursor was directly calcined to prepare g-C₃N₄/ZrO₂ hybrid photocatalysts. The photocatalytic properties of the sample were characterized by degradation of rhodamine B (RhB) under visible light. The g-C₃N₄/ZrO₂ hybrid photocatalysts have better degradation performance than the pure g-C₃N₄ and ZrO₂. The prepared catalysts were characterized by various techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectroscopy (FT-IR), and photoluminescence spectroscopy (PL) and electrochemical tests. The reasons for the improvement of catalytic activity were investigated from the aspects of crystal structure, surface morphology and photoelectric properties, and the catalytic mechanism were studied. The results show that the ZrO₂ nanoparticles were coated with g-C₃N₄ to form a heterostructure. Compared with the pure g-C₃N₄ and ZrO₂, the g-C₃N₄/ZrO₂ hybrids reduce the charge transfer resistance and inhibit the recombination of electron–holes well. In addition, it affects the band structure and improves the absorption of visible-light. At the same time, the study found that the main active species in the catalytic process were h⁺ and ·O₂⁻.

Graphitic carbon nitride (g-C₃N₄) was prepared by high-temperature calcination of urea.

Layer-by-layer assembly into bulk-like g-C3N4via artificial manipulation of electrostatic forces

For the first time, protonated and oxygen doped g-C₃N₄ nanosheets were assembled via an electrostatic force into a bulk-like photocatalyst with superior hydrogen production ability.

Experimental and DFT Studies of Au Deposition Over WO3/g-C3N4 Z-Scheme Heterojunction

A typical Z-scheme system is composed of two photocatalysts which generate two sets of charge carriers and split water into H₂ and O₂ at different locations. Scientists are struggling to enhance the efficiencies of these systems by maximizing their light absorption, engineering more stable redox couples, and discovering new O₂ and H₂ evolutions co-catalysts. In this work, Au decorated WO₃/g-C₃N₄ Z-scheme nanocomposites are fabricated via wet-chemical and photo-deposition methods. The nanocomposites are utilized in photocatalysis for H₂ production and 2,4-dichlorophenol (2,4-DCP) degradation. It is investigated that the optimized 4Au/6% WO₃/CN nanocomposite is highly efficient for production of 69.9 and 307.3 µmol h^-1 g^-1 H₂ gas, respectively, under visible-light (λ > 420 nm) and UV-visible illumination. Further, the fabricated 4Au/6% WO₃/CN nanocomposite is significant (i.e., 100% degradation in 2 h) for 2,4-DCP degradation under visible light and highly stable in photocatalysis. A significant 4.17% quantum efficiency is recorded for H₂ production at wavelength 420 nm. This enhanced performance is attributed to the improved charge separation and the surface plasmon resonance effect of Au nanoparticles. Solid-state density functional theory simulations are performed to countercheck and validate our experimental data. Positive surface formation energy, high charge transfer, and strong non-bonding interaction via electrostatic forces confirm the stability of 4Au/6% WO₃/CN interface.

Bifunctional g-C3N4/WO3 Thin Films for Photocatalytic Water Purification

A bifunctional thin film photocatalyst consisting of graphitic carbon nitride on tungsten trioxide (g-C3N4/WO3) is introduced for the improvement of photocatalytic activity concerning hexavalent chromium reduction and methylene blue dye removal in water, compared to the bare, widely used WO3 semiconductor. A bilayered structure was formed, which is important for the enhancement of the charge carriers’ separation. The characterization of morphological, structural, optoelectronic, and vibrational properties of the photocatalysts permitted a better understanding of their photocatalytic activity for both dye degradation and Cr+6 elimination in water and the analysis of the photocatalytic kinetics permitted the determination of the corresponding pseudo-first-order reaction constants (k). Trapping experiments performed under UV illumination revealed that the main active species for the photocatalytic reduction of Cr+6 ions are electrons, whereas in the case of methylene blue azo-dye (MB) oxidation, the activation of the corresponding photocatalytic degradation comes via both holes and superoxide radicals.

Rapid removal of sulfamethoxazole from simulated water matrix by visible-light responsive iodine and potassium co-doped graphitic carbon nitride photocatalysts

An environment-friendly iodine and potassium co-doped g-C₃N₄ (IKC₃N₄) photocatalyst was synthesized via the co-pyrolysis of urea and potassium iodate. Various characterization techniques were employed to evaluate the physical, thermal and chemical characteristics of the as-synthesized photocatalyst. Sulfamethoxazole (SMX) was used as a representative antibiotic pollutant. SMX removal by IK-C₃N₄ photocatalysts exceeded 99% (∼23 times higher than that of pure g-C₃N₄) within 45 min of visible light irradiation. The kinetics of SMX removal was analyzed with respect to solution pH, photocatalyst dosage and initial SMX concentration. Experimental data was found to fit the pseudo-first order kinetics and the Langmuir-Hinshelwood kinetics. The reuse of the photocatalyst up to 3 consecutive photodegradation cycles gave a minimal decline in SMX removal while the structure and the crystallinity of the nanomaterials remained unchanged. Overall, morphology engineering of conventional bulk graphitic carbon nitride can produce highly efficient photocatalysts for the decontamination of antibiotics in the aqueous environment.

