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Proper interface and band alignment always play essential roles in the separation of photoexcited charge of photocatalysts. In this work, we prepared a homodispersed S-scheme carbon nitride homojunction with local electron structure difference by a facile pre-doping and two-step calcination approach. Boron doping into heptazine created extra acceptor impurity, and Phosphorus doping into heptazine created extra donor impurity, which eventually modulated the electronic structure of carbon nitride. As heptazines with different element doping were integrated into carbon nitride by recalcination, B-CN and P-CN formed a homodispersed homojunction and thus produced a rich interface. Meanwhile, caused by Fermi energy levels equilibrium, the band bending constructed an S-scheme homojunction, which stimulated photogenerated electrons to transfer from CB of B-CN to VB of P-CN. The homodispersed S-scheme homojunction structure led to efficient suppression of recombination of photoinduced charge and retained stronger redox charge. Consequently, the photocatalytic performance was dramatically boosted to 2620 μmol g-1 from 60 μmol g-1 of pure CN in four-hour hydrogen evolution from water. This novel method for electron structure engineering helped to provide a new strategy for designing homojunction photocatalysts with excellent photocatalytic performance.

Carbon nitride-based photocatalysts hold an enormous potential in producing hydrogen. A strategy to simultaneously create isotype heterojunctions and active sites in highly-crystallized carbon nitride is anticipated to significantly boost the photocatalytic activity, but is yet to be realized. Herein, we find that cobalt salt added in the ionothermal synthesis can promote the phase transition of heptazine-based crystalline carbon nitride (CCN) to triazine-based poly(triazine imide) (PTI), rendering the creation of single-atom cobalt coordinated isotype CCN/PTI heterojunction. Co-CCN/PTI exhibits an appreciable apparent quantum yield of 20.88% at 425 nm for photocatalytic hydrogen production with a rate achieving 3538 μmol h−1 g−1 (λ > 420 nm), which is 4.8 times that of CCN and 27.6 times that of PTI. The high photocatalytic activity is attributed to the Type II isotype highly-crystallized CCN/PTI heterojunction for promoting charge carrier migration, and the single-atom Co sites for accelerating surface oxidation reaction.

To prepare g-C3N4 for photocatalytic tetracycline (TC) removal, KCl was employed to mix with or cover the top of the precursors, namely, melamine and urea. The mixing of KCl with the precursor will result in the incorporation of KCl within the layer structure of g-C3N4, whereas KCl covering the top might not have such an effect. Different precursor ratios contributed to the formation of heptazine-rich or triazine-rich units in the g-C3N4 structure. Melamine applied alone as a precursor will undergo a phase transformation into melam and triazine-rich g-C3N4, whereas with the addition of urea, the mixture will polymerize to form melem and heptazine-rich g-C3N4. The KCl-incorporated, heptazine-rich g-C3N4 (KCN80m) exhibited an improved photocatalytic activity for TC removal (greater than 80% during a 120-min period for 50 mL of a 20-ppm TC solution). The enhanced activity can be attributed to the improved charge separation through an electron and hole transfer through the K+ and Cl- sites, respectively; the formation of a nanojunction between the triazine and heptazine units of g-C3N4; an increased number of photoexcited electrons, indicated by the electron paramagnetic resonance spectroscopy results. We varied the mixing conditions of KCl and precursor ratio to synthesize different KCl-incorporated heptazine-rich g-C3N4 samples for effective removal of TC from water through photocatalysis.

Exploiting highly-efficient and metal-free photocatalyst for CO2 conversion into useful chemicals is a promising pathway to solve the energy and environmental crises. In this work, through a facile exfoliation process, an ultra-thin and short-range order g-C3N4 nanosheet with rich terminal oxygen defects is successfully constructed, which presents total electron yield of 36.30 μmol g-1h−1, 3.05 times higher than that of bulk one. The results affirms that both the van der Waals forces between the C3N4 layers and the C N bonds on the periodic heptazine units could be disrupted during the sonication process, thus achieving the ultra-thin and ultra-small g-C3N4 nanosheet, which enables the improvement of optical absorption and carrier separation abilities. The π-conjugated triazine rings structure is still remained but the terminal active C radicals tend to transform into oxygen defects which become the sites to bind and activate CO2. The in-situ DRIFTS provides the direct evidence that the size regulation and oxygen-defects design strategy can effectively promote the CO2 adsorption and activation process upon the photocatalyst, thus turning out to boost the reactivity toward CO2.

In this work, a wealth of triazine units was created in carbon nitride through a facile molten salt method to bridge titania and carbon nitride for accelerating charge transportation and enhancing hydrogen production performance. The doping of triazine ring into C3N4 framework results in more exposure of − C N − and − C N bond and forms a homojunction (MCN), which favors photocatalysis by acting as photoresponse and active centers, respectively. Moreover, the triazine units can bridge the hybridized C3N4 and TiO2, forming a stable MCN/TiO2 homo-heterojunction. Attributed to the matched band energy structure of MCN and TiO2 and the structural characteristics of triazine/heptazine heterocyclic, the light response, charge separation and transfer as well as the lifetime of carriers on MCN/TiO2 hybrid are improved significantly. As a result, the MCN/TiO2 homo-heterojunction exhibits excellent activity and stability for photocatalytic hydrogen production performance, up to 2594 μmol∙g−1∙h−1 under simulated solar irradiation, which is 5.5 times higher than that of the bare g-C3N4.

