New carbon nitride close to C6N7 with superior visible light absorption for highly efficient photocatalysis

The rational design and construction of novel two-dimensional (2D) carbon nitrides (CNs) beyond g-C₃N₄ is a hot topic in the fields of chemistry and materials. Inspired by the polymerisation of urea, we have prepared a series of novel C-C bridged heptazine CNs UO_x (where x is the ratio of urea to oxamide, x = 1, 1.5, 2, 2.5, and 3), which are similar to (C₆N₇)_n, upon the introduction of oxamide. As predicted using density functional theory (DFT) calculations, the conjugated structure of UO_x was effectively extended from an individual heptazine to the entire material. Consequently, its bandgap was reduced to 2.05 eV, and its absorption band edge was significantly extended to 600 nm. Furthermore, its carrier transfer and separation were significantly enhanced, establishing its superior photocatalytic activity. The optimised UO₂ exhibits a superior photocatalytic hydrogen production rate about 108.59 μmol h^-1 (using 10 mg of catalyst) with an apparent quantum efficiency (AQE) of 36.12% and 0.33% at 420 and 600 nm, respectively, which is one of the most active novel CNs reported to date. Moreover, UO₂ exhibits excellent photocatalytic activity toward the oxidation of diphenylhydrazine to azobenzene with conversion and selectivity reaching ~100%, which represents a promising highly efficient 2D CN material. Regarding phenols degradation, UO₂ also displayed significantly higher activity and durability during the degradation of phenol when compared to traditional g-C₃N₄, highlighting its significant potential for application in energy, environment and photocatalytic organic reactions.

Acute administration of sulfur-doped g-C3N4 induces cognitive deficits and exacerbates the levels of glial activation in mouse hippocampus

During the last decades, graphitic carbon nitride (g-C3N4) has attracted increasing attention in several biomedical fields. In this study, the effects of sulfur-doped g-C3N4 (TCN) on cognitive function and histopathology of hippocampus were investigated in mice. The characteristics of synthetized sample were evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and energy dispersive X-ray (EDX). Twenty-four male NMRI mice received vehicle, TCN at doses of 50, 150, or 500 mg/kg via gavage for one week. Morris water maze test was done to assess the cognitive function at day 14 post TCN administration. Nissl staining was used to determine the number of dark cells in the hippocampus. Immunostaining against NeuN, GFAP, and Iba1 was done to evaluate the neuronal density and levels of glial activation, respectively. Behavioral tests indicated that TCN reduces the spatial learning and memory in a dose-dependent manner. Histological evaluations showed an increased level of neuronal loss and glial activation in the hippocampus of TCN treated mice at doses of 150 and 500 mg/kg. Overall, our data indicate that TCN induces the cognitive impairment that is partly mediated via its exacerbating impacts on neuronal loss and glial activation.

Preparation of Fe-MIL(100)-encapsulated magnetic g-C3N4 for adsorption of PPCPs from aqueous solution

In the present work, the Fe-MIL(100) was encapsulated on the outer surface of magnetic g-C₃N₄ through a simple method to synthesize a novel adsorbent. The as-prepared g-C₃N₄/MnFe₂O₄/Fe-MIL(100) was characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), BET specific surface area (BET), vibrating sample magnetometry (VSM), and plasma emission spectrometry (PES). The g-C₃N₄/MnFe₂O₄/Fe-MIL(100) possessed rough surface, large surface area (303.68 m²/g), mesoporous structure and magnetic properties, which exhibited excellent adsorption performance for ciprofloxacin (CIP), oxytetracycline (OTC) and indomethacin (IDM) with the maximum adsorption capacities reaching up to 45.51, 64.34 and 103.91 mg/g, respectively. The adsorption processes of all three PPCPs could be described by different kinds of isotherms and kinetic models. Additionally, the adsorption capacity of the resulting adsorbent could maintain 73.43% of the first adsorption capacity even after ten cycles. Finally, the possible adsorption mechanisms of g-C₃N₄/MnFe₂O₄/Fe-MIL(100) for CIP/OTC/IDM were proposed. Thus, g-C₃N₄/MnFe₂O₄/Fe-MIL(100) possessed excellent features of high adsorption capacity, fast removal rate, easy synthesis, salt resistance and magnetic separation, which showed great potential application to be used as an effective adsorbent for adsorptive removal of PPCPs in wastewater.

