Graphitic Carbon Nitride Materials: Sensing,Imaging and Therapy

Graphitic carbon nitrides (g‐C₃N₄) are a class of 2D polymeric materials mainly composed of carbon and nitrogen atoms. g‐C₃N₄ are attracting dramatically increasing interest in the areas of sensing, imaging, and therapy, due to their unique optical and electronic properties. Here, the luminescent properties (mainly includes photoluminescence and electrochemiluminescence), and catalytic and photoelectronic properties related to sensing and therapy applications of g‐C₃N₄ materials are reviewed. Furthermore, the fabrication and advantages of sensing, imaging and therapy systems based on g‐C₃N₄ materials are summarized. Finally, the future perspectives for developing the sensing, imaging and therapy applications of the g‐C₃N₄ materials are discussed.     

Formation of B?N?C Coordination to Stabilize the Exposed Active Nitrogen Atoms in g‐C3N4 for Dramatically Enhanced Photocatalytic Ammonia Synthesis Performance

It is an important issue that exposed active nitrogen atoms (e.g., edge or amino N atoms) in graphitic carbon nitride (g‐C₃N₄) could participate in ammonia (NH₃) synthesis during the photocatalytic nitrogen reduction reaction (NRR). Herein, the experimental results in this work demonstrate that the exposed active N atoms in g‐C₃N₄ nanosheets can indeed be hydrogenated and contribute to NH₃ synthesis during the visible‐light photocatalytic NRR. However, these exposed N atoms can be firmly stabilized through forming B? N? C coordination by means of B‐doping in g‐C₃N₄ nanosheets (BCN) with a B‐doping content of 13.8 wt%. Moreover, the formed B? N? C coordination in g‐C₃N₄ not only effectively enhances the visible‐light harvesting and suppresses the recombination of photogenerated carriers in g‐C₃N₄, but also acts as the catalytic active site for N₂ adsorption, activation, and hydrogenation. Consequently, the as‐synthesized BCN exhibits high visible‐light‐driven photocatalytic NRR activity, affording an NH₃ yield rate of 313.9 µmol g^?1 h^?1, nearly 10 times of that for pristine g‐C₃N₄. This work would be helpful for designing and developing high‐efficiency metal‐free NRR catalysts for visible‐light‐driven photocatalytic NH₃ synthesis.     

Tubular g‐C3N4 Isotype Heterojunction: Enhanced Visible‐Light Photocatalytic Activity through Cooperative Manipulation of Oriented Electron and Hole Transfer

A tubular g‐C₃N₄ isotype heterojunction (TCNH) photocatalyst was designed for cooperative manipulation of the oriented transfer of photogenerated electrons and holes to pursue high catalytic performance. The adduct of cyanuric acid and melamine (CA·M) is first hydrothermally treated to assemble into hexagonal prism crystals; then the hybrid precursors of urea and CA·M crystals are calcined to form tubular g‐C₃N₄ isotype heterojunctions. Upon visible‐light irradiation, the photogenerated electrons transfer from g‐C₃N₄ (CA·M) to g‐C₃N₄ (urea) driven by the conduction band offset of 0.05 eV, while the photogenerated holes transfer from g‐C₃N₄ (urea) to g‐C₃N₄ (CA·M) driven by the valence band offset of 0.18 eV, which renders oriented transfer of the charge carriers across the heterojunction interface. Meanwhile, the tubular structure of TCNH is favorable for oriented electron transfer along the longitudinal dimension, which greatly decreases the chance of charge carrier recombination. Consequently, TCNH exhibits a high hydrogen evolution rate of 63 μmol h^?1 (0.04 g, λ > 420 nm), which is nearly five times of the pristine g‐C₃N₄ and higher than most of the existing g‐C₃N₄ photocatalysts. This study demonstrates that isotype heterojunction structure and tubular structure can jointly manipulate the oriented transfer of electrons and holes, thus facilitating the visible‐light photocatalysis.     

