Novel carbon and defects co-modified g-C3N4 for highly efficient photocatalytic degradation of bisphenol A under visible light

Comparative study of photocatalysis with bulk and nanosheet graphitic carbon nitrides enhanced with silver

The main goal of this research is to investigate the effectiveness of graphitic carbon nitride (g-C3N4, g-CN) in both bulk and nanosheet forms, which have been surface-modified with silver nanoparticles (Ag NPs), as photocatalysts for the degradation of acid orange 7 (AO7), a model dye. The photodegradation of AO7 dye molecules in water was used to test the potential photocatalytic properties of these powder materials under two different lamps with wavelengths of 368 nm (UV light) and 420 nm (VIS light). To produce Ag NPs (Ag content 0.5, 1.5, and 3 wt%) on the g-CN materials, a new synthesis route based on a wet and low-temperature method was proposed, eliminating the need for reducing agents. The photodegradation activity of the samples increased with increasing silver content, with the best photocatalytic performances achieved for bulk g-CN samples and nanosheet silver-modified samples (with the highest content of 3 wt% Ag) under UV light, i.e., more than 75% and 78%, respectively. The VIS-induced photocatalytic activity of both examined series was higher than that of UV. The highest activities of 92% and 98% were achieved for the 1.5% Ag-modified g-CN bulk and nanosheet materials. This research presents an innovative, affordable, and environmentally friendly chemical approach to synthesizing photocatalysts that can be used for degrading organic pollutants in wastewater treatment.

Recent progress and prospect of graphitic carbon nitride-based photocatalytic materials for inactivation of Microcystis aeruginosa

The proliferation of harmful algal blooms is a global concern due to the risk they pose to the environment and human health. Algal toxins which are hazardous compounds produced by dangerous algae, can potentially kill humans. Researchers have been drawn to photocatalysis because of its clean and energy-saving properties. Graphite carbon nitride (g-C3N4) photocatalysts have been extensively studied for their ability to eliminate algae. These photocatalysts have attracted notice because of their cost-effectiveness, appropriate electronic structure, and exceptional chemical stability. This paper reviews the progress of photocatalytic inactivation of harmful algae by g-C3N4-based materials in recent years. A brief overview is given of a number of the modification techniques on g-C3N4-based photocatalytic materials, as well as the process of inactivating algal cells and destroying their toxins. Additionally, it provides a theoretical framework for future research on the eradication of algae using g-C3N4-based photocatalytic materials.

Decade Milestone Advancement of Defect-Engineered g-C3N4 for Solar Catalytic Applications

Over the past decade, graphitic carbon nitride (g-C3N4) has emerged as a universal photocatalyst toward various sustainable carbo-neutral technologies. Despite solar applications discrepancy, g-C3N4 is still confronted with a general fatal issue of insufficient supply of thermodynamically active photocarriers due to its inferior solar harvesting ability and sluggish charge transfer dynamics. Fortunately, this could be significantly alleviated by the "all-in-one" defect engineering strategy, which enables a simultaneous amelioration of both textural uniqueness and intrinsic electronic band structures. To this end, we have summarized an unprecedently comprehensive discussion on defect controls including the vacancy/non-metallic dopant creation with optimized electronic band structure and electronic density, metallic doping with ultra-active coordinated environment (M-Nx, M-C2N2, M-O bonding), functional group grafting with optimized band structure, and promoted crystallinity with extended conjugation π system with weakened interlayered van der Waals interaction. Among them, the defect states induced by various defect types such as N vacancy, P/S/halogen dopants, and cyano group in boosting solar harvesting and accelerating photocarrier transfer have also been emphasized. More importantly, the shallow defect traps identified by femtosecond transient absorption spectra (fs-TAS) have also been highlighted. It is believed that this review would pave the way for future readers with a unique insight into a more precise defective g-C3N4 "customization", motivating more profound thinking and flourishing research outputs on g-C3N4-based photocatalysis.

Enhanced Photocatalytic Activity of π-Conjugated Pyridine Rings-Modified C3N4/Bi@BiOCl Z-Scheme Heterogeneous Material

The enhanced photocatalytic properties of Z-Scheme Bi@BiOCl/C3N4-DPY heterojunction materials were successfully prepared by the ultrasonic-assisted coprecipitation method. The Bi@BiOCl/C3N4-DPY heterojunction exhibited remarkable photocatalytic activity under visible light irradiation, and the degradation rate of methyl orange (MO) was about 90.6% in 180 min. This impressive efficiency is mainly due to the Z-Scheme charge transfer mechanism in Bi@BiOCl/C3N4-DPY, resulting in the efficient separation of charge carriers and an increase in the REDOX potential of photogenerated electrons and holes. C3N4 was modified with a π-deficient conjugated pyridine ring, which caused the light absorption redshift, promoted the formation of oxidizing •O2-, and improved the photocatalytic activity. At the same time, a well-aligned heterojunction is formed at the interface between C3N4-DPY and BiOCl, facilitating the seamless transfer of light-induced electrons from the LUMO of C3N4-DPY to the CB of BiOCl. In addition, the addition of Bi introduces a unique band gap reduction effect, resulting in a change in the density of the band states, which further promotes charge transfer and separation. It is worth noting that the introduction of metallic bismuth (Bi) brings about a unique band gap reduction effect, resulting in a change in the density of states within the band, which ultimately promotes charge transfer and separation. The Z-scheme charge migration inside Bi@BiOCl/C3N4-DPY further promotes the efficient separation of photogenerated electron-hole pairs, greatly improving the overall efficiency of the material. The Z-structured photocatalyst developed in this study has great application potential in various fields of photocatalysis.

