Enhancing the Activity of a Carbon Nitride Photocatalyst by Constructing a Triazine-Heptazine Homojunction.

Establishing homojunctions at the molecular level between different but physicochemically similar phases belonging to the same family of materials is an effective approach to promoting the photocatalytic activity of polymeric carbon nitride (CN) materials. Here, we prepared a CN material with a uniform distribution of homojunctions by combining two synthetic strategies: supramolecular assemblies as the precursor and molten salt as the medium. We designed porous CN rods with triazine-heptazine homojunctions (THCNs) using a melem supramolecular aggregate (Me) and melamine as the precursors and a KCl/LiBr salt mixture as the liquid reaction medium. The triazine/heptazine ratio is controlled by varying the relative amounts of the chosen precursors, and the molten salt treatment enhances the structural order of the interplanar packing units for the THCN skeleton, leading to rapid charge migration. The resulting built-in electric field induced by the triazine-heptazine homojunction enhances photogenerated charge separation; the optimal THCN catalyst exhibits an excellent H2 evolution rate via photocatalytic water splitting, which is ∼24 times as high as that of reference bulk CN, with long-term stability.

Hierarchically Porous Few-Layer Carbon Nitride and Its High H+ Selectivity for Efficient Photocatalytic Seawater Splitting.

Photocatalysts for seawater splitting are severely restricted because of the presence of multiple types of ions in seawater that cause corrosion and deactivation. As a result, new materials that promote adsorption of H+ and hinder competing adsorption of metal cations should enhance utilization of photogenerated electrons on the catalyst surface for efficient H2 production. One strategy to design advanced photocatalysts involves introduction of hierarchical porous structures that enable fast mass transfer and creation of defect sites that promote selective hydrogen ion adsorption. Herein, we used a facile calcination method to fabricate the macro-mesoporous C3N4 derivative, VN-HCN, that contains multiple nitrogen vacancies. We demonstrated that VN-HCN has enhanced corrosion resistance and elevated photocatalytic H2 production performance in seawater. Experimental results and theoretical calculations reveal that enhanced mass and carrier transfer and selective adsorption of hydrogen ions are key features of VN-HCN that lead to its high seawater splitting activity.

Ligand Isomerization Driven Electrocatalytic Switching.

The prevailing view about molecular catalysts is that the central metal ion is responsible for the reaction mechanism and selectivity, whereas the ligands mainly affect the reaction kinetics. Here, we question this paradigm and show that ligands have a dramatic influence on the selectivity of the product. We show how even a seemingly small change in ligand isomerization sharply alters the selectivity of the well-researched oxygen reduction reaction (ORR) Co phthalocyanine catalyst from an indirect 2e- to a direct 4e- pathway. Detailed analysis reveals that intramolecular hydrogen-bond interactions in the ligand activate the catalytic Co, directing the oxygen binding and thus deciding the final product. The resulting catalyst is the first example of a Co-based molecular catalyst catalyzing a direct 4e- ORR via ligand isomerization, for which it shows an activity close to the benchmark Pt in an actual H2-O2 fuel cell. The effect of the ligand isomerism is demonstrated with different central metal ions, thus highlighting the generalizability of the findings and their potential to open new possibilities in the design of molecular catalysts.

Boron and Sodium Doping of Polymeric Carbon Nitride Photoanodes for Photoelectrochemical Water Splitting.

Polymeric carbon nitride is a promising photoanode material for water-splitting and organic transformation-based photochemical cells. Despite achieving significant progress in performance, these materials still exhibit low photoactivity compared to inorganic photoanodic materials because of a moderate visible light response, poor charge separation, and slow oxidation kinetics. Here, the synthesis of a sodium- and boron-doped carbon nitride layer with excellent activity as a photoanode in a water-splitting photoelectrochemical cell is reported. The new synthesis consists of the direct growth of carbon nitride (CN) monomers from a hot precursor solution, enabling control over the monomer-to-dopant ratio, thus determining the final CN properties. The introduction of Na and B as dopants results in a dense CN layer with a packed morphology, better charge separation thanks to the in situ formation of an electron density gradient, and an extended visible light response up to 550 nm. The optimized photoanode exhibits state-of-the-art performance: photocurrent densities with and without a hole scavenger of about 1.5 and 0.9 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE), and maximal external quantum efficiencies of 56% and 24%, respectively, alongside an onset potential of 0.3 V.