WO3/g-C3N4 composites: one-pot preparation and enhanced photocatalytic H2 production under visible-light irradiation

A series of WO₃/g-C₃N₄ composites with different WO₃ contents were prepared via a facile one-pot pyrolysis method, and showed notably enhanced visible-light-driven photocatalytic H₂-evolution activities, with the highest rate of 400 μmol h^-1 g_cat^-1 that was 15.0 times of that for pristine g-C₃N₄. Contents and sizes of WO₃ crystallites in the composites were easily adjusted by changing the molar ratios of (NH₄)₂WS₄ to C₃H₆N₆ in the feed reagents, thereby successfully optimizing the Z-scheme system constructed by WO₃ and g-C₃N₄ and thus effectively reducing the recombination of photogenerated charge carriers in g-C₃N₄. Moreover, pore volumes and surface areas of the composites were gradually enlarged by introducing WO₃ into g-C₃N₄ via the one-pot preparation strategy, therefore promoting the redox reactions to evolve H₂. This work presented an effective route to simultaneously optimize the phase compositions and textural structures of photocatalysts for enhanced H₂ evolution.

Mechanistic Characteristics of Surface Modified Organic Semiconductor g-C₃N₄ Nanotubes Alloyed with Titania

The visible-light-driven photocatalytic degradation of Bisphenol A (BPA) was investigated using the binary composite of alkaline treated g-C₃N₄ (HT-g-C₃N₄) deposited over commercial TiO₂ (Evonik Degussa GmbH, Essen, Germany). The existence and contribution of both TiO₂ and g-C₃N₄/HT-g-C₃N₄ in the composite was confirmed through various analytical techniques including powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflectance spectra (UV-vis-DRS), and photoluminescence (PL) analysis. The results showed that the titania in the binary composite exhibited both pure rutile and anatase phases. The morphological analysis indicated that the spongy "morel-like" structure of g-C₃N₄ turned to nanotube form after alkaline hydrothermal treatment and thereby decreased the specific surface area of HT-g-C₃N₄. The low surface area of HT-g-C₃N₄ dominates its promising optical property and effective charge transfer, resulting in a deprived degradation efficiency of BPA two times lower than pure g-C₃N₄. The binary composite of HT-g-C₃N₄/TiO₂ exhibited excellent degradation efficiency of BPA with 2.16 times higher than the pure HT-g-C₃N₄. The enhanced photocatalytic activity was mainly due to the promising optical band gap structure with heterojunction interface, favorable specific surface area, and good charge separation.

Construction of WO3–g-C3N4 composites as efficient photocatalysts for pharmaceutical degradation under visible light

Simple construction of WO₃–g-C₃N₄ Z-scheme heterojunctions as efficient photocatalysts to degrade sulfamethoxazole, which is one of the most commonly used pharmaceuticals.

Preparation of CoWO4/g-C3N4 and its Ultra-Deep Desulfurization Property

The ultra-deep desulfurization of fuel oil has become inevitable for environmental protection. Here, CoWO4/g-C3N4 was used as a catalyst, H2O2 as an oxidant, and 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][EtSO4], IL) as an extractant for the oxidative desulfurization of model oil. Scanning electron microscopy, FT-IR spectroscopy, N2 adsorption isotherms, and X-ray diffraction were used to confirm the morphology, structure, and properties of the catalysts. The influence of calcination temperature, loading dose of cobalt, amount of H2O2, reaction temperature, and other parameters were investigated. The removal rate of sulfide in model oil could reach 92.9 % at 80°C in 180 min under the optimal operation conditions (V(oil) = 5 mL, T = 80°C, m(catalyst) = 0.03 g, V(H2O2) = 0.4 mL, t = 180 min, V(IL) = 1.0 mL). In addition, the catalyst was reused five times with no significant reduction in the catalytic activity.