Methyl-terminated graphite carbon nitride with regulatable local charge redistribution for ultra-high photocatalytic hydrogen production and antibiotic degradation

Intramolecular-tailored graphite carbon nitride (g-C3N4) has great potential to greatly optimize the photo-response performance and carrier separation ability, but exquisite molecular structure engineering is still challenging. Firstly, a series of oxygen and terminal methyl moiety co-modified g-C3N4 (CNNx) has been systematically prepared by using N-Hydroxysuccinimide (HOSu) as a novel copolymerized precursor and urea. The density functional theory (DFT) calculations demonstrated that the presence of oxygen can lower the binding energy for the C-C bond to make the terminal modification easier. The terminal methyl and Oxygen not only caused abundant alveolar defects to break the periodic symmetry but also acted as an electron-accepting platform to tune the local charge redistribution within g-C3N4 molecular. The synthesized CNNx (CNN25) achieved ultra-high photocatalytic activity and chemical stability under visible light toward antibiotic degradation (99% tetracycline, 92% doxycycline, 65% ofloxacin and 74% sulfathiazole degradation within 30 min) and hydrogen production (an apparent quantum efficiency of 2.10% at 400 nm). CNN25 also maintains good efficiency in surface water and groundwater. Moreover, the TC solution treated with CNN25 had hardly any harm to the growth of E. coli. We believe our findings will provide a facile and green strategy for the preparation of non-metallic modified g-C3N4.

Microtubular cellulose-derived kapok fibre as a solid electron donor for boosting photocatalytic H2O2 production over C-doped g-C3N4 hybrid complexation

Cellulose continues to play an important and emerging role in photocatalysis, and its favourable properties, such as electron-rich hydroxyl groups, could enhance the performance of photocatalytic reactions. For the first time, this study exploited the kapok fibre with microtubular structure (t-KF) as a solid electron donor to enhance the photocatalytic activity of C-doped g-C₃N₄ (CCN) via ligand-to-metal-charge-transfer (LMCT) to improve hydrogen peroxide (H₂O₂) production performance. As confirmed by various characterisation techniques, the hybrid complex consisting of CCN grafted on t-KF was successfully developed in the presence of succinic acid (SA) as a cross-linker via a simple hydrothermal approach. The complexation formation between CCN and t-KF results in the CCN-SA/t-KF sample displaying a higher photocatalytic activity than pristine g-C₃N₄ to produce H₂O₂ under visible light irradiation. The enhanced physicochemical and optoelectronic properties of CCN-SA/t-KF imply that the LMCT mechanism is crucial in improving photocatalytic activity. This study promotes utilising the unique t-KF material's properties to develop a low-cost and high-performance cellulose-based LMCT photocatalyst.

Synthesis and up-to-date applications of 2D microporous g-C3N4 nanomaterials for sustainable development

In recent years, with the development of industrial technology and the increase of people's environmental awareness, the research on sustainable materials and their applications has become a hot topic. Among two-dimensional (2D) materials that have been selected for sustainable research, graphitic phase carbon nitride (g-C3N4) has become a hot research topic because of its many outstanding advantages such as simple preparation, good electrochemical properties, excellent photochemical properties, and better thermal stability. Nevertheless, the inherent limitations of g-C3N4 due to its relatively poor specific surface area, rapid charge recombination, limited light absorption range, and inferior dispersion in aqueous and organic media have limited its practical application. In the review, we summarize and analyze the unique structure of the 2D microporous nanomaterial g-C3N4, its synthesis method, chemical modification method, and the latest application examples in various fields in recent years, highlighting its advantages and shortcomings, with a view to providing ideas for overcoming the difficulties in its application. Furthermore, the pressing challenges faced by g-C3N4 are briefly discussed, as well as an outlook on the application prospects of g-C3N4 materials. It is expected that the review in this paper will provide more theoretical strategies for the future practical application of g-C3N4-based materials, as well as contributing to nanomaterials in sustainable applications.

Functionalized 2D defect g-C3N4 for artificial photosynthesis of H2O2 and synchronizing tetracycline fluorescence detection and degradation

Artificial photosynthesis of H₂O₂ is a clean production technology, which brings the synergistic effect to photodegradation of pollutants. Inspired by defect engineering, 2D defective carbon nitride (g-C₃N₄) photocatalyst was obtained via potassium ion assisted synthesis. Defective g-C₃N₄ is protonated and applied to photosynthesis of H₂O₂, H₂O₂ concentration produced reached 477.7 μM, which was approximately 5.27 times that by pristine g-C₃N₄. Additionally, defective g-C₃N₄ materials are borrowed to synchronizing tetracycline (TC) fluorescence detection and degradation, suggesting the catalyst existed bifunctional characteristics of TC detection and degradation. Meanwhile, metal impregnation engineering (molybdenum) was borrowed enhancing the electron-trapping ability in local region of defective g-C₃N₄, which takes advantages to the efficient degradation of TC. Furthermore, optical and electrical properties of photocatalysts were investigated in details by advanced material characterization testing. This work provides potential applications in the field of artificial photosynthesis and pollution degradation.

