Bridging the g-C3N4 Interlayers for Enhanced Photocatalysis

Photocatalytic inactivation of algae in a fluidized bed photoreactor with an external magnetic field

Practical applications of photocatalysis in algae removal often involve the use of photoreactors, which can be of many different configurations. In this study, a fluidized bed photoreactor (FBPR) with an external magnetic field was designed and constructed to achieve algae inactivation continuously and stably. Magnetic photocatalyst ZnFe₂O₄/Ag₃PO₄/g-C₃N₄ attached to Fe₃O₄ aggregate, was dispersed and fixed at the bottom of the reactor to form a flower-like structure, which can not only increase the effective irradiation area of the photocatalyst, but also enhances mass transfer by inducing flow disturbance. Under the optimal operating conditions, i.e., 0.04 m/s flow rate, 200 mT magnetic field strength, and 0.025 g photocatalyst loading, the photoreactor can effectively remove algae cells within 6 h. During the continuous operation experiment, the quality of the magnetic photocatalyst and aggregate did not decrease significantly, and there was still a 90% removal efficiency after 18 h of continuous operation. Furthermore, in the experiment where humic acid was added to simulate actual water environment, certain advantages can still be observed with the FBPR. As a continuous reactor using a magnetic photocatalyst, the FBPR has the characteristics of high availability, low cost, and low energy consumption.

Environment Friendly g-C3N4-Based Catalysts and Their Recent Strategy in Organic Transformations

Organic molecules synthesized in an environmentally friendly manner have excellent therapeutic potential. The entire preparation technique was examined in the existence of a light source, implying that light has been replaced by heating and the usage of dangerous chemicals has decreased, resulting in less pollution of the environment. The advantages of these nanocarbon catalysts include high efficiency, environmentally friendly synthesis, eco-friendly, inexpensive, and non-corrodible. In organic transformations, solid metal base/metal-free catalysts produce better results. Here, the metal-free semiconductor g-C₃N₄ was used to demonstrate the catalytic behavior of organic conversions. g-C₃N₄ is a two-dimensional material and a p‑type semiconductor to enhance the photocatalytic activity. The excellent properties of g-C₃N₄ sheet lead to the support of metals to form metal-organic frameworks. Most of the reactions gained positive response under visible light irradiation. This review will inspire readers in widen the applications of g-C₃N₄ based catalyst in various organic transformation reactions.

Polymeric ionic liquid with carboxyl anchored on mesoporous silica for efficient fixation of carbon dioxide

The utilization of carbon dioxide (CO₂) has drawn much attention because of the increasing serious environmental problems. In order to promote the cycloaddition reaction of CO₂ to epoxides, a new synthesis strategy for friendly nonmetal catalyst to combine polymeric ionic liquid (PIL) with mesoporous silica (mSiO₂) was proposed. By thorough characterizations, those catalysts (mSiO₂-PIL-n, n = 1, 2, 3, 4) were verified that PIL with multiply catalytic active sites such as carboxyl group, imidazole ring and Br^-, was mainly anchored in mesoporous SiO₂ structures. Therefore, mSiO₂-PIL-n exhibited excellent catalytic activity for CO₂ cycloaddition reaction to epoxides under solventless and cocatalyst-free conditions. Typically, the appropriate PIL loading and specific surface area guaranteed mSiO₂-PIL-2 could efficiently catalyze the cycloaddition reaction with 96% yield and 99% selectivity to the target product of propylene carbonate under the conditions of 120 °C, 2 MPa and 6 h. Additionally, the mSiO₂-PIL-2 catalyst showed superior recyclability and there was no catalytic activity decrease for 10 runs of recycling due to the tightly anchored PIL on mesoporous SiO₂ by copolymerization. And the catalytic activity to other substituted epoxides over mSiO₂-PIL-2 was also expanded. Therefore, PIL anchored on mesoporous SiO₂ by copolymerization could be a promising synthetic strategy for the efficient catalyst to combine multiple active components in a single catalyst, meanwhile, mSiO₂-PIL-n exhibited an appealing catalyst candidate for the effective fixation and utilization of CO₂.

Tube wall delamination engineering induces photogenerated carrier separation to achieve photocatalytic performance improvement of tubular g-C3N4

Morphology adjustment is a feasible method to change the physicochemical properties of photocatalysts. The issue that excessively thick tube wall of tubular g-C₃N₄ is not conducive to the electron migration from inside to the surface thus inhibiting the separation of photogenerated carriers has always been ignored. Potassium ions were used to regulate the structure of the tubular supramolecular precursor by breaking hydrogen bonds, thereby promoting the synthesis of delaminated laminar tubular g-C₃N₄ (K-CN), which not only shortened the transfer distance of photogenerated electrons but also provided abundant reaction active sites. Experiments and DFT calculations were combined to reveal the details of the physicochemical properties of K-CN. The photocatalytic capacity of K-CN for tetracycline hydrochloride (TCH) degradation and H₂O₂ generation were 83% and 133 μM, respectively. This work not only synthesized a novel delaminated tubular g-C₃N₄ but also provided a strategy and inspiration for structure and performance optimization for tubular g-C₃N₄.

