In-situ Monitoring of Photocatalysis on Polymeric Carbon Nitride Thin Films.

Polymeric carbon nitride has attracted significant interest in heterogeneous photocatalysis due to its activity under visible-light irradiation. Herein, we report on using carbon nitride-coated NMR tubes for in-situ studies of photocatalytic reaction mechanisms. In a first step, we exploited carbon nitride-coated crimp vials as batch photoreactors for visible photocatalytic fluorinations of unactivated C(sp3)-H bonds, with moderate to excellent yields and reusability over multiple cycles. Eventually, carbon nitride-coated NMR tubes were used as a photoreactor by coupling them with optical fiber irradiation directly inside the spectrometer. This enabled us to follow the reaction with in-situ NMR spectroscopy identifying reactive intermediates otherwise elusive in conventional analyses. The method provides advantages for the study of photocatalytic mechanisms of complex reactions and substantially reduces the need of comparative tests for depicting reaction intermediates and conversion pathways.

Optimizing reaction conditions for the light-driven hydrogen evolution in a loop photoreactor.

Photocatalytic hydrogen production from water is a promising way to fulfill energy demands and attain carbon emission reduction goals effectively. In this study, a loop photoreactor with a total volume of around 500 mL is presented for the photocatalytic hydrogen evolution using a Pt-loaded polymeric carbon nitride photocatalyst under 365 nm irradiation in the presence of sacrificial reducing agents. The fluid flow pattern of the developed photoreactor was characterized experimentally and the photon flux incident to the loop photoreactor was measured by chemical actinometry. The system displayed exceptional stability, with operation sustained over 70 hours. A design of experiment (DOE) analysis was used to systematically investigate the influence of key parameters - photon flux, photocatalyst loading, stirring speed, and inert gas flow rate - on the hydrogen generation rate. Linear relationships were found between hydrogen evolution rate and photon flux as well as inert gas flow rate. Photocatalyst loading and stirring speed also showed linear correlations, but could not be correctly described by DOE analysis. Instead, linear single parameter correlations could be applied. Notably, the loop photoreactor demonstrated an external photon efficiency up to 17 times higher than reported in literature studies, while scaling the reactor size by a factor of 10.

Twisty C-TiO2 /PCN S-Scheme Heterojunction with Enhanced n→π * Electronic Excitation for Promoted Piezo-Photocatalytic Effect.

Herein, a twisty C-TiO2 /PCN (CNT) Step-scheme (S-scheme) heterojunction is fabricated and applied to degrade ciprofloxacin (CIP) with the assistance of ultrasonic vibration and visible light irradiation. The nitrogen-rich twisty polymeric carbon nitride (PCN) can not only induce a non-centrosymmetric structure with enhanced polarity for a better piezoelectric effect but also provide abundant lone pair electrons to promote n→π* transition during photocatalysis. Its hybridization with C-TiO2 particles can construct S-scheme heterojunction in CNT. During the piezo-photocatalysis, the strain-induced polarization electric field in the heterojunction can regulate the electron migration between the two components, resulting in a more effective CIP degradation. With the synergistic effect of ultrasonic vibration and visible light irradiation, the reaction rate constant of CIP degradation by CNT increases to 0.0517 min-1 , which is 1.86 times that of photocatalysis and 6.46 times that of ultrasound. This system exhibits a stable CIP decomposition efficiency under the interference of various environmental factors. In addition, the in-depth investigation found that three pathways and 12 major intermediates with reduced toxicity are produced after the reaction. Hopefully, the construction of this twisty CNT S-scheme heterojunction with enhanced piezo-photocatalytic effect offers inspiration for the design of environmentally functional materials.

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.

Bioinspired Photocatalytic NADH Regeneration by Covalently Metalated Carbon Nitride for Enhanced CO2 Reduction.

