A critical review on graphitic carbon nitride (g-C3N4)-based materials: Preparation, modification and environmental application

Graphitic carbon nitride (g-C3N4)-assisted materials for the detection and remediation of hazardous gases and VOCs

Graphitic carbon nitride (g-C3N4)-based materials are attracting attention for their unique properties, such as low-cost, chemical stability, facile synthesis, adjustable electronic structure, and optical properties. These facilitate the use of g-C3N4 to design better photocatalytic and sensing materials. Environmental pollution by hazardous gases and volatile organic compounds (VOCs) can be monitored and controlled using eco-friendly g-C3N4- photocatalysts. Firstly, this review introduces the structure, optical and electronic properties of C3N4 and C3N4 assisted materials, followed by various synthesis strategies. In continuation, binary and ternary nanocomposites of C3N4 with metal oxides, sulfides, noble metals, and graphene are elaborated. g-C3N4/metal oxide composites exhibited better charge separation that leads to enhancement in photocatalytic properties. g-C3N4/noble metal composites possess higher photocatalytic activities due to the surface plasmon effects of metals. Ternary composites by the presence of dual heterojunctions improve properties of g-C3N4 for enhanced photocatalytic application. In the later part, we have summarised the application of g-C3N4 and its assisted materials for sensing toxic gases and VOCs and decontaminating NOx and VOCs by photocatalysis. Composites of g-C3N4 with metal and metal oxide give comparatively better results. This review is expected to bring a new sketch for developing g-C3N4-based photocatalysts and sensors with practical applications.

Ultrathin Carbon Nitride Nanosheets Exfoliated and In Situ Modified with a Nickel Bis(Chelate) Complex for Boosting Photocatalytic Performances

Exfoliation and interfacial modification of two-dimensional (2D) polymeric carbon nitride (CN) are considerably vital for applications in photo/electrocatalysis fields. Here, a grinding-ultrasonic route was designed to construct nickel bis(chelate) complex (Ni(abt)₂, abt = 2-aminobenzenethiolate)-modified CN ultrathin nanosheets. Under the assistance of the shear force derived from the grinding process, Ni(abt)₂ was implanted into the interlamination of bulk CN, resulting in the formation of ultrathin CN (UCN) nanosheets. Simultaneously, Ni(abt)₂ molecules were anchored on the surfaces of as-formed UCN nanosheets due to the π-π stacking interaction. Interestingly, compared with single Ni(abt)₂ and UCN, the as-obtained Ni(abt)₂/UCN nanosheets exhibited excellent photocatalytic hydrogen evolution capability. A molecule-semiconductor internal electron transmission mechanism was suggested for explaining the separation and transfer of electron-hole pairs. Density functional theory (DFT) calculations demonstrated that the interface-induced electron redistribution tuned the electron density and hydrogen adsorption of the active centers, thus enhancing the photocatalytic performance of the hybrid catalyst. In addition, the as-obtained Ni(abt)₂/UCN nanosheets could also catalyze the reduction of nitroaromatics in the presence of NaBH₄. It was found that under the simulated sunlight irradiation, the conversion efficiency of nitroaromatic compounds to amino aromatic ones was up to 97.3%, far higher than that under the condition without light irradiation (51.7%), suggesting that the photocatalytic-produced hydrogen took part in the reduction of nitroaromatic compounds.

Integrating K and P co-doped g-C3N4 with ZnFe2O4 and graphene oxide for S-scheme-based enhanced absorption coupled photocatalytic real wastewater treatment

Recently, there has been a significant increase in the interest of using photocatalysis for environmental clean-up applications. In this research, potassium, and phosphorus co-doped graphitic carbon nitride (KPCN) photocatalyst modified with graphene oxide (GO) and heterostructured with ZnFe₂O₄ was synthesized via the hydrothermal method (KPCN/GO/ZnFe₂O₄). The photoactivity of KPCN/GO/ZnFe₂O₄ photocatalyst was examined for the photocatalytic degradation of target pollutants such as methylene blue (MB) dye, rhodamine B (RhB) dye, and tetracycline (TC) antibiotic. Furthermore, the chemical oxygen demand (COD) removal efficiency for real wastewater was determined to explore the practical application of KPCN/GO/ZnFe₂O₄ photocatalyst. The degradation efficiencies of bare graphitic carbon nitride, KPCN, KPCN/GO, and KPCN/GO/ZnFe₂O₄ photocatalysts for tetracycline antibiotics were 30%, 42%, 57%, and 87% within 60 min, respectively. Moreover, KPCN/GO/ZnFe₂O₄ photocatalyst showed 71% COD removal efficiency within 240 min. The ^•OH and ^•O₂^- were the major reactive species in the photocatalytic process. Results showed that the degradation efficiencies of graphitic carbon nitride were greatly enhanced upon doping and further improved with the addition of GO and ZnFe₂O₄. Doping improved light harvesting, GO enhanced the adsorption ability and heterojunction with ZnFe₂O₄ enhanced the charge separation as well as the reusability of synthesized KPCN/GO/ZnFe₂O₄ photocatalyst.