K+ Ion-Doped Mixed Carbon Nitride: A Daylight-Driven Photocatalyst and Luminophore for Enhanced Chemiluminescence

Photocatalytic production of reactive oxygen species from O₂ at the interface of the photocatalyst is significant to convert luminous energy like daylight into chemical energy and could be momentous for a reactive oxygen species-based chemiluminescence system. Herein, we synthesized a novel K⁺ ion-doped tri-s-triazine/triazine mixed carbon nitride (MCN), in which K⁺ ions were intercalated into the layers in a bridging manner. After a mild daylight treatment for 30 min, the MCN suspension could produce long-lifetime reactive oxygen species and further directly produce intense and stable chemiluminescence emission in the presence of luminol. In particular, the chemiluminescence intensity was 780 times that of H₂O₂-luminol, and MCN could be recycled several times in the chemiluminescence system. The mechanism results revealed a large number of reactive oxygen species that were generated from O₂ on the surface of MCN through a temperate photocatalytic process. In the theoretical calculation, the charge density of N interacting with K⁺ ions was significantly more negative than that at the corresponding position in graphitic carbon nitride, which was beneficial to the adsorption and activation of oxygen, and the narrower band gap suggested that the doping of K⁺ ions was conducive to the intramolecular charge transfer interaction. Then, the long-lifetime reactive oxygen species triggered the conversion of luminol into an excited-state intermediate, which further transferred energy to MCN, producing strong chemiluminescence emission. The K⁺ ion-doped MCN might conduct as an efficient photocatalyst for reactive oxygen species generation, recyclable catalysts, and luminophores in the photoinduced chemiluminescence system.

A multifunctional protective layer with biomimetic ionic channel suppressing dendrite and side reactions on zinc metal anodes

A multifunctional graphitic carbon nitride (GCN) protective layer with bionic ion channels and high stability is prepared to inhibit dendrite growth and side reactions on zinc (Zn) metal anodes. The high electronegativity of the nitrogen-containing organic groups (NOGs) in the GCN layer can effectively promote the dissociation of solvated Zn²⁺ and its rapid transportation in bionic ion channels via a hopping mechanism. In addition, this GCN layer exhibits excellent mechanical strength to suppress the growth of Zn dendrites and the volume expansion of Zn metal anodes during the plating process. Consequently, the electrodeposited Zn presents a uniform and densely packed morphology with negligible side-product accumulation. As a result, the half-cell composed of the Cu-GCN anode can deliver a remarkable long-term cycling performance of 1000 h at 0.5 mA cm^-2 and 0.25 mAh cm^-2. A full cell assembled with MnO₂ cathode also displays improved long-term cycling performance (150 cycles at 200 mA g^-1) when the Cu-GCN@Zn composite anode is applied. This work deepens our understanding of the kinetics of ion migration in the interface layer and paves the way for next-generation high energy-density Zn-metal batteries (ZMBs).

Renewable biomass-derived carbon-supported g-C3N4 doped with Ag for enhanced photocatalytic reduction of CO2

Constructing noble metal-doped g-C₃N₄/carbon composites is a feasible route to overcome the intrinsic drawbacks of pristine g-C₃N₄ for enhanced activity of CO₂ photoreduction. Herein, a novel Ag-doped g-C₃N₄/biomass-derived carbon composite with hollow bird's nest-like (Ag-g-C₃N₄/BN-C) is designed and prepared via a simple yet effective one-step pyrolysis method. In the Ag-g-C₃N₄/BN-C, the highly-dispersed Ag nanoparticles (20-30 nm) with the surface plasmon resonance (SPR) effect act as a significant cocatalyst not only to efficiently trap the photogenerated electrons from g-C₃N₄ to boost the separation of photogenerated electron-hole pairs but also to produce additional active "hot electrons", while the conductive quasi-spherical hollow structure of BN-C doubles the specific surface area with multiple reflections of light, providing abundant active sites and more utilization efficiency of light energy. As a result, the Ag-g-C₃N₄/BN-C exhibits a remarkably enhanced CO evolution rate of 33.3 μmol·g^-1·h^-1 without addition of any sacrificial reagents and photosensitizers, superior to those of both the pristine g-C₃N₄ and many reported g-C₃N₄-based counterparts. The findings of this work demonstrate a good indication for integrating g-C₃N₄ with SPR-dependence noble metal and renewable biomass-derived carbon for enhanced CO₂ photoreduction, which may be extended to modify other semiconductor materials for more photocatalytic applications with enhanced activity.