Herein, we present a novel bottom-up preparation route for heptazine-based polymers (melon), also known as graphitic carbon nitride. The growth characteristics of isolated 1D melon strings in microporous templates are presented and studied in detail. Removal of the microporous silicate template via etching is accompanied by the self-assembly of a 1D melon to stacked 3D structures. The advantages and limitations of the bottom-up approach are shown by using microporous templates with different pore sizes (ETS-10, ZSM-5, and zeolite Y). In accordance with the molecular size of the heptazine units (0.67 nm), a 1D melon can be deposited in ETS-10 with a pore width of about 0.78 nm, whereas its formation in the smaller 0.47 nm pores of ZSM-5 is sterically impeded. The self-assembly of isolated 1D melon to stacked 3D structures offers a novel experimental perspective to the controversial debate on the polymerization degree in 2D sheets of graphitic carbon nitride as micropore sizes below 1 nm confine the condensation degree of heptazine to isolated 1D strands at a molecular level. The growth characteristics and structural features were investigated by X-ray diffraction, N2 physisorption, scanning transmission electron microscopy/energy-dispersive X-ray analysis, 13C CP-NMR spectroscopy, and attenuated total reflection–infrared spectroscopy., Isolated 1D melon strands grow in microporous silicate templates. Removal of the silicate template via etching goes along with the self-assembly of the 1D melon to stacked graphitic carbon nitride. The self-assembly of an isolated 1D melon offers a novel experimental perspective to the controversial debate on the polymerization of graphitic carbon nitride as pore sizes below 1 nm confine the condensation degree of heptazine to isolated 1D strands at a molecular level.

Background Hydrogen (H2) is viewed as a clean, green, and sustainable energy source. The photocatalytic H2 evolution using light-activated photocatalyst under light irradiation is attractive to convert light into chemical energy in a feasible way. In the present work, photocatalysis of H2 generation was explored, using graphitic carbon nitride (g-C3N4) synthesized from freeze-dried dicyandiamide (DICY). Methods The DICY underwent rapid dissolution-recrystallization during the freeze-drying process, enabling more complete polymerization of the heptazine units of g-C3N4 than occurred in the same compound synthesized from untreated DICY. After loading with Pt as a cocatalyst, and under light irradiation (metal halide lamp) in the presence of triethanolamine, g-C3N4 synthesized from freeze-dried DICY (DCN) showed an increased H2 evolution rate (∼20 μmol h‒1) compared with that using g-C3N4 derived from untreated DICY (CN) (12 μmol h‒1). In addition, DCN had a higher H2 generation rate than CN under light of different wavelengths (400, 450, and 550 nm). Significant Findings The improved activity of DCN could be attributed to inhibition of charge recombination (evidenced by photoluminescence), fast charge transfer (evidenced by electrochemical impedance spectra and photocurrent measurements), and a suitable energy bandgap (evidenced by Mott-Schottky and UV–vis measurements) resulting from better-polymerized heptazine rings (evidenced by nuclear magnetic resonance spectroscopy). In summary, DCN prepared in this study could be used as a visible-light activated and effective metal-free material for photocatalytic H2 generation.

The bridging N atom in g-C3N4 structure plays a decisive role in photo-generated charge transfer because it usually confines photo-generated electrons and holes in each heptazine, thus leading to severe recombination. In this work, a kind of 2-aminoterephthalic acid-derived benzene ring group with rich π-electrons was considered to integrate with bridging N to break the above-mentioned confining effect. On the basis of density-functional theory calculations and experimental analysis, this 2-aminoterephthalic acid-derived bridging structure facilitated to draw photo-generated charge out of heptazine unit, and its polarized asymmetric structure promoted the directional transfer of photo-generated charge carriers across adjacent heptazines, thus efficiently reducing the recombination. Meanwhile, the 2-aminoterephthalic acid-derived bridging structure also reinforced the connectivity of heptazine units in g-C3N4 framework and led to high degree of polymerization, which thus extended the π-conjugated electronic system of g-C3N4 and modulated the band structure favoring photocatalytic hydrogen production. Consequently, a high photocatalytic H2-production activity of 24,595 μmol h−1 gcat-1 was achieved on the bridging regulated g-C3N4 under visible light, with an apparent quantum yield of 48.7% at 425 nm.

This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Projektnummer 364549901 – TRR 234 CataLight [Projects A5, B6, B7, C2, C3 and Z2] and BE 5102/5-1. We acknowledge also support by Spanish Ministerio de Ciencia e Innovación (PID2020-113558RB-C41)., Mitoraj, D., Krivtsov, I., Li, C., Rajagopal, A., Im, C., Adler, C., Köble, K., Khainakova, O., Hniopek, J., Neumann, C., Turchanin, A., da Silva, I., Schmitt, M., Leiter, R., Lehnert, T., Popp, J., Kaiser, U., Jacob, T., Streb, C., Dietzek, B., Beranek, R.

Ab initio studies were conducted to evaluate the performance of hydrogen storage by Mg-decorated graphite carbon nitride (g-CN, heptazine structure). In our calculations, we found that each unit of this material can accommodate one Mg atom. Partial charges from Mg were transferred to the pristine material, making itself more electropositive. This is favorable for hydrogen storage, as the adsorbed H2 molecules can be easily polarized, and the electrostatic interactions can be enhanced. The configurations of the Mg-decorated g-CN with multiple adsorbed H2 molecules were presented in this study, and the related adsorption mechanisms were also discussed in details. Each unit can adsorb at most 7 H2 molecules with adsorption energies ranging from −0.276 eV to −0.130 eV. In addition, besides Mg, we also noticed that the nitrogen atoms also perform well in hydrogen adsorption. For this novel material, its highest capacity of hydrogen storage can reach to 7.8 wt%, highly surpassing the target value of 5.5 wt% set by the U.S. department of energy (DOE)[1]. The computational results provided in this study indicates a promising prospect for alkali metal functionalized 2D materials in energy storage; and through decent explorations, the performance of this class of materials can be largely improved.

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 UOx (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 UOx 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.