Improving the photocatalytic hydrogen production of Ag/g-C3N4 nanocomposites by dye-sensitization under visible light irradiation

Ag nanoparticles were deposited on the surface of g-C3N4 by a chemical reduction method to increase visible-light absorption via the localized surface plasmon resonance effect, resulting in the reduced recombination of photo-generated electron-holes and enhanced photocatalytic activity. The Ag/g-C3N4 composite with a Ag loading of 3 wt% has the optimum photoactivity that is almost 3.6 and 3.4 times higher than pure g-C3N4 and the same photocatalysis system which has been reported, respectively. Fluorescein was introduced as a photosensitizer and H2 evolution soared to 2014.20 μmol g(-1) h(-1) and the rate is even about 4.8 times higher than that of the 3 wt% Ag/g-C3N4 composite. The chemical structure, composites, morphologies and optical properties of the obtained products are well-characterized by XRD, FTIR, TEM, EDS, XPS and UV-Vis DRS. Meanwhile, the photocatalyst exhibits high stability and reusability.

Mesoporous carbon nitride-tungsten oxide composites for enhanced photocatalytic hydrogen evolution

Composites of mesoporous polymeric carbon nitride and tungsten(VI) oxide show very high photocatalytic activity for the evolution of hydrogen from water under visible light and in the presence of sacrificial electron donors. Already addition of very small amounts of WO3 yields up to a twofold increase in the efficiency when compared to bulk carbon nitrides and their composites and more notably even to the best reported mesoporous carbon nitride-based photocatalytic materials. The higher activity can be attributed to the high surface area and synergetic effect of the carbon nitrides and the WO3 resulting in improved charge separation through a photocatalytic solid-state Z-scheme mechanism.

Polymeric photocatalysts based on graphitic carbon nitride

Semiconductor-based photocatalysis is considered to be an attractive way for solving the worldwide energy shortage and environmental pollution issues. Since the pioneering work in 2009 on graphitic carbon nitride (g-C3N4) for visible-light photocatalytic water splitting, g-C3N4 -based photocatalysis has become a very hot research topic. This review summarizes the recent progress regarding the design and preparation of g-C3N4 -based photocatalysts, including the fabrication and nanostructure design of pristine g-C3N4 , bandgap engineering through atomic-level doping and molecular-level modification, and the preparation of g-C3N4 -based semiconductor composites. Also, the photo-catalytic applications of g-C3N4 -based photocatalysts in the fields of water splitting, CO2 reduction, pollutant degradation, organic syntheses, and bacterial disinfection are reviewed, with emphasis on photocatalysis promoted by carbon materials, non-noble-metal cocatalysts, and Z-scheme heterojunctions. Finally, the concluding remarks are presented and some perspectives regarding the future development of g-C3N4 -based photocatalysts are highlighted.

Graphitic carbon nitride based nanocomposites: a review

Graphitic carbon nitride (g-C(3)N(4)), as an intriguing earth-abundant visible light photocatalyst, possesses a unique two-dimensional structure, excellent chemical stability and tunable electronic structure. Pure g-C(3)N(4) suffers from rapid recombination of photo-generated electron-hole pairs resulting in low photocatalytic activity. Because of the unique electronic structure, the g-C(3)N(4) could act as an eminent candidate for coupling with various functional materials to enhance the performance. According to the discrepancies in the photocatalytic mechanism and process, six primary systems of g-C(3)N(4)-based nanocomposites can be classified and summarized: namely, the g-C(3)N(4) based metal-free heterojunction, the g-C(3)N(4)/single metal oxide (metal sulfide) heterojunction, g-C(3)N(4)/composite oxide, the g-C(3)N(4)/halide heterojunction, g-C(3)N(4)/noble metal heterostructures, and the g-C(3)N(4) based complex system. Apart from the depiction of the fabrication methods, heterojunction structure and multifunctional application of the g-C(3)N(4)-based nanocomposites, we emphasize and elaborate on the underlying mechanisms in the photocatalytic activity enhancement of g-C(3)N(4)-based nanocomposites. The unique functions of the p-n junction (semiconductor/semiconductor heterostructures), the Schottky junction (metal/semiconductor heterostructures), the surface plasmon resonance (SPR) effect, photosensitization, superconductivity, etc. are utilized in the photocatalytic processes. Furthermore, the enhanced performance of g-C(3)N(4)-based nanocomposites has been widely employed in environmental and energetic applications such as photocatalytic degradation of pollutants, photocatalytic hydrogen generation, carbon dioxide reduction, disinfection, and supercapacitors. This critical review ends with a summary and some perspectives on the challenges and new directions in exploring g-C(3)N(4)-based advanced nanomaterials.

In situ preparation of novel heterojunction BiOBr/BiVO4 photocatalysts with enhanced visible light photocatalytic activity

Heterojunction BiOBr/BiVO₄ photocatalysts were in situ synthesized through acid etching coupled with a hydrothermal process and they displayed higher photocatalytic activity than pure BiOBr and BiVO₄ for MB removal under visible light irradiation.