Unmasking the Role of an Amorphous/Amorphous Interface and a Crystalline/Amorphous Interface in the Transition of Charge Carriers on the CN/SiO2/WO3 Photocatalyst

Making heterojunctions between semiamorphous carbon nitride (CN) and other well-matched semiconductors (or even insulators) can solve many photocatalytic problems such as the recombination of charge carriers. However, many researchers encounter intrinsic problems including the lack of detailed information on contact boundaries in their heterojunctions, particularly in the amorphous/amorphous interface. In addition, the roles of contact boundaries in the photocatalytic mechanisms of many heterojunctions are still obscure. This study synthesized a novel CN/SiO₂/WO₃ photocatalyst having two different contact features by constructing an amorphous/amorphous (CN/SiO₂) interface and a crystalline/amorphous (WO₃/CN) interface to provide deep insights into heterojunction interfaces. SiO₂ plays an exceptional role as a major component in the separation and migration of charge carriers. It not only modifies the texture but also transfers electrons. Surprisingly, the amorphous/amorphous interface shows an unpredicted capability for decreasing the recombination of electron-hole pairs. Based on capturing experiments and photoluminescence investigations, the amorphous/amorphous interface is unprecedently present in the production of hydroxyl radicals, while the crystalline/amorphous interface gives more superoxide radicals. This work provides a platform that opens a new perspective on the selection of mutual photocatalysts. It extends boundaries of conventional heterojunctions.

Self-supporting Atmosphere-Assisted Synthesis of 1D Mo2 C-based Catalyst for Efficient Hydrogen Evolution

One-dimensional materials exhibit fascinating properties in electrocatalytic applications but their fabrication faces the challenge of tedious and complicated operations. We have developed a bottom-up strategy to construct a 1D metal carbide catalyst (Mo₂ C@NC) consisting of ultrafine Mo₂ C nanoparticles embedded within nitrogen-doped carbon layers by simply calcining a mixture of ammonium molybdate, urea and melamine. Experimental results and thermodynamic calculations demonstrate that the retainable pyrolysis-generated self-supporting atmosphere plays a crucial role in the crystalline phase and morphology of materials. When functioned as an electrocatalyst for the hydrogen evolution reaction (HER), the achieved Mo₂ C@NC presents an excellent catalytic activity as well as outstanding stability. This work could shed fresh light onto the facile synthesis of effective HER catalysts with 1D nanostructure.

Efficient photoelectrochemical overall water-splitting of MoS2/g-C3N4 n-n type heterojunction film

The construction of heterojunctions has attracted considerable attention among the various strategies of water-splitting for hydrogen evolution due to their band structure advantages. In this research, we combined chemical vapor deposition and pulsed laser deposition to fabricate MoS₂/g-C₃N₄ heterojunction films on indium-tin oxide glass substrates, and we studied the photoelectrochemical (PEC) performance. The x-ray diffraction, x-ray photoelectron spectroscopy (XPS), and scanning electron microscope characterizations suggested the successful preparation of MoS₂/g-C₃N₄ heterojunction films. In particular, the shifts of the peak positions in the XPS spectra indicated the formation of a strong interaction between the g-C₃N₄ and MoS₂ films. After depositing MoS₂ on the g-C₃N₄ film, the visible-light absorption was enhanced and broadened, the electrical conductivity improved, and the intensity of the photoluminescence peak decreased. As a result, the greater generation, faster transport, and lower recombination rate of electrons and holes caused the heterojunction films to show higher PEC performance. More importantly, the obtained MoS₂/g-C₃N₄ film was confirmed to be an n-n type heterojunction and to have a typical type-II band structure, which could indeed suppress the recombination and promote the separation, transfer, and transport of photogenerated electron-holes. Finally, the obtained MoS₂/g-C₃N₄ film successfully achieved the overall water-splitting and the H₂ evolution rate when the visible-light radiation reached 252 µmol/h.

Interface Engineering of a 2D-C3N4/NiFe-LDH Heterostructure for Highly Efficient Photocatalytic Hydrogen Evolution

Photocatalytic water splitting offers an economic and sustainable pathway for producing hydrogen as a zero-emission fuel, but it still suffers from low efficiencies limited by visible-light absorption capacity and charge separation kinetics. Herein, we report an interface-engineered 2D-C₃N₄/NiFe layered double hydroxide (CN/LDH) heterostructure that shows highly enhanced photocatalytic hydrogen evolution reaction (HER) rate with excellent long-term stability. The morphology and band gap structure of NiFe-LDH are precisely regulated by employing NH₄F as a structure-directing agent, which enables a fine interfacial tuning via coupling with 2D-C₃N₄. The formation of a type II interface in CN/LDH enlarges the active surface area and promotes the charge separation efficiency, leading to an HER rate of 3087 μmol g^-1 h^-1, which is 14 times higher than that of 2D-C₃N₄. This study highlights a rational interface engineering strategy for the formation of a heterostructure with a proper hole transport co-catalyst for designing effective water-splitting photocatalysts.