Photocatalytic O2 activation by metal-free carbon nitride nanotube for rapid reactive species generation and organic contaminants degradation

Advanced oxidation processes (AOPs) using oxygen (O₂) as an oxidant represent a low-cost and sustainable wastewater treatment process. Herein, a metal-free nanotubular carbon nitride photocatalyst (CN NT) was prepared to activate O₂ to degrade organic contaminants. The nanotube structure allowed for sufficient O₂ adsorption, while the optical and photoelectrochemical properties enabled photogenerated charge to be efficiently transferred to the adsorbed O₂ to trigger the activation process. The developed CN NT/Vis-O₂ system based on O₂ aeration degraded various organic contaminants and mineralized 40.7% of chloroquine phosphate within 100 min. In addition, the toxicity and environmental risk of treated contaminants were reduced. Mechanistic studies suggested that the enhanced O₂ adsorption capacity and fast charge transfer behavior on CN NT surface led to reactive·O₂^-, ¹O₂ and h⁺ generation, each of which played a distinct role in contaminants degradation. Importantly, the proposed process could overcome the interference from water matrices and outdoor sunlight, and the energy and chemical reagent savings reduced the operating cost to about 1.63 US$·m^-3. Altogether, this work provides insights into the potential application of metal-free photocatalysts and green O₂ activation for wastewater treatment.

Preparation of carbon self-doped g-C3N4 for efficient degradation of bisphenol A under visible light irradiation

In this study, visible-light-driven carbon self-doped graphitic carbon nitride photocatalyst was fabricated by a facile method with urea and ammonium citrate, and used for photodegradation of bisphenol A (BPA) in the aqueous environment. The experiments indicated that the prepared photocatalyst (C_0.02CN) showed high catalytic activity, and 96.0%, 93.2%, and 95.5% BPA could be photodegraded in 150 min under pH 3, 6, and 11, respectively. The photocatalytic degradation rate (0.018 min^-1) and mineralization (27.6%) of C_0.02CN for BPA were about 6.7 and 3.5 times higher than those of the g-C₃N₄ (0.0027 min^-1, 7.87%), respectively. C_0.02CN had high reusability with a photodegradation efficiency of 84.5% for BPA after 3 cycles. Moreover, C_0.02CN introduced additional carbon atoms, which generated C-O-C bonds in the g-C₃N₄ lattice. In contrast to g-C₃N₄, carbon doping enhanced the visible light absorption range of C_0.02CN, reduced its band gap, and improved the separation efficiency of photogenerated electron-hole pairs. Radical quenching experiment and ESR results revealed that superoxide radicals (•O₂^-) and photogenerated holes (h⁺) acted as important parts in the high photodegradation activity under visible light irradiation. This work puts forward a one-pot strategy for the preparation of carbon self-doped g-C₃N₄, displacing the high-energy consuming and complicated preparation technology with promising industrial applications.

Regulating the concentration of dissolved oxygen to achieve the directional transformation of reactive oxygen species: A controllable oxidation process for ciprofloxacin degradation by calcined CuCoFe-LDH

Different reactive oxygen species (ROS) tend to attack specific sites on pollutants, leading to the formation of intermediates with different toxic effects. Therefore, regulating the directional transformation of ROS is a new effective approach for safe degradation of refractory organic compounds in wastewater. However, the regulation mechanism and transformation path of ROS remain unclear. In this work, the dissolved oxygen (DO) content was controlled by aeration to generate different ROS through the activation of O₂ on the calcined CuCoFe-LDH (CuCoFe-300). ROS quantitative experiments and electron paramagnetic resonance proved that O₂ was mainly activated to superoxide radical (•O₂^-) and singlet oxygen (¹O₂) under low DO concentration (0.231 mmol/L) (O₂ → •O₂^- → ¹O₂). With the increasing of DO concentration (0.606 mmol/L), O₂ was inclined to convert into hydroxyl radicals (•OH) (O₂ → •O₂^- → H₂O₂ → •OH). The density functional theory and function model of active sites utilization and DO concentration built a solid proof for ROS conversion mechanism that increasing the DO concentration promotes the increase of active sites utilization on the CuCoFe-300 system. That is, the •O₂^- was more prone to convert to •OH, not ¹O₂ in thermodynamics under high active sites utilization condition. Hence, the ROS generation was controlled by regulating DO concentration, and the nontoxic degradation pathway of ciprofloxacin was well-designed. This work is dedicated to the in-depth exploration of the mechanism between DO concentration and ROS conversion, which provides an extremely flexible, low energy consumption, and environmentally friendly wastewater treatment method in a new perspective.

Fabrication of hybrid covalent triazine framework-zinc ferrite spinel to uplift visible light-driven photocatalytic organic pollutant degradation

The tunability of porous covalent triazine frameworks (CTFs) can mitigate poor photostability and rapid hole-electron recombination. Herein, an excellent improvement of visible light-driven photocatalytic pollutant degradation was achieved using a hybrid semiconductor of covalent triazine framework-zinc ferrite spinel catalysts (CTF-ZnFe₂O₄). The as-prepared CTF-ZnFe₂O₄ composites were fabricated using a facile one-pot ionothermal method. The hybrid photocatalysts were identified using X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive X-ray (SEM-EDX), X-ray photoelectron spectrometer (XPS), Brunauer-Emmett-Teller (BET), Fourier transform infrared (FTIR), and UV-visible diffuse reflection spectroscopy (UV-vis DRS) characterizations. The analysis reveals that hybridization successfully ensued and altered the crystallinity structure, morphology, surface area, and bandgap energy of hybrid material. It was found that CTF-ZnFe₂O₄ 90:10 is very effective for the degradation of MB in a UV-vis light photocatalytic process with the efficiency of 95.4% and k_obs of 0.421 min^-1 for degradation of 50 mg/L MB with 0.5 g/L dosages for 120 min. Additionally, the scavenger study, effect of additional oxidants, and stability were performed for the practical application of a hybrid photocatalyst. CTF-ZnFe₂O₄ 90:10 shows outstanding pollutant degradation in sunlight irradiation and high stability with only a 5.2% reduction after a five-times sequential recycling process. Moreover, the photocatalytic mechanism of as-prepared CTF-ZnFe₂O₄ was mainly influenced by [Formula: see text] radical compared to [Formula: see text] and [Formula: see text] radicals. Overall, The as-prepared CTF-ZnFe₂O₄ shows significant potential to be utilized for photocatalytic wastewater treatment.