Carbon Nitrides from Supramolecular Crystals: From Single Atoms to Heterojunctions and Advanced Photoelectrodes.

Carbon nitride materials (CN) have become one of the most studied photocatalysts within the last 15 years. While CN absorbs visible light, its low porosity and fast electron-hole recombination hinder its photoelectric performance and have motivated the research in the modification of its physical and chemical properties (such as energy band structure, porosity, or chemical composition) by different means. In this Concept we review the utilization of supramolecular crystals as CN precursors to tailor its properties. We elaborate on the features needed in a supramolecular crystal to serve as CN precursor, we delve on the influence of metal-free crystals in the morphology and porosity of the resulting materials and then discuss the formation of single atoms and heterojunctions when employing a metal-organic crystal. We finally discuss the performance of CN photoanodes derived from crystals and highlight the current standing challenges in the field.

Long-Term Characterization of Oxidation Processes in Graphitic Carbon Nitride Photocatalyst Materials via Electron Paramagnetic Resonance Spectroscopy.

Graphitic carbon nitride (gCN) materials have been shown to efficiently perform light-induced water splitting, carbon dioxide reduction, and environmental remediation in a cost-effective way. However, gCN suffers from rapid charge-carrier recombination, inefficient light absorption, and poor long-term stability which greatly hinders photocatalytic performance. To determine the underlying catalytic mechanisms and overall contributions that will improve performance, the electronic structure of gCN materials has been investigated using electron paramagnetic resonance (EPR) spectroscopy. Through lineshape analysis and relaxation behavior, evidence of two independent spin species were determined to be present in catalytically active gCN materials. These two contributions to the total lineshape respond independently to light exposure such that the previously established catalytically active spin system remains responsive while the newly observed, superimposed EPR signal is not increased during exposure to light. The time dependence of these two peaks present in gCN EPR spectra recorded sequentially in air over several months demonstrates a steady change in the electronic structure of the gCN framework over time. This light-independent, slowly evolving additional spin center is demonstrated to be the result of oxidative processes occurring as a result of exposure to the environment and is confirmed by forced oxidation experiments. This oxidized gCN exhibits lower H2 production rates and indicates quenching of the overall gCN catalytic activity over longer reaction times. A general model for the newly generated spin centers is given and strategies for the alleviation of oxidative products within the gCN framework are discussed in the context of improving photocatalytic activity over extended durations as required for future functional photocatalytic device development.

Polymeric Carbon Nitride with Chirality Inherited from Supramolecular Assemblies.

The facile synthesis of chiral materials is of paramount importance for various applications. Supramolecular preorganization of monomers for thermal polymerization has been proven as an effective tool to synthesize carbon and carbon nitride-based (CN) materials with ordered morphology and controlled properties. However, the transfer of an intrinsic chemical property, such as chirality from supramolecular assemblies to the final material after thermal condensation, was not shown. Here, we report the large-scale synthesis of chiral CN materials capable of enantioselective recognition. To achieve this, we designed supramolecular assemblies with a chiral center that remains intact at elevated temperatures. The optimized chiral CN demonstrates an enantiomeric preference of ca. 14 %; CN electrodes were also prepared and show stereoselective interactions with enantiomeric probes in electrochemical measurements. By adding chirality to the properties transferrable from monomers to the final product of a thermal polymerization, this study confirms the potential of using supramolecular precursors to produce carbon and CN materials and electrodes with designed chemical properties.