Fabrication of zirconium-based metal-organic frameworks@tungsten trioxide (UiO-66-NH2@WO3) heterostructure on carbon cloth for efficient photocatalytic removal of tetracycline antibiotic under visible light

Designing recyclable photocatalysts with high activity and stability has drawn considerable attention in the fields of sewage treatment. Herein, a series of heterojunctions constructed by zirconium-based metal-organic frameworks (UiO-66-NH₂) and tungsten trioxide (WO₃) is immobilized on carbon cloth via a facile solvothermal method, resulting in highly recyclable photocatalysts. Multiple characterization techniques, such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy, verify the successful synthesis of UiO-66-NH₂ nanospheres on the surface of needlelike WO₃ modified carbon cloth. Results show that the optimal heterojunction photocatalyst exhibits excellent photocatalytic degradation efficiency for the removal of tetracycline (TC) from water, for which nearly 100% of TC is degraded within 60 min under visible light. Trapping experiments and electron spin resonance (ESR) spectra analyses demonstrate that the superoxide radicals O₂^- and photogenerated hole h⁺ play a dominant role in the degradation process. Excellent photocatalytic activity is dominantly attributed to the effective separation of photoinduced carriers in this type-Ⅱ heterostructure system. Moreover, the possible photocatalytic oxidation degradation pathway is confirmed by analyzing intermediates using liquid chromatography mass spectrometry (LC-MS). This study offers a highly efficient strategy to design recyclable heterojunction photocatalysts for the degradation of refractory antibiotics in sewage.

Generation of singlet oxygen over CeO2/K, Na-codoped g-C3N4 for tetracycline hydrochloride degradation in a wide pH range

Singlet oxygen (1O2) were widely studied for catalytic oxidation and photo dynamic therapy (PDT) and so on due to its unique properties, such as its long lifetime, wide pH tolerance,...

Theoretical and experimental studies of high efficient all-solid Z-scheme TiO2-TiC/g-C3N4 for photocatalytic CO2 reduction via dry reforming of methane

All-solid Z-scheme heterojunction TiO2-TiC/g-C3N4 was proposed and synthesized successfully by a facile calcination method and used for photocatalytic CO2 reduction in the presence of CH4. Under sub-atmospheric pressure and room...

Construction of high-performance g-C3N4-based photo-Fenton catalysts by ferrate-induced defect engineering

A highly-efficient Fe–g-C3N4 photo-Fenton catalyst was prepared using ferrate as a critical iron source and defect control additive. The complementary synergistic effects of Fe species and defect sites promote photoelectron transfer.

Fabrication of Mn/P co-doped hollow tubular carbon nitride by a one-step hydrothermal–calcination method for the photocatalytic degradation of organic pollutants

Efficient photocatalytic degradation of trace organic pollutants in aqueous environment by a hollow tubular carbon nitride co-doped with manganese and phosphorus under visible light.

Photocatalysis triggered CVD synthesis of graphene at low temperature

Photocatalysis is employed to dissociate CH4 which facilitates low temperature CVD growth of graphene.

Tuning the surface and optical properties of graphitic carbon nitride by incorporation of alkali metals (Na, K, Cs and Rb): Effect on photocatalytic removal of organic pollutants

Alkali metals have been known for their intercalation properties and can be employed for the separation of stacking in sheet-like materials. In this work, alkali metals (Na, K, Rb and Cs) have been systematically incorporated in varying concentrations in g-C₃N₄ sheets and their effect on resulting optical, surface and photocatalytic properties have been explored in detail. It was observed that the optical, electronic and surface properties of g-C₃N₄ were altered upon the incorporation of different alkali metal ions. The band gap and specific surface area of resulting materials were decreased as compared to the pristine g-C₃N₄. In addition, the alkali metal incorporation in g-C₃N₄ sheets showed the formation of cyanide groups and nitrogen vacancies in the resulted materials. Further, the photocatalytic activity of g-C₃N₄ and alkali metal incorporated g-C₃N₄ was calculated by studying the degradation of acid red 94 dye under visible light irradiation. It was observed that the photocatalytic activity of pristine g-C₃N₄ sheets was decreased with an increase in the concentration of alkali salt used during the synthesis of alkali metal incorporated g-C₃N₄. This decrease in the activity could arise due to the decreased surface area, detrimental amount of nitrogen vacancies and high concentration of alkali metal ions incorporated in the structural framework of g-C₃N₄ sheets. This work provides a unique example of the adverse effect of alkali metal ions on photocatalytic activity of g-C₃N₄ and paves future directions for the improvement of the performance of g-C₃N₄ based materials.