Quantitatively regulating the ketone structure of triazine-based covalent organic frameworks for efficient visible-light photocatalytic degradation of organic pollutants: Tunable performance and mechanisms

As promising visible-light-responsive photocatalysts, triazine-based covalent organic frameworks (CTFs) still suffer from broad bandgap and high electron-hole recombination. As such, different contents of electron-rich ketone group were introduced to CTFs (X % keto-CTF), aiming to clarify the mechanism of quantitatively regulating ketone for enhanced visible-light photocatalytic performance of CTFs. As ketone content increased, the bandgap narrowed, electron-hole recombination decreased, charge transfer and quantum yield increased. As a result, keto-CTF outperformed other keto-CTFs in visible-light photocatalytic degradation of tetracycline, and apparent rate constant of TC (k_obs) was 3.69 times higher than that of CTF. Importantly, ketone tuning induced varied types and concentrations of reactive species. Integrated with quantitative structure-activity relationships (QSARs) analysis and density functional theory (DFT) calculations, this study unravels how ketone content regulates bandgap structure of CTF, affects the contribution of varied reactive species, and quantitatively enhances the photocatalytic performance of CTFs. It also provides novel insights into the precise design and synthesis of CTFs-based catalyst structures for high-efficient visible-light photocatalytic degradation of organic pollutants.

Facilitated Photocatalytic Degradation of Rhodamine B over One-Step Synthesized Honeycomb-Like BiFeO3/g-C3N4 Catalyst

In the present work, a facile one-step methodology was used to synthesize honeycomb-like BiFeO₃/g-C₃N₄ composites, where the well-dispersed BiFeO₃ strongly interacted with the hg-C₃N₄. The 10BiFeO₃/hg-C₃N₄ could completely degrade RhB under visible light illumination within 60 min. The degradation rate constant was remarkably improved and approximately three times and seven times that of pristine hg-C₃N₄ and BiFeO₃, respectively. This is ascribed to the following factors: (1) the unique honeycomb-like morphology facilitates the diffusion of the reactants and effectively improves the utilization of light energy by multiple reflections of light; (2) the charged dye molecules can be tightly bound to the spontaneous polarized BiFeO₃ surface to form the Stern layer; (3) the Z-scheme heterojunction and the ferroelectric synergistically promoted the efficient separation and migration of the photogenerated charges. This method can synchronously tune the micro-nano structure, surface property, and internal field construction for g-C₃N₄-based photocatalysts, exhibiting outstanding potential in environmental purification.

Photocatalytic degradation of organic pollutants in water by N-doping ZnS with Zn vacancy: enhancement mechanism of visible light response and electron flow promotion

In order to improve the visible light response, N-doping ZnS (N-ZnS) nanospheres with Zn vacancy and porous surface were prepared by a simple one-pot hydrothermal method. Characterizations and density functional theory simulations showed excellent visible light response of N-ZnS. N-doping introduced impurity energy levels, which led to orbital hybridization and changed the original dipole moment. The presence of ortho Zn vacancy (O-Znv) can effectively reduce e--h+ recombination and photocorrosion. Furthermore, O-Znv caused lattice distortion (twisted the -S-Zn-N-(O-Znv)-S-Zn-S- chemical bond chain), resulting in "vacancy effect" to accelerate e- flow. Under visible light, the photocatalytic degradation efficiency of tetracycline (TC) and 2,4-dichlorophenol (2,4-DCP) was 90.31% and 60.84%, respectively. TOC degradation efficiency was 31.4% and 25.6%, respectively. Combined with Fukui index and LC-MS methods, it was found that TC and 2,4-DCP were degraded under the constant attack of active substances such as ·OH. This work can provide a reference for the application of catalytic materials in the field of visible light photocatalysis.

Carbon nitride-based Z-scheme heterojunctions for solar-driven advanced oxidation processes

Solar-driven advanced oxidation processes (AOPs) via direct photodegradation or indirect photocatalytic activation of typical oxidants, such as hydrogen peroxide (H₂O₂), peroxymonosulfate (PMS), and peroxydisulfate (PDS), have been deemed to be an efficient technology for wastewater remediation. Artificial Z-scheme structured materials represent a promising class of photocatalysts due to their spatially separated charge carriers and strong redox abilities. Herein, we summarize the development of metal-free graphitic carbon nitride (g-C₃N₄, CN)-based direct and indirect Z-scheme photocatalysts for solar-driven AOPs in removing organic pollutants from water. In the work, the classification of AOPs, definition and validation of Z-schemes are summarized firstly. The innovative engineering strategies (e.g., morphology and dimensionality control, element doping, defect engineering, cocatalyst loading, and tandem Z-scheme construction) over CN-based direct Z-scheme structure are then examined. Rational design of indirect CN-based Z-scheme systems using different charge mediators, such as solid conductive materials and soluble ion pairs, is further discussed. Through examining the relationship between the Z-scheme structure and activity (charge transfer and separation, light absorption, and reaction kinetics), we aim to provide more insights into the construction strategies and structure modification on CN-based Z-schemes towards improving their catalytic performances in AOPs. Lastly, limitations, challenges, and perspectives on future development in this emerging field are proposed.