The Directional Crystallization Process of Poly (triazine imide) Single Crystals in Molten Salts.

Poly (triazine imide) photocatalysts prepared via molten salt methods emerge as promising polymer semiconductors with one-step excitation capacity of overall water splitting. Unveiling the molecular conjugation, nucleation, and crystallization processes of PTI crystals is crucial for their controllable structure design. Herein, microscopy characterization was conducted at the PTI crystallization front from meso to nano scales. The heptazine-based precursor was found to depolymerize to triazine monomers within molten salts and KCl cubes precipitate as the leading cores that guide the directional stacking of PTI molecular units to form aggregated crystals. Upon this discovery, PTI crystals with improved dispersibility and enhanced photocatalytic performance were obtained by tailoring the crystallization fronts. This study advances insights into the directional assembling of PTI monomers on salt templates, placing a theoretical foundation for the ordered condensation of polymer crystals.

Efficient Water Cleaning by Self-standing Carbon Nitride Films Derived from Supramolecular Hydrogels.

The design of efficient self-standing hybrid systems for water purification that combines good adsorption properties with high photodegradation ability is highly challenging owing to the difficulty in simultaneously controlling the band structure and porosity of a semiconductor while maintaining its self-standing nature. Here, we report the synthesis of carbon-rich carbon nitride self-standing filters from supramolecular hydrogels composed of melamine and cyanobenzoic acid. The influence of the chemical structure on the properties of the hydrogels and the final films was studied by tuning parameters such as monomer nature, molar ratio, and pyrolysis temperature. Thanks to their ability to combine the adsorption and photodegradation of organic pollutants, the prepared self-standing films showed remarkable activity and stability in flow conditions (>95 % efficiency after 10 consecutive cycles). Additionally, the photocatalytic activity of the films was assessed in the powder form for the hydrogen evolution reaction and photocurrent generation in a photoelectrochemical cell. The reported work opens opportunities for the controlled synthesis of multifunctional filters for water purification and other energy-related and sustainable technologies.

Titanium Vacancies in TiO2 Nanofibers Enable Highly Efficient Photodriven Seawater Splitting.

Photodriven seawater splitting is considered to be one of the most promising techniques for sustainable hydrogen production. However, the high salinity of seawater would deactivate catalysts and consume the photogenerated carriers. Metal vacancies in metal oxide semiconductors are critical to directed electron transfer and high salinity resistance; they are thus desirable but remain a challenge. We demonstrate a facile controllable calcination approach to synthesize TiO2 nanofibers with rich Ti vacancies with excellent photo/electro performances and long-time stability in photodriven seawater splitting, including photocatalysis and photo-electrocatalysis. Experimental measurements and theoretical calculations reveal the formation of titanium vacancies, as well as unidirectional electron trap and superior H+ adsorption ability for efficient charge transfer and resistance to corrosion by seawater. Therefore, atomic-/nanoscale characteristics and mechanism have been proposed to clarify the generation of titanium vacancies and the corresponding interfacial electron transfer.

Titanium Vacancies in TiO2 Nanofibers Enable Highly Efficient Photodriven Seawater Splitting.

Invited for the cover of this issue are Xiao-Yu Yang and co-workers at Wuhan University of Technology, Heinrich-Heine-Universität Düsseldorf, University of the Witwatersrand, and Ben-Gurion University of the Negev. The image depicts Ti vacancies in TiO2 as powerful drivers of photo- and photo-electrocatalytic seawater splitting for hydrogen production. Read the full text of the article at 10.1002/chem.202101817.

Highly Efficient Polymeric Carbon Nitride Photoanode with Excellent Electron Diffusion Length and Hole Extraction Properties.

