Surface defective g-C3N4-xClx with unique spongy structure by polarization effect for enhanced photocatalytic removal of organic pollutants

A critical review on non-metal doped g-C3N4 based photocatalyst for organic pollutant remediation with sustainability assessment by life cycle analysis

Photocatalysis is recognized to be one of the most promising ways to address energy and environmental issues by utilizing visible light. Graphitic carbon nitride (g-C3N4), with a moderate band gap (∼2.7 eV) has been the flashpoint in environmental photocatalysis as it can work better under visible light, can be synthesized by a facile synthesis process using low-cost materials, thermally and chemically stable. Still the photocatalytic performance of g-C3N4 is not satisfactory because of certain limitations such as insufficient visible light absorption capacity, low electron-hole separation efficiency, high recombination rate, poor surface area. Introduction of doping, band structure engineering, defecting and designing of heterojunction, composites etc. were investigated to amplify its applications. Among all these modifications, elemental doping is a suitable and successful alternative for the enhancement of the photocatalytic activity by changing the optical and electronic properties. This review emphasizes on advancement and trends of elemental doping and its application on photocatalytic organic pollutant remediation in aqueous medium. The fundamental photocatalytic activity of heterogeneous photocatalysis and specifically g-C3N4-based photocatalysis have been discussed. The benfits of non-metal doping, enhanced photocatalytic performance by doping element, mechanism invloved in doping, advantages of co-doping has been explained. Mono, bi, and tri non-metal doped g-C3N4 and their application for the removal of organic pollutants from water medium by visible light photocatalysis has been summerized. Life cycle assessment (LCA) of photocatalytic system has been highlighted. Future research should focus on the large-scale application of the photocatalysis process considering the economic aspects. A rigorous life cycle assessment for deploying the non-metal doped g-C3N4-based photocatalysis technology for successful commercial application is recommended.

Simultaneously Utilizing Excited Holes and Electrons for Piezoelectric-Enhanced Photoproduction of H2O2 from S-Scheme 2D S-Doped VOx/g-C3N4 Nanostructures

2D/2D step-scheme (S-scheme) piezo-photocatalysts for the production of fine chemicals, such as hydrogen peroxide (H2O2), have attracted significant attention of global scientists owing to the efficiency in utilizing surface piezoelectric effects from 2D materials to overcome rapid charge recombination in photocatalytic processes. In this research, we reported the fabrication of 2D S-doped VOx deposited on 2D g-C3N4 to produce H2O2 via the piezo-photocatalytic process with high production yields at 20.19 mmol g-1 h-1, which was 1.75 and 4.87 times higher than that from solely piezo-catalytic and photocatalytic H2O2 generation. The finding pointed out that adding sulfur (S) to VOx can help to improve the catalytic outcomes by modifying the electronic properties of pristine VOx. In addition, when coupled with g-C3N4, the presence of S limits the formation of graphene in the VOx/g-C3N4 composites, causing shielding effects and pushing the cascade reactions toward water generation in the materials. Besides, the research also sheds light on the charge transport between g-C3N4 and S-VOx under irradiation and how the composites work to trigger the formation of H2O2. The presence of S in the composite systems enhances charge transfer between two semiconductors by strengthening the internal electric fields (IEF) to drive electrons moving in one direction, as demonstrated by density functional theory (DFT) calculations. Moreover, the formation of H2O2 significantly relies on the reduction of oxygen to generate oxygenic radical species at the g-C3N4 sites. Meanwhile, S-VOx provides oxidative sites in the composites to oxidize water molecules to directly or indirectly generate H2O2 or O2, which will further participate in the reactions to produce the final products. This study confirms the validation of S-scheme piezo-photocatalysts, thus encouraging further research on developing heterojunction materials with high catalytic efficiency, which can be used in practical conditions.

A magnetically reusable Ce-MOF/GO/Fe3O4 composite for effective photocatalytic degradation of chlortetracycline

Herein, we report a novel 1/GO/Fe3O4 photocatalyst, comprising Ce(BTB)(H2O) (MOF-1, H3BTB = 1,3,5-benzenetrisbenzoic acid), graphene oxide (GO), and iron oxide (Fe3O4) for photocatalytic degradation of chlortetracycline (CTC). This design enables the effective transfer of electrons from the MOF to GO, thereby reducing the photoelectron-hole recombination rate. Therefore, the optimized 1/GO/Fe3O4 photocatalyst with H2O2 shows the highest photocatalytic activity toward CTC. The kinetic constant is 5.4 times that in the system of MOF-1 and hydrogen peroxide, which usually acted as efficient electron acceptors to improve the photocatalytic performance of MOFs. More importantly, light absorption is extended from the ultraviolet to the visible region. Furthermore, 1/GO/Fe3O4 can be quickly recycled under an applied magnetic field and displays outstanding stability and reusability. According to the radical trapping experiments and electron paramagnetic resonance results, hydroxyl radicals, superoxide radicals, and holes all contribute to excellent photocatalytic activity. The possible catalytic mechanism of 1/GO/Fe3O4 is tentatively proposed. This work aims to explore the synergistic effect between metal-organic frameworks (MOFs) and GO, and provide a theoretical basis for MOF-based composites to remove antibiotic contaminants in the environment.