Photocatalytic degradation of organic pollutants by carbon quantum dots functionalized g-C3N4: A review

Graphitic carbon nitride (g-C₃N₄) has received much attention due to its unique characteristics of stable physicochemical features, facile preparation, and inexpensive cost. However, the bulk g-C₃N₄ has a weak capacity for pollutant degradation and needs to be modified for real application. Therefore, extensive research has been done on g-C₃N₄, and the discovery of the novel zero-dimensional nanomaterials known as carbon quantum dots (CQDs) provided it with a unique modification option. In this review, the development for the removal of organic pollutants by g-C₃N₄/CQDs was discussed. Firstly, the preparation of g-C₃N₄/CQDs were introduced. Then, the application and the degradation mechanism of g-C₃N₄/CQDs were briefly described. And the discussion of the influencing factors on g-C₃N₄/CQDs' ability to degrade organic pollutants came in third. Finally, the conclusions of photocatalytic degradation of organic pollutants by g-C₃N₄/CQDs and future perspectives followed. This review will strengthen the understanding of the photocatalytic degradation of real organic wastewater by g-C₃N₄/CQDs, including their preparation, application, mechanism, and influencing factors.

One-Pot Synthesis of 2D-2D WO3 /g-C3 N4 Photocatalyst in Reverse Microemulsion System via Supercritical CO2 for Enhanced Hydrogen Generation

Construction of Z-scheme photocatalyst is an effective approach for using solar energy to produce hydrogen during water splitting. Herein, 2D/2D WO₃ /g-C₃ N₄ heterojunction photocatalyst was synthesized by a convenient and green method including exfoliation and heterojunction procedures, in the reverse microemulsion system via supercritical carbon dioxide (scCO₂ ). The resultant W/CN-10.3 composite exhibited enhanced photocatalytic activities towards the hydrogen evolution during water splitting with a hydrogen evolution rate of 688.51 μmol g^-1  h^-1 , which was more than 16 times higher than bulk g-C₃ N₄ with the same loading amount of Pt as cocatalyst. Due to its effective separation of photogenerated carriers and prolonged lifetime, more photoexcited electrons with high reduction ability could contribute to the production of H₂ . Possible formation mechanism of 2D-2D WO₃ /g-C₃ N₄ nanosheets via scCO₂ in the reverse microemulsion system by the one-pot method has been proposed. This work provides an efficient and green strategy to synthesize 2D-2D heterojunction for the utilization in solar-to-fuel conversion.

Photocatalytic Synthesis of Ammonia from Pinecone Graphite-Phase Carbon Nitride Loaded with MoS2 Nanosheets as Co-catalysts

Photocatalytic nitrogen fixation is a promising alternative to the Haber-Bosch process to alleviate the energy and environmental crises. Here, we designed a pinecone-shaped graphite-phase carbon nitride (PCN) catalyst supported with MoS₂ nanosheets by a supramolecular self-assembly method. The catalyst shows an excellent photocatalytic nitrogen reduction reaction (PNRR) due to the larger specific surface area and the enhancement of visible light owing to the reduced band gap. Under simulated sunlight, the sample of PCN loaded with 5 wt % MoS₂ nanosheets (MS5%/PCN) shows a PNRR efficiency of 279.41 μmol g^-1 h^-1, which is 14.9 times that of bulk graphite-phase carbon nitride (g-C₃N₄), 4.6 times that of PCN, and 5.4 times that of MoS₂, respectively. The unique pinecone-like structure of MS5%/PCN not only improves the ability of light absorption but also assists in the uniform loading of MoS₂ nanosheets. Likewise, the existence of MoS₂ nanosheets improves the light absorption ability of the catalyst and reduces the impedance of the catalyst. Furthermore, as a co-catalyst, MoS₂ nanosheets can efficiently adsorb nitrogen (N₂) and serve as active N₂ reduction sites. From the perspective of structural design, this work can offer novel solutions for the creation of effective N₂-fixing photocatalysts.

Synthesis of Sulfur@g-C3N4 and CuS@g-C3N4 Catalysts for Hydrogen Production from Sodium Borohydride

In this work, the S@g-C₃N₄ and CuS@g-C₃N₄ catalysts were prepared via the polycondensation process. The structural properties of these samples were completed on XRD, FTIR and ESEM techniques. The XRD pattern of S@g-C₃N₄ presents a sharp peak at 27.2° and a weak peak at 13.01° and the reflections of CuS belong to the hexagonal phase. The interplanar distance decreased from 0.328 to 0.319 nm that facilitate charge carrier separation and promoting H₂ generation. FTIR data revealed the structural change according to absorption bands of g-C₃N₄. ESEM images of S@g-C₃N₄ exhibited the described layered sheet structure for g-C₃N₄ materials and CuS@g-C₃N₄ demonstrated that the sheet materials were fragmented throughout the growth process. The data of BET revealed a higher surface area (55 m²/g) for the CuS-g-C₃N₄ nanosheet. The UV-vis absorption spectrum of S@g-C₃N₄ showed a strong peak at 322 nm, which weakened after the growth of CuS at g-C₃N₄. The PL emission data showed a peak at 441 nm, which correlated with electron-hole pair recombination. The data of hydrogen evolution showed improved performance for the CuS@g-C₃N₄ catalyst (5227 mL/g·min). Moreover, the activation energy was determined for S@g-C₃N₄ and CuS@g-C₃N₄, which showed a lowering from 47.33 ± 0.02 to 41.15 ± 0.02 KJ/mol.

