KCl-mediated dual electronic channels in layered g-C3N4 for enhanced visible light photocatalytic NO removal

Enhanced biomimetic catalysis via self-cascade photocatalytic hydrogen peroxide production over modified carbon nitride nanozymes for total antioxidant capacity evaluation

The peroxidase mimics usually requires the addition of exogenous hydrogen peroxide (H2O2), which greatly hinder their practical applications. Herein, through rational co-modification of multiple elements (potassium (K), chlorine (Cl) and iodine (I)), the modified carbon nitride nanomaterials (KCl/KI-CN) could serve as efficient bifunctional catalysts. The multiple elements doping and the incorporation of cyano groups (CN) are deemed to enhance their photocatalytic and peroxidase-like activity, respectively. Based on the photocatalytic function, H2O2 can be produced continuously and steadily via two-electron oxygen reduction over modified carbon nitride under visible light irradiation. Subsequently, the KCl/KI-CN could catalyze the chromogenic substrate by the in-situ produced H2O2. Taking advantage of the bifunctional properties of modified carbon nitride, we for the first time demonstrate a self-cascade catalytic process and apply successfully for the ascorbic acid (AA) detection and versatile total antioxidant capacity (TAC) evaluation. This paper not only prepares an efficiently bifunctional catalyst but also provides a new self-cascade photocatalytic H2O2 production strategy for the peroxidase-like application.

K-N Bridge-Mediated charge separation in hollow g-C3N4 Frameworks: A bifunctional photocatalysts towards efficient H2 and H2O2 production

The development of bifunctional photocatalysts for enhancing hydrogen (H2) and hydrogen peroxide (H2O2) production from water is essential in addressing environmental and energy issues. However, the practical implementation of photocatalytic technology is still constrained by the inadequate separation of photo-generated charge carriers. Herein, potassium (K) atoms are introduced into the interlayers of graphitic carbon nitride (g-C3N4) with a hollow hexagonal structure (K-TCN) and are coordinated with N atoms in adjacent layers. The presence of K-N coordination serves as a layer bridge, facilitating the separation of charge carriers. The hollow hexagonal structure reduces the distance over which photogenerated electrons migrate to the surface, thereby enhancing the reaction kinetics. Consequently, the optimized K-TCN exhibits a dramatically improved photocatalytic H2 (941.6 μmol g-1h-1 with platinum (Pt) as the cocatalyst) and H2O2 (347.6 μmol g-1h-1) generation as compared to hollow g-C3N4 (TCN) and bulk g-C3N4 nanosheet (CN) without K-N bridge under visible light irradiation. The unique design holds promising potential for developing highly efficient bifunctional photocatalysts towards producing renewable fuels and value-added chemicals.

Alkali functionalized carbon nitride with internal van der Waals heterostructures: Directional charge flow to enhance photocatalytic hydrogen production

Improving the charge separation and migration in graphitic carbon nitride (CN) is the critical issue to enhance its photocatalytic performance, but still remains very challenging. Herein, the alkali metals were introduced into the interlayer and intralayer of CN to tackle this challenge. The lithium sodium-modifying carbon nitride layer (LiNaCN2) and the adjacent CN layer formed a van der Waals heterostructures (VDWHs), while the potassium-intercalating served as interlayer charge transfer channels to induce the directional charge flow. Experiments and theoretical calculations indicated that such unique construction provided intrinsic driving force to obtain the electrons from LiNaCN2 to CN via directional potassium channels. In accordance with the theoretical prediction, a dramatically red-shift of the light absorption feature was achieved for interlayer potassium-intercalating and intralayer lithium sodium-modifying co-functionalized carbon nitride (LiNaCN-K-CN2) to show narrowed bandgap energy of 2.15 eV. This directional charge flow in CN resulted in the rapid transfer of charge carriers in both interlayer as well as intralayer of CN, which reduced the electronic localization as well as extended the π conjugative effect. Consequently, the LiNaCN-K-CN2 displayed stable and remarkable hydrogen production rate of about 2.46 mmol g^-1 h^-1 with apparent quantum yield (AQY) of about 13.68% at 435 nm, which was 22 folds higher than that of the pristine CN. This finding provides the feasible strategy to precisely tune the directions of charge transfer for high-performance CN-based photocatalysts.