Natural photosynthesis is a highly unified biocatalytic system, which coupled cofactor (NAD(P)H) regeneration and enzymatic CO2 reduction efficiently for solar energy conversion. Mimicking nature, a novel system with Rh complex covalently grafted onto NH2 -functionalized polymeric carbon nitride (NH2 -PCN) was constructed. The integrated connection of the light-harvesting and electron mediation modules as Rhm3 -N-PCN could promote the efficient NAD+ reduction to NADH. As a result, the integrated system exhibited a conversion of ∼66 % within 20 minutes. By further coupling in situ generated NADH with formate dehydrogenase (FDH), a photoenzymatic production of formic acid (HCOOH) from CO2 was accomplished. Moreover, by immobilizing FDH onto a hydrophobic membrane, an enhanced HCOOH production of ∼5.0 mM can be obtained due to the concentrated CO2 on the gas-liquid-solid three-phase interface. Our work herein provides an integrated strategy for coupling the anchored electron mediator with immobilized enzyme for enhanced artificial photosynthesis.

Improved Charge Separation in Poly(heptazine-triazine) Imides with Semi-coherent Interfaces for Photocatalytic Hydrogen Evolution.

The construction of heterojunctions is a promising manner to accelerate the separation and transfer of the charge carriers at the interface. Herein, a binary poly(heptazine-triazine) imides (PHI/PTI) with semi-coherent interfaces was fabricated via a facile two-step salt-melt synthetic process. The built-in electric fields at the semi-coherent interface endow prompt exciton splitting and charge carrier separation. Hence, the optimized PHI/PTI-based copolymer presents a high apparent quantum yield (AQY=64 %) for visible-light driven hydrogen production, by the aids of K2 HPO4 as charge transfer mediator. This study provides physical insights for the rational promotion of the photocatalytic performance from the viewpoint of interfacial engineering of photocatalytic junctions on crystalline carbon nitride based semiconductors.

Highly Selective Photocatalytic CO2 Methanation with Water Vapor on Single-Atom Platinum-Decorated Defective Carbon Nitride.

Solar-driven CO2 methanation with water is an important route to simultaneously address carbon neutrality and produce fuels. It is challenging to achieve high selectivity in CO2 methanation due to competing reactions. Nonetheless, aspects of the catalyst design can be controlled with meaningful effects on the catalytic outcomes. We report highly selective CO2 methanation with water vapor using a photocatalyst that integrates polymeric carbon nitride (CN) with single Pt atoms. As revealed by experimental characterization and theoretical simulations, the widely explored Pt-CN catalyst is adapted for selective CO2 methanation with our rationally designed synthetic method. The synthesis creates defects in CN along with formation of hydroxyl groups proximal to the coordinated Pt atoms. The photocatalyst exhibits high activity and carbon selectivity (99 %) for CH4 production in photocatalytic CO2 reduction with pure water. This work provides atomic scale insight into the design of photocatalysts for selective CO2 methanation.

Pt Atoms/Clusters on Ni-phytate-sensitized Carbon Nitride for Enhanced NIR-light-driven Overall Water Splitting beyond 800†nm.

Near-infrared (NIR) light-driven overall water splitting beyond 800 nm remains a high-priority target yet great challenge. Here we report that efficient utilization of photogenerated electrons in a photosensitized system prepared by site-selective photodeposition of platinum single atoms/clusters (Pt-SACs) on Ni-phytate (PA-Ni)-sensitized polymeric carbon nitride (PCN). The optimal catalyst presents simultaneous hydrogen (H2 ) and oxygen (O2 ) evolution with an H2 evolution amount of 1.4 µmol at λ>800 nm for 24 hours, which its activity was approximately 140 times higher than that of a system without Pt-SAC modification (PA-Ni1.1 @PCN). This work represents the first NIR-light responsive photosensitized system for overall water splitting, and may open an avenue for precisely manipulating cocatalyst positions at the atomic level to improve NIR-light-driven overall water splitting via photosensitization.

Polymeric Carbon Nitride-Derived Photocatalysts for Water Splitting and Nitrogen Fixation.