Graphitic carbon nitride (g-C3N4) has garnered much attention due to its potential as an efficient metal-free photocatalyst. This study examines the evolution of properties in zero-dimensional quantum dots up to sizable clusters that mimic extended g-C3N4 monolayers. We employ density functional theory to investigate systematically the structural, electronic, and optical properties of the g-C3N4-based melamine and heptazine building blocks using a "bottom-up" construction of polymeric monolayers. The results from our computations indicate that the melamine- and heptazine-based polymeric g-C3N4 systems must be reduced to at least 2.74 and 4.00 nm, respectively, to observe an increase of its optical gap with a size reduction. The present study also examines the nature of the electronic transitions exhibited by g-C3N4-based monolayers through full natural transition orbital and density of state analyses. The most promising sites for water splitting and subsequent chemical doping studies are identified, which generally correspond to the nitrogen and carbon atoms, respectively.

Integrated modification of polymeric carbon nitride (PCN) by crystallinity-improving and multi-element-doping was realized to advance the photocatalytic activity of PCN in this work. Specifically, K–Na co-doped crystalline PCN (K–Na–CCN) was facilely synthesized by calcining melon with NaCl and subsequent with KOH solution. It owned well-defined poly (heptazine imide) framework doped with both Na+ and K+. K–Na–CCN realized enhanced visible light absorption and reduced recombination of photogenerated electron-hole pairs, leading to favorable photocatalytic hydrogen evolution activity much superior to the melon-based bulk PCN and Na-doped crystalline PCN. The co-doping of K+ and Na+ in the crystalline carbon nitride framework was highlighted for this improvement. This work owns reference significance to the microstructure optimization of crystalline PCN.

Carbon nitride (CN) polymer is considered as a potential material for photocatalysis, but surfers from the low photoconversion efficiency due to its high defect density. Although the defects can be limited through the crystallization of CN, the photocatalytic activity of single-component photocatalyst is still restricted by the recombination of photoexcited carriers. In our work, the ordered Schottky heterojunction of heptazine-based crystalline carbon nitride (HCN) and Ti3C2 MXene was successfully prepared through ionothermal method. The HCN/Ti3C2 composites exhibit higher photocatalytic performance than pristine HCN. Particularly, the HCNT20 sample exhibits the highest photocatalytic H2 evolution activity, which is about 8 and 2 times higher than that of bulk CN and pristine HCN, respectively. The results of charge transfer dynamics and DFT calculation reveal that the photocatalytic enhancement mechanism is primarily attributed to the synergistic effects of crystallization of CN, excellent conductivity of Ti3C2 and well-constructed Schottky heterojunction.

Density functional theory (DFT) computational studies were conducted to explore the hydrogen storage performance of a monolayer material that is built on the base of carbon nitride (g-C3N4, heptazine structure) with decoration by magnesium (Mg). We found that a 2 × 2 supercell can bind with four Mg atoms. The electronic charges of Mg atoms were transferred to the g-C3N4 monolayer, and thus a partial electropositivity on each adsorbed Mg atom was formed, indicating a potential improvement in conductivity. This subsequently causes the hydrogen molecules’ polarization, so that these hydrogen molecules can be efficiently adsorbed via both van der Waals and electrostatic interactions. To note, the configurations of the adsorbed hydrogen molecules were also elucidated, and we found that most adsorbed hydrogen molecules tend to be vertical to the sheet plane. Such a phenomenon is due to the electronic potential distribution. In average, each adsorbed Mg atom can adsorb 1–9 hydrogen molecules with adsorption energies that are ranged from −0.25 eV to −0.1 eV. Moreover, we realised that the nitrogen atom can also serve as an active site for hydrogen adsorption. The hydrogen storage capacity of this Mg-decorated g-C3N4 is close to 7.96 wt %, which is much higher than the target value of 5.5 wt % proposed by the U.S. department of energy (DOE) in 2020 [1]. The finding in this study indicates a promising carbon-based material for energy storage, and in the future, we hope to develop more advanced materials along this direction.

Summary The extension of graphitic carbon nitride (g-CN) materials into optoelectronic applications beyond photocatalysis has long been anticipated due to their non-metallic composition, moderate electronic band gap, and excellent stability. However, large-scale synthesis of uniform two-dimensional (2D) g-CN films with high crystallinity and tunable thickness remains the bottleneck for their future applications in optoelectronic devices. Here, a vapor-phase transport-assisted condensation method has been developed for the wafer-scale synthesis of uniform 2D g-CN films with tunable thickness. First-principle calculations indicate the direct condensation from melem to heptazine-based carbon nitride enables the formation of high-quality g-CN films. A facile water-assisted transfer strategy is developed for subsequent fabrication on arbitrary substrate. A free-standing flexible photodetector array with strain-insensitive responsibility is demonstrated with the g-CN films as imaging pixels. This study provides a convenient route to wafer-scale growth of high-quality 2D g-CN films and paves the way to the g-CN-based electronic and optoelectronic devices.

The Frenkel exciton dissociation efficiency and the subsequent free charge carrier migration rate mainly determine the photocatalytic property of polymeric carbon nitride (PCN). Herein, by postcalc...

Graphitic carbon nitride (g-C3N4) has emerged as a remarkably promising photocatalyst for addressing environmental and energy issues; however, it exhibits only moderate photocatalytic activity because of its low specific surface area and high recombination of carriers. Preparation of crystalline g-C3N4 by the molten salt method has proven to be an effective method to improve the photocatalytic activity. However, crystalline g-C3N4 prepared by the conventional molten salt method exhibits a less regular morphology. Herein, highly crystalline g-C3N4 hollow spheres (CCNHS) were successfully prepared by the molten salt method using cyanuric acid-melamine as a precursor. The higher crystallization of the CCNHS samples not only repaired the structural defects at the surface of the CCNHS samples but also established a built-in electric field between heptazine-based g-C3N4 and triazine-based g-C3N4. The hollow structure improved the level of light energy utilization and increased the number of active sites for photocatalytic reactions. Because of the above characteristics, the as-prepared CCNHS samples simultaneously realized photocatalytic hydrogen evolution with the degradation of the plasticizer bisphenol A. This research offers a new perspective on the structural optimization of supramolecular self-assembly.