Theoretical verification on adsorptive removal of caffeine by carbon and nitrogen-based surfaces: Role of charge transfer, π electron occupancy, and temperature

Eliminating an emerging water pollutant, caffeine molecules, from an aqueous solution using carbon and nitrogen-based adsorbents is of significant interest to public health. These adsorbents have been shown to have decent adsorption capacity toward caffeine due to their surface functionality. Therefore, screening various carbon and nitrogen-based surfaces can be a better option for high-performance adsorbents to remove caffeine efficiently from wastewater. Herein, we present combined first principles and molecular dynamics quantification of the adsorption enthalpies of caffeine molecules on the possible active sites of carbon and nitrogen-based adsorbents (graphene, phagraphene, graphdiyne, single-wall carbon nanotube, fullerene, and graphitic carbon nitride) with the incorporation of Van der Waals interactions. From the DFT calculations, N-doped carbon surfaces show the highest adsorption energies of single and dimer CAF compared to pristine carbon-based adsorbents. A charge density difference and Bader charge analysis display that high charge transfer occurs between the caffeine's oxygen and the surface's nitrogen atoms. An abundance of π-electrons from the nitrogen atoms, composed of large electron clouds of aromatic rings on the graphitic carbon surface, tends to favor extensive π-π interactions with the caffeine molecule. The high value of pz electron occupancy (1.445) of N in the hexagonal ring of the graphitic surface transfers additional charge transfer, which leads to strong adsorption energy of CAF than pristine surfaces. Also, the g-C3N4 surface adsorbs the CAF molecule with higher adsorption than other N-doped carbon surfaces due to the high pz_eo (1.5448) of N atoms on the surface. At 310 K, the water molecules' kinetics aids the single and dimer caffeine molecules to adsorb with the highest adsorption energies on the active sites of g-C3N4 surfaces than graphene adsorbent.

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.

In-situ construction of Zr-based metal-organic framework core-shell heterostructure for photocatalytic degradation of organic pollutants

Photocatalysis is an eco-friendly promising approach to the degradation of textile dyes. The majority of reported studies involved remediation of dyes with an initial concentration ≤50 mg/L, which was away from the existing values in textile wastewater. Herein, a simple solvothermal route was utilized to synthesize CoFe2O4@UiO-66 core-shell heterojunction photocatalyst for the first time. The photocatalytic performance of the as-synthesized catalysts was assessed through the photodegradation of methylene blue (MB) and methyl orange (MO) dyes at an initial concentration (100 mg/L). Under simulated solar irradiation, improved photocatalytic performance was accomplished by as-obtained CoFe2O4@UiO-66 heterojunction compared to bare UiO-66 and CoFe2O4. The overall removal efficiency of dyes (100 mg/L) over CoFe2O4@UiO-66 (50 mg/L) reached >60% within 180 min. The optical and photoelectrochemical measurements showed an enhanced visible light absorption capacity as well as effective interfacial charge separation and transfer over CoFe2O4@UiO-66, emphasizing the successful construction of heterojunction. The degradation mechanism was further explored, which revealed the contribution of holes (h+), superoxide (•O2 -), and hydroxyl (•OH) radicals in the degradation process, however, h+ were the predominant reactive species. This work might open up new insights for designing MOF-based core-shell heterostructured photocatalysts for the remediation of industrial organic pollutants.

Photocatalytic degradation of paracetamol and bisphenol A by chitosan supported covalent organic framework thin film with visible light irradiation

Covalent Organic Frameworks (COFs) have attracted extensive attention for the photocatalytic degradation of emerging organic contaminants. The difficulty in separation and recovery after use yet would hinder the practical application of COFs in powder form. In present study, COFs in film form were fabricated via using chitosan as the film-substrate to support COFs (CSCF). We found that CSCF could effectively degrade two types of emerging organic contaminants under visible light irradiation. Particularly, CSCF could effectively degrade 99.8% of paracetamol (PCT) and 94.0% of bisphenol A (BPA) within 180 min under visible light irradiation. •O₂^- and h⁺ played dominant roles during the photocatalytic degradation process. Hydroxylation and cleavage were the main degradation processes. CSCF exhibited good photocatalytic degradation performance in a broad range of ionic strengths, in the presence of common coexisting ions including Cl^-, NO₃^- and SO₄^2-, in a wide range of pH (5-11), and in real water samples including tap water, river water and lake water. Moreover, CSCF could be easily collected after use and exhibited excellent degradation performance in five successive cycles. CSCF has potential applications to treat water with either PCT or BPA contamination. This study provided a new insight into the practical application of COFs.