Photocatalysis is a promising energy conversion and environmental restoration technology. The main focus of photocatalysis is the development and manufacture of highly efficient photocatalysts. Semiconductor-based photocatalysis technology based on harnessing solar energy is considered as an attractive approach to solve the problems of global energy shortage and environmental pollution. Since 2009 pioneering work has been carried out on polymeric carbon nitride (PCN) for visible photocatalytic water splitting, thus PCN-based photocatalysis has become a hot research topic, demanding significant research attention. This article reviews the physical and chemical properties, synthesis methods, and the methods to control the morphology, heteroatom doping, and construction of heterojunctions to improve the performance of PCN-based photocatalysts in water splitting and nitrogen fixation. Through different design strategies, the photo-generated electron-hole pair separation efficiency of PCN materials can be effectively improved, thereby improving their photocatalytic performance. Finally, the challenges of PCN-based photocatalysts in water splitting and nitrogen fixation applications are discussed herein. It is strongly believed that through different design strategies, efficient PCN-based photocatalysts can be constructed for both water splitting and nitrogen reduction. These excellent modification strategies can be used as a guiding theory for photocatalytic reactions of other promising catalysts and further promote the development of photocatalysis.

Photodepositing CdS on the Active Cyano Groups Decorated g-C3 N4 in Z-Scheme Manner Promotes Visible-Light-Driven Hydrogen Evolution.

g-C3 N4 /CdS heterojunctions are potential photocatalysts for hydrogen production but their traditional type-II configuration generally leads to weak oxidative and reductive activity. How to construct the novel Z-scheme g-C3 N4 /CdS counterparts to address this issue remains a great challenge in this field. In this work, a new direct Z-scheme heterojunction of defective g-C3 N4 /CdS is designed by introducing cyano groups (NC-) as the active bridge sites. Experimental observations in combination with density functional theory (DFT) calculations reveal that the unique electron-withdrawing feature of cyano groups in the defective g-C3 N4 /CdS heterostructure can endow this photocatalyst with numerous advantageous properties including high light absorption ability, strong redox performance, satisfactory charge separation efficiency, and long lifetime of charge carriers. Consequently, the resultant photocatalytic system exhibits more active performance than CdS and g-C3 N4 under visible light and reaches an excellent hydrogen evolution rate of 1809.07 µmol h-1 g-1 , which is 6.09 times higher than pristine g-C3 N4 . Moreover, the defective g-C3 N4 /CdS photocatalyst maintains good stability after 40 h continuous test. This work provides new insights into design and construction of Z-scheme heterojunctions for regulating the visible-light-induced photocatalytic activity for H2 evolution.

0D, 1D, 2D molybdenum disulfide functionalized by 2D polymeric carbon nitride for photocatalytic water splitting.

Photocatalytic activity of molybdenum disulfide structures with different dimensions (0D, 1D and 2D) functionalized with polymeric carbon nitride (PCN) is presented. MoS2nanotubes (1D), nanoflakes (2D) and quantum dots (0D, QDs) were used, respectively, as co-catalysts of PCN in photocatalytic water splitting reaction to evolve hydrogen. Although, 2D-PCN showed the highest light absorption in visible range and the most enhanced photocurrent response after irradiation with light from 460 to 727 nm, QDs-PCN showed the highest photocatalytic efficiency. The detailed analysis revealed that the superior photocatalytic activity of QDs-PCN in comparison with other structures of MoS2arose from (i) the most effective separation of photoexcited electron-hole pairs, (ii) the most enhanced up-converted photoluminescence (UCPL), (iii) the highest reactivity of electrons in conduction band. Moreover, a narrowed size of QDs affected shorter diffusion path of charge carriers to active reaction sites, higher number of the sites and higher interfacial area between molybdenum disulfide and PCN.

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.

A NIR-Responsive Phytic Acid Nickel Biomimetic Complex Anchored on Carbon Nitride for Highly Efficient Solar Hydrogen Production.