Impact of piezoelectric effect on the heterogeneous visible photocatalysis of g-C3N4/Ag/ZnO tricomponent

In recent years, the piezophotocatalytic mechanism had been intensively recognized as a potential and promising route to sewage treatment. Here we report the piezoelectric effect improved heterogeneous photocatalysis of g-C₃N₄/Ag/ZnO (g-CN/A/Z) tricomponent in rhodomine B (RhB) degradation. Initially, the nanomaterials were characterized for their physico-chemical and optoelectronic properties using analytical techniques such as x-ray diffraction (XRD), scanning & transmission electron microscopes (SEM & TEM), UV-vis spectrophotometer and photoluminescence spectroscopy (PL). In addition, the photoelectrochemical activity of determining the photocurrent density and electrochemical impendence response were also been conducted. The catalytic properties of the tricomponent, g-CN/A/Z was studied with the degradation of RhB with visible photons irradiation and ultrasonication. In piezophotocatalysis, degradation up to 89% of RhB was achieved with 1.26 folds synergetic effect on par to the photocatalysis and piezocatalysis.

Carbon quantum dots sensitized 2D/2D carbon nitride nanosheets/bismuth tungstate for visible light photocatalytic degradation norfloxacin

A novel carbon quantum dots (CQDs) sensitized 2D/2D carbon nitride nanosheets and bismuth tungstate composite (CQD-CNs/BWO) was successfully prepared via the facile hydrothermal method and used for the photocatalytic degradation of norfloxacin (NOR). During 120 min irradiation test, CQD-CNs/BWO exhibited 9 and 1.76 times higher photocatalytic activity than CNs and BWO, respectively. CQDs and constructed 2D/2D structure could not only improve the light harvesting but also promote the generation and separation of electron-holes. The existing inorganic ions in solution (e.g. bicarbonate ions, chlorine ions, and sulfate ions) could inhibit NOR degradation. Based on the electron spin resonance and free radicals inhibition tests, the holes and superoxide radicals rather than hydroxyl radicals were the main reactive species. The intermediates and possible pathways were proposed, and the antibacterial activity of the treated solution after the reaction was evaluated via bacteriostatic tests. The prepared composite material with high photocatalytic activity and stability is potentially effective for the degradation of antibiotics in wastewater.

Bi2S3 quantum dots in situ grown on MoS2 nanoflowers: An efficient electron-rich interface for photoelectrochemical N2 reduction

Photoelectrocatalysis is considered a green, environmentally friendly, sustainable technology for NH₃ synthesis. However, the low efficiency of ammonia synthesis is currently the primary problem in photoelectrochemical nitrogen reduction reactions (PEC NRR). Herein, a nanocomposite BQD/MS developed through the in-situ growth of Bi₂S₃ quantum dots (BQD) on MoS₂ (MS) nanoflowers was demonstrated as an efficient PEC NRR catalyst. Experimental results showed that the strong interaction between BQD and MS modulated the interfacial charge distribution and increased the electron density on the MS side. Meanwhile, the excellent structure of BQD/MS promoted the effective migration of photogenerated electrons from excited BQD to the MS surface. The electron-rich MS reaction interface was conducive to cleaving the stable NN bond and improving the N₂ reduction performance. As a result, the prepared BQD/15MS photocathode obtained an excellent Faradaic efficiency of 33.2% and an NH₃ yield of 18.5 μg h^-1 mg^-1, which was about three times that of bare MS.

Promotion mechanism of -OH group intercalation for NOx purification on BiOI photocatalyst

Bismuth oxyiodide (BiOI) is a traditional layered oxide photocatalyst that performs in a wide visible-light absorption band, owing to its appropriate band structure. Nevertheless, its photocatalytic efficiency is immensely inhibited due to the serious recombination of photogenerated charge carriers. Herein, this great challenge is addressed via a new strategy of intralayer modification by -OH groups in BiOI, which leads to enhancement of the reactants' activation capacity to promote photocatalytic activity and generate more active species. Furthermore, analysis via a combination of experimental and theoretical methods revealed that the -OH group-functionalized samples reduce the energy barriers for conversion of the main intermediate (NO₂), which is easily transformed to NO₂^-, thus accelerating the oxidation of NO to the final product (NO₃^-). This study gives insight into NO oxidation, improving the photocatalytic efficiency, and mastering the photocatalysis reaction mechanism to curb air pollution.

Size controlled silver nanoparticles on β-cyclodextrin/graphitic carbon nitride: an excellent nanohybrid material for SERS and catalytic applications