Bifunctional Polymeric Carbon Nitride via Tuning Fabrication Conditions for Photocatalysis

In this contribution, the hydrogen evolution reaction and photodegradation of Rhodamine B (RhB) dye were studied using urea-based polymeric carbon nitride (PCN) as photocatalyst. The effects of calcination temperature and heating rate of the PCN on structural, morphological, optical, photoelectrochemical, and photocatalytic properties were addressed. Different properties were found to be crucial in boosting photocatalytic performance dependending on the reaction type. The highest efficiency in hydrogen evolution was observed in the presence of PCN characterized by the superior charge transport and charge lifetime properties arising from higher degree of structural arrangement and lower defect content in comparison to that of other photocatalysts. However, photocatalytic degradation of RhB was the most powerful when the catalyst exhibited the highest specific surface area as a key parameter determining its efficiency, although it presented lower charge transport and charge carrier properties.

Tetracycline hydrochloride degradation over manganese cobaltate (MnCo2O4) modified ultrathin graphitic carbon nitride (g-C3N4) nanosheet through the highly efficient activation of peroxymonosulfate under visible light irradiation

Peroxymonosulfate (PMS) activation by heterogeneous transition metal oxides is an effective approach for treating emerging pollutants in water. However, the low PMS activation efficiency associated with the valency conversion rate of transition metals has been a major challenge to sulfate radical-based oxidation. In this work, manganese cobaltate (MnCo₂O₄) nanoparticles anchored on graphitic carbon nitride (g-C₃N₄) flakes (MnCo₂O₄/g-C₃N₄) were successfully prepared and showed high PMS activation efficiency under visible (Vis) light. The obtained catalysts degraded 96.1% of the tetracycline hydrochloride (TCH) through the synergistic effect of PMS and photocatalysis. The reaction rate constant (0.2505 min^-1) was 5.3 and 1.8 times higher in the MnCo₂O₄/g-C₃N₄/PMS/Vis system than in the pristine g-C₃N₄ (0.0471 min^-1) and MnCo₂O₄ (0.1435 min^-1) systems, respectively. The characterization results verified that g-C₃N₄, which functions as the electron donor in the photocatalytic heterojunction system, could transmit numerous photogenerated electrons to MnCo₂O₄, thereby increasing the cyclability of divalent-trivalent metal ions. The composites also showed good stability, cycling capability, and cation/anion tolerance. Tentative degradation mechanism and reaction pathways were proposed based on the reactive species and degradation products.

Coordination of π-Delocalization in g-C3N4 for Efficient Photocatalytic Hydrogen Evolution under Visible Light

g-C₃N₄ with π-delocalization was coordinated between urea and a small amount of 1,3,5-tris(4-aminophenyl)benzene (TAPB) (UCN-xTAPB) by a facile polymerization. Compared with pristine g-C₃N₄(UCN), the obtained materials, UCN-xTAPB, showed an extended delocalization with increased electrical conductivity, enhanced adsorption of visible light, and improved separation of photogenerated electron-hole pairs. The average H₂ evolution rate of UCN-4TAPB is about 10.55 mmol h^-1 g^-1 under visible-light irradiation (λ > 420 nm), which is much higher than reported data. Furthermore, density-functional theory (DFT) calculation confirms that the proposed structure with the incorporation of TAPB into the CN network shows the extended delocalization. Moreover, different structures of aromatic rings (anthroic acid, naphthoic acid and benzoic acid) are applied to verify the role of the enhanced π-delocalization in g-C₃N₄. By adopting different precursors (thiourea, dicyandiamide) to polymerize with TAPB, we further confirm the extension of optical absorption under visible-light irradiation and the improvement of hydrogen evolution rate, indicating the universality of the current strategy. Therefore, we believe that our work provides an efficient strategy for constructing the delocalized structure of g-C₃N₄ as effective visible-light-responsive photocatalysts.