A challenge in photocatalysis consists in improving the efficiency by harnessing a large portion of the solar spectrum. We report the design and realization of a robust molecular-semiconductor photocatalytic system (MSPS) consisting of an earth-abundant phytic acid nickel (PA-Ni) biomimetic complex and polymeric carbon nitride (PCN). The MSPS exhibits an outstanding activity at λ=940 nm with high apparent quantum efficiency (AQE) of 2.8 %, particularly λ>900 nm, as it outperforms all reported state-of-the-art near-infrared (NIR) hybrid photocatalysts without adding any noble metals. The optimum hydrogen (H2 ) production activity was about 52 and 64 times higher with respect to its pristine counterpart under the AM 1.5 G and visible irradiation, respectively, being equivalent to the platinum-assisted PCN. This work sheds light on feasible avenues to prepare highly active, stable, cheap NIR-harvesting photosystems toward sustainable and scalable solar-to-H2 production.

Exciton-Mediated Energy Transfer in Heterojunction Enables Infrared Light Photocatalysis.

Although a few semiconductors can directly absorb infrared light, their intrinsic properties like improper band-edge position and strong electron-hole interaction restrict further photocatalytic applications. Herein, we propose an exciton-mediated energy transfer strategy for realizing efficient infrared light response in heterostructures. Using black phosphorous/polymeric carbon nitride (BP/CN) heterojunction, CN could be indirectly excited by infrared light with the aid of nonradiatively exciton-based energy transfer from BP. At the same time, excitons are dissociated into free charge carriers at the interface of BP/CN heterojunction, followed by hole injection to BP and electron retainment in CN. As a result of these unique photoexcitation processes, BP/CN heterojunction exhibits promoted conversion rate and selectivity in amine-amine oxidative coupling reaction even under infrared light irradiation. This study opens a new way for the design of efficient infrared light activating photocatalysts.

Iodide-Induced Fragmentation of Polymerized Hydrophilic Carbon Nitride for High-Performance Quasi-Homogeneous Photocatalytic H2 O2 Production.

Polymeric carbon nitride (PCN) as a class of two-electron oxygen reduction reaction (2 e- ORR) photocatalyst has attracted much attention for H2 O2 production. However, the low activity and inferior selectivity of 2 e- ORR greatly restrict the H2 O2 production efficiency. Herein, we develop a new strategy to synthesize hydrophilic, fragmented PCN photocatalyst by the terminating polymerization (TP-PCN) effect of iodide ions. The obtained TP-PCN with abundant edge active sites (AEASs), which can form quasi-homogeneous photocatalytic system, exhibits superior H2 O2 generation rate (3265.4 µM h-1 ), far surpassing PCN and other PCN-based photocatalysts. DFT calculations further indicate that TP-PCN is more favorable for electron transiting from ß spin-orbital to the π* orbitals of O2 , which optimizes O2 activation and reduces the energy barrier of H2 O2 formation. This work provides a new concept for designing functional photocatalysts and understanding the mechanism of O2 activation in ORR for H2 O2 production.

A [001]-Oriented Hittorf's Phosphorus Nanorods/Polymeric Carbon Nitride Heterostructure for Boosting Wide-Spectrum-Responsive Photocatalytic Hydrogen Evolution from Pure Water.

Red phosphorus is a promising photocatalyst with wide visible-light absorption up to 700 nm, but the fast charge recombination limits its photocatalytic hydrogen evolution reaction (HER) activity. Now, [001]-oriented Hittorf's phosphorus (HP) nanorods were successfully grown on polymeric carbon nitride (PCN) by a chemical vapor deposition strategy. Compared with the bare PCN and HP, the optimized PCN@HP hybrid exhibited a significantly enhanced photocatalytic activity, with HER rates reaching 33.2 and 17.5 µmol h-1 from pure water under simulated solar light and visible light irradiation, respectively. It was theoretically and experimentally indicated that the strong electronic coupling between PCN and [001]-oriented HP nanorods gave rise to the enhanced visible light absorption and the greatly accelerated photoinduced electron-hole separation and transfer, which benefited the photocatalytic HER performance.