A nanohybrid (NH), having high dispersion of silver nanoparticles (AgNPs) on β-cyclodextrin (β-CD)/graphitic carbon nitride (g-CN), designated as AgNPs/β-CD/g-CN-NH, was synthesized and characterized. It was exploited for a couple of environmental remediation applications like SERS sensing and catalytic reduction of specific organic pollutants in water. It showed excellent SERS activity as a Raman probe for the detection of malachite green (MG). Its enhancement factor (EF) and detection limit for MG were equal to 7.26 × 10⁶ and 1 × 10^-9 M, respectively. The relative standard deviation (RSD) was equal to 3.8% which indicates high homogeneity of AgNP dispersion and signal reproducibility of the SERS substrate. The NH displayed high catalytic activity for the reduction of eosin yellow (EY) in the presence of NaBH₄ with the rate constant (k) of 0.1142 min^-1. A comparison of the present NH with other reported materials reveals better SERS and catalytic activities of the former than those of the latter. The SERS activity of the NH was also examined for sensing of other triphenylamine dyes like methyl violet (MV), and it was successful. The same NH also exhibited high catalytic activity towards the reduction of Congo red (CR). The results of both studies clarify that the NH is an excellent SERS substrate and efficient catalyst for the detection of organic environmental pollutants having structures similar to MG and their degradation. This is due to the distribution of the controlled size of AgNPs on g-CN promoted by β-CD. Therefore, we focus our attention on future environmental applications of the nanohybrid as a very cheap SERS substrate and a very active catalyst.

Point-Defect Engineering: Leveraging Imperfections in Graphitic Carbon Nitride (g-C3 N4 ) Photocatalysts toward Artificial Photosynthesis

Graphitic carbon nitride (g-C₃ N₄ ) is a kind of ideal metal-free photocatalysts for artificial photosynthesis. At present, pristine g-C₃ N₄ suffers from small specific surface area, poor light absorption at longer wavelengths, low charge migration rate, and a high recombination rate of photogenerated electron-hole pairs, which significantly limit its performance. Among a myriad of modification strategies, point-defect engineering, namely tunable vacancies and dopant introduction, is capable of harnessing the superb structural, textural, optical, and electronic properties of g-C₃ N₄ to acquire an ameliorated photocatalytic activity. In view of the burgeoning development in this pacey field, a timely review on the state-of-the-art advancement of point-defect engineering of g-C₃ N₄ is of vital significance to advance the solar energy conversion. Particularly, insights into the intriguing roles of point defects, the synthesis, characterizations, and the systematic control of point defects, as well as the versatile application of defective g-C₃ N₄ -based nanomaterials toward photocatalytic water splitting, carbon dioxide reduction and nitrogen fixation will be presented in detail. Lastly, this review will conclude with a balanced perspective on the technical and scientific hindrances and future prospects. Overall, it is envisioned that this review will open a new frontier to uncover novel functionalities of defective g-C₃ N₄ -based nanostructures in energy catalysis.

Selective graphene-like metal-free 2D nanomaterials and their composites for photocatalysis

From the viewpoint of sustainability, graphene-like metal-free 2D nanomaterials (GMFs) hold great potential in different photocatalytic fields due to their distinct structures and properties. Although their lattice structures are highly similar, the properties of these nanomaterials are in vast diversity owing to the uniqueness of particular atomic arrangement, thus giving rise to their multi-faceted functionalities in photocatalytic process. In this review, we summarize the latest progress of GMFs and their hybrid composites in photocatalytic field, including graphene and its derivatives, hexagonal boron nitride (h-BN), graphitic carbon nitride (g-C₃N₄), black phosphorus (BP) and emerging 2D covalent organic frameworks (COFs). Their unique 2D structure and key photocatalytic properties are firstly briefly introduced. Then a critical discussion on their multiple roles in the activity enhancement of composite photocatalysts is emphasized, which in turn points out the direction of maximizing their functions and guides our efficient construction of hybrid photocatalysts based on above 2D nanomaterials. On this basis, a summary about the hybridization of above 2D metal-free materials is presented, and the merits of 2D/2D hybrid systems are elaborated. Last, we wrap up this review with some summative remarks, covering understanding their own unique strengths and weaknesses by comparison and proposing the major challenges and perspectives in this emerging field.

Photocatalytic Air Purification Using Functional Polymeric Carbon Nitrides

The techniques for the production of the environment have received attention because of the increasing air pollution, which results in a negative impact on the living environment of mankind. Over the decades, burgeoning interest in polymeric carbon nitride (PCN) based photocatalysts for heterogeneous catalysis of air pollutants has been witnessed, which is improved by harvesting visible light, layered/defective structures, functional groups, suitable/adjustable band positions, and existing Lewis basic sites. PCN-based photocatalytic air purification can reduce the negative impacts of the emission of air pollutants and convert the undesirable and harmful materials into value-added or nontoxic, or low-toxic chemicals. However, based on previous reports, the systematic summary and analysis of PCN-based photocatalysts in the catalytic elimination of air pollutants have not been reported. The research progress of functional PCN-based composite materials as photocatalysts for the removal of air pollutants is reviewed here. The working mechanisms of each enhancement modification are elucidated and discussed on structures (nanostructure, molecular structue, and composite) regarding their effects on light-absorption/utilization, reactant adsorption, intermediate/product desorption, charge kinetics, and reactive oxygen species production. Perspectives related to further challenges and directions as well as design strategies of PCN-based photocatalysts in the heterogeneous catalysis of air pollutants are also provided.