Potassium Molten Salt-Mediated In Situ Structural Reconstruction of a Carbon Nitride Photocatalyst

Metal-free polymeric carbon nitride (PCN) materials are at the forefront of photocatalytic applications. Nevertheless, the overall functionality and performance of bulk PCN are limited by rapid charge recombination, high chemical inertness, and inadequate surface-active sites. To address these, here, we employed potassium molten salts (K⁺X^-, where X^- is Cl^-, Br^-, and I^-) as a template for the in situ generation of surface reactive sites in thermal pyrolyzed PCN. Theoretical calculations imply that addition of KX salts to PCN-forming monomers causes halogen ions to be doped into C or N sites of PCN with a relative trend of halogen ion doping being Cl < Br < I. The experimental results show that reconstructing C and N sites in PCN develops newer reactive sites that are beneficial for surface catalysis. Interestingly, the photocatalytic H₂O₂ generation rate of KBr-modified PCN was 199.0 μmol h^-1, about three times that of bulk PCN. Owing to the simple and straightforward approach, we expect molten salt-assisted synthesis to have wide exploration in modifying PCN photocatalytic activity.

Potassium-Derived Charge Channels in Boron-Doped g-C3N4 Nanosheets for Photocatalytic NO Oxidation and Hydrogen Evolution

The application of graphitic carbon nitride (g-C3N4) in photocatalytic NO oxidation was limited due to severe recombination of photogenerated carriers and low concentration of oxidizing species. In this work, K and B were introduced into the interlayer and in-plane framework of g-C3N4 to address this challenge through the thermal polymerization process. The synthesized K-doped B-g-C3N4 nanosheets exhibited expanded light absorption and low charge recombination efficiency. In addition, the doping of K and B reduced the band gap of g-C3N4, which corresponded to enhanced light absorption. B was introduced into the in-plane structure by replacing C atoms, which adjusted the in-plane electron distribution. K was inserted into the interlayer by binding to the N and C atoms of adjacent layers. K-derived electron transfer channels were constructed, which increased electron delocalization and expanded the π-conjugate system. More electrons were transferred through the interlayer channels and were involved in the reaction process. The severe carrier recombination and weak transfer were improved due to the synergistic effect of K and B doping. K-doped B-g-C3N4 nanosheets exhibited enhanced generation of superoxide radicals and hydroxyl radicals, which played a key role during NO oxidation. The photocatalytic NO oxidation efficiency of codoped g-C3N4 nanosheets reached 61%, which was 2.1 and 1.2 times of that of pristine g-C3N4 and B-doped g-C3N4, respectively. The codoped g-C3N4 sample still exhibited stable photocatalytic NO oxidation efficiency after five cycles. This result provided a potential idea for improving the charge distribution and transfer of layered materials by codoping metallic and nonmetallic elements and for photocatalytic NO oxidation.

Environmental application of chlorine-doped graphitic carbon nitride: Continuous solar-driven photocatalytic production of hydrogen peroxide

Solar-driven photocatalytic generation of H₂O₂ over metal-free catalysts is a sustainable approach for value-added chemical production. Here, we synthesized chlorine-doped graphitic carbon nitride (Cl-doped g-C₃N₄) through a solvothermal method to effectively produce H₂O₂ with a rate of 1.19 ± 0.06 µM min^-1 under visible light irradiation, which was improved by 104 times compared to pristine g-C₃N₄. Continuous net production of H₂O₂ was realized at a rate of 2.78 ± 0.10 µM min^-1 up to 54 h with isopropanol as the hole scavenger, whereas H₂O₂ production was only sustained for ~ 6 h without scavengers. Both molecular simulations and advanced spectroscopic characterizations elucidated that the Cl dopant increased the charge transfer rate, decreased the bandgap, and reduced the activation energy of the rate-limiting step of O₂ reduction, all of which favored H₂O₂ production. This work implemented a novel metal-free photocatalyst for sustainable H₂O₂ production and elucidated the mechanism for promoting H₂O₂ production that can guide future photoreactive nanomaterial design.

