Developing extended visible light responsive polymeric carbon nitrides for photocatalytic and photoelectrocatalytic applications.

Polymeric carbon nitride (CN) has emerged as an attractive material for photocatalysis and photoelectronic devices. However, the synthesis of porous CNs with controlled structural and optical properties remains a challenge, and processable CN precursors are still highly sought after for fabricating homogenous CN layers strongly bound to a given substrate. Here, we report a general method to synthesize highly dispersed porous CN materials that show excellent photocatalytic activity for the hydrogen evolution reaction and good performance as photoanodes in photoelectrochemical cells (PEC): first, supramolecular assemblies of melem and melamine in ethylene glycol and water are prepared using a hydrothermal process. These precursors are then calcined to yield a water-dispersible CN photocatalyst that exhibits beneficial charge separation under illumination, extended visible-light response attributed to carbon doping, and a large number of free amine groups that act as preferential sites for a Pt cocatalyst. The optimized CN exhibits state-of-the-art HER rates up to 23.1 mmol h-1 g-1, with an AQE of 19.2% at 395 nm. This unique synthetic route enables the formation of a homogeneous precursor paste for substrate casting; consequently, the CN photoanode exhibits a low onset potential, a high photocurrent density and good stability after calcination.

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.

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.

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.

Layered Boron-Nitrogen-Carbon-Oxygen Materials with Tunable Composition as Lithium-Ion Battery Anodes.

The insertion of heteroatoms with different electronegativity into carbon materials can tune their chemical, electronic, and optical properties. However, in traditional solid-state synthesis, it is challenging to control the reactivity of monomers, and therefore, the amount and position of heteroatoms in the final materials. Herein, a simple, scalable, and general molten-state route to synthesize boron-nitrogen-carbon-oxygen (BNCO) materials with tunable boron-nitrogen-carbon composition, as well as electronic and optical properties, is reported. The new synthetic approach consists of polycyclic aromatic hydrocarbons (PAHs) and ammonia-borane as reactants that form a clear liquid-state stage spanning a wide temperature range, before the solid-state reaction. The molten-state stage enhances the control over the synthetic intermediates and final materials, owing to improved monomer miscibility and reactivity. The BNCO composition and optical properties are tuned by the PAH selection and final reaction temperature. The advantages of this method are demonstrated herein through the tunable optical properties, excellent stability to oxidization, facile deposition on substrates, and good activity as an anode material in lithium-ion batteries.

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.

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.