Triazine-based Carbon Nitrides for Visible-Light-Driven Hydrogen Evolution

Improved Photocatalytic H2 Evolution over G-Carbon Nitride with Enhanced In-Plane Ordering

A series of rod-like porous graphitic-carbon nitrides (short as CNs) with enhanced in-plane ordering have been fabricated through self-assembled heptazine hydrate precursors for the first time. By controlling the calcination of the preformed precursors with different temperature-rising rates, the resulted CNs (SAHEP-CNs-1) with the most ordered and least stacked graphitic planar are showing a tremendously improved hydrogen evolution rate of 420 μmol h^-1 under visible light and a remarkable apparent quantum efficiency of 8.9% at 420 nm, which is among the highest values for C₃ N₄ -related photocatalysts in the literature. This work discloses that enhancing in-plane ordering is one critical factor for improving the photocatalytic H₂ evolution of carbon nitride, which is an effective solution to prolong the lifetime of charge carriers by accelerating the charge transport and separation within the graphitic planar. This finding would present a facial strategy for the designing of efficient organic semiconductors for photocatalysis.

In Situ Solid-State Synthesis of a AgNi/g-C3 N4 Nanocomposite for Enhanced Photoelectrochemical and Photocatalytic Activity

A graphitic carbon nitride (g-C₃ N₄ ) polymer matrix was embedded with AgNi alloy nanoparticles using a simple and direct in situ solid-state heat treatment method to develop a novel AgNi/g-C₃ N₄ photocatalyst. The characterization confirms that the AgNi alloy particles are homogeneously distributed throughout the g-C₃ N₄ matrix. The catalyst shows excellent photoelectrochemical activity for water splitting with a maximum photocurrent density of 1.2 mA cm^-2 , which is the highest reported for doped g-C₃ N₄ . Furthermore, a detailed experimental study of the photocatalytic degradation of Rhodamine B (RhB) dye using doped g-C₃ N₄ showed the highest reported degradation efficiency of approximately 95 % after 90 min. The electronic conductivity increased upon incorporation of AgNi alloy nanoparticles on g-C₃ N₄ and the material showed efficient charge carrier separation and transfer characteristics, which are responsible for the enhanced photoelectrochemical and photocatalytic performance under visible light.

Self-Assembled PDINH Supramolecular System for Photocatalysis under Visible Light

A self-assembled perylene-3,4,9,10-tetracarboxylic diimide(PDINH) supramolecular system consisting of all-organic PDINH molecule building blocks through non-covalent interactions works as a visible light photocatalyst with high activity.

Rational design of carbon nitride photocatalysts by identification of cyanamide defects as catalytically relevant sites

The heptazine-based polymer melon (also known as graphitic carbon nitride, g-C₃N₄) is a promising photocatalyst for hydrogen evolution. Nonetheless, attempts to improve its inherently low activity are rarely based on rational approaches because of a lack of fundamental understanding of its mechanistic operation. Here we employ molecular heptazine-based model catalysts to identify the cyanamide moiety as a photocatalytically relevant ‘defect'. We exploit this knowledge for the rational design of a carbon nitride polymer populated with cyanamide groups, yielding a material with 12 and 16 times the hydrogen evolution rate and apparent quantum efficiency (400 nm), respectively, compared with the unmodified melon. Computational modelling and material characterization suggest that this moiety improves coordination (and, in turn, charge transfer kinetics) to the platinum co-catalyst and enhances the separation of the photogenerated charge carriers. The demonstrated knowledge transfer for rational catalyst design presented here provides the conceptual framework for engineering high-performance heptazine-based photocatalysts.

Graphitic carbon nitride is a promising hydrogen evolution photocatalyst, although there is limited understanding of its mechanistic operation. Here, the authors employ molecular heptazine-based model catalysts to identify catalytically relevant defects and to rationally design a highly active carbon nitride photocatalyst.

Modelling materials for solar fuel synthesis by artificial photosynthesis; predicting the optical, electronic and redox properties of photocatalysts

In this mini-review, we discuss what insight computational modelling can provide into the working of photocatalysts for solar fuel synthesis and how calculations can be used to screen for new promising materials for photocatalytic water splitting and carbon dioxide reduction. We will extensively discuss the different relevant (material) properties and the computational approaches (DFT, TD-DFT, GW/BSE) available to model them. We illustrate this with examples from the literature, focussing on polymeric and nanoparticle photocatalysts. We finish with a perspective on the outstanding conceptual and computational challenges.

Two physical strategies to reinforce a nonmetallic photocatalyst, g-C3N4: vacuum heating and electron beam irradiation

Herein, we demonstrated two physical strategies, namely, vacuum heating and electron beam irradiation, to reinforce a nonmetallic photocatalyst, g-C₃N₄.

Covalent organic frameworks based on Schiff-base chemistry: synthesis, properties and potential applications

Covalent organic-frameworks (COFs) are an emerging class of porous and ordered materials formed by condensation reactions of organic molecules.