Detection and removal technologies for ammonium and antibiotics in agricultural wastewater: Recent advances and prospective

With the extensive development of industrial livestock and poultry production, a considerable part of agricultural wastewater containing tremendous ammonium and antibiotics have been indiscriminately released into the aquatic systems, causing serious harms to ecosystem and human health. In this review, ammonium detection technologies, including spectroscopy and fluorescence methods, and sensors were systematically summarized. Antibiotics analysis methodologies were critically reviewed, including chromatographic methods coupled with mass spectrometry, electrochemical sensors, fluorescence sensors, and biosensors. Current progress in remediation methods for ammonium removal were discussed and analyzed, including chemical precipitation, breakpoint chlorination, air stripping, reverse osmosis, adsorption, advanced oxidation processes (AOPs), and biological methods. Antibiotics removal approaches were comprehensively reviewed, including physical, AOPs, and biological processes. Furthermore, the simultaneous removal strategies for ammonium and antibiotics were reviewed and discussed, including physical adsorption processes, AOPs, biological processes. Finally, research gaps and the future perspectives were discussed. Through conducting comprehensive review, future research priorities include: (1) to improve the stabilities and adaptabilities of detection and analysis techniques for ammonium and antibiotics, (2) to develop innovative, efficient, and low cost approaches for simultaneous removal of ammonium and antibiotics, and (3) to explore the underlying mechanisms that governs the simultaneous removal of ammonium and antibiotics. This review could facilitate the evolution of innovative and efficient technologies for ammonium and antibiotics treatment in agricultural wastewater.

Tailoring Advanced N-Defective and S-Doped g-C3 N4 for Photocatalytic H2 Evolution

Although challenges remain, synergistic adjusting various microstructures and photo/electrochemical parameters of graphitic carbon nitride (g-C₃ N₄ ) in photocatalytic hydrogen evolution reaction (HER) are the keys to alleviating the energy crisis and environmental pollution. In this work, a novel nitrogen-defective and sulfur-doped g-C₃ N₄ (S-g-C₃ N₄ -D) is designed elaborately. Subsequent physical and chemical characterization proved that the developed S-g-C₃ N₄ -D not only displays well-defined 2D lamellar morphology with a large porosity and a high specific surface area but also has an efficient light utilization and carriers-separation and transfer. Moreover, the calculated optimal Gibbs free energy of adsorbed hydrogen (ΔG_H* ) for S-g-C₃ N₄ -D at the S active sites is close to zero (≈0.24 eV) on the basis of first-principle density functional theory (DFT). Accordingly, the developed S-g-C₃ N₄ -D catalyst shows a high H₂ evolution rate of 5651.5 µmol g^-1  h^-1 . Both DFT calculations and experimental results reveal that a memorable defective g-C₃ N₄ /S-doped g-C₃ N₄ step-scheme heterojunction is constructed between S-doped domains and N-defective domains in the structural configuration of S-g-C₃ N₄ -D. This work exhibits a significant guidance for the design and fabrication of high-efficiency photocatalysts.

Photocatalytic Synthesis of Hydrogen Peroxide from Molecular Oxygen and Water

Hydrogen peroxide is a powerful and green oxidant that allows for the oxidation of a wide span of organic and inorganic substrates in liquid media under mild reaction conditions, and forms only molecular water and oxygen as end products. Hydrogen peroxide is therefore used in a wide range of applications, for which the well-documented and established anthraquinone autoxidation process is by far the dominating production method at the industrial scale. As this method is highly energy consuming and environmentally costly, the search for more sustainable synthesis methods is of high interest. To this end, the article reviews the basis and the recent development of the photocatalytic synthesis of hydrogen peroxide. Different oxygen reduction and water oxidation mechanisms are discussed, as well as several kinetic models, and the influence of the main key reaction parameters is itemized. A large range of photocatalytic materials is reviewed, with emphasis on titania-based photocatalysts and on high-prospect graphitic carbon nitride-based systems that take advantage of advanced bulk and surface synthetic approaches. Strategies for enhancing the performances of solar-driven photocatalysts are reported, and the search for new, alternative, photocatalytic materials is detailed. Finally, the promise of in situ photocatalytic synthesis of hydrogen peroxide for water treatment and organic synthesis is described, as well as its coupling with enzymes and the direct in situ synthesis of other technical peroxides.

Insights into engineered graphitic carbon nitride quantum dots for hazardous contaminants degradation in wastewater

Increased environmental pollution is a critical issue that must be addressed. Photocatalytic, adsorption, and membrane filtration methods are suitable in environmental governance because of their high selectivity, low cost, environment-friendly nature, and excellent treatment efficiency. Graphitic carbon nitride (g-C₃N₄) quantum dots (QDs) have been considered as photocatalysts, adsorbents, and membrane materials for wastewater treatments, owing to their stability, adsorption capacity, photochemical properties, and low toxicity and cost. This review summarizes g-C₃N₄ QD synthesis techniques, operating parameters affecting the removal performance in the treatment process, modification effects with other semiconductors, and benefits and drawbacks of g-C₃N₄ QD-based materials. Furthermore, this review discusses the practical applications of g-C₃N₄ QDs as adsorbents, photocatalysts, and membrane materials for organic and inorganic contaminant treatments and their value-added product formation potential. Modified g-C₃N₄ QD-based material adsorbents, photocatalysts, and membranes present potentially applicable effects, such as removal of most waterborne contaminants. Excellent results were obtained for the reduction of methyl orange, bisphenol A, tetracycline, ciprofloxacin, phenol, rhodamine B, E. coli, and Hg. Overall, this paper provides comprehensive background on g-C₃N₄ QD-based materials and their diverse applications in wastewater treatment, and it presents a foundation for the enhancement of similar unique materials in the future.