Nitrogen Vacancy-Modulated Peroxymonosulfate Nonradical Activation for Organic Contaminant Removal via High-Valent Cobalt-Oxo Species

Rapid generation of high-valent cobalt-oxo species (Co(IV)═O) for the removal of organic contaminants has been challenging because of the low conversion efficiency of Co(III)/Co(II) and the high activation energy barrier of the Co(II)-oxidant complex. Herein, we introduced nitrogen (N) vacancies into graphite carbon nitride imbedded with cobalt carbonate (CCH/CN-V_n) in a peroxymonosulfate (PMS)/visible light system to break the limitations of a conventional two-electron transfer path. These N vacancies enhanced the electron distribution of the Co 3d orbital and lowered the energy barrier to cleave the O-O bond of PMS in the Co(II)-PMS complex, achieving the modulation of major active species from ¹O₂ to Co(IV)═O. The developed synergistic system that exhibited adsorption and oxidation showed remarkable selectivity and contaminant removal performance in inorganic (Cl^-, NO₃^-, HCO₃^-, and HPO₄^-) organic (HA) and even practical aqueous matrices (tap water and secondary effluent). This study provides a novel mechanistic perspective to modulate the nonradical path for refractory contaminant treatment via defect engineering.

Fluorescent graphitic carbon nitride with photocatalytic oxidase-like activity for anti-counterfeiting application

Herein bulk phenyl- and carbon-modified graphitic carbon nitride (PCCN) powders with tunable fluorescent emission from green-color to yellow-color were prepared by copolymerization of 2,4-diamino-6-phenyl-1,3,5-triazine and 2,2,6-triaminopyrimidine. The corresponding nanosheets with blue-color to green-color fluorescence were obtained by the oxidation of their bulk powders in sulfuric or nitric acid and then ultrasonic exfoliation. The typical PCCN_0.6 nanosheets not only displayed strong green-color fluorescence but also exhibited photocatalytic oxidase-like activity, which can catalyze the oxidation of substrates 3,3',5,5'-tetramethylbenzidine and Amplex UltraRed by O₂ to produce blue-color colorimetric product and pink-color fluorescent product, respectively. By taking advantage of green-color fluorescence and photocatalytic activity of PCCN_0.6 nanosheets, a prototype for high-level anti-counterfeiting application was demonstrated by using the mixture of PCCN_0.6 nanosheets and Amplex UltraRed as the fluorescent ink.

Insight into the role of charge carrier mediation zone for singlet oxygen production over rod-shape graphitic carbon nitride: Batch and continuous-flow reactor

As a new approach of creating the photo-exited electron (e^-) and hole (h⁺) mediation zone for highly selective singlet oxygen (¹O₂) production, the rod-type graphitic carbon nitride (NCN) has been synthesized from the nitric acid-modified melamine followed by the calcination. The NCN exhibited a higher surface area and surface oxygen adsorption ability than bulk graphitic carbon nitride (BCN). The increment of CO and NH_x groups on NCN corresponded to e^- and h⁺ mediation groups, respectively, resulting in higher production of ¹O₂ than BCN. Moreover, those mediation groups on NCN result in higher recombination efficiency and longer e^- decay time. As a result, the optimized NCN-0.5 (derived from 0.5 M of nitric acid-modified melamine) displayed 5.8 times higher kinetic rate constant of atrazine (ATZ) removal under UVA-LED irradiation compared to BCN. This study also evaluated the ATZ degradation pathways and toxicity effect of by-products. In addition, continuous flow experiments using NCN-0.5 showed superior ATZ removal performance with a hybrid concept between a slurry photocatalysis and a continuous stirred tank reactor system using actual effluent obtained from a wastewater treatment plant. Thus, this work provides an insight into the strategy for highly selective ¹O₂ production and the potential for water purification application.

Exfoliation method matters: The microstructure-dependent photoactivity of g-C3N4 nanosheets for water purification

Exfoliation of carbon nitride (g-C₃N₄) into an ultrathin nanostructure significantly improves its photoactivity. However, the effects of the exfoliation method on the microstructure and photocatalytic performance of g-C₃N₄ nanosheets remain largely unknown. Herein, several typical strategies, such as thermal, chemical, ultrasonic and one-step exfoliation, were applied to exfoliate g-C₃N₄ nanosheets for photocatalytic applications. A procedure capable of controlling the morphology, microstructure, light-absorption property, and visible light photoactivity of g-C₃N₄ nanosheets was attempted. We found that nanosheets prepared from one-step exfoliation present superior photocatalytic efficiency under visible light than those fabricated by thermal exfoliation and ultrasonic exfoliation. The kinetic constants for bisphenol A (BPA) photodegradation over these samples were determined to be 6.5, 4.5 and 2.3 times higher than bulk g-C₃N₄, respectively. For chemical exfoliation, excessive oxidation by H₂SO₄ can lead to the structural defects and deactivation of urea-derived g-C₃N₄ nanosheets. Carbon nitride nanosheets synthesized by one-step exfoliation exhibited high specific surface area, optimal band gap energy structure, and high charge separation efficiency, thereby increasing visible-light photoactivity. Enabling cost-effective production of ultrathin and robust g-C₃N₄ nanosheets, one-step exfoliation offers a potential strategy to exploit high-performance g-C₃N₄ for water purification applications.