Direct Atomic-Scale Structure and Electric Field Imaging of Triazine-Based Crystalline Carbon Nitride

Crystalline carbon nitrides (CNs) have recently attracted considerable attention owing to their superior photocatalytic activity. However, the electron-beam-sensitive nature of crystalline CNs hinders atomic-resolution imaging of their local structures by conventional (scanning) transmission electron microscopy ((S)TEM) techniques. Here, the atomic structure of a triazine-based crystalline CN, poly(triazine imide) (PTI) incorporated with lithium and chloride ions, is unambiguously revealed using the emerging imaging technique of differential phase contrast STEM under a low dose. The lightest-element Li/H configuration is resolved within framework cavities of PTI and significantly affects the electronic structure for photoabsorption. The atomic electric field of PTI crystal directly determined in real space provides a fundamental evidence for the chemical bonding of Li ions and adjacent atoms for the migration of photogenerated carriers. These results facilitate the comprehension on local atomic configuration and chemical bonding state of crystalline CNs and can lead to a deeper understanding of the photocatalytic mechanism.

Experimental and theoretical investigation for the cycloaddition of carbon dioxide to epoxides catalyzed by potassium and boron co-doped carbon nitride

Much endeavor has been devoted to efficient heterogeneous catalysts for carbon dioxide (CO₂) conversion to high-value chemicals. Meanwhile, the cycloaddition of CO₂ to epoxides is considered as a green and atom-economy reaction to produce cyclic carbonates. Herein, a series of K, B co-doped CN with various doping contents (K, B-CN-X) were developed by simple one-step calcination of melamine and KBH₄. B was confirmed to replace the C site and KN bond was formed, which was verified by XPS (X-ray photoelectron spectroscopy) and DFT (density functional theory) calculation. Particularly, K, B-CN-4 displayed the optimal catalytic performance in the presence of Bu₄NBr (tetrabutylammonium bromide) cocatalyst for the CO₂ cycloaddition with propylene oxide. Besides, K, B-CN-4/Bu₄NBr catalyst exhibited good substrate versatility to various epoxides and excellent recycling performance. According to the DFT calculation on CO₂ adsorption and experimental results, K, B-CN-4 presented satisfactory catalytic activity due to the enhanced CO₂ adsorption after K and B dopings then the possible reaction mechanism was proposed. The promising K, B-CN-X catalyst presented an attractive application due to the simple, eco-friendly synthesis route for the efficient fixation of CO₂.

Engineering of graphitic carbon nitride with simultaneous potassium doping sites and nitrogen defects for notably enhanced photocatalytic oxidation performance

Graphitic carbon nitride (g-C₃N₄) offers exciting opportunities for sustainable photocatalytic oxidation of organic pollutants but suffers from drawbacks of insufficient oxidation driving force and low quantum efficiency. To over the drawbacks, here a simple and effective strategy was developed to engineer g-C₃N₄ with simultaneous interstitially embedded potassium dopant and nitrogen defects, and the process included supramolecular preorganization followed by KOH-assisted thermal polycondensation. In the prepared D_N-K-CN catalysts, potassium doping level and the amount of nitrogen defects were both controllable. With the increment of potassium doping level, the bandgap of the D_N-K-CN became narrow, along with continuously downshifted valence band position. The D_N-K-CN showed greatly enhanced visible-light photocatalytic oxidation performance with respect to g-C₃N₄ in the degradation of emerging phenolic pollutants, acetaminophen and methylparaben; meanwhile, the oxidation performance of D_N-K-CN depended on potassium doping level and the amount of nitrogen defects. Combination of experimental findings and theory calculations it is confirmed that the enhanced photocatalytic oxidation performance of D_N-K-CN was attributed to the synergistic effect of potassium dopant and nitrogen defects, which resulted in the generation of plentiful active oxygen species and the improvement of oxidation driving force of valence holes. The influence of potassium dopant and nitrogen defects on the electronic and band structures of g-C₃N₄ was revealed; simultaneously, mechanism of the enhanced photocatalytic oxidation performance of g-C₃N₄ after the introduction of potassium dopant and nitrogen defects was studied. The present work provided new insights into the electronic and band structure tuning for the improvement of the photocatalytic oxidation performance of g-C₃N₄.