Revisiting the roles of dopants in g-C3N4 nanostructures for piezo-photocatalytic production of H2O2: a case study of selenium and sulfur

The sustainable production of hydrogen peroxide (H₂O₂) from oxygen and water has become an exciting research hotspot in the scientific community due to the importance of this fine chemical in various fields. Besides, piezo-photocatalysis is an emerging star for generating H₂O₂ from these green reagents. For developing catalysts for this specific application, doping heteroatoms into carbon-based materials such as graphitic carbon nitrides (g-C₃N₄) is a growing fascination among worldwide scientists. However, systematic study on the effects of doping precursors on the catalytic results is still rare. Herein, we fabricated sulfur (S) and selenium (Se) doped g-C₃N₄ with various doping precursors to evaluate the effects of these agents on the production of H₂O₂ under light and ultrasound irradiation. Based on the results, Se-doped g-C₃N₄ gave an outstanding catalytic performance compared to S-doped g-C₃N₄, even in a significantly low quantity of Se. In order to fully understand the chemical, physical, optical, and electronic properties of pristine g-C₃N₄ and its derivatives, the as-prepared materials were thoroughly analyzed with various tools. Thus, this study would give more profound insights into doping techniques for carbon-based materials and encourage further research on the design and development of piezo-photocatalysts for practical applications.

O and S co-doping induced N-vacancy in graphitic carbon nitride towards photocatalytic peroxymonosulfate activation for sulfamethoxazole degradation

Doping-induced vacancy engineering of graphitic carbon nitride (GCN) is beneficial for bandgap modulation, efficient electronic excitation, and facilitated charge carrier migration. In this study, synthesis of oxygen and sulphur co-doped induced N vacancies (OSGCN) by the hydrothermal method was performed to activate peroxymonosulfate (PMS) for sulfamethoxazole (SMX) antibiotic degradation and H₂ production. The results from experimental and DFT simulation studies validate the synergistic effects of co-dopants and N-vacancies, i.e., bandgap lowering, electron-hole pairs separation, and high solar energy utilization. The substitution of sp² N atom by O and S co-dopants causes strong delocalization of HOMO-LUMO distribution, enhancing carrier mobility, increasing reactive sites, and facilitating charge-carrier separation. Remarkably, OSGCN/PMS photocatalytic system achieved 99.4% SMX degradation efficiency and a high H₂ generation rate of 548.23 μ mol g^-1 h^-1 within 60 min and 36 h, respectively under visible light irradiations. The SMX degradation kinetics was pseudo-first-order with retained recycling efficiency up to 4 catalytic cycles. The results of EPR and chemical scavenging experiments revealed the redox action of reactive oxidative species, wherein ¹O₂ was the dominant reactive species in SMX degradation. The identification of formed intermediates and the SMX stepwise degradation pathway was investigated via LC-MS analysis and DFT studies, respectively. The results from this work anticipated deepening the understanding of PMS activation by substitutional co-doping favoring N-vacancy formation in GCN lattice for improved photocatalytic activity.

Organocatalysis with carbon nitrides

Carbon nitrides, a distinguished class of metal-free catalytic materials, have presented a good potential for chemical transformations and are expected to become prominent materials for organocatalysis. This is largely possible due to their low cost, exceptional thermal and chemical stability, non-toxicity, ease of functionalization, porosity development, etc. Especially, the carbon nitrides with increased porosity and nitrogen contents are more versatile than their bulk counterparts for catalysis. These N-rich carbon nitrides are discussed in the earlier parts of the review. Later, the review highlights the role of such carbon nitride materials for the various organic catalytic reactions including Knoevenagel condensation, oxidation, hydrogenation, esterification, transesterification, cycloaddition, and hydrolysis. The recently emerging concepts in carbon nitride-based organocatalysis have been given special attention. In each of the sections, the structure-property relationship of the materials was discussed and related to their catalysis action. Relevant comparisons with other catalytic materials are also discussed to realize their real potential value. The perspective, challenges, and future directions are also discussed. The overall objective of this review is to provide up-to-date information on new developments in carbon nitride-based organic catalysis reactions that could see them rising as prominent catalytic materials in the future.

Enhanced Photocatalytic Degradation of Emerging Contaminants Using Ti3C2Tx MXene-Supported CdS Quantum Dots

The synthesis of efficient and stable catalysts for photocatalytic reactions is still a challenge. In this study, a new photocatalyst composed of two-dimensional titanium carbide (Ti₃C₂T_x) and CdS quantum dots (QDs) was fabricated, in which CdS QDs were intimately anchored on the Ti₃C₂T_x sheet surface. Due to the specific interface characteristics of CdS QDs/Ti₃C₂T_x, Ti₃C₂T_x can considerably facilitate the generation of photogenerated charge carriers, their separation, and their transfer from CdS. As expected, the obtained CdS QDs/Ti₃C₂T_x exhibit outstanding photocatalytic performance for carbamazepine (CBZ) degradation. Moreover, the quenching experiments demonstrated that superoxide radicals (^•O₂^-), H₂O₂, ¹O₂, and ^•OH are the reactive species involved in CBZ degradation, while ^•O₂^- made a major contribution. In addition, the sunlight-driven CdS QDs/Ti₃C₂T_x photocatalytic system is widely suitable for the elimination of different emerging pollutants in various water matrices, suggesting its potential practical environmental applications.

Electro-adsorption of Cs(I) ions from aqueous solution by capacitive deionization using ACC/MoO3 composite electrode

The separation of Cs⁺ from radioactive wastes is of paramount importance, but still a challenge. In this paper, ACC/MoO₃ composite electrode was prepared and used for the separation of Cs⁺ by capacitive deionization (CDI). The electrode materials were characterized by SEM-EDS, FTIR, XPS before and after adsorption experiments. The composite electrode was composed of ACC and hexagonal tunnel structure of MoO₃, which had a mesoporous structure. The specific surface area, average pore diameter, total pore volume and maximal specific capacitance were 170.4 m² g^-1, 2.127 nm, 0.906 cm³ g^-1 and 76.3 F g^-1, respectively. The adsorption kinetics, isotherms, and the mass transfer process were analyzed, and the possible adsorption mechanism was proposed. The removal efficiency of Cs⁺ increased with the increase of voltage and the decrease of Cs⁺ concentration, which reached 44.7 % after 240 min when voltage was 1.2 V and Cs⁺ concentration was 5 mg L^-1. The Cs⁺ adsorption onto the ACC/MoO₃ composite was multi-layer adsorption and the adsorption to active sites (AAS) was the rate-limiting step. Overall, the ACC/MoO₃ composite was a potential electrode for Cs⁺ separation.