Improving the photocatalytic activity of s-triazine based graphitic carbon nitride through metal decoration: an ab initio investigation

Through density functional theory calculations, we attempted to tune the electronic band structure of poly s-triazine based graphitic carbon nitride by decorating it with different metal atoms and clusters for improving its photocatalytic activity towards solar water splitting.

A facile one-step synthesis of ZnO quantum dots modified poly(triazine imide) nanosheets for enhanced hydrogen evolution under visible light

PTI/ZnO-QDs, a novel heterostructured photocatalyst, has been synthesized via a “one-pot” protocol, and exhibits significantly enhanced photocatalytic activity.

Air activation by a metal-free photocatalyst for “totally-green” hydrocarbon selective oxidation

We demonstrate that polytriazine imide (PTI) is an efficient photocatalyst for cyclohexane selective oxidation using air and water without any other oxidants, initiators and solvents.

Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts

Linear poly(p‐phenylene)s are modestly active UV photocatalysts for hydrogen production in the presence of a sacrificial electron donor. Introduction of planarized fluorene, carbazole, dibenzo[b,d]thiophene or dibenzo[b,d]thiophene sulfone units greatly enhances the H₂ evolution rate. The most active dibenzo[b,d]thiophene sulfone co‐polymer has a UV photocatalytic activity that rivals TiO₂, but is much more active under visible light. The dibenzo[b,d]thiophene sulfone co‐polymer has an apparent quantum yield of 2.3 % at 420 nm, as compared to 0.1 % for platinized commercial pristine carbon nitride.

Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts

Linear poly(p‐phenylene)s are modestly active UV photocatalysts for hydrogen production in the presence of a sacrificial electron donor. Introduction of planarized fluorene, carbazole, dibenzo[b,d]thiophene or dibenzo[b,d]thiophene sulfone units greatly enhances the H₂ evolution rate. The most active dibenzo[b,d]thiophene sulfone co‐polymer has a UV photocatalytic activity that rivals TiO₂, but is much more active under visible light. The dibenzo[b,d]thiophene sulfone co‐polymer has an apparent quantum yield of 2.3 % at 420 nm, as compared to 0.1 % for platinized commercial pristine carbon nitride.

A tunable azine covalent organic framework platform for visible light-induced hydrogen generation

Hydrogen evolution from photocatalytic reduction of water holds promise as a sustainable source of carbon-free energy. Covalent organic frameworks (COFs) present an interesting new class of photoactive materials, which combine three key features relevant to the photocatalytic process, namely crystallinity, porosity and tunability. Here we synthesize a series of water- and photostable 2D azine-linked COFs from hydrazine and triphenylarene aldehydes with varying number of nitrogen atoms. The electronic and steric variations in the precursors are transferred to the resulting frameworks, thus leading to a progressively enhanced light-induced hydrogen evolution with increasing nitrogen content in the frameworks. Our results demonstrate that by the rational design of COFs on a molecular level, it is possible to precisely adjust their structural and optoelectronic properties, thus resulting in enhanced photocatalytic activities. This is expected to spur further interest in these photofunctional frameworks where rational supramolecular engineering may lead to new material applications.

Production of hydrogen via the photocatalytic reduction of water is an attractive source of energy, but the catalysts are often expensive and possess little room for modification. Here, the authors show that covalent organic frameworks can be tuned for optimal photocatalytic performance.

Covalent Triazine-Based Frameworks as Visible Light Photocatalysts for the Splitting of Water

Covalent triazine-based frameworks (CTFs) with a graphene-like layered morphology have been controllably synthesized by the trifluoromethanesulfonic acid-catalyzed nitrile trimerization reactions at room temperature via selecting different monomers. Platinum nanoparticles are well dispersed in CTF-T1, which is ascribed to the synergistic effects of the coordination of triazine moieties and the nanoscale confinement effect of CTFs. CTF-T1 exhibits excellent photocatalytic activity and stability for H2 evolution in the presence of platinum under visible light irradiation (λ ≥ 420 nm). The activity and stability of CTF-T1 are comparable to those of g-C3 N4 . Importantly, as a result of the tailorable electronic and spatial structures of CTFs that can be achieved through the judicial selection of monomers, CTFs not only show great potential as organic semiconductor for photocatalysis but also may provide a molecular-level understanding of the inherent heterogeneous photocatalysis.

Polymeric photocatalysts based on graphitic carbon nitride

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

Structural distortion in graphitic-C3N4 realizing an efficient photoreactivity

Introducing structural distortion to semiconductors can dramatically modify their electronic structures, resulting in efficient separation of electron-hole pairs and achieving high photocatalytic activity of catalysts. Herein, we systematically studied the role that structural distortion played in the photocatalytic process by taking graphitic-C3N4 (g-C3N4) as an example, where the structural distortion can be introduced by elemental doping and heat treatment. Through the controllable structural distortion engineering, the photocatalytic activity of g-C3N4 can be significantly improved, which benefits from the effective separation of photogenerated electron-hole pairs, showing intriguing structural distortion-dependent photocatalytic activity. This study not only offers a new insight into the in-depth understanding of the effect of structural distortion on the photoreactivity of catalysts, but also provides a new pathway for designing advanced photocatalysts.