Polymeric carbon nitride (CN) has emerged as a promising semiconductor in photoanodes for photoelectrochemical cells (PECs) owing to its suitable electronic structure, tunable band gap, high stability, and low price. However, the poor electron diffusion within the CN layer and hole extraction to the solution still limit its applicability in PECs. Here, we report the fabrication of a CN photoanode with excellent electron diffusion length and remarkable hole extraction properties by careful design of its electronic interfaces. We combine complementary synthetic approaches to grow tightly packed CN layers forming a type-II heterojunction, which results in a CN photoanode with excellent charge separation, high electronic conductivity, and remarkable hole extraction efficiency. The optimized CN photoanode displays excellent PEC performance, reaching up to 270 µA cm-2 in a 0.1 M KOH solution at 1.23 V vs RHE, extremely low onset potential (∼0.0012 V), and long-term stability up to 18 h.

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.

Electronic Structure Engineering of Carbon Nitride Materials by Using Polycyclic Aromatic Hydrocarbons.

The design of charge separation sites under illumination in semiconductors is a standing challenge for their utilization as photo(electro)catalysts. Here, the synthesis of modified carbon nitride materials (CNs) with donor-acceptor (D-A) domains, with altering electronic structure, is reported. To do so, new monomers based on polycyclic aromatic hydrocarbons (PAH)-substituted 1,3,5-triazine were designed, which were then embedded within cyanuric acid-melamine supramolecular assemblies to form CN precursors. The conjugation degree of PAHs was systematically changed, from single benzene ring up to pyrene unit, elucidating the role of the conjugation degree on the morphology, structure and electronic properties as well as photo(electro)catalytic activity. The careful design of the D-A sites results in excellent photocatalytic activity as well as long-term stability for the hydrogen evolution reaction. Moreover, PAH-CNs films exhibit enhanced charge separation, optical absorption, electrochemical surface area and electronic conductivity, leading to an outstanding photoelectrochemical (PEC) activity compared to pristine CN.

Carbon Nitride Materials for Water Splitting Photoelectrochemical Cells.

Graphitic carbon nitride materials (CNs) have emerged as suitable photocatalysts and heterogeneous catalysts for various reactions thanks to their tunable band gap, suitable energy-band position, high stability under harsh chemical conditions, and low cost. However, the utilization of CN in photoelectrochemical (PEC) and photoelectronic devices is still at an early stage owing to the difficulties in depositing high-quality and homogenous CN layer on substrates, its wide band gap, poor charge-separation efficiency, and low electronic conductivity. In this Minireview, we discuss the synthetic pathways for the preparation of various structures of CN on substrates and their underlying photophysical properties and current photoelectrochemical performance. The main challenges for CN incorporation into PEC cell are described, together with possible routes to overcome the standing limitations toward the integration of CN materials in PEC and other photoelectronic devices.

Coordination-Directed Growth of Transition-Metal-Crystalline-Carbon Composites with Controllable Metal Composition.

Transition-metal-carbon (CTM) composites show ample activity in many catalytic reactions. However, control of composition, distribution, and properties is challenging. Now, a straightforward path for the synthesis of transition-metal nanoparticles engulfed in crystalline carbon is presented with excellent control over the metal composition, amount, ratio, and catalytic properties. This approach uses molten monomers that coordinate metals ions at high temperature. At high temperatures, strong coordination bonds direct the growth of carbon material with homogeneous metals distribution and with negligible losses, owing to the liquid-like reaction compared to the traditional solid-state reaction. The strength of the approach is demonstrated by the synthesis of mono, binary, and trinary transition-metal-crystalline-carbon composites with tunable and precise elemental composition as well as good electrochemical properties as oxygen evolution reaction electrocatalysts.

Carbon and Nitrogen Based Nanosheets as Fluorescent Probes with Tunable Emission.