Structure modulation of g-C3N4 in TiO2{001}/g-C3N4 hetero-structures for boosting photocatalytic hydrogen evolution

Structure design of photocatalysts is highly desirable for taking full advantage of their abilities for H2 evolution. Herein, the highly-efficient TiO2{001}/g-C3N4 (TCN) heterostructures have been fabricated successfully via an in situ ethanol-thermal method. And the structure of g-C3N4 in the TCN heterostructures could be exfoliated from bulk g-C3N4 to nanosheets, nanocrystals and quantum dots with the increase of the synthetic temperature. Through detailed characterization, the structural evolution of g-C3N4 could be attributed to the enhanced temperature of the ethanol-thermal treatment with the shear effects of HF acid. As expected, the optimal TCN-2 heterostructure shows excellent photocatalytic H2 evolution efficiency (1.78 mmol h-1 g-1) under visible-light irradiation. Except for the formed built-in electric field, the significantly enhanced photocatalytic activity of TCN-2 could be ascribed to the enhanced crystallinity of TiO2{001} nanosheets and the formed g-C3N4 nanocrystals with large surface area, which could extend the visible light absorption, and expedite the transfer of photo-generated charge carriers further. Our work could provide guidance on designing TCN heterostructures with the desired structure for highly-efficient photocatalytic water splitting.

Highly polymerized linear polyimide /H3PW12O40 photocatalyst with full visible light region absorption

The degree of polymerization in polyimides has significant effects on their photoelectric properties. Increase the degree of polymerization in polyimide can improve its light absorption capability as well as reduce the recombination rate of photogenerated electrons and holes. However, it is difficult to promote the polymerization of polyimides by conventional approaches, such as increasing the polymerization temperature since polyimides are susceptible to high temperature and can be decomposed. In this paper, a suitable proportion of phosphotungstic acid was introduced to increase the degree of polymerization of polyimide at a lower polymerization temperature, so as to improve the light absorption capability of the composite and inhibit the recombination of electron and hole. The p-phenylenediamine based polyimide with the one-dimensional linear polymer chain has excellent charge transfer ability, so the POM-π effect formed with phosphotungstic acid is stronger, correspondingly, the light absorption capacity of the composite photocatalyst formed is stronger than that of the cross-linked polyimide/phosphotungstic acid.

Hydrophilic and underwater superoleophobic porous graphitic carbon nitride (g-C3N4) membranes with photo-Fenton self-cleaning ability for efficient oil/water separation

Due to the great fouling resistance property, (super)hydrophilic/underwater superoleophobic membranes are prevalent candidates for oil-polluted wastewater treatment. Even so, membrane fouling inevitably occurs during long-term operation. Therefore, it is of great significance to construct anti-fouling membranes with robust flux recovery. Herein, a polyvinyl pyrrolidone (PVP) coated porous potassium-doped g-C₃N₄ (PKCN) membrane was fabricated for the first time by vacuum filtration. The as-prepared membrane displays enhanced hydrophilicity and underwater superoleophobicity. The permeability of the membrane increased significantly after sonication treatment, which is attributed to the increased pore volume and small nanosheets size that shorten the transport pathway of water molecules. Importantly, owing to the high photo-Fenton activity, the PKCN membrane exhibits fast (within 15 min) and excellent flux recovery (96.5%) after the photo-Fenton cleaning process. Furthermore, after 10 repeated usages, the PKCN membrane still keeps stable permeability and excellent purification efficiency. This work opens a door for developing self-cleaning membranes with the superior anti-fouling ability for effective oil/water separation.

Combined the Photocatalysis and Fenton-like Reaction to Efficiently Remove Sulfadiazine in Water Using g-C3N4/Ag/γ-FeOOH: Insights Into the Degradation Pathway From Density Functional Theory

Sulfadiazine (SDZ) is a common antibiotic pollutant in wastewater. Given that it poses a risk as an environmental pollutant, finding effective ways to treat it is important. In this paper, the composite catalytic material g-C₃N₄/Ag/γ-FeOOH was prepared, and its degradation performance was studied. g-C₃N₄/Ag/γ-FeOOH had a superior degradation effect on SDZ than g-C₃N₄ and γ-FeOOH. Compared with different g-C₃N₄ loadings and different catalyst dosages (5, 10, 25, and 50 mg/L), 2 mg/L g-C₃N₄/Ag/γ-FeOOH with a g-C₃N₄ loading of 5.0 wt% has the highest degradation promotion rate for SDZ, reaching up to 258.75% at 600 min. In addition, the photocatalytic enhancement mechanism of the catalyst was studied. Density functional theory (DFT) calculations indicated that the enhancement of photocatalytic activity was related to the narrowing of the forbidden band and the local electron density of the valence band. The bandgap of the catalyst was gradually narrowed from 2.7 to 1.05 eV, which can increase the light absorption intensity and expand the absorption edge. The density of states diagram showed that the local resonance at the interface could effectively improve the separation efficiency of e⁻-h⁺ pairs. Four degradation paths of SDZ were speculated based on DFT calculations. The analysis confirmed that the degradation path of SDZ primarily included Smiles-type rearrangement, SO₂ extrusion, and S-N bond cleavage processes.