Oxygenated Triazine-Heptazine Heterostructure Creates an Enormous Ascension to the Visible Light Photocatalytic Hydrogen Evolution Performance of Porous C3 N4 Nanosheets

A highly efficient g-C₃ N₄ photocatalyst is developed by a novel one-pot thermal polymerization method under a salt fog environment generated by heating the aqueous solution of urea and mixed metal salts of NaCl/KCl, namely SF-CN. Thanks to the synergistic effect of the oxygenation and chemical etching of the salt fog, the obtained SF-CN is an oxygenated ultrathin porous carbon nitride with an intermolecular triazine-heptazine heterostructure, meanwhile, shows enlarged specific surface area, greatly enhanced absorption of visible light, narrowed band gap with a lower conduction band, and an increased photocurrent response due to the effective separation of photogenerated holes and electrons, comparing to those of pristine g-C₃ N₄ . The theoretical simulations further reveal that the triazine-heptazine heterostructure possesses better photocatalytic hydrogen evolution (PHE) capability than pure triazine and heptazine carbon nitrides. In turn, SF-CN demonstrates an excellent visible light PHE rate of 18.13 mmol h^-1  g^-1 , up to 259.00 times of that of pristine g-C₃ N₄ .

In Situ Polycondensation Synthesis of NiS-g-C3N4 Nanocomposites for Catalytic Hydrogen Generation from NaBH4

The nanocomposites of S@g-C₃N₄ and NiS-g-C₃N₄ were synthesized for catalytic hydrogen production from the methanolysis of sodium borohydride (NaBH₄). Several experimental methods were applied to characterize these nanocomposites such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and environmental scanning electron microscopy (ESEM). The calculation of NiS crystallites revealed an average size of 8.0 nm. The ESEM and TEM images of S@g-C₃N₄ showed a 2D sheet structure and NiS-g-C₃N₄ nanocomposites showed the sheet materials that were broken up during the growth process, revealing more edge sites. The surface areas were 40, 50, 62, and 90 m²/g for S@g-C₃N₄, 0.5 wt.% NiS, 1.0 wt.% NiS, and 1.5 wt.% NiS, respectively. The pore volume of S@g-C₃N₄ was 0.18 cm³, which was reduced to 0.11 cm³ in 1.5 wt.% NiS owing to the incorporation of NiS particles into the nanosheet. We found that the in situ polycondensation preparation of S@g-C₃N₄ and NiS-g-C₃N₄ nanocomposites increased the porosity of the composites. The average values of the optical energy gap for S@g-C₃N₄ were 2.60 eV and decreased to 2.50, 2.40, and 2.30 eV as the NiS concentration increased from 0.5 to 1.5 wt.%. All NiS-g-C₃N₄ nanocomposite catalysts had an emission band that was visible in the 410-540 nm range and the intensity of this peak decreased as the NiS concentration increased from 0.5 to 1.5 wt.%. The hydrogen generation rates increased with increasing content of NiS nanosheet. Moreover, the sample 1.5 wt.% NiS showed the highest production rate of 8654 mL/g·min due to the homogeneous surface organization.

Carbon-Based Nanomaterials for Catalytic Wastewater Treatment: A Review

Carbon-based nanomaterials (CBM) have shown great potential for various environmental applications because of their physical and chemical properties. The unique hybridization properties of CBMs allow for the tailored manipulation of their structures and morphologies. However, owing to poor solar light absorption, and the rapid recombination of photogenerated electron-hole pairs, pristine carbon materials typically have unsatisfactory photocatalytic performances and practical applications. The main challenge in this field is the design of economical, environmentally friendly, and effective photocatalysts. Combining carbonaceous materials with carbonaceous semiconductors of different structures results in unique properties in carbon-based catalysts, which offers a promising approach to achieving efficient application. Here, we review the contribution of CBMs with different dimensions, to the catalytic removal of organic pollutants from wastewater by catalyzing the Fenton reaction and photocatalytic processes. This review, therefore, aims to provide an appropriate direction for empowering improvements in ongoing research work, which will boost future applications and contribute to overcoming the existing limitations in this field.

Direct Z-scheme P-TiO2/g-C3N4 heterojunction for the photocatalytic degradation of sulfa antibiotics

The construction of direct Z-scheme heterojunctions with high photocatalytic degradation ability is important for wastewater treatment, but there are still many unsolved challenges. In this article, we report the fabrication of a Z-scheme P-TiO₂/g-C₃N₄ (CNPT-X) heterostructure by the calcination method. Under simulated sunlight, CNPT-X composites are found to show excellent degradation performance against sulfonamide antibiotics sulfadiazine (SD), sulfamethazine (SM2), sulfamonomethoxine (SMM), and sulfamethoxazole (SMZ). CNPT-3 (400 mg L^-1) can be used to degrade four sulfa antibiotics within 90 min, with a degradation rate as high as 99%, which is higher than that for P-TiO₂ and g-C₃N₄ alone. The internal electron transfer paths and mechanisms for the composites are revealed by ESR radical detection experiments, XPS energy spectrum shifts, valence band positions and active material quenching experiments. Furthermore, the degradation products are analyzed by GC-MS, and four possible degradation pathways for sulfonamide pollutants are proposed. This photocatalyst provides new insights into the fundamental aspects of the photocatalytic degradation mechanism for composite pollutants, as well as new ideas for practical environmental applications.