Efficient NO removal and photocatalysis mechanism over Bi-metal@Bi2O2[BO2(OH)] with oxygen vacancies

Photocatalysis technology prevails as a feasible option for air pollution control, in which high-efficiency charge separation and effective pollutant activation are the crucial issues. Here, this work designed Bi-metal@ Bi₂O₂[BO₂(OH)] with oxygen vacancies (OVs) catalyst for photocatalytic oxidation of NO under visible light, to shed light on the above two processes. Experimental characterizations and density functional theory (DFT) calculations reveal that a unique electron transfer covalent loop（[Bi₂O₂]²⁺ → Bi-metal → O^2-）can be formed during the reaction to guide the directional transfer of carriers, significantly improving the charge separation efficiency and the yield of active oxygen species. Simultaneously, the defect levels served by OVs also play a part. During the NO purification process, in-situ DRIFTS assisted with DFT calculations reveal that Bi metals could be functioned as electron donors to activate NO molecules and form NO^-, a key intermediate. This induces a new reaction path of NO → NO^- → NO₃^- to achieve the harmless conversion of NO, effectively restraining the generation of noxious intermediates (NO₂, N₂O₄). It is expected that this study would inspire the design of more artful photocatalysts for effective charge transfer and safe pollutants purification.

Metal-Doped Graphitic Carbon Nitride Nanomaterials for Photocatalytic Environmental Applications—A Review

In the current world situation, population and industrial growth have become major problems for energy and environmental concerns. Extremely noxious pollutants such as heavy metal ions, dyes, antibiotics, phenols, and pesticides in water are the main causes behind deprived water quality leading to inadequate access to clean water. In this connection, graphite carbon nitride (GCN or g-C3N4) a nonmetallic polymeric material has been utilized extensively as a visible-light-responsive photocatalyst for a variety of environmental applications. This review focuses on recent developments in the design and photocatalytic applications of metal-doped GCN-based nanomaterials in CO2 photoreduction, water splitting toward hydrogen production, bacterial disinfection, and organic pollutant degradation. Additionally, this review discusses various methods of using GCN-based materials to optimize dye sensitization, metal deposition, ion doping, and their environmental applications.

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.

Visible-light-driven photocatalytic disinfection by S-scheme α-Fe2O3/g-C3N4 heterojunction: Bactericidal performance and mechanism insight

High-performance photocatalytic applications require to develop heterostructures between two semiconductors with matched band energy levels to facilitate charge-carrier separation. The S-scheme photocatalytic system has great potential to be explored, in terms of the improvement of charge separation, however, small efforts have been made in photocatalytic disinfection application. In this study, a non-toxic and low-cost S-scheme photocatalytic system composed of α-Fe₂O₃ and g-C₃N₄ was fabricated by in-suit production of g-C₃N₄ and firstly applied into water disinfection. The α-Fe₂O₃/g-C₃N₄ junction demonstrated an enhanced activity for photocatalytic bacterial inactivation, with the complete inactivation of 7 log₁₀ cfu·mL^-1 of Escherichia coli K-12 cells within 120 min under visible light irradiation. Its logarithmic bacterial inactivation efficiency was nearly 7 times better than that of single g-C₃N₄. The experimental results suggested that the effective prevention of charge-carrier recombination led to an improved generation of reactive oxygen species (ROSs), resulting in impressive disinfection performance. Moreover, the DNA gel electrophoresis experiments validated the reason for the irreversible death of bacteria, which was the leakage and destruction of chromosomal DNA. In addition, this S-scheme heterojunction also showed excellent photocatalytic disinfection performance in authentic water matrices (including tap water, secondary treated sewage effluent, and surface water) under visible light irradiation. Hence, the α-Fe₂O₃/g-C₃N₄ composite has great potential for sustainable and efficient photocatalytic disinfection applications.

Tuning the p-Orbital Electron Structure of s-Block Metal Ca Enables a High-Performance Electrocatalyst for Oxygen Reduction

Most previous efforts are devoted to developing transition metals as electrocatalysts guided by the d-band center model. The metals of the s-block of the periodic table have so far received little attention in the application of oxygen reduction reactions (ORR). Herein, a carbon catalyst with calcium (Ca) single atom coordinated with N and O is reported, which displays exceptional ORR activities in both acidic condition (E_1/2  = 0.77 V, 0.1 m HClO₄ ) and alkaline condition (E_1/2  = 0.90 V, 0.1 m KOH). The CaN, O/C exhibits remarkable performance in zinc-air battery with a maximum power density of 218 mW cm^-2 , superior to a series of catalysts reported so far. X-ray absorption near-edge structure (XANES) characterization confirms the formation of N- and O-atom-coordinated Ca in the carbon matrix. Density functional theory (DFT) calculations reveal that the high catalytic activity of main-group Ca is ascribed to the fact that its p-orbital electron structure is regulated by N and O coordination so that the highest peak (E_P ) of the projected density of states (PDOS) for the Ca atom is moved close to the Fermi level, thereby facilitating the adsorption of ORR intermediates and electron transfer.