The emerging class of heptazine-based polymeric materials has shown potential candidature as photocatalyst materials for hydrogen evolution. At the same time, they have shown promising application as solid base materials to catalyse various organic transformations. Thus, the material design rationale needs to be developed around the heptazine-based polymeric frameworks in order to specifically design task specific materials. Herein, we utilised controlled reaction conditions to synthesize the desired polymeric networks with trichloroheptazine as precursor. Material design strategy employed nitrogen rich [tris(2-aminoethylamine) and hydrazine] as soft linkers to understand the effect on band structure of developed heptazine-based polymeric networks. The developed polymeric networks were explored as platform to study systematically the effect on their respective photophysical properties and understand their surface basicity. The framework having aminoalkyl linker showed superior activity in photocatalysis as well as heterogeneous base catalysis. Further, model catalysts revealed the importance of N-atoms as active basic sites in these systems.

In spite of the enormous promise that polymeric carbon nitride (PCN) materials hold for various applications, the fabrication of high-quality, binder-free PCN films and electrodes has been a largely elusive goal to date. Here, we tackle this challenge by devising, for the first time, a water-based sol−gel approach that enables facile preparation of thin films based on poly(heptazine imide) (PHI), a polymer belonging to the PCN family. The sol−gel process capitalizes on the use of a water-soluble PHI precursor that allows formation of a non-covalent hydrogel. The hydrogel can be deposited on conductive substrates, resulting in formation of mechanically stable polymeric thin layers. The resulting photoanodes exhibit unprecedented photoelectrochemical (PEC) performance in alcohol reforming and highly selective (∼100 %) conversions with very high photocurrents (>0.25 mA cm−2 under 2 sun) down to, This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)–Projektnummer 364549901–TRR 234 [Projects B6, B7, C2, C4 and Z2] and BE 5102/5-1. C.N and A.T. acknowledge financial support of the DFG through the project TU 149/8-2 “Towards photo-active membranes for artificial photosynthesis” as well as the DFG through a research infrastructure grant INST 275/257-1 FUGG. S.G.G. acknowledges support by Spanish MINECO (MAT2016-78155-C2-1-R) and Gobierno del Principado de Asturias (GRUPIN-ID2018-170). L.d.S.G. acknowledges Juan de la Cierva Formación scholarship. I.K. acknowledges the support of the Alexander von Humboldt Foundation through the Humboldt Research Fellowship. Open access funding enabled and organized by Projekt DEAL.

Construction of alternating donor–acceptor (D–A) units in g-C3N4 is an effective strategy to achieve more effective charge carrier separation towards improved photocatalytic performance. However, the charge carrier kinetics, especially the vague intramolecular charge transfer (ICT) details and structure–performance relationship remain unknown. Herein, a series of D–A structured g-C3N4 photocatalysts with improved performance were synthesized by introducing 2-aminobenzothiazole (ABT) into the g-C3N4 framework. Remarkably, up to 3.83-fold improvement in photocatalytic hydrogen evolution was realized at 0.8 wt% ABT incorporation. It shall be due to the broadened visible light absorption range (λ ≤ 700 nm) and accelerated charge carrier separation which originated from the ultrafast ICT (τ1 = 112.20 fs) between the benzothiazole group and heptazine ring. The boosted performance is highly associated with the purposely introduced D–A structure, which may shed light on designing of analogues with better photocatalytic hydrogen evolution performance.

Ultrafast transient absorption anisotropy spectroscopy (TAA) reveals the orientational dynamics of light-induced excitations in a water soluble poly(heptazine imide). The results provide insights into the fast charge transfer processes in the material.

Photoreforming of biomass to value-added chemicals and fuels is a chemical approach to extract photosynthetically-trapped energy in complex biomolecules which otherwise disintegrate naturally in the environment. Designing precise photocatalytic materials that can selectively break the sturdy, nature-designed biomass with multiplex chemical composition/bonding and inaccessible sites is central to deploying this technology. Polymeric carbon nitride (CN) comprised of a 2D network of condensed heptazine/triazine (C6N7/C3N3) core has shown great promise for photoreforming of biomass derivatives due to intriguing physicochemical and optical properties. This review comprehensively summarizes the state-of-the-art applications of CN-based photocatalysts for the conversion of lignocellulosic biomass derivatives. Various chemical and structural modifications in CN structure such as doping, surface functionalization, hybridization entailing to higher selectivity and conversion have been discussed aiming at providing valuable guidance for future CN-based materials design.

Photocatalytic H2 production via water splitting holds great potential for the conversion of solar energy into renewable and storable chemical fuels in a sustainable manner, but this up-hill reaction is mainly hindered by the water oxidation process. This reaction is sluggish and relies on high overpotentials (wasted energy), while only oxygen is generated as a low value oxidation by-product. Coupling hydrogen generation to a less unfavorable oxidation reaction creating higher value products is therefore a recommendable issue. Here we found that poly(heptazine imide) (PHI), a stable and reactive allotrope of crystalline carbon nitride, supported with highly dispersed Pt nanoparticles (PtNPs) could achieve efficient and persistent H2 and CH4 production by photocatalytic reforming of biomass derived alcohols under visible light irradiation. The results reveal that PHI with low grain boundary defects restrains the bulk charge carrier recombination and largely promotes photocatalytic efficiency. The apparent quantum efficiency (AQE) of the optimized material for visible light H2 evolution could reach values as high as 15% at 420 nm. The DFT results reveal that the activity is associated with the Tafel step rather than the Volmer–Tafel step. This study highlights the development of a new covalent scaffold for the simultaneous generation of storable and valuable chemical fuels (H2 and CH4) in a feasible photocatalytic manner.