Polymeric Photocatalysts Based on Graphitic Carbon Nitride

Semiconductor‐based photocatalysis is considered to be an attractive way for solving the worldwide energy shortage and environmental pollution issues. Since the pioneering work in 2009 on graphitic carbon nitride (g‐C₃N₄) for visible‐light photocatalytic water splitting, g‐C₃N₄‐based photocatalysis has become a very hot research topic. This review summarizes the recent progress regarding the design and preparation of g‐C₃N₄‐based photocatalysts, including the fabrication and nanostructure design of pristine g‐C₃N₄, bandgap engineering through atomic‐level doping and molecular‐level modification, and the preparation of g‐C₃N₄‐based semiconductor composites. Also, the photo­catalytic applications of g‐C₃N₄‐based photocatalysts in the fields of water splitting, CO₂ reduction, pollutant degradation, organic syntheses, and bacterial disinfection are reviewed, with emphasis on photocatalysis promoted by carbon materials, non‐noble‐metal cocatalysts, and Z‐scheme heterojunctions. Finally, the concluding remarks are presented and some perspectives regarding the future development of g‐C₃N₄‐based photocatalysts are highlighted.     

One‐Pot Exfoliation of Graphitic C3N4 Quantum Dots for Blue QLEDs by Methylamine Intercalation

Here, a simplified synthesis of graphitic carbon nitride quantum dots (g‐C₃N₄‐QDs) with improved solution and electroluminescent properties using a one‐pot methylamine intercalation–stripping method (OMIM) to hydrothermally exfoliate QDs from bulk graphitic carbon nitride (g‐C₃N₄) is presented. The quantum dots synthesized by this method retain the blue photoluminescence with extremely high fluorescent quantum yield (47.0%). As compared to previously reported quantum dots, the g‐C₃N₄‐QDs synthesized herein have lower polydispersity and improved solution stability due to high absolute zeta‐potential (?41.23 mV), which combine to create a much more tractable material for solution processed thin film fabrication. Spin coating of these QDs yields uniform films with full coverage and low surface roughness ideal for quantum dot light‐emitting diode (QLED) fabrication. When incorporated into a functional QLED with OMIM g‐C₃N₄‐QDs as the emitting layer, the LED demonstrates ≈60× higher luminance (605 vs 11 Cd m^?2) at lower operating voltage (9 vs 21 V), as compared to the previously reported first generation g‐C₃N₄ QLEDs, though further work is needed to improve device stability.     

Oxidized g‐C3N4 Nanospheres as Catalytically Photoactive Linkers in MOF/g‐C3N4 Composite of Hierarchical Pore Structure

A unique composite of the copper‐based metal–organic framework (Cu‐benzene tricarboxylic acid (BTC)) with oxidized graphitic carbon nitride nanospheres is synthesized. For comparison, a hybrid material consisting of g‐C₃N₄ and Cu‐BTC is also obtained. Their surface features are analyzed using Fourier transform infrared spectroscopy, X‐ray diffraction, sorption of nitrogen, thermal analysis, scanning electron microscopy, photoluminescence, and diffuse reflectance UV–Vis spectroscopy. The results suggest that the formed nanospheres of oxidized g‐C₃N₄ act as linkers between the copper sites, playing a crucial role in the composite building process. Their incorporation to the Cu‐BTC framework causes the development of new mesoporosity. Remarkable alterations in the optical properties, as a result of the coordination of oxygen containing functional groups of the oxidized graphitic carbon nitride to the copper atoms of the framework, suggest an increase in photoreactivity. On the other hand, for the hybrid material consisting of Cu‐BTC and g‐C₃N₄, the unaltered pore volume and optical properties support the formation of a physical mixture rather than of a composite. The tests on reactive adsorption and detoxification of G‐series organophosphate nerve agent surrogate show the enhanced performance of the composite as catalysts and photocatalyst in visible light.     