Visible-light-assisted peroxymonosulfate activation by metal-free bifunctional oxygen-doped graphitic carbon nitride for enhanced degradation of imidacloprid: Role of non-photochemical and photocatalytic activation pathway

Bifunctional oxygen-doped graphitic carbon nitride (OCN) was fabricated to activate peroxymonosulfate (PMS) for degrading imidacloprid (IMD). The modulated electronic structure of OCN promoted the adsorption, electron transfer, and formation of the redox site of PMS. The light absorption capacity, and the separation and migration speed of photogenerated carriers of OCN were increased. Consequently, 94.5% of IMD (3.0 mg/L) was removed by OCN-10/PMS process in 2.0 h. Compared with g-C₃N₄/PMS (0.048 h^-1), the IMD degradation rate constant of OCN-10/Vis/PMS system (1.501 h^-1) was increased by 30.3 times. The PMS oxidation on electron-deficient C atoms and holes, the PMS reduction around electron-rich O atoms and photogenerated electrons, and the multiple reactions of superoxide radical were the sources of the main active species singlet oxygen. Moreover, even under different pH conditions, coexisting anions, humic acid, and other neonicotinoid pesticides, the OCN-10/Vis/PMS system still showed acceptable applicability. Finally, mass spectrometry identified that hydroxylation and N-dealkylation of amines were the primary degradation pathways of IMD. This paper demonstrates an environmental-friendly combined activation strategy of PMS that can be operated day and night with low energy consumption, aiming to pave the way for developing metal-free photocatalysts for high-efficient environmental purification based on advanced oxidation coupling technology.

Synthesis of an iso-type graphitic carbon nitride heterojunction derived from oxamide and urea in molten salt for high-performance visible-light driven photocatalysis

Thrice-modified g-C3N4 with cyano groups and an asymmetric planar heptazine/triazine-based iso-type heterojunction structure (MOCN) exhibits significantly higher photocatalytic activity.

Nanostructure Engineering of Graphitic Carbon Nitride for Electrochemical Applications

Graphitic carbon nitride with ordered two-dimensional structure displays multiple properties, including tunable structure, suitable bandgap, high stability, and facile synthesis. Many achievements on this material have been made in photocatalysis, but the advantages have not yet been fully explored in electrochemical fields. The bulk structure with low conductivity impedes charge-transfer kinetics during electrochemical processes. Excessive nitrogen content leads to insufficient charge transfer, while bulk structures produce tortuous channels for mass transport. Some attempts have been made to address these issues by nanostructure engineering, such as ultrathin structure design, heterogeneous composition, defect engineering, and morphology control. These structure-engineered nanomaterials have been successfully applied in electrochemical fields, including ionic actuators, flexible supercapacitors, lithium-ion batteries, and electrochemical sensors. Herein, a timely review on the latest advances in graphitic carbon nitride through various engineering strategies for electrochemical applications has been summarized. A perspective on critical challenges and future research directions is highlighted for graphitic carbon nitride in electrochemistry on the basis of existing research works and our experimental experience.

Constructing Co3O4/g-C3N4 Ultra-Thin Nanosheets with Z-Scheme Charge Transfer Pathway for Efficient Photocatalytic Water Splitting

Photocatalytic water splitting for hydrogen generation is a significant pathway for sustainable energy conversion and production. The photocatalysts with a Z-scheme water splitting charge transfer pathway is superior due to the good separation and migration ability of photoexcited charge carriers. Herein, Co₃O₄/g-C₃N₄ photocatalysts with Z-scheme charge transfer pathway were successfully constructed by an electrostatic interaction-annealing method. The as-prepared Co₃O₄/g-C₃N₄ ultra-thin nanosheets were tested and analyzed by XRD, EA, ICP, SEM, TEM, AFM, XPS, UV-Vis DRS, PL and photoelectrochemical measurements. Moreover, the influences of fabrication parameters on performance of Co₃O₄/g-C₃N₄ catalysts were investigated, and 0.5% Co₃O₄/g-C₃N₄ exhibited the optimal activity. Based on the characterization and catalytic performance, the Z-scheme charge transfer pathway of Co₃O₄/g-C₃N₄ was established and put forward. To further improve the catalytic performance of Co₃O₄/g-C₃N₄, 0.5% Pt was added as a co-catalyst. The obtained Pt/0.5% Co₃O₄/g-C₃N₄ was recyclable and remained the original catalytic water splitting performance within 20 h. The modification of Co₃O₄ and Pt improved the separation and migration of e⁻ and h⁺, and induced the increased hydrogen evolution rate of g-C₃N₄.

High-efficiency removal of tetracycline by carbon-bridge-doped g-C3N4/Fe3O4 magnetic heterogeneous catalyst through photo-Fenton process

Carbon-bridge-modified malonamide (MLD)/g-C₃N₄ (CN) was prepared by copolymerization of MLD with urea and melamine and loaded with Fe₃O₄ for the high-efficiency removal of tetracycline (TC) in water under photo-Fenton. The prepared catalysts were characterized by SEM, TEM, N₂ adsorption-desorption analysis, XPS, XRD, and FTIR, which proved that the modification method successfully introduced the C bridge into the carbon nitride molecular system and increased the structural defects of the catalyst. The Carbon-bridge-modified MLD/CN/Fe₃O₄ also had good visible-light response and charge-separation and transport abilities in the photoelectrochemical test. Degradation results showed that the photo-Fenton degradation of TC reached 95.8%, and the mineralization rate was 55.7% within 80 min at 80 mM H₂O₂ dosage, 0.5 g/L catalyst dosage, and near-neutral pH by 0.8MLD/CN/Fe₃O₄. Moreover, the oxidation products and mineralization pathways of TC were explored by LC-MS. Toxicity analysis indicated low environmental threat of the intermediates in TC mineralization. EPR analysis and H₂O₂ decomposition efficiency analyses showed an improvement in the H₂O₂ decomposition performance of 0.8MLD/CN/Fe₃O₄. This work could provide a valuable insight for the application of heterogeneous photo-Fenton technology in wastewater treatment.