Polymeric carbon nitride (PCN) has been widely used as a metal-free photocatalyst for solar hydrogen generation from water. However, rapid charge carrier recombination and sluggish water catalysis kinetics have greatly limited its photocatalytic performance for overall water splitting. Herein, a single-atom Ni terminating agent was introduced to coordinate with the heptazine units of PCN to create new hybrid orbitals. Both theoretical calculation and experimental evidence revealed that the new hybrid orbitals synergistically broadened visible light absorption via a metal-to-ligand charge transfer (MLCT) process, and accelerated the separation and transfer of photoexcited electrons and holes. The obtained single-atom Ni terminated PCN (PCNNi), without an additional cocatalyst loading, realized efficient photocatalytic overall water splitting into easily-separated gas-product H2 and liquid-product H2O2 under visible light, with evolution rates reaching 26.6 and 24.0 μmol g-1 h-1, respectively. It was indicated that single-atom Ni and the neighboring C atom served as water oxidation and reduction active sites, respectively, for overall water splitting via a two-electron reaction pathway.

Hydrogen (H)-bonded C3 symmetrical discotic liquid crystals (DLCs) (Hpz-Cn-TP, n = 6, 8, 9) forming segregated columnar assembly are described. The columnar architectures are formed by electronically decoupled donor (D) (triphenylene) and acceptor (A) (heptazine) units linked through complementary H-bonding. Unlike triazine, tris(triazolyl)triazine and tris-benzimidazolyl benzene which have been explored in depth to form H-bonded DLCs, a new heptazine core has been explored, herein, for the first time to develop functional supramolecular materials of technological importance. Among Hpz-Cn-TP complexes, both Hpz-C8-TP and Hpz-C9-TP showed ambipolar charge transport with the former demonstrating a maximum hole mobility of 1.60 cm2 V−1 s−1 and the latter showing a maximum electron mobility of 2.90 × 10−3 cm2 V−1 s−1, measured through the space-charge limited current technique. Not only discrete conduction channels but also highly preorganized homeotropically aligned conduction pathways are observed in the reported complexes, resulting in efficient ambipolar charge transport. The adopted H-bonded synthetic approach does not only contribute to the electronic decoupling of D and A units but also offer an easy synthetic route to achieve DLC materials with high purity that have widespread use in commercial organic electronics.

Van der Waals (vdW) heterostructures of two-dimensional monolayers are a relatively new class of materials with highly tunable band alignment, bandgap energy, and bandgap transition type. In this study, we performed density functional theory calculations to investigate how a vdW heterostructure of heptazine-based graphitic carbon nitride (hg-C3N4) and graphitic zinc oxide (g-ZnO) monolayers is formed (hg-C3N4/g-ZnO). This heterostructure is a potential solar-driven photocatalyst for the water-splitting reaction. Upon the formation of the heterostructure, a type-I indirect bandgap (Eg = 2.08 eV) is created with appropriate conduction band minimum and valence band maximum levels relative to the oxidation/reduction potentials for the water-splitting reaction. In addition, a very large electrostatic potential difference of 11.18 eV is generated across the heterostructure, leading to a large, naturally-formed, built-in electric field directing from hg-C3N4 to g-ZnO. The produced electric field forces photogenerated electrons in g-ZnO to transfer toward hg-C3N4, leading to a decrease in the electron–hole recombination rate. We also found that both g-ZnO and hg-C3N4 synergistically lead to higher light absorption of the heterostructure (λmax = 387 nm). Furthermore, band alignment, bandgap energy, and transition type of the heterostructure can be tuned by applying external perpendicular electric fields and biaxial strains. It was found that a strain of +2% leads to a Z-scheme band alignment (Eg = 2.34 eV, direct) and an electric field of 1 V A−1 leads to a type-II heterostructure (Eg = 2.29 eV, indirect), which are both beneficial for efficient water-splitting photocatalysis.

Graphitic carbon nitride, as a very promising two-dimensional structure host for single atom catalysts (SACs), has been studied extensively due to its significant confinement effects of single atoms for photocatalytic applications. In this work, a systematic investigation of g-C3N4 confining noble metal single atoms (NM1@g-C3N4) will be performed by using DFT calculations. The geometric structure calculations indicate that the most favorable anchored sites for the NM1 is located in the six-fold cavity, and the deformed wrinkle space of g-C3N4 helps the NM1 to be stabilized in the six-fold cavity. The electronic structure calculations show that the conduction band of NM1@g-C3N4 moved down and crossed through the Fermi level, resulting in narrowing the band gap of the NM1@g-C3N4. Moreover, the confined NM1 provide a new channel of charge transport between adjacent heptazine units, resulting in a longer lifetime of photo-generated carriers except Ru, Rh, Os and Ir atoms. Furthermore, the d-band centres of NM1 in NM1@g-C3N4 show that Rh1@, Pd1@, Ir1@ and Pt1@g-C3N4 SACs may have better photocatalytic performance than other NM1@g-C3N4 SACs. Finally, Pt1@g-C3N4 SACs are considered to have higher photocatalytic activity than other NM1@g-C3N4 SACs. These results demonstrate that the confinement effects of noble metals on monolayer g-C3N4 not only makes the single atom more stable to be anchored on g-C3N4, but also enhances the photocatalytic activity of the system through the synergistic effect between the confined NM1 and the monolayer g-C3N4. These detailed research may provide theoretical support for engineers to prepare photocatalysts with higher activity.

A hierarchical heterojunction polymer aerogel (HPA) composed of a oxygen- and nitrogen-linked heptazine-based polymer was rationally designed as a donor–acceptor (D–A) light harvester by a facile two-step self-assembly method. The HPA features a three-dimensional (3D) continuous network of 1D holey microfibers assembled from 2D nanomesh. The collaborative engineering of the D–A homojunction and 3D hierarchical open-pore nanoarchitecture in HPA causes a narrowed bandgap, improved hydrophilicity, and dramatically increased charge transfer and separation, and therefore the synthesis of sub-1 nm ultrafine Pt cocatalysts associated with countless bonding sites. It exhibits a high H2 evolution rate of 103.79 μmol h−1 (12 times higher than that of the conventional g-C3N4 powder), and an excellent internal quantum yield of 29.4% at 400 nm, much higher than those of most of the aerogel-based photocatalysts reported.