Photoassisted Construction of Holey Defective g‐C3N4 Photocatalysts for Efficient Visible‐Light‐Driven H2O2 Production

Holey defective g‐C₃N₄ photocatalysts, which are easily prepared via a novel photoassisted heating process, are reported. The photoassisted treatment not only helps to create abundant holes, endowing g‐C₃N₄ with more exposed catalytic active sites and crossplane diffusion channels to shorten the diffusion distance of both reactants from the surface to bulk and charge carriers from the bulk to surface, but also introduces nitrogen vacancies in the tri‐s‐triazine repeating units of g‐C₃N₄, inducing the narrowing of intrinsic bandgap and the formation of defect states within bandgap to extend the visible‐light absorption range and suppress the radiative electron–hole recombination. As a result, the holey defective g‐C₃N₄ photocatalysts show much higher photocatalytic activity for H₂O₂ production with optimized enhancement up to ten times higher than pristine bulk g‐C₃N₄. The newly developed synthetic strategy adopted here enables the sufficient utilization of solar energy and shows rather promising for the modification of other materials for efficient energy‐related applications.     

Crafting Mussel‐Inspired Metal Nanoparticle‐Decorated Ultrathin Graphitic Carbon Nitride for the Degradation of Chemical Pollutants and Production of Chemical Resources

The development of efficient photocatalysts for the degradation of organic pollutants and production of hydrogen peroxide (H₂O₂) is an attractive two‐in‐one strategy to address environmental remediation concerns and chemical resource demands. Graphitic carbon nitride (g‐C₃N₄) possesses unique electronic and optical properties. However, bulk g‐C₃N₄ suffers from inefficient sunlight absorption and low carrier mobility. Once exfoliated, ultrathin nanosheets of g‐C₃N₄ attain much intriguing photocatalytic activity. Herein, a mussel‐inspired strategy is developed to yield silver‐decorated ultrathin g‐C₃N₄ nanosheets (Ag@U‐g‐C₃N₄‐NS). The optimum Ag@U‐g‐C₃N₄‐NS photocatalyst exhibits enhanced electrochemical properties and excellent performance for the degradation of organic pollutants. Due to the photoformed valence band holes and selective two‐electron reduction of O₂ by the conduction band electrons, it also renders an efficient, economic, and green route to light‐driven H₂O₂ production with an initial rate of 0.75 × 10^?6 m min^?1. The improved photocatalytic performance is primarily attributed to the large specific surface area of the U‐g‐C₃N₄‐NS layer, the surface plasmon resonance effect induced by Ag nanoparticles, and the cooperative electronic capture properties between Ag and U‐g‐C₃N₄‐NS. Consequently, this unique photocatalyst possesses the extended absorption region, which effectively suppresses the recombination of electron–hole pairs and facilitates the transfer of electrons to participate in photocatalytic reactions.     

Faster Electron Injection and More Active Sites for Efficient Photocatalytic H2 Evolution in g‐C3N4/MoS2 Hybrid

Herein, the structural effect of MoS₂ as a cocatalyst of photocatalytic H₂ generation activity of g‐C₃N₄ under visible light irradiation is studied. By using single‐particle photoluminescence (PL) and femtosecond time‐resolved transient absorption spectroscopies, charge transfer kinetics between g‐C₃N₄ and two kinds of nanostructured MoS₂ (nanodot and monolayer) are systematically investigated. Single‐particle PL results show the emission of g‐C₃N₄ is quenched by MoS₂ nanodots more effectively than MoS₂ monolayers. Electron injection rate and efficiency of g‐C₃N₄/MoS₂‐nanodot hybrid are calculated to be 5.96 × 10⁹ s^?1 and 73.3%, respectively, from transient absorption spectral measurement, which are 4.8 times faster and 2.0 times higher than those of g‐C₃N₄/MoS₂‐monolayer hybrid. Stronger intimate junction between MoS₂ nanodots and g‐C₃N₄ is suggested to be responsible for faster and more efficient electron injection. In addition, more unsaturated terminal sulfur atoms can serve as the active site in MoS₂ nanodot compared with MoS₂ monolayer. Therefore, g‐C₃N₄/MoS₂ nanodot exhibits a 7.9 times higher photocatalytic activity for H₂ evolution (660 µmol g?¹ h^?1) than g‐C₃N₄/MoS₂ monolayer (83.8 µmol g^?1 h^?1). This work provides deep insight into charge transfer between g‐C₃N₄ and nanostructured MoS₂ cocatalysts, which can open a new avenue for more rationally designing MoS₂‐based catalysts for H₂ evolution.     