Photodepositing CdS on the Active Cyano Groups Decorated g-C3 N4 in Z-Scheme Manner Promotes Visible-Light-Driven Hydrogen Evolution

g-C₃ N₄ /CdS heterojunctions are potential photocatalysts for hydrogen production but their traditional type-II configuration generally leads to weak oxidative and reductive activity. How to construct the novel Z-scheme g-C₃ N₄ /CdS counterparts to address this issue remains a great challenge in this field. In this work, a new direct Z-scheme heterojunction of defective g-C₃ N₄ /CdS is designed by introducing cyano groups (NC-) as the active bridge sites. Experimental observations in combination with density functional theory (DFT) calculations reveal that the unique electron-withdrawing feature of cyano groups in the defective g-C₃ N₄ /CdS heterostructure can endow this photocatalyst with numerous advantageous properties including high light absorption ability, strong redox performance, satisfactory charge separation efficiency, and long lifetime of charge carriers. Consequently, the resultant photocatalytic system exhibits more active performance than CdS and g-C₃ N₄ under visible light and reaches an excellent hydrogen evolution rate of 1809.07 µmol h^-1 g^-1 , which is 6.09 times higher than pristine g-C₃ N₄ . Moreover, the defective g-C₃ N₄ /CdS photocatalyst maintains good stability after 40 h continuous test. This work provides new insights into design and construction of Z-scheme heterojunctions for regulating the visible-light-induced photocatalytic activity for H₂ evolution.

Recent progress on strategies for the preparation of 2D/2D MXene/g-C3N4 nanocomposites for photocatalytic energy and environmental applications

This review summarizes the possible synthetic routes, optical and morphological features to explore the 2D/2D interface and mechanism path in 2D/2D MXene/g-C₃N₄ nanocomposites for photocatalytic applications.

Carbon-Graphitic Carbon Nitride Hybrids for Heterogeneous Photocatalysis

Polymeric graphitic carbon nitride (g-C₃ N₄ ) and various carbon materials have experienced a renaissance as viable alternates in photocatalysis due to their captivating metal-free features, favorable photoelectric properties, and economic adaptabilities. Although numerous efforts have focused on the integration of both materials with optimized photocatalytic performance in recent years, the direct parameters for this emerging enhancement are not fully summarized yet. Fully understanding the synergistic effects between g-C₃ N₄ and carbon materials on photocatalytic action is vital to further development of metal-free semiconductors in future studies. Here, recent advances of carbon/g-C₃ N₄ hybrids on various photocatalytic applications are reviewed. The dominant governing factors by inducing carbon into g-C₃ N₄ photocatalysts with involving photocatalytic mechanism are highlighted. Five typical carbon-induced enhancement effects are mainly discussed here, i.e., local electric modification, band structure tailoring, multiple charge carrier activation, chemical group functionalization, and abundant surface-modified engineering. Photocatalytic performance of carbon-induced g-C₃ N₄ photocatalysts for addressing directly both the renewable energy storage and environmental remediation is also summarized. Finally, perspectives and ongoing challenges encountered in the development of metal-free carbon-induced g-C₃ N₄ photocatalysts are presented.

0D/2D Heterojunctions of Ti3C2 MXene QDs/SiC as an Efficient and Robust Photocatalyst for Boosting the Visible Photocatalytic NO Pollutant Removal Ability

In this work, a novel heterojunction catalyst was constructed by introducing Ti₃C₂ MXene quantum dots (QDs) into SiC. The Ti₃C₂ MXene QDs/SiC composite showed 74.6% efficiency in NO pollutant removal under visible light irradiation, which is 3.1 and 3.7 times higher than those of the bare Ti₃C₂ MXene quantum dots and SiC, respectively. The Ti₃C₂ MXene quantum dots existing in SiC can function as a channel for electron and hole transfer. The enhanced visible light absorption, increased superoxide radical, and strong oxidization ability endow the Ti₃C₂ MXene QDs/SiC composite with a superior photocatalytic performance for NOx removal. The increased superoxide radical formation and enhanced oxidization ability of Ti₃C₂ MXene QDs/SiC were demonstrated by theoretical calculations. The robust stability in both photocatalytic performance and crystal structures was revealed in the Ti₃C₂ MXene QDs/SiC composite using the cycling test, transient photocurrent response, XRD, and TG.