F,O,S-Codoped Graphitic Carbon Nitride as an Efficient Photocatalyst for the Synthesis of Benzoxazoles and Benzimidazoles

Graphitic carbon nitride (g-C3N4) is a metal-free heterogeneous catalyst that has attracted attention because of its good chemical stability, cost-effectiveness, and environmental friendliness. In the work presented herein, F,O,S-codoped carbon nitride was synthesized by heating a mixture of melamine cyanurate and trifluoromethanesulfonic acid at 550 °C for 50 min. The obtained product was characterized by transmission electron microscopy, infrared spectroscopy, X-ray powder diffraction, CHNS elemental analysis, total combustion-ion chromatography, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, and UV-Vis spectroscopy. Results point to an F,O,S-codoped g-C3N4. The material was applied as a photocatalyst for the formation of benzoxazoles and benzimidazoles by condensation–aromatization of 2-aminophenol or 1,2-phenylenediamine with suitable aldehydes (viz. benzaldehyde, 4-chlorobenzaldehyde, 2-naphthaldehyde, 2-hydroxybenzaldehyde, and 2-methoxybenzaldehyde), obtaining yields of up to 90% in 15 min under visible light irradiation, with good selectivity and reusability. Thus, the reported findings suggest that this F,O,S-codoped g-C3N4 may hold promise as a metal-free photocatalyst for the rapid synthesis of 2-arylbenzoxazoles and 2-arylbenzimidazoles.

Facile synthesis of CuO/g-C3N4 nanolayer composites with superior catalytic reductive degradation behavior

The cupric oxide (CuO) loaded graphitic carbon nitride (g-C₃N₄) nanocomposites (CuO/g-C₃N₄) were prepared by a facile calcination method. The formation of monoclinic CuO nanocrystals along with g-C₃N₄ was confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopic analysis. X-ray photoelectron spectral (XPS) analysis further confirms the formation of CuO/g-C₃N₄. Distribution of CuO stone-like crystalline nanoparticles on g-C₃N₄ nanosheets was observed by transmission electron microscopic images. The influence of CuO loading on the optical property of g-C₃N₄ was determined by ultraviolet (UV)-visible absorption and photoluminescence (PL) spectral analysis. Band gap was decreased from 2.7 to 2.3 eV by the addition of CuO nanoparticles. The catalytic performance of the synthesized samples in 4-nitrophenol (4-NP) and methyl orange (MO) reduction was evaluated. The 5 wt% CuO/g-C₃N₄ showed 99.5% (7 min) and 99.7% (4 min) reduction efficiency for 4-NP and MO respectively. The 5 wt% CuO/g-C₃N₄ could become a potential catalyst in the chemical treatment of organic pollutants.

Emerging and Promising Multifunctional Nanomaterial for Textile Application Based on Graphitic Carbon Nitride Heterostructure Nanocomposites

Nanocomposites constructed with heterostructures of graphitic carbon nitride (g-C₃N₄), silver (Ag), and titanium dioxide (TiO₂) have emerged as promising nanomaterials for various environmental, energy, and clinical applications. In the field of textiles, Ag and TiO₂ are already recognized as essential nanomaterials for the chemical surface and bulk modification of various textile materials, but the application of composites with g-C₃N₄ as a green and visible-light-active photocatalyst has not yet been fully established. This review provides an overview of the construction of Ag/g-C₃N₄, TiO₂/g-C₃N₄, and Ag/TiO₂/g-C₃N₄ heterostructures; the mechanisms of their photocatalytic activity; and the application of photocatalytic textile platforms in the photochemical activation of organic synthesis, energy generation, and the removal of various organic pollutants from water. Future prospects for the functionalization of textiles using g-C₃N₄-containing heterostructures with Ag and TiO₂ are highlighted.

Synergy of MoO2 with Pt as Unilateral Dual Cocatalyst for Improving Photocatalytic Hydrogen Evolution over g-C3 N4

Pt is usually used as cocatalyst for g-C₃ N₄ to produce H₂ by photocatalytic splitting of water. However, the photocatalytic performance is still limited by the fast recombination of photo-generated electrons and holes, as well as the poor absorption of visible light. In this work, MoO₂ /g-C₃ N₄ composites were prepared, in which MoO₂ synergetic with Pt photo-deposited during H₂ evolution reaction worked as unilateral dual cocatalyst to improve the photocatalytic activity. Within 4 hours of irradiation, the hydrogen production rate of MoO₂ -Pt dual cocatalyst modified g-C₃ N₄ reached 3804.89 μmol/g/h, which was 120.18 times of that of pure g-C₃ N₄ (GCN, 31.66 μmol/g/h), 10.98 times of that of MoO₂ modified g-C₃ N₄ (346.39 μmol/g/h), and 9.18 times of that of Pt modified g-C₃ N₄ (413.64 μmol/g/h). Characterization results demonstrate that the deficient MoO₂ not only promoted visible light absorption of g-C₃ N₄ , but also worked as a "electron pool" to capture and transfer electrons to Pt.

Recent Advances in g-C3N4-Based Materials and Their Application in Energy and Environmental Sustainability

Graphitic carbon nitride (g-C₃N₄), with facile synthesis, unique structure, high stability, and low cost, has been the hotspot in the field of photocatalysis. However, the photocatalytic performance of g-C₃N₄ is still unsatisfactory due to insufficient capture of visible light, low surface area, poor electronic conductivity, and fast recombination of photogenerated electron-hole pairs. Thus, different modification strategies have been developed to improve its performance. In this review, the properties and preparation methods of g-C₃N₄ are systematically introduced, and various modification approaches, including morphology control, elemental doping, heterojunction construction, and modification with nanomaterials, are discussed. Moreover, photocatalytic applications in energy and environmental sustainability are summarized, such as hydrogen generation, CO₂ reduction, and degradation of contaminants in recent years. Finally, concluding remarks and perspectives on the challenges, and suggestions for exploiting g-C₃N₄-based photocatalysts are presented. This review will deepen the understanding of the state of the art of g-C₃N₄, including the fabrication, modification, and application in energy and environmental sustainability.