Synergy of Dopants and Defects in Graphitic Carbon Nitride with Exceptionally Modulated Band Structures for Efficient Photocatalytic Oxygen Evolution

Electronic structure greatly determines the band structures and the charge carrier transport properties of semiconducting photocatalysts and consequently their photocatalytic activities. Here, by simply calcining the mixture of graphitic carbon nitride (g‐C₃N₄) and sodium borohydride in an inert atmosphere, boron dopants and nitrogen defects are simultaneously introduced into g‐C₃N₄. The resultant boron‐doped and nitrogen‐deficient g‐C₃N₄ exhibits excellent activity for photocatalytic oxygen evolution, with highest oxygen evolution rate reaching 561.2 µmol h^?1 g^?1, much higher than previously reported g‐C₃N₄. It is well evidenced that with conduction and valence band positions substantially and continuously tuned by the simultaneous introduction of boron dopants and nitrogen defects into g‐C₃N₄, the band structures are exceptionally modulated for both effective optical absorption in visible light and much increased driving force for water oxidation. Moreover, the engineered electronic structure creates abundant unsaturated sites and induces strong interlayer C–N interaction, leading to efficient electron excitation and accelerated charge transport. In the present work, a facile approach is successfully demonstrated to engineer the electronic structures and the band structures of g‐C₃N₄ with simultaneous introduction of dopants and defects for high‐performance photocatalytic oxygen evolution, which can provide informative principles for the design of efficient photocatalysis systems for solar energy conversion.     

Hierarchical Porous O‐Doped g‐C3N4 with Enhanced Photocatalytic CO2 Reduction Activity

Artificial photosynthesis of hydrocarbon fuels by utilizing solar energy and CO₂ is considered as a potential route for solving ever‐increasing energy crisis and greenhouse effect. Herein, hierarchical porous O‐doped graphitic carbon nitride (g‐C₃N₄) nanotubes (OCN‐Tube) are prepared via successive thermal oxidation exfoliation and curling‐condensation of bulk g‐C₃N₄. The as‐prepared OCN‐Tube exhibits hierarchically porous structures, which consist of interconnected multiwalled nanotubes with uniform diameters of 20–30 nm. The hierarchical OCN‐Tube shows excellent photocatalytic CO₂ reduction performance under visible light, with methanol evolution rate of 0.88 µmol g^?1 h^?1, which is five times higher than bulk g‐C₃N₄ (0.17 µmol g^?1 h^?1). The enhanced photocatalytic activity of OCN‐Tube is ascribed to the hierarchical nanotube structure and O‐doping effect. The hierarchical nanotube structure endows OCN‐Tube with higher specific surface area, greater light utilization efficiency, and improved molecular diffusion kinetics, due to the more exposed active edges and multiple light reflection/scattering channels. The O‐doping optimizes the band structure of g‐C₃N₄, resulting in narrower bandgap, greater CO₂ affinity, and uptake capacity as well as higher separation efficiency of photogenerated charge carriers. This work provides a novel strategy to design hierarchical g‐C₃N₄ nanostructures, which can be used as promising photocatalyst for solar energy conversion.     