2D carbon and nitrogen based semiconductors (CN) have attracted widespread attention for their possible use as low-cost and environmentally friendly materials for various applications. However, their limited solution-dispersibility and the difficulty in preparing exfoliated sheets with tunable photophysical properties restrain their exploitation in imaging-related applications. Here, the synthesis of carbon and nitrogen organic scaffolds with highly tunable optical properties, excellent dispersion in water and DMSO, and good bioimaging properties is reported. Tailored photophysical and chemical properties are acquired by the synthesis of new starting monomers containing different substituent chemical groups with varying electronic properties. Upon monomer condensation at moderate temperature, 350 °C, the starting chemical groups are fully preserved in the final CN. The low condensation temperature and the effective molecular-level modification of the CN scaffold lead to well-dispersed photoluminescent CN thin sheets with a wide range of emission wavelengths. The good bioimaging properties and the tunable fluorescence properties are exemplified by in situ visualization of giant unilamellar vesicles in a buffered aqueous solution as a model system. This approach opens the possibility for the design of tailor-made CN materials with tunable photophysical and chemical properties toward their exploitation in various fields, such as photocatalysis, bioimaging, and sensing.

Unprecedented Centimeter-Long Carbon Nitride Needles: Synthesis, Characterization and Applications.

Free standing centimeter-long 1D nanostructures are highly attractive for electronic and optoelectronic devices due to their unique photophysical and electrical properties. Here a simple, large-scale synthesis of centimeter-long 1D carbon nitride (CN) needles with tunable photophysical, electric, and catalytic properties is reported. Successful growth of ultralong needles is acquired by the utilization of 1D organic crystal precursors comprised of CN monomers as reactants. Upon calcination at high temperatures, the shape of the starting crystal is fully preserved while the CN composition and porosity, and optical and electrical properties can be easily tuned by tailoring the starting elements ratio and final calcination temperature. The facile manipulation and visualization of the CN needles endow their direct electrical measurements by placing them between two conductive probes. Moreover, the CN needles exhibit good photocatalytic activity for hydrogen production owing to their improved light harvesting properties, high surface area, and advantageous energy bands position. The new growth strategy developed here may open opportunities for a rational design of CN and other metal-free materials with controllable directionality and tunable photophysical and electronic properties, toward their utilization in (photo)electronic devices.

Covalent Functionalization of Carbon Nitride Frameworks through Cross-Coupling Reactions.

A new and simple synthetic route is introduced to covalently functionalize the carbon nitride (CN) framework by the implementation of halogenated phenyl groups (Cl, Br and I), which serve as a chemically reactive center, within the CN framework. The covalent modification is demonstrated here by substituting phenyl and tert-butyl propionate onto the modified-CN framework through Suzuki and reductive-Heck cross-coupling reactions, respectively. The effective functionalization leads to a facile exfoliation of the CN framework into thinner layers and greatly enhances the dispersibility in many solvents as well as the photocatalytic activity compared to the unmodified CN. The general covalent modification opens the possibility for tailor-made design of dispersible CN materials, including their photophysical and chemical properties, toward their exploitation in many fields, such as photocatalysis, bio-imaging, sensing, and heterogeneous catalysis.

Tough high modulus hydrogels derived from carbon-nitride via an ethylene glycol co-solvent route.

High concentration formulations of graphitic carbon nitride (g-CN) are utilized as photoinitiator and reinforcer for hydrogels. In order to integrate significant amounts of g-CN, ethylene glycol (EG) is employed as a co-solvent for the gel formation, which enables stable dispersion of up to 4 wt% g-CN. Afterwards, EG can be removed easily via solvent exchange to afford pure hydrogels. The diverse gels possess remarkably high storage moduli (up to 650 kPa for gels and 720 kPa for hydrogels) and compression moduli (up to 9.45 MPa for 4 wt% g-CN EG gel and 3.45 MPa for 4 wt% g-CN hydrogel). Full recovery without energy loss is observed for at least 20 cycles. Moreover, gel formation can be performed in a spatially controlled way utilizing photomasks with desired shapes. Therefore, the suggested method enables formation of hybrid gels by optical lithography with outstanding mechanical properties very similar to natural cartilage and tendon, and opens up opportunities for future applications in photocatalysis, additive manufacturing of biomedical implants and coating materials.