Salt-resistant nanosensor for fast sulfadimethoxine tracing based on oxygen-doped g-C3N4 nanoplates

A novel oxygen-doped g-C₃N₄ nanoplate (OCNP) structure that can serve as an efficient sulfadimethoxine (SDM) sensing platform has been developed. Taking advantage of its inherent oxygen-containing functional groups and 2D layered structure with π-conjugated system, OCNP exhibits effective radiative recombination of surface-confined electron-hole pairs and efficient π-π interaction with SDM. This causes rapid fluorescence response and thus ensures the fast and continuous monitoring of SDM. Based on the fluorescence experiments and band structure calculation, the mechanism of the SDM-induced quenching phenomenon was mainly elucidated as the photoinduced electron transfer process under a dynamic quenching mode. Under optimized conditions, the as-proposed nanosensor, which emitted strong fluorescence at 375 nm with an excitation wavelength at 255 nm, presents an excellent analytical performance toward SDM with a wide linear range from 3 to 60 μmol L^-1 and a detection limit of 0.85 μmol L^-1 (S/N = 3). In addition, this strategy exhibits satisfactory recovery varied from 94 to 103% with relative standard derivations (RSD) in the range 0.9 to 6.8% in real water samples. It also shows marked tolerability to a series of high concentrations of metals and inorganic salts. This strategy not only broadens the application of oxygen-doped g-C₃N₄ nanomaterial in antibiotic sensing field but also presents a promising potential for on-line contaminant tracing in complex environments.

A dual-signal sensor for the analysis of parathion-methyl using silver nanoparticles modified with graphitic carbon nitride

A highly sensitive and selective method was developed for both UV-vis spectrophotometric and fluorimetric determination of organophosphorus pesticides (OPs). This method used silver nanoparticles (AgNPs) modified with graphitic carbon nitride (g-C3N4). The AgNPs reduced the fluorescence intensity of g-C3N4. Acetylthiocholine (ATCh) could be catalytically hydrolyzed by acetylcholinesterase (AChE) to form thiocholine, which induces aggregation of the AgNPs. This aggregation led to the recovery of the blue fluorescence of g-C3N4, with excitation/emission peaks at 310/460 nm. This fluorescence intensity could be reduced again in the presence of OPs because of the inhibitory effect of OPs on the activity of AChE. The degree of reduction was found to be proportional to the concentration of OPs, and the limit of fluorometric detection was 0.0324 μg/L (S/N = 3). In addition, the absorption of the g-C3N4/AgNPs at 390 nm decreased because of the aggregation of the AgNPs, but was recovered in presence of OPs because of the inhibition of enzyme activity by OPs. This method was successfully applied to the analysis of parathion-methyl in real samples.

Construction of polymeric carbon nitride and dibenzothiophene dioxide-based intramolecular donor-acceptor conjugated copolymers for photocatalytic H2 evolution

Polymeric carbon nitride (g-C₃N₄) has succeeded as a striking visible-light photocatalyst for solar-to-hydrogen energy conversion, owing to its economical attribute and high stability. However, due to the lack of sufficient solar-light absorption and rapid photo-generated carrier recombination, the photocatalytic activity of raw g-C₃N₄ is still unsatisfactory. Herein, new intramolecular g-C₃N₄-based donor-acceptor (D-A) conjugated copolymers have been readily synthesized by a nucleophilic substitution/condensation reaction between urea and 3,7-dihydroxydibenzo[b,d]thiophene 5,5-dioxide (SO), which is strategically used to improve the photocatalytic hydrogen evolution performance. The experimental results demonstrate that CNSO-X not only improves light utilization, but also accelerates the spatial separation efficiency of the photogenerated electron-hole pairs and increases the wettability with the introduction of SO. In addition, the adsorption energy barrier of CNSO-X to H* has a significant reduction via theoretical calculation. As expected, the CNSO-20 realizes the best photocatalytic H₂ evolution activity of 251 μmol h^-1 (50 mg photocatalyst, almost 8.5 times higher than that of pure CN) with an apparent quantum yield of 10.16% at 420 nm, which surpasses most strategies for the organic molecular copolymerization of carbon nitride. Therefore, this strategy opens up a novel avenue to develop highly efficient g-C₃N₄ based photocatalysts for hydrogen production.

Low-Temperature Synthesis of Solution Processable Carbon Nitride Polymers

Carbon nitride materials require high temperatures (>500 °C) for their preparation, which entails substantial energy consumption. Furthermore, the high reaction temperature limits the materials' processability and the control over their elemental composition. Therefore, alternative synthetic pathways that operate under milder conditions are still very much sought after. In this work, we prepared semiconductive carbon nitride (CN) polymers at low temperatures (300 °C) by carrying out the thermal condensation of triaminopyrimidine and acetoguanamine under a N2 atmosphere. These molecules are isomers: they display the same chemical formula but a different spatial distribution of their elements. X-ray photoelectron spectroscopy (XPS) experiments and electrochemical and photophysical characterization confirm that the initial spatial organization strongly determines the chemical composition and electronic structure of the materials, which, thanks to the preservation of functional groups in their surface, display excellent processability in liquid media.