Predicting the trend and utility of different photocatalysts for degradation of pharmaceutically active compounds: A special emphasis on photocatalytic materials, modifications, and performance comparison

The rapid rise in the healthcare sector has led to an increase in pharmaceutically active compounds (PhACs) in different aqueous bodies. The toxicity of the PhACs and their ability to persist after conventional treatment processes have escalated research in the field of photocatalytic treatment. Although different photocatalysts have been successful in degrading PhACs, their inherent drawbacks have severely limited their application on a large scale. A substantial amount of research has been aimed at overcoming the high cost of the photocatalytic material, low quantum yield, the formation of toxic end products, etc. Hence, to further research in this field, researchers must have a fair idea of the current trends in the application of different photocatalysts. In this article, the trends in the use of various photocatalysts for the removal of different PhACs have been circumscribed. The performance of different groups of photocatalysts to degrade PhACs from synthetic and real wastewater has been addressed. The drawbacks and advantages of these materials have been compared, and their future in the field of PhACs removal has been predicted using S-curve analysis. Zinc and titanium-based photocatalysts were efficient under UV irradiation, while bismuth and graphene-based materials exhibited exemplary performance in visible light. However, iron-based compounds were found to have the most promising future, which may be because of their magnetic properties, easy availability, low bandgap, etc. Different modification techniques, such as morphology modification, doping, heterojunction formation, etc., have also been discussed. This study may help researchers to clarify the current research status in the field of photocatalytic treatment of PhACs and provide valuable information for future research.

Boosting the activation of molecular oxygen and the degradation of tetracycline over high loading Ag single atomic catalyst

Photocatalytic activation of molecular oxygen (O₂) is a promising way in oxidative degradation of organic pollutants. However, it suffers from low efficiency mainly due to the limited active sites for O₂ activation over traditional photocatalysts. Therefore, we established a single atomic Ag-g-C₃N₄ (SAACN) catalyst with 10 wt% loading of Ag single sites for boosting the O₂ activation during the degradation of tetracycline (TC), and 10 wt% loading of nanoparticle Ag-g-C₃N₄ (NPACN) was studied as a comparison. When using SAACN, the accumulative concentration of superoxide (•O₂^-), hydroxyl radical (•OH), singlet oxygen (¹O₂) reached up to 0.66, 0.19, 0.33 mmol L^-1h^-1, respectively, within 120 min, 11.7, 5.7 and 4.9 times compared with those using NPACN, representing 17.24% of dissolved O₂ was converted to reactive oxygen species (ROS). When additionally feeding air or O_2, the accumulative concentrations of •O₂^-, •OH, ¹O₂ were even higher (air: 4.21, 0.97, 2.02 mmol L^-1 h^-1; O₂: 17.13, 1.32, 9.00 mmol L^-1 h^-1). The rate constants (k) for degrading the TC were 0.0409 min^-1 over SAACN and 0.00880 min^-1 over NPACN, respectively (mineralization rate: 95.7% vs. 59.9% after 3 h of degradation). Moreover, the degradation ability of SAACN did not decrease in a wide range of pH value (4-10) or under low temperature (10 °C). Besides the high exposure of Ag single sites, other advances of SAACN were: 1(O₂ was more energetic favorable to adsorb on single atomic Ag sites; 2) Positive Ag single sites were easier to obtain the electrons from the surrounding N atoms, and facilitated electron transfer towards adsorbed O₂.

Facile fabrication of AgI/Sb2O3 heterojunction photocatalyst with enhanced visible-light driven photocatalytic performance for efficient degradation of organic pollutants in water

The construction of heterojunction is considered as a promising approach to designing highly effective visible-light driven photocatalysts. In this research, the AgI/Sb₂O₃ heterojunction photocatalyst was synthesized by a simple in situ deposition-precipitation procedure, which was supported by XPS results. Among the prepared samples, the 60% AgI/Sb₂O₃ samples exhibited the best ARG degradation ratio (98.3%) in 1 h under visible light irradiation, while the pure Sb₂O₃ and AgI exhibited almost none photocatalytic performance. The trapping experiments and EPR proved that the photo-generated ·O₂^- and ·OH made major contributions to the photocatalytic degradation of ARG by the 60% AgI/Sb₂O₃ samples. The enhanced photocatalytic performance of AgI/Sb₂O₃ heterojunction photocatalysts was ascribed to that the e^- produced in the CB of AgI would be transferred to the empty CB of Sb₂O₃, which could effectively promote separation of photo-induced carries. More importantly, the transfer of electrons from AgI to Sb₂O₃ would be in favor of restraining the reduction of Ag⁺ to Ag⁰ resulting in the good stability of heterojunction photocatalysts. The heterojunction photocatalyst provided in this work might be a prospective candidate for decontamination of water.

First-Principles Evaluation of Volatile Organic Compounds Degradation in Z-Scheme Photocatalytic Systems: MXene and Graphitic-CN Heterostructures

It is a formidable challenge to use the traditional trial-and-error method to identify suitable catalysts for the photocatalytic degradation of volatile organic compounds (VOCs). In this work, by performing density functional theory calculations, we designed three Z-scheme g-CN/M₂CO₂ (M = Hf, Zr, and Sc) heterostructures, which not only exhibit favorable structure stability but also show promising ability for photocatalytic degradation of VOCs. The enhancement of the photocatalytic activity of these three Z-scheme systems can be ascribed to the low recombination rate of electron-hole pairs because photoelectrons migrated from the g-CN layer to the M₂CO₂ layer as well as the internal electric fields in the Z-scheme heterojunction. Among the three heterostructures, only g-CN/Zr₂CO₂ presents favorable spectra utilization under photoirradiation as well as the direct band gap. As a result, in the Z-scheme g-CN/Zr₂CO₂ heterostructure, the electrons in the conduction band of g-CN migrate to the holes in the valence band of the Zr₂CO₂ layer, which improves extraction and utilization of photogenerated electrons in the g-CN sheet. Moreover, the Z-scheme g-CN/Zr₂CO₂ system shows superior performance for photocatalytic VOC degradation in comparison with individual g-CN and Zr₂CO₂, which can be attributed to the enhanced VOC adsorption capacity as well as excellent ability to photoactivate O₂ and H₂O into ^•O₂^- and ^•OH radicals. Our findings pave a new promising way to facilitate the application of MXene-based materials for VOC photocatalytic degradation.