Graphitic carbon nitride (CN) is a promising candidate for use in photocatalytic pollutant degradation, but it only shows moderate activity because of its sluggish photocarrier transfer and insufficient light absorption. Herein, CN@DMAP-X (DMAP is 4-dimethylaminopyridine and X is the amount of DMAP) samples were synthesized using a simple polycondensation strategy via grafting DMAP onto the edge of a g-C3N4 framework. The nucleophilic pyridine ring induces the delocalization of the local electrons of the large number of π-bonds, which alters the electron distribution and avoids in-plane carrier recombination. The steric hindrance effects of the 4-dimethylamino groups on DMAP limit the random growth of the g-C3N4 framework and contribute more layered structures, which can enhance visible light absorption via providing more scattering of light, and this can also provide more accessible active sites for the sunset yellow (SY) macromolecule. CN@DMAP-200 showed optimum visible light degradation efficiency toward SY, up to 2.51 times that of pristine g-C3N4. 1O2 was determined to be the dominant active species for SY degradation, whereas ˙O2− and h+ played secondary roles in photodegradation.

To better understand how hydrogen bonding influences the excited-state landscapes of aza-aromatic materials, we studied hydrogen-bonded complexes of 2,5,8-tris (4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a molecular photocatalyst related to graphitic carbon nitride, with a variety of phenol derivatives (R-PhOH). By varying the electron-withdrawing character of the para-substituent on the phenol, we can modulate the strength of the hydrogen bond. Using time-resolved photoluminescence, we extract a spectral component associated with the R-PhOH-TAHz hydrogen-bonded complex. Surprisingly, we noticed a striking change in the relative amplitude of vibronic peaks in the TAHz-centered emission as a function of R-group on phenol. To gain a physical understanding of these spectral changes, we employed a displaced-oscillator model of molecular emission to fit these spectra. This fit assumes that two vibrational modes are dominantly coupled to the emissive electronic transition and extracts their frequencies and relative nuclear displacements (related to the Huang-Rhys factor). With the aid of quantum chemical calculations, we found that heptazine ring-breathing and ring-puckering modes are likely responsible for the observed vibronic progression, and both modes indicate decreasing molecular distortion in the excited state with increasing hydrogen bond strength. This finding offers new insights into intermolecular excited-state hydrogen bonding, which is a crucial step toward controlling excited-state proton-coupled electron transfer and proton transfer reactions.

Graphitic carbon nitride (gCN) has only recently experienced a renaissance in a myriad of domains despite existing as a long-established material described in the chemical literature. Notwithstandi...

As a state-of-the-art conjugated polymer photocatalyst, graphitic carbon nitride(abbreviated as g-C3N4) has shown great potential in photocatalytic cofactor(reduced form of nicotinamide adenine dinucleotide, NADH) regeneration. Herein, Fe-doped g-C3N4 was engineered for photocatalytic NADH oxidation. The π-π interaction between the NADH molecule and the conjugated heptazine building block facilitates the adsorption of NADH onto the framework, as revealed by density functional theory(DFT) calculations. Furthermore, iron doping promoted the oxidation kinetics of NADH under blue LED illumination. The conversion ratio of NADH to its oxidized form could be up to 85.7% in 20 min, comparing with 59.4% for metal-free counterpart. Enzyme assay employing formate dehydrogenase(FDH) further verified the selectivity of the products, with 67.5%±2.6% of enzymatically active 1,4-NADH being regenerated following the oxidation process. Scavenger experiments suggest the dominant role of photo-induced electrons in the oxidation of NADH. This work could shed light on developing a novel cofactor regeneration route through the synergistic effect between the metal doping and noncovalent interaction based on the conjugated polymer.

Photocatalysis is a facile and eco-friendly approach for the removal of dyes and organic compounds, water splitting, and CO2 reduction and fixation. Herein we report the first use of P,S,O-co-doped graphitic carbon nitride (g-C3N4) to produce a photocatalyst hydrogel. The hydrogels are generated by photoinitiated crosslinking of N,N-dimethylacrylamide and N,N-methylenebis(acrylamide) with modified g-C3N4 materials and the photocatalytic activity is evaluated by monitoring the degradation of methyl blue (MB) solutions. S- or O-doping creates C‒S or C‒O bonds, respectively, by replacement N atoms, while P-doping produces P‒N bonds by substitution of C in the heptazine rings of g-C3N4. Based on a combination of theoretical calculations and experimental analyses, phosphorus, sulfur, and oxygen doping considerably facilitates charge separation across the heptazine rings and attracts photoexcited electrons, thus contributing to enhanced photocatalytic activity of doped g-C3N4. The P,S,O-co-doped g-C3N4 hydrogel exhibited high photocatalytic activity for MB removal under simulated solar irradiation and could be easily isolated and cleaned for reuse. Thus, this study reports the formation of a state-of-the-art photocatalyst hydrogel from P,S,O-co-doped g-C3N4 with enhanced photocatalytic activity and demonstrated reusability.

In this work, the crystalline carbon nitride photocatalysts were synthesized by an ionothermal technique with varied synthesis temperature of 500, 550, and 600 °C, and synthesis time of 2, 4, and 6 h. Fourier transform infrared spectra showed the successful formation of the prepared carbon nitrides from their characteristic vibration peaks. X-ray diffraction patterns suggested that the same phase of poly(triazine imide) and heptazine could be observed, but with different crystallinity. The optical properties showed that different temperatures and synthesis time resulted in the different band gap energy (2.72–3.02 eV) as well as the specific surface area (24–73 m2 g–1). The transmission electron microscopy image revealed that the crystalline carbon nitride has a near-hexagonal prismatic crystallite size of about 50 nm. Analysis by high-performance liquid chromatography showed that the best photocatalytic activity for phenol degradation under solar light simulator was obtained on the crystalline carbon nitride prepared at the 550 °C for 4 h, which would be due to the high crystallinity, suitable low band gap energy (2.82 eV), and large specific surface area (73 m2 g–1). Controlling both the temperature and synthesis time is shown to be important to obtain the best physicochemical properties leading to high activity.