Enhanced visible-light photocatalysis of clofibric acid using graphitic carbon nitride modified by cerium oxide nanoparticles

Recently, the emerging pharmaceutical micropollutants have become an environmental concern. Herein, we report an efficient elimination of clofibric acid (CA) using visible light-driven g-C₃N₄/CeO₂ prepared by hydrothermal method. Among the catalysts with different compound ratios, g-C₃N₄/CeO₂-3 (1.2 g g-C₃N₄ with 3 mmol Ce(NO₃)₃∙6H₂O) exhibited the best photocatalytic performance. The effect of catalyst dosage was investigated and the optimal value was determined as 0.5 g L^-1. The effect of initial pH (pH₀) showed CA elimination decreased with increasing pH₀. The underlying mechanism for CA oxidation was proposed based on synthetical analysis of photoluminescence emission spectra, transient photocurrent responses, electron paramagnetic resonance, chemical quenching experiments and band edge potential of g-C₃N₄ and CeO₂. Photogenerated hole was primarily responsible for CA elimination while singlet oxygen played an auxiliary role. The products of CA oxidation were detected using liquid chromatography mass spectrometry (LC-MS) method and a possible pathway was put forward. Various organics were used as target contaminants to assess photocatalytic performance of g-C₃N₄/CeO₂ heterojunction under acidic and alkaline pH conditions. The analysis of relationship between the oxidation peak potential (E_OP) and the reaction rate constant indicated that photocatalysis using as prepared g-C₃N₄/CeO₂-3 heterojunction is apt to oxidize contaminants with electron withdrawing group under acid condition.

Ultrathin S-doped graphitic carbon nitride nanosheets for enhanced sulpiride degradation via visible-light-assisted peroxydisulfate activation: Performance and mechanism

The wide use and distribution of sulpiride (SP) has caused potential threats to the water environment and human health. In this study, ultrathin S-doped graphitic carbon nitride nanosheets (US-CN) was successfully synthesized and characterized, and its SP removal efficiency was evaluated under various conditions via the visible-light-assisted peroxydisulfate (PDS) activation method. The degradation pathways and mechanism were also discussed through quenching experiments, density functional theory (DFT) calculations, and intermediate products detection. After sulfur doping and ultrasonic treatment, graphitic carbon nitride (CN) possessed an ultra-thin and porous structure, which facilitated the electronic distribution and more photocurrent, thus resulting in the excellent stability and removal efficiency for SP via PDS activation upon visible light irradiation. The singlet oxygen (¹O₂) generated by the US-CN/PDS/VL system played a significant role in SP degradation. Based on the bonds of electron-rich atoms fracturing and the SO₂ extrusion, the SP degradation pathway was proposed. This work provides a useful information for the SP photocatalytic degradation via PDS activation.

Controllable Generation of Reactive Oxygen Species on Cyano-Group-Modified Carbon Nitride for Selective Epoxidation of Styrene

The controlled generation of reactive oxygen species (ROS) to selectively epoxidize styrene is a grand challenge. Herein, cyano-group-modified carbon nitrides (CNCY _x and CN-T _y ) are prepared, and the catalysts show better performance in regulating ROS and producing styrene oxide than the cyano-free sample. The in situ diffuse reflectance infrared and density functional theory calculation results reveal that the cyano group acts as the adsorption and activation site of oxygen. X-ray photoelectron spectroscopy and NMR spectrum results confirm that the cyano group bonds with the intact heptazine ring. This unique structure could inhibit H₂O₂ and ^⋅OH formation, resulting in high selectivity of styrene oxide. Furthermore, high catalytic activity is still achieved when the system scales up to 2.7 L with 100 g styrene under solar light irradiation. The strategy of cyano group modification gives a new insight into regulating spatial configuration for tuning the utilization of oxygen-active species and shows potential applications in industry.

What does graphitic carbon nitride really look like?