Tubular g-C3N4/carbon framework for high-efficiency photocatalytic degradation of methylene blue

The preparation of high-efficiency, pollution-free photocatalysts for water treatment has always been one of the research hotspots. In this paper, a carbon framework formed from waste grapefruit peel is used as the carrier. A simple one-step chemical vapor deposition (CVD) method allows tubular g-C₃N₄ to grow on the carbon framework. Tubular g-C₃N₄ increases the specific surface area of bulk g-C₃N₄ and enhances the absorption of visible light. At the same time, the carbon framework can effectively promote the separation and transfer of charges. The dual effects of static adsorption and photodegradation enable the g-C₃N₄/carbon (CNC) framework to quickly remove about 98% of methylene blue within 180 min. The recyclability indicates that the tubular g-C₃N₄ can stably exist on the carbon framework during the photodegradation process. In the dynamic photocatalytic test driven by gravity, roughly 77.65% of the methylene blue was degraded by the CNC framework. Our work provides an attractive strategy for constructing a composite carbon framework photocatalyst based on the tubular g-C₃N₄ structure and improving the photocatalytic performance.

Tubular g-C₃N₄ grown on a carbon framework increased the surface area of bulk g-C₃N₄, enhanced the absorption of visible light and promoted the photocatalytic performance.

Photochemical Transformation and Catalytic Activity of Dissolved Black Nitrogen Released from Environmental Black Carbon

Biomass combustion results in the formation and wide distribution of black carbon (BC) in soils, wherein the dissolved fractions are among the most active components. Although the presence of dissolved black nitrogen (DBN) in BC has been identified, its environmental behavior and implication are not understood. This study investigated the photochemical transformation and catalytic activity of DBN under simulated solar irradiation. DBN is more easily transformed than dissolved BC due to its photoactive heteroaromatic N structure, and the half-life of DBN produced at 500 °C (8.6 h) is two times shorter than that of the dissolved BC counterpart (23 h). Meanwhile, solar irradiation is favorable for the homoaggregation of DBN. During irradiation, DBN generates not only reactive oxygen species (e.g., ¹O₂, O₂^-, and ^•OH) but also reactive nitrogen species (mainly ^•ON), which account for its higher photocatalytic degradation of bisphenol A than dissolved BC. These findings shed new light on the impact of heteroatoms on the phototransformation and activity of BC as well as cycling of N in terrestrial systems.

Metal-organic frameworks derived C/TiO2 for visible light photocatalysis: Simple synthesis and contribution of carbon species

A series of in-situ carbon-doped TiO₂ (C_x/TiO₂) composites with a porous and crystalline structure were successfully synthesized via one-step and low-temperature calcination of titanium metal-organic framework (MOF), MIL-125(Ti). The resultant materials were comprehensively investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), N₂ adsorption-desorption measurements, UV-vis diffuse reflectance spectrum (DRS), photoluminescence (PL) spectra and photoelectrochemical measurements, and their photocatalytic activities for bisphenol A (BPA) degradation were assessed. Compared with the benchmark TiO₂ photocatalyst (P25), the C_x/TiO₂ composite material with high specific surface, lower band gap, and reduced photogenerated electron hole ratio exhibited outstanding photodegradation activity and durability for BPA, which could be attributed to the combined effect of co-doping of multiple carbon species (substituent carbon and carbonate) and porous structure. During BPA degradation, the holes and superoxide radicals were the primary role oxidative species in the reaction process. Therefore, this new efficient photocatalyst is promising candidate for photodegradation of organic pollutants.

One-step hydrothermal synthesis of hierarchical nanosheet-assembled Bi2O2CO3 microflowers with a {001} dominant facet and their superior photocatalytic performance

Using citric acid (CA) and 1,5-naphthalenedisulfonic acid (NDSA) as the structure-directing agent, a hierarchical flower-like Bi₂O₂CO₃ product is successfully prepared via a simple one-step hydrothermal synthesis, which is spirally assembled by the {001} facet-dominated nanosheets. It is testified that the additive CA plays an important inducing role in forming the chemical composition of Bi₂O₂CO₃, the nanosized sheet-type subunits, and the exposure of the {001} facet, while the NDSA greatly improves the dispersity and porous structure of the Bi₂O₂CO₃ microflower. Due to the nano-size effect and distortion of surface Bi-O bonds, the Bi₂O₂CO₃ microflower could be excited by the visible light to exhibit a superior photocatalytic performance in the degradation of tetracycline (TC). Besides, it is found the exposed {001} facet of Bi₂O₂CO₃ would preferentially generate holes during the illumination process, thus enhancing the photooxidative activity of the Bi₂O₂CO₃ microflower. Finally, the structural and optical features of the Bi₂O₂CO₃ microflower have been discussed in detail, and its photocatalytic mechanism has also been proposed in this work.