In situ synthesis of g-C3N4/WO3 heterojunction plates array films with enhanced photoelectrochemical performance

g-C₃N₄/WO₃ heterojunction plate array films with enhanced photoelectrochemical (PEC) performance were successfully synthesized through a combination of hydrothermal and dipping-annealing methods.

Highly efficient visible-light photocatalysts: reduced graphene oxide and C3N4 nanosheets loaded with Ag nanoparticles

A novel ternary Ag/C₃N₄/RGO photocatalyst exhibits superior degradation activity under visible-light irradiation.

Improved photocatalytic activity of g-C3N4 derived from cyanamide–urea solution

This paper describes the fabrication of g-C₃N₄ by the polymerization of cyanamide–urea solution at elevated temperatures.

ZnO nanoparticle-functionalized WO3 plates with enhanced photoelectrochemical properties

ZnO nanoparticle-functionalized WO₃ plates were prepared via an electrodeposition and an electrochromism reaction of WO₃, and the composites improve the separation of photogenerated electrons and holes.

New insights into fluorinated TiO2 (brookite, anatase and rutile) nanoparticles as efficient photocatalytic redox catalysts

In this paper, we develop a fluorination methodology and optimize various synthesis conditions. We also demonstrate that photocatalytic redox activity is affected by the synergistic effect between surface fluorination and phase structure.

Synthesis high specific surface area nanotube g-C3N4 with two-step condensation treatment of melamine to enhance photocatalysis properties

High specific surface area nanotube g-C₃N₄ was fabricated by a simple two-step condensation method.

Enhanced visible-light-induced photocatalytic performance of a novel ternary semiconductor coupling system based on hybrid Zn–In mixed metal oxide/g-C3N4 composites

Ternary semiconductor coupling system based on hybrid Zn–In mixed metal oxide/g-C₃N₄ composites exhibited significantly enhanced visible-light-induced photocatalytic performance.

In situ growth of TiO2 nanocrystals on g-C3N4 for enhanced photocatalytic performance

Highly dispersed TiO₂ nanocrystals with (001) facets were successfully grown in situ on g-C₃N₄ through a facial method. The resultant composite exhibits remarkably enhanced photocatalysis compared to pure TiO₂ or g-C₃N₄ or mechanically mixed TiO₂/g-C₃N₄.

Multifunctional g-C(3)N(4) nanofibers: a template-free fabrication and enhanced optical, electrochemical, and photocatalyst properties

We have developed a facile, scale up, and efficient method for the preparation of graphitic-C3N4 nanofibers (GCNNFs) as electrodes for supercapacitors and photocatalysts. The as-synthesized GCNNFs have 1D structure with higher concentration of nitrogen that is favorable for higher conductivity and electrochemical performance. Secondly, the high surface area of GCNNF provides a large electrode-electrolyte contact area, sufficient light harvesting and mass transfer, as well as increased redox potential. Thus, the GCNNF supercapacitor electrode shows high capacitance of 263.75 F g(-1) and excellent cyclic stability in 0.1 M Na2SO4 aqueous electrolyte with the capacitance retention of 93.6% after 2000 cycles at 1 A g(-1) current density. GCNNFs exhibit high capacitance of 208 F g(-1) even at 10 A g(-1), with the appreciable capacitance retention of 89.5%, which proves its better rate capability. Moreover, the GCNNF shows enhanced photocatalytic activity in the photodegradation of RhB in comparison to the bulk graphitic-C3N4 (GCN). The degradation rate constant of GCNNF photocatalyst is almost 4 times higher than GCN. The enhanced photocatalytic activity of GCNNF is mainly due to the higher surface area, appropriate bandgap, and fewer defects in GCNNF as compared to GCN. As an economical precursor (melamine) and harmless, facile, and template-free synthesis method with excellent performance both in supercapacitors and in photodegradation, GCNNF is a strong candidate for energy storage and environment protection applications.

Synthesis and characterization of a ZrO2/g-C3N4 composite with enhanced visible-light photoactivity for rhodamine degradation

A ZrO₂/g-C₃N₄ composite was prepared by directly heating a ZrO₂–melamine mixture and showed excellent activity in rhodamine photodegradation.

The synergistic effect between WO3 and g-C3N4 towards efficient visible-light-driven photocatalytic performance

The photocatalyst showed efficient enhanced photocatalytic performance, which is about 3.65 and 3.72 times greater than pure WO₃ and g-C₃N₄ respectively.

Z-scheme photocatalytic hydrogen production over WO3/g-C3N4 composite photocatalysts

WO₃/g-C₃N₄ catalysts exhibit excellent photocatalytic performance for H₂ production from aqueous solution through the Z-scheme mechanism, which results in the efficient charge separation.