Photocatalytic Degradation and Antibacterial Properties of Fe3+-Doped Alkalized Carbon Nitride

Discovering novel materials and improving the properties of existing materials are the main goals in the field of photocatalysis to increase the potential application of the materials. In this paper, a modified graphitic carbon nitride (g-C₃N₄) photocatalyst named Fe³⁺-doped alkalized carbon nitride, which couples the photocatalytic reaction with the Fenton reaction, is introduced to demonstrate its Rhodamine B (RhB) degradation and antibacterial properties. Under visible-light irradiation, the degradation rate of RhB was 99.9% after 200 min, while the antibacterial rates of Pseudomonas aeruginosa (P. aeruginosa), Escherichia coli (E. coli), and Staphylococcus aureus (S. aureus) after 300 min were 99.9986%, 99.9974%, and 99.9876%, respectively. Moreover, the repetitive experiments of RhB degradation demonstrate that the proposed photocatalysts have excellent stability and reusability. The active free radical trapping experiments reveal that the superoxide radical (·O2−) is the dominant reactive oxygen species. In addition, the Fenton reaction is introduced into the photocatalytic system due to the doping of Fe³⁺, and the hydroxyl radical (·OH) produced from the Fenton reaction further enhances the photocatalytic performance. The remarkable improvement in photocatalytic performance of the proposed photocatalyst can be attributed to its broader UV–visible absorption characteristic and the occurrence of the Fenton reaction.

Synthesis and fabrication of g-C3N4-based materials and their application in elimination of pollutants

With the fast development of industrial and human activity, large amounts of persistent organic pollutants, heavy metal ions and radionuclides are released into the natural environment, which results in environmental pollution. The efficient elimination of the natural environment is crucial for the protection of environment to against the pollutants' toxicity to human beings and living organisms. Graphitic carbon nitride (g-C₃N₄) has drawn multidisciplinary attention especially in environmental pollutants' cleanup due to its special physicochemical properties. In this review, we summarized the recent works about the synthesis of g-C₃N₄, element-doping, structure modification of g-C₃N₄ and g-C₃N₄-based materials, and their application in the sorption, photocatalytic degradation and reduction-solidification of persistent organic pollutants and heavy metal ions. The interaction mechanisms were discussed from advanced spectroscopic analysis and computational approaches at molecular level. The challenges and future perspectives of g-C₃N₄-based materials' application in environmental pollution management are presented in the end. This review highlights the real applications of g-C₃N₄-based materials as adsorbents or photocatalysts in the adsorption-reduction-solidification of metal ions or photocatalytic degradation of organic pollutants. The contents are helpful for the undergraduate students to understand the recent works in the elimination of organic/inorganic pollutants in their pollution management.

Construction of Embedded Heterostructured SrZrO3/Flower-like MoS2 with Enhanced Dye Photodegradation under Solar-Simulated Light Illumination

SrZrO3/flower-like MoS2 composites with an embedded heterostructure were synthesized via a simple two-step hydrothermal method and their performance was evaluated by photodegradation of methylene blue (MB) under solar-simulated light irradiation. The sandwiched flower-like MoS2 with a high Mo(VI) ratio was adopted as the matrix, and SrZrO3 was grown between the MoS2 layers, forming an intense contact interface, which promotes the efficient separation and transport of photoinduced carriers. The enhanced photocatalytic degradation of 99.7% after 80 min of irradiation is exhibited over the MS5 sample (5 wt % SrZrO3 loading amount on the MoS2 matrix). Moreover, the ratio of Mo(VI) and the superoxide radical plays a crucial role in the photodegradation of MB, and the higher the ratio the better the performance. This work provided a strategy to design a new kind of photocatalyst for photocatalysis and indicated that MoS2 is preferably adopted as a matrix rather than as a loading component.

CdZnS nanorods with rich sulphur vacancies for highly efficient photocatalytic hydrogen production

A 1D CdZnS nanorod solid solution with rich sulfur vacancies achieved excellent photocatalytic hydrogen production under visible irradiation.