Graphitic carbon nitride (g-CN) has emerged as a promising metal-free photocatalyst, while the catalytic mechanism for the photoinduced redox processes is still under investigation. Interestingly, this heptazine-based polymer optically behaves as a “quasi-monomer”. In this work, we explore upstream from melem (the heptazine monomer) to the triazine-based melamine and melam and present several lines of theoretical/experimental evidence where the catalytic activity of g-CN originates from the electronic structure evolution of the C–N heterocyclic cores. Periodic density functional theory calculations reveal the strikingly different electronic structures of melem from its triazine-based counterparts. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy also provide consistent results in the structural and chemical bonding variations of these three relevant compounds. Both melam and melem were found to show stable photocatalytic activities, while the photocatalytic activity of melem is about 5.4 times higher than that of melam during the degradation of dyes under UV–visible light irradiation. In contrast to melamine and melam, the frontier electronic orbitals of the heptazine unit in melem are uniformly distributed and well complementary to each other, which further determine the terminal amines as primary reduction sites. These appealing electronic features in both the heterocyclic skeleton and the terminated functional groups can be inherited by the polymeric but quasi-monomeric g-CN, leading to its pronounced photocatalytic activity.

Recently, a derivative of the heptazine (tris-triazine) molecule, trianisole-heptazine (TAHz), was synthesized and was shown to catalyze the oxidation of water to hydroxyl radicals under 365 nm LED light in a homogeneous reaction (E. J. Rabe et al., J. Phys. Chem. Lett. 2018, 9, 6257-6261). The possibility of water photo-oxidation with a precisely defined molecular catalyst in neat solvents opens new perspectives for clarifying the fundamental reaction mechanisms involved in water oxidation photocatalysis. In the present work, the effects of chemical substituents on the three CH positions of Hz on the photocatalytic reactivity were explored with wave function-based ab initio electronic-structure calculations for hydrogen-bonded complexes of Hz and three selected Hz derivatives (TAHz, trichloro-Hz, and tricyano-Hz) with a water molecule. While anisole is an electron-donating substituent, Cl is a weakly electron-withdrawing substituent and CN is a strongly electron-withdrawing substituent. It is shown that the barrier for the photoinduced abstraction of an H atom from the water molecule is raised (lowered) by electron-donating (electron-withdrawing) substituents. The highly mobile and reactive hydroxyl radicals generated by water oxidation can recombine with the reduced chromophore radicals to yield photohydrates. The effect of substituents on the thermodynamics of the photohydration reaction was computed. Among the four chromophores studied, TAHz stands out on account of the metastability of its photohydrate, which suggests self-healing of the photocatalyst after oxidation of TAHzH radicals by OH radicals. In addition, the effect of substituents on the H atom photodetachment reaction from the reduced chromophores, which closes the catalytic cycle, has been investigated. The energy of the repulsive 2πσ* state, which drives the photodetachment reaction is lowered (raised) by electron-donating (electron withdrawing) substituents. All four chromophores exhibit inverted S1/T1 gaps. This feature eliminates long-lived triplet states and thus avoids the activation of molecular oxygen to highly reactive singlet oxygen.

We demonstrate chemical tuning and laser-driven control of intermolecular H atom abstraction from protic solvent molecules. Using multipulse ultrafast pump-push-probe transient absorption (TA) spec...

Small organic radicals are ubiquitous intermediates in photocatalysis and are used in organic synthesis to install functional groups and to tune electronic properties and pharmacokinetic parameters of the final molecule. Development of new methods to generate small organic radicals with added functionality can further extend the utility of photocatalysis for synthetic needs. Herein, we present a method to generate dichloromethyl radicals from chloroform using a heterogeneous potassium poly(heptazine imide) (K-PHI) photocatalyst under visible light irradiation for C1-extension of the enone backbone. The method is applied on 15 enones, with γ,γ-dichloroketones yields of 18–89%. Due to negative zeta-potential (−40 mV) and small particle size (100 nm) K-PHI suspension is used in quasi-homogeneous flow-photoreactor increasing the productivity by 19 times compared to the batch approach. The resulting γ,γ-dichloroketones, are used as bifunctional building blocks to access value-added organic compounds such as substituted furans and pyrroles., Long-lived carbon nitride radicals have been used in several photocatalytic reactions. Herein, long-lived potassium poly(heptazine imide) radicals enable synthesis of γ,γ-dichloroketones from enones by addition of CHCl2 moiety, generated from chloroform, to the C=C bond.

The graphitic carbon nitride (g-C3N4) nanosheets have attracted much attention for its excellent performance in photocatalysis, but the verification of the detailed experimental geometry structure ...

The insufficient oxidation capacity, high carrier recombination rate and limited sunlight absorption seriously suppress the photocatalytic activity of pure g-C3N4. Using state-of-the-art hybrid density functional theory, we report an efficient method to tackle all aforementioned issues of g-C3N4 by metal-nonmetal (S and K) co-doping here. We find the adsorption of K atom on hollow site causes dynamic strain of g-C3N4. The S + K co-doping not only shifts the band edges downwards to achieve a much large overpotential of ca. 0.76 V, but also significantly extends the visible-light absorption threshold of g-C3N4. More importantly, the newly established channel between neighboring heptazine units in the doped structure is highly favorable for the separation of charge carriers. Our results help the design of high-performance visible-light-responsive g–C3N4–based photocatalyst for solar water splitting.