Photoelectrochemical water oxidation over TiO2 nanotubes modified with MoS2 and g-C3N4

TiO₂ nanotube arrays (TNAs) have been studied for photoelectrochemical (PEC) water splitting. However, there are two major barriers of TNAs, including a low photo-response and the fast charge carrier recombination in TNAs, leading to poor photocatalytic efficiency. Through a comparison of MoS₂/TNAs and g-C₃N₄/TNAs, it was found that TNAs modified with MoS₂ and g-C₃N₄ exhibited a current density of, respectively, 210.6 and 139.6 μA·cm^-2 at an overpotential of 1.23 V vs RHE, which is 18.2 and 12 times higher than that of pure TNAs under the same conditions. The stability of the MoS₂/TNAs heterojunction is higher than that of g-C₃N₄/TNAs.

Direct Z-Scheme g-C3N5/Cu3TiO4 Heterojunction Enhanced Photocatalytic Performance of Chromene-3-Carbonitriles Synthesis under Visible Light Irradiation

In order to make the synthesis of pharmaceutically active carbonitriles efficient, environmentally friendly, and sustainable, the method is regularly examined. Here, we introduce a brand-new, very effective Cu3TiO4/g-C3N5 photocatalyst for the production of compounds containing chromene-3-carbonitriles. The direct Z-Scheme photo-generated charge transfer mechanism used by the Cu3TiO4/g-C3N5 photocatalyst results in a suppressed rate of electron-hole pair recombination and an increase in photocatalytic activity. Experiments showed that the current method has some advantages, such as using an environmentally friendly and sustainable photocatalyst, having a simple procedure, quick reaction times, a good product yield (82–94%), and being able to reuse the photocatalyst multiple times in a row without noticeably decreasing its photocatalytic performance.

Influences of different carbon substrates on the morphologies of carbon/g-C3N4 photocatalytic composites and the purification capacities of different composites in the weak UV underwater environment

In order to explore the influence of various carbon introduction on the morphology and photodegradation performance of C/g-C₃N₄ composites, three kinds of different carbon materials: carbon nanotubes (CNT), graphene (GN) and carbon fibers (CF) were introduced to modify g-C₃N₄, and the morphologies, light absorption capacities and the underwater purifications of the composite photocatalysts were investigated. Results showed that the composites synthesized with different carbon substrates shows great differences in growth morphology. In addition, the introduction of various carbon sources also has a great impact on the physical and chemical properties of the composites. Compared with GN/g-C₃N₄ and CF/g-C₃N₄, CNT/g-C₃N₄ shows strong light absorption ability, especially in long-wavelength region (570-660 nm). To further study the difference of degradation ability of the composites in the underwater environment, the purification performance of modified g-C₃N₄ at different water depths were carried out. The results show that under 40 cm of water, where the light intensity and ultra violet spectral are seriously attenuated, the purification efficiency of CNT/g-C₃N₄ at 40 cm is 3.35 times than that of g-C₃N₄. This work provides insight in the design of highly efficient metal-free photocatalysts for the environmental remediation.

Photosynthesis and growth of Amaranthus tricolor under strontium stress

Amaranthaceae are effective plants for cleaning soil contaminated by heavy metals and radionuclides. In this paper, Amaranthus tricolor was used to investigate the response of the plant photosynthesis to various concentration of strontium ions (0.2, 0.6, 3 and 6 mM), in order to determine the possibility of A. tricolor to remediate strontium contamination. The results showed that strontium ions (0.2-6 mM) had effect on light energy conversion and utilization in A. tricolor. Low level of strontium (0.2 mM) promoted the energy utilization in A. tricolor, while higher Sr concentration (3 mM or higher) increased the excess light energy in the plants. Under strontium stress of 6 mM, the acceptor side of PSII in A. tricolor leaves was more vulnerable to strontium stress than the donor side. Furthermore, strontium stress led to accumulation of Q_A^- and block in Q_B downstream of the electron transfer chain in PSII of A. tricolor leaves. The tolerance ability of A. tricolor to strontium and remediation is also reflected in its biomass and strontium content in plants. Strontium at 3 mM or below promoted the growth of A. tricolor, while higher concentration inhibited the plant growth, but without obvious wilting or curling of leaves. The maximal dry weight increased by 36.29% in shoots, and 60.14% in roots when the spiked-strontium concentration reached 0.2 mM. The maximal strontium content achieved 8.75 mg/g dry wt in shoots, and 1.71 mg/g dry wt in roots respectively, when strontium concentration was 6 mM. Transfer factors (TFs: ratio of Sr content in shoots to that in roots) of strontium in A. tricolor ranged from 2.85 to 5.93, while bio-concentration factors (BCFs: ratio of Sr content in shoots to that in solutions) ranged from 22.57 to 49.66. In summary, A. tricolor showed the excellent potential to remediate strontium contamination.