Graphitic carbon nitrides (g-CNs) have become popular light absorbers in photocatalytic water splitting cells. Early theoretical work on these structures focused on fully polymerized g-C3N4. Experimentally, it is known that the typically employed melamine polycondensation does not go toward completion, yielding structures with ∼15 at% hydrogen. Here, we study the conformational stability of "melon", with the [C6N9H3]n structural formula using DFT. Referencing to a 2D melon sheet, B3LYP-dDsC and PBE-MBD computations revealed the same qualitative trend in stability of the 3D structures, with several of them within 5 kJ mol-1 per tecton. Fina's orthorhombic melon is the most stable of the studied conformers, with Lotsch' monoclinic melon taking an intermediate value. Invoking a simple Wannier-Mott-type approach, Fina's and Lotsch' structures exhibited the lowest optical gaps (2.8 eV), within the error margin of the experimental value (2.7 eV). All conformers yielded gaps below that of the monolayer's (3.2 eV), suggesting Jelley-type ("J") aggregation effects.

An efficient broad spectrum-driven carbon and oxygen co-doped g-C3N4 for the photodegradation of endocrine disrupting: Mechanism, degradation pathway, DFT calculation and toluene selective oxidation

In this study, a new type of carbon and oxygen co-doped g-C₃N₄ (PACN) was successfully synthesized by a one-step thermal polymerization method for the photodegradation of Bisphenol A (BPA) and selective oxidation of toluene to benzaldehyde. The degradation rate of BPA was 23.58 times higher than that of pristine g-C₃N₄ and the efficiency benzaldehyde formation rate without the need of any solvent increased to 5.43 times that of g-C₃N₄. At the same time, the band structure calculation of its simulated structure is performed by DFT, which shows that the introduction of oxygen linking band can adjust its band structure and obtain a smaller band gap. In addition, the PACN displays an enhanced photocatalytic degradation of BPA under the long wavelength (λ ≥ 550 nm) and NIR light irradiation (λ ≥ 760 nm), which indicates that the synthesized materials have a broad spectrum of photocatalytic activity. According to the results of secondary ion mass spectrometry (SIMS) and nuclear magnetic resonance spectroscopy (NMR), C atoms and O atoms were introduced into the original g-C₃N₄ skeleton. In addition, the intermediate products were detected by mass spectrometry (HPLC-MS), and the BPA degradation pathway was proposed. A feasible photocatalytic reaction mechanism was also proposed.

Carbon nitride quantum dots tethered on CNTs for the electrochemical detection of dopamine and uric acid

A 0D–1D CNQDs/f-CNT architecture composed of 0D CNQDs tethered on a 1D functionalized multiwalled carbon nanotube (f-CNT) network was used for dopamine sensing.

Graphitic carbon nitride: a sustainable photocatalyst for organic pollutant degradation and antibacterial applications

Recently, graphitic carbon nitride (GCN) has been found to be of great interest in various sustainable applications. In this study, a simple preparation method using urea was utilized to synthesize GCN. In order to understand various morphological, structural, and optical aspects of the as-prepared sample, GCN was characterized using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, Brunauere-Emmette-Teller (BET), scanning electron microscopy (SEM), and diffused reflectance spectra (DRS) analysis. The visible-light-driven photocatalytic activity of prepared GCN was analyzed for various cationic dyes (Crystal violet, rose bengal, rhodamine B, auramine O, methylene blue) and anionic dyes (phenol red, xylenol orange, cresol red, methyl orange). The calculated efficiencies of degradation and values of apparent rate constant for all dye samples suggested that cationic dyes are more actively degraded using GCN than anionic dyes. In addition, GCN was further analyzed for its splendid antibacterial activity against pathogenic bacteria (Klebsiella pneumonia and Escherichia coli). The synthesized photocatalyst holds a bright scope for the efficient remediation of organic pollutants and bacterial disinfection in wastewater. Graphical abstract.

Metal-free n/n-junctioned graphitic carbon nitride (g-C3N4): a study to elucidate its charge transfer mechanism and application for environmental remediation

Graphitic carbon nitride (g-C₃N₄) has been regarded as a promising visible light-driven photocatalyst ascribable to its tailorable structures, thermal stability and chemical inertness. Enhanced photocatalytic activity is achievable by the construction of homojunction nanocomposites to reduce the undesired recombination of photogenerated charge carriers. In the present work, a novel g-C₃N₄/g-C₃N₄ metal-free homojunction photocatalyst was synthesized via hydrothermal polymerization. The g-C₃N₄/g-C₃N₄ derived from urea and thiourea demonstrated admirable photocatalytic activity towards rhodamine B (RhB) degradation upon irradiation of an 18 W LED light. The viability of the photoreaction with a low-powered excitation source highlighted the economic and environmental benefits of the process. The optimal g-C₃N₄/g-C₃N₄ homojunction photocatalyst exhibited a 2- and 1.8-fold increase in efficiency in relative to pristine g-C₃N₄ derived from urea and thiourea respectively. The enhanced photocatalytic performance is credited to the improved interfacial transfer and separation of electron-hole pairs across the homojunction interface. Furthermore, an excellent photochemical stability and durability is displayed by g-C₃N₄/g-C₃N₄ after three consecutive cycles. In addition, a plausible photocatalytic mechanism was proposed based on various scavenging tests. Overall, experimental results generated from this study is expected to intrigue novel research inspirations in developing metal-free homojunction photocatalysts to be feasible for large-scale wastewater treatment without compromising economically. Graphical abstract.