Enhanced photocatalytic activity of g-C3N4/MnO composites for hydrogen evolution under visible light

In this work, a range of g-C3N4/MnO composites were constructed using g-C3N4 nanosheets modified with MnO, and the photocatalytic performance for hydrogen evolution was evaluated by using these as-prepared g-C3N4/MnO composites as photocatalysts. It was found that the photocatalytic activity of the g-C3N4/MnO composites for hydrogen evolution is significantly enhanced compared with that of pristine g-C3N4 since the formation of heterojunctions between the MnO nanoparticles and g-C3N4 nanosheets through coordination covalent bonds promotes the charge carrier transfer and separation abilities of the composites. The loading mass of MnO also has a large influence on the photocatalytic activity of the g-C3N4/MnO composites. Particularly, the g-C3N4/MnO-5 composite with 5 wt% MnO shows superior photocatalytic activity with a hydrogen evolution rate of 559 μmol h-1 g-1 under visible light, which is about 9 times that of the bulky g-C3N4. These findings demonstrate that the combination of metal oxides and g-C3N4 to construct composite photocatalysts is an effective method to improve the photocatalytic performance.

Preparation of MnOx Supported LiOH Activated Soybean Oil Sludge Catalyst and Its Analysis in Denitration Mechanism of Selective Catalytic Oxidation (SCO)

Treatment with selective catalytic oxidation (SCO) is an effective technology applied recently for conversion of nitrogen oxides pollution control. In order to solve the problems of high cost and difficulties in practical application of SCO catalyst, it was put forward using the solid waste sludge from soybean oil plant as catalyst carrier to prepare denitration catalyst. The sludge was treated by alkaline activation and then MnOx-based sludge was prepared by impregnation. Finally, MnOx-based sludge was calcined in the muffle furnace. The effects of activation and calcination conditions on catalyst activity were investigated. Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the activity of the sludge based denitration catalyst, and the structure and activity of the sludge based denitration catalyst were furtherly confirmed. According to the achieved results, (1) after activated by LiOH with a mass concentration of 15% for 4 hours, the surface of the sludge catalyst has more alkali functional groups, making the denitration of sludge catalyst the best; (2) the MnOx-based catalyst calcined in the muffle furnace with calcination temperature of 450 °C for 4 hours has obvious denitration efficiency.

Constructing a novel family of halogen-doped covalent triazine-based frameworks as efficient metal-free photocatalysts for hydrogen production

Halogens, as typical non-metal dopants, have attracted intensive interests for developing highly active photocatalysts. However, the essential factors and underlying mechanism of halogen modification are still unclear. Herein, we systematically report the development of halogen (F, Cl and Br)-doped covalent triazine-based frameworks (CTFs) via a facile thermal treatment of CTFs and an excess of ammonium halide. The introduction of halogen atoms endowed CTFs with multiple superior effects such as improved optical absorption, promoted charge migration, narrowed band gaps and tuned band positions. The newly developed halogen-doped CTFs showed remarkable photocatalytic activities for H₂ evolution under visible-light irradiation. Notably, the most enhanced photocatalytic performance was obtained with Cl-doped CTFs, which exhibited 7.1- and 2.4-fold enhancements compared to un-doped CTFs and Cl-doped g-C₃N₄, respectively. The electronegativity and atomic radius of the halogen atoms affected the modification of the optical and electronic properties, leading to different photocatalytic performances of F-, Cl- and Br-doped CTFs. The conclusions presented in this work will provide some new insights into the understanding of the doping effect for the improvement of the photocatalytic activity of halogen-doped CTF photocatalysts.

Metallic 1T-phase MoS2 quantum dots/g-C3N4 heterojunctions for enhanced photocatalytic hydrogen evolution