Cl-based functional group modification MIL-53(Fe) as efficient photocatalysts for degradation of tetracycline hydrochloride

MIL-53(Fe) catalyst has been widely used to treat the pollutants in water. However, the limited number of electrons in MIL-53(Fe) catalyst has always affected the rate at which Fe³⁺ can be reduced to Fe²⁺. We modulated iron-based metal-organic frameworks (MOFs) using organic ligands modified with chlorine functional groups. The characterization results indicate that the 2Cl-MIL-53(Fe) catalyst exhibited the optimal photoelectric properties while maintaining the original structural characteristics. The experimental analyses and the first-principles study suggest that the introduction of a chlorine functional group not only reduced the band gap width and enhanced the visible-light absorption capacity, but also significantly enhanced the electron cloud density of Fe-O clusters. This could further accelerate the redox cycle of Fe(III)/Fe(II), beneficial for H₂O₂ activation. The constructed Cl-MIL-53(Fe) catalyst exhibited a 3.8 times higher reaction rate constant than pure MIL-53(Fe) catalyst. The specific TCH degradation pathway and mechanism of 2Cl-MIL-53(Fe) treatment are proposed. This study provides a new strategy for iron-based MOFs as a heterogeneous photo-Fenton catalyst to degrade pollutants in water.

The tremendous boost for photocatalytic properties of g-C3N4: regulation from polymerization kinetics to crystal structure engineering

Graphite carbon nitride (g-C3N4) has become research hotspot owing to its special electronic structure and excellent chemical stability. Although g-C3N4 has made great progress in the field of photocatalysis, its...

Enhanced degradation of chloramphenicol through peroxymonosulfate and visible light over Z-scheme Photocatalysts: Synergetic performance and mechanism insights

Effective removal of antibiotics in the environment can be a demanding issue concerning the ecosystem and human health. Photocatalysis and peroxymonosulfate (PMS) oxidation have become important methods to effectively remove stubborn pollutants. In this work, by integrating these two technologies, an efficient system for degrading chloramphenicol (CAP) in water was proposed. The system was constructed by coupling strontium-doped lanthanum cobaltate (LSCO5) with chlorine-doped carbon nitride (CGCN). By doping, the increase of oxygen vacancy and the adjustment of bandgap were realized. Photoluminescence and electrochemical impedance experiments showed that the heterojunction can promote electron transfer and photogenerated carrier separation. Under the synergistic effect of PMS oxidation and photocatalysis, the prepared composite with an optimal loading of 40% LSCO5 can degrade 95.6% of CAP within 20 min. Degradation experiments on different pollutants proved the versatility of the catalytic system. The enhanced degradation mechanism of CAP was explored based on the assessment of the degradation efficiency of CAP, electron paramagnetic resonance (EPR), and quenching experiments. Through liquid chromatography-mass spectrometry (LC-MS) analysis, a possible route for CAP degradation was also proposed. This research provides some inspiration for the remediation of polluted water with perovskite-based catalyst under the synergistic effect of PMS and photocatalysis.

A facile oxygen vacancy and bandgap control of Bi(OH)SO4·H2O for achieving enhanced photocatalytic remediation

The development of highly efficient photocatalysts is crucial for the remediation of organic pollutants. Herein, we reported a facile synthesis of oxygen vacancy rich Bi(OH)SO₄·H₂O photocatalyst by the control of precursor. The samples were characterized by XRD, scanning electron microscope, electron paramagnetic resonance, X-ray photoelectron spectroscopy etc. With more oxygen vacancies introduced, the photocatalytic activity on the degradation of RhB and tetracycline was significantly boosted. Density functional theory calculation was used to further reveal the influence of oxygen vacancy on the band structure of Bi(OH)SO₄·H₂O. The results and finding of this work are helpful for the development of sustainable environmental protection.