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

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

Sulfuric Acid Treated g-CN as a Precursor to Generate High-Efficient g-CN for Hydrogen Evolution from Water under Visible Light Irradiation

Modifying the physical, chemical structures of graphitic carbon nitride (g-CN) to improve its optoelectronic properties is the most efficient way to meet a high photoactivity for clean and sustainable energy production. Herein, a higher monomeric precursor for synthesizing improved micro-and electronic structure possessing g-CN was prepared by high-concentrated sulfuric acid (SA) treatment of bulk type g-CN (BCN). Several structural analyses show that after the SA treatment of BCN, the polymeric melon-based structure is torn down to cyameluric or cyanuric acid-based material. After re-polycondensation of this material as a precursor, the resulting g-CN has more condensed microstructure, carbon and oxygen contents than BCN, indicating that C, O co-doping by corrosive acid of SA. This g-CN shows a much better visible light absorption and diminished radiative charge recombination by the charge localization effect induced by heteroatoms. As a result, this condensed C, O co-doped g-CN shows the enhanced photocatalytic hydrogen evolution rate of 4.57 µmol/h from water under the visible light (>420 nm) by almost two times higher than that of BCN (2.37 µmol/h). This study highlights the enhanced photocatalytic water splitting performance as well as the provision of the higher monomeric precursor for improved g-CN.

Rare-Earth Single-Atom La-N Charge-Transfer Bridge on Carbon Nitride for Highly Efficient and Selective Photocatalytic CO2 Reduction

Photocatalytic CO₂ conversion into valuable solar fuels is highly appealing, but lack of directional charge-transfer channel and insufficient active sites resulted in limited CO₂ reduction efficiency and selectivity for most photocatalytic systems. Herein, we designed and fabricated rare-earth La single-atoms on carbon nitride with La-N charge-transfer bridge as the active center for photocatalytic CO₂ reaction. The formation of La single-atoms was certified by spherical aberration-corrected HAADF-STEM, STEM-EELS, EXAFS, and theoretical calculations. The electronic structure of the La-N bridge enables a high CO-yielding rate of 92 μmol·g^-1·h^-1 and CO selectivity of 80.3%, which is superior to most g-C₃N₄-based photocatalytic CO₂ reductions. The CO production rate remained nearly constant under light irradiation for five cycles of 20 h, indicating its stability. The closely combined experimental and DFT calculations clearly elucidated that the variety of electronic states induced by 4f and 5d orbitals of the La single atom and the p-d orbital hybridization of La-N atoms enabled the formation of charge-transfer channel. The La-N charge bridges are found to function as the key active center for CO₂ activation, rapid COOH* formation, and CO desorption. The present work would provide a mechanistic understanding into the utilization of rare-earth single-atoms in photocatalysis for solar energy conversion.

Emerging Concepts in Carbon Nitride Organic Photocatalysis

Carbon nitrides encompass a class of transition-metal-free materials possessing numerous advantages such as low cost (few Euros per gram), high chemical stability, broad tunability of redox potentials and optical bandgap, recyclability, and a high absorption coefficient (>10⁵  cm^-1 ), which make them highly attractive for application in photoredox catalysis. In this Review, we classify carbon nitrides based on their unique properties, structure, and redox potentials. We summarize recently emerging concepts in heterogeneous carbon nitride photocatalysis, with an emphasis on the synthesis of organic compounds: 1) Illumination-Driven Electron Accumulation in Semiconductors and Exploitation (IDEASE); 2) singlet-triplet intersystem crossing in carbon nitride excited states and related energy transfer; 3) architectures of flow photoreactors; and 4) dual metal/carbon nitride photocatalysis. The objective of this Review is to provide a detailed overview regarding innovative research in carbon nitride photocatalysis focusing on these topics.