An Autotransferable g‐C3N4 Li+‐Modulating Layer toward Stable Lithium Anodes

Commercial deployment of lithium anodes has been severely impeded by the poor battery safety, unsatisfying cycling lifespan, and efficiency. Recently, building artificial interfacial layers over a lithium anode was regarded as an effective strategy to stabilize the electrode. However, the fabrications reported so far have mostly been conducted directly upon lithium foil, often requiring stringent reaction conditions with indispensable inert environment protection and highly specialized reagents due to the high reactivity of metallic lithium. Besides, the uneven lithium‐ion flux across the lithium surface should be more powerfully tailored via mighty interfacial layer materials. Herein, g‐C₃N₄ is employed as a Li⁺‐modulating material and a brand‐new autotransferable strategy to fabricate this interfacial layer for Li anodes without any inert atmosphere protection and limitation of chemical regents is developed. The g‐C₃N₄ film is filtrated on the separator in air using a common alcohol solution and then perfectly autotransferred to the lithium surface by electrolyte wetting during normal cell assembly. The abundant nitrogen species within g‐C₃N₄ nanosheets can form transient Li? N bonds to powerfully stabilize the lithium‐ion flux and thus enable a CE over 99% for 900 cycles and smooth deposition at high current densities and capacities, surpassing most previous works.     

Nanogap Engineered Plasmon‐Enhancement in Photocatalytic Solar Hydrogen Conversion

Graphitic carbon nitride modified with plasmonic Ag@SiO₂ core–shell nanoparticles (g‐C₃N₄/Ag@SiO₂) are proposed for enhanced photocatalytic solar hydrogen evolution under visible light. Nanosized gaps between the plasmonic Ag nanoparticles (NPs) and g‐C₃N₄ are created and precisely modulated to be 8, 12, 17, and 21 nm by coating SiO₂ shells on the Ag NPs. The optimized photocatalytic hydrogen production activity for g‐C₃N₄/Ag@SiO₂ is achieved with a nanogap of 12 nm (11.4 μmol h⁻¹) to be more than twice as high as that of pure g‐C₃N₄ (5.6 μmol h⁻¹). The plasmon resonance energy transfer (PRET) effect of Ag NPs is innovatively proved from a physical view on polymer semiconductors for photoredox catalysis. The PRET effect favors the charge carrier separation by inducing electron–hole pairs efficiently formed in the near‐surface region of g‐C₃N₄. Furthermore, via engineering the width of the nanogap, the PRET and energy‐loss Förster resonance energy transfer processes are perfectly balanced, resulting in considerable enhancement of photocatalytic hydrogen production activity over the g‐C₃N₄/Ag@SiO₂ plasmonic photocatalyst.     

Creating Graphitic Carbon Nitride Based Donor‐π–Acceptor‐π–Donor Structured Catalysts for Highly Photocatalytic Hydrogen Evolution

Conjugated polymers with tailored donor–acceptor units have recently attracted considerable attention in organic photovoltaic devices due to the controlled optical bandgap and retained favorable separation of charge carriers. Inspired by these advantages, an effective strategy is presented to solve the main obstructions of graphitic carbon nitride (g‐C₃N₄) photocatalyst for solar energy conversion, that is, inefficient visible light response and insufficient separation of photogenerated electrons and holes. Donor‐π–acceptor‐π–donor polymers are prepared by incorporating 4,4′‐(benzoc 1,2,5 thiadiazole‐4,7‐diyl) dianiline (BD) into the g‐C₃N₄ framework (UCN‐BD). Benefiting from the visible light band tail caused by the extended π conjugation, UCN‐BD possesses expanded visible light absorption range. More importantly, the BD monomer also acts as an electron acceptor, which endows UCN‐BD with a high degree of intramolecular charge transfer. With this unique molecular structure, the optimized UCN‐BD sample exhibits a superior performance for photocatalytic hydrogen evolution upon visible light illumination (3428 µmol h^?1 g^?1), which is nearly six times of that of the pristine g‐C₃N₄. In addition, the photocatalytic property remains stable for six cycles in 3 d. This work provides an insight into the synthesis of g‐C₃N₄‐based D‐π–A‐π–D systems with highly visible light response and long lifetime of intramolecular charge carriers for solar fuel production.     