Optimizing the Electronic Structure of ZnS via Cobalt Surface Doping for Promoted Photocatalytic Hydrogen Production

Developing highly efficient semiconductor photocatalysts for H₂ evolution is intriguing, but their efficiency is subjected to the following three critical issues: limited light absorption, low carrier separation efficiency, and sluggish H₂ evolution kinetics. Element surface doping is a feasible strategy to synchronously break through the above limitations. In this study, we prepared a series of Co-surface-doped ZnS photocatalysts to systematically investigate the effects of Co surface doping on photocatalytic activity and electronic structure. The implantation of Co results in the emergence of the impurity level above the valence band (VB) and the upshifted conduction band (CB) and enhances its visible light absorption. Co gradient doping inhibits the combination and facilitates the migration of carriers. S atoms are proven to be reactive active sites for photocatalytic H₂ evolution over both ZnS and Co-doped ZnS. Co doping alters the surface electronic structure and decreases the absolute value for the hydrogen binding free energy (ΔG_H) of the adsorbed hydrogen atom on the catalyst. As a consequence, Co-surface-doped ZnS shows boosted photocatalytic H₂ evolution activity relative to the undoped material. This work provides insights into the mechanistic understanding of the surface element doping modification strategy to developing efficient photocatalysts.

Photocatalytic degradation of tetracycline by Phosphorus-doped carbon nitride tube combined with peroxydisulfate under visible light irradiation

Photocatalysis has been regarded as a kind of environmentally friendly advanced oxidation process to eliminate pollutants. In this work, Phosphorus-doped carbon nitride tube (PCN) was synthesized via a hydrothermal calcination method and applied to degrade tetracycline (TC) through combing with peroxydisulfate (PDS) under visible light irradiation. Experimental results showed that the optimized catalysts PCN-5 exhibited superior degradation performance and reusability for TC degradation. 96.4% TC could be degraded for optimal PCN-5 with 0.3 g·L^-1 catalysts and 1.0 g·L^-1 PDS under visible light within 60 min. In addition, the degradation rate constant for TC of PCN + PDS + Vis system was still above 85% after five uses. Radical trapping experiment indicating that O₂^·- is the dominant radical for TC degradation. The findings of this work revealed the potential application of the PCN + PDS + Vis system toward degrading contaminants in wastewater.

Iodide-Induced Fragmentation of Polymerized Hydrophilic Carbon Nitride for High-Performance Quasi-Homogeneous Photocatalytic H2 O2 Production

Polymeric carbon nitride (PCN) as a class of two-electron oxygen reduction reaction (2 e^- ORR) photocatalyst has attracted much attention for H₂ O₂ production. However, the low activity and inferior selectivity of 2 e^- ORR greatly restrict the H₂ O₂ production efficiency. Herein, we develop a new strategy to synthesize hydrophilic, fragmented PCN photocatalyst by the terminating polymerization (TP-PCN) effect of iodide ions. The obtained TP-PCN with abundant edge active sites (AEASs), which can form quasi-homogeneous photocatalytic system, exhibits superior H₂ O₂ generation rate (3265.4 μM h^-1 ), far surpassing PCN and other PCN-based photocatalysts. DFT calculations further indicate that TP-PCN is more favorable for electron transiting from β spin-orbital to the π* orbitals of O₂ , which optimizes O₂ activation and reduces the energy barrier of H₂ O₂ formation. This work provides a new concept for designing functional photocatalysts and understanding the mechanism of O₂ activation in ORR for H₂ O₂ production.

Synthesis and electrochemical properties of Zn2Ti3O8/g-C3N4 composites as anode materials for Li-ion batteries

Zn₂Ti₃O₈/g-C₃N₄ (0, 1, 3 and 8 wt%) composites were prepared via a simple solvothermal method, and their physical and electrochemical properties were systematically analyzed. SEM and HRTEM results show that the Zn₂Ti₃O₈/g-C₃N₄ spherical structures with width sizes of about 500-700 nm are plump and uniform. Moreover, g-C₃N₄ with a large specific surface area can effectively buffer the deformation of Zn₂Ti₃O₈ and reduce the resistance of Zn₂Ti₃O₈ charge transfer and Li⁺ diffusion, thus improving the conductivity of Zn₂Ti₃O₈. The results reveal that Zn₂Ti₃O₈/g-C₃N₄ (3 wt%) had the most outstanding electrochemical performance of all samples. It can deliver discharge (charge) capacities of 444.6 (387.9), 284.5 (280.8), 197.5 (199.9), 149.9 (149.3), 119.2 (118.7) and 81.4 (81) mA h g^-1 cycled at 50, 100, 300, 600, 900, and 1500 mA g^-1, respectively. At the same current densities, pure Zn₂Ti₃O₈ only provides discharge (charge) capacities of 325.4 (283.5), 223.7 (219.5), 142.9 (141.8), 95.4(94.8), 69.4 (69.4) and 38.3 (39.3) mA h g^-1. The results verify that Zn₂Ti₃O₈/g-C₃N₄ materials are expected to be remarkable anode materials for Li-ion batteries.