Rethinking of the intraparticle diffusion adsorption kinetics model: Interpretation, solving methods and applications

Adsorption is a widely used unit process in various fields, such as chemical, environmental and pharmaceutical, etc. The intraparticle diffusion adsorption kinetics model is one of the most widely used adsorption kinetics models. However, the application and solving method of this model have yet to be discussed. This model has two forms (q_t = kt^1/2 and q_t = kt^1/2 + constant, where q_t is the adsorption capacity at time t, k and constant are the model parameters), which have not been unified yet. Moreover, the interpretation of this kinetics model lacks a theoretical basis (if the line passes through the origin point (0, 0), the adsorption is dominated by the intraparticle diffusion; if not, it is a multiple adsorption process). In this study, we analyzed the proper equations of the intraparticle diffusion model and their applications, discussed the interpretation of the mass transfer steps revealed by this model, and provided the solving methods. The result indicated that the piecewise function q_t = k₁t^1/2 (0 ≤ t ≤ t₁); q_t - q_t = t1 = k₂(t - t₁)^1/2 (t₁ < t ≤ t₂) is the proper form of this model. The adsorbate diffusion in the pores inside the adsorbent is the mass transfer step revealed by this model. The statistical parameters should be used to evaluate the fitting results instead of judging whether the model lines pass through the origin point (0, 0). We provide the solving methods to use the Origin and Microsoft EXCEL software to solve the model. Our study established the method for application of the intraparticle diffusion model.

Plasma Ag-Modified α-Fe2O3/g-C3N4 Self-Assembled S-Scheme Heterojunctions with Enhanced Photothermal-Photocatalytic-Fenton Performances

Low spectral utilization and charge carrier compounding limit the application of photocatalysis in energy conversion and environmental purification, and the rational construction of heterojunction is a promising strategy to break this bottleneck. Herein, we prepared surface-engineered plasma Ag-modified α-Fe₂O₃/g-C₃N₄ S-Scheme heterojunction photothermal catalysts by electrostatic self-assembly and light deposition strategy. The local surface plasmon resonance effect induced by Ag nanoparticles broadens the spectral response region and produces significant photothermal effects. The temperature of Ag/α-Fe₂O₃/g-C₃N₄ powder is increased to 173 °C with irradiation for 90 s, ~3.2 times higher than that of the original g-C₃N₄. The formation of 2D/2D structured S-Scheme heterojunction promotes rapid electron-hole transfer and spatial separation. Ternary heterojunction construction leads to significant enhancement of photocatalytic performance of Ag/α-Fe₂O₃/g-C₃N₄, the H₂ photocatalytic generation rate up to 3125.62 µmol g^-1 h^-1, which is eight times higher than original g-C₃N₄, and the photocatalytic degradation rate of tetracycline to reach 93.6%. This thermally assisted photocatalysis strategy improves the spectral utilization of conventional photocatalytic processes and provides new ideas for the practical application of photocatalysis in energy conversion and environmental purification.

g-C3N4-Based Direct Z-Scheme Photocatalysts for Environmental Applications

Photocatalysis represents a promising technology that might alleviate the current environmental crisis. One of the most representative photocatalysts is graphitic carbon nitride (g-C3N4) due to its stability, cost-effectiveness, facile synthesis procedure, and absorption properties in visible light. Nevertheless, pristine g-C3N4 still exhibits low photoactivity due to the rapid recombination of photo-induced electron-hole (e−-h+) pairs. To solve this drawback, Z-scheme photocatalysts based on g-C3N4 are superior alternatives since these systems present the same band configuration but follow a different charge carrier recombination mechanism. To contextualize the topic, the main drawbacks of using g-C3N4 as a photocatalyst in environmental applications are mentioned in this review. Then, the basic concepts of the Z-scheme and the synthesis and characterization of the Z-scheme based on g-C3N4 are addressed to obtain novel systems with suitable photocatalytic activity in environmental applications (pollutant abatement, H2 production, and CO2 reduction). Focusing on the applications of the Z-scheme based on g-C3N4, the most representative examples of these systems are referred to, analyzed, and commented on in the main text. To conclude this review, an outlook of the future challenges and prospects of g-C3N4-based Z-scheme photocatalysts is addressed.

Waste Biomass Selective and Sustainable Photooxidation to High-Added-Value Products: A Review

Researchers worldwide seek to develop convenient, green, and ecological production processes to synthesize chemical products with high added value. In this sense, lignocellulosic biomass photocatalysis is an excellent process for obtaining various outcomes for the industry. One issue of biomass transformation via heterogeneous catalysis into valuable chemicals is the selection of an adequate catalyst that ensures high conversion and selectivity at low costs. Titanium oxide (TiO2), is widely used for several applications, including photocatalytic biomass degradation, depolymerization, and transformation. Graphite carbon nitride (g-C3N4) is a metal-free polymeric semiconductor with high oxidation and temperature resistance and there is a recent interest in developing this catalyst. Both catalysts are amenable to industrial production, relatively easy to dope, and suited for solar light absorption. Recent investigations also show the advantages of using heterojunctions, for biomass derivates production, due to their better solar spectrum absorption properties and, thus, higher efficiency, conversion, and selectivity over a broader spectrum. This work summarizes recent studies that maximize selectivity and conversion of biomass using photocatalysts based on TiO2 and g-C3N4 as supports, as well as the advantages of using metals, heterojunctions, and macromolecules in converting cellulose and lignin. The results presented show that heterogeneous photocatalysis is an interesting technology for obtaining several chemicals of industrial use, especially when using TiO2 and g-C3N4 doped with metals, heterojunctions, and macromolecules because these modified catalysts permit higher conversion and selectivity, milder reaction conditions, and reduced cost due to solar light utilization. In order to apply these technologies, it is essential to adopt government policies that promote the use of photocatalysts in the industry, in addition to encouraging active collaboration between photooxidation research groups and companies that process lignocellulosic biomass.