Elements doping is a powerful way to alter the electronic structure and enhancing the photo catalytic activity of materials by relaxing the surrounding chemical bonds and forming new chemical bond. In this work, we have performed, the first principle density functional theory calculations to investigate the geometric, electronic and optical properties of pristine, Na-doped and P-doped as well as Na and P (Na/P) co-doped heptazine based monolayer graphitic carbon nitride (g-C3N4). The co-doping process results in significantly narrow band gap of g-C3N4. The optical absorption shows better visible-light response compare to pristine g-C3N4. After doping the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) show strong delocalization and indicates photo generated electron/hole (e-/h+) pair disunion abilities of doped systems are superior than pristine heptazine based monolayer g-C3N4. Thus the co-doping with Na and P elements is an effective technique to boost the photocatalytic performance of heptazine based monolayer g-C3N4.

A precursor-engineering strategy coupled with a microwave molten-salt process (PE-MWMS) is developed to synthesize graphitic carbon nitride (g-C3 N4 ) with an isotype triazine/heptazine-based g-C3 N4 heterojunction as a photocatalyst for the hydrogen evolution reaction (HER) under visible light illumination. Four hybrid precursor combinations-thiourea/melamine, thiourea/dicyandiamide, urea/melamine, and urea/dicyandiamide-are used to synthesize g-C3 N4 heterojunctions by the PE-MWMS process. Control experiments indicate that the precursor components and microwave treatment have a great effect on the HER performance of the g-C3 N4 samples. Samples synthesized with the optimal molar ratios of thiourea/melamine (2:1), thiourea/dicyandiamide (2:1), urea/melamine (3:1), and urea/dicyandiamide (3:1), exhibit the highest HER rates of 3135, 2519, 2844, and 2565 μmol g-1 h-1 , respectively. The amounts of heptazine and triazine units in the g-C3 N4 samples can be easily adjusted by changing the ratios of the hybrid precursors and play a decisive role in improving the photocatalytic HER activity. Because of the unique composition and microstructure, the efficient separation of electron-hole pairs, the broadened photo-absorption edges, and the narrowed band gaps, the as-obtained triazine/heptazine-based g-C3 N4 nanostructures exhibit promising activity for HER application.

The adsorption of the Fe2+ and Fe3+ cations on four surface models of graphitic carbon nitride quantum dots, including planar and corrugated nanosheets of triazine and heptazine structures has been explored using density functional theory. The adsorption was most favorable on hollow sites but with different coordination numbers and geometrical configurations. A remarkable decrease in the energy gap of the semiconductor (e.g., from 4.10 to 0.01 eV with the buckled triazine-type sensor) delineated the high capability of the 2D material toward the detection of iron cations, particularly the Fe3+ analyte. Remarkable charge transfers (in the range of 0.688–0.908 e and 1.692–1.794 e for Fe2+ and Fe3+, respectively) from the surface to the analyte were identified from the QTAIM charge analysis. Ignoring the surface curvature could overestimate the adsorption energy while underestimating the energy gap and charge transfer of both types of graphitic carbon nitride. The LOL profiles and density of electronic states revealed the mechanism and nature of the interactions.

Graphite carbon nitride (GCN), which can be regarded as a nitrogen heteroatom-substituted graphite framework, has attracted great attention as a new 2D layered structure material with semiconductor electronic characteristics. Using molecular dynamics simulations, the in-plane thermal conductivity and cross-plane thermal resistance of two GCN structures (i.e., triazine-based and heptazine-based) are investigated. Our results show that the in-plane thermal conductivities of the triazine-based and heptazine-based GCN monolayers along the armchair direction are 55.39 and 17.81 W m-1 K-1, respectively. The cross-plane thermal resistance decreases with increasing layer number and reaches asymptotic values of 3.6 × 10-10 and 9.3 × 10-10 m2 K W-1 at 40 layers for triazine-based and heptazine-based GCN, respectively. The in-plane thermal conductivity can be effectively manipulated by changing the temperature and applying strain, while it is insensitive to the number of layers, which is in sharp contrast to that of graphene. Moreover, the cross-plane thermal resistance decreases monotonically with temperature and coupling strength, and can be modulated by external strain. Surprisingly, the cross-plane tensile strain can reduce the thermal resistance of the heptazine-based GCN. Our study serves as a guide to groups interested in the physical properties of GCN.

Transient FTIR measurements were performed on graphitic carbon nitride using a step-scan method, facilitating temporal resolution of a few nanoseconds. Variations in the location and the intensity of specific peaks and emerging of new peaks were observed during the first 170 nanoseconds after excitation. A comparison was made between graphitic carbon nitride prepared at two temperatures: 510 °C and 650 °C. The material prepared at 650 °C revealed very strong non-specific absorption beginning 35 nanoseconds after excitation and lasting for 20 nanoseconds. This phenomenon, termed “IR-blackening” was observed neither in g-C3N4 prepared at 510 °C nor in g-C3N4 prepared at 650 °C that was exposed to hole scavengers (ethanol and benzyl alcohol). In contrast, exposure of material prepared at 650 °C to an electron scavenger (methyl viologen) hardly alter the “IR-blackening” phenomenon. The results were explained by a mechanism, predicting higher reductive activity for materials having imperfect heptazine polymerization, as indeed was found in the photocatalytic degradation of 4-nitrophenol.

Conventional polymerization for the synthesis of carbon nitride usually generates amorphous heptazine-based melon with an abundance of undesired structural defects, which function as charge carrier recombination centers to decrease the photocatalytic efficiency. Herein, a fully condensed poly (triazine imide) crystal with extended π-conjugation and deficient structure defects was obtained by conducting the polycondensation in a mild molten salt of LiCl/NaCl. The melting point of the binary LiCl/NaCl system is around 550 °C, which substantially restrain the depolymerization of triazine units and extend the π-conjugation. The optimized polymeric carbon nitride crystal exhibits a high apparent quantum efficiency of 12 % (λ=365 nm) for hydrogen production by one-step excitation overall water splitting, owing to the efficient exciton dissociation and the subsequent fast transfer of charge carriers.