Molecular Design of Polymer Heterojunctions for Efficient Solar–Hydrogen Conversion

Semiconducting photocatalytic solar–hydrogen conversion (SHC) from water is a great challenge for renewable fuel production. Organic semiconductors hold great promise for SHC in an economical and environmentally benign manner. However, organic semiconductors available for SHC are scarce and less efficient than most inorganic ones, largely due to their intrinsic Frenkel excitons with high binding energy. In this study the authors report polymer heterojunction (PHJ) photocatalysts consisting of polyfluorene family polymers and graphitic carbon nitride (g‐C₃N₄) for efficient SHC. A molecular design strategy is executed to further promote the exciton dissociation or light harvesting ability of these PHJs via alternative approaches. It is revealed that copolymerizing electron‐donating carbazole unit into the poly(9,9‐dioctylfluorene) backbone promotes exciton dissociation within the poly(N‐decanyl‐2,7‐carbazole‐alt‐9,9‐dioctylfluorene) (PCzF)/g‐C₃N₄ PHJ, achieving an enhanced apparent quantum yield (AQY) of 27% at 440 nm over PCzF/g‐C₃N₄. Alternatively, copolymerizing electron‐accepting benzothiadiazole unit extended the visible light response of the obtained poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole)/g‐C₃N₄ PHJ, leading to an AQY of 13% at 500 nm. The present study highlights that constructing PHJs and adapting a rational molecular design of PHJs are effective strategies to exploit more of the potential of organic semiconductors for efficient solar energy conversion.     

A Metal‐Free,Free‐Standing,Macroporous Graphene@g‐C3N4 Composite Air Electrode for High‐Energy Lithium Oxygen Batteries

The nonaqueous lithium oxygen battery is a promising candidate as a next‐generation energy storage system because of its potentially high energy density (up to 2–3 kW kg^?1), exceeding that of any other existing energy storage system for storing sustainable and clean energy to reduce greenhouse gas emissions and the consumption of nonrenewable fossil fuels. To achieve high energy density, long cycling stability, and low cost, the air electrode structure and the electrocatalysts play important roles. Here, a metal‐free, free‐standing macroporous graphene@graphitic carbon nitride (g‐C₃N₄) composite air cathode is first reported, in which the g‐C₃N₄ nanosheets can act as efficient electrocatalysts, and the macroporous graphene nanosheets can provide space for Li₂O₂ to deposit and also promote the electron transfer. The electrochemical results on the graphene@g‐C₃N₄ composite air electrode show a 0.48 V lower charging plateau and a 0.13 V higher discharging plateau than those of pure graphene air electrode, with a discharge capacity of nearly 17300 mA h g^?1 _(composite). Excellent cycling performance, with terminal voltage higher than 2.4 V after 105 cycles at 1000 mA h g^?1 _(composite) capacity, can also be achieved. Therefore, this hybrid material is a promising candidate for use as a high energy, long‐cycle‐life, and low‐cost cathode material for lithium oxygen batteries.     

Impact of Interfacial Electron Transfer on Electrochemical CO2 Reduction on Graphitic Carbon Nitride/Doped Graphene

Effective electrocatalysts are required for the CO₂ reduction reaction (CRR), while the factors that can impact their catalytic activity are yet to be discovered. In this article, graphitic carbon nitride (g‐C₃N₄) is used to investigate the feasibility of regulating its CRR catalytic performance by interfacial electron transfer. A series of g‐C₃N₄/graphene with and without heteroatom doping (C₃N₄/XG, XG = BG, NG, OG, PG, G) is comprehensively evaluated for CRR through computational methods. Variable adsorption energetics and electronic structures are observed among different doping cases, demonstrating that a higher catalytic activity originates from more interfacial electron transfer. An activity trend is obtained to show the best catalytic performance of CRR to methane on C₃N₄/XG with an overpotential of 0.45 V (i.e., ?0.28 V vs reverse hydrogen electrode [RHE]). Such a low overpotential has never been achieved on any previously reported metallic CRR electrocatalysts, therefore indicating the availability of C₃N₄/XG for CO₂ reduction and the applicability of electron transfer modulation to improve CRR catalytic performance.     