Designing a 0D/2D S-Scheme Heterojunction over Polymeric Carbon Nitride for Visible-Light Photocatalytic Inactivation of Bacteria.

Constructing heterojunctions between two semiconductors with matched band structure is an effective strategy to acquire high-efficiency photocatalysts. The S-scheme heterojunction system has shown great potential in facilitating separation and transfer of photogenerated carriers, as well as acquiring strong photoredox ability. Herein, a 0D/2D S-Scheme heterojunction material involving CeO2 quantum dots and polymeric carbon nitride (CeO2 /PCN) is designed and constructed by in situ wet chemistry with subsequent heat treatment. This S-scheme heterojunction material shows high-efficiency photocatalytic sterilization rate (88.1 %) towards Staphylococcus aureus (S. aureus) under visible-light irradiation (λ≥420 nm), which is 2.7 and 8.2 times that of pure CeO2 (32.2 %) and PCN (10.7 %), respectively. Strong evidence of S-scheme charge transfer path is verified by theoretical calculations, in situ irradiated X-ray photoelectron spectroscopy, and electron paramagnetic resonance.

Electron Configuration Modulation of Nickel Single Atoms for Elevated Photocatalytic Hydrogen Evolution.

The emerging metal single-atom catalyst has aroused extensive attention in multiple fields, such as clean energy, environmental protection, and biomedicine. Unfortunately, though it has been shown to be highly active, the origins of the activity of the single-atom sites remain unrevealed to date owing to the lack of deep insight on electronic level. Now, partially oxidized Ni single-atom sites were constructed in polymeric carbon nitride (CN), which elevates the photocatalytic performance by over 30-fold. The 3d orbital of the partially oxidized Ni single-atom sites is filled with unpaired d-electrons, which are ready to be excited under irradiation. Such an electron configuration results in elevated light response, conductivity, charge separation, and mobility of the photocatalyst concurrently, thus largely augmenting the photocatalytic performance.

Artificial Photosynthesis with Polymeric Carbon Nitride: When Meeting Metal Nanoparticles, Single Atoms, and Molecular Complexes.

Artificial photosynthesis for solar water splitting and CO2 reduction to produce hydrogen and hydrocarbon fuels has been considered as one of the most promising ways to solve increasingly serious energy and environmental problems. As a well-documented metal-free semiconductor, polymeric carbon nitride (PCN) has been widely used and intensively investigated for photocatalytic water splitting and CO2 reduction, owing to its physicochemical stability, visible-light response, and facile synthesis. However, PCN as a photocatalyst still suffers from the fast recombination of electron-hole pairs and poor water redox reaction kinetics, greatly restricting its activity for artificial photosynthesis. Among the various modification approaches developed so far, decorating PCN with metals in different existences of nanoparticles, single atoms and molecular complexes, has been evidently very effective to overcome these limitations to improve photocatalytic performances. In this Review article, a systematic introduction to the state-of-the-art metal/PCN photocatalyst systems is given, with metals in versatility of nanoparticles, single atoms, and molecular complexes. Then, the recent processes of the metal/PCN photocatalyst systems in the applications of artificial photosynthesis, e.g., water splitting and CO2 reduction, are reviewed. Finally, the remaining challenges and opportunities for the development of high efficiency metal/PCN photocatalyst systems are presented and prospected.

Polymeric Donor-Acceptor Heterostructures for Enhanced Photocatalytic H2 Evolution without Using Pt Cocatalysts.

Polymeric carbon nitride (CN) is a promising material for photocatalytic water splitting. However, CN in its pristine form tends to show moderate activity due to fast recombination of the charge carriers. The design of efficient photocatalytic system is therefore highly desired, but it still remains a great challenge in chemistry. In this work, a pyrene-based polymer able to serve as an electron donor to accelerate the interface charge carrier transfer of CN is presented. The construction of donor-acceptor (D-A) heterojunction was confirmed to significantly restrain the charge recombination and, thus, improve the proton reduction process. This study provides a promising strategy to achieve solar H2 production in an efficient and low-cost manner.