Facile synthesis of graphitic carbon nitride via copolymerization of melamine and TCNQ for photocatalytic hydrogen evolution

Graphitic carbon nitride (g-C₃N₄) has been regarded as an intriguing photocatalyst applying to hydrogen generation but suffering rapid recombination of photoinduced electron-hole pairs and insufficient absorption under visible light. We developed a novel one-pot thermal copolymerization method of melamine as a precursor and 7,7,8,8-tetracyanoquinodimethane (TCNQ) as a comonomer to synthesize modified g-C₃N₄ (abbreviated as X% TCNQ) for the first time, aiming to directly incorporate TCNQ molecular into carbon nitride skeleton for the substitution of low-electronegative carbon for high-electronegative nitride atom. Results revealed that the as-prepared photocatalysts by copolymerization of melamine with TCNQ retained the original framework of g-C₃N₄, and dramatically altered the electronic and optical properties of carbon nitride. Various measurements confirmed that as-synthesized samples exhibited larger specific surface areas, faster photogenerated charge transfer and broader optical absorption by decreasing the π-deficiency and extending the π-conjugated system, thus facilitating the photocatalytic activity. Specifically, the 0.3% TCNQ exhibited as high as seven times than the pristine g-C₃N₄ on photocatalytic H₂ generation and kept its photoactivity for five circles. This work highlights a feasible approach of chemical protocols for the molecular design to synthesize functional carbon nitride photocatalysts by copolymerizing appropriate g-C₃N₄ precursor and comonomers.

Elucidating the structure of the graphitic carbon nitride nanomaterials via X-ray photoelectron spectroscopy and X-ray powder diffraction techniques

By using the most popular method of thermal condensation of dicyandiamide in a semi-closed system, graphitic carbon nitrides (gCNs) were synthesized at 500, 550, and 600 °C. The resulting materials were comprehensively analyzed via X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD)techniques. We show that the use of routine analytical methods provides an insight into the structure of the carbon nitride materials. The analysis of geometric linear structures and fully condensed structure of polymeric carbon nitrides was performed and the ranges within which the contents of different nitrogen species (pyridine, amine, imine and quaternary nitrogen) can change were determined. This analysis, in combination with quantitative XPS studies, permits to state that the carbon nitride structure prepared by the thermal condensation of dicyandiamide is closer to the structure in which poly(aminoimino)heptazine subunits are linked into chains rather than the structure involving fully-condensed polyheptazine network. The XRD analysis proved that the 3D crystal structure of carbon nitride is described more correctly by the orthorhombic cell and space group P21212 applied to condensed chains of poly(aminoimino)heptazine (melon) and not by the hexagonal cell with the space group P6m2.

Direct growth of uniform carbon nitride layers with extended optical absorption towards efficient water-splitting photoanodes

A general synthesis of carbon nitride (CN) films with extended optical absorption, excellent charge separation under illumination, and outstanding performance as a photoanode in water-splitting photoelectrochemical cells is reported. To this end, we introduced a universal method to rapidly grow CN monomers directly from a hot saturated solution on various substrates. Upon calcination, a highly uniform carbon nitride layer with tuned structural and photophysical properties and in intimate contact with the substrate is obtained. Detailed photoelectrochemical and structural studies reveal good photoresponse up to 600 nm, excellent hole extraction efficiency (up to 62%) and strong adhesion of the CN layer to the substrate. The best CN photoanode demonstrates a benchmark-setting photocurrent density of 353 µA cm⁻² (51% faradaic efficiency for oxygen), and external quantum yield value above 12% at 450 nm at 1.23 V versus RHE in an alkaline solution, as well as low onset potential and good stability.

Photoelectrochemical cells (PEC) can convert sunlight and water directly to a hydrogen fuel. Here a robust metal-free carbon nitride-based layer is used as an efficient photoanode for water-splitting PEC.

Platinum and zinc oxide modified carbon nitride electrode as non-enzymatic highly selective and reusable electrochemical diabetic sensor in human blood

Development of non-enzymatic glucose sensor is essential to reduce the cost of diabetes regular monitoring. Here, graphitic carbon nitride (g-C₃N₄) is modified with platinum and zinc oxide for non-enzymatic electrochemical glucose sensing in physiological conditions for the first time in the literature. The interactions between Pt, g-C₃N₄ and the ZnO are studied using different physicochemical characterization techniques. The Electrochemical glucose sensing at the ZnO-Pt-gC₃N₄ occurs at low applied potential of +0.20 V (vs. Ag/AgCl) with high sensitivity 3.34 μA/mM/cm² and fast response (5 s) time. This sensor exhibited a wide linear range 0.25-110 mM with lower limit of detection of 0.1 µM. The architectured sensor was evaluated in human blood, serum and urine samples. The sensor is 4 time reusable in whole blood without activity deterioration. This reusable surface helps to reduce the cost of strip.