Design of highly efficient g-C3N4-based metal monoatom catalysts by two extra-NM1 atoms: density functional theory simulations

Graphitic carbon nitride (g-C3N4) is recognized as a favorable substrate for monoatom catalysts due to its uniform nanoholes for anchoring metal monoatoms, while the oxygen evolution reaction (OER) overpotential (ηOER) values of g-C3N4-based metal monoatom catalysts are still large. To reduce the ηOER values, a class of novel TM1NM1NM1/g-C3N4 was designed via density functional theory simulations, where TM1 = Fe1, Co1 or Ni1 and NM1 = C1, N1 or O1. Contributing by two extra-NM1 atoms, the OER catalytic activities of these materials were effectively improved owing to the shortened TM1-NM bonds and weakened chemical activity of TM1 atoms. Based on the volcano activity relationship between the theoretical overpotential (ηOER) and d band center of the TM1 atom (εd), the chemical activity of TM1 atoms needs to be adjusted to a suitable magnitude (εd near -4.883 eV) for a good catalytic activity. The designed Fe1C1O1/g-C3N4 with the εd of -4.893 eV exhibited an excellent OER catalytic activity of ηOER = 0.219 V. This strategy was applied to devise the reaction active sites and highly efficient catalysts by adjusting the chemical activity of the TM1 atom with suitable extra-NM1 atoms.

Fabrication of heterostructured Ag/AgCl@g-C3N4@UIO-66(NH2) nanocomposite for efficient photocatalytic inactivation of Microcystis aeruginosa under visible light

In this work, a novel Ag/AgCl@g-C₃N₄@UIO-66(NH₂) heterojunction was constructed for photocatalytic inactivation of Microcystis aeruginosa (M. aeruginosa) under visible light. The photocatalyst was synthesized by a facile method and characterized by XRD, SEM, TEM, BET, XPS, FT-IR, UV-vis DRS, PL and EIS. The nanocomposite can not only provide lots of active sites, but also improve capacities to utilize visible-light energy and effectively transfer charge carriers, thus enhancing removal efficiencies of cyanobacteria (99.9% chlorophyll a was degraded within 180 min). Various factors in photodegradation of chlorophyll a were studied. Besides, changes on cellular morphologies, membrane permeability, physiological activities of M. aeruginosa during photocatalysis were investigated. Moreover, the cycle test indicated that Ag/AgCl@g-C₃N₄@UIO-66(NH₂) exhibits excellent reusability and photocatalytic stability. Finally, a possible mechanism of M. aeruginosa inactivation was proposed. In a word, Ag/AgCl@g-C₃N₄@UIO-66(NH₂) can efficiently inactivate cyanobacteria under visible light, thus providing useful references for further removal of harmful algae in real water bodies.

Copolymerization of urea and murexide for efficient photocatalytic hydrogen evolution and tetracycline degradation

A g-C₃N₄ based material modified with murexide has been prepared and used for photocatalytic hydrogen production and degradation of tetracycline.

High crystallinity and conjugation promote the polarization degree in O-doped g-C3N4 for removing organic pollutants

High crystallinity and extended conjugated system improve the polarization of O-doped g-C₃N₄, which efficiently promotes carrier separation.

High carrier separation efficiency for a defective g-C3N4 with polarization effect and defect engineering: mechanism, properties and prospects

Different types of defects in g-C3N4 induce polarization effect to promote the separation of charge carriers and improve the photocatalytic efficiency.

A fluorine induced enhancement of the surface polarization and crystallization of g-C3N4 for an efficient charge separation

A synergy of high crystallinity and surface polarization constructed by F doping dramatically promotes charge separation efficiency, significantly enhancing photocatalytic activity.

Ag2O and NiO Decorated CuFe2O4 with Enhanced Photocatalytic Performance to Improve the Degradation Efficiency of Methylene Blue

Dye wastewater is a serious threat to human health and life. It is an important task for researchers to treat it efficiently. Among many treatment methods, the photo-Fenton method can rapidly degrade organic pollutants. In this study, a ternary photocatalyst, Ag2O-NiO/CuFe2O4, was prepared and applied for a photo-Fenton reaction to degrade methylene blue (MB). MB had the best degradation effect when 10 mg of the catalyst were used in an 80 mL reaction system for measurement. The degradation rate of MB was up to 96.67% in 60 min with a high degradation rate constant k=5.67×10-2min-1. The total organic carbon (TOC) degradation rate was 78.64% with a TOC degradation rate constant of k=2.57×10-2min-1. Therefore, this study fully proves that Ag2O-NiO/CuFe2O4 can catalyze the photo-Fenton reaction and effectively degrade MB.

Surface defect-abundant one-dimensional graphitic carbon nitride nanorods boost photocatalytic nitrogen fixation

Defective g-C₃N₄ nanorods enable to boots the adsorption and cleavage of N₂ molecules to achieve higher photocatalytic nitrogen fixation performance.