Recently, molybdenum disulfide (MoS₂) has been regarded as an efficient non-precious-metal co-catalyst for photocatalytic hydrogen (H₂) evolution, however, its inherent low-density active site and poor electron transfer efficiency have essentially limited its photocatalytic properties. Here we report that 1T-MoS₂ quantum dots (QDs) can act as co-catalysts in assisting the photocatalytic H₂ evolution to form heterostructures with g-C₃N₄ nanosheets (denoted as 1T-MoS₂ QDs@g-C₃N₄). Benefiting from the abundance of exposed catalytic edge sites and the excellent intrinsic conductivity of 1T-MoS₂ QDs, an optimized 1T-MoS₂ QD@g-C₃N₄ composite (15 wt%) exhibits an extraordinary photocatalytic H₂ evolution rate of 1857 μmol h^-1 g^-1 under simulated solar light irradiation, apparently 37.9 times higher than that of pure g-C₃N₄ NSs (49 μmol h^-1 g^-1). Meanwhile, the 1T-MoS₂ QD@g-C₃N₄ composites exhibit a good stability in the cyclic runs for the photocatalytic H₂ production. The high efficient photocatalytic activity and stability of the 1T-MoS₂ QD@g-C₃N₄ composite is primarily attributed to the following reasons: (1) the introduction of 1T-MoS₂ QDs results in a stronger light absorption capability in comparison with pure g-C₃N₄; (2) the tiny particle size of 1T-MoS₂ QDs, in which edges and basal surface are catalytically active, provides a proliferated density of catalytically active sites; (3) 1T-MoS₂ QD co-catalysts with metallic characteristics could act as efficient electron acceptors, which builds up a highly efficient pathway for photo-generated electrons from the CB of g-C₃N₄ NSs to 1T-MoS₂ and thus realizes rapid spatial charge separation. The improved light harvesting ability, increased catalytically active sites, as well as increased separation of charge carriers could be responsible for the improved photocatalytic H₂ evolution. This work will provide new insight for the design and fabrication of smarter, cheaper and more robust artificial photocatalysts for photocatalytic H₂ evolution.

High-yield lactic acid-mediated route for a g-C3N4 nanosheet photocatalyst with enhanced H2-evolution performance

Facile and novel strategies to prepare g-C3N4 nanosheets are required to greatly improve their photocatalytic H2-production activity. In this study, a lactic acid-mediated synthesis route has been developed to prepare g-C3N4 nanosheets, which includes the preassembled formation of lactic acid-melamine co-monomers, followed by direct high-temperature calcination. In this case, it is found that during high-temperature calcination, the lactic acid molecules can greatly prevent the serious polymerization of melamine molecules, resulting in the formation of g-C3N4 nanosheets. Moreover, owing to the strong coupling with melamine molecules, lactic acid can also significantly increase the production rate (ca. 35.16 wt%) of g-C3N4 nanosheets from the melamine precursor via preventing the rapid sublimation of melamine and its intermediates during the calcination progress compared with the well-known two-step calcination method. Photocatalytic experimental data reveal that the resultant g-C3N4 nanosheet photocatalysts show a greatly improved H2-production rate, and the g-C3N4 (500 μL) sample exhibits the best photocatalytic performance, which is obviously two times higher than that of the conventional bulk g-C3N4. In addition to lactic acid, it is very interesting to find that acetic acid can also be used to prepare g-C3N4 nanosheets via a similar formation mechanism, strongly suggesting the universality and versatility of the present lactic acid-mediated synthesis route. The present synthesis strategy may broaden the horizons for the synthesis of high-efficiency photocatalysts.

Modified carbon nitride nanozyme as bifunctional glucose oxidase-peroxidase for metal-free bioinspired cascade photocatalysis

Nanomaterials-based biomimetic catalysts with multiple functions are necessary to address challenges in artificial enzymes mimicking physiological processes. Here we report a metal-free nanozyme of modified graphitic carbon nitride and demonstrate its bifunctional enzyme-mimicking roles. With oxidase mimicking, hydrogen peroxide is generated from the coupled photocatalysis of glucose oxidation and dioxygen reduction under visible-light irradiation with a near 100% apparent quantum efficiency. Then, the in situ generated hydrogen peroxide serves for the subsequent peroxidase-mimicking reaction that oxidises a chromogenic substrate on the same catalysts in dark to complete the bifunctional oxidase-peroxidase for biomimetic detection of glucose. The bifunctional cascade catalysis is successfully demonstrated in microfluidics for the real-time colorimetric detection of glucose with a low detection limit of 0.8 μM within 30 s. The artificial nanozymes with physiological functions provide the feasible strategies for mimicking the natural enzymes and realizing the biomedical diagnostics with a smart and miniature device.