In Situ Synthesis of Few‐Layered g‐C3N4 with Vertically Aligned MoS2 Loading for Boosting Solar‐to‐Hydrogen Generation

In artificial photocatalytic hydrogen evolution, effective incident photon absorption and a high‐charge recombination rate are crucial factors influencing the overall efficiency. Herein, a traditional solid‐state synthesis is used to obtain, for the first time, novel samples of few‐layered g‐C₃N₄ with vertically aligned MoS₂ loading (MoS₂/C₃N₄). Thiourea and layered MoO₃ are chosen as precursors, as they react under nitrogen atmosphere to in situ produce the products. According to the quasi‐Fourier transform infrared reflectance and X‐ray diffraction measurements, the detailed reaction process is determined to proceed through the confirmed formation pathway. The two precursor units MoS₂ and C₃N₄ are linked by Mo? N bonds, which act as electronic receivers/conductors and hydrogen‐generation sites. Density functional theory is also carried out, which determines that the interface sites act as electron‐accumulation regions. According to the photoelectrochemical results, MoS₂/C₃N₄ can achieve a current of 0.05 mA cm^?2, which is almost ten times higher than that of bare g‐C₃N₄ or the MoS₂/C₃N₄‐R reference samples. The findings in the present work pave the way to not only synthesize a series of designated samples but also thoroughly understand the solid‐state reaction.     

Proton‐Functionalized Two‐Dimensional Graphitic Carbon Nitride Nanosheet: An Excellent Metal‐/Label‐Free Biosensing Platform

Ultrathin graphitic carbon nitride (g‐C₃N₄) nanosheets, due to their interesting two‐dimensional graphene‐like structure and unique physicochemical properties, have attracted great research attention recently. Here, a new approachis developed to prepare, for the first time, proton‐functionalized ultrathin g‐C₃N₄ nanosheets by sonication‐exfoliation of bulk g‐C₃N₄ under an acid condition. This method not only reduces the exfoliation time from more than 10 h to 2 h, but also endows the nanosheets with positive charges. Besides retaining the properties of g‐C₃N₄, the obtained nanosheets with the thickness of 2–4 nm (i.e., 6–12 atomic monolayers) also exhibit large specific surface area of 305 m² g^?1, enhanced fluorescence intensity, and excellent water dispersion stability due to their surface protonation and ultrathin morphology. The well‐dispersed protonated g‐C₃N₄ nanosheets are able to interact with negatively charged heparin, which results in the quenching of g‐C₃N₄ fluorescence. A highly sensitive and highly selective heparin sensing platform based on protonated g‐C₃N₄ nanosheets is established. This metal‐free and fluorophore label‐free system can reach the lowest heparin detection limit of 18 ng mL^?1.     

Constructing Built‐in Electric Field in Ultrathin Graphitic Carbon Nitride Nanosheets by N and O Codoping for Enhanced Photocatalytic Hydrogen Evolution Activity

Codoping of N and O in ultrathin graphitic carbon nitride (g‐C₃N₄) nanosheets leads to an inner electric field. This field restrains the recombination of photogenerated carriers and, thus, enhances hydrogen evolution. The layered structure of codoped g‐C₃N₄ nanosheets (N‐O‐CNNS) not only provides abundant sites of contact with the reaction medium, but also decreases the distance over which the photogenerated electron–hole pairs are transported to the reaction interface. Quantum confinement in the ultrathin structure results in an increased bandgap and makes the photocatalytic reaction more favorable than bulk g‐C₃N₄. Under visible light irradiation, N‐O‐CNNS with 3 wt% Pt achieves a hydrogen evolution rate of 9.2 mmol g^?1 h^?1 and a value of 46.9 mmol g^?1 h^?1 under AM1.5 with 5 wt% Pt. Thus, this work paves the way for designing efficient nanostructures with increased separation/transfer efficiency of photogenerated carriers and, hence, increased photocatalytic activities.