Increased active sites and their accessibility of a N-doped carbon nanotube carbocatalyst with remarkably enhanced catalytic performance in direct dehydrogenation of ethylbenzene

This work presents a facile, low-cost, but efficient strategy for synthesizing HN-CNTs with enlarged active sites and their accessibility to reactants for the direct dehydrogenation of ethylbenzene.

Nitrogen-doped nanotubes-decorated activated carbon-based hybrid nanoarchitecture as a superior catalyst for direct dehydrogenation

Nitrogen-doped activated carbon-based nanoarchitecture fabricated through a facile pyrolysis approach demonstrates enhanced catalysis in the oxidant- and steam-free dehydrogenation of ethylbenzene.

C3N4-sensitized TiO2 nanotube arrays with enhanced visible-light photoelectrochemical performance

A C₃N₄-sensitized TiO₂ nanotube array-based photoanode was designed and fabricated via the in situ growth of C₃N₄ on the surface of TiO₂ nanotube, which shows stable and significantly improved PEC activity for the hydrogen generation under visible light irradiation.

Ultra-thin C3N4 nanosheets for rapid charge transfer in the core–shell heterojunction of α-sulfur@C3N4 for superior metal-free photocatalysis under visible light

Enhanced visible-light-driven photocatalytic ability is obtained by fabricating an α-S@C₃N₄ heterojunction with ultra-thin C₃N₄ nanosheet as the means of rapid charge transfer.

Photocatalytic hydrogen production using polymeric carbon nitride with a hydrogenase and a bioinspired synthetic Ni catalyst

Solar-light-driven H2 production in water with a [NiFeSe]-hydrogenase (H2ase) and a bioinspired synthetic nickel catalyst (NiP) in combination with a heptazine carbon nitride polymer, melon (CN(x)), is reported. The semibiological and purely synthetic systems show catalytic activity during solar light irradiation with turnover numbers (TONs) of more than 50,000 mol H2(mol H2ase)(-1) and approximately 155 mol H2 (mol NiP)(-1) in redox-mediator-free aqueous solution at pH 6 and 4.5, respectively. Both systems maintained a reduced photoactivity under UV-free solar light irradiation (λ>420 nm).

Photocatalytic Hydrogen Production using Polymeric Carbon Nitride with a Hydrogenase and a Bioinspired Synthetic Ni Catalyst

Solar-light-driven H₂ production in water with a [NiFeSe]-hydrogenase (H₂ase) and a bioinspired synthetic nickel catalyst (NiP) in combination with a heptazine carbon nitride polymer, melon (CN_x), is reported. The semibiological and purely synthetic systems show catalytic activity during solar light irradiation with turnover numbers (TONs) of more than 50 000 mol H₂ (mol H₂ase)⁻¹ and approximately 155 mol H₂ (mol NiP)⁻¹ in redox-mediator-free aqueous solution at pH 6 and 4.5, respectively. Both systems maintained a reduced photoactivity under UV-free solar light irradiation (λ>420 nm).

Highly efficient photocatalytic H₂ evolution from water using visible light and structure-controlled graphitic carbon nitride

The major challenge of photocatalytic water splitting, the prototypical reaction for the direct production of hydrogen by using solar energy, is to develop low-cost yet highly efficient and stable semiconductor photocatalysts. Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g-C3N4) from a low-cost precursor, urea, is reported. The g-C3N4 exhibits an extraordinary hydrogen-evolution rate (ca. 20,000 μmol h(-1) g(-1) under full arc), which leads to a high turnover number (TON) of over 641 after 6 h. The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5% under visible light, which is nearly an order of magnitude higher than that observed for any other existing g-C3N4 photocatalysts. Furthermore, it was found by experimental analysis and DFT calculations that as the degree of polymerization increases and the proton concentration decreases, the hydrogen-evolution rate is significantly enhanced.

g-C3N4-Based Photocatalysts for Hydrogen Generation

Graphitic carbon nitride (g-C3N4)-based photocatalysts have attracted dramatically increasing interest in the area of visible-light-induced photocatalytic hydrogen generation due to the unique electronic band structure and high thermal and chemical stability of g-C3N4. This Perspective summarizes the recent significant advances on designing high-performance g-C3N4-based photocatalysts for hydrogen generation under visible-light irradiation. The rational strategies such as nanostructure design, band gap engineering, dye sensitization, and heterojunction construction are described. Finally, this Perspective highlights the ongoing challenges and opportunities for the future development of g-C3N4-based photocatalysts in the exciting research area.