Hydrogen peroxide-assisted synthesis of oxygen-doped carbon nitride nanorods for enhanced photocatalytic hydrogen evolution

Polymer-derived carbon nitrides based photocatalysts are very promising for solar water splitting, CO₂ reduction and environmental remediation. However, these photocatalysts still suffer from low visible light utilization efficiency, rapid recombination of photogenerated charge carriers and slow transfer kinetics. Herein, we report a hydrogen peroxide-assisted hydrothermal strategy to synthesize one-dimensional oxygen-doped carbon nitrides (OCN) for photocatalytic hydrogen evolution. A possible self-assembly mechanism is discussed. Experimental results and theoretical calculations indicate that the as-synthesized one-dimensional OCN possess narrowed band gap energy and optimized band structure, which may allow more effective visible-light harvesting and facilitate photogenerated electron–hole pair separation and transfer. As a result, the photocatalytic hydrogen evolution rates improve from 10.4 μmol h⁻¹ to 74.0 μmol h⁻¹ under visible light (λ > 400 nm), which is among the best of the reported CN-based photocatalysts for visible-light-driven hydrogen evolution. This study provides a new avenue toward the development of highly efficient carbon nitrides based photocatalysts for photocatalytic applications.

One-dimensional oxygen-doped carbon nitride nanorods synthesized via a hydrogen peroxide-assisted process exhibit enhanced hydrogen evolution under visible light.

Emerging trends in sensors based on carbon nitride materials

A new class of functional materials, carbon nitrides, has recently attracted the attention of researchers.

One-step facile synthesis and high H2-evolution activity of suspensible CdxZn1-xS nanocrystal photocatalysts in a S2-/SO32- system

For a CdS-based photocatalyst, both the photocorrosion resistance and the rapid H₂-production reaction are highly required for improving its photocatalytic H₂-production performance. In this study, a facile strategy was reported to simultaneously realize an improved photocorrosion resistance and rapid interfacial H₂-evolution reaction of Cd_xZn_1-xS solid-solution photocatalysts in a sulfur-rich S^2-/SO₃^2- solution. Here, the suspensible Cd_xZn_1-xS nanocrystal photocatalysts are prepared by a one-step co-precipitation route through the direct introduction of Zn²⁺/Cd²⁺ mixing ions in a sulfur-rich Na₂S-Na₂SO₃ solution, and the resultant Cd_xZn_1-xS nanocrystals (ca. 5 nm) display a suspensible structure owing to the numerous and selective adsorption of S^2-/SO₃^2- on the surface of these Cd_xZn_1-xS nanocrystals. It is found that the bandgap structure of Cd_xZn_1-xS (from 2.25 to 3.52 eV) nanocrystals can be easily controlled by adjusting the Cd²⁺/Zn²⁺ molar ratio. The photocatalytic experimental results suggested that the suspensible Cd_xZn_1-xS nanocrystal photocatalysts clearly displayed an excellent photocatalytic H₂-production performance, and the suspensible Cd_0.6Zn_0.4S nanocrystals exhibit the highest photocatalytic H₂-generation performance of 717.19 μmol h^-1, a value higher than that of the sole CdS (320.99 μmol h^-1) and ZnS (5.89 μmol h^-1) by a factor of 2.2 and 121.8 times, respectively. Based on the experimental results, a possible S^2- active site-mediated mechanism accounted for the high H₂-production activity of the suspensible Cd_xZn_1-xS nanocrystals, namely the numerous adsorbed S^2- ions not only function as efficient hole scavengers to rapidly consume the photogenerated holes, resulting in an improved photocorrosion resistance of suspensible Cd_xZn_1-xS nanocrystals, but also serve as effective H⁺-capturing active sites to accelerate the interfacial H₂-production reaction. Meanwhile, an optimum bandgap structure of suspensible Cd_xZn_1-xS nanocrystals is also extremely required for promoting the photocatalytic H₂-production activity. This research may provide advanced insights for developing stable and high-activity photocatalytic materials.

Converting CO2 into fuels by graphitic carbon nitride-based photocatalysts

A metal-free photocatalyst, graphitic carbon nitride (GCN) with a moderate band gap catering for visible-light excitation, shows amazing potential in various photocatalytic applications. Carbon dioxide reduction using diversified photocatalysts has been attracting increasing public attention and the extensively studied GCN is one of the most promising photocatalysts. However, because of the low concentration and high recombination rate of photogenerated carriers, and some other disadvantages of the pristine GCN photocatalyst, the solar-to-fuel conversion efficiency is too low for practical use. Modifications or optimizations of GCN are therefore important to enhance its CO₂ photocatalytic conversion performance. This review summarizes the research progress made during the past five years on GCN-based photocatalysts in two main areas, which includes pristine GCN and its molecular modifications, and heterostructure composite photocatalysts based on GCN, for CO₂ reduction. It is expected that this review may benefit the development of GCN-based photocatalysts for CO₂-to-fuel conversion.